(Nanowerk Spotlight) Chemical manufacturing underpins modern civilization – fuels, plastics, fertilizers and pharmaceuticals all rely on it. Yet many legacy production routes exact heavy tolls on the planet and human health. New catalytic techniques aim to leapfrog current processes with drastically lower carbon footprints and less waste.
Take hydrogen peroxide, a versatile oxidant finding uses from oven cleaning solutions to semiconductor etching. Conventional anthraquinone-based factories operate in a linear fashion: natural gas feeds boilers and furnaces heating reactions up to 130 °C in hydrogen rich atmospheres to afford peroxide in just 50-70% yields. Extensive distillations and solvent extractions purify the product from contaminating acetone and other organics.
What if localized solar and wind electricity could instead electrochemically convert water and oxygen to hydrogen peroxide without high temperatures or unwanted byproducts? That sustainable vision seems within reach thanks to rapid advances marrying metal-organic frameworks (MOFs) and graphene. But formidable obstacles to efficiently pairing those technologies still remain.
As crystalline compounds comprising metal nodes linked by organic molecules, MOFs boast incredibly high internal surface areas rivaling the best activated carbons. That asset translates into abundant catalytic active sites tunable simply by substituting different metals or organic ligands. Cobalt-containing MOFs, in particular, balance activity and selectivity for the two-electron oxygen reduction half reaction to hydrogen peroxide. Yet their poor electrical conductivity hampers performance. They also readily dissolve and degrade over time in solutions.
Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, boasts remarkable conductivity, surface area and mechanical strength. Those properties led scientists to consider it as a support structure for fixing MOF particles. The high surface area grants ample sites to anchor the MOF while improving conductivity. Sandwiching MOFs between graphene layers may also bolster chemical resilience.
Unfortunately, earlier fabrication attempts showed limited success. Most methods demand extreme temperatures, pressures or caustic chemicals to produce graphene-MOF composites. MOF particles failed to bind uniformly across graphene surfaces as well. And the harsh processing conditions themselves diminished the coveted properties of both components.
Seeking a simpler path, a research team at the University of New South Wales turned to vertical graphene – a material composed of perpendicular sheets grown on substrates via plasma-enhanced chemical vapor deposition. The technique creates abundant defect sites on graphene edges and surfaces rather than the pristine basal planes. And the vertical alignment ensures full access and reactions with solutions.
Schematic illustration of the preparation of VG-ZIF-67 from the vertical graphene via a single-step impregnation method. (Image: Reprinted with permission by Wiley-VCH Verlag)
Reporting their findings in Advanced Materials (“Graphene and MOF Assembly: Enhanced Fabrication and Functional Derivative via MOF Amorphization”), the researchers discovered that by merely dipping vertical graphene samples into MOF precursor solutions for minutes at room temperature, uniform coatings self-assembled. Three different MOF varieties – ZIF-7, ZIF-8 and ZIF-67 – all successfully attached as 20-130 nm particles on the graphene without surfactants or other processing aids.
The crucial role of atomic hydrogen defects on vertical graphene driving MOF assembly became clear when the team repeated experiments with samples subjected to annealing to remove defects. Far fewer MOF nanoparticles attached afterwards. The abundant hydrogen defects are thought to energetically favor adsorption and crystallization of MOF precursors.
But perfect MOF crystals pose challenges for catalyzing electrochemical reactions. So, the group examined converting anchored ZIF-67 particles into amorphous films. Adding an ionic liquid as a stabilizing agent before heating to 400 °C generated a 30 nm coating maintaining short-range molecular bonding yet losing long-range order. This architecture prevented fracturing while keeping essential chemical motifs for reactivity.
When tested for the two-electron oxygen reduction half-reaction to hydrogen peroxide, the composite catalyst paired high activity and exceptional selectivity above 95%. It also proved stable for over 20 hours. The durability arises from strong chemical affinity between the vertical graphene and amorphized MOF components mitigating dissolution.
Graphene-MOF catalysts hold much promise across application spaces from renewable chemical production to batteries to carbon capture. But unlocking their potential at commercial scales requires competitive manufacturing costs. This research puts that goal within reach by simplifying preparation. Merely dipping inexpensive, defect-laden vertical graphene into readily available MOF precursor solutions reliably generates intricate hybrid architectures. Tuning subsequent processing conditions further customizes structures and properties as needed for given reactions or operating environments.
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