Gas-Uptake, Methanation, and Microcalorimetric Measurements on the Coadsorption of CO and H2over Polycrystalline Ru and a Ru/TiO2Catalyst

Abstract

The adsorption, methanation, and heat evolved over a Ru/TiO2catalyst were found to be quite different than that over a polycrystalline Ru sample, when exposed to CO+H2(1:4) pulses at different temperatures in the range 300–470 K. The coadsorbed H2is found to have a large promotional effect on the CO uptake by the Ru/TiO2catalyst, 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/TiO2the ratio H2(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 ∼5±0.5 in the case of ruthenium metal. The exposure of Ru metal to CO+H2(1:4) pulses gave rise to a differential heat of adsorption (qd)∼50 kJ mol−1at all the reaction temperatures under study, which corresponded to adsorption of CO and H2molecules at distinct metal sites and in 1:1 stoichiometry. In the presence of excess H2, aqdvalue of ∼180–190 kJ mol−1was observed at the reaction temperatures above 425 K, suggesting the simultaneous hydrogenation of Csspecies formed during CO dissociation. Contrary to this, aqd∼115 kJ mol−1was observed for the CO+H2(1:4) pulse injection over Ru/TiO2at 300 K, the value reducing to ∼70 kJ mol−1at higher reaction temperatures. Furthermore, a lowerqdvalue (∼50 kJ mol−1) was observed during CO adsorption over Ru/TiO2at 300 K in the presence of excess H2, which increased to ∼250 kJ mol−1for the sample temperatures of 420 and 470 K. These data are consistent with the FTIR spectroscopy results on CO+H2adsorption over Ru/TiO2catalyst, showing the formation of Ru(CO)n, RuH(CO)n, and RuH(CO)n−1type 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 multicarbonyl 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/TiO2catalyst, arises due to dissociation of the linearly or bridge-bonded CO molecules. The Ru–Cnand 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.