Enhancement of N₂O catalytic decomposition over Ca modified Co₃O₄ catalyst

Abstract

A series of Ca modified Co₃O₄ catalysts with different Ca/Co molar ratios were synthesized by the co-precipitation method and applied to N₂O catalytic decomposition. The experimental results showed that the performance of N₂O catalytic decomposition was obviously enhanced by the addition of Ca into the Co₃O₄ catalyst. The Ca modified Co₃O₄ catalyst with Ca/Co molar ratio of 1 : 2 exhibited the highest catalytic performance and almost 100% N₂O conversion was achieved at 400 °C. The characterization results showed that the addition of suitable calcium composition could promote the growth of the 111 crystal plane of Co₃O₄ and provide abundant surface oxygen on the surface of the catalyst. The kinetics studies confirmed that the activation energy of the Ca modified Co₃O₄ catalyst with Ca/Co molar ratio of 1 : 2 (E_a = 17.84 kJ mol⁻¹) was lower than that of pure Co₃O₄ (E_a = 43.21 kJ mol⁻¹), implying that the addition of Ca into the Co₃O₄ was beneficial to the catalytic decomposition of N₂O.

---

1. Introduction

Nitrous oxide (N₂O) is a trace component of the troposphere with a lifetime of 131 years, and its concentration is increasing by 0.2–0.3% yearly.¹ Nitrous oxide (N₂O) is regarded as a major environmental pollutant since it could largely contribute to the greenhouse effect and the stratospheric ozone destruction.^2,3 N₂O is regarded as the third most important greenhouse gas after CO₂ and CH₄.⁴ Due to its continuously increasing concentrations and long residence time in the atmosphere, the catalytic removal of N₂O attracts extensive attention.^5,6

Various methods were employed for abatement of N₂O emission. Among these approaches, direct catalytic decomposition of N₂O to nitrogen and oxygen was considered as one of the most economical and effective method. Nowadays, various types of catalysts have been used for catalytic decomposition of N₂O, such as noble metals supported systems,^7–9 pure and mixed metal oxides,^10,11 and ion exchanged zeolites.^12–14 Although the noble metal catalysts exhibited excellent catalytic performance, the applications of such catalysts were considerably limited by its high cost. The ion exchanged zeolites catalysts also had many limitations, they could be largely deactivated in the presence of excess oxygen and water steam.¹⁵ However, among the metal oxides, cobalt-based catalysts containing Co₃O₄ spinel presented high catalytic performance.^16–19 Since Co₃O₄ presented relatively high redox character, it was proved to be active for the decomposition of N₂O.²⁰ The N₂O catalytic decomposition activity of Co₃O₄ can be improved by different kinds of modification, such as the partial replacement of Co²⁺ by Ni²⁺, Zn²⁺, Mg²⁺ and Fe²⁺ over Co₃O₄ spinel oxide.^17,19,21 The rare earth (Ce and La) was proposed as the component could promote the catalytic activity of Co₃O₄ spinel by increasing the specific area and redox properties.²² It was reported that the alkali promoter (using K₂CO₃ precursor) showed higher catalytic activities than that of pure Co₃O₄, since potassium can facilitate the activation of N₂O and promote the recombination of surface oxygen.²³ However, the potassium that existed in the catalysts would outflow easily under the water steam condition.

Among the alkaline earth metals, calcium was considered as an attractive catalytic material for its low price and almost nontoxic property.²⁴ It was suggested by the previous literature that the catalytic performance could be improved after Co₃O₄ impregnated with calcium.¹⁰ In this work, the Ca modified Co₃O₄ catalysts with different molar ratio were prepared by the co-precipitation method. The effect of the addition of CaCO₃ into Co₃O₄ on the N₂O decomposition was studied. The XRD, H₂-TPR, XPS and TEM techniques were employed to investigate the activity–structure relationship of CaCoO_x catalyst.

2. Experiment

2.1 Catalysts preparation

The catalysts were prepared using the co-precipitation method described below. The known amounts of cobalt nitrate and calcium nitrate were dissolved in distilled water. Then 15 wt% K₂CO₃ solution was added dropwise into the obtained solution at room temperature until the pH reached 9. After 30 min stir, the solution was aged for 3 h. The resultant precipitate was collected by filtration and washed with distilled water until the pH of the filtrate reached 7. The cake was dried overnight at the temperature of 100 °C, and then calcined in air at the temperature of 500 °C for 2 h. The obtained catalysts were denoted as CaxCoy (x/y = 1 : 1, 1 : 2, 1 : 3, 1 : 5, which x/y represented the molar ratio of Ca/Co), Co₃O₄, CaCO₃.

2.2 Catalysts characterization

Powder X-ray diffraction (XRD) patterns were measured on Bruker D8 Advance X-ray diffraction meter (Germany), which were equipped with the CuKa radiation and operated at 40 kV and 40 mA. Diffraction patterns were recorded in a 2θ range between 10° and 90°.

X-ray photoelectron spectroscopy (XPS) experiments were carried out on an ULVAC PHI 5000 Versa Probe-II equipment (Japan). All the electron binding energies were referenced to the C1s peak at 284 eV.

TEM and high resolution TEM (HRTEM) of the catalyst samples were measured on FEI Tecnai G220 transmission electron microscope, which used an accelerating voltage of 200 kV.

H₂-temperature programmed reduction (H₂-TPR) was carried out on gas chromatography equipped with a quartz reactor and TCD detector. Before the test, 30 mg samples were pretreated at 400 °C for 40 min in N₂ (30 ml min⁻¹), and then cooled to 100 °C in N₂. The TPR analysis were performed under a 5% H₂/Ar (30 ml min⁻¹) using a heating rate of 10 °C min⁻¹ until the temperature reached to 550 °C.

Oxygen-temperature programmed desorption (O₂-TPD) was performed on gas chromatography equipped with a quartz reactor and TCD detector. Before the TPD experiments, the catalysts (270 mg) were pretreated at 400 °C for 1 h in He (20 ml min⁻¹), and then the sample was cooled to 120 °C in He (20 ml min⁻¹). O₂ adsorption was performed in 4% O₂/N₂ (20 ml min⁻¹) for 1 h at the temperature of 120 °C for removal of physical adsorption. Then, the sample was heated from 120 °C to 600 °C at a heating rate of 10 °C min⁻¹ in He (20 ml min⁻¹).

2.3 Catalytic measurements of N₂O decomposition

The catalytic activity measurement was carried out in a fixed-bed quartz flow reactor (6 mm i.d.) at the atmospheric pressure, and 0.2 ml (40–60 mesh) catalyst was used in each test. The reactant mixture was consisted of 1000 ppm N₂O, 0 or 5 vol% O₂, 0 or 3 vol% H₂O, Ar was used as balance gas. A total flow rate of feed gas was set at 200 ml min⁻¹, the gas hourly space velocity (GHSV) was 60 000 h⁻¹. The experiments were conducted at the temperatures between 250 °C and 500 °C in intervals of 25 °C. In order to ensure the reaction remained in a steady state, the reaction temperature was kept for 30 min before each measurement. The effluent gas concentrations were analyzed by gas chromatography equipped with ECD detector. The N₂O conversion was calculated according the formula below:

3. Results and discussion

3.1 XRD analysis

The XRD patterns of Co₃O₄ with different calcium content were shown in Fig. 1. As revealed by XRD patterns, the diffraction peaks for Co₃O₄ catalyst appeared at 31.23°, 36.88°, 45.03°, 59.36° and 65.21° corresponding to (111), (220), (311), (400), (511) and (440) planes of the spinel structure of Co₃O₄, respectively. No other phases, like Co₂O₃ or CoO, were found in these catalysts. In addition, the peaks belong to CaCO₃ (JCPDS86-0174) could also be observed on CaCoO_x curves. Because of the low calcination temperature, the Ca₉Co₁₂O₂₈ that formed by the solid state interaction between CaO and Co₃O₄ was not observed in the XRD patterns. Compared to the diffraction peaks of the pure Co₃O₄ oxides, the diffraction peaks of Co₃O₄ for CaCoO_x catalysts were weaker and broader, implying that the addition of CaCO₃ into Co₃O₄ restrained the crystallization of Co₃O₄ species.

Fig. 1 XRD patterns of different catalysts.

The diffraction peaks of the Co₃O₄ presented lower intensity compared to other literature,²² which can be explained that the Co₃O₄ was primary existed in the form of nanoparticles. Tang et al. revealed that the diffraction peaks intensity of the Co₃O₄ octahedral was stronger than those of the Co₃O₄ nanoparticles.²⁵ It has been found that the peaks of the Co₃O₄ became broader by the addition of K ions, the reason was that the K ions were highly dispersible in the matrix of the Co₃O₄.²⁶ But Dean reported that the radius of Ca⁺ (r_Ca²⁺ = 0.099 nm) was much larger than the radius of the Co²⁺and Co³⁺, so the Ca⁺ could not incorporate into the Co₃O₄ lattice.²⁷ Zhang et al. found that the introduction of Ba to NiO caused NiO diffraction peaks sharply weakened, because BaCO₃ could suppress crystal growth of NiO.²⁸ Therefore, the similar effect could be expected that the introduction of CaCO₃ could restrain the crystal growth of the Co₃O₄ in the catalyst samples. In previous studies, many authors believed that the N₂O decomposition activity was obviously influenced by the crystallite size of catalyst.¹⁸

3.2 Catalytic activity of CaCoO_x with different compositions for N₂O decomposition

The catalytic performance results of N₂O decomposition over the CaCoO_x with different molar ratio were shown in the Fig. 2. It was found that the pure CaCO₃ was inactive for N₂O decomposition. By contrast, pure Co₃O₄ showed certain catalytic activity for N₂O decomposition and nearly 100% conversion was achieved at 450 °C. However, it was established by the Fig. 2 that the addition of Ca into Co₃O₄ could largely improve the catalytic performance for N₂O decomposition. This implied that Co₃O₄ was the active component of CaCO₃–Co₃O₄ catalysts, while Ca acted as a promoter. The catalytic performance was strongly dependent on the Ca/Co molar ratio, as the increasing of Ca loading, N₂O conversions were increased sharply. However, excessively increased the loading of Ca could inhibit the catalytic decomposition of N₂O. The Ca1Co2 showed the best catalytic performance of N₂O decomposition, and almost complete conversion of N₂O decomposition could be obtained at 400 °C. But the further increase of Ca/Co molar ratio had a negative effect on the catalytic activity. Thus, Ca which existed in the form of CaCO₃ was proposed to be used as the alkaline earth metals for the improvement of the catalytic activity in N₂O decomposition procedure.

Fig. 2 The N₂O conversion over different catalysts with 1000 ppm N₂O/Ar.

It was established that the oxygen desorbing from active sites was the rate-determining step,²² thus the N₂O decomposition activity strongly depended on the oxygen desorption capability. However, the process of oxygen desorption could be inhibited in the presence of excess O₂. It was reported that the water vapor was the other important factor causing the inferior catalytic activity of N₂O decomposition.^6,10 Therefore, it was significant to investigate the effects of O₂ or H₂O on N₂O catalytic decomposition reaction. The effects of O₂ (5 vol%) or H₂O (3 vol%) on N₂O decomposition over Ca1Co2 and Co₃O₄ catalysts were illustrated in Fig. 3.

Fig. 3 N₂O conversion over Ca1Co2 and Co₃O₄ catalysts with feed gas of 1000 ppm N₂O/Ar + 5 vol% O₂ or 1000 ppm N₂O/Ar + 3 vol% H₂O.

As shown in Fig. 3, when 5 vol% O₂ was added to the feed gases, the N₂O decomposition activities of the two samples were both inhibited. However, the catalytic activity of the Ca1Co2 was obviously higher than the pure Co₃O₄. This indicated that the introduction of Ca into Co₃O₄ can significantly enhance the catalytic activity of N₂O decomposition even in the presence of 5 vol% O₂. As can be seen in Fig. 3, with the addition of 3 vol% water vapor, the catalytic activities were slightly decreased over Ca1Co2 and Co₃O₄ catalysts. It can be seen that the N₂O conversions over Ca1Co2 and Co₃O₄ at 400 °C were 95.7% and 58.2% respectively in the absence of H₂O, however, the N₂O conversions decreased to 83.4% and 42.6% respectively when 3 vol% H₂O was added in the system. The decreased activity in the presence of O₂ or H₂O was mainly caused by the competing adsorption of molecular oxygen, water vapor and reaction gases on the active sites of the catalysts, thus restrained the catalytic decomposition of N₂O.

3.3 H₂-TPR measurements

It was reported that the reduction of Co³⁺ to Co²⁺ was a very important process for N₂O decomposition over cobalt containing catalysts especially for the desorption of surface oxygen species resulting from the catalytic decomposition of N₂O.²⁹ In the present study, the H₂-TPR was used to investigate the cobalt species on the Co₃O₄ and CaCoO_x catalysts. As shown in Fig. 4, all the samples presented two H₂ consecutive peaks in the temperature of 100–500 °C. The first reduction peak at the region of 220–320 °C was due to the reduction of Co³⁺ to Co²⁺, and the second reduction peak in the range of 320–470 °C was owing to the reduction of Co²⁺ to Co⁰.³⁰ As can be seen in Fig. 4, when the CaCO₃ species was added into Co₃O₄, the reduction temperatures of Co³⁺ to Co²⁺ slightly shifted to higher temperature (ΔT₁ = 22 °C), suggesting that the reduction of Co³⁺ was slightly hindered. But the reduction temperatures of Co²⁺ to Co⁰ shifted to a lower temperature (ΔT₂ = 43 °C), indicating that reduction of Co²⁺ was improved. It also can be seen that almost similar reduction temperature of Co³⁺ to Co²⁺ was observed over the CaCoO_x catalyst with different Ca/Co molar ratios, it was obvious that the Ca/Co molar ratios showed no visible influence on the reduction behavior of Co³⁺ to Co²⁺. These results pointed out that the reduction reducibility of Co³⁺ to Co²⁺ on the Co₃O₄ catalyst was decreased by the addition of Ca species. On the contrary, the addition of Ca species into Co₃O₄ catalyst with suitable Ca/Co molar ratios greatly enhanced the catalytic performance of N₂O decomposition. This suggested that the reducibility of Co³⁺ to Co²⁺ was not the key factor for the decomposition of N₂O over the CaCoO_x catalysts in this study.

Fig. 4 H₂-TPR profiles of different catalysts.

The Fig. 4 also showed that the reduction temperature of Co²⁺ to Co⁰ shifted to a lower temperature after the addition of Ca species into Co₃O₄ (ΔT₂ = 43 °C), which revealed that the reducibility of Co²⁺ to Co⁰ on the Co₃O₄ catalyst was increased by the introduction of Ca species. The increased reducibility of Co²⁺ to Co⁰ may offset the decreased reducibility of Co³⁺ to Co²⁺ caused by addition of Ca species into Co₃O₄. Therefore, no negative effects on N₂O decomposition were observed, on the contrary, the greatly improved effect on N₂O decomposition was observed by the addition of Ca species into Co₃O₄. It is clear that the improved reducibility of Co₃O₄ especially for the reduction of Co²⁺ to Co⁰ was conductive to the N₂O decomposition by the addition of Ca species.

3.4 O₂-TPD

It was reported that the oxygen desorbing from active sites was considered as the rate-determining step.²² The O₂-TPD experiments were performed over Ca1Co2 and Co₃O₄ to better study the oxygen desorption. As shown in Fig. 5, there was an O₂ desorption peak at 150–400 °C. The O₂ desorption peak at temperature of 150–400 °C was ascribed to the desorption of surface oxygen species such as O₂⁻ and O⁻.²² The peak can be correlated with the number of oxygen vacancies produced during materials synthesis.³¹ From the Fig. 5, it can be seen that the Ca1Co2 catalyst presented higher intensity of desorption peak than the pure Co₃O₄. At the same time, the O₂ desorption peak of Ca1Co2 was broader than that of the individual Co₃O₄. The O₂-TPD analysis indicated that the addition of CaCO₃ in the sample can increase the molar amount of desorbed O₂ compared to Co₃O₄. In other words, the Co₃O₄ with the addition of CaCO₃ possessed higher amount of O₂-vacancies on the surface.

Fig. 5 O₂-TPD profiles of over Ca1Co2 and Co₃O₄ catalysts.

3.5 XPS analysis

The XPS was performed to study the oxidation states of various Co species on the surface.³² The XPS spectra of Co2p of the pure Co₃O₄ and Ca1Co2 were demonstrated in Fig. 6a. As can be seen in Fig. 6a, for the pure Co₃O₄, the Co2p peak can be fitted into two binding energy peak, corresponding to Co2p3/2 (779.7 eV) and Co2p1/2 species (779.7 eV). The addition of Ca showed no observable influence on the valence state of cobalt oxide. At the same time, the spin–orbit splitting (ΔE) of the Co2p peaks (ΔE) was both 15.0 eV, which indicated that the formation of Co₃O₄ spinel phase. The result was in good agreement with the reported results.^33,34 Two small peaks at 789.3 eV and 804.6 eV were also observed in the Co2p region, which were assigned to the Co²⁺ shake-up satellite peaks of Co₃O₄.³⁴ The analysis results of O1s peak provided the additional information of the two samples. The two contributions arose at 529.7 eV and 531.1 eV on the O1s peak could attribute to the lattice oxygen in oxides and adsorbed oxygen on the catalyst surface that weakly bounded to the catalyst surface.^35,36 According to Fig. 6b, it could be seen that the shoulder in the curve of Ca1Co2 was more obvious than that of the pure Co₃O₄. The shoulder position was approximately at 531.1 eV, Liu et al. reported that the higher binding energy shoulder was attributed to the presence of oxygen vacancies or surface oxygen species.³⁷ It was clear that there were more surface oxygen or oxygen vacancies in Ca1Co2 than that of the pure Co₃O₄, which may promote the decomposition of N₂O. The results were in accordance with the O₂-TPD analysis. Zamudio et al. compared the N₂O catalytic activity of MgCo₂O₄ prepared by solution combustion synthesis and co-precipitation methods, the catalyst prepared by co-precipitation method provided a higher amount of surface oxygen which showed better N₂O catalytic activity.³⁸ It was established that the desorption of the surface oxygen that weakly bounded to the surface of catalyst leaded to the formation of anionic vacancies, and the anionic vacancies potentially activate the adsorption and decomposition of N₂O.³⁹ It was well known that the weaker adsorption oxygen could benefit the surface mobility of oxygen on the catalyst. Because there were larger amounts of surface oxygen in the Ca1Co2, therefore, the addition of Ca species enhanced the removal of surface oxygen of Co₃O₄. It was believed that the Ca was conductive to desorption of oxygen, which was the rate determination step in the N₂O decomposition.

Fig. 6 X-ray photoelectron spectra of: (a) Co2p and (b) O1s core levels of Ca1Co2 and Co₃O₄ catalyst samples.

3.6 TEM analysis

The further insight into the morphology of the pure Co₃O₄ and Ca1Co2 catalysts were revealed by TEM characterization. Typical low magnification TEM images were presented in Fig. 7a and b, it can be seen that the Co₃O₄ particles were all nanoparticles with a diameter size of approximately 10–50 nm. The particle size distribution were displayed in the Fig. 7a′ and b′, the mean diameter size of Ca1Co2 (22.88 nm) were smaller than that of the pure Co₃O₄ (26.18 nm). This result showed that the addition of Ca species into Co₃O₄ could decrease the average size of the samples. Moreover, the Fig. 7a illustrated that the addition of Ca species could also improve the dispersion of Co₃O₄ compared to the pure Co₃O₄. The previous work revealed that the dispersity of the cobalt oxides in the cobalt and cerium binary oxide sample was higher than in the individual cobalt oxide, the addition of CeO₂ into the cobalt oxide can prevent the sintering of cobalt oxide microcrystals.⁴⁰ Furthermore, Zhou et al. reported that the introduction of CeO₂ to metal oxides such as the Co₃O₄, CuO, and NiO could provide higher surface area and prevented the metal oxides from agglomeration.^41,42 Qiao et al. suggested that the addition of Ca species can increase the dispersity of the NiO species on the sample surface.⁴³ A similar interaction might exist between the Ca and Co₃O₄. Due to Fig. 7, it was proved that the excellent dispersion of Co₃O₄ can contribute to the remarkable N₂O catalytic decomposition activity.

Fig. 7 TEM and HRTEM images of as-synthesized Ca1Co2 (a and c) and Co₃O₄ (b and d). The (a′) and (b′) are the size distribution.

Fig. 7c and d showed the high resolution TEM (HR-TEM) images of the Ca1Co2 and pure Co₃O₄ catalysts. The Ca1Co2 lattice spacing of 0.468 nm, 0.292 nm corresponded to the (111), (220) crystal planes of typical Co₃O₄ spinel structure, which was in according with the corresponding XRD pattern. The crystal planes of CaCO₃ were found in the cross lattice in the edge of the TEM image of Ca1Co2. The CaCO₃ crystal planes were marked by circling. From the Fig. 7c, it can be observed that the lattice spacing of d = 0.23–0.25 nm corresponded to facets of the CaCO₃. It belonged to 113 lattice plane of the CaCO₃.⁴⁴ The lattice spacing of (220) crystal plane of the pure Co₃O₄ was corresponding to 0.284 nm. From all the HR-TEM pictures of the pure Co₃O₄, no any (111) crystal plane of Co₃O₄ was observed. It could be speculated that the Ca could promote the lattice of (111) crystal planes exposing on the catalysts surface.

From the results discuss above, it was clear that the addition of CaCO₃ into Co₃O₄ catalyst influenced the reducibility of the catalyst. The H₂-TPR results also confirmed that the addition of CaCO₃ into Co₃O₄ could promote the reduction of Co²⁺ to Co. It has been reported that active sites with better reducibility exhibited higher activities in N₂O decomposition.⁴⁵ Therefore, it could be concluded that the CaxCoy catalysts with stronger reducibility showed higher activities than the pure Co₃O₄. Simultaneously, it has been reported that the higher amount of oxygen sites on the surface of catalyst contributed to the N₂O decomposition.³⁸ From Fig. 6b, it could be easily seen that the Ca1Co2 exhibited higher amount of surface oxygen than that of the individual Co₃O₄. The similar phenomenon was also observed in the O₂-TPD. As a consequence, the introduction of CaCO₃ into Co₃O₄ enhanced the catalytic activity of N₂O decomposition.

3.7 Steady-state kinetics studies

The rates of N₂O decomposition (r_m) were calculated using the following equation:
where V was the real volume flow rate, m_cat was the amount of the catalyst, [N₂O]_in and [N₂O]_out were respectively the inlet and outlet N₂O concentrations. The rates of N₂O decomposition were calculated at temperatures of 400 °C and 450 °C, the concentration of N₂O was varied from 600 to 1400 ppm. The calculated rates were presented in Fig. 8, it can be seen that the rate of N₂O decomposition over Ca1Co2 and Co₃O₄ increased linearly with the increase of N₂O concentration. This suggested that the N₂O decomposition was a first order reaction. The result was in good agreement with conclusions of other studies.^36,43

Fig. 8 Dependence of N₂O conversion rate on N₂O inlet concentration on Ca1Co2 and Co₃O₄ catalysts at 400 °C and 450 °C.

The activation energy (E_a) was calculated using the Arrhenius equation:

where k was the first-order rate constant, T was the reaction temperature, A was the pre-exponential factor, E_a was the activation energy. The plot of ln(k) versus 1/T was drawn according to the data obtained from steady-state reaction.

The Arrhenius plots of N₂O decomposition over Ca1Co2 and Co₃O₄ catalysts were shown in Fig. 9. The addition of Ca species into the Co₃O₄ showed a significant effect on the activation energy. It was clear that the activation energy of the Ca1Co2 (E_a = 17.84 kJ mol⁻¹) was much lower than that of the Co₃O₄ (E_a = 43.21 kJ mol⁻¹), which indicated that the existence of Ca species on the surface could greatly reduce the activation energy in the decomposition of N₂O. It was thus indicated that the addition of Ca species into Co₃O₄ could offer more active species during N₂O catalytic decomposition. According to previous discussion that Co₃O₄ was the active sites for N₂O decomposition over the Ca1Co2 catalyst. Based on the XRD and TEM results, the introduction of the Ca species can increase the dispersion of the Co₃O₄. Compared with Co₃O₄, the Ca1Co2 catalyst presented a better dispersion of metal species in catalyst, which may provide more active sites to take part in the reaction of the N₂O decomposition. It was established that the adsorption of the N₂O and desorption of oxygen were the rate-determining step. Due to the large difference of activation energy between the Ca1Co2 (E_a = 17.84 kJ mol⁻¹) and the Co₃O₄ (E_a = 43.21 kJ mol⁻¹), it might imply that the N₂O-adsorptive property was an essential feature that can affect the rate of N₂O decomposition. Kondratenkoa et al. reported that N₂O adsorption should be taken into consideration for catalytic N₂O decomposition.⁴⁶ The lower activation energy over the Ca1Co2 catalyst, suggesting that the addition of Ca species may accelerate the N₂O adsorbing on the active sites of Co₃O₄ catalyst.

Fig. 9 Arrhenius plot of N₂O decomposition on Ca1Co2 and Co₃O₄ catalysts.

4. Conclusion

The addition of CaCO₃ into Co₃O₄ could significantly enhance the catalytic performance for N₂O decomposition. The species of Ca was mainly presented in the form of CaCO₃ in all CaCoO_x samples. The addition of CaCO₃ into the Co₃O₄ catalyst could suppress the crystal growth of Co₃O₄ and promote the reduction of Co²⁺ to Co, which can benefit the N₂O decomposition. Moreover, the Co₃O₄ doped with Ca species presented smaller particle diameter and provided a higher amount of surface oxygen. The kinetics studies revealed that the N₂O catalytic decomposition reaction was first-order for the CaCoO_x catalysts. The activation energy for Ca1Co2 catalyst (E_a = 17.84 kJ mol⁻¹) was lower than the pure Co₃O₄ (E_a = 43.21 kJ mol⁻¹), suggesting that the addition of Ca enhanced the catalytic decomposition of N₂O.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21307047, no. U1137603).

References

1. H. B. Zhou, P. L. Hu, Z. Huang, F. Qin, W. Shen and H. L. Xu, Ind. Eng. Chem. Res., 2013, 52, 4504–4509.
2. P. Kustrowski, L. Chmielarz, A. Rafalska-Łasocha, B. Dudek, A. Pattek-Janczyk and R. Dziembaj, Catal. Commun., 2006, 7, 1047–1052.
3. L. Obalová, K. Karásková, K. Jirátová and F. Kovanda, Appl. Catal., B, 2009, 90, 132–140.
4. T. J. Wallington and P. Wiesen, Atmos. Environ., 2014, 94, 258–263.
5. M. A. Zamudio, S. Bensaid, D. Fino and N. Russo, Ind. Eng. Chem. Res., 2011, 50, 2622–2627.
6. R. Amrousse and T. Katsumi, Catal. Commun., 2012, 26, 194–198.
7. S. Imamura, R. Hamada, Y. Saito, K. Hashimoto and H. Jindai, J. Mol. Catal. A: Chem., 1999, 139, 55–62.
8. T. N. Angelidis and V. Tzitzios, Ind. Eng. Chem. Res., 2003, 42, 2996–3000.
9. J. P. Dacquin, C. Dujardin and P. Granger, J. Catal., 2008, 253, 37–49.
10. C. Ohnishi, K. Asano, S. Iwamoto, K. Chikama and M. Inoue, Catal. Today, 2007, 120, 145–150.
11. K. S. Chang, H. Song, Y. S. Park and J. W. Woo, Appl. Catal., A, 2003, 273, 223–231.
12. M. F. Fellah and I. Onal, Catal. Today, 2008, 137, 410–417.
13. A. H. Øygardena and J. Pérez-Ramírez, Appl. Catal., B, 2006, 65, 163–167.
14. N. Liu, R. D. Zhang, B. H. Chen, Y. P. Li and Y. X. Li, J. Catal., 2012, 294, 99–112.
15. F. Kapteijn, J. Rodriguez-Mirasol and J. A. Moulijn, Appl. Catal., B, 1996, 9, 25–64.
16. R. Amrousse, A. Tsutsumi, A. Bachar and D. Lahcene, Appl. Catal., A, 2013, 450, 253–260.
17. L. Yan, T. Ren, X. L. Wang, Q. Gao, D. Ji and J. S. Suo, Appl. Catal., B, 2003, 45, 85–90.
18. B. M. Abu-Zied, S. A. Soliman and S. E. Abdellah, J. Ind. Eng. Chem., 2015, 21, 814–821.
19. L. Yan, T. Ren, X. L. Wang, D. Ji and J. S. Suo, Catal. Commun., 2003, 4, 505–509.
20. W. Piskorz, F. Zasada, P. Stelmachowski, A. Kotarba and Z. Sojka, Catal. Today, 2008, 137, 418–422.
21. G. Maniak, P. Stelmachowski, J. J. Stanek, A. Kotarba and Z. Sojka, Catal. Commun., 2011, 15, 127–131.
22. L. Xue, C. B. Zhang, H. He and Y. Teraoka, Appl. Catal., B, 2007, 75, 167–174.
23. G. Maniaka, P. Stelmachowski, A. Kotarba, Z. Sojka, V. Rico-Pérez and A. Bueno-López, Appl. Catal., B, 2013, 136–137, 302–307.
24. V. Gölden, S. Sokolov, V. A. Kondratenko and E. V. Kondratenko, Appl. Catal., B, 2010, 101, 130–136.
25. X. F. Tang, J. H. Li and J. M. Hao, Mater. Res. Bull., 2008, 43, 2912–2918.
26. K. H. Asano, C. Ohnishi, S. Iwamoto, Y. Shioya and M. Inoue, Appl. Catal., B, 2008, 78, 242–249.
27. J. A. Dean, Lange's handbook of chemistry, McGraw-Hill, New York, 15th edn, 1999, pp. 318–322.
28. F. F. Zhang, X. P. Wang, X. X. Zhang, M. Turxun, H. B. Yu and J. J. Zhao, Chem. Eng. J., 2014, 256, 365–371.
29. X. Li, H. He, C. Liu, C. B. Zhang and B. Zhang, Environ. Sci. Technol., 2009, 43, 890–895.
30. C. Y. Ma, Z. Mu, C. He, P. Li, J. J. Li and Z. P. Hao, J. Environ. Sci., 2011, 23, 2078–2086.
31. F. Tanja and P. Regina, Appl. Catal., B, 2015, 176–177, 298–305.
32. Y. F. Wang, C. B. Zhang, F. D. Liu and H. He, Appl. Catal., B, 2013, 142–143, 72–79.
33. T. Warang, N. Patel, R. Fernandes, N. Bazzanella and A. Miotello, Appl. Catal., B, 2013, 132–133, 204–211.
34. Q. Y. Yan, X. Y. Li, Q. D. Zhao and G. H. Chen, J. Hazard. Mater., 2012, 209–210, 386–391.
35. J. P. Dacquin, C. Dujardin and P. Granger, J. Catal., 2008, 25, 37–49.
36. Q. Shen, L. D. Li, J. J. Li, H. Tian and Z. P. Hao, J. Hazard. Mater., 2009, 163, 1332–1337.
37. Q. Liu, L. C. Wang, M. Chen, Y. Cao, H. Y. He and K. N. Fan, J. Catal., 2009, 263, 104–113.
38. M. A. Zamudio, S. Bensaid, D. Fino and N. Russo, Ind. Eng. Chem. Res., 2011, 50, 2622–2627.
39. J. P. Dacquin, C. Lancelot, C. Dujardin, P. DaCosta, G. Djega-Mariadassou, P. Beaunier, S. Kaliaguin, S. Vaudreuil, S. Royer and P. Grange, Appl. Catal., B, 2009, 91, 596–604.
40. S. O. Soloviev, P. I. Kyriienko and N. O. Popovych, J. Environ. Sci., 2012, 24, 1327–1333.
41. H. B. Zhou, Z. Huang, C. Sun, F. Qin, D. S. Xiong, W. Shen and H. L. Xu, Appl. Catal., B, 2012, 125, 492–498.
42. H. B. Zhou, P. L. Hu, Z. Huang, F. Qin, W. Shen and H. L. Xu, Ind. Eng. Chem. Res., 2013, 52, 4504–4509.
43. D. S. Qiao, G. Z. Lu, D. S. Mao, Y. Guo and Y. L. Guo, J. Mater. Sci., 2011, 46, 641–647.
44. L. Obalova and V. Fıla, Appl. Catal., B, 2007, 70, 353–359.
45. P. F. Xie, Z. Ma, H. B. Zhou, C. Y Huang, Y. H. Yue, W. Shen, H. L. Xua, W. M. Hua and Z. Gao, Microporous Mesoporous Mater., 2014, 191, 112–117.
46. E. V. Kondratenkoa, V. A. Kondratenkoa, M. Santiago and J. Pérez-Ramírez, J. Catal., 2008, 256, 248–258.

---