Christos Kordulis

The Role of the Liquid¿Solid Interface in the Preparation of Supported Catalysts

The contribution of the Interface Science to the preparation of supported catalysts during the last two decades is presented. It is illustrated how the concepts and the methodologies of the Interface Science could be effectively used for an in‐depth understanding of the phenomena involved in the initial preparation step. This, extremely critical step, concerns the deposition of transition metal species containing the active elements, from an impregnation solution, onto the surface of common catalytic supports. Moreover, the aforementioned concepts and methodologies allow the regulation of the mode of interfacial deposition and the local structure of the deposited species and, thus, the surface characteristics and the catalytic behaviour of the resulted catalysts.

Titanium Dioxide (Anatase and Rutile): Surface Chemistry, Liquid–Solid Interface Chemistry, and Scientific Synthesis of Supported Catalysts

Solid Interface Chemistry, and Scientific Synthesis of Supported Catalysts Kyriakos Bourikas,† Christos Kordulis,‡,§ and Alexis Lycourghiotis*,‡ †School of Science and Technology, Hellenic Open University, Tsamadou 13-15, GR-26222 Patras, Greece ‡Department of Chemistry, University of Patras, GR-26500 Patras, Greece Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH/ICE-HT), P.O. Box 1414, GR-26500 Patras, Greece

Nano-tubular cellulose for bioprocess technology development.

Delignified cellulosic material has shown a significant promotional effect on the alcoholic fermentation as yeast immobilization support. However, its potential for further biotechnological development is unexploited. This study reports the characterization of this tubular/porous cellulosic material, which was done by SEM, porosimetry and X-ray powder diffractometry. The results showed that the structure of nano-tubular cellulose (NC) justifies its suitability for use in “cold pasteurization” processes and its promoting activity in bioprocessing (fermentation). The last was explained by a glucose pump theory. Also, it was demonstrated that crystallization of viscous invert sugar solutions during freeze drying could not be otherwise achieved unless NC was present. This effect as well as the feasibility of extremely low temperature fermentation are due to reduction of the activation energy, and have facilitated the development of technologies such as wine fermentations at home scale (in a domestic refrigerator). Moreover, NC may lead to new perspectives in research such as the development of new composites, templates for cylindrical nano-particles, etc.

Effect of pressure and temperature on alcoholic fermentation by Saccharomyces cerevisiae immobilized on γ-alumina pellets

Saccharomyces cerevisiae was immobilized on γ-alumina pellets and used for repeated batch fermentations in glucose medium (16.5 g/100 mL) at various temperatures and pressures. An increase in pressure from 3 to 7 atm and a decrease in temperature from 30 to 20 °C reduced the ethanol productivity by about 50% and 70%, respectively. Increasing concentrations of volatile by-products were observed at lower fermentation temperatures, while the pressure influence on the concentrations of these by-products was proved to be more complex. Mathematical expressions were established to allow the calculation of the fermentation rate at various pressures and sugar concentrations when the corresponding rate at atmospheric pressure is known. The study showed that the height of bioreactors has to be limited to 19.5 m due to hydrostatic pressure shock at higher fill levels.

Decomposition of N2O on Fe2O3/Al2O3 catalysts. Relationships between physicochemical and catalytic properties

The influence of the iron(III) content and the calcination temperature on the dispersion of supported Fe3+ species on alumina, on the semiconducting properties and on the catalytic activity of Fe2O3/Al2O3 catalysts has been studied, with the decomposition of N2O into N2 into N2 and O2 being used as a probe reaction. Above 250 °C iron(III) is located on the surface of the carrier as α-Fe2O3 and ‘strongly associated iron(III)’(denoted by Fe3+–S). The ratio of the amounts of these species remains constant from X= 0.172 to X= 0.615 mmol Fe3+ per g of alumina and then changes favouring Fe3+–S. The transformation of α-Fe2O3 into Fe3+–S is not accompanied by any detectable change in the Fe3+ dispersion but causes an increase in the activation energy of conduction. A rise in the calcination temperature and iron(III) content brings about an increase in the size of supported α-Fe2O3 crystals shown by a decrease in the dispersion of Fe+. It has been demonstrated that α-Fe2O3 and Fe3+–S exhibit similar activity. This increases linearly with the dispersion of the Fe3+ ions. Moreover, no relationship was found to exist between the semiconducting properties of the catalysts estimated by the activation energy of conduction and the catalytic activity. X-ray photoelectron spectroscopy was used to estimate the dispersion of the supported iron(III), conductivity experiments were performed to determine the activation energy of conduction and catalytic tests were carried out to determine catalytic activity.

Molybdena catalysts prepared on modified carriers: Regulation of the symmetry and valence of the molybdenum species formed on γ-Al2O3 modified with alkali cations

Abstract Five series of modified carriers were prepared by the dry impregnation of γ-Al 2 O 3 with various amounts of MNO 3 (M + ≡ Li + , Na + , K + , Rb + and Cs + ). Each modified carrier was used as a support in the preparation of three MoO 3 (6%, 12% and 18%) supported catalysts. The combined use of X-ray powder spectroscopy, diffuse reflectance spectroscopy and the classical analysis of NO 3 − and NO 2 − allows the structure of the modified supports to be investigated as well as the effects of the type of modifier and the modifier concentration on the molybdenum species formed on the carrier surface and on their valence and symmetry. It was found that a large quantity of MNO 3 remained on the carrier surface at temperatures below 400 °C. An increase in the calcination temperature from 400 to 500 °C produced a partial dissociation of MNO 3 to MNO 2 . A further increase in the temperature from 500 to 600°C resulted in the formation of Li 2 Al 2 O 4 and/or LiAl 5 O 8 and the complete disappearance of LiNO 3 and LiNO 2 . At this temperature some of the other alkali cations were still present on the γ-Al 2 O 3 surface in the form MNO 3 and MNO 2 . A critical alkali concentration must be present before surface crystallization of MNO 3 and MNO 2 commences. The symmetry and valence of the molybdenum species can be regulated by changing the doping parameters. At 300 °C all the modifiers inhibited the reduction of molybdenum(VI) to molybdenum(V) and this effect increased with the size of the dopant cation and with the dopant concentration. At 300 and 600 °C, all the alkali cations except Li + produced a transition from octahedral to tetrahedral coordination symmetry in the molybdenum(VI) species. The magnitude of this effect increased with increasing alkali content. These effects were independent of the MoO 3 concentration. A mechanism explaining the doping effects is developed.

On the synergy between tungsten and molybdenum in the W-incorporated CoMo/γ-Al2O3 hydrodesulfurization catalysts

The aim of this study is to investigate the influence of incorporation of tungsten on an already prepared CoMo/γ-Al2O3 hydrodesylfurization catalyst (base catalyst). The evaluation of W-incorporated CoMo/γ-Al2O3 is described, in relation to the base catalyst, concerning their activities for hydrodesulfurization (HDS) of thiophene and hydrogenation (HYG) of its unsaturated products towards butane. The main conclusion of this study is that incorporation of a low amount of tungsten onto CoMo/γ-Al2O3 catalyst increases its HDS and HYG activities. A synergy has not been found between Mo- and W-phase. The W-loading for which the maximum activity is observed highly depends on the details of the preparation method used.

Kinetics of N2O decomposition on Fe3+ supported on pure and Li-modified Al2O3

The kinetics of N2O decomposition on the surface of Fe3+ species supported on pure and Li+-doped Al2O3 have been studied. The supported FeIII species cause a transformation of the rate law from zero order, followed on pure support, to first order. Moreover, it has been concluded that the relatively high activity exhibited by the supported Fe2O3/Al2O3 catalysts compared with the activity of the Fe2O3 and Al2O3 should be attributed to the increase of the specific rate constant caused by the supported FeIII active phase. Finally, it was found that the deactivation observed after Li+ doping should be attributed to both the increase in the oxygen adsorptivity and the decrease in the dispersion of the supported FeIII species caused by the Li+ cations.

Towards the local structure of the Co(II), Ni(II), Cr(VI) and W(VI) ionic species formed upon impregnation on titania

The mode of interfacial deposition and the local structure of the Co(II), Ni(II), Cr(VI) and W(VI) ionic species formed upon impregnation on titania surface has been studied using various techniques provided by the interface science.

Fullerene C60 supported on silica and γ-alumina catalyzed photooxidations of alkenes

Deposition of fullerene C60 (2% w/w) on silica and γ-alumina provokes a two orders-of-magnitude increase of its activity for the liquid-phase photooxidation of 2-methyl-2-heptene. Kinetic studies concerning the above photooxidation showed a first-order dependence of the reaction rate on the alkene concentration. The corresponding reaction-rate constant was found to be higher in the case where γ-alumina was used as carrier. The nature of the carrier does not influence the mechanism and the selectivity of the reaction. High dispersion of the supported fullerene is achieved on the surface of the carriers, which increase the fullerene light absorbance especially in the visible range.