Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS)

The in situ combination of electrochemistry and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) has been used for the first time to investigate the surface structure sensitivity of asymmetric catalytic hydrogenation at single-crystal Pt electrodes. The adsorption and hydrogenation behavior of aqueous ethyl pyruvate (EP) at a range of modified and unmodified Pt{hkl} electrodes was measured both by cyclic voltammetry and by recording Raman spectra at hydrogen evolution potentials. Two primary surface intermediates were observed, including the previously reported half hydrogenation state (HHS), formed by addition of a hydrogen atom to the keto carbonyl group, as well as a new species identified as intact chemisorbed EP bound in a mu(2)(C,O) configuration. The relative populations of these two species were sensitive to the Pt surface structure; whereas the mu(2)(C,O) EP adsorbate was dominant at pristine Pt{111} and Pt{100}, the HHS was only observed at these electrodes after the introduction of defects by electrochemical roughening. Intrinsically defective Pt{110} and kinked Pt{321} and Pt{721} surfaces exhibited behavior similar to that of electrochemically roughened basal surfaces, indicating the requirement for low coordination sites for observation of the HHS. Rationalization of the differing behaviors is given on the basis of density functional theory (DFT) calculations, which indicate that the mu(2)(C,O) EP adsorbate is considerably more stable on basal {111} than on {221} stepped surfaces. A mechanism is proposed in which the mu(2)(C,O)-bound species is a precursor to the HHS but the rate of the first hydrogen atom addition is slow, leading to a low steady-state population of the HHS at terrace sites. The implications of this in the context of enantioselective hydrogenation at chirally modified Pt are discussed.