原位拉曼光谱研究CO还原α-Fe_(2)O_(3)过程的晶面效应

Probing into the crystal plane effect on the reduction ofα-Fe_(2)O_(3) in CO by Operando Raman spectroscopy

氧化铁作为铁基费托合成主催化剂的前驱体,其还原活化过程对催化剂整体性能有着至关重要的影响。α-Fe_(2)O_(3)作为一种晶体材料,其暴露晶面对铁基催化剂的还原和活化过程有重要影响,但目前对此仍然缺乏必要的研究。本研究合成了六角片、立方体和菱面体三种不同形貌的α-Fe_(2)O_(3)单晶纳米颗粒,其对应主暴露面分别为(001)、(102)和(104)晶面,然后利用原位拉曼光谱(ORS)研究了CO还原过程中α-Fe_(2)O_(3)晶体结构的转变过程。结果发现,与(104)和(102)晶面相比,(001)晶面具有更好的还原活性。对于三种晶面上的CO吸附和CO_(2)脱附,SEM、TEM和XPS等表征以及DFT理论计算结果表明,CO_(2)脱附是还原过程的决速步骤;(001)晶面对氧原子的束缚能力较弱,导致了其表面CO_(2)更容易脱附,从而促进了整个还原过程。

For the Fe-based catalysts in Fischer-Tropsch synthesis,the reduction and activation process ofα-Fe_(2)O_(3)precursor has a significant effect on the catalytic performance.As a crystalline material,the reduction and activation ofα-Fe_(2)O_(3)is assuredly influenced by the exposed crystal plane;however,there is a lack of necessary research in this regard.In this work,α-Fe_(2)O_(3)nanocrystals of three different morphologies,viz.,pseudo-cubic,hexagonal-plate and rhombohedra,were synthesized,which mainly expose the crystal planes of(102),(001)and(104),respectively.The evolution ofα-Fe_(2)O_(3)crystal structure was then investigated in CO atmosphere by using the Operando Raman spectroscopy(ORS).The results show that theα-Fe_(2)O_(3)(001)plane has a better reductive activity in comparison to the(104)and(102)planes.The SEM,TEM,XPS and XRD characterization and DFT calculation results reveal that CO_(2)desorption is a decisive step for the reduction ofα-Fe_(2)O_(3);owing to the weak binding ability of(001)crystal plane to oxygen atoms,the desorption of CO_(2)on the(001)crystal plane is much easier,which can promote the reduction process.