Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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10.2 Preparations 335

onto supports [37 – 39] , and/or by grafting the nanoparticles onto the support [40] .

Supported metal nanocatalysts can also be fabricated lithographically, using

electron beam lithography [41, 42] .

10.2.1

Silver Nanocatalysts

By using the above - described methods, silver materials with zero - , one - , or two -

dimensional nanostructures, including monodisperse nanoparticles, nanowires,

nanodisks, nanoprisms, nanoplates, and nanocubes, have each been prepared and

are recognized as having great potential for applications in optics, catalysis, and

other ﬁ elds [43 – 47] .

10.2.2

Copper Nanocatalysts

Copper nanoparticles have been synthesized and characterized by different

methods. Notably, chemical reduction, pulsed laser ablation, radiolytic reduction,

and the reduction of copper ions using supercritical ﬂ uids have been developed

to synthesize spherical and different - shaped nanoparticles [48 – 50] . Stability and

reactivity are the two important factors that impede the use and development of

metal clusters. In contrast to noble metals, such as Ag and Au, pure metallic

copper particles usually cannot be obtained via the reduction of simple copper

salts (e.g., copper chloride or copper sulfate) in aqueous solution, because the

reduction tends to stop at the Cu

O stage due to the presence of a large number

of oxygenous water molecules. However, this problem can be overcome by the

addition of other reagents carrying functional groups that can form complexes

with copper ions, or by using soluble surfactants as capping agents to prepare

copper particles in aqueous solution. Although zero - valent copper forms initially

in the solvent, ultimately it can be transformed relatively easily into oxides, in

solvents with high dipole moments and under ambient conditions. The use of

reverse micelles as microreactors and protecting shells has also helped to over-

come some of these complications. Likewise, electrolytic techniques have been

used to synthesize a variety of transition metal colloids of either decahedral or

isohedral shape, by controlling the electrode potential. The extreme air - sensitivity

of copper nanoparticles requires that care be taken during such preparation in

order to avoid oxidation.

10.2.3

Gold Nanocatalysts

Beyond preparing nanoscale materials in the form of colloids, considerable effort

has been expended in the preparation of supported heterogeneous catalysts, and

in particular the nature of the support and the process to immobilize an active

metal on the support (mostly metal oxides and active carbon). The use of gold

336 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

nanocatalysts is cited as an example here when introducing different methods for

preparing nanocatalysts.

Due to the lower melting point of gold, and its poor afﬁ nity for metal oxides, it

is difﬁ cult to prepare stable gold catalysts that are well dispersed on metal oxides.

The typical impregnation methods that are widely used to prepare supported Pd

or Pt catalysts are ineffective in the case of gold, because the presence of chloride

ions can cause a signiﬁ cant enhancement in the coagulation of gold particles

during the calcination of HAuCl

Haruta has summarized the methods used to prepare supported gold catalysts

(Table 10.1 ), and categorized them into four groups [51] :

• The ﬁ rst group includes coprecipitation [54] , amorphous alloying [55] , and

cosputtering [56] . These procedures generally consist of two steps: (i) the

preparation of well - mixed gold/metal oxide precursors; and (ii) transformation

of the gold precursor into gold particles, normally by calcinations in air above

550 K. Well - mixed precursors and high - temperature calcination are equally

Table 10.1 Preparation techniques for nanoparticulate gold catalysts [51] .

Categories Preparation techniques Support materials Reference(s)

Preparation of mixed

precursors of Au and

the metal component

of supports

Coprecipitation

(hydroxides or

carbonates) (CP)

Be(OH)

, TiO

, Fe

, Co

NiO, ZnO, In

SnO

[52 – 54]

Amorphous alloy

(metals) (AA)

ZrO

[55]

Cosputtering (oxides) in

the presence of O

(CS)

[56]

Strong interaction of

Au precursors with

support materials

Deposition – precipitation

(HAuCl

in aqueous

solution) (DP)

Mg(OH)

, Al

TiO

, Fe

, Co

NiO, ZnO, ZrO

CeO

, Ti - SiO

[57]

Liquid - phase grafting

(organogold complex in

organic solvents) (LG)

TiO

, MnOx, Fe

[58, 59]

Gas - phase grafting

(organogold complex)

(GG)

All types, including

SiO

, Al

- SiO

, and

activated carbon

[60, 61]

Mixing colloidal Au

with support materials

Colloid mixing (CM) TiO

, activated carbon [13]

Model catalysts using

single crystal supports

Vacuum deposition (at

low temperature) (VD)

Defects are the sites

for deposition, MgO,

SiO

, TiO

[62 – 64]

10.3 Selective Oxidation of Carbon Monoxide (CO) 337

important to ensure a strong contact between the Au particles and the crystalline

metal oxides.

• The strategy for the second group is based on the concept of depositing or

adsorbing Au compounds onto metal oxide surfaces. Among the three methods

referred to here, deposition – precipitation ( DP ) is widely used to produce active

Au catalysts. By controlling the pH and concentration of the HAuCl

solution,

the deposition of Au(OH)

can be controlled on the surfaces of the support metal

oxides so as to prevent precipitation in the liquid phase. Aggregation of the gold

nanoparticles, induced by chloride ions, can be prevented by washing the gold

compound before drying, and this represents one of the main reasons for the

high activity of these catalysts. The primary limitation here is that DP can only

be applied to metal oxides with an isoelectric point > 5. Although previously it

was shown that Au(OH)

could not be deposited on SiO

and active carbon,

recent studies have found that this constraint may be overcome by correct

surface modiﬁ cation [65] .

• In the third group, the procedure involves the direct immobilization of Au

colloids on modiﬁ ed metal oxide surfaces. In theory, this method could be

applied to all metal oxides, and the catalysts prepared would normally have a

good gold particle size distribution. However, there is often a relatively poor

contact between the gold particles and the support.

• In the fourth group, vacuum deposition is considered to be an important method

for preparing model catalysts that play a critical role when studying reaction

mechanisms, and especially the active sites of the supported gold catalysts. Au

anion clusters can be deposited with homogeneous dispersion at relatively low

temperatures [62, 64] on single crystals of MgO and TiO

(rutile). Surface defects

or speciﬁ c surface cages have been suggested as possible sites for stabilizing the

Au clusters [62, 63] .

10.3

Selective Oxidation of Carbon Monoxide ( CO )

10.3.1

Gold Catalysts

During the 1980s, Haruta et al . [66] found that gold nanoparticles, when supported

on α - Fe

, were highly active in the oxidation of CO, and especially at very

low temperatures, although this surprisingly high activity was not replicated

by other metals (Figure 10.1 ). In a later series of investigations conducted by the

same group [52] , Au/TiO

was found to be an equally effective catalyst, and this

in turn led to extensive studies of gold nanocatalysts supported on a variety of

metal oxides.

338 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Figure 10.1 CO conversion over various

catalysts as a function of temperature. Curve

1, Au/ α - Fe

(Au/Fe = 1/19, coprecipitation,

400 ° C); Curve 2, 0.5 wt% Pd/ γ - Al

(impregnation, 300 ° C); Curve 3, ﬁ ne Au

powder; Curve 4, Co

(carbonate, 400 ° C);

Curve 5, NiO (hydrate, 200 ° C); Curve 6,

α - Fe

(hydrate, 400 ° C); Curve 7, 5 wt%

Au/ α - Fe

(impregnation, 200 ° C); Curve 8,

5 wt% Au/ γ - Al

(impregnation, 200 ° C).

Reproduced with permission from Ref. [66] ;

Owing to the possible applications of polymer electrolyte fuel cells to automo-

biles and also to residential electricity - heat delivery systems, the low - temperature

water - gas - shift reaction continues to attract renewed interest. When compared to

commercial catalysts that are based on Ni or Cu and operated at 900 K or 600 K,

respectively, supported Au catalysts appear to have a clear operational advantage

in that they function at temperatures as low as 473 K [67] . During the course of

investigating the hydrogenation of CO

over supported Au catalysts, it was found

that Au/TiO

was selective towards the formation of CO, in that the reverse water -

gas - shift reaction could be conducted at a temperature as low as 473 K [67] . Later,

Au/TiO

was conﬁ rmed also to be active for the water - gas - shift reaction [68] .

The oxidation of CO is a typical reaction for which Au catalysts are extraordinar-

ily active at room temperature, and indeed are much more active than other noble

metal catalysts at temperatures below 400 K. One focal point of recent studies has

been the elaboration of the mechanism for CO oxidation [69, 70] . Although the

available data on this topic are vast – and occasionally contradictory – several pieces

of information were identiﬁ ed that were critical to developing an understanding

of the mechanism. For example, active catalysts always contain metallic Au parti-

cles which produce a CO absorption band at 2112 cm

− 1

, whereas oxidic Au species

that produce a CO absorption band at 2151 cm

− 1

are not responsible for steady -

state, high catalytic activity [71] . However, as the smooth surfaces of metallic Au

do not adsorb CO at room temperature [72] , this indicates that CO is adsorbed

only on steps, edges, and corner sites. As a consequence, the smaller metallic Au

particles are preferable [73] .

A theoretical calculation [74] has been used to explain why the smooth surface

of Au is noble in the dissociative adsorption of hydrogen. However, when Au is

deposited as nanoparticles on metal oxides by means of coprecipitation and DP

techniques, it exhibits a surprisingly high catalytic activity for CO oxidation at a

10.3 Selective Oxidation of Carbon Monoxide (CO) 339

Figure 10.2 High - resolution transmission

electron microscopy images of the 2.8% Au/

CeO

sample. (a) The white lines correspond

to the (202) Ce

O11 (3.3 Å ) and the (200)

CeO

(2.7 Å ) lattice spacing; (b) A hexagonal

faceted (111) Au crystal is indicated.

Reproduced with permission from Ref. [75] ;

KGaA, Weinheim.

temperature as low as 200 K [12, 52] . During the 1990s, this ﬁ nding led to many

research groups conducting extensive investigations into the catalysis of Au.

One remarkable study among many was conducted by Corma and coworkers [75] ,

who showed that gold nanoparticles supported on nanocrystalline CeO

, in conjunc-

tion with the DP method, proved to be a very active catalyst for CO oxidation (Figure

10.2 ). Indeed, the catalysts were found to be an order of magnitude more active for

CO oxidation than comparable catalysts prepared using a non - nanocrystalline

support. The Au/CeO

catalyst also showed excellent selectivity for CO oxidation in

the presence of H

at 60 ° C (close to the operating temperature of fuel cell), where

the selectivity of normal Au active catalysts would be negatively affected [76] .

Most gold catalysts which have been reported as active for CO oxidation were

prepared using the DP method, which provides catalysts with a strong interaction

between the gold and the metal oxide matrix. However, a major drawback of this

method is that it cannot be used to deposit gold at metal oxides with an isoelectric

point ( IEP ) < 5, such as SiO

. Subsequently, Sheng Dai and coworkers [65] success-

fully deposited gold at the surface of mesoporous SiO

by using the DP method

following a sol – gel surface modiﬁ cation with TiO

. The results showed the gold

nanoparticles (0.8 – 1.0 nm) in the mesopores to be highly active in terms of CO

oxidation (Figure 10.3 ).

Today, whilst it is widely recognized that these supported gold nanocatalysts

show a high activity in the low - temperature oxidation of CO, many questions

remain unanswered concerning the relatively simple reaction of CO oxidation. The

most notable of these are “ What is the reaction mechanism? ” , and “ What is the

nature of the active site? ”

340 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Figure 10.3 (a) Scheme of preparation of

gold/mesoporous material catalysts; (b)

Z - contrast TEM image of ultrasmall gold

nanoparticles on ordered mesoporous

materials. The bright spots (0.8 – 1.0 nm)

correspond to gold nanoparticles. Reproduced

Publications, Washington.

A substantial proportion of the research into CO oxidation catalyzed by sup-

ported gold has been motivated by the goal of identifying those catalyst properties

which affect the activity. In this respect, two early classes of observation were

important in determining the approaches used recently to investigate supported

gold catalysts: (i) that various preparation routes lead to catalysts with different

activities [52, 77] ; and (ii) that catalysts consisting of gold supported on reducible

metal oxides (e.g., Fe

, CeO

, TiO

) are typically more active than those

10.3 Selective Oxidation of Carbon Monoxide (CO) 341

supported on nonreducible metal oxides (e.g., γ - Al

, MgO, SiO

). Such observa-

tions led to a wide acceptance of the inferences that the preparation method

inﬂ uenced activity [12] , and that the support played a role in the catalysis.

Bond and Thompson [77] , in their review of the literature which extended to

the year 2000, and in an attempt to reconcile some apparently contradictory

hypotheses, proposed a mechanism for CO oxidation that was catalyzed by

supported gold (Scheme 10.1 ). The proposed active site consisted of nanoparticles

incorporating both zero - valent and cationic gold, with the latter positioned at

the metal – support interface. The suggestion by Bond and Thompson of the pres-

ence of cationic gold was based on observations by various authors of ν CO infrared

(IR) bands that were characteristic of CO bonded to cationic gold. However,

evidence was lacking not only of any such species in working catalysts, but also

of the suggestion that cationic gold was a “ glue ” which held the nanoclusters

to the support.

In 2002, Haruta [78] presented a review of the literature and proposed, on

the basis of measurements of the kinetics of CO oxidation catalyzed by supported

gold, that there were three temperature regions, each with different kinetics and

activation energies of the CO oxidation reaction. Haruta suggested that, at tem-

peratures below 200 K, the reaction catalyzed by Au/TiO

took place at the surfaces

of small gold nanoparticles dispersed on the support, but at temperatures above

300 K the reaction occurred at gold atoms at the perimeter sites of the supported

gold nanoparticles.

Scheme 10.1 Schematic representation of an active site and

possible reaction mechanism for CO oxidation catalyzed by

supported gold. Reproduced with permission from Ref. [77] ;

342 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

10.3.2

Silver Catalysts

Silver catalysts also have a relatively high activity for the selective oxidation of

CO at low temperatures. A silver catalyst was deactivated remarkably following

pretreatment in H

at high temperatures, but could be reactivated by treatment in

oxygen at similarly high temperatures. Interestingly, these changes in activities

were mostly reversible. The structures of the silver particles were seen to experi-

ence massive changes during the course of various pretreatments, and the exist-

ence of subsurface oxygen resulting from an oxygen treatment at high temperatures

was shown to be crucial for high selectivity and activity in CO selective oxidation

[79, 80] . As CO oxidation is generally claimed to be a structure - sensitive reaction,

restructuring of the silver particles is likely to exert an inﬂ uence on the activity of

the catalyst. Yang and Aoyama [81, 82] studied the thermal stability of uniform

silver clusters supported on oxidized silicon or aluminum surfaces in both oxidiz-

ing and reducing atmospheres, and found the thermal stability of the silver

clusters to be signiﬁ cantly lowered under oxidizing conditions. Moreover, heating

above 350 ° C under oxidizing condition could induce a migration of the

silver clusters.

Size selectivity in catalysis was reported for propylene partial oxidation and low -

temperature CO - oxidation, with Ag nanoparticles of < 5 nm diameter being shown

to have equal activity as the Au nanoparticles. In contrast, for ethylene epoxidation

only those Ag particles > 30 nm could catalyze the reaction [83] . Recently, much

attention has been focused on the use of spherical or undetermined - shape nano-

particles for catalyzing reactions. Very few studies have been undertaken in which

catalysis was conducted with nanoparticles of known shapes [84] , for example,

using truncated octahedral Pt nanoparticles to catalyze the electron - transfer reac-

tion, and cubic Pt nanoparticles in the decomposition of the oxalate capping agent.

The formation of different oxygen species, depending on the Ag particle size sput-

tered on the highly ordered pyrolytic graphite ( HOPG ) surface, resulted in a vari-

ation in catalytic activity of CO oxidation using oxygen under ( ultra - high vacuum )

UHV conditions revealed CO oxidation to be sensitive towards the size of particle

[85] . The oxygen uptake of a smaller Ag nanoparticle was seen to be signiﬁ cantly

higher than that of a larger particle and a bulk - like Ag which enhanced the reactiv-

ity of CO oxidation.

10.3.3

Gold – Silver Alloy Catalysts

The recent progress in polymer electrolyte membrane fuel cells has particularly

motivated the search for a highly efﬁ cient catalyst for CO selective oxidation at low

temperatures. Thus, combinations of metals in the forms of alloys, core – shell and

“ decorated ” surfaces (Pt, Pd, Rh, Ru, Au, Ag, Cu, Co, Fe, In, Ga) with different

supports, such as zeolite, Al

, SiO

, and activated carbon, have produced active

catalysts for the CO reaction. One such alternative catalyst, namely gold – silver

10.3 Selective Oxidation of Carbon Monoxide (CO) 343

Figure 10.4 CO conversion over reaction temperatures at

various molar ratios of Au : Ag. 䉱 , ratio 3 : 1;

䊏

, ratio 1 : 1;

䊉

ratio 1 : 0;

䉲

, ratio 0 : 1. Reproduced with permission from

alloy nanoparticles deposited on MCM - 41, demonstrated an exceptionally high

catalytic activity which was comparable to the most active catalysts (Figure 10.4 )

such as Au/TiO

and Au/Fe

[86, 87] . The alloying of Au and Ag showed a strong

synergistic effect in promoting the low - temperature oxidation of CO [88] . The alloy

catalyst activation was shown to depend on the composition (the Ag ratio was

crucial), the aluminum content in the support, and the pretreatment conditions.

10.3.4

Copper Catalysts

Copper nanoparticles are also active for the selective oxidation of CO. The majority

of studies with copper particles have been performed with ﬁ nely dispersed copper

on various supports, and have demonstrated high catalytic activities for CO oxida-

tion. Likewise, CuO mixed with ZnO or with CeO

have also shown promise as

catalysts. The results of a recent density functional theory ( DFT ) study showed that

gold and copper had a lower barrier for CO oxidation than for H

oxidation.

Ceria has a promoting effect on the activity of the Au/Al

catalyst in CO oxida-

tion [89] . The addition of Li

O and/or CeOx to copper, silver, and gold catalysts of

3 nm size on γ - Al

for the preferential oxidation of CO in a hydrogen atmosphere

[90] , have shown signiﬁ cant changes in the conversion. The nanoscale metal par-

ticles or metal complexes in polymer matrices show quite interesting chemical

and catalytic reactivity towards a variety of small gas molecules under relatively

mild conditions that differ from those of the corresponding free transition metal

complexes, or from those in inorganic oxide - supported systems. The incorporation

of copper nanoparticles into cellulose acetate, and the subsequent oxidation of

small gas molecules (e.g., CO, H

, D

, O

, NO, and oleﬁ ns) over a temperature

range of 25 to 160 ° C, has also been examined [91] . Nanoparticles of various sizes

prepared by different routes and hosted in the channels of SBA - 15, exhibited a

344 10 Oxidation Catalysis by Nanoscale Gold, Silver, and Copper

Figure 10.5 CO conversions versus reaction temperature over

Cu/SBA - 15 (post grafted) calcined at 500 ° C and reduced at

different temperatures. Reproduced with permission from

high catalytic activity for CO oxidation, with complete conversion at 190 ° C

(Figure 10.5 ) [92] . Such high catalytic activity was mainly inﬂ uenced by the size

and dispersion of the Cu particles.

10.4

Epoxidation Reactions

10.4.1

Gold Catalysts

Since the ﬁ rst recognition by Hayashi [93] that Au supported on TiO

could

catalyze the epoxidation of propylene in the gas phase containing O

and H

, the

catalytic properties of Au/TiO

and related systems have attracted interest not only

from the chemical industries but also from academia. Today, propylene oxide ( PO )

is recognized as one of the world ’ s most important bulk chemicals, and is used

in the production of polyurethane and polyols. The current industrial processes

utilize two - staged chemical reactions, using either Cl

or organic peroxides to yield

the byproducts stoichiometrically.

From both environmental and economic points of view, the direct synthesis of

PO by using molecular oxygen has long been a major academic challenge, although

supported noble metal catalysts such as Ag/carbonates and/or titanates, Pd/TS - 1,

Pd – Pt/TS - 1 [94 – 96] , and Au/TiO

(Figure 10.6 ) [93, 97] , have each been reported

to be active in this process.

In 1998, using the DP technique, Haruta and coworkers [97] produced nanoscale

gold catalysts that showed a high activity towards CO oxidation, based on the