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Institute
CxNy
(2021)
The search for metal-free and visible light-responsive materials for photocatalytic applications has attracted the interest of not only academics but also the industry in the last decades. Since graphitic carbon nitride (g-C3N4) was first reported as a metal-free photocatalyst, this has been widely investigated in different light-driven reactions. However, the high recombination rate, low electrical conductivity, and lack of photoresponse in most of the visible range have elicited the search for alternatives. In this regard, a broad family of carbon nitride (CxNy) materials was anticipated several decades ago. However, the attention of the researchers in these materials has just been awakened in the last years due to the recent success in the syntheses of some of these materials (i.e., C3N3, C2N, C3N, and C3N5, among others), together with theoretical simulations pointing at the excellent physico-chemical properties (i.e., crystalline structure and chemical morphology, electronic configuration and semiconducting nature, or high refractive index and hardness, among others) and optoelectronic applications of these materials. The performance of CxNy, beyond C3N4, has been barely evaluated in real applications, including energy conversion, storage, and adsorption technologies, and further work must be carried out, especially experimentally, in order to confirm the high expectations raised by simulations and theoretical calculations. Herein, we have summarized the scarce literature related to recent results reporting the synthetic routes, structures, and performance of these materials as photocatalysts. Moreover, the challenges and perspectives at the forefront of this field using CxNy materials are disclosed. We aim to stimulate the research of this new generation of CxNy-based photocatalysts, beyond C3N4, with improved photocatalytic efficiencies by harnessing the striking structural, electronic, and optical properties of this new family of materials.
The preparation of stable and efficient electrocatalysts comprising abundant and non-critical row-materials is of paramount importance for their industrial implementation. Herein, we present a simple synthetic route to prepare Mn(ii) sub-nanometric active sites over a highly N-doped noble carbonaceous support. This support not only promotes a strong stabilization of the Mn(ii) sites, improving its stability against oxidation, but also provides a convenient coordination environment in the Mn(ii) sites able to produce CO, HCOOH and CH3COOH from electrochemical CO2 reduction.
Carbon nanomaterials doped with some other lightweight elements were recently described as powerful, heterogeneous, metal-free organocatalysts, adding to their high performance in electrocatalysis. Here, recent observations in traditional catalysis are reviewed, and the underlying reaction mechanisms of the catalyzed organic transformations are explored. In some cases, these are due to specific active functional sites, but more generally the catalytic activity relates to collective properties of the conjugated nanocarbon frameworks and the electron transfer from and to the catalytic centers and substrates. It is shown that the !earnings are tightly related to those of electrocatalysis; i.e., the search for better electrocatalysts also improves chemocatalysis, and vice versa. Carbon-carbon heterojunction effects and some perspectives on future possibilities are discussed at the end.
Electrochemical reduction stands as an alternative to revalorize CO2. Among the different alternatives, Ni single atoms supported on carbonaceous materials are an appealing catalytic solution due to the low cost and versatility of the support and the optimal usage of Ni and its predicted selectivity and efficiency (ca. 100% towards CO). Herein, we have used noble carbonaceous support derived from cytosine to load Ni subnanometric sites. The large heteroatom content of the support allows the stabilization of up to 11 wt% of Ni without the formation of nanoparticles through a simple impregnation plus calcination approach, where nickel promotes the stabilization of C3NOx frameworks and the oxidative support promotes a high oxidation state of nickel. EXAFS analysis points at nickel single atoms or subnanometric clusters coordinated by oxygen in the material surface. Unlike the wellknown N-coordinated Ni single sites selectivity towards CO2 reduction, O-coordinated-Ni single sites (ca. 7 wt% of Ni) reduced CO2 to CO, but subnanometric clusters (11 wt% of Ni) foster the unprecedented formation of HCOOH with 27% Faradaic efficiency at - 1.4 V. Larger Ni amounts ended up on the formation of NiO nanoparticles and almost 100% selectivity towards hydrogen evolution.
Carbon suboxide (C3O2) is a unique molecule able to polymerize spontaneously into highly conjugated light-absorbing structures at temperatures as low as 0 degrees C. Despite obvious advantages, little is known about the nature and the functional properties of this carbonaceous material. In this work, the aim is to bring "red carbon," a forgotten polymeric semiconductor, back to the community's attention.
A solution polymerization process is adapted to simplify the synthesis and control the structure.
This allows one to obtain this crystalline covalent material at low temperatures. Both spectroscopic and elemental analyses support the chemical structure represented as conjugated ladder polypyrone ribbons.
Density functional theory calculations suggest a crystalline structure of AB stacks of polypyrone ribbons and identify the material as a direct bandgap semiconductor with a medium bandgap that is further confirmed by optical analysis.
The material shows promising photocatalytic performance using blue light.
Moreover, the simple condensation-aromatization route described here allows the straightforward fabrication of conjugated ladder polymers and can be inspiring for the synthesis of carbonaceous materials at low temperatures in general.