(312d) Water-Gas Shift Reaction on Catalysts Prepared Using Supercritical Fluid Deposition

Catalysts composed of noble metals and reducible metal oxides are of great significance for modern energy applications.  These applications include key reactions in hydrogen production such as water-gas shift (WGS) and preferential oxidation (PROX), as well as fuel cell electrodes.  A major impediment to implementing hydrogen fuel cell technology is the need for safe, reliable, and efficient catalytic hydrogen generation for logistical small-scale operations.  WGS is a crucial step in hydrogen enrichment, and effective catalysts that have been identified are platinum or gold in conjunction with ceria or other partially reducible oxides.  For economic feasibility, minimizing the amount of platinum or gold required is vital.   Recent research provides evidence that catalysts having extremely low loadings of these metals can still exhibit high activity, as the degree of reaction rate enhancement depends on interfacial area between active metal sites and oxides.  Thus, methods of synthesizing supported catalysts with intimate contact between the two are essential.

This paper focuses on the synthesis and characterization of Pt/ceria WGS catalysts prepared using supercritical fluid deposition (SCFD), a technique by which organometallic precursors are adsorbed from a supercritical fluid solvent on to a support.  Supercritical fluids (SCFs) offer unique advantages for interfacial deposition processes.  Liquid-like density promotes dissolution of metal precursors; viscosity and diffusivities more akin to those of gases facilitate both transfer through the SCF and surface adsorption.  These benefits make possible the reproducible, uniform distribution of organometallic precursors on the support.  In our work, the support is high-surface area alumina to which ceria has been added by insipient wetness followed by calcination.  The platinum is deposited using SCFD of platinum acetylacetonate from carbon dioxide, with subsequent calcination in air and reduction in hydrogen.   The nominal loadings are 5% ceria and 1% platinum.  Variations in preparation include reductive pretreatment of the ceria before Pt deposition, intended to disrupt the surface with oxygen vacancies that will ultimately enhance the deposition of Pt precursor on ceria-rich areas of the alumina surface.

Results of surface analyses, including temperature programmed reduction and carbon monoxide pulse titrations, indicate that the preparation results in both small nanoparticles (1-3 nm) and highly dispersed platinum.  Further evidence for this is obtained using transmission electron microscopy.  Catalyst activity testing takes place a flow reactor system using a simulated reformate stream containing CO, H₂, H₂O, and N₂.  Turnover frequencies and activation energies for WGS on catalysts will be presented and correlated with variables in synthesis method.