Tolerance and regeneration versus SO2 of Ba0.9A0.1Ti0.8Cu0.2O3 (A = Sr, Ca, Mg) LNT catalysts
Graphical abstract
Introduction
Direct-injection spark ignition engines (DISI) are a lean burn engine alternative to diesel engines because of their low fuel consumption and, for hence, their low CO2 emissions level [1]. Nevertheless, it has been reported that DISI engines increase the NOx and particulate matter (PM) emissions [2] compared to spark ignition (SI) engines [3]. The operating mode of a DISI engine (lean burn condition) prevents the complete abatement of NOx emissions by the TWC used for SI engines, and for this reason, the addition of a de-NOx after-treatment system in the exhaust is mandatory [4,5]. Two technologies are mainly proposed to control NOx emission in lean burn engines: Selective Catalytic Reduction (SCR) and “Lean NOx Trapping” (LNT) [6,7]. The conventional LNT catalysts, fitted in diesel cars, are composed of platinum-group metals (PGM) and alkaline or alkaline-earth oxides (BaO or K2O), both supported on a high surface area material (Al2O3, TiO2…) [8]. Recently, it has been demonstrated that interesting results for NOx emissions control in a DISI engine can be achieved by combining LNT and TWC technologies. However, some issues must be still addressed to match these technologies, such as: i) enhancing the NOx storage capacity of the conventional LNT catalyst at high temperature, as working range temperature for DISI engines is higher than for diesel engines [9] and ii) improving the tolerance and regeneration versus sulfur of the catalyst, without losing NOx storage capacity [10].
Perovskites oxides (ABO3) are currently proposed as a promising alternative to the PGM catalyst for NOx abatement, due to their high thermal stability, low cost and easy control of their catalytic and redox properties by partial substitution of A and/or B cations [11,12]. Li et al [13] suggested perovskites as LNT catalysts as they show a catalytic performance close to that reported for Platinum Group Metal (PGM) catalysts, and even higher tolerance versus sulfur dioxide. Kim et al [14] concluded that lanthanum-based perovskites can be considered viable automotive catalysts, reaching the maximum NOx conversion at 400 °C, even though the addition of a smaller amount of a PGM is mandatory to fulfill the sulfur resistance required. In addition, Shen et al [15] and Righini et al [16], studied the stability of potassium titanates as NOx adsorbents and concluded that the incorporation of potassium shifts the NOx adsorption to higher temperatures, though the mobility and reactivity of potassium at the required working temperatures made this catalyst not suitable for LNT application. Therefore, despite all these studies, more efforts must be done to improve the NOx conversion and tolerance and regeneration versus sulfur dioxide before perovskites are ready to be used as LNT catalysts at high temperature.
In previous articles, the authors concluded that by partial substitution of Ti by Cu, the BaTi0.8Cu0.2O3 perovskite becomes active for NOx storage in the temperature range of the lean burn engines application [17,18]. According to these results, and to try to overcome some of the drawbacks of LNT catalysts above-mentioned, the aim of this paper is to determine the effect of partial substitution of the A cation on the NOx storage capacity and tolerance versus sulfur dioxide of the BaTi0.8Cu0.2O3 perovskite. Considering literature reviewed: i) strontium and calcium doping may enhance tolerance versus sulfur of a commercial LNT catalyst [19] as the corresponding sulfates, that block the NOx storage sites, are easily regenerated; ii) magnesium, as textural promoter, may improve the resistance to sintering [20,21], since the use of reducing environments during regeneration processes could lead to a collapse of the perovskite structure and, for hence, to a loss of activity [14,22,23]. Therefore, in this paper, the effect of the partial substitution of barium by strontium, calcium or magnesium in the performance of BaTi0.8Cu0.2O3 catalyst has been analyzed.
Section snippets
Catalysts synthesis and characterization
Ba0.9A0.1Ti0.8Cu0.2O3 catalysts (A = Sr, Ca, Mg) were prepared by the modified Pechini’s sol-gel method in aqueous media described elsewhere [17,18]. In brief, a 0.1 M citric acid solution with 1:2 M ratio with respect to titanium was prepared and heated up to 60 °C. Titanium isopropoxide was hydrolyzed and the resulting solid species were filtered and dissolved in the citric acid solution. The pH of the resultant solution was increased to 8.5 with ammonia solution. Afterwards, the
Characterization of fresh catalysts
The actual metal content of the Ba0.9A0.1Ti0.8Cu0.2O3 catalysts (determined by ICP-OES and featured in Table 2, including the catalysts nomenclature and the specific surface area) confirms the incorporation of almost all the nominal amount of A and Cu metals into the catalysts. The specific surface area of the catalysts (measured by N2-adsorption) reveals that all the perovskite show almost negligible porosity, as it is expected for these mixed oxides [22].
Conclusions
Based on the results obtained, it can be concluded that SO2 inhibits both NO oxidation and NOx adsorption sites on the catalysts surface, but, a promoting effect on the tolerance and regeneration versus SO2 is observed for strontium and magnesium.
In the BTCuO_Sr catalyst, due to the substitution of Ba (II) by Sr (II) in the A site, the amount of the NOx adsorption active sites is larger than in the bare catalyst (BTCuO) and, consequently, this catalyst shows the highest NSC in the long-term
Acknowledgments
The authors thank Spanish Government (MINECO) and UE (FEDER Founding) (project CTQ2015-64801-R) and Generalitat Valenciana (project PROMETEO II/2018/076) for the financial support. V. Albaladejo-Fuentes thanks the University of Alicante for his Ph.D. grant.
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