Nuclear magnetic resonance studies of simple molecules on metal surfaces

Abstract

We have used NMR to study the structure and reactions of acetylene and ethylene adsorbed on small catalyst particles. By monitoring $\sp{13}$C-$\sp{13}$C and $\sp{13}$C-$\sp1$H dipolar couplings we found the carbon-carbon bond length to be 1.44 $\pm$ 0.02A for acetylene adsorbed on Rh clusters at room temperature and the surface species to consist mostly of CCH$\sb2$ with a small amount of CCH. Using $\sp2$H NMR we studied the conversion of ethylene adsorbed at low temperatures on Pt clusters to the stable room temperature species (CCH$\sb3$) and also the hydrogen-deuterium exchange in this CCH$\sb3$ species. We found an activation energy of 15.2 $\pm$ 2.0 kcal/mole and preexponential factor of 10$\sp{7{\pm}1.7}$ sec$\sp{-1}$ for the conversion reaction and an activation energy of 14.3 $\pm$ 2.5 kcal/mole and preexponential factor of 4 $\times$ 10$\sp{7{\pm}2}$ sec$\sp{-1}$ for the exchange reaction. We compare these results with those obtained by other researchers. We also studied the formation of CCH$\sb3$ from the stable room temperature species of acetylene adsorbed on Pt (CCH$\sb2$) and found an activation energy of 9.3 $\pm$ 2.5 kcal/mole and a preexponential factor of 10$\sp{3{\pm}2}$ sec$\sp{-1}$torr$\sp{-1}$. In addition, we used measurements of $\sp{13}$C-$\sp1$H dipolar couplings to monitor the hydrogen population for the surface species formed from acetylene adsorbed on Pt and Ir. We were able to show that the CCH$\sb2$ species do not lose hydrogen prior to carbon-carbon bond scission on Ir but that there is a gradual dehydrogenation below the temperature of carbon-carbon bond scission for CCH$\sb2$ species on Pt.