N2O decomposition over Cu-ZSM-5 catalysts prepared by the solid-state ion exchange between NH4-ZSM-5 and copper(ii) coordination polymer

Abstract

The second non-CO₂ greenhouse gas mentioned under the Kyoto Protocol of the United Nations Convention on Climate Change (December 1997) was N₂O. According to this protocol, the European Union, Canada, and Japan were committed to reducing their collective greenhouse gas (GHG) emissions by slightly more than 5 percent below their 1990 levels. Developed countries, also, we are asked to reduce our GHG emissions to at least 5 percent of the 1990 levels, and this is to be achieved till 2012. Since in many situations, N₂O formation is inevitable in the applied processes, catalysts offer a route to N₂O abatement. Over the past several years, more articles have been released that have tried to propose paths through which reduction can be achieved. Nitrous oxide emission, one of the suggested solutions, involves the direct catalytic decomposition of N₂O into its elements. Transition metal exchange zeolites, such as Cu-ZSM-5, Co-ZSM-5, and Fe-ZSM-5, have been reported to exhibit high catalytic activity towards N₂O direct decomposition. This study focused on the activity of a series of Cu-ZSM-5 catalysts, with copper exchange levels of 10–150%, for nitrous oxide (N₂O) decomposition. Different Cu-ZSM-5 catalysts were prepared by applying the solid state ion exchange (SSIE) method through the interaction between NH₄-ZSM-5 and the copper(II) complex. Thermal events emerged during the heat treatment, from ambient temperature to 700 °C, of the two precursors, which were followed by thermal analyses, viz. TGA and DTA. Based on the obtained thermal analysis profiles, different mixtures containing NH₄-ZSM-5 and copper(II) coordination polymers were calcined at 550 °C for 3 h in a static air atmosphere. Various integrated physicochemical techniques were used in the characterization of the obtained Cu-ZSM-5 catalysts, including XRD, FT-IR spectroscopy, SEM, N₂-adsorption and electrical conductivity analyses. It was found that the suggested preparation route led to the formation of Cu-ZSM-5, where no CuO or reaction precursors were detected in the 550 °C calcined samples. However, the crystallinity of the obtained Cu-ZSM-5 catalysts was found to decrease with the copper exchange level. The activity of the different Cu-ZSM-5 catalysts was tested for N₂O decomposition in the temperature range of 150–500 °C. The highest activity was exhibited by the catalyst with a copper exchange level of 75%.