Understanding the Electrochemically Induced Conversion of Urea to Ammonia for Direct Ammonia SCR Processes
Authors: Lu Fei , Botte Gerardine G 
Published: February 2020

Background

Nitrogen oxides (NOx) emissions are much concerned these days. NO2is not only one of the air pollutants, but it will also react in the atmosphere to form ozone and acid rain. An effective method to remove NOx is the selective catalytic reduction (SCR) process and ammonia is a known effective reductant for this process.
The urea SCR, where urea solution is injected into an SCR chamber to produce ammonia and then remove NOx on the SCR catalyst, faces challenges with lower exhaust temperature and harmful deposits on SCR catalyst. In contrast, direct NH3 SCR expands the SCR temperature “window” while eliminating deposits [1]. However the storage of pressurized ammonia becomes an issue. To confront these issues, Botte has proposed a new method, the electrochemically induced conversion of urea to ammonia (eU2A) process [2, 3]. The eU2A reactor provides the flexibility to decouple the ammonia generation from the SCR process at low temperature as indicated in Fig. 1. The eU2A process not only uses urea storage instead of ammonia but also provides direct NH3dosing with co-generation of hydrogen, which could potentially enable better efficiencies of the SCR catalyst.
Experimental results have shown that generation of ammonia from urea could be promoted by an applied voltage on Ni based catalysts [2], resulting in higher yield of ammonia when compared to thermal hydrolysis of urea to ammonia (THU) as shown in Fig. 2 [3]. However, the mechanism of the urea conversion to ammonia via the electrochemical process is still unknown. In this work, we implement in-situspectroscopy techniques to provide a better understanding of the eU2A process in alkaline media.

Methodology

A three-electrode reactor was designed and constructed at the Center for Electrochemical Engineering Research (CEER) for the production of ammonia from urea using chemical and electrochemical methods. Nickel based catalysts were used as working and counter electrodes for the cell.

Results

The eU2A experiments were conducted and compared with the THU process for different parameters including applied voltage and concentration of supporting electrolyte (KOH). In-situ Raman spectroscopy was employed to identify and monitor the intermediates and products formed during the reaction on the catalyst surface. Ex-situFTIR spectroscopy was utilized to identify the intermediates and products in the bulk solution during the reaction. By comparing time-dependent Raman and FTIR spectra for the eU2A experiments with the THU experiments, a possible reaction pathway for the conversion of urea to ammonia in the eU2A process is proposed. The findings and the proposed mechanism will be presented at the meeting.

References

Johannessen, T., “3rd Generation SCR System Using Solid Ammonia Storage and Direct Gas Dosing: Expanding the SCR window for RDE.” in Directions in Engine-efficiency and Emissions Research (DEER). 2012. Dearborn.

G. G. Botte, “Electrolytic cells and methods for the production of ammonia and hydrogen.” US Patent 0095636A1, 2009.

Lu, F. and G. G. Botte, “Electrochemically Induced Conversion of Urea to Ammonia.” ECS Electrochemistry Letters, 2015. 4(10): p. E5-E7.

Figure 1