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 143
absent from those of corundum sesquioxides. The spectra of the surface OH
groups of stoichiometric spinels, such as magnesium, zinc, cobalt and nickel alu-
minates and ferrites [118, 119], show these bands, but unsplit, confi rming the
above assigment with the additional information that the presence of cation vacan-
cies in the spinel structure is associated with this splitting.
3.6.2.4 Surface Hydroxyl Groups of Rock - Salt - Type Metal Oxides
Rock - salt - type oxides generally show only one predominant band: 3745 cm
− 1
for
MgO (see Figure 3.13 c), 3682 cm
− 1
for MnO [101] , 3680 cm
− 1
for CoO [127] and
NiO [128] . These bands are due to OH groups located above cations in overall
octahedral coordination.
3.6.2.5 Surface Hydroxyl Groups of the Oxides of Tetravalent Metals
The surface hydroxyl groups of the TiO
2
polymorphs anatase and rutile have been
the subject of several IR studies. Anatase shows a more complex spectrum, with
a complex OH stretching band above 3700 cm
− 1
not found for rutile [129] . The
surface hydroxyl groups of ZrO
2
polymorphs, tetragonal and monoclinic, have also
been the object of several studies. A doublet near 3770 and 3670 cm
− 1
is typically
found at the surface of monoclinic zirconia [130] . For tetragonal zirconia, a
main band is usually found between 3690 and 3660 cm
− 1
, with additional weaker
components (see Figure 3.18 , right) [131] . Ceria, CeO
2
, presents a main band near
3640 cm
− 1
and a weaker one near 3710 cm
− 1
, together with a third broader
feature at 3505 cm
− 1
(see Figure 3.18 ).
3.6.2.6 The Surface Hydroxyl Groups of Sulfated and Tungstated Oxides
Much interest has been devoted recently to tungstated oxides mainly in relation
to their use as active components of vanadia catalysts for the selective catalytic
reduction of NO
x
by ammonia and to their activity in the paraffi n skeletal isomeri-
zation reaction. Anatase and tetragonal zirconia give rise to better catalysts than
rutile and monoclinic zirconia.
A particular feature of tungsta - based catalysts concerns the possible reduction
of tungsten oxide to lower oxidation states. Residual surface OH groups of such
materials do not present well defi ned sharp bands, but quite broad features [132] .
They have been attributed to the hydrated form of surface tungstyl species, see
Scheme 3.4 . It has been suggested that stronger Br ø nsted acid sites are generated
when tungsten is reduced to lower valency [133, 134]
Scheme 3.4 Proposed structures for anhydrous and hydrated
forms of surface tungstyl species (left) and of the bridging
OH groups of zeolites.
144 3 The Use of Infrared Spectroscopic Methods
Figure 3.19 FTIR spectra of zeolite samples, all activated at 723 K.
Also sulfation of metal oxides introduces quite strong Br ø nsted acidity and, in
general, enhances the catalytic activity in acid - catalyzed reactions. Zirconia (tetrag-
onal more than monoclinic), when sulfated becomes very active for some hydro-
carbon conversion reactions. IR spectra show a band in the range 3645 – 3630 cm
− 1
,
to which the catalytic activity is attributed. [93, 135] . In sulfated titania, the strong
Br ø nsted acidity is associated to an OH stretching band at 3695 cm
− 1
[129] . Sul-
fation of silica gives rise to an unstable material, which apparently does not present
characteristic OH groups.
3.6.2.7 The Surface Hydroxyl Groups of Protonic Zeolites
Protonic zeolites fi nd industrial applications as acid catalysts in several hydrocar-
bon conversion reactions. The excellent activity of these materials is due to two
main properties: a strong Br ø nsted acidity of bridging Si
–
(OH) – Al sites (Scheme
3.4 , right) generated by the presence of aluminum inside the silicate framework
and shape selectivity effects due to the molecular sieving properties associated with
the well defi ned crystal pore sizes, where at least some of the catalytically active
sites are located.
The spectra of protonic zeolites (Figure 3.19 ) show, in addition to a band of
weakly acidic external terminal silanol groups at 3745 – 8 cm
− 1
, well defi ned and
strong bands in the region between 3650 and 3500 cm
− 1
, due to the bridging
hydroxyl groups. The position of this band is dependent on the size of the zeolite
cavities, ν OH being generally the lower the smaller the cavity. The OH stretching
band position and width can be infl uenced by H - bonding within the cavity. In the
case of zeolites with more than one type of quite different cavities, splitting of the
3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 145
band of the bridging hydroxyl groups can be observed. For the same zeolite struc-
ture, the intensity of ν OH strongly depends on the Al content, its position being
also slightly affected by composition. Data concerning protonic zeolites that fi nd
practical applications in the industry, such as H - FER, H - MFI, H - MOR, H - BEA,
H - MWW, H - MAZ and H - FAU, have been reviewed [100] .
3.6.2.8 The Surface Hydroxyl Groups of Microporous and Mesoporous
Silica - Aluminas ( SA s )
The IR spectra of silica - alumina s ( SA s), both microporous and mesoporous,
always present a very sharp band near 3747 cm
− 1
, as shown in Figure 3.20 for a
mesoporous MCM - 41 type SA sample with Si/Al atomic ratio 20. This band is
certainly due to terminal silanols, spectroscopically very similar to those of
pure silicas and of any silica - containing material. A tail towards lower frequencies
is likely due (as for pure silica) to H - bonded and geminal silanols. Several papers
reported the complete absence of bands assignable to bridging OH groups.
Gora - Marek and coworkers [136, 137] re - proposed recently the existence of
bridging OH groups in mesoporous SAs, reported in a few previous investigations.
In contrast, Cr é peau and coworkers [138] did not fi nd any other species except
terminal silanols on their strongly Br ø nsted acidic SA samples. Garrone and
coworkers [139] found that a weak band at 3611 cm
− 1
is associated with molecular
water adsorbed on mesoporous SA, and found a signifi cant acidity for adsorbed
H
2
O.
Figure 3.20 FTIR spectra of a MCM - 41 type mesoporous
silica - alumina (a) after activation by outgassing at 773 K;
(b) in contact with 1 Torr pyridine vapour; after outgassing at
room temperature for (c) 10 min; (d) 1 h (d) and (e) after
outgassing at 523 K for 10 min.
146 3 The Use of Infrared Spectroscopic Methods
Bevilacqua and coworkers [113] re - investigated the surface hydroxyl groups and
the surface acidity of silica, silicalite, mesoporous and microporous SAs, silicated
aluminas, aluminated silicas and silicalite, and of some zeolites, by IR spectros-
copy. Extremely small bands near 3610 cm
− 1
may be found on some SA samples
(mostly prepared in organic media) and on aluminated silicas after activation, thus
not being associated to adsorbed water. These bands certainly correspond to very
few OH groups, and impurities (such as bicarbonates) might contribute to their
formation. Bevilacqua and coworkers [113] suggested that, in disordered mesopo-
rous or microporous amorphous materials, zeolite - like pores may accidentally
form and host zeolite - like bridging hydroxyl groups.
3.6.3
Spectroscopic Characterization of the Strength of the Surface Br ø nsted Acid Sites
Different methods have been proposed to characterize the acid strength of the
surface hydroxyl groups on solid oxides. Basic probes allow detection of several dif-
ferent interactions with acidic sites, as shown in Scheme 3.5 . Species I depicts a
typical Lewis interaction, where the base coordinates as such on Lewis acid sites
located at the solid ’ s surface. Species II to IV depict interactions of bases at surface
Br ø nsted - type acid sites. Species II occurs when the acid – base interaction is very
weak, and only results in a hydrogen bonding. Strictly speaking, this phenomenon
does not enter into the Lowry – Bronsted defi nition of acidity. However, it can be
demonstrated that this is what occurs when a true base (which can be protonated
by an acid) interacts with a too weak acid or, alternatively when a true Br ø nsted acid
interacts with a too weak base. Species IV depicts the true Br ø nsted acid – base
interaction, with a total transfer of the proton from the Br ø nsted acidic OH to the
base, resulting in its protonation and in the formation of its conjugated acid. Species
III is an intermediate case that gives rise to a partial proton transfer and the forma-
tion of a “ symmetric hydrogen bond ” with the proton nearly halfway between the
base and the acid ’ s conjugated base. Species similar to V (protonated dimeric species
of the base) are well known in homogeneous inorganic chemistry.
Reasons for the choice of one or other probe have been discussed previously [101,
102] . In Table 3.8 data are reported concerning the basicity of some probe molecules
Scheme 3.5 Adsorption modes of a base (in this case
pyridine) on the surface Lewis (I) and Br ø nsted sites (in this
case terminal) on a solid oxide surface.
3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 147
expressed as p K
a
of the conjugated acid and as proton affi nity ( PA ). The former
parameter, p K
a
, is relevant with respect to acid – base interactions occurring in water
solution, which, consequently, also implies solvation with water of both the base
and its conjugated acid. The latter parameter, PA, is more appropriate for gas - phase
interactions. Interestingly, the two scales are not coincident, for example ammonia
is associated with a higher p K
a
and lower PA than pyridine. This means that
ammonia is more basic than pyridine in water, but is expected to be less basic than
pyridine in the gas phase. The greater solvation of the ammonium ion in water
(with the formation of four H - bonds) compared with the pyridinium ion could be
the reason for this invertion of their relative acidity in water with respect to the gas
phase. Some kind of solvation also occurs on oxide surfaces and, in particular, in
the micropores of zeolites. Our own data indicate that on metal oxide surfaces
ammonia is at least sometimes more easily protonated than pyridine.
The results of these studies for some solid oxide materials have been summa-
rized in Table 3.9 .
3.6.3.1 Techniques for the Evaluation of the Strength of Surface
Br ø nsted Acid Sites
The Hydrogen - Bonding Method
The strength of the acid – base interaction depends
on the acid strength of the acid as well as on the basic strength of the base. When
the acid or the base is too weak to allow proton transfer, the interaction results in
hydrogen bonding. In intermediate cases, the proton transfer may be only partial
Table 3.8 Molecular probes applied for surface acidity characterization.
Base Conjugated
acid
Base strength
Family Example p K
a
P A
Cyclic amines piperidine C
5
H
10
NH
CH NH
510 2
+
11.1 933
Alkyl amines n - butylamine n
–
C
4
H
9
–
NH
2
n−−
+
CH NH
49 3
10.9 916
Ammonia NH
3
NH
4
+
9.2 857
Phosphines trimethyl - phosphine (CH
3
)
3
P (CH
3
)
3
PH
+
8.65 957
Phosphine oxides trimethyl - phosphine oxide
(CH
3
)
3
P
=
O (CH
3
)
3
P
=
OH
+
907
Heterocyclic amines pyridine C
5
H
5
N C
5
H
5
NH
+
5.2 928
Ketones acetone
(CH
3
)
2
C
=
O (CH
3
)
2
C
=
OH
+
− 7.2
824
Ethers dimethylether (CH
3
)
2
O (CH
3
)
2
OH
+
∼ − 3
792
Nitriles acetonitrile
CH
3
–
C
≡
N CH
3
–
C
≡
NH
+
− 10.4
783
Hydrocarbons benzene C
6
H
6
CH
67
+
750
ethylene
H
2
C
=
CH
2
CH -CH
32
+
680
Carbon monoxide CO [HCO]
+
598
Nitrogen N
2
477
Hydrogen H
2
422
Argon Ar 369
148 3 The Use of Infrared Spectroscopic Methods
Table 3.9 Evaluation of the Br ø nsted acid strength of surface hydroxy - groups on catalytic materials by different IR techniques. The Br ø nsted acid strengths
roughly decrease from top to bottom.
Catalysts
↓
PA →
∆ ν O H ∆ ν OH
cm
− 1
Behavior
with
Polymerization of Protonation of
ν OH cm
− 1
CO Aceto
nitrile
n - C
4
H
8
C
2
H
4
C
3
H
6
i - C
4
H
8
C
4
H
6
Acetonitrile Pyridine NH
3
n - Butyl
amine
Piperidine
594 783
∼ 750
680 752 802 783 783 912 846 933
H - zeolites 3650 – 3500
∼ 300 – 340
ABC polym +
isom
yes yes yes yes yes
a)
yes yes yes yes
SO oxide
4
2−
3650 – 3630
∼ 140
ABC polym yes yes yes yes no yes yes yes yes
WO
3
/oxide Broad ABC polym tr yes yes yes no yes yes yes yes
SiO
2
–
Al
2
O
3
∼ 3745 ∼ 70 – 150
∼ 300
ABC polym no yes yes yes no yes yes yes yes
H
3
PO
4
/SiO
2
∼ 3660 ∼ 180 – 200
ABC polym no no yes yes no yes yes yes yes
Nb
2
O
5
· H
2
O 3740 – 3705
∼ 500
no yes yes
3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 149
Catalysts
↓
PA →
∆ ν O H ∆ ν OH
cm
− 1
Behavior
with
Polymerization of Protonation of
ν OH cm
− 1
CO Aceto
nitrile
n - C
4
H
8
C
2
H
4
C
3
H
6
i - C
4
H
8
C
4
H
6
Acetonitrile Pyridine NH
3
n - Butyl
amine
Piperidine
594 783
∼ 750
680 752 802 783 783 912 846 933
AlF
3
3730
3655
∼ 150 > 500
yes yes no tr yes yes yes
SiO
2
–
TiO
2
∼ 3740 ∼ 70 – 150 ∼ 450
polym tr no no tr no no yes yes yes
Al
2
O
3
–
B
2
O
3
3800 – 3650
∼ 70 – 150
480 – 420
330 – 280
butoxide
tr
no no yes no no yes
b)
yes yes
Silicated
γ - Al
2
O
3
∼ 3740 ∼ 70 – 150
480 – 420
330 – 280
∆ ν OH
200 – 300
no no no no no yes
b)
yes yes
γ - Al
2
O
3
3800 – 3650
∼ 70 – 100
450 – 400
330 – 280
∆ ν OH
200 – 300
no no no yes no no yes
b)
yes yes
TiO
2
- anatase 3750 – 3650
∼ 110 – 150 < 300
no no no no no no tr tr yes
am - SiO
2
∼ 3745 ∼ 70 – 150
400
∆ ν OH
150 – 200
no no no no no no no no no
a Likely formation of protonated bridging species [CH
3
CN . . . H . . . NCCH
3
]
+
.
b Likely disproportionation 2NH
3
→ NH
2
−
+ NH
4
+
; isom = isomerization; polym = polymerization; tr = traces.
150 3 The Use of Infrared Spectroscopic Methods
so that symmetrical strong hydrogen bonding occurs. These two situations can be
detected and distinguished spectroscopically from protonation and interaction
with Lewis sites.
In both these cases, the spectrum of the adsorbed base is only weakly perturbed
with respect to the liquid. However, hydrogen bonding causes the shift down and
the broadening of the OH stretching bands of the surface hydroxyl groups. Simple
hydrogen bonding gives rise to the formation of well defi ned although broad OH
stretching bands, and the shift of the maximum of the ν OH band upon interaction
can be taken as a measure of the H - bonding interaction. If the same base is used
on different surfaces, the shifts measure the acid strengths of the corresponding
protonic centres, according to the Bellamy – Hallam – Williams relation [140] . Very
weak bases that can be protonated only with diffi culty, such as nitriles or hydro-
carbons (e.g. ethylene or benzene), can be used for this purpose. Low - temperature
adsorption of even weaker “ bases ” , such as hydrogen or CO, is also useful experi-
mentally in this respect. In particular, the low - temperature adsorption of CO has
been particularly widely used in recent years.
In Figure 3.21 the effect of low - temperature adsorption of CO on the spectrum
of the surface OH groups of silica, silica - alumina and a zeolites (H - MFI) is shown.
The adsorption of CO on the silanol groups present on silica results in the forma-
Figure 3.21 FTIR spectra of amorphous silica (Aerosil from
Degussa), silica - alumina (Strem, Al
2
O
3
13%), and H - MFI
zeolite (Zeolyst, Si/Al = 25) after activation (full lines) and
after contact with CO gas at 140 K (broken lines).
3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 151
tion of H - bonded Si
–
OH · · · CO complexes with ν CO at 2155 cm
− 1
(i.e. shifted up
around 18 cm
− 1
with respect to liquid CO), and ν OH at 3676 and 3590 cm
− 1
, that
is,with downshifts of ∆ ν OH ∼ 70 cm
− 1
and ∼ 155 cm
− 1
. In the case of SA, additional
components are found at ν OH 3415 cm
− 1
( ∆ ν OH > 300 cm
− 1
) and at ν CO 2173 cm
− 1
,
showing the existence of a fraction of silanol groups with stronger acidity. In the
case of the H - MFI zeolite the hydroxyl group 3615 cm
− 1
is shifted down to 3304 cm
− 1
( ∆ ν OH ∼ 310 cm
− 1
), and ν CO is at 2175 cm
− 1
, again showing very strong acidity.
In Figure 3.22 , the spectra relative to the interaction of acetonitrile with a com-
mercial microporous SA are shown. Two kinds of H - bonded complexes are formed,
one of which is characterized by ν OH at 3415 cm
− 1
( ∆ ν OH ∼ 330 cm
− 1
), which, as
in silica, is due to normal silanols, and the other by ν OH ∼ 2950 cm
− 1
( ∆ ν OH
∼ 800 cm
− 1
), confi rming that a fraction of silanol groups of SAs have strong
acidity.
With stronger bases, the formation of even stronger “ symmetrical ” hydrogen
bonding causes the appearance of an almost continuous absorption in the overall
spectrum, with three maxima and two minima, the so - called ABC contour. The
minima correspond to the fi rst overtones of the MOH deformation modes, the
in - plane deformation δ OH, and the out - of - plane deformation, γ OH. The overall
ABC contour is due to the Fermi resonance of the ν OH mode (strongly shifted
downwards and broadened) with these overtones, both strongly shifted upwards
Figure 3.22 FTIR spectra of H - MOR (Zeolyst, Si/Al = 10), up,
and of an amorphous silica alumina (Strem, Al
2
O
3
13%), after
activation (full lines) and in contact with 5 Torr acetonitrile
vapour (broken lines) and after outgassing at room temperature
30 min (point line). A, B, C denote the components associated
to quasi - symmetrical hydrogen bonding.
152 3 The Use of Infrared Spectroscopic Methods
like the corresponding fundamentals. This occurs, for example, when acetonitrile
adsorbs on H - MOR zeolite (Figure 3.22 , upper spectra), but also when pyridine
adsorbs on mesoporous SA (Figure 3.20 ) where there is a complex distribution of
OH groups having different Bronsted acidity, some of which form “ quasi sym-
metrical ” H - bonds with pyridine.
The Basic Strength Method The presence of Br ø nsted acid sites can be detected
“ indirectly ” by studying the interaction of bases of appropriate strength and by
monitoring the formation of the corresponding protonated species. This is per-
formed easily with quite strong bases, when the spectroscopic features of the base
are readily distinguishable from those of its protonated conjugate acid. This occurs
in particular with ammonia and pyridine, which are in fact generally used, as well
as with other amines such as, for example, n - butylamine, piperidine, lutidine,
quinoline, and so on. In Figure 3.23 the spectra of pyridine adsorbed on H - MFI
zeolite (left) and on mesoporous SA (right) are compared. The spectrum recorded
on H - MFI after outgassing, with bands at 1636, 1625, 1547, 1491 cm
− 1
is due to
pyridinium ions (8a, 8b, 19a and 19b modes), formed by pyridine protonation. In
contrast, on SA the predominant features after outgassing (1622, 1578, 1492,
1456 cm
− 1
), are due to molecular pyridine coordinated over Lewis sites (again 8a,
8b, 19a and 19b modes), with small amounts of pyridinium ions (1638, 1547 cm
− 1
).
The bands observed at higher coverages (1595, 1578, 1480, 1440 cm
− 1
) are due, in
both cases, to molecular pyridine involved in H - bonding.
Figure 3.23 FTIR spectra of: left, pyridine
adsorbed on H - MFI zeolite (a) in contact with
1 Torr pyridine vapour and after outgassing
(b) at room temperature for 10 min, (c) at
373 K for 10 min, (d) at 473 K for 10 min,
(e) at 573 K for 10 min; right, an MCM - 41 type
mesoporous silica - alumina (a) after activation
by outgassing at 773 K; (b) in contact with
1 Torr pyridine vapour; after outgassing at
room temperature for (c) 10 min, (d) 1 h;
(e) after outgassing at 523 K for 10 min, see
Figure 3.16.