Size-dependent selectivity in vinylacetylene hydrogenation over Pd/Al2O3: a DFT study.

The hydrogenation of C4 alkynes to tunable products is of significant industrial importance. In this study, the size effects of Pd/Al2O3 catalysts on the selective hydrogenation of vinylacetylene were investigated by using density functional theory (DFT) calculations. Pd(111), Pd13/Al2O3, and Pd1/Al2O3 models were constructed as model systems for Pd particles, Pd clusters, and Pd single-atom catalysts, respectively. Metal-support interactions between Pd species and Al2O3 drive electron transfer from Pd to the Al2O3 surface. As a result, Pd species in the Pd13/Al2O3 model become slightly positively charged, and those in the Pd1/Al2O3 model are notably electron-deficient. The Pd13/Al2O3 model, with its d-band center closest to the Fermi level, exhibits the strongest adsorption of reaction intermediates and the highest H2 activation ability, whereas the Pd1/Al2O3 model shows the weakest in both respects. Mechanistic studies reveal that the selectivity of Pd/Al2O3 catalysts strongly depends on the size of Pd. Pd single atoms favor 1,3-butadiene, Pd particles preferentially yield 1-butene, and Pd clusters promote complete hydrogenation to butane.