(10e) Water-Gas Shift Catalysis Over Supported Platinum Nanoparticles

The water-gas shift (WGS) reaction (CO + H₂O → CO₂ + H₂) is an important industrial chemical process for hydrogen production. Promoted supported Pt catalysts have turnover rates (TOR’s) comparable to the industrial Cu/ZnO/Al₂O₃ catalysts but are more robust.

The WGS TOR’s, normalized by surface Pt atoms, for Pt/Al₂O₃, Pt/SiO₂, Pt/ZrO₂, and Pt/TiO₂ catalysts are independent of average Pt particle size. The apparent reaction orders for Ptcatalysts do not vary with particle size. Thus, all surface Pt atoms exhibit the same rate. The WGS TOR and H₂O order (in parenthesis) vary as Pt/Al₂O₃ (0.93) ~ Pt/SiO₂ (0.84) < Pt/TiO₂ ~ Pt/ZrO₂ ~ Pt/CeO₂ (0.66-0.72) at 300 °C. The CO, CO₂ and H₂ orders do not vary with the support. Further, the TORs for Pt/ZrO₂ and Pt/TiO₂ catalysts decrease by 125 and 10 times their original TOR, respectively, upon addition of 70 Atomic Layer Deposition (ALD) cycles of Pt, ~ 75 wt% Pt. This is due to excess Pt coverage, as shown from transmission electron microscopy (TEM) images, that limits the number of support sites available for H₂O activation. We interpret these data to show that the support plays a direct role in activating H₂O.

In addition, the WGS reaction rate per total mole of Pt for Pt/Mo₂C is 160 times higher than Pt/CeO₂ at 120 °C. Unlike the oxide supports, Mo₂C by itself has a WGS reaction rate per gram of catalyst (3×10^-7 mol H₂ (g cat)^-1 (s)^-1) comparable to the commercial Cu/ZnO/Al₂O₃ catalyst at 120 °C, 7% CO, 8.5% CO₂, 22% H₂O, 37% H₂ and balance Ar. This rate is promoted by a factor of 5-6 by the deposition of Pt over Mo₂C. The comparison of WGS reaction kinetics over Mo₂C and Pt/Mo₂C indicates a modification of nature of the dominant active sites, from Mo₂C to the Pt or Pt-Mo₂C interface for Pt/Mo₂C catalyst.