The catalyst of environmental catalysis 英语论文.doc

The catalyst of environmental catalysis Molecular chemisorption at manganese oxide/liquid interfaces has attracted increased interest due to its importance in the development of heterogeneous catalysts, microbial fuel cells and selective adsorption. Recently, inexpensive manganese oxide based catalysts instead of noble metal catalysts have been investigated in the catalytic wet oxidation (CWO) of phenolic wastewater. However, the adsorption of organic compounds on manganese oxide surfaces, which plays an important role in electron transfer and oxygen reduction, has not fully been explored. Phenol has attracted more attention in wastewater treatment because of its toxicity and wide occurrence in wastewater. Several conversion processes, currently under development for the production of synthetic fuel from coal, discharge substantial amounts of phenol (0.2-0.5g/L). Moreover, phenol is taken as a model compound for the study of adsorption and oxidation because phenol is an intermediate product in the oxidation of higher-molecular-weight aromatic compounds and phenol also converts to a series of intermediates on oxidation.Cryptomelane-type manganese oxide is an octahedral molecular sieve (OMS) composed of 2x2 edge-shared Mn06 octahedral chains, which are corner shared to form onedimensional tunnels. Potassium ions and water are located in the tunnels (designated K-OMS-2). Mixed-valent manganese ions (Mn2+ and Mn3+) are situated in the octahedral sites of cryptomelane. Transition metal ions can be incorporated into the framework or tunnel sites. Sizes and charges of M determine which sites are occupied. The synthetic counterpart of metal ion doped cryptomelane is known asoctahedral molecular sieve M-K-OMS-2 (M doped metalions). In recent years, some work has been published on the syntheses of cryptomelane-type manganese oxides and their applications in the oxidation of benzene, carbon monoxide, and other hydrocarbons. K-OMS-2 was found to possess excellent hydrophobicity and strong affinity for volatile organic compounds in the oxidation of benzene. Little has been reported on doping effects on the adsorption of substrates or on catalytic properties.This research focuses on structure-property relationships for doped cryptomelane. These doped porous cryptomelane materials have high surface areas (40-100m2/g), more oxygen vacancies compared with undoped K-OMS-2, and more surface bonded OH groups. The substitution of framework Mn2+ with bivalent (Cu2+) and trivalent ions (Co3+) can create more oxygen vacancies to maintain an overall charge. Surface bonded OH groups on OMS-2 react with solute molecules and increase the adsorption of phenolic compounds. Solute molecules tend to attach to oxygen vacancies. Subsequently, electron and/or oxygen transfer between the metal oxide surfaces and solutes take place at these sites.Doped K-OMS-2 materials have dual roles in the removal of aqueous phenol, including adsorption and oxidation of phenol at moderate temperatures (298-423 K), pressures (80-150 psi), and pH (6-9). When phenol is adsorbed, phenol is oxidized to CO2 and H2O in the catalytic wet oxidation with oxygen. The advantages of this process over microbial activated sludge processes are that this process withstands high phenol loading rates and the fluctuations in phenol loading and does not produce any biological sludge. We show that the chemisorbed phenolic compounds on doped K-OMS-2 can reach up to 2-5 times the weight of doped K-OMS-2. Compared with activated carbon (AC) adsorption, phenolic compounds on K-OMS-2 can be oxidized by oxygen or air with the catalysis of doped K-OMS-2 at 573 K. The products are CO2 and H2O. Doped K-OMS-2 adsorbent and catalysts are regenerated.The overall results in this research showed that doped M-K-OMS-2 catalysts had increased activity over that of undoped K-OMS-2 catalysts. Doped K-OMS-2 shows potential application in the removal of aqueous phenol. Structure activity relationships provide some scientific background for the development of highly efficient and low cost heterogeneous catalysts for applications in wastewater treatment.Doping Effect on Adsorption and Oxidation of Phenol. M-K-OMS-2 doped with Co3+, Ce4+, and Cu2+ have greater adsorption of phenol and higher catalytic activity than that of undoped K-OMS-2. These doped OMS-2 catalysts have high BET surface areas (40-63 m2/g). High surface area generally increased the adsorption and activity. Other doping and structure factors described here showed greater effects on the adsorption and oxidation than surface area. Although XRD patterns reveal that Co3+, Ce4+, and Cu2+ doped K-OMS-2 materials have the same 2x2 tunnel structure as K-OMS-2, the dopants make a distinct difference in the ability to adsorb and oxidize phenol. These metal ion dopants greatly enhance the adsorption properties and catalytic activities of the doped K-OMS-2 materials.Considering the structures, the doped ions either locate in the frame work to replace Mn3+ and Mn4+ or in the tunnel sites to replace K+. In a priori incorporation of metal ions, Pauling's Rules suggest the crystal radius might determine whether the ion is doped into the framework or in the tunnel sites. Justified by the crystal radii, Co3+ would be in the framework positions of K-OMS-2 due to their almost equivalent ionic radii to those of the Mn3+ ions. Ce4+ ions were presumed to be in the tunnel positions of K-OMS-2 because of their much largerionic radii compared to those of the Mn3+/Mn4+ ions. Cu2+ ions may primarily be in the framework of K-OMS-2.K-OMS-2 is a mixed-valent MnO2 in which charge imbalance on the octahedral framework due to reduction of Mn3+(VI and VIII represent the coordination numbers in the lattices).Doping K-OMS-2 catalysts with Ce4+, Co3+, and Cu2+significantly increases the activity of doped M-K-OMS-2 incomparison to undoped K-OMS-2.The highest efficient catalyst tested here is Co-K-OMS-2 catalyst with a phenol removal rate of 3.7kg phenol/h.kg catalyst. These catalysts have shown high catalytic activities and stability in heterogeneous reactions at moderate temperatures. Doped K-OMS-2 catalysts have larger adsorption capacity of phenol than activated carbon (0.24g phenol/5days.gAC).Doped K-OMS-2 is more stable in combustion and in regeneration than activated carbon. Although the price of doped K-OMS-2 is higher than activated carbon, the total operation cost of doped K-OMS-2 can be compensated by its capacity and longevity. The catalysts tested here indicate that doped K-OMS-2 has potential application in the removal of aqueous phenol from the synthetic fuel and other industries.