Activity and segregation behavior of Pd75%Ag25%(1 1 1) during CO oxidation – An in situ NAP-XPS investigation.

[Display omitted] • CO oxidation over Pd(1 1 1) and Pd 75% Ag 25% (1 1 1) studied by NAP-XPS. • Dynamic changes in the Pd/Ag surface composition observed for Pd 75% Ag 25% (1 1 1). • Pd 75% Ag 25% (1 1 1) significantly less active than Pd(1 1 1). • Reactant activation mechanistically different on Pd 75% Ag 25% (1 1 1) compared to Pd(1 1 1). The surface reactivity during CO oxidation over Pd(1 1 1) and Pd 75% Ag 25% (1 1 1) single crystals has been investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under excess oxygen conditions (p ≈ 2 mbar, O 2 :CO=10). The (√6×√6) Pd 5 O 4 surface oxide is present on Pd(1 1 1) when the surface is highly active under the applied conditions. The CO 2 formation profile follows a hysteresis upon temperature cycling between 150 and 450 °C that depends on the temperature ramp rate. The behavior is different for Pd 75% Ag 25% (1 1 1), with the CO 2 formation rate considerably lower, no surface oxide and only chemisorbed oxygen present at the surface at high temperature, reversible scrambling of the (near) surface Pd/(Pd+Ag) ratio, and no hysteresis behavior observed. The results suggest that the reactant activation is affected by both surface composition and surface termination, i.e. mechanistically different on Pd 75% Ag 25% (1 1 1) than Pd(1 1 1), and less efficient on (1 1 1) as compared to the (1 0 0) alloy counterpart. [ABSTRACT FROM AUTHOR]