Low-Temperature CO Oxidation and Long-Term Stability of Au/In 2 O 3 -TiO 2 Catalysts Vicente Rodrı ´guez-Gonza ´lez, † Rodolfo Zanella,* ,‡ Lina A. Calzada, ‡ and Ricardo Go ´mez § Departamento de Ecomateriales y Energı ´a, UANL, Instituto de Ingenierı ´a CiVil, AV. UniVersidad S/N, San Nicola ´s de los Garza, NueVo Leo ´n, C.P. 66451, Mexico, Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´gico, UniVersidad Nacional Auto ´noma de Me ´xico, Circuito Exterior S/N, Ciudad UniVersitaria A. P. 70-186, Delegacio ´n Coyoaca ´n, C.P. 04510, Me ´xico D. F. Mexico, and Departamento de Quı ´mica, UniVersidad Auto ´noma Metropolitana-Iztapalapa, AV. San Rafael Atlixco No 186, Me ´xico 09340, D.F. Mexico ReceiVed: NoVember 12, 2008; ReVised Manuscript ReceiVed: December 25, 2008 In the present work, the preparation of Au/In 2 O 3 -TiO 2 catalysts with loadings of 1, 6, and 12 wt % of In is reported. In 2 O 3 -TiO 2 supports prepared by the sol-gel method allow the formation of solids with high specific surface area. By means of the urea deposition-precipitation method, gold nanoparticles in the range of 2.4-3.6 nm were obtained on the supported gold catalysts. Catalysts tested in the CO oxidation showed a very high activity at the subambient reaction temperature (0 °C). The indium oxide catalysts (6 and 12 wt % of In) are more active than the Au/TiO 2 parent catalyst. Full conversion was achieved at temperatures lower than 150 °C on the Au/In-TiO 2 (6 wt % of In). It is shown that under reaction conditions, the Au/In-TiO 2 catalysts are more stable than the Au/TiO 2 parent catalyst. This behavior is related to the strong anchoring of the gold particles on the well-dispersed indium oxide and on the structural defects of the support caused by the doping with In of the anatase support. 1. Introduction Since the discovery in the late eighties that gold could be catalytically active when it is dispersed as small particles (<5 nm) on an oxide support, the preparation of gold-based catalysts has been widely studied in order to prepare active and stable catalysts. 1-3 Gold catalysts have increasingly attracted the attention of researchers due to their potential applications to the environmental pollution abatement. The most remarkable application of supported gold has been obtained in the CO oxidation at subambient temperature. 4 On supported gold catalysts, it has been found that the gold particles are more reactive when they are supported on reducible metal oxides such as TiO 2 , Co 3 O 4 , and Fe 2 O 3 5,6 and when they are in the nanometric range of 2-5 nm. 6-8 Thus, it is important to stabilize the gold nanoparticles in this size range. It is well-known that the improvement of the catalytic activity of supported gold catalysts depends on the preparation method, 9-12 the synthesis conditions, 9,13,14 the pretreatments, 15,16 and the nature of the supports. 17 One of the most important limitations in the use of gold catalysts is that they deactivate during the catalytic test, showing low resistance to sintering. 18,19 In this way, the support plays a major role for the stability of the gold particles that depends on both its structure and the specific interaction occurring between the gold particles and it. It has been observed that the sintering of gold particles occurs even in stored catalysts. Supported gold metal particles are very sensitive when they are exposed to a light source or atmospheric conditions. 16,20-23 Their high mobility in the presence of chlorides 24 and the low melting point of gold nanoparticles (400 °C) are drawbacks that may constrain their practical applica- tions. 25-27 Under the reaction conditions, the long-term stability during the CO oxidation is also a key feature since the activity progressively decreases with time. 28-30 In this case, the deac- tivation of gold-supported catalysts is produced by the adsorption of CO on either the gold particles or the support, forming carbonates that block the active sites coparticipating in the catalytic reaction. 28,29,31,32 The deactivation produced in this way is reversible, and after heating the catalyst, the activity concern- ing the formation of CO 2 is restored. Additionally, the ag- glomeration of the small Au particles giving larger but less active particles 19,28,33,34 produces, in this case, an irreversible and more important deactivation. Many studies have been done in order to stabilize the supported gold nanoparticles. It has been suggested that the use of binary mixed oxides as gold supports could be a good solution for the stabilization of gold nanoparticles. 35 The gold particles can be anchored to the support, which stabilizes them and prevents their sintering. 36-45 For example, it has been shown by DFT calculations that in Au/IrO 2 /TiO 2 catalysts, the binary mixed oxide allows the formation of an active Au-oxide interface, which increases the resistance of the gold nanoparticles to be sintered. 35 Yan et al., 45 prepared a highly stable catalyst by impregnation of gold over an alumina thin layer on TiO 2 (anatase) (Al 2 O 3 /TiO 2 ); the catalysts showed high activity for the CO oxidation even after calcination of the catalyst at 773 K. The HRTEM observations showed that the size of the Au particles increases markedly at high temperatures on TiO 2 but slightly on Al 2 O 3 /TiO 2 . 45 On the other hand, Venezia et al. 44 and Tai et al. 43 reported similar findings using TiO 2 /SiO 2 mixed oxides as gold supports. Although Al 2 O 3 used as an additive on TiO 2 45 and as Al 3+ in the Al 2 O 3 -SiO 2 mixed oxide 46,47 could stabilize the gold particles, no significant promotional effect has been observed on Al 2 O 3 coatings dispersed on SiO 2 48,49 or on Al 2 O 3 -CeO 2 mixed oxides. 50 On the other hand, it has been reported that addition of iron to TiO 2 , SnO 2 , or CeO 2 diminishes the deactivation rate. 51 * To whom correspondence should be addressed. E-mail: rodolfo.zanella@ ccadet.unam.mx. Tel.: +52(55)56228602, ext. 1115. Fax: +52(55)56228651. † Instituto de Ingenierı ´a Civil. ‡ Universidad Nacional Auto ´noma de Me ´xico. § Universidad Auto ´noma Metropolitana-Iztapalapa. J. Phys. Chem. C 2009, 113, 8911–8917 8911 10.1021/jp8099797 CCC: $40.75  2009 American Chemical Society Published on Web 04/29/2009