The knowledge obtained about chemisorption on semiconductor oxides makes pos-sible a better understanding of the behavior of these materials as oxidation catalysts. An oxidation reaction consists of several steps (Hagen, 1999):
1) Formation of an electron bond between the starting material to be oxidized (e. g., a hydrocarbon) and the catalyst; chemisorption of the starting material;
2) Chemisorption of oxygen;
3) Transfer of electrons from the molecule to be oxidized (the donor) to the acceptor (O2) by the catalyst;
4) Interaction between the resulting ion, radical, or radical ion of the starting material and the oxygen ion with formation of an intermediate (or the oxidation product);
5) Possible rearrangement of the intermediate;
6) Desorption of the oxidation product.
Hence the oxidation catalyst must be capable of forming bonds with the reactants and transferring electrons between them. Oxides of the p-type, with their tendency to adsorb oxygen up to complete saturation of the surface, are more active than n-type oxides. Unfortunately, activity and selectivity mostly do not run parallel, and the p-type semiconductors are less selective than the n-type semiconductors. The p-type semiconductors can often cause complete oxidation of hydrocarbons to CO2 and H2O, while the n-type semiconductor oxides often allow controlled/selective oxidation of the same hydrocarbons to be performed.
The ratio of adsorbed oxygen to hydrocarbon on p-type semiconductor oxides is generally high and is difficult to control even at low partial pressures of oxygen. The result is often complete oxidation of the hydrocarbon. In contrast the amount of adsorbed oxygen on n-type semiconductors is generally small and can readily be controlled by means of the nature and amount of dopant (nobel metals, Sb), making selective hydrocarbon oxidation possible.
Simple two-step oxidation/reduction mechanisms are often used to explain in-dustrial reactions. The oxidation of a molecule X (e.g. hydrocarbons) can proceed by two mechanism (Hagen, 1999):
½ O2(G) O* (5) X* + O* products
X* + O (lattice) products + lattice (6) ½ O2(G) + lattice vacancy O (lattice)
In case of (Eq. 5), O2 is more rapidly adsorbed than X, and X* reacts to remove this „excess” oxygen. Oxidation then proceeds through to the final products: CO2 and H2O.
In case (Eq. 6), adsorbed molecules of the starting material react with lattice oxygen. The result is selective oxidation, as is observed for partially oxidized molecules such as carbonyl compounds and unsaturated species in particular.
Selective catalysts that react according to this Mars-van Krevelen mechanism
(Krenzke and Keulks, 1980, 64) formally contain a cation with an empty or filled d orbital, for example:
Mo6+ V5+ Sb5+ Sn4+
4d° 3d° 4d10 4d10
Those metals in their highest oxidation states readily release lattice oxygen, formally as O2-. The pathway involving lattice oxygen is commonly referred to as the redox mechanism because the catalyst itself acts as the oxidizing agent. Gas-phase oxygen then serves only to reoxidize the reduced catalyst. There is a considerable amount of data which support the redox concept (Krenzke and Keulks, 1980, 61). Peacock and co-authours (Peacock et al.,1969) have shown that a bismuth molybdate catalyst can oxidize propylene in the absence of gas-phase oxygen and that the oxygen appearing in the products is from the lattice. The amount of oxygen removed during reduction corresponds to the participation of many sublayers of oxide ions. Report in-volving other oxide systems such as Bi-W-O, Sn-Sb-O, and Sn-P-O (Niwa and Murakami, 1972) have also concluded that these catalysts have the capacity to act as a source of active oxygen. Although these result strongly support the redox mechanism,
the most compelling evidence for the participation of lattice oxygen comes from studies of propylene oxidation in the presence of isotopic oxygen, 18O2 (Krenzke and Keulks, 1980, 61), (Ciuparu and Pfefferle, 2002), (Ciuparu et all., 2001).
Keulks (Keulks,1970) reported that during the oxidation of propylene over bismuth molybdate at 753 K in the presence of gas-phase 18O2, only 2 to 2.5% of the oxygen atoms in the acrolein and carbon dioxide produced were isotopically labeled. This lack of extensive incorporation of 18O2 into the reaction products implies the participation of lattice oxide ions in both the selective and nonselective oxidation reactions (Krenzke and Keulks, 1980, 64).
The heterogeneously catalyzed gas-phase oxidations of unsaturated hydrocarbons (e.g propylene) are large-scale industrial processes (Bettahar et al., 1996). Economic operation of these processes requires a selectivity of at least 60%. In the selective oxidation of propylene, metal oxides are mainly used as catalysts and many different products are obtained (Fig. 2), depending on the catalyst used. The catalytic oxidation of propylene leads preferentially to formation of acrolein (Bettahar et al., 1996):
H2C=CH-CH3 + O2 H2C=CH-CHO + H2O
Carbon dioxide, acetaldehyde, and acrylic acid are formed as side products
CH3-CH=CH2 +O2
Sn, Bi, Mo oxides Acrolein Acrylic Acid Acetone 1,5-Hexadiene Benzene Acetic acid Propylene oxide CO2 Mo,V oxides
Sn, Mo oxides Bi, P oxides Bi, Sb oxides Ti, V oxides Te, W oxides
Cu, Cr oxides
Fig. 2. Oxidation of propylene on various metal oxide catalysts (Hagen, 1999), (Bettahar et al., 1996).
The mechanism of selective oxidation of propylene to acrolein can be summarized as a succession of redox and acid-base steps (Burrington et. al., 1984),
(Fig.3). The π-allyl anion should be first formed by proton abstraction on a basic active site and then oxidized to the π-allyl cation on a redox active site. Through a nucleophilic attack by a lattice O2- anion, the latter species forms the σ-allylic species which, in turn, gives rise to acrolein by a hydride abstraction in a redox step. The intervention of a σ-allylic species is invoked to explain the obtaining of 1-d1 and 3,3-d2, acrolein molecules from 1,1-d2, propylene (Fig.3) in the same proportions (30%
and 70% respectively), (Fig.3). These results suppose that the interconversion between π and σ species takes a place (Burrington et. al., 1984), (Bettahar et al., 1996).
D2C CH CH2 O-
Fig.3. Mechanism of the catalytic oxidation of propylene to acrolein on metal oxides through allylic surface species (Bettahar et al., 1996).