The local structures and photo-catalytic activity of supported niobium oxide catalysts

Catalysis Today, 8 (1990) 67-75 Elsevier Science Publishers B.V., Amsterdam

The Local

Structures

67

and Photo-catalytic

Activity

of

Supported

Niobium

Oxide

Catalysts

S. Yoshida,

Y. Nishimura,

T. Tanaka,

H. Kanai and T. Funabiki

Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606 (Japan)

Photo-oxidation of propene has been performed over Nb205/Si02 catalysts. By phosphorescence spectroscopy, it was concluded that the active species is a photo-excited Nb=O bond dispersed on the catalyst surface. The oxygen of the Nb=O bond was transferred to propene directly, producing ethanal as a major product, and acetone, propanal and propenal as minor products. XANES and EXAFS spectra indicate that the surface niobium oxide species is dominantly a tetrahedral species in a highly dispersed state, while an aggregated species of a square pyramidal cluster is dominant in a low dispersed state. The change in the selectivity of the photo-oxidation can be correlated with the dispersion. The difference in the selectivity between Nb205/Si02 and V,O,/SiO, is discussed from a quantum chemical view point. INTRODUCTION In

previous

works,

vanadium

oxide

teristic

of producing

we

supported

studied on

photo-catalytic

silica

aldehydes

ing a catalyst

of vanadium active

l-3).

selectively

the other hand, bulk V203 exhibits fore, dispersion

(ref.

for the selective

structures.

The local

structure species

V=O bond in a highly dispersed belongs

niobium

table,

activity

niobium

catalysts

oxide

metathesis (ref.

8).

niobium

oxide

the

on supports

catalytic

activity

clarified.

In the present

0920-5861/90/$03.50

and

group

on

must

have

silica

been

studies are the

that is

scarce

studied

work,

0 1990 Elsevier

the

structure

XANES

Science

affect

the catalytic

ac-

to be a photo-excited

for

vanadium to

in the

exhibit

reactions

structures relationship of niobium studies

B.V.

periodic a

similar

photo-reactions such

7) and oxidation

and EXAFS

Publishers

of

Recently,

(ref.

and

area of active

species with particular

expected

of the local

local

of an ac-

(ref. 5). as

decomposition

for prepar-

Dispersion

is established

to that of V,O,/SiO,.

(ref. 6), water However,

same

supported

photo-catalytic

oxide

V04 cluster

to the

oxide

of metal

of V203/SiO2

On

(ref. 4). There-

the surface

of surface

over

the charac-

of conversion.

is important

photo-oxidation.

does not only increase

olefins

has

a quite poor photo-activity

but can also result in formation

As niobium

level

oxide on a silica support

tive metal oxide on a carrier

of

catalyst

at a high

metal oxide

tivity and the active

oxidation The

as

over olefin

of 2-propanol

of highly

dispersed

between

the photo-

oxide have

has

not

been carried

been out

68 for

the elucidation

silica,

local

and the correlation

and the local chemical

structure

comparison

EXPERIMENTS Catalyst

AND METHODS

support

cination

between

of niobium

the activity

of niobium

oxide

grade)

was

prepared

physisorption wt%) were

distilled

area

was

prepared and

pretreated

with

spectra.

composed

at over

prior

to the hydrolysis.

by the B.E.T.

of the silica

K

for

in dry 2 h

system

with

air

method

reaction

elsewhere

(ref.

of 1.3 KPa. U.V.

or

light was

irradiated

with a congas

to the reactor

at room

were the same as those in a previous

of was

recording

2). A mixed

( X > 290 nm) for 30 min. Procedures

N2

solution

K. A catalyst

of 500 mg were performed

described

using

0.66, 4.6, 10

an ethanol

at 773 each

before

cal-

(extra-

tempera-

for collecting

work

(ref. 2).

studies

out with

perature.

twice

alkoxide

mol of propene and oxygen was introduced

pressure

spectra catalyst

mentioned

Laboratory

than

the catalyst

circulating

Photoluminescence

carried

a quantum

and following

The commercial

(Nb-x: x stands for Nb content:

663

at 77 K. A powder as

more

oxygen

and the analysis

Spectroscopic

of propene

Furthermore,

of Si(OC,H5)4

as 565 m* g-l

calcination

ture from a 500 W Xe lamp products

vacua

by impregnation

of 50 and 100

at an initial

in

following

closed

for photo-oxidation

on

procedure

Catalysts

The reactions

ventional

dispersed

oxide catalysts.

by hydrolysis

determined

at 77 K.

Nb(OC2H5)5

highly

OF MO CALCULATIONS

and reaction

was

oxide

is discussed.

at 773 K for 5 h in a dry air stream.

The surface

manner

structure

was made with vanadium

preparation

Silica

pure

of

above.

a facility

for High Energy A sample

were was

was

recorded placed

X-Ray

by a Hitachi

in a quartz

absorption

of 10-B station

Physics

sealed

(KEK-PF)

in a glass

850

spectrofluorometer

cell and conditioned

spectroscopic at Photon

Factory

in a transmission cell after

in the

experiments

were

in the National

mode at room tem-

the standard

conditioning

procedure.

Models

and MO calculations

It is now generally V04 mono-oxo active

tetrahedral

species

in

tetrahedral

cluster,

Nb) cluster

for MO

silica

accepted

support.

species

thus,

calculations with

GAUSSIAN

is

truncation

were

species

Hydrogen

out

programs

to

also

are

the 10).

restricted All

the

Nb04

is a the

mono-oxo

by an M04H3

in detail by Sauer

within

a

located

for representation

(ref.

in V205/Si02

in the later section, be

was modeled

atoms

technique

is discussed

carried

80/82

concluded

active

calculations.

very common and the validity initio

the

species

(ref. 5). As shown

Nb205/Si02

This

theory

that the photo-active

(M= V,

to represent of silica

the

is now

(ref. 9). Ab Hartree-Fock

basis

sets

are

SCF of a

69

split valence type and the same as those used in a previous work for discussion of the ground state of vanadium and niobium oxide supported on silica and alumina (ref. 11).

RESULTS AND DISCUSSION

XANES and EXAFS In order to investigate the local structure of niobium oxide on silica, XANES and EXAFS spectra were recorded for prepared catalysts as well as for reference compounds, KNb03 and YbNbOh. Figures 1 and 2 show K-edge XANES spectra normalized to the height of edge jump of reference compounds and catalysts, respectively.An important characteristicof XANES of transition metal oxides is the pre-edge peak caused by the so-called s-d transition. The intensity depends on the coordination symmetry of oxygen atoms around the central metal atom. The peak is high and clear for a tetrahedral coordination and low and vague for a square pyramidal or an octahedral coordination (ref. 12). Although the local structure of YbNbOh is not clarified by X-ray diffraction analysis, a niobium atom in YNbOh is in a tetrahedron symmetry. The XANES of YbNb04 resembles very much that of

YNbOh reported by Nakai et al., suggesting

that a niobium atom is in a tetrahedral symmetry (ref. 13). The Raman spectrum

lx

supported the suggestion (ref. 14). The local structure of KNb03 is determined

Nb-0.66

by X-ray diffraction as that of a distorted octahedral one (ref. 15). The pre-edge peak in XANES of KNb03 is observed only as a shoulder, reflecting the octahedral symmetry around a niobium atom. The XANES of Nb-0.66 suggests that a niobium

(4

+ < \ : 9 o f <

r

Nb-4.6

I

b)

atom exists in an oxygen tetrahedron, judging 5-

from a similar pre-edge

r

i”

.z 3"

/

18.95

Nb-10

,

19.00

19.05

.8.$

peak to that of YbNb04. The peak becomes small with Nb loading. However, the peak of Nb-10

Photon energy / keV

Photon energy / keV

Fig. 1. XANES of YbNb04(a) Fig. 2. XANES of Nb 05/Si02 and !
70

is still clearer than that of KNb03. Thus, it can be concluded that the major species is a tetrahedral one in a catalyst of very low loading, that is, in a highly dispersed state and changes to a square-pyramidalone with the loading by aggregating tetrahedral species. The EXAFS of samples were analyzed by the method described in a previous paper (ref. 16). Fourier transforms of k3-weighted EXAFS (F.T.) of reference compounds are shown in Fig. 3 (without phase shift correction). A peak appearing at l-2 i is

due

to

0

Nb-0 bonds

2

6

4

R/P:

and peaks at 2-4 i are due to neighboring niobium atoms. A sharp peak due to the h'b-0 bonds for YbNbO4 shows

Fig. 3. Fourier transforms of k3-weighted EXAFS of YbNbO4(a) and KNbO3(b).

Fig. 4. Fourier transforms of k3-weighted EXAFS of Wb205/SiO2 catalysts.

little divergence in the length of each Nb-0 bond, while the peak broad and low in the peak height for KSbO reflects some divergence of the Nb-0 bond lengths (2x1.87, 2x1.99, "3 2x2.17 r\)(ref. 15). TABLE 1 Figure 4 shows the F.T.s of EXAFS of Structural parameters for Nb-0 catalysts. Obviously, the first large peak shells. is due to Nb-0 bonds. The peaks at long distances are not so large as those for reference crystalline compounds, indicating

catalysts

1.3 2.7

1.77 1.96

Nb-4.6

1.3 4.1

1.79 1.96

Nb-10

1.4 3.9

1.79 1.97

Predicted (tetrahedron)

1.0s) 3.0a)

1.72b) l-gob)

parameters for the first Nb-0 shell are extracted from the first peak and shown in Table 1. The theoretical phase shifts and backscatteringamplitudes given by Teo and Lee (ref. 17) were used for the analysis. The accuracy of the estimated

R/Z

Nb-0.66

that ordering of atoms in the catalysts is limited in a short range. The structural

CN

a) Assumed. b) Values predicted by MO calculations.

71 parameters is expected to be 0.5-1.0 % for the distances and lo-30 % for the coordination (ref. 18). The validity of this method is verified by analysis of EXAFS of KNb03.

Table 1 indicates that a niobium atom in Nb-0.66 is in a

distorted tetrahedron as suggested from the XANES. A short bond (1.77 1) possibly has a double bond character. On the other hand, the structural parameters for Nb-4.6 or Nb-10 indicate the presence of square-pyramidal Nb05 species. There is also one short bond (1.79 i). A peak observed at 2-3 i in Fig. 4 is due to the scattering from neighboring Nb atoms, showing some aggregation of the tetrahedral or square-pyramidal clusters.

The peak height

is a little larger for Nb-4.6 and Nb-IO than that for Nb-0.66, indicating an increase in the ordering of atoms with Nb loading. Simultaneously, the peak position shifted to a longer distance. Generally, a bond length shortens in a disordered compound. Thus, the degree of ordering, that is, the degree of aggregation, in Nb-10 is considered to be higher than that in Nb-4.6. From the results mentioned

WAVEISNCTE

/

mu

above, it is concluded that Fig. 5. Phosphorescence spectra of Nb-4.6. Nb-0.66 comprises of dominantly highly dispersed tetrahedral species, and Nb-4.6 and Nb-10 have mainly square-pyramidal species in an aggregated state.

Phosphorescencespectra It is well known that V205/Si02 emits phosphorescence when it is irradiated by a light of about-300 nm and the emission center is assigned to a charge transferred V=O bond in a triplet state. The present Nb205/Si02 catalysts also emit phosphor-

0

2

4 6 8 10 Nb content / wt:'.

escence and the spectrum is shown in Fig. 5. The spectrum was recorded at 77 K with irradiation of 290 nm light. The emission center can be assigned to a charge

Fig. 6. Yields normalized to Nb content in photo-oxidationof propene over Nb205/Si02 catalysts. @,a,~, Ostand for CH3CH0, CH3COCH3, CH3CH2CH0 and CH2CHCH0, respectively.

transferred Nb=O bond, in analogy with V,O,/SiO, mentioned above, and V205/PVG or Mo03/PVG (ref. 5).

The phosphorescencewas quenched by contact with propene

as shown in the figure. The same phenomenon was observed for V205/Si02 and an MO study showed a stabilization of an ethene molecule on an excited V=O bond by an interaction of C=C n-bond with the oxygen atom (ref. 11). Thus, we reasonably expect a similar interaction between the excited Nb=O bond and a propene molecule.

Photo-oxidationof propene No reaction proceeded in the dark at room temperature. Under irradiation of light ( X> 290 nm), the photo-oxidationwas observed with production of ethanaf as the most abundant product with other minor products of acetone, propanal and propenal. Yields normalized to 5 mg of Nb (corresponding to 1 wt% of the mass of catalyst used) are shown in Figure 6. The total yield was considerably lower than that over V205/Si02 (ref. 1). Another remarkable

difference from the

results over V205/Si02 is a very low selectivity to C3 compounds, showing that C=C double bond fission occurred very easily over Nb205/Si02 catalysts. The total yield decreased with the loading of Nb. The decrease is mainly brought about by a decrease in the yield of ethanal,

while the yields of acetone,

propanal and propenal changed a little, Referring to the results of XANES and EXAFS, it can be concluded that the photo-oxidationof propene over Nb205/Si02 proceeds on a photo-excited tetrahedral Nb04 species, which produces ethanal preferentially.The square pyramidal species exhibits low activity and probably produces acetone, propanal and propenal preferentially. Over V,O,/SiO, catalysts, the photo-oxidation hardly occurred in the absence of 02" indicating that activation of the V=O bond by adsorbed 02 is very important. On the contrary, over the Nb205/Si02 (Nb-4.6) in the absence of 02, aldehydes were produced in a comparable amount to that under a condition of existence of 02 as shown in Table 2. Neither ethene nor butene was detected, indicating that metathesis reaction did not occur. This is different from the results over Nb*O~/PVG reported by Morikawa et al. (ref. 6). The yield of TABLE 2 Photo-oxidationof propene over silica-supportedniobium oxide. products / umol catalyst

reactants CH3CHO propene

CH3COCH3

CH3CH2CHO

CH2CHCHO

3.17

0.26

0.0

0.0

3.10

0.47

0.47

0.18

Nb-4.6 propene + Cl2

Propene : 50 Umol, 02 : 100 umol.

73 ethanal is almost the same in both cases, but the yield of acetone is about half and propanal and propenal were not detected in the reaction without oxygen gas.

The results show that photo-excited lattice oxygen of Nb205/Si02 has

higher oxidizing activity than that of V205/Si02 and is transferred to propene to produce aldehydes. The presence of 02 did not affect the activity of Nb205/Si02 so profoundly as that of V,O,/SiO,. The O2 enhanced the selectivity to

C3 compounds presumably by affecting the electronic state of the excited

Nb=O bond.

Comparison between vanadium and niobium oxide catalvsts by MO calculations As mentioned above, there are some differences in catalytic features for propene photo-oxidationbetween V,O,/SiO, and Nb205/Si02. We concentrate on the nature of MO4 (M= V, Nb) species as this is the main active species. In order to clarify the difference between vanadium and niobium from a quantum chemical view point, ab initio MO calculations were performed for MO4H3 clusters. Geometrical structures were fully optimized in the ground state and the results are shown in Fig. 7. There is a short M-O bond for both clusters, indicating a double bond character (M=O bond). The O-M-O bond angle indicates that the MO4 unit of clusters is almost tetrahedral. The Nb=O and Nb-0 bond lengths are longer than the V=O and V-O bond lengths by 0.14-0.17

1,

respectively. These

parameters are in good agreement with the results of EXAFS mentioned above.

0

0 0

I M

15

Fig. 7. Models of V and Nb oxide clusters. Bond lengths and angles are obtained by optimization in MO calculations.

A

*

Fig. 8. Qrbital patterns for the TI-and TT-like MO of M=O bond.

For the triplet state, only the M=O bond length was optimized. For both clusters, the length was longer than that in the ground state by 0.3 i. The triplet state wave functions were constructed by one electron excitation from a bonding n-like MO (approximatelylocalized in the M=O bond) to an anti-bonding n*-like MO which spreads in the same plane as the rr-likeMO as shown in Fig. 8. Thus, two singly occupied orbitals were included in the triplet state. The atomic orbital coefficient at oxygen is dominant in the n-m) while that at metal is dominant in ~1*-MO,indicating a charge transfer from the oxygen to the metal atom by photo-excitation. Charges at atoms calculated by Mulliken's

population Table

analysis

are shown in

3. In both states,

atom in the niobium cluster negative

TABLE 3

the oxygen

Charges

is more

than that in the vanadium

cluster

and the metal atom in the

niobium

cluster

M

is more positive

than that in the vanadium

cluster,

indicating

state in

more polarized

the Nb=O bond than in the V=O bond. The negative the Kb=O

charge of oxygen

bond

adsorption lower

higher

of

than

olefins

activity

olefins

are adsorbed,

A difference is that

significantly

Another

remarkable

indicating

more

considerable

detailed

transfer charge

coefficient

-0.51 -0.43

-1.03 -1.06

bond

should

donation,

polarized

nature

be a disadvantage

and On

of propene

can

oxidize oxygen

propene

of Nb205/Si02 in

enhance

the highly

bond

electrons

and Nb205/Si02

the

of

so

nature

of adsorbed

V=O bond (ref. 19).

selectivity

to ethanal,

adsorbed

in R-MO

of gaseous activity

polarized

an assistance

of the excited

C=C

the once

olefins.

is the high

the

for

in

hand,

in the absence

not

reflect

polarization

the other

over V205/Si02

does

This would

result

in the Nb=O bond than that in

to the adsorbed

gaseous

would

propene.

of propene.

The

The MOs

with the rr*-MO of propene are those of the same orbital MO of M=O bond in the present

is dominant

weakening

+1.54 +I.49

that of V205/Si02.

weakening

discussion.)

the

Nb04H3(singlet) (triplet)

about by accepting

As discussed

in the

of electrons in

-0.91 -0.97

In the case of V,O,/SiO,,

feature

and n-like

at oxygen

of

for enough

is brought

which can interact symmetry

than

Nb205/Si02

Nb=O bond.

is necessary

-0.38 -0.37

in the V=O

as for V205/Si02.

of the excited

+I.29 +1.34

T-electron

the higher

presence

oxygen

weakening

that

through

in the photo-oxidation

the

3(OH)

V04H3(singlet) (triplet)

higher reactivity

the excited and

0

in

of.Nb205/Si02

the V=O bond suggests

oxygen

on the atom(s).

n-like

is expected

excited

Nb=O

at oxygen in the

above,

(Refer to ref. 19 orbital

for

coefficient

MO. The larger the value is, the larger

to the C=C

bond

than

n-like

of a C=C bond in adsorbed

cases.

the atomic

n* orbital.

the

V=O

bond

The larger negative indicates

MO. Thus, over the excited propene

the

larger

Nb205/Si02,

the

is expected.

CONCLL'SIONS Following

conclusions

1) The local structure a catalyst one

with

very

with the niobium

2) The active in a triplet

can be deduced of niobium

low

loading

from the present

oxide

on silica

of niobium

study.

is dominantly

and changes

tetrahedral

in

to a square-pyramidal

loading.

center for propene

photo-oxidation

is the photo-excited

Nb=O bond

state.

3) The excited

Nb=O

bond can oxidize

a propene

molecule

without

an assistance

of adsorbed oxygen because of the significantly polarized nature. 4) The low activity would result from the high electron density at the oxygen atom in the excited Nb=O bond. 5) The high selectivity to ethanal in propene photo-oxidation can be explained by preferential electron transfer to rr*MO of the C=C double bond of propene from n-like MO of the Nb=O bond.

ACKNOliLEDGMENT The X-ray absorption experiments were carried out under the approval of Photon Factory Program Advisory Committee (PAC) (Proposal No. 87-122).

REFERENCES 1 S. Yoshida, Y. Magatani, S. Noda and T. Funabiki, J. Chem. Sot. Chem. Comm., (1981) 601. 2 S. Yoshida, T. Tanaka, M. Okada and T. Funabiki, J. Chem. Sot. Faraday Trans. 1, 80 (1984) 119. 3 T. Tanaka, M. Ooe, T. Funabiki and S. Yoshida, J. Chem. Sot. Faraday Trans. 1, 82 (1986) 35. 4 P. Pichat, J.-M. Herrmann, J. Disdier and M.-N. Mozzanega, J. Phys. Chem., 83 (1979) 3122. 5 M. Anpo and Y. Kubokawa, Rev. Chem. Intermediates,8 (1987) 105. 6 A. Morikawa, T. Nakajima, I. Nishiyama and K. Otsuka, Nippon Kagaku Kaishi, (1984) 239. 7 K. Domen, A. Kudo, M. Shibata, A. Tanaka, K. Maruya and T. Onishi, J. Chem. Sot. Chem. Comm. (1986) 1706. 8 Y. Dada and A. Morikawa, Bull. Chem. Sot. Jpn., 60 (1987) 3509. 9 J. Sauer, Chem. Rev., 89 (1989) 199. 10 D.J. Binkley, R.A. Whiteside, R. Krishnan, R. Seeger, D.J. DeFrees, H.B. Schlegel, S. Topiol, L.R. Kahn and J.A. Pople, QCPE, 13 (1981) 406. 11 H. Kobayashi, M. Yamaguchi, T. Tanaka, Y. Nishimura, H. Kawakami and S. Yoshida, J. Phys. Chem., 92 (1988) 2516. 12 J.A. Horsley, I.E. Wachs, J.M. Brown, G.H. Via and F.D. Hardcastle, J. Phys. Chem., 91 (1987) 4014. 13 I. Nakai, M. Imafuku, J. Akimoto, R. Miyawaki and Y. Sugitani, Photon Factory Activity Rep. KEK Jpn, 4 (1986) 183. 14 J.-M. Jehng, personal communication. 15 L. Katz and H.D. Megaw, Acta. Cryst., 22 (1967) 639. 16 T. Tanaka, H. Yamashita, R. Tsuchitani, T. Funabiki and S. Yoshida, J.C.S. Faraday Trans. 1, 84 (1988) 2987. 17 B.-K. Teo and P.A. Lee. J. Am. Chem. Sot.. 101 (1979) 2815. 18 B.-K. Teo, EXAFS: Basic Principles and Data Analysis; Springer-Verlag, Berlin, 1986, p. 181. 19 H. Kobayashi, M. Yamaguchi, T. Tanaka and S. Yoshida, J. Chem. Sot. Faraday Trans. 1. 81 (1985) 1513.