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.High resolution transmission electron microscopy (HRTEM) was carried out using a JEOL 2010F electron microscope equipped with a field emission source at an accelerating voltage of 200 kV.Samples were deposited on holey Cu grids. The point-to-point resolution achieved was 0.19 nm and the resolution between lines was 0.14 nm. X-ray photoelectron spectroscopy (XPS) was performed on a SPECS system equipped with an Al anode XR50 source operating at 150 mW and a Phoibos MCD-9 detector. The pass energy of the hemispherical analyzer was set at 25 eV and the energy step was set at 0.1 eV. The pressure in the analysis chamber was kept below 10−7 Pa. The area analysed was about 2 mm × 2 mm. Data processing was performed with the CasaXPS program (Casa Software Ltd., UK).The interaction of NO/O2 with the catalytic surface has been investigated by IR spectroscopy and temperature-programmed desorption.NO/O2 mixture (1000 ppm NO + 3% O2 in He) has been fed to the IR reactor for the analysis of the surface species. It consists in a "Sandwich" IR cell containing the catalyst (15 mg) in form of selfsupported wafer; the gases leaving the cell were analyzed by mass spectrometer (ThermoStar TM...

Pt8 cluster on alumina under a pressure of hydrogen: Support-dependent reconstruction from first-principles global optimization

“…Sub nano Pt clusters supported on alumina in a hydrogen environment are important catalysts for many important catalytic reactions, including hydrogenation [20][21][22] , NO x removal 23 , CO oxidation 24,25 , dehydrogenation 26 , and reforming 27,28 . However, these are challenging systems, because the catalytic properties of Pt@Al 2 O 3 are sensitive to the environment, and the optimal performance requires a careful setting of reaction conditions.…”

Measures to Reduce the N2O Formation at Perovskite-Based Lean NOx Trap Catalysts under Lean Conditions

“…By the addition of 0.125 wt.% Rh the NO x storage capacity above 300 • C is slightly improved (Figure 3A). Though an auxiliary effect of the Rh coating at higher temperatures seems to be plausible, as the studies of Kubiak et al [51], Andonova et al [52] and Castoldi et al [53] showed appreciable NO oxidation abilities and NO x storage capacities for Rh-based BaO/Al 2 O 3 catalysts above 350 • C. Figure 3B compares the N 2 O evolution on LSFT_Pt and LSFT_PtRh. The observed N 2 O formation increase caused by the Rh addition could be related to an interplay between the generally high light-off temperatures for NO oxidation and N 2 O decomposition of Rh-based catalysts on the one hand, and the slight NO x adsorption capacity at low temperatures proceeding via the nitrite route on the other hand.…”