Claudia Zlotea

Size-Dependent Hydrogen Sorption in Ultrasmall Pd Clusters Embedded in a Mesoporous Carbon Template

Hydrogen sorption properties of ultrasmall Pd nanoparticles (2.5 nm) embedded in a mesoporous carbon template have been determined and compared to those of the bulk system. Downsizing the Pd particle size introduces significant modifications of the hydrogen sorption properties. The total amount of stored hydrogen is decreased compared to bulk Pd. The hydrogenation of Pd nanoparticles induces a phase transformation from fcc to icosahedral structure, as proven by in situ XRD and EXAFS measurements. This phase transition is not encountered in bulk because the 5-fold symmetry is nontranslational. The kinetics of desorption from hydrogenated Pd nanoparticles is faster than that of bulk, as demonstrated by TDS investigations. Moreover, the presence of Pd nanoparticles embedded in CT strongly affects the desorption from physisorbed hydrogen, which occurs at higher temperature in the hybrid material compared to the pristine carbon template.

Understanding the mechanism of hydrogen uptake at low pressure in carbon/palladium nanostructured composites

The hydrogen sorption/desorption mechanism below 1 bar at room temperature in porous carbons loaded with nanosized metal particles is not well understood and remains a controversial subject in the literature. The aim of this work is to provide a comprehensive view on the hydrogen sorption/desorption process on C/Pd composites by carefully analysing the phenomena involved during the hydrogen cycles in relation with the material characteristics (amount, size and chemical surface state of the palladium nanoparticles). The C/Pd composites consist of templated microporous carbon in which nanosized palladium particles were homogeneously dispersed. Depending on the synthesis condition, the amount of Pd loaded ranges between 1.4 and 12 wt% and the particle size is ranging from 3 to 15 nm. The presence of a palladium oxide layer on the Pd particle surface is revealed by XPS; the amount of this layer depends on the particle size. The hydrogen sorption/desorption measurements indicate that the total hydrogen amount sorbed on the C/Pd composite exceeds the amount required for the formation of β-palladium hydride. This hydrogen sorption excess is attributed in the literature to the spillover effect. To verify this assumption, we performed a careful exploitation of the hydrogen isotherms along with in situ and ex situ characterizations on the C/Pdx composites and a palladium oxide powder. For the first sorption/desorption cycle, two hydrogen sorption steps were identified and the sorbed hydrogen volume in each step was quantified. The first step which is irreversible is assigned to the reduction of PdO leading to the formation of Pd and water. The second step corresponds to the formation of the palladium hydride (PdHy), a step which is influenced by the presence of water. The processes involved in these two steps are strongly dependent on the Pd particle size. The results presented here clearly demonstrate that the PdO reduction is the predominant phenomenon, explaining the hydrogen uptake excess measured in the first cycle. Although a spillover effect cannot be excluded, the experimental data indicate that its possible contribution would remain significantly much weaker than the contribution due to the PdO reduction. To our knowledge, this is the first time that the effect of the Pd oxide layer on the hydrogen sorption (at low pressure and room temperature) on C/Pdx composites has been experimentally proved and quantified.

Synthesis of small metallic Mg-based nanoparticles confined in porous carbon materials for hydrogen sorption

MgH2, Mg-Ni-H and Mg-Fe-H nanoparticles inserted into ordered mesoporous carbon templates have been synthesized by decomposition of organometallic precursors under hydrogen atmosphere and mild temperature conditions. The hydrogen desorption properties of the MgH2 nanoparticles are studied by thermo-desorption spectroscopy. The particle size distribution of MgH2, as determined by TEM, is crucial for understanding the desorption properties. The desorption kinetics are significantly improved by downsizing the particle size below 10 nm. Isothermal absorption/desorption cycling of the MgH2 nanoparticles shows a stable capacity over 13 cycles. The absorption kinetics are unchanged though the desorption kinetics are slower on cycling.

Composition and size dependence of hydrogen interaction with carbon supported bulk-immiscible Pd–Rh nanoalloys

In-depth clarification of hydrogen interaction with noble metal nanoparticles and nanoalloys is essential for further development and design of efficient catalysts and hydrogen storage nanomaterials. This issue becomes even more challenging for nanoalloys of bulk-immiscible metals. The hydrogen interaction with bulk-immiscible Pd-Rh nanoalloys (3-6 nm) supported on mesoporous carbon is studied by both laboratory and large scale facility techniques. X-ray diffraction (XRD) reveals a single phase fcc structure for all nanoparticles confirming the formation of nanoalloys in the whole composition range. In situ extended x-ray absorption fine structure (EXAFS) experiments suggest segregated local structures into Pd-rich surface and Rh-rich core coexisting within the nanoparticles. Hydrogen sorption can be tuned by chemical composition: Pd-rich nanoparticles form a hydride phase, whereas Rh-rich phases do not absorb hydrogen under ambient temperature and pressure conditions. The thermodynamics of hydride formation can be tailored by the composition without affecting hydrogen capacity at full hydrogenation. Furthermore, for hydrogen absorbing nanoalloys, in situ EXAFS reveals a preferential occupation of hydrogen for the interstitial sites around Pd atoms. To our knowledge, this is the first study providing insights into the hydrogen interaction mechanism with Pd-Rh nanoalloys that can guide the design of catalysts for hydrogenation reactions and the development of nanomaterials for hydrogen storage.

First Evidence of Rh Nano-Hydride Formation at Low Pressure.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

Superior hydrogen storage in high entropy alloys.

Metal hydrides (MHx) provide a promising solution for the requirement to store large amounts of hydrogen in a future hydrogen-based energy system. This requires the design of alloys which allow for a very high H/M ratio. Transition metal hydrides typically have a maximum H/M ratio of 2 and higher ratios can only be obtained in alloys based on rare-earth elements. In this study we demonstrate, for the first time to the best of our knowledge, that a high entropy alloy of TiVZrNbHf can absorb much higher amounts of hydrogen than its constituents and reach an H/M ratio of 2.5. We propose that the large hydrogen-storage capacity is due to the lattice strain in the alloy that makes it favourable to absorb hydrogen in both tetrahedral and octahedral interstitial sites. This observation suggests that high entropy alloys have future potential for use as hydrogen storage materials.

Experimental Challenges in Studying Hydrogen Absorption in Ultrasmall Metal Nanoparticles

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.

Nanoconfinement of Mg6Pd particles in porous carbon: size effects on structural and hydrogenation properties

Mg6Pd nanoparticles as small as 4 nm have been synthesized inside the pores of porous carbon. They are formed by infiltration of Mg on previously formed Pd nanoparticles dispersed into carbon. Their crystalline structure, as evaluated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), differs from bulk Mg6Pd since their particle size is close to the large crystal cell (∼2 nm) of this intermetallic compound. Indeed, as compared to bulk Mg6Pd, the nanoparticles exhibit a simpler crystallographic arrangement and a higher atomic disorder. Both thermodynamic and kinetic H-sorption properties of Mg6Pd nanoparticles differ from those of bulk Mg6Pd. The H-kinetics of the Mg6Pd nanoparticles are significantly faster than bulk and are stable for at least 10 sorption cycles. Thermodynamic destabilization of the hydrided state is also observed for Mg6Pd nanoparticles. Changes in the hydrogenation properties are attributed to nanosizing as well as to the modified structure of the nanoparticles as compared to bulk Mg6Pd.

Hydrogen absorption in 1 nm Pd clusters confined in MIL-101(Cr)

We report here the unprecedented modification of the hydrogen absorption/desorption properties of 1 nm Pd clusters relative to the bulk and nanoparticles down to 2–3 nm. These metal clusters have been synthesized by a facile double solvent impregnation method. They contain on average 33 atoms and are confined/stabilized into a metal-organic-framework with different metal loadings (5–20 wt%). This is the first time, to the best of our knowledge, that 1 nm Pd clusters are effectively confined into a MOF for high metal loadings. Such ultra-small nanoparticles are crystalline with the archetypical fcc structure of the bulk metal, as confirmed by both HR-TEM and in situ EXAFS. Hydrogen absorption/desorption properties of 1 nm Pd clusters have been characterized by both laboratory and synchrotron facilities. Under ambient conditions, 1 nm Pd clusters absorb hydrogen forming solid solutions instead of a hydride phase, as usually encountered for the bulk and Pd nanoparticles down to 2–3 nm. This can be understood by a decrease of the critical temperature of the two-phase region in the Pd–H phase diagram below room temperature. Moreover, the activation energy of hydrogen desorption from Pd clusters strongly decreases relative to bulk Pd. This suggests a change in the rate limiting step from surface recombination or β → α phase transformation usually encountered in bulk Pd to hydrogen diffusion into α and β phases in 1 nm clusters.

Optimization of the synthesis of Pd-Au nanoalloys confined in mesoporous carbonaceous materials

Pd-Au nanoalloys confined in mesoporous carbonaceous materials were synthesized by a rapid one-pot microwave assisted approach. Green polymer resins based on phloroglucinol/glyoxylic acid or glyoxal were co-assembled in the presence of a template and metallic salts followed by microwave treatment between 40 and 80°C and subsequent thermal annealing, allowing simultaneous formation of mesoporous carbonaceous materials with in-situ confined Pd-Au nanoparticles. Several Pd-Au compositions were prepared (PdxAu100-x, where x=90; 80; 70 and 50) and their impact on the alloy structure and particle size/distribution evaluated. For Pd90Au10, homogeneously dispersed nanoalloy particles (∼8nm) are obtained in the carbonaceous framework. The increase in the Au content in the alloy gradually induces an increase in the particle size and agglomeration of the particles along with the formation of multiphased alloys, i.e., segregated Pd- and Au-rich nanoparticles. The particle agglomeration was avoided by decreasing the thermal annealing time. The homogeneity of the alloy structure was found to strongly depend by two parameters, the chelating/cross-linker agents and the microwave temperature, i.e., the chelating/cross-linker agents containing carboxylic groups and the higher temperatures inducing more heterogeneous structures. The hydrogen absorption in Pd90Au10 particles with different homogeneity degree was studied at room temperature up to 1bar. Generally, hydrogen absorbs in Pd-rich nanoalloys forming a hydride phase whereas Au-rich phases do not absorb hydrogen under the present conditions.