Tuneable bimetallic PdxCu100-x catalysts for selective butadiene hydrogenation

Abstract

Pd-based catalysts play an important role in selective hydrogenation reactions for the removal of polyunsaturated hydrocarbon traces (e.g. alkadienes) from alkene feedstocks. To improve the selectivity, Pd can be diluted with a more selective metal such as Cu, Ag or Au. We report a detailed study on the effect of the Pd:Cu ratio on the catalytic performance of carbon-supported Cu-rich catalysts for the selective hydrogenation of butadiene in an excess of propylene. Bimetallic PdxCu100-x nanoparticles of 7–8 nm diameter with tuneable Pd content of 0.6–13 % were prepared colloidally. Catalytic turnover frequencies for butadiene hydrogenation increased with Pd-fraction up to 1.0 s−1 for Pd7Cu93 and Pd13Cu87. The butene selectivity, measured at 90 % conversion, was roughly 80 % for the catalysts with a Pd fraction above 3 % and slightly increased with lower Pd concentrations. Operando X-ray absorption spectroscopy identified an electron density transfer from Cu to Pd in the bimetallic catalysts and a slight preferential clustering of Pd. The trend in catalytic activity was ascribed to an increased Pd ensemble size, indicated by higher Pd-Pd coordination numbers. For bimetallic PdxCu100-x/C catalysts, a Pd content of 3–7 % retains a high selectivity of 78 % at 90 % conversion, while improving the activity 3–4 orders of magnitude compared to pure Cu catalysts. These insights on how to control the activity-selectivity balance through metal nanoparticle compositions contributes to the rational design of bimetallic catalysts for selective hydrogenation reactions.