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

Among AEOs, MgO is the most studied material and can be considered as a

model system. High surface area MgO powders, outgassed at high temperature,

exhibit emission bands when excited by UV light at 260 – 400 nm, with a lower

energy than the band gap of MgO single crystals [3, 84] . PL was strongly quenched

by the adsorption of oxygen conﬁ rming that the emitting sites are located at the

surface of the oxide. Moreover, the absorption band of powdered MgO was found

at lower frequencies than that observed for bulk MgO single crystals [44, 45] . The

photoluminescence of MgO was ascribed to the charge - transfer processes involv-

ing surface ions in low coordination present at the MgO surface.

The lifetimes of emitting species are linked to energy transfers, which affect the

kinetics of emission processes. In addition, lifetimes also depend on the local

environment of the oxide ions involved in PL. Analysis of the lifetimes of the

photoluminescent species has revealed two types of emitting sites in MgO oxide.

One has a comparatively short lifetime ( ∼ 10

− 5

s), and is associated with the charge -

transfer processes involving low coordinated surface ions, and the other a long

lifetime (1 – 10

4

s), ascribed to the presence of surface F

s

+

centers [85] . The PL yield

was found to change from sample to sample and to depend on the pre - treatment

temperature. Moreover, outgassed MgO exhibited a photocatalytic behavior in the

cis – trans isomerization of 2 - butene [86] . The correlation between the intensity of

the short - lifetime PL and the rate of photocatalyzed isomerization indicated that

coordinatively unsaturated surface ions play a relevant role in the photocatalytic

activity of MgO [86] . More recently, the lifetimes of PL sites of MgO nanocubes

Figure 2.19 Excitation and emission spectra of alkaline earth

oxides outgasssed at 1200 K. Dashed lines show quenching by

oxygen. Reprinted from ref. [83] , with permission from Elsevier.

2.4 UV-Vis-NIR Photoluminescence Spectroscopy 83

84 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

have been modeled by means of a kinetic energy - transfer model [87] . This is a

complementary approach to the one based on analysis of the position and shape

of PL signals, and conﬁ rms that the emission and excitation bands are associated

with low - coordinated O

LC

2−

oxide ions. Moreover, lifetime measurements also sug-

gested that phosphorescence takes place in the emission process of nanocubic

MgO oxide.

PL spectroscopy can also be successfully applied to the characterization and

detection of particular molecular structures formed as a result of a reaction on the

catalyst surface. This was the case in a PL study of the reaction of pyridine and

oxygen with thermally activated SrO [88] . SrO powder is white and the PL spectra

at 300 K showed a broad emission band at 475 nm (Figure 2.20 curve a) and an

excitation band with at least two maxima at 280 and 315 nm (Figure 2.20 curve b).

The sample turned blue - violet upon pyridine adsorption and a very weak PL signal

was detected (Figure 2.20 curves c and d). The spectra were not modiﬁ ed by out-

gassing the excess of pyridine at 300 K and no color change was observed. After

admission of oxygen at 300 K, the color changed to yellow - pink and the emission

spectrum at 300 K showed a new maximum at 550 nm and a slight increase of

the intensity in the 400 – 450 nm range (Figure 2.21 , curves a and b). The emission

band recorded at 77 K after addition of O

2

(curve c) is more intense than the one

recorded at 300 K (curve a) and showed a ﬁ ne structure with three main peaks at

429, 457 and 476 nm. The corresponding excitation spectrum has a maximum at

275 nm (Figure 2.21 , curve d). The well deﬁ ned structure is attributed to the emis-

sion of bipyridyl species formed on the surface. To identify which isomer of

Figure 2.20 Photoluminescence spectra of SrO recorded at

300 K before and after pyridine (Py) adsorption. (a) emission

spectrum of SrO; (b) excitation spectrum of SrO; (c) emission

spectrum after Py adsorption; (d) excitation spectrum after Py

adsorption. Reprinted from ref. [88] , with permission from the

Royal Society of Chemistry.

bipyridyl (2 – 2 ′ or 4 – 4 ′ - Bpy) is formed on the SrO surface, the emission spectra of

the two bipyridyls adsorbed on SrO were recorded after admission of oxygen

(Figure 2.22 ). The spectrum of 4,4 ′ - bipyridyl (curve a) is quite similar to the spec-

trum obtained after adsorption of pyridine and oxygen on SrO (Figure 2.21 , curve

c) both in the shape and in the spacing of the vibronic structure. This study has

allowed the clariﬁ cation of the product formed by the interaction of pyridine with

oxygen on a SrO surface, namely 4,4 ′ - bipyridyl.

2.4.4.2 Investigations of Highly Dispersed Transition Metal Ions in Oxides or

Zeotype - Systems by PL Spectroscopy

PL spectroscopy, combined with lifetime measurements, is a powerful tool for

determination of the structure, coordination and oxidation state of TMIs dispersed

on different types of support, used as heterogeneous catalysts. This spectroscopic

technique is particularly useful for characterizing materials with a very low metal

loading (below 1 – 2 wt%), owing to its high sensitivity. Anpo and coworkers [11]

have reviewed the applications of PL to the characterization of catalysts and pho-

tocatalysts, covering a wide range of different catalytic systems. Different types of

TMI - containing catalysts can be characterized by PL [89] . Systems investigated

include vanadium - containing zeolites or mesoporous materials [22, 90 – 93] , molyb-

denum dispersed on silica [94] or in mesoporous silica [95] , chromium oxides

anchored on Vycor glass or silica [96, 97] , titanium - containing zeolite [98, 99] ,

titanium grafted on mesoporous systems [100, 101] and copper/ZSM - 5 catalysts

[102 – 105] . Here, we focus on the PL features of vanadium - containing materials,

Figure 2.21 Photoluminescence spectra of the SrO + Py + O

2

system. (a) emission spectrum of SrO after Py adsorption at

300 K; (b) emission spectrum of SrO after adsorption of O

2

on

preadsorbed Py at 300 K; (c) and (d) emission and excitation

spectra of of SrO after adsorption of O

2

on preadsorbed Py at

77 K. Reprinted from ref. [88] , with permission from the Royal

Society of Chemistry.

2.4 UV-Vis-NIR Photoluminescence Spectroscopy 85

86 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

owing to the richness of information provided by the spectra associated with the

vibrational ﬁ ne structure that can be recorded at low temperature (77 K). This

particular application allows the determination of detailed and speciﬁ c informa-

tion on the presence of different vanadium species.

In the case of vanadium - containing systems, PL only detects tetrahedral V

V

species and, owing to its high sensitivity to the local environment of lumine -

scent centers, allows different types of tetrahedral V

V

sites [11, 106 – 108] to be

distinguished.

Vanadium oxides anchored on oxide supports are important heterogeneous

catalysts for both the vapor - phase selective oxidation of alkylaromatic hydrocar-

bons, alkenes and alkanes and the selective reduction of NO [109, 110] . Vanadyl

groups (V

=

O) are considered to be the PL species in vanadium - containing materi-

als. The absorption and emission (phosphorescence) of the UV light arose from

the charge - transfer processes on the surface vanadyl groups of the tetrahedrally

coordinated VO

4

species involving an electron transfer from O

2 −

to V

5+

and a

reverse radiative decay from the charge - transfer triplet state (Scheme 2.1 ) [11] .

The nature of the oxidic support plays an important role in modulating the

nature of vanadium species and the catalytic behavior of the materials. In fact,

changes in the electron density of the vanadyl double bond can be produced by

the inductive effect through the bonds anchoring the tripodal vanadyl groups to

the oxidic support [111] . These electronic effects can be clearly demonstrated using

Figure 2.22 Photoluminescence spectra of the SrO + Bpy +

O

2

system at 77 K. (a) and (b) emission and excitation spectra

of SrO after adsorption of O

2

on preadsorbed 4,4 ′ - Bpy;

(c) and (d) emission and excitation spectra of SrO after

adsorption of O

2

on preadsorbed 2,2 ′ - Bpy. Reprinted from

ref. [88] , with permission from the Royal Society of Chemistry.

Figure 2.23 Emission and excitation spectra recorded at 300 K

of vanadyl group anchored on metal oxide. (a) hydrated 0.8V/

SiO

2

; (b) 3V/TiO

2

; (c) 2V/Al

2

O

3

; (d) 20V/MgO. Reprinted from

ref. [111] , with permission from Springer.

PL spectroscopy. A PL study of vanadium oxide supported on different oxides,

such as SiO

2

, TiO

2

, Al

2

O

3

, K - doped Al

2

O

3

, ZnO and MgO, was performed by Garcia

and coworkers [111] . Excitation in the 250 to 400 nm range led to PL bands in the

550 – 700 nm range from all solids, and a consistent shift in the emission maximum

was evident depending on the acid/base character of the oxide support (Figure

2.23 ). If the wavelengths of the emission maxima are plotted against the isoelectric

point or zero point of charge ( PZC ) of the oxides (Figure 2.24 ), a linear correlation

is observed. The PZC of the supports can be related to the acid/base character of

the oxides [110, 112] , meaning that this parameter has a direct inﬂ uence on the

PL, and hence on the molecular orbitals, of the vanadyl groups.

Vanadium loading only modiﬁ es the intensity of the emission band and not its

position, which depends only on the nature of the support. A signiﬁ cant decrease

Scheme 2.1 Schematic representation of the CT processes on

the surface vanadyl groups of the tetrahedrally coordinated

VO

4

species.

2.4 UV-Vis-NIR Photoluminescence Spectroscopy 87

88 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

of the emission intensity is, in fact, observed as the vanadium loading increases,

owing to the formation of di - , tri - and polyvanadium aggregates anchored on the

support surface instead of isolated vanadyls, which are the most intense emission

centers.

PL spectra collected at low temperature (77 K) allow the observation of the vibra-

tional proﬁ le of the vanadyl groups. This particular application of PL is extremely

useful for distinguishing different kinds of tetracoordinated vanadium sites, as

reported by Dzwigaj and coworkers for V

5+

species incorporated in a β - zeolite

framework [93, 113] . The emission spectra collected at 77 K with λ

ex

= 250 nm of

three V - β samples (VSi β as - synthesized, C - VSi β calcined and C - Hyd - VSi β calcined

and then hydrated) are reported in Figure 2.25 . All the solids exhibit emission

Figure 2.24 Variation of the emission maximum wavelength

with the isoelectric point or pcz of the oxide support in

supported vanadia catalysts. Reprinted from ref. [111] , with

permission from Springer.

Figure 2.25 Photoluminescence spectra,

recorded at 77 K, of (a) VSi β , evacuated at

353 K for 2 h; (b) C - VSi β , evacuated at 473 K

for 2 h; (c) C - Hyd - VSi β , evacuated at 473 K for

2 h. Reprinted with permission from ref. [113] ;

Figure 2.26 Second derivative of the photoluminescence

spectra, recorded at 77 K, of (a) VSi β , evacuated at 353 K for

2 h; (b) C - VSi β , evacuated at 473 K for 2 h; (c) C - Hyd - VSi β ,

evacuated at 473 K for 2 h. The decay curves (insets a, b, and c,

logarithmic scale) were monitored at 77 K. Reprinted with

Society.

2.4 UV-Vis-NIR Photoluminescence Spectroscopy 89

maxima at ca. 500 nm together with a vibrational ﬁ ne structure. The ﬁ ne structure

corresponds to transitions from the lowest vibrational level of the excited triplet

state T

1

(V

IV

–

O

−

) to the vibrational levels of the ground singlet state S

0

(V

V

=

O

2 −

) [11,

106 – 108] . The shapes, positions of maxima and intensities of the components of

the vibrational proﬁ les depend on the pre - treatment of the samples. In fact, three

vibrational ﬁ ne structures, ascribed to three different kinds of tetrahedral V

V

sites

( α , β , γ ), can be observed in the emission spectra of the VSi β samples (Figure 2.25 ).

Since the separation of the vibrational bands is not well resolved owing to the

superposition of different sets of vibrational structure involving tetrahedral V

V

sites with different symmetries, analysis of the second derivative of the PL spectra

allows the elucidation of the precise structure of the different vanadium sites

(Figure 2.26 ). The second derivative of the VSi β sample (curve a) shows the pres-

ence of two ( α and β ) main tetrahedral V species. The vibrational energy of the β

species is 1054 cm

− 1

, indicating a short V

=

O bond length (1.54 Å ). On the contrary,

the γ type species is predominant in the C - VSi β sample (curve b). The vibrational

energy of the γ species is 1036 cm

− 1

, indicating a longer V

=

O bond length (1.56 Å )

90 2 The Application of UV-Visible-NIR Spectroscopy to Oxides

and a higher symmetry than the β one. Finally, in the second derivative spectrum

of the C - Hyd - VSi β sample (curve c), the main contribution of the α - type tetrahe-

dral V species is evident. The vibrational energy of the α - species corresponds to

1018 cm

− 1

, which indicates a longer V

=

O bond length (1.58 Å ) and a higher sym-

metry compared to γ sites. Both the V

=

O bond length and the symmetry of V sites

decrease in the following order: α > γ > β along with a concomitant increase of

the O

=

V

–

O(Si, H) bond angle ( θ in Scheme 2.1 ), on the basis of VSEPR argu-

ments which apply for d

0

systems where there is no stabilization effect due to the

crystal ﬁ eld [114] .

The lifetime of the excited triplet state depends on the degree of distortion of

the symmetry of transition metal ions from the ideal tetrahedral one [115] . The

decay curves of the PL of the three V samples are reported in the inset of Figure

2.26 . The lifetimes are 49 ms for the γ sites, 28 ms for α sites and 88 ms for β sites.

The increase in the lifetimes from 28 ms to 88 ms for the sequence α , γ and β

species can be attributed to an increasing distortion of the tetracoordinated V sites

and to a decrease in the V

=

O bond length. On the basis of these results, combined

also with the DR UV - Vis data, Dzwigaj and coworkers proposed that the three

types of tetrahedral V species arise from three different framework sites. In

particular, the formation of each type of V site is related to the state of vacant T

atom sites (silanol nests). For α species, the authors proposed hydroxylated

(SiO)

2

(HO)V

=

O sites (structure A Scheme 2.2 ), for β species, non - hydroxylated

(SiO)

3

V

=

O sites (structure B in Scheme 2.2 ) and for γ species non - hydroxylated

(SiO)

3

V

=

O sites (structure C in Scheme 2.2 ) [93] .

Although based on a selected example, this demonstrates the richness of

physical - chemical insights that can be obtained through a deep analysis of PL

spectra of catalysts bases on highly dispersed TMI.

2.5

Conclusions

Both fundamental aspects and representative examples of types of investigation

and analyses of spectral data for different kinds of oxide - based materials of interest

in heterogeneous catalysis and photocatalysis have been reviewed. Hopefully, we

Scheme 2.2 Schematic representation of α , β and γ V sites in VSi β catalyst. Adapted from ref. [93] .

References 91

have increased awareness that the use of UV - Vis - NIR spectroscopy, in both absorp-

tion and emission modes, could signiﬁ cantly assist in the provision of chemical

and structural insight. The development of a detailed description of the nature and

properties of surface sites, which is of crucial importance for catalytic materials,

may demand some (stimulating and challenging) effort, owing to the multiplicity

of factors that can affect the actual features of an electronic spectrum. Of course,

it is important to keep in mind that information provided by UV - Vis - NIR spec-

troscopy is one of the “ tesserae ” of the complex (but fascinating) mosaic of comple-

mentary techniques, both spectroscopic and non - spectroscopic, required to achieve

a satisfactory understanding of catalytic materials.

Acknowledgments

The authors acknowledge ﬁ nancial support by Regione Piemonte (Progetto

NANOMAT, Docup 2000 - 2006, Linea 2.4a).

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

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