Nanowire Morphology of Mono- and Bidoped α-MnO2 Catalysts for Remarkable Enhancement in Soot Oxidation

In the present work, nanowire morphologies of alpha-MnO2, cobalt monodoped alpha-MnO2, Cu and Co bidoped alpha-MnO2, and Ni and Co bidoped a-MnO2 samples were prepared by a facile hydrothermal synthesis. The structural, morphological, surface, and redox properties of all the as-prepared samples were investigated by various characterization techniques, namely, scanning electron microscopy (SEM), transmission and high resolution electron microscopy (TEM and HR-TEM), powder X-ray diffraction (XRD), N-2 sorption surface area measurements, X-ray photoelectron spectroscopy (XPS), hydrogen-temperature-programmed reduction (H-2-TPR), and oxygen-temperature-programmed desorption (O-2-TPD). The soot oxidation performance was found to be significantly improved via metal mono- and bidoping. In particular, Cu and Co bidoped alpha-MnO2 nanowires showed a remarkable improvement in soot oxidation performance, with its T-50 (50% soot conversion) values of 279 and 431 degrees C under tight and loose contact conditions, respectively. The soot combustion activation energy for the Cu and Co bidoped MnO2 nanowires is 121 kJ/mol. The increased oxygen vacancies, greater number of active sites, facile redox behavior, and strong synergistic interaction were the key factors for the excellent catalytic activity. The longevity of Cu and Co bidoped alpha-MnO2 nanowires was analyzed, and it was found that the Cu/Co bidoped alpha-MnO2 nanowires were highly stable after five successive cycles and showed an insignificant decrease in soot oxidation activity. Furthermore, the HR-TEM analysis of a spent catalyst after five cycles indicated that the (310) crystal plane of a-MnO2 interacts with the soot particles; therefore, we can assume that more-reactive exposed surfaces positively affect the reaction of soot oxidation. Thus, the Cu and Co bidoped alpha-MnO2 nanowires provide promise as a highly effective alternative to precious metal based automotive catalysts.