OPOpenPithMake curiosity visibleChemistry lab / Catalysts

About this lab

Five topics: a catalyst moves neither the start nor the finish, only the road.

A catalyst takes part in the reaction and comes back unchanged at the end. It opens a road with a lower activation energy, so more collisions become effective. It speeds the forward and reverse reactions by the same factor, so it does not move the equilibrium. It picks a road, so it is selective. It works through active sites on its surface, so poisons that take those sites stop it.

Every reading and every curve comes from the same model. For why molecules must clear the activation energy at all, see the Microscopic world of reactions.

Five findings

Five findings

  • Another way over: I⁻ lowers the Ea of peroxide decomposition from 75 to 56 kJ/mol, while ΔH stays −98 kJ/mol. At 298 K the rate ratio is about 2140.
  • The surface cycle: adsorb, break the bond, add hydrogen one at a time, desorb. Coverage is 50 % at 50 bar and 85.7 % at 300 bar; with limited sites the rate levels off.
  • Equilibrium stays put: at 723 K and 200 bar both reactors end at 27.4 % NH₃; the catalysed one reaches 95 % of it in 3.6 s, the other in 18.0 s.
  • Selectivity: at 523 K nickel gives 90.9 % CH₄ and copper–zinc oxide gives 96.9 % CH₃OH.
  • Poisoning: a trace of H₂S kills 45 % of the sites for good; the same level of oxygen takes only 14 %, and the rate returns to 99.6 % after purification.

Model and sources

Model and sources

  • Energy profile: Ea values are those commonly quoted (75 uncatalysed, 56 with I⁻, 23 with catalase, kJ/mol) and ΔH = −98.0 kJ/mol; the intermediate energies are schematic. The rate ratio assumes both roads share the same pre-exponential factor.
  • Surface: Langmuir adsorption θ = KP/(1+KP) with K = 0.02 bar⁻¹; turnover r = kθ with k = 1 per site per second; both are teaching assumptions. The eight stages of the tracked N₂ are laid out in time.
  • Ammonia equilibrium: ideal gas; Kp(723 K) = 6.4 × 10⁻⁵ bar⁻² and ΔH = −110 kJ/mol, back-calculated from equilibrium ammonia of about 39 % at 400 °C and 27 % at 450 °C under 200 atm (teaching assumption). The rate law only expresses “both directions alike”; it is not real kinetics.
  • Selectivity: the Ea of each road on each catalyst is a teaching assumption that only sets which is lower (methanation 10 kJ/mol lower on nickel, methanol 15 kJ/mol lower on copper–zinc oxide); 24 turnovers are drawn by selectivity with a fixed seed.
  • Poisoning: 64 sites; poison uptake dθ/dt = ka·c·(1 − θ), with extra desorption kd·θ for oxygen; ka, kd and the levels are teaching assumptions.

check:kinetics recomputes every reading and every number in these findings from the model.

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