High-Pressure Sorption of Hydrogen in Urea

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 υ_s. 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.