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 agent.

Next, the exhaust gas, now containing NOx and ammonia, passes over a catalyst. The catalyst, commonly made from vanadium-titanium oxide or zeolites, provides active sites where the reactions can occur at lower temperatures than without a catalyst, significantly increasing the reaction rate and selectivity. The 'selective' aspect means that the ammonia preferentially reacts with NOx rather than with oxygen, which is also present in the exhaust gas.

The main chemical reactions involved in NOx reduction depend on the specific ratio of NO to NO₂ present in the exhaust gas:

1. Standard SCR Reaction: This is the primary reaction when nitric oxide (NO) is the dominant NOx component. Ammonia reacts with NO in the presence of oxygen to produce nitrogen gas and water vapor:
4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

2. Fast SCR Reaction: This reaction is highly efficient and occurs much faster than the standard SCR reaction when both nitric oxide (NO) and nitrogen dioxide (NO₂) are present in roughly equal molar proportions (a NO:NO₂ ratio close to 1:1). In this case, ammonia reacts with both forms of NOx simultaneously:
2NO + 2NO₂ + 4NH₃ → 4N₂ + 6H₂O
This reaction is highly desirable due to its rapid kinetics, leading to excellent NOx conversion rates.

3. NO₂ SCR Reaction: When nitrogen dioxide (NO₂) is the dominant NOx component in the exhaust gas, ammonia reacts directly with NO₂ to form nitrogen gas and water vapor. This reaction generally proceeds at a slower rate compared to the fast SCR reaction:
6NO₂ + 8NH₃ → 7N₂ + 12H₂O

In all these reactions, the harmful nitrogen oxides are chemically transformed into harmless, inert diatomic nitrogen gas, which makes up about 78% of the Earth's atmosphere, and water vapor. The catalyst itself is not consumed during the process, facilitating continuous NOx reduction.