Infrared spectroscopy measurements of the kinetics and
decomposition pathways of aqueous urea
((NH2)2CO, 200−300 °C, 275 bar) and guanidinium nitrate
([(NH2)3C]NO3, 240−300
°C, 275 bar) are described. A
Pt/Ir alloy flow cell with diamond wafer windows was used, and heat and
fluid transport models show that
isothermal and plug flow conditions exist. The hydrothermolysis of
urea was modeled by the conversion of
urea to NH4
+ + OCN- followed
by hydrolysis to CO2 + 2NH3. These
reactions are a subset of those for
hydrothermolysis of guanidinium nitrate. Decomposition of
guanidinium nitrate is catalyzed by the formation
of NH3. The reaction scheme involves deprotonation of
the guanidinium ion by NH3 to produce neutral
guanidine, which hydrolyzes to form urea as the rate-determining step.
The subsequent hydrothermolysis
chemistry follows that of urea. Thus, although the overall
decomposition rate of guanidinium nitrate is slower
than that of urea, the Arrhenius parameters for the step for formation
of CO2 from guanidinium nitrate and
urea are similar [E
a ≅ 66 kJ/mol, ln
A (s-1) ≅ 19]. Only a
small difference in these values is incurred by
using a 316 stainless steel−sapphire cell in place of the
Pt/Ir−diamond cell. Hence, wall effects appear to
be small for this reaction.
The temperature dependence of the infrared absorptivity of the asymmetric stretch of CO2 and N2O dissolved in H2O was determined at 300–600 K under 275 atm. These results are essential for using these species as internal calibrants of the rate of many hydrothermal reactions by infrared spectroscopy. The absorptivity (band area) for ν3(CO2) at constant number density increases from 1.58 × 104 cm/mmol at 300 K to 2.68 × 104 cm/mmol at 600 K. The absorptivity of ν3(N2O) is 8.57 × 103 cm/mmol at 300 K and 1.33 × 104 cm/mmol at 525 K. The absorptivity is suppressed in the presence of H2O by a factor of about 5 compared to results for the gas phase. The absorptivity increases, however, with increasing temperature in H2O solution, which is opposite the trend for the gas phase. The Lorentzian line shape in H2O solution provides a global relaxation time of <1 ps, which is more consistent with relaxation by vibrational energy transfer among associated molecules than by collisions or stochastic modulation by the surrounding H2O field.
Methods are described for determining equilibria and reactions in H 2 O by FT Raman spectroscopy and stainless steel and titanium flow cells operated up to 500 K and 275 bar. Semiquantitative correlations were achieved between the Raman scattering and concentration of CO 2 , N 2 O, and NO 3 -under these conditions. The CO 3 2-, HCO 3 -, CO 2 , and NH 2 CO 2 -components of aqueous (NH 4 ) 2 CO 3 were observed directly and reveal a preference for neutral species (CO 2 and NH 3 ) at higher temperature. The exothermic decomposition of aqueous [NH 3 -OH]NO 3 (HAN) was investigated at a pressure of 275 bar as a function of temperature, concentration, and flow rate. The flow reactor appears to be most useful when the Damköhler number is 1-2. From the induction times-to-exotherm in the Ti cell, apparent activation energies of 129 ( 29 kJ/mol for 0.87-1.52 m HAN and 66 ( 8 kJ/mol for 1.58-1.74 m HAN were obtained. Arrhenius preexponential factors are estimated. The apparent activation energies are compared to previous estimates at different conditions, and are consistent with the formation of a critical concentration of a species which catalyzes the exothermic process.
The first kinetic measurements for aqueous
[NH3OH]NO3 that are based on
species concentrations at
hydrothermal conditions are discussed. The decomposition rate of
0.1 and 0.2 m [NH3OH]NO3
was determined
in real time by flow-cell IR spectroscopy at 463−523 K and 27.5 MPa.
The loss of NH3OH+ (2730
cm-1
bending combination) and formation of N2O (ν3
at 2230 cm-1) occur at the same rate under
these conditions
and were fit to a first-order rate expression. Using all of the
data, E
a = 103 ± 21 kJ/mol and
ln(A/s) = 21
± 5. The decomposition rate resembles that of solid
[NH3OH]NO3 in a similar temperature
range, but the
kinetics for dilute solutions and the solid state are too slow to apply
to the combustion rate of a more
concentrated solution of
[NH3OH]NO3.
Spectrokinetic analysis by IR spectroscopy was performed on
1.07 m carbohydrazide,
(NH2NH)2CO, in a
Pt/Ir flow cell with diamond windows. The reaction rate at five
temperatures between 503 and 543 K at 275
bar pressure was obtained. A lumped kinetic scheme involving two
rate constants is proposed for the conversion
of carbohydrazide to CO2 and N2H4.
In the first step carbohydrazide decays by pseudo-first-order
kinetics
(E
a = 88 ± 1 kJ
mol-1,
ln(A/s-1) = 18).
ΔS
⧧ = −108 J
mol-1 K-1, which is
consistent with the involvement
of H2O in the cleavage of the C−N bond. The main
intermediate thus formed is proposed to be the hydrazinium
salt of hydrazacarboxylate, which can exist in equilibrium forms.
This intermediate decomposes in the second
stage of the reaction to N2H4 and
CO2 with global Arrhenius parameters of
E
a = 57 ± 1 kJ
mol-1 and
ln(A/s-1) = 10 for the
initial 10−15% of conversion to CO2. At higher
percentage of conversion, the rate of
formation of CO2 greatly accelerates, which suggests
autocatalysis. The catalyst is proposed to be CO2
by
its effect on the hydrogen ion concentration.
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