Novel non-noble bimetallic Cu-Zn/TiO2 catalysts for selective hydrogenation of butadiene

Oxide-supported copper catalysts are active for the selective hydrogenation of polyunsaturated hydrocarbons with high selectivity to alkenes formation, but a low catalytic stability limits its application. The goal of the work was to investigate whether alloying Cu with Zn will improve catalyst stability, and whether such a non-noble bimetallic catalyst could compete with noble metal-based catalysts for this type of reaction. TiO2-supported mono-metallic Cu and Zn and bimetallic Cu-Zn (Cu/Zn atomic ratio of 1 and 3) were prepared by deposition-precipitation with urea (DPU) and deposition-precipitation at pH ≈ 8 (DP8). Small metal nanoparticles (<2 nm) with uniform particle sizes were obtained from DP8 samples, while two different particle size ranges (∼3 nm and >15 nm) were found in the DPU samples after calcination at 400 °C and then reduction at 350 °C. The bimetallic character of the nanoparticles was attested by XRD and STEM-HAADF coupled with EDS analysis, as Cu3Zn alloy was found in DPU CuZn 1:1/TO2 sample and an "intermediate" Cu-Zn alloy (Cu0.9Zn0.1) was found in DPU CuZn 3:1/TiO2 sample. UV-visible spectroscopy showed that alloying of copper with zinc inhibits the reoxidation of copper by contact with air. The catalytic results showed that alloying Cu with Zn slightly decreased the activity in butadiene selective hydrogenation, but had almost no effect on the selectivity to alkenes. The important result is that the bimetallic Cu-Zn/TiO2 catalysts displayed much higher stability under isothermal reaction than Cu/TiO2, which deactivated rapidly in the first few hours. This higher stability was ascribed to the formation of a lower amount of carbonaceous species on the bimetallic catalysts, as revealed by TGA analyses. Moreover, the DP8 Cu-Zn/TiO2 samples showed lower T100% but higher stability than the corresponding DPU samples. Surprisingly, change in the composition of the Cu-Zn alloy was observed during the catalytic reaction at 105 °C, with a transition from Cu3Zn to Cu0.7Zn0.3 alloy for the DPU CuZn 1:1/TiO2 sample.