Hexagonal and Monoclinic Phases of LaOCO Nanoparticles and Their Phase-Related CO Behavior.

In this study, we prepared hexagonal and monoclinic phases of LaOCO nanoparticles by different wet preparation methods and investigated their phase-related CO behavior through field-emission scanning microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, CO-temperature programmed desorption, and linear sweeping voltammetry of CO electrochemical reduction. The monoclinic LaOCO phase was synthesized by a conventional precipitation method via La(OH)CO when the precipitation time was longer than 12 h. In contrast, the hydrothermal method produced only the hexagonal LaOCO phase, irrespective of the hydrothermal reaction time. The La(OH) phase was determined to be the initial phase in both preparation methods. During the precipitation, the La(OH) phase was transformed into La(OH)CO owing to the continuous supply of CO from air whereas the hydrothermal method of a closed system crystallized only the La(OH) phase. Based on the CO-temperature programmed desorption and thermogravimetric analysis, the hexagonal LaOCO nanoparticles (HL-12h) showed a higher surface CO adsorption and thermal stability than those of the monoclinic LaOCO (PL-12h). The crystalline structures of both LaOCO phases predicted by the density functional theory calculation explained the difference in the CO behavior on each phase. Consequently, HL-12h showed a higher current density and a more positive onset potential than PL-12h in CO electrochemical reduction.