The interaction of gold clusters with methanol molecules: Ab initio molecular dynamics of Aun+CH3OH and AunCH3OH

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.

Methanol C–O Bond Activation by Free Gold Clusters Probed via Infrared Photodissociation Spectroscopy

The activation of methanol (CD3OD and CD3OH) by small cationic gold clusters has been investigated via infrared multiphoton dissociation (IR-MPD) spectroscopy in the 615–1760 cm−1 frequency range. The C–O stretch mode around 925 cm−1 and a coupled CD3 deformation/C–O stretch mode around 1085 cm−1 are identified to be sensitive to the interaction between methanol and the gold clusters, whereas all other modes in the investigated spectral region remain unaffected. Based on the spectral shift of these modes, the largest C–O bond activation is observed for the mono-gold Au(CD3OD)+ cluster. This activation decreases with increasing the cluster size (number of gold atoms) and the number of adsorbed methanol molecules. Supporting density functional theory (DFT) calculations reveal that the C–O bond activation is caused by a methanol to gold charge donation, whereas the C–D and O–D bonds are not significantly activated by this process. The results are discussed with respect to previous experimental and theoretical investigations of neutral and cationic gold-methanol complexes focusing on the C–O stretch mode.

Probing the 2D-to-3D structural transition in gold clusters with a single sulfur atom: AuxS0,±1 (x = 1–10)

Gold sulfur clusters have received much attention because of the dramatic effect that the gold–sulfide interaction produces in thiol-passivated gold nanoparticles.

Theoretical Study of Methanol Decomposition Mediated by Au3+, Au3 and Au3−: Mechanism and Effect of Charge State of Gold on its Catalytic Activity

By performing density functional theory calculations, we studied methanol decomposition promoted by neutral, anionic and cationic Au trimers, which represent the three simplest prototypes of Au-cluster-based catalysts with different charge states. The results show that the Au3– and Au3+-mediated reactions proceed via four successive single dehydrogenation steps, CH3OH → CH3O → CH2O → CHO → CO, while the Au3–-mediated reaction occurs through two double dehydrogenation steps, CH3OH → CH 2O → CO. The additional negative charge reduces remarkably the binding capability of CO (the completely dehydrogenated product of methanol) on the cluster, and is thus beneficial in reducing poisoning of the catalyst by CO. In contrast, the neutral and positively charged clusters present strong interactions with CO, making the catalyst readily poisoned by CO. Furthermore, the reaction promoted by the cationic cluster shows a much higher energy barrier than those by the neutral and anionic clusters. So selecting suitable substrates that make Au nanoparticles negatively charged may be a promising strategy for promoting methanol oxidation.

Detection of OH Stretching Mode of CH3OH Chemisorbed on Ni3+ and Ni4+ by Infrared Photodissociation Spectroscopy

Structures of nickel cluster ions adsorbed with methanol, Ni3+ (CH3OH)m (m = 1-3) and Ni4+ (CH3OH)m (m = 1-4) were investigated by using infrared photodissociation (IR-PD) spectroscopy based on a tandem-type mass spectrometer, where they were produced by passing Ni3,4+ through methanol vapor under a multiple collision condition. The IR-PD spectra were measured in the wavenumber region between 3100 and 3900 cm-1. In each IR-PD spectrum, a single peak was observed at a wavenumber lower by approximately 40 cm-1 than that of the OH stretching vibration of a free methanol molecule and was assigned to the OH stretching vibrations of the methanol molecules in Ni3,4+ (CH3OH)m. The photodissociation was analyzed by assuming that Ni3,4+ (CH3OH)m dissociate unimolecularly after the photon energy absorbed by them is statistically distributed among the accessible modes of Ni3,4+ (CH3OH)m. In comparison with the calculations performed by the density functional theory, it is concluded that (1) the oxygen atom of each methanol molecule is bound to one of the nickel atoms in Ni3,4+ (defined as molecular chemisorption), (2) the methanol molecules in Ni3,4+ (CH3OH)m do not form any hydrogen bonds, and (3) the cross section for demethanation [CH4 detachment from Nin+ (CH3OH)] is related to the electron density distribution inside the methanol molecule.

Density functional investigation of the interaction of acetone with small gold clusters

The structural evolution of Au(n) (n=2, 3, 5, 7, 9, and 13) clusters and the adsorption of organic molecules such as acetone, acetaldehyde, and diethyl ketone on these clusters are studied using a density functional method. The detailed study of the adsorption of acetone on the Au(n) clusters reveals two main points. (1) The acetone molecule interacts with one gold atom of the gold clusters via the carbonyl oxygen. (2) This interaction is mediated through back donation mainly from the spd-hybridized orbitals of the interacting gold atom to the oxygen atom of the acetone molecule. In addition, a hydrogen bond is observed between a hydrogen atom of the methyl group and another gold atom (not involved in the bonding with carbonyl oxygen). Interestingly, the authors notice that the geometries of Au(9) and Au(13) undergo a significant flattening due to the adsorption of an acetone molecule. They have also investigated the role of the alkyl chain attached to the carbonyl group in the adsorption process by analyzing the interaction of Au(13) with acetaldehyde and diethyl ketone.

Ab Initio Molecular Dynamics Study of Methanol Adsorption on Copper Clusters

The preferential structures of small copper clusters Cun (n=2-9) and the adsorption of methanol molecules on these clusters are examined with first principles, molecular dynamics simulations. The results show that the copper clusters undergo systematic changes in bond length and bond order associated with altering their preferential structures from one-dimensional structures, to two-dimensional and three-dimensional structures. The results also indicate that low coordination number sites on the copper clusters are both the most favorable for methanol adsorption and have the greatest localization of electronic charge. The simulations predict that charge transfer between the neutral copper clusters and the incident methanol molecules is a key process by which adsorption is stabilized. Importantly, the changes in the dimensionality of the copper clusters do not significantly influence methanol adsorption.

Size-Dependent Carbon Monoxide Adsorption on Neutral Gold Clusters

We report on experiments probing the reactivity of neutral Au(n) clusters, n = 9-68, with carbon monoxide. The gold clusters are produced in a pulsed laser vaporization cluster source, operated at room temperature (RT) or at liquid-nitrogen temperature (LNT), pass through a low-pressure reaction cell containing CO gas, and are subsequently laser ionized. The reaction probabilities are determined by recording mass abundance spectra with time-of-flight mass spectrometry. The main observations are a strong temperature dependence and a remarkable size dependence. Upon cooling of the cluster source to LNT, the reactivity increases substantially. At LNT, the reaction probabilities for Au(n) with the first CO molecule are about a factor 10 higher than at RT. Moreover, adsorption of two, three, and even four CO molecules is observed, in contrast to RT clusters which at most adsorb one CO molecule. This temperature dependence is related to the lifetime of the cluster-molecule complexes, being much longer for cold clusters. The observed striking size dependence is similar at both temperatures and is discussed in terms of the electronic structure effects.

Detaching thiolates from copper and gold clusters: which bonds to break?

The interaction of alkanethiolates with small coinage metal clusters of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu(n)-thiolate (n = 1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n = 1 to 1.4 eV for n = 9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu(n) cluster. For the gold analogues, this effect is not observed and fragmentation at the S-C bond experiences only a slight 10% destabilization as n increases from 3 to 9. The relativistic origin of this difference between Cu and Au is discussed, and an analysis of bonding considerations is presented.

Binding of propene on small gold clusters and on Au(111): Simple rules for binding sites and relative binding energies

We use density functional theory (DFT) to investigate the bonding of propene to small gas-phase gold clusters and to a Au(111) surface. The desorption energy trends and the geometry of the binding sites are consistent with the following set of rules. (1) The bond of propene to gold is formed by donation of electron density from the highest occupied molecular orbital (HOMO) of propene to one of the low-lying empty orbitals [denoted by LUMO1, LUMO2, em leader (LUMO-lowest unoccupied molecular orbital)] of the gold cluster. (2) Propene binds to a site on the Au cluster where one of the low-lying LUMOs protrudes in the vacuum. Different isomers (same cluster, but different binding sites for propene) correspond to sites where different low-lying LUMOs protrude in space. (3) The desorption energy of the lowest energy isomer correlates with the energy of the lowest empty orbital of the cluster; the lower the energy of that LUMO, the higher the desorption energy. (4) If the lowest-lying LUMO protrudes into space at two nonequivalent sites at the edge of a cluster, propene binds more strongly to the site with the lowest coordination. These rules are consistent with the calculated bond energies and geometries for [Au(n)(C(3)H(6))](q), for n=1-5 and n=8 and q=-1, 0, +1. Based on them we have made a number of predictions that have been confirmed by DFT calculations. The bond of propene to gold is strengthened as the net charge of the cluster varies from -1, to zero, to +1. Compared to a gas-phase cluster, a cluster on a support binds propene more strongly if the support takes electron density from the cluster (e.g., a Au cluster on a gold surface) and more weakly if the support donates electron density to the cluster (e.g., a Au cluster on an oxygen vacancy on an oxide surface).