Electrochemically Induced Urea to Ammonia on Ni Based Catalys

Ammonia is a crucial chemical used in many fields such as the fertilizer, textile, and food industries. Nowadays, ammonia is commonly used as a reducing agent to reduce nitrogen oxide (NOx) to non-harmful nitrogen gas and water which is called the selective catalytic reduction (SCR) process. For the on-site production of ammonia, two traditional methods are urea hydrolysis and urea pyrolysis. However both of them require high temperature and pressure which are not accessible in mobile engines. A novel electrochemically induced urea to ammonia (eU2A) process in alkaline media was introduced and investigated in this investigation. Nickel beads are employed as the working catalyst in a two-electrode eU2A reactor using 7.0 M KOH as the supporting electrolyte at 70 °C. The ammonia generation rate per effective volume of catalyst in the eU2A process is ∼28 times higher than the thermal hydrolysis of urea (THU). The eU2A operates at low temperature and pressure which is suitable for SCR process in mobile engines and saves energy. The eU2A process is a promising technique that finds applications on the SCR process for the removal of nitride oxide from combustion systems (e.g., diesel vehicles and power plants).;In addition, the mechanism of eU2A in the alkaline medium using nickel electrodes was investigated in a classical three-electrode reactor with a mercury/mercury oxide reference electrode. In this investigation, the intermediates and products in the bulk solution were monitored by Fourier Transform Infrared spectroscopy, and the intermediates and products on the catalyst surface in eU2A process were analyzed by in-situ Raman spectroscopy. Urea electrolysis and THU (in the bulk solution) take place in parallel with the eU2A process in the eU2A reactor. It was found that the ammonia production rate depends strongly on the amount of nickel oxyhydroxide and the concentration of OH- ions. In addition, the redox couple of Ni2+ and Ni 3+ ions played important roles in ammonia generation in the eU2A process. Based on these observations and analyses, an eU2A mechanism is proposed in this dissertation.;The catalytic effect of Ni based working electrode could also be found when using K2CO3 as the supporting electrolyte during the eU2A process. However, Ni electrodes were found to corrode when the supporting electrolyte was changed to K2CO3. To deal with this problem, a novel graphene-coated Ni electrode was developed to improve its ammonia corrosion resistance while unexpectedly enhancing the ammonia generation rate in the eU2A process. The development of a corrosion resistant electrode is crucial for the eU2A reactions since the concentration of ammonia is inevitably high on the surface of the electrode in the heterogeneous ammonia generating process, leading to severe corrosion of the electrode and the loss of generated ammonia as well. In this research, the graphene was derived from raw coal by using the chemical vapor deposition method and then self-lifted onto a Ni electrode to form a protective layer for corrosion prevention. Transmission electron microscopy showed that the synthesized graphene had few layers and in-situ Raman spectroscopy indicated that the coating of graphene was stable during the eU2A process. As a result, the ammonia corrosion of the Ni electrode was dramatically reduced by ∼ 20 times with the graphene coating method. More importantly, a higher ammonia generation rate (∼ 2 times) was achieved using the graphene-coated Ni working electrode compared to a bare Ni electrode in the eU2A process.