120
carbon dioxide conversion, but the selectivity to methanol is affected, to the benefit
of carbon monoxide, formed by reverse water-gas shift reaction.
The use of zirconium oxide as support for Cu–ZnO to catalyze the hydrogenation
of carbon dioxide towards methanol has been studied for many years. The main
effect of zirconium oxide was first reported to be a higher dispersion of copper, due
to a better specific surface area of zirconia compared to alumina (Matsumura and
Ishibe 2011; Jeong et al. 2012), which allowed a better activity per mass of copper
(Li et al. 2014a, b). Zhou and co-workers (1999) demonstrated that this improved
dispersion was due to a high Cu/ZrO 2 interface, favored by oxygen vacancies at the
surface of zirconia. Other authors described this interface as microcrystalline copper particles stabilized by amorphous zirconia (Koeppel et al. 1992). Ro and coworkers (2016) observed that the Cu–ZrO 2 interfacial sites led to a turnover
frequency for the formation of methanol of one order of magnitude higher compared to Cu sites. Initially, it was proposed that the active sites of Cu–ZnO–ZrO 2
catalysts could be Cu
+
–ZnO species stabilized by zirconia (Xu et al. 1991). More
recently, Arena and co-workers (2007) showed that zirconia played a catalytic role
and participated in the carbon dioxide adsorption step, whereas for the conventional
catalysts based on alumina, the oxide support acts only as dispersant of the Cu–ZnO
actors (Sun et al. 1998), as illustrated in Fig. 5.5. It also has been reported that the
zirconia could enhance the basicity of the catalyst, thus increasing the carbon dioxide activation (Gao et al. 2013a, b). A complete kinetic study of the reaction of
carbon dioxide hydrogenation was reported by Portha and co-workers (2017) on
Cu–ZnO–alumina and Cu–ZnO–ZrO 2 catalysts. They clearly establish the positive
impact of zirconia on the selectivity to methanol. Recently, Kim and co-workers
(2018) studied the quaternary system Cu–Zn–Zr–Al and proposed that the presence
of Al promotes catalytic activity to methanol by directing the coprecipitation of the
mixed precursors in solution towards the formation of zincian malachite which
induce high copper surface in the final catalyst.
A better adsorption of hydrogen was found with cerium oxide-doped zirconia
support, which was already known for its beneficial effect on the formation of methanol from mixtures of carbon monoxide and hydrogen (Pokrovski and Bell 2006;
Shen et al. 2005). Recently, the doping of alumina support by ceria for copper catalysts was reported for the synthesis of methanol from mixtures of carbon dioxide and
hydrogen (Li et al. 2019). Optimal ceria content leads to a strong surface basicity due
to a strong metal–support interaction effect induced by the formation of a copper–
cerium oxide interface, beneficial for the formation of methanol (Li et al. 2019).
More than the overall formulation of the catalytic systems, the way they are synthesized has a deep impact on their catalytic activity, as the nature of the interfaces
at the atomic level is strongly influenced by the preparation method. The preparation method most often used for methanol synthesis catalysts is coprecipitation.
This method generally involves metal nitrates which are precipitated under the form
of mixed carbonates or hydroxycarbonates, precursors of the final catalysts. Many
operational parameters such as temperature, pH, concentration, aging time, and
washing procedure have been reported to be crucial for the activity of the final catalytic materials.
D. P. Minh et al.
carbon dioxide conversion, but the selectivity to methanol is affected, to the benefit
of carbon monoxide, formed by reverse water-gas shift reaction.
The use of zirconium oxide as support for Cu–ZnO to catalyze the hydrogenation
of carbon dioxide towards methanol has been studied for many years. The main
effect of zirconium oxide was first reported to be a higher dispersion of copper, due
to a better specific surface area of zirconia compared to alumina (Matsumura and
Ishibe 2011; Jeong et al. 2012), which allowed a better activity per mass of copper
(Li et al. 2014a, b). Zhou and co-workers (1999) demonstrated that this improved
dispersion was due to a high Cu/ZrO 2 interface, favored by oxygen vacancies at the
surface of zirconia. Other authors described this interface as microcrystalline copper particles stabilized by amorphous zirconia (Koeppel et al. 1992). Ro and coworkers (2016) observed that the Cu–ZrO 2 interfacial sites led to a turnover
frequency for the formation of methanol of one order of magnitude higher compared to Cu sites. Initially, it was proposed that the active sites of Cu–ZnO–ZrO 2
catalysts could be Cu
+
–ZnO species stabilized by zirconia (Xu et al. 1991). More
recently, Arena and co-workers (2007) showed that zirconia played a catalytic role
and participated in the carbon dioxide adsorption step, whereas for the conventional
catalysts based on alumina, the oxide support acts only as dispersant of the Cu–ZnO
actors (Sun et al. 1998), as illustrated in Fig. 5.5. It also has been reported that the
zirconia could enhance the basicity of the catalyst, thus increasing the carbon dioxide activation (Gao et al. 2013a, b). A complete kinetic study of the reaction of
carbon dioxide hydrogenation was reported by Portha and co-workers (2017) on
Cu–ZnO–alumina and Cu–ZnO–ZrO 2 catalysts. They clearly establish the positive
impact of zirconia on the selectivity to methanol. Recently, Kim and co-workers
(2018) studied the quaternary system Cu–Zn–Zr–Al and proposed that the presence
of Al promotes catalytic activity to methanol by directing the coprecipitation of the
mixed precursors in solution towards the formation of zincian malachite which
induce high copper surface in the final catalyst.
A better adsorption of hydrogen was found with cerium oxide-doped zirconia
support, which was already known for its beneficial effect on the formation of methanol from mixtures of carbon monoxide and hydrogen (Pokrovski and Bell 2006;
Shen et al. 2005). Recently, the doping of alumina support by ceria for copper catalysts was reported for the synthesis of methanol from mixtures of carbon dioxide and
hydrogen (Li et al. 2019). Optimal ceria content leads to a strong surface basicity due
to a strong metal–support interaction effect induced by the formation of a copper–
cerium oxide interface, beneficial for the formation of methanol (Li et al. 2019).
More than the overall formulation of the catalytic systems, the way they are synthesized has a deep impact on their catalytic activity, as the nature of the interfaces
at the atomic level is strongly influenced by the preparation method. The preparation method most often used for methanol synthesis catalysts is coprecipitation.
This method generally involves metal nitrates which are precipitated under the form
of mixed carbonates or hydroxycarbonates, precursors of the final catalysts. Many
operational parameters such as temperature, pH, concentration, aging time, and
washing procedure have been reported to be crucial for the activity of the final catalytic materials.
D. P. Minh et al.