Water-gas shift catalysis over supported gold and platinum nanoparticles
Abstract
The water-gas shift (WGS) reaction (CO + H2O → CO 2 + H2) is an important industrial chemical process for hydrogen production. Supported Au and Pt catalysts have turnover rates (TORs) comparable to the industrial Cu/ZnO/Al2O3 catalysts but are more robust. Our work focuses on determining the (1) active sites and (2) effect of support for supported Au and Pt nanoparticles. The WGS reaction rate per total mole of Au varies with average Au particle size (d) as d-2.7±0.1 for Au/TiO2, d-3.2±0.4 for Au/ZrO2, d-2.9±0.2 for Au/CeO 2, d-2.6±0.2 for Au/ZnO, and d-2.2±0.2 for Au/Al2O3 catalysts. The variations of reaction rate and apparent reaction orders with particle size were used to show that the active sites are low coordinated metallic corner and perimeter Au atoms. On the other hand, the WGS TORs, normalized by surface Pt atoms, for Pt/Al2O3, Pt/SiO2, Pt/ZrO2, and Pt/TiO2 catalysts are independent of average Pt particle size. Unlike for Au catalysts, the apparent reaction orders for Pt catalysts do not vary with particle size. Thus, all surface Pt atoms exhibit the same rate. The addition of Br at a level of 16% of the surface moles of Au to a 2.3%Au/TiO2 catalyst decreased its WGS reaction rate by six times. The addition of Br did not result in an appreciable change in the average Au particle size, apparent activation energy, or the reaction orders. From operando Fourier transform infrared (FTIR) spectroscopy experiments, the WGS reaction rate is proportional to the normalized peak area of CO adsorbed on metallic Au (IR peak at 2100 cm-1). Corner Au atoms were counted as the active sites by transient isotopic switch experiments. Thus, it was confirmed that metallic corner Au atoms are the dominant active sites for Au/TiO2 catalysts. The WGS reaction rate per total mole of Au and H2O order (in parenthesis) vary as Au/Al2O3 (∼ 0.6) < Au/CeO 2 (∼ 0.3) < Au/ZrO2 (∼ 0.0) < Au/TiO2 (∼ -0.3) at the same Au particle size at 120 °C. Similarly, for Pt catalysts, 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/TiO 2 catalysts decrease by 125 and 10 times their original TOR, respectively, upon addition of 70 atomic layer deposition (ALD) cycles, ∼ 75 wt.% Pt. This is due to excess Pt coverage that limits the number of support sites, shown from their transmission electron microscopy (TEM) images, available for H2O activation. Density functional theory (DFT) results show that the H2O activation barrier on rutile sites adjacent to Au (0.25 eV) is lower than on corner sites of unsupported Au nanoparticles (1.48 eV) and clean rutile TiO2 (110) surface (0.33 eV). We interpret these data to show that the support plays a direct role in activating H 2O. In conclusion, metallic, low-coordinated corner and perimeter Au atoms and metallic surface Pt atoms were identified as the active sites of supported Au and Pt catalysts for the WGS reaction, respectively. The WGS reaction kinetics and DFT results were used to determine that the effect of support on WGS reaction rates over supported Au and Pt catalysts is caused by its direct participation in activating water molecules.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.