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Visualizing surface redox dynamics in VOx/TiO2 catalysts

Ek, Martin LU orcid ; Arnarson, Logi ; Godiksen, Anita ; Moses, Poul Georg ; Skoglundh, Magnus ; Olsson, Eva ; Rasmussen, Søren B and Helveg, Stig (2022) 9th Nordic Symposium on Catalysis, 2022
Abstract
The reactivity of oxide surfaces is often attributed to formation and annihilation of oxygen deficient sites, which undergo complex structural rearrangements in the process. A prototypical example is VOx/TiO2 catalysts, widely used for e.g. selective catalytic reduction (SCR) of NOx emissions, where the possible effect of the supporting TiO2 surface structure has been much debated. Here, we present in situ atomic-resolution transmission electron microscopy (TEM) and electron energy loss (EEL) spectroscopy observations revealing variations in structure and oxidation state over individual nanoparticles.

The TEM image in Fig. 1a illustrates the catalyst particle morphology, with multiple surface facet types. For conditions relevant... (More)
The reactivity of oxide surfaces is often attributed to formation and annihilation of oxygen deficient sites, which undergo complex structural rearrangements in the process. A prototypical example is VOx/TiO2 catalysts, widely used for e.g. selective catalytic reduction (SCR) of NOx emissions, where the possible effect of the supporting TiO2 surface structure has been much debated. Here, we present in situ atomic-resolution transmission electron microscopy (TEM) and electron energy loss (EEL) spectroscopy observations revealing variations in structure and oxidation state over individual nanoparticles.

The TEM image in Fig. 1a illustrates the catalyst particle morphology, with multiple surface facet types. For conditions relevant to the SCR reaction (excess O2 (1 mbar) with traces (0.1 mbar) of NH3 and NO), the average oxidation state, measured using EEL spectroscopy in TEM mode, was found to be in the V(V)-V(IV) range. In scanning TEM-mode (STEM), EEL spectra could be recorded from individual facets, as illustrated in Fig. 1b. Surprisingly, these measurements revealed that {001} facets retained oxidation states closer to V(V), whereas {101} facets on the same nanoparticle were reduced toward V(IV).[1] Increasing the reducing potential of the gaseous environment (lowering O2 or introducing CO) revealed also reversible structural alterations in the surface VOx, as shown in Fig. 1c.[2] The restructuring was found to depend on the supporting facet; lower oxygen coordination and density of the surface cations results in high mobility and greater ability to accommodate large changes in oxidation states.

The present work shows that facet-dependent redox dynamics must be included in the description of SCR catalysis over nanostructured VOx/TiO2 catalysts. In turn, the present hybrid in situ electron microscopy approach should be generally applicable to resolve structure-sensitivity in oxide catalysis.

This work was supported by Innovation Fund Denmark, Danish National Research Foundation (DNRF146), and the Swedish Energy Agency.

References
[1] M. Ek, L. Arnarson, P.G. Moses, S.B. Rasmussen, M. Skoglundh, E. Olsson, and S. Helveg Nanoscale, 13, 7266 (2021).
[2] M. Ek, Q. M. Ramasse, L. Arnarson, P.G. Moses, and S. Helveg, Nat. Commun., 8, 305 (2017).
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9th Nordic Symposium on Catalysis, 2022
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Espoo, Finland
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2022-06-06 - 2022-06-08
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English
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2024-01-25 16:36:00
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@misc{e017c8de-7699-4685-a2f0-594ef9d34224,
  abstract     = {{The reactivity of oxide surfaces is often attributed to formation and annihilation of oxygen deficient sites, which undergo complex structural rearrangements in the process. A prototypical example is VOx/TiO2 catalysts, widely used for e.g. selective catalytic reduction (SCR) of NOx emissions, where the possible effect of the supporting TiO2 surface structure has been much debated. Here, we present in situ atomic-resolution transmission electron microscopy (TEM) and electron energy loss (EEL) spectroscopy observations revealing variations in structure and oxidation state over individual nanoparticles.<br/><br/>The TEM image in Fig. 1a illustrates the catalyst particle morphology, with multiple surface facet types. For conditions relevant to the SCR reaction (excess O2 (1 mbar) with traces (0.1 mbar) of NH3 and NO), the average oxidation state, measured using EEL spectroscopy in TEM mode, was found to be in the V(V)-V(IV) range. In scanning TEM-mode (STEM), EEL spectra could be recorded from individual facets, as illustrated in Fig. 1b. Surprisingly, these measurements revealed that {001} facets retained oxidation states closer to V(V), whereas {101} facets on the same nanoparticle were reduced toward V(IV).[1] Increasing the reducing potential of the gaseous environment (lowering O2 or introducing CO) revealed also reversible structural alterations in the surface VOx, as shown in Fig. 1c.[2] The restructuring was found to depend on the supporting facet; lower oxygen coordination and density of the surface cations results in high mobility and greater ability to accommodate large changes in oxidation states.<br/><br/>The present work shows that facet-dependent redox dynamics must be included in the description of SCR catalysis over nanostructured VOx/TiO2 catalysts. In turn, the present hybrid in situ electron microscopy approach should be generally applicable to resolve structure-sensitivity in oxide catalysis.<br/><br/>This work was supported by Innovation Fund Denmark, Danish National Research Foundation (DNRF146), and the Swedish Energy Agency. <br/><br/>References<br/>[1]	M. Ek, L. Arnarson, P.G. Moses, S.B. Rasmussen, M. Skoglundh, E. Olsson, and S. Helveg Nanoscale, 13, 7266 (2021).<br/>[2]	M. Ek, Q. M. Ramasse, L. Arnarson, P.G. Moses, and S. Helveg, Nat. Commun., 8, 305 (2017).<br/>}},
  author       = {{Ek, Martin and Arnarson, Logi and Godiksen, Anita and Moses, Poul Georg and Skoglundh, Magnus and Olsson, Eva and Rasmussen, Søren B and Helveg, Stig}},
  language     = {{eng}},
  month        = {{05}},
  title        = {{Visualizing surface redox dynamics in VOx/TiO2 catalysts}},
  year         = {{2022}},
}