What Is Chemisorption? Principles, Analysis & Industrial Applications
What Is Chemisorption?
Chemisorption is the process by which gas molecules adsorb onto a solid surface through the formation of chemical bonds — typically covalent or ionic bonds with surface atoms. Unlike physisorption (physical adsorption driven by weak van der Waals forces), chemisorption involves electron transfer or sharing between the adsorbate molecule and the surface, resulting in much stronger binding with adsorption enthalpies typically exceeding 40 kJ/mol (compared to <20 kJ/mol for physisorption).
Chemisorption is the fundamental mechanism of catalytic activity — it is how reactant molecules bind to active metal surface sites before reacting to form products. Characterising chemisorption — measuring the number, type, and strength of surface active sites — is therefore essential for catalyst development, optimisation, and quality control in the chemical processing, petroleum refining, and emissions control industries.
Chemisorption vs. Physisorption
| Property | Chemisorption | Physisorption |
| Bond type | Chemical (covalent/ionic) | Van der Waals forces |
| Binding enthalpy | 40–400 kJ/mol | 5–40 kJ/mol |
| Temperature range | High (often >25°C) | Low (near boiling point of gas) |
| Specificity | Highly specific to surface site type | Non-specific, universal |
| Surface layers | Monolayer only | Multi-layer possible |
| Reversibility | Often irreversible | Generally reversible |
Chemisorption Measurement Methods
Pulse Chemisorption
The most widely used industrial method. Pulses of a probe gas (H₂ for metals, CO for metal carbonyls, NH₃ for acid sites, CO₂ for basic sites) are injected into a flow of inert carrier gas passing over the catalyst at defined temperature. Each pulse is progressively consumed by chemisorption until the surface is saturated. The total gas consumed equals the number of surface active sites — expressed as metal dispersion, surface area, and active site density.
Probe gases and applications:
- H₂ chemisorption (ASTM D3908): Measures metallic surface area and metal dispersion of supported platinum group metal (PGM) catalysts (Pt, Pd, Rh, Ru) used in petroleum reforming and emissions catalysts
- CO chemisorption: Measures copper, cobalt, and nickel surface area in methanol synthesis and hydrogenation catalysts
- N₂O chemisorption: Specifically measures surface copper active site density
- NH₃ chemisorption: Measures total acid site density and strength distribution in zeolites and acid catalysts
Temperature-Programmed Reduction (TPR)
TPR heats the oxidised catalyst in a H₂/inert gas mixture at a defined rate and measures hydrogen consumption as a function of temperature. The reduction temperature profile characterises the oxidation state and reducibility of the active metal phase — identifying whether the metal is in an easily reducible surface oxide or a strongly bound lattice oxide form.
Temperature-Programmed Desorption (TPD)
After chemisorption saturation of the surface, the temperature is raised at a defined rate and desorbing probe molecules are measured. The desorption temperature of each peak reflects the bond strength of that chemisorption site — providing site strength distribution data. NH₃-TPD characterises acid site strength in zeolites; CO₂-TPD characterises base site strength.
BET Surface Area and Physisorption
Before chemisorption, catalyst total surface area and pore structure are characterised by N₂ physisorption (BET method, ASTM D3663) — providing the framework for normalising chemisorption data by active metal surface area and total surface area.
Industrial Applications of Chemisorption Characterisation
Petroleum reforming catalysts (Pt/Al₂O₃): H₂ chemisorption measures platinum dispersion — high dispersion (many small Pt clusters) provides more active sites per unit Pt mass, maximising catalytic activity for fuel reforming. Automotive emissions catalysts (TWC — three-way catalyst): CO chemisorption and H₂ chemisorption characterise precious metal utilisation in catalytic converter washcoat optimisation. Zeolite acid catalysts for petrochemicals: NH₃-TPD characterises the acid site distribution critical for selectivity control in cracking and isomerisation reactions.
Why Choose Infinita Lab for Chemisorption and Catalyst Characterisation?
Infinita Lab provides pulse chemisorption, TPR, TPD, and BET surface area analysis for catalyst materials through our nationwide accredited surface science and catalyst characterisation laboratory network.
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Frequently Asked Questions (FAQs)
What is metal dispersion and how is it measured by chemisorption? Metal dispersion is the fraction of metal atoms exposed at the surface (accessible to reactants) divided by the total metal atoms in the catalyst. High dispersion (small metal particles, many surface atoms) maximises catalytic activity per unit metal mass. Dispersion is calculated from H₂ or CO chemisorption uptake and the stoichiometry of adsorption per surface metal atom (typically H:Pt = 1:1 and CO:Pt = 1:1).
Why is H₂ chemisorption specific to platinum group metals? H₂ chemisorbs strongly and specifically on PGM surfaces (Pt, Pd, Rh, Ru, Ir) at moderate temperatures (25–50°C) but does not chemisorb significantly on support oxides (Al₂O₃, SiO₂, TiO₂) under the same conditions. This specificity enables H₂ chemisorption to measure only the metallic active phase surface area — independent of the support surface.
What does a TPR profile tell you about a catalyst? TPR peaks at lower temperatures indicate easily reducible species (surface metal oxides, well-dispersed metal clusters); peaks at higher temperatures indicate more bulk-like or strongly bound oxide phases that require higher temperatures or more severe reduction conditions. The total H₂ consumption quantifies the degree of catalyst oxidation and reducibility — critical for optimising reduction (activation) conditions before service.
How does NH₃-TPD characterise zeolite acid catalysts? NH₃ adsorbs strongly on both Brønsted acid sites (OH groups) and Lewis acid sites (coordinatively unsaturated Al atoms) on zeolites. TPD of adsorbed NH₃ shows desorption peaks: low-temperature (~200°C) and high-temperature (~400°C) peaks correspond to weak and strong acid sites, respectively. Peak areas give quantitative acid site densities; peak temperatures indicate site strength — correlating with catalytic activity for acid-catalysed reactions.
What is the ASTM standard for hydrogen chemisorption of catalysts? ASTM D3908 — Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts by Volumetric Vacuum Method — provides the standardised procedure for measuring platinum surface area and dispersion by H₂ pulse or static volumetric chemisorption. It specifies pre-treatment conditions, dosing protocol, and calculation methods for reproducible results.
