Homogeneous and catalytic oxidation of hydrogen and methane.

摘要

Catalytic oxidation of hydrogen and methane is studied, with emphasis on the influence of catalytic and homogeneous chemistries on bifurcation behavior and their impact on reactor operation. In order to simplify this complex problem, only the homogeneous problem was first studied through detailed modeling. Subsequently, the full catalytic problem was addressed with experiments and simulations, using a platinum catalyst. Simulation results were analyzed using a variety of numerical techniques to understand the role and interplay of chemistry, reaction exothermicity, and transport on bifurcation behavior. Experimental data was used primarily for model validation and refinement.; Investigation of only homogeneous oxidation (by modeling a perfectly mixed reactor) revealed that at relatively low pressures, gas-phase H ₂ ignition is kinetically driven, while at higher pressures, reaction exothermicity is essential for multiplicity and is significantly influenced by the transport of HO₂ and H₂O₂. In contrast, gas-phase CH₄ ignition shows a strong dependence on thermal feedback, regardless of reactor parameters, even when purely kinetic ignition is possible. Although oxidation of CH₄ is chemically more complex than H ₂, the dependence of CH₄ ignition on pressure and species transport is less complex than that of H₂. Mechanism reduction (for hydrogen and methane) and ignition criterion development (for hydrogen) were also performed to explore the possibilities of reducing the complexity of the problem.; Catalytic combustion of H₂ over platinum was subsequently modeled using a stagnation-point flow geometry. The study showed that at low temperatures, a distinct set of catalytic ignitions, extinctions, and autotherms occurs, attributed to surface reactions alone. The onset of homogeneous chemistry was seen at temperatures greater than ∼1000 K. Using turning points as the limits of stability, an engineering map indicating parameters for catalytic and complete oxidation was generated. In addition to the modeling efforts, catalytic ignition and autothermal temperatures were also measured experimentally. These results validate the model predictions reasonably well, showing that the model is capable of capturing the intriguing features of catalytic oxidation, such as autotherms, ignition, multiplicity, and the surface stoichiometric point.; Finally, reaction mechanism synthesis was studied. Using a semi-empirical approach, a general methodology for predicting the kinetic parameters for catalytic reactions was developed, which is thermodynamically consistent and takes into account adsorbate-adsorbate interactions. The H₂ oxidation over platinum has been chosen as a model system to test this methodology. Comparison with a variety of available experimental data in the literature shows that the proposed surface mechanism is capable of quantitatively capturing all the important features of the published experiments.