A-Level Chemistry: Catalysis and Catalytic Mechanisms | A-Level 化学：催化作用与催化机制

📚 A-Level Chemistry: Catalysis and Catalytic Mechanisms | A-Level 化学：催化作用与催化机制

Catalysis is one of the most fundamental concepts in chemistry, underpinning both industrial processes and biological systems. In this article, we explore the definitions, mechanisms and applications of catalysts for the CIE A-Level Chemistry syllabus, with particular attention to exam-relevant detail.

催化作用是化学中最核心的概念之一，支撑着工业过程和生物体系。本文将围绕 CIE A-Level 化学考纲，深入探讨催化剂的定义、机理与应用，并特别关注与考试相关的细节。

1. What Is a Catalyst? | 什么是催化剂？

A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy. Crucially, the catalyst is chemically unchanged at the end of the reaction, meaning it can be recovered and reused.

催化剂是一种通过提供活化能更低的新反应途径来加快化学反应速率的物质。关键在于，催化剂在反应结束时自身化学性质保持不变，因此可以被回收并重复使用。

Key features of a catalyst:

催化剂的关键特征：

It is specific in its action — different reactions require different catalysts.
它具有良好的选择性——不同反应需要不同的催化剂。
It does not alter the position of equilibrium in a reversible reaction; it only helps the system reach equilibrium faster.
它不改变可逆反应的平衡位置，仅仅使系统更快地达到平衡。
It is not consumed in the overall reaction, although it may participate in intermediate steps.
它在总反应中不被消耗，尽管它可能参与中间步骤。

2. Activation Energy and Reaction Profiles | 活化能与反应能量图

In an uncatalysed reaction, reactant molecules must overcome a high energy barrier known as the activation energy (Eₐ). A catalyst lowers this barrier by offering a different pathway, often involving the formation of an intermediate species that would not form in the uncatalysed reaction.

在无催化反应中，反应物分子必须跨越一个高能垒，即活化能（Eₐ）。催化剂通过提供一条不同路径——通常涉及无催化反应中不会形成的中间物种——来降低这一能垒。

Eₐ(catalysed) < Eₐ(uncatalysed) → rate increases

On a reaction profile diagram, the catalysed pathway shows a lower maximum peak. Consequently, a greater fraction of reactant molecules possess sufficient kinetic energy to cross the lower barrier, so the frequency of successful collisions increases dramatically.

在反应能量图中，催化路径的峰值更低。因此，更大比例的反应物分子具有足够的动能越过较低的能垒，有效碰撞的频率随之大幅增加。

It is essential to note that the enthalpy change (ΔH) remains identical for both catalysed and uncatalysed reactions — a catalyst does not affect the overall thermodynamics of the reaction. It only changes the kinetics.

必须注意，无论催化与否，反应的焓变（ΔH）完全相同——催化剂不影响反应的整体热力学，只改变其动力学。

3. Homogeneous Catalysis | 均相催化

In homogeneous catalysis, the catalyst is in the same physical state — usually aqueous or gaseous — as the reactants. This allows intimate mixing and rapid interaction between the catalyst and reactant molecules.

在均相催化中，催化剂与反应物处于相同的物理状态——通常为水溶液或气相。这使得催化剂与反应物分子之间能够充分混合并快速相互作用。

Example: The decomposition of hydrogen peroxide

示例：过氧化氢的分解

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

Iodide ions (I⁻) catalyse this decomposition. The mechanism involves a two-step process with a hypoiodite ion intermediate:

碘离子（I⁻）催化该分解反应。其机理为涉及次碘酸根离子中间体的两步过程：

H₂O₂ + I⁻ → H₂O + IO⁻ (slow)

H₂O₂ + IO⁻ → H₂O + O₂ + I⁻ (fast)

Notice that I⁻ is consumed in the first step and regenerated in the second step — this is the hallmark of homogeneous catalysis. The intermediate IO⁻ appears in the mechanism but not in the overall equation.

注意 I⁻ 在第一步中被消耗，在第二步中又再生——这正是均相催化的标志。中间体 IO⁻ 出现在机理中，但不出现于总方程式中。

Other examples include acid-catalysed esterification of carboxylic acids with alcohols, and the use of nitrogen monoxide (NO) as a homogeneous catalyst in the lead chamber process for sulfuric acid manufacture.

其他示例包括羧酸与醇的酸催化酯化反应，以及铅室法制硫酸中使用一氧化氮（NO）作为均相催化剂。

4. Heterogeneous Catalysis | 多相催化

In heterogeneous catalysis, the catalyst is in a different phase from the reactants — typically a solid catalyst with gaseous or liquid reactants. The reaction occurs on the catalyst surface at active sites.

在多相催化中，催化剂与反应物处于不同相态——通常为固体催化剂作用于气态或液态反应物。反应发生在催化剂表面的活性位点上。

Example: The Haber process

示例：哈伯法（合成氨）

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Finely divided iron is used as the catalyst at approximately 450 °C and 200 atm. The iron surface adsorbs nitrogen and hydrogen molecules, weakening their intramolecular bonds. Promoters such as Al₂O₃ and K₂O are added to increase the surface area and enhance the efficiency of the iron catalyst.

在约 450 °C 和 200 atm 条件下，使用细粉状铁作为催化剂。铁表面吸附氮气和氢气分子，削弱其分子内部化学键。常加入 Al₂O₃ 和 K₂O 等助催化剂以增大比表面积并提高铁催化剂的效率。

Example: The Contact process

示例：接触法（硫酸工业）

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Vanadium(V) oxide (V₂O₅) is the heterogeneous catalyst. In the actual mechanism, V₂O₅ is first reduced to V₂O₄ by SO₂, and then V₂O₄ is re-oxidised back to V₂O₅ by O₂.

五氧化二钒（V₂O₅）是该反应的多相催化剂。在实际机理中，V₂O₅ 首先被 SO₂ 还原为 V₂O₄，随后 V₂O₄ 被 O₂ 重新氧化为 V₂O₅。

SO₂ + V₂O₅ → SO₃ + V₂O₄

O₂ + 2V₂O₄ → 2V₂O₅

5. Adsorption in Heterogeneous Catalysis | 多相催化中的吸附

Adsorption is the key step in heterogeneous catalysis. In chemisorption, reactant molecules form chemical bonds with surface atoms of the catalyst. This weakens the bonds within the reactant molecules, making them more reactive.

吸附是多相催化的关键步骤。在化学吸附中，反应物分子与催化剂表面原子形成化学键，从而削弱反应物分子内部的化学键，使其更具反应活性。

The stages of a heterogeneous catalytic reaction can be summarised as:

多相催化反应的步骤可归纳为：

Diffusion of reactant molecules to the catalyst surface.
反应物分子向催化剂表面扩散。
Adsorption of reactants onto active sites via chemisorption.
反应物通过化学吸附在活性位点上吸附。
Reaction between adsorbed species — bonds are broken and new bonds formed.
吸附物种之间发生反应——旧键断裂，新键生成。
Desorption of product molecules from the surface.
产物分子从表面脱附。
Diffusion of products away from the catalyst surface.
产物从催化剂表面扩散离开。

Adsorption is an exothermic process. Although energy is required to break the adsorption bonds during desorption, the overall lowering of activation energy leads to a substantial net increase in reaction rate.

吸附是一个放热过程。尽管脱附时需要能量来断裂吸附键，但活化能的总体降低带来了反应速率的大幅净增长。

6. Catalytic Converters | 催化转化器

Catalytic converters in vehicle exhaust systems use heterogeneous catalysts — typically platinum, palladium and rhodium supported on a ceramic honeycomb structure. These metals provide a large surface area for gaseous reactions.

汽车尾气系统中的催化转化器使用多相催化剂——通常将铂、钯和铑负载于陶瓷蜂窝结构上。这些金属为气相反应提供巨大的表面积。

The converter simultaneously removes three classes of pollutants:

催化转化器同时去除三类污染物：

2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)

2NO(g) + 2NO₂(g) → 2N₂O(g) + O₂(g) (partial reduction)

CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O

Notice that carbon monoxide is oxidised to carbon dioxide while nitrogen monoxide is reduced to nitrogen — this is a redox coupling occurring on the same catalyst surface. This is an important exam point: the same catalyst can promote both oxidation and reduction simultaneously.

注意一氧化碳被氧化为二氧化碳，同时一氧化氮被还原为氮气——这是在同一催化剂表面上发生的氧化还原耦合。这是重要考点：同一种催化剂可以同时促进氧化和还原反应。

7. Enzyme Catalysis | 酶催化

Enzymes are biological catalysts, predominantly proteins, that exhibit remarkable specificity and efficiency. Each enzyme has an active site — a three-dimensional pocket with a shape complementary to its specific substrate. This is described by the lock-and-key model, later refined into the induced-fit model, in which

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A-Level Chemistry: Catalysis and Catalytic Mechanisms | A-Level 化学：催化作用与催化机制