Interaction of Hydrogen with Unsupported and Supported Nickel

Institute of Physical Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland, and ... Copyright © 2006 American Chemical Societ...
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Langmuir 2006, 22, 8758-8763

Interaction of Hydrogen with Unsupported and Supported Nickel Leszek Znak† and Jerzy Zielin´ski*,†,‡ Institute of Physical Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland, and Institute of Chemistry, Warsaw UniVersity of Technology, Łukasiewicza 17, 09-400 Płock, Poland ReceiVed February 28, 2006. In Final Form: July 6, 2006 The adsorption of molecular hydrogen on Ni powder and on Ni/Al2O3 and Ni/SiO2 catalysts was studied by the temperature-programmed desorption (TPD) method. The examinations were performed in the flow system, starting the TP measurements at low temperatures of 100 or 78 K, which resulted in the formation of complete characteristics of the interaction of hydrogen with nickel. Generally, three forms of chemisorbed hydrogen were distinguished: R, adsorbed on Ni surface, β, adsorbed in the “second layer”, and γ, located in the subsurface region of nickel. The comparison of the results of this work with those obtained in vacuum systems for various Ni surfaces has led to the conclusion that the same form of hydrogen desorbs from nickel above 200 K in vacuum systems but above 300 K in flow systems. The examinations performed for Ni/Al2O3 and Ni/SiO2 samples show that alumina suppresses but silica enhances the formation of the β-form of hydrogen.

Introduction Hydrogen is considered a prime adsorbate used for measuring the dispersion of supported metal catalysts. This view was not questioned until the end of 1970s, when it was discovered that some supports considerably affect the adsorption of hydrogen on metals.1-3 The phenomenon, termed the “strong metal-support interaction”, became the subject of extensive studies that showed that the magnitude of the effect depends on both the kind of metal and support and the preparation/pretreatment of the catalyst.4-9 These results showed the limited applicability of hydrogen to measuring the dispersion of nickel; however, at the same time, they implied that the interaction of hydrogen with supported metals may be a valuable quality in characterizing the effect of support on chemical/catalytic properties of metal catalysts. Temperature-programmed desorption of preadsorbed hydrogen (TPD-Hads) is commonly used for characterization of supported metal catalysts.10-14 As a rule, the measurements are carried out in flow systems and at a linear temperature rise. The preadsorption of hydrogen is usually performed at room temperature, whereupon the reactor with the examined sample is flushed with a stream * Corresponding author. E-mail: [email protected]. † Polish Academy of Science. ‡ Warsaw University of Technology. (1) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29. (2) Tauster, S. J.; Fung, S. C.; Garten, S. C. J. Am. Chem. Soc. 1978, 100, 170. (3) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. (4) Stevenson, S. A.; Dumesic, J. A.; Baker, R. T. K. Metal-Support Interaction in Catalysis, Sintering, and Redispersion; Van Nostrand-Reinhold: New York, 1987 and references therein. (5) Christman, K. R. In Hydrogen Effect in Catalysis; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 3. (6) Geus, J. W. In Hydrogen Effect in Catalysis; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 85. (7) Menon, P. G. In Hydrogen Effect in Catalysis; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 117. (8) Bartholomew, C. H. In Hydrogen Effect in Catalysis; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 139. (9) Burch R. In Hydrogen Effect in Catalysis”; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 347. (10) Weatherbee, G. D.; Bartholomew, C. H. J. Catal. 1984, 87, 55. (11) Bartholomew, C. H. Catal. Lett. 1990, 7, 27 and references therein. (12) Bartholomew, C. H. Catalysis 1994, 11, 93 and references therein. (13) Zielin´ski, J. Polish J. Chem. 1995, 69, 1187. (14) Kanervo, J. M.; Reinikainen, K. M.; Krause, A. O. I. Appl. Catal., A 2004, 258, 135.

of an inert gas to remove weakly adsorbed hydrogen, and the remaining hydrogen is examined by the TPD method. As a result of such a procedure, commonly obtained TPD-Hads spectra characterize only the hydrogen adsorbed irreversibly at room temperature and leave aside the less strongly adsorbed hydrogen. The examinations presented in this paper characterize not only the strongly adsorbed but also the weakly adsorbed hydrogen on nickel. Generally, the preadsorption of hydrogen was carried out at a temperature gradually lowered from an elevated value to 100 K, whereupon the reactor was flushed with a He stream and TPD examination was carried out. Consequently, the obtained spectra supply a more complete characteristic of the interaction of hydrogen with nickel. Similar examinations of supported nickel catalysts were performed by Konvalinka et al.15 and Stockwell et al.;16 however, the obtained results were not supplemented with suitable physicochemical interpretation. Experimental Section Apparatus. The measurements were carried out in a glass flow system17 equipped with a gradientless microreactor.18 A temperature controller maintained the reactor temperature within 1 K and provided linear temperature programming in the range of 73-1073 K. Low temperature was attained by immersing the electrical furnace with the reactor in liquid nitrogen. Hydrogen, helium, and argon were of 99.999% purity, and neon was of 99.95% purity. Hydrogen was further purified by a Trienco Hydrogen Purifier equipped with thickwalled palladium alloy tubing. The gas stream required was fed to the measuring system by a five-way selection valve and, before entering the reactor, it was additionally purified from traces of oxygen and water by passing through a MnO/SiO2 column. In the case of He and Ne streams, the column was maintained at 78 K, which lowered the content of impurities below 0.1 ppm. The composition of the gas stream leaving the reactor was monitored by means of a thermal conductivity detector (TCD) cell, and the results were collected by a computer-controlled system. Materials. Ni powder of high purity (batch no. S.94566A) from Johnson Matthey was first precalcined in oxygen at 773 K to remove traces of carbon, and it was subsequently reduced in an H2 stream. (15) Konvalinka, J. A.; van Oeffelt, P. H.; Sholten, J. J. F. Appl. Catal. 1981, 1, 141. (16) Stocwell, D. M.; Bertucco, A.; Coulston, G. W.; Bennett, C. O. J. Catal. 1988, 113, 317. (17) Zielin´ski, J. J. Catal. 1982, 76, 157. (18) Zielin´ski, J. React. Kinet. Catal. Lett. 1981, 17, 69.

10.1021/la0605541 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/09/2006

Hydrogen Interaction with Unsupported/Supported Ni The treatments, as well as the subsequent TPD examinations, were carried out for the sample mixed with broken quartz glass (1.0-1.5 mm), which diminished the inevitable sintering of Ni powder and thereby eliminated by-passing of a stream flowing through the examined sample. Ninety percent Ni/Al2O3 and 30% Ni/Al2O3 precursors were obtained by the coprecipitation method, mixing an aqueous solution of Na2CO3 with an aqueous solution of Ni(NO3)2 and Al(NO3)3.19 The precipitates were thoroughly washed, filtered, dried at 378 K, and finally calcined in the air at 673 K for 2.5 h. The former specimen was thoroughly examined previously.20 After reduction at 773 K, the size of the Ni crystallites calculated from O2 adsorption, H2 adsorption, and X-ray diffraction (XRD) measurements was 8.4, 17.0, and 5.8 nm, respectively. The 20% Ni/Al2O3 and 1% Ni/Al2O3 precursors were obtained by impregnation of γ-alumina (surface area 111 m2/g and grain size 0.15-0.35 mm) with an aqueous solution of nickel nitrate of ultrahigh purity. The alumina, prepared by hydrolysis of aluminum isopropoxide, was calcined for 12 h at 973 K prior to the impregnation.17 In the case of the former specimens, the size of the Ni crystallites determined from O2 chemisorption and the small-angle X-ray scattering method was 3.9 and 4.9 nm, respectively.17 Twenty-five percent Ni/SiO2 (EuroNi-1) was obtained by the coprecipitation method.21-23 The material was received in dry form, and the calcination was performed in the air at 673 K for 2.5 h. The 25% Ni/SiO2 specimen was thoroughly examined previously.24 After reduction at 773 K, the size of the Ni crystallites calculated from O2 adsorption and XRD measurement was 3.6 and 2.7 nm, respectively. One percent Ni/SiO2 was obtained by impregnation of silica with an aqueous solution of Ni(NO3)2 of ultrahigh purity. The silica, Davison 62 of surface area 340 m2/g, was washed with diluted HCl, dried at 393 K for 21 h, and calcined at 723 K for 4 h.25 After the treatment, the content of Fe, Na, and Ca was lowered to 0.02, 0.06, and 0.02 wt %, respectively. Measurement Procedure. As a rule, 2000 mg of Ni powder and 50 mg of supported catalysts were used for the examinations. Before TPD-Hads measurement, the examined sample was prereduced in situ in a H2 stream of 1 cm3/s. The reduction was carried out at a linearly increasing temperature of 0.17 K/s from room temperature to a chosen temperature (673 K for Ni powder and 773 K for supported nickel), whereupon the reduction was continued at the final temperature for 2.5 h. Then, the sample was purged from hydrogen, passing a He stream (0.5 cm3/s, 0.5 h, 673 K), and, subsequently, the preadsorption of hydrogen and its TPD examination were carried out. Generally, the preadsorption of hydrogen was performed at atmospheric pressure in three steps: (1) adsorption at constant temperature (293 K for Ni powder and 423 K for supported nickel catalysts) for 0.25 h, (2) adsorption at gradually decreasing temperature from the initial temperature to about 100 K, and (3) adsorption at 100 K for 0.25 h. After H2 adsorption, the reactor was flushed with an Ar stream (0.5 cm3/s, 100 K, 0.25 h) to remove weakly adsorbed hydrogen, and the remaining hydrogen was examined by the TPD method. Occasionally, H2 preadsorption was performed by the pulse method, admitting 4.5 µmol H2 portions into an Ar stream (0.5 cm3/s) flowing over the sample maintained at a chosen temperature, until saturation was attained. TPD-Hads was carried out in an Ar stream of 0.5 cm3/s with the temperature rising at 0.17 K/s from 100 to 673 K for Ni powder and to 773 K for supported catalysts. The stream leaving the reactor was passed through a 195 K trap, and the evolution of hydrogen was (19) Zielin´ski, J. Appl. Catal. 1993, 94, 107. (20) Znak, L.; Stol-ecki, K.; Zielin´ski, J. Catal. Today 2005, 101, 65. (21) Coenen, J. W. E.; Wells, P. B. In Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; p 801. (22) Coenen, J. E. Appl. Catal. 1989, 54, 65. (23) Coenen, J. E. Appl. Catal. 1991, 75, 193. (24) Zielin´ski, J. Catal. Lett. 1995, 31, 47. (25) Bonarowska, M.; Pielaszek, J.; Juszczyk, W.; Karpin´ski, Z. J. Catal. 2000, 195, 304.

Langmuir, Vol. 22, No. 21, 2006 8759 measured. At the end of each run, the reactor with the examined sample was cooled to determine the baseline, and calibration of the system was performed. The surface area of the Ni powder was determined from the measurement of argon adsorption performed at 78 K from Ar-He stream, using the Brunauer-Emmett-Teller (BET) equation. The measurements were performed in the same system used for the adsorption studies. A long column was placed between the reactor with the examined sample and the TCD cell to separate the response produced by Ar desorption from the response produced by temporary fluctuations in Ar-He flow rate. In the calculation of the nickel surface area and the number of surface Ni atoms, it was assumed that one Ar atom and one surface Ni atom occupied 0.169 nm2 11 and 0.0645 nm2,26 respectively.

Results and Discussion It is known that the shape and position of the TPD-Hads spectrum depends not only on the interaction of hydrogen with the examined sample but also on the experimental conditions of the measurement.27-28 This fact implies that TPD-Hads spectra may be used for characterization of the interaction, provided that (i) experimental conditions of the experiments are properly selected and (ii) undesired effects of these measurements are minimized. The most important point of the latter requirement is designing a TPD experiment free of mass transfer effects.29 The requirement is fulfilled when H2 desorption is slow in comparison with (i) H2 diffusion in the gas stream flowing through the catalyst bed and (ii) H2 diffusion inside porous particles of the examined sample.28 Appropriate calculations indicate that the former criterion equals about 0.08 for Ni powder and 0.02 for supported catalysts, and the latter criterion equals about 0.003 for Ni powder and 0.01 for supported catalysts. These values are smaller than the respective values indicated by Demmin and Gorte (0.1 and 0.05),28 which shows that the measurements of this work are free from mass transfer effects. The TPD-Hads spectra presented below indicate the complex character of the interaction of hydrogen with nickel. Such a state is self-evident if we take into account that (i) the interaction depends on the structure of the nickel surface and the kind of support, (ii) both adsorption and desorption depend on hydrogen coverage, and (iii) the desorption is accompanied by readsorption processes. This complexity implies that full interpretation of TPD-Hads spectra is an unfeasible task. Readsorption of hydrogen is an intrinsic process of TPD-Hads examination. At low H2 pressure in the course of TPD measurement, H2 readsorption is slow in comparison to H2 desorption, and the obtained TPD-Hads spectrum characterizes the kinetics of the desorption process. On the other hand, at high H2 pressure in the gas phase, the rate of H2 readsorption approaches the rate of hydrogen desorption, and the obtained TPD-Hads spectrum characterizes the equilibrium of the interaction. Determination of the effect of H2 readsorption on the TPD-Hads spectrum is a difficult task, as it needs special TPD-Hads examination of the very same sample at a negligible H2 concentration. In the case of hydrogen desorption from nickel, it is generally assumed that TPD examinations performed in vacuum systems, at a H2 pressure of ∼10-6 Torr, characterize the kinetics of the desorption, while the examinations performed in flow systems, at a H2 pressure of ∼1 Torr, characterize the equilibrium of the interaction. Ni Powder. Figure 1 shows TPD-Hads examinations of hydrogen preadsorbed on Ni powder. The spectrum of hydrogen (26) Iglesia, E.; Boudart, M. J. Catal. 1983, 81, 204. (27) Konvalinka, J. A.; Scholten, J. J. F.; Rasser, J. C. J. Catal. 1977, 48, 365. (28) Demmin, R. A.; Gorte, R. J. J. Catal. 1984, 90, 32. (29) Efstathiou, A. M.; Bennett, C. O. J. Catal. 1990, 124, 116.

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Figure 1. Desorption of hydrogen from Ni powder: (a) adsorption at constant temperature 293 K; (b) adsorption at 293-100 K.

Figure 2. Desorption of hydrogen from various nickel surfaces: (a) Ni powder in a flow system; (b) polycrystalline Ni foil in a vacuum system;31 (c) Ni(100) plane;33 (d) Ni(111) plane.34

preadsorbed at 293 K (curve a) represents a broad R-profile in the 300-650 K range, similar to those reported previously.10,13,30 At the same time, the spectrum of hydrogen preadsorbed at 293100 K (curve b) additionally exhibits two well-formed peaks, denoted as β and γ, at 327 and 186 K, respectively. These peaks, suggesting chemical rather than physical adsorption of hydrogen, have not been reported before. The TPD-Hads spectrum obtained in this work for Ni powder hardly resembles the spectra recorded in vacuum systems for various nickel surfaces (Figure 2). The difference very likely arises not only from the difference in these surface structures, but also from the difference in the experimental conditions of the measurements. Thus, the preadsorption of hydrogen in this work was performed at a H2 pressure of ∼760 Torr and exposure at ∼1012 L (in the case of pulse adsorption, at a H2 pressure of ∼10 Torr and exposure at ∼109 L), but the preadsorption of hydrogen in vacuum systems is usually carried out at a H2 pressure of ∼10-6 Torr and exposure at mostly 103 L. Additionally, TPD examinations of this work were performed at a rate of temperature rise of 0.17 K/s and a H2 pressure of ∼1 Torr, but the examinations in a vacuum system are usually carried out at a rate of temperature rise of 1-10 K/s and a H2 pressure ∼10-6 Torr. It is commonly accepted that the adsorption of molecular hydrogen on nickel is a fast process. The examinations in Figure 3 confirm that view partly. The decrease in H2 preadsorption (30) Znak, L.; Zielin´ski, J. Pol. J. Chem. 2004, 78, 2179.

Znak and Zielin´ ski

Figure 3. Effect of temperature and pressure/exposure on H2 adsorption on Ni powder: (a) static adsorption at 293-100 K; (b) static adsorption at 100 K; (c) pulse adsorption at 100 K.

temperature to 100 K reduces the quantity of adsorbed hydrogen (curve b), and the scale of the effect is different for individual forms of hydrogen. The decrease is small for the R-form, large for the β-form, and moderate for the γ-form. Additionally, when the adsorption was performed by the pulse method, that is, at low H2 pressure/exposure, the decrease was also large for the γ-form (curve c), which implies that the β- and γ-forms of hydrogen do not appear on nickel when H2 preadsorption is performed under vacuum conditions. Therefore, it is concluded that the R-profile obtained for Ni powder reflects much the same form of hydrogen as the profiles recorded in vacuum systems (Figure 2). At the same time, it is symptomatic that the R-profile for Ni powder consists of two closely overlapped peaks, similarly as it was recorded for the Ni(111) plane in a vacuum system (see Figure 2, curve d).32,34 The examinations in Figure 3 show that the adsorption of molecular hydrogen on nickel is fast in the case of the formation of the R-form, but it is rather slow in the case of the formation of the β- and γ-forms of hydrogen. Accordingly, it is supposed that the readsorption of hydrogen hardly affects the position of the β- and γ-peaks, but strongly affects the position of the R-profile, shifting the profile to high temperature. Figure 4 presents TPD-Hads spectra for Ni powder at divergent values of Ar flow rate and thereby at divergent H2 pressure over the examined sample. In agreement with the above suggestion, the measurements show that the decrease in H2 pressure does not affect the position of the β- and γ-peaks, but clearly shifts the R-profile to low temperature. The shift in Figure 4 is not large, but it is supposed that, at negligible H2 pressure, the profile should appear at the same temperature as the profile recorded for polycrystalline Ni foil in a vacuum system (Figure 2, curve b). The complete TPD-Hads spectra for Ni powder (Figures 1 and 4) show that the γ-peak sets up at the very beginning of the measurement, which might suggest that a part of that form of hydrogen evolves in the course of the standard flushing at 100 K, and it does not participate in the formation of the γ-peak. This doubt was removed by supplementary TPD-Hads examination in which, instead of argon, neon was used as a carrier gas, which allows H2 preadsorption and beginning the TPD examination at (31) Raupp, G. B.; Dumesic, J. A. J. Catal. 1985, 95, 587. (32) Johnson, A. D.; Maynard, K. J.; Daley, S. P.; Yang, O. Y.; Ceyer, S. T. Phys. ReV. Lett. 1991, 67, 927. (33) Kammler, Th.; Wehner, S.; Ku¨ppers, J. Surf. Sci. 1995, 339, 125. (34) Premm, H.; Po¨lzl, H.; Winkler, A. Surf. Sci. 1998, 401, L444.

Hydrogen Interaction with Unsupported/Supported Ni

Figure 4. Effect of argon flow rate on hydrogen desorption from Ni powder: (a) 1.5 cm3/s; (b) 0.167 cm3/s.

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Figure 6. Evolution of H/Nis stoichiometry in the course of a TP run for Ni powder: (a, a*) adsorption at 293-100 K; (b, b*) adsorption at 293 K.

Figure 5. Desorption of hydrogen from Ni powder: (a) Ar stream and start of TP run at 100 K; (b) Ne stream and start of TP run at 78 K.

a lower temperature of 78 K. The spectra in Figure 5 evidences that the decrease of initial temperature of TPD examination to 78 K does not enlarge the γ-peak, which proves that the peak reflects the whole of that form of hydrogen. The TPD-H2 spectra obtained for Ni powder (Figure 1), completed with BET examinations, were used for calculation of the H/Nis stoichiometry in the course of these runs (Figure 6). In the calculations it was assumed that, at the terminal point of the TPD-Hads run, the amount of hydrogen adsorbed on nickel falls to a small value corresponding to a H/Nis ratio of 0.1. For H2 preadsorption at 293 K, the initial value of the H/Nis ratio equals 1.15 (curve b*). This result is close to that reported by Weatherbee and Bartholomew.10 In turn, in the case of H2 preadsorption at 293-100 K, the initial value of the H/Nis ratio is close to 1.8 (curve a*), which suggests that hydrogen atoms locate not only on nickel surface. The high value of the stoichiometry may be related to (i) dissolution of hydrogen in nickel,31 (ii) adsorption of hydrogen in the subsurface region of nickel,28-30 and (iii) adsorption of hydrogen in the so-called “second layer”. The H/Nis relations in Figure 6 show that the quantity of R-, β-, and γ-forms of hydrogen corresponds to H/Nis ratios of about 1.0, 0.4, and 0.4, respectively. The value found for the R-form coincides with the stoichiometry accepted for H2 adsorption on individual Ni(111) and Ni(100) planes.32-34 The agreement supports the above suggestion that just the same form of hydrogen

Figure 7. Effect of time and initial temperature on H2 adsorption on Ni powder: (a) 293 K; (b) 293 K for 24 h; (c) 423 K; (d) 423 K for 24 h; (e) 673 K.

evolves above 200 K in a vacuum system, but above 300 K in the flow system. Figure 7 presents complementary studies of the effect of time and initial temperature on the interaction of hydrogen with Ni powder. In the case of H2 preadsorption at 293 K (Figure 7A), the extension of the time of the interaction to 24 h slightly enlarges the R-profile, but does not affect the β- and γ-profiles. At the same time, the increase in the temperature of H2 preadsorption to 423 K considerably enlarges the quantity of hydrogen desorbed in the 450-650 K range (Figure 7B, curve c), and the enlargement is more significant after 24 h of preadsorption (curve d). The

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Figure 8. Desorption of hydrogen from a 90% Ni/Al2O3 catalyst: (a) adsorption at 423-293 K; (b) adsorption at 423-100 K.

effect was ascribed to the dissolution of hydrogen in bulk nickel. It was confirmed that the increase corresponds to the solubility of hydrogen in bulk nickel at 423 K.35 At a higher preadsorption temperature of 673 K, the solubility of hydrogen in nickel is evidently larger;35 however, this fact was not recorded by TPDHads examination (curve e), very likely because a part of hydrogen, dissolved at 673 K, has desorbed from nickel in the course of gradual cooling of the sample. The shape and position of the γ-peak for Ni powder (Figure 1) are identical to those found in vacuum systems for Ni(111) and Ni (100) planes when, instead of molecular, atomic hydrogen was preadsorbed.32-34 Thorough examination of the interaction showed that (i) the peak reflects hydrogen located in the subsurface region of nickel, (ii) the quantity of that form of hydrogen depends on the experimental conditions of the interaction and may attain a few atomic layers, (iii) hydrogen occupies octahedral interstices, as in nickel hydride, and (iv) the appearance of that form of hydrogen is possible due to high free enthalpy of atomic hydrogen. Standard free enthalpy of the formation of nickel hydride from nickel and molecular hydrogen has positive value, about 23.6 kJ/mol H2.36-37 However, rough calculation (based on the thermodynamic data collected above room temperature36) shows that the enthalpy assumes a negative value at low temperatures, bellow ∼80 K, which indicates that nickel hydride may be formed as a result of the interaction of molecular hydrogen with nickel. Additional experiments performed in this work have not confirmed the suggestion: the extension of time of the interaction does not enlarge the size of γ-peak, which indicates that the quantity of that form of hydrogen attains some saturation value corresponding to a H/Nis ratio of about 0.4. Therefore, it is concluded that the γ-peak represents hydrogen located just below the nickel surface, and properties of that form of hydrogen differ from that of the hydrogen in bulk nickel hydride. The examinations in Figures 3 and 6 show that the β-form of hydrogen appears on Ni powder when (i) H2 pressure/exposure is high and (ii) the H/Nis ratio exceeds unity. These results suggest that the β-form represents hydrogen adsorbed on a nickel surface already occupied with hydrogen, forming the so-called second layer. It seems that the additional adsorption occurs mainly on less densely packed Ni planes and also on edges and corners of Ni crystallites. Alumina-Supported Nickel. It is known that adsorption of hydrogen on supported metal differs from that on unsupported (35) Smith, G. C. Hydrogen in Metals; American Society for Metals: Metals Park, OH, 1963. (36) Tkacz, M. J. Chem. Thermodyn. 2001, 33, 891. (37) Baranowski, B.; Filipek, S. M. Pol. J. Chem. 2005, 79, 789.

Znak and Zielin´ ski

Figure 9. Desorption of hydrogen from various Ni/Al2O3 catalysts: (a) 90% Ni/Al2O3 coprecipitated; (b) 30% Ni/Al2O3 coprecipitated; (c) 20% Ni/Al2O3 impregnated; (d) 1% Ni/Al2O3 impregnated; (e) Al2O3 support.

metal, and the difference may arise from two reasons: (i) an effect of the support on the adsorption properties of dispersed metal and (ii) the adsorption of hydrogen on the support itself. Attempting to explain the former phenomena, the experimental conditions of this work were chosen to minimize hydrogen adsorption on the support13. The effect of alumina on hydrogen adsorption on supported nickel may be connected with the size and structure of Ni crystallites (electronic properties of small particles, different proportion of Ni planes, steps, edges, kinks, corners, and various defects)38 and decoration of Ni crystallites with alumina species. The decoration is an experimentally established fact,39,40 but distribution of the decorants on Ni crystallites and their role in the modification of a Ni surface are still unknown. It is supposed that the decorants locate primarily on the corners and edges of Ni crystallites.41 Figure 8 presents TPD-Hads spectra obtained for a 90% Ni/ Al2O3 catalyst. The complete spectrum exhibits an R-profile and a γ-peak, similar as those for Ni powder, but does not exhibit a β-peak. The similarity indicates that the R- and γ-forms reflect hydrogen fixed on nickel. At the same time, the lack of a β-peak suggests that the presence of alumina hinders the adsorption of hydrogen weakly adsorbed on nickel, that is, the hydrogen located in the second layer and on edges and corners of Ni crystallites. The comparison of the spectra obtained for various Ni/Al2O3 catalysts (Figure 9) shows that neither the content of alumina nor the way of preparation of these specimens influence the interaction of hydrogen with nickel, which indicates that hydrogen is a proper adsorbate for the measurement of the metal surface area of these catalysts. At this point, it is important to note that the adsorption of hydrogen on alumina is small (curves d and e), which confirms the previous studies.13 In addition, it is important that the presence of nickel does not stimulate hydrogen adsorption, which indicates that the spillover of hydrogen on alumina is negligible in our studies. Besides, it is convenient that, at ambient temperature, the desorption of hydrogen from these catalysts is low (Figure 9), which facilitates measuring the quantity of strongly chemisorbed hydrogen, commonly used for evaluation of nickel dispersion. (38) Somorjai, G. A. Surf. Sci. 1994, 299/300, 849. (39) Lamber, R.; Schulz-Ekloff, G. Surf. Sci. 1991, 258, 107. (40) Lamber, R.; Schulz-Ekloff, G. J. Catal. 1994, 146, 601. (41) Zielin´ski, J. J. Mol. Catal. 1993, 83, 197.

Hydrogen Interaction with Unsupported/Supported Ni

Figure 10. Effect of initial temperature and time on hydrogen adsorption on a 90% Ni/Al2O3 catalyst: (a) 173 K; (b) 293 K; (c) 293 K for 40 h; (d) 673 K.

Figure 10 characterizes the effect of temperature and time on TPD-Hads spectra obtained for the 90% Ni/Al2O3 catalyst. The examinations show that the increase in temperature of H2 preadsorption from 173 to 673 K, as well as the extension of time of the adsorption to 40 h, insignificantly affects the obtained spectra. These results demonstrate that (i) the quantity of strongly chemisorbed hydrogen depends on experimental conditions only insignificantly and (ii) dissolution of hydrogen in bulk nickel is negligible for alumina-supported nickel catalysts. Silica-Supported Nickel. Figure 11 presents TPD-Hads spectra obtained for the 25% Ni/SiO2 catalyst. The complete spectrum (curve b), consisting of a few closely overlapping features, resembles the spectra reported by Konvalinka et al.15 Comparison of the spectrum obtained for the 25% Ni/SiO2 catalyst with the spectra recorded for Ni/Al2O3 catalysts (Figures 9 and 10)

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Figure 11. Desorption of hydrogen from Ni/SiO2 catalysts: (a) 25% Ni/SiO2, adsorption at 423-293 K; (b) 25% Ni/SiO2, adsorption at 423-100 K; (c) 1% Ni/SiO2; (d) SiO2 support.

indicates that, contrary to alumina, silica considerably enlarges the quantity of the β-form of hydrogen for this catalyst, which leads to close overlapping of the individual features of hydrogen. Complementary TPD-Hads measurements for silica support and for the 1% Ni/SiO2 catalyst (curves c and d in Figure 11) indicate that silica does not adsorb hydrogen, which demonstrates strong effect of silica on adsorption properties of nickel in 25% Ni/SiO2 catalyst. At the same time, it is unfavorable that the desorption of hydrogen from Ni/SiO2 attains high values at ambient temperatures (Figure 11, curve b), which makes it difficult to measure the amount of strongly chemisorbed hydrogen, commonly used for the evaluation of nickel dispersion. LA0605541