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A microcalorimeter equipped with gas circulation cells and coupled atoutlet to a gas chromatograph was used for the simultaneous measurements of the uptake and the differential heat (qd) evolvedduring the adsorption of CO and H2pulses over polycrystallineruthenium metal and a Ru/TiO2catalyst in the temperature range300–475 K and as a function of surface coverage. The initialdifferential heat for the adsorption of CO and H2overpolycrystalline ruthenium at 300 K was 120 and 65 kJ mol−1,respectively, the corresponding values in the case of Ru/TiO2beingaround 130 and 57 kJ mol−1. With the rise in sampletemperature, theqdfor CO adsorption over Ru metal remained almost constant, while inthe case of Ru/TiO2it decreased substantially. The fraction of COor H2adsorbed, conversion of COadto CO2, and the correspondingvalues of heat evolved showed different trends, when these sampleswere exposed to the successive CO or H2pulses at differenttemperatures. The H2adsorption is found to be suppressed onRu/TiO2, particularly at the low sample temperatures. Also, the COadsorption over Ru/TiO2at temperatures above 400 K resulted in thepartial reduction of the support, and this is facilitated by the heatevolved at the metal/support interfaces during CO chemisorption. Onthe other hand, the CO dissociation followed by CO(ad)+O(ad)reaction was a predominant step giving rise to CO2formation in thecase of Ru metal. This study also confirms that, for both thesamples, while the CO adsorption remains uninhibited by thepreadsorbed H2, the catalyst surface covered with the CO wascompletely inaccessible to subsequent H2adsorption. |
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