The role of thermal analysis in the development of high-iron-content kaolinite-based photocatalysts
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The role of thermal analysis in the development of high‑iron‑content kaolinite‑based photocatalysts Veronika Vágvölgyi1 · Katalin Győrfi2 · Balázs Zsirka2 · Erzsébet Horváth2 · János Kristóf1 Received: 3 September 2019 / Accepted: 13 January 2020 © The Author(s) 2020
Abstract Dynamic and controlled-rate thermogravimetric analyses have been carried out on acid-treated (11 and 5.8 M HCl), highiron-content kaolinites as potential photocatalysts. The mineral contaminants were determined by XRD, while the defect sites of reduced coordination number obtained by surface treatments were identified with 27Al MAS NMR spectroscopy. Upon heating, water is evolved from the surface-treated samples in three main stages: (1) removal of adsorbed water up to ~ 200 °C, (2) goethite dehydroxylation between 200 and 350 °C and (3) dehydroxylation of the clay in the 300–700 °C temperature range. Identification of water released from the above mass loss steps is difficult due to the significant overlap of steps 2 and 3, as well as to the presence of coordinated water at broken edges and defect sites (–OH+2 groups). As a result, the thermal behavior of surface-treated kaolinites should be taken into account both in the preparation of hybrids/composites and in the acid–base characterization of the catalytic surface. Keywords Kaolinites · Photocatalysis · Surface modification · CRTA
Introduction Kaolinite is a 1:1 type layered silicate with the general formula Al2Si2O5(OH)4 consisting of a two-dimensional arrangement of Si-centered tetrahedra and a two-dimensional arrangement of Al-centered octahedra. There are three types of OH groups located along octahedral edges: The socalled inner OH groups (iOHs) are in the plane common to both the tetrahedral and octahedral sheets and are not accessible by reagent molecules. The so-called inner-surface OH groups (isOH) form strong hydrogen bonds with the oxygen sheet of the next double layer. The OH groups located on the outermost surface of the particles and along broken edges are called as outer-surface OH groups (osOHs). The double layers held together via H bonds show a parallel orientation along the ‘c’ axis (book-type arrangement). The basal distance is around 7.1 Å. With suitable reagents (e.g., with short-chain fatty acids), the H bonds holding together the * Erzsébet Horváth [email protected] 1
Department of Analytical Chemistry, University of Pannonia, Egyetem u. 10, Veszprém 8200, Hungary
Institute of Environmental Engineering, University of Pannonia, Egyetem u. 10, Veszprém 8200, Hungary
2
double layers can be broken up, leading to the formation of intercalation complexes. (The double layers are separated by individual reagent monolayers.) With cascade intercalation, the clay can be separated to individual double layers. (This process is called as exfoliation.) The OH groups along octahedral edges result in polar TO layers of hydrophilic nature [1]. In the last decade, the results on the composites and hybrids of the 1:1 type clay minerals along with their novel
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