Mari Helene Farstad

Near Ambient Pressure XPS Investigation of CO Oxidation Over Pd3Au(100)

The CO oxidation behavior under excess oxygen and near stoichiometric conditions over the surface of Pd3Au(100) has been studied by combining near-ambient pressure X-ray photoelectron spectroscopy and quadrupole mass spectrometry and compared to Pd(100). During heating and cooling cycles, normal hysteresis in the CO2 production, i.e. with the light-off temperature being higher than the extinction temperature, is observed for both surfaces. On both Pd3Au(100) and Pd(100) the (√5 × √5)R27° surface oxide structure is present during CO2 production under excess oxygen conditions (O2:CO = 10:1), while at near stoichiometric conditions (O2:CO = 1:1) the surfaces are covered with atomic oxygen. Au as alloying element hence induces only minor differences in the observed hysteresis and the active phase compared to pure Pd. Alloying with Au thus yields a different behavior compared to Ag, where reversed hysteresis is observed for CO2 production over Pd75Ag25(100) at similar conditions [Fernandes et al., ACS Catal. (2016) 4154].

Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100)

Thin films of TiOx on Pd(100) and Pd(111) have been investigated with respect to their properties after oxidation and reduction cycles. High-resolution photoemission spectroscopy (HRPES) and low energy electron diffraction (LEED) have been applied to characterize the thin film oxidation states and structure before and after oxidation and reduction under ultrahigh vacuum conditions. Fully oxidized TiO2 films were formed on both surfaces. These structures display Moiré patterns in LEED, in one dimension for Pd(100) and in two dimensions for Pd(111), and they have previously not been reported for TiO2/Pd. The oxidation process causes strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiOx thin films on Pd(111) and Pd(100) was possible.