Unique reaction mechanism of preferential oxidation of CO over intermetallic Pt3Co catalysts: surface-OH-mediated formation of a bicarbonate intermediate

Abstract

A mechanistic study was performed regarding the preferential oxidation of CO in excess H₂ (PROX) over a Pt₃Co intermetallic compound supported on various metal oxides (Pt₃Co/MO_x: MO_x = SiO₂, Al₂O₃, MgO, CaO, and La₂O₃). The Pt₃Co/MgO catalyst exhibited the highest catalytic activity (97% CO conversion at 100 °C). CO chemisorption analysis revealed that (1) the Pt dispersion differed depending on the nature of the support and preparation conditions, and (2) the catalytic activity strongly depended on the Pt dispersion but not on the acid–base properties of the support. The kinetic study suggested that CO and O₂ adsorbed competitively on Co sites. In situ Fourier transform infrared analysis using D₂ indicated that, at a low temperature (<60 °C), a bicarbonate species was formed as a reaction intermediate from CO, O₂, and hydrogen that was supplied by the surface hydroxyl groups, not gas phase H₂, followed by its decomposition to CO₂ and H₂O in a 1 : 1 ratio. At a high temperature (>80 °C), gas phase H₂ is likely to participate in the formation of the bicarbonate-like intermediate. This study provides the first spectroscopic evidence of bicarbonate formation as an intermediate and the contribution of surface hydroxyl groups toward catalysis. The proposed reaction mechanism based on bicarbonate formation is unique compared with those reported for other PROX systems.