Deactivation of platinum catalysts by lead

B. Delmon and G.F.Froment (Eds.) Caralysi Deaciivaiion 1994 Studies in Surface Science and Catalysis, Vol. 88 0 1994 Elsevier Science B.V. All rights reserved,

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Deactivation of platinum catalysts by lead E. Lamy-Pitrra, B. Alfatimi and J. Barbier

Lab. d e Catalyse en C h i d e Organique, URA CNRS 350, Universiti de Poitiers

40, Avenue du Recteur Pineau, Poitiers 86022, France

ABSTRACT Platinized platinum catalysts were modified by submonolayers of lead, deposited and characterized by electrochemical techniques. Their activity was tested in liquid phase hydrogenation of C=C bonds. High toxicities were obtained with very low lead coverages (epb < 0.05), equal to about 20-50 atoms of platinum deactivated by one lead adatom. Such high toxicities cannot be explained neither by ensemble effects nor by ligand effects. A fast diffusion of lead adatoms on the platinum surface could account for this result. A plateau in activity is found, for medium lead coverages (0.05 < epb < 0.30-0.50) which could be ascribed to the formation of lead islands on the platinum surface. The formation of surface Pb-Pt alloys can be also suggested, by analogy to results reported in the literature. INTRODUCTION Lead is generally known as a poison of platinum catalysts (1,2), whose toxicity, for different catalytic reactions, depends on the way it is deposited. However, for some electrocatalytic reactions, such as the electrooxidation of alcohols, aldehydes o r acides, and also the electroreduction of oxygen, lead adatoms can exhibit a promoting effect (3-7). Moreover, lead can change the selectivity in the case of electrocatalytic reductions of nitrocompounds (S), whereas it inhibits the adsorption of hydrogen on platinum (9,lO). In this paper, the effect of lead adatoms on the activity of platinum catalysts, in liquid phase hydrogenation of olefinic compounds, is presented.

630 EXPERIMENTAL The catalyst used in this work was a platinized platinurn wire prepared electrolytically (roughness factor r = 70); Lead adatoms were deposited on the platinum a t controlled potential, under conditions of underpotential deposition leading to the adsorption of fractions of monolayer (9-11). The characterization of pure platinurn catalysts and of Pt catalysts modified by lead was achieved <
The effect of lead adatoms on the catalytic activity of platinum, for the hydrogenation of maleic acid and methylsubstituted maleic acids, was determined (Fig.1).

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Fig. 1 Dependence of the intrillsic activity of platinum catalysts nil the degree of coverage by lead, for the hydrogenation o f : 1) maleic acid ; 2) niethylmaleic acid and 3) dinietllylinaleic acid

In the experimental conditions used (pH2 = 1 atm, olefin concentration Cw = 10” M, room temperature) the kinetical orders, with respect to the olefin and to the hydrogen, are both equal to one (1516). The diffusion rate of hydrogen was calculated by use of the limiting diffusion current, associated to the electrocatalytic oxidation of hydrogen : A2 ---> 2H+ + 2e(11)

63 1 T h e obtained values a r e higher, by a factor of 5 to 10, than the measured hydrogenation rates (11). As it can be remarked o n figure 1, the deactivation o f the catalyst a t very low lead coverages (8pb < 0.05) is very important. The extrapolation of the tangents of these curves, traced a t the zero poisoning point (8pb = 0), u p to X axis, indicates that the catalyst would be completely deactivated when 5-10”/0 of platinum surface is covered with lead. Thus, taking into account that each lead atom blocks two platinum atoms, the initial toxicity of lead is very high, of the order o f 20-40 platinum atoms deactivated by one lead atom. F o r medium lead coverages, 0.05 < 8pb < 0.30, the catalytic activity of platinum remains stable and independent of Opt,, in all cases. 2. Effect of the temperature on the deactivation of platinum catalysts by lead

As it is reported in ref.12, there is no iriiportant modification of the electronic which coiild accoiint for the high toxicity of this properties of platinum by lead poison. Moreover, this result and also the form of the deactivation curve (Fig.1) cannot be explained by an eiisenible effect (see next chapter : discussion). An explanation is suggested based on the mobility of lead on the surface of platinum. Following G.E. Rhead et al (17,18) the diffusion rate of a metal depends 011 the ratio TM/T,(TMbeing the melting temperature of the metal a n d T the temperature of the experiment) : the lower this ratio, the higher the diffusion rate of the metal under consideration, Following this statement, the high toxicities of nietals having low melting points (Pb, Hg, S,; TI), as found in catalysis (1,19), coirld be defined by the magnitude of their diffusion rate on the surface of the metal catalyst, in cornparison to t h e reaction rate. Yet, direct measure of the diffusion rate of an adatom o n the surface of another metal is not easy to perform. Therefore, an indirect proof of the occiirrence of a fast diffusion of lead adatoms could be the depcndeiice of their toxicity 011 the teniperature of the catalytic hydrogenation reaction, with the assumption that the activation energy of lead diffusion should be low in comparison with those of chemical reactions (20). I n this case, the toxicity of lead for the catalytic hydrogenation o f maleic acid, should change with the temperature. T h e evolution o f the catalytic activity of p l : l t i i i u i n , modified by lead with different Opb, a t three different temperatures (276,296 arid 308 K), is sliown i n Fig.2.

Fig.2

Evolution of the iiitriilsic activity of platiiliinl catalysts, for the hydrogenatioii of maleic acid, as a function of the degree of coverage by lead, a t ditrerent temperatures : a) 276 K ; b) 296 K and c) 308 I<.

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The same type of deactivation curves is obtained a t 276 and 296 K. A high deactivation is obtained with low lead coverages ( e p b = 2-3'%). The initial toxicity (calculated by tracing the tangent a t the origin of the curve) is equal to 40 and 27 platinum atoms deactivated by one lead atom, a t 276 and 296 K respectively. At 308 K, the initial toxicity is very low, since the activity remains stable up to €Jpb = 0.15, and then it decreases.It should be remarked also that the plateau of activity, obtained a t medium coverages (0.05 < epb < 0.30) is located higher a t 296 K than a t 276 K, whereas a t 308 K it is located still higher starting a t lower lead coverages (at Opl, = 0). DISCUSSION As it is pointed out in chapt 2, the high initial toxicities and the form of deactivation curves, obtained by deposition of lead adatoms o n platinum catalysts, are unexpected on tlie basis of the usually obtained deactivation ciirves by different poisons (21,22,23). Indeed, the results obtained with copper and sulfiir poisons (22,23), deposited in the sanie way as lead on the platinum surface (e.g. in u.p.d. conditions, allowing to obtain some fractions of a monolayer of deposited adatoms), and tested i n the same conditions and for the same test-reactions, a r e consisted with the classical deactivation curves and can be explained by the statistical law : a = so (I-€J,,)". In the case of the hydrogenation of maleic acid, the exponent n was found equal to 5 i 1, for copper and sulfur poisons (15,16,22). This value was confirmed by the adsorption stoichiometry of msleic acid on platinum, as evaluated by electrocliemica1 techniques (14). Thus, neither the initial toxicity of lead adatoms, not the shape of the deactivation curves (Fig.1 and 2), can be explained by an ensemble effect . Moreover, tlie modification of the electronic properties of platinum by lead being insignificant (12 ) could not account for the high initial toxicity of lead. The obtained dependence of tlie toxicity of lead o n tlie temperature (Fig.2) supports the statement about a high rate surface diffusion of this adatoni on platinum. Indeed, tlie obtained decrease of the toxicity of lead with the raise of the temperature confirms the assumption that the activation energy of the hydrogenation reaction should be higher than that of tlie surface diffusion of lead. However, the absence o f deactivation by lead at 308 K is difficult to explain. It could be ascribed to the forniation of surface complexes between tlie olefins and tlie deposited lead, which would be active for the hydrogenation reaction (23). Otherwise, the absence of toxicity of lead a t 308 K could be attributed to its diffusion into the bulk of platinum, as it is evidenced in the case of tin adiitollis deposited o n platinurn a t room temperature (19). The mobility of lead adatoms on platinrim could favoiir tlie formation of alloys, even a t room temperature, as it has been shown by W.J. Lorenz et al (24) in the case of Pt-Pb and by T. Mallat et al (25) in tlie case of Pd-Pb. This suggestion is supported by the shape of the deactivation curves, found i n the case of Ni-Cu alloys (26) which is the same as that obtained i n this work and also i n the case of Pt-Sn (19). The absence of deactivation, for 0.03 < €lpb < 0.35 (Fig.1 and 2) could be explained by the formation of P b islands or clusters, which should be favoured by the raise of temperature. Indeed, the plateau i n tlie deacti\ ation curves (Fig.2) is located at

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56% of the initial activity a,, for 276 K, at 71% of a, for 296 K and at 100% of a, at 308 K. The formation of mobile islands is reported also by M.P. Green et al (27) in the case of underpotentially deposited lead on Au(lll), by STM and electrochemical techniques. I n conclusion, these results, concerning metal adatoms of low melting points (Pb, Sn, Hg...) confirm the important role of surface diffusion in the deactivation of platinum catalysts, in agreement with the theory of G.E. Rhead et al(17,18). REFERENCES

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