添加碱土金属化合物对CeO2甲烷氧化偶联催化性能的影响

天然气中甲烷的利用,特别是氧化偶联(OCM)制乙烯过程近年来成为多相催化研究的热点之一.自从1982年Keller和Bhasin提出该课题以来许多学者已进行了大量催化剂的研制工作.尽管在这方面已取得重要进展,但催化剂的性能距工业化要求还有较大差距.

Effects of Alkaline Earth Oxides on the Catalytic Performance of CeO_2 for Oxidative Coupling of Methane (OCM) Reaction

CeO2 is a nonstoichiometric oxide and contains several kinds of oxygen species in the bulk and on the surface. It was discovered that CeO2can generate CH3 radicals efficiently by abstracting hydrogen atoms from CH4 molecule, but the C2-selectivity for the OCM reaction is poor due to its high reactivity with respect to secondary reaction of CH3 radicals. In this paper CeO2 and alkaline earth metal oxide modified CeO2 catalysts were investigated. The experimental results of pure CH4 pulses showed that the lattice oxygen on the catalyst surface participates in both the selective and deep oxidation of CH4. The XRD results revealed that Ca/ CeO2 consisted of CeO2 and CaO phases and the d spacings of each phase arc almost the same as that of pure phase; while Sr/Ca/CeO2 system is composed of CaO, CeO2 and a spinel Sr2CeO4 phases, and the d spacings of each phase have relatively large shifts. So we concluded that CaO, SrO or Sr2CeO4 in the composite catalysts may enhance the p-type semiconductor characteristics of the system and increase the oxygen vacancies, as a result, strengthen the adsorbing ability for gaseous oxygen to form oxygen intermediates. In mixture pulse experiments we found that on composite catalysts the produced CO2 always has some delay than the other effluents at the initial stage, which suggests the existence of an equilibrium between gaseous CO2 and the carbonates on the surface due to the adsorption of CO2 on the basic centers. It is believed that the equilibrium may be accompanied by the transformation of oxygen species from deep oxidation type to selective oxidation one . As CO2-TPD spectra showed, no basic center is present on CeO2, while on composite catalysts strong basic centers exist. The basic CaO and SrO probably have the following effects on CeO2: (1) dispersing or covering the deep oxidation sites for CH4on CeO2; (2) modifying the structure and surface oxygen species, therefore resulting in more structural defects and oxygen vacancies and sequently strengthening the ability of adsorbing gaseous oxygen to form selective oxygen species.