Adsorption dynamics of hydrogen and carbon monoxide on V/Pd(111) alloy surfaces

We have studied the adsorption dynamics of hydrogen and carbon monoxide on a clean Pd(1 1 1 ) plane and on V/Pd(1 1 1) alloy surfaces using a supersonic molecular beam. The alloy surfaces were prepared by deposition of small amounts of vanadium on the palladium crystal through evaporation at a temperature of 570 K. At this temperature the adsorbed vanadium diffuses into the second layer forming a subsurface alloy. For molecular hydrogen the subsurface vanadium leads to a strong decrease of the initial sticking coefficient compared to the clean Pd surface. In contrast to Pd(1 1 1), where strong rotational effects in the adsorption of H-2 are observed, the modified Pd surfaces exhibit no influence of the rotational state of the impinging molecule on the adsorption. In addition, no isotope effect between H-2 and D-2 can be seen. In the case of carbon monoxide the binding energy of the molecules is drastically reduced on the V-Pd surface alloy compared to the clean Pd. An interpretation of the observed effects is given in terms of the modified electronic structure of the alloy surface. (C) 2001 Elsevier Science B.V. All rights reserved.