Cu−Dinitrosyl Species in Zeolites: A Density Functional Molecular Cluster Study

Selective Formation of Cu Active Sites with Different Coordination States on Pseudospinel CuAl2O4 and Their NO Reduction Catalysis

In the spinel framework, copper (Cu) in two distinct coordination states exhibits catalytic activity for NO reduction through different mechanisms. However, detailed exploration of their respective catalytic properties, such as the redox behavior of Cu and substrate molecule adsorption, has been challenging due to difficulties in their separate formation. In this study, we present the controlled formation of pseudospinel CuAl2O4, containing exclusively tetrahedrally or octahedrally coordinated Cu, achieved by manipulating aging temperature and O2 concentration. Through these materials, we observed that in the CO-NO reaction, the step primarily determining the rate differs: NO reduction dominates with octahedrally coordinated Cu, whereas carbon monoxide (CO) oxidation is prominent with tetrahedrally coordinated Cu. The lower coordination number of Cu significantly benefits NO reduction but negatively impacts the CO-NO reaction, albeit positively influencing NO reduction in three-way catalytic reactions.

Flexible bonding between copper and nitric oxide: infrared photodissociation spectroscopy of copper nitrosyl cation complexes: [Cu(NO)n]+ (n = 1-5)

The infrared spectra of mass-selected mononuclear copper nitrosyl cation complexes [Cu(NO)n](+) with n = 1-5 and their argon tagged complexes are measured via infrared photodissociation spectroscopy in the nitrosyl stretching frequency region in the gas phase. The experimental spectra provide distinctive patterns allowing the determination of the geometries and electronic structures of these complexes by comparison with the predicted spectra from density functional theory computations. The argon tagged [Cu(NO)2Ar2](+) and [Cu(NO)3Ar](+) complexes as well as the higher n = 4 and 5 complexes each involve a bidentate (NO)2 dimer ligand, suggesting that ligand-ligand coupling plays an important role in the bonding of these cation systems. The results also show that argon tagging has a strong influence on the geometric and electronic structures of the n = 2 and 3 complexes. The [Cu(NO)4](+) cation is the most intense peak in the mass spectrum, which is characterized to be the fully coordinated ion with a D2d structure involving two (NO)2 units but with only 14-valence electrons on Cu. The [Cu(NO)5](+) cation complex is determined to involve a [Cu(NO)4](+) core ion that is coordinated by an external NO ligand.

Mono- and dinitrosyls on copper(I) site in a zeolite model: effects of static correlation

Multiconfigurational RASSCF/RASPT2 approach has been applied to investigate bonding of one and two nitric oxide (NO) molecules to a simple model of Cu(I) site in zeolite environment, Cu(I)[Al(OH)(4)]. Two binding modes were considered for the mononitrosyls and four alternative structures for the dinitrosyls (each one in either singlet or triplet state). Stabilities of the mono- and dinitrosyl complexes obtained from the multireference calculations were compared to the previously reported coupled cluster CCSD(T) results, as well as to DFT calculations performed here with various functionals, either hybrid or nonhybrid ones. RASSCF calculations provided also a qualitative insight into the electronic structure of the studied complexes, concerning mainly the interaction between the Cu and the NO ligand, and between the two NO fragments. Whereas the electronic structure of the mononitrosyls is dominated by a single configuration, the dinitrosyls have a considerably multireference character. Various effects of nondynamical correlation have been pointed out for these interesting species, trying to assess their impact on performance of the tested DFT methods.

Linkage isomerization in heme-NOx compounds: understanding NO, nitrite, and hyponitrite interactions with iron porphyrins

Nitric oxide (NO) and its derivatives such as nitrite and hyponitrite are biologically important species of relevance to human health. Much of their physiological relevance stems from their interactions with the iron centers in heme proteins. The chemical reactivities displayed by the heme-NOx species (NOx = NO, nitrite, hyponitrite) are a function of the binding modes of the NOx ligands. Hence, an understanding of the types of binding modes extant in heme-NOx compounds is important if we are to unravel the inherent chemical properties of these NOx metabolites. In this Forum Article, the experimentally characterized linkage isomers of heme-NOx models and proteins are presented and reviewed. Nitrosyl linkage isomers of synthetic iron and ruthenium porphyrins have been generated by photolysis at low temperatures and characterized by spectroscopy and density functional theory calculations. Nitrite linkage isomers in synthetic metalloporphyrin derivatives have been generated from photolysis experiments and in low-temperature matrices. In the case of nitrite adducts of heme proteins, both N and O binding have been determined crystallographically, and the role of the distal H-bonding residue in myoglobin in directing the O-binding mode of nitrite has been explored using mutagenesis. To date, only one synthetic metalloporphyrin complex containing a hyponitrite ligand (displaying an O-binding mode) has been characterized by crystallography. This is contrasted with other hyponitrite binding modes experimentally determined for coordination compounds and computationally for NO reductase enzymes. Although linkage isomerism in heme-NOx derivatives is still in its infancy, opportunities now exist for a detailed exploration of the existence and stabilities of the metastable states in both heme models and heme proteins.

Theoretical investigation of dinitrosyl complexes in Cu-zeolites as intermediates in deNOx process

The structure and stability of nitrosyl complexes formed in Cu-FER zeolite were investigated using a periodic DFT model. The reliability of both DFT methods and cluster models when describing the Cu(+) interaction with NO molecules was examined. The relative stabilities of mononitrosyl complexes on various Cu(+) sites in Cu-FER are governed by the deformation energy of the particular site. Three types of dinitrosyl complexes with different coordination on the Cu(+) cation were identified: (i) four-fold tetrahedral, (ii) four-fold square-planar and (iii) three-fold trigonal-planar complexes. The most stable dinitrosyl complex, formed when the two NO molecules interact with Cu(+)via the N atom, has a tetrahedral coordination on Cu(+). The cyclic adsorption complex, having a square-planar arrangement of ligands on Cu(+) and interaction via O atoms, is only about 10 kJ mol(-1) less stable than the N-down dinitrosyl complex. This cyclic dinitrosyl complex is suggested to be the key intermediate in the deNO(x) process taking place in Cu-zeolites.

A computational study on N2adsorption in Cu-ZSM-5

The present computational study investigates the adsorption of N(2) by Cu-ZSM-5, with particular regard to the interaction with pairs of Cu(+) ions, employing simple cluster models in the calculations. It shows that several interaction patterns between N(2) and couples of Cu(+) sites are possible within the Cu-ZSM-5 structure. In particular, when pairs of Cu(+) ions are located at opposite sides of ten-membered rings, in the region where linear and sinusoidal channels intersect each other, a quasi-linear Cu-N-N-Cu adsorption occurs. Although lattice restraints cause small deviations from linearity, such interaction turned out to be more favourable than other adsorption patterns within the Cu-ZSM-5 structure. The linearity of the Cu-N-N-Cu fragment and the relatively low concentration of the related sites cause a low extinction coefficient for the N-N IR stretching mode, which is usually detected with very low intensity or not detected at all. The results of the present calculations may explain the experimental evidence for a nearly IR-silent fraction of nitrogen strongly adsorbed in the Cu-ZSM-5 catalyst which, as shown in a previous work, is linearly related to the number of active sites for NO decomposition.

On the site-specificity of polycarbonyl complexes in Cu/zeolites: combined experimental and DFT study

The preferred Cu(+) sites and formation of mono-, di-, and tricarbonyl complexes in the Cu-FER were investigated at the periodic density functional theory level and by means of FTIR spectroscopy. The site-specificity of adsorption enthalpies of CO on Cu-FER and of vibrational frequencies of polycarbonyl complexes were investigated for various Cu(+) sites in Cu-FER. Large changes in the Cu(+) interaction with the zeolite framework were observed upon the formation of carbonyl complexes. The dicarbonyl complexes formed on Cu(+) in the main channel or on the intersection of the main and perpendicular channels are stable and both, adsorption enthalpies and CO stretching frequencies are not site-specific. The fraction of Cu(+) ions in the FER cage, that cannot form dicarbonyl can be determined from IR spectra (about 7% for the Cu-FER with Si/Al = 27.5 investigated here). The tricarbonyl complexes can be formed at the Cu(+) ions located at the 8-member ring window at the intersection of main and perpendicular channel. The stability of tricarbonyl complexes is very low (DeltaH degrees (0 K)>or=-4 kJ mol(-1)).

EPR Spectroscopy of Cu(I)−NO Adsorption Complexes Formed over Cu−ZSM-5 and Cu−MCM-22 Zeolites

The Cu(I)-NO adsorption complexes were formed over copper exchanged and autoreduced high siliceous Cu-ZSM-5 and Cu-MCM-22 zeolites and studied by EPR spectroscopy at X-, Q-, and W-band frequencies. The spin Hamiltonian parameters of the Cu(I)-NO species are indicative of a nitrogen-centered radical complex with a bent geometry and a significant contribution of the Cu(I) 4s atomic orbital to the wave function of the unpaired electron. Two different Cu(I)-NO species were found in both zeolites. It has been confirmed by comparing the experimental data with the results of previous theoretical studies that the presence of two different species is due to the formation of Cu(I)-NO adsorption complexes from two different Cu(I) sites in the zeolite matrix with different numbers of oxygen coligands. The structure of the two sites in the Cu-ZSM-5 and Cu-MCM-22 zeolites must be similar as the spin Hamiltonian parameters are found to be almost independent of the zeolite matrix, where the Cu(I)-NO complex is formed. The EPR signal intensity of the Cu(I)-NO species was studied as a function of the NO loading, and the formation of diamagnetic Cu(I)-(NO)(2) species with rising NO pressure at the expense of paramagnetic Cu(I)-NO monomers could be demonstrated for both systems at low temperatures.