React. Kinet. Catal. Lett., Vol. 32, No. 2, 469-474 (1986)

ELECTRONIC SPECTROSCOPY OF THE STATE OF

Cu 2+ IONS IN CuO-TiO 2 CATALYSTS

Sh.A. Talipova and V.N. Vorobiev

Polytechnical Institute, Tashkent 700143, USSR

Received September 25, 1985 Accepted January 8, 1986

Diffuse reflectance spectroscopy of supported CuO-

TiO 2 catalysts have revealed that, depending on the

CuO content and the calcination temperature, either

Cu 2+ ions incorporated into the TiO 2 lattice, or

surface oxide clusters, primary oxide aggregates and

CuO1+ x phase bonded to the support by Ti - 0 - Cu

bridges, are formed.

C HOMO~bD 3~eKTp0HHOM CHeKTp0CKOHMM H O K a 3 a H 0 , q T 0 B H p o -

~ e c c e n p H r o T o s n e H H s HaHeCeHHblX C u 0 - T i 0 2 K a T a n H 3 a T 0 p 0 B B

3aBHCHM0CTH 0T C 0 C T a B a H T e M n e p a T y p N n p o K a n H s a H ~ s 0 6 p a -

Cu 2+ 3yDTCH HOHBI , BHe~peHHble B pemeTKy TiO2, a TaKme nO-

BepxHOCTHble OKHCHble K~acTepbl, nepBHqHNe OKMCHble aFpeFaTbl

H ~asa CuO1+x, B3aHMo~e~CTBym~Me C HOCMTeneM nocpe~cTSOM

Ti-O-CU CBSSe~.

Pronounced reducing properties of TiO 2 surface [I] p~rmit

us to use it as a promising carrier and modifying agent of ca-

talysts for hydrogenation and oxidation processes, which are

stable towards coke formation and deactivation by carbon con-

densation products [2]. Therefore, in the present study the

genesis of copper ion sites in the oxide form of supported

CuO-TiO 2 catalysts has been studied by diffuse reflectance spec-

troscopy and X-ray analysis.

Akad~miai Kiadd, Budapest

TALIPOVA, VOROB/[EV: ELECTRONIC SPECTROSCOPY

CuO-Ti02 samples were prepared by impregnation of hydrated

titanium d~oxide (TiO 2" xH20) with water solutions of pure grade

Cu(NOr " H20. Sizes of the primary crystallites were determined

a~ording to the broadening of X-ray linewidths [3].

It has been shown previously [4] that, depending on the pH

of TiO 2" xH20 precipitation, after its thermal treatment at tem-

peratures ranging within 300-500~ it can transform either to

anatase or to futile. The results obtained indicate that the

introduction of Cu 2+ ions exerts an additional effect on futile

formation at rather low temperatures in accordance with the pre-

vious data [5]. Initial TiO 2. xH20 precipitated at pH=6.0-7.0

is amorphous (Fig.1.1). Upon calcining at 300~ anatase nuclei

are formed with the sizes of primary crystallites of about 35A

(Fig. 1.2). Upon calcining at 600~ their size grows up to

about 80 A (Fig. 1.3). At temperatures of 800-1200~ anatase

transforms into well-developed crystals of rutile whose mean

size is > 500 A. If TiO 2. xH20 precipitated at pH=6.0-7.0 is im-

pregnated with a cupric nitrate solution, irrespective of the

latter concentration the support is crystallized at 300~ to

futile particles with the size of about 90 A (Fig. 1.4). At

600~ their size grows to ~ 900 A (Fig. 1.5) and to >1000 A at

1200~ (Fig. 1.6). If TiO2- xH20 is precipitated at pH=9.0,

cupric nitrate impregnation does not influence the normal way 300~ 700~ futile. of its transformations: TiO 2- xH20 �9 anatase

The formation of various compounds with Cu 2+ ions is dis-

tinctly seen from diffuse reflectance spectra. According to

these spectra, for the CuO-TiO 2 system one can conventionally

distinguish concentration ranges that are responsible for the

similar regularities in the observed transformations of copper

sites.

Samples with 0.1 - 0.5 wt.% CuO. After impregnation and

drying of the sam~es, the following adsorption bands are ob-

served in their spectra: a wide intense asymmetric band with a

maximum at 11.0 kK due to the 2Eg �9 2T2g electron trans-

fer in Cu 2+ ions in the distorted octahedral coordination; a

poorly resolved band at 24.0 kK* due to the charge transfer

* I kK= I03cm -I

470

TALIPOVA, VOROBIEV: ELECTRONIC SPECTROSCOPY

X

x • �9 6 X •215 • X �9

I 5 x •215 ~ �9 •

x L.

A ~ ~ ~ J L f 3

2

L I i /.,I 0 , �9 70 60 50 30 20

28

Fig. I. X-ray diffractograms of: I-3:TiO2.xH20 and

CuO-TiO 2 catalysts containing: 4:25.0 wt.%

CuO, and 5-6: 25 wt.% CuO. Calcination tem-

perature {~ 1-20, 2,4-300, 3-5-600 and

6-1200 ( ~ - anatase, x -rutile, �9 ~CuO)

band in Cu 2+ - O - Cu 2+ associaties and a narrow band with a

maximum at 6.9 kK caused by an overtone of the stretching vi-

brations of hydroxy groups. The 2E 2E2g g , band is considerably 2+ displaced with respect to that of the initial [Cu(H20) 6 ] ob-

served at 12.4 kK [7] Hence one can suggest that during ir~reg-

nation and drying of CuO, low concentration samples, aqua com-

[Cu(H20) 612+ interact with surface hydroxy groups of plexes of

TiO 2 to form surface Ti - O - Cu

and Ti - 0 --- Cu bridges

that distort the octahedral symmetry of Cu 2§ ions. Owing to the

thermal instability, after treatment at 300~ the supported Cu 2+

aqua complexes lose their water molecules and counterions and

partially incorporate into the support lattice. With increasing

471

TALIPOVA, VOROBIEV: ELECTRONIC SPECTROSCOPY

8 60 n , '

I

AO

t ' - - ' - - ' " ~ -

12" 0,/1/-..0 15.0 8~27 0 ~

,,,,o , ,/

5.2 ", 70/

I; .':.Y. l Ii-TJ/

5 i ~ % l I00 ~ l

80 69 11.0 /vf n 5 / I

' \ Jl

12o\ / / /

,,,_,I I I I I I

5 10 15 20 25 30 (kK)

100

8O

60 8 n -

l

A0

20

Fig. 2. Diffuse reflectance spectra of: 6-CUO1+0.006

and CuO-TiO 2 samples contairing: 1-4-L0~t.%CuO

and 5,7-9-10.0 wt.%CuO. Calcination temperature

(~ 1,5 - 95; 2,7 - 300; 9 - 600; 3 - 1000

and 4,8 - 1200

the temperature from 300 to 1200~ the degree of incorporation

grows, which is confirmed by the spectra (Fig. 2.2-4) wherein

the 2Eg �9 2T2g band is displaced from 11.0 to 13.5 kK with a

gradual temperature rise up to 1200~ According to Ref. [8],

the adsorption band at 13.0-15.0 kK is characteristic of Cu 2+

ions in TiO 2. Alongside with the displacement and intensity de-

crease of the 2Eg �9 2T2g band, one can observe an increase

in the intensity of the charge transfer band and its displacement

from 24.0 to 22.5 kK and a rise in the absorption level in the

region of 15.0-18.0 kK that is responsible [9] for the charge

transfer band of Cu + ions. Hence one can suggest that with in-

472

TALIPOVA, VOROBIEV: ELECTRONIC SPECTROSCOPY

creasing the calcination temperature, Cu 2+ ions are incorporated

into the TiO 2 lattice and surface oxide clusters (-Cu-O-Cu-O-)n

are formed on TiO 2 due to the sharp decrease of its surface

area. Intensity decrease of the band at 13.0-13.5 kK

and the increase in the absorption level in the region of 15.0-

18.0 kK can also imply the partial transformation of Cu 2+ ions,

but it must be tested by additional experiments.

Samples with 5-25 wt.% CuO. After impregnation and drying

of high-CuO samples one can observe the formation of surface

associates and aggregates of Cu(NO3)2.3H20 on hydrated TiO2.This

is confirmed by the similar spectra of CuO-TiO 2 samples (Fig.2.5)

and of crystalline Cu(NO3)2-3H20.

Above 250~ Cu(NO3)2-3H20 completely decomposes to nonst~-

chiometric CUOl+ x wherein with increasing the calcination tem-

perature from 250 to 900~ concentration of superstoichio-

metric oxygen x determined like in Ref.[10] changes from 0.006

to 0.001. As a result of the decomposition of cupric nitrate

aggregates on the TiO 2 surface, a CuO film chemically bonded to

the support surface by Ti-O-Cu bridges, is formed. Difference

of the surface film from CuO1+ x particles is seen from the X-ray

diffract0grams of the samples calcined at 300~ (Fig. 1.4). De-

spite the presence of 10-25 wt.% CuO, they demonstrate only low

intensity broad lines of rutile, whereas no CuO lines are ob-

served at all. Consequently, the surface CuO film is X-ray amor-

phous. On the other hand, CuO1+ x phase is a typical semiconductor

and has a broad structureless absorption band with a long-wave

edge near 14.6 kK (Fig. 2.6), while the spectrum of the surfaoe

CuO film exhibits a distinct band at 13.0 kK due to the 2Eg---2T2g

transfer of Cu 2+ ions, whereas the level of structureless absorp-

tion is rather low (Fig. 2.7). Hence one can suggest that the

surface CuO film formed after calcining at 300~ possesses

properties of a dielectrics. Surface CuO film stabilized to the

carrier does not dissociate to Cu20 even after calcining at

1200~ (Fig. 2.8). In the case of nonbonded CuO1+x, CuO trans-

forms to Cu20 at 960~ Hence the chemical bonding of the surface

CuO film to the carrier surface inhibits its crystallization to

473

TALIPOVA, VOROBIEV: ELECTRONIC SPECTROSCOPY

yield bulk Cu01+ x and its thermal dissociation to Cu20.

Above 450~ intense crystallization of the support starts,

its surface area diminishes and the most of the surface CuO film

transforms to primary oxide Cu01+ x aggregates bonded to the sup-

port by Ti-O-Cu bonds and also to particles of Cu01+ x, which is

observed in the diffractograms as lines with d=2.52, 2.32, 1.87,

1.50 and 1.38 A (Fig. 1.5,6). As a result of the above process,

the intensity of the band at 13.0 kK decreases and instead there

appears a large structureless absorption band with a long-wave

edge at 14.6 kK characteristic ~r the Cu01+ x phase.

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