Tanya Petrova

This study investigates the effect of dilute organic acid treatment on the structural, textural, electronic, and catalytic properties of layered La–Sr–Fe–O Ruddlesden–Popper (R–P) oxides using XRD, TEM, BET, Mössbauer spectroscopy, XPS, H₂-TPR, C₂H₆-TPR and catalytic testing. XRD and TEM confirm that the overall layered Ruddlesden–Popper structure is preserved after acid treatment and during catalysis, with minor changes in phase composition, including a decrease in the n = 1 phase and a relative increase in the n = 2 phase. BET analysis shows increased specific surface area and pore volume, forming a more accessible mesoporous structure that is retained under reaction conditions. Mössbauer spectroscopy and XPS reveal an increased Fe⁴⁺ fraction and formation of hydroxylated and carbonated surface species stabilizing active Fe sites. During catalysis, a dynamic Fe³⁺/Fe⁴⁺ redox cycle occurs, along with surface restructuring and involvement of non-lattice oxygen, while the bulk electronic structure remains largely unchanged. Catalytic tests show improved activity, with a 40–60 °C reduction in operating temperature for all acid-treated samples, independent of acid type. This enhancement is mainly attributed to surface-related modifications, including removal of surface Sr-containing species, improved surface accessibility, and enhanced mass transport, while the overall R–P structural remains preserved.

Catalysts with a Ruddlesden–Popper structure (LaSrFeO₄, La_0.5Sr_1.5FeO₄, and La_1.5Sr_0.5FeO₄) were successfully synthesized and tested for the complete oxidation of C₁–C₄ hydrocarbons (methane, ethane, propane, and n-butane) as representative compounds of volatile organic compounds (VOCs), which are responsible for the global warming process. The samples were characterized by nitrogen physisorption, XRD, TEM, Mössbauer spectroscopy, XPS, O₂-TPD, and “depletive oxidation”. The catalyst La_0.5Sr_1.5FeO₄ showed the highest activity in the VOCs oxidation processes. This activity was connected with the enhanced strontium content in the catalyst, leading to high surface Fe⁴⁺ concentration and increased Fe⁴⁺/Fe³⁺ ratio, which promoted oxygen mobility and surface oxidation. Kinetic studies, along with physicochemical characterization, indicated that the ethane combustion proceeds via the Mars–van Krevelen mechanism.

Nanocrystalline materials with the composition of (Cu_0.5Ni_0.5)_yFe_3−yO₄ and a spinel structure were synthesized by the auto-combustion sol–gel method. The materials were characterized by powder X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and N₂ physisorption. A decrease in the unit cell parameter and increase in the crystallite size with a decrease in the copper and nickel content in ferrites were evidenced. Mössbauer analysis determined that iron ions are in the 3+ states in all compositions. Transmission electron microscopy showed that synthesized ferrite materials consisted of nanoparticles with narrow size distributions. The catalytic properties of synthesized ferrites were studied in the reaction of ethyl acetate oxidation and methanol decomposition. The conversion of ethyl acetate and CO₂ selectivity increased with temperature, and this effect was most pronounced for (Cu_0.5Ni_0.5)_0.5Fe_2.5O₄, for which the main part of the particles possessed sizes below 10 nm, and the mean diameter was calculated to be 4.3 nm. The catalytic activity in the reaction of methanol decomposition was the highest for (Cu_0.5Ni_0.5)_0.25Fe_2.75O₄, and it decreased with the increase in Cu and Ni content in the samples. The analysis of the samples after the catalytic test indicated significant reduction transformations within the catalysts. Under the reaction medium, the spinel phase decomposed through the formation of Hägg carbide.

The water–gas shift (WGS) reaction (CO + H₂O ↔ CO₂ + H₂) plays an important role in the hydrogen economy because it is an effective way to reduce the carbon release to net-zero CO₂ emissions. The general goal of this research is to develop nanosized oxo-rhenium catalyst formulations promoted by K and Co components for the WGS process. Rhenium, as a low-cost catalyst component, is a good choice compared to platinum group metals. A surface density of 2 Re atoms/nm² on a γ-Al₂O₃ support as well as cobalt (3 wt.% CoO) and potassium (5 wt.% K₂O) amounts were chosen to match the composition of our own active sour WGS KCoRe catalyst developed some years ago. An initial evaluation of the impact of replacing half of the rhenium with molybdenum, which is more affordable, was also studied. The purpose of this study is to explore the catalytic ability of CoRe, K-CoRe, CoReMo, and K-CoReMo formulations in the WGS reaction and elucidate the effect of a CO/Ar reaction mixture used in an activation–reduction pretreatment to form active catalyst structures. Oxo-K-Co-Re(Mo) entities formed in synthesized samples and their reducibility were analyzed via several physicochemical methods, such as N₂ physisorption, PXRD, UV-vis DRS, and H₂-TPR. In summary, the selected potassium- and cobalt-promoted Re-containing formulations have potential as catalysts for the classical WGS reaction. The selection of an appropriate procedure for activation–reduction, involving the reducing gas (CO or H₂), temperature, and duration, was needed for tuning the K-CoRe catalyst’s high activity for the WGS reaction with structural stability and longevity.

Tri-metallic NiMoW catalysts prepared by impregnating mesoporous aluminas (pore sizes of ~9 nm and surface areas of ~225 m²/g) obtained by sol-gel (NiMoW/Al) and hydrothermal (NiMoW/Al_HYDT) processes were investigated in the hydrodesulfurization (HDS) of thiophene and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at H₂ pressures of 1 MPa and 5.0 MPa, respectively. The supports and catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The NiMoW/Al_HYDT catalyst, which was the most active in both test HDS reactions, was characterized by a pore size of 7.5 nm, whereas the pore size of the catalyst on sol-gel alumina (NiMoW/Al) was only 4.8 nm. Moreover, the NiMoW/Al_HYDT catalyst exhibited reduction peaks shifted to a lower temperature during TPR, indicating weaker metal support interactions, a higher degree of Mo (79%) and W (48%) sulfidation, and an optimal layer slab length distribution of Mo(W)S₂ nanocrystals preferentially between 2–4 nm with an average layer stacking of 1.7 compared to the NiMoW/Al counterpart.