Boosting Activity and Selectivity of CO2 Electroreduction by Pre-Hydridizing Pd Nanocubes

Article abstract of DOI:10.1002/smll.202005305

The electrochemical CO₂ reduction reaction (CO₂RR) to syngas represents a promising solution to mitigate CO₂ emissions and manufacture value-added chemicals. Palladium (Pd) has been identified as a potential candidate for syngas production via CO₂RR due to its transformation to Pd hydride under CO₂RR conditions, however, the pre-hydridized effect on the catalytic properties of Pd-based electrocatalysts has not been investigated. Herein, pre-hydridized Pd nanocubes (PdH_0.40) supported on carbon black (PdH_0.40 NCs/C) are directly prepared from a chemical reduction method. Compared with Pd nanocubes (Pd NCs/C), PdH_0.40 NCs/C presented an enhanced CO₂RR performance due to its less cathodic phase transformation revealed by the in situ X-ray absorption spectroscopy. Density functional theory calculations revealed different binding energies of key reaction intermediates on PdH_0.40 NCs/C and Pd NCs/C. Study of the size effect further suggests that NCs of smaller sizes show higher activity due to their more abundant active sites (edge and corner sites) for CO₂RR. The pre-hydridization and reduced NC size together lead to significantly improved activity and selectivity of CO₂RR.

Full text of DOI:10.1002/smll.202005305

Full PaPer
www.small-journal.com
Boosting Activity and Selectivity of CO Electroreduction
2
by Pre-Hydridizing Pd Nanocubes
Qiaowan Chang, Jeonghyeon Kim, Ji Hoon Lee, Shyam Kattel,* Jingguang G. Chen,*
Sang-Il Choi,* and Zheng Chen*
from the utilization of fossil fuels.[1] The
electrochemical CO2 reduction reac-
The electrochemical CO reduction reaction (CO RR) to syngas represents a
2
2
promising solution to mitigate CO emissions and manufacture value-added
chemicals. Palladium (Pd) has been identified as a potential candidate for
tion (CO RR), combined with renew-
2
2
able energy sources (i.e., solar and wind),
[
2–5]
represents a promising solution.
In
syngas production via CO RR due to its transformation to Pd hydride under
2
CO RR, CO2 can be converted into CO,
2
CO RR conditions, however, the pre-hydridized effect on the catalytic proper-
2
hydrocarbons, or oxygenates. Since the
reaction is generally performed under
cathodic conditions in aqueous electrolyte,
ties of Pd-based electrocatalysts has not been investigated. Herein, pre-hydrid-
ized Pd nanocubes (PdH0.40) supported on carbon black (PdH0.40 NCs/C)
are directly prepared from a chemical reduction method. Compared with Pd
H is inevitably produced as a byproduct
2
from the competing hydrogen reduction
reaction (HER). While significant efforts
have been made on prohibiting HER by
designing sophisticated electrocatalysts,
electrodes, or cell structures, low selec-
nanocubes (Pd NCs/C), PdH0.40 NCs/C presented an enhanced CO RR per-
2
formance due to its less cathodic phase transformation revealed by the in situ
X-ray absorption spectroscopy. Density functional theory calculations revealed
different binding energies of key reaction intermediates on PdH0.40 NCs/C
and Pd NCs/C. Study of the size effect further suggests that NCs of smaller
sizes show higher activity due to their more abundant active sites (edge and
tivity toward CO RR remains a critical
2
[
6–8]
challenge.
Instead of suppressing
HER, combining the CO RR and HER
2
corner sites) for CO RR. The pre-hydridization and reduced NC size together
2
together has been considered as a viable
approach, which produces syngas (CO and
H ) with tunable CO/H molar ratios that
lead to significantly improved activity and selectivity of CO RR.
2
2
2
can be readily used to manufacture value-
added chemicals using exiting thermochemical processes, such
1
. Introduction
[
9–12]
as the Fischer–Tropsch and methanol synthesis reactions.
Palladium (Pd) has been identified as a potential candidate
Efficient CO storage and utilization technologies are urgently
needed to mitigate the global climate change and ocean acidi-
2
[
9,13–17]
for the production of syngas via CO RR.
While Pd itself is
2
fication issues caused by the continuing increase of CO levels
a good HER catalyst, it was considered to be unsuitable for CO
2
Q. Chang, Prof. Z. Chen
Prof. J. H. Lee, Prof. J. G. Chen
Department of Chemical Engineering
Columbia University
Department of NanoEngineering
University of California San Diego
La Jolla, CA 92093, USA
New York, NY 10027, USA
E-mail: zhengchen@eng.ucsd.edu
E-mail: jgchen@columbia.edu
Q. Chang, Prof. Z. Chen
Prof. S. Kattel
Program of Chemical Engineering
University of California San Diego
La Jolla, CA 92093, USA
Department of Physics
Florida A&M University
Tallahassee, FL 32307, USA
E-mail: shyam.kattel@famu.edu
J. Kim, Prof. S.-I. Choi
Department of Chemistry and Green-Nano Materials Research Center
Kyungpook National University
Daegu 41566, Republic of Korea
E-mail: sichoi@knu.ac.kr
Prof. J. G. Chen
Chemistry Division
Brookhaven National Laboratory
Upton, NY 11973, USA
Prof. J. H. Lee
Prof. Z. Chen
School of Materials Science and Engineering
Kyungpook National University
Daegu 41566, Republic of Korea
Sustainable Power, and Energy Center
University of California, San Diego
La Jolla, CA 92093, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202005305.
DOI: 10.1002/smll.202005305
Small 2020, 2005305
2
005305 (1 of 10)
© 2020 Wiley-VCH GmbH

www.advancedsciencenews.com
www.small-journal.com
production in CO RR due to its strong CO binding that leads
morphology of monodispersed Pd and PdH0.40 NCs. After the
hydrogenation conversion from Pd to PdH0.40 NCs, the orig-
inal Pd cubic morphology could be retained, indicated by the
similar average edge length of Pd (7.1 ± 0.8 nm) and PdH0.40
(7.3 ± 0.5 nm) NCs, and their high shape uniformity (>90%)
(Figure S1, Supporting Information). The high-resolution TEM
(HRTEM) images for an individual Pd and PdH0.40 NC pre-
sented dominant Pd(200) facet exposed on both the NC surfaces
(Figure 1c,d). Due to the insertion of H into the Pd lattice, the
lattice spacing for the Pd(200) plane was increased from 1.94 Å
(Pd NCs) to 1.99 Å (PdH0.40 NCs) as shown in Figure 1e–f,
in good agreement with the X-ray diffraction (XRD) results
(Figure S2, Supporting Information). The elemental composi-
tion of PdH0.40 NCs was determined by the linear relationship
2
to CO poisoning.[ Sheng et al. revealed that Pd can be trans-
18]
formed into palladium hydride (PdH ) that presents decreased
1
*
CO and *H binding energies, which in turn adjusts the CO/H2
ratios from the CO RR and thus makes Pd a potential electro-
2
[9]
catalyst for syngas production. Moreover, the CO/H ratio can
2
be further improved by using Pd-based bimetallic catalysts and/
or supporting Pd on metal carbides and nitrides.[
11,13,19]
The key
to tune the CO/H ratio over Pd-based electrocatalyst is to con-
2
trol the formation of the PdH phase, specifically, the hydride
1
formation potentials during the CO RR. Lee et al. found that by
2
alloying with first-row transition metals, the hydride formation
potentials of PdM (M = Cu, Ni) were cathodically shifted, ena-
bling the syngas production with the CO/H ratios between 1
2
[
13]
[21]
and 2 at 0.9 V (versus reversible hydrogen electrode or RHE).
of lattice distance with hydrogen content (Equation S1 and
In spite of these prior investigations, the electrocatalytic per-
Table S1, Supporting Information). The XRD patterns showed
no peak shift after storing PdH0.4 NCs at room temperature in
air even for 12 months (Figure S3, Supporting Information),
formance of PdH has been rarely explored with different H
x
contents, especially in a viewpoint of the structure–property–
activity relationship. Moreover, it is not a trivial task to further
enhance its CO RR activity and tune the CO/H ratio in a range
[
22]
indicating their good stability.
The electronic structures of the Pd and PdH0.40 NCs were
revealed by the core-level and valence band spectra from X-ray
photoelectron spectroscopy (XPS). Compared with Pd NCs,
the Pd 3d core level spectrum of PdH0.40 NCs was shifted to
a higher binding energy (Figure S4a, Supporting Information),
and its valence band spectrum presented a decreased band-
width and emergence of two small shake-up peaks (marked as
red arrows, Figure S4b, Supporting Information). These phe-
nomena could be attributed to the reduction of the density of
state near the Fermi level of PdH0.40 relative to the Pd coun-
terpart, specifically, the charge transfer between fully filled Pd
2
2
of 1–2 (desired for the thermochemical processes) in the low
overpotential range (between −0.5 and −0.7 V).[
13,17]
In this work, we succeeded in overcoming the above chal-
lenges via a prehyridization process, using PdH0.40 nanocubes
supported on carbon black (PdH0.40 NCs/C) that were directly
prepared from a chemical reduction method to in situ elucidate
the structure–property–activity relationship of PdH during the
x
CO RR. The prehydridization process not only makes PdH
2
0.40
NCs/C readily for the in situ hydride phase transformation
during CO RR but also optimizes the hydride formation poten-
2
[
23–24]
tials. Compared with Pd nanocubes supported by carbon (Pd
NCs/C), the CO/H ratios and CO RR activity of PdH NCs/C
4d orbital and half-empty H 1s orbitals.
confirmed the formation of Pd hydride associated with the
charge transfer between Pd and H.
These XPS results
2
2
0.40
[
20]
with a similar size were enhanced by 77% and 220% at the low
overpotential (i.e., −0.6 V), respectively. Moreover, to achieve
To perform an electrochemical test, 7 nm Pd and PdH0.40
NCs were supported on carbon black (Pd NCs/C and PdH0.40
a CO/H ratio between 1 and 2, the suitable potential range
2
for syngas production with 7 nm PdH0.40 NCs/C catalyst was
extended by 300 mV (from −0.5 to −0.8 V) compared with Pd
NCs/C. In situ X-ray absorption fine structure (XAFS) analysis
showed that the extended potential with more favorable CO/H2
ratios for PdH0.40 NCs/C was due to its less cathodic phase
NCs/C). The CO RR activity and selectivity of 7 nm Pd and
2
PdH0.40 NCs/C catalysts toward gaseous products were com-
pared by the chronoamperometry (CA) test and gas chromatog-
raphy (GC) quantification under the operating potential range
between −0.5 and −1.0 V (Figure S5, Supporting Information).
Figure 2a showed that the total Faradaic efficiencies (FEs) of
transformation to the stoichiometric PdH . Density functional
1
theory (DFT) calculations revealed that the binding energies
CO (FECO) and H (FEH2) were in the range of 88–100% for
2
of key CO RR intermediates, such as *HOCO and *CO, were
both samples, indicating that CO and H were the major prod-
2
2
different on PdH0.40 and Pd, leading to enhanced activity and
selectivity on PdH0.40. We also found a strong size effect for
both Pd and PdH0.40 NCs/C, showing an increased CO/H2
ratios with a smaller particle size. Our work demonstrates the
critical role of the prehydridization over Pd-based electrocata-
ucts. With increasing overpotential, the trends for the changes
in FEH2 and FECO were different. From −0.6 V, both electro-
catalysts presented decreased FECO and increased FEH2 toward
higher potential, leading to reduced CO/H ratios (Figure 2b).
2
The decrease of FECO could be ascribed to the limited CO sol-
2
lyst for CO RR, opening up new directions for improving the
ubility during the CO RR, thus incurring the mass transport
2
2
activity and selectivity for syngas production.
limitation of reactant to the electrode surface, while for FEH2,
the reaction kinetics of HER was not limited by the diffusion of
the proton donor in the aqueous electrolyte.
When comparing the CO RR performance between Pd and
PdH0.40 NCs/C, the advantage of the prehydridized structure
2
2
. Results and Discussion
PdH0.40 NCs were obtained from the hydrogenation of as-pre-
(PdH0.40 NCs/C) was manifested by the tunable CO/H ratios
2
pared Pd NCs with the assistance of N,N-dimethylformamide
for syngas production with a significantly enhanced partial
(
DMF) and polyvinyl pyrrolidone (PVP) (see the details in sup-
Transmission electron microscopy
TEM) images (Figure 1a,b) showed the well-controlled cubic
current density of CO. For example, at −0.6 V the CO/H ratio
obtained over Pd NCs/C was 0.95, which was increased to 1.68
over PdH0.40 NCs/C. To obtain the syngas production with the
2
[20]
porting information).
(
©
2020 Wiley-VCH GmbH
Small 2020, 2005305
2005305 (2 of 10)

www.advancedsciencenews.com
www.small-journal.com
Figure 1. TEM images of a) Pd and b) PdH0.40 NCs of 7 nm in norminal size. HRTEM images of c) a Pd and d) a PdH0.40 NCs. Intensity profiles recorded
from the area indicated by the rectangular box in HRTEM image for e) a Pd and f) a PdH0.40 NC.
CO/H ratio in the range of 1 to 2, the suitable potential for Pd
2.6 times of the Pd counterparts at −0.6 V. Considering the
smaller electrochemical surface area (ECSA) of PdH0.40 NCs/C
due to the exposed surface *H species (Figure S7, Supporting
Information), the JCO based on ECSA of Pd (jCO-Pd) of 7 nm
PdH0.40 was further increased by 2.2-fold compared with that
of Pd NCs/C at −0.6 V, suggesting a significantly improved
2
NCs/C was allowed at −0.5 V only, while for PdH0.40 NCs/C,
the potential range was extended from −0.5 to −0.8 V, making
it more favorable to the syngas production from CO RR. More-
2
over, the partial current density of CO (JCO) based on geometric
electrode area of PdH0.40 NCs/C also showed considerable
enhancement compared with Pd NCs/C (Figure S6, Supporting
Information). For instance, the JCO of PdH0.40 NCs/C was
CO RR activity. Both Pd and PdH
NCs/C presented good
2
0.40
stability at −0.7 V (Figure S8, Supporting Information).
Small 2020, 2005305
2
005305 (3 of 10)
© 2020 Wiley-VCH GmbH

www.advancedsciencenews.com
www.small-journal.com
Ex situ XAFS analysis was first performed to compare the
chemical nature of Pd and PdH0.40 NCs/C. Compared with Pd
NCs/C, PdH0.40 NCs/C showed negative shift of X-ray absorp-
tion near edge structure (XANES) profile and longer PdPd
bond distance from extended X-ray absorption fine structure
(EXAFS) result, demonstrating the successful formation of Pd
hydride (Figure S9 and Table S2, Supporting Information). In
an attempt to monitor the change of the local chemical environ-
ment of Pd and PdH0.40 NCs/C during the CO RR, in situ XAFS
2
analysis was then conducted (Figure 3) to investigate the origin
of the enhanced CO RR performance of PdH
NCs/C. Upon
2
0.40
decreasing the potential for the electrochemical reduction from
.6 V to −0.9 V, both Pd and PdH0.40 NCs/C showed the gradual
0
shift in the XANES profiles (Figure 3a,b) toward lower energy
concurrently with an increase in the Pd–Pd interatomic distance
as shown in the EXAFS profiles (Figure 3c–d, and Figure S10,
Supporting Information). This can be attributed to the H uptake
and consequent lattice expansion associated with the PdH for-
1
mation. However, the prehydridization process was found to
play a role in the onset potential for the PdH formation. The
1
phase transition for Pd NCs/C took place at −0.7 V as evidenced
by the shifts in both XANES and EXAFS profiles. For the PdH0.40
NCs/C, this phase transition occurred at less cathodic potential
of −0.3 V, indicating that the PdH0.40 NCs more readily accepted
the additional H compared to Pd NCs. The different H uptake
manners could be more explicitly displayed in the change of
the Pd–Pd interatomic distance for both catalysts as a function
of applied potential (Figure 3e), illustrating the distinct electro-
chemical onset potentials toward the PdH formation.
1
The facile transformation of PdH0.40 to PdH can be inter-
1
preted by several factors. One can speculate that the H uptake
involved in the PdH formation entails an energy penalty related
to (1) the mix of Pd and H orbitals, (2) the increase in the Pd–
Pd distance, and (3) the resultant multiple H diffusion into
the Pd lattice. Therefore, the prehydridization process should
be efficient in mitigating the penalty by mixing the orbitals of
Pd and H and increasing the Pd–Pd interatomic distance in
advance, thus allowing the PdH formation to occur at a lower
potential (reduced by 300 mV). This is further supported by the
linear sweep voltammetry (LSV) curves (Figure S11, Supporting
Information). Both catalysts showed the broad redox peaks
associated with the PdH formation. Compared with Pd NCs/C,
the peak in PdH0.40 NCs/C was shifted positively by ≈40 mV.
Furthermore, the earlier PdH1 formation endowed PdH0.40
with better tolerance against CO-poisoning, thus leading to a
more rapid response of voltammetric current and an improved
CO RR activity of PdH NCs/C after −0.8 V, even though both
2
0.40
Figure 2. Electrocatalytic performance of 7 nm Pd and PdH0.40 NCs/C
electrocatalysts presented the same stoichiometry of PdH . The
1
from −0.5 to −1.0 V. a) Faradaic efficiency of CO and H . b) CO/H ratio.
2
2
CO stripping experiment was conducted to investigate the CO
anti-poisoning capability for both samples. (Figure S12, Sup-
porting Information). Compared with Pd NCs/c, the main
CO oxidation peak of PdH0.40 NCs/C was negatively shifted by
≈20 mV, implying its enhanced CO anti-poisoning effect from
the hydride structure. We also evaluated the CA of the PdH0.40
in 0.1 M KOH solution at 1.2 V for 5 min and compared the
phase and composition before and after the test using XRD
(Figure S13, Supporting Information). The results showed no
peak shift during CA test, indicating that PdH phase is stable
under the whole CO-stripping potential.
c) Partial current density of CO based on ECSA (jCO-Pd).
It is well-established that Pd can be readily transformed to
[9]
PdH at the electrochemical potentials for the CO RR. Accord-
1
2
ingly, the active phase of both Pd and PdH0.40 NCs/C in the
CO RR should be PdH NCs. In that case, one should expect
2
1
similar CO RR performance for the Pd and PdH
NCs/C
2
0.40
electrocatalysts. However, experimental results showed sig-
nificantly different CO RR performance, with PdH
NCs/C
2
0.40
being far superior to Pd NCs/C.
©
2020 Wiley-VCH GmbH
Small 2020, 2005305
2005305 (4 of 10)

www.advancedsciencenews.com
www.small-journal.com
2
Figure 3. In situ XAFS profiles measured at Pd K-edge. Normalized XANES profiles for a) 7 nm Pd NCs/C and b) PdH0.40 NCs/C. k -weighted EXAFS
spectra for c) 7 nm Pd NCs/C and d) PdH0.40 NCs/C. e) The interatomic distance (RPdPd) profiles for Pd and PdH0.40 NCs/C as a function of applied
potential determined from in situ EXAFS analysis.
It is observed that an optimal catalyst particle size exists in
all PdH0.40 catalyst exhibited enhanced CO/H ratios and J
2 CO
compared with the corresponding Pd NCs of similar sizes
(Figures S17 and S18, Supporting Information). The pheno-
[8,15]
many catalytic reactions.
Thus, the size effect of Pd and
PdH0.40 NCs for CO RR was also investigated. By adjusting
2
the amount of KCl and KBr used for the synthesis of Pd NCs,
menon of earlier PdH peak formation from PdH
shown by
1
0.40
1
3 and 19 nm Pd NCs were obtained (Figures S14–S16, Sup-
LSV curves was also observed in 13 and 19 nm NCs (Figure S19,
Supporting Information). Moreover, 13 and 19 nm PdH0.40 NCs
also presented improved tolerance against CO poisoning as
shown from the negatively shifted CO oxidation peak in the CO
stripping experiment (Figure S20, Supporting Information). It
porting Information). These Pd NCs were also transformed to
PdH0.40 NCs under the same condition as that for 7 nm PdH0.40
(Figures S14–S16, Supporting Information). Figure 4 shows
that the beneficial role of the prehydridization process of the
Pd NC catalysts in CO RR was still valid regardless of the par-
was noted that with increased particle size, the CO/H ratios of
2
2
ticle size. When the particle size increased to 13 and 19 nm,
Pd and PdH0.40 NCs/C both decreased (Figure 4a,b), implying
Small 2020, 2005305
2
005305 (5 of 10)
© 2020 Wiley-VCH GmbH

www.advancedsciencenews.com
www.small-journal.com
Figure 4. Size-dependent CO RR performance from −0.5 to 1.0 V. CO/H ratios obtained from a) Pd NCs b) PdH NCs. c) Partial current density of
2
2
0.40
CO based on ECSA (jCO-Pd) of all the prepared electrocatalysts.
a hindered CO RR rate for the large NCs. Due to the different
DFT calculations were performed to determine the lat-
tice parameter of PdHx with various H concentrations (x =
0–100%). In agreement with the experimental measurements,
the DFT results (Table S5, Supporting Information) predicted
that the lattice parameter and Pd–Pd bond length increased
2
ECSAs of Pd and PdH0.40 NCs/C, the jCO-Pd of all samples was
also compared and showed the similar trends for both Pd and
PdH0.40 NCs: 7 nm > 13 nm > 19 nm (Figure S21 and Table S3,
Supporting Information).
The above results suggest that in addition to the contribu-
with increasing H content in the PdH bulk structures. Addi-
x
tion of the hydride structure, the CO RR activity and selectivity
tional DFT calculations were carried out to obtain the binding
2
of Pd NC catalysts also strongly depend on the particle size.
energies of intermediates of CO RR (i. e. *CO and *HOCO)
2
The smaller NC size, the higher CO RR activity, and selectivity
and HER (i. e. *H) on Pd(100), PdH0.5(100), and PdH (100) sur-
2
1
towards CO production, which can be also supported by a pre-
vious study on demonstrating the different active sites (edge,
faces (Table S6, Supporting Information). A PdH0.5(100) surface
generated using a bulk PdH0.5 structure, computationally much
simpler to model compared to the PdH0.4 phase, was modeled
to represent the surface of PdH0.4 nanocubes.
[
15]
corner, and terrace sites) of Pd for CO RR and HER. For
2
CO RR, the adsorption of CO and the formation of key reac-
2
2
tion intermediate *HOCO are more preferable to the edge and
corner sites, while for HER, the formation of H* remains sim-
ilar on all three types of sites. As the smaller NCs size leads to
The DFT calculated binding energies of the intermediates
in their most favorable sites (shown in Figure 5a–d and listed
in Table S6, Supporting Information) revealed a weakened
more abundant active sites (edge and corner sites), the CO RR
binding on PdH (100) compared to Pd(100). Free energy dia-
2
x
performance in terms of CO production can be significantly
improved. In addition, the enhancement factor from the pre-
hydridization effect also decreased with the increased size.
For example, at −0.8 V, 7 nm PdH0.40 NCs/C presented ≈160%
increased jCO-Pd relative to the Pd counterpart. While for 19 nm
PdH0.40 NCs/C, it only showed ≈44% improvement compared
to the corresponding Pd NCs/C. The possible reason might be
that the prehydridization effect is also correlated to different
sites, which might be more significant on the edge and corner
sites than the terrace ones.
grams were constructed using the DFT calculated binding ener-
gies in Table S6, Supporting Information. The calculated free
energy diagrams (Figure 5e–f) showed a thermodynamically
favorable formation of the HER and CO RR intermediates on
2
Pd(100). However, the CO RR activity on Pd(100) should be lim-
2
ited by a large desorption barrier of *CO (i. e. *CO + H O(g)→
2
CO(g) +H O(g) + *) due to a strong binding affinity of Pd(100)
2
for the CO intermediate. In contrast, as shown in Figure 5e–f,
the weakened binding of absorbates resulted in the forma-
tion of the *H intermediate for HER and *HOCO for CO RR
2
©
2020 Wiley-VCH GmbH
Small 2020, 2005305
2005305 (6 of 10)

www.advancedsciencenews.com
www.small-journal.com
Figure 5. DFT optimized structures of a) PdH0.5(100) surface, b) *H, c) *CO, and d) *HOCO adsorbed at the most favorable site on PdH0.5(100). Black
solid lines show the unit cell used in the DFT calculations and the dotted circle shows the adsorbed *H atom on PdH0.5(100). Pd: gray, H: blue, O: red,
and C: brown. DFT calculated free energy diagrams of e) HER, and f) CO RR on Pd(100), PdH(100), and PdH0.5(100) surfaces at a potential (U) = 0 V.
2
Small 2020, 2005305
2
005305 (7 of 10)
© 2020 Wiley-VCH GmbH

www.advancedsciencenews.com
www.small-journal.com
being uphill on PdH0.5(100) and PdH(100). It was noticed that
relatively small applied external potential should be sufficient
to make the formation of *H and *HOCO downhill in energy.
Furthermore, the weaker binding of *CO made the overall
were collected with X-ray diffracter (PANalytical X’pert PRO-MPD/MRD)
using a Cu-Kα radiation at 40 kV and 25 mA. TEM and XRD instruments were
conducted at Korea Basic Science Institute (KBSI, Daegu Korea). XPS was
measured through Scientific ESCALAB 250Xi system (ThermoFisher) with
Al Kα X-ray (1486.6 eV) as the light source, which was conducted at
National NanoFab Center (NNFC, Daejeon, Korea). Metal contents in
nanocrystals were determined by inductively coupled plasma-atomic
emission spectrometry (ICP-AES, Perkin-Elmer, Optima 7300DV).
CO RR favorable on PdH (100) surfaces. Thus, in line with
2
x
the experimental findings, DFT results indicated that the for-
mation of PdH should enhance the CO RR compared to Pd.
x
2
Electrochemical Measurements: The air-tight H-cell was used for the
electrochemical measurement. The 0.5M sodium bicarbonate (NaHCO3)
solution prepared from CO -saturated 0.25M sodium carbonate
2
3
. Conclusions
(Na CO ) solution was utilized as an electrolyte. The catalyst ink was
2
3
prepared by dispersing 2.5 mg of electrocatalysts in a mixture solution
containing 2.0 ml of Milli-Q water, 0.5 ml of isopropanol, 10 µl of Nafion
(5%), and then 100 µl of each catalyst ink was deposited on a working
electrode (carbon paper: Toray carbon paper 060). The graphite paper
In summary, combined results from electrochemical measure-
ments, in situ XAFS characterization, and DFT calculations
show the benefit of prehydride PdH0.40 NC over the Pd coun-
terpart for the syngas production from the CO RR. Under the
CO RR conditions, both of the PdH
transformed into the stoichiometric PdH . Nevertheless, com-
pared with Pd NCs/C, PdH0 NCs/C presented less cathodic
phase transformation (to PdH ) by 300 mV, leading to an
enhanced CO RR performance of PdH
ratios and CO RR activity of 7 nm PdH
nificantly improved compared with that of Pd NCs/C with the
similar size at the low overpotential (i.e., −0.6 V), respectively.
Also, PdH0 NCs/C showed extended suitable potential range
for syngas production (CO/H ratios between 1 and 2) in com-
parison with Pd NCs/C. We also discovered that the size effect
also played a key role in determining the CO RR performance,
with smaller NC size showing higher activity. These results
suggest strategies to design more active and selective electro-
−
and Ag/AgCl (3M Cl ) were used as the counter and reference electrode,
2
respectively. The potentials were calibrated to a reversible hydrogen
electrode (RHE). During the test, the electrolyte was vigorously stirred
and Pd NCs would be
2
0.40
1
to mitigate the mass transport limitation of dissolved CO . To obtain
2
.40
stable electrochemical CO2 reduction data, the working electrode was
−
first held at −0.05 V for 60 s to remove the capping agents (PVP, Br ) of
1
−1
NCs/C. The CO/H2
NCs/C were sig-
0.40
electrocatalysts and then cycled in 0.02–0.8 V for three times at 50 mV s .
2
0.40
After purging CO for additional 10 min, the CA test was used to evaluate
2
2
the CO RR performance of electrocatalysts by holding each potential
2
(from −0.5 to −1.0 V) for a designated duration. After finishing the CA
test at specified potential, 100 µL of gas in the empty head space (≈17 ml)
of cathodic compartment was injected into gas chromatography (GC,
S46 Agilent, Agilent 7890B) to obtain the gaseous product distribution.
The CO stripping experiment was performed to obtain the CO anti-
poisoning capability of electrocatalysts. The CO adsorption was achieved
by holding the pre-cleaned electrode potential at 0.05 V for 10 min in a
CO-saturated 0.1 M KOH. The excess CO was cleared by purging Ar for
.40
2
2
3
0 min. The CO stripping curves were then taken with a scanning rate of
catalysts for CO RR with tunable product compositions.
−1
2
50 mV s . The Cu underpotential deposition (Cu UPD) was conducted in
an Ar-saturated 50 mM H SO + 50 mM CuSO solution at 10 mV s . See
−1
2
4
4
Supporting Information for the details about the calculation methods.
In Situ XAFS Measurements: A lab-made acryl kit was used for the
in situ XAFS measurement. The areal mass loading of the working
4
. Experimental Section
Size-Controlled Synthesis of Pd NCs (Size of 7, 13, and 19 nm): To
−
2
electrode was ≈5 mg cm to achieve sufficient XAFS signal. During the
control the size of Pd NCs, the amounts of KCl and KBr were varied. [
25]
measurements, CO gas was continuously bubbled into the electrolyte.
2
The reaction mixture including 60 mg of AA, 105 mg of PVP (Mw ≈
The hydrogen electrode (Hydroflex, E-DAQ) was used as a reference
electrode. The other electrochemical conditions were the same as in
the electrochemical measurements. In situ XAFS measurements were
conducted on the 7-BM (QAS) beamline in National Synchrotron Light
Source-II (NSLS-II) at Brookhaven National Laboratory. The XAFS signal
was recorded with a fluorescence mode using a Canberra PIPS detector.
The electrocatalytic performance was measured by stepwise potential
sweeping from 0.6 to −0.9 V. Each potential was held for 10 min using
chronoamperometry technique during the XAFS measurement. The
typical duration for a single spectrum was ≈45 sec and the last ten
spectra measured at each potential were merged to get high signal-
to-noise spectrum. During the measurements, the reference spectrum
of Pd foil was recorded at the same time, and was further used in
5
5 000), different amount of KCl (150 mg for 7 nm and no addition for
1
6
3 and 19 nm in nominal size), different amount of KBr (5, 300, and
00 mg for 7, 13, and 19 nm in nominal size, respectively) and 8 mL of
DI water was placed in a 25 mL glass vial, which was pre-heated at 80 °C
for 10 min in an oil bath under magnetic stirring. Then, 3 mL of DI water
containing 57 mg of Na PdCl was added into the pre-heated mixture.
2
4
The vial was maintained at 80 °C for 3 h. The final product was collected
by centrifugation at 12000 rpm for 10 min. The resulting Pd NCs were
re-dispersed in ethanol or DMF.
Conversion of Pd NCs into Pd Hydride NCs (PdH0.40 NCs): After
centrifugation of Pd NCs, they were dispersed in 30 mL of DMF
−
1
(
0.67 mgPd mL ) solution containing 30 mg of PVP. The reaction mixture
was heated at 160 °C for 16 h in an oil bath under magnetic stirring. The
final product was collected by centrifugation at 12000 rpm for 10 min.
The resulting PdH0.40 NCs were re-dispersed in ethanol.[
calibrating the edge energy (E ) of the samples under analysis.
0
The obtained spectra were processed using the ATHENA and
20]
[26–28]
ARTEMIS software in IFFEFIT package.
EXAFS analyses were
Preparation of Pd NCs or PdH0.40 NCs Catalysts by Carbon Loading:
Suspension of 20 mg Pd NCs or PdH0.40 NCs and 80 mg of Vulcan XC-72
carbon black were dispersed in 15 mL of ethanol. Then the mixture was
sonicated for 2 h and the product was collected by centrifugation. Each
catalyst was washed five times with ethanol and dried by Argon (Ar) gas.
Material Characterization: Transmission electron microscopy (TEM)
images were taken using an HT-7700 microscope (HITACHI, Japan)
operated at 100 kV by drop-casting the nanoparticle dispersions on carbon-
coated copper grids and drying under ambient conditions. High-resolution
TEM studies were carried out in a Titan G2 ChemiSTEM Cs Probe
microscope (FEI, USA) operated at 200 kV. X-ray diffraction (XRD) patterns
conducted by using the ARTEMIS software. The EXAFS spectrum (χ(k))
2
was weighted with k value to intensify the signal in the high k-regime.
The Hanning window was utilized for the Fourier-transform. All of the
EXAFS fittings were done in the R-space. The goodness of fitting was
evaluated based on the reliable factor (R-factor) and reduced chi-square
2
(reduced χ ). The amplitude reduction factor for Pd was obtained from
the reference Pd foil. For the EXAFS fitting, FCC Pd (a = 3.900 Å) model
was used. All of the data were co-refined using this model. A single-
scattering path between Pd and its first neighbor Pd was taken into
consideration for all EXAFS fitting. The fitting results were tabulated in
Table S4, Supporting Information.
©
2020 Wiley-VCH GmbH
Small 2020, 2005305
2005305 (8 of 10)

www.advancedsciencenews.com
www.small-journal.com
Computational Methods: Periodic DFT[29–30] calculations were
Facility operated for the DOE Office of Science by Brookhaven National
Laboratory under Contract No. DE-SC0012704 with assistance from the
Synchrotron Catalysis Consortium (DE-SC0012653). The DFT calculations
were performed using computational resources at the Extreme Science
and Engineering Discovery Environment (TGCHE190032), which is
supported by National Science Foundation Grant number ACI-1548562
and at the National Energy Research Scientific Computing Center
(NERSC), a U.S. Department of Energy Office of Science User Facility
operated under Contract No. DE-AC02- 05CH11231.
performed using the plane wave Vienna Ab Initio Simulation Package
[
31–32]
(
VASP) code.
The core electrons were described using the projector
augmented wave (PAW) potentials with the generalized gradient
approximation (GGA),[
33–34]
using PW91 functionals.
[35]
A plane-
wave basis set with a kinetic energy cutoff of 400 eV was used in all
calculations. The Brillion zone integration was performed using a 3 × 3 × 1
[
36]
Monkhorst-Pack grid.
The Pd(100) surface was modeled using a six-layer 3 × 3 surface
slab. Similarly, the H-terminated PdH(100) and PdH0.5(100) surfaces
were modeled using six-layer 3 × 3 surface slabs using the NaCl crystal
structure for bulk PdH and PdH0.5 (with 50% fewer H atoms compared
to bulk PdH), respectively. A vacuum layer of ≈14 Å thick was added in Conflict of Interest
the slab cell along the direction perpendicular to the surface to minimize
The authors declare no conflict of interest.
the artificial interactions between the surface and its periodic images.
During geometry optimization, atoms in the top three layers were
allowed to relax while atoms in the bottom three layers were fixed until
−
1
the Hellman–Feynman force on each ion was smaller than 0.01 eV Å .
The binding energy (BE) of adsorbate was calculated as:
Keywords
carbon dioxide reduction reaction (CO RR), density functional theory
2
BE
(
adsorbate
)
=E
(
slab+adsorbate
)
−E
(
slab
)
−E
(
adsorbate
)
(1)
(DFT), nanocubes, palladium hydride (PdH ), syngas
1
where E(slab + adsorbate), E(slab), and E(adsorbate) were the total
energy of slab with adsorbate, clean slab, and adsorbate in the gas
phase, respectively. E(H) was taken as one-half of the total energy of the
Received: August 28, 2020
Revised: October 18, 2020
Published online:
H molecule during the calculation of hydrogen BE.
2
The Gibbs free energy (G) was calculated as[37]
G =E +ZPE −TS
(2)
[
1] M. Meinshausen, N. Meinshausen, W. Hare, S. C. Raper, K. Frieler,
R. Knutti, D. J. Frame, M. R. Allen, Nature 2009, 458, 1158.
Here, E was the total energy obtained from DFT calculations, ZPE and
S were the zero-point energy and entropy respectively, and T = 298.15 K.
[
2] C. Long, X. Li, J. Guo, Y. Shi, S. Liu, Z. Tang, Small Methods 2019,
The free energy diagram of CO RR to CO has been calculated by
2
3
, 1800369.
3] M. Gattrell, N. Gupta, A. Co, J. Electroanal. Chem. 2006, 594, 1.
4] Y. i. Hori, in Modern aspects of electrochemistry, Springer, New York
008, p 89.
5] B. M. Tackett, E. Gomez, J. G. Chen, Nat. Catal. 2019, 2, 381.
[8]
considering the following sequential steps:
[
[
+
−
CO2
(
g
)
+(H +e )+∗ →∗HOCO
(3)
2
[
+
−
∗
HOCO+(H +e )→∗CO+H O
(
g
)
(4)
(5)
[6] Y. Pan, R. Lin, Y. Chen, S. Liu, W. Zhu, X. Cao, W. Chen, K. Wu,
2
W.-C. Cheong, Y. Wang, J. Am. Chem. Soc. 2018, 140, 4218.
[
7] S. Liu, H. Tao, L. Zeng, Q. Liu, Z. Xu, Q. Liu, J.-L. Luo, J. Am. Chem.
Soc. 2017, 139, 2160.
( )
∗CO → CO g +∗
In aqueous electrolytes, the HER inevitably takes place via the
[8] W. Zhu, R. Michalsky, O. n. Metin, H. Lv, S. Guo, C. J. Wright,
following sequential steps,[ and competes with the CO RR:
10]
2
X. Sun, A. A. Peterson, S. Sun, J. Am. Chem. Soc. 2013, 135, 16833.
[
9] W. Sheng, S. Kattel, S. Yao, B. Yan, Z. Liang, C. J. Hawxhurst, Q. Wu,
J. G. Chen, Energy Environ. Sci. 2017, 10, 1180.
+
−
H +e +∗ →∗H
(
Volmer step
)
(6)
(7)
[
[
10] Q. Lu, J. Rosen, Y. Zhou, G. S. Hutchings, Y. C. Kimmel, J. G. Chen,
+
−
F. Jiao, Nat. Commun. 2014, 5, 3242.
∗
H+H +e →H
(
g
)
(
+∗ Heyrovsky step
)
2
[
11] J. Wang, S. Kattel, C. J. Hawxhurst, J. H. Lee, B. M. Tackett,
K. Chang, N. Rui, C. J. Liu, J. G. Chen, Angew. Chem. 2019, 58, 6271.
12] I. Wender, Fuel Process. Technol. 1996, 48, 189.
[
13] J. H. Lee, S. Kattel, Z. Jiang, Z. Xie, S. Yao, B. M. Tackett, W. Xu,
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
N. S. Marinkovic, J. G. Chen, Nat. Commun. 2019, 10, 3724.
[
14] Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta 1994,
9, 1833.
3
[
[
[
15] D. Gao, H. Zhou, J. Wang, S. Miao, F. Yang, G. Wang, J. Wang,
X. Bao, J. Am. Chem. Soc. 2015, 137, 4288.
16] W. Zhu, S. Kattel, F. Jiao, J. G. Chen, Adv. Energy Mater. 2019, 9,
Acknowledgements
Q.C., J.K., and J.H.L. contributed equally to this work. Z.C. and Q.C.
acknowledge the support from the ACS Petroleum Research Fund
1802840.
17] H. Huang, H. Jia, Z. Liu, P. Gao, J. Zhao, Z. Luo, J. Yang, J. Zeng,
Angew. Chem. 2017, 56, 3594.
[
[
18] J. K. Nørskov, F. Abild-Pedersen, F. Studt, T. Bligaard, Proc. Natl.
Acad. Sci. U. S. A. 2011, 108, 937.
19] Y. Liu, D. Tian, A. N. Biswas, Z. Xie, S. Hwang, J. H. Lee, H. Meng,
J. G. Chen, Angew. Chem. 2020, 59, 11345.
(
#59989-DNI5) and Jacob School of Engineering at UC San Diego. S.-I.C.
and J.K. acknowledge support by the National Research Foundation of
Korea (NRF-2018R1C1B6004272, 2009-0082580). Authors at Columbia
University acknowledge support by the U.S. Department of Energy,
Office of Science, Catalysis Science Program (DE-FG02-13ER16381). This
research used resources at 7-BM (QAS) of the National Synchrotron Light
Source II, a U.S. Department of Energy (DOE) Office of Science User
[20] M. K. Kabiraz, J. Kim, W.-J. Lee, B. Ruqia, H. C. Kim, S.-U. Lee, J.-R. Kim,
S.-M. Paek, J. W. Hong, S.-I. Choi, Chem. Mater. 2019, 31, 5663.
[21] J. Eastman, L. Thompson, B. Kestel, Phys. Rev. B 1993, 48, 84.
Small 2020, 2005305
2
005305 (9 of 10)
© 2020 Wiley-VCH GmbH

www.advancedsciencenews.com
www.small-journal.com
[
22] Z. Zhao, X. Huang, M. Li, G. Wang, C. Lee, E. Zhu, X. Duan,
[29] P. Hohenberg, W. Kohn, Phys. Rev. 1964, 136, B864.
Y. Huang, J. Am. Chem. Soc. 2015, 137, 15672.
[30] W. Kohn, L. J. Sham, Phys. Rev. 1965, 140, A1133.
[31] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 1996, 6, 15.
[32] G. Kresse, J. Hafner, Phys. Rev. B 1993, 48, 13115.
[33] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758.
[34] P. E. Blöchl, Phys. Rev. B 1994, 50, 17953.
[35] J. P. Perdew, Y. Wang, Phys. Rev. B 1992, 45, 13244.
[36] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1976, 13, 5188.
[37] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin,
T. Bligaard, H. Jonsson, J. Phys. Chem.B 2004, 108, 17886.
[
[
23] P. Bennett, J. Fuggle, Phys. Rev. B 1982, 26, 6030.
24] C. T. Chan, S. G. Louie, Phys. Rev. B 1983, 27, 3325.
25] M. Liu, Y. Zheng, L. Zhang, L. Guo, Y. Xia, J. Am. Chem. Soc. 2013,
[
[
[
135, 11752.
26] S. Kelly, D. Hesterberg, B. Ravel, Methods of Soil Analysis Part
5
—Mineralogical Methods 2008, 5, 387.
27] B. Ravel, M. Newville, J. Synchrotron Rad. 2005, 12, 537.
28] J. J. Rehr, R. C. Albers, Rev. Mod. Phys. 2000, 72, 621.
[
©
2020 Wiley-VCH GmbH
Small 2020, 2005305
2005305 (10 of 10)