13.3 Catalysis of Pt Clusters
225
generally associated with the energy of the d-band center [44, 45]. This shows a
range from −9.0 eV (the ionization potential of a Pt atom) to −5.32 eV (the Fermi
energy of bulk platinum) [130]. Based on this, the increase in the catalytic activity
from Pt 8 to Pt 15 can be rationalized with the decrease of the ionization potential and
the change in the central position of the d-band, as well as the concomitantly enhanced
resonance with the antibonding π g * state of O 2 [130]. The maximum catalysis for
Pt 15 suggests its largest back-donation. Further increasing the cluster size lowers the
cluster’s HOMO and hence results in a weaker resonance with the antibonding π g *
state of O 2 ; while for the very small clusters (including Pt atom) the back-donation
is small because the HOMO energy of the cluster mismatches with π g * of oxygen
(Fig. 13.7C).
These studies have made the catalysis of Pt n clusters being widely studied;
however, the efficiency is relatively low considering a per-metal atom basis as only the
surface active-site atoms are used. As a precious and expensive metal, maximum atom
efficiency of Pt catalysts with single-atom dispersions is highly desired. However,
the challenge involves not only the limited catalytic efficiency of single Pt atoms
as demonstrated above, but also technical difficulty to make it in practical use.
Recently researchers have found a new solution to this challenge and synthesized a
single-atom catalyst that consists of only isolated single Pt atoms anchored to the
surfaces of iron oxide nanocrystallites (Fig. 13.8) [34]. This single-atom catalyst
shows excellent stability and high activity for both CO oxidation and preferential
oxidation of CO in H 2 ; also it has extremely high atom efficiency of Pt. Density
functional theory (DFT) calculations showed that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt
atoms, which helps to reduce both the CO adsorption energy and the activation barrier
for CO oxidation in forming CO 2 [34].
13.4 Catalysis of Copper-Related Systems
Some other catalysts such as copper, Cu/ZnO (/Al 2 O 3 ) binary system and even a
ternary system Cu/ZnO/Al 2 O 3 were widely used in industrial production of methanol
[142–146], with a worldwide demand of ~50 Mtons per year, from gas mixtures
(H 2 /CO 2 /CO) at elevated pressures and temperatures. Binary systems (Cu/ZnO
and Cu/Al 2 O 3 ) and ternary Cu/ZnO/Al 2 O 3 catalysts were important in methanol
synthesis with respect to their catalytic activity and stability within the reactions.
While the industrial catalysts containing low amounts of a refractory oxide displayed
improved catalysis [146], the key to high performance is a largely accessible Cu
surface area [142, 147]. Similar catalytic systems also attract interest for potential
use of methanol as a sustainable synthetic fuel obtained by hydrogenation of captured
CO 2 [148]. While the phenomenological optimization of the preparation of active
catalyst on these systems is well improved, the fundamental understanding of its
catalytic activity is still illusive to be further explored [149–151].
225
generally associated with the energy of the d-band center [44, 45]. This shows a
range from −9.0 eV (the ionization potential of a Pt atom) to −5.32 eV (the Fermi
energy of bulk platinum) [130]. Based on this, the increase in the catalytic activity
from Pt 8 to Pt 15 can be rationalized with the decrease of the ionization potential and
the change in the central position of the d-band, as well as the concomitantly enhanced
resonance with the antibonding π g * state of O 2 [130]. The maximum catalysis for
Pt 15 suggests its largest back-donation. Further increasing the cluster size lowers the
cluster’s HOMO and hence results in a weaker resonance with the antibonding π g *
state of O 2 ; while for the very small clusters (including Pt atom) the back-donation
is small because the HOMO energy of the cluster mismatches with π g * of oxygen
(Fig. 13.7C).
These studies have made the catalysis of Pt n clusters being widely studied;
however, the efficiency is relatively low considering a per-metal atom basis as only the
surface active-site atoms are used. As a precious and expensive metal, maximum atom
efficiency of Pt catalysts with single-atom dispersions is highly desired. However,
the challenge involves not only the limited catalytic efficiency of single Pt atoms
as demonstrated above, but also technical difficulty to make it in practical use.
Recently researchers have found a new solution to this challenge and synthesized a
single-atom catalyst that consists of only isolated single Pt atoms anchored to the
surfaces of iron oxide nanocrystallites (Fig. 13.8) [34]. This single-atom catalyst
shows excellent stability and high activity for both CO oxidation and preferential
oxidation of CO in H 2 ; also it has extremely high atom efficiency of Pt. Density
functional theory (DFT) calculations showed that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt
atoms, which helps to reduce both the CO adsorption energy and the activation barrier
for CO oxidation in forming CO 2 [34].
13.4 Catalysis of Copper-Related Systems
Some other catalysts such as copper, Cu/ZnO (/Al 2 O 3 ) binary system and even a
ternary system Cu/ZnO/Al 2 O 3 were widely used in industrial production of methanol
[142–146], with a worldwide demand of ~50 Mtons per year, from gas mixtures
(H 2 /CO 2 /CO) at elevated pressures and temperatures. Binary systems (Cu/ZnO
and Cu/Al 2 O 3 ) and ternary Cu/ZnO/Al 2 O 3 catalysts were important in methanol
synthesis with respect to their catalytic activity and stability within the reactions.
While the industrial catalysts containing low amounts of a refractory oxide displayed
improved catalysis [146], the key to high performance is a largely accessible Cu
surface area [142, 147]. Similar catalytic systems also attract interest for potential
use of methanol as a sustainable synthetic fuel obtained by hydrogenation of captured
CO 2 [148]. While the phenomenological optimization of the preparation of active
catalyst on these systems is well improved, the fundamental understanding of its
catalytic activity is still illusive to be further explored [149–151].