Understanding the Role of SnO2 Support in Water-Wolerant Methane Combustion: In situ Observation of Pd(OH)2 and Comparison with Pd/Al2O3

Article abstract of DOI:10.1002/cctc.201901744

Quantitative analysis of poisonous Pd hydroxide phase formation during methane combustion in the presence of water is performed via in situ X-ray absorption spectroscopy from 200 to 500 deg C for palladium catalysts supported on tin and aluminum oxides. Water presence inhibits oxidation of the metallic Pd; for the Pd/Al₂O₃ catalyst the major oxidation product is Pd(OH)₂, with PdO being the dominant phase on the Pd/SnO₂ system. Tin dioxide is reduced during methane oxidation in the presence of water, regardless of the excess oxygen in the feed. Temperature-programmed surface reactions at anaerobic and low-oxygen conditions revealed that the Pd/SnO₂ catalyst has an inability to catalyze methane steam reforming, as opposed to Pd/Al₂O₃. We suggest that tin oxide extracts hydroxyls from the PdO surface, which makes the latter more active in methane combustion in the presence of water but inactive in methane steam reforming.

Full text of DOI:10.1002/cctc.201901744

Accepted Article
Title: Understanding the role of SnO2 support in water-tolerant
methane combustion: In situ observation of Pd(OH)2 and
comparison with Pd/Al2O3
Authors: William Barrett, Jing Shen, Yongfeng Hu, Robert E. Hayes,
Robert W.J. Scott, and Natalia Semagina
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Understanding the role of SnO2 support in water-tolerant methane
combustion: In situ observation of Pd(OH)2 and comparison with
Pd/Al2O3
William Barrett,#[b] Jing Shen,#[a] Yongfeng Hu,[c] Robert E. Hayes,[a] Robert W.J. Scott,*[b] and Natalia
Semagina*[a]
Abstract: Quantitative analysis of poisonous Pd hydroxide phase
formation during methane combustion in the presence of water is
performed via in situ X-ray absorption spectroscopy from 200 to 500
deg C for palladium catalysts supported on tin and aluminum oxides.
Water presence inhibits oxidation of the metallic Pd; for the Pd/Al2O3
catalyst the major oxidation product is Pd(OH)2, with PdO being the
dominant phase on the Pd/SnO2 system. Tin dioxide is reduced during
methane oxidation in the presence of water, regardless of the excess
oxygen in the feed. Temperature-programmed surface reactions at
anaerobic and low-oxygen conditions revealed that the Pd/SnO2
catalyst has an inability to catalyze methane steam reforming, as
opposed to Pd/Al2O3. We suggest that tin oxide extracts hydroxyls
from the PdO surface, which makes the latter more active in methane
combustion in the presence of water but inactive in methane steam
reforming.
by other groups for the supports with high oxygen-storage
capacity[8] or hydrophobicity.[3] Our earlier in situ X-ray absorption
spectroscopy study provided evidence of the lack of oxygen at Pd
active sites in the wet feed at low temperatures in Pd/Al2O3
system,[9] which was supported by others.[10]
Among a myriad of reported catalysts, SnO2-supported Pt-
group materials have a rather extraordinary resistance to water
poisoning in wet methane combustion.[11,12] We have previously
shown (Figure 1[13]) that Pd/SnO2 can surpass even Pd-Pt/Al2O3
in its activity in the presence of water; studies by others[11] and
ourselves[13] also report insignificant variations in methane
conversion when the water content in the feed was increased up
to 20%. A kinetic study[11] revealed a more negative enthalpy of
H2O adsorption on Pd/Al2O3 than on Pd/SnO2, which was
suggested as a reason of high water tolerance of the latter system.
The SnO2 reducibility was also noted as an important contributor
to the Pd/SnO2 activity because of hypothesized lattice oxygen
spillover, although in the absence of added water. [14]
Introduction
Our selective attention to Pd/SnO2 was also fuelled by the fact
that SnO2 is used as sensors for CO and (less frequently) CH4 or
H2 detection via catalytic oxidation mechanisms.[15–32] The
humidity effect has been known in that field since 1970’s[16–18,24–
One of the major requirements of an efficient catalyst for
exhaust aftertreatment from lean-burn natural gas vehicles is its
water tolerance at low operating temperatures (below 500 oC).
Conventional Pd/Al2O3 catalysts are known for their instability at
such conditions due to the water and hydroxyl accumulation on
the active sites and/or support. [1] Deactivation of PdO active sites
is accepted to occur via the formation of inactive Pd(OH)2 at
temperatures below 450 oC (for Pd/Al2O3).[2,3] Pfefferle and
colleagues advocated that water impedes oxygen mobility on the
support, thus, its detrimental effect may be balanced by the use
of a support with high oxygen mobility,[4–7] which was corroborated
32]
as it influences the sensor signal by electron generation: H2O
+ Snlattice + Olattice ó (HO-Snlattice) + (OlatticeH)+ + e-.[19] SnOx surface
has a high affinity to water that competes with CO adsorption;[27]
[a]
[b]
[c]
Prof. N. Semagina, Dr. J. Shen, Prof. R.E. Hayes
Department of Chemical and Materials Engineering
University of Alberta, 9211-116 St. Edmonton AB T6G 1H9, Canada
E-mail: semagina@ualberta.ca
Prof. R.W.J. Scott, W. Barrett
Department of Chemistry, University of Saskatchewan
110 Science Place, Saskatoon SK S7N 5C9 Canada
E-mail: robert.scott@usask.ca
Dr. Y. Hu
Canadian Light Source Inc.
Figure 1. (a) Ignition tests of methane combustion in the presence of 5%
water; and (b) hydrothermal ageing test (the temperature was varied
between 400 and 500 oC, after which it was held constant). The tests are
done at the same total Pd loading in the reactor. Metal dispersion in
44 Innovation Boulevard, Saskatoon SK S7N 2V3 Canada
Pd/SnO2, Pd/Al2O3 and PdPt/Al2O3 catalysts is 11%, 43% and 10%,
[13]
# co-first authors, contributed equally
respectively. Reprinted from
Copyright 2018.
by permission from Springer Nature,
Supporting information for this article can be found on-line.
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the concentration of hydroxyl groups increases in the presence of
gas-phase water and O2.[20] In H2 sensing, in the presence of O2,
water formation occurs as expected, while at anaerobic conditions,
hydrogen reacts with the metal oxide surface to create “rooted”
hydroxyl groups and there is no detected desorbed water
formation.[21] In Pd/SnO2 systems, CO activated on PdOx reacts
with hydroxyls on SnO2 surface with CO2 formation.[23] CHx
reaction intermediates on Pt-group active sites can be oxidized by
OHads on adjacent oxophilic Sn surface.[33]
In this work we follow the Pd speciation by X-ray absorption
spectroscopy (XAS) of Pd/Al2O3 and Pd/SnO2 catalysts during
methane combustion in the wet feed to observe the presence of
Pd(OH)2 hydroxyl species on Pd/PdO particles and to understand
better the influence of the support on the water tolerance.
Typically, hydroxyl formation in methane combustion has been
determined by operando or in situ infrared spectroscopy; however
that does not typically allow one to distinguish hydroxyl groups
presents on the Pd and the support surface.[3,4,8] Herein, we were
able to track the metallic Pd, PdO and Pd(OH)2 via in situ Pd L
edge and K edge XAS studies at temperatures applicable to lean-
burn methane combustion in the presence of water. The XAS data
are complemented by reactions at different O2/CH4 in the feed (0
through 1.9 mol/mol) that also demonstrate a strikingly unusual
and unexpected feature of the Pd/SnO2 catalyst as compared to
Pd/Al2O3 with respect to water activation.
Figure 2. TPR profiles of SnO2, Pd/SnO2 and Pd/Al2O3 (0.5 g each).
Heating rate is 10 oC/min. H2 consumption was monitored by a calibrated
on-line mass spectrometer.
of surface adsorbed oxygen species, lattice oxygen on SnO2
surface, and amorphous SnO2, respectively [29]. Alternatively, the
low temperature shoulder reduction peaks of SnO2 at low
temperatures were attributed to the reduction of surface adsorbed
oxygen and Sn4+ to Sn2+.[38] Pd addition to SnO2 expectedly shifts
the reduction onset to lower temperatures due to hydrogen
spillover from reduced Pd. The low-temperature PdO reduction,
Pd hydride formation and decomposition occurs on both Pd/Al2O3
and Pd/SnO2 catalysts. Thus, to exclude the effect of labile
support oxygen on the speciation during in situ methane
combustion, we prereduced both catalysts at 300 oC.
Figure S1 in the Supporting Information presents XPS profiles
of the Pearlman’s catalyst Pd(OH)2/C and fresh (calcined)
Pd/SnO2 that served as Pd(OH)2 and PdO standards for the in
situ XAS studies. The Pearlman’s catalyst is characterized by
Albers et al.,[39] so the 3d5/2 peak at ~337.5 eV can be assigned to
Pd(OH)2. As expected after the calcination, the Pd/SnO2 sample
is primarily PdO (~336.3 eV; 89%) with contributions from Pd
(~335.3 eV; 5%), and Pd(OH)2 (~337.5 eV) 6%. The palladium
metal is created via the reduction of PdO by X-rays under vacuum,
suggesting the concentration of PdO on the surface could be up
to 94%.
Results and Discussion
In situ X-ray absorption study (XAS): pretreatment and
selection of standards
When a PdO catalyst is exposed to methane combustion in
the wet feed, it progressively becomes reduced, as observed by
Monaie et al. [8] via in situ X-ray absorption spectroscopy over 80
o
min on stream at 600 C, while PdO remained oxidized with no
water addition to the feed. The rather slow phenomenon is in line
with the Pd/Al2O3 deactivation profile from Figure 1b. Our earlier
in situ XAS study of the reduced Pd/Al2O3 catalyst in lean-fuel
methane combustion without added water in the feed
demonstrated that the metallic Pd was completely oxidized by the
start of the catalytic test (200 oC).[9] In the presence of water in the
feed, the Pd(0) fraction remained dominant up to about 350 oC,[9]
which confirmed the earlier hypothesis of Pfefferle and colleagues
that water inhibits oxygen migration and exchange.[7] PdO is
accepted as a more active phase than metallic Pd in methane
combustion,[8,34] or it is at least required in pair with Pd(0) for C-H
activation.[35] Since the presence of water and nature of the
support were suggested to influence the onset and the rate of
Pd(0) reoxidation process,[5] we reduced the studied calcined
PdO/Al2O3 and PdO/SnO2 catalysts prior to the in situ XAS
analysis.
In situ XAS: Pd speciation during methane combustion in the
wet feed
In the current spectroscopic study of the water effect during in
situ methane combustion, we use Pd K (and complement with Pd
L edge data) to differentiate the evolution of Pd(0), PdO, and
Pd(OH)2 species with temperature. The Pd K edge is primarily
transitions from 1s to 4p and, therefore, is sensitive to speciation
and relatively insensitive to species interacting with the surface
(primarily 4d interactions). Note that the XANES region is not
significantly temperature sensitive, which allows for linear
combination fitting of the data over multiple temperatures. Due to
conformational and oxidation state differences between the
materials we were able to distinguish between species via the
XANES portion of the XAS data. In situ Pd K edge data for the
Pd/SnO2 catalyst in wet conditions are shown in Figure 3, along
with the reference spectra used for linear combination fitting.
Figure 3a presents changes in the Pd/SnO2 spectra as a function
Figure 2 shows temperature-programmed reduction (TPR)
profiles of the catalysts and the pure SnO2 support. Bulk SnO2
reduction is known to occur above 500 °C,[36,37] while the lower-
temperature peaks (a, b and g) can be assigned to the reduction
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system that is absent in the SnO2 system. We do note that at 400
oC the intensity of the white line of the Pd/Al2O3 sample is above
even that of the Pd(OH)2 standard, which suggests that another
4d-electron withdrawing species is present on the surface. This
may be due to the presence of formate groups on the surface, as
others have shown that formate species can be long living
intermediates in the methane combustion pathway.[40]
Figure 3. Pd K edge XAS spectra of Pd/SnO2 (a) under in situ wet methane
oxidation conditions, and (b) Pd standards.
of temperature and the inset highlights the XANES region. Two
trends are obvious, as shown by vertical and horizontal arrows.
The horizontal arrows indicate the white line is shifting to higher
energy, while the vertical arrows show the growth in the feature at
about 24360 eV and loss of the feature at about 24380 eV. These
changes are consistent with Pd oxidation as can be seen by
comparing with Pd(0), PdO and Pd(OH)2 reference spectra in
Figure 3b. There are no major changes in spectra at low
o
o
temperatures (100 C – 200 C), followed by slow oxidation at
moderate temperatures (200 C - 350 oC) and nearly complete
o
o
o
oxidation at higher temperatures (400 C - 450 C). The spectra
of the Pd/SnO2 sample heated at 450 oC under methane oxidation
conditions closely resembles the spectra of the PdO standard.
Previously obtained in situ data of the K-edge of Pd/Al2O3[9] shows
a similar trend consistent with Pd oxidation at higher temperatures.
Figure 4 shows the Pd L3 edge data for Pd on both SnO2 and
Al2O3 supports. The white line at the L3 edge of Pd probes the
2p3/2 to 4d transition, and thus the occupancy of the valence 4d
band of Pd, and therefore is sensitive to both Pd speciation, and
to intermediates interacting with the Pd surface. Figure 4a shows
that from 100 oC – 200 oC, the original white line broadens slightly
and the peak maxima is relatively static. At moderate
temperatures, the spectra shift to slightly higher energies, with a
Figure 4. Pd L3 edge XAS spectra of (a) Pd/SnO2 and (b) Pd/Al2O3 under
in situ wet methane oxidation conditions, and (c) Pd standards.
Figure 5 presents the LCF fitting of the Pd K edge for both
Pd/SnO2 and Pd/Al2O3 samples. Data used for the Pd/Al2O3 data
is from our previous study, which used similar catalysts studied
under similar reaction conditions.[9] LCF was not done with Pd L3
edge data, given the possibility that surface speciation and/or
photoreduction changes may be influencing the spectra in the Pd
L3 edge. The Pd/SnO2 catalyst data in Figure 5a shows that Pd
metal slowly oxidizes with the formation of both PdO as a
dominant oxidation product and Pd(OH)2 as a minor product. At
higher temperatures, a complete dehydration of Pd(OH)2 is seen,
which is consistent with the dehydration onset suggested in the
literature based on kinetic studies.[2,3] The maximum observed
Pd(OH)2 fraction on Pd/SnO2 of 15% at 350 oC closely resembles
the Pd surface area of the catalyst (16% metal dispersion as
determined by CO chemisorption). This suggests coexistence
and competition between PdO and Pd(OH)2 for the surface area
o
slight increase in the white line intensity. Finally, beyond 350 C
the white line shifts slightly to lower energy and increases slightly
in intensity. On comparison with Pd standards in Figure 4c, this
suggests mild oxidation of the samples at higher temperature,
which is consistent with K edge data, although the Pd L3 edge
data suggests a lesser degree of Pd oxidation. One possible
rationale for the differences seen in the Pd L3 and K edge data for
Pd/SnO2 samples is that for lower energy L3 edge studies, the
gaseous environment needed to be switched to He during
measurements to prevent X-ray scattering of the gaseous
atmosphere, whereas for K edge measurements, this change in
atmosphere was not necessary. It is possible that some Pd
photoreduction may occur while under the He atmosphere.
Pd L3 edge data for the Pd/Al2O3 sample are shown in Figure
4b. As the temperature increases, the white line drifts right and
increases significantly in intensity, indicative of a fast oxidation of
the sample and consistent with major Pd speciation changes. As
noted in Figure 4c, the measured white line intensity of Pd(OH)2
standards is much higher than that seen for PdO, which suggests
the Pd in Pd(OH)2 is more 4d electron deficient. Thus the Pd/Al2O3
data suggests that significant Pd(OH)2 is formed in the Al2O3
o
below 350 C. Earlier transient studies also showed that in that
temperature region the water effect has not reached equilibrium.[1]
For the Pd/Al2O3 system with 40% Pd dispersion, as Figure
5b demonstrates, Pd(OH)2 is the only observed Pd(0) oxidation
o
product up to 350 C, when PdO also appears. This marks the
onset of catalytic activity (Figure 1), that occurs at higher
temperature as compared to Pd/SnO2 system where PdO exists
already at 100 oC.
Indeed, PdO is accepted as a more active phase than metallic
Pd in methane combustion.[8,34,41,42] One should keep in mind,
however, that the Pd (re)-oxidation is kinetically limited, and both
the
kinetics
and
thermodynamics
are
particle-size
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dependent.[41,43,44] A recent operando XAS study of Pd/Al2O3
catalysts with monodispersed Pd particles (3-16 nm range) in wet
methane combustion[41] confirmed that at least partially oxidized
surface is required for methane combustion. Our study lacks the
size uniformity of the Pd particles, but the starting Pd colloidal
solution contains 2-nm Pd particles, which sinter to larger particles
with 40% dispersion on Al2O3 and 16% on SnO2 with a lower
surface area. It cannot be excluded that the higher achieved
degree of Pd hydroxylation on Al2O3 (up to 70%) is due to the
smaller Pd particle size. However, at temperatures below 300 oC
(Figure 5), both Pd/Al2O3 and Pd/SnO2 catalysts are oxidized to
the same extent of approximately 25%, which is insignificantly
below the Pd dispersion on Al2O3 or above the dispersion on SnO2.
Dramatic changes in the Pd speciation happen around the Pd
dispersion values (within 10-15%, which may account for the
particle polydispersity). For such surface/near-sub-surface
oxidation, the two catalysts are distinctly different: Pd(OH)2
dominates the surface of Pd/Al2O3 catalyst, before even the
appearance of PdO, while PdO is prevalent on the Pd/SnO2
surface before the appearance of Pd(OH)2. Thus, the support
dramatically alters the observed speciation of oxidized Pd species
and affects the catalytic behavior.
Figure 6. Sn L3 edge XAS data for (a) Pd/SnO2 and (b) LCF fit of data
using SnO and SnO2 standards.
Oxygen and water effects on the Pd/SnO2 and Pd/Al2O3
catalysts
Figure 5 shows an expected trend of Pd(0) decrease with the
temperature increase, and the metal oxidation rate is suppressed
by the presence of water.[7,9,10] However, the SnO2/SnO support
promotes the formation of PdO, while mostly Pd(OH)2 is observed
on the Al2O3 support. To evaluate the metal oxide interaction with
water, we calculated standard Gibbs free energies of endothermic
dehydroxylation reactions R(1)-R(3) at 300 oC. Constant heats of
reaction and validity of short-cut van’t Hoff’s equation were
assumed; the thermodynamic properties at 298.15 K for Al(OH)3
are from [45], for Sn(OH)2 are from [46], for Pd(OH)2 are from [47,48]
.
R1). 2/3 Al(OH)3 ⇌ 1/3 Al2O3 + H2O DGo573K = - 36 kJ/molH2O
R2). Pd(OH)2 ⇌ PdO + H2O
R3). Sn(OH)2 ⇌ SnO + H2O
DGo573K = - 25 kJ/molH2O
DGo573K = - 19 kJ/molH2O
Figure 5. LCF of Pd K edge XAS data for Pd/SnO2 and (b) Pd/Al2O3 under
in situ wet methane oxidation conditions. Pd dispersion is 16% for Pd/SnO2
and 40% for Pd/Al2O3.
The dehydroxylation trend indicates that the hydroxylated
inactive Pd(OH)2 may coexist with dehydroxylated Al2O3, while in
the Pd/SnO2 system SnO can potentially serve as a sink for water
released from Pd(OH)2, followed by Sn(OH)2 dehydroxylation,
which potentially can explain the speciation observed in Figure 5.
To further clarify the oxygen and water effect on methane
conversion for the two catalysts, we performed a series of
experiments with various substoichiometric O2/CH4 ratios (0 – 1.9
mol/mol) and monitored evolved species by an on-line mass
spectrometer calibrated for CH4, O2, H2O, CO2, CO and H2. As
seen from Figures 7a and 7c, upon the O2 exhaustion in the feed,
the Pd/Al2O3 catalyst was able to continue methane conversion in
the presence of CO2 and H2O. The nearly constant CO2
concentration and increasing CO and H2 concentration at the
expense of H2O and CH4 are characteristics of methane steam
reforming, as expected for Pd/Al2O3 catalyst.[49] The Pd/SnO2
catalyst shows a strikingly unusual behavior (Figures 7b and 7d).
Upon complete oxygen consumption, no methane conversion
was observed for any of the tested O2/CH4 ratios. The mixture of
CH4, H2O, CO2 and minor CO remained unreactive at
temperatures between 400 and 550 oC, where normally Pd is
expected to catalyze the methane steam reforming. Although Pd
poisoning by thin tin oxide shell may occur,[24,50,51] this could not
explain the observed behavior because every presented ignition
In situ XAS: Sn L3 edge during wet methane combustion
The Sn L3 edge is presented in Figure 6. Due to the larger
particle size of the SnO2 support (44 µm) the fitting of this species
is significantly easier, however, is less sensitive. This is because
the surface, the most active and complex part of a fit, is not a
substantial percentage of the SnO2 particles and therefore, the
technique is not significantly sensitive to the surface speciation.
Commercial SnO and SnO2 were used as standards and the
o
overlay indicates SnO2 to SnO conversion between 100 C and
250 oC above which the system is relatively stable. The in situ
analysis of Sn L-edge (Figure 6) demonstrates approximately
o
10% reduction of SnO2 to SnO at 100 C, as expected from the
catalyst pretreatment, which increased to 30% at 250 oC and
o
stabilized up to 400 C. The reduction could occur by electrons
liberated during SnO2 hydroxylation as H2O + Snlattice+ Olattice ó
[27]
(HO-Snlattice) + (OlatticeH)+ + e-.
Note that this reduction occurs
in situ at excess oxygen in the feed.
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curve was measured five times in each set, and two sets of
experiments were performed in variable methane concentration
order; all tests systematically show less than 3% conversion
deviation. The blocking film cannot appear above 400 oC and
disappear at lower temperatures every time at multiple cycling
while producing highly repeatable kinetic curves. The only
possible scenario for the observed behavior is that the Pd/SnO2
catalyst is not able to activate H2O for methane steam reforming
(and we know that CH4 activation on Pd is possible because of
the successful combustion while O2 supplies lasted, Figure 7c).
This property would explain the catalyst tolerance to water
for the wet methane combustion (Figures 1 and 5b). Apparently,
SnO2 indeed extracts hydroxyls from PdO, which Al2O3 was not
able to achieve, which was seen by the in situ XAS (Figure 5). In
Pd/Al2O3 catalyst after the feed oxygen exhaustion Pd(OH)2 and
activated CH4*-Pd can coexist on Pd surface and lead to methane
steam reforming, while in Pd/SnO2 system the hydroxyls were
formed on the tin surface instead of Pd and could not participate
in the reforming reaction.
There is also a hypothesis in the literature that tin oxide may
provide its lattice oxygen to Pd.[14] However, Figure 7d shows that
the minor amount of CO (formed while molecular O2 supplies
lasted) was not converted to CO2 when no O2 was present in the
feed. To further clarify the possible oxygen donation from SnO2 to
Pd, we performed temperature-programmed surface reactions
(TPSRs) with methane at anaerobic conditions (Figure 8).
Figure 7. Ignition curves (a,b) for CH4 combustion at constant O2 and
variable CH (5000 – 9000 ppm) concentrations, and (c, d) product
4
distribution for O2/CH4 ratio of 1.0 mol/mol on Pd/Al2O3 and Pd/SnO2
catalysts. 10 °C/min heating rate. See Table 1 for other reaction conditions.
Figure 8. TPSR with CH4 at anaerobic conditions at 5 o/min heating rate; 0.5 g of the calcined samples. Solid lines are experimental data measured by
calibrated mass spectrometer; dash lines are theoretical data calculated using CH4 consumption (for complete oxidation of CH4).
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The TPSRs in Figure 8 showed the same product distributions
as was found in low-O2 reactions (Figure 7): Pd/Al2O3 was able to
convert methane to CO, H2 and H2O while Pd/SnO2 did not
produce any of CO and H2. Water evolution on Pd/Al2O3 was
concomitant with the onset of methane conversion, and some
water was used for steam reforming. The Pd/SnO2 catalyst also
converted some CH4 but only to CO2. As seen by comparing the
theoretical and experimental water release, H2O release was
significantly delayed on the Pd/SnO2 catalyst, but when it started,
all water was released from the catalyst, without converting to CO
and H2O. This again confirms that Pd/SnO2 catalyst can retain
water on SnO2 and prevent its activation on Pd.
- A variety of temperature-programmed reactions in low O2
environment showed the striking feature of Pd/SnO2 system of its
inability to catalyze methane steam reforming, as opposed to
Pd/Al2O3. The simplified bulk thermodynamic analysis of Pd, Sn
and Al hydroxides dehydroxylation suggests that tin oxide can
serve as a sink for hydroxyls extracted from Pd(OH)2, while
Al(OH)3 can provide hydroxyls to Pd.
- The study suggests that supports with more favourable
dehydroxylation ability than Pd(OH)2 are able to extract poisonous
hydroxyls from Pd/PdO particles and increase their activity in the
wet methane combustion but decrease the methane steam
reforming function.
Based on the mass balance of exiting CO, H2O and CO2, we
were able to calculate the amount of consumed catalyst-
originated oxygen. For both catalysts it matched the Pd molar
amount indicating that it was the oxygen stored in the catalyst as
PdO. Neither Al2O3, nor SnO2 donated any appreciative amount
of lattice oxygen under the conditions studied. Temperature-
programmed desorption of O2 under He flow performed up to 1100
oC (Figure 9) demonstrated peaks only for the PdO/Pd transition.
Only minor peaks were observed for SnO2: the low temperature
oxygen desorption peak centered at 610 °C can be ascribed to
surface adsorbed O- or O2- species; while the high temperature
peaks spanning from 700 – 1100 °C are the dissociation of lattice
oxygen on SnO2 surface.[52,53] The total area under the peaks for
SnO2 corresponds only to 0.04% of oxygen present in SnO2. This
finding is in line with metal-oxygen dissociation energy, which is
512 kJ/mol for Al-O (at 298K), 548 kJ/mol for Sn-O and 234 kJ/mol
for Pd-O.[54] It is thus unlikely that SnO2 donates its lattice oxygen
to Pd, which explains that the amount of oxygen consumed for
CH4 conversion under anaerobic conditions matches the PdO
content in the Pd/SnO2 catalyst.
- SnO2-supported Pd catalysts are recommended for a follow-
up investigation in a simulated multicomponent exhaust from
natural-gas fuelled engines to evaluate their performance for
potential commercialization. Melting points of SnO2 and SnO (1,
630oC and 1,080oC, respectively) restrict their possible use to
fuel-lean operations.
Experimental Section
Catalyst synthesis and pretreatment
The Pd/Al2O3 and Pd/SnO2 catalysts were synthesized from preformed Pd
nanoparticles deposited on commercial g-Al2O3 (particle size 150 mesh,
average pore size 58 Å, surface area 155 m2/g, Sigma Aldrich) and tin(IV)
dioxide (SnO2, particle size 325 mesh, 99.9%, Sigma Aldrich). The
nanoparticles were synthesized from palladium(II) acetate (98%, Sigma
Aldrich) via polyol method and stabilized by polyvinylpyrrolidone (PVP,
MW 40 000, Sigma Aldrich), as described earlier.[13] The obtained powders
were calcined in static air at 550 oC for 16 h.
In situ XAS
An in situ XAS cell was used at the Canadian Light Source (CLS). The cell
allows measurements of materials through transmission and/or
fluorescence measurements at different temperatures and in different
gaseous environments using both hard and soft X-Rays. The cell
description can be found in the Supporting Information (Figure S2). At the
beginning of the XAS experiment the samples were reduced by heating to
300 oC under a flow of 5% H2 in N2 for 10 minutes. The system was then
cooled to 100 oC at which point the gaseous environment was changed to
methane/air mixtures for the in situ experiment. 0.1% CH4/N2 and dry air
were used at flow rates of 154 and 103 mL/min respectively, the dry air
flowed through two bubblers to saturate it before mixing with the methane
mix.[34] Temperature intervals generally started at 100 oC and were
increased by 50 oC until 450 oC. The ramping rate was 10 oC/min, and
once at the respective temperature the system was allowed to equilibrate
for 10 minutes before measurements.
Figure 9. Oxygen desorption in He flow at 5 o/min heating rate. 0.5 g of
the calcined samples.
Conclusions
The Pd L edge and K edge spectra were collected at the Canadian Light
Source on the Soft X-ray Microcharacterization Beamline (SXRMB)
beamline and the Hard X-ray Microanalysis (HXMA) beamline,
respectively, while Sn L edge data was collected on the SXRMB beamline.
In the case of L edge data, the system was flushed with He immediately
before the measurements were taken to reduce scattering on the incident
and fluorescing photons (and the in situ conditions were resumed
immediately after data collection). After normalization and energy
calibration of the data, the data were analyzed by linear combination fitting
(LCF) using IFEFFIT in the Demeter software package (Athena).[55] Three
species were used as references for fitting: reduced Pd on metal oxide,
fully oxidized PdO on metal oxide, and Pearlman’s Catalyst Pd(OH)2/C
- In situ temperature-programmed XAS of reduced Pd/Al2O3
catalyst during methane combustion in the wet feed with excess
oxygen revealed that the major Pd(0) oxidation product was
Pd(OH)2 with minor amounts of PdO appearing at 350 C. The
dehydroxylation occurred above 450 oC.
- The reduced Pd/SnO2 catalyst oxidized mainly to PdO, with
significantly lower contributions from Pd(OH)2 as compared to
Pd/Al2O3. This explains the known higher water tolerance of the
tin system in methane combustion.
o
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10.1002/cctc.201901744
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(Sigma, 20 wt.%), metal oxide referring to SnO2 or Al2O3 depending on the
species being fit. The fit boundaries were set from -15 to +40 eV in the X-
Ray Absorption Near Edge Structure (XANES) region of the XAS spectra.
mL/min) between 120 and 550 oC at a 5 oC/min heating rate. The amount
of donated oxygen was calculated from mass balance of downstream CO,
½ CO2 and H2O.
For the in situ XAS studies the samples were 0.3 wt.% Pd/Al2O3 with 40%
Pd dispersion and 0.2 wt.% Pd/SnO2 with 16% Pd dispersion. The metal
loadings were determined by neutron activation analysis and CO
Temperature-programmed reduction (TPR) and oxygen
desorption
The TPR was performed with the same Autochem unit; hydrogen
consumption was monitored by the calibrated mass spectrometer. 0.5 g of
the calcined catalysts were pretreated in situ by reduction and oxidation at
550 oC. The TPR analyses were performed in 5% H2/He, 20 mL/min, up
to 550 oC with a ramping rate of 10 °C/min.
chemisorption (1:1 CO:Pd stoichiometry) as described earlier [13]
.
X-ray photoelectron spectroscopy (XPS)
XPS was performed using a Kratos AXIS Supra system. A 500 mm
Rowland circle monochromated Al K-α (1486.6 eV) source was utilized in
tandem with a hemi-spherical analyzer and spherical mirror analyzer. A
300x700 micron spot size was used. High resolution scans utilized 0.05
eV step sizes with a pass energy of 20 eV. Accelerating voltage was 15
keV with an emission current of 15 mA. XPS calibration was performed on
adventitious carbon 1s peaks aligning carbon-carbon bonding to 284.8 eV.
The temperature-programmed oxygen desorption (TPD) was also done
with 0.5 g catalysts with the same setup. The samples were calcined in
10% O2/He at 1100 °C for 10 min, cooled down to 120 oC and flushed with
He to remove physisorbed O2. The TPD analyses were carried out up with
a heating rate of 5 °C/min in He (50 mL/min), the concentration of evolved
O2 was quantified by the mass spectrometer.
Methane combustion at substoichiometric O2/CH4 ratios
All gas mixtures in this work were ordered from Praxair. Low-oxygen
methane combustion tests were carried out in a AutoChem 2950 HP
(Micromeritics) with an on-line mass spectrometer for product analysis
(Pfeiffer Vacuum Thermostar GSD 320). The tested samples contained
0.4 wt% Pd each (confirmed by neutron activation analysis at Bureau
Veritas, Canada). The metal dispersions measured by CO chemisorption
on the same instruments before and after the catalytic tests were 9%
(before) and 5% (after) for Pd/g-Al2O3 and 8% (before) and 5% (after) for
Pd/SnO2. Prior to the dispersion measurements, the catalysts were
reduced at 150 oC in H2/He, followed by CO pulse adsorption and
calcination in O2/He at 550 °C for 1 h.
Acknowledgements
We thank Dr. Danielle Covelli at the University of Saskatchewan’s
Saskatchewan Structural Sciences Centre (SSSC) for assistance
with XPS acquisition. We thank Dr. Hanieh Nassiri (Univeristy of
Alberta) for the data in Figure 1, and Dr. Ning Chen and Weifeng
Chen on the HXMA beamline for assistance with high energy XAS
measurements. The XAS work was performed at the Canadian
Light Source, which is supported by the Canadian Foundation for
Innovation, Natural Sciences and Engineering Research Council
of Canada, the University of Saskatchewan, the Government of
Saskatchewan, Western Economic Diversification Canada, the
National Research Council Canada, and the Canadian Institutes
of Health Research. Financial support from Natural Sciences and
Engineering Research Council of Canada (NSERC Strategic
grant STPGP 478979-15) is appreciated.
The catalytic tests were performed by temperature increase from 200 to
550 °C on the oxidized samples, 0.5 g each, 50 °C each step, 10 °C/min
ramping rate, followed by cool down. Table 1 shows the test order and
reaction mixture compositions. The mass spectrometer was calibrated for
oxygen (32 m/z), methane (15 m/z), water (18 m/z), carbon monoxide (28
m/z) and carbon dioxide (44 m/z), hydrogen (2 m/z) and helium as an
internal standard (4 m/z) prior to each test. Mass balance for He-exclusive
components was above 92% in all tests. Every test was performed five
times in each set, and two sets of tests were performed in variable
methane concentration order. All tests systematically show less than 3%
conversion deviation.
Keywords: heterogeneous catalysis • methane combustion • tin
oxide • palladium • X-ray absorption spectroscopy
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Entry for the Table of Contents
FULL PAPER
William Barrett, Jing Shen, Yongfeng
Hu, Robert E. Hayes, Robert W.J.
Scott*, Natalia Semagina*
Page No. – Page No.
Understanding the role of SnO2
support in water-tolerant methane
combustion: In situ observation of
Pd(OH)2 and comparison with
Pd/Al2O3
Hydroxyls away! Tin oxide extracts hydroxyls from Pd and liberates it for methane
combustion but renders Pd inactive for methane steam reforming, as opposed to
Pd/Al2O3. Quantitative analysis of metallic Pd, PdO, and Pd(OH)2 is presented for
Pd/Al2O3 and Pd/SnO2 catalysts as a function of temperature during methane-lean
combustion in the presence of water, as found by in situ X-ray absorption
spectroscopy.
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