Elsevier

Applied Catalysis A: General

Volume 581, 5 July 2019, Pages 43-57
Applied Catalysis A: General

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

https://doi.org/10.1016/j.apcata.2019.05.013Get rights and content

Highlights

  • The adsorption of NO/O2 over Pt- and Rh-containing catalysts leads to nitrites at low T and of nitrates at high T.

  • Pt-based catalysts shows superior NOx storage properties.

  • Pt-based catalysts shows superior NO oxidizing properties.

  • Pt-based catalysts are more active that Rh/Al2O3 in the reduction of stored NOx.

  • Rh- based catalysts show higher activity in the NH3 decomposition reaction and steam reforming of propylene.

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.

Introduction

The push for better fuel economy and lower vehicular CO2 emissions greatly increased the lean-burn engines (e.g. diesel) market share, especially in Europe where today diesel cars accounts for more than 50% of new vehicles. However, exhaust gases from these engines contain NOx and excess O2, which renders NOx reduction into N2 impractical over conventional three-way catalysts (TWCs) [1]. Consequently, two main technologies have been developed for mobile lean NOx removal, i.e. selective catalytic reduction (SCR) of NOx using urea as a reductant for heavy duty diesel applications [2,3], and lean NOx trap (LNT) catalysts (otherwise known as NOx adsorber or NOx storage-reduction catalysts, NSR) for light duty applications [4]. NSR operation is cyclic: during the lean phase, NOx is trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N2 and restore the original catalyst surface [5]. Typical NSR compositions include noble metals (Pt, Rh, and sometime Pd) for oxidation/reduction purposes, storage components like BaO and CeO2, and γ-alumina as high surface area support. Among noble metals, Pt is considered to be essential for both the storage and the reduction phase, whereas the role of Rh is not yet fully understood and clarified, although it is generally believed that Rh improves the regeneration and the overall catalyst efficiency [6].

In a previous work of some of us [7] we have investigated the reactivity of Rh in the NOx storage-reduction catalysis. By means of NO2 adsorption studies and of lean/rich cycles with H2 as reducing agents we have concluded that the Rh-based sample exhibits lower NOx storage ability than the Pt-based systems; on the other hand, the role of Rh in the reduction of stored NOx was not completely clear. In this work, we have considered the effect of the addition of Rh in typical Pt-Ba/alumina formulations focusing the attention on the fundamental aspects of the reactions occurring during the typical lean-rich cycling of the catalyst. For this purpose, a Pt/Rh-Ba/Al2O3 bimetallic catalyst, and the corresponding monometallic Pt- and Rh-based reference samples, have been prepared and deeply characterized by means of BET, XRD, XPS and TEM analysis. FT-IR has been used to characterized the nature of the stored NOx species, while temperature programmed desorption (TPD) and isotopic exchange (TPIE) have been considered to analyze the thermal stability and the reactivity of these species. Finally, isothermal lean-rich cycles using a complex reducing mixture (i.e. H2 + CO + C3H6) have been used to address the reactivity in the NOx removal and the selectivity of the NOx reduction process.

Section snippets

Materials and methods

Model Pt/Rh-Ba/Al2O3 (1/0.5/20/100 w/w/w/w) bimetallic sample has been prepared by incipient wetness impregnation of a commercial γ-alumina (Versal 250 UOP) support with Pt(NO2)2(NH3)2 (Strem Chemicals, 5% w/w), Rh(NO3)3 (Sigma Aldrich, 10% w/w) and Ba(CH3COO)2 (Sigma Aldrich, 99% w/w) aqueous solutions. After each impregnation step, the samples have been dried at 80 °C overnight and calcined at 500 °C for 5 h.

Model Pt-Ba/Al2O3 (1/20/100 w/w/w) and Rh-Ba/Al2O3 (0.5/20/100 w/w/w) monometallic

Morphological analysis

The nominal composition of the prepared catalysts, the specific surface area, pore volume and pore sizes are reported in Table 1, along with the metal dispersion. Pt-Ba/Al2O3 catalyst shows a specific surface area of 140 m2  g−1 with a final metal dispersion near 60%; Rh-Ba/Al2O3 exhibits a specific surface area of 150 m2 g−1 with a Rh dispersion near 7%. The bimetallic system shows similar morphological characteristics as the monometallic ones; it was not possible to measure the metal

Conclusions

In this work the reactivity of Pt- and Rh-containing NSR catalysts has been analysed in the adsorption of NOx and in their reduction by using different reductants. It has been found that significant amounts of NOx can be stored on all the investigated samples at all considered temperatures (in the range 150–350 °C). The nature of the adsorbed NOx species strongly depends on temperature, being mostly chelating nitrites at the lowest investigated temperature (150 °C) and nitrates (both bidentate

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