Shaping nanofiltration channels in a carbonaceous membrane via controlling the pyrolysis atmosphere

Abstract

This work investigates the effect of atmosphere on pyrolysis of a polymer matrix (precursor) for directing its transformation towards more disordered graphene species and smaller graphitic nanograins. These two structural characteristics are crucial to the generation of nano-channels (NCs) pertinent to nanofiltration (NF). Two measures are explored hereby to conduct the study: varying the pyrolysis atmosphere and implementing highly dispersed nickel atomic clusters (Ni-clusters) in the coating matrix undergoing pyrolysis. A thermally reactive polymer precursor is developed to allow the above two measures to act more effectively. The various pyrolysis atmospheres employed include inert Ar, a reducing H₂/N₂ gas mixture, and weak oxidizing CO₂. In the absence of the Ni-clusters, the H₂/N₂ atmosphere restrains the extent of graphitization through a chain transfer effect of H₂ that ceases the free radical chain propagation, whereas CO₂, owing to its high critical temperature (T_c) nature, shows the capability to reduce nanograin sizes. As for the catalytic roles of the embedded Ni-clusters, they vary with the pyrolysis atmosphere applied: offering coke nuclei for the growth of carbonaceous grains in Ar, enhancing gasification of carbon in CO₂, and repressing the extent of aromatization via hydrogenation in H₂/N₂. The carbonaceous membranes (CnMs) obtained under the above pyrolysis conditions are distinguished by the distribution density and structure of NCs evolved, which locate primarily in the boundaries of nanograins. The NF of an aqueous solution of methylene blue (MB, 10 ppmw) is utilized to assess these CnMs to show impacts of the NCs on the separation performance.