Isothermal Kinetic Study of Nitric Oxide Adsorption and Decomposition on Pd(111) Surfaces: Molecular Beam Experiments Kandasamy Thirunavukkarasu, † Krishnan Thirumoorthy, † Jo 1 rg Libuda, ‡ and Chinnakonda S. Gopinath* ,† Catalysis DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India, and Department of Chemical Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany ReceiVed: February 16, 2005; In Final Form: May 30, 2005 The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient (s NO ) were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument. NO dissociation and N 2 productions were observed in the transient state from 425 K and above on Pd(111) surfaces with selective nitrogen production. Maximum nitrogen production was observed between 475 and 500 K. It was found that, at low temperatures, between 300 and 350 K, molecular adsorption occurs with a constant initial s NO of 0.5 until the Pd(111) surface is covered to about 70-80% by NO. Then s NO rapidly decreases with further increasing NO coverage, indicating typical precursor kinetics. The dynamic adsorption - desorption equilibrium on Pd(111) was probed in modulated beam experiments below 500 K. CO titration experiments after NO dosing indicate the diffusion of oxygen into the subsurface regions and beginning surface oxidation at g475 K. Finally, we discuss the results with respect to the rate-limiting character of the different elementary steps of the reaction system. 1. Introduction In the last 2 decades increasing governmental regulations all over the world have stimulated tremendous growth in the research on environmental catalysis and green chemistry. 1-3 In particular, the automotive catalyst technology has improved, assisted by better petrochemical refining to yield cleaner fuels. Improvements in internal combustion engines and the above developments have led to fuel-efficient vehicles. The present technology of three-way catalytic converters 1,2 meets the requirements of oxidation of CO. More problematic is the oxidation of nonvolatile organic species to CO 2 and, in particular, the reduction of NO to N 2 , especially under the highly oxidizing exhaust gas conditions of fuel-efficient lean-burn engines. 4 Although Rh is very active for NO reduction under fuel-rich conditions, this is not the case under net oxidizing conditions, since the excess oxygen inhibits the NO reduction activity of Rh by oxidizing it. Pd has also been added to catalytic converters as one of the active metals for NO reduction for the past few years; 5 however, the mechanistic details of Pd are not fully understood yet. There were few new technologies 6,7 introduced in the past decade, like NO x storage or zeolite based catalysts, which provided different insights into the NO reduc- tion. However, regarding long-term stability, low and high- temperature NO reduction is still a challenge. 4 The above points clearly indicates that there is an urgent need to develop catalysts for NO reduction under net oxidizing conditions. In this respect, obtaining molecular-level insights into underlying catalysis aspects of NO reduction is a major challenge for current research. Palladium has been suggested as an alternative active element for NO reduction due to its NO dissociation capacity and its stability under high temperature and oxidizing conditions. 8-10 However, unlike Rh, 11-15 Pd has not been subjected to intense research, and its capability for NO reduction is not fully understood. In the recent past, there have been numerous reports on the NO reduction over Pd-based catalysts on different faces of single crystals and a variety of supported systems. 16-29 Despite that, there is a lack of fundamental understanding of NO reduction on the Pd surfaces. The present work of NO adsorption and dissociation on Pd- (111) focuses on fundamental kinetic aspects under isothermal conditions in a molecular beam instrument. 30-34 It was suggested that NO dissociates only at defect sites on Pd(111) without significant inherent activity on Pd(111); 17-20 nonetheless, there are reports 22-26,28,29,35 on steady-state NO dissociation to N 2 and N 2 O on Pd(111) as well as on Pd on supported oxides with reductants, like CO and H 2 , indicating inherent catalytic activity toward NO dissociation. Our present studies show that an inherent NO dissociation activity exists on Pd(111) surfaces, and the same can be observed clearly in the transient state. The present work on the dissociation of NO on Pd(111) surfaces is a part of our continuing study of the NO reduction reactions in our laboratory, and the NO + CO reaction on Pd(111) is reported in ref 35. 2. Experimental Section All kinetic and TPD experiments reported in this manuscript were performed using an extension of the so-called King and Wells collimated beam method. The home-built molecular beam instrument (MBI) used consists a 12 L capacity stainless steel ultrahigh vacuum (UHV) chamber evacuated with a 210 L/s turbo-molecular drag pump (Pfeiffer, TMU261) to a base pressure of about 3 × 10 -10 Torr. The MBI is equipped with a * Corresponding author. E-mail: cs.gopinath@ncl.res.in. Fax: 0091-20- 2589 3761. † National Chemical Laboratory. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft. 13283 J. Phys. Chem. B 2005, 109, 13283-13290 10.1021/jp050813f CCC: $30.25 © 2005 American Chemical Society Published on Web 06/21/2005