Improved Arsine Removal Efficiency Over MnO_<i>x</i> Supported Molecular Sieves Catalysts via Micro-Oxygen Oxidation

Abstract

With the aim of improving the arsine (AsH 3 ) removal efficiency, 5A molecular sieves were modified by the transition metal oxides and used as catalysts. MnO x catalysts supported on 5A molecular sieves (Mn-5A) were prepared by an impregnation method and subsequently tested toward the catalytic oxidation of AsH 3 under micro-oxygen conditions at 150 °C. The effects of the metal oxide types that supported, the impregnate concentration of manganese nitrate, the calcination temperature, the oxygen content, and the fixed bed reaction temperature on the AsH 3 removal efficiency were studied. The optimum catalyst was prepared by a solution impregnation method with a 0.5 mol·L –1 Mn 2+ aqueous solution and subsequently calcined at 500 °C. This catalyst showed the highest activity toward the oxidation of AsH 3 among the samples prepared herein (100% conversion) over a good low-temperature operation window (120–160 °C) and under low oxygen concentration (0–2.0%) conditions. The majority presence of Mn 4+ active species is believed to be one predominant factor accounting for the excellent AsH 3 removal efficiency of this catalyst. The dominant active Mn 4+ species of Mn-5A is found to be a mixture of MnO 2, Mn 3 O 4, and Mn 2 O 3 . BET area analysis coupled with EDS measurement show that after oxidizing AsH 3 with O 2, the resulting As 2 O 5 seems to block the surface of Mn-5A leading to a decrease of the activity. XPS data indicates that the oxidation state of MnO x species on Mn-5A was not affected after reaction. So the main deterioration mechanism of Mn-5A on AsH 3 removal under our reaction condition is mainly due to physical blocking effect. According to the characterizations results, the Mn-5A catalyst possessed a higher number of active sites than the rest of samples and formed active intermediates during reaction, both factors leading the highly efficient removal of AsH 3 under low-temperature micro-oxygen conditions.