Methanol and methane formation over palladium/rare earth oxide catalysts

Abstract

Lanthanide rare earth oxide (REO)-supported Pd systems can provide very good CO hydrogenation catalysts, especially for CH 3OH formation. At low temperature and 1 atm, the major product was methanol but the selectivity shifted to methane at higher temperatures because of equilibrium constraints. The turnover frequency (TOP) values for CH 4 varied more than tenfold, but all, except for Pd Eu 2O 3 , were more than an order of magnitude higher on these catalysts than on Pd SiO 2 or pure Pd metal. Activation energies for CH 4 on these Pd REO samples were consistently near 32 kcal/mole and very close to the value on Pd powder, indicating that the methanation reaction mechanism may be similar on all these catalysts. At 14.6 atm total pressure, the major product over the Pd REO catalysts was methanol and the selectivity for oxygenates ranged from 78 to 96% at 225 °C. Activation energies for CH 3OH varied from 17 to 21 kcal/mole and were close to the value obtained for Pd powder. Pd La 2O 3 and Pd Nd 2O 3 were the most active CH 3OH catalysts whereas Pd CeO 2 was the least active. All the Pd REO catalysts deactivated initially at high pressure but reached steady state after about 30 hr on stream, with the stabilized activity being approximately half the initial activity in all cases. In contrast, the activity of Pd powder increased with time before reaching a steady state. The selectivity of Pd powder for oxygenates was lower compared to REO-supported Pd, and the CH 3OH TOP over Pd powder was an order of magnitude lower than the least active Pd REO catalyst. The role of these supports in CH 3OH synthesis is consistent with that of promoting the concentration of formyl/formate intermediates on the surface, particularly at the Pd-support interface.