Estimation of Nitrogen Oxides (NOx) Removal Efficiency for TiO2 Concrete

Abstract

Nitrogen oxide (NOx) is a significant precursor of particulate matter (PM), particularly in high-traffic areas. Accordingly, this study aimed to reduce the presence of nitrogen oxide (NOx) along roadsides. Titanium dioxide (TiO2) is used as a photocatalyst to remove NOx through a chemical reaction. Typically, concrete and TiO2 are mixed to create TiO2 concrete. However, air pollutants or UV rays cannot be allowed to come into contact with a significant amount of TiO2. Thus, the TiO2 surface penetration method was used to fix TiO2 to the surface of the concrete. In this method, surface penetrants and TiO2 are combined and sprayed onto the concrete surface, enabling the possibility of NOx reduction using relatively less TiO2. When the fixation method is applied to vertical concrete structures, however, a peeling issue arises. To address this, a pressurized TiO2 fixation method was applied to vertical concrete structures. This method uses external force to penetrate and fix TiO2 to a specific depth. In the instance of the pressurized TiO2 fixation method, which was tried for the first time, penetration depth was used to ensure long-term durability as well as NOx removal efficiency. To investigate TiO2 distribution characteristics, the penetration depth and mass ratio of TiO2 particles in TiO2 concrete were measured using a scanning electron microscope (SEM/EDX). In addition, NOx removal efficiencies were evaluated using the NOx analyzing system (ISO 22197-1 standard). The experimental results showed that NOx removal efficiency increased with an increasing TiO2 mass ratio. When the TiO2 fixation method was used, the NOX removal efficiency was 32% when the TiO2 mass ratio at the surface was 50%, and the efficiency was 61% when the TiO2 mass ratio was 70%. This is attributed to the increase in NOx removal efficiency as TiO2 content at the concrete surface increases. This study analyzed and forecasted the NOx removal efficiency of TiO2 concrete based on the mass ratio of TiO2 on the surface. As the TiO2 mass ratio increased, the NOx removal efficiency improved, and it was determined that the surface TiO2 mass ratio significantly influences the NOx removal efficiency. Consequently, this study developed an equation to estimate NOx removal efficiency, making it possible to determine a suitable maintenance interval for TiO2 concrete.