Operando XAS Study of Pt-Doped CeO2 for the Nonoxidative Conversion of Methane

The methane to olefins, aromatics, and hydrogen (MTOAH) process via Pt/CeO$_2$ catalysts poses an attractive route to improve yield and stability for the direct catalytic conversion of methane. In this study, two sets of samples, one composed of PtO$_x$ single sites on ceria and the other with additional Pt agglomerates, were prepared. Both sets of samples showed enhanced catalytic activity for the direct conversion of methane exceeding the performance of pure ceria. Pulsed reaction studies unraveled three reaction stages: reduction of the ceria support during activation, an induction phase with increasing product formation, and finally, stable running of the catalytic reactions. The reduction of ceria was confirmed by X-ray absorption spectroscopy (XAS) after conducting the MTOAH reaction. Operando X-ray absorption spectroscopy at challenging reaction temperatures of up to 975 °C in combination with theoretical simulations further evidenced an increased Pt–Ce interaction upon reaction with CH$_4$. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra proved decoration and encapsulation of the Pt particles by the CeO$_2$/Ce$_2$O$_3$ support or a partial Ce–Pt alloy formation due to the strong metal–support interaction that developed under reaction conditions. Moreover, methyl radicals were detected as reaction intermediates indicating a reaction pathway through the gas-phase coupling of methyl radicals. The results indicate that apart from single-atom Pt sites reported in the literature, the observed Pt–Ce interface may have eased the activation of CH$_4$ by forming methyl radicals and suppressed coke formation, significantly improving the catalytic performance of the ceria-based catalysts in general

Exploring the interaction kinetics of butene isomers and NOx_{x} at low temperatures and diluted conditions

The oxidation of 1-butene and i-butene with and without addition of 1000 ppm NO was experimentally and numerically studied primarily at fuel-rich (ϕ = 2.0) conditions under high dilution (96% Ar) in a flow reactor operated at atmospheric pressure in the low temperature range of approximately 600-1200 K. Numerous intermediate species were detected and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). An elementary-step reaction mechanism consisting of 3996 reactions among 682 species, based on literature and this work, was established to describe the reactions and interaction kinetics of the butene isomers with oxygen and nitrogenous components. Model predictions were compared with the experimental results to gain insight into the low- and high-temperature fuel consumption without and with NO addition and thus the respective interaction chemistry. This investigation firstly contributes a consistent set of temperature-dependent concentration profiles for these two butene isomers under conditions relevant for engine exhaust gases. Secondly, the observed oxidation kinetics is significantly altered with the addition of NO. Specifically, NO promotes fuel consumption and introduces for i-butene a low-temperature behavior featuring a negative temperature coefficient (NTC) region. The present model shows reasonable agreement with the experimental results for major products and intermediate species, and it is capable to explain the promoting effect of NO that is initiated by its contribution to the radical pool. Further, it can describe the observed NTC region for the i-butene/NO mixture as a result of the competition of chain propagation and chain terminating reactions that were identified by reaction flow and sensitivity analyses

Investigation on the pyrolysis and oxidation of toluene over a wide range conditions. I. Flow reactor pyrolysis and jet stirred reactor oxidation

International audienc

Experimental and kinetic modeling study of styrene combustion

International audienc

Investigation on the pyrolysis and oxidation of toluene over a wide range conditions. II. A comprehensive kinetic modeling study

International audienc

Operando XAS Study of Pt-Doped CeO2CeO_2 for the Nonoxidative Conversion of Methane

The methane to olefins, aromatics, and hydrogen (MTOAH) process via Pt/CeO$_2$ catalysts poses an attractive route to improve yield and stability for the direct catalytic conversion of methane. In this study, two sets of samples, one composed of PtO$_x$ single sites on ceria and the other with additional Pt agglomerates, were prepared. Both sets of samples showed enhanced catalytic activity for the direct conversion of methane exceeding the performance of pure ceria. Pulsed reaction studies unraveled three reaction stages: reduction of the ceria support during activation, an induction phase with increasing product formation, and finally, stable running of the catalytic reactions. The reduction of ceria was confirmed by X-ray absorption spectroscopy (XAS) after conducting the MTOAH reaction. Operando X-ray absorption spectroscopy at challenging reaction temperatures of up to 975 °C in combination with theoretical simulations further evidenced an increased Pt–Ce interaction upon reaction with CH$_4$. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra proved decoration and encapsulation of the Pt particles by the CeO$_2$/Ce$_2$O$_3$ support or a partial Ce–Pt alloy formation due to the strong metal–support interaction that developed under reaction conditions. Moreover, methyl radicals were detected as reaction intermediates indicating a reaction pathway through the gas-phase coupling of methyl radicals. The results indicate that apart from single-atom Pt sites reported in the literature, the observed Pt–Ce interface may have eased the activation of CH$_4$ by forming methyl radicals and suppressed coke formation, significantly improving the catalytic performance of the ceria-based catalysts in general

Plasma-assisted low-temperature oxidation of n-butane: A synchrotron photoionization mass spectrometry and kinetic modeling study

The chemical kinetics of plasma-assisted low-temperature oxidation of n-butane (n-C4H10, 340 K, 30 Torr) is investigated by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and kinetic modeling. Species measurements are conducted in a flow reactor activated by a nanosecond discharge using SVUV-PIMS. Detailed species identification and quantification are reported in the present work. Fuel-specific oxygenated species including 2-butenal (2-C3H5CHO), methyl vinyl ketone (CH3COC2H3), butanone (CH3COC2H5), butanal (n-C3H7CHO), 2-methyloxetane (-CH(CH3)CH2CH2O-), 1,2-epoxybutane (-CH(C2H5)CH2O-), tetrahydrofuran (-CH2CH2CH2CH2O-), 1-/2-butanol (1-/2-C4H9OH), and butyl hydroperoxide (C4H9O2H) are detected. A kinetic model for n-C4H10 plasma is developed, which well predicts the mole fractions of most species observed in experiments. Kinetic modeling reveals that electron-impact reactions of n-C4H10 are the main sources for various C1-C4 alkenes, alkynes, and radicals. Therefore, these reactions have a significant effect on the fuel consumption pathways and species pool formation. Reactions involving alkyl peroxy (RO2), especially the self- and cross-reactions of RO2, are essential for RO2 depletion and the formation of alcohols, ketones, and aldehydes. On the other hand, the misprediction of the species detected in the system, such as C4 cyclic ethers, ethyl formate (HCOOC2H5), and acrolein (C2H3CHO), indicates that their formation pathways in the current mechanism are incompletely described. Further experimental and numerical research on these subjects is desired for the development of the kinetic models

Thermal decomposition of furans with oxygenated substituents : a combined experimental and quantum chemical study

A

Combined Experimental and Theoretical Study on Photoionization Cross Sections of Benzonitrile and o/m/p-Cyanotoluene

Toluene is one of the most abundant aromatic compounds in the atmosphere of Titan, and CN group has been regarded as an indicator for observing aromatic compounds in interstellar medium due to the large dipole moments resulted from the CN substituents. In the present work, the photoionization cross-sections (PICS) of benzonitrile and o/m/p-cyanotoluene, which are the products of CN + toluene were obtained at the photon energy ranging from ionization threshold to 14 eV combining experimental and theoretical methods. The synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was employed, and theoretical calculations based on the frozen-core Hartree-Fock approximation and Franck-Condon simulations were carried out to cross-verify the measurements. In addition, the generalized charge decomposition analysis was used to investigate the characteristics of ionized molecular orbitals. Our results are valuable for quantifying these key species (especially by the SVUV-PIMS technique) and estimating related parameters such as dissociation rates in interstellar space