Abstract
Hydrogen sorption in urea C(NH2)2O has been probed by direct measurements in Sievert’s apparatus at 7.23 and 11.12 MPa as well as by Raman spectroscopy for the sample compressed and heated in a high-pressure gas-loaded diamond-anvil cell up to 14 GPa. Both these methods consistently indicate the occurrence of small nonstoichiometric sorption of hydrogen in urea phase I. The compression of urea in hydrogen affects the Raman shifts of the C–N bending mode δ and the stretching mode υs. The sorption affects the H2 vibron position too. The sorption of 1.3 × 10–2 at 11.12 MPa corresponds to a stochastic distribution of H2 molecules in channel pores of urea. The mechanism leading to this stochastic sorption involves strong correlations between the swollen nanodot regions around the pores accommodating H2 molecules and the squeezed neighboring pores too narrow to act as possible sorption sites. This study on the hydrogen-bonded framework (HOF) of urea marks the smallest pores capable of absorbing hydrogen documented so far. This observation also reveals a new class of compounds, which is located between those that absorb large stoichiometric amounts of certain guest molecules and those that do not absorb them at all, namely, the group of compounds that absorb the guests in a stochastic manner.
Highlights
The application of hydrogen fuels has become an important task aimed at environment-friendly technologies
The voids in the urea crystal are only slightly smaller than the H2 molecule and the voids are connected into channels, the assumption that the H2 molecules can be absorbed at high pressure.[5,6]
Our sorption and desorption experiments in Sievert’s apparatus measured up to 11.12 MPa as well as the Raman spectra recorded with the use of diamond-anvil cell up to 14.0
Summary
The application of hydrogen fuels has become an important task aimed at environment-friendly technologies. In the ferrocene crystals, the shortest intermolecular H···H distance is reduced from 2.599 Å at 0.1 MPa to 2.151 Å at 2.89 GPa;[11] in the urea crystal, the shortest H···H contact is reduced from 2.766 to 2.118 Å between 0.1 MPa and 2.75 GPa.[12,13] On average, the shortest H···H contacts in molecular crystals display the compressibility (βd = −1/d·∂d/∂p) equal to 0.068 GPa−1 This value has been based on the structural data of hydrostatically compressed crystals of organic compounds between 0.1 MPa and 0.5 GPa. Presently, we have investigated the consequences of the van der Waals radii compression for the voids in the urea structure as well as their accessibility at high pressure when all dimensions of the urea and H2 molecules are squeezed. 11.12 MPa, the desorption was much slower compared to the quicker and stronger sorption process, which is an indication of different mechanisms of the sorption of H2 in different pressure ranges
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