Redistribution of Supported Vanadium Oxide Catalysts by Pattern Formation

Abstract

The behavior of ultra-thin (\1 ML) films of vanadium oxide on Rh(111) during the H 2 ? O 2 reaction was studied in the 10 -6 -10 -4 mbar range with photoemission electron microscopy (PEEM). One observes that under reaction conditions the V-oxide condenses forming a stripe pattern of macroscopic dimensions on the surface. The pattern is characterized by an intrinsic wave length which obeys a power law dependence on the reactants pressure. The origin of the pattern is discussed within the concept of reactive phase separation, i.e. the strongly attractive interactions between V and O and/or changes in the interfacial energies caused by reduction are presumably the main driving forces for condensation.

Dynamics of ultrathin V-oxide layers on Rh(111) in catalytic oxidation of ammonia and CO

“…In an earlier study of the O 2 + H 2 reaction on Rh(111)/VO x it was concluded that chemisorbed oxygen acts as kind of surfactant for VO x reducing the line tension between a VO x island and the surrounding bare metal surface. 17,18 If we assume that chemisorbed oxygen stabilizes the free V n O m clusters on the bare metal surface then in turn a reduced oxygen coverage would have a low concentration of free V-clusters as consequence. This could explain why no nucleation of VO x islands is seen in the bright elliptical area.…”

Self-Organization of Ultrathin Vanadium Oxide Layers on a Rh(111) Surface during a Catalytic Reaction. Part I: A PEEM Study

“…It should be pointed out here that due to beam-damaging effects an erroneous coverage calibration was given in a preceding paper, i.e., instead of 0.1 MLE given there, the actual coverage was ∼0.2 MLE. 11 The UHV chamber was operated as a continuous flow reactor, introducing the reactant gases through leak valves. Pressures were measured with an uncalibrated ion gauge; the different ionization probabilities of O 2 and H 2 were taken into account multiplying the p(H 2 ) reading by 2.…”

LEEM and PEEM as Probing Tools to Address Questions in Catalysis

“…Due to its wide applicability, it is important to rationally understand the working principle of the catalyst under different experimental conditions. In this sense, Lovis et al [29,30,46] have used LEEM and PEEM to address the properties of a V x O y /Rh(111) model system, establishing a representative example of what can be done and what kind of information can be extracted from a model system with this approach. For instance, their studies were focused on the dynamic changes that occur on the oxide phase when the model catalyst is exposed to reaction condition, in this case being the hydrogen oxidation reaction.…”

“…For instance, their studies were focused on the dynamic changes that occur on the oxide phase when the model catalyst is exposed to reaction condition, in this case being the hydrogen oxidation reaction. Some selected results extracted from reference [46] are shown in Fig. 5.…”

“…As a consequence, covered and uncovered regions on the surface of model catalysts or even regions having different adsorbates can exhibit different contrasts based on the differences in the local work function, making the surface chemical properties of the material visible in some way. On one side, it is well known that not all sites on a surface offer the same energetics for adsorption for (a-g) were successively recorded: with a at t = 0, b directly before it changes to c at t = 50 s, d t = 30 s, e directly before it changes to f at t = 50 s, g at t = 65 s. Reprinted with permission from reference [46] a given molecule. For instance, under-coordinated sites like atoms at step edges or kinks are more reactive (i.e.…”