In-situ investigation of gas phase radical chemistry in the catalytic partial oxidation of methane on Pt

Abstract

The catalytic partial oxidation of methane on platinum was studied in situ under atmospheric pressure and temperatures between 1000 and 1300 °C. By combining radical measurements using a molecular beam mass spectrometer and threshold ionization with GC, GC-MS and temperature profile measurements it was demonstrated that a homogeneous reaction pathway is opened at temperatures above 1100 °C, in parallel to heterogeneous reactions which start already at 600 °C. Before ignition of gas phase chemistry, only CO, H 2 , CO 2 and H 2 O are formed at the catalyst surface. Upon ignition of gas chemistry, CH 3 ⋅ radicals, C2 coupling products and traces of C3 and C4 hydrocarbons are observed. Because the formation of CH 3 ⋅ radicals correlates with the formation of C2 products it can be concluded that C2 products are formed by coupling of methyl radicals in the gas phase followed by dehydrogenation reactions. This formation pathway was predicted by numerical simulations and this work presents an experimental confirmation under high temperature atmospheric pressure conditions.

Methyl Radicals in Oxidative Coupling of Methane Directly Confirmed by Synchrotron VUV Photoionization Mass Spectroscopy

“…Using a matrix isolation electron spin resonance (MIESR) system, Lunsford et al have detected surface-generated, gas-phase methyl radicals in the OCM reaction catalyzed by various catalysts891011121314151617, but MIESR is not a technique capable of directly detecting gas-phase radicals. Schlögl's group developed molecular-beam mass spectroscopy with threshold ionization using the electron impact ionization technique for the in-situ investigation of gas-phase transient intermediates in heterogeneous catalytic reactions1920. Recently synchrotron VUV photoionization combined with molecular-beam mass spectroscopy, also referred as synchrotron VUV photoionization mass spectroscopy (SVUV-PIMS), was developed to study low-pressure premixed laminar flames21.…”

Photoionization Mass Spectrometry for Online Detection of Reactive and Unstable Gas‐Phase Intermediates in Heterogeneous Catalytic Reactions

“…The MBMS with EI ionization was also used in heterogeneous catalysis studies for detecting short‐lived intermediates. For instances, via adjusting the energy of the ionizing electrons to realize the threshold ionization that ionize targeted species prior to the appearance of fragments, Schlögl et al . developed a MBMS system to online characterize gas‐phase transient intermediates in the catalytic reaction of CH 4 and NH 3 to form HCN and catalytic partial oxidation reaction of CH 4 over Pt catalyst using a catalytic wall reactor.…”

“…[24][25][26][27][28] The MBMS with EI ionization was also used in heterogeneous catalysis studies for detecting short-lived intermediates. For instances, via adjusting the energy of the ionizing electrons to realize the threshold ionization that ionize targeted species prior to the appearance of fragments, Schlögl et al [29,30] developed a MBMS system to online characterize gas-phase transient intermediates in the catalytic reaction of CH 4 and NH 3 to form HCN and catalytic partial oxidation reaction of CH 4 over Pt catalyst using a catalytic wall reactor. Methylamine (CH 3 NH 2 ) and methylenimine (CH 2 =NH) were successfully observed in the gas phase contributing to the HCN synthesis, and gas-phase CH 3 * radicals were directly detected in the CH 4 oxidation process and were correlated with the formation of C 2 products.…”

The Solar Refinery

“…A schematic representation for the energy-relevant process of methane combustion [34,35] is given in Figure 10. Here a projection of the energy barriers on a selection of reaction coordinates is shown.…”