INTRODUCTION AND BACKGROUND Sales of oxidation catalysts account for about 18% of total world sales of catalysts in the process industries. It is an enormous scale. Oxidation reactions produce the most versatile commodities. Thermodynamics favour complete combustion but selective oxidation products can be intercepted “kinetically”. Metal oxides, especially transition metal oxides, form the basis of selective oxidation catalysts. The catalyst performance in terms of activity and selectivity is strongly related to the lattice structure. Most selective oxidation reactions kinetics can be described in terms of the “REDOX” mechanism. Until recently, few studies focussed on the catalyst surface dynamics in relation to the “REDOX” properties. The secret is there.

What happens on the surface of a selective oxidation catalyst? The best catalysts for selective oxidation are mixed oxides (binary or multicomponent). The various phases present co-operate in a special way not only to promote selectivity but also to sustain the “REDOX” cycles. The chemical oxidation reaction takes place on the surface following chemisorption of the hydrocarbon molecule. This leaves an oxygen vacancy on the surface The free energy of the surface changes according to: where  Hcoh = surface energy, Zs = number of missing nearest neighbour on surface, Z coordination number in the bulk, Ns = density of atoms in the surface One can expect the surface to restructure by inward relaxation of reduced metal ions in order to minimise the surface free energy. Upon reoxidation of the reduced surface metal, the former equilibrium position can be reinstated. How can we obtain experimental evidence of this surface restructuring? How can we exploit this phenomenon in quantitative terms?

An example of oxide catalyst structure: The Bismuth Molybdate lattice The metal atoms on the surface have a different co-ordination number than those in the bulk. They are special Surface O 2- species lost to the hydrocarbon can be recovered from transport of O 2- from the nearest oxide layers in the bulk Balance of evidence suggests that site for catalyst reduction is Bi and site for catalyst reoxidation is Mo Mo oxygen polyhedra constitute oxygen “reservoir” that fills oxygen vacancies on the surface. No evidence for gas phase oxygen involvement in filling surface vacancies.

Potential tools to study surface dynamics qualitatively and quantitatively Our understanding of selective oxidation processes has been gradual and often the biggest problem was reconciliation of kinetic data with mechanistic information. The surface chemical and physical phenomena models were the seeds of most current research activities in the field - Imagine - Test - Fit - Try again…. The combination of spectroscopic techniques and kinetic data offered a real opportunity to unravel the secrets of selective catalysis. This began with a thorough investigation of oxide lattice structure. Initially catalyst samples were studied under vacuum, then single crystals were investigated under reaction conditions, and recently “in-situ” studies are being conducted using real catalysts. The experiments are very difficult to conduct and are very costly. Only co-operation/collaboration between well resourced groups can achieve credible results. Recent studies have identified novel means of studying oxide surface dynamics both qualitatively and quantitatively. This approach is based on the established “REDOX” concept and the surface restructuring induced by surface thermodynamics always aiming at minimising the free energy. Results published in the literature on the oxidation of CO on transition metals revealed the existence of oscillatory reactions that closely match the periodic catalyst reduction/reoxidation. Standing wave patterns accompanying harmonic kinetic oscillations were observed by spectroscopy.

Spectroscopic techniques, catalyst systems and conditions

Where to now? Selective oxidation yields the most versatile chemicals and will continue to expand. It deserves a new approach. The new commercial success of Dupont with the moving bed reactor technology clearly suggests that catalyst surface dynamics must be the basis for any development of kinetic models for selective oxidation. The author is working on a project that is addressing this important issue. We are aiming at developing both kinetic and reactor models for selective oxidation under special reaction regimes. This a major shift from the traditional approach.