Adaptivity of depth distribution of two metals in Pd-Ag/HOPG catalyst to external conditions in the course of mild CO oxidation

Bimetallic Pd-Ag nanoparticles supported on surface of highly oriented pyrolytic graphite have been studied by mass-spectrometry and near ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS) techniques in the stoichiometric reaction mixture of CO oxidation at a total pressure of 0.25 mbar in a range of temperatures 25–300 °C to establish the correlation between their catalytic properties and evolution of the surface composition therein. Under the reaction mixture at room temperature, Pd segregation on the surface took place, which became more apparent upon a temperature increase to 150 °C. The intensity of the Pd-CO adsorption state identified in Pd3d X-ray photoelectron spectra gradually decreased from its maximum at room temperature to zero at 200 °C. Simultaneously, the Pd/Ag atomic ratio on the surface also dropped down due to the progressive CO desorption. Mass-spectrometry data showed that the reaction did not proceed at a temperature below 150 °C. Indeed, a CO2 (reaction product) signal appeared in Mass-spectrometry only above 150 °C, when the Pd-CO adsorption state in XPS essentially disappeared. Moreover, the thermal stability of Pd-Ag nanoparticles under the experimental conditions applied has been analyzed using scanning tunneling microscopy, which is also discussed in the paper.