Enhanced Stability of Pd/ZnO Catalyst for CO Oxidative Coupling to Dimethyl Oxalate: Effect of Mg2+ Doping

The catalytic performances of supported Pd nanoparticles (NPs) are strongly dependent on the support materials for CO oxidative coupling to dimethyl oxalate (DMO). Herein, hierarchical flower-like ZnO microspheres composed of porous nanosheets are employed as a new support material for a Pd catalyst, which exhibits excellent catalytic activity for CO oxidative coupling to DMO. The conversion of CO and the selectivity to DMO reach up to 67% and 98% at 130 degrees C, respectively. Unfortunately, the high activity of Pd/ZnO catalyst gradually deteriorates within 100 h. To resolve the poor stability, we further introduce Mg2+ ions into the ZnO support. It is exciting that the catalytic activity of the Mg2+-doped-ZnO-supported Pd nanocatalyst (Pd/Mg-ZnO) can be maintained for at least 100 h without obvious decay. Catalytic stability is greatly improved by the doping of Mg2+ ions. XRD, UV-visible diffuse reflectance spectra, and high-angle annular dark field scanning transmission electron microscopy characterizations demonstrate that a small portion of Mg2+ ions are successfully incorporated into the lattice of the ZnO support to form a Zn-Mg oxide solid solution. XPS, in situ diffuse reflectance Fourier transform infrared spectroscopy, and H-2-temperature-programmed reduction results reveal that the introduction of Mg2+ ions into the ZnO support leads to a strong metalsupport interaction caused by electron transfer from the ZnO substrate to the Pd NPs, which can effectively restrain the sintering of the active Pd NPs; retard the growth of Pd NPs; and thus, enhance the catalytic stability.