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Study of the oversolubility of gases in liquids of nanometric volume

Final Activity Report Summary - OVERSOL-NANO (Study of the oversolubility of gases in liquids of nanometric volume)

The main goal of this project was to measure gas solubility in liquids at the nanoscale. To this aim, we obtained a bank of H2, CH4 and CO2 solubility data in a series of mesoconfined solvents, such as water, ethanol, CCl4, CHCl3, n-hexane and acetone. The confining solids were mesoporous coarse-grained ?-alumina and silica, mesostructured silica’s, e.g. MCM and SBA, and silica aerogel. Gas solubility was measured by micro-volumetry at room temperature, ranging between 288 and 293 K, in the pressure range of 101 to 505 kPa. In these experiments the solid, previously outgassed under secondary vacuum, was soaked with a large volume of solvent and was subsequently partially evaporated in situ by the action of primary vacuum down to the desired loading, as monitored by weight change. As soon as the saturation pressure of the liquid was stabilised, perfectly known H2 doses were submitted to the cell at controlled pressure.

These studies revealed a dramatic increase of H2 solubility in nanoliquids for mean sizes lower than 15 nm as long as the gas-liquid interface was confined within the porous network. In all cases, although Henry’s law did not appear to apply at the nanoscale, the H2 concentration evolved linearly with pressure, involving an increased H2 solubility. The solubility turned into bulk values when the whole solid was soaked by the liquid. These results suggested an important role of mesoconfined gas and liquid interfaces in enhancing gas solubility. The measurements carried out using the new micro-volumetry apparatus were in good keeping with those obtained in the past via hydrogen-one nuclear magnetic resonance (1H-NMR). In the case of the system H2, n-hexane and aerogel, the observed solubility was 60 times higher than the corresponding bulk value.

Furthermore, a mass balance based model was conceived to account for the increased gas solubility in nanoliquids. This model assumed that the volume of a mesoconfined gas-liquid interface in a nanoliquid could not be neglected compared to a bulk liquid, where the interfacial volume could be omitted. Accordingly, in the case of a nanoliquid, the H2 surface excess concentration adsorbed at the gas-liquid interface might contribute significantly to the observed solubility. This model was in good keeping with the observed experimental trends of gas solubility with the nanoliquid size, as well as with two recent modelling studies published by Luzar and Bratko. However, in the case of aerogels, the extraordinary increase of gas solubility also suggested an influence of possible liquid reconstruction in the mesoporous cavities, involving stronger H2 adsorption or solvation by the liquid molecules.

Moreover, we proceeded to analyse the role of nanoliquids in the enhanced catalytic performance of interfacial gas and liquid catalytic membrane contactors (CMRs). As a matter of fact, the first experiments carried at Dalmon’s group in this field using nitrobenzene hydrogenation as a model reaction showed zero-order kinetics for the gas reactant, contrary to what was observed in slurry type reactors. This observation suggested that the catalyst H2 coverage was much higher in the former case. This hypothesis was verified during the stay for certain gas and liquid configurations.

Finally, on the basis of our body of results obtained during this project, we could anticipate two interesting applications of nanoliquids:

1. near-room hydrogen storage and
2. CO2 capture.

The first application was patented and some promising H2 storage results at 60 bar and room temperature were obtained, funded by two new French projects. Three projects on the second concept, one of which to the French Research Agency (ANR), were submitted and we were waiting for their acceptance by the time of this project completion.