Routes of formation and composition of NOx complexes adsorbed on palladium-promoted tungstated zirconia

Abstract

Surface species obtained during the adsorption of NO and NO/O2 coadsorption at room temperature on Pd-free (WZ) and Pd-promoted tungstated zirconia (Pd/WZ) are identified by means of in situ FT-IR spectroscopy. The WZ and Pd/WZ samples have a tetragonal structure and contain randomly distributed mesoporous phase. Dispersed palladium(II) species are present in two different environments: (i) Pd2+ ions, which have only Zr4+ ions in their second coordination sphere and (ii) Pd2+ ions, which are linked to both zirconium and tungsten ions via oxygen bridges. On the Pd/WZ sample, NO undergoes oxidation to various NOx species depending on the temperature. The compounds formed at room-temperature oxidation are adsorbed N2O3 and products of its self-ionization, NO+ and NO2-. In this process W(VI) is involved, being reduced to W(IV). At high temperature N2O3 decomposes, restoring the WO species. Under these conditions, NO undergoes oxidation to NO2 by the Pd(II) ions, which are reduced to Pd(I). The nitrosyls of Pd(I) display high thermal stability and do not disappear upon evacuation at 623 K. During NO/O2 coadsorption on the Pd/WZ catalyst at room temperature, the amounts of surface nitrates and NO2/N2O4 formed in the gas phase are significantly lower than those observed under identical conditions in the presence of tungstated zirconia. It is concluded that promotion of tungstated zirconia with palladium(II) suppresses the oxidation of NO by molecular oxygen at room temperature due to the elimination of acidic protons involved in the process. © 2005 Elsevier B.V. All rights reserved.