The influence of subsurface vanadium on the adsorption of hydrogen and carbon monoxide on Pd(111)

We have investigated the adsorption kinetics of hydrogen and carbon monoxide on both a clean and a vanadium modified Pd(111) plane using a supersonic molecular beam and thermal desorption spectroscopy. One-third of a monolayer vanadium was deposited on the palladium crystal through evaporation at a temperature of 573 K. At this temperature the adsorbed vanadium is transferred into the second atomic layer forming a subsurface alloy. For both adsorbates the subsurface vanadium leads to a marked decrease in the adsorption energy compared to the clean Pd(l 1 1) plane. The initial sticking coefficiant for CO is almost unity on both surfaces. The coverage dependence of the sticking coefficients indicates the involvement of an extrinsic precursor. In contrast to CO adsorption the initial sticking coefficient of molecular hydrogen on the alloy is drastically reduced to 0.32 compared to 0.61 on the clean Pd(1 1 1) surface. No isotope effects between H-2 and D-2 could be observed. (C) 2001 Elsevier Science Ltd. All rights reserved.