A novel Pd/MgAlOx catalyst for NOx storage-reduction

A novel NOx storage-reduction catalyst consisting of Pd dispersed on a magnesium-aluminum oxide (MgAlOx) support is described. The MgAlOx support was prepared from a synthetic Mg/Al layered double hydroxide, and Pd bis-acetylacetonate (acac) was adsorbed onto the freshly calcined support from toluene. The Pd/MgAlOx catalyst was pretreated in either O-2 or H-2 at 500 degrees C to decompose the adsorbed [Pd(acac)(2)] and remove organic residues. NOx adsorption experiments were conducted at 300 degrees C using two gas mixtures: 500 ppm NO2, 6% O-2 and 10% CO2 (bal. He) and 500 ppm NOx 500 ppm N-2 and 5% O-2 (bal. He). Temperature-programmed desorption (TPD) and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) evidence that NO2 adsorption on MgAlOx produces primarily surface nitrate species. The adsorption capacity of the support for NO + O-2 is four-fold lower than for NO2. Temperature-programmed reduction (TPR) in flowing H-2 demonstrates that surface nitrate and nitrite species on MgAlOx are reduced to N-2 at 300-400 degrees C. The adsorption capacity of Pd/MgAlOx for NO + O-2 is almost four-fold greater than MgAlOx evidencing a catalytic role of Pd in the NO, storage mechanism. TPR in flowing H-2 indicates that the adsorbed NO, species are removed at very low temperatures (similar to 50 degrees C) suggesting that they are located near the catalytically active Pd sites. (c) 2006 Elsevier B.V. All rights reserved.