Ostwald step in nitric acid manufacture:\nWhich catalyst is used for the high-temperature oxidation of ammonia to nitric oxide?

Introduction / Context:
The first step of the Ostwald process oxidizes ammonia (NH3) with air over a precious-metal catalyst to form nitric oxide (NO), which is further processed to nitric acid. Correct identification of the catalyst is fundamental to understanding nitric acid manufacture and the broader nitrogen fertiliser value chain.

Given Data / Assumptions:

• Reaction: NH3 + O2 → NO + H2O (followed by oxidation and absorption).
• Operating conditions: high temperature, short contact time, gauze catalyst.
• Catalyst must resist sintering and maintain high selectivity to NO.

Concept / Approach:
Industrial practice uses woven gauzes of platinum alloyed with rhodium. Rhodium improves mechanical strength, resistance to creep, and selectivity under harsh conditions. Alternative catalysts like vanadium pentoxide are used in different processes (e.g., SO2 oxidation in sulfuric acid manufacture), not in ammonia oxidation. Cobalt–molybdenum sulfide is a hydrotreating catalyst and irrelevant here.

Step-by-Step Solution:

Verification / Alternative check:
Plant diagrams and catalyst supplier literature consistently cite Pt–Rh gauze packs for NH3 oxidation, sometimes with palladium catchment for platinum loss control.

Why Other Options Are Wrong:

• Pt–Be or Pt–Mo: non-standard for Ostwald and lack the required proven performance.
• Co–Mo sulfide: hydrodesulfurization, not NH3 oxidation.
• V2O5: used for SO2 → SO3, not NH3 → NO.

Common Pitfalls:
Confusing catalysts between sulfuric and nitric acid industries; overlooking that Ostwald’s step is highly specialised.

Final Answer:
Platinum–rhodium gauze