Study of Pt-Ba/Al2O3 LNT catalysts for the simultaneous removal of NOx and soot: new insights on the role of stored NOx in the soot combustion

Soot or particulate matter (PM) and NOX are the main pollutants in diesel engine emissions and represent health and environmental problems. Therefore the automotive industry is currently facing serious challenges to meet the specific requirements of future regulations concerning both NOx and particulate emissions. Recently the potential use of lean NOx Traps (LNT) in the simultaneous removal of soot and NOx has been proposed. These systems accomplish the reduction of soot and NOx under cyclic conditions by switching between fuel-lean and fuel-rich conditions. The aim of the present study is to analyze the effects of the presence of soot on the behaviour of LNT catalysts. Besides, we want to provide new insights on the role in the soot combustion of NOx stored onto the catalytic surface. A homemade Pt-Ba/Al2O3 (1/20/100 w/w) catalyst was prepared by impregnation of -alumina with Pt and Ba soluble salts. The effect of the presence of soot on the NOx storage and reduction activity was studied by performing lean-rich cycles over both the pure catalyst sample and soot/catalyst mixture. For this purpose rectangular step feeds of NO (1000 ppm) in He + 3% v/v O2 and of H2 (4000 ppm) in He were alternatively fed to the reactor in the presence of CO2 and H2O in the feed stream at different temperatures. The interaction between soot and the stored NOx species has been investigated also by TPD and TPO experiments (heating rate 10 °C/min in He or in O2 (3%) from 250°C up to 500°C), in the presence and in the absence of soot. The results obtained during the lean NOx adsorption over the Pt-Ba/Al2O3-soot mixture showed that the presence of soot decreased the NOx storage capacity of the catalyst; in particular the presence of roughly 10% w/w of soot decreased by nearly 30% the amounts of stored NOx with respect to the soot-free catalyst. The decrease of the NOx adsorption capability is possibly related to the decrease of the NO2 concentration, due to its reaction with soot. However the presence of soot does not appreciably affect the behavior of the PtBa/Al2O3 catalyst in the reduction by H2 of the stored nitrates, being in all cases N2 the major reaction product. TPD results pointed out that the thermal stability of the adsorbed nitrates is also decreased in the presence of soot. This indicates an interplay between adsorbed NOx species and soot: soot affects the NOx storage but the adsorbed nitrates favor the oxidation of soot. These species actively oxidize soot by direct interaction due to their mobility on the surface and/or participate in the soot oxidation upon release of NO2 and O2 during decomposition. The participation of the adsorbed NOx species in the soot oxidation has also been clearly pointed out by TPO experiments showing that the oxidation of soot is greatly enhanced by the presence of adsorbed NOx species. Indeed larger amounts of CO2 are produced upon soot oxidation when the reaction is carried out in the presence of stored NOx.