Describe the chemical reactions involved in Selective Catalytic Reduction (SCR) for NOx control.

Selective Catalytic Reduction (SCR) is a post-combustion exhaust gas treatment technology that converts nitrogen oxides (NOx), primarily nitric oxide (NO) and nitrogen dioxide (NO₂), into harmless diatomic nitrogen (N₂) and water vapor (H₂O). The core chemical reactions occur on the surface of a catalyst using a reducing agent, typically ammonia (NH₃) or a precursor like urea. First, the reductant is introduced into the hot exhaust gas stream. If urea is used, it undergoes thermal decomposition and hydrolysis upon injection, forming ammonia and carbon dioxide. For example, urea (CO(NH₂)₂) decomposes into ammonia and isocyanic acid, which then hydrolyzes to form more ammonia and carbon dioxide: CO(NH₂)₂ + H₂O → 2NH₃ + CO₂. This generated ammonia then serves as the active reducing a....