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 O_br 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 O_brH, promotes the increase of the positive charge of H atoms from 0.133 to 1.47 e, which increases the O_brH bond dipole moment from 1.136 to 1.400 Debye, resulting in the shortening of the H-bond distances between NH₃ and O_brH (1.858 vs. 1.945 Å of only NH₃) and enlarging the O_brH bonds (0.980 vs. 1.120 Å). This reduces the activation energy barriers of O_brH 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.