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The effects of the nature and the structure of platinum group catalysts on their poisining and regeneration
Surface Technology, 13 (1981) 257 - 264
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THE E F F E C T S OF THE NATURE A N D THE STRUCTURE OF PLATINUM GROUP CATALYSTS ON THEIR POISONING A N D REGENERATION
A. A. SUTYAGINA and G. D. VOVCHENKO Department of Chemistry, Moscow State University, 117234 Moscow (U.S.S.R.)
(Received September 22, 1980)
Summary The poisoning of platinum group catalysts (platinum, rhodium, palladium, ruthenium, osmium and Rh-Os alloys) with mercury, sulphur and lead was investigated. A number of examples were given to show that the capability of being poisoned depends not only on the nature of the catalyst but also on its structure and surface morphology. We established that chemical compounds are formed when platinum, rhodium and palladium catalysts are poisoned, whereas no such interaction takes place when ruthenium and osmium catalysts are poisoned. The role of structure manifests itself in the poisoning of mixed catalysts, i.e. Rh-Os alloys. Differences in the type of interaction and the distribution of the poison over the surface, which are determined by the type of catalyst and its structure, were revealed in the poisoning of platinum and rhodium catalysts with sulphur and lead. Electrochemical methods such as anodic polarization at specific potentials and anodic-cathodic cyclic polarization in 0.1 N H2SO 4 solutions (with electrolyte replacement) were found to be effective in the regeneration of poisoned catalysts. The choice of the method was determined by the type of catalyst and the type of poison, by the interaction of the poison with the catalyst surface which is dependent on the structure and by the behaviour of the poison during the electrochemical t r eat m ent of the catalyst.
1. I n t r o d u c t i o n Problems of cur r ent interest in theoretical investigations of catalytic phenomena include the effect on the catalyst and the catalytic process of additives which, depending on the nature of their action, are called contact poiso.ns or promoters. In recent years, modification of the catalyst surface by cation adsorption has been widely used in catalytic processes carried out in the liquid phase. The phenomenon of surface modification was first reported by Roginsky [1] who showed that the same substance can act as both poison 0376-4883/81/0000-0000/$02.50
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and p r o m o t e r depending on its c o n c e n t r a t i o n . He concluded t h a t this action was incompatible with the t h e o r e t i c a l description of the poisoning of h o m o g e n e o u s and h e t e r o g e n e o u s surfaces in terms of the simple blocking of active centres. When the additives act on the c a t a l y s t a more f u n d a m e n t a l c h a n g e takes place in its surface as well as in its adsorptive and c a t a l y t i c properties. The a d s o r p t i o n of different atoms and ions on platinum group metals, and the effect of these adsorbents on the s t r u c t u r e of the electrical double layer, the e n e r g y state of the surface, the a d s o r p t i o n of h y d r o g e n and o r g a n i c substances, and the c a t a l y t i c properties of the metals have been reviewed by a n u m b e r of workers [2 - 7]. It has been noted t h a t when cations are adsorbed the a d s o r p t i o n of h y d r o g e n is m a r k e d l y reduced and the s t r e n g t h of its bonding with the surface changes. The c a t a l y s t also acquires new properties with respect to oxygen adsorption. Changes in these parameters also affect the rate and the selectivity of the e l e c t r o c a t a l y t i c processes. However, it should be noted that, while some progress has been made in u n d e r s t a n d i n g the mechanisms of c a t a l y s t modification, c a t a l y s t poisoning and r e g e n e r a t i o n has received little a t t e n t i o n recently. Nevertheless, it has been established t h a t when the m e c h a n i s m s of' c a t a l y s t poisoning are studied the n a t u r e of the catalyst, the state of its surface and the c h a r a c t e r of the d i s t r i b u t i o n and i n t e r a c t i o n of the poison with the surface should be t a k e n into account. If this is not done it is impossible to explain the complicated mechanisms of c a t a l y s t poisoning or to d e t e r m i n e reliable methods of c a t a l y s t r e g e n e r a t i o n . W h e n the influence of adatoms on the e l e c t r o c a t a l y t i c properties of metal catalysts is studied it is often assumed t h a t they are uniformly distributed over the surface, and the possibility t h a t they may p e n e t r a t e into the lattice or i n t e r a c t with the c a t a l y s t atoms and the differences in the c h a r a c t e r of c r y s t a l l i z a t i o n on the surface are not t a k e n into account. Data have been published [3 - 9] which confirm t h a t the a d s o r p t i o n of cations is a c c o m p a n i e d by the f o r m a t i o n of strong c h e m i s o r p t i o n bonds with the metal. The results of the i n v e s t i g a t i o n of the poisoning of platinum, rhenium, palladium, r u t h e n i u m and osmium with mercury, s u l p h u r and lead t h a t are r e p o r t e d in this paper show t h a t the state of the poison, the s t r e n g t h of the poison c a t a l y s t bond and the distribution of the poison over the surface are d e t e r m i n e d not only by the n a t u r e of the poison and the c a t a l y s t but also by the c a t a l y s t structure. Both the fine s t r u c t u r e (lattice defects, microstresses, block sizes etc.) and the m o r p h o l o g y (crystal type, presence of macrodefects such as c r a c k s etc.) of the c a t a l y s t may affect poisoning. These questions have not been a d e q u a t e l y studied. An investigation of the b e h a v i o u r of the adatoms d u r i n g the o p e r a t i o n of a specific c a t a l y s t u n d e r different conditions (potential, medium etc.) makes it possible to find the limits of applicability of a given additive as well as to d e t e r m i n e methods of r e g e n e r a t i n g the c a t a l y s t and removing the undesirable substances causing its poisoning.
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2. E x p e r i m e n t a l details The i n v e s t i g a t i o n was c o n d u c t e d on c a t a l y s t s deposited on p l a t i n u m from 1% solutions of the c o r r e s p o n d i n g salts at different c u r r e n t densities i k. The s t r u c t u r e of the c a t a l y s t surfaces was examined using a JSM-V3 s c a n n i n g microscope. The i n t e r a c t i o n between the poison and the catalysts, the particle size and the crystal s t r u c t u r e of the poison were investigated using X-ray phase analysis, e l e c t r o n diffraction e x a m i n a t i o n and electron m i c r o s c o p y t o g e t h e r with local X-ray spectral analysis. The d a t a on the state of the poison on the surface and its e l e c t r o c h e m i c a l b e h a v i o u r were o b t a i n e d from p o t e n t i o d y n a m i c c u r r e n t - p o t e n t i a l (I-Or) curves m e a s u r e d using a P-5848 potentiostat. A c o m p a r a t i v e estimate of the s t r u c t u r a l defects was made using the thermo-e.m.f, m e t h o d [10, 11], and the q u a n t i t y of poison on the surface was d e t e r m i n e d by chemical methods. C u m u l a t i v e poisoning of the c a t a l y s t s with m e r c u r y , lead and s u l p h u r was carried out using solutions of the a p p r o p r i a t e c o n c e n t r a t i o n s of HgC12, Pb(NO3) 2 and t h i o u r e a in 0.1 N H2SO ~. The poisons were deposited at a p p r o p r i a t e values of ~b,. and V or by c u r r e n t polarization. The ability of the c a t a l y s t to adsorb h y d r o g e n and its activity in the e l e c t r o r e d u c t i o n of a n u m b e r of compounds (nitromethane, a c e t o n i t r i l e and a c e t o a c e t i c ether) were used as criteria for the degree of poisoning.
3. R e s u l t s and d i s c u s s i o n Rhodium and platinum catalysts with two different s t r u c t u r e s were investigated. At small values of ik (up t o 2 mA cm 2) compact light-grey deposits with surfaces formed by " s o m a t o i d s " were obtained. At high values of ik (15 - 20 mA c m - 2) loose dark deposits with surfaces composed of dendrites or fine needle-like crystals were formed [12]. M i c r o p h o t o g r a p h s show t h a t t h e r e are microdefects (cracks) on the surfaces of compact deposits of both p l a t i n u m and rhodium. A 1% PdC12 solution acidified with HC1 to pH 1 was used to obtain the palladium deposits. Compact deposits of palladium were obtained at ik = 0.5 mA cm 2, and loose deposits were o b t a i n e d at ik = 6 m A c m 2 [13]. The thermo-e.m.f, d a t a showed t h a t the lattices of both p l a t i n u m [11] and r h o d i u m [14] deposited at positive potentials (small ik) c o n t a i n e d m a n y defects. X-ray investigations revealed a high dispersivity and the presence of microstresses in such electrodeposited catalysts. Changes in the s t r u c t u r e produced a c h a n g e in the true surface area of the c a t a l y s t s as m e a s u r e d by h y d r o g e n adsorption. For the same q u a n t i t y of metal the surface area is g r e a t e r in catalysts with the loose s t r u c t u r e . The q u a n t i t y of adsorbed h y d r o g e n was estimated by electrochemical m e t h o d s [15]. The i n v e s t i g a t i o n of the m e c h a n i s m s of m e r c u r y poisoning of electrodeposited p l a t i n u m (Pt/Pt), r h o d i u m (Rh/Pt) and palladium (Pd/Pt) c a t a l y s t s showed t h a t chemical compounds are formed. The diffraction lines cor-
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r e s p o n d i n g to the c h e m i c a l c o m p o u n d s RhHg~÷~, w h e r e 0 < x ~< I [14], and P t H g 4 [16] a p p e a r e d in the s p e c t r a o b t a i n e d from loose samples w h e n a b o u t 60% - 80% of the surface was poisoned. The p e r c e n t a g e p o i s o n i n g of the s u r f a c e was e s t i m a t e d by a s s u m i n g t h a t one m e r c u r y a t o m is adsorbed on one a t o m of the surface metal. As the degree of p o i s o n i n g increased, the i n t e n s i t y of the diffraction lines increased. The i n t e r a c t i o n of the poison w i t h the c o m p a c t c a t a l y s t s is a m o r e c o m p l i c a t e d process. C h e m i c a l c o m p o u n d s of the M e H g 4 type were o b s e r v e d only w h e n the p o i s o n i n g was excessive (200% and more). H o w e v e r , the c o m p o u n d P t H g 2 was formed on p l a t i n u m w h e n 50% - 60°/, of the s u r f a c e was poisoned [17]. The role of s t r u c t u r e in p o i s o n i n g was even m o r e m a r k e d for rhodium. W h e n 30% - 40% of the s u r f a c e of c o m p a c t r h o d i u m was poisoned the intensitY of the diffraction lines changed. At the o n s e t of p o i s o n i n g t h e r e was no c h e m i c a l i n t e r a c t i o n and m e r c u r y p e n e t r a t e d into the r h o d i u m bulk. This c a n be e x p l a i n e d [14] by the p r e s e n c e of a l a r g e r n u m b e r of defects, m i c r o s t r e s s e s and m i c r o c r a c k s t h a n in the r h o d i u m with the loose s t r u c t u r e . This c o n c l u s i o n is based on c h a n g e s o b s e r v e d in the lattice period of the c o m p a c t r h o d i u m deposits. The p e n e t r a t i o n of' m e r c u r y into the c r y s t a l l a t t i c e and the possibility of the f o r m a t i o n of solid s o l u t i o n s is also i n d i c a t e d by the c h a n g e s o b s e r v e d in the lattice period. No such effect was o b s e r v e d in the r h o d i u m c a t a l y s t with the loose s t r u c t u r e . The p e n e t r a t i o n of m e r c u r y into the b u l k c o m p e t e s w i t h the f o r m a t i o n of i n t e r m e t a l l i c c o m p o u n d s on the surface. The diffusion and m i g r a t i o n of the m e r c u r y depends on the m i c r o r e l i e f of the surface [18]. W h e n p a l l a d i u m c a t a l y s t s are poisoned with m e r c u r y the c o m p o u n d P d H g is fbrmed [13]. T I (mA)
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Fig. I. Anodic potentiodynamic ! @,. curves for Rh/Pt e]ectrodes: (a) ik
2 mA cm 2: (b)
15 mA em 2, Curves 1 and 1', unpoisoned electrodes; curves 2, 2', 3, 3'. 4, 4' and 5', removal of the catalyst poison in successive steps. A potential scanning velocity of 3.33 mV s i was used. ik =
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F i g u r e s l(a) and l(b) show the anodic p o t e n t i o d y n a m i c I - G c u r v e s o b t a i n e d in a 0.1 N H2SO 4 s o l u t i o n for R h / P t e l e c t r o d e c a t a l y s t s deposited at ik = 2 mA c m - 2 and at ik = 15 mA c m - 2 respectively. C u r v e s 1 and 1' c o r r e s p o n d to u n p o i s o n e d electrodes, and c u r v e s 2 a n d 2' were o b t a i n e d i m m e d i a t e l y a f t e r m e r c u r y p o i s o n i n g (95% poisoning). A f t e r poisoning, the a d s o r p t i o n of h y d r o g e n and o x y g e n is suppressed. The c o m p a c t c a t a l y s t s lose t h e i r ability to a d s o r b h y d r o g e n at a l o w e r p e r c e n t a g e p o i s o n i n g t h a n the loose c a t a l y s t s do. F i g u r e 1 shows t h a t the s h a p e of the c u r v e s changes. S e v e r a l new c u r r e n t p e a k s c o r r e s p o n d i n g to the o x i d a t i o n of m e r c u r y c a n be seen in c u r v e s 2 and 2' a n d are i n d i c a t i v e of a r a t h e r b r o a d s p e c t r u m of m e r c u r y s t a t e s and differences in the s t r e n g t h of the b o n d i n g with r h o d i u m surface. The first p e a k at 0.7 - 0.8 V, w h i c h h a s a m u c h g r e a t e r h e i g h t for the c o m p a c t deposits t h a n for the loose deposits, is a t t r i b u t e d [19] to the d i s s o l u t i o n of the free m e r c u r y p r e s e n t on the s u r f a c e in the form of droplets. This p e a k is n o t o b s e r v e d on r e p e a t c u r v e s or c u r v e s o b t a i n e d for r h o d i u m w h i c h h a s been k e p t in a desiccator. The second p e a k on c u r v e s 2 a n d 2' w h i c h o c c u r s at 0.9 - 1.2 V c o r r e s p o n d s to the o x i d a t i o n of m e r c u r y t h a t h a s b e e n a d s o r b e d on the r h o d i u m and h a s p a r t i a l l y p e n e t r a t e d into the bulk. T h e third p e a k at 1.3 - 1.6 V w h i c h h a s a l a r g e r a r e a c o r r e s p o n d s to the o x i d a t i o n of m e r c u r y w h i c h h a s formed c h e m i c a l c o m p o u n d s . The difference in the a r e a s u n d e r this p e a k for loose and c o m p a c t r h o d i u m with the s a m e degree of p o i s o n i n g i n d i c a t e s t h a t the d e n d r i t i c r h o d i u m deposit is m o r e c h e m i c a l l y a c t i v e in r e a c t i o n s w h i c h form s u r f a c e c o m p o u n d s with m e r c u r y . This is confirmed by the X-ray p h a s e a n a l y s i s d a t a [14]. Investig a t i o n s of c h a n g e s in t h e s e c h e m i c a l c o m p o u n d s w i t h time w h i c h were m a d e u s i n g a n o d i c or cyclic a n o d i c - c a t h o d i c I--Or c u r v e s shows t h a t the third p e a k decreases. This i n d i c a t e s a c o m p a r a t i v e l y low stability. The decomp o s i t i o n of the c h e m i c a l c o m p o u n d s a p p e a r s to be due m a i n l y to the processes of m e r c u r y diffusion and m i g r a t i o n into the bulk. Cyclic anodic c a t h o d i c p o l a r i z a t i o n w i t h r e p l e n i s h m e n t of the elect r o l y t e (0.1 N H2SO4) a f t e r the anodic c u r v e h a s been o b t a i n e d can be used to r e g e n e r a t e Pt/Pt, R h / P t and P d / P t electrode c a t a l y s t s since the quantit:¢ of m e r c u r y b o t h on the surface and in the b u l k d e c r e a s e s d u r i n g the cycle. It h a s p r e v i o u s l y b e e n e s t a b l i s h e d [20] t h a t m e r c u r y is r e m o v e d m o r e easily from c a t a l y s t s w i t h a loose s t r u c t u r e t h a n from c a t a l y s t s with a c o m p a c t s t r u c t u r e . The r e m o v a l of m e r c u r y by the anodic p o l a r i z a t i o n of electrodes in a n acid s o l u t i o n in the p o t e n t i a l r a n g e 1.0 - 1.5 V p r o v e s to be even m o r e effective d e p e n d i n g on the degree of poisoning, the n a t u r e of the m e t a l and the s t r u c t u r e . In view of the fact t h a t p l a t i n u m , r h o d i u m , palladium, r u t h e n i u m a n d o s m i u m differ in t h e i r ability to a m a l g a m a t e with and dissolve in m e r c u r y [21] it is of i n t e r e s t to c o m p a r e the d a t a r e p o r t e d in the f o r e g o i n g with the r e s u l t s of i n v e s t i g a t i o n s of r u t h e n i u m and osmium. The n a t u r e of the m e t a l was found to e x e r t a m a r k e d influence on the p o i s o n i n g m e c h a n i s m . Merc u r y a d s o r b e d on r u t h e n i u m and o s m i u m is c h a r a c t e r i z e d by a g r e a t e r h o m o g e n e i t y of states. The first and the second p e a k s on the I-(/~,. c u r v e s due
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to m e r c u r y o x i d a t i o n are not clearly defined, and the third p e a k is absent. No c h e m i c a l i n t e r a c t i o n s h a v e been d e t e c t e d by X-ray p h a s e a n a l y s i s [22, 23]. The s t r e n g t h of the m e r c u r y - - m e t a l bond d e c r e a s e s in the o r d e r H g Pd ~> Hg Pt > H g - - R h > H g - - R u ~ H g - - O s . The ability to form s u r f a c e c o m p o u n d s decreases in the s a m e order. T h e r e f o r e p o i s o n i n g of r u t h e n i u m and o s m i u m t a k e s place m o r e slowly t h a n t h a t of palladium, p l a t i n u m and rhodium. The role of s t r u c t u r e in p o i s o n i n g processes is of p a r t i c u l a r i n t e r e s t w h e n m i x e d - c a t a l y s t alloys are used. Alloys of the R h - O s system with a b r o a d r a n g e of c o m p o s i t i o n s w h i c h were o b t a i n e d by e l e c t r o d e p o s i t i o n h a v e been i n v e s t i g a t e d [24], and it was s h o w n t h a t t h e i r s u s c e p t i b i l i t y to poisoning depends not only on the o s m i u m c o n t e n t (osmium does not i n t e r a c t with m e r c u r y ) but also on the c a t a l y s t s t r u c t u r e . C a t a l y s t s with a loose s t r u c t u r e and a low o s m i u m c o n t e n t are less s u s c e p t i b l e to p o i s o n i n g t h a n c a t a l y s t s with a c o m p a c t s t r u c t u r e and a high o s m i u m content. We also i n v e s t i g a t e d the p o i s o n i n g of p l a t i n u m and r h o d i u m c a t a l y s t s with the less mobile poisons s u l p h u r and lead. Possible r e g e n e r a t i o n m e t h o d s were also examined. We found t h a t , as in the case of m e r c u r y , the ability to adsorb h y d r o g e n was inhibited at a h i g h e r p e r c e n t a g e p o i s o n i n g on r h o d i u m and p l a t i n u m deposits w i t h a loose s t r u c t u r e t h a n on deposits with a compact structure. E l e c t r o n diffraction e x a m i n a t i o n and m e a s u r e m e n t of the I-q5 r c u r v e s showed t h a t t h e r e were s u b s t a n t i a l differences in the s t a t e of the s u l p h u r deposited on the surfaces of the two metals. On p l a t i n u m m o r e of the s u l p h u r was in the a d s o r b e d form and some of it was c o m b i n e d in the sulphide PtS 2. The o x i d a t i o n c u r r e n t s t h a t a p p e a r e d on the curves for r h o d i u m poisoned with s u l p h u r were small c o m p a r e d with those for sulphurpoisoned p l a t i n u m and indicate t h a t the R h - - S bond is s t r o n g e r t h a n the Pt S bond. The sulphide Rh2S 3 was detected on the r h o d i u m surface. Cyclic anodic c a t h o d i c p o l a r i z a t i o n p r o v e d to be an efficient m e t h o d of r e g e n e r a t i o n w h i c h c o m p l e t e l y r e s t o r e d the p r o p e r t i e s of p l a t i n u m poisoned with sulphur. By using this m e t h o d it was possible to oxidize m o s t of the s u l p h u r to s u l p h a t e in the anodic cycle and to r e d u c e the sulphide in the c a t h o d i c cycle [25] so t h a t both the a d s o r p t i v e and the c a t a l y t i c p r o p e r t i e s of the p l a t i n u m were restored. This m e t h o d p r o v e d to be less efficient for r h o d i u m ; a significant a m o u n t of poison r e m a i n e d on the s u r f a c e and the ability to adsorb h y d r o g e n was r e s t o r e d by only 30% - 50% [26]. S e p a r a t e anodic and c a t h o d i c p o l a r i z a t i o n of poisoned electrodes u s i n g c u r r e n t s of different values proved to be an inefficient m e t h o d of r e m o v i n g the poisons in all cases. The role of the s t r u c t u r e and the m o r p h o l o g y of the s u r f a c e was also studied and is exemplified by the p o i s o n i n g of r h o d i u m c a t a l y s t s with lead. Lead p o i s o n i n g c a n be used to e s t i m a t e the n u m b e r of a c t i v e c e n t r e s in c a t a l y s t s [27]. It has been e s t a b l i s h e d [28] t h a t no c h e m i c a l i n t e r a c t i o n s t a k e place b e t w e e n the lead deposits and rhodium. L e a d and t e t r a g o n a l lead oxide ( P b Q , ) p h a s e s were found on the surface which, in a g r e e m e n t with
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the results of ref. 27, indicates the presence of three-dimensional crystallization in the lead deposits. E x a m i n a t i o n u s i n g electron m i c r o s c o p y and local X-ray spectral analysis shows t h a t c r y s t a l l i z a t i o n leads to a n o n - u n i f o r m d i s t r i b u t i o n of lead phases over the surface w h i c h depends on the surface m o r p h o l o g y [29]. Differences in the s t r u c t u r e and the m o r p h o l o g y of the r h o d i u m surface do not affect the c o m p o s i t i o n of the deposit phases, but the lead phases a p p e a r at a lower (about 20%) p e r c e n t a g e poisoning on c o m p a c t electrodes t h a n on electrodes with a loose s t r u c t u r e (about 65% poisoning) [28].
4. C o n c l u s i o n The a p p l i c a t i o n of the p o i s o n i n g m e t h o d to determine the conc e n t r a t i o n of active centres on the surface of c a t a l y s t s is of limited use, and the chemical and physical properties of the c a t a l y s t should be t a k e n into a c c o u n t w h e n it is applied. The limitations on this m e t h o d are imposed by the n o n - u n i f o r m d i s t r i b u t i o n of the poison owing to its chemical i n t e r a c t i o n s with the surface, its p e n e t r a t i o n into the bulk, its c r y s t a l l i z a t i o n etc., i.e. by factors t h a t are not directly c o n n e c t e d with the d i s t r i b u t i o n of active centres on the c a t a l y s t surface. W h e n the m e c h a n i s m s of poisoning and the n a t u r e of the i n t e r a c t i o n s of poisons with c a t a l y s t surfaces are i n v e s t i g a t e d the m o r p h o l o g y and the fine s t r u c t u r e of the metal w h i c h affect the n a t u r e of the d i s t r i b u t i o n and the i n t e r a c t i o n of the poison with the c a t a l y s t should be t a k e n into a c c o u n t in addition to its chemical nature.
References 1 S.Z. Roginsky, Adsorption and Catalysis on Non-uniform Surfaces, Nauka, Moscow, 1948. 2 P. Delahey, Double Layer and Kinetics of Electrode Processes, Mir, Moscow, 1967. 3 D.V. Sokolsky and A. M. Sokolskaya, Metallic Catalysts of Hydrogenation, Nauka, Alma-Ata, 1970. 4 D.V. Sokolsky and G. D. Zakumbayeva, Adsorption and Catalysis on Metals of Group VHI in Solution, Nauka, Alma-Ata, 1973. 5 A.N. Frumkin, Adv. Electrochem. Electrochem. Eng., 3 (1963) 287. 6 N.A. Balashova and V. E. Kazarinov, Usp. Khim., 34 (1965) 1721. 7 N.A. Balashova and V. E. Kazarinov, Electroanal. Chem., 3 (1969) 155. 8 G.N. Mansurov, N. A. Balashova and V. E. Kazarinov, Elektrokhimiya, 4 (1968) 641. 9 A.N. Frumkin, G. N. Mansurov, V. E. Kazarinov and N. A. Balashova, Collect. Czech. Chem. Commun., 31 (1966) 806. 10 Yu. M. Polukarov and Yu. D. Gamburg, Elektrokhimiya, 7 (1971) 717. 11 Yu. D. Gamburg, R. P. Petukhova, B. I. Podlovchenko and Yu. M. Polukarov, Elektrokhim@a, 10 (1974) 751. 12 A.A. Sutyagina, T. V. Kulchitskaya and G. D. Vovchenko, Elektrokhimiya, 11 (1975) 469. 13 A.A. Sutyagina, V. A. Perepelitsa and G. D. Vovchenko, Elektrokhimiya, 14 (1978) 1712.
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14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
T . V . K u l c h i t s k a y a , A. A. S u t y a g i n a , V. I. F a d e e v a and G. D. V o v c h e n k o . Vestn. Mosk. Univ., Khim., 16 (1975) 441. A . I . P l e t y u s h k i n a , G. P. K h o m c h e n k o and G. D. V o v c h e n k o , Catalytic Reactions in the Liquid Phase, N a u k a , Alma-Ata, 1963, p. 295. A . A . S u t y a g i n a , I. N. G o l y a n i t s k a y a , V. I. F a d e e v a and G. D. Vovchenko, Vestn. Mosk. Univ., Khim., 14 (1973) 177, A . A . S u t y a g i n a , T. V. K u l c h i t s k a y a , V. K. P o r t n o y and G. D. Vovchenko. V I N I T I 842-77, 1977. V . I . L i k h t m a n , E. D. S h c h u k i n a n d P. A. Rebinder, Physico-chemieal Mechanics of Metals, N a u k a , Moscow, 1962. Yu. M. Maksimov, V. E. K a z a r i n o v and O. A. Petriy, Elektrokhim@a, 8 (1972) 1254. A . A . S u t y a g i n a , T. V. K u l c h i t s k a y a and G. D. V o v c h e n k o , Elektrokhimiya. 13 (1977) 1757. L . F . Kozin, Tr. Inst. Khim. Nefti Prir. Solei, Akad. Nauk Kaz. S.S.R., 9 (1962) 104. A . A . S u t y a g i n a , T. V. K u l c h i t s k a y a and G. D. V o v c h e n k o , V I N I T I 3005-77, 1977. A . A . S u t y a g i n a , V. I. Fadeeva, I. N. G o l y a n i t s k a y a and G. D. V o v c h e n k o , Zh. Fiz. Khim., 45 (1971) 1782. A . A . S u t y a g i n a , T. V. K u l c h i t s k a y a a n d G. D. V o v c h e n k o . Elektrokhim@a, 12 (1976) 1171. A . A . S u t y a g i n a . N. I. Ilchenko, M. I. S e m e n e n k o and G. D. Vovchenko, Elektrokhimiya, 15 (1979) 1094. A . A . S u t y a g i n a , N. I. Ilchenko, M. N. S e m e n e n k o a n d G. D. V o v c h e n k o , Zh. Fiz. Khim., 13 (1979) 1865. A . N . Maltsev, N. I. Kobozev and L. V. V o r o n o v a , Zh. Fiz. Khim., 42 (1968) 1967. A . A . S u t y a g i n a . T. V. K u l c h i t s k a y a , V. K. P o r t n o y and G. D. V o v c h e n k o , Vestn. Mosk. Univ., Khirn., 19 (1978) 165. A . A . S u t y a g i n a , T. V. K u l c h i t s k a y a , A. D. Aliyev and G. D. V o v c h e n k o , Elektrokhim@a, 14 (1978) 100.
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THE E F F E C T S OF THE NATURE A N D THE STRUCTURE OF PLATINUM GROUP CATALYSTS ON THEIR POISONING A N D REGENERATION
A. A. SUTYAGINA and G. D. VOVCHENKO Department of Chemistry, Moscow State University, 117234 Moscow (U.S.S.R.)
(Received September 22, 1980)
Summary The poisoning of platinum group catalysts (platinum, rhodium, palladium, ruthenium, osmium and Rh-Os alloys) with mercury, sulphur and lead was investigated. A number of examples were given to show that the capability of being poisoned depends not only on the nature of the catalyst but also on its structure and surface morphology. We established that chemical compounds are formed when platinum, rhodium and palladium catalysts are poisoned, whereas no such interaction takes place when ruthenium and osmium catalysts are poisoned. The role of structure manifests itself in the poisoning of mixed catalysts, i.e. Rh-Os alloys. Differences in the type of interaction and the distribution of the poison over the surface, which are determined by the type of catalyst and its structure, were revealed in the poisoning of platinum and rhodium catalysts with sulphur and lead. Electrochemical methods such as anodic polarization at specific potentials and anodic-cathodic cyclic polarization in 0.1 N H2SO 4 solutions (with electrolyte replacement) were found to be effective in the regeneration of poisoned catalysts. The choice of the method was determined by the type of catalyst and the type of poison, by the interaction of the poison with the catalyst surface which is dependent on the structure and by the behaviour of the poison during the electrochemical t r eat m ent of the catalyst.
1. I n t r o d u c t i o n Problems of cur r ent interest in theoretical investigations of catalytic phenomena include the effect on the catalyst and the catalytic process of additives which, depending on the nature of their action, are called contact poiso.ns or promoters. In recent years, modification of the catalyst surface by cation adsorption has been widely used in catalytic processes carried out in the liquid phase. The phenomenon of surface modification was first reported by Roginsky [1] who showed that the same substance can act as both poison 0376-4883/81/0000-0000/$02.50
~ Elsevier Sequoia/Printed in The Netherlands
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and p r o m o t e r depending on its c o n c e n t r a t i o n . He concluded t h a t this action was incompatible with the t h e o r e t i c a l description of the poisoning of h o m o g e n e o u s and h e t e r o g e n e o u s surfaces in terms of the simple blocking of active centres. When the additives act on the c a t a l y s t a more f u n d a m e n t a l c h a n g e takes place in its surface as well as in its adsorptive and c a t a l y t i c properties. The a d s o r p t i o n of different atoms and ions on platinum group metals, and the effect of these adsorbents on the s t r u c t u r e of the electrical double layer, the e n e r g y state of the surface, the a d s o r p t i o n of h y d r o g e n and o r g a n i c substances, and the c a t a l y t i c properties of the metals have been reviewed by a n u m b e r of workers [2 - 7]. It has been noted t h a t when cations are adsorbed the a d s o r p t i o n of h y d r o g e n is m a r k e d l y reduced and the s t r e n g t h of its bonding with the surface changes. The c a t a l y s t also acquires new properties with respect to oxygen adsorption. Changes in these parameters also affect the rate and the selectivity of the e l e c t r o c a t a l y t i c processes. However, it should be noted that, while some progress has been made in u n d e r s t a n d i n g the mechanisms of c a t a l y s t modification, c a t a l y s t poisoning and r e g e n e r a t i o n has received little a t t e n t i o n recently. Nevertheless, it has been established t h a t when the m e c h a n i s m s of' c a t a l y s t poisoning are studied the n a t u r e of the catalyst, the state of its surface and the c h a r a c t e r of the d i s t r i b u t i o n and i n t e r a c t i o n of the poison with the surface should be t a k e n into account. If this is not done it is impossible to explain the complicated mechanisms of c a t a l y s t poisoning or to d e t e r m i n e reliable methods of c a t a l y s t r e g e n e r a t i o n . W h e n the influence of adatoms on the e l e c t r o c a t a l y t i c properties of metal catalysts is studied it is often assumed t h a t they are uniformly distributed over the surface, and the possibility t h a t they may p e n e t r a t e into the lattice or i n t e r a c t with the c a t a l y s t atoms and the differences in the c h a r a c t e r of c r y s t a l l i z a t i o n on the surface are not t a k e n into account. Data have been published [3 - 9] which confirm t h a t the a d s o r p t i o n of cations is a c c o m p a n i e d by the f o r m a t i o n of strong c h e m i s o r p t i o n bonds with the metal. The results of the i n v e s t i g a t i o n of the poisoning of platinum, rhenium, palladium, r u t h e n i u m and osmium with mercury, s u l p h u r and lead t h a t are r e p o r t e d in this paper show t h a t the state of the poison, the s t r e n g t h of the poison c a t a l y s t bond and the distribution of the poison over the surface are d e t e r m i n e d not only by the n a t u r e of the poison and the c a t a l y s t but also by the c a t a l y s t structure. Both the fine s t r u c t u r e (lattice defects, microstresses, block sizes etc.) and the m o r p h o l o g y (crystal type, presence of macrodefects such as c r a c k s etc.) of the c a t a l y s t may affect poisoning. These questions have not been a d e q u a t e l y studied. An investigation of the b e h a v i o u r of the adatoms d u r i n g the o p e r a t i o n of a specific c a t a l y s t u n d e r different conditions (potential, medium etc.) makes it possible to find the limits of applicability of a given additive as well as to d e t e r m i n e methods of r e g e n e r a t i n g the c a t a l y s t and removing the undesirable substances causing its poisoning.
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2. E x p e r i m e n t a l details The i n v e s t i g a t i o n was c o n d u c t e d on c a t a l y s t s deposited on p l a t i n u m from 1% solutions of the c o r r e s p o n d i n g salts at different c u r r e n t densities i k. The s t r u c t u r e of the c a t a l y s t surfaces was examined using a JSM-V3 s c a n n i n g microscope. The i n t e r a c t i o n between the poison and the catalysts, the particle size and the crystal s t r u c t u r e of the poison were investigated using X-ray phase analysis, e l e c t r o n diffraction e x a m i n a t i o n and electron m i c r o s c o p y t o g e t h e r with local X-ray spectral analysis. The d a t a on the state of the poison on the surface and its e l e c t r o c h e m i c a l b e h a v i o u r were o b t a i n e d from p o t e n t i o d y n a m i c c u r r e n t - p o t e n t i a l (I-Or) curves m e a s u r e d using a P-5848 potentiostat. A c o m p a r a t i v e estimate of the s t r u c t u r a l defects was made using the thermo-e.m.f, m e t h o d [10, 11], and the q u a n t i t y of poison on the surface was d e t e r m i n e d by chemical methods. C u m u l a t i v e poisoning of the c a t a l y s t s with m e r c u r y , lead and s u l p h u r was carried out using solutions of the a p p r o p r i a t e c o n c e n t r a t i o n s of HgC12, Pb(NO3) 2 and t h i o u r e a in 0.1 N H2SO ~. The poisons were deposited at a p p r o p r i a t e values of ~b,. and V or by c u r r e n t polarization. The ability of the c a t a l y s t to adsorb h y d r o g e n and its activity in the e l e c t r o r e d u c t i o n of a n u m b e r of compounds (nitromethane, a c e t o n i t r i l e and a c e t o a c e t i c ether) were used as criteria for the degree of poisoning.
3. R e s u l t s and d i s c u s s i o n Rhodium and platinum catalysts with two different s t r u c t u r e s were investigated. At small values of ik (up t o 2 mA cm 2) compact light-grey deposits with surfaces formed by " s o m a t o i d s " were obtained. At high values of ik (15 - 20 mA c m - 2) loose dark deposits with surfaces composed of dendrites or fine needle-like crystals were formed [12]. M i c r o p h o t o g r a p h s show t h a t t h e r e are microdefects (cracks) on the surfaces of compact deposits of both p l a t i n u m and rhodium. A 1% PdC12 solution acidified with HC1 to pH 1 was used to obtain the palladium deposits. Compact deposits of palladium were obtained at ik = 0.5 mA cm 2, and loose deposits were o b t a i n e d at ik = 6 m A c m 2 [13]. The thermo-e.m.f, d a t a showed t h a t the lattices of both p l a t i n u m [11] and r h o d i u m [14] deposited at positive potentials (small ik) c o n t a i n e d m a n y defects. X-ray investigations revealed a high dispersivity and the presence of microstresses in such electrodeposited catalysts. Changes in the s t r u c t u r e produced a c h a n g e in the true surface area of the c a t a l y s t s as m e a s u r e d by h y d r o g e n adsorption. For the same q u a n t i t y of metal the surface area is g r e a t e r in catalysts with the loose s t r u c t u r e . The q u a n t i t y of adsorbed h y d r o g e n was estimated by electrochemical m e t h o d s [15]. The i n v e s t i g a t i o n of the m e c h a n i s m s of m e r c u r y poisoning of electrodeposited p l a t i n u m (Pt/Pt), r h o d i u m (Rh/Pt) and palladium (Pd/Pt) c a t a l y s t s showed t h a t chemical compounds are formed. The diffraction lines cor-
260
r e s p o n d i n g to the c h e m i c a l c o m p o u n d s RhHg~÷~, w h e r e 0 < x ~< I [14], and P t H g 4 [16] a p p e a r e d in the s p e c t r a o b t a i n e d from loose samples w h e n a b o u t 60% - 80% of the surface was poisoned. The p e r c e n t a g e p o i s o n i n g of the s u r f a c e was e s t i m a t e d by a s s u m i n g t h a t one m e r c u r y a t o m is adsorbed on one a t o m of the surface metal. As the degree of p o i s o n i n g increased, the i n t e n s i t y of the diffraction lines increased. The i n t e r a c t i o n of the poison w i t h the c o m p a c t c a t a l y s t s is a m o r e c o m p l i c a t e d process. C h e m i c a l c o m p o u n d s of the M e H g 4 type were o b s e r v e d only w h e n the p o i s o n i n g was excessive (200% and more). H o w e v e r , the c o m p o u n d P t H g 2 was formed on p l a t i n u m w h e n 50% - 60°/, of the s u r f a c e was poisoned [17]. The role of s t r u c t u r e in p o i s o n i n g was even m o r e m a r k e d for rhodium. W h e n 30% - 40% of the s u r f a c e of c o m p a c t r h o d i u m was poisoned the intensitY of the diffraction lines changed. At the o n s e t of p o i s o n i n g t h e r e was no c h e m i c a l i n t e r a c t i o n and m e r c u r y p e n e t r a t e d into the r h o d i u m bulk. This c a n be e x p l a i n e d [14] by the p r e s e n c e of a l a r g e r n u m b e r of defects, m i c r o s t r e s s e s and m i c r o c r a c k s t h a n in the r h o d i u m with the loose s t r u c t u r e . This c o n c l u s i o n is based on c h a n g e s o b s e r v e d in the lattice period of the c o m p a c t r h o d i u m deposits. The p e n e t r a t i o n of' m e r c u r y into the c r y s t a l l a t t i c e and the possibility of the f o r m a t i o n of solid s o l u t i o n s is also i n d i c a t e d by the c h a n g e s o b s e r v e d in the lattice period. No such effect was o b s e r v e d in the r h o d i u m c a t a l y s t with the loose s t r u c t u r e . The p e n e t r a t i o n of m e r c u r y into the b u l k c o m p e t e s w i t h the f o r m a t i o n of i n t e r m e t a l l i c c o m p o u n d s on the surface. The diffusion and m i g r a t i o n of the m e r c u r y depends on the m i c r o r e l i e f of the surface [18]. W h e n p a l l a d i u m c a t a l y s t s are poisoned with m e r c u r y the c o m p o u n d P d H g is fbrmed [13]. T I (mA)
(a)
/~
05
I0
15 ~r (V)-
I(mA) 30
1' 5' t.'
(b) 0
If)
0s
2'
l,'
f
Io
15 ~,(v}-"
Fig. I. Anodic potentiodynamic ! @,. curves for Rh/Pt e]ectrodes: (a) ik
2 mA cm 2: (b)
15 mA em 2, Curves 1 and 1', unpoisoned electrodes; curves 2, 2', 3, 3'. 4, 4' and 5', removal of the catalyst poison in successive steps. A potential scanning velocity of 3.33 mV s i was used. ik =
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F i g u r e s l(a) and l(b) show the anodic p o t e n t i o d y n a m i c I - G c u r v e s o b t a i n e d in a 0.1 N H2SO 4 s o l u t i o n for R h / P t e l e c t r o d e c a t a l y s t s deposited at ik = 2 mA c m - 2 and at ik = 15 mA c m - 2 respectively. C u r v e s 1 and 1' c o r r e s p o n d to u n p o i s o n e d electrodes, and c u r v e s 2 a n d 2' were o b t a i n e d i m m e d i a t e l y a f t e r m e r c u r y p o i s o n i n g (95% poisoning). A f t e r poisoning, the a d s o r p t i o n of h y d r o g e n and o x y g e n is suppressed. The c o m p a c t c a t a l y s t s lose t h e i r ability to a d s o r b h y d r o g e n at a l o w e r p e r c e n t a g e p o i s o n i n g t h a n the loose c a t a l y s t s do. F i g u r e 1 shows t h a t the s h a p e of the c u r v e s changes. S e v e r a l new c u r r e n t p e a k s c o r r e s p o n d i n g to the o x i d a t i o n of m e r c u r y c a n be seen in c u r v e s 2 and 2' a n d are i n d i c a t i v e of a r a t h e r b r o a d s p e c t r u m of m e r c u r y s t a t e s and differences in the s t r e n g t h of the b o n d i n g with r h o d i u m surface. The first p e a k at 0.7 - 0.8 V, w h i c h h a s a m u c h g r e a t e r h e i g h t for the c o m p a c t deposits t h a n for the loose deposits, is a t t r i b u t e d [19] to the d i s s o l u t i o n of the free m e r c u r y p r e s e n t on the s u r f a c e in the form of droplets. This p e a k is n o t o b s e r v e d on r e p e a t c u r v e s or c u r v e s o b t a i n e d for r h o d i u m w h i c h h a s been k e p t in a desiccator. The second p e a k on c u r v e s 2 a n d 2' w h i c h o c c u r s at 0.9 - 1.2 V c o r r e s p o n d s to the o x i d a t i o n of m e r c u r y t h a t h a s b e e n a d s o r b e d on the r h o d i u m and h a s p a r t i a l l y p e n e t r a t e d into the bulk. T h e third p e a k at 1.3 - 1.6 V w h i c h h a s a l a r g e r a r e a c o r r e s p o n d s to the o x i d a t i o n of m e r c u r y w h i c h h a s formed c h e m i c a l c o m p o u n d s . The difference in the a r e a s u n d e r this p e a k for loose and c o m p a c t r h o d i u m with the s a m e degree of p o i s o n i n g i n d i c a t e s t h a t the d e n d r i t i c r h o d i u m deposit is m o r e c h e m i c a l l y a c t i v e in r e a c t i o n s w h i c h form s u r f a c e c o m p o u n d s with m e r c u r y . This is confirmed by the X-ray p h a s e a n a l y s i s d a t a [14]. Investig a t i o n s of c h a n g e s in t h e s e c h e m i c a l c o m p o u n d s w i t h time w h i c h were m a d e u s i n g a n o d i c or cyclic a n o d i c - c a t h o d i c I--Or c u r v e s shows t h a t the third p e a k decreases. This i n d i c a t e s a c o m p a r a t i v e l y low stability. The decomp o s i t i o n of the c h e m i c a l c o m p o u n d s a p p e a r s to be due m a i n l y to the processes of m e r c u r y diffusion and m i g r a t i o n into the bulk. Cyclic anodic c a t h o d i c p o l a r i z a t i o n w i t h r e p l e n i s h m e n t of the elect r o l y t e (0.1 N H2SO4) a f t e r the anodic c u r v e h a s been o b t a i n e d can be used to r e g e n e r a t e Pt/Pt, R h / P t and P d / P t electrode c a t a l y s t s since the quantit:¢ of m e r c u r y b o t h on the surface and in the b u l k d e c r e a s e s d u r i n g the cycle. It h a s p r e v i o u s l y b e e n e s t a b l i s h e d [20] t h a t m e r c u r y is r e m o v e d m o r e easily from c a t a l y s t s w i t h a loose s t r u c t u r e t h a n from c a t a l y s t s with a c o m p a c t s t r u c t u r e . The r e m o v a l of m e r c u r y by the anodic p o l a r i z a t i o n of electrodes in a n acid s o l u t i o n in the p o t e n t i a l r a n g e 1.0 - 1.5 V p r o v e s to be even m o r e effective d e p e n d i n g on the degree of poisoning, the n a t u r e of the m e t a l and the s t r u c t u r e . In view of the fact t h a t p l a t i n u m , r h o d i u m , palladium, r u t h e n i u m a n d o s m i u m differ in t h e i r ability to a m a l g a m a t e with and dissolve in m e r c u r y [21] it is of i n t e r e s t to c o m p a r e the d a t a r e p o r t e d in the f o r e g o i n g with the r e s u l t s of i n v e s t i g a t i o n s of r u t h e n i u m and osmium. The n a t u r e of the m e t a l was found to e x e r t a m a r k e d influence on the p o i s o n i n g m e c h a n i s m . Merc u r y a d s o r b e d on r u t h e n i u m and o s m i u m is c h a r a c t e r i z e d by a g r e a t e r h o m o g e n e i t y of states. The first and the second p e a k s on the I-(/~,. c u r v e s due
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to m e r c u r y o x i d a t i o n are not clearly defined, and the third p e a k is absent. No c h e m i c a l i n t e r a c t i o n s h a v e been d e t e c t e d by X-ray p h a s e a n a l y s i s [22, 23]. The s t r e n g t h of the m e r c u r y - - m e t a l bond d e c r e a s e s in the o r d e r H g Pd ~> Hg Pt > H g - - R h > H g - - R u ~ H g - - O s . The ability to form s u r f a c e c o m p o u n d s decreases in the s a m e order. T h e r e f o r e p o i s o n i n g of r u t h e n i u m and o s m i u m t a k e s place m o r e slowly t h a n t h a t of palladium, p l a t i n u m and rhodium. The role of s t r u c t u r e in p o i s o n i n g processes is of p a r t i c u l a r i n t e r e s t w h e n m i x e d - c a t a l y s t alloys are used. Alloys of the R h - O s system with a b r o a d r a n g e of c o m p o s i t i o n s w h i c h were o b t a i n e d by e l e c t r o d e p o s i t i o n h a v e been i n v e s t i g a t e d [24], and it was s h o w n t h a t t h e i r s u s c e p t i b i l i t y to poisoning depends not only on the o s m i u m c o n t e n t (osmium does not i n t e r a c t with m e r c u r y ) but also on the c a t a l y s t s t r u c t u r e . C a t a l y s t s with a loose s t r u c t u r e and a low o s m i u m c o n t e n t are less s u s c e p t i b l e to p o i s o n i n g t h a n c a t a l y s t s with a c o m p a c t s t r u c t u r e and a high o s m i u m content. We also i n v e s t i g a t e d the p o i s o n i n g of p l a t i n u m and r h o d i u m c a t a l y s t s with the less mobile poisons s u l p h u r and lead. Possible r e g e n e r a t i o n m e t h o d s were also examined. We found t h a t , as in the case of m e r c u r y , the ability to adsorb h y d r o g e n was inhibited at a h i g h e r p e r c e n t a g e p o i s o n i n g on r h o d i u m and p l a t i n u m deposits w i t h a loose s t r u c t u r e t h a n on deposits with a compact structure. E l e c t r o n diffraction e x a m i n a t i o n and m e a s u r e m e n t of the I-q5 r c u r v e s showed t h a t t h e r e were s u b s t a n t i a l differences in the s t a t e of the s u l p h u r deposited on the surfaces of the two metals. On p l a t i n u m m o r e of the s u l p h u r was in the a d s o r b e d form and some of it was c o m b i n e d in the sulphide PtS 2. The o x i d a t i o n c u r r e n t s t h a t a p p e a r e d on the curves for r h o d i u m poisoned with s u l p h u r were small c o m p a r e d with those for sulphurpoisoned p l a t i n u m and indicate t h a t the R h - - S bond is s t r o n g e r t h a n the Pt S bond. The sulphide Rh2S 3 was detected on the r h o d i u m surface. Cyclic anodic c a t h o d i c p o l a r i z a t i o n p r o v e d to be an efficient m e t h o d of r e g e n e r a t i o n w h i c h c o m p l e t e l y r e s t o r e d the p r o p e r t i e s of p l a t i n u m poisoned with sulphur. By using this m e t h o d it was possible to oxidize m o s t of the s u l p h u r to s u l p h a t e in the anodic cycle and to r e d u c e the sulphide in the c a t h o d i c cycle [25] so t h a t both the a d s o r p t i v e and the c a t a l y t i c p r o p e r t i e s of the p l a t i n u m were restored. This m e t h o d p r o v e d to be less efficient for r h o d i u m ; a significant a m o u n t of poison r e m a i n e d on the s u r f a c e and the ability to adsorb h y d r o g e n was r e s t o r e d by only 30% - 50% [26]. S e p a r a t e anodic and c a t h o d i c p o l a r i z a t i o n of poisoned electrodes u s i n g c u r r e n t s of different values proved to be an inefficient m e t h o d of r e m o v i n g the poisons in all cases. The role of the s t r u c t u r e and the m o r p h o l o g y of the s u r f a c e was also studied and is exemplified by the p o i s o n i n g of r h o d i u m c a t a l y s t s with lead. Lead p o i s o n i n g c a n be used to e s t i m a t e the n u m b e r of a c t i v e c e n t r e s in c a t a l y s t s [27]. It has been e s t a b l i s h e d [28] t h a t no c h e m i c a l i n t e r a c t i o n s t a k e place b e t w e e n the lead deposits and rhodium. L e a d and t e t r a g o n a l lead oxide ( P b Q , ) p h a s e s were found on the surface which, in a g r e e m e n t with
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the results of ref. 27, indicates the presence of three-dimensional crystallization in the lead deposits. E x a m i n a t i o n u s i n g electron m i c r o s c o p y and local X-ray spectral analysis shows t h a t c r y s t a l l i z a t i o n leads to a n o n - u n i f o r m d i s t r i b u t i o n of lead phases over the surface w h i c h depends on the surface m o r p h o l o g y [29]. Differences in the s t r u c t u r e and the m o r p h o l o g y of the r h o d i u m surface do not affect the c o m p o s i t i o n of the deposit phases, but the lead phases a p p e a r at a lower (about 20%) p e r c e n t a g e poisoning on c o m p a c t electrodes t h a n on electrodes with a loose s t r u c t u r e (about 65% poisoning) [28].
4. C o n c l u s i o n The a p p l i c a t i o n of the p o i s o n i n g m e t h o d to determine the conc e n t r a t i o n of active centres on the surface of c a t a l y s t s is of limited use, and the chemical and physical properties of the c a t a l y s t should be t a k e n into a c c o u n t w h e n it is applied. The limitations on this m e t h o d are imposed by the n o n - u n i f o r m d i s t r i b u t i o n of the poison owing to its chemical i n t e r a c t i o n s with the surface, its p e n e t r a t i o n into the bulk, its c r y s t a l l i z a t i o n etc., i.e. by factors t h a t are not directly c o n n e c t e d with the d i s t r i b u t i o n of active centres on the c a t a l y s t surface. W h e n the m e c h a n i s m s of poisoning and the n a t u r e of the i n t e r a c t i o n s of poisons with c a t a l y s t surfaces are i n v e s t i g a t e d the m o r p h o l o g y and the fine s t r u c t u r e of the metal w h i c h affect the n a t u r e of the d i s t r i b u t i o n and the i n t e r a c t i o n of the poison with the c a t a l y s t should be t a k e n into a c c o u n t in addition to its chemical nature.
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