The Study Of PdO-CeO₂Catalysts For Co And CH₄Catalytic Oxidation

The PdO/Ce1-xPdxO2-δ catalyst prepared by a solution-combustion method contained free surface PdOx species and PdOx species in Ce1-xPdxO2-δ solid solution, whereas the PdO/CeO2catalyst prepared by an impregnation method contained only free surface PdOx species. The free surface PdOx species could be removed by nitric acid. All catalysts were tested for CO and CH4oxidation. The catalysts were further characterized by X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET) technique, chemisorption of CO, H2-Temperature Programmed Reduction (H2-TPR), X-Ray Fluorescence (XPF), Raman, DRIFT, and TEM technique etc., get the information about the nature and active phase of the PdO/Ce1-xPdxO2-δ, Ce1-xPdxO2-δ and PdO/CeO2catalysts for CO and CH4oxidation.XRD, H2-TPR and TEM results indicated that the PdO/Ce1-xPdxO2-δ catalyst contained free surface PdOx species and the surface Pd contents are low or the PdO species may be highly dispersed. The bulk of the PdO/Ce1-xPdxO2-δ catalyst are Ce1-xPdxO2-δ solid solution. The acid treatment hardly changes the bulk structure of the PdO/Ce1-xPdxO2-δ catalyst and only removes the surface PdOx species. Concerning the PdO/CeO2catalyst, the lattice parameter is the same as the pure CeO2, implying the absence of Ce1-xPdxO2-δ solid solution in this sample. Contributions of different PdOx species to catalytic CO oxidation were quantitatively evaluated. The free surface PdOx species in the PdO/Ce1-xPdxO2-δ catalyst had the highest activity, those in the PdO/CeO2catalyst had medium activity, and the PdOx species in the Ce1-xPdxO2-δ solid solution lattice had the lowest activity. Raman spectrum suggested that the oxygen vacancy concentration of Ce1-xPdxO2-δ solid solution was higher than pure CeO2. DRIFT results indicating that CO molecules can hardly chemisorb on the surface of pure Ce1-xPdxO2-δ solid solution. Synergetic effects of PdOx species were responsible for the enhanced reactivity of the PdO/Ce1-xPdxO2-δ catalyst, as the free surface PdOx species provided CO chemisorption sites and the Ce1-xPdxO2-δ solid solution generated more oxygen vacancies for oxygen activation.It was also tested all catalysts activity of CH4oxidation, and contributions of the PdOx species were also quantitatively evaluated. For CH4oxidation, the situation is quite different. The specific reaction rate of the Pd4+ions in the Ce1-xPdxO2-δ solid solution (7.5μmolCH4gPd-1s-1) was highest than that of the PdO/Ce1-xPdxO2-δ catalyst (2.8μmolCH4gPd-1s-1) and the PdO/CeO2catalyst (1.7μmolCH4gPd-1s-1), implying that Pd4+species in the Ce1-xPdxO2-δ solid solution contribute more to the reaction than the surface PdOx species. Therefore, it can be seen that different active sites are required for CO and CH4oxidation reactions. The surface PdOx species favor CO oxidation reaction, while Ce1-xPdxO2-δ solid solution favor CH4oxidation. TEM, XPS and DRIFT results found that partial Pd4+ions in the Ce1-xPdxO2-δ lattice were unstable and migrated to the surface to form surface PdOx species under high temperature calcination (500℃).In order to investigate the effect of metal doping on PdO/Ce1-xPdxO2-δ catalytic activity, Zr was mixed to the PdO/Ce1-xPdxO2-δ catalyst. XRD results found that diffraction peaks due to tetragonal ZrO2phase are observed, and the cell parameter of the PdO/Ce1-x-yPdxZryO2-δ catalyst was lower than PdO/Ce1-xPdxO2-δ, indicated that only a portion of Zr were successfully doped in the Ce1-xPdxO2-δ lattice. Pd inhabits Zr penetrated into the CeO2lattice. For CO oxidation, the activity PdO/Ce1-x-yPdxZryO2-δ catalyst improved after Zr mixed, and the specific rate of the surface PdOx species (1434.2μmolco gPa-1s-1) is2.1times higher than that of the PdO/Ce1-xPdxO2-δ catalyst (673.4μmolco gPa-1s-1), due to that the oxygen vacancy concentration of the former is higher than the later. While the specific rate of PdOx in the Ce1-x-yPdxZryO2-δ catalyst (2.0μmolCO gPd-1s-1) is lower than that of the Ce1-xPdxO2-δ catalyst (2.7μmolCO gPd-1s-1). For CH4oxidation, Zr doping depresses the activity. The specific rate of Ce1-x-yPdxZryO2-δ catalyst (5.8μmolCH4gPa-1s-1) is lower than that of Ce1-xPdxO2-δ catalyst (7.5μmolcH4gPd-1s-1), attributing to that the existence of tetragonal ZrO2phase reduces the amount of the surface Pd4+species in the Ce1-x-yPdxZryO2-δ solid solution. Furthermore, the specific rate of PdO/Ce1-x-yPdxZryO2-δ catalyst (2.3μmolCH4gPd-1s-1) is lower than that of PdO/Ce1-xPdxO2-δ catalyst (2.8μmolCH4gPd-1s-1) and Ce1-xPdxO2-δ catalyst (7.5μmolCH4gPd-1s-1), due to the fact that co-existence of ZrO2phase and surface PdOx species decrease the amount of Pd4+species on the surface of the Ce1-x-yPdxZryO2-δ solid solution. Pd4+species in the Ce1-xPdxO2-δ solid solution is favor of CH4oxidation.