触媒作用

The study of reaction mechanisms and kinetics helps researchers to optimize the speed of a reaction. An increase in temperature or concentration will accelerate the rate but can also lead to decomposition or side products when increased too much. Alternatively, a chemical that accelerates the reaction rate without being permanently altered or consumed can be added to the reaction mixture. These substances are called catalysts and affect the rate constant by changing the activation energy. Recall that the higher the activation energy, the slower the reaction rate. Hence, many chemical reactions, like the decomposition of aqueous hydrogen peroxide, progress slowly. The addition of a catalyst lowers the activation energy by providing an alternative reaction mechanism without affecting the energy states of reactants and products. There are two types of catalysts—homogeneous and heterogeneous. A homogeneous catalyst exists in the same phase as the reactants. For instance, sodium bromide is added to aqueous hydrogen peroxide to accelerate its decomposition into gaseous oxygen and water. First, the bromide ions react with hydrogen peroxide in an acidic medium to form an orange solution of aqueous bromine and water. Next, aqueous bromine and hydrogen peroxide react and release bromide ions indicated by a colorless solution. Characteristic for a catalyst, bromide ions accelerate the reaction but remain unconsumed and do not appear in the net balanced equation. A heterogeneous catalyst exists in a different physical state compared to the reactants. Heterogeneous catalysis is characterized by four underlying steps—adsorption, diffusion, reaction, and desorption. For instance, the hydrogenation reaction of unsaturated ethene gas to ethane is accelerated by finely dispersed palladium on charcoal. Here, the catalyst exists in its solid-state, while reactants are in their gaseous forms. During this heterogeneous catalysis, ethene and hydrogen molecules adsorb on the catalyst surface. The hydrogen-hydrogen bond breaks allowing the hydrogen atoms to diffuse on the catalyst surface. When diffusing hydrogen atoms encounter adsorbed ethene molecules, they react to form saturated ethane. Subsequently, the products desorb and move away from the metal surface, leaving the intact catalyst behind.