Remarkable nanosize effect of zirconia in Au/ZrO2 catalyst for CO oxidation

Article abstract of DOI:10.1021/jp050645r

Nanosize effect of ZrO₂ in Au/ZrO₂ catalyst was studied by deposition-precipitation of Au nanoparticles in similar sizes (4-5 nm) on ZrO₂ nanoparticles of varying sizes. The catalysts were characterized with XRD, TEM, XPS, and nitrogen adsorption to understand the effect of ZrO₂ particle size on the catalytic nanostructures. Nanocomposite Au/ZrO₂ catalysts consisting of comparably sized Au-metal (4-5 nm) and ZrO₂ (5-15 nm) nanoparticles are found advantageous over those containing similarly sized Au-metal but larger ZrO ₂ (40-200 nm) particles for CO oxidation. This finding may have important implications on the designed preparation of advanced nanostructured catalysts and other chemical materials. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp050645r

9678
J. Phys. Chem. B 2005, 109, 9678-9683
Remarkable Nanosize Effect of Zirconia in Au/ZrO2 Catalyst for CO Oxidation†
Xin Zhang, Hai Wang, and Bo-Qing Xu*
InnoVatiVe Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering, Department of
Chemistry, Tsinghua UniVersity, Beijing 100084, China
ReceiVed: February 5, 2005; In Final Form: March 17, 2005
Nanosize effect of ZrO2 in Au/ZrO2 catalyst was studied by deposition-precipitation of Au nanoparticles in
similar sizes (4-5 nm) on ZrO2 nanoparticles of varying sizes. The catalysts were characterized with XRD,
TEM, XPS, and nitrogen adsorption to understand the effect of ZrO2 particle size on the catalytic nanostructures.
Nanocomposite Au/ZrO2 catalysts consisting of comparably sized Au-metal (4-5 nm) and ZrO2 (5-15 nm)
nanoparticles are found advantageous over those containing similarly sized Au-metal but larger ZrO2 (40-
200 nm) particles for CO oxidation. This finding may have important implications on the designed preparation
of advanced nanostructured catalysts and other chemical materials.
1. Introduction
parably sized Ni-metal (10-15 nm) and ZrO2 (7-25 nm)
nanocrystals, which show excellent catalytic performance for
the reforming reactions of methane (CH4 + CO2 ) 2CO + 2H2
and/or CH4 + H2O ) CO + 3H2).16-19 We are extending the
work to study the effect of reducing oxide particle size on the
catalysis of Au/oxide catalyst. Reported herein is our study on
the nanosize effect of zirconia on the catalytic performance of
Au/ZrO2 catalysts for CO oxidation. We prove that the catalytic
activity of Au nanoparticles (4-5 nm) in Au/ZrO2 can be
enhanced remarkably with reducing the particle size of zirconia
“support” in the range of 5-100 nm, which unravels a new
dimension in tailoring the performance of metal catalysts.
The interest in oxide-supported Au catalysts has increased
substantially since Haruta et al. discovered that small Au
nanoparticles are exceptionally active for low-temperature CO
oxidation.1-5 Exploration of the catalytic performance of Au
nanoparticles on the basis of sizes has been the main focus of
most research in this field.1-9 It was reported that the catalytic
activity by turnover frequency (TOF) of Au catalyst on different
oxide supports (TiO2, R-Fe2O3, Co3O4) increases with decreasing
the particle size of Au nanoparticles in the range of 2-10 nm.6
Goodman et al. found that Au nanoparticles of about 3.5 nm
showed the highest activity in a series of Au/TiO2 model
catalysts.8 Although the active catalysts commonly contain small
(2-5 nm) Au nanoparticles, the size of Au particles alone does
not seem to be a sufficient factor for the high activity of
nanosized Au catalysts.10-14 For instance, Bollinger and Vannice
found that deposition of TiOx overlayers onto inactive Au
powders (10 µm) can also produce very active TiO2-Au catalyst
for CO oxidation.10 In the attempt of Rolison et al., the loading
of ca. 6 nm Au nanoparicles onto TiO2 sized to 10-12 nm
produced composite Au/TiO2 catalyst that exhibited high activity
for CO oxidation, while Au particles of similar sizes were found
inactive on commercial TiO2 powders.13,14 While the nature of
active sites for CO oxidation on supported Au catalysts is still
being elucidated, increasingly gained experimental evidence
continues to show that the interaction between Au particles and
the supporting oxide support at their contact boundaries plays
an important role in determining the catalytic activity of the
Au nanoparticles.3,5,6,10 It can be expected that such interaction
at the metal-oxide boundaries can be properly tuned by
systematically reducing the particle size of the support oxides.
With advances occurring in the synthesis of nanoparticle
supports with controlled size, researchers should be encouraged
to explore the size effect of oxide-support nanoparticles, as well
as metal nanoparticles.15 For instance, our earlier exploration
on the size effect of ZrO2 on the catalysis of Ni metal has
generated nanocomposite Ni/ZrO2 catalysts consisting of com-
2. Experimental Section
2.1. Preparation and Catalytic Testing of Au/ZrO2 Cata-
lyst. ZrO2 powders were derived from a single source of ZrO-
(OH)2 hydrogel according to the methods described previ-
ously.16,17 The hydrogel was obtained by conventional hydrolysis
of zirconyl chloride in aqueous ammonia water of pH )10.
Following our earlier sample coding,16-19 ZrO2-CP-973 is the
sample prepared by calcination for 5 h in flowing air of the
ZrO(OH)2 hydrogel at 973 K. Also, ZrO2-AN-T (T ) 673, 873,
973, and 1073 K) samples were prepared by thermal processing
at temperature T in flowing nitrogen of an alcogel of ZrO(OH)2,
which was obtained by washing the hydrogel with anhydrous
ethanol several times. The alcogel was digested in anhydrous
ethanol under reflux (350 K) before it was processed to 973 K
in flowing nitrogen to give the ZrO2-AD-973 sample. Au/ZrO2
catalysts were prepared by the well-established deposition-
precipitation method.6,7 ZrO2 powders were dispersed in a 0.1
wt % HAuCl4 solution at room temperature, followed by
deliberately adjusting with 0.2 M NH4OH the solution pH to
ca. 9.0 ( 0.1. This aqueous dispersion was stirred for 6 h and
aged for 2 h at the adjusted pH, and then suction filtered.
Extensive washing with deionized water was then followed until
it was free of chloride ions (i.e., until conductance of the washed
solution in the final wash was less than 10-5 S m-1). Vacuum
drying at 383 K and air calcination at 673 K for 5 h of the
sample finalized the preparation of Au/ZrO2 catalysts.
* Corresponding author. Tel./Fax: (+86)10-6279-2122. E-mail: bqxu@
mail.tsinghua.edu.cn.
The activity in catalytic CO oxidation was measured in a
continuous flow, fixed-bed U-shape quartz reactor (i.d. 4 mm)
† Part of this work was orally presented at the 13th International Congress
on Catalysis, Paris, July 11-16, 2004.
10.1021/jp050645r CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/19/2005

Nanosize Effect of Zirconia in Au/ZrO2 Catalyst
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9679
TABLE 1: Size and Texture Properties of Zirconia
Nanoparticles
BET
pore
XLBA
sizea
(nm)
TEM
size
(nm)
ZrO2-
phaseb
(%)
surface volume
sample
(m2/g) (cm3/g)
ZrO2-CP-973
ZrO2-AN-973
ZrO2-AD-973
ZrO2-AN-673
ZrO2-AN-873
ZrO2-AN-1073
23
36
58
162
61
20
0.11
0.26
0.32
0.36
0.28
0.19
M22
40-200 M92/T8
M21/T20 20-25 M92/T8
M12/T11 10-15 M92/T8
M6/T5
M12/T11 12-17 M88/T12
M30 25-40 M97/T3
5-10
M76/T24
a Average crystal size obtained with the Scherrer equation by using
the (1h11) diffraction (2θ ) 28.5°) for monoclinic and the (111)
diffraction (2θ ) 30.4°) for tetragonal crystals; M and T represent the
monoclinic and tetragonal phase, respectively. b Percentage of mono-
clinic phase ) IM(1h11)/(1.6IM(1h11) + IT(111)).
Figure 1. Powder X-ray diffraction patterns of zirconia samples: (a)
ZrO2-AD-973; (b) ZrO2-CP-973; (c) ZrO2-AN-1073; (d) ZrO2-AN-973;
(e) ZrO2-AN-873; (f) ZrO2-AN-673.
CP-973. The presence of diffraction at 2θ ) 30.4° indicates
the presence of tetragonal crystals, although the XRD patterns
are dominated by monoclinic crystals. The indistinguishable
difference in the XRD patterns of ZrO2-AD-973, ZrO2-AN-973,
and ZrO2-CP-973 samples in the figure clearly indicates that
the crystal phase composition of ZrO2 is independent of the
sample history (CP, AN, or AD preparation) when the final
calcination temperature was 973 K. For the ZrO2-AN-T samples
obtained by varying the calcination temperature (T), the percent-
age of the monoclinic phase increased from 76% of ZrO2-AN-
673 to 97% of ZrO2-AN-1073, respectively.
The size and texture data of zirconia powders used for the
preparation of Au/ZrO2 catalysts are given in Table 1. It is
apparent that the particle sizes by TEM of ZrO2-AN-973 and
ZrO2-AD-973 are much smaller, and their surface area and pore
volumes are higher than that of ZrO2-CP-973. The similar sizes
obtained from XLBA and TEM measurements of the alcogel
derived ZrO2-AN-973 and ZrO2-AD-973 samples verify our
earlier observation that these samples are composed of discrete
ZrO2 nanocrystals.16-19 In contrast, the difference in the XLBA
(22 nm) and TEM (40-200 nm) sizes of the conventional ZrO2-
CP-973 sample indicates that primary crystals in this sample
have agglomerated more or less to form larger hard aggregates.
In the preparation of ZrO2-AN powders, the processing tem-
perature in flowing nitrogen of the ZrO(OH)2 alcogel was
changed between 673 and 1073 K to vary the particle sizes of
the powders, which produced the samples with particle sizes
of 5-10 nm (ZrO2-AN-673), 12-17 nm (ZrO2-AN-873), 20-
25 nm (ZrO2-AN-973), and 25-40 nm (ZrO2-AN-1073),
respectively. Again, the agreement in XLBA and TEM sizes of
these ZrO2-AN samples indicates that they are composed of
discrete ZrO2 nanocrystals.
Shown in Table 2 are properties of the calcined Au/ZrO2
catalysts. Nitrogen adsorption measurement indicated that the
loading of ca. 0.7% Au by deposition-precipitation has little
effect on the texture of zirconia. XRD patterns of the Au/ZrO2
catalysts were found similar to those of the corresponding “pure”
zirconia, and no diffraction for metallic Au was observed.
XPS study was carried out to obtain information on the state
of Au in the calicined Au/ZrO2 catalysts; the results are shown
in Figure 2 for Au/ZrO2-CP-973, Au/ZrO2-AD-973, Au/ZrO2-
AN-673, and Au/ZrO2-AN-973 catalysts. All of these Au/ZrO2
catalysts showed two XPS signals with binding energies near
83.5 and 87.3 eV. According to ref 21, the binding energies of
Au 4f electrons for Au, Au2O3, and Au(OH)3 are at 83.9, 86.3,
under atmospheric pressure (0.1 MPa). The reaction feed (1.0
vol % CO in dry air) was introduced into the reactor with a
space velocity of 20 400 mL/(h × g-cat). Before the reaction,
the catalysts (100 mg, diluted with 500 mg of quartz sand to
ca. 0.6 mL) were in-situ pretreated with H2 at 523 K for 2 h.
The reactor effluent was on-line analyzed with an HP-6890 gas
chromatograph equipped with a molecular 5A column and a
thermal conductivity detector (TCD).
2.2. Sample Characterizations. The crystal structures of
ZrO2 and Au/ZrO2 samples were characterized with powder
X-ray diffraction (XRD) on a Bruker D8 Advance X-ray
diffractometer using the Ni-filtered Cu KR radiation source
at 40 kV and 40 mA. The percentage of monoclinic phase
(M%) in the oxide “support” was measured according to the
equation:20 M% ) IM(1h11)/(1.6IM(1h11) + IT(111)), where IM(1h11) and
IT(111) are the diffraction intensities of the monoclinic (1h11)
(2θ ) 30.4°) and tetragonal (111) (2θ ) 28.5°) planes,
respectively. The average crystal size of ZrO2 was measured
with the X-ray broadening analysis (XLBA) by using the well-
known Scherrer equation: d ) 0.89λ/(B(2θ) cos θ), where B(2θ)
is the width of the XRD pattern line at half peak height in
radians, λ is the wavelength of the X-ray, 0.15406 nm, θ is the
angle between the incident and diffracted beams in degrees, and
d is the crystal size of the powder sample in nanometers.
Surface areas of the samples were measured with nitrogen
adsorption at 77 K on a Micromeritics ASAP 2010C instrument.
The samples were dehydrated under vacuum at 523 K for 5 h
before the adsorption measurement. TEM and HRTEM analyses
were performed on JEM-2010F (200 kV) and Tecnai F30 (300
kV), respectively. The samples were first dispersed carefully
in ethanol using an ultrasonic bath, and then deposited on a
polymer coated copper grid. X-ray photoelectron spectra (Au
4f) of the catalysts were recorded with an SKL-12 spectrometer
equipped with Mg KR radiation. The residual pressure in the
analytical chamber was maintained below 10-8 Torr during data
acquisition. The binding energies of Au 4f were corrected for
surface charging by referencing them to the energy of C 1s peak
of contaminant carbon at 285.0 eV. Au loading was determined
by XRF analysis on an XRF-1700 instrument.
3. Results and Discussion
All of the ZrO2 samples used in the preparation of Au/ZrO2
catalysts were derived from the same source of ZrO(OH)2
hydrogel. The thermal processing/calcination of ZrO2 samples
is known to produce crystals with two different phases, that is,
monoclinic (M) and tetragonal phases (T). Figure 1 compares
the XRD patterns of the alcogel derived ZrO2-AD-973 and ZrO2-
AN-T samples with that of the conventionally prepared ZrO2-
and 87.7 eV for 4f7/2 and 87.7, 89.6, and 91.4 eV for 4f5/2
,
respectively. Thus, the XPS data in Figure 2 are characteristic
of 4f7/2 and 4f5/2 electrons in the metallic Au of the catalysts.
As compared to those of the bulk Au metal,21 the lower binding

9680 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Zhang et al.
TABLE 2: Properties and Catalytic Activities of Au/ZrO2 Catalysts for CO Oxidation at 343 K
a
BET surface
(m2/g)
Au-loading
(wt %)
DAu
conv.
(%)
rate
TOFb
catalyst
(nm)
×10-7 mol/(s g-cat)
×10-3 s-1
Au/ZrO2-CP-973
Au/ZrO2-AN-973
Au/ZrO2-AD-973
Au/ZrO2-AN-673
Au/ZrO2-AN-873
Au/ZrO2-AN-1073
22
34
55
142
53
18
0.74
0.63
0.71
0.77
0.70
0.70
4.0
5.3
4.7
5.4
4.7
5.4
4.6
12.9
29.9
31.8
18.5
7.1
1.1
2.9
6.8
7.2
4.2
1.6
2.8
9.2
18.9
18.5
11.9
4.6
a Au particle size was determined by TEM. At least 200 particles were chosen to determine the mean diameter of Au particles according to
equation d ) ∑nidi/ni; the size scattering was within 3-7 nm. b The turnover frequencies (TOF) were calculated on the basis of the number of Au
atoms exposed at the surfaces, which were estimated from the mean diameters and actual loadings of the Au particles. In this calculation, the shape
of Au particles was assumed to be closed-shell particles of nearly spherical shape.2
catalysis of Au/ZrO2. Also included in Table 2 are the catalytic
activities for the CO oxidation reaction by CO conversion,
specific mass reaction rate, and TOF based on the Au atoms
exposed at the surfaces. Figure 4 shows the effect of reaction
temperature (323-373 K) on CO conversion over Au/ZrO2-
CP-973, Au/ZrO2-AN-973, and Au/ZrO2-AD-973 catalysts.
Clearly, the catalyst activity can be ranked in the order of Au/
ZrO2-AD-973 > Au/ZrO2-AN-973 > Au/ZrO2-CP-973, indicat-
ing that the smaller the size of zirconia nanoparticles in the
Au/ZrO2 catalyst, the higher the catalyst activity. We found that
all of our Au/ZrO2 catalysts showed stable catalytic activity for
the CO oxidation reaction (e.g., Figure 5). Under similar reaction
conditions, the steady-state activity of the present Au/ZrO2-CP-
Figure 2. XPS spectra of Au 4f in Au/ZrO2 catalysts: (a) Au/ZrO2-
973 catalyst agreed well with that of the Au/ZrO2 catalyst
CP-973; (b) Au/ZrO2-AN-973; (c) Au/ZrO2-AD-973; (d) Au/ZrO2-AN-
reported by Haruta et al.; for example, the present Au/ZrO2-
673.
CP-973 catalyst produced a CO conversion of 47% at 373 K
(Figure 5), while the CO conversion over the catalyst in Haruta
energies of Au 4f in these catalysts may indicate a modification
et al.’s work was 50%.6 The differences in the catalyst activity
of the electronic structure of Au due to the interaction between
are more pronounced when one compares the TOF in Table 2.
the nanosized metallic Au and zirconia particles.
To ensure accuracy, the TOF were calculated with the catalytic
Figure 3 shows the representative TEM images of all of the
data at 343 K because at this temperature the CO conversion
Au/ZrO2 catalysts tabulated in Table 2; Au particles in these
levels can be limited to less than ca. 30%. The TOF and the
catalysts are narrowly sized (scattering between 3 and 7 nm),
specific reaction rate over Au/ZrO2-AD-973 are 6 times larger
and their average sizes are in the range of 4-5 nm. Noticeably,
than that over Au/ZrO2-CP-973 at 343 K.
the particle sizes of ZrO2-CP-973 (40-200 nm) are 1 order of
magnitude larger than the sizes of Au nanoparticles (ca. 4.0
nm) in Au/ZrO2-CP-973 (Figure 3A). The images of Figure 3A
clearly show that a single particle of zirconia (ZrO2-CP-973) is
Figure 6 shows the dependence of CO conversion on the
reaction time-on-stream (TOS) at 343 K over the Au/ZrO2-AN-T
catalysts. Interestingly, the catalytic activity of Au/ZrO2-AN-T
was clearly seen to decrease with increasing the processing
capable of holding/supporting a number of the Au particles and
temperature of ZrO2-AN. The specific reaction rates and TOF
the chance is very low for the Au particles to make contact
of the most active Au/ZrO2-AN-673 are about 4 times higher
than that of the least active Au/ZrO2-AN-1073 catalyst (Table
2). While there are differences in the phase composition and
crystallinity, and consequently uncertainties in the surface
properties of the ZrO2-AN nanoparticles prepared with different
processing temperatures, particle size effect of the oxide support
(ZrO2-AN) can still be crucial in determining the catalytic
activity because a similar size effect of the oxide support also
appeared in the Au/ZrO2-CP-973, Au/ZrO2-AN-973, and Au/
ZrO2-AD-973 catalysts that contain differently sized zirconia
particles of similar phase composition and crystallinity (Figure
1 and Table 1). Alternatively, the present data clearly indicate
that the activity of Au/ZrO2 catalyst is greatly improved with
reducing the size of ZrO2 nanoparticles.
with more than one zirconia particle in Au/ZrO2-CP-973.
Obviously, the reduction in the particle sizes of zirconia from
ZrO2-CP-973 (40-200 nm) to ZrO2-AN-973 (20-25 nm) and
further to ZrO2-AD-973 (10-15 nm) resulted in increased
chance for Au particles (4-5 nm) to make contact with multiple
zirconia particles in the Au/ZrO2 catalysts (Figure 3A-C).
Reduction of the particle size of ZrO2-AN by changing the
processing temperature of the alcogel also produced a similar
effect in the Au/ZrO2-AN samples (Figure 3D, E, B, and F). In
fact, the Au/ZrO2-AD-973 and Au/ZrO2-AN-673 samples
(Figure 3C and D) were evolving to show the feature for Au/
ZrO2 nanocomposites because the sizes of Au particles (4-5
nm) were getting to match with the particle sizes of the zirconia
“support” (ZrO2-AD-973, 10-15 nm and ZrO2-AN-673, 5-10
nm), allowing a factor of ca. 2 for the size deviation.13,14,16,17
Thus, the present attempt of reducing the particle size of zirconia
to approach size comparability of the Au and zirconia nano-
particles has generated nanocomposite-like Au/ZrO2 catalyst,
for example, Au/ZrO2-AN-673, possessing a large amount of
the Au-ZrO2 contact boundaries or Au-ZrO2 junctions.
The comparable sizes of Au nanoparticles in the catalysts
make it possible to study the nanosize effect of ZrO2 on the
A referee of the present paper considered that the activity
difference between Au/ZrO2-AN-973 and Au/ZrO2-AD-973 was
not very large and may also be explained by the size effect of
Au particles. He/she might be correct if one just looks at these
two data without considering comprehensively all of the catalytic
data (Tables 1 and 2, and Figures 3, 4, and 6). The activity
difference is actually significant: Au/ZrO2-AD-973 is twice
more active than Au/ZrO2-AN-973 and 6 times more than Au/

Nanosize Effect of Zirconia in Au/ZrO2 Catalyst
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9681
Figure 3. TEM images of the Au/ZrO2 catalysts: (A) Au/ZrO2-CP-973; (B) Au/ZrO2-AN-973; (C) Au/ZrO2-AD-973; (D) Au/ZrO2-AN-673; (E)
Au/ZrO2-AN-873; (F) Au/ZrO2-AN-1073. The encircled particles are Au particles. Insets: HRTEM images of the selected areas.
ZrO2-CP-973. The average size of Au in Au/ZrO2-CP-973 (4.0
nm) is the smallest in comparison with those in Au/ZrO2-AN-
973 (5.3 nm) and Au/ZrO2-AD-973 (4.7 nm); if the size effect
of Au was more pronounced, the Au/ZrO2-CP-973 catalyst
would show the highest activity, but its activity is actually just
one-sixth of Au/ZrO2-AD-973. As shown in Table 2, Au/ZrO2-
AN-973 (Au: 5.3 nm), Au/ZrO2-AN-673 (Au: 5.4 nm), and
Au/ZrO2-AN-1073 (Au: 5.4 nm) have basically the same size
for Au particles, but the activity of Au/ZrO2-AN-673 having
the smallest zirconia particles is 4 times that of Au/ZrO2-AN-
1073, and twice that of Au/ZrO2-AN-973. More importantly,
our data show that the activity of Au/ZrO2 increases systemati-
cally/monotonically with the decrease of the “support” particle
size for all of the samples in Figures 4 and 6 (also see Table
2), strongly suggesting that the “support” size effect is far more
remarkable.
It seems unreasonable that the observed activity enhancement
in the present study may be due to the size scattering of Au

9682 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Zhang et al.
in the same range (4-6 nm). It is thus conclusive that the
activity difference in our Au/ZrO2 catalysts (Table 2, Figures 4
and 6) is mainly due to the size effect of the nanosized zirconia
support particles rather than the size scattering of Au particles.
The supporting material size effect on Au catalyst was also
addressed by Rolison et al.13,14 and Corma et al.;24 they showed
13,14
24
only two extreme cases for Au/TiO2
and Au/CeO2
catalysts, respectively. However, this present investigation seems
more systematic and has provided a general trend for the support
size effect in a series of zirconia samples.
It is unlikely that CO oxidation over Au/ZrO2-AD-973 and
Au/ZrO2-AN-673 catalysts involves a radically different chemi-
cal mechanism from that over Au/ZrO2-CP-973 and Au/ZrO2-
AN-1073 catalysts. Although the nature of active sites and the
reaction mechanism have not yet been clearly identified, many
researchers agreed that the contact boundaries between Au and
the support oxide should play an important role in the catalytic
reaction.2-6,10-12,25 The Au-oxide boundaries or contact peri-
meters function most probably as active sites for the activation
of the oxygen reactant; CO molecules adsorbed on Au may need
to migrate to react with the activated oxygen at those
boundaries.2-6,10-12,25 Thus, the main factors affecting the
catalyst performance should be the properties of the Au-oxide
contacts as well as the nature and particle size of the oxide
support as reviewed by Haruta.5,25 Specific to the case of
zirconia support, some recent studies have shown that zirconia
nanoparticles can hold a lot of oxygen vacancies,26-28 and in
particular the number of surface vacancies associated with the
paramagnetic F-centers was found to increase with decreasing
the size of zirconia nanoparticles smaller than 50 nm.28 Such
vacancy sites were found important in the formation of electron-
rich Au particles as active sites for the partial hydrogenation of
acrolein over Au/ZrO2 catalysts.27 According to Corma et al.,24
increased oxygen vacancies in nanosized small (3-4 nm) ceria
“support” were crucial in Au/CeO2 catalyst for the activation
of oxygen in the CO oxidation reaction. We believe that a
similar mechanism may prevail in the catalysis of CO oxidation
over the present Au/ZrO2 catalysts. The continuous reduction
in the sample particle size of the ZrO2 “support” not only leads
to continuously increased Au-oxide contact boundaries in
the Au/ZrO2 catalyst but also modifies the chemical properties
at such boundaries due to the presence of a larger number
of oxygen vacancies at the surfaces of smaller ZrO2 nano-
particles,26-28 creating a situation where oxygen can become
easily activated24 and then move more easily (the increased Au-
zirconia contact junctions mean minimized diffusion distances
for the reactive surface species) to react with the adsorbed CO
molecules, which accounts for the data that the activity of Au/
ZrO2 catalyst is greatly enhanced with reducing the size of ZrO2
nanoparticles.
Figure 4. Catalytic activity of Au nanoparticles supported on zirconia
nanocrystals: (2) Au/ZrO2-CP-973; (0) Au/ZrO2-AN-973; (b) Au/
ZrO2-AD-973.
Figure 5. Catalytic stability of Au/ZrO2-CP-973 and Au/ZrO2-AD-
973 catalysts at 373 K: (2) Au/ZrO2-CP-973; (b) Au/ZrO2-AD-973.
Figure 6. CO conversion at 343 K over Au/ZrO2-AN-T catalysts: ([)
Au/ZrO2-AN-673; (2) Au/ZrO2-AN-873; (0) Au/ZrO2-AN-973; (b)
Au/ZrO2-AN-1073.
particles. Comprehensive reviews by Haruta5,7,25 on the catalytic
CO oxidation over Au nanoparticles supported by various oxides
(e.g., TiO2, Fe2O3, Co3O4) have shown that the catalytic activity
by TOF of Au changed slowly when the particle size of Au is
larger than 4 nm (refer to Figure 9 in ref 6), although the rate
increased sharply when the size of Au particles becomes smaller
than 4 nm.6 For example, the TOF of Au/TiO2 catalyst decreased
by a factor of 2 (from 2.5 to 1.2 s-1) when the average size of
Au was increased from 4 to 6 nm (refer to Figure 2 in ref 4).4,8
Because ZrO2 is, in comparison with TiO2, a support of poorly
active for Au nanoparticles,22,23 the effect due to the size
scattering of Au particles can be much smaller when the average
size of Au is changed between 4 and 6 nm. The average size of
Au nanoparticles in the present Au/ZrO2 catalysts is within the
range of 4-6 nm, and their actual size scattering is reasonably
narrow enough (i.e., between 3 and 7 nm). The activity
difference among the present Au/ZrO2 catalysts goes up to 6
times (Table 2), as discussed in the last paragraph, and is clearly
more significant than the size effect of Au nanoparticles in the
even more active Au/TiO2 catalysts with the average Au sizes
It seems therefore that there are two approaches in increasing
Au-oxide contact boundaries for the Au catalyst. The first
approach is to reduce the size of Au nanoparticles as shown in
many earlier reports;1-9 the second is to reduce the particle size
of the oxide support as demonstrated in this work. In the second
approach, the small spherical Au nanoparticles, having sizes of
4-5 nm in diameter (Figure 3), are composed of 2000-4000
Au atoms;2,29 about 24-32% of the atoms are exposed on the
surface and can potentially have direct contact/interaction with
ZrO2. The increased contact boundaries between Au and ZrO2
by reducing the sizes of ZrO2 nanoparticles are mainly due to
the generation of Au-ZrO2 junctions with which a single Au
nanoparticle can make contacts with multiple ZrO2 particles
(Figure 3). This approach is in contrast to the creation of more

Nanosize Effect of Zirconia in Au/ZrO2 Catalyst
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9683
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metal-oxide boundaries by generating smaller metal particles
on a single oxide-particle in the first approach.
In contrast to conventional oxide-supported metal catalysts
with metal “flea” riding an oxide “boulder”, as in the case of
Au/ZrO2-CP-973 and Au/ZrO2-AN-1073, the similar sizes of
the ZrO2 (5-15 nm) and Au (4-5 nm) nanoparticles in the
highly active Au/ZrO2-AD-973 and Au/ZrO2-AN-673 catalysts
(Figure 3C and D) identify them as nanocomposites of Au and
ZrO2 nanoparticles.17 In addition to the effect of tailoring the
activity of Au nanoparticles and the reactivity at the Au-ZrO2
contact boundaries, the formation of multiple Au-ZrO2 contact
junctions for each of the Au nanoparticles in the nanocomposite
catalysts can also lead to many more channels and minimized
distances for the surface diffusion of reactive CO and/or oxygen
species. The fact that both of the nanocomposite Au/ZrO2
catalysts exhibit the highest activity for the CO oxidation (Table
2) proves again the feasibility of designing metal/oxide nano-
composite for advanced catalysts.16,17
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In conclusion, the present data indicate that the catalytic
activity of Au nanoparticles in Au/ZrO2 for CO oxidation can
be greatly improved by reducing the particle size of zirconia
nanoparticles. In particular, nanocomposite Au/ZrO2-AD-973
and Au/ZrO2-AN-673 catalysts consisting of comparably sized
Au-metal (4-5 nm) and ZrO2 (5-15 nm) nanoparticles are
found advantageous over those containing similarly sized Au-
metal but larger ZrO2 particles. This finding may have important
implications in the designed preparation of advanced Au and
other heterogeneous metal catalysts; it could also be informative
for the improvements of many other related nanostructured
chemical materials.
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Acknowledgment. We are indebted to the NSF of China
(grants 20125310, 20443008, and 20590360) and the National
Basic Research Project of China (grant 2003CB615804) for
financial support of this work. We also thank the reviewers for
their very helpful comments.
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