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

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higher refraction index than the other, and the incidence angle is greater than the

limit for refraction.

In the ﬁ rst case, it has been recognized that if molecules are adsorbed on the

surface of a metal, only part of the grazing incident radiation is reﬂ ected, part

being absorbed by the adsorbed species. This absorption is greatly increased if the

incident radiation is polarized perpendicular to the metal surface. This is the basis

of so - called Infrared Reﬂ ection Absorption Spectroscopy ( IRRAS ) [27], which is

applied widely to surface studies on metal surfaces. In these conditions, an addi-

tional selection rule applies:

∂

⎛

⎝

⎜

⎞

⎠

⎟

≠

⊥

(3.17)

which means that only the adsorbate vibrations associated with a change in the

dipole moment perpendicular to the crystal surface are detectable. The total reﬂ ec-

tion technique can be applied to detect the growth of oxidic layers over metal sur-

faces (i.e. the skeletal bands of the oxide layers) and this is a widely applied

technique in corrosion and electrochemical research, colloid chemistry and coating

technology.

The second case refers to the so - called Internal Reﬂ ection Spectroscopy, that is

used in the so - called Attenuated Multiple Total Internal Reﬂ ection technique

Figure 3.1 (a) FTIR reﬂ ectance spectrum of a

MgO monocrystal (incidence angle 26.5 ° ,

face [001]). (b) FTIR and FTFIR absorption /

transmission spectra of MgO powder

(reprinted with permission from G. Busca

and C. Resini, “ Vibrational spectroscopy

for the analysis of geological and inorganic

materials ” , in Encyclopedia of Analytical

Chemistry , Robert A. Meyers ed., Wiley,

Chichester, 2000, pp. 10984 – 11020).

3.2 Experimental Techniques 103

104 3 The Use of Infrared Spectroscopic Methods

( ATR ) [28], where an absorbing layer is deposited on one or two external surfaces

of a prism. The light penetrates the prism from a free surface and is multiply

reﬂ ected by the other faces. During the reﬂ ection, part of the light penetrates the

external layer and is absorbed by it. Thus, in the case of the internal reﬂ ection

techniques the reﬂ ectance is given by the following relation:

RA cd=− =−11ε

(3.18)

where d is the thickness of the layer penetrated by the radiation, c the molar con-

centration of the absorbing species in that layer and ε the molar absorption coef-

ﬁ cient of those species. An interesting variation of this technique is the so - called

Cylindrical Internal Reﬂ ection technique ( CIR ), invented by Wilks [29] . In this

case, for example, a hydrogel slurry can be analyzed using cylindrical crystals

immersed in the medium so that IR radiation is multiply transmitted and reﬂ ected

at the internal surface. This technique could be successfully applied to heteroge-

neous catalytic systems at the water – solid interface, or in stages of catalyst prepara-

tion (for example zeolite syntheses).

Another interesting variant of the total reﬂ ection technique is the so - called

Surface Electromagnetic Wave Spectroscopy ( SEWS ), which consists of the genera-

tion of a surface plasmon on a substrate by frustrated total internal reﬂ ection in

a prism located a few microns from the surface. This plasmon is decoupled by a

second prism. Some interesting data relating to surface modes on alumina have

been reported with this technique [30] .

3.2.4

The Diffuse Reﬂ ectance Technique

In recent years, the use of IR spectroscopy of powders in the diffuse reﬂ ectance

( DR ) mode has grown strongly, assisted by the commercialization of appropriate

attachments and cells. This technique is particularly attractive because it does not

require much effort in sample preparation (the powder is simply deposited in a

sample holder). Moreover, this technique, in contrast to the above transmission

technique, takes advantage of light scattering, and hence is very useful for studies

of surface chemistry.

This technique requires the collection, with appropriate collecting mirrors (such

as an integrating sphere), of the radiation scattered by the sample. Obviously, most

photons are essentially simply scattered but those corresponding to the energies

of vibrational transitions are potentially absorbed. The interpretation of the DR

spectra is based on the phenomenological theory of Kubelka and Munk [31, 32]

who deﬁ ned the so - called Kubelka – Munk ( KM ) function as follows:

fR R R ks

∞∞∞

()

=−

()

=12

(3.19)

where R

∞

is the reﬂ ectance of an “ inﬁ nitely thick ” layer and may in practice be

substituted by RR

∞∞

−

′

(i.e. by the reﬂ ectance spectrum from which the reﬂ ectance

of a reference transparent material such as KBr has been subtracted). The KM

function depends linearly on the absorption coefﬁ cient k (which can be expressed

as k = 2.303 ε c , where ε is the molar absorption coefﬁ cient and c is the molar con-

centration) and inversely on the scattering factor s . Obviously, the greater the

absorption coefﬁ cient and the smaller the scattering, the higher the KM function.

An approximation of the KM function is that k is treated as a variable but s is

assumed constant. This is not true in the vicinity of strong absorptions.

More realistic and complex theories consistent with the quite complex nature

of the phenomena involved in the DR of light were developed later. Experimental

studies have shown that this technique is affected by particle size, granulometric

distribution and the refractive index of the particles, which has an important role

when the particle size is near the wavelength of the IR radiation. Diffuse reﬂ ec-

tance Fourier transform ( DRIFT ) studies in the FIR region allow detection of the

skeletal spectra of materials, such as mixed oxide catalysts, pigments and metal

halides.

One important practical problem in DR measurement is the need to cancel the

specular reﬂ ectance from the front surface of the sample, which generates “ nega-

tive ” bands in the DRIFT spectrum, so causing apparent shifts in the true absorp-

tion bands. This is achieved by using appropriate cell designs in commercial

DRIFT attachments.

3.2.5

The Emission Technique

According to Kirchoff ’ s law:

εα==II

em b

(3.20)

the emittance, ε , (i.e. the ratio of the light emitted by the sample I

with respect

to that emitted by a black - body at the same temperature, I

) is equal to the absor-

bance, α , of the sample. The emissivity is proportional to the fourth power of the

temperature difference between the emitting sample and the detector (Stefan ’ s

law). This implies that emissivity is sufﬁ ciently strong at relatively high tempera-

tures to give a good signal - to - noise ratio in a large part of the IR spectrum. In fact,

the blackbody emission at T < 1000 K shows its maximum in the medium IR

region.

A number of problems arise in connection with the use of emission IR spec-

troscopy ( IRES ). One of them arises from the existence of temperature gradients,

which can cause self - absorption of the emitted radiation by the colder outer parts

of the sample itself; another is concerned with the selective reﬂ ection that occurs

in the vicinity of strong absorption bands. This reduces the absorptance and hence

the emittance. Moreover, perturbations can be created by reﬂ ections and emission

by the cell elements. These problems, however, can in part be overcome so that

IR emission spectra can be successfully recorded and are widely used, for example,

in the ﬁ elds of polymer and corrosion science and mineralogy. Some uses of IRES

3.2 Experimental Techniques 105

106 3 The Use of Infrared Spectroscopic Methods

of metal oxides and in the ﬁ eld of surface chemistry and catalysis were reviewed

some years ago by Sullivan and coworkers [33] . These authors reported several

examples of emission spectra of oxide catalysts and of adsorbates on supported

metals, and their review cites at least 11 papers concerning metal oxide surfaces.

Emission studies on minerals and catalytic materials [34] and on adsorption on

metal oxide gas sensors [35] have been published more recently. Some of the

advantages of this technique are the very easy sample preparation and its easy

applicability at high temperatures (150 – 400 ° C). It can be applied to investigate the

temperature - dependence of the radiative properties of materials including, for

example, glasses.

3.2.6

Photoacoustic and Photothermal Techniques

When an IR beam is incident on a solid surface it can be absorbed in part, and

this leads to its conversion into heat. If the beam is modulated (as in interferom-

eters like those of FTIR instruments) and the solid is in contact with a gas (air,

He, Ar, etc.), its conversion to heat gives rise to an acoustic signal. In fact, the

periodic temperature rise so obtained causes a periodic modulation of a gas pres-

sure in the cell, and this can be detected by a sensitive microphone. This acoustic

signal will be the more intense the stronger is the absorption at a particular wave-

length. Reﬂ ected and scattered light are not absorbed and hence do not cause a

photoacoustic signal. However, light absorbed by the gas over the sample causes

signals. This makes necessary the use of monatomic non - absorbing gases (He, Ar,

etc.) The photoacoustic effect, discovered as early as in 1880 by Bell [36] , could be

successfully applied only after the work of Rosencwaig and Gersho [37] in 1976.

The main limits of this technique are that (i) it requires a gaseous atmosphere;

(ii) the cell needs a microphone close to the sample, so the sample cannot be

heated and otherwise activated conveniently; (iii) the technique has an intrinsically

low signal - to - noise ratio. On the other hand, the photothermal effect is much more

efﬁ cient for species in the vapor phase than for bulk and surface species. Photo-

acoustic spectrometry ( PAS ) is not exactly a surface spectroscopy, because the

penetration of the thermal effect is always signiﬁ cant. The extent of this depends

on the modulation frequency, which varies with wavenumber in a FT spectrum.

The PAS technique has found successful application in several ﬁ elds including

heterogeneous catalysis, as reviewed in ref. [38] .

An alternative technique is the so - called Photothermal Beam Deﬂ ection Spec-

troscopy [ PBDS ], based on the so - called “ mirage ” effect ﬁ rst reported by Boccarra

and coworkers [39, 40] . In this case, the periodic temperature rise caused by the

absorption of the modulated IR radiation (i.e. the photothermal effect) is detected

optically because it causes periodic deﬂ ections of a laser beam passing close to the

surface of the solid sample. The PBDS technique has some advantages over the

PAS technique, because of its lower limits of sample dimensions, but it has dis-

advantages because of the critical geometric setup. Like PAS, PBDS can have

advantages with respect to traditional IR technique for the detection of surface

vibrations in very opaque materials. This has resulted in its application to carbons

and coals.

3.3

The Vibrational Modes of Molecular Species and of Inorganic Solids

3.3.1

“ Isolated ” Molecular Species

The total degrees of freedom associated with a chemical species containing N

atoms are 3 N . If this chemical species is a molecule in the gaseous state, three of

these degrees of freedom are associated with its translations and another three

with its rotations, so that, in the most general case, six modes are associated with

“ external ” motions (rotations and translations). The remaining 3 N − 6 degrees of

freedom are associated with “ internal ” vibrational modes. However, if the mole-

cule is a linear one, only two degrees of freedom are associated with rotations,

because no rotational freedom exists around the molecular axis. Thus, in this

particular case, the degrees of freedom associated to vibrations are 3 N − 5. A

complete treatment of the principles of vibrational spectroscopy is beyond the

scope of the present chapter. The methods for the determination of the number

and the optical activity of the vibrational modes of molecular species can be found

in several books [41 – 43] .

The 3 N − 6 (or 3 N − 5) degrees of vibrational freedom give rise to vibrational

modes that differ in their symmetry with respect to the symmetry elements of the

molecular point group and in their multiplicity. The non - degenerate modes are

denoted as A or B in relation to their symmetry or antisymmetry with respect to

the rotation about the principal symmetry axis. Doubly degenerate modes and

triply degenerate modes are denoted E and F respectively. The subscripts g and u

denote modes that are symmetric or antisymmetric, respectively, with respect to

the center of symmetry (if any), the superscript symbols ′ and ″ distinguish modes

that are symmetric or antisymmetric, respectively, with respect to symmetry planes

not containing the principal axis ( σ ≠ σ

), while 1,2,3 subscripts are related to the

symmetry with respect to other symmetry axes.

Character tables, which can be found in several vibrational spectroscopy books,

allow the determination, for any molecular point group, of the species (or irreduc-

ible representations) in relation to the symmetry elements typical of that group.

As further cited below, the classiﬁ cation in terms of a particular symmetry species

determines the activity (IR activity, Raman activity, both IR and Raman activity or

inactivity) of any mode.

In the gas phase, the vibrational transitions couple with the rotational ones,

giving rise to rotovibrational spectra . The different rotovibrational contours depend

on the symmetry of the vibration in relation to the symmetry of the molecule, and

on the resolution of the rotational components. In some cases, the energy of the

“ pure ” vibrational transition corresponds to the minimum of the absorption band:

3.3 The Vibrational Modes of Molecular Species and of Inorganic Solids 107

108 3 The Use of Infrared Spectroscopic Methods

in fact the “ pure ” vibrational transition is forbidden because the rotational quantum

number does not change. This is the case for gaseous CO

and CO, as shown in

Figure 3.2 . In other cases the energy of the pure vibrational transitions must be

determined using the sharp maximum in the middle. In Figure 3.3 the spectrum

of gaseous CO is compared with that of CO adsorbed on different solids. The

complex structure due to the rotovibrational contour present in the gas phase

spectrum is essentially lost upon adsorption. When weakly adsorbed, such as on

the OH group of silicalite, some rotational features may still be present (see aster-

isks in the ﬁ gure) because the molecule can still rotate around its main axis. The

CO stretching frequency for gaseous CO (2143 cm

− 1

) shifts a little down in the liquid

phase (2137 cm

− 1

) where the rotational structure has disappeared. Liquid CO is

frequently observed upon adsorption on solids at low temperature (such as on sili-

calite in the ﬁ gure). Relevant shifts upwards and downwards in the adsorbed state

are relative to the different interaction on cationic sites and hydroxyl groups, or on

reduced metal centers, on which the use of CO as a probe is based (see below).

3.3.2

Crystalline Solids

When a crystalline solid is considered, the N atoms present in the smallest (primi-

tive) Bravais cell must be taken into consideration to count the fundamental

vibrational modes. They give rise to 3 N total degrees of freedom, three of which

Figure 3.2 FTIR spectra of gaseous CO

, CO, NO and CH

(IR active fundamental modes) and of CH

OH (C

–

O stretching

mode only).

give rise to translational modes of the cell as a whole, which are the acoustic

modes. Thus, the vibrational degrees of freedom (optical modes) are, in this case,

3 N − 3 .

If the solid is molecular, the molecules (considered to be formed by M atoms,

where M = N / r and r is the number of molecules in the smallest Bravais cell) can

be treated as for the gas phase, so giving rise to 3 M − 6 (or 3 M − 5 if linear) vibra-

tions for each molecule. The degrees of freedom associated with the “ external ”

modes of every molecular unit (6 r for non - linear molecules and 5 r for linear mol-

ecules) give rise to lattice vibrations ( “ frustrated translations and rotations ” ) and

to three acoustic modes. On the other hand, the “ internal ” vibrations of each

molecules should in principle give rise to r - fold splitting, owing to the coupling of

the vibrations within its primitive unit cell as a whole.

Analysis of the symmetry of the chemical species under study (i.e. the point

group for a “ free ” molecule, the space and factor groups for a crystal [44] ), accord-

ing to the site symmetry of every atom, allows the determination of the irreducible

representation of the total modes and, after the subtraction of the translational

and rotational modes (the acoustic modes for the crystals), the irreducible repre-

sentation of the vibrational (or “ optical ” ) modes can be obtained. This means that

the number of vibrational modes belonging to the symmetry species associated

with the molecular or crystal symmetry can be counted. Consequently, the number

of active modes can be counted, according to the symmetry selection rules of the

different techniques (in particular IR and Raman).

Figure 3.3 FTIR spectra of gaseous CO and of CO adsorbed

on silica - alumina, on silicalite (150 K), on Au/CeO

and on

Pt - Pd alloy nanoparticles on MCM41 mesoporous

silica - alumina.

3.3 The Vibrational Modes of Molecular Species and of Inorganic Solids 109

110 3 The Use of Infrared Spectroscopic Methods

Every vibrational mode is due to motions of the entire chemical species (the

molecule or the primitive unit cell) as a whole. In principle, molecular and crystal

dynamics calculations should deﬁ ne rigorously the motions of every atom of the

chemical species upon a vibrational mode. This approach gives rise to a very

complex picture (at least for large and complex chemical species), so that the

results are sometimes not easily interpreted and comparison between the vibra-

tional behavior of similar species is frequently difﬁ cult.

However, a group approximation can be frequently used. Thus, the vibrational

modes can often be approximately attributed to the movements of small groups

of atoms. This is possible in particular if very different bonds are present in the

molecule, so that the coupling of their movements is negligible. In this way, it is

possible to “ dissect ” the chemical species under study into “ pieces ” and to consider

separately the vibrations of groups of few atoms (like the functional groups in

organic compounds). This makes possible a satisfactory, although approximate,

interpretation of the vibrational spectra of polyatomic molecules and of some

crystals, and allows easy comparison between the vibrational features of related

chemical species.

In the case of crystalline solids, more than one equivalent structural unit may be

present in the primitive cell. This results in splittings of the fundamental vibrational

modes of these units. In the case of many crystalline solid materials “ covalent ”

units (e.g. oxo - anions for oxo - salts) are present, together with other groups bonded

by ionic bonds (e.g. the cations in the oxo - salts). According to the above group

approximation, the internal vibrations of the covalent units can be considered sepa-

rately from their external vibrations ( hindered rotations and translations of the group

that ﬁ nally contribute to the lattice vibrations and to the acoustic modes of the unit

cell) and those of the other units. The presence of a number of covalent structural

units in the primitive cell, causes their internal modes to split.

On the other hand, when the unit cell is centrosymmetric the mutual exclusion

rule is valid, so that Raman active modes are IR - inactive and vice versa. In practice,

for centrosymmetric cells containing N oxo - anions, every internal vibrational

mode of the oxo - anion gives rise to N /2 IR active modes and N /2 Raman active

modes.

Additionally, the TO/LO splitting may be relevant and makes more complex the

identiﬁ cation of the vibrational modes of crystalline solids.

3.3.3

Amorphous Solids

Amorphous solids differ from crystalline solids because no long - range order

occurs. So, variable coupling exists between the vibrational modes of similar or

equivalent structural units. Consequently, amorphous solids can be treated in the

same way as liquids and gases. The vibrational spectra of amorphous materials

can present a smaller number of broader features than those of corresponding

crystalline materials, where crystal coupling effects can produce multiple sharp

features.

3.4

The Skeletal IR Spectra of Metal Oxides

IR spectroscopy is largely used for the characterization of metal oxide catalysts in

relation to their structural features, with additional possible information on their

morphology. Several collections of IR, Raman or both IR and Raman spectra of

inorganic materials and minerals have been published, and are available electroni-

cally. In the following we will brieﬂ y review some of the applications of vibrational

spectroscopies in the characterization of such materials.

3.4.1

Crystalline Simple Anhydrous Oxides

In network oxide structures the metal – oxygen bonds are all almost equivalent, so

that their movements couple extensively. Also stretching and deformation vibra-

tions mix. As a result of this, assignments of the different modes to a particular

vibration can be very uncertain. However, the highest frequency vibrational fea-

tures can be generally associated with stretching modes. The position of these

modes can be assumed to be primarily dependent on the coordination of the metal

ions involved and on the extent of the condensation of the resulting polyhedra, as

shown in Table 3.1 . This approach has been proposed by Tarte [45, 46] and is a

useful approximation for the interpretation of the IR and Raman skeletal spectra

of oxides of unknown structure. On the other hand, and perhaps more correctly,

analysis of the spectra suggests that the higher frequency modes are mainly to be

assigned to motions of the lighter oxygen atoms, so that they can be assigned to

stretchings of oxygen in different coordination states. This alternative approach

proposed ﬁ rst by Beattie and Gilson [47] , is shown in Table 3.2 .

Most simple oxide structures have been the object of extensive studies and

complete vibrational analysis. Some examples are summarized in Table 3.3 .

3.4 The Skeletal IR Spectra of Metal Oxides 111

Table 3.1

Absorption range (cm − 1) of some elements and coordinated compounds.

Atom X Coordination

Tetrahedral XO

Octahedral XO

Isolated Pure oxide Isolated complex

800 – 700 700 – 800

∼ 500

1000 – 650 600 – 300 500 – 400

III

850 – 700 700 – 300 500 – 350

III

750 – 600 600 – 350

III

450 – 300

III

650 – 550 650 – 300 400 – 300

700 – 550 450 – 300

650 – 400

< 300

< 350

112 3 The Use of Infrared Spectroscopic Methods

Table 3.2 Typical oxygen atom vibrational modes for metal oxide structures.

Structure No. of

oxygen

atoms

No. of

vibrational

modes

Stretchings Frequency

region

(cm

− 1

)

Deformations

Terminal

1 3 1 1500 – 800 1 degenerate

2 6 asym +

sym

1300 – 700 bending twisting Wagging Rocking

3 9 asym(2) +

sym

1300 – 700 2 asym.

bendings

sym.

bending

2 rockings Torsion

Bridging

–

M 1 3

Bent asym +

sym

1200 – 600 rocking

Linear asym bending degenerate

Triply

bridging

O 1 3

Pyramidal asym(2) +

sym

< 600

Planar asym(2) out - of - plane

3.4.1.1 Oxides of Divalent Elements

According to its very high symmetry, the rock - salt structure taken by several diva-

lent metal oxides, (space group Fm m O3

≡

, with Z = 4, and with one formula

unit only in the smallest Bravais mcell) only gives rise to one triply degenerate IR

active mode. The TO quoted for MgO at 401 cm

− 1

(see Figure 3.1 a), corresponds

to the lower frequency limit of the reﬂ ectance band while the LO corresponds to

the higher energy limit of the reﬂ ectance, and is quoted at 718 cm

− 1

for MgO (see

Figure 3.1 a). The transmission/absorption IR spectrum of the MgO powder shows

the maximum slightly above ν

(407 cm

− 1

in our spectrum of a polyethylene disk)

and a shoulder near ν

. Other components arise from particles with different

morphologies although the microcrystal powder spectrum of MgO is also affected

by surface relaxation. ν

is found near 330 cm

− 1

for MnO (manganosite) and near

405 cm

− 1

for NiO powder, which also have a rock - salt type structure.