Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.

Catalysis with two-dimensional materials and their heterostructures

Major challenges encountered when trying to replace precious-metal-based electrocatalysts of the hydrogen evolution reaction (HER) in acidic media are related to the low efficiency and stability of non-precious-metal compounds. Therefore, new concepts and strategies have to be devised to develop electrocatalysts that are based on earth-abundant materials. Herein, we report a hierarchical architecture that consists of ultrathin graphene shells (only 1-3 layers) that encapsulate a uniform CoNi nanoalloy to enhance its HER performance in acidic media. The optimized catalyst exhibits high stability and activity with an onset overpotential of almost zero versus the reversible hydrogen electrode (RHE) and an overpotential of only 142 mV at 10 mAcm(-2), which is quite close to that of commercial 40% Pt/C catalysts. Density functional theory (DFT) calculations indicate that the ultrathin graphene shells strongly promote electron penetration from the CoNi nanoalloy to the graphene surface. With nitrogen dopants, they synergistically increase the electron density on the graphene surface, which results in superior HER activity on the graphene shells.

Enhanced Electron Penetration through an Ultrathin Graphene Layer for Highly Efficient Catalysis of the Hydrogen Evolution Reaction

Selective catalytic hydrogenation has wide applications in both petrochemical and fine chemical industries, however, it remains challenging when two or multiple functional groups coexist in the substrate. To tackle this challenge, the "active site isolation" strategy has been proved effective, and various approaches to the site isolation have been developed. In this review, we have summarized these approaches, including adsorption/grafting of N/S-containing organic molecules on the metal surface, partial covering of active metal surface by metal oxides either via doping or through strong metal-support interaction, confinement of active metal nanoparticles in micro- or mesopores of the supports, formation of bimetallic alloys or intermetallics or core@shell structures with a relatively inert metal (IB and IIB) or nonmetal element (B, C, S, etc.), and construction of single-atom catalysts on reducible oxides or inert metals. Both advantages and disadvantages of each approach toward the site isolation have been discussed for three types of chemoselective hydrogenation reactions, including alkynes/dienes to monoenes, α,β-unsaturated aldehydes/ketones to the unsaturated alcohols, and substituted nitroarenes to the corresponding anilines. The key factors affecting the catalytic activity/selectivity, in particular, the geometric and electronic structure of the active sites, are discussed with the aim to extract fundamental principles for the development of efficient and selective catalysts in hydrogenation as well as other transformations.

Selective Hydrogenation over Supported Metal Catalysts: From Nanoparticles to Single Atoms

The oxygen evolution reaction (OER) is recognized as a key half-reaction in water electrolysis for clean hydrogen energy, which is kinetically not favored and usually requires precious metal catalysts such as IrO2 and RuO2 to reduce the overpotential. The major challenge in using non-precious metals in place of these precious metal catalysts for OER is their low efficiency and poor stability, which urgently demand new concepts and strategies to tackle this issue. Herein, we report a universal strategy to directly synthesize single layer graphene encapsulating uniform earth-abundant 3d transition-metal nanoparticles such as Fe, Co, Ni and their alloys in a confined channel of mesoporous silica. The single atomic thickness of the graphene shell immensely promotes the electron transfer from the encapsulated metals to the graphene surface, which efficiently optimizes the electronic structure of the graphene surface and thereby triggers the OER activity of the inert graphene surface. We investigated a series of non-precious 3d metals encapsulated within single layer graphene, and found that the encapsulated FeNi alloy showed the best OER activity in alkaline solutions with only 280 mV overpotential at 10 mA cm−2, and also possessed a high durability after 10 000 cycles. Both the activity and durability of the non-precious catalyst even exceed that of the commercial IrO2 catalyst, showing great potential to replace precious metal catalysts in the OER.

Single layer graphene encapsulating non-precious metals as high-performance electrocatalysts for water oxidation

Nitroarene reduction: a trusted model reaction to test nanoparticle catalysts

A nanocomposite able to function as a hydrogenation catalyst under strongly acidic conditions without the presence of noble metals is synthesized and thoroughly studied. This specially designed catalyst possesses a unique structure composed of carbon nitride (CN) with underlying nickel, in which the nickel endows the CN with new active sites for hydrogen adsorption and activation while it itself is physically isolated from the reactive environment and protected from poisoning or loss. The CN is inert for hydrogenation without the help of nickel. The catalyst shows good performance for hydrogenation of nitro compounds under strong acidic conditions, including the one-step hydrogenation of nitrobenzene in 1.5 M H2SO4 to produce p-amoniophenol, for which the acid in the reaction system has restricted the catalyst only to noble metals in previous studies. Further characterization has demonstrated that the nickel in the catalyst is in an electron-deficient state because some of its electron has been donated to...

Acid-Resistant Catalysis without Use of Noble Metals: Carbon Nitride with Underlying Nickel

A series of catalysts made of tungsten oxide loaded on SiO 2 , γ-Al 2 O 3 , SiO 2 –Al 2 O 3 and silica deposited γ-alumina are tested for 1-butene metathesis. Among these catalysts, the catalyst 6W/20SiO 2 /Al 2 O 3 gives the highest activity for 1-butene metathesis reaction with 1-butene conversion up to 71 mol% and the yield of propene up to 21 mol%. The excellent catalytic activity is related to the moderate dispersion of tungsten oxide and the suitable acidity of the support. The dispersion of WO x species and the acidity of supports were studied by characterization of XRD, Raman spectra, UV–vis, H 2 -TPR and NH 3 -TPD in detail. The surface properties of silica modified γ-alumina leads to the moderate aggregation of supported tungsten oxide, which appears to be more effective for 1-butene metathesis at the low temperature of 453 K. Optimized activity was realized by tuning the dispersion of tungsten species on silica deposited alumina.

Optimizing activity of tungsten oxides for 1-butene metathesis by depositing silica on γ-alumina support

The conversion of sugars to chemicals in aqueous-phase is especially important for the utilization of biomass. In current work, zirconium phosphate obtained by hydrothermal methods using organic amines as templates has been examined as a solid catalyst for the dehydration reaction of xylose to furfural in aqueous-phase. The use of dodecylamine and hexadecylamine in the synthesis process results in mesoporous zirconium phosphate with uniform pore width of ∼2 nm and in morphology of nanoaggregates, which is characterized by powder X-ray diffraction, N2 isothermal sorption, NH3 temperature-programmed desorption, FT-IR, and 31P MAS NMR spectroscopy. When used as a catalyst for xylose dehydration to furfural in aqueous-phase, the mesoporous zirconium phosphate presents excellent catalytic performance with high conversions up to 96% and high furfural yields up to 52% in a short time of reaction. Moreover, the catalyst is easily regenerated by thermal treatment in air and shows quite stable activity. The open structure with numerous active sites of the Bronsted/Lewis acid sites is responsible for the high catalytic efficiency of mesoporous zirconium phosphate.

High performance mesoporous zirconium phosphate for dehydration of xylose to furfural in aqueous-phase

Direct conversion of corn cob to formic and acetic acids over nano oxide catalysts

1-Butene, abundant but inexpensive in industry, can be used to produce more valuable propene and ethene through the well-known metathesis reaction. In terms of catalyst system of molybdena, controlling the interaction between the molybdena and the support is quite crucial. A special support, i.e., well-shaped one-dimensional γ-Al2O3 with uniform diameters of ∼10 nm and lengths of ∼100 nm, has been synthesized via oleylamine-assisted hydrothermal method. The molybdenum oxides are well controlled in sizes of moderate aggregation suitable for 1-butene metathesis with a fairly high rate of propene production (38.9 × 10−9 mol m−2 s−1, 30 mol% yield in a single pass) under mild conditions. The status of MoOx species in the catalyst has been investigated by solid state 29Si MAS NMR, Raman, UV–vis, H2-TPR and NH3-TPD techniques. The catalyst, highly stable after twelve times of regeneration, is important for practical utilization for the metathesis reactions.

Yuming Cui

Papers

Acid-Resistant Catalysis without Use of Noble Metals: Carbon Nitride with Underlying Nickel

Optimizing activity of tungsten oxides for 1-butene metathesis by depositing silica on γ-alumina support

High performance mesoporous zirconium phosphate for dehydration of xylose to furfural in aqueous-phase

Direct conversion of corn cob to formic and acetic acids over nano oxide catalysts

Efficient self-metathesis of 1-butene on molybdenum oxide supported on silica modified one-dimensional γ-Al 2 O 3