Oxidative degradation properties of Co-based catalysts in the presence of ozone

Abstract

Four series of cobalt-based catalysts, such as bare Co₃O₄ and CoO, CoO_x–CeO₂ mixed oxides, CoO_x supported over alumina and alumina–baria and CoMgAl and CoNiAl hydrotalcites have been synthesized and investigated for the oxidative degradation of phenol in the presence of ozone. Characterizations were obtained by several techniques in order to investigate the nature of cobalt species and their morphological properties, depending on the system. Analyses by XRD, BET, TPR, UV–visible diffuse reflectance spectroscopy and TG/DT were performed.

The CoNiAl hydrotalcite exhibits, after 4 h of reaction, the highest phenol ozonation activity followed by Co(3 wt%)/Al₂O₃–BaO and CoMgAl. The samples Co(1 wt%)/Al₂O₃–BaO and Co(1 and 3 wt%)/Al₂O₃ show a comparable medium activity, while the oxidation properties of bare oxides Co₃O₄, CoO and CoO_x–CeO₂ are really low. Leaching of cobalt ions in the water solution was detected during the reaction, the amount varied depending on the nature of catalysts. A massive release was observed for the CoMgAl and CoNiAl hydrotalcites, while cobalt catalysts over alumina and alumina–baria look much more stable. The recycle of CoO_x/Al₂O₃ and CoO_x/Al₂O₃–BaO was studied by performing three consecutive cycles in the phenol oxidation. Because of the potential interest of the cobalt-supported catalysts in the ozonation process, the oxidative degradation of naphtol blue black was also investigated.

On the basis of TPR and UV–visible results it appears that highly dispersed Co²⁺ ions especially present over Co(3 wt%)/Al₂O₃–BaO are the main active sites for phenol and naphtol blue black oxidative degradation by ozone.

Introduction

Removing pollutants from industrial process waters and wastewaters is becoming an important area of research as the amount and quality of freshwater available in certain regions of the world continues to decrease due to growing water demands and/or long periods of drought. Moreover, increasingly stricter wastewater discharge standards continue to be introduced worldwide in an effort to reduce the environmental impacts of industrial processes. Nowadays several technologies are available for reclamation of industrial process waters and wastewaters.

In the last years, many efforts have been devoted to the heterogeneous wet oxidation [1], [2], [3], [4]. Oxidative degradation is a powerful methodology, in this context the oxidizing agent may be H₂O₂, O₂ or O₃. Ozone, a powerful oxidizing agent (under acidic conditions E(O₃/O₂) = 2.07 V), is effective for the mineralization of refractory organic compounds. However, it reacts slowly with aromatic organic compounds and, in many cases, it does not cause the complete oxidation. Catalysis combined with the ozonation (oxidation by ozone) process improves the degradation of organic compounds and nowadays finds wide application in the field of wastewater treatments [5], [6], [7], [8], [9], [10], [11]. These processes are generally characterized by the production of OH radicals. A typical homogeneous catalytic ozonation involves the use of iron salts [12] or coupled with UV radiation [13]. The irradiated Fe(III) species in aqueous solution undergo a photo-redox process which gives rise to Fe(II) and hydroxyl radicals via photo-Fenton reaction. Simultaneously, the re-oxidation of Fe(II) into Fe(III) by oxidizing species in solution such as O₃, H₂O₂ or HO₂ confers an interesting catalytic aspect to the process and larger amount of oxidizing HO can be formed.

Heterogeneous catalytic ozonation dating back from the 1970s [14] is now, again, attracting interest. The major advantage of a heterogeneous over a homogeneous catalytic system is the ease of catalytic retrieval from the reaction media. However, the stability and durability of the catalyst under operating condition is important. Leaching of the catalytic active species or poisoning of the active sites or fouling of the catalyst surface by intermediate reaction products are important factors, which determine the stability and durability of the catalyst.

During the last decade, many studies have been focused on the removal of refractory organic compounds from water such as phenol, hydrocarbons, carboxylic acids, etc. [6], [13], [15], [16], [17], [18]. Co-based catalysts, such as Co/SiO₂ [19] or CoO_x/Al₂O₃ [20] have been used in ozone decomposition studies. The samples containing highly dispersed Co²⁺ species over the silica surface [19] have been recognized as the most efficient systems for the degradation of organic pollutants by ozone through a redox process:

Co²⁺ + O₃ + H₂O → Co(OH)²⁺ + HO + O₂

HO + O₃ → HO₂ + O₂

HO + organic pollutants → products (CO₂ + H₂O)

HO₂ + Co(OH)²⁺ → Co²⁺ + O₂ + H₂O

In the case of CoO_x/Al₂O₃ catalyst [20] the high oxidation activity in presence of ozone has been ascribed to the high content of active and mobile oxygen which strongly depends on the preparation method.

Even if cobalt oxides have shown high catalytic activities, to our knowledge, very few works deal with aqueous heterogeneous catalytic ozonation with cobalt oxide catalysts. One of these studies dealt with the removal of carboxylic acids, such as formic acid with different catalysts supported on activated carbon or SiO₂ [21].

In a more recent work, ozone and a cobalt catalyst supported over alumina have been used to oxidize oxalic acid from water at acidic pH [22]. Despite the high catalytic efficiency, the catalyst leached some cobalt into the water. As a result, the removal of oxalic acid was due to both heterogeneous and homogeneous catalytic ozonation.

Therefore, the attainment of active and stable catalysts for oxidative degradation of organic pollutants in water is still a challenge.

Recently, we have reported that different cobalt species (Co₃O₄, surface Co²⁺ ions, CoAl₂O₄ spinel) can be formed over the surface of alumina and alumina–baria supports, depending on the cobalt content and calcination temperature [23]. In particular, the presence of barium oxide in the alumina network is effective in the stabilization of well-dispersed Co²⁺ species, as well as in preventing sintering of alumina and diffusion of cobalt into the bulk upon treatment at high temperature [23], [24], [25]. Furthermore, our latest findings confirm that the mixed oxide Co₃O₄ is effective for the oxidation of methane at relatively low temperature and its activity is promoted by interaction with ceria which enhances the surface and bulk oxygen mobility [26], [27], [28].

On the other hand hydrotalcite-like compounds have received in the recent years an increasing attention in many different fields, such as catalysis [29], [30]. This wide range of applications comes from some special features of the hydrotalcites, bi-dimensional layered compounds containing in the network divalent cations (Mg²⁺, Co²⁺, Ni²⁺, Zn²⁺, Mn²⁺, Cd²⁺) and trivalent cations (Al³⁺, Fe³⁺, Cr³⁺, Ga³⁺) which positive charge is compensated by anions in the interlayer region [31]. The presence of transition metal ions which are redox sites gives to the hydrotalcites good catalytic activity in oxidation reactions [6].

On these bases, in the present work we considered worthwhile to investigate the oxidative effectiveness of four series of cobalt catalysts, as a function of the preparation method and nature of the support.

Here, the synthesis, characterization (XRD, BET, TPR, UV–visible spectroscopy, TGA/DTA) and catalytic activity are reported for pure Co₃O₄ and CoO oxides, CoO_x–CeO₂ mixed oxides, CoO_x supported on alumina and alumina–baria, CoMgAl and CoNiAl hydrotalcites. The oxidative degradation of aqueous phenol solutions and aqueous naphtol blue black solutions in the presence of ozone were investigated as test reactions. Phenol is generally taken as a model compound being present in many industrial wastewaters, naphtol blue black is a typical toxic azo-dye used in textile industries.

For two selected samples, Co(3 and 1 wt%)/Al₂O₃–BaO, investigation of cobalt species by TPR was performed also after three cycles of phenol oxidation.