Effect of the Catalyst Support on the NOX Formation During Combustion of NH3 SOFC Off-Gas

Ammonia has attracted much interest as a potential green and renewable hydrogen carrier or energy vector. Compared to hydrogen, ammonia offers several advantages. For example, ammonia has a significantly higher energy density and can be liquefied at room temperature at a moderate pressure of 8 bars. While ammonia can be cracked to supply hydrogen, it is also possible to convert it directly into high-temperature solid oxide fuel cells (SOFCs) to generate electricity. The Ship-FC project aims to install an ammonia-fed 2MW SOFC system on board the vessel Viking energy to demonstrate the feasibility of zero CO2 emission shipping. For this NH3 SOFC system, a catalytic afterburner is required to remove the hydrogen and ammonia present in the SOFC off-gas and to recover heat. The current study analysed the effects of different catalyst supports, with a focus on NOX formation through the combustion of an SOFC off-gas surrogate. The study investigated the performance of catalysts based on the active metals, platinum and iridium, as well as the catalyst supports, Al2O3, SiO2, and TiO2. The results were correlated with catalyst characterisation data and ammonia TPD results. The investigations showed that the formation of NOX was clearly affected by the nature of the catalyst support. The highest selectivity towards NOX was observed for Al2O3, followed by SiO2, and the lowest selectivity was observed for TiO2. This trend was evident for the supported platinum and iridium catalysts and for the samples exclusively containing the support. The trend for N2O formation was opposite to that of NOX formation (TiO2 > SiO2 > Al2O3) in both the presence and absence of platinum or iridium.