Lean NOx reduction with methanol over supported silver catalysts

Doctoral thesis, 2015

The oxygen rich and relatively cold exhaust gases from fuel-efficient combustion engines bring challenges for the reductive aftertreatment of NOx. In this context selective catalytic reduction over silver/alumina has shown positive results, especially with oxygenated reducing agents. In the present work, methanol, which is considered a promising renewable fuel, is evaluated as reducing agent for NOx over supported silver catalysts. The aim is to gain increased understanding of the catalytic processes, especially focusing on low-temperature activity and selectivity. The role of the supported silver species is studied, together with the influence of the support material and the gas composition, including the formation of hydrogen. For this purpose model catalysts were prepared, characterized and evaluated in flow-reactor experiments. The results show that the low-temperature activity for lean NOx reduction with methanol over silver/alumina is highly dependent on the composition of supported silver species and their interplay with adsorbed and gas phase species. This work provides new insights in the role of small silver species for the selectivity to N2 and the importance of somewhat larger silver species for catalytic activity at low temperature. These results were achieved by comparing silver/alumina samples of similar silver loading, but with different composition of silver species, as revealed by UV-vis spectroscopy, TEM and H2-TPR. Furthermore, comparisons of different support materials during methanol-SCR conditions, show a higher NOx reduction for silver supported on alumina than on ZSM-5. The NOx reduction over the alumina based catalyst is found to improve when the C/N ratio is moderately increased. The influence of the silver loading was studied and the results show that sol-gel silver/alumina with 3 wt% silver gives a high NOx reduction with methanol in a broad temperature interval, relevant for lean exhaust gases. Moreover, the observed formation of H2 from methanol gives an indication of that the high low-temperature activity associated with oxygenated reducing agents may be connected to the abstraction of hydrogen from the oxygenate, studied here for methanol by DRIFT spectroscopy. One effect of hydrogen, observed in the present work by UV-vis spectroscopy, is reduction of silver species.