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

other gases apart from the argon carrier were detected during the cool - down

period. However when 2% O

2

/Ar was passed over the sample at room temperature

butane, butenes and butadiene were evolved. This desorption process can be

viewed as an oxidative displacement, resulting in recombination of reaction inter-

mediates (hydrogen and alkyl, alkenyl and alkadienyl) from the surface of reduced

VO

x

species. The polynuclear aromatics were only removed when the catalyst was

subjected to a full regeneration involving high temperature oxidation [31] .

The regeneration of samples of V/ θ - Al

2

O

3

coked from low partial pressure

butane dehydrogenation via oxidation was also followed by UV Raman spectros-

copy as well as gas chromatography ( GC ). The Raman spectroscopy indicates that

the structure of the VO

x

species can be restored by oxidation of the coke deposits

up to 873 K. UV - Vis DRS, however, suggests that only high loaded samples are

fully restored.

51

V MAS NMR conﬁ rms the re - oxidation of reduced vanadium

species and, as such, spectra of the regenerated catalyst can be acquired (Gladden

and McGregor, unpublished results). Figure 15.4 b shows the spectrum of the

8 wt% catalyst after reaction at 973 K and regeneration at 873 K (note the peaks to

negative intensity correspond to Al atoms in the support). By comparison with

Figure 15.4 a it can be seen that the spectrum of the regenerated catalysts indi-

cates the presence of a greater quantity of ordered V

2

O

5

- like species than were

present in the as - prepared catalyst. However, one must take care as each tech-

nique has its “ blind spots ” and it is possible that it is the inability of each of

the techniques to “ see ” all of the vanadia species that leads to apparent differ-

ences. At low butane partial pressure, TPO quantiﬁ cation results showed that the

amount of coke formed in butane dehydrogenation follows the sequence: poly-

meric VO

x

> monomeric VO

x

> V

2

O

5

, Al

2

O

3

. However, at 1 bar butane pressure

the order is changed slightly with V

2

O

5

> polymeric VO

x

> monomeric VO

x

> Al

2

O

3

.

After regeneration the surface area of an 8%V catalyst was reduced by 50%, a

Figure 15.8 Desorption of butane at room temperature in a

ﬂ ow of 2% O

2

/Ar from vanadia/alumina catalysts after butane

dehydrogenation at 873 K and 1 bar.

15.5 Vanadia 609

610 15 Alkane Dehydrogenation over Vanadium and Chromium Oxides

change assigned to transformations in the nature of the support. A similar effect

was observed after propane dehydrogenation over a vanadia/alumina catalyst [73] ,

where a conversion of the γ - alumina support to α - alumina was conﬁ rmed by

XRD.

27

Al triple - quantum ( 3Q ) MAS NMR studies have also demonstrated that

the nature of the support undergoes a transformation. Figure 15.5 c shows the

27

Al 3Q MAS NMR spectrum of 8V after reaction and regeneration. Comparison

with Figure 15.5 b shows the loss of an isotropic resonance; this resonance was

assigned to 5 - coordinate Al and the loss of this species suggests a transformation

towards a more crystalline state in agreement with the reduced surface area.

It is rare for a structure – function relationship to be shown unambiguously in

heterogeneous catalysis. However, a structure – coke relationship has been estab-

lished in the dehydrogenation of butane on VO

x

/ θ - Al

2

O

3

catalysts by combined

reactivity, UV - Vis DRS, NMR, TEOM and UV Raman investigations. In this case,

both the nature and amount of coke show a dependence on the structure of the

VO

x

/ θ - Al

2

O

3

catalysts.

15.6

Conclusions

In this chapter we have reviewed the use of chromia and vanadia in alkane dehy-

drogenation. In both systems there is a rich and complex chemistry that is still

not yet fully understood, although considerable advances have been made over the

last few years. Both patent and academic literature are active and it can be expected

to be so for many years to come.

References

1 Frey , F.E. and Huppke , W.F. ( 1933 )

Industrial and Engineering Chemistry , 25 ,

54 .

2 Kearby , K.K. ( 1955 ) Catalysis , Vol. 3 (ed.

P.H. Emmet ), Reinhold , New York ,

p. 453 .

3 Hornaday , G.F. , Ferrell , F.M. and Mills ,

G.A. ( 1961 ) Advances in Petroleum

Chemistry and Reﬁ ning , Vol. 4 ,

Interscience , Paris , p. 451 .

4 Waddams , A.L. ( 1978 ) Chemicals From

Petroleum , 4th edn , Gulf Publishing

Company , Houston , p. 1980 .

5 Bhasin , M.M. , McCain , J.H. , Vora , B.V. ,

Imai , T. and Pujado , P.R. ( 2001 ) Applied

Catalysis A: General , 221 , 397 .

6 http://www.sud - chemie.com/scmcms/

web/page_en_4504.htm (May 2008 )

7 http://www.sud - chemie.com/scmcms/

web/page_en_4506.htm (May 2008 )

8 http://www.cbi.com/lummus/process -

technology/pdfs/catoﬁ ndehydrogenation.

pdf (May 2008 )

9 Fridman , V. and Rokicki , A. ( 2005 )

WO 2,005,040,075 , assigned to Sud Chemie .

10 Zaki , M.I. , Fouad , N.E. , Leyrer , J. and

Knozinger , H. ( 1986 ) Applied Catalysis , 21 ,

359 .

11 Cavani , F. , Koutyrev , M. , Triﬁ r ò , F. ,

Bartolini , A. , Ghisletti , D. , Iezzi , R. ,

Santucci , A. and Del Piero , G. ( 1996 )

Journal of Catalysis , 158 , 236 .

12 Puurunen , R.L. and Weckhuysen , B.M.

( 2002 ) Journal of Catalysis , 210 , 418 .

13 Jackson , S.D. , Matheson , I.M. , Naeye ,

M. - L. , Stair , P.C. , Sullivan , V.S. , Watson ,

S.R. and Webb , G. ( 2000 ) Studies in Surface

Science and Catalysis , Vol. 130 (eds A.

Corma , F.V. Melo , S. Mendioroz , J.L.G.

Fierro ), Elsevier , Amsterdam , p. 2213 .

References 611

14 Mentasty , L.R. , Gorriz , O.F. and Cad ù s ,

L.E. ( 2001 ) Industrial and Engineering

Chemistry Research , 40 , 136 .

15 Vuurman , M.A. , Wachs , I.E. , Stufkens ,

D.J. and Oskam , A. ( 1993 ) Journal of

Molecular Catalysis , 80 , 209 .

16 Grzybowska , B. , Sloczy ñ ski , J. ,

Grabowski , R. , Wcislo , K. , Kozlowska , A. ,

Stoch , J. and Zieli ñ ski , J. ( 1998 ) Journal

of Catalysis , 178 , 687 .

17 Sullivan , V.S. , Jackson , S.D. and Stair ,

P.C. ( 2005 ) Journal of Physical Chemistry

B , 109 , 352 .

18 Rahman , A. , Mohamed , M.H. , Ahmed ,

M. and Aitani , A.M. ( 1995 ) Applied

Catalysis A: General , 121 , 203 .

19 Ashmawy , F.M. and McAuliffe , C.A.

( 1984 ) Journal of the Chemical Society –

Faraday Transactions I , 80 , 1985 .

20 De Rossi , S. , Ferraris , G. , Fremiotti , S. ,

Garrone , E. , Ghiotti , G. , Campa , M.C.

and Indovina , V. ( 1994 ) Journal of

Catalysis , 148 , 36 .

21 Kanervo , J.M. and Krause , A.O.I. ( 2002 )

Journal of Catalysis , 207 , 57 .

22 De Rossi , S. , Casaletto , M.P. , Ferraris ,

G. , Cimino , A. and Minelli , G. ( 1998 )

Applied Catalysis A: General , 167 , 257 .

23 Cutrufello , M.G. , De Rossi , S. , Ferino , I. ,

Monaci , R. , Rombia , E. and Solinas , V.

( 2005 ) Thermochimica Acta , 434 , 62 .

24 Weckhuysen , B.M. and Schoonheydt ,

R.A. ( 1999 ) Catalysis Today , 51 , 223 .

25 Hakuli , A. , Kytokivi , A. and Krause ,

A.O.I. ( 2000 )

Applied Catalysis A: General ,

190 , 219 .

26 Jackson , S.D. and Stitt , E.H. ( 2003 )

Current Topics in Catalysis , 3 , 245 .

27 Pass , G. , Littlewood , A.B. and Burwell ,

R.L. , Jr ( 1960 ) Journal of the American

Chemical Society , 82 , 6281 .

28 van Reijin , L.L. , Sachtler , W.M.H. ,

Cossee , P. and Brouwer , D.M. ( 1965 )

Proceedings of the 3rd International

Congress on Catalysis , Vol. 2

(eds W.M.H. Sachtler , G.C.A. Schuit

and P. Zwietering ), North Holland ,

Amster dam , p. 829 .

29 Weckhuysen , B.M. , Wachs , I.E. and

Schoonheydt , R.A. ( 1996 ) Chemical

Reviews , 96 , 3327 .

30 Jackson , S.D. , Grenfell , J. , Matheson ,

I.M. and Webb , G. ( 1999 ) Reaction

Kinetics and the Development of Catalytic

Processes , Studies in Surface Science and

Catalysis, 122 (eds G.F. Froment and K.C.

Waugh ), Elsevier , Amsterdam , pp. 149 – 55 .

31 Jackson , S.D. and Rugmini , S. ( 2007 )

Journal of Catalysis , 251 , 59 .

32 Espinat , D. , Dexpert , H. , Freund , E. ,

Martino , G. , Couzi , M. , Lespade , P. and

Cruege , F. ( 1985 ) Applied Catalysis , 16 , 343

33 Airaksinen , S.M.K. , Ba ñ ares , M.A. and

Krause , A.O.I. ( 2005 ) Journal of Catalysis ,

230 , 507 .

34 Wachs , I.E. and Weckhuysen , B.M. ( 1997 )

Applied Catalysis A: General , 157 , 67 .

35 Harlin , M.E. , Niemi , V.M. and Krause ,

A.O.I. ( 2000 ) Journal of Catalysis , 195 , 67 .

36 Gao , X. , Banares , M.A. and Wachs , I.E.

( 1999 ) Journal of Catalysis , 188 , 325 .

37 Ruitenbeek , M. , Van Dillen , A.J. , de Groot ,

F.M.F. , Wachs , I.E. , Geus , J.W. and

Koningsberger , D.C. ( 2000 ) Topics in

Catalysis , 10 , 241 .

38 Wachs , I.E. and Chan , S.S. ( 1984 ) Applied

Surface Science , 20 , 181 .

39 Harlin , M.E. , Niemi , V.M. , Krause , A.O.I.

and Weckhuysen , B.M. ( 2001 ) Journal of

Catalysis , 203 , 242 .

40 Wachs , I.E. , Jehng , J. - M. , Deo , G. ,

Weckhuysen , B.M. , Guliants , V.V. ,

Benziger , J.B. and Sundaresan , S. ( 1997 )

Journal of Catalysis , 170 , 75 .

41 Wachs , I.E. , Jehng , J. - M. , Deo , G. ,

Weckhuysen , B.M. , Guliants , V.V. and

Benziger , J.B. ( 1995 ) Proceedings of the

5th European workshop meeting on

selective oxidation by heterogeneous

catalysis . Catalysis Today , 32 , 47 .

42 Sun , Q. , Jehng , J. - M. , Hu , H. , Herman ,

R.G. , Wachs , I.E. and Klier , K. ( 1997 )

Journal of Catalysis , 165 , 91 .

43 Mul , G. , Banares , M.A. , Garcia Cortez , G. ,

van der Linden , B. , Khatib , S.J. and

Moulijn , J.A. ( 2003 ) Physical Chemistry

Chemical Physics , 5 , 4378 .

44 Banares , M.A. , Cardoso , J.H. , Agullo -

Rueda , F. , Correa - Bueno , J.M. and Fierro ,

J.L.G. ( 2000 ) Catalysis Letters , 64 , 191 .

45 Christodoulakis , A. , Machli , M. ,

Lemonidou , A.A. and Boghosian , S. ( 2004 )

Journal of Catalysis , 222 , 293 .

46 Olthof , B. , Khodakov , A. , Bell , A.T. and

Iglesia , E. ( 2000 ) Journal of Physical

Chemistry B , 104 , 1516 .

47 Banares , M.A. and Wachs , I.E. ( 2002 )

Journal of Raman Spectroscopy , 33 , 359 .

48 Li , C. ( 2003 ) Journal of Catalysis , 216 ,

203 .

612 15 Alkane Dehydrogenation over Vanadium and Chromium Oxides

49 Gao , X. and Wachs , I.E. ( 2000 ) Journal of

Physical Chemistry B , 104 , 1261 .

50 Xie , S. , Iglesia , E. and Bell , A.T. ( 2000 )

Langmuir , 16 , 7162 .

51 Xie , S. , Iglesia , E. and Bell , A.T. ( 2001 )

Journal of Physical Chemistry B , 105 ,

5144 .

52 Argyle , M.D. , Chen , K.D. , Iglesia , E. and

Bell , A.T. ( 2005 ) Journal of Physical

Chemistry B , 109 , 2414 .

53 Dupuis , A.C. , Abu Haija , M. , Richter , B. ,

Kuhlenbeck , H. and Freund , H.J. ( 2003 )

Surface Science , 539 , 99 .

54 Argyle , M.D. , Chen , K.D. , Bell , A.T. and

Iglesia , E. ( 2002 ) Journal of Catalysis , 208 ,

139 .

55 Xiong , G. , Li , C. , Li , H.Y. , Xin , Q. and

Feng , Z.C. ( 2000 ) Chemical Communi-

cations , 677 .

56 Chua , Y.T. , Stair , P.C. and Wachs , I.E.

( 2001 ) Journal of Physical Chemistry B ,

105 , 8600 .

57 Xiong , G. , Li , C. , Feng , Z.C. , Ying , P.L. ,

Xin , Q. and Liu , J.K. ( 1999 ) Journal of

Catalysis , 186 , 234 .

58 Xiong , G. , Feng , Z. , Li , J. , Yang , Q. , Ying ,

P. , Xin , Q. and Li , C. ( 2000 ) Journal of

Physical Chemistry B , 104 , 3581 .

59 Li , C. and Stair , P.C. ( 1996 ) 11th

International Congress on Catalysis – 40th

Anniversary , Studies in Surface Science

and Catalysis , 101 (Pt. B) (eds J.W.

Hightower , W.N. Delgass , E. Iglesia and

A.T. Bell ), Elsevier , Amsterdam , p. 881 .

60 Stair , P.C. and Li , C. ( 1997 ) Journal of

Vacuum Science & Technology. A, Vacuum,

Surfaces, and Films , 15 , 1679 .

61 Tian , H. , Ross , E.I. and Wachs , I.E.

( 2006 ) Journal of Physical Chemistry B ,

110 , 9593 .

62 Catana , G. , Rao , R.R. , Weckhuysen ,

B.M. , Van Der Voort , P. , Vansant , E. and

Schoonheydt , R.A. ( 1998 ) Journal of

Physical Chemistry B , 102 , 8005 .

63 Wu , Z. , Kim , H.S. , Stair , P.C. , Rugmini ,

S. and Jackson , S.D. ( 2005 ) Journal of

Physical Chemistry B , 109 , 2793 .

64 Jackson , S.D. , Rugmini , S. , Stair , P.C.

and Wu , Z. ( 2006 ) Chemistry - Engineering

Journal , 120 , 127 .

65 Kanervo , J.M. , Harlin , M.E. , Krause ,

A.O.I. and Banares , M.A. ( 2003 ) Catalysis

Today , 78 , 171 .

66 Khodakov , A. , Olthof , B. , Bell , A.T. and

Iglesia , E. ( 1999 ) Journal of Catalysis , 181 ,

205 .

67 Arena , F. , Frusteri , F. , Martra , G. ,

Coluccia , S. and Parmaliana , A. ( 1997 )

Journal of the Chemical Society – Faraday

Transactions , 93 , 3849 .

68 Albonetti , S. , Cavani , F. and Triﬁ ro , F.

( 1996 ) Catalysis Reviews – Science and

Engineering , 38 , 413 .

69 Mamedov , E.A. and Corberan Cortes ,

V. ( 1995 ) Applied Catalysis, A: General ,

127 , 1 .

70 Blasco , T. and Lopez Nieto , J.M. ( 1997 )

Applied Catalysis, A: General , 157 , 117 .

71 Cavani , F. and Triﬁ ro , F. ( 1995 ) Catalysis

Today , 24 , 307 .

72 Madeira , L.M. and Portela , M.F. ( 2002 )

Catalysis Reviews – Science and Engineering ,

44 , 247 .

73 Jackson , S.D. , Lennon , D. , Webb , G. and

Willis , J. ( 2001 ) Catalyst Deactivation ,

Studies in Surface Science and Catalysis ,

139 , Elsevier , Amsterdam , p. 271 .

74 Volpe , M. , Tonetto , G. and de Lasa , H. ,

( 2004 ) Applied Catalysis, A: General , 272 ,

69 .

75 Wu , Z. and Stair , P.C. ( 2006 ) Journal of

Catalysis , 237 , 220 .

76 Chua , Y.T. and Stair , P.C. ( 2003 ) Journal of

Catalysis , 213 , 39 .

77 Baruya , A. , Gerrard , D.L. and Maddams ,

W.F. ( 1983 ) Macromolecules , 16 , 578

78 Noda , L.K. and Sala , O. , ( 2000 )

Spectrochimica Acta Part A – Molecular and

Biomolecular Spectroscopy , 56 , 145 .

79 Ukei , H. , Hirose , T. , Horikawa , S. , Takai ,

Y. , Taka , M. , Azuma , N. and Ueno , A.

( 2000 ) Catalysis Today , 62 , 67 .

80 Kuba , S. and Knozinger , H. ( 2002 ) Journal

of Raman Spectroscopy , 33 , 325 .

81 Asher , S.A. ( 1993 ) Analytical Chemistry , 65 ,

201A .

82 Davis , S.M. , Zaera , F. and Somorjai , G.A.

( 1982 ) Journal of Catalysis , 77 , 439 .

83 Tinnemans , S.J. , Kox , M.H.F. , Nijhuis ,

T.A. , Visser , T. and Weckhuysen , B.M.

( 2005 ) Physical Chemistry Chemical Physics ,

7 , 211 .

84 Huang , Z. , McGregor , J. , Steiner , P. ,

Gladden , L.F. , Rugmini , S. and Jackson , S.

D. ( 2007 ) Magnetic Resonance Imaging ,

25 ,

562 .

Properties, Synthesis and Applications of Highly Dispersed

Metal Oxide Catalysts

Juncheng Hu , Lifang Chen , and Ryan Richards

613

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

ISBN: 978-3-527-31815-5

16

16.1

Introduction

Metal oxides can be found in the form of single crystals (pure or defective), powder

(with large numbers of crystals), polycrystalline samples (crystals with various

orientations) or thin ﬁ lms and constitute an important class of materials covering

the entire range from metals to insulators. The insulating oxides are made up of

the metals from the far left and right sides of the periodic table. Typical examples

of insulating oxides include MgO, CaO, Al

2

O

3

and SiO

2

. The oxides of the metals

in the middle of the periodic table (Sc to Zn) make up semiconducting and metallic

oxides. Typical examples include ZnO, TiO

2

, NiO, Fe

2

O

3

and Cr

2

O

3

. Additionally,

transition metal oxides, which include the oxides of Ru, Mo, W, Pt, V and so forth,

exhibit a wide - ranging array of properties and phenomena. The diverse structures

adopted by metal oxides demonstrate the relationship between structure and prop-

erties. Correlation of structure and physical properties of transition metal oxides

requires an understanding of the valence electrons that bind the atoms in the solid

state [1] . Band theory and ligand ﬁ eld theory have been invoked to explain the

electronic properties of transition metal oxides. The important properties of metal

oxides that are of interest are magnetic, electrical, dielectric, optical, Lewis acid –

base and redox behaviour. The transition metal oxides are of particular interest for

applications in catalysis, sensor materials and other potential applications. Indeed,

metal oxides are the key components for a variety of catalytic reactions, functioning

directly as reactive components or as supports for dispersed active metal species,

or as additives or promoters to enhance the rate of catalytic reactions. The impor-

tance of metal oxides in the ﬁ eld of catalysis is thus profound. Hitherto, the main

interest in heterogeneous catalysis had centered on the kinetics of the reactions

rather than on their mechanisms; now it is the catalyst itself and, in particular, its

surface, that has become the focus of attention. In considering the action of a cata-

lyst and its surface, two particular factors have been explored. One is the geometri-

cal factor, for which experiments with chemisorption and catalysis on the various

faces of single crystals have helped to deﬁ ne the role. The second factor is the role

614 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

of electrons in the catalytic process, and the surface science of metal oxides has

received tremendous attention during recent years. The advent of several advanced

techniques to probe surfaces has led to a new understanding of their surface

properties and structures. This has helped catalyst scientists tailor the properties

of the oxide materials for appropriate applications.

This chapter presents an overview of structures, properties, synthetic methods

and applications in catalysis for highly dispersed metal oxides, with special atten-

tion given to recent developments in nano - scale metal oxides.

16.2

Properties

In the case of nano - scale materials one of the most inﬂ uential factors towards their

properties is the large number of atoms at the surface. For example, spherical

nanoparticles with a diameter of 3 nm have nearly 50% of the atoms or ions on

the surface, allowing the possibility of manipulation of bulk properties by surface

effects and allowing near - stoichiometric chemical reaction [2] . Delocalization can

vary with size when strong chemical bonding is present, and can lead to different

chemical and physical properties [3] . The formation of ultra - small particles is

facilitated by the refractory nature of most of the metal oxides [2] . For some materi-

als, especially MgO, Al

2

O

3

, ZrO

2

and TiO

2

, their highly ionic nature allows the

formation of many stable defect sites, including edges, corners and anion/cation

vacancies. MgO and CaO were found to be attractive materials to study because

they are highly ionic and have high melting points and it would be expected that

samples of very small particle size might be stable and isolable. Furthermore,

reactive surface sites on these oxides have been extensively studied for macro-

scopic and conventionally prepared samples.

Metal oxides prepared via aerogel methods have found various applications,

including as detectors for radiation, super insulators, solar concentrators, coatings,

glass precursors, catalysts, insecticides and destructive adsorbents. Nanoparticles

of crystalline substances have been found to possess about 10

19

interfaces cm

− 3

and surface areas up to 800 m

2

g

− 1

. Upon compaction, but without growing the

nanocrystals, solids with multitudinous grain boundaries are formed. It has been

proposed that further work in the area of consolidated nano - phase materials may

lead to ceramics with increased ﬂ exibility, less brittleness and perhaps greater

strength [2] . It may also be possible to form materials with a large fraction of atoms

at grain boundaries, perhaps in unique arrangements.

16.2.1

Structure and Bonding

Characterization of the exact nature of structure and bonding in nanomaterials is

particularly difﬁ cult because they generally consist of very small crystallites or are

amorphous. Advances in crystallography for powders and crystals employing

X - ray, electron and neutron diffraction have provided insight into the structures

of metal oxides. Metal oxides crystallize in a variety of structures and the bonding

in these materials can range from ionic (MgO, Fe

1 - x

O) to metallic (TiO, ReO

3

)

[1] .

An understanding of both crystal structure and bonding, as well as the local

microstructures that are due to defects, is necessary to understand the structure

of complex transition metal oxides. Of course, on the nanometer scale the number

of defects due to edges, corners, “ F ” centers and other surface imperfections is

greatly enhanced by the large surface area (Figure 16.1 ) [4] . The pursuit of an

understanding of the structure/property relationships at an atomic or molecular

level is integral to the understanding of the unique properties observed on the

nano - scale.

Five types of crystals found in bulk structures can be deﬁ ned based on bonding

considerations: covalent, ionic, metallic, molecular (van der Waals) and hydrogen

bonded. These are also present on the nano - scale. However, the number of atoms

at the surface must also be considered when examining the structure of nano - scale

materials. When highly electronegative and highly electropositive elements are

combined in a lattice, ionic crystals are formed. It has been found that the ionic

model is a poor approximation for crystals containing large anions and small

cations (e.g. oxides and sulﬁ des) where the covalent contribution to bonding

becomes signiﬁ cant [5] . Van der Waals interactions play a crucial role in many

transition metal oxides, especially those with layered structures. In many oxide

hydrates or hydroxy oxides, hydrogen bonding also contributes to the cohesive

energy. In most transition metal oxides the bonding is only partly ionic; in other

words, there is a considerable overlap between the orbitals of the cations and

anions. However, there are several examples of metal oxides that are primarily

ionic, such as MgO and CaO. Many transition metal oxides also exhibit metallic

properties. Inorganic compounds of the formula AB can have the rock salt (B1),

Figure 16.1 A representation of the various defects present on metal oxides (from [4] ).

16.2 Properties 615

616 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

CsCl (B2), zinc blende (B3), Wurtzite (B4), or NiAs (B8) structure [6] . Alkaline

earth metal oxides such as MgO and monoxides of 3d transition metals as well as

of lanthanides and actinides such as TiO, NiO, EuO and NpO, exhibit the rock salt

structure with 6

: 6 octahedral coordination.

16.2.2

Defects

The physical and chemical properties of metal oxides are dominated by defects.

Thermodynamic considerations imply that all crystals must contain a certain

number of defects at temperatures above absolute zero. Structural defects can

arise from a variety of causes: they may be thermally generated or they may arise

in the course of fabrication of the solid, incorporated either unintentionally or

deliberately. Defects are important because they are much more abundant at

surfaces than in bulk, and in oxides they are usually responsible for many of the

catalytic and chemical properties. Because of the number of atoms at the surface

and the limited number of atoms within the lattice, the chemistry and bonding

of oxide nanoparticles is greatly affected by the defect sites present. Point defects

in crystals, such as vacancies and interstitials described by Schottky and Frenkel,

account for the transport properties of ionic solids [6] . However, it appears that

the point defect model is valid only when the defect concentration (or the devia-

tion from stoichiometry) is extremely small. The defects that occur in ionic solids

are grouped into the following classes: point, linear, planar and volumetric

defects. Point defects are a result of the absence of one of the constituent atoms

(or ions) on the lattice sites, or their presence in interstitial positions. Foreign

atoms or ions present in the lattice represent another type of point defect. Point

defects cause displacements on neighboring atoms or ions because of polariza-

tion in the surrounding region. A cationic vacancy in an ionic solid will have an

electronegative charge, causing displacements of neighboring anions [7] . The

energy of formation of a point defect depends primarily on the atomic arrange-

ment in the immediate environment of the corresponding rows of atoms that do

not possess the proper coordination. Boundaries between small crystallites (grain

boundaries), stacking faults, crystallographic shear planes, twin boundaries and

anti - phase boundaries are planar defects. Three - dimensional volumetric defects

are a result of segregating point defects. The common point defects in ionic

solids are Schottky pairs (pairs of cation and anion vacancies) and Frenkel defects

(cation or anion interstitial plus a vacancy) [8] . When there is a large concentra-

tion of Schottky pairs, the measured pyknometric density of the solid is consider-

ably lower than the density calculated from the X - ray unit cell dimensions (e.g.

VO

x

) [5] . Creation of defects is generally an endothermic process with the forma-

tion energies of vacancies in ionic solids generally 2 eV or more. Therefore, the

intrinsic defect concentration in these solids is extremely low even at high tem-

peratures [7] . The surface of a crystal constitutes a planar, 2 - D defect. The envi-

ronment of atoms or ions on the surface of a crystal is considerably different

from that in the bulk. In polycrystalline materials, there are grain boundaries

between the particles. The interface between two solid phases is an important

factor in determining the course of reactions, crystal growth, and so on. An

interface may be coherent, incoherent or semicoherent. It is coherent when

the interface matches perfectly between the contact planes of two solid planes.

Epitaxial growth occurs even when there is considerable mismatch (semicoherent

interface). In close packed solids, one also often encounters stacking faults. For

example, in a solid with cubic close packing, ABC ABC ABC, there can be a fault

such as ABC AB ABC. Other types of planar defects include tilt boundary ( array

of periodically spaced edge dislocations ), twist boundary ( array of screw disloca-

tions ), twin boundary ( a layer with mirror plane symmetry with respect to the

rotation of one part of the crystal, on a speciﬁ c plane, with respect to another )

and antiphase boundary ( boundary across which the sublattice occupation

becomes interchanged ).

The radius of the oxide anion (1.44 Å ) given by Shannon and Prewitt is larger

than most cations. However, in crystals, the anions are subjected to a positive

Madelung potential, which gives rise to a contraction of the charge cloud, while

cations are subjected to negative potential causing an opposite effect [7] . Generally,

phase purity is hard to achieve, especially for phases containing more than one

cation [9] . Often, phase segregation occurs and so the particles are a mixture of

other possible phases derivable form the precursors.

The high surface areas of nano - scale particles yield a number of defect sites.

There have been numerous studies of their surfaces in an attempt to clarify the

type of defect sites that can exist [10 – 13] . The most common defects are coordina-

tively unsaturated ions arising from planes, edges, corners, anion/cation vacancies

and electron excess centers. Such sites are often attributed as the active sites for

many useful and interesting reactions, including methane activation [14] , D

2

- CH

exchange [15] , methanol oxidation [16, 17] , CO oligomerization [18] , oxygen

exchange in CO

2

[7] and H

2

O [8] .

16.2.3

Acid – Base Properties of Metal Oxides

Metal oxides are often hard acids or bases and thus possess sites capable of cata-

lyzing acid – base chemistry. Acidic metal oxides include Al

2

O

3

, TiO

2

, CeO

2

, V

2

O

5

,

WO

3

, etc. Basic metal oxides include MgO, CaO, SrO, BaO, Li

2

O, etc. The interac-

tions of the oxide surfaces with gases or liquids, which are relevant in the ﬁ elds

of adsorption and catalysis, are mainly governed by acid – base interactions. Acidic

properties of metal oxides sometimes play an important role in the catalytic oxida-

tions of hydrocarbons, where reactions, especially the selectivities to the target

products, are strongly affected by the functions of acidic sites.

Acid – base characterizations of metal oxide surfaces are often used to explain

their catalytic behavior. Extensive studies have been performed on the interaction

of acid – base probe molecules with powders or supported metal oxides. The adsorp-

tion of NH

3

at cation sites has been used to characterize the Lewis acidity of metal

oxides. The systematic use of CO adsorption at room temperature as a probe for

16.2 Properties 617

618 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

(strong) Lewis acidity at the surface of a number of metal oxides (TiO

2

, ZrO

2

, HfO

2

and Al

2

O

3

) of interest in catalysis was also illustrated. The adsorption of CO

2

, a

standard acidic probe molecule, has also been used to characterize the Lewis basic-

ity of the oxygen anions on metal oxides.

Several insulating oxides and oxide composites were found to be potential cata-

lysts for a variety of important reactions as a result of their surface basicity or

acidity [14, 19 – 22] . Some selected reactions typical to metal oxides include dehy-

dration of alcohols, cracking of hydrocarbons, isomerization of oleﬁ ns and

parafﬁ ns, dehydrohalogenation, alkylation and esteriﬁ cation. Several metal oxides,

such as MgO, CaO and SrO, exhibit surface basic behavior while others, such as

Al

2

O

3

and WO

3

, are considered to be acidic solids that possess more and stronger

acidic sites on their surfaces. Acid – base behavior and the presence of several types

of deﬁ ciencies in the lattice and on the surface are two major driving forces for

surface reactivity of metal oxides. When metal oxides are prepared on the nano -

scale, the percentage of coordinatively unsaturated ions, especially on edges and

corners, increases signiﬁ cantly. Consequently, surface chemistry effects, which

are barely noticeable in macroscopic systems, become prevalent in nanoparticle

systems. These effects are demonstrated by enhanced surface reactivities and

catalytic potentials possessed by many nanoparticle systems of metal oxides

[23 – 27] . As indicated earlier, two of the most intensively studied nanoparticulate

systems of the metal oxides are MgO and CaO. In particular, two types of nano-

crystalline oxides have been prepared and thoroughly studied: a “ conventional

preparation “ ( CP ) and an “ aerogel preparation “ ( AP ). Nanocrystalline MgO pre-

pared by a modiﬁ ed AP yields a ﬁ ne, white powder of 400 – 500 m

2

g

− 1

and 4 nm

average crystallite size. High - resolution transmission electron microscope ( TEM )

imaging of a single crystallite indicated a polyhedral structure suggesting the

presence of high surface concentrations of edge/corner sites and various exposed

crystal planes (such as (002), (001), (111)) [28] . Conversely, CP yields particles with

surface areas of 150 – 200 m

2

g

− 1

and 8 nm average crystallites. If intrinsic surface

chemistry differences due to size are to be uncovered, it is important to consider

that in bulk MgO the effective ionic charges are close to +2, whereas the MgO

molecule is much more covalent with effective charges close to +1. Lower coor-

dination surface ions such as Mg

c3

2+

, Mg

c4

2+

, O

c3

2−

and O

c4

2−

are expected to have effec-

tive charges between +1 and +2. Surface sites on crystalline and powdered MgO

have been probed by theoretical as well as experimental efforts. Ab initio calcula-

tions with H

2

have been used to probe perfect crystal surfaces and various defect

sites. On the perfect (100) MgO surface, H

2

has a small adsorption energy and

does not dissociate. However, temperature programmed desorption methods have

shown that polycrystalline samples do dissociate H

2

, probably on O

3c

– Mg

3c

sites.

These sites are apparently very active for heterolytic H

2

dissociation. The micro-

faceted (111) surface of MgO is particularly reactive, and steps, kinks and point

defects (ion vacancies and substitutions) are also important. Indeed, the unique

catalytic properties of defective MgO surfaces also depend on a plethora of unusual

coordination sites.

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

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