study of chemisorption and hydrogenation of ethylene on platinum by temperature-programmed...

JOURN.11, OF C.kT.kLYSIS 50, 1-7 (1977)

Study of Chemisorption and Hydrogenation of Ethylene on Platinum by Temperature-Programmed Desorption

Susu.\lu TSUCHIYA ANI) RIASA~II ~AKA~IUHA’

Department of Industrial Chemistry, Faculty of Engineering, I‘amaguchi C’niversity, Tokiwadai, ITbe, Japan 75.5

Received November 23, 1976

The temperature-programmed desorption (TPD) chromatograms of ethylene from platinum black comprised three peaks, A, B, and C, with peak maxima at about 370, 500, and higher than 720 K. Peak A was identified to be ethane only, or to be a mixture of et,hylene and ethane. Peaks B and C were methane formed from chemisorbed ethylene by decomposit,ion as the t,emperature was raised during the subsequent TPD. The reactivity of various types of chemisorbed hydrogen previously det,ect,ed by TPD was also investigated for the hydrogenation of ethylene. Two t,ypes of chemisorbed hydrogen, presumed to be present on the surface in the form of hydrogen atoms chemisorbed on top of platinum atoms and in the bridge form of molecular hydrogen, were found to react with ethylene.

INTRODUCTION

The existence of four different species of hydrogen chemisorbed on platinum surface has previously been reported (I), being detected as four separate peaks in the temperature-programmed desorption (TPD). These peaks, with peak maxima at about 170, 250, 360, and 570 K, are referred to as CX, p, y, and 6, respectively. They were tentatively assumed to correspond to two types of molecularly chemisorbed hydrogen: one in a bridge form @) and the other in linear form (a), and two types of hydrogen atoms: one adsorbed right on top of metal atoms (y) and the other in the inter- stices between the metal atoms (6). Their participation in the Hz-D2 exchange reaction was also examined (2), and only a! and y, were found to readily undergo surface exchange at 173 K.

The present work has been carried out to

1 Present address: Central Glass Co., Ube, Japan 75.5.

obtain more information on the differences in the reactivities of the forms of chemisorbed hydrogen, and on the chemisorption and hydrogenation of ethylene.

EXPERIMENTAL METHODS

Apparatus

The apparatus used for the present study was essentially the same as used previously (2), except for minor modification. It consisted basically of two parts : an adsorption and reaction system, in which adsorption and reaction were measured in a conven- tional manner, and a TPD system, in which adsorbed gas was later desorbed into a carrier gas stream (Nz or He) by programmed heating of the catalyst. The concentration of desorbed gas was measured and recorded by a thermal conductivity detector. When the ambient gas consisted of hydrocarbons, a gas chromatograph with a thermal conductivity detector, instead of a Pirani gauge, was employed to estimate

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2 TSUCHIYA AND NAKAMURA

the pressure. A mercury manometer and a McLeod gauge were occasionally used to calibrate the detector, but were then iso- lated from the reaction system by means of a stopcock to avoid contamination of the catalyst by mercury vapor. An oil diffusion pump behind the rotary oil pump was used for evacuation of the system.

Materials

The catalyst used in the present study was a platinum black obtained from Wako Pure Chemical Industries Ltd., and was pressed into tablets about 2-mm thick and 4 mm in diameter. Atomic absorption spectrophotometric analysis showed that the largest metallic impurity was Fe (130 ppm), followed by Rh (80 ppm), and Pd (20 ppm). The catalyst was first reduced in a hydrogen stream at atmospheric pressure for 30 h at 470 K, and then for 5 h at 680 K. Its surface area, estimated by the BET method with carbon dioxide adsorption at 150 K, was 0.306 m2/g, and the amount used was 2.53 g.

Cylinder hydrogen (99.975%) supplied from Osaka Hydrogen Co. was stored in a reservoir as a reactant after passing over platinieed silica at 720 K and then through a liquid nitrogen trap. The same cylinder hydrogen could also be fed through to the reactor for reduction of the catalyst. Mathe- son’s deuterium (99.5%) passed through a liquid nitrogen trap was also used as a reactant. Ethylene obtained from Taka- chiho Chemical Industry Ltd. was purified by repeated distillation with liquid nitrogen traps, and its gas chromatographic analysis indicated a purity of 99.9%. High purity helium (99.995%), and high purity nitrogen (99.999%), obtained from Seitetsu Kagaku Co. Ltd., were used as carrier gases for TPD after being passed through a molecular sieve trap cooled in liquid nitrogen.

Procedure

Before each run, the catalyst was treated with hydrogen for 4 h at 680 K and atmo-

spheric pressure, and then heated up to 820 K in the stream of carrier gas; all hydrogen chemisorbed on the catalyst was removed by this desorption. The catalyst was then cooled down to a selected temperature in the carrier gas, and the reactor was evacuated for 30 min, unless otherwise stated, using the oil diffusion pump to remove the carrier gas. The catalyst ac- tivity fell at first, but became reasonably stable after about fifty runs. All data reported here were obtained with the stabi- lized catalyst.

In a typical experiment with ethylene, a known amount of ethylene was adsorbed for 30 min at the experimental temperature. The reactor was then cooled down to 246 K with ethylene present in the gas phase, and kept for 15 min. Following this, the carrier gas, helium, was diverted from the bypass into the reactor to remove the ethylene in the gas phase, and after 5 min the catalyst was heated in a programmed manner. Before heating, hydrogen was sometimes admitted to react the adsorbed ethylene after helium was removed by 5 min of evacuation. The reaction time in these cases was 10 min.

TPD chromatograms were obtained on the recorder chart and showed peaks due to the gas desorbed from the catalyst at different temperature. A more detailed desorption of the TPD method has been given elsewhere (3). The desorption products were analyzed by gas chromatography, a liquid nitrogen trap being used after the detector, if necessary. The heating was stopped at about 820 K, and the catalyst was kept in the helium stream for 20 min at the same temperature before cooling. Hydrogen was then admitted into the reactor as described before. Reproducible results were usually obtained by this procedure. When the results were not reproducible, however, the catalyst was treated with 20 to 30 kPa of oxygen for 1 h at about 770 K to oxidize any organic contamination, and was again reduced by hydrogen as

HYDROGEN AND ETHYLENE ON PLATINUM 3

mentioned above. This treatment improved the reproducibility considerably.

In the hydrogenation of ethylene, various types of chemisorbed hydrogen were first prepared by TPD technique as described previously (1, a>, and then ethylene was admitted to the reaction system to start the reaction. Reaction time was 10 min in most cases. Helium was added to dilute the ethylene, when the circulation pump in the reaction system was operated. After the reaction, the system except for the reactor was evacuated through a liquid nitrogen trap to collect any condensable hydrocarbons, which were transferred to a sampler for analysis. The carrier gas, nitrogen or helium, was then diverted into the reactor, and materials adsorbed on the catalyst were removed by TPD to the chromatograph as in the case mentioned above.

Analysis

The reactants, the reaction mixtures, and the desorption products were analyzed by gas chromatography. For the hydrocarbons, a 2-m alumina column coated with 3 parts of liquid paraffins with respect to 100 parts of solid support was used at

room temperature. A 2-m molecular sieve 13X column was also employed at room temperature for analyzing hydrogen, helium, nitrogen, oxygen, and methane, the purity of which was confirmed. Deutero- hydrocarbons obtained after the reaction were analyzed by a JEOL JMS D-100 mass spectrometer. Before subjecting the reaction mixtures to mass spectrometric analysis, they were separated gas chromato- graphically to avoid mutual interference of mass spectra.

RESULTS AND DISCUSSION

Temperature-Programmed Desorption

The TPD chromatograms of hydrogen chemisorbed on the platinum black used in the present study are essentially the same as those observed previously (1, @, although the reduction conditions of platinum black were different with nitrogen being used as a carrier gas. Four different peaks, with peak maxima at about 170,250, 360, and 570 K, were observed, and are referred to as a, p, y, and 6, respectively.

Figure 1 represents typical TPD chromatograms taken after ethylene was adsorbed on the platinum black in the absence or presence of hydrogen as indicated, where

I I I 1 I

270 370 470 570 670 720 770

TEMPERATURE U/K)

FIG. 1. TPD chromatograms of ethylene on platinum. (a) CzH4 only was adsorbed. (b) H, was preadsorbed and CzH4 was subsequently admitted. (c) CzHl was preadsorbed and Hz was subsequently admitted. (d) HS and CXHI were simultaneously admitted.

4 TSUCHIYA AND NAKAMURA

helium was used as a carrier gas. The pressure of ethylene or hydrogen admitted was 5.3-6.0 X lo2 Pa, and the reaction temperature was 246 K, while the ethylene was adsorbed beforehand at room temperature in the experiment for the TPD chromatogram c. These four TPD chromatograms resemble each other, and are character- istically comprised of three peaks appear- ing at about 370, 500, and higher than 720 K. These peaks are referred to as A, B, and C in order of appearance with in- creasing temperature. By gas chromatographic analysis, peak A was identified to be ethane, except for TPD chromatogram b, which consisted of ethane and ethylene, and peaks B and C were both methane. The methane may be formed by decompo- sition from the adsorbed ethylene heated by TPD as was suggested by Komers et al. (4) for a similar system using silica-supported platinum as catalyst. In order to observe the desorbing hydrogen more clearly, nitrogen was used as a carrier gas for the TPD, and only d-hydrogen was observed in the chromatogram. The other forms of hydrogen may have been removed from the surface when ethylene was admitted, probably by the displacement in the adsorption and/or in the surface hydrogenation to ethane. On the surface, accordingly, only the admitted ethylene along

with a-hydrogen was adsorbed in the form of ethylene and/or ethane. The results suggest that ethylene is more strongly adsorbed on platinum than hydrogen.

The ethane, peak A in TPD chromatogram a, where hydrogen was absent, is probably a result of self-hydrogenation which occurred at elevated temperature during TPD, as was concluded by Komers et al. (4). However, at this stage it is not very clear whet,her the ethane observed in the other cases resulted from self-hydrogenation, or simply represented desorption of ethane formed by hydrogenation, or both.

Reactivity of Chemisorbed Hydrogen

Table 1 shows the results of the surface hydrogenation experiments with hydrogen preadsorbed on the platinum black in various forms (the surface reactant), and gaseous ethylene subsequently admitted into the reaction system (the gas phase reactant). The amounts of ethane formed in series B with b-form hydrogen practically agreed with those in series A, in which ethane was formed by a self-hydrogenation reaction. Apparently, hydrogen chemisorbed in &form, the strongest chemisorption, hardly reacts with the gas phase reactant under the experimental conditions.

TABLE 1

Hydrogenation of Ethylene on Platinum Blacka

Series Surface reactant (type of chemisorption and estimated amount*; cm: STP)

A B 1

2 c 1

2 D 1

2

B 7 8

- - 0.0120 - - 0.0170 - 0.0075 0.0110 - 0.009.5 0.0175

0.0015 0.0225 0.0245 0.0026 0.0260 0.0230

Gas phase reactant

Pressure CaHe formed (lo* Pa) (cm3, STP)

5.9 0.0002 6.7 0.0002 5.9 0.0004 6.1 0.0077 5.3 0.0080 6.1 0.0180 6.1 0.0220

a Catalyst weight 2.53 g. Reaction temperature 246 K, and reaction time 10 min. *Estimated from TPD chromatograms in comparable experiment.s,


In series C, with y- and d-forms of hydrogen preadsorbed on the surface, however, relatively larger amounts of ethane were produced than those in series B. Taking account of the self-hydrogenation shown in series A, the amounts of ethane formed agreed well with those of preadsorbed y-hydrogen. The reaction apparently reached the end point estimated from the initial amounts of the preadsorbed y-hydrogen and of the ethylene admitted to the system, the equilibrium constant for the hydrogenation of ethylene being ex- tremely large at the temperature. In the TPD chromatogram obtained after ethylene was admitted to the preadsorbed hydrogen, where nitrogen was used as a carrier gas, y-hydrogen disappeared, while h-hydrogen was still present and its amount was scarcely different. It is therefore clear that y-hydrogen is involved in the hydrogenation reaction.

In series D, in which three forms of hydrogen (& y, and 6) were preadsorbed, the amounts of ethane formed were again in approximate agreement with those of -y-hydrogen, suggesting that only preadsorbed y-hydrogen reacted. The amount of preadsorbed b-hydrogen, however, was much smaller than that of y-hydrogen, and the contribution of P-hydrogen to the hydrogenation could not be observed. It was accordingly not clear at this stage whether only y-hydrogen participates in the hydrogenation.

To elucidate the point the preadsorbed hydrogen was partially displaced by deuterium before ethylene was introduced, and the deuteroethanes formed by hydrogenation were then analyzed. The participation of the different types of chemisorbed hydrogen is consequently elucidated. For simplicity, the word “hydrogen” in the present paper will hereafter stand for both isotopes, protium and deuterium, and, where necessary, the individual isotopes will be designated as H and D. The types of surface used, on which hydrogen was

preadsorbed, were as follows: (i) &Hydro- gen was displaced by deuterium, and y- and P-hydrogen were protium (H-H-D) ; (ii) d-hydrogen was protium, and y- and P-hydrogen were deuterium (D-D-H); (iii) deuterium only was adsorbed (D-D-D); (iv) /3- and B-hydrogen were deuterium, and y-hydrogen was protium (D-H-D). These surfaces were prepared by the TPD technique as mentioned above with the following results, previously obtained (I, .2), being considered : Preadsorbed cu-deut’erium (or protium) was easily removed by the evacuation at more than 197 K before protium (or deuterium) was subsequently admitted. Preadsorbed ,& and d-deuterium (or protium) were not, but preadsorbed y-deuterium (or protium) was exchange- able with protium (or deuterium) in the gas phase even to the equilibrium at 197 K. y-Deuterium (or protium) could be mostly displaced by protium (or deuterium), if the amount of protium (or deuterium) admitted afterwards was sufficiently large.

The results of hydrogenation are shown in Table 2. The reaction was carried out at 246 K, and the other conditions were similar to those employed in the series D-2 ; the amounts of preadxorbed p- and y-hydrogen were 0.0026 and 0.0260 cm3, respectively. Deuteroethanes have individu- ally different sensitivity and mass spectra depending not only on the mass spectrometer used, but also on the degree of deutera- tion (5). The mass spectra obtained were not converted to the composition of individual isotopes. The mass spectra of ethylene, the reactant, were omitted in the table, because the m/e > 28 could not be observed in the mass spectra corrected for 13C contributions.

Obviously, ethylene is deuterated to deuteroethanes over the surfaces D-D-H, D-D-D, and D-H-D, but not over the surface H-II-D. These results agree quite well with the above suggestions that &hydrogen does not and y-hydrogen does participate in the hydrogenation. In addi-

TSUCHIYA AND NAKAMURA

TABLE 2

Comparison of Mass Spectraa

m/e Deuteroethanes formed (surface employed) CzHs

(H-H-D) b (D-D-H)b (D-D-D)” (D-H-D)”

28 414 125 29 78 151

30 100 100 31 - 63 32 - 16 33 - 7 34 - 4 35 - 2

36 - 1

128 410 391 145 104 75 100 100 100

60 7 - 19 - -

8 - - 5 - - 3 - - 1 - -

a Mass spectra were taken with a JEOL JMS D-100 mass spectrometer at I.V. = 75 V. b Reaction temperature was 246 K, and 0.0026 cm3 of 8- and 0.0260 cm3 of r-hydrogen were preadsorbed. c Reaction temperature was 223 K, and 0.0107 cm3 of /3- and 0.0400 cm3 of r-hydrogen were preadsorbed.

tion, the participation of P-hydrogen was also suggested, which was not detected before. In order to prevent preadsorbed @-hydrogen from desorbing before ethylene was subsequently introduced, the reaction was carried out at 223 K over the surface D-H-D, and the reaction conversion was accordingly lower.

On the mechanism of hydrogenation of ethylene, extensive studies (6) have been reported so far. It is premature at present to discuss the matter on the basis of our results and to add further conclusions other than those already reported.

CONCLUSION

While some of the information could perhaps be obtained otherwise, the TPD technique in many cases can be used with unique advantage. Although the information that ethylene is more strongly adsorbed than hydrogen on a platinum catalyst had already been suggested on the basis of kinetic data (‘Y), it was more clearly shown by TPD, and, in addition, the amounts of adsorbed species could be estimated. Using isotopes, the individual adsorbed species can be differently labeled,

and the reactivity of each is separately measurable. The participation of various types of chemisorbed hydrogen in the ethylene hydrogenation reaction was different from that in the surface exchange reaction between protium and deuterium. y-Hydrogen, presumed to be present on the surface in the form of hydrogen atoms chemisorbed on top of platinum atoms, was found to be involved in both reactions. However, @-hydrogen, presumed to be molecular hydrogen chemisorbed in a bridge form, also reacted with ethylene in the gas phase. The information on the other reactions still awaits further investigation.

ACKNOWLEDGMENTS

The authors are grateful to Mr. S. Fujisaki of this faculty for obtaining the mass spectra, and also to Professor T. Yashima and Dr. N. Takahashi of Tokyo Institute of Technology for the atomic absorption spectrophotometric analysis of the catalyst. The work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan (No. 73-855240), for which the authors’ thanks are due.

REFERENCES

1. Tsuchiya, S., Amenomiya, Y., and Cvetanovib, R. J., J. Catal. 19, 245 (1970).


z?. Tsuchiya, S., Amenomiya, Y., and Cvetanovic, 5. Amenomiya, Y., and Pottie, R. F., Canad. J. R. J., J. Cutul. 20, 1 (1971). Chew 46, 1735, 1741 (1968).

8. Cvetanovib, R. J., and Amenomiya, Y., in “Ad- vances in Catalysis” (D. D. Eley, H. Pines,

6. Bond, G. C., ‘Catalysis by Metals,” p. 229.

and P. B. Weisz, Eds.), Vol. 17, p. 103. Aca- Academic Press, New York, 1962.

demic Press, New York, 1967. ‘7. Thomson, S. J., and Webb, G., “Heterogeneous 4. Komers, R., Amenomiya, Y., and Cvetanovic, Cat.alysis,” p. 118. Oliver and Boyd, Edinburgh

R. J., J. Catal. 15, 293 (1969). and London, 1968.

study of chemisorption and hydrogenation of ethylene on platinum by temperature-programmed...

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