Photocatalytic NOx removal with TiO2-impregnated 3D-printed PET supports

In this work, we investigated the photocatalytic removal of NOx using 3D-printed supports. Monolithic supports with internal channels were fabricated by Fused Modelling Deposition (FDM) using PET as the filament feedstock. The printing parameters of the supports were optimized to maximize the exposure of the photocatalyst to UV light throughout the monolithic PET printed supports. The removal experiments were carried out in a continuous gas phase flow reactor, which was custom designed in-house incorporating a 3D printed PET support impregnated with TiO2 as photocatalyst. The impregnated and non-impregnated supports were characterized by diffuse reflectance spectrometry, SEM and AFM. The effect of several key-factors on the NOx removal capacity was investigated, including the type of PET filament (native recycled, BPET vs. glycol-modified, PETG), the type of TiO2 (P25 vs. Hombikat UV-100), the UV light source (LED vs. tubular lamps), and the number of deposited TiO2 layers. The highest NO and NOx removal were achieved by using PETG supports coated with a single layer of Hombikat UV-100 and irradiating the flat reactor from both sides using two sets of black light lamps. However, the highest selectivity toward nitrate formation was obtained when using P25 under the same experimental conditions. This work demonstrates that 3D printing is a reliable and powerful technique for fabricating photocatalytic reactive supports that can serve as a versatile platform for evaluating photocatalytic performance.

Photocatalytic fixation of NOx in soils

Nitrogen oxides (NOx = NO + NO2) are important atmospheric pollutants that are directly harmful to human health. Recently in urban and industrial areas, synthetic materials have been developed and deployed to photocatalytically oxidize NOx to nitrate (NO3-) in order to improve air quality. We show that the natural presence of small amounts (≤5%) of titanium oxides, such as anatase and rutile, can also drive NOx oxidation to nitrate in soils under UV-visible irradiation. The NO uptake coefficients ranged between 0.1 × 10-6 for sandy soils to 6.4 × 10-5 in the case of tropical clay soils; the latter comparable in efficiency to current industrial man-made catalysts. This photocatalytic N-fixation mechanism offers a new strategy for NOx mitigation from the atmosphere by transforming it into nitrate, and simultaneously provides an energy efficient source of essential fertilizer to agriculture.

Photocatalytic NOx Removal in Bismuth-Oxyhalide (BiOX, X = I, Cl) Cement-Based Materials Exposed to Outdoor Conditions

Cement-based materials modified with 3D BiOX (X = I, Cl) microspheres at different percentages (1, 5 and 10% by weight of the cement binder) were prepared to investigate the durability of the photocatalytic NOx removal under outdoor conditions. Weathering—corresponding to a period of 13 months outdoors—was studied in terms of NO removal efficiency under visible and UVA light irradiation for BiOI and BiOCl mortars, respectively. Following this period, the samples were protected from the environment for four years, and NOx removal and selectivity to nitrates were assessed. BiOI and BiOCl mortar samples were initially photocatalytically active; NOx removal performance increased as BiOX content increased. There was good photocatalyst dispersion, and compressive strength was not significantly impacted. The BiOI mortars had nearly completely lost their activity after 5 years from casting, whereas mortars containing 10% BiOCl had maintained about 7% of initial performance. The results suggest that mortar deactivation is due to surface dirt and nitrates accumulation from NOx oxidation on the surface rather than carbonation. An internal self-deactivation mechanism that affects BiOI in mortar matrix has also been postulated.

Ozone Formation during Photocatalytic Oxidation of Nitric Oxides under UV Irradiation with the Use of Commercial TiO2 Photocatalytic Powders

The application of photocatalytic materials has been intensively researched in recent decades. The process of nitric oxide (NO) oxidation during photocatalysis has been observed to result in the formation of nitric dioxide (NO₂). This is a significant factor of the photocatalysis process, as NO₂ is more toxic than NO. However, it has been reported that ozone (O₃) is also formed during the photocatalytic reaction. This study analyzed the formation and oxidationof O₃ during the photocatalytic oxidation of NO under ultraviolet irradiation using commercial photocatalytic powders: AEROXIDE^® TiO₂ P25 by Evonik, KRONOClean^® 7050 by KRONOS^®, and KRONOClean^® 7000 by KRONOS^®. An NO concentration of 100 ppb was assumed in laboratory tests based on the average nitric oxide concentrations recorded by the monitoring station in Warsaw. A mix flow-type reactor was applied in the study, and the appropriateness of its application was verified using a numerical model. The developed model assumed an empty reactor without a photocatalytic material, as well as a reactor with a photocatalytic material at its bottom to verify the gas flow in the chamber. The analysis of the air purification performance of photocatalytic powders indicated a significant reduction of NO and NO_x and typical NO₂ formation. However, no significant formation of O₃ was observed. This observation was verified by the oxidation of pure ozone in the process of photocatalysis. The results indicated the oxidation of ozone concentration during the photocatalytic reaction, but self-decomposition of a significant amount of the gas.

Preparation and Properties of Magnesium Cement-Based Photocatalytic Materials

Photocatalytic oxidation is a technology developed in recent years for the degradation of indoor air pollutants. In this study, magnesium cement-based photocatalytic material (MPM) was prepared by loading TiO2 photocatalysts onto a SiO2-modified basic magnesium chloride whisker (BMCW) surface, and was subsequently sprayed evenly on the surface of putty powder to form a photocatalytic functional wall coating (PFWC) material. Then, by introducing Ag, visible light photocatalytic functional wall coating (VPFWC) materials were also prepared. The results show that TiO2 and SiO2 form Ti–O–Si bonds on the BMCW surface, and the PFWC presents a promising degradation effect, with a photocatalytic removal rate of 46% for gaseous toluene, under ultraviolet light for 3 h, and an MPM coating concentration of 439 g/m2. This is related to the surface structure of the functional coating, which is formed using putty powder and MPM. The visible light photocatalytic efficiency of the VPFWC increased as the spraying amount of the AgNO3 solution increased, up to 16.62 g/m2, and then decreased with further increasing. The gaseous toluene was degraded by 28% and 73% in 3 h, by the VPFWC, under visible light and ultraviolet light irradiation, respectively. In addition, the photocatalytic performance of the PFWC/VPFWC also showed excellent durability after being reused five times.

Titania-Clay Mineral Composites for Environmental Catalysis and Photocatalysis

The use of titania-based composite materials in the field of heterogeneous catalysis and photocatalysis has a long and rich history. Hybrid structures combining titania nanoparticles with clay minerals have been extensively investigated for nearly four decades. The attractiveness of clay minerals as components of functional materials stems primarily from their compositional versatility and the possibility of using silicate lamellae as prefabricated building blocks ready to be fitted into the desired nanoconstruction. This review focuses on the evolution over the years of synthetic strategies employed for the manufacturing of titania–clay mineral composites with particular attention to the role of the adopted preparative approach in shaping the physical and chemical characteristics of the materials and enabling, ultimately, tuning of their catalytic and/or photocatalytic performance.

Visible light-driven novel Bi2Ti2O7/CaTiO3 composite photocatalyst with enhanced photocatalytic activity towards NO removal

Photocatalysis is regarded as a promising technology for removal of nitrogen oxide (NO), however, the low photocatalytic efficiencies under visible light irradiation and the deactivation of the photocatalyst are as yet the significant issues that should be addressed. In this work, visible-light-driven Bi₂Ti₂O₇/CaTiO₃ heterojunction composites were synthesized by a facile in-situ hydrothermal method. The Bi₂Ti₂O₇/CaTiO₃ composites displayed superior visible light photocatalytic activity than pure CaTiO₃ and pure Bi₂Ti₂O₇ in the removal of NO at the 600 ppb level in air. Among all the composites, Bi₂Ti₂O₇/CaTiO₃ containing 20 wt% Bi₂Ti₂O₇ exhibited the best photocatalytic activity, achieving a maximum removal efficiency of 59%. The improved photocatalytic performance is mainly attributed to the strong visible-light-absorbing ability, the presence of an appropriate density of oxygen vacancy defects and the formation of heterojunction between CaTiO₃ and Bi₂Ti₂O₇, resulting in an efficient charge separation at the interface as proven by photoluminescence (PL) and photo-induced current measurements. According to trapping experiments and spin-trapping ESR analysis, the ^•O₂^- and h⁺ are the principal reactive species involved in the photocatalytic NO removal. In addition, the as-obtained Bi₂Ti₂O₇/CaTiO₃ composite showed good chemical stability, which is beneficial for practical applications in air pollution removal.

Insight into the role of copper in the promoted photocatalytic removal of NO using Zn2-xCuxCr-CO3 layered double hydroxide

In this work the ability of Zn_2-xCu_xCr-CO₃ layered double hydroxides (LDHs) as highly efficient DeNOx photocatalysts was studied. LDHs with x = 0, 0.2 and 0.4 were prepared using a coprecipitation method. The samples were characterized by different techniques such as XRD, XPS, FT-IR, ICP-MS, TG, SBET, SEM and Diffuse reflectance (DR). The increased amount of copper ions in the LDH layers gave rise to slight changes in the structure and morphology and an important variation of the optical properties of the LDHs. The prepared ZnCuCr-CO₃ photocatalysts exhibited favourable conversion efficiency (51%) and an extraordinary selectivity (97%) for the photochemical NO abatement. The photochemical mechanism was elucidated from DOS, EPR, Femtosecond transient absorption and in-situ DRIFTS studies. The results suggested that the presence of Cu²⁺ ions in the LDH framework introduced new states in the valence band states, thus favouring the production and mobility of e^-/h⁺ charge carriers and a greater production of ⋅O₂^- and ⋅OH.

A Review of Photocatalytic Materials for Urban NOx Remediation

NOx is a pervasive pollutant in urban environments. This review assesses the current state of the art of photocatalytic oxidation materials, designed for the abatement of nitrogen oxides (NOx) in the urban environment, and typically, but not exclusively based on titanium dioxide (TiO2). Field trials with existing commercial materials, such as paints, asphalt and concrete, in a range of environments including street canyons, car parks, tunnels, highways and open streets, are considered in-depth. Lab studies containing the most recent developments in the photocatalytic materials are also summarised, as well as studies investigating the impact of physical parameters on their efficiency. It is concluded that this technology may be useful as a part of the measures used to lower urban air pollution levels, yielding ∼2% NOx removal in the immediate area around the surface, for optimised TiO2, in some cases, but is not capable of the reported high NOx removal efficiencies >20% in outdoor urban environments, and can in some cases lower air quality by releasing hazardous by-products. However, research into new material is ongoing. The reason for the mixed results in the studies reviewed, and massive range of removal efficiencies reported (from negligible and up to >80%) is mainly the large range of testing practices used. Before deployment in individual environments site-specific testing should be performed, and new standards for lab and field testing should be developed. The longevity of the materials and their potential for producing hazardous by-products should also be considered.

New Holistic Conceptual Framework for the Assessment of the Performance of Photocatalytic Pavement

Despite serious health and environmental burdens associated with air pollution by NO_x, the emission ceilings have been systematically exceeded in big European cities for several years. Photocatalytic technology can be an efficient solution for the removal of chemical air pollutants. Because diesel engine exhaust is the main source of NO_x emissions, the application of a photocatalyst onto road pavement appears to be an effective NO_x abatement method due to the large surface area, proximity to the emission source, and relatively good solar irradiance. Several laboratory-scale studies provided evidence demonstrating that most harmful contaminants can be readily mineralized. Furthermore, several projects were aiming to scale up this technology to pilot and real scales. Although the photocatalytic performances of selected materials in real urban environments were determined in some of these studies, the data are not conclusive for evaluating the overall performance because other material characteristics relevant to their functionality were not assessed. The lack of conformity criteria suitable for the evaluation of the overall performance of photocatalytic pavement under real operational conditions has generated skepticism and mistrust among public authorities and relevant stakeholders, which constrains the widespread implementation of this promising technology. In this context, the project LIFE-Photoscaling was focused on developing a new holistic conceptual framework to assess the photocatalytic pavement performance using the decision tool "Photoscaling Decision Maker" based on a set of quantitative indicators. For this purpose, a large volume of data obtained for 10 types of photocatalytic pavement materials was systemized on both the laboratory and pilot plant scales and three main indicators were defined: (1) photocatalytic performance effectiveness, (2) intrinsic performance, and (3) undesired secondary effects. Each top-level indicator includes several low-level subindicators associated with specific material characteristics. Finally, the ranges of the main indicators and subindicators and methods for their assessment were determined. These methods include standard, adopted, and original characterization techniques, which were selected based on the criteria such as simplicity, cost- and time-effectiveness, and relevance regarding the operational conditions.

Photocatalytic Activity of Zn x Mn3-x O4 Oxides and ZnO Prepared From Spent Alkaline Batteries

Oxides with Zn x Mn3-xO4 stoichiometries and ZnO were synthesized from the "black mass" material recovered from spent alkaline batteries. The oxides were characterized by XRF, XRD with Rietveld refinement, SEM, and TEM methods. Optical characterization included diffuse reflectance (DRS) and photoluminescence (PL) measurements. ZnO presented a clear band edge in the UV region, and PL signals were detected. The Zn/Mn oxides showed strong absorption in the UV region and a continuous absorption band in the Vis-IR regions. There is a non-detected PL signal due to excited charges being trapped on sub-band energy states and/or transfer by non-radiative paths. Photocatalytic activity under both irradiation conditions was evaluated using the resazurin dye test, terephthalic acid fluorescence probe method, and NOx air purification evaluation. In the three photoactivity tests, ZnO performed well under both UV and Vis irradiation, whereas no evidence of any appreciable photocatalytic activity was observed for the Zn/Mn oxides. The results are discussed in terms of the findings of previously reported optical measurements.

NOx removal efficiency of urban photocatalytic pavements at pilot scale

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 NO_x concentrations with enough accuracy. In this work, two pilot scale demonstration platforms were built at two different locations to assess the photocatalytic NO_X 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.

Oxidation of NO x Using Hydrogen Peroxide Vapor over Mo/TiO2

xMo/TiO₂ catalysts (x = 1, 2, 3, and 4%) were prepared using the coprecipitation method in the present study. The coprecipitation method was used in the thermal catalytic decomposition of H₂O₂ steam to treat NO _x at a low temperature range (80-160 °C). Several characterization techniques have been employed, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller measurements, transmission electron microscopy (TEM), scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM-EDXS), and Fourier transform infrared spectroscopy. The activity tests showed that the incorporation of molybdenum into TiO₂ led to a significant increase in the catalytic oxidation of NO, and under the condition of H₂O₂/NO = 6:1 (molar ratio), the NO _x removal rate of 2% Mo/TiO₂ is the highest, reaching 92.56%. XRD, TEM, and SEM-EDXS analyses showed that Mo was well dispersed on the surface of an anatase-phase TiO₂. XPS analysis indicated that Mo mixed with slag mainly existed in the form of Mo⁶⁺. Moreover, in comparison with the mostly reported SCO catalysts, used for the elimination of NO, the prepared Mo/TiO₂ catalyst showed excellent stability and sulfur resistance.

Photochemical emission and fixation of NOX gases in soils

Gaseous nitrogen oxides (NOx), which result from the combustion of fossil fuels, volcanic eruptions, forest fires, and biological reactions in soils, not only affect air quality and the atmospheric concentration of ozone, but also contribute to global warming and acid rain. Soil NOx emissions have been largely ascribed to soil microbiological processes; but there is no proof of abiotic catalytic activity affecting soil NO emissions. We provide evidence of gas exchange in soils involving emissions of NOx by photochemical reactions, and their counterpart fixation through photocatalytic reactions under UV-visible irradiation. The catalytic activity promoting NOx capture as nitrate varied widely amongst different soil types, from low in quartzitic sandy soils to high in iron oxide and TiO₂ rich soils. Clay soils with significant amounts of smectite also exhibited high rates of NOx sequestration and fixed amounts of N comparable to that of NO (nitric oxide) losses through biotic reactions. In these soils, a flux of 100 µg N_NO m^-2 h^-1, as usually found in most ecosystems, could be reduced by these photochemical reactions by more than 60%. This mechanism of N fixation provides new insight into the nitrogen cycle and may inspire alternative strategies to reduce NO emissions from soils.

Air Purification Performance of Photocatalytic Concrete Paving Blocks after Seven Years of Service

This paper presents the results of laboratory tests on photocatalytic pavement blocks from a bicycle lane in Poland after seven years of service. Air purification performance was tested on dusty and clean samples using different light sources and setups, with non-laminar gas circulation. Secondary Electrons Secondary Ions (SESI) and InLens detectors combined with SEM–EDS and X-ray analyses were applied to confirm the presence of TiO2 in the studied blocks. The obtained results show that TiO2 was present in the form of agglomerates with a diameter of 0.25–5 µm and was bonded to the cement matrix components. The tested samples still maintained nitric oxide (NO) removal capability with a NO reduction rate of 4–45%, depending on light source and surface cleanliness.

CdTe Quantum Dot-Functionalized P25 Titania Composite with Enhanced Photocatalytic NO2 Storage Selectivity under UV and Vis Irradiation

Composite systems of P25 (titania) functionalized with thioglycolic acid (TGA)-capped CdTe colloidal quantum dots (QDs) were synthesized, structurally characterized, and photocatalytically tested in the photocatalytic NO _x oxidation and storage during NO(g) + O₂(g) reaction. Pure P25 yielded moderate-to-high NO conversion (31% in UV-A and 40% in visible (vis)) but exhibited extremely poor selectivity toward NO _x storage in solid state (25% in UV-A and 35% in vis). Therefore, P25 could efficiently photooxidize NO(g) + O₂(g) into NO₂; however, it failed to store photogenerated NO₂ and released toxic NO₂(g) to the atmosphere. CdTe QD-functionalized P25 revealed a major boost in photocatalytic performance with respect to pure P25, where NO conversion reached 42% under UV-A and 43% under vis illumination, while the respective selectivity climbed up to 92 and 97%, rendering the CdTe/P25 composite system an efficient broad-band photocatalyst, which can harvest both UV-A and vis light efficiently and display a strong NO _x abatement effect. Control experiments suggested that photocatalytic active sites responsible for the NO(g) + O₂(g) photooxidation and formation of NO₂ reside mostly on titania, while the main functions of the TGA capping agent and the CdTe QDs are associated with the photocatalytic conversion of the generated NO₂ to the adsorbed NO _x species, significantly boosting the selectivity toward solid-state NO _x storage. Reuse experiments showed that photocatalytic performance of the CdTe/P25 system can be preserved to a reasonable extent with only a moderate decrease in the photocatalytic performance. Although some decrease in the photocatalytic activity was observed after aging, CdTe/P25 could still outperform P25 benchmark photocatalyst. Increasing CdTe QDs loading from the currently optimized minuscule concentrations could be a useful strategy to increase further the catalytic lifetime/stability of the CdTe/P25 system with only a minor penalty in catalytic activity.

Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NO x abatement

Semiconductor photocatalysis could be an effective means to combat nitrogen oxides (NO x ) based air pollution through mineralisation of NO x to nitrate. However, most of the typically TiO2-based catalysts employed show a much higher reactivity towards NO than NO2, leading to an accumulation of this unwanted and toxic intermediate. By grafting the photocatalyst with small amounts (≤0.1 at%) of isolated iron(iii) ions, the reactivity towards NO2 is increased by the factor of 9, bringing it up to par with the NO-reactivity and alleviating the problem with intermediate accumulation. Consequently, the observed selectivity of the reaction is dramatically increased from less than 40% to more than 90%. The paper also discusses possible mechanisms for this very beneficial behavior.

A Novel Method for the Combined Photocatalytic Activity Determination and Bandgap Estimation

The ability of a photocatalyst to degrade a target pollutant is a commonly used method to assess its effectiveness for environmental applications, while ultraviolet-visible (UV-vis) spectroscopy and spectroscopic ellipsometry are conventional techniques for the estimation of a semiconductor band gap. In this work, an array of six light-emitting diodes (LEDs), characterized by different emission peaks between 470–370 nm and absorbed power of 3 W, was implemented into an existing standard testing apparatus for the testing of nitrogen oxides degradation in air. The abatement indexes, obtained under different LEDs irradiation, were firstly compared to the ones determined according the standard and, secondly, correlated with the measured LED emission spectrum, in order to estimate the photocatalyst band gap. Results suggest that this expeditious technique can be easily implemented into existing testing apparatus for the estimation of the band gap and for the appraisal of photocatalytic materials under realistic conditions.

Photocatalytic activity and charge carrier dynamics of TiO2 powders with a binary particle size distribution

The effects of the particle size distribution on the charge carrier dynamics and the photocatalytic activity of mixed titanium dioxide (TiO₂) powder samples were investigated in this work.

Superior NOx photocatalytic removal over hybrid hierarchical Bi/BiOI with high non-NO2 selectivity: synergistic effect of oxygen vacancies and bismuth nanoparticles

Oxygen vacancies and bismuth nanoparticles over Bi/BiOI simultaneously contribute to the deep oxidation of NO_x and remarkable non-NO₂ selectivity.

Mesocrystalline anatase nanoparticles synthesized using a simple hydrothermal approach with enhanced light harvesting for gas-phase reaction

The most selective De-NO_x process reached by mesocrystalline anatase nanoparticles was associated with the developed mesoporosity, visible light photoactivity and mesocrystallinity.

Different Roles of Water in Photocatalytic DeNOx Mechanisms on TiO2: Basis for Engineering Nitrate Selectivity?

The nitrate selectivity of TiO₂ has important consequences for its efficiency as a NOx depollution photocatalyst. Most emphasis is typically given to photocatalyst activity, a measure of the rate at which NOx concentrations are reduced, but a reduction in NOx concentration (mainly NO + NO₂) is not necessarily a reduction in atmospheric NO₂ concentration because the catalytic process itself generates NO₂. With NO₂ being considerably more toxic than NO, more emphasis on nitrate selectivity, a measure of the NOx conversion to nitrate, and how to maximize it, should be given in engineering photocatalytic systems for improved urban air quality. This study, on the importance of adsorbed water in the photocatalytic oxidation of NOx, has identified important correlations which differentiate the role that water plays in the oxidation of NO and NO₂. This observation is significant and offers insights into controlling nitrate selectivity on TiO₂ and the potential for increased effectiveness in environmental photocatalyst applications.

Purely Visible-Light-Induced Photochromism in Ag-TiO2 Nanoheterostructures

We report titania nanoheterostructures decorated with silver, exhibiting tuneable photochromic properties for the first time when stimulated only by visible white light (domestic indoor lamp), with no UV wavelengths. Photochromic materials show reversible color changes under light exposure. However, all inorganic photochromic nanoparticles (NPs) require UV light to operate. Conventionally, multicolor photochromism in Ag-TiO₂ films involves a change in color to brownish-gray during UV-light irradiation (i.e., reduction of Ag⁺ to Ag⁰) and a (re)bleaching (i.e., (re)oxidation of Ag⁰ to colorless Ag⁺) upon visible-light exposure. In this work, on the contrary, we demonstrate visible-light-induced photochromism (ranging from yellow to violet) of 1-10 mol % Ag-modified titania NPs using both spectroscopic and colorimetric CIEL*a*b* analyses. This is not a bleaching of the UV-induced color but a change in color itself under exposure to visible light, and it is shown to be a completely different mechanism-driven by the interfacial charge transfer of an electron from the valence band of TiO₂ to that of the Ag_xO clusters that surround the titania-to the usual UV-triggered photochromism reported in titania-based materials. The quantity of Ag or irradiation time dictated the magnitude and degree of tuneability of the color change, from pale yellow to dark blue, with a rapid change visible only after a few seconds, and the intensity and red shift of surface plasmon resonance induced under visible light also increased. This effect was reversible after annealing in the dark at 100 °C/15 min. Photocatalytic activity under visible light was also assessed against the abatement of nitrogen oxide pollutants, for interior use, therefore showing the coexistence of photochromism and photocatalysis-both triggered by the same wavelength-in the same material, making it a multifunctional material. Moreover, we also demonstrate and explain why X-ray photoelectron spectroscopy is an unreliable technique with such materials.

Performance of ZnO synthesized by sol-gel as photocatalyst in the photooxidation reaction of NO

ZnO samples were prepared by sol-gel method applying a factorial design in order to improve the photocatalytic properties of the semiconductor oxide in the NO photooxidation reaction. The concentrations of zinc acetate and ammonium hydroxide were selected as critical variables in the synthesis of ZnO. Nine samples of ZnO were obtained as product of the factorial design and were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, and N₂ adsorption-desorption isotherms. The photocatalytic activity of ZnO samples was associated with the physical properties developed by each sample according to its respective conditions of synthesis. Some photocatalytic reaction parameters, such as mass of photocatalyst, irradiance, and relative humidity, were modified in order to evaluate its effect in the photocatalytic conversion of NO. As a relevant point, the relative humidity played an important role in the increase of the selectivity of the NO photooxidation reaction to innocuous nitrate ions when ZnO was used as photocatalyst.

On the underlying mechanisms of the low observed nitrate selectivity in photocatalytic NOxabatement and the importance of the oxygen reduction reaction

Semiconductor photocatalysis could be an effective means to combat air pollution, especially nitrogen oxides, which can be mineralized to nitrate.

Completely integrated wirelessly-powered photocatalyst-coated spheres as a novel means to perform heterogeneous photocatalytic reactions

Photocatalytic reactions can be efficiently driven by completely integrated photocatalyst–light emitter units which are wirelessly powered using resonant inductive coupling.

Catalyst tolerance to SO2and water vapor of Mn based bimetallic oxides for NO deep oxidation by ozone

Improving the catalyst stabilities under different conditions (water vapor, SO₂, both water vapor and SO₂) is important for industrial applications regarding catalytic NO deep oxidation by ozone.

Enhanced activity of α-Fe2O3 for photocatalytic NO removal

Unique electrospun α-Fe₂O₃ fibers of singular nano-architecture were obtained exhibiting a highly enhanced NO conversion photocatalytic efficiency

One-step synthesis of hierarchical Ni–Fe–Al layered double hydroxide with excellent sensing properties for NOx at room temperature

Hierarchical flower-like Ni–Al-layered and Ni–Fe–Al-layered double hydroxide intercalation compounds were synthesized and showed good sensing for NO_x at room temperature.

Advances in photocatalytic NOx abatement through the use of Fe2O3/TiO2 nanocomposites

Supported Fe₂O₃/TiO₂ nanocomposites were prepared for the first time by a plasma-assisted route and successfully tested in photocatalytic NO_x abatement driven by solar illumination.