Published: September 30, 2011 r2011 American Chemical Society 11960 dx.doi.org/10.1021/ie201474p | Ind. Eng. Chem. Res. 2011, 50, 11960–11969 ARTICLE pubs.acs.org/IECR Structural Characterization and Catalytic Evaluation of Supported CopperCeria Catalysts for Soot Oxidation Komateedi N. Rao, P. Venkataswamy, and Benjaram M. Reddy* Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 607, India ABSTRACT: The present investigation was undertaken with the aim of designing a noble-metal-free diesel soot oxidation catalyst that can work at relatively low temperatures. Accordingly, a series CeO 2 Al 2 O 3 , CeO 2 ZrO 2 , and CeO 2 SiO 2 mixed-oxide- supported copper catalysts were prepared by a modified depositionprecipitation method from ultradilute aqueous solutions and evaluated for soot oxidation at normal atmospheric pressure. The structural and electronic properties were investigated by various physicochemical techniques, namely, X-ray diffraction (XRD), Raman spectroscopy, temperature-programmed desorption (TPD), temperature-programmed reductionoxidation (TPRTPO), temperature-programmed mass spectroscopy (TP-MS), transmis- sion electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Formation of a Ce 1x Cu x O 2 solid solution was inferred from XRD, TPR, and Raman spectroscopy studies. The TEM results revealed the presence of copperceria nano-oxides of ∼57-nm size in highly dispersed form. The TPRTPO results suggested a profound influence of mixed-oxide supports on the reduction behavior of copper oxide. In particular, the TP-MS results provided direct evidence for labile oxygen generation. Among the investigated catalysts, the CuO/CeO 2 ZrO 2 combination exhibited excellent catalytic activity for soot oxidation, with T 1/2 = 611 K. Incorporation of Cu 2+ into the ceriazirconia solid solution favored the creation of more structural defects, which accelerate the oxygen diffusion and induce more of the surface-active oxygen species that are responsible for the enhanced soot oxidation activity at lower temperatures. 1. INTRODUCTION Soot particles are hazardous environmental pollutants that account for a major fraction of fine air particulate matter in urban areas. They can cause and enhance respiratory, cardiovascular, and allergic diseases. 15 The large-volume diesel engine market and its ever-increasing demand for heavy-duty engines have led to a large amount of carbon particulate (CP) emissions. Among several techniques that have been developed for reducing emis- sions from diesel engines, filtration followed by catalytic oxida- tion is one of the promising methods. In this catalytic approach, the system has to achieve onset regeneration at significantly lower temperatures. The solid soot particles are immobile when deposited, and because of their large size, they cannot penetrate into the catalyst’s pores. The temperature in the exhaust system also varies in a wide range from 473 to 873 K. 68 Therefore, an ideal catalyst should exhibit good activity at low temperatures and high thermal stability. In general, the catalytic oxidation of soot is relatively slow. It is also complicated because of the complex chemical nature of soot and the different size distribu- tions of the particulates. Several studies have emphasized the significance of redox properties of catalysts for soot oxidation. Ceria (CeO 2 ) is a well-known oxygen-storage/-release material in three-way catalytic converters employed for CO, hydrocarbon, and NO x abatement. 2,3,912 CeO 2 has the potential to increase the soot oxidation rate by involving the participation of “active oxygen” species and thereby lower the CP combustion tem- perature. 1,1315 The ability of ceria to provide exchangeable oxygen species is weakened by sintering and decreased specific surface area at high temperatures. Therefore, several studies have focused on supported ceria catalysts, which exhibit significantly enhanced thermal stability, better redox properties, and a high catalytic activity. Most oxidation catalysts in the international markets are based on noble metals (Pt, Pd, and Rh), which are highly expensive and which, because of their low abundance, are vulnerable to further price increases as demand grows. Therefore, exploration for noble-metal-free catalysts or catalysts containing low amounts of noble metals is receiving global importance. The primary objective of this study was to design a noble-metal-free catalyst for soot oxidation at relatively low temperatures. Recent reports suggest that the activity of ceria for complete oxidation reactions could be enhanced by the incorporation of transition metals, in particular, copper. Copper-based catalysts have attracted much attention recently in heterogeneous catalysis because of their good catalytic activity toward NO reduction, CO and hydro- carbon oxidation, watergas shift reaction, methanol synthesis, and wet oxidation of phenol. 1019 Further, copper-based cata- lysts are environmentally compatible and economically advanta- geous. More recently, efforts have been made to elucidate the strong metalsupport interaction of CuO and CeO 2 , which can significantly affect the catalytic properties. Of course, the redox properties and the catalytic activity of supported metal-oxide catalysts are strongly dependent on the nature of the support, the preparation method, and the calcination temperature. 1922 The present investigation was undertaken against this back- ground to explore the significance of interactions between the active copper oxide species and the ceria-based mixed-oxide supports. To improve the thermal stability, mechanical strength, specific surface area, and catalytic activity of CuO, different Received: July 10, 2011 Accepted: September 19, 2011 Revised: September 4, 2011