Hydrogenation of acetylene over various group viii metals: effect of particle size and carbonaceous deposits

Journal of Molecular Catalysis,

60 ( 1990) 99-108

99

HYDROGENATION OF ACETYLENE OVER VARIOUS GROUP VIII METALS: EFFECT OF PARTICLE SIZE AND CARBONACEOUS DEPOSITS A. J. DEN HARTOG, M. DENG*, F. JONGERIUS AND V. PONEC** Gorla.eus hbomtories,

Leiden University, P.O. box 9502,230OR.A LAden (The Netherhds)

(Received July 8,1989; revised October 3,1989)

Supported Pt, Pd, Rh and Ir catalysts have been studied in the hydrogenation of acetylene and the effect of the particle size on this reaction was established. The metals studied can be subdivided into two groups, Pd and Pt on the one hand and Ir and Rh on the other. The latter group shows effects that can most likely be explained by an influence of the particle size on the intermediate of direct hydrogenation to ethane, whereas in the case of Pd and Pt the effect of deposition of carbonaceous layers prevails.

Introduction Particle size effects and effects of alloying are attractive means to manipulate catalyst activity and selectivity. They influence the reaction that is studied either directly, or indirectly by influencing the side reactions, for example the deposition of carbonaceous layers 11-51. Extensive carbon deposition is predominantly found on flat catalyst surfaces that are well populated with large particles [Sl. Reactions that occur preferentially on these surfaces are then suppressed by the deposition of carbon. Small metal particles are less sensitive to self-poisoning since they contain relatively fewer flat faces. Thus both the carbon deposition on large particles and the decrease in the relative amount of flat faces with small particles, result in an effect on the reactions that are sensitive to it. Thus primary particle size effects can be simulated or obscured. In this paper, Pd, Pt, Rh and Ir are compared. These metals each exhibit characteristic behaviour with regard to hydrocarbon reactions. In the low temperature hydrogenation of olefms, of all active metals, these metals exhibit the lowest coverage of firmly bound, highly dehydrogenated species. Palladium and platinum are least likely to form metal-carbon multiple bonds and to break C-C bonds, the behaviour likely prerequisite to the formation of unreactive carbonaceous layers 171, 181. However, when the *Present address: Chinese Academy of Sciences, 52 San Li He Road, Beijing, China. **Author to whom correspondence should be addressed. 0 Elsevier Sequoia/Printed in The Netherlands

100

temperature increases an unreactive carbonaceous layer is also formed on Pd and Pt, and in steady-state high temperature reactions (hydrogenolysis, skeletal rearrangement) the extent of such blocking of the surface is even higher on Pd and Pt than on Ir and Rh and other metals. Upon hydrogenation one observes, in addition to the obvious products ethane and ethylene, also benzene and some higher hydrocarbons (predominantly C,). Certain surface intermediates of these reactions have been already identified [9] and one can assume that they are formed from the molecular non-dissociative adsorption of acetylene or, as Slavin et al. assume, from unreactive carbonaceous species, by dehydrogenation of ethylidyne-like species [lo]. The metals studied in this paper differ in their selectivities in the acetylene hydrogenation reactions. Selectivity to ethylene is highest with Pd and lowest with Ir 1111. Therefore, factors influencing the chemisorption of acetylene and ethylene have more room to improve the selectivity of the latter metal, Ir, with which the most pronounced effects can be expected.

Experimental

The catalysts were prepared from the metal salts as supplied by Johnson Matthey. H&(OH)6, (NH&.IrC~, PdCL and @JH,)&hCI, were used as precursors. As a support Kieselgel was used (SiOz, Merck, surface area 480 m2 g-l). The smallest particles were prepared by homogeneous impregnation as described by Ceus 1121. The catalysts with medium particles were prepared by wet impregnation, and the catalysts with the largest metal particles by a mechanical mixture of the Si02 support with the appropriate metal salt. The Ir and Rh catalysts with the smallest particles were prepared by means of ion-exchange with NaY zeolite as a support (Rh-1 and Ir-1 in Table 1). All catalysts were calcined in technical air at 350 “C

= C==C/(C-C

+ C==C) x 100%

(1)

101 TABLE 1 Characterization of the catalysts used Catalyzt

%Mb

TEW

XRD”

CO/Md

Ir-1” Ir-2 h-3 Ir-4 Rh-1” Fth-2 Rh-3 Rh-4

0.6 0.75 2.5 10.5 1.1 2.2 5.5 12.0

1 2.5
-

-

Pt.1 Pt.2 Pt.3 Pt-4 Pt.5 Pt.6 Pt.7 Pd.1 Pd.2 Pd.3 Pd-4

5.0 2.5 1.0 1.0 0.5 5.0 10 4.0 2.0 10.0 10.0

60 50 16

5.3 13 24 3.5 6.3 22 18 25 24 -

0.3 0.42 0.43 0.41 0.15 0.16 0.02 -

“Ion-exchanged NaY zeolites. b% M: metal loading in wt.%. ‘Average particle size (nm) from XFUI line broadening or TEM. dcO/M as determined in flow of Hz.

where C=C stands for the partial pressure of ethylene and C-C for that of ethane. Mass spectrometry used in this study did not allow quantitative determination of all higher hydrocarbons (although GC analysis has shown that most of the oligomerization products are C,), so that the total of the peaks at m/e = 39-45 and m/e = 50-60 (common to all higher hydrocarbons) has been taken as a measure of the extent of the oligomerization. In this paper a second selectivity is used to characterize the contribution of ethylene in the presence of other products (oligomerization and aromatization). This selectivity parameter is calculated by means of the total peak intensities of the relevant products of the MS signals: S*(C=C)

=Z(C=C)/{Z(C-C)

+ Z(C=C) + Z(olig) + Z(benzene)} X 100% (2)

where Z(product) stands for the total intensity of the fragmentation pattern of the product in question. Selectivities to the oligomerization and to benzene are calculated with similar equations. In this case the peak intensities in the mass spectrum are used rather than the absolute amounts, since it was not possible to distinguish between various oligomerization products and thus not possible to determine absolute amounts of these products anyway. For this reason these selectivities can only be compared in a qualitative way.

102

R.esults The results of the characterization of the catalysts (CO adsorption, XRD and electron ~c~~opy) are presented in Table 1. The hydrogenation of acetylene was followed in a static apparatus, which implies that the conversion (and the rate) changes with time, and in principle the selectivity varies also, until a steady state has been reached. ‘IFhedisadvantage of a static apparatus is in this respect obvious, but the advantage is that one can monitor the process leading to the steady state. Let us start with the results obtained with the Pd catalysts. Figure 1 shows the evolution of the selectivity to ethylene (S) as a function of conversion, for the first catalytic run with a given sample of the catalyst (virgin surfaces), for catalysts with varying average particle size. After a certain time (or after a certain conversion has been reached) the selectivity reaches a pseud~s~ady-site vahre. This value is indicated for a catalyst with a 5 nm diameter of Pd particles. On first glance, the pseudo-steady-state selectivity is a function of the particle size, as is illustrated in Fig. 2. However, when the reaction is repeated with the same catalyst (reaction mixture removed by pumping, new reaction mixture admitted), a higher selectivity is achieved for the small particle catalyst in the repeated runs (see Fig. 3). This effect is negligible with catalysts containing large metal particles, whereas for the catalysts containing small particles the effect is more pronounced (the values in the graph are again the ‘pseudo-steadystate’ values, the very slight variation in selectivity compared to Fig. 2 is due to the fact that the figures concern two different experiments). Platinum catalysts achieve an almost steady-state value ah-eady during the first runs. The selectivity to ethylene is plotted in Fig. 4 as a 100

s(“;:,,_

---

fly

_

?5-

r

!j”0

/ 50

100 convP/d

Fig. 1.Selectivity to ethylene as a function of the cmvereion (in the first reaction run) for eeveral Pd catalysts with varying particle size. T,,,= O'c;(+I24nm, (A, 6nm, 03) 5nm, (0) 1.6 nm.

103

5oo’: 200

250

b(a)

Fig. 2. Selectivities of the Pd catalysts as functions of the particle size. (T,, = 0 “C). Data for the selectivities achieved at constant value at the end of the first reaction ruu with a given catalyst (see text for definition of S and S*). (0) S (ethylene), (A) S* CC,,), (Cl) S* (Ar).

function of the CO/M ratio. When the CO/M values are converted into particle size, using the curves calculated by Bond 1131, the diameters vary from 2 to 20 run, in reasonable agreement with XRD data (catalyst Pt-2 deviates probably due to a bimodal particle size distribution). The selectivity is almost constant, with a slight tendency to lower selectivities at higher dispersions (smaller particles) as was found with Pd. It should be noted that Pt catalysts are much less active in the oligomerization reaction than Pd (see C,, in Figs 2 and 4 for Pd and Pt respectively). In the case of Ir the selectivity to ethylene increases when the particle size decreases. This is demonstrated in Fig. 5. When the particle size of the metal crystallites decreases below cu. 60 A, the selectivity increases considerably. Ir shows little formation of gaseous oligomerization products of acetylene, resembling Pt in this behaviour.

100 SW) 75

50

25

---x--+ CO”” WA a)

50

0 b)

25

convP/d

0

Fig. 3. Effect of repeated reactions on the selectivity of a Pd catalyst (d = 1.6 nm). (a) Selectivity S to ethylene aa a function of the conversion; (0) 6rst experiment, (Cl) second experiment; (b) S* (C,,) as a function of the conversion; (0) first experiment, (Cl) second experiment.

104 100

S*t%)

aLt

‘5x%)

LO

/A75 $_--

*, "+

0

:: b

20

~~

0

025

05 CO/M

Fig.4. Selectivity S to ethylene for Various Pt catalysts as functions of the CO/M ratio; (0) room temperature, (0) 50% (At 75*C, (+f S* CC,,) at 50°C. 75, SW

50-

25-.

.L_,__ 0

5

‘O &nm)

Fig. 5. Selectivity S to ethylene for Ir catalysta as a function of the particle size. (T,, temperature).

a

10

;

I

5

2.

&nm)

I

lo

= loom

f 40

&nmf

Selectivity S* for the oligomerization as a fknction of the particle size for Rh (Cl) and Ir catalysts (0). The lines connect points which belong together; no other meaning; should be inferred. T,,=50%!. Fig.

105

However, with Ir no effect of the particle size on these products is observed (no aromatization is observed, see also Fig. 6). Rhodium resembles Pt in that it shows an intermediate sensitivity to particle size. With Pt the selectivity to ethylene decreases slightly with increasing particle size, whereas for Rh the selectivity increases, although the effect on Rh is actually hardly significant. The selectivity of the Rh catalysts is 70-75%. Rh is rather active in the oligomerization reaction (that is, in the formation of gaseous products of an oligomerization reaction) but no effect of the particle size on this is observed. This selectivity in the oligomerization for Rh and Ir catalysts as function of the particle size is illustrated in Fig. 6. Discussion Hydrogenation of acetylene can be assumed to proceed in two steps, schematically: C=C’c!=C

2

The rate of step 2 is sometimes as much as two orders of magnitude higher than that of step 1 and yet the selectivity to ethylene can be high (reaching almost 100% on Pd catalysts), even at high conversions of acetylene. This has been explained by Bond 1111, who introduced the term of the thermodynamic selectivity factor: the intermediate ethylene is displaced from the surface by acetylene so that the high selectivity should be related to a high ratio of the heats of adsorption of acetylene and ethylene. However, the dependence of the selectivity on the temperature, the effect of hydrogen pressure and the very low (conversion-independent) selectivity of iridium 1141 lead to speculations that two pathways are involved in the hydrogenation of acetylene to ethane: a consecutive one, as indicated above, and a direct one, so that the scheme should be as follows, involving a direct pathway 115,201. C=CL,C+C2-C-r 3 \ Most likely, the intermediates in the reaction steps 1 and 3 are different. Moses et al. 1151 speculated that ethylidyne (M&J-CH,) is the intermediate of pathway 3, whereas Al-Ammar and Webb 1161 suggest that a carbonaceous layer is the active site for the hydrogenation; the ethylidyne intermediate bound to the metal is assumed by these and other authors 1173 to be unreactive. Godbey et al. 1181 assume that the ethylidyne has the function that Al-Ammar and Webb ascribe to the carbonaceous layer. Actually, if one accepts that the intermediates of 1 and 2 are the weakly bound molecules (e.g. a ;TG-complex), all dissociatively adsorbed forms -G&!-, =CLCHs, etc.) as well as the strongly bound di-a+C=CH, bound olefins and alkenes (*-HC=CH-*, *-H2CLCHz-* and similar) are potential candidates for the intermediates of pathway 3. At the moment

106

it is impossible to identify this intermediate exactly, but whatever the indicated species would be, it can be expected that: (a) Pathway 3 is more important for metals such as Ir, which are very active in breaking C-H bonds and forming multiple metal-carbon bonds than for metals such as Pt and Pd which are less active in this respect. (b) Any modification which suppresses the C-H bond breaking, the formation of metal-C multiple bonds or the formation of species such as ethylidyne, ethylidene and d&a-bound hydrocarbons, which are all more easily (maybe exclusively) formed on sites (ensembles of atoms) with more than one surface metal atom, would lead to the enhancement of the selectivity to ethylene, if pathway 3 is important with the metal in question at all. It is known from our earlier studies that alloying 1191 and particle size reduction [3] suppress the formation of multiple M-C bonds with metals such as Ir, Pt and Ni. For example, a modification of the surface of Ir by Au leads to almost 100% formation of CHsD upon CH4/D2 exchange (i.e. singly bound species are strongly favoured on the bimetallic catalysts) and in conjunction with this the same modification leads to an increase in the selectivity to ethylene 151 in acetylene hydrogenation. A recent paper 1203 similarly shows that the formation of the multiply bound ethylidyne is made less easy when the Pd surface is modified by Cu or blocked by preadsorbed oxygen or carbonaceous species (C-rich, H-lean). Therefore, it is not surprising that the selectivity of Ir to ethylene is enhanced by alloying with a less active metal or, as demonstrated here, by reducing the particle size. Small particles are, in general, more reluctant to form multiple metal-carbon bonds and they contain fewer well developed crystallographic planes, e.g. the (111) face, which best promote the formation of ethylidyne. Rh approaches Ir in this respect 12, 3, 111. With Pd (and Pt), the situation is different. Whereas with the other metals studied, the steady state selectivity (the value of selectivity after repeated runs) is reached in a rather short period of time, with Pd, not only is acetylene hydrogenated on a virgin Pd catalyst but a considerable part is converted into desorbable oligomerization products (such as C,, and benzene, at low temperatures), or adsorbed still more strongly forming a selectivity-modifying layer. Only when the formation of this layer is more or less complete (Fig. 31, does the selectivity to ethylene reach a constant (high) value. When the reaction vessel is evacuated after the first reaction with the Pd catalyst, and a second reaction is started, the effects of apparent particle size variations vanish. Both small and large particles then exhibit a high selectivity to ethylene, whereas in the transient state a particle size sensitivity is observed. This slow approach of the steady state values might be related to the lower propensity of the small Pd particles to form carbonaceous layers at lower temperatures 121I. The small particles are in our view less sensitive to deactivation by deposits. When the reaction vessel is evacuated and the adsorbed hydrocarbons are more deeply dehydrogenated by this treatment, a carbonaceous layer is finally formed and the selectivity

107

to ethylene enhanced. When the formation of the layer is completed the particle size effect vanishes. Ir catalysts do not show such an effect, and the selectivity varies with conversion to a lesser extent. Experiments which have been performed at higher pressures (-2 bar) in a pulse flow system [221 confirm this idea. The clean Pd catalysts show little effects of particle size variations (if anything, a slight increase in the steady-state selectivity to ethylene is found upon decreasing particle size). It also appeared from pulse flow experiments with Ni that this metal shows a selectivity which is constant from the beginning of the deposition of carbonaceous layers. In this respect the difference between Pd (and Pt) on the one hand and Ni and probably other metals on the other is quite clear. Literature can be found which relates the low selectivity, wherever found, to the absorbed or ‘occluded’ hydrogen [23]. Since Pd is known to absorb hydrogen more readily than other metals, it might be asked whether the non-steady-state behaviour observed with Pd (and to a much lesser extent with Pt) should not be related to the non-steady-state absorption of hydrogen. The answer is as follows. In the pressure range of our experiments, the equilibrium Pd: H stoichiometry (bulk) is about 1: 0.0055 1241. This should be compared with the Pd(surfVPd(total) ratio of about 0.5. It is not expected that the relatively small additional amount of absorbed hydrogen should play a decisive role in the incipient selectivity. On the other hand, we know from pulse flow experiments that much of the first acetylene pulses is retained on the surface. Therefore we relate the selectivity-time variations to the acetylene and not to the hydrogen adcab)-sorption. Moreover, small particles dissolve less hydrogen and the apparent non-steady-state selectivity of smaller particles is lower. In conclusion, in the hydrogenation of acetylene there are at least two important effects that determine the selectivity to ethylene. The first is the ensemble size effect, which leads to an increase in the selectivity to ethylene when the average ensemble size is diminished. This can be accomplished either by decreasing the particle size or by alloying with an inactive metal (Au). This effect was found to be important with supported Ir catalysts. The second effect is due to the deposition of carbonaceous layers on the metal surface. An apparent ensemble size effect is induced by these layers. The deposition of carbonaceous layers results in an increase in the selectivity to ethylene and is itself also particle size sensitive.

Acknowledgements

The investigations were supported by the Netherlands Foundation for Chemical research (SON) with fmancial aid from the Netherlands Organisation for Scientific Research @IWO). The authors would like to thank Mr J. I. Dees for drawing the final version of the figures. Johnson Matthey kindly supplied the noble metals used in this study.

108

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