Perovskite-Based Catalysts as Efficient, Durable, and Economical NO x Storage and Reduction Systems

: Diesel engines operate under net oxidizing environment favoring lower fuel consumption and CO 2 emissions than stoichiometric gasoline engines. However, NO x reduction and soot removal is still a technological challenge under such oxygen-rich conditions. Currently, NO x storage and reduction (NSR), also known as lean NO x trap (LNT), selective catalytic reduction (SCR), and hybrid NSR–SCR technologies are considered the most efficient control after treatment systems to remove NO x emission in diesel engines. However, NSR formulation requires high platinum group metals (PGMs) loads to achieve high NO x removal efficiency. This requisite increases the cost and reduces the hydrothermal stability of the catalyst. Recently, perovskites-type oxides (ABO 3 ) have gained special attention as an efficient, economical, and thermally more stable alternative to PGM-based formulations in heterogeneous catalysis. Herein, this paper overviews the potential of perovskite-based formulations to reduce NO x from diesel engine exhaust gases throughout single-NSR and combined NSR–SCR technologies. In detail, the effect of the synthesis method and chemical composition over NO-to-NO 2 conversion, NO x storage capacity, and NO x reduction efficiency is addressed. Furthermore, the NO x removal efficiency of optimal developed formulations is compared with respect to the current NSR model catalyst (1-1.5 wt % Pt–10-15 wt % BaO/Al 2 O 3 ) in the absence and presence of SO 2 and H 2 O in the feed stream, as occurs in the real automotive application. Main conclusions are finally summarized and future challenges highlighted.


Introduction
The concern about pollutants released by internal combustion engines has increased significantly since the late 1900s. Currently, with more than 600 million automobiles worldwide, vehicle exhaust is one of the main causes of air pollution, especially in urban areas. Unburned hydrocarbons (HCs), carbon monoxide (CO), particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), and soot are the main pollutants in the exhaust [1]. However, exhaust composition depends on the engine operation principle. In this sense, two types of engines can be differentiated, gasoline engines and diesel or lean-burn gasoline engines. The latter operate with high air-to-fuel ratios (A/F = 20-65), which favors fuel economy, engine efficiency, driving performance, and limits CO2, CO, and HCs emissions with respect to stoichiometric gasoline engines. These advantages have motivated the increasing implementation of diesel or lean-burn engines during the last years [2]. Nevertheless, the excess of oxygen introduced leads to a net oxidizing environment, which limits the simultaneous removal of NOx and soot [3]. As a result, the control of NOx, PM and soot is still a technological challenge in diesel and lean-burn gasoline engines. This fact, together with the progressive implementation of stringent standards regarding NOx and soot emissions (currently Euro VI legislation in Europe), has driven the development of a complex exhaust treatment system. The system is usually composed of diesel oxidation catalyst (DOC) followed by a diesel particulate filter (DPF) and NOx reduction catalyst (NRC) in series. This system is usually complemented by ammonia slip catalyst (ASC) placed downstream [4]. The limited NOx reduction efficiency of diesel engines has driven that recent efforts have been mainly focused on the development of an efficient NRC. NOx storage and reduction (NSR), selective catalytic reduction (SCR), and the hybrid NSR-SCR technologies have been developed in order to mitigate NOx emissions in diesel engines [5].
NOx storage and reduction (NSR) technology was introduced by Toyota in the mid-1990s [6]. This alternative operates cyclically under fuel-lean and fuel-rich conditions. The duration of the former period is in the order of few minutes while the latter period duration is few seconds. During the long lean period NOx are trapped over catalytic surface. Then, in the subsequent short-rich period, stored NOx are reduced preferentially to N2. For that, the NSR catalyst compositions usually contain active sites for NO-to-NO2 oxidation and NOx-to-N2 reduction, as well as storage components for NOx adsorption during lean period [7][8][9]. In this context, Pt-Ba/Al2O3 catalyst emerges as the model NSR formulation [10,11]. Alternatively, SCR technology achieves NOx reduction under steady oxidizing conditions by the injection of a selective chemical reductant of the NOx. The ammonia generated from urea decomposition, which is stored on-board in a specific reservoir is the usual reductant in this technology (NH3-SCR) [12,13]. In this case, catalysts based on Cu or Fe exchanged on different zeolites are widely adopted as NH3-SCR formulation [14][15][16].
Both alternatives, the NSR technology and the SCR technology, show some drawbacks that limit their extended implementation, as summarized in Table 1. On the one hand, NSR system requires high Pt loads (1-2 wt %) which increase the costs and limits the hydrothermal and sulfur resistance. Furthermore, some extent of NH3 and N2O may be formed during the short-rich period. On the other hand, NH3-SCR involves a continuous admission of NH3 to reduce NOx, which is formed from the thermal decomposition of urea stored in on-board tank. This fact limits the implementation of SCR technology in small vehicle due to the cost and space requirement. Taking into account that NSR system generates some NH3 as byproduct during the rich phase, a reasonable interest in linking both technologies has grown. Hybrid NSR-SCR technology emerges as a potential solution of some of the main drawbacks previously reported for stand-alone NSR and SCR technologies [17]. Specifically, in the hybrid NSR-SCR system the NOx removal efficiency is increased notably with a simultaneous decrease in the NH3 slip. This operation principle allows circumventing the implementation of an onboard NH3 generation unit. The coupled NSR-SCR technology is composed of NSR and SCR catalysts operating cyclically in a similar way to single-NSR technology. SCR catalyst can be placed downstream of the NSR catalyst in sequential NSR-SCR configuration or alternatively in a singlebrick composing the dual-layer architecture. In most cases, the NOx removal efficiency of hybrid NSR-SCR systems has been analyzed using model NSR formulation ((1-2 wt %) Pt-(10-15 wt %) BaO/Al2O3). However, the amount of noble metal (Pt) should be reduced or replaced by less expensive and thermally more stable materials.
Taking into account these drawbacks, during the last decade, a great interest in developing perovskite-based formulation for NOx removal in diesel engines has grown. Particularly, high structural stability and low cost qualify perovskite oxides as potential alternative to Pt-based catalysts, widely implemented as model NSR formulations [18]. Therefore, the main objective of this review is to overview the recent progress in the application of perovskite-based materials in the stand-alone NSR and combined NSR-SCR systems. First, we shall focus on the general application of perovskite-based formulations to NOx removal in diesel engines. Then, a general outlook on different preparation methods and chemical compositions used during the application of the perovskite-based formulation to the single-NSR system is provided. A major emphasis is devoted to the viability of perovskite-based formulation as alternative to Pt-based NSR catalyst. For that, the thermal and sulfur resistance of both type formulations is also compared. Finally, a brief look of the viability of the perovskite-based formulations as NSR system in the combined NSR-SCR technology is included.

Perovskite-Based Catalysts in Automotive Exhaust Catalytic Converters
Perovskite-type oxides have attracted attention as promising catalyst for exhaust control in automotive applications since Libby [19] and Voorhoeve et al. [20] explored perovskites firstly in early 1970s. The term of perovskite is the general name for oxides with ABO3 and/or A2BO4 structure. In the ideal cubic crystalline unit cell of perovskite (Figure 1), the larger cation A is located in the center edge of the structure; meanwhile the smaller cation B is located in the center of the octahedron. O is an anion that bonds both cations [21]. A cation (coordinated by 12 oxygen), can be a rare earth, alkaline, or alkaline-earth cation. The B cation (surrounded by six oxygen in octahedral coordination) can be any transition metal ions from 3d, 4d, or 5d configuration. In the perovskite structure, A cation plays an essential role as responsible for the stabilization of the structure, while B cation is responsible for the catalytic activity. Perovskite-type oxides can accommodate a wide number of components in A and B sites and can stabilize various distorted structures. The wide range of possible cationic substitutions in the perovskite family generates great flexibility in terms of structure, allowing it to be adjusted appropriately to the process. The stability of the perovskite structure is governed by geometric considerations summarized by the Goldschmidt tolerance factor, Equation 1, where rA, rB, and rO are the respective radii of A, B and oxygen ions. The tolerance factor must be in the range of 0.75 to 1.00 so that the oxide can crystallize with perovskite-type structure. As above mentioned, the flexibility to modulate the catalytic properties of the perovskite structure by the partial or total substitution of A and B cations allows to better adapting to the desired automotive applications [18,22]. Indeed, perovskite-based materials have been widely implemented as low-cost alternative to the catalysts composed of platinum-group metals (PGMs) in automotive catalytic converters [19,20,23,24]. Perovskites have shown excellent activity in oxidation reactions in their implementation as diesel oxidation catalyst (DOC) [23,[25][26][27][28][29][30][31][32]. Furthermore, these oxides demonstrated excellent efficiency in the joint mitigation of NOx and soot emissions from diesel engines. Thus, these materials have been implemented in diesel particulate NOx reduction filter (DPNR) [33][34][35][36][37][38][39]. On the other hand, the catalytic decomposition of nitrous oxide or nitric oxide has been reported as a one their potential applications [40][41][42][43][44][45][46]. Finally, perovskites have been widely implemented for NOx reduction in both stoichiometric gasoline engines (three-way catalyst, TWC) [33,[47][48][49][50][51][52][53] and diesel or lean-burn gasoline engines (NSR and SCR systems). Recently, the latter application has gained special attention due to the increasing necessity of developing an efficient NOx reduction system in diesel engines. This fact became more evident for NSR alternative to overcome the requirement of high platinum-group metals (PGMs) loads to maximize NOx removal efficiency. Therefore, a great range of studies have focused on developing perovskite-based formulations as economical and more durable alternative to Pt-based model catalyst.

Perovskite-Based Catalysts for NSR Technology
NSR technology is considered as a promising approach to control NOx emissions in diesel engines. This alternative operates cyclically under fuel-lean and fuel-rich conditions. During the lean period, platinum oxidizes NO-to-NO2, which is then adsorbed over Ba in the form of nitrites/nitrates. During the subsequent short-rich period, a reductant, such as CO, H2, or HC, is used to release and reduce the stored NOx. Thus, NO-to-NO2 conversion is considered a critical step in improving the NOx removal efficiency in the model NSR formulation. However, this reaction requires high Pt loadings, compound very costly and with limited thermal stability [23]. As a result, the applicability of perovskites-based materials to NSR technology is initially related to the capacity to oxidize NOto-NO2 during the lean period. After that, the NO2 form should be efficiently trapped over catalytic surface during the oxidizing period. Finally, the perovskite-based catalyst should selectively reduce the stored-NOx-to-N2. In order to cover efficiently the consecutive stages in the NSR process different alternatives have progressively been explored. Some authors modified the physicochemical properties of the perovskites-based formulations by partial substitution of A and B cations. Alternatively, other studies supported perovskites over high surface area materials, whereas in other cases additional components were incorporated over perovkite-based formulations. This allows tailoring the catalytic properties of perovskite-based materials for automotive applications. The final goal is to develop a perovskite-based formulation with similar or even higher NOx removal efficiency, with more sulfur resistance and hydrothermal stability than the Pt-based model catalyst.

NO-to-NO2 Conversion
As previously discussed, NO2 plays a decisive role as an intermediate species in the NSR process. Thus, a primary prerequisite to explore the real applicability of perovskite-oxides to NSR technology is to develop a perovskite-based formulation with high NO oxidation capacity. Generally speaking, Choi et al. [54] reported that the catalytic activity in oxidation reactions is strongly influenced by molecular and atomic interactions of oxygen with the perovskite surface. In this sense, many authors suggested that the catalytic oxidation over metal oxides (M) follows a Mars-van Krevelen mechanism [55,56]. As a result, the adsorption of dissociated oxygen is facilitated by vacancies (⬚) in the oxide lattice as schematizes the following reaction In order to obtain a formulation with high NO oxidation capacity, different preparation methods and perovskite compositions have been prepared and tested, as summarized in Table 2. Coprecipitation and especially citric acid method are the more explored synthesis routes due to their simplicity, ease of scale-up, and appropriate textural properties [22]. The synthesis conditions-such as citrate to nitrate molar ratio in the starting solution, pH of the gel precursor dissolution, and calcination protocol-have shown significant influence on NO-to-NO2 conversion activity of the material prepared by citric acid method [25,57]. Regarding perovskite composition, LaCoO3 and LaMnO3 perovskites and their doped modifications have been investigated extensively for NO-to-NO2 conversion due to the excellent performance on other oxidation reactions [58,59].
On the developed formulations, at low temperatures, low conversions were attained, due to kinetic limitations. With increasing temperatures NO-to-NO2 conversion began to increase until 300-350 °C, where the conversion began to drop due to thermodynamic limitations and the reaction pathway then followed the equilibrium curve, as observed in Figure 2. Both LaCoO3 and LaMnO3 benchmark systems show excellent NO-to-NO2 conversion efficiencies [58]. Based on the Mars-van Krevelen mechanism above described (Equations 2 and 3), the excellent activity of these materials for oxidation reactions can be related to some specific structural properties, such as change of oxidation state of B cation, active oxygen mobility, and ion vacancy defect [59]. Indeed, the promotion of oxygen vacancy density seems to be the key factor to maximize oxidation efficiency [60][61][62][63][64]. La 3+ partial substitution by Ca 2+ , Ba 2+ , or Sr +2 , is accepted as a simple way to alter the main physico-chemical properties of perovskite (crystallinity, average crystal size, specific surface area, and redox properties). Among them, Sr 2+ is the most explored cation for this approach. The introduction of lower oxidation state Sr 2+ in substitution of La 3+ in LaMnO3 and LaCoO3 lattice generates a net charge imbalance that may be compensated by alteration of the oxidation state of a fraction of transition metal, leading to Mn 4+ or Co 4+ formation. Alternatively, the oxidation state of transition metal could be maintained unaltered (Mn 3+ or Co 3+ ), but instead oxygen vacancies could be generated in the lattice to attain the charge balance. Even a mixed situation showing altered oxidation state of transition metal along with oxygen vacancies in the lattice could be expected. As suggested by Kim et al. [23] and more recently in our study [25] charge imbalance associated to strontium (Sr 2+ ) incorporation in the perovskite lattice in substitution of lanthanum (La 3+ ) was preferentially balanced by Mn 4+ promotion in La1−xSrxMnO3 perovskites, whereas formation of oxygen vacancies seems to be the mechanism for charge compensation in La1−xSrxCoO3 perovskites, where Co remained as Co 3+ ions. The preferential formation of oxygen vacancies explains the higher NO-to-NO2 conversion efficiencies for La1−xSrxCoO3 perovskites ( Figure  2). Indeed, the designed perovskite-based materials could potentially rival Pt-based model catalyst (Pt-Ba/Al2O3). Thus, La1−xSrxMnO3 and La1−xSrxCoO3 perovskites can be considered efficient approaches to promote NO-to-NO2 conversion in automotive catalysis. Different authors carried out kinetic studies on NO-to-NO2 oxidation process with LaMnO3 and LaCoO3-type perovskites. The results obtained lead to similar conclusions. On the one hand, reaction rate could be linearly accelerated by increasing the concentration of NO and O2 [57], whereas the NO oxidation rate of LaMnO3 and LaCoO3-type perovskites is limited in the presence of NO2 [54,72,74]. Indeed, Constantinou et al. [76] determined a NO, O2, and NO2 orders near to 1, 1 and -1, respectively for LaMnO3 perovksite. Regarding the apparent activation energies of these materials, their values were in the range of 31-45 kJ/mol for LaxMnO3 perovskites and in the range of 50-100 kJ/mol for La1-xSrxCoO3 perovskites.
Some of the studies reported in Table 1 have been focused on understanding the electronic structure of perovskites [54,69,71]. Specifically, Density Functional Theory (DFT) calculations have been carried out to analyze the NO-to-NO2 reaction on LaCoO3-type perovskites. These studies try to develop theoretical model to describe oxygen exchange process during NO-to-NO2 oxidation. Indeed, they concluded that the NO-to-NO2 reaction is favored by Cu [71] or Sr [54] doping due to a decrease of the energy of extralattice oxygen, favoring oxygen vacancies formation. Furthermore, as observed by kinetic experiments, the formation of NO2 seems to limit NO oxidation.

NOx Adsorption under Oxidizing Conditions
Once NO is oxidized to NO2, the nitrogen dioxide formed should be efficiently trapped over the catalytic surface. In agreement with above described, A-site elements of perovskite are usually alkali/alkaline earth metals, which serve as ideal adsorption sites for the NOx storage. As a result, perovskite-type oxides were also implemented as efficient lean NOx trap materials. For the first time in the scientific literature, Hodjati et al. [77][78][79] reported the NOx storage capacity (NSC) of different perovskite-type catalysts (with A = Ca, Sr or Ba; and B = Sn, Zr or Ti). Their studies showed that for the A-site cations NSC was in the order of Ca > Sr > Ba; while the influence of the B-site cations on the NSC was in the order of Ti > Zr > Sn. Thus, the BaSnO3 perovskite had the largest NOx storage capacity. In a series of consecutive studies BaCoO3 [80,81] and BaFeO3 [82][83][84] perovskites were also explored as alternatives with high NOx storage capacity and notable sulfur resistance. In these materials, the presence of BaCO3 as an impurity promoted the NOx adsorption capacity; however, this phase limited the regeneration capacity after SO2 poisoning. More recently, perovskites BaFe0.8Ti0.2O3 [85] and BaFe0.8Cu0.2O3 [75,86] have been proposed as alternative formulations.
Alternatively, La-based perovskites have been extensively studied in recent years due to their excellent NO oxidation conversion and structural stability. As observed for NSR model catalysts, Labased perovskites show volcano-type dependence of the NOx storage capacity with temperature, showing maximum NSC around 350-400 °C. At higher temperatures, NOx adsorption capacity tends to decrease due to the lower stability of NOx adsorbed species together with the lower NO-to-NO2 conversion (Figure 2), which is the limiting step during the NOx storage step. In agreement with that observed for NO oxidation capacity, the modification of physicochemical properties by the partial substitution of A and B cations could promote NOx adsorption efficiency. In this case, La 3+ was partially substituted by other cations with high basicity, such as K + , Ca 2+ , Ba 2+ , or Sr +2 , due to their high NOx adsorption capacity in the conventional NSR formulations. In fact, some studies demonstrated that Sr 2+ rather than La 3+ cations preferred to migrate from the bulk to the surface during the NOx storage period. This migration enhanced the perovskite NSC and catalytic performance due to the presence of higher amount of basic sites at the surface [87][88][89]. Similar process can be expected for the other explored cations. Ueda et al. [90] observed that the partial replacement of 30% of La 3+ by Ba 2+ practically tripled the NOx storage capacity of perovskite LaFe0.97Pd0.03O3. Meanwhile, Li et al. [34] proposed the La0.9K0.1Co0.9Fe0.1O3-δ perovskite as lean NOx trap material. More recently, Li et al. [66] observed how doping LaCoO3 perovskite with 30% of Sr (La0.7Sr0.3CoO3) maximized the storage capacity of La1-xSrxCoO3 perovskites. The best NOx adsorption efficiency was assigned to a best balance between NO-to-NO2 oxidation efficiency and NOx adsorption sites concentration at the surface. Based on the results of DRIFT, they also proposed three main storage routes in these solids: monodentate nitrates (1440 cm -1 ), free nitrate ions (1384 cm -1 ), and nitrates in perovskite (1362 cm -1 ). An increase in temperature above 300 °C favors the storage of NOx as free nitrate ions formed on strontium carbonates or surface structural strontium, as well as the adsorption of the other two species (Figure 3). López-Suarez et al. [91] observed similar NOx storage mechanism for the SrTi0.89Cu0.11O3 perovskite. Otherwise, Dong et al. [57] analyzed the influence of synthesis conditions on the NOx storage capacity of perovskites La0.7Sr0.3MnO3. In this case they observed that the adsorption of NOx at 350 °C occurs mainly in the form of free nitrate ions, which is favored by a greater specific surface area and a more homogeneous distribution of the different components. As described for NO oxidation conversion, the synthesis conditions also influence the NOx adsorption capacity of these perovskites. In this sense, Peng et al. [89] have controlled the selective dissolution of Sr inside the structure La0.5Sr0.5CoO3 by treating the catalyst with HNO3. The migration of Sr promotes the NO oxidation and NOx storage capacities. On the other hand, other authors analyzed the effect of partial substitution of B cation by Pt or Pd [70,92]. The incorporation of these noble metals over perovskite surface has been also explored [92,93]. In both cases the promotion of NOx adsorption capacity is assigned to generation of structural defects and especially to promotion of NOx adsorption sites regeneration during the short-rich period.
One of the main drawbacks of bulk perovskites is the crystal growth of the oxide due to the calcination at high temperature during the synthesis. As a result, bulk perovskites usually possess low specific surface areas (usually below 25 m 2 g −1 ) and limited NOx storage sites accessibility [72,76]. Both factors limit NOx adsorption capacity during lean conditions. Two potential solutions were explored to overcome this limitation: synthesis of mesostructured perovskites via nanocasting method and synthesis of supported perovskite by their distribution over high-surface area materials [94].
Regarding supported perovskites, He et al. [94] found that the distribution of 20 wt % of LaCoO3 perovskite over ZrTiO4 support limited sintering of perovskite. As consequence of the higher accessibility of the perovskite the NOx storage capacity was promoted. In fact, the perovskite-based catalyst exhibited higher NSC than Pt-based catalysts due to the promoted NO-to-NO2 oxidation behavior. Alternatively, You et al. [95,96] observed that the impregnation of a 10 wt % of LaCoO3 perovskite over ceria and Ce0.75Zr0.25O2 supports provides high NOx storage and reduction capacity. More conventional supports with lower price and higher surface area have been also explored. Ding et al. [97] confined the La0.7Sr0.3CoO3 perovskite nanoparticles (60 wt %) on mesoporous silica. This sample significantly increased the NOx adsorption capacity per sample mass unit at 300 °C with respect to bulk sample. Recently, we have prepared alumina-supported perovskites (10-50 wt % La0.7Sr0.3CoO3/Al2O3) by the impregnation of the La0.7Sr0.3CoO3 over γ-Al2O3 [87]. As observed in Figure 4, the distribution of the bulk perovskite over alumina support inhibited the agglomeration of the former. The higher distribution of perovskite phase favors the diffusion of intermediate compounds from oxidation to NOx adsorption sites. The intermediate loading (30 wt % La0.7Sr0.3CoO3/Al2O3) maximized the efficient use of perovskite phase. This fact was as a consequence of a best balance between well-developed perovskite phase and NO oxidation as well as NO adsorption sites (oxygen vacancies, structural La and Sr at the surface, and segregated SrCO3) distribution. In fact, the NOx storage capacity normalized per perovskite mass unit (NSC = 305.8 μmol NOx (gLSCO) -1 ) was three times higher than that of bulk perovskite (NSC = 115.0 μmol NOx (gLSCO) -1 ).
Regarding perovskites with ordered structures, the studies are scarce and the preparation methods are more complex. Ye et al. [65] compared the catalytic behavior of LaCoO3 bulk perovskite and a mesoporous-ordered LaCoO3 perovskite prepared by nanocasting. The latter was obtained by nanocasting using SBA-15 silica as hard-template. As can be observed in Figure 5, the agglomeration of perovskite is significantly inhibited for the sample prepared by nanocasting. As a result, the NSC increases from 93 μmol NOx g -1 for the sample prepared by conventional method to 252 μmol NOx g -1 for the sample prepared by nanocasting. The higher NSC can be attributed to the following aspects: (1) more active sites are exposed due to its much larger specific surface area; (2) easier transportation of reactants and products during reactions due to the mesoporous structure; (3) larger number of oxygen vacancies; (4) presence of high-valence cobalt ions (Co 3+δ ). Alternatively, the conformation of La0.8Cs0.2Mn0.8Ga0.2O3 perovskites with cotton-like morphology consisting of nanoparticles and nanorods [98] or ZnO nanorod array supported Pt:La0.8Sr0.2MnO3 lean NOx traps [99], emerge as more complex alternatives of improvement of the storage capacity of NOx. Recently, Alcalde-Santiago et al. [100] prepared macroporous SrTi1-xCuxO3 perovskites with high NOx adsorption capacities.  As previously reported, NSR catalysts usually contain strong basic components to promote the trapping of the acidic NOx molecules. This fact motivated the incorporation of small amounts of basic components over perovskite-based formulation, or alternatively, the mixing of the perovskite with a phase with high NOx trapping efficiency. Ye et al. [65] observed that the incorporation of K (5 wt % of K2CO3) into the mesostructured LaCoO3 perovskite increased the adsorption capacity from 252 μmol NOx g -1 to 981 μmol NOx g -1 at 350 °C, due to the presence of a higher concentration of surface basic sites. On the other hand, Qi et al. [72,76] in their studies ball-milled LaMnO3 perovskite with Ba/Al2O3 catalyst to promote NOx adsorption efficiency during the lean period. Alternatively, Wen et al. [101] loaded LaCoO3 perovskite (25 wt %) on Al2O3 support by mechanical mixing. Subsequently, K2CO3 (16 wt %) and Pt (0, 0.3, and 1 wt %) were loaded on LaCoO3/Al2O3 by the conventional impregnation method. Meanwhile, You et al. [95,96] incorporated increasing contents of K2CO3 (x = 1-8 wt %) on LaCoO3/CeO2 and LaCoO3/Ce0.75Zr0.25O2 catalysts. In this case, NOx storage capacity was maximized for the samples with 3-5 wt % of K2CO3 due to the presence of higher concentration of homogeneously distributed NOx adsorption sites at the surface. This activates a new NOx storage pathway in form of nitrates and/or nitrites formed due to the reaction between K2CO3 and NOx.

NOx Reduction
Perovskites have demonstrated as promising materials for NO oxidation and NOx storage in an oxygen-rich atmosphere. However, the final aim of NSR system is to reduce efficiently the NOx stored during the lean period preferentially to N2 in the subsequent short-rich period. Thus, perovskites should also be good NOx reduction catalysts in a fuel-rich atmosphere. In this sense, few works analyze the NOx removal efficiency of perovskite-based formulations. Table 3 summarizes the most relevant results found in the scientific literature related to the utilization of perovkite-based formulations as NSR catalyst. BaFeO3-x [84] and La0.7Ba0.3Fe0.776Nb0.194Pd0.03O3 [90] were the alternatives firstly explored; however, these perovskites did not show high NOx removal efficiency. Kim et al. [23] were the first that observed comparable results to conventional formulations in the application of perovskite-based catalysts as DOC and also as LNT. Specifically, their monolithic catalyst based on La0.9Sr0.1MnO3 perovskite ball-milled with Pd-Rh/BaO/CeO2-ZrO2 catalyst, with noble metal contents (1.8 Pd/0.2 Rh, g L -1 ) somewhat lower than a commercial catalyst (1.6 Pt/0.3 Pd/0.2 Rh, g L -1 ), showed a NOx removal efficiency similar to the commercial catalyst in the presence of CO2 and H2O. More recently, two consecutive studies carried out by researchers from General Motors [72,76] analyzed the catalytic behavior and reaction mechanism of a monolithic LaMnO3 + 4 wt % Pd/Al2O3 + 2 wt % Rh/CeO2-ZrO2 prepared by ball-milling. Figure 6 shows the NO, NO2 and ammonia concentration profiles at 350 °C under cycling lean (60 s)/rich (5 s) periods over Pd/Rh/LaMnO3/BaO/Al2O3 catalyst with a SV = 25,000 h −1 . The lowest NOx outlet concentration is observed at the beginning of the lean period due the NOx adsorption over basic sites (Feed composition: 400 ppm NO, 10% O2, H2O, CO2, and N2 as balance). As increasing lean period duration, NOx trapping sites become gradually saturated and thus NOx concentration at the reactor outlet increased. In any case, a significant amount of NOx was adsorbed on the catalyst during the lean period. In fact, a maximum NO outlet concentration of 25 ppm upon switching to rich period was reached after six cycles. This indicates that more than 90% of NOx are stored. During the short-rich period, oxygen was replaced by a mix of reductant gases (1% H2, 3% CO). As a result, the stored NOx are released as NO with a very small amount of ammonia upon switching to a rich feed. This indicates that the reaction between desorbed NOx and reductant gases (CO + H2) occurred. Indeed, this formulation showed NOx conversions above 85% and selectivity towards NH3 around 25%. Taking into account the mechanism of the process, numerous similarities with the Pt-based model catalysts were confirmed. In fact, this formulation showed similar temperature dependence of NOx storage and reduction to the conventional Pt-Ba/Al2O3 catalyst. However, Constantinou et al. [76] found some differences in their study: (i) a greater resistance to diffusion of nitrates at low temperature, (ii) a faster decomposition of nitrates in reducing environment, and (iii) a greater inhibition of the OSC (oxygen storage capacity) by nitrates.
Alternatively, other studies tried to develop new alternatives based on simpler perovskitebased formulations. In the study carried out by Li et al. [66], the improvement of the NOx storage capacity for the catalyst La0.7Sr0.3CoO3 after Sr doping was also accompanied by an improvement of the reduction of NOx with C3H6. This catalyst showed NOx conversion of 71% and selectivity towards N2 of 100%. However, the results were obtained with relative high rich/lead periods ratio (60 s/180 s), and continuous admission of the reducing agent (C3H6). In order to obtain higher NOx reduction efficiency, small contents of noble metals, especially Pd, are incorporated on perovskite-based formulation. Two preparation methods were explored for the synthesis of noble metal containing perovskite-based formulations: impregnation or doping the perovskite structure. Li et al. [67] proposed the incorporation of Pd inside the perovskite structure (La0.7Sr0.3Co0.97Pd0.03O3) by doping as a simple way to improve the NOx reduction efficiency of the catalyst. In their study, the Pd accommodation within the lattice was demonstrated by the results obtained by XRD, XPS, and EXAFS experiments. This formulation achieved high NOx conversions and selectivity towards N2 above 90%, feeding C3H6 (0.1%) only during the rich period. In fact, the NOx removal efficiency was similar to that observed for conventional NSR formulation, as observed in Figure 7. However, the duration ratio of rich/lead periods remained high (60 s/120 s).  For the first in the literature, Zhao et al. [92] compared the NOx removal efficiency of two Pdbased La0.7Sr0.3CoO3 perovskites prepared by impregnation or doping the perovskite structure. As a general trend, NOx storage and reduction efficiency is significantly promoted after the incorporation of Pd. The improvement of the NOx removal efficiency is assigned to a promotion of NOx adsorption sites regeneration and NOx reduction rate during rich period (Figure 8). Thus, the higher accessibility of Pd obtained by impregnation method further promoted these steps for Pd-impregnated sample. More recently, we synthesized different catalysts with increasing palladium loadings (0.75, 1.5, 2.25 and 3.0 wt %) incorporated by both methodologies over 30 wt % La0.7Sr0.3CoO3/Al2O3 formulation [102]. As a general trend, Pd-impregnated samples showed higher NOx-to-N2 conversion than Pddoped samples. In agreement with the observed by TEM-EDX mapping and XRD analysis, this fact was ascribed to the partial accommodation of Pd inside the perovskite structure observed for Pddoped sample, which limits Pd accessibility during lean-rich periods. Among Pd-impregnated samples, the 1.5 wt % Pd-30 wt % La0.7Sr0.3CoO3/Al2O3 variant achieved the best balance between NOx storage and reduction activity and minimum palladium content. Specifically, their NOx conversion and nitrogen production were as high as 86% and 70%, respectively. In fact, this formulation achieved comparable DeNOx activity to the model NSR catalyst (1.5 wt % Pt-15 wt % BaO/Al2O3). These results confirm the potential of the 1.5 wt % Pd-30 wt % La0.7Sr0.3CoO3/Al2O3 catalyst for NOx removal in diesel automobile applications. On the other hand, Wang et al. [70] and Wen et al. [101] analyzed the effect of Pt on NOx removal efficiency of LaCo0.92Pt0.08O3 and 0.3 wt % Pt/K2CO3/LaCoO3/Al2O3 formulations, respectively. These alternatives showed NOx-to-N2 removal efficiencies comparable or superior to 1 wt % Pt-16 wt % Ba/Al2O3 catalyst. However, the Pt load in the former was high (~ 6.0 wt % Pt), whereas the catalyst composition was too complex in the later, which makes them less promising than the previously described. Taking into account results reported for NOx storage capacity, the incorporation of perovskites on high surface area supports is presented as an alternative for partial or total replacement of Pt in these formulations. You et al. [95,96] observed how the catalyst 5 wt %K2CO3-10 wt % LaCoO3/S (with S = Ce0.75Zr0.25O2 doped with 5% Y) provided NOx reduction efficiencies of 98% and selectivities towards N2 of 99% at 350 °C. The high activity of these formulations (even in the presence of CO2 in the feed) is assigned to the high oxidation capacity of NO, and dispersion of the NOx storage centers (K), which favors a good contact between both phases and the diffusion of intermediate compounds.
However, although the results obtained are apparently promising, the reducing periods used in their experiments were too long (60 s) and the space velocities too low (45,000 h -1 ). In addition, the results were obtained in absence of NO in the rich period. As observed for NOx adsorption, the conformation of mesoporous perovskites with ordered structure has also been analyzed. The 5 wt % K2CO3/LaCoO3 formulation prepared by nanocasting and sequential impregnation of K2CO3 showed NOx reduction efficiencies of 97% and N2 selectivities of 97% at 350 °C [65]. However, again the reducing periods used are considered long (60 s), and the results were obtained in absence of NO in the rich period and feeding reducing agent (C3H6) in both periods. On the other hand, the ordered macroporous SrTi1-xCuxO3 perovskites showed a limited NOx reduction capacity and a high influence of CO2 and H2O, which limits their actual application [100].

SO2 and Hydrothermal Resistance
Based on above described results, perovskite-based formulations can be proposed as an economical alternative to 1.5 wt % Pt-15 wt % BaO/Al2O3 model catalyst. However, one of the essential characteristics of the NSR catalyst for application in the exhaust aftertreatment of diesel engines is the durability, basically referring to hydrothermal resistance. In this sense, Pt-based model catalyst shows poor hydrothermal stability and limited sulfur resistance [103,104]. Thus, to accomply the characteristics required for real application, the perovskite-based formulations should show high NOx removal efficiency with simple and complex feedstreams, appropiate hydrothermal stability, and sulfur resistance. In order to have a more realistic vision on these aspects, some of the works previously reported also analyzed the hydrothermal and sulfur resistance of the corresponding perovskite-based formulations.
Regarding sulfur resistance, it is widely accepted that the decrease of NOx removal efficiency after sulfur poisoning is derived from a significant decrease of NOx adsorption capacity during lean period. This fact is assigned to the formation of very stable sulfates over basic components of the NSR catalyst. Kim et al. [23] observed that their monolithic catalyst based on La0.9Sr0.1MnO3 perovskite ball-milled with Pd-Rh/BaO/CeO2-ZrO2 catalyst showed higher sulfur resistance and regenerability after SO2 poisoning. More recently a new concept has emerged based on the results reported by Nishihata et al. [105]. In that work, the authors observed for Pd-doped perovskites a self-regeneration of Pd 0 /Pd 2+ in and out of perovskite lattices when switching between oxidizing and reducing atmospheres. This behavior could improve the hydrothermal and sulfur resistance of perovskitebased materials. Taking these results as reference, Li et al. [67] also explored the sulfur resistance of La0.7Sr0.3Co0.97Pd0.03O3 perovskite. This formulation shows excellent sulfur tolerance. Based on the results obtained by EXAFS, XPS, and XRD analysis, this behavior was related to the Pd mobility in the perovskite structure, as outlined in Figure 9. In contrast, the Pt-BaO/Al2O3 catalyst was readily poisoned by sulfur (NOx conversion dropping from 99% to 55%, N2 selectivity dropping from 96% to 85%) and could not recover its initially catalytic activity by reducing in H2 at mild temperatures, such as 325 °C. Apparently, the Pd-doped perovskite provides a new possibility for overcoming the problems caused by sulfur poisoning for the LNT systems. More recently, Wang et al. [70] compared the sulfur resistance of LaCo0.92Pt0.08O3 and 1 wt % Pt-16 wt % Ba/Al2O3 catalysts. As observed in Figure 10, both formulations are quickly deactivated in the presence of 100 ppm of SO2, but LaCo0.92Pt0.08O3 shows higher regeneration ability than Pt-Ba/Al2O3 in the lean period. This fact is assigned to the formation of surface and bulk cobalt sulfate on LaCo0.92Pt0.08O3 perovskite, which is less stable than bulk barium sulfate under reducing conditions. The sulfur resistance of LaCoO3 perovskites was also highlighted in the simultaneous removal of NOx and soot [106]. Regarding hydrothermal resistance, the limited stability of the NSR model formulation is ascribed to Pt progressive agglomeration during reactions as well as the formation of BaAl2O4 phase. Wang et al. [70] compared the thermal stability of LaCo0.92Pt0.08O3 and model NSR catalyst (Pt-Ba/Al2O3). For that, samples were first treated in a muffle at 850 °C for 40 h and then submitted to stability test. As shown in the left side of the Figure 11, LaCo0.92Pt0.08O3 has a much better thermal stability than conventional Pt-Ba/Al2O3. As observed in Figure 11, the particle size of Pt-Ba/Al2O3 catalyst increased from 4−6 nm to 26−46 nm after hydrothermal aging. In contrast, fine Pt particles (about 4−7 nm) accommodated in the perovskite structure have no obvious change after LaCo0.92Pt0.08O3 was thermally aged under the same conditions. Thus, Pt particles sintering at high operating temperature explains the lower stability of Pt-Ba/Al2O3 catalyst. However, good redox property of LaCo0.92Pt0.08O3 can well maintain its perovskite-type structure. Thus, the selfregeneration of noble metal particles in and out of perovskite lattices when switching between oxidizing and reducing atmospheres seems to prevent it from agglomeration during reactions. On the other hand, as the real feed stream usually contents H2O, the NOx removal activity was also compared incorporating a 10% of H2O in the feed stream (right side of Figure 11). The presence of H2O results in a quick decrement of NOx conversion for both formulations. However, the NOx conversion is rapidly restored after cutting off the supply of water. More recently, Wen et al. [101] also analyzed the hydrothermal and sulfur resistance of 0.3 wt % Pt/K2CO3/LaCoO3 catalyst. This alternative showed NOx-to-N2 reduction efficiency, resistance to poisoning with SO2, regenerability, and durability comparable or superior to Pt-Ba/Al2O3 model catalyst.
The results analyzed suggest that the segregated metallic Pd/Pt from perovskite in fuel-rich atmospheres plays a significant role in obtaining promising achievements in reducing deactivation. This provides a new possibility for the application of perovskite-based formulations as alternative to NSR model catalyst and for solving the problems caused by simultaneous sulfur poisoning and noble metal aggregation at high temperatures.

Perovskite-Based Catalysts for Combined NSR-SCR Technology
Results above reported demonstrate that perovskite-based formulations are able to obtain NOx removal efficiency, hydrothermal stability, and sulfur resistance similar of even higher than conventional NSR model catalalyst (Pt-BaO/Al2O3). However, as above mentioned, single-SCR and NSR technologies have some drawbacks that limit their global application in diesel vehicles. These limitations have been partially solved by the implantation of hybrid NSR-SCR, discovered by the Ford Motor company [107]. Since its discovery hybrid NSR-SCR systems have been subjected to a continuous development. Indeed, different system architectures, catalytic formulations and operation control have been analyzed [108][109][110][111]. However, the NSR formulation in the combined NSR-SCR configuration is usually based on the Pt-model formulation. As a result, the cost of the hybrid NSR-SCR system increases, whereas its hydrothermal stability decreases. Taking into account the results above reported, the application of perovskite-based formulations in combined NSR-SCR systems can be considered as an improvement of the conventional NSR-SCR configuration.
In a preliminary study, we analyzed the applicability of perovskite-based materials to hybrid NSR-SCR configuration [112]. Figure 12 shows how evolved NOx (NO + NO2), and NH3 outlet concentration profiles during two consecutive lean-rich periods, as well as mass spectroscopy N2 signal for the single-NSR and combined NSR-SCR configurations at 300 °C. NSR catalyst correspond to 0.5 wt % Pd-30 wt % La0.5Ba0.5CoO3/Al2O3 formulation, whereas SCR system is composed of conventional 4% Cu/SAPO-34 formulation. As can be observed in Figure 12a, the single-NSR system shows the typical NOx outlet concentration profile previously described in Figure 6 [8]. However, when SCR system is placed downstream the NOx (Figure 12a) and NH3 (Figure 12b) outlet concentrations decrease drastically in comparison to single-NSR system. These results suggest that most NH3 formed during regeneration of the NSR catalyst is adsorbed over SAPO-34 zeolite and then, in the subsequent lean period, reacts with the NOx slipping NSR system following SCR reactions [113]. The occurrence of SCR reactions over the Cu/SAPO-34 catalyst is confirmed by the evolution of N2 signal measured by MS (Figure 12c). As can be observed, N2 signal is detected in the rich and lean periods when the reaction was carried out with the combined NSR-SCR system, whereas N2 formation is only detected during the rich period for the single-NSR systems. When the gas mixture is switched to lean conditions, practically all NOx is trapped on the NSR catalyst, and consequently, is not available to carry out the SCR reactions with the NH3 stored in the Cu/SAPO-34 downstream. However, as the NSR catalyst becomes saturated, the gradual increase of NOx leaving the NSR system promotes the N2 production by the reduction of non-adsorbed NOx with the NH3 previously stored over the acid sites of the zeolite [114]. This fact explains the similar evolution of the NOx concentration and N2 signal at the outlet of the combined NSR-SCR system. In summary, the applicability of 0.5 wt % Pd-30 wt % La0.5Ba0.5CoO3/Al2O3 catalyst to mixed NSR-SCR system is confirmed. Based on the reported results, the implementation of these types of perovskite-based materials in coupled NSR-SCR configurations can be considered as a promising evolution of the conventional NSR-SCR systems. As a result, this opens a new scope in the development of perovskite-based formulation as a new generation of NSR catalyst to overcome NOx removal environmental issue in diesel and lean-burn gasoline engines.

Conclusions
Diesel engines offer higher energy efficiency and lower CO2 emissions than gasoline engines. However, NOx emission removal diesel engine exhaust gases remains as an unsolved environmental issue. To overcome this technological challenge, two main potential solutions have been developed: NOx storage and reduction (NSR) and selective catalytic reduction using NH3 (NH3-SCR). However, these technological alternatives show some limitations for extensive application. NSR system shows some NO non-converted as well as large quantities of NH3 generated during the rich period as byproduct. Furthermore, the catalyst needs large quantities of expensive Pt to obtain high NOx removal efficiencies. This fat also limits the thermal stability of the system. Otherwise, NH3-SCR system requires the urea feeding system for the ammonia injection as chemical reductant. This additional system increases the cost and limits its implementation to light-duty vehicles. Moreover, the limited NO conversion at low temperature and NH3 slip are some of main of limitations. Recently, combined NSR-SCR configurations have been explored to overtake individual limitations of the stand-alone NSR or SCR systems. In fact, the developed hybrid NSR-SCR systems are able to increase the temperature operational window and NO conversion, and avoid the need for a urea dosing system. Nevertheless, the conventional Pt-Ba/Al2O3 NSR catalyst is the most adopted formulation in coupled NSR-SCR configurations. Thus, a new generation of catalysts and an evolution of the current technologies are essential to reduce NOx emissions below EURO VI standards.
Perovskite-type oxides can adopt a great range of stoichiometries and crystal structures maintaining a high thermal stability. Indeed, the physico-chemical properties can be controlled by the modification of their composition substituting partially A and B cations. This allows qualify them for their automotive application. As a consequence, perovskite-based materials have been extensively explored as economical and more durable alternative to Pt-based model catalyst for NSR system. Labased formulations, especially LaCoO3 and LaMnO3 perovskites, have been the most explored alternative due to their excellent efficiency in NO-to-NO2 conversion, which is considered a primary step in the NSR process. La 3+ partial substitution by other cations, such as Ca 2+ , Ba 2+ , or Sr +2 , is widely accepted as a simple way to improve NO oxidation conversion and NO storage capacity during lean period. The improvement of the NO oxidation activity is closely related to a higher oxygen vacancy density at the surface, which promotes the active oxygen mobility. Meanwhile, the promotion of NOx storage capacity is ascribed to the promotion of NO-to-NO2 conversion together with the presence of higher concentration of NOx adsorption sites at the surface. Unfortunately, bulk perovskites are not efficient enough as NOx storage and reduction catalyst due to their crystallization at high temperature. Supporting perovskite over high surface area materials, such as Al2O3, CeO2, Ce0.75Zr0.25O2, SiO2, or ZrTiO4, has demonstrated to be an efficient approach. Alternatively, the incorporation of small amounts of basic components, such as Sr, Ba, or K, over perovskite-based formulation, or alternatively, the mixing of the perovskite with a phase with high NOx trapping efficiency also improves NOx storage and reduction efficiency. Nonetheless, perovskite-based materials show limited NOx reduction at low and intermediate temperatures. The incorporation of noble metal (Pt or Pd) low contents by impregnation over perovskite-based formulation or doping perovskite structure emerge as efficient solutions. The former seems to be more appropriate to maximize NOx removal efficiency. Meanwhile, the latter partially inhibits the agglomeration of noble metal during reactions and promotes sulfur resistance. This fact is ascribed to the self-regeneration of noble metal particles in and out of perovskite lattices during lean-rich cycles. Indeed, these formulations show similar or even higher NOx removal efficiencies, and hydrothermal and sulfur resistance than conventional Pt-BaO/Al2O3 catalyst.
The promising results discussed in the application of perovskite-based formulations to standalone NSR system motivated their implementation in hybrid NSR-SCR configurations. The preliminary results have shown almost complete NOx conversion to N2 without NH3 slip. These results are even more promising considering that the noble metal content in the NSR catalyst is significantly lower than in conventional NSR-SCR configuration.
The results reported in this review reveal that perovskites-based materials have emerged as a new generation of material for diesel automotive applications. In upcoming years, more comprehensive studies focused on understanding the mechanism involved in NOx storage and reduction over perovskite-based formulation, are required. Furthermore, the efficiency of the system during cyclic lean-rich periods can be further promoted. It is worth to mention that the application of perovskite composed materials to the combined NSR-SCR system is very recent. Thus, this opens a new horizon on diesel engines aftertreatment systems with ample room for improvement. Efforts should be focused on exploring different catalyst architectures (i.e., segmented zones or dual layer monoliths), reducing the cost of the catalyst and developing of detailed kinetic model. Funding: Support from the Spanish Ministry of Economy and Competiveness (Project CTQ2015-67597-C2-1-R), the Basque Government (IT1297- 19), and the University of the Basque Country acknowledged. One of the authors (JAOC) was supported by a PhD research fellowship provided by the Basque Government (PRE_2014_1_396).

Conflicts of Interest:
The authors declare no conflict of interest.