Electrocatalytic Reactions for Converting CO2 to Value-Added Products: Recent Progress and Emerging Trends

Carbon dioxide (CO₂) emissions are an important environmental issue that causes greenhouse and climate change effects on the earth. Nowadays, CO₂ has various conversion methods to be a potential carbon resource, such as photocatalytic, electrocatalytic, and photo-electrocatalytic. CO₂ conversion into value-added products has many advantages, including facile control of the reaction rate by adjusting the applied voltage and minimal environmental pollution. The development of efficient electrocatalysts and improving their viability with appropriate reactor designs is essential for the commercialization of this environmentally friendly method. In addition, microbial electrosynthesis which utilizes an electroactive bio-film electrode as a catalyst can be considered as another option to reduce CO₂. This review highlights the methods which can contribute to the increase in efficiency of carbon dioxide reduction (CO₂R) processes through electrode structure with the introduction of various electrolytes such as ionic liquid, sulfate, and bicarbonate electrolytes, with the control of pH and with the control of the operating pressure and temperature of the electrolyzer. It also presents the research status, a fundamental understanding of carbon dioxide reduction reaction (CO₂RR) mechanisms, the development of electrochemical CO₂R technologies, and challenges and opportunities for future research.

Metal Oxides as Catalyst/Supporter for CO2 Capture and Conversion, Review

Various carbon dioxide (CO2) capture materials and processes have been developed in recent years. The absorption-based capturing process is the most significant among other processes, which is widely recognized because of its effectiveness. CO2 can be used as a feedstock for the production of valuable chemicals, which will assist in alleviating the issues caused by excessive CO2 levels in the atmosphere. However, the interaction of carbon dioxide with other substances is laborious because carbon dioxide is dynamically relatively stable. Therefore, there is a need to develop types of catalysts that can break the bond in CO2 and thus be used as feedstock to produce materials of economic value. Metal oxide-based processes that convert carbon dioxide into other compounds have recently attracted attention. Metal oxides play a pivotal role in CO2 hydrogenation, as they provide additional advantages, such as selectivity and energy efficiency. This review provides an overview of the types of metal oxides and their use for carbon dioxide adsorption and conversion applications, allowing researchers to take advantage of this information in order to develop new catalysts or methods for preparing catalysts to obtain materials of economic value.

Self-supported CdSe nanowire/nanosheet photoanodes on cadmium foil via in situ hydrothermal transformation of CdSe(en)0.5 complex nanostructures

To solve energy crisis, the engineering of highly efficient and cost-effective photoanodes is urgently required for clean fuel generation. Herein, CdSe(en)_0.5 (en = ethylenediamine) hybrid photoanodes were synthesized by a solvothermal approach. It was revealed that a second in situ hydrothermal treatment successfully converts cadmium foil-based inorganic-organic CdSe(en)_0.5 (en = ethylenediamine) hybrid nanosheets to an oriented cadmium hydroxide crowned CdSe nanowire-decorated porous nanosheet (Cd(OH)₂/CdSe NW/NS) heterostructure by dissolution and regrowth mechanisms. The alteration in second hydrothermal reaction conditions could modify the morphology and optical properties of the Cd(OH)₂/CdSe NW/NS heterostructure photoanodes. The possible growth mechanism of the Cd(OH)₂/CdSe NW/NS porous structure is studied at various second hydrothermal times using the control experiments of the synthesis. The optimized 3D porous Cd(OH)₂/CdSe NW/NS photoanodes exhibited an outstanding photocurrent density of 6.1 mA cm^-2 at 0 V vs. Ag/AgCl, which is approximately 7.6 times higher than that of the inorganic-organic CdSe(en)_0.5 hybrid under light irradiation (>420 nm cut off filter). A mechanism is proposed to explain the enhanced charge separation at the Cd(OH)₂/CdSe NW/NS photoanode/electrolyte interface, which is supported by PL and photoelectrochemical analyses. These findings open an avenue of phase and morphology transmutation for efficient formation of other hierarchical structures of metal selenides and sulfides. Additionally, the Al₂O₃ co-catalyst can act as effective hole trapping sites and improves the stability of the photoelectrode through the timely consumption of photogenerated charges, particularly holes.

Biocatalytic production of Ɛ-caprolactone using Geotrichum candidum cells immobilized on functionalized silica

Immobilization of the Geotrichum candidum (CCT 1205) cell with functionalized silica creates promising biocatalysts for production of ɛ-caprolactone. The results obtained by immobilization of the whole cell on SiO₂-NH₂ and SiO₂-SH supports indicate that the presence of reactive functional groups on the support may promote effective chemical bonds with the cell walls resulting the decreased dehydrogenases enzyme activity (5% yield in less than 2h) and consequently, increased Baeyer-Villiger monooxygenases enzyme activity with redacting of 25% of time reaction when is used SiO₂-NH₂ as support and 50% through use of SiO₂-SH as support relative to free cells when cyclohexanone is used as a substrate. The catalysts SiO₂-NH₂-Geotrichum candidum and SiO₂-SH-Geotrichum candidum were recycling and reused in the ɛ-caprolactone synthesis from cyclohexanone, and the biocatalysts promoted a quantitative conversion up to the eighth reaction cycle. KEY POINTS: • Immobilized microorganism is more efficient than free cell in the caprolactone synthesis. • The reaction times for amino and thiol groups in support were 3 h and 2 h, respectively. • These catalysts showed higher ɛ-caprolactone conversion at higher concentrations.

Chemoselective reduction of nitro and nitrile compounds using an Fe3O4-MWCNTs@PEI-Ag nanocomposite as a reusable catalyst

Chemoselective reductions by an Fe₃O₄-MWCNTs@PEI-Ag nanocomposite.

Enhanced CO₂ Adsorption by Nitrogen-Doped Graphene Oxide Sheets (N-GOs) Prepared by Employing Polymeric Precursors

Nitrogen-doped graphene oxide sheets (N-GOs) are prepared by employing N-containing polymers such as polypyrrole, polyaniline, and copolymer (polypyrrole-polyaniline) doped with acids such as HCl, H₂SO₄, and C₆H₅-SO₃-K, which are activated using different concentrations of KOH and carbonized at 650 °C; characterized using SEM, TEM, BET, TGA-DSC, XRD, and XPS; and employed for the removal of environmental pollutant CO₂. The porosity of the N-GOs obtained were found to be in the range 1-3.5 nm when the KOH employed was in the ratio of 1:4, and the XRD confirmed the formation of the layered like structure. However, when the KOH employed was in the ratio of 1:2, the pore diameter was found to be in the range of 50-200 nm. The SEM and TEM analysis reveal the porosity and sheet-like structure of the products obtained. The nitrogen-doped graphene oxide sheets (N-GOs) prepared by employing polypyrrole doped with C₆H₅-SO₃-K were found to possess a high surface area of 2870 m²/g. The N-GOs displayed excellent CO₂ capture property with the N-GOs; PPy/Ar-1 displayed ~1.36 mmol/g. The precursor employed, the dopant used, and the activation process were found to affect the adsorption property of the N-GOs obtained. The preparation procedure is simple and favourable for the synthesis of N-GOs for their application as adsorbents in greenhouse gas removal and capture.

Critical review of existing nanomaterial adsorbents to capture carbon dioxide and methane

Innovative gas capture technologies with the objective to mitigate CO₂ and CH₄ emissions are discussed in this review. Emphasis is given on the use of nanoparticles (NP) as sorbents of CO₂ and CH₄, which are the two most important global warming gases. The existing NP sorption processes must overcome certain challenges before their implementation to the industrial scale. These are: i) the utilization of the concentrated gas stream generated by the capture and gas purification technologies, ii) the reduction of the effects of impurities on the operating system, iii) the scale up of the relevant materials, and iv) the retrofitting of technologies in existing facilities. Thus, an innovative design of adsorbents could possibly address those issues. Biogas purification and CH₄ storage would become a new motivation for the development of new sorbent materials, such as nanomaterials. This review discusses the current state of the art on the use of novel nanomaterials as adsorbents for CO₂ and CH₄. The review shows that materials based on porous supports that are modified with amine or metals are currently providing the most promising results. The Fe₃O₄-graphene and the MOF-117 based NPs show the greatest CO₂ sorption capacities, due to their high thermal stability and high porosity. Conclusively, one of the main challenges would be to decrease the cost of capture and to scale-up the technologies to minimize large-scale power plant CO₂ emissions.

Anchoring di and tri-metallic nanoparticles on an amorphous functionalized surface for inducing photocatalytic activity

Surface functionalization chemistry was applied to immobilize di and tri metal nanoparticles on amorphous particles for an economically low-cost photocatalyst.

High Adsorption of Sulfamethoxazole by an Amine-Modified Polystyrene-Divinylbenzene Resin and Its Mechanistic Insight

Sulfamethoxazole (SMZ) adsorption by a series of amine-modified polystyrene-divinylbenzene resins (PSA/B/C/D) was investigated. All resins showed a similar pH dependent adsorption of SMZ but their capacities were linearly related with the contents of primary amines (-NH2) rather than secondary amines (-NH-). Mechanisms of SMZ adsorption by PSA (highest -NH2 content) were discussed as an example. Due to comparable pKa, H-bonding interactions of -NH2(0) with SMZ(0) (regular H-bond) and SMZ(-) (negative charge-assisted H-bond, (-)CAHB) successively contributed most adsorption (pH 4-9). At weakly acidic pH, -NH2(0) was partially protonated and electrostatic attraction between -NH3(+) and SMZ(-) occurred concurrently, but could be hindered by increased loading of SMZ(0). Hydrophobic/ π-π interactions were not major mechanisms as phenanthrene and nitrobenzenes had little effect on SMZ adsorption. At alkaline pH, where SMZ(-) and -NH2(0) prevailed, adsorption was accompanied by the stoichiometric (∼1.0) proton exchange with water, leading to OH(-) release and the formation of (-)CAHB [SO2N(-)···H···NH2]. The interaction and SMZ spatial distribution in the resin-phase were further confirmed by FTIR and Raman spectra. SMZ was uniformly adsorbed on external and interior surfaces. SMZ adsorption by PSA had low-interference from other coexistent matter, but high stability after multiple regenerations. The findings will guide new adsorbent designs for selectively removing target organics.

Dual-responsive nanoparticles that aggregate under the simultaneous action of light and CO2

Metallic nanoparticles co-functionalised with monolayers of UV- and CO₂-sensitive ligands were prepared and shown to respond to these two types of stimuli reversibly and in an orthogonal fashion.

Highly dispersed platinum nanoparticles supported on silica as catalyst for hydrogen production

A synthetic approach is developed to produce highly disperse, low loading (3.28 wt%) Pt nanoparticles incorporated silica (Pt-NP–S) with average diameter of 3.5 nm using economical and simple surface chemical modification and reduction processes.