Phenol hydrogenation on unsupported Pd nanostructures showed structure sensitivity with respect to particle shape and insensitivity with respect to particle size. The hydrogenation reaction was performed over Pd cubes (~25nm, Fig.1(a)), octadehra (~27nm, Fig.1(b)) and spheres (~6nm). While Pd cubes (exposing only the (100) facets) were observed to selectively (100%) produce cyclohexanone, octahedra (exposing only the (111) facets) were 100% selective towards cyclohexanol. Interestingly, Pd spheres of varying size (&lt;5nm to 6nm) and exposing both (111) and (100) facets were observed to produce cyclohexanone only. Density functional theory (DFT) calculations showed that the difference in the binding modes of phenol on Pd(111) (Eb=-101kJ/mol) and Pd(100) (Eb=-135kJ/mol), primarily lead to the difference in the selectivity (Fig.1(c) and 1(d)). On Pd(111), cyclohexanol was produced by phenol dissociation to phenoxy and subsequent hydrogenation (Fig.1(e)), whereas on Pd(100) cyclohexanone was produced by direct phenol hydrogenation (Fig.1(f)). Further hydrogenation of cyclohexanone to produce cyclohexanol on Pd(100) was observed to be prohibitive (Ea=125kJ/mol) at reaction conditions. Interestingly, a mixture of cube and octahedra with 40:60 ratio, resembling a spherical supported Pd nanoparticle was observed to yield cyclohexanone with 100% selectivity even on increasing the phenol weight in the reaction from 50mg to 450mg. However, a small fraction (8% selectivity) of cyclohexanol was detected when phenol concentration was raised to 600mg. The shift in selectivity was attributed to the reduction in the adsorption energy of phenol on the (100) facets (Eb=-97kJ/mol at 0.6ML coverage) which became comparable to the (111) facet due to increased lateral interactions at high phenol coverage. The reaction however remained structural insensitivity on varying the size of the Pd sphere.

Unraveling structure sensitivity in phenol hydrogenaton on Pd nanostructures

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.