Question

Describe briefly why a clean nickel surface (fcc structure) should not be regarded as comprising a perfect closepacked array of atoms, Indicate the arrangements of atoms that an adsorbate might encounter on the surface, and suggest possible modes of attachment for CO.

Short answer

A clean nickel surface is not perfectly close-packed due to surface imperfections and lower coordination number, allowing adsorbates like CO to attach at various sites such as atop, bridge, or hollow.

Step-by-step solution

Understanding the Structure

A clean nickel surface has a face-centered cubic (fcc) structure. In this structure, the atoms are arranged in layers, where each atom is surrounded by twelve others. This arrangement appears close-packed, but surface atoms have different coordination than interior atoms as they have fewer neighbors.

Recognizing Surface Imperfections

Despite the close-packed nature of the bulk structure, the surface may have imperfections such as steps, kinks, and vacancies. These imperfections mean that the surface cannot be regarded as a perfect array and affect adsorption.

Surface Atom Coordination

The atoms at the surface have a lower coordination number compared to those inside the bulk, i.e., surface atoms are not as tightly packed as their bulk counterparts. Therefore, the surface presents different arrangements like terrace, step, and kink sites.

Atom Arrangements for Adsorbate

An adsorbate like CO can encounter different atomic sites: atop (on top of a single atom), bridge (between two atoms), or hollow sites (surrounded by three or more atoms). These sites offer various possibilities for adsorption.

Modes of CO Attachment

CO can bond to the nickel surface in multiple modes. It may bond linearly at atop sites, bridge between two nickel atoms, or coordinate in a hollow site where it interacts with three surrounding nickel atoms.

Key concepts

Adsorption

Adsorption is a crucial concept in surface chemistry, particularly when discussing clean nickel surfaces with a face-centered cubic (fcc) structure. It is the process whereby atoms, ions, or molecules from a gas or liquid adhere to a surface. In the context of nickel, adsorbates such as carbon monoxide (CO) attach themselves to the metal surface.
One key element of adsorption is the interaction between the adsorbate, like CO, and the surface atoms of the nickel. Due to the specific arrangement of atoms in the fcc structure, certain areas on the surface are more favorable for adsorption. These areas include different sites like atop, bridge, and hollow. Each of these sites offers different arrangements and coordination, which significantly influences how strongly and in what orientation a molecule will bind.
Understanding adsorption is essential for applications like catalysis, where the efficiency of reactions is often determined by how well the reactants can adhere to the metal surface. Adsorption characteristics are significantly influenced by the physical and chemical properties of both the adsorbate and the substrate.

Surface Imperfections

Even though a nickel surface in its fcc structure may seem perfectly ordered, it is often far from it. Surface imperfections are ubiquitous on real-world surfaces and play a significant role in adsorption behaviors. These imperfections include features such as steps, kinks, and vacancies.
- **Steps** are linear features where the surface level changes, slightly elevating or lowering the arrangement.
- **Kinks** are sudden changes in the step edge direction, creating angular points on the surface.
- **Vacancies** are missing atoms in what would otherwise be a perfect array, leaving behind a void.
These imperfections can act as active sites for adsorption because they disrupt the regular atomic arrangement, often creating sites where atoms can more readily bond due to the lower coordination number. While a perfectly smooth surface might not provide many binding opportunities, these imperfections greatly enhance the surface reactivity and adsorption potential.

Atom Coordination

The concept of atom coordination is pivotal in understanding the structure of a nickel surface and its interaction with adsorbates. In the bulk of the metal, every atom is surrounded by twelve others, and this is known as the coordination number. At the surface, however, atoms have fewer neighboring atoms than those in the bulk, making them more reactive and available for bonding with adsorbates.
The reduction in coordination number from bulk to surface is due to the fact that surface atoms are only surrounded by neighboring atoms on one side. As a result, these surface atoms exhibit different electronic and geometric properties than atoms in the interior. This difference is especially evident in distinct features on the surface, such as terraces where flat planes cut through the structure, steps where discontinuities occur, and kinks which are angular protrusions.
These variations in atom coordination create diverse sites that can lead to differing adsorption behaviors for molecules like CO.

Adsorbate Binding Sites

When an adsorbate like CO binds to a surface, it can do so at various binding sites, each with unique structural and chemical properties. On a nickel surface, these binding sites are primarily categorized as atop, bridge, and hollow sites.
- **Atop sites** are situated directly above a single atom, making available one-to-one interactions.
- **Bridge sites** are located between two adjacent atoms, allowing a molecule like CO to interact with both simultaneously.
- **Hollow sites** can be further divided into threefold and fourfold, where the adsorbate interacts with three or more atoms, respectively. This site is often considered the most stable due to the increased interactions.
These sites provide numerous possibilities for how an adsorbate can attach, influencing factors such as the orientation of the molecule and the strength of adhesion. The preference for a specific site by a molecule like CO is influenced by factors such as the geometry of the molecule, the electron distribution, and the overall surface energy.