Gas cleaning in air pollution control Which of the following materials acts as an effective adsorbent for removing nitrogen oxides (NO and NO2) from gas or air streams in industrial pollution-control practice?

Introduction / Context:
Nitrogen oxides (NO and NO2, collectively NOx) are key gaseous pollutants emitted from combustion sources. Industrial gas cleaning often uses adsorption as a polishing step, especially for intermittent or low-flow exhausts. This question asks which common sorbent is actually used for capturing NOx from air/gas streams.

Given Data / Assumptions:

• Target pollutants: NO and NO2 (NOx) in an industrial exhaust.
• Single-step adsorption choice among common media.
• Standard temperature ranges; no catalytic reduction specified unless media are impregnated.

Concept / Approach:
Unmodified activated carbon has limited capacity for NO; however, activated carbon (active carbon) is widely used when appropriately impregnated (e.g., with alkali or oxidants) to oxidise/convert NO to NO2 or nitrate/nitrite species that are retained. In practice, “activated carbon” is the correct umbrella choice among listed materials for adsorptive NOx control in gas cleaning. Other listed media are either unsuitable or typically used for other contaminants (moisture, H2S, SO2).

Step-by-Step Solution:

Verification / Alternative check:
Chemisorptive carbons are specified in vendor literature for NOx service; field systems combine oxidation on the carbon surface with adsorption of reaction products, confirming practical effectiveness.

Why Other Options Are Wrong:

• Silica gel: A desiccant for water vapour; not a NOx sorbent.
• Bog iron (iron oxide): Commonly used for H2S removal, not NOx.
• Pulverised limestone: Used for SO2 neutralisation, not NOx adsorption.
• Molecular sieves only: Zeolites are not standard NOx adsorbents for typical stacks without special chemisorption routes.

Common Pitfalls:
Assuming any porous solid will adsorb NOx effectively; NO is poorly adsorbed unless oxidised or chemically bound. Activated carbon impregnations solve this gap.

Final Answer:
Active carbon