First Evidence of Rh Nano-Hydride Formation at Low Pressure Claudia Zlotea,* Yassine Oumellal, Mariem Msakni, Julie Bourgon, Ste ́ phane Bastide, Christine Cachet-Vivier, and Michel Latroche Institut de Chimie et des Mate ́ riaux Paris-Est, CNRS UMR 7182, UPEC, 2-8, rue Henri Dunant, 94320 Thiais, France ABSTRACT: 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. KEYWORDS: Rh, nanoparticles, hydrogen sorption, hydride phase T he downscaling of metal particles to nanometer range has become an important issue for the design of new materials for hydrogen storage, electrochemical sensors, and conver- sion. 1-4 Downsizing the metal particles to few nanometers can introduce fundamental changes as compared to the bulk state and therefore may overcome the drawbacks encountered in the latter materials such as, unfavorable thermodynamics and slow kinetics. For small particles, the ratio of surface to bulk atoms becomes important and surface effects may influence the total energy and modify the thermodynamics of hydrogen-metal interactions. The surface atoms are in contact with the outer medium leading to extremely developed surface area and therefore to strongly enhanced surface reaction kinetics especially for solid-gas reactions. Finally, due to the smaller size, reaction paths involving atomic diffusion within the particles are reduced allowing very fast reaction rates. Among all noble metals, Pd is the only element that absorbs hydrogen at ambient temperature and pressure forming an interstitial metallic hydride PdH 0.67 . 5 For this reason, bulk Pd is the most studied element for hydrogen storage and is one of the best understood metal-hydrogen system. At low pressure, Pd forms a solid solution with limited solubility with hydrogen occupying randomly the interstitial sites. At higher pressure, a new thermodynamically stable hydride phase is formed with larger hydrogen solubility. The formation of the hydride is directly evidenced by the presence of a plateau pressure in the pressure-composition isotherms (as a consequence of the Gibbs’ phase rule). The nanosized Pd has become the model material to study the nanosize/scaffold effect on metal-hydrogen interaction. 6,7 Contrary to Pd, other bulk noble metals do not absorb hydrogen at ambient pressure but only under very high pressure conditions. 8 For example, bulk Rh metal absorbs hydrogen under 4 GPa hydrogen pressure at room temperature and forms the monohydride RhH. 9 The hydrogen absorption is endothermic, i.e. the enthalpy of bulk hydride formation is positive ΔH= 17 kJ/mol H 2 , as calculated by Tkacz. 9 Because of the endothermic character of the reaction, hydrogen absorption measurements are usually carried out mainly at high temperature. 8,10 The hydrogen desorption is therefore an exothermic reaction. At nanoscale, the interfacial or surface energy may contribute to the thermodynamics as an induced additional pressure increasingly important with decreasing the particle size. 11 The latter term has been quantified for carbon at nanoscale and predicts an exponential increase of the additional pressure with decreasing the size. For example, an additional pressure of 2 and 4 GPa is calculated for nanocrystals of carbon with 4 and 2 nm radius, respectively. This effect can be also understood in terms of the lattice strain on the surface of nanomaterials. Similar exponential increase of the lattice strain with decreasing the size was demonstrated for Ag clusters. 12 This nanoscale effect might induce a comparable behavior to bulk materials under high pressure conditions, as we recently proved for Mg-Ni based nanoparticles. 13 As a consequence, nonabsorbing hydrogen noble elements at nanoscale might Received: April 28, 2015 Revised: June 17, 2015 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.5b01766 Nano Lett. XXXX, XXX, XXX-XXX