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Surface Catalysis on Metals
Published in Arthur T. Hubbard, The Handbook of Surface Imaging and Visualization, 2022
Fabio H. Ribeiro, Gabor A. Somorjai
A catalyst is a substance that can accelerate the rate by which a chemical reaction reaches equilibrium without being consumed in the process. An example of a heterogeneous catalytic reaction is the synthesis of NH3 from N2 and H2 with iron as the catalyst. The reaction between N2 and H2 does not proceed to equilibrium in the gas phase possibly because the high activation energy required for the homogeneous gas phase steps makes the rates too slow. However, when a catalyst is introduced (e.g., iron), N2 and H2 can dissociate on the surface and then recombine in a sequence of low activation energy steps and consequently high rates (Fig. 56.1).14
Research influence of light-grained concrete modifiers on local aggregate
Published in Evgeny Rybnov, Pavel Akimov, Merab Khalvashi, Eghiazar Vardanyan, Contemporary Problems of Architecture and Construction, 2021
A. Karapetyan, M. Badalyan, A. Ghahramanyan, G. Arakelyan
Quantitatively, the rate of chemical reactions is usually characterized by a change in the concentration of reacting substances per unit time. Since in the interaction of binders with water, direct determination of changes in their concentrations and emerging new compounds is difficult and the rate of hydration of binders is often estimated by various indirect indicators: the amount of bound water; the amount of calcium hydroxide released during the hydration of Portland cement; the amount of heat released; the magnitude of the contraction effect during hardening of the binder. Petrographic, X-ray, thermographic and other analyzes are also used. In addition to the kinetics of hydration, the degree of hydration is determined, characterized by the amount of binder that has reacted with water over a certain period, and expressed as a percentage, as well as the depth of hydration, measured by the thickness of the surface layer of the binder grain, hydrated at a given time.
Applied Chemistry and Physics
Published in Robert A. Burke, Applied Chemistry and Physics, 2020
Slow oxidation is a chemical reaction. Chemical reactions may produce heat; reactions that produce heat are considered exothermic. If the heat is insulated from dissipating to the outside of the material, it will continue to build up. As the heat builds, the material is heated from within. The process continues until the ignition temperature of the material is reached and ignition occurs.
Effect of mineral phases on the leaching efficiency of Ti slag
Published in Canadian Metallurgical Quarterly, 2023
Haibo Wang, Ke Sun, Bin Wang, Ruifang Lu, Xiaoping Wu
The activation energy of the leaching reaction in this work was determined to be 69.87 kJ/mol. For comparison, the activations energy values reported previously are listed in Table 5. Activation energy is the minimum amount of energy needed to start a chemical reaction. It can be seen from Table 5, under the moderate temperature and moderate acid concentration conditions, the leaching reactions of titanium slag or ilmenite are restricted by chemical reaction and other possible factors such as phase boundaries. The activation energy values are high, ranging from 64.4 kJ/mol to 90 kJ/mol for ilmenite and Ti-bearing slag. However, under the conditions of high temperature and high acid concentration, HTBBF slag becomes highly reactive, and the activation energies of these leaching reactions are reduced very significantly.
Photocatalytic treatment of real liquid effluent from hydrothermal carbonization of agricultural waste using metal doped TiO2/UV system
Published in Journal of Environmental Science and Health, Part A, 2023
Ekkachai Kanchanatip, Pradabduang Kiattisaksiri, Arthit Neramittagapong
The rate of reaction is the speed at which a chemical reaction takes place. In order to evaluate how fast the degradation occurred over different photocatalysts, the plots of ln(TOC/TOC0) versus the irradiation time were drawn and are shown in Figure 4b, where the slope of which upon linear regression equals to the apparent first-order rate constant k. The photocatalytic degradation of TOC in HTC effluent could be fitted well with the pseudo-first-order kinetic model (R2 > 0.9). The rate constant for photocatalytic degradation was determined from the following equation: where TOC0 and TOC are the initial TOC concentration and the TOC concentration at time t, respectively, and k is the apparent first-order rate constant. The rate constant of the photocatalysts is listed in Table 4, showing values in a range of 0.0148 min−1 to 0.0242 min−1. The rate constants (k) can be ranked in the order of Fe/A > Fe/P > Cu/A > Ni/A > Cu/P > Ni/P. Considering the metal dopants, Fe exhibited highest performance, followed by Cu and Ni, respectively. This high performance was possibly due to the regeneration of Fe3+ by some reactive species produced by TiO2 photocatalysis.[36] Moreover, the redox potentials for the Fe3+/Fe2+ was higher than those of Cu2+/Cu+ and Ni3+/Ni2+.[34,37]
Pyrolysis of four waste biomasses and elucidation of reaction kinetics and pyrolytic products
Published in Combustion Theory and Modelling, 2022
Two iso-conversional methods (FWO and KAS) were used to accurately evaluate the activation energy (E) at three heating rate (10, 20, 30°C/min). Activation energy refers to the minimum energy required for the chemical reaction to occur. Figure 4. shows the fitting lines with different conversion degrees when the conversion rate was in the range of 0.05–0.80 and the interval is 0.05. Meanwhile, Table 3 lists the E values and fitting correlation coefficients () of the four samples. The values calculated by THE and methods showed a small difference, as shown in Figure 5. The activation energy curves and the mean activation energy bar graphs of the four samples at different conversion rates were respectively. The mean activation energy of the selected interval ( = 0.05–0.8) was respectively: SCG was 238.1J/mol and 241.4J/mol; CMR was 200.8kJ/mol and 202.5 kJ/mol; VI was 236.6J/mol and 240.6kJ/mol; COS was 174.3kJ/mol and 174.7kJ/mol. However, was between 0.9784 and 0.9999, showing a good correlation.