Redispersion Reactions of Platinum/Alumina Reforming Catalysts

[摘要] Platinum metal supported on alumina is used industrially as a reforming catalyst, converting linear hydrocarbons to unsaturated cyclised and isomerised species. With prolonged use the platinum tends to agglomerate and carbon is deposited on the surface of the alumina, decreasing the catalytic activity. The catalyst can be reactivated by burning off the carbonaceous residues in oxygen and redispersing the platinum by an oxychlorination treatment. Two models have been proposed to explain the redispersion of platinum, The crystallite splitting model suggests that large platinum particles split. The second model suggests that redispersion occurs via the spreading of a layer of platinum oxide on the alumina due to the decrease in surface tension as platinum is oxidised to platinum oxide. Previous studies in this department suggest that no redispersion occurs following an oxychlorination treatment. X-ray diffraction data suggests that most of the platinum exists at the contrast limit of the electron microscope and below the resolution of X-ray diffraction analysis. The aim of the present study is to identify any sub-10A platinum particles following oxychlorination treatment and to gain a better understanding of the optimum regeneration treatment and to propose a model for the redispersion of platinum following oxychlorination treatment. X-ray diffraction line broadening, chemisorption, transmission electron microscopy have been used and catalytic measurements from a lab-scale micro-reformer have been used to study the regeneration and dispersion of the catalyst following oxychlorination treatments. Small sub-10A platinum particles have been identified by high resolution electron microscopy following oxychlorination treatment indicating that redispersion occurs. Following oxychlorination over 50% of the platinum particles detected by TEM were 15A in diameter. Quantitative XRD has shown that 80% of the platinum exists in a highly dispersed state following oxychlorination treatment. Hollow Cone Illumination was used to resolve small platinum particles. A method was attempted using a Te compound to 'stain' the pIatinum but this was unsuccessful in since it combined with the support as well as the platinum and hence was non-selective. Conditions of oxychlorination treatment were varied to find optimum conditions and to gain an understanding of the mechanism. CCl4 concentration was found to increase the redispersion indicating that a platinum oxychloride complex may be responsible for redispersion. Redispersion was found to increase with time of oxychlorination, although some large platinum particles still remain after the attempted regeneration treatments. With the higher metal loading (C3 0.8% by weight Pt) the size of the large particles,as determined by XRD, increases with the time of oxychlorination. The splitting model is therefore not a good representation of platinum redispersion as energetically large particles split more readily than smaller particles. A model is proposed where redispersion occurs via the spreading of a layer of a platinum oxide due to the decrease in surface tension as the platinum is oxidised to platinum oxide. Large particles may be protected by a layer of platinum oxide and require very long redispersion times to completely spread out. A sintering mechanism is proposed which competes with the redispersion mechanism and so large platinum particles co-existing with the highly dispersed material grow at the expense of the smaller particles. CCl4 must be injected into the gas stream during regeneration to give better metal dispersion. Catalytic data compare well with physical measurements of particle size and dispersion. The catalytic activity decreases following the ageing treatment and increases following the regeneration treatments. The catalytic activity is less stable following oxychlorination for 18 hours. An optimum redispersion treatment is therefore oxychlorination for 9 hours.

[发布机构] University:University of Glasgow

[关键词] Chemical engineering