Hierarchical Petal-on-Petal MnO2/Vertical Graphene Foam for Postplasma Catalytic Decomposition of Toluene with High Efficiency and Ultralow Pressure Drop

Review of Vertical Graphene and its Applications

Vertical graphene (VG) is a thin-film complex material featuring hierarchical microstructures: graphene-containing carbon nanosheets growing vertically on its deposition substrate, few-layer graphene basal layers, and chemically active atomistic defect sites and edges. Thanks to the fundamental characteristics of graphene materials, e.g. excellent electrical conductivity, thermal conductivity, chemical stability, and large specific surface area, VG materials have been successfully implemented into various niche applications which are strongly associated with their unique morphology. The microstructure of VG materials can be tuned by modifying growth methods and the parameters of growth processes. Multiple growth processes have been developed to address faster, safer, and mass production methods of VG materials, as well as accommodating various applications. VG's successful applications include field emission, supercapacitors, fuel cells, batteries, gas sensors, biochemical sensors, electrochemical analysis, strain sensors, wearable electronics, photo trapping, terahertz emission, etc. Research topics on VG have been more diversified in recent years, indicating extensive attention from the research community and great commercial value. In this review article, VG's morphology is briefly reviewed, and then various growth processes are discussed from the perspective of plasma science. After that, the most recent progress in its applications and related sciences and technologies are discussed.

Solar-Enhanced Plasma-Catalytic Oxidation of Toluene over a Bifunctional Graphene Fin Foam Decorated with Nanofin-like MnO2

In this work, we propose a hybrid and unique process combining solar irradiation and post-plasma catalysis (PPC) for the effective oxidation of toluene over a highly active and stable MnO₂/GFF (bifunctional graphene fin foam) catalyst. The bifunctional GFF, serving as both the catalyst support and light absorber, is decorated with MnO₂ nanofins, forming a hierarchical fin-on-fin structure. The results show that the MnO₂/GFF catalyst can effectively capture and convert renewable solar energy into heat (absorption of >95%), leading to a temperature rise (55.6 °C) of the catalyst bed under solar irradiation (1 sun, light intensity 1000 W m^-2). The catalyst weight (9.8 mg) used in this work was significantly lower (10-100 times lower) than that used in previous studies (usually 100-1000 mg). Introducing solar energy into the typical PPC process via solar thermal conversion significantly enhances the conversion of toluene and CO₂ selectivity by 36-63%, reaching ∼93% for toluene conversion and ∼83% for CO₂ selectivity at a specific input energy of ∼350 J L^-1, thus remarkably reducing the energy consumption of the plasma-catalytic gas cleaning process. The energy efficiency for toluene conversion in the solar-enhanced post-plasma catalytic (SEPPC) process reaches up to 12.7 g kWh^-1, ∼57% higher than that using the PPC process without solar irradiation (8.1 g kWh^-1), whereas the energy consumption of the SEPPC process is reduced by 35-52%. Moreover, the MnO₂/GFF catalyst exhibits an excellent self-cleaning capability induced by solar irradiation, demonstrating a superior long-term catalytic stability of 72 h at 1 sun, significantly better than that reported in previous works. The prominent synergistic effect of solar irradiation and PPC with a synergistic capacity of ∼42% can be mainly attributed to the solar-induced thermal effect on the catalyst bed, boosting ozone decomposition (an almost triple enhancement from ∼0.18 g_O3 g^-1 h^-1 for PPC to ∼0.52 g_O3 g^-1 h^-1 for SEPPC) to generate more oxidative species (e.g., O radicals) and enhancing the catalytic oxidation on the catalyst surfaces, as well as the self-cleaning capacity of the catalyst at elevated temperatures driven by solar irradiation. This work opens a rational route to use abundant, renewable solar power to achieve high-performance and energy-efficient removal of volatile organic compounds.