Chemisorption

Chemisorption is a specific type of adsorption involving a chemical reaction. It occurs when a substance, called the adsorbate, reacts with a surface, known as the adsorbent. Unlike physisorption, where particles merely sit on a surface, chemisorption creates new chemical bonds. These bonds are either ionic or covalent in nature. This strong interaction creates new types of electronic bonds between the species. Because these bonds are so powerful, chemisorption can actually change the structure of the surface itself.

The process of chemisorption follows a specific sequence of steps. First, the adsorbate particle must come into contact with the surface. If the particle hits the surface and bounces away elastically, it returns to the gas. However, if the particle loses momentum through an inelastic collision, it becomes trapped. This creates a precursor state where the particle is bonded by weak forces. The particle then diffuses across the surface until it finds a deep chemisorption potential well. Once it reaches this well, it reacts with the surface to form a permanent bond.

There are two main models used to describe how surface reactions occur. The first is the Langmuir–Hinshelwood mechanism. In this model, both reacting species are adsorbed onto the surface before they interact. The second is the Eley–Rideal mechanism. In this case, only one species is adsorbed, and the second reacts with it directly from the gas phase. These mechanisms help scientists understand how molecules meet and transform on a solid surface. Understanding these paths is vital for studying heterogeneous catalysis, where reactants and catalysts are in different phases.

Chemisorption can also involve the breaking of molecules, known as dissociative chemisorption. This is common with diatomic gases like hydrogen, oxygen, or nitrogen. One way to describe this is through precursor-mediation. The molecule is first adsorbed into a precursor state on the surface. It then moves across the surface to find specific chemisorption sites. At these sites, the molecule breaks its internal bond to form new bonds with the surface. This process often requires energy from the particle's translational or vibrational movement.

Scientists use specific terms to measure and model these energetic interactions. The energetic threshold between physisorption and chemisorption is generally accepted to be about 0.5 eV per species. To describe the energy of physisorption, scientists use a Lennard-Jones potential. For chemisorption, they use a Morse potential. Researchers also use a multidimensional potential energy surface (PES) to describe how a surface affects absorption. This surface helps account for the electronic degrees of freedom and ion interactions during the reaction.

Real-world examples demonstrate the incredible strength and utility of these bonds. For instance, oxygen can form very strong bonds with metals like Cu(110). These bonds can reach an energy of approximately 4 eV. Such strong bonding can cause a large restructuring of the metal surface. Another example is the formation of self-assembled monolayers (SAMs). This happens when reactive reagents, such as thiols, are chemisorbed onto gold surfaces. This process forms strong Au-SR bonds and releases hydrogen gas, creating a densely packed protective layer.

Chemisorption is also essential to the process of hydrogenation. In this reaction, hydrogen and an alkene bond to the atoms of a solid catalyst. This is a key part of heterogeneous catalysis, which is used to create various chemical products. The process relies on chemisorbed intermediates that combine to form a product. Once the product is formed, it desorbs, or leaves, the surface. This cycle allows the catalyst to remain available for more reactions, making it a fundamental concept in physical chemistry.