Photocatalytic and electrocatalytic reduction of CO₂ by MXene-based nanomaterials: A review

Recently, transition metal carbide or nitride (MXene)-based nanomaterials have been broadly investigated as new photocatalysts and electrocatalysts for the reduction of CO2 into valuable energy-rich fuels due to their unique properties such as rich surface chemistries, flexible morphologies, bandgap structures, considerable electrical conductivities, thermal stabilities, and significant specific surface areas. Nevertheless, only a few reviews have been reported on the application of MXenes or MXene-based nanomaterials as advanced photocatalysts and/or electrocatalysts for CO2 reduction, which do not cover new findings and the current development in the application of MXene-based nanomaterials for CO2 reduction. Accordingly, herein, we present a comprehensive review of current findings on the photocatalytic and electrocatalytic reduction of CO2 by various MXene-based nanomaterials. Particularly, this review focuses on the (i) photocatalytic reduction of CO2 by functionalized Ti3C2, TiO2/Ti3C2, g-C3N4/Ti3C2, and other/Ti3C2 catalysts, (ii) electrocatalytic CO2 reduction; (iii) CO2 reduction associated with photothermal catalysis and hydrogenation, and (iv) stability of MXene-based photoelectrocatalysts. Additionally, we have briefly explored the challenges in the large-scale fabrication of MXene-based nanomaterials and proposed the future research prospects of MXene-based nanomaterials.</p

Modeling the Transport of the “New-Horizon” Reduced Graphene OxideMetal Oxide Nanohybrids in Water-Saturated Porous Media

Little is known about the fate and transport of the “new-horizon” multifunctional nanohybrids in the environment. Saturated sand-packed column experiments (n = 66) were therefore performed to investigate the transport and retention of reduced graphene oxide (RGO)metal oxide (Fe3O4, TiO2, and ZnO) nanohybrids under environmentally relevant conditions (mono- and divalent electrolytes and natural organic matter). Classical colloid science principles (Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and colloid filtration theory (CFT)) and mathematical models based on the one-dimensional convection-dispersion equation were employed to describe and predict the mobility of RGO-Fe3O4, RGO-TiO2, and RGO-ZnO nanohybrids in porous media. Results indicate that the mobility of the three nanohybrids under varying experimental conditions is overall explainable by DLVO theory and CFT. Numerical simulations suggest that the one-site kinetic retention model (OSKRM) considering both time- and depth-dependent retention accurately approximated the breakthrough curves (BTCs) and retention profiles (RPs) of the nanohybrids concurrently; whereas, others (e.g., two-site retention model) failed to capture the BTCs and/or RPs. This is primarily because blocking BTCs and exponential/hyperexponential/uniform RPs occurred, which is within the framework of OSKRM featuring time- (for kinetic Langmuirian blocking) and depth-dependent (for exponential/hyperexponential/uniform) retention kinetics. Employing fitted parameters (maximum solid-phase retention capacity: Smax = 0.0406–3.06 cm3/g; and first-order attachment rate coefficient: ka = 0.133–20.6 min–1) extracted from the OSKRM and environmentally representative physical variables (flow velocity (0.00441–4.41 cm/min), porosity (0.24–0.54), and grain size (210–810 μm)) as initial input conditions, the long-distance transport scenarios (in 500 cm long sand columns) of the three nanohybrids were predicted via forward simulation. Our findings address the existing knowledge gap regarding the impact of physicochemical factors on the transport of the next-generation, multifunctional RGOmetal oxide nanohybrids in the subsurface

Application of a Ti₃C₂T_<i>X</i> MXene-Coated Membrane for Removal of Selected Natural Organic Matter and Pharmaceuticals

Ti3C2TX MXene was used for surface modification of membranes by vacuum-assisted filtration. Owing to its higher hydrophilicity, negatively charged surface, and lower molecular weight cutoff, the Ti3C2TX MXene-coated membrane showed great performance for the treatment of organic contaminants. Humic acid (HA)/tannic acid mixtures were selected as the target natural organic matter (NOM). Owing to weakened hydrophobic interaction and improved size exclusion upon using Ti3C2TX MXene, it was difficult for HA to pass through the membrane. Membrane performance was tested for two different charged pharmaceuticals (amitriptyline and ibuprofen) under three pH conditions. The water permeabilities of pure water and both pharmaceuticals showed similar trends. This indicates that separation is affected by electrostatic interactions because the membrane surface is more negatively charged after Ti3C2TX MXene coating. Additionally, the reusability of the Ti3C2TX MXene-coated membrane was evaluated in three filtration cycles for NOM. After the first and second cleanings, recoveries of water permeabilities were 95.5% and 91.6% for HA. Although NOM can act as a foulant, HA caused reversible fouling. These findings indicate that the Ti3C2TX-coated membrane can be engineered to effectively treat various organic contaminants with high water permeability, retention performance, and antifouling capability