Sintering of alumina-supported nickel particles under amination conditions: Support effects Johan Lif a,b, * , Ingemar Odenbrand c , Magnus Skoglundh a a Department of Chemical and Biological Engineering, Applied Surface Chemistry Chalmers University of Technology, S-412 96 Go ¨teborg, Sweden b Akzo Nobel Functional Chemicals AB, S-444 85 Stenungsund, Sweden c Lund University, Department of Chemical Engineering LTH, Box 124, S-221 00 Lund, Sweden Received 13 April 2006; received in revised form 3 October 2006; accepted 4 October 2006 Available online 17 November 2006 Abstract The sintering of alumina-supported nickel particles has been studied after heat-treatment in ammonia + hydrogen at 523 K and 250 bar. The investigated samples were nickel supported on g-alumina, transalumina and a-alumina, and co-precipitated nickel oxide-alumina. The sintering process was mainly followed by hydrogen chemisorption. The samples were also characterised by specific surface area measurements, X-ray diffraction, temperature programmed desorption of ammonia, in situ FTIR spectroscopy and temperature programmed reduction. For nickel supported on g-alumina, up to 40% of the initial metal surface area remained after the heat-treatment in ammonia + hydrogen compared with a-alumina or transalumina where only 10–20% of the initial metal surface area remained after the heat-treatment. The sintering can be correlated to the bond strength between the metal particle and the support. The larger the number of low-coordinated surface aluminium sites is, as for g-alumina, the stronger the metal–support interaction is and this in turn suppresses diffusion of nickel particles and/or atoms. # 2006 Elsevier B.V. All rights reserved. Keywords: Sintering; Nickel; Alumina; Lewis acids sites; Coordination number; Amination catalysis 1. Introduction Heterogeneous catalysed amination of alcohols is established as the most important industrial process for manufacture of different aliphatic and aromatic amines [1–3]. A typical catalyst used for this application is nickel dispersed on a support material like silica or alumina [3]. Although supported nickel catalysts usually have high initial activity and selectivity for amination reactions the activity will decrease with time-on-stream, i.e. the catalyst is deactivated. Deactivation is a problem of great and continuing concern in industrial catalysis and has thus been a subject of many reviews [4–7]. One of the most serious aspects of deactivation of supported catalysts is thermal degradation with concomitant loss of active metal surface area [5,8–16]. This phenomenon has proven to be the major deactivation mechanism of supported nickel catalysts in ammonia/hydrogen atmosphere, i.e. under amination conditions [17]. This type of degradation is normally referred to as sintering since small metal particles are converted into larger ones with lower surface-to-volume ratio. The driving force for sintering is to minimise the surface energy by increasing the co-ordination number of the surface atoms. Larger particles are more stable than smaller ones due to the fact that the metal–metal bond energies usually are considerably higher than the metal–support interactions. The detachment of a nickel atom from a nickel cluster requires typically 431 kJ/mol [18] whereas the binding energy between metal particles and the support is in the range of 5–15 kJ/mol [4,19]. Factors affecting the stability of the metal particle towards sintering include temperature, surrounding atmosphere (gas composition), support material, metal–support interaction, particle shape, particle size, support roughness, pore size and impurities of the support or in the metal. The most pronounced factor of these is the temperature, the process is strongly temperature dependent and sintering is generally observed for temperatures above 700 K [4,20,21]. The so-called Hu ¨ttig and Tamman temperatures indicate the temperature at which sintering starts [6]. When T Hu ¨ttig is reached atoms at defects www.elsevier.com/locate/apcata Applied Catalysis A: General 317 (2007) 62–69 * Corresponding author at: Akzo Nobel Functional Chemicals AB, S-444 85 Stenungsund, Sweden. Tel.: +46 303 853 29; fax: +46 303 887 86. E-mail address: johan.lif@akzonobel-chemicals.com (J. Lif). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.10.003