How it Works
This simulation models a 2D catalyst surface as a grid of discrete adsorption sites. Gas-phase molecules of species A and B randomly strike the surface and adsorb onto empty sites with probabilities set by their partial pressures P_A and P_B (Langmuir adsorption). Adsorbed particles do not move, but every timestep each occupied A site checks its four neighboring sites: if an adsorbed B is found there, the pair can react with probability k, forming product AB which instantly desorbs and frees both sites. If no reaction occurs, adsorbed particles can also desorb spontaneously with rates d_A or d_B, returning to the gas phase unreacted.
This is the Langmuir-Hinshelwood mechanism: both reactants must adsorb first, and only adjacent adsorbed pairs can react. Because surface sites are a shared, limited resource, raising one reactant's pressure too far starves the surface of the other — a self-poisoning effect that makes the reaction rate rise, peak, and fall again as the pressure ratio P_A/P_B is swept, producing the characteristic "volcano" curve shown on the right.
Surface reaction: A(ads) + B(ads) → AB(gas) [rate constant k]
LH rate law: r = k · θ_A · θ_B
Coverage balance: θ_A + θ_B + θ_empty = 1
Frequently Asked Questions
What is heterogeneous catalysis and why does it matter industrially?
Heterogeneous catalysis occurs when a solid catalyst (often a metal like Pt, Pd, or Ni) speeds up a gas- or liquid-phase reaction at its surface without being consumed. It underlies catalytic converters (removing CO and NOx from car exhaust), the Haber-Bosch process (ammonia synthesis for fertilizer), and Fischer-Tropsch synthesis (converting syngas into liquid fuels).
What does the Langmuir-Hinshelwood mechanism specifically require?
Both reactants must first adsorb onto the catalyst surface at separate empty sites. Only when an adsorbed A and an adsorbed B happen to occupy adjacent sites can they react to form the product, which then desorbs and frees both sites for the next cycle.
How does Langmuir-Hinshelwood differ from the Eley-Rideal mechanism?
In Eley-Rideal kinetics, only one reactant adsorbs onto the surface; the second reactant strikes it directly from the gas phase and reacts without ever adsorbing itself. Langmuir-Hinshelwood requires both species to be adsorbed neighbors before reacting, which is why surface coverage and site availability matter so much more here.
What does surface coverage θ mean?
Surface coverage θ is the fraction of catalytic sites occupied by a given adsorbed species, ranging from 0 (empty) to 1 (fully covered). In this two-reactant system, θ_A + θ_B + θ_empty = 1, and reaction can only proceed where occupied A and B sites happen to be adjacent.
What is the Langmuir adsorption isotherm?
The Langmuir isotherm describes equilibrium coverage as θ = Kp/(1 + Kp), where p is the partial pressure of the gas and K is an adsorption equilibrium constant. It assumes a fixed number of identical, independent sites that each hold at most one molecule — the same assumption this simulation's grid makes.
Why does the reaction rate have a maximum at an intermediate pressure ratio rather than increasing monotonically?
If pressure of A is too low relative to B (or vice versa), the surface fills almost entirely with B, leaving too few A sites for A-B neighbor pairs to form — the reaction is starved of one partner. This self-poisoning effect means the rate rises with pressure ratio, peaks near where θ_A ≈ θ_B, and then falls again as the dominant species crowds out the other.
What is catalytic poisoning in general?
Catalytic poisoning is the irreversible or long-lived blocking of active sites by a species (such as sulfur or lead) that adsorbs strongly but does not react or desorb, permanently reducing available surface area. This is distinct from the self-poisoning saturation modeled here, where an excess of one reactant merely outcompetes the other for sites but can still desorb and free them again.
How does a real catalytic converter work?
Catalytic converters use platinum, palladium, and rhodium coated onto a ceramic honeycomb. CO and unburned hydrocarbons adsorb and react with adsorbed oxygen atoms via Langmuir-Hinshelwood-type surface steps to form CO2 and H2O, while rhodium sites reduce NOx back to N2 and O2.
Why do catalysts increase reaction rate without being consumed?
Catalysts provide an alternative reaction pathway with lower activation energy by stabilizing the adsorbed transition state on their surface. Once the product desorbs, the catalytic site is regenerated exactly as it was, ready to bind fresh reactants — so the catalyst undergoes no net chemical change over a full cycle.
What is the difference between self-poisoning and irreversible poisoning?
Self-poisoning (modeled in this simulation) happens when one reactant's high partial pressure saturates the surface, crowding out the other reactant and killing the rate — but it is reversible simply by lowering that pressure. Irreversible poisoning happens when a contaminant binds so strongly it never desorbs, permanently removing that site from catalytic service.