Your browser does not fully support modern features. Please upgrade for a smoother experience.
Subject:
All Disciplines Arts & Humanities Biology & Life Sciences Business & Economics Chemistry & Materials Science Computer Science & Mathematics Engineering Environmental & Earth Sciences Medicine & Pharmacology Physical Sciences Public Health & Healthcare Social Sciences
Sort by:
Most Viewed Latest Alphabetical (A-Z) Alphabetical (Z-A)
Filter:
All Topic Review Biography Peer Reviewed Entry Video Entry
Topic Review
Advanced Oxidation Processes
Advanced oxidation processes are water-treatment methods that destroy organic contaminants through the action of strongly oxidising radical species, hydroxyl and sulfate radicals above all, generated in situ rather than dosed. Because the radicals react rapidly and with little selectivity, these processes come into their own where biological treatment fails, on compounds that are toxic, refractory or present at low concentration; the target is mineralisation, or at least conversion into intermediates that a biological step can then remove [1]. The classical route is the Fenton reaction, in which iron activates hydrogen peroxide; variants invoke ozone, ultraviolet light, semiconductor photocatalysis or electrochemical generation, and the choice depends on the water matrix, since carbonate and natural organic matter scavenge the radicals and raise the dose required [2]. Coupling one process with another, or with a biological or membrane stage, is generally more efficient than any single treatment, which is why integration is now the main direction of development [3]. Heterogeneous catalysts avoid the sludge produced by dissolved iron, and perovskite oxides activate peroxides at near-neutral pH [4]; iron oxide minerals catalyse the same chemistry without added iron [5]. Energy demand and the formation of oxidation by-products remain the main objections.
  • 9
  • 23 Sep 2026
Topic Review
Hydrogen Evolution Reaction (HER)
Hydrogen evolution reaction (HER) is the cathodic half-reaction in which protons or water are reduced to molecular hydrogen at an electrode. On platinum the exchange current is high and the overpotential small; the search for cheaper catalysts therefore concentrates on earth-abundant transition metals and their compounds, and bifunctional materials that also drive the oxygen evolution reaction simplify the overall device [1]. Two-dimensional transition metal carbides and nitrides have been examined as a class with a tunable surface and metallic conductivity [2]. Nickel chalcogenides, nickel selenide among them, reach appreciable activity in alkaline media [3]. Which phase is actually present under operation is not always the one that was synthesised, and in situ and operando measurements on nickel-based catalysts have shown the near-surface region reconstructing under cathodic bias [4]. Doping a metal phosphide with a second cation modifies the electronic structure and the hydrogen binding energy together, which is how samarium-doped nickel phosphide is understood [5].
  • 9
  • 28 Sep 2026
Topic Review
Heterogeneous Photocatalysis
Heterogeneous photocatalysis is the acceleration of a chemical reaction by light absorbed in a solid that is not consumed, so that the reaction takes place at the surface of the solid rather than in the surrounding phase. Absorption of a photon with energy above the band gap generates an electron-hole pair; if the pair separates and reaches the surface before recombining, it can reduce an acceptor and oxidise a donor there, and the quantum yield is therefore a competition between transfer and recombination. Titanium dioxide remains the reference material, and iron doping has been examined as a way to extend its absorption into the visible and to slow recombination [1]. Doping with rare-earth elements allows the simultaneous removal of an organic dye and of hexavalent chromium under visible light [2]. Whether an observed disappearance is photocatalytic at all needs a control, as the comparison of direct photolysis with photocatalytic treatment of carbamazepine shows [3]. Supporting the oxide on cellulose avoids the recovery step that a slurry requires [4]. Reactor geometry then decides how much of the absorbed light is actually turned into chemistry [5].
  • 8
  • 28 Sep 2026
Topic Review
Emulsion Separation
Emulsion Separation is the removal of the dispersed phase of an emulsion by a membrane, by a demulsifier, or by a field, and the membrane route dominates when the droplet is smaller than a few micrometres. A hydrogel nanofibrous membrane separates an oil-in-water emulsion by squeezing coalescence, so the water passes while the oil is held at the fibre [1]. A heterogeneous wettability, with a hydrophilic patch beside a hydrophobic one, breaks the film between two droplets and lets a coalesced droplet through [2]. A helical carbon nanotube grown inside a ceramic channel raises the flux without closing the pore [3]. A piezoelectric membrane turns a pressure fluctuation into a surface charge, and that charge destabilises the film at the interface [4]. A hyperbranched polyether demulsifier grafted on a polyvinylidene fluoride membrane adds a chemical route to the same end [5].
  • 7
  • 23 Oct 2026
Topic Review
Hybrid Membrane
A hybrid membrane, also termed a mixed matrix membrane, consists of molecular sieving particles dispersed in a continuous polymer, so transport is governed by both phases [1]. The filler contributes a selective path whose pore size is fixed by its crystal structure, while the polymer supplies cohesion and processability, and the effective permeability then depends on filler loading, on particle-matrix contact and on interfacial voids or rigidified layers [1]. At low loading the particles stay isolated and transport follows a Maxwell-type average, whereas above the percolation threshold a connected filler network short-circuits the polymer and selectivity is set by the filler alone [2]. A predictive model therefore needs the intrinsic permeability of each phase and the adhesion across the boundary [2]. Functionalising the particle surface suppresses void formation and delays the loss of selectivity at high loading [3].
  • 5
  • 06 Nov 2026
Topic Review
CO2 Reduction Reaction
CO2 reduction reaction is the electrochemical conversion of carbon dioxide into reduced carbon products at a cathode, driven by an applied potential and competing with hydrogen evolution in the same potential range. Which product appears is set early in the reaction path, where the first intermediate either binds through carbon or through oxygen, and on indium the exposed crystal plane decides that branching: surfaces favouring formate and surfaces favouring carbon monoxide differ only in their geometry [1]. Modifying the surface with an electronegative heteroatom shifts the binding energies of the intermediates and moves the product distribution away from what the unmodified metal gives [2]. Oxygen introduced into a copper-based catalyst has the same redirecting effect on the product spectrum [3]. Three independent handles, the local pH, the electrolyte cation and the surface structure, can be adjusted together to steer selectivity, which is why reported distributions depend so strongly on the cell [4]. Identifying which intermediate is present while the electrode is held at potential is the direct way to test these pictures [5].
  • 4
  • 23 Sep 2026
Topic Review
Cadmium Sulfide
Cadmium sulfide is a II-VI compound semiconductor with a direct band gap in the visible, examined mostly as a photocatalyst and as a quantum dot. Immobilising the sulfide on a polymer support was an early route to photocatalytic hydrogen sulfide splitting [1]. Embedding gold quantum dots in hollow sulfide spheres raises the hydrogen evolution rate by shortening the path to the surface [2]. Interfacial charge transfer from a sulfide quantum dot into a monolayer dichalcogenide can be followed directly [3]. Pairing a zinc-cadmium sulfide with graphitic carbon nitride gives a type-II heterojunction [4]. Sequential proton-coupled electron transfer at a sulfide surface converts nitrobenzene to aniline [5].
  • 4
  • 14 Oct 2026
Topic Review
Glycolysis
Glycolysis is the transesterification of a polyester with a glycol, and in polymer chemistry it is the most established route for depolymerising poly(ethylene terephthalate) back to monomers. The reaction cleaves the ester linkage with an excess of ethylene glycol and gives bis(2-hydroxyethyl) terephthalate, which can be repolymerised directly [1]. A catalyst is required to reach a useful rate, and zinc, manganese, and titanium compounds are the usual choices, while an ionic liquid can replace the conventional system [2]. The product is a mixture of oligomers whose composition depends on the glycol-to-polymer ratio and on the temperature, so the reaction has to be driven to completion before the monomer can be recovered [3]. The process tolerates coloured and slightly contaminated feedstock, which distinguishes it from mechanical recycling, but the purification of the product stream adds cost [4]. The same chemistry applied to polyurethane and to polycarbonate is less developed, because the alcohol has to be separated from a more complex mixture [5].
  • 3
  • 19 Oct 2026
Topic Review
Ceramic Membranes
Ceramic membranes are porous inorganic separation layers in which a metal oxide, carbide or nitride forms the selective barrier [1]. The support is a coarse asymmetric body of compatible ceramic, and successive coating and firing steps deposit layers of decreasing pore size, so the structure is graded from a macroporous support to a microporous skin [1]. Alumina, zirconia, titania and their mixtures set the chemical and thermal envelope, because the oxide decides resistance to acid, to alkali and to temperatures above those that polymer membranes tolerate [2]. Pore size and its distribution, not a dense lattice, separate the ultrafiltration and microfiltration grades from dense ceramic electrolytes [3]. The oxide surface carries hydroxyl groups and a pH-dependent charge, so ionic retention also depends on the electrolyte [1].
  • 3
  • 06 Nov 2026
  • Page
  • of
  • 27
Featured Entry Collections
>>

Featured Reprints

>>
Encyclopedia of Digital Society, Industry 5.0 and Smart City
Encyclopedia of Engineering
Encyclopedia of Social Sciences
Encyclopedia of Medieval Royal Iconography
Write an Entry
Feedback