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

Mechanistic insights into the dehydrogenation of formaldehyde, formic acid and methanol using the Pt4 cluster as a promising catalyst

Reaction mechanisms of the dehydrogenation of formaldehyde, formic acid and methanol on the Pt₄ cluster were computationally investigated using density functional theory (DFT) with the B3LYP functional in the conjunction with the aug-cc-pVTZ basis sets for H, C and O atoms, and the cc-pVDZ-PP basis set for Pt. Herein, the key mechanistic aspects of three possible pathways of the dehydrogenation of these compounds are summarized. The results indicate that the formation of H₂ and CO or CO₂ molecules is more energetically favorable than the generation of H and H₂O, HCHO products. Generally, the formation of H₂ molecule in the presence of catalysts is more favorable than the direct decomposition of either HCHO, HCOOH or CH₃OH molecule. The use of Pt₄ catalyst significantly reduces the energy barriers for C-H and O-H bond cleavage of all three compounds to 14, 9 and 12 kcal/mol, respectively. The decomposition of HCOOH is found to be the most energetically favorable. In addition, the mechanistic insights of the reactions confirm the reduction of the energy barriers of the gas-phase dehydrogenation by 67-82 kcal/mol and bring it to the values smaller than 14 kcal/mol in the presence of the Pt₄ catalysts.

Methanol C–O Bond Activation by Free Gold Clusters Probed via Infrared Photodissociation Spectroscopy

The activation of methanol (CD3OD and CD3OH) by small cationic gold clusters has been investigated via infrared multiphoton dissociation (IR-MPD) spectroscopy in the 615–1760 cm−1 frequency range. The C–O stretch mode around 925 cm−1 and a coupled CD3 deformation/C–O stretch mode around 1085 cm−1 are identified to be sensitive to the interaction between methanol and the gold clusters, whereas all other modes in the investigated spectral region remain unaffected. Based on the spectral shift of these modes, the largest C–O bond activation is observed for the mono-gold Au(CD3OD)+ cluster. This activation decreases with increasing the cluster size (number of gold atoms) and the number of adsorbed methanol molecules. Supporting density functional theory (DFT) calculations reveal that the C–O bond activation is caused by a methanol to gold charge donation, whereas the C–D and O–D bonds are not significantly activated by this process. The results are discussed with respect to previous experimental and theoretical investigations of neutral and cationic gold-methanol complexes focusing on the C–O stretch mode.

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.