物理化学学报

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以类水滑石为前驱体的Cu/ZnO/Al2O3催化剂用于COx加氢合成甲醇:CO在反应混合物中的作用

刘影1,2, 刘晓放1, 夏林1, 黄超杰1,2, 吴兆萱1, 王慧1,2, 孙予罕1,2,3   

  1. 1 中国科学院上海高等研究院,中国科学院低碳转化科学与工程重点实验室,上海 201210;
    2 中国科学院大学,北京 100049;
    3 上海科技大学,物质科学与技术学院,上海 201203
  • 收稿日期:2020-02-17 修回日期:2020-03-13 录用日期:2020-03-27 发布日期:2020-03-31
  • 通讯作者: 吴兆萱, 孙予罕 E-mail:wanghh@sari.ac.cn;sunyh@sari.ac.cn
  • 基金资助:
    国家自然科学基金(21776296, 21905291), 国家重点研发计划(2017YFB0602203), 中国科学院战略性先导科技项目(XDA21090201), 中国科学院(ZDRW-ZS-2018-1-3)和上海市扬帆计划(19YF1453000)资助项目

Methanol Synthesis by COx Hydrogenation over Cu/ZnO/Al2O3 Catalyst via Hydrotalcite-Like Precursors: the Role of CO in the Reactant Mixture

Ying Liu 11,2, 21, Xiaofang Liu 11, Lin Xia 11,2, Chaojie Huang 11, 21,2, Zhaoxuan Wu 11,2,3   

  1. 1 CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, P. R. China;
    2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China;
    3 School of Physical Science and Technology, Shanghai Tech University, Shanghai 201203, P. R. China
  • Received:2020-02-17 Revised:2020-03-13 Accepted:2020-03-27 Published:2020-03-31
  • Contact: Hui Wang, Yuhan Sun E-mail:wanghh@sari.ac.cn;sunyh@sari.ac.cn
  • Supported by:
    This work was supported by the National Natural Science Foundation of China (21776296, 21905291), National Key Research and Development Program of China (2017YFB0602203), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA21090201), the Chinese Academy of Sciences (ZDRW-ZS-2018-1-3), and the Shanghai Sailing Program, China (19YF1453000).

摘要: 近年来,催化CO2加氢合成甲醇被视为有望解决温室效应和燃料枯竭的有效途径。目前,铜基催化剂因具有较高的反应活性被广泛应用于工业生产。然而,竞争逆水煤气变换反应产生的CO导致甲醇选择性较低,同时副产物水引起Cu发生不可逆烧结,进而降低甲醇产率。众所周知,CO能够调整分子的表面竞争吸附和活性位的氧化还原行为,本工作拟向原料气中掺入具有还原性的CO以抑制逆水煤气变换反应和防止表面氧化中毒。另一方面,通常认为铜基催化的CO2加氢制甲醇是结构敏感性反应,不同的前驱体能够显著影响催化剂结构和形貌,进而影响催化活性。因此,我们首先通过共沉淀法和蒸氨法制备了含有类水滑石前驱体(CHT-CZA)和复合物前驱体(CNP-CZA)结构的Cu/ZnO/Al2O3催化剂。随后,为探究CO掺杂后反应机理,在250 °C,5 MPa的反应条件下,含有不同比例CO的原料气中 (CO2 : CO : H2 : N2 = x : (24.5− x) : 72.5 : 3)评价两种催化剂对甲醇合成的性能。评价结果显示两种催化剂反应性能趋势相同,随着CO含量增加,CO2转化率和STYH2O不断降低,STYMeOH逐渐增加。X射线光谱(XPS)显示随CO含量增加,催化剂表面还原性Cu比例增加。评价和表征结果说明CO引入抑制了逆水煤气变换反应的发生,通过还原被H2O氧化的活性Cu表面,促使更多的活性Cu位点暴露参与甲醇合成。另一方面,透射电镜(TEM)显示掺杂的CO会过度还原而引起颗粒团聚,导致催化剂逐渐失活。相比之下,含有水滑石前驱体的催化剂在任何气氛下均表现出更加优越的反应性能和长周期稳定性。这可归因于类水滑石前驱体独特的片层结构通过结构限域作用有效避免了因CO过度还原而导致的金属颗粒团聚,从而减少活性位点损失。

关键词: CO2氢化, 甲醇合成, Cu/ZnO/Al2O3催化剂, CO掺杂, 水滑石前驱体

Abstract: Catalytic hydrogenation of CO2 to methanol has attracted considerable attention due to its potential in alleviating global warming and mitigating the dependence on fossil fuels. Cu-based catalysts are widely used in industry because of their high activity for methanol production. However, the reaction still suffers from low methanol selectivity because of the generation of CO as a by-product via the reverse water gas shift reaction (RWGS). The formation of another by-product H2O leads to inevitable Cu sintering, which decreases the methanol production rate. It is well known that CO can alter competitive molecular adsorption on the surface and the redox behavior of the active sites; hence, CO doping in feed gas might not only inhibit the RWGS but also minimize surface poisoning by the adsorbed oxygen. On the other hand, CO2 hydrogenation to methanol over Cu-based catalysts is a structure-sensitive reaction, and a change in the precursor can have a remarkable influence on the structure and morphology of the catalyst, and ultimately, the catalytic performance. In this work, Cu/ZnO/Al2O3 catalysts have been prepared via a hydrotalcite-like precursor (CHT-CZA) and a complex phase precursor (CNP-CZA) using co-precipitation and ammonia evaporation methods. Subsequently, the performance of the two types of catalysts with different CO contents (CO2: CO : H2 : N2 = x : (24.5 − x) : 72.5 : 3) is compared at 250 °C and 5 MPa in order to explore the role of CO. The evaluation results show that both catalysts follow a similar trend in the conversion of CO and CO2 as well as the space-time-yield (STY) of MeOH and H2O. The conversions of CO2 and STYH2O decrease gradually with an increase in the CO volume, but STYMeOH is positively correlated with the CO volume. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis reveals that the amount of reduced Cu species on the surface increases with increasing CO content. Judging from these results, the introduction of CO inhibits the RWGS and enhances the methanol yield for both catalysts by removing the surface oxygen as the reducing agent and thereby facilitating the exposure of the active reduced Cu species. On the other hand, transmission electron microscopy (TEM) observations indicate the doped CO may cause agglomeration of particles due to over-reduction, leading to gradual catalyst deactivation. Compared with the traditional CNP-CZA, the catalyst derived from hydrotalcite-like compounds exhibits better activity and long-term stability under all atmospheres, at different CO doping levels. This is because the hydrotalcite-like layer structure helps maintain the active metal state and confine the structure by limiting the agglomeration of Cu species.

Key words: CO2 hydrogenation, Methanol synthesis, Cu/ZnO/Al2O3catalyst, CO doping, Hydrotalcite-like precursors

MSC2000: 

  • O643