One-pot synthesis of core-shell Au@CeO2-δ nanoparticles supported on three-dimensionally ordered macroporous ZrO2 with enhanced catalytic activity and stability for soot combustion

Catalysis

Science & Technology

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One-pot synthesis of core–shell Au@CeO₂

−δ

nanoparticles supported on three-dimensionally

Cite this: Catal. Sci. Technol., 2013, 3,

958

2

ordered macroporous ZrO with enhanced catalytic

2

activity and stability for soot combustion†

a

Yuechang Wei, Zhen Zhao,* Xuehua Yu, Baofang Jin, Jian Liu,* Chunming Xu,

ab

a

c

Aijun Duan, Guiyuan Jiang and Shuhua Ma*

A series of multifunctional catalysts of three-dimensionally ordered macroporous (3DOM) ZrO

2

-supported core–

shell structural Au@CeO2−δ nanoparticles were successfully synthesized by the one-pot method of gas bubbling-

assisted membrane reduction–precipitation (GBMR/P). All the catalysts possess a well-defined 3DOM structure with

interconnected networks of spherical voids, and the Au@CeO2−δ core–shell nanoparticles with different molar

ratios of Au/Ce are well dispersed and supported on the inner wall of the uniform macropore. 3DOM support facil-

itates the contact efficiency between solid reactant and catalyst, and the Au@CeO2−δ core–shell nanoparticles with

strong metal-oxides interaction improve the amount of active oxygen species and the sintering resistance of

supported Au nanoparticles due to the optimization of the interface area by formation of the metal-oxides core–

shell (MOCS) nanostructure particles. 3DOM Au@CeO2−δ/ZrO

for diesel soot oxidation. Among the as-prepared catalysts, 3DOM Au@CeO2−δ/ZrO

thickness of CeO2−δ nanolayer shell shows the highest catalytic activity for soot combustion, i.e., its T50 is 364 °C.

In summary, 3DOM Au@CeO2−δ/ZrO catalysts are excellent systems for catalytic combustion of solid particles or

2

catalysts exhibit high catalytic activity and stability

Received 15th April 2013,

Accepted 23rd May 2013

2

-2 catalyst with the moderate

2

DOI: 10.1039/c3cy00248a

macromolecules, and the design concept and facile synthesis method of 3DOM oxide-supported MOCS nanoparti-

cle catalysts can be extended to other metal/oxide compositions.

www.rsc.org/catalysis

3

4

1

. Introduction

metal, alkaline metal oxides, perovskite or perovskite-like

5

6

type oxides and ceria-based materials have been widely stud-

ied for diesel soot combustion. Especially, Ce-based materials

are one of the key components in automotive catalysts due to

their redox properties and oxygen storage/release capacity.

To design and synthesize high-performance materials in

terms of catalytic activity, understanding the properties

affecting catalytic performance is of great importance. The

nature of catalyzing soot combustion is a kind of deep oxida-

tion reaction which takes place at the three-phase boundary

among a solid catalyst, a solid reactant (soot) and gaseous

reactants (O

Particulate matter (PM, mainly containing soot particles)

emitted from diesel engines has caused acute human health

and environmental problems. This calls for efficient exhaust

gas treatment systems for diesel engines. The combination of

traps and oxidation catalysts in the continuously regenerating

particulate trap (CRT) appears to be one of the most efficient

after-treatment techniques. The key challenge is to find a

catalyst that can lower the oxidation temperature of soot as

much as possible. Several kinds of catalysts such as precious

1

6,7

8

2

9

2

, NO). Thus, the catalytic activity for diesel soot

combustion is affected by two factors: the contact efficiency

between the soot particle and the catalyst, and the intrinsic

activity of the material. These general catalysts have smaller

a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18#,

Fuxue Road, Chang Ping, Beijing 102249, China. E-mail: zhenzhao@cup.edu.cn,

liujian@cup.edu.cn; Fax: +86-10-89731586(O)

10

b

pore sizes (<10 nm) than soot particles whose diameters

range from 20 nm to 1 μm. Thus, the soot particles are diffi-

cult to enter inner pores of these catalysts with high surface

Guangxi Key Laboratory of Petrochemical Resource Processing and Process

Intensification Technology, Guangxi University, Nanning 530004, China

c

Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical

1

Materials, School of Chemistry and Chemical Engineering, University of Jinan,

Jiweilu 106, Jinan, Shandong, 250022, China. E-mail: chm_mashh@163.com

area. And the contact area and the exposed active site

amounts are limited, which affects the performance of cata-

lysts. So how to effectively make use of the inner surface for

increasing the contact between catalyst and soot particle, is a

key factor affecting the catalytic activity in heterogeneous

†

Electronic supplementary information (ESI) available: Detailed synthesis

procedures of PMMA microspheres and 3DOM ZrO , the digital photos of

2

GBMR device, SEM-EDX spectra and TEM images of the Au/ZrO catalyst cal-

cined at 500 °C. See DOI: 10.1039/c3cy00248a

2958 | Catal. Sci. Technol., 2013, 3, 2958–2970

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Paper

catalysis reactions. Three-dimensionally ordered macroporous

structural Au@CeO₂NPs on 3DOM ZrO₂support (3DOM

−δ

Au@CeO₂/ZrO ) is firstly designed and synthesized by a

−δ 2

1

2

(3DOM) oxides with big pore size (>50 nm) can remarkably

improve the catalytic activity for diesel soot combustion

due to the increasing contact efficiency between catalyst

one-pot method of the gas bubbling-assisted membrane

reduction–precipitation (GBMR/P) just developed in our labo-

ratory. This technique has also enabled the preparation of

metal–oxide nanocrystal over oxide supports with an uncon-

ventional morphology that cannot be achieved otherwise.

We carried out a diesel soot combustion reaction to explore

1

3

and solid particle. However, the catalytic performances of

DOM oxides for soot combustion are limited by their intrin-

sic activity.

Since the pioneering studies by Haruta and Hutchings,

3

supported gold catalysts have become one of the most exten-

sively researched issues in the field of heterogeneous cataly-

sis. Au nanoparticles (NPs) as active components are possible

choices for enhancing the intrinsic catalytic performance of

the catalytic activity and stability of 3DOM ZrO

2

-supported

Au@CeO₂core–shell nanoparticle materials. The well-

−δ

14

defined 3DOM structures consisting of interconnected net-

works of spherical voids with a big pore size (>50 nm) are

favorable for improving contact between catalysts and solid

3

DOM oxides, because the strong metal (Au)–support (oxide)

interaction (SMSI) is able to promote the adsorption–activation

property for reactant molecule (O ) and to increase the

reactants. And the Au@CeO₂core–shell nanostructure parti-

−δ

2

cles on the inner wall of uniform macropores supply the high

&

−

2−

−

amount of active oxygen species O

(O₂and O ) on the

density of active sites for O activation. The multifunctional

2

1

5

surface of catalysts. Thus, 3DOM oxides-supported gold cat-

alysts, which combine both advantages of the good contact

between diesel soot and catalyst by 3DOM supports and the

abundant active sites by gold clusters with suitable nanosize,

nanostructure material of 3DOM ZrO -supported Au@CeO

2 2−δ

core–shell nanoparticles is an excellent system for heteroge-

neous catalytic reaction or surface chemical process that

occurs at the interface of reactant and catalyst.

1

6

exhibit super catalytic performance for soot oxidation.

Unfortunately, the stabilities of supported Au-based catalysts

are poor. Therefore, improving the stability and catalytic

activity of 3DOM oxides-supported Au-based catalysts is the

key issue in development of relevant catalysis field.

2

. Experimental section

2.1 Sample preparation

Synthesis of monodispersed polymethyl methacrylate (PMMA)

microsphere was synthesized using a modified emulsifier-free

emulsion polymerization technique with water–oil biphase

double initiators. The hard template (PMMA) was formed by

centrifugation at 3000 rpm for 10 h. 3DOM ZrO support was

prepared by the colloidal crystal template (CCT) method using

ethylene glycol (EG)–methanol solution of ZrOCl ·8H O as

precursor solution. It is similar to that described previously.

Excess liquid was removed from the impregnated micro-

sphere templates via a Buchner funnel connected to vacuum.

The obtained precursor–template mixture was dried in air at

50 °C for 24 h. And then the dried sample was calcined to

remove the CCT at an air flow of 80 ml min in a tube fur-

nace. The temperature-rising rate was 1 °C min from room

temperature to 650 °C, and the calcination at 650 °C was kept

for 8 h. Finally, 3DOM ZrO support was obtained. Synthesis

of monodispersed PMMA microsphere, assembly of template,

and preparation of 3DOM ZrO

(CCT) method are shown in the ESI† in detail.

3DOM Au@CeO₂/ZrO catalysts were synthesized by one-

It is well-known that the catalytic performance of complex

nanostructures often shows strong structure and size depen-

dencies duo to so-called “SMSI” occurring at the interface

1

7

between the active (metal) phase and the support. Many

ideas have been proposed in the catalysis literature on the

origin of such support effects. These include electronic inter-

actions between both components, stabilization or destabiliza-

tion of certain particle shapes or sizes, surface transportation

of adsorbates through the boundary (spillover, reverse spill-

over, capture zone effects), and migration of partially reduced

oxides onto the active phase or the stabilization/destabilization

2

1

3

18

−1

of oxidized active phases by the support. In principle, the

combination of oxide (CeO ) NPs in a metal (Au) matrix can

−

1

2

produce materials with novel structure and/or electronic prop-

erties for the coordinatively unsaturated ceric (CUC) sites

2

1

9

which are active centers in the catalytic oxidation reactions.

It is attributed to the fact that the SMSI could be optimized

2

by colloidal crystal template

by formation of the Au–CeO₂core–shell nanostructure cata-

−δ

lysts due to the increasing interface areas between metal

−δ

2

0

active component and oxide support. However, assembly

of multifunctional nanostructure material with ordered

macroporous and high density of CUC sites is still a great

challenge because of the poor control over these parameters

in the fabrication of much more elaborate nanocrystals.

Therefore, the shape-controlled synthesis of multifunctional

nanostructure material with Au@CeO2−δ core–shell NPs

deposited on the surface of 3DOM oxides at the nanoscale is

a potential route for enhancing their catalytic activity and

stability for soot combustion.

pot process of the gas bubbling-assisted membrane reduction–

precipitation (GBMR/P) method just developed in our labora-

tory. The schematic diagram of the preparation method is

presented in Scheme 1 and the reagent specifications are shown

in Table 1. The detailed information about experimental appa-

ratus is shown in Fig. S2 (ESI†). The typical preparative proce-

dures were described as follows: the poly(N-vinyl-2-pyrrolidone)

(PVP) solution as a stabilizer was added into HAuCl solution

4

(beaker I). Then 3DOM ZrO₂support (1 g, 8.13 mmol) and

3 3 2

Ce(NO ) ·6H O were also introduced into the solution. The mix-

Motivated by the above considerations, the multifunc-

tional nanostructure material that consists of the core–shell

ture solution (100 ml) was driven by a peristaltic pump (BT00-

600M, Baoding Lange Co., Ltd, P.R. China) at ∼360 ml min

−

1

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always 4 wt%. The catalysts are generically named as 3DOM

Au@CeO₂/ZrO -n, where n is the theoretical molar ratio of

−δ

2

CeO

2

to Au.

For comparison, 3DOM Au/ZrO catalyst was prepared

2

1

6

by GBMR method. But 3DOM Au/ZrO₂catalyst was just

calcined in an oven at 200 °C for 1 h in order to obtain the

similar sizes of supported Au NPs with those of 3DOM

Au@CeO₂/ZrO catalysts.

−δ

2

2.2 Physical and chemical characterization

Scheme 1 Schematic diagram of the gas bubbling-assisted membrane reduction–

2

precipitation (GBMR/P) device for synthesis of 3DOM ZrO -supported Au@CeO2−δ

core–shell nanoparticle catalysts.

The nitrogen adsorption/desorption isotherm was obtained

1

6b

at −196 °C by using an automated gas sorption analyzer

(

Quantachrome Autosorb-iQ, USA) after having degassed the

sample for 4 h at 300 °C. The specific area was calculated

^f^r^o^m^t^h^e^B^r^uⁿ^a^u^e^r^–^E^m^m^e^t^t^–^T^e^l^l^e^r⁽^B^E^T⁾^e^q^u^a^tⁱ^oⁿ^u^sⁱⁿ^g^P^/^P0

Table 1 Reagent specifications for the preparation of 3DOM Au@CeO2−δ/ZrO

2

catalysts

values between 0.05 and 0.3. Mercury porosimetry experi-

ments were carried out on a Micromeritics AutoPoreIV 9500

apparatus in the pressure range 0–30 000 psi. XRD patterns

were measured on a powder X-ray diffractometer (Shimadzu

XRD 6000) using Cu Kα (λ = 0.15406 nm) radiation with a

Nickel filter operating at 40 kV and 10 mA in the 2θ range of

a

Sample

3DOM)

Support HAuCl

4

Ce(NO

(mM)

3

)

3

PVP

NaBH

4

NH

3

(mM)

(

(mM)

(mM) (mM)

ZrO

Au/ZrO

Au@CeO2−δ

ZrO -2

Au@CeO2−δ

ZrO -5

2

8.13

—

0.20

—

0

0.40

—

20

—

1.00

—

0

4.0

2

/

2

15–90°. The UV-Vis DRS experiments were performed on a

8.13

0.20

1.00

2.00

20

1.00

10.0

20.0

UV-Vis spectrophotometer (Hitachi U-4100) with the integra-

tion sphere diffuse reflectance attachment. The surface mor-

phology of the samples was observed by SEM (FEI Quanta

2

Au@CeO2−δ

ZrO -10

2

a

In monomer unit.

200F). The TEM and HRTEM images were carried out using a

JEOl JEM 2100 electron microscope equipped with an LaB₆

electron gun at an accelerating voltage of 200 kV. The average

particle diameter was calculated from the mean diameter fre-

flow rate to form a tubal cycling flow. A reductant solution

NaBH , 50 ml) in beaker II was injected to the membrane

(

4

quency distribution with the formula: d  n d / n , where

reactor with two ceramic membrane tubes (∅ 3 × 160 mm) by a

i

constant flow pump (HLB-2020, Satellite Manufactory of

i i

n is the number of particles with particle diameter d in a

−

1

Beijing, P.R. China) at a flow rate of 1 ml min . Meantime,

the hydrogen gas was also injected by the two other mem-

brane tubes. The metal precursor solution flowed in the glass

certain range. The actual content of Au in catalysts was deter-

mined by inductively coupled plasma atomic emission spec-

trometry (ICP-AES) (PE, OPTIMA 5300DV). X-ray photoelectron

spectra (XPS) were recorded on a Perkin-Elmer PHI-1600 ESCA

spectrometer using Mg Kα (hν = 1253.6 eV, 1 eV = 1.603 ×

tube reactor and outside the ceramic tubes. The NaBH solu-

4

tion was infiltrated through the abundant holes (d = 40 nm)

on the wall of the ceramic tubes into the glass tube reactor,

where the reduction of metal ions occurred immediately when

the two solutions met. The hydrogen bubbling-assisted stir-

−

19

10

J) X-ray source. The binding energies were calibrated

using C 1s peak of contaminant carbon (BE = 284. 38 eV) as

an internal standard. Temperature-programmed reduction

with H (H -TPR) measurements were performed in a conven-

−

1

ring operation (40 ml min ) was developed to vigorously stir

the solution and to make the reaction homogenous. After

2

tional flow apparatus. 100 mg sample was pretreated on a

fixed-bed tubular quartz system (ϕ = 8 mm) in an Ar atmo-

sphere at 200 °C for 1 h and subsequently cooled to 30 °C.

complete consumption of the NaBH solution, the precipita-

4

3 2

tion agent (NH ·H O) was added into beaker II and was

−

1

injected to the membrane reactor same as the reductant agent

Then 10% H /Ar flow (40 ml min ) was passed through the

2

−

1

by a constant flow pump at a flow rate of 1 ml min . The syn-

thesis process was not stopped until complete consumption

of the precipitation agent solution. The reaction system was

further vigorously bubbled with hydrogen gas for 1 h, and

subsequently kept static for 1 h. Then, the product was fil-

catalyst bed while the temperature was ramped up from 30 to

−

1

900 °C at a heating rate of 10 °C min . The hydrogen consump-

tion signal was monitored by a thermal conductivity detector

(TCD). Before the outlet gases entering the TCD, a cooling trap

and a filter packed with 5 Å molecular sieve (60–80 meshes)

were used to remove H O and CO . Raman spectra of the sam-

−

tered and washed with distilled water at 60 °C until the Cl

2

−

1

was completely removed according to a test with AgNO . The

ples were collected in the anti-Stokes range of 200–2000 cm

3

final products were calcined in an oven at 500 °C for 2 h and

using a inVia Reflex-Renishaw spectrometer. The sample

has been excited using a He–Gd laser (excitation wavelength of

325 nm).

the desired 3DOM Au@CeO₂/ZrO catalysts were obtained.

−δ

2

The initial weight percents of Au to ZrO in the catalysts were

2

2960 | Catal. Sci. Technol., 2013, 3, 2958–2970

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.3 Activity tests

Paper

2

size of holes on the ceramic membrane tubes. The highly

homogeneous dispersion of reductant/precipitant may be an

operative means to regulate the nucleation process so as to

produce a mass of Au or Ce(OH) nucleus. The steady H gas

The catalytic activities of all catalysts were evaluated with a

temperature-programmed oxidation reaction (TPO) on a

fixed-bed tubular quartz system (ϕ = 8 mm), and each TPO

run from 150 to 650 °C at a 2 °C min temperature-rising

rate. The soot was Printex-U (diameter of 25 nm, purchased

from Degussa). The catalyst (100 mg) and soot (10 mg) were

mixed at a weight ratio of 10 : 1 with a spatula in order to

reproduce the loose contact mode. Reactant gases containing

3

2

−

1

bubbling can promote mass transfer, reduce dead space and

concentration gradient of reductant, and improve homogene-

ity of the precursor solution. When the synthesis process

begins, numerous small crystallites are initially generated

nucleation) on the surface of the support, and they tend to

quickly aggregate together to form larger and more thermo-

The addition of

organic stability reagent into precursor solution may be an

operative means to regulate the growth process. Conse-

(

5

% O and 0.2% NO balanced with Ar were passed through a

2

−

1

dynamically stable particles (growth).

mixture of the catalyst and soot at a flow rate of 50 ml min .

The outlet gas compositions were analyzed with an on-line

gas chromatograph (GC, Sp-3420, Beijing) by using FID detec-

quently, the highly dispersed and uniformed Au@CeO

2−δ

tors. The catalytic activity was evaluated by the values of T10

50, and T90, which were defined as the temperatures at 10%,

0%, and 90% of soot conversion during the TPO process,

,

core–shell NPs are supported on the surface of 3DOM ZrO

2

T

5

support. The GBMR/P method can be divided into two parts:

GBM-R and GBM-P processes. Firstly, the GBM-R process can

respectively. The selectivity to CO₂formation (S_C_O₂) was

defined as that the CO₂outlet concentration (C_C_O₂) divided

control the dispersion and size of supported Au NPs on the

−

by the sum of the CO

2

and CO outlet concentration, i.e.,

surface of 3DOM ZrO

ions are adsorbed and anchored on surface [OH

the ZrO support, because the pH of the mixture solution

∼4.0) is lower than the isoelectric point (IEP) of the ZrO

2

. In the synthesis of catalysts, [AuCl ]

4

+

m

2

] groups of

S

CO₂= CCO₂/(CCO + CCO₂). S_C_O₂was denoted as SCO₂at the maxi-

2

mum temperature corresponding to the highest rate of the

soot-burnt. In all TPO experiments, the reaction was not fin-

ished until the soot was completely burnt off.

(

2

−

support (∼6.7). Meanwhile, the hybrid between [AuCl

4

] ions

and capping ligands of the poly(N-vinyl-2-pyrrolidone) (PVP)

might form. When NaBH₄solution was injected into the

3

. Results and discussion

tubular reactor through the holes of ceramic membrane

−

3

.1 One-pot synthesis mechanism of 3DOM Au@CeO2−δ/ZrO

2

tubes, the reduction of [AuCl

4

] occurred immediately to give

catalysts via the GBMR/P method

an Au nucleus (nucleation) with convenient protection (PVP),

resulting in restriction of the growth of crystal nucleus on the

surface of 3DOM ZrO . Therefore, the uniform distribution

2

size of Au NPs core on 3DOM ZrO₂can be obtained. Sec-

ondly, the GBM-P process can control the nanolayer thick-

ness of CeO2−δ core on the surface of Au NPs core. When

The effective control of the dispersion and structure of

supported metal@oxides core–shell NPs on the surface of

3

DOM oxides is a primary goal of catalyst design and is quite

challenging. Generally, the nucleation and growth of metal or

oxide nanocrystal govern the particle size and morphology in

the synthesis process by precipitation or reduction. The

synthesis mechanism of one-pot GBMR/P method for 3DOM

2

1

precipitation agent (NH

·H O) was injected into the tubular

3 2

reactor through the holes of ceramic membrane tubes, the

3

+

precipitation of Ce occurred immediately to give Ce(OH)

3

Au@CeO₂/ZrO catalysts is shown in Scheme 2. The device

−δ

2

on the surface of 3DOM Au/ZrO . The flow rate of precipita-

2

core of GBMR/P method is the ceramic membrane reactor

consisting of four ceramic membrane tubes with abundant

holes (d = 40 nm) on the wall, two of which are used for

highly homogeneous dispersion of reductant/precipitant and

the other two used for dispersion of H gas bubbling instead

2

of mechanical stirring (Scheme 1). The dispersion rates of

the reductant/precipitant solutions can be controlled by the

tion agent by a constant flow pump can control the deposi-

tion rate of Ce(OH) , and the molar ratio of Ce/Au can adjust

3

the thickness of CeO₂nanolayer shells. Finally, the disper-

−δ

sion and structure of supported Au@CeO2−δ core–shell NPs

on the surface of 3DOM ZrO

the GBMR/P method.

2

can be effectively controlled by

3.2 Catalyst characterization

Fig. 1 shows SEM images of 3DOM ZrO₂and Au@CeO

/

2−δ

2

ZrO catalysts with different Ce/Au ratios obtained by one-pot

process of the GBMR/P method. The images exhibit that the

materials are macroporous in structure, containing periodic

voids with an average diameter of 280 ± 10 nm and a wall

thickness of 30 ± 5 nm, which corresponds to a shrinkage

of 20–30% compared with the initial size (350 nm) of

PMMA microspheres. The macroporous structure of 3DOM

Scheme 2 Schematic representation for the one-pot synthesis of Au@CeO2−δ

Au@CeO /ZrO materials are similar to that of 3DOM ZrO

2

core–shell NPs supported on the surface of 3DOM ZrO

2

carrier by the GBMR/P

2−δ

2

method.

support. It indicates that the synthesis process of the GBMR/

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Table 2 Porosities, average diameters, pore volumes and pore areas of 3DOM

ZrO

2

, Au/ZrO

2

and Au@CeO2−δ/ZrO

2

catalysts

a

b

c

d

e

Catalyst

3DOM)

V

p

V

Hg

A

p

D

p

Porosity

−1

2

−1

(

(ml g

)

(ml g

)

(m g

)

(nm) (%)

ZrO

Au/ZrO

Au@CeO2−δ/ZrO

2

0.09

0.08

0.09

2.6

2.5

2.2

1.8

35.7

34.6

35.1

33.8

291

289

283

213

91.1

91.0

89.6

86.1

2

-2

2

-10 0.11

a

b

Pore volume determined by BET method. Total pore volume for

c

d

pores. Total pore area determined by Hg porosimetry. The average

pores diameter. Determined by Hg porosimetry.

e

observed by SEM image. With increasing Ce/Au ratio, the

Fig. 1 SEM images of 3DOM ZrO

2

, Au/ZrO

2

and Au@CeO2−δ/ZrO

2

catalysts

most probable pore diameters of 3DOM Au@CeO₂/ZrO cat-

−δ

2

prepared by GBMR/P method. (A) ZrO

2

;

(B) Au/ZrO

-10.

2

;

(C) Au@CeO2−δ/ZrO -2;

2

(

D) Au@CeO2−δ/ZrO -5; (E) Au@CeO2−δ/ZrO

2

alysts become small. It is consistent with the results observed

from SEM images. The porosities, average diameters, total

pore volumes and pore areas of 3DOM ZrO , Au/ZrO and

2

P method with a gas bubbling-assisted stirring operation is

less destructive to the 3DOM structure. The next layer is

highly visible in the SEM image and the voids are inter-

connected through an open window, ca. 75 ± 5 nm in diame-

ter (Fig. 1A). However, with increasing Ce/Au ratio, the size of

the open window decreases. It indicates that the CeO2−δ

Au@CeO2−δ/ZrO

the results are shown in Table 2. The total pore volume of

2

catalysts were analyzed and compared, and

3DOM ZrO

2

support determined by

N

2

physisorption

−

1

−1

(∼0.09 ml g ) is less than 5% of that (∼2.6 ml g ) deter-

mined by MIP, indicating the main contribution of

macropores to the total porosity in these materials. And its

average pore diameter and porosity are about ∼291 nm

and beyond 90%, respectively. It indicates that the solid reac-

tants can easily diffuse in the materials. After supported

Au@CeO2−δ core–shell NPs, the porosities, average diameters,

total pore volumes and pore areas of the catalysts have an

apparent decline, which is attributed to the deposition

2

nanolayer deposited on the surface of the 3DOM ZrO sup-

port, and too much CeO₂nanolayer can affect the diffusion

−δ

of reactant in the macroporous materials.

The presence of macropores in 3DOM ZrO

2 2

, Au/ZrO and

Au@CeO₂/ZrO catalysts was assessed by means of mercury

−δ

2

intrusion porosimetry (MIP). Fig. 2 displays the differential

curves of pore size distribution of concretes. The diameter

corresponding to the peak value of a different curve is

regarded as the most probable pore diameter, whose size

occupies the largest proportion of pores compared with other

pore diameters. It can be seen from Fig. 2 that the macropores

of CeO2−δ nanolayer on the inner wall of 3DOM ZrO

It is in good agreement with the results of SEM images.

Fig. 3 shows the XRD patterns of 3DOM ZrO , Au/ZrO

and Au@CeO2−δ/ZrO catalysts. The diffraction peaks (2θ) at

2

support.

2

30.3, 35.2, 50.4, 60.2, 63.0, 74.5 and 82.0° can be indexed to

(

those in the 50–200 nm range) in this sample are very regu-

lar, and the most probable pore diameter of 3DOM ZrO is

5.5 nm, which is consistent with the size of open window

the (011), (110), (112), (121), (202), (220) and (123) crystal faces

of 3DOM ZrO with a tetragonal structure (PDF# 50-1089),

2

7

Fig.

2

Pore size distribution of 3DOM ZrO

2

,

Au/ZrO

2

and Au@CeO2−δ/ZrO

(c) Au@CeO2−δ/ZrO -2;

2

Fig. 3 XRD patterns of 3DOM ZrO

prepared by GBMR/P method. (a) ZrO (b) Au/ZrO

2

(d) Au@CeO2−δ/ZrO -5; (e) Au@CeO2−δ/ZrO -10.

2

, Au/ZrO

2

and Au@CeO2−δ/ZrO

2

catalysts

catalysts determined by MIP. (a) ZrO

2

;

(b) Au/ZrO

2

;

2

;

2

;

(c) Au@CeO2−δ/ZrO -2;

2

(d) Au@CeO2−δ/ZrO

2

-10.

2

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respectively. And the weak diffraction peaks (2θ) at 28.2 and

Fig. 4 shows the TEM and HRTEM images of 3DOM Au/ZrO₂

and Au@CeO₂/ZrO catalysts. 3DOM structure with overlapped

pores can be clearly observed by TEM images after loading Au

31.5° can be indexed to the (−111) and (111) crystal faces of

DOM ZrO with a monoclinic structure (PDF# 37-1484),

−δ

2

3

2

respectively. It indicates the coexistence of the two crystal

phases structures for 3DOM ZrO₂support. Compared with

and Au@CeO₂NPs. The walls of the macroporous samples

are crystalline according to the selected area electron diffrac-

tion (SAED) (inset in Fig. 4A). The characteristic SAED rings of

−δ

3

2 2

DOM ZrO , the XRD patterns of 3DOM Au@CeO2−δ/ZrO cata-

lysts display a weaker diffraction feature, even the monoclinic

ZrO oxide from inside to outside are indexed as (011), (110),

2

phase of 3DOM ZrO support disappears. It may be attributed

(112) and (121) planes of a tetragonal structure, respectively.

It is in accordance with the result of the XRD characterization.

As shown in Fig. 4B, a bare Au nanoparticle deposited on the

2

to the formation of Ce–ZrO

2

solid solution at the interface of

4+

CeO₂nanolayer and ZrO support due to Ce ion entering

−δ

2

the lattice of ZrO and improving the lattice symmetry of

surface of the 3DOM ZrO support can be clearly observed in

2

3

2

ZrO .

As shown in Fig. 3, the characteristic diffraction peaks

the HRTEM image. The clear lattice fringes of one particle in

the HRTEM image were measured to be 2.3 Å indexed as (111)

crystal facet of gold particle with a face-centered-cubic (fcc)

structure, indicating that supported Au particles are crystal-

line with an exposed fcc (111) plane. And the lattice fringes of

another particle in HRTEM image were measured to be 3.0 Å

belonging to Ce-based oxides were not detected, even the molar

ratio of Ce to Au is 10. And the characteristic diffraction peaks

of Au NPs at ∼38.1° corresponding to (111) lattice face of FCC

structure were also not detected. It indicates that the Au NPs of

nanometer-scale are covered by CeO₂nanolayer or nanoparti-

−δ

cle on the surface of the 3DOM ZrO

2

support, and the nanopar-

2

indexed as (011) planes of the ZrO particle. As shown in

ticle sizes of Ce-based oxides may be beyond the detection

Fig. 4D, two Au NPs on the surface of 3DOM ZrO are covered

2

limit of the XRD spectra (<4 nm). The average crystalline size

by the CeO2−δ nanolayer and formed the core–shell structural

Au@CeO2−δ NPs. And the thickness of the CeO2−δ nano-shell

compared with Au NPs is in the range of 1–2 nm. With

increasing Ce/Au ratio, the thickness of the CeO2−δ nano-shell

deposited on the surface of Au NPs increases remarkably

(Fig. 4F). The lattice spacings of the support and the

of 3DOM ZrO

about 19.8 nm. No obvious change of its crystalline size is

observed after Au and CeO NPs supported on 3DOM ZrO . As

shown in Table 3 and Fig. S4 (ESI†), the BET surface area of

2

oxide determined by the Scherrer equation is

2

−1

3

DOM ZrO oxide support is only 28 m g . With increasing

2

Ce/Au ratio, the BET surface area increases remarkably. It is

attributed to the increasing roughness of the CeO nanolayer

2

or nanoparticles on the surface of 3DOM ZrO support. The

2

actual content of Au ion in catalysts was determined by ICP-

AES and the data is also shown in Table 3. The Au actual

content in the catalysts are close to the initial value, i.e., the

percent of the Au actual to initial contents is about 80%.

It indicates that most of the Au in solution is deposited on

the surface of 3DOM ZrO support by GBMR/P method. And

2

2 2

the surface molar ratios of CeO to ZrO were determined

by SEM-EDX spectra and the results are shown in Table 3 and

Fig. S3 (ESI†). The actual ratios of CeO to ZrO on the surface

2

of the catalysts are far higher than those of the initial values

Table 1), indicating that the deposition of CeO2−δ nanolayer

(

is enriched on the surface of the 3DOM ZrO support.

2

2 2

Table 3 Average crystal parameters and crystal sizes of 3DOM ZrO , Au/ZrO

and Au@CeO2−δ/ZrO

2

catalysts

a

D

Au

size

(nm)

Au

content

(wt%)

Ce/Zr

atom

ratio

b

c

d

Catalyst

(nm)

(ZrO

S

BET

2

−1

e

(3DOM)

2

)

(m g

)

ZrO

2

19.8

19.9

19.6

—

3.5

3.8

28

29

36

—

3.0

3.1

—

1 : 12

Au/ZrO

Au@CeO2−δ

ZrO -2

Au@CeO2−δ

ZrO -5

Au@CeO2−δ

ZrO -10

Determined by XRD using Scherrer equation with the intensity of

2

/

2

19.2

19.6

3.7

3.6

41

50

3.3

3.4

1 : 5

1 : 3

2

a

b

c

the most prominent peak, (011). Determined by HRTEM. Surface

area obtained by BET method. Determined by ICP-AES. Determined

by SEM-EDX spectra.

Fig. 4 TEM and HRTEM images of 3DOM Au/ZrO₂(A and B), Au@CeO /ZrO -2

2−δ

2

d

e

(

2 2

C and D), Au@CeO2−δ/ZrO -5 (E and F) and Au@CeO2−δ/ZrO -10 (G and H)

catalysts. The inset in TEM image of (A) is the corresponding SAED pattern.

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nanolayer shell (Fig. 4H), determined to be 0.30 and 0.31 nm,

reflectance spectrum of 3DOM Au/ZrO₂catalyst displays

a distinct and well-defined surface plasmon absorption

band centered at about 573 nm, which is the characteristic

are in agreement with the values for the ZrO (011) plane and

2

for the CeO (111) plane, respectively. The ordered nanostruc-

2

5

ture with Au@CeO₂core–shell-type NPs would provide

of nanocrystalline Au particle (diameter ∼4 nm). With

increasing CeO shell content, the intensity of the plasmon

−δ

abundant catalytic activity sites with low-coordinatively unsat-

urated ceric cation and improve the sintering resistance of

2

−δ

2

absorption band of 3DOM Au@CeO2−δ/ZrO catalysts is

Au NPs due to the maximal contact region of CeO₂(shell)–

remarkably weaker and even disappears, but the position of

the plasmon absorption band is almost unchanged. In the

case of a perfect Au@CeO2−δ core–shell structure, no gold

plasmon is expected, and this is indeed very similar to the

−δ

Au(core)–ZrO (support) nanojunction system at the atomic

2

level. All Au@CeO2−δ core–shell NPs are highly dispersed and

uniform in size (Fig. S5, ESI†). The sizes of Au@CeO₂core–

−δ

shell NPs over 3DOM ZrO support are in the range of 3–11 nm

case for 3DOM Au@CeO /ZrO -2 catalyst. It indicates that

2−δ 2

2

with a narrow distribution, and mean diameters of Au core

are 3.7 ± 0.1 nm by statistical analysis of more than

the surface of supported-Au NPs core is covered with CeO2−δ

shell, which is in accordance with the results of TEM

images. In addition, the absorption peak below 400 nm can

be assigned to the ligand-to-metal charge transfer (LMCT)

transitions of the Ce(IV)–O lattice and f–d electron transfer-

3

00 Au@CeO₂core–shell NPs to the sample and the results

−δ

are shown in Table 3. It indicates that the size of supported

Au NPs could be effectively controlled by the GBMR/P method.

3

+

For 3DOM Au/ZrO catalyst, the Au mean diameter calcined at

ring of Ce ion, leading to the absorption band widening

2

6

2

00 °C is 3.5 nm, but the mean diameter of supported Au NPs

and the peaks being indistinguishable. And with increasing

calcined at 500 °C is 9.1 nm (Fig. S6, ESI†), indicating that the

deposition of CeO2−δ nanolayer can improve the sintering

resistance of supported Au NPs.

CeO₂shell content, the intensity of the absorption band,

which exceeds 573 nm, increases. It can be assigned to the

−δ

2

interfacial interactions between Au and CeO , resulting in

As a source of complementary information on the charac-

terization of supported Au@CeO2−δ core–shell NPs, UV-Vis

DRS spectroscopy has been considered to be one of the most

appropriate techniques due to the characteristic surface

plasmon resonance (SPR) effect of supported Au NPs. The

surface plasmon adsorption of Au NPs arises from the collec-

tive oscillations of the free conduction band electron induced

by the incident electromagnetic radiation. Such resonances

are seen when the wavelength of the incident light far exceeds

the electron and energy transfer from the metal surface to

the semiconductor materials.

2

7

2

4

3.3 Activity test results

Diesel soot combustion reaction is a typical heterogeneous

catalytic reaction containing solid particle as reactant. The

corresponding activity and selectivity of 3DOM ZrO , Au/ZrO

2

and Au@CeO2−δ/ZrO catalysts for soot combustion were eval-

the particle diameter. However, CeO₂outer shell shows a

uated by temperature-programmed oxidation (TPO) reaction

and the results are shown in Fig. 6 and Table 4. For compari-

son, the TPO result of bare soot (i.e., without catalyst) is also

−δ

different plasmon resonance from that of the supported

Au NPs. Therefore, the core–shell structural Au@CeO2−δ NPs

can be confirmed by UV-Vis DRS. Fig. 5 shows the UV-Vis DRS

included. As shown in Fig. 6A, the peak temperature (T ) of

p

spectra of 3DOM Au/ZrO

using BaSO as reference sample. For comparison, the UV-Vis

spectrum of 3DOM ZrO catalyst is also included. The diffuse

2

and Au@CeO2−δ/ZrO

2

catalysts

CO₂concentration for soot combustion without catalyst is

more than 600 °C. In the CO concentration profile of soot

2

combustion, the T_pis the temperature corresponds to the

4

2

fastest burning rate of soot in one TPO measurement. All the

catalysts can promote soot combustion, and the CO concen-

2

tration peaks for soot combustion over the catalysts shifts

down to the lower temperature. For 3DOM ZrO catalyst, the

2

T

p

of CO

lyst (T_p= 485 °C) has higher catalytic activity than that of

DOM ZrO₂catalyst, which is attributed to the adsorption

and activation for O by supported Au NPs. After introduction

2

concentration is ∼500 °C. And 3DOM Au/ZrO

2

cata-

3

2

of the CeO₂nanolayer shell in catalysts, the peak tempera-

−δ

tures of CO concentration remarkably shift to the lower tem-

2

perature and their intensity also increases. Among the

catalysts, 3DOM Au@CeO₂/ZrO -2 catalyst exhibits the low-

−δ

2

est peak temperature (T = 380 °C) for soot combustion. It is

p

similar to the result of CeO

2

–Ag catalyst for soot combustion

under the similar condition reported by K. Yamazaki and

2

8

T. Kayama et al. However, with increasing of the Ce/Au

molar ratio, the peak temperatures of CO concentration shift

to the higher temperature. It may be attributed to the

decreasing of porosity and porous window sizes (Fig. 2 and

2

Fig. 5 The UV-Vis DRS spectra of 3DOM ZrO

alysts prepared by GBMR/P method. (a) ZrO ; (b) Au/ZrO

d) Au@CeO2−δ/ZrO -5; (e) Au@CeO2−δ/ZrO -10.

2

, Au/ZrO

2

and Au@CeO2−δ/ZrO

2

cat-

-2;

2

; (c) Au@CeO2−δ/ZrO

2

(

2

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site density of adsorption and activation for O

catalytic activities of 3DOM Au@CeO2−δ/ZrO

2

. However, the

-n catalysts will

2

decrease when n is greater than 2. Additionally, we noted that

m

CO₂

the S

values of 3DOM ZrO -supported Au and Au@CeO

2 2−δ

nanoparticle catalysts for soot combustion (>90%) are mark-

edly increased compared with that of ZrO catalyst (<80%).

2

m

CO₂

The very high S

results are attributed to supported-Au NPs

1

4a

which has super catalytic performance for CO oxidation.

It

can ensure that the CO gas derived from vehicle exhaust is

immediately removed under the practical temperature of diesel

engine emissions.

It is generally known that the thermal stability of supported

Au-based catalysts for deep oxidation reaction is poor, and it

has been proved that the average particle size (9.1 nm) of

supported Au NPs increased remarkably by the calcined at

5

2

00 °C in comparison with that (3.5 nm) of the calcined at

00 °C. However, the CeO2−δ nanolayer shell can improve the

sintering resistance of supported Au nanoparticles. We further

examined the consecutive stability of 3DOM Au@CeO₂/ZrO -5

−δ

2

catalyst used for three cycle tests of TPO experiments and the

results are shown in Fig. 6B and Table 4. As shown in Fig. 6B,

the CO

catalyst have no dramatic changes in the three cycles of TPO

measurement, indicating that 3DOM Au@CeO2−δ/ZrO -5

catalyst maintains its high catalytic activity under the condi-

tions of loose contact between catalysts and soot particles.

2 2

concentration profiles of 3DOM Au@CeO2−δ/ZrO -5

Fig. 6 The CO

catalysts (A) and stability test of 3DOM Au@CeO2−δ/ZrO

soot combustion. (a) ZrO ; (b) Au/ZrO ; (c) Au@CeO2−δ/ZrO

e) Au@CeO2−δ/ZrO -10; (f) pure soot (without catalyst).

2

concentration profiles of 3DOM ZrO

2

, Au/ZrO

2

and Au@CeO2−δ/ZrO

2

-5 (B) with three cycle tests for

2

-2; (d) Au@CeO2−δ/ZrO

2

-5;

(

2

For example, the T , T and T₉₀at third cycles are 337, 409

1

0

50

m

CO₂

and 454 °C, respectively. Additionally, the S

values of 3DOM

Table 4 The temperature and the selectivity to CO

2

of soot combustion over

Au@CeO2−δ/ZrO

surement are higher than 90%. According to the TEM images

of 3DOM Au@CeO2−δ/ZrO -5 catalyst used for three cycles in

2

-5 catalyst used for three cycles of TPO mea-

catalysts and without catalyst (loose contact condition)

m

Catalysts (3DOM)

T10/°C

T50/°C

T90/°C

S

CO₂

2

Fig. 7, the 3DOM structure is not destroyed after the TPO

experiment, and the mean size (3.8 nm) of Au NPs on the sur-

face of 3DOM CZO support is not remarkably changed in

comparison with that of the fresh sample (3.7 nm). The result

Soot (without catalyst)

482

367

332

316

332

346

336

337

585

485

467

364

407

422

409

646

543

524

422

451

466

453

454

55.0

76.9

90.5

93.1

92.6

92.0

91.6

91.5

ZrO

Au/ZrO

Au@CeO2−δ/ZrO

2

-2

-5-first

-10

-5-second

-5-third

a

indicates that the supported Au@CeO₂nanoparticle cata-

−δ

a

lysts have good thermal and consecutive stabilities for soot

combustion due to the interface confinement effect of the

CeO2−δ nanolayer shell. It suggests that the CeO2−δ nanolayer

coated on the surface of Au core can not only improve the

catalytic activity for soot combustion, but also improve the

stability of Au-based catalysts.

a

−1

2

Reaction condition: 5% O , 0.2% NO in Ar, 50 ml min ; the mass

a

ratio (catalyst/soot) is 10 : 1. The cycle tests of catalytic stability for

soot combustion under the same condition of TPO.

Table 2) which will affect the contact efficiency between soot

and catalyst. In order to clearly compare the catalytic activity

of the catalysts for soot combustion, we have defined the

temperature values T , T , T at which the 10%, 50%, and

3

.4 Discussion on the catalytic activity for soot combustion

For metal oxide catalysts, H -TPR measurement can simulta-

neously reflect the reducibility of the metallic ion with a high

2

10

50

90

9

0% of soot conversion were obtained, respectively, and the

results are shown in Table 4. Among the catalysts, 3DOM

Au@CeO2−δ/ZrO

2

-2 catalyst exhibits the highest catalytic activ-

m

CO₂

ity, i.e., T , T , T and S

are 316, 364, 422 °C and 93.1%,

1

0

50

90

respectively. After introduction of the CeO2−δ nanolayer shell

in catalysts, the T50 of 3DOM Au@CeO2−δ/ZrO -2 catalyst sig-

nificantly reduced by 103 °C compared with that of 3DOM

Au/ZrO catalyst. It is attributed to that the strong Au(core)–

2

−δ 2

Fig. 7 TEM (A and B) and HRTEM (C) images of 3DOM Au@CeO₂/ZrO -5

CeO₂(shell) interaction is favourable for enhancing the active

catalyst used for three cycle tests.

−δ

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valence converting to the ion with a low valence or metallic

interface and sub-interface of lattice oxygen at the nanoscale

atom, and the potential to remove or take up oxygen, i.e., the

in the CeO₂nanolayer. Therefore, the moderate thickness of

CeO2−δ nanolayer shell is favorable for improving the catalytic

performance for soot combustion, it is in accordance with the

−δ

2

−

29

mobility of the lattice oxygen (O ). In this work, the tem-

perature of reduction peak (T_r_e_d) is taken as a measure for

evaluating the redox ability of the catalyst. The lower the T_r_e_d

highest catalytic activity of 3DOM Au@CeO /ZrO -2 catalyst

2−δ

2

is, the stronger the redox ability of the catalyst. The H

2

-TPR

for soot combustion.

curves of 3DOM ZrO , Au/ZrO and Au@CeO /ZrO catalysts

obtained by the GBMR/P method are shown in Fig. 8. There

is one weak reduction peak centred at 695 °C in the curve of

The catalytic performance of materials for oxidation reac-

tion is correlated with the capability to activate oxygen. How

do the supported Au@CeO2−δ nanoparticle catalysts interact

with surface oxygen species? To examine the electronic prop-

erties of the surface elements, 3DOM ZrO , Au/ZrO and

2

2−δ

2

ZrO support (a), of which the reduction peak ranging from

2

4

+

5

50 to 800 °C is attributed to the outer reduction of Zr to

2

3

+

Zr . After the introduction of Au NPs, the intensity of the

reduction peak over 3DOM Au/ZrO₂catalyst increases

slightly, and its position is unchanged, indicating that

2

Au@CeO2−δ/ZrO catalysts were studied by XPS and the

results are shown in Fig. 9 and Table 5. As shown in Fig. 9A,

the XPS spectra in the Zr 3d region of the catalysts have not

appeared as new peaks after supported Au and Au@CeO2−δ

core–shell NPs, and the intensity of Zr 3d peaks located at

181.9 and 184.3 eV remarkably decreases due to the increas-

ing thickness of the CeO2−δ nanolayer shell. It indicates that

the electronic properties of the Zr-element have not been

affected by supported Au and Au@CeO2−δ core–shell NPs.

As shown in Fig. 9(B), the O1s binding energies are located

at 529.8, 531.5 and 532.9 eV, which are assigned to lattice

2

supported Au NPs could improve the reducibility of ZrO sup-

port. For 3DOM Au@CeO₂/ZrO₂catalysts, there are two

−δ

apparent reduction peaks in the H -TPR profiles, i.e., the rela-

2

tively lower temperature peak (100–350 °C) and the relatively

higher temperature peak (500–700 °C). The reduction peak of

3

DOM Au@CeO2−δ/CeO

uted to the reduction of the chemisorbed oxygen species

Au–O to Au or Ce–O –Au to Au), which is related to the weak-

2

catalysts centered at 156 °C is attrib-

(

x

2

−

ening of the Ce–O bond induced by gold atoms due to the

strong interaction between Au NPs core and CeO2−δ nanolayer

oxygen species (O ) and chemisorption's oxygen species

30

shell. With increasing of the Ce/Au molar ratio, the reduc-

tion peak centered at 156 °C shifts to a higher temperature at

2

77 °C which may be attributed to the reduction of the outer-

4

+

3+

most layers of Ce to Ce . It indicates that the activity of the

surface oxygen species for H have changed due to the

2

increasing thickness of CeO2−δ nanolayer shell deposited on

the surface of Au core. The reduction peaks ranging from

4

+

5

00 to 700 °C can be attributed to the reduction of outer Ce

4

+

layer (545 °C) and inner (605 °C) Ce layer or lattice oxygen

bulk reduction) belonging to the CeO nanolayer shell on

the surface of Au core. Their positions of reduction peaks

have no obvious change with increasing of the Ce/Au molar

ratio, and their intensity decrease, demonstrating that the

interaction between Au core and CeO2−δ shell affects only the

(

2

−δ

3

1

Fig. 9 X-ray photoelectron spectra (XPS) of Zr 3d (A), O 1s (B), Au 4f (C) and Ce

3

d (D) regions for 3DOM ZrO

2

, Au/ZrO

2

and Au@CeO2−δ/ZrO

2

catalysts. (a) ZrO

2

;

(

b) Au/ZrO ; (c) Au@CeO2−δ/ZrO

2

-2; (d) Au@CeO2−δ/ZrO

2

-5; (e) Au@CeO2−δ/ZrO

2

-10.

Table 5 Surface compositions and oxidation states of O, Au and Ce species

derived from XPS analyses

O species (%)

Au species (%) Ce species (%)

2⁻

2²⁻

2−

0

&+ a

3+

4+

Sample (3DOM)

ZrO

O

Au

Ce

2

3.7 19.5 76.8

—

38.7

31.8

31.3

—

61.3

68.2

68.7

Au/ZrO

2

5.4 20.6 74.0 87.4 12.6

-2 6.9 35.9 57.2 82.8 17.2

-5 6.4 34.8 58.8 86.5 13.5

Au@CeO2−δ/ZrO

2

Fig.

8

The

H

2

-TPR profiles of 3DOM ZrO

; (b) Au/ZrO ; (c) Au@CeO2−δ/ZrO

e) Au@CeO2−δ/ZrO -10.

2

,

Au/ZrO

2

and Au@CeO2−δ/ZrO

2

Au@CeO₂/ZrO -10 6.1 34.6 59.3 86.7 13.3

−δ

&+

2

catalysts. (a) ZrO

2

-2; (d) Au@CeO2−δ/ZrO

2

-5;

a

+

3+

The Au values are equal to the sum of Au and Au

.

(

2

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2

−

&−

2−

(

O₂and O ), respectively. The content ratio of O

(O₂

2

−

and O ) in catalysts increases slightly after the introduction

2

of Au NPs on ZrO support as shown in Table 5, which may be

attributed to the activation of supported Au NPs for O . After

2

the introduction of CeO nanolayer on Au NPs, the content

2

−δ

&

−

ratios of O

2

in catalysts increase remarkably, which is attrib-

uted to the effect of strong metal–oxide interaction on the

amount of active oxygen species. Among the catalysts, 3DOM

Au@CeO2−δ/ZrO

2

-2 catalyst exhibited the largest content ratio

&

−

of O₂. Increasing the amount of surface active oxygen spe-

−

2−

cies (O , O ) would give rise to unusual catalytic properties

2

for deep oxidation reactions.

Fig. 10 UV-Raman spectra of 3DOM ZrO

catalysts with a He–Gd laser (325 nm). (a) ZrO ; (b) Au/ZrO

d) Au@CeO2−δ/ZrO -10.

2

,

Au/ZrO

2

and Au@CeO2−δ/ZrO

; (c) Au@CeO2−δ/ZrO -2;

2

As shown in Fig. 9C, the deconvolutions of the Au 4f XPS

spectra indicate that both metallic and ionic gold species are

present in supported Au NP catalysts, i.e., the gold species of

2

(

2

0

+

3+

metal (Au ) and ion (Au and Au ) are identified on the surface

32

3+

of catalysts, and the tiny content ratio of Au species can be

ignored. Among the catalysts, 3DOM Au@CeO2−δ/ZrO -2 cata-

2

This result is in good agreement with those of H

measurements.

2

-TPR

&+

lyst exhibited the largest content percent of Au species

+

3+

(

Au and Au ) to the total Au species, which is 17.2%. The

In contrast to the cation-dominated information yielded

by XRD, Raman spectra of Ce-related crystal phases are domi-

nated by lattice oxygen vibrations, and are obviously sensitive

to the crystalline symmetry of surface oxides. Thus, it is a use-

ful tool to obtain the additional structural information of

&+

0

Au species are considered to be more active species than Au

3

for oxidation reactions. It is deduced that the appearance of

&+

Au may be derived from the increasing amount of surface

−

active oxygen species (O

2

, O ) due to the decrease in the

d-electron density of the Au atoms and the enhanced onset of

both M–O bond arrangement and lattice defects of the CeO

2

−δ

3

4

37

reactivity to oxygen in air, and also derived from the strong

interaction between gold core and CeO2−δ nanolayer shell.

Fig. 9D shows the XPS spectra in the Ce 3d region. The

four main 3d5/2 features at 882.3, 885.6, 888.5, and 898.2 eV

corresponding to V, V′, V″, and V‴ components, respectively.

And the 3d₃_/₂features at 900.8, 903.5, 906.8, and 916.6 eV cor-

responding to U, U′, U″, and U‴ components, respectively.

nanolayer. The UV-Raman spectra of 3DOM ZrO

and Au@CeO2−δ/ZrO catalysts with a He–Gd laser (325 nm)

are presented in Fig. 10. For 3DOM ZrO and Au/ZrO cata-

2 2

, Au/ZrO

2

lysts, there are four Raman bands located at ∼330, 375, 475

−

1

and 630 cm , which could be attributed to the Raman-active

1

3a

mode of ZrO₂.

the Raman bands change remarkably, indicating that the sur-

face of ZrO is covered by the CeO2−δ nanolayer. The spectra

of 3DOM Au@CeO /ZrO catalysts are dominated by a prom-

After introduction of the CeO

nanolayer,

2−δ

3

+

The signals V′ and U′, characteristics of Ce , exist evidently

2

3

5

in 3DOM Au@CeO₂/ZrO catalysts. With decreasing of the

−δ

2

2−δ

2

3

+

−1

Au/Ce ratios, the Ce species ratios increase from 31.3%

Au@CeO2−δ/ZrO -10) to 38.7% (Au@CeO2−δ/ZrO -2) in Table 5.

inent band at 295 cm , a weak band centered at 420 cm

and a strong band centered at 645 cm . The appearance of

the prominent band at ∼295 cm could be attributed to dis-

placement of oxygen atoms from their ideal lattice posi-

tions. The weak band centered at 420 cm could be

associated with the first-order symmetric stretching mode

−

1

(

2

3

+

−1

The increasing Ce content contributed to the interaction

between Au NPs core and CeO2−δ nanolayer shell which leads

to an increase of the oxygen vacancies and may lead to a

transference of the lattice oxygen in ceria and maintaining

cerium at the low-valent state. However, with the increasing

of the CeO2−δ nanolayer thickness, the effect of the strong

3

8

−1

3

9

(F2g) of the oxygen atoms around cerium ions. The strongest

−

1

peak at 645 cm , corresponded to a double degeneration into

Au–CeO₂interaction on the density of oxygen vacancies

the longitudinal optical mode of CeO nanolayer, predicting

−δ

2−δ

became weaker. To sum up, we can deduce that Au atoms pre-

fer to interact with cerium atoms in comparison with zirco-

nium atoms, and the strong interaction between Au core and

the possibility of the existence of a stable t-phase in the mixed

oxide system, and this band could be linked to oxygen vacan-

cies. It is attributed to the strong Au–CeO

interactions

2

−δ

4+

3

+

&+

0

CeO2−δ shell leads to increasing amounts of Ce and Au in

DOM Au@CeO2−δ/ZrO catalysts due to the electron

transformed from the Au metal core to Ce in the nanolayer

inducing the electronic transfer from Au to Ce , which leads

3

+

4+

3

2

to the two Ce ions (ionic radius 1.143 Å) replacing two Ce

4

+

40

ions (0.970 Å) for each oxygen vacancy created. The change

in UV-Raman spectra illustrates the corresponding lattice

modifications among the stable and metastable phases. In

addition, with increasing Ce/Au molar ratios, the intensity

36

shell.

Therefore, the amount of active oxygen species

increases due to the oxygen transference from the inner layer

to the outermost layer of the CeO₂shell and the increasing

−δ

−

1

−1

density of the oxygen vacancy. In summary, the chemical or

structural order may be responsible for the catalytic activity of

ratios of the peak between at ∼645 cm and at ∼295 cm

obviously decreases, could be ascribed to the amount of

change of oxygen vacancies. This result demonstrates that the

supported Au@CeO2−δ core–shell NPs with the moderate

3

DOM Au@CeO₂/ZrO catalysts for soot combustion, which

−δ 2

2

−

increases the amount of active oxygen species (O

2

and O ).

Catal. Sci. Technol., 2013, 3, 2958–2970 | 2967

View Article Online

Paper

thickness of the CeO₂nanolayer shell increases the density

of active oxygen species obviously. Therefore, we can deduce

that a strong metal–support interaction between Au atom and

Ce atom may exist, and the SMSI is beneficial to the creation

of the oxygen vacancies and the active oxygen species. It is

accordance with the results of TPO experiments.

Catalysis Science & Technology

−δ

4

1

3

.5 Catalytic mechanism of 3DOM Au@CeO₂/ZrO catalysts

−δ 2

for soot combustion

The nanoscale structure of catalysts as well as the physical

state can influence the catalytic activity of supported Au NPs

catalysts for soot combustion. The catalytic reaction of soot

combustion is a typical heterogeneous catalytic reaction,

which takes place on the three-phase boundary among a

solid catalyst, a solid reactant (soot), and gaseous reactants

2 2 2

Scheme 3 Mechanism illustration of 3DOM ZrO , Au/ZrO and Au@CeO2−δ/ZrO

catalysts for soot combustion.

increasing the density of active sites due to the increasing

(

O , NO). The efficiency of catalyst for solid–solid reaction is

interfaces area between Au and CeO . In this work, the

2

strongly influenced by the contact efficiency between soot

and catalyst in the process of catalysis. Since the number of

contact points is proportional to the catalyst concentration,

one can expect an increase in the activity with the higher con-

tact area. In many studies, the catalytic performances for soot

combustion are excellent under the conditions of tight con-

tact between soot and catalyst because of their good contact.

However, it is loose contacts between catalysts on the surface

of the filter and soot particles under practical conditions.

Thus, it is significantly important to study and design active

catalysts to improve the contact capacity for soot combustion

under loose contact conditions.

multifunctional nanostructure materials with Au@CeO2−δ

core–shell NPs deposited on the surface of 3DOM ZrO were

2

firstly synthesized by a one-pot process of the GBMR/P

method. Based on the results of TPO measurements

described above, the multifunctional nanocatalysts of 3DOM

2

ZrO -supported core–shell structural Au@CeO2−δ NPs with

moderate thickness of CeO2−δ nanolayer shell show a higher

catalytic activity for soot oxidation compared with bare Au

nanoparticles. For example, the catalytic activity of 3DOM

2

Au@CeO2−δ/ZrO -2 catalyst (T50 = 364 °C) was higher than

that of 3DOM Au/ZrO₂catalyst (T₅₀= 467 °C). It is mainly

attributed to the nanostructure with Au@CeO2−δ core–shell

NPs being favorable for increasing the density of active oxy-

gen species determined by XPS (Table 5). The high stability

To meet the increasing needs of contact area, 3DOM

materials with a uniform pore size and periodic voids inter-

connected through open windows are designed and synthe-

sized by the colloidal crystal template method. In our previous

3

+

2

of the Ce cation in Au@CeO2−δ/ZrO catalysts is a conse-

quence of their nontypical coordination mode and the effect

of Au–O–Ce bonding interactions. As shown in Scheme 3, the

schematic of the model systems vividly displays the nano-

structure effect of 3DOM ZrO -supported Au@CeO2−δ core–

2

shell NPs on the density of active sites and the catalytic activ-

ity for soot combustion. This result is consistent with the

1

6c

work,

it had been proved that soot particles can easily

enter the interior of 3DOM catalysts with the help of the air-

flow in the reaction process, and have less resistance going

through the catalyst structure. 3DOM structure is a desirable

feature for diesel soot combustion. The number of contact

points between soot and catalysts increases dramatically,

and the reaction distance between soot particle and catalyst

reduces in the reaction process whatever the role of the cata-

2

results of H -TPR and XPS analyses. In summary, 3DOM

Au@CeO₂/ZrO₂catalysts with multifunctional nanostruc-

−δ

tures, which combine both the advantages of good contact

between soot and catalyst by 3DOM supports and high den-

1

6c

lyst is be it direct or indirect. However, the structural fea-

ture of 3DOM catalyst is only one of the controlling factors

for soot combustion. Since the reaction of soot combustion

is a type of deep oxidation reaction, the catalysis nature of

the reaction is the redox process. The active sites of 3DOM

sity of active sites by the core–shell structural Au@CeO₂

−δ

NPs, have high catalytic activity and stability for diesel soot

combustion.

CeO –Au nanoparticle catalysts for soot combustion are

often coordinatively unsaturated metal cations (Ce ) which

2

4. Conclusions

3

+

are able to promote adsorption–activation of reactant mole-

The multifunctional catalysts of 3DOM ZrO -supported core–

2

cule (O and NO). Therefore, the intrinsic catalytic activities

of the catalysts are related to the density of active sites

shell structured Au@CeO₂NPs with a controlled thickness

of CeO2−δ nanolayer shell were firstly synthesized by the one-

pot GBMR/P method. The macropore structures of 3DOM

2

located at the Au–CeO boundaries.

2

It is critical to the design and synthesis of the ordered

nanocatalyst with high density of the active sites for enhanc-

ing the catalytic activity of soot combustion. In principle,

Au@CeO₂/ZrO₂catalysts are highly ordered and inter-

connected to each other by small pore windows. They exhibit

large total pore volume and high porosity. The core–shell

−δ

the Au–CeO₂

core–shell nanostructure is favorable for

structural Au@CeO

NPs are highly dispersed on the inner

−δ

2−δ

2968 | Catal. Sci. Technol., 2013, 3, 2958–2970

View Article Online

Catalysis Science & Technology

Paper

walls of uniform macropores. The nanolayer thickness of the

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CeO₂shell over Au NPs can be controlled at the nanoscale

−δ

level by the Ce/Au molar ratio. The Au@CeO2−δ core–shell

nanostructure can be favorable for enhancing the density of

&

−

3+

active sites (O and Ce ) due to the increasing interface area

between the Au core and the CeO2−δ shell. 3DOM Au@CeO

_δ/ZrO₂catalysts, which have both the advantages of good

2

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−

contact between the soot and the catalyst by 3DOM supports

and the high density of active sites for activation of O by the

2

core–shell structured Au@CeO₂NPs, exhibited high cata-

−δ

lytic activities and stabilities for diesel soot oxidation. In

summary, the preparation of metal@oxides core–shell nano-

structures on 3DOM oxides surface is straightforward via

the one-pot GBMR/P method. The method can potentially

be extended to other nanoparticle cores with different com-

positions, sizes and shapes and to other shell compositions

on the surface of 3DOM support. The fabrication of

metal@oxides core–shell nanostructure on the 3DOM oxides

surface could be widely applied in various heterogeneous

oxide–metal catalytic systems and could illustrate a promis-

ing and efficient way to the design active sites and improve

the stability for supported Au-based nanocatalysts.

Acknowledgements

9

(a) J. Liu, Z. Zhao, J. Wang, C. Xu, A. Duan, G. Jiang and

Q. Yang, Appl. Catal., B, 2008, 84, 185; (b) I. Atribak,

A. Bueno-López, A. García-García, P. Navarro, D. Frías and

M. Montes, Appl. Catal., B, 2010, 93, 267.

This work was financially supported by NSFC (Nos. 21177160,

2

1173270 and 21073235), 863 program (2013AA065302F), the

Science Foundation of China University of Petroleum-Beijing

No. YJRC-2013-13) and the Dean Project of Guangxi Key Lab-

(

10 (a) A. Setyan, J.-J. Sauvain and M. J. Rossi, Phys. Chem.

Chem. Phys., 2009, 11, 6205; (b) Y. Yu, M. Meng and F. Dai,

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Article Doi

One-pot synthesis of core-shell Au@CeO2-δ nanoparticles supported on three-dimensionally ordered macroporous ZrO2 with enhanced catalytic activity and stability for soot combustion

DOI: 10.1039/c3cy00248a

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Article abstract of DOI:10.1039/c3cy00248a

Full text of DOI:10.1039/c3cy00248a

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