Kinetics of Carbon–NO Reaction Studied by Scanning Tunneling Microscopy on the Basal Plane of Graphite

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.180, Iss.2, 1998-12, pp. : 245-257

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Abstract

The carbon–NO reaction was studied by scanning tunneling microscopy (STM) on the basal plane of graphite (HOPG) samples in a wide temperature range of 500–900°C. Monolayer etch pits, multilayer (deep) etch pits, and nascent etch pits were observed for the graphite–NO reaction in different reaction temperature regimes. The formation of deep pits and nascent pits became prevalent at higher temperatures, e.g., 850°C with 1% NO. No temperature “break” point in the Arrhenius plot of the turnover frequency (TOF) rates was observed. The activation energy for the graphite–NO reaction was 132 kJ/mol, which was relatively low compared to literature data. Careful analysis of basal plane STM features revealed that the often-reported (by others) temperature “break” point in the Arrhenius plot was caused by the contribution of NO basal plane c-attack in the higher temperature range, which forms nascent etch pits and deep pits. The formation of nascent and deep pits with higher activation energies (than that for monolayer etching) contributed to the higher apparent activation energy in the high temperature range. In global rate measurements for the NO-carbon reaction, it is not possible to distinguish between the monolayer etching process and basal plane c-attack. The TOF measured by the STM method were true intrinsic rates for edge site monolayer recession. For the NO-graphite reaction, the reaction is first order with respect to NO. The NO–graphite reaction is at least 15 times faster than the O₂-graphite reaction. The NO-graphite and N₂O–graphite reactions might have the same rate-limiting step in the low temperature range, but have different rate-limiting steps in the high temperature range.