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Production of Silver Bimetallic Catalysts by Liquid-Phase Reduction
G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts II! © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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PRODUCTION OF SILVER BIMETALLIC CATALYSTS BY LIQUID-PHASE REDUCTION K.P. DE JONG and;J.W. GEUS Department of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht, The Netherlands
ABSTRACT The possibilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts are investigated. Although the method is more generally applicable, we here concentrate on one combination of metals, viz. Pt and Ag. The catalysts are-characterized by electron microscopy and infrared spectra of adsorbed CO. PtAg/Si02 catalysts are prepared using a 6 wt.% Pt/Si0 2 catalyst as the starting material. Silver was deposited selectively onto the Pt particles by reduction of Ag(NH 3); ions dissolved in an aqueous suspension of the platinum-loaded silica. Using formalin as a reducing agent bimetallic particles having sizes from 30 to 50 ~ result, whilst infrared spectroscopy indicates the platinum surface to be covered almost completely by silver. The selective deposition of Ag onto the Pt particles during preparation appears to be due to the suppression of nucleation of Ag particles in the liquid phase on the one hand, and the catalyzing effect of Pt or metallic Ag on the reduction, on the other hand. INTRODUCTION In the production of supported bimetallic catalysts, besides a high dispersion to provide a large active surface area, an intimate contact between the two metal phases is important to bring about alloy formation at low temperatures. Impregnation and drying procedures have been successfully utilized with the preparation of supported bimetallic catalysts of two group VIII metals [1,2]. Preparation of catalysts containing both a group VIII and a group IB metal appears to be much more difficult. Boudart and coworkers [3,4] succeeded to prepare highly dispersed PdAu/Si02 catalysts by carefully applying an ion-exchange method. To this end they used a silica with a very high surface area (700 m2/g). Ion-exchange at the silica surface called for positively charged metal complexes. Many examples in the literature exist, however, where the deposition of a group VIII and IB metal onto a support using simple impregnation and drying procedures led to two separate metal phases. Preparing RhAg/Si0 2 by co-impregnation, Anderson et al. [5] observed both very small Rh particles (50 ~) and silver crystallites of
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500 ~. X-ray patterns of PdAg/SiO Z catalysts prepared by Soma-Noto and Sachtler [6J revealed the presence of considerably larger Ag crystallites besides small PdAg particles. O'Cinneide and Gault [7] had to calcine their PtAu/SiO Z catalysts at 770 0C in air to obtain alloy particles. These examples convincingly demonstrate the problems encountered with the production of bimetallic catalysts by impregnation and drying. Anderson [8J concludes that separation of the two metals already starts during the impregnation step. Usually, the group VIII metal ions will adsorb onto the support, whereas the IB metal ions remain dissolved. After drying and hydrogen reduction the result is obvious: the transition metal has formed very small particles due to the atomic dispersion after impregnation, whereas the IB metal has formed much larger crystallites in the pores of the support. of conventional methods, we will explore the possiBecause of the drawback~ bilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts. This work deals with liquid-phase reduction of noble metal ions to the metallic state in an aqueous suspension of the support. Concerning monometallic catalysts we have reported previously on the preparation of silicaand alumina-supported silver catalysts by reduction of complex silver ions [9). Favourable preparation conditions to enhance the silver dispersion were observed to be: homogeneous addition of the reducing agent, the absence of micropores in the carrier and an appreciable interaction between the soluble metallic complex and the support. In this paper attention will be paid to the difficult task of preparing small alloy particles of a group VIII and a group 18 metal. The preparation method comprises selective deposition of the second metal onto particles of the first component already present on the support. In an aqueous suspension of the support covered by particles of the first metal, a compound of the second metal is dissolved and the corresponding metal is subsequently deposited by addition of a reducing agent, e.g. formalin, hydrazine or gaseous hydrogen. Since the particles of the first component catalyze the reduction reaction, the second metal is deposited exclusively onto the metal particles. As a result the two metal phases are now in an intimate contact after the reductive deposition and alloy formation can be easily achieved. With a Pt/SiO Z catalyst as the starting material, the preparation of PtAg/SiOZ catalysts will be described in detail. To demonstrate the general applicability of the method, the preparation of RuAg/SiOZ will also be reported. The catalysts produced will be compared with a sample prepared by impregnation and drying. The catalysts will be characterized by electron microscopy, infrared spectra of adsorbed CO, and X-ray diffraction. EXPERIMENTAL Platinum-silver alloy catalysts were prepared by using a 6 wt. % Pt/SiOZ
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catalyst as the starting material. This catalyst has been prepared by Johnson Matthey and proposed as a common standard (Eurocat) by a research group sponsored by the Council of Europe. The Pt/SiO Z catalyst was powdered and suspended in an aqueous solution containing Ag(NH 3)z+ ions*. Unless stated otherwise, the reduction of the silver complex was performed at 50C by injection of a solution of formalin or hydrazine into the suspension through a capillary tube having its end below the level of the liquid. During the injection the suspension was vigorously agitated. After addition of the reducing agent the suspension was slowly heated to room temperature. This reduction procedure took two hours. The composition of the alloy catalyst produced was varied by depositing different q~antities of silver on an identical batch of Pt/SiOZ' A RuAg/SiOZ catalyst was produced by first preparing a Ru/SiOZ sample via impregnating silica (Aerosil ZOOV) with a RuC13 solution followed by drying and reduction with HZ at 400 0C. Next,silver was deposited onto this sample as described for Pt/SiOZ' The'reducing agent used was hydrazine. Because under the above experimental conditions oxygen is able to reoxidize the deposited metallic silver, air was excluded during the liquid-phase reduction by working in an NZ atmosphere. The concentration of silver ions in the solution duriRg precipitation was continuously measured by means of a combination of a silver-ion selective electrode (Philips IS 550-Ag+) and a reference electrode (Philips R44/Z-SD/l). Characterization of the samples dried at lZOoC by X-ray diffraction was done using a Debye ,-Scherrer camera. A Philips EM301 and a Jeol ZOOC microscope were used for detailed examination of the catalysts by electron microscopy. We estimated the surface composition of the PtAg/SiOZ catalysts from transmission infrared spectra of adsorbed CO. To that end a powdered catalyst was pressed into a self-supporting disk and transferred to an in situ infrared cell. As a standard treatment the catalyst was oxidized at 4000C in 1 atm of Oz to equilibrate the alloy particles. Subsequently the disk was reduced and evacuated at 4000C followed by oxygen adsorption (10 Torr OZ) at the same temperature. Short evacuation was followed by cooling down the catalyst to room temperature. The thus mildly oxidized catalyst was exposed to 100 Torr CO at room temperature and spectra were recorded using a Perkin-Elmer 580B spectrophotometer. Gas phase absorption was compensated for by an identical cell placed in the reference beam of the spectrophotometer. The spectra were corrected for background absorption of the SiOZ carrier.
*In the absence of stirring, silver-ammonia solutions spontaneously form extremely explosive silver-nitrogen compounds.
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RESULTS The BET surface of the Pt/Si02 catalyst used as a starting material was established to be 189 m2/g. At room temperature the reduced platinum catalyst adsorbed 3.30 ml(STP)H2/9 catalyst (H2 pressure 10 Torr). Using a surface stoichiometry H/Pts = 1.8 [10] we arrived at a surface-mean particle size of 23 ~. An electron micrograph of the catalyst has been reproduced in fig.la. This representative micrograph ~hows a very homogeneous distribution of Pt particles of a fairly narroW size distribution, which displayed a maximum around 20 ~. Silver was deposited onto Pt/Si02 by reduction of Ag(NH3); with either formalin or hydrazine at 50C. A survey of PtAg/Si0 2 samples produced is found in table 1. Catalysts prepared by formalin or hydrazine are designated by F or H, respectively. followed by a serial number. Using formalin as a reducing agent. it is important to carry out the liquidphase reduction below room temperature. As dealt with before [9]. formalin does not react with A9(NH3); at 50C in a suspension of unloaded Si0 2. In the presence of Pt/Si02. however. the reduction takes place very fast at 50C as was inferred from a rapid decline of the silver concentration measured by the ion-selective electrode. It is obvious that the platinum particles do catalyze the reduction of the silver complex more effectively than does the pure carrier. It has to be expected that silver will be deposited very selectively on the Pt particles TABLE 1. Survey of PtAg/Si02 catalysts produced by liquid-phase reduction of Ag(NH 3); in the presence of Pt/Si0 2. One catalyst (15) has been prepared by impregnation of Aerosil 200V with a mixed solution of silver and platinum nitrate. Catalyst
Pt/Si0 2 F1* F2* F3 F6 H4 15
Loading (wt.%Ag)
Alloy composition At.%Pt At.%Ag
2.0 5.5 5.7 1.5 5.3 5.7
100 62 36 36 69 37 36
0 38 64 64 31 63 64
Particle size from TEM (~) 20 30 50 50-100 + 20-300++
t See text for explanation.
* Total amount of formalin added at once. + Some large clustered particles besides very small ones. ++ Bimodal particle size distribution.
Absorbancet at 2090 cm- 1 1.68 0.08 0.61 0.31
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Fig.I. Transmission electron micrograph of the 6 wt.% Pt/Si0 2 catalyst (a); micrographs of the PtAg/Si02 catalysts FI (b), F2 (c), F3 (d). leaving the support uncovered. Fig.I.b shows the experimental facts supporting this reasoning. This electron micrograph of a freshly prepared and dried (I20 0C) PtAg/Si02 sample shows irregularly shaped metal particles, sometimes rod-like, whereas the original Pt particles (fig. I.a) definitely exhibit regular shapes. Obviously, metallic silver has been deposited exclusively adjacent to the Pt particles. Fig.I further reveals the absence of large silver particles (~ 300 ~) characteristic of the Ag/Si02 samples produced by liquid-phase reduction [9]. The alloy particles produced have a narrow size distribution exhibiting a maximum around 50 ~. PtAg/Si02 catalysts prepared by hydrazine reduction display a broad particle size distribution if compared with F-type catalysts (table 1). Hydrazine is so
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Fig.2. Transmission electron micrograph of PtAg/Si02' sample 15. The catalyst has been reduced at 400 0C in H2. fast a reducing agent that it does produce Ag both on the Pt particles and in the liquid phase. TEM revealed clustered silver particles the number of which was scarce, however. For comparison, one PtAg/Si02 sample was prepared by impregnation and drying (table 1, catalyst 15). An electron micrograph of this catalyst (fig.2) revealed a bimodal particle size distribution. Both particles of 20 to 50 ~ and of 100 to 300 ~ can be seen. The X-ray pattern of catalyst 15 showed fairly sharp Ag lines and no Pt lines, while there was no evidence of alloy formation. These observations strongly suggest that the smallest particles mainly consist of Pt, whereas the larger ones consist of Ag. Adsorption of platinum ions and silver ions remaining dissolved during impregnation readily explain these phenomena. We have thus shown that the liquid-phase reduction technique is superior to the impregnation and drying method for the production of PtAg/Si02. A most elegant way to demonstrate the success of our preparation method is by infrared spectra of adsorbed CO. Besides results obtained with the PtAg/Si02 catalysts F2 and F6 results for Pt/Si0 2 have been added for comparison (fig.3 ). To appreciate the results shown it is necessary to know that spectra of CO adsorbed on reduced and mildly oxidized Pt/Si0 2 catalyst are almost identical [11,12]. Apparently, CO is able to remove adsorbed oxygen from a Pt surface already at room temperature. On the contrary, the PtAg catalysts show largely different spectra for reduced and oxidized samples [12], which demon-
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Fig.3. Infrared spectra of CO adsorbed on oxidized catalysts. Samples F2 ( - ) , F6 (- - -) and Pt/Si0 2 (- . -). strates strongly held oxygen. This lower reducibility of PtAg compared with Pt has been used to estimate the surface composition of our alloy catalysts. The absorption band at 2090 cm- 1 shown in fig.3 is ascribed to CO adsorbed on reduced Pt sites [11,12] while the band at 2175 cm- 1 is due to CO bound to Ag+ ions [12,13]. The intensity of the 2090 cm- 1 band is positively correlated with the amount of Pt surface not covered by Ag; see fig.3 and table 1. From the intensity the almost complete covering of the Pt particles by silver in catalyst F2 can be inferred. At this point the reader should note that we start our preparation with pure Pt particles; every Pt particle not covered with Ag during the liquid-phase reduction will contribute to the band at 2090 em-I. Whereas catalyst F2 hardly exposes free Pt surface, catalyst F6 displays a significant absorption around 2090 cm- 1 (fig.3 ). Due to the low silver loading, not all of the Pt particles in catalyst F6 have been covered with Ag. Comparison of catalysts F2 and H4 (table 1) shows that hydrazine is less successful than is formalin to deposit uniformly Ag onto the Pt particles. To show that our preparation method is not limited to PtAg catalysts we produced a RuAg/Si0 2 sample by liquid-phase reduction. Transmission electron micros-
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copy of both Ru/Si02 and RuAg/Si0 2 yielded results similar to those obtained with the PtAg catalysts. RuAg particles ranging in size from 50 to 100 ~ have been distributed homogeneously over the support. DISCUSSION Concerning the preparation of Ag/Si02 catalysts [9], l'~uid-phase reduction of Ag(NH3); with formalin at temperatures ranging from 20 to 500C typically produced a bimodal particle size distribution. Particles nucleated on the support displayed sizes of 35 to 70 ~, whereas particles formed in the liquid phase were considerably larger (200-400 ft). A more or less fundamental lower limit of the average particle size for the silver-silica system of about 60 ft was recognized and put together with the high mobility of silver particles over silica surfaces. This mobility, which is especially high in the presence of water [14], was though" to reflect a weak metal-support interaction. Clearly, a strong interaction between silver and the support is a prerequisite to keep the deposited Ag particles small. The work presented here shows that the presence of Pt (or Ru) particles effectively enhances the silver-silica interaction. The enhanced interaction can be inferred from the fast reduction of the silver ions at low temperatures (SoC) and from the very high dispersion of the silver metal deposited. We emphasize two factors contributing to this high dispersion. First of all the absence of large silver particles, which nucleate in the liquid phase, is important. The absence of the large particles (~ 300 ~) is due to the low reaction temperature utilized where the reaction proceeds exclusively on the catalytically active Pt particles. Another contribution to the silver dispersion of the Pt particles is the anchoring of metallic silver on the support. Apparently, Pt particles are strongly bound to silica and are therefore effective in stabilizing the deposit, silver. The reader will take for granted that the strong interaction between silver and the platinum-loaded support is due to strong intermetallic bonds between tr two metals. Intermetallic bonding is much stronger than the physical interacti< between Ag and Si0 2. Strong interaction between two metals leading to small nUl of the deposited metal was observed also by Wassermann and Sander [15]. These authors deposited iron onto rocksalt and gold substrates kept at 80 K. Whereas many isolated iron crystallites were observed on rocksalt, an almost continuou layer was obtained with gold as a substrate which points to a much higher dens of iron nuclei. Besides the anchoring of silver, it is worthwhile to examine more closely 1 reduction of the silver complex. From the transmission electron micrographs (cf. figs. l.a and l.b ) it appears to us that the deposition of metallic si
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mainly occurs lateral on the support. This phenomenon can be explained by assuming adsorption of the complex silver ions on the support prior to the reduction step. An appreciable interaction between the silver complex and the support appeared to be favourable to enhance the dispersion of monometallic silver catalysts [9]. In a forthcoming paper we will show that the extent of metallic complex adsorption is an important parameter which controls the production of supported PtAu bimetallic particles [16]. The above results demonstrate that the concept of increasing the interaction of a group IB metal with a silica support by introduction of a second metal has been used to produce excellent bimetallic catalysts. PtAg and RuAg particles of 30 ~ on silica, homogeneously distributed over the support, can easily be produced using this technique. The method has been shown to be superior to impregnation and drying procedures. The reduction of a metal complex in the presence of a loaded supp~rt can be considered to be both a method to prepare bimetallic catalysts and a method to improve the dispersion and thermal stability of monometallic catalysts. The latter application calls for less expensive metals strongly adhering to the support. ACKNOWLEDGEMENTS The authors are indebted to Mr. R. Hendriks for preparing the greater part of the catalysts. The investigations were supported by the "Netherlands Foundation of Chemical Research" (SON) with financial aid from the "Netherlands Organi zat i on for the Advancement of Pure Research" (ZWO). REFERENCES 1 C.H. Bartholomew and M. Boudart, J. Catal., 25 (1973) 173-181. 2 J.H. Sinfelt and G.H. Via, J. Catal., 56 (1979) 1-11. 3 Y.L. Lam and M. Boudart, J. Catal., 50 (1977) 530-540. 4 E.L. Kugler and M. Boudart, J. Catal., 59 (1979) 201-210. 5 J.H. Anderson, P.J. Conn and S.G. Brandenberger, J. Catal., 16 (1970) 404-406. 6 Y. Soma-Noto and W.M.H. Sachtler, J. Catal., 32 (1974) 315-324. 7 A. O'Cinneide and F.G. Gault, J. Catal. 37 (1975) 311. 8 J.R. Anderson, Structure of Metallic Catalysts, Academic Press, London, 1975, p. 176. 9 K.P. de Jong and J.W. Geus, Applied Catalysis, submitted. 10 J.-P. Candy, P. Fouilloux and A.J. Renouprez, J. Chern. Soc., Faraday Trans. I, 76 (1980) 616-629. 11 H. Heyne and F.C. Tompkins, Trans. Faraday Soc., 63 (1967) 1274-1285. 12 K.P. de Jong, Ph. D. Thesis, Utrecht, 1982. 13 G.W. Keulks and A. Ravi, J. Phys. Chern., 74 (1970) 783-786. 14 L. Bachmann and H. Hilbrand, in R. Niedermeyer and H. Mayer (Eds.), Basic Problems in Thin Film Physics, Van den Hoeck and Rupprecht, Gottingen, 1966, p. 77. 15 E.F. Wassermann and W. Sander, J. Vac. Sci. Technol., 6 (1969) 537-539. 16 K.P. de Jong, R.C. Verkerk and J.W. Geus, in preparation.
120 DISCUSSION H. CHARCOSSET drag en at 400°C ?
What happens during heating your PtAg/Si02 catalyst in hyIs there some interdiffusion of Pt and Ag in these conditions?
K.P. de JONG A freshly prepared sample of PtAg/Si02 (catalyst F2) has been studied after reduction at 120°C. It was observed that adsorption of CO only led to a weak band in the IR spectrum. As Co hardly adsorbs on silver, we conclude that the Pt particles are still covered up by silver and that no interdiffusion of Pt and Ag has taken place at 120°C. However, after reduction at 400°C this sample adsorbed a considerable amount of CO, which shows the presence of Pt sites at the surface of the alloy particles. Apparently, the interdiffusian of the two elements within the alloy particles has been effected at 400°C. S. VASUDEVAN adsorption?
Why do you oxidize your catalysts before the IR study of CO Does the CO adsorb on the metal or on the metal oxide ?
K.P. de JONG 1. An extensive IR study of CO adsorption on the reduced samples showed a considerable shift of the vibrational frequency of adsorbed CO with the silver content of the alloy particles. Moreover, adsorbed CO caused a serious surface segregation of Pt. Both factors contribute to the difficulty of estimating the composition of the bimetallic particles after reduction. It was shown that oxidation caused segregation of silver (ions) to the surface. Under these conditions adsorbed CO hardly led to segregation of platinum to the surface, while the IR band of CO adsorbed on Pt sites did not shift considerably (fig. 3). Due to these three factors a fair impression of the bulk composition of the alloy particles could be obtained by studying the oxidized catalysts. 2. At room temperature CO was able to remove to a large extent adsorbed oxygen from the platinum surface. After this removal CO adsorbed on metallic Pt sites which led to the IR band at 2090 cm- 1. Co adsorption on platinum oxide would give rise to a band at 2120 cm- 1 (ref. 11). A. MIYAMOTO: On the basis of your method, the surface of Pt is covered with Ag. Then, is it possible to cover the Pt surface with Rh ? K.P. de JONG Whether a second metal, e.g. Rh, will be deposited selectively onto Pt particles already present on the support, depends on three factors: - Is there a considerable interaction between the metallic precursor and the support ? - What is the nature of the alloy produced, viz. exothermic or endothermic? Preliminary experiments with supported PtAu have shown that the preparation of this endothermic alloy is much more diffi~ult than of PtAg, which is an exothermic alloy. - Is the deposition reaction catalyzed by the metal itself? The catalytic action of Pt should be more pronounced than that of the second metal. If this is not true, the second metal will agglomerate and not be distributed homogeneously over the Pt particles. J. MARGITFALVI 1. what is the initial form of your Pt particles? Are they oxidized or are they covered with hydrogen ? 2. In what phase of the preparation will the contact between two metal phases be formed ? 3. What is the practical use of your PtAg/Si0 2 catalysts? K.P. de JONG: 1. Our starting material is been produced by ion-exchange and subsequent Because the catalyst has contacted air prior Pt particles will be covered with a layer of inferred from an induction period during the correlates with the removal of this adsorbed
the EUROPT-1 catalyst which has reduction with hydrogen at 400°C. to the use in our experiments, the adsorbed oxygen. This could be liquid-phase reduction, which oxygen.
121 2. As shown by electron microscopy, the two metal phases already are in contact after preparation and drying at 120°C (fig. 1). 3. Thepractical use of our PtAg/Si0 2 catalysts is twofold. In this study the catalysts served as a model system to elucidate general factors which control this liquid-phase reduction method. The preparation technique can now be used for other (practical) bimetallic systems. Secondly, silver is immobilized in the PtAg/Si02 catalysts by underlying platinum metal. With conventional supports, e.g. Si02 and a-A1203' silver metal is very mobile which leads to extensive sintering at elevated temperatures. Our preparation method offers the possibility to prepare thermostable, highly dispersed silver catalysts. L. GUCZI: On support not containing Pt, large silver particles are formed due to some additional migration on the surface. Bearing in mind this mechanism, some other can be proposed in addition to the migration. That is, on Pt/Si02 the nucleation rate is higher, due to the presence of Pt and the immediate formation of bimetallic particles. On the other hand, with silver alone, the nucleation rate is slower; thus, either particles can be grown prior to reduction, or, more simply, the already existing particles can be increased by being attached with non-reduced silver precursor. K.P. de JONG: Both mechanisms are indeed operative in the formation of particles in Ag/Si0 2 samples. From an extensive study of the preparation of supported monometallic Ag catalysts (1), the bimodal particle size distribution observed could be related to these two mechanisms. Large particles (~ 300 ~) are formed by reduction of silver ions from the liquid-phase at the surface of metallic silver particles present on the support and/or in the liquid-phases. The small particles (35-70 ~) have been formed from silver ions adsorbed on the support. Literature data show a similar particle size (40-80 ~) for catalysts produced by ion-exchange and H2 reduction (2). In the latter case an initial atomic distribution was obtained, but again a high mobility of the silver particles led to a particle size similar to that of particles produced by liquidphase reduction of adsorbed silver ions. (1) K.P. de JONG and J.W. GEUS, Applied Catalysis, in press. (2) M. JAF~OUI, B. MORAWECK, P.C. GRAVELLE and S.J. TEICHNER, J. Chim. Phys. Physicochim. Bioi. 75 (1978), 1060.
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PRODUCTION OF SILVER BIMETALLIC CATALYSTS BY LIQUID-PHASE REDUCTION K.P. DE JONG and;J.W. GEUS Department of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht, The Netherlands
ABSTRACT The possibilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts are investigated. Although the method is more generally applicable, we here concentrate on one combination of metals, viz. Pt and Ag. The catalysts are-characterized by electron microscopy and infrared spectra of adsorbed CO. PtAg/Si02 catalysts are prepared using a 6 wt.% Pt/Si0 2 catalyst as the starting material. Silver was deposited selectively onto the Pt particles by reduction of Ag(NH 3); ions dissolved in an aqueous suspension of the platinum-loaded silica. Using formalin as a reducing agent bimetallic particles having sizes from 30 to 50 ~ result, whilst infrared spectroscopy indicates the platinum surface to be covered almost completely by silver. The selective deposition of Ag onto the Pt particles during preparation appears to be due to the suppression of nucleation of Ag particles in the liquid phase on the one hand, and the catalyzing effect of Pt or metallic Ag on the reduction, on the other hand. INTRODUCTION In the production of supported bimetallic catalysts, besides a high dispersion to provide a large active surface area, an intimate contact between the two metal phases is important to bring about alloy formation at low temperatures. Impregnation and drying procedures have been successfully utilized with the preparation of supported bimetallic catalysts of two group VIII metals [1,2]. Preparation of catalysts containing both a group VIII and a group IB metal appears to be much more difficult. Boudart and coworkers [3,4] succeeded to prepare highly dispersed PdAu/Si02 catalysts by carefully applying an ion-exchange method. To this end they used a silica with a very high surface area (700 m2/g). Ion-exchange at the silica surface called for positively charged metal complexes. Many examples in the literature exist, however, where the deposition of a group VIII and IB metal onto a support using simple impregnation and drying procedures led to two separate metal phases. Preparing RhAg/Si0 2 by co-impregnation, Anderson et al. [5] observed both very small Rh particles (50 ~) and silver crystallites of
112
500 ~. X-ray patterns of PdAg/SiO Z catalysts prepared by Soma-Noto and Sachtler [6J revealed the presence of considerably larger Ag crystallites besides small PdAg particles. O'Cinneide and Gault [7] had to calcine their PtAu/SiO Z catalysts at 770 0C in air to obtain alloy particles. These examples convincingly demonstrate the problems encountered with the production of bimetallic catalysts by impregnation and drying. Anderson [8J concludes that separation of the two metals already starts during the impregnation step. Usually, the group VIII metal ions will adsorb onto the support, whereas the IB metal ions remain dissolved. After drying and hydrogen reduction the result is obvious: the transition metal has formed very small particles due to the atomic dispersion after impregnation, whereas the IB metal has formed much larger crystallites in the pores of the support. of conventional methods, we will explore the possiBecause of the drawback~ bilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts. This work deals with liquid-phase reduction of noble metal ions to the metallic state in an aqueous suspension of the support. Concerning monometallic catalysts we have reported previously on the preparation of silicaand alumina-supported silver catalysts by reduction of complex silver ions [9). Favourable preparation conditions to enhance the silver dispersion were observed to be: homogeneous addition of the reducing agent, the absence of micropores in the carrier and an appreciable interaction between the soluble metallic complex and the support. In this paper attention will be paid to the difficult task of preparing small alloy particles of a group VIII and a group 18 metal. The preparation method comprises selective deposition of the second metal onto particles of the first component already present on the support. In an aqueous suspension of the support covered by particles of the first metal, a compound of the second metal is dissolved and the corresponding metal is subsequently deposited by addition of a reducing agent, e.g. formalin, hydrazine or gaseous hydrogen. Since the particles of the first component catalyze the reduction reaction, the second metal is deposited exclusively onto the metal particles. As a result the two metal phases are now in an intimate contact after the reductive deposition and alloy formation can be easily achieved. With a Pt/SiO Z catalyst as the starting material, the preparation of PtAg/SiOZ catalysts will be described in detail. To demonstrate the general applicability of the method, the preparation of RuAg/SiOZ will also be reported. The catalysts produced will be compared with a sample prepared by impregnation and drying. The catalysts will be characterized by electron microscopy, infrared spectra of adsorbed CO, and X-ray diffraction. EXPERIMENTAL Platinum-silver alloy catalysts were prepared by using a 6 wt. % Pt/SiOZ
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catalyst as the starting material. This catalyst has been prepared by Johnson Matthey and proposed as a common standard (Eurocat) by a research group sponsored by the Council of Europe. The Pt/SiO Z catalyst was powdered and suspended in an aqueous solution containing Ag(NH 3)z+ ions*. Unless stated otherwise, the reduction of the silver complex was performed at 50C by injection of a solution of formalin or hydrazine into the suspension through a capillary tube having its end below the level of the liquid. During the injection the suspension was vigorously agitated. After addition of the reducing agent the suspension was slowly heated to room temperature. This reduction procedure took two hours. The composition of the alloy catalyst produced was varied by depositing different q~antities of silver on an identical batch of Pt/SiOZ' A RuAg/SiOZ catalyst was produced by first preparing a Ru/SiOZ sample via impregnating silica (Aerosil ZOOV) with a RuC13 solution followed by drying and reduction with HZ at 400 0C. Next,silver was deposited onto this sample as described for Pt/SiOZ' The'reducing agent used was hydrazine. Because under the above experimental conditions oxygen is able to reoxidize the deposited metallic silver, air was excluded during the liquid-phase reduction by working in an NZ atmosphere. The concentration of silver ions in the solution duriRg precipitation was continuously measured by means of a combination of a silver-ion selective electrode (Philips IS 550-Ag+) and a reference electrode (Philips R44/Z-SD/l). Characterization of the samples dried at lZOoC by X-ray diffraction was done using a Debye ,-Scherrer camera. A Philips EM301 and a Jeol ZOOC microscope were used for detailed examination of the catalysts by electron microscopy. We estimated the surface composition of the PtAg/SiOZ catalysts from transmission infrared spectra of adsorbed CO. To that end a powdered catalyst was pressed into a self-supporting disk and transferred to an in situ infrared cell. As a standard treatment the catalyst was oxidized at 4000C in 1 atm of Oz to equilibrate the alloy particles. Subsequently the disk was reduced and evacuated at 4000C followed by oxygen adsorption (10 Torr OZ) at the same temperature. Short evacuation was followed by cooling down the catalyst to room temperature. The thus mildly oxidized catalyst was exposed to 100 Torr CO at room temperature and spectra were recorded using a Perkin-Elmer 580B spectrophotometer. Gas phase absorption was compensated for by an identical cell placed in the reference beam of the spectrophotometer. The spectra were corrected for background absorption of the SiOZ carrier.
*In the absence of stirring, silver-ammonia solutions spontaneously form extremely explosive silver-nitrogen compounds.
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RESULTS The BET surface of the Pt/Si02 catalyst used as a starting material was established to be 189 m2/g. At room temperature the reduced platinum catalyst adsorbed 3.30 ml(STP)H2/9 catalyst (H2 pressure 10 Torr). Using a surface stoichiometry H/Pts = 1.8 [10] we arrived at a surface-mean particle size of 23 ~. An electron micrograph of the catalyst has been reproduced in fig.la. This representative micrograph ~hows a very homogeneous distribution of Pt particles of a fairly narroW size distribution, which displayed a maximum around 20 ~. Silver was deposited onto Pt/Si02 by reduction of Ag(NH3); with either formalin or hydrazine at 50C. A survey of PtAg/Si0 2 samples produced is found in table 1. Catalysts prepared by formalin or hydrazine are designated by F or H, respectively. followed by a serial number. Using formalin as a reducing agent. it is important to carry out the liquidphase reduction below room temperature. As dealt with before [9]. formalin does not react with A9(NH3); at 50C in a suspension of unloaded Si0 2. In the presence of Pt/Si02. however. the reduction takes place very fast at 50C as was inferred from a rapid decline of the silver concentration measured by the ion-selective electrode. It is obvious that the platinum particles do catalyze the reduction of the silver complex more effectively than does the pure carrier. It has to be expected that silver will be deposited very selectively on the Pt particles TABLE 1. Survey of PtAg/Si02 catalysts produced by liquid-phase reduction of Ag(NH 3); in the presence of Pt/Si0 2. One catalyst (15) has been prepared by impregnation of Aerosil 200V with a mixed solution of silver and platinum nitrate. Catalyst
Pt/Si0 2 F1* F2* F3 F6 H4 15
Loading (wt.%Ag)
Alloy composition At.%Pt At.%Ag
2.0 5.5 5.7 1.5 5.3 5.7
100 62 36 36 69 37 36
0 38 64 64 31 63 64
Particle size from TEM (~) 20 30 50 50-100 + 20-300++
t See text for explanation.
* Total amount of formalin added at once. + Some large clustered particles besides very small ones. ++ Bimodal particle size distribution.
Absorbancet at 2090 cm- 1 1.68 0.08 0.61 0.31
115
Fig.I. Transmission electron micrograph of the 6 wt.% Pt/Si0 2 catalyst (a); micrographs of the PtAg/Si02 catalysts FI (b), F2 (c), F3 (d). leaving the support uncovered. Fig.I.b shows the experimental facts supporting this reasoning. This electron micrograph of a freshly prepared and dried (I20 0C) PtAg/Si02 sample shows irregularly shaped metal particles, sometimes rod-like, whereas the original Pt particles (fig. I.a) definitely exhibit regular shapes. Obviously, metallic silver has been deposited exclusively adjacent to the Pt particles. Fig.I further reveals the absence of large silver particles (~ 300 ~) characteristic of the Ag/Si02 samples produced by liquid-phase reduction [9]. The alloy particles produced have a narrow size distribution exhibiting a maximum around 50 ~. PtAg/Si02 catalysts prepared by hydrazine reduction display a broad particle size distribution if compared with F-type catalysts (table 1). Hydrazine is so
116
Fig.2. Transmission electron micrograph of PtAg/Si02' sample 15. The catalyst has been reduced at 400 0C in H2. fast a reducing agent that it does produce Ag both on the Pt particles and in the liquid phase. TEM revealed clustered silver particles the number of which was scarce, however. For comparison, one PtAg/Si02 sample was prepared by impregnation and drying (table 1, catalyst 15). An electron micrograph of this catalyst (fig.2) revealed a bimodal particle size distribution. Both particles of 20 to 50 ~ and of 100 to 300 ~ can be seen. The X-ray pattern of catalyst 15 showed fairly sharp Ag lines and no Pt lines, while there was no evidence of alloy formation. These observations strongly suggest that the smallest particles mainly consist of Pt, whereas the larger ones consist of Ag. Adsorption of platinum ions and silver ions remaining dissolved during impregnation readily explain these phenomena. We have thus shown that the liquid-phase reduction technique is superior to the impregnation and drying method for the production of PtAg/Si02. A most elegant way to demonstrate the success of our preparation method is by infrared spectra of adsorbed CO. Besides results obtained with the PtAg/Si02 catalysts F2 and F6 results for Pt/Si0 2 have been added for comparison (fig.3 ). To appreciate the results shown it is necessary to know that spectra of CO adsorbed on reduced and mildly oxidized Pt/Si0 2 catalyst are almost identical [11,12]. Apparently, CO is able to remove adsorbed oxygen from a Pt surface already at room temperature. On the contrary, the PtAg catalysts show largely different spectra for reduced and oxidized samples [12], which demon-
117
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Fig.3. Infrared spectra of CO adsorbed on oxidized catalysts. Samples F2 ( - ) , F6 (- - -) and Pt/Si0 2 (- . -). strates strongly held oxygen. This lower reducibility of PtAg compared with Pt has been used to estimate the surface composition of our alloy catalysts. The absorption band at 2090 cm- 1 shown in fig.3 is ascribed to CO adsorbed on reduced Pt sites [11,12] while the band at 2175 cm- 1 is due to CO bound to Ag+ ions [12,13]. The intensity of the 2090 cm- 1 band is positively correlated with the amount of Pt surface not covered by Ag; see fig.3 and table 1. From the intensity the almost complete covering of the Pt particles by silver in catalyst F2 can be inferred. At this point the reader should note that we start our preparation with pure Pt particles; every Pt particle not covered with Ag during the liquid-phase reduction will contribute to the band at 2090 em-I. Whereas catalyst F2 hardly exposes free Pt surface, catalyst F6 displays a significant absorption around 2090 cm- 1 (fig.3 ). Due to the low silver loading, not all of the Pt particles in catalyst F6 have been covered with Ag. Comparison of catalysts F2 and H4 (table 1) shows that hydrazine is less successful than is formalin to deposit uniformly Ag onto the Pt particles. To show that our preparation method is not limited to PtAg catalysts we produced a RuAg/Si0 2 sample by liquid-phase reduction. Transmission electron micros-
118
copy of both Ru/Si02 and RuAg/Si0 2 yielded results similar to those obtained with the PtAg catalysts. RuAg particles ranging in size from 50 to 100 ~ have been distributed homogeneously over the support. DISCUSSION Concerning the preparation of Ag/Si02 catalysts [9], l'~uid-phase reduction of Ag(NH3); with formalin at temperatures ranging from 20 to 500C typically produced a bimodal particle size distribution. Particles nucleated on the support displayed sizes of 35 to 70 ~, whereas particles formed in the liquid phase were considerably larger (200-400 ft). A more or less fundamental lower limit of the average particle size for the silver-silica system of about 60 ft was recognized and put together with the high mobility of silver particles over silica surfaces. This mobility, which is especially high in the presence of water [14], was though" to reflect a weak metal-support interaction. Clearly, a strong interaction between silver and the support is a prerequisite to keep the deposited Ag particles small. The work presented here shows that the presence of Pt (or Ru) particles effectively enhances the silver-silica interaction. The enhanced interaction can be inferred from the fast reduction of the silver ions at low temperatures (SoC) and from the very high dispersion of the silver metal deposited. We emphasize two factors contributing to this high dispersion. First of all the absence of large silver particles, which nucleate in the liquid phase, is important. The absence of the large particles (~ 300 ~) is due to the low reaction temperature utilized where the reaction proceeds exclusively on the catalytically active Pt particles. Another contribution to the silver dispersion of the Pt particles is the anchoring of metallic silver on the support. Apparently, Pt particles are strongly bound to silica and are therefore effective in stabilizing the deposit, silver. The reader will take for granted that the strong interaction between silver and the platinum-loaded support is due to strong intermetallic bonds between tr two metals. Intermetallic bonding is much stronger than the physical interacti< between Ag and Si0 2. Strong interaction between two metals leading to small nUl of the deposited metal was observed also by Wassermann and Sander [15]. These authors deposited iron onto rocksalt and gold substrates kept at 80 K. Whereas many isolated iron crystallites were observed on rocksalt, an almost continuou layer was obtained with gold as a substrate which points to a much higher dens of iron nuclei. Besides the anchoring of silver, it is worthwhile to examine more closely 1 reduction of the silver complex. From the transmission electron micrographs (cf. figs. l.a and l.b ) it appears to us that the deposition of metallic si
119
mainly occurs lateral on the support. This phenomenon can be explained by assuming adsorption of the complex silver ions on the support prior to the reduction step. An appreciable interaction between the silver complex and the support appeared to be favourable to enhance the dispersion of monometallic silver catalysts [9]. In a forthcoming paper we will show that the extent of metallic complex adsorption is an important parameter which controls the production of supported PtAu bimetallic particles [16]. The above results demonstrate that the concept of increasing the interaction of a group IB metal with a silica support by introduction of a second metal has been used to produce excellent bimetallic catalysts. PtAg and RuAg particles of 30 ~ on silica, homogeneously distributed over the support, can easily be produced using this technique. The method has been shown to be superior to impregnation and drying procedures. The reduction of a metal complex in the presence of a loaded supp~rt can be considered to be both a method to prepare bimetallic catalysts and a method to improve the dispersion and thermal stability of monometallic catalysts. The latter application calls for less expensive metals strongly adhering to the support. ACKNOWLEDGEMENTS The authors are indebted to Mr. R. Hendriks for preparing the greater part of the catalysts. The investigations were supported by the "Netherlands Foundation of Chemical Research" (SON) with financial aid from the "Netherlands Organi zat i on for the Advancement of Pure Research" (ZWO). REFERENCES 1 C.H. Bartholomew and M. Boudart, J. Catal., 25 (1973) 173-181. 2 J.H. Sinfelt and G.H. Via, J. Catal., 56 (1979) 1-11. 3 Y.L. Lam and M. Boudart, J. Catal., 50 (1977) 530-540. 4 E.L. Kugler and M. Boudart, J. Catal., 59 (1979) 201-210. 5 J.H. Anderson, P.J. Conn and S.G. Brandenberger, J. Catal., 16 (1970) 404-406. 6 Y. Soma-Noto and W.M.H. Sachtler, J. Catal., 32 (1974) 315-324. 7 A. O'Cinneide and F.G. Gault, J. Catal. 37 (1975) 311. 8 J.R. Anderson, Structure of Metallic Catalysts, Academic Press, London, 1975, p. 176. 9 K.P. de Jong and J.W. Geus, Applied Catalysis, submitted. 10 J.-P. Candy, P. Fouilloux and A.J. Renouprez, J. Chern. Soc., Faraday Trans. I, 76 (1980) 616-629. 11 H. Heyne and F.C. Tompkins, Trans. Faraday Soc., 63 (1967) 1274-1285. 12 K.P. de Jong, Ph. D. Thesis, Utrecht, 1982. 13 G.W. Keulks and A. Ravi, J. Phys. Chern., 74 (1970) 783-786. 14 L. Bachmann and H. Hilbrand, in R. Niedermeyer and H. Mayer (Eds.), Basic Problems in Thin Film Physics, Van den Hoeck and Rupprecht, Gottingen, 1966, p. 77. 15 E.F. Wassermann and W. Sander, J. Vac. Sci. Technol., 6 (1969) 537-539. 16 K.P. de Jong, R.C. Verkerk and J.W. Geus, in preparation.
120 DISCUSSION H. CHARCOSSET drag en at 400°C ?
What happens during heating your PtAg/Si02 catalyst in hyIs there some interdiffusion of Pt and Ag in these conditions?
K.P. de JONG A freshly prepared sample of PtAg/Si02 (catalyst F2) has been studied after reduction at 120°C. It was observed that adsorption of CO only led to a weak band in the IR spectrum. As Co hardly adsorbs on silver, we conclude that the Pt particles are still covered up by silver and that no interdiffusion of Pt and Ag has taken place at 120°C. However, after reduction at 400°C this sample adsorbed a considerable amount of CO, which shows the presence of Pt sites at the surface of the alloy particles. Apparently, the interdiffusian of the two elements within the alloy particles has been effected at 400°C. S. VASUDEVAN adsorption?
Why do you oxidize your catalysts before the IR study of CO Does the CO adsorb on the metal or on the metal oxide ?
K.P. de JONG 1. An extensive IR study of CO adsorption on the reduced samples showed a considerable shift of the vibrational frequency of adsorbed CO with the silver content of the alloy particles. Moreover, adsorbed CO caused a serious surface segregation of Pt. Both factors contribute to the difficulty of estimating the composition of the bimetallic particles after reduction. It was shown that oxidation caused segregation of silver (ions) to the surface. Under these conditions adsorbed CO hardly led to segregation of platinum to the surface, while the IR band of CO adsorbed on Pt sites did not shift considerably (fig. 3). Due to these three factors a fair impression of the bulk composition of the alloy particles could be obtained by studying the oxidized catalysts. 2. At room temperature CO was able to remove to a large extent adsorbed oxygen from the platinum surface. After this removal CO adsorbed on metallic Pt sites which led to the IR band at 2090 cm- 1. Co adsorption on platinum oxide would give rise to a band at 2120 cm- 1 (ref. 11). A. MIYAMOTO: On the basis of your method, the surface of Pt is covered with Ag. Then, is it possible to cover the Pt surface with Rh ? K.P. de JONG Whether a second metal, e.g. Rh, will be deposited selectively onto Pt particles already present on the support, depends on three factors: - Is there a considerable interaction between the metallic precursor and the support ? - What is the nature of the alloy produced, viz. exothermic or endothermic? Preliminary experiments with supported PtAu have shown that the preparation of this endothermic alloy is much more diffi~ult than of PtAg, which is an exothermic alloy. - Is the deposition reaction catalyzed by the metal itself? The catalytic action of Pt should be more pronounced than that of the second metal. If this is not true, the second metal will agglomerate and not be distributed homogeneously over the Pt particles. J. MARGITFALVI 1. what is the initial form of your Pt particles? Are they oxidized or are they covered with hydrogen ? 2. In what phase of the preparation will the contact between two metal phases be formed ? 3. What is the practical use of your PtAg/Si0 2 catalysts? K.P. de JONG: 1. Our starting material is been produced by ion-exchange and subsequent Because the catalyst has contacted air prior Pt particles will be covered with a layer of inferred from an induction period during the correlates with the removal of this adsorbed
the EUROPT-1 catalyst which has reduction with hydrogen at 400°C. to the use in our experiments, the adsorbed oxygen. This could be liquid-phase reduction, which oxygen.
121 2. As shown by electron microscopy, the two metal phases already are in contact after preparation and drying at 120°C (fig. 1). 3. Thepractical use of our PtAg/Si0 2 catalysts is twofold. In this study the catalysts served as a model system to elucidate general factors which control this liquid-phase reduction method. The preparation technique can now be used for other (practical) bimetallic systems. Secondly, silver is immobilized in the PtAg/Si02 catalysts by underlying platinum metal. With conventional supports, e.g. Si02 and a-A1203' silver metal is very mobile which leads to extensive sintering at elevated temperatures. Our preparation method offers the possibility to prepare thermostable, highly dispersed silver catalysts. L. GUCZI: On support not containing Pt, large silver particles are formed due to some additional migration on the surface. Bearing in mind this mechanism, some other can be proposed in addition to the migration. That is, on Pt/Si02 the nucleation rate is higher, due to the presence of Pt and the immediate formation of bimetallic particles. On the other hand, with silver alone, the nucleation rate is slower; thus, either particles can be grown prior to reduction, or, more simply, the already existing particles can be increased by being attached with non-reduced silver precursor. K.P. de JONG: Both mechanisms are indeed operative in the formation of particles in Ag/Si0 2 samples. From an extensive study of the preparation of supported monometallic Ag catalysts (1), the bimodal particle size distribution observed could be related to these two mechanisms. Large particles (~ 300 ~) are formed by reduction of silver ions from the liquid-phase at the surface of metallic silver particles present on the support and/or in the liquid-phases. The small particles (35-70 ~) have been formed from silver ions adsorbed on the support. Literature data show a similar particle size (40-80 ~) for catalysts produced by ion-exchange and H2 reduction (2). In the latter case an initial atomic distribution was obtained, but again a high mobility of the silver particles led to a particle size similar to that of particles produced by liquidphase reduction of adsorbed silver ions. (1) K.P. de JONG and J.W. GEUS, Applied Catalysis, in press. (2) M. JAF~OUI, B. MORAWECK, P.C. GRAVELLE and S.J. TEICHNER, J. Chim. Phys. Physicochim. Bioi. 75 (1978), 1060.