Journal of AUoys and Compounds, 181 (1992) 527-534 527 JAL 8109

Effect of heavy rare earths on the catalytic and magnetic properties of platinum re-forming catalysts

Lu Weiqi and Li Fengyi Department of Chemistry, Jiangxi University, Nanchang (China)

Li J ianhui Department of Chemistry, University of Toronto, Toronto, Ont. M5S 1A1 (Canada)

Abstract

The magnetic properties of Pt-RE (RE=Gd, Tb, Dy, Ho, Tm) catalysts supported on T- A12Oa were studied using a Faraday magnetic balance. The conversion of cyclohexane, n-hexane and n-heptane on these catalysts has been investigated using a pulsed catalytic flow microreactor. The results show that heavy rare earths can improve the activity, stability and magnetic properties of platinum re-forming catalysts. The relationship between the adsorption and magnetic susceptibility of Pt-Gd/T-A1203 catalysts was also investigated under different conditions.

1. Introduct ion

Pt/AleOa is an impor tan t ca ta lys t for h y d r o c a r b o n convers ion . Prev ious s tudies showed that the activity and magnet ic suscept ibi l i ty of su p p o r t ed p la t inum catalysts were changed remarkab ly by adding rare ear th e lements [1 -3 ] . However , the effect of rare ear ths on the catalyt ic and magnet ic p rope r t i e s of the re - forming cata lysts has no t b een invest igated yet . In o rde r to e x t e nd the appl ica t ion o f ra re ear ths to catalyt ic re - forming react ions , p l a t i n u m - r a r e ear th catalysts were p r e p a r e d to s tudy the effect o f heavy ra re ea r ths on the activity, stabili ty and magne t i sm of p la t inum catalysts . It is our h o p e to find out why rare ear ths can improve the catalyt ic p roper t i e s and wha t the re la t ionship is be tween the activity and magne t ic p roper t i e s of the cata lysts in o rde r to p rov ide a scientific basis fo r se lec t ing g o o d p l a t i n u m - r a r e ear th catalysts.

2. E x p e r i m e n t a l detai l s

2.1. P r e p a r a t i o n o f ca ta lys t s P l a t i n u m - h e a v y ra re ear th cata lysts were p r e p a r e d by impregnat ion ,

ca lc inat ion and reduct ion . The con ten t s o f p la t inum and chlor ine were 0.5 and 1.0 wt.% respec t ive ly and the ra re ear th con ten t was in the range 0 . 0 5 - 1 . 5 wt.0/0.

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2.2. Evaluat ion o f catalysts The reaction conditions in the pulsed microcatalytic reactor were as

follows: temperature for cyclohexane, 583 K; n-hexane or n-heptane, 773 K. The catalytic reaction in the MR-GC-80 catalytic flow microreactor gas chromatography-chromatographic data microprocessor combination set was carried out under two sets of conditions: (1) temperature 773 K, pressure 10 kg cm -2 and hydrogen-to-hydrocarbon volume ratio 1200:1; (2) tem- perature 803 K, pressure 3 kg cm -~" and hydrogen-to-hydrocarbon volume ratio 800:1. The aromatization yield was the criterion of activity.

2.3. Measurement o f magnet ic properties o f catalysts The magnetic susceptibility of the catalysts was measured by a Faraday

magnetic balance. The magnetic susceptibility of a sample in the reduced or adsorbed form was measured in a specially made sample tube [4] which was set in a special Dewar flask. After the disturbance of ferromagnetic impurities had been corrected by the Honda-Owen formula [5], the apparent magnetic susceptibility and susceptibility of the metal components were obtained using the Weidemann law [6]. When the susceptibility of the catalysts at different temperatures had been measured, the Weiss constant of the sample was calculated by the Curie-Weiss law

C XM---- T - O

where X~ is the molar susceptibility, C is the Curie constant of the substance, T is the absolute temperature and ~ is the Weiss constant.

3. R e s u l t s an d d i s c u s s i o n

3.1. Effect o f heavy rare earths on activi ty o f catalysts The effect of gadolinium, terbium, dysprosium, holmium and thulium

on the catalytic activity for cyclohexane conversion can be seen in Table 1. The catalytic activity varied with different rare earth (RE) elements and different contents of the same element. Among a series of Pt-RE containing REs in different amounts, the catalytic activity was highest when the Pt-RE catalysts contained the following RE contents: 0.1% Gd, 0.6% Tb, 0.7% Dy, 0.1% Ho, 0.2% Tin. Pt-RE catalysts containing optimum RE content possessed the following dehydrogenation activity order:

P t -Gd 0.1%> Pt-Dy 0.7%> Pt-Ho 0 . 1 % > P t - T m 0 .2%>Pt -Tb 0.6%

These results suggest that the heavy rare earth affects the centres of metallic activity and that there is an interaction between platinum and RE in the Pt-RE catalysts.

3.2. Effect o f heavy rare earths on stabil i ty o f catalysts 3.2.1. Stabil i ty o f p la t inum, P t -Tm and Pt-Gd catalysts Table 2 lists the stability of platinum, P t -Tm and Pt -Gd catalysts under

conditions (2) of Section 2.2. The activity of the platinum catalyst decreased

TABLE 1

Catalytic activities for cyclohexane conversion

529

Catalyst Activity

Pt--Gd Pt-Tb Pt-Dy Pt-Ho Pt-Tm

Pt-RE(0%) 50 50 50 50 50 Pt-RE(0.05%) 75 48 48 70 60 Pt-RE(0.1%) 79 52 50 73 66 Pt-RE(0.2%) 58 59 70 70 Pt-RE(0.3%) 74 64 62 64 68 Pt-RE(0.5%) 71 68 58 60 Pt-RE(0.6%) 71 68 55 Pt-RE(0.7°/0) 70 63 74 Pt-RE(1.0%) 69 60 70

TABLE 2

Stability of catalysts under conditions (2)

Catalyst Activity decrease rate (%)

Reaction time (h) 5 7 9 11 13

Pt 8.0 19.0 28.9 36.1 41.0 Pt-Tm 2.5 7.1 13.6 15.7 15.9 Pt-Gd 3.5 9.2 15.9 18.1 18.7

quickly with increas ing reac t ion time, while tha t of the P t - T m and P t - G d cata lys ts dec reased slowly. After about 11 h the activity of the P t - T m and P t - G d catalysts was a lmos t stable but that of the p la t inum catalyst con t inued to decrease . Both thul ium and gadol in ium improved the catalytic stability.

3.2.2. Stability of regenerated Pt-Gd catalysts Table 3 shows tha t the activities of both cyc lohexane and n -hexane on

r egene ra t ed cata lys ts decreased . However, when gadol in ium was added to the p la t inum catalyst , the activity decrease rate became lower. The h igher the rare ear th content , the lower was the activity decrease rate.

Our p rev ious r e sea rch results showed that CS2 can be used to po i son the ca ta lys t until the convers ion yield of the dehydrogena t ion o f cyc lohexane is equal to zero. It was found that the amoun t of CS2 required to po i son P t - R E cata lys ts was m o r e than that for the p la t inum catalyst, which means that the n u m b e r of active cen t res in P t - R E catalysts is grea ter than that in the p la t inum cata lys t because their number s of active a tom combina t ions are a lmos t the same. The interact ion be tween RE and p la t inum acts to increase the n u m b e r s o f active cent res and to prevent those act ive cent res f rom being poisoned . Therefore heavy rare ear ths can improve no t only the

530

TABLE 3

Activities of regenera ted pla t inum and P t - G d catalysts for conversion of cyclohexane and n-hexane

Hydrocarbon Activity decrease rate (%)

Gadolinium conten t (wt.%) 0 0.05 0.1 0.3 0.6 1.5

Cyclohexane 21.3 10.3 9.1 5.8 3.6 1.4 n-Hexane 15.3 12.0 8.9 5.1 1.8 1.1

TABLE 4

Susceptibilities of P t -RE catalysts in different prepara t ion periods

Pt Pt--Gd P t -Dy P t -Ho P t - T m

X× 106 (uncalcined) --0.41 0.25 0.77 0.60 0.28 X× 106 (calcined) -- 0.36 0.32 1.01 0.95 0.35 X× 106 (reduced) 0.04 1.30 1.89 1.27 0.70

stability of resistance to carbon formation but also the stability of resistance to sulphide poisoning on catalysts.

3.3. Susceptibility of catalysts in preparation periods Table 4 shows that the magnetic susceptibility of Pt-RE catalysts follows

the sequence of different preparation periods:

Xtmcalcined ~ Xcalcined ~ )(reduced

The reason for the susceptibility increase is that the catalyst structure has been changed after calcination at 773 K [7-9] and the resulting metal oxide has subsequently been reduced under hydrogen atmosphere at 773 K. Platinum oxide, for example, was converted to dispersed platinum metal, which resulted in a remarkable change in susceptibilities. The reduced platinum and the interaction between platinum and RE could form the active centre of de- hydrogenation.

3.4. Weiss constants of Pt-Gd catalysts Weiss constants, which reflect the interaction among neighbouring metal

particles of a catalyst, are listed in Table 5. The results in Table 5 show that the rare earth element gadolinium has affected the Weiss constants of the catalysts strongly. This is direct evidence of the interaction between platinum and RE in Pt-RE catalysts. The Weiss constants are negative when the gadolinium content in Pt-RE catalysts is equal to 0.05%-0.10%. According to Heisenberg theory, there is an exchange of antiferromagnetism among paramagnetic particles of the catalyst. On the other hand, the Weiss constants

TABLE 5

Weiss constants of platinum and Pt-Gd catalysts

531

Gadolinium content (wt.%) 0 0.05 0.1 0.5 1.0 1.5 100

W (K) 45 -25 - 5 0 10 12 15 20

0. l

. i× 0.3 I :).2

0.-;

-3 c~ £,

i i I i i I

0 0 .4 0 .8 1.2

Gd G - - ~ Fig. 1. Correlation between relative magnetic susceptibility (Ax/x) and gadolinium content.

o f Pt/A120a and Gd/A120a catalysts are 45 and 20 K respect ively , which means tha t there is no exchange of an t i f e r romagne t i sm am o n g p la t inum a toms or gadol in ium ions themselves . The Weiss cons tan t b e c o m e s posi t ive with increas ing gadol in ium con ten t (see Table 5). These resul ts show that an ex c ha nge of an t i f e r romagne t i sm o c c u r r e d be tween p la t inum a toms and gad- ol inium ions. In addit ion, the resul ts of t ransmiss ion e lec t ron m ic ro sco p y (TEM) show tha t the ensembles on the sur face of P t - R E catalysts are m u ch f iner than those on the p la t inum cata lys t and that rare ear th e lements make the p la t inum ensembles on the sur face m u c h eas ier to disperse, which is p e r h a p s a resul t of the in te rac t ion be tween p la t inum and RE.

3.5. Adsorpt ion on Pt -Gd catalysts P t - G d catalysts possess a ve ry s t rong t e n d e n c y for adsorp t ion because

the gadol in ium e lec t rons o f half-full 4f orbitals have a s t rong in terac t ion with the ou te r orbi tal e lec t rons of pla t inum. It was found that the magnet ic suscept ibi l i ty o f P t - G d cata lys ts d e c r e a s e d af te r the catalysts s ta r ted to adsorb (see Fig. 1).

W he n the gadol in ium con t en t is equal to 0.1°/% the dec rease in catalytic suscept ibi l i ty is largest , which indica tes tha t the re are m o re unpa i red e lec t rons or d holes in the P t - G d catalyst . The catalyt ic activity for the catalyst is h ighes t because the gadol in ium is in the di luted magne t i sm state and there is an exchange of an t i f e r romagne t i sm be tween p la t inum a toms and gadol inium ions.

532

3.6. Activity and magnetic properties of Pt-RE catalysts Figures 2 and 3 show tha t the a roma t i za t i on yield of n - h e p t a n e and

c y c l o h e x a n e var ied with ra re ea r t h content . W h e n the d y s p r o s i u m o r thu l ium con ten t was 0 .7% or 0 .2% respec t ive ly , the ac t iv i ty of the ca ta lys t s was highest . The ac t iv i ty i nc reased w h e n the m a g n e t i c suscep t ib i l i ty inc reased . Fo r 0.7% Dy and 0 .2% T m bo th XPt-Dy and XPt-Tr. a re h ighes t . P t - G d , P t - T b and P t - H o ca ta lys t s s h o w the s a m e re la t ionship . This c o r r e s p o n d i n g rela-

0 ~pt-DY

I * YO

A Yn-CTHI~

,o I

6 0 t ~ -I 100

V / 50 ~ / * 80

4Oh, a , , , "iO 0 0.3 0.6 0.9 i .2 [.5

D~%--*-

Fig. 2. Correlation among susceptibility of Pt-Dy catalyst (Xe~-~), aromatization yield (Y) and dysprosium content.

f ~

~g

v

9° L 80

70

6O

5O

~0 0

• YO

• Yn-C~HI~

I I I I I I

0.2 0.4 0,6

"r .% "

250

20a

!50 X

~L00

~3

Fig. 3. Correlation among susceptibility of Pt-Tm catalyst (Xr~-T,,), aromatization yield (Y) and thulium content.

TABLE 6

L constants, susceptibilities and aromatization yields of catalysts

533

Pt-Gd Pt-Tb Pt-Dy Pt-Ho P t -Tm

X× l0 s (RE2Oa) 157.4 232 265.7 264.1 153.6 X× 106 (catalyst) 251.8 179.6 370 245.0 131 L 1.60 0.77 1.39 0.93 0.85 Y 79 68 74 73 7O Ranking 1 5 2 3 4

tionship shows that the catalytic activity is related to the electronic factor and hole size of the catalysts. In the field we investigated, the formation of holes favours the aromatic reaction of n-heptane or cyclohexane, while the heavy rare earth favours the formation of holes.

In order to compare the activities of a series of Pt -RE catalysts which contain the same amount of heavy rare earth, we define a new term here, the L constant. The L constant is the ratio of the susceptibility in P t -RE catalysts to the susceptibility in RE203. The L constant can be the difference between the state of rare earth ions in P t -RE catalysts and in REs. We found that the higher the L constant, the stronger is the interaction between platinum atoms and RE ions and the higher is the activity of the catalysts (see Table 6).

The L constants and the aromatization yields of cyclohexane, Y, for Pt -RE catalysts are found in the following orders:

Lpt-Gd >Lpt--Dy > Lpt-Ho > Lpt--Wm > Lpt_Wb

YPt-Gd > YPt-Dy > YPt-Ho > YPt-Wm > YPt--Wb

The last row corresponds with the activity order given in Section 3.1. If L constants are applied to deduce the re-forming activity of n-hexane

or n-heptane, care should be taken, because although the aromatization of cyclohexane needs only metal active centres, the aromatization of n-hexane or n-heptane needs both metal active centres and acidic centres on which n-hexane or n-heptane can isomerize. When the effect of rare earth elements on acidic centres is close to enough, L constants can be used conveniently.

4. C o n c l u s i o n s

(1) Heavy rare earths can improve the activity and stability of platinum re-forming catalysts and affect the magnetic behaviour of such catalysts.

(2) The magnetic susceptibilities of these catalysts follow the order of different preparat ion periods:

,)(uncalcined ( Xcalcined ~ )(reduced

534

(3 ) T h e r e is a c o r r e s p o n d i n g r e l a t i o n s h i p b e t w e e n t h e m a g n e t i c su s - c e p t i b i l i t y o f P t - R E c a t a l y s t s a n d t h e a r o m a t i z a t i o n y i e l d o f c y c l o h e x a n e o r n - h e p t a n e o n t h e c a t a l y s t s .

Acknowledgments

W e w i s h t o t h a n k P r o f e s s o r s L iu L i a n g t a n a n d L u o L e i t a o f o r t h e i r a s s i s t a n c e a n d t h e N a t u r a l S c i e n c e F o u n d a t i o n o f C h i n a f o r i t s f i n a n c i a l h e l p .

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(1986) 89. 3 Li Fengyi, Liu Liangtan, Luo Leitao and Jiang Sufang, in Xu Guangxian and Xiao Jimei

(eds.), New Frontiers in Rare Earth Science and Application~ Vol. 1, Science Press, Beijing, 1985, p. 660.

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Academic, New York, 1968, p. 409. 6 P. W. Selwood, Magnetochemistry, Interscience, New York, 1956, p. 149. 7 Li Jianhui, Li Fengyi, Li Xiancai and Luo Leitao, J. Less-Common Met., 148 (1989) 405. 8 Li Xiancai, Lao Leitao and Li Fengyi, Chin. J. Rare Earths, 8 (1990) 127. 9 A. K. Potapervich, G. M. Senkov, V. N. Gaeuskii, V. I. Yakerson and A. A. Slinkin, Kinet.

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