Mechanistic Study on Water Splitting Reactions by Small Silicon Clusters Si₃X, X = Si, Be, Mg, Ca

Interaction, dissociation, and dehydrogenation reactions of water monomer and dimer with pure and mixed tetrameric silicon clusters Si₃X with X = Si, Be, Mg, Ca were investigated using high accuracy quantum chemical calculations. While geometries were optimized using the DFT/B3LYP functional with the aug-cc-pVTZ basis set, reaction energy profiles were constructed making use of the coupled-cluster theory with extrapolation to complete basis set, CCSD­(T)/CBS. Cleavage of the O–H bond in water dimer is found to be more favored than that of water monomer in the reaction with Si₄. The water acceptor monomer in water dimer performs as an internal catalyst facilitating H atom transfer to form H₂. Adsorption of water dimer on Si₃X clusters mostly takes place upon interaction of the donor water molecule with Si cluster. Water dimer adsorbs more strongly on Si₃M than on Si₄. The most stable complexes obtained upon interaction of water dimer with Si₃M mainly arise from M–O interaction in preference over a Si–O connection. Substitution of a Si atom in Si₄ by an earth alkaline metal induces a substantial reduction of the energy barrier for the (rate-limiting) first O–H bond cleavage of water dimer. The most remarkable achievement upon doping is a disappearance of the overall energy barrier for the initial O–H bond cleavage in water dimer. Of the three binary Si₃M clusters considered, dehydrogenation of water dimer driven by Si₃Be is the most kinetically and thermodynamically favorable pathway. In comparison to another cluster such as Al₆ and nanoparticles Ru₅₅, energy barriers for water dimer dissociation on Si₃M are much lower. The mixed clusters Si₃M turn out to be as efficient alternative reagents for O–H dissociation and hydrogen production from water dimer. This study proposes further searches for other mixed silicon clusters as realistic gas phase reagents for crucial dehydrogenation processes in such a way they can be prepared and conducted in experiment

Theoretical Study of Silicon Monoxide Reactions with Ammonia and Methane

High-accuracy calculations were performed to study the mechanisms of the reactions between the diatomic silicon monoxide (SiO) with NH₃ and CH₄. These reactions are relevant to the SiO-related astrochemistry and atmospheric chemistry as well as the activation of the N–H and C–H bonds by the SiO triple bond. Energetic data used in the construction of potential energy surfaces describing the SiO + NH₃/CH₄ reactions were obtained at the coupled-cluster theory with extrapolation to the complete basis set limit (CCSD­(T)/CBS) using DFT/B3LYP/aug-cc-pVTZ optimized geometries. Standard heats of formation of a series of small Si-molecules were predicted. Insertion of SiO into the N–H bond is exothermic with a small energy barrier of ∼8 kcal/mol with respect to the SiO + NH₃ reactants, whereas the C–H bond activation by SiO involves a higher energy barrier of 45 kcal/mol. Eight product channels are opened in the SiO + NH₃ reaction including dehydrations giving HNSi/HSiN and dehydrogenations. These reactions are endothermic by 16–119 kcal/mol (calculated at 298.15 K) with the CCSD­(T)/CBS energy barriers of 21–128 kcal/mol. The most stable set of products, HNSi + H₂O, was also the product of the reaction pathway having lowest energy barrier of 21 kcal/mol. Ten product channels of the SiO + CH₄ reaction including decarbonylation, dehydration, dehydrogenation, and formation of Si + CH₃OH are endothermic by 19–118 kcal/mol with the energy barriers in the range of 71–126 kcal/mol. The formation of H₂CSiO + H₂O has the lowest energy barrier of 71 kcal/mol, whereas the most stable set of products, SiH₄ + CO, is formed via a higher energy barrier of 90 kcal/mol. Accordingly, while SiO can break the N–H bond of ammonia without the assistance of other molecules, it is not able to break the C–H bond of methane

Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters Si₂M with M = H, Li, Na, Cu, and Ag

High-accuracy quantum chemical calculations were carried out to study the mechanisms and catalytic abilities of various mixed silicon species Si₂M with M = H, Li, Na, Cu, and Ag toward the first step of methanol activation reaction. Standard heats of formation of these small triatomic Si clusters were determined. Potential-energy profiles were constructed using the coupled-cluster theory with extrapolation to complete basis set CCSD­(T)/CBS, and CCSD­(T)/aug-cc-pVTZ-PP for Si₂Cu and Si₂Ag. The most stable complexes generated by the interaction of methanol with the mixed clusters Si₂M possess low-spin states and mainly stem from an M–O connection in preference to Si–O interaction, except for the Si₂H case. In two competitive pathways including O–H and C–H bond breakings, the cleavage of the O–H bond in the presence of all clusters studied becomes predominant. Of the mixed clusters Si₂M considered, the dissociation pathways of both O–H and C–H bonds with Si₂Li turns out to have the lowest energy barriers. The most remarkable finding is the absence of the overall energy barrier for the O–H cleavage with the assistance of Si₂Li. The breaking of O–H and C–H bonds with the assistance of Si₂H, Si₂Li, and Si₂Na is kinetically preferred with respect to the Si₂Cu and Si₂Ag cases, apart from the case of Si₂Na for O–H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu₃, Pt₃, and PtAu₂, the energy barriers for the O–H bond activation in the presence of small Si species, especially Si₂H and Si₂Li, are found to be lower. Consequently, these small mixed silicon clusters can be regarded as promising alternatives for the expensive metal-based catalysts currently used for methanol activation particularly and other dehydrogenation processes of organic compounds. The present study also suggests a further extensive search for other doped silicon clusters as efficient and more realistic gas-phase catalysts for important dehydrogenation processes in such a way that they can be experimentally prepared and implemented

Structures, Thermochemical Properties, and Bonding of Mixed Alkaline-Earth-Metal Silicon Trimers Si₃M^+/0/– with M = Be, Mg, Ca

The ground state geometries, electronic structures, and thermochemical properties of binary alkaline-earth-metal silicon clusters Si₃M with M = Be, Mg, Ca in neutral, cationic, and anionic states were investigated using quantum chemical computations. Lowest-lying isomers of the clusters were determined on the basis of the composite G4 energies. Along with total atomization energies, thermochemical parameters were determined for the first time by means of the G4 and coupled-cluster theory with complete basis set CCSD­(T)/CBS approaches. The most favored equilibrium formation sequences for Si₃M clusters emerge as follows: all Si₃M^+/0/– clusters are formed by attaching the M atom into the corresponding cation, neutral and anion silicon trimer Si₃^+/0/–, except for the Si₃Mg⁺ and Si₃Ca⁺ where the metal cations are bound to the neutral Si₃. The resulting mixed tetramers exhibit geometrical and electronic features similar to those of the pure silicon tetramer Si₄^+/0/–. Electron localization function (ELF) and ring current analyses point out that the σ-aromatic character of silicon tetramer remains unchanged upon substituting one Si atom by one alkaline-earth-metal atom