(226g) Insight into the Structure Evolution of Iron-Based Catalysts for Hydrogenation of CO/CO2 Using Operando Techniques

Insight into the structure evolution of iron-based catalysts for hydrogenation of CO/CO₂ using Operando techniques

Yulong Zhang^a, Yang Sun^a, Zhengpai Zhang^a, Pengfei Tian^a, Jing Xu^a ,Hui-Ping Li ^b and Yi-Fan Han^a,b*

^a State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

^b Research Center of Heterogeneous Catalysis and Engineering Sciences, School

of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001,

China

* Corresponding author phone:  (+86) 21-64251928; fax:  (86) 21-64251928;

E-mail: yifanhan@ecust.edu.cn

Abstract:

Operando techniques have opened up new vistas for visualization of the active phases/sites evolution during reactions, especially at high pressures and high temperatures. The rational design and controllable synthesis should still be the ultimate goal for chemists and chemical engineers. The greatest challenge is how to establish the relationship between structure and performance of industrial catalysts.  Recently, with the combination of operando Raman spectroscopy (ORS) and some other techniques, such as in situ XRD, Temperature-Programmed techniques and in situ DRIFTS, we studied the complex reaction systems like F-T synthesis and CO₂ hydrogenation under harsh conditions For the FTS reaction, during activation process in different atmospheres, the phase transition of α-Fe₂O₃--Fe₃O₄--Fe₅C₂ was observed in both CO and syngas (H₂/CO) atmosphere. Moreover, various iron oxide/carbides aiming at different products have been also investigated by operando Raman spectroscopy during the CO₂ hydrogenation reaction. The preliminary relationship of catalyst structure and performance of iron catalysts under harsh working conditions was established. This work directs us to explore the reaction mechanism and design the next generation industrial catalysts.

Keywords: FTS, CO₂ hydrogenation, iron catalysts, operando techniques, structure and performance

1.     Introduction

Catalysis is a fascinating science for people highly involved. What’s more, more than 85% of the fuels and chemicals are produced from catalytic processes [1]. Up to now, the catalyst R&D is mainly based on trial-and-error works. As Schlögl declared “In-situ investigations with complementary methods are not a ‘luxury’ but rather essential parts of the formulating a hypothesis on the function of materials” [2], it is necessary to use the operando techniques to enhance the catalyst development effective. In all existing in situ techniques, operando experiment is a straightforward way to determine the dynamic structure of catalysts and can offer reliable information for the rational design of catalysts by characterizing the catalytic under real reaction conditions.

For instance, high temperature usually leads to the sintering and agglomeration of individual catalyst particles and the reduction on the surface adsorption sites, then further deteriorates the catalytic performance [3, 4]. Hence, for the study on dynamic structures of industrial catalysts, the development of operando techniques operating under harsh conditions is of paramount importance.

The operando Raman spectroscopy is naturally born for this aim not only because it is insensitive to CO₂ and H₂O, but also for those species have hampered most of spectroscopy applied in operando measurements [5].

In this paper, after long-term continuous efforts, operando Raman spectroscopy has been applied to monitor the structural evolution of iron-based catalysts in the FTS and CO₂ hydrogenation reaction under high temperature (260^oC~350^oC) and high pressure (2.5~3.0 MPa) which is typical harsh conditions in the process of industrial reactions.

2.     Experiment Sections

Catalysts: High purity of α-Fe₂O₃& γ-Fe₂O₃(Aladdin, 99.5%) and Fe₃O₄ (Aladdin, 99.5%) were used as reference samples. Also, a high purity of α-Fe₂O₃& γ-Fe₂O₃ (Aladdin, 99.5%) were used as the catalyst precursors for the F-T synthesis and CO₂ hydrogenation reaction.

Iron carbides: Fe₅C₂ and Fe₃C were synthesized on-line by exposing the α-Fe₂O₃ (Aladdin, 99.5%, 30 nm) to a pure CO flow (99.995%, 25 mL/min) and heating up to 350^oC and 450^oC at the temperature rate of 2^oC/min and keeping for 2 h, respectively.

ORS activation and reaction: The laser Raman spectra were recorded with a home-build operando setup using a confocal Raman spectroscopy (Lab RAM HR, Horiba J.Y.) equipped with a high-grade Leica microscope (long working distance objective 50x).

XRD patterns were recorded using Riguku D/max 2550 diffractometer with accelerated voltage 40 kV and detector current 100 mA. Cu-Kα (λ=51.540589 Å) radiation was used for a continuous scanning with the step-size of 0.01°over a 2 theta range 10-80°with a scan speed of 4^o/min.

3.     Results and discussion

Using the operando Raman spectroscopy (ORS), we have successfully determined active phases for the iron-based FTS reaction under industrial conditions. The structure of α-Fe₂O₃ catalyst was observed to experience extensive restructuring processes under activation and reaction processes (Figure 1). α-Fe₂O₃ was transformed into Fe₃O₄ slowly in a H₂ flow and the main phase during reaction is the mixture of Fe₃O₄ and Fe. CO- and H₂/CO-activated catalysts showed a transformation of α-Fe₂O₃→Fe₃O₄→Fe₅C₂; Meanwhile Fe₃O₄ is the key transition state during reaction. The catalytic activity followed an order of CO-activated > H₂/CO-activated ≈H₂-activated(Figure 2). The relationships of structure and performance indicate: (1) iron carbides should be responsible for its lower FTS activity; (2) Fe₃O₄ plays an important role for FTS; (3) carbonaceous species involved positively or negatively affect the performance of iron-based FTS catalysts.

Figure 1. In situ Raman spectra of α-Fe₂O₃ during activation processes in (a) 10%H₂/Ar; (b) 10%CO/Ar; (c) 5%CO/5%H₂/Ar. The spectra at 350°C were recorded in the time interval of 30 min with the flow of 8 mL·min^-1, corresponding to a gas hourly space velocity (GHSV) of ≈30000 h^-1.

Figure 2. Operando Raman spectra of α-Fe₂O₃ (260 ^oC, 3.0 MPa) were recorded in the time interval of 30 min for 7 h with the flow of 3.2 mL·min^-1, corresponding to a gas hourly space velocity (GHSV) of 12000h^-1; (a, b) 10%H₂/Ar; (c, d) 10%CO/Ar; (e, f) 5%CO/5%H₂/Ar. (a, c, e) represent the structural evolution of the catalyst and (b, d, f) represent the corresponding catalytic performance.

Moreover, with the combination of operando Raman spectroscopy (ORS) and some other techniques, such as in situ XRD, Temperature-Programmed techniques and in situ DRIFTS, we also studied the complex reaction systems under harsh conditions---CO₂ hydrogenation. Various iron oxide/carbides aiming at different products have been investigated by operando Raman spectroscopy during the CO₂ hydrogenation reaction (Figure 3.).

Figure 3. Operando Raman spectra of (a) 10%H₂/Ar reduced γ-Fe₂O₃ (b) 10%H₂/Ar reduced α-Fe₂O₃ during the reaction processes (5%CO₂/15%H₂/Ar, 350^oC, 25 bar) were recorded in the time interval of 30 min for 10 h with the flow of 3 mL/min, corresponding to a GHSV of 6000 ml/(g·h).

Also, in situ XRD was applied for CO₂ hydrogenation reaction under 1 bar and 350^oC. Figure 4 shows the formation of iron carbides within 5 hours, and no iron oxide was observed during all the reaction process, which ensure the inference that iron carbides could be directly transformed from metallic iron. The preliminary relationship of catalyst structure and performance of iron catalysts under harsh working conditions was established (scheme 1).

Figure 4. In situ XRD patterns for metallic iron during the 5%CO₂/15%H₂/Ar (1bar, 350^oC) reaction

Scheme 1. Structure evolution of α-Fe₂O₃& γ-Fe₂O₃ during both activation and reaction processes.

4.     Conclusions

The relationship of structure and performance of iron-based catalysts under harsh reaction conditions (F-T synthesis and CO₂ hydrogenation reaction) have been built by using operando techniques. For the FTS reaction, during activation process in different atmospheres (10%H₂/Ar, 10%CO/Ar and 5%H₂/5%CO/Ar), the phase transition of α-Fe₂O₃--Fe₃O₄--Fe₅C₂ was observed in both CO and syngas atmospheres. Various iron oxide/carbides aiming at different products have been also investigated by operando Raman spectroscopy.

The application of operando techniques will greatly enhance the establishment of the relationship of structure and performance of a catalyst, which is of paramount importance to understand the origin of active sites and reaction pathways.

5.  References

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5. Fu,D.; Dai,W.; Xu,X.; Mao,W.; Su,J.; Zhang,Z.; Shi,B.; Smith,J.; Li,P.; Xu,J.; Han,Y.F.*  Chem Cat Chem, 2015, 7(5), 752.