Olga Šolcová

Notes on the photo-induced characteristics of transition metal-doped and undoped titanium dioxide thin films.

This study reports the preparation of thin nanoparticulate films of titanium dioxide and its modified version doped with a transition metal. The behavior of prepared films was described by means of their photocatalytic and photo-induced electrochemical properties. The TiO(2) and M/TiO(2) (M=Ag, Zr, Fe) thin films were produced via a standard sol-gel method using titanium n-butoxide, acetylacetone, and transition metal acetylacetonates as precursors. Prepared films were analyzed by a series of techniques involving XRD, Raman spectroscopy, SEM, AFM, and XPS. Their photocatalytic activity was monitored with the aid of decomposition of the model compound Rhodamine B in water. All films were then tested for their photo-induced electrochemical properties based on evaluation of polarization curves (photocurrents). The highest reaction rate constant (0.0101min(-1)), which was even higher than that for pure TiO(2), was obtained for the Ag/TiO(2) sample. The highest quantum yield of the charge collection was determined for the undoped TiO(2) film.

In-field experimental verification of cultivation of microalgae Chlorella sp. using the flue gas from a cogeneration unit as a source of carbon dioxide

A complex treatment of agricultural waste including the following major steps: anaerobic fermentation of suitable waste, cogeneration of the obtained biogas and growth of microalgae consuming the CO2 from biogas and flue gas was verified under field conditions in a pilot-scale photobioreactor. The growth kinetics of microalgae Chlorella sp. consuming mixture of air and carbon dioxide (2% (v/v) of CO2), or flue gas (8—10% (v/v) of CO2) was investigated. The results obtained in the pilot photobioreactor were compared with results previously measured in laboratory photobioreactors. The field tests were performed in a pilot-scale outdoor solar-bubbled photobioreactor located at a biogas station. The pilot-scale photobioreactor was in the shape of a flat and narrow vertical prism with a volume of 300 L. The microalgae growth rates were correlated with empirical formulas. Laboratory analyses of the produced microalgae confirmed that it meets the strict EU criteria for relevant contaminants level in foodstuffs. Utilization of flue gases from cogeneration therefore was not found to be detrimental to the quality of microalgal biomass, and may be used in these types of bioreactors.

Wavelength Effect on Photocatalytic Reduction of CO2 by Ag/TiO2 Catalyst

Photocatalytic reduction of CO2 by water was performed in the presence of a Ag/TiO2 catalyst under illumination by lamps with different wavelengths (254, 365, and 400 nm). The yields of the main products (methane and methanol) were higher with the 254 nm lamp than with the 365 lamp while no products were observed with the 400 nm lamp. This was because the electron-hole generation rate increased with increasing energy of irradiation (decreasing wavelength) and there were higher densities of electron states at higher energies in TiO2. The increased efficiency of electron-hole generation with a shorter wavelength irradiation increased the efficiency of the catalyst. The energy of the electrons excited by visible light (400 nm) was too low for CO2 photocatalytic reduction.

Multicomponent counter-current gas diffusion in porous solids: the Graham's-law diffusion cell

Abstract A version of the Wicke–Kallenbach diffusion cell that utilizes the validity of Grahams law was used for counter-current diffusion measurements. Diffusion in binary and ternary inert gas systems (consisting of hydrogen, helium, nitrogen, argon) was followed via determination of net volumetric diffusion fluxes. In agreement with Grahams law these fluxes are significantly influenced by the composition of gases in diffusion cell compartments. Fitting of experimental data to the Mean Transport-Pore Model provides model parameters (transport parameters) that are independent of the gas kind and composition. Ternary diffusion extends the volume of experimental information and the confidence region of transport parameters shrinks significantly. The internal consistency of diffusion data verifies the validity of the multicomponent version of Grahams law.

Gas transport in porous media under dynamic conditions

Abstract The dynamic version of the Wicke-Kallenbach diffusion cell with one compartment closed and equipped with a sensitive pressure gauge was used for determination of sets of Mean Transport Pore Model and Dusty Gas Model parameters for an industrial catalyst (ICI 52-1 in reduced form). The dynamic pressure responses due to gas composition step changes of inert gases (H2, He, N2, Ar) were measured and fitted to the system of partial differential equations which describe the transport (mass balances with Maxwell-Stefan constitutive equations). The optimum textural parameters were obtained by simultaneous matching of all experiments. The model parameters are material constants of the porous solid and, thus, do not depend on temperature, pressure and kind of the transported gases. Both diffusion models gave a good agreement between experiments and calculations. The parameter reliability is discussed.

Strategy for predicting effective transport properties of complex porous structures

Abstract Measurements of effective transport properties of porous media, such as effective diffusivity and permeability, are well established by several experimental techniques. Effective transport properties can be also calculated from the spatially 3D reconstructed porous media, where the morphology characteristics required for the reconstruction are obtained from electron microscopy images. Here we demonstrate the reconstruction of porous alumina catalyst carrier with bimodal pore size distribution. Multi-scale concept is employed for the computation of effective diffusivity and permeability of reconstructed porous media and calculated effective transport properties are compared with transport parameters experimentally determined in Graham diffusion and simple permeation cell. The limitations of current state-of-the-art reconstruction techniques for porous media with broad pore size distribution are discussed. We show that the contribution of nano-pores towards the total diffusion flux is significant and cannot be neglected, but it is reasonable to neglect the contribution of nano-pores towards the sample permeability.

The influence of various deposition techniques on the photoelectrochemical properties of the titanium dioxide thin film

Thin sol–gel TiO2 layers deposited on the conductive ITO glass by means of three various deposition techniques (dip-coating, inkjet printing and spray-coating) were used as photoanode in the three-compartment electrochemical cell. The thin TiO2 films were treated at 450 °C and after calcination all samples possessed the crystallographic form of anatase. The relationship between surface structure and photo-induced conductivity of the nanostructured layers was investigated. It was found that the used deposition method significantly influenced the structural properties of prepared layers; mainly, the formation of defects and their quantity in the prepared films. The surface properties of the calcined layers were determined by XRD, Raman spectroscopy, SEM, AFM, UV–Vis analyses and by the optical microscopy. The photo-induced properties of nanoparticulate TiO2/ITO photoanode were studied by electrochemical measurements combined with UV irradiation.

Prediction and Evaluation of Time-Dependent Effective Self-diffusivity of Water and Other Effective Transport Properties Associated with Reconstructed Porous Solids

We reconstructed pore structures of three porous solids that differ from each other in morphology and topology of pore space. To achieve this, we used a stochastic method based on simulated annealing and X-ray computed microtomography. Simulated annealing was constrained by the following microstructural descriptors sampled along the principal and diagonal directions: the two-point probability function for the void phase and the lineal-path functions for both void and solid phases. The stochastic method also assumed the isotropic pore structures in accordance with a recent paper (Čapek et al. in Transp Porous Media 88(1): 87–106 (2011)). With the exception of the solid with the widest pores, we made tomographic volume images in high and low resolution, which enabled us to study the effect of resolution on microstructural descriptors and effective transport properties. A comparison of the two-point probability function and the lineal-path function sampled in the principal directions revealed that the pore structures derived from the tomographic volume images were slightly anisotropic, in opposition to the assumption of the stochastic method. Besides the anisotropy, other microstructural descriptors including the pore-size function and the total fraction of percolating cells indicated that the morphological and topological characteristics of the pore structures depended on the reconstruction method and its parameters. Particularly, the pore structures reproduced using the stochastic method contained wider pores than those obtained using X-ray tomography. Deviations between the pore structures derived from low- and high-resolution tomographic volume images were also observed and imputed to partial volume artefacts. Then, viscous flow of incompressible liquid, ordinary diffusion, Knudsen flow and self-diffusion of water in the reconstructed pore spaces were simulated. As counterparts, experimental data were measured by means of permeation and Wicke–Kallenbach cells and pulsed field gradient NMR. Deviations between the simulated quantities on the one hand and experimental data on the other hand were generally acceptable, which corroborated the pore-space models. As expected, the predictions based on the tomographic models of pore space were more successful than those derived from the stochastic models. The stationary effective transport properties, i.e. the effective permeability, the effective pore size and the geometric factor, were sensitive to a bias in long-range pore connectivity. Furthermore, the time-dependent effective diffusivity was found to be especially sensitive to relatively small morphological deviations between the real and reconstructed pore structures. It is concluded that the combined predictions of the effective permeability, the effective pore size, the geometric factor and time-dependent effective self-diffusivity of water are needed for the reliable evaluation of pore-space reconstruction.

Gas permeation in porous solids: Two measurement modes

Two types of permeation cells and measurement methods for determination of transport characteristics of porous solids were compared. The dynamic method is based on unsteady permeation of single gases in a porous medium. The pseudostationary method uses equilibration of a small gas pressure difference across the porous medium. For evaluation of data from the dynamic cell full and simplified solutions are formulated and compared. Transport parameters of the porous solid are obtained by nonlinear matching of data to theory. Both the full and simplified descriptions of the pressure transients give comparable transport characteristics. From the pseudostationary cell data the transport parameters are obtained by simple linear regression, again in good agreement with the dynamic method. In both cases it is possible to determine the significance of Knudsen and viscous gas transport mechanisms. Permeation measurements were performed with hydrogen, helium, nitrogen, and argon on one monodispersed and one bidispersed type of porous pellet.

Dynamics of pressure build-up accompanying multicomponent gas transport in porous solids: adsorbable gases

The dynamics of countercurrent transport of binary and ternary gas mixtures through a porous medium accompanied by a spontaneous temporary build-up of pressure inside the porous medium was experimentally studied. From the measurements, it follows that if a lighter gas replaces a heavier gas the pressure increases and vice versa. The larger the difference between molecular weights of the transported gases the larger the change of the pressure. The spontaneous pressure build-up can be satisfactorily described by the Mean Transport Pore Model (MTPM) or the Dusty Gas Model (DGM). Both models contain three parameters (transport parameters), which represent material constants of the porous medium, i.e., are independent of the kind of transported gases and conditions under which the transport takes place (temperature, pressure). Transport parameters have to be determined experimentally, e.g., by measurements similar to those performed in this study. With the use of obtained transport parameters it is possible to predict the transport under different conditions.