Tuning the photocatalytic performance of mesoporous silica-titanium dioxide and cobalt titanate for methylene blue and Congo red adsorption/photodegradation: Impact of azo dyes concentration, catalyst mass, wavelength, reusability and kinetic properties

Abstract

The present investigation explored the utilization of ordered mesoporous silica SBA-15 as a supporting matrix for photoactive materials, specifically TiO2 as anatase and mixed oxide TiO2/CoTiO3 forms. The resultant photoactive composites, denoted as SBA-15_1, SBA-15_2, and SBA-15_3, were synthesized with varying proportions of TiO2 and CoTiO3 and subjected to comprehensive physicochemical characterization. The findings unequivocally established the successful preparation of photocatalytic composites, accompanied by a discernible reduction in textural properties as the TiO2/CoTiO3 content increased. Notably, the presence of Ti(III) cations as Ti2O3 and Co(II) as dopant in the form of CoTiO3 was identified through X-ray photoelectron (XPS) and energy-dispersive X-ray (EDX) spectroscopy, contributing to an enhanced photocatalytic activity relative to commercial Aeroxide® P-25 titanium dioxide. The analysis of kinetic properties confirmed the effective and fast photodegradation of azo dyes (methylene blue (MB) and Congo red (CR)). SBA-15_1 exhibited a 30 % higher efficiency in the photodegradation of methylene blue than P-25. Furthermore, the initial concentration of the azo dye significantly affected the photodegradation process. SBA-15_1 degraded 1.1 mg g−1 at an initial concentration of 10 mg L−1 and 27.6 mg g−1 at 50 mg L−1 of methylene blue, corresponding to 11 % and 55.2 % degradation, respectively. Congo red displays similar trends with values of 1.2 mg g−1 at 10 mg L−1 and 10.9 mg g−1 at 50 mg L−1, representing degradation of 12.2 % and 21.8 % after 30 min. The degradation of the azo dye mixture proved a competitive reaction characterized by a high degree of adsorption and subsequent degradation of CR, followed by MB. The synergistic effect was manifested by an increased amount of azo dyes removed by the adsorption/photodegradation mechanism, 27.8 mg g−1 for MB and 25.5 mg g−1 for CR in 1: 1 (w: w, 25 mg: 25 mg) mixture at 254 nm after 30 min. The influence of the radiation wavelength used was also studied, while better photocatalytic results were observed using UV radiation (254 nm) compared to near-visible light (380 nm). Furthermore, the recycling of the SBA-15_1 material demonstrated its reusability, with catalytic activity maintained after five cycles. The obtained results significantly increase the economic and ecological prospects associated with the use of such composite materials in practice.