Preferential oxidation of CO in rich H2 over CuO/CeO2: Operando-DRIFTS analysis of deactivating effect of CO2 and H2O

Article abstract of DOI:10.1016/j.jcat.2009.02.012

A catalyst of copper oxide supported on nanostructured ceria has been examined with the aim of exploring deactivating effects produced by CO₂ or H₂O presence on its activity for preferential oxidation of CO in a H₂-rich st

Full text of DOI:10.1016/j.jcat.2009.02.012

Journal of Catalysis 263 (2009) 189–195
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Journal of Catalysis
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Preferential oxidation of CO in rich H2 over CuO/CeO2: Operando-DRIFTS analysis
of deactivating effect of CO2 and H2O
∗
Daniel Gamarra, Arturo Martínez-Arias
Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, Campus Cantoblanco, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A catalyst of copper oxide supported on nanostructured ceria has been examined with the aim of
exploring deactivating effects produced by CO2 or H2O presence on its activity for preferential oxidation
of CO in a H2-rich stream. For this purpose, the catalyst is explored by means of operando-DRIFTS
experiments. The results allow determining most relevant deactivating effects induced by CO2 and H2O.
These are mainly related to modifications of interfacial sites upon formation of specific carbonates
and a blocking effect induced by the presence of adsorbed molecular water, respectively, which limit
redox/catalytic activity of the interfacial zone of the catalyst active for CO oxidation. Such modifications
are directly evidenced by the difficulties of ceria to promote the generation of partially reduced states at
interfacial sites of the dispersed copper oxide particles or to propagate the reduction over such particles,
which affects to the hydrogen oxidation activity of the catalyst, leading on the whole to a general
decrease of the CO-PROX performance of the system.
Received 14 November 2008
Revised 16 January 2009
Accepted 12 February 2009
Available online 24 February 2009
Keywords:
CuO–CeO2 catalysts
CO-PROX
Operando-DRIFTS
CO2
H2O
Deactivation
© 2009 Elsevier Inc. All rights reserved.
1. Introduction
associated degree of interaction with ceria [11,17,18,28]. Reaction
models for CO oxidation (competing or not with H2 oxidation, as it
occurs under CO-PROX conditions) have been proposed on the ba-
sis of indirect analysis of redox or catalytic properties for this type
of catalysts [29,30], while direct evidence on redox changes tak-
ing place in the catalyst under the reactant atmosphere, providing
details on the nature of active sites for the processes taking place
under CO-PROX conditions in this type of catalysts, has been re-
cently reported on the basis of operando spectroscopic studies [11,
31,32]. This has allowed establishing redox–catalytic correlations
in oxidised copper–ceria catalysts, which are basically shown to
involve ceria promoting effects on the partial reduction of inter-
facial dispersed CuO entities, apparently constituting active sites
for CO oxidation; in turn, a separate process of extended reduction
of such entities can apparently provide sites active for H2 oxida-
tion [32].
Nevertheless, such analyses have been typically performed upon
employing simple CO-PROX reactant mixtures without including
gases like CO2 and/or H2O also present in the reforming-WGS
stream [3]. In this respect, catalytic tests have demonstrated deac-
tivating effects of both molecules on the CO-PROX performance of
CuO–CeO2 catalysts [3,12,28,33,34]. On the basis of such catalytic
results, Mariño et al. proposed that gradual CO oxidation activity
and CO2 selectivity decrease with increasing CO2 content in the
stream up to 15% could be related to competitive adsorption of
CO2 on the active copper sites and/or inhibition of oxygen mobil-
ity upon formation of carbonates on the ceria support [12]. In turn,
Avgouropoulos et al. observed that the deactivating effect induced
Production of H2 for polymer fuel cells (PEMFC) is usually
accomplished by a multi-step process that includes catalytic re-
forming of hydrocarbons or oxygenated hydrocarbons followed by
water–gas shift (WGS) [1,2]. The gas stream obtained after these
processes presents in most cases a relatively high CO concentration
that disallows efficient handling of the fuel by the Pt alloy anode
usually employed in the PEMFC. Preferential (or selective) oxida-
tion of CO in the H2-rich stream resulting from such processes
(CO-PROX) has been recognised as one of the most straightforward
and cost-effective methods to achieve acceptable CO concentra-
tions (below ca. 100 ppm), particularly when dealing with small
size stationary units or on-board H2 production [3–8].
Catalysts based on closely interacting copper oxide and ceria
have exhibited promising properties in terms of activity and selec-
tivity, while their lower cost with respect to formulations based on
noble metals could make them strongly competitive [3,4,6,7,9–16].
The particular ability of this class of catalysts for the CO-PROX or
related processes has been essentially attributed to the synergistic
redox properties produced upon formation of copper oxide–ceria
interfacial sites [4,6,10,11,17–27]. In this sense, generally speaking,
the properties of copper oxide entities for CO oxidation promo-
tion apparently depend strongly on their dispersion degree and/or
Corresponding author.
E-mail address: amartinez@icp.csic.es (A. Martínez-Arias).
*
0021-9517/$ – see front matter © 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.02.012

190
D. Gamarra, A. Martínez-Arias / Journal of Catalysis 263 (2009) 189–195
Fig. 1. DRIFTS spectra of CuO/CeO2 recorded under the simple CO + H2 + O2 CO-PROX mixture. As indicated in the left part, the bottom spectrum corresponds to the initial
one prior to introduction of the reactant mixture; the rest corresponds to spectra recorded every 20 K from 303 to 463 K, from bottom to top, except for the top one which
was recorded at 498 K. The top left inset expands the zone corresponding mainly to hydroxyl species.
by 15% CO2 apparently became more intense with the increase of
copper content in CuO–CeO2 catalysts prepared by hydrothermal
method while additional 10% water present in the feed suggested
water as a stronger CO oxidation inhibitor than CO2 [28,34]. Park
et al. proposed in their analysis of CuO–CeO2/Al2O3 CO-PROX cat-
alysts, on the basis of catalytic activity/stability tests and CO2/H2O
TPD studies, that the main cause for the decrease in catalytic ac-
tivity with CO2 and H2O in the feed is related to competitive CO
and CO2 adsorption as well as blockage of active sites by water
at low reaction temperature [33]. Nevertheless, direct under work-
ing conditions examination of such deactivating effects has not, to
the best of our knowledge, been reported. The present work aims
to get details on such deactivating effects by using DRIFTS spec-
troscopy under reaction conditions with on line MS gas analysis.
298 K under the same atmosphere followed by extensive purging
under inert gas at room temperature before introducing the reac-
tion mixture and heating in a stepped way, recording one spectrum
−1
(average of 25 scans at 4 cm
resolution) typically every 20 K af-
ter the signal of the different monitored gases (analysed on line
by means of a quadrupole mass spectrometer Pfeiffer Omnistar)
becomes constant at every reaction temperature (i.e. steady condi-
tions). Gas mixtures (a simple reference one with 1% CO + 1.25%
O2 + 50% H2 in He and two other having additionally either 15%
CO2 or 10% H2O) were prepared using mass flow controllers and a
−1
perfusion syringe with ca. 100 cm3 min
passing through the cat-
alyst bed at atmospheric pressure. Analysis of the spectra has been
carried out by using OriginPro 7 SR4 software.
3. Results and discussion
2. Experimental
DRIFTS experiments recorded under simple CO + H2 + O2
CO-PROX mixture are displayed in Fig. 1. Changes produced upon
contact with the reactant mixture are observed in three distinct
spectral zones. The first zone at higher wavenumber displays
bands corresponding mainly to hydroxyl species. These include,
on the basis of previous analyses of ceria-related materials, iso-
A sample of copper oxide supported on CeO2 (Cu wt% of 1.0)
was prepared by incipient wetness impregnation of a nanostruc-
tured CeO2 support prepared by microemulsion (in a similar man-
ner as described above) with copper nitrate aqueous solutions.
Following impregnation, the sample was dried overnight at 373 K
and finally calcined under air at 773 K for 2 h to yield a sample
with SBET = 107 m2 g−1. Characterisation details for both the sup-
port employed and the final catalyst are available elsewhere [21,
31,35].
lated hydroxyls giving rise to relatively narrow bands at ca. 3712–
−1
3690 cm
(mono-coordinated to surface cerium cations), as well
−1
as a series of bands or shoulders in the 3670–3629 cm
range
(attributable to bi- or tri-coordinated species bonded on more or
DRIFTS spectra were carried out using a Bruker Equinox 55 FTIR
spectrometer fitted with an MCT detector. The DRIFTS cell (Harrick)
was fitted with CaF2 windows and a heating cartridge that allowed
samples to be heated to 773 K. Aliquots of ca. 100 mg were cal-
cined in situ under diluted oxygen at 773 K and then cooled to
less unsaturated cerium cations); additionally, associated species
−1
giving a broad band extending from ca. 3800 to 3000 cm
are
detected [36,37].
−1
A second spectral zone below 1700 cm
exhibits bands due to
carbonate or related species, basically chemisorbed on ceria [37–

D. Gamarra, A. Martínez-Arias / Journal of Catalysis 263 (2009) 189–195
191
Fig. 2. MS evolution of the indicated gases during the run performed under the
simple CO-PROX CO–H2–O2 mixture performed with the DRIFTS cell (corresponding
to spectra presented in Fig. 1). Adequate corrections for CO2 contributions to m/e =
28 signal have been made in order to quantify CO, according to previous calibration
measurements.
40]. Most intense peaks in this zone are ascribed to bidentate
carbonates at ca. 1583 and 1297; a combination band at ca. 2880
−1
cm
(see the highest wavenumber zone at the left of Fig. 1), par-
ticularly apparent in lower temperature spectra, is also attributed
−1
to these species. A band at ca. 1478 cm
may correspond to
the antisymmetric stretching of the terminal CO bonds in poly-
or mono-dentate carbonates (the corresponding symmetric mode
would appear as an overlapped band at ca. 1350 cm−1) [37]; never-
−1
theless, it cannot be discarded that the band at ca. 1478 cm
may
belong to a particular type of carbonite (CO22−) species [41]. The
band at 1216 cm−1, along with that at 1399 cm
and a shoulder
−1
at ca. 1600 cm−1, are attributed to hydrogen carbonate species [37,
40]; this is confirmed by the presence of a sharp OH stretching vi-
−1
bration at ca. 3618 cm
also belonging to these species which
Fig. 3. MS evolution of CO (top) and H2O (bottom) during CO-PROX runs performed
with the DRIFTS cell under simple (CO + H2 + O2) reactant mixture or mixtures
additionally containing either CO2 or H2O (note, in this latter experiment, H2O evo-
lution could not be determined neither directly nor indirectly, due to the relatively
high H2O and H2 concentrations employed).
must be formed upon interaction of CO (or CO2) with monoden-
tate hydroxyls (giving rise to a band at ca. 3712–3690 cm
−1
in
the spectrum of the original sample which disappears upon con-
tact with the reactant mixture) [37,40]. In addition, the formation
of formate species at temperatures above ca. 463 K is evidenced by
bands at 2940 and 2840 cm−1, related to C–H stretching mode or
a combination band, while other intense features of these species
temperature, most likely related to such low temperature reduc-
tion process [43].
Fig. 2 displays, as an example, the evolution of all the gases
that have been monitored on line by mass spectrometry (MS), in
this case corresponding to the DRIFTS experiments displayed in
Fig. 1. In turn, Fig. 3 compares the effects of introducing either
CO2 or H2O in the reactant mixture during experiments performed
with the DRIFTS cell and for gases representative of CO and H2 ox-
idation reactions. The results reveal differences in the respective
deactivating effects as a function of reaction temperature. While
CO2 apparently induces a strong CO oxidation activity decrease
during the whole temperature range, the deactivating effect of H2O
is particularly strong at temperatures below ca. 380 K, at which CO
oxidation is strongly hindered, being relatively weaker above that
temperature.
Explanations for these behaviours can be achieved upon com-
parative analysis of the DRIFTS spectra as a function of the specific
CO-PROX reactant mixture employed. Fig. 4 displays the results ob-
tained in the presence of CO2 in the mixture. A first spectrum was
taken under CO2–H2–O2 mixture (i.e. prior to introducing CO in
the feed). In this condition, the generation of various carbonate-
type species was observed, mainly hydrogen carbonate (bands or
correspond to symmetric and asymmetric νOCO modes which must
−1
yield bands at ca. 1330–1370 and 1550 cm
[39], apparently over-
lapped on bands due to other carbonate-type species.
The other spectral zone (at intermediate frequencies) shows the
formation of CO2(g), evolving in accordance with CO oxidation ac-
+
tivity (vide infra), and a carbonyl species (a Cu -carbonyl giving
rise to a band at ca. 2115 cm−1, which apparently shifts gradually
−1
to the red down to ca. 2107 cm
with increasing temperature
up to 383 K, practically disappearing above that temperature, most
likely due to thermal stability effects [19]; note the assignment is
in accordance with previous reports, in which details can be found
+
to justify it [19,21,42]). The presence of these Cu -CO species al-
ready upon initial contact with the reactant mixture at 303 K is
consistent with the easy (ceria-promoted) reduction of copper in
the catalyst, considering that a fully oxidised state of copper is
present in the initial calcined catalysts [31]. In this sense, it must
be noted that room temperature reduction of copper in this cat-
alyst upon interaction with CO has been previously demonstrated
by EPR and XPS [19,21,31], while recent CO-TPR results on cata-
lysts of this type evidence an important CO consumption at room

192
D. Gamarra, A. Martínez-Arias / Journal of Catalysis 263 (2009) 189–195
Fig. 4. DRIFTS spectra of CuO/CeO2. The bottom one corresponds to the initial spectrum recorded prior to introduction of the gas mixture. The rest were recorded at the
temperatures indicated in the left part (the same for those in the right part from bottom to top). Following calcination and recording of the initial spectrum, the one just
above it was recorded under 1.25% O2 + 50% H2 + 15% CO2 (i.e. with no CO in the mixture). The rest were recorded under CO–H2–O2–CO2 CO-PROX mixture.
−1
upon introduction of CO in the mixture,
shoulders at 1214, 1399–1410 and ca. 1600 cm−1) and bidentate
carbonate (bands at ca. 1583 and 1297 cm−1). Formation of this
type of species in the absence of CO suggests that their formation
is basically related to CO2 interaction with the ceria surface of the
catalyst; note in this sense that analogous species detected upon
contact with the simple (without CO2) CO-PROX mixture at 303 K
(Fig. 1) could be related to CO2 formed by CO oxidation (note
a small but measurable activity is produced at that temperature)
which could justify its formation from the very beginning in such
case. Some small modification of such carbonate-type species is
produced when CO is introduced in the mixture, according to the
of the band at 1478 cm
it is observed the appearance of the Cu carbonyl at 2120 cm
+
−1
,
somewhat shifted to the blue with respect to the homologous
spectra recorded in the absence of CO2 in the reactant stream
(Fig. 1). This suggests a lower influence of the copper–ceria interac-
+
tion on the red shift of the Cu carbonyl frequency in this case [19,
21,42], taking into account comparison with corresponding stable
species detected on pure Cu2O [44]. This could be caused by the
presence of the carbonate and/or carbonite species giving rise to
−1
+
the band at 1478 cm
in the environment of the Cu adsorp-
tion sites, strongly predominant, as a difference with experiments
performed under simple CO–H2–O2 mixture, in the presence of
CO2 in the feed, as will be examined in more detail in a sepa-
rate contribution in the context of analysis of the CO oxidation
reaction over catalysts of this type [45]. Besides this, other differ-
ences between spectra in the absence or presence of CO2 in the
reactant stream (Figs. 1 and 4) are related to the relatively higher
apparent broadening detected in the most intense bands appearing
−1
at ca. 1600 cm
(this could also tentatively explain an increase of
a shoulder at ca. 1245 cm−1, that could be related to some modifi-
cation in the chemical environment of such species). Nevertheless,
the main change induced by introduction of CO in the mixture in
the carbonate region of the spectrum is related to the increase of
a band at 1478 cm−1, which, as mentioned above, could be related
to carbonate (poly- or mono-dentate species, according to discus-
sion elsewhere [37]; in such case, the corresponding symmetric
stretching mode would correspond to the band at 1354 cm−1, most
apparent at higher reaction temperature) or carbonite species. In
case such species corresponds to carbonate entities, it would most
likely be formed upon CO2 adsorption [37], and on ceria sites at
the CuO–CeO2 interface as a consequence of a CuO-promoted re-
ductive process produced upon interaction with CO; evidences for
occurrence of such ceria reduction at this relatively low temper-
ature can be found elsewhere [21]. In contrast, carbonite species
would most likely be formed upon CO interaction with reduced
ceria [41], most probably at interfacial sites at which low tempera-
ture reduction would be CuO-promoted [21]. Along with formation
intensity of carbonate-related species detected (as a rough estima-
−1
tion, the integrated intensity in the 1700–1000 cm
range is ca.
1.7 times higher in the presence of CO2 when comparing spectra
+
recorded at 303 K) and lower Cu carbonyl intensity in the pres-
ence of CO2 in the stream (Fig. 5). This latter was indeed shown
in former contributions to be a fingerprint of CO oxidation activ-
ity in this type of catalysts [11,31,32], as it is also accomplished
in this case according to comparison between Figs. 3 and 5 (note
+
the Cu carbonyl intensity decrease with increasing reaction tem-
perature must basically reflect thermal stability effects [19]). On
the basis of these results, the deactivating effect induced by CO2
presence in the feed over CO oxidation activity must be mainly
related to the hindering of interfacial redox activity induced by
formation of carbonates and/or carbonites at interfacial ceria sites,

D. Gamarra, A. Martínez-Arias / Journal of Catalysis 263 (2009) 189–195
193
+
Fig. 5. Intensity of the Cu carbonyl as a function of reaction temperature under
the indicated CO-PROX mixtures. Overlapping CO(g) contributions were subtracted
as required for this determination by using a reference run performed over inert
KBr under the same experimental conditions.
Fig. 6. DRIFTS spectra of the CeO2 support recorded under the simple CO + H2
+
O2 CO-PROX mixture at the indicated temperatures.
droxyls at that active region could somewhat limit the interfacial
redox processes and consequently the CO oxidation activity, the
deactivation effect appears lower than that observed in the pres-
ence of CO2 in the mixture (Fig. 3), thus indicating the greater
hindering of interfacial redox activity induced by carbonate-type
species.
The region of the spectra related to carbonate-type species re-
flects also the significant hydroxylation degree present over the
sample, leading to frequency modifications for this type of species
or formation of new type of complexes [38,39], as well as a general
which limits the ceria-promoted reduction leading to generation of
partially reduced CuOx active sites at such interfaces, taking into
account our previous hypothesis on the nature of active sites for
this reaction [32]. Evidence for the interfacial location of the car-
bonate species formed can be based on differences with spectra
recorded under CO-PROX conditions for the copper-free ceria sup-
port, Fig. 6, for which practically no oxidation activity was detected
below ca. 453 K. Thus, while the carbonate-related species formed
for copper-free ceria appear fairly similar to those detected for
CuO/CeO2 (basically bidentate species giving most intense bands
−1
intensity decrease for this type of species (ca. half integral inten-
at 1574 and 1306 cm
and poly- or mono-dentate ones at 1493
−1
and 1387 cm−1; note there is a certain shift in these values with
respect to those observed for CuO/CeO2, Fig. 1), they begin to ap-
pear at comparatively higher temperature in the absence of copper,
thus revealing a promoting effect of copper oxide on its forma-
tion and its possible location at ceria positions at or close to the
oxide–oxide interface (which could also explain the shift in bands
positions).
sity in the 1700–1000 cm
range, comparing spectra at 303 K
in the absence and presence of water in the feed). Thus, bands
−1
at 1554 and 1325 cm
may correspond to bidentate carbon-
ates while above ca. 383 K bands are formed at ca. 1547 and
−1
1372 cm−1, which along with those at 2935 and 2845 cm
can
correspond to formate species [39]; we do not believe this indi-
cates onset of WGS at that temperature since, as mentioned above,
WGS activity is residual below ca. 473 K in this type of catalysts
and furthermore requires copper reduction to a metallic state [47],
likely far from being achieved under the employed conditions ac-
cording to XANES analysis of another catalyst of this type [46]. It
On the other hand, the presence of CO2 in the reactant mix-
ture apparently has also an important detrimental influence on
H2 oxidation, according to Fig. 3. This can be related to a CO2-
induced impediment for propagation of the reduction (also pro-
moted to some extent, smaller than the effect produced over in-
terfacial sites, by ceria) over the dispersed CuO particles, which
could provide most active sites for such reaction [32], as suggested
by recent operando-XANES investigation on another catalyst of this
type [46].
−1
cannot be fully discarded that the band detected at ca. 1345 cm
may correspond to the mentioned formate species too, although
it shows a parallel evolution with a band at 1438 cm−1, suggest-
ing that it may correspond to mono- or poly-dentate carbonates
(most intense terminal CO stretching modes) [37,38], somewhat
shifted with respect to analogous bands detected in the absence of
water in the feed as a consequence of the relatively higher hydrox-
The low CO oxidation activity detected at low temperature in
the presence of water in the feed (Fig. 3) is reflected by the
+
ylation degree of the sample surface. Other smaller contributions
strong hindering of formation of Cu carbonyls (Figs. 5 and 7).
−1
are detected at ca. 1510 and 1305 cm
which could be related
The DRIFTS spectra evidence a strong hydroxylation degree for the
to the formation of carboxylate species [38], while small amounts
of hydrogen-carbonate species (bands or shoulders at ca. 1600 and
1399 cm−1) can also be formed at relatively high temperature. It is
noteworthy that, as it was observed in the presence of CO2 (Fig. 4),
onset of CO oxidation (above ca. 373 K) is accompanied by gen-
surface of the sample under this condition (Fig. 7). This is evi-
−1
denced by the strong intensity of a broad band at ca. 3317 cm
,
−1
which along with weaker ones at ca. 3692 and 1625 cm
can
be attributed to the presence of undissociated water molecules on
the sample surface (the two former correspond to OH stretching
modes and the latter to HOH bending [37]). The significant de-
crease of these species at temperatures above ca. 373 K allows
onset of CO oxidation (Fig. 3). Therefore, it appears that the re-
action onset shift is mainly related to a water-induced blocking
phenomenon which prevents access of the reactant molecules to
the interfacial active sites for consequent reductive activation and
reaction. Although above that temperature, the formation of hy-
−1
eration of the band at 1438 cm
(1478 cm−1without water in
the feed) suggesting, as outlined above, that formation of the cor-
responding carbonate (or carbonite, see above discussion) species
can be associated to the interfacial reduction processes associated
to generation of active sites for the reaction. Further experiments
are in course in order to corroborate and get details on this type
of correlation.

194
D. Gamarra, A. Martínez-Arias / Journal of Catalysis 263 (2009) 189–195
Fig. 7. DRIFTS spectra of CuO/CeO2. The bottom one corresponds to the initial spectrum recorded prior to introduction of the gas mixture. The rest were recorded at the
temperatures indicated in the left part (the same for those in the middle or right parts from bottom to top) under CO–H2–O2–H2O CO-PROX mixture.
4. Conclusions
the active sites, according to proposals for the basic role of the
ceria support as oxygen-handling component in the scientific liter-
ature [6,10,19].
A CO-PROX ceria-supported copper oxide catalyst has been ex-
amined with respect to deactivating effects induced by the pres-
ence of CO2 or H2O in the reactant stream. On the basis of
Acknowledgments
+
operando-DRIFTS examination, a hindering of formation of Cu
-
D.G. thanks the Ministerio de Educación y Ciencia (MEC) for
a FPI PhD grant under which his contribution to this work was
done. Thanks are due to the MEC (project CTQ2006-15600/BQU),
and Comunidad de Madrid (project ENERCAM S-0505/ENE/000304)
for financial support.
carbonyls, fingerprint of CO oxidation activity over this type of sys-
tems [32], is detected in the presence of any of the two molecules
in the feed. This is related to a decrease of CuO–CeO2 interfacial
redox activity as a consequence of the formation of carbonates on
interfacial ceria sites in the presence of CO2 in the reactant stream,
which brings as a consequence a limitation in the capability for re-
dox promotion by ceria of the generation of partially reduced CuOx
interfacial sites most active for CO oxidation; in turn, apparent de-
crease of H2 oxidation activity in the presence of CO2 in the feed
can be related to limitations in the ceria promoting effects towards
propagating the reduction over the dispersed CuO particles, taking
into account previous hypothesis for the nature of active sites for
such reaction [32]. On the other hand, the deactivating effect of
water in the feed appears to be mainly related to a blocking effect
induced by the presence of relatively large amounts of adsorbed
molecular water, limiting access of the reactant molecules to the
sample surface for activated generation of the active sites for the
process and further reaction. It may be noted that experimental
evidences for the deactivating effects of both molecules have basi-
cally been related to the partial hindering of generation of active
partially reduced copper oxide species active for the process [32].
However, given that carbonate or hydroxyl-molecular water species
responsible for such effect can be mainly located on ceria sites,
we cannot discard that inhibiting effects on the oxidation activity
are partially or alternatively related to issues involving hindering
of oxygen activation or transport of activated oxygen species to
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