Al4 Clusters Research Articles

Sulfonylcalix[4]arene-based Al4 clusters: three aluminum oxo clusters with different coordinated solvents for the CO2 cycloaddition reaction

By using a macrocyclic H4BTC4A-SO2 ligand, we successfully synthesized three Al4 clusters with different solvents, which exhibit different potentials to expose metal active sites for the catalytic reaction. of cycloaddition of carbon dioxide.

Oxygen Activation on Four-Atom Metal Clusters and Alloys

The dissociation of oxygen molecules and generation of two separated oxygen atoms is an important step in fuel cells. The activation of the bond linking two oxygen atoms in the oxygen molecule usually needs large activation barriers on metals utilized as catalysts. In the present study, the characterization and catalytic performance of four-atom Al nanocluster (Al4) as well as XAl3 (X = Ti and Sc) nanoalloys in activation of the oxygen molecule are investigated using density functional theory. Our results show that the tetrahedral isomer of the Al4 nanocluster lies lower in energy (Ecoh = −33.35 kcal/mol) than does the corresponding rhombus isomer (Ecoh = −31.05 kcal/mol). Also, transition metal (Ti and Sc) doping reduces the reactivity of the Al4 nanocluster. In consistent with the suggestion of reactivity descriptors used in the present study, our results reveal that the maximum catalytic performance in activation of the oxygen-oxygen bond is related to the pristine Al4 clusters. The energy barrier of O2 activation on the Al4 system (21.26 kcal/mol) exhibits that the activation of the bond linking of two oxygen atoms on Al4 is kinetically preferable. In this respect, our study shows that the Al4 nanocluster can be considered as an excellent catalyst for activation the oxygen molecule.

Oxygen activation on four-atom metal clusters and alloys

The dissociation of oxygen molecules and generation of two separated oxygen atoms is an important step in fuel cells. The activation of the bond linking two oxygen atoms in the oxygen molecule usually needs large activation barriers on metals utilized as catalysts. In the present study, the characterization and catalytic performance of four-atom Al nanocluster (Al4) as well as XAl3 (X = Ti and Sc) nanoalloys in activation of the oxygen molecule are investigated using density functional theory. Our results show that the tetrahedral isomer of the Al4 nanocluster lies lower in energy (Ecoh = –33.35 kcal/mol) than does the corresponding rhombus isomer (Ecoh = –31.05 kcal/mol). Also, transition metal (Ti and Sc) doping reduces the reacti­vity of the Al4 nanocluster. In consistent with the suggestion of reactivity descriptors used in the present study, our results reveal that the maximum catalytic performance in activation of the oxygen-oxygen bond is related to the pristine Al4 clusters. The energy barrier of O2 activation on the Al4 system (21.26 kcal/mol) exhibits that the activation of the bond linking of two oxygen atoms on Al4 is kinetically preferable. In this respect, our study shows that the Al4 nanocluster can be considered as an excellent catalyst for activation the oxygen molecule.

Interaction of Boron Nitride Nanotubes with Aluminium: A Computational Study.

The interaction of boron nitride nanotubes (BNNTs) with Al has been investigated by means of quantum chemical calculations. Two model structures were used: a BNNT adsorbing a four atom Al4 cluster, and a BNNT adsorbed on Al surfaces of different crystallographic orientations. The BNNTs were modeled as: (i) pristine, and (ii) having a boron (B-) or a nitrogen (N-) vacancy defect. The results indicated that the trends in binding energy for Al4 clusters were, similar to those of the adsorption on Al surfaces, while the Al surface orientation has a limited effect. In all cases, the calculations reveal that Al binding to a BNNT was strongly enhanced at a defect site on the BNNT surface. This higher binding was accompanied by a significant distortion of the Al cluster or the Al lattice near the respective vacancy. In case of a B-vacancy, insertion of an Al atom into the defect of the BNNT lattice, was observed. The calculations suggest that in the Al/BNNT metal matrix composites, a defect-free BNNT experiences a weak binding interaction with the Al matrix and tthe commonly observed formation of AlN and AlB2 was due to N- or B-vacancy defects within the BNNTs.

Molecular Aluminum Additive for Burn Enhancement of Hydrocarbon Fuels.

Additives to hydrocarbon fuels are commonly explored to change the combustion dynamics, chemical distribution, and/or product integrity. Here we employ a novel aluminum-based molecular additive, Al(I) tetrameric cluster [AlBrNEt3]4 (Et = C2H5), to a hydrocarbon fuel and evaluate the resultant single-droplet combustion properties. This Al4 cluster offers a soluble alternative to nanoscale particulate additives that have recently been explored and may mitigate the observed problems of particle aggregation. Results show the [AlBrNEt3]4 additive to increase the burn rate constant of a toluene-diethyl ether fuel mixture by ∼20% in a room temperature oxygen environment with only 39 mM of active aluminum additive (0.16 wt % limited by additive solubility). In comparison, a roughly similar addition of nano-aluminum particulate shows no discernible difference in burn properties of the hydrocarbon fuel. High speed video shows the [AlBrNEt3]4 to induce microexplosive gas release events during the last ∼30% of the droplet combustion time. We attribute this to HBr gas release based on results of temperature-programmed reaction (TPR) experiments of the [AlBrNEt3]4 dosed with O2 and D2O. A possible mechanism of burn rate enhancement is presented that is consistent with microexplosion observations and TPR results.

White-Light Optimal Control of Photoinduced Processes

The photofragmentation of Cu3 and Al4 clusters during charge reversal with white-light pulses is optimized by using an evolutionary algorithm and a pulse shaping setup. By controlling the spectral phase over the full spectral range of the supercontinuum, tailored pulses are generated with the aim of adapting the energetic and temporal structure of the pulse to the characteristic dynamics and electronic manifold of the investigated systems. In this context, maximizing the ratio of the cationic yield of a selected fragment (Cu2+ and Al+) and the parent ion is chosen as an optimization target. Significant enhancement factors are achieved, demonstrating a high selectivity in populating specific points on the potential energy surface, which facilitates the targeted photofragmentation of the investigated systems. The optimal pulse shapes indicate that both vibronic as well as electronic wave packets are probed. Additional laser-induced dissociation experiments suggest that fragmentation of the Cu3 clusters occu...

'All-Metal' Aromatic Sandwich Molecules: An Electronic Structure and Transport Study.

Electronic structure and transport is theoretically studied for neutral, all-metal aromatic sandwich molecules, Al4MAl4 (M = Cr, Mo) along with Al4 and Al4M (M = Cr, Mo) clusters in two-probe setups with silver and gold electrodes. Detailed electronic structure and transport studies of metallaromatic Al4MAl4 molecules show high electronic conductance [2.5*10(-4) S for Al4CrAl4 and 2.9*10(-4) S for Al4MoAl4] and three conduction channels simultaneously contribute to the total transmission probability. The study of transport properties of the bare Al4 cluster and Al4M cluster also show very high electronic conductance. The neutral Al4 cluster, when connected parallel to the electrodes, four Al atoms couple to the electrode atoms and at least eight electron conduction pathways contribute simultaneously to the conductance whereas for perpendicular connectivity only three conduction channels operate. All the molecules couple strongly to the electrodes by well-defined metal-metal bonds owing to their metallic nature indicating an easier electrode integration process, and the calculated current voltage curves are almost linear till applied voltages of 1 V for Al4MAl4 (M = Cr, Mo). The electronic transport of the clusters studied here resembles the values found for metal atomic chains in break junction studies.

Theoretical Study of the O2 + Al4 (Tetrahedral) System in Its Singlet State and Comparisons with Its Triplet State

There is an unresolved discrepancy between theory and experiment about the observed hindrance in dissociative adsorption of O2 on Al(111). In an attempt to understand the hindrance, we have investigated the interaction of O2 with a tetrahedral Al4 cluster (the apex considered as adatom) in a total singlet state by employing both the multireference configuration interaction (MRCI) and the density functional theory (DFT) methods. For an approach of O2 facing an apex of the Al4 pyramid and parallel to its base, both the MRCI and the DFT calculations show that after an early barrier the system goes through a narrow local minimum before it reaches a deep minimum of ∼203 kcal/mol near the pyramid base. The O2 molecule opens continuously, followed by a small decrease in the O–O distance around the deep minimum, forming antibonding orbitals with the pyramid base, whereas the O–O distance starts increasing in further increasing the incident energy. These results differ from previous results obtained for the same system but in a total triplet state, which showed two similar minima (one local narrow and one deep) but where O2 is dissociatively adsorbed to the pyramid base, forming bonding orbitals with it. The reason for this difference is discussed. Total triplet is an excited state where the bound O–O becomes spinless and the spin density is transferred to the pyramid apex. Similar transfers occur on an Al16 cluster formed by extending the pyramid base to a (111) arrangement. Our findings are contrasted with known results about the interaction of O2 with Si clusters. The effect of the spin state on the interaction of O2 with the planar Al base suggests that spin should be considered in an analysis of O2 adsorption on aluminum surfaces as well. To our understanding, to increase the sticking probability, free O2 must be, before incidence, prepared in singlet.

Theoretical Study of the O2 Interaction with a Tetrahedral Al4 Cluster

Employing both multireference configuration interaction (MRCI) and density functional theory (DFT) methods, we have studied the interaction of O₂ with a tetrahedral Al₄ cluster in the total spin triplet state. For a parallel to the base approach of O₂ facing an apex of the pyramid, the O₂ adsorption is hindered by a barrier. Both the MRCI and the DFT calculations show that after a small barrier, there are two local energy minima: a shallow one just above the apex atom and another deeper one below the apex atom. The latter corresponds to dissociative O₂ adsorption. We discuss the implications of these findings for the understanding of O₂ adsorption on defect sites of Al surfaces.

Competition between ionization and fragmentation of small aluminum cluster anions

The competition between delayed fragmentation and delayed ionization of laser excited Al-4 clusters was measured by simultaneous detection of charged and neutral fragments.

Interaction of Dioxygen with Al Clusters and Al(111): A Comparative Theoretical Study

We have studied the interaction of an oxygen molecule with Al clusters and Al(111) using both wave-function-based quantum chemistry methods and density functional theory (DFT). These calculations were motivated by the fact that molecular beam experiments indicate that the adsorption of O2 on Al(111) should be activated whereas periodic DFT calculations yield purely attractive adsorption paths for almost all impact configurations of O2 on Al(111). On small Al4 clusters, accurate wave-function-based quantum chemistry methods find a non-vanishing barrier in the O2 adsorption. The DFT calculations for slabs and larger Al clusters confirm the important role of spin effects for the O2 dissociation barrier on Al. The results indicate that exchange-correlation effects play a crucial role for the determination of the adsorption barrier in the O2/Al system but their determination is hampered by serious technical problems that are discussed in detail.

27Al NMR Study of the Metal Cluster Compound Al50C120H180

We present an 27Al NMR study of the metal cluster compound Al50Cp*12 which is composed of (identical) Al50 clusters, each surrounded by a Cp* ligand shell, and arranged in a crystalline 3D array (here Cp* = pentamethylcyclopentadienyl = C5(CH3)5). The compound is found to be non-conducting, the nuclear spin-lattice relaxation in the temperature range 100–300 K being predominantly due to reorientational motions of the Cp* rings. These lead to a pronounced maximum in the relaxation rate at T ∼ 170 K, corresponding to an activation energy of about 850 K. Data for the related compound Al4Cp*4, containing very much smaller Al4 clusters are also presented. A comparison is drawn with the quadrupolar relaxation recently observed for the non-conducting fraction of Ga84 molecules in the metal cluster compound Ga84[N(SiMe3)2]20-Li6Br2(thf)20·2toluene.