We have developed a simple plasma reactor for functionalizing powders. The reactor uses thermo-convective cells to mix the powder during the treatment and to ensure high homogeneity. The reactor has been used to treat batches of 50-150 mg of carbon black in a gas mixture of He/N-2 at 40 Torr. By using repetitive treatments on fractions of carbon black recovered from the cap of the treatment vessel (due to its fluidization during the treatment), we were able to fix a concentration of nitrogen functionalities of 13% at. N/C (as measured by XPS). We report the working conditions of the reactor and the results obtained.