Evidence of Enhanced Low Temperature Water-Gas Shift Rate with Sodium Promoted Pt/Zirconia-Based Catalysts Discovered by Combinatorial Methods
John M. Pigos1, Christopher J. Brooks1, Gary Jacobs2, and Burtron H. Davis2. (1) Honda Research Institute, 1381 Kinnear Rd, Columbus, OH 43212, (2) Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511
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.