Hydrogen adsorption states on silica-supported Ru-Ag and Ru-Cu bimetallic catalysts investigated via microcalorimetry

The nature of hydrogen adsorption states on silica supported Ru, Ru-Ag and Ru-Cu was investigated using microcalorimetry and selective hydrogen chemisorption. The differential heats of hydrogen adsorption as a function of hydrogen coverage were determined for Ru and bimetallics with varying Ag and Cu contents. The initial heats of hydrogen adsorption (differential heat at coverages approaching zero) were found to be similar on all three catalyst systems suggesting the absence of effects that directly alter the chemisorptive bond (e.g. electron transfer between Ru and the added metal). The addition Ag to Ru significantly reduced the total amount of hydrogen adsorbed per surface Ru atom. That is, the stoichiometry of adsorption was altered by the addition of Ag even though Ag does not adsorb hydrogen. Similarly, the addition of Cu reduced the amount of hydrogen adsorbed per surface metal atom. In both cases, the reduced amount of hydrogen adsorbed per surface atom (adsorptive stoichiometry) was associated with the loss of hydrogen adsorbed from weak and intermediate energy adsorption states (<80 kJ/mol). The population of sites with adsorption energies greater than or equal to 80 kJ/mol were unaffected. From atomistic simulations it is known that, at low concentrations, Ag and Cu preferentially occupy the low coordination metal sites. The loss of the intermediate and low adsorption energy sites is correlated with replacement of Ru atoms at edge, corner and other defectlike sites by Ag and Cu. Possible reasons for this structure sensitivity are discussed. (C) 1998 Elsevier Science B.V.