Understanding remarkable promotional effects of Zn on alumina in catalytic hydrolysis of perfluorocarbon

Abstract

Perfluorocarbons (PFCs) such as CF4 are extensively used as etchants and cleaning agents in semiconductor manufacturing. Because of their significant global warming potential, it is important to develop efficient technologies to decompose PFCs. In this study, we synthesized a series of Zn-promoted ?-Al2O3 catalysts with varying Zn contents for CF4 hydrolysis. Physical characterizations and kinetic studies indicated that the Lewis acid sites of unpromoted ?-Al2O3 predominantly adsorb H2O compared to CF(4 )under the reaction conditions, resulting in limited catalytic activity (H2O poisoning). As the Zn/Al molar ratio increased up to 0.1, nanocrystalline ZnAl2O4 was selectively formed on the surface of ?-Al2O(3) crystallites, generating a core-shell-like structure. The Lewis acid sites on the ZnAl2O(4) surfaces exhibited enhanced CF4 adsorption and weakened H2O adsorption compared to those on ?-Al2O3, thereby alleviating the detrimental effect of H2O poisoning. Consequently, despite the reduction in the total number of Lewis acid sites upon Zn addition, Zn-promoted ?-Al2O3 catalysts exhibited significantly enhanced catalytic activity. Furthermore, the ZnAl2O4 covering the ?-Al2O3 crystallites inhibited their phase transition to a-Al2O3, improving catalytic stability. When the Zn content was increased excessively (Zn/Al > 0.1), bulk ZnAl2O4 crystallites with low Lewis acidity were formed, resulting in a substantial loss of catalytic activity.