The role of lanthana loading on the catalytic properties of Pd/Al 2O3-La2O3 in the NO reduction with H2

Abstract

The role of La2O3 loading in Pd/Al2O 3-La2O3 prepared by sol-gel on the catalytic properties in the NO reduction with H2 was studied. The catalysts were characterized by N2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H 2 molar ratio (GHSV = 72,000 h-1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60-75%) at temperatures lower than 250°C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La 2O3. DTA shows that the phase transformation takes place at temperatures of 280-350°C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (∼1.5 times). La 2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250°C) favoring the formation of N2. © 2004 Elsevier B.V. All rights reserved.