Paper Summary

Citation: Lennard-Jones, J. E. (1932). Processes of Adsorption and Diffusion on Solid Surfaces. Transactions of the Faraday Society, 28, 333-359. https://doi.org/10.1039/tf9322800333

Publication: Transactions of the Faraday Society, 1932

What kind of paper is this?

This is a foundational theoretical and conceptual paper. It introduces a new physical model to describe the interaction of gases with solid surfaces. It is not an experimental or computational paper in the modern sense but rather a work that provides a unifying theoretical framework to explain a range of experimental observations.

What is the motivation?

The primary motivation was to reconcile conflicting experimental evidence regarding the nature of gas-solid interactions. At the time, it was observed that the same gas and solid could interact weakly at low temperatures (consistent with van der Waals forces) but exhibit strong, chemical-like bonding at higher temperatures, a process requiring significant activation energy. The paper seeks to provide a single, coherent model that can explain both “physical adsorption” (physisorption) and “activated” or “chemical adsorption” (chemisorption) and the transition between them.

What is the novelty here?

The core novelty is the application of quantum mechanical potential energy surfaces to the problem of surface adsorption. The key conceptual breakthroughs are:

  1. Dual Potential Energy Curves: The paper proposes that the state of the system must be described by at least two distinct potential energy curves as a function of the distance from the surface:

    • One curve represents the interaction of the intact molecule with the surface (e.g., H₂ with a metal). This corresponds to weak, long-range van der Waals forces.
    • A second curve represents the interaction of the dissociated constituent atoms with the surface (e.g., 2H atoms with the metal). This corresponds to strong, short-range chemical bonds.

  2. Activated Adsorption via Curve Crossing: The transition from the molecular (physisorbed) state to the atomic (chemisorbed) state occurs at the intersection of these two potential energy curves. For a molecule to dissociate and chemisorb, it must possess sufficient energy to reach this crossing point. This energy is identified as the energy of activation, which had been observed experimentally.

  3. Unified Model: This model elegantly unifies physisorption and chemisorption into a single continuous process. A molecule approaching the surface is first trapped in the shallow potential well of the physisorption curve. If it acquires enough thermal energy to overcome the activation barrier, it can transition to the much deeper potential well of the chemisorption state. This provides a clear physical picture for temperature-dependent adsorption phenomena.

What experiments were performed?

This paper does not report any new experiments. It is entirely theoretical, building upon the principles of quantum mechanics and interpreting the vast body of experimental sorption data available at the time. The work’s strength lies in its ability to provide a plausible physical mechanism that explains previously puzzling experimental results from others.

What were the outcomes and conclusions drawn?

  • Outcome: The paper introduced the now-famous Lennard-Jones diagram for surface interactions, which plots potential energy versus distance from the surface for both molecular and dissociated atomic species. This graphical model became a cornerstone of surface science.

  • Conclusions:

    • The nature of adsorption is determined by the interplay between two distinct potential states (molecular and atomic).
    • “Activated adsorption” is the process of overcoming an energy barrier to transition from a physically adsorbed molecular state to a chemically adsorbed atomic state.
    • The model predicts that the specific geometry of the surface (i.e., the lattice spacing) and the orientation of the approaching molecule are critical, as they influence the shape of the potential energy surfaces and thus the magnitude of the activation energy.
    • The reverse process—recombination of atoms and desorption of a molecule—also requires activation energy to move from the chemisorbed state back to the molecular state.
    • This entire mechanism is proposed as a fundamental factor in heterogeneous catalysis, where the surface acts to lower the activation energy for molecular dissociation, facilitating chemical reactions.

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