Applied Catalysis B: Environmental 102 (2011) 484–495 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Determination of kinetics and controlling regimes for H 2 oxidation on Pt/Al 2 O 3 monolithic catalyst using high space velocity experiments Saurabh Y. Joshi, Yongjie Ren, Michael P. Harold ∗ , Vemuri Balakotaiah ∗∗ Department of Chemical and Biomolecular Engineering, University of Houston, 4800 Calhoun Road, Houston, TX 77204, United States article info Article history: Received 22 May 2010 Received in revised form 13 December 2010 Accepted 14 December 2010 Available online 21 December 2010 Keywords: Hydrogen Platinum Water Monolith reactor Mass transfer controlled regime Washcoat diffusion Kinetic regime abstract Recently developed criteria [S.Y. Joshi, M.P. Harold, V. Balakotaiah, Chemical Engineering Science 65 (2010) 1729–1747] are used to characterize the various controlling regimes (kinetic, pore diffusion and mass transfer controlled) during H 2 oxidation on Pt/Al 2 O 3 monolithic catalyst. The hydrogen conversion was measured over a wide range of temperatures and space velocities. Experiments at typical space velocities (10 4 –10 5 h -1 ) revealed that complete conversion can be achieved at ambient temperature, a result of fast catalytic kinetics. The high activity complicates the analysis of the relative importance of reaction, washcoat diffusion and mass transfer. High space velocity operation with accompanying the- oretical analysis enabled an efficient determination of the intrinsic catalytic kinetics, quantification of the heat and mass transport coefficients and determination of the resistances due to reaction, washcoat diffusion and external mass transport processes. Specifically, we investigated the effects of catalyst aging, space velocity and catalyst temperature on the regime transition. The analysis reveals that washcoat dif- fusion regime is dominant over a wide range of temperatures for the aged catalyst whereas the monolith transitions to a mass transfer controlled regime above 90 ◦ C for the high dispersion fresh catalyst. The analysis also reveals that the aging of the catalyst due to sintering of Pt crystallites not only reduces the pre-exponential factor but also the observed activation energy. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The platinum-catalyzed oxidation of hydrogen to water is among the most-studied reaction systems. A large number of liter- ature studies have appeared on the kinetics of hydrogen oxidation on Pt [1–8]. It principally occurs as a side reaction in a variety of systems, including vehicular emission abatement in the catalytic converter and during oxidative reforming of hydrocarbons, among other examples. Over the years its principal utility has been as a model reaction. Hydrogen oxidation on Pt is known to be very fast at room tem- perature and above due to an unusually low activation energy. It has been reported that chemisorbed oxygen and hydrogen react on Pt with measurable rate even at temperatures as low as 120 K [1,2]. Fisher et al. [2] studied formation of water from atomic hydrogen and oxygen coadsorbed on the Pt(1 1 1) surface by x-ray photoe- mission spectroscopy and temperature programmed reaction. It was shown that the reaction can be driven to completion at very low temperatures. The authors concluded that the reaction has a low activation energy with an upper limit of 34 kJ/mol. Hanson ∗ Corresponding author. Tel.: +1 713 743 4307; fax: +1 713 743 4323. ∗∗ Corresponding author. Tel.: +1 713 743 4318; fax: +1 713 743 4323. E-mail addresses: mharold@uh.edu (M.P. Harold), bala@uh.edu (V. Balakotaiah). and Boudart [3] studied hydrogen oxidation over Pt/SiO 2 catalysts with Pt dispersions varying between 14% and 100%. The atmo- spheric pressure reaction was studied between 273 K and 373 K. They reported that in excess oxygen, the catalytic activity was inde- pendent of particle size (or platinum dispersion) and the reaction was first order in hydrogen and zero order in oxygen. A value for the activation energy of 7.5 kJ/mol was reported. Younis [4] studied hydrogen oxidation in packed bed reactor and reported an activation energy of 10 kJ/mol on Pt. Brown et al. [5] stud- ied oxidation of H 2 on Pt in excess air and reported a value of 16.1 kJ/mol. Rinnemo et al. [6] investigated catalytic ignition for hydrogen oxidation reaction on Pt. They explored experimentally and numerically the Frank-Kamenetskii condition for catalytic igni- tion, the sudden transition from a kinetically to a mass transfer controlled regime for an exothermic catalytic reaction. An appar- ent activation energy of 45.6 kJ/mol was estimated from the data. Fassihi et al. [7] studied the dependence of ignition temperature on the H 2 /O 2 ratio. At ignition, the temperature and reaction rate self- accelerate due to released heat of reaction. Pacia and Dumesic [8] investigated the low-pressure oxidation of hydrogen on a polycrys- talline Pt ribbon and reported an activation energy of 8.4 kJ/mol. In summary, the reported values of activation energy vary over a wide range (7–46 kJ/mol). In spite of the large number of studies of this reaction sys- tem cited above, very few studies have appeared in which the 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.12.030