Gold/titania: Synthesis, characterization and photocatalytic performance

In earlier studies at R&CE, TUDelft the photocatalytic oxidation of cyclohexane to cyclohexanone was found to be highly selective towards cyclohexanone, but unfortunately the conversion rate is low due to deactivation of the catalyst. The aim of this study is to improve the conversion and in the same time to reduce the deactivation of the catalyst during photo oxidation by increasing the oxidation activity on the TiO2 catalyst surface by deposition of metal gold. Metal gold deposition as nanoparticles on the TiO2 surface decreases the electron-hole recombination of the TiO2 semiconductor, resulting in a more efficient charge separation. This better charge separation would improve the photocatalytic activity of the catalyst. Several TiO2 supports (Degussa P25, Hombicat, Aldrich and Merck) and two different methods for reduction of Au3+ to Au0 , i.e. calcinations and chemical reduction by NaBH4 were applied. All catalysts were characterized for specific surface area and for pore distribution by nitrogen physisorption, for active sites by ammonia-TPD, for Au particle size by TEM, for anatase/rutile ratio of TiO2 by RAMAN, XRD, for adsorbed water and OH-groups by TGA and for absorbance capacity of light by UV/VIS. The Au/TiO2 catalysts were tested on their photo catalytic activity in three different oxidation reactions, namely cyclohexane oxidation to cyclohexanone (liquid-organic oxidation), degradation of methylene blue (liquid-aqueous oxidation) and oxidation of propane and propylene (gas-organic oxidation). During cyclohexane photo oxidation, high selectivity to cyclohexanone is obtained (cyclohexanone/cyclohexanol 90-97%). For all catalysts the character of the kinetics is very similar. The photo oxidation of cyclohexane follows two reaction stages, an initial stage and a steady-state stage. After an initial stage of 45-60 minutes of illumination the reaction rate decreases significantly. The decrease of reaction rate is attributed to deactivation of the TiO2 catalyst by oxidation of cyclohexanol to carboxylates, which adsorb at the active sites of the catalyst surface. Decarboxylation reactivates the catalyst and is assumed to be the rate determining step in the steady state stage. A two-loop reaction model is proposed to understand the kinetics. Gold deposition of the TiO2catalysts did not result in higher reaction rates, but in a decrease of both initial as steady state reaction rates. The stronger oxidation activity of Au/TiO2 will increase the kinetics of the cyclohexanol loop of the reaction model. Consequently, oxidation of cyclohexanol leads to accumulation of carboxylates on the catalyst surface. Further research and development of methods, which might speed up the decarboxylation step would be worthwhile for improvement of the photo oxidation rate in the steady state stage. The kinetics of the decomposition of MB appears to be pseudo first order as well for decomposition with pure TiO2 catalysts as for decomposition with Au/TiO2 catalyst. A two-cycle model is proposed, of which each cycle has two pathways, one involving a superoxide radical and the other involving an adsorbed hydroxyl ion. The cycles describe the decomposition of MB and the decomposition of MB degradation products respectively. All catalysts show fast decomposition in the initial state at a rate around 0.03/min. This rate appears to be independent of the active site concentration. This leads to the conclusion that the adsorption step is the rate determining process of MB decomposition by photo oxidation in the initial state. The photocatalytic oxidation of MB appears to be a two-stage process, with a decomposition rate in the initial stage, which is a factor 3-8 higher than in the steady state stage. The reduction of this rate is due to the production of degradation products, which compete with MB for absorption sites. In the steady state stage the desorption of degradation products becomes rate limiting. Pure Hombicat shows an initial delay of MB decomposition, which is explained by the presence of sulphites, which scavenge radicals formed by photo oxidation. After all sulphites are consumed MB decomposition can start. Gold deposition increased the decomposition rate for Aldrich catalyst only. For Au/Hombicat and Au/P25 the lower rate is believed to be the direct and/or indirect effect of lower active site concentration, in combination with limited diffusivity of MB due to pore diameter restrictions. The significant reduction of active sites could be due to sodium poisoning after the reduction by NaBH4. Photo oxidation of propane to acetone appears to occur with visible light, although at low conversion rates. Unfortunately, the challenging idea of the possibility of direct photo oxidation of propylene to propylene oxide in the presence of water has a negative outcome. No epoxies are formed, but a lot of other species like acetone, carboxylates and carbonates are detected by FTIR. In conclusion, the photo catalytic activity could be enhanced by gold deposition. However, competing reactions and catalyst deactivation could have large negative effects on the overall kinetics. As shown in this study, each combination of catalyst, photo oxidation reaction and environment has its own specific limitations. E.g. reaction mechanisms in liquid/gas phase or in aqueous/non-aqueous environment could be different for the same catalyst. In order to exploit the advantage of high selectivity, each specific photo oxidation process needs to be optimised for catalyst deactivation and competing reactions. Increasing the oxidation activity by gold deposition only is not enough to enhance the kinetics. Some ideas to study are selective active sites by catalyst modification, continuous removal of formed compounds to avoid competing reactions, and reaction medium that contains selective scavengers, which can prevent active sites blockage.