A density functional study on the adsorption of hydrogen molecule onto small copper clusters

Solvation of cationic copper clusters in molecular hydrogen

Multiply charged superfluid helium nanodroplets are utilized to facilitate the growth of cationic copper clusters (Cun+, where n = 1-8) that are subsequently solvated with up to 50 H2 molecules. Production of both pristine and protonated cationic Cu clusters are detected mass spectrometrically. A joint effort between experiment and theory allows us to understand the nature of the interactions determining the bonding between pristine and protonated Cu+ and Cu2+ cations and molecular hydrogen. The analysis reveals that in all investigated cationic clusters, the primary solvation shell predominantly exhibits a covalent bonding character, which gradually decreases in strength, while for the subsequent shells an exclusive non-covalent behaviour is found. Interestingly, the calculated evaporation energies associated with the first solvation shell markedly surpass thermal values, positioning them within the desirable range for hydrogen storage applications. This comprehensive study not only provides insights into the solvation of pristine and protonated cationic Cu clusters but also sheds light on their unique bonding properties.

Atom by atom built subnanometer copper cluster catalyst for the highly selective oxidative dehydrogenation of cyclohexene

The effect of particle size and support on the catalytic performance of supported subnanometer copper clusters was investigated in the oxidative dehydrogenation of cyclohexene. From among the investigated seven size-selected subnanometer copper particles between a single atom and clusters containing 2–7 atoms, the highest activity was observed for the titania-supported copper tetramer with 100% selectivity toward benzene production and being about an order of magnitude more active than not only all the other investigated cluster sizes on the same support but also the same tetramer on the other supports, Al2O3, SiO2, and SnO2. In addition to the profound effect of cluster size on activity and with Cu4 outstanding from the studied series, Cu4 clusters supported on SiO2 provide an example of tuning selectivity through support effects when this particular catalyst also produces cyclohexadiene with about 30% selectivity. Titania-supported Cu5 and Cu7 clusters supported on TiO2 produce a high fraction of cyclohexadiene in contrast to their neighbors, while Cu4 and Cu6 solely produce benzene without any combustion, thus representing odd–even oscillation of selectivity with the number of atoms in the cluster.

Molecular Adsorption of H2 on Small Neutral Silver-Copper Bimetallic Nanoparticles: A Search for Novel Hydrogen Storage Materials

In the search for novel hydrogen storage materials, neutral silver-copper bimetallic nanoparticles up to the size of eight atoms (Cu _m Ag _n : m + n ≤ 8) have been computationally studied. Density functional theory with the B3LYP exchange-correlation functional and the combined basis sets of LanL2DZ and aug-cc-pVQZ were used in all of the calculations. H₂ adsorption studies on the most stable cluster geometries of all of the neat and heterogeneous entities found that 12 potential candidates, CuAg₄, Cu₆, Cu₅Ag, Cu₄Ag₂, Cu₃Ag₃, Cu₂Ag₄, CuAg₆, Cu₅Ag₃, Cu₄Ag₄, Cu₃Ag₅, Cu₂Ag₆, and CuAg₇, fall within the recommended physisorption range of -18 to -6 kJ mol^-1. A correlation in the behavior of binding energy, vibrational frequency, average bond distance, highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, and chemical hardness with H₂ adsorption was observed. This analysis further revealed that the H₂ adsorption to the cluster was either a parallel or a perpendicular alignment. The analysis of the electron configuration of each atom in the cluster and the H₂ molecule and the charge transfer analysis of these 12 clusters also showed that the physisorption in the perpendicular mechanism is due to an induced dipole interaction, while that in the parallel mechanism is due to a weak ionic interaction. The clusters identified with perpendicular adsorption, CuAg₄H₂, Cu₆H₂, Cu₃Ag₃H₂, and Cu₂Ag₄H₂, polarized the H₂ molecule but had no charge transfer with the H₂ molecule and those identified with parallel adsorption, Cu₅AgH₂, Cu₄Ag₂H₂, CuAg₆H₂, Cu₅Ag₃H₂, Cu₄Ag₄H₂, Cu₃Ag₅H₂, Cu₂Ag₆H₂, and CuAg₇H₂, pulled the electrons from the H₂ molecule and had charge transfer with the H₂ molecule. The shapes of the frontier molecular orbital diagrams of the HOMO and LUMO also followed this observation.

A comparison of CO oxidation on cleaned ZnO [Formula: see text] surface and defective ZnO [Formula: see text] surface using density functional theory studies

In this work, we employed continuously the DFT calculations to study CO oxidation reaction on the defective ZnO [Formula: see text] surface. The oxygen (O) atom was removed from cleaned surface ZnO [Formula: see text] (CS-ZnO) to form the defective ZnO [Formula: see text] surface (DS-ZnO), which contained an O vacancy defect. Hereafter, the formation of oxygen vacancy was found to increase the adsorption abilities of O₂ and CO on DS-ZnO, in comparison to those on CS-ZnO. Many steps of elementary reactions including O₂ and CO adsorption, reacting between CO and O to form CO₂, and CO₂ desorption on DS-ZnO were investigated and calculated in terms of the configurations, activation energy, and reaction energy, to which the reaction pathway of CO oxidation has been found. Based on this pathway, the calculation results of the rate controlling step of 0.84 eV corresponding to the exothermic reaction energy of 4.11 eV on DS-ZnO indicated that the CO oxidation on DS-ZnO was more thermodynamically favorable and less kinetically desirable than that on CS-ZnO. In addition, the natural bonds of O₂ and CO adsorptions on DS-ZnO were also analyzed by the partial density of state (PDOS) and the electron density difference (EDD) contour plots.

IR Spectroscopic Characterization of H2 Adsorption on Cationic Cun+ (n = 4-7) Clusters

IR spectra of cationic copper clusters Cu_n⁺ (n = 4–7) complexed with hydrogen molecules are recorded via IR multiple-photon dissociation (IRMPD) spectroscopy. To this end, the copper clusters are generated via laser ablation and reacted with H₂ and D₂ in a flow-tube-type reaction channel. The complexes formed are irradiated using IR light provided by the free-electron laser for intracavity experiments (FELICE). The spectra are interpreted by making use of isotope-induced shifts of the vibrational bands and by comparing them to density functional theory calculated spectra for candidate structures. The structural candidates have been obtained from global sampling with the minima hopping method, and spectra are calculated at the semilocal (PBE) and hybrid (PBE0) functional level. The highest-quality spectra have been recorded for [5Cu, 2H/2D]⁺, and we find that the semilocal functional provides better agreement for the lowest-energy isomers. The interaction of hydrogen with the copper clusters strongly depends on their size. Binding energies are largest for Cu₅⁺, which goes hand in hand with the observed predominantly dissociative adsorption. Due to smaller binding energies for dissociated H₂ and D₂ for Cu₄⁺, also a significant amount of molecular adsorption is observed as to be expected according to the Evans–Polanyi principle. This is confirmed by transition-state calculations for Cu₄⁺ and Cu₅⁺, which show that hydrogen dissociation is not hindered by an endothermic reaction barrier for Cu₅⁺ and by a slightly endothermic barrier for Cu₄⁺. For Cu₆⁺ and Cu₇⁺, it was difficult to draw clear conclusions because the IR spectra could not be unambiguously assigned to structures.

Ab initio investigation of the role of the d-states occupation on the adsorption properties of H2, CO, CH4 and CH3OH on the Fe13, Co13, Ni13 and Cu13 clusters

Here, we report a theoretical investigation, based on density functional theory calculations, into the role of the occupation d-states on the adsorption properties of CH₄, CO, H₂ and CH₃OH on 3d 13-atom transition-metal (TM₁₃) clusters (TM = Fe, Co, Ni, Cu). Except for Cu₁₃, a gradual increase in the occupation of the d-states, i.e., from Fe₁₃ to Ni₁₃, increases the magnitude of the adsorption energy almost linearly for the H₂/TM₁₃ and CO/TM₁₃ systems, which can be explained by the enhancement of the sp-d hybridization due to the shift of the d-states towards the highest occupied molecular orbital (HOMO). For Cu₁₃, the d-states are located well below the HOMO, which reduces the sp-d hybridization, and hence, a smaller adsorption energy is obtained. However, this picture does not hold for CH₄/TM₁₃ and CH₃OH/TM₁₃, where the adsorption energy has nearly the same value for all TM₁₃ clusters, which can be explained by electrostatic effects such as local polarization of the molecules and nearby TM atoms, and hence, the basic features of physisorption systems. Based on the electron density difference, the polarization effects are slightly larger for systems with empty d-states.

Computational screening of carbon monoxide (CO) adsorption over neutral and charged Al7 clusters

A density functional theory study on the structures and chemical bonding of charged (Al7 + and Al7 -) and neutral Al7 clusters is presented. A distorted octahedral structure with an aluminum atom decorating one of the aluminum faces of the octahedron is predicted for these clusters. The AdNDP analysis reveals double (σ- and π-) aromatic and antiaromatic characteristics of Al7 + and Al7 - clusters, respectively. The UV-Vis Spectra of these clusters are also investigated using TD-DFT method. The molecular adsorption of carbon monoxide on the mentioned clusters is also explored. It is found that, the binding of CO through its carbon atom on considered clusters is a physical adsorption and Al7 - cluster shows the most tendency for the CO adsorption. The NBO analysis and density of states spectra confirm the weak interaction between carbon atom of CO and the aluminum atom of these clusters.

A Systematic Investigation on CrCun Clusters with n = 9-16: Noble Gas and Tunable Magnetic Property

A systematic investigation on structure, dissociation behavior, chemical bonding, and magnetic property of Cr-doped Cun clusters (n = 9-16) is carried out using the mean of density functional theory calculations. It is found that CrCu12 is a crucial size, preferring an icosahedral Cu12 cage with the central Cr dopant. Smaller cluster sizes appear as on the way to form the CrCu12 icosahedron while larger ones are produced by attaching additional Cu atoms to the CrCu12 core. The presence of Cr dopant obviously enhances the stability of CrCun clusters in comparison to that of pure counterparts. Exceptionally stable CrCu12 has an 18-electron closed-shell electronic structure, mimicking a noble gas in the viewpoint of superatom concept. Analysis on cluster electronic structure shows that the interplay between 3d orbitals of Cr and 4s orbitals of Cu has a vital role on the magnetic properties of CrCun clusters.

Insights into the structural, electronic and magnetic properties of V-doped copper clusters: comparison with pure copper clusters

The structural, electronic and magnetic properties of Cu_n+1 and Cu_nV (n = 1–12) clusters have been investigated by using density functional theory. The growth behaviors reveal that V atom in low-energy Cu_nV isomer favors the most highly coordinated position and changes the geometry of the three-dimensional host clusters. The vibrational spectra are predicted and can be used to identify the ground state. The relative stability and chemical activity of the ground states are analyzed through the binding energy per atom, energy second-order difference and energy gap. It is found that that the stability of Cu_nV (n ≥ 8) is higher than that of Cu_n+1. The substitution of a V atom for a Cu atom in copper clusters alters the odd-even oscillations of stability and activity of the host clusters. The vertical ionization potential, electron affinity and photoelectron spectrum are calculated and simulated for all of the most stable clusters. Compare with the experimental data, we determine the ground states of pure copper clusters. The magnetism analyses show that the magnetic moments of Cu_nV clusters are mainly localized on the V atom and decease with the increase of cluster size. The magnetic change is closely related to the charge transfer between V and Cu atoms.