Water-gas shift reaction over supported Au nanoparticles

Abstract

The water–gas shift (WGS) reaction rates per total mole of Au at 120 °C, 7% CO, 22% H2O, 8.5% CO2, 37% H2 decrease in the order Au/Anatase ∼ Au/Anatase001 (uniform anatase TiO2 single crystals with 64 per cent of the more {001} facets) ∼ Au/P25 ∼ Au/P25-WGC (obtained from the World Gold Council) ∼ Au/Rutile > Au/ZrO2 > Au/CeO2 > Au/ZnO when compared at the same number average Au particle size (d) and vary as ∼ d-3. From high resolution transmission electron microscopy images, the geometry of Au nanoparticles on these catalysts resembled truncated cubo-octahedra. A physical model of Au nanoparticles as truncated cubo-octahedra was used to calculate that the fractions of surface, perimeter and corner sites to the total Au sites vary as d-0.7, d-1.8 and d-2.9, respectively. Thus, the variation in the WGS reaction rate per total mole of Au (∼d-3) correlates well with the corner sites (d-2.9) allowing us to determine that the dominant active sites for these catalysts are the low coordinated metallic corner Au sites. In addition, as the apparent H2O order increases and the apparent activation energy decreases, the WGS reaction rate per total mole of Au systematically decreases for Au nanoparticles supported on anatase, anatase001, P25, rutile, ZrO2, CeO2, ZnO and Al2O3 at near 120 °C. Density functional theory calculations were carried out over Au nano-rods supported on rutile TiO2(100) and α-Al2O3(0001) surfaces to elucidate the differences in reactivity for the most reactive (Au/TiO2) and least reactive (Au/Al2O3) catalysts. Water preferentially adsorbs and dissociates at the Lewis acid- Lewis base Ti4+-Ob and Al3+-Ob site pairs at the Au/TiO2(110) and Au/α-Al2O3 interfaces with activation energies of 0.25 eV and 0.20 respectively. These barriers are significantly lower than those to activate water on the corner sites of unsupported Au nanoparticles (1.48 eV). The prediction that both the TiO2 and Al2O3 readily dissociate water supports the experimental findings that the support plays an important role in activating the water in the WGS reaction. The subsequent oxidation of CO appears to proceed at the Au/support interface via the reaction of CO adsorbed on Au with the OH on the support. DFT results indicate that OH binds 0.4 eV stronger to Al3+ sites at the Au/Al2O3 interface perimeter than to Ti4+ sites at the Au/TiO2 interface perimeter. As such, the barrier for OH to react with CO at an adjacent Au site is 0.52 eV higher for Au/Al2O3 (0.85 eV) than for Au/TiO2 (0.33 eV). The theoretical results suggest that the oxidation of CO is rate-limiting on Al2O3 whereas both the water dissociation as well as CO oxidation limit the rate on TiO2. This is consistent with the near first order CO dependence of the measured rates over both Au/TiO2 and Au/Al2O3 as well as the changes in the water reaction orders over Au/TiO2 (−0.2 to −0.5) and Au/Al2O3 (0.5 to 0.8).