Spectrokinetic investigation of reverse water-gas-shift reaction intermediates over a Pt/CeO2 catalyst

The reactivity of the surface species present over a 2%Pt/CeO2 catalyst during the reverse water-gas-shift (RWGS) reaction was investigated by a detailed operando spectrokinetic analysis. A single reactor common for the kinetic and the spectroscopic measurements was used. The reactor employed was a modified high-temperature diffuse reflectance FT-IR (DRIFT) cell from SpectraTech. The reactivity of the surface species was monitored by DRIFT spectroscopy (DRIFTS) and mass spectrometry (MS) using steady-state isotopic transient kinetic analysis (SSITKA) techniques, i.e., switching between 1% CO2 + 4% H-2 reaction mixtures containing either (CO2)-C-13 or (CO2)-C-12. The combination of these techniques allowed time-resolved simultaneous monitoring of the variation of the coverage of C-12 and C-13-containing surface intermediates and the concentration of the gas-phase products (CO)-C-12(g) and (CO)-C-13(g) due to the isotope exchange. These results clearly indicated that surface formates observed by DRIFTS were not the main reaction intermediates for the formation of CO(g) over the present catalyst under these experimental conditions, although the formation of CO(g) from formates was likely to occur to a limited extent. A quantitative analysis of the number of reactive surface species also showed that Pt-bound carbonyls could not be the only reaction intermediate. Surface carbonates are shown as being a main surface intermediate in the formation of CO(g). A reaction scheme involving a direct reoxidation of the ceria support by the CO2 via surface carbonates is suggested. A parallel between these results and mechanisms previously proposed for CO2 hydrogenation and CO2-reforming (i.e., dry-reforming) of methane on redox oxide-supported noble metal is made. In a more general perspective, the present data underlines the feasibility and appropriateness of the DRIFT-MS-SSITKA technique based on a single reactor in providing critical information about the nature of surface species (e.g., kinetic intermediate as opposed to spectator) on catalysts when the surface species are observable by DRIFT spectroscopy.