Revisiting active sites in heterogeneous catalysis: Their structure and their dynamic behaviour

• Published in: Applied catalysis. A, General General, 2014-03, Vol.474, p.40-50
• Main Author: VEDRINE, Jacques C
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Chemical Sciences; Chemistry; Dynamic behaviour; Dynamics; Exact sciences and technology; General and physical chemistry; Heterogeneous catalysts; Mathematical models; Oscillations; Solid surfaces; Steady state; Structure and characterisation of active sites; Structure sensitivity; Surface chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2013.05.029

Active sites in heterogeneous catalysis are composed of ensembles of atoms on the surface of the solid of different sizes and chemical compositions depending of the reaction (nature and size of reactants, reaction mechanism, etc.) and the catalyst structure. They could be unique leading to high selectivity or heterogeneous, resulting in low selectivity. Characterisation by real in situ physical methods and theoretical approach help in describing them. Catalytic reactions may be structure sensitive (demanding) or not (facile) whatever on metals, metal oxides or sulfides. •Active sites in heterogeneous catalysis.•Structure sensitivity for metals and metal oxides/sulfides.•Dynamic behaviour of surface layers under reaction.•Flexibility and disorganised surface layers.•Single site approach. This short review article aims at revisiting the description of active sites in heterogeneous catalysis on solid surfaces and their role in catalyst activity and selectivity. Special emphasis is brought to: (i) structure of the solid surface; (ii) importance of active site isolation on the surface; (iii) dynamic behaviour under reaction conditions and (iv) importance of active phase-support interaction. After a general view on active sites in heterogeneous catalyst, this article describes different case studies of metal oxide catalysts, namely MoO3 and VPO catalysts, to exemplify the concepts, including active site description, structure and/or electronic sensitivity, synergy effects and dynamic surface behaviour during catalytic reaction. Static and dynamic models are discussed considering the many and different views expressed during the years and all considering active sites as atoms or ensembles of atoms as suggested by Taylor in 1925 and instable catalytic reaction intermediates as proposed by Sabatier in 1913. However, with the years, complex structures and dynamic behaviour with interaction between the active sites and the adsorbates (reactants and products) during catalytic reactions have been identified and characterised. In the latter case surface metalloinorganic or metalloorganic entities, designated as chemadphase, constitute the basis of reaction intermediates and are submitted to continuous oscillations on the surface under steady state. Recent developments in physical techniques used to characterise catalysts, in particular during catalytic reaction, and theoretical approach and modelling are also presented and discussed in view of improved description of ac