**1. Introduction**

Diesel engines are a type of lean burn engine, operating at upper stoichiometric air-to-fuel ratios (A/F>14.7/1), which grew in popularity at the end of 20th century as they offered higher fuel efficiency and less CO2 emissions respect to gasoline engines [1]. However, these engines show a highly relevant drawback since they generate large amounts of NOx and soot [2,3]. In order to minimize the level of these pollutants, more stringent standards were progressively established all over the world. Nowadays, it is accepted that the current EURO VI standard regarding NOx emissions is not met by just improving the quality of the fuel, by modifying the engine, or by using three-way-catalysts (TWCs). Consequently, alternative catalytic strategies are mandatory in order to avoid the disappearance of vehicles fitted with a diesel engine [4,5].

Two methodologies have been proposed to control NOx emission in lean burn engines: selective catalytic reduction (SCR) and NOx storage and reduction (NSR), also called lean NOx trapping (LNT). LNT technology involves the adsorption of NOx under lean conditions, followed by the periodic regeneration of the catalyst by reduction under rich conditions [6]. The conventional LNT catalysts (fitted in diesel cars) are composed of a platinum-group metal and an alkaline or alkaline-earth oxide (BaO or K2O) supported on a high surface area material (Al2O3, TiO2, ... ). It has been found that LNT technology matched with a TWC in a direct-injection spark ignition (DISI) engine can feature interesting results for NOx control emission [7].

Nevertheless, these conventional LNT catalysts present some drawbacks [8], with the high cost of noble metals (mainly Pt) being one of the most relevant. In fact, an interesting challenge has been highlighted in a recent EU report, regarding the need to develop alternatives to the use of critical raw materials such as precious metals [9]. In this line, the potential of perovskite base catalysts is being largely illustrated in the literature for environmental applications [10–12].

In previous studies [11,12], titanium was partially substituted by copper in the BaTiO3 perovskite structure, showing the resulting BaTi1−xCuxO3 perovskites a high activity for NOx storage, which was attributed to the presence of oxygen vacancies (created on the catalyst surface as a consequence of the copper incorporation into the structure) and to the segregation of some phases (mainly BaCO3 and Ba2TiO4, but also CuO). It was also concluded that the BaTi0.8Cu0.2O3 catalyst presents a NOx storage capacity (NSC) at 420 ◦C in the range of levels reported for noble metal-based catalysts (around 300 μmol/g) [13], and hence could be proposed as a potential component of high-temperature LNT systems for lean burn engines, such as gasoline direct injection engines. Moreover, in the literature, other metals such as Mn, Fe, or Co have been proposed as promising B cations in the perovskite used as catalysts for NOx and soot removal [14–16]. Thus, the aim of this paper is to determine the effect of Ti partial substitution by Mn, Fe, and Co in the NSC of the BaTi0.8B0.2O3 LNT catalyst. The results will be analyzed with respect to the performance of the previously studied BaTi0.8Cu0.2O3 catalyst [11,12].
