The Ostwald Process & Catalytic Oxidation of Ammonia

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Ammonia is difficult to oxidize, but needs to be oxidized to form important products used in fertilizers. In this lesson we will learn how a catalyst can help ammonia be oxidized into nitric acid.

Fertilizer Production

If you have driven through the Midwest states in America you probably noticed field upon fields of corn and other plants. All of these fields of plants require large amounts of fertilizer in order to grow and produce in large quantities. Historically fertilizer was obtained from composting plant materials and from animal wastes. But you can only get so much fertilizer from these methods, and it often isn't as clean and safe as modern methods. If we still relied on plant and animal waste as sources of fertilizer, then food today would be a lot more prone to pathogenic microbes, and we wouldn't have as much cheap food as we see today. Over a century ago, a method was developed for creating fertilizer using nitrogen from the air, making it much easier to produce.

General Catalytic Oxidation

This method of making fertilizer, or the fertilizer ingredient nitric acid, is called the catalytic oxidation of ammonia. In general, the catalytic oxidation of ammonia uses a metal catalyst, such as platinum, copper, or nickel. This metal catalyst is heated, so that ammonia can reduce it. Oxygen added to the system can then oxidize the ammonia forming nitric oxide. When nitric oxide is mixed with water, nitric acid is formed. Nitric acid is the main ingredient in fertilizers.

Heat is necessary to drive the reaction forward. It occurs around 440 o F and at increased pressure, around 4-10 atmospheres. It needs to occur at such high temperatures and increased pressure because ammonia is not readily oxidized. It likes having three hydrogen atoms, and doesn't want to switch those out for oxygen atoms. But, the added heat drives the reaction, allowing it to occur. Once this reaction starts, it is highly exothermic, so it does start producing some of its own heat.

The type of apparatus that may be used to oxidize ammonia. Many different metal catalysts could be used. Platinum is shown here.
Ostwald process diagram

The complete reaction doesn't actually include the metal catalyst, because it only acts as a catalyst. It doesn't actually end up in any of the products. Ammonia and oxygen combine to form nitric oxide and water. As oxygen continues being added, the nitric oxide is further oxidized into nitrogen dioxide. When this is added to liquid water, some of it is further oxidized into nitric acid, while some is reduced back into nitric oxide. So, this process slowly oxidizes nitrogen, until it is fully oxidized.

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