Determination of the role of nanoparticle active sites in catalytic hydrogenation reactions by cyclic voltammetry and novel in-situ surface spectroscopy

Cyclic Voltammetry, Surface Enhanced Raman Spectroscopy and SHell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) have been used to investigate two types of heterogeneously catalysed hydrogenation reactions – hydrogenation of selected alkynes and the Orito reaction. Using a spectro-electrochemical flowcell, which is designed and built in-house, the selectivity and reactivity of the reactions on platinum surfaces have been truly discovered. By studying hydrogenation of a range of alkyne molecules, including 2-butyne-1,4-diol, 2-pentyne, 4-octyne, propargyl alcohol and 2-methyl-3-butyn-2-ol, on different platinum surfaces, it was found that alkyne adsorption on Pt defect sites produces a long-lived di-sigma/pi- alkene complex which may undergo further hydrogenation to produce alkane. This complex may form on different surfaces with various orientations. However, depending on the specific molecules, the intermediate may not survive on some surfaces due to the steady state conditions arising from the catalytic reaction whereby adsorption of alkyne and hydrogenative desorption of reaction intermediates determines the overall surface coverage of intermediate as a function of potential. Alkene selectivity can be increased by blocking defect sites, using polyvinylpyrrolidone or bismuth, leaving only platinum{111} terraces available for catalysis. By studying ethyl pyruvate (EP) adsorption on different surfaces of platinum and palladium, it was found that EP half-hydrogenated state (HHS) is a critical intermediate of the hydrogenated product which only forms on the step sites of the platinum surface at the hydrogen evolution reaction potential. However, another new intermediate adsorbate, which is believed to be a η1 species and is believed to be a precursor of the HHS only forms on the terraces of the surface. By studying the surface intermediates formed during hydrogenation of EP at palladium modified platinum single crystal electrodes, it was found that there was no unequivocal evidence for bands pertaining to EP adsorption on palladium could be identified under hydrogenating conditions due to the reconstruction of the palladium adlayer to reveal Pt subsurface sites