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The chemisorption properties of differently prepared model gold/titania interfaces have been compared with the aim of gaining a better understanding of the synergistic interplay between the constituents in gold/titania catalysts used in low-temperature CO oxidation. The structurally different gold/titania interfaces were prepared using various techniques, including wet chemical deposition (dip coating) and physical vapor deposition of TiO2 on flat and highly oriented Au(111)/mica films and immobilization of gold colloids on TiO2/Au(111)/mica films as well as on TiO2 powders. The low-temperature activity of small gold colloids anchored on films was corroborated by DRIFTS measurements. CO, CO2, and O2 adsorption/desorption studies were performed on the flat model catalysts with TDS, XPS, and ISS. All flat model systems did not show any significant CO adsorption. Oxygen desorption was evidenced by TDS. The adsorptive properties of powder model catalysts were investigated with DRIFTS, pulse thermal analysis, XPS, and ISS. CO adsorption on gold was weak and reversible in all cases and not significantly influenced by the presence of TiO2. Temperature-programmed desorption of CO2 indicated that CO2 was adsorbed if the systems were treated ex situ in oxygen at 673 K. The observed chemisorptive properties of the structurally different gold/titania interfaces support a mechanistic model for CO oxidation which is based on oxygen adsorption on vacancy sites of titania and CO adsorption on gold.

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The structure of IrO2 during the oxygen evolution reaction (OER) was studied by operando X-ray absorption spectroscopy (XAS) at the Ir L3-edge to gain insight into the processes that occur during the electrocatalytic reaction at the anode during water electrolysis. For this purpose, calcined and uncalcined IrO2 nanoparticles were tested in an operando spectroelectrochemical cell. In situ XAS under different applied potentials uncovered strong structural changes when changing the potential. Modulation excitation spectroscopy combined with XAS enhanced the information on the dynamic changes significantly. Principal component analysis (PCA) of the resulting spectra as well as FEFF9 calculations uncovered that both the Ir L3-edge energy and the white line intensity changed due to the formation of oxygen vacancies and lower oxidation state of iridium at higher potentials, respectively. The deconvoluted spectra and their components lead to two different OER modes. It was observed that at higher OER potentials, the well-known OER mechanisms need to be modified, which is also associated with the stabilization of the catalyst, as confirmed by in situ inductively coupled plasma mass spectrometry (ICP-MS). At these elevated OER potentials above 1.5 V, stronger Ir–Ir interactions were observed. They were more dominant in the calcined IrO2 samples than in the uncalcined ones. The stronger Ir–Ir interaction upon vacancy formation is also supported by theoretical studies. We propose that this may be a crucial factor in the increased dissolution stability of the IrO2 catalyst after calcination. The results presented here provide additional insights into the OER in acid media and demonstrate a powerful technique for quantifying the differences in mechanisms on different OER electrocatalysts. Furthermore, insights into the OER at a fundamental level are provided, which will contribute to further understanding of the reaction mechanisms in water electrolysis.