Formaldehyde (HCHO) is a major indoor pollutant and long-term exposure to HCHO may cause health problems such as nasal tumors and skin irritation. Photocatalytic oxidation is considered as the most promising strategy for the decomposition of HCHO. Herein, for the first time, a direct g-C3N4–TiO2 Z-scheme photocatalyst without an electron mediator was prepared by a facile calcination route utilizing affordable P25 and urea as the feedstocks. Photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic oxidation decomposition of HCHO in air. It was shown that the photocatalytic activity of the prepared Z-scheme photocatalysts was highly dependent on the g-C3N4 content. At the optimal g-C3N4 content (sample U100 in this study), the apparent reaction rate constant was 7.36 × 10−2 min−1 for HCHO decomposition, which exceeded that of pure P25 (3.53 × 10−2 min−1) by a factor of 2.1. The enhanced photocatalytic activity could be ascribed to the formation of a g-C3N4–TiO2 Z-scheme photocatalyst, which results in the efficient space separation of photo-induced charge carriers. Considering the ease of the preparation method, this work will provide new insights into the design of high-performance Z-scheme photocatalysts for indoor air purification.

Enhanced photocatalytic performance of direct Z-scheme g-C3N4–TiO2 photocatalysts for the decomposition of formaldehyde in air

Nitrogen-doped titanium dioxide as visible-light-sensitive photocatalyst: designs, developments, and prospects.

Photocatalytic purification of volatile organic compounds in indoor air: A literature review

Photocatalysis has been attracting much research interest because of its wide applications in renewable energy and environmental remediation. There are many materials that are found to show good photocatalytic activity in the presence of ultraviolet (UV) and visible light. However, the applications of these materials are limited to the UV portion of sunlight. α-Fe 2 O 3 has an advantage over the other conventional materials like TiO 2 , ZnO, etc . in using solar energy for photocatalytic applications due to its lower band gap ∼2.2 eV value. As a result of which Fe 2 O 3 is capable of absorbing a large portion of the visible solar spectrum (absorbance edge ∼600 nm). Also its good chemical stability in aqueous medium, low cost, abundance and nontoxic nature makes it a promising material for photocatalytic water treatment and water splitting applications. Except these advantages the usage of Fe 2 O 3 has been restricted by many anomalies such as higher e–h recombination effect, low diffusion length and VB positioning (VB is positive with respect to H + /H 2 potential). This article reviews the research that has been carried out to overcome these basic limitations and to enhance the photocatalytic activity of α-Fe 2 O 3 .

α-Fe2O3 as a photocatalytic material: A review

TiO2 photocatalyst for water treatment applications

The generation of harmful by-products during photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) is a bottleneck problem for the application of PCO technology in indoor environment. Toluene is a typical VOC found in indoor air. In this work, the by-products at ppb level were studied during PCO decomposition of toluene in a plate-type reactor and identified using PTR-MS (proton transfer reaction-mass spectrometry) and GC–MS (gas chromatography–mass spectroscopy). The results indicated that benzaldehyde, methanol, acetaldehyde, acetone/propionaldehyde, formic acid/ethanol and acetic acid were the main by-products in the gas phase. By adjusting the concentration of water vapor, some compounds adsorbed on the TiO 2 surface were ascertained, which resulted in the deactivation of TiO 2 . They were benzoic acid, benzene, acrylaldehyde, butyraldehyde and pentanal. Some of these by-products have not been reported in the literature. Possible photocatalytic oxidation pathways of toluene were proposed. A health-related index (HRI) was introduced to assess the risk level to human health of these ppb-level by-products. It concludes that although some undesired by-products (even carcinogenic) are generated during PCO decomposition of toluene, it seems that these by-products do not have negative effects to human health because of their low concentrations.

Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene

Benzene, toluene and xylenes in newly renovated homes and associated health risk in Guangzhou, China

It was found that there were unwanted by-products in the photocatalytic oxidation (PCO) of indoor ppb-level toluene, a typical volatile organic compound (VOC) in indoor air. However, up to now the control mechanism of the generation of the products has not been clear. In this study we address the problem: a titania-coated glass-plate reactor was applied to study the by-products and decomposition rate; the by-products generated under a series of water vapor concentrations were instantaneously identified by proton transfer reaction-mass spectrometer (PTR-MS); the results indicate that water vapor has a significant effect not only on the photocatalytic decomposition rate of toluene, but also on its by-products generation; the competitive adsorption mechanism between water vapor, toluene and its by-products was analyzed. By-products may make the typical Langmuir–Hinshelwood model unfeasible in real application. A health risk assessment of the by-products was also introduced. The results show that the maximum decomposition efficiency does not always lead to minimal by-products and lowest health risk. What it does show is that when evaluating the performance of photocatalytic air purification, health risks posed by the by-products should be the primary concern rather than the decomposition efficiency.

Effect of water vapor on the by-products and decomposition rate of ppb-level toluene by photocatalytic oxidation

Thermal catalytic oxidation (TCO) technology can continuously degrade formaldehyde at room temperature without added energy. However, there is very little knowledge on the TCO kinetic reaction mech...

Qiujian Xu

Papers

Photocatalytic purification of volatile organic compounds in indoor air: A literature review

Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene

Benzene, toluene and xylenes in newly renovated homes and associated health risk in Guangzhou, China

Effect of water vapor on the by-products and decomposition rate of ppb-level toluene by photocatalytic oxidation

Indoor formaldehyde removal by thermal catalyst: kinetic characteristics, key parameters, and temperature influence.