The State of the Working Catalyst on Alkali-Promoted Pt/Al2O3 and Pt/TiO2 for the Water Gas Shift Reaction
The search for active and stable low-temperature catalysts for the production of hydrogen has been subject of investigation due to the increasing interest in commercializing portable proton exchange membranes fuel cells (PEM) for power generation. The water gas shift (WGS) reaction (H2O + CO -> H2 + CO2) on noble metal (Pt, Au, Pd) catalysts is a suitable process for such technology as it offers greater stability during start-up and shut-down operations. However, these catalysts require developing formulations that can surpass the performance of the commercially available Cu-based catalysts.
Here, we report on the promotional effect of alkali additives (Na, Li, K) and we examined the nature of such promotion by combining kinetic analysis, x-ray absorption (XAS), transmission electron microscopy (TEM) and steady state isotopic transient methods (SSITKA). Among the alkali, Na showed the most exceptional enhancement of the turnover rate (TOR, 250 °C) up to 107 times that of Pt/Al2O3 and up to 4 times the TOR of Pt/P25. The alkali-promoted catalysts can therefore exhibit rates of reaction higher than the conventional Cu/ZnO/Al2O3 under fuel cell operating conditions. We found that the reaction kinetics for either support were identical upon the addition of Na, suggesting the creation of the same active sites. In situ XAS experiments under WGS conditions indicate that Pt remained metallic under WGS conditions in most of the catalysts. On samples where PtO was present no correlation was found between the rate and the fraction of Pt oxides. Transmission infrared experiments done in operando mode revealed important changes in the adsorption of CO on Pt, characterized by the appearance of low frequency bands. Additional bands were observed in the formate region, which we attributed to sodium formates. SSITKA measurements show that these formate species exchange 13C at the same rate as the product 13CO2, invoking a formate-type mechanism. Lastly, the estimation of the size of the “carbon pool” by SSITKA suggests that the role of Pt is to provide CO to other intermediates in the alkali-modified support where the rate determining step occurs. As the amount of alkali increases, the probability of Pt being surrounded by alkali increases and the size of the carbon pool largely exceeds the available surface area of Pt. Therefore, the promotion by alkali can be regarded simply as a support-driven effect.