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Microcalorimetric and catalytic investigations of transition metal nanoparticles intercalated in graphite
Colloids and Surfaces A: Physicochemical and Engineering Aspects 141 (1998) 397–403
Microcalorimetric and catalytic investigations of transition metal nanoparticles intercalated in graphite ´ . Mastalir a,*, Z. Kira´ly b, I. De´ka´ny b, M. Barto´k a A a Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, Jo´zsef Attila University, Do´m Te´r 8, 6720 Szeged, Hungary b Department of Colloid Chemistry, Jo´zsef Attila University, Aradi vt. tere 1, 6720 Szeged, Hungary Received 17 January 1997; received in revised form 20 June 1997; accepted 4 August 1997
Abstract The distribution of the metal content of pristine and thermally treated (medium-temperature reduction, MTR) Pt-, Pd- and Rh-graphite intercalation compounds (graphimets) was investigated by transmission electron microscopy, H titration and sorption microcalorimetry. It was established that MTR resulted in a migration of interlayer metal 2 atoms to the graphite surface accompanied by a moderate aggregation of the surface nanoparticles. Despite the increased metal content available for H adsorption after MTR, the catalytic activities decreased substantially. Besides 2 structural changes, the loss of catalytic performance was related to the increase of metal–hydrogen bond strengths suggested by the calorimetric data. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Catalytic hydrogenation; Graphimets; H titration; Sorption microcalorimetry; Transition metals; 2 Transmission electron microscopy
1. Introduction The formation of graphite intercalation compounds (GICs) is based on the layered structure of graphite and the difference in layer and interlayer bond strengths. The value of the in-plane C–C distance in graphite (0.141 nm) corresponds to the bond length of aromatic hydrocarbons. On the other hand, the interplanar C–C distance (interlayer spacing, 0.335 nm) is related to the formation of a weak van der Waals interaction between the graphite sheets. Intercalation takes place by insertion of various guest species (alkaliand alkali earth metals, acids, halogens or metal halides) between layers of the graphite host [1–5]. * Corresponding author.
Considering the widespread catalytic applications of transition metals in organic reactions, the present paper is focused on transition metal GICs (graphimets). Although graphite cannot be directly intercalated with transition metals, the required catalysts may be obtained by insertion and subsequent reduction of the corresponding metal salts [2,6 ]. The in situ reduction of intercalated transition metal halides can be performed by different reducing agents (H , NaBH , LiAlH , Na/NH , 2 4 4 3 Li, Na and K biphenyl ) [6,7]. Although the reduction results in the formation of finely divided subcolloidal metal particles between the graphite layers [8,9], the catalytic activity is mostly attributed to the presence of surface metal particles [10– 12]. Graphimets are claimed to be thermally unstable, so that the intercalated species may leave the interlayer space and migrate to the surface of
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graphite, even under relatively mild conditions [13]. The occurrence of such a deintercalation process upon medium-temperature reduction (MTR) has been reported previously [14,15]. The aim of the present study was to obtain more information on the effect of thermal treatment on the location and arrangement of the subcolloidal metal content of Pt-, Pd- and Rh-graphimets by different physicochemical methods generally used for catalyst characterization [16 ].
2. Experimental 2.1. Materials Pt-, Pd- and Rh-graphimet (1% metal in graphite each) were purchased from Alfa Chemical Company ( Karlsruhe, Germany). For each sample, the corresponding metal chlorides were intercalated into graphite in a chlorine stream and reduction was carried out with lithium-biphenyl [17]. MTR was effected in flowing H 2 (40 cm3 min−1) at 573 K for 1 h. The samples were then stored under He until use. 2.2. Methods 2.2.1. Transmission electron microscopy The particle size distribution and the average particle diameter of the surface metal particles were determined with an OPTON 902 electron microscope (Oberkochen, Germany) at 80 kV. Prior to measurement, the samples were ultrasonically dispersed in toluene and then deposited on a plastic film supported by a Cu grid. Magnifications were in the range 5×104–8.5×104 times. 2.2.2. H titration 2 Measurements were carried out in a flow reactor by using 9.9995% pure argon as the carrier gas. Titration was performed by 1.221 cm3 H pulses 2 at 298 K after preadsorbing O on the samples 2 over 2 weeks. From preliminary kinetic measurements, this period of time was found to be sufficient to measure D, the total number of exposed metal atoms accessible for H relative to the total number 2 of metal atoms. For Pd-graphimet, the temperature
of measurement was adjusted to 348 K in order to prevent hydride formation [18]. The D dispersion value was calculated for a stoichiometry M:H= 1:3 derived from the titration reaction M(O)+1.5 H M(H )+H O [19,20]. The n chemisorption 2 2 s capacity of H was calculated using n =wD/2A, 2 s where w is the metal content of graphimet and A is the atomic weight of the metal. The dispersion values of surface metal particles D (exclusive of s interlayer metal particles) were determined after preadsorbing O on the graphimets for 10 min. 2 This exposure time safely measures D for carbon supported transition metal catalysts. 2.2.3. Isothermal sorption microcalorimetry The schematic representation of the experimental apparatus designed in our laboratory for microcalorimetric investigations is shown in Fig. 1. The equipment consisted of three major units: a highvacuum system; a gas handling system; and the batch unit of an LKB 2107 isothermal sorption microcalorimeter, each connected to the main vacuum line. Details of the calorimeter vessel suitable for high vacuum operation have been described elsewhere [21]. Before use, both the pristine and MTR samples were pretreated with static H at 353 K for 1 h, followed by evacuation 2 at the same temperature for 2 h, and then for another 2 h at 298.15 K. This procedure was necessary to remove preadsorbed oxygen from the sur-
Fig. 1. Schematic representation of the equipment used for microcalorimetric measurements. HVS, high vacuum system ( p=10−4 Pa); ODP, oil diffusion pump; PM, Penning and Pirani manometers; SP, sample preparation port; GDS, gas dosing system; MM, membrane manometers (10−1–105 Pa); GB, gas burettes; CAL, LKB 2107 isothermal sorption microcalorimeter; T, ultrathermostate; CU, calorimeter control unit; PC, personal computer.
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face. Evacuation proceeded until the pressure decreased to 1.3×10−3 Pa. Measurements were made by the cumulative method at 298.15±0.02 K from 0 to 6 kPa (adsorption) and then backwards (desorption). Step-by-step hydrogen dosing was performed by using small pressure increments. The calorimeter power signal was continuously recorded against time and the heats of adsorption were calculated from the peak areas. The data collection and analysis were computer controlled. The integral molar enthalpy of chemisorption Q was calculated using Q =Q /n where int int p0 s Q is the heat of chemisorption (extrapolated to p0 zero pressure) per unit mass of the solid. 2.2.4. Catalytic test reaction The catalytic activity of Pt-, Pd- and Rh-graphimet was investigated via the reaction of cyclohexene hydrogenation in a static recirculation reactor system [22]. Cyclohexene, a Fluka product, was freshly distilled before use, and H was 2 obtained from a Matheson 8326 hydrogen generator equipped with a Pd membrane. Pretreatment was effected in 13.3 kPa H at 298 K for 2 h for 2 the pristine samples, and at 573 K under otherwise the same conditions for the MTR samples. The only product formed was cyclohexane. The time dependence of cyclohexene conversion was measured at 298 K from which the reaction rates were calculated. The catalytic performances were characterized via the turnover frequencies ( TOF ) which express the number of reactant molecules transformed during 1 s on one active center: TOF=rA/(6×1021 m D , where r is the reaction s rate and m is the weight of graphimet.
3. Results and discussion Transmission electron microscopy ( TEM ) measurements provided direct information on the surface metal content of graphimets. The particle size distributions determined from the electron micrographs of Pt-, Pd- and Rh-graphimet indicate that for the pristine samples, the size of most surface particles was 1–2 nm and only a limited number of aggregates were present (Figs. 2–4). However, the size distributions of the MTR samples clearly
Fig. 2. The particle size distributions of pristine and MTR Pt-graphimet.
Fig. 3. The particle size distributions of pristine and MTR Pd-graphimet.
demonstrate that thermal treatment resulted in a remarkable change in the size and location of metal nanoparticles. On the one hand, MTR increased the number of small (0–1 nm) surface metal crystallites to an appreciable extent, for Ptand Pd-graphimet in particular. This is solely attributed to the migration of interlayer metal atoms on the surface of graphite. The small size of deintercalated metal nanoparticles rules out the possibility of interlayer metal aggregation. On the other hand, the surface metal crystallites of the pristine samples underwent aggregation upon
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Fig. 4. The particle size distributions of pristine and MTR Rh-graphimet.
MTR. In fact, sintering is likely to take place by atomic migration [23]. Whereas only a moderate sintering effect was observed for Pd- and Rh-graphimets, the MTR Pt-graphimet was found to be highly polydispersed, as its TEM micrograph revealed the appearance of some large aggregates (24–270 nm, Fig. 2). Since deintercalation and surface aggregation produced an opposite effect on the particle size, it is not surprising that the number average diameters of metal nanoparticles calculated for the MTR samples are only slightly higher than those of the pristine ones. In other words, the average particle size is far less informative than the size distribution itself. The values listed in Table 1 also suggest a relatively low mobility of Rh crystallites, as Rh-graphimet was the least susceptible to migration and sintering of the three samples examined.
The occurrence of deintercalation was also confirmed by a comparison of the dispersion data of pristine and MTR graphimets. According to Table 1, the highest amount of exposed metal atoms was obtained for Pt-graphimet. It has been previously shown that both H and O may enter 2 2 the interlamellar space of graphite and undergo adsorption, although the rate of diffusion is smaller for O than for H [11,12]. Therefore, the D 2 2 dispersion values collected in Table 1 were determined after a prolonged oxidation time of 2 weeks for most samples. The results of the microcalorimetric investigation of Pt-, Pd- and Rh-graphimet are displayed in Figs. 5–7. The integral enthalpy isotherms of H adsorption are remarkably similar for Pt- and 2 Rh-graphimets. After chemisorption took place, adsorption was found to be reversible for both the pristine and MTR samples, that is, physisorption occurred. In this region, no important differences between the pristine and MTR samples can be observed. In contrast, for each Pd-graphimet
Fig. 5. Enthalpy isotherms of H sorption on Pt-graphimet at 2 298.15 K, before and after MTR.
Table 1 Characteristic data obtained for Pt-, Pd- and Rh-graphimet by different physicochemical methods and turnover frequencies determined for cyclohexene hydrogenation Sample
d (nm)
D (%)
D (%) s
Q (kJ mol−1) int
TOF (s−1)
Pt-graphimet Pt-graphimet, MTR Pd-graphimet Pd-graphimet, MTR Rh-graphimet Rh-graphimet, MTR
4.7 5.4 4.5 5.8 3.1 3.6
64 90 19 20 17 22
10.5 13.2 3.8 — 5.8 —
30 34 77 102 41 58
2.63 0.15 2.20 0 6.61 0
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Fig. 6. Enthalpy isotherms of H sorption on Pd-graphimet at 2 298.15 K, before and after MTR.
Fig. 7. Enthalpy isotherms of H sorption on Rh-graphimet at 2 298.15 K, before and after MTR.
sample, H chemisorption is followed by an 2 S-shaped absorption curve superimposed on the first plateau of the isotherm; this is due to the successive formation of a- and b-hydride phases [24–26 ]. In the two-phase region, a hysteresis loop can be observed for the pristine and MTR samples, but the shape and location of the loops are different. A detailed interpretation of the hydride formation of Pd-graphimets has been reported recently [21]. Therefore, it is not needed to analyze this phenomenon any further within the scope of the present work. The higher Q values obtained for the MTR p0 samples may either be attributed to an increase in the metal content available for H chemisorption 2 after MTR, or an increase of the strength of the metal–hydrogen bond, or both. To decide, the
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integral molar heats of chemisorption were calculated from a combination of the H titration and 2 calorimetric data. The values are listed in Table 1. Although no reference data were available for Rh-graphimet, the magnitudes of the measured heats for Pt- and Pd-graphimet are in agreement with those reported in the literature [27,28]. Apart from Pd-graphimet, the values of Q were relaint tively small. Nevertheless, the occurrence of H 2 chemisorption is established since the maximum value of the heat of H physisorption does not 2 exceed 10 kJ mol−1, due to the small size of the adsorbate [29]. The increase of the integral molar heats upon MTR indicate that not only deintercalation took place (as concluded from TEM and H titration data), but also the surface properties 2 of the metal changed. It is worth mentioning that the integral molar heats are average values over the entire surface coverage, and do not characterize surface heterogeneity. The expected large chemisorption enthalpies on the most active surface sites at low surface coverage are counterbalanced by the smaller heats associated with the less active sites at increasing surface coverages. The catalytic test reaction of cyclohexene hydrogenation provided additional information on the distribution of the active metal content of pristine and MTR graphimets. It should be stressed that preliminary experiments ruled out the possibility of cyclohexene diffusion into the interlayer space of graphite. Therefore, the active sites considered in the above reactions are restricted to the surface metal particles of all samples investigated [11,22]. The TOFs collected in Table 1 (related to D , the s dispersion of surface metal particles) reveal that the catalytic activity in the hydrogenation reaction considerably diminished after MTR for each sample. Nevertheless, the extent of activity loss varied with the metal content. For MTR Pt-graphimet, a decrease of one order of magnitude was found, whereas for the thermally treated Pdand Rh-graphimet the initial activities decreased to zero. It seems that the catalytically active metal sites of these two samples disappeared altogether after MTR. Although the effect of sintering may be a suitable reason for activity loss, the extent of aggregation indicated by TEM measurements was not so significant as to justify such a dramatic change. Apparently, the initial activities of Pd-
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and Rh-graphimet were due to the presence of small, 1–2 nm, surface metal crystallites. On MTR, the active centers became inaccessible, possibly by encapsulation into the graphite host. The additional small surface crystallites formed by migration of inserted metal nanoparticles proved to be catalytically inactive. Hence, MTR stabilized catalytically inactive crystal planes. On the other hand, the remaining activity of MTR Pt-graphimet makes the possibility of Pt encapsulation less likely. Therefore, the deactivation of the sample may be mostly attributed to the occurrence of a pronounced surface sintering (Fig. 2). Besides, the rearrangement of interlayer metal content should also be taken into account. As we have established above, MTR increased the ratio of either surface or inserted subcolloidal metal particles available for H and O adsorption. It seems likely that 2 2 most of the internal Pt atoms migrated from originally inaccessible positions to positions already available for H and O chemisorption, 2 2 but not for cyclohexene. A comparison between the heats of hydrogen chemisorption and the hydrogenation activities before and after MTR may lead to important consequences. The general mechanism of alkene hydrogenation reported by Horiuti and Pola´nyi has been recognized for a long time. Among other reaction steps, the mechanism implies the dissociative adsorption of H on the active center, regard2 less of the nature of the catalyst [30]. It follows that the resulting metal–H bond strength has an influence on the catalytic activity. The heat effect of H chemisorption on transition metals is closely 2 related to the corresponding metal–H binding energy, which is strongly affected by the surface rearrangement of metal atoms [31]. In the present case, restructuring of the metal content of graphimets upon MTR resulted in increased heats of H 2 chemisorption, which, in turn, implies an enhancement of the metal–H bond strength. Since weakly held hydrogen is considered as a chemically reactive species [31], the formation of strong metal–H bonds may contribute to the decrease of the hydrogenation activities experienced after MTR. In fact, if the pristine and MTR samples of a given metal are compared, an inverse relationship between Q and TOF may be concluded from int Table 1. It appears that such a relation does not
hold if samples of different metals (e.g. pristine Pd-graphimet and MTR Pt-graphimet) are considered. However, it makes little sense to seek for a correlation between the metal–H bond strength and the hydrogenation activity for different metals unless the corresponding metal–substrate bond strengths are also available. The authors are currently working on the technical details of such a microcalorimetric study. The increase of the heat of chemisorption was the least pronounced for Pt-graphimet which compares well with the remaining activity of the MTR sample. As the difference in Q increased for the int other two graphimets, their hydrogenation activities vanished completely. It should be emphasized that on account of their internal metal content, graphimets are far more complex materials than conventional supported metal catalysts. Additional information on the relationship between their structure and reactivity could be obtained from the measurement of the heat of cyclohexene adsorption. Further investigations on the correlation between the hydrogenation efficiency and the heat of chemisorption are being undertaken.
4. Conclusions It was concluded that the structure of the active metal content of Pt-, Pd- and Rh-graphimet underwent significant changes on MTR. Restructuring of both the interlayer and surface metal particles resulted in an increased number of ultrasmall surface particles accompanied by a moderate sintering. Despite the augmentation of the chemisorption capacities and the integral molar heats of H chemisorption, MTR considerably 2 reduced the catalytic activities of graphimets in cyclohexene hydrogenation. A working hypothesis is proposed for the correlation between the increased enthalpies of H chemisorption and the 2 decreased hydrogenation activities.
Acknowledgment The authors wish to thank the Hungarian Scientific Research Foundation for financial sup-
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port through Grants No. OTKA T016109 and T026430.
References [1] M.S. Dresselhaus, Mater. Sci. Eng. B 1 (1988) 259. [2] M.E. Volpin, N. Novikov, N.D. Lapkina, V.I. Kasatochkin, T. Struchkov, M.E. Kazakov, R.A. Stukan, A. Povitskij, S. Karimov, A.V. Zvarikina, J. Am. Chem. Soc. 97 (1975) 3366. [3] L.B. Ebert, J. Mol. Catal. 15 (1982) 275. [4] S.C. Moss, R. Moret, in: H. Zabel, S.A. Solin (Eds.), Structural Properties and Phase Transitions, Springer Series in Materials Science Vol. 14: Graphite Intercalation Compounds I. Springer-Verlag, Berlin, 1990, pp. 5–58. [5] E. Csuk, B.I. Gla¨nzer, A. Fu¨rstner, Adv. Organomet. Chem. 28 (1988) 83. [6 ] A. He´rold, Springer Ser. Solid State Sci. 38 (1981) 7. [7] H. Klotz, A. Schneider, Naturwiss. 49 (1962) 448. [8] D.J. Smith, R.M. Fisher, L.A. Freeman, J. Catal. 72 (1981) 51. [9] R. Erre, A. Messaoudi, F. Be´guin, Synth. Met. 23 (1988) 493. [10] A. Fu¨rstner, F. Hofer, H. Weidmann, Carbon 29 (1991) 915. ´ . Mastalir, M. Barto´k, J. Catal. 134 [11] F. Notheisz, A (1992) 608. ´ . Mastalir, F. Notheisz, M. Barto´k, Z. Kira´ly, I. De´ka´ny, [12] A J. Mol. Catal. A. Chem. 99 (1995) 115. [13] K. Aika, T. Yamaguchi, T. Onishi, Appl. Catal. 23 (1986) 129.
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´ . Mastalir, F. Notheisz, M. Barto´k, Catal. Lett. 35 [14] A (1995) 119. ´ . Mastalir, F. Notheisz, M. Barto´k, J. Phys. Chem. Solids [15] A 57 (1996) 899. [16 ] M.A. Martin, J.A. Pajares, L. Gonza´lez Tejuca, J. Colloid Interface Sci. 107 (1985) 540. [17] Lalancette, U.S. Patent 3847963 (1974). [18] J.E. Benson, H.S. Hwang, M. Boudart, J. Catal. 30 (1973) 146. [19] J.J.F. Scholten, A.P. Pijpers, A.M.L. Hustings, Catal. Rev. Sci. Eng. 27 (1985) 151. [20] P. Ehrburger, O.P. Mahajan, P.L. Walker, J. Catal. 43 (1976) 61. ´ . Mastalir, F. Berger, I. De´ka´ny, Langmuir 13 [21] Z. Kira´ly, A (1997) 465. ´ . Mastalir, F. Notheisz, J. Ocsko´, M. Barto´k, React. [22] A Kinet. Catal. Lett. 56 (1995) 69. [23] E. Ruckenstein, I. Sushumna, in: Z. Paa´l, P.G. Menon ( Eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York, 1988, p. 259. [24] J.F. Lynch, T.B. Flanagan, J. Phys. Chem. 77 (1973) 2628. [25] D.H. Everett, Calorimetry Thermometry Thermal Analysis (1982) 29. [26 ] D.H. Everett, P.A. Sermon, Z. Phys. Chem. Neue Folge 114 (1979) 109. [27] N. Cardona-Martinez, J.A. Dumesic, Applications of Adsorption Microcalorimetry to the Study of Heterogeneous Catalysis, Adv. Catal. 38, Academic Press, New York, 1992, p. 219. [28] P. Chou, M.A. Vannice, J. Catal. 104 (1987) 1. [29] P. Fejes, in: Z.G. Szabo´, D. Kallo´ ( Eds.), Enthalpy Changes Accompanying Adsorption, Contact Catalysis Vol. 1, Elsevier, Amsterdam, 1976, p. 184. [30] I. Horiuti, M. Pola´ny, Trans. Faraday Soc. 30 (1964) 1164. [31] K.R. Christmann, in: Z. Paa´l, P.G. Menon ( Eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York, 1988, p. 50.
Microcalorimetric and catalytic investigations of transition metal nanoparticles intercalated in graphite ´ . Mastalir a,*, Z. Kira´ly b, I. De´ka´ny b, M. Barto´k a A a Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, Jo´zsef Attila University, Do´m Te´r 8, 6720 Szeged, Hungary b Department of Colloid Chemistry, Jo´zsef Attila University, Aradi vt. tere 1, 6720 Szeged, Hungary Received 17 January 1997; received in revised form 20 June 1997; accepted 4 August 1997
Abstract The distribution of the metal content of pristine and thermally treated (medium-temperature reduction, MTR) Pt-, Pd- and Rh-graphite intercalation compounds (graphimets) was investigated by transmission electron microscopy, H titration and sorption microcalorimetry. It was established that MTR resulted in a migration of interlayer metal 2 atoms to the graphite surface accompanied by a moderate aggregation of the surface nanoparticles. Despite the increased metal content available for H adsorption after MTR, the catalytic activities decreased substantially. Besides 2 structural changes, the loss of catalytic performance was related to the increase of metal–hydrogen bond strengths suggested by the calorimetric data. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Catalytic hydrogenation; Graphimets; H titration; Sorption microcalorimetry; Transition metals; 2 Transmission electron microscopy
1. Introduction The formation of graphite intercalation compounds (GICs) is based on the layered structure of graphite and the difference in layer and interlayer bond strengths. The value of the in-plane C–C distance in graphite (0.141 nm) corresponds to the bond length of aromatic hydrocarbons. On the other hand, the interplanar C–C distance (interlayer spacing, 0.335 nm) is related to the formation of a weak van der Waals interaction between the graphite sheets. Intercalation takes place by insertion of various guest species (alkaliand alkali earth metals, acids, halogens or metal halides) between layers of the graphite host [1–5]. * Corresponding author.
Considering the widespread catalytic applications of transition metals in organic reactions, the present paper is focused on transition metal GICs (graphimets). Although graphite cannot be directly intercalated with transition metals, the required catalysts may be obtained by insertion and subsequent reduction of the corresponding metal salts [2,6 ]. The in situ reduction of intercalated transition metal halides can be performed by different reducing agents (H , NaBH , LiAlH , Na/NH , 2 4 4 3 Li, Na and K biphenyl ) [6,7]. Although the reduction results in the formation of finely divided subcolloidal metal particles between the graphite layers [8,9], the catalytic activity is mostly attributed to the presence of surface metal particles [10– 12]. Graphimets are claimed to be thermally unstable, so that the intercalated species may leave the interlayer space and migrate to the surface of
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graphite, even under relatively mild conditions [13]. The occurrence of such a deintercalation process upon medium-temperature reduction (MTR) has been reported previously [14,15]. The aim of the present study was to obtain more information on the effect of thermal treatment on the location and arrangement of the subcolloidal metal content of Pt-, Pd- and Rh-graphimets by different physicochemical methods generally used for catalyst characterization [16 ].
2. Experimental 2.1. Materials Pt-, Pd- and Rh-graphimet (1% metal in graphite each) were purchased from Alfa Chemical Company ( Karlsruhe, Germany). For each sample, the corresponding metal chlorides were intercalated into graphite in a chlorine stream and reduction was carried out with lithium-biphenyl [17]. MTR was effected in flowing H 2 (40 cm3 min−1) at 573 K for 1 h. The samples were then stored under He until use. 2.2. Methods 2.2.1. Transmission electron microscopy The particle size distribution and the average particle diameter of the surface metal particles were determined with an OPTON 902 electron microscope (Oberkochen, Germany) at 80 kV. Prior to measurement, the samples were ultrasonically dispersed in toluene and then deposited on a plastic film supported by a Cu grid. Magnifications were in the range 5×104–8.5×104 times. 2.2.2. H titration 2 Measurements were carried out in a flow reactor by using 9.9995% pure argon as the carrier gas. Titration was performed by 1.221 cm3 H pulses 2 at 298 K after preadsorbing O on the samples 2 over 2 weeks. From preliminary kinetic measurements, this period of time was found to be sufficient to measure D, the total number of exposed metal atoms accessible for H relative to the total number 2 of metal atoms. For Pd-graphimet, the temperature
of measurement was adjusted to 348 K in order to prevent hydride formation [18]. The D dispersion value was calculated for a stoichiometry M:H= 1:3 derived from the titration reaction M(O)+1.5 H M(H )+H O [19,20]. The n chemisorption 2 2 s capacity of H was calculated using n =wD/2A, 2 s where w is the metal content of graphimet and A is the atomic weight of the metal. The dispersion values of surface metal particles D (exclusive of s interlayer metal particles) were determined after preadsorbing O on the graphimets for 10 min. 2 This exposure time safely measures D for carbon supported transition metal catalysts. 2.2.3. Isothermal sorption microcalorimetry The schematic representation of the experimental apparatus designed in our laboratory for microcalorimetric investigations is shown in Fig. 1. The equipment consisted of three major units: a highvacuum system; a gas handling system; and the batch unit of an LKB 2107 isothermal sorption microcalorimeter, each connected to the main vacuum line. Details of the calorimeter vessel suitable for high vacuum operation have been described elsewhere [21]. Before use, both the pristine and MTR samples were pretreated with static H at 353 K for 1 h, followed by evacuation 2 at the same temperature for 2 h, and then for another 2 h at 298.15 K. This procedure was necessary to remove preadsorbed oxygen from the sur-
Fig. 1. Schematic representation of the equipment used for microcalorimetric measurements. HVS, high vacuum system ( p=10−4 Pa); ODP, oil diffusion pump; PM, Penning and Pirani manometers; SP, sample preparation port; GDS, gas dosing system; MM, membrane manometers (10−1–105 Pa); GB, gas burettes; CAL, LKB 2107 isothermal sorption microcalorimeter; T, ultrathermostate; CU, calorimeter control unit; PC, personal computer.
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face. Evacuation proceeded until the pressure decreased to 1.3×10−3 Pa. Measurements were made by the cumulative method at 298.15±0.02 K from 0 to 6 kPa (adsorption) and then backwards (desorption). Step-by-step hydrogen dosing was performed by using small pressure increments. The calorimeter power signal was continuously recorded against time and the heats of adsorption were calculated from the peak areas. The data collection and analysis were computer controlled. The integral molar enthalpy of chemisorption Q was calculated using Q =Q /n where int int p0 s Q is the heat of chemisorption (extrapolated to p0 zero pressure) per unit mass of the solid. 2.2.4. Catalytic test reaction The catalytic activity of Pt-, Pd- and Rh-graphimet was investigated via the reaction of cyclohexene hydrogenation in a static recirculation reactor system [22]. Cyclohexene, a Fluka product, was freshly distilled before use, and H was 2 obtained from a Matheson 8326 hydrogen generator equipped with a Pd membrane. Pretreatment was effected in 13.3 kPa H at 298 K for 2 h for 2 the pristine samples, and at 573 K under otherwise the same conditions for the MTR samples. The only product formed was cyclohexane. The time dependence of cyclohexene conversion was measured at 298 K from which the reaction rates were calculated. The catalytic performances were characterized via the turnover frequencies ( TOF ) which express the number of reactant molecules transformed during 1 s on one active center: TOF=rA/(6×1021 m D , where r is the reaction s rate and m is the weight of graphimet.
3. Results and discussion Transmission electron microscopy ( TEM ) measurements provided direct information on the surface metal content of graphimets. The particle size distributions determined from the electron micrographs of Pt-, Pd- and Rh-graphimet indicate that for the pristine samples, the size of most surface particles was 1–2 nm and only a limited number of aggregates were present (Figs. 2–4). However, the size distributions of the MTR samples clearly
Fig. 2. The particle size distributions of pristine and MTR Pt-graphimet.
Fig. 3. The particle size distributions of pristine and MTR Pd-graphimet.
demonstrate that thermal treatment resulted in a remarkable change in the size and location of metal nanoparticles. On the one hand, MTR increased the number of small (0–1 nm) surface metal crystallites to an appreciable extent, for Ptand Pd-graphimet in particular. This is solely attributed to the migration of interlayer metal atoms on the surface of graphite. The small size of deintercalated metal nanoparticles rules out the possibility of interlayer metal aggregation. On the other hand, the surface metal crystallites of the pristine samples underwent aggregation upon
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Fig. 4. The particle size distributions of pristine and MTR Rh-graphimet.
MTR. In fact, sintering is likely to take place by atomic migration [23]. Whereas only a moderate sintering effect was observed for Pd- and Rh-graphimets, the MTR Pt-graphimet was found to be highly polydispersed, as its TEM micrograph revealed the appearance of some large aggregates (24–270 nm, Fig. 2). Since deintercalation and surface aggregation produced an opposite effect on the particle size, it is not surprising that the number average diameters of metal nanoparticles calculated for the MTR samples are only slightly higher than those of the pristine ones. In other words, the average particle size is far less informative than the size distribution itself. The values listed in Table 1 also suggest a relatively low mobility of Rh crystallites, as Rh-graphimet was the least susceptible to migration and sintering of the three samples examined.
The occurrence of deintercalation was also confirmed by a comparison of the dispersion data of pristine and MTR graphimets. According to Table 1, the highest amount of exposed metal atoms was obtained for Pt-graphimet. It has been previously shown that both H and O may enter 2 2 the interlamellar space of graphite and undergo adsorption, although the rate of diffusion is smaller for O than for H [11,12]. Therefore, the D 2 2 dispersion values collected in Table 1 were determined after a prolonged oxidation time of 2 weeks for most samples. The results of the microcalorimetric investigation of Pt-, Pd- and Rh-graphimet are displayed in Figs. 5–7. The integral enthalpy isotherms of H adsorption are remarkably similar for Pt- and 2 Rh-graphimets. After chemisorption took place, adsorption was found to be reversible for both the pristine and MTR samples, that is, physisorption occurred. In this region, no important differences between the pristine and MTR samples can be observed. In contrast, for each Pd-graphimet
Fig. 5. Enthalpy isotherms of H sorption on Pt-graphimet at 2 298.15 K, before and after MTR.
Table 1 Characteristic data obtained for Pt-, Pd- and Rh-graphimet by different physicochemical methods and turnover frequencies determined for cyclohexene hydrogenation Sample
d (nm)
D (%)
D (%) s
Q (kJ mol−1) int
TOF (s−1)
Pt-graphimet Pt-graphimet, MTR Pd-graphimet Pd-graphimet, MTR Rh-graphimet Rh-graphimet, MTR
4.7 5.4 4.5 5.8 3.1 3.6
64 90 19 20 17 22
10.5 13.2 3.8 — 5.8 —
30 34 77 102 41 58
2.63 0.15 2.20 0 6.61 0
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Fig. 6. Enthalpy isotherms of H sorption on Pd-graphimet at 2 298.15 K, before and after MTR.
Fig. 7. Enthalpy isotherms of H sorption on Rh-graphimet at 2 298.15 K, before and after MTR.
sample, H chemisorption is followed by an 2 S-shaped absorption curve superimposed on the first plateau of the isotherm; this is due to the successive formation of a- and b-hydride phases [24–26 ]. In the two-phase region, a hysteresis loop can be observed for the pristine and MTR samples, but the shape and location of the loops are different. A detailed interpretation of the hydride formation of Pd-graphimets has been reported recently [21]. Therefore, it is not needed to analyze this phenomenon any further within the scope of the present work. The higher Q values obtained for the MTR p0 samples may either be attributed to an increase in the metal content available for H chemisorption 2 after MTR, or an increase of the strength of the metal–hydrogen bond, or both. To decide, the
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integral molar heats of chemisorption were calculated from a combination of the H titration and 2 calorimetric data. The values are listed in Table 1. Although no reference data were available for Rh-graphimet, the magnitudes of the measured heats for Pt- and Pd-graphimet are in agreement with those reported in the literature [27,28]. Apart from Pd-graphimet, the values of Q were relaint tively small. Nevertheless, the occurrence of H 2 chemisorption is established since the maximum value of the heat of H physisorption does not 2 exceed 10 kJ mol−1, due to the small size of the adsorbate [29]. The increase of the integral molar heats upon MTR indicate that not only deintercalation took place (as concluded from TEM and H titration data), but also the surface properties 2 of the metal changed. It is worth mentioning that the integral molar heats are average values over the entire surface coverage, and do not characterize surface heterogeneity. The expected large chemisorption enthalpies on the most active surface sites at low surface coverage are counterbalanced by the smaller heats associated with the less active sites at increasing surface coverages. The catalytic test reaction of cyclohexene hydrogenation provided additional information on the distribution of the active metal content of pristine and MTR graphimets. It should be stressed that preliminary experiments ruled out the possibility of cyclohexene diffusion into the interlayer space of graphite. Therefore, the active sites considered in the above reactions are restricted to the surface metal particles of all samples investigated [11,22]. The TOFs collected in Table 1 (related to D , the s dispersion of surface metal particles) reveal that the catalytic activity in the hydrogenation reaction considerably diminished after MTR for each sample. Nevertheless, the extent of activity loss varied with the metal content. For MTR Pt-graphimet, a decrease of one order of magnitude was found, whereas for the thermally treated Pdand Rh-graphimet the initial activities decreased to zero. It seems that the catalytically active metal sites of these two samples disappeared altogether after MTR. Although the effect of sintering may be a suitable reason for activity loss, the extent of aggregation indicated by TEM measurements was not so significant as to justify such a dramatic change. Apparently, the initial activities of Pd-
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and Rh-graphimet were due to the presence of small, 1–2 nm, surface metal crystallites. On MTR, the active centers became inaccessible, possibly by encapsulation into the graphite host. The additional small surface crystallites formed by migration of inserted metal nanoparticles proved to be catalytically inactive. Hence, MTR stabilized catalytically inactive crystal planes. On the other hand, the remaining activity of MTR Pt-graphimet makes the possibility of Pt encapsulation less likely. Therefore, the deactivation of the sample may be mostly attributed to the occurrence of a pronounced surface sintering (Fig. 2). Besides, the rearrangement of interlayer metal content should also be taken into account. As we have established above, MTR increased the ratio of either surface or inserted subcolloidal metal particles available for H and O adsorption. It seems likely that 2 2 most of the internal Pt atoms migrated from originally inaccessible positions to positions already available for H and O chemisorption, 2 2 but not for cyclohexene. A comparison between the heats of hydrogen chemisorption and the hydrogenation activities before and after MTR may lead to important consequences. The general mechanism of alkene hydrogenation reported by Horiuti and Pola´nyi has been recognized for a long time. Among other reaction steps, the mechanism implies the dissociative adsorption of H on the active center, regard2 less of the nature of the catalyst [30]. It follows that the resulting metal–H bond strength has an influence on the catalytic activity. The heat effect of H chemisorption on transition metals is closely 2 related to the corresponding metal–H binding energy, which is strongly affected by the surface rearrangement of metal atoms [31]. In the present case, restructuring of the metal content of graphimets upon MTR resulted in increased heats of H 2 chemisorption, which, in turn, implies an enhancement of the metal–H bond strength. Since weakly held hydrogen is considered as a chemically reactive species [31], the formation of strong metal–H bonds may contribute to the decrease of the hydrogenation activities experienced after MTR. In fact, if the pristine and MTR samples of a given metal are compared, an inverse relationship between Q and TOF may be concluded from int Table 1. It appears that such a relation does not
hold if samples of different metals (e.g. pristine Pd-graphimet and MTR Pt-graphimet) are considered. However, it makes little sense to seek for a correlation between the metal–H bond strength and the hydrogenation activity for different metals unless the corresponding metal–substrate bond strengths are also available. The authors are currently working on the technical details of such a microcalorimetric study. The increase of the heat of chemisorption was the least pronounced for Pt-graphimet which compares well with the remaining activity of the MTR sample. As the difference in Q increased for the int other two graphimets, their hydrogenation activities vanished completely. It should be emphasized that on account of their internal metal content, graphimets are far more complex materials than conventional supported metal catalysts. Additional information on the relationship between their structure and reactivity could be obtained from the measurement of the heat of cyclohexene adsorption. Further investigations on the correlation between the hydrogenation efficiency and the heat of chemisorption are being undertaken.
4. Conclusions It was concluded that the structure of the active metal content of Pt-, Pd- and Rh-graphimet underwent significant changes on MTR. Restructuring of both the interlayer and surface metal particles resulted in an increased number of ultrasmall surface particles accompanied by a moderate sintering. Despite the augmentation of the chemisorption capacities and the integral molar heats of H chemisorption, MTR considerably 2 reduced the catalytic activities of graphimets in cyclohexene hydrogenation. A working hypothesis is proposed for the correlation between the increased enthalpies of H chemisorption and the 2 decreased hydrogenation activities.
Acknowledgment The authors wish to thank the Hungarian Scientific Research Foundation for financial sup-
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port through Grants No. OTKA T016109 and T026430.
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