Supported cobalt oxide on MgO: Highly efficient catalysts for degradation of organic dyes in dilute solutions

Abstract

Cobalt oxide catalysts immobilized on various oxides (MgO, ZnO, Al₂O₃, ZrO₂, P25, SBA-15) were prepared for degradation of organic dyes in dilute solutions via a sulfate radical approach. Their efficiency in activation of peroxymonosulfate (PMS) was investigated for the degradation of methylene blue (MB). Among the catalysts employed, the Co/MgO catalyst was found the most active. The complete degradation of MB occurred in <7 min when the Co/MgO catalyst with an optimum Co₃O₄ loading of 5 wt% was used. The performance of the Co/MgO catalyst is found better than both the homogeneous cobalt ions and heterogeneous Co₃O₄ catalyst. XPS analysis indicates that the surface of the MgO support is extensively covered by the hydroxyl groups. Hence, it is suggested that the alkaline MgO support plays several important roles in (i) dispersing the cobalt oxide nanoparticles well, (ii) minimizing the leaching of cobalt ions into the liquid phase, and (iii) facilitating the formation of surface Co–OH complex which is a critical step for PMS activation. Besides MB, other organic dyes such as orange II and malachite green, can also be degraded within a few minutes using the Co/MgO catalyst. It is believed that the highly efficient and environmentally benign Co/MgO catalyst developed in this work can be widely applied in advanced oxidation technologies towards degradation of organic pollutants.

Introduction

Due to more stringent statutory regulations and increasing public concerns on harmful organic substances, there is an imperative need to develop efficient technologies for the complete removal of organic pollutants from wastewater effluents. Over the past years, sulfate radicals based-advanced oxidation technologies (SRs-AOTs) have emerged as a promising way leading to the total mineralization of most organic pollutants [1], [2], [3]. Oxone (peroxymonosulfate, PMS, as the active component) has been widely used in oxidation of organic pollutants due to its stability and ease of handling compared to H₂O₂ [2], [4], [5], [6], [7], [8], [9], [10], [11], [12]. This highly active oxidant has also been used in biochemistry [13] and polymer [14] industry to provide reaction initialization radicals. It has been reported that the coupling of transition metal ions like Fe²⁺, Cu²⁺, Mn²⁺ and Co²⁺, to PMS leads to the accelerated generation of SRs and hence higher oxidation efficiencies [3], [7], [15]. The various combinations between the dissolved transition metal ions and oxidants have been systematically evaluated for the generation of active radicals by Anipsitakis and co-workers. It was found that PMS coupled with Co²⁺ ions is the best combination for the generation of SRs for degrading 2,4-dichlorophenol (2,4-DCP) [3]. However, the potential health hazards caused by the dissolved Co²⁺ ions in water render such a homogeneous Co²⁺/PMS system with limited use. Therefore, the activation of PMS by heterogeneous cobalt sources has been given great attention recently. Dionysiou and co-workers studied both the unsupported Co₃O₄ and supported Co₃O₄ on several common supports (Al₂O₃, SiO₂, TiO₂) as the heterogeneous activators for PMS to degrade 2,4-DCP [10], [11], [16]. Kiwi and co-workers used polytetrafluoroethylene flexible film as the support for Co₃O₄ nanoparticles to decompose organic dyes under light irradiation [9]. Chen et al. evaluated the performance of unsupported Co₃O₄ nanoparticles [12].

Besides playing important roles in dispersing the Co₃O₄ nanoparticles and minimizing the leaching of cobalt ions into the liquid phase, support materials also facilitate the formation of surface Co–OH complex which is the critical step for heterogeneous activation of PMS [11], [12], [16]. In particular, it has been reported that TiO₂ plays a significant role in promoting this step [11]. As the surface properties vary extensively among different support materials, it is worthwhile to further study the effects of different supports for the development of efficient oxidation catalysts. To meet the ever rising environmental challenges, the desirable heterogeneous cobalt catalysts should offer higher activation efficiencies than the homogeneous Co²⁺ ions, as well as good operational stability.

MgO is frequently employed as an alkaline catalyst for some reactions of commercial importance [17], [18], [19]. The abundant surface basic sites on MgO are expected to generate the surface functional Co–OH complex. Herein, we report cobalt oxide supported on MgO (Co/MgO) as an efficient heterogeneous catalyst for activation of PMS. A series of other commonly available support materials including Al₂O₃, P25–TiO₂, SBA-15 (mesoporous SiO₂), ZnO and ZrO₂ were used for comparison. Their activities were measured and compared in the degradation of methylene blue (MB), which is a cationic, non-biodegradable and toxic dye used extensively in dying industry [20], [21], [22], [23]. It has been found that the Co/MgO catalyst is able to completely degrade MB and other organic dyes in dilute solutions within a very short duration of a few minutes without light irradiation. The current catalyst system is therefore expected to be highly desirable for competing with the conventional nondestructive treatment processes.