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J. Phys. Chem. B 1999, 103, 6036-6046
r-Alumina-Supported Nickel Catalysts Prepared with Nickel Acetylacetonate. 1. Adsorption in the Liquid Phase R. Molina,† M. A. Centeno,‡ and G. Poncelet*,† Unite´ de Catalyse et Chimie des Mate´ riaux DiVise´ s, UniVersite´ Catholique de LouVain, Place Croix du Sud 2/17, 1348 LouVain-la-NeuVe, Belgium, and Departamento de Quimica Inorganica y Instituto de Ciencias de Materiales de SeVilla-CSIC, UniVersidad de SeVilla, P.O. Box 878, 41071 SeVilla, Spain ReceiVed: February 15, 1999; In Final Form: May 15, 1999
Nickel acetylacetonate was used as a precursor for the preparation of Ni/Al2O3 catalysts with metal content between 0.4 and 3 wt %. The nature of the interactions between the metal complex and the support has been investigated by X-ray photoelectron (XPS) and infrared (IR) spectroscopies. The spectral features point to modifications of the precursor in the adsorbed state. IR results indicate that the interaction between acetylacetone and the support involves coordinately unsaturated (cus) Al3+ sites, hydroxyls of the support surface, and probably also basic oxygens. A model for these interactions is proposed. Monolayer coverage is achieved at a Ni loading of about 0.8-1.0%. Piling in multilayers occurs at higher content.
Introduction The mechanism of ligand substitution in transition metal acetylacetonates adsorbed on the surface of dispersed oxides is of prime importance to the understanding of the processes of catalyst activation on a molecular level.1 The nature of the interaction between those metal precursors and supports has been investigated in both vapor2-7 and liquid phases.8-13 Supported metal oxide catalysts prepared with metal acetylacetonates are obtained in two steps: irreversible adsorption of the complex on the support and decomposition of the adsorbed complex. At the adsorption step, the interaction may take place via hydrogen bonding or involve a ligand exchange mechanism, depending on a number of factors such as configuration (spherical, planar) and stability of the metal complex, nature of the metal and its coordination, nature of the support surface,3,8-12 and orientation of the acac ligands with respect to the surface hydroxyl groups of the support.10 The carbon/metal ratio of the supported complex has been used as an aid to assess the type of interaction mechanism.6,7,10-12 For stable complexes, the carbon/metal ratio of the adsorbed complex is similar to that of the precursor, while adsorbed unstable complexes present lower carbon/metal ratios and their structures differ from that of the parent complex. van Veen et al.9 investigated the reactivity of transition metal acetylacetonates with respect to the reactive surface groups of γ-Al2O3. The authors came to the following conclusion: (i) complexes that are stable in acid and base as, for example, Pt(acac)2, Pd(acac)2, and Co(acac)3, only react with coordinately unsaturated surface (cus) Al3+, with a transfer of acac ligands to Al3+; (ii) complexes that decompose in a base but not in an acid, for example, MoO2(acac)2 and VO(acac)2, react with both coordinately unsaturated (cus) Al3+ sites and basic OH groups of the support; (iii) complexes that are sensitive to acid react to a small extent with acidic OH groups, for example, Fe(acac)3, or they do not react at all, as Ru(acac)3. * Corresponding author. E-mail:
[email protected]. † Universite ´ Catholique de Louvain. ‡ Universidad de Sevilla.
Supported Ni catalysts prepared with nickel acetylacetonate have been scarcely investigated, most studies dealing principally with catalytic reactions14-19 and/or metal particle size measurements.20,21 The few studies of the interaction of nickel acetylacetonate with oxide supports (silica, alumina) prepared in the liquid phase are those of van Veen et al.8,9 who concluded that no well-defined chemisorption (in a well-defined way) occurred because of the formation of a precipitate. In recent studies, Ni(acac)2/γ-alumina6,7,22 and Ni(acac)2/silica6 have been prepared by vapor-phase deposition (VPD), and the reaction of the metal complex with surface hydroxyl groups has been investigated. The main objective of the present study is to complement the information on Ni(acac)2/alumina compositions prepared in the liquid phase. The results obtained for Ni(acac)2/R-Al2O3 systems are discussed in relation with the adsorption mechanism and monolayer formation. The thermal decomposition of the adsorbed species will be treated in a separate article.23 Experimental Section A commercial R-Al2O3 prepared by calcination of a transition alumina (from Rhone-Poulenc) was supplied by (former) Catalysts and Chemicals Europe. Small amounts of κ and θ transition phases were identified in the X-ray diffraction pattern. The nitrogen BET surface area was 42 m2 g-1 with a total pore volume of 0.21 cm3 g-1. The weight loss between 500 and 1000 °C amounted to 0.082%, which corresponds to 1.28 × 1018 OH m-2. The 27Al MAS NMR spectrum showed two signals at 11.5 ppm (octahedral Al) and 63.8 ppm (tetrahedral Al), in a proportion of 82-18%.24 Prior to the adsorption experiments, alumina was calcined at 500 °C for 16 h in air in order to remove adsorbed water. Required amounts of nickel acetylacetonate (fraction less than 100 µm, from Merck) were dissolved in benzene (highest purity, from UCB) under stirring for 15 min before addition of the calculated weight of alumina (fraction less than 100 µm). The solution/alumina ratio was 2.1 mL g-1 (10 times the total pore volume). The slurry was stirred at room temperature for 48 h. After filtration, the solid was washed several times with pure benzene (total volume: 25 mL g-1) and dried at room
10.1021/jp9905427 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
R-Alumina-Supported Ni Catalysts
J. Phys. Chem. B, Vol. 103, No. 29, 1999 6037
TABLE 1: Chemical Analysis Data
sample 0.4 0.8 1.0 1.6 3.0 Al2O3 Al2O3 + benzene Hacac/Al2O3 Ni(acac)2 a
Ni(acac)2 supplied (µmol/(g Al2O3)) Ni acac 85 169 209 361 682
168 335 414 716 1350
Ni/solid (%)
Ni uptake (%)
0.45 0.82 1.00 1.59 2.92
