Gas-uptake, methanation, and microcalorimetric measurements on the coadsorption of CO and H-2 over polycrystalline Ru and a Ru/TiO2 catalyst

The adsorption, methanation, and heat evolved over a Ru/TiO2 catalyst were found to be quite different than that over a polycrystalline Ru sample, when exposed to CO + H-2 (1:4) pulses at different temperatures in the range 300-470 K. The coadsorbed H-2 is found to have a large promotional effect on the CO uptake by the Ru/TiO2 catalyst, the extent of which depended on the catalyst temperature and the surface coverage. No such effect was observed in the case of Ru metal. Thus, while using Ru/TiO2 the ratio H-2(ad)/CO(ad) increased progressively from 0.7 to 4 with the rise in catalyst temperature from 300 to 470 K, it was almost constant at similar to 5 +/- 0.5 in the case of ruthenium metal. The exposure of Ru metal to CO + H-2 (1:4) pulses gave rise to a differential heat of adsorption (q(d)) similar to 50 kJ mol(-1) at all the reaction temperatures under study, which corresponded to adsorption of CO and H-2 molecules at distinct metal sites and in 1:1 stoichiometry. In the presence of excess H-2, a q(d) value of similar to 180-190 kJ mol(-1) was observed at the reaction temperatures above 425 K, suggesting the simultaneous hydrogenation of C-s species formed during CO dissociation. Contrary to this, a q(d) similar to 115 kJ mol(-1) was observed for the CO + H-2 (1:4) pulse injection over Ru/TiO2 at 300 K, the value reducing to similar to 70 kJ mol(-1) at higher reaction temperatures. Furthermore, a lower q(d) value (similar to 50 kJ mol(-1)) was observed during CO adsorption over Ru/TiO2 at 300 K in the presence of excess H-2, which increased to similar to 250 kJ mol(-1) for the sample temperatures of 420 and 470 K. These data are consistent with the FTIR spectroscopy results on CO + H-2 adsorption over Ru/TiO2 catalyst, showing the formation of Ru(CO)(n), RuH(CO)(n), and RuH(CO)(n-1) type surface complexes (n = 2 or 3) in addition to the linear or the bridge-bonded CO molecules held at the large metal cluster sites (RuxCO). The relative intensity of IR bands responsible to these species depended on the catalyst temperature, the RuxCO species growing progressively with the temperature rise. In the case of Ru metal, the formation of only linearly held surface species is envisaged. Arguments are presented to suggest that the CO molecules adsorbed in the multi-carbonyl form require lesser energy to dissociate and are therefore responsible to the observed low temperature (<450 K) methanation activity of Ru/TiO2. On the other hand, the activity at the higher reaction temperatures, both for the Ru metal and for the Ru/TiO2 catalyst, arises due to dissociation of the linearly or bridge-bonded CO molecules. The Ru-C-n and Ru-C species formed during dissociation of multicarbonyls and linearly bonded CO, respectively, are envisaged to have different rates of graphitization, the former species causing a rapid catalyst deactivation at the lower temperatures. (C) 1997 Academic Press.