Mechanistic Study on Water Splitting Reactions by Small Silicon Clusters Si 3 X, X = Si, Be, Mg, Ca.

Interaction, dissociation, and dehydrogenation reactions of water monomer and dimer with pure and mixed tetrameric silicon clusters Si3 X 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 Si3 X clusters mostly takes place upon interaction of the donor water molecule with Si cluster. Water dimer adsorbs more strongly on Si3 M than on Si4 . The most stable complexes obtained upon interaction of water dimer with Si3 M 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 Si3 M clusters considered, dehydrogenation of water dimer driven by Si3 Be 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 Si3 M are much lower. The mixed clusters Si3 M 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.