The H2O Effect on Cu Speciation in Cu-CHA-Catalysts for NH3-SCR Probed by NH3 Titration

The present work is focused on the effect of water NH3 adsorption over Cu-CHA SCR catalysts. For this purpose, samples characterized by different SAR (SiO2/Al2O3) ratios and Cu loadings were studied under both dry wet conditions. H2O adversely affects Lewis acid sites (Cu ions) all tested catalysts, as indicated decreased desorption at low temperature during TPD. Interestingly, NH3/Cu ratio, herein regarded an index for speciation cations, fell in range 3–4 (in presence gaseous NH3) or 1–2 (no conditions, line with formation NH3-solvated species (e.g., [CuII(NH3)4]2+ [CuII(OH)(NH3)3]+ NH3, [Z2CuII(NH3)2]2+ [ZCuII(OH)(NH3)]+ without NH3). When was fed to system, contrary, ratio always close 3 (or 1), while Brønsted acidity slightly increased. These results are consistent competition between hydrolysis a fraction Z2CuII into ZCuIIOH.