Optimizing Adsorbate Selection, Adsorption Conditions, and Stoichiometry in Chemisorption

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Choice of Adsorbate

The choice of adsorbate is critical in temperature-programmed desorption (TPD) experiments. The selected gas should chemisorb selectively on the metal, avoiding sorption on the support or other catalytic components. Ideally, the adsorbate should form a stable monolayer and avoid irreversible reactions with either the metal or the support.

Table 1: suitable adsorbates

Example: CO can react with nickel to form volatile—and hazardous—nickel carbonyl (Ni(CO)₄),

making it unsuitable for certain Ni catalyst systems.

The adsorbate–metal interaction can be visualized using potential energy diagrams (Figure 1).

The initial physisorption step involves a small activation barrier (ΔE₁) and a minor energy release

(ΔH₁). Transitioning to a chemisorbed state requires overcoming a second activation barrier,

which may be small (ΔE₂) or large (ΔE₃). The heat of chemisorption (ΔH₂) is independent of this

barrier.

If the activation barrier is high, the process is classified as activated chemisorption, which

proceeds slowly and may require higher temperatures or longer adsorption times for full surface

coverage.

Case Example:

Hydrogen chemisorption on supported cobalt metal is activated. As shown in Figure 2:

• Curve A: Room temperature adsorption yielded minimal desorption.
• Curve B: Elevated temperature adsorption, followed by cooling in hydrogen, resulted in

full site coverage and a significant TPD signal.

Because theoretical guidance is limited, adsorbate selection typically relies on literature

precedent and practical experience. Table 1 provides a summary of suitable adsorbates for

common catalytic metals.

Choice of

Adsorption Conditions

Adsorption conditions must balance complete surface coverage with minimal side reactions.

Key Considerations:

• Temperature: Sufficiently high to ensure adsorption, but below levels that promote

undesirable reactions (e.g., CO disproportionation to CO₂ and carbon).

• Time: Long enough to allow for equilibrium.

Spillover Warning:

Spillover, where adsorbates migrate from metal crystallites to the support (see Figure 3), is a

kinetically slow process that can distort TPD data if adsorption times or temperatures are too

high.

Recommended Starting Conditions:

• Temperature: 100–200°C
• Time: 30–90 minutes
• Post-adsorption flush: At low temperature to remove weakly held species.

Adsorbate Stoichiometry

Interpreting TPD results requires knowledge of the adsorbate-metal stoichiometry. While direct

measurement is not possible in simple TPD experiments, stoichiometry can be estimated by:

• Comparing chemisorption surface area to BET surface area.
• Infrared (IR) spectroscopy.
• Crystallite size measurements via TEM or XRD.

General Rules:

• Hydrogen & Oxygen: Typically dissociative adsorption (stoichiometry = 0.5).
• CO: Stoichiometry varies widely (0.5 to 2), depending on metal and crystallite size.

In some cases (e.g., very small Rh or Ir crystallites), H₂/M stoichiometries of 1 have been reported,

but these are rare.

When precise stoichiometry cannot be determined, CO uptake can still be used as a relative basis

for comparing catalysts.

Advanced Stoichiometry Determination Using the AMI-300IR

For IR-active adsorbates (such as CO, NO, and selected hydrocarbons), the AMI-300IR provides a

superior method for stoichiometry determination.

By integrating in-situ IR spectroscopy with TPD and chemisorption analysis, the AMI-300IR

enables:

• Quantitative analysis: Peak areas correlate directly to adsorbed species concentrations.
• Speciation: Differentiation between linear, bridged, or multidentate adsorption geometries.

• Dynamic monitoring: Real-time observation of adsorbate behavior during adsorption/desorption cycles.

Example:

During CO adsorption, the AMI-300IR can distinguish linear CO on atop sites from bridged CO

species. This capability enhances stoichiometric precision and provides deeper insight into

adsorption mechanisms.

The AMI-300IR is particularly valuable for:

• Validating adsorption models.
• Confirming stoichiometries in complex or supported catalysts.
• Enhancing the accuracy of TPD interpretations.

References

1. "Chemisorption and Catalysis on Supported Metals", AMI Notes.
2. "Temperature-Programmed Desorption of Adsorbed Species from Catalyst Surfaces", AMI Notes

3. "Measuring Acidity in Zeolites Using TPD", AMI Notes
4. "Surface Area Measurement from Temperature-Programmed Desorption Data", AMI Notes