| AVS 63rd International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS-TuA |
| Session: | Structure and Characterization of Oxides |
| Presenter: | Stig Koust, iNANO, Aarhus University, Denmark |
| Authors: | S. Koust, iNANO, Aarhus University, Denmark L. Arnarson, iNANO, Aarhus University, Denmark P.G. Moses, Haldor Topsøe Research Lab, Denmark I. Beinik, iNANO, Aarhus University, Denmark J.V. Lauritsen, iNANO, Aarhus University, Denmark S. Wendt, iNANO, Aarhus University, Denmark |
| Correspondent: | Click to Email |
Tighter regulations concerning nitrogen oxides (NOx) and an increased public concern, highlighted recently by a study from ICCT [1], demonstrating that new diesel cars emit more than seven times the allowed NOx, has clearly shown the urgent need for the development of more effective catalysts for the removal of NOx. The Selective Catalytic Reduction (SCR) is widely used to reduce NOx into N2 and H2O in flue and exhaust gasses. This reaction is best catalyzed using a TiO2-anatase supported sub-monolayer VOx-based catalyst.
Unfortunately, the detailed reaction mechanism(s) are still debated, and the nature of the active site is uncertain [2]. To tackle these issues, the preparation and characterization of good model catalyst model systems may provide new fundamental insights.
Here we present atomically resolved STM images of sub-monolayer vanadium (V) supported on anatase TiO2 (101). Upon V deposition at liquid nitrogen temperature (LT), the surface is covered with small isolated V clusters, distributed homogeneously on the terraces. Further characterization with XPS revealed the oxidation state of V being 2+, indicating a preferred binding between V clusters and surface oxygen atoms. This conclusion is further supported by the observed reduction of the titanium surface atoms.
Surprisingly, our STM studies revealed an embedding of vanadium into the near-surface region already at room temperature (RT). A significant decrease in the density of V clusters is observed after annealing at RT and new features in the STM images appeared, which we assign to monomeric V atoms at regular titanium lattice sites, substituting the surface titanium. This change in the surface is accompanied by a shift of the V2p XPS feature to higher binding energy, revealing the oxidation of the vanadium to be 3+/4+ as compared to only 2+ upon LT deposition. The V2p area is unaltered after annealing at RT, suggesting no loss of V due to re-evaporation or migration into the bulk. Our DFT calculations confirm the substitution of vanadium with surface titanium atoms.
Additionally we present STM and XPS studies of vanadia (V2O5) depositedon a-TiO2 (101) in comparison to metallic vanadium on the same surface. Vanadia displays weaker interaction with the surface compared to vanadium and we observe diffusion in to the sub-surface for vanadia after annealing at ~700K, however subsequent oxidation pulls vanadia back out to the surface.
1. Vicente, B., et al. REAL-WORLD EXHAUST EMISSIONS FROM MODERN DIESEL CARS . 2014.
2. Busca, G., et al., Applied Catalysis B: Environmental, 1998. (1–2): p. 1-36.