Rational Design of Ag/Ce x Ti1– x O2 ( x = 0.3, 0.5, and 0.7) Monolithic Catalysts for CO Oxidation

Abstract The diffusional resistance offered by packed-bed reactors is a severe limitation for CO oxidation catalysts. To minimize this engineering limitation, Ag/CexTi1–xO2 (AgCeTi-x, x = 0.3, 0.5, and 0.7) monolithic catalysts were synthesized by loading the catalyst through a solvothermal method on a cordierite monolithic substrate. The Ce/Ti molar ratio was systematically varied to interrogate its influence on Ce3+ concentration, oxygen vacancy density, and Ag+/Ag0 concentration, as characterized by XRD, BET surface area, FE-SEM, XPS analysis, and H2-TPR analysis. XRD analysis confirmed the formation of a multiphase Ag/CexTi1–xO2 composite involving the coexistence of crystalline phases of metallic Ag, CeO2, and TiO2, and a unique broccoli-like structure with Ag as the floret and the metal oxide as the stem is evident for the AgCeTi-0.7 sample from FE-SEM and EDX analysis. Upon loading the catalyst onto the monolithic cordierite structure, the major diffraction peaks correspond to the cordierite phase, and the corresponding morphological modification was observed. Both powder and monolithic catalysts were compared for CO oxidation performance, and an inverse performance trend was observed for Ce-rich (x = 0.7) and Ti-rich (x = 0.3) samples, with the x = 0.3 (T50% = 100 °C) sample showing better results among the powder samples and the x = 0.7 (T50% = 87 °C) sample showing better results among the monolithic samples. Significant improvement in monolithic catalyst performance could be related to overcoming the transport limitations despite having lower Oβ vacant oxygen and lower dual-site Ag+/Ag0 ratios. This work demonstrates that material composition along with reactor architecture must be co-optimized as an inseparable design unit for efficient catalyst performance.