Pulsed-Field Gradient NMR Spectroscopic Studies of Alcohols in Supported Gold Catalyststs

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.