Cluster size effects on hydrazine decomposition on Ir n /Al 2 O 3 /NiAl(1 1 0) Chaoyang Fan, Tianpin Wu, William E. Kaden, Scott L. Anderson * Department of Chemistry, University of Utah, 315 S. 1400 E. Rm 2020, Salt Lake City, UT 84112-0850, United States Received 18 July 2005; accepted for publication 24 October 2005 Available online 7 December 2005 Abstract A series of planar model catalysts were prepared by deposition of size-selected Ir þ n on Al 2 O 3 /NiAl(1 1 0), and hydrazine decomposition chemistry was used to probe their size-dependent chemical properties. Small Ir n (n 6 15) on Al 2 O 3 /NiAl(1 1 0) are able to induce hydra- zine decomposition at temperatures well below room temperature, with significant activity first appearing at Ir 7 . Both activity and prod- uct branching are strongly dependent on deposited cluster size, with these small clusters supporting only the simplest decomposition mechanism: dehydrogenation and N 2 desorption at low temperatures, followed by H 2 recombinative desorption at temperatures above 300 K. For Ir 15 , we begin to see ammonia production, signaling the onset of a transition to clusters able to support more complex chemistry. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Iridium; Catalysis; Hydrazine; Surface chemical reaction; Nanocluster 1. Introduction Decomposition of hydrazine on metal surfaces and sup- ported metal particles is important in monopropellant thrusters and gas generators, and potentially interesting as a source of pure hydrogen for fuel cells. Catalysts for thruster and gas generator applications must be stable at high temperatures, and are typically refractory transition metals dispersed on a refractory oxide support. Iridium dis- persed on a high surface area aluminum oxide support is the benchmark commercial catalyst (Shell 405). Issues of importance include particle sintering, support degradation, and activity over a wide temperature range. Even though the thrusters and gas generators operate at high tempera- tures, it is important to have good low temperature activity to insure smooth cold starts in intermittent operation in spacecraft environments. As part of a program to explore fundamental properties of such catalysts, we are studying size-selected Ir particles supported on planar epitaxial Al 2 O 3 supports. Here we report hydrazine decomposition results under temperature-programmed desorption (TPD) conditions, for Ir n , n < 15. There are a number of studies in the literature that pro- vide interesting points of comparison with this work. Hydrazine decomposition has been studied under UHV conditions for metal surfaces such as Ir(1 1 1) [1], Ru(0 0 0 1) [2], Pd(100) [3], Pt(111) [4], and Ni(111) [5]. There are also a number of temperature-programmed desorption studies of NH 3 and H 2 on various bulk metal surfaces that provide important insights for interpretation of the hydrazine results [6–9]. It would be nice to compare our results for Ir n /Al 2 O 3 /NiAl with results for real high surface area catalysts. The only detailed work on hydra- zine/Ir/Al 2 O 3 high surface area catalysts is that of Falconer and Wise [10], who studied TPD and reactions under stea- dy flow conditions, but only above room temperature and for hydrazine exposures much higher than in our experi- ments. We previously reported a study of TPD of hydra- zine on a model catalyst prepared by depositing enough Ir on a planar Al 2 O 3 /NiAl(1 1 0) support to grow some dis- tribution of non-size-selected Ir clusters [11], and found chemistry qualitatively similar to that on bulk metal sur- faces. We also reported preliminary results for reaction of 0039-6028/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2005.10.054 * Corresponding author. Tel.: +1 801 585 7289; fax: +1 801 581 8433. E-mail address: anderson@chem.utah.edu (S.L. Anderson). www.elsevier.com/locate/susc Surface Science 600 (2006) 461–467