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

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3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 133

pounds which ﬁ nd a number of applications in several ﬁ elds. Its thermal decom-

postion gives rise to a mixed oxide whose virtual composition is 5 MgO · MgAl

although these phases give rise to partial solid solutions depending on decomposi-

tion temperature. Such materials ﬁ nd several applications in today ’ s industrial

heterogeneous catalysis. Similar materials are obtained using other divalent (Ni,

Co, Zn, Cu) and trivalent (Ga, Fe, Cr) elements with different anions (nitrate,

chloride, etc.) Hydrotalcite - type solids crystallize in the

Rm D

3 166

≡≡n. space

group. The full rhombohedral unit cell contains three cations. However, owing to

the presence of three lattice points in the overall rhombohedral unit cell, the small-

est Bravais cell only contains one cation, that is, one of the above formula units.

Cation distribution in the 3a Wyckoff sites may be considered random. The struc-

ture is formed by brucite - type layers with the formula M(OH)

with carbonate ions

in the interlayer region to balance the charge excess due to the substitution of the

trivalent cation for the divalent one. Obviously, there are two trivalent cations per

carbonate ion. The water content is also related to the proportion of trivalent

cations. For x = 0.33 (for every three cations, one is trivalent) there is half a water

molecule and one sixth of a carbonate ion per smallest Bravais cell. The brucite -

type layers are made of ﬂ attened octahedral MO

and triply bridging hydroxyl

groups. Both water molecules and carbonate ions stay planar centrally between

the layers and parallel with respect to them. The distance between the protons of

both hydroxyls and water and the oxygen atoms of the carbonate ions and water

are such that hydrogen bonding should be considered to be negligible. The IR

spectra show the features of hydroxyl groups and of the anions [78] .

3.5.4

Impure Metal Oxides

Commercial metal oxide powders may be contaminated by molecules from the

atmosphere as well as those arising from the preparation procedure. Among

typical contaminants, water may produce hydroxides while carbon dioxide can

produce surface or bulk carbonates. This is the case of the “ MgO ” sample whose

spectrum is reported in Figure 3.13 a, which shows the OH stretching band near

3700 cm

− 1

of Mg(OH)

and the strong bands in the region 1500 – 1400 cm

− 1

due to

the C

O stretching of carbonate species. Both features disappear on heating under

vacuum, owing to decomposition to MgO. Other frequent contaminants are sul-

fates and hydrocarbon species.

3.6

The IR Spectroscopy of Adsorbed Probe Molecules for Surface

Chemistry Characterization

IR spectroscopy of adsorbed probe molecules is mostly performed with either the

transmission/absorption technique or with the DRIFT technique. In the transmis-

sion/absorption technique, self - supporting pressed disks of the pure oxide powders

134 3 The Use of Infrared Spectroscopic Methods

are prepared and put into the IR beam, in an appropriate cell that allows heating,

cooling and gas/vapor manipulation. Activation is mostly performed by outgassing

at relatively high temperature. In the case of DRIFT experiments the catalyst is

deposited on the sample holder, with gentle pressure, and activation is mostly

performed by ﬂ owing in inert dry gas.

The transmission/absorption IR spectrum of the oxide disk has a baseline slope

increasing towards higher frequency, reﬂ ecting the scattering of the beam, which

increases with increasing frequency (wavenumber), as shown in Figure 3.13 b and

c, where the spectra of pressed discs of a “ MgO ” commercial powder are reported.

The slope depends on the particle size and may be very steep for powders that are

constituted by quite large particles (near 1 µ m). Thus, transmission for large particle

size oxides may be zero at 4000 cm

− 1

or even lower frequencies. In the case of

DRIFT, the reﬂ ectance actually increases by increasing scattering, so that the base-

line is ﬂ at or even decreasing. The absorptions due to the surface hydroxyl groups

are superimposed on this line, in the region 3800 – 3000 cm

− 1

. In some cases, absorp-

tions due to overtones of bulk or surface metal oxide stretching vibrations are also

superimposed on the scattering line. On the low - frequency side, below 1300 –

600 cm

− 1

depending on the skeletal spectrum of the solid, the absorptions due to

bulk skeletal fundamental vibrations cut off the spectra of the pure powder disks.

The advantages of the DR technique over the transmission/absorption tech-

nique in the ﬁ eld of the surface chemistry of oxides are:

1) easier sampling ;

2) applicability to powders that scatter too much for the transmission/absorption

technique, assuming the surface area is sufﬁ ciently high to detect surface

vibrations with a sufﬁ ciently high signal - to - noise ratio ;

3) slightly lower sensitivity to bulk conduction phenomena, because of a higher

surface - to - bulk sensitivity ratio.

In effect, for general purposes (adsorption studies on relatively small - crystal - size

powders) the main advantage of the DR technique is the ﬁ rst one, with the disad-

vantages of: (i) less easy optical setting up; (ii) the requirement to work in ﬂ ow

rather than in vacuum (mainly because the sample is not pressed and powders

are highly mobile in vacuum, and because of a more difﬁ cult evacuation of the

cell, due to their design and size) with a consequently more difﬁ cult sample

activation.

The spectra of adsorbed and surface species obtained using transmission and

DR techniques are very similar in quality, in regard to resolution, signal - to - noise

ratio and sensitivity.

3.6.1

Infrared Characterization of Surface Element – Oxygen Bonds

3.6.1.1 Surface M – O – M Bridges

The adsorption of probe molecules may provide evidence of the existence of

metal – oxygen bonds at the surface of a metal oxide. In Figure 3.14 , the subtraction

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 135

spectra relative to the adsorption of pyridine on β - Ga

are reported. Besides the

sharp “ positive ” peaks due to adsorbed pyridine, a broad “ negative ” band is

observed in the range 850 – 880 cm

− 1

, with a tail towards higher frequencies. A

similar absorption is found in several other cases, such as on aluminas [79] and

aluminates [80] just above the cut - off due to the skeletal modes, and is attributed

to the relaxation of surface metal – oxygen bonds by adsorption of molecular

probes.

The existence of surface metal – oxygen bonds can also be indirectly deduced by

the reactivity of these bonds with molecules from the gas phase, such as the

surface hydration producing new hydroxyls, the surface carbonation producing

carbonates, and also some more complex reactivity, such as the reactivity with

alkoxysilanes producing surface alkoxides that may later be converted to surface

hydroxides by elimination [81] .

3.6.1.2 Surface M = O “ Double ” Bonds on Binary Oxides

Metals and other elements in very high oxidation states can give rise to element –

oxygen double bonds in their oxides. This is the case for vanadyl, niobyl,

molybdenyl, chromyl and tungstyl groups, as well as of P

O bonds present in

oxo - compounds of the corresponding elements. The location at the surface of

Figure 3.14 FTIR spectra of the surface species produced by

adsorption of pyridine on β - Ga

activated at 673 K: (a) in

the presence of 1 Torr of the vapor and after outgassing at

(b) room temperature, (c) 373 K, (d) 523 K. The spectrum of

the activated sample has been subtracted from those

recorded after adsorption.

136 3 The Use of Infrared Spectroscopic Methods

such bonds has been observed in the cases of V

O bonds (1038 cm

− 1

) at the

surface of V

[82] , Nb

O bonds at the surface of niobic acid (Nb

n H

[83] and W

O bonds at the surface of WO

(950 or 1040 cm

− 1

) [84] . These fea-

tures are more clearly observed as negative bands in the subtraction spectra

after adsorption of probe molecules (spectrum recorded after adsorption from

which the spectrum of the “ clean ” sample has been subtracted), showing that

such surface species are perturbed upon adsorption. Note that double bonds

also exist in the bulk but adsorb at lower frequencies in vanadia and niobia,

while they do not exist in the bulk of tungsta (a distorted ReO

structure with

octahedral coordination for the cations) but are formed at the surface as in the

structure of several tungstate species.

3.6.1.3 Surface Oxo - Anions

Surface Carbonate, Nitrate and Borate Species As already reported (Table 3.5 ), free

carbonate, nitrate and borate ions, as a result of their trigonal D

3 h

symmetry, have

one characteristic strong IR vibration ( ν

; asymmetric XO stretch) found near

1480 cm

− 1

for borates, 1415 cm

− 1

for carbonates and near 1380 cm

− 1

for nitrates,

together with two lower frequency IR - active deformation modes.

The lowering of the symmetry connected with coordination causes the splitting

of the doubly degenerate ν

vibration, as well as the IR activation of ν

, the Raman -

active symmetric deformation mode. Four different types of carbonate ions are

usually considered as possible surface species. They can be represented by the sim-

pliﬁ ed models I – IV (Scheme 3.1 ). Type I (symmetrical) refers to a surface species

whose spectroscopic features correspond to those of a non - coordinated ion. Mono-

dentate, bridging, chelating and polydentate structures may also be formed. The

structures may be distinguished considering in parallel the stability (polydentate >

bidentate > chelating > monodentate) and the extent of the splitting of ∆ ν

(bidentate

≥ chelating > monodentate ≥ polydentate) [85] . Some carbonate species, character-

ized by very large ∆ ν

and very weak stability have been denoted as “ covalent ” or

“ organic - like ” , but are most likely due to bent very strongly perturbed CO

molecules

[86] . Bicarbonate ions [HCO

]

−

can also be formed by adsorption of or contamination

by CO

. These species are characterized by ν OH modes near 3620 cm

− 1

, δ OH modes

near 1300 – 1200 cm

− 1

, as well as by two ν C

O modes ( ∼ 1600 and ∼ 1450 cm

− 1

Surface Silicate and Hydrogen - Silicate Species The addition of silica to oxides such as

alumina and titania is sometimes due to the need to enhance surface area and thermal

Scheme 3.1 Posible coordinations of carbonate ions and

other trigonal anions (nitrates, borates): I: trigonal symmetric;

II: monodentate; III bidentate (bridging); IV chelating;

V: polydentate.

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 137

stability and retard phase transitions [87] . Surface silication using silicon alkoxides is

also sometimes performed in order to modify the surface acidity [88, 89] . These treat-

ments generally give rise mostly to surface hydrogen - silicate species [HOSiO

]

3 −

, well

characterized by the characteristic OH stretching of surface silanol groups near

3735 cm

− 1

, and strong Si

–

O stretching modes in the region around 1000 cm

− 1

. In the

case of basic oxides, such as MgO and CaO, bulk orthosilicate species form easily

[90] . This also occurs by the reactive adsorption of methylsiloxanes [91] .

Surface Sulfates Tetrahedral oxo - anions give rise to nine internal vibrations plus

six external vibrations contributing to lattice vibrations and to acoustic modes of

the solid. When the ion takes its highest symmetry (T

point group) the symmetric

stretching and the symmetric deformation are Raman active, the last being doubly

degenerate. The asymmetric stretching and deformation are both IR and Raman

active and are triply degenerate. By lowering of the symmetry, as occurs on sur-

faces (Scheme 3.2 ), the degeneracies are broken and all modes can become IR and

Raman active. Scheme 3.2 shows possible geometries of these species, when they

are located on surfaces.

Spectroscopic studies showed that the sulfate ions [92] on ionic oxides in dry

conditions at low coverage, are tetracoordinated with one short S

O bond (mono -

oxo structure), characterized by very strong S

O bands in the range 1420 –

1350 cm

− 1

. In Figure 3.15 the spectrum of a sulfated ceria - alumina is presented;

the band at 1380 cm

− 1

is in fact due to this S

O stretching mode. S

–

O single - bond

stretching modes are observed below 1250 cm

− 1

, with the corresponding overtone

modes in the region 2500 – 1900 cm

− 1

. At higher coverage, disulfate species are

assumed to exist [93, 94] , although real proof of this probably does not exist.

However, surface sulfate species are strongly sensitive to hydration. The spectrum

changes extensively and the species probably convert into dioxo species.

Surface Vanadate, Molybdate and Tungstate Species The pure and mixed oxides

and the salts of vanadium, molybdenum and tungsten in their higher oxidation

states are used widely as heterogeneous catalysts, for selective oxidation as well as

for acid catalysis. Similarly, supported chromia and rhenium oxides ﬁ nd wide

application in different catalytic processes.

For most of the oxide - supported “ monolayer ” oxides (e.g. vanadia, molybdena

and tungsta supported on alumina), titania, zirconia and silica surface species are

Scheme 3.2 Left: isolated truly tetrahedral metallate species:

the four M

–

O bonds are equally long and the O

–

O angle

is near 109 ° 28 ′ ; the point group is T

; middle, “ deformed ”

fourfold coordinated metallate species, i.e. di - oxo species, the

point group is C

; right, “ deformed ” fourfold coordinated

metallate species, i.e. mono - oxo species, the point group is C

138 3 The Use of Infrared Spectroscopic Methods

observed (at least at low loadings, i.e. well below the loading corresponding to the

“ theoretical geometric monolayer ” ) which are spectroscopically characterized after

treatments which allow the desorption of water [42, 47] . Features include:

(i) a very high position of the very intense band observed in IR and Raman

activity, similar to that of gaseous mono - oxo species;

(ii) the coincidence of the position of the band detected in Raman and IR spectra

and its non - complexity in both cases;

(iii) the detection in the IR spectrum of a single overtone band.

Partial

O exchange experiments showed a simple splitting like that found,

for example, in the cases of WO

/Al

[95] and V

/TiO

catalysts [96] .

These data are a strong demonstration that the species are isolated and mono -

oxo. In Figure 3.15 , the fundamental (right) and ﬁ rst overtone region (left) for a

/ZrO

acid catalyst and of a V

- WO

/TiO

SCR. DeNOx catalyst are shown.

The strong band at 1003 cm

− 1

and the weaker one at 1996 cm

− 1

are due to the

stretching mode (fundamental and ﬁ rst overtone, respectively) of W

O “ double

bonds ” of mono - oxo tungstyl species of the WO

/ZrO

catalyst. The same modes

are found at ∼ 990 and 1973 cm

− 1

for the V

- WO

/TiO

catalyst where the bands

at 1035 and 2041 cm

− 1

are due to the fundamental and the ﬁ rst overtone, respec-

tively, of the V

O double bonds.

On the other hand, a further demonstration of the “ isolated ” nature of these

species is expected from the absence of modes assignable to M

–

M asymmetric

Figure 3.15 FTIR spectra of pure powder pressed disks of

sulfated ceria - alumina, V

- WO

- TiO

catalyst and WO

- ZrO

catalyst.

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 139

stretchings. These modes are expected to be very strong in the IR spectra, but

unfortunately the region of these modes is not available for alumina and titania

supported catalysts because of the opacity of the pressed disks in this region. In

the case of zirconia supported catalysts, where this region is in part available, this

band is not present, at least for low loaded molybdena [97] and tungsta [98]

samples. This topic has been discussed in detail previously [99] . At high loadings,

most authors agree that polymeric anions form.

3.6.2

Spectroscopic Detection of Surface Br ø nsted Acid Sites

The fragments arising from the dissociative adsorption of water on the surface of

metal oxides give rise to hydroxyl groups that are potentially active Br ø nsted acid

sites [100] or basic sites. Such surface hydroxyl groups can be detected directly,

recording the IR spectra of the oxide catalyst powders after treatments that allow

the desorption of molecularly adsorbed water, in the region 3800 – 3000 cm

− 1

, where

the O

–

H stretching modes ( ν OH

) fall. The position and shape of the ν OH bands

of the surface hydroxyl groups is informative on their coordination. Covalent oxide

components [101, 102] usually give rise to very typical strong sharp peaks due to

covalently bonded terminal OH groups. For example, silica - containing oxides (e.g.

any silicas and silicated oxides, see Figure 3.16 ) almost invariably show a peak in

the range 3750 – 3730 cm

− 1

while P

- containing samples contain a sharp band at

3670 – 3650 cm

− 1

observed in supported phosphoric acid samples [103 – 105] and in

several bulk phosphates [69, 83, 106 – 108] . Similarly, B

–

OH bonds give a sharp

band at 3710 – 3690 cm

− 1

[109, 110] , Ge

–

OH bonds at 3685 – 3670 cm

− 1

Conversely, in ionic oxides bridging and triply bridging hydroxyl groups are also

formed at the surface. Thus, the OH spectrum is frequently more complex (see,

for example, Figure 3.17 for alumina and Figure 3.18 for gallias, zirconias and

ceria). It is usually agreed that the OH stretching is highest for terminal OH

Figure 3.16 FTIR spectra of the surface hydroxyl groups of

SiO

samples after outgassing at 473 K (broken lines) and 673 K

(full lines).

140 3 The Use of Infrared Spectroscopic Methods

Figure 3.17 FTIR spectra of the surface hydroxyl groups of

γ - Al

samples: left aluminum oxide C from Degussa after

outgassing at 673 K and 873 K; right, samples from Akzo (A),

Sud Chemie (B), Engelhardt (C) and Condea (D), all having

surface area in the range 180 – 200 m

− 1

Figure 3.18 FTIR spectra of the surface hydroxyl groups of

α - Ga

, β - Ga

, monoclinic ZrO

, tetragonal ZrO

and

CeO

, all after outgassing at 773 K.

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 141

groups, intermediate for bridging and lowest for triply bridging OH groups (see

Scheme 3.3 ) This approach was ﬁ rst systematically proposed by Tsyganenko and

Filimonov [111] . Interesting support for this approach was given more recently by

the group of Lavalley [112] who showed a relationship between the OH stretching

bands of the surface hydroxyl groups and the C

–

O stretching bands of the surface

methoxy groups of methoxylated oxides, namely zirconia, ceria and thoria.

3.6.2.1 Hydroxyl Groups in Silica

Structurally, amorphous silica is quite a covalent material [101, 102] whose surface

behavior is dominated by the chemistry of the terminal silanol groups, characterized

by a sharp and strong IR OH stretching band at 3748 – 3730 cm

− 1

(Figure 3.16 ). The

IR band of silanol groups always presents a tail to lower frequencies, which has

been in part attributed to geminal silanols. Near - lying terminal silanol groups also

make hydrogen bonds with each other, giving rise to chain - bonded hydroxyls. These

groups, responsible for broad OH stretching bands near 3650 and 3530 cm

− 1

, con-

dense upon outgassing at high temperature, to produce gas - phase water and surface

siloxane bridges. The relative amount of isolated and H - bonded silanols depends

on porosity [113] , as shown in Figure 3.16 where the spectra of a highly porous silica

gel are compared with those of an almost non - porous material (Aerosil). The silanols

make the surface of highly hydroxylated silicas strongly hydrophilic, and their wet

surfaces are even more active in adsorption. It is well known that hydrogen bonds

also occur between the silanol groups and non - polar molecules such as hydrocar-

bons, allowing the use of silicas for adsorption of these compounds.

Silicalite - 1 is a fully siliceous zeolite, with the MFI structure. Its crystalline

framework, composed of SiO

tetrahedra, has an essentially covalent and hydro-

phobic character. When well crystalline, hydrophilic silanols, whose acidity is

comparable with that of silica [113, 114] , are present essentially at the external

surface (Figure 3.16 ). However, when prepared in a “ defective ” form, nests of H -

bonded silanol exist, giving rise to a quite complex IR spectrum in the OH stretch-

ing region with a prominent band at 3500 cm

− 1

and several sharp maxima in the

range 3745 – 3650 cm

− 1

; they are at least in part located in the channels [115] and

make the structure more hydrophilic.

3.6.2.2 Hydroxyl Groups in Alumina

Many studies have been devoted to the multiplicity of the surface hydroxyl groups

of aluminas. After the work of Peri [116] , and of Tsyganenko and Filimonov [111] ,

Scheme 3.3 Structure of hydroxyl groups on metal oxides:

I: covalent, terminal; II: ionic, terminal, III: ionic, bridging;

IV: ionic, triply bridging.

142 3 The Use of Infrared Spectroscopic Methods

Kn ö zinger and Ratnasamy reported a very popular model of the different exposed

planes of spinel type aluminas [117] . This model has been later modiﬁ ed by Busca

and coworkers [80, 118] and reviewed by Morterra and Magnacca [119] . More

recently, additional investigations have been published by Tsyganenko and Mar-

dilovich [120] and, on the basis of theoretical calculations by Fripiat and coworkers

[121], by Digne and coworkers [122] , who also attempted to model the interaction

of probe molecules. Lambert and Che [123] further reviewed these models and

demonstrated that the problem is still not solved. At least ﬁ ve components are

usually present in the IR spectrum of the hydroxyl groups of aluminas (Figure

3.17 ), at about 3790, 3770, 3740 – 3720, 3700 – 3670 and 3580 cm

− 1

, although in many

cases the observed peaks are multiple. The available data show that the hydroxyl

groups absorbing at higher frequency (above 3700 cm

− 1

) are available for interac-

tion with molecular probes, while those absorbing below 3700 cm

− 1

appear to be

substantially inactive [124, 125] . Liu and Truitt [126] emphasized the close proxim-

ity of surface OH groups of aluminas to Lewis acid sites. The bands below 3700 cm

− 1

are more sensitive to outgassing than those above 3700 cm

− 1

(Figure 3.13 , left)

suggesting that they can be hydrogen bonded. To our knowledge, a complete

investigation of the accessibility of these sites has not yet been published.

3.6.2.3 Surface Hydroxyl Groups of Other Sesquioxides and of Spinel - Type

Mixed Oxides

The spectra of the surface hydroxyl groups of oxides with similar structures show

some parallelism [118, 119] . In Figure 3.18 , left, the spectra of the surface hydroxyl

groups of gallia polymorphs α - Ga

and β - Ga

are shown. In Table 3.7 the

observed bands for these solids are compared with those observed for the corre-

sponding alumina polymorphs α - Al

and θ - and γ - Al

, as well as for the poly-

morphs of ferric oxide α - Fe

and γ - Fe

. α - Al

, α - Fe

and α - Ga

are

isostructural as is α - Cr

(corundum – haematite structure) with only octahedral

coordination for the cations, while β - Ga

, θ - and γ - Al

and γ - Fe

are all

structures derived from non - stoichiometric spinels with both tetrahedral and octa-

hedral coordination for the cations. The IR spectra of the surface hydroxyl groups

show some similarity, supporting the assignments of the higher frequency band

to terminal OH groups on cations in a tetrahedral - like environment. These bands

are always split, and are present in the spectra of the spinel - type sequioxides, but

Table 3.7 Surface OH groups on sesquioxides.

γ - Al

α - Al

γ - Fe

α - Fe

β - Ga

α - Ga

I 3790 – 3740 – 3745

II 3770 – 3725 – 3738

I 3730 3730 3675 3680, 3660 3685 3688

Bridging 3680 3690 3640 3640 3670 3660

Triply bridging 3580 3550 3450 3480, 3440 3550