A New One-Pot Sequential Reduction-Deposition Method for the synthesis of Silica-supported NiPt and CuPt Bimetallic Catalysts

Article abstract of DOI:10.1016/j.apcata.2019.117371

A one-pot redox deposition method is described to prepare silica-supported bimetallic NiPt and CuPt catalysts using differences in standard reduction potentials. The synthesis involved the initial reduction of non-noble metal ions (Ni²⁺ and Cu²⁺) by NaBH₄, followed by reduction of Pt ions by these reduced metals. Alloy formation and phase purity were established by X-ray diffraction (XRD), bulk composition by X-ray fluorescence (XRF), and surface composition by X-ray photoelectron spectroscopy (XPS). Alloy formation was enhanced by increase in the amount of base metals available to reduce Pt, or through the use of excess NaBH₄ after redox deposition. The catalysts were evaluated for methane oxidation at 300-400 °C and atmospheric pressure. Reference samples prepared using the incipient wetness impregnation (IW) method had inferior performance and characteristics. The redox deposition method provides a simple and versatile protocol for the preparation of bimetallic alloys between noble metals and base metals.

Full text of DOI:10.1016/j.apcata.2019.117371

Journal Pre-proof
A New One-Pot Sequential Reduction-Deposition Method for the
synthesis of Silica-supported NiPt and CuPt Bimetallic Catalysts
VibinVargheese, I. Tyrone Ghampson, Gwang-Nam Yun, Yasukazu
Kobayashi, Atsushi Takagaki, S. Ted Oyama
PII:
S0926-860X(19)30526-5
DOI:
https://doi.org/10.1016/j.apcata.2019.117371
APCATA 117371
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
5 August 2019
13 November 2019
4 December 2019
Please cite this article as: VibinVargheese, Ghampson IT, Yun G-Nam, Kobayashi Y, Takagaki
A, Ted Oyama S, A New One-Pot Sequential Reduction-Deposition Method for the synthesis
of Silica-supported NiPt and CuPt Bimetallic Catalysts, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117371
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A New One-Pot Sequential Reduction-Deposition Method for the Synthesis of Silica-
supported NiPt and CuPt Bimetallic Catalysts
a
a
b
a
VibinVargheese , I. Tyrone Ghampson , Gwang-Nam Yun , Yasukazu Kobayashi , Atsushi
a,d
a,b,c*
Takagaki , S. Ted Oyama
oyama@vt.edu
a
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
b
Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061, United States
c
School of Chemical Engineering, Fuzhou University, Fuzhou 350116, China
d
Present address: Department of Applied Chemistry, Kyushu University, Fukuoka 819-
0
935, Japan
*
Corresponding author.
S. Ted Oyama).
(
1

Graphical abstract
Highlights

A series of SiO -supported NiPt and CuPt samples was prepared by differences in
2
reduction potentials.

Single phase NiPt and CuPt alloys are formed at room temperature and retained after
2
H reduction.


The catalysts were tested for methane combustion under lean oxygen conditions.
Conventional catalysts prepared by co-impregnation had inferior characteristics and
performance.
Abstract
A one-pot redox deposition method is described to prepare silica-supported bimetallic
NiPt and CuPt catalysts using differences in standard reduction potentials. The synthesis
2
+
2+
involved the initial reduction of non-noble metal ions (Ni and Cu ) by NaBH , followed by
4
reduction of Pt ions by these reduced metals. Alloy formation and phase purity were
established by X-ray diffraction (XRD), bulk composition by X-ray fluorescence (XRF), and
surface composition by X-ray photoelectron spectroscopy (XPS). Alloy formation was
2

enhanced by increase in the amount of base metals available to reduce Pt, or through the use
of excess NaBH after redox deposition. The catalysts were evaluated for methane oxidation
4
at 300-400 °C and atmospheric pressure. Reference samples prepared using the incipient
wetness impregnation (IW) method had inferior performance and characteristics. The redox
deposition method provides a simple and versatile protocol for the preparation of bimetallic
alloys between noble metals and base metals.
Keywords: Bimetallic catalyst; nickel platinum; copper platinum; noble metal and base metal
alloy; reduction potential; redox deposition method
3

1
. Introduction
Supported bimetallic alloys are important materials because of their many uses as
catalysts for a variety of reactions at reducing [1,2,3,4,5,6] and oxidizing [7,8] conditions,
and their applications as electrodes in electrochemical devices [9,10]. There has been
extensive research on the use of bulk bimetallic alloy catalysts from the first half of the 20th
century with early reports by Hedvall [11] and Rienacker and Bommel [12], and other studies
associated with names like Schwab [13], Dowden [14], Eley [15], Bond [16], Emmett [17],
and Hall [18]. Subsequent investigations of supported bimetallic alloy catalysts were reported
by Sinfelt and co-workers [19] and the groups of Boudart [20], Swift and Bozik [21], and
Anderson [22].
These studies, primarily in the area of catalytic reforming of petroleum fractions,
stimulated interest in the use of bimetallic alloy catalysts for other reactions [3, 23]. The
applications of bimetallic alloys take advantage of the fact that they display catalytic
properties that are distinct from those of their two metal components. Studies since the 1960s
suggested that the unique catalytic properties of bimetallic alloys were associated with
changes in their electronic and geometric structures [24,25]. The electronic contribution was
denoted as a ligand effect [26], in analogy to the function of substituents on metallic
complexes. The geometric contribution was called an ensemble effect [26], to account for the
participation of multiple surface atoms, and is related to structure-sensitivity [27]. The
preparation of bimetallic alloy catalysts is challenging because of separate entities of the
individual metal components can form. Various synthesis methods have been investigated to
achieve intimate metal-metal interactions [23, 28,29], such as impregnation [30,31],
deposition-precipitation [29], electrostatic adsorption [32], colloidal deposition [33], ion-
exchange [34,35], and electroless deposition [36]. The methods are all effective for the
preparation of supported bimetallic samples.
4

The electroless deposition (ED) method involves the selective deposition of a
secondary metal salt onto the surface of an activated supported primary metal by
electrochemical reduction. The synthesis of catalysts with bimetallic surfaces [37,38,39] by
the use and control of an electroless bath, reducing agent, bath temperature, and pH reveal
bimetallic core-shell structures with strong surface interactions between the two metals [39,
4
0]. However, simultaneous nucleation of the depositing metal into monometallic particles
can occur unless stable precursor complexes are used in the electroless bath.
The galvanic displacement-reduction (GDR) method is a variation of the ED method
where the surface of a noble metal with a low reduction potential, activated with a suitable
liquid-phase reducing agent and in the presence of protecting polymers, reduces another
noble metal with a high reduction potential. This method has been used for the synthesis of
PdAu [41], PtAu [42] and PdPt [43] bimetallic compounds. Galvanic replacement has been
applied extensively in the field of electrocatalysis to prepare bimetallic catalysts active for the
oxygen reduction reaction [44,45]. A key motivation has been to reduce the amount of
expensive Pt used in the alloys and to synthesize core-shell nanoparticles [45,46,47].
The present paper reports a redox deposition (RD) method that is a variation of the
GDR method that allows the use of base metal components. The RD method differs from the
GDR method in two ways. In the RD method, the precursor salt of the non-noble metal is
initially deposited on the support, negating the use of protecting polymers, and the
subsequent treatment to decompose the polymer. The RD method also uses NaBH instead of
4
ethanol or ethylene glycol as the reducing agent. Other groups have previously utilized
relative reduction potentials to prepare noble metal-base metal pairs [48,49,50,51,52,53]. Bai
and coworkers synthesized hollow Pt nanospheres by using citrate-capped Co nanoparticles
as sacrificial templates [48]. Maillard and coworkers prepared hollow PtNi nanoparticles
through the combined action of galvanic displacement and the nanoscale Kirkendall effect,
5

and notably observed the preferential reduction of Ni ions by NaBH
Ni and Pt precursors [49]. Sarkar and Manthiram synthesized Pt@Cu core-shell nanoparticles
by using excess amount of NaBH for the stepwise reduction of Cu and Pt ions [50]. Konya
4
in a mixed solution of
4
and coworkers synthesized porous CuPt and CuPd nanotubes by exposing preformed Cu
nanowires to Pt and Pd precursors in a separate reaction vessel [51]. Yamashita and
coworkers prepared CoPd nanoparticles by using synthesized Co seeds to reduce Pd ions in a
different vessel [52]. Yang and coworkers synthesized Cu-containing bimetallic nanoparticles
at elevated temperatures (>100 °C) and with excess amount of oleylamine [53]. These studies
used sacrificial templates [49,49], excess reductants [49,50,53], or multiple reaction vessels
[
51,52] and in all cases prepared the bimetallic alloys in solution without a support or prior to
deposition on a support making the syntheses less controllable or less efficient.
The use of base metal components requires stronger reductants than are typically
employed for readily reducible noble metal systems. Results presented in this work for NiPt
and CuPt systems highlight key variables and provide relevant guidance for the controlled
synthesis of bimetallic alloys via redox mechanism. In this paper, the one-pot synthesis of
NiPt and CuPt bimetallic alloys by redox deposition is presented. Initially, silica-supported
2
+
2+
Ni or Cu seed metals are formed by the reduction of Ni and Cu with NaBH at room
4
temperature, followed by the reduction of Pt ions by Ni or Cu metal. Different base metal-to-
Pt ratios were explored to determine their effect on bimetallic alloy formation and to explain
the steps involved in redox deposition. Reference samples were also prepared for NiPt and
CuPt by sequential impregnation method. The catalysts were then evaluated for methane
oxidation at 300-400 °C and atmospheric pressure in a fixed-bed reactor.
2
.
Experimental
6

2
.1. Materials
The support used in this study was a fumed silica (Cabosil, L-90, SBET = 90 m /g).
The metal precursors used were NiCl ·6H O (Wako, 99.9%), Ni(NO ·6H O (Aldrich, 99.9
), CuCl ·2H O (Aldrich, 99.99 %), Cu(NO ·3H O (Wako, 99.9 %) and H PtCl ·6H
Wako, 98.5 %). The reducing agent used was NaBH (TCI, > 95 %). The gases used for
2
2
2
3
)
2
2
%
2
2
3
)
2
2
2
6
2
O
(
4
reactivity tests were methane (99.999 %), 20 % oxygen in helium (certified mixture grade),
and argon (99.9 %). The gas used for catalyst pretreatment was hydrogen (99.999 %). The
gases were all purchased from Tokyo Koatsu Yamazaki Co. Ltd.
2
.2. Catalyst preparation by the redox deposition (RD) method
Silica-supported M Pt (M = Ni, Cu) bimetallic catalysts with different molar ratios
were prepared by varying the amounts of Ni or Cu while maintaining the loading of Pt
constant at 2 wt.% (0.10 mmol/gsupport). The samples were denoted as M Pt (4:1), M Pt (2:1),
and M Pt (1:1), where the numbers in parentheses are molar ratios. The catalysts were
A
A
A
A
A
prepared using a one-pot redox deposition method as illustrated in Fig. 1. Initially, 2.5 g of
silica (41.6 mmol) was dispersed in 100 mL of deionized water and stirred at 400 rpm for 10
min. Then based on the desired molar ratios for M
appropriate amount of NiCl ·6H O or CuCl ·2H O (0.25-1.0 mmol) was dissolved in 15 mL
of deionized water and added to the silica/water mixture under continuous stirring. The
system was continuously purged with argon. To reduce the M precursor use was made of a
stoichiometric amount of NaBH (0.51-2.0 mmol) dissolved in 15 mL of deionized water.
A
Pt as summarized in Table 1, an
2
2
2
2
A
4
The solution was added dropwise to the mixture under Ar and this led to a change in color
from green to black for Ni or light blue to wine red for Cu. After stirring for 1.5 h, an
aqueous solution of H
2
PtCl
6
·6H O (0.25 mmol) was slowly added to the suspension and
2
stirred for 1.5 h under Ar atmosphere. The pH of the suspension at each stage was measured
7

using a Horiba D-51 pH meter. The solid samples were recovered by filtration and washed
several times in order to remove sodium, chlorine, and boron ions. The solid sample was then
dried overnight at 110 °C and then pelletized and sieved to a size of 650-1180 µm. Finally,
3
-1 -1
the catalysts were reduced at 400 °C under hydrogen (1000 cm min g catalyst) for 2 h and
3
-1 -1
then passivated under 0.2 % O /He flow (100 cm min g catalyst) for 3 h. Monometallic Ni
2
and Cu supported over silica were also prepared with nominal metal loadings similar to those
of NiPt (4:1) and CuPt (4:1) prepared by the redox deposition method.
Silica supported bimetallic NiPt (IW) and CuPt (IW) reference samples were prepared
by conventional co-impregnation using the incipient wetness (IW) method. Metal loadings
were chosen to match those measured by XRF (Table 2) for NiPt (4:1) and CuPt (4:1) catalysts
synthesized by the redox deposition method.
For the IW method use was made of aqueous solutions of 0.080 g of Ni(NO
0.28 mmol) and 0.13 g of H PtCl ·6H O (0.098 mmol) or 0.11 g of Cu(NO ·3H O (0.45
mmol) and 0.13 g of H PtCl ·6H O (0.097 mmol) on 2.5 g of silica (41.6 mmol), followed by
drying overnight at 110 °C and calcination at 400 °C for 4 h. The measured contents of NiPt
IW) and CuPt (IW) agree with the nominal loadings (Table 2). The samples were then
3
)
2
·6H
2
O
(
2
6
2
3
)
2
2
2
6
2
(
pelletized and sieved to a size of 650-1180 µm. Finally, the samples were reduced and
passivated under the same conditions used for the samples prepared by the redox deposition
method.
Another series of NiPt samples were also prepared by the redox deposition method
with additional NaBH
4
added to fully reduce metal ions. The nominal loadings were the same
as those of NiPt (4:1), NiPt (2:1) and NiPt (1:1). The method used was similar to the previous
except 1.5 h after the addition of Pt (Fig. 1 (d)), an aqueous solution of NaBH
4
was again
8

added. The amount was just enough to reduce all the remaining metal ions in the solution
(Table 1). The motivation behind synthesizing these catalysts was to ensure full reduction of
*
both metals, and will be discussed later. These catalysts were denoted as NiPt (4:1) , NiPt
*
*
(
2:1) and NiPt (1:1) ..
2
.3. Characterization
Temperature-programmed reduction (TPR) was carried out under H
2
with a TPD-1-
AT BEL equipped with a quadruple mass spectrometer. A quantity of 0.10 g of the as-
prepared catalyst was placed inside a quartz reactor and was treated at 200 °C for 2 h under
He and then cooled to 50 °C under He. The sample was then heated from 50 to 800 °C at a
3
-1
heating rate of 3 °C/min under H
m/z = 18) to obtain the catalyst reduction profile.
The BET surface area of the samples after reduction and passivation was determined
from the linear portion (P/P =0.01-0.20) of the N adsorption isotherms at 77 K using a
2
(100 cm min ) while monitoring the water signal
(
○
2
BELSORP mini II micropore size analyzer. Prior to the analysis, all the samples were dried
and evacuated at 120 °C for 3 h.
X-ray diffraction (XRD) patterns were measured for each catalyst before and after
reduction with a diffractometer (Rigaku RINT 2400) operated at 40 kV and 100 mA, using a
Cu Kα monochromatic X-ray source. The crystallite size (D
calculated from the line broadening of the most intense peak at 2θ = 39.9-41.8° (111) using
the Scherrer equation [54], D = K λ /β cos(θ), where K is the constant taken as 0.9, λ is the
c
) of the reduced catalysts was
c
wavelength of the X-ray radiation (0.1541 nm), β is the width of the (111) peak at half-
maximum corrected for instrumental broadening (0.1°), and 2θ is the Brag angle.
Uptakes of carbon monoxide (CO) were used to titrate the surface metal atoms with
the same equipment used for the TPR analysis. The samples were first reduced at 400 °C for
9

2
h in H flow and then cooled in He to 50 °C, following which, calibrated pulses of CO in a
2
He flow (3 % CO/He gas) were then introduced and monitored by a mass spectrometer.
X-ray photoelectron spectroscopy (XPS) was used to analyze the surface of the
samples using a PHI 5000 Versaprobe equipped with a Mg Kα X-ray source (1486.7 eV)..
The samples were reduced at 400 °C under H
2
for 2 h and passivated under 0.2 % O /He for 3
2
h prior to the XPS measurement.
Metal contents for the as-prepared catalysts were determined by X-ray fluorescence
XRF) spectroscopy using a Shimadzu Rayny EDX-800HS. Although the samples were
(
exposed to the atmosphere during handling, the metal ratios were likely not affected greatly
because the samples were passivated.
2
.4. Activity tests
The catalytic tests for methane combustion were carried out in a 1.0-cm o.d/0.75-cm
i.d, 27.5 cm long quartz tubular fixed-bed reactor in a temperature range of 300-400 °C at
atmospheric pressure. A quantity of 0.3 g of catalyst was loaded into a 2 cm section in the
center of the reactor. Prior to the reaction, the catalyst was pretreated in hydrogen for 2 h at
3
-1
4
00 °C. The reactant gas at a total flow rate of 100 cm (NTP) min consisted of 20 mol%
methane, 3 mol% oxygen, 11.6 mol% helium (internal standard) and argon (balance gas). The
reaction temperature was varied in the following order: 400, 350, 300, 325 and 375 °C. The
obtention of smooth conversion curves following this order is evidence for catalyst stability
and lack of deactivation.
The catalyst was first stabilized at 400 °C and atmospheric pressure for 4 h under the
reactant stream. The products were analyzed on-line using two gas chromatographs: a
Shimadzu GC-2010 equipped with a barrier discharge ionization detector (BID) and a Restek
capillary column (30 m × 0.32 mm i.d.× 10 µm film diameter) and a Shimadzu GC-8A
1
0

equipped with a thermal conductivity detector (TCD) and a molecular sieve 5A (3 m × 3 mm
i.d) column. Methane conversion was calculated from the moles of carbon of all observed
products divided by the initial moles of methane. Oxygen balance closed to 100 ± 5 %.
3
.
Results and discussion
.1. Synthesis method
As described in the experimental section, the catalyst synthesis started with the
suspension of the SiO support (Fig. 1 (a)) in distilled water with the pH of this suspension
being 6.1. This was followed by the addition of an aqueous solution of the primary metal
3
2
2
+
precursor (NiCl
2
·6H
Cu ions. The addition of Ni and Cu precursors decreased the pH of the suspensions to 4.6
and 4.3. Then, a stoichiometric amount of NaBH was added to the mixture under Ar
atmosphere to form metallic Ni or Cu particles adhered to the silica, as in the deposition
precipitation method. The addition of NaBH increased the pH of the Ni/SiO and Cu/SiO
suspensions to 5.8 and 5.6, which was attributed to the alkalinity of Na.
The next step consisted of the addition of the secondary metal precursor
PtCl ·6H O) to the solution containing the preformed Ni or Cu metallic clusters (Fig. 1
d)). The platinum metal precursor was dissociated into platinum chloride ions ([PtCl
2
O or CuCl
2
·2H O) (Fig.1 (b)) to provide a source of dissociated Ni or
2
2
+
4
4
2
2
(H
2
6
2
2
-
(
6
] ),
which were subsequently reduced by the metallic Ni or Cu clusters. The addition of Pt
precursor reduced the pH of the NiPt/SiO suspension from 5.8 to 5.1 and the CuPt/SiO
suspension from 5.6 to 4.8, which was attributed to the increase in H ion concentration due
to the dissociation of H PtCl ·6H O.
The equations below illustrate the reduction of [PtCl
2
2
+
2
6
2
2
-
6
] ion to Pt metal by Ni metal,
where the standard electrode potentials vs. the standard hydrogen electrode (SHE) are given
in parenthesis [55]:
1
1

+
−
°
2
Ni → 2Niꢀ ꢁ 4 e
(퐸 = ꢁ0.25 V)
ꢂꢃ
−
−
ꢀ−
°
[
[
PtCl ]ꢀ ꢁ 2e → [PtCl ] ꢁ 2Cl− (퐸
ꢊꢋ = ꢁ0.ꢌ8 V)
6
ꢄ
[ꢅꢆꢇꢈꢉ]
PtCl ]ꢀ ꢁ 2e → Pt ꢁ 4Cl
−
−
−
(퐸
°
ꢊꢋ = ꢁ0.75 V)
ꢄ
[ꢅꢆꢇꢈꢍ]
PtCl ]ꢀ ꢁ 2Ni → Pt ꢁ 2Niꢀ+ ꢁ ꢌCl− ꢎ∆퐸°
−
= ꢁ1.ꢌ8 Vꢏ
Toꢆaꢈ
[
6
As evident from the above, the total reduction potential for the NiPt system was
greater than zero (∆EoTotal = +1.68 V) and was favorable for the reaction to proceed as written.
Similar equations can also be written for the CuPt system with a total reduction potential
(
∆EoTotal = +1.09 V) as shown below:
+
−
°
2
Cu → 2Cuꢀ ꢁ 4 e
(퐸 = ꢑ0.34 V)
ꢇꢐ
−
−
ꢀ−
°
[
[
PtCl ]ꢀ ꢁ 2e → [PtCl ] ꢁ 2Cl−
(퐸
ꢊꢋ = ꢁ0.ꢌ8 V)
6
ꢄ
[ꢅꢆꢇꢈꢉ]
PtCl ]ꢀ ꢁ 2e → Pt ꢁ 4Cl
−
−
−
(퐸
°
ꢊꢋ = ꢁ0.75 V)
= ꢁ1.09 Vꢏ
Toꢆaꢈ
ꢄ
[ꢅꢆꢇꢈꢍ]
−
[
PtCl ]ꢀ ꢁ 2Cu → Pt ꢁ 2Cuꢀ+ ꢁ ꢌCl− ꢎ∆퐸°
6
Hence, the mechanism involved the reduction of the secondary metal ions by the
2
+
primary metals. The oxidized primary metal ion then dissolved in the solution: Ni (Ni →
2
+
-
2+
2+
-
Ni + 2e ) or Cu (Cu → Cu + 2e ). Fig. 2 depicts the proposed mechanism for redox
deposition.
For the M
A
Pt (2:1) and M Pt (1:1) catalysts, the redox equation indicates that all the
A
amount of primary metals should be used to reduce the Pt ions, in agreement with the XRF
results which showed that more than 95 % of the primary metals were dissolved when
compared to their nominal loading. For the M
A
Pt (4:1) samples, half of the initially added
1
2

2
+
2+
primary metal should be oxidized to Ni and Cu ions (Fig. 2 (c)), consistent with XRF
results that half of the original amounts of Ni and Cu of these samples were dissolved. The
less drastic loss of Ni and Cu cations in M
A
Pt (4:1) compared to M
A
Pt (2:1) is consistent with
the higher amount of M seed metals available to reduce Pt.
A
*
This sequence can be further understood from the XRF results of NiPt (4:1) , NiPt
*
*
(
2:1) and NiPt (1:1) catalyst, prepared by adding NaBH
4
to the solution after completion of
the redox deposition method (Fig. 1(d)). This was done to reduce all the dissolved Ni and Pt
*
ions in the solution. For example, to prepare the NiPt (4:1) , after synthesis of NiPt (4:1)
enough NaBH (1.0 mmol) was added to the catalyst solution (Table 1) to reduce all the
4
dissolved Ni metal ions in the solution. Since the synthesis of NiPt (4:1) started with 1.0
mmol of Ni to reduce 0.25 mmol of Pt ion, in the process, 0.50 mmol of Ni metal were
2
+
4+
oxidized to Ni ions while reducing the 0.25 mmol of Pt ion (2 moles of Ni to reduce 1
2
+
mole of Pt ion). To reduce these 0.50 mmol of Ni ions an amount of 1.0 mmol of NaBH
4
2
+
-
was added to the catalyst solution (Ni + 2BH
4
+ 6H
2
O → Ni + 2H
3
BO
3
+ 7H
2
). The same
*
*
reasoning was used to prepare NiPt (2:1) and NiPt (1:1) samples (Table 1). A comparison of
*
*
*
the measured Ni contents for NiPt (4:1) , NiPt (2:1) and NiPt (1:1) to their NiPt
counterparts (Table 2) shows that the addition of NaBH helped recover displaced Ni species
4
2
+
through re-reduction of Ni ions in the solution onto the support. A comparison of the Pt
contents of the two series provides additional insights. For the samples with higher Ni
contents, no significant difference in Pt content was observed, suggesting that almost all of
the platinum ions were reduced by Ni metal prior to the addition of NaBH
in the NiPt*
4
catalyst. In the case of NiPt (1:1), however, there was not enough reduced Ni metal (0.10
mmol) to reduce all the Pt ions (0.10), as evident from the measured Pt content of 0.063
*
mmol compared to the nominal 0.10 mmol. For NiPt (1:1) , the addition of NaBH
4
helped to
retain close to 0.098 mmol of Pt metal, suggesting that loss of Pt in NiPt (1:1) was due to
1
3

insufficient Ni seed metal atoms to reduce all the Pt ions. This is consistent with the
stoichiometric equation, demonstrating that the XRF results are in good agreement with the
proposed synthesis steps. The XRF measurements of all the samples did not show the
presence of Na, Cl or B, indicating that they were removed during washing.
3
.2. X-ray diffractions (XRD)
Fig. 3 shows the XRD patterns for the NiPt and CuPt catalysts prepared by the redox
deposition method. The diffraction pattern for monometallic Pt before reduction (Fig. 3 (a),
b)) showed peaks at 2θ = 39.7° and 46.2° corresponding to the (111) and (200) planes in fcc
Pt (PDF#04-0802). The diffraction patterns for the NiPt and CuPt catalysts before reduction
Fig. 3(a) and 3(b)) showed clear shifts to higher 2θ values compared to those of
(
(
monometallic Pt. The shifts to higher angle agree with previous results of NiPt [56,57,58,59]
and Cu-Pt alloys [60,61] catalysts, and give evidence for the formation of alloys even before
the reduction treatment. The shifts to higher angle are consistent with the smaller metallic
radius of Ni (125 pm) or Cu (128 pm) compared to metallic Pt (139 pm) [62], causing
contraction of the Pt lattice structure and increasing the diffraction angles. The formation of
NiPt and CuPt alloys at room temperature is due to the selective, redox-driven reduction of Pt
by Ni and Cu, as well as the favorable enthalpy of mixing between Pt and Ni (-15 kJ mol-1
-1
[
63]) and Pt and Cu (-16 kJ mol [64]). Fig. 3(a) and 3(b) also show that the shifts became
larger as the Ni or Cu content increases, in line with the decrease in the Pt lattice constant
Table 3).
(
Fig. 3(c) and (d) show the diffraction patterns for the samples after hydrogen
reduction at 400 °C. The diffraction pattern for monometallic Pt after H reduction at 400 °C
2
1
4

(Fig. 3(c), (d)) showes peaks at 2θ = 39.7° and 46.2° corresponding to the (111) and (200)
planes in fcc Pt metal. For the NiPt samples, the diffraction peaks became sharper and were
shifted toward higher diffraction angle. The sharpening of the diffraction peaks indicates the
growth of metal particles by the thermal treatment which also favors alloy formation. This
behavior was also observed for the CuPt series (Fig. 3(d)). Thermal treatment of the sample
increases the contribution to the entropy of mixing and favors alloy formation [65].
In addition to the NiPt and CuPt samples, NiPd and CuPd samples were prepared
using the redox deposition method to demonstrate its application to other base metal-noble
metal systems. The XRD patterns of these samples (Fig S2, Supplementary Information) give
clear evidence for the formation of phase-pure CuPd and NiPd alloys.
*
Fig. 4 compares XRD patterns of the NiPt series with NiPt catalysts prepared by
adding NaBH after completion of the redox deposition to reduce the oxidized Ni and Cu
4
cations in the solution. The shift to higher angles from the positions for Pt metal for the as-
*
prepared and reduced NiPt samples is enhanced compared to their NiPt counterpart. The
enhanced shift to higher angles is consistent with the incorporation of re-adsorbed Ni in the
Ni Pt alloy.
Reference samples for NiPt (4:1) and CuPt (4:1) were also prepared using the
incipient wetness impregnation method and were denoted as NiPt (IW) and CuPt (IW). The
samples were prepared with identical metal loadings as those determined by XRF analysis,
and were reduced and passivated under the same conditions used for the samples prepared by
the redox deposition method. Fig. 5 shows the diffraction patterns for the reduced samples.
The peaks for the samples prepared by the impregnation method were broader and less
intense, indicating that the particles were smaller and less crystallized. The NiPt (IW) sample
also showed a peak corresponding to the (111) reflection of metallic Ni, indicating the
1
5

formation of individual Ni clusters in addition to the NiPt alloys. For the CuPt (IW) catalyst
Fig. 5), the asymmetrical diffraction peak was deconvoluted into two features at 2θ = 41.3°,
corresponding to CuPt, and at 2θ = 42.1°, corresponding to Cu Pt [66]. This indicates that
(
3
the samples prepared by the IW method produces a mixture of phases.
The crystallite sizes calculated from the Scherrer equation were 5 nm (CuPt) and 11
nm (Cu Pt). The diffraction peak for CuPt (4:1) indicates the presence of a single CuPt phase.
3
Thus, the redox deposition method is more favorable for the formation of phase-pure
bimetallic alloys compared to traditional impregnation method.
Table 3 summarizes the lattice parameters for the catalysts after reduction using the
XRD peak positions of the (111) and (200) planes of the catalyst synthesized by the RD
method and IW method. The lattice parameters were used to calculate the lattice contractions
in the bimetallic samples compared to monometallic Pt:
퐿푎푡푡푖푐푒 푐표푛푡푟푎푡푖표푛 표푓 푏푖푚푒푡푎푙 (%)
ꢒ퐿푎푡푡푖푐푒 푝푎푟푎푚푒푡푒푟 (푎 )– 푙푎푡푡푖푐푒 푝푎푟푎푚푒푡푒푟 ꢎ푎(ꢔꢕꢖꢗꢓꢘꢙ)ꢏꢚ
푃ꢓ
=
× 100
푙푎푡푡푖푐푒 푝푎푟푎푚푒푡푒푟 (푎푃ꢓ)
The lattice contraction in NiPt samples decreased in the following order NiPt (4:1) ˃ NiPt
2:1) ˃ NiPt (1:1), as expected for the larger content of the smaller sized base metal atoms.
(
The lattice contractions for NiPt (4:1) and NiPt (IW) were comparable, but these were larger
than those in NiPt (2:1) and NiPt (1:1). The lattice contractions in CuPt catalyst followed the
same trend as NiPt: CuPt (4:1) ˃ CuPt (2:1) ˃ CuPt (1:1). There is a significant difference in
the lattice contraction between NiPt and CuPt samples. This is because the radius of Ni (125
1
6

pm) is farther from Pt (139 pm) than Cu (128 pm), and this may have limited the movement
of Ni atoms by diffusion in Pt. Table 3 also summarizes the theoretical values of lattice
parameter (atheor) for the homogeneously alloyed bimetals calculated from Vegard’s law,
which assumes a linear relationship between the alloy content and lattice parameter. These
theoretical values were calculated using the metal content from the XRF measurement. The
deviation of the lattice parameters calculated from the XRD peak positions (aexp) from the
values predicted by Vegard’s law [(aexp-atheor)/atheor] was then used to calculate the degree of
alloying in the bimetallic materials (Table 3) [67,68]. The degree of alloying was high for all
the samples, which is consistent with the favorable enthalpy of formation of NiPt [63] and
CuPt [64] alloys.
Other synthesis methods have been reported similar to the RD method described here.
One of these methods is the electroless deposition method (ED) [36,37,39]. However, the use
of an electroless bath containing the reducing agent and the more noble metal precursor
presents practical challenges because it might prevent the reduction of cations by the
reducing agent prior to deposition. To address this, Iglesia and co-workers combined the
colloidal and surface redox methods for a one-pot galvanic displacement synthesis of PdAu
[
41], PtAu [42Error! Bookmark not defined.] and PdPt [43] bimetallic compounds. The
method involved the use of a mild reducing agent (ethanol or ethylene glycol) to first reduce
solvated Pd or Pt ions which in turn then reduced Au cations with subsequent re-deposition of
the displaced Pd or Pt cations. The authors demonstrated that the use of a mild reducing agent
1
7

ensured that Au cations were reduced by Pd or Pt instead of by the reducing agent. It has
been verified in this study that this protocol fails to produce base metal-noble metal
compositions because the base metal cannot be reduced. . In addition, the use of a protecting
polymer to prevent agglomeration of the colloids in solution necessitates thermal treatment
prior to their use as catalysts which can lead to aggregation and sintering. Therefore, a
surface redox method that does not require the use of an electroless bath and a protecting
polymer simplifies the synthesis of bimetallic alloys, particularly of those involving base
metal-noble metal combinations. The synthesis of bimetallic NiPt [49,56, 69,70] and CuPt
[
50, 71] alloys by the galvanic displacement method has been reported. Although the
preparations involved the use of NaBH to first reduce Ni or Cu, followed by the addition of a
Pt precursor, as done here, the methods employed excess reducing agent. To avoid the
reduction of Pt by NaBH an amount of NaBH corresponding to the stoichiometric amount
of Ni or Cu precursor needs to be used , as we have done. However, the previously reported
studies used excess amounts of NaBH , strongly suggesting that Pt cations were
preferentially reduced by NaBH instead of by the Ni clusters in the solution.
4
4
4
4
4
It has been reported that the formation of bimetallic clusters via a redox mechanism is
dictated by differences in reduction potentials of the two component metals, as well as their
mixing enthalpies [41,42,43]. Although the preparation is not optimized, the findings
presented here for NiPt and CuPt systems provide guidelines for the controlled synthesis of
noble metal-base metal pairs.
Although the particle size of the bimetallic particles obtained by the RD method was
relatively large, and improvements are needed, this drawback is offset by the formation of
phase pure bimetallic nanoparticles at room temperature using a method that can be extended
to other compositions (NiPt, CuPt, NiPd, and CuPd).
1
8

3
.3. Temperature programmed reduction
Fig. 6 shows the evolution of water (m/z =18) during the reduction of NiPt and NiPt*
samples with different Ni content. No reduction peaks were observed for the bare SiO
2
support, indicating that the peaks from the profiles were associated with the reduction of the
metal constituents. The TPR traces were fitted with Gaussian functions to extract the peak
maxima. The profile for monometallic Pt showed a shoulder and a reduction peak at 350 °C.
The shoulder is attributed to the reduction of surface platinum oxide species and the higher-
temperature peak is ascribed to the reduction of bulk platinum oxide interacting with the
support [72,73]. The shoulder, which is present in the profiles of all Pt-containing samples,
has been previously observed in silica-supported NiPt alloys [74]. The profile for
monometallic Ni revealed a broader reduction feature at 421°C. The lower reduction
temperature of the monometallic Pt sample compared to the monometallic Ni sample reflects
the known easier reducibility of platinum [75,76] over nickel [47]. The TPR profiles for NiPt
(1:1) and NiPt (2:1) showed a peak at 348 and 349 °C, corresponding to the reduction of
platinum, while the peak associated with the reduction of nickel was not observed. The
intensity of the reduction peak for platinum is linked to the amount of Ni available to reduce
platinum metal ions during the redox reaction. For the NiPt (1:1) sample, the relatively small
reduction peak for platinum is due to the low Pt content as there is not enough nickel
4
+
available in NiPt (1:1) to reduce all the platinum since 1 mole of Pt ions requires 2 moles of
2
+
-
Ni metal (2Ni → 2Ni + 4e ). Fig. 6 shows a small but obvious shift to lower temperature of
the reduction peak for platinum in NiPt (1:1) and NiPt (2:1) compared to monometallic Pt.
Although the shift is small it likely originated from the presence of a small amount of Ni in
the Pt structure which altered the reducibility of platinum oxide. A similar shift to lower
temperature has been observed before [77]. As Ni content is increased, a peak associated with
the reduction of Ni is observed. This indicates that the Pt and Ni components the alloys
1
9

reduce at different temperatures. Moreover, the reduction peak of Ni for NiPt (4:1) shifted to
lower temperature compared to monometallic Ni catalyst, attributed to hydrogen spillover
2
+
from Pt to Ni species in close proximity [77]. These shifts in temperatures are consistent
with the formation of Ni-Pt alloys [77,78].
*
The reduction profiles of the NiPt samples provide further evidence of the effect of
higher Ni content on Ni-Pt alloy formation. Fig. 6 shows a more intense reduction peak and
*
an enhanced shift to lower temperature of the Pt reduction peak for NiPt (1:1) compared to
*
NiPt (1:1). This is linked to the higher amount of Ni metal atoms available in NiPt to reduce
*
*
Pt ions. The profiles for the higher Ni contents, (NiPt (2:1) and NiPt (4:1) ), showed a shift
to higher temperature of the Pt and Ni reduction peaks. The intensity of the Ni reduction
feature is enhanced compared to the corresponding peak of the NiPt counterpart. As was
previously explained, the addition of NaBH at the end of the synthesis re-reduced displaced
4
Ni cations in solution which were subsequently incorporated into the bimetallic alloy.
*
Notably, the higher reduction temperature peak of NiPt (1:1) is not visible even after the
addition of NaBH which further confirms that the amount of Ni in a 1:1 molar ratio is
4
insufficient to reduce all the Pt during redox deposition. The TPR profile for NiPt (IW) had a
sharp maximum at 234 °C and a lower-temperature shoulder, and was consistent with a
previous result for silica-supported NiPt prepared using the incipient wetness method [77].
The sharp peak was at a lower temperature than any of the reduction peaks from NiPt
catalysts, probably due to the smaller sizes of NiPt (IW) particles (Table 4) [76].
Fig. 7 shows the TPR profiles for the as-prepared CuPt series with different Cu
content prepared by the redox deposition method. In contrast to the Ni-containing series, the
Cu-containing samples had reduction peaks with diminished intensity, suggesting that the
components in the CuPt alloys were already in reduced form. This behavior is reasonable.
2
0

Since the samples prepared by the redox deposition method were exposed to the ambient
prior to TPR measurement, the difference in TPR profiles between the two series is probably
related to the lower oxidation potential for Cu/Cu (E Cu = -0.32 V) compared to Ni/Ni2+
2+
o
(
EoNi = +0.25 V) which made Cu in the as-prepared CuPt samples less susceptible to re-
oxidation. The profiles for the CuPt samples showed a broad reduction feature with a
maximum between the reduction temperature of monometallic Cu and monometallic Pt. This
suggests that Pt and Cu species in the alloy were reduced simultaneously. There is a
consistent shift to higher temperature with increasing Cu content, suggesting that the
reduction of the components became more difficult due to Cu-Pt interactions. The shifts are
evidence for alloy formation and are consistent with a favorable heat of formation of Cu-Pt
alloys [79]. Two reference samples were prepared by incipient wetness impregnation and
used to ascertain the reduction temperature for Cu and Pt. The Cu (IW) sample showed two
main reduction peaks centered at 200 °C, attributed to the reduction of Cu O, and 230 °C,
2
assigned to the reduction of CuO [80,81]. The CuPt (IW) sample had a low-temperature
shoulder and a prominent reduction peak at 178 °C. Although Cu and CuPt (4:1) prepared by
the RD method have similar contents to Cu (IW) and CuPt (IW), the TPR profiles were
different. The absence of the reduction peaks of Cu (IW) and CuPt (IW) from the CuPt
samples confirms that Cu and Pt components in the CuPt samples are already in their reduced
form.
3
.4. BET surface areas and CO uptakes
Table 4 shows the BET surface areas (m g ) and CO uptake (µmol gmetal-1) of the
2
-1
fresh and spent catalysts. The fresh catalysts are the samples reduced in hydrogen at 400 °C.
2
-1
The specific surface areas were between 75 to 92 m g and were close to that of the support
2
1

2
-1
(
86 m g ). The BET surface area did not change significantly after reactivity testing,
indicating that the textural properties of the catalyst were unaffected during the reaction. The
CO uptake for monometallic Ni catalyst was higher than for monometallic Pt, suggesting
higher dispersion of Ni on silica. For the NiPt series, CO uptakes increased with increasing
Ni content which is consistent with the formation of an alloy with Ni preferentially occupying
the surface of Pt. The CO uptakes for the CuPt series were an order of magnitude lower than
the NiPt series, a consequence of the intrinsic property of Cu. It is known that CO adsorbs
0
+
reversibly on metallic Cu surface and irreversibly on Cu surface [82,83]; therefore, the
minimal CO uptake capacity is consistent with Cu existing mainly in the metallic form in Cu-
containing samples as suggested by TPR (section 3.3) and XPS (section 3.5). The results
suggest surface enrichment by Cu in CuPt alloys and agree with previous spectroscopic [84]
and low-energy ion scattering [86] measurements of bimetallic CuPt nanoparticles after
hydrogen treatment. Therefore, the CO uptake values listed are an underestimation of the
available metal sites on the surface of CuPt catalysts. The CO uptake of the spent catalysts
decreased after the methane oxidation reaction, indicating that the catalysts sintered during
the reaction.
The crystallite sizes (D
equation are somewhat large, and this is part to the low surface area of the silica support. The
values show that the preparation method is not optimized. Improvements can be
c
) from XRD line-broadening calculated by the Scherrer
D
c
envisioned in the concentration of the active components used and the temperature of
synthesis. The average particle size from XRD analysis for NiPt (4:1) is 3 nm and for CuPt
2
2

(
4:1) is 7 nm, and these are lower than those obtained from transmission electron microscopy
TEM) images (Fig. S3), as the latter cannot detect smaller particles.
(
3
.5. X-ray photoelectron spectroscopy
Fig. 8(a) shows the Pt 4f core-level spectra of reduced (400 °C in hydrogen) and
passivated Pt and NiPt catalysts. Table 5 lists the binding energies for Pt 4f5/2, Pt 4f7/2, Ni
1/2, Ni 2p3/2, Cu 2p1/2 and Cu 2p3/2 core levels. The Pt 4f5/2 and Pt 4f7/2 signals for
2
p
monometallic Pt appeared at 74.2 eV and 70.8 eV, and are assigned to metallic Pt [59, 86]. In
2
+
addition, shoulder peaks at 74.4 eV and 71.4 eV are assigned to Pt in the passivation layer
or interacting with the yttria support [86, 87]. The Pt 4f signals for both the NiPt (4:1) and
NiPt (IW) samples shifted slightly to higher binding energy compared to those for
monometallic Pt. The positive shift is attributed to charge transfer between Pt and Ni.
Although Pt (2.28) is more electronegative than Ni (1.91) [88], electron transfer from Pt to Ni
is not unreasonable if the surface Pt was in reduced form and the surface Ni a was in oxidized
form before reduction by hydrogen. It can be inferred from the standard redox potentials of
Ni and Pt that, upon exposure to the ambient after redox deposition, that the Ni component on
the surface of the NiPt alloy is more likely to be in oxidized form than the Pt component. For
that reason, the electron transfer from Pt to Ni may have occurred during reduction by
2
+
hydrogen, reminiscent of Pt-induced reduction of Ni under reducing conditions previously
reported for the synthesis of bimetallic NiPt alloys [89,90]. Based on the mechanism for the
noble metal induced reduction (NMIR) method [3,89,90], it is postulated that Pt attracts and
2
+
polarizes Ni , inducing a partial positive charge on Pt. As a consequence, there is a shift in
2
+
electron density from Pt to Ni until reduction of Ni is completed. The charge transfer
2
3

indicates that Pt and Ni must be in close proximity and this gives evidence of Ni-Pt alloy
formation, consistent with the findings from XRD and TPR. Fig. 8(b) shows the Ni 2p core-
level spectra of the Ni-containing samples. The Ni 2p signal for the monometallic Ni sample
showed Ni 2p1/2 and Ni 2p3/2 contributions at 874.0 eV and 856.4 eV, which could be
2
+
assigned to Ni [91]. In addition, broad shake-up satellite peaks at 880.8 eV and 862.2 eV
2
+
are observed due to multielectron excitation which are characteristics of divalent Ni species
[
92,93]. These results indicate that surface Ni was mostly in oxidized form and was more
susceptible to oxidation than Pt. For the bimetallic samples, the signal intensities for Ni were
low because it has a lower X-ray cross-section compared to Pt [94]. The reference NiPt (IW)
2
+
sample showed a broad Ni 2p3/2 feature at 854.9 eV assigned to Ni and a satellite peak. An
obvious negative shift for the Ni 2p3/2 peak compared to monometallic Ni is in line with the
positive shift for the Pt 4f3/2 and 4f5/2 lines compared to monometallic Pt, and points to
electron transfer from Pt to Ni. The NiPt samples prepared by redox deposition showed no
visible Ni signals, suggesting low coverage of Ni on the surface and gives evidence for
surface Pt enrichment in NiPt catalysts. This result is inconsistent with deductions made from
CO chemisorption results that Ni preferentially occupies the surface of Pt in the alloy. A
plausible reason is that Pt and Ni co-exist on the surface of the alloy. This is supported by
predictions by Nørskov and co-workers [95], that the surface of a NiPt alloy will likely have
neighbouring Ni and Pt atoms.
Fig. 9 (a) shows the Pt 4f core-level spectra for the CuPt samples. There was a shift to
higher binding energy of the Pt 4f5/2 and Pt 4f7/2 lines similar to that observed for the NiPt
samples. Fig. 9 (b) shows the Cu 2p core-level spectra for the Cu-containing samples. The
deconvolution of the spectra revealed two contributions for both the Cu 2p1/2 and 2p3/2 peaks.
The Cu 2p1/2 signal at 952.3 eV and the Cu 2p3/2 peak at 932.4 eV are close to the binding
2
4

+
0
energies previously reported for partially oxidized Cu or Cu . The shoulder peaks at 954.2
2
+
+
0
eV and 933.9 eV can be assigned to Cu [96,97]. Although Cu and Cu cannot be
distinguished by XPS due to spectral overlap [96,97], it was deduced from CO chemisorption
and TPR results that the majority of the surface Cu is in metallic form. Thus, Cu is more
resistant to oxidation than Ni after superficial passivation. A small shift to higher binding
0
energy of the Cu peak indicates that partial oxidation occurred, although to a lesser extent
than the CuPt catalyst prepared by impregnation. In comparing the XPS results of the CuPt
and NiPt catalysts, the higher signal intensity for Cu suggests high coverage of Cu in CuPt as
a consequence of surface migration, and is likely the reason for the lower CO uptakes of the
Cu-containing samples. Surface enrichment of Cu in CuPt catalysts has been previously
reported [82,84].
3
.6. Reactivity
Methane combustion activities were evaluated over the NiPt (4:1), CuPt (4:1), Pt/SiO
Cu/SiO , Ni/SiO catalysts prepared by the redox deposition method and the NiPt (IW), CuPt
IW) catalysts obtained by the impregnation method. These tests were performed to compare
2
,
2
2
(
the activities of catalysts with identical metal loadings prepared by the redox deposition method
and the impregnation method. The (4:1) samples were chosen for the reactivity studies because
it was desired to examine the effect of the base metal on the Pt and because the samples had
similar compositions as the IW catalysts (Table 2). For all catalysts, the only products observed
were CO
and Arrhenius plots for the various catalysts. The conversions in all cases were lower than 10
in order to be in the kinetic regime. The slope from the Arrhenius plots was used to calculate
activation energies.
2
and H O. Fig. 10 shows turnover frequencies (TOFs) based on CO uptake values
2
%
2
5

The results for the Ni-Pt system are shown in Fig. 10(a) and (c). The activities were in
the order of NiPt (4:1) >NiPt (IW) > Ni > Pt, although the differences were not large. Both
bimetallic NiPt catalysts showed higher TOF values than the separate pure metal catalysts at
the highest temperature of 400 ℃. The E
a
values were similar for the Ni-containing samples
values were nearly the same for all
and were higher than those of the Pt catalyst. Since the E
a
the nickel-based catalysts, it is considered that the active species could be nickel metal for the
methane combustion. Even though the XPS measurements suggest electron transfer from Pt to
Ni (Fig. 8), there is probably little ligand effect since the TOFs and activation energies for the
alloys were similar to those of pure Ni.
The results for the Cu-Pt system are shown in Fig. 10(b) and (d). The activities were in
the order of CuPt (4:1) > Cu >CuPt (IW) > Pt. The bimetallic CuPt (4:1) catalyst prepared by
the redox deposition method showed a higher TOF than a pure Cu catalyst whereas the CuPt
(
IW) catalyst that was composed of a mixture of CuPt and Cu Pt (Fig. 5) had a lower TOF,
3
which was close to that of Pt. This difference in activity between the CuPt (4:1) and CuPt (IW)
catalysts cannot be explained by differences in particle size because the calculated crystallite
size of the catalysts were similar (Table 4). The difference could be due to the presence of
different CuPt phases in the catalysts, as determined by XRD analysis. Thus, these results
suggest that the CuPt phase is more active than Cu
3
Pt phase for methane combustion. Previous
Pt and CuPt,
studies have reported a difference in the electronic states of Cu and Pt in both Cu
3
arising from the change in their d-band states due to electronic (ligand) and geometric (strain)
effects [98,99]; these changes in the electronic states might be responsible for the difference in
catalyst activity. For the E
a
values, the order was CuPt (4:1) >CuPt (IW) > Cu > Pt. Apart from
values were different from one another among the copper-
bimetallic NiPt catalysts, the E
a
based catalysts, indicating a possible ligand effect. The activation energies of the bimetallic
NiPt and CuPt catalysts compared to the monometallic Pt catalyst was high. As discussed
2
6

above, XPS results indicate electrons transfer from Pt to Ni and Cu in the bimetallic system,
suggesting that the electronic structure of Pt in the NiPt and CuPt catalysts differs from that of
Pt in the monometallic Pt catalyst. The change in the electronic behavior probably modified
the catalytic sites, leading to different activation energies.
Previous investigations were conducted with high O
Pt noble metal catalysts showed higher activities than NiO and CuO catalysts (Table 6). The
results here were obtained at much lower O /CH ratios and the activity order was Cu > Ni >
Pt, with lower E values. This order is the reverse from that at high O /CH ratios. According
to previous work, the O /CH ratio significantly affects the methane catalytic combustion on
Pt [100] and Cu catalysts [101]. At O /CH ratio > 2, the surface active sites are totally saturated
by chemisorbed oxygen and the reaction rate strongly depends on methane pressure.
2
/CH
4
ratios under which Pd and
2
4
a
2
4
2
4
2
4
On the other hand, under oxygen-deficient conditions (O
active sites are less occupied by chemisorbed oxygen and more with other intermediates, so
the rate then depends on both methane and oxygen. Also, the E values are lower than that at
high O /CH ratio. Accordingly, the reaction might proceed by a different mechanism on Cu,
Ni, Pt metals at low O /CH ratio where Cu, Ni in the metallic state might be able to dissociate
CH in the vicinity of adsorbed oxygen. Therefore, the result suggests that cheap Cu, Ni and
their bimetallic CuPt, NiPt alloys can be promising alternatives as combustion catalysts at low
/CH ratio conditions.
2
/CH
4
ratio < 2), the surface
a
2
4
2
4
4
O
2
4
4
.
Conclusions
This work was carried out to demonstrate a new one-pot synthesis method for silica-
supported NiPt and CuPt catalysts by means of differences in reduction potentials of the two
2
7

components. Evidence for the formation of NiPt and CuPt alloys was obtained from XRD
patterns, and this was supported by shifts in reduction peak positions from TPR profiles, shifts
in binding energies from XPS spectra, and differences in uptakes between monometallic and
bimetallic samples from CO chemisorption measurements. Other Ni-Pt and Cu-Pt structures
were not detected in the XRD patterns, suggesting the formation of a single phase alloy.
Conversely, multiphase species were present in the reference silica-supported NiPt and CuPt
catalysts prepared by incipient wetness impregnation. A proposed redox formation mechanism,
in which a fraction of Ni or Cu acts as sacrificial seed metal atoms to reduce Pt, was supported
by XRF results. The fraction of Ni or Cu lost and the extent of bimetallic mixing was found to
be strong functions of the amount of Ni or Cu seed metals available to reduce Pt. Additional
amounts of NaBH were used to recover a majority of the displaced oxidized cations after the
4
redox reaction, which increased the number of base metals available for bimetallic alloy
formation. The similarity of the XRD features for both series indicated that NiPt and CuPt
alloys have similar bulk characteristics. The relative intensities of XPS signals of the
components and the disparity in CO uptakes, however, suggested that NiPt and CuPt have
different surface compositions: the coexistence of Ni and Pt atoms on the surface of NiPt alloys
and Cu-enrichment on the surface of CuPt alloys.
In the methane combustion, prepared NiPt (4:1) and CuPt (4:1) alloy catalysts showed
higher TOF values than NiPt (IW) and CuPt (IW) catalysts prepared by a conventional
impregnation method as well as separate pure metal catalysts. This activity enhancement was
suggested to be due to single phase alloy formation of NiPt and CuPt by the redox deposition
method developed in this study.
Author contributions
2
8

A New One-pot Sequential Reduction-deposition Method for the Synthesis of Silica-
supported NiPt and CuPt Bimetallic Catalysts
VibinVargheese, Carried out most of the experimental work, wrote the first draft
Tyrone Ghampson, Helped with the synthesis of catalysts and the writing of the paper
Gwang-Nam Yun, Helped with the testing of the catalysts and the figures
Yasukazu Kobayashi, Helped with the literature review on reactivity and the writeup
,
Atsushi Takagaki Helped with the synthesis and the directing of the research
S. Ted Oyama, Directed the research and wrote the paper
Acknowledgments
This work was supported by the Japan Science and Technology Agency under the
CREST program, Grant Number JPMJCR16P2 and the US Department of Energy, Basic
Energy Sciences under grant DE-FG02-96ER14669. We also acknowledge the assistance of
Dr. Norihisa Fukaya (AIST) for XRF measurements.
References
[
1] W. Yu, M.D. Porosoff, J.G. Chen, Review of Pt-based bimetallic catalysis: From model
surfaces to supported catalysts, Chem. Rev. 112 (2012) 5780-5817.
2
9