Electrochemical nitrogen reduction for ammonia synthesis (2015)

Ammonia NH3 is one of the most important chemicals worldwide and quantitatively the second largest heterogeneously catalyzed chemical after H2SO4. Around 80% of the produced NH3 is used as fertilizer precursor to supply the growing world population with sufficient amounts of food. Starting from N2 and H2, NH3 nowadays is typically produced by the Haber process applying an Fe-based catalyst. The worldwide production capacity for NH3 is about 140 million tonnes per year. Disadvantageously, the Haber process is one of the largest industrial energy consumers and for each ton of NH3 produced two tonnes of CO2 are emitted as well. An environmentally more friendly alternative synthesis process is desired, which can help to produce NH3 in a sustainable and ecological way in the future.In the present thesis, the electrochemical NH3 synthesis in an electrochemical membrane reactor (ecMR) was investigated. An ecMR consists of two compartments, the anodic and the cathodic half-cell, which are separated by a cation exchange membrane (CEM). The core of the ecMR is the membrane electrode assembly (MEA) which consists of two electrodes pressed into the CEM. To be environmentally friendly, the needed H+ for the NH3 synthesis were generated by the oxidation of H2O at the anode. An electrical potential was applied to the ecMR as driving force to create an electrical field, in which the H+ migrate through the membrane to the cathode. Nitrogen gas was fed to the cathodic department and was reduced to NH3 at the cathode. On a large scale application, renewable energy sources such as wind or solar power can drive the process. At the anode a state-of-the-art IrMMO catalyst was applied for the oxidation of H2O. Based on recently published density functional theory (DFT) calculations Ti, Fe and Ru were chosen as potential cathodic catalysts. Titanium and Fe were commercially available in the form of randomly structured felts and used as received as cathode in the ecMR. Since Ru is an expensive noble metal, a new galvanic coating process of Ru on Ti felts was investigated. The so-prepared Ru electrodes were tested in the ecMR as well as Ti and Fe electrodes. All three catalysts showed a high activity for the electrochemical NH3 synthesis, while Ru gave the best results with respect to production rate of NH3 and current efficiency. Parallel to the NH3 synthesis in gas phase in the ecMR also first experiments with Ti were performed in an one- and two-compartment liquid phase setup. Compared to the results obtained in gas phase, the liquid phase system was less stable and the results were less predictable. To answer the question if an electrochemical NH3 synthesis process can compete with the state-of-the-art Haber process, a complete synthesis process with N2 production via air separation, NH3 synthesis in an ecMR and downstream product separation was modelled and optimized in Aspen+. For the ecMR a new model was developed in Aspen Custom Modeler and implemented in the complete synthesis process in Aspen+.The results obtained in the present thesis encourage further research of the electrochemical NH3 synthesis. Particularly new electrocatalysts need to be investigated to further develop and optimize the ecMR to be the technology of choice in the future for the environmentally friendly synthesis of NH3.