Surface Chemistry of Thin Palladium Films

Abstract

The 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.

Reactions of Methanethiol on Cobalt-Covered Mo(110)

“…The lack of sensitivity of reaction methanethiol is different from other studies of the chemistry of ultrathin metal films. For example, the electronic and chemical properties of Pd deposited on Nb(110), Mo(100), Ru(0001), and Re(0001), as well as Ni on Mo(110), were extremely sensitive to the admetal coverage and structure. The investigations of Pd thin films used molecules with π-bonds, such as ethylene and CO, as probes for chemical differences.…”

Alcohol Chemistry as a Probe of Mixed-Metal Phases: Reactions of 2-Propanol on Cobalt-Covered Mo(110)

“…Previous studies of other metal overlayers deposited on metal substrates have shown that molecules like CO, H 2 , NO, and ethylene have desorption characteristics on the monolayer films that are different than on the bulk metals. 4,[6][7][8][9][10]68,69 It is interesting to note that the molecules for which a sensitivity to surface structure has been reported have π bonds. These results suggest that the degree of chemical sensitivity to the surface geometric structure depends strongly on the reactant bound to the surface.…”

“…Similar results have been reported for CO adsorption on several other thin metal films supported on refractory metals. [4][5][6][7][8][9][10] Herein, we discuss the reactions of 2-propanol on Co-covered Mo(110). Cobalt overlayers on Mo(110) have been previously investigated in detail, and the lattice spacing was found to depend on Co coverage.…”

“…Furthermore, substantial differences in the adsorption properties of carbon monoxide have been reported for thin metal films, such as Pd on Nb(110), which have larger lattice constants than the bulk materials. Similar results have been reported for CO adsorption on several other thin metal films supported on refractory metals. − …”

“…Indeed, synergistic effects between Co and Mo have been proposed based on theoretical studies to explain Co promotion in hydrodesulfurization catalysis. 3 Furthermore, substantial differences in the adsorption properties of carbon monoxide have been reported for thin metal films, such as Pd on Nb(110), 4 which have larger lattice constants than the bulk materials. Similar results have been reported for CO adsorption on several other thin metal films supported on refractory metals.…”