Development of porosity upon physical activation of grape seeds char by gas phase oxygen chemisorption–desorption cycles

•Novel method of activation by cycles permits good control of porosity development.•SBET values above 1000 m2/g can be obtained with burn-off values lower than 40%.•The low burn-off helps to maintain granular morphology of the activated carbons.•Carbons with unique hollow core structure and wall thickness of 250 μm are produced.•The activated carbons showed a good mechanical strength during attrition test.

Activation of grape seeds char upon cyclic oxygen chemisorption–desorption permits a controlled development of porosity versus burn-off using air as a cheap activation agent. In this work the influence of chemisorption and desorption temperature and the number of cycles is investigated. A fast increase of BET surface area (SBET) is obtained in the two first cycles; that increase becomes then lower although the SBET continues increasing upon the successive cycles. Regarding the Dubinin–Astakhov surface area (SDA) a slow increase was observed from cycle to cycle. The activation process led to the development of both micro and mesoporosity. Under the optimum conditions for surface area development, i.e. an oxidation temperature of 275 °C and desorption temperatures between 850 and 950 °C, values of 1129–1256 and 1339–1219 m2/g were obtained for SBET and SDA, respectively. Porosity was found to increase mainly during the desorption stage, although chemisorption also led to some surface area development. SEM characterization showed that the activated carbon maintained the granular morphology of the seeds even after 10 cycles showing the egg-shell structure of the precursor with longer and deeper cracks at the outer surface. The activated carbons showed a good mechanical strength during attrition tests.