Abstract
This study aims to identify the role of the various electronic states of gold in the catalytic behavior of Au/MxOy/TiO2 (where MxOy are Fe2O3 or MgO) for the liquid phase oxidation of n-octanol, under mild conditions. For this purpose, Au/MxOy/TiO2 catalysts were prepared by deposition-precipitation with urea, varying the gold content (0.5 or 4 wt.%) and pretreatment conditions (H2 or O2), and characterized by low temperature nitrogen adsorption-desorption, X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDX), scanning transmission electron microscopy-high angle annular dark field (STEM HAADF), diffuse reflectance Fourier transform infrared (DRIFT) spectroscopy of CO adsorption, temperature-programmable desorption (TPD) of ammonia and carbon dioxide, and X-ray photoelectron spectroscopy (XPS). Three states of gold were identified on the surface of the catalysts, Au0, Au1+ and Au3+, and their ratio determined the catalysts performance. Based on a comparison of catalytic and spectroscopic results, it may be concluded that Au+ was the active site state, while Au0 had negative effect, due to a partial blocking of Au0 by solvent. Au3+ also inhibited the oxidation process, due to the strong adsorption of the solvent and/or water formed during the reaction. Density functional theory (DFT) simulations confirmed these suggestions. The dependence of selectivity on the ratio of Brønsted acid centers to Brønsted basic centers was revealed.
Highlights
IntroductionNumerous oxidizing reagents (including toxic, expensive, stoichiometric oxidants [3,4]), along with the use of harmful solvents and harsh reaction conditions, have been employed to accomplish this transformation
The selective oxidation of alcohols to valuable carbonyl compounds is one of the most crucial transformations in the chemical industry, for both laboratory and industrial manufacturing [1,2].Traditionally, numerous oxidizing reagents, along with the use of harmful solvents and harsh reaction conditions, have been employed to accomplish this transformation
The results showed that the gold content, support nature and the pretreatment atmosphere significantly affected the catalytic properties of gold catalysts in the liquid phase oxidation of n-octanol
Summary
Numerous oxidizing reagents (including toxic, expensive, stoichiometric oxidants [3,4]), along with the use of harmful solvents and harsh reaction conditions, have been employed to accomplish this transformation. These methods lead to environmental pollution and economic problems [5,6,7,8]. Supported gold catalysts have proved to be efficient in the liquid phase oxidation of alcohols, due to their higher selectivity and better resistance to deactivation, compared to their conventional noble metal counterparts [14,15,16,17]. The effect of a number of parameters, such as gold content, particle size of gold, the preparation method, gold electronic state, influence of support modifiers, redox pretreatment, etc., have not been investigated in detail
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