In an ultra high vacuum heterogeneous gas reactions such as 2 C + O_2~G -> 2 CO~G and C + + CO_2~G -> 2 CO~G often show a less complicated reaction path than under normal pressure. In the present study the graphite sample is a commercial light-bulb filament, which is extremely resistant against oxidation. Under low pressure the combustion reaction is of first order with a continuous transition to zero order at temperatures between 900 and 1300 K depending on the oxygen pressure. The Boudouard reaction behaves similarly but obeys zero order only around 2000 K and for CO_2 pressures around 1 bar. The measured and the statistically calculated reaction rates agree well. An insight of the details of the reaction and chemisorption is determined by the evaluation of experiments carried out in an ultra high vacuum using isotopes such as ~(13)CO_2~G. The Boudouard equilibrium for graphite can be reached only catalytically, e.g., with sponge iron in statu nascendi.
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