Morphology control of ceria nanocrystals for catalytic conversion of CO2 with methanol

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catalyst with a special morphology such as nano-rods,20,21 nanocubes,22,23 nano-tubes24 has drawn great attention. Ceria catalysts with different morphologies possess different crystal planes and surface properties,30 which can inuence the interaction between ceria surface and the adsorbed reactants. There are a number of investigations on the morphology dependence of CeO2 and the morphology effects on many catalytic reactions.19,25–29,32,36 For example, Zhou et al. found that CeO2 nanorods showed better catalytic performance in CO oxidation than irregular nanoparticles because of the exposed reactive crystal planes.19 Liu et al. reported on the morphology effects of nanoscale ceria on the activity of CuO/CeO2 catalysts for NO reduction and found that CuO/CeO2 nano-rods presented the best activity because of the strong synergetic interactions between CuO and CeO2 nano-rods.26 Dai et al. investigated the catalysis oxidation of 1,2-dichloroethane and ethyl acetate over ceria catalysts with morphologies of nanorods, nanocubes and nano-octahedrons with ceria nanorods having the best catalytic activity, followed by nanocubes and nano-octahedrons.36 Fan et al. found that spindle-like CeO2 showed a high catalytic activity on CO conversion.32 Though CeO2-based catalysts have been applied in DMC synthesis in previous work,17 the morphology and crystal plane effect of ceria materials is not clear. Therefore, this paper describes the synthesis of ceria catalysts with different surface morphologies via a hydrothermal method without the addition of templates. Catalytic tests were performed for ceria materials and a series of characterization techniques including BET, X-ray diffraction (XRD), temperature-programmed desorption of ammonia and carbon dioxide (NH3-TPD and CO2-TPD), eldemission scanning electron microscopy (FE-SEM), transmission
This paper describes the synthesis of ceria catalysts with octahedron, nanorod, nanocube and spindle-like morphologies via a template-free hydrothermal method. The surface morphologies, crystal plane and physical-chemical structures were investigated via field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and temperature-programmed desorption of ammonia and carbon dioxide (NH3TPD and CO2-TPD). The catalytic performance over these ceria catalysts with different exposed planes were tested for dimethyl carbonate (DMC) synthesis from CO2 and methanol. The results showed that the spindle-like CeO2 showed the highest DMC yields, followed by nano-rods, nano-cubes and nanooctahedrons. A synergism among the exposed (111) plane, defect sites, and acid-basic sites was proposed to be crucial to obtaining the high reactivity of DMC formation.
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Published on 22 April 2013. Downloaded by Shanghai Normal University on 21/06/2013 02:50:49.
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Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, P. R. China. E-mail: jlgong@. cn; xbma@ School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, P. R. China † Electronic supplementary 10.1039/c3nr00831b information (ESI) available. See DOI:
Hale Waihona Puke Received 17th February 2013 Accepted 14th April 2013 DOI: 10.1039/c3nr00831b /nanoscale
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Introduction
Dimethyl carbonate (DMC) is an important green chemical raw material. Due to the wide applications in the use of polymer synthesis, fuel additives and solvents, etc., the research and development of DMC is drawing increasing attention.1–3 Several routes have been reported for DMC production, among which the process of direct synthesis from CO2 and methanol is an attractive approach, as it follows the principles of green chemistry.2,3 This process has another advantage for the utilization of low cost and widespread CO2 considering the current demanding concerns on the greenhouse effect. Various catalysts have been investigated for the direct synthesis of DMC from CO2 and methanol, such as organometallic compounds,4 potassium carbonate,5 metal(IV) tetra-lakesides,6 ZrO2,7–10 CexZr1ÀxO2 solid solutions11–15 and Cu–Fe bimetal catalyst16 et al. Among these catalysts, CeO2 catalysts show excellent catalytic performance in the formation of DMC.12,17 Ceria, one of the most important rare earth materials, is widely applied in heterogeneous catalytic reactions, such as three-way catalysis and selective oxidation.2,18,26–29 CeO2 in a cubic uorite structure possesses three low-index planes: (110), (111) and (100); the (111) plane is the most stable, followed by (110) and (100) facets.19,26,33 Recently, the synthesis of CeO2