Surface science on catalytically important metals: I. The mechanism of hydrodechlorination of palladium: Catalysis and TPD studies. II. The structure of disordered ethylene on platinum(111): A diffuse LEED study

The chemistry of metal catalysis was investigated with a view towards understanding the interrelationships among structure, reactivity and bonding. Two systems were investigated, hydrodechlorination over palladium catalysts and ethylene on the close-packed Pt(111) surface.; Hydrodechlorination shows a lack of structural sensitivity. Three different catalysts, Pd(111) and Pd(100) single crystals and polycrystalline Pd foil, all gave the same turnover frequencies and selectivities towards the various products formed in the reaction of CF{dollar}sb3{dollar}-CFCl{dollar}sb2{dollar} (CFC-114a). The rates are all around 0.6-0.7 molecules per surface Pd atom per second under standard conditions of 50 Torr H{dollar}sb2{dollar}, 50 Torr CF{dollar}sb3{dollar}-CFCl{dollar}sb2{dollar}, 0.1 Torr HCl, and 150{dollar}spcirc{dollar}C. The products typically formed (with selectivities) are CF{dollar}sb3{dollar}-CFH{dollar}sb2{dollar} ({dollar}sim{dollar}85%), CF{dollar}sb3{dollar}-CFHCl ({dollar}sim{dollar}10%) and CF{dollar}sb3{dollar}-CH{dollar}sb3{dollar} ({dollar}sim{dollar}5%). The kinetics were found to be first order in the reactant (CF{dollar}sb3{dollar}-CFCl{dollar}sb2{dollar}) and inverse first order in the product, HCl. The reaction was found to be zero order in hydrogen over the single crystals and half order over the foil. These kinetics implied a mechanism for the reaction which implicated adsorption of CF{dollar}sb3{dollar}-CFCl{dollar}sb2{dollar} onto a free surface site as the rate-determining step.; Thermal desorption data indicated that the molecules CF{dollar}sb3{dollar}-CFCl{dollar}sb2{dollar} and CF{dollar}sb3{dollar}-CFHCl{dollar}sb2{dollar} both physisorb to Pd(100) at low temperatures. The physisorbed layer desorbed around 160-170 K, at which point part of the adsorbed layer was inferred to have decomposed, based on the observed desorption of the hydrogenated product, CF{dollar}sb3{dollar}-CFH{dollar}sb2{dollar}. Adsorbed CF{dollar}sb3{dollar}-CFCl{dollar}sb2{dollar} yielded product at 235 K in high ({dollar}sim{dollar}50%) yield, while adsorbed CF{dollar}sb3{dollar}-CFHCl yielded product at 185 K in low ({dollar}sim{dollar}5%) yield. This is understood based on the intermediates believed to be formed.; The structure of disordered ethylene on Pt(111) was investigated at 200 K by diffuse LEED. Contrary to long-standing assumptions, the molecule was found to adsorb at the three-fold hollow site with a tilt of the molecular axis of about 22{dollar}spcirc{dollar} away from the plane of the surface. The fcc and hcp hollow sites both gave equally satisfactory agreement with the data (Pendry R-factor, R{dollar}rmsb{lcub}P,{rcub}{dollar} of 0.23 and 0.25 respectively), while the bridge site was found not to be correct (R{dollar}rmsb{lcub}p{rcub}{dollar} = 0.31, just outside the error bars). Incoherent mixing of the fcc and hcp site occupations was found to improve the fit, suggesting that the true structure might be a mixture of both sites. By contrast, mixing of either site with the bridge site yielded no significant improvement.