Surface Chemistry of Thin Palladium Films

ABSTRACTThe properties of Pd monolayers on several transition metal substrates are dramatically different from the surface properties of bulk Pd single crystals. The changes include a large rehybridization of the Pd atoms to a noble metal electronic configuration and a concomitant reduction in CO and H2 chemisorption bond strengths. We present here temperature programmed desorption (TPD) studies of CO, NO, H2, and C2H4 chemisorption on thin epitaxial Pd films on Nb(110). Our resuAs confirm for all of these gases a large reduction in the chemisorption bond energy for adsorption on Pd thin (<2 ML) films. Our TPD results for CO, which give a CO binding energy of about 10 kcal/mole, confirm previous UPS studies which show that a Pd monolayer forms only a weak chemisorption bond to CO. Thicker Pd films ( > 5 ML ) have CO TPD spectra that agree qualitatively with previous studies on Pd(111) single crystals, where the CO binding energy is about 30 kcal/mole. These results for Pd/Nb(110) will be discussed with respect to recent work on related systems involving thin Pd films on other transition metal substrates.