MeOx/Al2O3 and MeOx/CeO2 (Me = Fe, Co, Ni) catalysts for high temperature N2O decomposition and NH3 oxidation

Article abstract of DOI:10.1039/c5cy01381j

A series of MeO_x/Al₂O₃ and MeO_x/CeO₂ (Me = Fe, Co, Ni) compounds was prepared by incipient wetness impregnation and characterized by XRD, XPS, UV-vis DRS, differential dissolution phase analysis (DDPA). Their activity towards N₂O decomposition and ammonia oxidation at 750-900 °C was shown to depend on oxygen mobility in the sample as characterized by steady state isotopic transient kinetic analysis (¹⁸O SSITKA) and dispersion of MeO_x (Me = Fe) species. Obvious reverse activity-FeO_x dispersion dependence was related to the change of contribution from oxygen transfer from the support to active sites through the Me-ceria or not improbably, the Me-alumina interface. It was shown that the high efficiency of CeO₂ based samples in deN₂O can be additionally increased by CeO₂ supporting onto high surface area alumina.

Full text of DOI:10.1039/c5cy01381j

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MeOx/Al2O3 and MeOx/CeO2 (Me = Fe, Co, Ni) catalysts for high
temperature N2O decomposition and NH3 oxidation
L. G. Pinaeva,*a I. P. Prosvirin,a,b L. S. Dovlitovaa, I. G. Danilovaa, E. M. Sadovskayaa,b and L. A.
Isupovaa
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of MeOx/Al2O3 and MeOx/CeO2 (Me = Fe, Co, Ni) was prepared by incipient wetness impregnation and
characterized by XRD, XPS, UV-Vis DRS, differential dissolution phase analysis (DDPA). Their activity towards N2O
decomposition and ammonia oxidation at 750 – 900 oC was shown to depend on oxygen mobility in the sample as
characterized by steady state isotopic transient kinetic analysis (18O SSITKA) and dispersion of MeOx (Me = Fe) species.
Obvious reverse “activity - FeOx dispersion” dependence was related to change of contribution from oxygen transfer from
the support to active sites through Me-ceria or, it is not improbable, Me-alumina interface. It was shown that high
efficiency of Fe/CeO2 samples in deN2O can be additionally increased after CeO2 supporting onto high surface area
www.rsc.org/
alumina, ammonia oxidation to NOx being the same
.
by its reaction with NOx. Therefore, development of bi-functional
catalyst that is capable of efficient N2O abatement and NH3
oxidation to NOx becomes urgent for further optimization and
reducing of harmful emissions of the process of HNO3 production.
An idea of development of oxide-based catalyst that is capable to
convert slipping ammonia with high selectivity to NOx was
successfully realized by using dual bed system in which Fe2O3-Al2O3
based extruded monolith followed fewer number of platinum
gauzes [13]. Later studies showed their efficiency of both slipping
NH3 oxidation to NOx and N2O abatment [14]. At the same time,
Al2O3 based honeycomb catalysts with the same geometry and
FeOx based active component supported by impregnation were
substantially more active in both reactions and retained high
activity after the run in dual-bed system for 3 months [14].
Earlier we have shown that catalytic activity of Sr substituted La
manganites in high-temperature N2O decomposition correlated
with the coefficient of lattice oxygen self-diffusion [5]. At using the
materials with high bulk oxygen mobility the rates of oxygen
transfer in the two-phase system ‘‘O2gas–surface–catalyst bulk’’
characterized by 16O/18O exchange and N2O decomposition
additionally increased either at their decoration by structures
characterized by higher rate of heteroexchange (composites
LaSrFeO4 (surface)–La0.4Sr0.6FeO3 (bulk) [6]), or at modification of
near surface layers by hetero cations (LaFeO3-CeO2 composites
with Fe3+ ions inserted into fluorite lattice [15]). In the former case
[6] “activity – oxygen mobility” correlation was observed for
reaction of NH3 oxidation as well. Pérez-Ramírez et al [16] showed
that the reaction of NH3 oxidation to NOx over oxide catalysts
(Fe2O3, CeO2, Cr2O3) follows a Mars–van Krevelen-type scheme with
participation of lattice oxygen and reoxidation of the so-formed
vacancies by both gas-phase O2 and bulk lattice oxygen. Due to this,
the efficiency of NH3 oxidation to NOx, especially in oxygen
deficient conditions, depended on the rate of bulk oxygen diffusion
to surface vacancies.
1. Introduction
Secondary abatement processes of N2O formed as undesirable by-
product at the catalytic oxidation of ammonia into nitric oxide
implies catalyst set up in the ammonia burner operating at 800-900
°C directly after Pt-Rh gauzes [1]. Bulk or supported mixed metal
oxides were developed for catalytic N2O decomposition and put
into practice in nitric acid plants before 2000 (Co2AlO4/CeO2,
CuO/Al2O3, La0.8Ce0.2CoO3 and Fe/ZrO2) [1]. Later, alternative
catalytic formulations were evaluated as well, such as yttrium-
doped zirconia [2], ceria-zirconia [3], A1−x A’xB1−yB’yO3 perovskites
with A = La or Ca, B = Mn, Co or Fe, A’ = Sr, B’ = Cu or Ni [4-6],
metal-substituted hexaaluminates [7], Fe2O3-MeOx (Me= Al, Zr, Ce,
La, Cu, Cr) obtained by co-precipitation [8, 9], Co3O4 [10] and CoOx-
CeO2 [11] obtained by calcinations of mixtures of individual oxides.
The pilot-plant reactor and real plant studies [8] confirmed high
activity and very good mechanical stability of the Fe2O3/Al2O3
catalysts prepared by co-precipitation as well as no decomposition
of nitric oxide. Several aspects with respect to the reaction
mechanisms, the structure−activity correlations, the role of various
gas inhibitors as well as the strategies followed to adjust the local
surface structure of above mentioned noble metals free metal
oxides were analyzed in the review of Konsolakis [12].
Ageing of Pt-Rh gauzes is accompanied by decrease of activity and
yield of NO+NO2 (NOx), while N2O production rises. With account of
tendency of last decade to reduce platinum metal loading to the
reactor (including number of gauzes and wire thickness), at gauzes
deactivation noticeable slip of ammonia becomes possible followed
a.Boreskov Institute of Catalysis, Novosibirsk, Russia.
b.Novosibirsk State University, Novosibirsk, Russia.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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In accordance with these findings, the idea to combine high activity All Me/support samples (Me=Fe, Co, Ni) were prepared by incipient
towards N2O decomposition and NH3 oxidation (at high selectivity wetness impregnation of corresponding support by water solution
to NOx) in the same sample was checked. In addition to of Me nitrate with added citric acid in 10 wt.% excess to the
Fe/Al2O3(CeO2) samples, Co- and Ni- supported onto Al2O3 and stoichiometric amount to form the corresponding citrates and
°
CeO2 were tried as well since a vast number of patent applications ethyleneglycole, dried in air at 150 C for 3 h and then calcined at
have claimed a relatively high NO selectivity (90–95%) using Co and 900 °C for 4 h. In the commonly used abbreviation nMe/support “n”
Ni single metal oxides. Numerous attempts were done to corresponded to Me weight content in the sample in terms of
commercialize Co3O4 based catalysts, but substantially lower corresponding metal, although oxide compounds obviously formed.
activity and reversible deactivation of Co-systems, due to reduction For CeO2 and Al2O3-1200 the samples with Me concentration
of Co3O4 by NH3 to the inactive CoO under reaction conditions in 6.7*1019 at/m2 have been prepared which corresponded to Me
the upper parts of the bed excluded replacement of Pt gauzes by content around 2.5-2.7 wt% (Al2O3-1200) and 0.86-0.91 wt%
oxide catalysts. Nevertheless, these systems could be tried as the (CeO2). For all supports we varied as well Fe content in the samples
second catalyst in the dual bed system, especially provided from 0.86 to 19.8 wt%. Bulk MeOx were obtained by calcination of
stabilizing effect of CeO2 in Co3O4-CeO2 mixed oxides with low Co chemical purity corresponding nitrates at 900 °C for 6 h.
content is well-known [11]. Since copper catalyzes ammonium
2.2. Samples characterization
nitrate decomposition, an extremely important safety problem may
arise if copper leaches from the catalyst and accumulates in the
fertilizer product [1]. This undoubtedly completely excluded any Cu
based system for using in the conditions of ammonia burner,
although Cu/CeO2 revealed high efficiency deN2O at lower
temperatures [17, 18] and more probably can be located after the
absorption column. In addition, it is well known that under these
conditions Cu-containing systems oxidize NH3 to N2 with high
selectivity.
X-ray powder diffraction (XRD) patterns were recorded using
Bruker D8 diffractometer with CuK monochromatic radiation.
α
Each sample was scanned in the range of 2θ from 10° to 70° with a
step 0.05° of 2θ. Surface composition of the samples was
investigated by X-ray photoelectron spectroscopy (XPS) using
spectrometer SPECS with Al K irradiation (h
ν
= 1486.6 eV).
α
Binding energy (BE) scale was preliminarily calibrated by the
position of the peaks of Au 4f7/2 (84.0 eV) and Cu 2p3/2 (932.67 eV)
core levels. UV-Vis diffuse reflectance spectra (UV-Vis DRS) were
recorded using a UV-2501 PC Shimadzu spectrometer with an IRS-
250A diffusion reflection attachment in the 11000-54000 cm-1
range using the BaSO4 as a reference. The edge energy (or band
gap, Eg) for allowed transitions was determined by finding the
intercept of the straight line for the low-energy rise of a plot of
On the first stage we showed that intrinsic catalytic activity of
different Me/Al2O3 (Me
= Fe, Co, Ni) samples obtained by
impregnation towards N2O decomposition and NH3 oxidation was
substantially lower than for corresponding Me/CeO2 samples with
close surface Me concentration and correlated with the rate of
oxygen exchange in the samples. Variation of Fe content in the
Fe/Al2O3(CeO2) samples revealed noticeable reverse dependence of
deN2O and NH3 oxidation activity due to size effect. In accordance
with this, we supported FeOx onto specially developed Al2O3 [19]
characterized by high specific surface area, and formation of highly
dispersed FeOx at increased Fe content in the sample finally
resulted in N2O and NH3 conversion rise. For such samples we
focused on Fe oxide as active component because in the case Me =
Co, Ni low active MeAl2O4 or NiOx were formed on the surface of
alumina. Using of the effect of high oxygen mobility in CeO2 for this
purpose was restricted by its low surface area at temperature of
reaction, independently on preparation method [3, 20, 11].
Therefore, an attempt was done to disperse CeO2 using
precipitation onto high surface area Al2O3.
[F(R∞) h
F(R∞) is the Kubelka-Munk function.
ν ν [21], where hν is the incident photon energy,
]2 versus h
The method of differential dissolution phase analysis (DDPA) was
used to reveal the composition of the phases (including poorly
crystallized, but without information about oxygen content therein)
formed in the supported samples, including possible decoration of
one phase by another [22]. For this about 10 mg of the sample was
loaded in a microreactor and dissolved in the water based solvent
with the composition changing from the lower towards higher
acidity in the following order: 1) HCl (pH = 2), 2) (1÷3) M HCl, while
°
the temperature increased continuously from 20 °C to 90 C, 3) (3.6
M) HF and flowing the reactor with a flowrate at 3.6 ml/min.
Change of the outlet mixture composition in time was analyzed by
an atomic emission spectroscopy (BAIRD spectrometer) using
following spectral lines of the elements (nm): Fe – 238.2, Al – 308.2,
Ce – 413.8, Co – 238.8 with 5% accuracy of measurements at
sensitivity level 10-3 µg/ml.
2. Experimental
2.1. Samples preparation
CeO2 and Al2O3 samples used as the support was obtained by
calcination of corresponding nitrate of chemical purity (99.5%) at
Kinetics of oxygen exchange was characterized by steady state
isotopic transient kinetic analysis (SSITKA). For SSITKA experiments,
sample (g = 0.025 g) loaded into a reactor (quartz tube, i.d. = 3 mm)
°
900 C for 36 h. Al2O3 based supports were prepared by calcination
°
of: 1) Al(NO3)3*9H2O of chemical purity (99.5%) at 1000 C or 1200
°C for
6 h (denoted below as Al2O3-1000 and Al2O3-1200,
°
was heated to 800 C in 0.5% vol.16O2+He flow and kept at this
respectively), or 2) product of centrifugal thermal activation of
commercial-grade alumina hydrate (hydrargillite) [19] at 1000 °C for
temperature for 30 minutes, whereupon the gas mixture was
replaced stepwise by the same one containing 18O2 and Ar (1 vol.%)
as an inert tracer. Gas flow rate for all mixtures amounted to 16.7
cm3/s. All responses were analyzed as time variation of the 18O
19
h (Al2O3-C). To obtain Ce/Al2O3 support, the product of
thermochemical activation of hydrargillite was first calcined at 500
°C (SBET = 210 m2/g); then CeO2 was precipitated onto this product
from 0.75 M Ce(NO3)3*6H2O water solution using 1 M (NH4)2CO3 as
a precipitation agent, so that Ce : Al ratio in the final product was 1
: 1 (mole). The resultant precipitate was washed until the pH of the
atomic fraction in the gas phase αg(t)=(16O18O+218O2)/(2
Σ
OiOj),
where
Σ
OiOj = 16O2 + 16O18O + 18O2 [23].
The catalytic activity for samples with particles of 250 – 500 μm in
size was measured in a fixed-bed U-shaped reactor (3 mm i.d.
quartz tube) at ambient pressure in the temperature range
°
filtrate was 7, dried at 60 C overnight, followed by calcination at
900 °C for 5 h.
2 | J. Name., 2012, 00, 1-3
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°
750
÷
900 C and contact time ~10-3s. For NH3 oxidation, mixture of
species was about the same as in Al2O3-C, as follows from close
~67°
-Al2O3 -Al2O3), but cannot be
1%NH3 in air or 1%NH3 + 2%O2 in N2 was fed to the reactor charged widening of peaks at 2
θ
(γ
+ θ
by 0.015 or 0.043 g of the sample (see details in the text) with a calculated more precisely because of overlapping of its more
flow rate 25 l/h. Outlet mixture composition was measured by IR
intense peak at 2θ
~32.9° with that of CeO2.
spectroscopy. For N2O decomposition, a gas mixture consisting of
0.15 vol% N2O (+3%O2+3%H2O) in He flowed the reactor with a flow
rate 60 l/h. For samples characterized by high specific surface area
(Al2O3-C, Ce/Al2O3 based) we failed to reach reasonably low values
of N2O conversion at 8000C. Therefore, the data obtained at the
same sample loading (0.038 g) and flow rate (60 l/h) were
considered to analyze the effect of Fe concentration. Outlet
mixture composition was analyzed by on-line gas chromatograph
with Porapack T (i.d. = 3mm, l = 3 m) and NaX (i.d. = 3mm, l = 2 m)
columns.
Although Co3O4 and Co-Al-O spinels are characterized by the same
symmetry group, increased lattice parameters of Co compounds in
2.7Co/Al2O3-1200 sample (Fig.1A) (a=b=c=8.104 (1) Å) compared
with those in 0.91Co/CeO2 one or bulk Co3O4 (a=b=c=8.084 (0) Å)
favor formation of CoAl2O4 in the former [24], probably in the
mixture with Co3O4. In the 2.7Ni/Al2O3-1200 and 2.5Fe/Al2O3-1200
samples, additional NiO with admixture of highly dispersed Ni-Al-O
compounds of spinel structure and
α-Fe2O3 were respectively
detected. The lattice parameters of NiO and Fe2O3 in the supported
samples were exactly the same as for bulk MeOx (not presented in
the Table 1 for shortness). However, slight increase of
α-Al2O3
lattice parameters in all Me/Al2O3 samples points to the probability
of non-substantial Men+ penetration into alumina lattice. With
account of slightly higher radii of Me3+ ions compared with that of
Al3+ (69
÷74 pm and 67.5 pm, respectively), such assumption looks
3. Results and discussion
3.1. Phase composition
quite reasonable.
3.1.1. XRD analysis
FeOx supporting onto Al2O3-C resulted in continuous decrease of
Al2O3-1200 (Fig. 1a), Al2O3-1000 and CeO2 used as the supports
were single phase samples (Table 1), which values of coherent
scattering area dXRD calculated by the Scherrer equation were much
less than the particle size estimated from the BET surface area
(dBET). For example, for Al2O3-1200 dXRD = 360 Å, while dBET = 3800 Å;
for CeO2-900 – 680 Å and 6400 Å, respectively. TEM images of CeO2
sample revealed mixture of agglomerates composed of concreted
crystallites of either 100 nm or 10 nm in a size [15]. The same can
be reasonably supposed for Al2O3-1200 and Al2O3-1000 samples as
well to explain the difference between dBET and dXRD values. Al2O3-C
the peaks corresponding to
θ
-,
κ
-,
γ
- Al2O3 with rise of Fe content
-Al2O3
from 2.5 to 13.2 wt% (Fig.1b). Instead of them second
α
phase with increased lattice parameter (a=b=4.795 Å, c=13.057 Å)
becomes evident and increases in the intensity (Table 1) that more
obviously points to efficient incorporation of Fe3+ ions into the
lattices of low temperature aluminas thus promoting their
transition to
α-Al2O3 at lower temperature [25]. This fact agrees
well with decrease of SBET value from 70 m2/g for Al2O3-C to 22
m2/g after supporting 19.8% of Fe (pattern of the last sample not
presented for shortness).
represented mixture of
dispersed -Al2O3, -Al2O3 and traces of
sample CeO2 phase with the same lattice parameters but expected
more dispersed, than in CeO2 sample, species, -Al2O3 and traces of
-Al2O3 were detected (Fig.1C, Table 1). Dispersion of
α
- Al2O3 (Fig.1B, Table 1) with more
In the patterns of (0.86-0.91)Me/CeO2 samples traces of crystalline
θ
γ
κ-Al2O3. In the Ce/Al2O3
MeOx were observed only for the case Me = Co, and some α-Fe2O3
was detected for substantially higher Fe content (sample
2.5Fe/CeO2). Nevertheless, unlike to alumina based samples,
θ
γ
θ
-Al2O3
A
C
B
κ+θ
γ+α
α
α
α
N- NiO
NA - NiAl
O
4
2
CA - Co3O4(CoAl
O )
4
2
CA
γ− γ−Al
O
3
α
F - Fe
O
3
2
2
θ− θ−Al
O
3
CA
2
α
α
CA
θ+γ
CA
CA
α− α−Al
O
CA
2
3
γ+θ
F
−
Fe
O
θ+γ
2
3
4
3
θ
κ− κ−Al
O
θ
γ+θ+κ
γ
θ
α
2
3
F
N+NA
NA
κ+θ
γ
F
θ+γ
α
θ
F
NA
N
2
θ+γ
θ
N
θ+γ
γ
F
F
γ
F
F
γ
F
0%Fe
F
F
F
F
F
F
F
2.5%Fe
γ+θ
2
1
1
6.6%Fe
13.2%Fe
F
F
F
F
F
F
F
20
30
40
50
60
70
20
30
40
50
60
70
20
30
40
50
60
70
2
Θ
2
Θ
2Θ
Fig. 1 XRD patterns of: (A) Al2O3-1200 (1) and (2.5-2.7)Me/Al2O3-1200 (Me = Co(1), Fe (2), Ni(3)) samples, (B)
Fe/Al2O3-C samples with different Fe content, and (C) Ce/Al2O3 (1) and 9.9Fe/Ce/Al2O3 (2) samples.
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Table 1 Phase composition, structural parameters, BET surface area and crystallite size estimated from XRD (dXRD) of Al2O3,
CeO2 and MeOx/CeO2(Al2O3) samples.
Sample
Al2O3-1200
Al2O3-1000
Al2O3-C
SBET, m2/g
Phase composition
dXRD, Å
370
4
7
α
α
α
-Al2O3 (a=b=4.756 Å, c=12.986 Å)
-Al2O3 (a=b=4.760 Å, c=12.997 Å)
300
71
480
-Al2O3 (a=b=4.771Å, c=13.020 Å)
-Al2O3, -Al2O3, -Al2O3
-Fe2O3 (a=b=5.035(0) Å, c=13.741 Å)
-Al2O3 (a=b=4.761 Å, c=12.997 Å)
CoAl2O4 or Co3O4 (a=b=c=8.104 Å)
θ
κ
γ
100 (
γ
-Al2O3), 140 (
260
θ-Al2O3)
2.5Fe/Al2O3-1200
4.0
α
500
260
520
240
α
2.7Co/Al2O3-1200
2.7Ni/Al2O3-1200
4.0
4.0
α
-Al2O3 (a=b=4.761 Å, c=12.997 Å)
NiO (a=b=c= 4.178 Å)
NiAl2O4
≤120
580
510
α
-Al2O3 (a=b=4.760 Å, c=12.996 Å)
2.5Fe/Al2O3-C
6.6Fe- Al2O3-C
60
46
α-Al2O3 (a=b=4.778 Å, c=13.039 Å)
highly dispersed
θ
-Al2O3, -Al2O3, -Al2O3
κ
γ
510
α
-Al2O3: 31% - a=b=4.770 Å, c = 13.030 Å
470
highly dispersed
510
69% - a=b=4.795 Å, c=13.057 Å
θ-Al2O3, κ-Al2O3, γ-Al2O3
13.2Fe- Al2O3-C
33
α
-Al2O3: 18% - a=b=4.760 Å, c = 13.000 Å,
520
82% - a=b=4.795 Å, c=13.057 Å,
highly dispersed
traces of θ-Al2O3, κ-Al2O3, γ-Al2O3
Fe2O3 (a=b=5.032 Å, c=13.702 Å)
250
CeO2
0.91Co/CeO2
1.4
1.4
CeO2 (a=b=c=5.412 Å)
CeO2 (a=b=c=5.412 Å)
Co3O4 (a=b=c=8.084 Å)
CeO2 (a=b=c=5.411 Å)
CeO2 (a=b=c=5.411Å)
CeO2 (a=b=c=5.412 Å)
680
630
460
550
600
650
0.86Fe/CeO2
0.91Ni/CeO2
2.5Fe/CeO2
1.4
1.4
1.4
-
400
traces
CeO2 (a=b=c=5.411 Å)
-Al2O3 and traces of -Al2O3
CeO2 (a=b=c=5.411Å)
-Al2O3 and traces of -Al2O3
α-Fe2O3
CeO2/Al2O3
63
35
highly dispersed
460
θ
θ
γ
9.9Fe/Ce/Al2O3
highly dispersed
320
γ
α
-Fe2O3 (a=b=5.035Å, c=13.741 Å)
neither following sintering of CeO2, nor change of CeO2 lattice observed by XRD, stabilization of Ce3+ is more probably due to
parameters was noted after Me supporting (Table 1). By interaction with OH groups of alumina. Absence of such alumina-
considering the radius of Fe3+ ions (0.049-0.078 pm, depending on bonded hydroxyl groups can prevent penetration of Fe3+ into the
coordination and spin state) is smaller than that of Ce4+ ions in the bulk of alumina and phase transition observed in the case of Al2O3-C
typical cubic fluorite lattice (0.097-0.101 nm), expected lattice sample.
shrinkage took place at substantial Fe content in the mixed Fex-Ce1-
x oxides [26]. However, at x<0.05 even slight unit cell expansion was 3.1.2. DDPA.
observed and related by authors to partial Ce4+ reduction to larger
(1.23 Å) ion Ce3+ [27] during calcination in Fe3+ presence. Therefore, DDPA of (2.5-2.7)Me/Al2O3-1200 and (0.86-0.91)Me/CeO2 samples
penetration of supporting Me to the CeO2 lattice cannot be has been performed to reveal: 1) possible modification of support
excluded, especially with account of concreted nanocrystallites lattice due to Me insertion, and 2) formation of Co-Al spinels. Al2O3-
present in the initial support.
In the 9.9Fe/Ce/Al2O3 sample
parameter and dXRD value close to that in 13.2Fe/Al2O3-C and
2.5Fe/Al2O3-1200 samples. Although Fe supporting onto Ce/Al2O3
resulted in noticeable drop of the surface area, like in Al2O3-C based
1200 supported samples were quite stable towards dissolution. So,
about 5 wt% and 2 wt% of the 2.5Fe/Al2О3-1200 and 2.7Сo/Al2О3-
1200 samples respectively dissolved during all steps (Table 2).
Nevertheless, all Fe supported onto Al2О3-1200 passed into the
solution (Fig.2A): as individual oxide/hydroxide (about 18.4 mole%
of the probe dissolved in HCl), while the rest Fe (~43.5 mole% of
the probe) - in HF together with some Al. Since some dissolution of
the last proceeded even after all Fe was removed from the sample,
formation of Fe-Al solid solution is excluded, and the structure with
FeOx species located on the surface or on the intergrain boundaries
of concreted alumina micro-crystallites and linked to the last, more
probably, due to penetration of some Fe3+ ions to the disordered
α-Fe2O3 was detected with lattice
samples, it didn’t facilitate formation of α-Al2O3 (Tables 1-2, Fig.1C).
For similar CeO2/Al2O3 samples it was shown that this is Ce in the
lower oxidation state that stabilizes alumina toward the formation
of low-surface-area phases up to 1100 °C or 1200 °C K under
oxidizing and reducing conditions, respectively [28]. Since CeAlO3
crystallites at high temperature and under reducing conditions were
4 | J. Name., 2012, 00, 1-3
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Table 2 Chemical composition of dissoluble part of Me/Al2O3-1200(CeO2) samples (Me = Co, Fe, Ni).
View Article Online
DOI: 10.1039/C5CY01381J
ARTICLE
Sample
Sample
dissolved, of the sample in
weight% dissolved portion
Me, weight%
Element Distribution of elements dissolved during different steps
but O, mole %
total
HCl (pH=2)
(3.6)M HF
(1
÷3)M HCl
2.5Fe/Al2О3-1200
2.7Сo/Al2О3-1200
5
2.53
Al
Fe
Al
Co
Ce
Ce
Co
Ce
Fe
38.1
61.9
82.1
17.9
100
98.2
1.8
-
-
-
-
38.1
43.5
82.1
17.5
100
-
1.41
-
0.74
18.4
2
0.43
-
-
0.4
-
-
CeO2
0.91Co/CeO2
4
95
-
-
0.93
98.2
0.39
96.8
1.86
traces
0.86Fe/CeO2
0.91Ni/CeO2
54
11
1.0
96.8
3.2
0.60
0.81
Ce
Ni
91.3
8.7
-
-
91.3
0.9
traces
7.8
near subsurface layers can be considered. In the case of
2.7Сo/Al2О3-1200 only 0.43 wt% of Co dissolved in all steps, and
A
B
HCl (pH=2 3M, T=25 800C)
3.6M HF (T = 80 900C)
HCl
(pH=2)
3.6M HF (T = 80 900C)
HCl (1 3M, T=25 800C)
1.0
0.8
0.6
0.4
0.2
0.0
overwhelming majority
– as the solid solution with Co0.3Al1
1.0
0.8
0.6
0.4
0.2
0.0
stoichiometry reasonably related to the mixed Co-Al-O spinel like
compounds located in the near surface layer of the sample. The
nature of insoluble Co compounds still remained open.
CeO2 didn’t dissolve in HCl, and very slow dissolution (about 4% of
the total sample weight for 7 minutes) took place after HF feeding
(Fig.2B, Table 2). Me/CeO2 samples dissolved in substantially milder
conditions, but the character and the rate of their dissolution
depended on Me nature. So, already 95 weight% of 0.91Co/CeO2
sample dissolved in these conditions. Increase of solubility can be
related first of all to formation of Co-Ce-O solid solution with
2.5Fe/Al2O3
CeO2
Al
Fe
0
20
40
60
0
5
10
15
HCl (1 3M,
T=25 800C)
HCl
(1 3M,
25 400C)
HCl (pH=2)
3.6M HF (T = 80 900C)
3.6M HF (T = 40 900C)
HCl
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(pH=2)
0.91Co/CeO2
2.7Co/Al2O3
Al
Co
Ce
Co
Co/Ce = 0.004 and including all soluble Ce (dissolved in (1÷3) M
HCl). This obviously means that Co3+ ions are capable to insert into
more dispersed (~10 nm) concreted microcrystallites, and modify
the lattice of larger (~100 nm) particles increasing their solubility.
The rest of Co dissolved slowly as an oxide in the most rigid
0
20
40
0
20
40
Time, min
60
Time, min
Fig. 2 DDPA spectra of: (A) 2.5Fe(2.7Co)/Al2O3-1200, (B)
CeO2 and 0.91Co/CeO2 samples.
conditions (3÷6M HF), and, more probably, can be related to well-
crystallized Co3O4 detected by XRD (Table 1). Absence of any Co in
the same flow during dissolution of 2.7Сo/Al2О3-1200 sample
points to the formation of well-crystallized Co-Al spinel therein.
Quantity of dissolved 0.86Fe/CeО2 sample was some lower (54 wt%
of the total) and, like in 2.5Fe/Al2О3-1200 case, all Fe passed into
the solution (Fig.S1). It was included into Fe oxide/hydroxide which
solubility in HCl (pH=2) and at the substantially higher acidity (3.6
M HF) can be due to different degree of crystallinity or defect
structures formed. The parallel dissolution of remaining Fe and Ce
8
B
A
1
6
3
0
4
5
2
4
3
3
in the flow of (1÷3)M HCl with the stoichiometry Fe/Ce = 0.02
points to formation of Fe-Ce-O solid solution, more probably, with
participation of CeO2 composites characterized by smaller sizes of
crystallites. Unmodified ceria is stable even in 3.6 M HF and can be
related to composites with largest crystallites. We cannot exclude
that both unmodified CeO2 and Ce-Fe-O solid solution are
decorated by highly defective Fe oxide/hydroxide species dissolved
in the weak HCl. In the case of 0.91Ni/CeO2 sample (Fig. S1) most of
2
1
4
0
20000
30000
40000
20000
30000
40000
-1
Wavenumber,cm-1
Wavenumber, cm
Fig. 3 UV-vis DR spectra of: A – 2.9Co/Al2O3-1200 (1),
CoAl2O4 supplied by Aldrich (2), 0.93Co/CeO2 (3), CeO2
(4), Co3O4 (5); and B – FeOx (1), 0.86Fe/CeO2 (2),
2.7Fe/Al2O3-1200 (3), CeO2 (4).
supported Ni dissolved individually in (1
parallel Ni and Ce dissolution took place in the more rigid
conditions (3 6 M HF) and included only 10 weight% of the sample.
÷3) M HCl, while the
÷
Therefore, in the 2.7Сo/Al2О3-1200 sample Al3+ ions partially
replace Co3+ in the spinel structure, forming both highly disordered
and well-crystallized Co-Al-O species, while incorporation of Fe3+
ions into the boundaries of the micro-composites results in the
anchoring of Fe2O3 crystallites to alumina surface. Me-ceria
interaction results in formation of Me-Ce-O solid solutions in
Me/CeO2 samples, their content and degree of disorder in the
sample change in the following order: 0.91Co/CeO2 > 0.86Fe/CeO2
> 0.91Ni/CeO2
≥ CeO2.
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3.2. Microstructure
710.4
Co2p
Fe2p
A
B
Ni2p
C
711.7
UV-Vis DR spectra of different Co- and Fe- based catalysts have
been presented in the Fig.3a and Fig.3b, respectively. CeO2 was
characterized by strong a.b. at 36500 and 29000 cm-1 attributed to
←
Ce4+ O2- charge transfer and the interband transitions,
779.9
781.8
854.1
856.0
3.8Fe/Ce/Al2O3
0.86Fe/CeO2
CoOx
786.5
2.5Fe/Al2O3-1200
0.91Ni/CeO2
0.91Co/CeO2
correspondingly [29], and band gap energy (Eg) value at 3.35 eV
which is consistent with the reported value 3.3 eV [29-31]. The
spectra of 0.91Co/CeO2 (Fig.3a) and 0.86Fe/CeO2 (Fig.3b) samples
are in fact a superposition of those for corresponding bulk oxides
and CeO2, excluding the red shift of absorption edge of CeO2 (Eg
value at 3.18 and 3.22 eV for 0.91Co/CeO2 and 0.86Fe/CeO2,
respectively) and additional absorption band at ~20000 cm-1 (Co
case) and 23500 cm-1 (Fe case). Minimal value of red shift of
absorption edge compared to that in the support was evaluated for
0.91Ni/CeO2 sample (Eg value at 3.27 eV, spectrum not shown for
brevity). Earlier we already observed these spectral characteristics
for Fe/CeO2 sample prepared by the same way [15] and explained
their appearance by O2 adsorption on the structural defect, more
obviously, clusters of oxygen vacancies arose after entrance of
dopant ions into the bulk of CeO2. Like proposed in [30], change of
the value of absorption edge indicate that either (1) Me ions in
Me/CeO2 samples substitute into the CeO2 lattice; or (2) the
isolated Me ions form mixed Me-Ce oxide and strongly interact with
CeO2 leading to the electronic structure changes. In our case the
value of the red shift correlated well with content of Me-Ce mixed
solution in the sample, as determined by DDPA.
856.3
Ni2+ - satellite
2.7Co/Al2O3-1200
2.7Ni/Al2O -1200
3
Co2AlO4
780
790
800
810
850
855
860
865
870
705
710
715
720
725
730
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Fig. 4 Co2p (A), Ni2p (B), Fe2p (C) spectra of Me/Al2O3-
1200(CeO2) samples (Me = Fe, Co, Ni) with close (~7*1019
at/m2) concentration of Me, and Fe 2p spectrum of
3.8Fe/Ce/Al2O3 sample (C).
in HCl at pH=2), therefore, most of surface Co included in the Co-Al
spinel structures. To ensure the correctness of such assignment,
spectrum of CoAl2O4 supplied by Aldrich and additionally calcined
at 900 °C for 5 h has been presented as well. In 0.91Co/CeO2
sample the position of the main Co 2p3/2 peak shifted to lower BE
values (779.9 eV), while the shoulder at 781.9 eV still was present
resulting in the ratio of their intensities as ~3:2. Definite
assignment of the peak at 779.9 eV to individual CoO, Co3O4 or
CoOOH was impossible since presence of satellites at 786.7 and
790.8 eV agrees with formation of any of these compounds. In
addition, in the XPS spectrum of bulk Co3O4 sample characterized
by the same lattice parameters as for 0.91Co/CeO2 sample (Table 1)
intensity of satellite peak was substantially lower. Hence, different
oxide/hydroxide-like Co compounds, including those in Co-Ce-O
solid solution, have been formed on the surface of 0.91Co/CeO2
sample. In particular, Co(OH)2 in this case could result from
hydration of highly dispersed species.
In the spectrum of 2.9Co/Al2O3-1200 sample an intense triplet of
bands at about 15900, 17000 and 18200 cm-1 with the shoulder at
13800 cm-1 were detected and related to the transitions of
→
4A2 4T1(4P) of the tetrahedral Co2+ ions in Co-Al-O spinel and
cobalt oxide, respectively [32]. Presence of the same triplet in the
reference CoAl2O4 sample (Aldrich) evidences formation of Co-Al-O
spinel with admixture of Co3O4.
3.3. Surface composition.
Ni 2p3/2 peak at 856.3 eV in 2.7Ni/Al2O3-1200 sample (Fig. 4b) can
be due to both NiAl2O4 (BE ranging from 855.2 to 857.2 eV) and
Ni(OH)2 (855.6 - 856.6 eV) [41-45], the former being detected as
well by XRD (Fig. 1, Table 1). NiO state characterized by lower BE
value (854.2 eV) [45] was detected in the 0.91Ni/CeO2 sample
As follows from X-Ray photoelectron spectra of Ni 2p, Fe 2p, and Co
2p levels in Me/Al2O3-1200 and Me/CeO2 samples presented in the
Fig. 4, the nature of Me surface compounds depended on the
support nature. Usually, no difference between various Co
compounds can be reliably withdrawn from binding energies of
either Co 2p1/2 or Co 2p3/2 signals [10] without accounting for the
position of their satellites arising due to the shake-up process of
Co2+ compound in the high spin state. Indeed, BE values
characterizing Co 2p3/2 peak position in CoO, CoOOH and Co3O4 are
indistinguishable (~780.0 eV). Higher but also very close BE values
have been reported for CoAl2O4 and Co(OH)2 (781.9 and 781.2 eV,
respectively) [33-38]. However, for CoO, Co(OH)2 or CoAl2O4 the
satellite - main signal distances fall in the range 5-6.5 eV [34, 35, 39,
40], while the satellites in the spectrum of pure Co3O4 and CoOOH
should be located approximately 9–10 eV away from the main
signals [34,36,39]. In this regard, BE value of Co 2p3/2 signal at 781.6
eV with satellite at 786.6 eV in 2.7Co/Al2O3-1200 sample (Fig.4a)
excluded CoO, Co3O4 and CoOOH from our consideration, and
either Co(OH)2 [33] or CoAl2O4 [34,37,38] could form on the
surface. According to DDPA data, Co(OH)2 could be responsible for
about 0.4% of Co compounds (Table 2, corresponds to Co dissolved
together with Ni(OH)2 (NiO : Ni(OH)2)
≈ 50 : 50) resulting in the
widening of both main Ni 2p peak and energy shake up satellite
peak about 6 eV above the main line. Like for 0.91Co/CeO2 sample,
Ni(OH)2 detection could evidence presence of highly dispersed NiO
species both in Al2O3 and for CeO2 based samples. In this case, their
relative content in the samples is higher compared with Co case.
While the BE value of the Fe 2p3/2 spectra in 0.86Fe/CeO2 sample at
710.4 eV (Fig. 4c) evidences preferential Fe3+ state in the oxide, its
high energy shift (BE = 711.8 eV) detected in both 2.5Fe/Al2O3-1200
specimen and all Al2O3-1000, Al2O3-1200 and Al2O3-C supported
samples can be related to formation of mainly Fe3+ oxyhydroxide
therein [46]. In the 3.8Fe/Ce/Al2O3 sample both states of Fe3+ have
been detected as well, but fraction of Fe3+ state in the oxide is
some higher than in Fe/Al2O3 based samples. Therefore, FeOx
species anchor both to Al2O3 and CeO2 surface.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
Table 3 Physicochemical properties of different Me/Al2O3,
Me/CeO2 and 3.8Fe/Ce/Al2O3 samples.
6x1020
4x1020
2.5Fe/Al2O3-1200
Sample
SBET, Mes, at.%
Feexp
,
DMe,
2.7Co/Al2O3-1200
0.86Fe/CeO2-900
0.91Co/CeO2-900
CeO2-900
2x1019
m2/g (by XPS) rel. units rel. units
CeO2 based
0.91Ni/CeO2-900
2.5Fe/Al2O3-C
0.86Fe/CeO2
0.91Co/CeO2
0.91Ni/CeO2
2.5Fe/CeO2
1.3
1.3
1.3
1.3
1.0
1.2
2.0
1.3
-
1.5
1.8
1x1019
2x1020
-
2.7
2.1
0.72
1.5
0
0
0
2
4
6
8
10 12 14 16 18 9496
time, s
Al2O3-1200 and Al2O3-1000 based
2.5Fe/Al2O3-1200
2.7Co/Al2O3-1200
2.7Ni/Al2O3-1200
0.86Fe/Al2O3-1200
2.5Fe/Al2O3-1000
4
4
4
4
7
1.1
1.0
1.4
1.1
1.4
4.4
-
-
4.4
9.8
1.6
1.5
2.1
4.8
3.7
Fig. 5 Time dependence of the quantity of exchanged
oxygen (No) in 18O SSITKA experiments for different
Me/Al2O3 and Me/CeO2 samples.
Al2O3-C based
2.5Fe/Al2O3-C
6.6Fe/Al2O3-C
13.2Fe/Al2O3-C
19.8Fe/Al2O3-C
61
46
33
22
0.46
1.3
2.2
28.1
59.8
72.6
35.2
10.5
8.5
5.1
1.7
3.0x10-5
support
Fe/support
Co/support
Ni/support
2.5x10-5
2.0x10-5
1.6
CeO2/Al2O3 based
50 1.3
1.5x10-5
8.0x10-6
3.8Fe/CeO2/Al2O3
65.0
16.0
In the Ce 3d spectra of 3.8Fe/Ce/Al2O3 sample the normal complex
form due to shake-down satellites from an O1s to Ce 4f electron
transfer observed for CeO2 and 0.86Fe/CeO2 was supplemented by
the features marked as v’ and u’ due to anion defects and Ce3+ (Fig.
S2) [47]. This means that, besides smaller crystallized CeO2 species,
X-ray amorphous CeAlO3 and probably isolated Ce3+ species can
reasonably form after CeO2 precipitation onto high surface area
alumina [48,49], thus stabilizing alumina toward the formation of
low-surface-area phases, like proposed in [27].
Therefore, spinel-like surface structures including Al have been
formed on the surface of 2.7Co/Al2O3-1200 sample, and probably
their mixture with Ni(OH)2 - in 2.7Ni/Al2O3-1200 sample. It is quite
reasonable to suppose that above 7000C surface Fe oxyhydroxide
formed after Fe supporting onto Al2O3 and all Ni- and Co-containing
hydroxylated species convert to corresponding oxides, and thus
close types of surface compounds participate in the reaction in
both Al2O3-1200, CeO2 and CeO2/Al2O3 based samples.
4.0x10-6
0.0
750
800
850
900
T, 0C
Fig. 6 Temperature dependence of the activity of
different (2.5-2.7)Me/Al2O3-1200 (open symbols) and
(0.86-0.91)Me/CeO2-900 (solid symbols) samples (Me =
Co, Fe, Ni) in the reaction of N2O decomposition.
t
2CO
U
g
input
2
N
o
=
(α
g
−
α )dt
g
No = NA
∫
SBET
0
input
where αg
- isotope fraction in the inlet mixture (0.95), CO2 - O2
– flow rate of the reaction mixture
Avogadro number. It is seen that for (2.5-
concentration (mol/mol),
(mole/s), NA
U
Table 3 lists the data on the surface concentration of Me in the
different Al2O3-1200 and CeO2 based samples. Close values were
measured in (2.5-2.7)Me/Al2O3-1200 and 0.91Co(0.86Fe)/CeO2
samples with the same as related per surface unit (6.7*1019 at/m2)
content of Me therein that means close MeOx (Co-Al-O) species
dispersion (DMe) therein. Its relative value can be estimated as
–
2.7)Fe(Co)/Al2O3-1200 samples NO(t) dependencies (at higher rate
of exchange on 2.5Fe/Al2O3-1200, as determined from the slope of
No curves) reached the plateau after about 15 s after isotope
admission corresponding to exchanged 4.9·1020 and 3.2·1020
O
atoms/g for 2.5Fe/Al2O3-1200 and 2.7Co/Al2O3-1200 samples,
respectively. With account of the quantity of supported Me
(2.7·1020 atoms/g), one can suppose that, within the accuracy of
measurement, solely the oxygen of Fe2O3 and Co oxidic compounds
(Co3O4+Co3-xAlxO4) participated in the exchange. The integral
quantity of exchanged oxygen in 2.5Fe/Al2O3-C sample (7.9·1020
atoms/g) substantially exceeds that in supported Fe oxide.
Provided substantially more dispersed FeOx species have been
formed in this sample compared with those in 2.5Fe/Al2O3-1200
one (Table 3), it is more probably support oxygen that can transfer
to O2 molecule through developed Fe-alumina interface.
follows: DMe
= Mes/CFe, where Mes is the surface atomic
concentration of Me in the sample as measured by XPS (Table 3)
[%], CMe is total content of Fe in the samples [atoms/m2].
Noticeably higher content of Ni in the near surface layers of the
CeO2 based sample can be explained either by lower degree of Ni
penetration into CeO2 resulting in formation of Ni-Ce-O solid
solutions (Table 2), or by higher dispersion of NiOx species that
agrees well with high content of hydroxylated species.
3.4. 18O SSITKA.
Substantially more profound and faster oxygen exchange took
place on CeO2 based samples, obviously involving the bulk of the
support starting from the very low time after 18O2 admission. The
Fig. 5 shows the time dependencies of exchanged oxygen for
different samples as related to unit of the surface (NO) calculated
using the formulae
rate of exchange changed in the following order: 0.91Co/CeO2
≥
0.86Fe/CeO2 > 0.91Ni/CeO2 CeO2 and correlated with values of
≈
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red shift of CeO2 band gap energy (Eg) in turn depending on the increased mobility of oxygen is more prominent at lower quantity
content of Me-Ce mixed solution in the sample characterized by of supported Fe. However, activity drop was substantially more
increased degree of disorder. Therefore, oxygen transfer from prominent, although even higher Fes values were measured for the
fluorite lattice to surface by extended vacancies arisen after 2.5Fe/CeO2 sample than for the 0.86Fe/CeO2 one (Table 3).
insertion of Men+ ions to fluorite lattice can be responsible for Supposing the rate of oxygen transfer from the bulk to active sites
enforced rate of isotope exchange on Me/CeO2 (Me = Co, Fe) should depend on the length of MeOx-CeO2 interface, its decrease
samples compared with pure CeO2.
at enlargement of FeOx species (Table 3), revealed as start of
oxygen exchange at higher temperature (Fig. S4) should make an
additional negative contribution to the activity.
3.5. Catalytic activity
3.5.1. N2O decomposition
Effect of the support nature
Effect of the support surface area
In accordance with observed effect of FeOx dispersion onto activity,
Among Al2O3-1200 based samples with close as related to surface using of the support characterized by higher specific surface area
concentration of supported Me (6.7*1019 at/m2), catalytic activity allows obtaining more dispersed FeOx species at substantially
°
in N2O decomposition at T = 750-900 C can be ordered as follows: higher total Fe content (Table 3). This opens up additional
2.5Fe/Al2O3-1200 > 2.7Co/Al2O3-1200 > 2.7Ni/Al2O3-1200 > Al2O3- possibilities for increase of conversion as measured at the same
1200 (Fig. 6). (0.86-0.91)Me/CeO2 samples with quite close, both sample loading. Indeed, at close quantity of supported Fe, N2O
calculated (6.7*1019 at/m2) and real (Table 3), surface conversion increased with SBET value of Al2O3 (Fig. 7B).
concentration of Me as in (2.5-2.7)Me/Al2O3-1200 samples Unfortunately, for Fe/Al2O3-C samples we failed to perform
revealed substantially higher intrinsic activity. In the case of Co and catalytic tests at reasonably low conversion values for correct
Ni containing samples such rise of the activity could be partly calculation of the reaction rate values. This fact and substantial
related to formation of principally different state of active negative effect of the quantity of supported Fe on SBET value (Tables
component (oxides/hydroxides in CeO2 case instead of 2, 3) complicated analysis of parameters determining the activity.
corresponding Co(Ni)-Al-O spinels located on the surface of Al2O3). To pass over this problem, we calculated the concentration of
Nevertheless, similar effect observed for Fe based samples exposable FeOx species (Feexp) per weight unit by following
characterized by close type of active species on both supports formula: Feexp = Fes*SBET, where Fes is the surface concentration of
obviously points to the importance of increased oxygen mobility of Fe as measured by XPS (Table 3), and compared conversion values
CeO2 that in turn was substantially more active than Al2O3. measured for the same catalyst loading as dependent on both Fe
Provided the general scheme of N2O decomposition obeys content in the sample and Feexp. It turned out that N2O conversion
Langmuir-Hinshelwood mechanism:
decreased with increase of Fe content from 2.5% to 19.8% (Fig.7B),
or FeOx dispersion drop (Table 3), but this order cardinally differed
from that for Feexp (13.2Fe/Al2O3-C > 6.6Fe/Al2O3-C > 19.8Fe/Al2O3-
C > 2.5Fe/Al2O3-C). Therefore, similar to Fe/Al2O3-1200 samples, it
1)
2)
3)
N2O + S → N2O-S
N2O-S → N2 + O-S
2O-S ↔ O2 + 2S
where S is surface active site, N2O-S is adsorbed or chemisorbed is the size effect of FeOx species that seems to be the dominant
N2O, and O-S is adsorbed or chemisorbed O, the rate of O2 factor determining descending character of “conversion – Fe
desorption by step 3) in the case of (0.86-0.91)Me/CeO2 samples content” dependence. Similar reverse dependence was already
increases due to alternative fast supply of oxygen to reduced site S observed earlier for lower (500-650 °C) temperatures [51]. With
from the subsurface layers (Oss) of CeO2 characterized by increased account of SSITKA data obtained for 2.5Fe/Al2O3-1200 and
bulk oxygen mobility through Me-CeO2 interface containing a lot of 2.5Fe/Al2O3-C samples, higher activity of smaller FeOx species can
oxygen vacancies to obtain additional O-S site:
4) O-S ↔ Oss +S
be due to growing ability of oxygen supply to reduced active sites S
from the alumina regions adjacent to developed MeOx - alumina
Similar scheme considering two alternative pathways of interface (step 4). Oxygen (3 vol%) and H2O (3 vol.%) addition into
regeneration of active sites was also proposed for Cu/CeO2 samples reaction mixture, in accordance with considered above scheme of
tested in substantially lower temperature range (between 300 and the mechanism, decreased observed N2O conversion, but didn’t
550 °C) and included recombination step (if two oxidized copper change the order of activity (Fig. S5).
sites are close enough to each other) and the step of Cu2+−O−
species reduction by Ce3+ [50].
Effect of Fe content and FeOx dispersion
1.6x10-1
1.2x10-1
8.0x10-2
4.0x10-2
Al2O3-1200
CeO2-900
80
60
40
20
A
B
At variation of Fe content in Al2O3-1200 based samples from 0.89 to
6.6 wt% (Fe was chosen as the most active among Me) we
observed the inverse “activity (as related both to sample surface
unit and to one Fe atom) - Fe content” dependence (Fig. 7A). The
same Fes values for 0.86Fe/Al2O3-1200 and 2.5Fe/Al2O3-1200
samples (Table 3) evidence decrease of FeOx species dispersion
(DFe, Table 3) at higher Fe content. Finally, close rates of N2O
decomposition were measured for bulk Fe2O3 and all Fe/Al2O3-1200
samples (Fig. S3), although in the former of them Fes value should
be obviously higher. Therefore, activity of surface Fe sites
decreases substantially with enlargement of FeOx species.
Al2O3-1200
Al2O3-1000
Al2O3-C
CeO2
1.2x10-5
8.0x10-6
Ce/Al2O3
4.0x10-6
0.0
0
5
10
15
20
0
5
10
15
20
Fe, 1019 at/m2
Fe, weight%
Fig. 7 Reaction rate (A) and N2O conversion as measured
at the same sample loading and flow rate (B) at 800 C
depending on Fe content in Al2O3-1200, Al2O3-1000,
Al2O3-C, CeO2 and Ce/Al2O3 based samples.
At variation of Fe content in CeO2 based samples from 0.5 to 2.5
wt% (i.e. at close interval of supported Fe to that in the case of
Al2O3-1200 based samples as related per surface unit and thus
close dispersion of FeOx species) their higher activity due to
°
8 | J. Name., 2012, 00, 1-3
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Effect of CeO2 dispersion
same catalyst loading as for Al2O3-1200 based samples (0.015 g),
Using of the effect of high oxygen mobility of CeO2 for following i.e. at about 3 times lower number of Me sites therein (Table 3),
increase of the efficiency of N2O decomposition (Fig.7B) is higher values of NH3 conversion were observed for corresponding
restricted by its low surface area under the reaction conditions CeO2 based samples (Fig.8B). This increase, at least in the case of Fe
[3,20]. Therefore, in addition to traditionally used increase of based samples, is exclusively due to ability of the support to donate
concentration of active sites by supporting more active component, oxygen to FeOx, since substantially lower conversion values were
we tried to disperse CeO2 by supporting onto high surface area measured on CeO2 and bulk FeOx characterized by close SBET. N2O
Al2O3. Only Fe oxide was used in this case as well to avoid yield values were reasonably lower than on corresponding Al2O3-
formation low active Ni (Co)-Al spinels. These were 3.8Fe/Ce/Al2O3 1200 based samples. In addition, at 8000C the order of NH3
and 9.9Fe/Ce/Al2O3 samples that revealed the highest N2O
conversion (Fig. 7B) due to that part of FeOx therein was anchored
to the surface of higher dispersed ceria particles (Table 1, Fig.4c). At
the same time, the size effect, at least for the sample
3.8Fe/Ce/Al2O3, cannot be completely excluded (compare DFe
conversion and NOx yield (not shown for brevity) - 0.91Co/CeO2
0.86Fe/CeO2 > 0.91Ni/CeO2 CeO2 was exactly the same as for the
efficiency towards oxygen exchange therein (Fig.4).
≥
≈
Strong decrease of both NH3 oxidation [52] and N2O decomposition
[32] over Co oxide at high temperatures was shown to be due to
Co3O4 reduction to CoO. We didn’t observe such deactivation of
0.91Co/CeO2 sample with temperature both at N2O decomposition
(Fig.6), like in Co3O4-CeO2 mixed oxides with low Co content [11],
and at ammonia oxidation (Fig.S7B), although Co3O4 reduction
under TPD conditions was observed as well (Fig.S6). It obviously can
be explained by higher resistance of Co-Ce interface towards
reduction due to some oxygen supply from CeO2 by reaction (4).
Nevertheless, at lower temperature 0.91Co/CeO2 sample with Co
state closer to Co3O4 was already substantially more active than
0.87Fe/CeO2 both in N2O decomposition (Fig.S7A) and NH3
oxidation (Fig.S7B).
values in the Table
3 for the samples 3.8Fe/Ce/Al2O3 and
6.6Fe/Al2O3-C with close SBET).
3.5.2. NH3 oxidation
Effect of the support nature
Among (2.5-2.7)Me/Al2O3-1200 samples, 2.5Fe/Al2O3-1200 one was
the most active (Fig. 8A) and characterized by the highest
selectivity to NO+NO2 (86-88%). As the result, the orders for the
activity and the efficiency of NOx formation evaluated as NOx
yield=Conversion*NOx selectivity were close to that for the activity
of N2O decomposition: 2.5Fe/Al2O3-1200 >> 2.7Co/Al2O3-1200 >
2.7Ni/Al2O3-1200
~ Al2O3-1200. In addition, N2O yield value
Effect of Fe content and FeOx dispersion
calculated by similar manner as for NOx yield on 2.5Fe/Al2O3-1200
sample in this temperature interval was lower than 2% which is
comparable to that for Pt-Rh gauzes [1], while continuous increase
of N2O yield with temperature was noted for all other samples. At
lower O2 concentration in the inlet mixture (2.5%) yield of NOx
decreased to 77-78%, mainly because of lower selectivity. For Co-,
Ni- based samples and for pure Al2O3-1200 negative changes of
conversion and NOx selectivity were substantially more prominent.
We failed to calculate the rate of ammonia oxidation in all
temperature interval due to non-differential reactor operating
conditions. Hence, to compare intrinsic activity of samples
characterized by different SBET values, we tested catalytic properties
Descending dependence of NH3 conversion versus Fe content in
Al2O3-1200 and CeO2 based samples (0.015 g sample loading for all
these experiments) was observed (Fig. 9), similarly to the reaction
of N2O decomposition, and thus can be related to size effect. Again,
for Fe/CeO2 case it was more prominent due to attenuation of
oxygen transfer processes through Fe-CeOx interface (Fig.S4).
Nevertheless, unlike to reaction of N2O decomposition, FeOx
dispersion is not a predominant factor for NH3 conversion anymore.
So, at Fe content lower than 3*1019 atoms/m2 (case of Fe/Al2O3-C
and Fe/Ce/ Al2O3 samples) NH3 conversion value depended mainly
on the quantity of exposable Fe (Feexp).
At the same time, these were larger FeOx species that converted
NH3 to NOx more selectively in all Al2O3 based samples (Fig. S8),
finally resulting in quite close (77-81%, case of Fe/Al2O3-1200
samples) or growing with Fe content (up to 79% for Fe/Al2O3-C)
NOx yield values (Fig.9). In accordance with findings of Pérez-
Ramírez and Kondratenko [16], on bulk Fe2O3 the desired reaction
of (0.86-0.91)Me/CeO2 samples at catalyst loading 0.043
g
corresponding to the same surface area as for Al2O3-1200 based
samples. In these conditions already at 750 °C complete NH3
conversion was measured on all (0.86-0.91)Me/CeO2 samples, and
96%– for CeO2 that is obviously due to capability of CeO2 itself to
donate lattice oxygen for ammonia oxidation [16]. Even at the
follows
a
Mars–van Krevelen-type scheme involving the
100
100
B
A
100
100
90
80
70
60
50
80
60
40
20
90
80
70
60
50
80
60
Fe
Co
Ni
-
bulk FeOx
Al2O3-1200
Al2O3-1000
Al2O3-C
3
2
1
0
2
1
0
CeO2
Ce/Al2O3
0
5
10
Fe, 1019 at/m2
15
20
750
800
T,
850
900
750
800
T,
850
900
0
0
C
C
Fig. 8 Temperature dependence of NH3 conversion and
N2O yield for (2.5-2.7)Me/Al2O3-1200 (A) and (0.86-
0.91)Me/CeO2-900 (B) samples. Catalyst loading 0.015 g.
Fig. 9 Dependence of NH3 conversion (solid symbols) and
NOx yield (open symbols) on Fe content in different
Fe/(Al2O3,CeO2) samples. Catalyst loading 0.015 g.
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Journal Name
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oxidation on Al2O3 should be accounted as well.
We believe as well that more oxidized state of FeOx anchored to
403.
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Conclusions
We have shown that replacement of Al2O3 as the support for MeOx
(Me = Fe, Co, Ni) by CeO2 characterized by increased lattice oxygen
mobility resulted in substantial increase of the rates of 16O/18
O
exchange at 800 °C in corresponding samples. Oxygen transfer from
fluorite lattice to surface by extended oxygen vacancies arose after
insertion of Men+ ions to fluorite lattice was shown to be
responsible for enforced rate of isotope exchange.
18 M. Konsolakis, S. A. C. Carabineiro, E. Papista, G. E. Marnellos, P.
B. Tavares, J. Agostinho Moreira, Y. Romaguera-Barcelay, J. L.
Formation of low active spinel-like Me-Al-O structures restricted
application of Co and Ni oxides in any Al2O3 containing catalysts.
For CeO2- and Al2O3-based samples with supported FeOx an obvious
size effect was observed in both reactions. So, highly dispersed
FeOx were substantially more active due to change of contribution
of oxygen supply from the support (both ceria and, probably,
alumina) to reduced surface sites through Fe-CeOx interface thus
increasing the rate of O2 desorption. However, smaller species less
selectively oxidized NH3 to NOx.
Using of high surface area Al2O3-C the samples with high content of
dispersed FeOx in the weight unit were synthesized to increase
observable N2O conversion. To increase surface area of CeO2 and
thus efficiently use the promoting effect of oxygen mobility, the
last was dispersed onto alumina by precipitation. Fe/Ce/Al2O3
samples with optimal content of Fe revealed superior activity in
N2O decomposition and NH3 oxidation to NOx compared with
Fe/Al2O3 with close SBET and dispersion of FeOx species value.
Figueiredo, Catal. Sci. Technol., 2015, 5, 3714
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N2 + O2
N2O
N2 + O2
R3
R1
R2
Me-O
R1>R2>R3
Me-O
Al2O3
O
O
CeO2
O - M e
X1
O - M e
X1>X2>X3
X2
N2
X3
NO+NO2
NH3
NO+NO2