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Catalysts 2017, 7(1), 10; https://doi.org/10.3390/catal7010010

Article
N2O Direct Dissociation over MgxCeyCo1−xyCo2O4 Composite Spinel Metal Oxide
1
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
2
Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, China
*
Author to whom correspondence should be addressed.
Academic Editors: Enrique Rodríguez-Castellón, Agustín Bueno-López and Elisa Moretti
Received: 25 November 2016 / Accepted: 23 December 2016 / Published: 1 January 2017

Abstract

:
A series of Mg- and/or Ce-doped Co3O4 (MgxCo1−xCo2O4 CexCo1−xCo2O4, MgxCeyCo1−xyCo2O4) composite spinel metal-oxide catalyst was prepared by a coprecipitation method and evaluated for N2O direct decomposition. The activity measurement results suggest that Mg0.025Ce0.05Co0.925Co2O4 with a Mg/Ce mole ratio of 0.5 exhibited the highest N2O conversion activity, achieving 100% N2O conversion at T = 250 °C (35 vol % N2O balanced by He, gas hourly space velocity (GHSV) = 30,000 h−1). Characterizations using X-ray diffraction (XRD), Brunauer–Emmett–Teller method (BET), hydrogen temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) reveal that there were three main reasons for the excellent catalytic behavior of Mg0.025Ce0.05Co0.925Co2O4: (a) Mg and Ce co-doping could reduce the grain size of composite spinel metal oxide, which thereby significantly increased the BET specific surface area of Mg0.025Ce0.05Co0.925Co2O4 (111.2 g·m2 with respect to that of 32.5 g·m−2 for Co3O4); (b) Mg and Ce co-doping could improve the redox ability of Mg0.025Ce0.05Co0.925Co2O4, including reductions of Co3+ → Co2+ and Co2+ → Co0; and (c) Mg and Ce co-doping not only could improve the migration ability of surface atomic O, but also could increase the concentrations of surface atomic O.
Keywords:
N2O catalytic decomposition; composite spinel metal oxide; MgxCeyCo1−xyCo2O4

1. Introduction

Nitrous oxide (N2O) is one kind of colorless and weakly sweet gas, which was considered to be a nonpolluting gas for a long time, due to its harmlessness to human beings [1,2]. Recently, people have developed a much more profound understanding of N2O: (a) N2O has a global warming potential (GWP) about 310 times higher than that of CO2, which contributes to 6% of the global greenhouse effect [3]; (b) N2O can damage the ozone layer through a cyclic chain reaction system; (c) N2O has a very long atmospheric lifetime, nearly 150 years, as it is hard to decompose. In light of this, N2O emissions in some European countries have been bounded by the regulation of “Kyoto Protocol” [4]. However, the N2O content in the atmosphere is still rising. It has been mainly released from chemical industry processes, for example, adipic acid and nitric acid industry [5]. In addition to that, with the rapid increase of the numbers of vehicles, the N2O emitted from automobile engines also contributes to the increasing content of N2O in the atmosphere. Therefore, N2O emission control has become a global issue to protect our living environment.
Commonly, there are three ways for N2O abatement, including thermal pyrolysis [6], selective catalytic reduction [7,8,9], and direct decomposition [10,11,12,13,14,15,16,17]. The advantage of thermal pyrolysis is that it does not need catalyst. However, the high treatment temperature of ~800 °C not only can cause substantial energy consumption, but can also generate large amounts of NOx, resulting in a second air pollution problem. In addition to that, N2O decomposition (deN2O) efficiency of thermal pyrolysis method is low, which cannot meet increasingly strict legislations. As for the selective catalytic reduction method, its treatment temperature is much lower than that of the thermal pyrolysis method; however, further addition of reductant, hydrocarbons, and ammonia, for example, greatly increases the deN2O expense.
As for the direct decomposition method, it can directly decompose the N2O into N2 and O2 at relatively low temperatures. Moreover, it does not need any additional reductant. Therefore, the direct decomposition method serves as a candidate method for N2O abatement. The catalysts involved in N2O direct decomposition include a noble metal catalyst [10], metal-oxide catalyst [11,12,13,14,15], and zeolite catalyst [16,17,18]. Noble metal catalysts are expensive and can be readily contaminated by the coexistence of NO gas, which greatly hinders their practical application. Zeolite catalysts—especially Fe-ZSM-5, Fe-BEA, and Fe-FER—have been widely investigated for N2O direct decomposition due to their excellent N2O conversion activity; however, relatively low hydrothermal stability constitutes one of their biggest problems, especially during practical applications.
A type of metal-oxide catalyst, cobalt spinel (Co3O4) [11,13,14,15], has been proposed to be a promising candidate for N2O abatement, due to its excellent deN2O activity (achieving 100% N2O at T = 300 °C) and high thermal stability. As reported [14], the N2O decomposition over cobalt spinel is a classic redox reaction, involving N2O molecule activation by electron transfer from a cobalt (II) cation to a N2O molecule (Equation (1)). The produced oxygen species can migrate to the surface of the catalyst and finally recombine with each other, forming an O2 molecule (Equation (2)), which closes the catalytic cycle.
N2O + e → N2 + O
2O → O2 + 2e
According to the literature report, the specific surface area and catalytic activity can be effectively improved by doping Mg into Co3O4 [15]. Meanwhile, ceria (CeO2), which is commonly used as a catalytic support, possesses good oxygen storage capacity and thermal stability, which can also effectively enhance the deN2O catalytic activity of Co3O4 [14]. Therefore, in the present work, a series of single Mg- or Ce-doped Co3O4 (MgxCo1−xCo2O4 or CexCo1−xCo2O4, respectively) and Mg, Ce co-doped Co3O4 (MgxCeyCo1−xyCo2O4) spinel composite metal-oxide catalyst were prepared by a coprecipitation method, which was further evaluated for N2O direct decomposition. The effects of Mg-, Ce-doping were thereafter analyzed by various characterizations using X-ray diffraction (XRD), Brunauer–Emmett–Teller method (BET), hydrogen temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) to give detailed information on the crystal structure, grain size, specific surface area, redox ability, and surface chemical state of the prepared catalyst samples. Through the investigation of present work, a promising N2O direct decomposition candidate with high activity is proposed.

2. Results and Discussion

2.1. XRD and BET Results

XRD was conducted for samples of Mg0.2Co0.8Co2O4, Ce0.05Co0.95Co2O4, and Mg0.025Ce0.05Co0.925Co2O4, in order to investigate the effects of Mg-, Ce-doping on the crystal structure of Co3O4. The related XRD patterns are profiled in Figure 1, wherein the XRD pattern of Co3O4 was taken as a reference. It can be found that all the samples exhibited the characteristic diffraction peaks of the Co3O4 spinel phase (18.9°, 31.3°, 36.8°, 44.8°, 55.7°, 59.6°, 65.2°, JCPDS 80-1541). However, after Mg- and Ce-doping, the diffraction peaks significantly decreased, especially at 36.8°, which reveals that the grain size of Co3O4 decreased after Mg-, Ce-doping. As shown in Table 1, the calculated grain sizes (Scherrer equation of Equation (3)) decreased in the order of Co3O4 > Mg0.2Co0.8Co2O4 > Ce0.05Co0.95Co2O4 > Mg0.025Ce0.05Co0.925Co2O4. Similar findings were also observed by Xue et al. [13] and Stelmachowski et al. [15]. It was explained in [15] that introduction of Mg, Al into Co3O4 spinel could increase Co3O4 spinel lattice constant a, but decreased another lattice constant u (defining the position of oxygen anions in the spinel lattice), which led to final decreasing of the Co–O bond lengths in both the tetrahedral and octahedral units. Therefore, it could be deduced that the decreasing crystallization of prepared Mg-, and/or Ce-doped Co3O4 was due to the special interaction between doped metals (Mg, Ce), with Co located at octahedral and tetrahedral units.
The specific surface areas of Co3O4, Mg0.2Co0.8Co2O4, Ce0.05Co0.95Co2O4, and Mg0.025Ce0.05Co0.925Co2O4 are also listed in Table 1. It can be found that compared with pure Co3O4 (32.5 m2·g−1), a significant increase of the specific surface area was observed for Mg-, Ce-doped Co3O4 spinel metal oxides (111.2 m2·g−1 for Mg0.025Ce0.05Co0.925Co2O4). Meanwhile, the increasing order of BET surface area was consistent with the decreasing order of grain size: Co3O4 < Mg0.2Co0.8Co2O4 < Ce0.05Co0.95Co2O4 < Mg0.025Ce0.05Co0.925Co2O4. This indicates that the Mg- and Ce-doping could reduce the grain size of Co3O4, which consequently resulted in further increases of the related specific surface area.

2.2. H2-TPR

In this section, we describe the influence of Mg-, Ce-doping on the redox ability of Co3O4 (Co3+ → Co2+ and Co2+ → Co0), as studied by H2-TPR, with the results depicted in Figure 2. Two reduction peaks, respectively labeled as PH2-I (250–350 °C) and PH2-II (360–480 °C), were clearly observed for all investigated samples shown in Figure 2. PH2-I represents H2 reduction of Co3O4 to CoO (Co3+ → Co2+) and PH2-II represents H2 reduction of CoO to Co0 (Co2+ → Co0). As noted, there also existed a third reduction peak for the samples of Ce0.05Co0.95Co2O4 and Mg0.025Ce0.05Co0.925Co2O4 at T = 550 °C, which was probably related to the H2 reduction of CeOx [19].
In comparison with the H2 reduction profile of Co3O4 (Figure 2a), a much broader reduction peak located at PH2-II was observed for Mg0.2Co0.8Co2O4 (Figure 2b). This implies that the single Mg-doping could increase the Co2+ content on Mg0.2Co0.8Co2O4. It was reported that Co2+ oxidation by N2O to Co3+ was the rate-determining step during N2O direct dissociation over Co3O4 [20]. Therefore, in the present work, the increasing amount of Co2+ of Mg0.2Co2O4 is correlated with its superior deN2O activity with respect to that of pure Co3O4 (as will be described later). Different from the effect of Mg-doping, Ce-doping resulted in the reduction peak of PH2-II being shifted to much lower temperature (Ce0.05Co0.95Co2O4 of Figure 2c). This finding implies that the single Ce-doping could increase the redox ability of Co2+ (Co2+ → Co0), which can also well explain the much higher deN2O activity of Ce0.05Co0.95Co2O4 versus pure Co3O4. As shown in Figure 2d (Mg0.025Ce0.05Co0.925Co2O4), when Mg and Ce were simultaneously doped into the Co3O4, the reduction peaks of PH2-I and PH2-II all moved to the lowest temperature with respect to other samples. This reveals that the co-doping of Mg and Ce on Co3O4 could greatly improve the redox ability of Co3O4 (both Co3+ → Co2+ and Co2+ → Co0). In addition to that, according to the literature reports [21,22], the reduction of Co3+ → Co2+ was correlated with the oxygen desorption process. The lower the temperature needed for Co3+→ Co2 reduction, the easier it will be to remove the oxygen. Therefore, the H2-TPR of Mg0.025Ce0.05Co0.925Co2O4 could well explain why it shows the highest deN2O activity with respect to other metal-doped Co3O4 catalysts.

2.3. XPS

As discussed in this section, XPS was employed to investigate the surface chemical states of abovementioned the Mg-, Ce-doped Co3O4 composite spinel metal oxides. The related XPS spectra of Co2p, Ce3d, and O1s are shown in Figure 3A–C, respectively. As shown in Figure 3A, the binding energies of Co2p3/2 and Co2p1/2 in pure Co3O4 were located at 779.45 eV and 794.45 eV, respectively, with a Co2p3/2, Co2p1/2 spin-orbital splitting energy difference of 15 eV. After the single Mg-doping (Mg0.2Co0.8Co2O4 in Figure 3A), the related Co2p3/2 and Co 2p1/2 shifted by 0.8 eV to a lower binding energy with respect that of Co3O4; however, the difference in spin-orbital splitting energy remains 15 eV. Similar findings were also found for the samples of Ce0.05Co0.95Co2O4 and Mg0.025Ce0.05Co0.925Co2O4. The related Co2p3/2 and Co2p1/2 shifted to lower binding energies by 1.0 and 1.1 eV, respectively; however, the splitting energy difference remained 15 eV. In general, the decrease in the binding energy of an element means an increase in its outside nucleus electron density, which consequently results in a decreasing of the element valence state. Therefore, the band shift of Co2p to much lower binding energy for Mg-, Ce-doped Co3O4 samples implied that the Co2+ content increased after Mg-, Ce-doping. This finding was also verified by the peak analysis of Co2p3/2, discussed later. As noted, the increasing of Co2+ content means increasing of the O vacancy on the Mg-, Ce-doped samples. Increasing of O vacancy could then facilitate the mobility of atomic O, which was beneficial for the catalytic reaction. Therefore, the Mg0.025Ce0.05Co0.925Co2O4 sample, displaying the largest Co2p band shift (1.1 eV), would be much more active for N2O decomposition, as verified by the activity measurement study.
Figure 3B displays Ce 3d5/2 and Ce 3d3/2 of Ce0.05Co0.95Co2O4 and Mg0.025Ce0.05Co0.925Co2O4. It can be found that there were no obvious changes in the location of Ce3d for these two samples. This finding indicates that there is no obvious difference in Ce valence state on the surface of Ce0.05Co0.95Co2O4 and Mg0.025Ce0.05Co0.925Co2O4.
Figure 3C shows O1s of Mg-, Ce-doped Co3O4 samples, wherein the O1s of Co3O4 was taken as a reference. It can be found that (i) after Mg-, Ce-doping, the O1s of all metal-doped Co3O4 composite metal-oxide catalysts shifted to lower binding energy, with respect to that of pure Co3O4; (ii) Mg0.025Ce0.05Co0.925Co2O4 exhibited the largest O1s band energy shift. As reported [23,24], the decrease of O1s could reduce the interactions between Co and other metal components, which consequently enhance the catalytic activity of Co2+. In light of that, Mg0.025Ce0.05Co0.925Co2O4 displaying the lowest O1s binding energy can also explain its highest deN2O activity.
Table 2 lists the surface element content of the above-discussed Mg-, Ce-doped Co3O4 composite metal-oxide samples, which were measured by XPS. It can be found that after Mg-, Ce-doping, the surface content of atomic Co decreased; however, the content of atomic O increased. This indicates that addition of Mg and Ce into Co3O4 could increase concentration of the surface atomic O. As is well known, the surface atomic O exhibits much higher catalytic activity than that of lattice oxygen, especially for the oxidation–reduction reaction system [25]. As noted, the surface atomic O content of Mg0.025Ce0.05Co2O4 is highest (76.5%, as seen in Table 2) of all, which constitutes another main factor for its best deN2O catalytic performance.
As stated above, the Co2+ content of Me-, Co-doped Co3O4 samples increased due to a Co2p band shift to lower values. In order to determine the Co2+/Co3+ ratio on the surface of prepared Me-, Co-doped Co3O4, peak analyses were conducted for Co2p3/2, as shown in Figure 4. According to integral peak areas of Co2+, Co3+, and total surface Co content, the derived Co2+/Co3+ values were calculated and are listed in Table 2. It can be clearly seen that the Co2+/Co3+ value increased in the order of Co3O4 < Mg0.2Co0.8Co2O4 < Ce0.05Co0.95Co2O4 < Mg0.025Ce0.05Co0.925Co2O4, suggesting that Mg- and Ce-doping can increased the surface content of Co2+, and Mg0.025Ce0.05Co0.925Co2O4 possessed the highest Co2+ content with respect to other samples.

2.4. Activity Measurement

Figure 5 displays the activity measurement result of a series of Mg-doped Co3O4 samples (MgxCo1−xCo2O4, x = 0.2, 0.3, 0.4, 0.5). It can be found that the deN2O activity of MgxCo1−xCo2O4 gradually decreased as Mg-doping increased. Among the samples, Mg0.2Co0.8Co2O4 exhibited the highest deN2O activity, which could achieve 100% N2O conversion at T = 450 °C. Meanwhile, compared with pure Co3O4 (100% N2O conversion at T > 500 °C), much higher deN2O activity was observed for the MgxCo1−xCo2O4 sample, when the Mg-doping amount was below 0.4. Therefore, this activity measurement result suggests that a lower amount of Mg-doping is much more preferable.
Figure 6 depicts the activity measurement result of a series of Ce-doped Co3O4 samples (CexCo1−xCo2O4, x = 0.03, 0.05, 0.1, 0.2). The Ce-doping has an obvious promotion effect on deN2O activity, in comparison with that of Co3O4. Among the samples, Ce0.05Co0.95Co2O4 exhibited the highest N2O conversion activity, which could achieve 100% N2O conversion at T = 350 °C. Therefore, through this activity measurement, the optimum doping amount of Ce was chosen to be 0.05.
Based on the above activity measurement results, it can be concluded that the single Mg- or Ce-doping could efficiently improve N2O activity of the Co3O4 spinel catalyst, where: (i) decreasing Mg-doping content is beneficial for improving deN2O activity; and (ii) the optimum Ce-doping content is 0.05. In light of this, a series of Mg and Ce co-doping of Co3O4 (MgxCeyCo1−xyCo2O4) was further prepared and evaluated for N2O direct decomposition, as shown in Figure 7. Ce-loading was chosen to be 0.05 (y = 0.05), while Mg-loading varied from 0.015 to 0.15 (x = 0.015, 0.025, 0.05, 0.1, 0.15). It can be found that, except for Mg0.15Ce0.05Co0.8Co2O4, MgxCeyCo1−xyCo2O4 samples exhibit excellent deN2O activity, achieving T90 < 300 °C. Among them, Mg0.025Ce0.05Co0.925Co2O4, with a Mg/Ce ratio of 0.5, shows the best N2O conversion activity, which could achieve 100% N2O conversion at T = 250 °C. This indicates that the co-doping of Mg and Ce into Co3O4 could indeed improve the deN2O activity, and the best doping content of these samples was Mg0.025Ce0.05Co0.925Co2O4 with a Mg/Ce mole ratio of 0.5. In addition to that, the long lifetime (30 h) deN2O activity measurement was further conducted for the best-performing Mg0.025Ce0.05Co0.925Co2O4 sample at T = 250 °C, as shown in Figure 8. It can be found that the prepared sample exhibited stable deN2O activity, implying excellent durability of this sample.

2.5. Correlation between Characterization and Activity Measurement Result

In order to investigate Mg-, Ce-doping effects on the catalytic performance of Co3O4, various characterizations, including XRD, BET, H2-TPR, and XPS were conducted for the samples of single metal doping and Mg, Ce co-doping on Co3O4 (Mg0.2Co0.8Co2O4, Ce0.05Co0.95Co2O4). As revealed by XRD and BET, Mg-, Ce-doping could reduce the grain size of Co3O4, which in turn efficiently increases the related BET specific surface area: Co3O4 (32.5 m2·g−1) < Mg0.2Co0.8Co2O4 (77.8 m2·g−1) < Ce0.05Co0.95Co2O4 (106.5 m2·g−1) < Mg0.025Ce0.05Co0.925Co2O4 (111.2 m2·g−1). Therefore, the highest specific surface of Mg0.025Ce0.05Co0.925Co2O4 constitutes one of the important factors that led to the highest deN2O activity. In addition to that, the improved redox abilities of Mg-, Ce-doped Co3O4 samples were observed based on H2-TPR, and these also play an important role during N2O direct dissociation: (i) single Mg-doping could increase Co2+ content on Mg0.2Co0.8Co2O4; (ii) single Ce-doping could improve Co2+ redox ability; (iii) Mg-, Ce-doping could improve both Co3+ and Co2+ redox ability; (iv) compared with other Me-, Ce-doped Co3O4 samples, the highest redox ability of Mg0.2Co0.8Co2O4 is another reason for its showing the highest deN2O activity. The XPS, utilized to investigate the surface atomic component and valence state, suggested that Mg-, Ce-doping did not influence the valence status of Co. However, the O1s suggested that Mg-, Ce-doping could efficiently enhance the mobility as well as content of surface atomic O, which is favorable for N2O dissociation. Therefore, the highest mobility as well as content of surface atomic O on Mg0.2Co0.8Co2O4 constituted another important factor of its displaying the highest deN2O. In summary, the highest BET specific surface area, redox ability, surface atomic O mobility, as well as surface oxygen content of the prepared Mg, Ce co-doped Co3O4 (Mg0.025Ce0.05Co0.925Co2O4) constitute the main reasons for this sample having the highest deN2O activity, which also make this kind of catalyst promising for further practical applications.

3. Experimental

3.1. Preparation of Catalyst

The composite spinel metal oxide catalysts of the present work (MgxCo1−xCo2O4, CexCo1−xCo2O4, and MgxCeyCo1−xyCo2O4) were prepared by a coprecipitation method. Firstly, a certain amount of metal nitrate solutions (magnesium nitrate, cerium nitrate, cobalt nitrate) were mixed together and placed in a 70 °C water bath, which was stirred for 20 min. Then, an appropriate amount of Na2CO3 was dissolved into 20 mL deionized water in a 70 °C water bath, which was stirred for 20 min. The above two solutions were mixed and the pH value was adjusted to 9. After filtration, drying (120 °C for 2 h), and calcination (400 °C for 3 h), the final catalyst samples were obtained.

3.2. Characterization of Catalysts

The XRD patterns of the prepared samples have been recorded in 2θ ranges from 5° to 80°, based on X-ray diffractometer (Rigaku, Tokyo, Japan, D/max2500VB2) and with a radiation of Cu Kα (λ = 1.5406 Å). The crystal phases were confirmed according to the JCPDS reference. The crystallite sizes were calculated according to the Scherrer equation (Equation (3)) [3]. In this formula, K is Scherrer constant; β is the half width of the diffraction peak; and θ is a diffraction angle.
D = K λ β cos θ
The specific surface area (SBET) of the prepared catalyst sample was measured via a Sorptomatic 1990 instrument (Thermo Electron, Waltham, MA, USA) through nitrogen adsorption/desorption at 77 K and was calculated by the BET method. H2-TPR was performed using Thermo Electron TPD/R/O 1100 Series instrument equipped with a thermal conductivity detector (TCD). Before the experiment, samples were in turn pretreated by the following procedures: (1) heated at 100 °C for 1 h in a He (>99.999%) stream; (2) cooled down to 30 °C under above atmosphere; (3) H2-TPR started with a ramp of 10 °C/min from 30 to 750 °C. The flow rate of reducing gas (5.06% H2 balanced by N2) was 30 mL/min. XPS was conducted on a Thermo Fisher ESCALAB 250 system (Thermo Fisher Scientific, Waltham, MA, USA) with Al Kα radiation under ultrahigh vacuum (UHV), calibrated internally by carbon deposit C (1s) having a binding energy (BE) of 284.6 eV.

3.3. Activity Measurement

N2O direct decomposition was conducted in a fixed bed quartz tube reactor (Φ7 × 1 × L300 mm) under atmospheric pressure. Pelletized catalyst (0.2 g) with particle sizes of 250–425 μm was placed in the constant temperature zone of a vertical tubular reactor. Afterwards, a gas mixture consisting of 35 vol % N2O balanced by 65 vol % He was made according to the components of tail gas from related industrial plants as previously described [3] and fed into the reactor under atmospheric pressure. A total flow rate of 140 mL/min was employed for the activity test, corresponding to a GHSV (gas hourly space velocity) of 30,000 h−1. The gaseous products were discontinuously analyzed by gas chromatograph (GC-4000A, Dongxi Co. Ltd., Beijing, China) equipped with a TCD and a TDX-01 column for the separation of N2O, N2, and O2. The N2O conversion was designed as follows:
Conversion   of   N 2 O   ( % )   = 1 [ N 2 O ] [ N 2 O ] + [ N 2 ] × 100 %
where [N2O] and [N2] are the concentration of N2O and N2, respectively, as measured by the chromatograph. Additionally, the outlet gases of the reactor were further analyzed a five-way gas analyzer (SIEMENS VS5067-5D). This analyzer has an infrared optical sensor and is able to detect NO, CO, CO2, O2, and hydrocarbons (HCs). The detection result suggested that N2O was directly decomposed into N2 and O2 without generation of NOx over the investigated samples.

4. Conclusions

In the present work, a series of Mg-, Ce-doped Co3O4 composite spinel metal-oxide catalysts was prepared and evaluated for N2O direct dissociation. It was found that the Mg, Co co-doped Co3O4 with an optimum Mg/Ce mole ratio of 0.5 (Mg0.025Ce0.05Co0.925Co2O4) exhibited the highest N2O conversion activity, which could achieve 100% N2O conversion at T = 250 °C. Various characterizations using XRD, BET, H2-TPR, and XPS were conducted to gain deeper insight into the Mg-, Ce-doping effect. Three main factors were found to contribute to the Mg0.025Ce0.05Co0.925Co2O4 sample obtaining the highest deN2O activity: (a) Mg and Ce co-doping could reduce the grain size of composite spinel metal oxide, resulting in a significant increase of the BET specific surface area from 32.5 g·m−2 of pure Co3O4 to 111.2 g·m−2; (b) Mg and Ce co-doping could improve the redox ability of both Co3+ → Co2+ and Co2+ → Co2+; (c) Mg and Ce co-doping could improve the mobility as well as the concentration of surface atomic O.

Acknowledgments

The authors thank the National Natural Science Foundation of China (No. 21477007, 21407007, and Major Program 91534201), Natural Science Foundation of Jiangsu Province of China (No. BK20140268), the Fundamental Research Funds for the Central Universities (YS1401), BUCT Fund for Disciplines Construction and Development (Project No. XK1504).

Author Contributions

R.Z. and N.L. conceived and designed the experiments; P.C. performed the experiments; N.L. and P.C. analyzed the data; R.Z. and Y. L. contributed reagents/materials/analysis tools; N.L. wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction (XRD) patterns of MgxCeyCo1−xyCo2O4 composite spinel metal oxide: (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4.
Figure 1. X-ray diffraction (XRD) patterns of MgxCeyCo1−xyCo2O4 composite spinel metal oxide: (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4.
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Figure 2. Hydrogen temperature-programmed reduction (H2-TPR) profiles of MgxCeyCo1−xyCo2O4 composite metal oxides: (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4. PH2-I: H2 reduction of Co3O4 to CoO; PH2-II: H2 reduction of CoO to Co0
Figure 2. Hydrogen temperature-programmed reduction (H2-TPR) profiles of MgxCeyCo1−xyCo2O4 composite metal oxides: (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4. PH2-I: H2 reduction of Co3O4 to CoO; PH2-II: H2 reduction of CoO to Co0
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Figure 3. X-ray photoelectron spectroscopy (XPS) spectra of (A) Co2p; (B) Ce3d; and (C) O1s for the composite metal-oxide catalysts: (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4.
Figure 3. X-ray photoelectron spectroscopy (XPS) spectra of (A) Co2p; (B) Ce3d; and (C) O1s for the composite metal-oxide catalysts: (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4.
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Figure 4. Analysis of Co2p3/2 on composite metal-oxide catalysts (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4.
Figure 4. Analysis of Co2p3/2 on composite metal-oxide catalysts (a) Co3O4; (b) Mg0.2Co0.8Co2O4; (c) Ce0.05Co0.95Co2O4; (d) Mg0.025Ce0.05Co0.925Co2O4.
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Figure 5. Activity measurement results of N2O direct dissociation over MgxCo1−xCo2O4 (x = 0.2, 0.3, 0.4, 0.5) composite spinel metal-oxide catalysts. Reaction conditions: N2O:He = 35:65, gas hourly space velocity (GHSV) = 30,000 h−1; (a) Mg0.2Co0.8Co2O4; (b) Mg0.3Co0.7Co2O4; (c) Mg0.4Co0.6Co2O4; (d) Mg0.5Co0.5Co2O4; (e) Co3O4.
Figure 5. Activity measurement results of N2O direct dissociation over MgxCo1−xCo2O4 (x = 0.2, 0.3, 0.4, 0.5) composite spinel metal-oxide catalysts. Reaction conditions: N2O:He = 35:65, gas hourly space velocity (GHSV) = 30,000 h−1; (a) Mg0.2Co0.8Co2O4; (b) Mg0.3Co0.7Co2O4; (c) Mg0.4Co0.6Co2O4; (d) Mg0.5Co0.5Co2O4; (e) Co3O4.
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Figure 6. Activity measurement results of N2O direct dissociation over CexC1−xCo2O4 (x = 0.03, 0.05, 0.1, 0.2) composite spinel metal-oxide catalysts. Reaction conditions: N2O:He = 35:65, GHSV = 30,000 h−1; (a) Ce0.05Co0.95Co2O4; (b) Ce0.03Co0.97Co2O4; (c) Ce0.1Co0.9Co2O4; (d) Ce0.2Co0.8Co2O4; (e) Co3O4.
Figure 6. Activity measurement results of N2O direct dissociation over CexC1−xCo2O4 (x = 0.03, 0.05, 0.1, 0.2) composite spinel metal-oxide catalysts. Reaction conditions: N2O:He = 35:65, GHSV = 30,000 h−1; (a) Ce0.05Co0.95Co2O4; (b) Ce0.03Co0.97Co2O4; (c) Ce0.1Co0.9Co2O4; (d) Ce0.2Co0.8Co2O4; (e) Co3O4.
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Figure 7. Activity measurement results of N2O direct dissociation over MgxCeyCo1−xyCo2O4 (x = 0.015, 0.025, 0.05, 0.1, 0.15; y = 0.05) composite spinel metal-oxide catalysts. Reaction conditions: N2O:He = 35:65, GHSV = 30,000 h−1; (a) Mg0.015Ce0.05Co0.935Co2O4; (b) Mg0.025Ce0.05Co0.925Co2O4; (c) Mg0.05Ce0.05Co0.9Co2O4; (d) Mg0.1Ce0.05Co0.85Co2O4; (e) Mg0.15Ce0.05Co0.8Co2O4; (f) Co3O4.
Figure 7. Activity measurement results of N2O direct dissociation over MgxCeyCo1−xyCo2O4 (x = 0.015, 0.025, 0.05, 0.1, 0.15; y = 0.05) composite spinel metal-oxide catalysts. Reaction conditions: N2O:He = 35:65, GHSV = 30,000 h−1; (a) Mg0.015Ce0.05Co0.935Co2O4; (b) Mg0.025Ce0.05Co0.925Co2O4; (c) Mg0.05Ce0.05Co0.9Co2O4; (d) Mg0.1Ce0.05Co0.85Co2O4; (e) Mg0.15Ce0.05Co0.8Co2O4; (f) Co3O4.
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Figure 8. Long lifetime (30 h) activity measurement of N2O direct dissociation over Mg0.025Ce0.05Co0.925Co2O4 at T = 250 °C. Reaction conditions: N2O:He = 35:65, GHSV = 30,000 h−1.
Figure 8. Long lifetime (30 h) activity measurement of N2O direct dissociation over Mg0.025Ce0.05Co0.925Co2O4 at T = 250 °C. Reaction conditions: N2O:He = 35:65, GHSV = 30,000 h−1.
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Table 1. Brunauer–Emmett–Teller (BET) specific surface area and crystal size of the catalysts.
Table 1. Brunauer–Emmett–Teller (BET) specific surface area and crystal size of the catalysts.
CatalystsBET Specific Surface Area (m2·g–1)Crystal Size (nm)
Co3O432.522.4
Mg0.2Co0.8Co2O477.814.6
Ce0.05Co0.95Co2O4106.511.8
Mg0.025Ce0.05Co0.925Co2O4111.210.1
Table 2. XPS surface atomic percentage.
Table 2. XPS surface atomic percentage.
SampleSurface Atomic Score Levels (%)Co2+/Co3+
CoMgCeO
Co3O435.60064.40.65
Mg0.2Co0.8Co2O432.72.3065.10.76
Ce0.05Co0.95Co2O417.607.474.90.91
Mg0.025Ce0.05Co0.925Co2O417.52.43.776.51.11
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