物理化学学报 >> 2010, Vol. 26 >> Issue (07): 1879-1886.doi: 10.3866/PKU.WHXB20100732

催化和表面结构 上一篇    下一篇

ZrO2晶相对MoOx/ZrO2催化剂结构及其甲醇选择氧化性能的影响

张胜红, 张鸿鹏, 李为臻, 张伟, 黄华, 刘海超   

  1. 北京大学化学与分子工程学院, 分子动态与稳态国家重点实验室, 北京分子科学国家实验室, 北京 100871
  • 收稿日期:2010-04-09 修回日期:2010-05-17 发布日期:2010-07-02
  • 通讯作者: 刘海超 E-mail:hcliu@pku.edu.cn

Effects of Zirconia Crystallite Phases on the Structures of MoOx/ZrO2 Catalysts and Their Properties in the Selective Oxidation of Methanol

ZHANG Sheng-Hong, ZHANG Hong-Peng, LI Wei-Zhen, ZHANG Wei, HUANG Hua, LIU Hai-Chao   

  1. Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Stable and Unstable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
  • Received:2010-04-09 Revised:2010-05-17 Published:2010-07-02
  • Contact: LIU Hai-Chao E-mail:hcliu@pku.edu.cn

摘要:

以纯的单斜氧化锆(m-ZrO2)和四方氧化锆(t-ZrO2)为载体, 采用浸渍法分别合成了具有不同MoOx表面密度的MoOx/m-ZrO2和MoOx/t-ZrO2催化剂, 并结合粉末X射线衍射, Raman光谱和H2程序升温还原等技术表征了不同ZrO2晶相对MoOx分散状态、结构以及甲醇氧化反应性能的影响. 在低于锆表面MoOx的单层分散阈值(~5 nm-2) 时, m-ZrO2比t-ZrO2能够更有效地分散MoOx, 形成高分散的孤立或二维结构的MoOx物种, 避免了晶相MoO3的出现. 当Mo表面密度超过单层分散阈值后, 经过600 ℃焙烧, MoOx与ZrO2载体发生固相反应生成晶相ZrMo2O8, m-ZrO2比t-ZrO2更有利于ZrMo2O8的生成. 提高Mo表面密度, 催化剂表面的酸性随之增强, 说明晶相ZrMo2O8比分散的MoOx物种具有更强的酸性. t-ZrO2与MoOx作用形成的强酸中心更有利于催化甲醇脱水生成二甲醚, 但m-ZrO2使得MoOx具有更高的氧化还原能力和催化甲醇选择氧化反应的活性. ZrO2晶相对MoOx/ZrO2催化剂影响的研究结果将有助于研究VOx等其它金属氧化物催化剂以及发展酸性和氧化还原性双功能催化剂体系.

关键词: 选择氧化, 单斜氧化锆, 四方氧化锆, 晶相效应, 氧化钼, 甲醇

Abstract:

MoOx supported on pure monoclinic zirconia (m-ZrO2) and tetragonal zirconia (t-ZrO2) with different Mo surface densities were prepared and their structures were characterized by X-ray diffraction, Raman spectroscopy and temperature-programmed reduction in H2. At Mo surface densities lower than the theoretical monolayer coverage (~5 nm-2), isolated MoOx and two-dimensional polymolybdates were dominant on the m-ZrO2 surface while MoO3 crystallites were still present on the t-ZrO2 surface indicating a favorable dispersion of MoOx on m-ZrO2. As the Mo surface densities increased to more than monolayer coverage, MoOx tended to form ZrMo2O8 on the ZrO2 surface by a solid reaction at 600 ℃, which was more prevalent on m-ZrO2 than on t-ZrO2. The acidity of the MoOx/ZrO2 catalysts increased with an increase in the Mo surface density indicating that ZrMo2O8 is more acidic than dispersed MoOx. Catalysts prepared using t-ZrO2 resulted in more acidic MoOx/ZrO2 and consequently higher selectivity for dimethyl ether during the selective oxidation of methanol was achieved. The MoOx structures were more reducible on the m-ZrO2 surface leading to improved activity for the oxidation of methanol to formaldehyde, dimethoxymethane and methylformate. An understanding of the effect of ZrO2 crystallite phase on the structures of the MoOx species and their catalytic properties for the selective oxidation of methanol provides useful information for the study of other metal oxides such as VOx, and the design of catalysts that are bifunctional in terms of acidity and redox activity.

Key words: Selective oxidation, Monoclinic zirconia, Tetragonal zirconia, Crystallite phase effect, Molybdena, Methanol

MSC2000: 

  • O643