Kinetics of the a-C₃H₅+ O₂reaction, investigated by photoionization using synchrotron radiation

Abstract

The kinetics of the combustion-relevant reaction of the allyl radical, a-C3H5, with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring, using the CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence Spectroscopy) setup. The ability to measure threshold photoelectron spectra enables a background-free detection of reactive species as well as an isomer-specific analysis of reaction products. Allyl was generated by direct photodissociation of allyl iodide at 266 nm and 213 nm and indirectly by the reaction of propene with Cl atoms, which were generated by photolysis from oxalyl chloride at 266 nm. Experiments were conducted at room temperature at low pressures between 0.8 and 3 mbar using Ar as the buffer gas and with excess O2 to maintain nearly pseudo-first-order reaction conditions. Whereas allyl was detected by photoionisation using synchrotron radiation, the main reaction product allyl peroxy was not observed due to dissociative ionisation of this weakly bound species. From the concentration-time profiles of the allyl signal, second-order rate constants between 1.35 × 1011 cm3 mol-1 s-1 at 0.8 mbar and 1.75 × 1011 cm3 mol-1 s-1 at 3 mbar were determined. The rates obtained for the different allyl radical generation schemes agree well with each other, but are about a factor of 2 higher than the ones reported previously using He as a buffer gas. The discrepancy is partly attributed to the higher collision efficiency of Ar causing a varying fall-off behavior. When allyl is produced by the reaction of propene with Cl atom, an unexpected product is observed at m/z = 68, which was identified as 1,3-butadienal in the threshold photoelectron spectrum. It is formed in a secondary reaction of allyl with the OCCl radical, which is generated in the 266 nm photolysis of oxalyl chloride.

New analytical tools for advanced mechanistic studies in catalysis: photoionization and photoelectron photoion coincidence spectroscopy

“…73 Furthermore, FC factors can be routinely calculated in the double harmonic approximation, and can reliably aid the interpretation and assignment of even crowded spectra. 74 No comparable approaches exist for the interpretation of PI spectra, which explains why iPEPICO provides superior isomerselectivity for reactive intermediates in pyrolysis studies, 75,76 flame sampling, 77,78 kinetic experiments, 79 and, more recently, also in catalysis. 14,15,61,62 Figure 7a shows ms-TPE spectra of the three xylylene isomers together with FC simulations in excellent agreement.…”

“…In combination with calculated ionization energies and FC simulations, PEPICO data can provide definitive, isomer-selective assignment of reactive intermediates. In the past decade this approach has been tested for a large set of different reactive intermediates ranging from smaller reactive species, such as methyl, 75 amidogen, 82 the Crigee intermediate, 83 propargylic, 84,85 and allylic 79,84,86 species up to medium-sized benzylic, 70, 87 picolyl, 88 and even anilino 89 radicals. In the following, we will briefly review how the selectivity and universality of ms-TPE spectroscopy offered mechanistic insights into catalytic lignin valorization approaches at the VUV beamline of the Swiss Light Source.…”

Photoelectron spectroscopy of boron-containing reactive intermediates using synchrotron radiation: BH2, BH, and BF

“…24,26,27 In recent years PEPICO has also evolved as a reliable analytical tool to detect elusive intermediates in reactive environments such as model flames 28 , catalytic reactors 29 or in kinetic experiments. 30,31 In its ground electronic state (X 2 A 1 ) BH 2 is a bent, near prolate asymmetric top (C 2v ) with bond angle ~129⁰, while in the first excited state (A 2 Π u ) it is linear (D ∞h ) with slightly shorter B-H bonds. [32][33][34] With only seven electrons, BH 2 is an attractive system for high-level calculations and hence there are numerous reports on the potential energy surfaces, including rovibronic constants and predictions of the energy barrier to linearity.…”

Metamorphic meta isomer: carbon dioxide and ketenes are formed via retro-Diels–Alder reactions in the decomposition of meta-benzenediol

“…We have used molecular beam sampling of an in-vacuum flash pyrolysis microreactor coupled to photoelectron photoion coincidence (PEPICO) detection. This technique has been proven to be a sensitive and multiplexed tool to study chemical reactions and detect elusive species in the gas phase 13 upon photolysis, 27,28 fluorine abstraction, 29,30 as well as in combustion, 31-33 catalysis 8, 34-36 and pyrolysis processes. 24,37,38…”