Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene.

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O2/Ar and 10 vol % H2/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au3+ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au3+ possesses the optimal activity with 1.8 × 10-2 s-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au3+ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au3+ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H2 via constructing frustrated Lewis pairs with O.

Enhancement of acetylene hydrogenation activity over Ni-Zn bimetallic catalyst by doping with Au.

A series of modified Ni-Zn bimetallic catalysts were prepared by depositing different kinds of 4 wt% metals (Ir, Pt, Au, Cu, Ag) on the Ni-Zn-Al hydrotalcite (NZAH) and tested in the selective hydrogenation of acetylene. The activity was enhanced by 3-fold over the 4 wt% Au/NZAH, meanwhile, the yield of C2H4 was also increased more than 4 times compared with the NZAH. Characterization by means of H2-TPR, XRD, SEM, and TEM revealed that Au was highly dispersed on the catalyst, and the ternary Au-Ni-Zn alloy was formed during the H2 reduction at 500 degrees C. Doping Au in Ni-Zn bimetallic catalyst weakens the adsorption of acetylene and also hinders the coke deposition on the catalyst, which leads to the enhanced activity in acetylene hydrogenation.