Single-Atom Catalysts Based on the Metal-Oxide Interaction.

Metal atoms dispersed on the oxide supports constitute a large category of single-atom catalysts. In this review, oxide supported single-atom catalysts are discussed about their synthetic procedures, characterizations, and reaction mechanism in thermocatalysis, such as water-gas shift reaction, selective oxidation/hydrogenation, and coupling reactions. Some typical oxide materials, including ferric oxide, cerium oxide, titanium dioxide, aluminum oxide, and so on, are intentionally mentioned for the unique roles as supports in anchoring metal atoms and taking part in the catalytic reactions. The interactions between metal atoms and oxide supports are summarized to give a picture on how to stabilize the atomic metal centers, and rationally tune the geometric structures and electronic states of single atoms. Furthermore, several directions in fabricating single-atom catalysts with improved performance are proposed on the basis of state-of-the-art understanding in metal-oxide interactions.

Controllable and scalable synthesis of ordered mesoporous silica nanosheets by using acidified g-C3N4 as a lamellar surfactant.

Two-dimensional (2D) mesoporous nanomaterials are required in catalysis, separation, adsorption, and energy storage fields due to their outstanding mass transfer performance. However, their fabrication via the 'bottom up' strategy has been rarely reported and is limited by the difficulties in obtaining a versatile and accessible structure-directing agent. Here, ultrathin mesoporous silica nanosheets (MSN) were successfully synthesized by employing acidified g-C3N4 as a structural directing agent owing to its natural layered structure, stoichiometric solubility, and amphiphilicity. The thickness of MSN is readily adjustable by tuning the dosage of acidified g-C3N4 during the fabrication process, and when the mass ratio of silica/acidified-g-CzN4 is 10, the thickness of the MSN is 6-9 nm. TEM, SAXRD, and BET analysis demonstrated the mesoporous characteristics of MSN with a long-range ordered hexagonal arrangement symmetry, a uniform pore size distribution around 2.9 nm, and high BET surface areas of 1000-1150 m2 g-1. The superior mass-transfer performance of MSN in catalysis applications, which was derived from its special structure, was confirmed by the outstanding methane combustion activity of MSN supported Co3O4 catalysts. This work provides a controllable and scalable 'bottom up' fabrication method for 2D porous material, and also opens up an alternative application for g-C3N4.

Modulating the strong metal-support interaction of single-atom catalysts via vicinal structure decoration.

Metal-support interaction predominately determines the electronic structure of metal atoms in single-atom catalysts (SACs), largely affecting their catalytic performance. However, directly tuning the metal-support interaction in oxide supported SACs remains challenging. Here, we report a new strategy to subtly regulate the strong covalent metal-support interaction (CMSI) of Pt/CoFe2O4 SACs by a simple water soaking treatment. Detailed studies reveal that the CMSI is weakened by the bonding of H+, generated from water dissociation, onto the interface of Pt-O-Fe, resulting in reduced charge transfer from metal to support and leading to an increase of C-H bond activation in CH4 combustion by more than 50 folds. This strategy is general and can be extended to other CMSI-existed metal-supported catalysts, providing a powerful tool to modulating the catalytic performance of SACs.

Size-dependent strong metal-support interaction in TiO2 supported Au nanocatalysts.

The strong metal-support interaction (SMSI) has long been studied in heterogonous catalysis on account of its importance in stabilizing active metals and tuning catalytic performance. As a dynamic process taking place at the metal-support interface, the SMSI is closely related to the metal surface properties which are usually affected by the size of metal nanoparticles (NPs). In this work we report the discovery of a size effect on classical SMSI in Au/TiO2 catalyst where larger Au particles are more prone to be encapsulated than smaller ones. A thermodynamic equilibrium model was established to describe this phenomenon. According to this finding, the catalytic performance of Au/TiO2 catalyst with uneven size distribution can be improved by selectively encapsulating the large Au NPs in a hydrogenation reaction. This work not only brings in-depth understanding of the SMSI phenomenon and its formation mechanism, but also provides an alternative approach to refine catalyst performance.

A scalable chemical route to soluble acidified graphitic carbon nitride: an ideal precursor for isolated ultrathin g-C3N4 nanosheets.

We propose an efficient method to synthesize large-scale soluble acidified graphitic carbon nitride (g-C3N4). The as-prepared material exhibits the characteristics of a poly-ammonium salt and is soluble in several solvents with good dissolution-recrystallization reversible equilibrium. The pH value- and temperature-dependent solubility of the acidified g-C3N4 facilitates its separation and purification. After dissolution, acidified g-C3N4 forms isolated ultrathin nanosheets, making it an ideal precursor for large quantities of g-C3N4 nanosheets. This study raises the possibility of liquid assembly for g-C3N4 nanosheets based composite materials, expanding the functionalization and application of g-C3N4.