Uso de hematita potencializada com sítios ácidos em processos tipo-Fenton

Abstract

In this work, a catalyst was prepared by sulfating the surface of iron oxides for application in advanced oxidative processes and used in the degradation of different organic dyes. A precursor, goethite, was prepared using the coprecipitation technique and, subsequently, this precursor was impregnated with an aqueous solution of sulfuric acid, followed by heat treatment at 500 ° C for 3 hours. When undergoing this heat treatment, goethite is converted to the hematite phase. The catalysts were characterized by Raman Spectroscopy and X-Ray Diffraction, which made it possible to identify the iron phases. Before sulfation, goethite was the main phase identified, and after the procedure with sulfuric acid, hematite prevailed. The scanning electron microscopy coupled with an X-rays spectroscope by dispersive energy allowed to analyze the morphology of the catalysts, which are constituted by a non-segregated distribution of the main constituent elements in the samples. The Molecular Absorption Spectroscopy studies in the Infrared Region identified the functional groups present in the catalyst structure, in which it was possible to verify characteristic bands of the goethite and hematite phases, as well as the characteristic bands of the bonding of the sulfate group with the oxide metal. The Thermal Analyzes (TG, DTG, and DTA), allowed to analyze the thermal stability of the catalysts. Goethite is stable up to 40 °C, from which temperature water molecules start to leave. Hematite, on the other hand, presents a gradual loss of mass, attributed to the dehydroxylation of Fe-OH groups. Sulfated hematites showed a loss of mass close to 450 ° C related to sulfate decomposition. With the study of Nitrogen Physisorption Isotherms, it was possible to measure the specific surface area of the catalysts, and those submitted to more than one consecutive cycle had the highest area values. Regarding the surface acidity, obtained by the back-titration of the acid groups, it was possible to determine the number of surface acidic sites of the catalysts, and after sulfation, the acidity of the oxide increased about 18 times. The degradation kinetics of the dyes showed a 90% reduction in methylene blue in 9 minutes and for crystal violet and black Remazol B, 87% and 80%, respectively, in 18 minutes. The catalyst applied in a heterogeneous phase was very promising, as it maintained its activity for more than one consecutive reaction cycle, that is, after the 20 minutes of reaction, the catalyst still had catalytic activity. The leachate tests showed that the functional groups added to the catalyst surface were not leached to a homogeneous phase, thus guaranteeing the operational viability of the process. After the loss of activity in the fifth cycle, the catalyst could be regenerated by heat treatment in an oxidizing atmosphere, from a new charge of aqueous sulfuric acid solution. After regeneration, the catalyst showed excellent catalytic capacity, being able to carry out several cycles of methylene blue degradation.