Tailoring the efficiency of an active catalyst for CO abatement through oxidation reaction: The case study of samarium-doped ceria

In the present study, for the first time we tailored organic chemistry traditional synthesis tools towards preparation of CexSm1-xO2 (x = 0.2, 0.5, 0.8, 1) fluorite-type mixed oxides which are used for CO abatement. In this study, microwave radiation has been used as stand-alone heating method (conventional microwave), in the presence of oxalic acid acing as complexing agent (OX), or it has been coupled with simultaneous air cooling (AC), reflux (RE) and on/off cycles of the magnetron power (Cl and C5) in the so called enhanced microwave synthesis (EMW) approach. The EMW methods were applied in the case of Ce0.5Sm0.5O2 composition and enlightened different aspects of the rational design of its morphology (flower-like, rod-shaped, random aggregates), as well as the engineering of its oxygen vacant sites. CO oxidation reaction, in the presence and absence of H-2, was used as a model probe reaction towards the evaluation of the catalysts. Different characterization techniques, namely, XRD, BET, ICP-MS, Raman, SEM, FTIR, TEM, XPS, H-2-TPR, and CO2-TPD, have been employed to understand the synthesis structure-properties relationship of the catalysts. Interestingly, the EMW synthesis conditions affect the crystalline structure of the catalysts (XRD), and the crystal growth, giving rise to CexSm1-xO2 particles with a crystallite size in the range of 8-26 nm. A rational manipulation of the morphology of the CexSm1-xO2 was achieved under the different EMW synthesis conditions and reported herein for the first time in such a systematic way, where the Ce0.5Sm0.5O2 catalyst was crystallized in octahedral rods (RE), random aggregates (Cl and C5), and elongated flower-like shapes (AC). The co-presence of Ce3+ and Sm3+ in the metal oxides (XPS studies) facilitates the presence of oxygen vacant sites, as confirmed by the Raman spectra (550-590 cm(-1)), the mobility of which, though, differs as has been proved by H-2-TPR, following the order: Ce0.5Sm0.5 (RE) > Ce0.5Sm0.5 (AC) > Ce0.5Sm0.5 (C1). A tensile lattice strain in the 0.6-2.4% range of the cubic CexSm1-xO2 lattice was found to be strongly correlated with the CO oxidation performance. Two of the enhanced microwave methods, RE and AC, led to catalysts with improved catalytic performance, lowering the T-50 by 130 degrees C (or 27%) and 30 degrees C (or 6%), respectively, compared to the Ce0.5Sm0.5O2 (OX). The superior performance is discussed in terms of oxygen vacancies, oxygen mobility, lattice strain and CO2-affinity of the surfaces involved.