Adsorption Configuration and Dissociative Reaction of NH3 on Anatase (101) Surface with and without Hydroxyl Groups

This study investigates the possible adsorption configurations and dissociative reactions of NH3 on the anatase (101) surface by employing the first principles calculations. In addition, the hydroxyl group effect is also included to study how this effect influences the adsorption and the dissociative reactions. Without the presence of the hydroxyl group, the most stable adsorbate is the bidentate adsorbate Ti-N-O (E-ads = 44.9 kcal/mol), and the second is the bidentate adsorbate Ti-(H)N-O (E-adds = 40.8 kcal/mol). NH3 can also be adsorbed on 5c-Ti, forming H3N-Ti, which is the third most stable adsorbate (E-adds = 27.5 kcal/mol). The hydroxyl group present on the surface has the effect of significantly enhancing the adsorption of the monodentate adsorbates H2N-Ti and HN-Ti. However, such a presence only slightly enhances the bidentate adsorbate Ti-N-O. In addition, the adsorption energy increases as the number of hydroxyl groups on the surface increases. The hydroxyl group also has the effect to diminish the adsorption for bidentate adsorbate Ti-(H-2)N-O and monodentate adsorbate H-O, or to simultaneously enhance and diminish adsorption for Ti-(H)N-O depending on the location and number of the hydroxyl groups. However, the effect of the hydroxyl group on these two bidentate adsorbates (Ti-(H-2)N-O and Ti-(H)N-O) is not as significant as for monodentate adsorbates (H2N-Ti and HN-Ti). Two reaction pathways are found to reach two final products, Ti-N-O-c2+3(H-O) and N-Ti-c3+3(H-O), with the energetics of 89.0 and 83.2 kcal/mol, respectively. In addition, the maximum reaction energy barriers required to reach these two final products are 76.0 kcal/mol for the pathway where H2N-Ti dissociates into Ti-(H)N-O, and 126.9 kcal/mol for the pathway where HN-Ti dissociates into N-Ti. All of the reactions, except the forming of H3N-Ti, are endothermic. The hydroxyl group was found to lower or raise the energetics. The energetics of H2N-Ti+H-O and HN-Ti+2(H-O) are significantly lowered; however, the energetics of Ti-(H)N-O+2(H-O) and Ti-N-O+3(H-O) are slightly raised, as compared to those energetics without the presence of the hydroxyl group. Finally, the reaction pathway to N-Ti+3(H-O) is only found when considering the hydroxyl group effect.