Georgi N. Vayssilov

Georgi N. Vayssilov
Sofia University "St. Kliment Ohridski" · Faculty of Chemistry and Pharmacy

Dr. Sci.

About

175
Publications
15,332
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5,476
Citations
Citations since 2016
65 Research Items
2922 Citations
20162017201820192020202120220100200300400500
20162017201820192020202120220100200300400500
20162017201820192020202120220100200300400500
20162017201820192020202120220100200300400500

Publications

Publications (175)
Article
Organic solar cells based on wide band gap polymers and nonfullerene small-molecule acceptors have demonstrated remarkably good device performances. Nevertheless, a thorough understanding of the charge-transfer process in these materials has not been achieved yet. In this study, we use Fano resonance signals caused by the interaction of broad elect...
Preprint
Full-text available
We report the formation of a new Lewis acid Al site in H-FAU zeolite upon 650 ⁰C thermal treatment, unknown for any zeolite. Spectroscopy and DFT calculations reveal this site is a naked Al+3 ion which is charge balanced by a triplet of adjacent framework oxygens with net charge of -1 for each Si-O-Al moiety. This is the first reported observation...
Preprint
Full-text available
Atom trapping allows to prepare catalysts with atomically dispersed Ru ions Ru1O5 on (100) facets of ceria, as identified by EXAFS, infra-red spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in industrially importan...
Article
Full-text available
Cu/zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction steps remain unknown. Herein, using a combined spectroscopic, catalytic and DFT approach, we show that nitrosyl ions (NO+) in zeolitic micropores are the key intermediates for NO reduct...
Article
The hydroxyls, Brønsted acid sites (BAS) and silanols, provide key contributions in the global acidity of zeolites and have significant impact on their properties and applications. In this work, we present the acidity of BAS and silanols in zeolites depending on their configurations in zeolite nanoparticles. The acidity was evaluated based on the d...
Article
Full-text available
The interactions between Na+ or Mg2+ ions with different parts of single-stranded RNA molecules, namely, the oxygen atoms from the phosphate groups or the guanine base, in water solution have been studied using first-principles molecular dynamics. Sodium ions were found to be much more mobile than Mg2+ ions and readily underwent transitions between...
Article
Pd‐based catalysts are the most widely used for CO oxidation because of their outstanding catalytic activity and thermal stability. However, the fundamental understanding of the detailed catalytic processes on Pd‐based catalysts under realistic conditions is still not fully understood. In this study, we investigated CO oxidation on metallic Pd clus...
Preprint
Full-text available
Cu/Zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction steps remain unknown. Herein, using combined spectroscopic, catalytic and DFT approach we show that nitrosyl ions (NO+) in the zeolitic micropores are the key intermediates for NO reduc...
Preprint
Full-text available
Ceria nanoparticles supported on alumina have low activity for industrially relevant NO reduction by CO. We discovered that treatment of these catalysts at temperatures between 750 and ~1,000 ºC under the flow of CO and NO in the presence of steam, which typically leads to catalyst deterioration and sintering, in fact, leads to dispersion of ceria...
Article
The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH4F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by z...
Article
We report computational study of the distribution of germanium ions among the double four-membered rings (D4Rs) in SCM-14 germanosilicate and the influence of the structure-directing agent (SDA) on the stability...
Article
Full-text available
We used computational modeling, based on Density Functional Theory, to help understand the preference for the formation of silanol nests and the substitution of Si by Ti or Al in different crystallographic positions of the MSE-type framework. All these processes were found to be energetically favorable by more than 100 kJ/mol. We suggested an appro...
Article
Full-text available
YNU-5 zeolite has the YFI-type framework with a 12-12-8-ring system (2-dimensional 12-oxygen membered ring pores 3-dimensionally connected by twin 8-ring channels), and isolated 8-ring channels separated from the system by a thin (mono-atomic silicate) wall. In this study, the acidic property of YNU-5 zeolite was analyzed mainly by means of the amm...
Article
Full-text available
CO oxidation is of importance both for inorganic and living systems. Transition and precious metals supported on various materials can oxidize CO to CO2. Among them, few systems, such as Au/TiO2, can perform CO oxidation at temperatures as low as −70 °C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes under ambi...
Article
Zeolite Y and its ultra-stabilized hierarchical derivative (USY) are the most widely used zeolite-based heterogeneous catalysts in oil refining, petrochemisty, and other chemicals manufacturing. After almost 60 years of academic and industrial research, their resilience is unique as no other catalyst displaced them from key processes such as FCC an...
Article
The amount and location of acid sites (strong Brønsted and weak silanols) in zeolites are crucial for their applications. In this work we revealed the enigma of the complex H-bonded...
Article
Zeolite Y and its ultra‐stabilized hierarchical derivative (USY) are the most widely used zeolite‐based heterogeneous catalysts in oil refining, petrochemisty, and other chemicals manufacturing. After almost 60 years of academic and industrial research, their resilience is unique as no other catalyst displaced them from key processes such as FCC an...
Preprint
Full-text available
Atom trapping allows to prepare catalysts with atomically dispersed Ru ions anchored to the ceria support. The resulting catalysts free of expensive noble metals such as Pt, Pd, Rh (whose prices are ~8-60 times higher than Ru on the per-molar basis) with Ru loadings of only 0.25-0.5 wt% show excellent activity in industrially important catalytic NO...
Preprint
Atom trapping allows to prepare catalysts with atomically dispersed Ru ions anchored to the ceria support. The resulting catalysts free of expensive noble metals such as Pt, Pd, Rh (whose prices are ~8-60 times higher than Ru on the per-molar basis) with Ru loadings of only 0.25-0.5 wt% show excellent activity in industrially important catalytic NO...
Article
Rh(C2H4)2 species grafted on the HY zeolite framework significantly enhance the activation of H2 that reacts with C2H4 ligands to form C2H6. While in this case, the simultaneous activation of C2H4 and H2 and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C2H6, the results obtained for...
Article
Silica nanoparticles incorporating magnetic iron oxides (MS) with sizes around 20 nm were developed. The MS material was modified with NH2-groups and then grafted by PEG chains. The anticancer drug miltefosine was loaded into the NH2-modified and PEGylated MS nanoparticles by impregnation procedure resulting in 11–28% of drug content. PEG chains we...
Article
Structural properties and reducibility of zirconium-doped cerium dioxide systems were studied using periodic plane-wave calculations based on density functional theory. A systematic analysis of the results for nanoparticles of two sizes, Ce40−nZrnO80 ∼ 1.5 nm large and Ce140−nZrnO280 ∼ 2.4 nm large, in comparison with slab model data for Ce1−xZrxO2...
Article
Full-text available
Magnetic iron oxide containing MCM-41 silica (MM) with ~300 nm particle size was developed. The MM material before or after template removal was modified with NH2- or COOH-groups and then grafted with PEG chains. The anticancer drug tamoxifen was loaded into the organic groups’ modified and PEGylated nanoparticles by an incipient wetness impregnati...
Article
Nitrates formed on ceria surface were investigated by FTIR spectroscopy and DFT methods. Isotope labelling (¹⁵N, ¹⁸O) was utilized to strengthen the conclusions. We found that the structures usually associated with monodentate nitrates (IR bands at ca. 1460, 1300 and 1030 cm¹), although bound to surface most strongly via one oxygen atom, are triden...
Preprint
p>CO oxidation is of importance both for organic and inorganic systems. Transition and precious metals on various supports can oxidize CO to CO<sub>2</sub>. Among them, few systems, like Au/TiO<sub>2</sub>, can perform CO oxidation at the low temperature of -70 ⁰C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes...
Article
We provide the first observation of super-electrophilic metal cations on a solid support. For Pd/SSZ-13 the results of our combined experimental (FTIR, XPS, HAADF-STEM) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd+3 and Pd+4, are in fact present as super electrophilic Pd+2 species (charge-transfer...
Article
Full-text available
Iron-containing HZSM-5 zeolites are materials with important industrial applications as catalysts. Their characterization is difficult due to the various possible Fe-containing species which can exist in the pores of HZSM-5 zeolites and their dependence on the preparation technique. Three Fe–HZSM-5 samples were prepared by ion-exchange technique us...
Article
Using periodic density functional calculations we clarified the variations of the electronic structure of platinum clusters and nanowires upon their deposition on reducible and on non-reducible oxide as well as upon adsorption of carbon monoxide. Three models of platinum clusters and nanowires deposited on ceria surface and one on gamma-alumina sur...
Article
NaY samples in the crystalline form with Si/Al ratios from 2.15 to 2.40 were synthesized as adsorbents for paraquat. Their surface areas, particle size, concentration of basic sites decreased with an increase of Al content in the synthesis gel. The interaction between paraquat molecule and NaY zeolite was investigated by periodic DFT calculations t...
Article
ZSM-5/KIT-6 and ZSM-5/SBA-15 nanoparticles were synthesized and further modified by a post-synthesis method with (CH2)3SO3H and (CH2)3NHCO(CH2)2COOH groups to optimize their drug loading and release kinetic profiles. The verapamil cargo drug was loaded by incipient wetness impregnation both on the parent and modified nanoporous supports. Nanocarrie...
Article
Cerium is a key component in many DeNO x catalysts. An important property of ceria related to these applications is its high oxygen storage capacity. Indeed, in some reaction steps, cerium switches its oxidation state between 3+ and 4 + . To design new and effective cerium-based DeNOx catalysts, it is necessary to know in details the interaction be...
Article
Full-text available
The assembly of highly hydrophobic nanosized tungsten containing MFI type zeolite nanocrystals (W-MFI) in films, and their further use for selective exhaust gas (CO, CO2, NO and NO2) detection was investigated by by operando IR spectroscopy. Due to the hydrophobic nature and the presence of tungsten in the framework, the W-MFI films showed excellen...
Preprint
div> The results of our combined experimental (FTIR, XPS, HAADF-STEM, EXAFS) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd<sup>+3</sup> and Pd<sup>+4</sup>, are in fact present as super electrophilic Pd<sup>+2</sup> species (ion pairs with the negatively charged framework oxygens). In this contrib...
Article
The conversion of C2H4 in the absence and presence of H2 was examined over dealuminated HY30, HY15, and HY2.6 zeolites with different Si/Al ratios under ambient conditions (1 atm, 25°C). Only in the presence of H2, these materials catalyze the conversion of C2H4 into C2H6 with measurable rates but show no sign of C2H4 dimerization. The C2H4 hydroge...
Article
The present study evaluates the potential of encapsulated doxorubicin to reduce both the viability of melanoma cells and the tumor growth in a mouse melanoma model. The prepared doxorubicin loaded chitosan/alginate nanoparticles possessed mean diameter around 300 nm and negative zeta-potential. Classical molecular dynamic simulations revealed that...
Article
Full-text available
Carbon Cn moieties on late transition metals are historically regarded as poisoning agents for their use in heterogeneous catalysis. Recent experimental studies combining ab initio simulations showed the promoting catalytic role of subsurface C atoms in Pd surfaces, plus their existence was well established in Ni and Pt surfaces. Here we show, adjo...
Article
Carbon Cn moieties on late transition metals are historically regarded as poisoning agents for their use in heterogeneous catalysis. Recent experimental studies combining ab initio simulations showed the promoting catalytic role of subsurface C atoms in Pd surfaces, plus their existence was well established in Ni and Pt surfaces. Here we show, adjo...
Article
The synthesis of mesoporous KIL-2 and KIT-6 nanoparticles (40 and 60 nm, respectively) was performed. Post synthesis procedure was applied for the modification of the initial mesoporous silicas with amino groups. Incipient wetness impregnation or solid state reaction were used for curcumin loading in the parent and amino-modified nanoporous support...
Article
Atomically dispersed palladium in small‐pore zeolites is highly effective for simultaneous low temperature NO and CO abatement. In their Communication (DOI: 10.1002/anie.201809343) J. Szanyi and co‐workers develop a modified incipient wetness impregnation method for the synthesis of transition‐metal‐ion loaded small‐pore zeolites. Pd/SSZ‐13 with at...
Article
Palladiumatome in kleinen Zeolithporen binden NO und CO effektiv bei niedrigen Temperaturen. In ihrer Zuschrift (DOI: 10.1002/ange.201809343) stellen J. Szanyi und Mitarbeiter eine benetzungsbasierte Synthesemethode vor, um kleinporige Zeolithe mit Übergangsmetallionen zu beladen. Pd/SSZ‐13 mit dispergierten Metallatomen speichert NOx und CO bei ni...
Article
The majority of harmful atmospheric CO and NOx emissions comes from vehicles’ exhaust. Although there has been success addressing NOx emissions at temperatures above 250 ºC with the aid of selective catalytic reduction technology that relies on the Cu/SSZ‐13 material and sacrificial urea source, emissions during vehicle cold start (when the tempera...
Article
The majority of harmful atmospheric CO and NOx emissions comes from vehicles’ exhaust. Although there has been success addressing NOx emissions at temperatures above 250 ºC with the aid of selective catalytic reduction technology that relies on the Cu/SSZ‐13 material and sacrificial urea source, emissions during vehicle cold start (when the tempera...
Article
Full-text available
Using periodic density functional calculations, we studied the local structure and preferred locations of yttrium cations and oxygen vacancies in Y-doped cerium dioxide. We employed three kinds of models – a slab of the CeO2(111) surface and two ceria nanoparticles of different sizes and shapes. In the slab models, which represent the (111) surface...
Poster
Full-text available
Surface azides (N3‐) are formed on reduced ceria after adsorption of NO. They are characterized by antisymmetric N‐N‐N stretching modes at 2045‐2042 cm‐1. The N3‐ species are thermally stable up to 300 oC but the stability decreases in presence of water. Similarly to NCO‐, azides are inert toward pure NO or O2 at ambient temperature but readily int...
Poster
Full-text available
At the initial stages of NO adsorption on activated (oxidized) ceria nanoparticles mainly symmetric nitrites and trans‐hyponitrites ([N2O2]2−) are simultaneously produced as a result of NO disproportionation. At further stages non‐symmetric NO2‐ species, N2O and small amount of nitrates are formed. In the presence of small amounts of oxygen, the hy...
Article
The composite system comprising mesoporous silica nanoparticle KIT-6 with I3ad symmetry and polyelectrolyte complex (κ-carrageenan/chitosan/κ-carrageenan) was utilized as a promising carrier for the effective delivery of quercetin. KIT-6 silica nanoparticles with sizes around 45 nm were synthesized and modified with amino groups by post-synthesis m...
Article
Full-text available
Alkaline and alkaline earth ions, namely Na+, K+, Mg2+ and Ca2+, are critical for the stability, proper folding and functioning of RNA. Moreover, those metal ions help to facilitate macromolecular interactions as well as the formation of supramolecular structures (e.g. the ribosome and the ribozymes). Therefore, identifying the interactions between...
Article
Adsorption of NO and co-adsorption of NO and O2 on ceria have been re-investigated by FTIR spectroscopy. To provide unambiguous assignments of the IR bands, adsorption of ¹⁵NO and co-adsorption of ¹⁴NO + ¹⁵NO isotopic mixtures have also been studied and DFT calculations performed. At the initial adsorption stages mainly symmetric nitrite species (2...
Article
Full-text available
This paper reports a quantum chemical study of all stages of a one-pot synthesis of pyrrolidinedione derivatives from nitromethane and coumarin, which includes Michael addition, migration of an oxygen atom (Nef-type rearrangement), and cyclization to a pyrrolidine ring. The energy barrier of deprotonated nitromethane addition to coumarin is 21.7 kJ...
Article
HY zeolite-supported Rh(CO)2 complexes were used as precursors for the surface mediated synthesis of Rh(NO)2 species. The results of FTIR, EXAFS, and Mass Spectrometry measurements, as well as DFT calculations show that the replacement of the CO ligands in the Rh(CO)2 complexes by NO is a facile substitution process which is not affected by the Si/...
Article
Pd is widely used to catalyse hydrogenation and dehydrogenation reactions. One of them is the hydrogenation of ethylene, which includes the transformation of ethyl species to ethane. Herein, by means of density-functional calculations we address several still insufficiently understood factors affecting the latter process. In particular, we shed lig...
Article
Dry reforming of methane (DRM) is a promising utilization process of greenhouse gases, namely CO2 and CH4. Nickel-based catalysts are the most popular ones for DRM, because they are inexpensive and relatively active but deactivate rapidly mainly due to carbon formation. Carbon gasification by either partial or complete oxidation is considered to be...
Article
The synthesis of nanostructured zeolites enables modification of catalytically relevant properties such as effective surface area and diffusion path length. Nanostructured zeolites may be synthesized either in alkaline media, and so contain significant numbers of hydrophilic silanol groups, or in expensive and harmful fluoride-containing media. Her...
Article
Experiments showed small Rh clusters, supported in zeolites, to be catalytically active in the hydrogenation of ethene. We report a computational study on the transformations of ethene over Rh4 clusters supported in a faujasite zeolite framework, in particular on the influence of the hydrogen loading of the clusters. Our density functional calculat...
Article
Since the catalytic behavior of supported platinum species strongly depends on their nuclearity under the reaction conditions, in this study we addressed the stability of platinum species in the presence of CO. We applied density functional modeling to clarify the effect of the CO coverage on the structure and stability of small platinum clusters d...
Article
The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functiona...
Article
Pt-doped CeOx thin films electrocatalysts have recently been shown to exhibit high activity and stability at the anode of proton exchange membrane fuel cells (PEM-FC). To identify the role of the Pt dopant and the origin of the high stability of Pt-CeOx films, we have applied electrochemical in-situ IR spectroscopy on Pt-CeOx model thin film cataly...
Article
SBA-16 and MCM-41 silica materials were synthesized and modified by post-synthesis method with different amounts of Zn (2 and 4 wt.%). Quercetin, a flavonol compound, was loaded by incipient wetness impregnation method on the pure silica and Zn-modified mesoporous MCM-41 and SBA-16 supports. The parent and drug loaded formulations were characterize...
Article
To design new deNOx materials and processes we need to know in details the mechanism of NO interaction with solid surfaces. Many NOx conversion catalysts contain ceria as a component and cerium changes its oxidation state in the course of redox catalytic processes. Here we show that the interaction between NO and reduced ceria leads to formation of...
Article
Regarding the heterogeneous catalytic hydrogenation of ethene on small rhodium clusters, anchored in faujasite, we computationally studied C2Hx (x = 2–5) intermediates on zeolite-supported Rh3 and Rh4 clusters with pre-loaded hydrogen. According to the calculated Gibbs free energies, at temperatures and hydrogen pressure values representing pertine...
Article
We report the structure and stability of ceria units deposited on the surface of γ-Al2O3 respectively incorporated in its cavities, as determined by periodic density functional calculations. Ceria species are modeled as CeO2 or Ce2O4 moieties or as a small nanoparticle, Ce13O26, on the (100) and (001) surfaces of a γ-Al2O3 slab. Among the studied s...
Article
Rh(CO)2 complexes anchored on various dealuminated HY zeolites were used as precursors for the surface-mediated synthesis of Rh(CO)(H)x species. The carbonyl ligands of the initial Rh(CO)2 complexes react with gas-phase C2H4 to form Rh(CO)(C2H4) species and subsequently well-defined Rh(CO)(H)x complexes when the former are exposed to H2 at room tem...
Article
We report our results for the structure and relative stability of mononuclear platinum species on a ceria nanoparticle Ce21O42 depending on reduction or oxidation of the system. The most stable platinum species is Pt2+ at small {100} facets, where the ion is coordinated in a square-planar complex with four oxygen anions as ligands. Partial reductio...
Article
The efficiency of zeolite X nanocrystals (FAU-type framework structure) containing different extra-framework cations (Li+, Na+, K+ and Ca2+) in slowing down the thermal oxidation of palm oil is reported. The oxidation study of palm oil is conducted in the presence of zeolite nanocrystals (0.5 wt.%) at 180 °C. Several characterization techniques suc...
Article
Full-text available
High-energy irradiation of protic solvents can transiently introduce excess electrons that are implicated in a diverse range of reductive processes. Here we report the evolution of electron solvation in water and in alcohols following photodetachment from aqueous hydroxide or the corresponding alkoxides studied by two- and three-pulse femtosecond s...
Article
Full-text available
We show that the first stages of interaction between NO and reduced ceria comprise the formation of azides, N3-, with simultaneous oxidation of Ce3+ to Ce4+. This finding impose revision of some current views of catalytic NO conversion and may contribute to design new deNOx materials and processes.
Article
Using a periodic density functional approach, we modeled the dissociative adsorption of hydrogen on small Rh clusters, supported inside the cavity of a faujasite-type zeolite. H spillover from a zeolite hydroxyl group to the metal cluster is favorable by at least 160 kJ/mol. Therefore, we used the complexes HRh3 and HRh4 in deprotonated zeolite as...
Article
Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show t...
Article
Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show t...
Article
Fourier transform infrared (FTIR), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS) measurements, and DFT calculations were used to characterize the species formed following reaction of a Rh(CO)2(acac) precursor with dealuminated HY zeolites with different Si/Al ratios. The results indicate the formation of t...
Article
MCM-41 silica with spherical morphology and small particle sizes (100 nm) was synthesized and modified by post-synthesis method with amino and/or carboxylic groups. Solid state reaction was applied for the first time for loading of poorly soluble drug mesalazine (5-aminosalicylic acid - 5-ASA). The non-loaded and drug loaded mesoporous silicas were...
Article
Local reactivity descriptors, such as atomic charges, atomic electrostatic potential and atomic Fukui indices were computed for a series of 3-substituted coumarin (2-oxo-2H-1-benzopyran) derivatives, using Density Functional Theory (DFT) and Möller-Plesset methods (MP2). The variation of those properties as a function of the substituents was compar...
Article
We evaluated the accuracy of periodic density functional calculations for adsorption enthalpies of water, alkanes, and alcohols in silicalite and HZSM-5 zeolites using a gradient-corrected density functional with empirical dispersion corrections (PBE-D) as well as a nonlocal correlation functional (vdW-DF2). Results of both approaches agree in acce...