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Sorption and Microcalorimetric Investigations of Palladium/Hydrogen Interactions on Palladium-Graphimet Intercalation Catalyst 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 May 3, 1996. In Final Form: November 13, 1996X Sorption and microcalorimetric investigations of Pd/hydrogen interactions on a 1% Pd-graphimet catalyst at 298.15 K indicated that medium-temperature reduction (MTR; 573 K, H2) caused variations in both the surface and bulk properties of the metal particles. The enthalpy of chemisorption increased from -77 to -102 kJ‚mol-1 as a consequence of MTR. The enhanced Pd/hydrogen binding energy contributes to the diminishing catalytic performance of the MTR sample in hydrogenation reactions, as reported previously (refs 1-3). The adsorption of hydrogen was followed by stepwise absorption to produce the R and β interstitial solid solutions. Although a pronounced hysteresis was found in the two-phase region for each sample, significant differences in the location and shape of the hysteresis loops were observed. The changes in the shape and location of the loop upon MTR were explained in terms of an increase in the compressive strain, rather than in the size of the metal crystallites. The enthalpy of β-hydride formation was found to be -36.5 kJ‚mol-1, irrespective of the compactness of the metal.
Introduction Pd occupies a peculiar position among the group VIII metals: it both adsorbs and absorbs hydrogen under mild conditions.4,5 The solubility of hydrogen in Pd (formation of R and β-hydride phases) modifies the state of the metal and therefore influences its surface reactivity.6,7 For bulk Pd (e.g. sheet, wire or powder), the number of surface Pd atoms is negligible with respect to the number of bulk Pd atoms, and sorption8 and calorimetric studies9 on the formation of the R- and β-hydrides are therefore relatively simple. Hydrogen sorption on bulk Pd exhibits a hysteresis phenomenon in the two-phase region: a higher two-phase pressure is observed during its formation than during its decomposition. It has been argued that it is the β/R decomposition pressure on the desorption isotherm, rather than the R/β breakthrough pressure on the absorption isotherm, which may be regarded as the equilibrium pressure.8 The decomposition pressure, which increases with increasing temperature, has a value of 0.80 kPa at 298 K,8 and the corresponding saturation H/Pd atomic ratios are 0.015 and 0.58 in the R and β-phases, respectively.10 Since hydrogen tends to absorb at very low pressure, differentiation between R-phase formation and weak chemisorption is not completely straightforward * Corresponding author. † Department of Colloid Chemistry. ‡ Department of Organic Chemistry. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Mastalir, A Ä .; Notheisz, F.; Barto´k, M. J. Phys. Chem. Solids 1996, 57, 899. (2) Mastalir, A Ä .; Notheisz, F.; Ocsko´, J.; Barto´k, M. React. Kinet. Catal. Lett. 1995, 56, 69. (3) Mastalir, A Ä .; Notheisz, F.; Barto´k, M.; Haraszti, T.; Kira´ly, Z., De´ka´ny, I. Appl. Catal. A: General 1996, 144, 237. (4) Lewis, F. A. The Palladium/Hydrogen System; Academic Press: London, New York, 1967. (5) Hydrogen Effects in Catalysis. Fundamentals and Practical Applications (state-of-the-art reviews, presented in 28 chapters by 38 authors); Paa´l, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988. (6) Carturan, G.; Facchin, G.; Cocco, G.; Enzo, S.; Navazio, G. J. Catal. 1982, 76, 405. (7) Palczewska, W. in ref 5, Chapter 14. (8) Wicke, E.; Nernst, G. H. Ber. Bunsenges. Phys. Chem. 1964, 68, 224. (9) Lynch, J. F.; Flanagan, T. B. J. Chem. Soc. Faraday Trans. 1 1974, 70, 814. (10) Lynch, J. F.; Flanagan, T. B. J. Phys. Chem. 1973, 77, 2628.
for highly dispersed Pd particles.10 The calorimetric enthalpies of absorption have been reported to be -23 kJ‚mol-1 in the R-phase and -27 to -46 kJ‚mol-1 in the β-phase.9 The enthalpy of strong chemisorption varies from -60 to -120 kJ‚mol-1,11 while in the weak chemisorption region it ranges from -36 to -45 kJ‚mol-1.10 Literature data available on the heats of chemisorption of hydrogen by Pd (and other transition metals) on various supports were collected and their catalytic implications analyzed by Cardona-Martinez and Dumesic.11 Calorimetric investigations of hydrogen sorption on various refractory oxide-supported Pd catalysts revealed that the energetics of Pd/hydrogen interaction is not appreciably affected either by the nature of the support or by the Pd crystallite size, provided that the latter is larger than ca. 3 nm.12 Wunder et al. recently reported similar results for oxide-supported Pd samples, but they found no chemisorption on carbon-supported Pd materials at 300 K.13 It was concluded that only bulk absorption exists for carbon-supported particles indicating that the “desorption” temperature for surface hydrogen is less than that for hydrogen absorbed in bulk Pd. If so, the application of temperature-controlled H2-O2 titration and back-sorption experiments14,15 on carbon-supported Pd would fail to provide the number of surface metal atoms relative to the total number of metal atoms (briefly termed dispersion), an important parameter in heterogeneous catalysis. In contrast with carbon-supported metal catalysts, transition metal graphimets possess atomic dispersions or small clusters between the layers of the graphite host.16,17 Additionally, a considerable number of highly dispersed nanoparticles are situated on the external (11) Cardona-Martinez, N.; Dumesic, J. A. Applications of Adsorption Microcalotimetry to the Study of Heterogeneous Catalysis (Adv. Catal. Vol. 38); Academic Press: New York, 1992; pp 219-227. (12) Chou, P.; Vannice, M. A. J. Catal. 1987, 104, 1. (13) Wunder, R. W.; Cobes, J. W.; Phillips, J.; Radovic, L. R.; Lopez Peinado, A. J.; Carrasco-Marin, F. Langmuir 1993, 9, 984. (14) Aben, P. C. J. Catal. 1968, 10, 224. (15) Benson, J. E.; Hwang, H. S.; Boudard, M. J. Catal. 1973, 30, 146. (16) Csuk, R.; Gla¨nzer, B. I.; Fu¨rstner, A. Adv. Organomet. Chem. 1988, 28, 85. (17) 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. Am. Chem. Soc. 1975, 97, 3366.
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Figure 1. The gas sorption vessel designed for the LKB 2107 isothermal microcalorimeter (schematic): (1) calorimeter; (2) measuring block; (3) sorption vessel; (4) Swagelok flange; (5) stainless steel pipe; (6) calorimeter inlet port; (7) gas-dosing valve; (8) gas-handling system; (9) paraffin oil; (10) sorbent.
graphite surface. Even medium-temperature reduction (MTR; 573 K, H2) increases the surface metal content via the migration of interlayer particles.1,3,18 Experimental evidence indicates that the surface metal plays the dominant role in catalytic transformations, and in many hydrogenation and isomerization reactions, graphimets are regarded as supported heterogeneous catalysts.18,19 It seemed of interest, therefore, to perform calorimetric investigations of the sorption of hydrogen on Pd-graphimet in order to compare the results with those on carbon-supported Pd catalysts. Materials and Methods Pd-Graphimet. Pd-graphimet (1% Pd in graphite) was a product of Alfa Chemical Co.; it had a BET surface area of 10.2 m2 g-1. MTR was effected at 573 K for 2 h under 13.3 kPa H2. Before use, both the pristine and the MTR samples were pretreated with static hydrogen at 353 K for 1 h, followed by evacuation for 2 h, and then for a further 2-h period at 298.15 K (p
