Semi-hydrogenation of alkynes at single crystal, nanoparticle and biogenic nanoparticle surfaces: The role of defects in Lindlar-type catalysts and the origin of their selectivity

For the first time, the method of shell-isolated nanoparticle Raman spectroscopy (SHINERS) is used in combination with cyclic voltammetry (CV) and reactivity studies to investigate the adsorption behaviour of a series of three alkynes undergoing hydrogenation on nanoparticle, single crystal and bacteria/graphite-supported platinum surfaces. It is found that a strong association of alkynes with defect sites to produce a long-lived di-sigma/pi-alkene surface complex allows for deep hydrogenation of this intermediate to the alkane product. In contrast, when platinum surface defect sites are blocked by either bismuth or polyvinylpyrrolidone (PVP) (and thus leaving behind only Pt{111} terrace adsorption sites), large increases in selectivity to the semi-hydrogenation product are observed for all three alkynes. This finding is consistent with SHINERS collected from both well-ordered and roughened Pt{111} electrodes which revealed that the di-sigma/pi-bonded surface intermediate is hardly formed at all on Pt{111} unless defect sites are introduced via electrochemical roughening. As a general method of producing selective catalysts, the elimination of toxic heavy metals from Lindlar-type catalyst, used commonly in organic chemistry, and their replacement by more benign, organic species adsorbed at defect sites is discussed.