Abstract
The addition of H2 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 H2-promoted HC-SCR has been the subject of a very limited number of investigations. The H2-HC-SCR earlier studies reported mostly on Ag/Al2O3 samples containing about 2wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H2. The present study highlights for the first time that the H2-C3H6-SCR catalytic performances of Ag/Al2O3 samples improved in the 150–550°C temperature domain as the Ag loading (Ag surface density: x (Ag/nmAl2O32)) decreased well below 2wt%. A detailed kinetic study of H2-C3H6-SCR was performed in which the reaction orders in NO, C3H6 and H2, and the apparent activation energies were determined for the reduction of NOx to N2 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/nmAl2O32 (Ag loading of 2.2wt%) 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 H2-C3H6-SCR reaction proceeds via the activation of H2 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/nmAl2O32 also suggest that the interaction between NOx and C3H6 adspecies would be rate determining in the C3H6-SCR process.
Highlights
Since the late 1970s, the preservation of the air quality has become one of the major concerns for the OECD (Organization for Economic Co-operation and Development) countries in order to minimize the impact of air pollutants on environment and health
This observation is consistent with earlier studies of Sayah et al [27] in which it was concluded that the use of ethanol to disperse Ag/Al2O3 samples on TEM grids should be avoided, as doing such leads to the reduction of highly dispersed oxidized Ag species by ethanol and the corresponding formation of large Ag particles which are found to be not representative of the state of Ag in the investigated samples
In contrast to the C3H6-SCR reaction, the concept of optimum Ag surface density at 0.7 Ag/nm2Al2O3 (Ag loading of 2.2 wt%) [18] did not apply to the H2-promoted C3H6-SCR reaction over a broad temperature domain
Summary
Since the late 1970s, the preservation of the air quality has become one of the major concerns for the OECD (Organization for Economic Co-operation and Development) countries in order to minimize the impact of air pollutants on environment and health. In Europe in particular, the Euro 6 standards, expected to enter into force in January 2014, will mainly further reduce the NOx (NO + NO2) emissions from diesel automotives, from 180 to. To meet these ever more stringent standards, further improvements in the efficiency of the catalytic converters are required. Among the catalytic formulations evaluated to date, Ag/Al2O3 has been reported to be the most promising catalyst in the pioneering work by Miyadera [3]. It was shown that NOx could be efficiently and selectively reduced to N2 by various hydrocarbons in the presence of water and in a large excess of O2. The use of such Ag/Al2O3 catalysts is limited because of
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