Mechanistical investigations of CO2 hydrogenation to methanol over Cu/CeO2 catalysts

大气中二氧化碳（CO2）含量的上升一方面导致温室效应引起的全球变暖，另一方面导致海洋酸化。减少二氧化碳排放的一种可能性是将二氧化碳和氢气（H2）在催化剂上转化为可用的化学物质，如甲醇。甲醇可作为溶剂、替代燃料和生产石油化工产品的中间化学品。在工业上，甲醇是通过Cu/ZnO/Al2O3催化剂在50-100巴和175-325℃下合成的。这个过程需要很高的压力来获得足够的甲醇收率。然而，由于CO2是一种稳定的分子，它的活化与高温有关，因此一方面成本高，另一方面由于热力学平衡的改变而导致甲醇产率降低。因此，开发一种在较低温度下工作的催化剂是非常必要的。在较低温度下，负载铜的铈（Cu/CeO2）表现出与Cu/ZnO/Al2O3催化剂相当的活性。由于其Ce(III)/Ce（IV）氧化还原体系，铈具有产生缺陷的能力。这些缺陷反过来又有助于甲醇合成中CO2的活化。本研究的目的是阐明载铜氧化铈（Cu/CeO2）上CO2加氢制甲醇的机理。由于对二氧化碳在氧化铈上的加氢作用知之甚少，我们将主要研究纯氧化铈，然后再研究含铜氧化铈。特别关注的将是通过operando光谱建立结构-活性关系和甲醇合成过程中活性中间体的证明，而活性中间体将通过同位素的使用来澄清。如果已知甲醇合成的活性中间体，则可以设计具有这些活性物质吸附位点的催化剂，从而可以优化该过程。为了优化催化剂，(i)通过合成不同面[(100),(110),(111)]的二氧化铈对吸附位置的影响，（ii）研究不同铜负载壳的影响。然后将实验结果与DFT计算结果进行比较。此外，还分析了压力和温度对甲醇合成的影响。因此，将开发一种新的operando测量装置，该装置由拉曼光谱和漂移光谱以及质谱相结合组成，可以对高达30 bar的压力进行测量。借助这种新的operando设置，催化剂表面（漂移光谱）和亚表面（拉曼光谱）的变化将与各自的气相组成（质谱）相关联。

Rising atmospheric levels of carbon dioxide (CO2) led on the one hand to global warming caused by greenhouse effect and on the other hand to an acidification of the ocean. A possibility to reduce CO2 emissions is the conversion of CO2 and hydrogen (H2) over a catalyst to a usable chemical such as methanol. Methanol can be used as a solvent, alternative fuel and intermediate chemical for producing petrochemicals. In Industry, methanol is synthesized over a Cu/ZnO/Al2O3 catalyst at 50-100 bar and 175-325°C. This process requires a high pressure to gain a sufficient methanol yield. However, since CO2 is a stable molecule, its activation is associated with high temperatures and therefore on the one hand with high costs and on the other hand with a lower methanol yield caused by the shift in thermodynamic equilibrium. Therefore, development of a catalyst that works at lower temperatures is highly desirable. While working at lower temperatures, copper-loaded ceria (Cu/CeO2) shows comparable activities to Cu/ZnO/Al2O3 catalysts. Ceria has the ability to create defects due to its Ce(III)/Ce(IV) redox system. These defects in turn assist the activation of CO2 for methanol synthesis. The aim of this project is to elucidate the mechanism of CO2 hydrogenation to methanol over copper-loaded ceria (Cu/CeO2). Since little is known about CO2 hydrogenation over ceria, primarily pure ceria and then copper-loaded ceria will be investigated. A special attention will be focused on the establishment of structure-activity-relationships by operando spectroscopy and proof of active intermediates during methanol synthesis, while active intermediates shell be clarified by the use of isotopes. If the active intermediates of the methanol synthesis are known, a catalyst with adsorption sites for these active species can be designed and consequently the process can be optimized. For optimization of the catalyst, (i) the influence of adsorption sites by synthesis of ceria with different facets [(100), (110), (111)] and (ii) the influence of various copper-loadings shell be investigated. Experimental results will then be compared with DFT calculations. Furthermore, the influence of pressure and temperature in methanol synthesis will also be analysed. Therefore, a new operando measurement setup consisting of a combination of Raman and DRIFT spectroscopy coupled with mass spectrometry will be developed, which allows investigations for pressures up to 30 bar. With the aid of this new operando setup changes on the surface (DRIFT spectroscopy) and in the subsurface region (Raman spectroscopy) of the catalyst will be correlated with the respective gas phase composition (mass spectrometry).