DOI: 10.1002/cctc.200900302 Support Effects in the Gold-Catalyzed Preferential Oxidation of CO Svetlana Ivanova, [a] VØronique Pitchon, [a] Corinne Petit, [a] and ValØrie Caps* [b] Introduction The selective oxidation of carbon monoxide in the presence of hydrogen, also called preferential oxidation (PROX), is a key step in the hydrogen purification process in promising polymer electrolyte fuel cell (PE-FC) technology, which has been afford- ed increasing scientific interest in the recent years. [1] The cata- lysts have to operate downstream water–gas shift reactors and allow oxidation of the residual carbon monoxide fractions without consumption of the hydrogen fuel. In this way, the CO concentration, which causes poisoning of the electrocatalysts used to convert hydrogen to electricity, can be lowered to ac- ceptable levels and the overall fuel cell efficiency is improved. PROX catalysts need to be effective in the range of operating temperature of the PE-FC (80–120 8C), highly selective, and resistant to deactivation, particularly when water and carbon dioxide are present in the effluent gas. [2] The study of the PROX reaction is strongly related to oxida- tion catalysis, which explains why the first studied catalyst for this reaction was Pt/Al 2 O 3 . [3] Other alumina- or silica-supported metal catalysts have since been studied, such as Pt, Rh, Pd, Ru, Co–Cu, Ni–Co–Fe, Ag, Cr, Fe, Mn, [4] and, more recently, alloy catalysts. [5] However, none of these materials are as active at the temperature of interest for a PE-FC application as support- ed gold nanoparticles. The great oxidation potential of gold systems was revealed by the pilot study of Haruta et al. [6] on the low-temperature oxidation of carbon monoxide. Since then, numerous studies have highlighted the unique perform- ances of gold catalysts in mild oxidation reactions, in particular the PROX reaction, in which a high selectivity towards CO 2 can be achieved due to the higher oxidation rate of CO, as com- pared to H 2 , at low temperature. [7] Although Au/MnO x was ini- tially thought to be the best candidate for the PROX reaction, [8] other mineral oxides, such as TiO 2 , [9] Fe 2 O 3 , [10] CeO 2 , [11] and Al 2 O 3 , [12] have since been used as supports for gold nano- particles in this reaction. To date, the support effect in the gold-catalyzed PROX reac- tion is not clear. Some groups [13] have reported a significant effect of the support on the catalytic performances, with one order of magnitude difference between the most active material, Au/Fe 2 O 3 , and the least active, Au/Al 2 O 3 . By contrast, Rossignol et al. [14] reported similar CO oxidation rates for Au/TiO 2 -, Au/ZrO 2 -, and Au/Al 2 O 3 -catalyzed PROX reactions. Hence, significant improvement of these PROX catalysts can be expected through a deeper understanding of the role of the support in the reaction. However, there is still a general consensus in the scientific community on the importance of the gold particle size [15] and structure, [16] which are functions of the preparation method and the support chemical nature, for oxidation reactions. Hence a rigorous study of the support effect requires the use of catalysts with similar particle size distributions on all supports considered. Such catalysts can be obtained by laser vaporization [17] or colloidal deposition. [18] These methods rely The study of support effects on the gold-catalyzed preferential oxidation of carbon monoxide in the presence of hydrogen (PROX reaction) is possible only with careful control of the gold particle size, which is facilitated by the application of the direct anionic exchange method. Catalytic evaluation of ther- mally stable gold nanoparticles, with an average size of around 3 nm on a variety of supports (alumina, titania, zirconia, or ceria), clearly shows that the influence of the support on the CO oxidation rate is of primary importance under CO+O 2 con- ditions and that this influence becomes secondary in the pres- ence of hydrogen. The impact of the support surface structure on the oxidation rates, catalyst selectivity, and catalyst activa- tion/deactivation is investigated in terms of oxygen vacancies, oxygen mobility, OH groups, and surface area on the oxidation rates, catalyst selectivity and catalyst activation/deactivation. It allows the identification of key morphological and structural features of the support to ensure high activity and selectivity in the gold-catalyzed PROX reaction. [a] Dr. S. Ivanova, # Dr. V. Pitchon, Dr. C. Petit LMSPC, Laboratoire des MatØriaux, Surfaces et ProcØdØs pour la Catalyse UMR 7515 CNRS-ECPM 25 rue Becquerel, F-67087 Strasbourg Cedex 2 (France) [b] Dr. V. Caps + IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon UMR5256 CNRS/University of Lyon 2 avenue Albert Einstein, 69626 Villeurbanne (France) [ + ] Present address: KAUST Catalysis Center (KCC) 4700 King Abdullah University of Science and Technology Thuwal 23955–6900 (Kingdom of Saudi Arabia) E-mail : valerie.caps@kaust.edu.sa [ # ] Present address: Departamento de Química Inorgµnica and Instituto de Ciencias de Materiales de Sevilla CSIC-US, Avda. AmØrico Vespucio 49, 41092, Sevilla (Spain) 556  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2010, 2, 556 – 563