Reasons for the disintegration of microspherical precursor MnCO3 for soot combustion catalysts Ptδ+/MnOx

Abstract

Supported Pt^δ+/MnO_x catalysts were prepared for diesel soot combustion in NO_x/O₂. During the deposition–precipitation (DP) of Pt(NO₃)₂ at pH ≥ 9, microspherical precursor MnCO₃ showed an interesting disintegration. To determine the underlying reasons for this phenomenon, other wet-chemical treatments (Pt(NO₃)₂ impregnation or NH₃·H₂O-treatment) toward MnCO₃ and the DP of Pt(NO₃)₂ on calcined MnO_x were designed and conducted, demonstrating that interactions with Pt(NO₃)₂ and/or accelerated dissolution at high pH (∼11) could induce MnCO₃ disintegration. Such facile wet-chemical treatments also gave final calcined products with different phase compositions and surface chemical properties, resulting in different catalytic activities for soot combustion. Among them, the DP of Pt(NO₃)₂ at moderate pH (∼9) and Pt(NO₃)₂ impregnation both led to refined particle sizes and satisfying Pt dispersion, which exposed abundant active sites. Furthermore, oxygen species desorbed at moderate temperatures and NO_x species were found to be directly related to the specific catalytic activity. The key factor was found to be the refined particle size (namely, the catalyst–soot contact efficiency). Furthermore, plausible reaction pathways were proposed for soot combustion over Pt^δ+/MnO_x-type catalysts.