2. Experimental and computational
details
Calculation method
In this paper, the Gaussian12 software package was
used to study the thermal decomposition and fire-extinguishing mechanism
of CF3I by means of Ab initio quantum chemistry and DFT.
The geometrical configuration optimization and vibration analysis of all
stationary points involved in the reactions were obtained at the
B3LYP/LanL2DZ level, and at the same level group, the relationship
between reactants, products, intermediates and transition states (TS)
was analyzed by using the theory of intrinsic reaction coordinate (IRC),
and the correctness of each reaction path was verified. For the analysis
for transition states, TS, QST2 and QST3 methods are employed when
necessary. In order to obtain more accurate energy value, a more
accurate method was used to calculate the single point energy of the
stationary point optimized in the reaction at the level of CCSD/LanL2DZ
basis set, and the accurate energy barrier value was obtained. All bond
dissociation energies (BDEs) and energy barriers are corrected by zero
point energy (ZPE).
In addition, on the basis of calculating the energy barrier of reaction
path, the classical variational transition state theory (CVT) method,
which is the most widely used in VTST, is used to calculate the reaction
rate constants of each reaction path under the condition of considering
Eckart tunneling effect, to further verify the possibility of each
reaction path. The above calculation was completed by the
Kisthelp13 package.
2.2 Experimental method
2.2.1 Thermal decomposition
analysis
The thermal decomposition of CF3I (Yuji Tech, 99%) was
conducted in a quartz tube reactor under argon (Tianjin sizhi gas Co.
Ltd, 99.9%) flow (Fig.S1 ). In the experiment,
CF3I was first premixed with the argon carrier gas in a
mixing chamber, and their entering flow rates were adjusted separately
by standard mass flowmeters. At the outlet of the chamber, samples were
extracted for GC-MS (Thermo Fisher Scientific, Trace 1310) instrument
equipped with a DB-VRX column (Agilent, 30 m × 0.25 mm i.d., 1.4um film
thickness). Then, CF3I was further carried into the
tubular furnace, and its residence time was regulated by the gas flowing
rate. The residence time was chosen to be 10 s and the volume fraction
of CF3I was set as 20%. The pyrolysis temperature
ranged from 200 to 800℃. After the thermal paralysis in the quartz tube,
the decomposition products were analyzed by GC-MS. At different
pyrolysis temperatures, the sampling and GC-MS detection were repeated
three times at a time interval of 30 min, to ensure the reliable and
reproducible of the results. The temperature was increased at a ramping
rate of 5℃/min to the next targeting temperature and maintained stable
for 30 min.
2.2.2 Fire extinguishing concentration
measurement
The FEC of CF3I for extinguishing methane-air flame and
propane-air flame were measured by the cup burner (Fig.S2 )
method according to ISO14520-1 and NFPA 2001
standards14, 15. Volumetric flow rates of synthetic
air (Tianjin sizhi gas Co. Ltd, O2 20.9%,
N2 79.1%) and gas fuel were fixed at 40 L/min, 356
mL/min (methane), and 118 mL/min (propane), respectively, to achieve a
visible flame length of 80 mm. After the flame was pre-burned for 60 s,
the extinguishing agent of CF3I was delivered into the
flame burner, until flame blow-off occurred. The mean FEC was determined
based on five consecutive test trial results. All the flow meters
deployed in this paper were calibrated by the soap-film method or the
drainage method before the experiments, and the total uncertainty of FEC
obtained by the experiment was estimated as 5%. The flame extinguisher
was recorded by using a high-speed camera (Phantom Miro LAB110) which
operated at 500 frames per second, with an exposure time of 40μs.