Effect of catalyst preparation for the simultaneous removal of soot and NOx

Accepted Manuscript Title: Your article is registered as a regular item and is being processed for inclusion in a regular issue of the journal. If this is NOT correct and your article belongs to a Special Issue/Collection please contact [email protected] immediately prior to returning your corrections.–>Effect of catalyst preparation for the simultaneous removal of soot and NOx Authors: Laura Ur´an, Jaime Gallego, Li W.-Y., Alexander Santamar´ıa PII: DOI: Reference:

S0926-860X(18)30535-0 https://doi.org/10.1016/j.apcata.2018.10.029 APCATA 16858

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Applied Catalysis A: General

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9-6-2018 31-8-2018 23-10-2018

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Effect of catalyst preparation for the simultaneous removal of soot and NOx Laura Urán1, Jaime Gallego1, Li, W.-Y.2, Alexander Santamaría*,1 1

Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de Ciencias Exactas y

2

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Naturales, Universidad de Antioquia UdeA, Calle 70 No.52-21, Medellín, Colombia

Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Taiyuan, Shanxi, China

*

Corresponding author: [email protected]

Phone: 574+2196611

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Graphical abstarct

Highlights

Silver inclusion and Ago surface segregation are needed for soot-NO removal MW-assisted synthesis facilitates both silver inclusion and Ago segregation Silver inclusion correlates with oxygen vacancies formation Short MW-irradiation time is enough to promote metallic silver segregation Solid- state synthesis favored silver segregation as Ag2O which is less active

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    

Abstract La0.7Ag0.3MnO3 perovskite-like catalyst was prepared using three synthetic routes, microwaved-assisted (MW) microwaved-assisted hydrothermal (HMW) and solid state (SS) with the aim of studying catalyst preparation

method effect on the simultaneous removal of soot and NOx under loose contact condition. The materials were characterized using various analytical techniques, such as XRD, XPS, H2-TPR and O2-TPD. Among the prepared solids, the MW catalyst showed the best catalytic performance for the simultaneous removal since it was the one with the lowest soot ignition temperature (160 °C), the lowest temperature for the maximum NOx reduction (328 °C), the highest NOx reduction efficiency (60 %); and an intermediate T50 (371 °C) for soot

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oxidation. Results further showed that a rapid synthesis using short microwave irradiation time (6 min) for the precursor solution and short calcination times for the final solid (1 h) produced smaller particles (13 nm) with a high silver (Ag+) content incorporated into perovskite lattice structure thus facilitating the formation of oxygen vacancies as active sites. In fact, the increment in the oxygen vacancies correlated with the increment of surface oxygen content (α-oxygen) that along with the segregated metallic silver amount allowed inferring that both active sites are needed for the simultaneous NO and soot removal. Finally, based on our results it was suggested that both NO and O2 are adsorbed from the gaseous phase on the active sites (oxygen vacancies and Ago) of the

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catalyst surface to form NO2 as an intermediate that quickly reacts with soot to yield CO2 and N2.

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Keywords: Soot-NOx simultaneous removal; preparation method; LaAgMnO perovskite-like; oxygen

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vacancies; metallic silver.

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1. Introduction

The emission regulations for diesel engines are becoming increasingly stricter worldwide due to the common concern about air quality and health problem. In fact, particulate matter with sizes down to 2.5 µm can penetrate

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into the lungs causing serious health problems because of the presence of adsorbed hydrocarbons on its surface [1–3]. In addition, nitrogen oxides are major air pollutants that cause photochemical smog formation and acid

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rain. Many engine modifications and exhaust-after-treatment devices have been proposed and tested to reduce the NOx and soot emissions from diesel exhausts engines. However, the simultaneous reduction of NOx and soot particulate emissions cannot be accomplished by engine modifications alone, and the implementation of diesel

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particle filter combined with catalytic combustion technology appears to be the most practical method to reduce the emission of these harmful substances. To promote the NO conversion and reduce the soot combustion temperature, several catalysts have been proposed including noble metal catalysts, transition metal oxides, alkaline metal oxides, ceria-based oxides, and perovskite-like oxides [4–8]. The noble metal catalysts exhibit an excellent catalytic activity to soot oxidation (T50 = 325 °C and T90= 368 °C) and NO removal (200 °C
°C) [8]. However, they are very expensive, scarce and sensitive to be poisoned. So, many researches are looking for alternatives to replace them [9–14].

Currently, perovskite-structured oxides are probably the most studied mixed-oxide system in the field of heterogeneous catalyst attracting much scientific and application interest owing to their low price, adaptability,

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and thermal stability [9]. Perovskites can be described by the general formula ABX3 where A-cation is often a rare earth coordinated to 12 oxygen anions and B cation is in octahedral interstices in the X framework, where X is commonly oxygen. One of the advantages of the perovskite-like structure is the possibility to adopt a wide range of different compositions, changing either the A or the B cation or partially substituting each cation by other cations of the same or different valences [9]. Despite the extensive uses, the commercial success for this class of perovskite-based catalytic materials has not been achieved for vehicle exhaust emission control or for many other environmental applications. The content of oxygen and defects can vary depending on the

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composition due to a large stability range of the structure or the way this solid can be prepared which could

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increase catalytic active sites exposure. Owing to their exceptional redox properties (multiple valence states of

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the cations) coupled to a high oxygen mobility, perovskites can be remarkable oxidation catalysts.

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Different methodologies have been proposed to prepare perovskite-like materials as catalysts, solution chemistry methods (sol-gel, freeze-drying, spray-drying, co-precipitation, etc.) can generally produce highly crystalline materials at lower temperature (700 °C –800 °C) than solid-state reaction (900 °C –1000 °C)

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[9,15,16]. The physical properties of perovskite-like materials are greatly dependent on synthesis methods. In

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general, perovskites are commonly prepared by the solid-state method (or ceramic) through diffusion processes. However, the diffusion in the solid state is slow; thus, to complete the reaction and obtain a pure perovskite phase, high reaction temperatures (typically higher than 1000 °C) and long reaction times are required [16].

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Since 1980s several new synthesis routes, commonly classified as solution-mediated methods, were developed, such as co-precipitation, complexation, sol−gel, and freeze/spray drying [9]. These methods, based on

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dissolving precursors in a liquid media followed by drying and calcination steps, rapidly replaced solid−solid routes. Solution-mediated processes allow crystallization of pure perovskite phases at temperatures largely below those used in the ceramic methods [9]. Recent preparation routes, offering the material change from large

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nano- to small nanoscale or highly porous solids [9]. For instance, Microwave assisted methods have the potential to improve the crystallization process of different materials, offering many advantages, such as, (i) very short synthesis time, (ii) clean process with limited use of solvents, and (iii) high energy efficiency. The extremely short crystallization times are due to the very rapid temperature increase under microwave heating [17]. On the other hand, the combustion route, also called self-propagating high-temperature synthesis, is proposed as an efficient alternative to synthesize nanocrystalline mixed oxides [9]. Hydrothermal and

solvothermal synthesis of perovskites was reviewed by Modeshia et al. [16], who highlighted that they are very efficient to generate a few nanometer-sized oxide and mixed-oxide materials. Despite the low crystal sizes achieved, some authors evidenced some differences in particle morphology and homogeneity [5,9,15].

Pure ceria catalysts were prepared by a facile modified homogeneous co-precipitation method, and their catalytic

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performance was evaluated in the soot oxidation reaction [18]. It is shown that CeO2 samples prepared by the improved co-precipitation method exhibit obviously better catalytic activity than those prepared by the conventional co-precipitation method. The sample named CeO2-CP4-F showed the best activity, for which the peak temperatures, Tp, for soot combustion were around 465 °C in O2 and 430 °C in a NO/O2 stream under soot/catalyst “loose contact” conditions. Moreover, after high temperature aging, the activity of CeO2-CP4-A becomes even better, for which the Tp value of soot combustion is lowered to 445 °C in O2 air gas. The catalytic activity is not well associated with the physicochemical properties of BET surface area, particle size and reducibility at low temperatures. The number of

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lattice oxygen and its mobility in the series of CeO2 may be the crucial factor to decide the overall catalytic

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performance. According to the HR-TEM result, the excellent migration of lattice oxygen may result from the preferential exposure of more-reactive planes, which may be the essential reason to explain the good performance of

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CeO2 catalysts in soot oxidation [18].

Ceria catalysts with different morphologies have been successfully prepared by hydrothermal (CeO2-nanorods) and solvothermal (CeO2-nanoparticles and CeO2-nanoflakes) methods and their catalytic behaviors were evaluated for

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soot-NOx combustion [5]. CeO2 with different morphologies exhibit different physicochemical properties influencing

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further their catalytic activity toward soot oxidation. The CeO2 nanorods with width of 250 nm and length of about 2 μm exhibited the excellent catalytic activity for soot combustion under loose and tight contact conditions in presence/absence of NO. Its activity is higher than that of some CeO2 catalysts that reported in the literatures [5], and

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even reached a level of a precious metal catalyst. The characterization results prove that the difference in catalytic activities is attributed to their surface physicochemical properties derived from different morphologies of CeO2, in

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which the adsorbed oxygen species on the surface should be significant factors in soot combustion. In addition, high BET surface area may also be a positive effect on soot oxidation activity under the loose contact conditions.

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The catalyst activity of a series of SmMn2O5 oxides prepared by different methodologies (citric acid, hydrothermal, coprecipitation and combustion) was evaluated during the NOx-assisted soot combustion tests. The SmMn2O5 catalyst prepared by combustion method possessed a distinct morphology with slabs separated by interconnected macropores, and exhibited the overall highest catalytic activity for soot combustion compared to the other synthesized solids. The corresponding T50, and CO2 selectivity were 368 °C and 99.6%, respectively. Also, O2-TPD and H2-TPR measurements demonstrated higher mobility and better reducibility of surface-adsorbed oxygen for this catalytic

system. The soot combustion was greatly accelerated by the NO2-assisted mechanism under a NO + O2 atmosphere and further enhanced by the interconnected macropores of the catalyst, facilitating an intimate contact between the soot and the catalyst [19].

Previously, it has been found that some silver species could not only increase the carbon gasification rate

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[20,21], but also show remarkable performances in NOx abatement [22,23]. Wang et al. [24], synthesized by solid state method a highly active catalytic system (La0.7Ag0.3MnO3) for the simultaneous removal of NOx/diesel soot. The results indicated that the partial substitution of La3+ by Ag+ at A-site of the perovskite enhanced the catalytic activity due to 1) the increase of oxygen vacancies concentration required for the adsorption and activation of NO or molecular oxygen and 2) the over-stoichiometry oxygen content that accelerates the mobility of oxygen throughout the catalyst. They have also pointed out that the existence of metallic silver (Ago) on the catalyst surface speeds up the combustion rate of soot particulates and also promotes the reduction rate of

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NO. The lower temperature of soot particulates combustion and the higher conversion of NO towards N 2 are

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obtained as a result of the synergetic effect of those factors. Taking this into account the benefits of microwave assisted methodologies it is possible to develop highly effective catalysts for the simultaneous NOx and soot

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removal by improving the exposure of the required active sites (oxygen vacancies and Ag o). The purpose of this

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study is to investigate the catalytic performance of the (La0.7Ag0.3MnO3) perovskite-like catalysts synthesized by three different methodologies: microwave assisted (MW), hydrothermal microwave-assisted (HMW), and solid

2. Experimental

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state (SS), on the simultaneous removal of NOx, and diesel soot particulates under loose contact condition.

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2.1. Perovskite-like catalyst preparation Through literature we have found the La0.7Ag0.3MnO3 perovskite-like catalyst synthesized by the solid state (SS) methodology showed an excellent performance for the simultaneous removal of soot and NOx [24]. So, this

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catalyst and its preparation methodology were taken as starting point to evaluate the effect of other two methodologies: microwave-assisted (MW) and microwave-assisted hydrothermal (HMW). For the catalytic

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system preparation as described below, the reagent-grade metal nitrates: La(NO3)3 6H2O (Merck), AgNO3 (Merck) and Mn(NO3)2 4H2O (Merck) were mixed to prepared 5 g of perovskite keeping constant a molar ratio of La: Ag: Mn = 0.7: 0.3: 1, in all cases.

2.1.1. Solid state synthesis (SS) The precursor powders were weighted and mixed in the corresponding stoichiometric ratio and grinded continuously for 20 min in a mortar. Taking into account the theoretical weight, 20% in excess of solid NaOH

was required to complete the reaction (the mixture turns black). Then, the synthesized solid was washed with deionized water to remove the excess of base and filtered. The solid obtained was dried at 100 °C for 24 h, followed by a calcination step in air at 600 °C for 10 h using a heating rate of 5 °C/min [24].

2.1.2. Microwave-assisted synthesis (MW)

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To synthesize the MW solid we followed the procedure proposed in reference [25]. This procedure was chosen because it is simple, fast and energy efficient to produce perovskite with better crystallization and less impurities. For this synthesis, the required amount of metallic precursors was initially dissolved in 20 mL of deionized water, under ultrasound irradiation for 30 min. Then, a stirring process was performed during 6 h at 40 °C to improve crystallization. The resulting solution was subjected to microwave irradiation in a simple domestic oven (AS-HM-1.1 ME GRILL INOX) operating at 2.45 GHz with an output power of 700 W for 6 min. In this methodology, the solution´s solvent was evaporated during the microwave treatment, so, the washed

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and filtered steps for the obtained solid were no needed. Afterward, the solid sample was calcinated in air at 600

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°C for 1 h at a heating rate of 5 °C/min for cleaning purposes only since the perovskite phase was already formed. Although the procedure described in [25] implies that the solid calcination step can be achieved in-situ

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due to the high temperature reached in microwave, a further calcination step of the already formed perovskite

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for longer annealing time (>1h) must be avoided due to loss of the catalytic activity (see Fig. S1).

2.1.3. Microwave-assisted hydrothermal synthesis (HMW)

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For this synthesis, the pre-treatment steps involving the metallic precursor’s dissolution and the stirring process of the solution were similar to the one used for the microwave-assisted method. Afterward, certain amount of

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1M KOH solution (mineralizer) was added to the previous mixture to adjust its pH at 10. The mixture was transferred into sealed Teflon autoclaves and then placed in an advanced Lab-station microwave (Pyro Touch system- Milestones, Italy) which is operated at 2.45 GHz with a power supply of 1000 W. The reaction mixture

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was heated up to 200 °C for 30 min keeping the pressure inside the vessels at 15 bars. The resulting mixture was cooling down to room temperature and the products were centrifuged and washed with deionized water, and then dried at 100 °C for 15 h in a conventional oven. Finally, the solid sample was calcined at 600 °C for 10 h at a heating rate of 5 °C/min.

2.2. Catalyst characterization

The structural and textural characterization of the prepared catalysts was carried out by X-ray diffraction and low temperature N2-sorption method (BET). Phase and crystalline structure identification were carried out by Xray Panalytical X’PERT PRO MPD diffractometer with Cu Kα radiation (λ = 1.5406 Å), operated at 45 kV and 40 mA. The data were obtained in a continuous scan mode from 10° to 70° (2θ) with a scan step size of 0.013° and step time of 59.31 s. The diffractograms obtained were compared with those found in the JCPDS database

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for phase identification. The BET surface area (SBET) was measured by N2 physisorption at -196 °C on a Micromeritics ASAP 2010 unit, after sample degasification at 280 °C for 6 h. Morphology of samples were observed by scanning electron microscopy using a JEOL 7100F Field Emission Gun-Scanning Electronic Microscope (FEG-SEM) at 15 kV. As sample preparation, the powder were mounted on a copper stub using a double stick carbon tape and then coated with an ultra-thin film of gold (Au) in a sputter coating process. Particle diameter was determined by transmission electron microscopy using a TEM – Tecnai F20 Super Twin

sonication and deposited on a TEM lacey carbon grid.

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TMP with a resolution of 0.1 nm, at 200 kV as accelerating voltage. Powder was suspended in ethanol by

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The chemical characterization of the solids in terms of reducibility capacity, surface oxygen and active species

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was carried out. The reducibility of the samples was investigated by temperature programmed reduction (H2TPR) using a Micromeritics Autochem II equipment. A U-quartz tube reactor was loaded with 80 mg of sample

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and pretreated with 30 mL/min of 10% O2/He mixture at a heating rate of 10 °C/min up to 600 °C for 1 h. After cooling down to room temperature, a purge procedure under He atmosphere at a flow rate of 30 mL/min during

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1 h was done. Then, samples were heated again up to 600 °C at a heating rate of 10 °C/min under a 10% H 2/He atmosphere flowing at 50 mL/min. Quantitative hydrogen consumption were obtained by using TPR calibration

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running with a standard silver oxide sample (Micromeritics) in place of the catalysts and water generated during

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sample reduction was removed using a cold trap before reaching the TCD detector. The α-oxygen species concentration on catalysts was obtained by oxygen temperature programmed desorption

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(O2-TPD) using the same Micromeritics Autochem II equipment. The sample (80 mg) was introduced into the U quartz tube and was pretreated and purged with the same mixture gases, flow rates, heating rates and temperatures used for H2-TPR analysis. Then, the oxygen species desorption was carried out under He

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atmosphere by heating the samples to 600 °C at a heating rate of 10 °C/min. For oxygen desorption quantification, a calibration curve was performed by establishing the thermal conductivity detector (TCD) signal response when different oxygen concentrations in the gas mixture (oxygen-He) were used.

X-ray photoelectron spectroscopy (XPS) analysis was conducted to establish the near-surface composition of the materials before and after reaction. The equipment was a Thermo Fisher Scientific with an Al Kα X-ray

source at an operating voltage of 20 kV and a current of 20 mA. All the binding energies were calibrated with the C1s binding energy fixed at 284.6 eV as an internal reference.

2.3. Catalytic activity experiments 2.3.1. Simultaneous soot and NOx removal test:

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The catalytic performance of the prepared catalysts for both soot oxidation and NO reduction were evaluated in a fixed bed tubular quartz micro reactor. The reactor was mounted vertically in a split open furnace. A sootcatalyst-sand mixture (catalytic bed diameter 10 mm and height 20 mm) was placed on a thin layer of quartz fiber as support. A K-type thermocouple probe, with 1/16” diameter, was inserted into the quartz tube axially from the top all the way down to the bed surface for temperature measurement and control during reactions. Another K-type thermocouple was placed inside the furnace to control its temperature; the difference between

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the catalytic bed and the furnace temperature was around 50 °C.

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Previous to the catalytic experiments, 5 mg of carbonaceous material and 45 mg catalyst (1:9 weight ratio) were mixed with spatula (loose contact) during 5 min. Then, the resulting soot-catalyst mixture was mixed with 150

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mg of sand (SiO2) for another 5 minutes. The soot model used in this study consisted of a carbon black, FW200,

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from Orion Engineering. The soot-catalyst-sand mixture was exposed at a constant heating rate of 5 °C/min up to 600 °C, under a 2000 ppm NO/ 10% vol. O2/ He atmosphere at a flow rate of 100 mL/min. The evolution of CO2, CO, NO, NO2, N2, and O2 gases with temperature were followed with a mass spectrometer Pfeiffer 300c,

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using the signal intensities of the corresponding m/z = 12, 14, 16, 28, 30, 44, and 46. Also, a KIGAZ 310 gas

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analyzer was used as complement to measure the NOx and CO concentration, since sometimes they are difficult to follow by mass spectrometry due to signals overlapping. On the other hand, soot conversion and selectivity

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were defined according to Eq. (1) and Eq. (2), respectively. 𝑠𝑜𝑜𝑡 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 (%) = 𝐴𝐶𝑂2

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𝑆𝐶𝑂2 =

𝐴𝑇𝑂𝑇

𝐴𝑇

𝐴𝑇𝑂𝑇

× 100

(1)

(2)

Where; 𝐴𝑇 corresponding to the integrated area of CO2 and CO at a given temperature, 𝐴𝑇𝑂𝑇 is the integrated

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total CO2 and CO areas obtained during the whole experiment and ACO2, corresponds to the total area of CO2. 𝑖𝑛 𝑜𝑢𝑡 The NOx conversion efficiency was established according to Eq. (3), where 𝐶𝑁𝑂 and 𝐶𝑁𝑂 are the concentration

of NO measured at the inlet and the outlet of combustion reactor, respectively [11,26]. 𝜂𝑁𝑂 (%) =

𝑖𝑛 𝑜𝑢𝑡 𝐶𝑁𝑂 −𝐶𝑁𝑂 𝑖𝑛 𝐶𝑁𝑂

× 100

(3)

For a better understanding of the soot and NOx elimination process, two blank tests were carried out. 1) The soot combustion test with O2 (or NOx-free test) was done keeping the same experimental conditions and sootcatalysts and ratio used to prepare the reaction bed for the simultaneous removal tests described above. The soot-free combustion tests used to evaluate the oxidation capacity of NO to NO2 were performed over a catalytic bed composed of 45 mg of catalyst and 155 mg of sand under the same reaction conditions settled in section

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2.3.1. The outlet gas composition was monitored using mass spectroscopy and a KIGAZ 310 analyzer to measure NO and NO2. Also, gas chromatography using a 490-MicroGC equipped with a CP-Molsieve 5A column was used to corroborate the production of N2 (See Fig. S2), no additional nitrogen compounds were detected. NO to NO2 conversion was determined according to Eq. (3). Also, a stability test for some of the catalyst was carried out using a 5 cycle program (the results of these tests can be found in the supporting

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material).

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3. Results and discussion

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3.1. Structural and textural properties (XRD, surface area, morphology and particle size distribution) Fig. 1 shows the X-ray diffraction patterns of the prepared materials. All catalysts presented the diffraction

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peaks corresponding to the perovskite-like structure (LaMnO3, ICDD 32-0484) indicating that that the addition of silver did not modify the parent structure due to the similarity of the ionic radius between La3+(1.36Å) and

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Ag+(1.28Å). However, phase segregation was observed for HMW and SS materials; in the form of lanthanum

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oxide (La2O3, ICDD 2-0688), at 29° 2θ, and manganese oxide (MnO2, ICDD 4-0378) with some weak signals at 37° and 39° 2θ. Although it is also expected the formation of silver oxide as segregated phase, it is very difficult to ascertain its presence on manganites through XRD analysis since the diffraction peaks of Ag2O (ICDD 12-

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0793) overlap those of the perovskite and are very close to main diffraction peaks of the metallic silver (Ag0, ICDD 1-1167) [27]. Additionally, the 2θ signals seen in all synthesized solids at 38.3°, 44.5°and 64.6°

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corresponding to (111), (200) and (220) planes of fcc metallic silver indicate that not all silver was incorporated into the perovskite-like structure. The XRD pattern of MW sample did not show the presence of La2O3 and MnO2 oxides as segregated phases in the bulk, as the ones observed for HMW and SS samples. Thus, up to this

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point, we can only say that there is phase segregation with a certain level of collapse of the perovskite phase when the synthesis by HMW and SS were carried out.

Table 1 shows the BET surface areas results obtained for the three synthesized catalyst. As expected, the specific surface areas for this kind of materials, mixed oxides with negligible porosity, were relatively low [28–

30]. However, depending on the synthesis method some differences in the magnitude of the specific surface area among solids can be seen. The increase in the specific surface area observed for the prepared samples, SS (19 m2/g) < HMW (29 m2/g) < MW (34 m2/g), could be consequence of the time scale required for the synthesis. For instance, the solid prepared by microwave (MW) required only a few minutes to form the perovskite-like phase, while the solids synthesized by solid-state (SS) and microwave-assisted hydrothermal (HMW) methods

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took some hours to obtained the desired phase, leaving enough time for particle crystallization and growth. Also, it is important to highlight that although the microwave synthesis method allows obtaining the perovskite phase directly without performing the further calcination step, it is recommended to carry out a thermal cleaning procedure at 600 °C for 1 hour to remove some gaseous species resulting from precursor’s decomposition that can be re-absorbed on the surface of the catalyst. In this perspective, it is possible to think that the calcination time may also have some influence on the specific surface area development when solids are compared.

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Fig. 2 shows some representative SEM and TEM images of the solid samples synthesized by HMW, MW and

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SS methods with their corresponding particle diameter histograms. It is possible to see that the synthesis method has a clear effect on catalysts morphology and particle diameter distribution. While the HMW and SS solids

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clearly presented aggregates composed of primary particles with sphere-like shape, the MW solid showed

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aggregates with soft bumps on the surface making difficult to distinguish the limits of the primary particles. The average particle diameters, randomly estimated over 200 nanoparticles from several TEM images, were 29 nm ± 11 nm, 26 nm ± 9.6 nm and 13 nm ± 6.4 nm, for SS, HMW and MW samples, respectively, (Table 1). The MW

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sample has a narrower particle size distribution and an average particle diameter that is about half of the

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diameter found for the other two synthesized samples. In accordance with the specific surface area results, the catalyst with the lowest average diameter, (MW), was the one with the highest surface area. Here again, the irregular morphology and the lower particle size distribution observed for the MW solid could be explained by

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the differences on crystallization time during synthesis and calcination compared with the other two solids.

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Figure 2D evidenced the presence of surface spherical nanoparticles in the perovskites that according to EDX analysis correspond to metallic silver (See also Fig. S3). The estimated Ag nanoparticle size indicated that HMW sample has the widest particle distribution between 2 and 46 nm, centered at 15 nm; SS sample has an

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intermediate particle distribution between 2 and 25 nm, centered at 12 nm; and MW sample has the narrowest distribution between 2 and 10 nm, centering at 6 nm. The higher dispersion of Ag nanoparticles on the surface of MW could also explain by its higher surface area. Similar results have been reported by Hernandez et al. [27], who synthesized manganite and ferrite perovskites with different silver loadings.

3.2. Catalyst reducibility (H2-TPR) and surface oxygen species concentration (O2-TPD) Fig. 3 (A) illustrates the H2-TPR profiles for the perovskite-like catalyst (La0.7Ag0.3MnO3) prepared by different methodologies. For all cases, the main H2 consumption peak was obtained in the temperature window between 50 °C - 335 °C. However, the differences in the maximum hydrogen uptake temperatures observed in the raw

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TPR profiles depend on the way the catalyst was prepared that in this study followed this order: 165 °C, 207 °C and 216 °C for MW, SS and HMW, respectively. Also, what it is interesting to see is that the peak of the MW catalyst widely enlarge with temperature compared to the other two catalyst implying the contribution of different species. To elucidate the possible multicomponent contribution in the H2-TPR profiles we performed a deconvolution procedure using two gaussian cuves. The first curve in all profiles peaking below 150 °C has been mainly attributed to the reduction of segragated Ag2O to metallic silver (Ago) promoting the perovskite reduction due to spillover effect [27,31]. However, although some other authors have said that this signal can

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also be attributed to the reduction process of the silver-incorporated perovskite (Ag+) toward Ag0 there is not

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much evidence to support it [24,32,33]. The second cuve or main event, centered around 205 °C, has been attributed to the reduction of Mn4+ to Mn3+. Some studies have indicated that the manganese reduction process

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in the undoped LaMnO3 occurs above 400 °C [24,33]. But, when the partial substitution of La3+ by Ag+ were

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performed for the perovskite-like material prepared in this study, a decrease of the onset temperature of the first reduction peak and an enlargement in the Mn ions reducibility range were observed. According to Buchneva et

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al. [33], the doping with ions of the copper group (Cu,Ag,Au) leads to a weaking of the metal-oxygen bonds in the perovskite-like structure facilitating the manganese reduction and the pulling out of oxygen from the crystal

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framework. In fact, the migration of lattice oxygen to extraframework metallic silver becomes easier than that of manganese sugesting that samples synthsized by HMW and MW will show better catalytic performance than the

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one synthsized by SS. Of course, this migration mechanism is possible only after the insertion of silver ions into the perovskite-like structure [33].

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Table 1 summarizes the results associated to the amount of hydrogen uptaken for the three synthesized solids during Ag+ to Ag0 and Mn4+ to Mn3+ reduction stages. In the first reduction stage, the solids synthesized by HMW and MW showed the largest H2 uptake indicating more silver incorporation into the perovskite-like

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structure. While, in the second reduction stage, the H2 uptake for the sample synthesized by SS was the largest (650 μmol/gcat), indicanting a greatest concentration of Mn4+ species in this catalyst. This fact agress with the lowest silver incorporation into the perovskite-like structure for SS solid, because it leads A-site cation vacancies that could be compensated through an oxidation step from Mn3+ to Mn4+ to maintain structure electroneutrality, as other authors have indicated [14,32]. Additionally, when the H2 consumptions ratios representing the silver incorporation and manganese reducibility were calculated, the following descending

order in the synthesized solids was observed: MW (26 %) > HMW (19%)> SS (3%). This order follows a similar tendency when it is compared to the Ag+perov/Agtot atomic ratio obtained by the XPS (shown later).

Fig. 3 (B), shows the oxygen species desorption during O2-TPD experiments, the literature indicates that oxygen

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species can be classifiqued according to their desorption temperature, as follows: The ones desorbing below 550 °C are attributed to physical and chemical weakly adsorbed oxygen species on surface catalyst (O− or O− 2 ), named as α-oxygen. These oxygen species are of particular interest in catalysis area, because the increase in this kind of oxygen species is usually related with an improvement in the catalytic activity of a material, furthermore they are an indicative of surface oxygen vacancies [4,13,34,35]. There are other kind of oxygen species desorbing at temperatures higher than 550 °C (no shown in figure), known as β-oxygen species, corresponding

U

to the oxygens coming from the bulk of the perovskite (O2− ) [4,13,34].

N

As the oxygen species of interest for this study are those that are available at low temperature, the O2-TPD experiments were done up to 600 °C to avoid exceeding samples calcination temperature. Fig. 3 (B) presents the

A

oxygen desorption profiles for all synthetized samples. It has been found that of the oxygen desorption

M

temperature peak sligtly decreases with the increase in the amount of silver incorporated into perovskite structure beeing 110 °C, 122 °C and 128 °C for MW, HMW and SS samples, respectively. Considering what we have stated above, the incorporated silver facilites oxygen mobility throught the bulk oxydizing metallic silver.

D

Thus, HMW and MW samples meet all this criteria since they were the ones with the highest silver

TE

incorporation, the lowest oxygen desorption temperature and the highest α-oxygen desorption.

EP

3.3. Surface elemental composition and oxidation states (XPS) X-ray photoelectron spectroscopy (XPS) analysis provides surface elemental composition and oxidation states of each element. Binding energies of La 3d5/2, Ag 3d5/2 ,Mn 2p3/2 and O 1s core levels were recorded by XPS for

CC

the as prepared La0.7Ag0.3MnO3 perovskite-like samples prepared by the methodologies HMW, MW and SS (Fig. 4 and Fig. S4). The binding energies obtained for La 3d5/2 (833.3-834 eV) and (837.1-837.7 eV) core levels

A

are characteristics of La3+ in perovskite-like structures [13,36] and segregated lantanum as an oxide (La2O3) respectively [37] (see Fig. S4 in the supporting information).

Fig. 4 (A) presents the O1s spectra of all catalysts. The O1s spectra were deconvoluted into three peaks, the lower binding energy peak located in a range of 528.6 eV and 529.3 eV corresponds to lattice oxygen or Oβ; intermediate binding energy peak located around 531 eV corresponding to adsorbed oxygen (Oα) attributed to

weakly bonded O− or O− 2 (associated to surface defects, i.e. surface vacancies); and the peak at the highest binding energy around 533.0 eV, it is due to water and hydroxyl groups on the surface [4,7]. The Ag3d5/2 core level spectra obtained for the syntesized samples are presented in Fig. 4 (B). The spectra have three contributions: Ag0 in metallic form at 368.2 eV; Ag+ of Ag2O segregated on surface in the binding energy range of 367.7 eV and 367.9 eV; and Ag+ of silver incorporated into perovskite lattice at 367.3 eV with their

SC RI PT

corresponding shake up satellites [37]. The Mn2p3/2 spectra present three signals, in Fig. 4 (C), corresponding to Mn3+ (641.3 eV) and Mn4+ (642.3 eV) and a weak satellite (644.0 eV) of the Mn2p3/2. These manganese signals assignments are in agreement with previous works [4,14,38].

Table 2 shows the atomic surface ratios of different species from the three synthesized samples. The La/Mn atomic ratio evidences the La segregation on catalyst surfaces, with a higher La/Mn ratio than the stoichometric one, presented in braquets. According to La/Mn atomic ratios, the sample with most surface La-enrichment was

U

MW followed by HMW and SS samples. It is important to note that La segregation (as La2O3) over surface of

N

the HMW and SS samples was in agreement with XRD results. However, some discrepancies between XRD and XPS results were found for the MW sample since the XRD analysis indicated that this sample did not show

A

La2O3 segregation while the XPS analysis indicated the opposite. The difference in the response between the

M

two analysis could be associated to the difference in the sampling depth of the two analytical techniques since XRD is a bulk analysis, while XPS is surface analysis [37]. This means that La2O3 was segregate into the bulk

D

of SS and HMW samples while in MW sample, the La2O3 was segregate toward the surface.

TE

Similarly, the three synthesized samples presented an Ag/Mn atomic ratio higher than the stoichometric one, indicanting that part of the silver was segregated on catalyst surface. In fact, the segregated silver on catalyst surface was mainly discriminated as silver oxide (Ag2O) and metallic silver (Ag0). In particular, it was seen that

EP

the SS sample showed the highest silver segregation among the synthesized samples with a 52% as Ag2O and 6% as Ag0, which means low silver incorporation in the perovskite-like structure (~42%). However, for MW

CC

and HMW samples the silver content incorporation was 81% and 78% respectively, this result is in accordance with H2-TPR results that also suggested a higher silver incorporation for these two solids (see Table 1). Regarding the segregated silver content for the samples; it is interesting to know that even though the MW and

A

HMW samples had a similar silver oxide content (Ag2O), the MW sample had the double of Ag0 content on the catalyst surface compared with HMW. A possible explanation for the slightly increase of Ag0 could be due to the short calcination time used for the MW sample preparation.

In the light of these results, it seems that synthesis method have a great impact in the final characteristics of the solids. Particularly, the application of microwave irradiation to the previously treated precursor solution (30 min

of sonication and 6 h of stirring) improved the dispersion of the metal cations and the easy incorporation of the silver in the perovskite structure compared to the solid state method. On the other hand, the high metallic silver content observerd on MW solid sample could be associated to the short calcination time used to clean the surface, 1 h compared to the 10 h requiered for the perovskite phase formation in HMW and SS synthesis which

SC RI PT

means less time for segregated silver to be oxidized.

It has been widely reported that when the A ion of a perovskite-like structure is partially substituted with an ion of different oxidation state, as ocurred with the La0.7Ag0.3MnO3 catalyst prepared here where La3+ was partially substituted by Ag+, a charge compensation is requiered to achieve electroneutrality [9,13]. The compensation mechanism can be either by oxygen vacancies formation, evidenced with the presence of adsorbed oxygen species (Oα), or by the shifting of B cation toward higher valences (e.g., Mn3+ to Mn4+) [9,13,35]. In this study, it was observed that the increase of silver incorporation into perovskite-like lattice (Agperov/Agtot) followed a

U

linear trend with respect to the Oα/Otot atomic ratio, as can be seen in Fig. 5, indicating that the MW and HMW

N

solids are the ones with the higher quantity of oxygen vacancies. Albaladejo-Fuentes et al.2016 have found

A

similar results when they looked up the effect of copper content into the BaTi1-xCuxO3 perovskite-like material prepared by two different methodologies (sol-gel and hydrothermal) on the NO oxidation and NO storage

M

capacity tests. They stated that sol-gel methodology improved the incorporation of copper into the perovskitelike lattice causing a distortion of the tetragonal structure, as well as, an increase in oxygen vacancies, and more

D

active oxygen species on the catalyst surface avalable for NO oxidation, while the hydrothermal method favored

TE

the segregation of some species (CuO, BaCO3, TiO2 and Ba2TiO4) making it ideal for NO storage. [39].

On the other hand, the increase in quantity of manganese species with higher oxidation state (Mn3+  Mn4+) did

EP

not show any correlation neither with the oxygen surface species nor with the silver incorporation into the perovskite lattice since the Mn3+/Mntot and Mn4+/Mntot atomic ratios did not significantly change among the

CC

samples (see Table 2). So, considering the results presented above, the main charge compensation mechanism inferred for our samples was due to silver incorporation distorting the perovskite-like structure and creating oxygen vacancies which allows the formation of a high amount of oxygen surface groups instead of the shifting

A

of manganese cation toward higher valences.

3.4. Catalytic results for simultaneous removal of soot and NOx This section shows the results regarding the simultaneous removal of soot and NOx using the La0.7Ag0.3MnO3 perovskite-like catalyst synthesized by three different methodologies. The description of soot oxidation and NO

conversion was carried out in consequently way. Then, for a better understanding of the NOx influence on the catalytic oxidation of soot, some blank tests corresponding to NOx-free soot combustion and NOx-assisted soot combustion with and without catalyst were performed.

3.4.1 Simultaneous removal of soot and NOx

SC RI PT

Fig. 6 illustrates the species evolution profile (CO2, O2, NOx and N2) as function of temperature during the NOxassisted soot combustion over MW synthesized catalyst. From Fig. 6, it is possible to see that unlike the other two synthesized materials, the MW solid showed two evolution peaks for the CO2 signal, the one below 350 °C coincides with the maximum peak of NOx consumption and the maximum N2 evolution, while the peak at 400 °C coincides with the maximum consumption of O2 and the minimum of N2. This suggests that the first event is associated with the adsorption of NOx on silver nanoparticles, which becomes more evident as the percentage of metallic silver on the surface of the solid increases, as was seen in the MW solid. In this case, the NO-absorbed

U

species reacts with soot giving CO2 and N2 as products. On the other hand, the second event is mostly related to

N

the oxygen vacancies that also activate the NOx for soot oxidation through a mechanism that does not involve

A

the formation of N2 above 400 °C. For the other two solids (HMW and SS), containing less metallic silver content on their surface, the CO2 and N2 peaks coincide with the NOx and O2 depletion. It is worth to mention

M

that regardless of perovskite synthesis method, the main mechanism by which NOx elimination occurs were through the formation of N2. (See also the species profiles for HMW and SS solids in the supporting

TE

D

information, Fig. S5 and S6, respectively).

Fig. 7 shows CO2 evolution (A) and soot conversion (B) with temperature for the NOx-assisted catalytic soot oxidation experiments performed over the three synthesized solids. For comparison purposes, the non-catalytic

EP

test was introduced in Fig. 8 as reference. Soot ignition temperature (Tig), and the temperatures at which soot conversion was 50% and 90%, named as T50 and T90, respectively were evaluated. It is important to highlight

CC

that these parameters give us information about the catalytic performance since it has been stated that the lower these values, the higher the oxidation activity. For instance, Teraoka et al studied the catalytic performance of perovskite-like ABO3 and K2NiF4-like (A2BO4) oxides for the simultaneous removal of soot and NOx. They

A

found a promotion effect with the decreasing ignition temperature and the increasing selectivity with the appropriate substitution of lanthanum by potassium at A-sites; and that the lowering of the ignition temperature correlated to the higher oxidation activity [39].

From Fig. 7 (A) it can be seen that HMW and SS solids have a Tig of 180 °C and 210 °C respectively. Moreover, MW catalyst ignites soot easier with a Tig of 160 °C. Tig of the non-catalyzed system is 350 °C,

which means that MW and HMW catalysts were able to reduce in about a half soot ignition temperature. We also observed 100 % selective toward CO2 of all the tested catalyst, since no evidence of CO formation was seen; contrary to what was observed in the non-catalytic test where the selectivity to CO2 reached only 40%.

From Fig. 7 (B) it was found that the three synthesized solids had an important decrease in T50 compared with

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the non-catalytic test, having the following order: HMW (T50= 359 °C) < MW (T50= 371 °C) < SS (T50= 404 °C) < non-catalytic (T50= 578 °C). In fact, the solid with the best performance (HMW) showed an oxidation temperature shifting near to 220 °C compared to the non-catalytic one, while in the worst case (SS) a temperature shifting of 174 °C was found which is still considered promissory. By simply observation of Fig. 7 (B), it is possible to see that the catalytic tests have an increase in the slope of the curves in comparison with the non-catalytic one, giving an idea of a higher reaction velocity when the system have a catalyst. T90 have the same increasing order of T50: HMW (400 °C) < MW (430 °C) < SS (450 °C) confirming that HMW is the

U

material with the best catalytic activity for soot oxidation. Even though MW sample did not present the lowest

N

T50 and T90, we found an important catalytic activity of this material below 320 °C, which could be more interesting for a diesel exhaust treatment application. The samples with the best catalytic activity (HMW and

A

MW), were thoses with the highest α-oxygen content and with the highest Oα/Otot atomic ratio. Indicating a clear

M

relationship between the availability of surface oxygen species (Oα) and soot oxidation activity. According to the XPS results, the α-oxygen content was higher when the degree of silver incorporated into the perovskite-like

TE

obtained for the SS solid.

D

lattice increases. Indeed, the Agperov/Agtot atomic ratio for MW and HMW samples were almost twice to the one

The results of this study are in accordance with other authors that have studied Ag doped perovskite-like materials for catalytic oxidation. For instance, Kucharczyk et al.[37] reported a decrease of 30 °C in T50 for

EP

methane oxidation when the lanthanum position in LaCoO3 perovskite-like was replaced by 10% of silver (La0.9Ag0.1CoO3). Pecchi et al. [40] and Dinamarca et al. [13] reported something similar by increasing the silver

CC

content into the (La(1-x)Agx)Mn0.9Co0.1O3 perovskites-like catalyst used for soot oxidation. In fact, they observed that the perovskite-like material with higher silver incorporation allowed a reduction of 87 °C in the temperature of maximum combustion rate (Tm) compared with the undoped solid. So, it is evident from these results that

A

there is an improvement of soot oxidation activity with the increase of Ag inclusion into the perovskite-like structure and the increase of oxygen vacancies, as other authors have pointed out [34,35,39,41,42].

Fig. 8 shows the NOx profile during the catalytic soot combustion tests assisted by NOx. Here in, the dash dotted line represents the NOx concentration at the gas inlet. According to F. Bin et al. [43], the events above and

below the NOx-inlet concentration line imply the occurrence of NOx adsorption/desorption or reduction processes. For the HMW and SS solids at least two main desorption peaks for NOx species were observed at temperature below 200 °C. It has been recognized that this two-step desorption process is associated to different forms of nitrate/nitrite species including unidentate nitrates, bidentate nitrites, monodentate nitrates, chelating nitrates, and bridging nitrates, as well as, weakly adsorbed NO (nitrosyl) [43]. A slightly different profile was

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found for the MW catalyst at temperature below 200 °C where a desorption step was followed by an adsorption process. This last event can be attributed to thermal stable NO storage sites associated to nitrite and free ionic nitrate species having important adsorption behavior after 100 °C.

Consequently, as temperature increases above 200 °C, a sharp NOx adsorption/reduction peak was immediately promoted by the exothermic event associated to the soot oxidation process, which is followed by a clear

U

decrease of NOx concentration down to the dash dotted line. For example, the NOx reduction stage for MW catalyst began at 207 °C and achieved a maximum NOx conversion at about 328 °C. At this temperature, we

N

found an important peak at m/z = 28, suggesting N2 as the main product of NOx reduction process (see Fig. 6),

A

since no other reduction species, like N2O, was detected. In fact, both CO2 and N2 were formed at the same time

M

as the major products (see Fig. 6), which is wholly consistent with a simultaneous removal of NOx and soot. It can also be noticed that the lower the onset temperature for the NOx reduction step (MW, 207 °C < HMW, 237 °C < SS, 264 °C), the lower the temperature for the NOx reduction peak (MW, 328 °C < HMW, 375 °C < SS,

D

400 °C), and the higher the NOx conversion efficiency (MW 60 % > HMW 41 % > SS 38 %). The decrease in

TE

the maximum NOx reduction temperature and the increment in the NOx reduction efficiency could be related to synergetic effects between the sites that were active for the NOx adsorption process as will be discussed below. Regarding the active sites for the NOx adsorption/reduction stage, different authors report that NO could be

EP

adsorbed on metallic species segregated on catalyst surface [26] and/or oxygen vacancies [44–47]. These active sites adsorb and dissociate NO to form N2 in the presence of carbonaceous materials as it was previously

CC

described by Shangguan [47] (See Equations 4-8):

(4)

NOad + Cf  C*[ N,O]

(5)

C*[N,O] + NOad + 2Cf  CO2(g) + 2C*[N]

(6)

2C*[N]  N2(g) + 2Cf

(7)

C*[N,O] +NOad-  CO2 (g) + N2 (g)

(8)

A

NO(g)  NOad

First, NO from the gas phase adsorbs on the catalyst surface. Then, the NOad reacts with an active carbon site giving the formation of C*[N,O] complexes on soot particle surface. These complexes react with another NOad species adsorbed on catalyst releasing CO2 to the gas phase and increasing the accumulation of N-containing intermediate on soot surface which in a further reaction step can be release as molecular nitrogen (N2). An alternative for equation (6) and (7), the simultaneous dissociation toward CO2 and N2 can also take place

SC RI PT

(Equation 8). This mechanism is only accepted when soot oxidation reaction is taken place under NO atmosphere where it was found that the N2/CO2 evolution were temperature dependable [47] (Equations 9-11). NO(g) + ½ O2(g)  NO2(g)

(9)

NO2( g)  NOad + Oad

(10)

Cf + Oad  C*[O]

(11)

U

However, when soot oxidation reaction is taken place under NO/O2 atmosphere the mechanism for the

N

simultaneous removal is not straightforward. Some studies have reported that the oxidation of soot was enhanced by the coexistence of NO and O2 favoring the formation of NO2 in the gas phase (Equation 9) [7,47].

A

Then, this oxygenated specie can be dissociatively adsorbed on the catalyst surface to form adsorbed NOad and

M

Oad species (Equation 10). The reaction between the catalyst-adsorbed species and reactive Cf species give the C*[O] intermediate which is reactive toward the reduction of adsorbed or gaseous NO species. Taking into

D

account the catalysts characterization and discussion proposed above about the mechanism, it is necessary to evaluate the following aspects: 1) Is there some synergistic effect between the silver active sites and the oxygen

EP

mechanism?.

TE

vacancies in the simultaneous removal reaction?, and 2) what is the role of NO2 formation in the reaction

To aboard the first question, the three catalysts synthesized in this study presented at least two types of active sites including metallic silver (Ag0) and oxygen vacancies, both evidenced in XPS analysis. Ag2O does not seem

CC

to be an important active site for NO adsorption, because if so, it would be expected that the catalyst with the highest amount of segregated Ag2O (synthesized by SS) were the most active for the NO elimination, but it was

A

not.

Fig. 9 (A) illustrates the individual and combined contribution of the evaluated active sites (Ag0 and oxygen vacancies) in the conversion of NOx. It can be seen that although the individual contribution of both active sites responds positively to NOx conversion, the sites associated with superficial metallic silver presented a better response, particularly for the solid synthesized by MW. Since both active sites seem to have an influence on the NOx reduction process, we suggest quantifying the synergy between both kinds of sites. To accomplish this, we

used an expression similar to the one reported by H. Zhao et al. [10] introducing the parameter “Φ”, defined in Equation (12). Where the first term is the atomic fraction of metallic silver with respect to the non-incorporated silver into perovskite-like lattice, and the second term is the atomic fraction of α-oxygen on catalysts. Φ=[

𝐴𝑔0

+ 𝐴𝑔𝐴𝑔 +𝐴𝑔0 2𝑂

]×[

𝑂𝛼 𝑂𝛼 +𝑂𝛽 +𝑂𝑊

]

(12)

SC RI PT

This expression creates a new variable (Φ) that accounts the superficial amount of both active sites in the catalysts (oxygen vacancies and segregated metallic silver). Fig. 9 (A) suggests that the synergistic effect of both active sites represented by Φ, not only satisfactory responds to changes in NOx conversion, especially for MW sample, but also to changes in the NOx maximum conversion temperature with a linear correlation of 0.99 (see Fig. 9 (B)). From this result, it is important to highlight that the best compromise of segregated metallic silver and oxygen vacancies to NOx reduction efficiency follows this order MW> HMW> SS.

U

Regarding the NOx-assisted soot oxidation process, it has been widely reported that the NO can be turn during

N

the catalytic test into NO2 which is even much more active for soot oxidation than O2 [7,11,48]. Therefore, to asset the influence of NO on the catalytic performance, we carried out some blank experiments including the

A

NOx-free soot oxidation and NOx-assisted soot oxidation with and without catalyst. For the catalytic experiment,

M

the MW solid was taken as reference. Fig. 10 shows the soot conversion profile as function of temperature for the blank experiments. It is evident that when NO was present during the catalytic test, there was a decrease in the T50 value sugesting a soot oxidation enhancement. When soot oxidation was assisted with NO over MW

D

catalyst, a temperature reduction of 169 °C on the T50 value was observed in comparison with the NO-free soot

TE

oxidation test performed on the same solid. For the non-catalytic experiments (without catalyst), the effect of using NO for soot oxidation did not cause a significant change in the T50, only a small downshift in temperature

EP

(~20 °C) was observed in comparison with the non-catalytic and NOx-free test (or Soot-O2). Based on these results, it is evident that there is a significant enhancement in soot oxidation when both, NO and catalyst, are present in the system. This clearly suggests that there were an activation of NO on catalyts surface. The soot

CC

oxidation enhancement due to NO presence during the non-catalytic test, suggests that there are some sites on soot surface that are able to active NO, however the effect of this sites is not as significant as the active sites on

A

catalyst surface.

It is believed that the NO2 is the intermediate specie that can explain the rapid oxidation of soot during the catalytic test. However, during the catalytic soot combustion experiment assisted by NO was not possible to trace it, in part, because once the NO2 is formed from the NO oxidation, it is immediately consumed during the soot ocombustion process. To confirm this, NO catalytic oxidation tests, i.e. soot-free experiments, were performed to establish the ability of three synthesized samples for oxidizing NO to NO2 (see Fig. 11 A). HMW

and MW samples had the maximun NO convertion (~ 66%) at 315 °C and 323 °C, respectively, followed by the SS catalyst with a NO convertion peak around 54 % at 339 °C. It is important to note that the temperature range of NO2 evolution in the soot-free catalytic test matches the temperature range for soot oxidation and NO reduction toward N2 during the simultaneous removal experiments. In our study, the NO to NO2 conversion was mostly a catalytic-mediated process since the high availability of oxygen species on the surface (α-oxygen) of

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the perovskite favors the oxydation of NO. This result is in accordance with the one presented by J.A. Onrubia et al [4]. The authors studied NO oxidation ability of Sr-doped LaMnO3 and LaCoO3 pervoskites materials as alternative for diesel exhaust treatment. They found that Sr incorporation of into the La1-xSrxCoO3 perovskitelike structure promoted the oxygen vacancies formation and improved NO conversion compared to the nonsubstituted formulations, specially when its molar fraction corresponds to x = 0.3. Additionally, our results agrees with the finding of C.Cao et al. [11], that studied diesel soot oxidation over potassium-promoted Co3O4 nanowires monolithic catalysts. In that work, the solid with 5% of mass fraction of potassium (5KCo-NW), the

U

one with the highest α-oxygen amount, presented the highest NO to NO2 oxidation ability, and showed the best

N

soot oxidation performance.

A

Figure 11B shows a column chart illustrating the NO oxidation capacity according to the catalyst preparation

M

method. It can be observed that the solids synthesized by microwave-assisted methodologies (HMW and MW) presented better NO oxidation capacity than the one prepared by solid state, which is in accordance with the order of increase in the amount of O reported in Table 2. This suggests that this type of oxygen is an active site

D

for the adsorption of NO leading to the formation of a nitrite complex (O-NOad) which easily desorbs as NO2

TE

above 200 °C. It is also important to note that the solids with the highest content of oxygen or higher NO 2 were

EP

the ones that presented the lowest temperature to reach 50% of soot conversion.

At the light of these results and taken into account the mechanism propposed by Shangguan et al. [47] we

CC

suggest the following alternative mechanistic model to explained the simultaneous removal of NO and soot (Equations 13-16).

(13)

NO(g) + Oad  O-NOad

(14)

O-NOad  NO2(g)

(15)

NO2(g) + Cf  C*[NO2]

(16)

½ O2(g) + Cf  C*[O]

(17)

C*[O] + NO(g)  C*[NO2]

(18)

A

½ O2(g)  Oad

Catalyst surface

Soot surface

C*[NO2] + C*[NO2]  2CO2(g) + N2(g) O2(g) + 2NO(g) + 2Cf

 2CO2(g) +

(19) N2(g)

(20)

Net reaction

Although some of the reactions proposed can occur simultaneously on both surfaces (catalyst and soot), we decided to separate them to facilitate the discussion. First, there must exist a competition between molecular

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oxygen and gaseous NO for the active sites present on the surfaces of both catalyst and soot particles at low temperature. This would explain the formation of the oxygenated complexes (Oad, C*[O]) described in equations 13 and 17, and the formation of nitrite-like complexes (O-NOad, C*[NO2]) described in equations 14 and 18 which could be obtained through the reaction of gaseous NO over the previously formed oxygenated sites. Then, when the temperature is above 200 °C, the O-NOad decomposition over the catalyst becomes feasible providing an alternative and much faster NO2 production pathway [49]. Some DRIFT studies have evidenced that this complex is thermically unstable and rapidly desorbs to the gaseous phase as NO2 as we could see when the soot–

U

free experiment (Cat-NO/O2), was carried out (Figure 11 A) [49,50]. Next, in the catalyst-soot interface, the

N

NO2 released from catalyst speeds up the nitrite complex (C*[NO2]) formation on the soot surface [Equation 16]. In fact, it has been proved that this complex is formed on the surface of soot particles when they are

A

exposed to NO2 and/or NO/O2 environments [51–53]. Finally, when two C*[NO2] complexes are close enough,

M

they can experience a molecular rearrangement to produce CO2 and N2. This molecular rearrangement is not so simple to follow it experimentally, so a computational chemistry study that at the moment is out of the scope of

D

this paper is required to obtain more information about this process.

TE

On the other hand, taking into account our experimental results and the mechanism proposed here, we can derive a rate law equation that fits the experimental results obtained by Shangguan et al. [47] who have

EP

evaluated a similar catalytic system. In this case, If we assume that the release of NO2 from the catalytic surface (equation 15) is the rate-determining step, the rate law equation obtained is: dPCO2/dt = K(PNO)(PO2)1/2; where; K = k13 k14 k15. Note that this expression agrees pretty well with the one obtained experimentally dPCO2/dt =

CC

K(PNO)(PO2)0.4. 4. Discussion

A

Fig. 12 summarizes some of the main parameters that should be taken into account to asset the performance of the three-prepared catalysts on the simultaneous soot and NOx removal. it can be seen that even though HMW catalyst presented the best soot oxidation catalytic activity, i.e. the lowest T50, the MW catalyst showed the best performance for the simultaneous removal showing the lowest soot ignition temperature (160 °C), the lowest temperature for the maximum NOx reduction (328 °C), the highest NOx reduction efficiency (60 %); and an intermediate T50 (371 °C) for soot oxidation. It is important to note, that the catalytic activity improvement of

the perovskite-like material prepared by MW correlates with the characterization results associated to highest silver incorporation into the perovskite lattice and the highest Oα/Otot atomic ratio implying the presence of oxygen vacancies, so there must exist a synergistic effect between these variables that along with the small particle size diameter (13 nm), giving an slightly increase in the surface area (34 m2/g) compared with the other

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solids, would explain its effectiveness in the simultaneous removal of soot and NOx.

Also, it is worth to mention that the stability of the catalysts improved with reaction cycles (See Fig. S7). Particularly, the solids prepared by microwaves (MW and HMW) improved the catalytic activity towards soot oxidation after the second cycle reaching a T50 value of around 330 °C in the fifth cycle. On the other hand, although the MW solid showed a slight deactivation in the NOx removal after the second reaction cycle and HMW improved after the fourth cycle, the NOx reduction efficiency in MW was still higher. This behavior

U

observed in the reaction cycles is due to the fact that part of the segregated metallic silver on the solid surface,

M

A

vacancies formation that favors soot oxidation.

N

which is necessary for NO activation, migrates towards the structure of the perovskite, increasing the oxygen

The dynamic process of the microwave-assisted synthesis, in terms of sonication, long stirring time, and short

D

microwave irradiation time lead a better dispersion of silver metal cations, resulting in a good Ag+ incorporation into the perovskite-like lattice thus facilitating the formation of oxygen vacancies as active sites. In fact, the

TE

increment in the oxygen vacancies correlated with the increment of surface oxygen content (α-oxygen) that along with the segregated metallic silver amount allowed inferring that both active sites are needed for the

EP

simultaneous NO and soot removal. Although the amount of silver incorporated into perovskite lattice and the quantity of oxygen vacancies in the MW and HMW samples were basically the same, MW sample had the double of metallic silver on its surface probably due to the lower calcination time leading less time to silver

CC

segregated to be oxidized toward Ag2O.

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5. Conclusions The La0.7Ag0.3MnO3 perovskite-like catalyst was prepared by three different routes, microwaved-assisted (MW) microwaved-assisted hydrothermal (HMW) and solid state (SS) with the aim of studying catalyst synthesis method effect on the simultaneous removal of soot and NOx under loose contact condition. Among the synthesized solids, the MW catalyst showed the best catalytic performance for the simultaneous removal since it was the one with the lowest soot ignition temperature (160 °C), the lowest temperature for the maximum NO x

reduction (328 °C) and the highest NOx reduction efficiency (60 %) toward N2 and an intermediate T50 (371 °C) for soot oxidation. Results suggest that the dynamic process of the microwave-assisted synthesis, in terms of sonication, long stirring time, and short microvawe irradiation time lead a better dispersion of silver metal cations, resulting in a good Ag+ incorporation into the perovskite lattice thus facilitating the formation of oxygen vacancies as active sites that along with the increase of metallic silver segregation on catalytic surface can help

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to explain the simultaneous removal of soot and NOx.

Declarations of interest: none

Acknowledgements

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The authors want to thank the University of Antioquia for the financial support received through the CODI

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project No 2015-7828. Special thanks are given to Professor Robinson Buitrago at the Advanced Materials and Energy Group of Instituto Tecnológico Metropolitano for his support in the SEM images acquisition. We also

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thank the 2016 China-LAC Young Scientist Exchange Program. L.U. thanks the Colombian Administrative

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Fig. 1. X-ray diffractogram of La0.7Ag0.3MnO3 synthesized materials by MW HMW and SS methodologies.

× 30,000

100 nm

× 30,000

100 nm

× 30,000

100 nm

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Fig. 2. (A-C) SEM and TEM images of the three perovskite catalysts with their corresponding particle diameter distributions and (D) segregation of silver particles on catalyst surface.

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Fig. 3. H2-TPR (A) and O2-TPD (B) profiles of La0.7Ag0.3MnO3 prepared samples.

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Fig. 4. Decomposed XPS spectra in the (A) O1s core level (B) Ag3d5/2 core level and (C) Mn2p3/2 core level of

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the prepared catalysts.

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Fig. 5. Correlation of silver incorporation into perovskite-like lattice with surface oxygen vacancies

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Fig. 6. CO2, N2, NO and O2 species during simultaneous removal of soot and NOx over the MW catalyst

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Fig. 7. Method of synthesis effect of the perovskite-like material La0.7Ag0.3MnO3 on soot oxidation under 2000

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ppm NOx/ 10% vol O2/ He atmosphere. (A) CO2 evolution during NO-assisted catalytic experiments. (B) Soot

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conversion to CO2.

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Fig. 8. NOx concentration evolution with temperature during catalytic tests

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Fig. 9. (A) Individual and combined (Φ) contribution of the different active sites toward NOx conversion

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efficiency, (B) relationship between Φ and the temperature of maximum NOx conversion. The original Φ values

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were multiplied by 10 to adjust graphic scale.

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Fig. 10. NO effect on soot oxidation process during the catalytic (MW) and non-catalytic tests

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Fig. 11. (A) NO to NO2 oxidation capacity of the three synthesized catalyst and (B) relationship between NO2

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oxidation ability and soot oxidation activity (T50).

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Fig. 12. Catalytic parameters of the simultaneous soot and NOx removal for HMW, MW, and SS prepared

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samples.

Table 1. Textural characterization, redox properties and α-oxygen species concentration of La0.7Ag0.3MnO3 synthesized for the different methodologies.

HMW MW SS

SSAa 29 34 19

PDp (nm)b

PDAg0 (nm)b

H2 Ag+ to Ag0 (µmol/gcat)

Tpeak(°C) Ag+/Ag0

26 13 29

15 6 12

93 81 21

150 121 114

H2 Mn3+ to Mn4+

(µmol/gcat) 495 307 650

Tpeak (°C) Mn4+/Mn3+

H2 ratioc

α-O2 (µmol/gcat)

Tpeak(°C) O2 –TPD

216 192 207

0.19 0.26 0.03

702 412 345

122 110 130

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Specific surface area obtained from N2 physisorption using the BET method (m2/g)

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PD = particle diameter of perovskite and Ag0 estimated by random measurement of about 200 nanoparticles from TEM images

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H2 uptake ratio between (Ag+ to Ag0) and (Mn4+ to Mn3+)

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Table 2. Surface atomic ratios obtained from the XPS spectra of La0.7Ag0.3MnO3 perovskite-like samples prepared by different methodologies.

MW HMW SS

Ag/Mn [0.3] 0.56 0.46 0.62

Agperov/ Agtot 0.81 0.78 0.42

Ag2O/ Agtot 0.13 0.19 0.52

Ag0/ Agtot 0.06 0.03 0.06

Ag0/ Agsa 0.30 0.14 0.10

Ags is the total silver non-incorporated into perovskite-like lattice, i.e. Ag2O+Ag0.

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La/Mn [0.7] 0.98 0.90 0.65

Mn3+/ Mntot 0.65 0.62 0.57

Mn4+/ Mntot 0.28 0.27 0.31

Oβ/Otot Oα/Otot 0.65 0.69 0.77

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Sample

0.30 0.27 0.18