ausblenden:
Schlagwörter:
Platinum; Methane; Partial Oxidation; CPO; Methyl radical;
Heterogeneous-homogeneous reactions
Zusammenfassung:
Goal of the present work was the analysis of the complex interplay between surface and gas phase reactions using the catalytic partial oxidation of methane as reference system. The focus of the work was the detection and quantification of reactive gas phase intermediates to verify or rebut existing reaction models.
The reaction was performed at industrial relevant conditions of temperatures up to 1300 °C, total flows of about 1000 ml/min and atmospheric pressure. The reactor consisted of a resistively heated platinum tube, which acted as reaction tube and catalyst simultaneously. Reactive species were analyzed using a Threshold Ionization Mass Spectrometer. It allows the detection of small amounts of analyte in a matrix of interfering species, appearing at the same m/z ratio, by its ionization potential. The stabilization of reactive species was achieved by expanding a small gas fraction from the atmospheric pressure reaction mixture into a surrounding vacuum through an orifice of about 125 µm. This step represented the first stage of a three stage pumped vacuum system, which creates by a skimmer and collimator arrangement a molecular beam, that couples the reactor to the mass spectrometer. By the resulting supersonic expansion radicals and other species were quenched. The molecular beam therefore represents, with some restrictions, the gas phase above the catalyst.
The first task was the validation of the experimental system in terms of analysis of several key data, as e.g. energy spread and offset of the MS, its detection limits and the separation effects, occurring inside a molecular beam.
The found energy offset of 1.1 eV and the energy spread of 0.6 eV were low enough to allow the unambiguous identification of all expected reaction intermediates, except for the OH. radical, as the not consumed 13CH4 will become ionized at subjacent energies. Using an internal standard allowed additionally the quantitative data analysis.
Temperature profile measurements and of gas GC analysis identified two independent reaction ignitions. The first could be described as the catalytic oxidation of methane with CO, CO2, H2O and H2 as only products. Depending on flow rate, temperature and gas composition a second ignition was observed at much higher temperatures, which was described by a more complex product distribution. With the appearance of C2 products also CH3. radical could be detected. Their molecular flow correlated with the concentration of the C2 products. Together with the simultaneous occurrence of higher, highly unsaturated hydrocarbons as diacetylene or several C3 species it was concluded that these molecules were formed by homogeneous reaction pathways. The change in the oxygen conversion from about 80 %, due to a laminar flow profile inside the tube, to 100 % and the appearance of flames supported this theory. Experiments with varying flow rates and reactant stoichiometries revealed the exclusive formation of the radical in the gas phase.
One can conclude that heterogeneous and homogeneous reactions can run in parallel. Surface bound reactions release heat into the surrounding gas phase and, at a certain temperature, pyrolysis may start which is responsible for the formation of radicals and coupling products. Additionally gas phase oxidation reactions ignite generating more COx and several radicals (H., CH3., OH.), which can have an impact onto the reaction mechanism. Unfortunately beside CH3. no other radicals could be observed due to the expected very low concentrations below the detection limit of the MS. At least for the methane CPO under the chosen conditions heterogeneous and homogeneous reactions are coupled by the exchange of heat, but not by reactive species themselves.
This work is the first experimental detection and quantification of methyl radicals under such reaction conditions.
