Langmuir 1998, 14, 1281-1282
1281
Calorimetric Study of Sorption of Hydrogen by Carbon-Supported Palladium Zolta´n Kira´ly,*,† A Ä gnes Mastalir,‡ Ferenc Berger,† and Imre De´ka´ny† Departments of Colloid Chemistry and Organic Chemistry, Attila Jo´ zsef University, H-6720 Szeged, Hungary Received September 10, 1997. In Final Form: November 24, 1997
Introduction Sorption and calorimetric investigations of the interactions of hydrogen with Pd supported on silica, alumina, and titania indicated that a pronounced chemisorption takes place prior to bulk hydride formation.1-3 In a recent paper we likewise observed the occurrence of hydrogen chemisorption on pristine and thermally treated Pdgraphimets at 298.15 K.4 On the other hand, chemisorption at ambient temperature was found not to occur on Pd supported on carbon black and graphitic carbon.3 The structure of Pd-graphimet differs substantially from that of carbon-supported Pd,5 and at least in principle, this may account for the difference in Pd/hydrogen interactions. Whereas Pd nanocrystallites are situated on the surface of the support material for various kinds of carbon supports, Pd-graphimets contain atomic dispersions of Pd between the graphite layers besides finely divided Pd nanoparticles on the external surface sites. One of the referees of our recent paper4 rightly asked whether the nonoccurrence of hydrogen chemisorption on carbonsupported Pd and the occurrence of chemisorption on Pdgraphimet might be related to intercalated Pd species. The motivation of the present work was to clarify this point by performing sorption and microcalorimetric investigations of Pd/hydrogen interactions on a carbonsupported Pd catalyst, using the same experimental setup as for Pd-graphimet.4 Experimental Section Carbon-supported Pd (Pd/C) with a nominal metal content of 5 wt % was purchased from Aldrich. This composition was confirmed by ICP-AES spectroscopy (Jobin Yvon). The BET specific surface area was found from N2 adsorption measurements at 77 K to be 680 m2/g. The Pd particle size distribution was determined by transmission electron microscopy at 80 kV (OPTON 902). Sorption and microcalorimetric investigations were performed at 298.15 K by applying the same cumulative procedure as reported previously.4 The material balance of hydrogen sorption was determined with an automated gassorption apparatus (Micromeritics Gemini 2375), and the enthalpy balance of hydrogen sorption was determined with an LKB 2107 isothermal microcalorimeter. Before use, the sample was treated with static hydrogen at 353 K for 1 h, followed by * Corresponding author. E-mail:
[email protected]. † Department of Colloid Chemistry. ‡ Department of Organic Chemistry. (1) Cardona-Martinez, N.; Dumesic, J. A. Applications of Adsorption Microcalotimetry to the Study of Heterogeneous Catalysis; Advances Catalysis 38; Academic Press: New York, 1992; p 221. (2) Chou, P.; Vannice, M. A. J. Catal. 1987, 104, 1. (3) Wunder, R. W.; Cobes, J. W.; Phillips, J.; Radovic, L. R.; Lopez Peinado, A. J.; Carrasco-Marin, F. Langmuir 1993, 9, 984. (4) Kira´ly, Z.; Mastalir, AÄ .; Berger, F.; De´ka´ny, I. Langmuir 1997, 13, 467. (5) Volpin, M. E.; Novikov, N.; Lapkina, N. D.; Kasatochkin, V. I.; Struchkov, T.; Kazakov, M. E.; Stukan, R. A.; Povitskij, V. A.; Karimov, S.; Zvarikina, A. V. J. Amer. Chem. Soc. 1975, 97, 3366.
Figure 1. Pd particle size distribution on 5% Pd/C.
Figure 2. Isotherm of the sorption of hydrogen by Pd on 5% Pd/C at 298.15 K: Adsorption path (O) and desorption path (0). evacuation at ≈1.3 × 10-3 Pa at the same temperature for 2 h, and finally for a further 2-h period at 298.15 K.
Results and Discussion The Pd particle size distribution for Pd/C is given in Figure 1. The average particle size is similar to those of the previously investigated Pd-graphimets:4 5.2, 4.5 and 5.8 nm for Pd/C, pristine and thermally treated Pdgraphimets, respectively. The isotherm of hydrogen sorption on Pd/C is shown in Figure 2, and the corresponding calorimetric enthalpy isotherm is shown in Figure 3. It is immediately apparent that a pronounced chemisorption takes place prior to bulk hydride formation. The results of the analysis of the sorption and calorimetric isotherms for Pd/C will be summarized briefly. For comparison, we recall the corresponding results for Pdgraphimet (1 wt %) before and after MTR (mediumtemperature reduction; 573 K, H2).4 In terms of the H/Pd ratio, the chemisorption capacity is ca. 0.18 for Pd/C, as compared to ca. 0.20 for Pdgraphimets. This indicates that the numbers of surface Pd atoms available for hydrogen chemisorption on the three samples are close to one another. When β-hydride formation is completed, the H/Pd ratio reaches a value of ca. 0.57 for Pd/C (Figure 2), as compared to ca. 0.96 for Pd-graphimets.4 The different compositions of the interstitial solid solutions may be due to the different degrees of compactness of the Pd. Marked differences in the absorption capacity have also been observed for refractory oxide-supported Pd samples.2
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Figure 3. Calorimetric enthalpy isotherm of the sorption of hydrogen by Pd on 5% Pd/C at 298.15 K: Adsorption path (O) and desorption path (0).
A simple combination of Figure 2 with Figure 3 allows calculation of the integral molar heat of chemisorption and the heat of β-hydride formation. The heat of chemisorption is 90 kJ‚mol-1 for Pd/C, as compared to 77 and 102 kJ‚mol-1 for Pd-graphimet before and after MTR, respectively. Consequently, the heat of chemisorption is very sensitive to the history of the surface Pd atoms. The heat of formation of the β-hydride phase is 37 kJ‚mol-1 for Pd/C, which is in good agreement with the values of 36.5 kJ‚mol-1 obtained for Pd-graphimets and 36 kJ.mol-1 reported for other Pd catalysts.2,3 Therefore, the heat of β-hydride formation seems not to be (or only slightly) dependent on the history of the parent metal. Temperature-programmed desorption studies of the chemisorption of hydrogen by Pd on Pd/C catalysts indicated the formation of subsurface hydrogen (34 kJ‚mol-1) in excess of the strongly held monolayer (90 kJ‚mol-1).6,7 The present results also suggest that weak (6) Konvalinka, J. A.; Scholten, J. J. F. J. Catal. 1977, 48, 374.
Notes
chemisorption is likely to occur around the knees on the isotherms, prior to bulk hydride formation. The ascending branch of the hysteresis loop (adsorption path) is broadened in the sequence of Pd-graphimet < MTR Pd-graphimet < Pd/C. Accordingly, the position of the R/β breakthrough pressure pb is shifted from 1.5 kPa to ca. 2 kPa in this sequence. These observations may be related to the different particle size distribution and/or the different compressive strains of the Pd particles under comparison. The descending section of the hysteresis loop (desorption path) is significantly steeper than the ascending section. Further, it is less steep for Pd/C than for Pd-graphimets. The β/R decomposition pressure pd is situated at 0.8-0.9 kPa. The location of pd is less sensitive than that of pb to the history of the Pd species. Conclusions We conclude that the characteristic features of the sorption isotherms and the enthalpy isotherms of the hydrogen/carbon-supported Pd and hydrogen/Pd-graphimet systems are similar. The finding of the nonoccurrence of hydrogen chemisorption by Pd supported on carbon3 was probably a consequence of insufficient outgassing after hydrogen pretreatment. Prior to measurements, Pd samples are treated with hydrogen in general, to remove surface-bound oxygen. The subsequent removal of water and excess hydrogen under high vacuum at moderate temperature (to prevent sintering) for a sufficient time is essential in order to obtain a “clean” Pd surface ready for measurements. If hydrogen pretreatment is followed by outgassing under “mild” conditions, the chemisorbed hydrogen will not be (appreciably) affected, and only the dissolved hydrogen will be removed.8 LA971026F (7) Paa´l, Z.; Menon, P. G. Catal. Rev.sSci. Eng. 1983, 25, 229. (8) Benson, J. E.; Hwang, H. S.; Boudart, M. J. Catal. 1973, 30, 146.
