📚 Catalysis | 催化作用
A catalyst is a substance that alters the rate of a chemical reaction without undergoing any permanent chemical change itself. Catalysts are fundamental in industrial chemistry, biological systems and environmental control because they allow reactions to proceed under milder conditions and with greater selectivity.
催化剂是一种能改变化学反应速率、而自身不发生永久化学变化的物质。催化剂在工业化学、生物体系和环境控制中至关重要，因为它们使反应能在更温和的条件下进行，并具有更高的选择性。
1. What is a Catalyst? | 什么是催化剂？
A catalyst provides an alternative reaction pathway with a lower activation energy, Ea. It increases the rate of both the forward and reverse reactions equally, so the position of equilibrium is unchanged.
催化剂提供一条活化能 Ea 较低的替代反应路径。它同等程度地加快正反应和逆反应速率，因此平衡位置不变。
Catalysts are not consumed in the overall reaction. They may participate in intermediate steps but are regenerated at the end of the catalytic cycle.
催化剂在总反应中不被消耗。它们可能参与中间步骤，但在催化循环结束时会被再生。
2. How Catalysts Work: Activation Energy | 催化剂如何起作用：活化能
A reaction can only occur when particles collide with energy greater than or equal to the activation energy. A catalyst lowers this energy barrier, allowing a larger fraction of collisions to be successful at a given temperature.
反应只有在粒子以大于或等于活化能的能量碰撞时才能发生。催化剂降低这一能垒，使给定温度下有更大比例的碰撞能够成功。
On a Maxwell-Boltzmann distribution, the catalyst does not change the shape of the curve; it moves the activation energy threshold to the left, increasing the shaded area representing productive collisions.
在麦克斯韦-玻尔兹曼分布中，催化剂不改变曲线形状；它将活化能阈值向左移动，增大代表有效碰撞的阴影面积。
3. Homogeneous vs Heterogeneous Catalysis | 均相与异相催化
In homogeneous catalysis, the catalyst is in the same phase as the reactants, usually in solution or the gas phase. In heterogeneous catalysis, the catalyst is in a different phase, typically a solid in contact with gaseous or liquid reactants.
均相催化中，催化剂与反应物处于同一相，通常为溶液或气相。异相催化中，催化剂处于不同相，通常为固体，与气态或液态反应物接触。
Homogeneous catalysts often work through the formation of intermediate species, while heterogeneous catalysts provide a surface on which reactant molecules adsorb and react.
均相催化剂通常通过形成中间体起作用，而异相催化剂提供一个表面，使反应物分子吸附并发生反应。
4. Heterogeneous Catalysis: Surface Adsorption | 异相催化：表面吸附
Heterogeneous catalysis involves at least three key steps: adsorption of reactant molecules onto the catalyst surface, reaction on the surface, and desorption of products.
异相催化至少包括三个关键步骤：反应物分子吸附到催化剂表面、在表面发生反应以及产物脱附。
Adsorption is not the same as absorption. In adsorption, molecules bind weakly through physisorption or strongly through chemisorption to active sites on the solid surface; chemisorption often weakens bonds within the reactant and lowers Ea.
吸附不同于吸收。在吸附中，分子通过物理吸附或化学吸附结合在固体表面的活性位点上；化学吸附通常会削弱反应物内部的化学键，从而降低活化能。
The strength of adsorption must be moderate: too weak gives little activation; too strong blocks the active sites and prevents product release.
吸附强度必须适中：太弱则活化作用小；太强则阻塞活性位点并阻止产物释放。
5. Industrial Example: Haber Process | 工业实例：哈伯法
In the Haber process, nitrogen and hydrogen react to form ammonia. An iron catalyst with promoters such as potassium oxide and aluminium oxide is used.
在哈伯法中，氮气和氢气反应生成氨。使用含氧化钾和氧化铝等促进剂的铁催化剂。
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹
The catalyst allows the reaction to reach equilibrium more quickly at a moderate temperature of about 450 °C, avoiding the need for excessively high temperatures that would reduce the equilibrium yield of ammonia.
该催化剂使反应在约 450 °C 的中等温度下更快达到平衡，避免过高温度导致氨的平衡产率下降。
6. Industrial Example: Contact Process | 工业实例：接触法
In the Contact process, sulfur dioxide is oxidised to sulfur trioxide. Vanadium(V) oxide, V₂O₅, acts as the heterogeneous catalyst.
在接触法中，二氧化硫被氧化为三氧化硫。五氧化二钒 V₂O₅ 作为异相催化剂。
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
The vanadium(V) oxide is reduced to vanadium(IV) oxide, V₂O₄, and then re-oxidised to V₂O₅ in a two-step redox cycle, enabling an alternative pathway with lower Ea.
五氧化二钒先被还原为四氧化二钒 V₂O₄，然后又被氧化回 V₂O₅，构成两步氧化还原循环，提供活化能更低的替代路径。
V₂O₅ + SO₂ → V₂O₄ + SO₃
V₂O₄ + ½O₂ → V₂O₅
7. Catalytic Converters | 催化转化器
Catalytic converters in car exhausts use a ceramic honeycomb coated with platinum, palladium and rhodium to convert harmful gases into less harmful products.
汽车尾气催化转化器使用涂有铂、钯和铑的陶瓷蜂窝载体，将有害气体转化为危害较小的产物。
They catalyse the oxidation of carbon monoxide and unburnt hydrocarbons to CO₂ and H₂O, and the reduction of nitrogen oxides, NOₓ, to N₂ and O₂.
它们催化一氧化碳和未燃烧碳氢化合物氧化为 CO₂ 和 H₂O，并将氮氧化物 NOₓ 还原为 N₂ 和 O₂。
2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)
8. Homogeneous Catalysis: Transition Metal Ions | 均相催化：过渡金属离子
Transition metal ions can act as homogeneous catalysts because they have variable oxidation states. A common example is the reaction between peroxodisulfate(VI) ions and iodide ions, catalysed by Fe²⁺/Fe³⁺.
过渡金属离子由于具有可变的氧化态，可作为均相催化剂。一个常见例子是过二硫酸根离子与碘离子之间的反应，由 Fe²⁺/Fe³⁺ 催化。
S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)
In the uncatalysed reaction, two negative ions repel each other, so Ea is high. Iron ions provide an alternative pathway in which Fe³⁺ oxidises iodide and Fe²⁺ reduces peroxodisulfate.
在未催化反应中，两种负离子相互排斥，因此活化能很高。铁离子提供一条替代路径：Fe³⁺ 氧化碘离子，Fe²⁺ 还原过二硫酸根离子。
2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻
9. Enzymes: Biological Catalysts | 酶：生物催化剂
Enzymes are biological catalysts, mostly proteins, which are highly specific. Their active site has a shape complementary to the substrate, explained by the lock-and-key and induced-fit models.
酶是生物催化剂，大多为蛋白质，具有高度专一性。酶的活性位点形状与底物互补，可用锁钥模型和诱导契合模型解释。
Enzymes operate effectively at body temperature and pH, but denature at high temperatures or extreme pH because their tertiary structure is disrupted.
酶在体温和适宜 pH 下高效工作，但在高温或极端 pH 下会因三级结构被破坏而变性。
Enzyme activity can be inhibited by inhibitors, which may be competitive or non-competitive, and is affected by substrate concentration up to maximum rate, Vmax.
酶活性可被抑制剂抑制，分为竞争性抑制和非竞争性抑制；底物浓度影响酶活性直至最大速率 Vmax。
10. Autocatalysis | 自催化
Autocatalysis occurs when a reaction product acts as a catalyst for the reaction itself. The rate of reaction increases as the product concentration builds up.
自催化是指反应产物本身充当该反应的催化剂。随着产物浓度增大，反应速率加快。
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