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N–TiO2−xnanoparticles (NPs) were selected as a basic construction unit because there are many Ti3+and O vacancies on its surface. When S2−and Sn4+are co-doped into the N–TiO2−xlattice, the O2−and Ti4+atoms can be replaced, respectively. The formation of dopant energy levels from Sn4+and S2−in Sn4+/S2−/N–TiO2−xcauses an obvious red shift to the visible light region. The recombination rate of the e−/h+pairs was significantly reduced due to the fast migration and separation of the electrons as a result of the energy level doping from Sn4+and S2−and the formation of electron traps with Sn4+. The N–TiO2−xNP band gap was significantly narrowed from 3.07 eV to 0.77 eV upon co-doping with Sn4+and S2−, resulting in the superior photocatalytic activity of the Sn4+/S2−/N–TiO2−xNPs. The photocatalytic reduction rate of Cr(vi) increased by 11.6 times and the photodegradation rate of doxycycline hydrochloride (DH) achieved about 5.7 times that of the original N–TiO2−xNPs. This study provided a simple approach to preparing metal and non-metal co-doped N–TiO2−xNPs, which could greatly improve the removal efficiency of contaminants viatheir synergistic effect.

A single-site bifunctional catalyst for the oxidation–Knoevenagel condensation tandem reaction was prepared by the immobilization of phosphotungstic acid (HPW) on mesoporous graphitic carbon nitride (mpg-C3N4) viaelectrostatic interaction (HPW/mpg-C3N4). The results of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), solid-state 31P nuclear magnetic resonance (solid-state 31P NMR), zeta potentials, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) demonstrated that HPW was successfully immobilized on the protonated mpg-C3N4by electrostatic interaction. The acid amounts of the catalysts were determined by NH3temperature programmed desorption (NH3-TPD). The textural properties and morphology of HPW/mpg-C3N4were characterized by N2adsorption–desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 30% HPW/mpg-C3N4shows the best catalytic performance in the tandem reaction with 98.4% benzyl alcohol conversion and 96.2% selectivity to benzylidene malononitrile. The excellent catalytic performance of 30% HPW/mpg-C3N4in the tandem reaction is due to the good catalytic performance of HPW in the oxidation and Knoevenagel condensation, respectively. Furthermore, protonated mpg-C3N4not only acts as a support to facilitate good dispersion of HPW but also promotes the Knoevenagel condensation reaction effectively. Moreover, the HPW/mpg-C3N4catalyst could be recycled easily without significant loss of catalytic activity.

H5PMo10V2O40(PMoV2) was immobilized on N,N-dimethylhexadecylamine functionalized chloromethylated polystyrene (DMA16-CMPS) by electrostatic interaction and used as a catalyst for the direct hydroxylation of benzene to phenol with H2O2. The results of FT-IR, XRD and TGA indicated that PMoV2was immobilized and finely dispersed on DMA16-CMPS, and the structure was well preserved. UV-vis DRS, EPR and XPS confirmed the presence of V4+in the catalyst due to electronic interactions between PMoV2and DMA16-CMPS. The textural and morphological properties of the catalyst were characterized by N2adsorption–desorption, SEM and TEM. The hydrophobicity of the catalyst was also characterized by contact angle measurement. PMoV2/DMA16-CMPS exhibited excellent catalytic performance with 22.1% conversion of benzene and a phenol yield of 21.9% at 65 °C. The high catalytic performance of PMoV2/DMA16-CMPS is attributed to high dispersion of PMoV2on mesoporous and macroporous DMA16-CMPS, good benzene adsorption and phenol desorption ability, and V5+/V4+redox pairs in the catalyst. Meanwhile, PMoV2/DMA16-CMPS has the advantages of facile recovery and recycling.