In situ spectroscopic and theoretical investigation of methane activation on IrO2 nanoparticles: Role of Ir oxidation state on C-H activation

Iridium oxide (IrO2) shows a good potential for CH4 activation at mild conditions evidenced by density functional theory (DFT) and surface science studies. In this study, IrO2 NP (nanoparticles) of around 7 nm are found to activate CH4 at a temperature as low as -110 degrees C when Raman bands of surface species at 1328 and 1581 cm(-1) are observed, accompanied by the partial reduction of IrO2. DFT modeling of methane activation on IrO2 indicates that CH3, CH2Obr, H2Obr, and HObr (O-br, bridging oxygen between two Ir) are the more abundance surface species and the calculated vibrational frequencies of these species consistently explain the observed results of in situ Raman and in situ DRIFTS. The CH4 activation temperature on Ir NP of around 5.4 nm is found nearly 200 degrees C higher than that over IrO2 NP, confirming the superior activity of IrO2. DFT modeling demonstrates the high CH4 reactivity over IrO2 (2 1 1) and the even higher activity of partially reduced IrO2 . This evidences the important role of the oxidation state of Ir in determining the catalytic activity and it implies that tuning Ir oxidation state and/or designing Ir-oxide interface may facilitate the fabrication of value-added chemicals from methane over iridium-based catalysts. (C) 2020 Elsevier Inc. All rights reserved.