Daria V. Glyzdova

The possibility of introducing nitrogen into the structure of the carbon material Sibunit by its high-temperature treatment in a flow of ammonia at 800–1000°C was studied. It was shown that preliminary oxidation of the Sibunit surface with a 5% nitric acid solution promotes more efficient nitrogen binding, which may be due to oxygen-containing groups and/or greater defectness on the surface of the oxidized carbon material. Palladium on Sibunit preoxidized and treated in NH₃ at 1000°C leads to an increase in the activity and selectivity of the Pd/Sibunit catalyst in the reaction of hydrogenation of acetylene to ethylene. It was established that the improvement of catalytic characteristics is associated with the increased availability of supported palladium due to its localization in larger pores of the N-modified support.

Features of the catalytic action of Pd–Ag, Pd–Cu, Pd–Au, Pd–Ga, and Pd–Zn bimetallic systems on the acetylene conversion to ethylene are discussed taking into account two factors that determine the effect of the second metal on palladium, namely, the “ensemble effect” (geometric effect) and the “ligand effect” (electronic effect). The relationship between the calculated thermodynamic and kinetic parameters of the adsorption interaction of major reaction medium components and intermediates with the catalyst surface, the structure of active ensembles, and characteristics determined in tests, such as the structural parameters of bimetallic phases, the electronic state of their components, and catalytic properties, is shown. Some examples are given to show that the modifier atoms can be incorporated into active ensembles and the sites formed by the modifier atoms can be involved in the catalysis of individual elementary steps.

The study investigates interactions between palladium and silver in Pd–Ag bimetallic catalysts supported on a mesoporous carbon material Sibunit and, in particular, the dependence of these interactions on the deposition sequence of the metal precursors. Using XRD and TEM results, it was shown that impregnating the support with an aqueous solution that contained nitrate salts of both metals, followed by hydrogen treatment at 500°C, generates uniformly sized Pd _0.6 Ag _0.4 particles ( d _av = 5.6 nm). These particles exhibit high selectivity (79%) in the reaction of acetylene hydrogenation to ethylene. The catalysts synthesized by sequential impregnation of the support with solutions of Pd and Ag nitrates interleaved with heat treatment in H ₂ exhibited a lower selectivity (68–73%) due to the formation of particles non-uniform both in composition and size (about 4 to 60 nm). The IR spectroscopy data suggest this effect is presumably associated with the removal of O-containing functional groups from the carbon surface during the reduction of the supported precursor. Given that O-groups act as anchoring sites for the precursors of active components and suppress the ability of Sibunit to reduce metals from their salt solutions, the subsequent deposition of the second metal salt causes a non-uniform distribution of this metal on the surface and the generation of larger particles.

Novel bimetallic Pd-Co catalysts supported on the carbon material Sibunit were synthesized by an incipient wetness impregnation method and used for ethylene production by selective acetylene hydrogenation. It has been established that an increase in the Pd:Co molar ratio from 1:0 to 1:2 in 0.5%Pd-Co/C catalysts, treated in hydrogen at 500 °C, leads to an increase in the ethylene selectivity from 60 to 67% (T = 45 °C). The selectivity does not change with a further increase in the modifier concentration. The catalysts were investigated by TPR-H₂, XRD, TEM HR, EDS, and XPS methods. It was shown that palladium and cobalt in the 0.5%Pd-Co/C samples form Pd_(1−x)Co_x phases of solid solutions with different compositions depending on the Pd:Co ratio. The cobalt concentration in the Pd-Co particles increases with an increase in the Pd:Co ratio up to 1:2 and then remains at a constant level. In addition, monometallic Co particles were present in the samples with the Pd:Co ratio higher then 1:2. The optimal combination of catalytic properties (the ethylene yield is 62–63%) is typical for catalysts with a Pd:Co molar ratio of 1:2–1:4. which is mainly due to the presence of bimetallic particles containing ~41–43% by at. of cobalt.

The MnO_х-ZrO₂-CeO₂ oxide catalysts were synthesized by co-precipitation method with varying (1) Zr/Zr + Ce molar ratio at constant manganese content of 0.3; (2) manganese content at constant Zr/Ce molar ratio of 1; (3) Mn/Mn + Zr molar ratio at constant Ce content of 0.5. Catalysts are characterized by XRD, N₂ adsorption, TPR, and XPS. The catalytic activity of all the series was tested in the CO and propane oxidation reactions. In contrast to the variation of the manganese content, the Zr/Zr + Ce molar ratio does not significantly affect the catalytic properties. The dependence of the catalytic activity in CO oxidation on the manganese content has a «volcano» shape, and the best catalytic performance is exhibited by the catalyst with Mn/(Zr + Ce) = 1. In the case of propane oxidation reaction, there is «sigma» like dependence, activity increases with increase of Mn/(Mn + Zr + Ce) molar ratio up to 0.3, stabilizing with a further increase in the manganese content. XRD and XPS have shown that with an increase of the Mn concentration in the MnO_x-ZrO₂-CeO₂ catalysts, the amount of crystalline manganese oxides such as Mn₂O₃ and Mn₃O₄, as well as the surface concentration of Mn cations, increases. While the content of Mn_xZr_yCe_1-x-yO₂ solid solution decreases, the concentration of manganese cations (x) in volume of Mn_xZr_yCe_1-x-yO₂ mixed oxide grows. The maximum activity in CO oxidation corresponds to the balance between the amount of the solid solution and the concentration of manganese cations in the volume of mixed oxide. The propane oxidation reaction is less sensitive to the state of manganese ion rather than to its amount. In this case, a decrease in the content of the Mn_xZr_yCe_1-x-yO₂ solid solution with increase in manganese amount in catalyst is compensated by an increase in content of crystalline manganese oxides and the surface concentration of manganese.

MnO_x-CeO₂, MnO_x-ZrO₂, MnO_x-ZrO₂-CeO₂ oxides with the Mn/(Zr + Ce + Mn) molar ratio of 0.3 were synthesized by coprecipitation method followed by calcination in the temperature range of 400–800 °C and characterized by XRD, N₂ adsorption, TPR, TEM, and EPR. The catalytic activity was tested in the CO oxidation reaction. It was found that MnO_x-CeO₂, MnO_x-ZrO₂-CeO₂, MnO_x-ZrO₂ catalysts, calcined at 400–500 °C, 650–700 °C and 500–650 °C, respectively, show the highest catalytic activity in the reaction of CO oxidation. According to XRD and TEM results, thermal stability of catalysts is determined by the temperature of decomposition of the solid solution Mn_x(Ce,Zr)_1−xO₂. The TPR-H₂ and EPR methods showed that the high activity in CO oxidation correlates with the content of easily reduced fine MnO_x particles in the samples and the presence of paramagnetic defects in the form of oxygen vacancies. The maximum activity for each series of catalysts is associated with the start of solid solution decomposition. Formation of active phase shifts to the high-temperature region with the addition of zirconium to the MnO_x-CeO₂ catalyst.

Structural features of bimetallic Pd−Zn catalysts supported on Sibunit carbon material were studied by EXAFS, TEM, XRD and XPS methods, depending on the heat treatment conditions in hydrogen. It was found that supported particles of PdZn intermetallic compound of tetragonal structure are formed in all samples. An increase in the duration of catalyst reduction in H₂ at 500 °C from 1 to 20 hours leads to an increase in the amount of PdZn intermetallic particles (d_av=2–3 nm), which is confirmed by the increasing of the Pd−Zn coordination number from 2.4 to 4.3 and by the decreasing of the surface Pd⁰/PdZn ratio from 0.63 to 0.18. The observed changes in the nature and structure of active sites provide an increase in the ethylene selectivity of Pd−Zn/C catalysts in liquid‐phase acetylene hydrogenation.