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X-ray photoelectron spectroscopy study of supported noble metalcatalysts prepared by spontaneous electrodeposition
Applied Catalysis, 61 (1990) 63-73 Elsevier Science Publishers B.V., Amsterdam -
63 Printed in The Netherlands
X-ray Photoelectron Spectroscopy Study of Supported Noble Metal Catalysts Prepared by Spontaneous Electrodeposition BOJANA D. ALEKSIC*, BOGDAN R. ALEKSIC and BRANISLAV 2. MARKOVIC Institute of Chemistry, Technology and Metallurgy-Znstitute of Catalysis and Chemical Engineering, NjegoSeva 12 YU-11000 Belgrade (Yugoslavia) TSVETANA S. MARINOVA and KRASIMIR L. KOSTOV Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, BG-1040 Sofia (Bulgaria) and DESANKA i. SUSNJEVIC Institute of Physical Chemistry, Faculty of Science, University of Belgrade, P.O. Box 550, YU-11000 Belgrade (Yugoslavia) (Received 17 March 1989, revised manuscript received 29 November 1989)
ABSTRACT X-ray photoelectron spectroscopy (XPS) was used to study the chemical composition of the noble metal coatings deposited on metallic supports under various conditions of the spontaneous (electrodeposition) applied in preparation of catalysts for the purification of enamelling furnace exhaust gases. The effect of the sequence of platinum and palladium deposition from separate baths on the noble metals content in the surface layers of the catalysts was observed and compared with the chemical composition of the coatings deposited from the bath with mixed noble metal solution. Addition of surfactant to the electrolyte resulted in a change in the relative concentrations of noble metals in the coatings formed by successive deposition. The presence of lead ions in the first electrolytic bath inhibited the second noble metal deposition under the electroplating conditions applied. Key words: exhaust gases, hydrocarbon oxidation, palladium-platinum/iron, (electrodeposition), catalyst characterization (XPS ).
catalyst preparation
INTRODUCTION
Deposition of noble metals onto metallic supports by spontaneous electrodeposition as a method of preparing catalysts is a technique as yet insufficiently defined. Numerous parameters could affect the properties of the catalytic layer so formed. The situation seems to be even more intricate if the layer
0166-9834/90/$03.50
0 1990 Elsevier Science Publishers B.V.
64
is to be composed of two noble metals in a predetermined weight ratio as is the case with metallic catalysts for the oxidation vapours in enamelling furnaces. The lack of the literature on this type of catalyst is evident. On the basis of published data on the oxide-supported noble metal catalysts used for hydrocarbon oxidation in waste gases, it may be considered that in the air-rich environment in the furnace at a temperature of about 850 K both palladium and platinum will exert a high oxidation activity, but they will differ in thermal stability and resistance to poisons [ 11.Thus the chemical composition of the noble metal mixture will affect the working life of the catalyst and the economy of the enamelling process. During our previous investigations of the metallic catalysts for enamelling furnace exhaust gas purification, some of the conditions of the catalyst preparation by spontaneous electrodeposition, such as the nature and the concentration of noble metal precursors in the electrolyte, pretreatment of the catalyst support, the temperature and the time of deposition, were determined [ 2,3]. The objective of the present research was to use X-ray photoelectron spectroscopy (XPS) for acquiring information about the effects of the sequence of the noble metal deposition, and of the presence of some additives such as surfactants of foreign metal ions in the electrolyte, on the chemical composition of the catalytic layer formed by spontaneous electrodeposition of palladium and platinum on a stainless-steel support. EXPERIMENTAL
Preparation of the catalyst Noble metals were deposited by immersing the ribbon shaped samples (0.2 cmx0.5 cmx 10 cm) of the domestic stainless steel, corresponding to DIN quality X 5 Cr Ni 18 9, into separate baths of aqueous solution of palladium chloride or platinum chloride, or into the bath containing mixed electrolyte, for 15 minutes at 60 ‘C; concentration of the noble metal in the electrolyte was 1 g/l. The support samples were previously cleaned by immersion in 2 M potassium hydroxide solution at 80’ C and rinsed with hot bid&tilled water. After plating, the samples were rinsed by bi-distilled water until1 the reaction on chlorine ions was negative and dried in a stream of hot air. In order to study the effect of surfactant in the electrolytic bath, the analogous series of catalyst samples were prepared from the noble metal solutions to which 0.001 wt.-% of Lisapol [trade name of ethylene oxide (nonyl) phenyl] was added. The effect of a foreign metal was examined by adding lead in the form of lead acetate at a concentration of 0.3 g/l. The reproducibility of catalyst preparation was checked by cyclic voltammetry on the basis of the corresponding peaks in the hydrogen region. The deviation found was within 5% for ten samples prepared under the same conditions [ 41. The investigated sam-
65 TABLE 1 Survey of prepared catalysts and the sequence of noble metal deposition from various electrolytic baths Symbols: + is the sequence of immersion in separate baths, metals in brackets are the bath with the additive or with the mixed electrolyte, Lis is Lisapol and AC represents lead acetate. Sample
Deposed metals
Sample
Deposed metals
M-l M-2 M-3 M-4 M-5 M-6 M-7 M-8 M-9 M-10
Pd Pt Pd-Pt Pt+Pd (Pd+Pt) (Pd+Lis) (Pt+Lis) (Pd+Lis)+Pt Pd- (Pt+Lis) (Pt+Lis)+Pd
M-11 M-12 M-13 M-14 M-15 M-16 M-17 M-18 M-19
Pt-t (Pd+Lis) (Pd+Pt+Lis) (Pd+Ac) (Pt+Ac) (Pd+Ac)APt Pd- (Pt+Ac) (Pt+Ac)+Pd Pt+ (Pd+Ac) (Pd+Pt+Ac)
ples, the sequence of noble metal deposition and the additives are shown in Table 1. XPS analysis The XPS analysis was performed by means of Escalab II spectroscope at a pressure of about 5.3. lo-’ Pa using Mg Kcwradiation with a photon energy of 1253.6 eV and a total instrumental resolution of 1 eV. An argon ion gun was used, for two minutes, to clean the catalyst surface, and for an extended period of time when the analysis of the noble metal content at various depths of the catalytic layer was performed. Experimental conditions for the XPS analysis and the determinations of particular elements in the catalytic layer are as previously described [ 51. Spectra of Pd3d,,,, PMf,,,, Fe2p,,,, Cr2p,,,, Ni2p,,,, C Is, Pb4f,,,, Cl 2p and 0 1s were recorded in order to evaluate the noble metal coverage on the surface of the support. RESULTS AND DISCUSSION
Deposition of a single noble metal The metal concentrations on the surface of the catalysts formed by deposition of either palladium or platinum from the corresponding electrolyte with or without added surfactant are summarized in Table 2. The samples are marked as shown in Table 1. The values given relate to the catalyst surface after argon ion bombardment.
66 TABLE
2
Surface composition of catalysts formed with a single noble metal, in at % Sample
Cl
C
0
Cr
Fe
Pt
Pd
Ni
M-l M-2
3 6
-
4 6.3
5.6 14.7
69.4
87.3 -
3.5
M-6 M-7
-
31.3
_ _
5.4
2 14.5
48.3
98 -
0.5
-
Pd 3d
.jrlru85 BINDING
ENERGY
(eV1
305
/c-__‘J
BINDING
PdO ENERGY
CeVl
355
Fig. 1. Pd 3d and Pt 4f regions of XPS spectra of samples M-l and M-2, respectively; (a) before cleaning the surface and (b) after cleaning.
Before cleaning, both the carbon and the oxygen contents varied between 30 and 60 at .-% whilst after cleaning these elements were not found in any of the samples formed in electrolytes without additives as shown by the XPS spectra taken in the region of Pd 3d (sample M-l ) and Pt 4f (sample M-l) (Fig. 1). It is noteworthy that in the deposited layer platinum was present before cleaning in metallic form only whilst palladium was detected both in metallic and oxide forms. In some of the samples prepared with the addition of Lisapol, carbon was present after argon cleaning which indicates that washing of samples after electrolysis was not always sufficient to remove the surface-active compounds. More rigorous washing was not applied to avoid possible damage to the deposited layer. Therefore the conclusion about the degree of support covering by noble metals on these samples may be drawn only approximately on the basis of detected elements which originated from the steel support. The results obtained prove the high degree of covering. Palladium, as compared to platinum, was spread over the support to a greater extent under the same experimental conditions (Table 2, samples M-l and M-2). This is in agreement with the reported phenomenon [6] that deposition of palladium from chloride solutions of the two noble metals was favoured to certain degree.
61
Adding Lisapol resulted in some positive effects on the degree of support covering by palladium (sample M-6) although it was already high without additives in the electrolyte (sample M-l). This is in agreement with the results obtained previously by electron microscopy: Lisapol affected the dispersity and the homogenity of the deposited metal layer [ 31. The surfactants added to the electrolyte may produce the following effects during the electroless metal deposition: firstly, they provide better wetting of the metal substrate by the electrolyte resulting in a homogeneous distribution of the deposited metal, and secondly, their destructive adsorption on the support might inhibit the dissolution of metals from the support as well as the deposition of noble metals from the electrolyte. The first effect was the reason for adding Lisapol and it was more apparent during the catalyst preparation. The base metal content in the sample M-7 being in the range of the sample M-2 indicate that deposition of platinum was not affected by Lisapol, if it is assumed that carbon from Lisapol covered part of the platinum layer surface in the sample M-7. Deposition of two noble metals Results obtained by XPS analysis of the surface layer of catalysts formed with two noble metals are given in Table 3. Formation of the catalytic layer from two noble metals confirmed the complexity of the process. When the successive procedure was applied the first metallic layer was not covered completely by the second metal (samples M-3 and M-4); from the mixed bath platinum was deposited later and was found in a higher concentration in the external part of the coating (sample M-5). This may be convenient for the starting activity of the catalyst but abrasion during the prolonged work in the enamelling furnace may cause a faster activity decay. Measurements of the catalytic activity of the prepared samples in the reaction of isopropene oxidation by air under kinetic conditions, using a differenTABLE 3 Surface composition of catalysts formed with two noble metals, in at % Sample
Cl
C
0
Cr
Fe
Pt
M-3 M-4 M-5 M-8 M-9 M-10 M-11 M-12
5.5 5.5
10.5 18.3 26.4 -
8 _ -
3.1 _
6 8.2 5 2.6 1.4 7.6
Pd
Ni
81
13
-
28.3 72 70 79 4.4 0.6 18
54.8 9.3 25 -
0.2 -
95.6 71.6 69
-
68
tial reactor with external recycling of gaseous reactants, did not show any regularity of the dependence of the catalyst activity on the palladium and platinum ratio nor on the position of noble metals in the deposited layer. All the samples prepared had a high oxidation activity, the 50% conversion was achieved at temperatures in the range 390& 10 K and conversion higher than 95% was achieved at 400-430 K [ 21. Crosswise monit.oring of the concentrations of palladium and platinum through the coating formed by successive deposition of metals in the sample M-3 was performed by exposing the sample to the argon ion beam over several intervals of time for 20 minutes, when corresponding layers of deposited metals were removed up to the depth of 1200 nm from the exterior surface toward the catalyst support. The results are shown in Fig. 2, as are values for the coverage of the support by noble metals (0)) estimated as: 8= (apt + aPd) /lOO, where apt and apd are the relative atomic concentrations of noble metals in the observed depth (d) of the deposited layer. The thickness of the deposited coating may be defined as the intersection of the curve s representing the sum (apt+ +d) with the X-axis. Fig. 3 depicts changes in the platinum-to-palladium ratio across the deposited layers in catalysts prepared by successive or simultaneous deposition of the noble metals (samples M-3 and M-5, respectively). In both catalysts platinum was present in higher concentrations in the outer part of the layer; however, the simultaneously deposited coating had a more homogeneous chemical composition. Thus, there is a possibility that the amount and the distribution of the deposited platinum may be controlled not only by the duration of deposition but also by using the appropriate sequence of deposition. This is important in cases when the activity, the life of catalyst, and its price have to be balanced. The presence of Lisapol in the first electrolyte resulted in a somewhat greater degree of the second noble metal deposition as compared to the analogous samples prepared by successive deposition without surface active agent (Table 3,
Fig. 2. Concentration of noble metals in the deposited layer and surface coverage (f3) of the catalyst M-3; curve s represents the sum (a,,+ uFd).
69
f Pt/ Pd 301
M3
1
20
1
i
\
cI---i
’
10
7;min
Fig. 3. Atomic ratio of noble metals in the layers formed by successive and simultaneous
i
325
BINDING
ENERGY
deposition.
IA’)
Fig. 4. Depth profiling of the catalyst
M-9.
samples: M-8 and M-3, M-10 and M-4, respectively). The effects of Lisapol in the second bath used in formation of samples M-9 and M-11 was not clearly evident because of the large amount of carbon remaining on the surface, although the results given in Table 3 indicate that the second metal was predominant in these samples. To obtain some additional information, concentrations of platinum and palladium were analysed throughout the deposited layer in the sample M-9. The XPS spectra recorded at intervals of two minutes after each argon ion bombardment and presented in Fig. 4 did not reveal palladium till a depth of about 60 nm from the outside. When a higher amplification of the spectra was applied some small amounts of palladium were visible as shown in curve f of Fig. 4, representing higher amplification of the intensities from curve e. It may
70 TABLE 4 Platinum-to-palladium atomic ratio in the surface layer of the catalyst M-12, after prolonged argon ion bombardment T (min) Pt-to-Pd ratio
2 1.2
4 0.8
6 0.6
8 0.6
10 0.6
be assumed that, in the presence of Lisapol in the first bath, a smooth surface of the first noble metal was formed over which the second metal was deposited more uniformly than was the case when the first layer had a spongy structure as formed without the surface active substance. The first porous coating allowed the second metal to penetrate into the holes resulting in formation of the mixed “bimetallic” composition of the surface layer. When present in the second bath Lisapol affected the formation of a homogeneous layer of the second noble metal so the first deposed metal was less detectable by the applied XPS method. Addition of Lisapol to the mixed bath resulted in formation of coating where palladium predominated in the outer parts of the layer (sample M-12). The platinum-to-palladium weight ratio calculated on the basis of XPS spectra obtained from five subsequent levels of deposited coating after argon ion bombardment in time intervals of two minutes is given in Table 4. Relative changes in the platinum-to-palladium ratio between the five levels are similar to those of sample M-5 prepared without Lisapol (Fig. 3) but the concentration of platinum was significantly lower in the catalyst M-12. On the basis of the results obtained in a previous study on noble metal deposition in the presence of Lisapol [3] one may conclude that platinum as slightly more electropositive metal was deposited first by covering a great part of the support. It was facilitated by a better wetting with the electrolyte in the presence of the surface-active substance. Palladium was deposited later and more slowly over the layer of platinum. Deposition in the presence of a foreign metal in the electrolyte The study of the effect of a foreign metal in the electrolyte on the electroless deposition of noble metals was undertaken with lead because it was present in some commercial platinum compounds as an impurity from the production process. Lead compounds are used, also, as additives in electrolytes for various coatings formed by spontaneous electrodeposition. It was found that lead ions are not concurrently deposited with the main metal but penetrate into the holes of a spongy metallic coating, behaving like a filler. They also accelerate the formation of the depositing metal crystallization centres. The process is still empirical and not fully understood [ 71 unlike the under-potential depo-
71
sition of lead on the noble metals and the preparation of Lindlar type catalysts by wet impregnation of oxide supports, which processes have been studied by many authors. It was shown that lead added to noble metal catalysts promoted, inhibited or changed the selectivity of the catalysts depending on the means of deposition [ 81. Results of XPS analysis of samples M-13 to M-19 prepared in the presence of lead acetate in the electrolyte are summarized in Table 5. The presence of the base metals in the spectra of the samples formed by deposition of a single noble metal indicate higher covering of the support by palladium compared to platinum covering (samples M-13 and M-14, respectively ) . For the relationship with the effect of lead additional investigation of the deposited layer is necessary due to the complex structure of the coating with encrusted foreign metal. The XPS spectra of the samples M-13 and M-14 before and after cleaning are given in Fig. 5 and show that in the surface layer of palladium the presence of lead was more extensive along with the presence of noble metal as compared with the sample with platinum. An increase in the intensity of the Pb 4f peak in the catalyst M-13 after cleaning may indicate a faster encrustation of lead in the palladium layer. XPS analysis of the samples obtained by successive deposition of two noble metals (Table 5, samples M-15, M-16, M-17 and M-18) reveal unexpectedly that the second noble metal was not deposited on the coating formed in the first bath with lead acetate regardless of the nature of the noble metal in the first coating (samples M-15 and M-17). When the second bath contained lead ions both noble metals were found in the deposited coating (samples M-16 and M-17 ) palladium being predominate. From the mixed bath with lead acetate (sample M-19)) both noble metals and lead were deposited and platinum predominated as in the sample M-5 obtained without additives in the electrolyte. The explanation, which is still to be checked, might be that during the successive procedure the rate of the second noble metal deposition was reduced to a certain degree exceeding the time of the catalyst preparation run, the reducTABLE 5 Surface composition
of catalysts
formed in the presence
of lead as a foreign ion, in at %
Sample
Cl
C
0
Cr
Fe
Pt
Pd
Pb
Ni
M-13 M-14
2.9 _
16.2 36.4
-
7.6
0.7 23.6
29.9
62 _
11.1 1.5
1
M-15
3.6
19.9
_
8.2
15.3
-
41.3
11.5
0.5
M-16
-
-
-
6
1
38
57
-
M-17
-
17.0
8.5
5.6
14.3
51.7
-
M-18
7.7
-
-
1.2
2.0
30.0
43.5
2.7
15.6
0.2 -
M-l&
1000
500
0 BINDING
ENERGY
IeV:
Fig. 5. XI’S spectra of the samples prepared by deposition of a single noble metal in the presence of lead ions in the electrolyte: (a) before cleaning the surface and (b) after cleaning.
tion being caused by changes in the surface electronic properties of the first deposited noble metal layer with adsorbed lead atoms as was reported previously [9]. In the mixed bath the rate of deposition of both noble metals was higher than the rate of lead adsorption so the inhibition effect of lead was not significantly pronounced. CONCLUSION
XPS study of the noble metal coatings deposited by spontaneous electrodeposition has shown that a good coverage of the metallic support was obtained when a single metal was deposited. The effect of surfactant on the degree of support coverage was not expressive. Depth profiling of the relative concentrations of platinum and palladium in the case of coating formed with two noble metals indicated both the effect of the sequence of noble metal deposition and of the presence of surfactant in the electrolyte. Addition of Lisapol to the electrolyte may promote formation of a more homogeneous metal layer deposited first as the result of better wetting
73
and hence the more uniform contact of the electrolyte and the metallic support surface. Although the effect of lead ions as a foreign metal in the electrolyte was not clearly defined, it was observed that under the electroplating conditions applied the deposition of the second noble metal was inhibited by lead ions being present in the first electrolyte bath. ACKNOWLEDGEMENT
This work was supported in part by the Funds for Research of the Republic of Serbia (Yugoslavia).
REFERENCES 1 G.R. Lester, J.F. Brennan and J. Hoekstra in J.E. McEvoy (Ed.) Catalysts for the Control of Automotive Pollutants, Adv. Chem. Ser. 143, American Chemical Society, Washington, D.C., 1975, p. 24. 2 B.D. AleksiC, D.Z. SuinjeviC, B.Z. Markovic, V.N. Mladenovif and B. DraSkoviC, Project “Air Protection”, National Science Foundation, SR Serbia, Ann. Reports 1986/87, Belgrade. 3 D.Z. SuinjeviC, B.D. Aleksic, B.R. AleksiC, B.Z. MarkoviC and D.J. CeroviC, Surf. Coat. Techno:, 29 (1986) 123. 4 D.Z. SuinjeviC, B. KitanoviC, B.D. Aleksic and B.Z. Markovii, unpublished results. 5 B.D. AleksiC, C.S. Marinova, B.R. AleksiC, B.Z. Markovit, K.L. Kostov and V.N. Kosanic, Appl. Catal., 23 (1986) 245. 6 Ya.V. Vasner and M.A. Dacoian, Tekhnologiya elektrokhimiche-skikh pokritii, (Technology of Electra-Chemical Coatings), Mashgiz, Moskva, 1962. (in Russian) 7 N.A. Shwab in Elektrodnye processy pri elektroosazhdenii i rastvorenii metallov, (Electrode Processes in Electrodeposition and dissolution of Metals), Sb. Nauch. Trudov, Institut Obschei i neorganicheskoj khimii AN U.S.S.R., Kiev, Naukova Dumka, 1978. p. 64. (in Russian) 8 E. Lamy, Pitara L. Lghouzovani, M. Piktas and J. Barbier, Appl. Catal., 44 (1988) 261. 9 A. Palazov, Ch. Bonev, G. Kadinov and P. Shopov, J. Catal., 71 (1981) 1.
63 Printed in The Netherlands
X-ray Photoelectron Spectroscopy Study of Supported Noble Metal Catalysts Prepared by Spontaneous Electrodeposition BOJANA D. ALEKSIC*, BOGDAN R. ALEKSIC and BRANISLAV 2. MARKOVIC Institute of Chemistry, Technology and Metallurgy-Znstitute of Catalysis and Chemical Engineering, NjegoSeva 12 YU-11000 Belgrade (Yugoslavia) TSVETANA S. MARINOVA and KRASIMIR L. KOSTOV Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, BG-1040 Sofia (Bulgaria) and DESANKA i. SUSNJEVIC Institute of Physical Chemistry, Faculty of Science, University of Belgrade, P.O. Box 550, YU-11000 Belgrade (Yugoslavia) (Received 17 March 1989, revised manuscript received 29 November 1989)
ABSTRACT X-ray photoelectron spectroscopy (XPS) was used to study the chemical composition of the noble metal coatings deposited on metallic supports under various conditions of the spontaneous (electrodeposition) applied in preparation of catalysts for the purification of enamelling furnace exhaust gases. The effect of the sequence of platinum and palladium deposition from separate baths on the noble metals content in the surface layers of the catalysts was observed and compared with the chemical composition of the coatings deposited from the bath with mixed noble metal solution. Addition of surfactant to the electrolyte resulted in a change in the relative concentrations of noble metals in the coatings formed by successive deposition. The presence of lead ions in the first electrolytic bath inhibited the second noble metal deposition under the electroplating conditions applied. Key words: exhaust gases, hydrocarbon oxidation, palladium-platinum/iron, (electrodeposition), catalyst characterization (XPS ).
catalyst preparation
INTRODUCTION
Deposition of noble metals onto metallic supports by spontaneous electrodeposition as a method of preparing catalysts is a technique as yet insufficiently defined. Numerous parameters could affect the properties of the catalytic layer so formed. The situation seems to be even more intricate if the layer
0166-9834/90/$03.50
0 1990 Elsevier Science Publishers B.V.
64
is to be composed of two noble metals in a predetermined weight ratio as is the case with metallic catalysts for the oxidation vapours in enamelling furnaces. The lack of the literature on this type of catalyst is evident. On the basis of published data on the oxide-supported noble metal catalysts used for hydrocarbon oxidation in waste gases, it may be considered that in the air-rich environment in the furnace at a temperature of about 850 K both palladium and platinum will exert a high oxidation activity, but they will differ in thermal stability and resistance to poisons [ 11.Thus the chemical composition of the noble metal mixture will affect the working life of the catalyst and the economy of the enamelling process. During our previous investigations of the metallic catalysts for enamelling furnace exhaust gas purification, some of the conditions of the catalyst preparation by spontaneous electrodeposition, such as the nature and the concentration of noble metal precursors in the electrolyte, pretreatment of the catalyst support, the temperature and the time of deposition, were determined [ 2,3]. The objective of the present research was to use X-ray photoelectron spectroscopy (XPS) for acquiring information about the effects of the sequence of the noble metal deposition, and of the presence of some additives such as surfactants of foreign metal ions in the electrolyte, on the chemical composition of the catalytic layer formed by spontaneous electrodeposition of palladium and platinum on a stainless-steel support. EXPERIMENTAL
Preparation of the catalyst Noble metals were deposited by immersing the ribbon shaped samples (0.2 cmx0.5 cmx 10 cm) of the domestic stainless steel, corresponding to DIN quality X 5 Cr Ni 18 9, into separate baths of aqueous solution of palladium chloride or platinum chloride, or into the bath containing mixed electrolyte, for 15 minutes at 60 ‘C; concentration of the noble metal in the electrolyte was 1 g/l. The support samples were previously cleaned by immersion in 2 M potassium hydroxide solution at 80’ C and rinsed with hot bid&tilled water. After plating, the samples were rinsed by bi-distilled water until1 the reaction on chlorine ions was negative and dried in a stream of hot air. In order to study the effect of surfactant in the electrolytic bath, the analogous series of catalyst samples were prepared from the noble metal solutions to which 0.001 wt.-% of Lisapol [trade name of ethylene oxide (nonyl) phenyl] was added. The effect of a foreign metal was examined by adding lead in the form of lead acetate at a concentration of 0.3 g/l. The reproducibility of catalyst preparation was checked by cyclic voltammetry on the basis of the corresponding peaks in the hydrogen region. The deviation found was within 5% for ten samples prepared under the same conditions [ 41. The investigated sam-
65 TABLE 1 Survey of prepared catalysts and the sequence of noble metal deposition from various electrolytic baths Symbols: + is the sequence of immersion in separate baths, metals in brackets are the bath with the additive or with the mixed electrolyte, Lis is Lisapol and AC represents lead acetate. Sample
Deposed metals
Sample
Deposed metals
M-l M-2 M-3 M-4 M-5 M-6 M-7 M-8 M-9 M-10
Pd Pt Pd-Pt Pt+Pd (Pd+Pt) (Pd+Lis) (Pt+Lis) (Pd+Lis)+Pt Pd- (Pt+Lis) (Pt+Lis)+Pd
M-11 M-12 M-13 M-14 M-15 M-16 M-17 M-18 M-19
Pt-t (Pd+Lis) (Pd+Pt+Lis) (Pd+Ac) (Pt+Ac) (Pd+Ac)APt Pd- (Pt+Ac) (Pt+Ac)+Pd Pt+ (Pd+Ac) (Pd+Pt+Ac)
ples, the sequence of noble metal deposition and the additives are shown in Table 1. XPS analysis The XPS analysis was performed by means of Escalab II spectroscope at a pressure of about 5.3. lo-’ Pa using Mg Kcwradiation with a photon energy of 1253.6 eV and a total instrumental resolution of 1 eV. An argon ion gun was used, for two minutes, to clean the catalyst surface, and for an extended period of time when the analysis of the noble metal content at various depths of the catalytic layer was performed. Experimental conditions for the XPS analysis and the determinations of particular elements in the catalytic layer are as previously described [ 51. Spectra of Pd3d,,,, PMf,,,, Fe2p,,,, Cr2p,,,, Ni2p,,,, C Is, Pb4f,,,, Cl 2p and 0 1s were recorded in order to evaluate the noble metal coverage on the surface of the support. RESULTS AND DISCUSSION
Deposition of a single noble metal The metal concentrations on the surface of the catalysts formed by deposition of either palladium or platinum from the corresponding electrolyte with or without added surfactant are summarized in Table 2. The samples are marked as shown in Table 1. The values given relate to the catalyst surface after argon ion bombardment.
66 TABLE
2
Surface composition of catalysts formed with a single noble metal, in at % Sample
Cl
C
0
Cr
Fe
Pt
Pd
Ni
M-l M-2
3 6
-
4 6.3
5.6 14.7
69.4
87.3 -
3.5
M-6 M-7
-
31.3
_ _
5.4
2 14.5
48.3
98 -
0.5
-
Pd 3d
.jrlru85 BINDING
ENERGY
(eV1
305
/c-__‘J
BINDING
PdO ENERGY
CeVl
355
Fig. 1. Pd 3d and Pt 4f regions of XPS spectra of samples M-l and M-2, respectively; (a) before cleaning the surface and (b) after cleaning.
Before cleaning, both the carbon and the oxygen contents varied between 30 and 60 at .-% whilst after cleaning these elements were not found in any of the samples formed in electrolytes without additives as shown by the XPS spectra taken in the region of Pd 3d (sample M-l ) and Pt 4f (sample M-l) (Fig. 1). It is noteworthy that in the deposited layer platinum was present before cleaning in metallic form only whilst palladium was detected both in metallic and oxide forms. In some of the samples prepared with the addition of Lisapol, carbon was present after argon cleaning which indicates that washing of samples after electrolysis was not always sufficient to remove the surface-active compounds. More rigorous washing was not applied to avoid possible damage to the deposited layer. Therefore the conclusion about the degree of support covering by noble metals on these samples may be drawn only approximately on the basis of detected elements which originated from the steel support. The results obtained prove the high degree of covering. Palladium, as compared to platinum, was spread over the support to a greater extent under the same experimental conditions (Table 2, samples M-l and M-2). This is in agreement with the reported phenomenon [6] that deposition of palladium from chloride solutions of the two noble metals was favoured to certain degree.
61
Adding Lisapol resulted in some positive effects on the degree of support covering by palladium (sample M-6) although it was already high without additives in the electrolyte (sample M-l). This is in agreement with the results obtained previously by electron microscopy: Lisapol affected the dispersity and the homogenity of the deposited metal layer [ 31. The surfactants added to the electrolyte may produce the following effects during the electroless metal deposition: firstly, they provide better wetting of the metal substrate by the electrolyte resulting in a homogeneous distribution of the deposited metal, and secondly, their destructive adsorption on the support might inhibit the dissolution of metals from the support as well as the deposition of noble metals from the electrolyte. The first effect was the reason for adding Lisapol and it was more apparent during the catalyst preparation. The base metal content in the sample M-7 being in the range of the sample M-2 indicate that deposition of platinum was not affected by Lisapol, if it is assumed that carbon from Lisapol covered part of the platinum layer surface in the sample M-7. Deposition of two noble metals Results obtained by XPS analysis of the surface layer of catalysts formed with two noble metals are given in Table 3. Formation of the catalytic layer from two noble metals confirmed the complexity of the process. When the successive procedure was applied the first metallic layer was not covered completely by the second metal (samples M-3 and M-4); from the mixed bath platinum was deposited later and was found in a higher concentration in the external part of the coating (sample M-5). This may be convenient for the starting activity of the catalyst but abrasion during the prolonged work in the enamelling furnace may cause a faster activity decay. Measurements of the catalytic activity of the prepared samples in the reaction of isopropene oxidation by air under kinetic conditions, using a differenTABLE 3 Surface composition of catalysts formed with two noble metals, in at % Sample
Cl
C
0
Cr
Fe
Pt
M-3 M-4 M-5 M-8 M-9 M-10 M-11 M-12
5.5 5.5
10.5 18.3 26.4 -
8 _ -
3.1 _
6 8.2 5 2.6 1.4 7.6
Pd
Ni
81
13
-
28.3 72 70 79 4.4 0.6 18
54.8 9.3 25 -
0.2 -
95.6 71.6 69
-
68
tial reactor with external recycling of gaseous reactants, did not show any regularity of the dependence of the catalyst activity on the palladium and platinum ratio nor on the position of noble metals in the deposited layer. All the samples prepared had a high oxidation activity, the 50% conversion was achieved at temperatures in the range 390& 10 K and conversion higher than 95% was achieved at 400-430 K [ 21. Crosswise monit.oring of the concentrations of palladium and platinum through the coating formed by successive deposition of metals in the sample M-3 was performed by exposing the sample to the argon ion beam over several intervals of time for 20 minutes, when corresponding layers of deposited metals were removed up to the depth of 1200 nm from the exterior surface toward the catalyst support. The results are shown in Fig. 2, as are values for the coverage of the support by noble metals (0)) estimated as: 8= (apt + aPd) /lOO, where apt and apd are the relative atomic concentrations of noble metals in the observed depth (d) of the deposited layer. The thickness of the deposited coating may be defined as the intersection of the curve s representing the sum (apt+ +d) with the X-axis. Fig. 3 depicts changes in the platinum-to-palladium ratio across the deposited layers in catalysts prepared by successive or simultaneous deposition of the noble metals (samples M-3 and M-5, respectively). In both catalysts platinum was present in higher concentrations in the outer part of the layer; however, the simultaneously deposited coating had a more homogeneous chemical composition. Thus, there is a possibility that the amount and the distribution of the deposited platinum may be controlled not only by the duration of deposition but also by using the appropriate sequence of deposition. This is important in cases when the activity, the life of catalyst, and its price have to be balanced. The presence of Lisapol in the first electrolyte resulted in a somewhat greater degree of the second noble metal deposition as compared to the analogous samples prepared by successive deposition without surface active agent (Table 3,
Fig. 2. Concentration of noble metals in the deposited layer and surface coverage (f3) of the catalyst M-3; curve s represents the sum (a,,+ uFd).
69
f Pt/ Pd 301
M3
1
20
1
i
\
cI---i
’
10
7;min
Fig. 3. Atomic ratio of noble metals in the layers formed by successive and simultaneous
i
325
BINDING
ENERGY
deposition.
IA’)
Fig. 4. Depth profiling of the catalyst
M-9.
samples: M-8 and M-3, M-10 and M-4, respectively). The effects of Lisapol in the second bath used in formation of samples M-9 and M-11 was not clearly evident because of the large amount of carbon remaining on the surface, although the results given in Table 3 indicate that the second metal was predominant in these samples. To obtain some additional information, concentrations of platinum and palladium were analysed throughout the deposited layer in the sample M-9. The XPS spectra recorded at intervals of two minutes after each argon ion bombardment and presented in Fig. 4 did not reveal palladium till a depth of about 60 nm from the outside. When a higher amplification of the spectra was applied some small amounts of palladium were visible as shown in curve f of Fig. 4, representing higher amplification of the intensities from curve e. It may
70 TABLE 4 Platinum-to-palladium atomic ratio in the surface layer of the catalyst M-12, after prolonged argon ion bombardment T (min) Pt-to-Pd ratio
2 1.2
4 0.8
6 0.6
8 0.6
10 0.6
be assumed that, in the presence of Lisapol in the first bath, a smooth surface of the first noble metal was formed over which the second metal was deposited more uniformly than was the case when the first layer had a spongy structure as formed without the surface active substance. The first porous coating allowed the second metal to penetrate into the holes resulting in formation of the mixed “bimetallic” composition of the surface layer. When present in the second bath Lisapol affected the formation of a homogeneous layer of the second noble metal so the first deposed metal was less detectable by the applied XPS method. Addition of Lisapol to the mixed bath resulted in formation of coating where palladium predominated in the outer parts of the layer (sample M-12). The platinum-to-palladium weight ratio calculated on the basis of XPS spectra obtained from five subsequent levels of deposited coating after argon ion bombardment in time intervals of two minutes is given in Table 4. Relative changes in the platinum-to-palladium ratio between the five levels are similar to those of sample M-5 prepared without Lisapol (Fig. 3) but the concentration of platinum was significantly lower in the catalyst M-12. On the basis of the results obtained in a previous study on noble metal deposition in the presence of Lisapol [3] one may conclude that platinum as slightly more electropositive metal was deposited first by covering a great part of the support. It was facilitated by a better wetting with the electrolyte in the presence of the surface-active substance. Palladium was deposited later and more slowly over the layer of platinum. Deposition in the presence of a foreign metal in the electrolyte The study of the effect of a foreign metal in the electrolyte on the electroless deposition of noble metals was undertaken with lead because it was present in some commercial platinum compounds as an impurity from the production process. Lead compounds are used, also, as additives in electrolytes for various coatings formed by spontaneous electrodeposition. It was found that lead ions are not concurrently deposited with the main metal but penetrate into the holes of a spongy metallic coating, behaving like a filler. They also accelerate the formation of the depositing metal crystallization centres. The process is still empirical and not fully understood [ 71 unlike the under-potential depo-
71
sition of lead on the noble metals and the preparation of Lindlar type catalysts by wet impregnation of oxide supports, which processes have been studied by many authors. It was shown that lead added to noble metal catalysts promoted, inhibited or changed the selectivity of the catalysts depending on the means of deposition [ 81. Results of XPS analysis of samples M-13 to M-19 prepared in the presence of lead acetate in the electrolyte are summarized in Table 5. The presence of the base metals in the spectra of the samples formed by deposition of a single noble metal indicate higher covering of the support by palladium compared to platinum covering (samples M-13 and M-14, respectively ) . For the relationship with the effect of lead additional investigation of the deposited layer is necessary due to the complex structure of the coating with encrusted foreign metal. The XPS spectra of the samples M-13 and M-14 before and after cleaning are given in Fig. 5 and show that in the surface layer of palladium the presence of lead was more extensive along with the presence of noble metal as compared with the sample with platinum. An increase in the intensity of the Pb 4f peak in the catalyst M-13 after cleaning may indicate a faster encrustation of lead in the palladium layer. XPS analysis of the samples obtained by successive deposition of two noble metals (Table 5, samples M-15, M-16, M-17 and M-18) reveal unexpectedly that the second noble metal was not deposited on the coating formed in the first bath with lead acetate regardless of the nature of the noble metal in the first coating (samples M-15 and M-17). When the second bath contained lead ions both noble metals were found in the deposited coating (samples M-16 and M-17 ) palladium being predominate. From the mixed bath with lead acetate (sample M-19)) both noble metals and lead were deposited and platinum predominated as in the sample M-5 obtained without additives in the electrolyte. The explanation, which is still to be checked, might be that during the successive procedure the rate of the second noble metal deposition was reduced to a certain degree exceeding the time of the catalyst preparation run, the reducTABLE 5 Surface composition
of catalysts
formed in the presence
of lead as a foreign ion, in at %
Sample
Cl
C
0
Cr
Fe
Pt
Pd
Pb
Ni
M-13 M-14
2.9 _
16.2 36.4
-
7.6
0.7 23.6
29.9
62 _
11.1 1.5
1
M-15
3.6
19.9
_
8.2
15.3
-
41.3
11.5
0.5
M-16
-
-
-
6
1
38
57
-
M-17
-
17.0
8.5
5.6
14.3
51.7
-
M-18
7.7
-
-
1.2
2.0
30.0
43.5
2.7
15.6
0.2 -
M-l&
1000
500
0 BINDING
ENERGY
IeV:
Fig. 5. XI’S spectra of the samples prepared by deposition of a single noble metal in the presence of lead ions in the electrolyte: (a) before cleaning the surface and (b) after cleaning.
tion being caused by changes in the surface electronic properties of the first deposited noble metal layer with adsorbed lead atoms as was reported previously [9]. In the mixed bath the rate of deposition of both noble metals was higher than the rate of lead adsorption so the inhibition effect of lead was not significantly pronounced. CONCLUSION
XPS study of the noble metal coatings deposited by spontaneous electrodeposition has shown that a good coverage of the metallic support was obtained when a single metal was deposited. The effect of surfactant on the degree of support coverage was not expressive. Depth profiling of the relative concentrations of platinum and palladium in the case of coating formed with two noble metals indicated both the effect of the sequence of noble metal deposition and of the presence of surfactant in the electrolyte. Addition of Lisapol to the electrolyte may promote formation of a more homogeneous metal layer deposited first as the result of better wetting
73
and hence the more uniform contact of the electrolyte and the metallic support surface. Although the effect of lead ions as a foreign metal in the electrolyte was not clearly defined, it was observed that under the electroplating conditions applied the deposition of the second noble metal was inhibited by lead ions being present in the first electrolyte bath. ACKNOWLEDGEMENT
This work was supported in part by the Funds for Research of the Republic of Serbia (Yugoslavia).
REFERENCES 1 G.R. Lester, J.F. Brennan and J. Hoekstra in J.E. McEvoy (Ed.) Catalysts for the Control of Automotive Pollutants, Adv. Chem. Ser. 143, American Chemical Society, Washington, D.C., 1975, p. 24. 2 B.D. AleksiC, D.Z. SuinjeviC, B.Z. Markovic, V.N. Mladenovif and B. DraSkoviC, Project “Air Protection”, National Science Foundation, SR Serbia, Ann. Reports 1986/87, Belgrade. 3 D.Z. SuinjeviC, B.D. Aleksic, B.R. AleksiC, B.Z. MarkoviC and D.J. CeroviC, Surf. Coat. Techno:, 29 (1986) 123. 4 D.Z. SuinjeviC, B. KitanoviC, B.D. Aleksic and B.Z. Markovii, unpublished results. 5 B.D. AleksiC, C.S. Marinova, B.R. AleksiC, B.Z. Markovit, K.L. Kostov and V.N. Kosanic, Appl. Catal., 23 (1986) 245. 6 Ya.V. Vasner and M.A. Dacoian, Tekhnologiya elektrokhimiche-skikh pokritii, (Technology of Electra-Chemical Coatings), Mashgiz, Moskva, 1962. (in Russian) 7 N.A. Shwab in Elektrodnye processy pri elektroosazhdenii i rastvorenii metallov, (Electrode Processes in Electrodeposition and dissolution of Metals), Sb. Nauch. Trudov, Institut Obschei i neorganicheskoj khimii AN U.S.S.R., Kiev, Naukova Dumka, 1978. p. 64. (in Russian) 8 E. Lamy, Pitara L. Lghouzovani, M. Piktas and J. Barbier, Appl. Catal., 44 (1988) 261. 9 A. Palazov, Ch. Bonev, G. Kadinov and P. Shopov, J. Catal., 71 (1981) 1.