Effect of Substitution Degree and the Calcination Temperature on the N 2 O Decomposition over Zinc Cobaltite Catalysts

In this paper, a series of zinc cobaltite catalysts with the general formula ZnxCo1−xCo2O4 (x = 0.25, 0.50, 0.75 and 1.0) has been prepared using the co-precipitation method. Thermal analyzes (TGA and DTA) were used to follow up the thermal events accompanying the heat treatment of the parent mixture. Based on these results, the various parent mixtures were calcined at 500 ̊C. The obtained solid catalysts were characterized by using XRD, FT-IR and N2-adsorption. The catalytic decomposition of N2O to N2 and O2 was carried out on the zinc-cobaltite catalysts. It was found that partial replacement of Co by Zn in Co3O4 spinel oxide led to a significant improvement in their N2O decomposition activity. Moreover, the catalytic activity was found to be depended on the calcination temperature utilized.


Introduction
In the last two decades, there has been considerable increased concern about the harmful effects of N 2 O on our atmosphere.N 2 O is recognized as a strong greenhouse gas and also severely destructs the ozone in the stratosphere [1] [2] [3] [4].
Moreover, it causes the formation of acid rains [4].The atmospheric lifetime of nitrous oxide is about 120 years and its present concentration is 326 ppbv, whereas its Global Worming Potential (GWP) is 310 times higher than that of CO 2 [1].The catalytic decomposition of N 2 O to its elements, i.e.N 2 and O 2 , is consi-dered as an efficient rout to minimize its emission to the atmosphere.
The catalytic decomposition of N 2 O was investigated over various catalysts formulations.An interesting review on this topic was published recently by Konsolakis [3].The reported state of the art catalytic systems is bare oxides, hexaaluminates, hydrotalcites, spinels, perovskites and mixed metal oxides under various effluent stream components (e.g., O 2 , NO and H 2 O) [3].Among these catalysts categories, metal oxide based spinel catalysts revealed the lowest light off temperature (temperature corresponded to the 50% conversion).Therefore, focusing our attention to this catalysts category, high N 2 O decomposition activity was exhibited by this class of catalysts [24]- [36].For instance, Russo et al. [24] studied N 2 O decomposition over a series of spinel oxide catalysts (chro- mites, ferrites and cobalities) being prepared by the solution combustion route.Their results indicated that the catalytic activity of the prepared spinel oxides essentially depended mostly on the B site metal (Cr, Fe and Co).The catalysts hosting cobalt at the B site presented the best N 2 O decomposition activity [24].
Yan et al. [25] [26] reported an excellent catalytic performance of M x Co  [27].Concurrently, Shen et al. [28] presented a detailed study on N 2 O decomposition over different oxide supported Co 3 O 4 spinel catalysts prepared via the co-precipitation method and found that Co 3 O 4 /MgO with cobalt loading of 15% showed the best activity, where a 100% conversion was obtained at temperatures higher than 425˚C.The activity of the metal cobaltittes during N 2 O decomposition is greatly enhanced by the presence of dopants.In this way, activity increase was reported on doping Co 3 O 4 with Sr 2+ and Ba 2+ [29].Concurrently, promotion effect was reported on doping MgCo 2 O 4 with Li + , Na + , K + and Cs + [30].
Recently, we have reported the effect of transition metal exchange as well as the calcination temperature on the N 2 O decomposition activity of Ni x Co 1−x Co 2 O 4 [31] and Cu x Co 1−x Co 2 O 4 [32] catalysts.Although the N 2 O decomposition over zinc cobaltite catalysts was previously reported by Yan et al. [26], their catalysts were calcined at low temperature (400˚C), which would not permit the activity measurement at higher temperatures.Therefore, and in a continuation of that work, our recent work [31] [32], the present paper attempts to prepare a series of Zn x Co 1−x Co 2 O 4 (x = 0.25, 0.50, 0.75 and 1.0) catalysts through the thermal decomposition reactions of their corresponding metal carbonates at higher temperature (500˚C).Our main goal is to study the oxygen evolution via N 2 O decomposition over this series of catalysts.The catalysts were characterized using TGA, DTA, XRD, FT-IR and nitrogen adsorption at −196˚C.Moreover, the experiments will be extended to check the influence of increasing the calcination temperature (up to 1000˚C) on the activity of the best catalyst in this series.

Catalysts Preparation
A series of catalysts with the general formula Zn x Co 1−x Co 2 O 4 (x = 0.0; 0.25; 0.50; 0.75 and 1.00) were synthesized by co-precipitation method [31] [32].An aqueous solution of K 2 CO 3 (1 M) was added drop-wise into a mixed aqueous solution containing known amounts of Co(CH 3 COO) 2 •4H 2 O and Zn(CH 3 COO) 2 •4H 2 O at room temperature under mechanical stirring until a pH value of 9.1 was reached.The precipitate was filtered, and then washed intensively with distilled water.Finally, the obtained cakes was dried overnight at 100˚C and then calcined in static air at 500˚C for 3 h.In order to investigate the influence of the calcination temperatures on the catalytic activity, two other catalysts (with x value = 0.75) were prepared employing the same procedure and calcined at 750 and 1000˚C.

Catalytic Activity Measurements
The catalytic performances of the various Zn x Co 1−x Co 2 O 4 catalysts were evaluated in a quartz tube fixed-bed reactor.A mixture of the reactant N 2 O (500 ppm) and the N 2 as a balance gas was fed at the constant rate of 200 ml•min −1 via two thermal mass flow controllers to the reactor, which is placed in an electric oven.For each experiment 0.5 g of the catalyst was used and pretreated in N 2 at 500˚C for 1 h, then cooled to desired temperature.The temperature in the reactor was measured by a K-type thermocouple placed in the center of the catalyst bed and was controlled by a Cole-Parmer temperature controller (type Digi-Sense 89000-00).The inlet and outlet gases concentrations were analyzed with non-dispersive infrared analyzer for N 2 O and NO components (ABB, AO2020-Uras 14) and amagnetic oxygen analyzer (ABB, AO2020-Magnos 106).All the experiments revealed the absence of NO in the reactor outlet gases.

Catalysts Characterization
Thermoanalytical measurements (TGA and DTA) were carried out using a Shimadzu DT-60 instrument apparatus.The sample (10 mg) was placed in a platinum crucible and heated at a heating rate of 10˚C min −1 in air flowing at a rate of 40 ml•min −1 .XRD was used to check the formation of the spinel oxides structure in the prepared solids.X-ray diffraction patterns were obtained at room temperature using a Philips X-ray diffractometer (type PW 103/00) employing copper radiation (λ = 1.5405Å).The X-ray tube was operated at 35 kV and 20 mA.The diffraction angle 2θ was scanned at a rate of 0.06 min −1 .The data were analyzed using JCPDS standards cards.The FT-IR spectra were recorded at room temperature for the prepared catalysts in the wavelength region 4000 -400 cm −1 using KBr disk technique.Nitrogen adsorption-desorption isotherms were constructed using a NOVA 3200e automated gas sorption system at liquid nitrogen temperature (−196˚C).Prior to the measurements, each sample was degassed for 3 h at 250˚C.The potassium ion concentrations in the various samples were determined by atomic adsorption using 210 VGP atomic absorption spectrophotometer.

Thermal Analyses
The thermal events accompanying the heat treatment, from ambient till 1000˚C, of the non-calcined zinc/cobalt mixture, for the parent with x = 0.75, where monitored using TGA and DTA analyses.Inspection of the obtained TGA thermogram, Figure 1 ZnCo 2 O 4 formation, using other precursors, at the same temperature range [42] [43].Finally, as reported for other similar systems [31] [32] [37] [38] [39] [40] [41], the observed endothermic peak at 915˚C could be attribute to the decomposition of ZnCo 2 O 4 spinel yielding a mixture of its constituent oxides.In addition, weak intensity reflections attributable to ZnO (JCPDS 80-0075) were also found with increasing the x-value.In this respect, it was demonstrated that heating zinc cabaltite at temperatures as such as 500˚C leads to its partial decomposition forming ZnO [27] [43].Moreover, doping ZnCo 2 O 4 with oxides like ZrO 2 enhances its thermal stability at 550˚C -750˚C temperature range [27].

X-Ray Diffraction
The catalysts crystallite sizes were calculated using Scherrer equation.The obtained values for the zinc-containing catalysts (Table 1) are lower than that of the bare Co 3 O 4 (28 nm) [31].The estimated potassium ion concentrations are, also, listed in Table 1.All the zinc-containing spinels exhibit higher potassium content compared to the bare spinel (0.18 mg•g −1 ) [31].N 2 O decomposition (vide infra), the study was extended to check the effect of increasing the calcination temperature on its activity.Figure 3 shows the XRD patterns for the Zn/Co mixture with x = 0.75, which are calcined at 500˚C, 750˚C and 1000˚C.It is evident that raising the calcination temperature from 500˚C to 750˚C leads to: 1) a marked decrease in the intensity of the reflections due to ZnO; and 2) an intensity increase of all the peaks due to the spinel oxide.
Further increase in the calcinations temperature to 1000˚C resulted in a dramatic change in the obtained XRD pattern.One can spot the fact that the intensity of all reflections showed marked decrease.In addition, new reflections emerged at: 1) 2θ = 36.72˚,42.32˚ and 61.42˚ attributable to CoO (JCPDS 75-0533); and 2) at 2θ = 31.75˚,34.44˚, 47.57˚, 56.62˚, 62.91˚ and 69.09˚ characterizing ZnO (JCPDS 80-0075).This picture suggests that zinc cobaltite decomposes to the oxides of its constituents, i.e., zinc and cobalt oxide.This goes paralleled with the observed WL at 911˚C in the relevant TG thermogram (Figure 1(a)).[47].In this regard, the detection of the carbonate absorptions for these samples goes parallel with the measured residual potassium ions concentration for the zinc containing catalysts (Table 1).The spectra of the two catalysts having x = 0.75 and 1.00 show a weak absorption at 480 cm −1 , which is characteristic of the ZnO phase [43] [48].This in turn, suggests the presence of ZnO as an impurity for these two catalysts.Such finding agrees well with the information gathered from the XRD analysis in the previous section.All the spectra show two other bands at 1642 and 3200 -3600 cm −1 , which are due to the δ (OH) and ν (O-H) modes of water molecules, respectively [27] [31] [32].

FT-IR Spectra
Figure 5 shows the FT-IR spectra of the catalyst with x-value of 0.75 being calcined at the 500˚C -1000˚C temperature range.Inspection of this Figure   reveals that all spectra show the two bands characterizing the Zn-Co spinel structure at 577 and 671 cm −1 .However, the intensity of these two absorptions decreases continuously with the calcination temperature rise.Concurrently, Kostova et al. [48] reported that the intensity of the Zn-Co spinel bands, ν 1 and ν 2 , increase with temperature increasing to as high as 700˚C.The intensities of these bands stop to increase at 800˚C and decrease after treatment at 900˚C.
Such result is in a good agreement with the XRD analysis for the same samples [48].All the obtained spectra (Figure 5) indicate the persistence of the absorptions due to the carbonate phase with the temperature raise.For the sample calcined at 750˚C, one can notice the disappearance of the absorption due to ZnO at 480 cm −1 , which suggests the presence of zinc as Zn-Co spinel only without ZnO impurities.Such suggestion is in a good agreement with the XRD results (Figure 3).The spectrum for the 1000˚C calcined catalyst shows a weak absorption at 553 which is due to the CoO [49] [50].

N2 Adsorption
N 2 adsorption data of the catalyst with x = 0.0 is published elsewhere [32].Nitrogen adsorption-desorption isotherm s of the zinc-containing catalysts being calcined at 500˚C are plotted in Figure 6.As it can be seen from that The specific surface areas of these adsorbents were calculated using the BET equation and the obtained values are tabulated in Table 2.It is obvious that S BET decreases as the x-value increases till x = 0.50 followed by an increase on further x-value increase till x = 1.00.1000˚C is accompanied by a continuous S BET and S t decrease.Moreover, Table 2 indicates that this decrease is accompanied by a continuous decrease in both the micropore and the total pore volumes.showed that the partial replacement of Co 2+ by Zn 2+ in Co 3 O 4 spinel oxide led to a significant improvement in the catalytic activity for the N 2 O decomposition, and the Zn 0.36 Co 0.64 Co 2 O 4 catalyst was the most active in their investigated samples.However, the precise origins of the observed high activities have not been reported in their studies.This observed difference in catalytic activity for N 2 O decomposition, between the data presented in Figure 8 and the report of Yan et al. [25], could be due to the difference of preparation method and post-synthesis treatment of the precursor compounds.

N2O Decomposition Activity
The obtained high activity of the Zn/Co catalysts compared to the bare Co 3 O 4 spinel oxide catalyst [31] [32] can be understood in terms of the following points: 1) From the catalysts characterization data, it was shown that the thermal reduction of the spinel phase to its components, for the catalyst with x = 0.75, occurs at 915˚C as shown by the endothermic peak in Figure 1 [32], Ag/Fe x Al 2-x O 3 [51] and SrCO 3 -and BaCO 3 -Co 3 O 4 [30] catalysts.
3) The high activity of the zinc containing catalysts can be attributed, again, to their higher potassium ions content, compared to the Co 3 O 4 [31], and the higher S BET values for the catalysts having x = 0.75 and 1.00.The reported mechanism for N 2 O decomposition over cobalt oxide spinel catalysts requires the presence of Co 2+ -Co 3+ surface-redox couples [29] [30] according to: rise.Pushing the calcination temperature to 1000˚C is accompanied by a sharp drop in the activity where the maximum conversion did not exceed 12% at 500˚C reactor temperature.The characterization results demonstrated that calcining this composition at 750˚C leads to the formation of the perfect spinel structure.Therefore, it is plausible to relate the observed activity decrease to the observed decrease in the BET surface area (Table 2) as well as the expected crystallite size increase at such temperature.Regarding the 1000˚C calcined catalyst, it was also concluded from the characterization data that at such pretreatment temperature zinc cobaltite decomposes to the oxides of its constituents, i.e., zinc and cobalt oxide, together with the Co 3+ → Co 2+ reduction.Thus, one can state safely that in addition to the sintering effects which predominate at high temperatures, the observed sharp activity decrease for the 1000˚C calcined catalyst is influenced by the structure modifications taking place at such high temperature.Such modifications would lead to a retardation of the Co 2+ → Co 3+ → Co 2+ redox cycle which is essential for N 2 O decomposition [29] [30] [31] [32].

Conclusion
This paper focuses on the preparation and activity evaluation of zinc substituted Figure 1.TGA (a) and DTA (b) thermograms obtained for zinc/cobalt mixture with x = 0.75.

X
-ray diffraction patterns were determined for the 500˚C calcination products of the Zn/Co mixtures.The obtained patterns (Figure 2) were matched with the authentic JCPDS data in order to characterize the phases formed during the calcination process.Our analysis showed that the obtained difractogrames matched well with those standards of Co 3 O 4 (JCPDS 78-1969) and ZnCo 2 O 4 (JCPDS 81-2299), which are reported by many research groups [27] [42] [43] [44] [45] [46].
Figure 2. XRD powder diffractograms obtained for the Zn-Co catalysts calcined at 500˚C.

Figure 3 .
Figure 3. X-ray powder diffractograms obtained for Zn 0.75 Co 0.25 Co 2 O 4 catalysts being prepared by the co-precipitation method and calcined at 500˚C, 750˚C and 1000˚C.

Figure 4 .
Figure 4. FT-IR spectra obtained for Zn x Co 1−x Co 2 O 4 (x = 0.25, 0.50, 0.75 and 1.00) being prepared by the co-precipitation method and calcined at 500˚C.

Figure 5 .
Figure 5. FT-IR spectra obtained for Zn 0.75 Co 0.25 Co 2 O 4 being prepared by the co-precipitation method and calcined at 500˚C, 750˚C and 1000˚C.
Figure the introduction of Zn to the Co 3 O 4 with x = 0.25 and 0.50 leads to the increase in the Type I character of the obtained isotherms.Further increase in the Zn content till x = 1.00 is accompanied by recovering of isotherms Type II character.

Figure 6 .
Figure 6.Nitrogen adsorption-desorption isotherms of the Zn x Co 1−x Co 2 O 4 catalysts calcined at 500˚C (closed symbols refer to adsorption branches whereas open ones refer to desorption branches).

Figure 7 (
Figure 7. V a-t plots (a) and Pore volume distribution curves (b) obtained for the Zn x Co 1−x Co 2 O 4 catalysts calcined at 500˚C.

Figure 8 (Figure 8 .
Figure 8(a) shows the variation of N 2 O conversion with x-value over Zn x Co 1−x Co 2 O 4 catalysts at 150˚C -500˚C temperature range.Inspection of this Figure reveals that all over the reactor temperature range increasing the zinc ions content, i.e., x-value, leads to a continuous activity increase till x = 0.75.Further increase in the Zn 2+ concentration, i.e., for the ZnCo 2 O 4 catalyst, results in a slight activity decrease.The relevant T 50 values of these catalysts together with that of the catalyst with x = 0.00 [31] [32] are shown in Figure 8(b).From the inspection of Figure 8(b), it appears that all the Zn-containing catalysts exhibit higher activity than Co 3 O 4 , x = 0.00, i.e., lower T 50 values, where the lowest value is exhibited by the catalyst with x = 0.75.This finding agrees well with the reported results for bare Co 3 O 4 measured under the same experimental conditions [31][32].The dependence of the activity promotion on the Zn-concentration was also reported for this system of catalysts by Yan et al.[25].Their results

Co 3 O
4 catalysts, Zn x Co 1−x Co 2 O 4 (x = 0.25, 0.50, 0.75 and 1.0) through the thermal decomposition reactions of their corresponding metal carbonates.Characterization techniques indicated that the prepared catalysts adopt the spinel structure, which decomposed at high temperatures (930˚C).These catalysts were tested for N 2 O-direct decomposition at 150˚C -500˚C reactor temperatures.The obtained results indicate that these catalysts are promising candidates for low temperature N 2 O abatement.The N 2 O conversion activity is influenced by various parameters which include the nickel content, the crystallite size, the residual potassium ions, catalyst surface area and the calcination temperature.The optimum balance of these parameters, which leads to the highest activity, is fulfilled by the 500˚C calcined Zn 0.75 Co 0.25 Co 2 O 4 catalyst.
1−x Co 2 O 4 (M = Mg 2+ , Ni 2+ and Zn 2+ ) spinel catalysts for the decomposition of nitrous oxide.The Zn 0.36 Co 0.64 Co 2 O 4 catalyst is the most active in the studied samples.Highest activity performances were observed for Mg 0.54 Co 0.46 Co 2 O 4 , Ni 0.74 Co 0.26 -Co 2 O 4 and Zn 0.36 Co 0.64 Co 2 O 4 compositions.It was shown that, Zr 4+ doping (0.05 -0.15 mol.%) of ZnCo 2 O 4 led to the stabilization of this spinel at high calcination temperatures and improved its activity during N 2 O decomposition

Table 1 .
Crystallites size and K + concentrations in the various Zn-Co mixtures calcined at 500˚C.

Table 2 .
Texture data obtained from the analysis of nitrogen sorption isotherms of the Zn x Co 1−x Co 2 O 4 catalysts being calcined at 500˚C.
transformation of Type II to Type I of the obtained isotherms.Following the variation of S BET values with the calcination temperature they obtained values,

Table 2 ,
manifests that, as expected, raising the calcination temperature to [31]In comparison to the bare Co 3 O 4 , which decomposes at 930˚C[34], it appears that the addition of zinc ions enhances the reduction of Co 3+ ions.This, again, supports the promotional role of the added transition metal cation (Zn 2+ ) during N 2 O decomposition throughout facilitating the redox cycle Co 2+ → Co 3+ → Co 2+ and thus increasing the catalytic activity[29] [30][31][32].2)Thecalculatedcrystallitesizes of the various catalysts (Table1).In comparison with the crystallite size of pure Co 3 O 4 catalyst, 28 nm[31], it is evident that the values of all Zn 2+ containing catalysts are lower than that of pure Co 3 O 4 catalyst.Moreover, the trend of variation in these values with x-values is similar to that observed during N 2 O decomposition over this series of catalysts.Therefore, it is plausible to suggest that, the observed activity patterns of this series of catalysts are also influenced by the spinel crystallite size.This finding is in a good agreement with the reported N 2 O decomposition increases upon decreasing catalysts crystallite size of Mg x Co 1−x Co 2 O 4 [29], Ni x Co 1−x Co 2 O 4 [31], Cu x Co 1−x Co 2 O 4