Eley-Rideal Mechanism: When a Gas Molecule Strikes an Adsorbed Partner

What the Eley-Rideal Mechanism Is and Where It Applies

The Eley-Rideal mechanism describes a two-species surface reaction with an asymmetry: one reactant (call it A) is chemisorbed on the catalyst, while the second reactant (B) collides with that adsorbed A directly from the gas phase and reacts on impact. B never becomes a proper, thermally equilibrated adsorbed species; it either reacts in a single encounter or bounces off.

The overall stoichiometry is simply:

• A(g) + * → A-* (adsorption of A onto a free site *)
• B(g) + A-* → AB(g) + * (gas-phase B hits adsorbed A; product leaves)

Because B does not need a vacant site, the mechanism thrives when the surface is saturated with A. It is genuinely important in a handful of real settings: H₂ formation on interstellar dust grains, hydrogen abstraction and recombination on graphite and metals, some selective oxidations, ammonia oxidation, and plasma/atom-beam etching where energetic gas-phase radicals strike adsorbed layers. It is less common than Langmuir-Hinshelwood, so a genuine E-R assignment usually demands molecular-beam or isotope evidence.

The Mechanism and Rate Law, Step by Step

Start from the two elementary steps and assume the surface reaction (step 2) is rate-determining, with adsorption of A in quasi-equilibrium.

Step 1 (fast equilibrium): A(g) + * ⇌ A-*. Langmuir adsorption gives the coverage of A:

θ_A = K_A·p_A / (1 + K_A·p_A)

where K_A is the adsorption equilibrium constant of A and p_A its partial pressure.

Step 2 (rate-determining): B(g) + A-* → products. The rate is proportional to the flux of B hitting the surface (∝ p_B) times the fraction of sites carrying A (θ_A):

r = k·p_B·θ_A = k·p_B·K_A·p_A / (1 + K_A·p_A)

Symbols: r = reaction rate, k = surface rate constant, p_B = partial pressure of gas-phase B, θ_A = fractional coverage of A. The signature is that r is first order in p_B at all coverages, and in the high-coverage limit (K_A·p_A » 1) it reduces to r ≈ k·p_B — the rate keeps climbing with p_B even when A saturates every site.

Key Quantities and a Worked Example

Consider H-atom recombination on a graphite surface, the textbook E-R case: H(g) + H(ads) → H₂(g). The released bond energy is the H–H bond dissociation energy, D(H–H) = 436 kJ/mol (4.52 eV), minus the chemisorption well depth of the bound H atom.

Worked example. Suppose K_A·p_A = 9 so that θ_A = 9/(1+9) = 0.90. If k = 2.0 × 10⁻³ (in consistent units) and p_B = 50 Pa, then:

r = k·p_B·θ_A = (2.0×10⁻³)(50)(0.90) = 0.090 (rate units).

Double p_B to 100 Pa and the rate doubles to 0.18 — the hallmark first-order-in-B response. Contrast this with Langmuir-Hinshelwood, where raising one pressure eventually lowers the rate by crowding out the partner.

• Sticking / reaction probability per E-R collision is often low, ~10⁻³ to 10⁻¹, and rises with the translational energy of B.
• Activation energy for the abstraction step is typically small (a few to tens of kJ/mol) because the incoming atom is 'hot.'

How Eley-Rideal Behavior Is Measured

Distinguishing E-R from Langmuir-Hinshelwood experimentally rests on a few classic probes:

• Molecular-beam / atom-beam scattering: fire a well-collimated beam of B at a surface pre-covered with A. If product AB appears promptly, tracks the beam flux, and shows a non-thermal (translationally hot, forward-scattered) velocity distribution, that is the E-R fingerprint — B never equilibrated with the surface temperature.
• Product energy analysis: E-R products carry excess translational and internal energy; time-of-flight and REMPI (resonance-enhanced multiphoton ionization) reveal rovibrationally excited AB, unlike the thermalized products of L-H.
• Pressure-dependence / kinetics: plot rate vs p_B at fixed high coverage of A. A straight line through the origin (first order, no maximum) argues for E-R.
• Isotope labeling: using D(g) on H(ads) surfaces shows whether the incoming atom retains gas-phase character.

In industrial reactor modeling, engineers fit measured rates to competing Langmuir-Hinshelwood-Hougen-Watson (LHHW) and E-R rate expressions and pick the form that reproduces the pressure and coverage dependence best.

How It Compares to Related Mechanisms

The three canonical surface mechanisms differ in how many partners must adsorb:

• Langmuir-Hinshelwood (L-H): both A and B chemisorb, then react as neighbors. Rate = k·θ_A·θ_B, which passes through a maximum as coverage of one species crowds out the other — the tell-tale volcano curve. This is by far the most common heterogeneous pathway (e.g., CO oxidation on Pt).
• Eley-Rideal: only A adsorbs; B strikes from the gas. Rate rises monotonically with p_B, no volcano.
• Mars-van Krevelen: a special oxidation case where lattice oxygen of the catalyst is the adsorbed reactant and is later replenished from gas O₂.

A subtle relative is the 'hot-atom' (Harris-Kasemo) mechanism: B lands, skitters across the surface in a transient non-equilibrated state, then reacts — intermediate between E-R (no accommodation) and L-H (full accommodation). Many real 'Eley-Rideal' observations are actually hot-atom in character, which is why careful beam experiments matter.

Exceptions, Significance, and Famous Cases

Astrochemistry's crown jewel. Molecular hydrogen — the most abundant molecule in the universe — forms on interstellar dust grains. At low grain temperatures physisorbed atoms diffuse and recombine by Langmuir-Hinshelwood, but for warm graphitic grains (T ≳ 25 K) physisorbed atoms evaporate too fast, and the Eley-Rideal channel through chemisorbed sites dominates. The nascent H₂ leaves rovibrationally excited, an observable signature.

Caveats and limits:

• Pure E-R is rarer than it looks — reaction probabilities per hit are usually small (<1%), so total throughput is often modest unless the gas flux is enormous.
• Many claimed E-R cases are hot-atom or precursor-mediated once probed by molecular beams.
• The simple rate law assumes ideal Langmuir adsorption of A; lateral interactions, multsite adsorption, or surface reconstruction break it.

Where it genuinely operates — plasma etching of silicon by atomic F/Cl, atomic-oxygen erosion of spacecraft polymers in low Earth orbit, and surface hydrogenation — the E-R route lets a saturated surface keep reacting, which is impossible for a coverage-limited L-H mechanism.

Eley-Rideal vs Langmuir-Hinshelwood vs Mars-van Krevelen surface mechanisms

Feature	Eley-Rideal	Langmuir-Hinshelwood	Mars-van Krevelen
Reactants adsorbed before reacting	One	Both	One + lattice O
Rate-law form	k·p_B·θ_A	k·θ_A·θ_B	k·θ_A·(1-θ_vac)
Behavior vs coverage of A	Increases monotonically	Passes through a maximum	Depends on lattice reoxidation
Order in incoming gas B	First order in p_B	Fractional / saturating	First order in reductant
Thermal equilibration of B	No (often 'hot')	Yes	Reductant not lattice-bound
Classic example	H + H(ads) -> H2	CO + O(ads) on Pt	Selective oxidation on V2O5