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

Abstract

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 MnO 2 /GFF (bifunctional graphene fin foam) catalyst. The bifunctional GFF, serving as both the catalyst support and light absorber, is decorated with MnO 2 nanofins, forming a hierarchical fin-on-fin structure. The results show that the MnO 2 /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 CO 2 selectivity by 36−63%, reaching ∼93% for toluene conversion and ∼83% for CO 2 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 MnO 2 /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 O 3 g −1 h −1 for PPC to ∼0.52 g O 3 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 energyefficient removal of volatile organic compounds.

The Adsorption of Ozone on the Solid Catalyst Surface and the Catalytic Reaction Mechanism for Organic Components

“…Because the Mn atoms, which had a higher electron cloud density, were supported on graphene aerogel, it could enhance the adsorption efficiency of active sites and could be beneficial to yield hydroxyl radicals [23a] . Bo team supported MnO 2 on graphene fin foam (GFF) to prepare the MnO 2 /GFF catalyst [23b] . GFF was bifunctional; that was, it not only acted as the catalyst support but also was a light absorber, which could effectively capture and convert solar energy into heat (absorption of >95 %) [23b] .…”

“…Bo team supported MnO 2 on graphene fin foam (GFF) to prepare the MnO 2 /GFF catalyst [23b] . GFF was bifunctional; that was, it not only acted as the catalyst support but also was a light absorber, which could effectively capture and convert solar energy into heat (absorption of >95 %) [23b] . With the synergistic effect of solar irradiation and MnO 2 /GFF catalyst, the ozone decomposition was boosted from about 0.18 to 0.52 g (O3) g −1 h −1 , as well as showed the self‐cleaning capacity to greatly prolongs the catalyst life [23b] …”

“…In the third type, ozone was directly decomposed into atomic oxygen at oxygen vacancies on the catalyst surface; or ozone formed a six‐member ring intermediate with hydroxyl groups produced by the adsorption of water molecules on oxygen vacancies, and then the intermediate decomposed to hydroxyl radicals [12b–e,l–o,q,s‐u,29d] . The fourth type was a photocatalytic reaction, that is, photogenerated electrons migrated from metal to ozone, causing ozone to decompose into free radicals [23b,24a,31] …”

MnO2‐Based Materials for Environmental Applications

“…g) Reproduced with permission. [ 293 ] Copyright 2020, American Chemistry Society. h) Schematic illustration of the fabrication route of the GMA.…”

“…Moreover, nanofin‐like MnO 2 /bifunctional graphene fin foam catalysts were fabricated by the same research group (Figure 17g). [ 293 ] In this catalytic system, bifunctional graphene foam, acting as catalyst carrier and light absorber, was modified by MnO 2 nanofins, forming a multilayer fin‐to‐fin structure. After introducing solar energy and plasma into the as‐fabricated catalytic system, the system showed excellent catalytic performance for toluene degradation.…”