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

cles embedded. To ensure these nanoparticles are inside the polymer spheres

instead of on the surface of the spheres, the specimen should be rotated and be

examined by TEM from different angles. HRTEM images of individual nanopar-

ticles can be obtained as shown in the inset of Figure 10.20 . In these nanocrystal-

lites, twin defect superstructures are often observed.

Nanoparticles often aggregate to form large clusters. An interesting phenome-

non is oriented aggregation, in which all the nanoparticles connect to each other

with exactly the same crystal orientation. When the early stage crystal growth of

zeolite analcime was investigated, it was found that the crystals grew from the

seeds up to some nanoplatelets with a size of about 20 nm in diameter. The growth

orientation of the nanoplatelets with the short axis along the [111] zone axis was

observed by SAED. These [111] - oriented nanoplatelets stack to form aligned discus -

shaped aggregates via an oriented aggregation mechanism [115] . In the aggregates,

boundaries of the nanoplatelets can be seen clearly with light contrast lines and

each discus may contain at least a few hundreds nanoplatelets as shown in Figure

10.21 . The corresponding SAED pattern shows a single crystal - like structure,

which can be further conﬁ rmed from HRTEM images [115] .

10.8.2

Nanorods, Nanowires and Nanobelts of Oxides

When nanocrystals grow with a preferential crystal orientation, nanorods may

be produced. Umar and coworkers demonstrated the synthesis of ZnO

nanonails [116] . For this type of nanocrystallite, HRTEM was used to determine

the crystal structure, the single crystal state for individual nanonails, the particle

morphology and also the long axis, that is, growth along the [0001] direction of

the wurtzite hexagonal - phase of ZnO. This material is of interest as a hydrazine

10.8 Low-Dimensional Oxide Crystals 477

Figure 10.20 TEM image of polymer nanspheres embedded

with both CdSe/ZnS QDs and nano - γ - Fe

2

O

3

. The inset is

HRTEM of a Fe

2

O

3

nanoparticle showing the (222) fringes (G)

with d = 0.24 nm and two d - spacings related to

super structures, d = 0.76 nm (H) and d = 2.71 nm (I).

478 10 Transmission Electron Microscopy

electrochemical sensor. It is not surprising to see the direction of growth of a

hexagonal phase along its [0001] zone axis. Zeolite analcime crystals have a per-

fectly icositetrahedral morphology arising from their cubic structure. The discov-

ery of a nanorod intermediate phase during the early stage of crystal growth was

very surprising [115] . All the nanorods grew along the [111] direction but the

reason for this has yet to be investigated. It is obvious that the ﬁ nal morphology

of the nanorods is affected by the synthetic conditions. Even for ZnO nanorods,

Yang and Wang reported a transverse growth, forming interpenetrative nanorods.

The ends of the ZnO nanorods were bullet shaped [117] .

Using a thermal vapor deposition method, WO

3 − x

nanorods were produced [118] .

These nanorods grew along the [001] axis and were believe to contain a large

number of stacking faults in the (200) planes as shown by HRTEM images.

Nanorods of oxide can also be made using a template. In this case, the morphol-

ogy of the nanorods would depend on the shape of the template. For example,

WO

3

can grow in mesoporous silica MCM - 41 which consists of an array of cylin-

drical pores without interconnections [119] . The growing orientation of the nano-

rods is variable.

When nanorods grow to further increase their length, nanowires are produced.

The surface - to - volume ratio increases greatly. The materials are very useful in

semiconducting nanodevices and catalysis. In

2

O

3

nanowires were prepared as a

gas - sensing material by Vomiero and coworkers [120] . The growing direction, the

[100] direction of its cubic structure, was determined by SAED and HRTEM

imaging. At low magniﬁ cation, TEM images of nanowires often show some dark

contrast features owing to bending of the nanowires. These dark features can move

along the wire when the specimen is tilted with a small angle. The HRTEM images

of In

2

O

3

presented by Vomiero [120] showed uniform fringe contrast, indicating

Figure 10.21 TEM image of oriented aggregation of

nanoplatelets of zeolite analcime. The inset (top) is the

corresponding SAED pattern. Another inset (bottom) is TEM

image of a single nanoplatelet.

a constant specimen thickness across the nanowires on the projecting direction.

This property reﬂ ected a regular (non - cylindrical) cross - section of the nanowires

and allowed the scientists to carry out convergent beam electron diffraction, from

which the structural symmetry of In

2

O

3

can be determined.

Many oxide nanowires have been characterized by HRTEM, such as very long

single crystalline β - Ga

2

O

3

nanowires with a zigzag surface termination [121] , a

large - scale fabrication of single crystalline Cu

2

O nanowires [122] , Er - doped poly-

crystalline SnO

2

nanowires [123] , and anodized aluminum oxide - templated single

crystalline TiO

2

nanowires [124] .

Nano - scale ZnO is one of the most popular nanomaterials investigated in the

last few years. It is partly because ZnO can form many different forms of nano-

structure, such as nanoparticles, nanonails, nanocombs, nanorings, nanosprings,

nanobows, nanobelts, nanocages and nanowires, and partly because of its interest-

ing physical properties [125] . The most attractive and beautiful example of HRTEM

investigation of nano - scale ZnO is the study of ZnO nanobelts, ﬁ rst demonstrated

by Wang and coworkers [126] . ZnO has a wurtzite structure, which is hexagonal

(space group C

6 mc

) with unit cell dimensions a = 0.3296 and c = 0.520 65 nm. Owing

to its polar surfaces, containing positively charged Zn - (0001) and negatively

charged

O- 000 1

()

surfaces, a ZnO nanobelt can form nanorings when the short-

est dimension is along the [0001] axis as shown in Figure 10.22 a – c [127, 128] . A

very detailed formation mechanism of this peculiar morphology has been reported.

A core – shell structure in ZnO nanobelts has also been observed and investigated

using HRTEM (Figure 10.22 d and e) [127, 129] .

10.8.3

Oxide Nanotubes

Many metal oxides have been made in the form of nanotubes, but most of them

have an amorphous wall. SEM is normally used to characterize these materials.

For some crystalline oxide nanotubes, HRTEM is the only technique to determine

their structures. Titania - related nanotubular material is a good example.

Any crystalline TiO

2

treated in a high - concentration NaOH aqueous solution at

130 ° C for a few days can undergo a phase transformation to nanotubes [130, 131] .

However, a later HRTEM study revealed that the structure is derived from H

2

Ti

3

O

7

instead of TiO

2

[132] . Figure 10.23 a is a HRTEM image of a nanotube viewed down

a proﬁ le direction. In addition to the single crystalline wall, it was found that the

numbers of the layers on two sides are different, three on the top side and four

on the bottom side. The cross - section image in Figure 10.23 b reveals that, unlike

carbon nanotubes, the nanotubes form from scrolling up a single sheet of titanium

oxide. Further studies with computer image simulations indicated that the struc-

ture is derived from H - form titanium oxide, H

2

Ti

3

O

7

, which consist of [Ti

3

O

7

]

2 −

sheets separated by some protons. The formation mechanism of the nanotubes

from scrolling up [Ti

3

O

7

]

2 −

layer by layer on the [001] surface of H

2

Ti

3

O

7

have been

conﬁ rmed by HRTEM examination of several specimens with different reaction

times and extensive ab initio calculations of the surface tension, elastic strain

10.8 Low-Dimensional Oxide Crystals 479

480 10 Transmission Electron Microscopy

energy, interlayer coupling energy and Coulomb force [133] . Because HRTEM is

unable to detect protons in the structures, the presence of protons in the above -

mentioned nanotubes was proved by solid - state NMR spectroscopy and measure-

ment of proton conductivity [134] .

It is proposed that the principal of exfoliating layered compounds into nano-

tubes can be used for many layered oxides. Potassium hexaniobate (K

4

Nb

6

O

17

) is

one such compound, consisting of [Nb

6

O

17

]

4 −

layers separated by K

+

cations. Using

the intercalating and exfoliating methods, the [Nb

6

O

17

]

4 −

layers can be peeled off

from the crystal layer by layer to form nanotubes. The interlayer distance in the

wall of the nanotubes is variable from 0.83 to 3.6 nm when different intercalating

molecules such as tetra - n - butylammonium hydroxide (TBA

+

OH

−

) and alkylamines

(C

n

H

2 n +1

NH

2

) with different lengths of alkyl chain. The HRTEM observations

indicated that the organic molecules stayed in the interlayer space. HRTEM also

indicated that the number of layers in the wall was in the range of 3 to 8 and the

rolling direction was around the [100] zone axis of K

4

Nb

6

O

17

, while a helical angle

larger than 10 ° was observed for some nanotubes [135, 136] .

Figure 10.22 (a) Seamless single - crystal

nanorings of ZnO; (b) Bright - ﬁ eld and

(c) dark - ﬁ eld TEM images of a nanoring.

The inset in (c) is an enlargement of a local

region, displaying the contrast produced by

the uni - radius, loop - by - loop, self - coiling

nanobelt. (d) A low magniﬁ cation TEM image

of a Zn – ZnO core – shell heteronanobelt.

(e) A [0001] high resolution TEM image

recorded near the right - hand edge of a nano-

belt, showing the Zn – ZnO overlap region

and the ZnO wall. The corresponding electron

diffraction pattern is given below (d) [127] .

10.9

Summary

In summary, as demonstrated within this chapter, HRTEM and associated tech-

niques are very powerful in the study of the microstructures of oxides and oxide

catalysts. The detailed characterization of superstructures, surface structures and

defects enriches our knowledge of the structures of oxides and their structure -

dependent physico - chemical properties. For characterizing nano - scale oxides,

HRTEM plays a particularly important and unique role. Further exciting develop-

ments resulting from this fascinating area can be anticipated.

Figure 10.23 (a) HRTEM image of a H

2

Ti

3

O

7

nanotube

showing three layers on one side and four on another side.

(b) HRTEM image of the cross - section of a nanotube.

(c) Schematic model showing the effect of the alkaline

environment and cleavage of the surface layer due to

hydrogen deﬁ ciency on the surface. (d) Three - dimensional

model of the H

2

Ti

3

O

7

nanotubes.

10.9 Summary 481

482 10 Transmission Electron Microscopy

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Oxidation Reactions over Supported Metal Oxide Catalysts:

Molecular/Electronic Structure – Activity/Selectivity

Relationships

Israel E. Wachs and Taejin Kim

487

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves

ISBN: 978-3-527-31815-5

11

11.1

Overview

Supported vanadium oxide catalysts ﬁ nd wide application as oxidation catalysts

and are the focus of this chapter. The highly dispersed nature of the surface vana-

dium oxide phases on oxide supports allows their detailed characterization and

the establishment of molecular/electronic structure – activity/selectivity relation-

ships. The surface vanadium oxide species exist on oxide supports under dehy-

drated conditions as isolated and polymeric surface VO

4

species with one terminal

V

=

O bond and three bridging V

–

O

–

M bonds, in which M represents either an

adjacent V or an oxide support cation. Fundamental oxidation studies with the

CH

3

OH chemical probe molecule over the well deﬁ ned supported vanadium oxide

catalysts show that the bridging V

–

O

–

V and terminal V

=

O bonds do not play a

critical role in this redox reaction. However, kinetic studies reveal that the bridging

V

–

O – Support bond dominates the overall reaction by controlling the number

of chemisorbed methanol molecules present on the catalyst surface during

steady - state reaction conditions. The number of chemisorbed methanol molecules

is determined by the electron density on the oxygen atom in the bridging

V

–

O – Support bond, which increases with decreasing electronegativity of the

support cation. This general trend is also observed for other oxidation reactions

involving one surface VO

4

site. For oxidation reactions involving multiple redox

or redox – acidic sites, both the speciﬁ c oxide support, via the bridging V

–

O –

Support bond, and the surface density of catalytic active sites control the surface

intermediate coverage during steady - state reaction. These observations are out-

lined in more detail in this chapter.

11.2

Introduction

Supported metal oxide catalysts consist of dispersed surface metal oxide species,

the catalytic active sites, which are supported on high - surface - area oxides [1 – 3] . The

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

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