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Highly active sulfated titania photocatalysts (SO42−/TiO2) were prepared using Ti(SO4)2 as precursor via a precipitation and peptization process. Precipitates obtained through adding NaOH solution into Ti(SO4)2 were peptized with HNO3, followed by calcination at varying temperatures to form SO42−/TiO2. The objective of the present work is to clarify the effect of calcination temperature on the structure and surface properties of SO42−/TiO2. A variety of characterization techniques, involving X-ray diffraction (XRD), Brunauer–Emmett–Teller analysis (BET), X-ray photoelectron spectroscopy (XPS), surface photovoltage spectroscopy (SPS), and electron spin resonance (ESR), were employed in this study. XRD revealed that well-crystallized titania was formed upon calcination at 300 °C (SO42−/TiO2-300). Owing to the stabilization effect of sulfate, SO42−/TiO2-300 exhibited small crystalline size and high surface area, which can account for its superior photocatalytic performance. Catalyst calcination at 600 °C (SO42−/TiO2-600) rendered rapid growth of crystalline size and drop of surface area. Meanwhile, such high-temperature calcination treatment gave rise to Ti3+ defects and oxygen vacancies. Although the photoactivity of SO42−/TiO2-600 was remarkably reduced as compared with SO42−/TiO2-300, it was still higher than that of Degussa P25 due very likely to the formation of Ti3+-oxygen vacancies-Ti3+ sites that are beneficial for the separation of photoinduced electrons and holes.

Sulfate radical-based advanced oxidation technologies (SR-AOTs) are attracting considerable attention due to the high oxidizing ability of SRs to degrade organic pollutants in aqueous environments. This study was carried out to respond to current concerns and challenges in SR-AOTs, including (i) need of heterogeneous activation of sulfate salts using transition metal oxides, (ii) nanoscaling of the metal oxide catalysts for high catalytic activity and promising properties with respect to leaching, and (iii) easy removal and recovery of the catalytic materials after their applications for water and wastewater treatments. In this study, we report a novel approach of using Fe–Co mixed oxide nanocatalysts for the heterogeneous activation of peroxymonosulfate (PMS) to generate SRs targeting the decomposition of 2,4-dichlorophenol, and especially focus on some synthesis parameters such as calcination temperature, Fe/Co contents, and TiO 2 support. The physicochemical properties of the catalysts were investigated using porosimetry, XRD, HR-TEM, H 2 -TPR, and XPS. Ferromagnetic CoFe 2 O 4 composites formed by thermal oxidation of a mixed phase of Fe and Co exhibited significant implications for the efficient and environmentally friendly activation of PMS, including (i) the cobalt species in CoFe 2 O 4 are of Co(II), unlike Co 3 O 4 showing some detrimental effects of Co(III) on the PMS activation, (ii) CoFe 2 O 4 possesses suppressed Co leaching properties due to strong Fe–Co interactions (i.e. Fe–Co linkages), and (iii) Fe–Co catalysts in form of CoFe 2 O 4 are easier to recover due to the unique ferromagnetic nature of CoFe 2 O 4 . In addition, the presence of Fe was found to be beneficial for enriching hydroxyl group content on the Fe–Co catalyst surface, which is believed to facilitate the formation of Co(II)-OH complexes that are vital for heterogeneous PMS activation.

In order to generate sulfate radicals (SRs) as oxidizing species for the degradation of 2,4-dichlorophenol (2,4-DCP) in water, we explored heterogeneous activation of peroxymonosulfate (PMS) by supported cobalt catalysts. More attention was given to the effect of support materials (Al2O3, SiO2, TiO2) and cobalt precursors (Co(NO3)2, CoCl2, CoSO4) on cobalt–support interaction, cobalt leaching, and reactivity of the catalysts. Especially, the feasibility of simultaneous generation of SRs and hydroxyl radicals (HRs) in PMS-Co/TiO2 systems was first studied under ultraviolet (UV) radiation. Much lower cobalt leaching was observed in Co/Al2O3 and Co/TiO2 systems than that of Co/SiO2 most probably due to their relatively strong cobalt-support interaction. Co/TiO2 catalyst prepared with Co(NO3)2, compared to CoCl2 or CoSO4 (where Cl− and SO42−, respectively, were not completely removed upon heat treatment at 500 °C), showed strong cobalt–support interaction, and thereby exhibited negligible cobalt leaching. Under UV radiation, Co/TiO2 at Co/Ti molar ratio of 0.001 showed significant improvement in the degradation of 2,4-DCP due to HRs. The effective generation of HRs in the system can be explained with Co(III)-mediated charge transfer from the photoinduced electrons to PMS, inducing facilitation of photoinduced electron-hole separation. However, high cobalt loading (i.e., Co/Ti molar ratio of 0.1) on TiO2 surface exhibited negligible enhancement of 2,4-DCP transformation under UV radiation since the penetration of UV light to TiO2 was prohibited by the cobalt.

Titania–silica mixed oxide (TiO 2 –SiO 2 ) was prepared from the alkoxides: titanium tetrabutoxide (TBOT) and tetraethyl orthosilicate (TEOS), by using an ammonia–water solution as the hydrolysis catalyst. Variables included synthesis route, aging time, the type of alcohol, and calcination temperature. The materials were then evaluated as photocatalysts for the decomposition of n -heptane. Experimental results revealed: (1) the photoactivity of TiO 2 –SiO 2 prepared via the route that Ti sol and Si sol were simultaneously and slowly added, was significantly enhanced due to its good homogeneity. However, for pure TiO 2 the route that Ti sol was slowly added into ammonia–water solution was effective in inhibiting the growth of crystalline size and therefore improving the photoactivity; (2) longer aging time promoted the hydrolysis of TEOS. As a result, the intimate interaction between titania and silica (Ti–O–Si hetero linkages) were strengthened and the photoactivity was increased; (3) TiO 2 –SiO 2 prepared by using ethanol as the solvent had the best combination of crystalline size, crystallinity, and homogeneity, thus, it exhibited the best photoactivity among the mixed oxides prepared via different alcohols; (4) 600 °C calcined TiO 2 –SiO 2 possessed excellent photoactivity as compared with that calcined at 400 or 800 °C due to the good crystallinity and the synergism between Ti–O–Si hetero linkages and surface hydroxyl groups.

Gas–solid heterogeneous photocatalytic oxidation (PCO) of heptane and toluene over UV-illuminated pure titania, titania–silica mixed oxides and sulfated titania was investigated at room temperature in a batch reactor. By comparing the photocatalytic activities of the different catalysts, we draw the following two conclusions: (1) there is no clear relationship between the crystalline phase and photocatalytic activity; (2) larger surface area is not always effective in the photocatalytic process and its effects on the photocatalytic activity strongly depend on the nature of the reactant used. In addition, the study on the behavior of catalyst deactivation and regeneration shows that titania–silica mixed oxide is deactivated more slowly than pure titania and sulfated titania, and for sulfated titania the regeneration temperature can be lowered due to the presence of SO42−.