Effect of metal particle shape on hydrogen assisted reactions

The effects of the metal particle size and shape (e.g. cubic, spherical, tetrahedral or octahedral) on catalytic activity, selectivity and stability were elucidated in selective hydrogenation of carbon-carbon triple and double bonds, and carbonyl bonds as well as in hydroisomerization, and hydrodeoxygenation. In several reactions, clear effects of the nanoparticles size and shape on catalytic behavior were demonstrated, e.g. cubic particles exhibited high turn-over frequencies in acetylene and stilbene hydrogenation. Furthermore, high index Pt(111) and Pd(111) facets were more selective towards hydrogenation of buta-1,3-diene and acetylene, respectively in comparison with other facets. Coking was observed to change selectivity, especially in acetylene hydrogenation due to formation of the Pd carbide phase. In hydrogenation of 2-methyl-3-butyn-2-ol to the corresponding alkenol, structure sensitivity was absent below 50 % conversion due to a high coverage of the reactant caused by strong adsorption of alkynol, while above 50 % conversion the reaction was structure sensitive. The highest selectivity to alkynol semihydrogenation was obtained with 18 nm cubic Pd particles exhibiting a low amount of edge surface sties. Furthermore, Pt(111) planes exhibited the highest selectivity towards cinnamylalcohol formation in hydrogenation of cinnamaldehyde, even in some cases Pt(100) particles supported on carbon exhibited the highest turnover frequency. Small octahedral Pt(111) particles were selective in hexadecane hydroisomerization. These data were also supported by different catalyst characterization results, such as in-situ XRD, temperature programmed desorption as well as theoretical DFT calculations and kinetic modelling. Structure selectivity is reaction specific and rationalization of the results is typically restricted to a specific reaction requiring further studies for various heterogeneous catalytic reactions.