Differences in the reduction kinetics of thin RuO2 layers in atomic and molecular hydrogen revealed by in situ photoemission

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The evolution of thin RuO 2 layers on Ru(0001) during exposure to atomic and molecular hydrogen was compared using in situ X-ray photoelectron spectroscopy (XPS) to improve the understanding of hydrogen uptake and the kinetics of oxide reduction. Time-resolved O 1s spectra separate a hydrogen-induced hydroxyl component from the intrinsic RuO 2 line shape obtained after anhydrous oxidation. The different peak position compared to bridging surface OH in literature, the large OH peak area, and its small probing depth dependence suggest the formation of subsurface OH species during H* exposure. During H 2 exposures at 2.5 × 10 −5 mbar, mild hydroxylation is observed at room temperature and at 80 °C, but it proceeds slowly and saturates after < 30 min. Exposure to atomic hydrogen cracked at the same H 2 background pressure yields significantly larger hydroxyl peaks, which continue to increase slowly even after more than three hours. At 105 °C, a 4.5 nm thick oxide layer is removed within 200 min by H 2 , whereas the oxide is removed in approximately 20 min by atomic hydrogen. The different (sub)surface species and their time evolution support prior reports that assign hydrogen splitting as rate-limiting step for low-temperature reduction of RuO 2 by H 2 , while highlighting that, for H*, oxygen mobility may already be relevant significantly below the reported peak temperature of water desorption.