Combined UHV-STM and AP-XPS study of Selective Catalytic Reduction (SCR) of NOx over a VOx/TiO2 based catalyst
- Kræn Christoffer Adamsen (iNANO, Aarhus University, Denmark)
- Monday 30 March 2020
- Hotel NH Noordwijk Conference Centre Leeuwenhorst
a Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Denmark, Gustav wieds vej 14, 8000 Aarhus C, DK.
Fundamental understanding of catalytic processes for Selective Catalytic reaction (SCR) of NOx 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. This happens though an Eley-Rideal mechanism, however the active site has yet to be fully understood . Here we present a fundamental study of a partially VOx covered anatase-TiO2 (101), the predominant facet on anatase-TiO2 nanoparticles. Here is presented how VOx particles supported on the anatase- TiO2 (101) interacted with H2O, NH3 and NO.
We observe a strong interaction between vanadium dimer clusters supported by the anatase-TiO2 (101) and molecular water. This interaction was measured by in-situ Scanning Tunnelling Microscopy (STM), where water was introduced in a vacuum chamber (up to 5·10-6mbar) while measuring the same area repetitively. Exposing the Vanadia dimer clusters to molecular water induces hydrolysis, splitting of the dimers into monomers, which are known to be less reactive than dimers (and polymers).
Interactions between the vanadia dimers and water induces three different states; the dimer structure, which dominates the surface prior to water exposure, this irreversibly transforms to a hydrated state upon water introduction. This hydrated state is then reversibly converted into a dehydrated state by removal of water (this is summarised in figure 1).
Near Ambient Pressure (NAP)-XPS enables us to follow the oxidation-state of vanadium and identify adsorbed surface species close to operando conditions (0.6 mbar). Figure 2 shows the development of the O1s-difference spectra with increasing temperature close to operando conditions. The O1s difference spectra shows the amount of hydroxylation first decrease, with desorption of water and later an increase. The increase of hydroxylation suggest that NO and NH3 is reacting, leaving the surface more hydroxylated. Theoretical models suggest that this hydroxylation happens together with the reduction of the vanadia cluster. Examination of the V2p region indeed also shows reduction.
Combining the knowledge gained from the NAP-XPS and STM we can start to determine the active-site for SCR, which has been debated for 3 decades .
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