A study of the palladium‒platinum‒hydrogen system over a wide range of hydrogen pressures

Measurements with an extensive series of palladium-platinum alloys, of changes in electrical resistivity due to direct absorption of hydrogen from hydrogen gas at pressures up to 30 kbar (3 GPa) have been made at 25 and 75 $^\circ$C. Interrelations at 25$^\circ$C between hydrogen content, electrical resistivity and hydrogen pressures of up to ca. 20 bar have also been derived, for the more palladium-rich alloys, from measurements of electrode potentials. Substantial changes of electrical resistivity, indicative of significant hydrogen absorption, have been found to occur over increasingly higher ranges of pressures with increasing platinum contents. However, the overall form of relations between electrical resistivity and equilibrium hydrogen pressure (or fugacity) remains essentially the same, showing a gradually accelerating increase of resistivity with increasing hydrogen pressure up to maximum values (corresponding to relative electrical resistivities R/R$_0$ of ca. 1.5-2) followed by a decrease back to close to initial hydrogen-free values. Possibilities are discussed of the association of ranges of the most marked changes of resistivity with regions of $\alpha$ and $\beta$ phase hydride coexistence or with 'supercritical' regions of continuous hydrogen solid solution. An extrapolation of a trend of experimental results suggests that it may be possible to attain a solid solution of hydrogen in pure platinum up to a composition of ca. PtH$_{0.25}$ at hydrogen pressures of ca. 100 kbar at 25$^\circ$C. Other factors discussed are: alterations of thermodynamic parameters with increasing platinum content, and decreasing hysteretic differences between relations derived during increases and decreases of hydrogen pressure. Particular comparisons are made with behaviour in the palladium-rhodium-hydrogen and palladium-nickel-hydrogen systems.