Comparative Evaluation of HFC-134a Decompositions Over Metal-Impregnated γ-Al2O3 Catalysts (M/γ-Al2O3) (M = Mg, Ni, Co, Zn, Cu)

Abstract

Al₂O₃, widely used in the catalytic decomposition of fluorocarbons, is known to be rapidly deactivated owing to its transformation to AlF₃. Accordingly, in this study, metal-impregnated γ-Al₂O₃ catalysts (M/γ-Al₂O₃) were synthesized and their long-term stabilities for HFC-134a decomposition were investigated under the conditions of 10,000 ppm HFC-134a in air balance. Although γ-Al₂O₃ and Mg/γ-Al₂O₃ demonstrated long-term activities, significantly longer than those of the other catalysts, activity of γ-Al₂O₃ rapidly decreased, whereas that of Mg/γ-Al₂O₃ gradually decreased. Although Mg/γ-Al₂O₃ exhibited the smallest Brunauer-Emmett-Teller specific surface area among those of the synthesized catalysts, it demonstrated excellent long-term stability and the lowest deactivation rate due to the high total amount of weak acid sites. Mg/γ-Al₂O₃ exhibited considerably smaller crystal size of AlF₃ compared to other catalysts. Additionally, after HFC-134a decomposition, the morphology of Mg/γ-Al₂O₃ was clearly less modified and metal agglomeration was lower than those of other catalysts. Finally, this was caused by the reaction of Mg with HF, forming MgF₂, which inhibited the complete conversion of γ-Al₂O₃ to AlF₃ and produced AlOF_X, an intermediate compound. These results suggest that Mg impregnation in γ-Al₂O₃-based catalysts is a suitable method for enhancing the performances of these catalysts in the long-term decomposition of HFC-134a.