Cerium Catalysts

The cerium catalyst refers to an elemental cerium or cerium compound which has a catalytic function. Cerium is a golden yellow metal with a low melting point and is very soft (its Mohs hardness is the lowest of all elements). The alkali metal (IA) family in the sixth cycle is extremely chemically active. Cerium is the most metallic of the known elements (including radioactive elements). The cerium produces a layer of gray-blue cerium oxide in the air, which ignites in less than a minute, emitting a deep purple-red flame, creating a complex oxide of cerium. Cerium can react violently with water. Put the cerium into the sink filled with water and an explosion will occur immediately. Cerium and ice with a temperature as low as -116 ℃ can react violently and the resulting cerium hydroxide is the most alkaline e non-radioactive alkali hydroxide. Metal cerium has no elemental form in nature, but is rarely distributed in the form of salt on land and in the ocean.

Figure 1. Cerium oxide catalyst

Applications:

At present, cerium catalysts are mainly used in the field of environmental protection.

Volatile organic compounds are widely found in life (such as toluene, formaldehyde, etc.) and are extremely harmful to humans and the environment. At present, methods for removing volatile organic compounds include absorption, combustion, and plasma purification. Among them, the low temperature plasma synergistic catalyst removes gas pollutants, which has the advantages of simple process, high efficiency and large processing capacity. Cerium oxide, as an oxide having a fluorescent structure, generally has good oxygen storage capacity and catalytic oxidation activity. The use of cerium oxide as a catalyst and the occurrence of low-temperature plasma synergy can be applied to the removal of volatile organic compounds.

Photocatalytic oxidation is a reaction that acts through a semiconductor photocatalyst excited by light to generate electron-hole pairs. The absorption threshold of TiO₂, a commonly used semiconductor material, is 387 nm. CeO₂ is an N-type semiconductor with a light absorption threshold of approximately 420 nm. The absorption threshold of CeO₂ is higher than that of TiO₂, indicating that it has good light absorption capacity. The forbidden band width of CeO₂ is 2.94 eV. When it is illuminated by light with a wavelength less than 420 nm, the photon energy is greater than the forbidden band width, and the electrons on the valence band are excited, jumping over the forbidden zone into the conduction band, and at the same time corresponding holes are formed on the belt. Due to the good oxygen storage capacity of cerium oxide, it has great application in the catalytic degradation of pollutants. In addition, when the semiconductor material ceria is used as a catalyst for catalytic oxidation, metal ions are usually doped. The purpose is to modify the cerium oxide, change its lattice structure, and move the absorption peak of cerium oxide to the visible light region, thereby enhancing the photocatalytic action of the semiconductor.

The nitrogen oxides (NOx: NO, NO₂ and N₂O) of air pollutants are receiving more and more attention due to their environmental impact. At present, selective catalytic reduction (NH₃-SCR) using ammonia is the most promising denitration technology. The catalyst with the transition element Mn active component exhibits good denitration performance. However, a single Mn-based low temperature SCR catalyst is easily poisoned and deactivated in flue gas containing SO₂. The addition of cerium to the Mn-based low-temperature SCR catalyst to form a manganese cerium catalyst increases the acidity of the catalyst, facilitates the formation of NH_4⁺, increases the activity of the catalyst, and increases the selectivity of N₂. Therefore, the supported MnCeO_x denitration catalyst can be used for the treatment of nitrogen oxides of air pollutants.

Figure 2. Cerium hydroxide catalyst

References

1. Gutierrez-Arzaluz, Mirellai. (2016). "Catalysts with cerium in a membrane reactor for the removal of formaldehyde pollutant from water effluents." Molecules 21(6), 668/1-668/13.
2. Wang, Peng; Bai. (2016), "Effect of the dimensions of carbon nanotube channels on copper-cobalt-cerium catalysts for higher alcohols synthesis." Catalysis Communications 75, 92-97.
3. Dobosz, Justyna. (2016), "Catalytic activity of cobalt and cerium catalysts supported on calcium hydroxyapatite in ethanol steam reforming." Polish Journal of Chemical Technology 18(3), 59-67.