Thermodynamics of the hybrid interaction of hydrogen with palladium nanoparticles

Abstract

Palladium-hydrogen is a prototypical metal-hydrogen system. It is therefore not at all surprising that a lot of attention has been devoted to the absorption and desorption of hydrogen in nanosized palladium particles. Several seminal articles on the interaction of H with Pd nanocubes and nanoparticles have recently been published. Although each article provides for the first time detailed data on specific aspects of hydrogen in nanoparticles, they individually do not contain enough information to draw firm conclusions about the involved mechanisms. Here, we show that the large body of data available so far in literature exhibits general patterns that lead to unambiguous conclusions about the processes involved in H absorption and desorption in Pd nanoparticles. On the basis of a remarkably robust scaling law for the hysteresis in absorption-desorption isotherms, we show that hydrogen absorption in palladium nanoparticles is consistent with a coherent interface model and is thus clearly different from bulk Pd behaviour. However, H desorption occurs fully coherently only for small nanoparticles (typically smaller than 50 nm) at temperatures sufficiently close to the critical temperature. For larger particles it is partially incoherent, as in bulk, where dilute α-PdHx and high concentration β-PdHx phases coexist.

Interface Enthalpy-Entropy Competition in Nanoscale Metal Hydrides

“…The NDs show a huge hysteresis of~100 at low temperature. The observation of a high hysteresis is quite common in constrained systems such as thin films clamped on a rigid substrate [10][11][12]. Figure 6 suggests, qualitatively, that the hysteresis rises with increasing dimensionality of the constraint (1D for thin films and 3D for NDs) and with decreasing confinement length (decreasing ND diameter).…”

“…The combination of these two effects does not result in a true destabilization of the NDs, that can only be claimed if is higher than the bulk equilibrium value [10]. The pressure hysteresis values measured on NPs and NDs are collected in Figure 6.…”

The Hydrogen Grand Challenge

“…Apart from shortening the diffusion path lengths, nanostructured hydrides possess a higher surface area than pure hydrides, which improves the surface reaction kinetics. There have been reports that nanostructuring can change the stability of the hydride (Lohstroh et al, 2010;Baldi et al, 2014;Griessen et al, 2016) and even its decomposition reaction path (Mueller and Ceder, 2010). Both effects might originate from an increased contribution from the surface energy relative to the volume energy (Mueller and Ceder, 2010;Fichtner, 2011;Griessen et al, 2016).…”

“…There have been reports that nanostructuring can change the stability of the hydride (Lohstroh et al, 2010;Baldi et al, 2014;Griessen et al, 2016) and even its decomposition reaction path (Mueller and Ceder, 2010). Both effects might originate from an increased contribution from the surface energy relative to the volume energy (Mueller and Ceder, 2010;Fichtner, 2011;Griessen et al, 2016). It is telling that most of the research on hydrogen in nanoparticles relies on nanostructuring of palladium (Baldi et al, 2014;Griessen et al, 2016): Pd is a hydrogen absorbing, and chemically inert material, and nanostructuring recipes exist since long (Adams and Chen, 2011).…”

“…Both effects might originate from an increased contribution from the surface energy relative to the volume energy (Mueller and Ceder, 2010;Fichtner, 2011;Griessen et al, 2016). It is telling that most of the research on hydrogen in nanoparticles relies on nanostructuring of palladium (Baldi et al, 2014;Griessen et al, 2016): Pd is a hydrogen absorbing, and chemically inert material, and nanostructuring recipes exist since long (Adams and Chen, 2011). The true challenge for materials science lies in finding and functionalization of non-precious metals with improved properties [e.g., as demonstrated by Lohstroh et al (2010), see also Figure 3].…”