Elsevier

Catalysis Communications

Volume 84, 5 September 2016, Pages 56-60
Catalysis Communications

Short communication
Core–shell structured Cu@m-SiO2 and Cu/ZnO@m-SiO2 catalysts for methanol synthesis from CO2 hydrogenation

https://doi.org/10.1016/j.catcom.2016.06.010Get rights and content

Highlights

  • The Cu and Cu/ZnO nanoparticles coated by mesoporous silica shells are prepared.

  • The Cu nanoparticle size is only 5.0 nm with the confined effect of silica shell.

  • The proportion of strongly basic site enhances markedly with the addition of ZnO.

  • The core–shell endows Cu particles trapped inside with excellent anti-aggregation.

  • The Cu/ZnO@m-SiO2 exhibits the best catalytic performance with high stability.

Abstract

The core–shell catalysts with Cu and Cu/ZnO nanoparticles coated by mesoporous silica shells are prepared for CO2 hydrogenation to methanol. With the confined effect of silica shell, the size of Cu nanoparticles is only about 5.0 nm, which results in high activity for CO2 conversion. The CH3OH selectivity is enhanced significantly with the introduction of ZnO. The core–shell structured catalysts endow the Cu nanoparticles trapped inside with excellent anti-aggregation and no deactivation is observed with time-on-stream. Therefore, the core–shell Cu/ZnO@m-SiO2 catalyst exhibits the maximum CH3OH yield with high stability.

Introduction

As a cheap, nontoxic and abundant C1 feedstock, chemical utilization of CO2 is a challenge and important topic. Synthesis of methanol from CO2 hydrogenation is of vital importance for both greenhouse gas control and fossil fuel substitution, if the needed hydrogen can be generated from water electrolysis with the energy from renewable and sustainable sources (solar energy, wind, hydroelectric, or even nuclear) [1], [2].

Cu/ZnO/Al2O3 catalysts are commercially used for methanol synthesis from syngas. However, the low activity and stability of Cu/ZnO/Al2O3 catalysts, which are partly ascribed to Cu sintering accelerated by the presence of the water vapor byproduct, create major barriers toward direct application to CO2 hydrogenation [3], [4]. Recently, various attempts have been made to improve the performance of Cu/ZnO-based catalysts for CO2 hydrogenation to methanol [4], [5], [6], [7]. It has been suggested that the high activity for CO2 hydrogenation was generated by the presence of surface defects of metallic Cu surface which can reduce the activation energy of hydrogen dissociation [8]. Rich surface defects can be achieved by decreasing the size of Cu nanoparticles with a simultaneous high dispersion. In addition, many researchers claimed that the copper particle size played an important role in the catalytic performance of copper-based catalysts [7], [9], [10]. Therefore, great efforts have been made to decrease the Cu particle size for Cu-based catalysts. However, the Cu nanoparticles are easily aggregated during reduction and reaction process, leading to deactivation and low stability.

Metal nanoparticles within a core–shell structure have important applications in catalysis [11]. The outer shells can prevent the sintering of core metal nanoparticles, even under harsh reaction, due to the confinement effects in those materials [12], [13]. Recently, considerable attention has been given to synthesizing core–shell nanocomposites using silicon dioxide (SiO2) as the shell, because SiO2 is easy to form uniform spheres with tunable sizes and possesses high thermal stability and good compatibility with other materials [14], [15]. Moreover, compared with conventional SiO2, mesoporous SiO2 (m-SiO2) favors metal dispersion and diffusion due to the higher specific surface area and enriched porosity [16]. Although many researches on SiO2 coatings on various core materials have been reported, the study of the Cu@m-SiO2 core–shell nanocomposites is – to the best of our knowledge – still lacking.

Herein, the core–shell structured Cu@m-SiO2 and Cu/ZnO@m-SiO2 nanocatalysts were prepared and tested for CO2 hydrogenation to methanol. For comparison, the mesoporous-SiO2 supported catalyst was also prepared by incipient wetness impregnation. The effect of the silica coating on the properties of Cu-based catalysts for methanol synthesis from CO2 hydrogenation will be investigated in detail.

Section snippets

Sample preparation

The synthesis for CuO@mesoporous-SiO2 nanocomposites was as follows, 0.49 g of Cu(Ac)2 was dissolved in 1350 mL ethanol under magnetic stirring, and then 9.8 g of polyvinylpyrrolidone (PVP) was added as a dispersant of nanoparticles. Afterwards, the solution was solvothermal treatment at 423 K for 8 h [17]. The obtained brown suspension was homogeneously dispersed in a mixture of distilled water (1350 mL), ethanol (1350 mL) and aqueous ammonia solution (28 wt%, 54 mL), then 4.92 g cetyltrimethylammonium

Textural and structural properties of the prepared nanocomposites

The actual metal composition of CuO@m-SiO2, CuO/ZnO@m-SiO2 and CuO/m-SiO2 materials deriving from ICP measurement is summarized in Table 1. It was evident that the copper content of the three materials was similar with each other, around 12 wt%. In addition, Zn content was 5.45 wt% for CuO/ZnO@m-SiO2 and the Cu2 +:Zn2 + atomic ratio was in close agreement with the nominal compositions (7:3) taken for the catalyst preparation.

The XRD patterns of the calcined and reduced CuO@m-SiO2, CuO/ZnO@m-SiO2

Conclusions

In summary, Cu@m-SiO2 and Cu/ZnO@m-SiO2 core–shell nanocatalysts were successfully synthesized with promising performance and high stability for CO2 hydrogenation to methanol. Several Cu or Cu/ZnO nanoparticles with a uniform particle size in around 5.0 nm were well encased within a silica shell, while each nanoparticle was separated by a thinner silica wall. The Cu dispersion of the core–shell nanocatalyst was much higher than those of the mesoporous-SiO2 supported catalyst. In addition, the

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21503260), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA02040602), and Shanghai Municipal Science and Technology Commission, China (14DZ1207600, 15ZR1444500).

References (35)

  • S. Saeidi et al.

    J. CO2 Util.

    (2014)
  • C. Li et al.

    Appl. Catal. A Gen.

    (2014)
  • F. Arena et al.

    Catal. Today

    (2013)
  • P. Gao et al.

    J. CO2 Util.

    (2013)
  • G. Bonura et al.

    Appl. Catal. B Environ.

    (2014)
  • Y.C. Liu et al.

    Catal. Commun.

    (2007)
  • R.Y. Xie et al.

    Catal. Commun.

    (2011)
  • P. Gao et al.

    J. CO2 Util.

    (2015)
  • P. Gao et al.

    Catal. Commun.

    (2014)
  • Y.X. Liu et al.

    Catal. Commun.

    (2010)
  • C. Tisseraud et al.

    J. Catal.

    (2015)
  • R. Ladera et al.

    Appl. Catal. B Environ.

    (2013)
  • M. Sahibzada et al.

    J. Catal.

    (1998)
  • P. Gao et al.

    Appl. Catal. A Gen.

    (2013)
  • X. Guo et al.

    J. Mol. Catal. A Chem.

    (2011)
  • L.Z. Gao et al.

    J. Catal.

    (2000)
  • Y.P. Zhang et al.

    Energy Convers. Manag.

    (2006)
  • Cited by (77)

    View all citing articles on Scopus
    View full text