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

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 163

band is no longer observed in the case of the reduced catalyst. This band is cer-

tainly due to carbonyl species adsorbed over oxidized Pd centers, which are reduced

by the treatment in hydrogen. The position of this band allows its assignment to

carbonyls bound to Pd

2+

sites.

For both oxidized and reduced surfaces, a strong band is found centered very

near 2100 cm

− 1

with an additional component envisaged as a shoulder in the region

near 2060 cm

− 1

. The maximum tends to shift in both cases to lower frequencies

by outgassing and decreasing CO coverage, down to 2085 cm

− 1

. These features can

be quite conﬁ dently assigned to terminal carbonyls adsorbed on zero valent Pd

atoms. In agreement with this, CO adsorption on Pd monocrystals show that ter-

minal carbonyls are found near 2095 cm

− 1

on both the (111) and (100) planes.

At lower frequencies only a very weak and broad absorption centered at 1930 cm

− 1

is found in the case of the oxidized catalyst. On the contrary, a very strong band

is observed in the case of the reduced catalyst with a main maximum centered at

1981 cm

− 1

, and a pronounced shoulder near 1900 cm

− 1

. By outgassing the main

compomnent decreases quickly in intensity and its maximum tends to shift down

to 1940 cm

− 1

, while the lower frequency shoulder does not decrease in intensity

and becomes predominant at lower CO coverages. The vibrational range of these

absorptions allows their assignment to bridging carbonyls over Pd metal particles.

According to Binet and coworkers [156] , the sharper band in the region 1980 –

1940 cm

− 1

should be assigned to CO on Pd particles exposing a (100) type face

while the broader and more strongly bonded band in the 1930 – 1850 cm

− 1

region

should be assigned to species adsorbed on Pd particles exposing a (111) type face.

Analysis of the intensity of the bands shows that the reduction treatment causes

the complete disappearance of the bands of oxidized Pd centers and the simulta-

nous strong growth of the bands of bridging carbonyls on zero valent Pd. However,

also, the bands due to terminal carbonyls on zero valent Pd grow by a factor of 2

during reduction. This suggests that zero valent Pd particles are already present

in the oxidized sample but grow in size and number upon reduction.

In Figure 3.26 the IR spectra of CO adsorbed at low temperature (130 – 230 K on

an uncompletely reduced Cu - ZnO - Al

2

O

3

catalyst are shown. The main maximum

is found at 2150 cm

− 1

, with smaller maxima at 2176 and 2120 cm

− 1

, with an evident

tail towards lower and higher frequencies. Upon outgassing during warming, the

band at 2150 cm

− 1

tends to disappear rapidly (see arrow in Figure 3.26 , right),

showing that it is due, at least in part, to a very weakly adsorbed species, possibly

physisorbed on the surface hydroxyl groups. However, a component now at

2161 cm

− 1

persists during low - temperature outgassing. This feature can be assigned

to CO adsorbed either on Zn

2+

or on Cu

2+

ions, although the assignment of at least

part of this band to the lower frequency component of Cu

+

dicarbonyls is also

possible. The higher frequency component decreases less in intensity and shifts

from 2176 to 2204 cm

− 1

with decreasing coverage. Its position and behavior agrees

with an assignment to CO adsorbed on Al

3+

cations. The lower frequency band is

found at 2120 cm

− 1

at the highest coverages but has a complex behavior. Its

maximum shifts to higher frequencies towards 2130 cm

− 1

by outgassing and

decreases in intensity, but at higher temperatures 200 – 230 K) the band broadens

164 3 The Use of Infrared Spectroscopic Methods

and moves to lower frequency again (2115 cm

− 1

). The position and stability of this

band strongly suggests that it is mainly due to the carbonyls of Cu

+

ions or of small

clusters of zerovalent copper. It is in fact well known that carbonyls on bulk Cu

metal are very weakly adsorbed and usually responsible for absorptions below

2100 – 2070 cm

− 1

. It cannot be excluded that such species are present at the lowest

temperatures and are responsible for the tail at lower frequency of this band. On

the other hand, to assign the bands due to adsorbed CO, we have also to take into

account that during CO desorption at 140 – 170 K a strong absorption is formed

and grows at 2340 cm

− 1

which is certainly due to adsorbed CO

2

(Figure 3.26 ). By

further warming and outgassing the band decreases in intensity and disappears

at 230 K. Under these conditions, however, a shoulder at 2355 cm

− 1

forms and

disappears. The latter components can be conﬁ dently assigned to CO

2

adsorbed

on Al

3+

ions. The formation, at so low a temperature, of adsorbed CO

2

is certainly

due to the existence of very active oxidizing sites, in spite of the reduction pre -

treatments. According to previous studies [157, 158] , Cu

2+

is very active for the

low - temperature oxidation of CO. Consequently we conclude that at least part of

the band at 2160 cm

− 1

is due to Cu

2+

carbonyls that convert into CO

2

complexes of

Cu

+

or Cu

0

. Some of the adsorbed CO relocates to the new reduced centers and

this may be at the origin of the sudden downwards shift of the band of adsorbed

CO observed after the desorption of CO

2

. During desorption, part of the CO

2

readsorbs on the stronger Lewis acidic Al

3+

ions.

Figure 3.26 FTIR spectra of CO adsorbed at 140 K (increasing

admissions, left) over a Cu - ZnO - Al

2

O

3

catalyst (5 : 52 : 43

Cu : Zn : Al atomic ratios) calcined at 450 ° C. Right: under

desorption at increasing temperatures (1 to 10 correspond to

increasing temperature in the range130 – 230 K).

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 165

Nitrogen monoxide has one electron more than CO, so that the same electron

conﬁ guration could apply with the additional electron in an antibonding π

*

- type

orbital. The bond order is decreased to 2.5 and this is reﬂ ected in the lower stretch-

ing frequency (1875 cm

− 1

for NO gas, see Figure 3.2 , upper right hand side). Thus,

the NO molecule is a radical and this makes its reactivity far higher. It easily

dimerizes to N

2

O

2

, is easily oxidized to NO

+

, to nitrites NO

2

− or to NO

2

, and is

easily reduced to NO

−

and to N

2

O. NO can also easily disproportionate and is

thermodynamically unstable towards decomposition to N

2

+ O

2

. This makes this

molecule very reactive, so its successful use as a probe for cationic centers is

limited.

However, NO can also interact with surface metal centres on metal oxides,

giving rise to surface mononitrosyl, gem - dinitrosyl and trinitrosyl species. When

the cation or the metal atom contains, besides empty orbitals, full or partly ﬁ lled

d - type orbitals, they can interact with the π

*

- type orbitals of NO molecules, via a

π - type electron back - donation. This gives rise to bent nitrosyls, where N is likely

sp

2

hybrid and the NO stretching frequency is decreased.

Adsorbed NO can be even more sensitive than adsorbed CO to the oxidation

state of the cations. The use of NO as a probe allows, for example, the determina-

tion of the oxidation state of Co ions in zeolites better than CO [159] , showing the

existence of both Co

3+

and Co

2+

species. In fact, the bands observed at 1901,

1817 cm

− 1

are due to [Co(NO)

2

]

2+

gem - dinitrosyl species, while that at 1945 cm

− 1

is

assigned to [Co - NO]

3+

mononitrosyl. The three bands due to adsorbed CO at 2204,

2198 and 2188 cm

− 1

arise from cobalt carbonyls, but assignment of the oxidation

state is difﬁ cult.

3.6.6

External versus Internal Cationic Sites and Complex Interactions in Cation

Exchanged Zeolites

The external versus internal surface has also been investigated in the case of Co -

H - zeolites active in the selective catalytic reduction of NO by methane [20] . The

highly hindered nitrile, ortho - toluonitrile ( oTN ) has been used, co - adsorbed with

CO and NO. The pre - adsorption of oTN poisons the external cationic sites of Co -

H - ZSM5. Successive adsorption of CO shows that some of the Co ions are on the

inner surface and some on the outer surface. The coadsorption of oTN and NO

shows that trivalent Co ions are actually located in the internal cavity surface of

Co - H - ZSM - 5 while divalent Co ions distribute almost equally in the internal and

external surface.

Similarly, Co ions at the external surface of Co - H - FER and Co - H - MOR have

been detected. Similar experiments with hindered nitriles show the location of

alkali ions at the external surface of alkali - MOR zeolites [160] . CO mainly adsorbs

at the external surface of the small pore zeolite 3A (K - LTA) while it enters the

cavities of zeolite 4A (Na - LTA) [161] .

The use of CO and nitriles as probes also reveal the possibility of interactions

of adsorbed molecules with other cations in zeolite channels [162, 163] .

166 3 The Use of Infrared Spectroscopic Methods

3.6.7

Characterization of Surface Basicity

In contrast to acidity characterization with basic probes, the use of “ acidic ” mole-

cules to probe surface basicity is far less satisfactory. In fact, all acidic (or electro-

philic) molecules (Table 3.12 ) also contain accessible nucleophilic (basic) atoms.

It seems impossible to ﬁ nd a molecule that actually only interacts speciﬁ cally with

basic sites. On the other hand, metal oxides that display signiﬁ cant surface basicity

Table 3.12 Molecular probes applied for surface basicity characterization.

Compound Formula Reactivity

Carbon dioxide CO

2

Carbonates, bicarbonates, linear coordination

on Lewis sites

Carbon monoxide CO Formates, carbonite, polymeric anionic

species, dioxycarbene

Carbon disulﬁ de CS

2

S

2

CO

−

,

CO

3

2−

, other species

Sulfur dioxide SO

2

Sulphites, disulphites, hydrogensulphite

Pyrrole H

4

C

4

NH H - bonding, pyrrolate anions

Chloroform H

–

CCl

3

H - bonding, coordination by chlorine on Lewis

acid sites, hydrolysis

Acetonitrile CH

3

–

CN

−

CH

2

–

CN anions, previous coordination by

nitrogen on Lewis acid sites

Alcohols R

–

OH

R

–

O

−

anions, undissociative coordination,

different H - bondings

Thiols R

–

SH

R

–

S

−

anions, undissociative coordination,

different H - bondings

Trimethylborate (CH

3

O)

3

B Tetrahedral borate, dissociation

Borontriﬂ uoride BF

3

F

3

B ← O

2 −

, dissociation

Ammonia NH

3

H - bonding,

−

NH

2

anions, coordination by

nitrogen on Lewis acid sites

Pyridine H

5

C

5

N

H

5

C

4

N

−

anions, coordination by nitrogen on

Lewis acid sites, dimerization

Nitromethane CH

3

NO

2

HCNO

22

−

, CNO

−

formed by decomposition

3.6 The IR Spectroscopy of Adsorbed Probe Molecules for Surface Chemistry Characterization 167

are always very ionic and also carry Lewis acidity, although weak [101, 102] . So, in

this case the acido - basicity more than pure basicity properties are relevant.

The oxides of alkaline earth metals are the strongest solid bases among stable

metal oxides. They present very high reactivity when treated in vacuum at very

high temperatures. These treatments generate highly exposed cations and anions

that induce very complex radical - type chemistry, such as that producing polymeric

CO species from carbon monoxide. In their normal use, however, alkaline earth

oxides are partly hydroxylated and carbonated, but still present high surface basic-

ity. Lanthanide oxides such as La

2

O

3

and Nd

2

O

3

are also strong bases as well as

lanthanum, or other rare earth, containing perovskites. This is essentially due to

the very low polarizing power of the corresponding cations, which have large size

and relatively low charge. Oxides of alkali metals should be even stronger bases.

Their basic reactivity is so high that they are essentially non - stable under normal

conditions, that is, in the presence of water vapor, which is sufﬁ cient to convert

them into the corresponding hydroxides. On the other hand, similar stability

problems occur also with pure alkaline earth oxides. For this reason, basic solid

oxide catalysts mostly contain alkali and/or alkaline earth cations in thermally

stable mixed oxide phases. Basic zeolites and systems based on oxides (mainly

alumina) containing alkali or alkaline earth ions are apparently the most used both

for industrial applications and in academic research [163, 164] . Even stronger

bases are the “ superbases ” where alkali metallic phases are produced, but these

will be not considered here.

EF material free, alkali exchanged zeolites are used as quite mild basic catalysts.

Light alkali and alkaline earth metal zeolites, such as Na - X, Na - Y [165] , alkali - MOR,

Na - A and Ca - A [166] , have a mild Lewis acid behavior and do not appear to have

strong basic character. The same occurs for Na - silica - alumina [167] . However,

heavy alkali metal zeolites such as Cs - Y actually act as base catalysts, or rather

as acid – base catalysts, for example for toluene side - chain alkylation. Stronger

basic character arises from impregnation of alkali zeolites with alkali salts, later

Figure 3.27 FTIR spectra of CO adsorbed at 140 K (a) and

after outgassing at 120 K (b), and of NO adsorbed at 300 K

(c) on Co - H - MFI zeolite.

168 3 The Use of Infrared Spectroscopic Methods

decomposed to occluded alkali oxide, as evidenced by CO

2

adsorption microcalo-

rimetry. The characterization of such materials is still quite poor.

Several studies have been devoted to the characterization of metal oxide carriers

impregnated by alkali and alkaline earth metal ions. Doping of most metal oxides

with large - size cations results in their accumulation at the surface. In fact, due to

their size, such cations cannot penetrate into the vacancy sites of the close packing

oxide arrays of most oxide carriers, such as alumina and titania. Alkali metal

cations remain exposed at the surface where their weak Lewis acidity (correspond-

ing to the strong acidity of the oxide anions) is easily detectable by conventional

probes. This has been shown using IR spectroscopy of adsorbed ammonia and

pyridine, for example, for the systems K

2

O - TiO

2

[168] , SrO - TiO

2

[169] and CaO -

Al

2

O

3

[170] . Such surface layers adsorb carbon dioxide very strongly so that bulk

carbonate particles can form, as in the case of the BaO - Al

2

O

3

system [171] .

In the case of Ba - β - alumina [172] and La - β - alumina, the surface apparently only

exposes the large low valency cations, detectable by adsorbing bases, and very basic

oxygen species that adsorb CO

2

in the form of carbonates. The decomposition

temperature of surface carbonates on Ba - β - alumina (BaAl

12

O

19

) has been followed

by IR under outgassing, and compared with the same experiment using MgO.

BaAl

12

O

19

appears to be far more basic than pure alumina and most pure oxides

but less basic than pure alkaline earth oxides.

Perovskite - type phases can form when small cations and large cations combine

in a mixed oxide. The surface, however, appears to be largely dominated by the

large cations and basic oxide anions, as in the cases of BaTiO

3

[173] , SrTiO

3

[171]

and several lanthanum perovskites [174] .

Magnesium aluminates produced by decomposition of hydrotalcites (the

layered double hydroxides with formula Mg

6

Al

2

(OH)

16

CO

3

· 4H

2

O) are currently

very popular base catalysts. Their thermal decompostion gives rise to a mixed oxide

whose virtual composition is 5 MgO · MgAl

2

O

4

, although these phases give rise to

partial solid solutions depending on the decomposition temperature. Stoichiomet-

ric MgAl

2

O

4

, is essentially a normal spinel phase with tetrahedrally coordinated

magnesium and octahedrally coordinated Al. Actually, due to partial inversion of

the spinel structure, low coordination Al cations typical of spinel - type aluminas

can be detected at the surface and produce a small density of very strong Lewis

acid sites [175] . Excess magnesium ions result in the segregation of large amounts

of MgO, causing the predominance of basic character. CO

2

adsorption forms

surface carbonates, which are stable up to near 773 K.

3.6.8

Use of Isotopically Labeled Molecules

The energy and the frequency of the molecular vibrational modes depend on the

mass of the atoms directly involved as well as on the bond energy. Consequently,

the position of the absorption peaks are shifted when isotopically labeled mole-

cules are used. This is a useful property for the conﬁ rmation of the assignment

of vibrational modes when needed. The adsorption of heavy water allows investiga-

tion of the surface hydroxyls, converted into deuteroxyls. Similarly, the use of

deuterated organics facilitates the distinction of CH deformation modes (which

are shifted when deuterated organics are used) from CO stretching modes that

can occur in similar positions. This has been used, for example, to provide a more

precise characterization of the adsorption modes of methanol on alumina, using

deuterated methanols [176] . Similarly,

16

O/

18

O exchange at the surface of oxides

can be performed with

18

O

2

or with H

2

18

O and this allows conﬁ rmation of the state

of surface metal – oxygen bonds [92, 95, 96] . Also the use of

13

C and

15

N labeled

molecules, for example

13

CO,

13

CO

2

and

15

NO, provide clariﬁ cation of the structure

of surface species.

3.7

Infrared Studies of the Mechanisms of Heterogeneously Catalysed Reactions

After reviewing the successful application of this technique for the characteriza-

tion of solid surfaces in 1984, Peri [13] , one of the pioneers of IR spectroscopy in

catalysis, stated that “ rather limited real success has been achieved to date in the

search for active intermediates on catalyst surfaces by IR methods. ” However, the

previously described work of Kokes [14, 15] and Tamaru [18] were cited as success-

ful examples. In an interesting book, Davydov summarized several examples of

the application of IR spectroscopy to determine the mechanisms of oxide - catalyzed

processes [177] . In a more recent review, published in 1996, the present author

reported several examples of studies of the surface reactivity of organic molecules

on metal oxides, providing information on the mechanisms of heterogeneously

catalyzed partial and total oxidation [178] . IR spectroscopy can be applied in static

conditions, looking at the evolution of adsorbed species at the surface. This

approach frequently allows the detection of the different steps of the catalytic reac-

tion, such as the adsorption of reactants, the step - by - step evolution to products

and, ﬁ nally, the desorption of the products. This provides information on the

pathways of several reactions such as, among recent work performed in our labora-

tory, the conversion of synthesis gas to methanol, methane and higher hydrocar-

bons [179] , the steam reforming of methanol [180] , the skeletal and double - bond

isomerization of oleﬁ ns [181, 182] and the reactions for the catalytic abatement of

NO

x

[183, 184] . These studies allow the development of understanding of the basic

chemistry of the catalytic phenomena.

In situ and operando studies [19 – 21] refer to investigations performed under ﬂ ow

conditions with the simultaneous detection of the reaction products (usually by

gas chromatography) and the measure of conversion/selectivity as a function of

the reaction parameters. These studies have the strong advantage of looking at the

state of the catalyst during reaction, but the simultaneous disadvantage of not

allowing the trapping of the active species, which may be present in very low con-

centration (if at all) during real catalysis, with inactive spectator species being

possibly much more evident. As discussed previously [179] , the most informative

approach, in the opinion of the present author, is the combination of operando

3.7 Infrared Studies of the Mechanisms of Heterogeneously Catalysed Reactions 169

170 3 The Use of Infrared Spectroscopic Methods

studies with surface science studies performed in static conditions or ﬂ ushing

inert gas [185] , following the evolution of the adsorbed species with the variation

of the temperature, and also in conditions where conversion does not occur at all

and the products are still not formed, that is, in conditions that are not

operando.

Lamberti and coworkers [186] have reviewed some successful application on

the use of IR spectroscopy on reaction mechanism studies performed in their

laboratory.

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

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