Ni-Cu alloy nanoparticles loaded on various metal oxides acting as efficient catalysts for photocatalytic H2 evolution

Article abstract of DOI:10.1039/c5ra04838a

Catalysis of Al₂O₃-SiO₂, TiO₂, SiO₂ and CeO₂ (MOx) impregnated with pre-formed Ni-Cu alloy nanoparticles (Ni-CuNPs/MOx) for photocatalytic hydrogen (H₂) evolution was compared with that of MOx impregnated with Ni²⁺ and Cu²⁺ ions followed by calcination and reduction (Ni-Cu/MOx). The photocatalytic H₂ evolution was conducted by photoirradiation (λ > 340 nm) of a deaerated mixed solution of a phthalate buffer (pH 4.5) and acetonitrile [1:1 (v/v)] containing β-dihydronicotinamide adenine dinucleotide (NADH), 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh⁺-NA), and Ni-CuNPs/MOx or Ni-Cu/MOx as an electron donor, a photosensitiser and an H₂-evolution catalyst, respectively. Ni-CuNPs/Al₂O₃-SiO₂ exhibited activity for the photocatalytic H₂ evolution, whereas Ni-Cu/Al₂O₃-SiO₂ showed no activity. Such precursor dependent catalysis can be elucidated by the ion-exchangeable nature and high surface area of Al₂O₃-SiO₂, on which Ni-Cu alloy particles hardly form from metal salts. On the other hand, Ni-Cu/TiO₂ and Ni-Cu/SiO₂ exhibited higher activity than Ni-CuNPs/TiO₂ and Ni-CuNPs/SiO₂, respectively, resulting from formation of smaller Ni-Cu alloy nanoparticles on TiO₂ and SiO₂ by reducing Ni²⁺ and Cu²⁺ on the surfaces. When CeO₂ was used as the support, no catalytic activity was observed for either Ni-CuNPs/CeO₂ or Ni-Cu/CeO₂. Kinetic study for thermal H₂ evolution suggested that Ni-CuNPs were severely deactivated for H₂ evolution by being loaded on CeO₂. This journal is

Full text of DOI:10.1039/c5ra04838a

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DOI: 10.1039/C5RA04838A
ARTICLE
Ni-Cu alloy nanoparticles loaded on various metal
oxides acting as efficient catalysts for
photocatalytic H2 evolution
Cite this: DOI: 10.1039/x0xx00000x
Yusuke Yamada,*ab Shinya Shikanoa and Shunichi Fukuzumi*acd
Received 00th March 2015,
Accepted 00th xxxx 2015
Catalysis of Al2O3-SiO2, TiO2, SiO2 and CeO2 (MOx) impregnated with pre-formed Ni-Cu alloy
nanoparticles (Ni-CuNPs/MOx) for photocatalytic hydrogen (H2) evolution was compared with that of
MOx impregnated with Ni2+ and Cu2+ ions followed by reduction (Ni-Cu/MOx). The photocatalytic H2
evolution was conducted by photoirradiation (λ > 340 nm) of a deaerated mixed solution of a
phthalate buffer (pH 4.5) and acetonitrile [1:1 (v/v)] containing β-dihydronicotinamide adenine
dinucleotide (NADH), 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+–NA), and Ni-CuNPs/MOx or Ni-
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cu/MOx as an electron donor,
a photosensitiser and an H2-evolution catalyst, respectively. Ni-
CuNPs/Al2O3-SiO2 exhibited activity for the photocatalytic H2 evolution, whereas Ni-Cu/Al2O3-SiO2
showed no activity. Such precursor dependent catalysis can be elucidated by the ion-exchangeable
nature and high surface area of Al2O3-SiO2, on which Ni-Cu alloy particles hardly form from metal salts.
On the other hand, Ni-Cu/TiO2 and Ni-Cu/SiO2 exhibited higher activity than Ni-CuNPs/TiO2 and Ni-
CuNPs/SiO2, respectively, resulting from formation of smaller Ni-Cu alloy nanoparticles on TiO2 and
SiO2 by reducing Ni2+ and Cu2+ on the surfaces. When CeO2 was used as the support, no catalytic
activity was observed for either Ni-CuNPs/CeO2 or Ni-Cu/CeO2. Kinetic study for thermal H2 evolution
suggested that Ni-CuNPs were severely deactivated for H2 evolution by being loaded on CeO2.
reduction of usage of precious metals and development of
alternative catalysts composed of cheaper metals are highly
1. Introduction
Hydrogen (H2) has been regarded as the most promising energy
carrier of the next generation,1-3 however, most part of H2
supplied to chemical industry is currently produced by the
steam forming of fossil fuels such as natural gas, which is a
large energy consuming process and emitting carbon dioxide as
a main by-product.4 It is highly desired that H2 is produced
from renewable chemicals by utilizing natural energy.5 In this
context, H2 production utilizing solar energy has attracted much
attention in this research field.6-13 Visible light-driven
demanded. For reduction of using amount of precious metals,
Ru nanoparticles supported on a silica support have been
developed.39 As a catalyst using a base metal, Ni nanoparticles
has been reported although catalytic activity was half that of Pt
nanoparticles.40 Recently, Ni and Cu loaded on silica or titania
prepared by
a conventional impregnation method using
Ni(NO3)2 and Cu(NO3)2 as precursors followed by reduction
have been reported to exhibit high catalytic activity with a
synergistic effect.41 Although the catalytic activity highly
depends on the preparation procedures and supports of the
catalysts,41 requisites to obtain the synergistic effect between Ni
and Cu and the origin of the support effect have yet to be
clarified.
photocatalytic H2-evolution systems composed of
a
photosensitiser, an electron relay and an H2-evolution catalyst
have so far been extensively studied.14–25 Recently, efficient
photocatalytic H2-evolution systems were developed by
employing electron donor–acceptor linked dyads,26–32 which
We report herein catalytic activity of Ni-Cu alloy
nanoparticles, which were prepared ex situ by thermal
form
long-lived
electron
transfer
states
under
photoirradiation.33–35 The long lifetime of the electron-transfer
states allows to eliminate electron relays in the H2-evolution
systems.27–32
decomposition of Ni(acac)2 and Cu(acac)2 (acac
=
acetylacetonato), loaded on a metal oxide support chosen from
SiO2, TiO2, Al2O3-SiO2 and CeO2 (MOx) for photocatalytic H2
evolution. The photocatalytic H2 evolution was conducted by
photoirradiation (λ > 340 nm) of a mixed solution of a
phthalate buffer (pH 4.5) and MeCN containing β-
Pt nanoparticles has been widely used as
a high
performance H2-evolution catalyst,36,37 however, Pt is a most
expensive metal because of its limited reserve.38 Thus,
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dihydronicotinamide adenine dinucleotide (NADH), 2-phenyl-
4-(1-naphthyl)quinolinium ion (QuPh+–NA) and Ni-
CuNPs/MOx as an electron donor, a photosensitiser and an H2-
evolution catalyst, respectively. The chemical structure of
QuPh+–NA and the overall photocatalytic cycle of H2 evolution
are depicted in Scheme 1. The catalytic activity of Ni-
CuNPs/MOx was compared with that of Ni-Cu/MOx, which are
prepared by the impregnation method using Ni(NO3)2 and
Cu(NO3)2 as precursors to clarify the requisites for high
catalytic activity with each support.
(a)ꢀ
(b)ꢀ
(d)ꢀ
(e)ꢀ
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DOI: 10.1039/C5RA04838A
100 nmꢀ
20 nmꢀ
(a)
(b)
(c)ꢀ
(f)ꢀ
QuPh•–NA•+
NADH
QuPh+–NA
hν
QuPh+–NA
x 1/2
2QuPh•–NA
H2
+
N
Ni-Cu NPs
on MOx
20 nmꢀ
2H+
NAD+ + H+
Me
QuPh+–NA
Fig. 1 TEM images and powder X-ray diffraction peaks of (a,d) Cu nanoparticles,
Scheme 1 (a) Molecular structure of QuPh+–NA and (b) the overall photocatalytic
(b,e) Ni-Cu alloy nanoparticles and (c,f) Ni nanoparticles from (111) plane.
cycle of H2 evolution
regarded as Ni-Cu alloy with the size of 15 nm.
The instability of CuNPs prevents their direct use for
photocatalytic H2 evolution. Thus, NiNPs, CuNPs and Ni-
CuNPs were loaded on TiO2 by the impregnation method to
compare catalysis of Ni-CuNPs/TiO2 for photocatalytic H2
evolution with those of NiNPs/TiO2 and CuNPs/TiO2. The
photocatalytic H2 evolution was conducted by photoirradiation
(λ > 340 nm) of a deaerated mixed solution (2.0 mL) of a
phthalate buffer (50 mM, pH 4.5) and acetonitrile (MeCN) [1:1
(v/v)] containing NADH (1.0 × 10–3 M), QuPh+–NA (8.8 × 10–4
M) and Ni-CuNPs/TiO2, NiNPs/TiO2 or CuNPs/TiO2 (100 mg
L–1). Continuous H2 evolution was observed for the reaction
system employing Ni-CuNPs/TiO2 with an H2-evolution rate of
0.9 µmol h–1, whereas no or insignificant amount of H2
evolution was observed for the reaction systems employing
CuNPs/TiO2 and NiNPs/TiO2 (Fig. 2). Thus, alloy formation of
2. Results and discussion
2.1 Effect of Ni-Cu alloy formation on catalysis
Ni and Ni-Cu alloy nanoparticles (NiNPs and Ni-CuNPs) were
prepared by thermal decomposition of Ni(acac)2 and/or
Cu(acac)2 in deaerated oleylamine and 1-ocatadecene at 230 °C
by changing the ratio of Ni/Cu between 1/0 and 5/5.42 Cu
nanoparticles (CuNPs) were obtained by Cu(OAc)2 reduction
by NaBH4 in the presence of polyvinylpyrrolidone (PVP).43
The molar ratio of Ni/Cu in the Ni-CuNPs determined by the
X-ray fluorescence measurements was 55/45. The particles
sizes of the as-prepared nanoparticles determined by dynamic
laser scattering (DLS) were 25, 15 and 50 nm for NiNPs, Ni-
CuNPs and CuNPs, respectively (Fig. S1). The sizes of NiNPs
and Ni-CuNPs agreed with those determined by the TEM
observation (Figs. 1b and 1c), however, the sizes of CuNPs
determined by TEM were larger than 100 nm (Fig. 1a), which
is double of those determined by DLS measurements. Growing
of CuNPs resulted from the exposure to the electron beam for
TEM observation, or intrinsic instability of CuNPs after
removal of excess stabiliser during washing process.
Alloy formation in Ni-CuNPs was clearly confirmed by
powder X-ray diffraction measurements. In general, both Cu
and Ni metals take the face–centred–cubic structure.44 The
reflection peaks from (111) surfaces of CuNPs and NiNPs
appeared at 2θ = 43.4° and 44.7°, respectively (Figs. 1d and 1f),
which are agreed with the values reported for Cu metal and Ni
metal, 43.38° and 44.53°, respectively,44 although the peak
position of CuNPs is inaccurate owing to concomitant
formation of Cu2O species as evidenced by the peak appeared
at 36.58°.44 The (111) diffraction peak of Ni-CuNPs appeared
at 43.5°, which is in between those from NiNPs and CuNPs,
without an accompanied peak. Thus, Ni-CuNPs used as a
precursor for preparation of Ni-CuNPs/MOx (vide infra) are
Fig. 2 Time courses of H2 evolution under photoirradiation (λ > 340 nm) of an N2-
saturated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1
(v/v)] containing QuPh+–NA (8.8 x 10–4 M), NADH (1.0 x 10–3 M) and 3.5 wt% Ni-
CuNPs/TiO2 (red circle), 1.3 wt% NiNPs/TiO2 (green square) or (c) 3.2 wt%
CuNPs/TiO2 (blue triangle) (100 mg L–1).
2 | J. Name., 2012, 00, 1-3
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Ni and Cu is necessary for high catalytic activity. The high photocatalytic H2 evolution was also examined by employing
catalytic activity could result from the optimisation of metal- Ni-CuNPs/TiO2 instead of Ni-Cu/TiO2 under otherwise the
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hydrogen bond strength, because the nickel-hydrogen bond same conditions (Fig. 4a, circle). The H2-evolution rate and H2
DOI: 10.1039/C5RA04838A
seems too strong, on the contrary, the copper-hydrogen bond yield were 0.82 µmol h–1 and 50%, respectively, suggesting that
seems too weak for H2 evolution as predicted by the density Ni-Cu/TiO2 is superior to Ni-CuNPs/TiO2 in terms of the H2-
functional theory (DFT) calculations.45 The induction period (5 evolution rate and H2 yield. The lower catalytic activity of Ni-
min) before H2 evolution may result from the reduction of CuNPs/TiO2 resulted from larger particles size as implied
native oxide formed on the surface of Ni-CuNPs.
previously where the optimum loading amount of Ni and Cu is
between 3-4 wt% and further increase in the loading amount
resulted in lower catalytic activity.41
2.2 Precursors effect of Ni–Cu alloy nanoparticles loaded on
TiO2 and SiO2 on photocatalytic H2 evolution
More obvious precursor effect was observed when SiO2 was
The surface structures of Ni-CuNPs/TiO2 and Ni-Cu/TiO2 were employed as the support. Ni-Cu/SiO2 prepared by using
observed by a transmission electron microscope (TEM). Ni- Ni(NO3)2 and Cu(NO3)2 exhibited high catalytic activity in
CuNPs/TiO2 were prepared by the impregnation method using terms of an H2 yield higher than 70% and an H2-evolution rate
pre-formed Ni-CuNPs as a precursor, on the other hand, Ni- of 1.3 µmol h–1 (Fig. 4b, square). On the other hand, Ni-
Cu/TiO2 samples were prepared by the impregnation method CuNPs/SiO2 exhibited a lower H2 yield of 20% and an H2-
using Ni(NO3)2 and Cu(NO3)2 as precursors with the reductive evolution rate of 0.2 µmol h–1 (Fig. 4b, circle). TEM
treatment. Particles with the size of 10-20 nm were observed on observations suggest that the sizes of Ni-CuNPs supported on
TiO2 surfaces of Ni-CuNPs/TiO2 (Fig. 3a), whilst smaller SiO2 were ~2 nm and 10-15 nm for Ni-Cu/SiO2 and Ni-
particles with the size of ~5 nm were observed on TiO2 surfaces CuNPs/SiO2, respectively (Fig. S2). Thus, decreasing the size
of Ni-Cu/TiO2 (Fig. 3c). Formation of Ni-Cu alloy for both of Ni-CuNPs would be important for catalysis improvement.
samples was evidenced by identical peak positions of the Additionally, the higher catalytic activity of Ni-CuNPs/TiO2
diffraction peaks around 43.5° in powder XRD patterns (Figs. than that of Ni-CuNPs/SiO2 may result from the higher
3b and 3d). The loading amounts of Ni-Cu alloy were reducibility of TiO2.41
determined by X-ray fluorescence measurements as 3.5 and 3.0
(a) ꢀ
(b) ꢀ
wt% for Ni-CuNPs/TiO2 and Ni-Cu/TiO2, respectively.
*ꢀ
(a)ꢀ
(b)ꢀ
50 nmꢀ
Fig. 4 Time courses of H2 evolution under photoirradiation (λ > 340 nm) of a
deaerated mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and MeCN [1:1
(v/v)] containing QuPh+–NA (8.8 x 10–4 M), NADH (1.0 x 10–3 M) and (a) 3 wt% Ni-
Cu/TiO2 (100 mg L–1, black square) and 3.5 wt% Ni-CuNPs/TiO2 (100 mg L–1, red
circle) and (b) 3 wt% Ni-Cu/SiO2 (100 mg L–1, black square) and 3.5 wt% Ni-
CuNPs/SiO2 (100 mg L–1, red circle).
(c)ꢀ
(d)ꢀ
*ꢀ
2.3 Photocatalytic H2 evolution with Ni-CuNPs supported on
Al2O3-SiO2
On the contrary to Ni-CuNPs/TiO2 and Ni-CuNPs/SiO2, Ni-
CuNPs/Al2O3-SiO2 exhibited higher catalytic activity for the
photocatalytic H2 evolution than Ni-Cu/Al2O3-SiO2. H2-
evolution rate was 0.6 µmol h–1 for the photocatalytic H2-
evolution system employing Ni-CuNPs/Al2O3-SiO2, which was
prepared by using pre-formed Ni-Cu nanoparticles, as an H2
evolution catalyst under otherwise the same reaction conditions
for Ni-CuNPs/TiO2. The H2-evolution rate was slightly slower
but comparable to that of Ni-CuNPs/TiO2 (0.9 µmol h-1). On
the contrary, Ni-Cu/Al2O3-SiO2, which was prepared by the
impregnation method using Ni(NO3)2 and Cu(NO3)2 as
precursors with the reductive treatment, showed insignificant
20 nmꢀ
Fig. 3 (a,c) TEM images and (b,d) powder X-ray diffraction peaks of Ni-Cu/TiO2
prepared by the impregnation method using (a, b) Ni-Cu nanoparticles or (c,d)
Ni(NO3)2 and Cu(NO3)2. The diffraction peaks with the “*” mark originate from the
TiO2 support.
Photocatalytic H2 evolution was performed by employing
Ni-Cu/TiO2 as an H2-evolution catalyst (Fig. 4a, square). The
average H2-evolution rate for the reaction time between 5 to 30
min was 1.5 µmol h–1 with an H2 yield higher than 80%. The
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activity for the photocatalytic H2 evolution (Fig. 5). Thus, the formation of metal particles including Ni, Cu and Ni-Cu alloy
structure of Ni and Cu species on the surface of Al2O3-SiO2 on Ni-Cu/Al2O3-SiO2. Thus, the low catalytic activity of Ni-
View Article Online
DOI: 10.1039/C5RA04838A
should be different for these catalysts.
Cu/Al2O3-SiO2 may be ascribed to little formation of Ni-Cu
alloy particles on Al2O3-SiO2.
Little formation of Ni-CuNPs on Ni-Cu/Al2O3-SiO2 can be
explained by the presence of cation exchange sites of Al2O3-
SiO2 with a high surface area. In an aqueous solution, Ni2+ and
Cu2+ ions can be adsorbed on cation exchange sites by
electrostatic interaction. The strongly adsorbed metal ions are
hardly reduced to form metal nanoparticles on the surfaces of
Al2O3-SiO2 with a high surface area (Scheme 2). When a
support has no cation exchange sites, physisorbed Ni2+ and
Cu2+, which weakly bound to the surfaces, can move around to
form Ni-Cu alloy particles during the heat and reduction
treatment.
Cation exchangeable surfaces: Al2O3-SiO2
strongly bound metal ions
Ni2+
no particle
formation
Fig. 5 Time courses of H2 evolution under photoirradiation (λ > 340 nm) of a
deaerated mixed solution of a phthalate buffer and MeCN [1:1 (v/v)] containing
QuPh+–NA (8.8 x 10–4 M), NADH (1.0 x 10–3 M) and 3 wt% Ni-Cu/Al2O3-SiO2 (100
mg L–1, black square) or 6.5 wt% Ni-Cu/Al2O3-SiO2 (100 mg L–1,red circle).
Cu2+
O
Ni2+
Cu2+
O
O
Si
O–
O
O
O–
O
Si
O
O
Si
O
O
Si
O
O–
Al3+
O
heat and reduction
treatment
O
O
Si
O–
O
O
O
O
Si
O
Al3+
O
Al3+
O
O
O
O
O
O
O
Al3+
O
O
O
O
O
O
O
The structure of Ni-CuNPs loaded on Al2O3-SiO2 was
confirmed by TEM observations. As shown in Fig. 6a,
deposition of Ni-CuNPs with the size of 10-20 nm on Al2O3-
SiO2 surfaces was observed as the cases of Ni-CuNPs/TiO2 and
Ni-CuNPs/SiO2. Fewer and much smaller Ni and/or Cu metal
Electroneutral surfaces: SiO2
Ni2+
O
weakly bound metal ions
Ni-Cu alloy
particle
O
O
O
Si
Cu2+
Si
O
Si
heat and reduction
treatment
O
O
O
O
O
Si
O
O
O
Si
O
Si
O
O
O
O
O
Si
O
O
O
Si
O
O
O
O
(a)ꢀ
(b)ꢀ
Scheme 2 Formation of Ni-CuNPs on electroneutral surfaces and non-formation
of Ni-CuNPs on cation exchangeable surfaces
*ꢀ
2.4 Photocatalytic H2 evolution with Ni-CuNPs supported on
CeO2
Both Ni-Cu/CeO2 and Ni-CuNPs/CeO2 exhibited very low
catalytic activity for the photocatalytic H2 evolution (Fig. 7),
50 nmꢀ
(c)ꢀ
(c)ꢀ
(d)ꢀ
*ꢀ
50 nmꢀ
2θ / degreeꢀ
Fig. 6 (a,c) TEM images and (b,d) powder X-ray diffraction peaks of Ni-Cu/Al2O3-
SiO2 prepared by the impregnation method using (a, b) Ni-Cu nanoparticles or
(c,d) Cu(NO3)2 and Ni(NO3)2. The diffraction peaks with the “*” mark originate from
the Al2O3-SiO2 support.
nanoparticles on Al2O3-SiO2 were observed for Ni-Cu/Al2O3-
SiO2 as the cases of Ni-Cu/TiO2 and Ni-Cu/SiO2. The XRD Fig. 7 Time courses of H2 evolution under photoirradiation (λ > 340 nm) of a
deaerated mixed solution of a phthalate buffer and MeCN [1:1 (v/v)] containing
peak around 2θ = 43° assignable to metal particles was
observed for Ni-CuNPs/Al2O3-SiO2, however, a weak shoulder
QuPh+–NA (8.8 x 10-4 M), NADH (1.0 x 10-3 M) and Ni-Cu/CeO2 (100 mg L-1)
prepared by impregnation method using Ni and Cu salts (black square, 3wt%),
peak was observed for Ni-Cu/Al2O3-SiO2, suggesting that little
and Ni-Cu nanoparticles (red circle, 2.4wt%).
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although TEM measurements for Ni-CuNPs/CeO2 indicated
that Ni-CuNPs were successfully loaded on CeO2 surfaces as
same as Ni-CuNPs/Al2O3-SiO2 (Fig. S3). However, the H2
yields calculated from the amount of NADH in the reaction
solutions were less than 5% and the H2-evolution rates were
lower than 0.1 µmol h–1 for both Ni-Cu/CeO2 and Ni-
CuNPs/CeO2. There are two possible reasons for such low
catalytic activity of Ni-Cu/CeO2. One is the overlapping of
absorption of CeO2 and QuPh+–NA (Fig. 8). Stronger light
absorption (< 460 nm) by CeO2 disturbs the light absorption by
QuPh+–NA to form QuPh•–NA. The other possible reason is
electronic interaction between CeO2 and Ni-CuNPs. High
mobility of lattice oxygen of CeO2 may result in oxidation of
Ni-CuNPs, which is unsuitable for proton reduction.
View Article Online
DOI: 10.1039/C5RA04838A
Fig. 9 Time courses of H2 evolution by the thermal reduction of protons by
QuPh•–NA (0.44 mM) in the presence of 3 wt% Ni-Cu/TiO2 (red circle) and Ni-
Cu/CeO2 prepared by the impregnation method using Ni-Cu nanoparticles (green
triangle, 2.4 wt%) or Ni(NO3)2 and Cu(NO3)2 as precursors (blue square, 3 wt%).
(a)ꢀ
(b)ꢀ
achieve high catalytic activity, pre-formed Ni-CuNPs (Ni-
Cu/Al2O3-SiO2) should be used as
a precursor of Ni-
CeO2ꢀ
CuNPs/Al2O3-SiO2. These results clearly indicate the
importance of Ni-Cu alloy formation, in which metal-hydrogen
bond strength may be optimized. When CeO2 was used as the
support, no active catalyst for H2 evolution was obtained
irrespective of precursors, suggesting that CeO2 is unsuitable
support for Ni-CuNPs acting as H2 evolution catalysts. By
QuPh+–NAꢀ
CeO2ꢀ
QuPh+-NAꢀ
Wavelength / nmꢀ
Fig. 8 (a) Diffuse reflectance UV-vis spectra of CeO2 (black) and QuPh+–NA (red)
employing pre-formed Ni-CuNPs as
a precursor of an
and (b) photographs of CeO2 and QuPh+–NA.
impregnation method, the synergistic effect between Ni and Cu
and the support effect can be considered separately.
The rate-determining step of the photocatalytic H2 evolution
is the proton reduction on a catalyst surface or H2 evolution. To
evaluate the proton reduction and H2-evolution catalysis of Ni-
CuNPs/CeO2 under thermal reaction conditions, the proton
reduction by QuPh•–NA was executed as follows: QuPh•–NA
4. Experimental section
4.1 Materials
All chemicals were obtained from chemical companies and
used without further purification. SiO2 (10-20 nm), TiO2
(anatase), nickel acetylacetonate [Ni(acac)2] and oleylamine
(70%) were purchased from Sigma-Aldrich. Copper nitrate,
nickel nitrate, copper acetate [Cu(OAc)2], sodium hydroxide
and sodium aluminate were obtained from Wako Pure
Chemical Industries. Copper acetylacetonate [Cu(acac)2], 1-
was generated by photoirradiation of
a mixed solution
containing QuPh+–NA in the presence of NADH. After QuPh•–
NA formed, an aliquot of dispersion containing an H2-evolution
catalyst such as Ni-CuNPs/CeO2 was injected to the solution in
the dark. When Ni-Cu/TiO2 was employed as the H2-evolution
catalyst, a considerable amount of H2 evolution was observed
within 15 min (Fig. 9, circle). On the other hand, much slower
H2 evolution was observed when Ni-Cu/CeO2 or Ni-
CuNPs/CeO2 was employed as the catalyst (Fig. 9, square and
triangle). The slower H2 evolution suggests that Ni-CuNPs are
deactivated on the surfaces of CeO2. Thus, electronic
interaction between CeO2 and Ni-CuNPs is the predominant
reason for the low catalytic activity of Ni-CuNPs/CeO2.
octadecene,
polyvinylpyrrolidone
(PVP)
and
β-
dihydronicotinamide adenine dinucleotide disodium salt
(reduced form) (NADH) were obtained from Tokyo Chemical
Industry. Acetonitrile was obtained from Nacalai Tesque. CeO2
was provided by Daiichi Kigenso Kagaku Kogyo Co., Ltd. 2-
Phenyl-4-(1-naphthyl)quinolinium perchlorate (QuPh+–NA),34
Ni-Cu alloy nanoparticles,42 Ni nanoparticles,42 Cu
46,47
nanoparticles,43 and Al2O3-SiO2
were prepared by literature
3. Conclusions
methods. Purified water was provided by using a Millipore
DIRECT-Q UV3 water purification system (18.2 MΩ cm).
4.2 Preparation of Ni-Cu alloy nanoparticles42
Ni-Cu alloy nanoparticles were prepared by a literature
method.42 Ni(acac)2 (26 mg, 0.10 mmol) and Cu(acac)2 (26 mg,
0.10 mmol) were dissolved in dry oleylamine (1.0 mL) at 85 oC
under an Ar atmosphere. A mixed solution of oleylamine (10
mL) and 1-octadecene (10 mL) in a 50 mL three-neck round-
bottom flask was degassed under reduced pressure for 30 min at
Various metal oxide catalysts supporting Ni and Cu alloy
nanoparticles (Ni-CuNPs) were prepared and employed for the
photocatalytic H2 evolution. Catalytically active smaller Ni-
CuNPs were formed on TiO2 and SiO2 by impregnation of
Ni(NO3)2 and Cu(NO3)2 followed by the reduction. On the
contrary, Ni-CuNPs formation was not observed when Al2O3-
SiO2 was employed as a support instead of TiO2 or SiO2. To
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o
o
140 C to remove water and oxygen, then heated to 230 C 4.7 Preparation of Ni-Cu alloy nanoparticles loaded on
(ramp rate: 10 C min–1) under an Ar atmosphere. The solution metal oxides
o
containing metal precursors was injected into the mixed A metal oxide support (50 mg) was immersed Viienw AartichleeOxnalninee
DOI: 10.1039/C5RA04838A
solvents in 20 s, and was allowed to further react for 10 min at solution (30 mL) containing Ni-CuNPs (1.5 mL of the stock
the temperature under an Ar atmosphere. After cooled to room solution) and sonicated for 30 min. The obtained catalyst
temperature, the obtained suspension was centrifuged (9000 precursor was dried at 60 °C in an oven and calcined at 350 °C
rpm, 10 min) to separate black precipitates. The black (ramp rate: 5 °C min−1) for 4 h in air. The obtained powder was
precipitates were washed three times by dispersion/precipitation soaked in an ethanol solution containing NaBH4 to reduce
(n-hexane/ethanol) cycles. The final product was dispersed in oxides of Cu and Ni before catalysis evaluation. The loading
n-hexane (5 mL).
4.3 Preparation of Ni nanoparticles42
amount of Ni-Cu was determined by X-ray fluorescence
measurements.
Ni(acac)2 (51 mg, 0.20 mmol) was dissolved in dry oleylamine 4.8 Transmission electron microscopy (TEM)
(1.0 mL) at 85 oC under an Ar atmosphere. A mixed solution of The sizes and shapes of nanoparticles were determined from
oleylamine (15 mL) and 1-octadecene (5 mL) in a 50 mL three- bright field images using a JEOL JEM-2100 that has a thermal
neck round-bottom flask was degassed under reduced pressure field emission gun with an accelerating voltage of 200 kV. The
o
for 30 min at 140 C to remove water and oxygen, then heated observed samples were prepared by dropping a dispersion of
o
o
to 230 C (ramp rate: 5 C min–1) under an Ar atmosphere. The catalysts and allowing the solvent to evaporate and then
solution of the nickel precursor was injected into the mixed scooped up with an amorphous carbon supporting film on a Cu
solvents in 20 s, and allowed to react for 5 min at this grid.
temperature under an Ar atmosphere. After cooled to room 4.9 Catalysts characterisation by powder X-ray diffraction,
temperature, the obtained suspension was centrifuged (9000 diffused reflectance UV-vis spectroscopy and dynamic laser
rpm, 10 min) to separate black precipitates. The black scattering
precipitates were washed three times by dispersion/precipitation X-ray diffraction patterns were recorded by a Rigaku MiniFlex
(n-hexane/ethanol) cycles. The final product was dispersed in 600. Incident X-ray radiation was produced by a Cu X-ray tube
n-hexane (5 mL).
4.4 Preparation of Cu nanoparticles43
operating at 40 kV and 15 mA with Cu Kα radiation of 1.54 Å.
The scanning rate was 2° min–1 from 10° to 80° in 2θ. The
Cu(OAc)2 (60 mg, 0.30 mmol) and PVP (0.40 g) were diffuse reflectance UV-vis spectra were recorded with a Jasco
dissolved in water (20 mL) in a 50 mL three-neck round-bottom V-670 spectrophotometer equipped with an integrating sphere
flask at room temperature. Then, an aqueous solution of NaBH4 module (SIN-768). Reflectance obtained for each sample was
(11 mg, 0.30 mmol) and NaOH (12 mg, 0.30 mmol) was converted to f(R∞) values according to the Kubelka-Munk
injected into the Cu(OAc)2/PVP aqueous solution and stirred theory, f(R∞) = (1– R∞)2 / 2R∞, where R∞ is the reflectance of
for 15 min under an Ar atmosphere at room temperature. After the sample layer. BaSO4 was used for back ground spectra
the reaction, acetone was added to cause flocculation and then measurements. Dynamic light scattering (DLS) measurements
the suspension was centrifuged (9000 rpm, 10 min) to separate were performed with a Zetasizer Nano ZS instrument (Malvern
black precipitates. The black precipitates were washed three Instruments Ltd., U.S.A.).
times by dispersion/precipitation (acetone/ethanol) cycles. The 4.10 N2 adsorption for BET surface area determination
final product was dispersed in ethanol (5 mL).
4.5 Preparation of Al2O3-SiO2
Nitrogen adsorption-desorption at 77 K was performed with a
Belsorp-mini (BEL Japan, Inc.) within a relative pressure range
46,47
SiO2 was suspended in an aqueous solution (50 mL) containing from 0.01 to 101.3 kPa. A sample mass of ~100 mg was used
sodium aluminate (1.3 g, 16 mmol) and stirred for 20 h at room for adsorption analysis after pretreatment at 120 °C for 1 h
temperature. The precipitate was collected by filtration and under vacuum conditions and kept in N2 atmosphere until N2-
dried at 120 °C. The dried sample was calcined at 550 °C (ramp adsorption measurements. The samples were exposed to a
rate: 5 °C min−1) for 6 h in air.
4.6 Preparation of Ni-Cu loaded on metal oxides by using amount of N2 was calculated from the change of pressure in a
Ni(NO3)2 and Cu(NO3)2 as precursors cell after reaching the equilibrium (at least 5 min). The surface
mixed gas of He and N2 with a programmed ratio and adsorbed
A typical procedure for the preparation of metal oxides loading area of each catalyst was determined by the Brunauer-Emmett-
Ni and Cu by an impregnation method is as follows: A metal Teller (BET) method for multiple N2 adsorption amounts under
oxide support (350 mg) was immersed in an aqueous solution the conditions of partial pressure less than 0.3. The Brunauer-
containing Ni(NO3)2·6H2O (27 mg, 92 mmol) and Emmett-Teller (BET) surface areas of TiO2, SiO2, SiO2-Al2O3,
Cu(NO3)2·3H2O (21 mg, 85 mmol) and sonicated for 30 min. CeO2 were determined to be 6.8, 52, 118 and 162 m2 g-1,
The obtained catalyst precursor was dried at 60 °C in an oven respectively.
and calcined at 350 °C (ramp rate: 5 ºC min–1) for 4 h under air. 4.11 Photocatalytic H2 evolution
The obtained powder was immersed in an aqueous solution A mixed solution (2.0 mL) of a phthalate buffer (pH 4.5) and
containing NaBH4 to reduce oxides of Cu and Ni before MeCN [1:1 (v/v)] containing QuPh+–NA (0.88 mM), NADH
catalysis evaluation.
(1.0 mM) and an H2-evolution catalyst was flushed with N2 gas.
The solution was then irradiated for a certain time with a xenon
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lamp (Ushio Optical, Model X SX-UID 500X AMQ) through a
colour filter glass (Asahi Techno Glass L39) transmitting λ >
340 nm at room temperature. After 1 min stirring in the dark,
gas in a headspace was analysed by Shimadzu GC-14B gas
chromatography (detector: TCD, column temperature: 50 ºC,
column: active carbon with the particle size 60-80 mesh, carrier
gas: N2 gas) to determine the amount of evolved H2.
5
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4.12 H2 evolution with QuPh•–NA in the dark39,41
Typically, a mixed solution (2.0 mL) of a deaerated phthalate
9
Z. Han and R. Eisenberg, Acc. Chem. Res., 2014, 47, 2537–2544.
buffer (pH 4.5) and MeCN [1:1 (v/v)] containing QuPh+–NA 10 A. J. Cowan and J. R. Durrant, Chem. Soc. Rev., 2013, 42, 2281-2293.
(0.44 mM) and NADH (1.0 mM) was photoirradiated for 5 min 11 D. G. Nocera, Acc. Chem. Res., 2012, 45, 767–776.
with a Xe lamp through a colour filter glass transmitting λ > 12 M. Wang, L. Chen and L. Sun, Energy Environ. Sci., 2012, 5, 6763-
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6778.
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Acknowledgements
This work was supported by Grants-in-Aid (Nos. 24350069 and
25600025) for Scientific Research from Japan Society for the
Promotion of Science (JSPS), an ALCA project from Japan
Science and Technology Agency (JST). We sincerely
acknowledge the Research Centre for Ultra-Precision Science
& Technology, Osaka University for TEM measurements.
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a
Engineering, Osaka University, ALCA and SENTAN, Japan Science and
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Which%is%the%be*er%precursor%to%be%an%H2%evolu5on%catalyst?ꢀ
View Article Online
DOI: 10.1039/C5RA04838A
vsꢀ
Cu2+ꢀ
Cu-Ni!
NPs!
Ni2+ꢀ
Neither!ꢀ
Both!ꢀ
NPs!ꢀ
Ions!ꢀ
CeO2ꢀ
TiO2ꢀ
SiO2ꢀ
Al2O3)SiO2ꢀ
Catalysis of Ni-Cu alloy nanoparticles loaded on
various metal oxides for photocatalytic H2 evolution
depends on preparation methods and supports.
This journal is © The Royal Society of Chemistry 2012
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