The development of water and oxygen tolerant catalysts for ammonia synthesis

Due to its essential use as a fertilizer, ammonia synthesis from nitrogen and hydrogen is considered to be one of the most important chemical processes of the last 100 years with the development of the high temperature and pressure Haber-Bosch process winning Fritz Haber and Carl Bosch the Nobel Prize in chemistry in 1918 and 1931 respectively. Since then an enormous amount of work has been undertaken in order to investigate and develop effective catalysts for this process. This has led to the discovery of various effective catalysts for the synthesis of ammonia, with high activities shown for iron, ruthenium and metal nitride catalysts. Although the catalytic synthesis of ammonia has been extensively studied in the last century, many new catalysts are still currently being developed in order to reduce the operating temperature and pressure of the process and to improve the conversion of reactants to ammonia. In this thesis, new promotor materials for ammonia synthesis catalysts have been developed. These promotor materials are BaZr0.1Ce0.7Y0.2O3, ZrO2/Zr, Ce0.8Sm0.2O2-δ, CeO2-xNy, and Ce0.5Sm0.5O2-xNy. Through catalytic activity measurements carried out at high temperature and pressure along with a range of characterisation techniques including; X-ray diffraction (XRD), Scanning electron microscopy (SEM), Element mapping (EDX), B.E.T. surface area analysis (B.E.T.), and X-ray florescence (XRF), the effect of properties including extrinsic oxygen vacancies and nitrogen content were analysed for their effect on the ammonia synthesis reaction. In our results it was found that the activity can be significantly increased by introducing these desirable properties in to the catalyst promotors. It was found that an increase from 9.7 mmol g-1 h-1 to 12.3 mmol g-1 h-1 can be achieved at 3 MPa and 450 oC when extrinsic oxygen vacancies are introduced to the CeO2 promotor through samarium doping. By introducing nitrogen in to the CeO2 promotor to create oxynitride material CeO2-xNy an increase in activity was seen at 450 oC and 3 MPa to a value of 17.2 mmol g-1 h-1. By introducing extrinsic oxygen vacancies in to the oxynitride material it was shown that an activity of 18.8 mmol g-1 h-1 can be achieved at a temperature of 400 oC and 3 MPa for the Ce0.5Sm0.5O2-xNy promoted Fe catalyst and an activity of 27.6 mmol g-1 h-1 at 400 oC and 3 MPa for the Ce0.5Sm0.5O2-xNy promoted Ru catalyst.. From these results the potential to develop promising industrial catalysts for both current ammonia plants as well as small scale low carbon ammonia synthesis processes using renewable electricity can be seen whether using these material or new promotor materials exhibiting similar properties.