Reactions of Laser-Ablated Gold with Nitric Oxide: Infrared Spectra and DFT Calculations of AuNO and Au(NO)2 in Solid Argon and Neon

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Various Bond Interactions between NO and Anionic Gold Clusters: A Theoretical Calculation

We studied the electronic and geometrical structures of AunNO- (n = 1-20) using the B3LYP method with relatively large basis sets to understand the size dependent reactivities of Aun- with...

Low-Temperature Disproportionation Reaction of NO on Au6-: A Mechanism Involving Three NO Molecules Promoted by the Negative Charge

Conversion of NO to other nitrogen oxides is an elementary step in its catalytic removal processes. On coinage metal surfaces, two kinds of NO activation mechanisms have been well documented: the unimolecular dissociation of NO generates two adsorbed atoms, and the dissociation of an adsorbed (NO)₂ unit generates an adsorbed O and a free N₂O. In this work, we observed a disproportionation mechanism involving three NO molecules on Au₆^- at a very low temperature (150 K), in which an adsorbed (NO)₂ reacts with a free NO forming an adsorbed NO₂ and a free N₂O. The density functional theory (DFT) calculations indicated that this disproportionation step is significantly exothermic and has a very low activation barrier. The charge distributions on the involved cluster complexes and the correlation between the activity and the electronic properties of Au₆^- indicate the important role of extra negative charge in all reaction steps. The disproportionation mechanism revealed in this work could possibly exist in the NO removal processes on real gold catalysts.

The adsorption and activation of NO on silver clusters with sizes up to one nanometer: interactions dominated by electron transfer from silver to NO

Evidence for NO unitary adsorption, the formation of (NO)₂ and the reduction to form N₂O is observed on silver clusters with sizes up to one nanometer. The adsorption and activation of NO are enhanced by electron transfer from silver to NO.

Infrared photodissociation spectroscopy of mass-selected silver and gold nitrosyl cation complexes

The [M(NO)n](+) cation complexes (M = Au and Ag) are studied for exploring the coordination and bonding between nitric oxide and noble metal cations. These species are produced in a laser vaporization supersonic ion source and probed by infrared photodissociation spectroscopy in the NO stretching frequency region using a collinear tandem time-of-flight mass spectrometer. The geometric and electronic structures of these complexes are determined by comparison of the distinctive experimental spectra with simulated spectra derived from density functional theory calculations. All of these noble metal nitrosyl cation complexes are characterized to have bent NO ligands serving as one-electron donors. The spectrum of [Au(NO)2Ar](+) is consistent with 2-fold coordination with a near linear N-Au-N arrangement for this ion. The [Au(NO)n](+) (n = 3-4) cations are determined to be a mixture of 2-fold coordinated form and 3- or 4-fold coordinated form. In contrast, the spectra of [Ag(NO)n](+) (n = 3-6) provide evidence for the completion of the first coordination shell at n = 5. The high [Au(NO)n](+) and [Ag(NO)n](+) (n ≥ 3 for Au, n ≥ 4 for Ag) complexes each involve one or more (NO)2 dimer ligands, as observed in the copper nitrosyl cation complexes, indicating that ligand-ligand coupling plays an important role in the structure and bonding of noble metal nitrosyl cation complexes.

THEORETICAL STUDY OF NITROGEN MONOXIDE ADSORPTION ON RHODIUM CLUSTERS AT DIFFERENT SITES

The adsorption of nitrogen monoxide NO with charged and neutral [Formula: see text] clusters at atop, bridge, and threefold hollow sites had been investigated by density functional theory calculations. The results showed that rhodium clusters had strong orbital interactions with NO and formed the complex [ Rh n NO ]-/0/+. The stretching vibrational frequencies of the N–O bonds changed with the different adsorption sites and clusters sizes. The interactions between rhodium clusters and NO molecular could be described through the donation and back-donation of their frontier orbitals. The more back donation from Rh to NO , the weaker the N–O bonds, exhibiting that the lengthening of the N–O bond length and the lowering of its vibrational frequency. In general, the donation and back-donation interactions followed the tendencies: anionic > neutral > cationic, big size > small size, threefold hollow site > bridge site > atop site.

Adsorption of molecular hydrogen and hydrogen sulfide on Au clusters

The authors present theoretical results describing the adsorption of H2 and H2S molecules on small neutral and cationic gold clusters (Au(n)((0/+1)), n=1-8) using density functional theory with the generalized gradient approximation. Lowest energy structures of the gold clusters along with their isomers are considered in the optimization process for molecular adsorption. The adsorption energies of H2S molecule on the cationic clusters are generally greater than those on the corresponding neutral clusters. These are also greater than the H2 adsorption energies on the corresponding cationic and neutral clusters. The adsorption energies for cationic clusters decrease with increasing cluster size. This fact is reflected in the elongations of the Au-S and Au-H bonds indicating weak adsorption as the cluster grows. In most cases, the geometry of the lowest energy gold cluster remains planar even after the adsorption. In addition, the adsorbed molecule gets adjusted such that its center of mass lies on the plane of the gold cluster. Study of the orbital charge density of the gold adsorbed H2S molecule reveals that conduction is possible through molecular orbitals other than the lowest unoccupied molecular orbital level. The dissociation of the cationic Au(n)SH2+ cluster into Au(n)S+ and H2 is preferred over the dissociation into Au(m)SH2+ and Au(n-m), where n=2-8 and m=1-(n-1). H2S adsorbed clusters with odd number of gold atoms are more stable than neighboring even n clusters.

Infrared Spectroscopic and Theoretical Studies on the Reactions of Copper Atoms with Carbon Monoxide and Nitric Oxide Molecules in Rare-Gas Matrices

Reactions of laser-ablated Cu atoms with CO and NO mixtures in solid argon and neon have been investigated using matrix-isolation infrared spectroscopy. Copper carbonyls and copper nitrosyls have been observed, whereas copper carbonyl nitrosyl complexes are absent from the present experiments. New products, (CuCO)2, [NO]Cu[NO], Cu2(mu2-NO), and Cu(NO)2Cu, have been formed in the copper experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and comparison with theoretical predictions. Density functional theory calculations have been performed on these copper carbonyls and copper nitrosyls, which support the identification of these products from the matrix infrared spectrum. A plausible reaction mechanism has been proposed to account for the formation of copper carbonyls and copper nitrosyls. Similar matrix experiments with Ag and Au produce no new species.

Theoretical Study on Silver- and Gold-Loaded Zeolite Catalysts: Thermodynamics and IR Spectroscopy

Density functional calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of NO, N(2)O, CO, O(2), and H(2)O, on a model for Ag(I) and Au(I) ion-exchanged ZSM-5 catalysts. Using statistical mechanics, the DeltaH and DeltaG values were calculated in order to evaluate the stability of the adsorbates on Ag(I) and Au(I) sites. The calculated vibrational frequencies are in reasonable agreement with the reported experimental values. The analysis of the results shows that at 475 degrees C the adsorption of two NO molecules and the direct N(2)O decomposition on AgZSM-5 are thermodynamically unfavorable. The adsorption of one NO molecule presents a small positive DeltaG value. On the contrary, in the case of AuZSM-5, the adsorption of one NO molecule and the direct N(2)O decomposition to produce N(2) are thermodynamically favorable. For both models, the N(2)O decomposition by AgO and AuO species is thermodynamically very favorable. The analysis of the interaction with H(2)O shows that water displaces the adsorbed NO on AgZSM-5 but not on AuZSM-5 which indicates that the AuZSM-5 catalyst is less sensitive to deactivation by H(2)O than the AgZSM-5 catalyst.

Theoretical study of nitric oxide adsorption on Au clusters

The adsorption properties of NO molecule on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory with the generalized gradient approximation, and with the hybrid functional. For anionic and cationic clusters, the charge transfer between the Au clusters and NO molecule and the corresponding weakening and elongation of the N-O bond are essential factors of the adsorption. The neutral Au clusters have also remarkable adsorption ability to NO molecule. The adsorption energies of NO on the cationic clusters are generally greater than those on the neutral and anionic clusters.