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Combining XPS and ToF-SIMS for assessing the CO oxidation activity of Au/TiO2 catalysts

Chenakin, Sergey, Kruse, Norbert
Journal of catalysis 2018 v.358 pp. 224-236
X-ray photoelectron spectroscopy, ambient temperature, carbon monoxide, catalysts, catalytic activity, gold, hydroxylation, mass spectrometry, nanogold, oxalates, oxidation, recipes, titanium dioxide
Au/TiO2 catalysts were produced by deposition-precipitation using different preparation conditions. Emphasis was laid on producing TiO2 supports via oxalate precursors in the presence or absence of alkali and/or alkali-earth elements to vary on purpose the catalysts’ morphological state, surface composition and electronic structure. The home-made catalysts and a commercial Au/TiO2 were analyzed in a comparative manner by mainly X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to identify the key parameters responsible for their high activity in the CO oxidation at room temperature. Among these parameters, which were strongly influenced by the catalyst preparation conditions, the abundance of reduced Ti³⁺ species was ascertained to be overarching along with the availability of negatively charged Au nanoparticles. The abundance of O 2p bonding orbitals in the valence band of TiO2, which was modified by the presence of Au particles, and the extent of the support hydroxylation were also found to have a positive effect on the reaction rate. We further demonstrate that it is the specific area occupied by gold particles on the support surface that has to be taken into account to establish a reliable dependence of the reaction rate on the XPS-derived Au/Ti atomic ratio. All these factors have to be considered when assessing the activity performance of Au/TiO2 catalysts prepared and activated according to different recipes. After-reaction studies, which were also performed with a selected home-made catalyst, show that a slight decrease in its catalytic activity with time-on-stream can be mainly attributed to an appreciable accumulation of various carbon-containing species on the support surface along with a loss of OH groups. In summary, we show that combined XPS/ToF-SIMS studies provide a reliable guide for tailoring the preparation of high-performance Au-supported catalysts.