Composition-dependent activity of Cu-Pt alloy nanocubes for electrocatalytic CO2 reduction

Article abstract of DOI:10.1039/c4ta06608a

A protocol has been developed for the synthesis of Cu-Pt alloy nanocubes with a relatively broad range of composition ratios. It enables the investigation of the composition-dependent activity of Cu-Pt alloys for electrocatalytic CO₂ reduction.

Full text of DOI:10.1039/c4ta06608a

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Materials Chemistry A
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Composition-dependent activity of Cu–Pt alloy
nanocubes for electrocatalytic CO reduction†
2
Cite this: DOI: 10.1039/c4ta06608a
Xueyu Zhao, Binbin Luo, Ran Long, Chengming Wang and Yujie Xiong*
Received 2nd December 2014
Accepted 22nd January 2015
DOI: 10.1039/c4ta06608a
www.rsc.org/MaterialsA
A protocol has been developed for the synthesis of Cu–Pt alloy gives them a crystallographic match. Thus the miscible bime-
nanocubes with a relatively broad range of composition ratios. It tallic combinations of Cu with Pt will allow us to maneuver the
enables the investigation of the composition-dependent activity of turnover rates of key steps and in turn improve the catalytic
Cu–Pt alloys for electrocatalytic CO
2
reduction.
activity and selectivity. Second, Cu nanoparticles tend to
undergo oxidation to form Cu(I) or Cu(II) during the synthesis
and storage process. Although Jin et al. have successfully
synthesized pure Cu nanocubes, the edge lengths of the nano-
Excessive consumption of fossil fuels has gradually led to the
depletion of these nite natural resources while increasing the
levels of atmospheric carbon dioxide (CO ) – a greenhouse gas.
2
16
cubes ranged from 50 to 200 nm. It is well recognized that the
1
surface atoms will become more dominant in the catalytic
To address this environmental issue, efforts are being made to
capture and sequester CO and to convert it into reusable
carbon forms. Among various approaches that thus far have
been developed for CO reactivation, electrochemical reduction
of CO represents a potentially “clean” technique only at the
17
reactions as the size of catalytic particles shrinks. Thus it is
2
imperative to keep the particle sizes small for boosting their
2
catalytic efficiency. Unfortunately, the Cu nanocubes at smaller
16
2
2
sizes are likely made of Cu O rather than Cu. Given that Pt is a
2
metal with high chemical stability, alloying with Pt would be a
promising approach to improve the resistance of Cu nano-
particles to oxidation.
3,4
expense of a sustainable supply of electric energy.
The major obstacle preventing efficient reduction of CO
the lack of catalysts that can readily couple with an electric
2
is
Cu–Pt alloy nanoparticles are thus ideal candidates for the
energy to achieve rapid and selective cleavage of C–O bonds in
electrocatalytic reduction of CO . However, it still remains
2
2
CO and formation of new bonds in the products. Towards this
challenging to develop Cu–Pt alloy nanocrystals with well-
dened shapes and controllable compositions. In the literature,
goal, various metal electrocatalysts have been screened experi-
5–9
10–12
mentally and analyzed computationally
to optimize their
Xu et al. presented the Cu
compositions that were synthesized via a colloidal method.
x
Pt100ꢀx nanocubes at different
activity and selectivity for CO reduction. Cu nanoparticles have
18,19
2
been demonstrated as valid low-cost electrocatalysts for the
Yet the range of controllable compositions (x ¼ 20–46) is not
broad enough to establish the composition–property relation-
ship for Cu–Pt alloys. As predicted, Cu can provide active sites
13,14
reduction of CO
2
into hydrocarbons;
however, a couple of
reasons have urged us to develop new electrocatalysts with Cu-
based alloys. First, incorporation of other transition metals into
Cu catalysts can modify the reaction activation barrier for
different steps. For instance, the alloying of Cu with Pt can lower
the decomposition activation energy of formate up to 13%. Cu
and Pt have a similar face-centered cubic (fcc) structure which
for CO reduction while Pt is benecial to improving sample
2
stability. If the relationship can be established, it would become
feasible to nd an optimal combination to achieve high
performance and minimize materials cost.
In this communication, we reinvent the wheel by analyzing
the bottleneck of existing protocols and identify the key factor
limiting the composition ratio of Cu to Pt in the synthesis of
Cu–Pt alloy nanocubes. As a result, we achieve the synthesis of
Cu–Pt alloy nanocubes with a relatively broad range of compo-
sition ratios while the cubic morphology is largely maintained,
providing an excellent platform for investigating the composi-
15
Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative
Innovation Center of Chemistry for Energy Materials and School of Chemistry and
Materials Science, University of Science and Technology of China, Hefei, Anhui
2
5
30026, P. R. China. E-mail: yjxiong@ustc.edu.cn; Fax: +86 551 63606657; Tel: +86
51 63606657
†
Electronic supplementary information (ESI) available: For materials synthesis tion–property relationship.
and characterization methods, and additional TEM and XPS data. See DOI:
0.1039/c4ta06608a
1
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19
In the established protocol, platinum(II) acetylacetonate
Pt(acac) ) and copper(II) acetylacetonate (Cu(acac) ) in solvent
(
2
2
1-octadecene (ODE) were co-reduced in the presence of 1,2-tet-
radecanediol (TDD), tetraoctylammonium bromide (TOAB),
oleylamine (OLA), and 1-dodecanethiol (DDT). The OLA and
TOAB served as stabilizing/capping agents for Cu and Pt
components, respectively, while the DDT was utilized as a
coordination chemical to balance the reduction rates of Pt and
Cu precursors. In particular, 1,2-tetradecanediol (TDD) is an
agent with stronger reducing capability than OLA, and thus
plays the role of a reductant in this protocol. However, it has two
neighbouring hydroxyl groups whose oxidation products may
coordinate to metal cations, which essentially slows down the
18,19
nucleation and growth.
We thus highly suspect that its
presence is the key to limiting the composition controlling
range. For this reason, we look into the possibility of whether
indeed the TDD impacts the composition control. In our
simplied protocol, we thus eliminate the use of TDD from the
reaction system, and instead, directly employ the capping agent
OLA as the reductant. As a result, we have been able to obtain
Cu–Pt alloy nanocubes with relatively consistent morphologies
and higher Cu concentrations in the alloys. In the synthesis, the
compositions of nanocubes are tailored by precisely tuning the
ratio of metal precursors and the amounts of stabilizing/coor-
dination agents (see experimental conditions in Table 1).
Fig. 1a–d show the transmission electron microscopy (TEM)
images of four typical samples with different compositions,
indicating that these alloy nanocubes have comparable cubic
shapes and size distributions despite their varied compositions.
Fig. 1 TEM images of (a) Cu22Pt78, (b) Cu32Pt68, (c) Cu68Pt32 and (d)
Cu85Pt15 nanocubes synthesized under the typical conditions listed in
Table 1. The reaction time was 30 min, counted from the point of
ꢁ
adding OLA and DDT at 110 C. (e) Size-distribution diagram and (f)
The particle size is estimated to be 8–10 nm (see Fig. 1e for the HRTEM image of the Cu68Pt32 nanocubes in panel c.
size distribution of a typical sample). Note that the composi-
tions of nanocubes are determined by inductively coupled
plasma mass spectrometry (ICP-MS). As revealed by high-reso- 1549) to bare Cu (JCPDS #04-0836) (see standard XRD patterns
lution TEM (HRTEM, Fig. 1f), the obtained cubic Cu Pt
in Fig. 2b). This set of data clearly demonstrates that our
x
100ꢀx
nanoparticles are single crystals and enclosed by {100} facets obtained nanocubes are Cu–Pt alloys.
˚
according to the lattice fringe spacing of 1.9 A (JCPDS #48-1549
It turns out that the removal of TDD from our system is the
for PtCu).
key to achieving a high ratio of Cu to Pt in the nal products. To
To further verify the alloy nature of the samples, we collected evaluate the effect of TDD on the synthesis, we have compared
X-ray diffraction (XRD) patterns (Fig. 2a). For the four samples the nucleation and growth process of our system with that of the
with different compositions, the diffraction peaks gradually TDD-present case, taking the Cu Pt synthesis as an example.
6
8
32
shi to higher 2q as the Cu composition increases, in agreement Judging from color changes, we recognize that the nucleation
20,21
with Vegard's law.
This peak shiing falls into the range of and growth of nanoparticles become substantially faster when
XRD peaks from bare Pt (JCPDS #65-2868) to PtCu (JCPDS #48- the TDD is taken out of the protocol. This viewpoint is
Table 1 Experimental conditions for preparing Cu
with different compositions
x
Pt100ꢀx nanocubes
a
Pt(acac)
2
Cu(acac)
2
TOAB
OLA
DDT
Cu22Pt78
Cu32Pt68
Cu68Pt32
Cu85Pt15
0.030
0.025
0.020
0.020
0.050
0.055
0.060
0.110
0.60
0.55
0.50
0.70
0.80
0.88
1.00
1.32
0.04
0.04
0.04
0.04
a
ꢁ
All values are in the unit of mmol. Solvent: 10 mL ODE; 230 C, 20 min
(
i.e., 30 min if counted from the point of adding OLA and DDT Fig. 2 (a) XRD patterns of Cu
at 110 C). The compositions of alloy nanocubes were measured by compositions. (b) Standard XRD patterns of Pt, PtCu, and Cu with fcc
x
Pt100ꢀx nanocubes with different
ꢁ
ICP-MS.
structures.
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Fig. 4 The schematic illustrating the growth mechanisms for the
Cu Pt100ꢀx nanocubes prepared in the presence or absence of TDD,
x
Fig. 3 TEM images of nanocubes prepared under the same conditions
as those for Cu68Pt32 in Fig. 1c, except for the presence of TDD and
varied reaction times: (a) with TDD, 10 min, (b) without TDD, 10 min, (c)
with TDD, 30 min, and (d) without TDD, 30 min. The reaction time was
taking the comparison of Cu68Pt32 (with TDD) with Cu53Pt47 (without
TDD) as an example.
ꢁ
counted from the point of adding OLA and DDT at 110 C.
Pt(II)/Pt(0) and Cu(II)/Cu(0) pairs. Thus when the reduction rates
are low in the case of TDD synthesis (top panel of Fig. 4), the
galvanic replacement would overwhelm the reduction of metal
precursors by the added reductants, leading to the gradually
decreasing ratio of Cu to Pt. In contrast, in the absence of TDD
supported by the TEM analysis (Fig. 3): at the intermediate stage
(
t ¼ 10 min), the particle sizes yielded by our protocol are
obviously larger than those in the presence of TDD (8 nm versus
nm). To decode the nature of this difference, we collected
(bottom panel of Fig. 4), the reduction of precursors is fast
6
enough so that the proportion of Cu in the alloy nanoparticles
increases as the synthesis proceeds, driven by the high
concentration of Cu precursor.
In addition to the absence of TDD, there are a few other
factors critical to obtaining high-quality nanocubes. OLA as the
capping/stabilizing agent for Cu is critical to the morphology
intermediate products from the synthesis and measured their
concentrations with ICP-MS. Based on the concentrations, we
can calculate the amounts of Cu and Pt that have been reduced
at each point of time in one single synthesis. As listed in Table 2,
two conclusions can be readily made: (1) the reduction rates for
both Cu and Pt in the absence of TDD are higher than those
using TDD at early stages; (2) as the synthesis proceeds, the
reduction rate of Pt dramatically exceeds that of Cu in the TDD
synthesis whereas this feature does not occur in the absence of
TDD. As the added amount of Cu(acac)2 is excessive in the
synthesis (0.060 mmol Cu(acac) versus 0.020 mmol Pt(acac) ),
the Cu reduction is supposed to become faster than the Pt
reduction along with the precursor consumption. Thus there
should be a certain process that has impeded the further
reduction of Cu.
It is known that galvanic replacement can take place between
Cu(0) and Pt(II) species due to the potential difference between
and dispersion of nal Cu Pt100ꢀx products. As shown in
x
Fig. S1,† formation of {100} facets on the surface cannot be
achieved when less OLA (e.g., 0.50 mmol) is used, resulting in
irregular shapes; however, excessive use of OLA (e.g., 1.50
mmol) reduces shape uniformity and dispersity. Similarly, a
certain amount of Pt capping/stabilizing agent TOAB is required
2
2
to dene the cubic shape of Cu Pt
nanocrystals while too
x
100ꢀx
much TOAB is disadvantageous for the dispersion of yielded
ꢀ
nanoparticles (Fig. S2†). Although the Br ions from TOAB are
the major components responsible for shaping the Cu Pt100ꢀx
x
18
nanocubes, the solubility of the capping agent is also crucial to
Table 2 Compositions of Cu68Pt32 nanocubes prepared with and without TDD for different reaction times (starting from the addition of OLA and
ꢁ
DDT at 110 C). All the other conditions were kept the same as those for Cu68Pt32 in Fig. 1c. The amounts of Cu and Pt here are measured by ICP-
MS and calculated for one single synthesis
With TDD (0.5 mmol)
Cu
Without TDD
Cu
t/min
x
Pt100ꢀx
Cu/mmol
Pt/mmol
x
Pt100ꢀx
Cu/mmol
Pt/mmol
10
20
30
Cu60Pt40
Cu55Pt45
Cu53Pt47
4.81
8.69
9.54
3.21
7.17
8.57
Cu62Pt38
Cu66Pt34
Cu68Pt32
6.69
10.58
13.12
4.10
5.45
6.17
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the efficacy of its capping effect. For instance, if TOAB is reference to the blank electrode, recorded at room temperature
replaced with the same amount of cetyltrimethylammonium in CO -purged 0.5 M KHCO solution (pH ¼ 7.3) at a scan rate of
2
3
ꢀ1
bromide (CTAB), the low compatibility of CTAB with the used 50 mV s between ꢀ2.0 and ꢀ0.5 V (vs. Ag/AgCl). To better
solvent ODE distinctly lowers the quality of nal products assess the performance of Cu Pt
alloy nanocubes, we have
x
100ꢀx
(Fig. S3†). The product morphology is also directly associated performed the electrocatalysis measurements benchmarked
with the amount of coordination agent DDT that can synchro- against pure Pt and Cu nanocubes that are synthesized by
nize the reduction of Pt and Cu ions during the reaction. An following the established protocols (see their morphologies in
16,19
excess of DDT (e.g., 0.12 mmol) turns the growth of nanocrystals Fig. S7†).
As expected, surface oxidation easily occurs on the
into branched structures (Fig. S4a†) although the compositions pure Cu nanocubes, so part of Cu(0) has been oxidized into Cu(I)
of nal products have not been altered. In the absence of DDT, and Cu(II) once their surface capping agents are removed, as
however, the reduction rates of Pt and Cu ions can no longer be revealed by the XPS spectra of nanocubes aer dispersion in
2
2
balanced so that the compositions turn out to be Cu Pt
water for 10 days (Fig. S8†). The electrochemical measure-
having less Cu. As a result, most of the resulted nanoparticles ments show that the Cu Pt100ꢀx nanocubes exhibit improved
are irregular in size and shape (Fig. S4b†). Overall, the success catalytic activities in the CO electroreduction reaction with
in broadening the composition controlling range relies on increasing Cu constituent, according to the gradually positively
6
5
35
x
2
2
3,24
adjusting the reduction rates by eliminating the use of TDD, shied onset potentials.
and meanwhile, the concentrations of other chemicals need to Pt
be tightly controlled for well-dened shapes.
The onset potentials of the
Cu100ꢀx nanocubes become more positive with the increase
of the copper constituent, while the onset potential of the blank
x
The incorporation of Pt into Cu nanocrystals should be electrode is the most negative. Moreover, at ꢀ1.45 V, the current
capable of improving the sample stability. As indicated by the density of the Cu Pt nanocubes is nearly 2.5 times higher
8
5
15
TEM images, no distinct morphological changes can be than that of the Cu22Pt78 nanocubes, indicating the enhanced
observed in Cu68Pt32 nanocubes aer they have been dispersed catalytic activity by increasing copper constituent. Accordingly,
in hexane for 3 months (Fig. S5†). In order to further verify this the catalytic activity follows the order of pure Pt > Cu85Pt15
>
feature, we carried out X-ray photoelectron spectroscopy (XPS) Cu68Pt32 > Cu32Pt68 > Cu22Pt78 > pure Cu. It is believed that the
measurements on the Cu68Pt32 nanocubes aer the 3 month low electrocatalytic activities of pure Cu nanocubes are ascribed
storage. As displayed in the XPS spectra (Fig. S6†), Cu LMM to their surface oxidation.
binding energy at ꢂ919 eV, Cu 2p binding energy at ꢂ932 eV,
In principle, the electrochemical reduction of CO2 in an
3
/2
and their summation at ꢂ1851 eV are characteristic of Cu(0), aqueous solution may produce different carbon products,
22
demonstrating that the Cu in the sample is free of oxidation.
depending on how reaction intermediates are stabilized by
10,11,25
Moreover, the Pt 4f binding energy at ꢂ71.4 eV is a character- catalytically active surfaces.
For instance, the conversion of
0
4,10,24
istic value for Pt . It conrms that the Cu
x 2
Pt100ꢀx alloy nano- CO to CO most likely follows the mechanisms below:
cubes have excellent long-term stability.
ꢀ
ꢀ
CO + H O + e + * / COOH* + OH (aq)
(1)
(2)
(3)
To identify samples with excellent stability, we further
explored Cu Pt nanocubes with four different composi-
2
2
x
100ꢀx
ꢀ
ꢀ
COOH* + e / CO* + OH (aq)
CO* / CO + *
tions in electrocatalytic CO reduction. Each sample is coated
2
ꢀ
2
on a Ti foil with a loading amount at 0.25 mg cm aer specic
cleaning procedures, and the electrocatalysis is performed in a
conventional H-cell separated by Naon® 117. Fig. 5a shows
electrochemical measurements of the four samples, in
where the asterisk denotes a catalytically active site. During the
CO electroreduction process, hydrogen evolution reaction
HER) may also take place, which appears as a major side
reaction oen observed in previous studies:
2
(
ꢀ
ꢀ
H O + e + * / H* + OH (aq)
(4)
2
ꢀ
ꢀ
H* + H
2
O + e /H
2
+OH (aq)+*or H* + H*/H
2
+ 2* (5)
To achieve high selectivity for CO reduction, one has to
suppress the HER side reaction. We believe that composition
control should be a versatile knob to tune this reaction selec-
tivity, as it is known that Cu is an active material for electro-
Fig.
5 (a) Electrochemical measurements for electroreduction
13,14
catalytic CO2 reduction
while Pt is a highly efficient
performance recorded on Cu
compositions benchmarked against pure Pt nanocubes and pure Cu
nanocubes with a blank electrode as a reference. (b) Faradaic effi-
ciencies of the different-composition Cu
measured at ꢀ1.75 V for 0.5 h, benchmarked against pure Pt nano-
cubes and pure Cu nanocubes. Electrolyte: 0.5 M KHCO , CO purged.
No alcohol or carboxylic acid products have been detected in the
liquid phase by the GC measurements.
x
Pt100ꢀx nanocubes with different
26–28
electrocatalyst for HER.
To prove this concept, we have detected the CO
products of Cu Pt100ꢀx nanocubes using gas chromatography
GC). CO is converted to CH by a nickel methanation reactor
and then analyzed by a ame ionization detector (FID), while H
2
reduction
x
Pt100ꢀx nanocubes
x
(
4
3
2
2
and CH are directly determined using a thermal conductivity
4
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detector (TCD) and a FID, respectively (see GC parameters in
Table S1† and GC pneumatic circuit diagram in Fig. S9†). We
have also analyzed the liquid products by the GC measure-
ments, and no alcohol or carboxylic acid products have been
found in the liquid phase. Based on the amounts of various
products, Faradaic efficiencies have then been evaluated
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above the detection limit of GC, the electrochemical reaction is
run for 0.5 h at ꢀ1.75 V where the currents reach the maximum.
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2
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. Given their high stability, the
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Acknowledgements
2
This work was nancially supported by the NSFC (no. 21101145,
2
1471141), Recruitment Program of Global Experts, CAS
Hundred Talent Program, and Fundamental Research Funds
for the Central Universities (no. WK2060190025,
WK2310000035).
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