J. Phys. Chem. 1996, 100, 7463-7470
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Spectroscopy of Hydrothermal Reactions. 2. Reactions and Kinetic Parameters of [NH3OH]NO3 and Equilibria of (NH4)2CO3 Determined with a Flow Cell and FT Raman Spectroscopy J. W. Schoppelrei, M. L. Kieke, and T. B. Brill* Department of Chemistry, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: April 4, 1995; In Final Form: February 7, 1996X
Methods are described for determining equilibria and reactions in H2O 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 CO2, N2O, and NO3- under these conditions. The CO32-, HCO3-, CO2, and NH2CO2- components of aqueous (NH4)2CO3 were observed directly and reveal a preference for neutral species (CO2 and NH3) at higher temperature. The exothermic decomposition of aqueous [NH3OH]NO3 (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 Damko¨hler 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.
I. Introduction Rapid exothermic reactions in H2O at high pressure and temperature could arise during the treatment of energetic materials by hydrothermal methods. Our intention is to develop semiquantitative or quantitative spectroscopic methods to characterize these and other reactions in real time. FT Raman spectroscopy has not been employed previously to characterize hydrothermal chemistry. With regard to technique development, flow reactor-Raman spectroscopy cells are described that are suitable for studying reactions in H2O at high pressure and temperature. Because this is our lead article on the subject, the description is given in detail. The method is demonstrated with two systems. First, Raman spectra of the equilibrium components of aqueous (NH4)2CO3 at 290-450 K reveal preference for the neutral species, NH3 and CO2, at high temperature and pressure. These results are consistent with the IR spectra.1 The (NH4)2CO3 equilibrium is important because CO2 and NH3 are probable final products of hydrothermolysis of many organic compounds below 675 K. Second, kinetic parameters were determined for the induction process leading to rapid exothermic decomposition of aqueous hydroxylammonium nitrate (HAN). The reactions of the component ions, NH3OH+ and NO3-, occur by complex mechanisms.2-8 The previous upper temperature limit of kinetic study of aqueous HAN was 308 K. Although the kinetic analysis of aqueous HAN at 425-475 K proved to be difficult, a combination of Raman spectral data and heat balance characteristics of the flow reactor enabled several kinetic parameters to be estimated. II. Description of the Experiments FT Raman Spectrometer. A Nicolet 910 FT Raman spectrometer was adapted to study hydrothermal reaction chemistry. A diode-pumped cw Nd:YAG laser with a maximum power of 1.5 W at 1.064 µm was employed for excitation. * Correspondence author. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)00966-X CCC: $12.00
Instrument holographic filters removed spurious YAG lines and Rayleigh scattered radiation. Plasma lines from the HeNe alignment laser and stray light were removed by a longwave pass filter on the detector window. The long term fluctuation of the laser power was (1 % of the total power. Short-term fluctuation was not detectable by the power meter, but adequate S/N values were obtained which indicates that any short-term power fluctuations do not affect the data. A Ge avalanche detector cooled by liquid N2 was employed. The software rejected interferograms that were corrupted by cosmic radiation. These occurred 3-15% of the time. It was experimentally found that the intensity of Raman scattering by each species was directly proportional to the YAG laser power in the 0.21.2 W range indicating that the photometric response of the spectrometer is linear over this power range. The sample compartment was modified to accept the control wires and feed lines of the heated flow cell. This involved rebuilding the compartment so that access was available during the experiment while minimizing stray light from the room. The collection configuration was 180° backscattering. The prism mirror that directs the source beam to the sample was bathed by a slow stream of cool Ar gas to prevent radiative heating by the cell. Because the system operates in the near-IR, the detector is sensitive to thermal radiation from the heated cell. Thus, saturation of the detector limits the upper temperature range of conventional FT Raman spectroscopy. Methods for removing the thermal emission by using a Q-switched laser and synchronous9 or asynchronous10 sampling have been demonstrated. We chose a less elaborate and, correspondingly, less effective method to reduce the thermal radiation that reaches the detector. Gold foil containing a light port was placed between the cell and the collection optics to serve as a cold baffle. Gold has relatively low IR emissivity and high IR reflectivity. The cold baffle reduced the thermal radiation reaching the detector by approximately 20%. A shortwave pass interference filter (Spectrogon SP-1420-S) was placed before the detector to block wavelengths above 1.419 µm. The shortwave pass filter enabled Raman spectra of intensely scattering species with shifts below © 1996 American Chemical Society
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Figure 1. Side and end views of the flow cell used for Raman spectroscopy.
2337 cm-1 to be detected up to 623 K while eliminating much of the thermal radiation. For the studies described herein, the practical upper limit of temperature is about 513 K for strongly scattering species and 473 K for weakly scattering species. Flow Cells. The corrosiveness of H2O at high temperatures toward most materials11-13 as well as the potential for reactions on the cell walls stimulated tests with different materials of containment. Flow cells having low volume and flow rate were fabricated from type 316 stainless steel (SS) and titanium. H2O at high temperature can leach ions from SS.11-13 On the other hand, SS cells are readily replaced. Ti was chosen because it is known not to be seriously corroded by aqueous hydroxylammonium nitrate.14 Spectral windows of sapphire and diamond are both satisfactory. The use of 0° sapphire is necessary to reduce corrosion by high-temperature H2O and to withstand the force necessary to seal the cell. Diamond-wafer windows potentially enable the intensity ratio of the Stokes and antiStokes shifts of diamond at 1330 cm-1 to be used to estimate the cell window temperature.15 Figure 1 illustrates the cell fabricated from SS and Ti. A cylindrical rod of metal was machined with (0.002 cm tolerance to create the sample cavity and accept the windows (sapphire shown) and endcaps. The entrance and exit feed tubes were offset both radially and longitudinally in the optical cavity to disrupt the flow and enhance heat transfer and mixing. A thermocouple well was drilled into the wall so that the 0.16 cm o.d. type 304 SS sheath of the thermocouple slightly protruded into the sample cavity. The sheath was affixed with a Swagelok fitting and contained an ungrounded type K thermocouple. It was found that a slight protrusion of the sheath into the fluid minimized the response time of the thermocouple to temperature changes. The internal volume, V, of the cell (the entrance tube and the optical cavity) is 0.044 cm3. The resulting residence time given by V/u, where u is flow rate, is 2.7 s when the flow rate is 1 mL/min. A ring of 0.051 cm thick Au foil was placed between the sapphire window and each metal face. Steel endcaps with hardened steel Allen bolts compressed the cell components to achieve the seal. The cell body was tightly wrapped with 0.16 cm o.d., insulated, 21.3 Ω/m heater wire and insulated with fiberglass tape. To preheat the feed solution to 373 K, the SS inlet tube was traced with heater wire and insulated up to the cell body starting about 15 cm away. This preheat stage is well below the reaction temperature. Separate PID controllers (Omega Engineering) were used on the cell and feed tube to measure and control the temperature to (0.5 K. The thermocouple sheath was insulated several inches beyond the cell body to reduce the temperature gradient. As noted earlier, a 0.013 cm thick sheet of gold was placed between the metal face of the cell and the collection optics to
Schoppelrei et al. reduce the amount of thermal radiation that enters the interferometer. The fact that H2O itself has relatively large IR emissivity limits the efficacy of this cold baffle method. These cells were supported by clamping the inlet and outlet tubes to a rigid holder so that alignment could be achieved by an xyz translation stage. The flow and pressure control systems were discussed before.1 The flow path contained no dead space and operated hydrodynamically. The flow rate was constant to within two percent of the overall rate, and the pressure was maintained at (0.7 bar throughout the data collection. The temperature of the cell containing stagnant H2O at 290 K rose less than 1 K in 5 min as a result of heating by the laser operating at 1.5 W. The effect of laser heating is probably less pronounced at elevated temperatures because the PID controller maintains the temperature to within (0.5 K. Attempted Quantitation of Species. Quantitation of Raman scattering can be difficult.16 It requires that the cell alignment and laser power be constant throughout all measurements and that the temperature and pressure in the flow reactor be constant during measurement of an individual data point. More fundamentally, there is still no assurance that Raman scattering by a species is linearly related to concentration, especially at different temperatures and pressures,17 because different degrees of molecular association and distribution of vibrational energy potentially exist. Consequently, the area and height changes of the Raman scattering intensity resulting from different experimental conditions were investigated. First, the relationship between the concentration of dissolved gaseous species and the Raman scattering intensity was determined. HPLC grade H2O (18 MΩ with 0.2 µm filtration) was sparged for 30 min with Ar gas at room temperature. This H2O was then drawn into a stirred autoclave containing the gas under study at about 0.5 bar. The head space of the autoclave was pressurized with N2O or CO2 and stirred vigorously for 120150 min. The solubility parameters were determined from literature data to be 0.0143 (at 40 bar) and 0.0290 bar/m for N2O18 and CO2,19 respectively. The Raman scattering intensities of the resulting solutions were recorded at 0.7 scans/s and 8 cm-1 resolution with 500 coadded scans. The intensities were normalized to a common laser power to correct for any longterm variation. The response function (SKBr/Sbb) of the instrument was obtained from the theoretical blackbody spectrum, Sbb, and the scattering spectrum, SKBr, of dried KBr powder in an NMR tube illuminated by a blackbody source at 1925 K. The Raman scattering of the research fluid was divided by the response function. Peak heights and areas were resolved with Peakfit software. It was necessary to scale data sets obtained at different times to a common function. This was accomplished by scaling the regression of all data sets to a common line with zero as the origin, so that the same concentration gave the same scattering intensity. With data managed as described above, Figure 2 reveals that the Raman scattering intensities of N2O (1284 cm-1) and CO2 (1380 cm-1) in H2O solution at 23 °C and in the concentration range of interest are linearly proportional to the molal concentration of the dissolved gas within the accuracy of the measurement. A similar determination of the relationship between the scattering intensity of ν1(NO3-) and the concentration of aqueous KNO3 is shown in Figure 2. A linear relationship exists up to the 2.0 m limit of the determination. At much higher concentrations (>5 m), contact ion pairing competes with solvent-separated ion pairs, which causes line shape and intensity deviations.20-22 Second, the relationship between the Raman scattering intensity of a fixed concentration of dissolved species and the
Spectroscopy of Hydrothermal Reactions. 2
Figure 2. Density-scaled Raman scattering intensities for KNO3, N2O, and CO2 dissolved in H2O as a function of concentration. Relative scattering characteristics were determined by comparison to the intensity of the 1031 cm-1 line of CH3OH which was flowed at 25 °C before and after the sample studies.
Figure 3. Raman scattering intensity of aqueous N2O as function of temperature. The intensity in each case was divided by concentration.
temperature of the solution was determined. For aqueous N2O and CO2, the solution was pumped from the autoclave through the cell at a flow rate of 0.25 mL/min and a pressure of 275 bar. The cell was then stabilized at the desired temperature. The resultant scattering intensity was corrected for fluctuations in the laser power and the decrease in density resulting from the fact that the temperature was increased while the pressure was held constant. The density of pure water was used23 because the deviation for 0.5 m CO2 at 631 K is only 1.7%.24 Because the alignment is difficult to reproduce exactly, a redundant data point was frequently recorded to check constancy during the measurements. Figures 3 and 4 show that Raman scattering by N2O (1284 cm-1) and CO2 (1384 cm-1) depends only slightly on temperature up to the 475 K limit of study, although deviations exist among the individual determinations. For this plot, the scattering intensity at each temperature was divided by the sample concentration. Clearly, multiple measurements must be obtained because little faith can be placed in an isolated measurement, especially when the temperature limit is approached. The leastsquares fits of the data points for N2O and CO2 in Figures 2-4 provide concentration estimates during reactions and in equilibria. For KNO3, the intensity of ν1(NO3-) in a 1.1 m solution was investigated as a function of ascending and descending temperature. Figure 5 shows that the intensity drops sharply above about 500 K because the NO3- shift is consumed by the
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Figure 4. Raman scattering of aqueous CO2 treated in the same manner as Figure 3.
Figure 5. Raman scattering intensity of ν1(NO3-) in 1.07 m KNO3 under 275 bar with the shortwave pass filter in place. The loss of intensity above 500 K is primarily the result of the effects of thermal radiation.
rising baseline resulting from thermal radiation. There is also evidence that the detector becomes less sensitive, even in energy ranges that are not overlapped by thermal radiation, whenever the cell is above about 475 K. This condition is manifested by loss of intensity of vibrational modes having shorter wavelengths than the thermal radiation edge. Useful spectra of NO3- were, therefore, not obtained above about 510 K. The fact that the scattering intensity recovers upon descent in temperature indicates that the intensity drop is not caused by decomposition of the NO3- ion in this case. The hot bands of the Fermi resonance diad (ν1 + 2ν2) of CO2 provide an approximate temperature of the solution above 665 K.25 Unfortunately, Raman shifts of these hot bands are too poorly resolved at 290-500 K to estimate temperature accurately, although the hot bands of the diad can be detected. Reaction Measurements. The shift in the complex equilibria of aqueous (NH4)2CO3 as a function of temperature was followed spectroscopically in 1.2 m (NH4)2CO3 (Johnson Matthey) in HPLC-grade H2O. The solution was first pressurized to 275 bar while flowing at a rate of about 0.25 mL/min and then heated to the desired temperature. At 8 cm-1 resolution, 500 interferograms were summed. Hydroxylammonium nitrate solutions were prepared from a 2.80 M stock solution (Southwestern Analytical Chemicals, Inc.). With the fluid path filled with H2O under 275 bar pressure, the HAN solution was pumped through until the NO3-
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scattering intensity was constant (30-50 mL). With the solution flowing against the back pressure, the temperature of the cell was raised at 8-10 K/min to a temperature 10-20 K below the expected reaction temperature. The temperature was then increased in 1 K increments and allowed to stabilize ((1 K) for at least 1 min. Beginning at least 10 K below the expected reaction temperature, 25 spectra were summed at 0.7 scans/s and 8 cm-1 resolution for each temperature step. The reaction temperature was identified by both a sharp 2-5 K temperature rise of the thermocouple and a distinct change in the spectrum. When a single flow rate and different temperatures were studied, data were always taken while ascending in temperature. Similar data could be obtained at constant temperature while carefully decreasing the flow rate. After reaction had been observed and prior to using a different set of conditions, the cell was flushed at 4 mL/min with the heaters off. III. Mixing and Potential Catalysis in the Flow Reactor A consideration in the use of this flow reactor is whether the solution is chemically mixed or not. Although a more sophisticated analysis could be conducted,26 a simple estimate based on Fick’s law suggests that sufficiently good mass transport exists within the cavity to assume that the solution is well mixed. By using a conservative value of the diffusion coefficient, D, of 2 × 10-3 cm2/s at the temperature of interest, Fick’s law (td ) ri2/2D) requires a diffusion time, td, of about 0.5 s for a component to transverse the distance of ri ) 0.16 cm from the center axis of the cell to the nearest wall. This time is comparable to the shortest residence time used in this work and is the “worst case” scenario because mixing has been enhanced through disturbance of the flow by off-setting of the inlet and exit tubes. Corrosion of the cell, which results in metal ions in solution as well as different surface area and composition of the cell wall, is a concern. Because aqueous HAN is known to react with metal ions that might occur from corrosion of SS,27,28 the metal ion content of 100 mL solutions of HAN heated to 475 K and flowing at 0.1 mL/min under 275 atm pressure was determined. By inductively coupled plasma emission spectroscopy, the molybdenum, chromium, manganese, and copper concentrations were found to be
