Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod Reactivity study of CO+NO reaction over Pd/Al 2 O 3 and Pd/CeZrO 2 catalysts Xiaoyin Chen, Yimeng Lyu, Uzoma Nwabara, Johannes W. Schwank ⁎ Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA ARTICLE INFO Keywords: Palladium CO+NO N 2 O Alumina Ceria-zirconia NCO ABSTRACT To elucidate the reactions involved in the three-way catalyst under stoichiometric conditions, the reactivity of CO + NO has been studied by continuous flow and surface reactions over Pd supported on Ce 0.75 Zr 0.25 O 2 (CZO) and Al 2 O 3 model catalysts with different Pd particles sizes and oxidation states. Pd/CZO showed higher activity with complete NO removal at 125 °C than Pd/Al 2 O 3 at 300 °C As a primary by-product, N 2 O was produced on both catalysts prior to reaching complete CO conversion in a 1:1 NO/CO feed ratio. NO inhibition through N 2 O formation was intensified when 2:1 NO/CO feed ratio was used, where the complete CO conversion can’t be achieved during the course of the reaction. When 1:1 NO/CO feed ratio was used over Pd/Al 2 O 3 , a concave feature in the CO light-off curve was observed, where the CO light-off curve showed a downward inflections after complete NO conversion had been reached, and then started to increase again with further increase in tem- perature. The inflection of CO conversion coincided with the maximum yield of N 2 O vs. reaction temperature. It was also found that the CO + NO reactivity was affected by the Pd oxidation state and particles size. in situ DRIFT experiments showed that the formation of NCO and NCO-derived N 2 O intermediate species is closely related to the inflection of CO conversion. A pathway of N 2 O formation via the NCO intermediate species has been proposed to be responsible for the fallback in CO conversion when the reaction temperature increased, as both interactions of “NCO ad +N ad ” and “NCO ad + NO ad ," which produce N 2 and N 2 O, respectively, consume NO but generate CO. 1. Introduction Carbon monoxide (CO) and nitric monoxide (NO) are the two major types of pollutants that result from automobile exhaust emissions [1]. CO is a toxic gas, and the emission of CO is a serious health concern [2]. Emission of NO has a severe negative effect on the environment as NO is a major source of acid rain and harmful to human health [3]. The elimination of CO and NO from automobile exhaust has become ex- tremely important [4]. Three-way catalysts (TWC), which are composed of platinum group metals (PGM, such as Pd, Pt, and Rh) as the active components, have been developed to eliminate CO, NO, and hydrocarbons simultaneously in the emissions [5] under conditions close to stoichiometric A/F ratio. CO oxidation by NO (or NO reduction by CO) is a critical reaction in- volved in the TWC catalysts. The overall CO + NO reaction follows Eq. (1). + → + NO CO N CO 1 2 2 2 (1) The formation of N 2 via Eq. (1) would occur preferentially on PGM active sites [6–8]. The use of Pd as a component and ceria as a support in the TWC significantly improves CO/HC oxidation activity and long- term durability [9–11,12]. The light-off of CO oxidation by O 2 on Pd- based catalysts shows a smooth S-shape curve with increasing reaction temperature [13–15]. For CO oxidation by NO (i.e. CO + NO) in the absence of molecular oxygen, a side reaction has been identified, in which the formation of N 2 O as an intermediate of N 2 has been verified [16,17], following Eq. (2) + → + NO CO NO CO 2 2 2 (2) N 2 O is a concern for stratospheric ozone layer depletion [18]. The formation of N 2 O from the CO + NO reaction is still a subject of interest [7,8,17,19–32], particularly during cold-start of engines, as the com- plete reduction of NO to N 2 is desired. Consequently, the CO + NO reaction over Pd-based TWC is crucial. It was reported that the re- activity of CO + NO can be significantly improved by use of ZrO 2 promoted CeO 2 (CZO) support [33]. However, a concave feature in the light-off curve of NO reduction with increasing temperature was https://doi.org/10.1016/j.cattod.2018.07.005 Received 5 March 2018; Received in revised form 21 June 2018; Accepted 4 July 2018 ⁎ Corresponding author. E-mail address: schwank@umich.edu (J.W. Schwank). Catalysis Today xxx (xxxx) xxx–xxx 0920-5861/ © 2018 Elsevier B.V. All rights reserved. Please cite this article as: Chen, X., Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.07.005