The effect of low temperature on the activity of diesel engine catalyst is essentially due to the dual changes of the chemical reaction and physical structure of the catalyst surface in the cold state. When the ambient temperature drops sharply, the energy threshold required for the catalytic reaction becomes difficult to cross at the initial start of the diesel engine, which makes the catalyst unable to function as efficiently as at room temperature or high temperature, thereby triggering a series of chain reactions, resulting in a significant decrease in activity.
From the perspective of chemical reaction kinetics, low temperature directly inhibits the redox reaction rate on the catalyst surface. Harmful components in diesel engine exhaust, such as nitrogen oxides, carbon monoxide and hydrocarbons, need to undergo oxidation or reduction reactions under the action of catalysts to be converted into harmless substances. These reactions require sufficient activation energy. At low temperatures, the movement speed of exhaust gas molecules slows down, and the collision frequency and energy with the active sites of the catalyst are reduced, making it difficult for the reaction to proceed fully. Even if the catalyst itself has active centers, it cannot trigger the reaction smoothly due to the lack of sufficient "vitality" between molecules, just like an engine without sufficient fuel is difficult to run continuously.
Low temperature also affects the physical properties of the diesel engine catalyst carrier. Catalysts are usually loaded on carriers with high specific surface area, and the pore structure of the carrier is crucial to the adsorption and diffusion of the reaction gas. In a low-temperature environment, the thermal stability of the carrier material decreases, and some pores may shrink or even close, making it difficult for exhaust gas molecules to enter the catalyst and contact the active sites. This change in physical structure is like blocking the "channel" for gas to enter the reaction site, which reduces the effective reaction area of the catalyst and makes a large number of active sites "idle", ultimately leading to a decrease in overall catalytic efficiency.
The condensation of water on the surface of diesel engine catalyst is also an important reason for the decrease in activity at low temperatures. Diesel engine exhaust contains a large amount of water vapor. At low temperatures, these water vapors are very easy to condense into liquid water or frost on the surface of the catalyst. Liquid water not only occupies the active sites of the catalyst and hinders the adsorption of exhaust gas molecules, but also may combine with acidic gases in the exhaust gas to form corrosive substances and destroy the active structure of the catalyst. The formation of frost will further change the microscopic morphology of the catalyst surface, affect gas diffusion, and make the activity of the catalyst worse.
Low temperature will also change the atomization and combustion characteristics of diesel. Under cold conditions, the viscosity of diesel increases and the atomization effect becomes worse, resulting in incomplete combustion and an increase in unburned hydrocarbons and particulate matter in the exhaust gas. These incompletely burned substances will deposit on the surface of the catalyst, forming carbon deposits or blocking pores, covering active sites, just like putting a layer of "shackles" on the catalyst, making it difficult to exert catalytic effect. Moreover, unburned fuel may also react with the catalyst, causing catalyst poisoning and further reducing its activity.
In addition, the electron transfer efficiency inside the catalyst will also be affected at low temperatures. The essence of catalytic reaction is the transfer process of electrons between reactants and catalysts, and low temperature will hinder the smooth movement of electrons. The reduction in electron transfer efficiency makes it difficult for the catalyst to quickly activate the reactant molecules, prolonging the reaction time and reducing the reaction rate. Just as the current transmission in the circuit is not smooth and the electrical equipment cannot work properly, the catalyst cannot efficiently complete the catalytic task due to the problem of electron transfer.
Although low temperature brings many challenges to the activity of diesel engine catalyst, researchers are trying to break through this problem by improving catalyst formula, optimizing carrier structure and developing new materials with strong low temperature adaptability. In the future, with the continuous advancement of technology, it is expected to develop catalysts that can still maintain high efficiency and activity in low temperature environments, so that diesel engines can also achieve clean emissions under cold conditions.
