Engineering pore morphology using silica template route over mesoporous cobalt oxide and its implications in atmospheric pressure carbon dioxide hydrogenation to olefins

https://doi.org/10.1016/j.apmt.2020.100586Get rights and content

Highlights

  • 3D m-Co-KIT-6 and 2D m-Co-SBA-15 mesoporous Co3O4 catalysts through hard template

  • Superior CO2 hydrogenation activity under atmospheric pressure condition for 3D mesoporous Co3O4.

  • Enhanced activity attributed to more active site, pore structure and lower mass diffusion limitations.

  • 3D mesoporous Co3O4 show selectivity for olefins compared to 2D mesoporous structure.

Abstract

Highly ordered mesoporous cobalt oxides (denoted as m-Co-KIT-6 and m-Co-SBA-15) with three dimensional and two dimensional pore morphology respectively have been synthesized using 3D KIT-6, and 2D SBA-15 as silica template via nanocasting route. CO2 hydrogenation activity was evaluated for these mesoporous materials under atmospheric pressure conditions. In comparison to nanoparticles of cobalt oxide (Co3O4-nano), mesoporous catalysts showed excellent activity for CO2 hydrogenation due to their higher number of exposed active sites and lower mass diffusion limitations. The ordered mesoporous structure of Co3O4 catalysts favored the chain growth of carbon atoms for the production of C2+ hydrocarbons while Co3O4 nanoparticles showed strong selectivity toward CH4. High selectivity for C2+ (∼ 25%) was obtained for both m-Co-KIT-6 and m-Co-SBA-15 catalysts at 320 °C. In addition, the 3D pore structure of m-Co-KIT-6 catalyst exclusively formed more olefins (54.9%) fraction.

Introduction

Conversion of CO2 to valuable products provides an acceptable solution for reduction of anthropogenic CO2 emissions. CO2 has been used as C1 feedstock for synthesis of various chemicals and fuel alternatives [1,2]. The hydrogenation of CO2 to hydrocarbons and oxygenates provides an alternative energy storage solution. Unfortunately, the hydrogenation of CO2 to hydrocarbons is a challenging task because of thermodynamic stability, chemical inertness and fully oxidized nature of the CO2 molecule which often required high pressure conditions for obtaining desirable products [3]. Among the products, the synthesis of paraffins and olefins from CO2 is valuable compared to methane, although methane production is thermodynamically more favorable [4,5]. The lower olefins, such as ethene (C2=) and propene (C3=), are important basic feedstock for industries and are considered to be value added products [6]. It has been widely accepted that the transformation of CO2 to hydrocarbons proceeds in two steps. First, CO is formed from hydrogenation of CO2 by reverse water gas shift reaction (RWGS) route. Then, CO undergoes further hydrogenation to form hydrocarbons by different types of intermediate species formation [7]. Cobalt based catalysts have been widely explored for synthesis of hydrocarbons from CO by Fischer-Tropsch process [8]. So far less attention has been focused on cobalt catalyst for CO2 hydrogenation under atmospheric pressure conditions, due to more selective methane formation tendency [9]. The relatively low heat of CO2 adsorption on catalyst surface favors the fast hydrogenation of surface-adsorbed intermediates, leading to the shift in CO2 hydrogenation selectivity towards methane and decreased C2+ hydrocarbon yield [10,11]. In order to obtain valuable hydrocarbons, it is highly desirable to suppress the fast hydrogenation activity of cobalt. The catalytic performance of cobalt based hydrogenation catalyst is largely affected by the Co surface species. The higher concentration of surface Co3+ species is responsible for high conversion and hydrocarbon selectivity [12]. In addition, partially reduced cobalt species retards activity for secondary reaction of olefins to form paraffin hydrocarbons [13,14]. Researchers have investigated promoted cobalt catalyst systems for the conversion of CO2 to hydrocarbons [[15], [16], [17]]. Owen et al. showed that the addition of noble metals and alkali promoters in cobalt based catalysts enhanced the hydrocarbon selectivity for CO2 hydrogenation under atmospheric pressure conditions [18]. However it is a difficult task to produce C2+ hydrocarbons by unpromoted cobalt catalyst under similar conditions. Catalytic materials containing same composition but different morphologies and structures show significant diversity in catalytic activity, selectivity due to varying extent of exposable active sites [[19], [20], [21], [22], [23]]. Co3O4 materials with different structures and morphologies have been widely reported as catalysts as well as support materials for noble metals for heterogeneous catalytic processes like CO oxidation, NOx reduction, glycerol reduction, water oxidation and oxidation of volatile organic compounds [24 and references therein]. Recently Jimenez et al. demonstrated that the reduction under pure hydrogen of different faceted Co3O4 particles generates active surface species enabling to tune the product selectivity during CO2 hydrogenation reaction. Co3O4 nanorods with exposed {110} facets exhibit higher methane yield by inhibiting the formation of site blocking formate species as compared to conventional Co3O4 nanoparticles with exposed {111} and {001} facets [25].

Hydrogenation of CO2 over mesoporous solid oxides is of great interest, not only due to their textural properties but also due to their lower deactivation and product distribution property [[26], [27], [28], [29]]. The ordered and open porosity can also enhance the mass transfer efficiency of the products through the mesoporous structure with less formation of deposited inactive coke [30,31]. Mesoporous metal oxides, especially cobalt oxide, have been well studied in the field of heterogeneous catalysis. Li et al. reported Pt promoted mesoporous Co3O4 catalyst for CO2 hydrogenation to higher alcohols under high pressure conditions [27]. It was observed that the ordered mesoporous structure promotes the carbon chain growth for production of higher alcohols and retards the rate of methane formation. The hard silica templating or nanocasting method is a versatile route for synthesis of mesoporous metal oxides. By changing the silica template pore structure, it is indeed possible to synthesize mesoporous metal oxides with different textural properties [32].

In this study, ordered mesoporous Co3O4 with tunable textural parameters are synthesized by using two dimensional longitudinal mesoporous channels containing SBA-15 and three dimensional interconnected mesoporous KIT-6 silica as hard templates. Obtained 2D and 3D replicas are evaluated in atmospheric pressure CO2 hydrogenation. In terms of ease of safe handling of hydrogen and for a hydrogen economy, development of a stable CO2 hydrogenation catalyst under atmospheric conditions is a boon. SBA-15 silica hard template generates two dimensional mesoporous Co3O4 (m-Co-SBA-15), while KIT-6 silica forms interconnected three dimensional mesoporous Co3O4 (m-Co-KIT-6). Thus by changing the pore structure of hard silica template, ordered mesoporous Co3O4 materials with different pore morphologies have been developed. For comparison, non porous Co3O4 nanoparticles were also prepared by a precipitation method. The mesoporous Co3O4 nanostructures as well as Co3O4 nanoparticles have been extensively characterized by N2-physisorption, XRD, TEM, H2-TPR and XPS techniques. Further, the importance of morphology of the catalyst for CO2 hydrogenation under atmospheric pressure conditions has been reported.

Section snippets

Catalyst preparation

Cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O), sodium hydroxide (NaOH), concentrated hydrochloric acid (HCl) and sodium carbonate (Na2CO3) were purchased from Thomas Baker Chemicals. The PEO-PPO-PEO Pluronic triblock copolymer (P123) and tetraethyl orthosilicate (TEOS) were purchased from Sigma-Aldrich Co. All chemicals used were of analytical grade and used in synthesis without further purification.

Textural characterization of catalysts

Fig. 1 shows the N2 adsorption-desorption isotherms and BJH (Barrett-Joyner-Halenda) pore size distribution plots of ordered mesoporous Co3O4 replicas and Co3O4 nanoparticles. Type IV adsorption-desorption isotherms were observed for both mesoporous samples, which suggest the successful replication of silica templates. The hysteresis loop of the Co3O4 mesoporous materials is observed at relative pressure (P/P0) range of 0.4–0.9 and reflects the capillary condensation phenomenon in the

Conclusion

Mesoporous m-Co-SBA-15 and m-Co-KIT-6 catalysts were prepared by the silica nanocasting method and nano-Co3O4 catalyst was prepared by the precipitation method. The order of CO2 hydrogenation activity is m-Co-KIT-6 > m-Co-SBA-15 > nano-Co3O4. The distribution of CH4, CO and C2+ hydrocarbon products over these mesostructured Co3O4 catalysts were found to be different from the Co3O4-nano catalyst. The latter showed only methane production with negligible amount of C2+ hydrocarbons fraction

Conflicts of interest

There are no conflicts to declare.

CRediT authorship contribution statement

Sharad Gupta: Data curation, Investigation, Validation, Writing - original draft. Carmen Ciotonea: Data curation, Writing - review & editing. Sébastien Royer: Funding acquisition, Writing - review & editing. Jean-Philippe Dacquin: Conceptualization, Supervision, Validation, Funding acquisition, Writing - review & editing. C.P. Vinod: Conceptualization, Supervision, Validation, Funding acquisition, Writing - review & editing.

Acknowledgements

S. Gupta acknowledges CSIR New Delhi for SRF fellowship and CNRS for the student exchange programme (LIA MATSUCAT) and the funding associated with it. CPV acknowledges generous support from CSIR-NCL through in-house grant (MLP030626). Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche et de l’Innovation, Région Hauts-de-France and FEDER are acknowledged for supporting and funding partially this work. We also thank Olivier Gardoll (Université de Lille, UCCS

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