Characterization of copper-silica catalysts prepared by ion exchange

379

Applied Catalysis, 2 (1982) 379-387 Elsevier Scientific Publishing Company,

CHARACTERIZATION

M. SHIMOKAWABE, Department

Amsterdam

OF COPPER-SILICA

N. TAKEZAWA

of Chemical

- Printed in The Netherlands

CATALYSTS

PREPARED

BY ION EXCHANGE

Hokkaido

University,

H. KOBAYASHI

and

Process

Engineering,

Sapporo

060,

Japan.

(Received

16 July 1981, accepted

18 January

1982)

ABSTRACT Copper-silica catalysts were prepared by ion exchange between the hydroxyl hydrogens on a silica surface and tetramnine copper (II) ions, and were characterized by a variety of analytical methods. It was concluded that copper ammine complexes held on the surface can exist in three different states, i.e. isolated and clustered diammines and free tetramnine copper (II) nitrate. The relative amounts of these species were strongly dependent upon the amount of copper loaded onto the silica. These ammine complexes decomposed around 300°C. The clustered species transformed into highly dispersed CuO clusters at 300°C in air and then further crystallized into bulk CuO at aroun 720°C. The isolated species, on the other hand, remained in part as isolated Cu B + ions on the support after calcination at temperatures above 3oo"c, while the tetrammine copper(II)‘nitrate rapidly transformed into bulk CuO at 500°C. Based on these results, a surface phase diagram of the catalyst was tentatively presented as a function of copper loading and calcination temperature.

INTRODUCTION The elucidation of a catalyst

of the effects

is of considerable

ization of catalysts this purpose

were fairly active hydrogen

prepared

[l]. In previous

atoms

in the methanol

into gaseous

catalysts

which were prepared

was markedly copper

affected

loading.

in the copper the reaction

under various work,

for methanol

converted

activity

were strongly

decreased

of the catalyst,

[6], some experiments

which were prepared

by ion exchange

OlSS-9~34/82/0000-0000/$02.76

precursor

when the copper depended

between

or supported

and the amount of

revealed

that anions and inhibited

at lower pH. On the

when the hydroxide copper formed

upon the copper

out over copper-silica

hydroxyl

0 1982 EIsevier Scientific

for

catalysts

was

in the course

loading was decreased.

markedly

were carried

required

with various metal oxides

was prepared

decreased

size of the metallic

appreciably therefore,

hydroxide

held in the catalyst

held markedly

character-

are effectively

of support-free

of the hydroxide

of the catalyst

at higher pH. The particle

More recently

copper

to a great extent when the hydroxide

of the reaction

is, therefore,

as well as in water,

by the pH of preparation

material

A precise

CH30H + H20 + CO2 + 3H2, so that

[4 - 81. The activity

by kneading

other hand, the amount of anions prepared

conditions

design.

it was shown that copper-containing

molecule,

hydrogen

upon the nature or the structure

for catalyst

steam reforming,

Characterization

starting

of preparation importance

The

loading. catalysts

protons on a silica gel surface

Publishing Company

380 and tetrammine the activity catalysts.

copper

conditions

parameters copper

affected

as well as

by the preparation

of the

In connection with these results, the present work was undertaken

study the states of catalysts various

It was shown that the selectivity

(II) cation.

of the reaction was strongly

which were prepared

by an ion exchange

and we show how the states of the catalyst

involved

in the preparation,

to

method at

varied with the

such as the calcination

temperature

and

loading on silica.

EXPERIMENTAL Catalysts The catalysts

were prepared

complex

solution was prepared

complex

cation was exchanged

Chromato

from copper nitrate with hydroxyl

method.

copper(I1)

and aqueous ammonia,

and the

protons on a silica gel surface

were dried at 110°C in air for 3 h. Copper contents

in the range 0.5 - 11.1 wt %. Some experiments

catalyst

Tetrammine

(Nihon

Ind. Co., surface area = 413 m2 g-') at a pH of 11 - 12. The catalysts

thus prepared varied

by an ion exchange

prepared

by the impregnation

method.

in the catalysts

were carried out over a

The catalyst

thus prepared

contained

21.1 wt % copper.

Characterization

of catalysts

DTA and TGA experiments equipment

were carried

The heating experiments,

a hydrogen-nitrogen

by gas chromatography of the catalysts

(Ohkura Electric

Infrared spectra of the catalysts

the determination on the surface. reference

cristobalite

spectra

sphere

(Hitachi 210-2101)

was attached.

with a Hitachi Model 260-50

(Vacuum Generators

ESCA 3) was employed

state of copper or for the assignment

infrared for

of ammonia

The 2p binding energy of Si (103.6 eV) in silica was used as the line for the XPS spectra.

was confirmed

of surface hydroxyls magnesium

Co., Model 701). Diffuse reflectance

were recorded

XPS spectroscopy of the valence

reduction)

regions were recorded with a Hitachi Model 330

to which an integrating

spectrophotometer.

programmed

mixture (4 % hydrogen) was passed over the catalyst -1 and the hydrogen consumption was determined

in the UV/VIS/NIR

spectrophotometer

with the aid of DTA

(Cahn Model RG), respectively.

rate was 5 - 10°C min -'. In the TPR (temperature

at a total flow rate of 50 ml min

energy

out in air or nitrogen

(Rigaku Denki 8441 Al) and an electrobalance

The presence

by XRD (X-ray diffraction,

on the silica gel surface was estimated

bromide and subsequent

determination

from the concentration

before and after the ion exchange.

solution was titrated with EDTA solution pyridylazo)-2-naphthol

as an indicator.

by reaction with ethyl

of the amount of evolved

The amount of copper on the support was estimated ion in the solution

of cupric oxide or

Rigaku Denki 2114). The number

The tetrammine

of known concentration

ethane

[91.

of cupric

copper

(II)

using PAN l-(2-

381 RESULTS

AND DISCUSSION

Figure

1 illustrates

the DTA curves of copper-silica

at 110°C prior to the experiments. two exothermic appreciable

when the copper loading exceeds or helium atmosphere,

gas analyses

revealed

265 - 3OO"C, respectively. were observed oxidation

catalysts

which were dried

peak is seen around

100°C while

peaks are seen around 265 - 300°C. The latter exothermic

out in a nitrogen although

An endothermic

that water and ammonia

Since exothermic

only in the presence

of copper and/or

5 wt %. When the experiments

only the peak around

of oxygen,

were carried

100°C was observed,

desorbed

peaks around

peak becomes

at 100°C and around

265 - 300°C and 720°C

these would be ascribable

to the

ammonia.

:: w

i =;

! 8 6 in air static

FIGURE

1

DTA curves of Cu/Si02 catalysts

Figure 2 illustrates

dried at 110°C in air.

XPS spectra of the catalyst.

A peak due to N 1s of ammonia

is seen around 400 eV. However, if nitrate was present. around

there is no peak at 407 eV as would be expected The satellite peak which is characteristic of Cu 2+ is seen

945 eV, together with the Cu 2P 3,2 peak of divalent

The peak ratio Cu2+ (satellite)/Cu2+ well with that obtained that divalent

copper

decrease

copper

(II)

for a number of cupric compounds

ions exist predominantly

to copper was estimated tetrammine

2p3,2 is estimated

copper around

to be 0.47 which agrees [lo]. This strongly

on the surface.

nitrate.

From TG or OTA analysis both around

suggests

The ratio of ammonia

to be 2.07 on the basis of the intensities

in the weight was observed

936.4 eV.

of XPS of

of the catalyst,

a sharp

100°C and 300°C. As described

in

382

410

Binding

FIGURE 2

energy

XPS spectra of Cu/Si02 catalyst

the results of gas analyses, the latter estimated

395

400

405

the former

is due to that of ammonia.

eV

(IO wt %).

is due to the desorption

to be twice that of copper atoms held on the support,

results obtained

by XPS. When the catalyst

which was ascribed by gas analysis

to N Is of ammonia

as expected

to half the number of hydroxyls

copper

by chemical

(II)

from the observations

ion and diammine

hydroxyl

hydroxyl

copper

groups.

protons

analysis

and it was found

at saturation.

on silica as determined

bromide and surface

that a pair of surface

any appreciable

the

and TG analysis.

that IO wt % of copper was held on the silica surface

conclude

lost was

confirming

was heated above 3OO"C, the XPS peak

vanished

The amount of copper held was determined

ethyl magnesium

of water while

The number of ammonia molecules

This corresponds

by the reaction

between

Based on these results,

is exchanged

(II) is formed on the surface without

amount of counter anion from the solution.

we

with one tetrammine

It decomposes

holding at 265 -

300°C. Table 1 shows the results obtained were calcined

in air at various

loading were calcined were observed obtained

together

electronic

after the catalysts

When the catalysts

with higher copper

at 800°C or 900°C in air, the XRD patterns with those of cristobalite,

for the catalyst

temperatures.

by means of the XRD method

temperatures.

The presence

with lower copper of isolated

spectra of the catalysts

loading

cupric

due to bulk CuO

while no bulk CuO patterns even calcined

ion was suggested

with lower copper

loading.

were

at these high

later by the

383

2

4

8

6

lo+e/"c

FIGURE 3

Temperature

programmed

b, 1 wt % Cu. The catalysts

reduction

were calcined

spectra

of Cu/Si02 catalyst.

a, 10 wt%

Cu;

in air at 500°C for 3 h prior to the

experiments.

TABLE

1

Change

in XRD patterns

catalyst

of Cu/SiO2 catalyst calcination

/wt %

temperature

110

500

700

800

/"C 900

Cu/Si02

(0.5)

a

a

a

a

a

CulSi02

(1.0)

a

a

a

a

(CUO)

Cu/Si02

(8.0)

a

a

a

(CUO)

cuoc

Cu/Si02

(10.4)

a

a

a

(CUO)

cuoc

Cu/SiO,

(11.1)

a

a

b

b

cuoc

ano CuO patterns b not determined 'cristobalite

Figure calcined with

were observed

was formed

3 illustrates

typical

TPR spectra obtained

at 500°C in air. A strong peak appeared

10 % copper.

The peak temperature

from the catalysts

which were

around 280 - 300°C for the catalyst

almost coincides

with that obtained

on bulkCu0.

384 The number of hydrogen molecules atoms loaded on the silica. was predominantly

consumed

was practically

From these results,

formed when the catalyst

with

since no bulk CuO was observed

which was calcined

at 5OO"C,

by DTA, it was concluded copper(I1)

In contrast

with the results

were formed by the decomposition DTA peak was observed.

on bulk CuO or small clusters

ions held are, at least in part, strongly

UV/VIS/NIR

catalysts

I

Ref. = A1203 (D)

lO%R

I

(A) temperature -air-drying _______-,,(yJc 500°C 900°C .i _/'-~iL,

--._z O-H 400

600 Wave length

FIGURE 4

UV/VIS/NIR

800

1000 /nm

spectra of Cu/Si02 catalysts.

which were

When 10 wt % copper was loaded on

1

-

of

interacted

is decreased.

spectra of copper/silica

---

peak

The latter TPR peak is located at

in air at various temperatures.

200

exothermic

two TPR peaks are seen at 280 - 300°C

to that observed

with silica when the amount of its loading

dried or calcined

In conjunction

the exothermic

1 wt % copper.

compared

that the cupric

Figure 4 illustrates

in air

of the catalyst

into bulk CuO at 720°C where another

to these results,

and 650°C on the catalyst with much higher temperature CuO, suggesting

state.

that these clusters

at 260 - 300°C where

They would further crystallize was observed.

in XRD patterns

the cupric oxide formed was very likely to exist as

small clusters of CuO in highly dispersed

of diammine

that cupric oxide

10 wt % copper was calcined

at 500°C. However,

obtained

the same as that of copper

it was concluded

1400

1600

385 silica,

the absorptions

absorptions

at 1400 and 1500 nm which are ascribed

of the hydroxyl

the absorption

which

groups

is ascribed

[11] greatly

decrease

to the N-H stretching

to the overtone

in their

vibration

intensities of amninia

and

appears

at 1530 nm. This was also confirmed by the infrared spectra of the catalysts. The -1 absorption at 960 cm , which is ascribed to the Si-0 stretching vibration of the SiOH group on the surface protons with tetrammine Figures

observed

oxygen

the spectra obtained

at 270 and 350 nm are ascribed

and the isolated

and clustered

band at 700 - 900 nm is assigned octahedral

due to the exchange

environment

to d-d transitions

or calcined

is present

cations.

the catalyst absorption

diammine

as divalent

appears

structureless

shows that diammine copper

(II)

680 nm according

complex

copper

In particular,

gave a characteristic

by the Kubelka-Munk

on catalysts

absorption

with

equation

loading.

in the charge transfer

became discernible, that the isolated

particularly species were

This since cis-

absorption

at 670 -

c171.

cu

F(R,)

= (1-R_)2/2R_

and wt % copper.

at 270 and 350 nm which were

[18] against

the weight

% of copper

It is seen that the isolated

less than 2 - 3 wt % copper,

in copper

the

clustered, 12

8

reflectivity

dried at room temperature.

with increase

at 110°C although

by the ion exchange

Figure 5 shows the plots of the reflectivities

catalyst

be noted

with 10 wt % copper.

wt.%,

estimated

it should

(II)

4

between

that copper

with 0.5 wt % copper when

or calcined

was formed

in an

is seen with all catalysts

to Bjerrum et al Cl63 and Grant and Kollrack

Relation

The absorption

on the catalyst

in solution

0

between

ions situated

at llO"C, indicating

l

FIGURE 5

transfer

of cupric

at 680 nm on the catalyst

was dried at room temperature

is somewhat

evidently

to charge

[I3 - 151. The latter absorption

predominantly

hydroxyl

below 1000 nm. The

ions [13], respectively.

which were dried at room temperature

that the absorption

of surface

ions.

4 (C) and (D) illustrate

absorptions surface

[12], decreases

copper(I1)

while the clustered

When the catalysts region became

species

species

in part clustered.

increase

were dried at llO"C, the

broader and that around

on the catalysts

in the

predominate

350 nm

with 1 - 3 wt % copper,

suggesting

However,

change

no appreciable

386 occurred

in the charge transfer

higher than this amount. copper

bands of other catalysts

When the catalyst

(II) held on the surface

diammine

transformed

kept their structure

with

loading,

bands

The spectra greatly

the isolated

The clustered

above. On the

species

at 500°C. This was illustrated

band around 270 nm together

the absorption

above.

10 wt % copper.

cupric oxide as discussed

with lower copper

even after heating

of the sharp absorption heated at 9OO"C,

as described

for the catalyst

into highly dispersed

other hand, for catalysts

lower or

was heated above 260 - 3OO"C, the diammine

decomposed

changed and became structureless

having copper

still

by retention

with that near 750 nm. When

in the region

270 - 350 nm became

indicates

that the isolated species had in part clustered. 2+ transition of Cu still remained.

However,

broader.

It

the d-d

10 -

8-

CuO(crvsta1)

+ (d)

- m-----m_ ,” 2 N

6-

z

4-

--N .. (d) Cu++(iso)

#

2 _ ,(a')diamnine-Cu(II)(iso.dehydrated) f '.+(b')diammine-Cu(II)(clus.dehydrate!i) (a')‘,

4

10

'8

6

Structure

of the CulSi02 catalyst.

When copper was impregnated ion exchange

method,

on silica

a strong absorption

at 1385 cm -' [19]. The excess amount nitrate.

in marked

contrast

in an amount

of copper loading

exceeding

which was ascribed

is probably

(clus) = clustered.

saturation

to nitrate

held as tetrammine it decomposed.

on the ion exchanged

by the

occurred

copper

(II)

By calcination

catalysts

This was

on which

CuO

720°C.

Based on the results held on the silica

16

due to bulk CuO were readily observed.

to the observations

was only formed above

14

(iso) = isolated,

When this species was heated above 3OO"C,

at 500°C in air, the XRD patterns

12

cu

wt.%, FIGURE 6

(a') + (b') + (c')

II I i (a)diammine-Cu(II)(iso)+(b)(clusi) (a)+(b)+(c)tetrammine-Cu(I1) 2

0

*. '.

+ (e) CuO (highly dispersed)

(d)

(a)

%

obtained

by various

analytical

gel surface are tentatively and calcination

temperature

methods,

summarized

the states of copper

in a diagram

as a function

as shown in Figure 6. It is evident

381 that the states are strongly lOO"C, the isolated

dependent

and clustered

upon these parameters.

diammines

to more than 10 wt % copper,

however,

After the dehydration

lOO"C, isolated

copper

ammines

ammine

transforms

remain

partly as isolated

dispersed However,

present

around

on the surface

exists

CuO clusters tetrammine

species

cupric

around

(II)

ion clusters,

rapidly

copper

nitrate.

(II)

All

265 - 300°C. The clustered while

ions, even after decomposition.

nitrate

below

corresponding

are in part clustered.

are heated above 72O"C, they transform

copper

At temperatures

The ammine

as free tetrammine

decompose

into highly dispersed cupric

are present.

decomposes

isolated

species

When highly

again

into bulk CuO.

into bulk CuO even at 500°C.

CONCLUSION Copper-silica a variety

were greatly

affected

of the catalyst. catalyst

catalysts

of methods.

prepared

by an ion exchange

by the amount of copper

On the basis of the results

was presented

method were characterized

It was shown that the states of copper

as a function

loaded and the calcination obtained,

of the parameters

by

held on the support

a surface involved

diagram

temperature of the

in the preparation

of the catalyst.

ACKNOWLEDGEMENT We would

like to thank Professor

M. Nagayama

for their help in the XPS measurements

and Dr. H. Konno, Hokkaido

University,

and discussion.

REFERENCES 1 2

J.R. Anderson, "Structure of Metallic Catalysts", Academic Press, New York, 1975 A. Sugier and 0. Bloch, Proc. Fourth Intern. Cong. Catalysis, Akademiai, Tiado, Budapest, 1971, p. 238. H. Kobayashi, N. Takezawa and C. Minochi, Chem. Lett., (1976) 1347. C. Minochi, H. Kobayashi and N. Takezawa, Chem. Lett., (1976) 507. K. Takahashi, N. Takezawa and H. Kobayashi, Applied Catal., 2 (1982) 363. H. Kobayashi, N. Takezawa, C, Minochi and K. Takahashi, Chem. Lett., (1980) 1197 H. Kobayashi, N. Takezawa and C. Minochi, Bull. Fat. of Eng., Hokkaido Univ., 102 (1981) 13. H. Kobayashi, N. Takezawa and C. Minochi, J. Catal., 69 (1981) 487. 98 M. Sato, T. Kanabayashi, N. Kobayashi and Y. Shima, J. Catal., 7 (1967) 342. 10 D.C. Frost, A. Ishitani and C.A. McDowell, Mol. Phys., 24 (1878) 861. 11 L.H. Little, "Infrared Spectra of Adsorbed Species", Academic Press, New York, 1966. R. Soga, Bull. Chem. Sot. Japan, 34 (1961) 1461. G.K. Boreskov, Proc. Sixth Intern. Cong. Catal., London, The Chemical Society, London, 1976, p. 204. R.M. Frieman, J.J. Freeman and K.W. Lytle, J. Catal., 55 (1978) 10. 1': F.N. Khasnov, V.N. Vorb'ev and G. Sh. Talipov, Kinet. i Katal., 19 (1978) 577. 16 J. Bjerrum, J. Ballhausen and C.K. Jorgenson, Acta Chem. Stand., 8 (1954) 1275. D.M. Grant and R. Kollrack, J. Inorg. Nucl. Chem., 23 (1961) 25. 1; P. Kubelka and F. Munk, 2. Tech. Phys., 12 (1931) 593. 19 F.A. Miller and C. Wilkins, Anal. Chem., 24 (1952) 1253.