(130c) The Use of Isotopic Transient Techniques to Investigate the Nature of Alkali Promotion for the Water-Gas Shift Reaction On Pt Catalysts

We report on the promotional effect of alkali additives (Na, Li, K) on the water gas shift (WGS) reaction by combining kinetic analysis, x-ray absorption spectroscopy (XAS) and steady state isotopic transient methods.  Among the alkali metals, 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 turnover rates higher than the conventional Cu/ZnO/Al₂O₃ under fuel cell operating conditions.  We found that the trends in reaction kinetics for catalysts on either Al₂O₃ or P25 (TiO₂) supports 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.  Therefore, the promotion by alkali can be regarded simply as a support-driven effect.

Operando IR experiments also confirmed that the majority of CO adsorbed on Pt are on metallic Pt for Na-doped and undoped Pt/Al₂O₃ under similar WGS conditions.  By increasing the Na loading, more linear bonded CO on Pt are changed to bridging bonded CO and tri-folded bonded CO.  Isotopic transient experiments with ¹³CO measured the carbon-related surface active intermediates, N_COx, under reaction conditions. The results show that N_COx increases from less than 1% of surface Pt for the Na-free catalyst to over 50% for the Na-doped sample corresponding to a Na:Pt mole ratio of 12 and about 100% of surface Pt for the Na-doped samples corresponding to a Na:Pt mole ratio higher or equal to 20.  This result indicates that only a small fraction of the surface of Pt on the Na-free samples participates in the WGS catalytic path.  The addition of alkali facilitates water activation and modifies the adsorption of CO, such that most of the Pt surface can be used to catalyze the reaction.