CuOx/CNTs催化剂上丙烯氧气环氧化反应

以尿素为沉淀剂，通过均相沉积 沉淀法合成了CuO/CNTs(CNTs即碳纳米管)复合材料，对CuO/CNTs在不同气氛和温度下进行预处理；并采用X射线衍射(XRD)和透射电子显微镜(TEM)对预处理后的CuO_x/CNTs催化剂的物相结构、微观形貌和粒径大小进行了表征，发现不同预处理方式可以控制Cu物种的价态分布；然后详细研究了Cu物种的价态对CuO/CNTs催化丙烯氧气环氧化反应性能的影响。结果表明：随着N₂预处理温度升高，Cu²⁺物种逐渐被CNTs还原为Cu⁺物种， Cu⁺物种进一步被还原为Cu⁰物种，但是由于CNTs的限域效应，Cu物种的形貌和粒径大小几乎没有发生变化；CuO_x/CNTs的丙烯环氧化反应活性随着N₂预处理温度的升高先增加后降低，当N₂预处理温度为350 ℃时，Cu⁺物种含量最高，此时环氧丙烷的形成速率最大；CuO_x/CNTs催化剂上丙烯环氧化反应活性的变化趋势与Cu⁺物种含量的变化趋势完全一致，Cu⁺物种最有利于环氧丙烷的生成，有可能是CuO_x/CNTs催化剂上丙烯氧气环氧化反应的活性中心。

Propylene Epoxidation by Oxygen Over CuOx/CNTs Catalysts

CuO/CNTs samples were synthesized by a homogeneous deposition precipitation method using urea as precipitation reagent. The crystalline phase, morphology and size of CuO_x/CNTs catalysts with different pretreatment atmospheres and temperatures were characterized by XRD and TEM, respectively. The valence state of Cu species is adjusted by different pretreatment methods, and the catalytic activities of CuO_x/CNTs catalysts with different valence states of Cu species in propylene epoxidation were also detailedly investigated. With elevating the N₂ pretreatment temperatures, Cu²⁺ species are gradually reduced to Cu⁺ species and then Cu⁺ species was further reduced to metallic Cu⁰ by CNTs. However, the morphology and size of Cu species remains almost unchanged due to the confinement effect of CNTs. Catalytic performance of CuO_x/CNTs catalysts in propylene epoxidation is increasing at first and then decreasing with enhancement of N₂ pretreatment temperatures. The propylene oxide formation rate and the content of Cu⁺ species both reach the highest value when CuO_x/CNTs is pretreated in N₂ at 350 ℃. The propylene epoxidation activity is in good agreement with the content of Cu⁺ species, hence Cu⁺ species are beneficial to produce propylene oxide and Cu⁺ species are the main active site for propylene epoxidation reaction over CuO_x/CNTs catalyst by oxygen.