Deep oxidation of benzene over LaCoO3 catalysts synthesized via a salt-assisted sol-gel process
Graphical abstract
Introduction
The abatement of volatile organic compounds (VOCs) [1], such as benzene, acetone and isopropanol have received growing attention since they were described as one of the major air pollutants which have been a significant threat to the environment as well as human health [[2], [3], [4]]. The catalytic oxidation of VOCs with low energy consumption and high purification efficiency is regarded as one of the most effective methods for the elimination of VOCs [5]. Perovskite-type oxides (PTOs), which are among the most promising catalysts for this treatment, are considered as alternatives for noble metals, mainly due to their ease of synthesis and low cost as compared to noble metals, while demonstrating good catalytic activity in deep oxidation of hydrocarbons [[6], [7], [8]]. PTOs are drawing more and more attention for their flexible electronic structure, which contributes to diverse physical and chemical properties, especially in catalysis [9]. In particularly, LaCoO3 exhibits excellent catalytic activity for oxidation [10,11].
However, LaCoO3 always tends to sinter at high crystallization temperature (above 600 °C) which causes its inherent low specific surface area (typically less than 5 m2/g) that limits its catalytic activity [6,12]. In order to improve the textural properties of LaCoO3, various approaches have been developed. For instance, perovskite LaCoO3 for CO gas sensor was prepared by an additional high energy ball milling step, resulting in an enhanced specific surface area (66 m2/g) [13]. Besides that, Wang et al. [14] used the mesoporous SiO2 (KIT-6, 916 m2/g) as template to fabricate mesoporous perovskite LaCoO3 with a high surface area of 270 m2/g. Yakovleva et al. [15] proposed a microwave treatment strategy for rapid crystallization and to limit the grain growth of LaCoO3, leading to a relatively high surface area of 34 m2/g. Among these methods, the hydroxy acid complexation route has the advantages of simple in operation and cheap in materials, making it a feasible method for scale-up preparation of PTOs with acceptable textural structure and physicochemical property [16].
On the other hand, metal ions, such as alkaline and alkaline earth metal ions, which serve as additives, have been widely used in perovskite to improve the catalytic performance. For instance, Ca2+ and Mg2+ have been extensively studied to substitute the metal in position A and/or B to improve the catalytic activity for oxidation reactions, such as the complete oxidation of CO [17], ethyl acetate [18], toluene [11], etc. Moreover, Na+ and K+ are not only substituted into the A site of PTOs lattice to generate more active surface oxygen species [[19], [20], [21]], but also are used as raw materials for molten-salt synthesis and salt-assisted solution combustion synthesis [22,23]. PTOs synthesized by the molten-salt method could reach a relatively high surface area (about 30 m2/g for LaCoO3) without using complexing agents during the process. However, this method consumes a large amount of nitrates (lanthanum nitrate, cobalt nitrate, NaNO3 and KNO3 were mixed in a molar ratio of 1: 1: 30: 30) as flux and requires a high calcination temperature (700 °C) [22]. Besides that, salt-assisted solution combustion synthesis is also adopted to prepare the PTOs. By adding fuels like ethylene glycol and sodium salt, PTOs could be obtained at a lower synthetic temperature (300 °C), but this method creates an impure crystal phase [24]. Furthermore, PTOs with porous structure could be obtained by the solution combustion synthesis with fuels like urea [25] and stearic acid [26]. However, in order to improve the crystallinity degree, additional calcination at high temperature is essential, but at the expense of decrease in the surface area of PTOs [23].
In our previous work, perovskite LaCoO3 with a relatively high surface area of 33 m2/g was biosynthesized at a low calcination temperature of 500 °C via the plant-mediated method. In this method, inorganic components (K+, Na+, Ca2+, Mg2+) in plant extract together with NO3− acted as combustion aids resulting in decreasing calcination temperature was demonstrated [27]. However, to the best of our knowledge, there is limited literature focusing on the role of alkaline or alkaline earth metal nitrates with a single component on the low-temperature synthesis of perovskite LaCoO3. In this work, based on our previous studies, we systematically studied the role of alkaline or alkaline earth metal nitrates in the synthesis of LaCoO3 by citric acid complexation method. Besides that, a salt-assisted sol-gel strategy to facile fabrication of pure perovskite LaCoO3 at a low calcination temperature of 500 °C was proposed. In addition, the roles of nitrate additives in altering the activity in the catalytic combustion of benzene were investigated.
Section snippets
Materials
Lanthanum nitrate, cobalt (II) nitrate, potassium nitrate, sodium nitrate, calcium nitrate, magnesium nitrate and citric acid (CA) were purchased from Aladdin Chemicals Co. Ltd. All chemicals are of analytical grades and were used without further purification.
Traditional sol-gel method
For the control samples, the LaCoO3 was prepared by the traditional sol-gel method. The details were as follows: lanthanum nitrate, cobalt nitrate and CA (the molar ratio is 1: 1: 0.4) were dissolved in 20 mL deionized water and stirred
The effect of nitrate additives on catalysts preparation
XRD tests were employed to determine the crystal phase composition of our synthesized samples, the results are given in Fig. 1. Firstly, as shown in Fig. 1a, the sample (LCO-500) synthesized with the traditional sol-gel method at 500 °C showed the main crystal phase of perovskite LaCoO3 (JCPDS No. 48-0123), while some excess peaks, especially at 2θ = 30° were clearly observed. However, a pure phase of LaCoO3 was obtained when the calcination temperature was raised to 700 °C (LCO-700),
Conclusion
In summary, by adding nitrate additives (KNO3, NaNO3, Ca(NO3)2 or Mg(NO3)2) into the sol-gel route, we developed a facile method to synthesize perovskite LaCoO3 with pure phase at a relatively low calcination temperature of 500 °C. The results of X-ray diffraction measurement and thermal gravimetric analysis demonstrated that nitrate additives (especially KNO3 or NaNO3) acted as combustion aids which aggravated the decomposition of CA accompanying with plenty of heat and gases released,
CRediT authorship contribution statement
Junjie Huang: Investigation, Data curation, Writing - original draft. Kuncan Wang: Investigation, Writing - original draft. Xintong Huang: Investigation, Validation. Jiale Huang: Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 41673088), the Fundamental Research Funds for the Central Universities (No. 20720182010) and the XMU Training Program of Innovation and Entrepreneurship for Undergraduates (No. 2019Y1590). The authors thank Qinghua Chen’s group in Fujian Normal University and Dr. Mingzhi Wang in Xiamen University for their enthusiastic help in some characterizations.
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