Journal of Catalysis 184, 189–201 (1999) Article ID jcat.1999.2417, available online at http://www.idealibrary.com on Dynamic Behavior of Adsorbed NO and CO under Transient Conditions on Pd/Al 2 O 3 Khalid Almusaiteer and Steven S. C. Chuang 1 Department of Chemical Engineering, The University of A kron, A kron, Ohio 44325-3906 Received October 22, 1998; revised January 4, 1999; accepted January 5, 1999 The dynamic behavior of adsorbed NO and CO under transient NO–CO reaction conditions on Pd/Al 2 O 3 has been studied by in situ infrared (IR) spectroscopy coupled with TPR and pulse reaction techniques in the 303–673 K range. Below the light-off temperature (i.e., 561 K), Pd 0 –NO and Pd 0 –CO are the dominant adsorbates on the Pd surface. Pd 0 –NO competes favorably over Pd 0 –CO for the same reduced Pd 0 site when the temperature is increased. Pulse reaction studies at 473 K suggest that Pd 0 –NO dissociates to form adsorbed nitrogen and adsorbed oxygen. Adsorbed oxygen further reacts with Pd 0 –CO to produce CO 2 . Concentration profiles of CO 2 and Pd 0 –COduring the pulse reaction studies indicate that removal of adsorbed oxygen from the Pd surface to produce CO 2 is the rate- limiting step. Prolonged exposure of the catalyst to the NO flow at 473 K results in oxidation of Pd 0 to Pd + and produces Pd–NO + ; the presence of gaseous CO reduces Pd + to Pd 0 and increases the surface coverage of Pd 0 –NO. Above the light-off temperature, Pd– NO + , Al–NCO, nitrate, and carbonate species are the dominant adsorbates. The presence of Pd–NO + indicates that the process for Pd 0 oxidation to Pd + by NO is faster than that of Pd + reduction to Pd 0 by CO. This study demonstrates that careful selection of transient IR techniques allows (i) determination of the modes of adsorbed NO and CO participating in the reaction and (ii) develop- ment of a comprehensive mechanism for the NO–CO reaction on Pd/Al 2 O 3 catalyst. c  1999 Academic Press Key Words: Pd (palladium); Pd/Al 2 O 3 ; infrared spectroscopy (IR);adsorbed NO;adsorbed CO;chemisorption;NO–CO reaction; NO reduction; pulse reaction; temperature-programmed reaction (TPR); mechanism; rate-limiting step; reaction dynamics; transient condition. INTRODUCTION Carbon monoxide (CO) and nitric oxide (NO) are con- sidered the major pollutants from automobile emissions. Removal of CO and NO from the automobile exhaust is an important subject in environmental catalysis (1–5). In a conventional catalytic converter, Rh and Pt are used for simultaneous control of CO, NO, and hydrocarbons (HC) 1 To whom correspondence should be addressed. E-mail: schuang@ uakron.edu. emissions. Due to limited supply and elevated cost of Rh, automobile producers have introduced the palladium cata- lyst as a substitute for Rh and Pt (4). Successful develop- ment of Pd-based catalysts for the catalytic converter has been attributed to the excellent durability of Pd compared to that of Rh/Pt. While the total cost of Rh/Pt catalytic con- verter isclose to that ofPd-onlycatalyticconverter,Pd-only automotive catalysts provide an alternative to the monop- olistic usage of Rh for automobile NO emission control. However, there are some drawbacks in using Pd, such as the susceptibility of Pd to poisoning by sulfur, phosphorus, lead, and zinc (4–6). Continuing improvement of Pd catalyst to meet increas- ingly stringent requirements for NO emission control re- quires a comprehensive understanding of the NO–CO re- action mechanism on Pd. Understanding of Pd catalysis of the NO–CO reaction has been achieved by spectroscopic studies of NO and CO adsorption and steady-state and sur- face science studiesofPd single crystaland supported metal catalysts (7–11). Spectroscopic studies have shown that NO chemisorbson a Pd surface ascationicNO [Pd–NO + ],linear NO [Pd 0 –NO], and bent NO [Pd–NO − ];CO chemisorbs as linear CO [Pd 0 –CO] and bridged CO [(Pd 0 ) 2 –CO] (7–12). Although these adsorbates on the Pd surface may serve as active precursors involved in surface reactions for N 2 and CO 2 formation, little attempt has been made to correlate the mode and concentration ofadsorbateswith the catalyst activity. Investigation of the dynamic behavior and reactivity of various modes of adsorbed NO and CO by transient tech- niquescouldprovidemechanisticinformationregardingac- tive and spectator adsorbatesaswellasthe change in behav- ior ofthe adsorbatesand catalyst surface due to variation in reactant concentration (13, 14). In the present work, tem- perature programmed reaction (TPR) and pulse reactions were used to determine the nature of active sites and re- activity of various adsorbates under transient conditions as well as to verify and improve the general form of the pro- posed mechanism for the NO–CO reaction on Pd-based catalysts. Fourier transform infrared (FTIR) spectroscopy was used to monitor the transient responses of adsorbates, 189 0021-9517/99 $30.00 Copyright c  1999 by Academic Press All rights of reproduction in any form reserved.