Chemical Science 2022, 13, 3140‒3146
Graphene is a carbon material made from planarly arranged hexagons, showing notable physical properties. When pentagons are introduced to the plain, fullerene as a 0-dimensional sphere could be obtained with a positive curvature. When wrapped cylindrically, carbon nanotubes as 1-dimensional materials could be obtained. On the contrary, introduction of heptagons and octagons into the graphene architecture will lead to a non-planar structure with a negative curvature, and the presence of the corresponding ordered 3-dimensional graphene materials has been predicted by Mackay and co-workers in 1992. Figure 1A shows the schematic of Minimum-Surface Graphene.
However, the synthesis of such an ideal 3D graphene material has yet to be achieved.
Many research groups have approached to this ideal material, and one of the state-of-the-art material would be zeolite-templated carbon (ZTC): Prof. Takashi Kyotani in Tohoku University reported the synthesis of ZTC by chemical vapor deposition (CVD) of propylene and other hydrocarbons, and ZTC showed an ordered structure of the unit cell with 1.2 nm periodicity as confirmed by XRD, which reflected the ordered structure of the corresponding zeolite template. However, detailed analysis of ZTC including Raman spectroscopy and temperature-programmed desorption of gases showed that ZTC is amorphous carbon with many edge-defects rather than "graphene." On the contrary, Dr. Takahiro Morishita, Prof. Michio Inagaki, and co-workers reported that the use of MgO as a template of CVD-based carbonization led to high-quality porous graphene materials. Later, we also reported that chemically stable methane (CH4) would be deposited onto the surfaces of alumina nanoparticles (Figure 1B) to give high-quality nanoporous graphene (NPG). A structural regularity of ZTC and a high-quality of graphene (NPG) should be achieved at the same time for the synthesis of "minimum surface porous graphene materials," but currently we have no template materials that can tolerate high-temperature CH4-CVD: We recognized that CH4-CVD at high temperatures led to the thermal collapse of the structures when many conventional templates were tested. If we reduce the reaction temperature of CH4-CVD by enhancing the reaction rate, the synthesis of "minimum surface porous graphenoid materials" could be ensured using the present template materials available. To this aim, we investigated detailed surface chemistry and kinetics during CH4-CVD on alumina nanoparticles, to elucidate the key factor governing the CH4-CVD reaction.
Practically, the kinetic analysis of CH4-CVD on γ‒alumina nanoparticles at various temperatures (Figure 1C,D) was coupled with computational chemistry using density functional theory (DFT) to elucidate the surface chemistry. As a consequence, we found the followings:
Thus, activation of methane on alumina nanoparticles was kinetically feasible to give porous nanographene (NPG) materials with no use of transition metal active centers by CH4-CVD conditions, and the early-stage activation of CH4 has been supported by both experimental and computational chemistry. This study shows that a surface oxygen defect formed by high-temperature reaction with CH4 plays a crucial role in CH4-CVD. Activation of chemically stable CH4 was initiated by the single proton transfer (PT) step, and the PT from CH4 is the rate-limiting step of the whole process in CH4-CVD. Advanced surface engineering for defects will improved the reactivity to CH4 and lowering the reaction temperature of CH4 activation on surfaces of metal oxides, and this will enable us to synthesizing "ideal 3D nanoporous graphenes" by CH4-CVD using a readily available template having ordered nanoporosity in the future.
This work was supported by Grants-in-Aid (19K15281) from Japan Society for the Promotion of Science (JSPS), the Ebara Hatakeyama Memorial Foundation, and Ensemble Grant for Early Career Researchers at Tohoku University.