H2Splitting on Pt/Ru Alloys Supported on Sputtered HOPG

Abstract

The equilibrium hydrogen exchange rate between adsorbed and gas phase hydrogen at 1 bar is measured on three Pt/Ru systems: Pt overlayers on Ru films, Ru overlayers on Pt films, and Pt/Ru bulk alloys, with different Pt/Ru compositions. The catalysts are prepared by electron beam physical vapor deposition and supported on a sputtered HOPG substrate. The hydrogen exchange rate is measured in the temperature range 40–200 °C at 1 bar, by utilization of the H-D exchange reaction. We find that the exchange rate, r, and the dissociative sticking probability, S, as a function of surface concentration exceed the values measured on the individual metals and have a maximum for a composition of 2/7 Pt in the bulk alloys. The Pt overlayers on Ru films give similar results for r and S, and maximum activity is found when 4 Å of Pt are deposited on 50 Å Ru films. ISS spectra show that both Ru and Pt are on the surface, which indicates the formation of a surface alloy already at 200 °C. We also find that the maximum activity is given by the alloys with a surface composition of Pt/Ru equal to a 1:1 ratio. Experiments are also carried out in the presence of 10 ppm CO for the Pt–Ru bulk alloys. We find that alloying Pt with Ru improves significantly the resistance toward CO poisoning with respect to pure Pt, and the resistance increases with an increasing amount of Ru in the bulk alloys. The faster hydrogen exchange rate with respect to the pure metals and the higher CO tolerance of the alloys are attributed to strain and ligand effects, caused by the compression of the surface due to the presence of the larger Pt atoms in the neighboring Ru atoms. The apparent energy of desorption at equilibrium, Eapp, for the three Pt–Ru systems is found to decrease with an increasing amount of Ru in the alloys, and it is attributed to geometrical ensemble effects.