The catalyst used for the oxidation of ammonia to produce nitric acid is the Platinum-Rhodium gauze.

The process of ammonia oxidation to nitric acid is known as the Ostwald process, and it involves several steps. The catalyst plays a crucial role in facilitating the reaction and increasing its efficiency.

Here is a detailed explanation of why the Platinum-Rhodium gauze is used as the catalyst for this reaction:

1. The Ostwald process:
- The Ostwald process is a multi-step chemical reaction that converts ammonia (NH3) to nitric acid (HNO3).
- The first step involves the oxidation of ammonia to nitric oxide (NO) and water (H2O).
- The second step is the oxidation of nitric oxide to nitrogen dioxide (NO2).
- The third step is the absorption of nitrogen dioxide in water to form nitric acid.

2. Catalyst requirements:
- The catalyst used in the ammonia oxidation process must have several key properties.
- It should promote the desired reaction while minimizing the formation of unwanted byproducts.
- It should have high activity and selectivity towards the desired reaction.
- It should be stable under the reaction conditions and resist deactivation.

3. Platinum-Rhodium gauze catalyst:
- The Platinum-Rhodium gauze is widely used as the catalyst in the Ostwald process.
- It is a mesh-like structure made of platinum and rhodium metals.
- The catalyst is typically supported on a ceramic or metal substrate.
- The Platinum-Rhodium gauze catalyst has several advantages for this reaction.

4. Advantages of Platinum-Rhodium gauze catalyst:
- High activity: Platinum and rhodium have high catalytic activity for the oxidation of ammonia.
- Selectivity: The catalyst promotes the desired oxidation reactions while minimizing side reactions.
- Resistance to deactivation: The Platinum-Rhodium gauze catalyst is stable under the reaction conditions and does not easily get deactivated.
- Surface area: The mesh-like structure of the catalyst provides a large surface area for the reactants to come into contact with the catalyst, enhancing the reaction rate.

In conclusion, the Platinum-Rhodium gauze catalyst is used for the oxidation of ammonia to produce nitric acid in the Ostwald process due to its high activity, selectivity, stability, and large surface area.