Oxidation of CO in H 2–CO mixtures catalyzed by platinum: alkali effects on rates and selectivity

Abstract

Turnover rates and selectivities for CO oxidation in H 2–CO–O 2 reactant mixtures are much greater on Pt clusters supported on Cs-modified SiO 2 than on clusters of similar size dispersed on SiO 2 or Al 2O 3 supports. Selectivity effects reflect the inhibition of spillover-mediated H 2 oxidation pathways on support surfaces. Infrared spectra showed that Cs, Rb, or Na titrates support hydroxyl groups required for these unselective pathways. These H 2 oxidation pathways are unaffected by competitive CO co-adsorption and can lead to significant selectivity losses during preferential CO oxidation in H 2-rich streams. CO oxidation selectivities on Pt are above 90%, even at H 2/CO ratios of ∼100; much lower selectivities reported previously reflect contributions from spillover-mediated H 2 oxidation pathways on supports, which are not inhibited by CO co-reactants. Cs and Rb also increased turnover rates for monofunctional oxidation of CO on Pt cluster surfaces, apparently by disrupting the growth of chemisorbed CO monolayers, which prevent activation of O 2 co-reactants. The addition of 1.6 Cs/nm 2 to SiO 2 led to CO oxidation turnover rates more than 10 times higher than those on Pt/SiO 2 catalysts with similar Pt dispersion. CO monolayer growth processes are evident from a gradual decrease in CO oxidation rates during the initial stages of H 2–CO–O 2 reactions, which occurs simultaneously with an increase in intensity for chemisorbed CO infrared bands measured during reaction. This densification of chemisorbed CO monolayers is consistent with the recovery of initial rates by thermal treatment in inert streams and with the observed monotonic increase in CO oxidation selectivity as catalysts gradually approach steady-state CO coverages during reaction. Alkali appears to disrupt CO monolayer growth by promoting the formation of unreactive chemisorbed carbon via CO dissociation and disproportionation reactions, the rates of which increase when alkali is added to Pt surfaces. Chemisorbed carbon blocks some active Pt surface atoms, but also introduces obstacles that inhibit the formation of dense CO monolayers, thus retaining exposed Pt atoms required for the activation of O 2 co-reactants. Rb and Cs cations, which are more electropositive than Na cations, increase CO oxidation turnover rates more strongly, consistent with the proposed mechanisms for the electronic promotion effects of alkali on CO dissociation.