Local hydrogen bonding environment induces the deprotonation of surface hydroxyl for continuing ammonia decomposition.

Abstract

There is still a paucity of fundamental understanding about the reaction of ammonia decomposition over TiO2, especially the role of water. Herein, FPMD and DFT calculations were used to address this concern. The results reveal that ammonia decomposition in pure ammonia causes the hydroxylation of the surfaces and reduction of the proton acceptor sites, making proton transfer (PT) difficult, increasing the distances between the NH3 and Obr sites and changing the adsorption configurations of NH3, which are not favourable for accepting protons from NH3 dissociation. When water is introduced, the local hydrogen bonding environment, consisting of NH3 and H2O with the H2O dynamically close to the ObrH, promotes the increase of the positive charge of H atoms from 0.133 to 1.47 e, which increases the ObrH bond dipole moment from 1.136 to 1.400 Debye, resulting in the shortening of the H-bond distances between NH3 and ObrH (1.858 vs. 1.945 Å of only NH3) and enlarging the ObrH bonds (0.980 vs. 1.120 Å). This reduces the activation energy barriers of ObrH deprotonation and causes the surfaces to have low hydroxyl coverage from 0.425 to 0.382 eV. Our study reveals the role of water and provides new insights into ammonia decomposition on TiO2.