Surface reactivity of oxide phases generated on Pd(111) during the growth vs. reduction of PdO(101) films

Abstract

We used temperature programmed reaction spectroscopy (TPRS) to investigate the adsorption and complete oxidation of propane on various surface oxygen phases which evolve during the growth and reduction of PdO(101) films on Pd(111). Desorption spectra reveal clear differences in the binding of molecular propane on the different oxygen phases investigated, including the p(2×2)-O chemisorbed phase on Pd(111), two-dimensional (2D) oxide(s) and multilayer PdO(101) thin films. We find that propane reacts to an immeasurable extent on the p(2×2)-O and 2D oxide phases during TPRS, while reaction is facile on the PdO(101) surface. For the partially oxidized and reduced surfaces, we find that the CO2 product yield obtained from propane-saturated surfaces increases continuously with increasing oxygen coverage above about 1 ML (monolayer), and observe a hysteresis in the surface reactivity such that more CO2 is produced when a given oxygen coverage is generated during the growth vs. reduction of a thin PdO(101) film. We show that the fractional areas of coexisting oxygen phases can be estimated by fitting propane TPD spectra obtained from partially oxidized surfaces with propane TPD spectra obtained from well-defined reference surfaces. Based on the estimated surface area fractions, we find that the CO2 product yields scale linearly with the amount of PdO(101) at the surface, irrespective of the types and amounts of coexisting phases, and show that the observed hysteresis arises from differences in the PdO(101) surface fractions obtained at a given coverage during growth and reduction of PdO(101) films. The estimated surface fractions also reveal that 2D oxide domains develop during the first stage of PdO(101) reduction, while metallic surface domains become prevalent only during the latter half of the oxide reduction process. A key finding is that the activation of propane on partially oxidized Pd(111) is relatively insensitive to the complexity of the surface phase distribution, and depends almost exclusively on the amount of PdO(101) domains that are present.