Research Article
pubs.acs.org/acscatalysis
Tuned Chemical Bonding Ability of Au at Grain Boundaries for
Enhanced Electrochemical CO2 Reduction
Kang-Sahn Kim,† Won June Kim,† Hyung-Kyu Lim,‡ Eok Kyun Lee,*,† and Hyungjun Kim*,‡
†
Department of Chemistry and ‡Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced
Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Korea
S Supporting Information
*
ABSTRACT: Electrochemical carbon dioxide (CO2) reduction is an emerging technology for efficiently recycling CO2
into fuel, and many studies of this reaction are focused on
developing advanced catalysts with high activity, selectivity,
and durability. Of these catalysts, oxide-derived metal
nanoparticles, which are prepared by reducing a metal oxide,
have received considerable attention due to their catalytic
properties. However, the mechanism of the nanoparticles’
activity enhancement is not well-understood. Recently, it was
discovered that the catalytic activity is quantitatively correlated
to the surface density of grain boundaries (GBs), implying that
GBs are mechanistically important in electrochemical CO2
reduction. Here, using extensive density functional theory
(DFT) calculations modeling the atomistic structure of GBs on the Au (111) surface, we suggest a mechanism of electrochemical
CO2 reduction to CO mediated by GBs; the broken local spatial symmetry near a GB tunes the Au metal-to-adsorbate πbackbonding ability, thereby stabilizing the key COOH intermediate. This stabilization leads to a decrease of ∼200 mV in the
overpotential and a change in the rate-determining step to the second reduction step, of which are consistent with previous
experimental observations. The atomistic and electronic details of the mechanistic role of GBs during electrochemical CO2
reduction presented in this work demonstrate the structure−activity relationship of atomically disordered metastable structures
in catalytic applications.
KEYWORDS: grain boundary, CO2 reduction, electrochemical reduction, density functional theory, Au surface
■
INTRODUCTION
Over the past decades, human beings have relied on fossil fuels
for over 80% of their total energy needs, and, at present, carbon
dioxide (CO2) accumulation in the atmosphere has led to the
demand for renewable energy sources to replace fossil fuels.1−3
Consequently, much research has focused on developing
various chemistries to transform excess CO2 into various
carbon frameworks with chemical energy. Among several
ongoing attempts, electrochemical conversion technology is
advantageous because of its potentially high reactivity and
efficiency at ambient conditions and its ease of scale-up to
large-scale processes when combined with renewable energy
harvesting technologies such as photovoltaics.4−7
In particular, the selective formation of carbon monoxide
(CO) has received much attention because not only is
electrochemical CO2 reduction to CO, which is a two-electron
process, the simplest reduction reaction pathway of the various
routes, but CO is also an industrially important gas product
with broad applications in chemical manufacturing,8 as a
bioregulator in medicine9 and as a lasing medium in highpowered infrared lasers.10 However, no satisfactory solution
with practical applicability has yet been found. A key obstacle is
the limited catalyst performance in terms of activity, selectivity,
© 2016 American Chemical Society
and stability. Although the thermodynamic potential for CO2
reduction to CO requires only −0.11 VRHE, the overpotentials
of selective CO-forming catalysts, e.g., Au and Ag, are as high as
610 mV and 840 mV, respectively.11
In 2012, Kanan et al. reported that electrochemically
oxidizing and then reducing an Au catalyst, which was denoted
an oxide-derived Au (OD-Au) catalyst, resulted in a dramatic
improvement in its catalytic activity and a substantial decrease
of ∼200 mV in the overpotential.12 Their follow-up study in
2015 further revealed that the grain boundary (GB) density on
the Au surface and the electrochemical catalytic activity are
strongly correlated.13 It should be noted that atomic disorder at
grain boundaries has been proposed to enhance the catalytic
activity in several different reactions.14−22 For example, the
abundance of grain boundaries and stacking faults was
discussed to lead to the high activity of silver-supported
catalysts in ethylene epoxidation.22 However, an atomistic and
electronic level of mechanistic understanding of the enhanced
catalytic activity in CO2 reduction at grain boundaries has not
Received: February 9, 2016
Revised: May 2, 2016
Published: May 31, 2016
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Au crystal and performing an inversion operation on the
exposed {112} face as plane of symmetry.
The model system consisted of 3 atomic layers containing a
total of 150 Au atoms. Due to the periodic boundary conditions
of the simulation cell, two domains in the model formed two
GB lines. It should be noted that the GB lines were not
identical; one (lower GB line in Figure 1b) was concave with
the atoms near the GB line slightly caved into the surface,
whereas the other (upper GB line in Figure 1b) was convex
with the atoms near the GB line slightly protruding from the
surface. In the following discussion, the atop Au catalytic sites
are labeled from t1 to t9 in increasing order, with t1 located
near the concave GB, t5 located in the middle, i.e., the bulk-like
regime, and t9 located near the convex GB.
The SIESTA program28 was employed for the large-scale
DFT calculations using the Perdew−Burke−Ernzerhof (PBE)
exchange-correlation functional29 and numerical atomic orbital
basis sets. The free energies were calculated by diagonalizing
the partial Hessian of the adsorbate, and the solvation energies
were taken into account using the Delphi program,30,31 which
solves the Poisson−Boltzmann equation numerically (further
simulation details are fully described in the Supporting
Information).
The electrochemical CO2 reduction pathway to CO includes
the following elementary steps23,32
yet been addressed. Therefore, fundamental questions about
the relationship between the metastable surface structure and
the catalytic activity remain.
In this work, first-principles density functional theory (DFT)
calculations were performed to model the atomistic structure of
GBs on the Au (111) surface and to investigate electrochemical
CO2 reduction to CO at GBs. Based on a theoretical model for
estimating the CO2 reduction potential on metal surfaces,23 the
potential variance along the GB surface was estimated, and the
atomic and electronic structures were further analyzed to
determine the role of the grain boundary. It was found that the
broken local spatial symmetry at the GB tunes the adsorbate-tometal σ-bonding and metal-to-adsorbate π-backbonding
strengths on the Au surface relative to those of the bulk Au.
This change in chemical bonding ability at the GB helps to
stabilize the key COOH intermediate, resulting in enhanced
catalytic activity in CO2 reduction.
■
RESULTS AND DISCUSSION
An atomistic model of the Σ3 {112} high-angle grain boundary
(HAGB) on the Au (111) surface was constructed as shown in
Figure 1. Of the many possible HAGBs, the Σ3 {112}
CO2 (aq) + [H+ + e−] → *COOH (aq)
(R1)
*COOH (aq) + [H+ + e−] → *CO (aq) + H 2O (aq)
(R2)
*CO (aq) → CO (g)
(R3)
where the asterisk (*) denotes adsorbed species on the catalyst
surface. In our previous study, we decomposed the reaction free
energy (ΔG°) of each elementary step ((R1), (R2), and (R3))
into the COOH and CO binding free energies (ΔGbCOOH and
ΔGbCO, respectively) under the bias potential U (VRHE):
ΔG°(R1) = 2.02 + U + ΔGbCOOH, ΔG°(R2) = −1.07 + U +
ΔGbCO − ΔGbCOOH, and ΔG°(R3) = −0.50 − ΔGbCO, in eVs.23
In this study, ΔGbCOOH and ΔGbCO were calculated using an
Au surface model with GBs and were then converted into ΔG°
(U = 0). The results are listed in Table 1, in which the most
thermodynamically unfavorable step of the three steps at the
zero-bias limit is highlighted. The COOH binding affinity for
Au atoms near the concave GB (t1 and t2 sites) is markedly
enhanced by 270−300 meV compared to the clean Au (111)
surface. The COOH binding affinity for Au atoms near the
convex GB (t7, t8, and t9 sites) is more profoundly enhanced
by 800−1300 meV, causing the first reduction step to be
spontaneous even under zero bias; however, the substantial
Figure 1. (a) Tilted view (2 × 1 × 1 supercell) and (b) side and top
views (1 × 1/2 × 1 supercell) of the atomistic model of the Σ3 {112}
high-angle grain boundary (HAGB) on the Au (111) surface. The
three atomic layers contain 150 atoms. Two bulk domains form two
GB lines (concave, convex), which are indicated by the brown and
ivory atoms, respectively. The 9 atop catalytic active sites are labeled as
t1−t9 and shown in magenta.
symmetric tilt HAGB was chosen for this study because Σ3
GBs are known to be the most abundant HAGB on Au surfaces
from experimental analyses of the grain boundary character
distribution (GBCD).24−27 Additionally, the Σ3 {112} GB is
the only Σ3 GB that can be constructed mathematically on a
clean Au (111) surface. The Σ3 {112} HAGB was constructed
by exposing the {112} face of a bulk face-centered cubic (FCC)
Table 1. Binding Free Energies and Zero-Bias Reaction Free Energies at t1−t9 Sitesa
a
The most unfavorable step of the three elementary steps (R1, R2, R3) is highlighted in red.
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like regime at the t5 site. At active sites near the GBs (t1, t7, t9
sites), however, the first reduction step becomes spontaneous,
whereas the second reduction or CO desorption step becomes
uphill (has a low rate; is rate-determining). Assuming that the
second electron transfer from the electrode to the molecule is
not finished before complete CO desorption from the catalyst
surface, the rate-determining step (RDS) changes from the first
reduction step to the second reduction step at the active sites
near the GBs, which is consistent with the experimentally
observed change in the Tafel slope from ∼120 mV·dec−1 for
polycrystalline Au to ∼60 mV·dec−1 for OD-Au.12
Additional calculations on the feasibility of further reduction
of CO to COH/CHO at t7 and t8 sites are shown in Figure S7.
The significant downhill process followed by uphill with
substantial energy cost (>0.3 eV) at catalytic active sites near
the convex GB (t7, t8, and t9 sites) in Figure 2a and Figure S7
implies that these sites will be immediately occupied by
intermediate species. The bound species being neither
desorbed nor further reduced, the sites near the convex GB
are expected to hardly participate in the electrochemical activity
during catalytic cycles. On the other hand, the thermodynamics
involved in the turnover process of CO2 at sites near the
concave GB (e.g., t1 and t2 sites) are feasible and relevant to
the experimentally observed electrochemical activity.
The reaction free energies of the CO2 reduction steps change
dramatically when a GB is introduced, because it alters the
correlation between ΔGbCOOH and ΔGbCO on metal surfaces.
Au atoms near GBs can more effectively stabilize COOH over
CO, resulting in a new correlation between ΔGbCOOH and
ΔGbCO, which is compared to the preexisting correlation line
for various clean metal (i.e., Zn, Ag, Au, and Cu) surfaces in
Figure 2b.
To understand the origin of the efficient stabilization of
COOH over CO at Au atoms near the GBs, the electronic
structures were analyzed. The density of states (DOS) of the
Au surface with GBs was compared to that of the clean Au
surface (Figure S1) in terms of peak positions, shapes, and
location of the d-band centers.34 No obvious differences are
observed, suggesting that the interaction between the metal
band and adsorbate (usually characterized using d-band theory)
cannot fully explain the specific COOH binding affinity for the
Au atoms near the GBs. Instead, the adsorbate-metal bonding
must be understood based on a more localized orbital−orbital
interaction concept.35 Indeed, the adsorbate binding results in
two localized states (sharp peaks in the DOS) at E-Ef = −7.3/−
6.3 eV for COOH binding (Figure 3a) and E-Ef = −8.1/−7.4
eV for CO binding (Figure 3b). Projected density of states
(PDOS) analysis further reveals that the lower-energy state is
due to the carbon pz orbital and Au dz2 orbital (Figure 3e and
Figure 3f), whereas the higher-energy state is due to the carbon
px/py orbital and Au dxz/dyz orbital (Figure 3c and Figure 3d)
(the surface normal direction was chosen as the z-direction).
Based on the metal−ligand bonding theory of organometallic
catalysts, the lower-energy state can be assigned to the
adsorbate-to-metal σ-bonding state, whereas the higher-energy
state is the metal-to-ligand π-backbonding state. These
assignments are further confirmed by the real-space visualization of these states in Figure S4.
The occupations of the σ-bonding and π-backbonding states
when COOH and CO are adsorbed at each catalytic site of t1−
t9 were calculated by integrating the DOS peaks. Figure 4a and
Figure 4b show the variation of relative state occupations with
the state occupations of clean surface as zero-base. It should be
increase in the CO binding affinity near the convex GB makes
either the second reduction step or the final CO desorption
step thermodynamically unfavorable.
To illustrate the effects of these changes in the binding
affinity near the GBs, the reaction free energy profiles at four
different active sites (t1, t7, and t9 near the GBs and t5 in the
middle) were constructed and are compared to that of the clean
Au (111) surface in Figure 2a. The first two steps in the energy
Figure 2. (a) Representative reaction free energy profiles for the
following catalytic sites under a finite bias potential of U = −0.4 VRHE:
t1 (red) near the concave GB, t7 (blue) and t9 (green) near the
convex GB, and t5 (magenta) in the bulk regime. For comparison, the
reaction free energy profile calculated for the clean Au (111) surface
with no GB is also shown. (b) Correlation between the COOH and
CO binding free energies (ΔGbCOOH and ΔGbCO, respectively). For
comparison, the correlation line for GB-free clean metal (Cu, Au, Ag,
and Zn) surfaces is presented,23 showing that the COOH intermediate
is more effectively stabilized relative to CO at GBs. The contour map
shows the theoretical reduction potential determined using ΔGbCOOH
and ΔGbCO following the model described in ref 23.
profile are reduction steps ((R1) and (R2)), and the last step is
CO desorption (R3). The bias potential of U = −0.4 VRHE was
chosen (cf. the operation potential of OD-Au was −0.35 VRHE
in the experiment12). Under finite bias potential, CO2 reduction
at the t1 site consists of two downhill processes followed by a
slight uphill process requiring 45 meV for CO desorption.
Considering the fact that the first reduction step on the clean
surface requires 260 meV, the decrease of ∼200 mV in the
overpotential observed in the experiments can be explained by
reduction at the t1 site.12
Because it is difficult to disrupt the stable sp-hybridization
symmetry of the carbon atom to bend the linear CO2
molecule,33 the first reduction step is usually rate-determining
as observed on the clean Au (111) surface and also in the bulk4445
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Figure 3. Au density of states (DOS) when (a, c, e) COOH and (b, d, f) CO are adsorbed at the t1 site located near the grain boundary. (a) and (b)
Au DOS at the t1 site before (black) and after (red) adsorption. The DOS of the C atom is also shown in blue. As shown by the projected density of
states (PDOS) in (c)-(f), the total Au DOS in this energy range is mostly due to the Au d-band, and the two sharp peaks that appear after adsorbate
binding are due to dz2-pz (lower-energy peak) and dxz/yz-px/y (higher-energy peak) interactions.
noted that the state occupation is proportional to the strength
of each binding mode.35−37 The σ-bonding strength remains
nearly constant at all the binding sites from those near the GBs
to that in the bulk regime (Figure 4a). In contrast, the πbackbonding strength increases significantly when the adsorbate binds to Au sites near the GBs, particularly in the case of
COOH binding (Figure 4b). Similar trends are also observed
for the site-dependent binding energies (Figure S5). To
confirm that the π-backbonding affinity near the GBs is
stronger, the change in the C−O bond length was calculated
and is shown in Figure 4c (a longer C−O bond corresponds to
stronger metal-to-adsorbate π-backbonding). The results follow
a similar trend to that of changes in the π-backbonding state
occupation.
In highly symmetric crystalline structures, the atomic orbitals
of the metal atoms exhibit substantial overlap, resulting in an
energy band as described by the linear combination of atomic
orbitals (LCAO) theory. To exploit the Au dz2 or dxz/dyz
orbitals to form a σ- or π-bond, respectively, with an adsorbate,
the atomic orbital from the band must be localized, which
requires energy. Based on the orbital shapes due to the Au
atom arrangement in the topmost layer of the (111) surface
(hexagonal close-packed layer in the xy plane), it is expected
that the in-plane orbital−orbital overlap of the dxz/dyz orbitals is
greater than that of the dz2 orbitals, suggesting that the energy
cost of ΔEloc. is larger for π-backbond formation. The
introduction of a GB is expected to break the local symmetry,
leading to a decrease in ΔEloc. for π-backbond formation at the
GB and thus an increase in the π-backbonding affinity. Figure
5a and Figure 5b show the average nearest-neighbor distances
for the surface Au atoms and the corresponding standard
deviations, respectively. The results clearly demonstrate that the
local symmetry of the close-packed surface is broken at the GB,
which leads to stronger metal-to-adsorbate π-backbonding. The
local symmetry is distorted more substantially near the convex
GB.
Figure 4. Relative state occupations of the (a) σ-bonding (lowerenergy peak in the DOS) and (b) π-backbonding (higher-energy peak
in the DOS) states. The relative state occupation of parts a and b
means the state occupation of each catalytic site minus the state
occupation of the clean surface. The π-backbonding strength increases
noticeably when COOH binds near a GB. (c) The C−O bond length,
another indicator of the π-backbonding strength, also demonstrates
the enhanced π-backbonding near the GBs.
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possibility exists that the remaining oxygen affects the catalytic
activity of oxide-derived metal catalysts, though presumably not
significant in the noble metal cases (e.g., Au, Ag). It will be of
interest in future studies. We further anticipate that the results
of this study provide new insight into the origin of the superior
(electro)chemical catalytic activity at GBs based on chemical
bonding concepts, which can be further utilized to design
advanced catalysts with controlled grain boundaries.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00412.
Computational details and method to construct an
atomistic model of grain boundary; changes of density
of states before and after COOH/CO adsorption; wave
function density maps of localized states (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
Figure 5. (a) Average nearest-neighbor distances and (b) corresponding standard deviations. The values (Å) are relative to those on the
clean surface. The local symmetry of the surface, which is composed of
top-layer nearest neighbors and second-layer nearest neighbors, is
broken to a greater degree at the convex GB than at the concave GB,
which leads to stronger COOH binding at the convex GB.
*E-mail: [email protected] (E.K.L.).
*E-mail: [email protected] (H.K.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the Global Frontier R&D Program
(2013M3A6B1078884) of the Center for Hybrid Interface
Materials (HIM) funded by the Ministry of Science, ICT &
Future Planning, and also by the New & Renewable Energy
Core Technology Program of the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) granted
financial resource from the Ministry of Trade, Industry &
Energy, Republic of Korea (20153030031720).
However, the presence of a GB has a greater effect on the
Au-COOH π-backbonding strength than on the Au-CO πbackbonding strength (see Figure 4b and Figure 4c), due to the
symmetry of the π* orbital of the adsorbate species. The linear
CO molecule, which has degenerate π*xx and π*yy orbitals,
adsorbs on an Au atom along the surface normal z-direction. In
contrast, the COOH π* orbitals are restricted to be either
parallel or normal to the COOH molecular plane. Metal-toadsorbate bond formation requires that the adsorbate π* orbital
be properly aligned with the metal dxz/dyz orbitals to maximize
the orbital−orbital overlap. In contrast to the dxz/dyz orbitals of
a single Au atom, the rotational invariance around the z-axis of
the dxz/dyz orbitals of an Au atom at the surface is broken
because of the local environment. Consequently, while the
degenerate CO π* orbitals can readily achieve maximal overlap
with the dxz/dyz orbitals in the presence or absence of a GB by
rotating the orbitals around the z-axis, the rotationally variant
COOH π* orbitals cannot. Therefore, the breaking of the local
symmetry near a GB is expected to have a more profound effect
on COOH adsorption than on CO adsorption.
■
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CONCLUSION
The origin of the superior catalytic activity in electrochemical
CO2 reduction observed at the Σ3 {112} high-angle grain
boundary on the Au (111) surface was elucidated using DFT.
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