Browsing by Author "Hadjiivanov, K."

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Open Access
Effects induced by interaction of the Pt/CeO x /ZrO x /γ-Al 2 O 3 ternary mixed oxide DeNO x catalyst with hydrogen
(Elsevier, 2020) Andonova, S.; Ok, Zehra Aybegüm; Özensoy, Emrah; Hadjiivanov, K.
Effects of H2/D2 adsorption on the surface chemistry of Pt/CeOx-ZrOx/γ-Al2O3 DeNOx catalyst were investigated. In-situ FTIR spectroscopy and NOx-TPD techniques were utilized to monitor changes in the surface chemistry of studied materials. Adsorption studies of CO and O2 revealed that the Pt/Ce-Zr/Al sample, initially reduced with H2 at 723 K, is characterized by the presence of oxygen vacancies in close vicinity of Ce3+ centres and metallic Pt sites. Adsorption of O2 occurred through the formation of superoxide (O2 −)ads species and oxidation of Ce3+ to Ce4+ ions. The ability of the catalyst to activate molecular O2 originates from its relatively high population of oxygen vacancies located on/near the surface. Interaction of Pt/Ce-Zr/Al system with H2 or D2 takes place through heterolytic dissociation at ambient temperature. D2 adsorption leads to the reduction of Ce4+ to Ce3+ ions and formation of adsorbed molecular heavy water and gradual D/H exchange with the existing surface hydroxyl groups. Generated D2O interacts with isolated hydroxyls/deuteroxyls through H-bonding and this provokes the formation of H-bonded OeH/OeD groups. These later species are relatively stable and gradually vanish with increasing temperatures above 523 K, leaving behind only isolated hydroxyls. Surfaces enriched with H-bonded hydroxyls are characterized with an enhanced NOx storage ability revealing their significant role in low-temperature NOx adsorption mechanism.
Open Access
FTIR study of low-temperature CO adsorption on Mn-ZSM-5 and MnY zeolites. Effect of the zeolite matrix on the formation of Mn2+(CO)x geminal species
(Elsevier, 2002-08) Hadjiivanov, K.; Ivanova, E.; Kantcheva, M.; Ciftlikli, E. Z.; Klissurski, D.; Dimitrov, L.; Knözinger, H.
Adsorption of CO on Mn-ZSM-5 zeolite at 85 K results in formation of physically adsorbed CO, several kinds of H-bonded CO and Mn2+ (CO)(x) geminal species (2202 cm(-1)). Decreasing the coverage during evacuation results in disappearance of the physically adsorbed CO and the H-bonded forms and in conversion of the dicarbonyls to linear Mn2+-CO Species (2214 cm(-1)). The latter are quite stable at 85 K. Coadsorption (CO)-C-12 and (CO)-C-13 reveals that the CO molecules in the geminal polycarbonyls behave as independent oscillators. In contrast, CO adsorption at 85 K on MnNaY zeolite only leads to formation of linear Mn2+-CO species (2210 cm-1) and mono- and di-carbonyls associated with residual sodium cations. The results are interpreted as evidence that site-specified geminal carbonyls are formed with cations possessing an ionic radius bigger than a critical value. This value is different for different positions in various zeolites and is bigger for cations in S-II positions in Y zeolites than is the case of cations in a ZSM-5 matrix. (C) 2002 Elsevier Science B.V. All rights reserved.
Open Access
Pt/CeOx/ZrOx/γ-Al2O3 ternary mixed oxide DeNOx catalyst: surface chemistry and NOx interactions
(American Chemical Society, 2018) Andonova, S.; Ok, Zehra Aybegüm; Drenchev, N.; Özensoy, Emrah; Hadjiivanov, K.
Surface chemistry and the nature of the adsorbed NOx species on a Pt/CeO2-ZrO2/Al2O3 catalyst were investigated by IR spectroscopy, X-ray diffraction, H2-temperature programmed reduction, and NOx-temperature programmed desorption. Parallel studies were also carried out with benchmark samples such as CeO2/Al2O3, ZrO2/Al2O3, CeO2-ZrO2/Al2O3 and Pt-supported versions of these materials. All samples were studied in their reduced and nonreduced forms. The use of CO as a probe molecule revealed that during the synthesis of the mixed-metal oxide systems, deposited zirconia preferentially interacted with the alumina hydroxyls, while deposited ceria was preferentially located at the Lewis acid sites. Despite the limited extent of Zr4+ ions incorporated into the CeO2 lattice, the reduction of ceria was promoted and occurred at lower temperatures in the presence of zirconia. When deposited on ZrO2/Al2O3, platinum formed relatively big particles and existed in metallic state even in the nonreduced samples. The presence of ceria hindered platinum reduction during calcination and yielded a high platinum dispersion. Subsequent reduction with H2 led to the production of metallic Pt particles. Consequently, NO adsorption on nonreduced Pt-containing materials was negligible but was enhanced on the reduced samples because of Pt0-promoted NO disproportionation. The nature of the nitrogen-oxo species produced after NO and O2 coadsorption on different samples was similar. Despite the high thermal stability of the NOx adsorbed species on the ceria and zirconia adsorption sites, the NOx reduction in the presence of H2 was much more facile over Pt/CeO2-ZrO2/Al2O3. Thus, the main differences in the NOx reduction functionalities of the investigated materials could be related to the ability of the catalysts to activate hydrogen at relatively lower temperatures.
Open Access
Structure and properties of KNi–hexacyanoferrate Prussian Blue Analogues for efficient CO2 capture: Host–guest interaction chemistry and dynamics of CO2 adsorption
(Elsevier, 2021-06-04) Andonova, S.; Akbari, S. S.; Karadaş, Ferdi; Spassova, I.; Paneva, D.; Hadjiivanov, K.
Potassium Nickel hexacyanoferrate Prussian Blue Analogues (K-NiFe-PBAs) offer an excellent platform for efficient CO2 capture due to their porous nature and accessible channels. Herein, the effect of Ni:K atomic ratio on the structure and the CO2 storage capacity was studied by employing K-NiFe-PBAs with Ni:K ratio of ca. 2.5 and 12. The porosity and the isosteric heat of CO2 adsorption can be modulated and optimized by varying the Ni:K atomic ratio in the PB framework and thus, covering the thermodynamic criterion for easy CO2capture and release with acceptable energy costs. The synthesized K-NiFe-PBAs containing only trace amounts of K+ ions (with Ni:K = 12) shows an adsorption capacity (∼3.0 mmol g–1 CO2 at 273 K and 100 kPa) comparable to other well established CO2 adsorbents. In situ FTIR spectroscopy was further employed to elucidate the host–guest interaction chemistry and the dynamics of K-NiFe-PBAs within CO2 and H2O. The analysis enabled, to the best of our knowledge, is the first FTIR spectroscopic observation of the high sensitivity of the material to structural distortions induced by small changes under water vapor pressure. It was found that H2O hardly affects CO2 adsorption and the materials are perspective for CO2 capture in the presence of water.