Preparation Of Titania-Supported Catalysts By Ion Exchange, Impregnation And Homogeneous Precipitation

311

G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation orCata/ysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PREPARATION OF TITANIA-SUPPORI'ED CATALYSTS BY ION EXCHANGE, IMPREGNATION AND

HCMJGENEDUS PRECIPITATION R. BURCH and A. R. FLAMBARD

Olenistry Department, The university, Whiteknights, Reading RG6 2AD, England

ABSTRACl'

Titania and silica-supported

cata~ysts

have been prepared by the techniques

of wet :inpregnation and ion exchange, and by precipitation either by the addition of alkali or hanogeneously by the hydrolysis of urea.

After drying,

the uncalcined materials have been investigated by X-ray diffraction, surface area rreasurernant, and tarperature-prograrrrned reduction.

Titania-supported

catalysts prepared by wet :inpregnation or ion exchange shewed little evidence of interaction with the support.

For hanogeneously precipitated catalysts, there

was sare indication of the formation of titanate-type species.

INl'roDOCTION It

is nOlrl recognised that oxide supports used to disperse metal particles are

rarely, if ever, inert.

In addition to the sanetimes extensive interaction

betv.een the deposited phases and the support which can occur during the early stages of catalyst preparation - to fonn specific canpounds such as silicates or aluminates (1,2) - it has also been found recently that a support may influence the catalytic properties of the rretal after reduction.

The most,

striking effects are found when reducible transition rretal oxides, such as titania, are used as supports. (3)

Such strong metal-support interactions (SMSI)

w:rre first noted for platinum metal/titania catalysts whose chenisorption and catalytic properties were modified after reduction at high tarperatures. (4,5) In earlier w:>rk (6,7,8) we have shown that Ni/titania catalysts can exhibit

special properties under conditions where SMSI are absent, and it became apparent that there was a need for a thorough investigation of the influence of sample history on catalytic properties. The precise way in which the active metal or its precursors are first

brought into contact with the support can affect the structure, reducibility, dispersion, and even the rrorphology of the catalyst.

The three methods of

preparation rrost carroonly encountered are wet impregnation, deposition/

312 precipitation, and ion exchange.

Impregnation may not be silrple as other effects,

such as adsorption may occur simultaneously. (9)

Frequently, poor dispersions

and broad particle size distributions are obtained. (10,11)

Often, there appears

to be little interaction between support and deposited phase, although this will depend on metal loading. (12)

The deposition/precipitation of metal precursors

onto a support may also be accanpanied by adsorption. (13, 14)

In the usual methcd

where the precipitant is added directly to a suspension of the support, local supersaturation and inharogeneity can result in poor dispersions. (14)

However,

the hcnoqenecus generation of the precipitant can yield high dispersions and

unifonn particle size distributions, often as a direct result of the fonnation of compounds with the support. (13-16)

Adsorption, or ion exchange, usually leads

to catalysts with high metal surface areas. (10-12)

Depending on the conditions

and system under investigation this mayor may not be a consequence of direct canpound fonnation between deposited phase and support.

Catalysts prepared by

this method are frequently rnuch more difficult to reduce than ilrpregnated catalysts. (10-12) In this paper

'lie

describe the preparation, using these three techniques, of

titania-supported Ni catalysts, and canpare these with silica-supported catalysts prepared as reference materials. EXPERIMENTAL

Materials and reagents The titania (Degussa P25) consisted of 80% anatase and 20% rutile, and had a surface area of 50 m2g-1. The silica (Davison grade 57) had a surface area of 2g-1, 290 m and was ground up before use.

The 35-60 mesh size fraction was used.

The source of nickel for all the preparations was nickel nitrate (Fisons).

Preparation of catalysts Wet impregnation.

Suspensions of the supports were contacted with solutions

of the appropriate Ni concentration for 0.25 h at 298 K before excess water was removed by rotary evaporation at 343 K.

Catalyst precursors 'J'Jere dried in air at

393 K for 16 h, and stored under vacuum until required. Deposition/precipitation. (i) addition of NaCE at 298 K. 3 of 0.0144 M Ni nitrate solution 'llere added to 1. 52 g of support, and the

200 an

pH adjusted to 2.4.

Snall doses of 0.01 M NaoH solution were then added by means

of a graduated burette. was allowed to

After the addition of each dose of NaaI, the solution

cx:me to equilibrium, and the

(ii) hydrolysis of urea at 353 K.

pH noted.

The reaction vessel was charged with 1.52 g

of support in 200 an3 of 0.0144 M nickel nitrate solution, and heated to 353 K.

The pH was adjusted to 2.4, 4.6 g of urea powder was added, and the change in pH with time recorded autanatically.

313 Ion exchange.

Solutions of the required nickel concentration were prepared

and arnronium hydroxide added to raise the pH to 11.

The adsorption of the hex-

anmino Ni(II) CCllplex (12,17) was performed by adding this solution to a suspension of the support, in water and leaving for a period of 500 h. materials 393 K.

~

After this time, the

filtered off, washed with dilute arnronium hydroxide, and dried at

The dried materials were stored under vacuum,

Techniques used in catalyst characterisation Samples were analysed for their Ni contents by atanic adsorption spectrophotanetry, after dissolution with HF. measured using a calm microbalance.

Nitrogen adsorption isotherms were

X-ray powder patterns were measured with a

Phillips horizontal diffractaneter using nickel filtered Cu radiation. Tar[lerature-prograrmed reduction profiles were measured in the usual way.

The

heating rate was 7 K minute-I, the gas mixture contained 5 or 25% H in argon, 2 3 -1 and the gas flow rate was 10 an minute • RESULTS AND DISCUSSION Our objective in this work has been to canpare the preparation of titania and

silica-supported Ni catalysts in order to gain insight into the nature of the interactions between titania and metals.

Three methods of catalyst preparation

have been used - impregnation, deposition/precipitation, and ion exchange - in order of increasing degree of interaction between the support and the metal precursor. Wet impregnation Little adsorption of Ni onto silica or titania surfaces is to be expected fran acidic solutions, and this has been confirmed under our conditions by a spectrophotanetric investigation of the adsorption equilibrium.

Therefore, the majority

of the Ni taken up by the support during wet impregnation consists initially of a deposit of nickel nitrate.

HClINeVer, when the materials are dried to rarove

excess solvent, dehydration of the nickel nitrate crystallites may lead to an interaction with the support as the Ni tries to recover its co-ordination sphere. Figure 1 shows the nitrogen adsorption isothenns for the supports and sane of the catalysts, and the pore size distribution (PSD) for the silica samples. sunmarises the relevant surface area and porosity data.

Table 1

2 catalyst has the sarre BEl' surface area as the original silica, but a snaller internal surface area. pores.

The Ni/Si0

The PSD shows that the catalyst has a larger fraction of narrow

This suggests that during drying the nickel nitrate tends to wet the

support and spread out.

In the case of titania, the support is non-porous, but

the introduction of nickel nitrate results in the fonnation of mesopores.

is accanpanied by a substantial reduction in the surface area.

This

These effects

are thought to occur because the deposited nickel nitrate CCllqJOUIlds act as an 'adhesive' to hold the primary titania particles together, as shown in Figure 2.

314

b.

Q

...

11)

....

Q)

O'l

-

0

~500

....0..0

"tJ

....

Q)

.Q

Q)

~250

~ ~

{l 0

~C\j

05

1·0

P/Po

1-0

Fig. 1. Nitrogen adsorption isothenns for silica (a) and titania (c) catalysts, and pore size distributions for silica samples (b).

TABLE 1 surface areas of the supports and the Ni catalysts 2 SBmI -1 m (g support)

sal 2 I -1 m (g support)

Silica

286

254

Sample

Ni9.8Si

285

230

Titania

52

30

Nil.OTi

48

29

Ni4.7Ti

46

26

Ni9.8Ti

33

22

a interna l surface area.

Fig.

2. Fonnation of a secondary titania structure after liIpregnation.

315 Thennogravimetric analysis of these catalysts shows (Table 2) that after drying, the average number of water molecules retained by the deposited Ni nitrate is in

the range 1-3, Le. the hexahydrate has becane dehydrated. TABLE 2 Experimental and calculated weight loss of Ni catalysts during reduction Sample

calculated wt , lossa/rng

Measured

wt. loss/rng

3

2

1

0

2.39

2.37

2.13

1.89

1.65

Ni4.8Si

10.12

11.42

10.26

9.11

7.96

Ni9.8Si

17.06

21.20

19.06

16.91

14.77

Nil.OTi

2.46

2.49

2.24

1.99

1.74

Ni4.7Ti

6.50

11.24

10.10

8.97

7.83

Ni9.8Ti

17.18

20.45

18.38

16.32

14.25

NiO.94Si

~umbers

refer to number of water molecules retained by the nickel.

X-ray diffraction of the Ni9.8Si catalyst indicated the presence of NiCOH)2.Ni(N03)2.2H20 (making up about 70% of the deposited material, Ni(OO)2 (about 5%), and a third phase which resenbled NiO.

For the Ni9.8Ti catalyst,

X-ray diffraction indicated that Ni (00) 2.Ni (00

2.2Hp made up about 40% of the 3) total nickel, Ni (00) 2 about 30%, and Ni (N03) 2' 4H the remainder. There was no 20 evidence of direct canpound formation between the titania and the nickel. Even allowing for the fact that the metal loading for a given surface area of

support differs for the two sets of catalysts, we conclude that for impregnated catalysts there is little interaction between nickel and titania. Deposition/precipitation ceus and co-workers (13,14) have dem:mstrated that useful information on the degree of interaction between a metal ion and a support can be obtained by monitoring the pH of the solution as hydroxide ions are gradually introduced.

We

have used their method to ccmpare the properties of silica and titania. (a) addition of NaCH Figure 3 shows the changes in pH as NaOH is added to distilled water, a nickel nitrate solution, and to suspensions of the supports in distilled water or nickel nitrate solution.

I f the addition of hydroxide ions did not lead to

the interaction of Ni ions with the support, then the measured pH curves should be equivalent to the sum of the curves for the nitrate solution and the support alone.

Figure 3 shows that the experimental curves differ markedly fran the

calculated curves (shown as broken lines in Figure 3), especially in the case of

316

g

Q

H2O

9

Ni 2 + 7

I

P

Si 02+ Ni

0-

2

+

5

3

Fig.

50

75

100 50 5 OH X 10 Imoles

75

-

100

3. 'l'itration curves for the silica and titania systems at 298 K.

titania.

It is inferred that for both supporta there is an interaction which

ccmrences at pH 5.2 for silica and pH 4.2 for titania.

By analogy with the work

of Geus and Hennans (14) on Ni/Si0

catalysts we conclude that the precipitation 2 of Ni hydroxide on the silica surface began at podnt; P on Figure 3 (a) . For the titania, there is a similar change in slope at a pH of 5.4.

These experiments

show that the adsorption of Ni ions onto a titania surface occurs at a pH of 4.0,

and that this is most probably followed by a smooth change over to give a

precipitate attached to the surface of the support.

XRD measurerrents on the

products of these preparations failed to'identify the structure of the precipitate, no lines could be detected.

Chemical analysis showed that the Ni content of roth

the silica and titani.a-suppor'ted materials was 0.9%, indicating that for the

addition of a given arrount of hydroxide the amount of nickel deposd.ted is independent of the support.. (b) hydrolysis of urea Figure 4 shows the change in pH with time as hydroxyl ions are generated by the hydrolysis of urea at 353 K.

The pH curve for water corresponds to the hydrolysis

of urea to give amronium carbonate. curve is obtained.

When silica is present, a broadly similar

With titania, however, the pH rises above the value for

317

6

9

Si0

HD

2

Ni + SiD2 +Ni 2+

2

5 4 I

c.

0·5 Fig.

1·0 Time Ih

1·5

4. Urea hydrolysis curves for the silica and titania systems at 353 K.

distilled water.

'!'his is thought to be due to the adsorption of carbonate ions

on the titania, which is known to contain basic hydroxyl groups. When the experiments are perfonned in the presence of nickel nitrate solution, quite differen\ curves are obtained, especially in the case of titania.

For both

systems the pH curves exhibit transient max.ilI1a (not illustrated in the titania case) characteristic of a nucleation barrier.

Such maxima were absent for Ni

nitrate in the absence of a support, and are stringly indicative of a supporcNi ion interaction.

XRD again failed to show evidence for any crystalline phases,

even th)ugh in this case the Ni content was about 3%. We conclude that this methcrl of preparation gives w=ll dispersed Ni both for silica (13-16) and for titania supports. Ion exchange Under the oonditions used in the ion exchange experiments it was found that the silica had adsorbed 95% and the titania 40% of the available nickel.

If

318 allowance is made for the different surface areas of the supports, these values correspond to 25% and 60% coverage. XRD

~

However, although for the silica catalyst

only very broad lines oorresponding to Ni silicate (12), in the case

of titania well defined lines oorresponding to Ni hydroxide, Ni Oxide, and Ni nitrate were observed.

It is apparent that this method of preparation leads to

extensive interaction between Ni ions and silica, presumably because under alkaline conditions the silica has a tendency to dissolve.

In contrast, the

interaction with thil titania appears to be limited to creation of a surface on which Ni oxide and Ni hydroxide can deposit.

There is no evidence for the

formation of Ni titanates. Temperature-prograrrrred reduction The influence of the method of preparation on the reducibility of sane of these catalysts has been investigated by TPR.

Fiqure 5 shows TPR profiles for

uncalcined sarrples of impregnated and ion exchanged catalysts.

These results

daronstrate rather well the differences between the Ni corpounda formed in the case of silica and titania.

In the case of the impregnated catalysts, the sharp

peaks at about 590 K have been identified as being due to the reduction of Ni oxide, the remainder of the profiles being indicative of the reduction of supported Ni oxide.

The ion exchanged Ni/Si0

catalyst is very difficult to

2 reduce, and is canparable in reduction characteristics to Ni silicate.

Significantly, however, for the ion exchanged Ni/ri0

catalyst reduction is much

2

more facile, confinning the absence of strong interactions with the support. CDNCLUSIOOS These experiments have demonstrated that a titania surface is much less reactive towards metal ions than a silica surface, which is often itself considered to be rooderately inert.

In particular, it has been shown that

titania-supported nickel catalysts, whether prepared by impregnation, deposition/precipitation, or ion exchange have little tendency for reactive interaction between the netal Lens and the support.

Even under oonditions

where there is a strong adsorption of Ni ions by the titania surface there is little direct evidence for the formation of titanates.

319

s

c 0

a. .~

~

..

E ::J Ul

C 0

u

c

~

Q)

Cl 0

-g, L.

::c

g

473

673

T/K

873

Fig. 5. 'I'E!rperature-progranrned reduction profiles for uncal.ctned , :impregnated and ion exchanged catalysts. (a), 10.7%Ni/Si0 i.rrpregnated, (b), 9.2%Ni/Si0 2 ion exchanged, 2 (c), 13.8%Ni/I'i0 inpregnated, (d), 4.0%Ni/I'i02 ion exchanged. 2

ACKNCMLEDGEMENI

R.B. thanks Amax Inc., and A.R.F. thanks the States of the Island of Jersey for financial support.

we

are grateful to Degussa and W.R. Grace for supplying

samples of the catalyst supports.

320

1 2 3 4 5 6 7 8 9

G.C.A. Schuit and L.L. van Reijen, Advances in Catalysis, 10(1958)242. K. r.brikawa, T. Shirasaki and M. Okeda, Advances in Catalysis, 20(1969)97. S.J. Tauster, S.C. FUng, R.T.K. Baker and J.A. Horsley, Science, 211(1981)1121. S.J. Tauster, S.C. FUng and R.L. Garten, J. Amer. Chem. Soc., 100(1978)170. P. ~iaudeau, B. Parmier and S.J. Teichner, C.R. Acad. Sci., C289(1979)395. R. Burch and A.R. Flambard, J. Chern. Soc. Chern. Communications, (1981)123. R. Burch and A.R. Flambard, React. Kinet. Catal. Letts., 17(1981)23. R. Burch and A.R. Flambard, suhnitted to J. Catal. (1982). J.R. Anderson, 'Structure of Metallic Catalysts', Academic Press, London, (1975)17110 V.A. DZis'ko: Kinet Catal., 21(1980)207. 11 M.S. Borisova, B.N. Kuznetsov, V.A. Dzis'ko, V.I. Kulikov and S.P. Noskova, Kinet. Catal., 16(1975)888. 12 M. Houalla, F. Dellanney, I. Matsuura and B. DelIron, J. Chern. Soc. Faraday I, 76 (1980) 2128. 13 J.A. van Dillen, J.W. Geus, L.A.M. He:rrnans and J. van der Meijden, Proc. 6th Int. Congr. Catal., London, 1976. (Eds. G.C. Bond, P.B. wells and F.C. Tompkins), 2(1976)677. 14 L.A.M. Hermans and J.W. Geus, Proc. 2nd Int. Sympositml, IJJuvain-1a-Neuve, 1978. (Eds. B. Delmon, P. Grange, P. Jacobs and G. Poncelet), (1979)113. 15 J.T. Richardson and R.J. Dubus, J. Catal., 54(1978)207. 16 J.T. Richardson, R.J. Dubls, J.G. Crump, P. Desai, U. Osterwalder and T.S. Cale, Proc. 2nd Int. Sympositml, IJJuvain-la-Neuve, 1978. (Eds. B. DelIron, P. Grange, P. Jacobs and G. Poncelet), (1979)131. 17 M. Primet, J.A. Dalmon and G.A. Martin, J. Catal., 46(1977)25. 18 M. Primet, P. Pichet and M.V. Mathieu, J. Phys. Chern., 75(1971)1221. >

321 DISCUSSION J. KIWI: 1. To which temperature did you heat the Ni(N03)2 on Ti02 and how long, since you report that no NiTi03 has been formed in your systems? 2. In Fig. 1 you report nitrogen adsorption isotherms for silica that at the left hand side evidence the outside BET area and towards the right show evidence for inner pores in this material. How did you assess the reported curve for the inner pores you report? Did you try titration methods to determine the contribution of the internal surface area since H20 has a diameter of 2.8 ~ as compared with N2 having 14 A2 in particle surface? A.R. FLAMBARD 1. The catalyst precursors discussed in this paper were all uncalcined, that is to say they were only subjected to a mild drying stage (16 h at 393 K). However, I have carried out separate experiments (Flambard A.R., Ph. D. Thesis, University of Reading 1982) into the effects of calcination on titania - supported nickel catalyst precursors and have not observed the presence of any detectable nickel titanate - type phases after calcination at temperatures up to 873 K. 2. The N2 BET isotherm for the silica shown in Fig. 1 (a) shows many of the characteristics of a Type IV isotherm, which is not uncommon for xerogels. The pore-size distribution curves of Fig. 1(b) were determined by use of the Kelvin equation, assuming cylindrical pores and a liquid-solid contact angle of zero. We did not investigated the possibility of using titration methods in order to determine microporosity. R. SIGG What is the difference between you BET surface area and the internal surface area? (Table 1). A.R. FLAMBARD: The difference between the BET surface areas and the internal surface areas listed in Table 1 arises from the use of two different models and hence equations (the BET and the Kelvin equations) for their determination. Although the surface area of a porous material is largely composed of contribution from the surfaces of the pores, the BET model will estimate the external surface as well. The Kelvin model will only estimate the surface due to the walls of the pores. J.W. JENKINS It would seem as though you are comparing the different surface reactivities of titania prepared by flame hydrolysis and silica prepared by precipitation. Have you had a chance to look at a flame hydrolyzed silica or a precipitated titania gel ? A.R. FLAMBARD: It is true that the titania and silica samples used as catalyst supports in this investigation were prepared by different methods. We have not investigated a flame hydrolyzed silica and only briefly looked at some precipitated titania gels because of the following reasons. Firstly, the precipitated titania gels that were available to us were all of a low surface area « 10 m2g- 1) and prepared from titanium (IV) sulphate. As the results presented in this paper represent only a fraction of those assimilated during an intensive investigation which was primarily geared to look at the active catalysts (i.e. after reduction), then these precipitated titanias were, for obvious reasons, considered unsuitable. Scondly, the function of the silica supported materials were to act as references. Therefore, in order to be in line with most of the literature, a precipitated silica was employed. I would consider a material prepared by flam hydrolysis to be perhaps more reactive than a material prepared by precipitation, so that it may be speculated that the use of a silica prepared by the former method would show up the differences between silica and titania even more effectively. G.C. BOND: With reference to the TPR plot of the ion-exchanged Ni/Ti02 catalyst (Fig. 5(d» ,it appears as if there may be an uptake of H2 in excess of that required to reduce Ni 2 + to Ni o• Is this your opinion also? If so, what degree of reduction of the Ti0 2 does it correspond to ?

322 A.R. FLAMBARD: We have carried out quite a detailed investigation into the reducibility of titania - supported nickel catalyst precursors, both before and after calcination. I am of the opinion that these measurements indicate that there is srnne surface reduction of the support. However, this does not imply the formation of'such phase as Ti4C7 (the Magneli phases) and indeed our results indicate that the support surface is only reduced as far as TiOl.98' This may have important consequences as far as the SMSI effect is concerned (Burch, R. and Flambard, A.R., submitted to J. catal. 1982). For the uncalcined catalyst whose TPR profile is shown in Fig. 5(d), this support reduction is difficult to observe directly because of the presence of peaks due to the reduction of nitrate decomposition products. The reduction of this nitrate is believed to account largely for the ~cess hydrogen consumption in this case.