Nanocatalysis on tailored shape supports: Au and Pd nanoparticles supported on MgO nanocubes and ZnO nanobelts

Article abstract of DOI:10.1021/jp0651034

Active gold and palladium nanoparticles supported on MgO nanocubes and ZnO nanobelts and transition-metal-containing MgO nanobelts were synthesized by combining evaporation and deposition-precipitation techniques. The high activity and stability of the Au/CeO₂ and Pd/CeO₂ nanoparticle catalysts deposited on the MgO cubes are remarkable and imply that a variety of efficient catalysts can be designed and tested using this approach. The significant increase in the concentration of corner and edge sites in MgO nanocubes make them well-defined supports to study the detailed mechanism of the catalytic activity enhancement.

Full text of DOI:10.1021/jp0651034

2
1387
2006, 110, 21387-21393
Published on Web 10/11/2006
Nanocatalysis on Tailored Shape Supports: Au and Pd Nanoparticles Supported on MgO
Nanocubes and ZnO Nanobelts
†
‡
Garry Glaspell, Hassan M. A. Hassan, Ahmed Elzatahry, Lindsay Fuoco,
†
Nagi R. E. Radwan, and M. Samy El-Shall*
Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2006
ReceiVed: August 8, 2006; In Final Form: September 15, 2006
Active gold and palladium nanoparticles supported on MgO nanocubes and ZnO nanobelts and transition-
metal-containing MgO nanobelts were synthesized by combining evaporation and deposition-precipitation
2 2
techniques. The high activity and stability of the Au/CeO and Pd/CeO nanoparticle catalysts deposited on
the MgO cubes are remarkable and imply that a variety of efficient catalysts can be designed and tested using
this approach. The significant increase in the concentration of corner and edge sites in MgO nanocubes make
them well-defined supports to study the detailed mechanism of the catalytic activity enhancement.
catalysts such Au and Pt.1
3-15
The increase in the catalytic
Nanocatalysis is a phenomenon of significant fundamental
research and important practical applications in a variety of
fields such as chemistry, physics, materials, and environmental
and atmospheric sciences in addition to its traditional signifi-
cance in advancing the petroleum field.1-3 It encompasses
supported or unsupported nanometer-scale metal catalytic
structures (spherical nanoparticles, nanorods, nanoplates,
nanocubes, etc.) where the catalytic phenomena are specific to
that length scale and, in general, are related to the high surface
area and the density of the unsaturated surface coordination sites
of the nanocatalysts.1 The origins of enhancements in activity
and selectivity can be explained by dispersion factors or by
quantum factors unique to this special length scale. Various
factors identified include the emergence of electronic and/or
atom-packing shell structures, along with fundamentally altered
interactions with the support.1
efficiencies has been attributed to the increased surface areas
and the well-dispersed nature of the carbon support and
catalyst. Herein, we report the synthesis and characterization
of Au and Pd nanoparticle catalysts supported on metal oxides
of different shapes and demonstrate their excellent catalytic
activities for the oxidation of carbon monoxide.
The low temperature oxidation of carbon monoxide is one
of the current important environmental issues, since small
exposure (ppm) to this odorless invisible gas can be lethal.16
Therefore, the discovery that Au nanoparticles between 2 and
5 nm are exceptionally active for low temperature CO oxidation
has stimulated extensive research to develop highly active
catalysts to remove even a small amount of CO from the local
1
5
-6
4-6,17-25
environment.
There is also a strong incentive to develop
-6
active supported catalysts that utilize small amounts of the noble
metals such as gold and palladium. The catalytic activity of the
Au-based catalysts depends on various factors such as the type
of the support, the Au precursor, the preparation conditions,
Most of the studies in homogeneous catalysis involve
spherical nanoparticles or nanoparticles of undetermined shapes.
However, recent studies have examined the effects of the
nanoparticles’ shape on the activation energy and the rates of
the catalytic reaction.7-11 The influence of particle shape on
the catalytic activity depends on the increase in the density of
the pretreatment conditions, and the catalytic reaction condi-
17-25
tions.
Among these factors, the nature and shape of the
support are expected to play a major role given the strong
tendency of Au nanoparticles to efficiently adsorb CO molecules
the active corner/edge sites which correlates with strong surface
activity.7
-11
This correlation is attributed to the fact that
17-19
but the inability to activate oxygen molecules.
nanocrystals of different shapes have different facets with
different fractions of atoms located at different corners, edges,
and different defect sites.12 Thus, nanoparticle catalysts such
as Au or Pd with identical sizes and shapes may exhibit different
activities when supported on metal oxides having different
shapes. Therefore, it is important to develop ways of tailoring
the design and formation of new nanocatalyst systems with
controlled sizes and shapes that result in high catalytic activity.
Recent approaches have utilized carbon nanotubes as supporting
materials for the dispersion and assembly of metal nanoparticle
Highly ionic metal oxide supports such as MgO and ZnO
exhibit both Lewis base and Lewis acid characteristics as usually
reflected in their high surface reactivity which depends on both
particle size and shape. MgO is considered a model system for
solid state and surface studies because of its simple rock-salt
crystal structure and highly ionic and nonconducting nature. It
26,27
also exhibits catalytic activity for a wide variety of reactions.
This activity has been attributed to the presence of defect sites
28,29
such as corners, edges, steps, and kinks.
Klabunde and co-
workers have prepared high surface area MgO with average
crystalline sizes of about 4 nm and demonstrated that these
materials are especially effective as “destructive adsorbents” for
†
Permanent address: Department of Chemistry, Suez Canal University,
Suez, Egypt.
2
8-31
‡
the destruction of toxic chemicals.
Recently, nanocubes of
Permanent address: Mubarak City for Scientific Research and Technol-
ogy Applications (MuCSAT), New Bourg El-Arab, Alexandria, Egypt.
MgO have been synthesized by chemical vapor deposition
1
0.1021/jp0651034 CCC: $33.50 © 2006 American Chemical Society

21388 J. Phys. Chem. B, Vol. 110, No. 43, 2006
Letters
Figure 1. TEM micrographs of (A) MgO nanocubes formed by thermal evaporation at 1200 °C and 760 Torr O
00 Torr O , (C) MgO wires formed by thermal evaporation at 10 Torr O
and (E and F) ZnO belts formed by thermal evaporation at 860 °C and 300 Torr O
2
, (B) same as part A but using
, (D) MgO nanoparticles formed by the LVCC method at 900 Torr O ,
2 2
1
2
2
.
followed by annealing at 1170 K under high vacuum condi-
dropwise addition of NaOH while stirring. After stirring the
solution for an hour, the nanoparticles formed on the support
were filtered and dried under vacuum.
3
2
tions. The significant increase in the concentration of corner
and edge sites in MgO nanocubes make them well-defined
supports to study the detailed mechanism of the catalytic activity
enhancement.
The Au/CeO2 and Pd/CeO2 supported on the MgO cubes and
ZnO belts were also prepared using microwave irradiation
3
5
In this letter, we report the synthesis of Au and Pd nanopar-
ticle catalysts supported on MgO nanocubes and ZnO nanobelts
and compare their catalytic activities for CO oxidation with the
catalysts supported on ceria nanoparticles. The results clearly
demonstrate that the combination of active and inactive supports
of controlled shapes can lead to remarkable enhancement of
the catalytic activity of the Au- and Pd-based nanoparticle
catalysts.
(MWI) synthesis. In this case, an appropriate amount of Ce-
(NO3)4 solution in ethanol and the metal precursor (HAuCl4 or
Pd(NO3)2) were added to the suspension of the MgO cubes or
the ZnO belts. While stirring, 10 N NaOH (Alfa Aesar) was
added dropwise until the pH of the resulting solution was ∼10.
The resulting solution was then placed in a conventional
microwave. The microwave power was set to 33% of 650 W
and operated in 30-s cycles (on for 10 s, off for 20 s) for 10
min. The resulting powder was washed with distilled water and
ethanol and left to dry.
The MgO nanocubes and the ZnO nanobelts were generated
by thermal evaporation of the metal powder (1190 °C for Mg
and 910 °C for Zn) in the presence of oxygen in a tube furnace
under controlled pressure and flow rate. MgO belts terminated
by transition metal (Fe, Co, Ni) tips were also synthesized by
combining laser ablation of the transition metal targets with the
thermal evaporation of Mg in the presence of oxygen (Sup-
porting Information). Dispersed Au and Pd nanoparticles
supported on the MgO nanocubes and ZnO nanobelts were
Transmission electron microscopy (TEM) and X-ray diffrac-
tion (XRD) were used to characterize the nanocrystalline
materials. TEM images were obtained using a Joel JEM-1230
electron microscope operated at 120 kV. XRD spectra were
obtained using an X’Pert Philips Materials Research Diffrac-
tometer, with Cu KR radiation.
For the CO catalytic oxidation, the sample was placed inside
3
3,34
35
prepared using the deposition-precipitation (DP) method.
a Thermolyne 2100 programmable tube furnace reactor. The
Specifically, the appropriate amount of the HAuCl4 or Pd(NO3)2
aqueous solution was added to a suspension of the MgO or ZnO
support. The pH of the solution was adjusted to 9 by the
sample temperature was measured by a thermocouple placed
near the sample. In a typical experiment, 4 wt % CO and 20 wt
% O2 in He were passed over the sample while the temperature

Letters
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21389
Figure 2. XRD patterns of (a) MgO nanocubes, (b) ZnO nanobelts, and (c and d) MgO and ZnO nanoparticles, respectively, prepared by the
LVCC method.
Figure 3. TEM micrographs of (A) 1% Au deposited on MgO nanocubes, (B) 2% Pd loaded ZnO nanobelts, (C and D) 1% Au/24% CeO
2
2
2
deposited on ZnO nanobelts, (E) 5% Au deposited on ZnO nanobelts, (F) 1% Au/24% CeO deposited on MgO nanocubes, (G) 1% Pd/24% CeO
deposited on ZnO nanobelts.
was ramped. The gas mixture was set to flow over the sample
at a rate of 100 cm /min controlled via MKS digital flow meters.
gas mixture for 15 min in order to remove moisture and
adsorbed impurities.
3
The conversion of CO to CO2 was monitored using an infrared
gas analyzer (ACS, Automated Custom Systems Inc.). All of
the catalytic activities were measured (using a 20 mg sample)
after a heat treatment of the catalyst at 300 °C in the reactant
Figure 1A displays a typical TEM image of the MgO
nanocubes formed via thermal evaporation in oxygen at 760
Torr. The as-synthesized nanocubes are quite uniform with an
average length of ∼130 nm. However, reducing the pressure

21390 J. Phys. Chem. B, Vol. 110, No. 43, 2006
Letters
Figure 4. CO oxidation on Au (A, C, and E) and Pd (B, D, and F) deposited on MgO nanocubes and ZnO nanobelts.
within the furnace to 100 Torr shrinks the length of the cubes
to ∼45 nm, as shown in Figure 1B. Thus, by carefully
controlling the pressure within the furnace, MgO nanocubes of
different sizes can be prepared. It should be noted, however,
that when the oxygen pressure is below 10 Torr, long wires
rather than cubes are formed, as shown in Figure 1C. Interest-
ingly, laser evaporation of a Mg target using the laser vaporiza-
tion controlled condensation (LVCC) method36 in the presence
of an oxygen atmosphere produces a mixture of very small cubes
Also, the XRD pattern of the MgO nanocubes prepared by
thermal evaporation is similar to that of the MgO nanoparticles
prepared by the LVCC method, as shown in Figure 2a and c.
Since the growth of the cubes occurs in three dimensions, the
XRD patterns for the MgO nanoparticles and nanocubes show
similar peak intensities. However, the XRD pattern of the ZnO
belts (Figure 2b) shows a significant enhancement in the
intensity of the diffraction (1 1 0) peak, indicating that the belts
3
7
are grown along the [1 1 0] direction.
(
1
less than 10 nm) and spherical particles, as shown in Figure
D. For ZnO, thermal evaporation of Zn in the presence of
oxygen at 300 Torr produces the ZnO nanobelts shown in Figure
E which appear to grow from the original tetrahedral cores
Figure 3 displays several representative TEM images of the
Au and Pd nanoparticle catalysts deposited on the MgO cubes
and the ZnO belts using the DP method. It is clear that the Au
and Pd nanoparticles are well dispersed on the MgO cubes and
the ZnO belts with no evidence of large aggregates of Au or
the Pd nanoparticles existing independently. The average size
of the deposited Au and Pd nanoparticles are ∼9 and ∼12 nm,
as determined from the analysis of several TEM images.
Figure 4 compares the catalytic activities of the bare MgO
nanocubes and ZnO nanobelts with the deposited nanoparticles
1
shown in Figure 1F. The average width of the ZnO nanobelts
is ∼30 nm.
The XRD patterns of the MgO nancubes and the ZnO
nanobelts, shown in Figure 2, match well with the ICCD
reference patterns 01-075-1525 and 01-079-0205, corresponding
to the crystal structures of the bulk MgO and ZnO, respectively.

Letters
J. Phys. Chem. B, Vol. 110, No. 43, 2006 21391
Figure 5. TEM micrographs of MgO wires and belts initiated by (A) Co, (B) Fe, and (C) Ni during the thermal evaporation of Mg at 1200 °C in
. (D) CO oxidation on the MgO belts incorprating transition metal oxides.
O
2
of Au, Pd, Au/CeO2, and Pd/CeO2 on the same supports. Clearly,
both the bare MgO cubes (130 nm) and ZnO belts (30 nm)
exhibit low activity for the CO oxidation, as indicated by the
to achieve good dispersion of the Au or Pd nanoparticles on
the MgO cubes.
Interestingly, the 5% (wt) Au nanoparticles supported on the
MgO cubes show a much higher activity (50% conversion at
5
0% conversions at 350 and 370 °C, respectively. However,
the ZnO belts exhibit significantly higher activity for the 100%
CO oxidation (381 °C) as compared to the MgO cubes (82% at
1
28 °C) than the same concentration of the Au nanoparticles
supported on the ZnO belts (50% conversion at 280 °C), as
shown in parts C and E of Figure 4, respectively. This indicates
that the activity of the Au catalyst is strongly dependent on the
type and shape of the metal oxide support. This is not the case
for the Pd catalyst where the performance of the 5% Pd
nanoparticles supported on the MgO cubes (100% conversion
at 154 °C) is nearly similar to that supported on the ZnO belts
(100% conversion at 162 °C). As compared to Au, the Pd
catalysts show very sharp conversion curves immediately past
the light-off temperatures.
4
35 °C).
Figure 4A and B compare the concentration effects (wt %)
of the Au and Pd nanoparticles supported on the MgO cubes.
In the case of Au nanoparticles, the activity increases signifi-
cantly with increasing concentration of the Au nanoparticles
within the 1-5% (wt) range. However, for Pd nanoparticles,
only a small enhancement in the activity is observed by
increasing the Pd loading from 2 to 5% (wt). The observed
concentration effect can be explained by increasing the number
of dispersed active sites before reaching the aggregation limit
where the catalytic activity is expected to decrease. In both
systems, higher concentrations of the metal nanoparticles (7.5
wt %) supported on the MgO cubes resulted in decreasing
catalytic activity. This suggests that the 5% (wt) seems to be
near the optimum concentration allowed using the DP method
Figure 4 shows also a comparison of the activities of the Au
and Pd nanoparticle catalysts deposited on the MgO cubes and
ZnO belts with the activities of the same catalysts supported
on CeO2 nanoparticles. For the Au catalyst, the activity is higher
on the CeO2 support than on the MgO cubes or the ZnO belts.
However, for the Pd catalyst, the difference between the

21392 J. Phys. Chem. B, Vol. 110, No. 43, 2006
Letters
activities on the different supports (CeO2 spherical particles,
MgO cubes, and ZnO belts) is considerably small. This suggests
that the influence of the shape of the support on the catalytic
activity varies depending on the nature of the metal-support
interaction.
exhibit 50% CO conversion in the temperature range -25 to
2
0
13 °C depending on the pretreatment conditions.
We have also explored the catalytic activity of the MgO
nanobelts containing transition metal oxide tips. These nanobelts
were synthesized by coupling the laser ablation of selected
transition metal targets with the thermal evaporation of the Mg
powder in the presence of an oxygen atmosphere within a tube
furnace (Supporting Information). As shown in Figure 5, the
TEM images suggest that the MgO nanobelts are grown from
attached transition metal nanoparticle tips through the vapor-
The most interesting result shown in Figure 4 is the
enhancement of the activity of the Au supported CeO2 nano-
particles upon the deposition on the MgO cubes. In this case,
the MWI synthesis of the Au/CeO2 catalyst in the presence of
the MgO cubes is expected to produce Au nanoparticles
supported on spherical CeO2 particles, as demonstrated in
4
0,41
liquid-solid (VLS) mechanism.
The diameters of the belts
35
were determined to be 10, 100, and 60 nm for the ablated Co,
Fe, and Ni samples, respectively. It is interesting that the
morphology of the tips depends on the nature of the metal
catalyst. The spherical clusters observed for the Co/MgO belts
previous work. These Au/CeO2 nanoparticles are then depos-
ited on the MgO cubes, although the possibility of depositing
Au nanoparticles directly on the MgO cubes cannot be
eliminated. However, the very different activity of the Au/CeO2/
MgO catalyst system as compared to the Au/MgO catalyst
indicates the presence of strong Au-CeO2 interaction in the
Au/CeO2/MgO catalyst system. By using only 1 (wt) % Au
supported on 24 (wt) % CeO2 nanoparticles supported on 75
(
Figure 5A) are similar to those observed for the Co-initiated
4
2
MgO fibers. However, in the case of Fe and Ni, cubic and
cylindrical tips were observed, as shown in parts B and C of
Figure 5, respectively. These transition metal oxide tips attached
to the MgO belts can provide suitable catalytic centers with
variable activities for CO oxidation. The CO conversion curves
over the MgO belts incorporating the transition metal oxides
are shown in Figure 5D. It is clear that the highest catalytic
conversion is observed for the Fe-initiated MgO nanobelts with
(wt) % MgO cubes, a remarkable improvement in the activity
of the Au/CeO2 catalyst is observed with a 70% CO conversion
and a full conversion (100%) achieved at 25 and 91 °C,
respectively. Furthermore, the long-term stability of this catalyst
that utilizes such a small concentration of the Au nanoparticles
is excellent, as indicated by no decrease in activity even after
1
00% CO conversion occurring at 293 °C. On the other hand,
Co/MgO belts show 50% CO oxidation at temperatures below
00 °C. Optimization of the parameters that control the
1
0 h of the reaction time. It should be noted that the same
2
catalytic activity is observed after storing the particles in air
for several days following a heat treatment of the catalyst at
dimensions of the belts and the shape of the attached transition
metal catalysts is expected to improve the catalytic properties
of these systems. The MgO belts containing terminal transition
metal oxide tips should prove to be interesting model systems
for studying size- and shape-dependent catalytic and magnetic
properties and also for a variety of applications in devices and
sensors and possibly biomedical applications.
In conclusion, active Au and Pd nanoparticle catalysts
deposited on MgO nanocubes and ZnO nanobelts were synthe-
sized by combining the thermal evaporation and deposition-
precipitation techniques. The high activity and stability of the
Au/CeO2 and Pd/CeO2 nanoparticle catalysts deposited on the
MgO cubes are remarkable and imply that a variety of efficient
catalysts can be designed and tested using this approach. The
higher crystallinity and larger size of the nanocubes and
nanobelts can provide higher thermal stability which ultimately
enhances the durability of the catalyst. The transition-metal-
containing MgO nanobelts provide good model systems for
studying the size- and shape-dependent catalytic and magnetic
properties in high temperature materials.
300 °C in the reactant gas mixture for 15 min in order to remove
moisture and adsorbed impurities.
Although the catalytic enhancement is also observed for the
Pd/CeO2 catalyst supported on the MgO cubes relative to the
Pd/CeO2 (spherical particles), the activity of the Pd/CeO2/MgO
catalyst is less than that of the Au/CeO2/MgO catalyst. This is
probably due to the differences in the metal-support interactions
of the Pd/MgO and Au/MgO systems. Since the remarkable
activity of the 1% Au/24% CeO2/75% MgO cubes is not
observed when 1% Au/24% CeO2 is deposited on 75% ZnO
belts, this may suggest that the enhancement is due to the
interaction with the cube-shaped support. This result demon-
strates that MgO nanocubes with active corners and edges can
lead to significantly enhanced catalytic activity.
It is important to compare the catalytic activity of the 1%
Au/24% CeO2 supported on the MgO cubes with other catalyst
systems used for the CO oxidation. The observed activities of
7
0 and 100% CO conversions achieved at 25 and 91 °C,
respectively, for the Au/CeO2/MgO (cubes) catalyst are much
better than those reported for the Au nanoparticles supported
on the surface of the CeO2 nanorods where 81% CO conversion
was achieved at 220 °C. The performance of the Au/CeO2/
MgO (cubes) catalyst is also better than that of the â-MnO2
Acknowledgment. The research described herein was sup-
ported by the National Science Foundation through the US-
Egypt Cooperative Research Program (OISE-0413971). Ac-
knowledgment is also made to the Donors of the American
Chemical Society Petroleum Research Fund (PRF#41602-AEF)
and to Philip Morris USA for the partial support of this research.
9
nanorods where with 90% CO conversions were achieved at
1
26 °C for the pure â-MnO2 nanorods and at 80-90 °C for the
10
Ag-doped MnO2 nanorods. The current Au/CeO2/MgO (cubes)
Supporting Information Available: Experimental setup for
the synthesis of MgO belts using transition metal catalysts. This
material is available free of charge via the Internet at http://
pubs.acs.org.
catalyst is also significantly superior to the quasicubic R-Fe2O3
3
8
nanoparticles which showed a 100% CO oxidation at 230 °C.
Furthermore, the activity of the Au/CeO2/MgO (cubes) catalyst
is much higher than that of the bimetallic PdAu nanoparticles
supported on TiO2 where the 1 and 100% CO conversions were
reported to occur at 150 and 250 °C, respectively.3 However,
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