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