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Activity tests

The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of diesel particulate matter with the catalysts are presented in Fig. 1. It is obvious that the mass loss rate peaks of the soluble organic fraction (SOF), soot precursor, and dry soot, shifted significantly to lower temperatures with the doping of K. The introduction of K significantly lowered the oxidation temperature of diesel exhaust particulates compared with the catalytic activity of Ce0.5Mn0.5O2. The catalyst K0.2–Ce0.5Mn0.5O2 (z = 0.2) displayed the best catalytic effect on diesel soot oxidation.

Figure 1

TG and DTG curves of diesel particulate matter with catalysts.

The weight loss characteristics of diesel particulate matter in the presence of various catalysts are shown in Table 2. It is noticeable in the table that K doping reduced the oxidation temperature of SOF. Specifically, with increasing K content, the SOF oxidation temperature decreased by 11, 19, and 18 °C, respectively. This indicates that the ability of the Ce–Mn solid solution to oxidize diesel particulate matter at low temperatures was improved by K doping. In the intermediate temperature range, the oxidation temperature of the soot precursor decreased by 15, 25, and 23 °C compared with the Ce0.5Mn0.5O2-catalyzed reaction, with K0.1–Ce0.5Mn0.5O2, K0.2–Ce0.5Mn0.5O2, and K0.3–Ce0.5Mn0.5O2 as the catalyst, respectively. As indicated by the data, K0.2–Ce0.5Mn0.5O2 displayed better relative activity for the catalytic oxidation of soot precursor. A similar trend was also observed in the high temperature range. K0.1–Ce0.5Mn0.5O2, K0.2–Ce0.5Mn0.5O2, and K0.3–Ce0.5Mn0.5O2 reduced the ignition temperature of dry soot by 17, 28, and 26 °C, respectively, compared with the value obtained using Ce0.5Mn0.5O2. Meanwhile, the maximum peak combustion temperature of dry soot was also lowered by 45, 61, and 57 °C, upon K doping (z = 0.1, 0.2, and 0.3, respectively). Therefore, K doping significantly enhanced the catalytic activity of the Ce–Mn solid solution for the oxidation of particulate matter, and lowered the temperature required for soot oxidation.

Table 2 Weight loss characteristics of diesel particulate matter.

The Coats–Redfern integral was determined for the catalyzed diesel particulates, and the linear fitting curves of ln[− ln(1 − α)/T2] versus 1/T are shown in Fig. 2. The fitting curves showed excellent linear regression, with the goodness of fit (R2) exceeding 0.99, indicating high accuracy of the fitting results. The activation energy and pre-exponential factor of each reactant can be obtained by calculating the reaction curve equation of each reactant with Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2. The calculation results are listed in Table 3. Doping with K led to a decreasing trend in reaction activation energy, implying that the energy required for catalytic oxidation of soot was lower with K-doped catalyst, which resulted in easier soot oxidation. The minimum observed activation energy of 27.46 kJ/mol was achieved for the reaction catalyzed by K0.2–Ce0.5Mn0.5O2, which is about 20 kJ/mol lower than that reported in relevant literature33,34. Moreover, the pre-exponential factor of soot oxidation was found to increase significantly with K-doped Ce0.5Mn0.5O2 as the catalyst. A larger pre-exponential factor represents more effective collisions between the catalyst and soot during the reaction process, which would facilitate soot oxidation.

Figure 2
figure2

Fitting curves of ln[− ln(1 − α)/T2] and of 1/T particles under catalysis.

Table 3 Effect of Kz–Ce0.5Mn0.5O2 catalyst on activation energy and pre-exponential factor.

Catalyst characterization

Some typical characterizations had been taken to investigate the mechanism of soot catalytic oxidation by Kz–Ce0.5Mn0.5O2 catalysts on the microstructure level. The XRD profiles of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 (z = 0.1, 0.2, and 0.3) are shown in Fig. 3. Compared with the standard XRD profile of pure CeO2 (JCPD 34-0394), the K-doped catalysts exhibited a typical fluorite structure, with the diffraction peaks of CeO2 locating at 28.5°, 33.1°, 47.5°, 56.3°, 59.0°, 69.6°, and 76.9°. The fluorite structure of the material was therefore not altered by K doping. However, the diffraction peaks of the K-doped catalysts shifted to lower angles. This is because the antifluorite structure of K2O enabled the formation of coordinating tetrahedra between K and Ce, with K partially entering Ce0.5Mn0.5O2 to form a solid solution. Since the ion radius of K+ (0.133 nm) is larger than that of Ce4+ (0.094 nm), Ce3+ (0.103 nm), Mn3+ (0.065 nm), and Mn4+ (0.053 nm), the K ions doped into the lattice induced lattice expansion and enlarged the unit cells in the fluorite structure, leading to the diffraction peaks shifting. In addition, Ce4+ was partially substituted by K+ during the doping process, which was accompanied by the transition of electrons between ions, generating oxygen vacancies.

Figure 3
figure3

XRD profiles of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 catalysts.

Figure 3 also indicates that upon K doping, the characteristic peaks of KNO3 appeared at 2θ = 23.5° and 41.8°. With the continuous increase of K content, a peak at 2θ = 33.8°, which is also attributed to KNO3, was observed for K0.3–Ce0.5Mn0.5O2. Meanwhile, the characteristic diffraction peak at 2θ = 15.5° of a new compound K2Mn4O8 also appeared for catalysts with higher K content. The K2Mn4O8 phase was absent and the peak positions of the Mn oxide phase were shifted to lower angles for the catalyst with low K content (K0.1–Ce0.5Mn0.5O2). The formation of K2Mn4O8 at elevated K content is attributed to the fact that in addition to forming the tetrahedral structure with Ce, K also combines with Mn oxides and dissolves in the solid solution. Therefore, at higher K concentrations, in addition to the K species covering the surface of the solid solution, residual K combined with the Mn oxide to form a new K2Mn4O8 phase.

The redox capacity is an important indicator of the catalytic performance of a catalyst, particularly in cases where the catalyst is applied for catalytic oxidation of diesel particulate matter. H2-TPR is an effective technique that reflects the reducing ability of a catalyst. Comparing the H2-TPR profiles of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 (Fig. 4) revealed that the catalytic peak positions varied depending on the amount of K doped. The reactive oxygen species present can be characterized using the temperatures corresponding to the reduction peaks, and the amount of reactive oxygen species represents the catalytic ability of the catalyst in the oxidation of particulate matter. In our previous work, Ce0.5Mn0.5O2 displayed relatively good reduction peak positions in the range of 100–500 °C, indicating high activity, which was a result of the conversion and electron transition between Mn4+/Mn3+ and Ce4+/Ce3+ ion pairs32. The peak positions of the H2-TPR profile varied with increasing K content (Fig. 4). Compared with the peak positions of Ce0.5Mn0.5O2 at 240 and 381 °C, the corresponding peak temperatures of K0.1–Ce0.5Mn0.5O2 dropped to 226 and 366 °C, respectively. Therefore, K doping resulted in the shift of the reduction peaks (< 400 °C) to lower temperatures.

Figure 4
figure4

H2-TPR curves of Kz–Ce0.5Mn0.5O2 catalysts.

The K+ ions doped into the material substituted some of the Ce4+ ions, which caused greater conversion of Mn3+ to the higher valence state (Mn4+), leading to a gradually increasing Mn4+/Mn3+ ratio with higher K+ content. Consequently, the surface charge of the catalyst became unbalanced, and the mobility of lattice oxygen was enhanced. The generation of oxygen vacancies then enhanced the adsorption of reactive oxygen species by the catalyst35. The H2 reduction peaks indicated that the K+ ions remained relatively stable without going through valence state conversion. Higher degrees of K doping led to a mounting in the amount of oxygen species adsorbed by the catalyst and significantly improved oxygen mobility. It is speculated that for the K-doped catalysts the active oxygen species on K sites could spill over to the soot surface, and react with the free carbon sites to form ketene species with C=C=O structure; the active oxygen species at K sites were then continuously supplied with gaseous oxygen via mobile lattice oxygen until the particulate matter was fully oxidized.

O2-TPD is a technique that indicates the activity of a catalyst and its selectivity for reactant molecules. Generally, there are three typical kinds of oxygen species on the surface of Ce-based composite catalysts, including adsorbed molecular oxygen (O2) that desorbs in the low temperature region, adsorbed atomic oxygen (O) that desorbs in the intermediate temperature region, and lattice oxygen (O2−) that desorbs in the high temperature region. The O2-TPD profiles of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 catalysts are presented in Fig. 5. The observation of weak low-temperature desorption taken for all of the catalysts, indicating a low proportion of adsorbed molecular oxygen (O2). K doping diversified the flow mode of oxygen species, and supplemented the cubic fluorite structure defects caused by Mn entering the CeO2 lattice, which facilitated the migration of oxygen species, the conversion of lattice oxygen to adsorbed oxygen, and the adsorption of surrounding oxygen molecules.

Figure 5
figure5

O2-TPD profiles of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 catalysts.

As shown in Fig. 5, K0.1–Ce0.5Mn0.5O2 (K content of z = 0.1) showed a significantly higher desorption peak intensity in the intermediate temperature region, indicating enhanced oxygen species activity and an increased amount of adsorbed atomic oxygen (O). The adsorbed atomic oxygen (O) performs a dominant part in the catalytic oxidation of diesel soot. However, the corresponding desorption temperature in the intermediate temperature region increased for K0.1–Ce0.5Mn0.5O2 compared with Ce0.5Mn0.5O2. This is thought to be caused by the improved stability among atoms induced by the small amount of K during the calcination of the catalyst, which inhibited the desorption capability of the adsorbed oxygen species. The oxygen desorption intensity was significantly enhanced with further increase of the K content. When z = 0.2, the adsorbed atomic oxygen (O) actively desorbed, displaying a distinct oxygen desorption peak at ~ 480 °C. In addition, the lattice oxygen desorption at 700 °C was also intensified.

The Raman spectra of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 catalysts are shown in Fig. 6. Ce0.5Mn0.5O2 exhibited a typical cubic fluorite structure, indicated by a vibration peak (447/cm), which was attributed to the typical F2g vibration of the CeO2 cubic fluorite structure. The peak shifted slightly upon K doping, however, the overall cubic fluorite structure of CeO2 was not altered, which is accordant with the XRD results. In our previous work we reported that the vibration peak at 641/cm was caused by Mn entering the CeO2 lattice. Characteristic peaks at the same position were also observed for Kz–Ce0.5Mn0.5O2 catalysts, which were attributed to the vibration of Mn–O. However, the peaks were broader and showed a slight red shift. This observation suggests the existence of a small amount of MnOx in the Kz–Ce0.5Mn0.5O2 catalysts, and that K doping led to the variation of the structural valence between Mn and Ce, which subsequently resulted in a change in the amount of oxygen vacancies, facilitating the migration of oxygen species on the catalyst surface and promoting the catalytic combustion of soot.

Figure 6
figure6

Raman spectra of Ce0.5Mn0.5O2 and Kz–Ce0.5Mn0.5O2 catalysts.

Table 4 exhibits the information of the three CeO2 samples, which are surface area, pore volume and average pore diameter. The specific surface area of the sample was calculated using BET (Brunauer–Emmett–Teller) method, and the pore volume and aperture were calculated by the isotherm adsorption branch using BJH (Barrett–Joyner–Halenda) model, where the pore volume was calculated using the adsorption volume at the relative pressure p/p0 = 0.99. It can be seen from Table 4 that the surface area of the prepared Kz–Ce0.5Mn0.5O2 is significantly greater than that of Ce0.5Mn0.5O2. The larger surface area indicates more surface active sites per unit mass of Kz–Ce0.5Mn0.5O2. Furthermore, there are more opportunities to achieve a closer contact between the catalysts and the reactants.

Table 4 Surface area, pore volume and pore diameter of CeO2 samples.

In situ FTIR results are shown in Fig. 7. It is found that ketene species may exist in this reaction (1388/cm). At first, the mixture of soot and K0.2–Ce0.5Mn0.5O2 was heated to 430 °C in O2 + He followed by cooling down to 200 °C with purging with He. In this step, some soot was depleted and thus a clear FTIR signal and lots of free carbon sites were obtained. The corresponding spectra are shown in Fig. 7 (20 min), illustrating the presence of chelating bidentate carbonate and ionic carbonate on K0.2–Ce0.5Mn0.5O2. Because diesel engine exhaust contains a large amount of NO, the real exhaust atmosphere is simulated. NO was introduced and switched off when the spectrum did not change significantly. As expected, the band of the ionic nitrite was observed in Fig. 7 (40 min). At this time, free carbon sites and ionic nitrite were abundant on the surfaces of soot and K0.2–Ce0.5Mn0.5O2, respectively.

Figure 7
figure7

In situ FTIR spectra of the mixture of soot and K0.2–Ce0.5Mn0.5O2 after heating at 500 °C.

The mixture was progressively heated up to higher temperatures in He (60 and 80 min). During this period, the band of the ionic nitrite gradually decreases in intensity, simultaneously with the formation of the ketene group. These facts suggest that the ionic nitrite may be consumed with the production of the ketene group. In other words, the ionic nitrite on K0.2–Ce0.5Mn0.5O2 interacts with the free carbon sites on the soot to form the ketene group, which can be described as36:

$$ {text{C}} = {text{C}}^{*} + {text{K}}^{ + } – {text{NO}}_{{2}}^{ – } to {text{C}} = {text{C}} = {text{O}} + {text{NO}} + {text{K}}^{ + } -$$

(1)

The ketene group has been identified as the intermediate of soot oxidation with O2 or NO2, which is a surface oxygen complex formed on the surface of soot with graphite structures. Likewise, the ketene group can serve as the intermediate of soot oxidation with NO. During this process, chelating bidentate carbonate (1,257/cm) and ionic carbonate (1,091/cm) are formed, which have been observed in soot oxidation with O2. These carbonates originate from the adsorption of the produced CO2 on potassium sites.

Catalytic characteristics

The catalytic oxidation of soot using K-doped Ce0.5Mn0.5O2 as the catalyst was analyzed by relating the characterization results indicating variations in structure and surface ions, to the redox capacity of K-doped Ce0.5Mn0.5O2, as shown in Fig. 8. For the K-doped Ce0.5Mn0.5O2 catalysts, the active oxygen species on the K sites spill over to the soot surface and react with the free carbon sites to form ketene species with C=C=O structure. Meanwhile, the K sites enable the supplementation of active oxygen species in the catalyst by activating surrounding gaseous oxygen and enhancing the mobility of lattice oxygen until the soot is fully catalytically oxidized to CO237. The activated oxygen on the surface of K site may overflow to the free carbon site on the soot, forming a carbon–oxygen complex, that is reaction intermediate, ketene group. The K effect is used to supplement the consumed surface oxygen by chemical adsorption and dissociation of gas-phase oxygen or surface lattice oxygen. In the absence of transient reactions, carbothermal reduction and gas phase oxygen, surface lattice oxygen participates in soot combustion. The ketene group is further oxidized to carbon dioxide by other active oxygen, which increases the number of exposed free carbon sites. The selectivity of soot combustion is due to the fact that these free carbon can be directly oxidized into CO by gas phase oxygen. The number of active sites increases with the increasing of K, which will occupy more free carbon sites and avoid combining with gas phase O2 to form CO, resulting in a small increase in CO2 selectivity. What’s more, K can promote the escape of oxygen to soot by forming ketene group.

Figure 8
figure8

Illustration of particle matter combustion with O2 on Kz–Ce0.5Mn0.5O2.



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