A review of the control of pore texture of phosphoric acid-activated carbons

Phosphoric acid activation is a chemical activation process that has been widely used to produce activated carbons from lignocellulose. The author systemically reviews the process from the role of the acid, and the pore structure, chemical properties and uses of these activated carbons. This review provides comprehensive information on the control of the pore structure and proposes that the interaction of the phosphoric acid and the lignocellulose results in the formation of phosphoric acid/biopolymer composites so that the activation can be simply regarded as two successive steps, which are formation and thermal decomposition of such composites. Based on this idea, the paper comprehensively analyzes the effect of many factors including the composition and texture of the raw materials and the impregnation parameters on the constituents and structures of the H₃PO₄/biopolymer composite and thus on the pore structure of the activated carbon. These complex factors include (1) the composition of the lignocellulose and pretreatment of the raw materials, (2) cellular structure and crystallinity, (3) impregnation parameters such as impregnation ratio, method, temperature and time. During the subsequent heat treatment of the H₃PO₄/biopolymer composite, the significant effect of the heating program on the formation and development of pore structure, including such parameters as temperature, heating rate and any intermediate isothermal treatment, and carbonization atmosphere including inert gases, oxidative gases and steam are reviewed. Finally, the roles of the oxidative atmosphere and other oxidants during activation are outlined, and two types of carbonization and their effect on pore development are also proposed and discussed.