Relationship between the activity and the excess free surface energy of noble metal catalysts

The relationship between the excess free surface energy (ΔF) of noble metal catalysts (Pt, Pd, Ru, and Ir), prepared by various methods, and their catalytic hydrogenation activity has been investigated. The excess free surface energy was determined by the measurement of the electrochemical potential of the degassified powder catalysts. The potential of the catalysts was measured in a 1 N HCl solution, 10−3 M with respect to its own ions, referred to the Ag/AgCl/HCl reference electrode. It has been established that for a given metal there exists a relationship between the different activities, depending on the mode of preparation and the free energy values; notably, higher activities correspond to larger excess free energies. The excess free surface energy of the various metals is characteristic primarily of the nature of the given metal. On comparing the excess free surface energies of various metals, a higher excess in free energy does not mean at the same time a higher catalytic activity, but a relationship was found between ΔF and the heat of sublimation.