Modeling the Formation and Propagation of VO_x Islands on Rh(111) under Reactive Conditions

Abstract

Phase transitions between different states of a system often involve a long-term maturation process known as ripening, whose mechanism can take various forms. Recent experimental investigations on phase transitions involving vanadium oxides on rhodium have shown the existence of a new ripening mechanism, during which micrometric islands of a dense VO x phase move and coalesce exclusively in the presence of a catalytic reaction. It was hypothesized that this new pathway is fueled by an underlying polymerization/depolymerization process. In this work, we develop a generic reaction-diffusion model containing the basic physicochemical ingredients of these systems. This model reproduces qualitatively the observed behaviors and confirms that it can be traced back to an interplay between intermolecular interactions and the nonequilibrium polymerization process.

Phase Separation within Vanadium Oxide Islands under Reaction Conditions: Methanol Oxidation at Vanadium Oxide Films on Rh(111)

“…The movement of islands of macroscopic size toward each other has been explained with a VO x polymerization/depolymerization process that is sensitive to gradients in the oxygen coverage. 16,19 The transition from a stripe pattern to a pattern of circular islands occurs in a temperature range from ≈900 to 1000 K. This transition is fully reversible, as demonstrated by the Tcycling experiment shown in Figure 4. Starting with a stripe pattern, increasing the temperature from 930 to 1030 K causes transformation of the stripe pattern into a pattern of circular islands.…”

“…Under oxidizing conditions, the initially immobile VO x islands start to move as the temperature exceeds 850 K. They coalesce ( t = 300 s), and two of the larger islands again coalesce ( t = 500 s), forming an island of roughly 200 μm diameter with a bright core region. The movement of islands of macroscopic size toward each other has been explained with a VO x polymerization/depolymerization process that is sensitive to gradients in the oxygen coverage. , …”

“…This problem is at the heart of the well-known pressure and materials gap in heterogeneous catalysis. Here we study the dynamic behavior of submonolayer coverages of VO x on Rh(111) during catalytic reactions. − …”

“…Mechanistically, the island coalescence and the size oscillations could be well explained. 16,19 The reaction-induced island coalescence was shown to be due to a polymerization/ depolymerization equilibrium that is controlled by the oxygen coverage between isolated V x O y -clusters on the metal surface and massive VO x -islands. The robustness of this mechanism has also been tested with other oxidation reactions, namely CO oxidation, ammonia oxidation and with the O 2 + H 2 reaction on VO x /Rh(111).…”

Dynamics of Ultrathin Vanadium Oxide Layers on Rh(111) and Rh(110) Surfaces During Catalytic Reactions

“…The island coalescence in the system CH 3 OH + O 2 /VO x /Rh(111) has been reproduced with a skeleton reactiondiffusion model (De Decker et al, 2019). This model considers a highly simplified reaction mechanism, but takes into account the PD mechanism including the adsorption of oxygen and the existence of energetic interactions between the adparticles.…”