Kristina Chakarova

Bulgarian Academy of Sciences | BAS · Institute of General and Inorganic Chemistry

Ceria and ceria-based catalysts are very important in redox and acid-base catalysis. Nanoceria have also been found to be important in biomedical applications. To design efficient materials, it is necessary to thoroughly understand the surface chemistry of ceria, and one of the techniques that provides such information about the surface is the vibr...

The interaction between O2 and reduced ceria nanocubes was mainly investigated using FTIR spectroscopy. Nanorods and nanoparticles were also studied for comparison. Adsorption of O2 at 100 K on unreduced ceria produces only O2 molecularly adsorbed on Ce4+ sites. The Ce3+ cations on ceria reduced by H2 at 773 K were monitored using the 2F5/2 → 2F7/2...

The interaction between O2 and reduced ceria nanocubes was mainly investigated by FTIR spectroscopy. Nanorods and nanoparticles were also studied for comparison. Adsorption of O2 at 100 K on unreduced ceria produces only O2 molecularly adsorbed on Ce4+ sites. The Ce3+ cations in H2-reduced ceria were monitored by the 2F5/2 → 2F7/2 electronic transi...

Adsorption of CO2 on NaY at ambient temperature results in formation of Na+-O=C=O adducts. Lowering temperature in presence of CO2 leads to two effects. First, CO2 molecules change their orientation. A second CO2 molecule then attaches to the same Na+ site thus forming geminal species. However, the formation conditions of the geminal species in NaY...

Efficient CO2 capture is necessary to solve the current environmental problems of global warming. Porous materials are promising for this purpose and adsorption capacity is a critical issue. Large cations in zeolites can often attach two small molecules, but it is not yet well established whether this applies to CO2. Here we present a detailed FTIR...

Coal fly ash zeolites (CFAZs) of type X with low (Na-X) and medium (Na-Ca-X) content of calcium were synthesized by alkaline conversion of lignite coal fly ash generated by combustion of lignite with lower and higher limestone amounts, extracted from different coal deposits and burned in separate thermal power plants. The concentration and state of...

The transition to a hydrogen economy requires the development of cost-effective methods for purifying hydrogen from CO. In this study, we explore the possibilities of Cu/ZSM-5 as an adsorbent for this purpose. Samples obtained by cation exchange from aqueous solution (AE) and solid-state exchange with CuCl (SE) were characterized by in situ EPR and...

Nickel-decorated mesoporous cerium–iron oxide composites were synthesized by a combination of incipient wetness impregnation and template-assisted hydrothermal techniques. The effects of the Fe/Ce ratio and the calcination temperature of cerium–iron oxides on the phase composition, texture, structure, and redox properties of the composites were stu...

The variety of functionalities and porous structures inherent to metal-organic frameworks (MOFs) together with the facile tunability of their properties makes these materials suitable for a wide range of existing and emerging applications. Many of these applications are based on processes involving interaction of MOFs with guest molecules. To optim...

Nano-scaled UiO-66 and UiO-66-NH2 crystals are synthesized via a solvothermal procedure and investigated regarding their applicability in CO2 sensing devices. The interaction between CO2 and UiO-66 as well as UiO-66-NH2 is studied via FTIR and dielectric measurements with a home-made sensing-cell contraption. The affinity towards low CO2 concentrat...

The variety of functionalities and porous structures inherent to metal-organic frameworks (MOFs) together with the facile tunability of their properties makes these materials suitable for a wide range of existing and emerging applications. Many of these applications are based on processes involving interaction of MOFs with guest molecules. To optim...

Metal-organic frameworks (MOFs) and MOF-based materials find increasing and diverse applicability. The performance of MOFs in adsorption and catalysis is strongly related to their acid-basic properties. Here we report on the development of acidity (protonic and Lewis) and basicity during thermo-vacuum treatment of two MOFs of practical importance,...

A porous scandium MOF denoted STA-27 (St Andrews Porous Material-27) has been synthesised solvothermally using the 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine anion (TCPP4-) as the tetratopic carboxylate linker. STA-27 possesses a unique scandium-based 1D rod secondary building unit (SBU) comprising corner-sharing scandium Sc2O11 dimers connected via...

A series of CexZr1-xO2 supports with different Ce/Zr molar ratios were used for the preparation of gold catalyst used in the selective oxidation of 5-hydroxymethyl-2-furfural to 2,5-furandicarboxylic acid. The used method of gold deposition allows the preparation of homogeneous gold particles size and shape distribution, a formulation very useful f...

A fully interpenetrated 8,3-connected zirconium MOF with the the-i topology type, STA-26, has been prepared using the 4,4',4"-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate (TMTB) tritopic linker with formic acid as a modulating agent. In the as-prepared form it possesses Im-3m symmetry compared with the Pm-3m symmetry of the non-interpenetrated a...

The purpose of this paper is to provide a clear and simple introduction to the application of isotopically labelled molecules in the characterization of various surfaces by vibrational spectroscopies. Although the possible applications are multifarious, we have tried to systematize the most important of them in several groups. Initially we provide...

Adsorption of CO2 on MIL-53(Al) and NH2-MIL-53(Al) has been studied by FTIR spectroscopy at different temperatures and equilibrium pressures. For better interpretation of the spectra ¹³CO2 was also utilized. It is established that with both samples at low coverages CO2 forms O-bonded complexes with the structural OH groups (OH...O¹²CO). These speci...

Metal cations that are exchanged in zeolites are generally characterized by a low coordination number and can adsorb more than one small molecule. For instance, Cu + ions exchanged in ZSM-5 (and in other zeolites) can bind, at low temperature, up to three CO and up to two NO molecules. Although mixed aqua-carbonyl complexes are easily formed, here...

A [Ge]FAU zeolite sample (2.36 wt. % Ge) was synthesized by a new approach comprising utilization of mother liquids instead of zeolite seeds and characterized by FTIR spectroscopy of adsorbed CO and N2 as probe molecules. A Ge-free sample with similar Al concentration was also prepared and studied for comparison. The background spectra of the H-[Ge...

FTIR spectra of (12)CO2 and (12)CO2 + (13)CO2 mixtures adsorbed on MIL-53(Al) reveal the formation of highly symmetric dimeric (CO2)2 species connected to two structural OH groups.

Acidity of solids is decisive for their interaction with guest molecules. One of the most used methods for measuring the acidity of surface hydroxyl groups is the hydrogen bond method based on the spectral shift of the OH stretching modes induced by the adsorption of weak bases. However, many materials of practical interest (e.g. metal organic fram...

Hydrogen dissociation and spillover on supported metal nanoparticles have received renewed interest because these chemical processes are closely related to applications in heterogeneous catalysis and hydrogen storage. In heterogeneous catalysis, spillover can control the reaction rate and selectivity of a wide range of reactions, e.g. hydrogenation...

FTIR spectra of CO adsorbed at 100 K reveal remarkable acid strength of the free NH groups of tridentate ammonium ions in NH4FER zeolite.

Adsorption of NO on an over-exchanged Cu-ZSM-5 sample (SiO2/Al2O3 = 23) leads to formation of the well-known nitrosyl species of Cu2+ and Cu+. In addition, two kinds of Cu3+–NO mononitrosyls are detected by IR bands at 1960 and 1947 cm−1. The Cu3+ species are produced as a result of disproportionation of Cu2+ ions from associated sites: Cu2+–O–Cu2+...

The acidity of the bridging OH/OD groups in high-silica H-ZSM-5 and D-ZSM-5 zeolites was measured using CD3CN as an IR probe molecule. We found that the OH groups are more acidic because the ν(CN) modes of the CD3CN···HO complexes formed with bridging hydroxyls appeared at frequencies higher by 1.1-1.2 cm− 1 than the respective modes of CD3CN···DO...

The coordination chemistry of Cu+ cations in zeolites raises a big interest. About 20 years ago, it was established that Cu+ sites in Cu-ZSM-5 can form mono- di- and tricarbonyls, as well as mono- and di-nitrosyls. Later, it was reported that mixed complexes of the Cu+(H2O)xCO, Cu+(NH3)xCO and Cu+(CH3COCH3)CO types can also exist. A possible reason...

Low temperature adsorption of N2 on an H-D-FER sample was studied by FTIR spectroscopy in order to obtain information on the acidity of the hydroxyl groups. Three kinds of zeolite bridging hydroxyls were clearly distinguished, at 3609, 3596 and 3555 cm−1 (3614, 3600 and 3556 cm−1 at 100 K). The respective OD groups were observed at 2662, 2653 and 2...

The OH/OD isotopic shift factors (i = νOH/νOD) of isolated silanols on SiO2 and [Si]BEA are between 1.3563 and 1.3568, values lower than the theoretical shift of 1.3744. However, i of the harmonic OH modes almost coincides with the theoretical value which indicates that the experimental deviations in this case are mainly due to anharmonicity. The a...

Low-temperature adsorption of CH4 and 15N2 on H–ZSM-5, SiO2, and HY is comparatively studied. Partly and fully deuteroxylated samples were also investigated. It was established that methane forms H bonds simultaneously with oxygen and hydrogen from the Si–OH groups, which reflects in enhanced shift of the OH modes to lower frequency as compared to...

In this review, some of the advantages of the use of isotopically labelled molecules for FTIR characterisation of porous materials are summarised. The most important sites determining the properties of these materials are the hydroxyl groups and the incorporated cations. D H exchange is very useful for characterisation of hydroxyls and here some ne...

In this paper we report on low-temperature CO isotopic scrambling ((12)C(16)O + (13)C(18)O →(12)C(18)O + (13)C(16)O). The reaction proceeds on a commercial silver-exchanged zeolite even at about 100 K and requires an optimal reduction degree of the catalysts.

A Au/SiO2 sample was prepared by ammonia-assisted grafting using HAuCl4 as a gold precursor. Gold on the sample evacuated at 673 K is essentially in metallic form: adsorption of CO at 100 K results in formation of Au0-CO species (IR band at 2122 cm-1 shifting to 2103 cm-1 at high coverage). Coadsorption of CO and O2 even at ambient temperature lead...

Adsorption of benzene on H-ZSM-5 leads, at low coverages, mainly to formation of 1:1 adducts with the zeolite bridging hydroxyl groups (3612 cm−1). As a result, the O–H modes are shifted to two discrete bands with maxima at ca. 3350 and 3250 cm−1. The appearance of two bands is attributed to Fermi resonance of the shifted O–H stretchings with the 3...

A Cu-[Al]MCM-41 sample was studied by FTIR spectroscopy of probe molecules (CO and NO) and methanol conversion test reaction. It was found that the parent MCM-41 material was characterized by the practical absence of strongly acidic hydroxyl groups (typical of zeolites), but possessed some hydroxyls with a moderate acidity. Introduction of copper t...

Adsorption of CO (13CO) and 15N2 at 100 K on H-ZSM-5 samples has been followed by FTIR spectroscopy. For better interpretation of the spectra, experiments with H-D-ZSM-5 and D-ZSM-5 were also carried out. It was established that zeolite bridging hydroxyls displaying at 100 K a band at 3616 cm−1 formed 1:1 adducts with CO, and as a result, the band...

Two infrared OH stretching bands are produced after interaction of CO with H-ZSM-5 bridging hydroxyls. These two bands are not due to heterogeneity of the acid sites but arise from Fermi-resonance.

Accessible iron in oxidized Fe–ZSM-5 and Fe–BEA zeolites is mainly in the form of Fe3+ species that do not interact with CO. Some exchanged cations are stabilized as Fe2+ species and form carbonyls detected in the 2205–2190 cm−1 region. Evacuation of the samples at 673 K provokes autoreduction of part of Fe3+ and creation of more Fe2+ sites detecta...

The heterogeneous catalytic oxidation of methanol on Au/Fe2O3 catalysts in the temperature range 50–300°C has been studied. A high activity of the catalyst with respect to the complete oxidation of methanol to carbon dioxide has been observed. The degree of conversion is higher than 50% at 100°C and almost 100% at 150°C. KeywordsAu/Fe2O3 catalysts...

This paper describes the application of TiO2 nano-particles (anatase form) for the solid-phase extraction of iron from coastal seawater samples. We investigated the adsorption processes by infra-red spectroscopy. We compared in batch and on-(mini)column extraction approaches (0.1 and 0.05 g TiO2 per sample, respectively), combined to external calib...

The state of the accessible cobalt ions in a Co–FER sample was studied by CO and NO as IR probe molecules. Both Co2+ and Co3+ sites were detected. With NO the Co3+ ions formed mononitrosyl species (band at 1934cm−1) and with CO, monocarbonyls (band at 2204cm−1). A part of the Co3+–CO species was converted into dicarbonyls (band at 2194.5cm−1) at lo...

The surface properties of calcium phosphate precursor (CP) and crystalline calcium hydroxyapatite (HA) prepared biomimetically have been studied by IR spectroscopy of adsorbed CO. Both samples are characterized by the absence of Brønsted acidity. Low-temperature CO adsorption on CP evacuated at 523 K leads to formation of only one family of Ca2+–CO...

Calcination and evacuation of a Au/FeOx/CeO2 catalyst at 573K leads to reduction of the deposited gold to metal. This metal state is stable under oxygen and only at 573K some metal atoms are oxidized to Auδ+ sites (Au+ cations situated on metal gold particles). However, even at room temperature, gold is readily oxidized in a CO+O2 mixture producing...

Two Au/TiO2 samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673K. TEM revealed gold particles of 3.2 and 3.9nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR...

Two platinum-containing zeolites were studied by XPS, TPR and IR spectroscopy of adsorbed CO. The sample Pt–NaY was prepared by ion-exchange and the Pt/NaY sample, by impregnation. XP spectra indicate a higher heterogeneity of platinum on the impregnated sample and enrichment of its outer surface in platinum, compared to the exchanged sample. Acces...

Different carbonyls are formed after CO adsorption at ambient temperature on a Pt-Na-mordenite (Pt-Na-MOR) sample. Pt(3+)(CO)(2) dicarbonyls (nu(s) at 2205 cm(-1) and nu(as) at 2167 cm(-1)) are decomposed without formation of monocarbonyls. The respective mixed-ligand species, Pt(3+)((12)CO)((13)CO), formed after (12)CO-(13)CO coadsorption, display...

In this work, we report on the formation of several non-classical carbonyls of platinum produced after CO adsorption on Pt/H–ZSM-5. These are: (i) two kinds of Pt3+(CO)2 dicarbonyls (νs at 2212cm−1 and νas at 2176cm−1; νs at 2196cm−1 and νas at ca. 2155cm−1), which are decomposed without producing monocarbonyls; (ii) Pt2+–CO (2150cm−1), which form...

This work reports the formation of some new non-classical carbonyls of platinum formed after CO adsorption on Pt–H–ZSM-5. These are: (i) Pt3+(CO)2 dicarbonyls (2211 and 2175 cm−1), which are decomposed without producing monocarbonyls; (ii) Pt2+(CO)2 (2165 and 2150 cm−1), decomposed via Pt2+–CO species (2113 cm−1); and (iii) Pt+(CO)3 complexes (2162...

Adsorption of CO on Ni/TiO2 sample results in the formation of Ni2+CO and Ti4+CO species which are observed in the 2210–2185 cm−1 region and are characterized by similar stabilities. This makes unambiguous assignment of the carbonyl bands problematic. An analogous situation is found with Ni/ZrO2 sample (carbonyls in the 2190–2155 cm−1 region), wher...

The aim of this work is to investigate the species produced after CO adsorption on cationic platinum sites in Pt­H­ZSM­5. To prevent formation of anionic carbonyls, we gently decomposed the platinum amine complexes on the parent sample by calcination in oxidizing atmosphere. We expected that the big cationic radius of Pt 2+ and/or Pt + cations woul...

The influence of mechanochemical treatment on the solid state reactions between alpha-Fe2O3 and V2O5 was studied. A stoichiometric mixture of both oxides was subjected to an intense milling in air for different time intervals. The mechanochemical treatment for 15 h leads to a drastic decrease of the temperature of synthesis of FeVO4 (from 1073 K to...

Low temperature CO adsorption on a Pd-H-ZSM-5 sample results in formation of Pd3+(CO)2 species (2221 and 2200 cm−1). The dicarbonyl structure has been proven by coadsorption of 12CO and 13CO: mixed Pd3+(13CO)(12CO) species absorb at 2210 and 2158 cm−1. Evacuation of the sample at increasing temperatures leads to reduction of palladium and formation...

The influence of mechanochemical treatment on the solid state reactions between α-Fe 2 O 3 and V 2 O 5 was studied. A stoichiometric mixture of both oxides was subjected to an intense milling in air for different time intervals. The mechanochemical treatment for 15 h leads to a drastic decrease of the temperature of synthesis of FeVO 4 (from 1073 K...

Adsorption of CO on Pd/WO3-ZrO2 sample at 85 K results in formation of carbonyls typical of the support as well as of Pd3+-CO species (2212 cm−1). The latter are masked by the strong Zr4+-CO band. Even at 90 K the Pd3+ sites are reduced to Pd2+ (the respective carbonyls absorbing at 2184 cm−1) and, at higher temperatures, to Pd+ (carbonyl bands at...

Some drawbacks of the phosphate modifier such as reagent blank contribution and background absorption in electrothermal atomic absorption spectrometric determination of cadmium and lead are substantially alleviated by application of small amounts of phosphate, approximately 0.2 mumol (25 mug of NH(4)H(2)PO(4) or (NH(4))(2)HPO(4)), on the integrated...