Adsorption and Microcalorimetric Measurements on the Interaction of CO and H2with Polycrystalline Ru and Ru/TiO2Catalyst

Abstract

A microcalorimeter equipped with gas circulation cells and coupled at outlet to a gas chromatograph was used for the simultaneous measurements of the uptake and the differential heat (q(d)) evolved during the adsorption of CO and H-2 pulses over polycrystalline ruthenium metal and a Ru/TiO2 catalyst in the temperature range 300-475 K and as a function of surface coverage. The initial differential heat for the adsorption of CO and H-2 over polycrystalline ruthenium at 300 K was 120 and 65 kJ mol(-1), respectively, the corresponding values in the case of Ru/TiO2 being around 130 and 57 kJ mol(-1). With the rise in sample temperature, the q(d) for CO adsorption over Ru metal remained almost constant, while in the case of Ru/TiO2 it decreased substantially. The fraction of CO or H-2 adsorbed, conversion of COad to CO2, and the corresponding values of heat evolved showed different trends, when these samples were exposed to the successive CO or H-2 pulses at different temperatures. The H-2 adsorption is found to be suppressed on Ru/TiO2, particularly at the low sample temperatures. Also, the CO adsorption over Ru/TiO2 at temperatures above 400 K resulted in the partial reduction of the support, and this is facilitated by the heat evolved at the metal/support interfaces during CO chemisorption. On the other hand, the CO dissociation followed by CO(ad) + O-(ad) reaction was a predominant step giving rise to CO2 formation in the case of Ru metal. This study also confirms that, for both the samples, while the CO adsorption remains uninhibited by the preadsorbed H-2, the catalyst surface covered with the CO was completely inaccessible to subsequent H-2 adsorption. (C) 1997 Academic Press.