Restructuring Effects in the Platinum-Catalysed Enantioselective Hydrogenation of Ethyl Pyruvate

The relative performance of Pt{111} and Pt{100} terraces in the enantioselective hydrogenation of ethyl pyruvate over cinchonidine-modified Pt/graphite has been investigated. Three series of 5% Pt/graphite catalysts have been prepared by sintering as-received material at temperatures in the range 400–1000 K. All underwent Pt particle growth and faceting as shown by transmission electron microscopy. Cyclic voltammetry has revealed the presence of {111}-terraces, {100}-terraces and related stepped features on these Pt surfaces. The performance of these surface structures as catalysts in the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate has been determined using cinchonidine as the chiral modifier. As reported previously, the as-received catalyst provided an enantiomeric excess of ~ 40%(R). Two series of Pt/graphite catalysts having different particle size distributions were prepared by sintering in 5% H2/Ar and one by sintering in pure Ar. Those sintered in H2/Ar showed progressive Pt particle growth with faceting and considerable surface restructuring, whereas that sintered in Ar showed particle growth with little or no faceting and restructuring. The enantiomeric excess provided by the two series sintered in H2/Ar showed a maximum of ~ 60%(R) [i.e. 80%(R)-lactate, 20%(S)-lactate] for catalysts sintered at 700 K, and this correlated with the fraction of Pt{111}-terraces in the surface which showed a coincident maximum. By contrast, the series sintered in pure Ar showed no such maxima, indicating the importance of restructuring in H2 during catalyst preparation. Two catalysts sintered in H2/Ar at 450 and 500 K showed unexpectedly low values of the enantiomeric excess and contained atypically high surface exposures of {100}-terraces, indicating that {100}-surfaces are deleterious to high catalyst performance. Poisoning of these {100}-surfaces by the selective adsorption of sulfur restored the enantioselectivity to its expected value. The two series of catalysts sintered in H2/Ar gave comparable catalytic performance, but the one composed of smaller Pt particles contained a higher fraction of surface Pt{111}-terraces than the other, leading to an expectation of higher enantioselectivity. This suggested that some of the Pt{111}-surfaces identified by cyclic voltammetry are too small to accommodate the adsorption of the 1:1 modifier-reactant complexes necessary for rate-enhanced enantioselection, and hence do not contribute significantly to the overall reaction. Future catalyst design should therefore concentrate on preparation procedures that maximise {111}-terrace formation or contain poisons that deactivate {100}-surfaces.