High-Pressure Sorption of Hydrogen in Urea

Abstract

Hydrogen sorption in urea C(NH 2 ) 2 O 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 H 2 vibron position too. The sorption of 1.3 × 10 –2 at 11.12 MPa corresponds to a stochastic distribution of H 2 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 H 2 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.

Driving and characterizing nucleation of urea and glycine polymorphs in water

“…Due to its industrial importance, urea has been studied extensively, and several crystalline structures have been synthesized and reported. These include polymorphs named I (space group: ), III ( P 2 12 12 1 ), IV ( P 2 12 12 ), and V( P m c n ), where polymorph I corresponds to the typical polymorph stabilized at ambient conditions, and the rest are high-pressure, high-temperature products ( 94 – 100 ). Moreover, an ever-enriched dictionary of polymorphism in urea has been established with computational methods using classical and quantum approaches ( 15 , 60 , 88 , 89 , 101 – 104 ).…”

Toward Automated Sampling of Polymorph Nucleation and Free Energies with the SGOOP and Metadynamics

“…One alternative is to define a step function to introduce the rotational invariance as achieved through the function f . This OP is developed for the purpose of differentiating form I and form IV along the direction of v 2 , where form IV ( P 2 1 2 1 2) is known to be a high-pressure product of urea in the experiment and form I ( P ̅42 1 m ) is the most stable structure found at ambient conditions. ,,, The main difference between these two polymorphs is illustrated by θ 2 as it equals 50° for form IV and 90° for form I (shown in Figure lower panel) . In this work, the threshold of the step function of measured angles is set to be 100°, which shows better performance than the cutoff of 90° in distinguishing form I from its collapsed version, and the cutoff of distance is set to be 0.64 nm, corresponding to the range of the first neighbor shell.…”

“…We illustrate the framework by studying urea polymorph nucleation from the melt, which is a small molecule commonly used as a fertilizer and an active pharmaceutical ingredient. The urea system has been widely studied both experimentally − and computationally. ,, For consistency, we used the same notations of polymorphs of urea as in ref . In particular, forms I and IV have been synthesized in lab and studied in detail, whereas forms A and B have been found only in simulations so far. ,, Reference suggests that OPs such as the orientational entropy play an important role in urea nucleation and in exploring the configuration space relevant to the processes at work.…”

Towards automated sampling of polymorph nucleation and free energies with SGOOP and metadynamics

“…This order parameter is developed for the purpose of differentiating form I and form IV along the direction of v 2 , where form IV (P 2 1 2 1 2) is known to be a high-pressure product of urea in experiment and form I ( P 42 1 m) is the most stable structure found at ambient conditions. 38,39,41,43 The main difference of these two polymorphs are illustrated by θ 2 , as it equals 50°for form IV and 90°for form I (shown in Fig. 1 lower panel).…”

“…25 We illustrate the framework by studying urea polymorph nucleation from the melt. The urea system has been widely studied both experimentally [36][37][38][39][40][41][42][43] and computationally. 35,44,45 For consistency, we used the same notations of polymorphs of urea as in Ref.…”