Metal-catalyzed graphitic nanostructures as sorbents for vapor-phase ammonia

Abstract

Activated carbons have long been used as substrates for the filtration of vapor-phase molecules, often with metal salts or oxides added to improve their sorption capacities for specific agents, but their real-world performance and applicability may be hindered by such factors as long-term stability and complex processing. En route to a new class of carbon-based sorbents, we have developed solid-state synthetic methods to produce bulk carbonaceous solids based on the pyrolysis of thermoset solids containing low concentrations (<1 wt%) of metal precursors (based on either Ni, Fe, or Co) that decompose in situ to catalyze the formation of a complex graphitic nanostructure. Selective combustion of residual amorphous carbon from the pyrolyzed solid generates a mesoporous network that facilitates diffusional transport of gas-phase molecules to the interior surfaces of the solid, and also converts the entrained metals to their respective metal oxide forms. We examine the ammonia-sorption properties of a series of these graphitic nanostructured compositions, and demonstrate that ammonia uptake is primarily determined by the type of residual metal oxide, with the Co-containing carbonaceous solid providing the best ammonia-sorption capacity (1.76 mol kg⁻¹). Thermal reduction of the Co-containing material drastically decreases its ammonia-sorption capacity, showing that the oxide form (in this case Co₃O₄) of the entrained metal nanoparticles is most active for ammonia filtration. The effects of the carbon–oxygen functionalities on the nanostructured graphitic surfaces for ammonia sorption are also discussed.