Structural Diversity and Flexibility of MgO Gas-Phase Clusters

Electronic structure and physicochemical properties of the metal and semimetal oxide nanoclusters

Clusters are physical entities composed of a few to thousands of atoms with capabilities to develop novel materials, like cluster-assembled materials. In this sense, knowing the electronic structure and physicochemical properties of the isolated clusters can be useful to understand how they interact with other chemical species by intermolecular forces, as free, embedded, and saturated clusters, and by intramolecular forces, acting as support clusters. In this way, in the present work, the electronic structure and physicochemical properties of metal oxide nanoclusters (MgO, Al₂O₃, SiO₂, and TiO₂) were studied by highly correlated molecular quantum chemistry methods. Through the electronic state's characterization, a semiconductor aspect was found for the titania oxide nanocluster (T_e < 0.8 eV) while the other agglomerates showed a characteristic of insulating material (T_e > 3.3 eV). From the stability index, the following stability order can be characterized: (SiO₂)₄ > (Al₂O₃)₄ > (MgO)₄ > (TiO₂)₃. Initial information of intermolecular and intramolecular forces caused by the studied clusters was calculated through the relative electrophilicity index, which classified the (MgO)₄ and (TiO₂)₃ clusters as the more reactive ones, in which the (MgO)₄ cluster was identified as a nucleophilic species, while the (TiO₂)₃ cluster as an electrophilic molecule.

A DFT study of the aldol condensation reaction in the processing of ethanol to 1,3-butadiene on a MgO/SiO2 surface

The ETB process on different sites of MgO/SiO2.

Synthesis Target Structures for Alkaline Earth Oxide Clusters

Knowing the possible structures of individual clusters in nanostructured materials is an important first step in their design. With previous structure prediction data for BaO nanoclusters as a basis, data mining techniques were used to investigate candidate structures for magnesium oxide, calcium oxide and strontium oxide clusters. The lowest-energy structures and analysis of some of their structural properties are presented here. Clusters that are predicted to be ideal targets for synthesis, based on being both the only thermally accessible minimum for their size, and a size that is thermally accessible with respect to neighbouring sizes, include global minima for: sizes n = 9 , 15 , 16 , 18 and 24 for (MgO) n ; sizes n = 8 , 9 , 12 , 16 , 18 and 24 for (CaO) n ; the greatest number of sizes of (SrO) n clusters ( n = 8 , 9 , 10 , 12 , 13 , 15 , 16 , 18 and 24); and for (BaO) n sizes of n = 8 , 10 and 16.

On the structure of superbasic (MgO)n sites solvated in a faujasite zeolite

Theory and experiment reveal the structure of magnesium oxide nanoclusters in a superbasic faujasite zeolite.

Electronic Effects on Room-Temperature, Gas-Phase C-H Bond Activations by Cluster Oxides and Metal Carbides: The Methane Challenge

This Perspective discusses a story of one molecule (methane), a few metal-oxide cationic clusters (MOCCs), dopants, metal-carbide cations, oriented-electric fields (OEFs), and a dizzying mechanistic landscape of methane activation! One mechanism is hydrogen atom transfer (HAT), which occurs whenever the MOCC possesses a localized oxyl radical (M-O^•). Whenever the radical is delocalized, e.g., in [MgO]_n^•+ the HAT barrier increases due to the penalty of radical localization. Adding a dopant (Ga₂O₃) to [MgO]₂^•+ localizes the radical and HAT transpires. Whenever the radical is located on the metal centers as in [Al₂O₂]^•+ the mechanism crosses over to proton-coupled electron transfer (PCET), wherein the positive Al center acts as a Lewis acid that coordinates the methane molecule, while one of the bridging oxygen atoms abstracts a proton, and the negatively charged CH₃ moiety relocates to the metal fragment. We provide a diagnostic plot of barriers vs reactants' distortion energies, which allows the chemist to distinguish HAT from PCET. Thus, doping of [MgO]₂^•+ by Al₂O₃ enables HAT and PCET to compete. Similarly, [ZnO]^•+ activates methane by PCET generating many products. Adding a CH₃CN ligand to form [(CH₃CN)ZnO]^•+ leads to a single HAT product. The CH₃CN dipole acts as an OEF that switches off PCET. [MC]⁺ cations (M = Au, Cu) act by different mechanisms, dictated by the M⁺-C bond covalence. For example, Cu⁺, which bonds the carbon atom mostly electrostatically, performs coupling of C to methane to yield ethylene, in a single almost barrier-free step, with an unprecedented atomic choreography catalyzed by the OEF of Cu⁺.

The influence of Sc doping on structural, electronic and optical properties of Be12O12, Mg12O12 and Ca12O12 nanocages: a DFT study

Density functional theory (DFT) calculations were used to study the effect of scandium doping on the structural, energetic, electronic, linear and nonlinear optical (NLO) properties of Be₁₂O₁₂, Mg₁₂O₁₂ and Ca₁₂O₁₂ nanoclusters. Scandium (Sc) doping on nanoclusters leads to narrowing of their E _g, which enhances their conductance greatly. Also, the polarizability (α) and first hyperpolarizability (β₀) of nanoclusters were dramatically increased as Be, Mg or Ca atoms are substituted with a Sc atom. Among all clusters, α and β₀ values for Sc-doped Ca₁₂O₁₂ were the largest. Consequently, the effect of the doping atom, as well as of cluster size, on electronic and optical properties was explored. Time dependent (TD)-DFT calculations were also carried out to confirm the β₀ values; the results show that the higher value of first hyperpolarizability belongs to Sc-doped Ca₁₂O₁₂, which has the smallest transition energy (ΔEgn). The results obtained show that these clusters can be candidates for using in electronic devices and NLO materials in industry.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

Nonlinear effects in infrared action spectroscopy of silicon and vanadium oxide clusters: experiment and kinetic modeling

Nonlinear effects in infrared action spectroscopy are experimentally quantified and successfully modeled for different inorganic clusters.

Pool-BCGA: a parallelised generation-free genetic algorithm for the ab initio global optimisation of nanoalloy clusters

The Birmingham cluster genetic algorithm is a package that performs global optimisations for homo- and bimetallic clusters based on either first principles methods or empirical potentials. Here, we present a new parallel implementation of the code which employs a pool strategy in order to eliminate sequential steps and significantly improve performance. The new approach meets all requirements of an evolutionary algorithm and contains the main features of the previous implementation. The performance of the pool genetic algorithm is tested using the Gupta potential for the global optimisation of the Au10Pd10 cluster, which demonstrates the high efficiency of the method. The new implementation is also used for the global optimisation of the Au10 and Au20 clusters directly at the density functional theory level.

Structure and magnetic properties of (Fe2O3)n clusters (n = 1–5)

Global minimum structures of (Fe₂O₃)_n clusters (n = 1–5) determined for the first time in this size range show weak dependence of the structure and relative stabilities of different isomers on their magnetic states.

Stability and metastability of clusters in a reactive atmosphere: theoretical evidence for unexpected stoichiometries of MgMOx

By applying a genetic algorithm and ab initio atomistic thermodynamics, we identify the stable and metastable compositions and structures of MgMOx clusters at realistic temperatures and oxygen pressures. We find that small clusters (M≲5) are in thermodynamic equilibrium when x>M. The nonstoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory with a hybrid exchange-correlation functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor density-functional theory with (semi)local exchange-correlation functionals is sufficient for even a qualitative prediction.

Gas-phase reactions of cationic vanadium-phosphorus oxide clusters with C2H(x) (x=4, 6): a DFT-based analysis of reactivity patterns

The reactivities of the adamantane-like heteronuclear vanadium-phosphorus oxygen cluster ions [V(x)P(4-x)O(10)](.+) (x=0, 2-4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas-phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT-based calculations; homolytic C-H bond activation constitutes the initial step, and for all systems the P-O(.) unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen-atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e(-) reduction of the cationic clusters.

Density functional study of water-gas shift reaction on M3O(3x)/Cu(111)

Density functional theory (DFT) was employed to study the water dissociation and water-gas shift (WGS) reaction on a series of inverse model catalysts, M(3)O(3x)/Cu(111) (M = Mg, Ti, Zr, Mo, W; x = 1, 2, 3). It has been found that the WGS reaction on Cu can be facilitated by introducing various oxides to lower the barrier of water dissociation. Accordingly, the calculated reaction energy for water dissociation was used as a scaling descriptor to screen the WGS activity of oxide-Cu model catalysts. Our calculations show that the activity towards water dissociation decreases in a sequence: Mg(3)O(3)/Cu(111) > Zr(3)O(6)/Cu(111) > Ti(3)O(6)/Cu(111) > W(3)O(9)/Cu(111), Mo(3)O(9)/Cu(111). It seems that Mg(3)O(3)/Cu(111) is the best WGS catalyst among the systems studied here, being able to dissociate water with no barrier. During the process, both Cu and oxides participate in the reaction directly. The strong M(3)O(3x)-Cu interaction is able to tune the electronic structure of M(3)O(3x) and therefore the activity towards water dissociation. Further studies of the overall WGS reaction on Mg(3)O(3)/Cu(111) show that water dissociation may not be the key step to control the WGS reaction on Mg(3)O(3)/Cu(111) and the removal of H from Mg(3)O(3) can be problematic. The strong interaction between H and O from Mg(3)O(3) blocks the O sites for further water dissociation and therefore the WGS reaction. Our study observes a very different behavior of oxide clusters in such small size from the bigger ones supported on Cu(111) and provides new insight into the rational design of the WGS catalysts.

Gas-Phase Neutral Binary Oxide Clusters: Distribution, Structure, and Reactivity toward CO

Neutral binary (vanadium-cobalt) oxide clusters are generated and detected in the gas phase for the first time. Their reactivities toward carbon monoxide (CO) are studied both experimentally and theoretically. Experimental results suggest that neutral VCoO4 can react with CO to generate VCoO3 and CO2. Density functional theory studies show parallel results as well as provide detailed reaction mechanisms.

Structure characterization of metal oxide clusters by vibrational spectroscopy: possibilities and prospects

This article summarizes the methodological progress that has been made in the vibrational spectroscopy of isolated polynuclear metal oxide clusters, with particular emphasis on free electron laser-based infrared action spectroscopy of gas phase clusters, over the last decade. The possibilities, limitations and prospects of the various experimental approaches are discussed using representative examples from pivotal studies in the field.

Structure determination of neutral MgO clusters--hexagonal nanotubes and cages

Structural information for neutral magnesium oxide clusters has been obtained by a comparison of their experimental vibrational spectra with predictions from theory. (MgO)(n) clusters with n = 3-16 have been studied in the gas phase with a tunable IR-UV two-color ionization scheme and size-selective infrared spectra have been measured. These IR spectra are compared to the calculated spectra of the global minimum structures predicted by a hybrid ab initio genetic algorithm. The comparison shows clear evidence that clusters of the composition (MgO)(3k) (k = 1-5) form hexagonal tubes, which confirm previous theoretical predictions. For the intermediate sizes (n≠ 3k) cage-like structures containing hexagonal (MgO)(3) rings are identified. Except for the cubic (MgO)(4) no evidence for bulk like structures is found.