An infrared study of the intermediates of methanol synthesis from carbon dioxide over Pd/β-Ga2O3

The interaction of CO2 and H-2/CO2 with pure beta-Ga2O3 and Pd/beta-Ga2O3 (1 wt% Pd) was studied by temperature-programmed reaction, between 323 K and 723 K at 0.1 MPa, using in situ FTIR spectroscopy. Under CO2(g), bicarbonate, bidentate, and polydentate carbonate species are formed over the surface of gallia at 323 K. When beta-Ga2O3 is exposed to H-2/CO2 only polydentate carbonate reacts with hydrogen, at T > 473 K (i.e., after the dissociative adsorption of H-2 on gallia), to give bidentate and monodentate formate species (b-HCOO and m-HCOO, respectively) which are further hydrogenated to methoxy groups, just over 523 K. It is proposed that the addition of Pd to the oxide support only increases the hydrogenation rate of all the carbon-containing species bonded to the beta-Ga2O3 surface, by spillover of atomic H from metallic Pd to gallia: (i) at 323 K (bi)carbonate groups are hydrogenated to m-HCOO and b-HCOO, and (ii) from 423 K upwards m-HCOO is further transformed to methoxy. A strong evidence of the interconversion between m-HCOO and b-HCOO was also found. (C) 2004 Elsevier Inc. All rights reserved.