structure, therefore, lattice engineering through alloy different metals could tuning
the above two factors and further improve the activity and selectivity of the catalyst.
Pd 7 Cu 1 supported on TiO 2 nanoplates with isolation Cu in Pd lattice for CO2PR was
reported by Long and coworkers [18]. In this research, when the Cu loading amount
is below 12.5%, the XAFS results show that the oxidation of Cu (absent of Cu-O)
could be inhibited effectively and Cu atoms were isolated in the Pd lattice (absent of
Cu–Cu bonds). During the CO2PR evaluation, the Pd 7 Cu 1 /TiO 2 sample shows the
optimal CO 2 reduction activity and CH 4 selectivity. In situ DRIFTS experiments
show the enhanced signals of HCO 3
À , CO 3
¼ , and CO 2
À species over samples with
isolation of Cu atoms; in addition, the first-principle theory also indicate the Pd–Cu
pairs could enhance the CO 2 adsorption. Both experimental and theoretical results
suggest the Pd-Cu pairs favor the CO 2 adsorption. The different d band centers of Cu
in Pd 7 Cu 1 and Pd 1 Cu 1 revealed the Pd-surrounded environment could tune the
electronic structure of Cu and improve the catalytic activity of Cu. Au–Cu alloy
NPs supported on TiO 2 (p25) reported previously also show enhanced performance
in CO2PR compared with Au/TiO 2 or Cu/TiO 2 (Fig. 12.10) [14].
To gain deep understanding of the reaction mechanism and intermediates along
the CO2PR, time-resolved in situ FTIR was applied. During the reaction, the
generation of Cu-CO band (2126 cm
À1 ) indicates the Cu favors the CO 2 reduction
instead of H 2 O reduction; also the CO 2
.- (1589 cm
À1 ) shows a continuously decreasing trend during the irradiation, which is assumed as a reactive species generated
from the surface Ti
3+ (Fig. 12.10c, d). Further studies were carried out by using two
light sources (visible light and UV light) to trigger CO2PR. Under the visible light
irradiation, CH 4 and H 2 were the main products over the optimal Au-Cu/TiO 2 ,
indicates the hot electrons generated from the surface plasma resonance of Au NPs
and reacts with the activated CO 2 to generate CH 4 . However, when using UV light as
the light source, the Au–Cu alloy NPs act as the electron sink and promote the charge
separation, which generate higher amount of H 2 .
Low-coordinated sites (i.e., edge or corner sites) in metal NP-supported catalyst
are often treated as active sites. Generally, these low-coordinated sites possess
unique properties like strong binding energy toward certain reaction intermediates
and low free energy which to some rate determines steps. Combining the experimental results with the DFT calculation, Mistry et al. [67] proposed that, in CO 2
electroreduction reaction, Au NPs show the size-dependent activity; the smallersized Au with more low-coordinated sites is more active in HER than CRR. Gao
et al. [68] studied the Pd NPs with variable size in CO 2 electroreduction, and the
result shows that low-coordinated sites of Pd are more suitable for COOH* generation but HER is insensitive to different surface sites. Zhu et al. [16] synthesized Pd
nanosheet with similar thickness but different size (TiO 2 -Pd NSs-s, small; TiO 2 -Pd
NSs-m, middle; TiO 2 -Pd NSs-l, large) and proposed the edge sites of Pd nanosheet
are the active site for CO2PR. Keeping the Pd loading amount as constant, with
decrease of the size of Pd nanosheet, results in increased Pd edge density, and the CO
and CH 4 yield increased as well. So the edge sites of Pd may act as the active site in
CO2PR; to further confirm this edge-dependent activity, the Pd nanorings with even
smaller size and higher density of edge sites were prepared and deposited on TiO 2
12.4 Roles and Properties of Different Cocatalysts
299
the above two factors and further improve the activity and selectivity of the catalyst.
Pd 7 Cu 1 supported on TiO 2 nanoplates with isolation Cu in Pd lattice for CO2PR was
reported by Long and coworkers [18]. In this research, when the Cu loading amount
is below 12.5%, the XAFS results show that the oxidation of Cu (absent of Cu-O)
could be inhibited effectively and Cu atoms were isolated in the Pd lattice (absent of
Cu–Cu bonds). During the CO2PR evaluation, the Pd 7 Cu 1 /TiO 2 sample shows the
optimal CO 2 reduction activity and CH 4 selectivity. In situ DRIFTS experiments
show the enhanced signals of HCO 3
À , CO 3
¼ , and CO 2
À species over samples with
isolation of Cu atoms; in addition, the first-principle theory also indicate the Pd–Cu
pairs could enhance the CO 2 adsorption. Both experimental and theoretical results
suggest the Pd-Cu pairs favor the CO 2 adsorption. The different d band centers of Cu
in Pd 7 Cu 1 and Pd 1 Cu 1 revealed the Pd-surrounded environment could tune the
electronic structure of Cu and improve the catalytic activity of Cu. Au–Cu alloy
NPs supported on TiO 2 (p25) reported previously also show enhanced performance
in CO2PR compared with Au/TiO 2 or Cu/TiO 2 (Fig. 12.10) [14].
To gain deep understanding of the reaction mechanism and intermediates along
the CO2PR, time-resolved in situ FTIR was applied. During the reaction, the
generation of Cu-CO band (2126 cm
À1 ) indicates the Cu favors the CO 2 reduction
instead of H 2 O reduction; also the CO 2
.- (1589 cm
À1 ) shows a continuously decreasing trend during the irradiation, which is assumed as a reactive species generated
from the surface Ti
3+ (Fig. 12.10c, d). Further studies were carried out by using two
light sources (visible light and UV light) to trigger CO2PR. Under the visible light
irradiation, CH 4 and H 2 were the main products over the optimal Au-Cu/TiO 2 ,
indicates the hot electrons generated from the surface plasma resonance of Au NPs
and reacts with the activated CO 2 to generate CH 4 . However, when using UV light as
the light source, the Au–Cu alloy NPs act as the electron sink and promote the charge
separation, which generate higher amount of H 2 .
Low-coordinated sites (i.e., edge or corner sites) in metal NP-supported catalyst
are often treated as active sites. Generally, these low-coordinated sites possess
unique properties like strong binding energy toward certain reaction intermediates
and low free energy which to some rate determines steps. Combining the experimental results with the DFT calculation, Mistry et al. [67] proposed that, in CO 2
electroreduction reaction, Au NPs show the size-dependent activity; the smallersized Au with more low-coordinated sites is more active in HER than CRR. Gao
et al. [68] studied the Pd NPs with variable size in CO 2 electroreduction, and the
result shows that low-coordinated sites of Pd are more suitable for COOH* generation but HER is insensitive to different surface sites. Zhu et al. [16] synthesized Pd
nanosheet with similar thickness but different size (TiO 2 -Pd NSs-s, small; TiO 2 -Pd
NSs-m, middle; TiO 2 -Pd NSs-l, large) and proposed the edge sites of Pd nanosheet
are the active site for CO2PR. Keeping the Pd loading amount as constant, with
decrease of the size of Pd nanosheet, results in increased Pd edge density, and the CO
and CH 4 yield increased as well. So the edge sites of Pd may act as the active site in
CO2PR; to further confirm this edge-dependent activity, the Pd nanorings with even
smaller size and higher density of edge sites were prepared and deposited on TiO 2
12.4 Roles and Properties of Different Cocatalysts
299