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

References 173

94 Manoilova , O. , Olindo , R. , Otero Are á n ,

C. , and Lercher , J.A. ( 2007 ) Catalysis

Communications , 8 , 865 – 70 .

95 Stencel , J.M. , Makovsky , L.E. , Diehl ,

J.R. and Sarkus , T.A. ( 1984 ) Journal of

Raman Spectroscopy , 15 , 283 .

96 Ramis , G. , Cristiani , C. , Forzatti , P. and

Busca , G. ( 1990 ) Journal of Catalysis ,

124 , 574 – 5 .

97 Payen , E. , Gengembre , L. , Maug é , F. ,

Duchet , J.C. and Lavalley , J.C. ( 1991 )

Catalysis Today , 10 , 521 – 39 .

98 Gutierrez - Alejandre , A. , Castello , P. ,

Ramirez , J. , Ramis , G. and Busca , G.

( 2001 ) Applied Catalysis A: General , 216 ,

181 – 94 .

99 Busca , G. ( 2002 ) Journal of Raman

Spectroscopy , 33 , 348 – 58 .

100 Busca , G. ( 2008 ) Chemical Reviews , 107 ,

5366 – 71 .

101 Busca , G. ( 1999 ) Physical Chemistry

Chemical Physics , 1 , 723 – 36 .

102 Busca , G. ( 2005 ) in Metal Oxides:

Chemistry and Applications (ed. J.L.G.

Fierro ), CRC Press , Boca Raton, Fl ,

pp. 247 – 318 .

103 Busca , G. , Ramis , G. , Lorenzelli , V. ,

Rossi , P.F. , La Ginestra , A. and

Patrono , P. ( 1989 ) Langmuir , 5 , 911 – 16 .

104 Ramis , G. , Busca , G. , Lorenzelli , V. ,

Rossi , P.F. , Bensitel , M. , Saur , O. and

Lavalley , J.C. ( 1988 ) Proceedings of the

9th International Congress on Catalysis,

Calgary, Canada , Vol. IV, pp. 1874 – 81 .

105 Ramis , G. , Yi , L. , Busca , G. , del Mart í n ,

M. , Arco , C. , Rives , V. and Sanchez

Escribano , V. ( 1998 ) Materials Chemistry

and Physics , 55 , 173 – 87 .

106 Busca , G. , Cavani , F. , Centi , G. and

Triﬁ r ò , F. ( 1986 )

Journal of Catalysis , 99 ,

400 – 14 .

107 Abbattista , F. , Delmastro , S. , Gozzelino ,

G. , Mazza , D. , Vallino , M. , Busca , G.

and Lorenzelli , V. ( 1990 ) Journal of the

Chemical Society – Faraday Transactions.

I , 86 , 3653 – 8 .

108 Ramis , G. , Busca , G. , Lorenzelli ,

V. , La Ginestra , A. , Galli , P. and

Massucci , M.A. ( 1988 ) Journal of the

Chemical Society – Dalton Transactions ,

881 – 6 .

109 DelMastro , S. , Gozzellino , G. , Mazza ,

D. , Vallino , M. , Busca , G. and

Lorenzelli , V. ( 1992 ) Journal of the

Chemical Society – Faraday Transactions I ,

88 , 2065 – 70 .

110 Armaroli , T. , Bevilacqua , M. , Trombetta ,

M. , Milella , F. , Guti è rrez Alejandre , A. ,

Ramirez Solis , J. , Notari , B. , Willey , R.J.

and Busca , G. ( 2001 ) Applied Catalysis A:

General , 216 , 59 – 71 .

111 Tsyganenko , A. and Filimonov , V.N.

( 1972 ) Spectroscopy Letters , 5 , 477 – 87 .

112 Montagne , X. , Lynch , J. , Freund , E. ,

Lamotte , J. and Lavalley , J.C. ( 1987 )

Journal of the Chemical Society – Faraday

Transactions I , 83 , 1417 – 25 .

113 Bevilacqua , M. , Montanari , T. , Finocchio ,

E. and Busca , G. ( 2006 ) Catalysis Today ,

116 , 132 – 42 .

114 Astorino , E. , Peri , J. , Willey , R.J. and

Busca , G. ( 1995 ) Journal of Catalysis , 157 ,

482 – 500 .

115 Zecchina , A. , Bordiga , S. , Spoto , G. ,

Marchese , L. , Petrini , G. , Leofanti , G. and

Padovan , M. ( 1992 ) Journal of Physical

Chemistry , 96 , 4991 – 7 .

116 Peri , J.B. ( 1965 ) Journal of Physical

Chemistry , 69 , 211 – 20 .

117 Kn ö zinger , H. and Ratnasamy , P. ( 1978 )

Catalysis Reviews – Science and Engineering ,

17 , 31 – 70 .

118 Busca , G. , Lorenzelli , V. , Ramis , G. and

Willey , R.J. ( 1993 ) Langmuir , 9 , 1492 – 9 .

119 Morterra , C. and Magnacca , G. ( 1996 )

Catalysis Today , 27 , 497 – 532 .

120 Mardilovich , A.A. and Tsyganenko , P.P.

( 1996 ) Journal of the Chemical Society –

Faraday Transactions , 92 , 4843 – 52 .

121 Fripiat , J. , Alvarez , L. , Sanchez Sanchez ,

S. , Martinez , E. , Morades , E. , Saniger , J.

and Sanchez , N. ( 2001 ) Applied Catalysis

A: General 215 , 91 – 100 .

122 Digne , M. , Sautet , P. , Raybaud , P. ,

Euzen , P. and Toulhoat , H. ( 2002 ) Journal

of Catalysis. , 211 , 1 – 5 .

123 Lambert , J.F. and Che , M. ( 2000 ) Journal

of Molecular Catalysis. A – Chemical , 162 ,

5 – 18 .

124 Malpartida , I. , Larrubia Vargas , M.A. ,

Alemany , L.J. and Busca , G. ( 2007 )

Applied Catalysis B: Environmental , 80 ,

214 – 25 .

125 Bevilacqua , M. and Busca , G. ( 2008 ) in

press.

126 Liu , X. and Truitt , R.E. ( 1997 ) Journal of

the American Chemical Society , 119 ,

9856 – 60 .

174 3 The Use of Infrared Spectroscopic Methods

127 Busca , G. , Guidetti , R. and Lorenzelli ,

V. ( 1990 ) Journal of the Chemical Society –

Faraday Transactions I , 86 , 989 – 94 .

128 Busca , G. , Lorenzelli , V. and Sanchez

Escribano , V. ( 1992 ) Chemistry of

Materials , 4 , 595 – 605 .

129 Busca , G. , Saussey , H. , Saur , O. ,

Lavalley , J.C. and Lorenzelli , V. ( 1985 )

Applied Catalysis , 14 , 245 – 60 .

130 Bensitel , M. , Saur , O. , Lavalley , J.C. and

Morrow , B.A. ( 1987 ) Materials Chemistry

and Physics , 17 , 249 – 58 .

131 Jacob , K. - H. , Kn ö zinger , E. and Benier ,

S. ( 1993 ) Journal of Materials Chemistry ,

3 , 651 – 7 .

132 Gutierrez - Alejandre , A. , Castillo , P. ,

Ramirez , J. , Ramis , G. and Busca , G.

( 2001 ) Applied Catalysis A: General , 216 ,

181 – 94 .

133 Baertsch , C.D. , Komala , K.T. , Chua ,

Y. - H. and Iglesia , E. ( 2002 ) Journal of

Catalysis , 205 , 44 – 57 .

134 Kuba , S. , Che , M. , Grasselli , R.K. and

Kn ö zinger , H. ( 2003 ) Journal of Physical

Chemistry B , 107 , 3459 – 63 .

135 Manoilova , O. , Olindo , R. , Otero Are á n ,

C. and Lercher , J.A. ( 2007 ) Catalysis

Communications , 8 , 865 – 70 .

136 G ó ra - Marek , K. , Derewinski , M. , Sarv ,

P. and Datka , J. ( 2005 ) Catalysis Today ,

101 , 131 – 8 .

137 G ó ra - Marek , K. and Datka , J. ( 2006 )

Applied Catalysis A: General , 302 , 104 – 9 .

138 Cseri , G. , Cr é pau , V. , Montouillout , A. ,

Vimont , L. , Mariey , T. and Maug é , F.

( 2006 ) Journal of Physical Chemistry. B ,

110 , 15172 – 85 .

139 Garrone , E. , Onida , B. , Bonelli , B. , Busco ,

C. and Ugliengo , P. ( 2006 ) Journal of

Physical Chemistry. B , 110 , 19087 – 92 .

140 Ramis , G. , Busca , G. and Lorenzelli , V.

( 1989 ) in Structure and Reactivity of

Surfaces (eds A. Zecchina , G. Costa and

C. Morterra ), Elsevier , pp. 777 – 86 .

141 Busca , G. ( 1998 ) Catalysis Today , 41 ,

191 – 206 .

142 Trombetta , M. , Busca , G. , Rossini , S.A. ,

Piccoli , V. and Cornaro , U. ( 1997 )

Journal of Catalysis , 168 , 349 – 63 .

143 Trombetta , M. and Busca , G. ( 1999 )

Journal of Catalysis , 187 , 521 – 3 .

144 Trombetta , M. , Busca , G. , Lenarda , M. ,

Storaro , L. and Pavan , M. ( 1999 ) Applied

Catalysis A: General , 182 , 225 – 35 .

145 Montanari , T. , Bevilacqua , M. , Resini , C.

and Busca , G. ( 2004 ) Journal of Physical

Chemistry. B , 108 , 2120 – 7 .

146 Trombetta , M. , Armaroli , T. , Guti é rrez -

Alejandre , A. , Ram í rez , J. and Busca , G.

( 2000 ) Applied Catalysis A: General , 192 ,

125 – 36 .

147 Montanari , T. , Bevilacqua , M. and Busca ,

G. ( 2006 ) Applied Catalysis A: General ,

307 , 21 – 9 .

148 Bevilacqua , M. and Busca , G. ( 2002 )

Catalysis Communications , 3 , 497 – 502 .

149 Armaroli , A. , Bevilacqua , M. , Trombetta ,

M. , Guti è rrez Alejandre , A. , Ramirez , J.

and Busca , G. ( 2001 ) Applied Catalysis A:

General , 220 , 181 – 90 .

150 Trombetta , M. , Armaroli , T. , Guti è rrez

Alejandre , A. , Gonzalez , H. , Ramirez

Solis , J. and Busca , G. ( 2001 ) Catalysis

Today , 65 , 285 – 92 .

151 Trombetta , M. , Busca , G. , Storaro , L. ,

Lenarda , M. , Casagrande , M. and

Lucchini , V. ( 2000 ) Physical Chemistry

Chemical Physics , 2 , 3529 – 37 .

152 Romero Sarria , F. , Blasin - Aube , V. ,

Saussey , J. , Marie , O. and Daturi , M.

( 2006 ) Journal of Physical Chemistry B ,

110 , 13130 – 7 .

153 Lundie , D.T. , McInroy , A.R. , Marshall , R. ,

Winﬁ eld , J.M. , Jones , P. , Dudman , C.C. ,

Parker , S.F. , Mitchell , C. and Lennon , D.

( 2005 ) Journal of Physical Chemistry B ,

109 , 11592 – 601 .

154 Morrow , B.A. and Cody , I.A. ( 1976 )

Journal of Physical Chemistry , 80 , 1995 – 8 .

155 Hadjiivanov , K.I. and Vayssilov , G.N.

( 2002 ) Advances in Catalysis , 47 , 307 – 511 .

156 Binet , C. , Jadi , A. and Lavalley , J.C. ( 1989 )

Journal De Chimie Physique , 86 , 451 – 70 .

157 Busca , G. ( 1987 ) Journal of Molecular

Catalysis , 43 , 225 – 36 .

158 Gallardo Amores , J.M. , Sanchez

Escribano , V. , Busca , G. and Lorenzelli ,

V. ( 1994 ) Journal of Materials Chemistry ,

4 , 965 – 71 .

159 Montanari , T. , Marie , O. , Daturi , M. and

Busca , G. ( 2005 ) Catalysis Today , 110 ,

339 – 44 .

160 Salla , I. , Montanari , T. , Salagre , P. ,

Cesteros , Y. and Busca , G. ( 2005 ) Physical

Chemistry Chemical Physics , 7 , 2526 – 33 .

161 Montanari , T. , Salla , I. and Busca , G.

( 2008 ) Microporous and Mesoporous

Materials , 109 , 216 – 22 .

References 175

162 Salla , I. , Montanari , T. , Salagre , P. ,

Cesteros , Y. and Busca , G. ( 2005 )

Journal of Physical Chemistry B , 109 ,

915 – 22 .

163 Weitkamp , J. , Hunger , M. and Rymsa ,

U. ( 2001 ) Microporous and Mesoporous

Materials , 48 , 255 – 70 .

164 Davis , R.J. ( 2003 ) Journal of Catalysis ,

216 , 396 – 405 .

165 Kozyra , P. , Salla , I. , Montanari , T. ,

Datka , J. , Salare , P. and Busca , G.

( 2006 ) Catalysis Today , 114 , 188 – 96 .

166 Montanari , T. and Busca , G. ( 2008 )

Vibration Spectroscopy , 46 , 45 – 51 .

167 Montanari , T. , Kozyra , P. , Salla , I. ,

Datka , J. , Salare , P. and Busca , G.

( 2006 ) Journal of Materials Chemistry , 16 ,

995 – 1000 .

168 Busca , G. and Ramis , G. ( 1986 ) Applied

Surface Science , 27 , 114 – 26 .

169 Gallardo Amores , J.M. , Sanchez

Escribano , V. , Daturi , M. and Busca , G.

( 1996 ) Journal of Materials Chemistry , 6 ,

879 – 86 .

170 Morterra , C. , Magnacca , G. , Cerrato , G. ,

Del Favero , N. , Filippi , F. and Folonari ,

C.V. ( 1993 ) Journal of the Chemical

Society – Faraday Transactions , 89 ,

135 – 50 .

171 Prinetto , F. , Ghiotti , G. , Nova , I. , Lietti ,

L. , Tronconi , E. and Forzatti , P. ( 2001 )

Journal of Physical Chemistry B , 105 ,

12732 – 5 .

172 Busca , G. , Cristiani , C. , Forzatti , P. and

Groppi , G. ( 1995 ) Catalysis Letters , 31 ,

65 – 74 .

173 Busca , G. , Buscaglia , V. , Leoni , M. and

Nanni , P. ( 1994 ) Chemistry of Materials ,

6 , 955 – 61 .

174 Daturi , M. , Busca , G. and Willey , R.J.

( 1995 ) Chemistry of Materials , 7 ,

2115 – 26 .

175 Rossi , P.F. , Busca , G. , Lorenzelli , V. ,

Waquif , M. , Saur , O. and Lavalley , J.C.

( 1991 ) Langmuir , 7 , 2677 – 81 .

176 Busca , G. , Rossi , P.F. , Lorenzelli , V. ,

Benaissa , M. , Travert , J. and Lavalley , J.C.

( 1985 ) Journal of Physical Chemistry , 89 ,

5433 – 9 .

177 Davydov , A.A. ( 1990 ) Infrared

Spectroscopy of Adsorbed Species on the

Surface of Transition Metal Oxide , John

Wiley & Sons, Inc .

178 Busca , G. ( 1996 ) Catalysis Today , 27 ,

457 – 96 .

179 Sanchez Escribano , V. , Larrubia Vargas ,

M.A. , Finocchio , E. and Busca , G. ( 2007 )

Journal of Materials Chemistry , 316 ,

68 – 74 .

180 Larrubia Vargas , M.A. , Busca , G. ,

Costantino , U. , Marmottini , F. ,

Montanari , T. , Patrono , P. , Pinzari , F.

and Ramis , G. ( 2007 ) Journal of

Molecular Catalysis. A: Chemical , 266 ,

188 – 97 .

181 Busca , G. , Finocchio , E. , Lorenzelli , V. ,

Trombetta , M. and Rossini , S.A. ( 1996 )

Journal of the Chemical Society – Faraday

Transactions I , 92 , 4687 – 93 .

182 Trombetta , M. , Busca , G. , Rossini , S.A. ,

Piccoli , V. and Cornaro , U. ( 1997 ) Journal

of Catalysis , 168 , 334 – 48 .

183 Busca , G. , Lietti , L. , Ramis , G. and Berti ,

F. ( 1998 ) Applied Catalysis B –

Environmental , 18 , 1 – 36 .

184 Busca , G. , Larrubia , M.A. , Arrighi , L. and

Ramis , G. ( 2005 ) Catalysis Today , 107 –

108 , 139 – 48 .

185 Tibiletti , D. , Goguet , A. , Reid , D. ,

Munier , F.C. and Burch , R. ( 2006 )

Catalysis Today , 113 , 94 – 101 .

186 Lamberti , C. , Groppo , E. , Spoto , G.

Bordiga , S. and Zecchina , A. ( 2007 )

Advances in Catalysis , 51 , 1 – 74 .

Resonance Raman Spectroscopy – Θ - Al

2

O

3

- Supported

Vanadium Oxide Catalysts as an Illustrative Example

Zili Wu , Hack - Sung Kim , and Peter C. Stair

177

4

4.1

Introduction

Raman spectroscopy is a powerful tool for characterizing the molecular structure

of catalytic materials under in situ conditions. In recent years, resonance Raman

studies of heterogeneous catalysts have added additional dimensions to this tech-

nique by providing the capability of probing the heterogeneity of surface catalytic

sites. Speciﬁ cally, the heterogeneously distributed catalytic phases (sites) with dif-

ferent electron absorptions can be selectively resonance enhanced in Raman spec-

troscopy by using appropriate laser excitation. Thus different catalytic sites can be

individually probed via varying laser excitations (multi - wavelength excitation). In

this chapter, we present an example of applying the resonance Raman spectros-

copy technique for investigating the structure of V/Al

2

O

3

catalysts under various

oxidation and reduction conditions as an illustrative example of the power of the

technique. The example shows that multi - wavelength - excited Raman spectroscopy

is able to provide not only qualitative information on the structure of different

surface VO

x

species but also quantitative information on the distribution of these

VO

x

species as a function of vanadium loading. The reduction behaviors of the

different VO

x

species (monovanadate, polyvanadate and V

2

O

5

) have been distin-

guished by this method. Raman spectroscopy application to butane dehydrogena-

tion catalyzed by VO

x

species is given in Volume 2 of this series. It is expected

that multi - wavelength - excited Raman spectroscopy will be employed for more

applications in catalysis and material science studies.

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

ISBN: 978-3-527-31815-5

178 4 Resonance Raman Spectroscopy – Θ-Al

2

O

3

-Supported Vanadium Oxide Catalysts

4.2

Structure of Al

2

O

3

- Supported Vanadia Catalysts

4.2.1

Introduction

Supported vanadia catalysts have attracted signiﬁ cant interest in recent decades

because of their favorable catalytic performance in many redox reactions [1 – 6] ,

such as dehydrogenation and oxidative dehydrogenation of alkanes to oleﬁ ns, oxi-

dation of methanol to formaldehyde, ammoxidation of aromatic hydrocarbons and

selective reduction of NO

x

. A better understanding of the catalytic properties of

supported vanadium oxides requires determination of the structure – reactivity rela-

tionships of the supported VO

x

catalysts. Raman spectroscopy has been success-

fully, and frequently, employed by many research groups for characterization of

the molecular structure of supported vanadium oxides [2, 4 – 18] . For vanadia sup-

ported on most oxide supports, for example SiO

2

, Al

2

O

3

, TiO

2

, ZrO

2

and MgO,

three types of surface vanadia species have been identiﬁ ed: isolated vanadia,

polymerized vanadia and crystalline V

2

O

5

. The presence of these species is a func-

tion of the method of preparation, the VO

x

loading, the nature of the support and

the ambient environment [2, 4 – 11, 15] . For dehydrated, supported VO

x

, three kinds

of bonds between vanadium and oxygen are present in these VO

x

species: V

=

O,

V

–

O

–

V and V

–

O

–

S (where S refers to the support). It is generally accepted that

the bridging V

–

O

–

S bond is critical for redox reactions catalyzed by supported

VO

x

[4 – 6, 13, 18] . The discrimination of different surface VO

x

species has usually

been based on their characteristic Raman features. Isolated and polymerized VO

x

species have been characterized by sharp V

=

O stretching bands in the range

990 – 1050 cm

− 1

, broad Raman features in the range 500 – 950 cm

− 1

have been attrib-

uted to V

–

O

–

V stretching in polymerized VO

x

and the presence of V

2

O

5

is indi-

cated by an intense V

=

O stretching band at 995 cm

− 1

. The catalytic properties of

supported VO

x

catalysts have been shown to be sensitive to the distribution of

these surface VO

x

species [4 – 6, 10, 13, 15] .

In addition to the structure in the dehydrated state, the structure of supported

vanadia catalysts under redox reaction conditions is directly related to the catalytic

performance. Vanadia catalysts are usually reduced to some extent during a redox

reaction, and the reduced vanadia species have been proposed as the active sites

[4, 19 – 24] . Therefore, information on the valence state and molecular structure of

the reduced vanadia catalysts is of great interest. A number of techniques have

been applied to investigate the reduction of supported vanadia catalysts, such as

temperature programmed reduction ( TPR ) [25 – 27] , X - ray photoelectron spectros-

copy ( XPS ) [21] , electron spin resonance ( ESR ) [22] , UV - Vis diffuse reﬂ ectance

spectroscopy ( UV - Vis DRS ) [18, 28 – 32] , X - ray absorption ﬁ ne structure spectros-

copy ( XAFS ) [11] and Raman spectroscopy [5, 26, 33 – 41] . Most of these techniques

give information only on the oxidation state of vanadium species. Although Raman

spectroscopy is a powerful tool for characterization of the molecular structure of

supported vanadia [4, 29, 42] , it has been very difﬁ cult to detect reduced supported

4.2 Structure of Al

2

O

3

-Supported Vanadia Catalysts 179

vanadia species with conventional (visible) Raman measurements [26, 33 – 39] . A

widely accepted explanation for this phenomenon is that the Raman cross - section

of reduced vanadium oxide species is very small or even near zero [43] . Thus, it

remains challenging to employ Raman spectroscopy for the determination of

information on the molecular state of reduced vanadia species.

It is notable that most Raman studies of supported VO

x

catalysts have been

carried out using a single excitation wavelength in the visible region (488, 514 or

532 nm) [3 – 15] . However, several investigations on supported transition metal

oxides [16, 17, 44, 45] , including vanadium oxides [16, 17] under ambient condi-

tions, using both UV and visible wavelength Raman excitations suggest that more

complete, and sometimes new, structural information on supported metal oxides

may be achieved by using multiple excitation wavelengths. The reason for this lies

in the strong electronic absorption in the UV and visible wavelength regions

exhibited by most transition metal oxides, which make it possible to measure

resonance - enhanced Raman spectra. Under circumstances where supported VO

x

species are present in a distribution of cluster sizes or coordination geometries, it

is likely that these species also possess a corresponding distribution of electronic

absorption wavelengths. Excitation of Raman spectra within the absorption region

will produce resonance - enhanced spectra from the subset of VO

x

species with

absorptions at the excitation wavelength. By measuring the Raman spectra at

several wavelengths, more information can be obtained about the various VO

x

species in the distribution. Moreover, when UV excitation is employed, Raman

spectra can even be obtained from supported VO

x

at low loadings ( < 1% wt) on

oxides with strong ﬂ uorescence because of the avoidance of ﬂ uorescence and

enhanced sensitivity [38, 46] . In addition, the decreased self - absorption effects in

the UV region indicated by in situ UV - Vis DRS studies of reduced VO

x

and CrO

x

[18, 28 – 31, 47, 48] suggests UV Raman spectroscopy may be capable of detecting

reduced supported metal oxides.

In this case study, we have carried out further investigation into the structure

of VO

x

supported on alumina under both oxidized (dehydrated) and reduced envi-

ronments using both UV - (244 nm) and visible - (488 nm) excited Raman spectros-

copy. Special attention has been directed towards the structure of supported VO

x

at extremely low surface density (down to 0.01 V nm

− 2

).

4.2.1.1 Dehydrated State

The visible and UV Raman spectra of dehydrated V/ θ - Al

2

O

3

are shown in Figure

4.1 . The visible Raman spectra (Figure 4.1 a) of V/ θ - Al

2

O

3

with a surface VO

x

density lower than 0.16 V nm

− 2

are dominated by ﬂ uorescence from the θ - Al

2

O

3

support. Samples with surface VO

x

densities above 0.16 V nm

− 2

start to show

Raman bands from VO

x

species: a weak broad band centered at 915 cm

− 1

, a V

=

O

vibration at 1023 cm

− 1

for 0.16 V, 1030 cm

− 1

for 1.2 V and 1035 cm

− 1

for 4.4 V and

8.8 V samples, and an intense band at 994 cm

− 1

characteristic of V

2

O

5

for 4.4 V and

above. The broad band at ∼ 915 cm

− 1

has previously been ascribed to V

–

O

–

V

stretching modes and has been used as a proof for the presence of polymerized

VO

x

species [5] . Here we assign it to the interface mode, V

–

O

–

Al, based on recent

180 4 Resonance Raman Spectroscopy – Θ-Al

2

O

3

-Supported Vanadium Oxide Catalysts

density functional theory calculations of the VO

x

–

Al

2

O

3

system [6] . This assign-

ment is conﬁ rmed by the following UV Raman results.

UV Raman spectra of dehydrated V/ θ - Al

2

O

3

are shown in Figure 4.1 b. It is strik-

ing to note that some Raman features due to surface VO

x

species can be distin-

guished for V/ θ - Al

2

O

3

samples at extremely low surface densities, even down to

Figure 4.1 Raman spectra of dehydrated V/ θ - Al

2

O

3

as

a function of surface VO

x

density (V nm

− 2

): (a) visible

( λ

excitation

= 488 nm); (b) UV ( λ

excitation

= 244 nm).

4.2 Structure of Al

2

O

3

-Supported Vanadia Catalysts 181

0.01 V nm

− 2

, namely a weak Raman band at 1013 cm

− 1

and a broad shoulder at

around 912 cm

− 1

. Detection of these Raman features, not seen in the visible Raman

spectra, result from the avoidance of ﬂ uorescence and possible enhancement by

the resonance Raman effect. The broad feature centered at 912 cm

− 1

is present with

medium intensity at all loadings, suggesting that this broad Raman feature cannot

be exclusively assigned to V

–

O

–

V stretching and it is consistent with the theoreti-

cal assignment to V

–

O

–

Al bonds [6] . The 1013 cm

− 1

band, assigned to the V

=

O

stretching mode of surface VO

x

species, shifts gradually to higher wavenumber as

the surface VO

x

density increases. This band remains observable at ca 1026 cm

− 1

for a surface VO

x

density of 14.2 V nm

− 2

, a result which is clearly different from

the visible Raman spectra (Figure 4.1 a) where the V

=

O band is above 1030 cm

− 1

for surface VO

x

densities greater than 1.2 V nm

− 2

and disappears above monolayer

loading (8 V nm

− 2

). The differences in Raman shift and in intensity versus VO

x

density between visible - and UV - excited Raman spectra suggest the presence of

different types of surface VO

x

species on the Al

2

O

3

support. At medium surface

VO

x

densities (0.16 – 4.4 V nm

− 2

), a weak broad feature appears near 625 cm

− 1

,

indicating the formation of polymerized VO

x

species on the surface. When the

surface VO

x

density is above 8 V nm

− 2

, the formation of crystalline V

2

O

5

is evident

from the characteristic band at 994 cm

− 1

and additional bands in the range

400 – 600 cm

− 1

.

The results from both UV and visible Raman measurements suggest an evolving

picture of surface VO

x

species on θ - Al

2

O

3

: V/Al

2

O

3

with a surface density lower

than 1.2 V nm

− 2

possesses mainly isolated VO

x

in tetrahedral coordination. At

surface densities higher than 1.2 V nm

− 2

, polymerized VO

x

participate to a larger

extent on the surface, and V

2

O

5

forms at a surface VO

x

density above 4.4 V nm

− 2

.

This picture is further supported by our UV - Vis DRS and TPR measurements (not

shown here) of V/ θ - Al

2

O

3

samples [49] .

Comparing the UV and visible Raman spectra of V/ θ - Al

2

O

3

samples, two kinds

of V

=

O band shifts can be distinguished: a shift as a function of excitation wave-

length for the same sample and a shift as a function of surface VO

x

density for a

series of samples. The latter shift, from 1012 to 1027 cm

− 1

as the surface VO

x

density increases from 0.01 to 14.2 V nm

− 2

in the case of UV - excited Raman spectra,

can be ascribed to structural changes of the surface VO

x

and possibly vibrational

coupling between V

=

O oscillators. The former shift, the same sample showing

V

=

O stretching bands at different frequencies, 8 – 10 cm

− 1

, in UV Raman and

visible Raman spectra, indicates that UV and visible excitation detect different VO

x

species present on the support. This kind of band shift can be explained in terms

of the resonance enhancement effect. The UV - Vis absorption spectra of V/ θ - Al

2

O

3

(Figure 4.2 ) show signiﬁ cant absorption bands at both 244 and 488 nm laser wave-

lengths for samples with surface densities above 1.2 V nm

− 2

. This implies that the

Raman spectra at both wavelengths are resonance enhanced. The longer wave-

length absorption increases in intensity with vanadia loading, relative to the short

wavelength absorption. This demonstrates that a mixture of surface vanadia

species is present and is consistent with a picture where polymerized VO

x

species

are resonance enhanced under visible laser excitation while isolated and less

182 4 Resonance Raman Spectroscopy – Θ-Al

2

O

3

-Supported Vanadium Oxide Catalysts

polymerized VO

x

are resonance enhanced under UV laser excitation. The selective

enhancement of VO

x

species with different cluster sizes is similar to the case of

resonance Raman spectroscopic characterization of conjugated polyene sequences

where different excitation wavelengths give different C

=

C stretching frequencies,

thus allowing conjugated polyenes with different sequence lengths to be detected

[50] . Interestingly, the higher V

=

O Raman shift measured under visible compared

to UV excitation is consistent with this picture by reference to the measurements

on single crystal model catalysts since the V

=

O infrared frequencies are found to

increase with increasing V

=

O surface density [51] . Thus, it would appear that

visible - excited Raman spectra are more sensitive to polymeric or cluster VO

x

and

UV - excited Raman spectra are more sensitive to isolated and less polymerized VO

x

species.

UV and visible Raman measurements of CrO

x

/ θ - Al

2

O

3

were also conducted to

see if the selective resonance Raman detection of different surface metal oxide

species is a general phenomenon (Figure 4.3 ). For simplicity, the comparison of

the spectra features focuses on the Raman band due to Cr

=

O stretching at

1015 cm

− 1

. In contrast to the case of VO

x

/ θ - Al

2

O

3

, the Cr

=

O stretching vibration

is observed at the same Raman shift in both UV and visible Raman spectra and

for all the CrO

x

/ θ - Al

2

O

3

samples. The absence of both excitation - energy - dependent

shifts and coverage - dependent shifts of ν (Cr

=

O) leads to the assumption that the

CrO

x

species are homogeneously distributed, that is only one type of CrO

x

species

is formed on the support surface, either monochromate, dichromate or polychro-

mate. However, the results in Figure 4.3 cannot rule out inhomogeneity in the

surface CrO

x

species. However, excitation - dependent shifts in the Raman band

Figure 4.2 UV - Vis DRS spectra of dehydrated V/ θ - Al

2

O

3

as a

function of surface VO

x

density (V nm

− 2

).

4.2 Structure of Al

2

O

3

-Supported Vanadia Catalysts 183

positions from inhomogeneously distributed systems are rarely observed for any

of a number of reasons, as detailed elsewhere [52] . The most important reason for

most systems [52] , and the likely one for CrO

x

/ θ - Al

2

O

3

, is that the changes in the

environment of species which lead to inhomogeneous broadening of the electronic

absorption are not correlated with corresponding changes in the Cr

=

O vibrational

frequency. If, for example, multiple CrO

x

species have overlapping electronic

absorptions, then the observed Raman shift will be an average over the species.

Additional Raman measurements of the CrO

x

/ θ - Al

2

O

3

system using a series of

excitation wavelengths could resolve this issue.

4.2.1.2 Reduced State

UV Raman spectra of VO

x

/ θ - Al

2

O

3

with low surface VO

x

density (0.06 and

0.16 V nm

− 2

) show that the bands due to θ - Al

2

O

3

at 746 and 841 cm

− 1

increase rela-

tive to those of surface VO

x

species with Raman bands at ca 915 and 1018 cm

− 1

after hydrogen treatment at temperatures of 673 K and above, which is an indica-

tion of the reduction of surface VO

x

species. This is consistent with in situ UV - Vis

DRS measurements from V/ θ - Al

2

O

3

, which show decreasing absorbance in the

UV region upon hydrogen reduction. This results in reduced self - absorption and

increased Raman intensity from θ - Al

2

O

3

upon hydrogen reduction of VO

x

/ θ - Al

2

O

3

.

A new Raman feature is observed near 880 cm

− 1

when the samples are reduced at

873 K, while the intensities of the bands due to oxidized VO

x

species decrease

greatly. The relative intensity change of the bands due to V

=

O ( ∼ 1018 cm

− 1

) and

V

–

O

–

Al ( ∼ 915 cm

− 1

) as a function of reduction temperature shows that the inten-

sity of the band due to V

–

O

–

Al bond decreases more rapidly than that due to

Figure 4.3 Raman spectra of dehydrated Cr/ θ - Al

2

O

3

as a

function of surface CrO

x

density (V nm

− 2

):

(a) UV ( λ

excitation

= 244 nm); (b) visible ( λ

excitation

= 488 nm).

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

Подождите немного. Документ загружается.