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

Domenii publicaţii > Chimie + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Mick D. Mantle, Dan I. Enache, Ewa Nowicka, Scott P. Davies, Jennifer K. Edwards, Carmine D’Agostino, Darren P. Mascarenhas, Lorraine Durham, Meenakshisundaram Sankar, David W. Knight, Lynn F. Gladden, Stuart H. Taylor, and Graham J. Hutchings

Editorial: J. Phys. Chem. C, 115 (4), p.1073-1079, 2011.

Rezumat:

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,
, 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.

Cuvinte cheie: Pulsed field gradient NMR spectroscopy, Effective diffusivity, Au-Pd-Titania catalysts, Alcohols oxidation

URL: http://pubs.acs.org/doi/abs/10.1021/jp105946q