Enhanced oxygenates formation in the Fischer?Tropsch synthesis over co- And/or Ni-containing Fe alloys: Characterization and 2D gas chromatographic product analysis

pubs.acs.org/acscatalysis

Research Article

Enhanced Oxygenates Formation in the Fischer−Tropsch Synthesis

over Co- and/or Ni-Containing Fe Alloys: Characterization and 2D

Gas Chromatographic Product Analysis

Mohamed I. Fadlalla, Sundaram Ganesh Babu, Thulani M. Nyathi, C. J. Kees-Jan Weststrate,

Nico Fischer, J. W. Hans Niemantsverdriet, and Michael Claeys *

Cite This: ACS Catal. 2020, 10, 14661 −14677

Read Online

sı

*

Metrics & More

Article Recommendations

Supporting Information

ACCESS

ABSTRACT: Transition metal alloys are receiving considerable

attention in heterogeneous catalysis as they hold promise to

combine advantageous properties of the constituting metals and,

therefore, provide attractive avenues for targeted catalyst design.

The present study concerns the eﬀect of Co and Ni substituents in

the ferrite (Fe₃O₄) structure used as a catalyst precursor for

medium-temperature Fischer−Tropsch (MTFT) synthesis, in

anticipation of enhanced oxygenate selectivities. The ferrites

were synthesized by co-precipitation and characterized in detail

before and after exposure to MTFT conditions, employing both

conventional ex situ and state of the art in situ techniques. The

complex product spectrum from the MTFT was analyzed by

combining oﬀ-line one-dimensional and on-line two-dimensional

gas chromatography. The latter was used speciﬁcally to investigate the formation of minority species, such as oxygenates, which are

often disregarded in literature. In situ XRD and magnetometry showed no notable change in the reduction behavior of the ferrites

with a cobalt substituent, but substituting with Ni decreased the reduction temperature drastically from 315 to 250 °C, most likely

due to the increased hydrogen dissociation activity of Ni. The activity, CO conversion, in MTFT increased in the order Fe ≪ CoFe

< NiFe < CoNiFe. Incorporation of Co and Ni in the catalysts makes them less prone to deposition of inactive carbon. The addition

of Ni speciﬁcally, also results in a signiﬁcant shift in selectivity toward a shorter average chain length, lower oleﬁnicity and higher

water−gas shift activity. Interestingly, these shifts are paralleled by a 76% or 170% increase in C₂₊oxygenates selectivity or yield,

respectively. The increase in hydrogenation activity of substituted (i.e., Co and/or Ni) Fe-based catalysts, plays a critical role in the

Fischer−Tropsch synthesis activity and selectivity to the diﬀerent product classes (i.e., paraﬃns, oleﬁns, and oxygenates) and the

ﬁndings reported here provide valuable insights of key importance for further development and optimization of FT catalysts.

KEYWORDS: medium-temperature Fischer−Tropsch, ferrites, alloys, GCxGC, oxygenate selectivity

preventing/limiting catalyst deactivation.⁷One attempt to

address these challenges is the use of bimetallic catalysts,

1. INTRODUCTION

The Fischer−Tropsch synthesis (FTS) is the catalytic

polymerization reaction of carbon monoxide and hydrogen

(i.e., syngas) to produce a broad range of valuable hydro-

mainly consisting of FTS active metals (i.e., Fe, Co and Ni),

for example, Ni−Fe/TiO₂,^8,9Ni−Fe/Al₂O₃,^10,11Fe−Ni/

SiO₂,¹²Fe−Co alloys,¹³⁻¹⁵Co−Fe/TiO₂,¹⁶⁻¹⁹and even

carbon-based products (predominantly linear paraﬃns and

trimetallic compositions.²⁰Combinations of iron with noble

oleﬁns, but also, for example, alcohols, aldehydes, carboxylic

acids, and aromatics).¹⁻³FTS is widely considered as an

metals such as Pt, Ir, and Pd generally convert syngas to

methanol,²¹⁻²⁵while the combination of iron and rhodium is

alternative to reﬁning crude oil for producing conventional

fuels (gasoline, diesel, and jet fuel) from various carbon-

known to produce C₂oxygenates as well.²⁴Higher CO

bearing feedstocks, including natural gas, coal, biomass, and

even syngas from sequestered CO₂and electrolytic H₂.⁴⁻⁶

Received: July 31, 2020

Revised: November 16, 2020

Therefore, its economic feasibility is directly related to the

global oil price. To reduce the FTS process dependency on the

oil price, extensive research eﬀorts are focused on signiﬁcantly

increasing the selectivity of speciﬁc high economic value

compounds or product classes (e.g., fuels or light oleﬁns) and

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

14661

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

conversion was observed with the bimetallic Ni−Fe catalyst in

comparison to the monometallic (i.e., Fe or Ni) catalysts for

both supported and unsupported systems.^8,9,26−28Moreover,

the ratio of Ni or Co to Fe inﬂuences activity (i.e., CO

conversion) and selectivity. Li et al.²⁷showed that an increase

of Ni in the unsupported Ni−Fe alloy increases the

hydrogenation and methanation activity of the catalyst, while

Ishihara et al.¹²reported similar trends with regard to

hydrogenation, but did not ﬁnd enhanced methanation activity

for TiO₂and SiO₂supported Ni−Fe alloys. For the Co−Fe

system, various studies report an increase in activity with

increasing Co content.^12,16,26In terms of selectivity, the higher

Co content results in higher paraﬃn content and a decreased

activity for the water−gas shift reaction.¹⁶Tihay et al.¹³⁻¹⁵

showed a synergistic eﬀect of the Co−Fe alloy and cobalt

ferrite composite on the stability of the metallic phase and a

higher oleﬁn selectivity in the C₂−C₄fraction. They also

showed by X-ray diﬀraction (XRD) that the alloy phase and

the spinel phase lose crystallinity under CO/H₂and CO₂/H₂

conditions.¹⁴

acids) from syngas. This is only achievable through better

understanding of the mechanism of oxygenates formation, the

nature of the active site and catalyst structure−activity/

selectivity correlation.

The present work focuses on the inﬂuence of substituents in

the ferrite structure of catalyst precursors on the activity and

selectivity in medium-temperature FTS, with the aim of

increasing the selectivity toward oxygenates. The substituents

used are cobalt, nickel, or a combination thereof. All catalysts

are formed from the spinel structure, thus, eliminating any

structural/support eﬀects. Activation of the catalysts by

reduction was investigated by means of in situ XRD and in

situ magnetometry, while the physical properties of the

catalysts were studied by means of nitrogen physisorption

and transmission electron microscopy (TEM). For product

analysis, we employ a combination of one- and two-

dimensional gas chromatography, the latter coupled with

mass spectrometry and a ﬂame ionization detector, which

greatly enhances product separation, identiﬁcation, and

quantiﬁcation.

Materials with a spinel structure (AB₂O₄) are extensively

used in ﬁelds such as magnetism²⁹and battery research,³⁰

owing to the possibility to modify their chemical and physical

properties via the variation of the cations in the A and B sites.

Furthermore, research on the use of spinel structures in

catalysis include alkane oxidative dehydrogenation,^31,32carbon

monoxide oxidation,³³and water−gas shift,³⁴as well as the

FTS.³⁵For the latter, Chonco et al.³⁵used ferrites to study the

eﬀect of copper as a promoter in iron-based catalysts. They

highlighted the importance of the spinel and the delafossite

structure (Cu^I+Fe^III+O₂) versus a physical mixture of CuO and

α-Fe₂O₃, with the former structures showing higher activity.

The authors suggested that the presence of copper in close

proximity to iron limits the extent of sintering, thus, enhancing

the carburization of the catalyst, resulting in a higher surface

area of the active carbide phase.

2. EXPERIMENTAL SECTION

2.1. Catalyst Synthesis. The catalysts were synthesized by

a co-precipitation method described in detail by Arulmurugan

et al.⁴⁹The synthesis of Fe₃O₄was carried out using near

boiling solutions of Fe(III) nitrate (0.2 M, Sigma-Aldrich) and

FeCl₂(0.1 M, Sigma-Aldrich) as precursors. Both solutions

(50 mL each) were added quickly to a near boiling NaOH

solution (35.35 g in 1.3 L of deionized H₂O). The addition of

the metal solutions resulted in an immediate formation of a

black precipitate. The mixture was maintained at 95 °C for 1 h

to allow suﬃcient time for the rearrangement of the hydroxide

into the spinel structure. The mixture was allowed to cool to

room temperature, and the precipitate was ﬁltered (vacuum

ﬁltration) and washed with deionized water until the ﬁltrate

reached neutral pH. The product was dried in air at 120 °C

overnight and subsequently characterized. The same synthesis

method was followed for the preparation of the substituted

ferrites, where an additional solution with the target

concentration (0.03 M in 50 mL) of the substituent (nickel(II)

nitrate and cobalt(II) nitrate, obtained from Sigma-Aldrich)

was added, and the Fe(II) solution concentration changed to

0.07 M in 50 mL.

2.2. Catalyst Characterization. The crystal phase

identiﬁcation was achieved by XRD analysis of the powder

sample using a Bruker D8 advance diﬀractometer equipped

with a LYNXEYE XE detector and a Co source (K_α1= 1.79 Å).

The elemental composition in the ferrites was determined by

inductively coupled plasma-optical emission spectroscopy

(ICP-OES). In preparation, the samples (∼20 mg) were

digested using a combination of HNO₃, HClO₃, and HCl/HF

(4:1). The obtained solutions were subsequently diluted with

deionized H₂O. A multistandard solution was prepared and

used for ICP-OES calibration for the elements (Co, Ni, and

Fe). The surface area and physisorption properties of the

catalysts were measured at liquid nitrogen temperature with

nitrogen as an absorbent in a Micromeritics TriStar instru-

ment. Prior to analysis, the catalyst was degassed at 200 °C

overnight in a Micromeritics FlowPrep 060 sample preparation

unit. The Raman spectra of the catalysts were measured in a

Renishaw inVia Raman spectrometer, operated with the Nutec

software package, and equipped with a green laser (512 nm).

The laser power and exposure time were varied to prevent

Oxygenates, mainly alcohols, are currently used in a variety

of industries, for example, ethanol is considered a valuable fuel

alternative that is biodegradable³⁶and butanol is viewed as an

important intermediate in the plastic and pharmaceutical

industries, while long chain alcohols are key in the surfactants

industry.³⁷Currently, alcohols are mainly produced via sugar

fermentation³⁸and the hydration of alkenes.³⁹Direct

conversion of syngas to alcohols is considered an attractive

route, which in comparison to the aforementioned two routes,

is versatile and environmentally friendly, since fewer operation

units are required, which also result in lower capital and

operation cost.³⁷Syngas to oxygenates, mainly alcohols, was

reported over catalysts based on diﬀerent active metals: (i)

cobalt, with an alcohol selectivity of 38 wt % over Co₂Cu at

240 °C,⁴⁰∼45 wt % over Co₄MnK_0.1at 220 °C,⁴¹and ∼15 C%

over Co/activated carbon at 220 °C;⁴²(ii) molybdenum, with

an alcohol selectivity of 50 C% over K-NiMoS₂/Al₂O₃-

montmorillonite at 280 °C⁴³and 32 C% at 250 °C over K-

Mo₂C/TiO₂;⁴⁴(iii) rhodium, with an ethanol selectivity of 24

C% at 240 °C over Rh/Al₂O₃;⁴⁵and (iv) iron, with alcohol

46

selectivity of 49 C% at 225 °C over Fe/Al₂O₃and 62% at

320 °C over K-FeCu/silica.⁴⁷The industrial FT process

produces oxygenates in the range of 7−12 C%, based on the

operating condition and the nature of the Fe catalyst.⁴⁸

However, the development of a catalyst(s) with higher activity,

selectivity, and stability is needed for the industrial production

of oxygenates (alcohols, aldehydes, ketones, and carboxylic

14662

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

laser-induced phase transformation/damage. The shape and

size of the catalysts particles were determined in a Tecnai FEI

T20 transmission electron microscope operated with a ﬁeld

emission gun at 200 kV. The particles were immobilized on a

carbon-coated copper grid.

ature implies that oxidic Fe (and its substituents) is being

reduced to metallic Fe (and its substituents).

Conventional H₂-temperature-programmed reduction (H₂-

TPR) analysis was carried out using a Micromeritics

AutoChem II 2920 instrument. The catalyst (∼100 mg) was

placed in a quartz U-tube reactor and heated at a rate of 10

°C/min from 60 to 900 °C under a 50 mL/min ﬂow of 5 vol %

H₂in Ar. The consumption of H₂during the reduction

experiment was analyzed using a thermal conductivity detector

(TCD).

The eﬀect of the substituents on the reduction behavior of

the diﬀerent ferrites was investigated using an in situ XRD

capillary cell, described in detail elsewhere,^50,51mounted on a

Bruker D8 advance diﬀractometer equipped with a Mo source

(K_α1= 0.71 Å) and a VANTEC detector. The total ﬂow of

pure hydrogen (used as the reducing gas) was set at 5 mL/min

using a mass ﬂow controller over a catalyst mass of ∼18 mg.

The catalyst was heated from 50 to 450 °C at 1 °C/min with

XRD patterns measured in 5 min intervals during ramping

(step size = 0.018°, time/step = 0.2 s, total scan time = 4 min

24 s and 2θ range of 13−30°). Further investigation of the

reduction temperature and pathway was carried out using an in

situ magnetometer⁵²⁻⁵⁴(developed by UCT and SASOL,

South Africa) with a magnetic ﬁeld strength of up to 20 kOe or

2.0 T. The H₂ﬂow was 27.7 mL/min controlled with a mass

ﬂow controller and the temperature was monitored with a N-

type thermocouple placed in the catalyst bed. The reduction

method for 100 mg of the catalyst diluted with 400 mg SiC

starts from 50 to 450 °C at a heating rate of 1 °C/min and

back to 50 °C at 2 °C/min under pure H₂ﬂow. Magnetic

measurements were taken every 5 min at 2, 0, and −2 T along

the heating ramp from 50 to 450 °C to monitor the degree of

reduction and during the cool down to 50 °C. The latter

measurements, cool down curves, served to ascertain if nickel

segregation occurred during reduction, which would show in a

change of magnetization around its Curie temperature (353

°C). Magnetization versus ﬁeld strength measurements (M−

H) were also recorded at 50 (fresh/ferrite phase), 450

(reduced/alloy phase), and 50 °C (reduced/alloy phase at

near room temperature) in 65 points between 2 and −2 T

(total time 40 min). The γ value indicates the weight

percentage of particles with sizes larger than the critical

diameter for the superparamagnetic behavior of Fe (d_crit= 8

nm at room temperature⁵⁵) and is calculated using the

expression:

2.3. FTS Testing and Product Analysis. The FTS testing

was carried out in a 1/2” stainless steel ﬁxed bed reactor. All

catalysts were tested following the same pretreatment and FTS

conditions. A total of 300 mg (size range 50−75 μm) of the

catalyst was diluted with 2 g of SiC (300 μm) and loaded in

the predetermined isothermal zone of the reactor. The catalyst

was then reduced at 400 °C for 5 h under a H₂ﬂow and cooled

down to reaction temperature. FTS was carried out at 20 bar

total pressure, 280 °C reaction temperature, a H₂/CO ratio of

2 and 36 mL/min of syngas and 4 mL/min of argon (internal

reference). The reactor was connected to a hot catch pot

(heated to 140 °C for wax collection) and a cold catch pot

(cooled via a H₂O chiller to 10 °C for oil and H₂O collection).

The catalyst activity (CO and H₂conversion) and selectivity to

CH₄and CO₂were analyzed on-line by a Varian CP 4900

micro GC-TCD equipped with three individual columns (10

and 20 m Mol Sieve 5A and 10 m Porapak Q). The light

hydrocarbon fraction was sampled using the ampoule

method,⁵⁷before the cold trap. The ampoules were analyzed

using a gas chromatograph (Varian GC 3900) equipped with a

ﬂame ionization detector (FID). To achieve eﬀective

separation of C₁and C₂species on the CP-Sil 5CB column,

the GC oven was initially cooled to −55 °C with CO₂before

heating gradually to 250 °C. A detailed analysis of the gas

phase was also carried out using a 2D GCxGC (LECO Pegasus

4D GC) equipped with a ﬂame ionization detector (FID) and

a time-of-ﬂight-mass spectrometer (TOF-MS) directly con-

nected to the reactor outlet gas after the hot trap. The system

was operated in reverse phase mode, where the primary

column has a polar stationary phase (Stabilwax, 30 m, 250 μm,

0.1 μm) and the secondary column a nonpolar one (RTX-5,

1.39 m, 180 μm, 0.2 μm). Modulation of the eﬄuent from the

primary column was achieved by cooling a dry N₂gas stream

with liquid nitrogen. The data analysis was performed using

the TOF Chrom software package. Detailed 1D GC and 2D

GCxGC methods and instrument parameters are provided in

the Supporting Information (Table S1). The carbon balance

for all FTS experiments was within the range of 92−95%.

2.4. Spent Catalyst Characterization. After the reaction,

the reactor was cooled to 50 °C and the catalyst was passivated

using 1% O₂in N₂at a ﬂow of 50 mL/min.⁵⁸XRD, Raman

spectroscopy, and thermogravimetric analysis (TGA) of the

spent catalysts was subsequently performed. The TGA analysis

was carried out using a Thermal Analyzers Discovery SDT 650,

with a 10 °C/min heating rate from 50 to 700 °C, under 50

mL/min air ﬂow to decompose any carbonaceous compounds

present on the catalyst surface and/or pores.

2·M

M_sat

γ(wt %) =

^rem·100

(1)

where γ is the percentage of non-superparamagnetic material,

M_remis the remnant (measured) magnetism, and M_satis the

saturation magnetism of the material at the temperature of

measurement.

Note that, in this work, samples with γ values below 10 wt %

are regarded as being superparamagnetic, as most of the

particles would have a diameter smaller than the critical

diameter for Fe. The remnant magnetization is measured in the

absence of an external magnetic ﬁeld (i.e., 0 T), while the

saturation magnetization is determined by extrapolation

beyond the maximum ﬁeld strength of the magnetometer

(i.e., >2 T). The magnetite phase is ferrimagnetic while

metallic iron is ferromagnetic,⁵⁶both of which can be detected

using the magnetometer and can be distinguished from one

another due to their diﬀerent saturation magnetizations. It is

worth noting that the magnitude of the magnetic moments

decreases with temperature; therefore, an increase in the

magnetization of oxidic iron samples with increasing temper-

3. RESULTS

3.1. Catalyst Characterization. 3.1.1. Ex Situ Catalyst

Characterization. All synthesized materials, after drying,

displayed the same diﬀraction patterns in XRD (Figure 1),

which matched the reference pattern of iron ferrite (ICDD

14663

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

all synthesized materials display irregular particle shapes

(Figure 2), with no obvious alteration upon Co and/or Ni

substitution.

Figure 1. XRD diﬀraction patterns of the ferrite structures synthesized

via co-precipitation, obtained with a Co source (λ = 1.79 Å). The

expanded section illustrates the change in the position of the (3 1 1)

reﬂection and d-spacing as a function of cation (i.e., Co, Ni, and Co−

Ni) substitution in the ferrite structure.

Figure 2. TEM images of ferrite and substituted ferrites synthesized

via co-precipitation: Fe₃O₄(a), Co_0.3Fe_2.7O₄(b), Ni_0.3Fe_2.7O₄(c), and

Co_0.3Ni_0.3Fe_2.4O₄(d).

PDF-2 entry 01−071−6336). The absence of additional

diﬀraction lines supports the successful incorporation of the

substituents into the ferrite lattice structure. Moreover, the d-

spacing of the (3 1 1) plane, especially in the case of

Co_0.3Ni_0.3Fe_2.4O₄, shifted to higher values (i.e., lower 2θ),

which indicates an increase in the lattice volume due to the

presence of Co and Ni. The concentration of the Co and Ni

substituents was determined by ICP-OES (see Table 1).

Average crystallite sizes were determined by Rietveld reﬁne-

ment as being 7, 7, 4, and 4 nm for Fe₃O₄, Co_0.3Fe_2.7O₄,

Ni_0.3Fe_2.7O₄, and Co_0.3Ni_0.3Fe_2.4O₄, respectively. Assuming

spherical particles, the theoretical crystallite sizes were

calculated (see Supporting Information for calculations)

using the BET surface area measurements of the diﬀerent

ferrites. The sizes were found to be 6, 6, 4, and 5 nm for Fe₃O₄,

Co_0.3Fe_2.7O₄, Ni_0.3Fe_2.7O₄, and Co_0.3Ni_0.3Fe_2.4O₄, respectively.

It is evident that there is strong concurrence between the

experimental and theoretical crystallite size, where Ni-bearing

ferrite demonstrated the smallest crystallite size. The same

trend is qualitatively observed in the BET surface area

measurements determined experimentally and theoretically

(Table 1), with the highest surface areas displayed by the Ni-

containing ferrites. A similar eﬀect has previously been

reported by Li et al.²⁷TEM characterization indicates that

Exceptional care must be taken when analyzing spinel-type

materials with Raman spectroscopy, as the transition metal

(iron, in the present case) inside the structure exhibits

bivalency (ferrous or ferric).⁵⁹A high-power laser source and

long exposure under aerobic conditions can oxidize the Fe²⁺in

the spinel structure to Fe³⁺and change the chemical and

crystal phase to hematite.⁵⁹The ﬁngerprint vibrational

stretching bands of pristine Fe₃O₄(Figure 3) conﬁrm the

presence of the magnetite structure.⁶⁰The Raman shifts at

350.4, 471.8, and 675.3 cm⁻¹correspond to the E_g, T_2g, and

A_1gmodes, respectively, which is in-line with previous

reports.^61,62The other two weak bands for the T_2gmode

appear at 225.7 and 553.7 cm⁻¹.^61,63The most intense band of

the spectra at 675.3 cm⁻¹(for the A_1gmode) can be assigned

to the vibration of the Fe−O bond.⁶¹Importantly, no notable

change was observed in the Raman spectra after the

incorporation of Co and/or Ni (Figure 3). This conﬁrmed

that the ferrite (spinel) crystal structure is maintained after the

incorporation, which is in good agreement with the powder

XRD results. The second order scattering band around 1303

Table 1. Eﬀect of Substituents on the Ferrite Surface Area, As Determined by BET Surface Area Measurements, and the Metal

Ratio in the Ferrites, As Determined by ICP-OES Analysis

a

size XRD

(nm)

surface area measurements

theoretical surface area

target substituent ratio

M/Fe

ICP-OES substituent ratio

M/Fe

catalyst

Fe₃O₄

Co_xFe_3−xO₄

Ni_xFe_3−xO₄

Ni_xCo_xFe_3−2xO₄

(m²g⁻¹

)

(m²g⁻¹

)

7

4

183

187

264

251

179

159

290

264

0.3/2.7

0.31/2.69

0.3/2.7

0.30/2.70

0.3/0.3/2.4

0.28/0.27/2.40

a

Assuming spherical particles with a crystallite size determined by XRD analysis (refer to Supporting Information for calculations).

14664

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

subsequently facilitate the reduction of adjacent Fe^x+in an

intraparticle hydrogen spillover mechanism. This intermediate

̈ ̈

phase, wustite or Ni/Ni+Co substituted wustite, is completely

reduced to the metallic/alloy phase at 325 °C. All samples

show a similar degree of sintering upon reduction, with all

resulting crystallite sizes ranging between 15 and 17 nm.

Magnetization Measurements. Magnetic properties of

substituted ferrites have extensively been investigated; the

inﬂuence of ferrite composition, crystallite size, shape and

architecture have been reviewed by Kolhatkar et al.⁶⁹Ferrite/

magnetite (Fe₃O₄) has a ferrimagnetically ordered structure, in

which in the absence of a magnetic ﬁeld, the atomic layers with

larger magnetic dipole moments align antiparallel with respect

to those with smaller magnetic dipole moments. In contrast,

metallic Fe is ferromagnetic, which diﬀers from ferrite in that

all magnetic dipole moments are equal in magnitude and are

aligned parallel to each other.

Figure 3. Raman spectra of fresh Fe₃O₄, Co_0.3Fe_2.7O₄, Ni_0.3Fe_2.7O₄,

and Co_0.3Ni_0.3Fe_2.4O₄catalysts. The dotted box area highlights the

2nd order scattering band position.

Diﬀerent iron phases and alloys exhibit diﬀerences in their

saturation magnetization and Curie temperature, as summar-

ized in Table 2. Therefore, magnetic properties can be used to

study the reduction of the substituted ferrites to determine the

inﬂuence of substituents on the reduction behavior and to

conﬁrm alloy formation. Since metallic iron has a higher

saturation magnetization than the ferrite phase (i.e., Fe₃O₄),⁵⁶

the onset of reduction is recognized as the temperature where

the magnetization increases. Table 2 shows that this is the case

for the alloys as well. Noteworthy, the FeO phase is

antiferromagnetic and thus undetectable in the magnetometer.

However, the formation of a FeO phase (un/substituted) may

otherwise be observed by a drop in the magnetization during

the reduction process. This was clearly observed in the case of

Fe₃O₄and Co_0.3Fe_2.7O₄by the drop in magnetization around

250 °C and to lesser extent for the Ni-bearing ferrites.

The substituents in the ferrite structure inﬂuence the

magnetization of the iron-based structure, with a slight

increase in magnetization upon inclusion of cobalt (from 70

to 80 emu/g) and a decrease with the inclusion of Ni (from 70

to 39 emu/g). The comparable magnetization of ferrite and

cobalt ferrite and a decrease in the magnetization over nickel

ferrite was also observed by Lee et al.⁷⁰and rationalized by the

change in the magnitude of the magnetic moment upon

inclusion of the substituents. In general, the saturation

magnetization values obtained in this study are comparable

with the reported literature values (see Table 2). The

diﬀerences may be due to the analysis temperature, material

composition, and/or particle size.⁶⁹

The magnetic measurements (Figure 5a) show that the

onset reduction temperatures from the ferrite phase to metallic

phase are about 310, 300, 245, and 260 °C, over Fe₃O₄,

Co_0.3Fe_2.7O₄, Ni_0.3Fe_2.7O₄, and Co_0.3Ni_0.3Fe_2.4O₄, respectively.

While these onset of reduction temperatures are in reasonable

agreement with those from the XRD analysis, we note that the

values obtained from the magnetic measurements are system-

atically lower. The reason is that XRD detects only crystalline

phases, while in principle all metal ions can contribute to the

magnetization and be detected with this technique. Hence, if

reduction of the oxide phase initially forms unordered

intermediates, these contribute to the overall magnetism, but

not to the XRD.

cm⁻¹is a characteristic peak for iron oxide; it represents an

overtone phenomenon or a combination of modes.⁶⁴However,

the incorporation of external ions masked this second order

scattering, and it is almost absent in Co_0.3Ni_0.3Fe_2.4O₄.

Ex situ XRD showed similar diﬀraction patterns for the

unsubstituted and substituted ferrites, indicating possible

incorporation of the substituents in the lattice structure of

the ferrite. However, this is not conclusive since XRD only

detects crystalline phases. Surface area measurement by BET

method showed an increase in the surface area, in particular

that of the Ni-bearing ferrite. The theoretically calculated

surface areas and crystallites size values were comparable with

those experimentally obtained.

3.1.2. In Situ Catalyst Characterization. X-ray Diﬀraction.

The eﬀect of the substituents in the ferrite structure on the

reduction temperature and pathway under H₂was studied in

an in-house developed in situ XRD cell.^50,51The changes in

phase composition and crystallite size were determined by

Rietveld reﬁnement using the software TOPAS 4.2⁶⁵(using

Fe₃O₄and Fe crystal structures for reﬁnements, with the d

spacing allowed to vary to account for the substituents). No

changes in the reduction onset temperature and reduction

pathway were observed with the introduction of cobalt into the

ferrite structure (Figure 4a,b). Both ferrite and cobalt ferrite

have an onset reduction temperature of 315 °C and reduce to

the wustite and metal phase. In general, the reduction pathway

̈

agrees well with Ding et al.⁶⁶The unchanged reduction

temperature after inclusion of cobalt can be explained by the

limited increase in the hydrogen dissociation activity of cobalt

compared to iron. Cobalt-based catalysts in FTS are, therefore,

commonly promoted with PGM metals to facilitate reduc-

tion.^67,68The average crystallite size increased from 6−7 nm to

15−17 nm during reduction. It is important to note that no

phase segregation was observed during reduction of the

Co_0.3Fe_2.7O₄phase, which suggests the presence of a CoFe

alloy in the reduced phase. The latter also showed a slight

increase in the d-spacing of the (1 1 0) plane, indicative of Co

incorporation in the Fe structure.

The introduction of Ni into the ferrite structure altered the

reduction behavior signiﬁcantly (Figure 4a,b). The onset

reduction temperature was lowered to 250 °C for both

Ni_0.3Fe_2.7O₄and Co_0.3Ni_0.3Fe_2.4O₄. We propose that the higher

hydrogen dissociation capacity of Ni is responsible for this

enhancement. Hydrogen can be activated on Ni sites and

It is important to note that both the in situ XRD and the

magnetic measurements show complete reduction of the

ferrites to the metallic phases, since (1) no other crystalline

phases were detected besides the metallic phase in XRD, and

14665

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

Figure 4. Compositional changes of diﬀerent ferrite structures (a) and an on-top view of the XRD patterns (b) obtained in situ during H₂

reduction (XRD: Mo K_α1= 0.71 Å).

(2) the magnetization of all catalysts reached a stable value just

above the temperatures where, according to XRD, full

reduction was reached. The slight decrease in magnetization

toward higher temperatures is due to thermal eﬀects.⁶⁹

The magnetization versus ﬁeld strength measurements (M−

H) were used to determine the magnetic nature of the particles

present in each sample, that is, whether superparamagnetic or

non-superparamagnetic. The M−H measurements of Figure

5b, obtained at 450 °C, show hysteresis behavior when the

applied magnetic ﬁeld approaches 0, depending on the metallic

phase composition. Ni-containing samples show the highest

contents of superparamagnetic material, indicated by the

14666

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

Table 2. Magnetic Properties of Relevant Iron Phases, Substituted Ferrites, Cobalt, Nickel and Iron Alloys

Curie temperature

saturation magnetism (emu g⁻¹), this

saturation magnetism (emu g⁻¹),

a

magnetism

(°C)

study

literature

ref

56

α-Fe

ferromagnetic

ferrimagnetic

antiferromagnetic

770

580 15

404

204

70

80

222

56

Fe₃O₄(magnetite)

CoFe₂O₄

NiFe₂O₄

FeO

90−82

68−72

55

70−72

73

517−570

39

a

Co_0.3Ni_0.3Fe_2.4O₄

48

74

72

75

Co

Ni

CoFe

Ni₂₀Fe₈₀

CoNiFe

1115

353−358

ferromagnetic

53

209−222

186

213

173

189

a

Saturation magnetization determined in this study at 50 °C before and after reduction for the (un)substituted ferrite and iron metallic/alloy phase,

respectively.

lowest γ values of 20.5 and 10.3 wt % for NiFe and CoNiFe,

respectively, compared with values of 39.5 and 49.2 wt % of

non-superparamagnetic material in the Fe and CoFe samples,

respectively. The small hysteresis and, consequently, the low γ-

values for Ni-containing alloys can be ascribed to the large Ni

critical diameter of 55 nm.⁶⁹

are detected in the magnetometer. Both in situ techniques

demonstrate that Ni acts as a reduction promoter, since

inclusion of Ni results in a decreased onset of the reduction

temperature from 310 °C (Fe₃O₄and Co_0.3Fe_2.7O₄) to 250 °C

(Ni_0.3Fe_2.7O₄and Co_0.3Ni_0.3Fe_2.4O₄). We suggest that the eﬀect

is caused by the higher reducibility of Ni²⁺to Ni⁰, which then

facilitates H₂dissociation and subsequent migration to the

adjacent iron and cobalt (via spillover), where initiation of the

reduction process is known to be relatively diﬃcult.⁷⁹

Magnetic measurements have been previously used to

conﬁrm alloy formation in CoNi catalysts.⁷⁴In the case of

alloy formation, the decrease in magnetization with an increase

in temperature is monotonic, and no signiﬁcant variation/

inﬂection point in the absolute ﬁrst derivative curve around the

Ni Curie temperature region should be observed (i.e., ∼353

°C, see Table 2). Figure 5c shows a monotonic decrease in

magnetization with an increase in the temperature and no

inﬂection point in the absolute ﬁrst derivative curve (Figure

5d). Thus, the Ni_0.3Fe_2.7O₄precursor reduced to a Ni−Fe alloy

structure, while the Co_0.3Ni_0.3Fe_2.4O₄reduced to an alloy

structure evidently containing Ni+Fe. Due to the high Curie

temperature of cobalt (∼1115 °C, see Table 2), the same

analysis cannot be done for the Co-containing alloys, as only

temperatures below 1000 °C can be realized using the current

magnetometry setup.⁵²⁻⁵⁴

3.2. Spent Catalyst Characterization. After the FT

reaction, the catalysts were passivated and analyzed by XRD to

determine their bulk crystalline phase composition (Figure 6).

̈

Rietveld reﬁnement (Table 3) showed increases in the Hagg

carbide concentration from 78 to 95−97 wt % for the multi-

metallic catalysts (CoFe, NiFe, and CoNiFe) compared to the

pure iron sample. This suggests either an improved

carburization of the ferrite as a result of incorporating Co,

Ni, and Co−Ni, or a suppression of the reoxidation behavior

which would be caused by the product water under reaction

conditions. Only the pure Fe sample showed an oxidic

component, namely, magnetite, in the spent sample. The

̈

increase in the Hagg carbide concentration and disappearance

The conventional H₂-TPR of the (un)substituted ferrites

(Figure S1) shows the presence of three reduction steps. The

ﬁrst reduction, between 200 and 350 °C, is attributed to the

reduction of Fe³⁺(in the ferrite structure) to Fe²⁺.⁷⁶This

reduction peak is less intense and broad in the TPR proﬁles of

the substituted ferrites, in particular, the Ni-bearing ones, and

seems to have a slightly lower onset temperature for these

catalysts. It has also been proposed that this peak can be due to

the reduction of hematite (α-Fe₂O₃) impurities in the ferrite.⁷⁷

Alternatively, these Fe₂O₃“impurities” may be present as

maghemite (γ-Fe₂O₃), which is indistinguishable from Fe₃O₄

in XRD characterization and which would explain why no

phase change is observed in the in situ XRD reductions (see

Figure 4), which correspond to these TPR peaks. The second

reduction step, between 350 and 550 °C, is assigned to the

reduction of the (un)substituted ferrite to a (un)substituted

of any iron oxide phase upon introduction of Co and/or Ni

may be caused by a higher hydrogenation activity, assisting in

the removal of oxygen from the catalyst surface, thus,

preventing reoxidation of the carbide phase, in agreement

with Unmuth et al.⁸⁰The Hagg carbide formed from the

̈

metallic iron has similar crystallite size, but in the bi- and tri-

̈

metallic catalysts, the Hagg carbide is signiﬁcantly smaller

(Table 3). This could be due to cleavage of the metallic phase

̈

and/or intermediate phase crystallites prior to Hagg carbide

phase formation. A similar observation was reported by

Chonco et al.³⁵during in situ XRD analysis of α-Fe₂O₃

activation in a carbon monoxide environment. The decrease

in the crystallite size of the metallic catalysts (see Table 3)

̈

could therefore be another reason for the increase in the Hagg

carbide content in the bi- and tri-metallic catalysts, since the

̈

carbon incorporation into the structure, to form Hagg carbide,

wu

°C, may represent the transformation of the (un)substituted

wu

stite to the (mono-, bi- or tri)metallic phase.⁷⁶⁻⁷⁸

̈

stite phase, and the third reduction, between 550 and 800

readily occurs in smaller crystallites size Fe-based materi-

als.^27,81−83

̈

A possible deactivation of Fe-based FTS catalysts is

deposition of “coke” on the catalyst surface.⁸⁵The passivated

catalysts were characterized by TEM and TGA in air to

determine the presence of carbon overlayers on the catalyst

surface and the mass loss corresponding to carbon oxidation,

The use of in situ XRD and magnetometry for investigating

the inﬂuence of substituents on the reduction behavior of the

ferrite, forms a powerful combination since XRD cannot detect

non-crystalline phases, which (as long as these are magnetic)

14667

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

̈

respectively. Noteworthy, Hagg carbide decomposes in the

same temperature range as that of “coke”; therefore, this may

̈

contribute to the mass change. However, since Hagg carbide is

the dominant phase in all catalysts, the contribution will be

comparable, and we assume that the main diﬀerence in the

TGA proﬁles obtained arises from the carbon deposition. The

TEM results (Figure 7) show the presence of an overlayer on

(un)substituted iron catalyst particles, which is most likely due

to carbon deposited during the FTS reaction. The TGA

analysis (Figure 8) of the spent catalysts under air ﬂow

demonstrated mass losses of 32, 23, 14, 12 wt % from Fe,

CoFe, NiFe, and CoNiFe spent catalysts, respectively. This

trend is similar to that obtained for the onset reduction

temperature and hydrogenation activity, where the Ni bearing

alloy demonstrated the lowest onset reduction temperature

and the highest hydrogenation activity. Therefore, it is possible

that the hydrogen-rich catalyst surface prevents carbon

deposition and/or promotes the in situ removal of carbon

via hydrogenation to methane.^86,87This could also explain the

increase in methane selectivity over the Ni-bearing alloys.

Furthermore, the carbonaceous material (coke) is classiﬁed in

two classes, “soft” and “hard” coke. “Soft” coke consists of

alkene-like compounds with an H/C ratio of 2.⁸⁸⁻⁹⁰These

compounds decompose under an air atmosphere in the

temperature range of 180−330 °C,⁸⁸while the “hard” coke

has a lower H/C ratio and decomposes in the temperature

range of 300−530 °C. The mass loss for all catalysts occurs in

the temperature range of 280−410 °C, possibly indicating a

mixture of “soft” and “hard” coke. Hence, the substituents in

the Fe catalyst only seem to inﬂuence the amount of coke

formed, but not the nature of the coke formed.

The nature of carbon deposits on the catalyst surfaces, as

monitored with Raman spectroscopy (Figure 9), shows a

similar diﬀerentiation as the XRD analysis. The spent Fe

catalyst shows two distinct bands at 1351 and 1576 cm⁻¹,

which correspond to the D- and G-bands of distorted and

graphitic carbon, respectively.⁹¹⁻⁹³The other three catalysts

display a broad band around 2000 cm⁻¹, suggesting the

presence of amorphous carbon not coupling eﬀectively with

the laser due to very small size and low concentration of

graphitic crystallites.^94,95It appears that the deposited carbon

on the alloys has a diﬀerent nature to that present on the pure

iron catalyst, and lacks the graphitic structure as observed on

the latter. This could be a further indication of the

hydrogenation activity of the substituted alloys, limiting carbon

deposition and growth on the catalyst surface.

Figure 5. Saturation magnetization as a function of temperature in a

H₂environment for all catalysts (a), M−H plots obtained at 450 °C

for all catalysts, with the enlarged section highlighting the hysteresis

behavior (b), cooling down curves (c), and absolute ﬁrst derivatives of

the cooling down curves of Ni-bearing alloys (d). Dotted area in (c)

and (d) indicates the Curie temperature of metallic nickel.

3.3. FTS Activity and Selectivity. The FTS activity (CO

conversion after 48h time on stream (TOS)) of the

synthesized ferrites after reduction (at 400 °C for 5 h under

a ﬂow of 100 mL/min H₂) increases with the introduction of

substituents in the ferrite structure from 23.9% over Fe

catalyst, to 33.6, 38.1, and 41.9%, over CoFe, NiFe, and

CoNiFe, respectively (see Table 4 and Figure 10). This can be

rationalized with the reported higher intrinsic CO activation

capacity of Ni and Co.^2,96The decrease in speciﬁc surface area

based on the crystallite size of the reduced metal, as measured

during in situ XRD, can only account for a 5−10% increase in

surface atoms and can, as such, not be solely responsible for

the improved activity. The CO₂selectivity (31.1, 26.9, 35.2,

and 35.0 C%, over Fe, CoFe, NiFe, and CoNiFe, respectively)

decreases when Co is incorporated and increases when Ni is

present. Generally, CO₂formation in the FTS is associated

with the water−gas shift (WGS) reaction,²with metallic iron

Figure 6. XRD patterns of standards and passivated spent catalysts

after FTS at 280 °C, 20 bar, H₂/CO = 2. (*) Reﬂections due to SiC

used as catalyst diluent.

14668

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

Table 3. Composition of Spent Catalyst (Determined via Rietveld Analysis) after 48 h TOS at 20 bar, 280 °C, H₂/CO Ratio of

2; Conversion of 24, 34, 38, and 42%, over Fe, CoFe, NiFe, and CoNiFe, Respectively

Fe

CoFe

NiFe

CoNiFe

a

magnetite phase (wt %)

mono-/bi-/tri-metallic phase (wt %)

14 (17.2 1.7 nm)

8 (58.7 5.8 nm)

78 (17.5 0.6 nm)

a

5 (28.4 4.1 nm)

95 (8.1 0.2 nm)

5.9

3 (27.8 8 nm)

97 (13.3 0.4 nm)

5.8

5 (35.8 6.5 nm)

95 (8.7 0.3 nm)

6.9

a

̈

Hagg carbide phase (wt %)

b

Rwp (%)

5.7

a

b

Numbers in parentheses are the crystallite sizes of the phase. Rwp indicates the quality of the ﬁt obtained after applying Rietveld reﬁnement, with

values between 2 and 10% indicating a good ﬁt.⁸⁴

Figure 9. Raman spectra of passivated spent catalysts after FTS at 280

°C, 20 bar, H₂/CO = 2.

Figure 7. TEM analysis of spent and passivated catalysts after 48 h

TOS at 20 bar, 280 °C, H₂/CO ratio of 2. Conversion of 24, 34, 38,

and 42%, over Fe, CoFe, NiFe, and CoNiFe, respectively.

CO adsorption and suppresses H₂adsorption, overall reducing

the CO dissociation/methanation activity and enhancing the

WGS reaction.⁹⁸⁻¹⁰⁰

The hydrocarbon selectivity of the catalysts shows clear

evidence of an increased hydrogenation activity in the presence

of the substituent metals, especially Ni. The methane

selectivity increases from 9.5 C% over the pure Fe catalyst,

to 12.1, 30.4, and 32.5 C% over CoFe, NiFe, and CoNiFe,

respectively. In parallel, the chain growth probability (C₃−C₈)

decreases from 66 to 49%, again with the Ni containing

samples showing the greatest change. The same eﬀect is seen

in the composition of the hydrocarbons, that is, the oleﬁn to

paraﬃn ratio (Table 4).

3.4. Oxygenate Formation. The detailed selectivity for

the diﬀerent oxygenates classes (e.g., alcohols, aldehydes,

ketones, and acids) was studied with the use of 2D GC,

allowing for baseline separation between the diﬀerent classes of

compounds based on the functional groups (i.e., paraﬃns,

oleﬁns, aldehydes ketones, alcohols, and acids) and the carbon

number (see Figure 11), as reported by Grobler et al.¹⁰¹Since

all the substituted ferrite catalysts have comparable conversion

(iso-conversion), its eﬀect on selectivity is assumed negligible.

The total oxygenates selectivity increased slightly by the

inclusion of cobalt in the Fe catalyst (from 10.4 to 11.7 C%).

Metallic cobalt is generally not regarded as highly selective

toward oxygenates in the FTS.^102−104Work reported by

Gnanamani et al.¹⁰⁵and Xiong et al.¹⁰⁶indicates that cobalt

carbide phases, potentially formed in small quantities under

reaction conditions, support oxygenate formation. In the

present study, there is no direct evidence of Co₂C formation,

but it can also not be fully excluded. The total oxygenate

selectivity increases signiﬁcantly from 10.4% for iron, to 15.6−

Figure 8. TGA/TPO analysis under air of passivated spent catalysts

after FTS at 280 °C, 20 bar, H₂/CO = 2.

and the magnetite phase are well-known to sustain the WGS

reaction.²Metallic cobalt only shows very little CO₂selectivity

under typical FTS conditions.^2,97In the present study, it was

not possible to elucidate if the observed drop in CO₂selectivity

in the presence of Co is actually based on the observed

suppression of the iron oxide phase in the active catalyst (see

Table 3) or through the presence of the less WGS active

metallic cobalt itself. On the other hand, pure Ni has been

studied as a WGS catalyst, and is usually not very eﬀective as

its CO dissociation and methanation activity dominate.

Diﬀerent promoting and alloying approaches have been

reported to steer selectivity of Ni catalysts toward the WGS

reaction, with the synthesis of NiFe alloys being one of them. It

is reported that the presence of Fe weakens the strength of the

14669

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

Table 4. FTS Activity and Selectivity (given in C%) after 48 h of TOS at 20 Bar, H₂/CO Ratio of 2, and 280 °C

Fe

CoFe

NiFe

CoNiFe

X_CO

α

23.9 1.1

0.66

33.6 1.6

0.64

38.1 0.6

0.49

41.9 1.6

0.49

CO₂selectivity

selectivity to organic products

CH₄

C₂₋₄paraﬃns

C₂₋₄oleﬁns

C₅₊paraﬃns

C₅₊oleﬁns

MeOH

C₂₊alcohols

aldehydes

ketones

acids

C₃₌/C₃₋

31.1 0.9

26.9 1.0

35.2 0.7

35.0 0.8

9.5 0.1

7.8 0.8

33.9 0.9

11.8 0.3

26.5 1.1

0.5 0.1

5.7 0.9

2.8 0.5

0.6 0.2

0.8 0.01

5.1

12.1 0.2

11.5 1.4

30.7 1.5

11.0 0.6

22.7 1.1

0.7 0.1

7.8 0.1

2.2 0.4

0.5 0.1

0.5 0.01

2.8

30.4 0.8

20.6 0.3

20.7 0.1

3.7 0.2

7.4 0.4

1.2 0.1

12.7 0.4

2.3 0.4

0.3 0.06

0.5 0.05

3.0

32.5 1.2

18.4 0.6

21.2 0.2

4.2 0.1

7.7 0.7

1.1 0.1

11.7 0.5

2.2 0.1

0.7 0.2

0.2 0.01

3.1

primary C₄₌/total C₄₌

0.95

0.9

0.88

0.9

Figure 10. FTS organic product selectivity in C% after 48 h of TOS at 20 bar, H₂/CO ratio of 2, and 280 °C. Highlights show oxygenate selectivity

in C%. Figure S2 (in Supporting Information) shows the selectivity, including CO₂.

16.8 C% for the Ni-containing alloys CoNiFe and NiFe. When

considering the valuable C₂₊alcohols and aldehydes only, a

relative increase of up to 76% in selectivity is observed for

NiFe and up to 170% improvement in yield is obtained for

CoNiFe. This observation is noteworthy as oxygenates are

generally believed to be prone to secondary hydrogenation

reactions resulting in paraﬃns,¹⁰⁷which one may expect to be

enhanced in the presence of Ni. However, the result can be

rationalized by the same hypothesis supporting an increased

WGS activity, that is, the addition of Fe to Ni suppresses the

hydrogen adsorption and thereby reduces the CO dissociation

probability. The adsorbed molecular CO can potentially be

incorporated into chain growth, yielding oxygenates, as

proposed by Pichler and Schulz.¹⁰⁸The increase in the

oxygenate content may therefore be due to the increased

formation of these primary compounds. For all catalysts, most

of the oxygenates are found in the C₂fraction, with 40−50 C%

of all oxygenates. The composition of this fraction (see Figure

12a) varies greatly. In the case of the pure iron catalyst, 56.2%

in the C₂oxygenates fraction is ethanol, with the balance being

made up by ethanal and acetic acid at 31.4 and 12.4%,

respectively. Upon introduction of higher hydrogenation

activity through incorporation of substituents, the concen-

tration of ethanol in the C₂oxygenates fraction increases to

76.5% in the presence of Co, and 82−84% while the catalysts

contain nickel, with the acetic acid and ethanal content

decrease below 4 and 20%, respectively. Overall, alcohols are

the dominating oxygenate species in the product, independent

14670

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

Figure 11. GCxGC-FID (logarithmic scale) online product analysis

48 h of TOS at 20 bar, H₂/CO ratio of 2, and 280 °C over Fe, CoFe,

NiFe, and CoNiFe. In the small graph, (1) paraﬃn, (2) oleﬁn, (3)

aldehydes and ketones, (4) alcohols, and (5) acids. The enlarged

sections display the retention time area of linear and branched

alcohols.

of catalyst composition. Their content increases in the

presence of substituents from 60% to approximately 80% of

the oxygenated products. Figure 12b shows the oxygenates

(i.e., alcohols, aldehydes and acids) content in the total

product (i.e., oxygenate, oleﬁn, and paraﬃn) with a similar

carbon number. Due to the unfavored thermodynamics of

methanol formation, the selectivity to this product is low. The

highest selectivity was obtained at C₂, which could be due to

OH addition to the CH₃−CH= catalyst surface species,¹⁰⁹

and/or due to the CO-insertion mechanism taking place, that

is, insertion of molecular CO into the growing chain.¹⁰⁸With

increasing carbon number, the selectivity to oxygenates (Figure

12b) and alcohol content in total alcohols (Figure 12c)

decreased, which could be rationalized by carbon number-

dependent secondary conversion.²The ratio of linear to

branched (n/iso) paraﬃns and alcohols (Table 5) shows

similar values for the two classes of compounds (i.e., paraﬃns

and alcohols) for each catalyst. Moreover, the inclusion of

substituents (Co and/or Ni) in the Fe structure results in a

drop in the normal to iso ratio. This correlation could be due

to a common intermediate(s) for the formation of both

paraﬃns/oleﬁns and oxygenates.

Figure 12. C₂oxygenate composition (a), content of alcohols and

aldehydes in the linear product fraction as a function of carbon

number (b), and alcohol content in total alcohols as a function of

carbon number (c), produced by FTS after a 48 h TOS at 280 °C, 20

bar, and H₂/CO ratio of 2.

Table 5. Linear to Branched Ratio (n/iso) in Paraﬃns and

Alcohols after 48 h of TOS at 20 bar, H₂/CO Ratio of 2, and

280 °C, over the Four Catalysts

4. DISCUSSION

Fe

CoFe

NiFe

CoNiFe

Fischer−Tropsch synthesis is a well-established technology for

the conversion of coal, natural gas, heavy oils, or biomass to

synthetic fuels and chemicals. In the future, it is believed to

play an essential role in the storage and conversion of

renewable electricity as well, where CO₂is the carbon source

and electrolysis of water will provide hydrogen (and oxygen).

In an eventually defossilized society, it is important to dispose

over ﬂexible carbon-neutral technology that not only provides

synthetic fuels, but also a range of chemicals. Exploring how

selectivities in the FTS can be manipulated by varying the

composition of catalysts is therefore of great importance. The

paraﬃns

alcohols

4.2

4.8

4.1

3.7

3.5

3.0

3.6

3.1

present work shows that alloying the most used FTS catalyst,

iron, with other abundant metals, cobalt and nickel, provides a

way to shift the product distribution toward oxygenates, with

subtle but important changes in the other product classes as

well.

The bi- and tri-metallic catalysts are, in fact, modiﬁed iron

catalysts, derived from magnetite (ferrite) in which 10% of the

14671

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

iron has been replaced by nickel or cobalt or, in the tri-metallic

case, 20% by Ni and Co.

conversion levels and markedly changed product distributions

during FTS.

The FTS activity of the catalysts increased in the order Fe ≪

CoFe < NiFe < CoNiFe. Several factors contribute to this

trend. In addition to the larger particle size of the unpromoted

iron catalyst, the degree of carburization is signiﬁcantly lower

than in the promoted catalysts, as the iron catalyst forms iron

oxide during FTS. In contrast to this, the bi- and tri-metallic

catalysts are almost fully reduced and carbided (Table 3).

Furthermore, Co and Ni exhibit higher intrinsic activity than

iron for CO hydrogenation.^96,111

Addition of Co and/or Ni into the ferrite structure via a co-

precipitation method yielded substituted ferrites with crystal-

lite sizes ranging between 4 and 7 nm. The inclusion of the

substituents in the ferrite lattice alters the position (i.e., d-

spacing) of the (3 1 1) reﬂection, which is the most intense

one. The XRD analysis (Figure 1) shows a slight increase in

the d-spacing upon the introduction of the substitutes: from

2.51 Å for Fe₃O₄, to 2.52 Å for Co_0.3Fe_2.7O₄and Ni_0.3Fe_2.7O₄,

and 2.53 Å for the tri-metallic Co_0.3Ni_0.3Fe_2.4O₄. The change in

d-spacings is small, in line with the comparable ionic radii of

Fe²⁺(0.83 Å), Co²⁺(0.82 Å), and Ni²⁺(0.78 Å).¹¹⁰

Also, the magnetic measurements in Table 2 indicate that

the incorporation of Ni and Co in the ferrite lattice did occur,

as the measured values for the saturation magnetization of the

ferrite and Co-substituted ferrites were comparable, and both

higher than those of the Ni-containing ferrites, in agreement

with literature values. Fe₃O₄and CoFe₂O₄are reported to have

similar measured magnetic moments of 4 μB,¹¹⁰while the

NiFe₂O₄measured magnetic moment is 2.3 μB.¹¹⁰These

values explain the comparable saturation magnetization for

ferrite and the Co-substituted ferrite, and both being higher

than those of the Ni-containing ferrites (bi- and tri-metallic

ferrite). The same eﬀect is observed in the reduced alloys.

Hence, the characterization results are consistent with the

notion that the substituting elements have indeed been

incorporated in the ferrite structure. This is also reﬂected in

the thermal evolution of the reduction process, as is clearly

revealed by the in situ XRD experiments of Figure 4, and the in

situ magnetization measurements of Figure 5. While the eﬀects

of cobalt incorporation in the ferrite for reduction are subtle

(but noticeable, Figure 4), nickel clearly facilitates the

reduction of the iron signiﬁcantly and reduces the temperature

needed for complete reduction by more than 50 °C.

As the Fe metal and the bi- and tri-metallic alloys exhibit

higher saturation magnetization than the corresponding ferrites

(Table 2), reduction can be monitored by following the

magnetization in situ. One should be aware that, for

superparamagnetic particles, an increase in crystallite size

during the reduction process also contributes to an increase in

saturation magnetization of the metallic phase. Figure 5

conﬁrms the trend revealed by the in situ XRD that

incorporation of Ni in the ferrites enhances their reducibility

signiﬁcantly, while Co has little eﬀect. Reduction of oxides

proceeds via the dissociation of H₂on metallic nuclei formed

at the surface of the oxide, a process that occurs more easily on

nickel oxide than on iron oxide. Once such metallic nuclei have

been established, H₂molecules can dissociate readily and H-

atoms can diﬀuse over the surface and eﬀectuate reduction of

the iron oxide. This process is often referred to as intraparticle

hydrogen spillover in the literature.⁷⁹

The product selectivity of the CoFe system resembles that of

iron, but incorporation of Ni has a signiﬁcant eﬀect on the

distribution of products. The NiFe and CoNiFe catalysts show

higher hydrogenation activity, as evident in the higher

selectivity for CH₄, a lower chain-growth probability, lower

oleﬁn/paraﬃn ratios in the C₂₋₄and C₅₊fraction, and a

doubling in the selectivity toward oxygenates, notably C₂₊

alcohols. It is well-known that the primary FTS product is rich

in oleﬁns, while subsequent secondary readsorption and

hydrogenation enhances the overall paraﬃn content.¹¹²The

oleﬁn to paraﬃn ratio in the product fraction with carbon

numbers of 2−4 drops from 4.3 to 2.9 when Co is

incorporated into the catalyst and to 1 in the presence of Ni.

The same qualitative trend is observed for the longer carbon

chain numbers at an overall lower oleﬁn content, a well-

documented observation associated to a longer residence time

of the primary oleﬁn and, therefore, a higher probability of

secondary hydrogenation.^112−114The increases in the paraﬃn

content, methane selectivity, and decreased chain growth

probability upon incorporation of nickel in the catalyst can be

attributed to the relatively high hydrogenation activity of

nickel, and is in fact also reﬂected in the enhanced reducibility

of the Ni-containing catalysts, as discussed above.

The oxygenate selectivities over Fe, CoFe, NiFe, and

CoNiFe were 10.4, 11.7, 17.0, and 15.9 C%, respectively

(Table 4 and Figure 10), the dominant oxygenates being

alcohols, followed by aldehydes. Ethanol is the predominant

product, representing 40−50% of all oxygenates. Within the C₂

oxygenates, the ethanol content increases from 56% on pure

Fe, to 76% on CoFe and 82−84% for the Ni-containing alloys.

The selectivity of valuable C₂₊alcohols plus aldehydes

increased from 8.5 to 15 C% which translates to a relative

increase of 76%. This eﬀect is even more substantial when

expressing the C₂₊oxygenates formation in terms of yields

(1.40, 2.45, 3.71, 3.78 C% over Fe, CoFe, NiFe, and CoNiFe,

respectively) as increases of 75, 165, and 170% were obtained

via Co, Ni, and Co+Ni modiﬁcation, respectively. The ﬁndings

are considered to be of key importance for further develop-

ment of active and selective catalysts for speciﬁc classes of

compounds. It is likely that additional promotion (e.g., with

alkali metals) can greatly enhance performance in terms of this

type of catalysts and bring them on par with state-of-the-art

catalysts for the production of oxygenates from synthesis

gas.^115,116

The iron metal and the corresponding alloys form the

precursor of the catalytically active phases, which form in situ

during the FTS reaction. All active samples contain the Fe₅C₂

There are diﬀerent proposed formation mechanisms for

oxygenates reported in literature.²Aldehydes and alcohols

form in the “enol” mechanism via a chain termination step,

while acids may form via the Cannizzaro reaction (i.e.,

secondary reaction of aldehydes).²Johnston and Joyner¹¹⁷

proposed that alcohol formation takes place via the coupling

reaction of alkyl and surface hydroxyl groups. However, the

most considered mechanism of oxygenate formation in FTS is

̈

or Hagg carbide, while it is not possible to ascertain whether

these carbides contain the Co and Ni as substituents on the Fe

positions or as segregated metal or carbide. Nevertheless, the

eﬀect of the substituents is obvious, in the (1) crystallite sizes

of the carbide phases, which are smaller (Table 3), (2) smaller

amount and diﬀerent nature of the carbon accrued by the

catalysts during FTS (Figures 8 and 9) and, (3) higher CO

14672

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

the CO insertion mechanism.^2,108In this mechanism, chain

growth takes place via CO insertion into a metal−alkyl group,

yielding a surface acyl species. As C₁species are believed to

dominate on the catalyst surface (as also reﬂected in methane

selectivities often larger than expected due to ideal Anderson−

Schulz−Flory kinetics),²this predominantly leads to the

formation of C₂oxygenates (mostly ethanol), as observed in

this study. The formation of paraﬃns and oleﬁns occurs by

termination of the growing chain through hydrogen addition

or β-H-elimination, respectively. Importantly, alcohols are

proposed to form via the reaction of RCHOH surface species

with hydrogen. Aldehyde formation can take place via β-H-

elimination of RCHOH, yielding enol species, which isomerize

to aldehydes.²Considering the widely accepted CO insertion

mechanism for higher alcohols formation, in particular,

ethanol,^118,119two types of active sites may be required to

facilitate chain propagation and CO non-dissociative adsorp-

tion, as well as insertion into the growing chain. The former

function is possibly carried out over the Fe sites, while the

latter two functions preferably occur over the alloy sites (Co-

and/or Ni-Fe site). The presence of the substituents therefore

aids in the formation of oxygenates. Oxygenates are also

reported to undergo secondary reactions, including hydro-

genation and incorporation into the growing chain.^120−122The

high oxygenates selectivity at the C₂position could be

explained by the high stability of ethanol, in comparison to

ASSOCIATED CONTENT

■

sı

* Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acscatal.0c03346.

Theoretical calculations of the catalyst surface area and

crystallite size, FTS activity and selectivity calculation/

formulas, and conventional H₂-TPR proﬁle, as well as

detailed 2D GCxGC analysis method is provided (PDF)

AUTHOR INFORMATION

■

Corresponding Author

Michael Claeys − Catalysis Institute and c*change (DSI-NRF

Centre of Excellence in Catalysis), Department of Chemical

Engineering, University of Cape Town, Rondebosch 7701,

South Africa; orcid.org/0000-0002-5797-5023;

Email: michael.claeys@uct.ac.za

Authors

Mohamed I. Fadlalla − Catalysis Institute and c*change (DSI-

NRF Centre of Excellence in Catalysis), Department of

Chemical Engineering, University of Cape Town, Rondebosch

7701, South Africa

Sundaram Ganesh Babu − Catalysis Institute and c*change

(DSI-NRF Centre of Excellence in Catalysis), Department of

Chemical Engineering, University of Cape Town, Rondebosch

7701, South Africa

Thulani M. Nyathi − Catalysis Institute and c*change (DSI-

NRF Centre of Excellence in Catalysis), Department of

Chemical Engineering, University of Cape Town, Rondebosch

7701, South Africa

C

₃₊alcohols, against secondary reactions¹²³and the possibility

of ethanol forming via an additional route, to CO insertion, by

the reaction of surface ethylidene with an OH group.¹²⁴In

general, the production of oxygenates, in particular, alcohols,

require a catalyst that would facilitate the chain growth/

propagation and CO insertion steps.¹²⁵Although the catalytic

material in this study does not provide high oxygenates

selectivity, it provides a route to at least enhance oxygenates

formation via a stabilized Fe-based catalyst with limited carbon

deposition and improving reducibility and carburization.

Finally, we note that the GCxGC product analysis in Figure

11 illustrates how rich the product distributions of the FTS

catalysts are. In the future, we intend to employ such

information for detailed mechanistic studies of chain growth

in the Fischer−Tropsch reaction.

C. J. Kees-Jan Weststrate − SynCat@DIFFER, Syngaschem

BV, 5600 HH Eindhoven, The Netherlands

Nico Fischer − Catalysis Institute and c*change (DSI-NRF

Centre of Excellence in Catalysis), Department of Chemical

Engineering, University of Cape Town, Rondebosch 7701,

South Africa; orcid.org/0000-0002-8817-3621

J. W. Hans Niemantsverdriet − Catalysis Institute and

c*change (DSI-NRF Centre of Excellence in Catalysis),

Department of Chemical Engineering, University of Cape

Town, Rondebosch 7701, South Africa; SynCat@DIFFER,

Syngaschem BV, 5600 HH Eindhoven, The Netherlands

5. CONCLUSION

The study determined the inﬂuence of Co and/or Ni

substituents in the ferrite structure and on the chemical and

physical properties, as well as FTS activity and selectivity,

particularly to oxygenated compounds. Characterization via in

situ XRD and magnetometry analyses under a H₂environment

showed that Ni acts as a reduction promoter by facilitating the

reduction of the ferrite at lower temperatures. Alloys were

formed upon reduction, but exposure to FT conditions caused

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.0c03346

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

̈

the formation of Hagg carbide, likely containing Co and Ni as

substituents. The modiﬁcation with Co and Ni further resulted

in smaller crystallites, less carbon deposition, and pronounced

positive eﬀects on FTS activity (i.e., CO conversion), as well as

increased selectivities to methane, paraﬃns and oxygenates. It

is likely that the modiﬁcation, in particular that with nickel,

leads to signiﬁcant changes in hydrogen availability on the

catalyst surface, which selectively aﬀects steps of product

formation ultimately promoting desorption as alcohols. The

ﬁndings of this study are of fundamental importance toward

the development of iron-based catalysts optimized for selective

product formation.

The authors acknowledge the ﬁnancial contributions from

c*change (DSI-NRF Centre of Excellence in Catalysis), the

National Research Foundation (NRF), and Syngaschem BV,

Eindhoven, The Netherlands. The latter is grateful for funding

from Synfuels China Technology Co. Ltd, Beijing-Huairou.

We further acknowledge the Aaron Klug Centre for Imaging

and Analysis at the University of Cape Town, Mr. Russell

Geland from the analytical laboratory for ICP-OES and BET-

surface area measurements, and Mr. Dominic de Oliveira and

Mr. Adli Peck for assistance with the magnetometer analysis.

14673

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

REFERENCES

pubs.acs.org/acscatalysis

Research Article

(20) Azizi, H. R.; Mirzaei, A. A.; Kaykhaii, M.; Mansouri, M.

Fischer −Tropsch synthesis: Studies effect of reduction variables on

the performance of Fe −Ni −Co catalyst. J. Nat. Gas Sci. Eng. 2014, 18,

484−491.

(21) Niemantsverdriet, J. W.; Van Grondelle, J.; Van der Kraan, A.

M. Methanol from synthesis gas over bimetallic FePd catalysts.

Hyperfine Interact. 1988, 41 (1), 677−680.

(22) Kimura, T.; Fukuoka, A.; Fumagalli, A.; Ichikawa, M. Fe-

Promoted selective methanol synthesis in CO hydrogenation

catalyzed on SiO₂-supported Ir₄Fe and Pd₆Fe₆bimetallic carbonyl

cluster-derived catalysts. Catal. Lett. 1989, 2 (4), 227−233.

(23) Fukushima, T.; Araki, K.; Ichikawa, M. Promoting effects of Fe

addition in MeOH synthesis catalysed by SiO₂-supported Pd. J. Chem.

Soc., Chem. Commun. 1986, No. 2, 148b−150.

(24) Bhasin, M. M.; Bartley, W. J.; Ellgen, P. C.; Wilson, T. P.

Synthesis gas conversion over supported rhodium and rhodium-iron

catalysts. J. Catal. 1978, 54 (2), 120−128.

(25) van Gruijthuijsen, L. M. P.; Louwers, S. P. A.; van Santen, R. A.;

Niemantsverdriet, J. W. CO bonding and hydrogenation activity of

promoted noble metal catalysts during restructuring in high-pressure

CO hydrogenation: FeIr/SiO₂. Catal. Lett. 1997, 43 (1), 45−49.

(26) Arai, H.; Mitsuishi, K.; Seiyama, T. TiO₂-supported Fe−Co,

Co −Ni, and Ni −Fe alloy catalysts for Fischer −Tropsch synthesis.

Chem. Lett. 1984, 13 (8), 1291−1294.

(27) Li, T.; Wang, H.; Yang, Y.; Xiang, H.; Li, Y. Study on an iron −

nickel bimetallic Fischer −Tropsch synthesis catalyst. Fuel Process.

Technol. 2014, 118, 117−124.

(28) Cooper, M. E.; Frost, J. New iron/nickel alloy catalyst for

Fischer −Tropsch synthesis. Appl. Catal. 1990, 57 (1), L5−L8.

(29) Modak, S.; Ammar, M.; Mazaleyrat, F.; Das, S.; Chakrabarti, P.

K. XRD, HRTEM and magnetic properties of mixed spinel

nanocrystalline Ni −Zn −Cu-ferrite. J. Alloys Compd. 2009, 473 (1),

15−19.

(30) Patoux, S.; Daniel, L.; Bourbon, C.; Lignier, H.; Pagano, C.; Le

Cras, F.; Jouanneau, S.; Martinet, S. High voltage spinel oxides for Li-

ion batteries: From the material research to the application. J. Power

Sources 2009, 189 (1), 344−352.

■

(1) van der Laan, G. P.; Beenackers, A. A. C. M. Kinetics and

Selectivity of the Fischer −Tropsch Synthesis: A Literature Review.

Catal. Rev.: Sci. Eng. 1999, 41 (3−4), 255−318.

(2) Claeys, M.; van Steen, E. Basic studies. In Stud. Surf. Sci. Catal.;

Steynberg, A., Dry, M., Eds.; Elsevier, 2004; Vol. 152, Chapter 8, pp

601−680.

(3) van de Loosdrecht, J.; Botes, F. G.; Ciobica, I. M.; Ferreira, A.;

Gibson, P.; Moodley, D. J.; Saib, A. M.; Visagie, J. L.; Weststrate, C. J.;

Niemantsverdriet, J. W. 7.20 - Fischer−Tropsch Synthesis: Catalysts

and Chemistry. In Comprehensive Inorganic Chemistry II, 2nd ed.;

Reedijk, J., Poeppelmeier, K., Eds.; Elsevier: Amsterdam, 2013; pp

525−557.

́

(4) Abello, S.; Montane, D. Exploring Iron-based Multifunctional

Catalysts for Fischer −Tropsch Synthesis: A Review. ChemSusChem

2011, 4 (11), 1538−1556.

(5) Jahangiri, H.; Bennett, J.; Mahjoubi, P.; Wilson, K.; Gu, S. A

review of advanced catalyst development for Fischer −Tropsch

synthesis of hydrocarbons from biomass derived syn-gas. Catal. Sci.

Technol. 2014, 4 (8), 2210−2229.

(6) Sapountzi, F. M.; Gracia, J. M.; Weststrate, C. J.; Fredriksson, H.

O. A.; Niemantsverdriet, J. W. Electrocatalysts for the generation of

hydrogen, oxygen and synthesis gas. Prog. Energy Combust. Sci. 2017,

58, 1−35.

(7) Calderone, V. R.; Shiju, N. R.; Ferre, D. C.; Rothenberg, G.

Bimetallic catalysts for the Fischer −Tropsch reaction. Green Chem.

2011, 13 (8), 1950−1959.

(8) Jothimurugesan, K.; Gangwal, S. K. Titania-Supported Bimetallic

Catalysts Combined with HZSM-5 for Fischer −Tropsch Synthesis.

Ind. Eng. Chem. Res. 1998, 37 (4), 1181−1188.

(9) van de Loosdrecht, J.; van der Kraan, A. M.; van Dillen, A. J.;

Geus, J. W. Metal-support interaction: titania-supported nickel-iron

catalysts. Catal. Lett. 1996, 41 (1), 27−34.

(10) Feyzi, M.; Khodaei, M. M.; Shahmoradi, J. Effect of sulfur on

the catalytic performance of Fe −Ni/Al₂O₃catalysts for light olefins

production. J. Taiwan Inst. Chem. Eng. 2014, 45 (2), 452−460.

(11) Feyzi, M.; Mirzaei, A. A.; Bozorgzadeh, H. R. Effects of

preparation and operation conditions on precipitated iron nickel

catalysts for Fischer −Tropsch synthesis. J. Nat. Gas Chem. 2010, 19

(3), 341−353.

(31) Menini, L.; Pereira, M. C.; Parreira, L. A.; Fabris, J. D.;

Gusevskaya, E. V. Cobalt- and manganese-substituted ferrites as

efficient single-site heterogeneous catalysts for aerobic oxidation of

monoterpenic alkenes under solvent-free conditions. J. Catal. 2008,

254 (2), 355−364.

(12) Ishihara, T.; Eguchi, K.; Arai, H. Hydrogenation of carbon

monoxide over SiO₂-supported Fe-Co, Co-Ni and Ni-Fe bimetallic

catalysts. Appl. Catal. 1987, 30 (2), 225−238.

(13) Tihay, F.; Pourroy, G.; Roger, A. C.; Kiennemann, A. Selective

synthesis of C₂-C₄oleﬁns on Fe-Co based metal/oxide composite

materials. In Stud. Surf. Sci. Catal.; Parmaliana, A., Sanﬁlippo, D.,

Frusteri, F., Vaccari, A., Arena, F., Eds.; Elsevier, 1998; Vol. 119, pp

143−148.

(32) Toledo-Antonio, J. A.; Nava, N.; Martínez, M.; Bokhimi, X.

Correlation between the magnetism of non-stoichiometric zinc

ferrites and their catalytic activity for oxidative dehydrogenation of

1-butene. Appl. Catal., A 2002, 234 (1), 137−144.

(33) Rashad, M. M.; Fouad, O. A. Synthesis and characterization of

nano-sized nickel ferrites from fly ash for catalytic oxidation of CO.

Mater. Chem. Phys. 2005, 94 (2), 365−370.

(14) Tihay, F.; Roger, A. C.; Kiennemann, A.; Pourroy, G. Fe −Co

based metal/spinel to produce light olefins from syngas. Catal. Today

2000, 58 (4), 263−269.

(34) Khan, A.; Smirniotis, P. G. Relationship between temperature-

programmed reduction profile and activity of modified ferrite-based

catalysts for WGS reaction. J. Mol. Catal. A: Chem. 2008, 280 (1),

43−51.

(35) Chonco, Z. H.; Lodya, L.; Claeys, M.; van Steen, E. Copper

ferrites: A model for investigating the role of copper in the dynamic

iron-based Fischer −Tropsch catalyst. J. Catal. 2013, 308, 363−373.

(36) Kumar, N.; Smith, M. L.; Spivey, J. J. Characterization and

testing of silica-supported cobalt −palladium catalysts for conversion

of syngas to oxygenates. J. Catal. 2012, 289, 218−226.

(15) Tihay, F.; Roger, A. C.; Pourroy, G.; Kiennemann, A. Role of

the Alloy and Spinel in the Catalytic Behavior of Fe-Co/Cobalt

Magnetite Composites under CO and CO₂Hydrogenation. Energy

Fuels 2002, 16 (5), 1271−1276.

(16) Duvenhage, D. J.; Coville, N. J. Fe:Co/TiO₂bimetallic catalysts

for the Fischer −Tropsch reaction: Part 4: A study of nitrate and

carbonyl derived FT catalysts. J. Mol. Catal. A: Chem. 2005, 235 (1),

230−239.

(17) Duvenhage, D. J.; Coville, N. J. Fe:CoTiO₂bimetallic catalysts

for the Fischer −Tropsch reaction I. Characterization and reactor

studies. Appl. Catal., A 1997, 153 (1), 43−67.

(18) Duvenhage, D. J.; Coville, N. J. Fe:Co/TiO₂bimetallic catalysts

for the Fischer −Tropsch reaction: Part 2. The effect of calcination

and reduction temperature. Appl. Catal., A 2002, 233 (1), 63−75.

(19) Duvenhage, D. J.; Coville, N. J. Fe:Co/TiO₂bimetallic catalysts

for the Fischer −Tropsch reaction: Part 3: The effect of Fe:Co ratio,

mixing and loading on FT product selectivity. Appl. Catal., A 2005,

289 (2), 231−239.

́

(37) Luk, H. T.; Mondelli, C.; Ferre, D. C.; Stewart, J. A.; Perez-

Ramírez, J. Status and prospects in higher alcohols synthesis from

syngas. Chem. Soc. Rev. 2017, 46 (5), 1358−1426.

(38) Bengelsdorf, F. R.; Straub, M.; Durre, P. Bacterial synthesis gas

̈

(syngas) fermentation. Environ. Technol. 2013, 34 (13−14), 1639−

1651.

(39) Nakagawa, Y.; Tajima, N.; Hirao, K. J. Comput. Chem. 2000, 21

(14), 1292−1304.

14674

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

(40) Xiang, Y.; Barbosa, R.; Kruse, N. Higher Alcohols through CO

Hydrogenation over CoCu Catalysts: Influence of Precursor

Activation. ACS Catal. 2014, 4 (8), 2792−2800.

(61) Shebanova, O. N.; Lazor, P. Raman spectroscopic study of

magnetite (FeFe₂O₄): a new assignment for the vibrational spectrum.

J. Solid State Chem. 2003, 174 (2), 424−430.

(62) Yue-Jian, C.; Juan, T.; Fei, X.; Jia-Bi, Z.; Ning, G.; Yi-Hua, Z.;

Ye, D.; Liang, G. Synthesis, self-assembly, and characterization of

PEG-coated iron oxide nanoparticles as potential MRI contrast agent.

Drug Dev. Ind. Pharm. 2010, 36 (10), 1235−44.

(63) Graves, P. R.; Johnston, C.; Campaniello, J. J. Raman scattering

in spinel structure ferrites. Mater. Res. Bull. 1988, 23 (11), 1651−

1660.

(64) Beattie, I. R.; Gilson, T. R. The single-crystal Raman spectra of

nearly opaque materials. Iron(III) oxide and chromium(III) oxide. J.

Chem. Soc. A 1970, No. 0, 980−986.

(65) Coelho, A. Indexing of powder diffraction patterns by iterative

use of singular value decomposition. J. Appl. Crystallogr. 2003, 36 (1),

86−95.

(66) Ding, M.; Yang, Y.; Wu, B.; Li, Y.; Wang, T.; Ma, L. Study on

reduction and carburization behaviors of iron phases for iron-based

Fischer −Tropsch synthesis catalyst. Appl. Energy 2015, 160, 982−989.

(67) Conner, W. C.; Falconer, J. L. Spillover in Heterogeneous

Catalysis. Chem. Rev. 1995, 95 (3), 759−788.

(68) Nabaho, D.; Niemantsverdriet, J. W.; Claeys, M.; van Steen, E.

Hydrogen spillover in the Fischer −Tropsch synthesis: An analysis of

platinum as a promoter for cobalt −alumina catalysts. Catal. Today

2016, 261, 17−27.

(69) Kolhatkar, A. G.; Jamison, A. C.; Litvinov, D.; Willson, R. C.;

Lee, T. R. Tuning the magnetic properties of nanoparticles. Int. J. Mol.

Sci. 2013, 14 (8), 15977−16009.

(70) Lee, J.-H.; Huh, Y.-M.; Jun, Y.-w.; Seo, J.-w.; Jang, J.-t.; Song,

H.-T.; Kim, S.; Cho, E.-J.; Yoon, H.-G.; Suh, J.-S.; Cheon, J.

Artificially engineered magnetic nanoparticles for ultra-sensitive

molecular imaging. Nat. Med. 2007, 13 (1), 95−99.

(71) Maaz, K.; Mumtaz, A.; Hasanain, S. K.; Ceylan, A. Synthesis

and magnetic properties of cobalt ferrite (CoFe₂O₄) nanoparticles

prepared by wet chemical route. J. Magn. Magn. Mater. 2007, 308 (2),

289−295.

(72) Gong, Y. X.; Zhen, L.; Jiang, J. T.; Xu, C. Y.; Shao, W. Z.

Preparation of CoFe alloy nanoparticles with tunable electromagnetic

wave absorption performance. J. Magn. Magn. Mater. 2009, 321 (22),

3702−3705.

(73) Nabiyouni, G; Fesharaki, M. J.; Mozafari, M; Amighian, J

Characterization and Magnetic Properties of Nickel Ferrite Nano-

particles Prepared by Ball Milling Technique. Chin. Phys. Lett. 2010,

27 (12), 126401−4.

(74) van Helden, P.; Prinsloo, F.; van den Berg, J.-A.; Xaba, B.;

Erasmus, W.; Claeys, M.; van de Loosdrecht, J. Cobalt-nickel

bimetallic Fischer −Tropsch catalysts: A combined theoretical and

experimental approach. Catal. Today 2020, 342, 88−98.

(75) Dijith, K. S.; Aiswarya, R.; Praveen, M.; Pillai, S.; Surendran, K.

P. Polyol derived Ni and NiFe alloys for effective shielding of

electromagnetic interference. Mater. Chem. Front. 2018, 2 (10),

1829−1841.

(76) Dumitru, R.; Papa, F.; Balint, I.; Culita, D. C.; Munteanu, C.;

Stanica, N.; Ianculescu, A.; Diamandescu, L.; Carp, O. Mesoporous

cobalt ferrite: A rival of platinum catalyst in methane combustion

reaction. Appl. Catal., A 2013, 467, 178−186.

(41) Xiang, Y.; Kovarik, L.; Kruse, N. Rate and selectivity hysteresis

during the carbon monoxide hydrogenation over promoted Co/

MnO_xcatalysts. Nat. Commun. 2019, 10 (1), 3953.

(42) Zhao, Z.; Lu, W.; Yang, R.; Zhu, H.; Dong, W.; Sun, F.; Jiang,

Z.; Lyu, Y.; Liu, T.; Du, H.; Ding, Y. Insight into the Formation of

Co@Co₂C Catalysts for Direct Synthesis of Higher Alcohols and

Olefins from Syngas. ACS Catal. 2018, 8 (1), 228−241.

(43) Liu, Y.; Murata, K.; Inaba, M. Synthesis of mixed alcohols from

synthesis gas over alkali and Fischer −Tropsch metals modified MoS₂/

Al₂O₃-montmorillonite catalysts. React. Kinet., Mech. Catal. 2014, 113

(1), 187−200.

(44) Wu, Q.; Christensen, J. M.; Chiarello, G. L.; Duchstein, L. D.

L.; Wagner, J. B.; Temel, B.; Grunwaldt, J.-D.; Jensen, A. D.

Supported molybdenum carbide for higher alcohol synthesis from

syngas. Catal. Today 2013, 215, 162−168.

(45) Ojeda, M.; Granados, M. L.; Rojas, S.; Terreros, P.; García-

García, F. J.; Fierro, J. L. G. Manganese-promoted Rh/Al₂O₃for C₂-

oxygenates synthesis from syngas: Effect of manganese loading. Appl.

Catal., A 2004, 261 (1), 47−55.

(46) Pijolat, M.; Perrichon, V. Synthesis of alcohols from CO and H₂

on a Fe/A1₂0₃catalyst at 8−30 bar pressure. Appl. Catal. 1985, 13

(2), 321−333.

(47) Ding, M.; Ma, L.; Zhang, Q.; Wang, C.; Zhang, W.; Wang, T.

Enhancement of conversion from bio-syngas to higher alcohols fuels

over K-promoted Cu-Fe bimodal pore catalysts. Fuel Process. Technol.

2017, 159, 436−441.

(48) Dry, M. E. High quality diesel via the Fischer −Tropsch process

− a review. J. Chem. Technol. Biotechnol. 2002, 77 (1), 43−50.

(49) Arulmurugan, R.; Jeyadevan, B.; Vaidyanathan, G.;

Sendhilnathan, S. Effect of zinc substitution on Co −Zn and Mn −

Zn ferrite nanoparticles prepared by co-precipitation. J. Magn. Magn.

Mater. 2005, 288, 470−477.

(50) Fischer, N.; Claeys, M. Phase changes studied under in situ

conditions A novel cell. Catal. Today 2016, 275, 149−154.

(51) Claeys, M. C. M.; Fischer, N. F. Sample presentation device for

radiation-based analytical equipment. U.S. Patent 8,597,598 B2, 2013.

(52) Fischer, N.; Clapham, B.; Feltes, T.; van Steen, E.; Claeys, M.

Size-Dependent Phase Transformation of Catalytically Active Nano-

particles Captured In Situ. Angew. Chem., Int. Ed. 2014, 53 (5), 1342−

1345.

(53) Claeys, M.; Dry, M. E.; van Steen, E.; du Plessis, E.; van Berge,

P. J.; Saib, A. M.; Moodley, D. J. In situ magnetometer study on the

formation and stability of cobalt carbide in Fischer −Tropsch

synthesis. J. Catal. 2014, 318, 193−202.

(54) Claeys, M.; Van Steen, E.; Visagie, J. L.; Van de Loosdrecht, J.

Magnetometer. U.S. patent 8,773,118 B2, 2014.

(55) Dalmon, J.-A. Magnetic Measurements and Catalysis. In

Catalyst Characterization: Physical Techniques for Solid Materials;

Imelik, B., Vedrine, J. C., Eds.; Springer US: Boston, MA, 1994; pp

585−609.

(56) Fischer, N.; Henkel, R.; Hettel, B.; Iglesias, M.; Schaub, G.;

Claeys, M. Hydrocarbons via CO₂Hydrogenation Over Iron

Catalysts: The Effect of Potassium on Structure and Performance.

Catal. Lett. 2016, 146 (2), 509−517.

(57) Schulz, H.; Geertsema, A. Erdol und Kohle 1977, 30, 313−321.

(58) Wolf, M.; Fischer, N.; Claeys, M. Effectiveness of catalyst

passivation techniques studied in situ with a magnetometer. Catal.

Today 2016, 275, 135−140.

(59) Song, K.; Lee, Y.; Jo, M. R.; Nam, K. M.; Kang, Y.-M.

Comprehensive design of carbon-encapsulated Fe₃O₄nanocrystals

and their lithium storage properties. Nanotechnology 2012, 23 (50),

505401−505406.

(60) Murugappan, K.; Silvester, D. S.; Chaudhary, D.; Arrigan, D. W.

M. Electrochemical Characterization of an Oleyl-coated Magnetite

Nanoparticle-Modified Electrode. ChemElectroChem 2014, 1 (7),

1211−1218.

(77) Jozwiak, W. K.; Kaczmarek, E.; Maniecki, T. P.; Ignaczak, W.;

Maniukiewicz, W. Reduction behavior of iron oxides in hydrogen and

carbon monoxide atmospheres. Appl. Catal., A 2007, 326 (1), 17−27.

(78) Braga, T. P.; Sales, B. M. C.; Pinheiro, A. N.; Herrera, W. T.;

Baggio-Saitovitch, E.; Valentini, A. Catalytic properties of cobalt and

nickel ferrites dispersed in mesoporous silicon oxide for ethylbenzene

dehydrogenation with CO₂. Catal. Sci. Technol. 2011, 1 (8), 1383−

1392.

(79) Hurst, N. W.; Gentry, S. J.; Jones, A.; McNicol, B. D.

Temperature Programmed Reduction. Catal. Rev.: Sci. Eng. 1982, 24

(2), 233−309.

14675

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

(80) Unmuth, E. E.; Schwartz, L. H.; Butt, J. B. Iron alloy Fischer −

Tropsch catalysts: I: Carburization studies of the Fe-Ni system. J.

Catal. 1980, 63 (2), 404−414.

(81) Cheng-Hua, Z.; Yong, Y.; Zhi-Chao, T. A. O.; Hong-Wei, X.;

Yong-Wang, L. I. Structural properties and reduction behavior of Ni

promoted FeMnK/SiO₂catalysts for Fischer−Tropsch synthesis. J.

Fuel Chem. Technol. 2006, 34 (6), 695−699.

(82) Li, S.; Li, A.; Krishnamoorthy, S.; Iglesia, E. Effects of Zn, Cu,

and K Promoters on the Structure and on the Reduction,

Carburization, and Catalytic Behavior of Iron-Based Fischer −Tropsch

Synthesis Catalysts. Catal. Lett. 2001, 77 (4), 197−205.

(83) Wu, B.; Tian, L.; Xiang, H.; Zhang, Z.; Li, Y.-W. Novel

precipitated iron Fischer −Tropsch catalysts with Fe₃O₄coexisting

with α -Fe₂O₃. Catal. Lett. 2005, 102 (3), 211−218.

(84) Kniess, C. T.; Lima, J. C. d.; Prates, P. B. The Quantiﬁcation of

Crystalline Phases in Materials: Applications of Rietveld Method. In

Sintering − Methods and Products; Shatokha, V., Ed.; InTech: Croatia,

2012; pp 293−316.

(85) de Smit, E.; Weckhuysen, B. M. The renaissance of iron-based

Fischer −Tropsch synthesis: on the multifaceted catalyst deactivation

behaviour. Chem. Soc. Rev. 2008, 37 (12), 2758−2781.

(86) Chen, W.; Kimpel, T. F.; Song, Y.; Chiang, F.-K.; Zijlstra, B.;

Pestman, R.; Wang, P.; Hensen, E. J. M. Influence of Carbon Deposits

on the Cobalt-Catalyzed Fischer −Tropsch Reaction: Evidence of a

Two-Site Reaction Model. ACS Catal. 2018, 8 (2), 1580−1590.

(87) Nishiyama, Y.; Tamai, Y. Deposition of carbon and its

hydrogenation catalyzed by nickel. Carbon 1976, 14 (1), 13−17.

(88) Pradhan, A. R.; Wu, J. F.; Jong, S. J.; Tsai, T. C.; Liu, S. B. An

ex situ methodology for characterization of coke by TGA and ¹³C CP-

MAS NMR spectroscopy. Appl. Catal., A 1997, 165 (1), 489−497.

(89) He, S.; Sun, C.; Yang, X.; Wang, B.; Dai, X.; Bai, Z.

Characterization of coke deposited on spent catalysts for long-chain-

paraffin dehydrogenation. Chem. Eng. J. 2010, 163 (3), 389−394.

(90) Sahoo, S. K.; Rao, P. V. C.; Rajeshwer, D.; Krishnamurthy, K.

R.; Singh, I. D. Structural characterization of coke deposits on

industrial spent paraffin dehydrogenation catalysts. Appl. Catal., A

2003, 244 (2), 311−321.

(91) Gopiraman, M.; Deng, D.; Ganesh Babu, S.; Hayashi, T.;

Karvembu, R.; Kim, I. S. Sustainable and Versatile CuO/GNS

Nanocatalyst for Highly Efficient Base Free Coupling Reactions. ACS

Sustainable Chem. Eng. 2015, 3 (10), 2478−2488.

(92) Gopiraman, M.; Ganesh Babu, S.; Khatri, Z.; Kai, W.; Kim, Y.

A.; Endo, M.; Karvembu, R.; Kim, I. S. Dry Synthesis of Easily

Tunable Nano Ruthenium Supported on Graphene: Novel Nano-

catalysts for Aerial Oxidation of Alcohols and Transfer Hydrogenation

of Ketones. J. Phys. Chem. C 2013, 117 (45), 23582−23596.

(93) Babu, S. G.; Gopiraman, M.; Deng, D.; Wei, K.; Karvembu, R.;

Kim, I. S. Robust Au −Ag/graphene bimetallic nanocatalyst for

multifunctional activity with high synergism. Chem. Eng. J. 2016, 300,

146−159.

(94) Scheibe, H.-J.; Drescher, D.; Alers, P. Raman characterization

of amorphous carbon films. Fresenius' J. Anal. Chem. 1995, 353 (5),

695−697.

(95) Dillon, R. O.; Woollam, J. A.; Katkanant, V. Use of Raman

scattering to investigate disorder and crystallite formation in as-

deposited and annealed carbon films. Phys. Rev. B: Condens. Matter

Mater. Phys. 1984, 29 (6), 3482−3489.

(96) Dry, M. E. FT catalysts. In Stud. Surf. Sci. Catal.; Steynberg, A.,

Dry, M., Eds.; Elsevier, 2004; Vol. 152, Chapter 7, pp 533−600.

(97) Patzlaff, J.; Liu, Y.; Graffmann, C.; Gaube, J. Studies on product

distributions of iron and cobalt catalyzed Fischer −Tropsch synthesis.

Appl. Catal., A 1999, 186 (1), 109−119.

(100) Watanabe, K.; Miyao, T.; Higashiyama, K.; Yamashita, H.;

Watanabe, M. High temperature water −gas shift reaction over hollow

Ni −Fe −Al oxide nano-composite catalysts prepared by the solution-

spray plasma technique. Catal. Commun. 2009, 10 (14), 1952−1955.

(101) Grobler, T.; Claeys, M.; van Steen, E.; Janse van Vuuren, M. J.

GC × GC: A novel technique for investigating selectivity in the

Fischer −Tropsch synthesis. Catal. Commun. 2009, 10 (13), 1674−

1680.

(102) Di Sanzo, F. P.; Lane, J. L.; Bergquist, P. M.; Mooney, S. A.;

Wu, B. G. Determination of total oxygenates in fischer-tropsch liquid

products. J. Chromatogr. A 1983, 281, 101−108.

(103) Anderson, R. R.; White, C. M. Analysis of Fischer −Tropsch

by-product waters by gas chromatography. J. High Resolut.

Chromatogr. 1994, 17 (4), 245−250.

(104) Nel, R. J. J.; de Klerk, A. Fischer −Tropsch Aqueous Phase

Refining by Catalytic Alcohol Dehydration. Ind. Eng. Chem. Res. 2007,

46 (11), 3558−3565.

(105) Gnanamani, M. K.; Jacobs, G.; Graham, U. M.; Ribeiro, M. C.;

Noronha, F. B.; Shafer, W. D.; Davis, B. H. Influence of carbide

formation on oxygenates selectivity during Fischer −Tropsch synthesis

over Ce-containing Co catalysts. Catal. Today 2016, 261, 40−47.

(106) Xiong, J.; Ding, Y.; Wang, T.; Yan, L.; Chen, W.; Zhu, H.; Lu,

Y. The formation of Co₂C species in activated carbon supported

cobalt-based catalysts and its impact on Fischer −Tropsch reaction.

Catal. Lett. 2005, 102 (3), 265−269.

(107) Tau, L. M.; Dabbagh, H. A.; Davis, B. H. Fischer −Tropsch

synthesis: carbon-14 tracer study of alkene incorporation. Energy Fuels

1990, 4 (1), 94−99.

(108) Pichler, H.; Schulz, H. Chem. Ing. Tech. 1970, 42 (18), 1162−

1174.

(109) Schulz, H.; Schaub, G.; Claeys, M.; Riedel, T. Transient initial

kinetic regimes of Fischer −Tropsch synthesis. Appl. Catal., A 1999,

186 (1), 215−227.

(110) Goldman, A. Crystal Structure of Ferrites. Modern Ferrite

Technology, 2nd ed.; Springer US: Pittsburgh, PA, 2006; pp 51−70.

(111) Botes, F. G.; Niemantsverdriet, J. W.; van de Loosdrecht, J. A

comparison of cobalt and iron based slurry phase Fischer −Tropsch

synthesis. Catal. Today 2013, 215, 112−120.

(112) Schulz, H.; Claeys, M. Reactions of α -olefins of different chain

length added during Fischer −Tropsch synthesis on a cobalt catalyst in

a slurry reactor. Appl. Catal., A 1999, 186 (1), 71−90.

(113) Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. Selectivity

Control and Catalyst Design in the Fischer−Tropsch Synthesis: Sites,

Pellets, and Reactors. In Adv. Catal.; Eley, D. D., Pines, H., Weisz, P.

B., Eds.; Academic Press, 1993; Vol. 39, pp 221−302.

(114) Kuipers, E. W.; Scheper, C.; Wilson, J. H.; Vinkenburg, I. H.;

Oosterbeek, H. Non-ASF Product Distributions Due to Secondary

Reactions during Fischer −Tropsch Synthesis. J. Catal. 1996, 158 (1),

288−300.

(115) Ma, W.; Kugler, E. L.; Dadyburjor, D. B. Promotional Effect of

Copper on Activity and Selectivity to Hydrocarbons and Oxygenates

for Fischer −Tropsch Synthesis over Potassium-Promoted Iron

Catalysts Supported on Activated Carbon. Energy Fuels 2011, 25

(5), 1931−1938.

(116) Zhang, X.; Liu, Y.; Liu, G.; Tao, K.; Jin, Q.; Meng, F.; Wang,

D.; Tsubaki, N. Product distributions including hydrocarbon and

oxygenates of Fischer −Tropsch synthesis over mesoporous MnO₂-

supported Fe catalyst. Fuel 2012, 92 (1), 122−129.

(117) Johnston, P.; Joyner, R. W. Structure-Function Relationships

in Heterogeneous Catalysis: the Embedded Surface Molecule

Approach and Its Applications. In Stud. Surf. Sci. Catal.; Guczi, L.;

́

Solymosi, F.; Tetenyi, P., Eds.; Elsevier, 1993; Vol. 75, pp 165−180.

(118) Zhao, L.; Mu, X.; Liu, T.; Fang, K. Bimetallic Ni −Co catalysts

supported on Mn −Al oxide for selective catalytic CO hydrogenation

to higher alcohols. Catal. Sci. Technol. 2018, 8 (8), 2066−2076.

(119) Davis, B. H. Fischer −Tropsch synthesis: current mechanism

and futuristic needs. Fuel Process. Technol. 2001, 71 (1), 157−166.

(120) Ozbek, M. O.; Niemantsverdriet, J. W. Methane, form-

aldehyde and methanol formation pathways from carbon monoxide

(98) Watanabe, K.; Miyao, T.; Higashiyama, K.; Yamashita, H.;

Watanabe, M. Preparation of a mesoporous ceria −zirconia supported

Ni −Fe catalyst for the high temperature water −gas shift reaction.

Catal. Commun. 2011, 12 (11), 976−979.

(99) Ashok, J.; Wai, M. H.; Kawi, S. Nickel-based Catalysts for High-

temperature Water Gas Shift Reaction-Methane Suppression.

ChemCatChem 2018, 10 (18), 3927−3942.

14676

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

ACS Catalysis

pubs.acs.org/acscatalysis

Research Article

and hydrogen on the (001) surface of the iron carbide χ -Fe₅C₂. J.

Catal. 2015, 325, 9−18.

(121) Spivey, J. J.; Egbebi, A. Heterogeneous catalytic synthesis of

ethanol from biomass-derived syngas. Chem. Soc. Rev. 2007, 36 (9),

1514−1528.

(122) Davis, B. H. Fischer −Tropsch Synthesis: Reaction mecha-

nisms for iron catalysts. Catal. Today 2009, 141 (1), 25−33.

(123) Gall, D.; Gibson, E. J.; Hall, C. C. The distribution of alcohols

in the products of the Fischer −Tropsch synthesis. J. Appl. Chem.

1952, 2 (7), 371−380.

(124) Schulz, H.; van Steen, E.; Claeys, M. Selectivity and

mechanism of Fischer−Tropsch synthesis with iron and cobalt

catalysts. In Stud. Surf. Sci. Catal.; Curry-Hyde, H. E., Howe, R. F.,

Eds.; Elsevier, 1994; Vol. 81, pp 455−460.

(125) Ao, M.; Pham, G. H.; Sunarso, J.; Tade, M. O.; Liu, S. Active

Centers of Catalysts for Higher Alcohol Synthesis from Syngas: A

Review. ACS Catal. 2018, 8 (8), 7025−7050.

14677

https://dx.doi.org/10.1021/acscatal.0c03346

ACS Catal. 2020, 10, 14661−14677

Article Doi

Enhanced oxygenates formation in the Fischer?Tropsch synthesis over co- And/or Ni-containing Fe alloys: Characterization and 2D gas chromatographic product analysis

DOI: 10.1021/acscatal.0c03346

Source and publish data:

Authors:

Article abstract of DOI:10.1021/acscatal.0c03346

Full text of DOI:10.1021/acscatal.0c03346

Products guided by the article

R&D Labs maybe for 124-38-9

Relevant to this article

Hot Product