DFT Calculations of the Adsorption States of O 2 on OH/H 2 O-Covered Pt(111)
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ORIGINAL RESEARCH
DFT Calculations of the Adsorption States of O2 on OH/H2O-Covered Pt(111) Juan A. Santana 1
# Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract The adsorption of O2 on Pt(111) was studied with density functional theory calculations. Various adsorbed states of O2 were evaluated on clean and OH/H2O-covered Pt(111) surfaces at the solid/gas and solid/liquid interfaces. The results reveal that the adsorption of O2 on OH/H2O-covered Pt(111) surface starts with the physical adsorption of O2. Two other adsorption states are reachable from the physisorbed state, the end-on, and bridging chemisorbed O2. The analysis of the energetics of these adsorption states shows that O2 physically adsorbed at the OH/H2O-covered Pt(111) surface is a high-energy state that requires activation to transition to the end-on chemisorbed O2 state. On the other hand, the end-on chemisorbed state can transition to the bridging chemisorbed state with only a small activation energy when a nearby Pt adsorption site is available. Frequency analysis of the physisorbed, end-on, and bridging adsorption states shows that adsorbed O2 stretching frequencies are close to 1400, 1300, and 900 cm−1, respectively. Keywords Electrochemical adsorption . ORR catalysis . Hydrogen fuel cell . Computational electrochemistry . Physisorbed O2
Introduction Many experimental and theoretical works have been conducted to elucidate the reaction mechanism of the oxygen reduction reaction (ORR) over metal surfaces. The research work has been mainly on Pt and Pt-based surfaces due to their high activity and stability toward the ORR [1]. The extensive experimental and theoretical data have revealed many atomiclevel details, in particular for the gas-phase adsorption and dissociation of O2 on Pt surfaces [2–6]. At the Pt electrode/ electrolyte interface, the experimental data concerning the adsorbed state of the initial intermediate species in the ORR (O2ads, O2−ads, and O22−ads) are more limited [7, 8]. Adzic and Wang studied the inhibition of ORR on Pt(111) electrodes with Ag adatoms and showed that O2 was adsorbed at a bridge site [8]. An important step forward to characterize the adsorbed state of the initial intermediate species at the Pt/ electrolyte interface has been the identification of an O2 stretching band at 1005–1016 cm−1 [7]. Employing surface-
* Juan A. Santana [email protected] 1
Department of Chemistry, University of Puerto Rico, P. O. Box 372230, Cayey, PR 00737-2230, USA
enhanced infrared reflection and adsorption spectroscopy (SEIRAS) in an attenuated reflection mode (ATR), Shao et al. studied the ORR on Pt electrodes in alkaline media [7]. They assigned the observed band at 1005–1016 cm−1 to superoxide intermediates (O2−ads) occupying a bridge site on the Pt surface. Their findings represented the first experimental evidence of O2 molecular entities adsorbed on Pt electrodes under electrochemical conditions. Kunimatsu et al. observed an infrared (IR) absorption band at 1400–1403 cm−1 associated with the
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