Thermodynamic Stability, Redox Properties, and Reactivity of Mn3O4

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Thermodynamic Stability, Redox Properties and Reactivity of MnO, FeO, and CoO Model Catalysts for NO Decomposition – Resolving the Origins of Steady Turnover 3

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Jan Kaczmarczyk, Filip Zasada, Janusz Janas, Paulina Indyka, Witold Piskorz, Andrzej Kotarba, and Zbigniew Sojka ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02642 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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ACS Catalysis

Thermodynamic Stability, Redox Properties and Reactivity of Mn3O4, Fe3O4, and Co3O4 Model Catalysts for N2O Decomposition – Resolving the Origins of Steady Turnover

Jan Kaczmarczyk, Filip Zasada*, Janusz Janas, Paulina Indyka, Witold Piskorz, Andrzej Kotarba and Zbigniew Sojka*

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Corresponding Author *[email protected], telephone number: +48 12 663 20 73 *[email protected], telephone number: +48 12 663 22 95

Keywords: spinel, deN2O, TPR, TPO, Ellingham diagrams, catalyst stability, reaction mechanism, reduction, oxidation

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ABSTRACT Manganese, iron and cobalt model spinel catalysts were systematically investigated for understanding the roots of their diverge performance in N2O decomposition. The catalysts were characterized by XRD, RS, N2-BET, SEM and STEM/EELS techniques before and after the reaction. Their redox properties and the thermodynamic stability range were thoroughly examined by survey and narrow scan TPR/TPO cycles. The results were accounted for by the constructed size dependent Ellingham diagrams. It was shown that Fe3O4 and Mn3O4 spinels exhibit redox labile Mn2+/Mn3+ and Fe2+/Fe3+constituents, and in the conditions of the deN2O reaction these catalyst have pronounced tendency for stoichiometric over-oxidation. Redox properties of Co3O4 are highly anisotropic with Co2+ being oxidation reluctant but Co3+ prone for easy reduction. Stability of the Co3O4 catalyst is then controlled by partial reduction of octahedral Co3+ cations, due to the surface oxygen release at elevated temperatures in lean oxygen environments. The N2O decomposition was studied by temperature programmed surface reaction (TPSR) and pulse experiments using

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O-labeling of the catalysts. It was shown that

Co3O4 provide a sustainable redox Co3+/Co4+ couple for catalytic decomposition of N2O, which operates along a reversible 1-electron process, leading to formation of O–surf. intermediates that recombine next into dioxygen. As the reaction temperature increases the deN2O mechanism evolves from supra-facial to intra-facial recombination of the oxygen intermediates. Fe3O4 decompose nitrous oxide in a stoichiometric way via irreversible 2-electron reduction of oxygen intermediates into O2–, giving rise to lattice expansion and formation of a γ-Fe2O3 shell, discerned by Raman spectroscopy. Post-reaction STEM/EELS imaging confirmed a magnetitecore and a maghemite-shell morphology of the catalyst grains. Similar tendency for autogenous oxidation was observed for Mn3O4, yet a rather weak thermodynamic driving force makes this


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catalyst kinetically more stable. At higher reaction temperatures, the incipient γ-Mn2O3 layer may be decomposed back to the parent Mn-spinel, when oxygen pressure is low. To quantify gradual oxidation of the investigated spinels during the N2O decomposition size dependent thermodynamic 3D-diagrams were developed, and used for rationalization of the experimental observations. The obtained results reveal dynamic nature of the investigated spinels in varying redox conditions, and explain a remarkable performance of Co3O4 in comparison to Fe3O4 and Mn3O4. Catalytic behavior of the two latter spinels is actually governed by a sesquioxide shell, produced spontaneously in the course of the deN2O reaction.


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1.Introduction Nitrous oxide is a well-known greenhouse gas with destructive effect on the stratospheric ozone layer. Due to its harmful behavior catalytic decomposition of N2O into N2 and O2 is one of the key challenges of environmental catalysis, and the subject of intensive fundamental and applied investigations.1,2,3,4 Low temperature decomposition of N2O present in the tail gases of nitric and adipic acid plants has been studied on various catalytic systems like simple and mixed oxides,5,6,7 perovskites,8 spinels,9,10,11 calcined hydrotalcites,12,13 mesoporous silica materials,14 zeolites,15 and a large range of supported catalysts.16,17,18,19 Among them, the mixed oxides of spinel structure exhibit the highest activity.9,20,21,22 Their catalytic performance is associated with the specific electronic and magnetic properties, which stem from the intrinsic multi-valence nature and two types of the coordination states of the constituent redox ions. Improvements of the catalyst performance have been achieved by bulk modification (doping with alien cations)23,24,25 or by tuning the surface properties of the catalyst with alkali promoters.26,27 Owing to their well-defined structure, easily controlled morphology, and high flexibility for modification of electronic properties by doping, spinels are also extensively used in model catalytic studies.2,3,9,28 The close packed cubic Fd-3m structure of the 2-3 spinels is characterized by divalent (M2+) and trivalent (M3+) cations distributed among the tetrahedral A (8a) and the octahedral B (16d) interstitials, depending on the degree of inversion x. For normal spinels, x = 0, the half-filled octahedral sites contain M3+ cations, whereas tetrahedral sites, exhibiting one-eight occupancy, are filled with M2+ cations.29 In the case of x = 1 (inverse spinels), the A sites are hosting the M3+ cations, whereas the B sites are occupied half and half by the M3+ and M2+ cations. As a result, the spinel structure can be regarded as a versatile matrix, being able to accommodate a wide


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range of transition metal cations and oxidation states.30,31 The resultant tunable redox properties are related with the particular electronic configuration of the constituent ions, where the valence and conduction bands result from the crystal field splitting of the 3d levels. The catalytic activity of the spinel catalysts depends also on the grain size and surface morphological features as well.32,33 The cationic redox mechanism of N2O decomposition reaction34 consists in two overall steps: N2O activation by interfacial electron transfer (N2O + e → N2 + O–), and diffusive recombination of the surface oxygen intermediates into dioxygen (O– → ½O2 + e), concerted with the back electron transfer that restores the oxidation state of the active sites. Since both steps exhibit a clear redox character, they strongly depend on the electronic properties of the catalyst surface, and, indeed, the N2O decomposition has long been regarded as a probe reaction for gauging ±

electron redox properties of the catalysts (N O N + 1⁄2 O ).35 In contrast to abundant literature on deN2O activity of spinels, there are only scarce reports that address mutual relation between the catalysts thermodynamic stability in various redox environments and their reactivity.3,35 In the present paper these points were systematically examined by XRD, Raman, TPR/TPO, SEM, STEM/EELS and TPSR techniques including 18Olabeling, to understand the thermodynamic redox requirements that are needed for sustainable N2O decomposition over a series of Mn3O4, Fe3O4 and Co3O4 model spinel catalysts, that are often used as generic host structures for wide-ranging functionalization by bulk doping. The obtained results may also help to select an optimal starting spinel for further deN2O redox tuning, resolve past controversies concerning the nature of the active sites, and reveal the origin of the superior performance of Co3O4 in catalytic oxidation processes in comparison to other transition metal spinels.


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2. Experimental Section Materials and samples: Commercially available high purity Fe3O4 and Mn3O4 (SigmaAldrich) samples were used, and cleaned in situ to remove excess oxygen by annealing in the helium flow. Co3O4 was synthesized by drop-wise addition of 1 M (NH4)2CO3 to 1 M Co(NO3)2 solution in room temperature (to obtain pH = 9). The precursor was filtered, dried overnight at 80 °C, and calcined in air at 700 °C for 3 hours. The isotopically labeled Co318O4 specimen was obtained by deep reduction of Co3O4 with CO, and its subsequent reoxidation with 18O2 back to the cobalt spinel phase. Catalyst characterization: XRD measurements using CuKα1 radiation were carried out by a Rigaku Miniflex X-ray diffractometer equipped with a DeTEX detector. Diffractograms were registered in the 2θ range of 15–85° with the resolution of 0.02°. Crystal phase of each sample was determined by comparison of the recorded diffractograms with the corresponding AMCSD database records. The three-point BET surface areas of the investigated samples were measured by low-temperature nitrogen adsorption using a Quantasorb apparatus. Raman spectra were recorded with an inVia Renishaw spectrometer equipped with a Leica microscope and the 785 nm laser excitation. SEM micro-imaging was carried out by a Tescan VEGA 3 microscope equipped with a LaB6 electron gun, using a 20 kV beam. The STEM/EELS (Scanning Transmission Electron Microscopy / Electron Energy Loss Spectroscopy) measurements were performed with a FEI Tecnai Osiris microscope operating at the acceleration voltage of 200 kV, equipped with a high-brightness field emission electron source (X-FEG), and an electron energyloss spectrometer (Gatan Quantum SE 963). Prior to microscopic analysis, the samples were ultrasonically dispersed in ethanol and dropped on a holey carbon film supported on a copper grid (Agar Scientific, 400 mesh). STEM imaging was performed using a high angle annular dark


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field (HAADF) detector (Fischione model 3000) with a ~1.5 nm probe size. Two-dimensional spectrum image (SI) technique with STEM-EELS measurements was applied to distinguish various iron oxide phases, and to reveal their spatial distribution within the spinel grains. The beam convergence angle was set to 7.59 mrad, and the collection angle was set to 43.9 mrad. The core loss spectra were recorded in 1s/pixel exposure with a step size of 1.9 nm/pixel and 0.25 eV dispersion, the energy resolution was kept below 1 eV. The electric conductivity and the Seebeck effect measurements were carried out in the own-made setup. Spinel Reduction, Oxidation and Catalytic studies: Temperature programed reduction and oxidation (TPR/TPO) measurements were carried out in an own-made set-up (operating in the LabView environment) equipped with a QMS detector (Hiden Analytical HPR20), Brooks mass controllers, automatic Valco switching valves, and a quartz plug-flow reactor. Reductionoxidation and oxidation-reduction cycles were performed using gas mixtures containing 2% CO (TPR) or 2% O2 (TPO) in He as a balance. After each TPO or TPR step, the catalysts were cooled down to room temperature in the helium atmosphere before the counter redox process (TPR/TPO) was performed. The stoichiometry of the investigated samples at each temperature was calculated on the basis of the sample weight (~100 mg, sieved fraction of 200–300 µm), gas flow (50 cm3/min) and the reactant consumption. Catalytic tests were performed in the same reactor with the same sample, using 2% N2O in helium (25 cm3/min). Conversion and Turnover Rates (TOR) values were calculated on the basis of the QMS signals calibrated against the helium balance gas. For isothermal pulse redox (CO/O2) and deN2O experiments a 2.5 cm3 loop was used for injections.


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3.Results and discussion 3.1. Characterization of Spinels XRD diffractograms of the investigated spinel catalysts together with the corresponding Rietveld simulation are shown in Figure S1 in the Electronic Supporting Information (ESI). The characteristic diffraction patterns indicate that for all the samples no other than crystalline spinel phases were present. The XRD lines were indexed within the Fd-3m space group (24210-ICSD) with a = 8.083 Å, u = 0.2632 for Co3O4, and a = 8.394 Å, u = 0.2583 for Fe3O4 (magnetite), whereas for α-Mn3O4 (hausmannite) within the tetragonally distorted I41amd structure with a = 5.763 Å, c = 9.4671 Å (JCPDS 24-0734). The corresponding values of the lattice constants a and c, and the oxygen parameter u were determined by the Rietveld simulation. The crystallites size (dXRD) and strain (ε) were calculated using the Williamson-Hall plot (see Figure S2, ESI), and the results are summarized in Table 1. For Co3O4 (dXRD = 82 nm) and Mn3O4 (dXRD = 73 nm) the crystallite dimensions are quite similar, whereas in the case of Fe3O4 they are smaller (dXRD = 28 nm). The lattice strain for all samples was below 0.3 %. The average grain size calculated from the BET surface area (SBET) assuming a spherical shape model, dBET = 6

(Vmol and

Mmol stands for molar volume and molar mass respectively), indicated that the iron and cobalt spinel crystallites are fairly well separated, whereas in the case of manganese spinel they form grains composed of few merged crystallites on average. These results are consistent with microscopic SEM investigations. The typical SE-images of the investigated catalysts, shown in Figure 1, indicate that the overall morphologies of all the spinels are quite similar. They are dominated by submicron aggregates of the Mn3O4, Fe3O4 and Co3O4 crystallites, the size of which is in the range of 30–100 nm.


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Table 1: XRD and BET characteristics of the spinel samples. a c dXRD u nm nm nm

ε %

SBET m2·g-1

dBET nm

Mn3O4

5.763

9.471

0.2229 0.3840

73

0.3

6

200

Fe3O4

8.394

-

0.2583

28

0.2

35

33

Co3O4

8.083

-

0.2632

82

0.2

10

98

Figure 1: SEM micrographs of the Mn3O4 (a), Fe3O4 (b) and Co3O4 (c) samples.

In order to eliminate possible presence of spurious impurity phases, all the samples were additionally examined by Raman techniques. The obtained Raman spectra of Mn3O4, Fe3O4 and Co3O4 are shown in Figure 2a-c. Among 14 Raman-active modes (2A1g, 2B1g, 4B2g, 6Eg) predicted by factor-group analysis for I41/amd space symmetry of Mn3O4,36 in the recorded Raman spectrum only five peaks with pronounced oscillator strength can be distinguished: an intense A1g peak at 650 cm-1, due to symmetric stretching of the Mn–O bonds, accompanied by weaker bands at 480 (T2g), 370 (Eg), 325 (T2g) and 295 (T2g) cm-1 (Figure 2a).37 In the investigated spectrum no clear features characteristic of manganese sesquioxides could be distinguished.


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Figure 2: Raman spectra of Mn3O4 (a), Fe3O4 (b) and Co3O4 (c) spinels together with the band assignment.

In the case of the Raman spectrum of the Fe3O4 spinel (Figure 2b), the factor group analysis predicts 5 Raman active A1g, Eg and 3T2g bands. According to previous studies based on good quality polarized RS measurements,38 we assigned the dominant peak at 670 cm-1 to a symmetric Fe–O breathing A1g mode. It is followed by much weaker T2g band at 550 cm-1, an Eg band at 310 cm-1, and a T2g band located around 190 cm-1, associated with the symmetric and asymmetric bending/wagging oscillations of the Fe and O atoms in the magnetite structure. The absence of any features around 400 cm-1, diagnostic of hematite, indicates that such impurities were not present. Indeed, the scattering power of hematite is much larger than that of magnetite, so even very small amounts of α-Fe2O3 could principally be detected.38 The Raman spectra of cubic Co3O4 (Fd-3m) are characterized by 5 resolved bands at 194 (F2g), 480 (Eg), 520 (F2g), 620 (F2g) and 690 (A1g) cm−1, predicted by the factor analysis39 (Figure 2c). Summarizing, all the Raman spectra reveal the absence of any spurious minor phases, and confirm their good


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crystallinity as well. Some differences in the reported Raman frequencies and previous literature data can be attributed to the temperature and grain size effects.38,40

3.2. Redox cycles Owing to the importance of redox properties of the catalyst for deN2O activity we investigated the reduction-oxidation cycles for the Mn3O4, Fe3O4 and Co3O4 catalysts by temperature programed reduction and oxidation with lean carbon monoxide and lean dioxygen, respectively (Figure 3a1-c1). Such redox conditions, concerning the chemical potential of oxygen, are in the broad range of those existing in typical deN2O and deCH4 processes, respectively. The metal to oxygen ratio for each temperature (Figure 3a2-c2) were determined by integration of the corresponding TPR/TPO profiles, and the resultant areas were calibrated against a sequence of the O2/CO pulses of known volume. The horizontal dashed lines indicate an apparent oxygen/metal ratio corresponding to the stoichiometries of the nearest reference phases, and do not imply that such distinct phases are formed actually. Generally, all the investigated spinel catalysts exhibit broad multimodal reduction profiles in the TPR survey scans. The reduction process for Mn3O4 and Fe3O4 leads to the corresponding sub-stoichiometric monoxides, whereas Co3O4 is reduced ultimately to the metallic cobalt (Figure 3a1-c1). In contrast to quite regular profile for manganese and cobalt spinels, a number of discrete peaks of various intensity indicate complex reduction behavior of Fe3O4. This fact may be, inter alia, associated with the particle size effect and the iron spinel inverse structure discussed below. The reducible Fe3+ cations present in both the tetrahedral and octahedral positions make, obviously, the reduction process intrinsically more involved: (Fe3+)[ Fe2+Fe3+O4] + xCO → (Fe3+(1-x))[Fe2+(1+2x)Fe3+(1-x)O4-x] + xCO2

(0 ≤ x ≤ 1),


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Figure 3: Temperature programmed reduction with lean CO (yellow background) and oxidation with lean O2 (green background) cycles performed for Mn3O4 (a1) Fe3O4 (b1) and Co3O4 (c1), together with the corresponding profiles of the oxygen to metal ratio (a2, b2, c2). After the TPR step the catalysts were cooled down to room temperature in the helium atmosphere before the TPO was performed. The CO2 and O2 QMS signals refer to 44 m/z and 32 m/z lines, respectively.


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Another complication in the magnetite reduction arises from complex mechanism caused by its particular

thermodynamics.41

At

temperatures

below

550–570 °C,

due

to

facile

disproportionation of a nascent wustite phase, 4FeO → Fe3O4 + Fe, magnetite can actually be reduced to iron. Yet, above this temperature FeO becomes thermodynamically stable, and may appear as a reaction product.42 As a result, assignment of the particular TPR peaks is indistinct as they clearly do not correspond to well separated reduction events. The overall reduction profile of hausmannite, apart from a narrow peak at 380 °C assigned previously to spurious Mn3+ in the tetrahedral sites,43 is rather featureless, and can be accounted for by the following continuous process: (Mn2+)[Mn3+Mn3+O4] + xCO → (Mn2+(1-x))[Mn3+(2-2x)Mn2+3xO4-x] + xCO2

(0 ≤ x ≤ 1),

where reduction of the octahedral Mn3+ cations is accompanied by relocation of the tetrahedral Mn2+ ions into the octahedral interstitials, giving rise to gradual segregation of the evolving manganese monoxide: (Mn2+(1-x))[Mn3+(2-2x)Mn2+3xO4-x] → 3x(Mn2+O)|(1-x)(Mn2+)[Mn3+2O4]. In the case of cobalt spinel, the observed main peaks can be associated with the following reduction and phase segregation44 processes: (Co2+)[Co3+Co3+O4] + xCO → (Co2+(1-x))[Co3+(2-2x)Co2+3xO4-x] + xCO2

(0 ≤ x ≤ 1)

(Co2+(1-x))[Co3+(2-2x)Co2+3xO4-x] → 3x(Co2+O)|(1-x)(Co2+)[Co3+2O4] The segregated CoO, above 350 °C, is next reduced to metallic cobalt: 3(Co2+O) + xCO → (Co2+O) (3-x)|Cox + xCO2

(0 ≤ x ≤ 3)

The spinel reduction starts slightly earlier for Co and Mn (~200 °C and ~250 °C, respectively) and is shifted to ~300 °C for iron. As can be judged from the associated composition plots (oxygen/metal ratio), the monoxide stoichiometry was achieved already at ~350 °C for Co3O4,


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but in the case of Mn3O4 and Fe3O4 at ~550 °C and ~750 °C, respectively. This shows distinctly more reluctant reducibility of magnetite and hausmannite, in comparison to cobalt spinel, which even in the narrower temperature range was readily reduced into Co0. The TPO survey profiles (Figure 3 right panels) for Mn3O4 and Co3O4 exhibit broad features only, which were associated with re-oxidation of MnO back to Mn3O4, and quite facile oxidation of metallic Co to CoO and above 350 °C to Co3O4. As expected, for iron spinel several distinct bands can be distinguished, reflecting again the complex redox chemistry of the involved iron oxide, discussed above. In the case of Mn and Fe samples the reoxidation process was accomplished at ~550 °C and ~520 °C, above the spinel stoichiometry, indicating their partial overoxidation into the incipient sesquioxides. The reoxidation of cobalt, in turn, despite of being more extended in the stoichiometric range, was terminated much earlier (~430 °C), and the overall Co3O4 phase composition was essentially not crossed. To probe the redox properties of the investigated spinels more thoroughly, narrow scan TPR/TPO and reversed TPO/TPR cycles were next carried out in the temperature range of M3O4 →MO1+δ and M3O4 →M2O3–δ redox transitions (Figure 4). The reduction curves (Figure 4a1) reveal the reducibility sequence Co(T10%,red = 350 °C) >> Mn(T10%,red = 480 °C) > Fe(T10%,red = 540 °C), as gauged by the temperature of 10% reduction (marked by the dotted horizontal lines). Reduction of cobalt spinel, once triggered, proceeds quite rapidly in comparison to moderate reaction progress observed for iron and manganese. The corresponding TPO profiles (Figure 4a2) reveal a reverse oxidation order with Fe (T10%,ox = 320 °C) >> Mn (T10%,ox = 600 °C) >> Co (virtually oxidation stable). In the case of the iron spinel its oxidation to maghemite occurs rapidly already above 250 °C, whereas for Mn3O4 its oxidation to γ-Mn2O3 (see below) is rather sluggish, and T10%,ox is reached at 600 °C only.


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Figure 4: Oxygen to metal ratio (O:M) profiles obtained from narrow scan TPR (a1) and TPO (a2) measurements for the Mn3O4, Fe3O4 and Co3O4 samples.

On the contrary to iron and manganese, in the oxidized conditions Co3O4 preserves its spinel stoichiometry in a rather wide temperature range. However, a more detailed pulse O2 adsorption studies (see Figure S3 in ESI) showed that until ~300 °C cobalt spinel surface is covered by various oxygen species, produced in the processes such as ½O2 + Co3+ = O– + h·(Co4+) or O2 + xCo3+ = O2x– + xh·(Co4+), x = 1, 2. Their presence was confirmed by titration with CO pulses (Figure S3a, ESI), and their chemical nature and structure have recently been unraveled by quantum chemical modeling, with the resultant electron holes (h·) are localized on the octahedral cobalt ions.3 This fact was confirmed by electric conductivity and Seebeck effect measurements (Figure S3b and c, ESI). The activation energy of 0.29 eV is characteristic for hole hoping between the octahedral cobalt cations (CoB3+ + CoB4+(h·) → CoB4+(h·) + Co3+),45 and positive value of the Seebeck coefficient confirms the hole nature of the charge carriers. The above observations indicate that in the oxidizing conditions Co3O4 preserves the spinel structure in contrast to the other spinels. It may be, however, faintly disordered in the surface region by formation of the CoB4+(h·) defects upon interaction with oxidants (O2 or N2O). As a


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result a Co3+/Co4+ redox couple associated with the octahedral core is established, playing a vital role in nitrous oxide decomposition (vide infra). Fe3O4, and to less extent, Mn3O4, are gradually transformed into the corresponding sesquioxides in such conditions, showing that the Fe2+ and Mn2+ states are unstable even at pO2/p0 = 0.02. As a result, the Fe2+/Fe3+ and Mn2+/Mn3+ redox pairs in the spinel host cannot operate in a sustainable way in the catalytic processes involving oxygen as a reactant. Such redox pairs lead to deep two electron reduction of ½O2 to O2– species, their incorporation by lattice expansion, and subsequent formation of new phases, as discussed below in more detail. The presented TPR/TPO results can be rationalized by means of the size dependent Ellingham diagrams (log(pO2/p0) vs. T),41 designed for two limiting grain sizes of r = 10 and r = 100 nm (Figure 5). The equilibrium oxygen pressures were calculated from the changes in the free enthalpies of the respective redox processes, –log(pO2/p0) = 2.303∆Gr(r)/RT, as explained in more detail in ESI (section 3, Table S1). The left panel (a1-c1) refers to the fine grain (10 nm), whereas the right one (a2-c2) to the coarse grain (100 nm) samples. The solid lines refer to phase boundaries between the most stable oxides, whereas the dashed black lines to the metastable ones. On accounting for the TPO experiments, the oxygen pressure of pO2/p0 = 10–2 was indicated by blue dashed lines, whereas in the case of TPR, following literature arguments46 a virtual oxygen pressure range (corresponding to the CO/CO2 mixture) was indicated by transparent yellowish horizontal stripes. As can be inferred from Table 1 and Figure 1, the particle size of the investigated spinels are closer to the coarse grain limit for Co and Mn, and to less extend for magnetite. We have checked, however, that the diagram constructed for d = 30 nm is closer to the that for d = 100 nm.


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Figure 5: Ellingham diagrams for the Mn-O (a), Fe-O (b) and Co-O (c) systems, calculated assuming spherical particles of 10 nm (left column) and 100 nm (right column) in diameter.

Such simple thermodynamic account, clearly reveals the regions of stability and uneven size dependence of redox properties among the investigated oxides. For Mn and Co (Figure 5a1-5a2 and 5c1-5c2, respectively) with the increasing oxygen pressure the same sequence of appearance


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of the corresponding oxide phases is observed for the fine and coarse grain samples at all temperatures. Whereas in the case of Mn3O4 the stability area is shrank upon decreasing the particle size, for Co3O4 the size effect is rather small. In the case of iron oxides, the sequence of the phase presence depends not only on the particle size, but also on the temperature. Wustite can appear only for larger particles at temperatures above 450 °C as a stable phase. The magnetite stability region is moved toward higher oxygen pressures in the case of larger particles, and is followed by formation of γ-Fe2O3 (small grains) or α-Fe2O3 (large grains) with the increasing oxygen pressure, in agreement with previous experimental results.47,48 In the case of coarse grain magnetite, although the Fe3O4/γ-Fe2O3 phase boundary line is shifted to higher oxygen pressure with respect to the Fe3O4/α-Fe2O3 (Figure 5b2), due to the structural compatibility with spinel, an incipient formation of the maghemite may be kinetically preferred. As implied by Figure 5a, for both grain sizes at oxygen pressures applied in the TPO experiments, hausmannite can be oxidized to Mn2O3 at low temperatures. However, with the increased temperature the Mn3O4/Mn2O3 oxygen equilibrium pressure is approaching the TPO level, indicating that the driving force for Mn3O4 oxidation is gradually ceasing, thus coarse grain Mn spinel may be stable above ~700 °C. Reduction of Mn3O4 should lead to MnO at T > 500 °C for fine and T > 700 °C for coarse grain samples, in nice agreement with the TPR results (vide supra). The equilibrium oxygen pressure of magnetite for its oxidation to Fe2O3 is much lower than the pO2(TPO) level (Figure 5b), providing a robust driving force for an efficient maghemization process in the whole investigated temperature range, regardless the particle size. Above ~700 °C, small grain Fe3O4 can be reduced directly into metallic Fe, whereas for larger ones reduction starts earlier (T > 500 °C), and is passing through kinetically hardly reducible wustite phase. It


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supports complex reduction of magnetite that is incomplete even above 700 °C, as observed in the TPR experiment (Figure 3b1). The Ellingham diagram of cobalt spinel (Figure 5c) implies its reduction into CoO already around 200 °C, and then (above 300 °C) to metallic cobalt for both fine and coarse grains, in an excellent agreement with the TPR data (Figure 3c1). The equilibrium oxygen pressure of Co3O4 oxidation overcomes the pO2(TPO) value dramatically, making cobalt spinel a stable phase even at high oxygen pressures. It is, however, worth noting that at elevated temperatures (above ~700 °C and ~500 °C for small and large particles, respectively) reduction of Co3O4 is possible even in the oxidizing TPO conditions. It involves surface oxygen release (2Co3+ + O2– → 2Co2+ + VO + ½O2),49 and may lead to participation of an interfacial recombination of surface oxygen intermediates produced in the deN2O reaction, especially when coarse grain catalyst is used (vide infra). As a result, the autogenous oxygen produced during the N2O decomposition is apparently beneficial for stabilization of the cobalt spinel structure, inhibiting formation of anionic vacancies. This point has been elucidated by us recently in more detail by means of DFT modelling.3 Summarizing,

the

TPR/TPO

results,

supported

by

the

presented

thermodynamic

considerations, showed that the reducibility of the trivalent to divalent cations increases in the order Fe3+ < Mn3+ > TOR(Mn3O4) = 0.005 µmol·m-2·s-1. In contrast to regular behavior for the Co3O4 catalyst with concurrent evolution of gaseous dioxygen and dinitrogen as final products, the deN2O profile of the iron


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spinel shows an unusual temperature dependence. In the low temperature range (300–400 °C) a well pronounced TOR peak of nitrous oxide, reaching almost 0.020 µmol·m-2·s-1, was observed. Above 400 °C the N2O conversion ceases completely, but at T > 500 °C it is restored, increasing with the temperature in a typical monotonous way. Inspection of the associated N2 and O2 profiles (Figure 6b) indicates that the low temperature TOR peak is accompanied by evolution of dinitrogen only. The oxygen produced during the N2O decomposition is thus completely retained on the catalyst surface below ~500 °C, indicating that the iron spinel catalyst is gradually oxidized in such conditions into maghemite, in accordance with the TPO data and the Ellingham diagram predictions. This observation is reinforced by comparison of the deN2O conversion profiles of the magnetite sample (pretreated in the flow of helium at 600 oC to remove oxygen excess), registered in the heating and cooling modes (Figure 7a red solid and dotted lines), with that of the Fe3O4 sample pretreated in the oxygen flow prior to the N2O decomposition test (Figure 7b green line). In the case of the Fe3O4(He) catalyst the low temperature peak is absent in the deN2O profile registered on cooling, whereas for the Fe3O4(O2) the conversion profiles obtained upon heating (solid lines) and cooling (dotted lines) are essentially the same. They are also similar to the N2O conversion curve observed for the reference α-Fe2O3 sample, implying that the sustainable turnover is associated with an autogenic formation of a sesquioxide shell during the stage of stoichiometric reaction of N2O with the magnetite surface. A quantitative description of this process is presented below. In the case of Mn3O4, the N2O TOR and corresponding N2, O2 evolutions profiles (Figure 6) are apparently regular, and the N2O decomposition curves observed upon heating and cooling are also close (Figure 7b green solid and dotted lines, respectively). However, for an initially oxidized sample (calcination under 1 bar of oxygen at 500 °C for 4 h implies formation of a


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Mn2O3 shell, see Figure 5) the profiles recorded for heating and cooling modes of N2O decomposition are significantly different (Figure 7a red solid and pink dotted lines). It indicates a distinct change in the surface state of the catalyst during the TPSR experiment. The deN2O activity of the oxidized Mn3O4(O2) sample is irregular, being initially noticeably enhanced, above ~700 °C it declines merging with that of the Mn3O4 sample pretreated in helium (green and red profiles). Since on cooling this effect was not reproduced, it may be associated with partial oxidation of the catalyst due to pretreatment in oxygen. Yet, at high temperatures the catalyst is spontaneously reduced back to the spinel, in accordance with the Ellingham diagram predictions (Figure 5a). The N2O conversion profiles of the reference α-Mn2O3 sample for the heating and cooling modes (gray solid and dashed lines), show that this oxide is stable in the reaction temperature window. This implies formation of a γ-Mn2O3 shell during partial oxidation of Mn spinel at 500 °C, confirmed by Raman spectra (see below).


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Figure 7: deN2O conversion profiles recorded upon heating (solid lines) and cooling (dashed lines) for the differently pretreated samples (flow of He or O2) of Fe3O4 (a) and Mn3O4 (b) catalyst. Reference conversion lines for α-Fe2O3 and α-Mn2O3 are also shown. The observed susceptibility of Co3O4 for reduction in the lean oxygen conditions at elevated temperatures is reflected in the deN2O reaction mechanism. The TPSR profile of nitrous oxide decomposition over the Co318O4 catalyst (Figure 8, top panel) shows that in the low temperature region, where cobalt spinel is fully stable, only unlabeled

16

O2 is formed via the suprafacial

recombination of the 16Osurf. intermediates produced upon N2O dissociation (16Osurf. + 16Osurf. → 16

O2). However, at T > 500 °C (where cobalt spinel starts to be liable for reduction)

accompanied by gradual evolvement of recombination with the

18

16

16

O2 is

O18O species, brought about by interfacial cross

O-labeled lattice oxygen of the catalyst surface (18OO +

16

Osurf. →


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16

O18O + VO). The oxygen vacancies may be refilled by oxygen atoms provided by N2O

dissociation (N216O + VO = 16OO + N2).

Figure 8: The TPRS profile of nitrous oxide decomposition over Co318O4 catalyst showing the evolution of oxygen 16O2, 16O18O and 18O2 isotopomers (top panel), together with the associated N2O pulse experiments performed at various temperatures (bottom panel).

The observed different redox behavior of the cobalt spinel catalyst in the deN2O reaction conditions is more distinctively illustrated by N2O/Co318O4 pulse experiments (Figure 8, bottom panel) performed at different temperatures. At 300 °C upon the N2O pulse only dinitrogen is released, indicating that at this temperature oxygen is entirely retained, and the catalyst surface is partially oxidized (N2O + Co3+ → Osurf.– + Co4+(·h) + N2), and covered by the oxygen Osurf.– intermediates. Upon increasing of temperature to 400 °C simultaneous appearance of both the reaction products, N2 and

16

O2, indicate that the catalytic cycle is closed and dioxygen is

produced along the suprafacial recombination pathway exclusively. Further increase in the temperature (500 °C) of the N2O injection is accompanied by growing contribution of

16

O18O


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dioxygen in the reaction products. Involvement of surface

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18

O-species in the oxygen

recombination step is made possible by thermodynamic incentive of the Co3O4 catalyst for reduction via surface oxygen release, revealed by the TPR/TPO experiments and the Ellingham diagram analysis. In practical conditions of N2O abatement, inhibitors such as water and dioxygen are typically present. The investigations into their effect is only relevant for cobalt spinel catalyst, which exhibits sustainable turnover, and the results (conversion versus temperature) are shown in Figure 9. Both water and dioxygen exhibit appreciable negative effect on N2O conversion, shifting the T50% by 30 °C and 45 °C, respectively in line with previous literature reviewed elswhere.2 Simultaneous presence of oxygen and water in the feed obstructs the N2O decomposition essentially to the same extend as in the case of water only. However, the nature of the inhibiting effect of water and oxygen is distinctly different. Whereas, in the case of water it consist in mere site blocking,6 the role of oxygen is more involved. In the first stage of the N2O reaction it competes with nitrous oxide for the cobalt active sites, and in the second step it is intermingled with the oxygen recombination step, creating idling cycles (O2(g) → Oads. + Oads. → O2(g)). It is also worth noting that the enhanced coverage of surface oxygen intermediates reduces the role of diffusion step in O2 recombination. This issue is currently investigated by kinetic Monte Carlo modelling in more detail. The rather minor effect of water can be accounted for by its low thermal stability on the Co3O4 surface as revealed by our previous ab initio thermodynamic investigations.50 On the dominant (100) faceting water is fully released already at T < ~250 °C, whereas for the second most abundant (111) at T < ~450 °C. Thus, the persistent lowering of the activity may be associated with the blocked participation of the (111) facet in the deN2O process.


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Figure 9: deN2O conversion profiles illustrating the effect of O2 (green line) H2O (blue line) and H2O + O2 (navy blue dotted line), together with the reference conversion plot for sole N2O (red line).


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3.4. Post mortem analysis of the catalysts The spinel catalysts after the N2O decomposition TPSR tests were examined by Raman spectroscopy, and the corresponding spectra are shown in Figure 10. Whereas for the Mn3O4 and Co3O4 catalysts the Raman spectra remain apparently unchanged after the reaction (cf. Figures 10a1/a2 and Figures 10c1/c2), in the case of Fe3O4 the corresponding spectrum is dramatically modified. The bands of magnetite (Figure 10b1) are strongly attenuated and new lines characteristic of γ-Fe2O3 appear (Figure 10b2), confirming that the magnetite is significantly oxidized during the catalytic reaction into the corresponding maghemite phase, in accordance with the Ellingham diagram (Figure 5b). For longer time on stream it is converted into hematite (α-Fe2O3) layer, featured by well-developed strong Raman lines (Figure 10b3). Comparison of the RS spectrum of the Mn3O4 sample after prolonged N2O decomposition at 500 °C (Figure 10a3) with the sample oxidized under 1 bar of dioxygen (Figure 10a4), shows that oxidation of hausmannite is much slower than for magnetite, yet feasible. Thus, in the Raman spectrum of the Mn3O4 samples recorded after the TPSR test, due to a limited time on stream the diagnostic features of the incipient γ-Mn2O3 are apparently too weak to emerge.


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Figure 10: Raman spectra of the investigated spinels upon deN2O reaction and treatment at various oxidative conditions. The Mn3O4 spectrum recorded before (a1), after (a2) N2O TPSR experiment, after prolonged N2O decomposition at 500 °C (a3), and upon oxidation at 1 bar of dioxygen at 500 °C, showing formation of a γ-Mn3O4 shell (a4). Fe3O4 spectrum recorded before (b1), after (b2) N2O TPSR experiment and after prolonged N2O decomposition at 500 °C (b3), showing formation of a γ-Fe3O4 shell and its transformation into α-Fe3O4. The Co3O4 spectrum recorded before (c1), after (c2) N2O TPSR experiment, confirming phase stability of the cobalt spinel.


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For corroborative STEM/EELS imaging of the catalyst oxidation state changes upon the reaction with N2O we selected the most labile iron spinel catalysts. The spectrum imaging technique (SI) was used to acquire the experimental data presented in Figure 11 for spatial imaging of the magnetite into maghemite transformation.

Figure 11. EELS spectrum image of the magnetite catalyst particle after deN2O reaction (a), together with background subtracted reference oxygen K-edge ‘fingerprints’ of magnetite (b1), and maghemite (b2), and the chemical composition map (magnetite – red, maghemite – green) showing a core-shell structure of the catalyst (c)

The presence of both phases can be revealed by monitoring the O-K peak, since the oxygen edge profiles are sensitive to the local valence and bonding status of the oxygen anions surrounding.51 The main diagnostic feature of the both iron oxides is the relative intensity of the O-K prepeak with respect to the absorption maximum, which changes from 0.15 for Fe3O4 to 0.20 for γ-Fe2O3. In order to reveal such differences the averaged O-K spectra for core and surface regions of the catalyst nanograin were extracted (Figure 11b1 and b2), and used as spectroscopic signatures. Spatially resolved EELS spectral fine-structure analysis was then carried out to reveal the differences in the oxidation state of iron between surface and core regions of the catalyst nanoparticle. The resultant composition map (Figure 11c) shows a clear


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magnetite-core and maghemite-shell morphology of the catalyst grain, consistent with a pronounced conversion of Fe3O4 into γ-Fe2O3, according to the reaction Fe3O4 + δ/2N2O → Fe3+[Fe2+1-δFe3+1+δ

3/8δ]O4+δ/2

+ δ/2N2. The core-shell structure of the catalyst shows that the

N2O catalytic decomposition is associated with the segregation of the sesquioxide layer, and therefore with surface Fe3+ ions only. Thus, it apparently involves a Fe3+/Fe4+ redox couple, similarly to that proposed for cobalt.


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3.5. Thermodynamic modeling of the spinel catalyst oxidation As discussed above, the interaction of N2O with Fe3O4 and Mn3O4 leads to formation of a nonstoichiometric M3-xO4 surface layer that with the reaction progress evolves into the corresponding γ-Fe2O3 or γ-Mn2O3 phase, as revealed by Raman and STEM/EELS techniques. Although for cobalt spinel the N2O decomposition occurs in a catalytic way since Co3O4 is stable under the reaction conditions (Figure 10c), we assessed also a possibility of its virtual transformation into a hypothetical γ-Co2O3 oxide, for the sake of completeness. For modeling the observed gradual oxidation of spinels during the catalytic reaction we developed a thermodynamic approach based on that proposed earlier for perovskites.52 Taking again the magnetite catalyst as an example, the governing reaction considered herein is the surface equilibration of Fe3O4 with the gas-phase N2O, i.e., a redox process leading to a complete reduction of the oxygen intermediates, combined with expansion of the spinel lattice and formation of cationic vacancies. In the Kröger–Vink notation such process can be expressed as: (×)

N O() + 2Fe

(∙)!

→ O× + 2Fe

2 %%% + V$ + N() 3

From the perspective of the involved defect chemistry, this process is equivalent to simplified magnetite oxidation by dioxygen produced in the course of N2O decomposition: (×)

O() + 4Fe

(∙)!

→ 2O× + 4Fe

4 %%% + V$ 3

Within an ideal solid solution approximation the corresponding oxidation constant, Kox, can be expressed as: '() =

4

4

%%% .! +,-× . /012(∙)! 3 +567 (×) 4

/012

3 8-9(:)


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An infinitesimal change in the magnetite non-stoichiometry can, in turn, be formulated in the following way: δ ! Fe! O4 + O → Fe! = AFe (=B?) Fe >

The resultant increment of the lattice sites (unit cells) due to the oxidation is equal to 1/2δ. Upon applying the electroneutrality and site conservation principles, the maghemization extent, δ, can be related to the oxygen partial pressure in the following way: 1 4 3 logI8-9 /8K L = − log '() − 4 log(1 − N) + 2 log O4 + NP + 4 log(N) + log O NP 2 3 8

The Kox values were calculated for the given spinel catalyst using the standard oxidation enthalpy and entropy values, taken from Table S1 (Supporting information section). On account for disorder in the octahedral sub-lattice, caused by the presence of the cationic vacancies in the formed γ-Fe2O3, we also included the configurational entropy term (see ESI section 5 and Figure S4). The final equation was translated into three-dimensional (δ, T, pO2), thermodynamic diagrams (Figure 12), which describe continuous evolution of M3O4 (δ = 0, blue regions) into γM2O3 (δ = 1, red regions), M = Mn, Fe, Co, as a function of oxygen pressure and temperature. The oxygen pressure corresponding to the maximum conversion of N2O, (log(pO2/p0) = –2, marked by white line), and oxygen pressure corresponding to actual conversion at given temperature (dashed orange line), were added to delineate the catalytic oxidation conditions of the deN2O experiments. The light blue lines indicate the temperature of the N2O conversion maxima.


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Figure 12: Thermodynamic 3D diagrams for M3O4 to γ-M2O3 transformation for Co (a), Fe (b) and Mn (c) systems, quantifying the oxidation of spinels (δ = 0) into the corresponding γsesquioxides (δ = 1) as a function of oxygen pressure and temperature. The maximum conversion of N2O and oxygen pressure corresponding to actual conversion at given temperature are indicated by white and dashed orange lines, respectively. The light blue lines indicate the temperature of the N2O conversion maxima. In the case of Co3O4 (Figure 12a) the oxygen pressure produced during N2O decomposition is several orders of magnitude lower than the equilibrium pressure corresponding to the onset of Co3O4 oxidation, in the whole investigated temperature range. Therefore, the spinel phase is thermodynamically oxidation stable in the applied catalytic conditions. The opposite situation is expected for magnetite (Figure 12b), since at oxygen pressures produced by deN2O reaction δ ~ 1, even at the incipient stage of the reaction at lowest temperatures. This


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implies thermodynamically favorable formation of the maghemite phase, and explains the experimental results (Raman, EELS and catalytic data) discussed above. For Mn3O4 the oxidation process is more intricate. At lower temperatures, albeit the conversion is still low, the resultant oxygen pressure is high enough for the appreciable oxidation of the catalyst to nonstoichiometric Mn3-xO4 phase (δ ~0.5). With the increasing temperature, despite that the oxygen pressure is increased by the growing conversion, the hausmannite non-stoichiometry decreases reaching δ ~0.1 at 800 °C. As a result, after the deN2O reaction carried until 700 °C, we may expect that the Mn3O4 catalyst is partly oxidized (as confirmed by the Raman spectrum in Figure 10a3 and a4). The presented thermodynamic diagrams allows for prediction of the excess oxygen effect on N2O decomposition over Mn and Fe spinels. The analysis of Figures 12a and 12c shows that addition of oxygen will not influence the observed change in the iron spinel state since even in the presence of lone N2O, it can be easily and fully oxidized to maghemite in the whole temperature range. Dioxygen can obviously accelerate this process. In the case of Mn3O4 the excess of oxygen (typically few percent) slightly enhances the stability of γ-Mn2O3, yet for appreciable effect oxygen pressures should be much higher (above ~0.5 atm). Summarizing, the developed 3D spinel oxidation diagrams provide a convenient thermodynamic background for detailed understanding the nature of different behavior and stability of the Mn3O4, Fe3O4 and Co3O4 catalysts in the redox environment of N2O decomposition. The stability of Co3O4 is governed by the diverged redox properties of its constituents. The tetrahedral Co2+ cations are strongly oxidation resistant, averting the spinel structure from structural changes (transformation into Co2O3). The octahedral Co3+ ions, however, are quite prone for reduction into Co2+ in lean oxygen environments at elevated


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temperatures. The resultant defects are accommodated by spinel structure to large extent. The mechanism of N2O decomposition may, thus, evolve from supra-facial to intra-facial recombination of the oxygen intermediates as the reaction temperature increases or when oxygen pressure is low (due to small conversion). These effects are expected to be more pronounced for large grain samples. In the case of easily oxidizable magnetite catalyst, its catalytic behavior can be attributed to the presence of a sesquioxide shell, produced spontaneously in the course of the deN2O reaction. Similar tendency for oxidized shell formation is expected for Mn3O4, however a smaller thermodynamic driving force makes this catalyst kinetically more stable in the deN2O reaction conditions. At higher temperatures, the γ-Mn2O3 layer maybe decomposed back to the parent Mn-spinel. As a result, the catalytic performance of the nominally iron and manganese spinel samples is actually related with the corresponding sesquioxides. Indeed, their activity profiles are very closed to that of the α-Fe2O3 and α-Mn2O3 references, since in all corresponding samples the equal trivalent active sites are operating. Taking the three investigated spinels as parent hosts for further redox tuning toward optimal performance in the N2O decomposition, Mn3O4 and Fe3O4 should be stabilized against undesired oxidation, whereas Co3O4 against reduction by appropriate doping. Alternatively, one can enhance the cobalt spinel activity, via apposite bulk or surface doping, to maintain the deN2O process within the limits of the catalyst stability. All the above experimental results and their thermodynamic account reveal clearly dynamic nature of the investigated spinels in varying redox conditions. It has important structural and chemical implications for mechanism of the catalytic N2O decomposition, controlling not only the sustainable versus stoichiometric turnovers, but also transition from the suprafacial into the interfacial reaction mechanism. An in-depth understanding of this complex and interlaced


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redox and structural dynamics can be exploited for amending the key factors of the spinel catalysts redox behavior and stability controlling their performance.


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4. Conclusions Stability and redox properties of Mn3O4, Fe3O4 and Co3O4, spinels were explored by TPR/TPO cycles and thermodynamic modelling, supported by XRD, Raman, SEM and STEM/EELS measurements. The revealed dynamic redox nature of the investigated spinel catalysts has important structural and chemical implications for mechanism of the catalytic N2O decomposition. It controls not only catalytic versus stoichiometric N2O turnover, but also the transition from suprafacial into interfacial recombination mechanism of the surface oxygen intermediates. Cobalt spinel being oxidation stable in the course of the deN2O reaction provides a sustainable redox Co3+/Co4+ couple for the catalytic decomposition of N2O along a reversible 1-electron process, leading to the formation of O– surface oxygen intermediates. The redox labile Fe3O4, and to the less extent Mn3O4, are unstable in oxidation conditions, giving rise to N2O decomposition in a stoichiometric way caused by irreversible 2-electron reduction of oxygen intermediates into O2–, leading to gradual formation of γ-Fe2O3 and γ-Mn2O3. The observed activity of Mn and Fe spinels is actually governed by the sesquioxide shell, produced spontaneously in the course of the deN2O reaction. The revealed complex redox and structural dynamics of spinels can be exploited for amending the key factors of the catalysts stability and redox behavior controlling their performance.


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Supporting Information. XRD analysis of investigated spinels; Pulse O2/CO experiments and conductivity measurements; Size dependent Ellingham diagrams construction details; M3O4/γ-M2O3 system configurational entropy calculations; This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *[email protected], telephone number: +48 12 663 2073 *[email protected], telephone number: +48 12 663 2295

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgment This work was supported by the financial support of Polish National Science Center grant no. DEC-2011/03/B/ST5/01564. The research was carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the


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Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). We thank Dr S. Witkowski for his help in Rietveld analysis, and Dr W. Zajac for help in electric conductivity measurements.

Abbreviations 3D; 3 dimensional, a.u.; arbitrary unit, EELS; Electron Energy Loss Spectroscopy, ESI; Electronic Supporting Information, HAADF; high angle annular dark field, RS; Raman Spectroscopy, SEM; Scanning Electron Microscopy, SI; Spectrum image, STEM; Scanning Transmission Electron Microscopy, TOR; Turnover Rate, TPO; Temperature Programmed Oxidation, TPR; Temperature Programmed Reduction, TPSR; Thermo-Programmed Surface Reaction, VO; oxygen vacancy.


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ACS Catalysis

References

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Thermodynamic Stability, Redox Properties, and Reactivity of Mn3O4

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