Quantitative analysis of the reactivity of formate species seen by DRIFTS over a Au/Ce(La)O₂ water-gas shift catalyst: First unambiguous evidence of the minority role of formates as reaction intermediates

The reactivity of the species formed at the surface of a Au/Ce(La)O₂ catalyst during the water-gas shift (WGS) reaction were investigated by operando diffuse reflectance Fourier transform spectroscopy (DRIFTS) at the chemical steady state during isotopic transient kinetic analyses (SSITKA). The exchanges of the reaction product CO₂ and of formate and carbonate surface species were followed during an isotopic exchange of the reactant CO using a DRIFTS cell as a single reactor. The DRIFTS cell was a modified commercial cell that yielded identical reaction rates to that measured over a quartz plug-flow reactor. The DRIFTS signal was used to quantify the relative concentrations of the surface species and CO₂. The analysis of the formate exchange curves between 428 and 493 K showed that at least two levels of reactivity were present. "Slow formates" displayed an exchange rate constant 10- to 20-fold slower than that of the reaction product CO₂. "Fast formates" were exchanged on a time scale similar to that of CO₂. Multiple nonreactive readsorption of CO₂ took place, accounting for the kinetics of the exchange of CO₂(g) and making it impossible to determine the number of active sites through the SSITKA technique. The concentration (in mol g^-1) of formates on the catalyst was determined through a calibration curve and allowed calculation of the specific rate of formate decomposition. The rate of CO₂ formation was more than an order of magnitude higher than the rate of decomposition of formates (slow + fast species), indicating that all of the formates detected by DRIFTS could not be the main reaction intermediates in the production of CO₂. This work stresses the importance of full quantitative analyses (measuring both rate constants and adsorbate concentrations) when investigating the role of adsorbates as potential reaction intermediates, and illustrates how even reactive species seen by DRIFTS may be unimportant in the overall reaction scheme.