Selective catalytic reduction of NO with NH3 over Cr-ZSM-5 catalysts: General characterization and catalysts screening

Cr exchanged ZSM-5 catalysts (Cr/Al = 1) prepared by solid-state ion exchange were tested in SCR of NO with ammonia. Since CrCl3 precursor sublimates during the solid-state reaction, chromate species were stabilized in the exchange sites of H+-ZSM-5 (Si/Al = 15) and Cr2O3 formation is, therefore, limited. However, using Cr nitrate and NH4+-ZSM-5 (Si/Al = 26) zeolite, massive particles of α-Cr2O3 occupied the catalyst surface. At the contrary, well-dispersed particles of Cr2O3 were detected with H+-ZSM-5 and Cr nitrate precursor. Starting from Cr acetate precursor, agglomerates of amorphous oxide were detected with H+-ZSM-5 like support. In the temperature range 50–300 °C, Cr chloride and Cr nitrate precursors and H+-ZSM-5 led to highly active catalysts, while Cr nitrate and NH4+-ZSM-5 led to a poorly active catalyst. However, catalysts issued from Cr acetate are promising in SCR but inefficient agglomerates of oxide (amorphous or crystalline chromia) could be avoided with NH4+-ZSM-5 like support.

Graphical abstract. Cr-ZSM-5 catalysts exhibited promising catalytic properties in SCR of NO with NH3 at low temperatures. Higher catalytic activities of Cr-ZSM-5 catalysts provided from a synergy between two crucial preparation parameters: Si/Al molar ratio and the nature of Cr precursor.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Cr-ZSM-5 catalysts (Cr/Al = 1) were tested in SCR of NO with NH3. ► Cr2O3 morphology was elucidated by UV–vis DR spectroscopy (B and μ-parameters). ► Agglomerates of crystalline and amorphous Cr2O3 are inefficient in SCR of NO. ► Poly and/or monochromates, Cr2O3 clusters and Cr oxo-cations are active species. ► Cr-ZSM-5 catalysts, being abandoned, are promising in low-temperature SCR.