571
Cu-Zn alloy on Zn/Cu (111) surface
• Existence of Cu
+
monovalent cations in ZnO matrix
• Stabilized Cu ions in morphologically active ZnO
• Formation of Cu-ZnO interface through oxygen (Cu-O-Zn)
1.4.6 Importance of ZnO
The significance of ZnO in methanol synthesis catalyst could be realized by the fact
that Cu metal alone is not able to provide active centers for the reaction to occur. In
fact, a study conducted by Herman et al. signifies the co-existence of ZnO in the
catalyst as the activity of the binary catalyst (Cu/ZnO) increased by three orders of
magnitude as compared to that of pure Cu metal. From the early findings of the
methanol synthesis catalysts to the present stage of the catalytic platform, the role
of ZnO to facilitate the easy reaction propagation has remained contrasting. A few
important roles of ZnO, as identified from literature, are as follows:
• ZnO is reported to enhance Cu dispersion, thereby elucidating a synergistic
effect with Cu species [76].
• Promotes the stability of active Cu
+
sites in the catalyst by allowing the incorporation of Cu
+
into the crystal lattice of ZnO [74].
• The main function of ZnO was identified to facilitate heterogeneous H 2 splitting
(H
δ+
/H
δ−
) and spillover which involves the migration of H
+
/OH
−
species over the
catalyst surface.
• Incorporation of ZnO in the catalyst matrix induces the shape changes in Cu
particles impregnated on the support depending upon the redox potential of surrounding gas phase as identified by atom-resolved TEM [77]. These changes in
shape of the particle results in irregularities on the surface which enhances the
catalytic activity.
Table 4 demonstrates the exemplary behavior of ZnO when exposed to different
feed streams. On the other hand, some studies demonstrated the negative or neutral
effect of ZnO addition into the Cu-based catalytic system. Chinchen et al. emphasized that there does not exist any significant difference in catalytic activity between
the Cu-supported catalyst on SiO 2 and MgO as compared to that supported on ZnO,
thereby totally denying the well-known Cu-ZnO synergy. Another controversial discussion related to Cu-ZnO interaction exists in the explanation of the Schottky junction effect existing between these two. The effect is known to increase the oxygen
vacancies in ZnO thrice when used alone, but no reaction intermediates were
observed when Cu is present along with ZnO. This suggests that the theory fails to
explain the increased activity of Cu-ZnO catalyst [78].
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…
Cu-Zn alloy on Zn/Cu (111) surface
• Existence of Cu
+
monovalent cations in ZnO matrix
• Stabilized Cu ions in morphologically active ZnO
• Formation of Cu-ZnO interface through oxygen (Cu-O-Zn)
1.4.6 Importance of ZnO
The significance of ZnO in methanol synthesis catalyst could be realized by the fact
that Cu metal alone is not able to provide active centers for the reaction to occur. In
fact, a study conducted by Herman et al. signifies the co-existence of ZnO in the
catalyst as the activity of the binary catalyst (Cu/ZnO) increased by three orders of
magnitude as compared to that of pure Cu metal. From the early findings of the
methanol synthesis catalysts to the present stage of the catalytic platform, the role
of ZnO to facilitate the easy reaction propagation has remained contrasting. A few
important roles of ZnO, as identified from literature, are as follows:
• ZnO is reported to enhance Cu dispersion, thereby elucidating a synergistic
effect with Cu species [76].
• Promotes the stability of active Cu
+
sites in the catalyst by allowing the incorporation of Cu
+
into the crystal lattice of ZnO [74].
• The main function of ZnO was identified to facilitate heterogeneous H 2 splitting
(H
δ+
/H
δ−
) and spillover which involves the migration of H
+
/OH
−
species over the
catalyst surface.
• Incorporation of ZnO in the catalyst matrix induces the shape changes in Cu
particles impregnated on the support depending upon the redox potential of surrounding gas phase as identified by atom-resolved TEM [77]. These changes in
shape of the particle results in irregularities on the surface which enhances the
catalytic activity.
Table 4 demonstrates the exemplary behavior of ZnO when exposed to different
feed streams. On the other hand, some studies demonstrated the negative or neutral
effect of ZnO addition into the Cu-based catalytic system. Chinchen et al. emphasized that there does not exist any significant difference in catalytic activity between
the Cu-supported catalyst on SiO 2 and MgO as compared to that supported on ZnO,
thereby totally denying the well-known Cu-ZnO synergy. Another controversial discussion related to Cu-ZnO interaction exists in the explanation of the Schottky junction effect existing between these two. The effect is known to increase the oxygen
vacancies in ZnO thrice when used alone, but no reaction intermediates were
observed when Cu is present along with ZnO. This suggests that the theory fails to
explain the increased activity of Cu-ZnO catalyst [78].
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…