Kinetics of formation of intermediate compounds and catalytic transformation of reagents on the surface of complex oxides and composite solid systems

Abstract

Chemical transformations on the surfaces of solids represent multistage processes in which intermediate compounds determine the formation of the final reaction products, while the study of the kinetic regularities of their emergence and decomposition at the phase boundary makes it possible to elucidate the mechanisms of catalytic action of complex oxide systems used in building materials technology for the synthesis of binder compositions and thermal treatment of ceramic masses. Complex oxides with transition metals ensure the adsorption of reagents on active sites and their transformation through unstable intermediates, with the catalytic activity being determined not only by the composition but also by the defective structure of the crystal lattice, surface morphology, and interphase interactions in composite materials. Composite solid systems exhibit synergistic effects associated with the redistribution of electron density, the formation of new active centers, and modification of the energy profile, which is observed during the hydration of cement clinker minerals, where the surface of aluminate and ferrite phases catalyzes the transformations of sulfate and carbonate components with the formation of ettringite, calcium monosulfoaluminate, and amorphous hydrosilicate phases, the rate of formation of which varies depending on temperature and reagent concentration. The kinetics of these processes does not always correspond to the Langmuir–Hinshelwood or Eley–Rideal models and may have an autocatalytic character, while on perovskite-like oxides the transformation of hydrocarbons and carbon oxides occurs through surface carbonates and formates with lifetimes ranging from milliseconds to minutes. For the perovskites LaCoO3, LaMnO3, and LaFeO3, the activation energies of intermediate formation are 34.2–52.3 kJ/mol, the rate constants at 573 K are 0.0187-0.0041 s-1, and for the spinels NiCo2O4, CuFe2O4, and ZnAl2O4 they are 28,6-61,4 kJ/mol and 0.0312-0.0023 s-1, demonstrating dependence on the nature of the cation and its positions in the structure. In binary compositions LaCoO3 – ZnAl2O4 with 85 % perovskite, a synergistic effect is observed with the conversion rate exceeding the additive value by 30.4 %, along with the maximum specific surface area and a change in selectivity. The temperature dependence on doped calcium aluminate follows the Arrhenius law with activation energies of 42,8 and 54,3 kJ/mol for the formation and decomposition of carbonates, leading to a decrease in the stationary concentration of the intermediate. In nanodispersed TiO2, a decrease in particle size from 247,6 to 7,3 nm reduces the activation energy by 43%, increasing the rate constant tenfold due to an increase in the proportion of defective centers, in accordance with an empirical relationship with a parameter of 18,3 nm. A compensation effect of Constable-Cremer with an isentropic temperature of 648 K was discovered, while an integral assessment of efficiency reveals the superiority of nickel-cobalt spinel. Such regularities open opportunities for regulating the kinetics of structure formation in cement and ceramic systems for the creation of next-generation materials with optimized properties.