Co-reporter:Jaruwan Amtawong, Suvrajit Sengupta, Michael T. Nguyen, Nicole C. Carrejo, Jin Guo, Everly B. Fleischer, Rachel W. Martin, and Kenneth C. Janda
The Journal of Physical Chemistry A September 28, 2017 Volume 121(Issue 38) pp:7089-7089
Publication Date(Web):September 11, 2017
DOI:10.1021/acs.jpca.7b08730
We report the formation kinetics of trifluoromethane clathrate hydrate (CH) from less than 75 μm diameter ice particles and CHF3 gas. As previously observed for difluoromethane and propane hydrate formation, the initial stages of the reaction exhibit a strong negative correlation of the reaction rate with temperature, consistent with a negative activation energy of formation. The values obtained for trifluoromethane, ca. −6 kJ/mol (H2O), are similar to those for difluoromethane, even though the two molecules have different intermolecular interactions and sizes. The activation energy is lesser per mole of H2O, but greater per mole of guest molecule, than for propane hydrate, which has a different crystal structure. We propose a possible explanation for the negative activation barrier based on the stabilization of metastable structures at low temperature. A pronounced dependence of the formation kinetics on the gas flow rate into the cell is observed. At 253 K and a flow rate of 15 mmol/h, the stage II enclathration of trifluoromethane proceeds so quickly that the overpressure, the difference between the gas cell pressure and the hydrate vapor pressure, is only 0.06 MPa.
Co-reporter:Jaruwan Amtawong; Jin Guo; Jared S. Hale; Suvrajit Sengupta; Everly B. Fleischer; Rachel W. Martin
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 13) pp:2346-2349
Publication Date(Web):June 8, 2016
DOI:10.1021/acs.jpclett.6b00982
The role of methanol as both an inhibitor and a catalyst for the formation of clathrate hydrates (CHs) has been a topic of intense study. We report a new quantitative study of the kinetics of propane CH formation at 253 K from the reaction of propane gas with <75 μm ice particles that have been doped with varying amounts of methanol. We find that methanol significantly accelerates the formation reaction with quite small doping quantities. Even for only 1 methanol molecule per 10 000 water molecules, the maximum uptake rate of propane into CHs is enhanced and the initiation pressure is reduced. These results enable more efficient production of CHs for gas storage. This remarkable acceleration of the CH formation reaction by small quantities of methanol may place constraints on the mechanism of the inhibition effect observed under other conditions, usually employing much larger quantities of methanol.
Co-reporter:Michael T. Nguyen
The Journal of Physical Chemistry C 2016 Volume 120(Issue 16) pp:8482-8489
Publication Date(Web):April 6, 2016
DOI:10.1021/acs.jpcc.6b01294
The effects of temperature and gas flow rate on the formation kinetics of difluoromethane clathrate hydrate from CF2H2 gas and <38 μm diameter ice particles were investigated. The early stage of the reaction exhibits a negative correlation with temperature that is characterized by an activation energy of −7.2 ± 0.7 kJ/mol(H2O) for the 6 mmol/h flow rate and −6.7 ± 1.5 kJ/mol(H2O) for 19 mmol/h. We surmise that the origin of the negative activation energy involves the existence of an unstable structure or gas adsorption to the ice surface. Although the effect of flow rate on the observed activation energy is modest, other aspects of the kinetics are more dramatically affected. Initially, the reaction is faster for the higher flow rate simply because more gas is available for the reaction. However, the reaction stalls for a lower percent yield for the faster flow rate. We postulate that the fast flow rate creates more uniform hydrate formation over the ice surface, resulting in the onset of stage III kinetics at a lower percent yield. Both the temperature and flow rate dependence of the formation kinetics have important implications for practical applications of clathrate hydrates for gas storage and transportation.
Co-reporter:Ramón Hernández-Lamoneda, Cristina Sanz-Sanz, Octavio Roncero, Jordan M. Pio, Molly A. Taylor, Kenneth C. Janda
Chemical Physics 2012 Volume 399() pp:86-93
Publication Date(Web):3 May 2012
DOI:10.1016/j.chemphys.2011.09.015
Abstract
We present the first comprehensive ab initio study of the Ne–Br2 potential energy surfaces and the non-adiabatic couplings between the valence excited electronic states. These ab initio results are used to obtain 3-D approximate potentials for each electronic state, and these potentials are used in a wave packet calculation of the competing electronic predissociation and vibrational predissociation dynamics. The results of this calculation are in excellent agreement with both experimental results and a previous empirical fit to the experiments. The calculations allow us to observe not only the competition between vibrational and electronic dynamics for the dimer, but also the competition between two different electronic channels. Coupling to the 2g state dominates for the levels studied here, but coupling to the C state is progressively more important for low vibrational levels, and may dominate at levels below which the current results pertain. The ability of ab initio surfaces and couplings to so accurately reproduce experimental data raises the hope of a complete understanding of the VP and EP dynamics for other Rg-halogen dimers. Success in the case presented here is largely due to the fact that the VP dynamics for the vibrational levels in this study are in the simple, direct regime. Understanding the simple case so thoroughly provides new hope that the more complicated examples, such as ArI2 and NeCl2, for which experiment and theory are not currently in accord, may yet yield to analysis.
Co-reporter:Joel J. Rivera and Kenneth C. Janda
The Journal of Physical Chemistry C 2012 Volume 116(Issue 36) pp:19062-19072
Publication Date(Web):August 14, 2012
DOI:10.1021/jp3035049
The effects of ice particle size and temperature on the conversion rate of ice to propane clathrate hydrate are presented in this work. Results from this study are interpreted in terms of the three-stage shrinking core model: stage I, initiation of the water and propane enclathration reaction at the surface of the ice particles; stage II, mass transfer through pores on the outer clathrate hydrate shell; and stage III, diffusion of propane molecules through the hydrate layer surrounding the inner core of ice. For the smaller ice particles studied, the initial growth rate and total percent conversion are both higher than for larger particles due to a higher surface to volume ratio. Surprisingly, the peak conversion rate of propane clathrate hydrate increases with decreasing temperatures over the temperature range studied in this work, while the total percent conversion increases for higher temperatures, most likely due to improved mass transfer and diffusion in the later stages of the reaction. For the smallest particles studied, an activation energy of −5.5 ± 0.1 kJ/mol (H2O) is measured for up to 20% conversion. This is the first reported value for the activation energy of propane hydrate formation from ice.
Co-reporter:Melissa R. Prado, Yazmin Cazares and Kenneth C. Janda
Industrial & Engineering Chemistry Research 2009 Volume 48(Issue 11) pp:5160-5164
Publication Date(Web):April 30, 2009
DOI:10.1021/ie900027q
Propane hydrate has been studied to investigate its use as a possible storage medium for methane gas. For practical purposes, hydrate formation needs to be efficient at near ambient conditions. Ice particles exposed to varying mixtures of propane and methane gas are found to form the double hydrate efficiently and may be quite effective for storing methane. For instance, exposure of the ice particles to a 2/1 methane/propane gas mixture for 18 h converts 66% of the ice to hydrate with 77% occupancy of the 512 cages and a final dissociation temperature of 287 K. The samples were then decomposed to determine the pressure versus temperature at a rate of 1 K/h. It was found that deviations from equilibrium were minor. The compositions of the hydrates were determined via gas chromatography. For the 180 μm particles employed in this study, 77% of the water was converted to double hydrate over 72 h. Additional experiments were performed in which the ice particles were sequentially exposed to propane and then methane to make the double hydrate. However, this technique did not prove to be as efficient as exposure to the mixed gas. Finally, gas composition measurements during decomposition of the double hydrates reveal that the two gases are released simultaneously.
Co-reporter:José Zúniga, Adolfo Bastida, Alberto Requena, Nadine Halberstadt, J. Alberto Beswick and Kenneth C. Janda
The Journal of Physical Chemistry A 2009 Volume 113(Issue 52) pp:14896-14903
Publication Date(Web):October 7, 2009
DOI:10.1021/jp905043t
The vibrational bound states of the He2Ne+ complex have been determined using a potential energy surface previously published by Seong et al. [J. Chem. Phys. 2004, 120, 7456]. The calculation was performed by sequential diagonalization−truncation techniques in a discrete variable representation using Radau hyperspherical coordinates. There are 52 bound levels. The ground state has an energy of 605.3 cm−1 above the absolute minimum and lies about half way to dissociation. The evaporation energy of one He atom is equal to 866.1 cm−1. Only four levels have energies below the classical energy for dissociation, and all the other 48 states are bound by the zero-point energy of the HeNe+ fragment. The implications of the properties of the eigenvalue spectrum and of the corresponding wave functions on the vibrational relaxation dynamics and infrared spectra of HeNNe+ clusters is discussed.
Co-reporter:Joanne A. Abbondondola, Everly B. Fleischer and Kenneth C. Janda
The Journal of Physical Chemistry C 2009 Volume 113(Issue 11) pp:4717-4720
Publication Date(Web):2017-2-22
DOI:10.1021/jp804515h
Experiments are reported that show propane is incorporated into clathrate hydrate cages much more rapidly using propane−xenon mixtures than for pure propane gas. Uptake rates for pure propane type II clathrate hydrate, pure xenon type I clathrate hydrate, and propane and xenon binary type II clathrate were studied for several different synthesis procedures. Upon adding a 0.92 xenon:propane ratio gas mixture to ice particles, the time required for achieving 62% of the theoretical yield of propane enclathration is 20 min, versus 3 days for pure propane. Although the acceleration of clathrate formation decreases as xenon is depleted, enhancement continues even after the composition falls below 3% Xe. It appears that xenon serves to nucleate the dodecahedral 512 cages while propane nucleates the larger 51264 cages. The type II xenon−propane structure is not only more thermodynamically stable than either pure hydrate; it is also formed much more quickly than propane clathrate, nearly as fast as type I xenon clathrate.
Co-reporter:Thomas Ruchti, Berton E. Callicoatt and Kenneth C. Janda
Physical Chemistry Chemical Physics 2000 vol. 2(Issue 18) pp:4075-4080
Publication Date(Web):12 Jul 2000
DOI:10.1039/B002051F
The
ionization and fragmentation of clusters containing 1100, 2200, 2800 or 3300 helium atoms, on average,
and between one and four Xe atoms are studied by electron impact mass spectrometry. The results are quite different
from those of earlier studies of helium clusters containing Ne and Ar atoms. The observation of
the Xe+
fragment is much more probable for ionization of clusters containing a single Xe atom than is the observation
of Ne+ or Ar+ for clusters that contain a single Ne or Ar atom. Also, clusters that contain two Xe atoms
are much less likely to yield Xe2+ compared to the analogous process for clusters containing two Ne or Ar
atoms. These differences are attributed to the fact that the charge transfer from He+ to Xe can lead to electronically
excited Xe+ ions. Charge transfer from He+ to Xe is not substantially more probable than from He+
to Ne or Ar. The changes of
the charge transfer
probability and the fragmentation patterns with cluster size are discussed.
.jpg)
