The catalysts of three-dimensionally ordered macroporous Ce 1-xZrxO2-supported gold nanoparticles for soot combustion: The metal-support interaction

Journal of Catalysis 287 (2012) 13–29

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Journal of Catalysis

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The catalysts of three-dimensionally ordered macroporous

Ce_1ꢀxZr_xO₂-supported gold nanoparticles for soot combustion:

The metal–support interaction

⇑

Yuechang Wei, Jian Liu, Zhen Zhao , Aijun Duan, Guiyuan Jiang

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18# Fuxue Road, Chang Ping, Beijing 102249, China

a r t i c l e i n f o

a b s t r a c t

Article history:

Three-dimensionally ordered macroporous (3DOM) Ce_1ꢀxZr_xO₂-supported gold nanoparticle catalysts

were successfully synthesized by the gas bubbling-assisted membrane reduction (GBMR) method. Au

nanoparticles with similar sizes are well dispersed and supported on the inner walls of uniform macrop-

ores. The active oxygen species (O₂^ꢀ, O^ꢀ) over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts are derived from the two

approaches: one is direct activation of oxygen on the surface of gold nanoparticles and the other is

derived from strong metal–support interaction in which Ce-based supports serve as a reservoir for oxy-

gen in the oxidation reaction. 3DOM Au/Ce_1ꢀxZr_xO₂catalysts exhibited good catalytic performance for

soot combustion, which is strongly related to the role of supports including the structures, the Ce/Zr

ratios and the active oxygen supply pathways. The reaction pathways of soot combustion can be classi-

ﬁed by two regions. At the low temperature, the soot particles are direct oxidized by active oxygen spe-

cies migrated from the surface of supported Au catalysts. At the relatively high temperature, the catalytic

activity for soot combustion is strongly related to intermediate reactant of NO₂derived from NO

oxidation.

Received 1 October 2011

Revised 5 November 2011

Accepted 7 November 2011

Available online 30 December 2011

Keywords:

Three-dimensionally ordered macroporous

materials

Gold

Ce_1ꢀxZr_xO₂

Soot combustion

Heterogeneous catalysis

Interaction

1. Introduction

particles are difﬁcult to enter the inner pores of these catalysts.

The process of catalytic soot combustion is limited due to the poor

Diesel engines are prevalent power sources for vehicles owing

to their efﬁcient energy conversion, superior fuel economy and

low CO₂emissions, although they present the environmental

drawbacks of particulate matter (PM, mainly containing soot)

and NO_xformations as combustion by-products. The combination

of traps and oxidation catalysts in the continuously regenerating

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

economic after-treatment techniques [1]. The key challenge is to

ﬁnd a catalyst that can lower down the combustion temperature

of soot as much as possible. Recently, several kinds of catalysts

such as precious metals [2], transition metal oxides [3], alkaline

metal oxides [4], perovskite-like type oxides [5] and ceria-based

oxides [6] have exhibited good catalytic performances for diesel

soot combustion. Especially, Ce_1ꢀxZr_xO₂solid solutions are one of

the key components in TWCs due to their excellent redox property,

oxygen storage and releasing capacity and thermal stability [7]. In

generally, catalytic approach of soot combustion is affected by two

factors: the contact condition between soot and catalyst, and the

intrinsic activity of catalyst. The conventional catalysts have smal-

ler pore sizes (<10 nm) than soot particles (>20 nm). Thus, soot

soot/catalyst contact and the restricted active site amounts. So how

to effectively make use of the inner surface for increasing the con-

tact area between catalysts and soot particles is a key factor affect-

ing the catalytic activity in heterogeneous catalysis reaction. In

view of the catalytic combustion of diesel soot, controlling the

porosity of catalysts is highly desirable. Three-dimensionally or-

dered macroporous (3DOM) perovskite or Ce_1ꢀxZr_xO₂oxide cata-

lysts [8,9] exhibited better catalytic performance for diesel soot

oxidation in comparison with disordered macroporous and nano-

particles samples. The big pore sizes (>100 nm) can permit solid

reactant to enter the inner pores of materials and to easily trans-

port and diffuse. Consequently, the tangible active points between

soot particles and catalysts will be increased dramatically. How-

ever, the catalytic performances of Ce_1ꢀxZr_xO₂solid solutions are

limited by their intrinsic activity. It is generally known that the

addition of metal (active site) to oxides can modify the intrinsic

catalytic properties of themselves due to the establishment of an

interaction between metal and support, which can enhance the re-

dox properties of the modiﬁed materials [10].

Since the pioneering studies by Haruta et al. [11] and Hutchings

[12], supported gold catalysts have become one of the most exten-

sively research issues in the ﬁeld of heterogeneous catalysis

[13,14]. Most of the studies have been focusing on the origin of

⇑

Corresponding author. Fax: +86 10 89731586 (O).

E-mail address: zhenzhao@cup.edu.cn (Z. Zhao).

doi:10.1016/j.jcat.2011.11.006

14

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

the unique catalytic activity of gold nanoparticles. The reasons are

that, on the one hand, it is of great scientiﬁc interest accounting for

chemical inert gold being changed to highly active materials. On

the other hand, it may support the development of new catalysts

in different ﬁelds such as low-temperature CO oxidation, selective

catalytic oxidation and water gas shift reaction, and so on [15,16].

The efﬁciency of supported Au nanoparticles in a low-temperature

reaction depends on the variety of factors, including the size and

shape of Au particles [17], the role of the support [18], the oxida-

tion state of the active gold species (metallic, Au⁺or Au³⁺) [19],

the cluster-support interactions, the oxygen supply pathways

[20], the preparation procedures and pretreatment conditions

[21]. It is commonly accepted that activation of O₂is the most crit-

ical step in catalytic oxidation reactions involving gold. The reac-

tivity of gold clusters with O₂is important for understanding the

remarkable catalytic effects. A critical question pertaining to the

gold catalyst is how gold nanoparticles interact with and activate

molecular oxygen in reactant gas. Among the commonly reported

reasons of the chemisorption and activation of O₂on Au-based cat-

alysts are the so-called metal and support interactions, that is, spe-

ciﬁc phenomena occurring at the interface between the active

metal phase (Au) and the support. Many ideas have been proposed

in the catalysis literature on the origin of such support effects.

These include electronic interactions between both components

[22], stabilization or destabilization of certain particle shapes or

sizes [23], surface transportation of adsorbates through the bound-

ary (spillover, reverse spillover, capture zone effects) [24], so-

called strong metal–support interactions involving migration of

partially reduced oxides onto the active phase [25] or the stabiliza-

tion/destabilization of oxidized active phases by the support [26].

However, the clear understanding for the effect of supports on cat-

alyst performance is difﬁcult due to interference with size effect in

the wide size distribution of gold nanoparticles obtained by the

conventional methods [27]. Up to date, an exact mechanism of gold

catalysts has not been fully revealed, especially reaction mecha-

nism for the reaction containing solid reactant is very rare.

as precursors. The preparation method was described in literature

[29].

2.2. Preparation of 3DOM Ce_1ꢀxZr_xO₂-supported gold nanoparticle

catalysts

The synthesis of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts was carried

out via the gas bubbling-assisted membrane reduction (GBMR)

method which was just developed in our laboratory [30]. The cat-

alysts will be generically named as 3DOM Au/Ce_1ꢀxZr_xO₂, where

the weight percent of gold in the catalyst in theory is 2 wt%,

and x is the molar ratio of Zr in Ce_1ꢀxZr_xO₂solid solution. The

schematic diagram of the GBMR method is presented in Fig. S2.

The detailed synthesis mechanism of 3DOM Ce_1ꢀxZr_xO₂-supported

gold nanoparticle catalysts is shown in Electronic Supportiong

Informantion Section, and the schematic diagram of synthesis

mechanism is presented in Fig. S3. The typical preparative proce-

dures were described as follows: The poly (N-vinyl-2-pyrroli-

done) (PVP) solution as a stabilizer was added into HAuCl₄

solution, which the [PVP]/[Au] ratio is 100. Then 3DOM Ce_1ꢀxZr-

_xO₂support (0.5 g) was also introduced into the solution

(50 ml). The mixture solution was driven by a peristaltic pump

(Baoding Lange Co., Ltd.) and the rotate speed at 200 rpm

(ꢂ360 ml min^ꢀ1) to form a tubal cycling ﬂow. A reductant solu-

tion (NaBH₄) (1 ml min^ꢀ1, 50 ml) was injected into the membrane

reactor with two ceramic membrane tubes (

U

3 mm ꢃ 160 mm,

Hyﬂux Group of Companies, Singapore) by a constant ﬂow pump

(HLB-2020, Satellite Manufactory of Beijing). Meantime, the

hydrogen gas was also ﬂow through the membrane rector with

the other two membrane tubes. The metal precursor solution ﬂo-

wed inside the glass tube reactor and outside the ceramic tubes.

The NaBH₄solution was inﬁltrated through the abundant holes

(d = 40 nm) on the wall of the ceramic tubes into the glass tube

reactor, where the reduction in metal ions occurred immediately

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

ring operation (40 ml min^ꢀ1) was developed to vigorously stir the

solution and to make the reaction homogenous. The synthesis

process was stopped after complete consumption of the NaBH₄

solution. The reaction system was further vigorously bubbled

with hydrogen gas for 1 h and then kept static for 1 h. Then,

the product was ﬁltered and washed with distilled water at

60 °C until the Cl^ꢀwas completely removed according to a test

with AgNO₃, and the ﬁnal products were dried in an oven at

200 °C for 1 h and the desired 3DOM Au/Ce_1ꢀxZr_xO₂catalysts

were obtained.

Motivated by the above considerations, 3DOM Ce_1ꢀxZr_xO₂-sup-

ported gold catalysts as a bifunctional material, which combine

both advantages of good contact between diesel soot and catalyst

by 3DOM supports and the highly active sites for the activation

of O₂by gold clusters with suitable nanosize, are designed and they

exhibit good catalytic performances for soot oxidation. The nature

of an interaction between supports and gold nanoparticles and

even its effect on the catalytic performance for soot combustion

are investigated. Moreover, the possible mechanism for soot com-

bustion over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts is proposed.

2.3. Physical and chemical characterization

2. Experimental

The Brunauer–Emmett–Teller (BET) method was used to mea-

sure the speciﬁc surface areas of the samples. Mercury porosime-

try experiments were carried out on a Micromeritics AutoPoreIV

9500 apparatus in the pressure range 0–30,000 psia. XRD patterns

were measured on a powder X-ray diffractometer (Shimadzu XRD

2.1. Preparation of 3DOM Ce_1ꢀxZr_xO₂solid solution supports

Synthesis of monodispersed PMMA microsphere, assembly

template and preparation of 3DOM Ce_1ꢀxZr_xO₂solid solution

supports by colloidal crystal template (CCT) method are similar

to that described previously [28]. The typical schematic diagrams

are shown in Fig. S1. The dried sample was calcined to remove

the CCT in a tube furnace with an air ﬂow (80 ml min^ꢀ1). The

temperature-rising rate was 1 °C min^ꢀ1from room temperature

to 550 °C, and the calcination at 550 °C was kept for 5 h, and

3DOM Ce_1ꢀxZr_xO₂solid solutions were obtained.

To compare the catalytic activity for soot combustion over dif-

ferent types of catalysts, the nano-Ce_1ꢀxZr_xO₂solid solution cata-

lysts were prepared by the micropore-diffused coprecipitation

method with the ammonia solution as a precipitation agent and

with various metal nitrates (Ce(NO₃)₃ꢁ6H₂O and Zr(NO₃)₄ꢁ3H₂O)

6000) using CuK

a (k = 0.15406 nm) radiation with a Nickel ﬁlter

operating at 40 kV and 10 mA in the 2h range of 20–80° at a scan-

ning rate of 4°/min. The patterns were compared with JCPDS ref-

erence data for phase identiﬁcation. The crystallite sizes of

obtained samples were calculated from the width of the (111)

line using the Scherrer equation (D = 0.9k/(bcosh), where k is

the wavelength of the radiation, b is the corrected peak width

at half-maximum intensity (FWHM) and h is the peak position).

The UV–Vis diffuse reﬂectance spectroscopy (UV–Vis DRS) exper-

iments were performed on a UV–Vis spectrophotometer (Hitachi

U-4100) with the integration sphere diffuse reﬂectance attach-

ment. The actual content of Au in catalysts was determined by

inductive coupled plasma atomic emission spectrometry (ICP-AES)

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

15

(PE, OPTIMA 5300DV). The surface morphology of the catalyst

was observed by ﬁeld emission scanning electron microscopy

(FESEM) on a Quanta 200F instruments using accelerating volt-

ages of 5 kV. Samples for SEM were dusted on an adhesive con-

ductive carbon belt attached to a copper disk and were coated

with 10 nm Au prior to measurement. The TEM and HRTEM

images were carried out using a JEOl JEM 2100 electron micro-

scope equipped with a ﬁeld emission source at an accelerating

voltage of 200 kV. The average particle diameter was calculated

from the mean diameter frequency distribution with the formula:

d = n_id_i/ n_i, where n_iis the number of particles with particle

diameter d_iin a certain range. Raman spectra of the samples were

collected in the anti-Stokes range of 100–2000 cm^ꢀ1using a Jobin

Yvon LabRam-HR spectrometer (HR800). The sample has been ex-

cited using a He–Gd laser (excitation wavelength of 325 nm). X-

ray photoelectron spectra (XPS) were recorded on a Perkin–Elmer

PHI-1600 ESCA spectrometer using Mg Ka (hv = 1253.6 eV,

1 eV = 1.603 ꢃ 10^ꢀ19J) X-ray source. The binding energies were

calibrated using C1s peak of contaminant carbon (BE = 284.6 eV)

as an internal standard.

Fourier transform infrared spectroscopy (FT-IR) absorbance

spectra were recorded in the wave numbers ranging from 6000

to 400 cm^ꢀ1via a FTS-3000 spectrophotometer manufactured by

American Digilab company. The measured wafer was prepared

as KBr pellet with the weight ratio of sample to KBr, 1/100. The

resolution was set at 2 cm^ꢀ1during measurements. For in situ

diffuse reﬂection infrared Fourier transformed (DRIFT) experi-

ments, it did not need to use KBr as diluent. The spectra were re-

corded in the range of 600–4000 cm^ꢀ1after 256 scans at a

resolution of 4 cm^ꢀ1. The powder sample was placed in a sample

holder assembly in a Harrick Praying Mantis DRIFT cell. The gases

were supplied by individual mass ﬂow controllers with a total

ﬂow rate of 50 ml/min. Before reactant gases entering, the sample

was pretreated with Ar at 300 °C for 30 min. The sample was then

cooled to reaction temperature and equilibrated in Ar. After the

sample had cooled to the reaction temperature, a spectrum of

the treated sample was taken as the background at that temper-

ature. The in situ experiment was performed by the introduction

of 2000 ppm NO and 5% O₂in Ar. Meanwhile, the IR spectra were

sequentially recorded at the different reaction temperatures of

30 °C, 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, and

450 °C, respectively. Temperature-programmed reduction with

H₂(H₂–TPR) measurements were performed in a conventional

ﬂow apparatus. 100 mg sample was pretreated under air atmo-

sphere by calcination at 200 °C for 1 h and subsequently cooled

to 30 °C. Afterwards, 10% H₂/Ar ﬂow (40 ml/min) was passed over

the catalyst bed while the temperature was ramped from 30 to

900 °C at a heating rate of 10 °C/min. The hydrogen consumption

signal was monitored by a thermal conductivity detector (TCD).

Before the outlet gases entering the TCD, a cooling trap and a ﬁl-

ter packed with molecular sieve 5A (60–80 meshes) were used to

remove H₂O and CO₂. To quantify the total amount of H₂con-

sumed during these experiments, CuO was used as a calibration

reference. O₂–TPD measurements were performed on a conven-

tional ﬂow apparatus. 100 mg catalyst was pretreated in the Ar

ﬂow at 200 °C for 1 h. Then the catalyst was adsorbed O₂for

30 min (40 ml min^ꢀ1) at room temperature and purged residual

oxygen. The catalyst was heated to 900 °C at 10 °C min^ꢀ1rate un-

der Ar ﬂow (40 ml min^ꢀ1). The desorbed oxygen was monitored

by TCD. NO temperature-programmed oxidation (TPO) experi-

ments were performed on the above ﬁxed beds for studying on

the formation of NO₂. Reactant gas compositions were 5% O₂

and 0.2% NO with He balanced gas. The reaction temperature

ranged from 100 °C to 500 °C. The catalyst used was 100 mg

and without soot during the reaction. The other reaction condi-

tions were the same as the following TPO experiment. The NO₂

concentration in the outlet gas was in situ detected by mass

spectrometry (MS, Quantachrome autosorb-1C).

2.4. Activity measurements

The catalytic performances of all the catalysts were evaluated

with a temperature-programmed oxidation reaction (TPO) on a

ﬁxed-bed tubular quartz reactor (/ = 8 mm), and each TPO run

from 150 to 650 °C at a 2 °C min^ꢀ1rate. The model soot was Prin-

tex-U (diameter 25 nm, purchased from Degussa) as shown in

Fig. S4. Elemental analysis of Printex-U particulates showed its car-

bonaceous nature with 92.0% C, 0.7% H, 3.5% O, 0.1% N, 0.2% S and

3.5% other [31]. 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 (50 ml min^ꢀ1) contain 5%

O₂and 0.2% NO balanced with Ar. The outlet gas compositions

were analyzed with an on-line gas chromatograph (GC, Sp-3420,

Beijing) by using FID detectors. Before entering the FID detector,

CO and CO₂were fully converted to CH₄by a convertor with Ni cat-

alyst at 380 °C. The catalytic activity was evaluated by the values of

T₁₀, T₅₀and T₉₀, which were deﬁned as the temperatures at 10%,

50% and 90% of soot conversion, respectively, were oxidized during

the TPO procedure. The selectivity to CO₂formation (S_CO2) was de-

ﬁned as that the CO₂outlet concentration (C_CO2) divided by the

P

sum of the CO₂and CO outlet concentration, i.e., S_CO2= C_CO2

/

(C_CO+ C_CO2). S^mwas denoted as S_CO2at the maximum temperature

CO2

corresponding to the soot-burnt rate was the highest. In all TPO

experiments, the reaction was not ﬁnished until the soot was com-

pletely burnt off.

The turnover frequency (TOF) is deﬁned as the ratio of reaction

rate to the active site density of catalysts. The isothermal reactions

for soot combustion were conducted for obtaining TOF value of

catalyst, at which a stable and small conversion of soot was

achieved in an approximate kinetic regime. The temperatures were

selected at 210 °C and 300 °C because the soot conversion is low

and the rate of soot conversion is nearly constant over time. Fur-

thermore, a series of experiments were performed to exclude inter-

nal and external diffusion and heat transfer. The uniform

macroporous structure and the small particle diameter (<40 lm)

of catalysts could exclude the intraparticle mass transport limita-

tions. Thus, the reaction rate and the TOF values of soot combus-

tion can be obtained. Therein, TOF_Auis on the basis of Au loading

amount determined by ICP-AES. The total ﬂow rate was 50 ml/

min, and reaction gas composition was same as the TPO reaction.

3. Results

3.1. Catalyst characterization

3.1.1. The results of SEM

The morphology and macroporous structure of all samples were

investigated by the means of ﬁeld emission scanning electron

microscope (FESEM). Fig. 1 shows SEM images of 3DOM Ce_1ꢀxZr_xO₂

solid solutions obtained by colloidal crystal template method and

Au/Ce_1ꢀxZr_xO₂catalysts (x = 0–1) obtained by GBMR method. The

images exhibit that the macroporous materials contain skeleton

surrounding uniform close-packed periodic voids with the average

diameter of 250 20 nm, which corresponds to shrinkage of 25–

30% compared with the initial size (350 nm) of PMMA micro-

spheres (Fig. S5). This shrinkage is caused by the melting of

polymer templates and sintering of the produced metal oxides. In

addition, the wall thicknesses observed from SEM images are

30 5 nm illustrates that the wall is composed of crystallite grains.

The next layer is highly visible in the SEM image, and the voids

are interconnected through the open window, ca. 65 5 nm in

16

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

Fig. 1. SEM images of the 3DOM Ce_1ꢀxZr_xO₂and Au/Ce_1ꢀxZr_xO₂catalysts. (a) Au/CeO₂, (b) Au/Ce_0.9Zr_0.1O₂, (c) Au/Ce_0.8Zr_0.2O₂, (d) Au/Ce_0.7Zr_0.3O₂, (e) Au/Ce_0.6Zr_0.4O₂, (f) Au/

Ce_0.5Zr_0.5O₂, (g) Au/Ce_0.3Zr_0.7O₂, (h) Au/ZrO₂, (i) CeO₂, (j) Ce_0.9Zr_0.1O₂, (k) Ce_0.8Zr_0.2O₂, (l) Ce_0.7Zr_0.3O₂, (m) Ce_0.6Zr_0.4O₂, (n) Ce_0.5Zr_0.5O₂, (o) Ce_0.3Zr_0.7O₂, (p) ZrO₂.

diameter. 3DOM Au/Ce_1ꢀxZr_xO₂materials are similar to the well-

deﬁned 3DOM structure of Ce_1ꢀxZr_xO₂supports obtained by colloi-

dal crystal template method. It indicates that the synthesis process

of GBMR method with gas bubbling-assisted stirring operation did

not destroy the 3DOM structures.

and Au/Ce_1ꢀxZr_xO₂catalysts (x = 0.2, 0.3) as representative exam-

ples. The diameter corresponding to the peak value of differentcurve

is regarded as the most probable pore diameter, whose size occupies

the largest proportion of pores compared with other pore diameters.

The narrower the most probable pore diameter, the ﬁner the pore

structure. It can be seen in Fig. 2 that the macropores (those in the

30ꢂ200 nm range) in this sample are very regular, and the most

probable pore diameters of 3DOM catalysts do not obviously change

after supporting gold nanoparticles, which is consistent with the

observation that the open windows interconnected macropores in

SEM images. The total speciﬁc pore volumes and the average pore

diameters of various concretes are analyzed and compared, and

the results are given in Table 1. The total pore volume of the materi-

als determined by N₂physisorption (ꢂ0.07 ml g^ꢀ1) is less than 3% of

the total pore volume (ꢂ3.4 ml g^ꢀ1) determined by mercury poros-

imetry, indicating main contribution of macropores to the total

porosity in these materials. The average pore diameter of 3DOM

materials is about ꢂ260 nm, which is in good agreement with the re-

sults of SEM images. The porosity of 3DOM materials is beyond 90%,

indicating that the solid reactants can easily diffuse in the materials.

Therefore, 3DOM Ce_1ꢀxZr_xO₂supported Au catalysts will have good

catalytic performance for the catalytic combustion of solid particles.

The BET surface areas of 3DOM Ce_1ꢀxZr_xO₂solid solutions and

Au/Ce_1ꢀxZr_xO₂catalysts (0 6 x 6 1) are listed in Table 2. The BET

surface areas of the 3DOM catalysts are 44–60 m²g^ꢀ1. As shown

in Table 2, no obvious changes of their BET surface areas are ob-

served after Au nanoparticle deposited on the surface of 3DOM

Ce_1ꢀxZr_xO₂solid solutions. As shown in Fig. S6, no organic residues

(PVP) over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts are detected by FT-IR

spectra.

3.1.2. The results of MIP and BET

The presence of macropores in the 3DOM catalysts was assessed

by the means of mercury intrusion porosimetry (MIP). Fig. 2 displays

the differential curves of pore size distribution of 3DOM Ce_1ꢀxZr_xO₂

0.025

Ce_0.8Zr_0.2O₂

Au/Ce_0.8Zr_0.2O₂

0.020

Ce_0.7Zr_0.3O₂

Au/Ce_0.7Zr_0.3O₂

0.015

0.010

0.005

0.000

10

100

1000

Pore size Diameter/nm

Fig. 2. Pore size distribution of the 3DOM Ce_1ꢀxZr_xO₂and Au/Ce_1ꢀxZr_xO₂(x = 0.2,

0.3) catalysts determined by MIP.

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

17

Table 1

Porosities, average diameters, pores volume and pore areas of 3DOM Ce_1ꢀxZr_xO₂and Au/Ce_1ꢀxZr_xO₂catalysts (x = 0.2, 0.3).

a

b

c

d

Catalyst

V_p(ml g^ꢀ1

)

A_p(m²g^ꢀ1

)

V_Hg(ml g^ꢀ1

)

D_p(nm)

Porosity^e(%)

Ce_0.8Zr_0.2O₂

Au/Ce_0.8Zr_0.2O₂

Ce_0.7Zr_0.3O₂

0.08

0.07

52.7

50.6

54.4

53.2

3.4

3.3

3.7

3.5

261

259

265

262

92.3

92.0

92.8

92.9

Au/Ce_0.7Zr_0.3O₂

a

b

c

Pore volume determined by BET method.

Total pore area determined by MIP.

Total pore volume for pores.

The average pore diameters.

Determined by Hg porosimetry.

d

e

Table 2

Average crystal parameters and crystal sizes of the 3DOM Ce_1ꢀxZr_xO₂and Au/Ce_1ꢀxZr_xO₂catalysts with different Au loading amounts.

c

Catalyst (3DOM)

D^a(nm) (Ce_1ꢀxZr_xO₂)

S_BET(m²g^ꢀ1

)

Au size^b(nm)

Au content^d(wt.%)

Without Au

With Au

Without Au

With Au

Au/CeO₂

13.1

10.2

8.9

8.7

9.3

9.9

11.7

21.7

13.1

10.2

8.8

8.6

9.3

10.0

11.7

21.7

60

54

56

59

51

50

47

44

62

57

58

53

50

49

44

3.4

3.3

3.4

3.2

1.3

1.5

1.4

1.3

1.2

1.3

1.2

Au/Ce_0.9Zr_0.1O₂

Au/Ce_0.8Zr_0.2O₂

Au/Ce_0.7Zr_0.3O₂

Au/Ce_0.6Zr_0.4O₂

Au/Ce_0.5Zr_0.5O₂

Au/Ce_0.3Zr_0.7O₂

Au/ZrO₂

a

b

c

Determined by XRD using Scherrer equation with the intensity of the most prominent peak, (111).

Determined by HRTEM.

Surface area obtained by BET method.

Determined by ICP-AES.

d

3.1.3. The results of XRD

For the 3DOM ZrO₂and Au/ZrO₂catalysts, all the diffraction peaks

of XRD pattern indicate the coexistence of the two crystal phases

structures including monoclinic (JCPDS 36-0420) and orthorhombic

(JCPDS 37-1413). However, no ZrO₂crystal phase was detected in all

the XRD proﬁles of Ce_1ꢀxZr_xO₂solid solutions (x > 0), which indicates

the formation of solid solutions. With increasing of x in 3DOM

Ce_1ꢀxZr_xO₂solid solutions (0 6 x 6 0.7), the diffraction peaks of 2h

show a slight upshift from 28.8 to 30.0 °C. The effect is primarily

due to Zr⁴⁺ion entering into the ﬂuorite-type lattice of ceria and

forming Ce_1ꢀxZr_xO₂solid solutions, leading to the shrinkage of the

lattice. The peaks of the XRD patternsof 3DOM Ce_1ꢀxZr_xO₂solidsolu-

tions and Au/Ce_1ꢀxZr_xO₂catalysts are sharp for the sample with low

ceria. As the ceria content increases, it becomes broad, attributing to

the presence of the smaller average crystallite sizes of the Ce_1ꢀxZr_xO₂

solid solutions. As shown in Fig. 3B, no characteristic diffraction

peaks (ꢂ38.1°) belonging to Au nanoparticles were detected after

3DOM Ce_1ꢀxZr_xO₂-supported gold nanoparticles, indicating the high

dispersion and small size of Au particles on the surface of 3DOM

Ce_1ꢀxZr_xO₂solid solution supports. The average crystallite sizes of

the 3DOM Ce_1ꢀxZr_xO₂solid solutions are calculated by Scherrer

equation using the intensity of the most prominent peak to (111)

crystal face, and the calculated values are summarized in Table 2.

The average crystallite size of the 3DOM Ce_0.7Zr_0.3O₂solid solution

shows that the smallest value is 8.7 nm, which is corresponding to

ca. 3–4 crystallites per 3DOM Ce_0.7Zr_0.3O₂solid solution wall thick-

ness. No obvious changes of their crystallite sizes are observed after

Au nanoparticle deposited on the surface of 3DOM Ce_1ꢀxZr_xO₂solid

solutions. Therefore, it can be concluded that the deposition of Au

nanoparticles on the surface of 3DOM Ce_1ꢀxZr_xO₂solid solutions

leads to less impact on the crystalline phase, crystalline size and

crystallinity of 3DOM Ce_1ꢀxZr_xO₂solid solutions.

To conﬁrm the formation and phase structures of 3DOM Ce_1ꢀx

Zr_xO₂solid solutions and Au/Ce_1ꢀxZr_xO₂catalysts (0 6 x 6 1), XRD

measurements were carried out and the XRD patterns are shown

in Fig. 3. All the diffraction peaks of the 3DOM CeO₂solid solution

and Au/CeO₂catalyst can be indexed to the (111), (200), (220),

(311), (400) and (331) crystal faces, which correspond to a face

centered cubic (FCC) ﬂuorite structure of CeO₂(JCPDS 43-1002).

(111)

(A)

(220)

(311)

(200)

(331)

(222)

(420)

(B)

80

(400)

x=0

x=0.1

x=0.2

x=0.3

x=0.4

x=0.5

x=0.7

x=1

Au(111)

x=0

x=0.1

x=0.2

x=0.3

x=0.4

x=0.5

x=0.7

x=1

20

30

40

50

2θ/^o

60

70

3.1.4. The results of TEM and HRTEM

Fig. 4 shows the TEM images of 3DOM Ce_1ꢀxZr_xO₂-supported Au

nanoparticle catalysts with same Au loading amount of 2 wt.%. The

Fig. 3. XRD patterns of the 3DOM Ce_1ꢀxZr_xO₂solid solutions (A) and Au/Ce_1ꢀxZr_xO₂

catalysts (B) (0 6 x 6 1).

18

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

Fig. 4. TEM images of the 3DOM Au/Ce_1ꢀxZr_xO₂catalysts. The ones in a, c, g, h images are the corresponding SAED pattern. (a) Au/CeO₂, (b) Au/Ce_0.9Zr_0.1O₂, (c) Au/Ce_0.8Zr_0.2O₂,

(d) Au/Ce_0.7Zr_0.3O₂, (e) Au/Ce_0.6Zr_0.4O₂, (f) Au/Ce_0.5Zr_0.5O₂, (g) Au/Ce_0.3Zr_0.7O₂, (h) Au/ZrO₂.

3DOM structure with overlapped pores can be clearly observed by

TEM images after the loading Au nanoparticles. The pore sizes of

3DOM Ce_1ꢀxZr_xO₂supports are 250 20 nm, and the voids are

interconnected through open windows with 65 nm in diameter,

which is in agreement with the observed by SEM images and

MIP. The walls of the macroporous samples are crystalline by se-

lected area electron diffraction (SAED) (inset in Fig. 4a, c, and g),

which the characteristic SAED rings of the Ce_1ꢀxZr_xO₂solid solu-

tions except for ZrO₂are indexed as (111), (200), (220) and

(311) planes of face centered cubic (fcc) ﬂuorite structure, and

the SAED rings of ZrO₂show the coexistence of the two crystal

phase structures, which are in accord with the results of XRD char-

acterization. The perfect macroporous structure provides the ideal

reaction space for solid reactants (diesel soot, as shown in Fig. S4).

The pore sizes of these catalysts are big enough to permit soot par-

ticles to enter their inner pores and reduce the diffusion resistance,

and the number of contact points between soot and catalysts will

be increased dramatically. Therefore, the catalytic activity for soot

combustion over 3DOM catalysts will be enhanced remarkably.

Fig. 5 shows the HRTEM images and size distribution of Au

nanoparticles over 3DOM Ce_1ꢀxZr_xO₂solid solutions. The spherical

gold nanoparticles precipitated on the surface of 3DOM Ce_1ꢀxZr_xO₂

supports can be clearly observed in HRTEM images, and all Au par-

ticles are highly dispersed and uniformed in size. The sizes of Au

nanoparticles over 3DOM Ce_1ꢀxZr_xO₂solid solutions are in the

range of 1–6 nm with a narrow size distribution, and the mean

diameters of Au nanoparticles for all catalysts are 3.3 0.1 nm by

statistic analysis of more than 300 Au particles for each sample.

The lattice fringes of gold particles and Ce_1ꢀxZr_xO₂solid solution

particles in HRTEM images were clearly observed, for example

the lattice fringes was measured to be 2.35 Å indexed as (111)

planes of fcc gold particle in Fig. 5a and g. It indicates that sup-

ported Au particles are crystalline and exposed fcc (111) plane.

The particle sizes of Ce_1ꢀxZr_xO₂solid solutions (x < 1) are in the

range of 5–15 nm, which is in accord with the result of XRD char-

acterization. Supported Au nanoparticles exhibited hemispherical

morphology on the surface of the Ce_1ꢀxZr_xO₂supports, and the

contact angle is about 90° in Fig. 5g. It is evident that the roughness

originated from the contact between the Au nanoparticles and the

Ce_1ꢀxZr_xO₂supports, which is an indication of the existence of a

strong metal–support interaction. The material of nanocomposite

structure in Fig. 6b, between nanometric crystal of Ce_1ꢀxZr_xO₂solid

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

19

Fig. 5. The HRTEM images and size distribution of Au nanoparticles over 3DOM Ce_1ꢀxZr_xO₂solid solutions. The lattice fringes in a image correspond to (111) plane of Au

particle at 2.35 Å. The one in g image shows an Au particle whose lattice fringes correspond to fcc (111) plane at 2.35 Å in fast Fourier transform (FFT) image. (a) Au/CeO₂, (b)

Au/Ce_0.9Zr_0.1O₂, (c) Au/Ce_0.8Zr_0.2O₂, (d) Au/Ce_0.7Zr_0.3O₂, (e) Au/Ce_0.6Zr_0.4O₂, (f) Au/Ce_0.5Zr_0.5O₂, (g) Au/Ce_0.3Zr_0.7O₂, (h) Au/ZrO₂.

solution supports (ꢂ8 nm) and Au nanoparticles active component

(ꢂ3 nm), should possess strong synergetic effect for redox ability

because the composite structure is favorable for the mobility of lat-

tice oxygen in catalysts [32]. The particle size and physical struc-

ture of the gold nanoparticles are very similar in 3DOM Au/

Ce_1ꢀxZr_xO₂catalysts, indicating that the interference of size effect

of Au particles may be ignored in the identiﬁcation of the inﬂuence

of the support on the catalytic activity.

3.1.5. The results of UV–Vis DRS

As a source of complementary information on the particle size of

supported gold particles, UV–Vis DRS spectroscopy has been consid-

ered to be one of the most appropriate techniques due to the surface

plasmon resonance (SPR) wavelength strongly depending on the

size of supported Au nanoparticles [33]. The surface plasmon

adsorption in the metal nanoparticles arises from the collective

oscillations of the free conduction band electron induced by the

20

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

552

450

445

474

a

b

c

412

321

631

d

e

f

a

g

h

b

300

400

500

600

700

800

100 200 300 400 500 600 700 800 900 1000

Raman Shift/cm^-1

Wavelength/nm

Fig. 6. The UV–Vis DRS spectra of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts recorded by using

3DOM Ce_1ꢀxZr_xO₂solid solutions as reference samples. (a) Au/CeO₂, (b) Au/

Ce_0.9Zr_0.1O₂, (c) Au/Ce_0.8Zr_0.2O₂, (d) Au/Ce_0.7Zr_0.3O₂, (e) Au/Ce_0.6Zr_0.4O₂, (f) Au/

Ce_0.5Zr_0.5O₂, (g) Au/Ce_0.3Zr_0.7O₂, (h) Au/ZrO₂.

Fig. 7. Raman spectra of 3DOM Ce_0.8Zr_0.2O₂(a) and Au/Ce_0.8Zr_0.2O₂(b) catalysts.

doubly degenerate into longitudinal optical mode of CeO₂, and this

band could be linked to oxygen vacancies due to the substitution of

Zr⁴⁺into the ceria lattice [40]. Compared with 3DOM Ce_0.8Zr_0.2O₂

catalyst, the Raman spectra of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst

exhibits the signiﬁcant difference between the curve (a) and (b).

The strongest peak centered at 445 cm^ꢀ1shifts by 15 cm^ꢀ1toward

the high wave number and the two shoulder peaks (412 and

474 cm^ꢀ1) disappeared suggests that a certain loss of anion-lattice

ordering leads to the low symmetry of the oxygen atoms around

cerium ions, because the interaction between Au atom and

incident electromagnetic radiation. Such resonances are seen when

the wavelength of the incident light far exceeds the particle diame-

ter. To verify the electronic state of the Au nanoparticles on the sur-

face of 3DOM Ce_1ꢀxZr_xO₂solid solution carriers, the UV–Vis DRS

spectra of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts using 3DOM Ce_1ꢀxZr_xO₂

solid solutions as reference sample were recorded, and the results

are shown in Fig. 6. The diffuse reﬂectance spectra of the supported

Au nanoparticle catalysts display well-deﬁned surface plasmon

absorption band centered at ꢂ552 nm with different catalysts,

which is the characteristic of nanocrystalline Au particles (diameter

<5 nm) and conﬁrm the low degree of their polydispersity. [34] It is

accord with the calculated results of HRTEM images on the size dis-

tribution of supported Au nanoparticles. Generally speaking, the

decreasing in particle size causes the increase in quantization of sur-

face plasmon oscillation, resulting in a blue shift of the plasmon

band [35]. It is known that the frequency and the strength of the res-

onance were determined by the size and shape of the particles as

well as the dielectric function of the surrounding medium. In this

study, the positionand shapeof theplasmon absorption band of sup-

ported gold nanoparticles are slightly different as judged from the

DRS data, which the shoulder peak intensity of the plasmon adsorp-

tion band at 450 nm decreased with the increasing of Zr-doped

amount. It indicates that the 3DOM Ce_1ꢀxZr_xO₂matrix has strong

chemical interaction with the gold surface, and the strong interac-

tion is related to the amount of Ce in supports. It was demonstrated

from the above results that well-deﬁned Au nanoparticles on the

3DOM Ce_1ꢀxZr_xO₂support could be successfully fabricated through

GBMR method.

supports induces

a localized defect or an impurity-derived

vibration mode of lattice oxygen. In addition, the intensity of the

peak at 631 cm^ꢀ1obviously increases could be ascribed to the

change of amount of oxygen vacancies. Therefore, we can deduce

that a strong metal–support interaction between Au atom and Ce

atom may exist, which in accordance with the observations by

HRTEM images and UV–Vis DRS, and the strong interaction is

beneﬁcial to the creation of the oxygen vacancies. The similar

effect observed by Raman spectra has been reported in Pd/Ceria

systems [41].

3.1.7. The results of H₂–TPR

For metal oxide catalysts, H₂–TPR measurement can simulta-

neously reﬂect the reducibility of metallic ion with high valence

converting to the ion with low valence or metallic atom, and the

potential to remove or take up oxygen, i.e., the mobility of the lat-

tice oxygen (O^2ꢀ) [42]. Thus, the temperature of reduction peak

(T_red) is taken as a measure to evaluate the redox ability of catalyst.

The lower T_redis, the stronger is the redox ability of catalysts. Since

the rate of soot oxidation at low temperature is more important

than that of at high temperature in practical applications, we espe-

cially focus on the lowest T_redof H₂–TPR. The H₂–TPR curves of

3DOM Ce_1ꢀxZr_xO₂solid solutions and Au/Ce_1ꢀxZr_xO₂catalysts are

shown in Fig. 8. As shown in Fig. 8A, there are two apparent reduc-

tion peaks ranging from 280 to 600 °C. The one reduction peak at

low temperature (ꢂ350 °C) is attributed to the reduction of the ad-

sorbed oxygen, and the other reduction peak (ꢂ503 °C) corre-

sponds to the outermost layers reduction of Ce⁴⁺–Ce³⁺. The

reduction peak ranging from 650 to 850 °C can be attributed to

the reduction of the inner Ce⁴⁺layer and lattice oxygen (bulk

reduction). With the increasing of Zr-doped content, the peak at

around 503 °C gradually shifts toward higher temperature and

the reduction peak at high temperature becomes weak, which

indicates the reduction of the reducibility of catalysts [43]. Among

these catalysts, the consumption amount of H₂for the peak rang-

ing from 280 to 600 °C over Ce_0.8Zr_0.2O₂catalyst is the largest

3.1.6. The results of Raman spectroscopy

In contrast to the cation-dominated information yielded by

XRD, Raman spectra of ﬂuorite-related phases are dominated by

lattice oxygen vibrations [36] and are obviously sensitive to the

crystalline symmetry. Thus, it is a useful tool to obtain the addi-

tional structural information of both M–O bond arrangement and

lattice defects [37]. The Raman spectra of 3DOM Ce_0.8Zr_0.2O₂carrier

and Au/Ce_0.8Zr_0.2O₂catalyst are presented in Fig. 7. For 3DOM

Ce_0.8Zr_0.2O₂solid solution, there are three Raman bands located

at ꢂ321, ꢂ445 and ꢂ630 cm^ꢀ1in Fig. 7a. The appearance of weak

band at ꢂ321 cm^ꢀ1could be attributed to displacement of oxygen

atoms from their ideal ﬂuorite lattice position [38]. The broad and

subtle band centered at ꢂ445 cm^ꢀ1could be attributed to the

symmetric stretching mode (F_2g) of the oxygen atoms around

cerium ions [39]. The band at ca. 631 cm^ꢀ1corresponded to a

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

21

consumption for the lowest temperature reduction peaks on

3DOM Au/Ce_0.8Zr_0.2O₂catalyst is the largest, which is 41.5 mol.

773

503

(A)

l

350

However, the total amount of H₂consumption at the lower tem-

peratures decreases with the increasing of Zr-doped content,

which can be assigned to the decreasing amount of surface oxygen

species over the surface of catalysts. It indicates that the redox

ability of catalyst is strongly related to the elements of Au and Ce.

The surface reducible species of the gold oxide (AuO_xor Au(CeO₂)_y),

which intimately interacted with the Ce_1ꢀxZr_xO₂solid solutions,

are formed by the diffusion of Au atoms into sublayers of

Ce_1ꢀxZr_xO₂solid solutions [46]. Therefore, the obtained results

suggest that the different supports-surface composition can

provide different reacting behaviors of Au toward H₂oxidation

and can enlighten different synergic effects between the 3DOM

Ce_1ꢀxZr_xO₂supports and Au nanoparticles [47].

CeO₂

Ce_0.8Zr_0.2O₂

Ce_0.5Zr_0.5O₂

Ce_0.3Zr_0.7O₂

ZrO₂

100

200

300

400

500

600

700

800

900

Temperature/^oC

3.1.8. The results of O₂–TPD

713

224

To further investigate the adsorption and activation of oxygen

on carriers and supported Au catalysts, O₂–TPD measurements

were carried out, and the results are displayed in Fig. 9. The O₂

desorption peaks of 3DOM Ce_1ꢀxZr_xO₂carriers are relatively weak.

The O₂–TPD proﬁle of 3DOM Ce_0.8Zr_0.2O₂solid solution shows that

there are two desorption peaks of oxygen at 66 and 171 °C, which

are assigned to the desorption of physical adsorption oxygen spe-

127

(B)

480

398

Au/CeO₂

Au/Ce_0.8Zr_0.2O₂

Au/Ce_0.5Zr_0.5O₂

Au/Ce_0.3Zr_0.7O₂

Au/ZrO₂

cies (O₂) and chemisorption’s oxygen species (O^ꢀ), respectively. It

2

indicates that the desorption oxygen capability of Ce_1ꢀxZr_xO₂solid

solutions will be inﬂuenced by the doping Zr content. As shown in

Fig. 9B, the O₂–TPD proﬁles of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts show

that there are four oxygen desorption peaks centered at 92, 190,

10

100 200 300 400 500 600 700 800 900

Temperature/^oC

66

(A)

97

8

CeO₂

Fig. 8. The H₂–TPR proﬁles of 3DOM Ce_1ꢀxZr_xO₂solid solutions (A) and Au/

Ce_1ꢀxZr_xO₂catalysts (B).

6

171

Ce_0.8Zr_0.2O₂

Ce_0.7Zr_0.3O₂

Ce_0.5Zr_0.5O₂

quantiﬁed by using CuO as a calibration reference (Table S1), which

is attributed to the increasing reduction extent of supports with

the moderate Zr-doped content into the ceria lattice.

4

2

0

ZrO₂

As shown in Fig. 8B, the H₂–TPR proﬁles of 3DOM Au/Ce_1ꢀxZr_xO₂

catalysts are obviously different from those of 3DOM Ce_1ꢀxZr_xO₂

solid solutions. The proﬁles of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts show

a series of strong peaks within the lower reduction temperatures

ranging from 50 to 350 °C compared with 3DOM Ce_1ꢀxZr_xO₂oxides.

The result suggests that a strong interaction between nanogold

particulates and supports occurs on 3DOM Au/Ce_1ꢀxZr_xO₂catalysts

[44]. They can be related to the reduction of the chemisorption’s

oxygen species on highly dispersion supported Au nanoparticles

(Au–O_xto Au) or the interface between Au particles and supports

(Ce–O_x–Au to Au), which is related to the weakening of the Ce–O

bond induced by gold atom [45]. With the increasing of the Zr-

doped content, the temperature of reduction peaks gradually shifts

to the higher temperature from 224 to 360 °C. Even, the Au/ZrO₂

catalyst does not exhibit a clear reduction peak related to gold at

the low reduction temperatures. This result indicates that the

interaction between Au and Zr is not strong as that of Au and

CeO₂. Among these catalysts, 3DOM Au/Ce_1ꢀxZr_xO₂catalysts

(x 6 0.2) display better reacting behaviors toward H₂oxidation at

the lower temperatures due to the strong metal (Au)-support

(Ce) interaction, indicating that they will give better catalytic per-

formances for soot oxidation. Table S2 shows the amount of H₂

consumption for the each peak and the total ﬁve peaks with differ-

ent temperatures and the total amounts for all the peaks on 3DOM

Au/Ce_1ꢀxZr_xO₂catalysts. It is noteworthy that the amount of H₂

0

200

400

600

800

Temperature/^oC

408

β

20

15

10

5

(B)

459

Au/CeO₂

667

γ

Au/Ce_0.8Zr_0.2O₂

Au/Ce_0.7Zr_0.3O₂

Au/Ce_0.5Zr_0.5O₂

92

190

α

Au/ZrO₂

600

0

200

400

800

1000

Temperature/^oC

Fig. 9. The O₂–TPD curves of 3DOM Ce_1ꢀxZr_xO₂solid solutions (A) and Au/

Ce_1ꢀxZr_xO₂catalysts (B).

22

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

408 and 667 °C. The ﬁrst peak at lowest temperature is assigned to

the desorption of physical adsorption oxygen species (O₂). The

desorption peaks in the temperature range from 180 to 500 °C

can be assigned to chemisorption’s surface active oxygen species

(O^ꢀ₂, O^ꢀ), and the peak at high temperature should be assigned to

lattice oxygen (O^2ꢀ) [48]. Compared with Ce_1ꢀxZr_xO₂, it is clearly

that the intensity of desorption peaks of physical adsorption oxy-

gen species on Au/Ce_1ꢀxZr_xO₂remarkably decrease, indicating that

the interaction between adsorption oxygen and catalyst becomes

strong after loading Au nanoparticles, and physical adsorption oxy-

gen specie may convert into chemisorption oxygen species. The

amount of surface active oxygen species over the catalysts de-

creases with the increasing of Zr-doped content, especially surface

active oxygen species (O^ꢀ₂, O^ꢀ) due to the decrease in the d-elec-

tron density of the Au atoms and the onset of reactivity to oxygen

in air [52], and also results in the strong interaction between gold

and supports. As shown in Fig. 10B, the O1s binding energies are

located at 528.8, 530.7 and 532.0 eV, which are assigned to lattice

oxygen species (O^2ꢀ) and chemisorption’s oxygen species (O^ꢀand

O^ꢀ₂), respectively. The content ratio of O^&₂^ꢀin catalysts remarkably

increases after the introduction Au on Ce_0.8Zr_0.2O₂support as

shown in Table 4, which may be attributed to the effect of strong

metal–oxide interaction to active oxygen species. Thus, the

increasing of surfaces active oxygen species (O^ꢀ₂, O^ꢀ) would gives

rise to unusual catalytic properties for deep oxidation reaction.

Fig. 10C shows the XPS spectra in the Ce 3d region. The four

main 3d_5/2features at 881.7, 884.6, 888.1 and 897.7 eV corre-

sponding to V, V⁰, V⁰⁰and V⁰⁰⁰components, while the 3d_3/2features

at 900.3, 902.7, 907.3 and 916.1 eV corresponding to U, U⁰, U⁰⁰and

U⁰⁰⁰components, respectively [53]. The signals V⁰and U⁰, character-

istics of Ce³⁺, exist evidently in the 3DOM Au/Ce_0.8Zr_0.2O₂catalyst

in comparison with those of 3DOM Ce_0.8Zr_0.2O₂catalyst [54]. The

value of R^b(the Ce species ratio of Ce³⁺/Ce⁴⁺) increases from

0.252 (Ce_0.8Zr_0.2O₂) to 0.433 (Au/Ce_0.8Zr_0.2O₂) in Table 4. The

increasing of Ce³⁺content is contributed to the interaction be-

tween cerium and the surrounding atoms, which leads to an in-

crease in the oxygen vacancies. It can be deduced that the

interaction between cerium and gold may lead to a transference

of the lattice oxygen in ceria over the surface of Au nanoparticles

and maintaining cerium at the low-valent state. Thus, the gold

atoms at the perimeter of the gold nanoparticles with the cerium

or the ultrasmall Au clusters located near an oxygen vacancy bear

a positive charge due to that Au atoms can be oxidized by the sur-

face Ce atoms [55]. Combined with the XPS result of Zr 3d in

Fig. 6D, the spectrum corresponding to level 3d of zirconium of

3DOM Au/Ce_0.8Zr_0.2O₂catalyst has no change comparing with

3DOM Ce_0.8Zr_0.2O₂catalyst. Therefore, we can deduce that Au

atoms prefer to interact with cerium atoms in comparison with zir-

conium atoms, and the synergetic effect between gold and cerium

leads to increasing the amounts of Ce³⁺and Au^&+in the 3DOM Au/

Ce_0.8Zr_0.2O₂catalyst. It indicates that there is a strong interaction

between Ce_1ꢀxZr_xO₂and gold due to the electron transformed from

Au metal to Ce⁴⁺in supports, and the amount of active oxygen spe-

cies also increases due to the oxygen transference from ceria to Au.

A strong metal–support interaction between Au atom and Ce atom

is also observed by Raman spectra in Fig. 7, and the strong interac-

tion is beneﬁcial to the creation of the oxygen vacancies [41]. This

is similar to the reported results on the strong Pt-ceria interaction

by Vayssilov et al. [32]. In summary, the chemical or structural

ordering may be responsible for the catalytic activity of Au/Ce_0.8

Zr_0.2O₂catalyst, which increases the amount of active oxygen

species (O^ꢀ₂, O^ꢀ). This result is in good agreement with those of

O₂–TPD and H₂–TPR measurements.

activated oxygen species (b) and lattice oxygen (c). The analysis re-

sults of O₂–TPD in Table 3 show that the amounts of surface active

oxygen species (O^ꢀ₂, O^ꢀ) over the catalysts are strongly depended

on the nature of supports. The amount of active oxygen species

(a) over 3DOM Au/Ce_0.8Zr_0.2O₂catalyst is the largest, and the

amounts of active oxygen species (b and

c) decrease with the

increasing of doping Zr content, even no

c

oxygen species for

3DOM Au/ZrO₂catalyst was found. It indicated that there is the

strong synergetic effect between Au nanoparticles and Ce_1ꢀxZr_xO₂

solid solutions, and the Ce/Zr ratio in 3DOM Au/Ce_1ꢀxZr_xO₂cata-

lysts is a critical factor for this interaction. Therefore, we deduce

that a distribution of electronic charges between a small cluster

of Au atom and support can weaken the Ce(Zr)–O bond, and the

presence of gold increases the amount of surface oxygen in

Ce_1ꢀxZr_xO₂solid solutions. This increase can occur through partial

occupation of vacant Ce sites with Au⁺, which will create additional

oxygen vacancies on the surface of the Ce⁴⁺–O₂ﬂuorite-type oxide

[49]. Consequently, the adsorption and activation of oxygen are

dependent on the capacity to supply oxygen of su_ꢀpport, especially

the amount of surfaces active oxygen species (O₂, O^ꢀ) [25]. It is

very consistent with the results of H₂–TPR characterization.

3.1.9. The results of XPS

The catalytic performance of materials for oxidation reaction is

correlated with the capability to activate oxygen. How do the sup-

ported Au nanoparticle catalysts interact with surface oxygen spe-

cies? To examine the electronic properties of the supported Au

nanoparticles and surface oxygen species, 3DOM Ce_0.8Zr_0.2O₂and

Au/Ce_0.8Zr_0.2O₂catalysts were studied by XPS, and the results are

shown in Fig. 10. The XPS spectra in the Au 4f region of 3DOM

Au/Ce_0.8Zr_0.2O₂catalyst are shown in Fig. 10A. Deconvolution of

the Au spectra indicates that both metallic and ionic gold species

are present in supported Au nanoparticle catalysts, i.e., the gold

species of metal (Au⁰) and ion (Au⁺and Au³⁺) are identiﬁed on

the surface of catalysts [50]. The concentrations of Au⁺species

measured by XPS are about 20.7 at.% (the atomic ratio of Au⁺to

the total Au atoms) on the Au/Ce_0.8Zr_0.2O₂catalysts, and Au^&+spe-

cies (Au⁺and Au³⁺) are considered to be the more active species

than Au⁰for oxidation reaction [51]. It is deduced that the appear-

ance of Au^&+may lead to the increasing the amount of surface

3.1.10. The results of NO oxidation

Fig. 11 shows the concentration change of NO₂during NO oxi-

dation over 3DOM Au/Ce_0.8Zr_0.2O₂catalyst by the means of MS

measurement. For comparison, the NO₂concentration proﬁles of

NO oxidation over 3DOM Ce_0.8Zr_0.2O₂catalyst and the case without

catalyst are also included. Almost no NO₂was detected in the case

without catalyst when the temperature was less than 500 °C, as

shown in plot (a). However, when 3DOM Ce_0.8Zr_0.2O₂catalyst

was present, the NO₂concentration remarkably increased, and it

was the largest on a typical catalyst of 3DOM Au/Ce_0.8Zr_0.2O₂cata-

lyst, as shown in plot (b) and (c). The NO₂concentration over

3DOM Au/Ce_0.8Zr_0.2O₂catalyst increases dramatically when the

temperature is higher than 260 °C. The peak temperature of NO₂

concentration proﬁle over 3DOM Au/Ce_0.8Zr_0.2O₂catalyst shifted

to the lower temperature by about 50 °C in comparison with that

Table 3

The analysis results of surface active oxygen species on the different catalysts by O₂–

TPD measurements.

Sample (3DOM)

a

(O^ꢀ₂

)

b (O^ꢀ)

c )

(O^2ꢀ

a

T (°C)

S

(10⁷) T (°C) S_b(10⁷) T (°C)

S

c

(107)

a

Au/CeO₂

190

188

187

188

2.03

2.10

1.76

1.73

1.51

408

420

430

443

459

13.35

13.25

11.53

8.56

703

664

665

667

–

23.25

22.10

20.86

15.28

–

Au/Ce_0.8Zr_0.2O₂

Au/Ce_0.7Zr_0.3O₂

Au/Ce_0.5Zr_0.5O₂

Au/ZrO₂

3.08

a

Determined by the peak area of surface oxygen species in O₂–TPD proﬁle.

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

23

Au⁰

528.8

(A)

(B)

Au 4f

O 1s

b

Au⁰

530.7

532.0

Au⁺

Au³⁺

a

82

84

86

88

90

526

528

530

532

534

Bingding Energy/ev

181.4

(D)

V'''

U

Zr 3d

Ce 3d

_U'''(C)

V

U'

U''

V'

183.9

V''

b

a

880

890

900

910

920

178

180

182

184

186

Bingding Energy/ev

Fig. 10. X-ray photoelectron spectra (XPS) of Au 4f (A), O 1s (B), Ce 3d (C) and Zr 3d (D) regions for the 3DOM Ce_0.8Zr_0.2O₂(a) and Au/Ce_0.8Zr_0.2O₂(b) catalysts.

Table 4

8.00E-012

7.00E-012

6.00E-012

5.00E-012

4.00E-012

3.00E-012

2.00E-012

1.00E-012

Surface composition and oxidation state of Au, Ce and O species dervied from XPS

analyses.

c

Sample

(3DOM)

Au species

Ce species

O species

O^2ꢀR^c

Au⁰Au⁺Au³⁺R^a

Ce³⁺Ce⁴⁺R^b

O₂^ꢀ

O²₂^ꢀ

b

Ce_0.8Zr_0.2O₂

–

20.1 79.9 0.252 13.7 23.1 63.2 0.582

Au/Ce_0.8Zr_0.2O₂69.3 20.7 10.0 0.443 30.2 69.8 0.433 14.2 34.1 51.7 0.934

a

b

c

Determined by the Au species ratio of the Au^&+(Au⁺+Au³⁺)/Au⁰.

Determined by the Ce species ratio of the Ce³⁺/Ce⁴⁺

.

Determined by the oxygen species ratio of the adsorbed oxygen (O^ꢀ₂+ O₂^2ꢀ)/

lattice oxygen (O^2ꢀ).

a

of 3DOM Ce_0.8Zr_0.2O₂catalyst, which is accorded with that of CO₂

concentration proﬁle over the catalyst at high temperature in

TPO process. It partially explains the high catalytic activity of

3DOM Au/Ce_0.8Zr_0.2O₂catalyst for soot combustion at relatively

high temperature (>250 °C).

150

200

250

300

350

400

450

500

Temperature /^oC

Fig. 11. The outlet NO₂ion current curves during NO oxidation process. (a) without

catalyst; (b) 3DOM Ce_0.8Zr_0.2O₂catalyst; (c) 3DOM Au/Ce_0.8Zr_0.2O₂catalyst.

3.2. Reactions

contact area between soot and catalyst under the condition of soot

embedded into the inner pores with the help of airﬂow. The cata-

lytic activity for soot oxidation is remarkably enhanced after Au

nanoparticles supported on the surface of 3DOM Ce_1ꢀxZr_xO₂solid

solutions, especially T₁₀and S_CO2. Among the different ratios of

Ce/Zr in 3DOM Au/Ce_1ꢀxZr_xO₂catalysts, Au/Ce_0.8Zr_0.2O₂catalyst

exhibits the best ignition catalytic performance under loost contact

condition, which might be assigned to the fact of the synergetic

effect of the interface between Au nanoparticle and Ce_0.8Zr_0.2O₂

carrier to dioxygen activation. However, there is a negative effect

on the activity (T₁₀) for soot combustion with the increasing of Zr

3.2.1. TPO reactions

The TPO experimental results of diesel soot oxidation over the

particle Ce_1ꢀxZr_xO₂, 3DOM Ce_1ꢀxZr_xO₂solid solutions and Au/

Ce_1ꢀxZr_xO₂catalysts are listed in Table 5. For comparison, the

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

and its T₁₀, T₅₀, T₉₀and S^mare 482, 585, 646 °C and 55.0%, respec-

CO2

tively. The TPO results show that all the catalysts can promote soot

oxidation, and the 3DOM Ce_1ꢀxZr_xO₂catalysts have higher catalytic

activity than those of particle catalysts, which are attributed to the

open and interconnected macropores structure giving more

24

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

Table 5

The temperatures and the selectivity to CO₂for soot combustion over without catalyst, the particle Ce_1ꢀxZr_xO₂, 3DOM Ce_1ꢀxZr_xO₂solid solutions and Au/Ce_1ꢀxZr_xO₂catalysts

(loose contact).

S^m_CO2(%)

Catalysts

T₁₀

T₅₀

T₉₀

Particles

3DOM

Au/3DOM

Particles

3DOM

Au/3DOM

Particles

3DOM

Au/3DOM

Particles

3DOM

Au/3DOM

CeO₂

370

364

381

382

384

390

398

470

482

337

329

328

352

357

355

367

247

240

232

248

263

299

322

353

445

442

449

451

461

465

475

580

585

390

378

394

403

415

416

422

485

349

361

365

375

377

383

407

430

479

487

491

493

505

515

520

639

646

432

420

432

446

457

458

467

543

396

407

418

419

423

426

452

477

98.0

98.3

97.1

96.5

95.2

92.3

84.1

60.2

55.0

97.3

85.7

88.4

84.8

82.6

80.1

78.7

76.9

99.8

99.7

99.8

99.7

99.5

98.5

97.8

Ce_0.9Zr_0.1O₂

Ce_0.8Zr_0.2O₂

Ce_0.7Zr_0.3O₂

Ce_0.6Zr_0.4O₂

Ce_0.5Zr_0.5O₂

Ce_0.3Zr_0.7O₂

ZrO₂

Soot

Reaction conditions: reaction gas, 5% O₂and 0.2% NO in Ar, 50 ml min^ꢀ1; the soot/catalyst mass ration is 1/10.

content above 20%. In addition, the regular catalytic activity (T₅₀

and T₉₀) of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts is different from the re-

sult of T₁₀. The 3DOM Au/CeO₂catalyst exhibits the best catalytic

performance for soot combustion at higher temperature, which

T₅₀and T₉₀are 349 and 396 °C, respectively. It indicates that there

is a negative effect on the activity (T₅₀and T₉₀) of 3DOM Au/

Ce_1ꢀxZr_xO₂catalysts with the increasing of Zr content. The TPO re-

sults of 3DOM Au/CeO₂and Au/Ce_0.8Zr_0.2O₂catalysts on three cy-

cles are shown in Table S3. The catalytic performance of the

catalyst remarkably becomes worse after one cycle due to the

low thermal stability of supported Au catalysts [28]. Thus, it is an

important issue to prevent the sintering of Au nanoparticles and

it is worth investigating in the further work.

To more clearly investigate the process of soot combustion, the

CO₂concentrations (C_CO2) as a function of reaction temperature are

shown in Fig. S7. The TPO patterns of soot combustion on the

3DOM Au/Ce_1ꢀxZr_xO₂catalysts exhibit two peaks of temperature

centered at ꢂ235 °C and ꢂ360 °C. The peak of the CO₂concentra-

tion proﬁles for soot combustion over the 3DOM Au/Ce_1ꢀxZr_xO₂

catalysts shifted to the lower temperature comparing with those

over the 3DOM Ce_1ꢀxZr_xO₂catalysts. And with increasing of the

content of Zr-doped in catalysts, the CO₂concentration peak at

235 °C decreased, and the position of the CO₂concentration peak

(ꢂ360 °C) shifted to the higher temperature. It is suggested that

the catalytic performances of supported Au catalysts are strongly

dependent on the nature of supports. And the catalytic mechanism

of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts at high temperature (>260 °C) is

different from that of the low temperature (<260 °C).

density of catalysts. The reaction rate of soot combustion is determined

by an isothermal reaction at 210 and 300 °C in the kinetic regime, and

the uniform macroporous structure of catalysts could exclude the

intraparticle mass transport limitations. The reaction rate for soot com-

bustion (the concentration of CO₂per unit time) was calculated accord-

ing to the slope of the lines, as shown in Fig. S9. The reaction rate for

soot combustion over 3DOM Ce_1ꢀxZr_xO₂catalysts at 210 and 300 °C

can be negligible in contrast to that of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts,

for example, the R₂₁₀over 3DOM Ce_0.8Zr_0.2O₂catalyst is only

0.022 ꢃ 10^ꢀ6mol min^ꢀ1and the R₃₀₀over 3DOM CeO₂catalyst is only

0.091 ꢃ 10^ꢀ6mol min^ꢀ1. Table 6 lists the quantiﬁed values of the reac-

tion rate, the total loading amount of Au, and the TOF for soot combus-

tion over the 3DOM Au/Ce_1ꢀxZr_xO₂catalysts under the same condition

as that of TPO reaction. The TOF values of the catalysts are quite large,

indicating that the Au sites are very active for the soot combustion. The

TOF value of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst (1.31 ꢃ 10^ꢀ3S^ꢀ1) was six

times as that of 3DOM Au/ZrO₂catalyst (0.21 ꢃ 10^ꢀ3S^ꢀ1) at 210 °C,

and the TOF value of 3DOM Au/CeO₂catalyst (2.70 ꢃ 10^ꢀ3S^ꢀ1) was

approximately 2.5 times as that of 3DOM Au/ZrO₂catalyst

(1.12 ꢃ 10^ꢀ3S^ꢀ1) at 300 °C, despite a very similar size distribution of

Au nanoparticles being observed by TEM images. Indeed, as the reac-

tion of soot oxidation occurs at same temperature and the total num-

bers of exposed surface atoms of Au nanoparticle with similar size

distribution are almost identical, other factors such as the nature of

the supports and activated surface oxygen species may be involved.

The oxygen storage ability of Ce-based supports is well-known and

this, allied to the ease with which the surface oxygen can be reduced,

may inﬂuence upon the reaction, in contrast to the non-reducible

ZrO₂supports. Among all the tested catalysts, the TOF values of

3DOM Au/Ce_0.8Zr_0.2O₂catalyst is largest at 210 °C, while that of

3DOM Au/CeO₂catalyst is largest at 300 °C, indicating that there are

different catalytic mechanisms for soot oxidation in the different reac-

tion temperature region.

The S^m_CO2in the soot oxidation over 3DOM Ce_1ꢀxZr_xO₂catalysts are

markedly decreased in contrast to particle-type catalysts in Table 5.

It is attributed to the open and interconnected macropores giving

less choice for the contact between CO and catalyst, and the CO

passed through the 3DOM catalysts faster than soot. To more clearly

observe the selectivity of CO₂in the process of soot combustion, the

S_CO2proﬁles of soot oxidation over the 3DOM Ce_1ꢀxZr_xO₂and Au/

Ce_1ꢀxZr_xO₂catalysts are shown in Fig. S8. The CO₂selectivities over

the 3DOM Ce_1ꢀxZr_xO₂catalysts were always less than 90% except for

CeO₂, and the higher value of S_CO2over 3DOM Ce_1ꢀxZr_xO₂catalysts

are in narrow temperature range (300 °Cꢂ450 °C). However, the

S_CO2is largely enhanced after Au nanoparticles supported on the sur-

face of 3DOM Ce_1ꢀxZr_xO₂solid solutions in Fig. S8B, and it is nearly

100% in wider temperature range (200–450 °C), which can ensure

that the CO emitted from cars is immediately removed under the

practical temperature of diesel engine emissions (150–400 °C).

And the S_CO2over the 3DOM Au/Ce_1ꢀxZr_xO₂catalysts is inﬂuenced

by the supports with the different ratios of Ce/Zr.

4. Discussion

4.1. The effect of the supports on catalytic performance for soot

combustion

It is reported that the nature of the support material can inﬂu-

ence the catalytic activity of the resulting gold catalysts [56]. The

effects of supports on catalytic performance for soot combustion

are mainly from two aspects: structure and redox property. The

reaction for catalytic combustion of soot is a typical heterogeneous

catalytical reaction, which takes place at the three-phase boundary

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

tants (O₂, NO). The efﬁciency of catalyst for solid–solid reaction

is strongly inﬂuenced by the contact between soot and catalyst.

In many studies, the outer surface of general catalysts is only avail-

able for soot combustion because their pore diameters (usually

3.2.2. Isothermal reactions

The intrinsic activity of catalyst is based on turnover frequency

(TOF), which is deﬁned as the ratio of reaction rate to the active site

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

25

Table 6

The total loading amount of Au, reaction rate and TOF values for soot combustion over the 3DOM Au/Ce_1ꢀxZr_xO₂catalysts at 210 and 300 °C.

a

Catalysts (3DOM)

Au actual content/wt.%

Au/mol ꢃ 10^ꢀ6

210 °C

R (mol min^ꢀ1)ꢃ10^ꢀ6

300 °C

R (mol min^ꢀ1)ꢃ10^ꢀ6

TOF_Au(S^ꢀ1ꢃ 10^ꢀ3

)

TOF_Au(S^ꢀ1ꢃ 10^ꢀ3

)

Au/CeO₂

1.3

1.5

1.4

1.3

1.2

1.3

1.2

6.6

7.6

7.1

6.6

6.1

6.6

6.1

0.48

0.51

0.60

0.48

0.39

0.28

0.16

0.08

1.20

1.29

1.31

1.12

0.97

0.77

0.39

0.21

1.07

0.92

0.89

0.82

0.74

0.59

0.54

0.41

2.70

2.34

1.95

1.92

1.87

1.62

1.37

1.12

Au/Ce_0.9Zr_0.1O₂

Au/Ce_0.8Zr_0.2O₂

Au/Ce_0.7Zr_0.3O₂

Au/Ce_0.6Zr_0.4O₂

Au/Ce_0.5Zr_0.5O₂

Au/Ce_0.3Zr_0.7O₂

Au/ZrO₂

Reaction conditions: reaction gas, 5% O₂and 0.2% NO in Ar, 50 ml min^ꢀ1; the soot/catalyst mass ratio is 1/10.

a

Determined by inductive coupled plasma atomic emission spectrometry.

<10 nm or even non-porous) are much smaller than the particle

diameter of soot (ꢂ25 nm) observed by the TEM image in Fig. S4.

Therefore, to meet the needs to increase contact area, 3DOM mate-

rials with uniform pore size (>50 nm) and periodic voids intercon-

nected through open windows are designed and synthesized by

colloidal crystal templates method. 3DOM Ce_1ꢀxZr_xO₂catalysts

have higher catalytic activities than the corresponding particle cat-

alysts. The pathway of reaction for soot combustion can be sum-

marized: the soot particles can easily enter the interior of 3DOM

catalysts and have less resistance going through the catalyst with

the help of the airﬂow in the reaction under loose contact between

soot and catalysts. The direct evidence is observed by TEM images

of the composition of 3DOM catalyst and soot with the same con-

dition of TPO reaction until 250 °C in Fig. 12. It is shown that 3DOM

structure still kept intact in the reaction process, and soot particles

entered the inner pores of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst with the

help of the airﬂow. The voids of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst are

ﬁlled with soot particles in the Fig. 12B and C, indicating that

3DOM structure is a desirable feature for diesel soot combustion.

The perfect macroporous structure provides the ideal reaction

space for solid reactants (diesel soot). The number of contact points

between soot and catalysts should be increased dramatically. How-

ever, the result also shows that 3DOM structure is adverse to

improving the selectivity of CO₂as shown in Table 5. The produc-

tion of relatively large amounts of CO during soot combustion is

because that the small molecule of CO is fast to pass through the

3DOM catalysts with the big pore size which is disadvantageous

to the contact between CO and catalyst. But 3DOM Au/Ce_1ꢀxZr_xO₂

bifunctional catalysts exhibited much higher S_CO2for soot oxida-

tion, which is easy for understanding. The supported Au catalysts

exhibit a surprisingly high activity for low-temperature CO oxida-

tion [11].

The catalytic activities of different materials with the same

3DOM structure and Au particle size for soot combustion are obvi-

ous different as shown in Table 5, indicating that the intrinsic cat-

alytic properties are strongly related to support themselves. The

reaction of soot combustion is a kind of deep oxidation reaction.

The catalysis nature of the reaction is the redox process. Therefore,

the intrinsic catalytic performances of catalysts are related to the

redox property determined by the nature of supports. As shown

in Fig. 8A, 3DOM Ce_0.8Zr_0.2O₂support showed better redox prop-

erty at low temperature (350 °C). It can be deduced that the mobil-

ity of lattice oxygen increases from the bulk to the surface with the

moderate Zr-doped content into the ceria lattice. It leads to the

change of the lattice and the increase in the channel diameter for

lattice oxygen migration, since the ion radius of Zr⁴⁺(0.086 nm)

is smaller than that of Ce⁴⁺(0.101 nm) [57]. The effectiveness of

the catalysts are related to their ability to deliver oxygen from

the lattice to the surface of soot in a wide temperature region,

and cerium-rich samples have a stronger ability to donate oxygen

for soot oxidation, i.e., the cerium-rich samples have high mobility

of lattice oxygen, which are consistent with the results of O₂–TPD

measurements. For example, in the O₂–TPD proﬁle of 3DOM

Ce_1ꢀxZr_xO₂catalysts, the mobility of oxygen species over Ce_0.8

Zr_0.2O₂sample is the highest, and the mobility of oxygen species

will be inﬂuenced by the Zr-doped content. The generation of

‘‘active oxgen species’’ induced by Zr cations would explain the

high activity of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst for soot combustion

at low temperature. Compared with the catalytic activities of 3DOM

Ce_1ꢀxZr_xO₂and Au/Ce_1ꢀxZr_xO₂catalysts, it could be concluded that

the better redox property of supports (different Ce/Zr ratios) can

improve the catalytic activity for soot combustion, especially at

the low temperatures.

4.2. The effect of the Au nanoparticle on catalytic performance for soot

combustion

The catalytic activities for soot oxidation are remarkably en-

hanced after the introduction of Au nanoparticles on the surface

of 3DOM Ce_1ꢀxZr_xO₂solid solutions, especially T₁₀and S_CO2. As

far as the role of supported Au nanoparticle in oxidation reaction

is concerned, it has not been well recognized whether the reaction

proceeds exclusively on metallic gold particles or it might occur

preferentially at the edges of gold particle involving either sites

on the adjacent support or on gold atoms inﬂuenced by the sup-

port. Generally speaking, the catalytic performances of gold cata-

lysts strongly depend on the particle size since Au nanoparticles

can activate molecule oxygen. It is not the ﬁrst time that the role

of oxygen is evoked particularly in oxidation reactions over gold

nanoparticles [52]. Small gold particles lose its metallic character

due to an increase of the d-electron density and also to a high de-

gree of surface de-saturation, which leads to a higher reactivity.

Our previous work [28] showed that the catalytic performance

for soot combustion was strongly depended on the size of sup-

ported Au nanoparticles, and the catalytic activity was correlated

with the active oxygen species on Au. In this study, the average

sizes of supported Au particles are equivalent, but the catalytic

activities for soot combustion over different supports are obviously

different, indicating that besides the particle size, the other factors

should also govern the catalytic performances of the catalysts. The

properties of supports would play an important role in catalyzing

soot oxidation. The analysis results in Fig. 13 showed that the T₁₀

and T₅₀values are closely correlated with the amount of surface ac-

tive oxygen species (a) and (b) which are varying from the Ce/Zr

ratios in catalysts. Based on the experimental observations by

H₂–TPR, O₂–TPD and XPS measurements, we can deduce that the

active oxygen species over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts may be

derived from the two approaches (or ‘‘generator’’): one is direct

activation of oxygen on the surface of gold nanoparticles. The other

is derived from the synergetic effect of metal (Au)-support (Ce)

26

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

Fig. 12. TEM (A–C) and HRTEM images (D) of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst after undergone the TPO reaction until 250 °C. Reaction conditions: reaction gas, 5% O₂and 0.2%

NO in Ar, 50 ml min; the soot/catalyst mass ratio is 1/10.

360

which the induced support may give rise to the fact that Ce-based

(A)

supports serve as a reservoir for oxygen in the oxidation reaction.

The interaction between support and gold may lead to a transfer of

the lattice oxygen in support or the adsorption of oxygen from the

support vacancies at the low temperatures, though this effect is

usually observed above 300 °C for pure supports [58]. The active

oxygen species derived from the metal–support interaction is more

important for soot combustion than the ﬁrst one. Therefore, 3DOM

Au/Ce_1ꢀxZr_xO₂catalysts (x 6 0.2) have good catalytic performance

for soot combustion due to a strong ability to donate oxygen of

cerium-rich samples. For instance, the TOF value of 3DOM Au/

Ce_0.8Zr_0.2O₂catalyst is six times as that of 3DOM Au/ZrO₂catalyst

at 210 °C. However, the active oxygen species predominating in

the oxidation reaction general stays at the surface of 3DOM Au/

Ce_1ꢀxZr_xO₂catalysts. Thus, the question is how the active oxygen

species migrate from the surface of catalyst to soot and what will

form?

2.2

2.0

1.8

1.6

1.4

340

320

300

280

260

240

220

Active oxygen (α)

T₁₀

0

0.2

0.3

0.5

1

x (Au/Ce_1-xZr_xO₂)

440

420

400

380

360

340

It can be found in the fact that the contact area for solid–solid

reactions, i.e., between catalytic sites and the soot, is of great

importance. Since the number of contact points is proportional

to the catalyst concentration, one can expect an increase in activ-

ity with higher surface loadings. In many studies, the catalytic

performances for soot combustion are very excellent under tight

conditions between soot and catalyst because of their good con-

tact. However, it is loose contacts between catalysts on the sur-

face of ﬁlter and soot particles under practical conditions. Thus,

it is signiﬁcantly important to study and design the active cata-

lysts for improving the contact capacity for soot combustion un-

der loose contact conditions. In our previous works [59], the high

activities have been related to the low melting points of active

site, which ensure a good contact between soot and catalyst.

The catalytic mechanism for soot oxidation has been reported

that there is a correlation between melting temperature of cata-

lyst and its catalytic activity [60]. As shown in Fig. 12D, Au nano-

particles supported on the inner surface of 3DOM structure

catalysts are able to further increase the contact between soot

and catalyst because of the ‘‘wetting’’ and surface mobility

capability of Au nanoparticles with low melting points [61].

The contact between soot and Au nanoparticles is a tight-like

contact type in the solid–solid reactions, in which supported Au

nanoparticle is the media center for tight-like contact between

(B)

14

12

10

8

Active oxygen (β)

T₅₀

6

4

2

0

0.2

0.3

0.5

1

x (Au/Ce_1-xZr_xO₂)

Fig. 13. The relation between the peak area of surface active oxygen species by O₂–

TPD and the soot combustion temperature over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts with

different values of x. T₁₀and the active oxygen specie (

oxygen specie (b) (B) as a function of x.

a) (A) and T₅₀and the active

Ce_0.8Zr_0.2O₂support and soot particles, indicating that the catalytic

performance for soot combustion would be enhanced because the

active oxygen species could migrate from the catalyst to the soot

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

27

particle by the way of spillover. Therefore, 3DOM Au/Ce_1ꢀxZr_xO₂

4.4. Mechanism consideration

catalysts exhibited super catalytic performance (T₁₀) for soot

combustion at the low temperatures.

Based on the above results and discussions, the following three

reasons can explain the high catalytic activities of 3DOM Ce_1ꢀx

Zr_xO₂-supported Au nanoparticle catalysts for soot combustion.

First of all, the redox property of catalyst determines its intrinsic

activity. The nanocomposite structure bifunctional materials, both

nanometric crystal of Ce_1ꢀxZr_xO₂supports and Au nanoparticles

active component, possess the strong redox ability. It is attributed

to that supported Au nanoparticles have the super capability of the

adsorption and activation of oxygen in air, and there is a synergistic

effect to donate the active oxygen species because the interaction

between cerium and gold may lead to transference of the lattice

oxygen in support and increase the amount of oxygen vacancies.

Therefore, the intrinsic activity of 3DOM Au/Ce_1ꢀxZr_xO₂catalysts is

very excellent. Secondly, the good contact between catalyst and soot

is a necessary external condition. It is signiﬁcantly important for the

migration of active oxygen species from catalyst to soot particle in

the typical reaction of heterogeneous catalysis. 3DOM structure is

a desirable feature for diesel soot combustion. The soot particles

can be diffused into the inner pores of 3DOM catalysts with the help

of airﬂow in the reaction underloose contact condition between soot

and catalysts. Thus, the surface area of inner pores is to reach the

maximum application. And the supported Au nanoparticle could

further improve the contact between catalyst and soot particle with

the tight-like contact. The good contact is a necessary condition for

the migration of active oxygen species from catalyst to soot particle

which is a crucial factor affecting the catalytic performance for soot

combustion. Thirdly, similarly to noble metal Pt catalysts, 3DOM Au/

Ce_1ꢀxZr_xO₂catalysts can efﬁciently catalyze NO to NO₂. The NO₂can

directly oxidize the soot particle to CO₂, which ensure the good indi-

rect contact performances between catalysts and soot particles. In

addition, supported Au catalysts exhibit the high selectivity to CO₂

production in soot combustion reaction which avoid secondary pol-

lution and suppress emission of CO. Therefore, 3DOM Au/Ce_1ꢀxZr_xO₂

catalysts are the good catalyst system for soot combustion under

loose contact conditions.

All the results above allow us to propose a mechanism for soot

combustion with NO and O₂over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts at

the different temperatures. The pathway of reaction for soot com-

bustion can be summarized and shown in Scheme 1. First of all, ac-

tive oxygen species over the surface of Au nanoparticle derived

from two approaches of which one is direct activation of oxygen

by Au nanoparticle (1) and the other is derived from the induced

support with the interaction between Au atom and Ce atom (2)–

(3). The active oxygen species would migrate from catalyst to the

carbon surface by the contact site between soot and catalyst. At

low temperatures (<240 °C) in Scheme 1A, Au nanoparticle can

contact with catalyst and soot particle, and active oxygen species

can spill over to the free carbon sites on soot to give off CO or

CO₂(4) [63]. Therefore, the catalytic performance of supported

Au catalyst is dependent on the ability to donate oxygen of sup-

ports (Ce/Zr ratio). The TOF value of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst

is six times as much as the value of 3DOM Au/ZrO₂catalyst at

210 °C since Ce_0.8Zr_0.2O₂support exhibit the stronger capability

to activate oxygen than that of ZrO₂. With the increasing of the

reaction temperature (240ꢂ260 °C) in Scheme 1B, the amount of

active oxygen species (O^ꢀ) over Au nanoparticles decreases be-

cause the rate of oxygen desorption increases conﬁrmed by O₂–

TPD results, and the part of active oxygen species are consumed

by the oxidation of NO–NO₂. There is no adequate active oxygen

species (O^ꢀ) to migrate to the surface of soot particle. Thus, the rate

of catalytic soot combustion decreases. At the relatively high tem-

peratures (>260 °C) in Scheme 1C, the large amount of NO can be

oxidized to NO₂over the catalyst (5). And NO₂can directly react

with the free carbon sites to give off CO or CO₂(6). Therefore, the

4.3. The effect of NO on the catalytic performance

In the famous CRT technology, NO_xis acting as an efﬁcient

mobile oxidizing agent, which accesses the soot combustion

and causes light-off of soot in an appropriate temperature range,

since the oxidizing ability of NO₂is stronger than that of O₂.

However, most of NO_xin diesel exhaust gas is NO, NO₂is almost

non-existent. In the CRT system, the removal of soot particles by

oxidation with NO₂is achieved by placing an oxidation catalyst

upstream of the trap in order to convert NO in the exhaust into

NO₂[2,62]. This mechanism subtly changed solid (soot)–solid

(catalyst) contact into solid (soot)–gas (NO₂)–solid (catalyst)

contact. A similar mechanism is found for soot combustion over

3DOM Ce_1ꢀxZr_xO₂-supported Au nanoparticle catalysts. As shown

in Fig. S10, the proﬁles of CO₂concentration over the typical

catalysts show a similar variation trends under the condition

of the presence or absence of NO. In the low temperatures range

(<250 °C), the C_CO2proﬁles are very similar to the cases with

or without NO. It suggested that the catalytic activity at the

low temperatures is not mainly correlated with the NO gas as

oxidizing agent. But at relatively high temperature (>250 °C),

the C_CO2peak in the presence of NO is higher and narrower than

the case without NO, indicating that the catalytic performances

of supported Au catalysts are strongly related to the NO gas. It

is attributed to that 3DOM Au/Ce_1ꢀxZr_xO₂catalysts can promote

NO oxidizing to NO₂which is used as intermediate to catalyze

soot oxidation. The strong oxidation ability of the catalyst for

NO oxidation is consistent with the results of in situ DRIFT

measurement in Fig. S11. The identiﬁcation of the adsorbed

species proves that NO_xis stored in the forms of nitrite/nitrates

on the catalyst surface, for which NO must be partially oxidized

to NO₂in a preceding step. This also explains that NO₂can be

released from the surface of catalyst at higher temperatures

(>250 °C).

The direct evidence of NO₂produced is detected by the means

of in situ MS measurement. As shown in Fig. 11, bare soot exhib-

ited no activity for NO₂production, and the NO₂level reached with

bare soot is the same as that measured in the initial time of the

reaction. In comparison, 3DOM Ce_0.8Zr_0.2O₂oxide was quite effec-

tive for NO oxidation to NO₂when reaction temperature greater

than 250 °C. The NO₂concentration obtained over this catalyst in-

creased with temperature rising until the thermodynamic equilib-

rium of the equation ðNO þ 1=2O₂$ NO₂Þ was fulﬁlled and then it

decreased at higher temperatures following thermodynamics pro-

ﬁle. The peak temperature for the production of NO₂on 3DOM Au/

Ce_0.8Zr_0.2O₂catalyst is about 350 °C, which is much lower than that

of 3DOM Ce_0.8Zr_0.2O₂oxide. The NO₂concentration over the 3DOM

Au/Ce_0.8Zr_0.2O₂catalyst is greater than that of the bare 3DOM

Ce_0.8Zr_0.2O₂oxide, resulting in a major contribution of the pro-

duced NO₂to the soot oxidation process. The strong oxidizing abil-

ity of 3DOM Au/Ce_0.8Zr_0.2O₂catalyst for NO oxidation is consistent

with the results of H₂–TPR. Thus, 3DOM Au/Ce_0.8Zr_0.2O₂catalyst

should give much higher catalytic activity for soot combustion.

In summary, according to the results of the effect of NO oxidation

experiment, in situ DRIFT and in situ MS detection, it was found

that NO is oxidized to NO₂over the catalyst. Then, NO₂acts as

the oxidizing agent for soot combustion. The NO₂concentration

over the 3DOM Au/Ce_0.8Zr_0.2O₂catalyst is higher than that of the

bare 3DOM Ce_0.8Zr_0.2O₂oxide. This can partially explain the strong

synergistic effect of gold and cerium in catalyst on the soot

oxidation.

28

Y. Wei et al. / Journal of Catalysis 287 (2012) 13–29

Scheme 1. Mechanism illustration of soot combustion over 3DOM Au/Ce_1ꢀxZr_xO₂catalysts at different temperature in the presence of O₂and NO.

rate of soot combustion at relatively high temperatures remarkably

increases owing to the combined effect of the active oxygen spe-

cies (O^ꢀ, O^2ꢀ) and NO₂, especially NO₂plays a dominant role in this

process. In addition, in the whole process of catalytic soot combus-

tion, the Au-based catalyst exhibits high catalytic activity for CO

oxidation (7). In summary, the whole reaction process for soot

combustion over the 3DOM Au/Ce_1ꢀxZr_xO₂catalysts may be de-

scribed as following equations:

catalysts for soot combustion are in accordance with the amount

of active oxygen species (O^ꢀ₂, O^ꢀ).

The reaction pathway for soot combustion over 3DOM Au/

Ce_1ꢀxZr_xO₂catalysts can be divided into two sections. At the low

reaction temperature, the soot particles are direct oxidized by ac-

tive oxygen species migrated from the surface of supported Au cat-

alysts. At the relatively high temperatures, the combined effects of

the active oxygen species and NO₂enhance the rate of soot com-

bustion, especially NO₂as an intermediate reactant plays a domi-

nant role.

O₂þ Au ! AuAO^ꢄ

ð1Þ

Ce^4þþ Au⁰! Ce^3þþ Au^þþ V_O

O₂þ V_O! O^ꢄ

O^ꢄþ C@C ! CO or CO₂

NO þ O^ꢄ! NO₂

ð2Þ

ð3Þ

ð4Þ

ð5Þ

ð6Þ

ð7Þ

Acknowledgments

This work was supported by the National Natural Science Foun-

dation of China (Nos. 20833011, 21177160, 21173270, and

20803093), the Program for New Century Excellent Talents in Uni-

versity of China (NCET-10-0811), the 863 program of China (No.

2009AA06Z313), CNPC Project (2011D-4604-0101), and Innovation

Foundation (2010D-5006-0402).

NO₂þ C@C ! NO_xþ CO or CO₂

CO þ AuAO^ꢄ! Au þ CO₂

Appendix A. Supplementary material

O^ꢄrepresents the surface activate oxygen species; V_Orepresents a

surface oxygen vacancy; C@C represents the free carbon (soot par-

ticle). The catalytic activity of soot combustion on 3DOM Au/

Ce_1ꢀxZr_xO₂catalysts in NO + O₂can be explained by the proposed

mechanism. The synergic effect between Au particle and support

play an essential role in the high catalytic activity for soot

combustion.

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.jcat.2011.11.006.

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Article Doi

The catalysts of three-dimensionally ordered macroporous Ce 1-xZrxO2-supported gold nanoparticles for soot combustion: The metal-support interaction

DOI: 10.1016/j.jcat.2011.11.006

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