Advanced nanomaterials for highly efficient CO2 photoreduction and photocatalytic hydrogen evolution

ABSTRACTAt present, CO2 photoreduction to value-added chemicals/fuels and photocatalytic hydrogen generation by water splitting are the most promising reactions to fix two main issues simultaneously, rising CO2 levels and never-lasting energy demand. CO2, a major contributor to greenhouse gases (GHGs) with about 65% of the total emission, is known to cause adverse effects like global temperature change, ocean acidification, greenhouse effects, etc. The idea of CO2 capture and its conversion to hydrocarbons can control the further rise of CO2 levels and help in producing alternative fuels that have several further applications. On the other hand, hydrogen being a zero-emission fuel is considered as a clean and sustainable form of energy that holds great promise for various industrial applications. The current review focuses on the discussion of the recent progress made in designing efficient photocatalytic materials for CO2 photoreduction and hydrogen evolution reaction (HER). The scope of the current study is limited to the TiO2 and non-TiO2 based advanced nanomaterials (i.e. metal chalcogenides, MOFs, carbon nitrides, single-atom catalysts, and low-dimensional nanomaterials). In detail, the influence of important factors that affect the performance of these photocatalysts towards CO2 photoreduction and HER is reviewed. Special attention is also given in this review to provide a brief account of CO2 adsorption modes on the catalyst surface and its subsequent reduction pathways/product selectivity. Finally, the review is concluded with additional outlooks regarding upcoming research on promising nanomaterials and reactor design strategies for increasing the efficiency of the photoreactions

Co3O4 microcubes with exceptionally high conductivity using a CoAl layered double hydroxide precursor via soft chemically synthesized cobalt carbonate

Cubic microparticles of Co3O4 spinel were synthesized by calcination of CoCO3 obtained using CoAl layered double hydroxide (LDH) as a unitary precursor through soft-chemical decomposition. The obtained cobalt spinel showed an exceptionally high electrical conductivity at room temperature. This is attributed to high concentrations of charge carriers (Co4+), unique morphology, high reduction temperature and low activation barrier

A facile photo-induced synthesis of COOH functionalized meso-macroporous carbon films and their excellent sensing capability for aromatic amines

A simple photo-induced approach is developed for the preparation of COOH functionalized meso-macroporous carbon films with tunable pores without using any inorganic mesoporous silica templates, which show excellent sensing selectivity for aniline and the selectivity can be enhanced upon increasing COOH functional groups

Synthesis of nitrogen-rich mesoporous carbon nitride with tunable pores, band gaps and nitrogen content from a single aminoguanidine precursor

Highly ordered mesoporous carbon nitride (CN) with an extremely high nitrogen content and tunable pore diameters was synthesized by using a new precursor with a high nitrogen content, aminoguanidine hydrochloride and mesoporous silica SBA-15 with different pore diameters as hard templates. Surprisingly, the N/C ratio of the prepared mesoporous CN (MCN-4: 1.80) was considerably higher than that of the theoretically predicted CN nanostructures (1.33). This is mainly due to the fact that the CN precursor easily undergoes polymerization at high temperature and affords a highly stable polymer composed of a diamino-s-tetrazine moiety with a six-membered aromatic ring containing six nitrogen atoms that are linked trigonally with the nitrogen atoms. The obtained materials were thoroughly characterized by means of XRD, nitrogen adsorption, high resolution TEM, electron energy loss spectra, high resolution SEM, X-ray photoelectron spectroscopy, FTIR, and C, N, O, and S analysis. The results show that the MCN-4 materials possess a well-ordered mesoporous structure similar to SBA-15 with a high specific surface area and tunable band gap in the range of 2.25-2.49 eV. Interestingly, the pore diameter of the materials can be finely tuned from 3.1-5.8 nm by increasing the pore diameter of the hard-template SBA-15. The reaction temperature plays a critical role for the formation of MCN, and we found that 400 °C is the best condition to obtain MCN-4 with a high nitrogen content. We have further investigated the catalytic application of the MCN-4 materials towards Friedel-Crafts hexanoylation of benzene and compared the results with the mesoporous CN with less nitrogen content (MCN-1) and nonporous CN. Among the materials studied, MCN-4 showed the highest activity, affording a high yield of hexanophenone within a few hours, which is mainly due to the presence of free amine groups on the wall structure of MCN-4

Selective sensing performance of mesoporous carbon nitride with a highly ordered porous structure prepared from 3-amino-1,2,4-triazine

We demonstrate the preparation of highly ordered and graphitic mesoporous carbon nitride with an ordered porous structure and a high nitrogen content (MCN-ATN) by a nano-hard-templating approach through a simple polymerization of 3-amino-1,2,4-triazine (ATN) inside the pore channels of a mesoporous silica template with an Ia3d symmetry and a 3D porous structure. Powder X-ray diffraction and high resolution transmission electron microscopy analysis show that the prepared materials exhibit well-ordered mesopores with a 3D porous network. Nitrogen adsorption measurements also confirm that the samples possess excellent physical parameters including high surface areas (472-635 m 2 g-1), large pore volumes (0.71-0.99 cm3 g-1) and tunable pore diameters (5.5-6.0 nm). One of the most important features of this work is that the cyclic aromatic precursor helps to preserve the nitrogen in the carbon matrix of the final product even after the carbonization process. The C/N ratio of the samples is ca. 0.92 which is much lower than that obtained for the samples prepared using ethylene diamine and carbon tetrachloride through a nano-hard templating process. X-ray photoelectron spectroscopy results reveal that MCN-ATN is mainly composed of sp2 hybridized carbon atoms bonded with nitrogen atoms, associated with the triazine moieties from the ATN molecules. Temperature programmed desorption of CO 2 over MCN-ATN demonstrates that the sample is basic which originates from the amine groups on the surface of the CN wall structure. Finally, the samples are mounted on the quartz crystal microbalance (QCM) and used for sensing both acidic and basic organic vapors. Among the samples studied, MCN-ATN with the largest pore diameter showed a highly selective sensing performance for acidic molecules, especially the formic acid thanks to the presence of weak basic sites on the CN walls

Facile synthesis and basic catalytic application of 3D mesoporous carbon nitride with a controllable bimodal distribution

Here we demonstrate a facile synthesis of 3D mesoporous carbon nitride with Ia3d symmetry (MCN-6) using mesoporous silica KIT-6 with 3D porous structure and different pore diameters as hard templates, and ethylenediamine and carbon tetrachloride as the sources for N and C, and C, respectively. The obtained materials possess bimodal pores that can be controlled with a simple adjustment of the pore diameter of the KIT-6 templates. The lower angle powder X-ray diffraction (XRD) patterns and high resolution transmission electron microscope (HRTEM) images confirm that the MCN-6 materials possess a well-ordered mesoporous 3D structure with a highly interwoven and a branched pore structure. The textural parameters such as the specific surface areas and specific pore volumes of the materials can also be controlled by tuning the pore diameter of the hard template. The specific surface area and the specific pore volume of the samples increase with increasing the pore diameter of the hard template. The C/N ratio of the MCN-6 is ca. 4.3 which is similar to that obtained for MCN-1 prepared from SBA-15 as template. FT-IR and XPS spectroscopy results reveal that samples contain a CN network with a lot of free NH groups which are originated from ethylenediamine and can offer the basic sites. The temperature programmed desorption of CO confirms that the samples are highly basic and the basicity of the sample is 0.195 mmol of CO per g which is higher than that of MCN-1 (0.14 mmol of CO per g). We tested the performance of MCN-6 materials in the base-catalyzed Knoevenagel condensation of benzaldehyde and malononitrile. The catalysts exhibit excellent activity and afford a high yield of the corresponding α,β-unsaturated nitrile in a short reaction time even at room temperature. In addition, catalysts are highly stable and can be recyclable several times without affecting their activity

Microwave-assisted synthesis of highly crystalline mesoporous hydroxyapatite with a rod-shaped morphology

Mesoporous hydroxyapatite (MHA) with a rod-shaped morphology has been successfully synthesized for the first time employing cetyltrimethylammonium bromide (CTAB) as a template and CaCl 2 and K 2HPO 4 as the precursors for hydroxyapatite under alkaline medium at the reaction temperature of 120°C via microwave method. The obtained material exhibits a disordered mesoporous structure with a high crystallinity and highly uniform rod-like morphology with an average size of ca. 25 nm in width and 100 nm in length

Transesterification of ethylacetoacetate catalysed by metal free mesoporous carbon nitride

The basic catalytic performance of the mesoporous carbon nitride (MCN) for the transesterification of ethylacetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol under heterogeneous reaction conditions without using any solvents was demonstrated. The catalyst was prepared by using a nano-hard templating approach through a simple polymerisation reaction between ethylenediamine (EDA) and carbon tetrachloride (CTC) in the mesochannels of the SBA-15 followed by the carbonisation and the silica removal by HF. The material was thoroughly analysed by sophisticated characterisation techniques such as X-ray diffraction (XRD), N adsorption studies, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy and CHN analysis. Structural investigation of the MCN by XRD, HRTEM and N adsorption revealed that the prepared catalyst exhibits highly ordered two-dimensional (2D) porous arrays with a high surface area and a large pore volume. The catalytic results revealed that the MCN was found to be an efficient catalyst in transesterifying long and short chain primary alcohols, and cyclic and aromatic alcohols to afford their corresponding β-keto esters in high yields. More importantly, the catalyst was highly active when 1-butanol was used. The influence of various parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream on the yield of the final product was studied in detail. In addition, the activity of the catalyst was also compared with pure mesoporous carbon and the results were discussed. The recyclability studies revealed that the MCN catalyst was highly stable under the rigorous reaction conditions and can be reused several times without any significant loss of catalytic activity. Crown Copyrigh

Cobalt oxide functionalized nanoporous carbon electrodes and their excellent supercapacitive performance

Nanoporous carbon (CMK-3-150) functionalized with different amounts of cobalt oxide (CoO) nanoparticles was synthesized by an incipient wetness impregnation technique for supercapacitor application. The characterization results reveal that the specific surface area and pore volume of the CoO functionalized CMK-3-150 marginally decrease upon increasing the amount of the CoO whereas the pore diameter and the structure of the CMK-3-150 were not affected even after the functionalization. The electrochemical measurements show that the specific capacitance of the electrodes was enhanced after the functionalization with CoO. Among the electrodes studied, CMK-3-150 functionalized with 15 wt% CoO shows an excellent cycling stability and specific capacitance of 331 F g, which is ca. two times higher than that of the pure nanoporous carbon. This enhanced performance is due to the combined contribution of electrical double layer capacitance and pseudocapacitance. A symmetric supercapacitor device based on the CMK-3-150-15Co electrode gives the maximum energy density of 29.67 W h kg at a power density of 0.07 kW kg. This journal i

Direct synthesis and characterization of highly ordered cobalt substituted KIT-5 with 3D nanocages for cyclohexene epoxidation

Highly ordered cobalt incorporated KIT-5 silica (Co-KIT-5) with different Co contents and a well-ordered three-dimensional cage type porous structure were prepared for the first time by using Pluronic F127 as structure directing agent at different molar water to hydrochloric acid (nHO/) ratio. The amount of Co content in the silica framework of KIT-5 can be finely controlled with a simple adjustment of the nHO/ ratio as it controls the concentration of the H ions in the synthesis gel. It has been found that the nHO/ ratio of 463 is the best condition to obtain Co-KIT-5 with a high Co content. The obtained materials were characterized by various techniques such as powder X-ray diffraction (XRD), N adsorption studies, field emission high resolution scanning electron microscopy (FE-HRSEM), high resolution transmission electron microscopy (HRTEM), ultraviolet-visible diffused reflectance (UV-Vis DRS), electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS). Characterization results revealed that Co atom can be introduced in the silica framework without affecting the structural order and the textural parameters of the samples. ESR, XPS and UV-Vis DR spectra confirmed that the Co atoms are indeed occupy the tetrahedral coordination with the silica framework of KIT-5. The catalytic performance of Co-KIT-5 with different Co contents in the cyclohexene epoxidation has been investigated using TBHP/HO as oxidants and acetonitrile as a solvent. Co-KIT-5 exhibited a high catalytic performance with TBHP as oxidant and remained inactive when HO was used. The effect of various reaction parameters such as reaction time, reaction temperature, and reactant feed ratio and oxidant, affecting the catalytic activity of Co-KIT-5 has also been studied. Among the catalysts studied, Co-KIT-5-0.90 was found to be the best catalyst, affording a high conversion of cyclohexene. In addition, the catalyst was found to be highly stable and can be reused several times without affecting its catalytic activity under the optimized reaction conditions