ABSTRACT The structure of chitin nanofibrils as a promising filler for bioresorbable suture mater... more ABSTRACT The structure of chitin nanofibrils as a promising filler for bioresorbable suture materials and matrixes for cellular technologies and tissue engineering is investigated via the methods of X-ray diffraction and scanning electron microscopy. It is shown that the powder microparticles obtained via lyophilization of an aqueous dispersion of chitin nanofibrils have a band structure with a cross-sectional size of 30 μm and a thickness of 0.1 μm. The bands consist of nanoparticles 25 nm in thickness and 400–500 nm in length. The chitin nanofibrils are composed of two crystallites with cross-sectional sizes of 11–12 nm and b axes perpendicular to the nanofibril axis. The chitin nanofibrils tend to form planar elements with a layered structure on both the microlevel and the nanolevel. The addition of chitin nanofibrils to a chitosan solution leads to a rise in its viscosity. However, the action of shear stresses leads to a substantial decrease in the chitosan-chitin solution viscosity, a phenomenon that is due to the presence of planar anisodiametric nanoparticles of chitin.
ABSTRACT The crystals of tetralead hexanitrate ethylenediaminetetraacetatoplumbate(II) tetrahydra... more ABSTRACT The crystals of tetralead hexanitrate ethylenediaminetetraacetatoplumbate(II) tetrahydrate Pb4(NO3)6[PbC10H12O8]µ4H2O were synthesized and studied by X-ray diffraction (MoKa radiation, 2388 reflections, RF = 0.0376, RW = 0.0348). The orthorhombic unit cell (space group Fdd2, a = 18.995(2), b = 40.649(4), c = 8.5178(8) Å, Z = 8) contains 16 complex cations [Pb2(NO3)3]2+, 8 complex anions [Pb(Edta)]2–, and 32 crystallization water molecules. The organic and inorganic fragments alternating along the y axis are united into a single frame by the bridging oxygen atoms and by hydrogen bonds. The (Edta)4– ligand, forming one ethylenediamine metallocycle and four glycinate ones, is compressed in the axial direction and stretched in the equatorial direction. The coordination polyhedron of the Pb2+ cation bound with the ligand is a strongly distorted trigonal antiprism. The inorganic fragment is a close-packed double layer of Pb atoms linked with each other by NO3 groups. Thus lead atoms perform a dual function in the structure, forming both complex anions and outer-spheric cations.
ABSTRACT The ratio of growth rates of the K2(S,Cr)O4 crystals in the [100] and [001] directions (... more ABSTRACT The ratio of growth rates of the K2(S,Cr)O4 crystals in the [100] and [001] directions (the anisometry coefficient) is proportional to the initial effective supersaturation of the solutions from which the crystals were grown. The values of the factors of asymmetry and excess of distribution of the crystal anisometry coefficient are affected by the composition of solution. The change in the values of these factors is caused by the change in the value of the initial supersaturation of solution according to the specific features of the solubility diagram.
ABSTRACT The investigation of crystal growth kinetics and phase equilibria in the system K2SO4–K2... more ABSTRACT The investigation of crystal growth kinetics and phase equilibria in the system K2SO4–K2CrO4–H2O was carried out using the method for the determination of the saturation temperature and the growth rate by microscopic observation of the seed crystal behaviour. The kinetics of mixed crystal growth can be characterized by the expression V = 450.24σ1,6,2, where σ is the value of effective supersaturation of the solution calculated by taking into account the specific features of the phase equilibria diagram. Use of this criterion allows the control of the rate of delta crystal growth from solutions of variable composition.[Russian Text Ignored].
ABSTRACT A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1−xAlx(O... more ABSTRACT A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1−xAlx(OH)2(CO3)x/2·m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 ≤ x ≤ 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn:Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O4 phase within the amorphous material is proposed. Samples with Zn:Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide “(CuO)x(ZnAl2O4)y” of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m2g−1. This can be explained by the unfavorable microstructure of the resulting Cu/ZnAl2O4 catalyst: The Cu particles are to a large extent embedded in a compact oxide matrix. The applicability as Cu/ZnO/Al2O3 catalysts and the role of htl precursor phases in course of industrial catalyst preparation are discussed.
ABSTRACT Deactivation behavior is an important topic in catalyst development. In case of methanol... more ABSTRACT Deactivation behavior is an important topic in catalyst development. In case of methanol synthesis the conventional Cu/ZnO/Al2O3 system is commonly known to be prone to sintering, however, information about the structural development during deactivation or the sintering mechanism(s) are scarce. We present a systematic deactivation study on three different Cu/ZnO/Al2O3 catalysts which are aged under constant conditions and periodically analyzed using kinetic measurements and N2O chemisorption. A power law model for the catalyst activity with time on stream is derived. Furthermore it is found, that the presence of water provokes a steep loss in active surface area and specific activity. Also, the TEM particle size distributions generated during the aging treatment are evaluated and discussed.
Angewandte Chemie (International ed. in English), 2014
Different surface sites of solid catalysts are usually quantified by dedicated chemisorption tech... more Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2.
ABSTRACT The stability of the surface termination of intermetallic Pd2Ga nanoparticles and its ef... more ABSTRACT The stability of the surface termination of intermetallic Pd2Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd2Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd2Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd2Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga2O3, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd2Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd2Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd2Ga particles.
ABSTRACT The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/Z... more ABSTRACT The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM2O4 (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM2O4 catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface.
A technique of contact angle measurement was applied to the nano-scale oxide-supported metal part... more A technique of contact angle measurement was applied to the nano-scale oxide-supported metal particles. For Cu supported on ZnO and ZrO2 the angles were found to increase and the work of adhesion to decrease with increasing particle size. Such a trend is interpreted as an effect of negative contact line tension of 2.1 x 10(-9) J m(-1) and 1.0 x 10(-9) J m(-1) in the Cu/ZnO and Cu/ZrO2 system, correspondingly. For the small-sized Cu particles the apparent work of adhesion on ZnO support is higher than that on ZrO2.
A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model ... more A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al(2)O(3) methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10-20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO.
ABSTRACT The structure of chitin nanofibrils as a promising filler for bioresorbable suture mater... more ABSTRACT The structure of chitin nanofibrils as a promising filler for bioresorbable suture materials and matrixes for cellular technologies and tissue engineering is investigated via the methods of X-ray diffraction and scanning electron microscopy. It is shown that the powder microparticles obtained via lyophilization of an aqueous dispersion of chitin nanofibrils have a band structure with a cross-sectional size of 30 μm and a thickness of 0.1 μm. The bands consist of nanoparticles 25 nm in thickness and 400–500 nm in length. The chitin nanofibrils are composed of two crystallites with cross-sectional sizes of 11–12 nm and b axes perpendicular to the nanofibril axis. The chitin nanofibrils tend to form planar elements with a layered structure on both the microlevel and the nanolevel. The addition of chitin nanofibrils to a chitosan solution leads to a rise in its viscosity. However, the action of shear stresses leads to a substantial decrease in the chitosan-chitin solution viscosity, a phenomenon that is due to the presence of planar anisodiametric nanoparticles of chitin.
ABSTRACT The crystals of tetralead hexanitrate ethylenediaminetetraacetatoplumbate(II) tetrahydra... more ABSTRACT The crystals of tetralead hexanitrate ethylenediaminetetraacetatoplumbate(II) tetrahydrate Pb4(NO3)6[PbC10H12O8]µ4H2O were synthesized and studied by X-ray diffraction (MoKa radiation, 2388 reflections, RF = 0.0376, RW = 0.0348). The orthorhombic unit cell (space group Fdd2, a = 18.995(2), b = 40.649(4), c = 8.5178(8) Å, Z = 8) contains 16 complex cations [Pb2(NO3)3]2+, 8 complex anions [Pb(Edta)]2–, and 32 crystallization water molecules. The organic and inorganic fragments alternating along the y axis are united into a single frame by the bridging oxygen atoms and by hydrogen bonds. The (Edta)4– ligand, forming one ethylenediamine metallocycle and four glycinate ones, is compressed in the axial direction and stretched in the equatorial direction. The coordination polyhedron of the Pb2+ cation bound with the ligand is a strongly distorted trigonal antiprism. The inorganic fragment is a close-packed double layer of Pb atoms linked with each other by NO3 groups. Thus lead atoms perform a dual function in the structure, forming both complex anions and outer-spheric cations.
ABSTRACT The ratio of growth rates of the K2(S,Cr)O4 crystals in the [100] and [001] directions (... more ABSTRACT The ratio of growth rates of the K2(S,Cr)O4 crystals in the [100] and [001] directions (the anisometry coefficient) is proportional to the initial effective supersaturation of the solutions from which the crystals were grown. The values of the factors of asymmetry and excess of distribution of the crystal anisometry coefficient are affected by the composition of solution. The change in the values of these factors is caused by the change in the value of the initial supersaturation of solution according to the specific features of the solubility diagram.
ABSTRACT The investigation of crystal growth kinetics and phase equilibria in the system K2SO4–K2... more ABSTRACT The investigation of crystal growth kinetics and phase equilibria in the system K2SO4–K2CrO4–H2O was carried out using the method for the determination of the saturation temperature and the growth rate by microscopic observation of the seed crystal behaviour. The kinetics of mixed crystal growth can be characterized by the expression V = 450.24σ1,6,2, where σ is the value of effective supersaturation of the solution calculated by taking into account the specific features of the phase equilibria diagram. Use of this criterion allows the control of the rate of delta crystal growth from solutions of variable composition.[Russian Text Ignored].
ABSTRACT A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1−xAlx(O... more ABSTRACT A series of hydrotalcite-like (htl) compounds of the general composition (Cu,Zn)1−xAlx(OH)2(CO3)x/2·m H2O was prepared with a fixed Cu:Zn ratio of 70:30. Phase pure samples could be obtained for 0.3 ≤ x ≤ 0.4. The htl precursors thermally decompose in multiple steps. After dehydration and dehydroxylation amorphous materials were obtained at 330 °C. Phase segregation during this mild calcination was only observed for samples with a Zn:Al ratio deviating strongly from 1:2. A mechanism for this low-temperature segregation process basing on the preformation of the ZnAl2O4 phase within the amorphous material is proposed. Samples with Zn:Al ratios near 1:2 form an amorphous carbonate-modified mixed oxide “(CuO)x(ZnAl2O4)y” of homogeneous microstructure. Crystallization occurs upon carbonate decomposition at temperatures higher than 500 °C. Despite the small size of the Cu nanoparticles (around 7 nm) formed upon reduction, the accessible Cu surface area is below 5 m2g−1. This can be explained by the unfavorable microstructure of the resulting Cu/ZnAl2O4 catalyst: The Cu particles are to a large extent embedded in a compact oxide matrix. The applicability as Cu/ZnO/Al2O3 catalysts and the role of htl precursor phases in course of industrial catalyst preparation are discussed.
ABSTRACT Deactivation behavior is an important topic in catalyst development. In case of methanol... more ABSTRACT Deactivation behavior is an important topic in catalyst development. In case of methanol synthesis the conventional Cu/ZnO/Al2O3 system is commonly known to be prone to sintering, however, information about the structural development during deactivation or the sintering mechanism(s) are scarce. We present a systematic deactivation study on three different Cu/ZnO/Al2O3 catalysts which are aged under constant conditions and periodically analyzed using kinetic measurements and N2O chemisorption. A power law model for the catalyst activity with time on stream is derived. Furthermore it is found, that the presence of water provokes a steep loss in active surface area and specific activity. Also, the TEM particle size distributions generated during the aging treatment are evaluated and discussed.
Angewandte Chemie (International ed. in English), 2014
Different surface sites of solid catalysts are usually quantified by dedicated chemisorption tech... more Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2.
ABSTRACT The stability of the surface termination of intermetallic Pd2Ga nanoparticles and its ef... more ABSTRACT The stability of the surface termination of intermetallic Pd2Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd2Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd2Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd2Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga2O3, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd2Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd2Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd2Ga particles.
ABSTRACT The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/Z... more ABSTRACT The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM2O4 (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM2O4 catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface.
A technique of contact angle measurement was applied to the nano-scale oxide-supported metal part... more A technique of contact angle measurement was applied to the nano-scale oxide-supported metal particles. For Cu supported on ZnO and ZrO2 the angles were found to increase and the work of adhesion to decrease with increasing particle size. Such a trend is interpreted as an effect of negative contact line tension of 2.1 x 10(-9) J m(-1) and 1.0 x 10(-9) J m(-1) in the Cu/ZnO and Cu/ZrO2 system, correspondingly. For the small-sized Cu particles the apparent work of adhesion on ZnO support is higher than that on ZrO2.
A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model ... more A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al(2)O(3) methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10-20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO.
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Papers by Igor Kasatkin