An Experimental Verification of the Physical Nature of Pt Adsorption onto Alumina

Abstract

In the past decade increasing attention has been paid to catalyst preparation, and in particular to the impregnation step. Various models have been employed to describe the uptake of dissolved metal complexes by oxide surfaces as occurs in catalyst impregnation. These models include “coordination chemistry” models, “chemical adsorption” models, and “physical adsorption” models. In this paper, an experimental discrimination between these models is undertaken. The comprehensive body of experimental evidence presented here suggests that the mechanism for adsorption of Pt complexes arising from chloroplatinic acid onto alumina, under the normal impregnation conditions of room temperature and short (1 h) contact times, is purely physical. That Pt uptake does not depend on Al solubility implies that the coordinative mechanism is not operative. That adsorption of Cl− is not the cause of Pt retardation at low pH contradicts the chemical mechanism of earlier models given for Pt adsorption onto alumina. All experimental data, including large pH shifts, are accounted for by a physical adsorption model which includes a realistic model of surface charging and a balance to account for proton transfer between the liquid and the surface. Since all aluminas have about the same PZC and charging parameters, they behave in an identical fashion regarding the uptake of Pt. All uptake-pH data, for the same amount of alumina surface, can be simulated for all aluminas using one theoretical revised physical adsorption (RPA) curve with no adjustable parameters.