Molecular Storage of Ozone in a Clathrate Hydrate: An Attempt at Preserving Ozone at High Concentrations

<div><p>This paper reports an experimental study of the formation of a mixed O₃+ O₂+ CO₂ hydrate and its frozen storage under atmospheric pressure, which aimed to establish a hydrate-based technology for preserving ozone (O₃), a chemically unstable substance, for various industrial, medical and consumer uses. By improving the experimental technique that we recently devised for forming an O₃+ O₂+ CO₂ hydrate, we succeeded in significantly increasing the fraction of ozone contained in the hydrate. For a hydrate formed at a system pressure of 3.0 MPa, the mass fraction of ozone was initially about 0.9%; and even after a 20-day storage at −25°C and atmospheric pressure, it was still about 0.6%. These results support the prospect of establishing an economical, safe, and easy-to-handle ozone-preservation technology of practical use.</p> </div

Results of the ozone preservation tests.

<p>This graph shows the time evolution of ozone fraction (mass basis) in each O₃+ O₂+ CO₂ hydrate stored under an aerated atmospheric-pressure (0.101 MPa) condition temperature-controlled at −25°C. The comparison of the ozone preservation test data obtained in this study (marked by closed symbols) and those from a previous study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0048563#pone.0048563-Nakajima1" target="_blank">[7]</a> (marked by open symbols) are compared. The legend inserted in the graph indicates the O₃+ O₂ versus CO₂ molar ratio in the feed gas and the system pressure <i>p</i> for each hydrate-forming operation. The error bar for each data point represents the uncertainty of the ozone-fraction measurement by iodometry.</p

The initial ozone fraction in the formed hydrate versus the gas-phase composition.

<p>The mole fraction of ozone, <i>X</i>_O3, shown here is for the gas phase inside the reactor when the hydrate formation ceased. The legend inserted in the graph indicates the system pressure <i>p</i> during each hydrate-forming operation. The error bar for each data point represents the uncertainty of the ozone-fraction measurement by iodometry.</p

PXRD profile of an O₃+ O2+ CO₂ hydrate at 98 K.

<p>The solid curve shows the intensities observed using Cu−Kα radiation. The top row of tick marks represent the calculated peak positions for the structure I hydrate, and the lower two rows represent those for the hexagonal ice Ih and cubic ice Ic, respectively. The hydrate sample (accompanied by ice crystals) used in this PXRD measurement was formed from a mixture of O₃+ O2 and CO₂ in a nearly 2∶ 8 molar ratio at the condition of <i>p</i> = 2.0 MPa and <i>T</i> = 0.1°C.</p

Schematic illustration of the experimental setup for forming the O₃+ O₂+ CO₂ hydrates.

<p>This setup consists of (a) an oxygen cylinder, (b) a carbon-dioxide cylinder, (c) an ozone generator, (d) a gas-mixing chamber, (e) a pressure gauge, (f) and (g) pressure gauges, (h) and (i) gas-pressurizing chambers, (j) a gas-sampling chamber, (k) a pressure gauge, (l) a hydrate-forming reactor, (m) a Pt-wire resistance thermometer, (n) a stirrer, (o) a data logger, (p) an immersion cooler, (q) a PID-controlled heater, (r) a vacuum pump, (s) an ozone monitor, (t) a vacuum pump, and (u) an ozone decomposer.</p