Ruthenium thin film under methanation atmosphere analyzed by x-ray photoelectron spectroscopy

Ruthenium thin films were prepared by DC magnetron sputtering on silicon (111) substrates. Samples were subjected to different temperature conditions (300, 400, 500, and 600 °C) under a flow of hydrogen and carbon dioxide with a 4:1 molar ratio to emulate the atmosphere of the methanation reaction. Each treatment was performed in situ. X-ray photoelectron spectroscopy was used to examine the surface chemical changes on Ru thin films before and after the treatments. Survey spectra, Ru 3d, and O 1s core level spectra were measured for each sample. Results showed that the O 1s spectra presented significant changes in all samples; before the thermal treatments, this signal has four different oxygen species, where it can be appreciated the existence of nonstoichiometric oxidized ruthenium. The latter disappears after the first heat treatment due to the high reducibility of ruthenium. Subsequently, in the following treatments, ruthenium oxide reappears, in smaller quantities when compared with the film without treatments. However, the highest amount of oxidized ruthenium can be observed in the temperature range (400–500 °C) during the treatments, in which the conversion of CO₂ is maximum for Ru supported catalysts.