Sharif Digital Repository

Tipping points occur in diverse systems in various disciplines such as ecology, climate science, economy, sociology, and engineering. Critical thresholds in system parameters or state variables at which a tiny perturbation can lead to a qualitative change in the system exist in many subsystems in complex systems. These thresholds are called tipping points, and these subsystems are called tipping elements. Additionally, many systems with tipping points can be modeled as networks of coupled multistable subsystems. Domino-like tippings are called tipping cascades. Considering that these tipping cascades are primarily unprecedented, it is essential to study the dynamics and control of these... 

Recent studies in various fields such as the environment, climate, economics, and neuroscience have shown that many systems exhibit dynamic responses based on catastrophic shifts—sudden and unexpected changes in the system's state and behavior in response to environmental changes. One of the simplest cases of these shifts occurs when the dynamical bifurcation diagram of a system has a tipping point, which serves as a critical threshold for system parameters, allowing small disturbances to lead to significant changes in the system's state. Research on many natural systems has revealed that tipping elements as the subsystems of a complex system are not independent, and the tipping of one... 

Ionic liquids due to their unique physical and chemical properties are widely used in various fields of chemistry and chemical engineering. Determining properties of ionic liquids can be difficult, time consuming or even impossible. In first step of this study the IL thermo database was chosen as source of experimental data of ionic liquids. Viscosity, density, surface tension and melting temperature of ionic liquids based on bis(trifluoromethylsulfonyl)imide anion with cations imidazolium, pyridinium, pyrrolidinium, piperidinium and ammonium were modeled using QSPR approach. Linear and nonlinear models were built by means of multiple linear regression (MLR), multilayer perceptron neural... 

Catalytic esterification of acetic acid with 1-hexanol was investigated over polyoxometalates. H3PW12O40 and H3PMo6W6O40 polyoxometalates with keggin structures were synthesized. Catalytic activity and recyclability of them were improved by loading on the γ-Al2O3 with impregnation method and Cs, K and NH4 salts preparing.Catalysts were characterized by XRD, SEM, FT-IR, BET, DRS and NMR techniques. According to XRD's results, catalysts have ace-centered cubic crystal systems. Loading on supports and preparing the salts do not destroy the structures of polyoxometalates. SEM images show the uniformity of catalysts' surface and catalysts' fixing on the support. It was seen that the keggin... 

In this study, CoFe2O4/CuO nanocomposite was synthesized by hydrothermal method and utilized for removal of metronidazole in electro-Fenton process. Nanocomposite was characterized by FT-IR, XRD, FE-SEM, VSM and BET methods. The Results of XRD patterns confirmed the monoclinic structure formation for copper(Ⅱ) oxide and the spinel structure for CoFe2O4. The FE-SEM images displayed the formation of CuO nanosheets for copper(Ⅱ) oxide and the decoration of cobalt ferrite nanoparticles on the surface of CuO nanosheets. Measurement of the surface area of nanocatalysts using BET isotherm represented the high surface area for CuO nanosheets and the increase of surface area of nanocomposite in... 

Phenolic compounds, which are one of the most dangerous compounds for the environment, are present in many effluents of various industries such as petrochemicals and pharmaceuticals, and the need to eliminate these compounds is increasingly felt. In this study, adsorption of three compounds of para-chlorophenol, para-nitrophenol and hydroquinone on polypyrrole adsorbent in the form of neutral, charged, copolymer with polyaniline and phosphorus-doped have been investigated using density functional theory (DFT). Calculations are performed at two levels of b3lyp/6-31+G(d,p) and wb97xd/631+G(d,p) for two different orientations of adsorbate molecules that the correlation-exchange function wb97xd... 

Today, fossil fuels cause countless environmental pollutants, so replacing fossil fuels with clean energy sources is very important. Water splitting using light and a semiconductor is one of the new methods of producing oxygen and hydrogen, which due to its simplicity and cheapness, has attracted a lot of attention today. In this study, ZnMn2O4 photocatalyst nanoparticles were first loaded on a graphitic carbon nitride support in different ratios of ZnMn2O4 and carbon support by hydrothermal method. Then the photocatalysts were evaluated for evolution of oxygen and hydrogen photocatalysts through water splitting under visible light. In ZnMn2O4 / g-C3N4 (80:20) nanocomposite, the highest... 

Among the xenobiotic compounds, chlorophenols are considered to be as the most dangerous compounds for the environment and living organisms. These compounds are abundantly found in the wastewater of many chemical industry factories. In this research, by using Gaussian software and density functional theory at the level of B3LYP / 6-31G (d, p),the adsorption of molecules such as phenol, 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol over graphene, nitrogen and boron doped graphene sheet are studied. The most stable configurations were determined and adsorption energies were calculated. In addition, to understand the adsorption mechanism, electron properties such as state density and... 

In this project, BiVO4 , Mn3O4 and BiVO4- Mn3O4 nanocatalysts were synthesized using the hydrothermal method. The properties of these nanocatalysts were determined by XRD, SEM, FT-IR, DRS, PL, ICP, EDX, BET and AA techniques. Analysys XRD patterns and SEM images of samples confirmed monoclinic structure for BiVO4 and tetragonal structure for Mn3O4. The DRS and PL spectra showed adctrease in the band gap and electron – hole pair recombination for BM 60:40 relative to BiVO4. Oxygen production was studied in the presence of different percentages of BiVO4 and Mn3O4 in BiVO4 - Mn3O4 nanocomposite and was observed that the 60:40 wt.% (Bismuth vanadate 60% and manganese oxide 40%) produced more... 

The use of energetic ionic liquids as hypergolic fuels, has low toxicity and environmental risk relative the hypergolic fuels such as the hydrazine. In this research by using Gauss view and Gaussian softwares and density functional theory and MP2 methods at the level of B3LYP/6-311G(d,p), the proton transfer from the cations to anions for a set of hypergolic ionic liquids in the gas and solution phases were studied. The most stable configuration of cations and anions are determined. The interaction energies of the selected cations and anions, proton affinities and basicity properties of these hypergolic anions are calculated. The result represents that the lower proton affinity and basicity... 

Nanocomposite FeVO4/CeO2 was synthesized by hydrothermal method and was applied for removal of methyl orange. This nanocomposite was characterized by SEM, FT-IR, XRD, BET methods. SEM images showed that FeVO4 nanorode decorated on CeO2 spherical particles. The results of XRD patterns confirmed the triclinic structure for Fe(III)vanadate and the cubic fluorite structure for cerium oxide. The surface area of FeVO4, CeO2 and FeVO4/CeO2 was determined by BET analysis which the results represented an increase in surface area in nanocomposite using cerium oxide as support. The electro-Fenton reaction was performed in undivided electrochemical reactor using stainless steel electrodes and the... 

In this project, metal oxides loaded on different supports were used in the heterogeneous electro-Fenton (EF) process for organic pollutants removal. At first, Fe3O4/AC was synthesized for catechol degradation as a phenolic compound. 98.2% removal of catechol was achieved by operational conditions optimization. The catalytic activity of Fe3O4/AC was remained constant after 6 cycles. In the other study, reduced graphene oxide was used as a support for the magnetite nanoparticle catalyst. The catalytic activity of Fe3O4/rGO in EF degradation of reactive red 195 (RR195) dye was studied and 93.34% removal was obtained. Response surface methodology (RSM) was used and a quadratic model with... 

Thermodynamic quantities such as proton affinity (PA) and molecular basicity (GB) for (CaO)n nanoclusters with n=2–16 have been calculated using three computational models of the DFT (B3LYP, M06, PW91, CAM–B3LYP and ωB97XD functionals), MP2, and HF with the cc-PVNZ (n = D and T) basis set in the gas phase. Absolute deviation error (AAD%) indicates that obtained PA and GB values using DFT model and the B3LYP method with mean percentage errors of 0.77 and 0.90 %, respectively, have the higher accuracy than the other methods and models. Owing to the unique physical and chemical properties of ILs, can be used as catalysts, promoters and modifiers of catalyst in many organic reactions. Therefore,...