Fundamental aspects of enantioselective heterogeneous catalysis: the surface chemistry of methyl pyruvate on Pt{111}

STM, NEXAFS, XPS, and TPR have been used to characterize the adsorption and rt activity of methyl pyruvate on Pt(1 1 1). In the absence of coadsorbed hydrogen, methyl pyruvate polymerizes at room temperature yielding polymer chains, partly dendritic. The STM and XPS data provide independent estimates of the average length, found to be similar to9 monomer units. NEXAFS shows that this polymer contains C=O bonds and no C=C bonds; the C=O bonds are inclined at 64 degrees +/- 5 degrees with respect to the metal surface. It is proposed that polymerization occurs by hydrogen elimination from the monomer, followed by an aldol condensation that involves elimination of methanol. This mechanism is in excellent accord with the intramolecular bending, shape, and reactivity of the polymer deduced from the NEXAFS, STM, and TPR results. Coadsorbed hydrogen completely suppresses polymerization. These findings suggest that irreversible deactivation during start-up or steady-state operation of Pt catalysts during enantioselective hydrogenation of alkyl pyruvates can be due to hydrogen starvation which results in polymerization of the prochiral reactant.