Enhanced heterogeneous activation of peroxymonosulfate by Ruddlesden-Popper-type La2CoO4+δ nanoparticles for bisphenol A degradation
Graphical abstract
Introduction
Bisphenol A (2,2-bis (4-hydroxyphenyl) propane, BPA) is intensively utilized as a key industrial chemical for the production of polycarbonate plastics and epoxy resins in the plastic industry [1]. Unfortunately, a significant amount of BPA is continuously discharged into the environment and detected in food, drinking water, and aquatic animals [2]. BPA mimics estrogenic activity in human and animal bodies causing disruption of the endocrine system through interfering with hormonal and homeostatic functions [3]. Therefore, the removal of BPA especially from aquatic environments is of great interest.
Recently, the Fenton reaction employing hydroxyl (OH) and sulfate radicals (SO4–) has attracted increasing interest as a promising method to oxidize organic pollutant in water [4], [5], [6]. Compared to the OH, SO4– has a longer half-life time, higher standard oxidation potential, and wider pH range applicability, which is more efficient for bisphenol A degradation [7]. Sulfate radicals are typically generated by peroxymonosulfate (PMS) or peroxydisulfate (PDS) activation using heat [8], ultrasound [9], UV [10], electrochemical processes [11], and transition metal ions (e.g., Co, Mn, Cu, Fe, and Ni) [12].
Homogenous transition-metal catalysis is one of the feasible and cost-effective ways to generate SO4–. Among the transition metals, Co2+ exhibits the most efficient homogeneous catalyst for PMS activation [13] and is catalytically regenerated during PMS activation, which enhances its catalytic activity [14]. However, Co-based homogeneous activation has serious drawbacks, especially secondary metal-ion pollution and high catalyst consumption [15]. Thus, heterogeneous, Co-based catalysts such as cobalt oxide [16], cobalt-based bimetallic oxide [17], [18], and supported cobalt oxide [19] nanoparticles have been investigated for PMS activation. Among them, transition metal-based perovskites (ABO3 with B being the transition metal) have attracted increasing attention in heterogeneous catalysis because of their flexibility in chemical composition and their ability for generating oxygen vacancies, thus enabling catalytic redox reactions [20]. More recently, some reports have focused on different transition metals La-based perovskites (LaMO3 (M: Co, Cu, Fe, and Ni)) for PMS activation [21]. Particularly, LaCoO3 perovskites have been considered being very effective to generate SO4– from PMS as Co–O– bonds enable a high number of reactive sites for PMS activation [22]. Moreover, the B-site cation deficiency in the LaCoO3−δ perovskite structures enhances their catalytic activity [23]. Although these perovskite catalysts have some advantages such as high catalytic activity, they suffer from metal leaching in the aqueous medium [24]. Thus, it is essential to explore highly stable heterogeneous catalysts for PMS activation.
Compared to transition metal-based perovskites (ABO3), Ruddlesden-Popper mixed oxides (A2BO4) nanoparticles (RP-MONp) exhibit higher stability and redox ability, because their crystal structure is more opened due to extra space between A-site atoms. They consist of alternating layers of perovskite (ABO3) and rock salt (AO) [25]. These layered structures possess good thermal stability [26] and show high ionic conductivity. Several studies on RP-based catalysts show that the RP-MONp reveal better oxygen diffusion coefficients and surface exchange than other ABO3 perovskite type. They have been reported as catalysts in various heterogeneously catalyzed reactions including oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) [27], photocatalytic water splitting [28], NO and CO removal [29], oxidation of urea and small alcohols [30], as a cathode for a solid oxide electrolysis cell [31], and as a catalyst in the Fenton reaction [32]. In this regard, La2CoO4+δ has been selected to be used as a PMS activator since its structure can tolerate excess amounts of oxygen upon oxidation at room temperature up to a δ of 0.16 [33]. Moreover, the oxidation state of the B-site cation in this structure can be controlled by tailoring oxygen non‐stoichiometry [34]. To summarize, La2CoO4+δ is a highly suitable material for advanced catalytic oxidation processes.
Several methods have been demonstrated to synthesize Ruddlesden-Popper mixed oxide catalysts including co-precipitation [35], sol–gel synthesis [36], combustion methods [37], hydrothermal flow synthesis [38], and solid-state reactions [39]. However, all these methods require high-temperature annealing (≥1000 °C) at an extended period, which promotes particle growth and sintering, leading to a reduced catalytically active surface area. As an alternative, spray-flame synthesis is an established method to produce non-sintered, homogeneously distributed perovskite nanoparticles down to 10 nm, and even below [40], [41]. Surface-sensitive measurements of spray-flame-synthesized LaCoO3 nanoparticles show a high content of Co2+ ions being present at the particles’ surface. These results are attributed to the local formation of oxygen-deficient or layered perovskite structures such as A2BO4 type Ruddlesden-Popper structures, which can stabilize Co2+ much better than the pure perovskite ABO3 [42]. Thus, specific interest is in the targeted formation and investigation of Co-containing Ruddlesden-Popper-type nanoparticles as highly active heterogeneous catalysts.
Herein, the objective of this study is to seek a one-step method for the synthesis of La2CoO4+δ nanoparticles using a spray-flame method with the purpose of improving their catalytic performance with a very low amount of metal leaching to achieve the environmental requirements in respect of organic pollutant degradation. The performance of the catalysts for PMS activation was evaluated in terms of BPA degradation. Moreover, effects of several important parameters such as initial pH value, inorganic anions (such as Cl−, NO3− H2PO4−, HCO3−, etc.), and water bodies (tap water, and drinking water) on their catalytic performances along with the reusability of the La2CoO4+δ nanoparticles were also investigated. The radical generation was identified by radical scavenging experiments and electron paramagnetic resonance studies, and a mechanism for PMS activation is proposed.
Section snippets
Materials
For the synthesis of La2CoO4+δ nanoparticles, La(CH3COO)3 1.05 H2O (99.9%; Sigma‐Aldrich) and Co(CH3COO)2 4 H2O (≥98%; Sigma-Aldrich) were used as the metal precursors while propionic acid (ACS reagent, >99.5%; Sigma‐Aldrich), propanol (anhydrous, 99.7%; Sigma‐Aldrich), ultrapure DI water (18.2 MΩ cm at 25 °C) and ethanol (>99.8%; Sigma‐Aldrich) were employed as solvents. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (>97.0%; TCI), Bisphenol A (≥99%), potassium peroxymonosulfate, sodium phosphate
Characterization of La2CoO4+δ nanoparticles
The XRD patterns of the pristine nanoparticles were analyzed to determine crystalline phases of La2CoO4+δ and LaCoO3–x (Fig. 2). The LaCoO3–x diffraction peaks show a good match to a LaCoO2.937 perovskite phase (Inorganic Crystal Structure Database (ICSD): 153995), while the XRD pattern of La2CoO4+δ can be mainly related to a Ruddlesden-Popper type phase of La2CoO4.13 (ICSD: 237238) with a minor content of La(OH)3 (ICSD: 31584). Notably, the diffraction peaks of La2CoO4+δ appear more broadened
Conclusions
In summary, we have demonstrated that La2CoO4+δ nanoparticles were successfully synthesized via spray-flame synthesis, and applied as novel catalysts for PMS activation. The spray-flame synthesis method has practical advantages such as scalability to produce large amounts of La2CoO4+δ nanoparticles in one step without high-temperature annealing, eco-friendliness, and cost-effectivity. Compared to LaCoO3–x catalyst, La2CoO4+δ nanoparticles exhibit a faster catalytic activity for BPA oxidation
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.
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