Supported mesoporous Cu/CeO2-δ catalyst for CO2 reverse water–gas shift reaction to syngas

https://doi.org/10.1016/j.ijhydene.2020.02.058Get rights and content

Highlights

  • Cu0–CeO2-δ interface structure can be formed on the supported Cu/CeO2-δ catalyst.

  • Surface rich Cu0 species can improve the synergistic effect between oxygen vacancies and Cu0 species.

  • The synergistic effect between metal Cu and oxygen vacancies contributed to the high RWGS activity.

  • Cu0–CeO2-δ interface structure contributed to the high CO2 hydrogenation activity.

Abstract

The design and development of a high performance hydrogenation catalyst is an important challenge in the utilization of CO2 as resources. The catalytic performances of the supported catalyst can be effectively improved through the interaction between the active components and the support materials. The obtained results demonstrated that the oxygen vacancies and active Cu0 species as active sites can be formed in the Cu/CeO2-δ catalysts by the H2 reduction at 400 °C. The synergistic effect of the surface oxygen vacancies and active Cu0 species, and Cu0–CeO2-δ interface structure enhanced catalytic activity of the supported xCu/CeO2-δ catalysts. The electronic effect between Cu and Ce species boosted the adsorption and activation performances of the reactant CO2 and H2 molecules on the corresponding Cu/CeO2-δ catalyst. The Cu/CeO2-δ catalyst with the Cu loading of 8.0 wt% exhibited the highest CO2 conversion rate in the RWGS reaction, reaching 1.38 mmol·gcat−1 min−1 at 400 °C. Its excellent catalytic performance in the RWGS reaction was related to the complete synergistic interaction between the active species via Ce3+-□-Cu0 (□: oxygen vacancy). The Cu/CeO2-δ composite material is a superior catalyst for the RWGS reaction because of its high CO2 conversion and 100% CO selectivity.

Introduction

CO2 can trigger a series of severe environmental problems and seriously threats to the survival of human beings. However, CO2 is also the most abundant and cheapest C1 resource, and the carbon content of global CO2 is ten times over that of coal, oil, and natural gas. Thus, with the development of C1 chemistry, the carbon capture, storage and utilization (CCSU) of CO2 is of great significance, which driven by the promise of addressing both energy shortages and the environmental problems. At the same time, the CO2 molecule is inert molecules with high thermodynamic and chemical stability [1,2]. The main difficulty for CO2 utilization is the preliminary activation of CO2. At this regards, CO2 catalytic hydrogenation reduction appears as an interesting strategy. Because H2 is considered as a high-energy electron donor and abundant H2 can be produced by water electrolysis and photo-assisted decomposition of water [3].

Currently, CO2 hydrogenation reduction mainly includes reverse water–gas shift (RWGS), MeOH synthesis, methanation, etc. Among of them, CO2 RWGS reaction (CO2 + H2 → CO + H2O) is considered as one of the most effective and economical method for CO2 hydrogenation reduction. Because the RWGS reaction can convert the inert CO2 into the more reactive CO. The CO and H2 via the F-T reaction can be used to produce long-chain hydrocarbons and other valuable chemicals (such as alkanes, olefin, alcohols, aldehyde, acid, etc.) that a reconventionally derived from petroleum [1]. In addition, the carbon neutral cycle can achieve via the RWGS reaction, which coupled to the renewable energy source (i.e. wind or solar etc.) to realize the zero emission of CO2 [4].

Catalysts for RWGS reaction have inevitably attracted increasing attention. Among of them, supported precious metal catalysts are preferred materials because of their high catalytic activity and selectivity toward CO [[3], [4], [5]]. Supported precious metals (Pt [4], Pd [5], Au [6], and Rh [7]) over metal oxides such as CeO2 or TiO2 have been extensively used to study CO2 RWGS reaction, which showed a high catalytic activity. However, due to the high cost and poor stability at high temperature, the availability of these metals is limited, which hampered their large-scale applicability. Therefore, transition metals as catalysts for CO2 RWGS reaction have received increasing attention. Among of the studied transition metals, the metal Ni has been applied to investigate the CO2 RWGS reaction because of its high activity in most of the catalytic hydrogenation reduction reaction. However, Ni-based catalysts for CO2 RWGS reaction exhibited poor CO selectivity due to the formation of by-product CH4 [8,9]. Recently, special attention has been paid to Cu-based catalysts system as a substitute for precious metal catalysts in CO2 RWGS reaction due to its high activity for the CO2 WGS reaction and high selectivity for product CO. However, Cu-based catalysts tend to sinter at high temperature, which easily result in poor CO2 RWGS reaction activity [10]. Therefore, it is a challenge to prepare the Cu-based catalysts with high stability and activity.

Cu loading on reducible supports with large specific surface area can improve thermal stability and catalytic activity of the catalyst due to the strong interaction between metal Cu and support [11]. CeO2 is a typical rare earth metal oxide with a face-centered cubic fluorite structure. In recent years, CeO2 as catalyst support has attracted increasing attention due to its high oxygen storage capacity and the reversible reducibility. Additionally, the mesoporous CeO2 materials with high surface area and developed pore structure can also be easily obtained by hard template method [12]. The mesoporous support is conducive to well disperse the active metals on the surface of support, which provides numerous reactive active sites [11]. In the processes of calcination and reduction, abundant pore structure and large surface area of the support can promote active metal particles to be stabilized on the surface of support to inhibit the aggregation and sintering of metal particles, thus, the catalytic reaction activity can be enhanced [13]. At the same time, abundant of surface oxygen vacancies can be formed on the surface of CeO2 support under reduction conditions [14]. And the formed surface oxygen vacancies can effectively inhibit surface active metal particles to be aggregated, which due to the interaction between surface oxygen vacancies and active metal particles [[15], [16], [17]]. Compared to precious metals, the Cu/CeO2 system showed lower cost and excellent catalytic performances in a lot of reduction reactions [18,19]. The reports mentioned that the strong metal–support interactions (SMSI) between Cu species and CeO2 can improve the reducibility and stability of the corresponding catalysts, which is beneficial for the catalytic reduction reaction. In addition, the generation of surface oxygen vacancies in CeO2 support will provide the additional driving force for the reduction of CO2 to CO in reducing atmosphere [20]. However, there are no reports about the investigation of the Cu/CeO2 catalyst system applied to CO2 RWGS reaction.

In this study, the mesoporous CeO2 support was prepared by hard template method, and the mesoporous Cu/CeO2-δ catalysts with different Cu content were prepared by wet impregnation method, which used to study CO2 RWGS reaction. The effect of Cu content on Cu/CeO2-δ catalysts was characterized by H2-temperature programmed reduction (H2-TPR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), CO2-temperature programmed desorption (CO2-TPD), and in-situ X-ray photoelectron spectroscopy (in-situ XPS).

Section snippets

Catalyst preparation

In a typical synthesis of mesoporous CeO2 by the hard-template method, 1.0 g each of cerium nitrate (Ce(NO3)3·6H2O) was dissolved in 20 mL of ethanol. KIT-6 mesoporous silica was then added. The silica template was prepared according to our previous report [21]. The mixture was stirred at room temperature until a dry powder was obtained. The power was dried at 100 °C for 24 h. Then, the dry power was calcinated at 400 °C for 2 h and at the heating ramp rate of 10 °C/min. The impregnation

H2-TPR studies

Fig. 1 shows the H2-TPR profiles of the 1CuO/CeO2, 3CuO/CeO2, 5CuO/CeO2, 8CuO/CeO2, 10CuO/CeO2, 12CuO/CeO2, and 15CuO/CeO2 precursors. In the TPR profiles of the 1CuO/CeO2, 3CuO/CeO2, and 5CuO/CeO2 precursors, two reduction peaks (α and γ) are observed at about 115–300 °C, while three reduction peaks (α, β, and γ) can be detected in the TPR profiles of Cu-richer (>5%) samples at about 115–300 °C. For the TPR profile of the pure CuO, only a strong reduction peak is observed at about 315 °C.

The

Conclusion

The interactions between CuO and CeO2 can improve the xCuO/CeO2 precursors to be reduced at lower temperatures compared with pure CuO and CeO2. Active Cu0 species and oxygen vacancies are formed as evidenced by XRD and in-situ XPS results. The synergistic effect of the surface oxygen vacancies and active Cu0 species can be realized, and the active Cu0 species can be stabilized because of the existence of Ce3+-□-Cu0 and Cu0–CeO2-δ interface structures. The existence of Ce3+-□-Cu0 and Cu0–CeO2-δ

Acknowledgements

This research is funded by Graduate Innovation Fun of Chongqing Technology and Business University (No. yjscxx2019-101-57); Chongqing Research Program of Basic Research and Frontier Technology (cstc2019jcyj-msxm1390).

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