A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy, without being consumed in the process. Catalysts work by facilitating the formation of the transition state, enabling the reaction to proceed more rapidly. They remain unchanged chemically and are not consumed during the reaction, allowing them to participate in multiple reaction cycles.

Two examples of catalysts are:

1. Enzymes: Enzymes are biological catalysts that facilitate and regulate biochemical reactions in living organisms. They are typically proteins that act as catalysts by lowering the activation energy required for specific reactions. Enzymes play a crucial role in various biological processes such as digestion, metabolism, and DNA replication. For example, the enzyme amylase catalyzes the hydrolysis of starch into smaller sugar molecules.
2. Platinum in Catalytic Converters: Platinum and other precious metals (such as palladium and rhodium) are commonly used as catalysts in catalytic converters of automobiles. They facilitate the conversion of harmful pollutants from exhaust gases into less harmful substances. For instance, platinum catalysts help to convert carbon monoxide (CO) into carbon dioxide (CO2) and nitrogen oxides (NOx) into nitrogen (N2) and oxygen (O2), reducing their environmental impact.

These examples demonstrate how catalysts can significantly enhance reaction rates and enable chemical transformations without being consumed in the process, making them crucial in various industrial, environmental, and biological applications.