Study Of Methane Catalytic Combustion On Rare Earth Oxides

Compared with traditional flame combustion, the methane catalytic combustion releases more heat and less pollutants, which is interesting to generator, internal combustion engine, and domestic heating. The noble metal catalysts are the most active catalysts for the methane combustion, but it is far from applicable because of its high price and poor thermal stability. Thus, many researchers have devoted to develop the cheap and highly active oxide catalysts. China owns abundant rare earth resources, and utilizing the unique catalytic property of rare earth elements to develop the rare earth catalysts for methane combustion would contribute to both the research of methane catalytic combustion and the efficient utilization of rare earths.Based on the utilization of rare earth oxides in methane catalytic combustion, in this paper the methane combustion on rare earth oxides (especially on light rare earth oxides) was systematically investigated, and the factors of affecting the catalytic activity of rare earth oxides for methane catalytic combustion were understood. The mechanism of methane catalytic combustion over rare earth oxides was understood by spectroscopy techniques and kinetics investigations. Based on these understandings, whole series of nanostructured rare earth oxides were prepared by a novel synthesis method, and these oxides have good activity for the methane catalytic combustion. The main research results are achieved as follows.1. The influence of the lattice constant on the catalytic activity of Pr6O11 was investigated, and the CO oxidation was used as the model reaction. Series of Pr6O11 nanorods with different lattice constant were synthesized by a hydrothermal method. With an increase of lattice constant, the catalytic activity of the Pr6O11 nanorods decreases, the reducibility of Pr6O11 nanorods decreases, and the strength of mid-strong base sites enhances. As the lattice constant increases, the morphology of Pr6O11 nanorods is getting more and more regular, and the surface defects are reduced, resulting in the decreasing of the reducibility of the Pr6O11 nanorods. Synchronously, as the lattice constant increases, the distance between the Pr and O atoms becomes larger, and the electron distribution of O becomes spatially diffuse, which facilitates the electrophilic attacking like CO2 absorption and finally leads to the increase of mid-high base strength.2. The difference of the products in methane oxidation under the same condition on Er2O3 and Mn3O4 was investigated, and it has been found that the M-O bond strength can influence the distribution of products. The M-O bond strength of Er2O3 is higher than that of Mn3O4, which induces the methane to be partially oxidized and hydrogen is produced on Er2O3, while water is produced on Mn3O4. Catalytic oxidation mechanism studies reveal that, methane first dissociates on the catalyst surface and reacts with the lattice oxygen to form hydroxyl species on Er2O3 or Mn3O4. Then the hydroxyl species can further transform to hydrogen or water depending on the bond energies of M-O and H-O. If the energy of M-O is higher than that of H-O, hydrogen is formed; otherwise, water is generated. Thus, the bond strength could alternate the mechanism of catalytic reaction on rare earth oxides.3. Series of oxides with different M-O bond strength are selected as the catalysts for methane combustion, and the isotopic oxygen tracer experiments are utilized to investigate the role of M-O bond strength in methane combustion. The results indicate that methane combustion reaction follows Mars-van Krevelen mechanism on those oxides. Further kinetics investigation suggests that the rate-determining step in the methane combustion is the extraction of lattice oxygen. A linear relationship between the reducibility of surface lattice oxygen and the catalytic activity is established. On the other hand, the reducibility of bulk lattice oxygen can also affect the catalytic activity by influence the ability of surface lattice oxygen refreshing. Thus, it is very significant to choose an oxide with active lattice oxygen for methane combustion.4. The influence of pretreatment condition on the catalytic activities of the palladium catalysts supported on Pr6O11 and Al2O3 was investigated, and the CO oxidation was used as the model reaction. The results indicate that, the pre-treating gas (H2, O2, and Ar) could greatly influence the catalytic activity of supported palladium catalysts. The abundant surface species on the catalysts could form after being pretreated, and the surface species can prevent the Pd particle from sintering. On the other hand, the reactive oxygen amount of supports can influence the stability of catalysts, and the larger the reactive lattice oxygen the higher the catalyst stability is. Because the Pr6O11 support has more active lattice oxygen, the Pr6O11 is a preferred support for palladium catalysts compared with the Al2O3 support.5. Based on the above understandings, a novel method for the synthesis of morphology and size controllable nanostructured material has been developed. In this method, lamellar liquid crystal is used as a template to synthesis oxide nanosheets. The morphology and thickness of the as-obtained oxide nanosheets are controllable. Moreover, the as-obtained material is rigid and thermal-stable. The nanosheet oxide shows unique luminescent property and outstanding catalytic activity compared with traditional rare earth oxides. Furthermore, this novel method could be utilized to prepare other metal compound nanosheets.