Chemisorption of gaseous molecules, especially hydrogen, is extensively used to estimate the dispersion of group VI11 (groups 8-10) metal cataly~ts.l-~Chemisorption methods are especially important for highly dispersed catalysts, because of the difficulty to establish their dispersion by other techniques, such as X-ray diffraction or electron microscopy.lWhen hydrogen chemisorption data are used to calculate metal surface areas, often a hydrogen-to-metal stoichiometry of one is assumed.However, H / M values exceeding unity have been obtained for supported Pt,293 Rh,495 and Ir6*7 systems.In these cases the dispersions cannot be calculated straightforwardly from the chemisorption results, because of the uncertainty in the adsorption stoichiometry.To solve this problem we used the EXAFS technique to determine the average metal-metal coordination number in the metal particles, which is related to the particle size.From the combined results of the chemisorption and EXAFS measurements we have been able to conclude that the high H / M values are due to the fact that more than one hydrogen atom can be attached to a surface metal atom and that the trend in stoichiometry is analogous to that observed in metal polyhydride complexes.The results of hydrogen chemisorptions and EXAFS meas u r e m e n t ~~ were compared for several Pt, Rh, and Ir catalysts supported on S O 2 , A1203, and TiO2I0 (Figure 1).Although a 'In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees.A and B notation is eliminated because of wide confusion.Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e&, I11 -3 and 13.)
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Kip et al. (1986) studied this question.