Questions: Surface Chemistry and Adsorption

Question 1

The Langmuir adsorption isotherm assumes: (1) a fixed number of equivalent adsorption sites, (2) each site holds at most one adsorbate molecule, (3) no interactions between adsorbed molecules. Under these assumptions, the fractional surface coverage theta = KP / (1 + KP), where K is the equilibrium constant and P is the gas pressure. At very high pressures, the Langmuir isotherm predicts that surface coverage continues to increase without limit.

• T. True
• F. False

Answer: False

At very high pressures (KP >> 1), the Langmuir isotherm predicts theta approaches 1 — complete monolayer coverage — not unlimited adsorption. This saturation behavior is a defining feature of the Langmuir model: once every site is occupied, additional gas pressure cannot increase coverage. The shape of the isotherm is hyperbolic: rapid initial increase at low pressure (each molecule easily finds an empty site), then a plateau as sites fill up. This saturation distinguishes the Langmuir isotherm from the BET isotherm, which allows multilayer adsorption and shows coverage increasing beyond a monolayer at pressures approaching the saturation vapor pressure.

Question 2

The BET method measures the surface area of a porous material by analyzing its nitrogen adsorption isotherm at 77 K. Why is nitrogen at 77 K used rather than, say, water vapor at room temperature?

Model answer: Nitrogen at 77 K (its boiling point) provides physisorption that is reversible, uniform, and well-characterized. At this temperature, N2 molecules physisorb on essentially all surface types with a known cross-sectional area (0.162 nm2 per molecule), enabling straightforward conversion from monolayer capacity to surface area. The interaction is weak enough to be reversible (allowing adsorption-desorption isotherms) but strong enough for measurable coverage. Water vapor is unsuitable because water chemisorbs on many oxide surfaces, forms hydrogen-bonded clusters rather than uniform monolayers, and has an irregular cross-sectional area that varies with surface chemistry — all of which violate BET assumptions.

The BET method transforms an experimental isotherm into a surface area by determining the monolayer capacity (volume of gas needed to cover the surface one molecule deep) and multiplying by the known area per molecule. The BET equation linearizes the multilayer isotherm in the relative pressure range P/P0 = 0.05-0.35, where the model is most reliable. Below this range, micropore filling distorts the isotherm; above it, capillary condensation in mesopores adds non-surface-area contributions. Despite its simplifying assumptions, BET surface area is the universally accepted metric for comparing porous materials — catalysts, adsorbents, battery electrodes, and construction materials.

Question 3

A material with a surface energy of 2000 mJ/m2 (e.g., a freshly cleaved metal oxide) is exposed to air. What happens to its surface, and why?

• A. Nothing — high surface energy materials are stable in air
• B. The surface immediately adsorbs water vapor, hydrocarbons, and other ambient molecules to lower its surface energy, forming a contamination layer within seconds. This is why ultra-high vacuum (UHV) is required for clean surface science experiments
• C. The surface energy causes the material to spontaneously fracture into smaller pieces
• D. The surface reconstructs by melting a thin layer at the surface

Answer: (B)

High surface energy is thermodynamically unfavorable — the system lowers its total energy by adsorbing molecules from the environment onto the surface, satisfying dangling bonds and replacing the high-energy solid-vacuum interface with lower-energy solid-adsorbate interfaces. A freshly cleaved metal oxide surface in air acquires a monolayer of water and hydrocarbons within seconds. This is why surface-sensitive techniques (XPS, LEED, STM) require ultra-high vacuum (< 10^-9 mbar): at atmospheric pressure, a surface is bombarded by ~10^23 molecules per cm2 per second, and even at 10^-6 mbar, a monolayer forms in about 1 second. Surface preparation (sputtering, annealing in UHV) is necessary to obtain atomically clean surfaces for study.