Full Length ArticleScreening and design of active metals on dendritic mesoporous Ce0.3Zr0.7O2 for efficient CO2 hydrogenation to methanol
Graphical abstract
Introduction
The increase of carbon dioxide (CO2) concentration in the atmosphere has posed enormous challenges about climate change and global warming [1], [2], [3]. These challenges have prompted researchers worldwide to develop green energy sources and their utilization to convert CO2 to value-added chemicals and/or fuels. Due to the inertness of CO2, it takes a reactant with high free energy such as hydrogen (H2) to activate CO2. Therefore, CO2 hydrogenation is one of the most logical and promising approaches. Indeed, CO2 hydrogenation has been employed to generate various of chemicals, such as liquid fuels, olefins, and aromatics. Among them, methanol, in particular, has been extensively investigated for its possible roles as a fuel, a hydrogen carrier, and a raw material for the production of various chemicals [4], [5], [6].
There have been tremendous growth and progress in the development of supported metal catalysts for CO2 hydrogenation, such as Cu-based catalysts, Cu/ZnO/Al2O3, Cu/ZrO2, and Pd/ZnO [7], [8], [9]. Although Cu/ZnO/Al2O3 has a high efficiency for CO2 hydrogenation to produce methanol, it tends to favor the competing reverse water gas shift (RWGS) reaction, limiting the methanol productivity [10]. Lately, catalysts of Cu incorporated with ZrO2 showed an excellent activity toward CO2 hydrogenation to methanol due to Cu-ZrO2 interphase interactions and formation of active sites that promote the activity of methanol synthesis through improving the reaction pathway to favor formate formation over reverse water gas shift reaction (RWGS) [11], [12]. More recently, mixed metal oxides based catalysts (Ga, Ni, Zn, and Cu) have drawn attention owing to their reported synergy properties resulting in improved activity and stability of the intermediates leading to methanol formation [9], [13], [14]. However, they suffer from lower methanol yields, and thus there is a need to carry out further development to improve and innovate novel materials to meet industrial requirements at mild operation conditions with high methanol yields [15], [16].
We have previously reported that the novel dendritic CZ support could boost the accessibility to the active site and promote hydrogen spillover of the incorporated PdCu active metals due to the high dispersion degree of active metals on the dendritic open-pore channels, which exhibited high performance for CO2 hydrogenation to methanol [17]. However, the methanol synthesis performance of PdCu/CZ needs to be further enhanced. Therefore, the study of different multifunctional metals incorporated onto the dendritic CZ for CO2 hydrogenation is indispensable.
In this study, different multifunctional metals were successfully loaded onto the dendritic CZ solid solution via the incipient-wetness impregnation method. The PdZn/CZ catalyst with distributed ZnO exhibited the highest CO2 conversions (25.7 %), methanol yield (6.9 %), and superior 100 h long-term stability than the rest of the series of multifunctional catalysts. The best CO2 hydrogenation activity of PdZn/CZ catalyst can be ascribed to the access of ZnO active sites that alter the reaction route to disfavor the RWGS pathway, improved hydrogen dissociation capability, generated more oxygen vacancies, and further utilized the dendritic CZ characteristics that have proven to support methanol synthesis.
Section snippets
Synthesis of hard template
Preparation of dendritic mesoporous spherical nanoparticles (DMSNs) hard templates were described in our previous work [18], [19], [20].
Synthesis of Ce0.3Zr0.7O2 support.
Dendritic CZ solid solution were prepared via EISA method using DMSNs as the hard template with the assistance of acetone solvent. 12 mmol (5.21 g) of Ce(NO3)3·6H2O and ZrOCl2·8H2O with Ce/Zr molar ratios (3:7) was dissolved into 15 mL of acetone. After the solution has been rapidly stirred, 2 g of DMSNs was added and then followed by ultrasonic dispersion that
Characterization of series catalyst
XRD patterns of the series of different active metals supported CZ catalysts are shown in Fig. 1. The characteristic peaks of the series catalysts and the pure CZ support maintained a similar pattern, indicating that the supports retained their original structures after loading different active metals .The series show four prominent characteristic peaks, which can be assigned to (1 1 1), (2 0 0), (2 2 0), and (3 0 0) crystal faces of CeO2 with face-centered cubic (fcc) fluorite structure [23]. PdCu/CZ,
Discussion
Different active metal-supported dendritic CZ catalysts were successfully fabricated for CO2 hydrogenation to methanol. PdZn/CZ catalyst with higher hydrogen dissociation capability and better CO2 adsorption property showed excellent CO2 conversion of 25.7 % and high methanol yield of 6.9 % at 250 °C, 5.0 MPa and GHSV of 3600 mL gcat−1 h−1 . The structure–function relationship of PdZn/CZ catalyst was given as follows:
The CZ composite with wrinkled surfaces (Fig. 2) possesses a higher specific
Conclusion
Novel PdZn/CZ catalyst with more oxygen vacancies thru the incorporation of ZnO can improve the adsorption and the activation ability of CO2 during CO2 hydrogenation process. The homogeneous dispersion of PdZn active metals on CZ support can boost the hydrogen spillover and the facilitation of the active hydrogen. ZnO active sites play a role in suppressing the RWGS reaction to promote methanol selectivity. Dendritic PdZn/CZ catalyst exhibited not only the highest CO2 conversion (25.6 %) and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by King Abdullah University of Science and Technology (KAUST).
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