多相催化Heterogeneous catalysis

多相催化一般分三步：反应物分子扩散到表面并吸附在活性位点上；吸附使分子内的键被削弱，反应物在表面相互反应，所需活化能低于无催化剂时；产物从表面脱附，空出活性位点供下一轮使用。吸附强度要适中：太弱则分子无法被活化，太强则产物不易脱附、占住活性位点。增大催化剂的表面积（做成细粉、多孔材料或负载在载体上）可以提高反应速率。

Heterogeneous catalysis usually proceeds in three steps: reactant molecules diffuse to the surface and adsorb on active sites; adsorption weakens bonds within them, and they react on the surface with a lower activation energy than without the catalyst; the products desorb, freeing the sites for the next cycle. Adsorption must be of moderate strength: too weak and the molecules are not activated, too strong and the products cannot leave and block the sites. Increasing the surface area (fine powders, porous materials, catalysts dispersed on a support) raises the rate.

哈伯法合成氨用铁催化剂：N₂ + 3H₂ ⇌ 2NH₃。接触法制硫酸用五氧化二钒催化 SO₂ 氧化为 SO₃。汽车尾气催化转化器用铂、钯、铑把 CO、NOₓ 和未燃烃转化为 CO₂、N₂ 和 H₂O，例如 2CO + 2NO → 2CO₂ + N₂。植物油加氢制人造黄油用镍催化剂。

The Haber process for ammonia uses an iron catalyst: N₂ + 3H₂ ⇌ 2NH₃. The Contact process for sulfuric acid uses vanadium(V) oxide to catalyse the oxidation of SO₂ to SO₃. Car catalytic converters use platinum, palladium and rhodium to convert CO, NOₓ and unburnt hydrocarbons into CO₂, N₂ and H₂O, e.g. 2CO + 2NO → 2CO₂ + N₂. Nickel catalyses the hydrogenation of vegetable oils to make margarine.

某些物质会强烈吸附在活性位点上而不脱附，使催化剂失活，称为催化剂中毒。例如铅会使汽车催化转化器失效，这是推广无铅汽油的原因之一；硫化物会使合成氨的铁催化剂中毒，因此原料气要先脱硫。

Some substances adsorb so strongly on the active sites that they never desorb, deactivating the catalyst; this is called catalyst poisoning. Lead, for example, ruins car catalytic converters, one reason unleaded petrol was introduced; sulfur compounds poison the iron catalyst in ammonia synthesis, so the feed gas is desulfurised first.

朗缪尔提出单分子层吸附模型，把吸附量与气体压力定量联系起来，为多相催化动力学奠定基础。二十世纪后期，超高真空和表面谱学技术使人们能够在原子尺度观察表面反应的各个步骤，埃特尔借此阐明了铁表面上合成氨的机理。

Langmuir's monolayer adsorption model related the amount adsorbed quantitatively to gas pressure and laid the basis for the kinetics of heterogeneous catalysis. In the late twentieth century, ultra-high-vacuum and surface-spectroscopy techniques made it possible to follow the individual steps of surface reactions at the atomic scale; Ertl used them to establish the mechanism of ammonia synthesis on iron.