Probing surface-sensitive redox properties of VOx/TiO2 catalyst nanoparticles.

Redox processes of oxide materials are fundamental in catalysis. These processes depend on the surface structure and stoichiometry of the oxide and are therefore expected to vary between surface facets. However, there is a lack of direct measurements of redox properties on the nanoscale for analysing the importance of such faceting effects in technical materials. Here, we address the facet-dependent redox properties of vanadium-oxide-covered anatase nanoparticles of relevance to, e.g., selective catalytic reduction of nitrogen oxides. The vanadium oxidation states at individual nanoscale facets are resolved in situ under catalytically relevant conditions by combining transmission electron microscopy imaging and electron energy loss spectroscopy. The measurements reveal that vanadium on {001} facets consistently retain higher oxidation states than on {10l} facets. Insight into such structure-sensitivity of surface redox processes opens prospects of tailoring oxide nanoparticles with enhanced catalytic functionalities.

Visualizing atomic-scale redox dynamics in vanadium oxide-based catalysts.

Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. These processes are often difficult to uncover due to changes in the surface stoichiometry and atomic arrangement. Here we employ high-resolution transmission electron microscopy to study vanadium oxide supported on titanium dioxide, which is of relevance as a catalyst in, e.g., nitrogen oxide emission abatement for environmental protection. The observations reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions. The transformation depends on the anatase titanium dioxide surface termination and the vanadium oxide layer thickness, suggesting that the properties of vanadium oxide are sensitive to the supporting oxide. These atomic-resolution observations offer a basis for rationalizing previous reports on shape-sensitive catalytic properties.Redox processes in metal oxide surfaces can exhibit structure sensitivities which are difficult to uncover. Here, the authors use atomic-resolution imaging to demonstrate facet dependent alterations in the surfaces of supported vanadium oxide upon reduction and oxidation.