Dariya Dontsova

A Stable Single-Site Palladium Catalyst for Hydrogenations

We report the preparation and hydrogenation performance of a single-site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three-phase hydrogenations of alkynes and nitroarenes in a continuous-flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.

Merging Single-Atom-Dispersed Silver and Carbon Nitride to a Joint Electronic System via Copolymerization with Silver Tricyanomethanide

Herein, we present an approach to create a hybrid between single-atom-dispersed silver and a carbon nitride polymer. Silver tricyanomethanide (AgTCM) is used as a reactive comonomer during templated carbon nitride synthesis to introduce both negative charges and silver atoms/ions to the system. The successful introduction of the extra electron density under the formation of a delocalized joint electronic system is proven by photoluminescence measurements, X-ray photoelectron spectroscopy investigations, and measurements of surface ζ-potential. At the same time, the principal structure of the carbon nitride network is not disturbed, as shown by solid-state nuclear magnetic resonance spectroscopy and electrochemical impedance spectroscopy analysis. The synthesis also results in an improvement of the visible light absorption and the development of higher surface area in the final products. The atom-dispersed AgTCM-doped carbon nitride shows an enhanced performance in the selective hydrogenation of alkynes in comparison with the performance of other conventional Ag-based materials prepared by spray deposition and impregnation-reduction methods, here exemplified with 1-hexyne.

Potassium Poly(heptazine imides) from Aminotetrazoles: Shifting Band Gaps of Carbon Nitride‐like Materials by 0.7 V for More Efficient Solar Hydrogen and Oxygen Evolution

Potassium poly(heptazine imide) (PHI) is a photocatalytically active carbon nitride material that was recently prepared from substituted 1,2,4‐triazoles. Here, we show that the more acidic precursors, such as commercially available 5‐aminotetrazole, upon pyrolysis in LiCl/KCl salt melt yield PHI with the greatly improved structural order and thermodynamic stability. Tetrazole‐derived PHIs feature long‐range crystallinities and unconventionally small layer stacking distances, leading to the altered electronic band structures as shown by Mott–Schottky analyses. Under the optimized synthesis conditions, visible‐light driven hydrogen evolution rates reach twice the rate provided by the previous gold standard, mesoporous graphitic carbon nitride, which has a much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared with ordinary carbon nitrides makes it an efficient water oxidation photocatalyst, which works even in the absence of any metal‐based co‐catalysts under visible light. To our knowledge, this is the first case of metal‐free oxygen liberation from water.

“The Easier the Better” Preparation of Efficient Photocatalysts—Metastable Poly(heptazine imide) Salts

Cost-efficient, visible-light-driven hydrogen production from water is an attractive potential source of clean, sustainable fuel. Here, it is shown that thermal solid state reactions of traditional carbon nitride precursors (cyanamide, melamine) with NaCl, KCl, or CsCl are a cheap and straightforward way to prepare poly(heptazine imide) alkali metal salts, whose thermodynamic stability decreases upon the increase of the metal atom size. The chemical structure of the prepared salts is confirmed by the results of X-ray photoelectron and infrared spectroscopies, powder X-ray diffraction and electron microscopy studies, and, in the case of sodium poly(heptazine imide), additionally by atomic pair distribution function analysis and 2D powder X-ray diffraction pattern simulations. In contrast, reactions with LiCl yield thermodynamically stable poly(triazine imides). Owing to the metastability and high structural order, the obtained heptazine imide salts are found to be highly active photocatalysts in Rhodamine B and 4-chlorophenol degradation, and Pt-assisted sacrificial water reduction reactions under visible light irradiation. The measured hydrogen evolution rates are up to four times higher than those provided by a benchmark photocatalyst, mesoporous graphitic carbon nitride. Moreover, the products are able to photocatalytically reduce water with considerable reaction rates, even when glycerol is used as a sacrificial hole scavenger.

Synthesis of an electronically modified carbon nitride from a processable semiconductor, 3-amino-1,2,4-triazole oligomer, via a topotactic-like phase transition

A thermally induced topotactic transformation of organic polymeric semiconductors is achieved using similarity of the chemical structures of two C,N,H-containing materials. Namely, the oligomer of 3-amino-1,2,4-triazole (OATA) is transformed into an electronically modified graphitic carbon nitride (OATA-CN) upon heating at 550 °C. During the transition, the flat band potential of the organic semiconductor is only slightly shifted from −0.11 eV to −0.06 eV, while the optical band gap is significantly expanded from 1.8 eV to 2.2 eV. The advantage of the suggested approach is the processability of the starting semiconductor combined with minor morphology changes during the heat-treatment that enable preservation of the original oligomer micro- and macrostructures in the resulting carbon nitrides. As an illustration, different OATA morphologies, including spherical nanoparticles, nanobarrels, nanowires and self-assembled macrospheres and composite sheets are synthesized and then transformed into OATA-CN with the retention of morphology. The surface area of the final carbon nitrides reaches 66 m2 g−1, without using any template, auxiliary reagent or post treatment. As a consequence, the photocatalytic activity of the obtained carbon nitrides in visible light driven hydrogen evolution is up to 5 times higher than that measured for the reference bulk carbon nitride prepared by pyrolysis of melamine.

Towards Organic Zeolites and Inclusion Catalysts: Heptazine Imide Salts Can Exchange Metal Cations in the Solid State

Highly crystalline potassium (heptazine imides) were prepared by the thermal condensation of substituted 1,2,4-triazoles in eutectic salt melts. These semiconducting salts are already known to be highly active photocatalysts, for example, for the visible-light-driven generation of hydrogen from water. Herein, we show that within the solid-state structure, potassium ions can be exchanged to other metal ions while the crystal habitus is essentially preserved.

Synthesis of efficient photocatalysts for water oxidation and dye degradation reactions using CoCl2 eutectics

A simple one-step procedure for the preparation of cobalt oxide–carbon nitride composites by carrying out the thermal condensation reaction of carbon nitride precursors in cobalt(II) chloride-containing salt melts is developed. This method enables us to control the structure of the polymer constituent by the proper selection of the second eutectic component. In this respect, alkali metal chlorides and zinc chloride give rise to poly(triazine imides), while tin(II) chloride delivers melon-based polymers. The crystallinity of the carbon nitride polymer phase can be influenced by varying the concentration of the precursor in the melt. On the other hand, cobalt oxide loading in the final products can be increased by decreasing the precursor concentration in the melt. The products are highly active photocatalysts for Rhodamine B degradation as exemplified by SnCl2/CoCl2-derived solids, and the water oxidation reaction (WOR). The activity of the products in the WOR is comparable with that of the bench composite photocatalyst prepared by using the three-step procedure that includes the synthesis of Co3O4 nanoparticles as a separate step, thus illustrating the advantages of the developed salt melt assisted approach.

Baking ‘crumbly’ carbon nitrides with improved photocatalytic properties using ammonium chloride

Ammonium chloride can serve as a green, unreactive and reusable template to prepare heptazine-based graphitic carbon nitrides with surface areas up to 30 m2 g−1 and up to 6 times higher photocatalytic activity, by a simplified procedure that comprises only pyrolysis of the reaction mixture containing precursor and salt.

Facile Synthesis of Potassium Poly(heptazine imide) (PHIK)/Ti-Based Metal–Organic Framework (MIL-125-NH2) Composites for Photocatalytic Applications

Photocatalytically active composites comprising potassium poly(heptazine imide) (PHIK) and a Ti-based metal-organic framework (MOF, MIL-125-NH2) are prepared in situ by simply dispersing both materials in water. The driving forces of composite formation are the electrostatic interactions between the solids and the diffusion of potassium ions from PHIK to MIL-125-NH2. This mechanism implies that other composites of poly(heptazine imide) salts and different MOFs bearing positive surface charge can potentially be obtained in a similar fashion. The suggested strategy thus opens a new avenue for the facile synthesis of such materials. The composites are shown to have a superior photocatalytic activity in Rhodamine B degradation under blue light irradiation. The reaction rate is doubled compared to that of pure MOF compound and is 7 times higher than the activity of the pristine PHIK. The results of the electron paramagnetic resonance (EPR) investigations and the analysis of the electronic structures of the solids suggest the electron transfer from MIL-125-NH2 to PHIK in the composite. The possible pathways for the dye degradation and the rationalization of the increased activity of the composites are elaborated.