Many reactions although thermodynamically allowed, do not occur because they are kinetically inhibited. In case the activation energy is too high, the reaction will not occur even if it is an exothermic reaction. In such situations, a catalyst is required to lower the activation energy by providing energetically favorable pathways in which the activation energies of all intermediates steps are lowered compared to the barrier of the gas phases reaction. The first process of heterogeneous catalysis is adsorption. It is always an exothermic reaction of at least one of the gases whose intramolecular bond are weakened or broken and after reacting with other species in several consecutive steps at the surface of the catalyst; the products desorbs into the gas phase and thereby generate the active sites for the catalytic cycle. A catalyst should therefore have a low affinity for the products to enable desorption and high affinity for the reactants.
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Heterogeneous catalysis is adsorption
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