均相催化Homogeneous catalysis

Fe²⁺ 催化过二硫酸根氧化碘离子：S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺，2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂。直接反应需两个阴离子相碰，彼此排斥而很慢；铁离子可以改变氧化态，在两步中先被氧化再被还原。大气中 NO₂ 催化 SO₂ 的氧化：SO₂ + NO₂ → SO₃ + NO，NO + ½O₂ → NO₂。酯化反应中的 H⁺ 和生物体内的酶也是均相催化剂。

Fe²⁺ catalyses the oxidation of iodide by peroxodisulfate: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺, then 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂. The direct reaction needs two anions to collide and they repel, so it is slow; iron can change oxidation state and is oxidised then reduced. In the atmosphere NO₂ catalyses the oxidation of SO₂: SO₂ + NO₂ → SO₃ + NO, then NO + ½O₂ → NO₂. H⁺ in esterification and enzymes in living things are also homogeneous catalysts.

催化剂分子全部参与反应，活性和选择性往往很高，反应条件温和；缺点是催化剂与产物处于同一相，分离回收较难。过渡金属能形成多种氧化态和配合物，是许多均相催化剂的核心，如威尔金森催化剂用于烯烃加氢。

Every catalyst molecule can take part, so activity and selectivity are often high and conditions mild; the drawback is that the catalyst is in the same phase as the product and harder to separate and recover. Transition metals, with their variable oxidation states and complexes, are at the heart of many homogeneous catalysts, such as Wilkinson's catalyst for hydrogenating alkenes.