ELSEVIER Catalysis Today 29 (1996) 367-372 j CATALYS I S TODAY Hydrogenation of carbon dioxide to methanol over palladium-promoted Cu/ZnO/A1203 catalysts M. Sahibzada, D. Chadwick *, I.S. Metcalfe Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technologyand Medicine, London SW7 2BY, UK Abstract The effect of Pd on a Cu/ZnO/A1203 catalyst for methanol synthesis from CO2/H 2 has been investigated. Activities of impregnated catalysts and physical mixtures were studied in an internal recycle reactor under 5 MPa, 250°C and a range of conversions. In all cases, the promotion of methanol production was greater at higher flow rates (lower conversions). The promotion achieved by use of Pd/AI203 + Cu/ZnO/Al203 physical mixtures was found to increase with Pd content. Greater promotion was observed over the Pd impregnated Cu/ZnO/Al203 catalysts, although this was insensitive to the particular Pd loadings used. The results are consistent with the proposal that hydrogen spillover is responsible for the observed promotion. The effectiveness of Pd as a promoter for the reduction of CuO in the catalysts was studied by TPR and was found to be related to the level of promotion in methanol production. Keywords: CO 2 hydrogenation; Pd catalysts; Methanol production 1. Introduction The recognition that the hydrogenation of carbon dioxide to methanol is a promising ap- proach to the recycling of emitted CO 2 has given rise to considerable interest in the devel- opment of selective catalysts. Inui and co- workers [1-3] have developed a two-stage pro- cess for the conversion of CO 2 to gasoline via methanol. In this context, they have investigated the effect on methanol synthesis of Rh, Ru, Pd and Ag addition to Cu/ZnO-based catalysts. Pd and Ag were found to promote significantly methanol synthesis from CO 2. In these studies, the promoting metals supported on A1203 were * Corresponding author. incorporated in physical mixtures and, since Pd/AI203 and Ag/A1203 did not show any activity for methanol synthesis from CO 2, it was concluded that the mechanism of promotion was by hydrogen spillover which served to maintain a reductive state of Cu. Subsequently, Fujimoto and Yu [4], investigating the effect of Pd and Pt on Cu/ZnO-based catalysts, arrived at a similar conclusion and in addition demon- strated the stability of the Pd promoted catalysts against poisoning of methanol synthesis activity by water. Lanthanide oxides [5] and TiO 2 [6] have also been shown to promote CO 2 conver- sion to methanol over Cu/ZnO-based catalysts, while Ag promotes Cu/ZrO 2 [7] and Pd on other supports has also been investigated [8-10]. There is also interest in methanol synthesis 0920-5861/96/$32.00 © 1996 Elsevier Science B.V. All rights reserved SSD! 0920-5861 (95)00306-1