(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.