Sub-10 nm Au-Pt-Pd alloy trimetallic nanoparticles with a high oxidation-resistant property as efficient and durable VOC oxidation catalysts

Article abstract of DOI:10.1039/c4cc04596c

Sub-10 nm AuPtPd alloy trimetallic nanoparticles (TMNPs) with a high oxidation-resistant property were prepared by photo-deposition followed by a high temperature (700-900 °C) air annealing process.

Full text of DOI:10.1039/c4cc04596c

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Sub-10 nm Au–Pt–Pd alloy trimetallic
nanoparticles with a high oxidation-resistant
property as efficient and durable VOC oxidation
catalysts†
Cite this: Chem. Commun., 2014,
50, 11713
Received 17th June 2014,
Accepted 30th July 2014
Peisheng Qiao,a Shaodan Xu,a Dejiong Zhang,b Renhong Li,c Shihui Zou,a
DOI: 10.1039/c4cc04596c
Juanjuan Liu,a Wuzhong Yi,a Jixue Lib and Jie Fan*a
www.rsc.org/chemcomm
Sub-10 nm AuPtPd alloy trimetallic nanoparticles (TMNPs) with a high temperatures above 300 1C. The challenge of preparing highly
oxidation-resistant property were prepared by photo-deposition active nanocatalysts based on sub-10 nm MNPs that have long-
followed by a high temperature (700–900 8C) air annealing process. term stability at high reaction temperatures is in high demand.8–10
Here, we report a successful synthesis of sub-10 nm AuPtPd
Trimetallic nanoparticles (TMNPs) have received considerable TMNPs by photo-deposition within ordered, extra-large mesoporous
attention due to their unique catalytic, electrical, magnetic and TiO2 (EP-TiO2) followed by a high temperature (700–900 1C) air
optical properties, which are very different from those of the annealing process. With respect to other Pd-containing BMNPs, the
monometallic or bimetallic ones due to synergistic effects Pd species in the TMNPs display a high oxidation-resistant property,
between multiple metal elements.1,2 Many methods have been showing a high content of the Pd0 oxidation state (490%) after
developed to prepare TMNPs, and most of them need complex thermal aging (up to 900 1C) and significantly enhanced activity
multi-step procedures and are in unsupported forms.3–5 The for volatile organic compound (VOC) oxidation.
coexistence of trimetallic precursors complicates the deposition or
The synthetic procedure is described in Scheme 1. The meso-
decomposition kinetics and thus makes it more difficult to ration- porous support (EP-TiO2) has a well-defined three-dimensional
ally design the growth process of nanocrystals. In addition, during cubic mesoporous structure (face-centered cubic) with a cage size
the subsequent thermal treatment performed to get metal nano- of 26.4 nm and a window size of 7.9 nm.11 To prepare AuPtPd
particles, the particles must have enough mobility to migrate on the TMNPs (noted as AuxPtyPdz/EP-TiO2, where x, y, and z represent a
support, interact with each other, and form trimetallic particles. nominal metal molar ratio of Au, Pt, and Pd, respectively), 10 mL
However, phase separation and sintering are two common of methanol solution containing 100 mg of EP-TiO2 and desired
problems, especially when the metal ratio falls in the miscibility amounts of metal precursors (H2PtCl6, HAuCl4, and PdCl2)
gap, or if the intended composition is not thermodynamically was dispersed in a Pyrex glass reactor. After removal of oxygen,
stable.6,7 Hence, the ability to synthesize supported trimetallic the mixture was subjected to the UV light irradiation for 2 h.
nanoparticles where there is a tighter control over the composi- By consecutive washing–centrifugation cycles, the product was
tion from particle-to-particle, while preserving the mean particle isolated and dried at room temperature (RT). Thermal annealing
size below 10 nm, still remains a significant synthetic challenge. at designed temperature is applied to prepare trimetallic alloy
On the other hand, catalytic studies of colloidal nano- nanoparticles. The detailed synthesis and material characteriza-
particles have shown that the thermal and chemical stabilities of tion are given in the ESI.†
MNP catalysts are crucial. Many industrially important catalytic
Fig. 1a shows the HAADF-STEM image of the Au25Pt25Pd50
processes, including CO oxidation, partial oxidation and cracking TMNPs after the photo-deposition step. The efficiency of the
of hydrocarbons and combustion reactions, are operated at
a Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry,
Zhejiang University, Hangzhou, China. E-mail: jfan@zju.edu.cn;
Fax: +86 571 87952338; Tel: +86 571 87952338
b Department of Materials Science and Engineering, Zhejiang University, Hangzhou,
China
c Key Lab of Advanced Textile Materials and Manufacturing Technology, Ministry of
Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, China
Scheme 1 Schematic description of the synthesis of AuPtPd alloy TMNPs
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
within EP-TiO2.
c4cc04596c
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Fig. 1 HAADF-STEM images and particle size distribution, typical HAADF-STEM images, corresponding EDS mappings and cross-sectional line-scanning
profiles of Au25Pt25Pd50/EP-TiO2 after the photo-deposition (a–d) and after annealing at 800 1C (e–h). The inset in (e) shows the typical HR-TEM image
of Au25Pt25Pd50/EP-TiO2 after annealing at 800 1C.
metal photo-deposition is more than 85% (Au, Pt, Pd), and the nanoparticles and then served as ‘‘seeds’’ for the growth of PtPd
actual composition (Au23Pt29Pd48) and metal loading concen- alloys on their surface. This fact is directly related to the faster
tration of the Au25Pt25Pd50/EP-TiO2 are very close to its nominal deposition kinetics of Au as compared to Pt and Pd precursors,
composition and the calculated loading content as determined which has been reported in other synthetic system.4,12
by ICP-AES analysis (Table S1, ESI†) and EDS (Fig. S1, ESI†).
Thermal annealing at temperatures up to 900 1C is applied
After photo-deposition, the Au25Pt25Pd50 TMNPs are uniformly to prepare Au25Pt25Pd50 alloy TMNPs. Fig. 2a shows the XRD
dispersed throughout mesoporous EP-TiO2, with an average patterns of Au25Pt25Pd50 TMNPs annealed at different tempera-
particle size of 6.8 Æ 1.7 nm. The EDS mappings and cross-sectional tures. Low-temperature annealing (350–700 1C) leads to the for-
line-scanning profiles of several individual Au25Pt25Pd50 TMNPs mation of PdO and AuPt alloys.5 When the annealing temperature
show that the Au signal only locates in the core of the nanoparticle increases to 700 1C, the original AuPt(111) peak shifts to a higher
with a spherical distribution, while Pt and Pd occupy the same angle (39.51) towards the Pd(111) peak (40.11), which is in agree-
spatial area with an irregular shape, indicating that the Au@PtPd ment with the shrinkage of the lattice parameter due to the smaller
core–shell structure is obtained (Fig. 1b–d, Fig. S1, ESI†).
unit cell parameter of Pd, indicating that the AuPtPd alloy is
More importantly, the EDS analysis of individual Au@PtPd formed. Meanwhile, the intensity of the PdO peak signal is
particles reveals that the actual particle-to-particle composition significantly reduced. When the annealing temperature is as high
(Au22Pt27Pd51) is also close to their nominal composition.
as 800 1C, the PdO signal almost disappears and AuPtPd diffraction
The corresponding XRD pattern of Au25Pt25Pd50 TMNPs peak continuously shifts to a high angle (39.71) as a result of the
formed after the photo-deposition shows two distinct diffraction reduced lattice distance due to incorporation of more Pd content.
peaks at 38.61 and 39.61, respectively, which could be assigned to Au The lattice fringes of 0.228 nm and 0.197 nm for individual
and PtPd alloys (Fig. 2a). The core–shell structure of Au@PtPd Au25Pt25Pd50 TMNPs are assigned to the (111) and (200) planes
TMNPs suggests that the Au species were first reduced into spherical of AuPtPd alloy nanoparticles, respectively, in good agreement
with the XRD result (Fig. 1e inset, Fig. S2, ESI†).13
The alloy structure of Au25Pt25Pd50 TMNPs formed by high-
temperature annealing is further confirmed by HAADF-STEM
and EDS analysis. The EDS mappings and cross-sectional line-
scanning profiles of several individual Au25Pt25Pd50 TMNPs show
that Au, Pt and Pd occupy the same spatial area with a spherical
shape after air annealing (Fig. 1f–h, Fig. S3a, ESI†). The EDS
analysis of individual AuPtPd alloy TMNPs reveals that the actual
particle-to-particle composition (Au28Pt27Pd45) is also close to their
nominal composition, suggesting that the high-temperature air
annealing does not change the composition of the TMNPs. Com-
bined with HAADF-STEM analysis and the corresponding EDS
mappings of a broad area (Fig. S3b, ESI†), supported Au–Pt–Pd
alloy TMNPs, where there is a tighter control over the composition
from particle-to-particle, have been successfully prepared.
One surprising positive feature of the prepared AuPtPd alloy
TMNPs is their sub-10 nm size even after such a hard thermal
Fig. 2 (a) XRD patterns of Au25Pt25Pd50/EP-TiO2 annealed at different
temperatures. For comparison, the XRD patterns of pure Au, Pt, Pd and
PdO from the JCPDS are presented. (b) Normalized XPS spectra of Pd 3d
treatment, which is very important for their catalytic applications.
The high-temperature annealing (800 1C) only leads to slight
peaks for different Pd-containing samples after air annealing at 800 1C.
The marked contents of Pd0 are determined by XPS data.
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growth of the size from 6.8 Æ 1.7 to 8.1 Æ 2.3 nm (Fig. 1a and e).
Both trimetallic alloy composition and the unique nanostructure
of EP-TiO2 are responsible for its excellent thermal stability. In the
case of monometallic and bimetallic nanoparticles of Au, Pt, and
Pd, serious sintering phenomena were observed (Fig. S4, ESI†).
The average sizes of Au50Pt50, Au33Pd67 and Pt33Pd67 calculated by
the Debye–Scherrer equation are 30.3 nm, 14.3 nm and 12.4 nm,
respectively, much larger than Au25Pt25Pd50 TMNPs. Besides, in
stark contrast to the situation using EP-TiO2 as the support, the
AuPtPd TMNPs supported on commercial TiO2 nanocrystals
(anatase) grow into large TMNPs (B20 nm), possibly through
the uncontrolled particle-migration process (Fig. S5, ESI†).14
However, the TMNPs can be fixed on the internal surface of the
cage-like mesoporous supports due to the significantly reduced
particle-migration probability, which greatly enhances thermal
stability of AuPtPd TMNPs within EP-TiO2.15
Another important feature of the supported AuPtPd alloy
TMNPs is the high oxidation-resistant property of the Pd
species. Many studies of Pd-based catalysts have confirmed
that reduced Pd0 is far more active than PdO in VOC oxidation.7
However, it is well known that Pd0 tends to be oxidized under
oxidation conditions when temperature is above the threshold
200 1C.16 From the XRD data, Au25Pt25Pd50 alloy TMNPs show
the lowest PdO signal as compared to those for Pd, PtPd and
AuPd samples after the same thermal treatment in air (Fig. S4,
ESI†). XPS analysis is performed to quantitatively evaluate the
Pd0 content of the TMNPs. It shows that the Pd0 content is as
high as 73% for Au25Pt25Pd50 alloy TMNPs, while almost all the
Pd species of pure Pd and Pt33Pd67 are in the PdO form (Fig. 2b,
Fig. S6, ESI†). This difference implies that the presence of the
Au composite is very important to stabilize the Pd0 oxidation
Fig. 3 (a) Immiscible area isotherms of the bulk Au–Pt–Pd phase diagram.
(b) XRD patterns of samples 1–5 annealed at 800 1C. (c) the reaction
temperatures for 50% (T50) and 15% (T15) conversion of n-hexane for
samples 1–6 (1 wt%) and 7–10 (4 wt%).
state. It has been reported that Au can be helpful in the PdO system, offering a new composition regulation opportunity for
decomposition at high temperature (B800 1C), resulting in the the alloy TMNP catalysts. It is worthy of note that almost all the
formation of AuPd and Pd0,1 which has been also confirmed by Pd species of supported Au50Pt25Pd25 alloy TMNPs are in the
XPS analysis in this study (Fig. 2b). Based on the bulk Au–Pt–Pd Pd0 state (B92%, Fig. 2b).
phase diagram (Fig. 3a), the high Pd content (450%) is the
One of the greatest challenges in MNP catalysis is the
miscible region to form AuPtPd alloys.2 The formation of AuPtPd deactivation of catalysts under realistic process conditions.18 In
alloy TMNPs stabilizes the Pd0 composite and suppresses its some realistic industrial reactions such as the catalytic combustion
re-oxidation during the cooling process.17 The stable and high Pd0 widely used in the tail gas processing and the oil refining process,
content of AuPtPd alloy TMNPs manifests the unique synergistic they can maintain the reaction at only 300–400 1C for most of the
effects between multiple metal elements and distinguishes them time, but sometimes the reaction temperature can reach as high as
from monometallic or bimetallic systems.
nearly 1000 1C at one moment owing to the inevitably excessive
We have also successfully prepared the AuPtPd alloy TMNPs combustion or other reasons. Although it may only last for several
that fall in the miscibility gap of the bulk AuPtPd system using seconds, the momentary process brings serious irreversible
the same method. Fig. 3b shows the XRD patterns of the degradation of the catalyst. So in the preparation process of
supported Au50PtyPdz TMNPs after air annealing at 800 1C. Only the industrial catalysts, the catalysts must be aged under harsh
Au50Pt5Pd45 falls in the miscibility region based on the bulk pretreatment conditions before use in order to make sure they
Au–Pt–Pd phase diagram, and its corresponding XRD pattern are durable and stable enough.
shows a single XRD diffraction peak at 39.11, indicating that an
In this study, the catalytic performance of AuPtPd TMNPs
alloy structure is formed. It is surprising to find that both has been investigated in the oxidation of 1000 ppm n-hexane in
Au50Pt15Pd35 and Au50Pt25Pd25 also formed thermally stable air. In order to evaluate their durability in long-term operation,
alloy structures, which were not miscible at temperatures below all samples were pre-treated by thermal aging at 800 1C in air
900 1C and 1000 1C, respectively. Continuously reducing the Pd for 5 h prior to the catalytic testing. The fresh Pt and Pd samples
content leads to the formation of Au-rich and Pt-rich AuPtPd phases. (4 wt% loading) show good catalytic activity towards n-hexane
This observation suggests that thermodynamically stable miscibility oxidation, with T50 less than 240 1C. However, after thermal
of the sub-10 nm AuPtPd system is different from that of the bulk aging at 800 1C in air, a sharp decrease of their oxidation
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activity was observed, and their T50 increased to above 400 1C useful for the synthesis of hitherto unknown composite multi-
(Fig. 3c, Fig. S7, ESI†). The activation temperatures (T15) of the Pt metallic nanoparticles structured on nanometer domains with new
and Pd catalysts also increased to 305 1C and 260 1C, respectively, chemical and electronic states, which may benefit their catalytic/
suggesting a very poor activity for n-hexane oxidation. Pd-based other properties.
bimetallic nanoparticles (Au33Pd67 and Pt33Pd67) also displayed
We are grateful for financial support from the National
poor oxidation activity after the thermal aging (Fig. 3c). In con- Science Foundation of China (21222307) and the Fundamental
trast, all AuPtPd alloy TMNPs showed good activity even though Research Funds for the Central Universities (2014XZZX003-02).
their metal loading concentration was only one-fourth of the Pt or
Pd catalyst. Among them, the Au50Pt25Pd25 sample showed the
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