Applied Catalysis A: General 400 (2011) 203–214 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Effect of metal particle size on sulfur tolerance of Ni catalysts during autothermal reforming of isooctane Joseph M. Mayne, Kevin A. Dahlberg, Thomas A. Westrich, Andrew R. Tadd, Johannes W. Schwank ∗ Transportation Energy Center, Department of Chemical Engineering, University of Michigan, 2300 Hayward, Ann Arbor, MI 48109, USA article info Article history: Received 11 January 2011 Received in revised form 21 April 2011 Accepted 25 April 2011 Available online 30 April 2011 Keywords: Nickel Particle size Sulfur poisoning Autothermal reforming Isooctane Thiophene abstract This paper describes to what extent Ni particle size affects the sulfur-tolerance of ceria-zirconia supported Ni catalysts during autothermal reforming (ATR) of isooctane. Particle size was isolated as an experimental variable by preparing catalysts with a range of Ni loadings that had nearly identical Ni surface areas. Under sulfur-free conditions, isooctane conversion and synthesis gas yield increased as the Ni particle size increased, contrary to the expectation that smaller particle sizes with lower Ni coordination would be more active. However, larger Ni particles proved to be more vulnerable to sulfur poisoning. The poor ATR activity of small Ni particles can be attributed either to a lack of sufficiently large nickel surface ensembles, or to their higher propensity to form nickel oxides under reaction conditions. This contribution suggests that, under typical ATR conditions, more highly dispersed Ni catalysts will not result in elevated sulfur tolerance. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The influence of metal particle size on catalyst performance has been demonstrated for a broad range of systems. In most cases, the goal of catalyst synthesis is to formulate a material with a stable particle size distribution skewed towards smaller particles [1]. Smaller particles maximize the number of surface metal atoms available for catalytic chemistry and additionally have active sites that have been shown to be chemically different compared to larger particles. Such chemical differences arise from an increased rel- ative concentration of higher energy crystallographic planes in smaller particles. These surfaces feature a lower average coordi- nation number and altered electronic structure [2,3], thus binding many adsorbing molecules more strongly. The effect of particle size was investigated when producing hydrogen from hydrocarbon fuels via autothermal reforming (ATR) over Ni-based catalysts. Ideally, this system couples oxidation and steam reforming pathways to produce an equilibrium-limited mix- ture of H 2 , CO, CO 2 , and CH 4 [4–6]. This reformate has several applications, including power generation via solid oxide fuel cells and automotive emissions control [7,8]. In practice, the long-term stability and activity of noble- and non-noble-metal based catalysts ∗ Corresponding author. Tel.: +1 734 764 3374; fax: +1 734 763 0459. E-mail address: schwank@umich.edu (J.W. Schwank). are limited by side reactions that produce carbon deposits and poi- soning of active sites by sulfur-containing compounds commonly found in hydrocarbon fuels [9–11]. Investigations into particle size effects have been applied pre- viously to certain aspects of this chemistry. For example, it has been well demonstrated that steam reforming reactions preferen- tially occur at lower coordinated kink and step sites. By contrast, these reactions proceed at much slower rates on highly coordinated terrace sites [12–17]. In addition, there is evidence that carbon deposition rates are dependent on metal particle size [18–20]. However, it is often difficult to control particle size independently of other variables such as metal loading and total number of surface sites. Therefore, many previous studies have failed to differentiate or account for these confounding effects. This study attempts to isolate the effect of particle size on the sulfur tolerance of Ni-based catalysts during the ATR of isooctane by varying particle size independently of the total number of active surface sites. To accomplish this a novel experimental strategy was employed, whereby a series of catalysts with different weight loadings were selectively pretreated to yield samples with various particle sizes but similar numbers of active surface sites as indi- cated by H 2 chemisorption measurements. The activities of these catalysts were then measured and compared for ATR of sulfur-free and thiophene-containing isooctane. These experiments clarify the role of particle size effects in the ATR system, and therefore provide guidance for the development of Ni catalysts that are more tolerant to sulfur exposure under ATR conditions. 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.04.039