Modelling of the dynamics of complex catalytic phenomena based on surface mobility processes and the remote control mechanism

Ruiz, Patricio;Li, YW;Gaigneaux, Eric;Delmon, Bernard
(1997) International Symposium on Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis — Location: ANTWERP(Belgium) (15.September.1997)

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  • Ruiz, PatricioUCLouvain
    Author
  • Li, YW
    Author
  • Gaigneaux, Ericorcid-logoUCLouvain
    Author
  • Delmon, BernardUCLouvain
    Author
Abstract
We report new results concerning the role of surface mobile species in catalytic processes. More precisely, the proposed contribution deals with cooperative effects due to diffusion of some surface species from one to another kind of surface. The understanding of atomic scale phenomena leads to new modelling and chemical engineering developments which will be outlined. With sulfide and oxide catalysts, new catalytic sites are created by the reaction of spillover species with the surface of a potentially active phase, thus modifying its activity and selectivity. This is the remote control (RC). A most conspicuous result is a strong catalytic synergy between two or several distinct solids. In addition, the active phase is protected against unwanted solid-state transformations leading to deactivation. When a RC operates, some part (phase) of the catalysts dissociates a molecule (O-2 or H-2) to form surface mobile species. These species flow, namely "spillover", over the other part. In this way, they create active sites, or regenerate sites deactivated by some infrequent but harmful processes occurring in the normal course of catalytic reactions. These may be failure of sulfur atoms to be eliminated by hydrogen in hydrotreating (HDS, HDN, etc.) or surface reduction in selective catalytic oxidation. Remotely controlled reactions lead to special kinetic equations, which consist of the product of a function determining the number of sites by another function reflecting the occupancy of sites and the intrinsic reaction rate (e.g. of the Langmuir-Hinshelwood type). Hydrotreating reactions are particularly interesting in this respect, because the remote control causes an interconversion between hydrogenation sites and sites responsible for the hydrogenolysis of carbon-heteroatom bonds. A new model will be shortly presented, which reflects all the specificities of hydrotreating reactions, including transient effects. In selective oxidations, the presence or absence of spillover oxygen has an effect on the facetting and reconstruction of MoO3 crystallites, as demonstrated using SEM and AFM. These results highlight the necessity to keep the surface structure in an optimal configuration during the whole catalytic cycle. A modified Mars-van Krevelen model can account better than previous ones for the reduction-oxidation cycle and the change in number of the active sites asa function of the reaction conditions. The outlooks for the development of new fundamental concepts or for a general chemical engineering approach to catalytic processes will be summarized. RC takes place in numerous reactions. Many aspects are influenced by consideration of the RC concept: activity and selectivity, catalyst formulation and "architecture", ageing processes, kinetic modelling, and process operation. A general philosophy emerging from the results presented is that full consideration of the dynamics of structural changes occurring at the catalyst surface permits conspicuous advances in both the atomic and molecular mechanisms and the macroscopic kinetic modelling.
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Ruiz, P., Li, Y., Gaigneaux, E., & Delmon, B. (1997). Modelling of the dynamics of complex catalytic phenomena based on surface mobility processes and the remote control mechanism. Studies in Surface Science and Catalysis, 109, 203-215. https://doi.org/10.1016/S0167-2991(97)80408-9 (Original work published 1997)