The palladium + hydrogen equilibrium at high pressures and temperatures

Abstract

Isotherms of the palladiumfhydrogen system have been determined up to 990 atm at supercritical temperatures from 326" to 477°C. At the highest pressures the atomic ratio H/Pd in the solid phase exceeds 0.59, assumed as a limiting value in Lacher's theory although the theoretical equation reproduces the general features of the isotherms, which show the persistence of H+H interaction above the critical region. The new isotherms are correlated with existing measurements for the critical region and for the same pressure range at low temperatures. The electrical resistance of the solid phase has also been measured.

Review of absorption and adsorption in the hydrogen–palladium system

“…Levine and Weale [17] found that the heat of absorption increases (becomes less exothermic) with increasing hydrogen to palladium ratio at higher pressures and at temperatures above the critical point (Table 4). According to Levine and Weale [17] the relationship between enthalpy of absorption and composition is linear, with a change in gradient at approximately 0.5, they attribute this behavior to lattice expansion.…”

“…Thus some researchers [17] have found that the heat of absorption varies with composition but the data of other authors [18,26] suggest that this is not true. Let us examine the data of Levine and Weale [17] in more detail.…”

“…However, the less exothermic subsurface adsorption can be explained by considering the expansion of the lattice during hydrogen absorption which would require work to be done on the lattice [17]. This also explains the greater difference between surface and subsurface adsorption calculated by Dong et al [5] who have allowed for Table 2 Heats of adsorption and absorption calculated for Pd (1 0 0) and Pd (1 1 0) relaxation as opposed to Paul and Sautet [9] who have considered a frozen palladium lattice, albeit with a very large lattice parameter.…”

“…In other words, the number of available sorption sites increases continuously as the lattice expands [17]. As a result of this continuous expansion of the palladium hydride lattice as the hydrogen content increases, the heat of absorption (or formation of palladium hydride) is thought to be a function of composition [25].…”

“…According to Levine and Weale [17] the relationship between enthalpy of absorption and composition is linear, with a change in gradient at approximately 0.5, they attribute this behavior to lattice expansion. The isosteric heats of sorption range from À46.1 to À11.7 kJ/mol.…”