Size-Dependent Electrocatalytic Reduction of CO2 over Pd Nanoparticles

Article abstract of DOI:10.1021/jacs.5b00046

Size effect has been regularly utilized to tune the catalytic activity and selectivity of metal nanoparticles (NPs). Yet, there is a lack of understanding of the size effect in the electrocatalytic reduction of CO₂, an important reaction that couples with intermittent renewable energy storage and carbon cycle utilization. We report here a prominent size-dependent activity/selectivity in the electrocatalytic reduction of CO₂ over differently sized Pd NPs, ranging from 2.4 to 10.3 nm. The Faradaic efficiency for CO production varies from 5.8% at -0.89 V (vs reversible hydrogen electrode) over 10.3 nm NPs to 91.2% over 3.7 nm NPs, along with an 18.4-fold increase in current density. Based on the Gibbs free energy diagrams from density functional theory calculations, the adsorption of CO₂ and the formation of key reaction intermediate COOH are much easier on edge and corner sites than on terrace sites of Pd NPs. In contrast, the formation of H for competitive hydrogen evolution reaction is similar on all three sites. A volcano-like curve of the turnover frequency for CO production within the size range suggests that CO₂ adsorption, COOH formation, and CO removal during CO₂ reduction can be tuned by varying the size of Pd NPs due to the changing ratio of corner, edge, and terrace sites.

Full text of DOI:10.1021/jacs.5b00046

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Communication
Size-Dependent Electrocatalytic Reduction of CO2 over Pd Nanoparticles
Dunfeng Gao, Hu Zhou, Jing Wang, Shu Miao, Fan Yang, Guoxiong Wang, Jianguo Wang, and Xinhe Bao
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Dunfeng Gao,
Hu Zhou, Jing Wang, Shu Miao, Fan Yang, Guoxiong Wang,* Jianguo
,‡
†
Wang,* and Xinhe Bao
†
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian,
116023, China.
‡
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310032, China.
§
University of Chinese Academy of Sciences, Beijing, 100039, China.
Supporting Information Placeholder
4
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to the competitive hydrogen evolution reaction (HER), etc.
ABSTRACT: Size effect has been regularly utilized to tune
Key points in addressing these issues, are the successive CO2
the catalytic activity and selectivity of metal nanoparticles
NPs). Yet, there has been a lack of understanding on size
6-9
10,11
adsorption,
intermediates formation
and product re-
(
1
2,13
moval on active sites during the electrocatalytic reaction.
Various metallic electrodes have been screened thorough-
ly, and it is found that the trends on product composition
and distribution are strongly dependent on the binding en-
effect in the electrocatalytic reduction of CO2, an important
reaction coupling with intermittent renewable energy stor-
age and carbon cycle utilization. We report here a prominent
size-dependent activity/selectivity in the electrocatalytic
reduction of CO2 over differently-sized Pd NPs, ranging from
2.4 nm to 10.3 nm. The Faradaic efficiency for CO production
varies from 5.8% at -0.89 V (vs. reversible hydrogen electrode)
over 10.3 nm NPs to 91.2% over 3.7 nm NPs, along with an
1
4
ergy of intermediates on metals. For instance, CO2 is con-
verted into CO on Au, Ag and into hydrocarbons on Cu due
1
4
to the adsorption strength of CO* intermediates on metals.
Varying the particle size has also shown a size effect on the
electrocatalytic reduction of CO2, where the dependence of
Faradaic efficiency and current density on particle size has
18.4 fold increase in current density. Based on the Gibbs free
energy diagrams from density functional theory calculations,
the adsorption of CO2 and the formation of key reaction
intermediate COOH* are much easier on edge and corner
sites than on terrace one of Pd NPs. In contrast, the for-
mation of H* for competitive hydrogen evolution reaction is
similar on all three sites. A volcano-like curve of the turnover
frequency for CO production within the size range suggests
that CO2 adsorption, COOH* formation and CO* removal
during CO2 reduction can be tuned by varying the size of Pd
NPs due to the changing ratio of corner, edge and terrace
sites.
15
16
17
18,19
been observed on Ag , SnO2 , Cu and Au
nanoparticles
(
NPs). However, there have been limited studies looking into
the size effect among NPs smaller than 10 nm, which are
most commonly used in catalysis. Recently, studies on the
CO2 reduction reactivity trends over Au NPs within a very
18
19
narrow size range of ~1-8 nm or 4-10 nm were reported.
Current density for both CO and H2 production increases
obviously with decreasing the size of Au NPs along with the
decrease in Faradaic efficiency toward CO production. Densi-
ty functional theory (DFT) calculations reveal that the edge
site on Au NPs is much more active than terrace and corner
sites for CO2 reduction to CO, while corner sites are highly
active for competitive HER. In contrast to the behavior of Au
NPs, we report here Faradaic efficiency for CO production
increases with decreasing Pd NP size in a size range of 2.4-
10.3 nm, which varies from 5.8% at -0.89 V (vs. reversible
hydrogen electrode, RHE) over 10.3 nm NPs to 91.2% over 3.7
nm NPs, along with an 18.4 fold increase in current density
for CO production. DFT calculations indicate that corner and
edge sites (small Pd NPs) facilitate the adsorption of CO2
and formation of key reaction intermediates COOH* during
CO2 reduction compared with terrace one (large Pd NPs),
while the formation of H* for competitive HER is similar on
all three sites. Furthermore, a volcano-like curve of the turn-
over frequency (TOF) for CO production within the size
range suggests that CO2 adsorption, COOH* formation and
CO* removal during CO2 reduction can be tuned on differ-
The accelerated depletion of fossil fuel resources leads to
increasing accumulation of greenhouse gas, CO2, in the at-
1
mosphere, which raises serious environmental concerns. To
reduce our dependence on the fossil fuel, renewable energy
sources, such as solar and wind energies, are growing up
rapidly, whereas, these renewable electricities are difficult to
be merged into the electricity grid due to their intermittent
and local nature, and further expansion is highly limited by
the lack of efficient energy storage and conversion tech-
2
niques. Electrocatalytic reduction of CO2 to fuel and chemi-
cal feedstock, powered by intermittent renewable electricity,
is an attractive route for simultaneous conversion of CO2
3
and renewable energy sources. However, there are several
fundamental challenges in the electrocatalytic reduction of
CO2, such as high overpotential, low Faradic efficiency due
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ently-sized Pd NPs due to the changing ratio of corner, edge
Controlled potential electrolysis of CO2 was performed in
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and terrace sites.
an H-cell (separated by Nafion 115) filled with 0.1 M KHCO3
solution (pH=6.8) as shown in Scheme S1. Under the report-
ed reaction conditions, CO and H2 were major products as
detected by an on-line micro gas chromatography (GC), and
only trace amount of formate with a Faradaic efficiency of
0.37-1.3% (Table S2 and Figure S4) was detected by nuclear
magnetic resonance (NMR). Figure 2a-b shows the applied
potential dependence of Faradaic efficiency and current den-
sity for CO production over Pd NPs. All the Faradaic efficien-
cies for CO production firstly increase with applied potential,
despite the size of Pd NPs. The competitive HER is facile at
low overpotentials and CO2 reduction is prone to occur with
increased applied potential. However, the Faradaic efficiency
decreases at high applied potentials, which is probably
caused by the limited mass transport of CO2 in 0.1 M KHCO3
solution. On 2.4 nm Pd and 3.7 nm-Pd, all the Faradaic effi-
ciencies at different potentials exceed 80%, and the maxi-
mum value of 91.2% is reached at -0.89 V (vs. RHE), which is
A series of carbon-supported Pd NPs were prepared using
sodium citrate as a stabilizing agent and NaBH4 as a reduc-
tive agent. The particle size of Pd NPs was controlled by ad-
justing the ratio of sodium citrate to PdCl2 and the reduction
temperature. For distinction, the average size of Pd NPs in
each sample was used in the sample name, for instance, 3.7
nm-Pd indicates that Pd NPs in the Pd/C catalyst has an av-
erage particle size of 3.7 nm. Figure 1 and Figure S1 show
transmission electron microscopy (TEM) and high resolution
TEM (HRTEM) images of six Pd/C catalysts with different
sizes. The NPs are uniformly deposited on carbon support,
and typical Pd(111) plane with the characteristic lattice spac-
ing of 0.23 nm is observed. Figure S2 shows the correspond-
ing histograms of particle-size distribution of six Pd/C cata-
lysts by counting more than 200 particles for each from the
TEM images. The particle size distribution gradually broad-
ens with increasing NP size. X-ray diffraction (XRD) patterns
of the Pd NPs are shown in Figure S3, and the average crys-
talline size of Pd NPs was calculated from the (220) diffrac-
tion peak (Table S1). Obviously, both TEM and XRD results
confirm that a series of Pd NPs in a narrow size range are
successfully prepared. The actual loadings of Pd in all the
Pd/C catalysts are 18.3±0.5 wt%, as measured by inductively
coupled plasma optical emission spectroscopy (ICP-OES) as
listed in Table S1. The consistent loading of Pd in Pd/C cata
lysts allows us to directly compare CO2 reduction activities
of different Pd/C catalysts and investigate the size effect of
Pd NPs.
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much higher than that over Pd wire electrode (<6%). The
Faradaic efficiencies for CO production over 2.4 nm Pd and
3.7 nm-Pd are also comparable to those over Au and Ag
1
5,18,19,21
NPs.
Figure 2. The applied potential dependence of (a) Faradaic
efficiencies and (b) current densities for CO production over
Pd NPs with different sizes.
The current density for CO production increases with ap-
plied potential (Figure 2b), indicative of accelerated reaction
rate for CO2 reduction at high applied potentials. Figure S5
summarizes the mass activities of Pd NPs for CO production
-1
at various potentials. The mass activity reaches 23.9 A g
at
Pd
-
4
0.89 V (vs. RHE) on 3.7 nm-Pd, which is higher than that on
nm-Au NPs in the case of a similar metal loading on car-
Figure 1. TEM image and HRTEM images of (a, b) 3.7 nm-Pd,
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(
c, d) 6.2 nm-Pd and (e, f) 10.3 nm-Pd.
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bon and is comparable to that on ultrathin Au nanowire. It
should be noted that the mass activity of Pd NPs was meas-
ured based on gas diffusion electrode, which can be directly
used in practical compact electrolysis cells. Therefore, these
Pd/C catalysts demonstrate promising applications for prac-
tical industrial CO2 electrocatalytic reduction processes.
Interestingly, the Faradaic efficiencies and current densi-
ties for CO formation over Pd NPs with different sizes are
quite different at various potentials. The current densities for
CO production on 2.4 nm-Pd are smaller than that on 3.7
nm-Pd with nearly the same Faradaic efficiency. Further in-
creasing the size of Pd NPs, both Faradaic efficiencies and
current densities for CO production decrease saliently. The
Faradaic efficiency decreases sharply from 91.2% to 5.8% over
a particle size ranging from 3.7 nm to 10.3 nm at -0.89 V (vs.
RHE). The current densities for CO production also change
in the same trend, and the current density on 3.7 nm-Pd has
an 18.4 fold increase compared to that over 10.3 nm-Pd,
S8c), since a stronger hydroxyl adsorption on defects (corner
and edge sites), corresponding to small Pd NPs, than on ter-
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race sites for large Pd NPs. The strong adsorption of CO on
Pd NPs could be probed by the electrochemical CO stripping
25
voltammetry method. As shown in Figure S9, a broad CO
stripping profile with a dominant peak around 1.03 V (vs.
RHE) occurs on 2.4 nm-Pd, 3.7 nm-Pd and 4.5 nm-Pd,
whereas there is a sharp peak around 0.9 V (vs. RHE) with a
shoulder peak around 1.03 V (vs. RHE) on 6.2 nm-Pd, 7.8 nm-
Pd and 10.3 nm-Pd. The negative shift on peak potential and
the reduced current density at 1.03 V (vs. RHE) with increas-
ing particle size suggest the weakened adsorption of CO* on
Pd NPs, in consistent with the DFT calculations (Figure S7).
From the calculated Gibbs free energy diagrams, the for-
mation of H2 has tiny differences on these models (Figure
S10), and the overall Gibbs free energy changes is much
smaller than that for the formation of CO*, indicating that
CO2 reduction is predominant under the present experi-
mental conditions.
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which is in contrast to the case of Au NPs.
To the best of
our knowledge, the unique particle size dependence of the
Faradaic efficiencies and current densities toward CO2 re-
duction has not been reported previously.
In order to understand the relationship between the CO2
reduction reactivity and the size of Pd NPs, DFT calculations
were conducted. It is still a computational challenge to di-
rectly simulate the CO2 reduction over Pd NPs with different
sizes by means of DFT calculations. However, it is possible to
simulate the process over three different reaction sites (ter-
race, edge and corner) of Pd NPs, represented by flat Pd(111)
surfaces, stepped Pd(211) surfaces and Pd55 (or Pd38) clusters,
respectively. Furthermore, according to the ratio of three
kinds of reaction sites (terrace, edge, and corner) on ideal Pd
NPs with different sizes, it is reasonable to build the intrinsic
22
relationship between catalytic properties and NPs size. CO2
weakly adsorbs on Pd(111), Pd(211), Pd55 with 0.23, -0.11 and -
0.21 eV respectively and CO2 prefers to be away from Pd(111)
(Figure S6). The calculated Gibbs free energy diagrams for
CO2 reduction into CO on Pd(111), Pd(211) , Pd55 and Pd38 at
0 V (vs. RHE) are shown in Figure 3 according to the meth-
23
odology proposed by Nørskov et al. It is seen that on Pd(111),
the formation of COOH*, a key reaction intermediate of CO2
reduction, is associated with an increase in free energy (0.22
eV). While on Pd(211) and Pd55, COOH* is very stable with a
release of -0.15 and -0.53 eV, respectively. Therefore, the for-
mation of reaction intermediate COOH* becomes easier
from terrace (Pd(111)), stepped (Pd(211)) to corner (Pd55).
Pd38 has a free energy similar to Pd55. On these models, the
formation of CO*, another reaction intermediate, are all ex-
ergonic with very similar Gibbs free energy changes with
Figure 3. (a) The adsorption of COOH and (b) Free energy
diagrams for CO2 reduction to CO on Pd(111), Pd(211), Pd55
and Pd38.
According to the DFT calculation results, the corner and
edge sites on Pd NPs are considered active for CO2 reduc-
tion, and the corner, edge and terrace sites are all active for
HER. TOF for CO or H2 production on Pd NPs at various
potentials was calculated according to the ratio of corner
and edge or the ratio of corner, edge and terrace in Pd NPs
those of COOH*, due to that the conversion of COOH* to
(
Figure S11). Figure 4 shows the size dependence of TOF for
21,24
CO* is typically facile.
Therefore, it is concluded that the
CO production at various potentials. The maximum TOF
varies from 3.7 nm-Pd to 4.5 nm-Pd when the applied po-
tential is shifted from -0.59 V to -0.89 V (vs. RHE) gradually.
The sluggish removal of CO* at a high coverage on 3.7 nm-
Pd, having a higher ratio of corner and edge sites, probably
retards the increase of TOF with the potential, compared
with that on 4.5 nm-Pd. The difference of TOF for CO pro-
duction over differently-sized Pd NPs is enlarged at high
applied potentials, revealing that the electrochemical step
for formation of HCOO* and CO* becomes dominant on
formation of COOH* is a key electrochemical step for CO2
reduction to CO. The main reason for different changes of
Gibbs free energies on different reaction sites is that the ad-
sorption of COOH and CO become stronger from terrace,
step to corner site (Figure 3 and Figure S7). These calcula-
tions well explain that Pd NPs have positive shifts in the po-
sition of the reduction peak of Pd(OH)2 in the cathodic
25
scan between 0.9 V and 0.5 V with increasing particle size
Figure S8). The reduction peak potential of Pd(OH)2 shows
a linear decrease with increasing the size of Pd NPs (Figure
(
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the reaction rate. The increase of TOF on 2.4 nm-Pd and
Author Contributions
These authors contributed equally.
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10.3 nm-Pd is much less sensitive with the applied potential
than that on Pd NPs with the middle sizes (Figure S12a),
which suggests that the non-electrochemical step plays very
important roles on 2.4 nm-Pd and 10.3 nm-Pd, probably as-
sociated with the strong adsorption of CO on step and cor-
ner sites and weak adsorption of CO2 on terrace site, re-
spectively. TOF for H2 production increases with the in-
creasing size of Pd NPs at various potentials (Figures S12b
and S13). The formation of H* is similar on corner, edge and
terrace sites, which participates both CO2 reduction and
‖
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the Minis-
try of Science and Technology of China (Grants 2012CB215500
and 2013CB733501), the National Natural Science Foundation
of China (Grants 21136001, 21103178, and 21176211).
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HER. The increase of TOF values for H production in the
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size range from 2.4 nm to 4.5 nm is sluggish because H* on
corner and edge sites (small Pd NPs) might mainly partici-
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Experimental details, TEM and HRTEM images, XRD pat-
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AUTHOR INFORMATION
Corresponding Author
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25) Xiao, L.; Zhuang, L.; Liu, Y.; Lu, J.; Abruna, H. D. J. Am. Chem.
Soc. 2009, 131, 602.
(26) Mayrhofer, K. J. J.; Blizanac, B. B,; Arenz, M.; Stamenkovic, V.
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wanggx@dicp.ac.cn
jgw@zjut.edu.cn
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