Selective oxidation of ethanol in the liquid phase over Au/TiO₂

Abstract

The selective oxidation of aqueous ethanol solutions with air over two commercial 1.5 and 1 wt% Au/TiO2 catalysts was investigated in six stirred mini-autoclaves operated in parallel. The catalysts were characterised by various techniques including elemental analysis, N2 physisorption, X-ray diffraction and transmission electron microscopy (TEM). Temperature, pressure, ethanol concentration, catalyst concentration and reaction time were varied in the batch experiments to study the reaction kinetics. It was possible to confirm the generally accepted mechanism of primary alcohol oxidation, in which acetaldehyde is a primary product of the oxidation that quickly undergoes further transformation to acetic acid. In presence of both acetic acid and ethanol the formation of ethyl acetate takes place until equilibrium conditions are reached. The high yields of acetic acid can be rationalised by the inhibited total oxidation of acetic acid under the applied reaction conditions. Improper storage of the gold catalysts in air exposed to light was found to lead to an irreversible change of the performance, which cannot be restored by means of recalcination. Sintering and blocking of surface sites by deposits were ruled out as possible causes for the deactivation

Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy

“…Gold nanoparticles were assumed to be hemispheres, which has been suggested as appropriate representation for 2-4 nm sized gold nanoparticles on rutile surfaces [55], also being in agreement with recent HR-TEM observations on Au/TiO 2 catalysts [26]. Our calculation (See Supplementary Information) yielded a distance of about 100 nm, so on the probed area in one line scan (200 spots of ~1 µm diameter), we expect to see approximately 1.6 × 10 4 Au nanoparticles. If we neglect mutual coverages of TiO 2 powder particles by each other, and further estimate the shape of TiO 2 nanoparticles as simple cubes, then possibly only one-sixth of these nanoparticles are on the top face (~2700 gold nanoparticles).…”

“…One sample was prepared similarly, but not calcined after the colloidal synthesis (Au/anatase uncalc.). To investigate the role of the chemicals used in the synthesis, anatase blank samples were generated in analogy to the original colloidal deposition method by exposing anatase to the same chemicals at the same concentrations of chloride, NaBH 4 , and PVA, but without using HAuCl 4 . A potential influence of chloride anions in the auric acid precursor was accounted for by the addition of the same chloride concentration using NaCl.…”

“…It was provided by STREM Chemicals Inc. and was not treated apart from being finely ground to powder in a mortar for about 10 min. The average size of the gold nanoparticles is in the range of 2-3 nm [4,48]. Details of the calculation of the average distance of the gold nanoparticles can be found in the supplementary information.…”

“…In this respect, the combination with gold nanoparticles as catalyst or cocatalyst is very common, for example in low temperature CO oxidation [2,3] or for the selective oxidation of alcohols [4][5][6][7]. Since TiO 2 can commonly exist in different crystal structures, with anatase and rutile being the two most common and most studied ones [1], there has been much debate about the (photo)catalytic properties of either of them.…”

“…Furthermore, the catalytically active sites could be located on the gold nanoparticles, on the support, or at the metal-semiconductor interface [3][4][5]. The question arises as to whether nanoparticles supported on the different TiO 2 modifications anatase and rutile differ in their catalytic behavior, and whether the support actually remains unchanged after noble metal deposition.…”