| Methane is the second most important anthropogenic greenhouse gas after carbon dioxide,and the global warming potential of methane is about 21 times higher than that of carbon dioxide.A lot of methane is directly emitted to the atmosphere every year in China and a large portion of these emissions come from the ventilation air methane(VAM)in coal mining.The concentration(0.1%?1%)of these tremendous amount of lean methane is under the lower flammable limit,so it is not possible to treat them or recover energy through conventional combustion.Catalytic combustion is a clean and efficient technology for the removal of the lean methane and has attracted wide attention because it can lower the total oxidation temperature of methane,and the emissions of polluting gases,such as nitrogen oxides and carbon monoxide during the treatment of methane can also be reduced.Palladium(Pd)-based catalysts are reported to be more active than other materials during the catalytic combustion of lean methane,and the catalytic activity can be further enhanced by the interaction between Pd and metal oxide support,but noble metal nanoparticles including Pd trend to sinter at higher temperature,resulting in the low stability of the catalysts.Novel structures such as dumbbell-like and core-shell structures can enhance the the activity and stability of catalysts in the combustion of lean methane,but the synthesis process of these novel structures is influenced by many factors.According to our summary,the seed size and size ratio could influence the epitaxial growth controllability,and the component sizes and size distribution could be influenced by the reaction temperature and reaction time.Moreover,the morphology of final products can be determined by many factors,including the solvent polarity,species and ratio of the precursors,the lattice mismatch between different components,the species of reducing or oxidizing agents,the surfactant concentration and the operating environment.Owing to the restrictions and harsh conditions in the synthesis process,these novel structures are limited to very few species of chemical elements,most of which are not suitable for the catalytic combustion of methane.Since the catalytic activity is mainly influenced by the interaction between noble metal and metal oxide,and the catalyst stability comes from the fixed position and segregation of the noble nanoparticles.So Pd-NiCo2O4/SiO2 catalysts in which Pd and NiCo2O4 nanoparticles uniformly distributed on the SiO2 support were firstly fabricated in this work,and every Pd or NiCo2O4 nanoparticle can be separated from each other.A series of characterizations including TEM,BET,XRD and XPS were used to analyze the catalysts,and the catalytic performances of these catalysts in the catalytic combustion of lean methane were evaluated.Results indicated that the specific surface areas of the synthesized catalysts were very high,there were enough adsorbed oxygen on the catalysts surface,Pd nanoparticles were uniformly distributed and most of the Pd existed in the form of PdO,which were all in favor of a high activity.Methane(1 vol%in air)can be totally oxidized by the Pd-NiCo2O4/SiO2 catalyst(Pd loading= 2.0 wt%)at 3780C when the space velocity was 30 000 ml·g··h-1,while the T90 of the catalyst without a uniform distribution was 56? higher.This result indicated that the interaction between different materials can enhance the catalytic activity.The catalytic activity of Pd-NiCo2O4/SiO2 was also higher than that of Pd-Co3O4/SiO2 with the same distribution and Pd loading,which was because the introduction of Ni3+ can facilitate the dissociation of CH4.The catalytic activity for methane oxidation did not decrease during the long time reaction and the TEM results showed no obvious change in the distribution of Pd and NiCo2O4,Pd was still separated by NiCo2O4 without agglomeration.So the uniform distribution can not only increase the stability of noble metal nanoparticles,but also can enhance the catalyst activity.In order to avoid the contact and agglomeration of the Pd nanoparticles on different support particles when the catalyst particles pile up,a novel catalyst of hierarchical porous flower-like NiCo2O4 supported Pd was then fabricated to further improve the activity and stability.Every flower-like NiCo2O4 was composed of porous nanoplates without overlapping,which is structurally similar to a rose in bloom,so Pd nanoparticles on different petal nanoplates will not come into contact with each other.Catalysts with different Pd loading were fabricated and tested,the catalytic activity was slightly enhanced with the increasing of Pd loading until the Pd loading came up to 3.0 wt%,however,further increasing of Pd loading did not give the higher activity.This result indicated that the catalytic activity is not determined only by the metal or the support,the interaction between different materials also plays an important role in the oxidation of methane.The catalyst of Pd/NiCo2O4 with 2.0 wt%Pd loading exhibited high activity at the space velocity of 30 000?75 000 ml g-1·h-1 with the T90 of 309?and total methane conversion temperature of 330?.Even at wet condition with the presence of 10 vol%water vapor,the activity was still acceptable with T90 of 366?.The catalytic activity for methane oxidation did not decrease during the long time reaction with the presence of 10 vol%water vapor and no obvious change in the catalyst structure was observed.The H2-TPR result showed that the addition of Pd can enhance the reducibility of NiCo2O4 and the reduction extent of NiCo2O4 was very high(up to 80%),this indicated that the porous NiCo2O4 material can give the chance of free flowing for the reactant gases,which can get into the inner active sites and the contact between catalyst materials and reaction gases can be increased and finally improve the catalytic activity.In addition to traditional characterization methods like TEM,SEM,XRD,BET and H2-TPR,quasi in situ XPS combined with in situ MS experiments were carried out to analyze the component changes on catalyst surface or in gas phase at different temperatures during the oxidation of methane.The results indicated that there are two different reaction pathways for the transformation of CHO to CO2:At lower temperature,the CHO tends to couple with oxygen atom to form the OCHO intermediate,and then dehydrogenated to CO2;at higher temperature,the CHO is transformed to CO2 by the pathway of CO oxidation,bypassing the OCHO intermediate. |
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