Water-gas shift: Effect of Na loading on Pt/m-zirconia catalysts for low-temperature shift for the production and purification of hydrogen

Recently, Honda Research USA, Inc. and the University of Kentucky Center for Applied Energy Research researchers showed that Na doping of Pt/ZrO₂ catalysts can significantly improve the rate of water-gas shift (WGS). This was attributed to the electronic weakening of the carbon-hydrogen (C-H) bond of formate, with formate being the suggested intermediate of an “associative mechanism.” In this work, we sought to determine (1) how the Na dopant loading affects the steam-assisted formate decomposition and WGS reaction rates and (2) the nature of the electronic effect that weakens the C-H bond of formate as a function of Na dopant loading. We observed that doping Na to up to 1% by weight slightly increased the density of defect-associated bridging hydroxyl groups, which are active sites for producing formate from CO. However, at 2.5% Na, the optimum loading, there was a distinct step-change shift of the band corresponding to C-H stretching of formate to lower wavenumbers, consistent with bond weakening. Pt-carbonyl bands were still able to form at this loading, indicating the availability of Pt sites for dehydrogenating formate during H₂O-promoted formate decomposition. At 5% Na loading, while the formate C-H band remained at lower wavenumbers, Pt-carbonyl bands were severely attenuated, indicating a lack of availability of Pt metal sites at the higher loading. Thus, both steam-assisted formate decomposition and low-temperature WGS rates reached maxima at an optimum Na dopant loading of 2.5% Na. The results suggest that the transition state of formate decomposition involves H₂O, with Pt assisting by providing a porthole for H₂ removal, and with Na exerting a weakening effect on the formate C-H bond. The cleaving of this bond is the proposed rate-determining step of the cycle.