Experimental Challenges in Studying Hydrogen Absorption in Ultrasmall Metal Nanoparticles

Abstract

Recent advances on synthesis, characterization, and hydrogen absorption properties of ultrasmall metal nanoparticles (defined here as objects with average size ≤3 nm) are briefly reviewed in the first part of this work. The experimental challenges encountered in performing accurate measurements of hydrogen absorption in Mg-and noble metal-based ultrasmall nanoparticles are addressed. The second part of this work reports original results obtained for ultrasmall bulk-immiscible Pd-Rh nanoparticles. Carbonsupported Pd-Rh nanoalloys in the whole binary chemical composition range have been successfully prepared by liquid impregnation method followed by reduction at 300°C. EXAFS investigations suggested that the local structure of these nanoalloys is partially segregated into Rh-rich core and Pd-rich surface coexisting within the same nanoparticles. Downsizing to ultrasmall dimensions completely suppresses the hydride formation in Pd-rich nanoalloys at ambient conditions, contrary to bulk and larger nanosized (5-6 nm) counterparts. The ultrasmall Pd90Rh10 nanoalloy can absorb hydrogen-forming solid solutions under these conditions, as suggested by in situ X-ray diffraction (XRD). Apart from this composition, common laboratory techniques, such as in situ XRD, DSC, and PCI, failed to clarify the hydrogen interaction mechanism: either adsorption on developed surfaces or both adsorption and absorption with formation of solid solutions. Concluding insights were brought by in situ EXAFS experiments at synchrotron: ultrasmall Pd75Rh25 and Pd50Rh50 nanoalloys absorb hydrogen-forming solid solutions at ambient conditions. Moreover, the hydrogen solubility in these solid solutions is higher with increasing Pd content, and this trend can be understood in terms of hydrogen preferential occupation in the Pd-rich regions, as suggested by in situ EXAFS. The Rh-rich nanoalloys (Pd25Rh75 and Pd10Rh90) only adsorb hydrogen on the developed surface of ultrasmall nanoparticles. In summary, in situ characterization techniques carried out at large-scale facilities are unique and powerful tools for in-depth investigation of hydrogen interaction with ultrasmall nanoparticles at local level.

Improving Reproducibility in Hydrogen Storage Material Research

“…Nanoporous materials, in contrast, only need to be degassed, although some are air and moisture sensitive and must be handled in an inert environment. [21] Examples include MOFs with open metal sites and any high surface area material doped with small metal clusters [52] that can easily oxidise. MOFs also usually have solvent molecules trapped in their pores, following synthesis, and these must be removed – a process also known as activation .…”

“…It is sometimes necessary to pretreat a sample with H 2 , prior to degassing at an elevated temperature, in order to help first reduce oxide layers in doped samples and then subsequently remove the water. [52] This will help prevent water formation during the subsequent hydrogen sorption measurements. Note that this is specific to the presence of oxidised metal particles, and that differing pretreatment procedures have been reported in the literature.…”

“…Note that this is specific to the presence of oxidised metal particles, and that differing pretreatment procedures have been reported in the literature. [ 52 , 103 ] Each sample must therefore be assessed on a case‐by‐case basis.…”

Rational Design of Nanosized Light Elements for Hydrogen Storage: Classes, Synthesis, Characterization, and Properties

“…However, at ultrasmall particle size (e.g. 3 nm) the coherence lengths in light hydrides including MgH 2 are too short to diffract X-rays [226] and in this case, assuming a material with a heterogeneous particle size distribution, only the largest nanoparticles will contribute to the coherent diffraction and the observed diffraction pattern. For core/shell nanoparticles, the decrease in intensity of the diffraction peaks can be the direct proof of the formation of a full shell screening the diffraction of the core.…”

Interactions of Hydrogen with Pd@MOF Composites

“…However, the characterisation of hydrogen interaction with ultrasmall Pd nanoparticles (size <3 nm), i. e . in the size range relevant for MOFs’ pores, is challenging and might need the help of techniques proposed at the large‐scale facilities …”