Applied Catalysis, 42 (1988) 229-237 Elsevier
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Publishers
B.V.,
229
Amsterdam
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Silica Supported Rhodium-Ruthenium Bimetallic Catalysts in Carbon Monoxide Hydrogenation I. Influence of the Method of Preparation and Methanation Behaviour ANITA
RAMACHANDRAN
and D.K.
CHAKRABARTY*
Solid State Laboratory, Department of Chemistry, Indian Institute of Technology, Pou;ai, Bombay 400 076 (India) (Received
13 October
1987, accepted
26 April 1988)
ABSTRACT Silica supported Rh-Ru bimetallic catalysts X-ray diffraction and hydrogen chemisorption
were prepared by impregnation, characterized by and their methanation behaviour studied over a
wide temperature range. The influence of preparation method - specifically the drying step and the effect of preoxidation prior to reduction -was examined. It was shown that poorly dispersed bimetallic
catalysts
led to the segregation
of the individual
metals and mixed metal clusters
were
not formed. But suppression of agglomeration in the preparation giving highly dispersed catalysts led to the formation of mixed metal clusters. The existence of mixed metal clusters was supported by the results
of carbon
monoxide
methanation
which was used as a probe reaction.
The addition
of rhodium to ruthenium gave catalysts which were more efficient for methanation of carbon monoxide at higher temperature. As the reaction temperature was raised, the methanation efficiency of ruthenium was lowered due to the competing water gas-shift reaction. Addition of rhodium helped methanation by suppressing the formation of carbon dioxide because of its better ability
to convert
carbon
dioxide to methane.
INTRODUCTION
Studies on supported bimetallic catalysts have engaged the attention of several research groups for nearly four decades. The initial period saw a focus on bimetallic catalysts, one metal component of which was known to be active and the other inactive for the reaction of interest [ 11. Investigations on bimetallic catalysts where both metals were known to be active were relatively few. Interest in combinations of two Group VIII metals has developed in the last fifteen years especially since the superiority of bimetallic Pt-Re/Al,O, over Pt/Al,O, was clearly recognized [ 21. The almost exclusive use of a supported Pt-Rh bimetallic catalyst in automotive exhaust pollution control has lent further importance to such bimetallic combinations [ 31.
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Of all the Group VIII metals, supported ruthenium showed the most outstanding activity for the methanation of carbon monoxide [4 1. At higher temperatures, it was also a very active catalyst for the conversion of carbon monoxide to carbon dioxide, a reaction that should be suppressed if high methanation efficiency over a wide temperature range is to be achieved [5J. Solymosi et al. [6] reported that supported rhodium showed excellent catalytic performance for the methanation of carbon dioxide. It was of interest, therefore, to study the properties of Rh-Ru bimetallic catalysts to see whether the addition of rhodium to ruthenium suppressed the conversion of carbon monoxide to carbon dioxide while retaining the high methanation activity characteristic of ruthenium. It cannot be assumed in the case of bimetallic catalysts that mixed metal clusters are formed rather than separate segregated particles. In such a situation a demanding or structure-sensitive catalytic reaction such as methanation can serve as a useful probe [ 11. Ruthenium is a much superior methanation catalyst compared to other Group VIII metals. Hence, formation of mixed metal clusters in a bimetallic combination of ruthenium and another Group VIII metal of relatively poor methanation activity should lead to a drastic reduction in the activity of the bimetallic catalyst. A preliminary report by Miura et al. [7] showed just such an abrupt fall in methanation activity when the Ru:M ratio fell below four (M stands for platinum, iridium or rhodium). The influence of catalyst pretreatment, particularly the effect of direct reduction of the impregnated mass as against its preoxidation followed by reduction, has been the subject of several studies [ 2 1. It was found, for example, that when a supported platinum catalyst was oxidised prior to reduction, the catalyst had a higher dispersion than the one directly reduced. Also, precalcination was often carried out under industrial conditions of operation to convert the metal chloride precursor to oxide which was then reduced to metal. This was done to prevent the formation of highly corrosive hydrochloric acid which is formed if the metal chloride is directly reduced in a flow of hydrogen. To examine all the above mentioned possibilities, we investigated the preparation and properties of silica supported Rh-Ru catalysts. So far, there has been only one preliminary report of a methanation study on this system. In this work, we report the influence of the method of preparation on the structure and methanation behaviour of these catalysts. EXPERIMENTAL
Materials RhC13.3H20 was obtained from Fluka Chemicals and RuCI,*xH,O from Johnson Mathey. The support silica was silicon dioxide (Alfa Products) with high specific surface (385 m* gg ’ ) . Hydrogen was of UHP grade (IOLAR-2 )
231
supplied by Indian Oxygen British Oxygen Company.
Limited.
Carbon
monoxide
was supplied
by the
Catalyst preparation
Catalysts were prepared by impregnation or co-impregnation. The total metal loading was maintained at 0.17 mmol of metal per gram of the catalyst. The appropriate amount of the metal salt was dissolved in doubly distilled water and the support was added to this solution to obt.ain a thin slurry. Two series of catalysts were prepared by varying (i) the method of drying this slurry and (ii) the subsequent pretreatment. Series SOR catalysts were dried by slowly heating the slurry with continuous stirring over a magnetic stirrer-cum-heater. The dried catalysts were ground to fine powder and calcined in a dry air flow at 623 K for one hour before being subjected to reduction in a hydrogen flow at 773 K for six hours. Three catalysts were prepared with the following rhodium to ruthenium ratio: 1: 0, 4: 1, 1: 1 and these were designated Rh-SOR, Rh,,Ru,,-SOR and RhbaRu&OR respectively. Series SVR catalysts were prepared by stirring the slurry for twelve hours and drying it under reduced pressure with regular stirring. The dried catalysts were ground to a fine powder and directly reduced in a flow of hydrogen for six hours at 723 K. Five catalysts were prepared with the following rhodium to ruthenium ratio: 1: 0,4 : 1,l: 1,1: 4,0: 1 and were designated Rh-SVR, RhRDRuZOSVR, Rh,,Ru,&VR, Rhl,ORu,,-SVR and Ru-SVR respectively. Apparatus
and procedure
X-ray diffraction (XRD) patterns of the reduced catalysts were recorded carefully on a Philips PW 1140 powder diffractometer using Cu KCYradiation. Very slow scanning speeds (l/2 to l/8 deg. mu-‘) were used. Mean particle sizes were calculated using the Scherrer equation with Warren’s correction [ 81 for instrumental broadening for samples where distinct diffraction bands were obtained. Metal dispersion by hydrogen chemisorption was determined using a conventional high-vacuum glass adsorption apparatus. Each sample was degassed at 623 K for two hours and cooled to room temperature prior to adsorption. Hydrogen adsorption was determined at room temperature and an equilibrium pressure of 100 Torr (13 kPa). The sample was then evacuated and hydrogen uptake was determined again at the same pressure. Metal dispersion was calculated from the difference of the two hydrogen uptake values which gave the irreversibly adsorbed hydrogen, assuming the surface metal to hydrogen ratio tobe 1:l [9]. Methanation of carbon monoxide was studied in a stainless steel pulsed-flow
232
microcatalytic reactor with swagelok@ fittings reported earlier [lo]. The catalyst was reduced in-situ at its preparation temperature and cooled in a flow of hydrogen. Methanation was studied by injecting a pulse of 0.5 cm3 of carbon monoxide in a stream of hydrogen at intervals of 25 K from 423 K upwards. The catalyst weight (0.5 g) and the hydrogen flow (50 cm” min-I) were kept fixed throughout. The products were analysed by an on-line gas chromatograph equipped with thermal conductivity detectors and CTR I column. A calibration mixture supplied by Altech was used at regular intervals.
RESULTS
AND DISCUSSION
Metal dispersion
Series SOR catalysts distinctly showed the (111) line of rhodium and/or the (101) and (100 ) lines of ruthenium indicating the presence of large crystallites of the individual metals. No new line due to any mixed metal cluster was observed. From Table 1 it can be seen that the rhodium particles were generally larger than the ruthenium particles. SVR catalysts were all X-ray amorphous, indicating that the metal particles were smaller than 5 nm, the detection limit for X-ray diffraction. Thus, the method of vacuum drying of the impregnated mass followed by direct reduction had the distinct advantage of giving smaller metal crystallites indicating better dispersion of these catalysts. The above conclusion was supported by the results of hydrogen chemisorption. It can be seen from Table 1 that SVR samples had much higher dispersion. The formation of mixed metal clusters is more likely in such highly dispersed catalysts [ 111. The absence of any metal lines in the XRD line proTABLE Particle size (from XRD ) and dispersion Rh-Ru/SiO, catalysts Catalyst
Rh-SOR Rh8,Ru,,-SOR Rhi,,Ru,,-SOR Rh-SVR Rh,,,Ru,,-SVR RhioRu,O-SVR Rh,,Ru,,-SVR Ru-SVR
Metal particle
(from hydrogen chemisorption)
size (nm)
Rhodium
Ruthenium
35.7 21.8 37.4
14.5 15.1
Dispersion
0.26 0.09 0.05 0.48 0.54 0.54 0.73 0.63
of metals in the various
233
files of these catalysts suggests that the particles were very small where mixed metal clusters may have been formed. Methanation
of carbon
monoxide
On the SOR catalysts, the percentage of methane formed varied linearly with metal dispersion in the catalyst over the entire temperature range (Fig. 1). The variation in methanation activity of these catalysts was then due to the number of metal sites on the surface. Fig. 2 shows conversion to methane as a function of temperature on the various compositions of the SOR catalysts. That the change in activity was not due to any cocluster formation in the bimetallic catalysts was evident from the fact that the XRD patterns showed very distinct lines due to individual metal particles. Ruthenium is known to be far more active than rhodium as a methanation catalyst. Hence the higher activity of the rhodium catalyst compared to that of the bimetallics is due to higher metal dispersion. A discussion of the relation between composition and activity for this series of catalysts would not be meaningful in view of the wide variation in dispersion. Conversion to methane as a function temperature for the SVR catalysts is
t
673K
METAL
Fig. 1. Dependence
DISPERSION
of the percent
(%)
of methane
-
formed on total metal dispersion
in the SOR cata-
1
548 598 TEMPERATURE
490
I
698
648 (K)-+
Fig. 2. Conversion to methane as a function of temperature: SOR, (A-A ) Rh,,,Ru,,,-SOR.
(o-o
) Rb-SOR,
(O-_O
7oc
) Rh,,Ru,,,m
o673K
I
f
a 5
Y
4%
2
c 448
498 548 TEMPERATURE
598 [K) 4
0
648
Fig. 3. Conversion to methane as a function of temperature; SVR, (0-E ) Rh,,Ru,,-SVR, (m-o ) Rh80Ry,,-SVR, Fig. 4. Conversion
to methane
as a function
20
CATALYST
(o-o (M-B)
40
) Ru-SVR, Rh-SVR.
of catalyst composition
60
COMPOSITION
(A-A
80
100
(%Ru)+
) Rh,,Ru,,-
at various temperatures.
235
shown in Fig. 3. Since most samples in this series had comparable dispersion, it is possible to discuss methanation activity as a function of catalyst composition more meaningfully. As expected, Ru-SVR was the most active catalyst at least up to 573 K. At higher temperatures, Rh,,Ru,,--SVR was the most active catalyst. The effect of catalyst composition on methanation activity is more clearly seen from Fig. 4. Up to 573 K, there was a sharp drop in conversion to methane as the rhodium-to-ruthenium ratio fell below four and little change was observed thereafter. Similar results were reported by Miura et al. [ 71 for turnover numbers (TON) vs. composition plots at 498 K. In agreement with these authors we are inclined to conclude that an ensemble of a minimum of four ruthenium atoms is necessary for good methanation activity. Dilution of ruthenium beyond this limit lowers the activity to a nearly constant level. If these catalysts contained segregated particles of the individual metals, methanation activity would not remain unaffected as it did but would fall systematically with the decrease in ruthenium content since the latter is a much superior methanation catalyst. Although methanation was a structure sensitive reaction, the effect of overall particle size was not a major one as at least three of the catalyst, Rh-SVR, Rh,,Ru,,-SVR and Rh50Ru50-SVR, had very close dispersion and Ru-SVR was not very much higher. From these results, it appears that the change in catalytic activity is related to the formation of mixed metal clusters in the SVR series. At higher temperatures, the bimetallic catalysts and even rhodium showed methanation activity comparable to that of ruthenium. This was because at 600 K and above, ruthenium catalyses the formation of carbon dioxide by the water-gas shift reaction. This was increasingly suppressed as the rhodium content increased. This is shown in Fig. 5. These results are in agreement with the report of Solymosi et al. [6] that supported rhodium is an excellent catalyst for the methanation of carbon dioxide. The role of rhodium in these catalysts is to convert the carbon dioxide formed at high temperature to methane. Thus, for methanation of carbon monoxide at higher temperatures, bimetallic RhRu catalysts are more advantageous than ruthenium alone. Influence of method of preparation A question pertinent to this study is why did mixed metal clusters not form in series SOR? There could be two contributing factors. First, during catalyst preparation the slurry was dried by slow heating, unlike series SVR whose drying was carried out at room temperature. That the formation of mixed metal clusters would strongly depend on the degree of dispersion and hence on the size of the metal crystallites, and that mixed metal surface sites would form on very small metal crystallites has been extensively discussed [ 111. Conditions conducive to the growth of metal particles would lead to phase segregation. In
236
2Q-
0 523
573 623 TEMPERATURE
673 (K)+
Fig. 5. Conversion to carbon dioxide as a function of temperature on the SVR catalysts; (o-o ) Ru-SVR, (A-A ) Rh20Ru80-SVR, (0-K ) Rh,,Ru,,-SVR, (o-o ) Rh8,Ry2&3VR, (m-m) Rh-SVR.
the case of SOR catalysts, the metal particle sizes were fairly large and dispersions low. The drying step itself could lead to segregation because of the higher temperature employed. The second contributing factor could be the preoxidation treatment. It was found that certain supported bimetallics segregate on oxidation but there was no uniform pattern in this respect. Oxygen treatment did not lead to segregation of the metals for Pt-Ir/Al*O, f 121 and Pt-Rh/SiO, [ 13 1, but segregation was observed in the case of Pt-Ru/SiO, [10,13], Pt-Re/ A1203 [ 121 and Pt-Ni/SiOz [ 15 1. Only specific studies on the influence of oxidation-reduction can unambiguously settle this question. However, it may be noted that some studies have established the sintering of ruthenium during calcination prior to reduction [ 141. It is significant that mixed metal clusters were formed in series SVR. The maximum solid solubility of ruthenium in rhodium is 20.3 atomic percent and that of rhodium in ruthenium is 19.7 atomic percent [ 161. Despite these severe miscibility constraints mixed metal clusters can be formed. Such formation of mixed metal clusters has been reported for several bimetallic systems consisting of a Group VIII and a Group IB metal of low bulk miscibility in a highly dispersed state. CONCLUSIONS
The following conclusions may be drawn from this study: (i) The formation of silica supported Rh-Ru bimetallic coclusters depended on the method of preparation. While heat drying of the impregnated support followed by preoxidation and reduction led to large segregated metal particles,
237
vacuum drying at room temperature followed by direct reduction resulted in highly dispersed mixed metal clusters. (ii) Addition of more than 20 percent rhodium to ruthenium led to a drastic fall in methanation activity of the bimetallic cluster catalysts justifying the earlier conclusion that an ensemble of four or more ruthenium atoms was necessary for good methanation activity. (iii) At higher temperatures, the bimetallics and even rhodium competed with ruthenium in methanation activity. This is because the conversion of carbon monoxide to carbon dioxide by water-gas shift reaction at higher temperature is suppressed in the presence of rhodium, probably due to conversion of carbon dioxide to methane.
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