(600by) Dispersed Pt/Ceria Catalysts for Water-Gas Shift Prepared Using Supercritical Fluid Deposition

Hydrocarbons are currently the most viable source of hydrogen.  Synthesis gas, or syngas, is a fuel gas generated through steam reforming, gasification, or partial oxidation of fossil fuels.  Syngas is composed largely of carbon monoxide and hydrogen, along with several other compounds depending on the source of the gas.  To use synthesis gas as a feed to a fuel cell, the carbon monoxide concentration must be reduced significantly, approximately 10 – 20 ppm, to prevent poisoning of the cell and maintain performance.  The water-gas shift (WGS) reaction reduces the CO content of the stream while simultaneously increasing H₂ content. 

The water gas shift reaction (WGSR), CO + H₂O ↔ H₂ + CO₂, is a slightly exothermic reaction that is very important in the processing of feed streams for fuel cells.  Industrial application of the WGS reaction is normally done in two steps, both a high temperature and low temperature shift.  At low temperatures, high conversions are achievable but are limited due to slow kinetics.  At high temperatures, reaction rates are faster; however, conversion is limited due to equilibrium constraints.  Current industrial catalysts are not suitable for fuel cell feed purification due to both technical and safety constraints.  The operating temperature of a fuel cell is far lower than the temperature required for efficent use of these catalysts.  To account for low activity, large amounts of catalyst must be used leading to non-viable reactor volumes for fuel cell applications.  The pyrophoric nature of these catalysts as well as their thermal instability also makes them ill-suited for fuel cell use.  There is a need to develop a catalyst that can operate nearer normal fuel cell operating conditions, normally in the range of 80°C – 120°C, while maintaining its surface integrity and activity for long term performance.

Noble metals have been shown to catalyze the WGS, and platinum, specifically, shows a higher activity.  The addition of ceria to platinum catalysts has been shown to have synergistic effects on the activity for WGS, including both increased activity and longer stability.  Research has shown that these enhancements are dependent on the degree of interface between platinum and ceria.  In this study, we investigate the use of supercritical fluid deposition (SCFD) to prepare catalysts of highly dispersed platinum on supported ceria.  SCFD takes advantage of the excellent transport properties of supercritical fluids to facilitiate the uniform adsorption of organometallic precursors on a support surface.  The precursors then undergo calcination and reduction treatments to form metal nanoparticles.

 In our work, we investigate the effects of pretreating alumina-supported ceria prior to depositing platinum precursor, with the goal of maximizing the contact area between platinum and ceria.  Time required for deposition to occur is related using atomic emission spectroscopy for elemental analysis. Temperature-programmed reduction is used to characterize metal/support interactions, and CO pulse titration is employed to evaluate the dispersion of platinum on the surface.   Catalyst activity for WGS is measured in a tubular flow reactor.  Both reaction rates and surface analyses are correlated with catalyst preparation variables.