Effect of hydrogen reduction on the CO adsorption and methanation reaction over Ru/TiO2 and Ru/Al2O3 catalysts

The C-O stretch vibrational bands developed over Ru/TiO2 and Ru/Al2O3 catalyst surfaces during adsorption of CO at different temperatures were investigated as a function of H-2 pretreatment given to a sample in the temperature range of 575-875 K. While the reduction at temperatures below 675 K had no significant effect, the higher temperature H-2 pretreatment resulted in the progressive annihilation of nu CO bands in the 2050-2145 cm(-1) region, identified with the multicarbonyl species bonded to Ru sites of different oxidation states. The removal of these bands showed a parallelism with the loss of catalyst activity at reaction temperatures below 500 K, whereas the activity at the temperatures above 550 K remained almost unaffected. We conclude that the Ru(CO)(n) species are formed over highly dispersed metal surfaces and are responsible to the low temperature catalytic activity. On the other hand, the catalytic activity at higher reaction temperatures is attributed to certain monocarbonyl moieties, the formation of which is rather independent of metal dispersion. Our results also reveal the occurrence of the reductive surface agglomeration of Ru metal at high temperatures and the reformation of the smaller crystallites on subsequent exposure of the catalyst to O-2.