Influence of inlet concentration and light intensity on the photocatalytic oxidation of nitrogen(II) oxide at the surface of Aeroxide® TiO2 P25

Sustainable nitrate production out of thin air: the photocatalytic oxidation of molecular nitrogen

Dinitrogen can be photocatalytically oxidized by TiO2 to nitrogen oxides and nitrates. This enables the sustainable production of fixed nitrogen essentially from thin air using sunlight.

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.

Intensification of Heterogeneous Photocatalytic Reactions Without Efficiency Losses: The Importance of Surface Catalysis

Advances in LED and photoreactor technology have brought semiconductor photocatalysis to the verge of feasibility of industrial application for the synthesis of value-added chemicals. However, the often observed efficiency losses under intensified illumination conditions still present a great challenge. This perspective discusses the origin of these efficiency losses and what needs to be done to prevent or counteract it and pave the way for efficient, intensified heterogeneous photocatalytic processes. The role of surface catalysis is particularly highlighted as one of the rate-limiting steps.

Graphic Abstract

A Holistic Approach to Model the Kinetics of Photocatalytic Reactions

Understanding and modeling kinetics is an essential part of the optimization and implementation of chemical reactions. In the case of photocatalytic reactions this is mostly done one-dimensionally, i.e., only considering the effect of one parameter at the same time. However, as discussed in this study, many of the relevant reaction parameters have mutual interdependencies that call for a holistic multi-dimensional approach to accurately model and understand their influence. Such an approach is described herein, and all the relevant equations given so that researchers can readily implement it to analyze and model their reactions.

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.

Photoelectrocatalytic activity of an ordered and vertically aligned TiO2 nanorod array/BDD heterojunction electrode

Rutile TiO₂ nanorod (TiNR) arrays were fabricated on a boron-doped diamond (BDD) substrate by a simple hydrothermal synthesis method. A fluorine-doped tin oxide (FTO) electrode grown with TiNR arrays was also prepared using the same technology for comparison. Field-emission scanning electron microscopy results show that oriented TiNR arrays can grow vertically on the surface of BDD and FTO electrodes. TiNR arrays grown on both electrodes had the same length (3μm). In comparison with the TiNR/FTO electrode, the TiNR/BDD electrode demonstrated a higher photoelectrocatalytic activity for the degradation of water and organic compounds, which is mostly attributed to the formation of a p-n heterojunction between the TiNR arrays and BDD at high potential, apart from the density of TiNR. A linear relationship between the photoelectrocatalytic current and the organic concentration can be observed on both electrodes. However, the linear range between net photoelectrocatalytic current values and organic compound concentrations for the TiNR/BDD electrode are much greater than those for the TiNR/FTO electrode, which makes the TiNR/BDD electrode a versatile material for the photocatalytic degradation and sensing of organic compounds.

Development of a versatile experimental setup for the evaluation of the photocatalytic properties of construction materials under realistic outdoor conditions

The interest on outdoor photocatalytic materials is growing in the last years. Nevertheless, most of the experimental devices designed for the assessment of their performance operate at controlled laboratory conditions, i.e., pollutant concentration, temperature, UV irradiation, and water vapor contents, far from those of real outdoor environments. The aim of the present study was the design and development of an experimental device for the continuous test of photocatalytic outdoor materials under sun irradiation using real outdoor air as feed, with the concomitant fluctuation of pollutant concentration, temperature, and water vapor content. A three-port measurement system based on two UV-transparent chambers was designed and built. A test chamber contained the photoactive element and a reference chamber to place the substrate without the photoactive element were employed. The third sampling point, placed outdoors, allowed the characterization of the surrounding air, which feeds the test chambers. Temperature, relative humidity (RH), and UV-A irradiance were monitored at each sampling point with specific sensors. NO x concentration was measured by a chemiluminescence NO x analyzer. Three automatic valves allowed the consecutive analysis of the concentration at the three points at fixed time intervals. The reliability of the analytical system was demonstrated by comparing the NO x concentration data with those obtained at the nearest weather station to the experimental device location. The use of a chamber-based reaction system leads to an attenuation of NO x and atmospheric parameter profiles, but maintaining the general trends. The air characterization results showed the wide operating window under which the photoactive materials should work outdoors, depending on the traffic intensity and the season, which are reproduced inside the test chambers. The designed system allows the measurement of the photoactivity of outdoor materials or the comparison of several samples at the same time. The suitability of the system for the evaluation of the DeNO x properties of construction elements at realistic outdoor conditions was demonstrated. The designed experimental device can be used 24/7 for testing materials under real fluctuations of NO x concentration, temperature, UV irradiation, and relative humidity and the presence of other outdoor air pollutants such as VOCs, SO x , or NH3. The chamber-based design allows comparing a photocatalytic material with respect to a reference substrate without the photoactive phase, or even the comparison of several outdoor elements at the same time.

Photocatalytic NOxabatement: why the selectivity matters

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

Purification of aromatic hydrocarbons via fibrous activated carbon/photocatalytic composite coupled with UV light-emitting diodes

In the present study, the applicability of light-emitting diodes (LEDs) to a fibrous activated carbon (FAC)/titania (TiO2) composite used for the purification of gas-phase benzene, toluene, ethyl benzene and xylene was evaluated. The surface and morphological properties of the FAC and prepared FAC/TiO2 composite were investigated by X-ray diffraction and scanning electron microscopy. The study protocol included two different tests, comparison of FAC and the FAC/TiO2 composite for BTEX removal efficiency and evaluation of the FAC/TiO2 composite for benzene and toluene removal efficiency under differing operation conditions. The time-series ratios obtained with the FAC/TiO2 composite showed longer removal times for all target compounds when compared to those of FAC alone. For both benzene and toluene, the breakthrough time decreased with the stream flow rate, whereas it increased with increasing light intensity under the experimental conditions employed in this study. The breakthrough time of benzene and toluene increased as increasing amounts of TiO2 were coated onto the surface of FAC between 11 and 229 mg-TiO2 (g-FAC)(-1), but decreased as the amount of TiO2 increased to 451 mg (g-FAC)(-1). These findings indicate that the optimal TiO2 weight for the removal process of benzene and toluene via the FAC/TiO2 composite is between 229 and 451 mg-TiO2 (g-FAC)(-1).

Chemistry of NOx on TiO2 Surfaces Studied by Ambient Pressure XPS: Products, Effect of UV Irradiation, Water, and Coadsorbed K(.)

Self-cleaning surfaces containing TiO2 nanoparticles have been postulated to efficiently remove NOx from the atmosphere. However, UV irradiation of NOx adsorbed on TiO2 also was shown to form harmful gas-phase byproducts such as HONO and N2O that may limit their depolluting potential. Ambient pressure XPS was used to study surface and gas-phase species formed during adsorption of NO2 on TiO2 and subsequent UV irradiation at λ = 365 nm. It is shown here that NO3(-), adsorbed on TiO2 as a byproduct of NO2 disproportionation, was quantitatively converted to surface NO2 and other reduced nitrogenated species under UV irradiation in the absence of moisture. When water vapor was present, a faster NO3(-) conversion occurred, leading to a net loss of surface-bound nitrogenated species. Strongly adsorbed NO3(-) in the vicinity of coadsorbed K(+) cations was stable under UV light, leading to an efficient capture of nitrogenated compounds.