Porosity Development in Activated Carbons Prepared from Walnut Shells by Carbon Dioxide or Steam Activation

The influence of carbon dioxide and steam as activating agents on the porosity development of activated carbons produced from walnut shells was investigated. The study was made covering a wide range of burnoff (12-76%) and employing different temperatures and times: in carbon dioxide activation, 850 degrees C varying the activation time in the range 60-480 min, and in steam activation, 700, 850, and 900 degrees C (for 30-120 min). It was found that the gasifying agent has a profound influence on the activated carbon porosity development. First, steam is more reactive and produces, in general, activated carbons with greater N-2 adsorption capacity. Second, the increase in the fraction of mesopores with activation time is more pronounced for steam. While steam generates micro-, meso-, and macropores from the early stages of the process, carbon dioxide produces highly microporous carbons, with broadening of the microporosity only for long activation times.