Adsorption and hydrogenation of CO on Pd(1 1 1) and Rh(1 1 1) modified by subsurface vanadium

On Pd(1 1 1) and Rh(1 1 1) a subsurface alloy of V atoms located mainly in the second metal layer is formed by vapor deposition of V at elevated temperatures. The topmost layer consists exclusively of Pd or Rh atoms in √3×√3R30° geometry. Neither reversible adsorption of CO under UHV conditions nor hydrogenation of CO at atmospheric pressure lead to surface segregation of V compounds. Numerical simulation and threshold temperature programmed desorption analysis of CO TPD series obtained from the clean and V modified samples showed that the presence of subsurface V significantly reduces the desorption energy of molecular CO over a wide coverage range and changes the coverage-dependence of the desorption energy. The presence of subsurface V atoms promotes CO hydrogenation on both Pd(1 1 1) and Rh(1 1 1). The enhanced catalytic activity is interpreted in view of the reduced CO desorption energies and of surface roughening induced by subsurface alloy growth.