Insights concerning the mechanism were obtained by fitting the data using a method developed for solidstate reactions (see Figures S-2 and S-3; details of the fitting procedure are provided in the Supporting Information). The bulk material exhibits two kinetic regimes at low conversion (a £ 0.5). Initially, the reaction is zeroth order, consistent with a phase nucleation process. This is followed by a first-order process that may be associated with a phase boundary-controlled reaction, which is consistent with previous experiments and modeling of internal interfacial effects. It also appears that a diffusioncontrolled reaction can be ruled out, which agrees with our prior work using scanning transmission X-ray microscopy (STXM) images indicating that a surface process controls the desorption rate. Hydrogen uptake by the nanoscale material is more complex, exhibiting three different first-order kinetic regimes.