| The CH4molecules have high symmetry and stability, and the electron transfer of the CO2have needed the energy of up to20.4eV, which similarly have strong self-stability. The CH4-CO2reforming is one of the most challenging projects in catalysis and energy research areas. The high-temperature reactivity and selectivity of the CH4-CO2reforming limited by the thermodynamic are needed to be solved by developing efficient catalyst. Currently, studies on the catalysts used in CH4-CO2reforming mainly focused on the noble metals and transition metals used. However, the noble metals are expensive and these resources are limited while the transition metals are easily deactivated due to carbon deposition, which has become the bottleneck of its industrial application.Based on the existing problems, semi-cokes were creatively proposed to be used as the catalyst in the process of the CH4-CO2reforming. The effects of Hongce lignite semi-cokes, Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes on the CH4-CO2reforming were investigated by using a fixed-bed reactor. The factors affecting the catalyst stability and activity were investigated. The semi-cokes were activated by the high-temperature and high-pressure during the hydrothermal reaction of ammonia and hydrogen peroxide, and the rules regarding the effect of activation conditions on the CH4-CO2reforming were investigated. The essence of the semi-cokes catalysts were revealed by analysing the changes of the surface properties and structure before and after reaction. The reaction characteristic of the CH4-CO2reforming was investigated under the condition of pressure. The mechanism of the CH4-CO2reforming over semi-cokes was studied, and the kinetics model was established. The conclusions are drawn as follows:(1) The CH4-CO2reforming has been studied over different semi-cokes. The CH4-CO2reforming shows the same conversion trend with different semi-cokes, where the CH4and CO2conversions are higher at the initial phase, and as the reaction progresses, the conversions gradually decrease and tend to be stable. Therefore it is shown that the reaction mechanism is the same in the CH4-CO2reforming over different semi-coke. When the reaction became stable, the order of the catalyst activity was Hongce lignite semi-cokes>Shenmu bituminous coal semi-cokes> Jincheng anthracite semi-cokes, indicating that the catalyst activity of the semi-cokes was lower with the increasing of coal rank. Hongce lignite semi-cokes have high catalyst activity because it has large specific surface area, low ash and rich oxygenic groups. The analysis of carbon balance shows part of semi-cokes catalyst participated in gasification reaction with CO2, and the semi-cokes catalyst is a consumptive catalyst.(2) The changes of functional groups and the surface structure of the semi-cokes before and after the reforming reaction have been analysed. After the reaction, the absorption peak near1450cm-1almost vanished for all three semi-cokes, the absorption peak at1023cm-1was obviously diminished for the Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes, and the absorption peak at1598cm-1for the carboxyl C=O function group almost disappeared. These changes indicate that the functional groups on the surface of the semi-cokes participated in the reforming reaction. The result shows that the coal rank and the BET are the main factors which influence the catalyst performance, and the functional groups on the surface of the semi-cokes and the types and amounts of alkaline metals have a certain type of influence on the activity of the reforming reaction. After480min of reaction, the diffraction peaks of the semi-cokes mainly emerge in carbon and silica. The peaks for the alkaline earth metals have almost completely diminished(3) The semi-cokes activated by H2O2under high temperature and high pressure have been carried out. The catalytic performance of Jincheng anthracite semi-cokes whose initial catalytic effect is poor increased significantly, with the conversions of CH4and CO2increasing by24.38%and21.73%, respectively. The catalytic performance of Hongce lignite semi-cokes whose initial catalytic effect is good increased little, with the conversions of CH4and CO2increasing by just7.47%and1.28%, respectively. After activated, the Hongce lignite semi-cokes absorption peak at3444cm-1is caused by OH increase significantly, The Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes absorption peak near1598cm-1is caused by C=O and the absorption peak near1023cm-1is caused by the organic functional groups such as fat clusters and alkylene oxides, etc. strengthen significantly. However, Jincheng anthracite semi-cokes absorption peak near1598cm-1caused by C=O increased more than others. At the same time, the content of acid and alkaline functional groups on the surface of semi-cokes has increased, and the content of alkaline functional groups increased higher than the acid functional groups, meaning that the content of alkaline has increased. The net amount of basicity of Hongce lignite semi-cokes, Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes increased by0.161mmol/g,0.103mmol/g and0.102mmol/g, respectively.(4) The semi-cokes activated by ammonia under high temperature and high pressure have been carried out. After being activated, the Hongce lignite semi-cokes and the Shenmu bituminous coal semi-cokes absorption peak near3444cm-1are caused by-OH, the peak near1589cm-1is caused by OO, that near1450cm-1is caused by nitro and-NH2, that near1087cm-1is caused by-C-N, that near797cm-1is caused by C-H. These absorption peaks were increased significantly, Jincheng anthracite semi-cokes absorption peak near1589cm-1is caused by C=O, that near1450cm-1is caused by nitro and NH2, that near1087cm-1is caused by-C-N. These absorption peaks were also increased significantly. After ammonia hydrothermal reaction under high temperature and high pressure, the content of alkaline functional groups on the surface of semi-cokes have been increased greatly, but the content of acid functional groups have been decreased. The amounts of basicity of Hongce lignite semi-cokes, Shenmu bituminous coal semi-cokes and Jincheng anthracite semi-cokes were increased by0.361mmol/g,0.311mmol/g and0.240mmol/g, respectively. But the amount of acidity was decreased by0.012mmol/g,0.014mmol/g and0.009mmol/g, respectively. On the one hand, ammonia has a weak alkaline, which can carry out a neutralization reaction with acid functional groups to reduce the content of acid functional groups. On the other hand, it becomes easy to be combined with phenol and at last it enhanced the content of alkaline functional groups on the surface of semi-cokes.(5) A small high temperature and high pressure reactor has been improved and built, and the rule of CH4-CO2reforming over semi-cokes under pressure has been gained. The conversion rate of methane decreased with the reaction pressure increased, but the conversion rate of carbon dioxide increased at the initial phase, then decreased and trend to stable at last; the reaction temperature and the gas-solid contact time have the same influence on the CH4-CO2reforming under atmospheric pressure, but the ratio of CO/H2decreased as the ratio of CH4/CO2increased and CO/H2>1, which is different from atmospheric pressure variation. After the reaction under pressure, the absorption peak of semi-coke at1700cm"1is caused by C-O and O-H declines significantly under pressure of0.7MPa and IMPa. Instead the absorption peak has changed little under pressure of2MPa and3MPa, which shows the catalyst activity of the semi-cokes decreased with increasing pressure; meanwhile it exhibited a strong absorption peak near2350cm-1which is just the physical characteristics of CO2absorption peak, and further validates the steps of catalytic reforming reaction. To be specific, at first reaction gas is adsorbed by the surface of catalyst, then it reacted on the surface of catalyst. The SEM figure shows a lot of granular objects are covered on the surface of semi-cokes catalyst after the pressurized reaction, and with the increase of reforming pressure, carbon deposit would agglomerate on the surface of semi-cokes catalyst, which means dispersed points accumulate gradually into the cover, especially under the pressure of2MPa and3MPa. The pores distributed in the catalyst surface have been completely covered by carbon deposition, and the pore structure on the surface of semi-cokes catalyst decreased progressively until it disappeared with the increase of reforming pressure, when pressure increased. The bulk density of carbon deposition on the surface of semi-cokes catalyst was improved, resulting in forming a dense layer on the surface of semi-cokes catalyst, which has prevented the reaction gas from contacting with the active sites of catalyst and reduced the activity of catalysts or even deactivated the catalysts.(6) The reaction mechanism of the CH4-CO2reforming over semi-cokes has been proposed. It is considered that the oxygenic functional groups on the surface of semi-cokes first dissociate activated oxygen atoms, and at the same time carbon dioxide can also dissociate activated oxygen atoms and carbon monoxide under high temperature; then the activated oxygen reacted with methane to form activated CHXO, and was decomposed into carbon monoxide and methylene CH2. Meanwhile methane reacted with semi-cokes active site to form gradually activated carbon atoms and H2. A part of carbon reacted with carbon dioxide which was adsorbed on the surface of catalyst, while the others which were too late to react would deposit on the surface of catalyst to form carbon deposition. It was not good for partially activated oxygen atoms to be free on the semi-cokes catalyst surface. When the above interaction reached dynamic balance, Semi-coke catalyst would reach steady state. The kinetic equation of the CH4-CO2reforming over semi-cokes has been built, and dynamics parameters have been gained, with the reaction rate constant. k=25.81exp(58.4±3.7kJ/RT). |
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