Cu-exchanged 3D-printed geopolymer/ZSM-5 monolith for Selective Catalytic Reduction of NOx

Selective Catalytic Reduction (SCR) is the most widely used deNOx process for NOx removal from both stationary and mobile sources involving the reduction of NOx to N2, generally with ammonia, over a suitable catalyst [1]. The location of the deNOx unit downstream the electric precipitation and desulfurization unit in power plants or the application of the SCR process to the after-treatment of exhaust gas of diesel engine, typically exiting at a temperature <350°C, require the development of catalysts operating at quite low temperature with respect to the "high dust" SCR process operating at 400-500°C with V2O5-WO3/TiO2 catalysts. Cu-CHA zeolite showed a superior deNOx activity over a wide temperature window and excellent hydrothermal stability as well [2]. Nevertheless, zeolite-type catalysts are currently limited by the inability to produce self-supported monoliths or foams. Zeolite washcoated ceramic substrate are generally produced with a consequent low zeolite loading and/or loss of the active phase due to poor adhesion and unmatched thermal expansion with the substrate especially in mobile applications. The combination of zeolite with geopolymer could provide the required geometrical structure and, at the same time, a hierarchical porosity (from macro to micro pores) which can be involved in the catalyzed reaction when a suitable active metal ion (Cu, Fe) is incorporated or exchanged in the composite catalyst. In this work 3D-printed geopolymer containing about 23% ZSM-5 was produced as potential structured zeolite-based catalyst for SCR reaction after the introduction of copper as exchanged cation. The 3D printing technique of the ZSM-5-geopolymer mixture is described and the preliminary procedure to reduce the strong basicity of the starting composite material, hindering the copper exchange, is discussed. The effect of pre-treatments on the physical and morphological structure of both geopolymer and zeolite and on the nature of copper was investigated using characterization techniques such as SEM, XRD, N2 physisorption, Temperature Programmed Reduction (TPR).