J. Chem. Tech. Biotechnol. 1979,29,480-486

The Role of Additives in the Impregnation of Platinum and Ruthenium on Alumina Catalysts Ahmed Kadry Aboul-Gheita

Petroleum Department, CoIlege of Engineering, Baghdad University, Baghdad, Iraq

(Paper received 4 September 1978 and accepted 7 February 1979)

Platinum- and ruthenium-alumina catalysts are prepared in the absence and presence of additives during metal impregnation. In the absence of additives, metal penetration and distribution in the support pellets is poor, whereas in the presence of a proper additive, uniform metal distribution can be achieved. The metal dispersion in the support is found to be a function of its distribution. In Pt/Al,O,, the addition of HCl improve the distribution and dispersion of the metal as its concentration increases. However, the use of monoethanolamine as an additive achieves an abrupt improve- ment in the distribution and dispersion of platinum, followed by a region of slight improvement (pH range 2.5-6.5). With a further increase in the concentration additive of the platinum distribution and dispersion deteriorates due to excessive metal penetration and accumulation in the centre of the pellet. In ruthenium preparations, citric acid appears to be the best additive. Oxalic acid also improves ruthenium impregnation but needs to be added in larger concentrations than citric acid. The hydrogenation activity of the catalysts reflected the improvement of the Pt and Ru distributions and dispersions in the support pellets.

1. Introduction

Supported noble metal catalysts are generally distinguished by their outstanding activities and selectivities for a multitude of chemical reactions even at low metal contents.'-3 For preparing these catalysts by impregnation of preformed supports with metal salt precursors, it is almost always intended to achieve deep metal penetration, distribution and dispersion. This depends greatly on the mode of interaction between the metal precursor and the support, particularly, at low metal concentrations.4 If the interaction is fast whereby the precursor is rapidly absorbed from solution, poor metal distribution in the support is produced, whereas slow metal uptake results in uniform metal penetration.6 Chemicals may be added to modify the metal-support interaction in order to achieve uniform impregnation. In platinum impregnations, the additives HF,6 HCl,7 citric acid* and NH4Cle have been used but their roles have not been clarified.

Although ruthenium-supported catalysts possess the highest specific activities among the Group VIII metals in the Fischer-Tropsch synthesislo and ammonia decomposition,ll additives for ruthenium impregnation are claimed only once.la Supported ruthenium catalysts impregnated in the absence of additives have poor metal distribution and d i spe r s i~n .~J~J~ This work is con- cerned with studying the role of certain additives and their behaviour at different concentrations on the penetration and dispersion of platinum and ruthenium in a pelleted y-AlaO3 support.

2. Experimental 2.1. Catalyst support Preformed y-AlaO3 in the form of 3 x 3.5 mm pellets was used as a support for all catalyst pre- parations. This alumina has the following properties: surface area, 213 ma g-1; total pore volume, 0.61 cm3 g-1; grain density, 1.09 g cm-3; structural density, 3.26 g ~ m - ~ .

Temporary address: Petroleum Research Institute, Madinet Nasr, Cairo, Egypt. 0142-0356/79/08o(M480 S02.00 0 1979 Society of Chemical Industry

480

mpregnation of Pt and Ru on alumina catalysts 481

2.2. Metal precursors Chloroplatinic acid (H2Pt Cl6.6H20) and ruthenium tetrachloride (RuC14) were used as metal precursors for preparing the platinum and ruthenium on alumina catalysts, respectively. Both salts were AnalaR grade.

2.3. Impregnations Aqueous solutions of each precursor, adjusted to obtain 0.35 wt- % of the metal in the final catalyst, were used as impregnating solutions either separately or after introducing a definite concentration of the additive selected. In all impregnations, the final volume of the impregnating solution was three times as large as the volume of the desired weight of alumina. The solution/alumina mixture was then left at room temperature, with intermittent shaking, till the solution was completely depleted.

2.4. Determination of the extent of metal distribution The impregnate was separated from the liquid, washed with distilled water, dried at 383 K for 1 h and left to cool to room temperature. Ten pellets were picked out randomly and cut perpindicular to their axes to expose new cross-sectional faces. In the case of platinum impregnates, the new faces were sprayed with an indicator solution composed of 2% KI in ethanol that transforms the faint yellow impregnated zone to deep reddish brown. This technique is simpler and gives more accurate results than the procedure of boiling the impregnated pellets in an aqueous SnClz solution.8 In case of ruthenium preparations, no indicator was used since RuC14 is black. The impregnated zone of pellets was measured by visual inspection under magnification and the percentage of the area penetrated by the metal was calculated as the extent of metal distribution. Each reported value was the mean of ten measurements from different pellets.

Table 1. Hydrogen chemisorption on the prepared catalysts

No CI-/(PtC16)2- additive 20 50 100 250 400 600

Additive HCI; precursor Hz PtCla6HzO HZ uptake

(cmsg-1atSTP) 0.0562 0.0943 0.110 0.116 0.144 0.181 0.190 (pmol g-9 2.51 4.21 4.92 5.19 6 .44 8.06 8.50

Fraction exposed 0.28 0.47 0.55 0.58 0 .72 0.90 0.95 (H/Pt)

No pH value additive 2 .5 4 . 5 6 . 5 7 . 5 9 . 0

Additive, MEA; precursor, HzPtCle6HzO Ha uptake

(cmsg-'at STP) 0.0762 0.171 0.186 0.188 0.128 0.0802 (pmol g-l) 3 . 4 0 7.61 8.32 8.41 5.73 3.58 Fractionexposed 0.38 0 .85 0.93 0 .94 0.64 0.40

(HIP8 ~~

No pH value additive 0.155 0.14 0.135 0.125

Additive, citric cid; precursor, RuCh HZ uptake

(cmSg-latSTP) 0.0388 0.124 0.151 0.186 0.233 (pmol g-9 1.73 5.54 6.75 8.30 10.38 Fractionexposed 0.10 0.32 0.39 0.48 0.60

( H / W

482 A. K. Aboul-Gheit

2.5. Determination of the metal fraction exposed (metal dispersion) The fraction of metal exposed was determined by hydrogen chemisorption using a pulse technique similar to that of Freel.l6 An amount of calcinated catalyst was heated in the furnace of the chemi- sorption apparatus at 773 K for 3 h in a flow of 50 cms min-l of ultra-pure hydrogen gas (reductant). The hydrogen flow was then replaced with oxygen-free nitrogen gas flowing at 30 cm3 min-l for 2 h at 773 K (degassing). The furnace was shut off and the catalyst container cooled down to room temperature. Hydrogen was then pulsed into the nitrogen carrier gas till saturation, i.e. appearance of hydrogen peaks equivalent to complete volumes of pulses. The hydrogen uptake or the volume of hydrogen adsorbed by the catalyst was calculated as hydrogen atoms adsorbed per total metal atoms (fraction exposed or metal dispersion) on basis of 1 : 1 stoichiometry.14 The hydrogen uptakes and the fraction exposed for all catalysts are given in Table 1.

2.6. Finishing and activity tesb of the catalysts After impregnation and washing, the impregnates were dried at 383 K for 2 h in a current of dry air and then reduced in the hydrogenation reactor for 12 h in a hydrogen current at atmospheric pressure. Benzene hydrogenation over the finished catalysts was carried out in a high pressure flow-type laboratory-scale reactor using 10 g of catalyst in all runs. The reactor system and technique is described elsewhere.’ The operating conditions were: 373 K; 30 atm; WHSV, 6 g g-l h-l; Ha/benzene ratio = 6. The hydrogenation products contained only benzene and cyclohexane that were analysed by gas-chromatography using a 20% silicon oil on chromosorb-W column.

2.7. Expression of the additives concentrations In platinum-alumina impregnations, hydrochloric acid and monoethanolamine were used, whereas in ruthenium-alumina impregnations, different additives were unsuccessfully tried, then, finally citric acid proved to be the best. The concentration of HCl was expressed by Cl-/(PtCle)Z-, whereas the Concentration of monoethanolamine and citric acid are expressed as pH value of the impreg- nating solutions. The pH values were measured by a glass calomel electrode attached to a pH meter. Equilibrium readings were taken after 15 min in all measurements.

3. Results and discurnion It is pointed out that the mode of impregnation carried out throughout this study is impregnation in excess solution. This mode may be defined as ‘ion exchange’e which differs from the simple wetting of the support with a volume of the impregnating solution just equal to the pore volume of support, which is frequently defined as ‘dry impregnation’.

Most impregnations that do not use additives result in poorly distributed metal, particularly, when the metal content is low. However, at high metal concentrations, diffusion transport of the precursor to the particle interior may occur if sufficient time is allowed.16 If the proper additive is used, it will competitively adsorb on the support and subsequently the metal precursor adsorbs and interacts, i.e. exchanges ions, in an accessing fashion till the metal reaches the centre of the support pellets.

3.1. Preparation of platinum-alumina catalysts

3.1.1. The use of hydrochloric acid as an additive In the absence of any additive, the impregnation of alumina pellets with chloroplatinic acid results in poorly distributed platinum (20 % distribution), associated with a metal fraction exposed (dis- persion) of 0.28. Progressive introduction of HCl is found to increase both distribution and dispersion of the metal. As a result the hydrogenation activity of the catalysts obtained are also increased (Figure 1). In all of the platinum preparations in the presence of HCl, small deviations of N 4% are obtained between the platinum distribution (determined by simple visual inspection) and the metal fraction exposed (determined by hydrogen chemisorption measurements-Table 1). Catalysts prepared at lower HCl concentrations (i.e. 2.5 x 102 of Cl-/(PtCl6)2- and lower) exhibit

I m p r ~ t i o n of Pt and Ru on alumina c a t d y h

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80 c g c 0

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-

-

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-

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80 c g c 0

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c V

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20 0 -

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in alumina and

20

0

relatively high hydrogenation activities (Figure 1) which may be attributed to the high intrinsic hydrogenating activity of platinum. However, the relatively low rates of activity increase at higher additive concentrations and may be attributed to retardation by diffusion limitation along the catalyst pores because a portion of the metal would be too deep in the pellet interior. The mechanism of interaction between chloroplatinic acid and alumina in the absence of HCl

may proceed as in equation (1) :

2A1+ + (PtCle)*-+Alz-PtCle (1) On addition of HCl, it will competitively interact with aluminium ions of the support as in

equation (2):

Al+ + C1- +A141 (2) and the chlorinated aluminium species then exchanges the chloroplatinate anions of the precursor as in equation (3):

2Al-C1+ (PtC16)a-+AlpPtC16 + 2C1- (3) Summers and Ausens have proved that chlorine ions are released in impregnating alumina with

solutions containing platinum- and palladium-chlorocomplexes, which may be supportive of equation (3).

3.1.2. The use of monoethanolamine as an additive Monoethanolamine (MEA) differs from HCl in its behaviour as an additive in the impregnation of alumina with chloroplatinic acid (Figure 2) in that MEA abruptly increases platinum distribution and dispersion from 20% and 0.28, respectively, to 80% and 0.85 by just changing the pH of solution from 2.0 to 2.5. Progressive MEA introduction, throughout increasing the pH from 2.5 to 6.5, improves the metal distribution and dispersion to 91.5% and 0.94, respectively. Beyond a pH of 6.5, a further increase in additive concentration results in a rapid decline of platinum distri- bution and dispersion, due to excessive migration of the metal into the interior of the pellets and leaving their marginal zone almost devoid of the metal. For instance, at a pH of 9.0, if a cross- section of an impregnated pellet is sprayed with the alcoholic KI indicator, it appears as a central dark brown spot surrounded by a faintly coloured ring.

32

e x w

20 0 -

I I I I I I

20

0

A. K, Aboul-Gheit

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0 : C b = I D .- 1 8

2 0 - ' O - d

c - L -

I 0 ' I I I I I I I

5 - 4 0

Q) N C

m 20

0

pH of impregnoling solutions

Figure 2. Effect of monoethanolamine concentration on the distribution and dispersion of platinum in alumina and on the hydrogenation activity of the prepared catalysts.

On correlating platinum distribution and catalytic activity (Figure 2), it is evident that the activity is always of a higher percentage than the metal distribution up to a pH value of 6.5, above which the declining activity becomes of a lower percentage than that of the declining platinum distribution. Here, the major portion of the metal has deeply penetrated and distributed to the pellet centre. Hydrogenation on such catalysts takes place after benzene diffusion along the catalytic pores to reach the deeper platinum sites. Hence, it can be assumed that diffusion retardation would have lowered the activity relative to metal distribution. Also, platinum dispersion is considerably lowered due to the formation of larger crystallites of platinum as a result of accumulation of the impregnated metal in the centre of the pellets.

The mechanism of impregnation may be visualised by equations (4) and (5). In equation (4), the acidic precursor reacts, preferentially, with the basic amine additive to produce an amine chloro- platinate complex, which then exchanges its amine cations with the aluminium cations of the support as given by equation (5).

(PtCle)a- + 2(Amine)+ +(Amine)nPtCle (4)

( 5 ) 2A1+ + (Amine)aPtCla +Al-PtCle + 2(Amine)+

This reaction seems to take place at a slow rate; i.e. much slower than the reaction represented by equation (3) that takes place between chloroplatinic acid and the Al-C1 species. Summers and Ausen5 have found that slow interaction between the precursor and support produces internally distributed metal, whereas rapid interaction produces shallow distribution at the outside of pellets. Therefore, efficient distribution is achieved by the first small additions of the amine; i.e. from pH 2.0-2.5. The deteriorative effect of large additions of the amine, due to excessive penetration of platinum, may be contributed to by the basicity of the amine and its chromatographic influence as an eluent.

3.2. Preparation of ruthenium-alumina catalysts 3.2.1. Reversibility and irreversibility of impregnation In impregnating platinum, palladium and iridium on al~mina,1.~*7J~ with no additive outer-shell impregnations are obtained, despite the fact that the metal-support interactions are reversible.

Impregnation of Pt and Ru on alumina cataly8'Bb 485

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On the other hand, ruthenium impregnations appear irreversible.'* Thus before calcination, Ru is not redistributed. RuC14 has proved to be more stable than RuCls and other ruthenium salts. Table 2 gives the hydrogenation activities of catalysts containing 0.6 wt-% Ru prepared with RuC14 in the presence of some acidic additives. Oxalic acid can produce complete uniformity of

100

80

c - a-"

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; 20

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Table 2. The percentage benzene hydrogenation on 0 . 6 % Ru- alumina catalysts impregnated with acidic additives

Additive

Non Hydrochloric acid

At pH 3 .0 At pH 0 . 4

At pH 0.1

At pH 0.135 At pH 0.125

Oxalic acid

Citric acid

Reaction temperature (K)

323

-

23 38.5

313

-

50 61.5

413

a

15 25

33

as 96

573

11.5

20 35

64

ruthenium distribution. Citric acid is superior to oxalic acid, in using lower concentrations which are preferable in catalyst preparations. Hydrochloric acid gives little improvements in activity and distribution of ruthenium-alumina preparations.

In the absence of an additive, impregnation gives a poor distribution such that the metal profile resembles a sharp shallow band located at the pellet's outer edge. This outer shell appears impervious, since it is formed immediately on contacting the precursor solution with the alumina pellets then no further absorption of the metal takes place even after contacting for several days.

486 A. K. Aboul-Gheit

3.2.2. The use of citric acid as an additive Figure 3 shows that as the concentration of citric acid increases (PH decreases), ruthenium distri- bution and dispersion are improved. Although complete distribution is achieved at a pH of 0.125, only 50% of the benzene is hydrogenated, indicating that the intrinsic hydrogenating activity of ruthenium is much lower than that of platinum,a and that higher reaction temperatures are required for higher conversions (Table 2).

Ruthenium dispersions have been found (Figure 3) to increase a$ a function of increasing the metal distribution. However, the dispersion values (H/Ru) obtained for ruthenium catalysts (Table 1) appear much lower than dispersions for respective platinum distributions. Fiedorow et al.14 have found that the pulse technique gives lower H/Ru values because hydrogen adsorption on ruthenium is an activated process and needs several hours to attain equilibrium.17 Nevertheless, Fiedorow et ale1* found that this technique results in reproducible hydrogen uptakes within k 5 %. These authors determine an H/Ru of 0.04 for a I wt-% Ru catalyst prepared by impregnating RuCb in alumina. However, GrenobleI3 determined a value of 0.21 H/Ru for a 1 wt-% Ru catalyst prepared by impregnating RuCls.xHa0 on y-Ah03, using a static chemisorption technique. Hence, it can be assumed that the H/Ru values given in Table 1 may be lower than the true dispersions, but can be employed qualitatively since all of the catalysts under investigation contain 0.35 wt- % Ru on the same alumina.

The role of citric acid may be the formation of a complex with RuC14, having a considerably slow rate of adsorption on the alumina than the RuC14 itself, which improves metal distribution and dispersion.

References 1. Aboul-Gheit, A. K.; Cosyns, J. J. appl. Chem. Biorechnol. 1976, 26, 536. 2. Aboul-Gheit, A. K. J. appl. Chem. Biorechnol. 1977, 27, 121. 3. Aboul-Mheit, A. K.; Cosyns, J. Rev. Insr. Mex. Petrol. 1975, 7(3), 61. 4. Taylor, W. F.; Sinfelt, J. H.; Yates, D. J. C. J. Phys. Chem. 1965, 69, 3857. 5. Summers, J. C.; Ausen, S. A. J. Caral. 1978, 52,445. 6. Michalko, E.; US Patents 3 259 454 and 3 259 589. 7. Aboul-Gheit, A. K.; Cosyns, J. J. Appl. Chem. Biotechnol. 1976, 26, 15. 8. Summers, J. C.; Hegedus, L. L. J. Caral. 1978, 51, 185. 9. Brunelle, J. P.; Sugier, A.; Le Page, J. F. J. caral. 1976, 43, 273.

10. Vannice, M. A. J. Card 1975, 38,449. 11. Friedlander, A. G.; Courty, P. R.; Montarnal, R. E. J. Caral. 1977, 48, 312. 12. Aboul-Gheit, A. K. Aromatics Hydrogenation on Supported Bimetallic Combinations Inst. Franc. Perrole,

Rep. Re$ 1973, No. 20,874. 13. Grenoble, D. C. J Catal. 1978,51,203. 14. Fiedorow, R. M. J.; Chahar, B. S.; Wanke, S. E. J. Caral. 1978, 51, 193. 15. Freel, J. J. caral. 1972, 25, 139. 16. Maatman, R. W.; Prater, C. D. Ind. Eng. Chem. 1957, 49,253. 17. Taylor, K. C. J. Card. 1975, 38,299.