Full Length Article Kinetic and catalytic analysis of mesoporous metal oxides on the oxidation of Rhodamine B Morena S. Xaba, Ji-Hyang Noh, Keabetswe Mokgadi, Reinout Meijboom ⇑ Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park, 2006 Johannesburg, South Africa article info Article history: Received 10 October 2017 Revised 24 January 2018 Accepted 27 January 2018 Available online 31 January 2018 Keywords: Heterogeneous catalysis Mesoporous metal oxide Rhodamine B Langmuir-Hinshelwood model Reaction kinetics Diffusion limitation abstract In this study, we demonstrate the synthesis and catalytic activity of different mesoporous transition metal oxides, silica (SiO 2 ), copper oxide (CuO), chromium oxide (Cr 2 O 3 ), iron oxide (Fe 2 O 3 ) cobalt oxide (Co 3 O 4 ), cerium oxide (CeO 2 ) and nickel oxide (NiO), on the oxidation of a pollutant dye, Rhodamine B (RhB). These metal oxides were synthesized by inverse micelle formation method and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), adsorption-desorption isotherms (BET) and H 2 -temperature programmed reduction (TPR). UV–vis spec- trophotometry was used to monitor the time-resolved absorbance of RhB at k max = 554 nm. Mesoporous copper oxide was calcined at different final heating temperatures of 250, 350, 450 and 550 °C, and each mesoporous copper oxide catalyst showed unique physical properties and catalytic behavior. Mesoporous CuO-550 with the smallest characteristic path length d, proved to be the catalyst of choice for the oxidation of RhB in aqueous media. We observed that the oxidation of RhB in aqueous media is dependent on the crystallite size and characteristic path length of the mesoporous metal oxide. The Langmuir-Hinshelwood model was used to fit the experimental data and to prove that the reaction occurs on the surface of the mesoporous CuO. The thermodynamic parameters, E A , DH # , DS # and DG # were cal- culated and catalyst recycling and reusability were demonstrated. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction Wastewater from industrial plants, in particular, textile, paper mills, colorant and dye manufacturers, produce large amounts of dyestuff which are then discarded in the natural streams [1–4]. The major constituents of these pollutants are heavy metals, pesti- cides, dyes, aliphatic and aromatic detergents, degreasing agents, volatile organics and chlorophenols [1]. Dyes are usually synthetic aromatic compounds, which are seriously harmful to the aquatic and marine life. Rhodamine B (RhB) (Fig. 1) is one of the most com- mon xanthene dyes which is applied in textile industries and has been found to be potentially toxic and carcinogenic [4]. In addition, wastewaters from dyeing and textile industries easily produce toxic trihalomethanes when chlorinated. RhB is toxic and resistant to biodegradation and direct photolysis, therefore, RhB has a considerable contribution to the environmental pollution [5]. Therefore, the removal of these pollutants from the environment is an important challenge in ecological systems, because of their toxicity and carcinogenic properties [5–7]. We, therefore, aim to demonstrate that mesoporous metal oxides can be used to degrade these toxic chemicals to less harmful ones. RhB can be detected by UV–Vis spectroscopy at a maximum wavelength of k max = 554 nm and it is this absorption wavelength that we take advantage of to elucidate the kinetics of catalytic reactions of our mesoporous metal oxides. Mesoporous metal oxides were first synthesized by a liquid- crystal template method in 1992 by Kresge and co-workers [8]. Mesoporous materials have been used as supports and catalysts for oxidation and reduction reactions [9,10]. For example, meso- porous titanium dioxide [11], zinc oxide [12], copper oxide [13], graphene oxide [14], iron oxide [15], cerium oxide [16] and cobalt oxide [17], have been used as catalysts. Rhodamine B degradation in the presence of an oxidizing agent such as hydrogen peroxide (H 2 O 2 ) has been studied previously [18], we also incorporated H 2 O 2 as it is an environmentally friendly oxidant. Metal nanoparti- cles have also been used to investigate the degradation of organic dyes [14,19,20]. However, nanoparticles are usually colloidal and homogeneous, which poses a potential drawback for separation of catalyst from products. The mesoporous copper oxide is interesting because it has tune- able properties such as pore size, pore volume, surface area, crys- tallite size, and thus can be used to control activity as desired. A https://doi.org/10.1016/j.apsusc.2018.01.241 0169-4332/Ó 2018 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail addresses: rmeijboom@uj.ac.za, reinout.meijboom@gmail.com (R. Meijboom). Applied Surface Science 440 (2018) 1130–1142 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc