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D. Allam et al. / C. R. Chimie xxx (xxxx) xxx
© 2019 Académie des sciences. Published by Elsevier Masson SAS. This is an open access
1. Introduction
synthesis reveals to be an attractive technique for the
synthesis of nanooxide powders.
The concentration of carbon dioxide (a greenhouse gas)
inexorably increases in the atmosphere. Various methods
are currently examined to decrease the CO2 concentration
or to convert it into valuable chemical products. The use of
CO2 as a feedstock for the synthesis of high added value
chemicals is a promising alternative for CO2 abatement
[1,2]. The simplest way to use carbon dioxide is the hy-
drogenation into valuable compounds, such as methanol
(MeOH) and DME (dimethyl ether) [3e8]. Such conversion
is often performed on copper-based catalysts. Industrial
methanol synthesis is performed by catalytic hydrogena-
tion of syngas (H2/CO/CO2) over Cu/ZnO/Al2O3-type cata-
lyst. Unfortunately, the industrial Cu/ZnO/Al2O3 catalyst is
neither active nor selective in CO2 hydrogenation. Previous
published research studies demonstrated that the use of
industrial catalysts brings to very low hydrogenation con-
versions of CO2 to methanol [9].
This issue can be overcome by developing suitable cat-
alysts, which can effectively convert carbon dioxide to
methanol [10e13]. Even if copper-based materials are
promising catalysts for the hydrogenation of CO2 [14e19],
further investigations are required for developing new
catalytic materials able to give high conversion of CO2 and
improved selectivity to MeOH.
To improve the catalytic performance of methanol
synthesis from H2/CO2 feeding gas, CuO/ZnO catalysts have
been widely modified by adding various activators or other
metals (Zr, Si, La, Ti, Cr, Ga, Ce, Fe, Nb, Pd, etc.) [20e25]. The
effect of the support was also extensively studied. The type
of support affects both CO2 conversion and methanol
selectivity, and, in general, basic oxides such as La2O3,
Sm2O3, Nb2O5, In2O3, and ThO2 [26,27] used as supports
favor the methanol formation. The preparation methods
have also a considerable influence on the catalytic perfor-
mance [28e30]. Several methods such as coprecipitation
[31e35], impregnation [36e38], and solegel [38,39] have
been developed to prepare copper-based oxide catalysts.
Moreover, the coprecipitation synthesis was improved by
addition of reducing agents such as chitosane [14] and
NaBH4 [15]. Surfactant-assisted coprecipitation [16],
solvent-free routine combustion [40], and microfluidic
coprecipitation [41] are novel synthesis methods that allow
obtaining a good repeatability of the synthesis and an
improved homogeneity of the phases present in the cata-
lyst. Other methods, such as impregnation and solegel, can
also produce catalysts with large specific surface areas and
high CuO dispersion [42]. Polyol synthesis represents a
good alternative to the classical synthesis methods. In
particular, it presents many advantages, such as the possi-
bility to precisely modulate the stoichiometric ratio, the
homogeneous mixing of the various components, the low
cost, and the short reaction time. Therefore, polyol
CuO/ZnO/Al2O3-type catalysts obtained by polyol syn-
thesis have been successfully applied in the reverse water
gas shift reaction [43], in the alcohol-assisted low tem-
perature methanol synthesis from syngas [44] and in
methanol reforming [45].
The polyol method [46,47] permits to synthesize nano-
sized metallic powders with uniform size distribution and
shape [46e48]. In recent years, the polyol method has been
studied by many researchers [49e52]. These investigations
showed that the crystallite size and shape [46] can be
controlled by varying the reduction temperature, the pH, and
the nucleation-protective agent concentration [44,49e52].
Until now, and to the best of our knowledge, no attempts to
prepare Cu/ZnO catalyst for methanol production using the
polyol method have been done. In a typical polyol synthesis,
polyols (ethylene glycol, diethylene, glycerol, and tetra-
ethylene glycol) act at the main time as the reaction medium
and as the reducing agent. The metal precursor is reduced
through a redox reaction between the metal precursors and
polyolic species. Therefore, the reaction temperature is an
important parameter, because the oxidation potential of
polyol chemicals decreases with the increase in the reaction
temperature [49,53,54]. Nucleation-protective chemicals
such as polyvinylpyrrolidone are occasionally used to pre-
vent sintering and agglomeration of metal particles [53,54].
The various published articles indicate that the choice of the
preparation conditions strongly affects the activity of cata-
lysts prepared by the polyol method.
In the present work, the so-called “polyol method” has
been used to obtain improved catalytic materials for the
hydrogenation reaction of CO2 to methanol. The influence
of metal dispersion, spinel formation, and surface proper-
ties of binary and ternary catalysts (CuOeCeO2, eZnOe
CeO2, CuOeZnOeCeO2, and CuOeZnOeAl2O3), prepared by
the polyol method using polyethylene glycol as a solvent, is
evaluated in the CO2 hydrogenation to methanol at atmo-
spheric pressure, used as a test reaction.
2. Experimental section
2.1. Preparation of binary and ternary polyol catalysts
Two binary catalysts (labeled ZnOeCeO2 and CuOe
CeO2, with Zn/Ce and Cu/Ce molar ratios equal to 1) and
two ternary catalysts (labeled CuOeZnOeCeO2 and
CuOeZnOeAl2O3, with molar ratios of Cu/Zn/Ce and Cu/Zn/
Al equal to 1/1/2) were prepared by the polyol method. The
reaction temperature and the choice of the solvent were
selected referring to the available investigations reported
[44,45]. These experimental conditions seem to favor the
formation of nanocrystallites. Different from the synthesis
reported in these published studies, nitrate-base
Please cite this article as: D. Allam et al., Improved Cu- and Zn-based catalysts for CO2 hydrogenation to methanol, Comptes
Allam, Djaouida
