Kinetic and thermodynamic studies of hydrogen adsorption on titanate nanotubes decorated with a Prussian blue analogue

In this paper, the kinetic and thermodynamic hydrogen adsorption characteristics of a novel composite comprising TiNT decorated with the Prussian blue analogue Cd₃Fe^III are investigated at high pressures and different temperatures. It is shown that boundary-layer (film) diffusion does not play a limiting role in the mass transport of hydrogen inside the composite material. The diffusion coefficient and time constant at different temperatures and pressures are calculated using an intra-particle diffusion model. The results suggest that molecular diffusion dominates Knudsen diffusion in the composite material. There are clear improvements in the mass transport characteristics compared to bulk Cd₃Fe^III. The Gibb's free energy is estimated by fitting isotherm equilibrium data to the Dubinin-Astakhov model and is used to calculate the enthalpy and the entropy of adsorption. The calculated value of enthalpy is characteristic of a physisorption process and is considerably higher than the activation energy for intraparticle diffusion, suggesting that the rate-limiting step of hydrogen is not mass transport to the adsorption sites.