Explanation:

Introduction:
Nitric acid (HNO3) is an important industrial chemical used in the production of fertilizers, explosives, and various other chemicals. One of the major methods used to produce nitric acid is the oxidation of ammonia (NH3) using a suitable catalyst.

The Catalyst:
The catalyst used for the oxidation of ammonia to produce nitric acid is a platinum-rhodium gauze. This catalyst is a combination of platinum (Pt) and rhodium (Rh) metals, typically in a ratio of 1:1 or 1:2.

Working Principle:
The oxidation of ammonia to nitric acid involves a two-step process. In the first step, ammonia is oxidized to nitrogen monoxide (NO):

4 NH3 + 5 O2 → 4 NO + 6 H2O

In the second step, nitrogen monoxide reacts with oxygen (O2) to form nitrogen dioxide (NO2):

2 NO + O2 → 2 NO2

The nitrogen dioxide (NO2) is then dissolved in water to form nitric acid (HNO3):

3 NO2 + H2O → 2 HNO3 + NO

Role of the Catalyst:
The platinum-rhodium gauze catalyst plays a crucial role in the ammonia oxidation process. It facilitates the conversion of ammonia to nitrogen monoxide by providing an active surface for the reaction to take place. The catalyst also helps in maintaining an optimal temperature range for the reaction.

The platinum-rhodium gauze catalyst has several advantages:
1. High Activity: The catalyst has a high surface area, which maximizes the contact between the reactants and the catalyst, leading to efficient conversion of ammonia to nitrogen monoxide.
2. Thermal Stability: The catalyst can withstand high temperatures without undergoing significant degradation, ensuring its long-term stability and effectiveness.
3. Selectivity: The catalyst promotes the desired reaction (ammonia oxidation) while minimizing side reactions, leading to high selectivity towards nitric acid formation.
4. Resistance to Poisoning: The catalyst is resistant to poisoning by impurities present in the reactants or the process stream, ensuring its prolonged use without significant loss in activity.

Conclusion:
The platinum-rhodium gauze catalyst is used for the oxidation of ammonia to produce nitric acid due to its high activity, thermal stability, selectivity, and resistance to poisoning.