The production of hydroxyl radicals (OH) by several photocatalytic cementitious materials was quantified by an adaptation of Terephthalic acid (TA) as probe method for fluorescence spectrometry ...(TA-FL method) for construction materials. Six different materials were evaluated for OH using the method developed, and the photocatalytic activity was compared with the NOx degradation rate, obtaining a positive relationship between both parameters even though the rate of NOx degradation was significantly larger than that of the OH detected. The difference could not be attributed exclusively to the contribution of O2−. Thus, it has been postulated that most of the NOx degradation takes place through the holes, trapped in the surface of the photocatalyst, in equilibrium with OH in the bulk solution, which still makes the quantification of OH as an effective indicator of the photocatalytic performance of cementitious materials.
Display omitted
•Nitroblue tetrazolium ink is reliable to assess the photocatalytic activity.•Emulsions coatings and cement-based materials were evaluated.•Ink progression can be monitored by ...reflectance and colour measurements.•NBT ink proposed test is comparable with oxidative NOx gas removal rate.•NBT ink proposed test is “simple-to-use”, rapid and inexpensive.
The photoactivity of TiO2-bearing construction materials was assessed with nitroblue tetrazolium (NBT) ink. The photocatalytic reaction was monitored with UV–vis diffuse reflectance spectroscopy, colorimetry and digital image processing. The results delivered by the three datasets were linearly related, even though the experimental setup was inexpensive and involved only handheld instruments. The NBT reduction and ISO 22197-1 oxidative NOx removal findings also proved to be acceptably linear, may, then, be promising alternatives to conventional NOx photocatalytic performance testing. The advantages of the procedure proposed over conventional testing include convenience of use, time savings, in-situ applicability, readily distinguishable colour change and low cost.
ZnO nanospheres were synthesised and then deposited by both single- and double-fire fast processes on as-prepared ceramic substrates. The photocatalytic degradation of resazurin ink was tested under ...UV light. The single-fired samples did not show any evidence of photocatalytic activity because the nanoparticles melted during sintering at 1210°C. The double-fire ZnO spray-coating method successfully produced glazed materials with an active ZnO surface layer despite the high sintering temperature. The influence of experimental parameters, including the ZnO nanoparticle loading (0.03 to 1 mg/cm2) and firing temperature (650 to 800°C), were also investigated. Samples with a ZnO loading of 1 g/cm2 fired at 650°C showed the best photocatalytic activity. Increasing the temperature to 700 and 800°C led to the coalescence of ZnO nanoparticles, which reduced the photocatalytic activity.
This work aims to study the effect of addition of photocatalyst on the characteristics of TiO2 modified mortars and the influence of type of binder, surface roughness and microstructure, on their ...photoactivity for self-cleaning of organic dyes (rhodamine B and methylene blue) and NOx degradation. Mortars with four different types of cements and three levels of roughness were prepared. From the results, it was found that the available active surface is a parameter more influential than surface roughness for assessing photocatalytic efficiency. Concerning the composition of the mixes, the classification according to photocatalytic efficiency was the same for both NOx and self-cleaning, being, in decreasing order: Portland cement (quite similar to calcium aluminate cement for NOx), fly ash and slag mortars. The difference has been explained on the basis of oxidation-reduction potentials and photoabsorption energy of the different constituents of cementitious matrix.
Display omitted
•Combination of TiO2 with cement binders leads to inclusion of energetic disorders.•Energetic disorders in the TiO2 cement samples leads to energetic tail bands.•Ti+3 or oxygen ...vacancies energetic disorders appear in TiO2 cement samples.•Energetic disorders in TiO2 cement materials can enhance the photoactivity.
Different mortar mixes using TiO2 P25 as photocatalyst have been prepared. The samples were characterized by UV–vis spectroscopy, X-ray photoelectron spectroscopy and Photoluminiscence spectroscopy. Also, determination of hydroxyl radicals and degradation tests of NOx under visible and UV light have been performed. The results obtained have allowed determining the band edges, Urbach energies and valence band tail sizes, directly related to the energy levels present in the TiO2-cement based materials. Therefore, the schema of the electronic structures of the reference TiO2-photocatalyst and the three TiO2-cement based samples have been constructed. The photocatalytic efficiency has been evaluated according to these energy levels, being able to conclude that in some cases, as the mortar with slags, the chemical composition seems to prevail over the effect of the optical parameter and electronic band structure.
•Alkali-activated materials with TiO2 nanoparticles additions were studied.•Alkali-activated binders showed good behavior and high compressive strength.•TiO2 additions in alkali-activated enhanced ...mechanical strength.•Self-cleaning properties was affected by the presence of nanoparticles.
Nowadays, alkali-activated binders have become one of the most viable options in the reduction of CO2 emissions; receiving considerable attention due to favorable mechanical properties and low energy consumption. Following from that, the effect of addition of TiO2 nanoparticles in alkali-activated binders based on metakaolin and ground granulated blast furnace slag on the mechanical properties, microstructure, formation of new reaction phases, and self-cleaning properties, was analyzed. The results indicated that synthesized materials showed an enhancement in compressive strength obtaining the best results with 0.5% of TiO2, furthermore, self-cleaning properties showed a slight modification in the presence of the nanoparticles.
Photocatalytic technology implemented in construction materials is a promising solution to contribute to alleviate air quality issues found in big cities. Photocatalysis has been proved able to ...mineralise most harmful contaminants. However, important problems associated with monitoring the efficiency of these solutions under real conditions still remain, including the lack of affordable analytical tools to measure NOx concentrations with enough accuracy. In this work, two pilot scale demonstration platforms were built at two different locations to assess the photocatalytic NOX removal efficiency of ten selected materials exposed outdoors for AQmesh low-cost sensor PODs were used to measure ground-level to measure NO and NO2 concentrations during nearly one year. The pollutant removal efficiency of the materials was then calculated based on a comparison with simultaneously concentration measurements carried-out on reference, non-active materials. It was found that the NO2 removal efficiency presented large variations across the seasons, with maxima during the warmer months, while NO efficiencies were comparatively steadier. Statistical analysis delivered evidence that the efficiencies significantly depend on different meteorological variables (irradiance and relative humidity) besides NO, NO2 ambient concentrations. Lower efficiencies were observed for higher concentration levels and vice versa. The influence of water vapour could be related to two different effects: a short-term contribution by the instantaneous air humidity and a long-term component associated with the hygroscopic state of the material. The contribution of wind to the pollutant removal efficiencies was principally related to the humidity of air masses moving above the location and to the advection of pollutants from specific emission sources.
Display omitted
•10 commercial photocatalytic pavements tested at pilot scale during 1 year•Measurements from low cost sensors post-processed by means of ML algorithms•Effect on NOx removal efficiencies of environmental variables assessed through GAMs•Removal efficiencies increase significantly as pollutant concentrations decrease.•Hygroscopic state of the material is a highly influential parameter for NO2 removal.
•Photocatalytic activity of iron based pigment-TiO2 mortar depends on electron transfer.•Electron transfer depends on the band positions of pigment-TiO2 heteroestructure.•Electron transfer depends on ...ratio iron-TiO2 of pigment-cement heteroestructure.•Electron transfer of iron based pigment-TiO2 mortar inhibited the recombination.•There is an optimum iron-TiO2 ratio that minimizes the electron transfer ability.
This paper analyses the effect of adding iron based pigments to produce photocatalytic colored mortars (Ph-CM) evaluating nitrogen oxide air purification-NOx, Rhodamine B self-cleaning-RhB and formation rate of hydroxyl radicals-OH. All Ph-CM tested shifted towards lower energy values with respect to the absorption-edge of the TiO2-mortar samples without pigments. However, different UV–Visible photoactivity behavior was obtained in function of the type and content of pigment. The results indicate that even though a decrease in the photocatalytic efficiency can occur, in some cases, an interfacial electron transfer between the conduction band of iron pigment and the photocatalytic mortar could also take place, resulting in an effective inhibition of electron-hole recombination, higher formation rate of OH, and thereby higher photocatalytic activity. This electron transfer ability directly depends on the band-edge position of pigment-cement heteroestructure and the iron to pigment ratio.
Health risks due to NO2 exposure commonly exceed acceptable levels in modern societies. Among the measures to reduce such risks, photocatalytic materials present a promising technology. However, ...while the pollutant remediation of such materials has been extensively validated in laboratory studies, the performance under real world environmental exposure conditions is still subject to controversy. Indeed, a comparison of available in-situ monitoring studies manifests non-conclusive and highly scattered results regarding the photocatalytic effectiveness observed. The reasons for this behaviour must be carefully explored in order to prevent non-efficient photocatalytic applications from being put into practice on a larger scale. This paper presents a comprehensive large-scale study for assessing the photocatalytic NO2 remediation by active pavements in a street of Madrid (Spain), comprising different in-situ monitoring techniques. The discussion is enriched by relating the obtained results to those of other large-scale studies. The discrepancies between these results may be traced back to different circumstances, among them the distance between the active pavement and the pollutant concentration sampling inlet, as well as to significant site-specific and time-dependent variations of pollutant concentrations and climatic parameters. Under due consideration of these influences, for materials with relatively high initial effectiveness, it was concluded that in most such applications, the average NO2 removal effectiveness, if evaluated at a typical inlet height of Air Quality Stations (3 m), will not exceed a value of 4% (averaged over a sufficiently large number of measurement points in the area of application and a sustained amount of time, i.e. several months). When considering more realistic human exposure conditions (lower heights and daytime), it might be justified to assume somewhat higher average effectiveness.
Display omitted
•A commercial photocatalytic material tested at street scale•Measurements from passive samplers and low cost sensors made•NO2 removal efficiencies assessed through statistical analysis•The photocatalytic effectiveness depended on the sampling point•Photocatalytic spatial scattering because of site-specific influences
Nowadays, a large proportion of photocatalytic oxidation (PCO) devices are being implemented in heating, ventilation and air-conditioning systems. However, no systematic studies have been carried out ...regarding the influence of inlet air preconditioning. To analyse the impact of the inlet air-conditions into photocatalytic efficiency, a simulated air-conditioning duct with flowing gas through inside was designed. Isobutylene was chosen as the target VOCs. The concentration in the gas phase was monitored using a photoionization detector. The influence of flow rate, relative humidity and temperature on the VOC removal efficiency was analysed. Experimental results were presented in terms of gas-removal efficiency (η) and clean air delivery rate (CADR) and analysed on a kinetic basis. From them, the weight of each parameter in the global process has been determined, from bigger to smaller contribution, flow>>temperature>relative humidity. Also, the relevance of the inlet air conditions has been illustrated in a model room in order to determinate the time necessary to obtain a threshold value accomplishing with enough air quality and the energy consumption of the device. Additionally, the photocatalytic decontamination has been assimilated to the “air exchange rate”, a parameter commonly used in indoor air quality studies. The results show that preconditioning of air can improve the efficiency of photocatalytic devices and bring important energy savings.
PDF
