Determination of the Au active site and surface active species via operando transmission FTIR and isotopic transient experiments on 2.3 wt.% Au/TiO ₂ for the WGS reaction

The kinetics and the active site for the water gas shift (WGS) reaction were investigated by operando FTIR during steady state and isotopic transient experiments on a 2.3 wt.% Au/TiO ₂ catalyst. The WGS rate at 120 °C and activation energy measured in the operando IR reactor for the WGS reaction (7% CO, 11% H ₂O, 9% CO ₂, and 37% H ₂) on the Au/TiO ₂ catalyst were 1.1 ± 0.1 × 10 ^-2 (mol H ₂) (mol ^-1 Au) s ^-1 and 56 ± 3 kJ mol ^-1, respectively. We have shown that the kinetics measured over a pressed wafer in a transmission FTIR were similar to those measured in a regular plug flow reactor (PFR). Operando IR data were used to follow the catalyst deactivation as a function of time, and the amount of CO adsorbed on metallic Au species (Au ⁰, 2100 cm ^-1) correlated with the WGS rates, while CO adsorbed on partially negatively charged Au species (Au ^δ-, 2040 cm ^-1) did not. The rate loss did not correlate with the amounts of formate- and carbonate-like species measured by IR but was consistent with a loss of active sites by sintering. Measurements by isotopic transient experiments ( ¹³CO- ¹²CO) on the Au/TiO ₂ catalyst with an average Au particle size of 3 nm showed that less than 3% of the total surface Au atoms on the Au nanoparticles were responsible for the majority of the catalytic activity. Time-resolved IR spectra collected during the isotopic transient experiments showed that CO adsorbed on Au ⁰ species were involved in the reaction pathway, while the surface formate and carbonate species were spectators under these WGS conditions. Based on these results, we conclude that metallic corner Au atoms are the dominant active sites for WGS on Au/TiO ₂ catalysts and that CO adsorbed on Au ⁰ is an active intermediate. In addition, we present a detailed description of the design, construction, and operation of a transmission FTIR cell capable of performing operando kinetic measurements and isotope switching experiments with a signal decay time of about 95% in 1.5 s.