Synthesis, Characterization and Catalytic Performance of Dedicated Ceria-Zirconia Catalysts for HCl Oxidation Reaction

The heterogeneously catalyzed oxidation of HCl to Cl2 (Deacon process) represents a sustainable route to the recovery of chlorine from HCl-containing streams in the chemical chemistry. It has been demonstrated that CeO2-based materials have the potential to act as effective catalysts for the HCl oxidation reaction, offering an economically viable alternative to those based on RuO2 commercial materials. The thesis continues to focus on CexZr1-xO2 mixed oxides and supported CeO2 on ZrO2 (CeO2@ZrO2) catalysts, with the aim of elucidating the relationship between structure, oxygen storage capacity and catalytic HCl oxidation activity and durability. Moreover, rapid and sensitive in situ deactivation and reactivation experiments are devised to gain insight into the “chlorination-dechlorination” process. Although the enhanced Deacon activity can be achieved by combining CeO2 with ZrO2 in solid solution reported by numerous studies, a clear-cut relationship between OSC and the activity may be obscured due to the fact that previous studies have suffered from the limitation that CexZr1-xO2 differ in several key parameters, such as specific surface area. Accordingly, in this thesis, we initially developed CexZr1-xO2 solid solutions as a function of the composition x through the co-precipitation method, maintaining a constant specific surface area at 46 ± 2 m2/g. It can therefore be stated that the OSCc value is an appropriate descriptor of the catalytic HCl and CO oxidation reaction. From the linear relationship between the oxidation activity of HCl and OSCc, which in turn is linearly related to the activity of CO oxidation, it can be concluded that the HCl oxidation reaction over the CexZr1-xO2 catalysts proceeds via the Mars-van Krevelen mechanism. Furthermore, the effect of calcination process on the structural properties and catalytic performance of CeO2@ZrO2 catalysts was systematically investigated. CeO2 with varying concentrations were loaded onto the surface of ZrO2 particles using the incipient wetness impregnation method and calcined at different temperatures. The catalyst, which was calcined at 600 °C for a period of 5 h, resulted in the formation of a highly dispersed CeO2 layer, which exhibited an enrichment of Ce3+ species. The observed 1-2 nm thick CeO2 wetting layer is responsible for the high specific activity observed in the catalytic oxidation of HCl. Moreover, enhanced stability of the CeO2 layer calcined at 600 °C can be attributed to the formation of a sharp interface with the ZrO2 support. Lastly, we applied two “deactivation-reactivation” cycles at 430 °C over fresh CeO2 and 20CeO2@ZrO2 catalyst respectively by switching the reaction gas mixture. It was unexpected that the activity of the 20CeO2@ZrO2 catalyst can be fully restored by oxygen exposure at 430 °C after the second deactivation/reactivation cycle, whereas the activity of the pure CeO2 catalyst decreases gradually after each cycle. Apparently, 20CeO2@ZrO2 catalyst exhibits superior regeneration performance to that of the CeO2 catalyst. Furthermore, a straightforward model is devised based on the Johnson-Mehl-Avrami-Kolmogorov approach to elucidate the reoxidation process of the catalyst. This observation is modelled by a faster nucleation rate in the supported 20CeO2@ZrO2 catalysts compared to the pure CeO2 catalyst. This can be explained by the abundant nucleation sites provided by the high surface area of ZrO2.

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Synthesis, Characterization and Catalytic Performance of Dedicated Ceria-Zirconia Catalysts for HCl Oxidation Reaction

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