Hydrocarbon Oxidation at High Temperatures

Chemisorbed oxygen or surface oxides steer the selectivity in Pd electrocatalytic propene oxidation observed by operando Pd L-edge X-ray absorption spectroscopy

Operando Pd X-ray absorption spectroscopy elucidates how the surface oxygen species steer the selectivity in the electrochemical oxidation of propene.

Multi-fuel surrogate chemical kinetic mechanisms for real world applications

The most important driving force for development of detailed chemical kinetic reaction mechanisms in combustion is the desire by researchers to simulate practical systems. This paper reviews the parallel evolution of kinetic reaction mechanisms and applications of those models to practical, real engines. Early, quite simple, kinetic models for small fuel molecules were extremely valuable in analyzing long-standing, poorly understood applied ignition and flame quenching problems, and later kinetic models have been applied to much more complex flame propagation, problems including autoignition in spark-ignition engines and issues related to octane numbers and knock in modern, high compression ratio and other engines. The recent emergence of very large, multi-fuel surrogate kinetic mechanisms that can address many different fuel types and real engine applications is discussed as a modern analytical tool that can be used for a wide variety of practical applications.

Sonolysis of aqueous 4-nitrophenol at low and high pH

The sonolysis of 4-nitrophenol in argon-saturated aqueous solution has been studied at 321 kHz. In order to evaluate separately the effect of OH radicals that are formed in the cavitational bubble and part of which react in the aqueous phase with this substrate, radiolytic studies in N2O-saturated solutions were carried out for comparison. A detailed product study of the sonolysis of 4-nitrophenol solutions shows that at pH 10, where 4-nitrophenol is deprotonated (pKa = 7.1), its sonolytic degradation is fully accounted for by OH-radical-induced reactions in the aqueous phase. At this pH, the sonolytic yield of H2O2 resulting from OH radical recombination in the solution, measured as a function of the 4-nitrophenol concentration, is reduced in line with the scavenging capacity of the 4-nitrophenolate. In contrast, at pH 4 the formation of H2O2 is already fully suppressed when the solution is 7 x 10(-4) mol dm-3 in 4-nitrophenol, and oxidative-pyrolytic degradation predominates, as exemplified by the large yields of CO and CO2 which are accompanied by a large H2 yield. The basis of this difference in behavior is a hydrophobic enrichment of 4-nitrophenol (which is undissociated at pH 4) at the interface of the cavitational bubble by a factor of about 80. The pH dependence of the yields of the pyrolytic products reflects the hydrolytic equilibrium concentration of 4-nitrophenol. The paper also demonstrates that the complexity of this sonochemical system precludes its use a gauge to determine the temperature in the interior of the cavitational bubble.

Temperature dependence of laser-induced fluorescence of nitric oxide in laminar premixed atmospheric-pressure flames

The temperature dependence of laser-induced NO A (2)?(+)-X (2)? fluorescence in the hot gases of natural gas-air flames, seeded with known quantities of NO, has been determined experimentally by means of a difference method. The flame temperature at three fixed equivalence ratios was changed when the mixture velocity was varied through a water-cooled, flat-flame burner and was measured by coherent anti-Stokes Raman spectroscopy. When the possible reburning of part of the seeded NO is allowed for, the results in the range 1700-2150 K are best described by the temperature dependence obtained from a model in which quenching corrections are neglected, as in the case of a saturated two-level system, when millijoule pulse energies are used. Measurements of the fluorescence intensity at constant seed concentration as a function of equivalence ratio between 0.75 and 1.3 also indicate that quenching corrections are unnecessary under these excitation conditions. Using the measured intensities of the seeded flame as a calibration factor, we determined the absolute NO concentrations as functions of the equivalence ratio at 1 cm above the burner. The results indicate that, with the calibration method presented here, a relative accuracy of 5% should be obtainable.

Études des réactions d'oxydation du n-heptane et de l'isooctane

The oxidation mechanisms of two alkanes that have quite different octane numbers: n-heptane (0) and isooctane (100), were investigated to obtain a better understanding of the chemical mechanism of autoignition phenomena and to improve the compatibility of the available fuels with the engines. The experimental study was performed at 923 K in a setup equipped with a jet-stirred reactor. The oxidation mechanisms of n-heptane and isooctane were simplified first by a purely kinetic analysis based on the product formation and then by using a software of simulation of reaction mechanisms. The very different behaviour of these two hydrocarbons was explained by the presence, in the products of isooctane oxidation, of alkenes, which would have an antiknock effect due to the formation of resonance-stabilized radicals. Key words: oxidation reaction, n-heptane, isooctane, autoignition, modelling.