Hydrogen Spillover in Ga-Pd/SiO2 Catalysts for Methanol Synthesis

D.L. CHIAVASSA; A.L. BONIVARDI; M.A. BALTANÁS

París, Francia

Congreso; 13th International Congress on Catalysis; 2004

The promoting effect of metal oxides on the activity and selectivity of supported Pd0 is usually attributed to the close intimacy between both components. Yet, the alternative, ´remote´ action of Pd0 as the provider of atomic hydrogen via spillover is also feasible on these systems, which therefore could actually be considered true bifunctional catalysts. Gallia-promoted Pd/SiO2 catalysts have been shown to be excellent materials for methanol synthesis from CO2/H2, as a consequence of the formation of formate surface species on Ga2Ox, which later hydrogenate via the participation of Pd0 as the main source of atomic hydrogen, Hs [1,2]. On these catalysts Ga suboxides are formed at low temperatures upon H2 reduction, but until now it is unknown whether metal-promoter intimacy is decisive in terms of the catalytic chemistry of the system.By means of mechanical mixtures of the active components, separately supported on silica, it was possible to study and demonstrate that indeed Hs can be generated by Pd far from Ga2Ox and move (spill-over) there to reach the other reactive species (formates) and complete the reaction cycle. The promotion of galia on the Pd/silica catalysts seems to occur due to two complementary effects: a) intimate Ga-Pd contact and b) H-spillover onto the support. The observed activity and selectivity are the result of the balance between both effects.