Active sites and influence of reaction conditions on the selective catalytic reduction of NOx over the silver-alumina catalyst

Doctoral thesis, 2018

Hazardous nitrogen oxides (NOx) are challenging to remove from fuel-efficient excess-oxygen operating engines. A promising solution is selective catalytic reduction (SCR) of NOx, using hydrocarbons (HC-SCR) or ammonia (NH3-SCR) as reducing agent. Highly efficient SCR of NOx requires a fully understood system, which includes both the catalyst and the reducing agent. In this work, Ag/Al2O3 has been evaluated as an SCR catalyst with HC as well as NH3 as reducing agents, in order to investigate the active sites and elucidate the influence of the nature of the reducing agent during lean NOx reduction. In order to investigate the role of the active phase, Ag/Al2O3 was compared to an In/Al2O3 catalyst, containing the equivalent molar amount of active phase. In addition, the effect of an uneven distribution of the reducing agent is evaluated. Catalyst samples for which the active phase was synthesized in the water pools of a reversed microemulsion, using methanol as reducing agent for the metal complexes, was also prepared and evaluated. This work shows that the Ag/Al2O3 catalyst in general exhibits superior activity for NOx reduction compared to In/Al2O3. However, since one of the hydrocarbon reductants was shown to reduce NOx more efficiently over In/Al2O3, the exact nature of the reducing agent was shown to be of uttermost importance for the catalytic activity. In addition, the In/Al2O3 catalyst provides a higher concentration of acidic sites, compared to the Ag/Al2O3 catalyst, which was shown to inhibit the NH3-SCR reaction over this catalyst. Furthermore, it was confirmed that the active phase can be tailored in a microemulsion synthesis containing a greener route than used in the past, resulting in catalytically active nanoparticles. Moreover, the position of the reductant injection spray in the exhaust pipe in front of the SCR catalyst was shown to significantly affect the NOx reduction and unwanted slip of the reductant. The results presented in this thesis contribute to the overall understanding of the interplay between various active sites and reductants in SCR of NOx, and may therefore help advance current technologies to improve the sustainability of future transports.