Improving the Selectivity of Photocatalytic NOx Abatement through Improved O2 Reduction Pathways Using Ti0.909W0.091O2Nx Semiconductor Nanoparticles: From Characterization to Photocatalytic Performance

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.

Fabrication of three-dimensional hierarchical porous 2D/0D/2D g-C3N4 modified MXene-derived TiO2@C: Synergy effect of photocatalysis and H2O2 oxidation in NO removal

A novel three-dimensional multi-level porous g-C₃N₄ modified MXene-derived TiO₂@C aerogel (g-C₃N₄/TiO₂@C aerogel) was synthesized for NO removal. Through SEM analysis, 2D g-C₃N₄ and 2D Ti₃C₂ nanosheets were constructed into an interconnected macroscopic framework with continuous macropores via ice template. OD TiO₂ nanoparticles uniformly covered 2D C nanosheets with irregular mesopores and macropores in in-situ oxidation of Ti₃C₂ nanosheets by calcination via TEM analysis. g-C₃N₄/TiO₂@C aerogel for photocatalytic activation of hydrogen peroxide (H₂O₂) had an excellent efficiency of 90.7% for NO removal at parts per million level. This efficiency was 4.9 times and 7.8 times that of g-C₃N₄/TiO₂@C aerogel and H₂O₂ individually, due to the synergy between photocatalysis and H₂O₂ oxidation. Meantime, g-C₃N₄/TiO₂@C aerogel exhibited an enhanced performance compared with g-C₃N₄ nanosheet (55.7%) and TiO₂@C aerogel (38.5%). It was attributed to the large specific surface area (93.82 m²/g) with hierarchical mesoporous and macroporous structure and the 2D/OD/2D heterojunction of g-C₃N₄/TiO₂@C aerogel, further enhancing electron-hole separation. The mechanism was hypothesized that g-C₃N₄/TiO₂@C aerogel activated H₂O₂ to generate hydroxyl radicals (·OH) and superoxide radicals (·O₂^-) for oxidation of NO.

A Combination of EPR, Microscopy, Electrophoresis and Theory to Elucidate the Chemistry of W- and N-Doped TiO2 Nanoparticle/Water Interfaces

The doping of TiO2-based nanomaterials for semiconductor-sensitised photoreactions has been a practice extensively studied and applied for many years. The main goal remains the improvement of light harvesting capabilities under passive solar irradiation, that in the case of undoped TiO2 is limited and restricted to relatively low latitudes. The activity and selectivity of doped TiO2 photocatalysts are generally discussed on the basis of the modified band structure; energetics of intrinsic or extrinsic band gaps including trapping states; redox potentials of band edges, including band bending at solid/fluid interfaces; and charge carriers scavenging/transfer by/to adsorbed species. Electron (and hole) transfer to adsorbates is often invoked to justify the formation of highly reactive species (e.g., HO. from water); however, a complete description of the nanoparticle surface chemistry dictating adsorption/desorption events is often missing or overlooked. Here, we show that by employing a surface electrochemical triple-layer (TLM) approach for the nanoparticles/water interface, in combination with electron paramagnetic resonance spectroscopy (EPR), transmission electron microscopy and electrophoretic measurements, we can elucidate the surface chemistry of doped TiO2 nanoparticles and link it to the nature of the dopants. Exemplifying it for the cases of undoped, as well as W- and N-doped and codoped TiO2 nanoparticles, we show how surface charge density; surface, Stern and ζ potentials; surface acidity constants; and speciation of surface sites are influenced by the nature of the dopants and their loading.

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.

Recent Progress in the Abatement of Hazardous Pollutants Using Photocatalytic TiO2-Based Building Materials

Titanium dioxide (TiO2) has been extensively investigated in interdisciplinary research (such as catalysis, energy, environment, health, etc.) owing to its attractive physico-chemical properties, abundant nature, chemical/environmental stability, low-cost manufacturing, low toxicity, etc. Over time, TiO2-incorporated building/construction materials have been utilized for mitigating potential problems related to the environment and human health issues. However, there are challenges with regards to photocatalytic efficiency improvements, lab to industrial scaling up, and commercial product production. Several innovative approaches/strategies have been evolved towards TiO2 modification with the focus of improving its photocatalytic efficiency. Taking these aspects into consideration, research has focused on the utilization of many of these advanced TiO2 materials towards the development of construction materials such as concrete, mortar, pavements, paints, etc. This topical review focuses explicitly on capturing and highlighting research advancements in the last five years (mainly) (2014-2019) on the utilization of various modified TiO2 materials for the development of practical photocatalytic building materials (PBM). We briefly summarize the prospective applications of TiO2-based building materials (cement, mortar, concretes, paints, coating, etc.) with relevance to the removal of outdoor/indoor NOx and volatile organic compounds, self-cleaning of the surfaces, etc. As a concluding remark, we outline the challenges and make recommendations for the future outlook of further investigations and developments in this prosperous area.

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.