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.

Atomically dispersed noble metal catalysts often exhibit high catalytic performances, but the metal loading density must be kept low (usually below 0.5%) to avoid the formation of metal nanoparticles through sintering. We report a photochemical strategy to fabricate a stable atomically dispersed palladium-titanium oxide catalyst (Pd1/TiO2) on ethylene glycolate (EG)-stabilized ultrathin TiO2 nanosheets containing Pd up to 1.5%. The Pd1/TiO2 catalyst exhibited high catalytic activity in hydrogenation of C=C bonds, exceeding that of surface Pd atoms on commercial Pd catalysts by a factor of 9. No decay in the activity was observed for 20 cycles. More important, the Pd1/TiO2-EG system could activate H2 in a heterolytic pathway, leading to a catalytic enhancement in hydrogenation of aldehydes by a factor of more than 55.

Ultrathin metal films can exhibit quantum size and surface effects that give rise to unique physical and chemical properties. Metal films containing just a few layers of atoms can be fabricated on substrates using deposition techniques, but the production of freestanding ultrathin structures remains a significant challenge. Here we report the facile synthesis of freestanding hexagonal palladium nanosheets that are less than 10 atomic layers thick, using carbon monoxide as a surface confining agent. The as-prepared nanosheets are blue in colour and exhibit a well-defined but tunable surface plasmon resonance peak in the near-infrared region. The combination of photothermal stability and biocompatibility makes palladium nanosheets promising candidates for photothermal therapy. The nanosheets also exhibit electrocatalytic activity for the oxidation of formic acid that is 2.5 times greater than that of commercial palladium black catalyst.

Noble metal nanoparticles stabilized by organic ligands are important for applications in assembly, site-specific bioconjugate labelling and sensing, drug delivery and medical therapy, molecular recognition and molecular electronics, and catalysis. Here we report crystal structures and theoretical analysis of three Ag 44 (SR) 30 and three Au 12 Ag 32 (SR) 30 intermetallic nanoclusters stabilized with fluorinated arylthiols (SR ¼ SPhF, SPhF 2 or SPhCF 3 ). The nanocluster forms a Keplerate solid of concentric icosahedral and dodecahedral atom shells, protected by six Ag 2 (SR) 5 units. Positive counterions in the crystal indicate a high negative charge of 4 À per nanoparticle, and density functional theory calculations explain the stability as an 18-electron superatom shell closure in the metal core. Highly featured optical absorption spectra in the ultraviolet-visible region are analysed using time-dependent density functional perturbation theory. This work forms a basis for further understanding, engineering and controlling of stability as well as electronic and optical properties of these novel nanomaterials.

Noble-metallic nanoparticles have attracted increasing research attention during the past decades due to their interesting sizedependent optical, electronic, and catalytic properties. 1,2 Nanoparticles with a narrow size distribution can further function as building blocks for the construction of higher-ordered superlattices that exhibit collective properties of individual nanoparticles. [3][4][5][6][7][8] Although several synthetic routes of noble-metallic nanoparticles have been developed, the challenge remains of obtaining monodisperse nanoparticles with size <10 nm on a large scale. Since the first report in 1994, the syntheses of metallic nanoparticles with size less than 10 nm have been dominated by the Brust method, a twophase protocol that can be easily scaled up to gram scale. However, the nanoparticles prepared by the Brust method and its variations typically have a continuous and broad size distribution in the range of 1-4 nm. 9,10 Similarly, the method based on the solvated metal atom dispersion technique is suitable for preparation of metal nanoparticles on the gram scale, 6,11 but post-heat treatment is generally required for good size dispersivity.Recently, efforts have been made to develop one-phase syntheses in which the reduction of metal takes place homogeneously in a selected organic solvent rather than at the two-phase interface as in the Brust method. 12-15 Even though these one-phase syntheses have been shown to significantly narrow the particle size distribution, to our best knowledge, monodisperse metallic particles with size dispersivity <5% have not yet been reported by using any one-phase synthesis without a subsequent size-selection process.We report here a facile one-step one-phase synthetic route to achieve a variety of metallic nanoparticles by using amine-borane complexes as reducing agents. With the use of different metal sources, both mono-and alloyed metallic nanoparticles with a narrow size distribution can be obtained in a single step on a gram scale. The synthesized nanoparticles are ready to function as building blocks for the formation of large colloidal crystals ( Figure 1) directly from the reaction mixtures.All syntheses were carried out in air by mixing metal source(s) and capping ligand (e.g., thiols) in an organic solvent, such as benzene, toluene, or chloroform. An amine-borane complex was then added to the mixture and stirred until the reduction was complete. As an example, dodecanethiol-capped gold nanoparticles were prepared as follows: 0.25 mmol AuPPh 3 Cl was mixed with 0.125 mL of dodecanethiol in 20 mL of benzene to form a clear solution to which 2.5 mmol of tert-butylamine-borane complex was then added. The color of the mixture darkened gradually and became purple-red after stirring at 55°C for 5 min. TEM samples were prepared by dipping carbon-coated Cu TEM grids directly into the solution and drying in air for at least 2 h. As shown in Figure 1A, long-range close-packed superlattices of 6.2 nm gold nanoparticles ( Figure 1A) can be obtained even without ...

Hybrid metal nanoparticles can allow separate reaction steps to occur in close proximity at different metal sites and accelerate catalysis. We synthesized iron-nickel hydroxide-platinum (transition metal-OH-Pt) nanoparticles with diameters below 5 nanometers and showed that they are highly efficient for carbon monoxide (CO) oxidation catalysis at room temperature. We characterized the composition and structure of the transition metal-OH-Pt interface and showed that Ni(2+) plays a key role in stabilizing the interface against dehydration. Density functional theory and isotope-labeling experiments revealed that the OH groups at the Fe(3+)-OH-Pt interfaces readily react with CO adsorbed nearby to directly yield carbon dioxide (CO2) and simultaneously produce coordinatively unsaturated Fe sites for O2 activation. The oxide-supported PtFeNi nanocatalyst rapidly and fully removed CO from humid air without decay in activity for 1 month.

Tuning the electronic structure of heterogeneous metal catalysts has emerged as an effective strategy to optimize their catalytic activities. By preparing ethylenediamine-coated ultrathin platinum nanowires as a model catalyst, here we demonstrate an interfacial electronic effect induced by simple organic modifications to control the selectivity of metal nanocatalysts during catalytic hydrogenation. This we apply to produce thermodynamically unfavourable but industrially important compounds, with ultrathin platinum nanowires exhibiting an unexpectedly high selectivity for the production of N-hydroxylanilines, through the partial hydrogenation of nitroaromatics. Mechanistic studies reveal that the electron donation from ethylenediamine makes the surface of platinum nanowires highly electron rich. During catalysis, such an interfacial electronic effect makes the catalytic surface favour the adsorption of electron-deficient reactants over electron-rich substrates (that is, N-hydroxylanilines), thus preventing full hydrogenation. More importantly, this interfacial electronic effect, achieved through simple organic modifications, may now be used for the optimization of commercial platinum catalysts.

High-index surfaces of a face-centered cubic metal (e.g., Pd, Pt) have a high density of low-coordinated surface atoms and therefore possess enhanced catalysis activity in comparison with low-index faces. However, because of their high surface energy, the challenge of chemically preparing metal nanocrystals having high-index facets remains. We demonstrate in this work that introducing amines as the surface controller allows concave Pt nanocrystals having {411} high-index facets to be prepared through a facile wet-chemical route. The as-prepared Pt nanocrystals display a unique octapod morphology with {411} facets. The presence of high-index {411} exposed facets endows the concave Pt nanocrystals with excellent electrocatalytic activity in the oxidation of both formic acid and ethanol.