Tuning butene selectivities by Cu modification on Pd-based catalyst for the selective hydrogenation of 1,3-butadiene

Pd-based bimetallic catalysts have shown unique selectivity to alkenes in the selective hydrogenation reactions, but often at the expense of activity. Therefore, the modification by a non-precious metal with similar or increased activity is crucial in lowering the catalyst cost. In the current study, γ-Al₂O₃ supported Pd–Cu catalysts with different metal ratios were investigated in the selective hydrogenation of 1,3-butadiene. The adsorption properties of hydrogen and C4 species over model surfaces Pd(1 1 1), Pd₁Cu₃(1 1 1) and Cu(1 1 1) were studied by DFT calculations, and it was predicted that the Pd–Cu bimetallic catalysts should be more selective to butenes than the Pd monometallic catalyst. Moreover, the γ-Al₂O₃ supported Pd–Cu catalysts were prepared by incipient wetness co-impregnation, and were characterized by TEM, CO pulse titration, TPR, N₂O chemisorption, FT-IR of adsorbed CO and XPS; the catalytic study was performed in a fixed-bed reactor as well as in a batch reactor equipped with FT-IR. By the addition of Cu, Pd active sites were diluted, and the particle sizes were reduced. The Pd–Cu bimetallic catalysts exhibited surprisingly higher activity than Pd at low temperatures. In consistent with the DFT results, Pd–Cu bimetallic catalysts showed higher selectivities to total butenes and 1-butene. By analyzing the characterization and the hydrogenation test results, the electronic effect and geometric effect should be responsible in enhancing activity and tuning selectivity.