ChemInform Abstract: THE KINETICS OF HYDROGEN ABSORPTION IN PALLADIUM (ALPHA‐ AND BETA‐PHASE) AND PALLADIUM‐SILVER‐ALLOYS

Abstract

The reaction H2 → 2 H (dissolved) on thin Pd and Pd‐Ag foils (2.5–5 μm) was studied, with the aid of resistance measurements, in a flow apparatus at temperatures in the range 20–150°C. The rate determining step is the dissociation of the H2‐molecule on the metal surface. It is assumed that in the formation of the activated complex one atom (proton) already enters an interstitial site in the lattice. For very low hydrogen concentrations xH in α‐phase Pd and in Pd‐Ag alloys, where Sieverts law applies, the following rate law is valid The rate law shows that there is a segregation equilibrium between dissolved atoms and atoms in the interstitial reaction sites. For α‐phase Pd the enthalpy of this segregation is ‐1.9 kcal/mol H, the activation energy of the forward reaction is 6.8 kcal and of the backward reaction 11.5 kcal, which yields the reaction enthalpy ‐4.7 kcal/mol H. In a range of higher hydrogen concentrations the forward reaction rate is accelerated by a factor exp(gxH). This leads to the decreased slope of the isotherm p H 21/2 vs. xH, that is known from thermodynamic studies. The acceleration is ascribed to the lattice expansion caused by the dissolved atoms. After the d‐band of the metal phase is filled by electrons from hydrogen (and silver), the forward reaction rate decreases with increasing xH and the backward reaction rate increases exponentially. This leads to the known steep rise of the isotherm p H 21/2 vs. xH in the ß‐phase region or in the Pd – Ag alloys after d‐band filling.

Thermodynamics and kinetics of hydrogen/deuterium absorption–desorption in Pd 0.77 Ag 0.23 alloy

“…To the best of author's knowledge, the kinetics of PdeAg alloy foil in the bi-phasic (a þ b) region is not reported in the literature for direct comparison. However, the activation energy of hydrogen absorption by Pd 0.77 Ag 0.23 obtained in the present study is considerably lower compared to both the literature values reported for single phase region of pure palladium and PdeAg alloys [24,25]. Since, kinetic data depends on many factors like method of measurement, sample size and shape, surface impurity and heat treatment; it is apparently difficult to compare results obtained by different authors.…”

“…The kinetics of hydrogen absorption in palladium and PdeAg alloys have been studied by Auer and Grabke [24] with the aid of resistance measurements in a flow apparatus in the temperature ranges 293e423 K. However, they have reported the kinetic data only for the single phase regions and concluded that the rate determining step is the dissociation of the H 2 molecule on the metal surface. The activation energy for hydrogen absorption reaction is 28.5 kJ/mol H 2 for pure palladium and in the range of 29.3e37.6 (kJ/mol H 2 ) for Pd 1Àz Ag z (z ¼ 0.1e0.4).…”

Substrate effect on the optical response of thin palladium films exposed to hydrogen gas

“…3(a) ). It can be seen that for both samples the data initially follow Sievert's law [1] characteristic of the α phase of PdH x , i.e. a linear relationship, before deviating from it for ffiffiffiffiffiffiffi P H 2 p in excess of 50.4 Pa 1/2 (or P H 2 ≈ 2.54 × 10 3 Pa).…”

“…With PdH x in the α phase, the absorbed H concentration n follows Sievert's law n = AP H 2 1/2 , where P H 2 is the hydrogen partial pressure and A is a proportionality constant. As P H 2 increases, interactions between absorbed H atoms cause deviations from Sievert's law [1] typically for pressures above 2.7 × 10 3 Pa at room temperature. The β-phase of PdH x is characterised with a saturation of the hydrogen concentration n with increasing H pressure following the expansion of the Pd lattice (α + β transition phase).…”

“…In particular Pd based compounds have received a lot of interest due to Pd's capability of absorbing 900 times its volume of hydrogen under specific conditions of pressure and temperature, inducing dramatic and in most cases reversible changes in its physical and structural properties. When Pd is exposed to a hydrogen gas atmosphere, molecular hydrogen (H 2 ) is chemisorbed on the Pd surface where it dissociates into two H atoms (dissociative chemisorption) [1]. After donating its electron to Pd, the atomic H diffuses into the Pd lattice as a proton [2,3].…”