الفهرس 
Nanomaterials based on metal oxidesOnly 14 pages are availabe for public view
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Abstract
Nanosized materials have attracted growing interest as a result of their distinctive structure and properties. Semiconductors with nanostructures have comprehensive applications in industrial catalysis and in green chemistry areas such as wastewater treatment and self-cleaning surfaces. Zinc oxide (ZnO) is among few nano-semiconductors that having good chemical, mechanical, photostability and oxidation properties. This nanomaterial has been studied and used as a photocatalytic, catalytic support and dopant material for many environmental applications. In this study the various roles of ZnO have been studied.
1-ZnO as a photocatalyst
For expansion in this field the effect of preparation method and doping with TiO2 and CeO2 on physicochemical properties of ZnO in photodegradation of Remazol Red RB-133 (RR) have been studied.
ZnO nanomaterials have been prepared by three different methods thermal decomposition, precipitation and sol-gel-combustion. Sol-gel-combustion method was carried out using metal nitrate and different fuels (urea, oxalic acid and citric acid). Molar ratios of citric acid to salt were (0.50, 0.75, 1.00, 1.25, and 1.50). These nanomaterials have been characterized by studying their structural, morphological, surface and optical properties. The photocatalytic activity was evaluated by photocatalytic degradation of Remazol Red RB-133 (RR) under UV-light irradiation. The results obtained illustrated that the photocatalytic efficiency was affected by preparation method, type and ratio of fuel to salt. The optimum was a gel precursor containing zinc nitrate and citric acid prepared in the molar ratio of 1 this sample showed the highest rate constant (5.18 x10-3 min-1) because of its small particle size and band gap. The highly active nanomaterial was applied for photocatalytic degradation of mixtures of two dyes (RR) and Methylene Blue (MB). It was clear that ZnO synthesized by sol-gel combustion method has the highest decolorization efficiency of dye mixtures in aqueous wastewater and considered as a promising photocatalyst.
The photocatalytic activity of ZnO was improved by doping with (TiO2 & CeO2). TiO2/ZnO and CeO2/ZnO nanoparticles with different titanium (0.005, 0.01, 0.025, 0.05 and 0.075) or cerium molar ratios (0.0075, 0.01, 0.015, 0.025 and 0.05), were prepared via impregnation method. The results showed that both TiO2 and CeO2 doping increased the photocatalytic degradation of RR. The promotion of photocatalytic activity was attributed to doping of ZnO with an appropriate amount of TiO2 or CeO2 has small crystal size, high surface area, small band gap that increased separation efficiency of the photogenerated electrons-holes in ZnO. Variation of TiO2 or CeO2 concentration changes the rate of dyes degradation. 0.025 TiO2/ZnO and 0.015CeO2/ZnO had the maximum photocatalytic activity among the others doping concentration because they have small Eg value. TiO2 / ZnO nanomaterial had higher activity than CeO2/ZnO because of its high surface area, more active sites and small band gap.
2- ZnO as a catalytic support
ZnO was used as a support for some selected catalytic systems, such as Co3O4/ZnO and MnO2/ZnO nanomaterials. The catalytic activities of these catalysts have been studied towards H2O2 decomposition 20–40ºC and expressed as the rate constant values. Co3O4/ZnO catalysts were prepared by impregnation method using Zn-samples which prepared from different precursors. The catalytic activity of Co3O4/ZnO catalyst was higher than that of the pure oxides.
The results showed that Co3O4/ZnO nanomaterials having higher surface area than those of the pure oxides. Additionally, Co3O4/ZnO sample in which ZnO produced via sol–gel combustion using citric acid (CoZnOSCI) showed the largest surface area while CoZnOP sample possessed the smallest value. The HR-TEM images confirmed that addition Co3O4 to ZnO followed by calcination affected the morphology. The most catalytic active sample was Co3O4/ZnO in which ZnO synthesized via thermal decomposition of zinc carbonate (CoZnOTC) while CoZnOSCI catalyst being the less active one. Different Co3O4 amounts were impregnated (0.008, 0.01, 0.03, 0.07, 0.08 and 0.1 moles) on zinc carbonate as precursor and calcined at 500oC. The catalytic activity increased with increasing the extent of cobalt oxide loading to 0.07 mol, above this concentration the catalytic activity decreased.
0.07CoZnOTC catalysts were calcined at different temperatures from 450 to 800°C, The catalytic activity of 0.07CoZnOTC increased with raise the calcination temperature to 500°C, above this temperature the catalytic activity was accompanied by a progressive decrease. XRD and TEM results showed that the crystallinity and crystallite sizes of all phases increased with increasing the calcination temperature from 500 to 800°C.
The effects of doping various amounts of Ag2O (0.010, 0.015, 0.020, 0.025 and 0.030 M) on the surface and catalytic properties of Co3O4/ZnO catalyst were investigated. The effect of calcinations temperature also was studied. The results showed that Ag2O doping increased the efficiency of Co3O4/ZnO catalyst towards the decomposition of H2O2. The maximum increase was 2.79 fold for the sample doped with 0.030 M Ag2O (2.72 mol%). Increasing the calcinations temperature increased the catalytic activity up to 600oC, above this temperature the catalytic activity began to decrease. Loading Ag2O resulted in a decrease the crystal size and limited increase in surface area as shown in XRD and surface area sections MnO2/ZnO nanomaterial containing different MnO2 loadings (0.08, 0.15, 0.20, 0.25, 0.30 and 0.35 mol) have been successfully prepared by impregnation method. The effect of the calcination temperature from 400 to 800 oC and Ag2O doping on the structural, physicochemical and catalytic properties of the 0.25MnO2/ZnO catalysts was investigated. The results showed that loading MnO2 on ZnO support calcined at 500 oC led to an increase in the surface area and pore volume and reduction in their crystallite sizes. The catalytic activity was accelerated by increasing MnO2 loading. Increasing the calcination temperature to 500 oC increased the catalytic activity, above this temperature catalytic activity decreased. Doping Ag2O with (2.34 mol%) led to an increase the catalytic activity of 0.25MnO2/ZnO with 2 fold.
3- ZnO as a catalytic dopant
ZnO was used as a dopant for Co3O4/MgO system. 0.07Co3O4/MgO catalyst was synthesized by different methods namely: traditional impregnation (IM), sol-gel (SG), co-precipitation using ammonia (PA), co-precipitation using urea (PU) and ultrasonic impregnation (US) methods. These catalysts were applied in catalytic decomposition of H2O2 at 20–40 oC. The HR-TEM results revealed that the preparation method affected on the morphology of catalysts. The catalysts prepared by sol–gel had the highest surface area and the pore volume while those prepared by traditional impregnation method had the smallest values. The catalytic activity of the synthesized catalysts expressed as reaction rate constant k (min-1) followed this order: 0.07CoMgOPu > 0.07CoMgOPA > 0.07CoMgOSG > 0.07CoMgOIM. It was noted that the catalyst prepared by co-precipitation method using urea had the highest catalytic activity; the catalyst prepared by traditional impregnation method had the smallest value.
ZnO doping for 0.07CoMgO nanomaterials resulted in a significant increase in the rate constant of H2O2 decomposition through creation an optimal electron-mobile environment by ZnO at the surface, which improved the redox properties of Co3O4. Ultrasound as a preparation was introduced to assist the impregnation process. The time of ultrasonication was varied between 5 and 60 min; ultrasonic time of 15 min was the most suitable time to prepare highly active 0.07CoMgO catalysts towards H2O2 decomposition. ZnO doping for 0.07CoMgOUS nanomaterials led to slight change in the catalytic activity for all ultrasonic times.
Increasing the temperature from 20 to 40oC accelerated the decomposition of hydrogen peroxide for all prepared catalysts. The values of activation energy of various nanomaterials towards H2O2 decomposition were calculated by Arrhenius equation. The obtained values of activation energy ΔE were considerably lower than (208 kJ/mol) which corresponded to (O-O) bond of hydrogen peroxide.