Preparation Of Oxy-acid Salt Photocatalyst And Removal Of Pollutants From Water:Principle And Performance

The problem of water pollution has become one of the most important restricting factors of economic and social development in China.In order to meet the demand of environmental water pollution control,a stable non-metallic oxysalt photocatalyst was used to avoid the external pollution of the catalyst.The photocatalytic oxidation and photocatalytic reduction were studied by using photogenerated holes and electrons.Combined with photoelectrochemistry,density functional theory,free radical scavenger,spectroscopy and other characterization methods,the relationship between the conduction band and valence band position,internal dipole moment,interface carrier transport and water pollution control was discussed,and the reason for its intrinsic high catalytic activity was unraveled.Our research could deepen the understanding of the non-metallic oxy-acid salt photocatalytic system,and provide experimental and theoretical basis for the design of specific redox and high-efficiency photocatalytic reaction system for water pollution control.In this study,we report the in-situ photochemical activation of sulfate（i-PCAS）to produce SO_<sup>4.-radicals with bismuth phosphate（BPO）serving as photocatalyst.The prepared BPO rod-like material could achieve remarkably enhanced degradation of 2,4-dichlorophenol（2,4-DCP）in the presence of sulfate,indicated by the first-order kinetic constant(k=0.0402 min^-1)being approximately 2.1 times that in the absence(k=0.019 min^-1)at pH-neutral condition.This presented a marked contrast with commercial TiO₂（P25）,the performance of which was always inhibited by sulfate.The impact of radical scavenger and electrolyte,combined with electron spin resonance（ESR）measurement,verified the formation of ·OH and SO₄·-radicals during i-PCAS process.According to theoretical calculations,BPO has a sufficiently high valence band potential making it thermodynamically favorable for sulfate oxidation,and weaker interaction with SO₄·-radicals resulting in higher reactivity toward target organic pollutant.To develop photocatalyst that is effective for degradation of organic pollutants,we fabricate a novel direct solid Z-scheme BiPO₄-Bi₂O₂（OH）（NO₃）（BPO-BHN）heterojunction structured hybrid.The BPO-BHN is shown to greatly promote the degradation of 2,4-dichlorophenol（2,4-DCP）under ultraviolet light.On the basis of pseudo-first-order kinetics,the apparent degradation rate constant（kapp）of 0.050 min^-1 obtained for BPO-BHN is approximately 3.33 and 12.5 times of that for individual BPO(k=0.015 min^-1)and BHN(k=0.004 min^-1),respectively.This suggests a virtually synergistic photocatalysis of BPO and BHN when they form a direct solid Z-scheme heterojunction structure,which is favorable for improving UV-light harvesting,hole/electron separation and oxidizing capability.In particular,as a novel non-linear optical（NLO）material,the BHN plays a significant role in the formation of Z-scheme structure for its unique ability of capturing photo-electrons from BPO by highpotential C+face in valence band.On the basis of the photocatalytic oxidation in the previous chapter,we hope to expand the response range of the photocatalytic oxidation system to the visible and near-infrared regions,so as to greatly improve the efficiency of photocatalytic light utilization.However,due to the fact that the oxidation-reduction potential of narrow band gap semiconductor is not enough to generate high energy photogenerated holes/electrons,the efficiency of water treatment is very low.Under this context,the present study reports the efficient production of SO_<sup>4.-radicals through visible-light-driven photocatalytic activation（VL-PCA）of PMS by using Cu₂（OH）PO₄ single crystal for enhanced degradation of a typical recalcitrant organic pollutant,i.e.,2,4-dichlorophenol（2,4-DCP）.It took only 7 min to achieve almost 100%removal of 2,4-DCP in the Cu₂（OH）PO₄/PMS system under visible-light irradiation and pH-neutral condition.The 2,4-DCP degradation was positively correlated to the amount of Cu₂（OH）PO₄ and PMS.Both·OH and SO_<sup>4.-radicals were responsible for enhanced degradation performance,indicated by radical scavenger experiments and electron spin resonance（ESR）measurements.The Cu₂（OH）PO₄ single crystal exhibited good cyclic stability and negligible metal leaching.According to density functional theory（DFT）calculations,the visible-light-driven transformation of two copper states between trigonal bipyramidal sites and octahedral sites in the crystal structure of Cu₂（OH）PO₄ facilitates the generation of ·OH and SO_<sup>4.-radicals from the activation of PMS and cleavage of O-O bondsIn photocatalytic reaction,photogenerated electrons have the same function as photogenerated holes,so we hope to use the reduction performance of photogenerated electrons to treat the oxidizing pollutants nitrate and bromate in water.The technical problems of traditional photocatalytic treatment of nitrates are low reduction efficiency and reduction selectivity,which is because most of the single semiconductors have low electron mobility rate and poor electron selective migration and enrichment.Based on the fourth chapter,nonlinear optical materials LiNbO3 was studied.The hydrotherm ally synthesized LiNbO3 powder could achieve efficient denitrification in water,evidenced by 98.4%nitrate removal and 95.8%nitrogen selectivity at reaction time of 120 min and pH-neutral condition.Based on the first-order kinetics of PCDN,the kinetic constant for LiNbO3 is almost three times as that of conventional TiO2（P25）under the same conditions.As suggested by the hole scavenger experiments,the LiNbO3 should proceed with photocatalytic reduction of nitrate through direct heterogeneous interaction with electrons at the conduction band of LiNbO3.This may represent a different mechanism from P25.The unique second harmonic generation（SHG）effects of NLO materials enable them to produce more electrons and minimize the electron-hole recombination,which improves the efficiency and stability of the PCDN process.As mentioned before,UV light only occupies a very small ratio of sunlight.If we can expand the UV reduction system to visible light reduction system,it has important research significance.We have developed the monoclinic bismuth vanadate（BiVO₄）single crystal as visible photocatalyst for effective removal of bromate.The BiVO₄ crystal photocatalyst with optimized（010）and（110）facets ratio could achieve nearly 100%removal of BrO3-stably at pH-neutral condition driven by visible light,accounting for the first-order kinetic constant of 0.0368 min^-1.The reduction half reaction with photo-generated electrons（BrO3-reduction）and oxidation half reaction（H2O oxidation）with photo-generated holes,which take place separately on the（010）and（110）facets under visible light irradiation,the spatially separated photo-generated electron and hole lead a high carriers separation and BrO3-reduction efficiency.