$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, $\Delta H = -92$ kJ/mol (exothermic). Conditions: temperature 400-500°C (compromise — higher T gives faster rate but lower yield; lower T gives higher yield but too slow). Pressure: 150-300 atm (higher pressure favours NH3 side, Le Chatelier — fewer moles of gas). Catalyst: finely divided iron (Fe) with promoters: K2O (electronic promoter — increases electron density on Fe surface) and Al2O3 (structural promoter — prevents Fe particles from sintering). Rate-determining step: dissociative adsorption of N2 onto Fe surface. This is the hardest step — N≡N triple bond (945 kJ/mol) must be cleaved. Importance: Haber process produces ~150 million tonnes NH3/year globally. NH3 used mainly for fertilisers (ammonium nitrate, urea, ammonium phosphate). Without Haber process: current world population could not be fed.

CH2=CH2 + 1/2 O2 → CH3CHO ($\Delta G < 0$, thermodynamically favourable). Catalyst system: PdCl2 + CuCl2 in aqueous HCl. Overall: CH2=CH2 + PdCl2 + H2O → CH3CHO + Pd0 + 2HCl. Regeneration: Pd0 + 2CuCl2 → PdCl2 + 2CuCl. 2CuCl + 2HCl + 1/2 O2 → 2CuCl2 + H2O. Net: CH2=CH2 + 1/2 O2 → CH3CHO. PdCl2 is the actual catalyst; CuCl2 is co-catalyst that regenerates Pd2+ from Pd0; O2 reoxidises Cu+. Mechanism involves: coordination of ethylene to Pd²⁺, nucleophilic attack by OH⁻, rearrangement to vinyl alcohol intermediate, isomerisation to acetaldehyde. Acetaldehyde is an important industrial chemical for acetic acid, ethanol acetate production.

[ClRh(PPh3)3] (chlorotris(triphenylphosphine)rhodium(I)) — prepared by RhCl3 + PPh3. Homogeneous catalyst for hydrogenation of alkenes and alkynes at mild conditions (room temperature, 1 atm H2 pressure). Mechanism: (1) Dissociation of PPh3 to give 14-electron species [ClRh(PPh3)2]. (2) Oxidative addition of H2 to give Rh(III) dihydride. (3) Coordination of alkene. (4) Migratory insertion of alkene into Rh-H bond. (5) Reductive elimination of product alkane. (6) Catalyst regenerated. Selectivity: less hindered double bonds react faster. Does not reduce aromatic rings (unlike heterogeneous catalysts). Nobel Prize 1973: Wilkinson and Fischer for organometallic chemistry including this catalyst. Modified Rh-BINAP catalyst: asymmetric hydrogenation (produces one enantiomer selectively — used to make L-DOPA for Parkinson's treatment).

Karl Ziegler (1953): TiCl4 + Al(C2H5)3 (triethylaluminium) catalyses polymerisation of ethylene at low pressure and temperature. Giulio Natta (1954): extended to stereospecific polymerisation of propylene. Nobel Prize 1963. Mechanism: chain growth at Ti-C bond. Alkene inserts between Ti and growing polymer chain. The crystal surface of TiCl4 provides specific geometry → stereocontrol. Isotactic polypropylene (all methyl groups on same side): crystalline, high melting point, rigid. Syndiotactic (alternating): also crystalline. Atactic (random): amorphous, soft. Ziegler-Natta gives isotactic/syndiotactic (useful, structured) vs free-radical polymerisation which gives atactic. Modern metallocene catalysts (Cp2ZrCl2/MAO) are second generation: even better stereocontrol, single-site catalysts.

2SO2 + O2 → 2SO3 (catalyst: V2O5/K2SO4 on SiO2). Temperature: 450-550°C (compromise). V2O5 reduces to V2O4 (accepting O from SO2), then reoxidised by O2 back to V2O5 — cyclic redox mechanism. SO3 + H2SO4 → H2S2O7 (oleum) → dilute to get H2SO4. Cannot absorb SO3 directly in water (too exothermic, forms acid mist). Global production ~250 million tonnes H2SO4/year — most produced chemical. Uses: fertilisers (superphosphate), pickling of steel, battery acid, detergents, dyestuffs, explosives. "Sulphuric acid production = measure of industrialisation of a country."

4NH3 + 5O2 → 4NO + 6H2O (Pt-Rh catalyst, 900°C). 2NO + O2 → 2NO2 (homogeneous, no catalyst). 3NO2 + H2O → 2HNO3 + NO (NO recycled). The first step (NH3 oxidation to NO) is the key. Pt-10%Rh gauze catalyst: very high surface area, high temperature stability. The reaction is exothermic and fast — contact time ~milliseconds. HNO3 uses: fertilisers (ammonium nitrate), explosives (TNT, RDX — nitration reactions), nylon (adipic acid), pharmaceuticals. Mixed acid (HNO3 + H2SO4) is used for nitration of benzene and other aromatic compounds in industry.

Homogeneous: catalyst and reactants in same phase. Advantages: every catalyst molecule accessible, highly selective. Disadvantages: separation difficult. Examples: acid-base catalysis (H+ in ester hydrolysis), Wilkinson, Wacker, enzyme catalysis. Heterogeneous: different phase. Advantages: easy separation, can withstand harsh conditions. Disadvantages: only surface exposed, harder to make selective. Examples: Fe (Haber), V2O5 (Contact), Pt (catalytic converter), Ni (hydrogenation of oils). Enzyme catalysis: biological catalysts. Protein with specific active site (lock-and-key). Extremely efficient (kcat up to 10^7 s-1). Specific: one enzyme, one reaction type. Examples: amylase (starch→sugars), protease (protein digestion), carbonic anhydrase (CO2 hydration), DNA polymerase (DNA replication). Autocatalysis: product catalyses its own formation (KMnO4 + oxalic acid — Mn2+ is catalyst).

Physical adsorption (physisorption): van der Waals forces. Low energy (<40 kJ/mol). Reversible. Multilayer. Increases with decreasing temperature. Decreases with increasing temperature. Chemisorption: chemical bond formation between adsorbate and surface. High energy (40-400 kJ/mol). Usually monolayer. Irreversible (unless desorbed by heating). Increases with temperature (activation energy needed) up to a maximum, then decreases. Adsorption isotherm: Freundlich: x/m = kP^(1/n). Langmuir: x/m = aP/(1+bP). Gibbs adsorption equation: $\Gamma = -(c/RT)(d\gamma/dc)$ where $\Gamma$ = surface excess, $\gamma$ = surface tension. Catalysis mechanism on surface: (1) Adsorption of reactants. (2) Activation (weakening of bonds). (3) Reaction on surface. (4) Desorption of products. (5) Surface regenerated.