Tuning the chemistry of metal surfaces: I. Adsorption and reaction of NO and N₂O on ultrathin Pd films on Ta(110)

Nitric oxide (NO) chemisorption is a sensitive chemical probe of the electronic structure and reactivity of metal surfaces. We have used NO, in conjunction with temperature programmed desorption and high resolution electron energy loss spectroscopy, to explore the altered reactivity of ultrathin (monolayer, bilayer, trilayer) Pd films deposited on Ta(110). The reactivity of the Pd-monolayer film is strongly altered from that of bulk-terminated Pd surfaces. NO is molecularly adsorbed on the Pd monolayer at 95 K, but the desorption activation energy is decreased to only 8 kcal/ mol, and N₂O is the primary desorption product. At low initial NO coverages, N₂O desorbs in a reaction rate-limited peak at 129 K, which grows and shifts up in temperature, with increasing coverage, to 159 K at saturation. NO desorption occurs at 135 K, in addition to N₂O, at high coverages. Separate N₂O adsorption experiments show that N₂O is weakly and reversibly bound to the Pd monolayer film, desorbing by 115 K. NO chemisorption and reaction was independent of the initial geometric structure of the Pd monolayer, i.e., nearly identical results were obtained when using a pseudomorphic-bcc(110) or incommensurate-fcc(111) Pd monolayer. However, the chemical properties and reactivity of the Pd films rapidly returned to that of bulk Pd(111) surfaces as the Pd film thickness was increased above one monolayer. "Tuning" of NO chemistry on these Pd films was possible for initial thicknesses of 2-3 layers. The interaction of NO with a Pd monolayer on Ta(110) closely resembles that of NO with a Ag(111) surface, supporting the interpretation that Pd-Ta bonding interactions lead to a filled Pd d-band resulting in more noble metal-like properties.