Pore geometry influence on the deactivation behavior of Ni-based catalysts for simultaneous production of hydrogen and nanocarbon J. Salmones a , J.A. Wang a , M.A. Valenzuela a, *, E. Sa ´ nchez a , A. Garcia b a Laboratorio de Catalı´sis y Materiales, ESIQIE-Instituto Polite ´cnico Nacional, Zacatenco, C.P. 07738, Me ´xico D.F., Mexico b UPIICSA, Instituto Polite ´cnico Nacional, Te 950 Col. Granjas-Me ´xico, 08400 Me ´xico D.F., Mexico 1. Introduction The demand for hydrogen is ever increasing due to a variety of utilization in chemical processing, electronics, food processing, metal manufacturing and fuel cells [1]. Steam reforming (SR), partial oxidation or autothermal reforming of hydrocarbons and alcohols have been the conventional methods to produce hydrogen [2]. A general disadvantage of these processes is the formation of carbon monoxide and carbon dioxide, which are difficult to separate from hydrogen [3]. Furthermore, for certain applications, like in fuel cell technology, carbon monoxide has to be removed from hydrogen in order to prevent the electro-catalyst from poisoning [4]. Recently, the catalytic decomposition of methane (CDM) has received extensive attention as a potential process for the production of high-purity hydrogen [5–16]. The reaction is moderately endothermic and the energy required for the produc- tion of 1 mol hydrogen (45.1 kJ mol 1 at 800 8C) is less than that required for SR of methane (48.0 kJ mol 1 ). Unlike the SR process, the CDM process does not include water gas shift reaction, selective oxidation of remained CO and pressure-swing adsorption stages, which considerably simplifies the process and reduces the operation cost [7,17]. The produced carbon may be used as the substitutes of carbon black, fibers, graphite, composites, carbon fillers in tires and plastics or be used as catalyst support [18]. Deposited carbon also can be oxidized with H 2 O and O 2 into CO, synthesis gas and CO 2 . The CDM into hydrogen and carbon followed by catalyst regeneration in oxidative atmosphere can be carried out in a cyclic manner in the same reactor [8,19] or by step- wise steam of carbon formed on the catalyst [20]. The most studied catalysts for the CDM are nickel supported on: SiO 2 , SiO 2 –Al 2 O 3 , SiO 2 –MgO, SiO 2 –CeO 2 , TiO 2 , graphite, ZrO 2 , Al 2 O 3 , MgO, ZnAl 2 O 4 , CeO 2 , TiO 2 , La 2 O 3 ,Y 2 O 3 , oxidized diamond, among others [5–16]. Other metals such as Fe, Pt, Pd, Cr, Ru, Mo or W on different supports have been successfully tested in the CDM [8,21,22]. Unfortunately, rapid deactivation of Ni-based catalyst takes place at temperatures above 500 8C, leading to a low yield of hydrogen [23]. The catalyst deactivation occurs when the metallic particles are encapsulated by non-reactive carbon compounds [24]. It is, therefore, a major challenge to develop a catalytic system that sustains its activity at high temperatures and drastic regeneration processes to avoid metal sintering. The nature of support usually shows important effect on the catalytic activity of the catalysts, as it is well known that the change of the structure or electronic state of the metal active species is correlated with the interaction between metals and support. The surface area and thermal stability of the catalysts support are intensely investi- gated; however, less attention is paid on the study of the pore geometrical effect of the support. For example, to date, in the reaction of CDM, contribution of the pore geometry of support to the catalytic stability of the Ni-containing catalysts has not been reported yet. Furthermore, the addition of MgO to the Ni-based catalysts induces the formation of Ni–Mg–O solid solutions and could confer specific features to the Ni active sites [25]. Certainly, the stabilization of small sized Ni crystallites with MgO causes a Catalysis Today 148 (2009) 134–139 ARTICLE INFO Article history: Available online 1 May 2009 Keywords: Methane decomposition Ni/Mg–Al–O Hydrogen production Pore geometry ABSTRACT Pore geometry of Ni-containing Mg–Al–O mixed oxide catalysts could be controlled by varying the Ni content in the synthesis. Low Ni content may lead to the catalysts having mesopores with shape cylinder and narrow pore size distribution; high Ni content results in the catalyst having pores with shape ink- bottle and a wide pore size distribution. In methane decomposition to produce hydrogen and nanocarbon, the 50 wt.% Ni/Mg–Al–O catalyst was rapidly deactivated after 2 h of reaction; however, the catalysts with 15 and 25 wt.% of Ni showed much longer lifetime, which can be explained by assuming a new modal related to pore geometry of the catalysts. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +52 55 5729 6000x55112; fax: +52 55 5586 2728. E-mail address: mavalenz@ipn.mx (M.A. Valenzuela). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2009.03.005