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Catalytic glycerol conversion by means of either photon or thermal energy is of great importance and can be realized by metal supported on TiO2 systems. Although various procedures have been employed to synthesize efficient metal/TiO2 catalysts, the promotional mechanisms for both reactions are still unclear due to the lack of well-defined systems. Here, we have deposited gold nanoparticles on a series of highly crystalline anatase TiO2 substrates with different crystallite sizes (7, 12, 16, 28 nm) by both direct precipitation and sol-immobilization methods to examine the effect of metal deposition methods and TiO2 sizes on both photo- and thermal catalytic glycerol reforming. For photocatalytic H2 evolution from glycerol, optimum performance was observed for the Au supported on 12 nm TiO2 for both deposition methods. For thermal catalytic glycerol oxidation, all catalysts show a similar selectivity to glycerate (>70%) regardless of the TiO2 size and metal deposition method; however, the metal deposition method significantly influences the catalytic activity. In situ UV-vis spectrometry reveals that the optimized photocatalytic performance originates from enhanced charge transfer kinetics and a more negative Fermi level for proton reduction, whereas electrochemical analysis reveals that the promoted glycerol oxidation is caused by the enhanced oxygen reduction half-reaction.

Liquid-phase catalytic oxidation of glycerol in aqueous solutions using porous solid catalysts represents a viable strategy for the sustainable production of fine chemicals from renewable resources. Various aspects of these novel types of reactions are still under investigation. Catalyst deactivation is one of those issues that need to be understood and addressed in order to make these processes commercially viable. In a previous study, it has been reported that the catalytic activity of Au/TiO2 catalysts for the oxidation of glycerol with O2 under basic conditions can be severely inhibited by some reaction intermediates or products. It was suggested that the presence of certain species blocks the active sites of the catalyst, preventing the adsorption of glycerol, which in turn results in a decrease of reaction rate. In this work, we used NMR relaxation time measurements in order to assess surface interactions of glycerol in a Au/TiO2 catalyst pretreated with aqueous solutions of various oxygenates, including intermediates and products of glycerol oxidation, under basic conditions, in particular, evaluating changes in glycerol adsorption properties. The NMR T1/T2 ratio of glycerol, which is indicative of the strength of interaction of glycerol with the catalyst surface, traces out well the trend in catalytic activity in the presence of different additives, suggesting that adsorption of glycerol onto the catalyst surface plays a crucial role in the reaction, which supports the hypothesis previously made in the literature. This experimental approach and the related results represent a significant advance in the understanding of liquid-phase catalytic reactions occurring over solid surfaces, which can be used to understand and optimize catalytic processes and the effect of intermediate and product inhibition.

In recent work, it was reported that changes in solvent composition, precisely the addition of water, significantly inhibits the catalytic activity of Au/TiO2 catalyst in the aerobic oxidation of 1,4-butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidation in methanol is generally valid, the solvent effect on the aerobic catalytic oxidation of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO2 catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed-field gradient nuclear magnetic resonance (PFG-NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Combined NMR diffusion and relaxation time measurements suggest that molecular diffusion and, in particular, the relative strength of diol adsorption, are important factors in determining the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non-invasive characterisation tools for catalytic materials, which complement conventional reaction data.

We report a pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopic study of the effective diffusivity of alcohols in catalysts comprising gold supported on silica, titania and ceria and gold−palladium alloy nanoparticles supported on titania. These catalysts are shown to be highly active for the selective oxidation of alcohols. However, we observe that molecules possessing hydroxyl functional groups in the 2-position exhibit very low reactivities. To help understand the nature of conversion and selectivity, we observe from traditional catalytic measurements involving gas chromatography of the reaction mixtures, we have studied the effective self-diffusivities, Deff, of 1-, 2-, and 3-octanols and 1,2- and 1,4-butanediols in Au−ceria, Au−silica, Au−titania, and Au−Pd−titania using PFG-NMR spectroscopy. The results show that the octanols diffuse approximately 35% slower on silica supports than on titania. In addition, a marked two-component diffusive behavior is seen for ceria-supported catalysts with the dominant component, for 1-, 2-, and 3-octanols, being close to that of the free bulk liquid, and the slower component being an order of magnitude slower. The values of the 1,2- and 1,4-butanediol self-diffusion coefficients for silica-based gold catalysts are closer to those of the bulk liquid 1,2- and 1,4-butanediols. Au−Pd−titania also showed reduced self-diffusivities when compared with the bulk liquids but were similar to their monometallic counterparts. A new parameter, ξ, the PFG-NMR interaction parameter, is introduced and is defined as the ratio of free liquid diffusivity to effective liquid diffusivity within the porous medium and accounts, collectively, for the functional group interaction of the probe molecule with itself and the porous medium. This parameter, along with reference tortuosity values determined by PFG-NMR gives new insight into the dynamics of hydrogen-bonded networks of different functional groups that exist within the porous catalyst matrix. The inhibition effect observed from traditional catalytic activity studies for the oxidation of 2-octanol is considered to result from competitive adsorption of the ketone product.