Quantum chemical modeling of the interaction in MeN(H2O)M (Me = Cu, Ag, Au; N = 1-3; M = 1, 2) system

Abstract

Purpose. Quantum chemical modelling of the interaction of IB metal clusters with water molecules was carried out within the framework of the density functional theory. The aim of this work is to study the adsorption process of H₂O molecules on small IB metal clusters.

Methods and methodology. Quantum chemical modeling of the IB metal clusters interaction with water molecules was carried out in terms of density functional theory (hybrid PBE0 functional).

Results. The structure, vibration frequencies, charge distribution and other parameters of Me_n(H₂O)_m (Me = Cu, Ag, Au; n = 1-3; m = 1, 2) isomers were calculated. The paper demonstrates that Cu₂ and Cu₃ particles are able to thermodynamically decompose water molecules to hydrogen and hydroxyl under standard conditions, while other examined clusters of IB metal are not able to do so. The standard enthalpy and Gibbs free energy of H₂O adsorption decrease in magnitude in the series Cu > Au > Ag. The results of the study show that if the adsorption enthalpy does not or very slightly depends on the temperature (the difference of values at 0 K and 298.15 K does not exceed 5 kJ/mol), the entropy factor plays a significant role. The value of TΔSº_ads reaches –40 kJ/mol and thus is comparable to the adsorption enthalpy by its absolute value. The geometry of the adsorbed water molecule is not much different from its geometry in an isolated state.

Conclusion. It was demonstrated that the frequencies of the symmetric and asymmetric O–H stretches of adsorbed water molecule are decreasing in all the studied clusters. This effect is more visible when H₂O is adsorbed on copper clusters, with frequency reduction reaching 86 cm^–1. The frequency of the bending mode does not change significantly. An increase in the intensity during adsorption is observed only in the symmetric stretching vibration O–H. Vibration frequencies identifying the Cu-H-Cu and Cu-H fragments of the adsorption complexes containing decomposed water molecule were calculated. Calculations allow to conclude that among studied IB metals clusters only Cu₃ and Cu₂ particles are able to decompose adsorbed H₂O molecules into hydrogen atom and hydroxyl.

CONFLICT OF INTEREST

The authors declare the absence of obvious and potential conflicts of interest related to the publication of this article.

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