Hydrodesulphurisation (HDS) of naphtha prior to reforming or ammonia synthesis uses which standard catalyst system?

Introduction / Context:
Sulphur poisons downstream catalysts (e.g., reforming, steam reforming, ammonia synthesis). Therefore, feed purification via hydrodesulphurisation (HDS) is standard. Recognising the correct catalyst system is basic process knowledge in fertiliser and petroleum refining technology.

Given Data / Assumptions:

• Naphtha contains organosulphur impurities.
• HDS converts sulphur compounds to H2S in the presence of H2.
• Industrial HDS catalysts are supported sulfides of transition metals.

Concept / Approach:
Co–Mo (cobalt–molybdenum) sulfide supported on alumina is the classic HDS catalyst for light and middle distillates, including naphtha. It provides high activity for C–S bond scission under typical temperatures and pressures. Alternatives like Ni–Mo are also used, but Pt–Rh and silica gel are not HDS catalysts; nickel alone is far less effective for complex feeds.

Step-by-Step Solution:

Verification / Alternative check:
Process schematics in ammonia plants show naphtha HDS reactors upstream of reformers, loaded with Co–Mo/Al2O3 catalysts and followed by ZnO for H2S removal.

Why Other Options Are Wrong:

• Pt–Rh: reforming/oxidation, not HDS.
• Silica gel: adsorbent, not a hydrotreating catalyst.
• Nickel only: insufficient for broad HDS at standard conditions.
• Cu–Zn–Al2O3: low-temperature shift catalyst, not HDS.

Common Pitfalls:
Confusing hydrotreating (sulfided Co–Mo/Ni–Mo) with reforming (Pt-based) or shift (Cu–Zn–Al2O3) catalysts.

Final Answer:
Co–Mo on alumina