Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate pyrochlores for higher alcohol synthesis

Article abstract of DOI:10.1016/j.cattod.2012.07.011

Two Rh-based lanthanum zirconate pyrochlores (La₂Zr ₂O₇; LZ) were prepared by the Pechini method and tested for the synthesis of higher alcohols via CO hydrogenation. In one, Rh was substituted into the pyrochlore lattice (LRZ, 1.7 wt%) while for the second, Rh was supported on an unsubstituted La₂Zr₂O₇ (R/LZ, 1.8 wt%). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) results show that the surface reducibility depends on whether the Rh is in (or supported on) the LZ pyrochlore. The total hydrogen consumption in TPR for LRZ (0.28 mg H₂/g_cat) is much greater than R/LZ (0.07 mg H₂/g_cat), likely due to the presence of a perovskite phase in the LRZ (LaRhO₃; identified by XRD), in which rhodium is more reducible than in R/LZ. The formation of the perovskite accompanies that of the pyrochlore in the synthesis process, which includes heat treatment up to 1000 °C. CO hydrogenation results show higher ethanol selectivity for R/LZ than LRZ, possibly due to the strong interaction between Rh and LZ on the R/LZ, forming atomically close Rh⁺/Rh ⁰ sites, which have been suggested to favor ethanol production.

Full text of DOI:10.1016/j.cattod.2012.07.011

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Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Synthesis, characterization, and catalytic activity of Rh-based lanthanum
zirconate pyrochlores for higher alcohol synthesis
Victor Abdelsayeda,b, Dushyant Shekhawata,∗, James A. Poston Jr.a, James J. Spiveya,c
a National Energy Technology Laboratory, U.S. Department of Energy, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA
b URS Corporation, 3610 Collins Ferry Rd., Morgantown, WV 26507, USA
c Louisiana State University, Cain Department of Chemical Engineering, Baton Rouge, LA 70803, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two Rh-based lanthanum zirconate pyrochlores (La2Zr2O7; LZ) were prepared by the Pechini method
and tested for the synthesis of higher alcohols via CO hydrogenation. In one, Rh was substituted into the
pyrochlore lattice (LRZ, 1.7 wt%) while for the second, Rh was supported on an unsubstituted La2Zr2O7
(R/LZ, 1.8 wt%). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR)
results show that the surface reducibility depends on whether the Rh is in (or supported on) the LZ
pyrochlore. The total hydrogen consumption in TPR for LRZ (0.28 mg H2/gcat) is much greater than R/LZ
(0.07 mg H2/gcat), likely due to the presence of a perovskite phase in the LRZ (LaRhO3; identified by XRD),
in which rhodium is more reducible than in R/LZ. The formation of the perovskite accompanies that of
the pyrochlore in the synthesis process, which includes heat treatment up to 1000 ◦C. CO hydrogenation
results show higher ethanol selectivity for R/LZ than LRZ, possibly due to the strong interaction between
Rh and LZ on the R/LZ, forming atomically close Rh+/Rh0 sites, which have been suggested to favor ethanol
production.
Received 28 February 2012
Received in revised form 20 June 2012
Accepted 23 July 2012
Available online xxx
Keywords:
Higher alcohol synthesis
Pyrochlore
CO hydrogenation
Syngas conversion
Ethanol synthesis
Pechini method
Microwave reduction
Rh nanoparticles
Published by Elsevier B.V.
1. Introduction
the most active and selective for higher alcohol synthesis compared
to alternatives based on modified copper, cobalt–molybdenum,
The conversion of syngas from natural gas, coal, or biomass into
C2+ alcohols has attracted increasing attention due to the potential
use of these oxygenates as neat fuels or fuel additives [1–3]. Addi-
tionally, these oxygenate compounds can be transported as liquids
and reformed into a hydrogen-rich syngas for use in solid oxide
fuel cells [4–7]. The stoichiometry of the higher alcohol synthesis
reaction by CO hydrogenation can be expressed as:
or promoted Fischer–Tropsch catalysts [16–21]. Literature shows
that the activity and selectivity of Rh-based catalysts are increased
by promoter(s) [22–24], choice of support [9,25–27], synthesis
method [28,29], and Rh precursors [30]. Optimum higher alcohol
formation requires a balance among the rates of CO dissociation,
hydrogenation, and CO insertion [31–33]. For example, promoters
such as rare earth metals [33,34], alkali metals [20,35], and other
transition metals [33,36,37] play an important role in these elemen-
tary steps. Usually, these promoters are used to activate the oxygen
atom of the absorbed CO molecule and hence weaken the C O bond,
leading to CO dissociation followed by a hydrogenation step to form
CHx species. The mechanism for C C bond formation leading to
higher alcohols also requires the atomic proximity of an activated,
associatively adsorbed CO that can react with the CHx species. Sub-
sequent hydrogenation of this initial C2 intermediate leads to the
final higher alcohol products. These two sites—one forming CHx and
the other an activated CO—are catalytically distinct, but need to be
atomically adjacent.
nCO + 2nH2 → CnH2n + 1OH + (n − 1)H2O; ꢀH < 0 and n = 2–8
The major byproducts are C1 compounds, including methanol,
and the thermodynamically favored products CO2 and methane
[1,8]. Therefore, the major challenge for higher alcohol synthesis by
CO hydrogenation is to develop a catalyst with high kinetic selec-
tivity towards higher alcohols, specifically catalysts that drive the
reaction toward the formation of the C–C bond.
Rh-based catalysts have been extensively studied for their abil-
ity to produce C2+ alcohols via hydrogenation of CO [9–11], CO2
[1,12,13], or both [2,14,15]. These catalysts have been shown to be
In addition to promoters, optimizing the chemical and the elec-
tronic nature of Rh species on the surface could improve the activity
and selectivity to higher oxygenates. However, there are contradic-
tory results on the electronic state of active Rh sites in the literature.
While Watson and Somorjai [38] claimed that oxidized Rh species
∗
Corresponding author.
E-mail address: dushyant.shekhawat@netl.doe.gov (D. Shekhawat).
0920-5861/$ – see front matter. Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.cattod.2012.07.011
Please cite this article in press as: V. Abdelsayed, et al., Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate
pyrochlores for higher alcohol synthesis, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.011

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in perovskite (LaRhO3) correlate with ethanol selectivity and are
associated with the stabilization of Rhn+ on the surface, Gysling
et al. [39] showed that oxygenate formation occurs on metallic Rh
sites in LaRhO3.
Recently, the presence of an atomically adjacent ionic and
metallic Rh species (Rh0–Rhn+) has been reported [30,40,41] to
enhance the coupling between undissociated CO and CHx and the
the pyrochlore lattice. The goal is to investigate systematically the
role of these Rh species on the synthesis of higher oxygenates.
2. Experimental
2.1. Catalyst synthesis
selective formation of ethanol via ketene [42,43] (H2C
C O) or
2.1.1. Materials
acetyl [42,44] (H3C O) intermediates. For example, Du et al. [34]
C
Zirconium oxynitrate (ZrO(NO3)2·xH2O), lanthanum nitrate
hexahydrate (La(NO3)3·6H2O), anhydrous citric acid (C6H8O7),
ethylene glycol (C2H6O2), rhodium chloride (RhCl3), rhodium
nitrate (Rh(NO3)3·xH2O), hydrazine monohydrate (N2H4·H2O), and
analytical grade ethanol (EtOH) were purchased from Alfa Aesar
and were used without any further purification.
show that a surface complex such as Fig. 1a enhances higher oxy-
genate selectivity via the formation of a “tilted” CO species in which
both the carbon and oxygen atoms are bound to the surface (P could
be a promoter cation or an oxygen vacancy).
A similar Rh0–Rhn+ pair has been postulated by Wang et al. [44].
They also suggest that a metal promoter (Mn in their case) is in
atomic proximity to the Rhn+ atom, forming a “tilted” CO, such as
the one postulated by Du et al. [34] in Fig. 1a. They show that ethanol
is formed by insertion of a carbene from the Rh0 site, as shown in
the second step of the mechanism (Fig. 1b).
2.1.2. Rh-substituted pyrochlores (LRZ)
Pyrochlores of lanthanum zirconate (LZ) and 2 wt% Rh-
substituted lanthanum zirconate (LRZ) catalyst materials were
prepared using a modified Pechini method [55,56]. Typically, citric
acid (CA), a metal complexing agent, was added to a mixed aqueous
solution of lanthanum nitrate, zirconium oxynitrate, and rhodium
chloride to form a clear solution. The molar ratio between CA and
total metal ions was kept at 1.2. The clear solution was heated to
70 ◦C on a stirred hot plate before ethylene glycol (EG) was added
such as the molar ratio of EG to CA is unity. The solution was kept
under stirring at this temperature for several hours to evaporate
water and accelerate polymerization between metal-chelated cit-
ric acid and ethylene glycol. A clear viscous gel was obtained, which
was further heated at 130 ◦C in a heating mantle until an amorphous
polyester-type resin was obtained. The solid material was collected
and calcined at 1000 ◦C for 8 h to breakdown the organic polymer
and burns the carbon off leading to a highly crystalline clean mixed
metal oxide.
Based on FTIR results, another type of site, shown in Fig. 1c, has
been proposed by Kiennemann et al. to account for the formation
of higher oxygenates [45]. This site consists of a Rh Ce site which
also binds CO through both the oxygen and carbon atoms. This is
postulated to lead to the formation of a formyl intermediate by
hydrogenation of the activated CO (presumably through H2 disso-
ciation on the Rh site), which they show to be a key intermediate
in the formation of higher oxygenates. However, Kiennemann et al.
[45] also note that this is not the only possible consequence of CO
activation on this type of site, which may either favor CO disso-
ciation (by weakening the carbon–oxygen bond), or promote CO
insertion (by modifying the electron density of the carbon atom).
It is not clear whether this sort of site would uniquely favor one or
the other of these options.
One way in which the type of site needed to produce higher oxy-
genates can be prepared is to use crystalline oxides into which an
active metal such as rhodium can be isomorphically substituted. In
this case, the rhodium coordination at the surface could be tailored
based on the level of substitution. Another option for producing
this type of site is to support rhodium clusters on an oxide to pro-
duce these sites by interaction with the oxide surface, in which case
the rhodium atoms closest to the surface would be more oxidized
while those farther away would be more metallic.
Among the class of crystalline oxides materials which might be
used to produce these sites are the pyrochlores (A2B2O7) [46–51],
where A is a rare earth trivalent element and B is a tetravalent
transition element. Although their use as an active support in
heterogeneous catalysis is rare [52,53], rhodium can be either sub-
stituted into the structure, or supported on the surface of these
oxides. To our knowledge, there are no reports in the literature
investigating this class of material specifically in CO hydrogena-
tion reactions. Of specific interest here are lanthanum zirconate
pyrochlores, which have been studied for reactions such as fuel
reforming [52–54], but not for CO hydrogenation. Here, we report
the preparation and characterization of three pyrochlore catalysts
for CO hydrogenation:
2.1.3. Rh-deposited pyrochlores (R/LZ)
Microwave-assisted chemical reduction method was used
to deposit 2 wt% Rh on the surfaces of pure un-substituted
(LZ)-pyrochlore catalysts (prepared by modified Pechini method
mentioned in Section 2.1.2). In a typical procedure, rhodium nitrate
was dissolved in a 100 ml of a H2O:EtOH (1:2) mixture such as the
Rh metal content is equivalent to 2 wt% Rh. The LZ powder was
dispersed in the above-mentioned solution for 3 h then hydrazine
monohydrate (500 ␮l) was added quickly at room temperature
under vigorous stirring. No color change was observed to the dis-
persion indicating that the Rh ions did not reduce under room
temperature conditions. The solution was kept under stirring for
another hour before being transferred into a sealed Teflon vessel
and placed on a turntable tray of an Anton Paar microwave reaction
system (Synthos 3000) equipped with pressure and temperature
sensors. The dispersion was microwaved for 5 min under contin-
uous stirring and 800 W of microwave power. After cooling the
dispersion, the particles were separated by centrifuge, washed with
ethanol and then dried under vacuum at 10−3 Torr for 48 h. After
drying, the R/LZ catalyst was calcined at 500 ◦C in air for 3 h before
being characterized or tested.
Microwave-assisted chemical reduction method, which is very
different from conventional impregnation methods, offers fast
and size-controlled particle nucleation and deposition on catalytic
supports. Microwave irradiation (MWI) provides a fast dielectric
heating to the reaction mixture, due to the difference in the reac-
tant and solvent dielectric constants, leading to enhancement in
the reduction rate of Rh nitrate by hydrazine hydrate [57–59]. A
microwave-induced homogeneous nucleation of metal clusters is
intended to produce a narrow size distribution of Rh on the catalyst
support.
(a) Rh substituted into the zirconate site (LRZ), with Rh atomically
bound to oxygen and lanthanum at the surface;
(b) Rh supported on the pyrochlore (R/LZ), with no covalent bond
between the Rh atoms and the pyrochlore; and
(c) Lanthanum zirconate (LZ) as a baseline catalyst.
LRZ and R/LZ are intended to have different types of
Rh–pyrochlore interaction; specifically, in LRZ all rhodium catalysts
are closely associated with the pyrochlore lattice, while on R/LZ, at
least some rhodium is presumably more metallic since it is not in
Please cite this article in press as: V. Abdelsayed, et al., Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate
pyrochlores for higher alcohol synthesis, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.011

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3
(a)
(b)
CH2
(Rh0 Rh+ )
C
O
CH2
C
O
CH2
O
+ H
(Rh0 Rh+ )
(Rhx0
Rhy+ )
P+
P+
x
y
P
x
y
O
O
CH2
Insertion
CH2
CH2
O
(Rh0 Rh+ )
P+
+ H
(c)
x
y
C
O
(Rh0xRh+ )
O
P+
y
O
+ CH3-CH2-OH
Rh
Ce
Fig. 1. (a) Postulated Rh sites corresponding to higher oxygenate selectivity [34], (b) mechanism of ethanol formation on promoted Rh [44], and (c) activation of CO on
Rh/ceria surface, leading to increased higher oxygenate selectivity [45].
Table 1
2.2. Catalyst characterization
Reaction conditions used to study the Rh-based pyrochlore catalysts.
2.2.1. X-ray diffraction (XRD)
Parameter
Value
Powder X-ray diffraction analysis was performed on a Pana-
lytical X’pert Pro (PW3040) X-ray diffraction system utilizing Cu
K␣radiation. Samples were placed on a zero diffraction Si holder
and were scanned from 10◦ to 90◦ (2ꢁ). Analysis was carried with
Highscore Plus Analysis software equipped with a standard ICDD
X-ray diffraction database supplied by Panalytical. Average crystal-
lite size was calculated from the diffraction pattern using Scherrer
equation [60] (d = k␭/(B cos ꢁ) where d is the average crystallite size,
k is a constant that depend on crystallite shape, ꢂ is the X-ray wave-
length, ꢁ is the diffraction angle, and B is the FWHM (full width at
half max) of the diffraction peak.
H2/CO
2
280
2.9
15,000
1.0
T (◦C)
P (MPa)
WHSV (cc/h/gcat
Cat. wt. (g)
)
2.2.4. X-ray photoelectron spectroscopy (XPS)
The elemental electronic states of the catalyst surface were
investigated using XPS. The measurements were carried out on
a Physical Electronics (PHI) spectrometer model 590. The photo-
electron kinetic energy was measured by OmniFocus III spherical
capacitance analyzer (SCA). Magnesium anode was used as the
source of X-ray radiation (MgK␣: 1253.6 eV). The pressure in the
analysis chamber was maintained between 10−8 and 10−9 Torr dur-
ing measurements. The binding energies were corrected utilizing
the reference binding energy (BE) of adventitious carbon C1s at
2.2.2. Surface area and ICP analysis
The BET surface area was calculated from the N2 isotherm data
using the Brunauer–Emmett–Teller (BET) model (P/Po = 0.1–0.3).
The data were collected using a Micromeritics ASAP 2020 unit.
Prior to analysis, the samples were first degassed at 300 ◦C for 3 h
before being cooled to room temperature for analysis. Each cata-
lyst was measured at least three times and the average values along
with the standard deviation (95% confidence limits) were reported.
The Barrett, Joyner, and Halenda (BJH) model was used to estimate
the pore size of the catalysts [61]. The bulk concentrations of La,
Zr, and Rh were determined by inductively coupled plasma opti-
cal emission spectrometer (ICP-OES) manufactured by PerkinElmer
(Optima 7300). Acid-digestion techniques were used to dissolve the
catalysts prior to the ICP analysis.
284.8 eV. The spectra from the regions related to Zr3d, La3d, Rh3d
1s, and O1s core levels were recorded and analyzed using “Auger-
Scan” analysis software developed by RBD Instruments.
,
C
2.2.5. Electron microscopy and X-ray microanalysis
Transmission electron microscopy (TEM) was conducted on
JEOL JEM-2100 operated at accelerating voltages of 200 kV and
equipped with a high resolution CCD camera. Samples were pre-
pared by placing a droplet of ethanol-dispersed particles on a
Formvar carbon-coated, 300-mesh copper grid (Ted Pella) and
allow it to evaporate in air at room temperature. Energy dis-
persive spectroscopic (EDS) analysis was obtained utilizing a
2.2.3. Temperature programmed reduction (TPR) and H2 pulse
chemisorption
JEOL FE-7600 scanning electron microscope (SEM) interfaced to a
Thermo-Electron Noran System Seven (NSS) X-ray microanalysis
system. The EDS detector utilized in the X-ray microanalysis was a
Thermo-Electron Ultradry Energy dispersive spectrometer, which
was calibrated utilizing the Cu K␣ line at 8.041 kV.
TPR and H2 pulse chemisorption experiments were carried
out using a flow system with a thermal conductivity detector
(Micromeritics Autochem 2910). Prior to each TPR run, the cata-
lysts (∼0.2 g) were heated to 950 ◦C under 5% O2/He (50 ml/min,
ramp 5 ◦C/min). After the catalyst cools down to 50 ◦C, the TPR anal-
ysis was then carried out by ramping the temperature to 950 ◦C
(5 ◦C/min) under 5.0% H2/Ar (30 ml/min).
2.3. Catalyst activity measurements
2.3.1. Experimental setup and reaction conditions
H2 pulse chemisorption was used to determine the percentage
of Rh metal dispersed on the surface of reduced catalyst at 50 ◦C.
First the catalyst was purged with Ar gas (50 ml/min) at 50 ◦C after
being reduced in H2/Ar at 950 ◦C then 5.0% H2/Ar gas pulses each
of 0.5377 ml was introduced to the sample until hydrogen saturate
the catalyst surface. The Rh metal dispersion was calculated using
the actual ICP concentration of Rh.
A continuous fixed-bed flow reactor was used to measure the
catalytic performance of the catalysts (Fig. 2). Table 1 shows the
reaction conditions used in the CO hydrogenation reaction over
Rh-based pyrochlore catalysts. In a typical run, the catalyst (1 g)
was first ground, then diluted with sand (5 g) and then placed in
the reactor before being in situ reduced under 10% H2/N2 flow
(500 sccm) at 500 ◦C for 2 h at atmospheric pressure. Once the
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[4 4 0], and [6 2 2] crystal planes respectively, with a face centered
cubic unit cell structure [50,49]. When 1.7 wt% Rh was substituted
in the catalyst material, new diffraction peaks at 2␪ = 31.88 and
45.71◦ corresponding to LaRhO3 perovskite (JCPDS:10-0305) were
observed in addition to the main pyrochlore peaks (Fig. 3c). The for-
mation of perovskite phase could be due to the presence of Rh in
excess of an amount that can be isomorphically substituted in the B-
site of pyrochlore [48,49] possibly due to the mismatch between the
4+
ionic radii of Rh and Zr cations [r(Rh3+, 0.805 A) < r(Zr , 0.860 A)].
This perovskite phase could also form during the calcination pro-
cess [62,63]. The catalysts were calcined for 8 h at 1000 ◦C in air
which could lead to the formation of LaRhO3 [62,63]. No other La,
Zr, or Rh crystalline metal oxides were observed in the XRD pat-
terns indicating that the Pechini method produces a homogeneous
mixed metal oxide phases with no single metal oxide segregations.
The average pyrochlore crystal sizes in LZ and LRZ were determined
from the XRD patterns parameters according to Scherrer equation
to be very similar (30 3 nm), while the perovskite crystal size was
smaller (19 2 nm).
˚
˚
The XRD pattern for the R/LZ (calcined in air at 500 ◦C) is shown
in Fig. 3b. No size change in the pyrochlore particles was observed
before and after Rh deposition with no diffraction peaks corre-
sponding to Rh metal or its oxide being observed. This could be
due to several factors, such as size, concentration and dispersion of
Rh metal particles. This catalyst has only 1.7 wt% Rh which forms no
crystalline phase detectable by XRD, which has a detectable limits
of ∼3 nm.
Fig. 2. Experimental setup used for CO hydrogenation reaction into alcohols.
reactor was cooled down to the desired reaction temperature
(280 ◦C), the pressure was adjusted to 2.9 MPa and the syngas mix-
ture (H2/CO = 2) was introduced.
Fig. 3d and e shows the XRD patterns for LRZ catalysts collected
after CO hydrogenation reaction and temperature programmed
reduction (TPR), respectively. No change in the diffraction peaks
of pyrochlore phase was observed after being subjected to these
conditions. However, the perovskite diffraction peaks completely
disappeared due to reduction of LaRhO3 into Rh and La2O3 at high
temperature by H2 or CO/H2 [39]. It is not clear whether the Rh
produced during perovskite reduction are incorporated into the
pyrochlore bulk structure or remained on its surface. In both cases,
the XRD could not detect any crystalline Rh peaks. On the other
hand the diffraction patterns corresponds to R/LZ after CO hydro-
genation reaction and TPR are shown in Fig. 3 f and g, respectively,
show a metallic Rh peak at 2ꢁ = 41.06◦. The broadening of the Rh
peak (FWHM) and hence its particle size varies after CO hydrogena-
tion reaction, where the catalyst was reduced at 500 ◦C compared
to after reduction at 950 ◦C by TPR. The higher the reduction tem-
perature, the larger the Rh particles formed on the LZ surface due
to aggregation and growth.
2.3.2. Product analysis
Oxygenates in the product gas steam (methanol, ethanol,
n-propanol, isopropanol, isobutanol, n-butanol, acetone, and
acetaldehyde) were analyzed in Hewlett Packard 5890 Gas
Chromatograph (GC) equipped with FID detector and Restek-
StabilwaxDA column (60 m, 0.32 mm ID). The GC data were
processed using TotalChrom Workstation software. Hydrocarbons
and other outlet gases (H2, CO, CO2, and CH4) were analyzed by a
ThermoONIX model Prima ␦B Mass Spectrometer (MS) equipped
with an electron impact ionization source. The MS analysis of the
products was conducted after condensing out any oxygenates in
the gas stream using a sample conditioner connected in series with
the MS as shown in Fig. 2. The MS and GC were calibrated with the
appropriate standard gas mixtures before each run. The CO con-
version, in all reactions studied, was kept intentionally under 1%
to avoid any possible oxygenate condensation before analysis and
to ensure that the produced oxygenates remain in the vapor phase
prior to GC injection port. The GC column (polar polyethylene glycol
columns) used in the study is not designed to handle high concen-
trations of water, one of the CO hydrogenation side products. The
product selectivity was calculated based on carbon efficiency and
is defined as:
3.1.2. Surface area and Rh metal dispersion
The BET surface area, total pore volume, and average pore
diameter are reported in Table 2 for freshly calcined materials.
Depositing Rh on LZ (R/LZ) increased the BET surface area slightly,
from 5.31 0.03 m2/g to 6.26 0.01 m2/g. This is possibly due to
the formation of Rh nanocrystals on the surface leading to a higher
total surface area per unit weight compared to pure LZ catalyst.
However, for LRZ, the surface area was essentially the same as for
LZ, as expected if rhodium is isomorphically substituted into the LZ
lattice. The average pore diameter of the R/LZ was slightly lower
than that of the LZ or LRZ catalysts, which is consistent with the
formation of rhodium clusters in the pores of the R/LZ.
niCi
ꢀ
S(%) = 100 ×
(niCi)
where ni and Ci is the number of carbon atoms and molar concen-
tration of the ith product, respectively.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. XRD analysis
The bulk Rh content and dispersion are displayed in Table 2.
The ICP analysis showed that both LRZ and R/LZ catalysts had a
similar bulk concentration of Rh at 1.7 and 1.8 wt%, respectively.
After both LRZ and R/LZ were subjected to TPR at 950 ◦C, the Rh
substituted pyrochlore catalyst (LRZ) shows a higher Rh dispersion
(10.3%) than R/LZ (6.0%). Using the following empirical relationship
The powder X-ray diffraction patterns for fresh-calcined LZ,
R/LZ, and LRZ catalysts are shown in Fig. 3a-c. Only a single crys-
talline phase, identified as La2Zr2O7 (JCPDS:17-0450) was observed
in LZ catalyst. The diffraction peaks located at 2ꢁ = 28.63, 33.15,
47.61, and 56.45◦ correspond to diffraction from [2 2 2], [4 0 0],
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5
O
Calcined
Reduced
O
O
O
O
O
O
O
O
O
O
(g)
(f)
#
O
O
O
O
O
O
O
O
O
(c)
(b)
*
*
(e)
(d)
(a)
20
30
40
50
60
70
80
90
20
30
40
50
60
70
80
90
2θ
2θ
Fig. 3. X-ray diffraction patterns for calcined: (a) LZ, (b) R/LZ, (c) LRZ, (d) spent LRZ after CO hydrogenation reaction, (e) LRZ after TPR, (f) spent R/LZ after CO hydrogenation
reaction and (g) R/LZ after TPR. (ꢀ) for pyrochlore, (∗) for perovskite and (#) for metallic Rh crystalline phase peaks.
Table 2
Surface properties and Rh metal dispersion of fresh pyrochlore catalysts.
Catalyst
BET surface area (m2/g)
Total pore volume (cc/g) × 103
Average pore diameter (nm)a
Rh loading (wt%)b
Rh dispersion (%)c
LZ
R/LZ
LRZ
5.31 0.03
6.26 0.01
5.64 0.02
44.9 0.70
44.3 0.38
45.1 2.48
34.8 0.89
29.5 0.26
32.0 2.08
–
1.8
1.7
–
6.0
10.3
a
The Barrett, Joyner, and Halenda (BJH) model [61] was used to estimate the pore size of the catalysts.
The Rh loading was determined using ICP-OES technique.
b
c
The Rh metal dispersion on the catalyst surfaces was determined by H2 pulse chemisorption technique after performing a TPR at 950 ◦C.
between percent dispersion (%D) and cluster diameter (d, nm) [64]:
due to Rh being substituted in both the pyrochlore and perovskite
crystal structures (Fig. 3). This would explain both the presence of
two overlapping reduction peaks in the TPR profile, and the fact that
they are close to one another, as would be expected for a pyrochlore
and perovskite.
108
d =
(%D)
Table 3 shows that the total H2 uptake is far greater for LRZ
(0.28 mg H2/gcat) than for R/LZ (0.07 mg H2/gcat) likely due to reduc-
tion of both the perovskite phase, which completely disappear upon
reduction (Fig. 3), and some reducible surface rhodium that remains
in or closely associated with the pyrochlore structure. Assuming
all rhodium in these catalysts was fully oxidized (e.g. present as
RhO2), Table 3 shows the maximum calculated H2 uptake is to be
The mean cluster size for rhodium is ∼18 nm for R/LZ and
∼10.5 nm for LRZ.
The higher Rh dispersion for LRZ than for R/LZ is likely due to
the reduction of the perovskite phase in as-prepared LRZ during
the reductive pretreatment (to 950 ◦C) before the dispersion was
measured. This pretreatment apparently produces a metallic (and
presumably well dispersed) surface rhodium species.
3.1.3. Temperature programmed reduction (TPR)
429 oC
380 oC
Fig. 4 shows the TPR results for the as-prepared pyrochlore cat-
alysts. Reduction of LZ is negligible when plotted on the same scale
as for LRZ and R/LZ, as reported by others for this same pyrochlore
[52,65]. This indicates that the La and Zr mixed oxide species are
very stable under reduction conditions and high temperatures.
A weak reduction peak appeared at 253 ◦C for R/LZ, which can be
assigned to reduction of Rh species on the LZ surface. This reduc-
tion peak temperature is much higher than that observed for Rh
supported on SiO2 and Al2O3 catalysts (70–136 ◦C) [33,52,66], sug-
gesting a strong Rh–pyrochlore interaction, even though rhodium
is not in the pyrochlore structure. The small Rh clusters on the sur-
face are unexpectedly stable to reduction on the pyrochlore surface,
consistent with the dispersion results (Table 2) showing signifi-
cantly more surface reducible rhodium in the LRZ than that in the
R/LZ.
LRZ
0.05
0.00
R/LZ
0.05
253 oC
0.00
LZ
0.05
0.00
Fig. 4 shows a major reduction peak at 429 ◦C for LRZ, suggesting
that rhodium is part of lattice and it is not simply supported on the
surface. This broad peak had two components, a lower temperature
shoulder at around 380 ◦C followed by a higher temperature peak at
around 429 ◦C. This indicates two different forms of rhodium on the
surface each with different coordination environment. This may be
0
200
400
600
800
1000
Temperature (ºC)
Fig. 4. TPR profiles for as-prepared calcined pyrochlore-based catalysts. All catalyst
were oxidized first at 950 ◦C (TPO) then run under 5%H2 balanced in Ar at a ramping
rate of 5 ◦C/min. Results were plotted on the same TCD signal scale.
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Table 3
Table 5
Catalysts reducibility obtained from TPR analysis for fresh and calcined catalysts.
Atomic ratios as determined by ICP, EDX, and XPS techniques as a function of Rh
location in the catalyst.
Catalyst
Total H2 uptake
Experimentala
Atomic ratio
Technique
Catalyst
LRZ
Calculatedb
(mg H2/gcat
Estimated fraction of
reduced Rh (%)
(mg H2/gcat
)
)
R/LZ
LZ
R/LZ
LRZ
0.02
0.07
0.28
–
0.70
0.66
–
10
42
La/Zr
ICP
1.1
1.3
1.3
0.05
0.07
0.01
1.0
1.2
0.9
0.05
0.10
2.50
EDX
XPS
ICP
EDX
XPS
Rh/La
a
Based on TPR measurements.
Based on the ICP concentration of Rh in the catalyst and the assumption of
b
complete reduction of Rh species (RhO2 + 2H2 → Rh + 2H2O).
0.66 mg H2/gcat for LRZ and 0.70 mg H2/gcat for R/LZ using the ICP
composition of Rh (Table 2). The reducible fraction of Rh can then
be calculated to be 42 and 10% for LRZ and R/LZ, respectively.
mixture of Rh0 and Rh+ [38,39,73,74]. These results indicate that
La and Zr ions in the pyrochlore lattice play a significant role in
stabilizing Rh0–Rh+ species at the Rh–pyrochlore interface.
3.1.4. X-ray photoelectron spectroscopy (XPS)
3.1.5. Transmission electron microscopy (TEM)
The XPS spectra for as prepared (calcined) catalysts and for spent
(after CO hydrogenation reaction) catalysts are displayed in Fig. 5.
The catalysts were reduced at 500 ◦C under 10% H2/N2 before CO
hydrogenation reaction took place at 280 ◦C. The intensity of the
Rh3d XPS spectra for R/LZ is much greater than that observed for
LRZ due to the nature, location and concentration of Rh species on
the surfaces found in these catalysts. This observation also agrees
with the preparation method adopted to introduce Rh species to
the catalyst. In LRZ, the rhodium is atomically distributed in the LZ
structure; bulk and surface, while in R/LZ, the rhodium particles are
intentionally deposited on the LZ surface.
Peaks at binding energies (BE) between 308.17–309.43 eV and
between 312.92–314.52 eV are assigned to Rh 3d5/2 and 3d3/2
regions, respectively [67,68]. The 3d5/2 binding energy of metal-
lic Rh0 is 307.0–307.4 eV [69–72], oxidized Rh+ is 307.6–309.6 eV
[68,73,74], Rh2+ is 308.4–309.3 eV [73], Rh3+ is 308.8–311.3 eV
[71,73–75] and Rh4+ is 309.3–309.6 eV [54,68,72] varies according
to the Rh binding environment. The spectra (Fig. 5) show that the
Rh 3d peaks are shifted to lower binding energies after CO hydro-
genation due to the reduction of the Rh species on the surface. The
difference in Rh 3d5/2 binding energy (ꢃ≡E) before and after CO
hydrogenation reaction is reported in Table 4. The ꢃ≡E signifi-
cantly increased from R/LZ to LRZ: R/LZ (0.33 eV) < LRZ (2.06 eV).
This indicates that Rh in LRZ is more reducible than Rh in R/LZ, con-
sistent with the TPR results (Fig. 4 and Table 3). This could also imply
that the interaction between surface Rh and lanthanum zirconate
is higher for R/LZ than for LRZ, possibly due to perovskite reduc-
tion in LRZ into metallic Rh and La2O3 [39]. The broadening of the
Rh 3d peaks observed in LRZ indicates the higher metal dispersion
on the surface [73] compared to a less broad peaks for R/LZ cata-
lyst consistent with the H2 pulse chemisorption results reported in
Table 2.
TEM was used to gain direct information about the size and
structure of the pyrochlore catalysts. Fig. 6a shows the TEM image
for the fresh calcined LZ catalyst. The average particle size is
between 25 and 40 nm, which agrees well with the estimated
values from the XRD analysis. Particles were found to be highly
agglomerated with a secondary particle size in the micrometer
scale range. These observations were also found with field emission
scanning electron microscopy (not shown here). After Rh (1.8 wt%)
was deposited on the LZ surfaces, the particle surface morphol-
ogy of calcined R/LZ changed (Fig. 6b). Disperse Rh nanoparticles
of less than 2 nm were observed covering LZ catalyst. The TEM
results confirm the presence of dispersed Rh nanocrystals on the
LZ surface which could explain the absence of Rh peaks in the XRD
patterns due to their small size, dispersion and concentration. The
results also indicate that the MWI method employed to deposit Rh
nanoparticles on the catalyst surface produces small Rh nanocrys-
tals with narrow particle size distribution. The larger rhodium
cluster size after TPR to 950 ◦C (∼18 nm for R/LZ; see discussion of
Table 2 dispersion results above and Fig. 3 XRD discussion) suggests
significant sintering of the clusters as a result of this treatment.
3.1.6. Bulk and surface composition
The compositional difference between substituted and
deposited Rh-pyrochlore catalysts is shown in Table 5. The
ICP analysis showed that the atomic ratios for La/Zr and Rh/La are
very similar for these two catalysts, with 1.1 and 0.049 for LRZ,
and 1.0 and 0.053 for R/LZ, respectively. However, at the surface,
EDX and XPS analysis show different atomic ratios with more La
and Rh on the surface than in the bulk especially for R/LZ catalyst.
The XPS Rh/La ratio for R/LZ catalyst compared to that obtained
by EDX shows much more Rh on the surface which is expected
due to the penetration depth difference associated with these two
techniques [76–78]. XPS has a shorter penetrating depth (∼5 to
10 nm) while EDX has a longer one (1–10 ␮m) [76–78].
Table 4 shows that the Rh 3d5/2 binding energy for freshly cal-
cined catalysts are assigned to nonstoichiometric Rh oxide species
(Rh3+ and Rh4+) [73–75] with more ionic nature for LRZ with a dif-
ference of binding energy of 0.29 eV between fresh R/LZ and LRZ
catalysts. On the other hand, after reaction the binding energies of
Rh 3d5/2 show that the LRZ has more metallic nature while those
for R/LZ still show some ionic nature. For example, spent R/LZ cat-
alyst has a peak at 307.84 eV which is assigned to a nonequimolar
3.2. Catalytic activity measurements
Selectivities for CO hydrogenation for LZ, R/LZ, and LRZ catalysts
are shown in Fig. 7. The main products are methanol, ethanol, n-
propanol, and methane. Only trace amounts of other oxygenates
such as butanol, acetone, and acetaldehyde were detected, and col-
lectively account for less than 1% selectivity. For LZ, the results show
no selectivity for alcohols. Methane and CO2 were the major prod-
ucts observed with LZ catalyst. The absence of oxygenate formation
on LZ confirms the essential role that Rh plays as an active catalyst
for CO catalytic hydrogenation into alcohols and other oxygenates
[1,79]. For LRZ and R/LZ, methanol, ethanol, and n-propanol were
observed.
Table 4
XPS binding energies for Rh 3d5/2 of fresh and spent catalysts.
Catalyst
Rh 3d5/2 (BE, eV)
Fresh
Spent
ꢃBE (eV)
R/LZ
LRZ
308.17
308.46
307.84
306.78
0.33
2.06
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7
(a) R/LZ
(b) LRZ
200 a.u.
After
After
2000 a.u.
200 a.u.
Before
Before
2000 a.u.
320
315
310
305
300
315
310
305
300
295
Binding Energy (eV)
Binding Energy (eV)
Fig. 5. XPS for fresh and spent catalysts (Rh 3d region) (a) R/LZ and (b) LRZ.
Fig. 6. TEM images for LZ catalyst before (a) and after (b) Rh was deposited on the surface (R/LZ). LZ was calcined at 1000 ◦C for 8 h and R/LZ was calcined after Rh was
deposited at 500 ◦C for 3 h.
The turnover frequencies (TOF) for LRZ and R/LZ, calculated
based on the Rh dispersion (Table 2), suggest a slightly higher
inherent activity for the R/LZ, in addition to its higher selectivity
to higher oxygenates. For R/LZ, the TOF was 10.9 s−1 (molecules of
CO reacted/Rh site) compared to 8.5 s−1 for LRZ.
Fig. 7 shows that R/LZ has the highest selectivity towards ethanol
and the lowest selectivity towards methanol and CO2. On the
other hand, LRZ has approximately four times higher methanol
selectivity with lower ethanol selectivity. The more metallic Rh
nature on the surface of the LRZ (Rh0 ꢁ Rh+) correlates with its
high methanol selectivity as observed by Gysing et al. [39], while
R/LZ, with more ionic Rh nature on the surface (Rh+ ꢁ Rh0) has the
highest ethanol selectivity. These results suggest that the electronic
state of Rh catalyst on the surface and the relative ratio between
Rh+/Rh0 species is a determining factor in the product selectiv-
ity and the C C coupling mechanism. For these two catalysts, the
Rh0/Rh+ ratio in the R/LZ leads to greater rates of C C bond forma-
tion and higher ethanol selectivity. This is consistent with the XPS
results shown above, and with literature showing stabilized ionic
Rh+ decreases the rate of hydrocarbon formation while increasing
that of oxygenate formation [67].
The methanol to ethanol ratio observed for LRZ and R/LZ is very
similar to what has been reported for LaRhO3 and Rh/La2O3 [34,38].
The variation in product selectivity are due to competing processes
of hydrogenation and carbonylation corresponding to varying con-
centrations of molecular and dissociated CO and hydrogen on the
surface [1,36,79]. At the metal–support interface region of reduced
R/LZ, the XPS results show that Rh binding energy (307.84 eV) is
between Rh0 at about 307 eV and Rh3+ at about 311 eV. This sug-
gests the presence of Rh in two oxidation states: Rh0 and Rhn+. The
proximity of these types of Rh atoms at the metal–support inter-
face region enhances the selectivity of C2 and higher oxygenates,
consistent with earlier studies [44,45].
The high ethanol to methanol ratio observed with R/LZ is
consistent with XPS and TPR results which suggest a strong
metal–support interaction between deposited Rh and LZ support.
Fig. 7. Catalytic carbon-based selectivity towards methanol, methane, CO2, ethanol
and n-propanol at WHSV of 15,000 cm3/g/h, in the syngas mixture of H2/CO = 2 and
at 2.9 MPa.
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Rh in LaRhO3 can be reduced to form Rh0, which they claim as
the active Rh species. Additionally, their measured difference in
activation energy between methanol (28 2 kcal/mole) and higher
oxygenates 16 3 kcal/mol [38] implies that higher oxygenates
originated from the common CHx unit formed by hydrogenation
of dissociatively adsorbed CO while methanol is formed by hydro-
genation of associatively adsorbed CO. The selectivity difference
between LRZ and R/LZ (Fig. 7) is the result of difference in the rates
of CO insertion and hydrogenation of CHx species on the surface.
4. Conclusion
Lanthanum zirconate pyrochlores with rhodium either substi-
tuted within the lattice (LRZ) or supported on a pure lanthanum
zirconate (R/LZ) were prepared using a modified Pechini method.
The catalytic selectivity of these materials for CO hydrogenation
into alcohols shows that the Rh oxidation state on the catalyst sur-
face plays an essential role in determining the product selectivity.
Atomically adjacent metallic and oxidized rhodium sites and their
relative proportion determine the selectivity to ethanol. TPR, H2
pulse chemisorption and XPS analysis show that surface rhodium
in the pyrochlore structure (LRZ) is significantly more reducible
than rhodium supported on an LZ pyrochlore support (R/LZ) due to
instability of secondary perovskite phase (LaRhO3) prepared along
with the main pyrochlore phase under reduced atmosphere and
high temperature conditions. The more metallic Rh sites in LRZ
favor the methanol formation compared to higher alcohols, with
the alcohol product distribution following the ASF chain growth
mechanism. However, R/LZ has mainly ionic Rh+ surface species
and shows far greater ethanol selectivity.
Fig. 8. Anderson–Schulz–Flory (ASF) plot for oxygenates produced over substituted
versus supported Rh on LZ catalysts.
The interface between Rh and LZ appears to play an important role
in balancing the rates of CO dissociation and CO insertion under
hydrogenation conditions to favor the formation of higher alcohols
[10,44,80]. The XPS results in Table 4 show that the surface Rh is
in a non-stoichiometric oxidation state, implying the presence of
Rh+/Rh0 pairs at the interface of the Rh clusters with LZ. The cou-
pling between ionic and atomic Rh in presence of La and Zr ions
found in the LZ crystal lattice structure may act as promoters for
C
C coupling and subsequently for the formation of higher alcohols
[10,44,80,81].
The LZ support appears to have two important roles in the reac-
tion process: (a) it provides centers for the Rh catalyst through its
basic La component on the surface, and (b) the interface between
Rh and LZ helps in decreasing the reducibility of Rh on the surface,
apparently producing the sort of Rh+/Rh0 sites needed for higher
oxygenate formation.
Acknowledgements
Funding provided by NETL-Fuels program, Research and Engi-
neering Support (RES), Contract No. FE0004000. We gratefully
acknowledge David Berry, Daniel Haynes, and Mark Smith for
useful discussions, Frank Thomas for reactor setup, and Kimberly
Carter and Jinesh Jain for ICP analysis.
The selectivities shown in Fig.
7 suggest that the reac-
tion mechanism on LRZ and on R/LZ catalysts maybe different.
The oxygenate product distribution on LRZ catalyst follows
Anderson–Schulz–Flory (ASF) chain growth mechanism with no
apparent parallel path for the formation of C2+ oxygenates (see
Fig. 8) [39].
On the other hand, for R/LZ the Rh oxidation state on the cat-
alyst surface is more ionic, and the rate of formation of ethanol
exceeds what would be predicted from an ASF mechanism based on
the selectivity to methanol and propanol. This suggests that either
some methanol condenses to form ethanol, or the rate of CO–CHx
coupling (leading ultimately to ethanol and propanol) exceeds that
of CO hydrogenation (which forms methanol). In the latter case, the
oxygenates are formed by parallel paths—one leading directly from
CO to methanol and one leading to higher alcohols. The observed
selectivity difference between LRZ and R/LZ is the result of compe-
tition between the two.
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Please cite this article in press as: V. Abdelsayed, et al., Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate
pyrochlores for higher alcohol synthesis, Catal. Today (2012), http://dx.doi.org/10.1016/j.cattod.2012.07.011