The production of synthetic ammonia remains dependent on the energy- and capital-intensive Haber–Bosch process. Extensive research in molecular catalysis has demonstrated ammonia production from dinitrogen, albeit at low production rates. Mechanistic understanding of dinitrogen reduction to ammonia continues to be delineated through study of molecular catalyst structure, as well as through understanding the naturally occurring nitrogenase enzyme. The transition to Haber–Bosch alternatives through robust, heterogeneous catalyst surfaces remains an unsolved research challenge. Catalysts for electrochemical reduction of dinitrogen to ammonia are a specific focus of research, due to the potential to compete with the Haber–Bosch process and reduce associated carbon dioxide emissions. However, limited progress has been made to date, as most electrocatalyst surfaces lack specificity towards nitrogen fixation. In this Review, we discuss the progress of the field in developing a mechanistic understanding of nitrogenase-promoted and molecular catalyst-promoted ammonia synthesis and provide a review of the state of the art and scientific needs for heterogeneous electrocatalysts. The artificial synthesis of ammonia remains one of the most important catalytic processes worldwide, over 100 years after its development. In this Review, recent developments in enzymatic, homogeneous and heterogeneous catalysis towards the conversion of nitrogen to ammonia are discussed, with a particular focus on how mechanistic understanding informs catalyst design.

Catalysts for nitrogen reduction to ammonia

The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. It is seen that Ru/MgAl2O4 is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si3N4. The activity differences cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT calculations have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is suggested that special B5-type sites are primarily responsible for the catalytic activity of the present supported Ru catalysts. It is shown how the number of such B5-type sites depends on the Ru crystal size for a given crystal morphology. We have found that the activity of the Ru/MgAl2O4 catalyst increases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0 nm due to sintering and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in supported ammonia synthesis catalysts. This is in agreement with a low number of B5-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphology of the Ru crystals and the resulting change in abundance of B5-type sites is the main cause for the significant activity variations observed for Ru catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring B5-type sites.

Structure sensitivity of supported ruthenium catalysts for ammonia synthesis

Adsorption of nitrogen on potassium promoted Fe(111) and (100) surfaces

Publisher Summary This chapter discusses the mechanism of hydrocarbon synthesis over Fischer-Tropsch catalysts. The superiority of liquid over gaseous energy carriers, in particular for automotive purposes, resides in the high energy density of liquids and the relatively low weight of the required container. When compared to solid fuels such as coal, liquids have the advantage that pollutants can be removed more easily, and transport through pipelines or in tankers is cheap and efficient. The chemistry of the catalytic processes is, of course, independent of the way in which the synthesis gas was manufactured; besides coal gasification the steam re-forming of natural gas is a well-known route to produce synthesis gas, although with a higher H 2 /CO ratio. It is convenient to distinguish methanation, where methane is the predominant product, and the Fischer- Tropsck process directed toward the manufacture of predominantly liquid hydrocarbons. However, as the product composition for a given catalyst largely depends on the conditions, the term Fischer-Tropsck catalysis is used also for those cases in which, owing to the variation of one or more of these parameters, the product is mainly gaseous or solid at room temperature and atmospheric pressure.

Mechanism of Hydrocarbon Synthesis over Fischer-Tropsch Catalysts

Preparation and characterization of chlorine-free ruthenium catalysts and the promoter effect in ammonia synthesis: 3. A magnesia-supported ruthenium catalyst

Support and promoter effect of ruthenium catalyst: II. Ruthenium/alkaline earth catalyst for activation of dinitrogen

Support and promoter effect of ruthenium catalyst: I. Characterization of alkali-promoted ruthenium/alumina catalysts for ammonia synthesis

Potassium-dinitrogen-ruthenium complex as an active catalyst for nitrogen fixation

Isotopic equilibration of nitrogen on potassium-promoted transition metal catalysts☆

Nitrogen uptake by metallic ruthenium is enhanced by the presence of potassium vapor, simultaneously increasing the stable incorporation of potassium, demonstrating a “binder action” of nitrogen. The molar ratio of nitrogen uptake to the stable potassium suggests a chemical formula (KN2Ru)n, as supported by formation of hydrazine on hydrolysis.

Asao Ohya

Papers

Support and promoter effect of ruthenium catalyst: II. Ruthenium/alkaline earth catalyst for activation of dinitrogen

Support and promoter effect of ruthenium catalyst: I. Characterization of alkali-promoted ruthenium/alumina catalysts for ammonia synthesis

Potassium-dinitrogen-ruthenium complex as an active catalyst for nitrogen fixation

Isotopic equilibration of nitrogen on potassium-promoted transition metal catalysts☆

Cooperative incorporation of potassium and nitrogen into metallic ruthenium