Investigation of dissociative CO2 adsorption properties on Pt-group surfaces with ambient pressure X-ray photoelectron spectroscopy

Abstract

Dissociative CO2 adsorption properties on Pt-group metal surfaces, i.e., Pt(111), Pd(111), Cu(111), and Pt3Sn(111), were investigated with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) under elevated CO2 pressure at room temperature (RT). Under low pressure regime, ~10-6 Torr CO2, no significant CO2 dissociation takes place on Pt-group metal surfaces. On the other hand, as CO2 pressure reaches 10-3 Torr CO2, dissociated CO adsorption occurs on Pt(111), Pd(111), and Pt3Sn(111) surfaces. On Cu(111) surfaces, CO2 directly adsorbs first, and dissociated O atoms start to adsorb under 25 mTorr CO2 gas pressure. Even though dissociative CO2 adsorption properties are different on each Pt-group surface, critical enhancement of adsorbate feature above certain pressure regime indicates the presence of pressure gap on Pt-group metal surfaces. After characterizing the dissociative CO2 adsorption behavior on Pt-group surfaces, CO2 hydrogenation reaction on Pt-group surfaces was monitored under 250 mTorr CO2 and 750 mTorr H2 at elevated temperature. During CO2 hydrogenation reaction, adsorbed CO species are removed from the surface with the formation of hydrocarbon and C0 species, implying the interaction between CO2 and H2 molecules occurs on Pt-group surfaces. ii However, formation of value-added product, e.g., CH4, CH3OH, COOH, etc., is not observed on the surface with AP-XPS and residual gas analyzer (RGA) because the amount of generated products is extremely limited. On Pt3Sn(111) alloy surfaces with (2×2) and (√3×√3) structure, desorption of adsorbed CO species begins at lower temperature, ~400 K. Desorption temperature of CO species on Pt3Sn(111) surface is much lower than that on Pt(111) surface, ~500 K, revealing that Sn incorporation plays a key role for modifying the strength of bond between dissociated CO and surface of the catalyst. Also, on Pt3Sn(111) oxide surface prepared by selective oxidation of Sn, both direct CO2 adsorption on the lattice oxygen site of SnO layer and dissociated CO adsorption on Pt atoms were observed even under 10-6 Torr CO2. It is identified that oxide layer can be an important reaction site for CO2 interaction. Overall, our results provide a fundamental understanding of dissociative CO2 adsorption characteristics on various Pt-group surfaces, which can contribute to fabrication of effective Pt-group-based catalysts for CO2 reduction.