Reactivity of NiO for 2,4-D degradation with ozone: XPS studies
Graphical abstract
Introduction
Catalytic ozonation with metal oxides has shown to enhance the degradation of recalcitrant compounds in water. Generally, two mechanisms are proposed during catalytic ozonation: (i) ozone decomposition on the catalyst surface enhancing hydroxyl radical (OH) generation, where the hydroxyl groups were active sites and (ii) forming surface complexes between the pollutants and the surface metal sites of the catalysts, which renders the coordinated pollutants more reactive than conventional ozone.
In recent years, several researchers have found that the degradation of organic compounds followed the OH oxidation pathway in the ozonation combined with heterogeneous catalysts, using nano-TiO2 [1], ceramic honeycomb [2], [3], Mn-ceramic honeycomb [4], synthetic goethite [5], and so on. It is believed that when introduced into water, metal oxides tend to strongly adsorb H2O molecules. The adsorbed H2O dissociates to OH−and H+, forming surface hydroxyl group with the surface metal and oxygen sites, respectively [6]. Therefore, ozone can react with the surface-bound OH, initiating the production of OH on the surface of the manganese dioxide [7]. Furthermore, it is also reported that dissolved ozone adsorbs into the catalyst's surface and subsequently decompose rapidly due to presence of adsorbed hydroxyl groups on the surface of aluminum oxide [8]. Specifically, the experimental results from Ma et al. using FeOOH and ceramic honeycomb impregnated with Mn and Cu showed that uncharged surface bound hydroxyl groups can induce the ozone decomposition to generate OH in aqueous solution [4], [5]. Nevertheless, the same research group proposed, in the case of ceramic honeycomb modified with metals: Zn, Ni and Fe system, a mechanism where an ozone molecule–surface −OH2+ groups interaction takes place and might be responsible of the catalytic activity, as a result of electrostatic and/or hydrogen bonding interactions [9].
Hydroxyl radical generation is useful in degrading a large number of water soluble organic compounds [10], [11], [12], [13], [14], [15]. Though, the presence of saturated organic matter (ozonation products) or bicarbonate/carbonate in the reaction medium caused that hydroxyl radical oxidation turns out to be ineffective. Therefore, catalytic ozonation through complexes formation pathway would be acceptable for the degradation of saturated compounds.
Recently, some publications refer that oxalic acid degradation using CeO2 supported palladium or copper oxide did not promote hydroxyl radical generation from ozone [16], [17]. These results expose that both, the support (CeO2) formed surface complexes with oxalic acid, while active phase (PdO) of the catalyst carried out a selective reaction and further decomposed ozone in diverse chemical species. The high activity of the catalyst increased the oxalate degradation in water due to the synergetic function of two oxides (PdO and CeO2) [16].
On the other hand, it has a great effort to evidence the reactive oxygen species generated by the interaction of the ozone with catalyst in aqueous solution. Lately, it has reported that FTIR [18], ATR-FTIR [19] and Raman spectroscopy [16] were used to reveal the formation of some surface complexes or new species originated during catalytic ozonation. In addition to previous cited techniques, X-ray photoelectron spectroscopy (XPS) appears as a powerful technique to clarify the surface phenomena carried out during the organic compound degradation in aqueous solution as a result of the interaction between 2,4-D and the catalysts. Furthermore, XPS can help to elucidate the reaction mechanism in the catalytic ozonation.
In previous works, it has been demonstrated the utility of XPS as a tool to determine the formation of new surface species, which would be responsible for the catalytic activity [20], [21]. Specially, we have found that the interaction of ozone and 2,4-D on the surface of Ni/TiO2 catalyst leads to the formation of O* and Ti* which together with the OH increase slightly the degradation of the pollutant compared with ozone alone [20]. Furthermore, in the degradation of di-n-butyl phthalate (DBP) with NiFe2O4 catalysts, it was shown by XPS and other techniques that oxidizing reaction occurred on the surface and Ni2+ provided the electrons, indicating the involvement of Ni2+ - Ni3+ - Ni2+ redox processed during the reaction.
In this study, four commercial catalysts (TiO2, SiO2, Al2O3 and NiO) were used in the 2,4-dichlorophenoxyacetic acid (2,4-D) elimination. 2,4-D is the most widely used herbicide worldwide to control broadleaf weeds in cereal and grain crops, recreational areas, golf courses and gardening. Moreover, the herbicide cannot be biodegraded effectively for concentrations higher than 1 ppm [22]. It has been detected as a major pollutant in ground and surface waters. Most publications about 2,4-D degradation in aqueous solution have been focused to the use of advanced oxidation processes (AOPs), for example: photo-Fenton [23], catalytic ozonation [15], [24] and photocatalytic process [25], [26]. The results of previous processes showed that the mineralization of 2,4-D needed a prolonged reaction time, thus a good option should be heterogeneous catalytic ozonation. The use of XPS allowed us to elucidate the formation of surface complexes and new species originated due to ozone exposure.
Section snippets
Materials and reagents
A model solution was prepared with 80 mg L−1 of 2,4-D (Alfa Aesar, 98%) in distillate water, the initial pH was 3.1. The pH was not controlled during the ozonation. All chemical used in the experiment were analytic grade reagents. TiO2 (Degussa, 99.5%), SiO2 (Cab-o-sil, 95%), nanoparticles of Al2O3 and NiO (Sigma–Aldrich, 99%) were used in the experiments as commercial catalysts.
Ozonation
The ozone concentration in gas phase was measured by Ozone Analyzer BMT 946BT (BMT Messtechnik, Berlin). The catalysts
Comparison of mineralization degree with all catalysts
Catalytic ozonation mechanism of metal oxides has been reported that starts with hydroxyl groups’ chemisorption on the catalyst surface [6]. When these groups get in contact with ozone, a chain reaction gives rise to OH generation. Hence, if the quantity of hydroxyl groups increase in the catalyst surface, a proportional increment in OH would also take place. In consequence, the organic compound would have a higher conversion in the medium. Considering this fact, Fig. 1 shows the mineralization
Conclusions
The present study investigated commercial catalysts for 2,4-D degradation by ozone in aqueous solution. It was found that NiO, in comparison with TiO2, SiO2, Al2O3, is a good candidate to mineralize high concentrations of the herbicide (80 mg L−1) in short reaction times (1 h) and with a high stability. Based on the experimental results and discussion, the following conclusions can be drawn.
- 1.
The higher ozone decomposition (27%) was achieved by NiO, which was explained in terms of its ability to
Acknowledgements
The author thanks the Department of Graduate Study, Investigation of the National Polytechnic Institute of Mexico (Project 20110185) and the National Council of Science and Technology of Mexico – CONACyT (Projects: 153356 and 156150)
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