Galina O. Bragina

The transformations of chemical states and structures occurring in the PdIn/Al₂O₃ catalyst upon redox treatments in different gaseous atmospheres at different temperatures are addressed by an assortment of in situ bulk- (XRD) and surface-sensitive (XPS and DRIFTS CO) techniques. Any desired state of the catalyst between two opposite extremes of highly dispersed oxide species and regularly ordered PdIn intermetallic compound could be set in fully controlled and reversible ways by selecting appropriate conditions for the reductive treatment starting from the fully oxidized state. Since mutual conversions of multi-atomic Pd_n centers into single-site Pd₁ centers are involved in these transformations, the methodology could be used to find an optimum balance between the activity and selectivity of the catalytic system.

The effect of support on the performance of Pd₁Ag₁₀/Al₂O₃ and Pd₁Ag₁₀/CeO₂–ZrO₂ catalysts in the selective hydrogenation of diphenylacetylene (DPA) was studied. Characterization of the catalyst by DRIFTS-CO and HRTEM revealed the formation of a PdAg single-atom alloy (SAA) structure on the surface of PdAg nanoparticles, with Pd₁ sites isolated by Ag atoms. It was found that the use of CeO₂–ZrO₂ as a carrier makes it possible to increase the activity of the Pd₁Ag₁₀ catalyst by a factor of three without loss of selectivity compared to the reference Pd₁Ag₁₀/Al₂O₃. According to the HRTEM data, this catalytic behavior can be explained by an increase in the dispersion of Pd₁Ag₁₀/CeO₂–ZrO₂ compared to its Pd₁Ag₁₀/Al₂O₃ counterpart. As evidenced by DRIFTS-CO data, the high selectivity of the Pd₁Ag₁₀/CeO₂–ZrO₂ sample presumably stems from the stability of the structure of isolated Pd₁ sites on the surface of SAA Pd₁Ag₁₀/CeO₂–ZrO₂.

Metal–hypercrosslinked polystyrene (HCP) composites active in the hydrogenation of unsaturated hydrocarbons are obtained by impregnation of HCP with palladium and rhodium compounds in a supercritical carbon dioxide (SC-CO₂) medium followed by their reduction to the metallic state with molecular hydrogen. The influence of the conditions of preparation of the composites on their activity and selectivity in the hydrogenation of diphenylacetylene (DPA) is evaluated. The adsorption of palladium hexafluoroacetylacetonate on HCP from an SC-CO₂ medium is studied for the first time using supercritical fluid chromatography; it is shown that the adsorption equilibrium is described by the Langmuir equation.

This research was focused on studying the performance of the Pd₁Ag₃/Al₂O₃ single-atom alloy (SAA) in the liquid-phase hydrogenation of di-substituted alkyne (1-phenyl-1-propyne), and development of a kinetic model adequately describing the reaction kinetic being also consistent with the reaction mechanism suggested for alkyne hydrogenation on SAA catalysts. Formation of the SAA structure on the surface of PdAg₃ nanoparticles was confirmed by DRIFTS-CO, revealing the presence of single-atom Pd₁ sites surrounded by Ag atoms (characteristic symmetrical band at 2046 cm⁻¹) and almost complete absence of multiatomic Pd_n surface sites (2) and alkyne concentration. It is remarkable that selectivity remains almost constant upon variation of 1-phenyl-1-propyne (1-Ph-1-Pr) conversion from 5 to 95–98%, which indicates that a direct alkyne to alkane hydrogenation is negligible over Pd₁Ag₃ catalyst. The kinetics of 1-phenyl-1-propyne hydrogenation on Pd₁Ag₃/Al₂O₃ was adequately described by the Langmuir-Hinshelwood type of model developed on the basis of the reaction mechanism, which suggests competitive H₂ and alkyne/alkene adsorption on single atom Pd₁ centers surrounded by inactive Ag atoms. The model is capable to describe kinetic characteristics of 1-phenyl-1-propyne hydrogenation on SAA Pd₁Ag₃/Al₂O₃ catalyst with the excellent explanation degree (98.9%).

To obtain a supported heterogeneous catalyst, laser ablation of metallic palladium in supercritical carbon dioxide was performed in the presence of a carrier, microparticles of γ-alumina. The influence of the ablation process conditions—including supercritical fluid density, ablation, mixing time of the mixture, and laser wavelength—on the completeness and efficiency of the deposition of palladium particles on the surface of the carrier was studied. The obtained composites were investigated by scanning and transmission electron microscopy using energy dispersive spectroscopy. We found that palladium particles were nanosized and had a narrow size distribution (2–8 nm). The synthesized composites revealed high activity as catalysts in the liquid-phase hydrogenation of diphenylacetylene.

Formation of PdIn intermetallic nanoparticles supported on α-Al₂O₃ was investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and hydrogen temperature-programmed desorption (H₂-TPD) methods. The metals were loaded as heterobimetallic Pd(μ-O₂CMe)₄In(O₂CMe) complex to ensure intimate contact between Pd and In. Reduction in H₂ at 200 °C resulted in Pd-rich PdIn alloy as evidenced by XRD and the disappearance of Pd hydride. A minor amount of Pd₁In₁ intermetallic phase appeared after reduction at 200 °C and its formation was accomplished at 400 °C. Neither monometallic Pd or in nor other intermetallic structures were found after reduction at 400–600 °C. Catalytic performance of Pd₁In₁/α-Al₂O₃ was studied in the selective liquid-phase diphenylacetylene (DPA) hydrogenation. It was found that the reaction rate of undesired alkene hydrogenation is strongly reduced on Pd₁In₁ nanoparticles enabling effective kinetic control of the hydrogenation, and the catalyst demonstrated excellent selectivity to alkene.