Platinum Metals Rev., 1977, 21, (4), 132
Control of the Penetration of Platinum into Alumina Pellets
Small cylindrical pellets of alumina are frequently used as supports for noble metals to be used as catalysts in fixed bed reactors. It has been known for some years that the observed efficiency of such catalysts depends upon the location of the metal within the pellet, and for many reactions it has been believed that a very thin layer of metal close to the pellet’ s surface produces optimum results. Recent theoretical discussions (1, 2) have led to a clearer understanding of the importance of the concentration profile of the metal through the pellet: if the reaction has an overall positive order it is better to have the metal near the surface, but if it has an overall negative order then it is preferable to concentrate the metal towards the centre of the pellet.
Such theoretical considerations are of little value unless they can be put to experimental test, and for this purposea means of controlling the location of the metal within the pellet is needed. A striking instance of how this can now bedone is provided by the recent work of T. A. Nuttall (3) working in the laboratories of the Council for Scientific and Industrial Research at Pretoria in a report on “Catalysts with Sub-surface Active Layers Prepared by Co-impregnation”. Nuttall used the method of competitive adsorption, by taking solutions of chloroplatinic acid containing various concentrations of citric acid.
The citric acid or the citrate ion apparently adsorbs in competition with the chloroplatinate ion on the active centres on the alumina surface, so that the higher the citric acid concentration, the further the chloroplatinate ions can penetrate. In this way it proved possible to produce pellets in which the platinum after the reduction either was close tothe surface, this being described as an “egg-shell” catalyst; or was confined to the centre of the pellet (an “egg-yolk” catalyst); or was located in an intermediate zone (an “egg-white” catalyst).
The effect of other variables, such as time of impregnation, drying and reduction conditions, on the concentration profile were also examined.
Thus, with improved procedures for controlling the location of the metal within catalyst particles, the way is now clear to experimental tests of theoretical models, and hence to a better appreciation of the role of diffusion within the pores of catalyst particles.
- 1E. R. Becker and J. Wei, J. Catalysis, 1977, 46, ( 3 ), 365
- 2S. Minhas and J. J. Carberry, J. Catalysis, 1969, 14, ( 3 ), 270
- 3T. A. Nuttall, CSIR Report CENG 182, published by the Council for Scientific and Industrial Research, Pretoria, South Africa