FULL PAPER 1 The effects of platinum dispersion and Pt state on catalytic properties of Pt/Al2O3 in NH3 oxidation Elena M. Slavinskaya, [a] Lidiya S. Kibis, [a] Olga A. Stonkus, [a] Dmitry A. Svintsitskiy, [a] Andrei I. Stadnichenko, [a] Elizaveta A. Fedorova, [a] Anatolii V. Romanenko, [a] Vasyl Marchuk, [b] Dmitry E. Doronkin, [b,c] Andrei I. Boronin* [a] [a] Dr. E.M. Slavinskaya, Dr. L.S. Kibis, Dr. O.A. Stonkus, Dr. D.A. Svintsitskiy, Dr. A.I. Stadnichenko, Dr. A.V. Romanenko, E.A. Fedorova, Prof. Dr. A.I. Boronin Boreskov Institute of Catalysis SB RAS Pr. Lavrentieva 5, 630090 Novosibirsk, Russia E-mail: boronin@catalysis.ru [b] V. Marchuk, Dr. D.E. Doronkin, Institute for Chemical Technology and Polymer Chemistry Karlsruhe Institute of Technology (KIT), Engesserstr. 20, 76131 Karlsruhe, Germany E-Mail: dmitry.doronkin@kit.edu [c] Dr. D.E. Doronkin Institute of Catalysis Research and Technology Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Supporting information for this article is given via a link at the end of the document. Abstract: Dependence of NH3 oxidation on the state and dispersion of Pt species in Pt/γ-Al2O3 catalysts was investigated. Prereduced Pt/γ-Al2O3 catalysts containing Pt 0 nanoparticles exhibited significantly higher activity than preoxidized ones with the same Pt dispersion. The most significant improvement of the catalytic activity (TOF increased by 30 times) was observed when the size of Pt 0 particles increased from ~1 to ~8 nm. N2 selectivity was found to be mainly determined by the reaction temperature, with a minor influence of Pt particle size. Preoxidized catalysts containing ionic Pt were activated by the reaction medium, while partial deactivation was observed for the prereduced ones. The activity improvement was associated with an increase in the ratio of Pt 4+ /Pt 2+ species on the surface of preoxidized catalysts. The activity decrease of the prereduced catalysts was due to the partial oxidation and subsequent redispersion of Pt particles. Introduction of Н2О and СО2 to the reaction mixture only moderately influenced NH3 oxidation activity shifting NH3 conversion curves by about +15°C. Introduction Suppressing NOx emissions from diesel engines is a challenge due to a lack of reducing agents in exhaust gas. [1] To solve this problem, the NH3-SCR process (selective catalytic reduction of NOx by ammonia), previously developed to remove NOx from stationary sources, is effectively applied. [2] However, the excess of ammonia is used under NH3-SCR conditions to maximize NOx conversion which results in environment pollution by NH3. [3] Integrated Heavy Duty Diesel (HDD) aftertreatment systems include diesel oxidation catalyst, catalyzed soot filter, NH3-SCR component, and an ammonia slip catalyst (ASC), used to reduce ammonia emissions. [4] ASCs are also used to remove residual ammonia during the implementation of the DeNOx process, for example, in case of neutralization of NOx produced by large utility boilers, industrial boilers, and municipal solid waste incinerators. [5] In this case, the use of the ASC catalyst should allow ammonia concentration in the exhaust gases of less than 10 ppm. [5] The state-of-art ASC is a complex system comprising catalyst layers for ammonia oxidation (AMOX) and selective NOx reduction (NH3-SCR). [6] For the oxidation of ammonia, a Pt/Al2O3 catalyst is usually applied providing high low-temperature activity. [6b, 7] However, such a catalyst possesses relatively low N2 selectivity due to the formation of N2O at temperatures below 250°C and the formation of NOx above 250°C. [7a, 8] To achieve maximal activity and reduce contribution from the by-products (N2O, NOx), it is necessary to establish factors governing the operation of Pt/Al2O3 during ammonia oxidation. At the same time it is important to reach the highest degree of Pt usage thus reducing the cost of aftertreatment systems. [9] Activity and selectivity of the Pt/Al 2O3 catalyst are affected by Pt dispersion because of the difference in the physicochemical properties of small and large Pt particles. For example, Pt metallic particles smaller than 3 nm exhibit semiconductor and dielectric properties due to the modification of electronic structure in comparison with bulk platinum. [10] Furthermore, the ratio of surface adsorption sites (vertices, edges, terraces) determining catalytic properties is different for small and large nanoparticles. In case of small particles, the fraction of surface atoms increases, while coordination numbers decrease. [10b] Note that tolerance towards reaction medium exposure (for example, excess of O2) substantially depends on the initial size of platinum nanoparticles. The strong influence of Pt crystallite size on catalytic properties of platinum catalysts in ammonia oxidation has been reliably established in the literature. [8, 11] It is known that small Pt particles exhibit lower activity compared to large crystallites. At the same time, N2 selectivity increases with decreasing Pt particle size. In the available reports [8, 11a, 11b] only a narrow temperature range was studied, where NO and NO2 were not formed. Besides, catalysts with different Pt content were compared. The catalytic data were obtained at high ratios of NH3:O2 (1:1 or 1:3, not relevant for exhaust aftertreatment) and/or at low temperatures under stationary conditions, which led to catalyst deactivation. [8b] Catalysts containing small Pt crystallites are most prone to deactivation due to oxidation of