The successful synthesis of ammonia from elemental nitrogen and hydrogen using high temperature and pressure in the presence of an efficient catalyst, by Fritz Haber and Carl Bosch, at the beginning of the 20th century, ranks as one of the most important discoveries in the history of chemistry, catalysis, and chemical engineering. They succeeded in overcoming engineering requirements, in discovering a suitable catalyst, and establishing optimal conditions, as well as industrial-scale, continuous flow high-pressure process. For these achievements they were awarded the Nobel Prize in Chemistry: Fritz Haber in 1918 "for the synthesis of ammonia from its elements" and Carl Bosch, 1931, together with Friedrich Bergius "for theirs services in developing chemical high-pressure methods". Regarding its industrial development and progress, ammonia has become the base from which almost all nitrogen-containing products are derived. It is particularly important in supplying ammonia derivatives for agricultural uses as the largest source of nitrogenous fertilizer, thus increasing tremendously food production and therefore contributing enormously to the survival of the fast growing world population. These discoveries have stimulated both basic and applied research all over the world, up to present times, leading to the development of the ammonia industry. Besides fertilizers, it is used as a raw material of nitrogen compounds, including nitric acid, ammonia salts, urea, acrylonitrile, amines, amides, and others. This review article on ammonia synthesis highlights the history of its discoveries from elements by Fritz Haber and Carl Bosch and their many associates. It also shows the development of this process and its recent progress. In addition, the development and application of new catalysts of ammonia synthesis, as ruthenium base catalyst and iron-catalyst from wustite (Fe1-xO) or Co-modified, is described. The research activities of ammonia synthesis under mild conditions using biological, electrochemical or nitrogen/transition metal intermediate complex formation methods, is also described. The important biographical data and scientific achievements of the main contributors for the described subject, Fritz Haber, Carl Bosch, Gerhard Ertl and Alwin Mittasch, are presented as well. Fritz Haber, a German chemist, was a professor at Technische Hochschule in Karlsruhe, besides his many scientific achievements, in 1908, he discovered ammonia synthesis from its elements on the high-pressure recycling reactor equipment using iron, osmium or uranium as catalyst. Carl Bosch, German chemical engineer, joined the BASF company in 1899 when he realized the industrial Haber-Bosch ammonia process. He also designed the new reactor equipment that could endure not only high pressure but also the influence of hydrogen on steel. His associate, Alwin Mittasch, German chemist, invented a multi-component catalyst by adding a minor quantity of metal oxides (up to 5 %) as promoters (that is, Al2O3, CaO and K2O) into pure iron catalyst for ammonia synthesis, and thus enhanced its performance dramatically. It is used still today, often under the name Mittasch catalyst. Gerhard Ertl, German physical chemist, in 2007, received a Nobel Prize for chemistry: "for his pioneering work in the discipline of surface chemistry". Among others, he refined the mechanism of Haber-Bosch process of ammonia synthesis, including the role of catalytic promoters. The first ammonia plant in the world using Haber-Bosch process was under stream in 1913 in Oppau, near Leverkusen, followed by a plant of a much larger capacity at Leunawerke near Merseburg in 1917. The worldwide production of ammonia, using Haber-Bosch process, today exceeds 150 million tons annually.
