Galina N. Baeva

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.

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%).

Local structure of Pd₁ single sites on the surface of Pd₁In₁ intermetallic nanoparticles supported on α-Al₂O₃ was investigated by the combination of CO-DRIFTS spectroscopy and DFT. CO-DRIFTS spectra of PdIn/Al₂O₃ catalyst exhibit only one asymmetric absorption band of linearly adsorbed CO comprising two peaks at 2065 and 2055 cm⁻¹ attributable to CO molecules coordinated to Pd₁ sites located at (110) and (111) facets of PdIn nanoparticles. The absence of bridged or hollow-bonded CO bands indicates that multipoint adsorption on PdIn nanoparticles is significantly hindered or impossible. DFT results show that on (110) facet multipoint CO adsorption is hindered due to large distance between neighboring Pd atoms (3.35 Å). On (111) facet multipoint CO adsorption on surface palladium atoms is impossible, since adjacent Pd atoms are located below the surface plane.

A series of oxidative treatments of PdIn-supported intermetallic nanoparticles at different temperatures were performed. The bulk and surface structure of catalyst during phase transformation was investigated by bulk- and surface-sensitive techniques (in situ XAFS, DRIFTS of adsorbed CO). It was found that comparison of palladium and indium fractions in bulk and on the surface suggests the formation of a «core-shell» structure. According to obtained results, the core consists of In-depleted intermetallic compound or inhomogeneous bimetallic phase with the inner core of metallic Pd, when a mixture of indium oxide, metallic palladium and small part of PdIn is present on the surface.

Volatile organic compounds (VOCs) have a negative effect on both humans and the environment; therefore, it is crucial to minimize their emission. The conventional solution is the catalytic oxidation of VOCs by air; however, in some cases this method requires relatively high temperatures. Thus, the oxidation of short-chain alkanes, which demonstrate the lowest reactivity among VOCs, starts at 250–350 °C. This research deals with the ozone catalytic oxidation (OZCO) of alkanes at temperatures as low as 25–200 °C using an alumina-supported manganese oxide catalyst. Our data demonstrate that oxidation can be significantly accelerated in the presence of a small amount of O₃. In particular, it was found that n-C₄H₁₀ can be readily oxidized by an air/O₃ mixture over the Mn/Al₂O₃ catalyst at temperatures as low as 25 °C. According to the characterization data (SEM-EDX, XRD, H₂-TPR, and XPS) the superior catalytic performance of the Mn/Al₂O₃ catalyst in OZCO stems from a high concentration of Mn₂O₃ species and oxygen vacancies.

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.

Pd−In/Al₂O₃ single‐site catalyst was able to show high selectivity (up to 98 %) in the gas phase semihydrogenation of propyne. Formation of intermetallic Pd−In compound was studied by XPS during reduction of the catalyst. FTIR−CO spectroscopy confirmed single‐site nature of the intermetallic Pd−In phase reduced at high temperature. Utilization of Pd−In/Al₂O₃ in semihydrogenation of propyne with parahydrogen allowed to produce ≈3400‐fold NMR signal enhancement for reaction product propene (polarization=9.3 %), demonstrating the large contribution of pairwise hydrogen addition route. Significant signal enhancement as well as the high catalytic activity of the Pd−In catalyst allowed to acquire ¹H MR images of flowing hyperpolarized propene gas selectively for protons in CH, CH₂ and CH₃ groups. This observation is unique and can be easily transferred to the development of a useful MRI technique for an in situ investigation of selective semihydrogenation in catalytic reactors.