A study of the kinetics and mechanism of the adsorption and anaerobic partial oxidation of n-butane over a vanadyl pyrophosphate catalyst
The interaction of n-butane with a ((VO){sub 2}P{sub 2}O{sub 7}) catalyst has been investigated by temperature-programmed desorption and anaerobic temperature-programmed reaction. n-Butane has been shown to adsorb on the (VO){sub 2}P{sub 2}O{sub 7} to as a butyl-hydroxyl pair. When adsorption is carried out at 223 K, upon temperature programming some of the butyl-hydroxyl species recombine resulting in butane desorption at 260 K. However, when adsorption is carried out at 423 K, the hydroxyl species of the butyl-hydroxyl pair migrate away from the butyl species during the adsorption, forming water which is detected in the gas phase. Butane therefore is not observed to desorb at 260 K after the authors lowered the temperature to 223 K under the butane/helium from the adsorption temperature of 423 K prior to temperature programming from that temperature to 1100 K under a helium stream. Anaerobic temperature-programmed oxidation of n-butane produces butene and butadiene at a peak maximum temperature of 1000 K; this is exactly the temperature at which, upon temperature programming, oxygen evolves from the lattice and desorbs as O{sub 2}. This, and the fact that the amount of oxygen desorbing from the (VO){sub 2}P{sub 2}O{sub 7} at {approximately}1000 K is the same as that required for the oxidation of the n-butane to butene and butadiene, strongly suggests (1) that lattice oxygen as it emerges at the surface is the selective oxidant and (2) that its appearance at the surface is the rate-determining step in the selective oxidation of n-butane. The surface of the (VO){sub 2}P{sub 2}O{sub 7} catalyst on which this selective oxidation takes place has had approximately two monolayers of oxygen removed from it by unselective oxidation of the n-butane to CO, CO{sub 2}, and H{sub 2}O between 550 and 950 K and has had approximately one monolayer of carbon deposited on it at {approximately}1000 K. It is apparent, therefore, that the original crystallography of the (VO){sub 2}P{sub 2}O{sub 7} catalyst will not exist during this selective oxidation and that theories that relate selectivity in partial oxidation to the (100) face of the (VO){sub 2}P{sub 2}O{sub 7} catalyst cannot apply in this case.
- Research Organization:
- UMIST, Manchester (GB)
- OSTI ID:
- 20014529
- Journal Information:
- Journal of Catalysis, Vol. 189, Issue 2; Other Information: PBD: 25 Jan 2000; ISSN 0021-9517
- Country of Publication:
- United States
- Language:
- English
Similar Records
Reactive lattice oxygen sites for C sub 4 -hydrocarbon selective oxidation over. beta. -VOPO sub 4
On the mechanism of n-butane oxidation to maleic anhydride on VPO catalysts. I. A kinetics study on a VPO catalyst as compared to VPO reference phases