Impregnated active carbons to control atmospheric emissions I.Influence of the impregnated species on the porous structure

摘要

Impregnated active carbons were prepared to be used as catalysts for complete oxidation, aiming at the reduction of atmospheric emission of volatile organic compounds. Good dispersion of the catalysts is required, as its regulates the conversion efficiency in the pores where pollutants can access to be converted. When impregnation is performed on the raw material or after activation, the influence of impregnated species on the sturcture and on the catalyst dispersion is already well studied. This paper aims to analyze the influence of impregnated species on the structure and on the catalyst dispersion when the impregnation step is performed after carbonization, as this knowledge is yet very scarce. Olive stones were used as raw material. On order to evaluate the influence of the methodology of impregnation with CoO, Co_3O_4, and CrO_3, the impregnation step was completed after activation and between carbonization and activation. In the first sequence, the impregnated oxides must be deposited on the internal surface, blocking part of the initial microporous structure. When impregnation is conducted after carbonization, metal species act as catalysts during the activation step. The structure developed strongly depends either on the chemical state of the catalyst or on its distribution in the carbonized material. A well-developed pore structure can appear just where metal species can access, remaining a micropore structure with pores of very small sizes. The better efficiency of Co_3O_4 as a catalyst of the activation step is conditioned by a worse distribution of the carbonized material, related to the bigger size of its crystals. CrO_3 is the least efficient catalyst for the activation step.