Athanasios K. Ladavos

Preparation of hydroxyapatite via microemulsion route.

Hydroxyapatite (HAp) was prepared using a microemulsion route in combination with the pH-shock wave method. The samples as received consisted of amorphous aggregated particles, which had remarkable mesoporosity with a narrow pore size distribution. After being heated at 650 degrees C, the A-type carbonate hydroxyapatite was crystallized at 635 degrees C in particles of similar size (40--120 nm) with no internal porosity. At a higher temperature (900 degrees C) a sintering process took place, resulting in network of a larger particles, consisting of HAp and beta-tricalcium phosphate (beta-TCP). The crystallization of HAp occurs at 635 degrees C with an activation energy of 62.7--72.2 kcalmol(-1).

Preparation, characterization, mechanical and barrier properties investigation of chitosan-clay nanocomposites.

In the current study the effect of dilution of chitosan acetate solution and of the use of a reflux-solution method on the morphology, the mechanical and water barrier properties of chitosan based nanocomposites is being investigated. Two series of nanocomposite films from two chitosan acetate solutions with 2 w/v% and 1 w/v% in chitosan were prepared, with 3, 5 and 10 wt% Na-montmorillonite (NaMMT) and/or 30 wt% glycerol. Intercalation of NaMMT was more effective in films based on 2 w/v% solutions which presented decreased hydrated crystallinity. Upon NaMMT addition an enhancement was found in stiffness and strength (up to 100%) and a remarkable decrease in the elongation at break (up to 75%) and water vapor permeability (WVP) (up to 65%). This enhancement was less pronounced in 1 w/v% systems. Addition of glycerol had a negative effect on the stiffness, strength and WVP, and a positive effect on the elongation at break and the absorbed water. Compared with the conventional solution cast method, the reflux treatment led to a significant improvement of the tested properties of nanocomposite films.

Variation of surface properties and textural features of spinel ZnAl2O4 and perovskite LaMnO3 nanoparticles prepared via CTAB-butanol-octane-nitrate salt microemulsions in the reverse and bicontinuous states.

Two binary oxides, a spinel, ZnAl2O4, and a typical perovskite, LaMnO3, have been prepared via CTAB-1-butanol-n-octane-nitrate salt microemulsion in the reverse and bicontinuous states. The exact point of the reverse and bicontinuous states of the microemulsion used in the synthesis was determined by conductivity experiments. The materials obtained after heating at 800 degrees C were characterized by XRD analysis for their crystal structure, N2 porosimetry for their surface area and porosity, and SEM and TEM photography for their texture. The ZnAl2O4 spinel obtained via the reverse microemulsion appears in SEM in a more fragmented form and with a higher specific surface area (143.7 m(2)g(-1)), compared to the corresponding solid prepared via the bicontinuous microemulsion, which appears more robust with lower surface area (126.7 m(2)g(-1)). Nevertheless both materials reveal in TEM a sponge-like structure. The perovskite materials LaMnO3 prepared via the reverse microemulsion showed in SEM a peculiar doughnut-like texture, each doughnut-like secondary particle having a diameter of 2 microm. The corresponding sample developed via the bicontinuous microemulsion showed in SEM uniform secondary particles of size approximately 0.2 microm. Both perovskite samples LaMnO3 appear well crystallized with relative low surface areas, 23.7 m(2)g(-1) for the reverse sample and 10.9 m(2)g(-1) for the bicontinuous one. The TEM photographs reveal that both of them, of reversed and bicontinuous origin, are made up of primary nanoparticles in the size range 40-100 nm. In SEM those materials showed a different secondary structure.

Methane combustion on LaSrCeFeO mixed oxides: bifunctional synergistic action of SrFeO3−x and CeOx phases

The catalytic deep oxidation of methane over perovskite-type mixed oxide catalysts is examined. Three groups of mixed oxides (i) La1−yCeyFeO3 (0.0<y≤0.5), (ii) La1−xSrxFeO3 (0.0<x≤0.5) and (iii) La1−x−ySrxCeyFeO3 (0.0<x,y≤0.3), containing La, Sr, Ce, Fe cations in various ratios were prepared and characterized by XRD, Mossbauer spectroscopy and O2-TPD measurements. The catalysts are active in the temperature range of 440–740°C. A maximum of activity appears for y=0.3, in the series La1−yCeyFeO3 in a wide temperature range. The increase of Sr content in the series La1−xSrxFeO3 results in the enhancement of the reaction rates which is related to the presence of SrFeO3−x crystal phase that favors the surface reaction. The double substituted solids La1−x−ySrxCeyFeO3 show higher catalytic activity as compared to the activity of the two other examined perovskitic series which is related to the bifunctional synergistic action of CeO2 and SrFeO3−x crystal phases in the solids.

Mechanistic aspects of NO+CO reaction on La2−xSrxNiO4−δ (x=0.00–1.50) perovskite-type oxides

Abstract The mechanistic aspects of NO+CO reaction on perovskite-type series La 2− x Sr x NiO 4−δ x=0.00, 0.25, 0.50, 0.75, 1.00, 1.25and1.50 ) prepared by the nitrate route have been studied. The catalysts were active between 380 and 580°C. The kinetics of NO+CO reaction exhibits a rather complex behaviour which is differentiated according to low and high reaction temperatures as well as to small and large degree of substitution x of La by Sr. The conversion of CO runs parallel to NO conversion with a hysteresis which leads to N 2 O production. The reactants NO and CO react in a 2 : 1 ratio at low reaction temperatures ( T≲480°C , 2NO+CO→products ) but they convert in a 1 : 1 ratio at high temperatures ( T≳480°C , NO+CO→products ). Plots of the ratio of the degrees of conversion X of NO and CO versus temperature, X NO /X CO =f(T) , show that the transition from one mechanism to the other is perfectly linear. An inverse relation is observed between the quantities of δ and Δ(X NO /X CO )/ΔT . From the kinetic analysis it is estimated that the difference of the apparent activation energy for samples with small x and large x , at low reaction temperatures corresponds to the enthalpy of adsorption of NO while at high reaction temperatures it corresponds to the enthalpy of adsorption of oxygen. The difference between the apparent activation energies corresponding to high and low temperature, for both reactants, passes from negative to positive values at the same point where the values of δ pass from negative to positive ones or from samples rich in oxygen to samples poor in oxygen.

Effects of substitution in perovskites La2−xSrxNiO4−λ on their catalytic action for the nitric oxide+carbon monoxide reaction

Abstract The catalytic activity of the perovskite series La 2− x Sr x NiO 4−γ ( x = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) has been studied for the reduction of nitric oxide by carbon monoxide in equimolecular mixture. The conversion ( X %) of reactants approaches the value X NO / X CO = 2 at low temperatures and low degree of substitution of lanthanum by strontium but tends to unity at high temperatures and high degree of substitution. This behaviour is due to the change of the rate determining step of the reaction which at low temperature is the oxygen formation through decomposition of nitric oxide while at high temperatures it is the reaction between the adsorbed oxygen species and carbon monoxide. The apparent activation energies for the nitric oxide elimination drop with the process of substitution from x = 0.00 to x = 1.00 and increase thereafter. The controlling factor of the catalytic activity of the series seems to be the strength of adsorption of oxygen on the perovskite surface. Thus it is shown that the sum of the enthalpies of the metal-oxygen bonds in the material is closely related to the temperature where reaction switches mechanism. Besides, the differential mean electronegativity of the solids is related both to the reaction rate as well as to the apparent activation energy for nitric oxide reduction.

Structure and catalytic activity of perovskites La-Ni-O supported on alumina and zirconia

Lanthanum nickelate binary oxidic species supported on alumina and zirconia were tested for their catalytic activity for the NO + CO reaction. The support took place by impregnation of Al2O3 or ZrO2 without any pretreatment or after addition of lanthana on the support surface. The zirconia based solids show a surface area (BET) of 5–7 m2 g−1, one order of magnitude smaller than those based on alumina (40–60 m2 g−1). The crystal phase developed on ZrO2 was only La2NiO4 while on Al2O3 the phases found by X-rays were LaNiO3 and LaAlO3. The solids La2NiO4/ZrO2 containing nickel in 2 + state, were more active than LaNiO3/Al2O3 containing Ni3+. The smaller activity of the La-Ni-O solids supported on Al2O3 is attributed to the stronger adsorption of oxygen, or oxygenated species, on Ni3+ as compared to the weaker adsorption on Ni2+ existed in the La-Ni-O solids supported on ZrO2. Especially the stronger adsorption of nitric oxide in the first case results in a reaction route where an excess of nitric oxide as large as 25–30% is eliminated as compared to the reacting carbon monoxide. The excess of eliminated nitric oxide undergoes disproportionation towards nitrous oxide and nitrogen, the former undergoing further decomposition towards nitrogen and oxygen. In the second case of ZrO2/La-Ni-O the weaker adsorption of nitric oxide results in a nearly equimolecular elimination of the two reactants and the excess of nitric oxide reacting does not exceed a 12–14% of carbon monoxide. The Arrhenius plots for the nitric oxide reduction on the alumina-based solids show a unique behaviour for the whole temperature range examined while similar plots for the zirconia-based solids show a two stage behaviour corresponding to different surface mechanism. The activation energies of nitric oxide elimination with carbon monoxide on the above solids are appreciably smaller than activation energies of nitrous oxide decomposition on the same materials, in accordance with previous observations. Finally the activity of La2NiO4/ZrO2 solids calculated per mass of perovskite is nearly five times that of La2NiO4 pure perovskites prepared through the nitrate or citrate routes although their surface areas are equal for the citrate and only double for the nitrate solids.

Surface characteristics and catalytic activity of Al-Pillared (AZA) and FeAl-pillared (FAZA) clays for isopropanol decomposition

Abstract Fractionated Al-pillared (code name AZA) and FeAl-pillared (code name FAZA) clays were examined for the dependence of their specific surface area on the particle size. The results indicate a very weak dependence and a fractal dimension of D ≈ 3 for both materials, characteristic of highly porous solids. The a s plots of the fractionated particles show that the part of the surface area due to the mesoporosity plus the external surface, decreases as the particle size increases and possess a fractal dimension equal almost to 2.8. For both kinds of solids, AZA and FAZA as well as zeolite-Y, the acidity estimated by NH 3 adsorption was found equal to 1.4 × 10 18 , 2.2 × 10 18 and 2.1 × 10 18 sites per m 2 respectively. The experimental results of catalytic activity of AZA, FAZA and zeolite-Y, using the decomposition of isopropanol as a probe reaction in the range of 60 to 130°C, suggest that AZA is catalytically more active compared to FAZA for the total conversion of isopropanol. The reaction rate calculated per g is one order of magnitude higher on the zeolite-Y, as compared to AZA and/or FAZA, due to its higher surface area but it is almost equal if the comparison is made per unit surface area. The products obtained were mainly propene and diisopropyl ether with a selectivity for propene laying between 0.8–0.9 irrespective of particle size of the solids or the temperature. On the contrary, zeolite-Y shows a high selectivity for propene (≈ 0.9) only at low conversions while at higher conversion selectivity tends to 0.5. The kinetic investigation of the reaction showed single Arrhenius plots for AZA and zeolite-Y with apparent activation energies ( E app ) around 80–90 kJ/mol for AZA and 70 kJ/mol for zeolite. On the contrary, FAZA showed two distinct regions in the Arrhenius plots. One at low temperatures ( T C ) with E app 25–35 kJ/mol and another at high temperatures ( T > 100° C ) with E app ≈ 70–80 kJ/mol, depending on the fraction of the solid. This alternation of catalytic behaviour in FAZA was attributed to the extensive adsorption of water at higher conversions that leads to kinetic expression inhibited by the adsorbed water, while at lower conversions first order kinetic is proposed. AZA on the other hand as well as zeolite-Y shows kinetic behaviour inhibited by water adsorption even at very low conversions.

Preparation, characterization, mechanical, barrier and antimicrobial properties of chitosan/PVOH/clay nanocomposites

In the current study low molecular weight poly(vinylalcohol) (PVOH) was used to prepare chitosan/PVOH blends and chitosan/PVOH/montmorillonite nanocomposites via a reflux - solution - heat pressing method. The effect of PVOH content and montmorillonite type (hydrophylic vs. organically modified) on the morphology, mechanical, thermomechanical, barrier and antimicrobial properties of the obtained polymer blends and nanocomposite films was studied. Higher amounts of PVOH (20 and 30%) resulted in plasticization of the films, with an increase in the elongation at break and decrease of the stiffness and the strength while effective blending between chitosan and PVOH chains was observed based on the XRD and DMA findings. Addition of PVOH was beneficial for water and oxygen barrier properties of the obtained films while it did not influence the antimicrobial activity of films against the growth of Escherichia coli. Intercalated structures were obtained after the addition of hydrophilic and organo-modified clays leading into stiffening of the nano-modified films and enhancement of their barrier and antimicrobial properties.

Preparation and characterization of acetylated corn starch-(PVOH)/clay nanocomposite films

Acetylated corn starch (ACS)-based clay (NaMMT) nanocomposite films, with or without addition of polyvinyl alcohol (PVOH), were prepared by casting with glycerol as a plasticizer. The obtained nanocomposite structure was ascertained by XRD study for all polymer-clay nanocomposites. XRD patterns are indicative of an intercalated nanocomposite structure. Mechanical and thermomechanical properties of polymer nanocomposites were studied. The addition of clay induces significant reinforcing effects in the thermoplastic ACS systems. Replacement of glycerol with PVOH in the ACS-NaMMT system results in superior mechanical strength, due to the creation of hydrogen bonds between the ACS and the PVOH chains. Enhancement in water barrier properties was observed for all nanocomposite films, which reaches up to 67% for acetylated starch-PVOH-clay nanocomposites in comparison to acetylated thermoplastic starch, as indicated by water vapor transmission measurements.