Catalysis

In short

A catalyst speeds up a reaction by offering a lower-energy path and is recovered unchanged at the end, so it changes the *rate* but never the equilibrium. Industrially, catalysis is how almost every bulk chemical is made — ammonia, sulphuric acid, petrol, polymers, fertilisers — and it is where chemical kinetics meets mass transfer, because on a porous pellet the reaction and diffusion compete. In GATE (CH) it is examined as Langmuir–Hinshelwood rate laws, the Thiele modulus and effectiveness factor, pore and film diffusion resistance, and deactivation. Placement interviews at IOCL, BPCL, HPCL, RIL, GNFC, RCF, Coromandel, Tata Chemicals, UPL and catalyst makers (Süd-Chemie India/Clariant, Johnson Matthey India) probe why a reactor's conversion fell and whether the catalyst is poisoned, coked or diffusion-limited.

Explained

What a catalyst is and is not

• Catalyst: a substance that increases the rate of reaction by providing an alternative mechanism of lower activation energy, and is not consumed stoichiometrically.
• It cannot shift equilibrium — it accelerates forward and reverse reactions by the same factor, so K remains fixed by thermodynamics. A "catalyst that improves yield" only does so for a *kinetically limited* or *selectivity-limited* system, not an equilibrium-limited one.
• It cannot make a thermodynamically impossible reaction go (ΔG > 0 at the operating condition).
• Inhibitor / negative catalyst: slows a reaction (e.g. hydroquinone stopping premature polymerisation of styrene in storage).
• Autocatalysis: a product accelerates the reaction, so the rate rises then falls (ester hydrolysis producing acid).
• Promoter: a small addition with little activity itself that raises activity or stability — K₂O and Al₂O₃ on the iron catalyst for ammonia (Al₂O₃ is a *structural* promoter preventing sintering; K₂O an *electronic* promoter).
• Poison: irreversibly adsorbs on active sites — sulphur on Ni and Pt, lead on three-way catalysts, CO on low-temperature fuel-cell Pt, chlorides on Cu.
• Support (carrier): high-area, cheap, thermally stable solid (γ-Al₂O₃, SiO₂, activated carbon, TiO₂, zeolite) that disperses expensive metal so a 0.3 wt% Pt gives a large metal area.
• Turnover frequency (TOF): molecules converted per active site per second — the true intrinsic activity measure, independent of loading.

Classification

• Homogeneous: every molecule of catalyst is accessible, so high activity and selectivity at mild conditions, well-defined single site, no diffusion limitation. Limitation: separation from product is costly, corrosion, thermal instability, cannot be used above ~200 °C easily.
• Heterogeneous: easy separation, continuous fixed-bed operation, robust at high temperature. Limitation: sites are non-uniform, and internal/external mass and heat transfer intrude.
• Enzymes: extraordinary selectivity (stereo- and regio-specific) at 30–40 °C and near-neutral pH, but narrow temperature/pH window and inhibition. Followed by Michaelis–Menten kinetics, which is mathematically the same form as Langmuir–Hinshelwood.

Adsorption — the basis of surface catalysis

• Physisorption: van der Waals, ΔH ≈ 5–40 kJ/mol, multilayer, reversible, fast, no bond breaking. Used for BET surface-area measurement with N₂ at 77 K.
• Chemisorption: chemical bond with the surface, ΔH ≈ 40–400 kJ/mol, monolayer only, often activated and specific. This is the step that activates the reactant. Selective chemisorption (H₂, CO) measures *active metal* area and dispersion.
• Langmuir adsorption isotherm assumptions: energetically uniform sites, monolayer, one molecule per site, no interaction between adsorbed molecules, adsorption–desorption at equilibrium.
• Other isotherms: Freundlich (empirical, heterogeneous surface, q = kC^1/n), Temkin (heat of adsorption falls linearly with coverage), BET (multilayer extension of Langmuir).
• Characterisation toolbox: BET (total area, m²/g), mercury porosimetry / N₂ desorption (pore size distribution), XRD (crystallite phase and size), TPR/TPD (reducibility, acid-site strength), NH₃-TPD and pyridine-FTIR (Brønsted vs Lewis acidity), TEM/SEM-EDX (dispersion), TGA (coke).

The seven steps on a porous catalyst pellet

For a fixed-bed reaction A → B, every molecule goes through:

1. External (film) diffusion of A from bulk fluid to the outer pellet surface.
2. Internal (pore) diffusion of A into the pore network.
3. Adsorption of A on an active site.
4. Surface reaction of the adsorbed species.
5. Desorption of B.
6. Pore diffusion of B out.
7. Film diffusion of B to the bulk.

• Steps 3–5 are *chemical*; 1–2 and 6–7 are *physical*. The slowest is the rate-controlling step, and the observed rate law looks like that step.
• Diagnostics: if the rate changes when only the stirrer speed or superficial velocity is raised (same contact time), external diffusion controls. If the rate per gram changes when pellet size is reduced, internal diffusion controls. If both are unaffected, the surface reaction (intrinsic kinetics) controls.
• Falsified kinetics: in the strong pore-diffusion regime the observed order becomes (n+1)/2 — a true second order appears as 3/2 order and a true zero order as ½ order, while first order stays first order — and the observed activation energy is about half the true value (E_obs ≈ E_true/2). In the external-diffusion regime the apparent order → 1 and E_obs ≈ 5–10 kJ/mol (diffusion's weak temperature dependence). An Arrhenius plot that bends over to a shallow slope at high temperature is the classic signature.

Langmuir–Hinshelwood–Hougen–Watson (LHHW) rate laws

Derivation outline (single-site surface reaction of A, product B, inert I):

1. Write each step as reversible and equate the non-rate-limiting steps to equilibrium.
2. Use the site balance C_t = C_v + C_A·s + C_B·s (total = vacant + occupied).
3. Eliminate the unknown surface concentrations and keep the rate-limiting step's rate.
4. Single-site surface reaction controlling: −r_A = k K_A P_A / (1 + K_A P_A + K_B P_B)
5. Dual-site (bimolecular, both adsorbed) — true Langmuir–Hinshelwood: −r_A = k K_A K_B P_A P_B / (1 + K_A P_A + K_B P_B)²
6. Eley–Rideal: one species adsorbed, the other reacts from the gas phase: −r_A = k K_A P_A P_B / (1 + K_A P_A + K_B P_B)
7. Adsorption of A controlling: −r_A = k(P_A − P_B/K)/(1 + K_B P_B)
8. Dissociative adsorption (H₂ → 2H·s) puts √(K_H P_H2) in numerator and denominator.
9. Shape of LHHW: at low P_A, rate ∝ P_A (first order); at high P_A, the surface saturates and the rate becomes zero order. For the dual-site form the rate passes through a maximum with P_A — one reactant crowds the other off the surface. Interviewers love this: *"why did conversion fall when I raised the reactant pressure?"*
10. Mechanism discrimination: fit candidate forms to data, reject any model giving a negative adsorption constant or a K that *rises* with temperature (adsorption is exothermic, so K must fall with T).

Internal diffusion: Thiele modulus and effectiveness factor

• Effectiveness factor η = actual rate with pore diffusion / rate if the whole interior were at the external surface concentration. η ≤ 1 (η can exceed 1 only in a strongly exothermic non-isothermal pellet).
• Thiele modulus φ compares reaction rate to diffusion rate inside the pellet. For first order in a sphere of radius R: φ₁ = R√(k ρ_c S_a/D_e) (or R√(k/D_e) with k per unit volume).
• Limits for a sphere, first order: φ < 0.4 → η ≈ 1 (kinetic control); φ > 4 → η ≈ 3/φ (strong diffusion control, reaction confined to a thin outer shell).
• Weisz–Prater criterion uses only observed data (no k needed): C_WP = φ²η = (−r_A,obs ρ_c R²)/(D_e C_As). C_WP ≪ 1 → no internal limitation; C_WP ≫ 1 → severe.
• Effective diffusivity D_e = D_AB·ε_p·σ/τ, where ε_p = pellet porosity, σ = constriction factor, τ = tortuosity (typically 2–6). In small pores Knudsen diffusion dominates (D_K ∝ d_pore√(T/M)) and D_e is pressure-independent.
• Remedies for diffusion limitation: smaller pellets (but higher pressure drop — Ergun), egg-shell (surface-impregnated) catalyst, bimodal macro+micro pore structure, monoliths and rings, lower temperature.
• Thermal effects: Prater number β = ΔT_max/T_s = (−ΔH)D_e C_As/(k_e T_s) gives the maximum internal temperature rise; a strongly exothermic pellet can run hotter inside than outside.

Deactivation and regeneration

• Deactivation rate law: activity a(t) = −r_A(t)/−r_A(t=0), with −da/dt = k_d aⁿ (and sometimes a concentration term). Order 0, 1 or 2 are the examined forms.
• Reactor responses: raise temperature along a planned trajectory to hold conversion (ammonia, hydrotreating); moving bed or CCR (continuous catalyst regeneration) for fast coking as in Platforming; fluidised riser + regenerator for very fast coking as in FCC (catalyst life on the order of seconds).
• Three-way catalytic converter (Pt-Pd-Rh on a cordierite monolith with ceria–zirconia oxygen storage) works only near stoichiometric air–fuel ratio, controlled by the lambda sensor; leaded petrol destroys it permanently — which is why unleaded petrol preceded catalytic converters in India.

Indian practice, codes and regulation

• Fixed-bed reactors and converters are pressure vessels designed to IS 2825 (unfired pressure vessels) or ASME Section VIII Div. 1, registered and inspected under the state Factories/industrial-safety machinery; note that the four Labour Codes in force from 21 November 2025 replaced the Factories Act, 1948 and the OSH&WC Code now carries the factory-safety provisions.
• Hazardous-chemical plants (ammonia, oleum, chlorine) fall under the MSIHC Rules, 1989 and the Chemical Accidents (EPPR) Rules, 1996 under the Environment (Protection) Act, 1986; consents to establish and operate come from the State Pollution Control Board under CPCB norms, and emissions from catalytic units are covered by industry-specific standards.
• PESO (Nagpur) licenses pressure/flammable-gas installations; BIS standards cover fuel and fertiliser product quality (urea, sulphuric acid).
• Vehicle catalytic converters must meet BS-VI (nationwide from April 2020; Phase 2 with RDE from April 2023) — the reason S in petrol and diesel was cut to 10 ppm.
• Spent catalyst containing Ni, Co, Mo, V is a hazardous waste under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016 — it must go to an authorised recycler/TSDF, and precious-metal recovery is routine commercial practice.
• GST on chemicals and catalysts generally falls in the 18% slab under the 5/18/40% structure effective 22 September 2025; verify the specific HSN before quoting.

Key points

• Catalyst — lowers activation energy, unchanged at the end, does not alter equilibrium conversion.
• Homogeneous vs heterogeneous — single phase, high selectivity, hard to separate vs solid, easy to separate, transport-limited.
• Promoter / poison / support — enhances, kills, disperses respectively; Al₂O₃+K₂O on Fe, S on Ni, γ-Al₂O₃ as carrier.
• Physisorption — weak, multilayer, measures total area by BET; chemisorption — strong, monolayer, measures active metal area.
• Langmuir isotherm — uniform sites, monolayer, no adsorbate interaction, equilibrium.
• Seven steps — film diffusion, pore diffusion, adsorption, surface reaction, desorption, pore diffusion out, film diffusion out.
• LHHW form — rate = (kinetic term × driving force)/(adsorption term)ⁿ; the denominator is site competition.
• Rate maximum with pressure — dual-site mechanism, one reactant displaces the other.
• Thiele modulus φ — reaction rate ÷ internal diffusion rate; η — penalty paid for pore diffusion.
• η ≈ 3/φ for a sphere when φ > 4 — rate becomes proportional to 1/R, so halving pellet size doubles the rate per gram.
• Falsified kinetics — apparent order (n+1)/2 and apparent E ≈ E_true/2 under strong pore diffusion.
• Weisz–Prater C_WP — observable test for internal limitation; Mears criterion for external.
• External diffusion diagnosis — rate depends on fluid velocity or stirrer speed at constant contact time.
• Deactivation — coking (reversible by burn-off), poisoning (usually permanent), sintering (permanent), attrition.
• Regeneration strategy follows deactivation speed — fixed bed (months/years), moving bed/CCR (days), fluid riser (seconds).
• TOF — true intrinsic activity per site, independent of loading and dispersion.
• Selectivity, not just activity, is usually the commercial reason a catalyst is chosen (zeolite shape selectivity, Ziegler–Natta stereospecificity).
• Spent catalyst — hazardous waste under the 2016 Rules; metals recovered by authorised recyclers.

Formulas and examples

Example 1 — Effectiveness factor and pellet size. A first-order reaction on spherical pellets of R = 3 mm has k = 2.0 s⁻¹ (per unit pellet volume) and D_e = 1.2 × 10⁻⁷ m²/s.

• φ = R√(k/D_e) = 3 × 10⁻³ × √(2.0/1.2 × 10⁻⁷) = 3 × 10⁻³ × √(1.667 × 10⁷) = 3 × 10⁻³ × 4083 = 12.25.
• φ > 4, so η ≈ 3/φ = 3/12.25 = 0.245 — only 24.5% of the pellet's intrinsic activity is realised.
• Crush to R = 0.75 mm (one-quarter): φ = 3.06, so use the full formula. coth(3.06) ≈ 1.0045; η = (3/3.06²)(3.06 × 1.0045 − 1) = (3/9.364)(2.074) = 0.664.
• Rate per gram rises by 0.664/0.245 = 2.7 times — at the cost of higher bed pressure drop.

Example 2 — Weisz–Prater test from plant data. Observed rate −r_A = 1.5 × 10⁻³ mol/(g_cat·s) = 1.5 mol/(kg·s); pellet density ρ_c = 1,400 kg/m³; R = 2 mm; D_e = 8 × 10⁻⁸ m²/s; surface concentration C_As = 50 mol/m³.

• C_WP = (1.5 × 1400 × (2 × 10⁻³)²)/(8 × 10⁻⁸ × 50) = (1.5 × 1400 × 4 × 10⁻⁶)/(4 × 10⁻⁶) = 8.4/4 × 10⁻⁶ ÷ 10⁻⁶ … compute cleanly: numerator = 1.5 × 1400 = 2100; 2100 × 4 × 10⁻⁶ = 8.4 × 10⁻³. Denominator = 8 × 10⁻⁸ × 50 = 4 × 10⁻⁶.
• C_WP = 8.4 × 10⁻³ / 4 × 10⁻⁶ = 2,100 ≫ 1 → severe internal diffusion limitation. Since C_WP = φ²η and η ≈ 3/φ, φ ≈ C_WP/3 = 700 and η ≈ 3/700 ≈ 0.0043 — the reaction is happening in a very thin outer shell, so an egg-shell catalyst would give almost the same rate with 1/200th of the metal.

Example 3 — Falsified activation energy. An Arrhenius plot of ln(−r_A,obs) vs 1/T gives a slope of −3,200 K in the high-temperature region, where the Weisz–Prater number is 150.

• E_obs = 3,200 × 8.314 = 26,600 J/mol ≈ 26.6 kJ/mol.
• Strong pore diffusion → E_obs ≈ E_true/2, so E_true ≈ 53 kJ/mol.
• Check: the intrinsic (low-temperature) branch of the same plot should show a slope about twice as steep, ≈ −6,400 K.

Example 4 — Fitting a Langmuir–Hinshelwood rate law. For A → B with only A adsorbing (single-site, surface reaction controlling), −r_A = kK_A P_A/(1 + K_A P_A). Data at 500 K:

• Linearise: P_A/(−r_A) = 1/(kK_A) + P_A/k.
• P_A/(−r_A): 0.625, 0.833, 1.622, 4.587.
• Slope between the last two points: (4.587 − 1.622)/(10 − 3) = 0.4236 → k = 1/0.4236 = 2.36 mol/(kg·s).
• Intercept = 1.622 − 0.4236 × 3 = 0.351 = 1/(kK_A) → K_A = 1/(2.36 × 0.351) = 1.21 bar⁻¹.
• Check at P_A = 1.0: −r_A = 2.36 × 1.21 × 1/(1 + 1.21) = 2.856/2.21 = 1.29 mol/(kg·s) vs 1.20 observed — acceptable fit. Note the rate plateaus near k = 2.36 as the surface saturates.

Example 5 — Deactivation and temperature compensation. A hydrotreater catalyst deactivates first order with k_d = 0.0025 day⁻¹. Intrinsic E = 90 kJ/mol; start-of-run temperature 340 °C (613 K).

• After 1 year: a = exp(−0.0025 × 365) = exp(−0.9125) = 0.402 — 60% of activity lost.
• To hold the same rate, k must rise by 1/0.402 = 2.49 times: ln 2.49 = (90,000/8.314)(1/613 − 1/T₂).
• 0.9125 = 10,825 × (1/613 − 1/T₂) → (1/613 − 1/T₂) = 8.43 × 10⁻⁵ → 1/T₂ = 1.6313 × 10⁻³ − 0.843 × 10⁻⁴ = 1.5470 × 10⁻³ → T₂ = 646 K = 373 °C.
• So a ~33 °C rise over the year — exactly the end-of-run temperature limit that decides when a reactor is shut for a catalyst change.

Where it is used

• Refining (IOCL, BPCL, HPCL, RIL, NRL, MRPL): FCC, hydrocracking, hydrotreating, catalytic reforming, isomerisation — catalyst selection, activity monitoring, end-of-run prediction and reactor turnaround planning are core process-engineer work.
• Fertilisers (RCF, GNFC, IFFCO, Chambal, Coromandel, NFL): ammonia synthesis loop, steam reforming, shift converters, ammonia oxidation on Pt-Rh gauze for nitric acid; guard beds to protect Ni and Cu catalysts.
• Bulk and fine chemicals: sulphuric acid contact plants, PTA oxidation, hydrogenation of oils (Ni) in vanaspati, UPL/Deepak Nitrite/Aarti Industries nitration and hydrogenation units.
• Polymers: Ziegler–Natta and metallocene catalysis in PP/PE plants (RIL, HMEL, OPaL); the catalyst determines tacticity, molecular-weight distribution and hence product grade.
• Environment and automotive: three-way converters and SCR for BS-VI vehicles, DeNOx SCR on thermal-power and cement stacks, catalytic incineration of VOCs under SPCB consent conditions.
• Job roles: process engineer, catalyst/technical-services engineer at Clariant/Johnson Matthey/Haldor Topsoe India offices, R&D at CSIR-NCL Pune, CSIR-IIP Dehradun and IOCL R&D Faridabad, and bio-catalysis roles in enzyme and pharma manufacturing.

Interview questions

• Q: Can a catalyst increase the equilibrium conversion of ammonia synthesis? A: No — it speeds up both directions equally, so K and the equilibrium conversion are set only by temperature and pressure; the catalyst merely lets you reach equilibrium at a lower temperature, which indirectly helps because K is larger there.
• Q: How would you tell, on a plant, whether a reactor is limited by intrinsic kinetics, pore diffusion or film diffusion? A: Raise the superficial velocity at constant space time — a rate change means film control; compare rate per gram for crushed versus whole pellets — a change means pore control; if neither moves, the surface reaction controls.
• Q: Why does the apparent activation energy halve under strong pore diffusion? A: The observed rate goes as η·k ≈ (3/φ)k ∝ √(k·D_e), so the temperature dependence of k enters as a square root and the Arrhenius slope is about half the true one.
• Q: What is the difference between a promoter and a support? A: A promoter is a small additive that raises the activity or stability of the active phase; a support is the bulk high-area carrier that disperses and anchors it — alumina can act as both (structural promoter and carrier).
• Q: Why does the rate of a dual-site reaction pass through a maximum with reactant pressure? A: The adsorption term is squared in the denominator, so beyond an optimum one reactant occupies so many sites that its partner cannot adsorb adjacently.
• Q: Difference between coking and poisoning, and why it matters commercially? A: Coke physically blocks pores and burns off in a regenerator, so the catalyst is reusable; a poison chemisorbs on the active site and is usually permanent, forcing a charge replacement — so the cure is a guard bed and feed treatment, not regeneration.
• Q: Why is FCC catalyst circulated continuously while an ammonia catalyst lasts years? A: Coking on zeolite in the riser is a matter of seconds, so the solid must be regenerated continuously; iron in the ammonia loop deactivates only by slow sintering and poisoning of a clean, desulphurised syngas.
• Q: What does BET surface area tell you, and what does it not? A: It gives the total (mainly internal) area from N₂ physisorption — but not how much of it is *active*; active metal area needs selective chemisorption of H₂ or CO.
• Q: An effectiveness factor greater than 1 — possible? A: Yes, only for a strongly exothermic reaction in a non-isothermal pellet, where the hotter interior reacts faster than the surface would; for isothermal pellets η ≤ 1.
• Q: Why does a catalytic converter need unleaded petrol and λ ≈ 1? A: Lead permanently poisons the noble metals, and the three-way function needs oxidation of CO/HC and reduction of NOx simultaneously, which only works in a narrow window around the stoichiometric mixture.
• Q: What is bifunctional catalysis, with an example? A: Two different kinds of site cooperate — in Pt-Re/Cl-Al₂O₃ reforming the metal dehydrogenates paraffins to olefins and the chlorided alumina acid sites isomerise and cyclise them.
• Q: Why does lowering pellet size help only up to a point? A: Once φ < 0.4 the pellet is already kinetically controlled, so further reduction buys no rate but raises pressure drop (Ergun) and attrition — and in a fluid bed, carryover.
• Q: What is the usual fate of a spent hydrotreating catalyst in India? A: It is a hazardous waste under the Hazardous and Other Wastes Rules, 2016, and goes to an authorised recycler for Mo, Ni and Co recovery or to a TSDF — it cannot be landfilled on site.

Common mistakes

• Saying a catalyst "shifts equilibrium" or "increases yield" without distinguishing a kinetically limited from an equilibrium-limited reaction.
• Mixing up physisorption and chemisorption, or using BET area as a measure of active sites.
• Reporting a fitted LHHW model with a negative adsorption constant, or one whose K increases with temperature — both are physically impossible, since adsorption is exothermic.
• Using the sphere formula η ≈ 3/φ outside the strong-diffusion regime (φ > 4), or applying a slab correlation to a sphere without changing the characteristic length (L = R/3 for a sphere if a generalised modulus is used).
• Forgetting unit consistency in the Thiele modulus: k must be per unit *pellet volume* (1/s) to pair with R and D_e; if k is per kg, multiply by pellet density.
• Quoting a measured activation energy as intrinsic when the Weisz–Prater number shows the data were taken under diffusion control.
• Confusing deactivation with a process upset — a falling conversion can equally be loss of feed purity, channelling, or bypass through a cracked basket, so always check the temperature profile and pressure drop before blaming the catalyst.
• Treating deactivation order as always first order; fit a(t) rather than assuming, and never extrapolate a short laboratory run to a multi-year commercial cycle.

Related topics

Chemical kinetics · Reactor design: batch, CSTR and PFR · Multiple reactions and selectivity · Non-ideal reactors and residence time distribution · Biochemical engineering basics · Mass transfer and diffusion