CO and soot oxidation activity of doped ceria: Influence of dopants

This article represents a comparative study of a series of doped ceria catalysts towards environmental applications like CO and soot oxidation catalysis. Transition and rare earth metals of varying size and reducibility property have been selected namely, zirconium (Zr), hafnium (Hf), iron (Fe), manganese (Mn), praseodymium (Pr), and lanthanum (La) as dopants. A facile coprecipitation approach has been used to incorporate the dopants into ceria lattice.The formation of homogeneous solid solutions and their respective physicochemical properties have been confirmed by employing XRD analysis, BET surface area measurements, TEM, Raman, UV-DRS, XPS, and TPR techniques. All the doped CeO2 samples exhibited smaller crystallite size, larger BET surface area, and higher amounts of oxygen vacancies than that of pure CeO2. CO oxidation has been performed in the presence of oxygen under atmospheric pressure, and 300–850 K temperature range in a fixed bed microreactor. Soot oxidation was carried out in presence of air using a thermo gravimetric analyzer within a much wider temperature window of 300–1273 K. The physicochemical properties of the doped ceria materials have been comparatively analyzed to correlate the influence of dopants with their improved behaviour in both the oxidation reactions. Vital role of ‘lattice oxygen’ in CO oxidation and ‘active oxygen species’ in soot oxidation on the catalyst surface has been considered, assuming that Mars and van Krevelen mechanism and active oxygen mechanism play the key role in CO and soot oxidation, respectively. The O 1s XP spectra confirmed that Mn doped ceria (denoted as CM) exhibited most loosely bound lattice oxygen and highest concentration of surface adsorbed oxygen species compared to other materials. Accordingly, a superior CO and soot oxidation activity have been observed for manganese doped ceria. Significant lowering of T50 (390 K and 669 K for CO and soot oxidation respectively) temperature have been observed in both the oxidation reactions; which is primarily attributed to the considerable lowering of lattice oxygen binding energy and higher concentration of surface adsorbed oxygen species.