Efficient Hydrogen Peroxide Generation Utilizing Photocatalytic Oxygen Reduction at a Triphase Interface

摘要

class="head no_bottom_margin" id="sec1title">IntroductionHydrogen peroxide (H2O2) is a valuable chemical with rapidly growing demand in a wide variety of industrial areas, including fuel cells, chemical oxidation, environment protection, and paper and textile industries (). The global H2O2 market demand is expected to reach 6,000 kilotons in 2024 (). Currently, industrial processes for H2O2 synthesis involve the multistep anthraquinone oxidation, which requires complex large-scale infrastructure and large amounts of energy. Thus, developing efficient and cost-effective alternative routes for H2O2 generation is of ongoing importance (, , , , ).The photocatalytic reduction of oxygen to H2O2 has received great attention as it requires only light, water, and O2 (, , , , , , , , href="#bib25" rid="bib25" class=" bibr popnode">Shiraishi et al., 2014, href="#bib27" rid="bib27" class=" bibr popnode">Sorcar et al., 2018, href="#bib30" rid="bib30" class=" bibr popnode">Teranishi et al., 2010, href="#bib31" rid="bib31" class=" bibr popnode">Teranishi et al., 2016, href="#bib32" rid="bib32" class=" bibr popnode">Wang et al., 2015). During the reaction photogenerated conduction band (CB) electrons reduce O2 to produce H2O2; O2 + 2e⁻CB + 2H⁺_aq→ H₂O₂ [(O₂/H₂O₂) = 0.695 V versus normal hydrogen electrode (NHE)]. However, to date resultant product concentrations have been quite limited. The low production rate can be ascribed to the following aspects, which may not be strictly independent of one another: first, the low concentration and slow diffusion rate of O₂ in liquid phase results in deficient accessibility of the photocatalysts to reactant; second, the recombination of electrons and holes limits the electron utilization efficiency, and such limitation becomes more serious in the presence of higher charge carrier concentrations associated with greater light intensities; third, the degradation of H₂O₂ by photogenerated charge carriers also reduces the product yield.The performance of interfacial catalytic reactions is generally governed by the interface environment. Herein, we simultaneously address these limitations by demonstrating a reaction system possessing an air-liquid-solid triphase reaction interface as illustrated in href="/pmc/articles/PMC6606954/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1A, where the nanostructured semiconductors are deposited on the top surface of a porous superhydrophobic substrate. Learning from nature, based on the cooperative effect between the low surface energy and rough surface structure, superhydrophobic substrates have been fabricated and used in a wide variety of fields (href="#bib1" rid="bib1" class=" bibr popnode">Aebisher et al., 2013, href="#bib6" rid="bib6" class=" bibr popnode">Deng et al., 2012, href="#bib9" rid="bib9" class=" bibr popnode">Feng et al., 2002, href="#bib8" rid="bib8" class=" bibr popnode">Feng and Jiang, 2006, href="#bib11" rid="bib11" class=" bibr popnode">Hong et al., 2007, href="#bib18" rid="bib18" class=" bibr popnode">Lafuma and Quéré, 2003, href="#bib19" rid="bib19" class=" bibr popnode">Lei et al., 2016, href="#bib29" rid="bib29" class=" bibr popnode">Su et al., 2016, href="#bib33" rid="bib33" class=" bibr popnode">Wooh et al., 2017, href="#bib34" rid="bib34" class=" bibr popnode">Wu et al., 2014, href="#bib35" rid="bib35" class=" bibr popnode">Yohe et al., 2012). When immersed in water the superhydrophobic substrate traps air within atmosphere-linked air pockets, resulting in an interface where solid, liquid, and air three phases coexist (href="#bib9" rid="bib9" class=" bibr popnode">Feng et al., 2002, href="#bib18" rid="bib18" class=" bibr popnode">Lafuma and Quéré, 2003). The triphase system allows reactant O₂ to diffuse directly from the air phase to the reaction interface, rather than by slow diffusion through the liquid. Benefiting from this interface architecture the accessibility of the photocatalyst to O₂ is greatly increased, which in turn (1) enhances the reaction rate between O₂ and photogenerated electrons, (2) suppresses the electron-hole recombination and increases the charge utilization efficiency, and (3) reduces the degradation reaction between H₂O₂ and photogenerated electrons, thus leading to much enhanced rates of H₂O₂ production.href="/pmc/articles/PMC6606954/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">class="inline_block ts_canvas" href="/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6606954_gr1.jpg" target="tileshopwindow">target="object" href="/pmc/articles/PMC6606954/figure/fig1/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC6606954/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">Figure 1Schematic Illustration of the Triphase Photocatalytic Reaction System(A) Photocatalysts are immobilized on the porous superhydrophobic substrate.(B) Enlarged view of the solid-liquid-air triphase reaction zone. Reactant O₂ is rapidly delivered from the air to the reaction interface resulting in a significantly enhanced rate of H₂O₂ production.