Experimental study of photocatalytic concrete products for air purification

Abstract

Air quality in inner-city areas is a topic which receives much attention nowadays but in the coming years, the overall interest on this topic will become even bigger. One major concern is caused by the reduction of the limiting values given by the European Council Directive 1999/30/EC [Relating to limit values for sulphur dioxide, nitrogen dioxide and oxides of nitrogen, particulate matter and lead in ambient air. Official Journal of the European Communities 1999, L 163/41–60] and increasing traffic rates especially for diesel powered passenger cars and freight vehicles.

A promising approach for solving the problem of nitrogen oxides (NO_x) is the photochemical conversion of nitrogen oxides to low concentrated nitrates due to heterogeneous photocatalytic oxidation (PCO) using titanium dioxide (TiO₂) as photocatalyst. A variety of products containing TiO₂ are already available on the European market and their working mechanism under laboratory conditions is proven. However, there is still a lack of transforming the experimental results obtained under laboratory conditions to practical applications considering real world conditions.

This paper presents the research conducted on photocatalytic concrete products with respect to the evaluation of air-purifying properties. The degradation process of nitric oxide (NO) under laboratory conditions is studied using a test setup for measuring the performance of photocatalytic active concrete products. The test setup uses the UV-A induced degradation of NO and is oriented on the ISO standard ISO 22197-1:2007. Besides the introduction of the test setup, a uniform measuring procedure is presented to the reader which allows for an evaluation and direct comparison of the performance of photocatalytic active concrete products. This kind of direct comparison was not possible so far. Furthermore, the results of a comparative study on varying photocatalytic concrete products of the European market will be discussed.

Introduction

Air pollution caused by road traffic and industry is one of the major problems in metropolitan and urban areas. Despite intensifying emission control requirements (e.g. [5]) and the increased installation of emission reduction systems, the air pollution and in particular the pollution by nitric oxides (NO) from diesel engines will remain a serious issue in the near future (cp. Table 1). The by far largest emissions are generated by local traffic and industrial flue gases (cp. Fig. 1). In this respect, attempts regarding the active reduction of NO can be found in forms of filter devices for industrial stacks (denitrogenization – DENOX plants) or active filter systems for e.g. ventilation outlets of tunnels. According to Matsuda et al. [18], a further solution is the photocatalytic oxidation of nitrogen oxides (NO_x) to low concentrated nitrates. The accruing reaction products in form of nitrate compounds are water soluble and will be flushed from the active concrete surface by rain. The nitrate compounds can be subsequently extracted from the rain water by a standard sewage plant.

When earlier work mainly focused on the treatment of waste water, PCO has received considerable attention regarding the removal of air pollutants during the last years. Since the middle of the 1990s efforts have been made, first in Japan, in large scale applications of this photocatalytic reaction for air-purifying purposes and self-cleaning applications. The construction industry provides several products containing photocatalytic materials since the middle of the 1990s on commercial basis. These products are for example window glass and ceramic tiles providing self-cleaning features. The utilization of the self-cleaning abilities of modified blends of cement was used for the first time in 1998 for the construction of the church “Dives in Misericordia” in Rome.

For the degradation of exhaust gases originated from traffic, a sheet-like application close to the source is desirable. Suitable surfaces for applications which are close to the source of the emission can be found in large illuminated surfaces in the direct road environment. These are for example noise barrier walls and the road or sidewalk surfaces itself. The production of the first concrete paving blocks specially designed for the degradation of exhaust gases started in 1997 in Japan. In 2002, investigations to the application of a cement based asphalt slurry seal have been conducted in Italy.

Titanium dioxide (TiO₂) in low concentration is applied as photocatalyst for the photocatalytic oxidation process. This photocatalyst appears to be the most suitable semiconducting material and is making use of the UV-A part of the sunlight for the chemical conversion of NO_x. TiO₂ is one of the oxides of titanium, also called rutile titanium white. It appears in remarkable extent in nature – the ninth most abundant element in the earth's crust. In solid state, TiO₂ can appear in three different crystalline modifications namely: rutile (tetragonal), anatase (tetragonal) and the seldom brookite (orthorhombic). However, only the optoelectronic properties of the anatase modification turn this crystalline modification to the best suitable photocatalytic material. On the one hand, the semiconductor band gap E_g of 3.2 eV is wide and on the other hand the oxidizing potential of the valence band is with 3.1 eV (at pH = 0) relatively high. Both facts lead to the conclusion that multitude of organic and inorganic molecules can be oxidized in the presence of TiO₂ and UV-A light. This also applies for low oxidizable molecules. Besides the high photocatalytic efficiency of TiO₂ due to its material properties, anatase is suitable for the photocatalytic degradation process because it is chemically stable, harmless and, compared to other semiconductor metal oxides, relatively cheap.

The PCO is induced by the transfer of electrons from the valence band to the conduction band by photons in the UV-A range. The UV-A absorption creates electron holes which are responsible for the formation of radicals and charged species such as ${\text{OH}}^{}$, ${\text{O}}_2^{-}$, ${\text{HO}}_2^{}$. The generation of hydroxyl radicals results from the presence of water at the surface of the photocatalyst. For this purpose, a certain amount of water molecules, supplied by the relative humidity, and electromagnetic radiation is required to start the degradation process. The electromagnetic radiation E is expressed by the product of Planck's constant h and the frequency f [9]:

$${\text{TiO}}_2\stackrel{hf}{\to }{\text{TiO}}_2+{\text{e}}^{-}+{\text{h}}^{+}$$

$${\text{H}}_2\text{O}\leftrightarrow {\text{OH}}^{-}+{\text{H}}^{+}$$

$${\text{OH}}^{-}+{\text{h}}^{+}\to {\text{OH}}^{}$$

The formed hydroxyl radicals act as a strong oxidant for organic and inorganic compounds in further reactions. According to Herrmann et al. [9], the most reactive species are hydroxyl radicals which are the best oxidizing species after fluorine. The further reactions are being subject to the heterogeneous photocatalysis and characterized by the adsorption of the precursor and the desorption of the reaction products.

In this paper, a setup and a measuring procedure for the evaluation of photocatalytic concrete products is described and employed. Using this setup, the effect of influencing parameters on the degradation of NO is studied and an appropriate measuring procedure for the direct comparison of photocatalytic concrete products is presented. The suggested measuring procedure was used for the assessment of photocatalytic concrete products which render a representative overview of the European market.