Active Center - Catalytic Center

A specific site on the surface of a solid catalyst that has catalytic activity. This is a basic concept proposed by HS Taylor in 1925. Until then, catalytic activity was thought to promote reactions based on an increase in the surface concentration of reactant molecules due to capillary condensation, but this theory states that the reaction proceeds when reactant molecules adsorb onto atoms with high degrees of unsaturation on the surface. This idea was supported by the fact that catalytic activity can be lost due to the binding of trace amounts of poison molecules (catalyst poisons) that cover only a small part of the surface, that catalytic activity is sensitive to surface structure, and activity changes significantly when treated by heating, and that the heat of adsorption decreases rapidly as the amount of adsorption increases. This idea of ​​active centers is in contrast to the multiplet theory proposed by AA Balandin and the lattice surface activity theory proposed by O. Beeck and others. The active center theory was developed into the idea of ​​lattice defect activity proposed by A. Smekal and the idea of ​​boundary line activity proposed by GM Schwab. The structure of the active sites is thought to differ depending on the reaction, based on its suitability for the adsorption and reaction of reactant molecules. Currently, it is believed that point defects such as vacancies and interstitial atoms, ends of dislocations, and surface steps and kinks can become active centers depending on the reaction, and the quantitative relationship with catalytic activity is being studied.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

固体触媒の表面にあって触媒作用をもつ特定の場所をいう．1925年，H.S. Taylorによって提唱された基本的概念である．触媒作用は，それまで毛管凝縮などによる反応分子の表面濃度の増大にもとづいて，反応を促進させるものと考えられていたが，この説により，表面上で結合の不飽和度の高い原子などの上に反応分子が吸着し，反応が進行するとされた．この考えは，表面の小部分を覆う程度の微量の毒物分子(触媒毒)の結合によって触媒作用が失われる場合があること，触媒作用が表面構造に敏感であり，加熱などの処理によりいちじるしく活性が変化することや，吸着量が増すにつれて，吸着熱が急激に減少する事実から支持された．この活性中心の考えはA.A. Balandinによる多重子説や，O. Beeckらを中心とした格子面活性説と対照的である．活性中心説はA. Smekalによる格子欠陥活性，G.M. Schwabによる界線活性の考えに発展した．活性点となる部分の構造は，反応分子の吸着や反応に適合することからみて，反応によって異なると考えられ，現在では，空格子点，格子間原子などの点欠陥や転位の末端，表面のステップやキンクが反応によって活性中心となるとみられ，触媒活性との定量的関係が研究されている．

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