Photocatalytic NOx Abatement Research Articles

Novel TiO2-Supported Gold Nanoflowers for Efficient Photocatalytic NOx Abatement.

In this study, we pioneered the synthesis of nanoflower-shaped TiO2-supported Au photocatalysts and investigated their properties. Au nanoflowers (Au NFs) were prepared by a Na-citrate and hydroquinone-based preparation method, followed by wet impregnation of the derived Au NFs on the surface of TiO2 nanorods (TNR). A uniform and homogeneous distribution of Au NFs was observed in the TNR + NF(0.7) sample (lower Na-citrate concentration), while their distribution was heterogeneous in the TNR + NF(1.4) sample (higher Na-citrate concentration). The UV-Vis DR spectra revealed the size- and shape-dependent optical properties of the Au NFs, with the LSPR effect observed in the visible region. The solid-state EPR spectra showed the presence of Ti3+, oxygen vacancies and electron interactions with organic compounds on the catalyst surface. In the case of the TNR + NF(0.7) sample, high photocatalytic activity was observed in the H2-assisted reduction of NO2 to N2 at room temperature under visible-light illumination. In contrast, the TNR + NF(1.4) catalyst as well as the heat-treated samples showed no ability to reduce NO2 under visible light, indicating the presence of deformed Au NFs limiting the LSPR effect. These results emphasized the importance of the choice of synthesis method, as this could strongly influence the photocatalytic activity of the Au NFs.

Fluoride-etched TiO2 microspheres modified with Al2O3 for enhanced photocatalytic conversion of NOx into nitrate ions

Photocatalysis holds promise for the removal of NOx from the air, yet its limited selectivity toward desired products (e.g. nitrate ions) hinders its widespread application. In this study, we synthesized and comprehensively characterized amorphous TiO2 microspheres (ATMS), fluoride-etched TMS (F-TMS), and Al2O3-modified F-TMS (Al2O3/F-TMS) photocatalysts. The photocatalytic NOx abatement tests revealed negligible photoactivity for the ATMS, with significant improvement observed after the fluoride ions etching using the hydrothermal method. However, the selectivity of F-TMS for NOx removal remained lower than that of TiO2 P25. The Al2O3/F-TMS photocatalyst exhibited a substantial increase in selectivity. The analysis of NOx abatement results indicated a 2.7-fold boost in selectivity compared to TiO2 P25. A reasonable correlation between the amount of chemically absorbed water and nitrate selectivity was observed. The electron paramagnetic resonance (EPR) disclosed that the fluoride etching induced the formation of intra-band-gap hole-trapping sites and thus enhanced the photocatalytic activity. Nevertheless, the selectivity improvement was mainly attributed to Al2O3's ability to stabilize the NOx oxidation intermediates at the surface for subsequent oxidation to nitrate ions. This study provides valuable insights for tailoring photocatalysts to enhance both activity and selectivity in photocatalytic NOx abatement.

2D-2D heterostructured composites Bi4O5Br2-SnS2 with boosted photocatalytic NOx abatement

A desirable photocatalytic system ought to feature efficient NOx removal and retardance of toxic NO2 generation. In this investigation, a series of novel two-denominational (2D)-2D composites Bi4O5Br2-SnS2 (X BS) were successfully fabricated through a simple mechanical ball-milling route and characterized systematically. The coexistence of both expected components was confirmed. 2D morphologies of both components were stacked in face-to-face manner and thus formed heterojunction structures, verified by SEM and TEM patterns. The visible-light absorption was enhanced in composites caused by the shrinkage of band gaps. With microstructural and optical merits, these binary composites showed reinforced photocatalytic abatement of NOx and attenuated generation of toxic NO2 under visible light, compared with bare Bi4O5Br2. Successive recycling catalytic experiments revealed the satisfactory reusability and structural stability of achieved composites. Ultimately, a reasonable type-II model was deduced to describe the migration and transfer of interface carriers based on experimental and analytical results. Such work designed and provided various bismuth-based composites constructed by the simple mechanical ball-milling for controlling and treating nitrogen oxides at ppb level in the atmospheric environment.

Methodology for Simultaneous Analysis of Photocatalytic deNOx Products

The ISO standard 22197-1:2016 used for the evaluation of the photocatalytic nitric oxide removal has a main drawback, which allows only the decrease of nitric oxide to be determined specifically. The remaining amount, expressed as “NO2”, is considered as a sum of HNO3, HONO NO2, and other nitrogen-containing species, which can be potentially formed during the photocatalytic reaction. Therefore, we developed a new methodology combining our custom-made analyzers, which can accurately determine the true NO2 and HONO species, with the conventional NO one. Their function was validated via a photocatalytic experiment in which 100 ppbv of either NO or NO2 dispersed in air passed over (3 L min−1) an Aeroxide© TiO2 P25 surface. The gas-phase analysis was complemented with the spectrophotometric determination of nitrates (NO3−) and/or nitrites (NO2−) deposited on the P25 layer. Importantly, an almost perfect mass balance (94%) of the photocatalytic NOx abatement was achieved. The use of custom-made analyzers enables to obtain (i) no interference, (ii) high sensitivity, (iii) good linearity in the relevant concentration range, (iv) rapid response, and (v) long-term stability. Therefore, our approach enables to reveal the reaction complexity and is highly recommended for the photocatalytic NOx testing.

Photocatalytic NOx abatement: Recent advances and emerging trends in the development of photocatalysts

The abatement of NOx emission, which is one of the most critical global environmental issues, is traditionally addressed using risky and costly approaches that require high reaction temperatures. To counter this drawback, much attention has been directed at the development of photocatalytic NOx abatement techniques, including photocatalytic oxidation, photocatalytic decomposition, and photo-selective catalytic reduction (photo-SCR), as they allow one to reduce the reaction temperature and increase conversion efficiency. Therefore, photocatalyst design is of crucial importance for the development of next-generation NOx abatement technologies. This review rationally summarizes and discusses recent progress in photocatalytic NOx abatement, highlighting catalyst type, removal pathway, and parameters governing the removal efficiency for each approach as well as considering strategies for the advancement of photocatalysts featuring high catalytic activities at low reaction temperatures for each process. Finally, the remaining challenges obstacles and directions for the future development of photocatalytic NOx abatement are provided.

A variational principle for the photocatalytic NOx abatement

Numerical study of NOx abatement in a photocatalytic reactor has been caught much attention recently. There are two ways for the numerical simulation, one is the CFD model, the other is the variational-based approach. The latter leads to a conservation algorithm with less requirement for the trial functions in the numerical study. In this paper we establish a variational principle for the problem, giving an alternative numerical method for NOx abatement.

Enhancement of photocatalytic NOx abatement on titania via additional metal oxide NOx-storage domains: Interplay between surface acidity, specific surface area, and humidity

In this work, we propose a simple and effective preparation procedure to obtain ternary mixed oxides composed of titania (TiO2, P25), alumina (γ-Al2O3) and calcium oxide (CaO) functioning as efficient photocatalytic NOx oxidation and storage (PHONOS) catalysts that are capable of facile NOx abatement under ambient conditions in the absence of elevated temperatures and pressures with UVA irradiation. In this architecture, titania was the photocatalytic active component and CaO and/or γ-Al2O3 provided NOx storage domains revealing dissimilar specific surface areas (SSA) and surface acidities. We show that photocatalyst formulation can be readily fine-tuned to achieve superior photocatalytic performance surpassing conventional P25 benchmark in short (1 h) and long term (12 h), as well as humidity-dependent photocatalytic tests. We demonstrate the delicate interplay between the surface acidity, SSA and humidity and provide detailed mechanistic insights regarding the origin of photocatalytic activity, selectivity and deactivation pathways.

Photocatalytic NOx abatement and self-cleaning performance of cementitious composites with g-C3N4 nanosheets under visible light

The combination of photocatalyst with cementitious materials for air pollution abatement and self-cleaning has gained considerable attention. However, the most popularly used photocatalytic cementitious composites based on TiO2 achieve the purification function under ultraviolet sunlight, significantly impeding a broader application of photocatalytic cementitious composites. Here, the photocatalytic cementitious composites were prepared by incorporating g-C3N4 nanosheets (CNNs) in Portland cement. With the increase of CNNs content, the photocatalytic NOx abatement and self-cleaning performance was notably improved, while the micro-hardness of cementitious composites increased first and then decreased. Among the three mixing dosages (0.5%, 1% and 2% by weight of cement), the cementitious composite admixed with 2% CNNs showed the highest photocatalytic NOx abatement efficiency of 227.3 μmol m−2 h−1, and could degrade the rhodamine B within 40 min under visible light. Additionally, its microscopic hardness was 5.4% higher than the one without CNNs, suggesting that a moderate amount of CNNs (below 2%) in cementitious composites was effective in achieving an improved photocatalytic depollution performance.

Photocatalytic NOx abatement: Mathematical modeling, CFD validation and reactor analysis.

A 2D CFD model was implemented for the numerical simulation of NOx abatement in a photocatalytic reactor, considering the effect of relative humidity (10-60%), light intensity (0.3-13 W⋅m-2) and inlet NO concentration (0.1-1.0 ppm). Significant differences of NOx concentration at the catalytic surface and bulk gas were found (Δmax of ∼12% and ∼16% for NO and NO2, respectively) and corrections were proposed to achieve intrinsic rate laws from a model available in the literature. An analysis of the reactor performance was conducted and a nonlinear behavior was observed when the channel height (H) was varied. A point of maximum for the integral rate of NO and NO2 consumption as a function of H was found (ΔNO of ∼2% and ∼-1% for H→2H→4H; [Formula: see text] of ∼46% and -8.5% for H→2H→4H). Additionally, the NO conversion decreased from ∼29% to ∼7% and the selectivity decreased from ∼85% to ∼80% (passing through a point of minimum at 2H) when the height was varied in the range H-4H. When comparing the results from the CFD simulations and the predictions of a plug flow model, deviations for NO conversion and selectivity increased with H (Δmax of ∼2% and ∼45%, respectively).

Titania nanosheet photocatalysts with dominantly exposed (001) reactive facets for photocatalytic NOx abatement

The particular role of reactive facets TiO2 on photocatalytic NOx removal was systematically explored. The photo selective catalytic reduction (photo-SCR) of NOx was studied using titania nanosheet photocatalysts. The titania nanosheets were synthesized by hydrothermal method with titanium n-butoxide (TBOT) and hydrofluoric acid (HF). The synthetic pathway of transformation from TBOT (Ti precursor) to TiO2 nanosheet was clearly revealed that TiOF2 is the intermediate during the hydrothermal process. The NO conversion of titania nanosheets was found to be higher than the conversion of commercial P25 and TiO2 synthesized by the sol-gel method. Moreover, one of the titania nanosheets, FT1.5, displayed the highest efficiency of NOx removal at 393K, but there was a trade-off relationship between NO conversion and NO2 selectivity. Our results suggest that the optimal synthetic process for highest efficiency of TiO2 nanosheet in photocatalysis is to use TBOT and HF with the F/Ti atomic ratio=1.5 (FT1.5) at 453K for 24h. In summary, the synthesized TiO2 nanosheet exhibits great potential to be applied in NOx abatement.

Durability of Cementitious Photocatalytic Building Materials

Photocatalytic building materials have been shown to be a promising remediation technique for air pollutants such as NOx and VOC’s. However, there are still doubts and questions about the durability of the air cleaning characteristic in time. In this respect, a pre-normative research is being undertaken at BRRC as a first step towards evaluation of the life time of these photo-active air purifying products. This is done by combining and adapting existing durability tests for cement-based materials with standardized test methods to determine the photocatalytic activity. First results of this research are presented here, including a state-of-the-art on accelerated ageing testing as well as the validation of applicable ageing procedures in combination with a normalized evaluation test for photocatalytic NOx abatement. Finally, recommendations for future work regarding durability testing on photocatalytic building materials are also given.

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 NOx abatement driven by solar illumination.

Photocatalytic NOx abatement by calcium aluminate cements modified with TiO2: Improved NO2 conversion

Photocatalytic activity of TiO2 was studied in two types of calcium aluminate cement (CAC) under two different curing regimes. The effect of the TiO2 addition on the setting time, consistency and mechanical properties of the CACs was evaluated. The abatement of gaseous pollutants (NOx) under UV irradiation was also assessed. These cementitious matrices were found to successfully retain NO2: more abundant presence of aluminates in white cement (w-CAC, iron-lean) helped to better adsorb NO2, thus improving the conversion performance of the catalyst resulting in a larger NOx removal under UV irradiation. As evidenced by XRD, SEM, EDAX and zeta potential analyses, the presence of ferrite in dark cement (d-CAC, iron-reach) induced a certain chemical interaction with TiO2. The experimental findings suggest the formation of new iron titanate phases, namely pseudobrookite. The reduced band-gap energy of these compounds compared with that of TiO2 accounts for the photocatalytic activity of these samples.

TiO2–Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement

TiO2–Al2O3 binary oxide surfaces were utilized in order to develop an alternative photocatalytic NOx abatement approach, where TiO2 sites were used for ambient photocatalytic oxidation of NO with O2 and alumina sites were exploited for NOx storage. Chemical, crystallographic and electronic structure of the TiO2–Al2O3 binary oxide surfaces were characterized (via BET surface area measurements, XRD, Raman spectroscopy and DR-UV-Vis Spectroscopy) as a function of the TiO2 loading in the mixture as well as the calcination temperature used in the synthesis protocol. 0.5 Ti/Al-900 photocatalyst showed remarkable photocatalytic NOx oxidation and storage performance, which was found to be much superior to that of a Degussa P25 industrial benchmark photocatalyst (i.e. 160% higher NOx storage and 55% lower NO2(g) release to the atmosphere). Our results indicate that the onset of the photocatalytic NOx abatement activity is concomitant to the switch between amorphous to a crystalline phase with an electronic band gap within 3.05–3.10eV; where the most active photocatalyst revealed predominantly rutile phase together and anatase as the minority phase.

Photocatalytic NOxabatement: why the selectivity matters

Photocatalytic NOxabatement is typically accompanied by the release of several potentially dangerous intermediates during the reaction. Herein, we discuss and offer solutions to this problem

Photocatalytic NOx abatement: The role of the material supporting the TiO2 active layer

The importance of the choice of a suitable substrate as supporting material for photoactive TiO2, either in the form of powders or thin films or in photoactive paints, is frequently disregarded. In this paper four different supports (stainless steel, sand-blasted stainless steel, Teflon and glass) are object of investigation. The final aim is to verify the presence of interactions between the photocatalyst (AEROXIDE® TiO2 P25 by Evonik Degussa Corporation) and the support, directly involved in the photocatalytic activity in the NOx abatement. The characterization results have been correlated with the photoactivity of the different samples. In particular, a coating of about 6–9μm seems to allow a photocatalytic result free from any positive or negative interference with the supporting material, therefore giving reliable results about the photoactivity of the TiO2 under investigation.