Synthesis Of Co₃O₄ And Co-Mn-O Composite Oxide Catalysts And Their Catalytic Application In Selective Oxidation Of CO In H₂-rich Gases

In this dissertation,Co₃O₄ with different morphologies and sizes were synthesized to study their catalytic performances for CO selective oxidation in H₂-rich streams.The effect of Co₃O₄ samples' morphologies and sizes on their catalytic activities was studied.Co-Mn-O composite oxides were also prepared and studied in details as the catalyst for the same reaction.Many methods were used to synthesize Co₃O₄ samples with different morphologies and sizes.Urea precipitation-hydrothermal reaction-air oxidation method was utilized to synthesize Co₃O₄ plates and spheres made up of nanoparticals. The morphologies of Co₃O₄ are found in accordance with the morphology of their precipitation precursors.Preparation conditions like hydrothermal time, concentrations,solvent property,and surfactant were found to have important effect in the morphology and size of Co₃O₄.Catalytic performance of Co₃O₄ synthesized with different morphologies was prepared.We found that Co₃O₄ by ammonia precipitation-hydrothermal reaction-air oxidation method was more highly active than commercial Co₃O₄.A series of Co-Mn-O composite oxides with different Co/Mn ratios and several other Co₃O₄-based binary oxides including Co-Cu,Co-Cr,Co-Mo,Co-V,Co-Ag oxides were prepared by a co-precipitation method.For the CO selective oxidation in H₂-rich gases,the Co-Mn-O(Co/Mn=8/1) composite oxide sample showed evidently higher catalytic performance than both Co₃O₄ and MnO₂.The Co-Mn-O(Co/Mn= 8/1) composite oxide catalyst was found most effective among the binary composite oxides.Results of characterization showed that only spinel structure of Co₃O₄ was confirmed for the Co-M-O(Co/M=8/1 or 8/2) composite oxides,but the crystallite size of these composite oxides decreased evidently.Mn may be entering into the cubic structure of Co₃O₄,possibly suggesting the formation of Mn_xCo_3-xO₄ solid solution. Co-Mn-O composite oxides samples exhibit different redox property in comparison with single Co₃O₄ and MnO₂,which suggests a strong interaction between Co and Mn species in the composite samples.From the O₂-TPD results,it is clear that incorporation of Mn in the structure of Co₃O₄ enhanced the capacity of active oxygen species and the mobility of lattice oxygen.We speculate the lattice oxygen mobility and higher CO adsorption capacity in Co-Mn composite oxides play an important role in the oxidation of CO.CO pulse was used to study the difference of oxygen species between Co₃O₄ and Co-Mn-O composite oxide.DRIFTS was used to investigate the relevancy between surface carbonate species and their reactivity.The oxidation of CO over the composite oxides was speculated in the following process:gas-phase CO was adsorbed on a cobalt site as carbonate species and converted into CO₂(g) on the surface.After CO₂(g) was released from carbonate species and an oxygen vacancy was created.The oxygen vacancy was recovered by the adsorption of O₂ in gas phase, and this is the rate-determining step for CO oxidation over Co₃O₄ catalyst.