Platinum redispersion

Foger and Jaeger (70, 71) studied the mechanism of platinum redispersion by chlorine, finding that redispersion occurs by a four-step process. First, one of a number of platinum chloride species forms. For successful redispersion it is critical that conditions be selected such that /J-PtCb forms. This process is influenced by temperature, concentration, and support materials. Second, the volatile / -PtCl2 is transported in the vapor phase. Third, /f-PtCh is readsorbed. 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

According to anomalous small angle X-ray scattering (ASAXS), this compound slowly decomposes to release redispersible platinum nanoclusters with a mean diameter of 1.2 nm. The protective shell is formed on site by excess aluminium organics. 

Particle splitting is not merely of academic interest. Recently, it has been demonstrated that three-way automotive exhaust catalysts can be made from supported palladium alone, rather than from rhodium and platinum. The opportunity results in part from improvements in technology that permit palladium to be redispersed by a particle-splitting process during lean-bum periods. Possibly this redispersion results from palladium particle splitting. 

It follows that regeneration may consist of either (i) removal of IS sometimes poisons, most often inhibitors or fouling agents, e.g., coke (hydrogenation catalysts, e.g., selective hydrogenation of pyrolysis gasoline) or (ii) redispersion of the active species (platinum catalysts) or (iii) both (hydrodesulfurization or catalytic reforming catalysts). 

Figure 10 Rate and CSD data for redispersion of model R/Y-AI2O3 in air at 773 K (a) normalized platinum surface area versus time (b) CSD from TEM of fresh catalyst (c) CSD from TEM of catalyst exposed to air for 1 h (d) CSD from TEM of catalyst exposed to air for 2 h. ...

The overall deactivation due to the HTR is reversed by oxygen treatment at 673 K. Hence sintering of the platinum particles is not responsible for the deactivation since redispersion is not likely to... 

In the case of supported platinum catalysts, a number of investigations about redispersion have been carried out and the mechanism... 

Pieck et al. studied the redispersion and chloride regulation of samples of commercial and laboratory Pt-Re/Al202 catalysts coked in plant and in laboratory, respectively. After coke burning, the samples had 0.4% to 0.5% chloride and were oxychlorinated with a stream of trichloroethylene-methanol-air. Figure 40 shows the total metallic dispersion as a function of catalyst chloride content at the end of oxychlorination for catalysts A (commercial) and B (laboratory) regenerated at different temperatures. The dispersive effect of chloride is evident for concentrations above 0.5% to 0.6% chloride. For chloride concentrations of approximately 0.9%, the total metallic dispersions are high and similar for both catalysts in spite of the different initial values. It was speculated that the species (Pt O Cly) responsible for the dispersion of platinum crystals are similarly produced on both catalysts for the same high... 

The same methodology could be applied to platinum using Pt2(dba)3 and various room-temperature ionic liquids, and led to metallic Pt nanoparticles with a mean size near 2.0-2.5 nm (Scheme 6). " The isolated Pt(0) nanoparticles could be redispersed in the ionic liquid or in acetone or else used in solventless conditions for liquid-liquid biphasic,... 

Lieske, H., Lietz, G., Spindler, H. Volter, J. Reactions of platinum in oxygen- and hydrogen-treated Pt/lgammal-AljOj catalysts I. Temperature-programmed reduction, adsorption, and redispersion of platinum. Journal of Catalysis 81, 8-16 (1983). 

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