A novel Pd/MgAlO x catalyst for NO x storage-reduction

Abstract

A novel NO x storage-reduction catalyst consisting of Pd dispersed on a magnesium–aluminum oxide (MgAlO x ) support is described. The MgAlO x 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/MgAlO x catalyst was pretreated in either O 2 or H 2 at 500 °C to decompose the adsorbed [Pd(acac) 2] and remove organic residues. NO x adsorption experiments were conducted at 300 °C using two gas mixtures: 500 ppm NO 2, 6% O 2 and 10% CO 2 (bal. He) and 500 ppm NO, 500 ppm N 2 and 5% O 2 (bal. He). Temperature-programmed desorption (TPD) and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) evidence that NO 2 adsorption on MgAlO x produces primarily surface nitrate species. The adsorption capacity of the support for NO + O 2 is four-fold lower than for NO 2. Temperature-programmed reduction (TPR) in flowing H 2 demonstrates that surface nitrate and nitrite species on MgAlO x are reduced to N 2 at 300–400 °C. The adsorption capacity of Pd/MgAlO x for NO + O 2 is almost four-fold greater than MgAlO x evidencing a catalytic role of Pd in the NO x storage mechanism. TPR in flowing H 2 indicates that the adsorbed NO x species are removed at very low temperatures (∼50 °C) suggesting that they are located near the catalytically active Pd sites.