Catalysts increase reaction rates and are the heart of the chemical industry, determining energy consumption, selectivity, and economic efficiency.
Basic Knowledge
Definition of a catalyst: A substance that increases the reaction rate but does not itself change before and after the reaction. It changes the reaction pathway by lowering the activation energy Ea.
Classification:
・Homogeneous catalysts: the same phase as the reactants (a dissolved catalyst in a liquid solution)
・Heterogeneous catalysts: a different phase (a solid catalyst with gaseous or liquid reactants)
・Enzyme catalysts: protein-based, highly substrate-specific, and effective under mild conditions
Historical industrial catalysts:
・Haber–Bosch process (N2+H2→NH3): Fe-K2O-Al2O3 (1909)
・Contact process (SO2→SO3): V2O5
・Ostwald process (NH3→HNO3): Pt-Rh (a platinum-rhodium alloy)
・Hydrogenation (C=C+H2→C-C): Ni, Pt, Pd
・Cumene process (benzene+propylene→cumene): phosphoric acid/zeolite
・Cross-coupling (C-C bond formation): Pd (Negishi, Suzuki, and Heck; 2010 Nobel Prize)
・Asymmetric synthesis (stereoselective synthesis): BINAP-Ru (Noyori; 2001 Nobel Prize)
・Olefin metathesis (rearrangement of C=C): Grubbs catalyst (2005 Nobel Prize)
Automobile catalysts (three-way catalysts): Pt-Pd-Rh dispersed on a support.
・Oxidation: 2CO + O2 → 2CO2, CxHy + O2 → CO2 + H2O
・Reduction: 2NO + 2CO → N2 + 2CO2
Three harmful gases (CO, HC, NOx) are removed simultaneously.
Parameters in reaction engineering: temperature, pressure, reaction time, stirring, flow rate, amount of catalyst, and concentration. Continuous-flow reactors (flow reactions) offer better controllability, safety, and scalability than batch reactions and are expanding in modern industry.
Catalyst (lowers activation energy and remains unchanged)
Homogeneous/heterogeneous (whether the phases are the same or different)
Three-way catalyst (automobile exhaust purification; Pt/Pd/Rh)
Zeolite (a porous crystalline aluminosilicate; a molecular sieve and catalyst)
Asymmetric catalyst (selectively synthesizes enantiomers; essential for pharmaceutical production)
Flow chemistry (continuous-flow reactions with excellent safety and controllability)
Further Study (Principles and Applications)
Surface chemistry of heterogeneous catalysts: Reactants are adsorbed onto the catalyst surface (by chemisorption or physisorption) → a surface reaction (bond breaking and re-forming) → desorption (products leave the surface). The Sabatier principle states that adsorption must be neither too strong nor too weak; maximum activity occurs at an intermediate bond strength.
Maximizing surface area: Catalysts are dispersed on powders and porous supports (SiO2, Al2O3, activated carbon, and zeolites), achieving several hundred to several thousand m2 per gram of surface area.
Deactivation: poisoning (S, P, As, and other substances cover active sites), sintering (particles aggregate at high temperature, reducing surface area), and coking (carbon deposition). Regeneration or replacement is necessary.
Water-electrolysis catalysts: Catalysts that produce H2 from H2O using renewable electricity, including catalysts based on earth-abundant elements that replace platinum, are keys to the green hydrogen economy. CO2-reduction catalysts: Conversion of CO2 + electrons into useful compounds such as CO/HCOOH/CH3OH/ethylene, providing a path toward carbon neutrality.
Artificial photosynthesis: Sunlight+H2O+CO2→fuels and chemicals; a dream technology since the latter half of the 20th century. Japan’s NEDO and artificial photosynthesis chemistry projects are promoting it as a national strategy.
- Catalysts lower Ea and remain unchanged before and after a reaction.
- The three classifications are homogeneous, heterogeneous, and enzyme catalysts.
- Historical catalysts: Fe (NH3), V2O5 (SO3), and Pt-Rh (HNO3).
- Automobile three-way catalysts use Pt-Pd-Rh.
- Asymmetric catalysts (Noyori and BINAP-Ru) are used in pharmaceutical production.
- Heterogeneous catalysis proceeds through adsorption → reaction → desorption.
- Water electrolysis, CO2 reduction, and artificial photosynthesis are future challenges.
Points to Note (Common Confusions and Frequent Mistakes)
① Catalysts do not shift equilibrium (they accelerate the forward and reverse reactions equally). The equilibrium constant K does not change. ② Positive and negative catalysts: “catalyst” normally means a positive catalyst, but negative catalysts (inhibitors) also exist. ③ Enzymes are catalysts: they are biopolymer catalysts with strong temperature and pH dependence and are characterized by the maximum velocity Vmax and Michaelis constant Km. ④ Many Nobel Prizes have involved catalysis: Noyori in 2001, Grubbs in 2005, Ertl in 2007, and Negishi, Suzuki, and Heck in 2010.
Practice
- Explain why a catalyst increases the reaction rate but does not shift equilibrium, from the perspective of activation energy.
- Write equations for the three reactions of an automobile three-way catalyst and state the reducing agent required for the reduction of NOx in each reaction.
- Give three major causes of deactivation of heterogeneous catalysts and describe countermeasures.