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•Recent advances on the use of transition metal complexes for PEC CO2 reduction.•Hybrid systems consisting of metal complexes supported on semiconductors for PEC CO2 reduction.•Key ...findings of the reported PEC systems using transition metal complexes and their hybrids for CO2 reduction.•Gaps and challenges in the PEC CO2 reduction using transition metal complexes.•Future recommendations for efficient PEC CO2 reduction systems using transition metal complexes.
The ever-increasing concentration of CO2 in the atmosphere has become an issue of grave concern mainly due to its adverse effect on the climate induced by the excessive greenhouse effect. Among the various possible mitigation strategies, utilization of CO2 for the production of commodity chemicals and fuel is a promising and environmentally friendly approach. However, the higher thermodynamic and kinetic stability of CO2 molecules requires extensive energy and extreme reaction conditions. In recent decades, alternative techniques like photocatalytic, electrocatalytic, and photo-electrocatalytic reduction of CO2 have been considered technically feasible, economically viable and environmentally friendly. Specifically, photoelectrocatalytic reduction of CO2 is highly important as it combines the advantages of photocatalysis and electrocatalysis such as use of renewable, inexhaustible solar energy, less electrical energy consumption, no sacrificial donor and higher conversion efficiency. This review systematically summarizes the recent advances in PEC CO2 reduction, primarily using molecular complexes of transition metals and their hybrids with semiconductors. After an initial discussion on the basic principal of PEC CO2 reduction, a detailed discussion of the different transition metal complexes, both mono-metallic and bimetallic of noble and non-noble metals, various process parameters for PEC CO2 reduction, along with the proposed mechanism and limitations are presented. In addition, the review highlighted the recent advancements in PEC CO2 reduction and critical evaluation of different issues to predict the direction to pursue the research for effective conversion of CO2 to valuable chemicals.
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•Single step synthesis of a ternary hybrid Ag-rGO/g-C3N4 via hydrothermal approach•Efficient reduction of nitrobenzene to aniline without using any reducing agents like hydrazine ...hydrate.•Dual function of methanol as hole scavenger and proton donor•A truly heterogeneous material without any detectable leaching of the active metal•The photocatalyst could be recycled for several runs with consistent efficiency.
In this report, a ternary Ag-rGO/g-C3N4 hybrid was synthesized by a simple hydrothermal approach for the photocatalytic reduction of nitroarene compounds into their corresponding amines under visible light. Importantly, the present method did not require reducing agents, like hydrazine hydrate; instead methanol has been used as a source of electrons and protons for the photoreduction process. After grafting of Ag NPs, a significant enhancement in the efficiency of the rGO/g-C3N4 for the reduction of nitrobenzenes was observed. Under optimized experimental conditions, the conversion of nitrobenzene and yield of aniline were determined to be 99% and 98%, respectively under visible light illumination for 4 h. The nitrobenzene compounds bearing both electron donating and withdrawing groups were selectively converted into their corresponding aniline products without altering the functionality. The enhanced performance of the developed photocatalyst attributed to the effective separation of photoexcited electrons on the photocatalyst surface and their subsequent transfer for the reduction of nitrobenzene molecules.
Visible light assisted photocatalytic transformations have been considered as an efficient and sustainable approach for the production of high-value chemicals. The present paper describes the ...synthesis of plasmonic silver nanoparticle incorporated reduced graphene oxide and its photocatalytic performance for the selective oxidation of various benzylamines to the corresponding imines using molecular oxygen as an oxidant under ambient temperature conditions. The developed photocatalyst was found to be highly stable and exhibited excellent photoactivity with a consistent recycling ability for several runs without a loss in activity. Moreover, to the best of our knowledge, the developed photocatalyst represents the first example of a graphene-based photocatalyst for the oxidative coupling of amines.
Visible light assisted photocatalytic reduction of nitro-compounds has been found as an efficient and sustainable approach for the production of amines.
The present study describes the synthesis, characterization, and evaluation of cobalt(ii) phthalocyanine embedded bismuth oxyhalide (CoPc@Bi24O31Br10) for the photoreduction of CO2 into methanol ...selectively using triethylamine (TEA) as a sacrificial donor under visible light illumination. The highest yield of methanol was found to be 3485.3 μmol per g of the photocatalyst with a methanol formation rate of 145.2 μmol h−1 g−1. The developed photocatalyst was found to be quite stable and showed comparable results for five recycling runs without any significant loss in efficiency. Importantly, no significant leaching of the complex was observed even after the fifth run.
A novel heteroleptic iridium complex supported on graphitic carbon nitride was synthesized and used for photoreduction of carbon dioxide under visible light irradiation. The methanol yield obtained ...after 24 h irradiation was 9934 μmol g−1cat (TON 1241 with respect to Ir) by using triethylamine (TEA) as a sacrificial donor, which was significantly higher as compared to the semiconductor carbon nitride 145 μmol g−1cat under identical conditions. The presence of triethylamine was found to be vital for the higher methanol yield. After the reaction, the photocatalyst could easily be recovered and reused for subsequent six runs without significant loss in photo activity.
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Carbon dioxide (CO
2
) utilization for the production of fuels and energy-rich chemicals is one of the attractive topics of research in modern-day chemistry. The main barriers in the CO
2
utilization ...include: 1) the cost associated in CO
2
capture, separation, purification, and transportation; 2) higher energy demand for conversion due to its higher kinetic and thermodynamic stability; 3) the lack of socio-economical driving forces. Valorization of CO
2
has been realized as a viable option for the mitigation of CO
2
as well as to create new business opportunities. Carbon dioxide is a nontoxic, renewable, and abundant C1 source; whereas carboxylation of olefins to carboxylic acids represents one of the most important transformations owing to the broad applicability of these chemicals in various domains, such as water-absorbing polymers, preservatives of food, fertilizers, building blocks for the manufacturing of cosmetics, soaps, detergents, rubbers, dyes, animal feed, plastics, agrochemicals and pharmaceuticals. The present review mainly focuses on the recent advances in the transition metal-catalyzed carboxylation of the olefins by using carbon dioxide as a feedstock. Firstly, the hydrocarboxylation or direct carboxylation using coordination complexes of transition metals such as rhodium, copper, cobalt, nickel, ruthenium, iron, palladium, and zirconium have been discussed in detail. In the next section, the hetero carboxylation reactions, for example, boracarboxylation, silacarboxylation, carbocarboxylation, thiocarboxylation and dicarboxylation have been presented to explore the wider scope of CO
2
utilization for carboxylation reactions. Lastly, the carboxylation of dienes and difunctionalization reactions have been discussed. Further, the recent trends and key challenges in the use of CO
2
as a carbxylating agent for carboxylation reactions have been described.
New graphene oxide (GO)‐tethered–CoII phthalocyanine complex CoPc–GO was synthesized by a stepwise procedure and demonstrated to be an efficient, cost‐effective and recyclable photocatalyst for the ...reduction of carbon dioxide to produce methanol as the main product. The developed GO‐immobilized CoPc was characterized by X‐ray diffraction (XRD), FTIR, XPS, Raman, diffusion reflection UV/Vis spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP‐AES), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR, XPS, Raman, UV/Vis and ICP‐AES along with elemental analysis data showed that CoII–Pc complex was successfully grafted on GO. The prepared catalyst was used for the photocatalytic reduction of carbon dioxide by using water as a solvent and triethylamine as the sacrificial donor. Methanol was obtained as the major reaction product along with the formation of minor amount of CO (0.82 %). It was found that GO‐grafted CoPc exhibited higher photocatalytic activity than homogeneous CoPc, as well as GO, and showed good recoverability without significant leaching during the reaction. Quantitative determination of methanol was done by GC flame‐ionization detector (FID), and verification of product was done by NMR spectroscopy. The yield of methanol after 48 h of reaction by using GO–CoPc catalyst in the presence of sacrificial donor triethylamine was found to be 3781.8881 μmol g−1 cat., and the conversion rate was found to be 78.7893 μmol g−1cat. h−1. After the photoreduction experiment, the catalyst was easily recovered by filtration and reused for the subsequent recycling experiment without significant change in the catalytic efficiency.
Very photoactive! Cobalt phthalocyanine grafted to the chemically functionalized graphene oxide was found to be an efficient heterogeneous visible‐light‐induced photoredox catalyst for the photoreduction of carbon dioxide to methanol in a very good yield. The developed photocatalyst exhibited superior activity compared with the existing photocatalytic systems and gave methanol as the major reaction product (see scheme).
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A substantially improved methanol yield was achieved from the photoreduction of carbon dioxide under visible light by using a hybrid photocatalyst consisting of molecular cobalt ...phthalocyanine tetracarboxylic acid (CoPc-COOH) complex immobilized to the organic semiconductor graphitic carbon nitride (g-C3N4) and triethylamine as sacrificial electron donor. The structural and morphological features of the hybrid photocatalyst determined by various techniques like FTIR, UV–Vis, Raman, XPS, TGA, BET etc. After 24 h of light irradiation, the methanol yield by using g-C3N4/CoPc-COOH photocatalyst (50 mg) was found to be 646.5 µmol g−1cat or 12.9 mmol g−1cat with conversion rate 538.75 µmol h−1 g−1cat. However, the use of homogeneous CoPc-COOH (6.5 µmol Co, equivalent to g-C3N4/CoPc-COOH) and g-C3N4 (50 mg) provided 88.5 μmol (1770 μmol g−1cat) and 59.2 μmol (1184 μmol g−1cat) yield of methanol, respectively under identical conditions. The improved photocatalytic efficiency of the hybrid was attributed to the binding ability of CoPc-COOH to CO2 that provided the higher CO2 concentration on the support. Further, the semiconductor support provided better electron mobility and charge separation with the integrated benefit of facile recovery and recycling of the material at the end of the reduction process.
Nickel nanoparticle-decorated phosphorous-doped graphitic carbon nitride (Ni@g-PC₃N₄) was synthesized and used as an efficient photoactive catalyst for the reduction of various nitrobenzenes under ...visible light irradiation. Hydrazine monohydrate was used as the source of protons and electrons for the intended reaction. The developed photocatalyst was found to be highly active and afforded excellent product yields under mild experimental conditions. In addition, the photocatalyst could easily be recovered and reused for several runs without any detectable leaching during the reaction.
The first report on the use of vanadium-based catalysts for oxidative cyanation of tertiary amines with molecular oxygen in the presence of sodium cyanide and acetic acid to afford the corresponding ...alpha-aminonitriles in good to excellent yields is described.
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