Enhancement by H2 of C3H8-SCR of NOx using Ag/γ-Al2O3

• H2 consumption tracks the conversion of NO at low temperature during H2-C3H8-SCR.
• N2 yield prevails at low and high temperature whereas NO2 is emitted between 200 and 260 °C.
• H2 promotes formation of Agn clusters and Ag° nanoparticles on surface catalyst.
• Clusters and nanoparticles of Ag presumably activate H2-C3H8-SCR at low temperature.

Addition of hydrogen to the feed stream during the lean NOx reduction with C3H8 catalyzed by 2 wt.% Ag/γ-Al2O3 lowers NO light-off to below 100 °C. With our reducing mixture NO reaches high conversion in two well-defined zones, the first at low temperature, between 80 and 180 °C, and a second one above 180 °C, extending to 500 °C without a significant variation in conversion. NO conversion drops in the intermediate range. H2 is responsible for the reaction of NO at low temperature, whereas C3H8 becomes the reductant above about 180 °C. N2 and NO2 are the main competing products, with the highest selectivity to N2 being obtained around 140 °C and above 380 °C. NO2 is formed at low temperature and is predominant (77–83 ppm) between 200 and 260 °C. N2O is detected in only small amounts (2 ppm) between 160 and 500 °C. Operation at high space velocities during H2-C3H8-SCR led to the formation of about 48 ppm NH3 at 500 °C and to an increase in the CO/CO2 ratio. Through UV–Vis spectroscopy we find that Ag moieties are very labile on the Ag/γ-Al2O3 catalysts even at low temperatures, and it appears that different Ag species intervene and are responsible for the variations in activity and selectivity during SCR. Ex-situ UV–Vis shows that Ag+ and Agnδ+ species are present after C3H8-SCR, and that Agn clusters and metallic Ag nanoparticles are also formed when hydrogen is present in the feed. X-ray diffraction confirmed the formation of metallic silver, presumably nanoparticles with size >5 nm, after H2-C3H8-SCR. Our results are consistent with a mechanism involving competition between O2 and the reductant for surface species that are precursors of NO2 on one side, but that can also be reduced to N2. Those intermediates are reduced by H2 at low temperature and by reaction with C3H8 above 180 °C.

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