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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
What happens on the surface of a selective oxidation
catalyst?
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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:
  H coh
Zs
Ns
Z
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
An example of oxide catalyst structure: The Bismuth
Molybdate lattice
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The metal atoms on the surface have a
different co-ordination number than
those in the bulk. They are special
Surface O2- species lost to the
hydrocarbon can be recovered from
transport of O2- 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.
(MoO2)2+
O2(Bi2O2)2+
O
2-
Bi3+
Mo6+
Potential tools to study surface dynamics qualitatively and
quantitatively
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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
Spectroscopic techniques, catalyst systems and conditions
Real catalyst systems Single crystal systems
Techniques suitable XRD, TP methods, IR IR, TP methods, STM,
and Raman, EXAFS,
AFM
for reaction
Mossbauer, ESR,
conditions
NMR, AFM
Vacuum only
techniques
XPS, SIMS, SNMS,
LEIS, RBS, TEM,
SEM
All surface science
techniques, including
methods not listed in
this table
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.