(39aj) DFT Study of Coverage-Depended Adsorption of NH3 On TiO2-B (100) Surface

The selective catalytic reduction (SCR) of NO_x with NH₃ to N₂ and H₂O is an efficient way to reduce the amount of NO_x from industrial plants and vehicles. V₂O₅/TiO₂ catalyst is the basis for the industrial reaction. Ammonia adsorption is critical for SCR of NO on the catalyst. It was reported that the SCR activity increases with the amount of NH₃ chemisorption on TiO₂ surfaces increasing [1]. However, the saturation coverage of NH₃ on TiO₂ surface such as rutile (110) surface was reported to be half a monolayer [2]. TiO₂ surface with high NH₃ chemisorption capacity is expected for SCR. Our previous study showed that TiO₂-B (100) surface is very unique. It is with high activity and a loose structure. The fully dissociative adsorption of water at 1 ML coverage on TiO₂-B (100) surface is still favored due to its unique open structure [3].

In order to investigate the NH₃ chemisorption capacity of TiO₂-B (100) surface, we studied the adsorption of ammonia on TiO₂-B (100) surface at coverages ranging from 1/6 to 1 ML using ab initio density functional calculations. The results show that the decrease rate of average molecular adsorption energy of NH₃ with coverage on TiO₂-B (100) surface is substantially lower than that on rutile (011) surface above 1/2 ML coverage due to its open structure. The variation pattern of average molecular adsorption energy with coverage on TiO₂-B (100) surface is different from that on other TiO₂ surfaces. It shows a linear dependence on the coverage of  on TiO₂-B (100) surface, but accelerated degradation on rutile (011). The possibility of NH₃ molecule onto the Ti_5c site is nearly equal to forming dimer with adsorbed NH₃ on TiO₂-B (100) surface at 5/6 ML coverage.

[1] Yamazoe S., et al., Development of the efficient TiO₂ photocatalyst in photoassisted selective catalytic reduction of NO with NH₃. Catalysis Today, 2006. 111(3-4): p. 266-270.

[2] Siu W.K., Bartynski R.A., and Hulbert S.L., The role of defects at low concentrations in the NH₃/TiO₂(110) adsorption system: An Auger-photoelectron coincidence spectroscopy study. The Journal of Chemical Physics, 2000. 113(23): p. 10697-10702.

[3] Liu W.J., et al., A shortcut for evaluating activities of TiO₂ facets: water dissociative chemisorption on TiO₂-B (100) and (001). Physical Chemistry Chemical Physics, 2010. 12(31): p. 8721-8727.