NH₃-NO SCR Catalysts for Engine Exhaust Gases Abatement: Replacement of Toxic V₂O₅ with MnO_x to Improve the Environmental Sustainability

In Table 2 the morphological properties of the different catalysts are listed. As shown, the MnO_x exhibited the highest surface area (119.1 m²/g), pore volume (0.36 cm³/g) and mean pore size around 11 nm. The surface area of as received TiO₂ was 56 m²/g, slightly decreasing to 52 m²/g after calcination. No significant changes occurred in the pore volume and mean pore size that remained around 0.27 cm³/g and 22.0 nm, respectively.

The WO₃ loading on TiO₂ produced some changes in the morphology by increasing its porosity (pore volume 0.47 cm³/g, pore size 29.2 nm) and surface area (61.7 m²/g). A similar trend occurred after the impregnation of manganese on the WTi oxide, for which SSA increased until 3wt% of manganese loading, while almost stable pore volume and mean pore size values were registered in comparison with the parent WTi. Such findings suggest high dispersion of amorphous MnO_x up to a loading of 3wt% in good accordance with the absence of crystalline phases for all xMnWTi catalysts, as reported in the XRD results. As far as 12MnWTi the SSA decreased in comparison with 1.5 MnWTi and as well the pore volume and mean pore size decreased, implying that some agglomeration of manganese oxide on the surface of the catalyst may occur (see also XPS characterization).

The reference commercial catalyst, VWTi, has SSA 55.6 m²/g, pore volume 0.21 cm³/g and mean pore size 15 nm [18].

Figure 1 displays the XRD patterns of 12MnWTi, WTi, pure MnO_x and the corresponding ICSD references. Regarding the bare MnO_x, the intense peaks at 2θ = 37.1° – 42.4° – 56.1° – 66.8° were ascribed to MnO₂ mainly as hexagonal phase (ICSD #29,561), though the coexistence of tetragonal phase (ICSD #759,758) cannot be excluded due to quite broad and weak peak around 2θ = 28.5° − 71.8°. In the case of WTi the sharp peaks were attributed to the anatase (ICSD #9854) and rutile (ICSD #9161) phases of titania, while no peaks corresponding tungsten oxide were observed, indicating the highly dispersion of the WO₃ on the surface. Compared with the pattern of WTi, the deposition of Mn did not produce any new crystalline phase, no characteristic peaks of MnO₂ were detected even at high loading (12MnWTi). As far as the VWTi reference, only peaks of the anatase phase were identified [18], being a commercial catalyst likely deposited over a less common titania.

The reducibility of xMnWTi samples was investigated by H₂-TPR experiments. Corresponding analyses were also performed on MnO_x and WTi used as reference materials. The H₂-TPR profiles of MnO_x, WTi, and xMnWTi catalysts are shown in Fig. 2 and the theoretical and experimental H₂ consumption values (mL/g) are listed in Table 3.

As reported in the literature, the MnO_x reduction can occur through two-step process MnO₂→Mn₂O₃→MnO [21, 22] according to the shape of the curve. In fact, two strong peaks at 323 and 462 °C with a slight shoulder at around 170 °C were observed for MnO_x, corresponding to the stepwise reduction above mentioned. Moreover, according with the literature, the shoulder reduction peak can be attributed to reducible small clusters of surface manganese oxide species [21]. Assuming that all manganese is present as MnO₂, the theoretical hydrogen consumption requested for the reduction of Mn⁴⁺ to Mn²⁺ is equal to 281 mL/g (Table 3). Being the experimental consumption equal to 210 mL/g it can be deduced that in the bulk MnO_x the manganese species are present mainly as Mn⁴⁺ along with Mn³⁺, as confirmed by XPS results. In accordance with this result, it was proposed that MnO₂ and Mn₂O₃ are the primary active phases for NO reduction in MnO_x catalysts for SCR reaction, especially MnO₂ [23].

The TiO₂ hydrogen consumption reported in Table 3 refers to the titania oxide that was calcined under the same conditions as for MnWTi catalysts. H₂-TPR of WTi catalyst (Fig. 2) showed an intense peak at 756 °C related to the reduction of W⁶⁺ to W⁴⁺ [24]. The reduction peak at 410 °C corresponds to the surface reduction Ti⁴⁺ → Ti³⁺ while the reduction of bulk titanium occurs at high temperature (over 800 °C) [25‐27].

After the deposition of bare MnO_x on WTi, the reduction peaks associated to MnO_x in 1.5-3-12MnWTi catalysts were shifted progressively at higher temperature indicating that MnO_x strongly interacts with the support and is less easy to reduce. In good accordance with the theoretical and experimental total hydrogen consumption values (see Table 3), manganese was stabilized as Mn³⁺ over WTi oxide, according to XPS data, and completely reduced to Mn²⁺.

X ray Photoelectron Spectroscopy has been used to evaluate the changes occurring upon deposition of manganese and tungsten oxides on the titania surface. Figure 3 shows the survey spectra of MnO_x, WTi and 12MnWTi. Besides the peaks due to C1s, arising by atmospheric contamination, only peaks due to Mn, Ti, W and O are visible on the surface, indicating that no precursor residues are present in the samples. Figure 4 shows the high-resolution region of Mn 2p for MnO_x, 1.5MnWTi and 12MnWTi with the two components of Mn 2p_3/2 and Mn 2p_1/2 separated by 11.5 eV. In Table 4 the binding energies corresponding to Mn 2p_3/2 are reported. Mn 2p_3/2 component for MnO_x can be deconvoluted with two peaks at 641.9 eV and 643.4 eV which can be attributed to Mn (III) and Mn (IV) respectively [28, 29]. The deposition of MnO_x on WTi stabilizes Mn (III) oxidation state as evidenced by the position of the peaks (see Fig. 4; Table 4) in both 1.5 and 12MnWTi. Tungsten position of W 4f_7/2 is, for all samples, indicative of W (VI) [30‐32]. Table 4 shows also the surface atomic ratios. Generally, the ratio measured is higher than the theoretical one, in accord with the fact that both W and Mn have been introduced by impregnation and are therefore localized on the surface. As far as 12MnWTi the surface Mn/Ti ratio is lower than the theoretical one and this is probably due to the high amount of Mn leading to the formation of some agglomerated particles. By looking at W/Ti surface atomic ratio it seems that the deposition of manganese occurs preferentially on titanium, as evidenced by the relative increase of the W/Ti with the increase of MnO_x on the sample.

The surface acidity properties of 1.5-3-12MnWTi and those of the bare MnOx and WTi oxides were investigated by NH₃-TPD analyses. The ammonia desorption curves of all catalysts are shown in Fig. 5 and ammonia uptakes are reported in Table 5. All the catalysts exhibited ammonia desorption within the temperature range of 175–600°C. The temperature of the desorption peak maxima gives indications about the nature and the relative strength of acidic sites, while the area under the ammonia desorption represents the number of surface acidic sites. The acid site density was also calculated (see Table 5) by dividing the ammonia uptake by the catalysts surface area. Among the analysed samples, the WTi was characterized by the highest acidity, conversely the bare MnO_x showed very low acidic behavior.

The so far reported data are in good agreement with our review article [33] where we reported that tungsten plays a considerable role increasing Brӧnsted and Lewis acidic sites. The number of acidic sites and acidic density exhibited the following order MnO_x < 12MnWTi < 3MnWTi < 1.5MnWTi < WTi. As discussed in NH₃-SCR performance section, by increasing manganese loading over WTi, the surface acidity promoted by tungsten oxide decreased and consequently the catalysts exhibited high activity in the low temperature range. Conversely, at low manganese loading the acidity of WTi was scarcely influenced resulting in high NO conversion of catalysts at high temperature. Summarizing, the manganese loading deposited over the surface of WTi played the key role of acidity moderator for the tungsten-based catalyst, influencing the catalytic properties of the MnWTi samples that exhibit tailored NO SCR activity at low, medium or high, temperatures.

Figures 6 and 7 show the NO conversion and N₂ selectivity curves for the prepared catalysts and the commercial reference, VWTi. Pure MnO_x was able to achieve considerable conversion values, above 80–90% at low temperature, in the range 100–200 °C. Then, the oxidation of ammonia was promoted at T > 200 °C and the NO SCR collapsed towards almost no conversion at 300 °C.

WTi exhibited a completely opposite trend showing extremely poor activity at low temperature and NO conversion between ~ 70–95% in a restricted range, 350–400 °C. This finding confirmed previous investigations about the relatively low efficiency of only WO₃ based catalysts [34, 35]. Concerning the xMnWTi samples, it was found that the MnO_x loading strongly influenced their catalytic activity. As shown in Figs. 3 and 6MnWTi was the best performing sample with NO conversion values over 80% up to ~ 98% in a wide temperature range, from 150 to 300 °C. In the case of the1.5MnWTi, the NO conversion started to increase at T > 150 °C achieving 92% at 300–350 °C, while the 12MnWTi catalyst reached almost full conversion at 200 °C, with slightly lower values near 90% at 150 and 250 °C, respectively. Based on the so far reported data it results that, as a function of the manganese content, it is possible to modulate the catalytic performances of the MnWTi catalysts in the range of low, medium or high temperature.

As previously discussed, and in agreement with our review [33], tungsten oxide plays a considerable role in NH₃-NO SCR by increasing Brӧnsted and Lewis acidity and stabilizing against sintering and transition to rutile the anatase phase of TiO₂ that synergically catalyzes the reaction. As reported by Zhu et al. [36] strong Lewis sites are active in the NO SCR at high temperatures (> 350 °C), weak Lewis acid sites contribute at low temperatures (< 200 °C) and Brӧnsted acid sites promote the conversion at medium temperature (250–300 °C). Therefore, it results that the acidity of the MnWTi catalysts increased or decreased as a function of a different Mn loading that affected the NO conversion efficiency in all range of temperature investigated, between 100 and 400 ° C.

Among all the xMnWTi catalysts, 3MnWTi displayed the highest activity in NOx conversion, but an ideal catalyst is expected to have high N₂ selectivity in order to avoid emission of harmful nitrogen oxides as NO₂ and N₂O [37, 38]. As shown in Fig. 7, the selectivity of MnO_x rapidly declined already at very low temperature, being less than 10% at 250 °C, while the WTi showed almost constant values not higher than 80–85% in the range 250–400 °C.

In the between of the above-mentioned catalysts, there are 12MnWTi and 3MnWTi showing a declining selectivity trend in the whole temperature window (100–400 °C). The 1.5MnWTi exhibited high selectivity, more than 90% up to 250 ° C with values between 80 − 70% at 350–400 °C. Therefore, such catalyst represents the best compromise between good catalytic performance in NO_x conversion at high temperature window (250–400 °C) and high selectivity.

The drop in selectivity observed for MnO_x, 3MnWTi and 12MnWTi catalysts was ascribed to the competitive oxidation reaction of ammonia catalysed by manganese, which produces NO, NO₂ and N₂O and consequently decreased the selectivity [5]. In order to confirm the occurrence of such reaction, in Fig. 8 the NH₃, NO, NO₂ and N₂O concentration (ppm) trends as a function of temperature were compared for two catalysts, the worse selective and the best one, respectively, MnO_x and 1.5MnWTi.

In the case of MnO_x, simultaneously with the decrease in the concentration of NO and NH₃ the signal of N₂O appears, already at 50 °C, and increased with the temperature up to a maximum of ~ 600 ppm at 200–250 °C; then above 250 °C the N₂O values decreased together with the increase of NO₂.

entration (ppm) as a function of temperature for MnO_x (a) and 1.5MnWTi (b) catalysts.

As for the 1.5MnWTi, that among the investigated catalyst is the most selective towards N₂ formation, the NO and NH₃ conversion was accompanied by low N₂O formation and almost no NO₂ was detected.

After identifying the best compromise between catalytic activity and N₂ selectivity in the 1.5MnWTi, we focused on comparing NO conversion and N₂ selectivity curves of such catalyst with those of the commercial reference (VWTi) (see Figs. 6 and 7).

It results that although 1.5MnWTi is less selective than the commercial catalyst, especially above 250 °C, it displayed similar or even enhanced (150–200 °C) NO conversion.

Considering the toxicity and low thermal stability of V₂O₅, that is the active phase in the commercial catalyst along with WO₃, the good performances achieved by the 1.5MnWTi represent an important result for the improvement of the environmental sustainability of NH₃-NO SCR devices.