Evaluation of Photon Absorption in an Aqueous TiO2 Slurry Reactor Using Monte Carlo Simulations and Macroscopic Balance

Kinetic modeling of the photocatalytic degradation of acetaminophen and its main transformation products

In this study, a kinetic model of the heterogeneous photocatalytic degradation of acetaminophen and its main transformation products is presented. Kinetic photocatalytic modeling and photon absorption rate modeling were included. Monte Carlo method was used to model the photon absorption process. Experiments were carried out in a reactor operated in batch mode and TiO2 nanotubes were used as photocatalyst irradiated with 254 nm UVC. Kinetic parameters were estimated from the experiments data by applying a non-linear regression procedure. Intrinsic expressions to the kinetics of acetaminophen degradation and its main transformation products were derived. Model, kinetics and photon absorption formulations and parameters proved to be affordable for describing the photocatalytic degradation of acetaminophen, but improvements should be done for better description of formation and oxidation kinetics of main transformation products. The model should be tested with other pharmaceuticals and emergent pollutants to calibrate it and evaluate its applicability in a wide range of compounds.

Model equations of light scattering properties and a characteristic time of light propagation for polydisperse colloidal suspensions at different volume fractions

We developed model equations of light scattering properties and a characteristic time of light propagation for polydisperse colloidal suspensions at different volume fractions. By the model equations, we examined numerical results using the first-order (dependent) scattering theory (FST) and radiative transfer theory in 600-980 nm wavelength. The model equations efficiently treat the interference of electric fields scattered from colloidal particles by a single effective coefficient, providing fast computation. Meanwhile, the FST provides accurate but complicated treatment. We found the interference effects on the scattering properties and characteristic time depend linearly on wavelength. Dimensionless analysis showed a simple mechanism of the interference effects, independently of wavelength and source-detector distance.

Synthesis and Performance of Photocatalysts for Photocatalytic Hydrogen Production: Future Perspectives

Photocatalysis for “green” hydrogen production is a technology of increasing importance that has been studied using both TiO2–based and heterojunction composite-based semiconductors. Different irradiation sources and reactor units can be considered for the enhancement of photocatalysis. Current approaches also consider the use of electron/hole scavengers, organic species, such as ethanol, that are “available” in agricultural waste, in communities around the world. Alternatively, organic pollutants present in wastewaters can be used as organic scavengers, reducing health and environmental concerns for plants, animals, and humans. Thus, photocatalysis may help reduce the carbon footprint of energy production by generating H2, a friendly energy carrier, and by minimizing water contamination. This review discusses the most up-to-date and important information on photocatalysis for hydrogen production, providing a critical evaluation of: (1) The synthesis and characterization of semiconductor materials; (2) The design of photocatalytic reactors; (3) The reaction engineering of photocatalysis; (4) Photocatalysis energy efficiencies; and (5) The future opportunities for photocatalysis using artificial intelligence. Overall, this review describes the state-of-the-art of TiO2–based and heterojunction composite-based semiconductors that produce H2 from aqueous systems, demonstrating the viability of photocatalysis for “green” hydrogen production.

Six Flux Model for the Central Lamp Reactor Applied to an External Four-Lamp Reactor

One of the difficulties of establishing the intrinsic kinetics of photocatalytic oxidation processes is due to the complex mathematical formula used to determine the rate of photon absorption. To solve this problem, some models have been proposed and checked, such as the Six Flux Model (SFM) confirmed in central lamp photoreactors. External lamp photoreactors are also one of the most used configurations to study the photocatalytic oxidation of contaminants in water, and complex mathematical solutions have been reported to solve the rate of photon absorption. In this work, SFM Equations already reported for the central lamp photoreactor have been adapted to determine the rate of photon absorption in an external four-lamp photoreactor. The results obtained show slight differences from those of the Monte Carlo method. Additionally, once the rate of photon absorption was validated, the intrinsic rate constant and scavenging factor of the photocatalytic oxidation of some contaminant compounds from results already published have been determined.

Absorbed radiation and kinetic model in photocatalysis by TiO2

The current work studies a novel and affordable methodology to estimate and quantify the photon flux absorbed and the amount of light that leaves from an illuminated photocatalytic system with TiO2 suspended in water. To achieve it, a new parameter B F S λ w a t $BF{S}_{\lambda }^{wat}$ is defined and presented. It indicates, for every wavelength, the fraction of the incoming radiation which is not absorbed by the system. B F S λ w a t $BF{S}_{\lambda }^{wat}$ was estimated by means of actinometric experiments in a jacketed reactor and a model based on Beer–Lambert law. For wavelengths below 388 nm and TiO2 concentrations between 0.05 and 2 g L−1, experimental values of B F S λ w a t $BF{S}_{\lambda }^{wat}$ were between 0.77 and 0.27. In the second part of the work, a simple kinetic model, which breaks down the effect of incident radiation and kinetic constant, is developed. For this, the photon flux absorbed by TiO2, previously determined, was included in the model. This new model was tested in the photocatalytic degradation of 2,4-dichlorophenol under different TiO2 concentrations. The kinetic model fits satisfactorily the experimental values and a new kinetic constant kʹ ap [mol·L−1 Einstein−1] was obtained, which is independent of the amount of catalyst loaded to the system. This achievement may be very useful for an easy initial comparison, design or scaling up of different photocatalytic reactors with similar geometry.

Design and performance evaluation of a photocatalytic reactor for indoor air disinfection

Since COVID-19 pandemic, indoor air quality control has become a priority, and the development of air purification devices effective for disinfecting airborne viruses and bacteria is of outmost relevance. In this work, a photocatalytic device for the removal of airborne microorganisms is presented. It is an annular reactor filled with TiO₂-coated glass rings and irradiated internally and externally by UV-A lamps. B. subtilis spores and vegetative cells have been employed as model biological pollutants. Three types of assays with aerosolized bacterial suspensions were performed to evaluate distinct purification processes: filtration, photocatalytic inactivation in the air phase, and photocatalytic inactivation over the TiO₂-coated rings. The radiation distribution inside the reactor was analysed by performing Monte Carlo simulations of photon absorption in the photocatalytic bed. Complete removal of a high load of microorganisms in the air stream could be achieved in 1 h. Nevertheless, inactivation of retained bacteria in the reactor bed required longer irradiation periods: after 8 h under internal and external irradiation, the initial concentration of retained spores and vegetative cells was reduced by 68% and 99%, respectively. Efficiency parameters were also calculated to evaluate the influence of the irradiation conditions on the photocatalytic inactivation of bacteria attached at the coated rings.

Computer Simulations of Photocatalytic Reactors

Photocatalysis has been considered future technology for green energy conversion and environmental purification, including carbon dioxide reduction, water splitting, air/water treatment, and antimicrobial purposes. Although various photocatalysts with high activity and stability have already been found, the commercialization of photocatalytic processes seems to be slow; it is thought that the difficulty in scaling up photocatalytic processes might be responsible. Research on the design of photocatalytic reactors using computer simulations has been recently intensive. The computer simulations involve various methods of hydrodynamics, radiation, and mass transport analysis, including the Monte Carlo method, the approximation approach–P1 model, and computational fluid dynamics as a complex simulation tool. This review presents all of these models, which might be efficiently used for the scaling-up of photocatalytic reactors. The challenging aspects and perspectives of computer simulation are also addressed for the future development of applied photocatalysis.

Assessment of an intrinsic kinetic model for TiO2–formic acid photodegradation using LEDs as a radiation source

Intrinsic kinetic parameters for two photocatalysts, one of them modified with carbon, were estimated to model HCOOH photodegradation using LEDs with different emission spectra.

Photocatalytic Hydrogen Production Under Near-UV Using Pd-Doped Mesoporous TiO2 and Ethanol as Organic Scavenger

Photocatalysis can be used advantageously for hydrogen production using a light source (near-UV light), a noble metal-doped semiconductor and an organic scavenger (2.0 v/v% ethanol). With this end, palladium was doped on TiO2 photocatalysts at different metal loadings (0.25 to 5.00 wt%). Photocatalysts were synthetized using a sol-gel method enhancing morphological properties with a soft template precursor. Experiments were carried out in the Photo-CREC Water II reactor system developed at CREC-UWO (Chemical Reactor Engineering Centre- The University of Western Ontario) Canada. This novel unit offers hydrogen storage and symmetrical irradiation allowing precise irradiation measurements for macroscopic energy balances. Hydrogen production rates followed in all cases a zero-order reaction, with quantum yields as high as 30.8%.

Braiding kinetics and spectroscopy in photo-catalysis: the spectro-kinetic approach

The combination of kinetic and spectroscopic tools has become a key scientific methodology for the understanding of catalytic behavior but its application in photocatalysis has inherent difficulties due to the nature of the energy source of the reaction. This review article provides an overview of its use by, first, presenting mechanistically derived kinetic formulations and spectroscopic data handling methods including intrinsic expressions for light and, second, highlighting representative examples of application. To do it we consider universal catalytic systems, particularly (although not exclusively) titania-based materials, and the most frequent hole and/or electron triggered reaction schemes. This review also provides a general framework to pave the way for the future progress of the spectro-kinetic approach in the photocatalysis area.

Photocatalytic degradation of an emerging pollutant by TiO2-coated glass rings: a kinetic study

This work presents the photocatalytic degradation of the pharmaceutical drug clofibric acid in a fixed-bed reactor filled with TiO₂-coated glass rings. Experiments were carried out under UV radiation. A kinetic model that takes into account radiation absorption by means of the local surface rate of photon absorption (LSRPA) has been developed. The LSRPA was obtained from the results of a radiation model. The Monte Carlo method was employed to solve the radiation model, where the interaction between photons and TiO₂-coated rings was considered. Data from experiments carried out with rings with different numbers of catalyst coatings and different irradiation levels were used to estimate the parameters of the kinetic model. A satisfactory agreement was obtained between model simulations and experimental results.

Photodegradation Efficiencies in a Photo-CREC Water-II Reactor Using Several TiO2 Based Catalysts

This study reports phenol degradation using several TiO2 photocatalysts (DP25, Anatase 1, Hombikat UV-100, Anatase 2) in a Photo-CREC Water-II Reactor. The physicochemical properties of the photocatalysts used, such as crystallinity, superficial area, and pore size distribution are reported. Reactor efficiencies are calculated using both Quantum Yields (QYs) and Photochemical and Thermodynamic Efficiency Factors (PTEFs). This is accomplished using phenol and phenol intermediate photoconversion rates. This allows the determination of hydroxyl radical consumption rates, at every step of the photodegradation process. With these data, and with the absorbed photon rates, energy efficiencies are calculated. It is shown that for the best performing photo catalysts the maximum QYs reach 50 % levels. These favourable photoconversion efficiencies confirm the critical importance of having available highly performing photocatalysts and photoreactors, such is the case of Photo-CREC Water-II Reactor unit.

Nanoparticle scattering characterization and mechanistic modelling of UV-TiO2 photocatalytic reactors using computational fluid dynamics

A computational fluid dynamic (CFD) model was developed to describe the process performance of a semi-batch annular TiO2-UV photoreactor in an Eulerian framework. The model accounted for the optical behaviour of titanium dioxide (TiO2) suspensions, the flow distribution and the oxalic acid degradation in the reactor. The scattering component of the optical model, explicitly included in the CFD simulations using a TiO2-specific scattering phase function integrated in the radiative transfer equation, was calibrated using an optical goniometer by comparing simulated scattering light profiles against irradiance measurements collected for various TiO2 concentrations and UV wavelengths and subsequently solved by the discrete ordinate (DO) radiation model. Several scattering phase functions were tested against the goniometric measurements confirming that the Henyey-Greenstein (HG) equation was the most appropriate angular distribution function at 254 and 355 nm, irrespective of the TiO2 concentration. Using the calibrated HG function, a new approach for quantifying the absolute values of absorption and scattering coefficients in TiO2 suspensions was proposed. It consists of iteratively solving, using the DO model, the radiative transfer equation for various combinations of absorption and scattering coefficients until the error between observed and predicted angular irradiance measurements is minimized. The accuracy of the optical parameters was verified with independent CFD simulations carried out for an annular photoreactor and already available in the literature. Predicted and simulated irradiance and oxalic acid degradation data were found to be in excellent agreement, confirming the considerable potential of the integrated modelling approach presented in this paper for the design, optimization and scale-up of photocatalytic technologies for water and wastewater treatment applications.

Kinetic modeling of the photocatalytic degradation of clofibric acid in a slurry reactor

A kinetic study of the photocatalytic degradation of the pharmaceutical clofibric acid is presented. Experiments were carried out under UV radiation employing titanium dioxide in water suspension. The main reaction intermediates were identified and quantified. Intrinsic expressions to represent the kinetics of clofibric acid and the main intermediates were derived. The modeling of the radiation field in the reactor was carried out by Monte Carlo simulation. Experimental runs were performed by varying the catalyst concentration and the incident radiation. Kinetic parameters were estimated from the experiments by applying a non-linear regression procedure. Good agreement was obtained between model predictions and experimental data, with an error of 5.9 % in the estimations of the primary pollutant concentration.

Determination of Kinetic Parameter in a Unified Kinetic Model for the Photodegradation of Phenol by Using Nonlinear Regression and the Genetic Algorithm

This study reports the kinetic parameter estimation in the photocatalytic degradation of phenol over different TiO2 catalysts by using the Genetic Algorithm (GA) and nonlinear regression. Reaction networks are based on a previously reported unified kinetic model (UKM) of the Langmuir–Hinshelwood type. Nonlinear least-squares fitting and GA are used to find the values for the kinetic constants. The computed parameters were found to predict experimental data for phenol photodegradation at different levels of concentrations. It is shown that both methods render close values for the kinetic constants. This suggests that UKM approach gives the global minimum and as a result, this method provides good and objective parameter estimates with low to moderate cross-correlation among kinetic constants and acceptable 95% Confidence Intervals (CIs). Global optimization by using GA requires extensive computer times of up to 5 minutes. Least square fitting provides the same results with computer times of seconds only. It is then concluded that the UKM approach effectively avoids overparameterization by finding the global optimum when optimizing the kinetic constants.