of 10 angstroms. Alternatively, metal dispersions equal to unity would be obtained if the metal clusters were raftlike or platelike structures consisting of single layers of metal atoms on the carrier. This alternative would require a strong interaction between the metal and the carrier to impart stability to clusters with such shapes.

An experimental estimate of S/M can be made from a measurement of the amount of gas chemisorbed on the metal clusters. The chemisorption must be selective, readily saturating the surfaces of the metal clusters with a monolayer but not occurring on the metal-free surface of the carrier. The chemisorption of a gas such as hydrogen, carbon monoxide, or oxygen at room temperature has been used effectively for this purpose. Use of the method requires knowledge of the stoichiometry of the chemisorption process, that is, the number of molecules chemisorbed per surface metal atom. The ratio of the number of chemisorbed molecules to the total number of metal atoms present in the catalyst, coupled with knowledge of the chemisorption stoichiometry, makes it possible to determine the metal dispersion S/M.

In hydrogen chemisorption on the Group VIII metals, it is generally accepted that the hydrogen molecule dissociates, so that hydrogen atoms are adsorbed on the surface. Typical data on the chemisorption of hydrogen at room temperature on a platinum-on-alumina catalyst¹¹ are shown in Figure 4. The isotherm labeled A is the original isotherm determined on the “bare” catalyst surface. The bare surface was prepared by evacuation of the adsorption cell at high temperature (725 K) subsequent to the reduction of the catalyst in flowing hydrogen at 775 K. The catalyst was cooled to room temperature in a vacuum, hydrogen was passed over it, and isotherm A was measured. The adsorption cell was again evacuated at room temperature for 10 min (to approximately 10^–6 torr), and a second isotherm, labeled B, was measured. Isotherm A represents the total chemisorption, and isotherm B represents the weakly chemisorbed fraction, since it is removed by simple evacuation at room temperature. Isotherm B includes adsorption on the alumina carrier. The difference isotherm, labeled A-B, is obtained by subtracting isotherm B from isotherm A and is independent of pressure over the range of pressures used in obtaining the isotherm. It represents the strongly chemisorbed fraction, that is, the amount that cannot be removed by evacuation at room temperature. The quantity H/M in the right-hand ordinate of Figure 4 represents the ratio of the number H of hydrogen atoms adsorbed to the number M of platinum atoms in the catalyst. If we assume a stoichiometry of one hydrogen atom per surface platinum atom in the case of the strongly chemisorbed fraction, the value of H/M determined from the dif-