NH3-SCR over Cu/SAPO-34 – Zeolite acidity and Cu structure changes as a function of Cu loading

•Cu exchanged SAPO-34 catalysts with various loading are prepared by the solid state ion exchange method.•The zeolite acidity properties as a function of Cu loading are characterized.•Three different types of Cu structures are proved to present in the samples.•The relationship between NH3-SCR performance and Cu structures are investigated.

A series of SAPO-34 catalysts with various Cu loadings (ranging 0.7–3.0 wt%) were prepared by the solid state ion exchange method (SSIE) and the acid properties of the catalysts were characterized by NH3-TPD and in situ DRIFTS. During synthesis, more Cu migrates and exchanges at the Brønsted acid sites with increasing amounts of Cu precursor during the SSIE, resulting in a monotonic reduction in the total number of Brønsted acid sites. Interestingly, the Lewis acid sites created by Cu ions linearly increased with increasing Cu loading up to 2.0 wt% and then only slightly increased with a further increase of Cu loading up to 3.0 wt%. XRD, H2-TPR, UV–vis and DRIFTS were used to probe Cu structures. From the DRIFTS, two different perturbed TOT vibrations were observed for both the zeolite skeleton and NH3 adsorption spectra, indicating two different types of exchanged Cu species exist. This is also consistent with the results of NO adsorption DRIFTS spectra, in which two different Cu2+ sites were observed to adsorb NO molecules. One of the sites was considered to be the isolated Cu2+ species and the other could be another type of exchanged Cu site, possibly CuxOy clusters (dimeric or oligomeric Cu species). The isolated Cu2+ on each sample was further identified by UV–vis and quantified by H2-TPR experiments. Based on these results, isolated Cu2+ species were found to be the active sites of NH3-SCR at low temperature. Furthermore, CuO phases were detected on the samples with high Cu loadings via XRD. The presence of CuxOy may promote NH3 oxidation by O2, leading to the observed decrease in standard SCR performance at high temperature.

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