Room-Temperature CO Oxidation over Au-Pd Monometallic and Bimetallic Nanoparticle-Supported MgO

In the present work, the modified impregnation approach was effectively used to prepare five catalysts that contain 1 wt % monometallic gold (Au), palladium (Pd), and bimetallic gold-palladium (Au-Pd) with different ratios supported by MgO. The structure of each catalyst was thoroughly examined using various techniques, including X-ray powder diffraction, nitrogen adsorption- desorption, transmission electron microscopy, scanning electron microscopy-energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy (XPS), and the carbon monoxide oxidation process was employed to assess their catalytic activity. The results indicated that loading Au and/or Pd nanoparticles (NPs) had no discernible effects on the crystal size, surface area, or pore radius of MgO. Additionally, the results demonstrated that gradual crystal growth occurs in the bimetallic catalysts when the Au/Pd ratio decreases, which causes a dramatic reduction in the dispersion percentage. The XPS results showed that most of Au NPs are distributed on the surface as Au0, with only traces of oxidized species (Au+) present. The Pd NPs, in contrast, were predominantly located as oxidized or partially oxidized species (Pd2+ and Pd delta+), with a minor amount of Pd0. Formation of Au/Pd alloy on the surface of the catalyst was clearly observed at a Au/Pd ratio of 1:1. Furthermore, in comparison to Pd-rich catalysts, Au-rich catalysts demonstrated superior catalytic performance toward CO oxidation. There was no indication of an Au/Pd synergistic effect, and the development of an Au/Pd alloy reduced the catalyst's ability to catalyze CO oxidation. More significantly, the findings showed that the higher activity was caused by both small particles of Au0 (rather than Pd0), which served as active sites on the surface for CO adsorption and oxidized/partially oxidized species (Au+, Pd2+, and Pd delta+), which provided the adsorbed CO molecule with active oxygen necessary for CO2 formation.