Nano-Crystalline Co3O4 Spinel Prepared by Combustion Method as a Catalyst for Direct Decomposition of N2O

Abstract

Combustion method has been used as a fast and facile method to prepare nano-crystalline Co3O4 spinel employing urea as a combustion fuel. Alkali-promoted catalysts were prepared by the incipient wetness impregnation of the prepared nano-material with the aqueous solutions of alkali carbonates (Li2CO3, Na2CO3, K2CO3, and Cs2CO3). N2O decomposition was investigated over pure and alkali promoted nano-crystalline Co3O4 catalysts. The beneficial influence of dopants on the catalyst activity increases in the order: un-promoted

Keywords

N₂O decomposition, Nano-crystalline Co₃O₄, Combustion.

Introduction

Nitrous oxide (N₂O) is a dangerous environmental pollutant because of its high greenhouse potential and its ozone-depleting property [1]. The Kyoto Protocol of the United Nations Convention on Climate Change (December 1997) cited N₂O as a second non-CO₂ greenhouse gas. N₂O is a greenhouse gas with a long life time of about 150 years in the atmosphere [2,3]. Although N₂O is not the major contributor to global warming, it is much more potent than either of the other two most common anthropogenic greenhouse gases, CO₂ and CH₄. N₂O is 310 and 21 times of the Global warming potential (GWP) of CO₂ and CH₄, respectively [2,4]. Furthermore, N₂O is an important source of stratospheric nitrogen oxides, which initiates a chain of cyclic reaction leading to stratospheric ozone destruction [2–5]. It is emitted from both natural and anthropogenic sources such as nitric acid and adipic acid plants and fluidized bed combustors for sewage-sludge or industrial wastes besides the medical exhaust as well as biological and agricultural emissions [6]. The concentration of nitrous oxide in the atmosphere continues to increase, and this increase appears to be caused mainly by human activities. With increasing concerns about protecting our environment, the catalytic removal of nitrous oxide from exhausts becomes very attractive [7].

Heterogeneous catalytic decomposition of N₂O, to its elements nitrogen and oxygen, is a suitable solution in many of the anthropogenic emission sources, and various catalytic formulations have been proposed, including supported and unsupported metals [8–11], pure and mixed oxides [12-15] and zeolites [16-19]. Supported noble metal catalysts, such as Rh catalysts [1], show high activities in N₂O decomposition reactions at 200-300°C. However, the high cost of the noble metal limits their applications. Transition metal ion exchanged zeolite catalysts such as Fe-ZSM-5 are more active in the selective catalytic reduction (SCR) of N₂O by hydrocarbons than in the decomposition of N₂O in a temperature range of 300-400°C [3]. Recently, spinel-type oxides have been a subject of increasing applied research [20-24] in addition to be used as efficient catalysts [25-28]. Among them cobalt oxide, Co₃O₄, which belongs to the family of complex-metal oxides known as spinels. Co₃O₄ adopt the normal spinel crystal structure based on a cubic close packing array of oxide atoms which have the general formula AB₂O₄ in which A is a divalent cation located at the tetrahedral 8a sites, and B atoms are trivalent cations occupying the octahedral 16d sites. Cobalt containing spinels was shown to be superior to other transition metal spinels towards the N₂O decomposition in presence or in absence of oxygen [7,9,13,15]. Co₃O₄ received paid attention due to its promising applications in gas sensing [20], low temperature CO oxidation [21], anode materials in Li-ion batteries [22], steam reforming of ethanol [23], oxidation of volatile organic compounds (VOCs) [24], N₂O decomposition [25], ammonia oxidation [26], Fischer-Tropsch synthesis (FTS) [27], and decomposition of hydrogen peroxide [28,29]. Much effort has been directed to prepare Co₃O₄ nano-particles, including sol-gel [30], hydrothermal [31], micro-emulsion [32], microwave-assisted hydrothermal [33], and combustion synthesis [29,34,35].

Recent studies indicated that the residual alkali metals in the Co-based catalysts, which were introduced during the precipitation, caused a significant increase of catalytic activity [36]. For example, Stelmachowski et al. [37] studied the N₂O decomposition over alkali-doped Co₃O₄ and found that the addition of alkali-promoters greatly enhance the catalytic activity for N₂O decomposition in the order: Li<<Na<K<Cs. In addition, Ohnishi et al. [38] found that the sodium content remaining in the Co₃O₄ catalyst was the determining factor controlling the activity and the optimal Na/Co molar ratio was 3.8 × 10^-3. Similarly, it is observed that the presence of residual K in Co₃O₄–CeO₂ which was prepared by co-precipitation with K2CO₃ as the precipitant improved largely the catalytic performance [39]. Motivated by these Experimental findings, Asano et al. [7] prepared K-doped Co₃O₄ catalyst by impregnation of KNO3 on CoCO₃, in which the alkali content could be finely tuned. They concluded that the number of active sites increased and desorption of oxygen was promoted by the addition of K to the Co₃O₄, which resulted in an enhanced activity for N₂O decomposition. In addition to the promotional effect of Na and K on Co₃O₄ catalyst, it is also observed that the addition of an appropriate amount of Cs to Co₃O₄ could weaken the Co–O bond, and the Cs-modified Co₃O₄ catalyst exhibited higher activity for N₂O decomposition compared to bulk Co₃O₄ [40].

In the present work, N₂O decomposition over bulk and alkali promoted (Li, Na, K and Cs) nano-crystalline Co₃O₄ catalysts, prepared using combustion method, was studied. We found that the catalyst activity increases in the order: unpromoted< Li<Na<K<Cs. Our study was extended to check the effect of changing both the Cs/Co molar ratio and calcination temperature on the catalyst activity towards N₂O decomposition.

Experimental

Preparation Procedure

The nano-crystalline Co₃O₄ was prepared using the combustion method as described elsewhere [29]. Urea was used as a combustion fuel. In a typical procedure, the required amounts of cobalt nitrate and urea for urea/cobalt nitrate molar ratio of 8 were weighed to the nearest milligram. Cobalt nitrate and urea were dissolved in 100 mL of distilled water to form a pink homogeneous solution; the solution was then transferred to a flask fitted with a water condenser, after that it was heated to 100°C with stirring. The reaction mixture was refluxed with stirring at that temperature for 12 hours. The product obtained was evaporated by heating over a hotplate at 100°C. Finally, the dried precursor was calcined in a Muffle furnace at 400°C for 3 hour in a static air atmosphere.

Alkali-doped catalysts were prepared by the incipient wetness impregnation of prepared nano-crystalline Co₃O₄ with the aqueous solutions of alkali carbonates (Li₂CO₃, Na₂CO₃, K₂CO₃, and Cs₂CO₃). The alkali carbonates were found to be more efficient precursors than the corresponding hydroxides [41]. Three series of samples were prepared. In the first one (series I), we have investigated the influence of adding alkali cations (Li⁺, Na⁺, K⁺, and Cs⁺) in promotion of N₂O decomposition. The alkali metal/ cobalt ratio was adjusted to 0.05. The obtained mixtures were dried at 90°C overnight and then calcined at 400°C for 3 h in a static air atmosphere. Based on N₂O measurements, in series II, we have investigated the influence of changing Cs/Co ratio, from 0.0125 – 0.200, on the catalysts activity towards N₂O decomposition. After that, In order to investigate the effect of calcination temperature, in series III, we calcined the most active sample, i.e., Cs/Co₃O₄ with Cs/Co 0.025 molar ratio, at different temperatures (i.e., 500°C, 600°C and 700°C).

Characterization

Simultaneous TGA and DTA curves were recorded with a Shimadzu DTG-60 apparatus using a heating rate of 10°C min-1 in air atmosphere (flow rate 40 ml/min). Powder X-ray diffraction (XRD) patterns were recorded using Philips diffractometer (Type PW 103/00) with CuKα radiation (λ=1.5405 ?) at 35 kV and 20 mA with a scanning rate in 2θ of 0.06 min-1. FTIR spectra were performed as KBr discs in the range 4000-400 cm^-1, using Thermo-Nicolet-6700 FTIR spectrophotometer. BET surface areas were determined on a Quantachrome (Nova 3200 series) multi-gas adsorption instrument with nitrogen physisorption at -196°C.

Activity Measurements

catalytic reaction was carried out in a fixed-bed quartz flow reactor, containing approximately 500 mg of the catalyst in all the experiments. The temperature in the reactor was measured using a thermocouple on the catalyst bed and was controlled within ± 0.2°C of the desired temperature by a Cole-Parmer temperature controller (model Digi-Sense 89000-00). The desired concentration of N₂O was added with the aid of thermal mass flow controllers (AALBORG, DFC2600) using N2 as a balance gas, the volume flow rate was 200 cm3/min (NTP). Analysis of the reactor inlet and outlet gases was carried out using a magnetic oxygen analyzer (ABB, AO2020-Magnos 106) and non-dispersive infrared analyzer for the components N₂O and NO (ABB, AO2020-Uras 14). Preliminary experiments for the decomposition of nitrous oxide over all the catalysts showed the absence of NO in the exit gas.

Results and Discussion

Catalysts Characterization

Thermal analysis: Figure 1 shows the TG profiles, in air atmosphere as a carrier gas, of the calcined precursors (alkalipromoted Co₃O₄ with alkali/Co ratio 0.05) at 400°C, all of which showed essentially the same profile, and two weight-decrease processes were observed at the temperature ranges of 50–280°C and 740–900°C. The former is due to dehydration of the adsorbed water, and the latter weight decrease is attributed to decomposition of the alkali-carbonates, releasing CO₂, and thermal reduction of Co₃O₄ to CoO [29,34,35].

X-ray diffraction: XRD patterns of un-promoted (a); and alkali-promoted (Li⁺ (b), Na⁺ (c), K⁺ (d), and Cs⁺ (e)) nano-crystalline Co₃O₄ catalysts having alkali/cobalt ratio of 0.05 being calcined at 400°C are shown in Figure 2. Only nano-crystalline Co₃O₄ spinel phase and no alkali-containing components can be observed due to the low content and mono-layer dispersion of alkali species on catalyst surface. The line profile, shown in Figure 2, is fitted for 8 peaks for nano-crystalline Co₃O₄ (JCPDS no. 80-1545) (2θ=18.94°, 31.30°, 36.82°, 38.50°, 44.80°, 55.66°, 59.38°, 65.20°) [34], marked by their indices planes [(111), (220), (311), (222), (400), (422), (511), (440)], respectively. The peaks became broader by the addition of alkali-cations but no peak shift was observed, indicating that alkali-cations are highly dispersed in the matrix of the nano-crystalline Co₃O₄ catalyst [7]. In addition, the intensities of the XRD peaks related to the nano-crystalline Co₃O₄ reflections are slightly decreased. The crystallite sizes (D) of alkali-doped nano-crystalline Co₃O₄ were calculated using Scherrer equation [35] from the full-width at half-maximum (FWHM) of the peak at 2θ=36.82°. The relevant values are listed in Table 1. From Table 1, it is evident that the Li/Co₃O₄ (0.05) catalyst had the largest Co₃O₄ crystallites, followed by the Na/Co₃O₄ (0.05) and K/Co₃O₄ (0.05) catalysts, while the Cs/Co₃O₄ (0.05) catalyst had rather small crystallites. Therefore, and based on N₂O activity, our study was extended to check the influence of changing the Cs/Co²⁺ molar ratio on the crystallite size and N₂O decomposition activity of Cs-promoted Co₃O₄.

Table 1: Physico-chemical properties of alkali-promoted nano-crystalline Co₃O₄ catalysts.

XRD patterns of the Cs promoted nano-crystalline Co₃O₄ with different Cs/Co ratios are shown in Figure 3. XRD pattern of the samples shows only the characteristic peaks of nano-crystalline Co₃O₄ at lower Cs/Co²⁺ ratios (i.e., <0.1). While, at higher Cs/ Co²⁺ ratios, peaks due to the crystalline Cs₂CO₃ phases (JCPDS no. 71-1981) are noticed, in addition to Co₃O₄. The intensity of these peaks was found to increase with increasing Cs loading. The XRD patterns suggest that at higher Cs content the crystallites of Cs₂CO₃ are formed on the surface of Co₃O₄.

In order to check the role of Cs/Co²⁺ ratio in changing the structural parameters of nano-crystalline Co₃O₄ such as lattice parameter (a) and crystallite size (D), the lattice parameters were computed using the d-spacing values and the respective (hkl) parameters. The obtained Results are plotted in Figure 4. The variation of the lattice parameter with Cs/Co²⁺ ratio follows the same trend as that observed for the crystallite size. Inspection of Figure 4 revealed that, the crystallite size of Cs/Co₃O₄ decreased with increasing the cesium/cobalt ratio from 0.0125 till 0.025, and then it shows a mild continuous increase upon further Cs/Co²⁺ increase.

Since the Cs/Co₃O₄ obtained by using Cs/Co²⁺ ratio of 0.025 exhibits the lowest crystallite size, the study was extended to check the influence of changing the calcination temperature on the crystallite size of Cs-promoted nano-crystalline Co₃O₄ at this ratio. XRD patterns of the as-prepared Cs/Co₃O₄ obtained via calcining Cs/Co²⁺ parents having molar ratio of 0.025 at 400- 700°C temperature range are shown in Figure 5. From this figure four points could be raised: (i) all diffraction peaks belong to one phase only (Co₃O₄) at the calcination temperatures range 400-700°C, (ii) these peaks showed a continuous decrease in their intensities with increasing the calcination temperature, (iii) No Cs-containing phase was found possibly due to the low content and mono-layer dispersion of Cs species on catalyst surface, and (iv) the diffraction peaks for Cs doped Co₃O₄ shift continuously to lower 2θ values upon increasing the calcination temperature from 400 till 700°C, indicating an increase in the spinel lattice parameters, characteristic for formation of a solid solution [42]. Figure 6 shows the lattice parameters of nano-crystalline Co₃O₄ phase at different calcination temperatures (400-700°C) and the crystallite size calculated from XRD data. From these data one can state safely that, the sample calcined at 400°C exhibits the lowest crystallite size (D=17 nm). With increasing the calcination temperature the average crystallite size is increased. This behavior is expected because the heating facilitates the diffusion and agglomeration of the particles.

Fourier Transformation Infrared Spectroscopy (FTIR): Cubic spinel structure of Co₃O₄ with Co²⁺ (3d7) and Co³⁺ (3d6) located at tetrahedral and octahedral sites, respectively, belongs to the space group (Fd3m) [34]. The group theory predicts the following modes in the spinel [35]: Γ=A_1g (R)+E_g (R)+F_1g (in)+3F_2g (R)+2A_2u (in)+2E_u (in)+4F_1u (IR)+2F_2u (in), where (R), (IR) and (in) represent Raman active vibrations, infrared-active vibration and inactive modes, respectively. The FTIR spectra of un-promoted and alkalipromoted nano-crystalline Co₃O₄ catalyst (alkali=Li⁺, Na⁺, K⁺ and Cs⁺ where alkali/Co²⁺ molar ratio=0.05), being calcined at 400°C are shown in Figure 7. In the investigated region (4000-400 cm^-1), the entire obtained spectra manifest the presence of two absorption bands at 567 (υ₁) and 662 (υ₂) cm^-1; which originate from the stretching vibrations of the metal-oxygen bond and confirm the formation of Co₃O₄ spinel oxide [34,35]. The υ1 band is characteristic of OB₃ (where B denotes the Co³⁺ in the octahedral hole) vibration and the υ₂ band is attributable to the ABO₃ (where A denotes the Co²⁺ in the tetrahedral hole) vibration in the spinel lattice. The intensities of such peaks decreased and became broader upon the addition of alkali-cations. For the un-promoted nano-crystalline Co₃O₄ catalyst, Figure 7 shows a broad band centered at 3400 cm^-1 and an absorption band at 1630 cm^-1, such absorption were assigned to the O-H stretching and bending modes of water [32], respectively, which indicates that the surface was hydroxylated because the sample was exposed to air and some water was adsorbed probably on the external surface of the samples during handling to record the spectra. For alkali-promoted nano-crystalline Co₃O₄, one can observe another set of absorption bands at 858, 1155 and 1523 cm^-1 corresponding to the different vibration modes of carbonate anion [25,33]. The band at 2360 cm^-1 was assigned to vibration of carbon dioxide molecule [33].

Figure 8 shows the FT-IR spectra of Cs/Co₃O₄ catalysts (Cs/Co=0.0125, 0.025, 0.05, 0.1, 0.2), being calcined at 400°C. Also, all the obtained spectra manifest the presence of two strong absorptions at 567 and 662 cm^-1. Such absorptions were assigned, respectively, to ν1 and ν2 stretching vibrations of the metal-oxygen bond in Co₃O₄ spinel oxide [34]. Moreover, one can observe another set of absorptions at 1400-1550 cm^-1. Such absorptions was assigned, again, to vibration mode of carbonate anion [25,33]. In this context, it is worth mentioning that the intensity of such absorption increases with increasing Cs/Co²⁺ molar ratio. At Cs/Co²⁺ molar ratio of 0.2, there are another two additional bands at 1008 and 831 cm^-1; also this can be assigned to the carbonate anion. For the same series of catalysts, Figure 8 shows absorption at 1668 cm^-1, this can be assigned to the H-O-H bending mode of vibration [32]. The band at 2360 cm^-1 was assigned to vibration of CO₂ molecule [33].

Catalytic Decomposition of N₂O

Figure 10 shows the dependence of the steady-state conversion of N₂O on the reactor temperature for pure and alkali-doped (where alkali cation/Co=0.05) nano-crystalline Co₃O₄ catalysts calcined at 400°C. All the alkali promoted catalysts exhibit a higher N₂O decomposition activity (T50 values of 351, 270, 260, and 253°C for the Li-, K-, Na-, and Cs-promoted catalysts, respectively) compared with that of the un-promoted Co₃O₄ (T50=387°C). In other words, the activity order is: un-promoted<Li<Na<K<Cs at reaction temperature of 350°C with N₂O conversion of 20.57, 48.2, 92.2, 98, and 99.6%, respectively. It is worth noting that for the most active sample Cs–Co₃O₄, the N₂O decomposition starts at a temperature as low as 150°C and the practically full conversion is reached already at the target temperature of 350°C. In our case the promotional effect of cesium is superior to that of the other cations reported previously [37,38,43].

The mechanism of the reaction between N₂O and the catalysts active centers is generally thought to be a charge donation from the catalyst to the antibonding orbitals of N₂O, which destabilizes the N–O bond and leads to scission [5]. Therefore, electron charge transfer from the metal ion to the N₂O molecule is a crucial step for N₂O decomposition [25]. Electron transfer occurs from a low oxidation state metal cation, which then increases its oxidation state. The recoverability of the ion from a high to a low oxidation state is also important for the regeneration of the active centers. Accordingly, the activity of the Co₃O₄ oxide spinel is attributed to the coexistence of a Co²⁺-Co³⁺ ion pair because of a facile one electron transfer between these ions during N₂O decomposition, as shown in the following mechanism [25]:

N₂O (g) + Co²⁺ → N₂O− (ads.) .....Co³⁺ ............ (1)

N₂O⁻ (ads.) .....Co³⁺ → N₂(g) + O⁻ (ads.) .....Co³⁺ ............ (2)

O⁻ (ads.) .....Co³⁺ → ½O₂(g) + Co²⁺ ............ (3)

In this mechanism, Eq. 1 is plausible since N₂O has positive electron affinity of 0.3–0.8 eV [44]. The existence of N₂O^- species was confirmed throughout infrared spectra of adsorbed N₂O on chromia [45]. In addition, the formation of O- species (Eq. 2) was identified by ESR studies during N₂O decomposition over CoO–MgO catalysts [46]. Generally, N₂O can be adsorbed onto the catalyst surface either through oxygen or nitrogen atoms [7]. Adsorption via oxygen atom would increase the N–N bond order and decreases the bond order between N–O atoms leading to the formation of nitrogen molecule and adsorbed oxygen atom (Eqs. 1,2). On the other hand, N₂O adsorption via its the nitrogen end Results in an increase in the bond order between nitrogen and oxygen atoms while decreasing the bond order of the two nitrogen atoms giving NO molecule as a reaction product. This is in a good agreement with the proposed mechanism, where no NO was detected in the reactor outlet (see Sect. 2.3).

Dell et al. [47] correlated the N₂O decomposition activity of a series of metallic oxides with their semi-conductivity behavior. Based on their classification, the highest activity was observed over p-type semi-conductors whereas the lowest activity was correlated with n-type semi-conductors. Because Co₃O₄ is a p-type semi-conductor [25], this could be another reason for its N₂O decomposition activity.

Figure 11 shows the DTA thermograms obtained by heating the 400°C calcined pure and alkali-doped (where alkali cation/ Co=0.05) nano-crystalline Co₃O₄ catalysts in air to 1000°C. For pure nano-crystalline Co₃O₄, one sharp endothermic peak was found at 915°C. This peak is attributed to the thermal reduction of Co³⁺ to Co²⁺. The Introduction of alkali-cations leads to a continuous shift of this peak toward lower temperature in this order: Cs<K<Na<Li<un-doped. For Cs/Co₃O₄ the maximum of this peak is at about 877°C. This peak becames broader with the addition of alkali-cations. It has been reported that the role of alkali cations in the increase of N₂O decomposition activity is to enhance the Co³⁺ to Co²⁺ reduction for the cobalt based catalysts, i.e., the regeneration of Co²⁺ ions that are required for N₂O adsorption and subsequent decomposition [36,39]. Based on the data shown in Figure 11, it is plausible to suggest that the role of the added alkali-cations is to enhance the Co³⁺ to Co²⁺ reduction under catalytic conditions and thus increases the catalytic activity. Moreover, in the open literature there are some particularly important contributions about the influence of catalysts crystallites sizes on their activity. Recently, Abu-Zied [11] reported an inverse relationship for Ag crystallite size and N₂O decomposition activity for a series of Ag/Al_xFe_2-xO₃ catalysts. In addition, Abu-Zied et al. demonstrated that the highest N₂O decomposition activity is obtained over catalysts that have a lower Co₃O₄ crystallite size for a series of SrCO₃- and BaCO₃-Co₃O₄ catalysts [25]. Table 1 reveals that the addition of alkali-cations from Li to Cs is accompanied by a continuous decrease in catalyst crystallite size. Therefore, the highest activity of the cesium-containing catalyst can be related, also, to its lower crystallite size. Finally and based on the above discussion, the observed higher N₂O decomposition activity of Cs/Co₃O₄ could be due to: (i) its small crystallite size, (ii) the change in the electronic properties of the Co₃O₄ as a result of adding Cesium, and (iii) the semi-conductivity character of Co₃O₄ since it is known to be a p-type semi-conductor. Accordingly, our study was extended to investigate the influence of changing the Cs/Co ratio towards N₂O decomposition activity.

Figure 12a depicts the dependence of N₂O conversion on the reactor temperature for nano-crystalline Co₃O₄ and its cesium containing Co₃O₄ catalysts (where Cs/Co=0.0125, 0.025, 0.05, 0.10, and 0.20) being calcined at 400°C. From the inspection of this figure it appears that, over the whole temperature range Cs-doping even at a small ratio (Cs/Co=0.0125) Results in a noticeable increase in activity. A further increase in the Cs/Co ratio is accompanied by a continuous increase in activity until Cs/ Co=0.025. After this ratio a continuous decrease in activity is obtained. However, all the Cs-doped catalysts have higher activity compared with the un-doped nano-crystalline Co₃O₄ catalyst.

Figure 12b shows the variation of the temperature of _50% N₂O conversion, T50, with the Cs/Co molar ratio for cesium doped nano-crystalline Co₃O₄ catalysts being calcined at 400°C. From this figure, the most pronounced effect, as gauged by the _50% conversion temperature, is observed for Cs/Co 0.025, (ΔT_50%=T_50% (un-promoted) - T_50% (alkali promoted)=165°C); for Cs/Co having a molar ratio of 0.05 it drops to ΔT_50%=134°C. Further increase in cesium ratio to 0.1 was accompanied by shift in ΔT_50% value towards higher temperatures by 120°C on average, i.e., ΔT_50%=43°C, whereas for Cs/Co having molar ratio of 0.20 ΔT_50%=24°C. It is noteworthy that, for the most active sample Cs_0.025–Co₃O₄, the N₂O decomposition starts at a temperature as low as 100°C and the practically full conversion is reached already below the target temperature of 350°C.

In a series of Cs promoted Co₃O₄ catalysts and tested their activity towards direct decomposition of N₂O, it was found that, Cs promoted Co₃O₄ catalysts exhibit higher activity for the decomposition of N₂O compared to bulk Co₃O₄ also in the presence of oxygen and steam in the feed stream [40]. The catalyst with Cs/Co ratio of 0.05 showed highest activity. The enhancement in catalytic activity of the Cs promoted catalysts was attributed to the change in the electronic properties of Co₃O₄ which, in turn, facilitate desorption of oxygen during the reaction. It was demonstrated, based on TPR Results, that the presence of small amount of Cs is effective in stabilizing the lower oxidation state of Co due to the interaction between Cs and Co₃O₄, which promotes the reduction of Co³⁺ to Co²⁺. At high Cs content, it appears that Cs migrates to Co surface leading to reduction inhibiting counter effect. Concurrently, it is suggested that the easy release of oxygen might be due to the weakening of Co–O bond in the presence of alkali cations [48]. Thus, the early desorption of O2 in the case of Cs promoted catalysts might be the reason for its high activity at low reaction temperatures.

Thus, in the light of this interesting literature data, it is plausible to suggest that the enhancement effect of the added cesium cations, during N₂O decomposition over Co₃O₄ catalyst, could be also related to the electronic interaction induced by these cations. Such interaction produces electron rich cobalt species that can donate electrons to N₂O molecules leading to the weakening of the N-O bond, thus increasing N₂O decomposition activity. In addition, Cs/Co₃O₄ having Cs/Co 0.025 exhibits the lowest crystallite size and the highest S_BET values among the catalysts of this series. Therefore, as was mentioned above, the observed higher activity of this catalyst (Figure 12a) could be, also, related to its lower crystallite size and higher surface area. Considering the activity decrease with cesium content increase, Pasha et al. [40] investigated cesium-promoted Co₃O₄ and related the observed decrease in activity for the catalysts with a Cs/Co>0.05 to the Hedvall effect [49]. In addition, from Table 1, it was shown that increasing the Cs/Co molar ratio from 0.025 to 0.20 leads to a continuous decrease the surface area as well as increase in the crystallite size. Therefore, we may, also, relate negative effect of Cs in promoting the nano-crystalline Co₃O₄ activity to its role in decreasing the catalysts surface areas as well as increasing the crystallite sizes.

From Figure 12a it was found that the best performance of cesium promoted nano-crystalline Co₃O₄ catalyst is that having Cs/Co ratio of 0.025. Accordingly, we have decided to check the influence of increasing the calcination temperature on its activity. The influence of calcination temperature on the N₂O decomposition activity was reported by many research groups [7,11,25,50] where the observed decrease in activity to BET surface area is correlated to the increase in the calcination temperature.

Figure 13 depicts the dependence of N₂O conversion on the reactor temperature over the cesium containing Co₃O₄ catalysts (where Cs/Co=0.025) being calcined at 400, 500, 600, and 700°C. From a close inspection of this figure one can easily abstract the following points: (i) the onset of the reaction starts at 150, 250, 275, and 375°C over Cs_0.025-Co₃O₄ catalysts calcined at 400, 500, 600, and 700°C, respectively. (ii) The catalyst calcined at 400°C exhibit the higher activity than that catalysts calcined at high temperatures (i.e., 500, 600, and 700°C). (iii) The maximum N₂O decomposition activity, 100%, was obtained over Cs_0.025- Co₃O₄ catalysts calcined at 400°C, at reaction temperature of 350°C. (iv) The T50 (temperature of _50% decomposition) showed a shift by 115, 165, and 205°C to higher temperature upon raising the calcination temperature from 400 to 500, 600, and 700°C, respectively. From Table 1, it was shown that with the increase in the calcination temperature S_BET surface area showed a sharp continuous decrease. The S_BET values decreased, from 34 to 2.7 m².g^-1, as a result of raising the calcination temperature from 400 up to 700°C. In addition, it was shown that the crystallite size of Cs/Co₃O₄ is increased with increasing the calcination temperature from 400 to 700°C. Such effect is a direct response of sintering effects which predominated at high temperatures. Thus, one can state safely that the observed dramatic activity decrease accompanying the calcination temperature rise from 400 to 700°C is attributed to the obtained crystallite size increase as well as the sharp surface area decrease. This, in turn, is accompanied by a decrease in the surface concentration of the Co³⁺-Co²⁺ redox couple necessary for the N₂O initiation and decomposition.

Conclusions

The present investigation includes, studying the promotional effect of doping alkali cations (Li, Na, K, and Cs) on the catalytic activity of nano-crystalline Co₃O₄, prepared via combustion method, in N₂O decomposition. The sequence of the promotional effect was found to be: un-promoted<Li<Na<K<Cs-promoted catalyst. The reason for the increase in activity for the added alkali cations was electronic in nature. Changing the Cs/Co ratio revealed that the maximum activity was obtained over the catalyst with a Cs/Co of 0.025. At higher Cs/Co ratios, a continuous decrease in activity was observed. The observed decrease in activity for the catalysts with Cs/Co>0.025 is related to the crystallite size increases and S_BET decreases.

Acknowledgments

The authors would like to gratefully acknowledge the Deutscher Akademischer Austausch Dienst (DAAD) for granting us the use of gas analyzers for these experiments.

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