Al2O3-supported Pt/Rh catalysts for NOx removal under lean conditions
Graphical abstract
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
References (27)
- et al.
Catal. Today
(2000) - et al.
Catal. Today
(1996) - et al.
Appl. Catal. B: Environ.
(2012) - et al.
Appl. Catal. B: Environ.
(2013) - et al.
Chem. Eng. J.
(2010) - et al.
J. Catal.
(2015) - et al.
Catal. Today
(2014) - et al.
Appl. Catal. B: Environ.
(2018) - et al.
Appl. Catal. B: Environ.
(2016) - et al.
Catal. Today
(2012)
Catal. Today
J. Catal.
Catal. Today
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Enhancing the thermal stability and activity of Pd–Rh/Al<inf>2</inf>O<inf>3</inf> catalyst for three–way catalytic reaction by introducing Pt and varying the synthesis method
2023, Journal of Environmental Chemical EngineeringEngine emissions with air pollutants and greenhouse gases and their control technologies
2022, Journal of Cleaner ProductionCitation Excerpt :Moreover, designing the SCR catalyst formulations to improve the oxidation activity of NO to NO2 could enhance the fast-SCR reaction at low-temperatures, for example, by loading manganese oxides (Li et al., 2021b), cobalt oxides (Irfan et al., 2008) or iron oxides (Zhang et al., 2021). The NO2/NOx ratio is also a determinative factor for NO storage in PNA and LNT because NO2 is more easily adsorbed on catalysts rather than NO. The catalyst loading noble metals, such as Pt (Theis and Lambert, 2015), Pd (Onrubia-Calvo et al., 2020), or Rh (Castoldi et al., 2019), are able to convert NO to NO2, increasing the capacity for NOx storage at low temperatures. Furthermore, some noble metal-free catalysts, for instance, Co-SSZ-13, Ce/BEA (Wu et al., 2022) and mesoporous LaCoO3 perovskite (Xie et al., 2022), were developed to enhance NOx storage in a similar way (see Table 4).
Rh/CeO<inf>2</inf>+Pt/Ba/Mn/Al<inf>2</inf>O<inf>3</inf> model NSR catalysts: Effect of Rh/Pt weight ratio
2021, Catalysis CommunicationsCitation Excerpt :In the region of 250–400 °C, by increasing the Rh/Pt weight ratio a clear advantage in N2 yield can be ibserved, corresponding to less N2O and NH3 production. Ammonia adsorbed species start to decompose into N2 and H2 at ~250 °C over the Rh-containing catalyst, which results in more N2 production [8]. N2 yield is improved with increasing Rh/Pt weight ratio due to the higher reactivity of Rh than Pt in the NH3 decomposition reaction [8].