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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. [3]
Homogeneous reactions are chemical reactions in which the reactants and products are in the same phase, while heterogeneous reactions have reactants in two or more phases. Reactions that take place on the surface of a catalyst of a different phase are also heterogeneous. A reaction between two gases or two miscible liquids is homogeneous.
An example of heterogeneous catalysis is the reaction of oxygen and hydrogen on the surface of titanium dioxide (TiO 2, or titania) to produce water. Scanning tunneling microscopy showed that the molecules undergo adsorption and dissociation. The dissociated, surface-bound O and H atoms diffuse together.
Reactions on surfaces are reactions in which at least one of the steps of the reaction mechanism is the adsorption of one or more reactants. The mechanisms for these reactions, and the rate equations are of extreme importance for heterogeneous catalysis .
Like with the fixed bed reactors, the liquid typically flows downward. At the same time, gas is flowing upward. The primary use for trickle-bed reactors is hydrotreatment reactions (hydrodesulfurization and hydrodemetalation of heavy crude oil, [2] hydrodeasphaltenization of coal tar [3]). This reactor is often utilized in order to handle feeds ...
Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, 18 O 2 – 16 O 2 and deuterium-alkane isotopic exchange, metal deposition, water detoxification, and gaseous pollutant removal.
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 the heterogeneous processes, gaseous substrate and oxygen (or air) are passed over solid catalysts. Typical catalysts are platinum, and redox-active oxides of iron, vanadium, and molybdenum. In many cases, catalysts are modified with a host of additives or promoters that enhance rates or selectivities.