Sebastian Jarczewski

The commercially available zeolite HY and its desilicated analogue were subjected to a classical wet impregnation procedure with NH₄VO₃ to produce catalysts differentiated in acidic and redox properties. Various spectroscopic techniques (in situ probe molecules adsorption and time-resolved propane transformation FT-IR studies, XAS, ⁵¹V MAS NMR, and 2D COS UV-vis) were employed to study speciation, local coordination, and reducibility of the vanadium species introduced into the hierarchical faujasite zeolite. The acid-based redox properties of V centres were linked to catalytic activity in the oxidative dehydrogenation of propane. The modification of zeolite via caustic treatment is an effective method of adjusting its basicity—a parameter that plays an important role in the ODH process. The developed mesopore surface ensured the attachment of vanadium species to silanol groups and formation of isolated (SiO)₂(HO)V=O and (SiO)₃V=O sites or polymeric, highly dispersed forms located in the zeolite micropores. The higher basicity of HY_deSi, due to the presence of the Al-rich shell, aided the activation of the C−H bond leading to a higher selectivity to propene. Its polymerisation and coke formation were inhibited by the lower acid strength of the protonic sites in desilicated zeolite. The Al-rich shell was also beneficial for anchoring V species and thus their reducibility. The operando UV-vis experiments revealed higher reactivity of the bridging oxygens V-O-V over the oxo-group V=O. The (SiO)₃V=O species were found to be ineffective in propane oxidation when temperature does not exceed 400 °C.

Supported manganese–cobalt spinel catalysts and the corresponding mesoporous carbon supports were examined in order to reveal the effect of electrical conductivity and surface oxygen functionalities on the oxygen reduction reaction (ORR) in alkaline media. Carbon replicas of comparable porosity were obtained from sucrose by nano-replication of a silica template followed by carbonization at different temperatures (650–1050 °C). The degree of their graphitization was assessed by deconvolution of the Raman spectra (1000–1800 cm⁻¹), whereas relative electrical conductivity measurements were performed by means of the contactless microwave cavity quality factor perturbation technique. The electrocatalytic activities of the obtained materials towards the ORR (gauged by the ORR onset potential and the number of transferred electrons) were tested in alkaline media using standard hydrodynamic methods. It was found that with increasing carbonization temperature, the electrical conductivity of the examined carbon materials increased, in parallel with their growing crystallinity. For the carbon samples calcined in the temperature range 650–850 °C, the ORR onset potential initially rises as the electrical conductivity increases, passes through a maximum at around 850 °C, and then falls. Such a volcano-type shape was found also in the case of manganese–cobalt spinel catalysts deposited on the examined carbon supports. The observed non-monotonous electrocatalytic behavior was accounted for by two descriptors: the carbon support electrical conductivity and the amount of surface carbonyl and quinone groups, changing in the opposite way with the carbonization temperature. It was shown that variation of the electrical conductivity multiplied by the amount of carboxyl and quinone groups with the carbon calcination temperature exhibits the same behavior as the number of electrons exchanged in the ORR, confirming that an optimal conjunction of these properties plays a decisive role. The prime advantage of the dispersed spinel active phase lies in a significant enhancement of the 4-electron reduction pathway. To the best of our knowledge, the role of electrical conductivity and the nature of the surface oxygen-bearing functional groups in the ORR were elucidated for the first time.

Mesoporous alumina with narrow pore size distribution centered in the range of 4.4–5.0 nm and with a specific surface area as high as 270 m²·g⁻¹ was prepared via the nanocasting approach using a CMK-3 carbon replica as a hard template. Based on this support, a series of catalysts containing 1, 5, 10, 20 and 30 wt % of chromium was prepared by incipient wetness impregnation, characterized, and studied in the dehydrogenation of propane to propene (PDH). Cr species in three oxidation states—Cr(III), Cr(V) and Cr(VI)—were found on the oxidized surface of the catalysts. The concentration of these species varied with the total Cr loading. Temperature-programmed reduction (H₂-TPR) and UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) studies revealed that Cr(VI) species dominated at the lowest Cr content. An increase in the Cr loading resulted in an appearance of an increasing amount of Cr(III) oxide. UV-Vis-DRS measurements performed in situ during the PDH process showed that at the beginning of the catalytic test Cr(VI) species were reduced to Cr(III) redox species. A crucial role of the redox species in the PDH process over the catalysts with the low Cr content was confirmed. The stability test for the catalyst containing 20 wt % of Cr showed that this sample exhibited the reproducible catalytic performance after the first four regeneration–dehydrogenation cycles. Moreover, this catalyst had higher resistance on deactivation during the PDH process as compared to the reference catalyst with the same Cr loading, but was supported on commercially available alumina.