Rate of Oxidation of a Cobalt Catalyst in Water and Water/Hydrogen Mixtures: Influence of Platinum as a Reduction Promot
- PDF / 344,495 Bytes
- 6 Pages / 595.276 x 790.866 pts Page_size
- 19 Downloads / 183 Views
Rate of Oxidation of a Cobalt Catalyst in Water and Water/Hydrogen Mixtures: Influence of Platinum as a Reduction Promoter E. L. Viljoen Æ E. van Steen
Received: 4 June 2009 / Accepted: 16 September 2009 / Published online: 25 September 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The effect of a reduction promoter, platinum, on the oxidation of a cobalt catalyst was studied. An induction period was observed for the unpromoted catalyst which was absent for the Pt-promoted catalyst. The rate of oxidation was similar for both the Pt-promoted and unpromoted catalyst in the range of degrees of reduction between 50 and 10%. The rate of oxidation of the remainder of metallic cobalt was, however, faster for the platinum promoted catalyst. Keywords Cobalt Fischer–Tropsch synthesis Oxidation Promoter Reduction
1 Introduction Cobalt is an active catalyst for Fischer–Tropsch synthesis especially if high chain growth probability is required, e.g. for the conversion of natural gas into liquid products [1, 2]. Metallic cobalt is believed to be the active phase in cobaltbased catalysts for Fischer–Tropsch synthesis, since the activity is directly proportional to the exposed metal surface area [3]. Due to the relative high cost of cobalt, a stable highly dispersed cobalt catalyst is required to ensure extended catalyst life time [4]. Promoters are added to cobalt catalysts to enhance the reducibility and dispersion [5].
E. L. Viljoen (&) E. van Steen Centre for Catalysis Research, Department of Chemical Engineering, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa e-mail: [email protected] E. van Steen e-mail: [email protected]
123
Therefore, factors like the addition of promoters, possibly influencing the stability of the cobalt catalyst, have to be investigated in order to tailor highly stable, highly active cobalt based Fischer–Tropsch catalyst. Water is the major product in cobalt catalysed Fischer– Tropsch synthesis. It has been reported that the thermodynamic expected behaviour of small crystallites (\8 nm) will deviate from bulk crystallites. They may oxidise upon exposure to water/hydrogen mixtures [3, 6, 7]. The threshold crystallite size for oxidation depends on the water/hydrogen partial pressure ratio [6, 7]. Reduction promoters, such as platinum [8, 9], in cobaltbased Fischer–Tropsch catalysts enhance the reduction yielding metallic cobalt. They may also enhance the oxidation of metallic cobalt with water, since good catalysts for the forward reaction are typically also good catalysts for the reverse reaction. It has been reported [10], that rhenium, which enhances the reduction process, also influences the rate and/or extent of oxidation of cobalt catalyst in water and in water/ hydrogen mixtures. The reduction promoter, palladium, was also found to increase the rate of oxidation of cobalt by oxygen when the cobalt is not fully covered by palladium [11]. A complete palladium shell around the cobalt was found to inhibit the oxidation of the cobalt [11]. The
Data Loading...
