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Electrochemistry of Fuel Cells
Published in Xianguo Li, Principles of Fuel Cells, 2005
Since it is generally a difficult task to determine a complex mechanism for a given electrode reaction under various conditions, the concept of rate-determining step of an overall reaction has been frequently used for the calculation of electrode reaction rate. The rate-determining step may be defined as the reaction step that determines the rate of the overall reaction. This concept holds both in the case of consecutive and of parallel reactions or a combination of the two types of the reactions. Further, it is known that many electrochemical reactions proceed by a consecutive mechanism, and few by a parallel-path mechanism. In other word, it may be regarded that the rate of an overall reaction is mainly influenced, or determined, by one step among many elementary reactions, called the rate-determining step. Such a concept considerably simplifies the analysis and calculation of the rate of an overall electrode reaction, and have been used extensively in fuel cell literature, although it is over-simplistic in the description of the complex electrode reactions. It is known that the most important factor in determining the power and efficiency of electrochemical energy conversion is the reaction rate of the rate-determining step, although other factors may be also important such as the adsorptive properties of reactants and intermolecular forces among the species adsorbed on the electrode.
A kinetic model and parameters estimate for the synthesis of 2-phenyloctane: a starting material of bio-degradable surfactant
Published in Indian Chemical Engineer, 2023
Sudip Banerjee, Md Aurangzeb, Amit Kumar
According to Froment et al. [23], the single-event is very useful for obtaining a smooth estimation procedure and significant value of parameters involved in large numbers in a model. The single-event methodology considers the change in symmetry of reactant in the elementary reaction step. Using this methodology, the kinetic rate constant (k) is expressed as the product of the single-event-rate coefficient (k′) of that step and the number of single-event (ne). The latter one is defined as the ratio of the global symmetry of the reactant and activated complex. After incorporating ne in the kinetic rate coefficient (k′) from the transition state theory, the kinetic rate constant is expressed as: Here, and denote the global symmetry of reactant and activated complex, respectively. The number of single-event captures the difference in structure between the reactant and activated complex.
A DFT Study of N2O Homogeneous and Heterogeneous Reduction Reaction by the Carbon Monoxide
Published in Combustion Science and Technology, 2022
Ruobing Wang, Chan Zou, Yue Zhang
In the second step (IM2→TS2→IM3), the O(6) atom gradually approaches to the C atom of CO. In this process, the intermediate IM2 transforms into the intermediate IM3 through the transition state TS2 after crossing an energy barrier of 10.43 kJ/mol. In addition, this elementary reaction step is an exothermic process with 128.15 kJ/mol. The distance between the C atom of CO molecule and O(6) decreases gradually for this elementary reaction step: 0.260 nm (IM2)→0.239 nm (TS2)→0.144 nm (IM3). In the last step (IM3→TS3→IM4), the N2 molecule breaks away from the char surface. This reaction process is activated by 129.37 kJ/mol and endothermic by 7.47 kJ/mol. According to the theory of reaction rate determining step, it can be seen that the rate-determining step of the heterogeneous reaction between N2O and CO on the char surface is from IM3 to IM4.
Extraction of Scandium and Iron from Red Mud
Published in Mineral Processing and Extractive Metallurgy Review, 2022
Ding Wei, Xiao Jun-Hui, Peng Yang, Shen Si-Yue, Chen Tao, Zou Kai, Wang Zhen
Hydrochloric acid concentration, leaching temperature, leaching time and solid/liquid ratio had a remarkable influence on the leaching efficiency of scandium from magnetic separation tailings. A maximum Sc leaching efficiency of 83.94% was obtained at optimum leaching conditions of 200 rpm agitation speed, 20% hydrochloric acid solution, solid/liquid ratio of 1:10 (g/mL), −74 µm sample particle size and 80°C for 3 h leaching duration. The apparent reaction activation energy of the acid leaching process was 26.77 kJ/mol and the frequency factor = 6.83 s−1. The process of leaching scandium with magnetic separation tailings conforms to the kinetic characteristics of multiphase liquid-solid region reaction, controlled by the chemical reaction step. The process achieved selective leaching, as most of Si and Ti were still present in the leaching residue, and the Si and Ti in the leaching residue can be recycled and reused by selecting an appropriate method. The utilization of red mud should comprehensively consider its material composition, chemical properties, production technology, etc., seek to expand the application fields of red mud, and produce high value-added products, so as to meet economic feasibility and minimize its environmental pollution.