Spectroscopy of Hydrothermal Reactions. 1. The CO₂−H₂O System and Kinetics of Urea Decomposition in an FTIR Spectroscopy Flow Reactor Cell Operable to 725 K and 335 bar

Abstract

A combined microflow reactor and short-path-length spectroscopy cell along with the accompanying process controls are described to obtain real-time, in situ transmission IR spectra of reaction components of aqueous solutions up to 725 K and 335 bar. Quantitation of the spectra was required to obtain kinetics and equilibrium constants. The extinction coefficient of CO2 in H2O at 275 bar was found to increase monotonically from 1.52 × 106 at 298 K to 2.26 × 106 cm2 mol-1 at 573 K. Also, CO2 dissolved in H2O was rotationally quenched on the IR time scale below 375 K but progressed into rotational diffusion around 625 K and finally essentially free rotation above 700 K. The kinetics and pathway of hydrothermolysis of urea to CO2 and NH3 were determined directly from spectral data at 473−573 K. Good agreement was obtained between experimental and calculated concentration−time data by using a reaction model consisting of (NH2)2CO → NH4 + + OCN- and NH4 + + OCN- + H2O → CO2 + 2NH3. The Arrhenius parameters for the first-order reaction are E a = 84.2 kJ mol-1 and ln A (s-1) = 17.5, and for latter pseudo-second-order reaction are E a = 58.5 kJ mol-1 and ln A (L mol-1 s-1) = 17.1. The global rate of formation of CO2 without the kinetic model is first-order and has different Arrhenius parameters. As part of this study, the species of 0.1 m (NH4)2CO3 equilibrium were determined at 298−650 K and 275 bar. The equilibrium shifted from the hydrolyzed ionic components at lower temperature to the neutral CO2, NH3, and H2O components at higher temperature. Therefore, the (NH4)2CO3 equilibrium does not influence the kinetic model of urea above about 475 K.

Detailed modeling of the evaporation and thermal decomposition of urea‐water solution in SCR systems

“…However, for simplicity purposes, we choose to include only the forward step of R1. The activation energies used in this modeling work for R1 and R10 are that recommended by Schoppelrei et al [21]. Schoppelrei et al [22] find a pre-exponential factor ranging between 2.19 × 10 7 and 3.98 × 10 7 s -1 depending on the flow cell used.…”

“…Numerous studies [21][22][23][24] indicate that urea decomposes readily in aqueous solution, yielding cyanate (NCO -) and ammonium (NH 4 + ) ions. The preferred reaction route seems to go through a zwitterionic intermediate, H 3 NCONH [25].…”

Simple hydrothermal synthesis method for tailoring the physicochemical properties of ZnO: morphology, surface area and polarity

“…Seeing that the urea content was identical in the three experiments, the lower pH values encountered at lower reaction temperature suggest slow urea decomposition rates and vice versa. According to the literature, the urea decomposition is a temperaturedependent reaction that is favored at relatively high temperatures, typically above 90 C. 40,41 At 70 C (lower pH) the urea hydrolysis is slow, so that the concentration of ammonium and bicarbonate ions is low and consequently less ZnO particles are formed leading to the quasi-ower structures of Fig. 10A.…”

In situ UV-VIS Detection of the Association of Water-Soluble Anionic Porphyrin and Aromatic Bases in Aqueous Solution at High Tempeatures Using a Capillary Flow Hydrothermal Reactor System

“…1 Several types of monitoring techniques using batch and flow reactors have been developed to study reaction behaviors of hydrothermal processes. [2][3][4][5][6][7] One difficulty of monitoring hydrothermal reactions is that the reaction rates under the hydrothermal conditions increase with increasing temperature. Thus, monitoring hydrothermal reactions at the millisecondsecond time range is frequently required.…”

“…To eliminate the influence of the decomposition of chemical species during monitoring a target reaction, the observation of the reaction should be performed within such a short time range. While in situ techniques have become popular using Raman and FT-IR spectroscopy, 2,4 UV-VIS spectroscopy is not yet generally practical. 3,5 Thus, we have developed a new monitoring method using a flow reactor, which enables in situ UV-VIS spectrophotometric measurement in aqueous solution at high temperatures, in the time range 0.08 -3.2 s at temperatures up to 300˚C (Capillary Flow Hydrothermal (CFH) UV-VIS spectrophotometry).…”