α-Al2O3 for ethyne hydrogenation in a large excess of ethene and hydrogen

Applied Catalysis, 58 (1990) 227-239

227

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Activity and Selectivity of Pd -AI203 for Ethyne Hydrogenation in a Large Exces§-of-Ethene and Hydrogen H.R. ADIJRIZ, P. BODNARIUK, M. DENNEHY and C.E. GIGOLA*

Planta Piloto de Ingenieria Quimica, Plapiqui, 12 de Octubre 1842, 8000 Bahia Blanca (Argentina) (Received 20 July 1989, revised manuscript received 18 October 1989)

ABSTRACT

The hydrogenation of ethyne in the presence of large amounts of hydrogen and ethene was studied at 1,520 kPa over Pc~/o~-A1203catalysts of varying dispersion. Both TON and selectivity for ethane/(Sc2Hs vmoles df ethane produced from both ethyne and ethene per mole of ethyne converted) were found to decrease as the palladium dispersion increased. The former result was consistent with a clear dependence of the rate order for ethyne on the particle size of the palladium: it was 0 for large particles and - 0 . 5 for small particles. This kinetic behaviour was ascribed to a stronger interaction of the ethyne on the small palladium particles. By increasing the temperature at constant space velocity, SC2H~ Was measured over a wide conversion range. The amount of ethane formed was high oi~largeOarticles hydrogen-to-palladium ratio ( < 0.2 ) and negligible on s~all particles. Using an ethene-free mixture, it was shawrr that the selectivity for ethene ~Sc2H,) is close to 100% up to high conversion. Moreover~ Sc2m d~es not depend on the particle ize of the palladium. Consequently, the higl~ Sc2m~-----o~serv~d.don]o~v-dispersion catalysts was due p thee parallel hydrogenation of ethene. Thes~s.result)/were int~ipreted by assuming that the presence of multiple adsorbed species on large palladium particles prevents the strong bonding of ethyne and therefore the simultaneous adsorption and reaction of ethene is permitted.

INTRODUCTION

The selective hydrogenation of ethyne in the presence of large amounts of ethene is a process of considerable importance for the manufacture of polymergrade ethene. From an industrial point of view, the main objective is the reduction of the ethyne concentration to less than 10 ppm without ethene hydrogenation. Two different hydrogenation processes are used for this purpose: the front-end and the tail-end configuration. The former involves the hydrogenation of the cracked gas mixture (with large amount of ethene and hydrogen) after caustic scrubbing to remove sulphur contaminants. In the tail-end process the hydrogenation is carried out after the addition of a stoichiometric amount of hydrogen as required by the C2H2--, C2H4 reaction. Commercial pal0166-9834/90/$03.50

© 1990 Elsevier Science Publishers B.V.

228 ladium catalysts with low metal loading and low dispersion are now used for both processes but carbon monoxide is continuously added in trace amounts to inhibit the undesirable hydrogenation of the alkenes [1,2]. The present investigation was performed to gain a better understanding of both the activity and the selectivity of ethyne hydrogenation on Pd/a-A1203 using a front-end reaction mixture without carbon monoxide. If the selectivity of palladium is determined only by the thermodynamic factor, we may simply expect to observe the conversion of ethyne, with no formation of ethane from ethene until most of the ethyne has been eliminated. However, literature results concerning the selectivity of palladium catalysts for the semi-hydrogenation of alkynes seem to vary depending on particle size and feed composition. According to Borodzirlski et al. [3], large particles favour the direct conversion of ethyne into ethane owing to the presence of the fl-hydride phase. In line with this observation, Carturan et al. [4] demonstrated that small palladium particles are more selective for the half-hydrogenation of phenylacetylene and it was concluded that this selectivity arises from the strong interaction between the alkyne and the palladium phase. In the presence of dissolved hydrogen the comparable strengths of the metal-alkyne and metal-alkene bonds lead to a lower selectivity. However, they also proposed, as suggested by Borodziriski et al. [3 ], that the direct conversion of alkynes to alkanes may occur in the presence of the fl-hydride phase. Both studies were conducted with alkyne-hydrogen mixtures at atmospheric pressure. Using isotopic labelling, McGown et al. [5 ] showed that the ethane produced during the hydrogenation of ethyne-ethene rich mixtures on commercial palladium catalysts is mainly formed from ethene. To explain this result, it was postulated that two types of sites exist on the metal surface. A similar technique was used by Weiss and co-workers [6-8] to demonstrate that on palladium black and Pd/A1203, ethyne is hydrogenatedto both ethene and to ethane, whereas ethene hydrogenation occurs at low ethyne concentration on the same active sites. It was also suggested that ethane formation from ethyne involves alkylidene species at intermediates. Using a simulated tail-end mixture [8] at 60% ethyne conversion the selectivity for ethane was in the range 2-10%. The effect of palladium dispersion on these reactions has also been investigated [9]. The selectivity for ethane formation from ethyne was observed to decrease by a factor of four whereas the dispersion increased from 2% to 60%. The effect of dispersion on the parallel hydrogenation of ethene was found to be similar. Again it was proposed that the dissociative adsorption of ethyne plus the presence of the fl-hydride phase on large particles leads to ethane formation. According to the authors, this mode of chemisorption does not block the simultaneous adsorption and hydrogenation of ethene, as expected on the basis of thermodynamic arguments. Although the influence of the fl-palladium hydride phase on selectivity has been invoked in several studies, the low hydrogen partial pressure in tail-end

229 hydrogenation mixtures (1% hydrogen + ethyne balanced by ethene) seems to exclude that possibility. The thermodynamics of the Pd-H system for bulk palladium indicates that a minimum pressure of 40 Torr (1 Tort = 133.322 Pa) is needed to form the fl-palladium hydride phase at 300 K [10]. When ethyne is removed by the front-end hydrogenation process (H2/C2H2=50; H2/ C2H4= 2 ), the high hydrogen pressure supports the formation of the fl-hydride phase. Under this condition the specific activity for ethyne hydrogenation was found to decrease with increasing dispersion [2]. In contrast, the results of S~irk~iny et al. [9] for tail-end hydrogenation demonstrated just the opposite effect. Consequently, it was considered interesting to compare the selectivity for ethane formation of these different reaction mixtures as a function of palladium particle size. This paper mainly reports the selective hydrogenation of a simulated frontend ethane cracker gas mixture on palladium catalysts of varying dispersion. A basic kinetic study was also performed. In contrast to previous work, the investigationwas conducted at high pressure in the absence of carbon monoxide. EXPERIMENTAL Pd/AI203 catalysts containing 0.058% and 0.09% of palladium were prepared by wet impregnation of a-alumina (Rh5ne Poulenc SCS9, 13 m 2 g-l) using palladium acetylacetonate dissolved in benzene as precursor. After 72 h the liquid was removed and the preparations were dried at 383 K followed by calcination for 1 h in air at 573 K. A portion of the 0.058% palladium catalyst was reduced in a flow of hydrogen by raising the temperature at 5 K min- 1up to 573 K and maintaining it for 1 h. The remaining part was sintered in hydrogen at 1073 K for 16 h. The 0.09% palladium catalyst was reduced at 773 K for 2 h and a very low dispersion sample was obtained owing to the higher metal loading. A commercial Pd/a-Al203 catalyst (IC138-1 ) was also available. The metal content in the reduced catalysts was determined by atomic absorption spectrometry. The palladium dispersion was calculated from measurements performed in a low-dead-volumeglass apparatus, following the back-sorption technique proposed by Boudart and Wang [11]. The samples were first reduced at 573 K and then evacuated at the same. temperature for 12 h. After measuring the sorption isotherms at room temperature, the samples were evacuated for 0.5 h at the same temperature and a second measurement provided the amount of absorbed and weakly bound hydrogen. The extent of hydrogen chemisorption was obtained by subtracting these contributions from the total uptake. A summary of the catalysts prepared and their characterization is given in Table 1. Several compressed reaction mixtures, listed in Table 2, were prepared by mixing CP-grade hydrocarbons (methane, ethyne and ethene from Matheson) with ultra-high-purity hydrogen and nitrogen (from AGA) in steel cylinders. They were rolled for several days before use. Mixtures 1-6 were used

230 TABLE 1 Composition and hydrogen adsorption data for Pd/a-A1203 Catalyst

P d (%)

H/Pd

L L L C

0.058 0.058 0.090 0.040

0.58 0.10 0.05 0.18

1-1 1-2 2 1 (ICI 38-1)

TABLE 2 Inlet composition of reaction mixtures (mol-%) Mixture

CH 4

C2H 2

C2H 4

H2

N2

1 2 3 4 5 6 7 8

6.37 8.99 10.92 11.71 11.30 9.20 -

0.49 0.64 0.84 0.90 0.98 0.19 0.85 0.85

42.14 60.36 72.24 77.48 64.70 68.40 4.00

51.00 30.00 16.00 9.90 23.10 22.20 15.33 17.33

83.80 77.80

as feed gases to obtain rate data at low conversion. The effect of ethyne conversion on selectivity was studied mainly with mixture 3. In order to compare our results with those of previous studies, ethene was excluded or added in small amounts in mixtures 7 and 8, respectively. Although ethane was excluded from all mixtures to facilitate the measurement of small amounts formed during reaction, it was present as an impurity at a level of 0.013%. The apparatus for the activity and selectivity measurements was essentially the same as that employed previously [2 ], except that a two-stage flow reactor with intermediate cooling was used to minimize thermal effects. Two SS Swagelok unions (1/8 in.) holding catalyst charges of 0.01 g were connected with 2 m of 1/16-in. stainless-steel tubing and immersed in a diethylene glycol bath. The feed mixtures and the reaction products were analysed using on-line gas chromatography (GC). In order to test the selectivity of unsupported palladium a special reactor, as shown in Fig. 1, was constructed. The reaction mixtures were passed over the external surface of a palladium tube (14 cm X 0.3 cm), sealed to a stainless-steel shell by Swagelok unions, while the internal surface was exposed to pure nitrogen. Fresh samples were pretreated under the reaction mixture for 4 h at 288 K and 1520 kPa. Following this pretreatment, conversion measurements at constant space velocity were carried out at increasing temperatures up to the point where the hydrogenation of ethene was

231

1

tl

t it t t

It

II

',!

s

tl

Fig. 1. Schematic diagram of catalytic wall reactor. 1 = Reactor inlet; 2 = reactor outlet; 3 = P d tube; 4 = N2 filling; 5 = Swagelok fitting.

the predominant reaction. In order to obtain kinetic data under differential conditions, the flow-rate was adjusted at selected temperatures. Steady-state operation was maintained for 1 h before changing the operating conditions to the next selected values. RESULTS

The rate of ethyne hydrogenation was measured at low conversion (15%) between 273 and 313 K. Linear Arrhenius plots were obtained, as shown in Fig. 2. An apparent activation energy of 16 _+2 kcal tool- 1 was obtained for all the catalysts tested. Using these data and the information in Table 1, the specific activity at 288 K was calculated and is presented in Table 3 as a function of dispersion. A clear trend of decreasing specific activity with increasing dispersion is observed. As reported previously [2 ], it decreases by one order of magnitude as the dispersion increases from 10% to 60%. Kinetic data obtained with mixtures 1-6 for samples L 1-1, L 2 and C 1 were fitted to a power-law rate equation. Using mixtures 5 and 6 the dependence on ethyne partial pressure was found to be zero and - 0 . 5 for catalysts C 1 and L 1-1, respectively. The main results are summarized in Table 4. Despite the five-fold change in ethyne concentration, the TON did not vary for the low-dispersion sample. In contrast, when sample L 1-1 was tested the rate increased as the concentration decreased, leading to a negative value of - 0.5. Consequently, the dependence

232

0 I-.-

k-1-1

0J 312

I

I

I

i

3.3

314

315

3.6

T-l,, 103 ( K -1 ) Fig. 2. Arrhenius plots for hydrogenation of ethyne on palladium catalysts with mixture 3. TABLE 3

Kinetic parameters for ethyne hydrogenation on Pd/t~-A1203 as a function of palladium dispersion Feed gas: mixture 3. Catalyst

H/Pd

E (kcal mo1-1)

T O N (s -1) at 288 K

L C L L

0.58 0.18 0.10 0.05

17 16 17 14

0.28 1.1 4.7 7.3

1-1 1 1-2 2

ma 1.6 1.3 1.3

aH 2 order from Fig. 3.

of TON on dispersion was less pronounced at low ethyne concentrations. Regarding the dependence on hydrogen partial pressure, linear plots of log (activity) vs. log(pHi) were obtained with mixtures 1-4, as can be seen in Fig. 3. The rate order for hydrogen was found to be between 1.3 and 1.6 for all the samples tested. The effect of increasing temperature on ethyne conversion and ethane formation, at constant flow-rate, is presented in Fig. 4. There is a clear distinction between low- and high-dispersion samples. For catalyst L 1-2 (H/Pd = 0.10)

233 TABLE 4

Dependence of reaction rate on ethyne concentration Feed gas: mixtures 5 and 6.

Catalyst

C2H2 (mol-%)

H2 (mol-%)

TON (s- 1) at 288 K

L 1-1

0.98

23.0

0.58

L 1-1 C 1

0.19 0.98

22.2 23.0

1.42 2.70

C 1

0.19

22.2

2.58

,0

n -0.5 0.0

[;;:0.0o,

A

'E

% I"1-

C z O I--

A

[::.:

10

PH2( at m ) Fig. 3. Dependence of reaction rate on hydrogen partial pressure. Effect of palladium dispersion. Feed compositions: mixtures 1-4; T = 288 K.

the amount of ethane formed increased continuously up to a value of 0.2% at 80% conversion. In contrast, for the high-dispersion catalyst L 1-1 ( H / Pd = 0.58) the ethane content was very low, less than 0.02 % up to 50% ethyne conversion. It should be mentioned that small changes in ethene concentration could not be followed owing to the large excess of this component in mixture 3. Consequently, only the selectivity for ethane (Scan6) was calculated from the GC data. This selectivity is plotted versus conversion in Fig. 5 for all the samples tested. There is a clear trend of decreasing SC2H6 with increasing dispersion. Selectivity values equal to or greater than 1 were obtained with unsupported palladium, although in this instance the ethyne conversion was limited to 10-15%. At higher conversion values there was a marked decrease in ethene concentration owing to hydrogenation, and isothermal conditions could

234

80

0 193

)

Z

0.053

o_ (./3 60 n" ILl Z

0

olol

o.o59

40 0029

£ 20

0.024

V/oo:'

I 0.014

0.014 I

283

I

293

303

313

T(K)

Fig. 4. Ethyne conversion ( % ) vs. T ( K ) at constant space velocity [ 29 c m 3 ( S T P ) g - c a t - 1 s - 1]. The numbers denote the ethane content ( m o l - % ) . Initial value: 0.013 m o l - % . Feed composition: mixture 3.

100

/

unsupported

o" Pd Pd = 0 . 0 5 )

J

L 1-2

/

20

( H/Pd=

~ 20

z,O

P

0.10 )

d 60

=0.,~) 80

t00

C2 H2 CONVERSION (%) Fig. 5. Ethane selectivity (%) vs. ethyne conversion (%). Effect of palladium dispersion. Feed composition: mixture 3. not be maintained. Low-dispersion catalysts L 1-2 and L 2 gave higher selectivities and ethane formation was detected even at low ethyne conversion. On the other hand, with the high-dispersion sample L 1-1, ethene was the main product of ethyne hydrogenation. In an attempt to determine whether the ethane was formed from ethyne or ethene, similar experiments were conducted with mixtures 7 and 8 over unsup-

235 TABLE 5 Effect of ethyne conversion on ethane and ethene selectivities over unsupported palladium Data obtained by increasing both temperature and flow-rate. Mixture 7

Mixture 8

C2H2 conversion (%)

Sc2H6

SC~H4

C2H2 conversion (%)

SC2H8

SC2H4

6.8 10.2 15.6 27.3 55.8 75.6 85.5

0.0 0.0 0.05 0.11 0.12 0.17 0.18

1.00 1.00 1.00 0.88 0.87 0.82 0.81

10.0 12.5 20.4 32.2 54.1 72.3 90.0

0.36 0.45 0.39 0.22 0.73 0.87 0.97

0.79 0.65 0.59 0.80 0.33 0.16 0.06

ported palladium (see Table 5). First the reaction was performed in the absence of ethene (mixture 7) while keeping the concentration of ethyne constant at the same values as for mixture 3. Then the concentration of ethene was increased to 4% (mixture 8). In these experiments the ethene selectivity was also calculated as moles of ethene formed or consumed per mole of ethyne converted. Table 5 shows that ethane was not formed in the absence of ethene; initially the reaction exhibited a selectivity of 100% towards ethene. When the ethyne conversion was higher than 15%, small amounts of ethane were formed, leading to a parallel decrease in Sc2m. Similar results were obtained with samples C 1 and L 1-1. On addition of 4% ethene, pronounced changes in these selectivities were observed. Within experimental error, the initial value of Sc2H6 was fairly high and it increased continuously over the whole conversion range. An opposite tendency was observed for SC2H4 but a net gain of ethene was maintained. On the basis of these observations, it is clear that ethyne hydrogenation proceeds exclusively in the direction of ethene formation. Consequently, ethane formation is mainly due to ethene hydrogenation and this reaction occurs only on large palladium particles. DISCUSSION

In agreement with previous data [2] the specific activity was found to decrease with increasing dispersion. This behaviour has already been observed in the hydrogenation of several alkynes and dienes such as 1-butyne, 1,3-butadiene, isoprene, valylene (2-methyl-l-buten-3-yne) and phenylacetylene [4,12,13]. The former reaction has also been studied on Pt/A1203 and Rh/

236 A1203 catalysts [14,15] with similar results. Consequently, the change in TON with particle size cannot be related to the formation of the fl-hydride phase. Boitiaux et al. [ 14 ] attributed this sensitivity to metal dispersion to a "multicomplexation" mechanism involvinga metal atom of low coordination number (small particles) and two hydrocarbon molecules. This interpretation was consistent with the observed variations in reaction order with particle size: for Pt/A1203 the apparent order with respect to 1-butyne varies between 0 and - 1 as the dispersion increases from 3% to 96%. Our kinetic data seem to follow this pattern. The rate order for ethyne decreased from 0 to - 0.5 as the dispersion increased from 18% to 58% (Table 4). Therefore, at a relatively low ethyne concentration (high conversion) the difference in TON between small and large particles would not be significant. At high ethyne concentration small particles could be deactivated owing to multiple adsorption of this hydrocarbon, as postulated by Boitiaux et al. [ 14 ]. Recently, on the basis of an X-ray photoelectron spectroscopic study, Hub et al. [ 16 ] argued that small "electrondefficient" palladium particles are responsible for this behaviour. Regarding the influence of the hydrogen partial pressure, our results are also consistent with those of Boitiaux et al. in the sense that the correspondent order does not change with dispersion. However, the value of 1.3-1.6 differs from the their reported first-order behaviour. Orders in hydrogen greater than 1 have been quoted by Bond et al. [17] for ethyne hydrogenation on platinum and palladium catalysts, under different experimental conditions. In addition, the data in Fig. 2 show that the apparent activation energy remained constant regardless of the metal dispersion. The value of 16 + 2 kcal mol-1 is in reasonable agreement with those reported for ethyne and valylene hydrogenation on supported palladium of 14 and 16 kcal mol-1, respectively [13,18]. These similarities of kinetic parameters at least suggest that the same mechanism and nature of the rate-limiting step are valid for different alkyne hydrogenation reactions. A clear effect of particle size on the overall selectivity for ethane formation is shown by the data in Fig. 5. It should be noted, however, that sample L 1-1 (60 % dispersion ) behaves differently from the others. The selectivity for ethane was less than 1% up to 50% conversion. Therefore, it seems convenient to discuss first the implications of this result. The very low concentration of ethane in the product stream indicates that ethyne hydrogenation on small palladium particles proceeds almost exclusively in the direction of ethene formation. In other words, the intrinsic selectivity for ethene is close to 100%. As generally accepted, this behaviour arises from the large difference in the strength of adsorption between ethyne and ethene. Supporting evidence for this explanation has been provided by a recent theoretical study [19]. Ethane formation becomes important at low ethyne concentration owing to the parallel hydrogenation of ethene. On the basis of our data, the hydrogenation of ethene is suppressed if the ethyne concentration remains above 0.3%. This result is con-

237

sistent with those obtained in earlier studies under different experimental conditions. S~irkEiny et al. [9], using atmospheric pressure, reported an overall ethane selectivity of 15% at 60-70% conversion using a comparable catalyst, an initial ethyne concentration of 0.3% and a larger excess of ethene. In this instance the amount of ethane produced directly from ethyne was of the order of 4%. Hub et al. [ 16 ] observed that small palladium particles give a constant selectivity of 98% towards 1-butene during the hydrogenation of 1-butyne, up to a limiting conversion of 70%. Consequently, our first conclusion is that the selectivity of high-dispersion palladium catalyst for the hydrogenation of ethyne-ethene mixtures at medium pressure is mainly governed by the relative heats of adsorption of these hydrocarbons. Returning to the curves in Fig. 5, it is seen that low-dispersion samples produce significant amounts of ethane at all conversion levels. An interesting feature of these results is the marked increase in ethane selectivity observed in the 0-10% dispersion range. In addition, the selectivity was constant up to a minimum value of conversion, and this limit decreased at low palladium dispersion. Both tendencies are in very good agreement with those studies cited above. S~irkEinyet al. [9] found a high ethane selectivity of 67% (at 60-70% conversion) in the low-dispersion range (7%). As the dispersion increased to 38% the selectivity decreased by a factor of 3. During the hydrogenation of 1butyne, Hub et al. [16] observed that the selectivity for 1-butene, for a 26% dispersion catalyst, was very high and constant (98%) up to 50% conversion whereas, as mentioned before, it was around 70% for high-dispersion samples. According to the results in Fig. 5, the undesirable formation of ethane at low conversion could be expected if the metal dispersion is less than 20%. An important question to address is whether this ethane is produced by ethyne or ethene hydrogenation. The experimental evidence in Table 4 strongly suggests that ethane formation on large palladium particles is due only to ethene hydrogenation. The fact that a similar activation energy was obtained for all the catalysts does not support a change in reaction mechanism with particle size. Consequently, we postulate that the selectivity for ethene in the C2H2+ H2 reaction is very high regardless of the metal dispersion. This implies that the parallel hydrogenation of ethene occurs more easily as the particle size increases, leading to a pronounced effect on the amount of ethane formed. One argument that could be invoked to explain this behaviour is the presence of /?-hydride phase which is due to the high hydrogen pressure used in our experiments. However, it is noted that a marked change in selectivity occurs when the dispersion falls below 20%, although the fl-hydride phase has been detected even on samples of 50% dispersion [20]. The C2H4-~ C2H6 reaction could also be ascribed to the presence of ethylidyne on the metal surface. Although several adsorbed species derived from ethyne and ethene have been identified on platinum, palladium and rhodium, it is difficult to prove that they play an active role in the hydrogenation processes [21,22]. In our experiments, the

238

initial activity for ethene hydrogenation observed on low-dispersion samples is consistent with the kinetic behaviour already discussed. The decrease in TON and reaction order with increasing dispersion indicates a stronger ethyne adsorption on the small palladium particles, which in turn precludes the hydrogenation of ethene. In addition, it has recently been suggested [22 ] that on large palladium particles the presence of adsorbed species such as ethylidyne or ethylidene inhibits the strong adsorption of ethyne to the metal surface. Consequently, the adsorption and reaction of ethene could be facilitated. Therefore, we can reconcile the effect of particle size with the activity and selectivity results. On the basis of this study, it would be necessary to inhibit the hydrogenation of ethene in order to improve the selectivity of low-dispersion Pd/a-A1203 catalysts. In commercial practice this is achieved by addition of carbon monoxide. Another approach involves the use of bimetallic catalysts. The beneficial effect of alloying palladium with lead has already been reported [231 for ethyne hydrogenation. CONCLUSIONS

The main result that emerges from this study is that the dependence of TON on palladium dispersion, previously observed for ethyne hydrogenation in the presence of a large excess of ethene and hydrogen, is less pronounced when the reaction is carried out at low ethyne concentration (ca. 0.2% ). Our kinetic data have demonstrated that the rate order for ethyne varies from 0 to - 0.5 as the dispersion increases. Consequently, the TON for the removal of trace amounts of ethyne from front-end mixtures does not depend on the palladium dispersion. In other words, the particle size is not an important variable for improving the hydrogenation activity of Pd/~-A1203. However, low-dispersion catalysts are used in industrial applications. Their lower rates per unit mass result in a more stable reactor operation as the occurrence of thermal effects is minimized. It has also been shown that the selectivity for ethene of Pd/o~-A1203 is very high regardless of the palladium dispersion and the high ethene and hydrogen partial pressures. Low-dispersion catalysts favour the undesirable formation of ethane via the parallel hydrogenation of ethene. This reaction is almost absent on high-dispersion samples owing to the strong and multiple adsorption of ethyne [ 14 ]. In commercial practice the addition of carbon monoxide inhibits the ethene hydrogenation, which in turn improves the selectivity.

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