Identification of active oxygen species for soot combustion on LaMnO₃ perovskite

Abstract

Active oxygen species for soot combustion with O₂ on LaMnO₃ were identified as O₂²⁻, O₂ⁿ⁻ (1 < n < 2) and O₂^m− (0 < m < 1) for the first time using in situ Raman spectroscopy during temperature-programmed oxidation (TPO) from room temperature to higher temperatures.

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Active oxygen species play a determining role in understanding catalytic combustion on metal oxides but are very difficult to detect. In the case of diesel soot combustion, which is used to control the emission of soot from diesel engines, superoxide (O₂⁻) has been observed on CeO₂-based oxides using in situ electron spin resonance (ESR) spectra at −196 °C.¹ However, diamagnetic species such as O₂²⁻ are not EPR-active. Furthermore, in situ EPR measurements are difficult to implement and give low signals at higher temperatures.^1bIn situ infrared spectroscopy is another useful and widely used technique. However, both metal oxide catalysts, such as perovskites, and reactant soot are black in color with high opacity. In contrast, in situ Raman spectroscopy is an excellent tool to study surface intermediates in reaction conditions. Sullivan et al. found a peak at 2100 cm⁻¹, which is assigned to Ce³⁺–CO intermediates, using in situ Raman measurements for a soot–Ce_0.8Zr_0.2O₂ tight-contact mixture subjected to a flow of He at 500 °C.^1c However, the declared peak at 2100 cm⁻¹ is too hard to be identified.

The ABO₃-type perovskite, where A is a rare-earth element and B is a transition metal, is an important catalyst for diesel soot combustion in view of its full oxidation activity, hydrothermal stability and favorable costs.² Among them, LaMnO₃ is proposed to be one of the most active catalysts.³ Unfortunately, the active oxygen species related to soot combustion on perovskites are still ambiguous. Fino et al. pointed out that the weakly chemisorbed oxygen on the surface plays a decisive role for LaCrO₃-based perovskites.⁴ Contrarily, bulk oxygen was also deduced to be responsible for the activity of LaCoO₃-based perovskites by the same group.⁵ Li et al. ascribed the active oxygen species for soot combustion on LaCoO₃-based perovskites to the surface lattice oxygen.⁶ Bassou et al. suggests that the key parameters for high activity oxide-based catalysts are associated with the presence of active and mobile surface oxygen species while high oxygen storage capacity and bulk oxygen ion mobility are not necessary.^3c No direct spectroscopic evidence of active oxygen species is given in these studies.

In this communication, soot combustion on LaMnO₃ was studied by temperature-programmed oxidation (TPO) reactions and isothermal reactions. The active oxygen species were characterized by temperature-programmed desorption (TPD) of O₂ and temperature-programmed reduction (TPR) of soot (the true reactant). The in situ Raman spectroscopy shows the presence of O₂²⁻, O₂ⁿ⁻ (1 < n < 2) and O₂^m− (0 < m < 1), which is the first case of direct spectroscopic detection of active oxygen species during soot combustion on perovskites. Moreover, the active oxygen was quantified using isothermal anaerobic titration reactions with soot as the probe molecule.⁷

The XRD pattern (Fig. S1, ESI†) shows a single-phase rhombohedral structure for the fresh sample after calcination at 700 °C for 5 h. The unit cell parameter, crystallite size, oxygen nonstoichiometry (δ) and surface area are summarized in Table S1 (ESI†). According to the oxidation state of Mn obtained via titration (See the Experimental section in the ESI†) and the principle of electroneutrality, the amount of oxygen nonstoichiometry (δ) is estimated to be 0.16, which indicates that LaMnO₃ is oxidatively nonstoichiometric (i.e., there is over-stoichiometric oxygen or cationic vacancies in LaMnO₃).⁸ The Brunauer–Emmett–Teller (BET) surface area is low, suggesting high amounts of sintering of the particles (Fig. S2, ESI†).

The O₂-TPD profile (Fig. 1) shows that LaMnO₃ desorbs a large amount of oxygen between 175–300 °C and from about 500 °C.⁹ The O desorption at the lower temperature is much weaker compared with that at the higher temperature, which can be attributed to weakly bonded surface oxygen and lattice oxygen, respectively. The XRD pattern (Fig. 1 inset) shows that the perovskite structure has changed from rhombohedral to orthorhombic after O₂-TPD.

Fig. 1 O₂-TPD profile of LaMnO₃ after pretreatment at 700 °C in O₂ for 1 h and the XRD pattern after O₂-TPD (inset).

The activity was first checked via TPO reactions (Fig. 2). LaMnO₃ decreases T₁₀ (the temperature at which 10% of the soot is converted) from 475 °C for noncatalytic combustion to 367 °C. Regarding the CO₂ selectivity, the noncatalytic combustion is only 69.3% while LaMnO₃ yields nearly 100% CO₂. Secondly, soot-TPR (Fig. 3) shows that a smaller amount of CO_x was formed in comparison with the amount in the presence of gas-phase O₂ (Fig. 2). The orthorhombic perovskite structure after soot-TPR (Fig. 3 inset) is similar to that after O₂-TPD. Furthermore, the rhombohedral perovskite can be restored after re-oxidation (Fig. 3 inset).

Fig. 2 TPO patterns of CO_x for soot combustion with O₂ under a tight-contact condition between the soot and the catalyst. The weight ratio of soot : LaMnO₃ is 1 : 9. Pure soot was also given for reference. The carbon mass balance is between 90 and 100%. The inset is the enlarged section of the TPO pattern for LaMnO₃ at 100–300 °C.

Fig. 3 Soot-TPR profiles and the corresponding XRD patterns after soot-TPR from room temperature to 850 °C and after re-oxidation at 700 °C with O₂ (inset). The weight ratio of soot : LaMnO₃ is 1 : 9.

Both the TPO and soot-TPR reactions suggest that the role of LaMnO₃ is expected to provide active oxygen to soot at a temperature lower than that of the direct utilization of gas-phase O₂ by soot particles. To elucidate the nature of the active oxygen species, in situ Raman spectra of the soot–LaMnO₃ tight-contact mixture with heating from room temperature to 450 °C in static air (lying in between the flowing air and the inert atmosphere) were performed (Fig. 4). For reference, in situ Raman measurements of LaMnO₃ without soot were also conducted under the same conditions (Fig. S3, ESI†). No band was observed for LaMnO₃ at room temperature (blue line in Fig. 4) and during the increasing temperatures (Fig. S3, ESI†). The Raman spectrum of soot (red line) is similar to that in the literature.¹⁰ Significantly, four new bands at 795, 913, 1070 and 1400 cm⁻¹ were observed for the soot–LaMnO₃ mixture from about 150 °C. The former two peaks can be assigned to O₂²⁻,¹¹ whereas the latter two were due to O₂ⁿ⁻ (1 < n < 2)¹² and O₂^m− (0 < m < 1),¹³ respectively. According to the relative intensity of the peaks, the percentage of active oxygen is in the sequence of O₂²⁻ ≫ O₂ⁿ⁻ > O₂^m− on LaMnO₃. After the reaction, soot combustion was complete.

Fig. 4 In situ Raman spectra of soot combustion in static air (the weight ratio of soot : LaMnO₃ is 1 : 20). (a) room temperature, (b) 150 °C, (c) 200 °C, (d) 250 °C, (e) 300°C, (f) 350 °C, (g) 400 °C, (h) 450 °C for 1 min, (i) 450 °C for 10 min, (j) 450 °C for 15 min, (k) 450 °C for 20 min, and (l) 450 °C for 120 min, (m) 450 °C for 130 min. Blue line: LaMnO₃; Red line: soot.

It is assumed that oxygen, upon adsorption from the atmosphere onto the catalyst surface, can accept electrons one by one in order to be activated:¹⁴

O₂ → O₂⁻ → O₂²⁻ (1)

The present results demonstrate the high reactivity of O₂²⁻, O₂ⁿ⁻ and O₂^m− towards soot oxidation even at a low temperature such as 150 °C, as shown in the Fig. 2 inset and Fig. 4.^1a Combined with the results of the O₂-TPD, TPO and soot-TPR, these weakly bonded active oxygen species are obtained either from the gas phase in the presence of O₂, as in the case of the TPO reactions, or from the lattice in the absence of gas-phase O₂, for instance in soot-TPR. The O₂ desorption above 500 °C demonstrated the transformation from lattice oxygen to active oxygen. Furthermore, the same structure change after O₂-TPD and soot-TPR confirmed the participation of lattice oxygen. Aneggi et al. made a similar conclusion for CeO₂-based oxides though no information on active oxygen was given.¹⁵ Hueso et al. reported that pervoskites can even react with soot at room temperature during the preparation of the catalyst and soot mixture simply by grinding. However, only an ambiguous concept of low-coordination oxygen species was provided, based on the O 1s X-ray photoemission spectroscopy.¹⁶ In our work, the active oxygen species were definitely identified as O₂²⁻, O₂ⁿ⁻ and O₂^m− using in situ Raman at 150–450 °C. Dai et al. reported similar phenomena on SrCl₂/Ln₂O₃ (Ln represents Sm and Nd) for ethane-selective oxidation to ethene even at 700 °C.¹⁷ These findings are exciting because it is generally accepted that superoxides and peroxides are able to be detected by ESR and/or Raman spectroscopy only at low temperatures.^1a

Finally, the active oxygen was quantified using isothermal anaerobic titrations with soot as a probe molecule.^7a As measured in the ESI, the amount of active oxygen is 3.7 × 10⁻⁵ mol g⁻¹. Based on a kinetic reaction rate of 8.3 × 10⁻⁸ mol s⁻¹ g⁻¹ at 280 °C, the turnover frequency (TOF) is calculated to be 2.24 × 10⁻³ s⁻¹, which is comparable to that for La_1−xK_xCoO₃ (x = 0–0.3) (2.06–2.17 × 10⁻³ s⁻¹) obtained at 320 °C ,¹⁸ suggesting that the quantification result is reliable.

In summary, the active oxygen species for soot combustion on LaMnO₃ were identified as O₂²⁻, O₂ⁿ⁻ (1 < n < 2) and O₂^m− (0 < m < 1) using in situ Raman spectroscopy at high temperatures. Furthermore, the amount of active oxygen can be measured by isothermal anaerobic titrations with soot as the probe molecule.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21077043, 21007019 and 21107030).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, structure and textural properties, XRD, TEM, in situ Raman spectra of LaMnO₃ in static air and the calculation of the active oxygen amount. See DOI: 10.1039/c2cy20353g

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