Subsurface hydrogen absorption and hydride phase transitions in highly dispersed catalytically active deposits of platinum metals

摘要

The forms of bonding of very high concentrations of hydrogen chemisorbed by highly dispersed surface deposits of platinum group metals continue to provide a subject area for considerable discussion. Nevertheless of all the platinum emtals, it is only for palladium that clear evidence exists of formation of a bulk hydride (alpha or beta') phase at low values of equilibrium hydrogen pressures at ambient temperatures. This feature together with associated high bulk lattice hydrogen solubilities and high hydrogen diffusion coefficients is a background factor in the employment of surface coatings of palladium as a technique for transmitting hydrogen into other metals and alloys. Evidence from electrode potential measurements at electrodepositions of palladium black also has suggested possibilities of surface or subsurface hydride transitions at much lower hydorgen chemical potentials than the alpha -> beta phase transition in bulk palaldium. Further studies have been carried out with different electrode substrates regarding the stability of such 'subsurface' hydrides to oxidation and thermal outgassing.