摘要
Ｍｎ２Ｏ３Ｎａ２ＷＯ４／ＳｉＯ２催化剂中，Ｍｎ２Ｏ３的引入起到了活化气相氧的作用．由于该催化剂中Ｗ，Ｍｎ间电子的迅速传递，在甲烷氧化偶联反应过程中，甲烷的选择氧化发生在Ｗ位，而气相氧转化为晶格氧发生在Ｍｎ位，从而建立了十分有效的氧化还原循环．这正是该催化剂较Ｎａ２ＷＯ４／ＳｉＯ２具有更高甲烷选择氧化活性的原因．
Mn 2O 3 Na 2WO 4/SiO 2 catalyst for oxidative coupling of methane has been studied by XANES, TPR, TPO and high temperature quenching ESR. XANES showed that the white line of 2 p→5d transition disappeared at the region of WL 3 edges due to the electron transfer from W ion to Mn ion. The reduction temperature of W ion in Mn 2O 3 Na 2WO 4/SiO 2 catalyst increased apparently in comparison with that in Na 2WO 4/SiO 2 catalyst. Meanwhile, the reoxidation of low valent Mn ion in the reduced sample became easier than that in Na + MnO x /SiO 2 catalyst. When molecular oxygen was added, the F center in Mn 2O 3 Na 2WO 4/SiO 2 catalyst, which formed under the condition of heating in vacuum, disappeared at the ice and salt bath temperature, but the disappearance of the F center in Na 2WO 4/SiO 2 catalyst was found at the temperature near 100℃. All these evidences indicate that owing to the rapid electron transfer between W and Mn ions in Mn 2O 3 Na 2WO 4/SiO 2 catalyst, the capability of gas phase oxygen adsorption on F center and the replenishing rate of lattice oxygen from gas phase oxygen are enhanced significantly by the addition of Mn 2O 3. Correlating the redox properties of Mn 2O 3 Na 2WO 4/SiO 2 catalyst with electron transfer between W and Mn ions, a possible reaction mechanism for selective oxidation of methane on this catalyst is proposed. The selective oxidation of methane may take place on the W sites, and the exchange of gas phase oxygen into lattice oxygen occur at the Mn sites. Thus, a more effective redox cycle is established in Mn 2O 3 Na 2WO 4/SiO 2 system that may explain why the activity for selective oxidation of methane on Mn 2O 3 Na 2WO 4/SiO 2 catalyst is much higher than that on Na 2WO 4/SiO 2 catalyst.
出处
《催化学报》
SCIE
CAS
CSCD
北大核心
1997年第5期378-383,共6页
基金
国家自然科学基金
关键词
甲烷
氧化偶联
钨酸钠
氧化锰
负载型催化剂
锰
Methane, Oxidative coupling, Sodium tungstate, Manganese oxide, Supported catalyst, Redox mechanism
