Modification Of Bismuth Molybdate By Alkali Treatment And Its Photocatalytic Pentachlorophenol Degradation Performance Under Visible Light

In the process of agricultural production,chemical pesticides are widely used to ensure the growth of crops.The effect of some pesticides are obvious,but their toxicity cannot be ignored.When used,they have caused residues in soil and water and resulted in increasingly serious environmental pollution problems.Also,they have threatened human health.Photocatalytic technology uses stable,non-toxic,and non-pollution semiconductor photocatalysts,and utilizes solar energy,which acts as abundant renewable energy in the natural world.This technology has its natural advantages and broad application prospects in solving environmental pollution.Different from the traditional photocatalyst Ti O₂,bismuth molybdate,as an important member of the bismuth-based photocatalyst family,can absorb visible light and has a typical layered structure.Due to its low cost,safety and non-toxicity,easy energy band regulation and stable structure,it has been extensively studied in recent years.It shows its unique advantages in environmental remediation and pollution control.In order to further improve the activity of bismuth molybdate,researchers have conducted a lot of research on the intrinsic properties of bismuth molybdate morphology,surface structure and energy band position,so a variety of modification methods have been adopted,such as controlling morphological structure,introducing defects,doping foreign elements,and constructing heterojunctions.Among them,since the reaction sites are mainly on the surface,changing the morphology and surface structure of the material can affect the photocatalytic activity,which is particularly evident.Therefore,this article designed and synthesized two kinds of photocatalyst materials as pH-adjusted surface-modified bismuth molybdate and ammonia-treated modified bismuth molybdate exposed with different crystal facets.The ammonia-treated surface modification method promoted some surface hydroxyl group grafted on the surface of the bismuth molybdate material.In turn,it affected the reaction mechanism of the substrate on the surface of the material and ultimately improved the photocatalytic performance.The specific research contents of this article are summarized as follows:1. A hydrothermal method was applied to synthesize unmodified pure bismuth molybdate material,and ammonia was used as a weak alkaline to perform secondary hydrothermal treatment on the bismuth molybdate material.By controlling the amount of additional nitric acid,the pH value of the hydrothermal process was adjusted,and different pH-adjusted bismuth molybdate materials were obtained.Through X-ray diffraction?XRD?,scanning electron microscopy?SEM?,high-resolution transmission electron microscopy?HRTEM?and element distribution spectroscopy?EDS?,it was confirmed that the modification process did not change the crystal structure and element composition of the bismuth molybdate materials.Subsequently,a visible light photocatalytic degradation test for pentachlorophenol was conducted,and it was found that under visible light conditions,the photocatalytic degradation reaction rate of bismuth molybdate material with pH=11 modified was increased by 5 times compared to the original one.And through the active species capture experiment and electron paramagnetic resonance?ESR?detection of free radicals,it was proposed that the main active species were photogenerated holes.In addition,Fourier transform infrared spectroscopy?FTIR?detection and X-ray photoelectron spectroscopy?XPS?detection confirmed the introduction of hydroxyl groups on the surface of the material.Combined with the substrate adsorption test and the quantitative experiment of·OH,the structure effect between surface hydroxyl introduction and photocatalytic activity was summarized.The principle is that the introduced surface hydroxyl group serves as the adsorption site for pentachlorophenol,which promotes the full contact of the generated active species with pentachlorophenol,and accelerates its degradation and mineralization process.2. By adjusting different reaction pH values???pH=6,10?,bismuth molybdate materials with?001?and?010?crystal facets exposed were obtained,and then the hydrothermal process with ammonia treatment was carried out to obtain bismuth molybdate materials with different exposed facets.X-ray diffraction?XRD?and Raman spectroscopy?Raman?characterization methods were used to confirm the successful construction of different exposed facets.Scanning electron microscopy?SEM?,high-resolution transmission electron microscopy?HRTEM?and element distribution spectroscopy?EDS?showed that the ammonia treatment process did not affect the structure and composition of those materials.Then,the experiment of visible light photocatalytic degradation of pentachlorophenol was carried out,and it was found that the activity of?001?facet exposed bismuth molybdate after modification was more significant than that of?010?facet exposed material.Immediately through active species capture experiments and electron paramagnetic resonance?ESR?detection,it was pointed out that the main active species in the degradation process were photogenerated holes.At the same time,combined with photocurrent,photoluminescence spectroscopy?PL?,electrochemical impedance spectroscopy?EIS?and other characterization tests,it was found that the ammonia modification process promoted the generation and migration of photo-generated carriers of?001?facet exposed bismuth molybdate materials.But for?010?facet exposed material,it was the opposite.Fourier transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?tests indicated that the surface hydroxyl groups of the?001?facet exposed material were easier to introduce.The adsorption test and H₂O₂quantitative experiment,combined with GC-MS,systematically analyzed the intermediate products of the degradation process,studied the synergy between the surface hydroxyl groups and the crystal plane,and speculated that when the degradation of pentachlorophenol was carried out by ammonia treatment modified bismuth molybdate under visible light It would undergo the degradation path of dechlorination as the main process and hydroxylation as the auxiliary process,and gradually opened the ring,and finally realized the complete mineralization of pentachlorophenol,and obtained non-toxic products.