The X-ray diffraction patterns of the manganese-alumina samples prepared are included in Figure 1. In addition, in order to analyze potential changes occurring on the alumina support during the preparation of the supported catalysts, the XRD patterns of the γ-Al₂O₃ support calcined at 400 °C and 600 °C have also been included.

The diffraction peaks of γ-Al₂O₃ appear at 2θ = 19.51°, 32.7°, 37.5°, 39.24°, 45.5° and 67.1°, being consistent with the diffractograms reported for this material (JCPDS: 00-046-1215) [17,18]. The XRD patterns of γ-Al₂O₃ are equal after calcinations at 400 °C and 600 °C, evidencing the thermal stability of this support in the range of temperatures analyzed.

The diffraction patterns of MnAlT400 and MnAlT400W present peaks assigned to the γ-Al₂O₃ support along with peaks at 2θ = 12.54° and 25.10°, which can be attributed to birnessite [19]. The expected peaks of potassium manganate were not detected by XRD in MnAlT400, probably due to its high dispersion on the support. However, the presence of potassium manganate was evidenced in a previous study performed with unsupported manganese samples (without alumina) after calcinations of potassium permanganate at 400 °C [6]. The XRD pattern changes significantly for MnAlW600, showing characteristics peaks of criptomelane (JCPDS 44-0141) [20].

Evidence of the presence of potassium manganate in MnAlT400 are the greenish color of the water used in the washing step performed to MnAlT400 and the increase of the intensity of the birnessite peaks of MnAlT400W with regard to those of MnAlT400. In addition, the K/Mn molar ratio determined from Atomic Absorption Spectrometer (AAS) analysis values (data in Table 1), is higher in MnAlT400 than in MnAlT400W, that is, decreases after the washing step due to potassium manganate removal. The SEM images included in Figure 2 also support the formation of potassium manganates upon calcination of γ-Al₂O₃-supported KMnO₄. γ-Al₂O₃ (Figure 2a) consists of quite spherical particles of 100–200 µm, the difference in contrast between the particles in this figure is assigned to “charging effect” on the surface of particles probably because this material is dielectric and the electrons are accumulated on it. These spherical particles are covered by potassium manganate needles of 15–40 µm length (Figure 2b). These needles are removed by water washing, and therefore, are neither observed in MnAlT400W nor in MnAlW600.

Figure 3 shows the FTIR spectra of the manganese-alumina samples together with those of the γ-Al₂O₃ support calcined at 400 °C and 600 °C. The bands between 3800–3680 cm⁻¹ and 2000–1500 cm⁻¹ are assigned to the O-H vibration of adsorbed water and/or to surface hydroxyl groups [21], and those between 980–550 cm⁻¹ are attributed to Al-O bond vibrations [22]. The spectra of γ-Al₂O₃ after calcinations at 400 °C and 600 °C are quite similar to each other, supporting that the γ-Al₂O₃ support is thermally stable in the range of temperatures studied, which is consistent with the XRD conclusions.

As can be observed in Figure 3, the band at 3745 cm⁻¹, assigned to adsorbed water and/or hydroxyl groups on γ-Al₂O₃, disappear upon KMnO₄ impregnation, and the position of those at 3460 and 1643 cm⁻¹ shift with respect to their position on bare alumina. The bands at 1400, 918 and 760 cm⁻¹ are attributed to KMnO₄, and the shift in position of these bands with regard to the typical positions reported for pure KMnO₄ [6,23] evidences a certain interaction between KMnO₄ and γ-Al₂O₃.

In sample MnAlT400, the IR bands below 700 cm⁻¹ are assigned to vibrational modes of Mn-O bonds [24] and the bands at 509 y 422 cm⁻¹ are characteristics of birnessite [25]. The band at around 1019 cm⁻¹, which can be ascribed to Mn=O vibrations, suggests the presence of manganate-type oxides (Mn(VI)Ox) [26]. This band disappears after washing the MnAlT400 sample (see IR spectra of MnAlT400W) due to the removal of this soluble specie. After calcinations at 600 °C of the washed sample (see IR spectra of MnAlTW600) a new band appears at 1022 cm⁻¹, which has been reported for the vibration mode of Mn³⁺–O [27], and finally some studies attribute the bands between 450 and 750 cm⁻¹ to the vibration of Mn–O bonds in cryptomelane [28].

Figure 4 and Figure 5 show the TGA and DSC results, respectively, and Table 2 compiles the weight losses determined from TGA profiles for different temperature ranges. The weight losses observed below 250 °C can be partially attributed to water release and the weight losses occurring above 500 °C are associated with structural changes that imply O₂ delivery [6]. For instance, in γ-Al₂O₃-supported KMnO₄, the endothermic DSC peaks at temperatures below 200 °C are due to the release of physisorbed water, and the exothermic peaks at 288 °C and 537 °C are associated to phase transitions occurring with oxygen release.

The weight loss suffered by γ-Al₂O₃-supported KMnO₄ between 250 and 500 °C is related to the decomposition of KMnO₄, considering that the γ-Al₂O₃ support is stable in the range of temperatures studied, as deduced from XRD and FTIR. This KMnO₄ decomposition is in accordance with previously reported DSC results [6].

The quantitative weight loss values included in Table 2 show that the weight loss suffered until 250 °C by the washed sample (MnAlT400W) is lower than that suffered by the counterpart non-washed sample (MnAlT400), which is attributed to the hygroscopic character of manganates, as it was reported by Kappenstein et al. [29]. The weight loss of MnAlW600 below 250 °C is due to the release of water adsorbed in the porous structure of cryptomelane. The structural changes related with the formation of manganese oxide phases, as determined by XRD and FTIR, are in accordance with the DSC profiles (Figure 5).

In conclusion, the TGA results discussed are consistent with those previously obtained with pure KMnO₄ (without alumina) [6]. Birnessite and potassium manganate are formed after KMnO₄ calcination at 400 °C, and transformation to cryptomelane occurs after potassium manganate removal by water washing and calcination at 600 °C.

N₂ adsorption–desorption isotherms are shown in Figure 6, and different parameters obtained from these isotherms are compiled in Table 3. All samples present type IV isotherms, according to IUPAC classification [30], with considerable N₂ adsorption at low relative pressures, which is indicative of microporosity [31], and hysteresis cycles. These profiles indicate that the materials present also a mesoporous structure and them shown capillary condensation above P/Po 0.3. According to the De Boer [30,31,32] classification, the hysteresis cycles present contributions of slit–shaped pores (B) and interconnected pores with bottleneck shape (E). The mesopores size distributions determined by the BJH method (inset in Figure 6) show peaks centered between 35.98 Å and 37.86 Å, without significant change among samples.

The micropore size distribution, obtained by the HK method, (Figure 7) is monomodal for MnAlT400, while bimodal for bare γ-Al₂O₃, MnAlT400W and MnAlW600, suggesting the presence of primary and secondary micropores [33]. These results evidence that potassium manganate on MnAlT400 blocks part of the γ-Al₂O₃ microporosity, in agreement with results reported by other authors [34], and the closed micropores becomes again open after removal of the soluble salts by water washing. The BET surface areas included in Table 3 are consistent with this conclusion. The BET surface area of γ-Al₂O₃ decreases after KMnO₄ impregnation, and increases again after the washing step (sample MnAlT400W).

H₂ reduction profiles for all samples prepared are included in Figure 8. According with the description of several authors [35,36,37], the reduction of supported manganese (IV) oxide occurs in several consecutive steps. The lower temperature peaks are attributed to the reduction of Mn⁴⁺ to Mn₃O₄ following the reactions sequence: MnO₂→Mn₂O₃→Mn₃O₄. The reduction step taking place at higher temperatures is attributed to Mn₃O₄ reduction to MnO.

The H₂ reduction profile obtained with MnAlW600 shows two main reduction peaks at 400 °C and 434 °C, which could indicate that the reduction proceeds following the sequence of steps KMn₈O₁₆ (cryptomelane)→Mn₃O₄→MnO. A similar H₂ reduction profile is obtained with MnAlT400W, but the temperatures of the main peaks are shifted to lower temperatures. The intensity of the second peak is always higher than that of the first one in both samples. On the contrary, the H₂ reduction profile of MnAlT400 (non-washed sample) differs from those of the washed samples (MnAlW600 and MnAlT400W) due to the presence of salts with manganese cations in oxidation states higher than +4. In conclusion, the H₂ reduction experiments suggest the presence of Mn cations with oxidation state near +4, as expected for birnessite and cryptomelane.

XPS characterization was used in order to obtain additional information about the oxidation state of manganese on the different samples by using a method reported elsewhere [6]. Figure 9 plots the formal oxidation state of manganese for three different commercial manganese oxides (MnO, Mn₃O₄ and MnO₂, by Aldrich) with regard to the difference between the binding energy of the Mn 2p^3/2 and Mn 2p^1/2 peaks measured experimentally by XPS with each oxide (+2, +2.7 and +4, respectively). These reference values allow us to drawn a calibration line, and thus, the formal oxidation state of manganese in MnAlT400, MnAlT400W and MnAlW600 can be estimated by introducing in this trend line the XPS values calculated by difference between the corresponding binding energies of the Mn 2p^3/2 and Mn 2p^1/2 transitions. The formal oxidation states estimated are +4.3, +3.5 and +3.7 for MnAlT400, MnAlT400W and MnAlW600, respectively. The value obtained with the unwashed sample (MnAlT400; +4.3) is slightly above +4, which is consistent with the presence of manganate salts (in addition to birnessite) where the oxidation state of manganese is +6. The values obtained with the washed samples are between +3 and +4 because the manganates have been removed and only the birnessite and/or cryptomelane phases remain on the samples. As expected, the formal oxidation state obtained with the sample calcined at 600 °C (MnAlW600; +3.7) is slightly higher to that obtained with the sample calcined at 400 °C (MnAlW600; +3.5). These XPS results are in agreement with the H₂-TPR conclusions and confirm the presence of manganese cations with the oxidation states expected for birnessite and cryptomelane phases, which are the phases identified by XRD and FTIR.

The catalytic combustion of soot was evaluated in O₂/N₂ and NO_x/O₂/N₂. The soot conversion profiles obtained in O₂/N₂, and included in Figure 10, show that only the sample MnAlT400 is able to lower the soot combustion temperature with regard to the uncatalyzed reaction in these reactions conditions. This is attributed to the presence of potassium manganates, that is, this is not a catalytic effect but a redox reaction where potassium manganate oxidises soot.

In experiments performed in NO_x/O₂/N₂ (Figure 11), the lowest soot combustion temperature is also achieved with MnAlT400. However, in the presence of NO_x, the washed samples (MnAlW600 and MnAlT400W) are also able to lower the soot combustion temperature with respect to the uncatalyzed reaction. These results are consistent with those previously obtained with unsupported manganese samples of similar composition. In the previous study [6], it was concluded that the combustion of soot assisted by birnessite and cryptomelane catalysts (once manganese salts have been removed) occurs by the NO₂ mechanism, that is, by acceleration of the NO oxidation to NO₂, which is much more oxidizing than NO and O₂. For comparison, the soot combustion profile obtained with a Pt/Al₂O₃ reference catalyst has been included in Figure 11. Note, that the onset of the soot combustion temperatures obtained with the manganese samples are not very different to that obtained with Pt. Therefore, these oxides are able to accelerate soot combustion and would be a valuable alternative for catalytic combustion of soot, since they are cheaper than Pt catalysts and easy to prepare.

In order to analyze in more detail the reaction mechanism by which these alumina-supported manganese catalysts (MnAlW600 and MnAlT400W) accelerate soot combustion in a NO_x/O₂ gas mixtures, blank experiments (only with the catalysts and without soot) have been performed with the NO_x/O₂/N₂ gas mixture. The NO₂ percentage (produced by NO oxidation) in total NO_x basis is plotted in Figure 12 as a function of temperature. These results confirm that the manganese catalysts prepared in this study are able to oxidize NO to NO₂, and the amount of NO₂ yielded is not very different to that produced by the Pt/Al₂O₃ reference catalyst (the amounts of NO₂ evolved are reported in Figure 12 in mmol/gr). These blank experiments confirm that the catalyzed combustion of soot takes place with the participation of the NO₂-assisted mechanism.

Alumina-supported manganese samples were synthesized by means of the wet impregnation method [38]. γ-Al₂O₃ (Carlo Erba) was impregnated with Commercial KMnO₄ (J.T. Baker, analytical grade) in solution and the solvent was removed at 90 °C. The nominal MnO₄⁻ loading was 4.5 wt%.

Three alumina-supported manganese samples, referred to as MnAlT400, MnAlT400W and MnAlW600, were prepared following different procedures:

In addition, a 1% Pt/γ-Al₂O₃ catalyst was prepared by excess-volume impregnation of γ-Al₂O₃ using Pt(NH₃)₄(NO₃)₂ as precursor. After impregnation, the catalyst is heated in static air at 110 °C for 12 h and calcined at 700 °C for 5 h.

The samples were characterised using the same techniques and the technical specifications reported by Becerra et al. [6] in brief: (i) Powder X-ray diffraction patterns were obtained in a Rigaku MiniFlex II diffractometer; (ii) infrared measurements were carried out using a NICOLET 380 spectrometer with a DTGS detector; (iii) thermogravimetric analysis (TGA) was performed in a thermo balance, by TA Instruments, model TGA Q500 and (iv) differential scanning calorimetric measurements were performed with a TA Instruments equipment, model DSC Q100.

Elemental analysis was performed in an Atomic Absorption Spectrometer (AAS) Perkin Elmer, model 3110. For this purpose, the samples were dissolved (even the alumina support) by a fusion process with lithium metaborate [39]. 50 mg of sample are mixed with 300 mg of lithium metaborate and calcined at 1000 °C for 15 min. The mixture is cooled down to room temperature, and 20 mL of distilled and deionized water and 5 mL of 37% HCl are added. This solution is heated until boiling under continuous stirring, and once the solids are dissolved, 1 mL of 15% lanthanum chloride is added. Finally, the solution is diluted with water for elemental analysis. The Mn and K contents were determined at wavelengths of 279.5 nm and 766.5 nm, respectively.

Scanning Electron Microscopy (SEM) images were recorded with a Jeol JSM 5910LV microscope, operated at 15 kV and 19 kV in high vacuum mode, with Back-Scattered Electrons (BES) and secondary electron imaging (SEI) for image generation.

N₂ adsorption-desorption isotherms were measured at −196 °C in a volumetric set up (Micromeritics, model ASAP 2020). The specific surface area was calculated by the Brunauer-Emmet-Teller (BET) method and porous distribution by Joyner-Hallenda (BJH) and Hovarth-Kawazoe (HK) methods.

XPS analysis was carried out in a VG-Microtech Multilab electron spectrometer using Mg-Kα (1253.6 eV) radiation source. H₂-TPR experiments were performed in a Micromeritics device; model Pulse ChemiSorb 2705, with a TCD detector.

Catalytic tests were performed in a fixed-bed reactor at atmospheric pressure under a gas flow (500 mL/min) containing 5% O₂ balanced with N₂ or 500 ppm NO_x + 5% O₂ balanced with N₂. The model soot used was a carbon black from Degussa S.A. (Printex-U). The experiments consisted of heating the soot-catalyst mixtures from 25 to 750 °C at 10 °C/min. The soot-catalyst mixtures contained 80 mg of catalyst + 20 mg of soot + 300 mg SiC, and were prepared with a spatula following the so-called “loose contact” procedure [40]. Blank experiments were performed only with the catalysts (without soot) under similar experimental conditions. The gas composition was monitored by specific NDIR-UV (Non Dispersive Infrared Ultra-Violet) gas analyzers for NO, NO₂, CO, CO₂ and O₂ and the soot conversion was determined from CO and CO₂ evolved.