Elsevier

Catalysis Today

Volume 339, 1 January 2020, Pages 274-280
Catalysis Today

Strong impact of cobalt distribution on the activity for Co3O4/CaCO3 catalyzing N2O decomposition

https://doi.org/10.1016/j.cattod.2018.10.036Get rights and content

Highlights

  • The stepwise precipitation made the Co3O4/BaCO3 catalyst a special structure.

  • It has an outstanding activity in the catalysts with the same composition.

  • The catalyst displayed better resistance to sintering at 800 °C.

  • It is also stable and highly active in the presence of O2 and H2O at 350 °C.

Abstract

For the Co3O4/CaCO3 catalysts used for N2O decomposition, the activity of the catalyst prepared by stepwise precipitation is much superior to the catalysts prepared by impregnation or coprecipitation in the same composition. HRTEM observation and EDX analysis indicate that the stepwise precipitation leads to cobalt existing as little Co3O4 crystallites tightly bound to the CaCO3 particles. The special structure made the Co3O4 ideal accessibility and better interaction with the support in the catalyst, and the CaCo2.5(SP) catalyst with this structure is much more active than the CaCo2.5 catalyst reported in literature prepared in traditional method. At 300 °C, N2O in the feed gas 2000 ppmv N2O/Ar was completely converted over the CaCo2.5(SP) at 20,000 h−1. Moreover, the CaCo2.5(SP) catalyst exhibited better resistance to sintering at 800 °C and quite high activity under the presence of 5 vol% O2 and 2 vol% H2O at 350 °C as well.

Introduction

Nitrous oxide (N2O) has attracted great attention due to its large impact on global warming and ozone depletion [[1], [2], [3]]. The N2O came from nitric and adipic acid manufactures as well as from vehicle exhaust are the main contributors that could be controlled [4,5]. Catalytically decomposing the N2O into harmless N2 and O2 is an effective technology to eliminate the N2O from the flue gases [6,7]. For the catalyst to be used for treating the flue gas in adipic acid production, which contains N2O in the range of 20–50 vol.%, the investigations are mainly focused on thermal stability of the catalysts, due to the exothermic process to be treated [2,8,9]. However, for the catalyst to treat the flue gas in nitric acid production, which contains N2O in the range of 0.03-0.35 vol.%, developing a catalyst with higher activity at low temperature in the presence of impurity gas (e.g. O2 and H2O) is the main challenging topic, because the temperature increasing due to the N2O decomposition is negligible in this case and the inhibition effect of the coexisting impurity gases on the activity of catalyst become severe at low temperature [2,10].

Generally, it is accepted that the catalytic decomposition of N2O over metal oxides could be approximately described by the following steps [2,5,[11], [12], [13]]N2O+*O*+N22OO2+2(L-H)N2O+OO2+N2+(E-R)Firstly, N2O is adsorbed on the surface of catalyst and then dissociate to N2 (g) and O∗ (i.e. adsorbed oxygen species). As the amounts of adsorbed N2O over the metal oxides are often negligible, the two steps are often described as the step (1). Secondly, the active sites (*) can be regenerated by the recombination of adsorbed oxygen species via the Langmuir-Hinshelwood (L-H) mechanism step (2) or by the reaction of N2O molecule with the O* site via the Eley-Rideal (E-R) mechanism step (3) [11,12]. Herein, it should be noted that the active sites just refer to the vacant sites that are not only capable of accepting the oxygen atom come from the N2O dissociation, but also capable of being regenerated via the step (2) or (3), as reported in the literature [2,5,[11], [12], [13], [14], [15], [16]]. The active sites regeneration is the rate-determining step for the N2O decomposition [5,17,18]. Clearly, to regenerate the active site by whichever of the two routes, breaking the Msingle bondO bond is required. Hence, how to weaken the Msingle bondO bond and therefor accelerate the rate-determining step is the most important topic for the research of the catalyst working at lower temperature.

So far, most of the investigations on the catalyst have used Co3O4 as active component due to the oxide possessing higher activity compared to the other transitional metal oxide. The Co3O4 has a typical spinel structure Co(II)Co(III)2O4, in which the Co(II) occupies the tetrahedral sites and the Co(III) occupies the octahedral sites [[19], [20], [21], [22]]. It is initially believed that the Co2+ over the catalyst surface works as the oxygen vacancy for Co-based spinel catalysts, such as MgxCo1-xCo2O4 [23], MxCo1-xCo2O4 (Mdouble bondNi, Mg) [24], Cosingle bondCe [25] and Ksingle bondCosingle bondCe [26] catalysts. Recently, Sojka and the coworkers proposed that the active site catalyzing the reaction is the octahedral Co(III) in the spinel structure. It is strongly supported by the relationships between their activity results and the structures for Co3-xFexO4 catalysts characterized by Mössbauer spectroscopy [27], and by the relationships between the reaction rates and the Raman shifts arisen from the A1g vibration of octahedral Co(III) for Co3O4 and different K-Co3O4 catalyst samples [28], and by the activity change arisen from non-redox Mg2+ and Al3+ cations replacing Co(II) or Co(III) in the Co3O4 as well as DFT calculations [29].

Alkali metals, particularly, Cs [30] and K [28,31,32] as the additive into Co3O4 significantly improve the catalytic activity. Nevertheless, it is reported that the activity was notably hindered by the coexisted impurity gases such as O2, H2O, NOx and CO2 [30,31]. On the other hand, for the Co3O4 catalysts using Ni [6,24], Cu [33], Zn [34] or Ce [25,26] as the additives, although the activity is not comparable to that using alkali metals as the additives when impurity gases were absence in the feed gas, the tolerance of catalysts to the impurity gases is much better. Moreover, Mg as the promoter of Co3O4 not only increased the specific surface area of the catalyst, but also enhanced the capability of the catalyst producing oxygen vacancies [23,35]. The other alkaline earth metals doping Co3O4 was also studied for the reaction by impregnation [36], it was found that Sr and Ba have positive effect whereas Ca has negative effect on the activity. However, recently, it was reported that addition of Ca into Co3O4 by coprecipitation greatly promoted the activity of the catalyst [37].

Xue et al. reported that promotional effect of Ce on the activity of Co3O4 is significantly dependent on the catalyst preparation method [26]. Abu-Zied et al. found that the rare earth metal ions, Pr3+, Sm3+ and Tb3+, which have larger ionic radius compared with that of Co2+, being added into Co3O4 by microwave assisted method largely enhanced the catalytic activity [38]. In our previous studies, we found that the coprecipitation was the best method for introducing Bi [39], Pb [40] and Ag [41] into the Co3O4 for improving the catalyst activity. These findings clearly indicated that the catalyst with high activity for the reaction must possess a relative ideal structure, with which the active component Co not only could contact better with the additive and therefore could be effectively benefited from electric interaction with the additive, but also could sufficiently expose to reactant molecules and therefor catalyzing them under the reaction conditions.

As mentioned before, Ca was reported to exert quite different impacts on the catalyst activity when used as additive of Co3O4 by impregnation or coprecipitation. Herein, we noticed that the solubility product constant of CaCO3 (4.9 × 10−9) is much higher than that of CoCO3 (1.4 × 10-13), which implies that in the coprecipitation process, CoCO3 would be precipitated prior to CaCO3, leading to undesired coverage of Co3O4 by CaCO3 in some extent. Thus, a more active Co3O4/CaCO3 catalyst is expected if a better structure favoring the accessibility of Co and the interaction of Co with Ca could be constructed. In this paper, we report that the Co3O4/CaCO3 catalyst was obtained by stepwise precipitation, and the special texture endues the catalyst an outstanding activity for the reaction.

Section snippets

Catalyst preparation

Co(NO3)2·6H2O, Ca(NO3)2·4H2O and Na2CO3 were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. and Sinopharm Group Chemical Reagent Co., Ltd., respectively. All of the reagents were directly used for the catalyst preparation without purification.

The CaCox catalysts (x = 0 ∼ 20) were generally prepared using the Ca(NO3)2·4H2O and Co(NO3)2·6H2O as precursors and Na2CO3 as precipitant in the following procedures: Ca(NO3)2 aqueous solution (0.2 mol/L) containing desired amount of Ca(NO3)2·4H2

Textural structure of the CaCox(SP)

The textural structure for the CaCox(SP) catalysts was studied by XRD, and TEM techniques. Fig. 1. shows X-ray diffraction patterns of the CaCox(SP) and the Co3O4(P). All of the CaCox(SP) catalysts exhibited characteristic diffraction peaks of the spinel structure of Co3O4 (JCPDS 43-1003) without peak shift, while the samples with higher Ca contents (Ca/Co 1/5) also exhibited the characteristic peaks of CaCO3 (JCPDS 47-1743). No the other peak could be observed on all of the samples. The

Conclusions

In summary, the CaCo2.5(SP) catalyst prepared by stepwise precipitation method has the following special structure. The active component cobalt exists as the little Co3O4 crystallites with average size of ca. 16 nm, which are tightly bound to the CaCO3 phase that plays the roles of both promotional additive and support in the catalyst. This structure not only significantly enlarged the accessibility of the active component to the reactant molecules, but also ensured the better interaction

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21777015 and 21277019).

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