Recent Progress and Current Status of Photocatalytic NO Removal

*Reshalaiti Hailili, Zelong Li, Xu Lu and Xiaokaiti Reyimu*

## **Abstract**

Air pollution has become a globally prominent environmental problem in which nitrogen oxide (NOx, 95% NO and NO2) has been considered as one of the most serious harmful gaseous pollutants that can cause haze, photochemical smog, and acid rain. Exposure to NO (ppb) harms human health with a risk of respiratory and cardiopulmonary diseases. As such, much attention is focused on the throughout removal, effective control, and precise monitoring of NO, especially for those of NO with low concentration (ppb). Semiconductor-based photocatalysis is a practical approach for pollutant treatments, especially for low concentrations but highly toxic ones, for example, NO (ppb) removal in indoor and outdoor atmospheres. This work aims to introduce the main process, methods and summarize the critical scientific issues during the photocatalytic NO treatment and review the latest progress in semiconducting materials. This work also surveys the newly emerged photocatalysts such as metal oxides, Bi-based semiconductors, including Bi2O2CO3, BiOX (X = Cl, Br, and I), Bi-metal-based defective photocatalysts, and other Bi-based catalysts with well-defined surface/interface characters for the complete NO removal, specific conversion mechanisms and controlling the generation of the toxic intermediate (NO2) is highlighted. The challenges/bottlenecks of the practical applications in the field are also highlighted at the end.

**Keywords:** photocatalysis, nitrogen oxides (NOx), visible light, microstructure, surface defects, selectivity

## **1. Introduction**

According to the World Health Organization (WHO) health report in 2021, air pollution caused about 7 million deaths worldwide, of which 4.2 million die prematurely every year by outdoor pollution, and 3.8 million by indoor pollution (**Figure 1a**) [1]. Air pollutants also affect local plants and animals'survival and cause environmental issues. Nitrogen oxides (NOx, mainly NO + NO2), primarily emitted from power stations, factories, and automobiles are regarded as the major source of atmospheric contaminations, which remarkably influence the tropospheric chemistry and become the main cause of the greenhouse effects, acid rain, photochemical smog, and PM2.5 [2]. Generally, with an unpaired electron {(σ2s)2 (σ\* 2s)2 (σ2px)<sup>2</sup> (π2py)2 (π2pz)<sup>2</sup> (π\* 2py)1 }, NO is chemically active and reacts with oxygen readily to generate NO2.

### **Figure 1.**

*(a) WHO 2021 report on the correlations between mortality and environmental pollution; (b) numbers and citations of publications about photocatalytic NO removal since 2010.*

However, the reaction activity significantly descends when the concentration of NO is lower than the ppm level and it can exist stably in the air for a long time. Long-term exposure of NO even at such a low level still causes serious respiratory diseases, such as lung cancer, emphysema, and asthma. Moreover, the concentration of NO in the indoor environment is up to 200 ppb, because of cooking or smoking activities. Concerning such severe harms, strict legislation and policies have been enacted to control the emissions of NO in many countries. For instance, in 2004, the USA claimed the maximum emission rate of NO is required not to exceed 553.5 mg m<sup>3</sup> for tangentially fired boilers [3]. The limit of NO from light-duty vehicles is set at 35 mg km<sup>1</sup> in China (GB 18352.6-2016) [4]. According to the WHO guideline, the lowest NOx concentration emission threshold under the current was set at 40 μg m<sup>3</sup> [5]. Thus, it is urgent to develop environmentally friendly, effective methods and evolution of technical measurements that could control emissions, decrease the concentrations, and reduce the harm of nitrogen oxides, for example, NO at the level of hundreds of ppb in indoor circumstances. Conventionally, methods such as physical/ chemical adsorption, advanced wet oxidation, and post-combustion reduction technologies such as selective catalytic reduction with NH3 (SCR-NH3) and hydrocarbons (SCR-HC), are used for minimizing the NO emission [6]. The chemical adsorption method refers to water or aqueous solutions of acids, bases, and salts to absorb the nitrogen oxides in the exhaust gas so that the exhaust gas can be purified. This method has low investment in equipment and low operating costs. However, the absorption efficiency is not high, and the purification effect is poor for the exhaust gas containing more NO, and it is not suitable for treating the exhaust gas with large volumes. SCR is also regarded as an efficient method to reduce the values of NO. For instance, Dong et al. reported the "anchoring (NH3/He plasma)-reduction (O2/N2 plasma)" plasmacatalytic circular system for the NO reduction at room temperature in the presence of O2 and revealed that NO was firstly oxidized to NO2 by plasma, then NO2 experienced part-disproportionation reactions during adsorption on oxygen vacancies, producing NO, N2O, N2, and nitrate [7]. However, these approaches require high temperatures, special handling systems, and sophisticated equipment to avoid NH3 slip, reducing agents, or cocatalysts, meanwhile suffering from high costs and yields of more toxic byproducts. Moreover, the above techniques are no longer suitable for the complete removal of dilute air pollutants, for example, gaseous NO at the ppb level. Utilizing endless solar light as the driving force, semiconductor-based photocatalysis has

### *Recent Progress and Current Status of Photocatalytic NO Removal DOI: http://dx.doi.org/10.5772/intechopen.112485*

received considerable attention to remove the atmospheric gaseous hazards from the atmosphere, especially for the ones in low concentration but highly toxic, for example, NO (ppb), in an economically attractive and environmentally friendly manner [8–10]. In view of the current state of NO removal, photocatalytic oxidation is the most studied way to reduce NO concentration from the air. As shown in **Figure 1b**, the number of papers on photocatalytic NO removal and their citations has rapidly increased in the past 13 years.
