Insights into the influence of the Ag loading on Al2O3 in the H2-assisted C3H6-SCR of NOx

The addition of H-2 has been reported to promote drastically the selective catalytic reduction of NOx by hydrocarbons (HC-SCR). Yet, the influence of the Ag loading on the H-2-promoted HC-SCR has been the subject of a very limited number of investigations. The H-2-HC-SCR earlier studies reported mostly on Ag/Al2O3 samples containing about 2 wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H-2. The present study highlights for the first time that the H-2-C3H6-SCR catalytic performances of Ag/Al2O3 samples improved in the 150-550 degrees C temperature domain as the Ag loading (Ag surface density: x (Ag/nm(2),1203)) decreased well below 2 wt%. A detailed kinetic study of H-2-C3H6-SCR was performed in which the reaction orders in NO, C3H6 and H-2, and the apparent activation energies were determined for the reduction of NOx to N-2 on a Ag(x)/Al2O3 catalysts series, for which Ag was found to be in a highly dispersed state by TEM and HAADF-STEM. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to 0.7 Ag/nm(Al2O3)(2) (Ag loading of 2.2 wt%) coinciding with the changes observed earlier in the NOx uptakes of the Al2O3 supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H-2-C3H6-SCR reaction proceeds via the activation of H-2 and C3H6 on Ag species and their further reaction with NOx adspecies activated on the Al2O3 support. The unexpected higher catalytic performances of the Ag samples with the lower Ag surface densities was attributed to the higher concentration of active sites on the Al2O3 supporting oxide able to chemisorb NOx species, in agreement with the NOx uptake data. The kinetic data obtained for Ag surface densities lower than 0.7 Ag/nm(Al2O3)(2),3 also suggest that the interaction between NOx and C3H6 adspecies would be rate determining in the C-3-H-6-SCR process. (C) 2014 Elsevier B.V. All rights reserved.