The state of palladium and copper on the surface of the PdCl2–CuCl2/γ-Al2O3 nanocatalyst for the low-temperature oxidation of CO by molecular oxygen was studied by various spectroscopic techniques. Using X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), freshly prepared samples of the catalyst were studied. The same samples were also evaluated after interaction with CO, O2, and H2O vapor in various combinations. It was shown that copper exists in the form of Cu2Cl(OH)3 (paratacamite) nanophase on the surface of the catalyst. No palladium-containing crystalline phases were identified. Palladium coordination initially is comprised of four chlorine atoms. It was shown by XAS that this catalyst is not capable of oxidizing CO at room temperature in the absence of H2O and O2 over 12 h. Copper(II) and palladium(II) are reduced to Cu(I) and Pd(I,0) species, respectively, in the presence of CO and H2O vapor (without O2). It was found by DRIFTS that both linear (2114 cm−1, 1990 cm−1) and bridging (1928 cm−1) forms of coordinated CO were formed upon adsorption onto the catalyst surface. Moreover, the formation of CO2 was detected upon the interaction of the coordinated CO with oxygen. The kinetics of CO oxidation was studied at 18–38 °C at an atmospheric pressure for CO, O2, N2, and H2O (gas) mixtures in a flow reactor (steady state conditions).
PdX2-organic solvent-water solutions, where X = halide (preferably bromide or iodide) and the organic solvent is 1,4-dioxane or tetrahydrofuran, are suggested as catalytic systems for CO oxidation. A number of coupled processes take place in these systems. The kinetics and mechanism of CO oxidation by oxygen in the PdI2-LiI-1,4-dioxane-water system are reported. Cyclohexene hydrocarboxylation into cyclohexanecarboxylic acid can be carried out as part of the coupled process occurring in the PdBr2-tetrahydrofuran-water system.
A systematic study of the kinetics of the low-temperature oxidation of carbon monoxide with oxygen on a PdCl2–CuCl2/γ-Al2O3 supported catalyst was carried out over a wide range of the partial pressures of oxygen, water, and CO in order to test hypotheses on the reaction mechanism. It was shown that, as the temperature was increased from 20 to 38°C, rate of formation of CO2 decreased and the apparent activation energy was about–40 kJ/mol. The hypotheses of different degrees of complexity concerning the reaction mechanism were formulated based on physicochemical data and a Langmuir–Hinshelwood model. Mechanisms in which carbon dioxide is formed on the interaction of the surface Pd(I) and Pd(II) complexes that include carbon monoxide and water with the surface complex of Cu(I) that coordinates oxygen were recognized as the most probable.
Main concepts of the Shilov–Ostwald theory of conjugate chemical reactions are analyzed from the standpoint of complex reaction kinetics, theory of reaction routes, and thermodynamics of irreversible processes. Classification of conjugate reactions and an algorithm of catalytic system design for conducting these processes are proposed.
Low-temperature oxidation of carbon monoxide on the Pd/γ-Al2O3 catalyst has been studied at room temperature and atmospheric pressure of air containing 100 mg/m3 CO. The catalyst became gradually deactivated during the process. Storage of the partially deactivated catalyst in air for 10–12 h resulted in a complete recovery of activity. According to XPS data, the material obtained by depositing Pd3(OAc)6 on γ-Al2O3 before contact with CO contains palladium compounds in the oxidation states I and 0. The specific catalytic activity of palladium compounds in different oxidation states was evaluated based on the correlation between the catalytic activity and the content of Pd(I) and Pd(0) compounds, and it was shown that the activity of the former is higher than that of the latter.
A Pt(1.5%)Sn(0.25%)/Al2O3 catalyst synthesized by the sequential impregnation of γ-alumina with aqueous solutions of hydroplatinic acid and tin chloride has been characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The kinetic laws governing methylcyclohexane dehydrogenation to toluene and hydrogen in a tubular flow reactor in the isothermal mode at atmospheric pressure have been studied, while varying the reactant concentrations, contact time, and temperature. An empirical mathematical model of kinetics, which includes three adsorption forms of a bifunctional active site, has been developed. The model adequately describes the test data for the forward and reverse reaction routes.