The structure of copper sites in Cu-SSZ-13 during NH3-SCR was unravelled by a combination of novel operando X-ray spectroscopic techniques. Strong adsorption of NH3 on Cu, its reaction with weakly adsorbed NO from the gas phase, and slow re-oxidation of Cu(I) were proven. Thereby the SCR reaction mechanism is significantly different to that observed for Fe-ZSM-5.

X-ray absorption spectroscopy (XAS) at the Cu K-edge is an important tool for probing the properties of copper centers in transition-metal chemistry and catalysis. However, the interpretation of experimental XAS spectra requires a detailed understanding of the dependence of spectroscopic features on the local geometric and electronic structure, which can be established by theoretical X-ray spectroscopy. Here, we present a systematic computational study of the Cu K-edge XAS spectra of selected Cu complexes based on time-dependent density-functional theory in combination with a molecular orbital analysis of the relevant transitions. For a series of Cu ammine model complexes as well as a comprehensive test set of 12 Cu(I) and 5 Cu(II) complexes, we revisit the dependence of the pre-edge region in Cu K-edge XAS spectra on oxidation state and coordination geometry. While our calculations confirm earlier experimental assignments, we can also reveal additional signatures of the ligand orbitals and identify the underlying orbital interactions. The comprehensive picture revealed by this study will provide a reliable basis for the interpretation of in situ Cu K-edge XAS spectra of catalytic intermediates.

We present a methodology for analyzing the dependence of molecular spectra calculated with quantum-chemical methods on the underlying molecular structure. This analysis is applied to investigate the structural sensitivity of calculated valence-to-core X-ray emission (VtC-XES) spectra for the test case of three iron carbonyl complexes, Fe(CO) 5 , [FeCp(CO) 2 (THF)] 1 (Cp 5 cyclopentadienyl, THF 5 tetrahydrofuran), and Fe(CO) 3 (cod) (cod 5 cyclooctadienyl). Based on this analysis, we discuss how the VtC-XES spectra depend on changes of metal-ligand bond distances and bond angles as well as on the structure of the ligands. The benefits of such an analysis of the structural sensitivity are discussed. Our methodology can serve as a first step toward quantifying and accounting for uncertainties due to the underlying model structure in theoretical spectroscopy.inverse quantum chemistry, theoretical spectroscopy, uncertainty quantification, X-ray spectroscopy 1 | I N TR ODU C TI ON Spectroscopy is an essential tool for unraveling molecular structure. However, only few spectroscopic techniques, most importantly nuclear magnetic resonance (NMR) spectroscopy or X-ray absorption fine structure spectroscopy (EXAFS), provide direct access to structural parameters such as interatomic distances or coordination numbers and thus allow for the direct determination of molecular structures and/or a direct structural refinement. In contrast, many important spectroscopic techniques such as vibrational spectroscopy, UV/Vis spectroscopy, or X-ray absorption and emission spectroscopy, only provide indirect access to structural information. In this case, spectroscopic experiments are often combined with quantum-chemical calculations [1] to connect them to specific features of the underlying molecular structure.For instance, vibrational spectroscopy in combination with quantum-chemical calculations can be used to determine the gas-phase structures of polypetides [2][3][4] or to assign the absolute configuration of chiral molecules. [5] The analysis of calculated vibrational spectra can provide detailed insights into the connection between specific spectral features and the underlying molecular structure. [6][7][8] For X-ray spectroscopy, quantumchemical calculations can provide an assignment of the peaks in X-ray emission and absorption spectra to occupied and unoccupied electronic states, and can connect those to the structure of the ligand environment in transition-metal complexes (see e.g., ).Here, we will focus on valence-to-core (VtC) X-ray emission spectroscopy (XES) [13,14] as an example. In combination with quantum-chemical calculations, VtC-XES can be used to obtain partial structural information, in particular in the vicinity of transition metal centers. VtC-XES makes it possible to identify which ligands are coordinated to a transition metal center, for instance in molecular transition metal complexes, [11,[15][16][17] for catalytic metal centers in zeolites, [18,19] and for metal clusters in enzymes. [20][21][22] The ability of...

While the Hieber anion [Fe­(CO)3(NO)]− has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe–NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d8 reference Fe­(CO)5 and the formal 3d10 reference [Fe­(CO)4]2– by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe­(CO)4]2–. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

We computationally investigate the mechanism of the reduction half-cycle of the selective catalytic reduction of nitrogen oxides with ammonia. We compare both Fe- and Cu-exchanged zeolite catalysts and aim at exploring all accessible reaction pathways. From our calculations, a comprehensive picture emerges that unifies several previous mechanistic proposals. We find that both for Fe and for Cu catalysts different reaction pathways are feasible but some of the possible reaction pathways differ in these two cases. Our computational results provide a basis for the interpretation of in situ spectroscopic investigations that can possibly distinguish the different mechanistic pathways.

X-ray absorption spectroscopy (XAS) at the Cu K-edge is an important tool for probing the properties of copper centers in transition metal chemistry and catalysis. However, the interpretation of experimental XAS spectra requires a detailed understanding of the dependance of spectroscopic features on the local geometric and electronic structure, which can be established by theoretical X-ray spectroscopy. Here, we present a systematic computational study of the Cu K-edge XAS spectra of selected Cu complexes based on time-dependent density-functional theory in combination with a molecular orbital analysis of the relevant transitions. For a series of Cu ammine model complexes as well as a comprehensive test set of 12 Cu(I) and 5 Cu(II) complexes, we revisit the dependance of the pre-edge region in Cu K-edge XAS spectra on oxidation state and coordination geometry. While our calculations confirm earlier experimental assignments, we can also reveal additional signatures of the ligand orbitals and identify the underlying orbital interactions. The comprehensive picture revealed by this study will provide a reliable basis for the interpretation of <i>in situ</i> Cu K-edge XAS spectra of catalytic intermediates.

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<div> <div> <div> <p>We computationally investigate the mechanism of the reduction half-cycle of the selective catalytic reduction (SCR) of nitrogen oxides with ammonia. We compare both Fe- and Cu-doped zeolite catalysts and aim at exploring all accessible reaction pathways. From our calculations, a comprehensive picture emerges that unifies sev- eral previous mechanistic proposals. We find that both for Fe and for Cu catalysts, different reaction pathways are feasible, but some of the possible reaction pathways differ in these two cases. Our computational results provide a basis for the inter- pretation of in situ spectroscopic investigations that can possibly distinguish the different mechanistic pathways. </p> </div> </div> </div>