Thermodynamic and kinetic isotope effects in H/D absorption by Zr-2.5Nb alloy: Insights into hydride stability and rate-limiting steps

Zr-2.5Nb alloy is susceptible to hydrogen embrittlement, yet the H/D isotope effects governing hydrogen absorption remain unquantified. In this study, the thermodynamic and kinetic isotope effects of H and D absorption were investigated using PCT and P-t measurements on activated Zr-2.5Nb powder over 573-1073 K, with thermodynamic comparison restricted to β-Zr (948-1023 K). Thermodynamically, the higher D 2 plateau pressure indicates a slightly larger ΔG(T) for D 2 absorption than that for H 2 , implying a slightly lower thermodynamic stability of the deuteride. Kinetically, hydrogen absorption consistently outpaces deuterium absorption across the entire temperature range, with a stronger kinetic isotope effect at lower temperatures. The apparent activation energy changes from negative in the α-Zr region to positive in the β-Zr region, indicating a switch in the rate-controlling mechanism from surface adsorption/dissociation to interfacial reaction. These findings provide mechanistic insight into the H/D isotope effects in Zr-2.5Nb.