Two-dimensional materials have emerged as highly efficient photocatalysts for the degradation of nitrogen oxides (NOx) under sunlight, offering advantages such as high surface area, tunable electronic properties, and enhanced light absorption. Among these, titanium dioxide (TiO₂) nanosheets and bismuth oxyhalides (BiOX, where X = Cl, Br, I) are particularly notable due to their unique structural and electronic characteristics that facilitate photocatalytic NOx removal. This article examines the charge carrier dynamics, surface adsorption mechanisms, and durability of these materials under humid conditions, while contrasting their performance with thermal catalytic reduction methods.

The photocatalytic activity of 2D materials is governed by their ability to generate and separate electron-hole pairs upon sunlight irradiation. TiO₂ nanosheets, particularly those with exposed {001} facets, exhibit superior charge separation due to their well-defined crystalline structure and reduced recombination rates. The high surface energy of these facets promotes the adsorption of oxygen and water molecules, which act as electron acceptors and hole scavengers, respectively. Under UV and visible light, photogenerated electrons reduce adsorbed oxygen to form superoxide radicals (•O₂⁻), while holes oxidize water to produce hydroxyl radicals (•OH). These reactive oxygen species (ROS) subsequently oxidize NO to NO₂ and further to nitrate (NO₃⁻), completing the degradation process.

BiOX materials, on the other hand, possess a layered structure with [Bi₂O₂] slabs interleaved by double halogen layers, which induces an internal electric field that enhances charge separation. The bandgap of BiOX can be tuned by varying the halogen component, with BiOBr and BiOI exhibiting visible-light absorption due to their narrower bandgaps compared to BiOCl. The valence band positions of BiOX are sufficiently positive to generate •OH radicals, while the conduction band levels allow for oxygen reduction. The anisotropic charge transport in BiOX reduces recombination losses, further improving photocatalytic efficiency.

Surface adsorption plays a critical role in NOx degradation, as the initial interaction between NOx molecules and the catalyst surface determines the subsequent reaction pathways. TiO₂ nanosheets exhibit strong adsorption of NO due to the presence of surface hydroxyl groups and oxygen vacancies, which act as active sites. The adsorption is further enhanced under humid conditions, as water molecules facilitate the formation of intermediate species such as nitrous acid (HNO₂) and nitric acid (HNO₃). BiOX materials, particularly BiOI, demonstrate high affinity for NOx adsorption due to their polar surfaces and the presence of iodide ions, which promote the formation of stable nitrate species.

Durability under humid conditions is a key consideration for practical applications. TiO₂ nanosheets maintain their photocatalytic activity over extended periods due to their chemical stability and resistance to photocorrosion. However, the accumulation of nitrate species on the surface can lead to deactivation over time, necessitating periodic washing or regeneration. BiOX materials, while highly active, are more susceptible to halogen loss under prolonged irradiation, especially in the presence of water. Strategies such as hybridization with carbon materials or the formation of heterojunctions have been employed to enhance their stability.

In contrast to photocatalytic degradation, thermal catalytic reduction of NOx typically involves the use of noble metals (e.g., Pt, Pd) or metal oxides (e.g., CeO₂, CuO) supported on high-surface-area carriers. These catalysts operate at elevated temperatures (200–400°C) and rely on reducing agents such as ammonia (NH₃) or hydrocarbons to convert NOx into nitrogen (N₂) and water. While thermal catalysis offers high conversion efficiencies, it is energy-intensive and suffers from catalyst poisoning due to sulfur oxides (SOx) and particulate matter in exhaust streams. Photocatalysis, by comparison, operates at ambient temperatures and utilizes sunlight as the energy source, making it more sustainable for decentralized applications.

The charge carrier dynamics in thermal catalysis differ significantly from photocatalysis. In thermal systems, the reaction proceeds via Langmuir-Hinshelwood or Eley-Rideal mechanisms, where adsorbed NOx and reducing agents react on the catalyst surface. The process is limited by mass transfer and the availability of active sites, whereas photocatalysis leverages light-generated charge carriers to drive redox reactions independently of external reducing agents. Additionally, thermal catalysts often require precise control of temperature and gas composition to avoid side reactions, whereas photocatalysts are more tolerant to fluctuating conditions.

Humidity effects also vary between the two approaches. In thermal catalysis, water vapor can compete with reactants for adsorption sites, reducing efficiency. In photocatalysis, water serves as a hole scavenger, enhancing the formation of •OH radicals and improving NOx degradation. However, excessive humidity can lead to the blocking of active sites or the dissolution of BiOX materials, necessitating careful optimization of reaction conditions.

The development of 2D material photocatalysts for NOx degradation represents a promising avenue for air pollution control, combining high activity with environmental sustainability. Future research should focus on improving the durability of these materials under realistic conditions, as well as scaling up synthesis methods for industrial applications. By leveraging the unique properties of TiO₂ nanosheets and BiOX, alongside advances in material design and reactor engineering, photocatalytic NOx removal can become a viable complement to traditional thermal catalytic methods.