On the mechanism of selective CO oxidation on nanosized Auγ-Al2O3 catalysts

Abstract

Nanometer-sized gold particles on oxide supports are efficient catalysts for the selective catalytic oxidation (SCO) of carbon monoxide under conditions compatible with the operation of PEM fuel cells. Nanosized Au/␥-Al 2 O 3 catalysts are able to oxidize CO between 20 and 70°C in an atmosphere of hydrogen combining high CO conversion and satisfactory selectivity to CO 2 (1). It is generally agreed that the catalytic activity of gold depends on the size of the gold particles, but the nature of support material, the preparation method, the activation procedure have also been suggested to play a key role (2). Sites at the gold support interface have also been claimed to be responsible for the activity in CO oxidation (3). Strain in the Au particles due to the mismatch of the lattices at the interface with the support (4) and the effect of low-coordinated sites and roughness (4) have also been suggested as important factors for high activity. The most important effect, concerning the catalytic activity of gold nanoparticles for low temperature oxidation of CO is related to the availability of many low-coordinated gold atoms on the small particles (5). Effects related to the interaction with the support may also contribute, but to a considerably smaller extent (5).CO oxidation over group VIII noble metals is the most studied catalytic reaction. However, selective oxidation of CO in hydrogen-rich conditions is not as well studied (6,7). CO (reactant) and CO 2 (product) sorption processes are fundamental elementary steps for SCO and moreover, the effect of hydrogen on their sorption over supported Au catalysts remains a subject of significant interest.Among the various supported Au catalysts, Au/Al 2 O 3 is perhaps one that has shown the widest variation for CO oxidation, ranging from being very inactive to practically as active as Au/TiO 2 (8). Compared to Au/␥-Al 2 O 3 , multicomponent gold-based catalysts also supported on ␥-Al 2 O 3 and combined with common metal oxides, such as: MnO x , MgO, FeO x (9,10) as well as gold catalysts supported on other materials such as Au/TiO 2 (11) and Au/FeO x (11,12) have been found to exhibit even more promising commercial performance for the SCO reaction. However, based on its simplicity, a well-studied Au/␥-Al 2 O 3 catalyst (1,9) has been utilized in the present work as model system, in order to gain fundamental information on the catalytic behaviour of gold nanoparticles and identify the active sites on Au/␥-Al 2 O 3 .In this paper, new findings which offer further information concerning the mechanism of CO preferential oxidation over nanosized Au are presented. The first finding concerns CO 2 formation in the absence of oxygen and hydrogen both in bare ␥-Al 2 O 3 as well as in Au/␥-Al 2 O 3 (called as CO decomposition). At high temperatures (>200 °C) in excess of H 2 , over the Au/␥-Al 2 O 3 catalyst, reversed water gas shift (RWGS) reaction results in CO 2 consumption towards CO and H 2 O formation. Finally, the kinetic measurements of the present work indicate that hydrog...

Gold on oxide-doped alumina supports as catalysts for CO oxidation

“…Similar results were obtained for the other analysed samples (images not shown). Other authors also found gold in the reduced state in doped alumina materials, as reported in the literature (Grisel et al 2000(Grisel et al , 2001(Grisel et al , 2002aGrisel and Nieuwenhuys 2001a, b;Gluhoi et al 2003Gluhoi et al , 2004Gluhoi et al , 2005aGluhoi et al , b, 2006aLin et al 2004;Gavril et al 2006;Gluhoi and Nieuwenhuys 2007a, b;Lippits et al 2007Lippits et al , 2008Lippits et al , 2009). …”

Support Effects in the Gold‐Catalyzed Preferential Oxidation of CO

“…However, reconstruction of the gold particles under reaction conditions can not be fully ruled out 33. However, since the amounts of water liberated and hydroxy groups lost directly depended on the specific surface area of the alumina support, and since this reactivity pattern is specific to high‐surface‐area alumina‐supported gold catalysts12 and was not detected in the activity patterns of gold nanoparticles supported on low‐surface‐area alumina,20b the involvement of OH and H 2 O is likely. Conversely, both the titania‐ and zirconia‐supported catalysts were activated under reaction conditions upon heating to 280 °C, despite the likely formation of poisonous surface carbonate species under these conditions 34.…”

“…42 The presence of water in the CO+O 2 feed is also known to directly promote the CO oxidation activity of an alumina‐supported gold catalyst, a lot more significantly than that of Au/TiO 2 ,32b potentially by hydroxylation of the high‐surface‐area material. Enhancement in the low‐temperature CO oxidation rate over alumina‐supported gold catalysts in PROX has also been attributed to the weakening of CO adsorption on gold with hydrogen in the feed 12. However, the promotion was maximal only after a whole heating step under PROX conditions, which suggests that there is more than just an effect of H 2 on the gas and adsorbed phases.…”

“…Since then, numerous studies have highlighted the unique performances 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 compared to H 2 , at low temperature 7. Although Au/MnO x was initially 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 nanoparticles in this reaction.…”

Small gold species supported on alumina. A computational study of α‐Al2O3(0001) and γ‐Al2O3(001) using an embedded‐cluster approach

“…Alumina has been classified as inactive support in contrast to materials like TiO 2 or Fe 2 O 3 , which exhibit oxidative abilities 5 as does CeO 2 29. Nevertheless alumina‐supported gold catalysts show very good catalytic performance 59, 30–32. The interaction of small gold clusters with surface sites of alumina is expected to affect the coordination of the metal atoms in the interface region and eventually the dispersion of supported metal species as larger metal particles form from such smaller adsorbates.…”