Fumihide Shiraishi

Photocatalytic decomposition of acetaldehyde over TiO2/SiO2 catalyst

To establish a promising method for the purification of air containing volatile organic compounds, photocatalytic decompositions of gaseous acetaldehyde over TiO2 deposited on porous silica (TiO2/SiO2 catalyst) and over a platinized TiO2/SiO2 catalyst (Pt-TiO2/SiO2 catalyst) have been investigated including the capture of intermediates on the catalyst surface and regeneration of the deactivated catalyst by heating. Results of kinetic analysis show that these photocatalytic decompositions obey Langmuir-Hinshelwood kinetics. A comparison between the amounts of acetaldehyde decomposed and CO2 produced reveals that about 10 % of acetaldehyde is missing. From the observation of the photocatalyst surface before and after the reaction by FT-IR spectroscopy, we conclude that this is due to the adsorption of intermediates such as formic acid and acetic acid on the porous catalyst as well as deposition of coke-like substances. When the Pt-TiO2/SiO2 catalyst is heated to a temperature above 473 K, these substances can be removed and discharged as CO2. A series of results obtained in the present work suggests that the use of a Pt-TiO2/SiO2 catalyst will enable us to construct a multifunctional reaction process for air purification, in which volatile organic compounds are photocatalytically decomposed. The harmful intermediates formed during the reaction are partly adsorbed on the porous catalyst, remain in the reactor system, and together with deposited coke-like substances are converted into CO2 by heat treatment of the catalyst. The catalyst is thus regenerated.

A rapid treatment of formaldehyde in a highly tight room using a photocatalytic reactor combined with a continuous adsorption and desorption apparatus

Abstract A novel air-purification system, consisting of the photocatalytic reactor with a parallel array of blacklight blue fluorescent lamps and the continuous adsorption and desorption apparatus with a cylindrical ceramic-paper honeycomb rotor retaining activated carbon or zeolite fine particles, was constructed and the performance of this system was investigated for treatment of gaseous HCHO at a concentration level of ppbv ( mg m −3 ) in a 10 m 3 highly tight closed room. With the zeolite rotor, it was very difficult to desorb the adsorbed HCHO by exposing the rotor to heated air and then to photocatalytically decompose the desorbed HCHO. In contrast, the activated carbon rotor provided an excellent performance. With this rotor, the indoor HCHO concentration was reduced to the neighborhood of the WHO guideline (0.1 mg m −3 ) in 10 min and to an almost zero value in 90 min . Needless to say, this surprisingly high performance is owing to the cooperative work by the activated carbon rotor to adsorb the indoor HCHO and the photocatalytic reactor to rapidly decompose the HCHO desorbed by heating the rotor. This system offers several other advantages. The adsorption rotor can be used semi-permanently because it is continuously regenerated. In addition, the HCHO adsorbed on the activated carbon rotor is readily released at a desorption temperature of 120°C. Under such a low temperature condition, little loss in the photocatalytic activity was caused.

Photolytic and photocatalytic treatment of an aqueous solution containing microbial cells and organic compounds in an annular-flow reactor

Abstract A method for simultaneous treatment of an aqueous solution to photolytically sterilize microbial cells and photocatalytically decompose organic compounds was studied using an annular-flow photocatalytic reactor with a 6-W germicidal lamp (wavelength; 254 nm) as a light source. All the experiment were carried out in a batch recirculation mode. When a thin film of titanium oxide was located on the outer surface of a quartz glass tube, which was set to protect the germicidal lamp from direct contact with an aqueous solution flowing through the annulus, i.e. , when the photocatalyst was located at the inner surface of the annulus, the sterilization and decomposition slowed down. This is due to the fact that the degree of transparency of UV lights with wavelengths less 300 nm is significantly decreased by the film of titanium oxide. When the film of titanium oxide was located at the outer surface of the annulus, on the other hand, both the rates of sterilization and decomposition became higher. An experimental result using an aqueous solution containing E. coli and formaldehyde showed that the sterilization of E. coli is dependent on the circulation rate of the solution but the photocatalytic decomposition of formaldehyde is independent of it and can be approximated by a perfectly-mixed reactor model. Consequently, it was found that the annular-flow photocatalytic reactor used in the present work makes it possible to simultaneously carry out the photolytic sterilization of microbial cells and the photocatalytic decomposition of organic compounds.

Photocatalytic decomposition of acetaldehyde in air over titanium dioxide

The photocatalytic decomposition of acetaldehyde in air at initial concentrations ranging from 3 to 200 mg m−3 has been studied in a semitransparent closed box with an inlet volume of 0.056 m3. The photocatalytic reactors consisted of a glass tube, 250 mm long with inside diameters of 28, 35, or 45 mm, whose inner surface was coated with a thin film of titanium dioxide, and a 6-W blacklight fluorescent lamp located at the axis of the glass tube. The decomposition of acetaldehyde was almost complete within 1–3 h and its main product was carbon dioxide. A kinetic study showed that the photocatalytic reaction obeys a Langmuir adsorption isotherm. Although the light intensity was certainly decreased with the distance from the light source, the degree of this decrease was much smaller than the degree of the decrease in the kinetic constants, which suggests that the light intensity is not simply proportional to the degree of the photo-excitation of TiO2 and the rate of the resulting photocatalytic decomposition of acetaldehyde. © 1999 Society of Chemical Industry

Experimental evaluation of the usefulness of equations describing the apparent maximum reaction rate and apparent Michaelis constant of an immobilized enzyme reaction.

To determine the usefulness of equations previously proposed for the apparent maximum reaction rate and apparent Michaelis constant of an immobilized enzyme, starch hydrolysis by glucoamylase immobilized on a porous ceramic support was considered as a model system. Initial reaction rates, v0, were measured for a wide range of initial starch concentrations, Sb0, to make a nonlinear plot of Sb0/v0 versus Sb0, and the apparent kinetic parameters were determined from the slopes and intercepts of tangents to the nonlinear plot at given values of Sb0. The equations were found to accurately express the diffusional effect on the kinetic parameters.

Characteristics of apparent kinetic parameters in a packed-bed immobilized enzyme reactor

The design equation for a packed-bed immobilized enzyme reactor is expressed in terms of apparent kinetic parameters and the relationship between the exit concentration of substrate consumed and the logarithm of the exit unconverted fraction of substrate is studied. Agreement of calculated values with experimental data indicate that this relationship is not linear but is convex downward. Furthermore, simulation is made to characterize the apparent kinetic parameters. The results show that with increasing flow rates of a reaction mixture, the apparent kinetic parameters decrease rapidly towards their respective intrinsic values. As the entrance substrate concentration increases, on the other hand, these values initially increase, pass through maximums, and finally decrease towards their intrinsic values. These variations are found to occur remarkably with the apparent Michaelis constant.

Highly accurate solution of the axial dispersion model expressed in S-system canonical form by Taylor series method

A numerical method for solving an axial dispersion model (two-point boundary value problem) with extremely high-order accuracy is presented. In this method, one first recasts fundamental differential equations into S-system (synergistic and saturable system) canonical form and then solves the resulting set of simultaneous first-order differential equations by the shooting method combined with a variable-order, variable-step Taylor series method. As a result, it is found that over wide ranges of systemic parameters (Peclet number, dimensionless kinetic constant, and reaction order), this method promises numerical solutions with the superhigh-order accuracy that is comparable to the machine accuracy of the computer used. The advantage of the numerical method is also discussed.

Enzymatic hydrolysis of soluble starch in a polyethylene glycol-dextran aqueous two-phase system

Abstract Hydrolysis of soluble starch by glucoamylase and β-amylase was investigated as a model reaction in an aqueous two-phase system consisting of polyethylene glycol (PEG) and dextran (DEX). Changes in glucose concentration observed in the batch reaction experiments with glucoamylase were almost identical for the aqueous two-phase and pure water systems, showing that the enzymic reactions investigated were not influenced by the presence of PEG and DEX. The partition of β-amylase into the DEX phase was insufficient compared to that of glucoamylase. Hence, the former enzyme was crosslinked with glutaraldehyde to increase its apparent molecular weight and, as a consequence, the partition coefficient, defined as the concentration ratio of the component partitioned into the PEG phase to that into the DEX phase, was decreased to 17% of that of the original enzyme. In the operation in which the enzyme and substrate are partitioned selectively into the DEX phase and allowed to react there while the product, thus transferring to the PEG phase, is recovered, the aqueous two-phase system with a smaller partition coefficient provided longer operational stability.

A reliable Taylor series-based computational method for the calculation of dynamic sensitivities in large-scale metabolic reaction systems: Algorithm and software evaluation

Dynamic sensitivity analysis has become an important tool to successfully characterize all sorts of biological systems. However, when the analysis is carried out on large scale systems, it becomes imperative to employ a highly accurate computational method in order to obtain reliable values. Furthermore, the preliminary laborious mathematical operations required by current software before the computation of dynamic sensitivities makes it inconvenient for a significant number of unacquainted users. To satisfy these needs, the present work investigates a newly developed algorithm consisting of a combination of Taylor series method that can directly execute Taylor expansions for simultaneous non-linear-differential equations and a simple but highly-accurate numerical differentiation method based on finite-difference formulas. Applications to three examples of biochemical systems indicate that the proposed method makes it possible to compute the dynamic sensitivity values with highly-reliable accuracies and also allows to readily compute them by setting up only the differential equations for metabolite concentrations in the computer program. Also, it is found that the Padé approximation introduced in the Taylor series method shortens the computation time greatly because it stabilizes the computation so that it allows us to use larger stepsizes in the numerical integration. Consequently, the calculated results suggest that the proposed computational method, in addition to being user-friendly, makes it possible to perform dynamic sensitivity analysis in large-scale metabolic reaction systems both efficiently and reliably.

A highly accurate numerical method for calculating apparent kinetic parameters of immobilized enzyme reactions: 1. Theory

Abstract To establish a highly reliable numerical method for calculation of the values of apparent kinetic parameters in various types of immobilized enzyme reactions, the performance of the shooting method combined with a variable-order, variable-step Taylor-series method has been discussed in a series of two papers. In this first paper, every formulae that are necessary to execute the numerical calculation are given and the procedure and technique for the calculation are described. As an example, the effectiveness factor is calculated for the Thiele modulus from 1 to 700 and the calculated values are found to have accuracies that are almost equal to the machine accuracy of the personal computer used. In the Taylor-series method, the step sizes are efficiently estimated from the ratio of the appropriate two of the first few terms in each Taylor-series solution to the relevant simultaneous differential equations, which contributes greatly to reduction in both the loss-of-significance error and the execution time.