Theoretical Research On The Enrichment, Determination And Transformation Of Some Environmental Pollutants

The pollution and destruction of the environment from human is increasing since the industrial revolution. And large scale pollution incidents occurred frequently especially in recent decades. The global warming, acid rain, species extinction and other issues caused by environmental pollution have already caused extensive and far-reaching impact for human. Therefore, scientists have been seeking different ways to achieve the enrichment, detection and treatment of the pollutants. The typical representative of environmental pollutants is persistent organic pollutants (POPs), including polychlorinated biphenyls (PCBs), dioxins, DDT and so on. Take PCBs as an example, thare are several effective measures reducing the PCBs pollution, including high temperature incineration, photoinduced degradation, electrochemical peroxidation, microbial degradation and so on. Since Szejtli first mentioned the possibility using of Cyclsdextrins (CDs) in soil remediation in 1992, CDs and its various derivatives have been applied quickly in the field of environmental protection. So far, there are two main areas of the treatment of PCBs using CDs:washing of contaminated soils with a relatively concentrated CD solution and using small amounts of CDs as a bioavailability-enhancing additive to accelerate the biodegradation of PCBs. These technologies are all based on the solubilization effect of CDs forming inclusion complexes with PCBs. Although great achievement has been got for the experimental research with the CDs controlling PCBs, so far there is no theoretical study about this topic. Our knowledge about CD-PCB complexes is so limited that the solubilization effect of CDs for PCBs is still unclear. In addition, most of the available PCB determination methods need expensive instrumentations. So it is highly desired that deteminating PCBs using routine analysis techniques including infrared and Raman spectra on appropriately treated PCB samples can be achieved.Carbon monoxide pollution in atmosphere is a threat to human health. CO catalytic oxidation at low temperature is the most simple and effective way to eliminate CO. Since Haruta's findings that highly dispersed gold nanoparticles have ultrahigh activity of CO catalytic oxidation in low temperature, gold nanopaticles and its alloys attract sustained experimental and theoretical interest in the world. During the past decades, extensive research interest has been paid to improve the catalytic activity of gold nanoparticles by tuning the particle morphology, modifying the substrate, controlling the pretreatment conditions and alloying gold nanoparticles. But our knowledge about the mechanism of CO oxidation catalyzed by nano gold and gold-silver alloy nanoparticles is sill far from complete. And whether the carbonate species is formed in the catalytic cycle is also uncertain.Considering the issues concerned above, we use quantum chemical and molecular dynamics study of the host-guest interaction between CDs and PCBs. We chose several high toxicity PCB molecules as models, such as PCB52 and PCB126, and research the electrostatic interaction and van der Waals interaction between PCBs and CDs. From the quantum chemical calculation and molecular dynamics simulations, we try to find the best molar ratio between PCBs and CDs. Spectra calculation was used to find whether the general infrared and Raman techniques are suitable for the direction of CD-modified PCBs. In addition, we do a research on the mechanism of the CO oxidation catalyzed by Au-Ag nanopaticles, and find the reaction details in the formation of carbonate. In short, our work provide useful information from the atom and molecule level for the inclusion of CDs to PCBs and the catalytic mechanism of nano gold and its alloys for CO oxidation. We expect that our theoretical study can provide understanding to some relevant experimental researches.The major innovative conclusions in this thesis are listed as follows:1. The research history and current state on PCBs and CO pollutions have been briefly reviewed. PCBs'toxicity, source and distribution are summarized. Give a detailed description of the treatment and detection of PCBs. For the CO pollution, first we introduce the use of common catalyst like Pt and Pd, and then depict the research history of nano gold and the technology about how to improve the catalytic activity of gold nanoparticles, like tuning the particle morphology, modifying the substrate, controlling the pretreatment conditions and alloying the gold nanoparticles. We also introduce the research progress of Au-Ag alloy nanopaticles on experiment and theory. Moreover, the theory of quantum chemistry and molecular dynamics simulations of this paper are summarized.2. DFT calculations and MD simulations were carried out on the inclusion complexes of the most common cyclodextrinsα-,β-, and y-CDs with the model molecule of PCBs (PCB52) to better understand the complexation of PCBs by CDs. The theoretical results confirm that the CDs are of ability accommodating PCB52 molecule. The stability of inclusion complexes depends on both the type of CD and host-guest stoichiometry ratio. While a-CD preferentially form 1:1 and 2:1 complexes andβ-CD stably include the guest molecule with 1:1,2:1, and 2:2 stoichiometries,γ-CD is proven to be of capability to form all four 1:1,1:2,2:1, and 2:2 complexes. The complex ofγ-CD is generally the most stable compared to those ofα- andβ-CD with the same stoichiometry ratio with the sole exception being for 1:1 complexes, of which a-CD-PCB52 is the energetically most stable. MD simulations give a clear picture of the scene that PCB52 molecule enters the cavity ofβ-CD. The IR spectra of 1:1 inclusion complexes mainly present the spectra features of the CDs and give only slight indication for bands of the guest molecule. In contrast, the characteristic Raman bands of the guest molecule are remarkably prominent in the Raman spectra of the inclusion complexes. Raman spectroscopy can be used for the identification of CD-modified PCBs, whereas IR spectroscopy is not suitable for such an application.3. Different ratios for PCB126 and beta-CD systems in water solvent have been investigated by molecular dynamics (MD) simulation. Computational results have revealed that PCB126 and beta-CD can form stable 1:1 inclusion complexes whatever from the big rim or the small rim of beta-CD. But the beta-CD has no ability to include the second PCB126 in the water solvent because of the small dimension of BCD's cavity. Two PCB126s and Two beta-CDs can't form 2:2 complex, but the product that combined by two 1:1 inclusion complexes. The IR spectra of 1:1 inclusion complexes mainly present the spectra features of the beta-CD and give only slight indication for bands of the guest molecule. In contrast, the characteristic Raman bands of the guest molecule are remarkably prominent in the Raman spectra of the inclusion complexes. The band of symmetric stretching vibration of PCBs'benzene ring is unique and impressive in complex's Raman spectra. Raman spectroscopy can be used for the identification of CD-modified PCBs, whereas IR spectroscopy is not suitable for such an application.4. Based on the DFT calculations, we present the mechanism details of the catalytic oxidation of CO by Au2-, focusing on whether the carbonate species is formed and if so, how it is formed during the reaction. The result shown that there are two pathways to form carbonate:the Au2--O intermediate pathway and the direct oxygen pathway. In Au2--O intermediate pathway, CO and O2 react and generate Au2--O intermediate, and then react with a new formed CO2 molecule and form Au2CO3- finally; In direct oxygen pathway, CO and O2 form carbonate on the Au2- directly. The energy barrier of the rate determining step of the direct oxygen pathway is lower than the Au2--O intermediate pathway and can happen much easier. Our calculation shown that the carbonate formation reaction follow an Eley-Rideal (ER) mechanism.5. To better understand the higher activity of Au-Ag bimetallic catalysts than pure metallic Au catalyst for low-temperature CO oxidation, we here presented a comparative theoretical study of the catalytic activity of Au2- and AuAg- dimers, which represents the simplest models for monometal Au and bimetallic Au-Ag nanoparticles. By performing DFT calculations, we have shown the mechanism details of CO oxidation over Au2- and Au-Ag- dimers, for which both the mono- and double-center catalytic mechanisms have been taken into account. It is found that Au2-and AuAg- catalyze CO oxidation according to the similar mono-center Eley-Rideal mechanism, which is different from the generally supposed mechanism picture of Au-Ag bimetallic catalysts, where both Ag and Au sites are proposed to participate in the reaction with Ag sites adsorbing and activating O2 molecules and Au sites interacting with the coming CO molecules. The catalytic reaction is shown to be of the multi-channel and multi-step characteristic, which can proceed via two or three elementary steps along four possible pathways, including the direct oxygen pathway, the carbonate intermediate pathway, the energetically most favorable O-C-O-O-C-O group involved pathway, and the O-O-C-O group mediated oxygen abstract pathway. For AuAg- dimer, the Au site is more active than the Ag site, and the calculated energy barrier values for each determining step for the Au-site catalytic reaction are remarkably lower than those for both the Ag-site catalytic reaction and the Au2-catalytic reaction. The present results provide assistance to some extent for understanding the experimentally observed exceptionally high catalytic activity of Au-Ag nanoparticles and nanoalloys for low-temperature CO oxidation.