Isotopic transient studies of sodium promotion of Pt/Al₂O₃ for the water-gas shift reaction

The promotional effect of sodium on Pt/Al₂O₃ catalysts for the water-gas shift (WGS) reaction was investigated with operando FTIR during steady-state and isotopic transient experiments. The highest turnover frequency (TOF) promotion per surface Pt occurred on a 0.82 wt.% Pt/Al₂O₃ catalyst with a 30:1 molar ratio of Na:Pt. This catalyst exhibited a TOF of 0.35 s^-1 at 250 °C and 7% CO, 11% H₂O, 9% CO₂, and 37% H₂ compared to 0.43 × 10^-2 s^-1 on Pt/Al₂O₃. Operando IR revealed that Na addition modifies CO adsorption on Pt to create more strongly bound, multiply-bonded CO species at 1768 cm^-1 and 1697 cm^-1 compared to predominantly linear CO on Na-free Pt/Al₂O₃ at 2060 cm^-1. Transient experiments with ¹²CO/¹³CO isotope switches showed that the number of carbon-containing active intermediates increased from less than 1% of the surface Pt for Na-free Pt/Al₂O₃ to nearly 100% of the surface Pt for 20Na:Pt/Al₂O₃, which indicates that only a small fraction of the Pt surface on the Na-free samples participates in the WGS reaction. From time-resolved IR spectra during transient WGS with ¹²CO/¹³CO and ¹²CO₂/¹³CO₂ isotope switches, we propose that surface formate species are spectators for all Na-promoted Pt/Al₂O₃ catalysts under these WGS conditions. The results suggest that Na promotes Pt/Al₂O₃ for WGS by modifying the local electronic properties of Pt and creating new H₂O activation sites, which provide greater availability of surface OH/H species to react with nearly all CO on the metallic Pt surface through a non-formate pathway.