Ostwald process

The Ostwald process is a major industrial method for producing nitric acid, a chemical of central importance in the manufacture of fertilisers, explosives, and numerous inorganic nitrates. Developed and patented in the early twentieth century, it remains the dominant commercial route owing to its efficiency, cost-effectiveness, and strong integration with the ammonia-producing Haber process. In modern chemical industries, the two processes operate in close succession, with ammonia serving as the essential feedstock for nitric acid synthesis.

Background and industrial significance

Nitric acid plays a crucial role in global agriculture, primarily through the production of nitrate-based fertilisers such as ammonium nitrate. The Ostwald process is valued for its high conversion efficiencies, manageable operating conditions, and compatibility with large-scale continuous production. Because it relies on ammonia derived from atmospheric nitrogen via the Haber process, it effectively enables the industrial fixation of nitrogen. Consequently, the Ostwald–Haber combination is often regarded as a foundation of twentieth-century chemical manufacturing.
The process occurs in two principal stages: the catalytic oxidation of ammonia to nitric oxide, followed by the subsequent oxidation and absorption of nitrogen oxides to form nitric acid. Both reactions are exothermic, contributing to the thermal efficiency of the method.

Initial catalytic oxidation of ammonia

The first stage involves oxidising ammonia with atmospheric oxygen. Ammonia is passed over a catalyst at elevated temperature and moderate pressure. A platinum gauze catalyst alloyed with approximately 10% rhodium is most commonly used, providing mechanical strength and enhanced nitric oxide yield. Alternatives such as platinum on fused silica wool, copper, or nickel may also be employed, though platinum-rhodium remains the industrial standard.
The catalytic reaction generates nitric oxide and steam, and proceeds rapidly after initiation due to its exothermic nature. Typical operational temperatures approach 900°C, and pressures may reach several atmospheres depending on plant design. Because the reaction produces large quantities of heat, the apparatus is constructed to manage thermal stresses and avoid catalyst degradation.
A number of side reactions limit the efficiency of nitric oxide formation. Some competing pathways convert ammonia directly to nitrogen gas. These secondary reactions are mitigated by carefully regulating contact time between the gas stream and catalyst surface. Another undesired reaction forms nitrous oxide, which reduces yield and carries environmental implications. Managing these by-products is therefore an important aspect of reactor optimisation.

Catalyst behaviour and degradation

The platinum-rhodium catalyst undergoes gradual deterioration due to prolonged exposure to extreme temperatures and reactive species. A characteristic mode of degradation, informally termed cauliflowering, refers to the formation of fragile porous growths on the catalyst surface. The precise mechanism remains uncertain. Proposed explanations include physical distortion through penetration of hydrogen atoms into the metal lattice or migration of metal atoms from within the catalyst towards its surface. Regular replacement of the gauze is a standard operational requirement, and recovery systems are employed to capture valuable platinum-group metals lost during use.

Secondary oxidation to nitrogen dioxide

The nitric oxide generated in the first step is subsequently cooled from around 900°C to approximately 250°C, a temperature favourable for its further oxidation. In this exothermic conversion, nitric oxide reacts readily with oxygen to form nitrogen dioxide. A related equilibrium reaction produces dinitrogen tetroxide, which interconverts with nitrogen dioxide depending on temperature and pressure. These transformations are important for the absorptive stage that follows, as nitrogen dioxide and its dimer serve as the primary reactive species for nitric acid formation.

Absorption and conversion to nitric acid

The second major stage consists of absorbing nitrogen oxides in water within an absorption column, often designed as a plate or packed tower. As nitrogen dioxide dissolves, it undergoes a sequence of redox and hydration reactions that yield dilute nitric acid. In this process, part of the nitrogen dioxide is reduced back to nitric oxide, which is subsequently recycled to the oxidation stage. The recycling loop improves overall nitrogen utilisation efficiency.
More than forty distinct absorption reactions involving oxides of nitrogen have been documented, though only a subset is industrially significant. If the final reaction occurs in the presence of air, additional oxygen contributes to the final oxidation steps. The dilute acid produced is commonly concentrated through distillation to meet industrial specifications.

Overall reaction and thermodynamic considerations

The overall stoichiometry may be derived by combining the individual steps. Without specifying the physical state of water, the summarised reaction shows that ammonia reacts with oxygen to yield nitric acid and is strongly exothermic. Plants often exploit this heat release for energy recovery, thereby improving thermal efficiency.
Where air participates in the final absorption phase, the combined reaction differs slightly but still demonstrates substantial heat evolution. Regardless of variations in implementation, the overarching thermodynamic profile facilitates continuous, energy-efficient operation.

Historical development

The Ostwald process was developed by Wilhelm Ostwald, a prominent chemist whose work contributed significantly to the foundations of physical chemistry. He secured a patent for the method in 1902, enabling widespread industrial production of nitric acid and profoundly influencing fertiliser manufacture, military technology, and inorganic chemical synthesis. Its integration with the Haber process positioned it as a cornerstone of the chemical industries of the twentieth century and beyond.