Infrared Characterization of Rh Surface States and Their Adsorbates during the NO-CO Reaction Khalid A. Almusaiteer and Steven S. C. Chuang* Department of Chemical Engineering, The UniVersity of Akron, Akron, Ohio 44325-3906 ReceiVed: June 29, 1999; In Final Form: January 4, 2000 Rh surface states and their adsorbates during the NO-CO reaction have been characterized by the in situ infrared (IR) coupled with temperature-programmed reaction (TPR) technique. The TPR profiles of adsorbates and CO 2 show that Rh surface states and their adsorbates are governed by the redox reaction cycle of NO- CO. Adsorbed oxygen from dissociated NO oxidizes Rh 0 to Rh + ; adsorbed CO reduces Rh + to Rh 0 . The extent of oxidation and reduction of Rh 0 /Rh + is in part reflected in the intensity of the adsorbates residing on these sites (i.e., Rh + (CO) 2 , Rh 0 -CO, Rh-NO + , and Rh 0 -NO - ). An increasing NO/CO ratio shifts the TPR profiles of Rh + (CO) 2 , NO conversion, and light-off to higher temperatures. The results reveal that a high NO/CO ratio or high concentration of oxidant enhances the extent of oxidation of Rh 0 to Rh + , resulting in low catalyst activity for NO reduction. Keeping the Rh surface in the Rh 0 state by a low NO/CO ratio decreases the Rh + (CO) 2 intensity and shifts the light-off to a lower temperature. O 2 ,H 2 , and C 3 H 8 present in simulated gas compete with NO and CO for the Rh site, lowering NO reduction activity. Introduction Changes in the catalyst state during reaction have long been recognized in catalysis. 1-8 In homogeneous catalysis, the catalyst precursor transforms to the active catalyst form through ligand addition and dissociation. 9 In heterogeneous catalysis, the catalyst surface state could undergo various forms of transfor- mation (e.g., oxidative disruption, reductive agglomerization, redispersion, sintering, etc.), depending on the reaction environment. 1-8,10 Despite the recognition of the possible changes in the catalyst surface state, understanding of catalyst surface states under reaction conditions has been limited. This is due to the difficulties in in situ characterization of catalyst surfaces under practical reaction conditions. The most investigated catalysts for surface reconstruction during adsorption and reaction processes are Rh catalysts 1,3,4,10-28 because of their important role in the automobile catalytic converter. Yang and Garland were the first to observe that adsorption of CO over Rh 0 crystallite on SiO 2 or Al 2 O 3 caused disruption of the Rh 0 crystallites. 1 Solymosi and co-workers have found that CO adsorption on Rh led to oxidative disruption of the Rh 0 crystallites to isolated Rh + at 300 K, while CO adsorption on isolated Rh + sites at temperatures above 448 K resulted in the formation of Rh 0 crystallites. 15,19,27 The former was termed as CO-induced oxidative disruption and the latter as CO-induced reductive agglomerization. 15 The CO-induced oxidative disruption process was further shown to be involved with surface OH and was facilitated on small Rh crystallites. 16 The process can be assisted by NO, which oxidizes Rh 0 to Rh + . 15 Changes in catalyst surface states undoubtedly have a great influence on the catalyst activity. Schmidt and co-workers have shown that in the pretreatment of Al 2 O 3 - and SiO 2 -supported Rh catalysts (i) NO increases the Rh dispersion, (ii) H 2 causes sintering of Rh particles, and (iii) CO has no effect on the Rh dispersion. 10,24 They also showed that decreases in dispersion cause a decrease in catalyst activity for the NO-CO reaction. 10,24 Hecker and Bell found that preoxidation of Rh/SiO 2 increases its activity for the NO-CO reaction, whereas prereduction of Rh/SiO 2 decreases its activity for the NO-CO reaction. 12 Although the role of CO and NO in modifying the Rh surface state and morphology has been clearly elucidated, little is known about the effect of NO and CO partial pressures on the Rh surface states and their adsorbates/activities during the NO- CO reaction. The present study is aimed at determining the oxidation state of Rh and its adsorbates as a function of temperature and NO/CO partial pressure during the NO-CO reaction. Infrared spectroscopy (IR) coupled with temperature- programmed reaction (TPR) is used to determine the adsorbate structure for elucidation of the Rh surface states and product formation during the NO-CO reaction and the reaction of a gas stream simulated to automobile exhaust. To determine the role of Rh + in the reaction, oxygen was added to the NO/CO stream to increase the number of Rh + sites. This study is expected to contribute to a better understanding of Rh surface states during the NO-CO reaction in automobile exhaust catalysis. Experimental Section The 2 wt % Rh/Al 2 O 3 catalyst was prepared by incipient wetness impregnation of RhCl 3 ‚2H 2 O (Alfa Chemicals) onto γ-alumina support (Alfa Chemicals, 100 m 2 /g). The catalyst was dried overnight in air at room temperature, calcined by flowing air at 723 K for 6 h, and then reduced by flowing H 2 at 723 K for 6 h. The average Rh crystallite size was determined to be 52 Å by H 2 chemisorption. The experimental apparatus including an in situ infrared (IR) reactor cell with CaF 2 windows has been reported in detail elsewhere. 29 The catalyst powder was pressed into three self- supporting disks. One (25 mg) was placed in the path of the IR beam in the center of the IR reactor cell; others (98 mg) were * To whom correspondence should be addressed. Phone: (330)972-6993. E-mail: schuang@uakron.edu. 2265 J. Phys. Chem. B 2000, 104, 2265-2272 10.1021/jp9922155 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/19/2000