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A study of the ruthenium–alumina system
Applied Catalysis A: General 170 (1998) 307±314
A study of the ruthenium±alumina system P. Betancourta,b, A. Rivesb, R. Hubautb, C.E. Scotta,*, J. Goldwassera a
Universidad Central de Venezuela, Centro de CataÂlisis PetroÂleo y PetroquõÂmica, Escuela de QuõÂmica, Facultad de Ciencias, Apartado Postal 47102, Los Chaguaramos, Caracas 1020-A, Venezuela b Laboratoire de Catalyse HeteÂrogeÁne et HomogeÁne, URA CNRS 402, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq CeÂdex, France Received 18 November 1997; received in revised form 10 February 1998; accepted 10 February 1998
Abstract Ru/alumina catalysts (0.21±5.11 wt% Ru) were characterized using temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and H2 and CO chemisorptions. The transformations of cyclohexene were used as test reactions. The TPR data showed, for the catalysts calcined at 5008C, two peaks at 1908C and 2238C. The high temperature peak becomes quantitatively more important as the Ru content is increased. With the aid of XPS and H2 and CO chemisorption, the low temperature peak is associated with a well-dispersed ruthenium phase while the high temperature peak is related to the reduction of RuO2 species. As expected from the dispersion measurements, the latter decreases with increasing Ru contents, in agreement with the literature. The catalytic results are in line with the characterization studies, showing an increase in the activity for the hydrogenolysis reaction (formation of methane) over the hydrogenation±dehydrogenation reactions, as the Ru content is increased. The latter can be explained in terms of the structural requirements of the hydrogenolysis reaction reported previously. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium/Al2O3; Dispersion; Reducibility; TPR; Hydrogenolysis
1. Introduction Ruthenium catalysts have been the subject of a variety of studies due to their relevance in hydrotreating [1±5] and Fischer±Tropsch [6,7] processes. Particularly, reduced ruthenium supported catalysts have been thoroughly investigated for over 30 years [8± 25,27±30]. These studies were conducted to understand better the catalytic and surface properties of
*Corresponding authors, solely on behalf of all equally contributing authors. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00061-1
these solids. Early results indicated that the speci®c rate for different catalytic reactions, as well as the adsorption properties of reduced ruthenium supported catalysts, showed a strong dependence on the metal dispersion [8±12]. For example, by studying cyclohexane conversion on supported ruthenium catalysts, Lam and Sinfelt [8] found that the ratio of dehydrogenation to hydrogenolysis increased by more than a factor of 10, by decreasing the metal particle size. Dalla Betta [9], and Yang and Goodwin [10] found that the CO/H ratio, calculated from the corresponding chemisorption experiments, increased with decreasing crystallite size. Infrared (IR) experiments also showed
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a variation in the carbon±oxygen stretching frequency as the crystallite size was modi®ed. Taylor [11], by using oxygen chemisorption and H2±O2 titration methods to measure the metallic dispersion over reduced ruthenium±alumina catalysts, concluded that it was inappropriate to use these methods for crystallites smaller than 4 nm, since signi®cant changes in adsorption stoichiometry occurred over these small metal particles. In a detailed study, King [13] reported that the speci®c reaction rate for the formation of methane (using COH2 as reactants) decreased with increasing Ru dispersion. A similar dependence with the metal dispersion was shown by the CO uptake. Kellner and Bell [14] studied the extent to which metal dispersion affects the activity and selectivity of reduced Ru/ Al2O3 catalysts used for CO hydrogenation. For dispersions over 0.7 a very rapid decline in the turnover frequency was observed. This fast decrease in speci®c activity was suggested to be due to changes in the electronic properties of the small crystallites or to particular interactions of metal crystallites with the support. For unsupported ruthenium samples, Don et al. [15] showed that the chlorine and oxygen strongly bound to the metal atoms in ruthenium(IV) oxide, can be considerably diminished by calcining it in air prior to reduction. More recent work by Lu and Tatarchuk [16,17] showed that hydrogen chemisorption over supported ruthenium catalysts became activated in the presence of chlorine adatoms apparently due to an electronic perturbation of the metal by chlorine. This effect did not occur in the absence of adsorbed chlorine and is increasingly more pronounced as the metal dispersion is increased. Kikuchi et al. [18] observed changes in the turnover frequency (TOF) for CO hydrogenation with the metallic dispersion over Ru/Al2O3 and Ru/V2O3. However, TOF over Ru/V2O3 was considerably higher than that over Ru/Al2O3, indicating that the TOF was dependent not only on the dispersion but also on the nature of the support. Hydrogenation of aromatic compounds to the corresponding cycloalkanes, using ruthenium catalysts, has attracted the attention of many research groups [19±25]. Catalytic hydrogenation of benzene over ruthenium catalysts has been carried out, either in the gas or liquid phase, over a wide range of experi-
mental conditions. It proceeds in agreement with the associative mechanism proposed by Horiuti and Polanyi [26], with formation of adsorbed half-hydrogenated species and desorption of cyclohexane and, scarcely, cyclohexadienes, into the gas phase. The high activity of ruthenium in benzene hydrogenolysis was ®rstly studied by Schuit and van Reijen [27], and Hartog et al. [28]. Other results by Liberman et al. [29] indicated that ruthenium is a highly active catalyst for hydrogenolysis of cyclohexane to aliphatic hydrocarbons. Kubicka [30] found that the TOF of different supported metals for hydrogenolysis of benzene decreased in the following order: Ru>Tc>Re. From the above discussion it can be seen that although reduced ruthenium±alumina catalysts have been thoroughly investigated in the past, certain features remain unclear. For example, how many reducible ruthenium species can be detected as the Ru loading is increased? Is there a relationship between these different Ru species and the metallic dispersion? Is the catalytic activity for the cyclohexene transformations (formation of cyclohexane, benzene and methane), related to the aforementioned Ru species? The main purpose of the present paper is to provide some insight into these questions. 2. Experimental 2.1. Catalysts The support used was a g-Al2O3 (AKZO, CK-300, surface area: 255 m2/g). The ruthenium salt (RuCl3, research grade) was supplied by Merck. The catalysts with different ruthenium contents were prepared by the wet impregnation method, using aqueous RuCl3 solutions. Subsequently, the impregnated solids were dried in air at 1208C. A portion of these samples was calcined under dry air at 5008C. A fraction of the 5.85 wt% ruthenium±alumina solid was used without previous calcination. The solids are designated as Rux, where x stands for the Ru wt%. 2.2. Methods Ruthenium contents were determined with the aid of an atomic emission spectrometer with an inductively coupled plasma (ICP) source attached to it. For
P. Betancourt et al. / Applied Catalysis A: General 170 (1998) 307±314 Table 1 Metal content and surface areas of the ruthenium±alumina catalysts Catalyst
Ru (wt%)
Temperature of calcination (8C)
Surface area (m2/g)
Ru0.2 Ru0.5 Ru0.8 Ru1 Ru2 Ru4 Ru6 Ru6
0.21 0.43 0.81 0.90 1.62 3.82 5.11 5.85
500 500 500 500 500 500 500 No calcination
222 219 207 203 194 188 180 224
the catalysts subjected to calcination, the wt% Ru was obtained after the calcination process. The different Ru contents correlated very well with the theoretical loadings expected from the impregnation method. The Ru contents for the calcined (5008C) and uncalcined Ru6 catalyst were 5.11 and 5.85 wt%, respectively, as shown in Table 1. Speci®c surface areas (SSA) were determined using a commercial Micromeritics ASAP 2400 surface area analyzer at liquid nitrogen temperature. H2 and CO chemisorption experiments were performed in a conventional BET system similar to that used in [31,32]. Catalyst samples of approximately 0.2 g were placed in a glass microreactor and reduced in situ with pure H2 (60 cm3/min) for 2 h at 5008C. After evacuating the catalysts at the same temperature for 1 h, two adsorption isotherms were obtained at room temperature. The linear region of the ®rst isotherm was extrapolated to zero pressure in order to calculate the amount of physisorbed and chemisorbed H2 or CO. After an evacuation period of 1 h, the second isotherm for both gases was obtained. The same procedure (as in the ®rst isotherm) was used to calculate the amount of physisorbed species. The irreversibly held H2 or CO was calculated from the difference between both values. A 1:1 H:Ru [9] stoichiometry was assumed to calculate the metallic dispersion (H/Ru). TPR experiments were carried out with the aid of a Micromeritics 2900 TPD/TPR apparatus. 100 mg of catalyst and a mixture 10% (v/v) H2±Ar (20 cm3 (STP)/min) were used. The sample was previously heated to 1508C under dry N2 (30 cm3 (STP)/min) for 1 h and subsequently cooled to room temperature. The temperature was then increased under the hydrogen
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mixture at 108C/min and the TPR pro®les were recorded. The temperature was raised to 5008C. XPS analysis were performed with a Leybold± Heraeus LHS10 spectrometer using a non-monochromatized Al K source (h1486.6 eV) operated at 300 W. Oxide catalyst samples were pressed onto an indium support. Binding energy corrections were made using the Al 2p photopeak as an internal standard (assuming a value of 74.0 eV for Al 2p in alumina). The Ru/Al intensity ratios were determined from the integrated intensities of Ru 3d5/2 peak (after a gaussian deconvolution) and the Al 2p peak, after subtraction of the linear background. Catalytic experiments were performed in a continuous ¯ow ®xed bed microreactor operated at atmospheric pressure. Prior to the reactions, the catalyst was reduced under a ¯ow of pure H2 (60 cm3 (STP)/min) for 2 h at 5008C. For the transformations of cyclohexene, the partial pressure of the hydrocarbon was kept constant at 40 Torr by means of a cold water bath (6.78C) that surrounded a saturator where hydrogen became saturated with the corresponding vapor pressure of cyclohexene. The reaction parameters used were: reaction temperature2508C, H2 ¯ow60 cm3/min, weight of catalyst20±40 mg. The conversion level was kept below 10%. The reaction products were analyzed by on-line gas chromatography, using a Hewlett-Packard 5890 chromatograph with a ¯ame ionization detector. An HP-1 (methyl silicone gum) column was used for separation purposes. 3. Results and discussion 3.1. Characterization studies Table 1 shows the metal contents and speci®c surface areas for the series of Ru/Al2O3 catalysts. It can be seen that the SSA, for the catalysts calcined at 5008C, decreases as the metal loading increases. The SSA for the Ru6 catalyst (calcined at 5008C) is about 70% of that of the pure alumina. The TPR pro®les are displayed in Fig. 1. The results are summarized in Table 2. There are signi®cant differences between calcined and the non-calcined sample. A sharp signal for the non-calcined Ru6 solid (trace (h), Fig. 1) located at a temperature at
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Fig. 1. TPR profiles for the Rux/Al2O3 catalysts: (a) Ru0.2; (b) Ru0.5; (c) Ru0.8; (d) Ru1; (e) Ru2; (f) Ru4; (g) Ru6 (calcined 5008C); (h) Ru6 (non-calcined). Table 2 TPR data Catalyst
Tm (8C)
A2/A1
Ru0.2 Ru0.5 Ru0.8 Ru1 Ru2 Ru4 Ru6
195±220 188±222 190±227 190±224 192±227 185±223 191±226
0.8 1.3 1.6 2.7 1.5 2.9 3.2
peak maximum (Tm) of 1328C was detected. This Tm is well within the range reported for the complete reduction of unsupported RuCl3 (1608C [33]). This result may suggest that the latter is the main species present on the uncalcined catalyst. A broad shoulder can also be observed at 1128C. It should be noted that this is the only catalyst where this shoulder appears. Its identi®cation is rather dif®cult and it can be due to the reduction of unknown ruthenium species present on the catalyst surface. In any case, the reduction process takes place at signi®cantly lower temperatures than for the calcined samples. This result may indicate the existence of different ruthenium species on the surface of the uncalcined and calcined ruthenium solids, vide infra. The TPR patterns for the calcined catalysts are shown in Fig. 1 (traces (a)±(g)). As observed two
peaks are always present. The Tm's for the high temperature peak are within the 22348C range. As bulk RuO2 displays only one peak in the thermogram, located at about 2178C [33], it can be assumed that the high temperature peak observed in the different thermograms (Fig. 1, traces (a)±(g)) corresponds to the reduction of bulk RuO2. The Tm's for the low temperature peak are located within the 19058C range. The relative areas of these two peaks change with the ruthenium content. Table 2 shows an increase in the ratio of the area of the high temperature peak with respect to that of the low temperature signal (A2/A1), as the ruthenium loading is increased. By just analyzing the TPR data it is very dif®cult to identify the species whose reduction process generates the low temperature peak. However, it can be safely stated that different ruthenium species are present on the catalysts calcined at 5008C. Their relative contribution can be estimated from the A2/A1 ratios. Table 3 shows the H/Ru and CO/Ru ratios for the calcined catalysts. As observed both ratios decrease as the ruthenium content is increased, in agreement with the literature [35,36], as the dispersion (H/Ru) of the ruthenium phase has been reported to decrease with increasing metal loading [9,16,17,34±36]. The last column of Table 3 shows the CO/H ratios. It can be seen that the CO/H ratios decrease as the Ru content is increased. This can be expected in terms of multiple adsorption of CO on small metal crystallites [35]. As the latter increases, for higher Ru contents, the formation of subcarbonyl species is no longer favored due to the relative decrease in the number of sites (corners, edges) that are capable of multiple CO adsorption. In fact, for the Ru4 and Ru6 solids, the CO/H ratio is lower than 1, suggesting the formation of bridge-type Ru±CO structures. It is well documented in the literature [35,41] that the formation of CO Table 3 Metal dispersion of Ru/Al2O3 catalysts calcined at 5008C Catalyst
H/Ru
CO/Ru
CO/H
Ru0.2 Ru0.5 Ru0.8 Ru1 Ru2 Ru4 Ru6
0.752 0.508 0.415 0.380 0.241 0.200 0.144
2.410 1.574 1.079 0.615 0.265 0.180 0.072
3.2 3.1 2.6 1.6 1.1 0.9 0.5
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bridged species is favored as the size of the metal particles increase. XPS was used to study the surface coverage of the ruthenium species on the unreduced catalysts calcined at 5008C. Fig. 2 shows a typical XPS spectrum for the
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Ru0.5 catalyst. Fig. 3 shows the experimental Ru 3d5/2/Al 2p intensity ratios plotted as a function of the atomic (bulk) Ru/Al ratio. Also included are the Ru/Al ratios predicted by the model of Kerkhof and Moulijn [37] assuming monolayer coverage (solid
Fig. 2. XPS spectrum of unreduced Ru0.5 catalyst.
Fig. 3. XPS Ru/Al intensity ratios as a function of Ru content.
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line). It can be seen that for Ru0.5 catalyst, the Ru/Al intensity ratio agrees with the value calculated by the monolayer model. For loadings 0.90 wt% Ru, the Ru/Al intensity ratios remain basically constant with increasing Ru/Al atomic ratio, indicating a decrease in the Ru dispersion with increasing Ru content. Similar XPS patterns have been reported for the cobalt±alumina system [38]. These results suggest the formation of a well-dispersed ruthenium species on the low content Ru catalysts (Ru0.5), and a loss of metallic dispersion as the Ru content is increased as the result of formation of bulk-type Ru species, e.g., RuO2, in agreement with the H2 and CO chemisorption and TPR data. The nearly constant value for the Ru/Al intensity ratios, obtained for catalysts with loadings above 0.9 wt% Ru, is at variance with the decrease in H/Ru or CO/Ru ratios (for increasing ruthenium loading above 0.9 wt% Ru) observed in the chemisorption experiments. This can be understood in terms of the following: No variation in the XPS intensity ratios would be expected for supported metal oxide crystallites having a particle size which is greater than the photoelectron escape depth of the metal component. Evidently, the chemisorption values are not limited by the latter. In light of the XPS and chemisorption results, the low temperature peak observed in the thermogram of the catalysts calcined at 5008C can be assigned to the reduction of the well-dispersed ruthenium species aforementioned, yielding well-dispersed metallic species. As the Ru loading is increased, the high
temperature peak becomes more important (Table 2) due to the formation of bulk RuO2 species. 3.2. Catalytic experiments Fig. 4 shows the results for the catalytic transformations of cyclohexene over the series of Ru/alumina catalysts calcined at 5008C. At low Ru content (Ru0.2), cyclohexane (hydrogenation product) is the sole product observed. As the ruthenium content increases, the cyclohexane decreases while benzene (dehydrogenation) and methane (hydrogenolysis) are formed. In fact, for contents 0.90 wt% Ru, only benzene and methane are observed in the product distribution. It can also be seen that, for the higher loading catalysts, the formation of methane becomes more important compared to that of benzene. The catalytic results shown in this work are in line with the characterization results shown in Section 3.1 and with different reports from the literature [8,30,33,34]. At low ruthenium contents, where the fraction of the well-dispersed Ru phase (Tables 2 and 3) is quantitatively important, the hydrogenation± dehydrogenation reactions are strongly favored over the hydrogenolysis reaction. As the Ru content increases (the fraction of metallic ruthenium generated by the reduction of RuO2 is now dominant (Table 2)), the hydrogenolysis reaction becomes increasingly more important. The latter has been thoroughly investigated [39,40,42] over a variety of
Fig. 4. Cyclohexene transformations for Ru/Al2O3 calcined at 5008C. Conversion (%) vs. wt% Ru. For experimental details see text.
P. Betancourt et al. / Applied Catalysis A: General 170 (1998) 307±314
metallic surfaces. It is now widely accepted that this reaction is structure sensitive and that metallic ensembles over a certain size are needed for the reaction to occur [8,42]. Consequently, as the Ru metal particle size increases, the hydrogenolysis reaction should be favored over the hydrogenation±dehydrogenation reactions. Our results agree with that of Lam and Sinfelt [8] who have shown that the ratio of the dehydrogenation rate to the hydrogenolysis rate for cyclohexane increases drastically with increasing ruthenium dispersion. Finally, the change in C6 product selectivity with increasing metal loading deserves a comment. It is noteworthy that on Ru0.2 and Ru0.5 catalysts cyclohexane is the main C6 product while for the other catalysts benzene prevails. A possible explanation is the type of adsorbed intermediates involved in the hydrogenation and dehydrogenation of cyclohexene. For the former an isolated double bond must be hydrogenated while for the latter the whole ring must be involved in order to produce benzene. Several reports have shown differences between the hydrogenation of cyclohexene and other reactions such as hydrogenation of benzene and dehydrogenation of cyclohexane (where cyclic intermediates have been proposed [42]). Early studies [43] have shown that the C6H12)C6H63H2 reaction was poisoned by tetrasubstituted silanes on Pt/C at 3008C. The catalyst was, however, still active for double bond hydrogenation. More recent work [44] on the transformations of cyclohexene over Ni, Rh and platinum supported catalysts, has shown that the hydrogenation reaction is less sensitive to poisoning by Et3SiH than the dehydrogenation reaction (formation of benzene). Rehban and Haensel [45] showed that a 0.001 wt% Pt/Al2O3 catalyst was active for cyclohexene hydrogenation but not for benzene hydrogenation. Seemingly, the well-dispersed ruthenium particles are more prone to hydrogenate cyclohexene than to be involved in the formation of cyclic intermediates needed for the formation of benzene. 4. Conclusions Ru/alumina catalysts (0.21±5.11 wt% Ru) were characterized using TPR, XPS and H2 and CO chemisorptions. The transformations of cyclohexene were
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used as test reactions. The TPR data showed, for the catalysts calcined at 5008C, two peaks at 1908C and 2238C. The latter becomes quantitatively more important as the Ru content is increased. With the aid of XPS and H2 and CO chemisorption, the low temperature peak is associated with a well-dispersed ruthenium phase while the high temperature peak is related to the reduction of bulk RuO2 species. As expected from CO and hydrogen chemisorption as well as from the XPS experiments, the dispersion decreases with increasing Ru contents, in agreement with the literature. The catalytic results are in line with the characterization studies, showing an increase in the activity for the hydrogenolysis reaction (formation of methane) over the hydrogenation±dehydrogenation reactions, as the Ru content is increased. The latter can be explained in terms of the structural requirements of the hydrogenolysis reaction reported previously. Acknowledgements We would like to thank CONICIT through Grant S1-2698 and Franco-Venezuelan PICS 324 for ®nancial support of this work. References [1] T.A. Pecoraro, R.R. Chianelli, J. Catal. 67 (1981) 430. [2] J.P.R. Vissers, C.K. Groot, E.M. Van Oers, V.H.J. de Beer, R.J. Prins, Bull. Soc. Chim. Belg. 93 (1984) 813. [3] P.C.H. Mitchell, C.E. Scott, J.P. Bonnelle, J.G. Grimblot, J. Catal. 107 (1987) 482. [4] Y.-J. Kuo, R.A. Cocco, B.J. Tatarchuk, J. Catal. 112 (1988) 250. [5] Y.-J. Kuo, B.J. Tatarchuk, J. Catal. 112 (1988) 229. [6] M.A. Vannice, Catal. Rev.-Sci. Eng. 14 (1976) 153. [7] M.A. Vannice, J. Catal. 44 (1976) 152. [8] Y.L. Lam, J.H. Sinfelt, J. Catal. 42 (1976) 319. [9] R.A. Dalla Betta, J. Phys. Chem. 79 (1975) 2519. [10] C.H. Yang, J.G. Goodwin, React. Kinet. Catal. Lett. 20 (1982) 13. [11] K.C. Taylor, J. Catal. 38 (1975) 299. [12] A. Sayari, H.T. Wang, J.G. Goodwin, J. Catal. 93 (1985) 368. [13] D.C. King, J. Catal. 51 (1978) 386. [14] C.S. Kellner, A.T. Bell, J. Catal. 75 (1982) 251. [15] J.A. Don, A.P. Pijpers, J.J.F. Scholten, J. Catal. 80 (1983) 296. [16] K. Lu, B.J. Tatarchuk, J. Catal. 106 (1987) 166. [17] K. Lu, B.J. Tatarchuk, J. Catal. 106 (1987) 176. [18] E. Kikuchi, H. Nomura, Y. Morita, Preprint of Sixth Japan± Soviet Catalysis Seminar, Osaka, 1981, p. 36. [19] M.M. Johnson, G.P. Nowack, J. Catal. 38 (1975) 518.
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A study of the ruthenium±alumina system P. Betancourta,b, A. Rivesb, R. Hubautb, C.E. Scotta,*, J. Goldwassera a
Universidad Central de Venezuela, Centro de CataÂlisis PetroÂleo y PetroquõÂmica, Escuela de QuõÂmica, Facultad de Ciencias, Apartado Postal 47102, Los Chaguaramos, Caracas 1020-A, Venezuela b Laboratoire de Catalyse HeteÂrogeÁne et HomogeÁne, URA CNRS 402, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq CeÂdex, France Received 18 November 1997; received in revised form 10 February 1998; accepted 10 February 1998
Abstract Ru/alumina catalysts (0.21±5.11 wt% Ru) were characterized using temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and H2 and CO chemisorptions. The transformations of cyclohexene were used as test reactions. The TPR data showed, for the catalysts calcined at 5008C, two peaks at 1908C and 2238C. The high temperature peak becomes quantitatively more important as the Ru content is increased. With the aid of XPS and H2 and CO chemisorption, the low temperature peak is associated with a well-dispersed ruthenium phase while the high temperature peak is related to the reduction of RuO2 species. As expected from the dispersion measurements, the latter decreases with increasing Ru contents, in agreement with the literature. The catalytic results are in line with the characterization studies, showing an increase in the activity for the hydrogenolysis reaction (formation of methane) over the hydrogenation±dehydrogenation reactions, as the Ru content is increased. The latter can be explained in terms of the structural requirements of the hydrogenolysis reaction reported previously. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium/Al2O3; Dispersion; Reducibility; TPR; Hydrogenolysis
1. Introduction Ruthenium catalysts have been the subject of a variety of studies due to their relevance in hydrotreating [1±5] and Fischer±Tropsch [6,7] processes. Particularly, reduced ruthenium supported catalysts have been thoroughly investigated for over 30 years [8± 25,27±30]. These studies were conducted to understand better the catalytic and surface properties of
*Corresponding authors, solely on behalf of all equally contributing authors. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00061-1
these solids. Early results indicated that the speci®c rate for different catalytic reactions, as well as the adsorption properties of reduced ruthenium supported catalysts, showed a strong dependence on the metal dispersion [8±12]. For example, by studying cyclohexane conversion on supported ruthenium catalysts, Lam and Sinfelt [8] found that the ratio of dehydrogenation to hydrogenolysis increased by more than a factor of 10, by decreasing the metal particle size. Dalla Betta [9], and Yang and Goodwin [10] found that the CO/H ratio, calculated from the corresponding chemisorption experiments, increased with decreasing crystallite size. Infrared (IR) experiments also showed
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a variation in the carbon±oxygen stretching frequency as the crystallite size was modi®ed. Taylor [11], by using oxygen chemisorption and H2±O2 titration methods to measure the metallic dispersion over reduced ruthenium±alumina catalysts, concluded that it was inappropriate to use these methods for crystallites smaller than 4 nm, since signi®cant changes in adsorption stoichiometry occurred over these small metal particles. In a detailed study, King [13] reported that the speci®c reaction rate for the formation of methane (using COH2 as reactants) decreased with increasing Ru dispersion. A similar dependence with the metal dispersion was shown by the CO uptake. Kellner and Bell [14] studied the extent to which metal dispersion affects the activity and selectivity of reduced Ru/ Al2O3 catalysts used for CO hydrogenation. For dispersions over 0.7 a very rapid decline in the turnover frequency was observed. This fast decrease in speci®c activity was suggested to be due to changes in the electronic properties of the small crystallites or to particular interactions of metal crystallites with the support. For unsupported ruthenium samples, Don et al. [15] showed that the chlorine and oxygen strongly bound to the metal atoms in ruthenium(IV) oxide, can be considerably diminished by calcining it in air prior to reduction. More recent work by Lu and Tatarchuk [16,17] showed that hydrogen chemisorption over supported ruthenium catalysts became activated in the presence of chlorine adatoms apparently due to an electronic perturbation of the metal by chlorine. This effect did not occur in the absence of adsorbed chlorine and is increasingly more pronounced as the metal dispersion is increased. Kikuchi et al. [18] observed changes in the turnover frequency (TOF) for CO hydrogenation with the metallic dispersion over Ru/Al2O3 and Ru/V2O3. However, TOF over Ru/V2O3 was considerably higher than that over Ru/Al2O3, indicating that the TOF was dependent not only on the dispersion but also on the nature of the support. Hydrogenation of aromatic compounds to the corresponding cycloalkanes, using ruthenium catalysts, has attracted the attention of many research groups [19±25]. Catalytic hydrogenation of benzene over ruthenium catalysts has been carried out, either in the gas or liquid phase, over a wide range of experi-
mental conditions. It proceeds in agreement with the associative mechanism proposed by Horiuti and Polanyi [26], with formation of adsorbed half-hydrogenated species and desorption of cyclohexane and, scarcely, cyclohexadienes, into the gas phase. The high activity of ruthenium in benzene hydrogenolysis was ®rstly studied by Schuit and van Reijen [27], and Hartog et al. [28]. Other results by Liberman et al. [29] indicated that ruthenium is a highly active catalyst for hydrogenolysis of cyclohexane to aliphatic hydrocarbons. Kubicka [30] found that the TOF of different supported metals for hydrogenolysis of benzene decreased in the following order: Ru>Tc>Re. From the above discussion it can be seen that although reduced ruthenium±alumina catalysts have been thoroughly investigated in the past, certain features remain unclear. For example, how many reducible ruthenium species can be detected as the Ru loading is increased? Is there a relationship between these different Ru species and the metallic dispersion? Is the catalytic activity for the cyclohexene transformations (formation of cyclohexane, benzene and methane), related to the aforementioned Ru species? The main purpose of the present paper is to provide some insight into these questions. 2. Experimental 2.1. Catalysts The support used was a g-Al2O3 (AKZO, CK-300, surface area: 255 m2/g). The ruthenium salt (RuCl3, research grade) was supplied by Merck. The catalysts with different ruthenium contents were prepared by the wet impregnation method, using aqueous RuCl3 solutions. Subsequently, the impregnated solids were dried in air at 1208C. A portion of these samples was calcined under dry air at 5008C. A fraction of the 5.85 wt% ruthenium±alumina solid was used without previous calcination. The solids are designated as Rux, where x stands for the Ru wt%. 2.2. Methods Ruthenium contents were determined with the aid of an atomic emission spectrometer with an inductively coupled plasma (ICP) source attached to it. For
P. Betancourt et al. / Applied Catalysis A: General 170 (1998) 307±314 Table 1 Metal content and surface areas of the ruthenium±alumina catalysts Catalyst
Ru (wt%)
Temperature of calcination (8C)
Surface area (m2/g)
Ru0.2 Ru0.5 Ru0.8 Ru1 Ru2 Ru4 Ru6 Ru6
0.21 0.43 0.81 0.90 1.62 3.82 5.11 5.85
500 500 500 500 500 500 500 No calcination
222 219 207 203 194 188 180 224
the catalysts subjected to calcination, the wt% Ru was obtained after the calcination process. The different Ru contents correlated very well with the theoretical loadings expected from the impregnation method. The Ru contents for the calcined (5008C) and uncalcined Ru6 catalyst were 5.11 and 5.85 wt%, respectively, as shown in Table 1. Speci®c surface areas (SSA) were determined using a commercial Micromeritics ASAP 2400 surface area analyzer at liquid nitrogen temperature. H2 and CO chemisorption experiments were performed in a conventional BET system similar to that used in [31,32]. Catalyst samples of approximately 0.2 g were placed in a glass microreactor and reduced in situ with pure H2 (60 cm3/min) for 2 h at 5008C. After evacuating the catalysts at the same temperature for 1 h, two adsorption isotherms were obtained at room temperature. The linear region of the ®rst isotherm was extrapolated to zero pressure in order to calculate the amount of physisorbed and chemisorbed H2 or CO. After an evacuation period of 1 h, the second isotherm for both gases was obtained. The same procedure (as in the ®rst isotherm) was used to calculate the amount of physisorbed species. The irreversibly held H2 or CO was calculated from the difference between both values. A 1:1 H:Ru [9] stoichiometry was assumed to calculate the metallic dispersion (H/Ru). TPR experiments were carried out with the aid of a Micromeritics 2900 TPD/TPR apparatus. 100 mg of catalyst and a mixture 10% (v/v) H2±Ar (20 cm3 (STP)/min) were used. The sample was previously heated to 1508C under dry N2 (30 cm3 (STP)/min) for 1 h and subsequently cooled to room temperature. The temperature was then increased under the hydrogen
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mixture at 108C/min and the TPR pro®les were recorded. The temperature was raised to 5008C. XPS analysis were performed with a Leybold± Heraeus LHS10 spectrometer using a non-monochromatized Al K source (h1486.6 eV) operated at 300 W. Oxide catalyst samples were pressed onto an indium support. Binding energy corrections were made using the Al 2p photopeak as an internal standard (assuming a value of 74.0 eV for Al 2p in alumina). The Ru/Al intensity ratios were determined from the integrated intensities of Ru 3d5/2 peak (after a gaussian deconvolution) and the Al 2p peak, after subtraction of the linear background. Catalytic experiments were performed in a continuous ¯ow ®xed bed microreactor operated at atmospheric pressure. Prior to the reactions, the catalyst was reduced under a ¯ow of pure H2 (60 cm3 (STP)/min) for 2 h at 5008C. For the transformations of cyclohexene, the partial pressure of the hydrocarbon was kept constant at 40 Torr by means of a cold water bath (6.78C) that surrounded a saturator where hydrogen became saturated with the corresponding vapor pressure of cyclohexene. The reaction parameters used were: reaction temperature2508C, H2 ¯ow60 cm3/min, weight of catalyst20±40 mg. The conversion level was kept below 10%. The reaction products were analyzed by on-line gas chromatography, using a Hewlett-Packard 5890 chromatograph with a ¯ame ionization detector. An HP-1 (methyl silicone gum) column was used for separation purposes. 3. Results and discussion 3.1. Characterization studies Table 1 shows the metal contents and speci®c surface areas for the series of Ru/Al2O3 catalysts. It can be seen that the SSA, for the catalysts calcined at 5008C, decreases as the metal loading increases. The SSA for the Ru6 catalyst (calcined at 5008C) is about 70% of that of the pure alumina. The TPR pro®les are displayed in Fig. 1. The results are summarized in Table 2. There are signi®cant differences between calcined and the non-calcined sample. A sharp signal for the non-calcined Ru6 solid (trace (h), Fig. 1) located at a temperature at
310
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Fig. 1. TPR profiles for the Rux/Al2O3 catalysts: (a) Ru0.2; (b) Ru0.5; (c) Ru0.8; (d) Ru1; (e) Ru2; (f) Ru4; (g) Ru6 (calcined 5008C); (h) Ru6 (non-calcined). Table 2 TPR data Catalyst
Tm (8C)
A2/A1
Ru0.2 Ru0.5 Ru0.8 Ru1 Ru2 Ru4 Ru6
195±220 188±222 190±227 190±224 192±227 185±223 191±226
0.8 1.3 1.6 2.7 1.5 2.9 3.2
peak maximum (Tm) of 1328C was detected. This Tm is well within the range reported for the complete reduction of unsupported RuCl3 (1608C [33]). This result may suggest that the latter is the main species present on the uncalcined catalyst. A broad shoulder can also be observed at 1128C. It should be noted that this is the only catalyst where this shoulder appears. Its identi®cation is rather dif®cult and it can be due to the reduction of unknown ruthenium species present on the catalyst surface. In any case, the reduction process takes place at signi®cantly lower temperatures than for the calcined samples. This result may indicate the existence of different ruthenium species on the surface of the uncalcined and calcined ruthenium solids, vide infra. The TPR patterns for the calcined catalysts are shown in Fig. 1 (traces (a)±(g)). As observed two
peaks are always present. The Tm's for the high temperature peak are within the 22348C range. As bulk RuO2 displays only one peak in the thermogram, located at about 2178C [33], it can be assumed that the high temperature peak observed in the different thermograms (Fig. 1, traces (a)±(g)) corresponds to the reduction of bulk RuO2. The Tm's for the low temperature peak are located within the 19058C range. The relative areas of these two peaks change with the ruthenium content. Table 2 shows an increase in the ratio of the area of the high temperature peak with respect to that of the low temperature signal (A2/A1), as the ruthenium loading is increased. By just analyzing the TPR data it is very dif®cult to identify the species whose reduction process generates the low temperature peak. However, it can be safely stated that different ruthenium species are present on the catalysts calcined at 5008C. Their relative contribution can be estimated from the A2/A1 ratios. Table 3 shows the H/Ru and CO/Ru ratios for the calcined catalysts. As observed both ratios decrease as the ruthenium content is increased, in agreement with the literature [35,36], as the dispersion (H/Ru) of the ruthenium phase has been reported to decrease with increasing metal loading [9,16,17,34±36]. The last column of Table 3 shows the CO/H ratios. It can be seen that the CO/H ratios decrease as the Ru content is increased. This can be expected in terms of multiple adsorption of CO on small metal crystallites [35]. As the latter increases, for higher Ru contents, the formation of subcarbonyl species is no longer favored due to the relative decrease in the number of sites (corners, edges) that are capable of multiple CO adsorption. In fact, for the Ru4 and Ru6 solids, the CO/H ratio is lower than 1, suggesting the formation of bridge-type Ru±CO structures. It is well documented in the literature [35,41] that the formation of CO Table 3 Metal dispersion of Ru/Al2O3 catalysts calcined at 5008C Catalyst
H/Ru
CO/Ru
CO/H
Ru0.2 Ru0.5 Ru0.8 Ru1 Ru2 Ru4 Ru6
0.752 0.508 0.415 0.380 0.241 0.200 0.144
2.410 1.574 1.079 0.615 0.265 0.180 0.072
3.2 3.1 2.6 1.6 1.1 0.9 0.5
P. Betancourt et al. / Applied Catalysis A: General 170 (1998) 307±314
bridged species is favored as the size of the metal particles increase. XPS was used to study the surface coverage of the ruthenium species on the unreduced catalysts calcined at 5008C. Fig. 2 shows a typical XPS spectrum for the
311
Ru0.5 catalyst. Fig. 3 shows the experimental Ru 3d5/2/Al 2p intensity ratios plotted as a function of the atomic (bulk) Ru/Al ratio. Also included are the Ru/Al ratios predicted by the model of Kerkhof and Moulijn [37] assuming monolayer coverage (solid
Fig. 2. XPS spectrum of unreduced Ru0.5 catalyst.
Fig. 3. XPS Ru/Al intensity ratios as a function of Ru content.
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line). It can be seen that for Ru0.5 catalyst, the Ru/Al intensity ratio agrees with the value calculated by the monolayer model. For loadings 0.90 wt% Ru, the Ru/Al intensity ratios remain basically constant with increasing Ru/Al atomic ratio, indicating a decrease in the Ru dispersion with increasing Ru content. Similar XPS patterns have been reported for the cobalt±alumina system [38]. These results suggest the formation of a well-dispersed ruthenium species on the low content Ru catalysts (Ru0.5), and a loss of metallic dispersion as the Ru content is increased as the result of formation of bulk-type Ru species, e.g., RuO2, in agreement with the H2 and CO chemisorption and TPR data. The nearly constant value for the Ru/Al intensity ratios, obtained for catalysts with loadings above 0.9 wt% Ru, is at variance with the decrease in H/Ru or CO/Ru ratios (for increasing ruthenium loading above 0.9 wt% Ru) observed in the chemisorption experiments. This can be understood in terms of the following: No variation in the XPS intensity ratios would be expected for supported metal oxide crystallites having a particle size which is greater than the photoelectron escape depth of the metal component. Evidently, the chemisorption values are not limited by the latter. In light of the XPS and chemisorption results, the low temperature peak observed in the thermogram of the catalysts calcined at 5008C can be assigned to the reduction of the well-dispersed ruthenium species aforementioned, yielding well-dispersed metallic species. As the Ru loading is increased, the high
temperature peak becomes more important (Table 2) due to the formation of bulk RuO2 species. 3.2. Catalytic experiments Fig. 4 shows the results for the catalytic transformations of cyclohexene over the series of Ru/alumina catalysts calcined at 5008C. At low Ru content (Ru0.2), cyclohexane (hydrogenation product) is the sole product observed. As the ruthenium content increases, the cyclohexane decreases while benzene (dehydrogenation) and methane (hydrogenolysis) are formed. In fact, for contents 0.90 wt% Ru, only benzene and methane are observed in the product distribution. It can also be seen that, for the higher loading catalysts, the formation of methane becomes more important compared to that of benzene. The catalytic results shown in this work are in line with the characterization results shown in Section 3.1 and with different reports from the literature [8,30,33,34]. At low ruthenium contents, where the fraction of the well-dispersed Ru phase (Tables 2 and 3) is quantitatively important, the hydrogenation± dehydrogenation reactions are strongly favored over the hydrogenolysis reaction. As the Ru content increases (the fraction of metallic ruthenium generated by the reduction of RuO2 is now dominant (Table 2)), the hydrogenolysis reaction becomes increasingly more important. The latter has been thoroughly investigated [39,40,42] over a variety of
Fig. 4. Cyclohexene transformations for Ru/Al2O3 calcined at 5008C. Conversion (%) vs. wt% Ru. For experimental details see text.
P. Betancourt et al. / Applied Catalysis A: General 170 (1998) 307±314
metallic surfaces. It is now widely accepted that this reaction is structure sensitive and that metallic ensembles over a certain size are needed for the reaction to occur [8,42]. Consequently, as the Ru metal particle size increases, the hydrogenolysis reaction should be favored over the hydrogenation±dehydrogenation reactions. Our results agree with that of Lam and Sinfelt [8] who have shown that the ratio of the dehydrogenation rate to the hydrogenolysis rate for cyclohexane increases drastically with increasing ruthenium dispersion. Finally, the change in C6 product selectivity with increasing metal loading deserves a comment. It is noteworthy that on Ru0.2 and Ru0.5 catalysts cyclohexane is the main C6 product while for the other catalysts benzene prevails. A possible explanation is the type of adsorbed intermediates involved in the hydrogenation and dehydrogenation of cyclohexene. For the former an isolated double bond must be hydrogenated while for the latter the whole ring must be involved in order to produce benzene. Several reports have shown differences between the hydrogenation of cyclohexene and other reactions such as hydrogenation of benzene and dehydrogenation of cyclohexane (where cyclic intermediates have been proposed [42]). Early studies [43] have shown that the C6H12)C6H63H2 reaction was poisoned by tetrasubstituted silanes on Pt/C at 3008C. The catalyst was, however, still active for double bond hydrogenation. More recent work [44] on the transformations of cyclohexene over Ni, Rh and platinum supported catalysts, has shown that the hydrogenation reaction is less sensitive to poisoning by Et3SiH than the dehydrogenation reaction (formation of benzene). Rehban and Haensel [45] showed that a 0.001 wt% Pt/Al2O3 catalyst was active for cyclohexene hydrogenation but not for benzene hydrogenation. Seemingly, the well-dispersed ruthenium particles are more prone to hydrogenate cyclohexene than to be involved in the formation of cyclic intermediates needed for the formation of benzene. 4. Conclusions Ru/alumina catalysts (0.21±5.11 wt% Ru) were characterized using TPR, XPS and H2 and CO chemisorptions. The transformations of cyclohexene were
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used as test reactions. The TPR data showed, for the catalysts calcined at 5008C, two peaks at 1908C and 2238C. The latter becomes quantitatively more important as the Ru content is increased. With the aid of XPS and H2 and CO chemisorption, the low temperature peak is associated with a well-dispersed ruthenium phase while the high temperature peak is related to the reduction of bulk RuO2 species. As expected from CO and hydrogen chemisorption as well as from the XPS experiments, the dispersion decreases with increasing Ru contents, in agreement with the literature. The catalytic results are in line with the characterization studies, showing an increase in the activity for the hydrogenolysis reaction (formation of methane) over the hydrogenation±dehydrogenation reactions, as the Ru content is increased. The latter can be explained in terms of the structural requirements of the hydrogenolysis reaction reported previously. Acknowledgements We would like to thank CONICIT through Grant S1-2698 and Franco-Venezuelan PICS 324 for ®nancial support of this work. References [1] T.A. Pecoraro, R.R. Chianelli, J. Catal. 67 (1981) 430. [2] J.P.R. Vissers, C.K. Groot, E.M. Van Oers, V.H.J. de Beer, R.J. Prins, Bull. Soc. Chim. Belg. 93 (1984) 813. [3] P.C.H. Mitchell, C.E. Scott, J.P. Bonnelle, J.G. Grimblot, J. Catal. 107 (1987) 482. [4] Y.-J. Kuo, R.A. Cocco, B.J. Tatarchuk, J. Catal. 112 (1988) 250. [5] Y.-J. Kuo, B.J. Tatarchuk, J. Catal. 112 (1988) 229. [6] M.A. Vannice, Catal. Rev.-Sci. Eng. 14 (1976) 153. [7] M.A. Vannice, J. Catal. 44 (1976) 152. [8] Y.L. Lam, J.H. Sinfelt, J. Catal. 42 (1976) 319. [9] R.A. Dalla Betta, J. Phys. Chem. 79 (1975) 2519. [10] C.H. Yang, J.G. Goodwin, React. Kinet. Catal. Lett. 20 (1982) 13. [11] K.C. Taylor, J. Catal. 38 (1975) 299. [12] A. Sayari, H.T. Wang, J.G. Goodwin, J. Catal. 93 (1985) 368. [13] D.C. King, J. Catal. 51 (1978) 386. [14] C.S. Kellner, A.T. Bell, J. Catal. 75 (1982) 251. [15] J.A. Don, A.P. Pijpers, J.J.F. Scholten, J. Catal. 80 (1983) 296. [16] K. Lu, B.J. Tatarchuk, J. Catal. 106 (1987) 166. [17] K. Lu, B.J. Tatarchuk, J. Catal. 106 (1987) 176. [18] E. Kikuchi, H. Nomura, Y. Morita, Preprint of Sixth Japan± Soviet Catalysis Seminar, Osaka, 1981, p. 36. [19] M.M. Johnson, G.P. Nowack, J. Catal. 38 (1975) 518.
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