Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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16.3 Synthesis 619

16.2.4

Redox Property of Metal Oxides

In some catalytic reactions, metal oxides often undergo reduction and re - oxidation

simultaneously by loss and gain of surface lattice oxygen to and from the gas

phase. This phenomenon is called redox catalysis. The redox property as well as

the acidic and basic nature are the most important properties of metal oxide

catalysis. It is well known that some simple metal oxides such as V

, CoO

, NiO,

MnO

, CeO

, MgO and some mixed metal oxides have redox properties. Mars and

van Krevelen ﬁ rst proposed a redox mechanism to describe oxidation of com-

pounds over oxide catalysts [29] . The Mars – van Krevelen mechanism ( MvK ) is now

commonly accepted. When an adsorbate is oxidized at the surface, the oxidant is

often a surface lattice oxygen atom, thus creating a surface oxygen vacancy. Surface

oxygen vacancies are proposed to participate in many chemical reactions catalyzed

by metal oxides. Vacancies also bind adsorbates more strongly than normal oxide

sites and assist in their dissociation. The redox property of a metal oxide catalyst

can be characterized by using the techniques of temperature - programmed

reduction/temperature - programmed oxidation ( TPR/TPO ).

16.3

Synthesis

The development of synthetic methods is one of the fundamental aspects to the

understanding and development of nano - scale materials. The novel properties and

numerous applications of nano - scale materials have encouraged many researchers

to invent and explore preparation methods that allow control over such parameters

as particle size, shape, size distributions and composition. While considerable

progress has taken place, one of the major challenges is the development of a

“ synthetic toolbox ” which would afford access to size and shape control of struc-

tures on the nano - scale and conversely allow scientists to study the effects these

parameters impart to the chemical and physical properties of the nanoparticles.

The two principal approaches to the preparation of nano - scale materials are

“ bottom - up ” and “ top - down ” . While “ top - down ” preparations involve approaching

the nano - scale by breaking down larger starting materials, the “ bottom - up ” prepa-

ration methods are of primary interest to chemists and materials scientists because

the fundamental building blocks are atoms or molecules, and these methods will

be the focus of this chapter. Among the most sought after goals of synthetic chem-

ists is gaining control over the way these fundamental building blocks come

together and form particles. Interest in “ bottom - up ” approaches to nano - scale

oxides and other materials is clearly indicated by the number of reports and

reviews on this subject [30 – 46] . There are, of course, numerous “ bottom - up ”

approaches to the preparation of nano - scale materials, and metal oxides are no

exception. Gas – solid (wet chemical) and liquid – solid (physical) transformations

620 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

are two different approaches to synthesizing nanomaterials by “ bottom - up ” prepa-

ration methods. Several physical aerosol methods have been reported for the

synthesis of nano - size particles of oxide materials including gas condensation

techniques [47 – 53] , spray pyrolysis [51, 54 – 60] , thermochemical decomposition of

metal – organic precursors in ﬂ ame reactors [53, 61 – 63] and other aerosol processes

named after the energy sources applied to provide the high temperatures during

gas – particle conversion. The sol – gel process is by far the most common and widely

used “ bottom - up ” wet chemical method for the preparation of nano - scale oxides.

Other wet chemistry methods including novel micro - emulsion techniques, oxida-

tion of metal colloids and precipitation from solutions have also been used.

The methods of sample preparation are critical as they determine the morphol-

ogy of the resulting material [64] . For example, burning Mg in O

(MgO smoke)

yields 40 – 80 nm cubes and hexagonal plates, while thermal decomposition of

Mg(OH)

, MgCO

and especially Mg(NO

)

yields irregular shapes often exhibiting

hexagonal platelets. Surface areas can range from 10 m

− 1

(MgO smoke) to

250 m

− 1

for Mg(OH)

thermal decomposition, but surface areas of about

150 m

− 1

are typical. In the case of calcium oxide, surface areas can range from

1 to 100 m

− 1

when prepared by analogous methods, but about 50 m

− 1

is typical.

In the following discussion, the most common methods for the synthesis of metal

oxides will be the focus.

16.3.1

Sol – Gel Technique

Sol – gel processing describes a type of solid materials synthesis procedure, per-

formed in a liquid and at low temperature (typically T < 100 ° C). The development

of sol – gel techniques has long been known for preparations of metal oxides and

has been described many times [30 – 38, 40 – 46, 65] . The process is typically used

to prepare metal oxides via the hydrolysis of reactive precursors, usually alkoxides

in an alcoholic solution, resulting in the corresponding hydroxide. It is usually

easy to maintain such hydroxide in a dispersed state in the solvent. Condensation

of the hydroxide molecules with loss of water leads to the formation of a network.

When hydroxide species undergo polymerization by condensation of the hydroxy

Figure 16.2 The sol – gel process: (a) sol, (b) gel.

16.3 Synthesis 621

Figure 16.3 A ﬂ ow chart of a typical sol – gel process for preparing

nano - scale metal oxide powder.

network, gelation is achieved and a dense porous gel is obtained. The gel is a

polymer with a three - dimensional skeleton surrounding interconnected pores,

and the gels that are obtained are termed colloidal gels (Figure 16.2 b). Removal

of the solvents and appropriate drying of the gel are important steps that result

in an ultra - ﬁ ne powder of the metal hydroxide. Heat treatment of the hydroxide

is a ﬁ nal step that leads to the corresponding ultra - ﬁ ne powder of the metal oxide.

Depending upon the heat treatment procedure, the ﬁ nal product may be in the

form of a nano - scale powder, bulk material or oxygen - deﬁ cient metal oxide. A

ﬂ ow chart of typical sol – gel processing of nano - scale metal oxides is shown in

Figure 16.3 .

The chemical and physical properties of the ﬁ nal product are primarily deter-

mined by the hydrolysis and drying steps.

622 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

16.3.1.1 Hydrolysis and Condensation of Metal Alkoxides

The initial species used in sol – gel processing are metal alkoxides (M(OR)

). Hydro-

lysis of metal alkoxides involves nucleophilic reactions with water as follows:

M OR H O M OR OH ROH

()

+↔

()()

−yyx

The mechanism of this reaction involves the addition of a negatively charged

δ −

group to the positively charged metal center (M

δ +

). The positively charged

proton is then transferred to an alkoxy group followed by the removal of ROH.

M + ROH

¦Ä-

+ M

¦Ä+

¦Ä-

M + ROH

Condensation occurs when the hydroxide molecules bind together as they

release water molecules and a gel/network of the hydroxide is obtained as shown

below:

MOH

MOMOH

H++

The rates at which hydrolysis and condensation take place are important param-

eters that affect the properties of the ﬁ nal product, as slower and more controlled

hydrolysis typically leads to smaller particle sizes and more unique properties.

Hydrolysis and condensation rates have been found to depend on the electronega-

tivity of the metal atom, the alkoxy group, the solvent system and the molecular

structure of the metal alkoxide. Those metals with higher electronegativities

undergo hydrolysis more slowly than those with lower electronegativities. For

example, the hydrolysis rate of Ti(OEt)

is about ﬁ ve orders of magnitude greater

than that of Si(OEt)

. Hence, the gelation times of silicon alkoxides are much

longer (on the order of days) than those of titanium alkoxides (few seconds or

minutes) [55] . The sensitivity of metal alkoxides toward hydrolysis decreases as the

OR group size increases, with smaller OR groups leading to higher reactivity of

the corresponding alkoxide toward water, in some cases resulting in uncontrolled

precipitation of the hydroxide.

The choice of solvents in sol – gel processes is very important because alcohol

interchange reactions are possible. As an example, when silica gel was prepared

from Si(OMe)

and heated to 600 ° C the surface area was 300 m

− 1

with a mean

pore diameter of 29 Å when ethanol was used as a solvent. However, when metha-

nol was used, the surface area dropped to 170 m

− 1

and the mean pore diameter

increased to 36 Å [32] . The rate of hydrolysis also becomes slower as the coordina-

tion number around the metal center in the alkoxide increases. Therefore, alkox-

ides that tend to form oligomers usually show slower rates of hydrolysis and,

16.3 Synthesis 623

hence, are easier to control and handle. n - Butoxide (O - n - Bu) is often preferred as

a precursor to different oxides including TiO

and Al

, because it is the largest

alkoxy group that does not prevent oligomerization [33] .

Because most metal alkoxides are highly reactive toward water, careful handling

in dry atmospheres is required to avoid rapid hydrolysis and uncontrolled precipi-

tation. For alkoxides that have low rates of hydrolysis, acid or base catalysts can

be used to enhance the process. The relatively negative alkoxides are protonated

by acids, creating a better leaving group and eliminating the need for proton

transfer in the transition state. Alternatively, bases provide better nucleophiles

(OH

−

) for hydrolysis; however, deprotonation of metal hydroxide groups enhances

their condensation rates. In the case of highly reactive compounds, controlling the

hydrolysis ratio may necessitate the use of non - aqueous solvents, where hydrolysis

is controlled by strict control of water in the system rather than by acids or bases.

Klabunde and coworkers have demonstrated the effectiveness of this approach in

the preparation of gels from Mg(OEt)

in methanol and methanol – toluene solvents

[66] .

16.3.1.2 Solvent Removal and Drying

Developments in the areas of solvent removal and drying have further facilitated

the production of nano - scale metal oxides with novel properties. When drying is

achieved by solvent evaporation at ambient pressure with moderate shrinkage, the

gel network shrinks as a result of capillary pressure, and the hydroxide product

obtained is referred to as xerogel. However, if supercritical drying is applied using

a high - pressure autoclave reactor at temperatures higher than the critical tempera-

tures of solvents, less shrinkage of the gel network occurs, as there is no capillary

pressure and no liquid – vapor interface, which allows the pore structure to remain

largely intact by avoiding the pore collapse phenomenon. In practice, supercritical

drying consists of heating the wet gel in a closed container, so that the pressure

and temperature exceeds the critical temperature, T

, and critical pressure, P

, of

the liquid entrapped in the pores inside the gel. The critical conditions are very

different depending on the ﬂ uid which impregnates the wet gel. A few values are

given in Table 16.1 [67] . The hydroxide product obtained in this manner, which is

the traditional drying technique, is referred to as an aerogel and is the origin of

the label “ aerogel ” . Aerogel powders usually demonstrate higher porosities and

larger surface areas than analogous xerogel powders. Aerogel processing has been

very useful in producing highly divided powders of different metal oxides [28, 68,

69] (Figures 16.4 – 16.6 ).

Sol – gel processes have several advantages over other techniques for the synthe-

sis of nano - scale metal oxides. Because the process begins with a relatively homo-

geneous mixture, the resulting product is generally a uniform ultra - ﬁ ne porous

powder. Sol – gel processing also has the advantage that it can be scaled up to

accommodate industrial - scale production.

Numerous metal oxide nanoparticles have been produced by making some

modiﬁ cations to the traditional AP method. One modiﬁ cation involved the

addition of large amounts of aromatic hydrocarbons to the alcohol – methoxide

624 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

Table 16.1 Critical point parameters of common ﬂ uids.

Fluid Formula T

( ° C) P

(MPa)

water H

O 374.1 22.04

carbon dioxide CO

31.0 7.37

Freon 116 (CF

)

19.7 2.97

acetone (CH

)

O 235.0 4.66

nitrous oxide N

O 36.4 7.24

methanol CH

OH 239.4 8.09

ethanol C

OH 243.0 6.3

Figure 16.4 TEM micrograph of the

nanostructure of CP - MgO. Note the absence

of porosity and that all of the nanocrystals

have agglomerated (from [68] ).

Figure 16.5 TEM micrograph of the

nanostructure of AP - MgO (supercritical

solvent removal). Porosity is formed by the

interconnected cubic nanocrystals of MgO

(from [28] ).

16.3 Synthesis 625

solutions before hydrolysis and alcogel formation. This was done in order to

further reduce the surface tension of the solvent mix and to facilitate solvent

removal during the alcogel – aerogel transformation [30, 64, 70] . The resulting

nanoparticles exhibited higher surface areas, smaller crystallite sizes and more

porosity for samples of MgO, CaO, TiO

and ZrO

[71, 72] .

Nano - structured MgO and CaO have been reported to be extremely effective for

the destructive adsorption of numerous environmental toxins and several chemi-

cal warfare agents. Figure 16.7 outlines brieﬂ y how nano - crystalline and micro -

crystalline MgO and CaO were prepared. For nano - crystalline MgO (AP - MgO)

surface areas ranged from 250 to 500 m

− 1

and for AP - CaO 100 – 160 m

− 1

. For

CP micro - crystalline MgO and CaO, the surface areas are 130 – 250 and 50 –

100 m

− 1

, respectively, whereas commercially available MgO and CaO had the

lowest surface areas. Richards and coworkers have reported a simple method for

preparing a sheet - like MgO with a thickness of less than 10 nm and, more interest-

ingly, exhibiting the highly ionic (111) facet as the major surface of the “ nanosheet ” .

In this system, a mixture of water and methanol was added slowly to hydrolyze a

magnesium methoxide/benzyl alcohol solution to produce the nanosheets. Theo-

retical studies suggest that water plays an important role as chemisorption of water

forms a hydroxyl surface which stabilizes the otherwise unstable (111) surface of

MgO [69] .

Mesoporous and spherical TiO

materials are interesting candidates for applica-

tions in catalysis, biomaterials, microelectronics, optoelectronics and photonics.

Zhang and coworkers have put forward a new method to synthesize mesoporous

TiO

as well as hollow spheres, using titanium butoxide, ethanol, citric acid, water

and ammonia [73] . The mesoporous solid and hollow spheres were formed with

the same reactants but adding them in a different order. The TiO

possessed a

mesoporous structure with the particle diameters of 200 – 300 nm for solid spheres

and 200 – 500 nm for hollow spheres. The average pore sizes and BET surface areas

of the mesoporous TiO

solid and hollow spheres are 6.8 and 7.0 nm and 162

Figure 16.6 TEM micrograph of MgO(111)

(supercritical solvent removal). Here the MgO

nanoplates possess the (111) surface on the

edges (from [69] ).

626 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

and 90 m

− 1

, respectively. Optical adsorption studies showed that the TiO

solid

and hollow spheres possess a direct band gap structure with the optical band gap

of 3.68 and 3.75 eV, respectively. The advantage of this route is that it does not use

surfactants or templates, as is more usual for synthesizing mesoporous materials.

In the proposed preparation mechanism (Figure 16.8 ), ammonium citrate forms

and plays a key role in the formation of the mesoporous solid [73] . The formation

of solid mesoporous TiO

or of hollow spheres was highly dependent on the extent

of TiO

condensation at the onset of ammonium citrate crystal growth. That is,

mesoporous solid spheres form when the TiO

condensation process takes place

simultaneously with the formation of ammonium citrate crystals; mesoporous

hollow spheres are produced when the nucleation and growth of ammonium

citrate crystals occurs before the TiO

condensation process.

The chemistry of transition metals differs from those systems previously dis-

cussed, and only a few transition metals exhibit metal – alkoxide chemistry ame-

nable to sol – gel synthesis. Cerda and coworkers prepared BaSnO

[74] by calcining

a gel formed between Ba(OH)

and K

SnO

at pH ≈ 11. The material possessed a

particle size of 200 – 500 nm and contained BaCO

as an impurity, which can be

eliminated by high temperature calcination. O ’ Brien and coworkers [75] synthe-

Figure 16.7 Schematic of the preparative scheme for nano -

crystalline MgO (CaO) labeled AP - MgO (CaO) and micro -

crystalline MgO (CaO) labeled CP - MgO (CaO).

16.3 Synthesis 627

sized samples of monodisperse nanoparticles of barium titanate with diameters

ranging from 6 to 12 nm by the sol – gel method. The technique was extended by

Meron and coworkers [76] to the synthesis of colloidal cobalt ferrite nanocrystals.

This synthesis involved the single - stage high - temperature hydrolysis of metal –

alkoxide precursors to obtain crystalline, uniform, organically coated nanoparti-

cles, which were well dispersed in an organic solvent. They were also able to form

Langmuir – Blodgett ﬁ lms consisting of a close packed nanocrystal monolayer. The

structural and magnetic properties of these nanocrystals were similar to those of

bulk Fe

with a very high coercivity at low temperatures. Other aerogel oxides,

such as V

[77, 78] MoO

[78] and MnO

[79, 80] , have also been prepared.

Nanocomposites of RuO

– TiO

[81] are of particular interest because of their

supercapacitor properties.

16.3.2

Co - precipitation Methods

One of the conventional syntheses to prepare nanoparticles is the precipitation of

sparingly soluble products from aqueous solutions, followed by thermal decomposi-

tion of those products to oxides [82 – 84] . This process involves dissolving a salt pre-

cursor, usually a chloride, oxychloride or nitrate, such as AlCl

to make Al

Y(NO

)

to make Y

and ZrOCl

to make ZrO

. The corresponding metal hydrox-

ides are usually formed and precipitated in water by the addition of a basic solution

such as sodium hydroxide or ammonia solution. The resulting chloride or nitrate

salts, such as NaCl or NH

Cl, are then washed away and the hydroxide is calcined

after ﬁ ltration and washing to obtain the ﬁ nal oxide powder. This method is useful

in preparing composites of different oxides by co - precipitation of the corresponding

Figure 16.8 Schematic illustration of the formation

mechanisms for mesoporous TiO

hollow and solid spheres

(from [73] ).

628 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

hydroxides in the same solution. One of the disadvantages of this method is the

difﬁ culty in controlling the particle sizes and size distribution. Very often, fast

(uncontrolled) precipitation takes place, resulting in large particles. In order to

overcome this shortcoming, some new co - precipitation methods have developed,

such as sonochemical co - precipitation and microwave - assisted co - precipitation.

16.3.2.1 Co - precipitation from Aqueous Solution at Low Temperature

The products of co - precipitation reactions are usually amorphous at or near room

temperature. It is difﬁ cult to determine experimentally whether the as - prepared

precursor is a single - phase solid solution or a multi - phase, nearly homogeneous

mixture of the constituent metal hydroxides, carbonates and oxides that react to

form a single phase mixed metal oxide when heated.

Many nano - particulate metal oxides are prepared by calcining hydroxide co -

precipitation products. As an example, the zirconia system is highlighted here

because of its interesting properties and applications. Zirconia has very interesting

properties as an acid – base catalyst, as a promoter for other catalysts and as an inert

support material [85, 86] . Sulfated ZrO

is of great interest on account of its high

activity as a solid acid catalyst in alkylation, while ZrO

has also been found to act

as a photocatalyst, owing to its n - type semiconductor nature.

The typical preparation consists of the calcination of a hydroxylated gel prepared

by hydrolysis of zirconium salts in various media [72, 87 – 94] . Amorphous zirconia

undergoes crystallization at around 450 ° C and hence its surface area decreases

dramatically at that temperature. At room temperature the stable crystalline phase

of zirconia is monoclinic while the tetragonal phase forms upon heating to 1100 –

1200 ° C For several applications, it is desirable to have the tetragonal phase with

a high surface area. However, preparation protocols are needed to obtain the

tetragonal phase at lower temperatures. Many researchers have tried to maintain

the high surface area ( HSA ) of zirconia by several means. Usually the ZrO

mixed with CaO, MgO, Y

, Cr

, or La

for stabilization of the tetragonal

phase at low temperature [95] . Bedilo and Klabunde [92, 93] have prepared HSA

sulfated zirconia by a supercritical drying technique and found that the resulting

material was active towards alkane isomerization reactions [92, 94] . Recently,

Chane - Ching and coworkers [96] reported a general method to prepare HSA

materials through the self - assembly of functionalized nanoparticles. This process

involves functionalizing the oxide nanoparticles with bifunctional organic anchors

such as aminocaproic acid and taurine. After the addition of a copolymer surfac-

tant, the functionalized nanoparticles will slowly self - assemble on the copolymer

chain through a second anchor site. Using this approach the authors prepared

several metal oxides, including CeO

, ZrO

and CeO

– Al(OH)

composites. The

method yielded ZrO

of surface area 180 m

− 1

after calcining at 500 ° C, 125 m

− 1

for CeO

and 180 m

− 1

for CeO

– Al(OH)

composites.

Table 16.2 [97 – 100] shows the literature data for zirconia obtained by different

processes and the resulting surface area obtained at different calcination tempera-

tures. Richards and coworkers [101] obtained stable ZrO

by the cetyltrimethylam-

monium chloride stabilization route and the data indicate that digesting the