13.4 Catalysis of Copper-Related Systems
227
Cu metal reference sample. While the ZnO-free Cu reference exhibited little activity,
the catalysts that performed best were prepared by following the industrial synthesis
method. The performance by the Cu surface areas resulting in the intrinsic activities,
normalized to the intrinsically most active catalyst for each temperature, is shown in
Fig. 13.9. The scatter of the data showed that the Cu surface area alone can do not
explain the differences in performance. The neutron scattering data permitted for a
sufficiently reliable fitting of the peak position 400 of the nanostructured Cu phase
(Fig. 13.9c). For the inactive pure Cu sample, both ratios fell near the expected ideal
value, while the catalytic materials showed a lower value for h = 1 and a higher one
for h = 2, which is consistent with the presence of stacking faults in the Cu clusters
(Fig. 13.9d) [142].
Utilizing aberration-corrected high-resolution transition electron microscopy
(HRTEM), Behrens et al. [142] examined the relation of bulk defects and surface
steps in the most active catalysts, as displayed in Fig. 13.10, which accords with
the model situations of stepped Cu(211) surface or the stacking fault-created step
shown in the inset of Fig. 13.9d. A vast majority of Cu nanoparticles was faulted and
exhibited planar extended defects, stacking faults, and twin boundaries. The curvature of the spherical particle causes the surface to intrinsically contain a set of steps.
The sample related to the HRTEM image of Fig. 13.10a showed a pattern of stepped
surface facets like (211) and (522) being responsible for the curvature at the lower
exposed side of the Cu nanoparticle, which is associated with an inward curvature
of the surface and does not occur on regular spherical or ellipsoidal fcc particles or
Wulff polyhedra. The other sample (Fig. 13.10b) showed planar bulk defects, seen
as the twin boundaries, reflected in changes of the surface faceting creating a local
inward curvature of the nanoparticle. Another sample (Fig. 13.10c, d) showed that
twin boundaries could create distinctive surface ensembles even if the Cu surface of
a larger nanoparticle appears essentially flat or the position stuck out of the regular
surface. These arrangements were described as a high-energy site created by the
termination of a planar defect at the surface of the Cu nanoparticles [142].
The undistorted pure Cu was quite inactive in the methanol synthesis experiment, which was also confirmed by DFT calculations for the flat Cu(111) surface.
Figure 13.11 presents the DFT-calculated energy diagrams for CO 2 and CO hydrogenation on close-packed (black curve), stepped (blue curve), and Zn substituted steps
(red curve) respectively. Essentially all the intermediates are thermodynamically less
stable than CO 2 and H 2 in the gas phase (Fig. 13.11B), but is clearly shown that the
flat Cu(111) surface bound intermediates more weakly than did Cu(211). Both the
energies of the intermediates and the transition-states were stabilized considerably
for the (211) surface compared with the (111) surface, rendering the steps more
vigorous than the terraces. Similarly, CO hydrogenation proceeded via an initial
hydrogenation of the carbon atom of CO, through a few intermediates such as HCO,
H 2 CO, H 3 CO and then the aim product methanol (H 4 CO). Both for the hydrogenation of CO 2 and CO, the last two intermediates are the same, and the order of activity
is CuZn(211) > Cu(211) > Cu(111) as the steps lower the adsorption energies of the
intermediates substantially compared with the flat surface [142].
227
Cu metal reference sample. While the ZnO-free Cu reference exhibited little activity,
the catalysts that performed best were prepared by following the industrial synthesis
method. The performance by the Cu surface areas resulting in the intrinsic activities,
normalized to the intrinsically most active catalyst for each temperature, is shown in
Fig. 13.9. The scatter of the data showed that the Cu surface area alone can do not
explain the differences in performance. The neutron scattering data permitted for a
sufficiently reliable fitting of the peak position 400 of the nanostructured Cu phase
(Fig. 13.9c). For the inactive pure Cu sample, both ratios fell near the expected ideal
value, while the catalytic materials showed a lower value for h = 1 and a higher one
for h = 2, which is consistent with the presence of stacking faults in the Cu clusters
(Fig. 13.9d) [142].
Utilizing aberration-corrected high-resolution transition electron microscopy
(HRTEM), Behrens et al. [142] examined the relation of bulk defects and surface
steps in the most active catalysts, as displayed in Fig. 13.10, which accords with
the model situations of stepped Cu(211) surface or the stacking fault-created step
shown in the inset of Fig. 13.9d. A vast majority of Cu nanoparticles was faulted and
exhibited planar extended defects, stacking faults, and twin boundaries. The curvature of the spherical particle causes the surface to intrinsically contain a set of steps.
The sample related to the HRTEM image of Fig. 13.10a showed a pattern of stepped
surface facets like (211) and (522) being responsible for the curvature at the lower
exposed side of the Cu nanoparticle, which is associated with an inward curvature
of the surface and does not occur on regular spherical or ellipsoidal fcc particles or
Wulff polyhedra. The other sample (Fig. 13.10b) showed planar bulk defects, seen
as the twin boundaries, reflected in changes of the surface faceting creating a local
inward curvature of the nanoparticle. Another sample (Fig. 13.10c, d) showed that
twin boundaries could create distinctive surface ensembles even if the Cu surface of
a larger nanoparticle appears essentially flat or the position stuck out of the regular
surface. These arrangements were described as a high-energy site created by the
termination of a planar defect at the surface of the Cu nanoparticles [142].
The undistorted pure Cu was quite inactive in the methanol synthesis experiment, which was also confirmed by DFT calculations for the flat Cu(111) surface.
Figure 13.11 presents the DFT-calculated energy diagrams for CO 2 and CO hydrogenation on close-packed (black curve), stepped (blue curve), and Zn substituted steps
(red curve) respectively. Essentially all the intermediates are thermodynamically less
stable than CO 2 and H 2 in the gas phase (Fig. 13.11B), but is clearly shown that the
flat Cu(111) surface bound intermediates more weakly than did Cu(211). Both the
energies of the intermediates and the transition-states were stabilized considerably
for the (211) surface compared with the (111) surface, rendering the steps more
vigorous than the terraces. Similarly, CO hydrogenation proceeded via an initial
hydrogenation of the carbon atom of CO, through a few intermediates such as HCO,
H 2 CO, H 3 CO and then the aim product methanol (H 4 CO). Both for the hydrogenation of CO 2 and CO, the last two intermediates are the same, and the order of activity
is CuZn(211) > Cu(211) > Cu(111) as the steps lower the adsorption energies of the
intermediates substantially compared with the flat surface [142].