Surface Characteristics, Hydrogen Sorption, and Catalytic Properties

High-Performance Pd-Based Hydrogen Spillover Catalysts for Hydrogen Storage. Brian D. Adams , Cassandra K. Ostrom , Shuai Chen , and Aicheng Chen...
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Surface Characteristics, Hydrogen Sorption, and Catalytic Properties of Pd-Zr Alloys Ferenc Berger,† Mo´nika Varga,‡ Gabriele Mulas,§ A Ä rpa´d Molna´r,*,‡ and † Imre De´ka´ny Departments of Colloid Chemistry and Organic Chemistry, University of Szeged, H-6720 Szeged, Hungary, and Department of Chemistry, University of Sassari, Via Vienna 2, I-07100 Sassari, Italy Received June 3, 2002. In Final Form: January 28, 2003 Significant differences are observed in the hydrogen sorption behavior of as-received, HF-treated, and oxidized PdZr alloys. Removal by HF treatment of the surface ZrO2 layer, which blocks sorption of hydrogen, and a part of the bulk Zr generates a porous surface layer of increased surface area. The resulting specimen sorbs hydrogen even at low H2 partial pressure. A thick oxide layer composed of a mixture of PdO and ZrO2 is formed by aerobic oxidation (553 K, 3 h). A diffusion controlled slow H2 sorption by Pd in the surface layer followed by a rapid and large H2 sorption by the bulk alloy is observed. This results in the fracturing of the surface oxide layer and the formation of the hydrides of the constituent elements. The pretreated alloy samples studied exhibit high catalytic activities and selectivities in the hydrogenation of compounds with multiple unsaturation (cycloocta-1,3-diene, phenylacetylene, 1-pentyne).

Introduction Selective partial hydrogenation (semihydrogenation) of compounds with multiple unsaturation (acetylenes and dienes) to the corresponding monoenes is an important transformation in the laboratory practice, fine chemicals production, and industrial polymerization processes. Palladium is known to be by far the most selective metal to achieve regioselective hydrogenation of dienes or the chemoselective hydrogenation of acetylenes. In addition, it is also capable of transforming nonterminal alkynes to cis-alkenes with very high selectivity. The interpretation of these outstanding catalytic properties with the aim of developing even more selective catalyst systems still attracts great attention.1 Palladium is able to adsorb and absorb hydrogen under mild conditions. At low hydrogen partial pressure the surface of Pd is saturated and then forms an R-hydride phase on increasing hydrogen pressure. At a certain hydrogen saturation, phase transformation is induced, resulting in the formation of the β-hydride phase.2-4 These hydride phases, particularly the β-hydride phase, affect the catalytic properties of Pd, although data available in the literature are quite controversial. According to Borodzinski,5 selectivity decreases in the presence of the β-hydride phase. Since the probability of the formation of the β-hydride phase decreases with increasing metal dispersion, selective semihydrogenation of alkynes was shown to require high dispersion and low hydrogen partial pressure.6,7 In contrast, high alkene * Corresponding author. Do´m te´r 8, Szeged. Telephone: 36 62 544277. Fax: 36 62 544200. E-mail: [email protected]. † Department of Colloid Chemistry, University of Szeged. ‡ Department of Organic Chemistry, University of Szeged. § University of Sassari.

(1) Molna´r, A Ä .; Sa´rka´ny, A Ä .; Varga, M. J. Mol. Catal. A: Chem. 2001, 173, 185. (2) Palczewska, W. Adv. Catal. 1975, 24, 245. (3) Lynch, J. F.; Flanagan, T. B. J. Chem. Soc., Faraday Trans. 1 1974, 70, 814. (4) Lewis, F. A. Platinum Met. Rev. 1994, 38, 112. (5) Borodzinski, A.; Du¨s, R.; Frak, R.; Janki, A.; Palczewska, W. Proc. th 6 Int. Congr. Catal. 1976, 150.

selectivities were observed in the presence of the β-hydride phase in recent studies.8,9 The ability of palladium to yield alkene selectively can be further improved by promoters (modifiers) and additives, such as CO, various organic bases, and sulfur compounds, and by adding a second, inactive metal to the catalyst. We have been engaged in recent years in studying various Pd-containing alloys in selective hydrogenations. PdZr, PdCuZr, and PdCuSi alloy ribbons and powders prepared by rapid quenching and high-energy ball milling, respectively, were used in the hydrogenation of 1,3cyclooctadiene, phenylacetylene, and isomeric pentynes.10-12 The as-received alloys usually exhibited negligible activity. Various pretreatments, therefore, were applied to transform these alloy precursors to active catalysts. Pretreatments affected both activities and selectivities. These observations led us to carry out further studies with the aim of acquiring information of the various factors which affect the catalytic properties. Here we disclose the results of a study with respect to the structural (surface and bulk) and hydrogen sorption properties and the characteristics in catalytic hydrogenation of a Pd25Zr75 alloy ribbon both in the as-received state and following various pretreatments. Experimental Section Materials. The Pd25Zr75 alloy ribbon was prepared from the melt of the pure metals by rapid quenching in argon atmosphere using the single roll technique. The alloy was used in the form (6) Carturan, G.; Facchin, G.; Cocco, G.; Enzo, S.; Navazio, G. J. Catal. 1982, 76, 405. (7) Guczi, L.; LaPierre, R. B.; Weiss, A. H.; Biron, E. J. Catal. 1979, 60, 83. (8) Pradier, C. M.; Mazina, M.; Berthier, Y.; Oudar, J. J. Mol. Catal. A: Chem. 1994, 89, 211. (9) Aduriz, H. R.; Bodnariuk, P.; Dennehy, M.; Gigola, C. E. Appl. Catal. 1990, 58, 227. (10) Varga, M.; Mulas, G.; Cocco, G.; Molna´r, A Ä . J. Therm. Anal. Calorim. 1999, 56, 305. (11) Varga, M.; Mulas, G.; Cocco, G.; Molna´r, A Ä .; Lovas, A. Mater. Sci. Eng. A 2001, 304-306, 462. (12) Varga, M.; Molna´r, A Ä .; Mohai, M.; Berto´ti, I.; Janik-Czachor, M.; Szummer, A. Appl. Catal., A 2002, 234, 167.

10.1021/la020516r CCC: $25.00 © 2003 American Chemical Society Published on Web 04/04/2003

Characterization of Pd-Zr Alloys of 50 mm long by 2 mm wide strips with a thickness of 0.05 mm. HF activation was performed by keeping the sample in 0.035 M HF solution with constant stirring for 1 h at 298 K. Oxygen treatment was carried out at 553 K for 3 h in air. The structure of the samples was studied by X-ray diffraction (XRD, Siemens D500 diffractometer using Cu KR radiation) and differential scanning calorimetry (DSC, Perkin-Elmer DSC-7 with scanning rate o20 K/min). Phenylacetylene (Aldrich) and cycloocta-1,3-diene (Aldrich) were distilled before use and were percolated through a short basic Al2O3 column to remove peroxide impurities. Similar purification was applied to heptane (Aldrich) used as solvent. 1-Pentyne (Aldrich) was used without further purification. Hydrogen was prepared with a Chrompack (UCAR) 8326 generator operating with a palladium membrane. Argon had a purity of 99.9990%. Methods. BET surface areas were determined by a Micromeritics Gemini 2375 sorption apparatus using nitrogen adsorption at 77 K. Microcalorimetric measurements were performed at 298 K using an LKB 2107 microcalorimeter. The calorimeter coupled with a gas-handling system and employing two membrane manometers allowed precision pressure measurements in the ranges from 0.13 to 133.3 Pa and from 13.3 Pa to 133 kPa, respectively. The system was equipped with an oil diffusion pump backed up by a rotary pump and liquid N2 traps, which produced an ultimate vacuum better than 1.3 × 10-3 Pa. The pressure was monitored by Penning and Pirani gauges. Calorimetric measurements were carried out using pretreated samples [393 K, 80 Torr H2 (1 Torr ) 1.33 kPa), 20 min] by sequentially introducing small doses of hydrogen until the sample became saturated. Then the procedure was repeated backward by gradually reducing the pressure for the desorption steps. The resulting heat response for each dose was recorded as a function of time. Data collection and analysis were computer-controlled. Two methods were used for catalytic reactions. Hydrogenation of phenylacetylene and cycloocta-1,3-diene were carried out in the liquid phase, in an all-glass apparatus at ambient temperature and pressure with stirring (2800 rpm). The reaction mixture consisted of 0.020 mg (in the hydrogenation of cycloocta-1,3diene) or 30 mg (in the hydrogenation of phenylacetylene) of catalyst, 100 µL of reactant, and 900 µL of heptane. The catalyst and solvent were kept in hydrogen for 30 min before reaction. Product composition was determined by gas chromatography (Carlo Erba Fractovap Mod G, 1.5 m CARBOWAX 1000 column, TCD detector). 1-Pentyne was studied in the gas phase, in a static circulation system (148.2 dm3) containing a glass reactor, which can be sealed and filled while maintaining vacuum in the rest of the system. The gas mixture was recirculated by an electromagnetic differential pressure circulation pump. The vacuum system consisted of an oil diffusion pump backed up with a rotary vacuum pump. Sampling was made by a loop attached to a Carlo Erba Fractovap 2150 chromatograph equipped with a flame ionization detector. Reactants and reaction products were analyzed at 413 K using a 50 m long Al2O3/KCl column. Prior to each catalytic test the catalyst sample was reduced under 150 Torr H2 at 423 K for 0.5 h. Subsequently, the reactor was evacuated and cooled to the desired temperature, and finally, the reaction mixture was introduced into the reactor. In all experiments the circulation system was filled up to 101.3 kPa using Ar as diluent gas. Reaction conditions: 4 mg of catalyst, 313 K, 10 Torr 1-pentyne, 20 Torr hydrogen.

Results and Discussion As-Received Alloy. The as-received alloy has a very small surface area (0.2 m2/g), which corresponds to the geometrical surface. Figure 1 shows the surface of the Pd25Zr75 alloy without pretreatment. A number of lines and grooves parallel to the edge of the ribbon observed on the inner surface are a characteristic feature of metallic glasses prepared by the melt quenching method with the single roll technique (Figure 1A). These heterogeneities are created by the inert gas used during fabrication, which is trapped between the wheel and the solidifying ribbon.

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Figure 1. Optical micrographs of the inner side (A) and outer side (B) of as-received PdZr.

Figure 2. XRD patterns of as-received (A) and HF-treated (B) PdZr.

The outer surface, in turn, is much smoother with some artifacts (Figure 1B). The alloy can be regarded as a partially crystallized amorphous alloy, as the XRD pattern of the ribbon indicates the presence of PdZr2 intermetallic compound in addition to the low-intensity halo of the amorphous materials (Figure 2A). XPS measurements carried out in an earlier study with this sample10 showed the presence of ZrO2 on the surface of the alloy, while Pd could be detected only in very low concentrations. It is well-known that alloys of electropositive and electronegative elements undergo surface segregation upon exposure to oxygen. An oxygen-induced Zr enrichment and selective oxidation of Zr at the surface were found in Ni-Zr and Cu-Zr alloys.13-16 The asreceived alloy did not show catalytic activity in the hydrogenation of hydrocarbons with multiple unsaturation, which is due to this inactive oxide layer, that is, to the almost complete lack of the active metal (Pd). Furthermore, exposure of the sample to hydrogen resulted in neither hydrogen uptake nor any calorimetric signal up to 200 Torr at 298 K. HF-Treated Alloy. Etching in HF solution is a successful method to activate Zr-containing alloys because HF acts as a selective reagent to dissolve zirconium species (13) Yu, X.-N.; Slapbach, L. Z. Phys. Chem. N. F. 1989, 165, 1171. (14) Sen, P.; Sarma, D. D.; Budhani, R. C.; Chopra, K. L.; Rao, C. N. R. J. Phys. F: Met. Phys. 1984, 14, 565. (15) Shina, S.; Badrinarayana, S.; Shina, A. P. J. Less-Common Met. 1986, 125, 85. (16) Wright, R. B.; Hankins, M. R.; Owens, M. S.; Cocke, D. L. J. Vac. Sci. Technol., A 1987, 5, 593.

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Figure 5. Adsorption-desorption isotherms of N2 on HFtreated PdZr alloy at 77 K.

Figure 3. Optical micrographs of the inner side (A) and outer side (B) of HF-treated PdZr.

Figure 6. Isotherm of hydrogen sorption by HF-treated PdZr at 298 K.

Figure 4. Differential pore volume distribution of the HFtreated PdZr sample.

from the surface.17-19 HF dissolution (treatment with 0.035 M HF for 1 h at 298 K) resulted in a weight loss of 36%. Since dissolution of Pd in HF is much slower than that of Zr/ZrOx, a composition of Pd40Zr60 for the etched sample can be calculated. The surface of the sample underwent significant changes. Both sides of the alloy ribbon show a rough structure with surface features more pronounced than those of the original sample (Figure 3), which qualitatively points to a possible increase in the surface area. Indeed, the BET surface area of the new sample is 5.5 m2/g with an average pore diameter of 3.2 nm (Figure 4). Nitrogen adsorption-desorption isotherms provide further proof for the formation of a porous surface structure. The shape and size of the hysteresis loop shown in Figure 5 indicate a porous surface with a rather narrow pore size distribution. In addition, the removal of the surface layer results in the appearance of Pd on the surface, as shown by our earlier XPS study.10 Substantial changes, however, take place not only on the surface but also in the bulk. As XRD measurement indicates (Figure 2B), Pd segregated from the matrix of the as-received alloy, and DSC experiments revealed complete crystallization.10 In summary, HF treatment generates a crystalline sample with a porous, Raney-type surface with Pd exposed for catalytic reaction. (17) Yamashita, H.; Yoshikawa, M.; Kaminade, T.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1986, 82, 707. (18) Yamashita, H.; Yoshikawa, M.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2883. (19) Katona, T.; Molna´r, A Ä .; Perczel, I. V.; Kopasz, Cs.; Hegedu¨s, Z. Surf, Interface Anal, 1992, 19, 519.

Figure 7. Calorimetric enthalpy isotherm of hydrogen sorption by HF-treated PdZr at 298 K.

The results of volumetric and microcalorimetric hydrogen sorption measurements are shown in Figures 6 and 7. Significant hydrogen uptake is detected already at low H2 pressure, and it still continues at 100 Torr (Figure 6), which indicates that Zr also contributes to the hydrogen sorption process. As is obvious from the shape of the adsorption isotherm, the R f β transformation occurring in the pressure range 5-15 Torr is not sharp, although it is somewhat more pronounced on the desorption curve. A similar phenomenon was also observed for dispersed Pd samples.20 The integral adsorption heats measured by microcalorimetry during adsorption and desorption are shown in Figure 7. The shape and size of the hysteresis loop are similar to those of the isotherms. It is important to note that most of the sorbed hydrogen could not be removed by prolonged evacuation. This fraction of the sorbed hydrogen may be considered to be chemisorbed hydrogen. (20) Chou, P.; Vannice, M. A. J. Catal. 1987, 104, 1.

Characterization of Pd-Zr Alloys

Figure 8. Differential sorption enthalpy of hydrogen as a function of specific adsorbed quantity by HF-treated PdZr at 298 K.

Figure 9. Optical micrographs of the inner side (A) and outer side (B) of oxidized PdZr.

Figure 8 shows the differential sorption enthalpy of hydrogen as a function of the specific adsorbed quantity calculated by taking into consideration the adsorptiondesorption and calorimetric data. It is seen that the differential heat of sorption decreases gradually from 100 to 40 kJ/mol. The value for the differential adsorption heat in the β-hydride region determined here for the HFtreated sample is 47.7 kJ/mol (literature data are in the range 23-46 kJ/mol.3). A calculation on the basis of the quantity of H2 sorbed at 100 Torr partial pressure according to Figure 6 gives a composition of Pd40Zr60H38 for our hydrogen-treated sample. This corresponds to a H/Pd ratio of 0.95 in contrast to about 0.6, which is the literature value for saturated Pd samples.2 If one considers that Pd, in all probability, is only partially available for hydrogen even after HF treatment, then the obvious conclusion is that Zr itself plays an important part in hydrogen sorption. Oxidized Alloy. Treating the as-received sample in air (553 K, 3 h) brings about a 6.5% weight increase corresponding to a composition of Pd25Zr75O39. In addition, a new, unique surface develops characterized by numerous grains (Figure 9) but without any significant increase in specific surface area. The sample thus prepared shows a rather unusual behavior. In contrast to all literature observations indicating the ability of pure Pd to adsorb and absorb hydrogen,2,21 and the results of the present study with respect to the HF-treated PdZr alloy, which shows that it sorbs substantial quantities of hydrogen at 100 Torr H2 pressure, no hydrogen uptake was detected with the oxidized alloy even at 150 Torr. At about 200 Torr H2 pressure, however, hydrogen sorption with peculiar kinetics was detected. We could measure a rapid initial hydrogen sorption in about 400 s, showing an (21) Cardona-Martinez, N.; Dumesic, J. A. Adv. Catal. 1992, 38, 219.

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Figure 10. Calorimetric signal at 200 Torr hydrogen pressure on oxidized PdZr alloy. Inset: solid line, 200 Torr; broken line, 300 Torr.

Figure 11. Specific integral adsorption heat and the specific adsorbed hydrogen quantity by oxidized PdZr at 298 K (initial H2 pressure: 200 Torr).

Figure 12. Specific sorption heat vs specific sorbed hydrogen quantity function (oxidized Pd alloy, 298 K, 200 Torr initial H2 pressure).

apparent saturation (Figure 10, inset, solid line). This is, however, only an induction period of a second, much more pronounced hydrogen sorption reaching completion in about 10 000 s (Figure 10). The further H2 uptake at 300 Torr is only negligible (Figure 10, inset, broken line), indicating that the sample was already saturated at 200 Torr. From data determined at 200 Torr initial H2 pressure, the specific integral adsorption heat and the specific adsorbed hydrogen quantity as a function of time were calculated (Figure 11). Numerical differentiation of the combination of these two curves (Figure 12) gives the differential heat of adsorption versus specific adsorbed hydrogen quantity function (Figure 13). The initial differential heat of adsorption is very close to the value of pure Pd,21 but after the induction period it increases rapidly and gives an average value of 110 kJ/mol. The quantitative interpretation of the large amount of adsorbed hydrogen detected in this case requires an

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Figure 15. Conversion of phenylacetylene as a function of time over HF-treated (O) and oxidized (b) PdZr alloys. Figure 13. Differential heat of adsorption vs specific adsorbed hydrogen quantity by oxidized PdZr at 298 K.

Figure 16. Change in the product composition in the hydrogenation of cycloocta-1,3-diene over HF-treated PdZr alloy: (2) cycloocta-1,3-diene; (b) cyclooctene; (9) cyclooctane. Figure 14. Optical micrographs of oxidized PdZr after hydrogen sorption.

estimation of the possible composition of the oxidized sample, specifically, the relative ratio of PdO and ZrO2. Our earlier XPS study10 and literature observations22,23 showed that both components of PdZr alloys undergo simultaneous oxidation to form PdO and ZrO2. The actual possibility, therefore, is a sample with the nominal composition of (PdO)5.6(ZrO2)16.7Pd19.4Zr58.3. Since the mild hydrogen treatment applied before the adsorption and calorimetric measurement reduces only PdO, we start with a sample of Pd25(ZrO2)16.7Zr58.3. By the end of the adsorption and calorimetric measurement (200 Torr after 10 000 s), this composition changes to Pd25(ZrO2)16.7Zr58.3H139. Taking into consideration that the fraction of hydrogen sorbed by Pd is the same as that for pure Pd (H/Pd ) 0.6) and the rest should react with Zr, we can deduce a final composition of (PdH0.6)25(ZrO2)16.7(ZrH2.1)58.3. The composition of zirconium hydride calculated in this way is very close to the stoichiometric ZrH2 composition. It follows that, in harmony with instrumental observations,10 complete transformation of the bulk did occur during H2 sorption. The endothermic heat effect, which can be detected during the desorption of hydrogen upon evacuation, was also measured. An initially fast and then slow and prolonged desorption was detected with an integral enthalpy effect of 276 J/g until 3600 s. This low value, when compared to the saturation value of -703 J/g (Figure 11), is indicative of a partial hydrogen desorption. The sample used in the adsorption-desorption measurements underwent visible changes; specifically, it became cracked. As optical microscopy shows (Figure 14), the cracked surface oxide layer peeled off, revealing the smooth matrix underneath. This phenomenon may offer (22) Baiker, A.; De Pietro, J.; Maciejewski, M.; Walz, B. In Structure Activity and Selectivity Relationship in Heterogeneous Catalysis; Grasselli, R. K., Sleight, A. W., Eds.; Elsevier: Amsterdam, 1991; p 169. (23) Takahashi, T.; Nishi, Y.; Otsuji, B.; Kai, T. Can. J. Chem. Eng. 1987, 65, 274.

an explanation to the unusual hydrogen adsorption behavior of the oxidized sample. As seen in the inset of Figure 10, exposure to 200 Torr hydrogen results in an immediate H uptake. This indicates that a substantial amount of Pd is readily available for hydrogen. The lack of H adsorption at lower pressure, in turn, shows that diffusion of hydrogen to Pd clusters formed during the reductive pretreatment of the oxidized sample embedded in ZrO2 is slow. Hydrogen sorption, therefore, is diffusion controlled and requires rather high partial pressure, and even at 200 Torr a temporary saturation is observed. Hydrogen sorption by Pd, however, results in an expansion of the crystal lattice. The strain thus generated breaks up the surface layer and opens up the underlying alloy matrix for further rapid and more significant H sorption. The bulk alloy, consequently, is almost completely transformed to the corresponding hydrides, PdH0.6 and ZrH2. Catalytic Hydrogenations. The HF-treated and oxidized PdZr alloy samples were applied in the hydrogenation of phenylacetylene and cycloocta-1,3-diene in the liquid phase and in the hydrogenation of 1-pentyne in the gas phase. The as-received alloy showed practically no activity in the hydrogenation of the three model compounds, which is in accordance with its inability to sorb hydrogen. In sharp contrast, pretreatments brought about highly increased activities, and the catalyst samples also exhibited high selectivities. The two pretreatments, however, resulted in some characteristic differences in catalytic properties. The HF-treated sample showed a considerably higher activity (higher reaction rate) in the hydrogenation of phenylacetylene than the sample after oxidation (Figure 15). Initial selectivity values (calculated by dividing the rate of alkene formation by the rates of formation of all products) are 82 and 80.1, respectively. Only the HFtreated sample was tested in the hydrogenation of cycloocta-1,3-diene, showing complete selectivity. In fact, after the consumption of 1 equiv of H2, the reaction stopped, allowing the selective synthesis of cyclooctene (Figure 16). The differences of the activities in the hydrogenation of

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Conclusions

Figure 17. Consumption of the starting material as a function of time in the hydrogenation of 1-pentyne over HF-treated (2) and oxidized (b) PdZr alloys.

1-pentyne are even more pronounced than those observed in the reaction of phenylacetylene (Figure 17). Initial selectivities, in this case, however, are much higher (97.8 and 96.3, respectively). The better catalytic properties (higher catalytic activity) found for the HF-treated sample can be attributed to the differences in the hydrogen sorption properties of the two samples studied. As mentioned, HF treatment removes the surface Zr/ZrOx layer and generates a catalyst of rough surface and increased surface area. This catalyst surface, consisting of pure, Raney-type Pd, is readily available for both hydrogen and the reacting organic molecules and allows fast reactions to take place. In contrast, the surface of the oxidized sample is covered by ZrO2. Even though this sample absorbs large amounts of hydrogen (consumed mainly to form ZrH2) and the oxide layer cracks and peels off after prolonged hydrogen treatment, the diffusion of hydrogen is slow. Even more severe diffusion limitations for the organic molecules between the solid-liquid and solid-gas interfaces, therefore, can be expected in hydrogenation experiments. The overall effect is the lower reaction rates observed for the oxidized sample.

Significant differences observed in the H sorption behavior and catalytic properties of an as-received, HFtreated, and oxidized PdZr alloy allow us to draw the following important conclusions. (i) The surface of the as-cast PdZr alloy is covered with a ZrO2 layer, which blocks the sorption of hydrogen and, therefore, hinders Pd-catalyzed transformations. (ii) Treatment by HF removes the surface oxide layer, and in addition, a part of the bulk Zr is also dissolved. This results in the generation of a porous surface and significantly increases the specific surface area. Hydrogen sorption, consequently, ensues even at low H partial pressure and is not completed even at 100 Torr. The R f β phase transformation is not sharp. (iii) The behavior of the oxidized sample is rather curious. The results show that aerobic oxidation (553 K, 3 h) brings about a thick oxide layer composed of a mixture of PdO and ZrO2 with a Pd-to-Zr ratio similar to that in the bulk phase. A diffusion-controlled slow H sorption by Pd in the surface layer followed by a rapid and large H sorption by the bulk alloy is observed with the concomitant fracturing of the surface oxide layer and the formation of the hydrides of the constituent elements. (iv) HF treatment and oxidation are useful activation procedures to transform the PdZr alloy samples to catalysts, which are active and selective in the hydrogenation of compounds with multiple unsaturation. The lower activity of the oxidized sample is attributed to diffusion limitations brought about by the oxide-covered surface. Acknowledgment. We are most grateful for the financial support to the Hungarian National Science Foundation (Grant OTKA T030156). LA020516R