Formation of Hydrogen Peroxide from O-(H2O)n Clusters.

Abstract

Hydrogen peroxide (H2O2) has recently received much attention as a safe and clean energy carrier for hydrogen molecules. In this study, based on direct ab initio molecular dynamics (AIMD) calculations, we demonstrated that H2O2 is directly formed via the photoelectron detachment of O-(H2O)n (n = 1-6) (water clusters of an oxygen radical anion). Three electronic states of oxygen atoms were examined in the calculations: O(X)(H2O)n (X = 3P, 1D, and 1S states). After the photoelectron detachment of O-(H2O)n (n = 1) to the 1S state, a complex comprising O(1S) and H2O, O(1S)-OH2, was formed. A hydrogen atom of H2O immediately transferred to O(1S) during an intracluster reaction to form H2O2 as the final product. Simulations were run to obtain a total of 33 trajectories for n = 1 that all led to the formation of H2O2. The average reaction time of H2O2 formation was calculated to be 57.7 fs in the case of n = 1, indicating that the reaction was completed within 100 fs of electron detachment. All the reaction systems O(1S)(H2O)n (n = 1-6) indicated the formation of H2O2 by the same mechanism. The reaction times for n = 2-6 were calculated to range between 80 and 180 fs, indicating that the reaction for n = 1 is faster than that of the larger clusters, that is, the larger the cluster size, the slower the reaction is. The reaction dynamics of the triplet O(3P) and singlet O(1D) potential energy surfaces were calculated for comparison. All calculations yielded the dissociation product O(X)(H2O)n → O(X) + (H2O)n (X = 3P and 1D), indicating that the O(1S) state contributes to the formation of H2O2. The reaction mechanism was discussed based on the theoretical results.