Catalyzed carbon-NO reaction studied by scanning tunneling microscopy and ab initio molecular orbital calculations

The STM technique has been employed to study the catalysis of the carbon-NO reaction by alkali, alkaline earth, and transition metal catalysts. The catalytic actions by the catalysts used in this study follow the well established modes of channeling, edge recession, and pitting. The turnover frequencies of the catalyzed reactions are calculated based on the channeling rates on the basal plane of graphite with 1% NO at 600 degrees C, and the relative catalytic activities of the catalysts are compared. NO is dissociatively chemisorbed by these catalysts, followed by diffusion of O atoms to the edge carbon sites, where breakage of C-C bonds takes place to free CO. The rate-limiting step for the gasification reaction by NO is the breakage of C-C bonds. Ab initio molecular orbital calculations are performed on model graphite substrates with -O and -O-M groups (where M = metal) bonded to the zigzag face. The surface C-C bonds in these structures are substantially weakened by adding -O or -O-M on the active carbon atom, leading to CO release. The extent of weakening in the C-C bond energy by different metals is in general agreement with the order of catalyst activities measured as turnover frequencies. The rank order of TOF by different catalysts is Cu > Ba = K > Sr > Ca > Mg > Na > Co > Fe = Li. (C) 1999 Academic Press.