A fundamental model of platinum impregnation onto alumina

Refined forms of the revised physical adsorption Model (RPA) proposed by Agashe and Regalbuto (J. Coll. Interf. Sci. 185 (1997) 174) and a proton transfer model by Park and Regalbuto (J. Coll. Interf. Sci. 175 (1995) 239) were employed to model five sets of data of platinum (from chloroplatinic acid, CPA) adsorption over alumina found in the literature. Through a detailed individual consideration of ionic strength, which is the main cause of adsorption retardation at pH extremes, the results were further improved such that adsorption could be predicted a priori to a very reasonable degree without adjustable parameters. A comprehensive simulation of uptake as a function of initial platinum concentration, initial solution pH and the surface loading (amount of oxide surface per solution volume) shows that the model reflects all important features of adsorption typically seen in experimental studies and illustrates how it can be used to optimize the design of a catalyst impregnation. Over a range of low–moderate surface loadings corresponding to wet impregnation of relatively small amounts of oxide in excess solution, the diprotic chloroplatinic acid itself creates favorable electrostatic conditions and little or no pH adjustment is required to obtain full uptake of the available platinum from the solution (high "uptake efficiency"). At higher surface loadings corresponding to dry impregnation or pore volume filling that are more representative of industrial catalyst preparation recipes, however, the pH buffering effect of the oxide becomes so pronounced that additional acid is required to sufficiently charge the oxide surface and facilitate full platinum uptake. This offers a new interpretation of the often-cited necessity of excess hydrochloric acid in pellet impregnation at high surface loading, where a homogeneous metal distribution in the pellet is desired; the protons rather than the chloride ions may be the important factor. Furthermore, excessive amounts of platinum in solution (corresponding to an excess of a monolayer of Pt complexes at the surface) lead to self-inhibition based on high ionic strength in the solution and a dramatic drop in noble metal efficiency.