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Heterogeneous Catalysts
In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. Phase distinguishes between not only solid, liquid, and gas components, but also immiscible mixtures (e.g. oil and water), or anywhere an interface is present. Heterogeneous catalysis typically involves solid phase catalysts and gas phase reactants. In this case, there is a cycle of molecular adsorption, reaction, and desorption occurring at the catalyst surface. Thermodynamics, mass transfer, and heat transfer influence the rate (kinetics) of reaction. Heterogeneous catalysis is very important because it enables faster, large-scale production and the selective product formation. Approximately 35% of the world's GDP is influenced by catalysis. The production of 90% of chemicals (by volume) is assisted by solid catalysts. The chemical an ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydrogenation On Catalyst
Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons. Process Hydrogenation has three components, the unsaturated substrate, the hydrogen (or hydrogen source) and, invariably, a catalyst. The reduction reaction is carried out at different temperatures and pressures depending upon the substrate and the activity of the catalyst. Related or competing reactions The same catalysts and conditions that are used for hydrogenation reactions can also lead to isomerization of the alkenes from ci ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lennard-Jones Potential
The Lennard-Jones potential (also termed the LJ potential or 12-6 potential) is an intermolecular pair potential. Out of all the intermolecular potentials, the Lennard-Jones potential is probably the one that has been the most extensively studied. It is considered an archetype model for simple yet realistic intermolecular interactions. The Lennard-Jones potential models soft repulsive and attractive ( van der Waals) interactions. Hence, the Lennard-Jones potential describes electronically neutral atoms or molecules. It is named after John Lennard-Jones. The commonly used expression for the Lennard-Jones potential is V_\text(r) = 4\varepsilon \left \left(\frac\right)^ - \left(\frac\right)^6 \right, where r is the distance between two interacting particles, \varepsilon is the depth of the potential well (usually referred to as 'dispersion energy'), and \sigma is the distance at which the particle-particle potential energy V is zero (often referred to as 'size of the particle'). Th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Catalyst Support
In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the material. Consequently, great effort is made to maximize the specific surface area of a catalyst. One popular method for increasing surface area involves distributing the catalyst over the surface of the support. The support may be inert or participate in the catalytic reactions. Typical supports include various kinds of carbon, alumina, and silica. Applying catalysts to supports Two main methods are used to prepare supported catalysts. In the impregnation method, a suspension of the solid support is treated with a solution of a precatalyst, and the resulting material is then activated under conditions that will convert the precatalyst (often a metal salt) to a more active state, perhaps the metal itself. In such cases, the catalyst support is usual ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dispersion (chemistry)
A dispersion is a system in which distributed particles of one material are dispersed in a continuous phase of another material. The two phases may be in the same or different states of matter. Dispersions are classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not precipitation occurs, and the presence of Brownian motion. In general, dispersions of particles sufficiently large for sedimentation are called suspensions, while those of smaller particles are called colloids and solutions. Structure and properties Dispersions do not display any structure; i.e., the particles (or in case of emulsions: droplets) dispersed in the liquid or solid matrix (the "dispersion medium") are assumed to be statistically distributed. Therefore, for dispersions, usually percolation theory is assumed to appropriately describe their properties. However, percolation theory can be applied only if t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mesoporous Silicates
Mesoporous silicates are silicates with a special morphology. Background Porous inorganic solids have found great utility as catalysts and sorption media because of their large internal surface area, i.e. the presence of voids of controllable dimensions at the atomic, molecular, and nanometer scales. With increasing environmental concerns worldwide, nanoporous materials have become more important and useful for the separation of polluting species and the recovery of useful ones. In recent years there has been great progress in applying environmentally friendly zeolites in heterogeneous reaction catalysis. The reason for their success is related to their specific features in converting molecules having kinetic diameter below 1 nm, but they become inadequate when reactants with sizes above the dimensions of the pores have to be processed. Research efforts to synthesize zeolites with larger pore diameter, high structural stability and catalytic activity have not given the expe ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Active Site
In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate ( binding site) and residues that catalyse a reaction of that substrate (catalytic site). Although the active site occupies only ~10–20% of the volume of an enzyme, it is the most important part as it directly catalyzes the chemical reaction. It usually consists of three to four amino acids, while other amino acids within the protein are required to maintain the tertiary structure of the enzymes. Each active site is evolved to be optimised to bind a particular substrate and catalyse a particular reaction, resulting in high specificity. This specificity is determined by the arrangement of amino acids within the active site and the structure of the substrates. Sometimes enzymes also need to bind with some cofactors to fulfil their function. The activ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Adsorb
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a fluid (the ''absorbate'') is dissolved by or permeates a liquid or solid (the ''absorbent''). Adsorption is a ''surface phenomenon'', while absorption involves the whole volume of the material, although adsorption does often precede absorption. The term ''sorption'' encompasses both processes, while ''desorption'' is the reverse of it. Like surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent or metallic) of the constituent atoms of the material are fulfilled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bon ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Surface Diffusion
Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters ( adparticles) at solid material surfaces.Oura, Lifshits, Saranin, Zotov, and Katayama 2003, p. 325 The process can generally be thought of in terms of particles jumping between adjacent adsorption sites on a surface, as in figure 1. Just as in bulk diffusion, this motion is typically a thermally promoted process with rates increasing with increasing temperature. Many systems display diffusion behavior that deviates from the conventional model of nearest-neighbor jumps. Tunneling diffusion is a particularly interesting example of an unconventional mechanism wherein hydrogen has been shown to diffuse on clean metal surfaces via the quantum tunneling effect. Various analytical tools may be used to elucidate surface diffusion mechanisms and rates, the most important of which are field ion microscopy and scanning tunneling microscopy.Oura, Lifshits, Saranin, Zotov, and Katayama ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chain Propagation
Chain propagation (sometimes referred to as propagation) is a process in which a reactive intermediate is continuously regenerated during the course of a chemical chain reaction. For example, in the chlorination of methane, there is a two-step propagation cycle involving as chain carriers a chlorine atom and a methyl radical IUPAC Gold Book which are regenerated alternately: :•Cl + CH4 → HCl + •CH3 :•CH3 + Cl2 → CH3Cl + •Cl The two steps add to give the equation for the overall chain reaction: :CH4 + Cl2 → CH3Cl + HCl. Polymerization In a |
Catalytic Reaction Coordinate
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst. Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category. Catalysis is ubiquitous in chemical industry of all kinds. Estimates are that 90% of all commercially produced chemical products involve catalysts at some stag ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dissociation (chemistry)
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid dissolves in water, a covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission, which gives a proton (H+) and a negative ion. Dissociation is the opposite of association or recombination. Dissociation constant For reversible dissociations in a chemical equilibrium :AB A + B the dissociation constant ''K''d is the ratio of dissociated to undissociated compound :K_d = \mathrm where the brackets denote the equilibrium concentrations of the species. Dissociation degree The dissociation degree \alpha is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol α. More accurately, degree of dissociation refers to the amount of solute dissociated ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
