NH3-SCR technology is commercially used to meet the NOx emission limits of power plants and Diesel engines. Among the catalysts used for this application, Cu-CHA systems show extraordinary activity and hydrothermal stability in a broad temperature range under SCR conditions. Our work aims at understanding how the Cu content and the SiO2/Al2O3 ratio (SAR) of Cu-CHA catalysts affects the NOx removal efficiency. Indeed, these two indexes are important to determine the location, mobility and types of Cu active sites present in the zeolite catalysts, e.g. ZCuIIOH and Z2CuII, which in turn have an impact on the SCR activity. Two sets of Cu-CHA model catalysts were studied: one with different copper loadings (0 - 3% w/w) at a fixed SAR (25) and the another one with different SAR (10 - 25) and a fixed copper loading (2% w/w). NH3 and NO2 adsorption and related TPD runs were performed to evaluate the NH3 and nitrate storage properties on the catalysts. From the comparison between the adsorption tests of the un-promoted and promoted catalysts, two main preliminary results were obtained: i) nitrates can be stored only on Cu sites; ii) the low-T peak present in the typical NH3-TPD of Cu-CHA catalyst is ascribed to Cu-ions only, while the high-T peak is due to the Brønsted sites of the zeolite. The number of NH3 molecules coordinated by each Cu atom was evaluated: we observed a decrease of the NH3/Cu ratio on increasing SAR and %Cu. This behaviour can be ascribed to the nature of the different Cu sites existing within the framework depending on the Cu:Al ratio of the catalysts. We found that Cu ions are able to adsorb 3 or 4 NH3 molecules. As SAR and Cu loading increased we also observed both a progressive increase in the number of adsorbed molecules and a greater stability of these species (illustrated by a shift of the NO2-TPD peak to higher T). This can be explained again by the presence of different Cu species. In fact, due to higher SAR and Cu loadings a higher fraction of Cu ions with a lower charge density can be present in the zeolite, which results in an easier accommodation and stabilisation of negatively charged nitrates. This speciation was confirmed by performing TPR runs feeding a mixture of NH3 and NO as reducing agent: the two Cu species showed different reducibility, which could potentially influence their SCR activity.

SiO2/Al2O3 ratio and Cu-loading effects on the NH3-SCR activity of Cu-Chabazite catalysts

2019 Spring National Meeting

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