Atmospheric pressure studies of selective 1,3-butadiene hydrogenation on well-defined Pd/Al2O3/NiAl(110) model catalysts: Effect of Pd particle size

The selective hydrogenation of 1,3-butadiene was studied on a series of Pd/Al2O3/NiAl(110) model catalysts with mean Pd particle sizes of 2–8 nm. While Pd nanoparticles ⩾4 nm exhibit a near zero-order reaction kinetics with respect to butadiene, the rate behavior on smaller Pd particles is more complex, and the rate increases with decreasing butadiene pressure. This indicates a change in the rate-limiting step from a regime governed by adsorption to a regime governed by the surface reactions. When the total number of Pd surface atoms is used for rate normalization, the turnover frequency (TOF) of 1,3-butadiene hydrogenation increases linearly with increasing particle size. But this is only an apparent particle size dependence. Considering a realistic structural model of the Pd nanoparticles [i.e., with incomplete (111) terraces] for rate normalization, 1,3-butadiene hydrogenation becomes particle size independent, even though the reaction is structure sensitive, as corroborated by reactivity studies on Pd(111) and Pd(110) single crystals. For 1,3-butadiene hydrogenation, well-faceted Pd nanoparticles ⩾4 nm behave like Pd(111).