Nitrogen oxides (NO_x) are known to be harmful to the environment. It is necessary to process gases containing these oxides or gases originating from motor vehicles or fixed engines to convert them into nitrogen. One known treatment is SCR selective reduction.

Building energy consumption generally accounts for more than one third of the total social energy consumption. At the same time, building energy contributes up to 25% of the world's greenhouse gas emissions, and is one of the major greenhouse gas emission reduction households. Glass windows are the main channel for building to exchange light and heat with the outside world, and 50% of building energy consumption is carried out through glass windows. The heat absorption of building exterior walls also aggravates the phenomenon of heat island in urban center.

Tungsten oxide film is a widely studied functional material. It has excellent short-wavelength transmission, and the forbidden band width is easily adjusted by doping. Through ion implantation, ultraviolet photon irradiation, and gas molecule adsorption, its optical and electrical properties will change significantly, so it is displayed on optical glass and flat panel. There are wide application prospects in the fields of photoelectric conversion, electrochromism and photochromism.

Oxidation of tungsten disulfide refers to the process in which tungsten disulfide reacts with oxygen to produce sulfur dioxide and tungsten trioxide when heated in air. As one product of the oxidation, tungsten trioxide can form a protective film during the lubrication process so as to reduce the wear of the metal surface.

Oxidation is a chemical reaction between a substance and oxygen. Slow reaction of substance and oxygen is called eremacausis while the intense oxidation is called burning.

When heated in air, tungsten disulfide reacts with oxygen to form sulfur dioxide and tungsten trioxide. Its initial rate of slow oxidation is 460°C, and the initial rate of fast oxidation is 802°C. At 427°C, it starts to lose sulfur. The oxidation of tungsten disulfide is affected by temperature and particle size. The higher temperature and the smaller tungsten disulfide particles lead to the more intense reaction.

There are two products of the oxidation of tungsten disulfide:

1.Sulfur dioxide

Sulfur dioxide (SO₂) is colorless transparent gas with a pungent smell. It is soluble in water, ethanol and ether. It has a property of bleaching and is often used to bleach pulp, wool as well as silk. As it can inhibit the growth of mold and bacteria, it is usually used as a preservative.

However, sulfur dioxide is one of the major pollutants in the atmosphere. It is produced by the burning of traditional fuels such as coal and oil. With the dissolution and oxidation, it can form sulfuric acid (the main component of acid rain), doing harm to the environment.

2.Tungsten trioxide

Tungsten trioxide (WO₃) is yellow powder, being insoluble in water, being soluble in alkali and being slightly soluble in acid. It is used for making tungsten wires, fireproof materials and alloys with high melting points.

During the lubrication, tungsten trioxide is formed by the slow oxidation of tungsten disulfide under high temperature. As a protective film, tungsten trioxide can inhibit the further oxidation and prevent the fast loss of tungsten disulfide. At the same time, it can protect the surface of metal with its low friction coefficient.

Thermal stability of tungsten disulfide varies in different gaseous mediums or gas states. Tungsten disulfide can be heated and react not only in air but also in vacuum. Usually, hydrogen, argon and nitrogen are needed in the preparation of tungsten disulfide.

Thermal stability, also known as heat resistance, refers to the deformation capacity of substance under the influence of temperature. The smaller the deformation, the higher the thermal stability. Generally, we can judge the thermal stability of a substance by observing the phenomena during the heating.

Thermal stability of tungsten disulfide is as follows:

1. In air (O₂)

When heated in air, tungsten disulfide reacts with oxygen to form sulfur dioxide and tungsten trioxide with low friction coefficient. The higher temperature and the smaller tungsten disulfide particles lead to the more active reaction.

2. In vacuum

When heated in vacuum, tungsten disulfide can release a small amount of gas. It can slowly decompose into sulfur and tungsten when heated to above 1100°C. And when heated to 1250°C, it is decomposed completely.

3. Hydrogen (H₂)

Tungsten disulfide produces tungsten and hydrogen sulfide when heated to about 80°C in hydrogen. The chemical reaction is as follows:

1/2 WS₂ (solid) +H₂ (gas) ←→H₂S (gas) +1/2 W (solid)

The relationship of the equilibrium constant K and reaction temperature T at low temperature is different from that at high temperature, which is caused by different crystalline states of tungsten disulfide.

4. Nitrogen (N₂)

Tungsten disulfide is stable in nitrogen.

5. Argon (Ar)

Tungsten disulfide loses a small amount of sulfur when heated to more than 1000°C in argon.

Nanomaterials have become one of the hotspots of current research due to their unique physical and chemical properties, such as high specific surface area, quantum size effect, small size effect and macroscopic quantum tunneling effect. In recent years, many Nano-Research centers have been established in China, and some remarkable achievements have been made in catalysis, gas sensing and other fields.

In the field of semiconductor photocatalysis, titanium dioxide has been widely studied, but its bandgap is wide (3.2ev). Only ultraviolet light can excite it to produce photogenerated carriers. In sunlight, the proportion of ultraviolet light is very small, so the utilization of sunlight in pure titanium dioxide photocatalysis process is very low.

Tungsten oxide is a kind of tungstic anhydride and a kind of tungstate product. Tungsten oxide includes trioxide and tungsten dioxide. Tungsten oxide is partly used in the production of chemical products, such as paints and coatings, petroleum industrial catalysts, etc. But tungsten oxide is more an intermediate product and is widely used in the preparation of tungsten metal powder and tungsten carbide powder. It is used in the production of tungsten metal products and a large number of tungsten alloy products.

When a semiconductor irradiates light with energy above bandgap, the electrons of valence band excite in the conduction band, generate holes in the valence band, and generate electrons in the conduction band. They have strong oxidation and reduction abilities respectively, and have redox effects on molecular species in contact with semiconductors. Such an action is called photocatalysis, and such semiconductors are called photocatalysts.

The active atoms of tungsten oxide are located on the surface of the crystal, which greatly enlarges the contact area between the crystal surface and the gas, and can effectively improve the gas sensing performance. Tungsten oxide with one-dimensional nanowire structure attracts many researchers because of its huge specific surface area.