Cu-CHA – a model system for applied selective redox catalysis

Abstract

We review the structural chemistry and reactivity of copper-exchanged molecular sieves with chabazite (CHA) topology, as an industrially applied catalyst in ammonia mediated reduction of harmful nitrogen oxides (NH3-SCR) and as a general model system for red-ox active materials (also the recent results in the direct conversion of methane to methanol are considered). Notwithstanding the apparent structural simplicity of the material, a crystalline zeolite with only one crystallographically independent T site, the Cu-SSZ-13 catalyst reveals a high degree of complexity that has been decrypted by state of the art characterization tools. From the reviewed data, the following important aspects in the understanding of the Cu-SSZ-13 catalyst clearly emerged: (i) the structural dynamics of the Cu-species require precise control of the environmental conditions during activation and characterization; (ii) the availability of a large library of well-defined catalysts with different Si/Al and Cu/Al compositional ratios is key in unravelling the red-ox properties of the active Cu sites; (iii) a multi-technique approach is required, combining complementary techniques able to provide independent structural, electronic and vibrational information; (iv) synchrotron radiation based techniques (EXAFS, XANES, XES and time-resolved powder XRD) played a relevant role; (v) operando methodology (possibly supported by advanced chemometric approaches) is essential in obtaining structure-reactivity relations; (vi) the support of theoretical studies has been indispensable for the interpretation of the experimental output from characterization and for a critical assessment of mechanistic models. The old literature that classified Cu-exchanged zeolites in the category of single-site catalysts has been partially disproved by the recent advanced studies where it has been shown that the active site in the low temperature NH3-SCR catalyst is a mobile Cu-molecular entity that "lives in symbiosis" with an inorganic solid framework. Only in the high temperature NH3-SCR regime do the mobile Cu-species lose their ligands and find docking sites at the internal walls of the zeolite framework, thus reflecting the idea of a single-site catalyst. After a brief introduction, the review is divided into three main parts devoted to characterization (Section 2), reactivity (Section 3), and industrial applications (Section 4), followed by some concluding remarks and providing a perspective of the field.

Understanding and Optimizing the Performance of Cu‐FER for The Direct CH4 to CH3OH Conversion

“…In the case of Cu‐MOR, and Cu‐ZSM‐5 the molCH 3 OH/molCu has been reported not to depend on the Cu loading. However, Cu–CHA exhibited a similar effect at low Cu/Al ratio as seen for Cu‐FER, which was explained by the coordination of Cu ions in the very stable Al 6r positions as bare Cu II , making the Cu species inactive for the conversion ,…”

“…The different activation times for laboratory tests (longer activation to enhance the productivity) and HERFD XANES experiments (shorter activation, dictated by the limited beam time availability) allow relevant qualitative correlations but hinder quantitative ones. Figure depicts the characteristic XANES features of the investigated Cu‐zeolites which were interpreted on the basis of the current Cu K‐edge XANES literature ,,,,…”

Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH3 at Low Temperature

“…Small pore Cu−CHA is thus the catalyst of choice for diesel engines SCR, thanks to its excellent performance and hydrothermal stability over a wide range of temperatures . Moreover, the structural simplicity of chabazite, allowing for very sophisticated experimental and computational studies, makes it a model system to solve fundamental issues of structure‐activity correlations in the wider context of transition metal‐exchanged zeolites …”

“…[1] Moreover, the structural simplicity of chabazite, allowing for very sophisticated experimental and computational studies, makes it a model system to solve fundamental issues of structure-activity correlations in the wider context of transition metal-exchanged zeolites. [2] The state of the Cu ions in CuÀ CHA depends on the Al distribution, on the Si/Al and Cu/Al ratios and on the presence of an oxidizing or reducing atmosphere. In an oxidizing atmosphere, Cu II ions can be stabilized in correspondence of a single Al framework atom, resulting in a Z[Cu II (OH)], or form a Z 2 Cu II site (being Z the framework negative charge induced by the presence of Al).…”

“…(1)] and the fast [Eq. (2)] SCR reactions can be divided in two distinct half-cycles: Cu II reduction (with release of N 2 and H 2 O) and Cu I re-oxidation, necessary to re-establish the active site for NO x decomposition. [4]…”