| Biofuel ethanol is widely used as an alternative fuel and additive in vehicles,which can increase the octane number of gasoline and reducing the emission of atmospheric pollutants(mainly CO and NOx).However,the exhaust gases emitted directly into the atmosphere still contain small amounts of ethanol and acetaldehyde,which are harmful to humans and involved in the formation of photochemical smog.Catalytic combustion is a clean and efficient technology that removes the atmospheric pollutants by lowering the reaction energy barrier and reaction temperature.To better eliminate ethanol,scientists have been working on catalyst preparation with high performance.The catalysts currently used in catalytic oxidation research can be mainly divided into two categories:precious metals and transition metals.Palladium-based and manganese-based catalysts are the most representative,mainly using loading and doping methods to enhance catalyst activity.The catalytic reaction mechanism is studied based on experiments and DFT calculations under lean combustion conditions.Catalytic reactions are generally coupled from gas-phase and catalytic reactions,and the understanding of the catalytic mechanism can be facilitated by elucidating the gasphase reaction mechanism.While the combustion mechanism for the gas phase is currently well established,the catalytic reaction mechanism on the surface has been obtained through early techniques such as X-ray photoelectron spectroscopy,lowenergy electron diffraction and infrared spectrometry,and there has been a lack of experimental means to carry out in situ detection of key species under realistic reaction conditions.Previous mechanistic studies have made important progress in the catalytic oxidation of ethanol on noble metals,but the mechanism of action of oxygen in the catalytic oxidation reaction have not been clearly identified,and conventional IR techniques are more complex in distinguishing all the species with ester groups.To further understand the complex mechanism of catalytic oxidation reactions under real conditions,this work investigated the reaction mechanisms of pyrolysis,oxidation,and catalytic oxidation promoted by palladium-based and manganese-based catalysts under rich combustion conditions in combination with conventional chromatography and synchrotron radiation photoionization mass spectrometry.The main content of each chapter is as follows:Firstly,the necessity for pollutant treatment in the context of the fossil energy crisis and double carbon is presented.While ethanol,as an important renewable alternative fuel and gasoline additive,has been extensively studied for its removal by catalytic combustion.In addition to the preparation of high-performance catalysts,research progress on catalytic reaction models and reaction mechanisms is also highlighted,followed by an explanation of the advantages of synchrotron radiation photoionization mass spectrometry and its application in catalytic systems,and finally the research content and research lines of this paper are presented.Based on the above research background,the experimental platform and analysis methods of gas-phase kinetic simulations are then mainly described.In addition to the flowing quartz tube reactor combined with a conventional gas chromatography-mass spectrometry experimental platform,a flowing tube reactor coupled with synchrotron radiation photoionization mass spectrometry experimental platform is also presented.Based on the different ionization energies of the components and the tunability of the synchrotron photon energy,product identification can be achieved by using different photon energies to ionize the experimental components.At the same time,simulations using the CHEMKIN PRO software allow for prediction and analysis of the conversion mechanisms of fuels and products.This paper focuses on the reaction of ethanol oxidation catalyzed by Pd/γ-Al2O3 and Mn3O4 catalysts on the basis of well-established gas-phase reaction kinetics.In the gas-phase pyrolysis reaction,the decomposition of ethanol is dominated by hydrogen abstraction reaction which is attacked by H atoms.And the pyrolysis reactions are dominated by the formation of methane and carbon monoxide from acetaldehyde intermediates via β-dissociation reactions at high temperature.In contrast,the hydrogen abstraction reaction of OH radicals attacking ethanol is the main consumption pathway in the oxidation experiments,while OH radicals are produced via the reaction sequence O2→ HO2→ H2O2→OH.It was found that the reaction between oxygen and ethanol almost completely formed HO2 radicals,and the production of methanol and formaldehyde species was positively associated with the concentration of CH3O radicals strongly influenced by HO2 through ROP analysis.Thus,these two species appear earlier and have higher concentrations than those in the pyrolysis reaction.The mole fraction of other highly unsaturated products such as acetylene,propylene,allene,propargyl,2-butyne,1-butene and benzene decreases with increasing oxygen concentration,suggesting that the low-temperature aerobic conditions may inhibit their generation and ultimately weaken or prevent the generation of soot.The catalytic oxidation mechanism on Pd/γ-Al2O3 was elucidated mainly by gasphase pyrolysis,oxidation reactions and catalytic pyrolysis reactions carried out in a flow tube reactor.Combining the trends of product concentrations with the calculation of apparent activation energies,it is confirmed that the decomposition of ethanol on γAl2O3 is successively dominated by bimolecular and unimolecular dehydration with increasing temperature,forming diethyl ether and ethylene.In contrast.Pd/γ-Al2O3 allows the acetaldehyde intermediate produced by the dissociation of ethanol to undergo a β-dissociation reaction to produce the same C1 product as in the gas phase,and the addition of oxygen will result in a significant decrease in the reaction temperature.Compared to catalytic pyrolysis,the conversion curves of ethanol in catalytic combustion indicate that the deactivation of both catalysts is due to different reasons.From the catalyst characterization results,it is hypothesized that the deactivation of γ-Al2O3 catalyst results from the addition of oxygen,triggering the carbon deposition.In addition,the deactivation of Pd/γ-Al2O3 is assumed to be the accumulation of various oxygen-containing byproducts on the surface as a result of a series of oxidation reactions,resulting in the coverage of the active site as the Pd particle size increases.Product analysis from chromatography and photoionization mass spectrometry suggested that the presence of oxygen is conducive to the catalytic oxidation of the acetyl group to acetic acid and subsequently ethyl acetate by Pd atoms.At the same time,the presence of vinyl ketone and formaldehyde demonstrated that the catalytic reaction undergoes the sequential dehydrogenation of the acetyl group similar to that in the gas phase,as well as that the acetyl decarbonation reaction occurring in the catalytic system.In this paper,three catalysts with different morphologies were also synthesized,including one-dimensional Mn3O4 nanorod,two-dimensional Mn3O4 nanoplate,and three-dimensional Mn3O4 nano-octahedra.It reveals the reasons for the differences in the catalytic performance of three catalysts with various characterizations.Among them,Mn3O4 nanoplate catalyst shows the best activity due to its large specific surface area,defect-rich crystal structure,abundant Mn4+and excellent low-temperature reducibility.In terms of the mechanism study,a clear desorption peak from surface lattice oxygen was measured in a Raman experiment with variable temperature,confirming that the catalytic reaction follows the MVK mechanism.The by-product of acetone was detected by synchrotron photoionization mass spectrometry,which probably originated from the binding of acetyl groups and methyl groups on the surface.By comparing the major products,it was revealed that the palladium-based and manganese-based catalysts promote decarbonation and dehydrogenation reactions to occur,respectively. |
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