Abstract
Recent advances on synthesis, characterisation and hydrogen absorption properties of ultra-small 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 ultra-small nanoparticles are addressed. The second part of this work reports original results obtained for ultra-small bulk immiscible Pd-Rh nanoparticles. Carbon supported 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 ultra-small dimensions completely suppresses the hydride formation in Pd-rich nanoalloys at ambient conditions, contrary to bulk and larger nanosized (5-6 nm) counterparts. The ultra-small Pd90Rh10 nanoalloy can absorb hydrogen forming solid solutions under these conditions, as suggested by in situ 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: ultra-small 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 ultra-small 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 ultra-small nanoparticles at local level.
Highlights
The control of materials at the nanoscale comprises a new frontier of opportunity in science and technology
The change in ΔR can be directly related to the hydrogen solubility in solid solutions, which is higher for Pd-rich compositions, in agreement with pressure–composition isotherms (PCI) curves (Figure 2). This can be understood in terms of hydrogen preferential occupation in the Pd-rich regions, as suggested by in situ EXAFS
The experimental challenges encountered in synthesis, handling, characterization, and accurate determination of hydrogen absorption properties of MgH2 and noble metal-based ultrasmall nanoparticles have been addressed
Summary
The control of materials at the nanoscale comprises a new frontier of opportunity in science and technology. From extensive research on the synthesis of nanoparticles, it became obvious that stabilized nanoparticles into light and porous supports/scaffolds are appropriate candidates to avoid/limit coalescence during utilization in the foreseen applications (de Jongh and Adelhelm, 2010; de Jongh et al, 2013; Zlotea and Latroche, 2013). On another perspective, future energy landscape stringently requires the use of less-pollutant energy resources and the reduction of greenhouse emissions from fossil fuels. Light and nanostructured materials (nanosized metals/alloys and hydrides along with nanoporous solids) are currently explored for their promising solid-state hydrogen storage and electrochemical conversion properties (Bérubé et al, 2007; de Jongh and Adelhelm, 2010; Cheng et al, 2012; Oumellal et al, 2014; Crivello et al, 2016; Sartori et al, 2016)
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