Structure-Controlled Chemical Properties of PdZn Near-Surface Alloys

PdZn catalysts have demonstrated high potential in methanol steam reforming, water-gas shift reaction, and propane dehydrogenation. In order to understand the reaction mechanisms, beta(1)-PdZn surface alloys were produced on Pd(111) and Pd(100) single crystal substrates via vapor deposition of diethylzinc. The structural properties and thermal stability of the surface alloys were investigated by X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM) techniques. Several phases of the PdZn alloy were observed on the Pd(111) surface depending on the preparation conditions. On Pd(111), atomically resolved STM images show a transition from a PdZn p(2 x 1) rowlike structure to the beta 1-PdZn(111) surface to a zigzaglike structure when annealed at increasingly higher temperatures. Similarly, a transition from the beta 1-PdZn(001) to the beta 1- PdZn(010) structure was observed when PdZn alloys were prepared on a Pd(100) single crystal. The adsorption of carbon monoxide and propylene was investigated by high-resolution electron energy loss spectroscopy. CO was found to bind exclusively in the linear position (no bridge configuration) for the beta(1)-PdZn(010) surface, while complete destabilization of CO was observed on all other beta 1 alloy surfaces. Propylene was found to no longer adsorb on any of the beta 1 alloy surfaces even at subambient temperatures, 130 K. To complement the experimental observation, DFT calculations have been performed, which point to lower binding energies of CO and propylene on PdZn surfaces compared to Pd surfaces. This fundamental surface science study depicts the inability of propylene to adsorb on the 1:1 alloy terrace surfaces, thereby leading to the inhibition of deeper dehydrogenation, hydrogenolysis, and coke formation. Hence, this can be considered as one of the major factors for the high alkene selectivity and stability of PdZn alloy catalysts for alkane dehydrogenation.