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Article
Role of hydroxyl groups in the preferential oxidation
of CO over copper oxide-cerium oxide catalysts
Arantxa Davó-Quiñonero, Miriam Navlani-García, Dolores
Lozano-Castello, Agustín Bueno-López, and James A. Anderson
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02741 • Publication Date (Web): 01 Feb 2016
Downloaded from http://pubs.acs.org on February 8, 2016
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1
Role of hydroxyl groups in the preferential oxidation
of CO over copper oxide-cerium oxide catalysts
Arantxa Davó-Quiñonero
1
, Miriam Navlani-García
1
, Dolores Lozano-Castelló
1,2
, Agustín
Bueno-López
1,2,*
, James A. Anderson
2
1
MCMA group. Inorganic Chemistry Department; University of Alicante.
2
Surface Chemistry and Catalysis Group, School of Engineering, University of Aberdeen.
KEYWORDS. PROX; copper; ceria, CO oxidation; H
2
purification.
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ABSTRACT
Model CuO/Ce
0.8
X
0.2
O
δ
catalysts (with X = Ce, Zr, La, Pr or Nd) have been prepared in order to
obtain CuO/ceria materials with different chemical features, and have been characterized by
XRD, Raman spectroscopy, N
2
adsorption and H
2
-TPR. CO-PROX experiments have been
performed in a fixed-bed reactor and in an operando DRIFTS cell coupled to a mass
spectrometer. The CO oxidation rate over CuO/Ceria catalysts correlates with the formation of
the Cu
+
-CO carbonyl above a critical temperature (90ºC for the experimental conditions in this
study) because copper-carbonyl formation is the rate limiting step. Above this temperature CO
oxidation capacity depends on the redox properties of the catalyst. However, decomposition of
adsorbed intermediates is the slowest step below this threshold temperature. The hydroxyl
groups on the catalyst surface play a key role in determining the nature of the carbon-based
intermediates formed upon CO chemisorption and oxidation. Hydroxyls favor the formation of
bicarbonates with respect to carbonates, and catalyst forming more bicarbonates produce faster
CO oxidation rates than those which favor carbonates.
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1.- Introduction
CO oxidation by molecular oxygen is probably one of the most studied catalyzed reactions,
and the selective oxidation of CO in H
2
-rich gas mixtures (CO-PROX) has become of special
relevance in recent years in order to provide sufficiently pure H
2
for proton exchange membrane
fuel cells (PEMFC).
1,2
H
2
is obtained by catalytic reforming of hydrocarbons followed by the
water gas shift reaction (WGS),
3
and this process yields a H
2
-rich gas stream with up to 1% of
CO. This high concentration is not acceptable because CO poisons the Pt electrocatalysts, and
CO concentration must be lowered below ca. 100 ppm.
4
The CO-PROX reaction is one of the
most suitable methods for the purification of such H
2
-rich gas streams, and copper oxide-cerium
oxide catalysts are promising materials
5-8
for CO-PROX that could replace expensive Pt or Au
catalysts.
9-13
The CO-PROX reaction is a competitive process where CO and H
2
compete with each other
for molecular oxygen
14
. The catalyst must be selective towards the CO-O
2
reaction with regard
to the undesired oxidation of H
2
, which decreases the net H
2
yield of the complete catalytic
reforming + WGS + CO-PROX series of reactions. A correlation was established between the
CO oxidation rate and the level of interfacial reduction of Cu
2+
to Cu
+
, and such interfacial
reduction can be correlated to the formation of Cu
+
-CO carbonyls.
15,16
On the other hand, the
oxidation of H
2
occurs once copper is significantly reduced, and the active species for the H
2
-O
2
reaction are partially reduced copper oxide nanoparticles
15
where dissociative chemisorption of
H
2
takes place.
17
The selectivity of copper oxide-cerium oxide catalysts can be related to the
preferential adsorption of CO on Cu
+
and hindered H
2
dissociation on oxidized sites.
17
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The evolution of the carbon-containing species on the catalysts surface during the course of the
reaction is complex and is not completely understood, but it has been reported that surface
hydroxyls on the catalyst have a positive effect on CO oxidation for noble metal catalyst.
18
This
aspect has been analyzed in detail in this study for CuO/Ceria, and model CuO/Ce
0.8
X
0.2
O
δ
catalysts (with X = Ce, Zr, La, Pr or Nd) have been prepared in order to obtain CuO/Ceria
materials with different chemical features. The objectives were to understand the role of the
carbon containing intermediates in the CO oxidation reaction, and to determine whether the
hydroxyl groups on the catalyst influence the formation of bicarbonates in preference to
carbonates, and to provide evidence to support a hypothesis that catalysts which preferentially
form bicarbonates are able to oxidize CO faster than those forming carbonates-type species.
2.- Experimental
2.1 Catalysts preparation.
Five metal oxides with composition Ce
0.8
X
0.2
O
δ
(X = Ce, Zr, La, Pr or Nd) were prepared
using the following metal precursors: Ce(NO
3
)
3
·6H
2
O (Alfa-Aesar, 99.5 %), ZrO(NO
3
)
2
·xH
2
O
(Fluka, ~ 27 % Zr), La(NO
3
)
3
·6H
2
O (≥ 99.0%), Pr(NO
3
)
3
·6H
2
O (99.9%) and Nd(NO
3
)
3
·6H
2
O (≥
99.9%).
The amounts of each precursor used were those required to obtain 5 g of metal oxide, and
aqueous solutions of these amounts were prepared with 10 ml of solvent. The dissolutions were
introduced in a muffle furnace previously heated at 200ºC, and after 1 hour, the temperature was
increased at 10ºC/min until 500ºC, holding the samples at this temperature for 2 hours.
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