Langmuir 1997, 13, 2735-2739
2735
Microcalorimetric Studies of CO and H2 Adsorption on Nickel, Nickel-Boride, and Nickel-Phosphide Catalysts J. Shen,†,‡ B. E. Spiewak,† and J. A. Dumesic*,† Department of Chemical Engineering, University of WisconsinsMadison, Madison, Wisconsin 53706, and Department of Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China Received September 3, 1996. In Final Form: March 19, 1997X Nickel powder, and nickel boride (Ni-B) and nickel phosphide (Ni-P) alloy particles prepared by aqueous chemical reduction, were studied by microcalorimetry. Treatment of Ni powder, Ni-B, and Ni-P in H2 at 623 K was sufficient to produce reduced nickel surfaces for chemisorption. After treatment, these samples were shown by X-ray diffraction to be crystalline. The presence of B and P increased the BET surface areas to values of 8.4 and 19.7 m2/g, respectively, compared to the value of 1.7 m2/g for Ni powder. Microcalorimetric measurements of CO and H2 adsorption performed at 308 K on reduced Ni powder exhibited initial heats of 120 and 85 kJ/mol, respectively. The presence of B decreased the initial heats of CO and H2 adsorption by 20 and 10 kJ/mol, respectively and decreased the saturation uptakes of these molecules per unit surface area. The presence of P decreased the initial heats of CO and H2 adsorption by 30 and 20 kJ/mol, respectively, and also decreased the saturation uptakes of these molecules per unit surface area.
Introduction Nickel catalysts are widely used in the chemical industry for hydrogenation reactions. Amorphous nickel alloys, such as nickel boride (Ni-B) and nickel phosphide (Ni-P), are particularly interesting as hydrogenation catalysts, since these materials may exhibit higher activity, greater selectivity, and better stability, than their crystalline counterparts.1-14 Moreover, recent advances in sample preparation have produced ultrafine amorphous alloy particles,15-21 having significantly higher surface area than amorphous alloy ribbons and films prepared by conventional melt-quench techniques. The attractive catalytic properties of these amorphous alloys have been attributed to a random arrangement of atoms in the amorphous phase and also to an electronic effect contributed by the metalloid elements (e.g., B and P).13 * To whom correspondence should be addressed. † University of WisconsinsMadison. ‡ Nanjing University. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Wade, R. C.; Holah, D. G.; Hughes, A. N.; Hui, B. C. Catal. Rev.Sci. Eng. 1976, 14, 211. (2) Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2027. (3) Okamoto, Y.; Nitta, Y.; Imanaka, T.; Teranishi, S. J. Catal. 1980, 64, 397. (4) Okamoto, Y.; Fukino, K.; Imanaka, T.; Teranishi, S. J. Catal. 1982, 74, 173. (5) Okamoto, Y.; Matsunaga, E.; Imanaka, T.; Teranishi, S. J. Catal. 1982, 74, 183. (6) Yokoyama, A.; et al. J. Catal. 1981, 68, 355. (7) Yoshida, S.; Yamashita, H.; Funabiki, T.; Yonezawa, T. J. Chem. Soc., Chem. Commun. 1982, 964. (8) Yoshida, S.; Yamashita, H.; Funabiki, T.; Yonezawa, T. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1435. (9) Yamashita, H.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Chem. Commun. 1984, 868. (10) Yamashita, H.; Yoshikawa, M.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2485. (11) Yamashita, H.; Kaminade, T.; Funabiki, T.; Yoshida, S. J. Mater. Sci. Lett. 1985, 4, 1241. (12) Yamashita, H.; Yoshikawa, M.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1771. (13) Molna´r, A.; Smith, G. V.; Barto´k, M. Adv. Catal. 1989, 36, 329. (14) Fan, Y.; et al. J. Mater. Sci. Lett. 1993, 12, 596. (15) van Wonterghem, J.; et al. Nature 1986, 322, 622. (16) Kim, S. G.; Brock, J. R. J. Colloid Interface Sci. 1987, 116, 431. (17) Linderoth, S.; Mørup, S. J. Appl. Phys. 1991, 69, 5256. (18) Shen, J.; et al. J. Appl. Phys. 1992, 71, 5217. (19) Shen, J.; Li, Z.; Yan, Q.; Chen, Y. J. Phys. Chem. 1993, 97, 8504. (20) Hu, Z.; et al. J. Non-Cryst. Solids 1993, 159, 88. (21) Shen, J.; Li, Z.; Chen, Y. J. Mater. Sci. Lett. 1994, 13, 1208.
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In previous work,22 we showed that the adsorption of CO, and to a lesser extent H2, on reduced nickel powders promoted with K and Cs, was a sensitive probe of the electronic effects contributed by electropositive K and Cs adatoms. In the present study, CO and H2 chemisorption measurements are performed on nickel metalloid samples to address the effects contributed by the metalloid elements. More specifically, microcalorimetric measurements of CO and H2 adsorption at 308 K were performed on Ni powder, Ni-B, and Ni-P to establish the effects of B and P on nickel. Moreover, X-ray diffraction (XRD) measurements were performed to establish the amorphous/ crystalline state of these samples, and X-ray photoelectron spectroscopy (XPS) was used to probe surface composition. Experimental Section Microcalorimetric measurements of CO and H2 adsorption were performed at 308 K using a Setaram C-80 heat-flux calorimeter. The calorimeter was connected to gas handling and volumetric systems employing a Baratron capacitance manometer for precision pressure measurement (0.5 ( 10-4 Torr). The ultimate dynamic vacuum of the volumetric system was 10-6 Torr, and the maximum apparent leak rate, including the calorimetric cells, was ca. 10-5 Torr/min in a system volume of approximately 40 cm3 (i.e., 10-5 µmol/min). In a typical microcalorimetric experiment, a sample (ca. 1-2 g) was loaded into the calorimetric cells and activated according to the treatment procedures described below. The cells containing the sample were then immersed in the calorimeter and allowed to equilibrate thermally. Once thermal equilibration had been achieved (ca. 5-6 h), small doses of adsorbate (1-10 µmol quantities) were admitted sequentially to the sample until it became saturated. The resulting heat response for each dose was recorded as a function of time and integrated to determine the energy released (mJ). The amount of gas adsorbed (µmol) was determined volumetrically from the dose and equilibrium pressures and the system volumes and temperatures. The differential heat (kJ/mol), defined as the negative of the enthalpy change of adsorption per mole of gas adsorbed, was then calculated as a function of the amount of gas adsorbed. Nickel powder for the microcalorimetric studies was prepared by heating in air an aqueous solution of nickel(II) nitrate hexahydrate (Aldrich, 99.999% purity). The resulting nickel powder was dried overnight in air at 390 K, calcined in flowing (22) Spiewak, B. E.; Shen, J.; Dumesic, J. A. J. Phys. Chem. 1995, 99, 17640.
© 1997 American Chemical Society
2736 Langmuir, Vol. 13, No. 10, 1997
Shen et al.
Table 1. Bulk Compositions and BET Surface Areas of Ni Powder, Ni-B, and Ni-Pa weight percent metalloid total
sample
Ni
Ni Ni-B Ni-P
100 93.18 94.89
a
0 6.58 6.44
atomic percent BET area Ni metalloid (m2/g)
100 100 99.76 72 101.33 89
0 28 11
1.7 8.4 19.7
Elemental analysis provided by Galbraith Laboratories, Inc.
O2 for 3 h at 623 K to form nickel oxide, and then reduced to metallic nickel in flowing H2 for 1 h at 523 K and for 1 h at 573 K. Nickel boride amorphous alloy particles were prepared by the reaction of nickel chloride with sodium borohydride in an aqueous solution, following the procedures of Shen et al.19 In particular, 580 mL of solution containing 11 g of NaBH4 (0.5 mol/L) was added over 10 min at room temperature into 840 mL of solution containing 20 g of NiCl2‚6H2O (0.1 mol/L) with vigorous stirring. The black precipitate (Ni-B) formed was filtered, washed with distilled water, dried with anhydrous ethanol under vacuum, and stored in a N2-containing glovebox. Nickel phosphide amorphous alloy particles were prepared by the autocatalytic reaction of nickel-chloride with sodium hypophosphite in an aqueous solution, as described by Hu and coworkers.20,23 In this procedure, 20 g of NiCl2‚6H2O and 18 g of NaH2PO2‚H2O were dissolved separately in distilled water and then mixed to form a 210 mL solution having Ni2+ and H2PO2concentrations of 0.4 and 0.8 mol/L, respectively. The pH of the solution was maintained at a value of 12 by the addition of aqueous NaOH (1 mol/L). One drop of diluted NaBH4 solution was then added to the basic solution to initiate the autocatalytic reaction between Ni2+ and H2PO2-. After several hours, the reaction was complete, and a black slurry (Ni-P) had formed. The slurry was then washed with NH4OH solution to remove unreacted Ni(OH)2 and other reaction residues, washed with distilled water to remove NH4OH, dried with anhydrous ethanol under vacuum, and placed in a N2-containing glovebox. In this manner, ultrafine Ni-P amorphous alloy particles were prepared having uniform size distribution.20 The bulk compositions of the Ni-B and Ni-P samples were determined by elemental analysis (Galbraith Laboratories, Inc.) and are shown in Table 1. Prior to the microcalorimetric measurements, the Ni powder, Ni-B, and Ni-P samples were loaded into the calorimetric cells and evacuated. Since Ni-B and Ni-P are pyrophoric, these samples were loaded into the calorimetric cells while in the N2containing glovebox (
