Reaction dynamics of metal/oxide catalysts: Methanol oxidation at vanadium oxide films on Rh(1 1 1) from UHV to 10 2 mbar Bernhard von Boehn a , Christopher Penschke b,1 , Xiaoke Li b , Joachim Paier b,⇑ , Joachim Sauer b , Jon-Olaf Krisponeit c,d , Jan Ingo Flege c,d,e , Jens Falta c,d , Helder Marchetto f , Torsten Franz f , Gerhard Lilienkamp g , Ronald Imbihl a,⇑ a Institut für Physikalische Chemie und Elektrochemie, Leibniz Universität Hannover, Callinstrasse 3A, 30167 Hannover, Germany b Institut für Chemie, Humboldt-Universität zu Berlin, 10099 Berlin, Germany c University of Bremen, Institute of Solid State Physics, 28359 Bremen, Germany d MAPEX Center for Materials and Processes, University of Bremen, 28359 Bremen, Germany e Brandenburgische Technische Universität Cottbus-Senftenberg, Fachgebiet Angewandte Physik und Halbleiterspektroskopie, 03046 Cottbus, Germany f Elmitec Elektronenmikroskopie GmbH, 38678 Clausthal-Zellerfeld, Germany g Institute of Energy Research and Physical Technologies, Technische Universität Clausthal, 38678 Clausthal-Zellerfeld, Germany article info Article history: Received 13 December 2019 Revised 9 March 2020 Accepted 16 March 2020 Keywords: Vanadium oxide Methanol oxidation Inverse catalyst Restructuring Near ambient pressure low-energy electron microscope Heterogeneous catalysis Pressure gap abstract Recent advances in in situ microscopy allow to follow the reaction dynamics during a catalytic surface reaction from ultra-high vacuum to 0.1 mbar, thus bridging a large part of the pressure gap. Submonolayer vanadium oxide films on Rh(1 1 1) have been studied during catalytic methanol oxidation in situ with spatially resolving imaging techniques. At 10 6 –10 4 mbar VO x condenses into macroscopic circular islands that exhibit a substructure, consisting of a reduced island core and an oxidized outer ring. This substructure arises due to an oxygen gradient inside the VO x islands, which results in different coex- isting 2D-phases of VO x on Rh(1 1 1). This substructure is also responsible for a ‘‘breathing-like” oscilla- tory expansion and contraction that the islands undergo under stationary conditions. Using density functional theory, the 2D-phase diagram of VO x on Rh(1 1 1) has been computed. The oscillatory behavior can be understood as a periodic phase transition between two 2D phases of VO x . With a newly developed near ambient pressure – low-energy electron microscope, it was shown that VO x islands disintegrate at 10 2 mbar, resulting in turbulent dynamics. Ó 2020 Elsevier Inc. All rights reserved. 1. Introduction Catalytic surfaces restructure under reaction conditions with the extent ranging from atomic-scale reconstructions to real mor- phological changes and chemical transformations [1–3]. As a con- sequence of dynamic processes governing the composition and structure of a catalyst in operation, a pressure gap exists between surface science studies conducted in UHV and ‘‘real catalysis”, operating typically in a pressure range from 1–100 bar [4,5].A key challenge in heterogeneous catalysis is therefore the develop- ment of in situ and operando techniques, which allow monitoring a catalyst in its active state. The reaction-induced restructuring processes, often associated with an activation of the catalyst, are well documented for metal surfaces, but comparatively little is known about the dynamics of oxide catalysts under reaction conditions. As a model system for supported oxide catalysts we investigate submonolayer vanadium oxide films on a Rh(1 1 1) surface. Vanadium oxide catalysts are among the most important catalysts in chemical industry, finding application in many partial oxidation reactions, e.g. formaldehyde production from methanol in the formox process [6,7], sulfuric acid production and the selective catalytic reduction (SCR) process in environmental chemistry [8]. The geometric and electronic struc- ture of pure and supported vanadium oxides, their reactivity, and the influence of the support material have been the subject of numerous studies [9–16]. The high catalytic efficiency of V-oxides can be attributed to the ability of vanadium to easily change the oxidation state between V 3+ ,V 4+ , and V 5+ under reaction conditions [8,9,14,17]. In industrial applications V-oxide is usually deposited on an oxidic support https://doi.org/10.1016/j.jcat.2020.03.016 0021-9517/Ó 2020 Elsevier Inc. All rights reserved. ⇑ Corresponding authors. E-mail addresses: joachim.paier@chemie.hu-berlin.de (J. Paier), imbihl@pci.uni- hannover.de (R. Imbihl). 1 Present address: Institut für Chemie, Universität Potsdam, 14476 Potsdam-Golm, Germany. Journal of Catalysis 385 (2020) 255–264 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat