Identification of Surface Species on Titania-Supported Manganese, Chromium, and Copper Oxide Low-Temperature SCR Catalysts Donovan A. Pen ˜ a, ² Balu S. Uphade, ‡ Ettireddy P. Reddy, ² and Panagiotis G. Smirniotis* ,² Chemical and Materials Engineering Department, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0012, and National Chemical Laboratory (NCL), Dr. Homi Bhabha Road, Pune-411 008, India ReceiVed: December 11, 2003; In Final Form: April 6, 2004 TiO 2 -supported transition metal oxides (Mn, Cr, and Cu) for the SCR of NO with NH 3 have been synthesized by wet impregnation. The adsorption and coadsorption of NH 3 , NO, and O 2 , in conjunction with in situ FT-IR spectroscopy, was used to elucidate the reaction mechanism as the samples were heated from 323 to 673 K. While Cr was the only transition metal that generated significant amounts of Brønsted acidity, strong Lewis acid sites were present over all of the materials. The peak strength corresponding to the δ s (NH 3 ) coordinated to Lewis acid sites decreased in the following order: Ti > Mn > Cr ∼ Cu. Similarly, the peak strength corresponding to the δ as (NH 3 ) coordinated to Lewis acid sites decreased as follows: Mn > Cr ∼ Cu. Exposing the catalysts to oxygen before the introduction of NO did not impact the adsorption of NO as nitrates on the catalysts, suggesting that labile lattice oxygen plays an important role in the formation of nitrates. Three types of nitrates were observed after the adsorption of NO. Monodentate and bidentate nitrates formed on the surface of all the materials tested, while bridged nitrates only formed on CrO x /TiO 2 . The in situ FTIR data collected resulted in the development of a reaction mechanism for MnO x /TiO 2 . A combination of moderately strong monodentate and bidentate nitrate species, along with a split in the symmetric deformation of NH 3 coordinated to Lewis acid sites, appear to be important for high activity and selectivity. The peak resulting from the vibrational mode of ammonia adsorbed on Lewis acid sites, which is located at ∼1170 cm -1 , is believed to be important in facilitating hydrogen abstraction to form amide species that react with bidentate nitrates (1620 cm -1 ). It is proposed that the reaction mechanism proceeds through the formation of nitrosamide and azoxy species, which most likely possess lifetimes as reaction intermediates that are too brief for detection. In contrast to MnO x /TiO 2 , the apparent participation of Brønsted acid sites for CrO x /TiO 2 suggests that a different reaction pathway is involved for this catalyst. Introduction The selective catalytic reduction (SCR) of NO with NH 3 in excess oxygen is a well-established commercial technology using V 2 O 5 and WO 3 (or MoO 3 ) supported on anatase TiO 2 . 1-2 The catalyst was designed to operate at medium temperatures (573-673 K). Consequently, the catalyst should be placed immediately after the boiler or at some other location upstream of the desulfurizer and/or particulate control device to avoid costly reheating of the flue gas. However, placement at that location leads to catalyst deactivation and SO 2 oxidation in coal- fired utility plants because of the high concentrations of SO 2 and particulate matter. Efforts have been made to develop catalysts capable of operating at low temperatures (353-523 K) 3-9 so that the catalyst bed may be moved downstream of the desulfurizer and/or particulate control device in what is known as the tail end configuration. 10 At the tail end (353- 523 K) the concentrations of SO 2 and particulate matter in the flue gas are drastically reduced, which will result in increased catalyst life and decreased oxidation of SO 2 to SO 3 . Recently, novel MnO x catalysts were developed 11 that provided high activity and nitrogen selectivity in the presence of 10 vol % water at low temperatures (373-473 K). Extensive in situ FT-IR studies have been reported in the past over vanadia/titania catalysts using NH 3 , NO and O 2 as probe molecules to understand the SCR reaction mechanism using NH 3 , 12-20 and the major mechanisms proposed have been reviewed by Busca et al. 21 Most of the studies concluded that NH 3 was readily adsorbed, while NO could not be adsorbed on the surface of vanadium pentoxide in the absence of oxygen. Earlier studies also suggested that NH 3 was adsorbed on Brønsted (as NH 4 + ) as well as Lewis (as NH x -like species; x ) 1-3) acid sites on V 2 O 5 /TiO 2 . However, most of the studies concluded that surface Brønsted acid sites are important active sites for the SCR reaction. 12,15-18 Additionally, the SCR of NO with NH 3 has been reported to follow both Langmuir- Hinshelwood 12 and Eley-Rideal 13 type mechanisms. In a series of papers, Busca and co-workers 21-23 reported FT- IR results from ammonia, hydrazine, and pyridine adsorption over various transition metals supported on anatase TiO 2 . In contrast to the work primarily performed by Topsøe and co- workers, 2,16-19 Busca and co-workers 24 concluded that Brønsted acidity plays no role in either SCR or SCO after studying various catalysts, including V 2 O 5 /TiO 2 . Although a great deal of literature is available on using FT-IR to study the adsorption and coadsorption of NH 3 , NO, and O 2 on V 2 O 5 /TiO 2 , few papers deal with other catalytic systems used in the SCR of NO with NH 3 . 23,25-29 In addition to V 2 O 5 /TiO 2 , other catalytic systems including Mn/γ-Al 2 O 3 28,30 and chromia 25 have also been studied. * To whom correspondence should be addressed. Telephone: (513) 556- 1474. Fax: (513) 556-3473. E-mail: panagiotis.smirniotis@UC.EDU. ² University of Cincinnati. ‡ National Chemical Laboratory (NCL). 9927 J. Phys. Chem. B 2004, 108, 9927-9936 10.1021/jp0313122 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/19/2004