A catalyst is a substance that converts reactants into products through an uninterrupted repeating cycle of elementary reaction steps, with the catalyst being regenerated to its original state in the final step of the cycle. More simply: “A catalyst is a substance that accelerates a chemical reaction without itself being consumed in the process.” Many types of substances can be used as catalysts, such as metals, metal oxides, organometallic complexes, and enzymes . Catalysis has become the core science for regulating the rate and direction of chemical reactions.
To understand the nature of catalysis, the first is to study the active center/active phase of the catalyst, that is, to unlock the secret of the so-called “black box”; Use various modern physical and chemical methods to obtain basic information at the atomic and molecular level in real time and real space for research.
In 1925, H.S.Taylor proposed the concept of active center, which means that the catalytic “site” is not the entire surface of the catalyst, but some specific “sites” of the catalyst, namely the active center/active phase.
Temperature Programmed Desorption (TPD), is to take a catalyst that has pre-adsorbed a certain gas molecule, and pass a gas (usually an inert gas, such as He gas) with a stable flow rate under the programmed heating temperature. Molecules on the catalyst surface are desorbed at a certain temperature, and the desorption speed increases as the temperature increases, and the desorption is completed after a high value. For the desorbed gas, the thermal conductivity detector can be used to detect the relationship between the concentration and the temperature, and the TPD curve can be obtained. The desorbed alkaline gas is absorbed by acid, and the amount of acid consumed can be obtained by titration, thereby obtaining the total acid amount of the catalyst. The titration step can also be automatically completed by the chemical adsorption instrument, and the accurate desorption amount can be directly obtained.
TPR (Temperature-Programmed Reduction) is developed on the basis of TPD. It can provide information on the interaction of metal oxides with each other or between metal oxides and supports during the reduction process of supported metal catalysts. The method and principle are as follows: a pure metal oxide has a specific reduction temperature, which can be used to characterize the properties of the oxide. If another oxide is introduced into the oxide, the two oxides are mixed together. If each oxide keeps its own reduction temperature constant during TPR, they have no effect on each other.
Conversely, if the solid-phase reaction of the two oxidations occurs, and the properties of the oxides change, the original reduction temperature will also change. This change can be recorded with the TPR technique. Therefore, TPR is an effective method to study the interaction between metal oxides and metal oxides and between metal oxides and supports in supported catalysts.
In the hydrocarbon reaction, hydrocarbons are reduced to carbon and deposited on the surface of the catalyst, and the deposited carbon is called coke. Due to carbon deposition, the catalyst activity is attenuated. Therefore, it is of great significance to study the kinetics and reaction mechanism of carbon deposits to reduce the occurrence of carbon deposits and prolong the life of catalysts. Models have been proposed for the study of the mechanism of carbon on the surface of single crystals. But for commonly used catalysts, the relationship between the metal surface structure and carbon deposition is more complicated due to the role of the carrier. TPO (Temperature Programmed0xidization) is a sensitive method to study catalyst coking and its correlation with reaction performance.
Solid surface acid sites can generally be regarded as active sites on the surface of oxide catalysts. In many catalytic reactions such as catalytic cracking, isomerization, polymerization and other reactions, hydrocarbon molecules interact with surface acid sites to form carbon ions, which are intermediate species of the reaction. The carbanion theory can successfully explain the reaction of hydrocarbons on acidic surfaces, and also provides a strong proof for the existence of acid sites.
To characterize the properties of solid acid catalysts, it is necessary to determine the type of surface acid sites (Lewis acid, Bronsted acid), strength and amount of acid. There are many methods for determining surface acidity, such as alkali titration, alkaline gas adsorption, thermal difference method, etc., but none of these methods can distinguish L acid and B acid sites. AMI-300IR in-situ infrared & temperature programmed chemical adsorption instrument is used to study the surface acidity of solid catalysts. It can effectively distinguish L acid and B acid. In this method, basic adsorbates such as ammonia, pyridine, trimethylamine, n-butylamine, etc. to characterize the acid site, among which pyridine and ammonia are widely used.
