Direct Formation of Acetate from the Partial Oxidation of Ethylene on a Au/TiO2 Catalyst

The partial oxidation of ethylene to form adsorbed acetate on a Au/TiO2 catalyst at temperatures as low as 370 K is reported here using Fourier transform infrared (FTIR) spectroscopy, gas chromatography-mass spectrometry (GC-MS) and density functional theory (DFT) calculations. Ethylene reacts with oxygen on Au/TiO2 to produce acetate on the TiO2 support as determined by the comparison with a blank TiO2 and Au/SiO2 catalyst. As shown by DFT calculations, O-2 dissociation occurs at the dual-perimeter Au-Ti4+ sites of Au/TiO2 catalysts. Surprising, no ethylene oxide on the catalyst surface or in the gas phase is detected by either FTIR or GC-MS techniques at temperatures up to 673 K. The reaction pathway to ethylene oxide involves a higher barrier (similar to 1.0-1.5 eV) than the pathway for acetate formation (similar to 0.1-0.6 eV). The rate-limiting step to form adsorbed acetate was found to be the protonation of the H2C*C(OH)O* intermediate to produce the bound acetic acid. The theoretical initial deuterium kinetic isotope effect is similar to 3 which is consistent with the experimental data.