Catalysis | 催化 - aleveler.com

📚 Catalysis | 催化

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. It provides an alternative reaction pathway with a lower activation energy, allowing a greater proportion of reactant particles to have sufficient energy to react at a given temperature. Understanding catalysis is essential for A-Level Edexcel Chemistry, as it links energetics, kinetics, and industrial processes.

催化剂是一种能提高化学反应速率而本身在反应过程中不被消耗的物质。它提供了一条活化能更低的替代反应路径，使得在给定温度下有更多反应物粒子具有足够的能量发生反应。理解催化对 A-Level Edexcel 化学至关重要，因为它将能量学、动力学和工业过程联系在一起。

1. What is a Catalyst? | 什么是催化剂？

A catalyst is a substance that alters the rate of a chemical reaction without undergoing any permanent chemical change itself. It remains chemically unchanged at the end of the reaction and can be recovered. Crucially, a catalyst does not alter the equilibrium position or the enthalpy change of a reaction; it only affects the rate at which equilibrium is reached.

催化剂是一种能改变化学反应速率而自身不发生永久性化学变化的物质。它在反应结束时保持化学性质不变并可回收。至关重要的是，催化剂不改变反应的平衡位置或焓变，只影响达到平衡的速率。

Catalysts work by providing an alternative route from reactants to products, one with a lower activation energy (Eₐ). This means that at a given temperature, a larger fraction of molecules have kinetic energy equal to or greater than Eₐ, so more successful collisions occur per unit time, increasing the reaction rate.

催化剂通过提供一条从反应物到产物的替代路径来起作用，该路径的活化能 (Eₐ) 更低。这意味着在给定温度下，有更大比例的分子具有大于或等于 Eₐ 的动能，因此单位时间内发生更多的有效碰撞，从而提高反应速率。

2. Types of Catalysis: Homogeneous vs. Heterogeneous | 催化类型：均相与非均相

Catalysis can be broadly classified into two types based on the physical states of the catalyst and the reactants. In homogeneous catalysis, the catalyst is in the same phase (usually liquid or gas) as the reactants. In heterogeneous catalysis, the catalyst is in a different phase, most commonly a solid catalyst with liquid or gaseous reactants.

催化可以根据催化剂与反应物的物理状态大致分为两类。在均相催化中，催化剂与反应物处于同一相（通常是液相或气相）。在非均相催化中，催化剂处于不同相，最常见的是固体催化剂与液体或气体反应物。

Homogeneous catalysts often work by forming an intermediate species that reacts further to give the product and regenerate the catalyst. Heterogeneous catalysts typically work by adsorbing reactant molecules onto their surface, where bonds are weakened or broken, facilitating reaction.

均相催化剂通常通过形成一种中间体来起作用，该中间体进一步反应生成产物并再生催化剂。非均相催化剂通常通过将反应物分子吸附到其表面上来起作用，在表面上化学键被削弱或断裂，从而促进反应。

3. How Catalysts Work: Energy Profiles | 催化剂如何工作：能量曲线

An energy profile diagram for a catalysed reaction shows two curves: the original uncatalysed pathway with a higher activation energy hump, and the catalysed pathway with a lower activation energy hump. The enthalpy change (ΔH) remains the same for both pathways, as the catalyst does not affect the energies of reactants or products.

催化反应的能量曲线图显示两条曲线：原始的非催化路径具有较高的活化能峰，而催化路径具有较低的活化能峰。两条路径的焓变 (ΔH) 相同，因为催化剂不会影响反应物或产物的能量。

For an exothermic reaction, the catalysed path has a smaller Eₐ(fwd) for the forward reaction. For a reversible reaction, the catalyst lowers the activation energy for both the forward and backward reactions equally, so the equilibrium constant Kc remains unchanged.

对于放热反应，催化路径的正向反应活化能 Eₐ(正向) 更小。对于可逆反应，催化剂同等程度地降低正向和逆向反应的活化能，因此平衡常数 Kc 保持不变。

4. Heterogeneous Catalysis: Surface Adsorption | 非均相催化：表面吸附

Heterogeneous catalysis involves a solid catalyst and gas or liquid reactants. The process generally follows these steps: reactant molecules adsorb onto active sites on the catalyst surface, forming weak bonds with surface atoms. This adsorption weakens intramolecular bonds in the reactants or holds them in a favourable orientation, so that reaction can occur with a lower activation energy. Finally, product molecules desorb from the surface.

非均相催化涉及固体催化剂以及气体或液体反应物。该过程通常遵循以下步骤：反应物分子吸附到催化剂表面的活性位点上，与表面原子形成弱键。这种吸附削弱了反应物分子内的化学键，或使它们保持在有利的取向上，从而可以以较低的活化能发生反应。最后，产物分子从表面脱附。

The effectiveness of a heterogeneous catalyst depends on the surface area available for adsorption. Finely divided metals or supporting the catalyst on a porous material such as silica or alumina greatly increases the active surface area and hence the catalytic activity.

非均相催化剂的有效性取决于可用于吸附的表面积。高度分散的金属或将催化剂负载在诸如二氧化硅或氧化铝等多孔材料上，会大大增加活性表面积，从而提高催化活性。

5. Examples of Heterogeneous Catalysts | 非均相催化实例

Haber Process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) uses a finely divided iron catalyst with promoters such as Al₂O₃ and K₂O. The iron surface adsorbs N₂ and H₂ molecules, weakening the strong N≡N triple bond and facilitating the formation of NH₃. Typical conditions are 400–450 °C and 200 atm.

哈伯法：N₂(g) + 3H₂(g) ⇌ 2NH₃(g) 使用铁基催化剂（细粉状）并加入 Al₂O₃ 和 K₂O 等促进剂。铁表面吸附 N₂ 和 H₂ 分子，削弱了牢固的 N≡N 三键，促进 NH₃ 的生成。典型条件为 400–450 °C 和 200 atm。

Contact Process: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) uses vanadium(V) oxide, V₂O₅, as a heterogeneous catalyst. The surface V⁵⁺ sites oxidise SO₂ to SO₃ and are reduced to V⁴⁺, which is then re-oxidised by O₂. This is an example of a redox catalysis mechanism on a solid surface.

接触法：2SO₂(g) + O₂(g) ⇌ 2SO₃(g) 使用五氧化二钒 (V₂O₅) 作为非均相催化剂。表面的 V⁵⁺ 位点将 SO₂ 氧化为 SO₃ 并被还原为 V⁴⁺，随后 V⁴⁺ 被 O₂ 重新氧化。这是固体表面氧化还原催化机制的一个例子。

Hydrogenation of alkenes: Nickel, palladium, or platinum solids catalyse the addition of H₂ across C=C bonds. The metal surface adsorbs both the alkene and H₂, dissociating H₂ into H atoms that add stepwise to the alkene to form an alkane.

烯烃加氢：镍、钯或铂固体催化 H₂ 在 C=C 双键上的加成。金属表面同时吸附烯烃和 H₂，将 H₂ 解离为 H 原子，再逐步加成到烯烃上生成烷烃。

6. Homogeneous Catalysis: Intermediate Formation | 均相催化：中间体形成

In homogeneous catalysis, the catalyst and reactants are in the same phase. The catalyst reacts with one or more reactants to form an intermediate species, which then reacts further to regenerate the catalyst and produce the final products. The activation energy of each step is lower than that of the uncatalysed direct reaction.

在均相催化中，催化剂和反应物处于同一相。催化剂与一种或多种反应物反应生成一种中间体，该中间体随后进一步反应再生催化剂并生成最终产物。每一步的活化能都低于非催化直接反应的活化能。

Because the catalyst is regenerated, only a small amount is needed. Homogeneous catalysts often allow better control of selectivity because the catalytic species is uniformly distributed at the molecular level throughout the reaction mixture.

由于催化剂可再生，因此只需少量即可。均相催化剂通常能更好地控制选择性，因为催化物种在分子水平上均匀分布于整个反应混合物中。

7. Examples of Homogeneous Catalysts | 均相催化实例

Ozone depletion by CFCs: Chlorine atoms (Cl•) from CFCs act as homogeneous catalysts in the stratosphere. Cl• + O₃ → ClO• + O₂; ClO• + O → Cl• + O₂. The chlorine radical is regenerated, so one Cl• can destroy thousands of ozone molecules.

CFC 导致的臭氧消耗：来自氯氟烃 (CFC) 的氯原子 (Cl•) 在平流层中充当均相催化剂。Cl• + O₃ → ClO• + O₂；ClO• + O → Cl• + O₂。氯自由基得以再生，因此一个 Cl• 可以破坏数千个臭氧分子。

Esterification: The reaction between a carboxylic acid and an alcohol to form an ester is catalysed by a strong acid like concentrated H₂SO₄. The H⁺ ions protonate the carbonyl oxygen, making the carbonyl carbon more electrophilic and facilitating nucleophilic attack by the alcohol. The H⁺ is regenerated at the end.

酯化反应：羧酸与醇反应生成酯的过程由浓 H₂SO₄ 等强酸催化。H⁺ 离子使羰基氧质子化，使得羰基碳更具亲电性，有利于醇的亲核进攻。H⁺ 在反应结束时再生。

Iodine–propanone reaction: CH₃COCH₃ + I₂ → CH₃COCH₂I + HI is catalysed by H⁺ ions. The acid protonates the ketone, enabling enol formation, which then reacts rapidly with I₂. The rate equation is rate = k[CH₃COCH₃][H⁺], and the reaction is zero order with respect to I₂.

碘-丙酮反应：CH₃COCH₃ + I₂ → CH₃COCH₂I + HI 被 H⁺ 离子催化。酸将酮质子化，使其形成烯醇，烯醇随后与 I₂ 快速反应。速率方程为 rate = k[CH₃COCH₃][H⁺]，该反应对 I₂ 为零级。

8. Autocatalysis | 自催化

Autocatalysis occurs when one of the reaction products acts as a catalyst for the reaction. This leads to a reaction rate that increases over time as the product concentration builds up, even though reactant concentrations are falling. The rate–time graph shows an initial slow phase, then a sharp increase, followed by a slowing down as reactants are used up.

当反应产物之一充当该反应的催化剂时，就会发生自催化。这导致反应速率随着时间的推移而增加，因为产物浓度在逐渐积累，即使反应物浓度在下降。速率-时间图显示初始缓慢阶段，然后急剧上升，最后随着反应物耗尽而减慢。

Example: The reaction between ethanedioate ions (C₂O₄²⁻) and manganate(VII) ions (MnO₄⁻) in acidic solution: 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O. The product Mn²⁺ ions catalyse the reaction. The reaction starts very slowly at room temperature, but once enough Mn²⁺ is formed, the rate increases dramatically, and the purple colour of MnO₄⁻ disappears rapidly.

实例：乙二酸根离子 (C₂O₄²⁻) 与高锰酸根离子 (MnO₄⁻) 在酸性溶液中的反应：2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O。产物 Mn²⁺ 离子催化该反应。室温下反应开始时非常缓慢，但一旦生成足够的 Mn²⁺，速率急剧增加，MnO₄⁻ 的紫色迅速消失。

9. Catalytic Converters | 催化转化器

Automobile catalytic converters use a combination of heterogeneous catalysts to reduce toxic emissions from internal combustion engines. They contain a ceramic honeycomb coated with platinum, palladium, and rhodium. The large surface area of the honeycomb ensures efficient contact with exhaust gases.

汽车催化转化器使用多种非均相催化剂的组合来减少内燃机的有毒排放物。它们包含一个陶瓷蜂窝载体，表面涂有铂、钯和铑。蜂窝结构的大表面积确保了与废气的有效接触。

Three key reactions occur: (1) Oxidation of CO to CO₂: 2CO + O₂ → 2CO₂; (2) Oxidation of unburnt hydrocarbons to CO₂ and H₂O: CₓHᵧ + (x+y/4)O₂ → xCO₂ + (y/2)H₂O; (3) Reduction of nitrogen oxides: 2NO + 2CO → N₂ + 2CO₂. This turns harmful pollutants into less harmful substances.

发生三个关键反应：(1) CO 氧化为 CO₂：2CO + O₂ → 2CO₂；(2) 未燃烧的烃类氧化为 CO₂ 和 H₂O：CₓHᵧ + (x+y/4)O₂ → xCO₂ + (y/2)H₂O；(3) 氮氧化物的还原：2NO + 2CO → N₂ + 2CO₂。这将有害污染物转化为危害较小的物质。

Catalytic converters are most effective when the engine runs at the stoichiometric air-to-fuel ratio (about 14.7:1 by mass for petrol), so that both oxidising and reducing reactions can occur simultaneously. Leaded fuel must be avoided because lead compounds poison the catalyst.

当发动机以化学计量空燃比（汽油约为质量比 14.7:1）运行时，催化转化器最有效，这样氧化和还原反应可以同时发生。必须避免使用含铅燃料，因为铅化合物会使催化剂中毒。

10. Enzymes: Biological Catalysts | 酶：生物催化剂

Enzymes are protein molecules that act as highly specific homogeneous catalysts in biological systems. Each enzyme has an active site with a specific three-dimensional shape that is complementary to its substrate, described by the lock-and-key model or the induced-fit model.

酶是在生物系统中作为高度特异性均相催化剂的蛋白质分子。每种酶都有一个活性位点，具有与其底物互补的特定三维形状，这可以用锁钥模型或诱导契合模型来描述。

Enzymes lower activation energy by binding the substrate(s) in the correct orientation, by providing a favourable microenvironment, and by putting strain on substrate bonds. They are sensitive to temperature and pH; extreme conditions can denature the enzyme, altering the active site shape and causing loss of catalytic activity.

酶通过以正确取向结合底物、提供有利的微环境以及使底物化学键产生张力来降低活化能。它们对温度和 pH 敏感；极端条件可使酶变性，改变活性位点形状，导致催化活性丧失。

The Michaelis-Menten model describes enzyme kinetics, where the initial rate v is related to substrate concentration [S] by v = Vmax [S] / (Km + [S]), where Vmax is the maximum rate and Km is the Michaelis constant. Enzyme inhibition can be competitive or non-competitive.

米氏模型描述了酶动力学，其中初始速率 v 与底物浓度 [S] 的关系为 v = Vmax [S] / (Km + [S])，Vmax 是最大速率，Km 是米氏常数。酶抑制可以是竞争性的或非竞争性的。

11. Catalyst Poisoning and Deactivation | 催化剂中毒与失活

Catalyst poisoning occurs when impurities bind irreversibly or strongly to active sites on a heterogeneous catalyst, blocking them from reacting with the intended reactants. This reduces or completely destroys catalytic activity. Common catalyst poisons include lead, sulfur compounds, and carbon monoxide for many metal catalysts.

当杂质不可逆地或强烈地结合到非均相催化剂的活性位点上，阻止它们与目标反应物反应时，就会发生催化剂中毒。这会降低或完全破坏催化活性。常见的催化剂毒物包括铅、硫化合物以及对许多金属催化剂而言的一氧化碳。

For example, in the Haber process, sulfur compounds must be removed from the hydrogen feedstock (derived from natural gas) before it enters the reactor; otherwise the iron catalyst is poisoned. Similarly, lead in petrol poisons the platinum/rhodium catalysts in catalytic converters.

例如，在哈伯法中，氢气原料（源自天然气）在进入反应器之前必须脱除硫化合物，否则铁催化剂会中毒。同样，汽油中的铅会使催化转化器中的铂/铑催化剂中毒。

12. Factors Affecting Catalytic Activity | 影响催化活性的因素

The activity of a heterogeneous catalyst is influenced by its surface area, dispersion of active metal, temperature, pressure, and the presence of promoters or inhibitors. Promoters are substances that increase the activity of a catalyst without themselves being catalysts for the reaction, for example Al₂O₃ in the iron Haber catalyst.

非均相催化剂的活性受其表面积、活性金属的分散度、温度、压力以及促进剂或抑制剂的存在等因素影响。促进剂是能提高催化剂活性而其本身并非该反应催化剂的物质，例如哈伯法铁催化剂中的 Al₂O₃。

For homogeneous catalysts, factors such as concentration, solvent, and pH can significantly affect catalytic efficiency. In enzyme catalysis, temperature and pH are critical; both have optimal values beyond which activity declines sharply due to denaturation or changes in ionisation state.

对于均相催化剂，浓度、溶剂和 pH 等因素可显著影响催化效率。在酶催化中，温度和 pH 至关重要；两者都有最佳值，超出这些值活性会因变性或电离状态改变而急剧下降。

The physical form of a heterogeneous catalyst is engineered to maximise surface area; for instance, Raney nickel is a porous, finely divided form of nickel used for hydrogenation reactions, with a high proportion of active surface atoms.

非均相催化剂的物理形态经过工程设计以最大化表面积；例如，雷尼镍是一种用于加氢反应的多孔、高度分散的镍，具有高比例的活性表面原子。

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Catalysis | 催化