Al2O3-supported Pt/Rh catalysts for NOx removal under lean conditions

Abstract

In this work the reactivity of Pt-Rh NOx storage-reduction (NSR) catalysts in the reduction of NOx under lean conditions is investigated. It is found that significant amounts of NOx are stored on both Rh- and Pt-based samples at all the investigated temperatures (in the range 150–350¿°C). Mostly chelating nitrites are adsorbed at the lowest investigated temperature (150¿°C), while nitrates (both bidentate and ionic) at higher temperatures. However, at all temperatures nitrites prevail at the beginning of the storage phase, while nitrates represent the most abundant adsorbed species after prolonged contact. Pt-containing catalysts (either monometallic Pt or bimetallic Pt/Rh) show higher NOx storage capacity than the Rh monometallic sample, possibly due to the higher dispersion of Pt vs. Rh and/or to the higher oxidizing capability of Pt vs. Rh.

The stored NOx species show relevant thermal stability, and decompose to NOx and O2 upon heating. In particular, nitrites disproportionate to gaseous NO and nitrates; these latter then decompose to NOx and O2. On the Rh-Ba/Al2O3 catalyst the disproportionation reaction is observed with a higher temperature onset if compared to the Pt-based samples. The analysis of the reactivity of the stored NOx species (probed by isotopic labeling experiments and reduction with H2 and NH3) showed the lower reactivity of the Rh-Ba/Al2O3 sample; however Rh shows activity in the ammonia decomposition reaction to N2 and H2, unlike Pt. The lower reactivity of the Rh-Ba/Al2O3 sample is also pointed out by experiments under cyclic lean-rich conditions. However, the presence of Rh increases the reactivity of the catalyst in the steam reforming of hydrocarbons, especially at high temperature, and accordingly the reactivity of the bimetallic Pt/Rh sample at high temperatures is higher than that of the Pt and Rh monometallic catalysts.