| AVS 65th International Symposium & Exhibition | |
| Surface Science Division | Monday Sessions |
| Session SS+HC+MI-MoM |
| Session: | Dynamical Processes at Surfaces |
| Presenter: | Kræn Christoffer Adamsen, Aarhus University, Denmark |
| Authors: | K.C. Adamsen, Aarhus University, Denmark S. Koust, Aarhus University, Denmark E.L. Kolsbjerg, Aarhus University, Denmark B. Hammer, Aarhus University, Denmark S. Wendt, Aarhus University, Denmark J.V. Lauritsen, Aarhus University, Denmark |
| Correspondent: | Click to Email |
Fundamental understanding of catalytic processes for NOx removal (Selective Catalytic reaction, SCR) is vital for improving existing catalysts and developing new. In the SCR cycle, NOx is known to react from gas-phase on adsorbed ammonia on VOx/TiO2 based catalysts , and adsorption of ammonia on such oxides is therefore of great importance for fundamental understanding of NOx-removal and SCR catalysis. Here we present a fundamental study of the static and dynamic behaviour of ammonia on anatase-TiO2 (101), the predominant facet on anatase-TiO2 nanoparticles. High resolution Scanning Tunnelling Microscopy (STM) of static adsorbed ammonia molecules at room temperature, indicates a strong binding to the surface. Through synchrotron radiation XPS ammonia was found to adsorb molecularly. The strong binding of ammonia was further quantified by Temperature Programmed Desorption (TPD) which also shows a highly coverage dependent binding energy, indicating molecular repulsion. All experimental obtained results are in accordance with a proposed theoretically calculated DFT-model of ammonia absorption.
Next, single ammonia molecule diffusion measured utilizing the high-speed Aarhus STM, show diffusibility to all neighbouring sites. Molecular repulsion also show a clear effect on static structures, where nearest neighbouring site occupation is rarely observed. Statistical analysis of intermolecular coordination supplied repulsion energies, which agree with observed values in TPD spectra and theory. For diffusion, we conclude that molecular repulsion increases the diffusibility for higher coordinated ammonia molecules. However when two ammonia occupy two nearest neighbour sites, they have the possibility of diffusing through a rolling effect, where ammonia can move more easily in one direction, this phenomena has also been seen for water on other oxide surfaces. Our analysis thus shows a surprisingly complex diffusion behaviour of NH3 on anatase TiO2(101), which however resembles water dimer diffusion of water dimers on Rutile-TiO2 (110).