Catalysis of Gold Nanoparticles Deposited on Metal Oxides (1)

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eature

102

Volume 6, no. 3, 2002

Catalysis of gold nanoparticles

deposited on metal oxides

Masatake Haruta

Research Institute for Green Technology

National Institute of Advanced Industrial Science and Technology (AIST)

16-1 Onogawa, Tsukuba 305-8569, Japan

tel: +81-298-61-8240

fax: +81-298-61-8240

e-mail : [email protected]

Gold in bulk is chemically inert and has often been regarded to be poorly active as a catalyst.

However, when gold is small enough—with particle diameters below 10 nm—it turns out to be

surprisingly active for many reactions, such as CO oxidation and propylene epoxidation. This is

especially so at low temperatures. Here, a summary of the catalysis of Au nanoparticles deposited

on base metal oxides is presented. The catalytic performance of Au is dened by three major

factors: contact structure, support selection, and particle size, the rst of which being the most

important because the perimeter interfaces around Au particles act as the site for reaction.

Heterogeneous catalysts currently used are classied into

three types of compounds: metal oxides, metal suldes, and

metals. Metal oxides are used mainly for selective oxidation

of hydrocarbons[1] and selective reduction of NOx with

NH3[2]. Metal suldes are used for hydrodesulfurization of

petroleum[3]. Metals are most widely used for a variety of

reactions[4], including hydrogenation, complete and partial

oxidation, and reduction of NOx with hydrocarbons[5].

Actua lly, catalytic metals are limited to 12 elements of

group VIII and Ib of the Periodic Table. Most widely used

are the 3d metals of Fe, Co, Ni and Cu, the 4d metals of

Rh, Pd and Ag, and the 5d metal of Pt. Ruthenium(4d) and

Ir(5d) have only limited applications. Osmium is excluded

as a catalyst component because its oxide is toxic. Gold

(5d) is the only exception and has usually been regarded

to be poorly active as a catalyst. The catalytic excellence of

group VIII metals can be ascribed to the optimum degree

of d-band vacancy. The elements of group Ib, the so called

coinage metals, Cu, Ag and Au, have fully occupied d-bands.

Owing to relatively low ionization potential, Cu and Ag

readily lose electrons to yield d-band vacancies. In fact, Cu is

used for methanol synthesis and Ag is used for ethylene oxide

synthesis in the chemical industry. On the other hand, Au has

a high ionization potential and accordingly has poor afnity

towards molecules. Surface science investigations[6~8] and

theoretical calculations[9] have proved that no dissociative

adsorption of H2 and O2 takes place over the smooth

surfaces of Au at temperatures below 473K, indicating that

Au is catalyt ically inactive for hydrogenation and oxidation.

Indeed, the conventional supported Au catalysts were much

less active than supported Pt group metal catalysts. But,

it should be noted that these supported Au catalysts were

not as highly dispersed as other supported noble metals.

When they were prepared by the impregnation method, Au

particles were usually larger than 30 nm, while Pt particles

were distributed at around 5 nm in diameter[10]. This is

because the melting point of Au is much lower than those

of Pd and Pt (Au:1336K, Pd:1823K, Pt:2042K). Due to

the quantum-size effect, the melting point of Au particles

with a diameter of 2 nm is lowered to 573K [11], and these

small Au nanoparticles tend to coagulate much more readily

than Pd and Pt nanoparticles during calcination of catalyst

precursors at temperatures above 573K. Accordingly, we did

not know whether the rough surfaces or nanoparticles of



103

Au, having a substant ial number of steps, edges and corners,

were really cata lytically inactive or not.

The rst hint that Au might not always be poorly

active when dispersed as small nanoparticles was presented

by Bond and Sermon, for hydrogenation over Au/SiO2 prepared by calcination at a temperature as low as 383~401

K [12,13], and by Paravano and his coworkers, for oxygen and

hydrogen transfer reactions over Au/MgO and Au/Al2O3

catalysts[14,15]. These landmarks, achieved prior to 1981,

are reviewed by Schwank [16]. It should be kept in mind

that most supported Au catalysts in the past may have been

severely contaminated by Cl- and/or Na+, the amount of

which depended on Au loadings.

We found in 1983, and patented in 1984, that once Au

is deposited as hemispherical nanoparticles on selected metal

oxides, it exhibits surprisingly high catalytic activity for CO

oxidation, even at 200K [17,18]. This nding has graduallyevoked renewed interest in Au catalysts[19–23]. The present

article deals with up-to-date progress in research on the

unique catalytic properties and mechanistic understandings

of Au nanoparticles deposited on metal oxides and activated

carbon. The ongoing and future applications of Au catalysts

are also presented.

Preparation of highly dispersed gold catalysts

Traditional Au catalysts were prepared by the impregnation

method (IMP). A metal oxide support is immersed in an

aqueous solution of HAuCl4 and then water is evaporated to

disperse HAuCl4 crystal lites over the support surfaces. The

dried precursor is calcined in air, usually at temperatures

above 473K, and is often reduced in a diluted H2 stream.

In this case, as shown in Figure 1[10], the size of Au

particles is larger than 30 nm, because the interaction of

HAuCl4 crystallites with the metal oxide support is weak,

and chloride remaining on the support surfaces markedly

promotes the coagulation of Au particles.

We have developed four techniques which can deposit

Au nanoparticles on various types of metal oxides[24]:

coprecipitation[17], co-sputtering[25], deposition-precipitation

(DP)[26], and gas-phase grafting (GG)[27,28]. In addition to

these, the amorphous alloy method[29] and the l iquid phase

grafting (LG) method[30,31] are a lso applicable.

In the case of Pt group metals, calcination of catalyst

precursors in air produces metal oxides which strongly

interact with the support metal oxides. Also, the reduction

in the H2 stream brings about the strong interaction of

metallic particles with the supports and removes Cl - as

HCl. Because Au does not form stable metal oxides during

calcination, in order to disperse Au as small nanoparticles—

preventing them from coagulation—Au precursors should

strongly interact with the support.

The above six techniques can be classied into two

categories. The rst is based on the preparation of well-

mixed precursors—for example, hydroxide, oxide and metal

mixtures of Au and the metal component of the support—by

coprecipitation, co-sputtering and alloying, respectively.

These precursor mixtures are then transformed during

calcination in air into metallic Au particles strongly attached

to the crystalline metal oxides, α-Fe2O3, Co3O4, and ZrO2,

respectively. The second technique is to utilize the depositionor adsorption of Au compounds, for example, Au hydroxide

by DP, and organogold complex by GG and LG.

Scheme 1 shows a detailed procedure for DP. Due to

the amphoteric properties of Au(OH)3, the pH of aqueous

HAuCl4 solution is adjusted at a xed point in the range from

6 to 10. Careful control of the concentration (around 10-3M),

pH(6~10), and temperatures(323K~363K) of the aqueous

HAuCl4 solution can lead to the selective deposition of

Figure 1 TEM micrograph of a Au/TiO2 catalyst prepared by the impregnation

method.

HAuCI4 aq.

pH = 2-3

HAuCI4 aq.

pH = 6-10

Au(OH)3 /support

Au /support

• wahsing

• drying

• calcination (573-673K)

support

NaOH aq.

Au(OH)4-

Scheme 1 Flow chart of the procedure in the deposition-precipitation method.



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Au(OH)3 only on the surfaces of support metal oxides without

precipitation in the liquid phase. Because the precursor can be

washed before drying, sodium and chloride ions are removed

down to a level of about 50 ppm. One of the constraints of DP

is that it is applicable only to metal oxides, the isoelectric points

of which are above 5. Gold hydroxide cannot be deposited on

SiO2(IEP=2), SiO2-Al2O3(IEP=1) and WO3(IEP=1).

In contrast, GG using dimethyl-gold acetylacetonate is

unique because it can deposit Au nanoparticles even on SiO2and SiO2-Al2O3. Figure 2[28] shows a TEM photograph for

Au nanoparticles deposited on MCM-41. With an increase in

Au loading, wire-like metal rods appear, suggesting that most

Au particles might be incorporated into the meso-tubes.

Fine structure of gold nanoparticles

Carefully prepared Au catalysts have a relatively narrow size

distribution of Au particles, giving mean diameters in the

range of 2 to 10 nm with standard deviation of about 30%. A

major reason why Au particles remain as nanopart icles even

after calcination at temperatures above 573K is the epitaxial

contact of Au nanoparticles with the metal oxide supports.Gold particles always exposed its most densely packed plane,

(111) plane, in contact with α-Fe2O3(110), Co3O4(111),

and anatase TiO2(112 ), and rutile TiO2(110).

Figure 3 shows a typical TEM image of Au particles

epitaxially attached on anatase TiO2[32]. The surface atomic

conguration is better matched for a Au(111) plane sitting

on the oxygen layer of anatase TiO2 than on the Ti

layer. 3-dimensional nanostructure analyses by electron

holography together with high-resolution TEM revealed

that smaller hemispherical Au particles with a thickness

under 2 nm had contact angles with the support that were

below 90° (wet interface), whereas larger Au particles with a5 nm thickness had that angle above 90° (dry interface)[33].

This difference in the wettability of Au particles may arise

from the change in the electronic state of the contact

interfaces with the particle size. When Au/TiO2 was calcined

at temperatures above 573K, Au particles coagulated with

each other forming larger particles, mostly gathered in the

valleys at the junctions between the TiO2 particles[34].

Factors controlling catalytic activity and

selectivity of gold

Except for H2 oxidation and hydrocarbon hydrogenations,

most reactions are remarkably structure-sensitive over

supported Au catalysts. Two typical reactions are CO

oxidation and propylene epoxidation. The oxidation of CO is

the simplest reaction and has been the most intensively studied

since Langmuir rst presented a theory of adsorption and

catalysis for this reaction[35]. It is also practically important in

connection with the purication of engine exhaust gases and

of hydrogen produced by steam reforming of methanol and

hydrocarbons for polymer electrolyte fuel cells. The direct

epoxidation of propylene is regarded as a Holy Grail of sorts

because current industrial processes need two-stage reactions.

Ordered hexagonal arrays

Mesoporous SiO2 MCM-41

Surface Area: 1100 m2 /g

3 n m

Figure 2 TEM micrograph of a Au/MCM-41 catalyst prepared by the gas-

phase grafting method.

(b)Au

<110>

{ 1 1 1 } 2 .3 5 Å { 1 1 2 } 2 .3

3 Å

<201>

TiO2

Figure 3 (a) TEM micrograph of a Au/TiO2 catalyst prepared by the deposition-

precipitation, and (b) its schematic model at the interface.



105

What characterizes Au catalysts in these two reactions are thethree factors which rule the activity and selectivity; strong

contact of Au particles with the support, suitable selection of

the support, and size control of Au particles.

(1) Strong contact of gold particles with the support

Figure 4 shows turnover frequencies (TOFs), the reaction

rate over one single surface metal atom per second,

of CO oxidation at 300K over Au/TiO2 and Pt/TiO2

catalysts prepared by DP, photocatalytic deposition and IMP

methods[36]. The DP method yields hemispherical metal

particles with their at planes strongly attached to the TiO2

support (see Figure 3), while photocatalytic deposition andIMP methods y ield spherical particles simply loaded on the

TiO2 support (see Figure 1) and, therefore, much larger

particles—particula rly in the case of Au. Over Pt/TiO2, the

reaction of CO with O2 can take place on the Pt surfaces

and the metal oxide support is not directly involved in the

reaction. This can account for why different preparation

methods do not make any appreciable difference in the TOF

of Pt catalysts. On the other hand, the TOF of Au/TiO2

markedly depends on the contact structure changing by

four orders of magnitude. The TOF of strongly attached

hemispherical Au particles exceeds that of Pt by one order

of magnitude.

The strong contact of Au particles is also indispensable

for the epoxidation of propylene in the gas phase containing

O2 and H2[37]. Figure 5 shows that spherical Au particles

simply loaded on TiO2 (prepared by IMP) needs higher

temperatures for reaction to occur and causes complete

oxidation to produce CO2. The yield of H2O is much larger

than that of CO2, showing that H2 oxidation takes place

much faster than propylene oxidation. On the other hand,

hemispherical Au particles strongly attached to the TiO2

support (prepared by DP) produce propylene oxide with

1

10-1

10-2

10-3

10-4

10-5

10-6

t u r n o v e r

f r e q u e n c i e s ( s - 1 )

TiO2

Au PtAu Pt

TiO2 TiO2 TiO2

platinumgold

Figure 4 Turnover frequencies for CO oxidation over spherical and

hemispherical particles of Au and Pt supported on TiO2.

A brief history of catalysis by goldintermezzo 1

Until 1975, catalysis by Au was often studied by using wire, foil,

lm, gauze and sponge and, accordingly, at high temperatures.

A landmark was the work of Wood and Wise, who demonstrated

that a Au lm was active for the hydrogenation of cyclohexene and

1-butene if dissociated hydrogen atoms were supplied through a

Pd-Ag alloy timble.

In the 1970s, G. C. Bond and P. A. Sermon, and also G. Paravano,

paid attention to the size effect of Au par ticles on the catalytic activity.

They tried to disperse small Au particles on MgO, Al2O3 and SiO2

by low-temperature calcination or by liquid-phase reduction. Some of

them exhibited appreciable catalytic activities for the hydrogenation

of linear alkenes, oxygen and hydrogen transfer reactions, and NO

reduction with H2 at moderate temperatures. Although supported

gold possessed unique selectivity, a constraint was that the catalytic

activity was always inferior to those of other noble metals.

Another approach toward the size effect was bottom-up,

starting from a single atom or clusters. In the late 1970s, Ozin

reported that atomic Au could react with a CO+O2 matrix at a very

low temperature (10K) and liberate CO2 when the temperature

was raised to 30-40K. Cox reported specic numbers of atoms in

small Au clusters (a few to twenty atoms) for reactions involving

H2, O2 and CH4.

These observations clearly showed that small gold particles

could exhibit noticeable activity for hydrogenation and oxidation

reactions. However, few people recognized at that time the

implications of these valuable contributions.

The interest in catalysis by Au was revitalized when Hutchings

predicted in 1985, and subsequently conrmed, that Au would be

the most active catalyst for the hydrochlorination of acetylene to

produce vinyl chloride.

almost 100% selectivity at a lower temperature, 323K. TheH2 consumption is only about three times that of propylene

conversion and appreciably smaller than that of spherical Au

particle catalysts.

The sharp contrast between the above two catalysts in

CO oxidation and propylene epoxidation suggests that the

reactions may take place at the perimeter interfaces around

Au particles. To conrm this hypothesis, Vannice prepared an

inversely supported catalyst, namely, TiO2 layers deposited on

a Au substrate, and observed appreciable catalytic activity [38].

We prepared the Au/TiO2 catalyst by mechanically mixing

a colloidal solution of Au particles 5 nm in diameter

with TiO2 powder, then enacting calcination in air atdifferent temperatures[111]. Calcination at 873K promotes

the coagulation of Au particles to form larger particles with



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diameters above 10 nm, but at the same time with stronger

contact (observed by TEM), leading to much higher catalytic

activity than calcination at 573K.

(2) Selection of suitable supportsFor CO oxidation, except for strongly acidic materials such as

SiO2-Al2O3 and activated carbon, many oxides can be

used as a support. Semiconductive metal oxides such as

TiO2, Fe2O3, Co3O4 and NiO provide more stable Au

catalysts than insulating metal oxides like Al2O3 and SiO2.

Among Au supported on Al2O3, SiO2 and TiO2, TOFs

at room temperature are nearly equal, indicating that the

contribution of metal oxide supports is more or less similar

in intensities[39]. The difference appears in the moisture

effect: Al2O3 and SiO2 need a higher H2O concentration

above 10 ppm for CO oxidation at room temperature than

TiO2[40]. Alkaline earth metal hydroxides such as Ba(OH)2

and Mg(OH)2 are the best to exhibit high activity at a

temperature as low as 196K. However, the Au catalysts are

stable for only 3 or 4 months [41].

For the combustion of hydrocarbons, Co3O4, which is

the most active base metal oxide for complete oxidation, gives

rise to the highest activity [42,43]. For nitrogen-containing

compounds, ferric oxide and nickel ferrites lead to the highest

activity owing to their good afnities to nitrogen[44]. The

oxidative decomposition of trimethylamine, which is a typical

odor compound, proceeds at temperatures below 373K,

yielding mainly N2 and CO2, while N2O is mainly produced

over Pd and Pt catalysts even at higher temperatures.

For the selective oxidation of hydrocarbons in the

co-presence of O2 and H2, as listed in Scheme 2, only TiO2

and Ti-silicates act as effective supports[45–49]. The support

requirements are very strict. Only anatase, neither rutile

nor amorphous, makes Au selective, although the reason is

still unclear. TEM observations showed that Au particles

were more often epitax ial ly contacted on anatase than on

rutile, indicating that the location of Ti cations around

Au particles is more regular on the anatase surfaces [32].

When Ti cations are isolated from each other on the

surface or in the bulk network of SiO2, Au is also selective

to epoxidation. The distance between Ti cations may be

important; on the surfaces of anatase and Ti silicate, Ti

cations are separated from each other at a distance of the

diameter of oxygen anion or farther, while on the rutile

surface, they are located closer.

For methanol synthesis, ZnO, which is also the best

support for commercial Cu catalysts, works as the best

support[50,51]

. For NO reduction with hydrocarbons in thepresence of O2 and H2O, Al2O3 gives the most selective

reduction catalysts[52].

(3) Size control of gold particles

Figure 6 shows that at a critical diameter of 2 nm, the main

product in the reaction of propylene with O2 and H2 switches

from propylene oxide to propane[37]. When Au particles are

smaller than 2 nm (clusters), they behave differently in the

presence of O2 from the bulk Au and become similar to Pd

and Pt. Over Pd and Pt catalysts the product is only propane,

irrespective of the presence and absence of O2, while propane

is formed over Au catalysts only when O2 is present. Thisimplies that a change in the surface property of Au clusters

is induced by O2 through electron abstraction from the Au

clusters to form negatively charged oxygen species, O2-. This

critical diameter corresponds to a layer of 3 or 4 atoms thick

if the Au clusters are hemispherical in shape. The work by

Goodman and his coworkers[53] shows that the electronic

state of Au clusters changes with layers of 2 or 3 atoms thick.

313 - 473 K

TiO2 (anatase)Ti-MCM41

Ti-β, TS-1, TS-2

TiO2 (rutile) TiO2 (amorphous)

CH3CH=CH2 + O2 + H2

CH3CH CH2

O

CH3CCH3, CO2

O

Scheme 2 Products of the reaction among propylene, oxygen and hydrogen

over gold catalysts supported on the various materials containing titanium.

reaction temperature (K)

0

1

2

3

4

5

6

7

8

9

10

320310 330 340 350 360

p r o d u c t y i e l d ( % )

CH3CH=CH2 + O2 + H2 CH3CH CH2 + H2O

impregnation

Au

Au

deposition - precipitation

H2O

H2O

CO2

O

CH3CH CH2

O

TiO2

TiO2

Figure 5 Product yields of the reaction among propylene, oxygen and

hydrogen over Au/TiO2 catalysts prepared by the deposition-precipitation and

impregnation methods.



107

Figure 7 plots the rates of CO oxidation per exposed

surface area of Au, which are equivalent to TOFs, over

unsupported Au powder and over Au/TiO2[54]. In the case

of Au/TiO2, the rates were about one order of magnitude

larger when measured by lowering the temperature from

353K than when measured by raising the temperature

from 203K. This difference is assumed to arise from

the accumulation of carbonate species on the surfaces of

the support at low temperatures resulting in the loss of

the activating power of the perimeter interfaces for O2.

Therefore, the rate over Au/TiO2 which was deactivatedduring experiments at lower temperatures is regarded to be

close to the rate of CO reaction with O2 over the surfaces

of Au part icles without the contribution of O2 activation at

the perimeter interfaces. The linear relation among the rates

over unsupported Au powder and the ones over deactivated

Au/TiO2 supports our assumption and shows that the

normalized rate increases with a decrease in the mean

diameter of Au particles by a factor of 2/3. This increase can

be explained if the active sites are edge, corner or step sites,

the fractions of which increase with a decrease in the size

of Au particles[61]. The one order of magnitude difference

in the rate between fresh (obtained by high temperature

Au particle size (nm)

0

1

2

3

1.6 2.0 2.82.4

p r o d u c t y i e l d ( % )

CO2

C3H6

Au/TiO2 (p-25)

353K

C3H6 / O2 / H2 / Ar = 1 / 1 / 1 / 7

SV : 4,000 h-1 · ml/g-cat

CH3CH CH2

O

Figure 6 Product yields of the reaction among propylene, oxygen and

hydrogen over Au/TiO2 as a function of Au particle size.

101

100

100 101 102 103

10-1

10-2

10-3

k C O ( m m o l m i n - 1 m

- 2 c m H g - 1 )


Au powder

Au/TiO2 273K

deactivated

fresh

Figure 7 Reaction rates for CO oxidation over fresh and deactivated Au/TiO2

catalysts as well as unsupported Au powder.

How metal support interactions affect the catalytic properties of metalsintermezzo 2

Metal support interactions can be classied into three types.

The rst is that metal particles work as the active phase, with

the support modifying the electronic structure of the metal. This

happens only when metal particles are small enough to be

substantially altered by some electron transfer from or to the

support. The critical size is quantum size, which is about 2.0

nm in diameter for spherical particles of noble metals. For such

sizes, the fraction of atoms exposed to the surface exceeds 50%.

The electronic structure of such metal particles deviates from

that of bulk metal even when they are unsupported. However, the

interaction with a metal oxide support, especially semiconductive

metal oxides, can enhance this change through electron transfer.

Goodman observed by scanning tunneling spectroscopy that the

electronic state of Au islands deposited on TiO 2 changes when

their thickness is less than 3 atomic layers.

The second type involves new reaction sites appearing at

the metal-support perimeter interface. The surface of the metallic

particles is, however, still indispensable to provide sites for the

adsorption of at least one of the reactants. For example, CO

on the edge and corner sites of Au nanoparticles. The supports

may also work as a reservoir for counter-reactants. For example,

O2 on metal oxide supports. Even though molecular O2 may not

dissociate by itself at the perimeter interface, it is speculated that

it can react with CO at the periphery sites to form CO2. An open

question is why the perimeter interface works as a reaction zone.

One possibility is that the interface is composed of Au oxide or

hydroxide, which is stabilized and recycled by interaction with

metal oxide supports.

The last type of effect is the reverse of the rst case.

Thin metal oxide layers covering the metal particles may work

as the catalytically active phase, in particular when the layer

thickness is smaller than a few atoms. This happens, for example,

when Pd/CeO2 is prepared by co-precipitation or deposition-

precipitation.



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measurements) and deactivated Au/TiO2 can be ascribed to

the contribution of the TiO2 support.

Among supported noble metal catalysts, Au supportedon Mg(OH)2 is the most active for CO oxidation at 196K,

giving 100% conversion at an hourly space velocity of

20,000h-1mg/g-cat. However, it suddenly dies after 3 to 4

months, losing activity even at 473K [55]. We have estimated

the most probable size distribution of Au particles by using

Debye Functional Analysis of X-ray scattered by Au particles

for fresh and aged catalysts. It is suggested that the active

Au species are 13-atom Au clusters and when they grow

to 55-atom clusters with truncated decahedral structure,

the catalytic activity is completely lost. Figure 8 shows

that among the two structures of 13-atom clusters, the

icosahedron is active, whereas the cubo-octahedron is

inactive[41]. It is hard to explain why such a drastic di fference

was observed. The coordination number of the Au atom is

5 for the icosahedron and 4 for the cubo-octahedron. The

active Au/Mg(OH)2 catalyst, which is mainly composed

of icosahedral Au clusters of 13 atoms, showed negative

apparent activation energy in the temperature range from

196K to 273K [56]. This can be explained by the enhanced

transformation of the icosahedron into the cubo-octahedron

with a rise in reaction temperature.

Heiz and coworkers prepared model catalysts by

depositing size-selected Au anion clusters onto a single crystal

of MgO. Appreciable size dependency of CO adsorption was

observed and the highest reactivity to CO was observed for

the anion clusters of eleven atoms [57]. It was also reported

that 8 and 11 are the smallest and the second smallest

number of atoms to exhibit the catalytic activity for CO

oxidation over the MgO support. Higher activity of Au

clusters on defect-rich MgO than on defect-poor MgO

was observed. Ab init io simulations indicated that part ial

electron transfer from the surface of the Au clusters, and

oxygen-vacancy defects in the support, play an essential rolefor the genesis of catalytic activity [58,59].

Kinetic behaviour and reaction mechanism

As typically shown in Figure 9 for Au/TiO2, the rate of

CO oxidation over Au/TiO2, Au/Fe2O3 and Au/Co3O4

is independent of the concentration of CO down to 0.1

vol% and only slightly dependent on the concentrat ion of

O2(0~0.25 order)[18]. Actually, this fact was a surprise to

us because the conversion of CO increased with a decrease

in the concentrations of CO and O2. Over unsupported Au

powder, with the mean diameters of primary Au particles at

17 nm, the rate is almost independent of the concentrationsof CO and O2

[54]. These independencies suggest that

CO and O2 are adsorbed on the catalyst surfaces nearly

to saturation and the reaction of CO with O2 is the rate

determining step.

Icosahedron Cubo-octahedron

Icos.:Cubo-oct.

58 : 42 7 : 93

100 % 7 : 93CO Conversion(-70˚C)

100

10

0.1 1 10

0.1

k C O ( 1 0 - 5

m o l C O / m i n . g - c a t )

Concentration of CO or O2 (vol%)

rate ∝ [CO]0.05[02]0.24

Au/TiO2

3.3 wt%

273 K

CO

O2

Figure 8 CO oxidation over Au13 clusters supported on Mg(OH)2 with dif ferent

composition of icosahedral and cubo- octahedral structures.

Figure 9 Dependence of the reaction rates for CO oxidation over Au/TiO2 on

the concentration of reactant gasses.

2200 2100 2000

a d s o r b a n

c e

wavenumbers (cm-1)

4 mbar CO at 90 K

.5A

2176

2154

2098

573K (2.5 nm)

473K (2.4 nm)

873K (10.6 nm)

d Au

Figure 10 FT-IR spectra for CO adsorbed on Au/TiO2 calcined at

473, 573 and 873 K.



109

1000/T (K-1)

10-1

10-2

10-3

3 4 5

region I E a 2 kJ/mol

region II

region III

k C O ( m m o l m i n - 1 m

- 2 c m H g - 1 )

100

101

Au/TiO2

E a 30 kJ/mol

Au powder

E a 5 kJ/mol

Figure 11 Dependence of the reaction rates for CO oxidation over Au/TiO2 aswell as unsupported Au powder on the reaction temperature.

Figure 10 shows FT-IR spectra for CO adsorpt ion at 90K

over Au/TiO2 calcined in air at different temperatures[60].

The most active sample (calcined at 573K, mean diameter of

Au part icles 2.5 nm) has the largest intensities of the peak

at 2110~2120cm-1, which can be assigned to CO linearly

adsorbed on the metall ic Au sites. When Au particles become

larger than 10 nm in diameter (sample calcined at 873K),

the intensity of this peak is markedly reduced, indicating

that CO adsorption may take place only on steps, edges and

corners of Au particles, not on the smooth surfaces. Thisagrees well with what is obtained by self-consistent density

functional theory calculations[61].

No direct experimental evidence has yet been presented

where oxygen is activated for reacting with CO adsorbed

on the Au surfaces, and whether the oxygen molecule

is dissociatively or non-dissociatively adsorbed. A TAP

(temporal analysis of products) study of O2 adsorption

and the reaction of O2 with CO[62,63], 18O2 isotope

experiments [62-64] and ESR measurements[64,65], indicate

that molecularly adsorbed O2, most likely O2- at the

perimeter interface, is involved in the oxidation of CO.

Figure 11 shows the Arrhenius plots in a wide range

of temperatures from 90K to 400K [66]. There are three

temperature regions where different kinetics are operating

with markedly dif ferent rates and apparent activat ion

energies. The probable pathways for CO oxidation are

schematically shown in Scheme 3.

At temperatures below 200K, the TiO2 surfaces and its

perimeter interfaces around Au particles are covered with

carbonate species formed by surface reactions of CO. Reaction

of CO with O2 takes place only on the surfaces of Au, more

specically, on the step, edge and corner sites, with apparent

Scheme 3 Reaction mechanisms proposed for CO oxidation over Au/TiO2.

O2 + Au/TiO2

Au/TiO2...O2 + 2 [O C-Au]

O C-Au

O

O

CO2

O2 (g)

slow O C-Au + CO2

CO + Au

Au/TiO2...O2

O C-Au

O O

TiO2

O O

O

O

O

OO

O

O

C

C

C

C

surface

perimeter intermediate

spectator

Au

activation energies of almost zero kJ/mol. This means that

when Au particles are small enough, the catalytic activity can

be detected at any temperature. Actually, unsupported Au

powder (primary particles with mean diameters of 17 nm)

exhibits measurable activity for CO oxidation at 200K with an

apparent activation energy of nearly zero[54]. At temperatures

above 300K, reaction takes place at the perimeter interfaces

between CO adsorbed on the surfaces of Au particles and

oxygen (most likely molecular) adsorbed at the support

surfaces. This reaction also gives nearly zero apparentactivation energy, but proceeds much faster by more than one

order of magnitude than the reaction over the Au surfaces. At

intermediate temperatures from 200K to 300K, the reaction

proceeds at the perimeter interfaces, which are partly covered

with carbonate species. The coverage of the species may

change depending on temperature, thus giving rise to an

apparent activation energy of around 30 kJ/mol.

There are still other arguments about the active species

of Au, especially in the case of Au/Fe2O3: oxidized Au

species, Au(I)[67], Auδ+[68] or metal oxide support surfaces

with modied reducibility by the interaction with Au

nanoparticles[69,70].

It is unlikely that oxidic Au species are major catalytically

active phases because the most active supported Au catalysts

are prepared by calcination in air at 573 K, where Au precursors

(hydroxides or organo complexes) can be transformed mostly

into metallic particles. A certain fraction of Au species

remains as atomically dispersed species in the matrix of the

support, which was proved by EXAFS[22,68,71,72], XPS[69],

Mössbauer[70,73] and IR for CO adsorbed[60,67]. However,

no correlation between the amount of oxidic Au species and

catalytic activity has yet been presented. It is speculated that



110

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ef f e c t

Figure 12 Dependence of the conversion for glycol oxidation over Au/Al2O3

and Au/C catalysts on the Au particle size[88].


0

20

40

60

80

100

43 5 6 7 8 9

c o n v e r s i o n (

m o l % )

Au/Al2O3

Au/C

the samples mainly consisting of oxidic Au could exhibit high

catalytic activity because oxidic Au species are transformed

into metallic particles during reaction.

Even though metallic Au particles are indispensable, a

question arises as to why the periphery of Au particles can

activate O2 molecules at low temperatures.

As proposed by Bond and Thompson[74], it is probable

that the perimeter interfaces contain oxidic Au(III) or Au(I)

species, most probably Au(OH)3

or Au(OH) under usual

conditions, where H2O is present at a concentration of 10

ppm. These hydroxides may be stabilized and reversibly formed

and decomposed with the aid of the metal oxide supports.

Another argument proposed by Goodman and his

coworkers is that the non-metallic nature of Au clusters

leads to the highest catalytic activity [53]. This conclusion is

questionable. The transition of electronic state was measured

on one specic Au cluster of a dened diameter by scanning

tunneling spectroscopy, whereas the catalytic activity was

measured by using a specimen of Au/TiO2 model catalyst

with a mean diameter of all Au clusters. A maximum in

catalytic activity, with respect to the mean diameter of Auclusters, was observed where the transition of electronic state

from metallic to non-metallic started. However, this fact can

be more reasonably explained by assuming that a maximum

in the total surface area or the number of step sites [61] of

metallic Au clusters is obtained at a certain diameter where

the transition to the non-metallic state starts.

Reactions catalyzed by gold

Supported Au catalysts can also catalyze many reactions other

than CO oxidation and propylene epoxidation. The following

reactions usually take place at much lower temperatures or with

much higher selectivities than over the other metal catalysts.

(1) Water Gas Shift Reaction and CO Removal from H2

Owing to ongoing applications of polymer electrolyte fuel

cells to automobiles and to residential electricity-heat delivery

systems, low-temperature water gas shift reaction is attracting

renewed interest. In comparison with commercial catalysts

based on Ni or Cu, which operate at 900K or at 600K

respectively, supported Au catalysts appear to be advantageous

in operation at a temperature as low as 473K. As a support,

TiO2[75] and ZrO2

[76] (IV A group metal oxides), and Fe2O3[76] are especially effective, and the crystallinity of these metal

oxides appreciably affect the catalytic activity [77]. Although

the stability may not be good, Au/NaY is reported to be

active for a water gas shift reaction at 373K [78]. The redox

cycle of Au+ and Au0 is assumed to operate for CO activation,

while the NaY support activates H2O.

For selective CO removal in an H2 stream, supported Au

catalysts are potentially advantageous over other noble metal

catalysts because only supported Au catalysts are much more

active for CO oxidation than H2 oxidation. Au/Mn2O3[79]

and Au/Fe2O3[80,81] in particular exhibit good stability as

well as high catalytic activity and selectivity.

(2) Hydrogenation of Unsaturated Hydrocarbon

A characteristic feature of Au catalysts for the hydrogenation of

unsaturated hydrocarbons is that partial hydrogenation takes

place very selectively: butadiene to butene and acetylene to

ethylene[12,13,82,83]. Hydrocarbon hydrogenations are known

to be structure-insensitive, proceeding at approximately the

same TOF on metal particles of various sizes[84]. It was

also the case in the hydrogenation of butadiene over Au

catalysts as far as Al2

O3

, SiO2

and TiO2

were concerned

as supports[82]. Turnover frequency (TOF) differed only by

a factor of three among Au particles with mean diameters

ranging from 3.5 to 7 nm which were deposited on Al2O3,

SiO2 and TiO2. These Au catalysts were perfectly selective

to partial hydrogenation to form butene(1-butene 70%,

2-butene 30% at 323K), whereas other noble metal catalysts

by-produced butane.

In the cases of acetylene hydrogenation over Au/Al2O3

and acrolein hydrogenation over Au/TiO2 and Au/ZrO2,

the catalytic activity per unit weight of Au increased with

a decrease in the size of Au particles down to 2 nm,

indicating that the metallic nature of Au is also important forhydrogenation.

In the hydrogenation of α,β unsaturated aldehyde,

selectivity to the hydrogenation of C=O versus that of C=C

was reported to reach 40~50% when Au particles deposited

on TiO2 or ZrO2 were larger than 2 nm in diameter[85,86].

It has been reported recently that Au/ZnO exhibits a

selectivity of about 80% to C=O hydrogenation to produce

the unsaturated alcohol, but-2-en-1-ol, from but-2-enal[87].

(3) Liquid Phase Oxidation and Hydrolysis

Prati and Rossi have found that Au supported on activated

carbon is more active and selective than other noble metalcatalysts for the oxidation in a Methanol-H2O(6:4) solvent of

glycols to α-hydroxy acids, which are used in the cosmetics

and food industries[88]. Not only activated carbon, but also

γ -Al2O3 and TiO2 make Au active and selective[88,89]. An

interesting feature in Figure 12 is that over activated carbon



111

support, a maximum activity is observed when the mean

diameter of Au particles is 7-8 nm, whereas the smaller Au

particles give higher activity over γ -Al2O3 and TiO2 support.

This can be explained as follows: smaller Au particles can

be easily xed on the internal surfaces of carbon, and

are consequently less accessible to reagents in the liquid

phase than larger Au particles located on the external

surfaces. It should also be noted that Au sols stabilized with

polyvinylpirrolidone or tetrakis(hydroxymethyl)phosphonium

chloride can give uniformly dispersed Au particles even on

activated carbon by simply dipping the support materials.

Unsupported Au nanoparticles also exhibit unique catalytic

properties in liquid phase reaction. N-methylimidazole-

functionalized Au particles are active for the hydrolysis of a

carboxylic acid ester to about 30 times as high as a reference

monomeric catalyst without Au particles[90].

(4) NO Reduction with Hydrocarbons

The reduction of NO with hydrocarbons to N2 in the

co-presence of excess O2 takes place over some supported

Au catalysts[52]

. Alkenes (C2H4, C3H6) are more effective asa reductant than alkanes (CH4, C2H6, C3H8) because the

former can adsorb on the Au surfaces more strongly. The

temperature and efciency for NO reduction depend on

the kind of metal oxide supports and increase in the order

of ZnO(523K, 49%), α-Fe2O3(523K, 12%), MgO(623K,

42%), TiO2(623K, 30%), Al2O3(673-723K, 80%). The NO

conversion to N2 obtained over Au/Al2O3 in the presence

of 5 vol% O2 and 10 vol% H2O is comparatively higher

than over the other catalysts reported so far[5]. The reaction

passes through NO oxidation with O2 to form NO2, which

then reacts with propylene. Therefore, enhanced activity is

obtained for a mechanical mixture of Au/Al2O3 with MnOx which is active for NO oxidation to NO2

[91].

Gold wire[92] and lm[93] were known to be active for

N2O decomposition to N2 and O2. Recently, Au/Co3O4 has

been reported to be active for this reaction even at 523K in

the co-presence of 10 vol%O2 and 5 vol%H2O[94].

(5) Reactions involving Halogens

Because of the stability of Au against halogens, some

supported Au catalysts have been reported to be more active

and stable for reactions involving halogens than other noble

metal catalysts. Gold supported on activated carbon is the

most active for the hydrochlorination of acetylene to produce

vinylchloride[95]. Gold on Co3O4 or Al2O3 is active for the

oxidative decomposition of CCl2F2 and CH3Cl[96]. Gold

supported on LaF3 is active for the synthesis of HCN by the

reaction of uorinated hydrocarbons with NH3[97].

Ongoing and future applications

The characteristic features of supported gold catalysts are:

active at low temperatures, activated by moisture and often

very selective. Gold deposited on Fe2O3, which is supported

on a zeolite wash-coated honeycomb, has been commercially

used as an odor eater in modern Japanese toilets since 1992.

Gold catalysts are more active at temperatures below 400K

for the oxidative decomposition of trimethlamine, a typical

odor compound, than Pd and Pt catalysts, and more selective

to N2 with the least formation of N2O[44]. It is probable that

odor eaters using gold catalysts will expand their applications

to air cleaners, home garbage treatment equipments, volatileorganic vapor treatments and so on.

Gold supported on Fe2O3 or La2O3 is the most active

among noble metal catalysts for the oxidative decomposition

of dioxin at temperatures below 473K. Recently, as shown

in Figure 13, we have integrated Ir, Pt and Au catalysts

supported on La2O3, SnO2 and TiO2, respectively, and 95%

decomposition of dioxin was obtained, even at 423K, at an

hourly space velocity of 12,000h-1·ml/g-cat.[98]. Owing to

the booster effect of Ir catalysts, the integrated noble metal

catalysts are applicable to the removal of dioxin from the

outlet gases of incinerators.

The third promising eld of application is in CO2 lasers[99]. It has long been known that coating the glass

inside-walls of a laser discharge tube with Au results in a

substantial improvement in the performance of sealed-off

direct current excited CO2 lasers[100]. The performance of

radio frequency excited CO2 lasers can also be enhanced by a

joint action of the Au coating of electrodes and the addition

of CO at a high concentration(4~8 vol%)[101,102]. The role

of CO is explained by the adsorption on the Au surfaces to

prevent them from being deactivated by oxygen species. This

phenomenon is consistent with the results obtained at 90K

for the reactivities of CO and oxygen preadsorbed; the latter

is more stably adsorbed and less reactive to form CO2[60].

It will take longer to utilize Au catalysts in the chemical

industry. There are possibilities of earlier commercialization

in selective partial oxidation in liquid phase for ne chemicals.

As for bulk chemical production, the direct epoxidation of

propylene can be commercialized if the yield of propylene oxide

(PO) is improved to 10% from the present level of 3%, and the

efciency of H2 is improved to 50%, at least, from 20~30%.

Gold supported on anatase TiO2 is selective only at

temperatures below 353K. Because this temperature is not

sufciently higher than the boiling point of PO (307K),

dioxin decomposition below 423 K

carrier

Pd SnO2 Ir La2O3Fe2O3Au

Cl Cl CO2CO HCl

HCHO etc.

O

O

Figure 13 Schematic structure model of the three-component integrated

catalyst for dioxin decomposition.



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the conversion of propylene remains small due to the slow

desorption of PO. Therefore, a variety of titanium silicates as

well as Ti deposited on SiO2 have been tested as a support

for Au particles[45-49,103]. Among titanosilicates such as TS-1,

Ti-β zeolite, Ti-MCM41 and Ti-MCM48, having different

sizes and structures of pores, Ti-MCM48 is proven to give

the best catalytic performance[104]. The atomic ratio of Ti/Si

should be within 1/100~3/100 to keep Ti cations isolated

from each other in the matrix. When these Ti/SiO2

support

materials are used, Au is selective to epoxidation up to

473K, yielding improved conversion almost to 5%[47,103,104].

A reaction pathway has been proposed in which H2O2 is

formed on the Au surfaces and then transformed into OOH

on Ti4+ tetrahedrally coordinated in SiO2 matrices to react

there with propylene[47,49,104].

Outlook

Gold catalysts have been attracting growing interest from the

point of view of both applications and fundamental study [23].

Owing to their excellent performance in complete oxidation

at ambient temperature with enhancement by moisture,

Au catalysts are expected to penetrate into our daily

life and make it more comfortable and healthy. Through

appropriate selection of support materials, gold can exhibit

high selectivity in hydrogenation and part ial oxidation, and

may contribute to the development of highly selective and

green chemical processes.

The genesis of catalysis by gold is, in most cases, ascribed

to the perimeter interfaces around Au particles. This presents

us with a new guiding principle to create a wide range of

new catalytic systems, because the combination of catalytic

metals with a variety of support materials gives rise to a

great deal of novel catalysts. Good examples are Pd/CeO2

and Pt/ZrO2, prepared by coprecipitation for the low-

temperature decomposition and synthesis of methanol[105].

Recently, fundamental work has been emerging to

understand the unique catalysis of Au and its size-

dependency. By means of surface science techniques using

well-dened catalytic materials, most often size-selected

gold clusters deposited on single crystalline metal oxides

such as MgO[57] and TiO2[53,106,107]. An atomic scale

understanding is being accumulated in combination with

questions and answers

When and how did you get involved in catalysis by gold?

In 1976 when the Japanese chemical industry was suffering

heavily from the second oil crisis and pollution problems,

fortunately, I was offered a permanent position at the

Osaka National Research Institute where I was involved in

the development of hydrogen catalytic combustors under anational project for new energy technologies.

I planned to develop inexpensive metal oxide catalysts

that could replace Pd and Pt. Firstly, I prepared transition

metal oxides by a variety of methods and then mixed metal

oxides. A few years later, stimulated by a publication of

Professor Moro-oka, I plotted the catalytic activities of metal

oxides for H2 oxidation against the metal-oxygen (M-O)

bonding energies and obtained a volcano-like relationship.

My idea was then to combine Ag or Au, having weaker

M-O bonding energies than those of Pd and Pt, with other

metals having stronger M-O bonding energies. I observed

that some Ag composite oxides, Ag-Mn and Ag-Mn-Co

oxides in particular, were more active than Ag oxide alone

and showed an activity comparable to that of Pd and Pt for

H2 oxidation. For CO oxidation, the composite oxides were

active at room temperature and superior to Pd and Pt.

I presented this work at the 3rd International

Symposium on Catalyst Preparation held at Louvain-la-

Neuve in 1982. Dr. C. S. Brooks, who had retired from

United Technology Co. Ltd., asked me whether I had

prepared the composite oxides of Au. I answered that Au

composite oxides might not be interesting because Au was

(at that time) more expensive than Pt. He changed my

mind by saying, “The cost does not matter at this stage

of research. It is important to nd evidence and conrm

your hypothesis that active catalysts may emerge from Agor Au. In the USA, even researchers working for industry

recognize a signicance in checking a hypothesis. Why don’t

you demonstrate that Au becomes cata lytically active when

combined with the oxides of base metals?”

The active sites responsible for the catalytic activity of

gold are located at either the accessible periphery of the

metal particles where the supporting oxide may also

contribute directly, or on the surface of the gold particles

whose properties are altered by the strong interaction

with the support. How does one or the other situation

prevail depending on the reaction type?

When gold is deposited as nanoparticles on a variety of metal

oxides and carbonaceous materials, it exhibits noticeably

high catalytic activity for many reactions. Among them, CO

oxidation and propylene epoxidation are structure sensitive

reactions, the rate and reaction products of which are

markedly dependent on the size of the Au particles, the type



113

H2O concentration (ppm)

10-7

0.1 1 10 100 1000 10000

270 K

k C O ( m o l / l

s g - c a t )

10-6

10-5

Au/TiO2

theoretical calculations[9,58,61]. In this approach, gold may

be the best target of research because gold itself is poorly

active, and the detectable activity is obtained only when the

specic structure and size of Au particles are given. What is

also advantageous in Au model catalysts is that the activity

can be often observed at lower temperatures where designed

structure can be maintained.

In order to make recent attempts more effective to

combine experimental work using real catalysts and model

catalysts with theoretical calculations[108], the effect of

moisture should be taken into account. Most surface science

work is carried out in an ultra-high vacuum, whi le catalytic

activity measurements for real catalysts are carried out in a

xed bed ow reactor using reactant gas containing moisture

at least 1 ppm, usually 10 ppm. The catalyst surfaces are

covered with OH groups and water molecules at room

temperature. For CO oxidation which does not produce

H2O and proceeds at temperatures below 373K, moisture

markedly changes the catalytic activity of metal oxides

and gold catalysts. Under dry conditions, with a H2O

concentration of 80 ppb, CO oxidation can take place even

Figure 14 Dependence of the reaction rates for CO oxidation over Au/TiO 2 on

the H2O concentration in the reactant gas.

of support and the structure of the interface. In these cases,

the periphery is working as reaction sites but metallic gold

particles are also necessary as sites for the adsorption of one

of the reactants.

On the other hand, the hydrogen oxidation and the

hydrogenation of alkenes and alkynes are structure insensit ive

reactions whose rates in terms of turnover frequency andproduct selectivity are almost independent of the size of the

Au part icles (in the range of 2 nm to 10 nm) and of the type

of support. This is because these reactions take place on the

surface of the Au particles, the electronic state and defect

density may change markedly at a critical size around 2 nm.

The activity of gold catalysts has been reported mainly

for reactions occurring below 500K. Do you know of any

example where gold is active at a higher temperature?

One of the characteristic features of catalysis by Au is that

the apparent activation energy is very low, usually below 40

kJ/mol. and sometimes even below 10 kJ/mol. This feature

may be a reection of the weak interaction of the reactants

with stepped or defect sites of Au. Such low activat ion

energies mean that Au catalysts are more competitive at

lower temperatures and especially useful for environmental

applications at ambient temperatures. Consequently, Au

catalysts are, in principle, inferior to other noble metal

catalysts at temperatures above 500K. The only known

exception, presently, is the selective reduction of NOx

with hydrocarbons. Gold supported on Al2O3 is active and

selective for NO reduction with propylene in the presence

of excess O2 and H2O at temperatures from 400K to

700K. An advantageous feature is that the conversion of

NO does not decrease with increasing temperature (inthis range) because the combustion of propylene is less

promoted over Au/Al2O3.

Because the melting point of Au is much lower than

those of Pd and Pt, and can be as low as 600K when the

particle diameter is 2 nm or less, it appears that the upper

reaction temperature limit for Au catalysts (which usual ly

contain Au particles with diameters of 3-4 nm) should

range from 600K to 700K.

at 210K over Co3O4 without Au[109,110], while supported

Au catalysts prefer moisture as shown in Figure 14 [40].



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acknowledgement

The author is grateful to Dr. M. Datè of AIST Kansai for his

critical reading and comments together with the drawing of

gures and schemes.

curriculum vitae

Masatake Haruta is Director of

the Research Institute for Green

Technology at the National Institute

of Advanced Industrial Science and

Technology. In April 2001, he moved

to Tsukuba from Ikeda, Osaka

prefecture, due to a reorganization

of Japanese national laboratories

into semi-autonomous bodies. He

graduated from Nagoya Institute of Technology, Department

of Industrial Chemistry, in 1970, and received his PhD

degree from Kyoto University in 1976 on the electrochemical

uorination of organic compounds. He then was granted

a tenure position at Osaka National Research Institute

(ONRI), at the Agency of Industrial Science and Technology

(AIST), where he was responsible for the development of

hydrogen-fueled catalytic space heaters. From 1981 until

1982 he was a visiting scientist at the Université Catholiquede Louvain, Belgium, and then joined, in 1984, the

headquarters of the Sunshine Project at AIST, Ministry of

International Trade and Industry. In 1990 he was promoted

to Head of the catalysis section and in 1994 he became a chief

senior researcher organizing a new interdisciplinary basic

research laboratory for research exploring the potential of

gold catalysts. In 1994 he was appointed as a guest professor

at the Technical University of Vienna and as an adjunct

professor of the graduate school of Osaka University, Faculty

of Science (until 2001). He became Director of the Energy

and Environment department of ONRI in 1999. From

2000 to this date, he has also been an adjunct professor at theCollaborative Research Centre for Advanced Science and

Technology at Osaka University. Haruta’s research interests

have moved from base metal oxides for catalytic combustion

to the preparation of homodispersed particles, gas sensors

using noble metals deposited on semiconductive metal oxides,

and eventually to the application of gold catalysts. Currently,

his research group at Ikeda investigates and develops further

the fundamentals and applied aspects of catalysis by gold

nanoparticles.

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Catalysis of Gold Nanoparticles Deposited on Metal Oxides (1)

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