Characterization of Nb-Containing MnO x -CeO 2 Catalyst for Low-Temperature Selective Catalytic Reduction of NO with NH 3 Maria Casapu, Oliver Kro ¨cher,* Max Mehring, Maarten Nachtegaal, Camelia Borca, Messaoud Harfouche, and Daniel Grolimund Paul Scherrer Institute, 5232 Villigen PSI, Switzerland ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: March 25, 2010 The reactivity in the selective catalytic reduction of the individual and binary components of NbO x -MnO x -CeO 2 catalysts has been studied with coated cordierite monoliths in the temperature range of 150-450 °C. FTIRS, DRIFTS, TA, XRD, BET, and XAS have been used to elucidate the structural and catalytic properties. The results confirmed the contribution of the manganese oxides, particularly to the low- temperature NO-to-NO 2 oxidation reaction. The significant increase of the surface acidity as a result of niobium oxide addition has been established. The data obtained revealed also the strong interaction between the manganese and niobium catalytic active sites. This phenomenon leads to a very good distribution of the oxidizing and acidic sites in the catalyst structure and also diminishes the unselective NH 3 oxidation at higher temperatures. However, in order to keep the low-temperature catalytic activity, an excess of manganese relative to the niobium content is needed. 1. Introduction Due to the strong polluting effects of nitrogen oxides, the regulations of the emission levels are becoming more and more stringent, especially within Europe, Japan, and the United States. Among the various technologies that have been proposed for the elimination of NO x emissions, the selective catalytic reduction (SCR) with ammonia is believed to be most suitable to fulfill this task. During the SCR process, the reducing agent (NH 3 , urea) is deliberately added by injection into the exhaust stream. Over the catalyst surface, ammonia reacts with NO x to form N 2 . For the SCR reaction of NO x with NH 3 , two cases have been distinguished: the “standard” reaction occurring between NH 3 and NO and the “fast” SCR reaction where both NO and NO 2 are involved. The second path has an important contribution, particularly at low temperatures where most of the conventional NH 3 -SCR catalysts exhibit a rather low activity. However, the “fast” SCR reaction requires the generation of NO 2 from NO upstream or over the SCR catalyst. Several catalysts have been found to be efficient for catalyzing both the oxidation of NO to NO 2 and the NH 3 -SCR reaction at 180-350 °C. In this respect, the systems containing manganese as active species have been intensively studied during the last years. Promising activity was determined for pure manganese oxides 1 and MnO x supported on Al 2 O 3 , 2 TiO 2 , 3,4 NaY zeolite, 5 CeO 2 , 6-10 and active carbon. 11 Recently, we have reported that, by doping the MnO x -CeO 2 catalyst with niobium oxide, a significant increase of the SCR activity and selectivity is obtained. Besides, it was shown that, due to the high sulfur sensitivity of the manganese-containing catalysts, their applicability for the low-temperature SCR reaction is possible only in a sulfur-free exhaust or downstream of a sulfur trap. The aim of the present work was to perform a systematic investigation of the factors that contribute to the improved catalytic activity and selectivity. In a first step, the individual and binary components of the MnO x -NbO x -CeO 2 catalyst have been tested for their SCR performance. A careful characteriza- tion of the catalyst was then accomplished by means of various physical and spectroscopic techniques, including diffuse reflec- tance infrared fourier transform spectroscopy (DRIFTS), thermal analysis (TA), fourier transform infrared spectroscopy (FTIRS), X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). 2. Experimental Section 2.1. Catalyst Preparation. The MnO x -NbO x -CeO 2 , MnO x -CeO 2 , NbO x -CeO 2 , and MnO x -NbO x samples were prepared by coprecipitation of manganese acetate, cerium acetate, and niobium chloride with (NH 4 ) 2 CO 3 , as described in detail in ref 12. The resulting precipitate was filtered, dried overnight at 120 °C, and calcined in air at 650 °C for 5 h. The molar ratios between the components were Mn/Nb/Ce ) 23: 23:54, Mn/Nb/Ce ) 11.5:23:65.5, Mn/Ce ) 30:70, Nb/Ce ) 30:70, and Mn/Nb ) 1:2. These samples will be referred to, in the following, as MnNbCe, 1:2-MnNbCe, MnCe, NbCe, and 1:2-MnNb. Two additional catalysts were prepared that contain 5 wt % MnO 2 in a formal composition of MnO 2 -CeO 2 or MnO 2 -Nb 2 O 5 -CeO 2 . This corresponds to molar ratios of Mn/ Ce ) 9.4:90.6 and Mn/Nb/Ce ) 8.9:24.9:66.2. These samples will be referred to as 5 wt %-MnCe and 5 wt %-MnNbCe. Pure manganese, cerium, and niobium oxide reference materials were made by the same procedure and denoted as MnO x , NbO x , CeO x . Thermal aging of the catalysts was studied by calcining the MnNbCe, 1:2-MnNbCe, and 1:2-MnNb samples at 650 °C for 4 days and at 800 or 1000 °C for 5 h. 2.2. Catalytic Activity Tests. The SCR tests were carried out with catalyst loaded (1.2-1.3 g) cordierite monoliths (4 cm ×1.7 cm ×1.2 cm) with a cell density of 400 cpsi, according to a preparation procedure described elsewhere. 13 The as-prepared monoliths were fixed into a quartz plug flow reactor with a ceramic fiber mat. Heating was accomplished with heating coils, and the temperature was regulated by two thermocouples, positioned up- and downstream of the catalyst. * To whom correspondence should be addressed. Tel: +41 56 310 20 66. Fax: +41 56 310 23 23. E-mail: oliver.kroecher@psi.ch. J. Phys. Chem. C 2010, 114, 9791–9801 9791 10.1021/jp911861q  2010 American Chemical Society Published on Web 05/12/2010