Al2O3-SnO2 systems as a support for metallic catalysts I. preparation and structure

Materiais

Chnistly

Al@@1102

and Physics,

SYsrEMS

I.PRElPARATlON

27 (1991)

117-128

AS ASUPPORT

117

FOR MFTALLIC

CATALYS’I8

AND Sl’RUCl’URE

P . KI~ZENSZ~N Faculty of Chemistry, AMichiewicz Received

University, Gruawaldzha

6,~780

Poznah,(Poland)

August 27,199O ; accepted October 1,199O

ABsTRAcr

The effects on phase imposition

and structure of adding tin to alumina were investi-

gated by X-ray difftaction and thermogravimetric

methods. Structural and phase transfor-

mation were observed to be functions of the calcination

temperature

and composition.

EVl’RODUCIlON

Many chemical transformations is proportional

are =catalyzed by metals. Sina

the activity of a catalyst

to its effective surface area exposed to reactants, considerable

efforts are

often made to maximize the surface area per unit mass of catalytic material maximum, and thus the use of highly dispersed metal is the traditional

approach to this problem. In praG

t&x, small metal particles cannot exist separately and therefore they have to be prevented from coming into contact with each other to avoid aggregation and sintering. A high degree of metal dispersion

on the surface area can be obtained by an appropriate

carrier [r-4], addition of another metal [5-81 or non-mctul[9], pretreatment

[12,13], and regeneration

cnviromcnts

selection of a

preparation methods [lO,llJ,

mpcricnccd

by the catalyst, The at-

tempts to explain the behaviour of the carrier are in the focus of our catalytic

interests.

Although the support material may be relatively inert towards the metal partkies which rest upon its sutface, total inertness pected. Metal-support interaction electronic

properties

has neither been attained, nor indeed it is to be ex-

can certainly affect both the particle morphology and the

of the metal particles [14-253. Introduction

of the admixture to the

carrier matrix Ieads to the transformation

of its structure, which then involves changes in

the quantity and strength of interactions

between the catrier and the met&c

posited on its surf&e. It should be remembered 0254-0584/91/$3.50

phase de-

that as a result of these changes, the

0 Elsevier

Sequoia/Printed

in The

Netherlands

118

physico-chemical

character

tural properties,

formation of a new phase and of other defects etc.), thus affecting (indc-

pendtntly

of the carrier will also change (e.g. acid-basic properties, tex-

of metal centers) the catalytic process.

Since under the conditions of catalytic reaction, the composition of the surface layer of catalyst will depend on the composition of the bulk composition

of catalyst on the other hand, therefore the way of incorporation

of the other metal, for bimetallic of different phenomena tion, and precipitation the appcarancc

of the reaction mixture on the one hand, and

catalysts, is of importance.

This may result in a number

such as the ordering of defects on the surface, surface reconstrucof new bidimtnsional

surface phases. These phenomena

of new types of active centers at the surfaa

may entail

of the catalyst, directing the

catalytic reaction into a new path. The application of PtSnly-Alz(>s concerning

as a reforming catalyst has stimulated many studies

the role of tin and its effects on catalytic properties.

studies was to determine

the geometric or electronic

modification

some of the papers the influence of tin is not resticted

The main focus of the of Pt by tin, though in

to the metallic phase [26j.

In mc& of the studies on the system, tin was introduced to the camlyt by the impregnation method, which limited its distribution report on the tin distributed

to only the surface layers. Only a few papers

all over the bulk of carrier and

consequently

attempt to

compare the effect of surface tin and of the tin localized in the bulk of Ah03 properties

of the Pt-Sn-AlzO3

coprecipitation

on the

system [27,28]. According to Sexton and co-workers [281,

methods give a uniform tin distribution

and Pt can then be easily distributed

to come into contact with the tin ions. The conclusions

support the ideas presented

Butch [29J that tin ions are a surface modifier of y-Al$Q most likely due to changes in the electronic interaction This paper is a continuation

by

and the reactivity changes are

between Pt and Sn(Il)@l$&

of our studies on modeling of the PtSn+l$J3

.

system

[39,311.

EWERIMENTAL

The

binary systems AhO3

synthesized by coprecipihtion in carbon tetmchloridc

-

SnO2 with

Ah103 / SnOz

molar ratio between

O-l wtre

methods. As starting materials, a solution of tin (IV) acetate

and a solution of aluminum isopropoxidc in isopropanol were used.

All the reactants used were analytical reagent grade and the trace amounts of water were removed from the applied solvents directly before their use. Homogcnous

mixtures of the reagents were subjected to hydrolysis at m

with the

pH value kept between 7.0 to 7.5 with the help of ammonia water (2%w/w). At this tem-

119

peraturc, precipitate

the obtained

solution

with a precipitate

was filtered, washed (with isopropanol

for 48 hrs, Finnaly, samples were obtained in air at a temperature

was aged for one week, The resulted and distilled water) and dried at 100°C

by annealing

the obtained

materials for 8 hrs

between Sotr12#°C.

The tin content of the prepared samples was determined aluminum gravimetrically

as Al(CgHK)N&

iodometxicaliy, and that of

The chemical composition

of the samples cx-

amincd as well as their symbols are given in Table I.

Table 1. Symbols of sample and chemical composition. Chemical

Sample

Expected Composition

Molar Ratio A1203

Al Sn-0.01 Sn-0.02 Sn-0.05 Sn-0.075 Sn-0.1 Sn-0.2 SD-O.5 !+I-1 Sn

:

snoz

1 1 1 1 1 1 1 1 1 0

0 0.01 0.02 0.05 0.075 0.1 0.2 OS 1 1

* After calcination

at temperature

The thcxmogravimctric thcrmobalancc force, a-alumina

composition

Al203

:

[ % w/w] 100 98.54 97.13 93.12 90.02 87.12 77.19 57.51 40.36 0

Determined

sno2

Ah&l3

f%w/w] 99.61 98.24 96.92 93.01 89.91 87.00 77.12 57.38 40.16 0

0 1.46 287 6.88 9.98 1288 2281 4249 59.64 100

:

Composition* SnO2 0 1.40 277 6.78 9.83 13.05 2294 4231 59.52 99.78

800°C for 24 hours.

and differential

thermal analyses were carried out in a OD-102

(MOM Budapest). For measurcmcnb

of differential thermal electromotive

was used as a reference mattriaL Specimens of 3tWUIO mg were heated

at a rate of 5 dcg min‘” under atmospheric pressure.

. . kmv dJilhwt ion aualvm The X-ray powder diffraction patterns of diffennt 62 (made in GDR) diffractometer

samples were taken on a TUR M

with Ni-filtered Cuba

radiation and a scanning speed

120

of 2Omin -I between 2Q = lO-&IO. Diffraction analysis of all samples was performed in the same conditions.

RESULTS

AND

DISCUSSION

For the mixtures of the maximum tin conccnttation

(Sn-0.5 and Sn-1 see Table I), at

the final stage of tin (IV) acetate addition into the alcohol solution of Al(OC3H7)3,

a

white precipitate was observed to appear (of a gel-like consistency) which dissolved in the’ process of mixing

Only in the case of the Sn-1 sample were we unable to reverse the

process. The reason for the observed opacity seems to be the pseudoesterification

and

different solubility of the transient products of the reaction in the system carbon tctmchlo ride - isopropanol Al(OC3H7)3

:

+ Sn(CH3CO0)4

~AI(OC~H~)Z(CH~COO) + Sn(CH3COO)3(OCbH7)

...

The results of chemical analysis shown in Table I prove that the actual composition of samples is consistent

with that expected and the insignificant

differences are due to the

adsorbed water. If not stated otherwise in the text, all samples prior to use have been dried at 100°C for 24 hrs. Figures 1-4 present diffraction spectra of studied systems subjected to thermal treatment at 500, 800, 1000 and lZMJ% in air. As expected [32,33], the X-ray reflections

from all

samples heated at SOO% are very weak, broad and blurred by the scattering due to various defects present in these poorly crystalline pattern of an unmixed sample y-Al203 . An introduction intensity

phases (Fig-l). All reflections

in a diffraction

(Al-500) are due to a poorly developed crystalline

of a tin component

leads to a decrease of the number

phase and

of the reflections due to the y-Al203 phase (Sn-0.01 to Sn-0.1) when compared

to the pure ptepamtion

(Al-SOO)

In diffraction patterns of samples

of higher tin concentration,

fraction pattern of the Sn-0.2 sample), new rtfltctions the phase +l@3,

indicating the formation of a new phase.

of the new reflections increases with increasing concentration tions due to yAlfl3

become undetectable

tion patterns with literature

The number

and intensity

of tin and finally the refleo

(Sn-1 sample). A comparison of these diffrac-

data [34] suggest the prcstnce

is formed as one of the transient

(starting from the dif-

appear, besides the ones related to

of the SnsOl&Ilz

products in thermal decomposition

phase

which

of fl -stannic acid.

The intensity and sharpness of reflections in a standard diffraction pattern, Fig.3 (Al-Soo) indicate that an increase in the temperature

of calcination

to soo”C only slightly affects

121

0 - Al203 Ir - Hp_SngOl6 Al-500

0

0

l

Sn-0.01

1

’

*

Sn-0.05

I

.,.!,,

I,,,,,1

Sn-0.075

4

,.,,,,

,,,,

4

/,,,,,,,.,,

.,,.,,,

3

2 b

,

3

.

1

0

,..,.,..,

2 .

4

e

! ,,,,.I,,,,,.,,,,,,,.,,,,,, 3 2 c 0 0 I "I""""'!" "" 3 1 2

/.,'." 0

"Z' 1

Sn-0.1

I ,""' 0

Sn-0.2

.

0

4

.

3

. I,,,,,,,,,

1

11

3

.

2

I Sn-0.02

.

8,

2

. I ,,,,,

0

o

I

ru

rA

4

t

4

3

2

1

4

1

=O. 1 I/I,

X-ray powder diagrams of the samples after calcination

at 5oO°C ,

Sn-1

.O

,,. .,.,,I,. 2

1

0

A

A

A

,,,~,,,,,, / 4 3 d(A)

F&l.

l .-yAI-800

i

!~~~....~./~~.

0

1..

1 Sn-0.05

I

1

i:

;etee .,..!...!.‘.,.........,

2

.

V -

Al203 Sn02

3

4

3

4

Q I

2

Sn-0.2

r., 0

...

97.

I...?'... .;..%......I 7 1 2 3

V

..I

..l 4

r

=O.l

I/I,

d (A1 Fig 2 X-ray powder diagrams of the samples after calcination

at 8W% .

0

2

1 Sn-0.05

0

I,,,,,!

0

4

3

0

ho $A A ,/,I,,,,,,,, !,,,,,,,,,,

1

2

3

1

2

3

4

Sn-0.2

0

4 d(A)

Fig.3. X-ray powder diagrams of the samples after calcination

0

2v

1 v

v

v

V.

yπ

i

1

4

V

Sn-0.05 0

3

‘I

at looo”C .

1

2

4

3 V V

v vv

Sn-0.2 ,.....""I"

0

1

""

v

v

A 3""'

2

v

v

I

=o. 1 I/I,

I

4

3 d (A)

Fig. 4. X-ray powder diagrams of the samples atIer calcination

at 120Cf’C.

123

the ordering of Al ions in the spine1 structure

y-Al203

of

and all the reflection in the

Al-800 diffraction pattern arc still only due to the phase y-A&03. Similarly as in the discussed above (SOO”c), in the region of low tin concentration the diffraction patterns the reflections

due to the

+UJO~

(Sn-0.01 to Sn-0.075 ) in phase arc fewer in number

and less intense. This suggests that tin oxide and alumina exhibit a mutual protective against crystallization.

scrks

action

In the samples including over 10% of admixture (e.g. G-O.1 and

SO

on) the phase SnO2 appears, besides the phase y-Alfl3. The diffraction pattern of the standard sample ~i~3~-1~), lOtM*C, rcvcals only the reflections characteristic

after its calcination

of the fLAlzO3

at

phase In this tcmper-

aturc series, even a small amount of tin admixture (Fig.3, Sn-0,OS) changes the shape of the diffraction pattern when compared to that of the standard sample (~-1~). the rcflcctions

from the a-Al203 phase WC can also detect those related to the l~-Alfl3

phase. This indicates

that an admixture

decrease in the temperature

of ~nsition

is a decrease in the temperature absented

after introducing

centration

Besides

of a foreign component from y-Ah03 to

of transition

in the series of

an admixture of CuO or Fc203

in this temperature

~Q203.

in this case leads to a A similar effect, that y-AlzO3 to ~-Al@3 was

[3S1 An increase in tin con-

series leads to the disappearance

of reflections due to the

phase K and the appearance of reflections characteristic of the phase SnOz (Fig.3, Sn-0.2). Futher increase in tin concentration

(sample Sn-0.5) results in a such a great increase in

intensity of the reflections from SnOn that the reflections from the aluminum oxide phases become undetectable. The changes in diffraction spectra of the samples calcined at 1200% are a little bit different, Fig4. As expected, the sample without tin admixture was ~r-Alfl3 An increase in Al ion ordering implying the transition

(eo~ndum}.

from a densely packed regular lattice

to a densely packed hexagonal lattice in u-Al203 gives intense reflections from this well crystallixcd form (Fig.4, Al-1200), In contradistinction cast even a small amount of tin admixture

to the above described series, in this

is immediately

detectable

in the diffraction

patterns as the phase SnOz, forming besides the n and a phases of alumina. Also for this tcmperaturc

s&es,

an intn>duccd tin admixture Eavoum the prcscncc of tht

phase (pure alumina at this temperature does not change

the

is a-A1~

IC-Al@3

). An increase in tin concentration

character of the diffraction patterns. With regard to the fact that the

above discussed systems can be used as carriers in the metallic phase of a bimetallic catalyst, frequently applied in a rcducting atmosphere

in the reforming process, we also subjected

the samples to thermal trcatmcnt with hydrogen. A few chosen samples were, after drying, subjected to calcination

in the atmosphere of Hz. Figure 5 presents an exemplary diffraction

pattern of a sample of Al : SD = 0.2 obtained (Sn-0.2rcduc) repeated

for 6 hours. Two phases, 9 -Al203

treatment

by reduction

with hydrogen at 1000°C

and metallic tin arc detectable in it A

of the same sample but in the oxidizing atmosphere

(lOOO%, air@)

124

0 -K&o3 0

Sn-0.2 (reduc.)

0

QQQi% I,,

0

,,

.t.f:.-’

1

c*

- Sn (metallic)

v -SnO2 00 0

,,I..

0 .r,

.../..,

2

,‘.“I

3

4

Lb-O.2 (reoxid.)

V I

1 0

Y& Ty?T;y p‘; o "' ..., .',.'..'.."I 1 2

i:

=o.

1 I/I,

,.I) '.""'1

Fig. 5. X-ray powder diagrams of t&c sample Sn-02 cakinatcd in tht atmosphere Hz and after mxidation.

c

unidentified

Vunlabeled

k Al203 -

Sn02

SnO*Al203-800

0

U:!l!t' 1

0

1

..,..:I......,......,,., 2 3i 4

2

3 d

iai

Fig 6. X-ray powder diagrams of the samples obtained by mechanical mixing of SnO or snoz with AI203 .

725

leads to the disappearance

of the reflections

from metallic tin and the appearance

of the

reflections from SnOz instead (Figs, Sn-O.Zreox.). This diffraction pattern is almost identical with that of the sample So-0.2, Fig.3. Since the final effect of thermal decomposition

significantly depends on the character

of the starting material [36,37J, for the sake of comparison

we prepared samples in which

the precursor sources of tin and aluminium were their oxides o/- Al203 and SnOz ) in the molar ratio 1 : 1. Diffraction patterns of these samples after their calcination

at 800,lOOO

and 1200% revealed only the reflections from the phase SnOz or from transient alumina. Only in the diffraction reflections

pattern of a sample annealed

from the phases SnOz and a-Al&

at 13OffC, apart from the

we observed the ones which could not be

ascribed to any simple oxide phase of tin or aluminium, to indicate that in the above described conditions, ium as for example the tin aluminate

phases of

Figd

The obtained results seem

direct bonding between tin and alumin-

suggested by Adkins and Davis [38], are either not

formed at all or are formed in undetectable

amounts.

In order to investigate the phase changes, the samples dried at 100°C (non-calcined) were subjected to thermogravimetric and TG curves obtained

measurements.

Figure 7 presents examples of DTA

for the samples : Al, Sn-0.01, Sn-0.1, Sn-1.0 and Sn.

TG

DTA

1

Sn Q-1

Sn Sn-i Sn-0.01

Al

0

200 400 600 800 ( "c 1

0

200 400 600 800 ( "c 1

Fig.7. DTA and TG cutvcs for samples: Al ; Sn-0.01 ; Sn-0.2 ; Sn-1 and Sn.

126

DTA CUIVCSof the sample without tin admix&n

and the sample with a low conctnrra-

tion of this admixture (&t-0&T) revcalcd three endothermic effcctq in tbt range from 100 to 14Q”C, at about 300°C and between 530 and 580°C.

The first one is r&ted to the liber-

ation of chemically unbound water, the loss of which calculated from the TG curve, is 4 %. A strong endothermic

peak with a maximum at 300°C was observed in the DTA curve.

The weigh loss calculated

from the TG curye at this tcmpcraturc

This peak may correspond to dchyd~~la~on ci&y

to the process of dehyd~~yla~n

was found to be 24%,

of the bydratcd alumina [39J, and mo’pt prc-

of bayerite f4t3, 411.

DTA and TG curves of a

preparation formed as a result of hydrolysis of the tin precursor (tin (IV) acetate) suggest that dehydm~tion

of the product of hy~ly~s

results of elemental and ~ermu~virne~c

is a multi-sue

process,

On the grounds of the

analyses, the process of dehy~~tion

can be

written as

4 SnzOsH2 -

8 Sn03H12 -

2 Sn&9Hs

This scheme is in a very good agreement d~h~d~~tion

-L

snao16H2

-

8sllo2

with that proposed by Laitincn

&& [34] for

of tin (lV) hydroxide to cassiterite.

The cbaractcr of DTA curoes of the binary systems depends on the amount of the introduced tin admixture. The .&apes of DTA cuxycs of p~pa~tions

including low amounts

of tin admixture (Sn-0.01) are almost identical with the DTA GUNCshape of the standard preparation

of AI, Fig.7.

With increasing tin conantmtion

the shapes of DTA curves become mom and more

similar to that of the standard prcparatioa

of Sn and the changes in its character are only

due to partial or complete overlapping of endothermicpeaks of dchydratation of both components. A fcaturc charactcriatic of all DTA CUNZSis tbc abacncc of cxothcrmic peaks where presence would be r-elated to a direct metion system leading to the formation

between the components of this binary

of a new phase. This confirms the conclusion

foliowing

from the analysis of diffraotion patterns as to t&t wexistcnec of oxid+ forms of aiuminium and tin in an unbound

form. On tbc other band, the formation of tin aluminate

[38], in

the surfacG layer is not excluded. Its appearance will be limited by the position of equilibrium of the r&ox process :

802

w

F0

S&O4

es8 Sn +

Sn

The movements atong the scheme depend mainly on the composition of the mixture subject to catalytic proc-eess (its reduction properties), the ccmperaturc of tbe prvwss and the environment of the tin component in the bulk of the catalyst The investigation car&i

out

127

at our laboratory

[43] proved the difference in reducibility

by coprecipitation

and their correspondents

A1203

between the systems obtained

obtained as mechanical mixtures of SnOz and

oxides, which would suggest some degree of interaction

between these two ingredi-

ents.

CONCLUSION

As a result of application of the ~pr~ipi~tion

method, a carrier was obtained in The effect of

which the tin admixture was distributed over the whole volume of the carrier.

tin admixture on phasal changes of alumina depends on both its amount, calcination perature and the atmosphere

in which calcination

Stabilization of lower temperature (lzoo”c)

tem-

proeeeds.

forms of alumina upon high-temperature

is a very favorable effect brought about by the introduced

caldnation

admixture.

Under the studied conditions no direct bonding between tin and alumina was found to form.

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