(435c) Evidence of Enhanced Low Temperature Water-Gas Shift Rate with Sodium Promoted Pt/Zirconia-Based Catalysts Discovered by Combinatorial Methods

Through combinatorial catalytic testing, we observed that adding the alkali metal Na to based Pt/ZrO₂ catalysts enhances the rate of the water-gas shifts reaction. Some research has proposed that the C-H bond breaking of the formate is the rate-limiting step of the low temperature water-gas shift mechanism. To understand the effect of Na on the Pt-promoted catalyst (and on the mechanism), we carried out DRIFTS analysis of the Pt/ZrO₂ and PtNa/ZrO₂ catalysts using three approaches. They included (1) dynamic formate coverage studies during steady state water-gas shift at 300°C and 225°C, (2) transient forward formate decomposition tests at low temperature in steam, and (3) formate H-D exchange studies in the absence of steam (i.e., reverse decomposition). First, steady state water gas shift testing at 225°C indicated that the coverage of formate was more limited for the PtNa/ZrO₂ catalyst, suggesting that it was reacting faster. Secondly, in transient formate decomposition tests at 130°C in steam, forward decomposition to carbonate species was observed, and the formates on PtNa/ZrO₂ decomposed to 20 % of their initial value in 1/2 the time in comparison with Pt/ZrO₂. Finally, the slow thermal (i.e., reverse) decomposition of formate was carried out at 225°C in the presence of D₂. The exchange of the formate C-H to C-D bond was followed. The half-life was 10 min. for the PtNa/ZrO₂, in comparison with the value of 20 min. obtained for the Pt/ZrO₂ catalyst. In each study, the decomposition rate of formate was much more facile for the Na promoted catalyst relative to Pt/ZrO₂ alone. The results suggest that electronic modification of formate was achieved, leading to an important weakening of the formate C-H bond.