Hydrogen adsorption and desorption on alumina supported platinum-multicomponent catalysts with a gas chromatographic pulse technique

Applied Catalysis. 72 11991 ) 153-163

153

Elsevier Science Publishers B.V., Amsterdam

Hydrogen adsorption and desorption on alumina supported platinum-multicomponent catalysts with a gas chromatographic pulse technique A. G e r v a s i n i * a n d C. Flego Emricerche S.p.A., Via Maritano 26, 20097 S. Donato Milanese-Ml (Italy), tel. (+39-2) 5207403, fax. (+39-2) 5204422

Received 23 October 1990. revised manuscript received 25 JanuaD' 1991

Abstract A stud)' of hydrogen chemisorption of multicomponent catalysts containing platinum as an active metal was performed by means of the pulse-flow chromatographic technique. Several series of adsorpt ion experiments, performed at various constant intervals of time between hydrogen pulses, were carried out on each catalyst. During the experiments hydrogen desorption phenomena wereobserved. Hydrogen desorption was kinetically studied. The rate coefficients of the desorption reaction relevant to each catalyst were evaluated. Desorption has been discussed taking into account spillover effects. The occurrence of desorption phenomena gave rise to apparent adsorption parameters I i.e. platinum dispersion, platinum surface area ), however proper parameters have been obtained by graphical extrapolation. Keyu,ords: platinum, pulse-flow method, adsorption I hydrogen ), desorption I'hydrogen~,dispersion.

INTRODUCTION G a s c h e m i s o r p t i o n o v e r s u p p o r t e d m e t a l c a t a l y s t s is a c o m m o n p h y s i c o c h e m i c a l m e t h o d used to e v a l u a t e t h e degree o f m e t a l d i s p e r s i o n and, by u s i n g a s u i t a b l e model, t h e a v e r a g e c r y s t a l l i t e d i m e n s i o n [ 1 ]. T h e s e p a r a m e t e r s are o f t e n r e l a t e d to t h e c a t a l y t i c a c t i v i t y of m e t a l c a t a l y s t s . As a c o n s e q u e n c e m a n y t e c h n i q u e s h a v e b e e n d e v e l o p e d in i n d u s t r i a l a n d u n i v e r s i t y l a b o r a t o r i e s [ 2 - 6 ] to p r o p e r l y d e t e r m i n e t h e m e t a l d i s p e r s i o n of s u p p o r t e d m e t a l c a t a l y s t s . Static v o l u m e t r i c t e c h n i q u e s are the m o s t a c c u r a t e a n d precise m e t h o d s which c a n be u s e d for t h e e v a l u a t i o n of t h e a b o v e p a r a m e t e r s . D y n a m i c t e c h n i q u e s (i.e. gas c h r o m a t o g r a p h i c m e t h o d s : pulse-flow m e t h o d , f r o n t a l a n a l y s i s m e t h o d ) , e v e n t h o u g h t h e y are less a c c u r a t e , offer p r a c t i c a l a d v a n t a g e s u n d e r e x p e r i m e n t a l c o n d i t i o n s . T h e p u l s e - f l o w gas c h r o m a t o g r a p h i c t e c h n i q u e perm i t s t h e a m o u n t of gas a d s o r b e d by a c a t a l y s t to be rapidly d e t e r m i n e d by the i n t r o d u c t i o n of successive pulses of a c t i v e gas diluted in a n inert c a r r i e r gas [ 7 - 9 ]. T w o c o n d i t i o n s are usually s t a t e d to be n e c e s s a r y in o r d e r to e n s u r e the

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© 1991 Elsevier Science Publishers B.V.

154

reliability of the method: (i) a fast rate of gas adsorption, and (ii) a very slow rate of gas desorption, negligible during the experiment. In this work we report on a hydrogen chemisorption stud)' which was performed by using the pulse-flow method on four metal multicomponent supported platinum catalysts, in order to evaluate the dispersion of the platinum active phase. Of the catalysts used one was platinum supported over alumina, one was a reforming catalyst, and two were catalysts used for the dehydrogenation of long chain alkanes. The occurrence of appreciable hydrogen desorption processes could severely compromise the interpretation of the experimental results of hydrogen adsorption. However, a graphical procedure has been applied to obtain adsorption parameters even in the presence of desorption processes. A kinetic interpretation of the hydrogen desorption reaction has also been performed. EXPERIMENTAL

Materials and catalysts Gases of special purity supplied from Siad were used: nitrogen, 99.9995% and hydrogen, 99.9999%. The gases flowed through an oxygen remover (Supelpure-O Trap Supelco) which reduced the oxygen to less than 5 ppm. Four catalysts were made. The first contained only platinum (Pt-1), the second (a reforming catalyst) contained platinum and tin (Pt-2), the others, namely Pt-3 and Pt-4 (catalysts for t he dehydrogenat ion of long chain alkanes [10,11]), contained platinum, tin, indium, and platinum, tin, indium, thallium, respectively. The same support (y-AleO:3) was employed to disperse the metal phase for all the samples. The incipient wetness impregnation technique was used to make the catalysts. Granular )'-A120:3 (Akzo 3992E, 20-40 mesh, surface area 196 m-'/g) was impregnated with solutions prepared by'using nitric acid (65%) and a H2PtCI6 solution (platinum, 2.64% w/w). After 24 h of imbibition the samples were dried and calcined at 500~C for 4 h. The starting acidic solution also contained SnCl4 in the preparation of Pt-2, SnC14, In (NO.3 I,.,, and LiOH in the preparation of Pt-3, and SnCl4, In(NO,3):3, TINO:~, and Mg( NO3)2 in the preparation of Pt-4. The composition of the catalysts is reported in Table 1. Three other samples, namely: Sn/y-AI20:3 (0.21% w/w ), In/y-Al20:3 (0.40% w/w), and Tl/)'-Al~O:3 (0.39% w/w), were made as has been described above in order to perform hydrogen sorption power tests on components other than platinum.

155 TABLE 1 Composit ion of catalysts Catalyst

Component ( g of metal / 100 g of support I Pt

Sn

Tl

In

Li

Pt- 1 Pt-2 Pt-3 Pt-4

0.40 0.55 0.40 0.40

0.48 0.45 0.45

0.40 0.33

0.49

0.08

Precursor

H~PtCI~

SnCI 4

TIN03

In(NO~)~

LiOH

Mg

0.80 Mg(NO~2

Apparatus and procedure A conventional gas chromatograph (Dani 3600) was modified in order to perform the chemisorption experiments. An external circuit of gases permitted the in-situ pretreatment of the samples as well as the introduction of pulses of the active mixture by means of a six port valve (Carlo Erba). The linearity of the TC-detector response was checked over a wide range of hydrogen partial pressures covering the values utilized in the experiments. The gas chromatograph was coupled to a Hewlett-Packard 3380A integrator to calculate the area of the chromatographic peaks (A). The samples (0.5-1.5 g} were placed in a glass U-tube, mounted in the original position of the gas chromatographic column. The gases were allowed to flow through the catalyst bed. Before the chemisorption experiments took place the samples were heated from room temperature to 500¢C (10~C/min) in a nitrogen stream (15 ml/min ). They were afterwards reduced in a hydrogen stream (35 m l / m i n ) at 500°C for 2 h, then kept under a stream of nitrogen (15 m l / m i n ) for 1 h at the same temperature to remove hydrogen, and eventually cooled to room temperature in an inert atmosphere. All the hydrogen chemisorption experiments were performed at 20 + 1 ¢C, holding the tube in a thermostated water bath. Successive pulses of a mixture of 0.27 ml of hydrogen and nitrogen, at a known hydrogen partial pressure, were sent to the catalyst by a constant stream of carrier gas (15 ml/min nitrogen). A series of chemisorption experiments, with pulses having different hydrogen partial pressures in the range 6-17 kPa, were carried out in order to check the attainment of the surface saturation. These experiments gave the same results ( + 2% ) for each catalyst, in terms of the amount of hydrogen adsorbed per gram of platinum calculated, as has been reported in the next section. In each series of kinetic experiments, the hydrogen partial pressure was kept constant, namely 17 kPa for Pt-1, 15 kPa for Pt-2 and 10 kPa for Pt-3 and Pt-

156

4. The time interval (At) between successive pulses was kept constant in each experiment and ranged from i to 20 min.

Calculation of the amount of hydrogen chemisorption From the experimental chromatographic results it was possible to evaluate the amount of hydrogen adsorbed per gram of platinum (VadJgpt). The hydrogen of the first pulses was sorbed until the surface was saturated with hydrogen, further pulses produced peaks of the same area on the chromatogram. The total amount of adsorbed hydrogen expressed in arbitrary units (Aad~) could be evaluated by calculating the difference between the mean value of the chromatographic peak areas at the saturation (,4~at) multiplied by the number of pulses sent to the sample before the attainment of the saturation (np), and the sum of the values of the chromatographic peak areas before the attainment of the saturation (A.o~): Aads = (/~at" np ) - A o o ,

From this value the total amount of hydrogen adsorbed (~d~/ml) was obtained by means of the following equation:

V," AaJ LL , where Vp (ml) is the amount of hydrogen of a pulse. RESULTS AND DISCUSSION

In the multicomponent platinum catalysts investigated (Pt-1, Pt-2, Pt-3, and Pt-4 ), platinum was the only active element able to chemisorb hydrogen, or in any case to promote the chemisorption, at 20~C. In fact, neither the YA1203 support nor the Sn/)'-Al203, In/y-Al203 and Tl/)'-AI.20:~ samples displayed any capability of hydrogen sorption under the conditions employed. V61ter et al. have already reported [ 12 ] that tin and lead over alumina do not adsorb hydrogen. During our experiments on hydrogen adsorption, desorption phenomena were present. These were witnessed by performing different series of chemisorption runs in which the pulses were sent to the sample at various, constant intervals of time (Jr). The area of the chromatographic peaks at saturation (,4.,at) decreased as a function of At, down to undetectable values for high At. In fact, each pulse had to restore the fraction of hydrogen desorbed during the At interval and the Ai~t values were smaller than that which should be obtained in the absence of desorption. Fig. 1 shows, as an example, the chromatograms relevant to Pt-3 from chemisorption runs carried out at different At, ranging from 1 to 5 rain. The decrease in the saturation peak area with At, cannot be ascribed to the

157

At.

1 2 3 4 5 6 7 B 9

'

4

L

.~

2

3

4

5

2

6

7

4

~2

3

El

~

4

6

6

1 min

9

7

B

6

.t=3...n

9

7

B

At--

45

rnin

Fig. 1. Chromatograms of hydrogen adsorption on Pt-3 at different time intervals between pulses (At).

presence of oxygen in the carrier gas that could react with hydrogen. The presence of oxygen, even when at the maximal permitted concentration by the carrier gas purity, could only remove a few percent of adsorbed hydrogen between two pulses, at the highest At utilized. Moreover, the amount of hydrogen removed should have been the same for each catalyst.

Kinetics of hydrogen desorption To determine the kinetic parameters relevant to the hydrogen desorption reaction, the dependence of the area of the chromatographic peak at the saturation on At has been taken into account (Table 2). The peak area can be related to the degree of saturation of the catalyst surface by eqn. 1. In the equation, A~ is the area of the peak at saturation for a At value; Ao and A~ are the analogous areas when At is 0 and ~ , respectively; Ot is the degree of catalyst surface coverage after a At time from a pulse at saturation; 0o and 0x are the analogous values when At is 0 and oo, respectively. Ao -At Ao - A.~.

Oo-0, Oo - O~

(i)

On the basis of the above definitions, 0o is the maximum saturation degree and is equal to one, and •:¢ can be considered equal to zero as well as A~, because they correspond to a complete removal of desorbable hydrogen. Thus eqn. 2 can be written as :

158

(2)

At O, A,, - O,,

If desorption followed a second order kinetics which is in agreement with the usual mechanism of adsorption/desorption of hydrogen on platinum: P t ( s ) + 1/2 H~ ( g ) ~ P t - H (ads)

(3)

the kinetic equations written in terms of 0 should be: - ~dO = k •0 2

(4)

1 1 -----=k.dt O, Oo

(5)

Eqn. 6 can be derived from eqn. 5 on the basis ofeqn. 2 and by considering that 0o= i. 1 A~

1 k _It Ao - A o

(6)

In contrast with eqn. 6, poor linear trends were observed by plotting 1/A, vs. At, as evidenced both by the poor linear correlation coefficients (r= 0.970 for TABLE 2 Dependence of the area of the chromatographic peak at saturation ~,.(~, ~ on intervals between hydrogen pulses (dt At

.4-~t (arbitraD' units)

~min) Pt- 1

Pt-2

Pt-3

Pt-4

1

17 900

15 600

8200

9000

2

16 300

14 800

8100

5800

2.5

-

3

15 000

4

13 700

-

6600

-

-

3900

5600

4100

3600

2400

3200

13 500

4.5

-

5

-

13 600

6

-

12 6O0

7 8

10 700

10

-

12

7750

15

-

20

3900

-

> 500

-

-

23O0

-

-

550

-

-

11 3 0 0 80OO -

-

-

-

-

-

-

-

-

-

159 10

9.5

9

q- ~ . .

e"

--

8.5

-¢

75

h

i

.i

2

4

6

i

i

L

J

i

i

i

8

10

12

14

16

18

20

Lt

22

(min)

Fig. 2. Linearization diagram tbr the evaluation of the kinetic coefficients: I - - I I - - ) ~ - - + - - ) Pt-2; ( " ( 2 " " ) Pt-3; I - - * - " ) Pt-4.

Pt-l:

TABLE3

Kinetic parameters of the hydrogen desorpt ion reaction Catalyst

kn~, (min - t )

r

t t .2 qmin)

p.

Pt- 1 Pt-2 Pt -:3 Pt-4

0.0785 +_0.0018 0.0451 _+0.0036 0.348 + 0.075 0.269 + 0.024

0.999 0.984 0.919 0.985

8.83 15.4 1.99 2.58

1 0.57 4.4:3 3.43

a p ---- k d e ~ ' P t - t / k d e ~ . ~ .

I"

Pt-1; 0.967 for Pt-2; 0.882 for Pt-3; and 0.979 for Pt-4 ) and by the non-random distribution of the experimental points along the straight line. On the other hand, if desorption were a first order reaction, the kinetic equations would be:

_dO_ k. dt-

0

(7)

0~ At In ~o----In ~,: ----k-,It

(8)

In A t

(9)

= - k. At +

constant

A very good linear correlation was obtained by plotting In At vs. A t (Fig. 2) in the case of Pt-1, and in the case of the other catalysts the linear correlation coefficients were satisfactory (Table 3 ). However, the correlation coefficients

160

were always higher when the first-order kinetic equation was employed. Moreover, in this case the experimental deviations were more randomly distributed. Desorption was clearly of first order only for Pt- 1, while for the other catalysts an intermediate order, closer to the first one, seemed to be more reliable. A first-order hydrogen desorption, not in agreement with mechanism (3), suggests that hydrogen desorbed from other sites than platinum. Hydrogen can migrate to other sites of the surface after chemisorption on platinum because of a spillover effect. The occurrence of a hydrogen spillover from the platinum to the alumina support is well known [13,14], and was also observed from platinum to tin in the Pt-Sn/AI20:3 catalysts [15]. In Table 3 the kinetic first-order coefficients of hydrogen desorption for the four catalysts and the ratios between these coefficients with respect to the one for Pt-1 are reported. These ratios reflect the influence of the other metals in the catalyst composition on the desorption of hydrogen. The presence of tin in Pt-2 gave rise to a rate of hydrogen desorption half that observed in the presence of only platinum, while the addition of further metals (indium in Pt-3, and indium and thallium in Pt-4) increased the rate 8 or 6 times with respect to the presence of both platinum and tin.

Evaluation of adsorption parameters By applying the calculation procedure described in the experimental part to the chromatographic results, different Vad~/gp~values have been obtained for TABLE 4 Chemisorbed a m o u n t of hydrogen and platinum dispersion Jt

Pt- 1

Pt-2

Pt-3

Pt-4

Imin) V~dJge,

Pt*/Pt

V.d~,,"get

ml/g)~

Pt*,,"Pt

t m l / g I~

I. ' ~ / g v t

Pt*/Pt

V.d.~g et ( m l / g )~

P t * ,"Pt

( m l / g Ia

1

40.5

0.70

12.8

0.22

25.7

0.45

15.7

0.26

'2

43.25

0.75

15.5

0.27

34.1

0.59

16.6

0.29

2.5

.

36.0

0.63

-

-

:3

41.0

4

43.0

4.5 5

. -

6

-

7

.

8

4:3.3

.

.

.

0.71 I).74 .

.

-

63.1

1.10

23.4

0.41

15.0

0.26

88.2

1.53

24.3

0.42

0.21

76.1 -

1.32 -

;33.7 19.75

0.58 0.34

-

-

21.4

0.37

21.4

0.37

.

-

11.9

.

-

13.0 .

0.23

.

0.75

.

.

.

.

10

-

-

12.4

12

43.1

0.75

.

.

15 20

44.;35

-0.77

1;3.5 .

.

.

0. '2'2

. .

. 0.24

aVolume is calculated under normal conditions.

. .

. .

. . . .

.

. . . .

.

. .

161 50m

_--

A 30-

>~ 20 10 0 0

1'2

1'6

2'0

A t {mln) 100 80

K >®

6O

40 20 ¸

L~t (rain)

Fig.3. Dependence of the amount of hydrogen adsorbed 4VadJge~) on the time interval between pulses (At). A: l l ) Pt-1; ( + ) Pt-2. B: ( . ) Pt-3: { + ) Pt-4. TABLE5 Chemisorbed amount of hydrogen extrapolated to At = 0 and plat inum dispersion parameters Catalyst

t'adJgpt ( ml/g )a

Pt*/Pt

Area I m-'/get )

Pt-1 Pt-2 Pt-3 Pt-4

41.5 11.3 24.5 15.0

0.72 0.235 0.43 0.26

199 65 118 72

"Volume is calculated under normal conditions. d i f f e r e n t At. T a b l e 4 r e p o r t s VadJgpt v a l u e s for all t h e s a m p l e s ; e a c h value was a v e r a g e d on at least t h r e e c h e m i s o r p t i o n r u n s (in m o s t cases d e v i a t i o n s were < 10% ). In T a b l e 4 the p l a t i n u m d i s p e r s i o n v a l u e s ( P t * / P t ) are also rep o r t e d . T h e y are calculated, as usual, s t a r t i n g f r o m t h e o b s e r v e d VadJge~ a n d t a k i n g into a c c o u n t t h e h y d r o g e n a d s o r b e d on t h e p l a t i n u m in a dissociative w a y [ 16 ]. O n e c a n notice t h e great r a n g e in the v a l u e s of t h e l a t t e r p a r a m e t e r o b t a i n e d for e a c h c a t a l y s t , the g r e a t e s t r a n g e s were f o u n d in the case of P t - 3 a n d P t - 4 on w h i c h t h e h y d r o g e n d e s o r p t i o n was m o r e considerable.

162 It should be impossible to obtain sound values of metal-dispersion because of the ihct that desorption leads to an overestimation of the amount of hydrogen adsorbed. Nevertheless the "true value" of the amount of hydrogen adsorbed could be obtained graphically by extrapolating the curves Vad~/ge~ VS. At to At=O, where desorption is absent. Actually, our Vads/gpt vS. At curves (Fig. 3) do not always show a regular increasing trend, but they do display more or less marked maxima. The strange shape of the Pt-2, Pt-3, and Pt-4 curves cannot be due to experimental artifacts, such as variations in pressure and flow-rate, since the experiments were fairly reproducible and such artifacts would also have affected the Pt-1 curve. In the case of Pt-2, the slope of the curve is slight, with a small maximum at At<5 min. By contrast, Pt-3 and Pt-4 displayed, as has already been noticed, a stronger dependence of l~dJgpt on Jt, with a marked maximum at a J t of approximately 4 min. When J t is sufficiently high, the curves relevant to the Pt-2 and Pt-4 take on a trend similar to the one observed for Pt-1 throughout the whole range of the J t investigated. However, the extrapolation to J t = 0 of the curves, both befbre and after the maximum, leads to the same values of V,d~/gp, (Fig. 3). The chemisorbed amount of hydrogen obtained by extrapolation, together with the calculated platinum dispersion and platinum surface area values, are collected in Table 5. CONCLUDING REMARKS It is difficult to achieve a sound interpretation for the occurrence of maxima in the Pt-2, Pt-3, and Pt-4 curves of Fig. 3. They could be due to the presence of different simultaneous desorption reactions with different rates. Hydrogen which is first adsorbed on platinum could desorb after spilling-over onto other sites, as is suggested by the first (or the nearly first ) order of desorption reaction. Thus, in the case of Pt-1 hydrogen should desorb only from alumina, while in the other cases it could also desorb from metal components other than platinum. The occurrence of desorption via spillover does not invalidate the graphical procedure used to obtain the adsorption parameters. In fact, the extrapolation leads to an ideal condition in which desorption is absent. The addition of metal components (tin, indium, and thallium ) to platinum catalysts led to a decrease in the observed platinum dispersion (Table 5 ). The loosening of the chemisorptive P t - H bond, due to alloy formation and electronic interaction with metals surrounding the platinum (ligand effect ) [ 12,1720], can explain a low apparent platinum dispersion.

16;3

REFERENCES 1 J.R. Anderson and K.C. Pratt, Introduction to Characterization and Testing of Catal~'ts, Academic Press. London, 1985. Chapter 1, and references therein. 2 H.L. Gruher, J. Phys. Chem., 66 ( 1962 ) 48. 3 H.L. Gruber. Anal. Chem., 34 (1962) 1828. 4 A. Hausen and H.L. Gruber, J. Catal., 20 ( 1971 ) 97. 5 F. Figueras Roca. L. de Mourgues and Y. Trambouze, J. Gas Chromatogr.. 6 11968 ) 161. 6 N.E. Buyanova. N.B. Ibragimova and A.P. Karnaukhov. Kinet. Catal.. 10 (1969) 322. 7 J. Freel. J. Catal.. 25 (1972) 1:39. 8 J. Freel, J. Catal.. 25 ( 1972 ) 149. 9 J. Prasad, K.R. Murthy and P.G. Menon. J. Cata]., 52 (1978) 515. l0 G. Bellussi, V. Fattore, G. Fornasari, G. Faraci and S. Palmer)', Ita]. Pat., 23 149A/87. 11 I. Tamotsu and H. Chi Wen, US Pat., 4 486 547 (1984). 12 J. ViSiter. G. Lietz, M. Uhlemann and N. Hermann. J. Catal., 68 ( 19811 42. 13 P.A. Sermon and G.C. Bond. Catal. Rev.. 8 ( 1973 ) 211. 14 W.C. Conner, Jr,,G.M. P a j o n g a n d S . J . Teichner. Adv. Catal.,34 (1986) 1. 15 A.C. Muller, P.A. Engelhard and J.E. Weisang. J. Catal., 56 ( 19791 65. 16 R.J. Matyi. L.H. Schwartz and J.B. Butt, Catal. Rev., 29 ( 1987 ) 41. 17 H. Verbeek and W.M.H. Sachtler, J. Catal.. 42 t 1976) 257. 18 H. Lieske and J. Viilter, J. Catal., 90 ( 1984 ) 96. 19 G.T. Baronetti, S.R. de Miguel, O.A. Scelza and A.A. Castro, Appl. Catal., 24 ( 19861 109. 20 H. Berndt, H. Mehner, J. ViSiter and W.Z. Meisel, Z. Anorg. Allg. Chem., 429 11977 ) 47.