FTIR, UV−Vis, and HRTEM Study of Au/ZrO2 Catalyst: Reduced Reactivity in the CO−O2 Reaction of Electron-Deficient Gold Sites Present on the Used Samples

Identification and Characterization of a Au(III) Reductase from Erwinia sp. IMH

Microorganisms contribute to the formation of secondary gold (Au) deposits through enzymatic reduction of Au(III) to Au(0). However, the enzyme that catalyzes the reduction of Au(III) remains enigmatic. Here, we identified and characterized a previously unknown Au reductase (GolR) in the cytoplasm of Erwinia sp. IMH. The expression of golR was strongly up-regulated in response to increasing Au(III) concentrations and exposure time. Mutant with in-frame deletion of golR was incapable of reducing Au(III), and the capability was rescued by reintroducing wild-type golR into the mutant strain. The Au(III) reduction was determined to occur in the cytoplasmic space by comparing the TEM images of the wild-type, mutant, and complemented strains. In vitro assays of the purified GolR protein confirmed its ability to reduce Au(III) to Au nanoparticles. Molecular dynamic simulations demonstrated that the hydrophobic cavity of GolR may selectively bind AuCl₂(OH)₂ ^-, the predominant auric chloride species at neutral pH. Density functional theory calculations revealed that AuCl₂(OH)₂ ^- may be coordinated at the Fe-containing active site of GolR and is probably reduced via three consecutive proton-coupled electron transfer processes. The new class of reductase, GolR, opens the chapter for the mechanistic understanding of Au(III) bioreduction.

CO Oxidation on Planar Au/TiO2 Model Catalysts under Realistic Conditions: A Combined Kinetic and IR Study

The oxidation of CO on planar Au/TiO2 model catalysts was investigated under pressure and temperature conditions similar to those for experiments with more realistic Au/TiO2 powder catalysts. The effects of a change of temperature, pressure, and gold coverage on the CO oxidation activity were studied. Additionally, the reasons for the deactivation of the catalysts were examined in long-term experiments. From kinetic measurements, the activation energy and the reaction order for the CO oxidation reaction were derived and a close correspondence with results of powder catalysts was found, although the overall turnover frequency (TOF) measured in our experiments was around one order of magnitude lower compared to results of powder catalysts under similar conditions. Furthermore, long-term experiments at 80 °C showed a decrease of the activity of the model catalysts after some hours. Simultaneous in-situ IR experiments revealed a decrease of the signal intensity of the CO vibration band, while the tendency for the build-up of side products (e. g. carbonates, carboxylates) of the CO oxidation reaction on the surface of the planar model catalysts was rather low.

Carbon-protected Au nanoparticles supported on mesoporous TiO2 for catalytic reduction of p-nitrophenol

With the introduction of carbon on Au/TiO₂, the reaction rate of C/Au/TiO₂ increased by 29% and the stability enhanced by about 3 times more than Au/TiO₂ in the p-nitrophenol reduction reaction. Carbon species enhanced the stability of Au nanoparticles and also increase the organic reactants adsorptive ability.

Highly dispersed gold on zirconia: characterization and activity in low-temperature water gas shift tests

Gold-loaded zirconia and sulfated zirconia catalysts were tested in the low-temperature water gas shift reaction. The samples were characterized by N2 adsorption analysis, temperature-programmed reduction, X-ray diffraction, pulse-flow CO chemisorption, FTIR spectroscopy, and high-resolution transmission electron microscopy. A reference catalyst, Au/TiO2, provided by the World Gold Council was investigated for comparison. CO chemisorption and FTIR data indicate the presence of only highly dispersed gold clusters on the sulfated sample and both small clusters and small particles on the non-sulfated sample. Both gold-zirconia catalysts are much more active than the Au/TiO2 reference sample over all the temperature range investigated. The sample prepared on sulfated zirconia exhibits higher stability than the catalyst on unmodified zirconia. The prominent role in the water gas shift reaction of gold clusters in close contact with the support was deduced.