Continuous Pyrolysis Microreactors: Hot Sources with Little Cooling? New Insights Utilizing Cation Velocity Map Imaging and Threshold Photoelectron Spectroscopy

Abstract

Resistively heated silicon carbide microreactors are widely applied as continuous sources to selectively prepare elusive and reactive intermediates with astrochemical, catalytic, or combustion relevance to measure their photoelectron spectrum. These reactors also provide deep mechanistic insights into uni-and bimolecular chemistry. However, the sampling conditions and effects have not been fully characterized. We use cation velocity map imaging to measure the velocity distribution of the molecular beam signal and to quantify the scattered, rethermalized background sample. Although translational cooling is efficient in the adiabatic expansion from the reactor, the breakdown diagrams of methane and chlorobenzene confirm that the molecular beam component exhibits a rovibrational temperature comparable with that of the reactor. Thus, rovibrational cooling is practically absent in the expansion from the microreactor. The high rovibrational temperature also affects the threshold photoelectron spectrum of both benzene and the allyl radical in the molecular beam, but to different degrees. While the extreme broadening of the benzene TPES suggests a complex ionization mechanism, the allyl TPES is in fact consistent with an internal temperature close to that of the reactor. The background, room-temperature spectra of both are superbly reproduced by Franck−Condon simulations at 300 K. On the one hand, this leads us to suggest that room-temperature reference spectra should be used in species identification. On the other hand, analysis of the allyl iodide pyrolysis data shows that iodine atoms often recombine to form molecular iodine on the chamber surfaces. Such sampling effects may distort the chemical composition of the scattered background with respect to the molecular beam signal emanating directly from the reactor. This must be considered in quantitative analyses and kinetic modeling.

Operando PEPICO unveils the catalytic fast pyrolysis mechanism of the three methoxyphenol isomers

“…37 The sharp peak at 9.241 eV is assigned to the doubly degenerate X ˜+ 2 E 1g cation ground electronic state of benzene. 37,38 The experimental spectrum of m/z 78 agrees well with the reference spectra of fulvene and benzene and a very similar composition is found in all three methoxyphenol isomers.…”

10 Years of the ACS PHYS Astrochemistry Subdivision

“…Similar relationships between deuterated methanol and temperature in Orion KL are also reported . One manuscript in this virtual special issue shows that resistively heated silicon carbide microreactors have little-to-no rovibrational cooling in the expansions, but the chemical composition of the background likely will differ from that of the molecular beam emanating directly from the reactor . Finally, two spectral databases are also reported in order to provide large amounts of spectral data for many molecules and rich spectral characterization of CO 2 .…”

“…43 One manuscript in this virtual special issue shows that resistively heated silicon carbide microreactors have little-to-no rovibrational cooling in the expansions, but the chemical composition of the background likely will differ from that of the molecular beam emanating directly from the reactor. 44 Finally, two spectral databases are also reported in order to provide large amounts of spectral data for many molecules 45 and rich spectral characterization of CO 2 . 46 All of these considerations for spectral analysis certainly enhance inferences that can be gleaned from these types of experiments and theoretical computations informing future astrochemical observations.…”

Mechanism and Kinetics of the Thermal Decomposition of Fe(C5H5)2 in Inert and Reductive Atmosphere: A Synchrotron‐Assisted Investigation in A Microreactor

“…This temperature-dependent red-shift was observed in the literature before, using similar reactors and was recently evaluated in detail. [54][55][56] While this fact leads to the appearance of the signal already at 9.2 eV, the majority of the ferrocene fragments appear only above 11.1 eV, which is shown by fitting a line to the linear increase of the signal. When adding the literature bond dissociation energy of the cation (Fe(C 5 H 5 ) 2 + → Fe(C 5 H 5 ) + + C 5 H 5 ) to the reported ionization threshold of ferrocene at 6.9 eV, [57] the appearance energy of the major fragment, Fe(C 5 H 5 ) + from ferrocene can be calculated to be 11.51 ± 0.43 eV (BDE: 4.61 ± 0.43 eV), using the values of Optiz and Härtner, [50] or from Rowland (11.16 ± 0.15 eV), which agrees slightly better.…”