New benzene absorption cross sections in the VUV, relevance for Titan’s upper atmosphere

This is a pre-print (pre-peer review) manuscript. It is moderately different from the accepted manuscript and from the published article. Citation of published article: Fernando J. Capalbo, Yves Bénilan, Nicolas Fray, Martin Schwell, Norbert Champion, Et-touhami Es-sebbar, Tommi T. Koskinen, Ivan Lehocki, Roger V. Yelle. Icarus, vol. 265, p. 95 - 109. February 2016. doi: 10.1016/j.icarus.2015.10.006.International audienceBenzene is an important molecule in Titan’s atmosphere because it is a potential link between the gas phase and the organic solid phase. We measured photoabsorption in the ultraviolet by benzene gas at temperatures covering the range from room temperature to 215 K. We derived benzene absorption cross sections and analyzed them in terms of the transitions observed. No significant variation with measurement temperature was observed. We discuss the implications of our measurements for the derivation of benzene abundance profiles in Titan’s thermosphere, by the Cassini/Ultraviolet Imaging Spectrograph (UVIS). The use of absorption cross sections at low temperature is recommended to avoid small systematic uncertainties in the profiles. We used our measurements, together with absorption cross sections from other molecules, to analyze four stellar occultations by Titan, measured by UVIS during flybys T21, T41, T41_II, and T53. We derived and compared benzene abundance profiles in Titan’s thermosphere between approximately 530 and 1000 km, for different dates and geographical locations. The comparisons of our benzene profiles with each other, and with profiles from models of the upper atmosphere, point to a complex behavior that is not explained by current photochemical models

Étude électrique et analyse par fluorescence induite par laser des décharges de Townsend à la pression atmosphérique dans N2, N2/N2O et N2/O2

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Hetero-/Homogeneous Combustion of Fuel-Lean CH₄/O₂/N₂ Mixtures over PdO at Elevated Pressures

The heterogeneous and homogeneous combustion of fuel-lean CH₄/O₂/N₂ mixtures over PdO was investigated experimentally and numerically at equivalence ratios φ=0.27-0.44, pressures 1-12 bar and surface temperatures 710-1075 K. In situ Raman measurements of major gas-phase species concentrations across the boundary layer of a channel-flow catalytic reactor assessed the heterogeneous reactivity, while planar laser induced fluorescence (LIF) of the OH radical monitored homogeneous combustion. Simulations were performed using a 2-D code with detailed heterogeneous and homogeneous reaction mechanisms. Comparisons between Raman-measured and predicted transverse profiles of major species mole fractions attested the atmospheric-pressure suitability of a detailed surface mechanism and allowed for the construction of a global catalytic step valid in the range 1-12 bar. The methane catalytic reaction rate exhibited an overall pressure dependence ~p1-n where the exponent n was itself a monotonically increasing function of pressure, rising from 0.58 at 3 bar to 1.02 at 12 bar. This resulted in a non-monotonic pressure dependence of the catalytic reaction rate in the range 1-12 bar, a behavior in stark contrast to other noble metals (Pt and Rh) where the methane reaction rates always increased with rising pressure. Surface temperatures remained well-below the PdO decomposition temperature at each corresponding pressure, owning largely to the self-regulating temperature effect of PdO, and this in turn mitigated homogeneous ignition. Simulations using the PdO decomposition temperatures as boundary conditions for the wall temperatures were further performed for practical CH₄/air catalytic reactors in power generation systems. It was shown that for p \u3c 7 bar (a range relevant to microreactors) homogeneous ignition was altogether suppressed. For higher pressures relevant to gas-turbine burners, however, gaseous combustion ought to be considered in the reactor design

Homogeneous Ignition during Fuel-Rich H₂/O₂/N₂ Combustion in Platinum-Coated Channels at Elevated Pressures

The hetero-/homogeneous combustion of fuel-rich H2/O2/N2 mixtures (equivalence ratios φ = 2.5-6.5) was investigated experimentally and numerically in a platinum-coated channel at pressures p = 1-14 bar. One-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of OH at pressures below ∼5 bar and of hot-O2 at pressures above ∼5 bar (wherein OH-LIF was not applicable) determined the onset of homogeneous ignition. Simulations were carried out using a 2-D code with detailed hetero-/homogeneous chemical reaction schemes and transport. Both Raman measurements and numerical simulations attested a transport-limited catalytic conversion of the deficient O2 reactant over the gas-phase induction zones. The agreement between measured and predicted homogeneous ignition distances was better than 12%, thus establishing the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/p to the homogeneous ignition distances. This outcome, in conjunction with the intricate pressure dependence of the gaseous ignition chemistry of hydrogen, yielded shorter homogeneous ignition distances at 14 bar compared to 1 bar. The practical implication for gas turbine burners utilizing the catalytic-rich/gaseous-lean combustion concept was that the high operating pressures of such systems promoted the onset of homogeneous ignition within the catalytic module. Sensitivity analysis has finally identified the key catalytic and gaseous reactions affecting homogeneous ignition

Experimental and Numerical Investigation of Fuel-Lean H₂/CO/Air and H₂/CH₄/Air Catalytic Microreactors

The catalytic combustion of fuel-lean H2/CO/air and H2/CH4/air mixtures (equivalence ratios φ = 0.3-0.5) was investigated experimentally and numerically in a 30 x 30 x 4 mm3 microreactor made of SiC and equipped with six 1.5-mm internal diameter platinum tubes. The goal was to demonstrate high surface temperatures ( \u3e 1200 K) with good spatial uniformity, for power generation applications in conjunction with thermophotovoltaic devices. Surface temperatures were measured with an infrared camera while exhaust gas compositions were assessed with a micro gas chromatograph. Three-dimensional simulations with detailed hetero-/homogeneous chemistry, conjugate heat transfer in the solid, and external heat losses complemented the measurements. The diverse transport (Lewis number), kinetic (catalytic reactivity), and thermodynamic (volumetric heat release rate) properties of the H2, CO, and CH4 fuels gave rise to rich combustion phenomena. Optimization of the channel flow directions mitigated the high spatial non-uniformities of temperature, which were induced by the low Lewis number of H2. Measured surface temperature distributions had mean values as high as 1261 K, with standard deviations as low as 10.6 K. Syngas or biogas (H2/CO mixtures) yielded lower wall temperatures compared to undiluted H2, even for small volumetric CO:H2 ratios (1:9 and 2:8). Although CO had a high catalytic reactivity when combusting in H2/CO mixtures, its larger than unity Lewis number did not allow for the attainment of high surface temperatures. Mixtures of H2/CH4 (such as fuels produced by natural gas decarbonization) were the least attractive due to the substantially lower catalytic reactivity of CH4

Spatio-temporal distribution of absolute densities of argon metastable 1s 5 state in the diffuse area of an atmospheric pressure nanosecond pulsed argon microplasma jet propagating into ambient air

International audienc

Mimicking Titan's upper atmosphere reactivity with a RF-capacitively coupled N2-CH4 plasma

International audienceTitan is the largest satellite of Saturn. Its upper atmosphere is well-known for the production of a phochemical organic smog. We simulate this ionospheric chemistry with a RF plasma setup. We present here the study of the gas phase composition, neutral and positively charged species, correlated with aerosol production efficienc

Volatile products controlling Titan's tholins production

International audienceA quantitative agreement between nitrile relative abundances and Titan's atmospheric composition was recently shown with a reactor simulating the global chemistry occurring in Titan's atmosphere [Gautier et al. (2011) Icarus, 213: 625]. Here we present a complementary study on the same reactor using an in-situ diagnostic of the gas phase composition. Various initial N2-CH4 gas mixtures (methane varying from 1 to 10%) are studied, with a monitoring of the methane consumption and of the stable gas neutrals by in-situ mass spectrometry. Atomic hydrogen is also measured by optical emission spectroscopy. A positive correlation is found between atomic hydrogen abundance and the inhibition function for aerosol production. This confirms the suspected role of hydrogen as an inhibitor of heterogeneous organic growth processes, as found in [Sciamma-O'Brien et al. (2010) Icarus, 209, 704]. The study of the gas phase organic products is focussed on its evolution with the initial methane amount [CH4]0 and its comparison with the aerosol production efficiency. We identify a change in the stationary gas phase composition for intermediate methane amounts: below [CH4]0=5%, the gas phase composition is mainly dominated by Nitrogen-containing species, whereas hydrocarbons are massively produced for [CH4]0>5%. This predominance of N-containing species at lower initial methane amount, compared with the maximum gas-to solid conversion observed in Sciamma-O'Brien et al. 2010 for identical methane amounts confirms the central role played by N-containing gas-phase compounds to produce tholins. Moreover, two protonated imines (methanimine CH2=NH and ethanamine CH3CH2=NH) are detected in the ion composition in agreement with Titan's INMS measurements, and reinforcing the suspected role of these chemical species on aerosol production