Background for Evaluating the Catalytic Properties of Small Supported Particles

Background for Evaluating the Catalytic Properties of Small Supported Particles 

In his interesting article on particle size effects, Bond (27) gives the results of model calculations based on spheres and on cubes with five faces exposed, for Ni, Pt, and Pd. For these simple systems, the fraction of total metal atoms exposed, FE, is related to the diameter or length of a side d (in the figures, d is written with a bar above it to highlight the fact that it is an average value) by the approximate formula 

We define the atomic rate of reaction AR as the rate of production of a given product, with the units s l, or moles of product per mole of total metal atoms per second. This is the rate that we strive to maximize for economic reasons, as far as catalyst development is concerned. However, it is convenient to decompose the atomic rate AR into two factors of a more fundamental nature  

We prefer the term fraction-exposed (96) to dispersion, since the latter conveys an idea of distribution in space that is not directly related to particle size and shape. The quantity FE is commonly measured by hydrogen chemisorption at room temperature, using the equation 

The quantity H/M, i.e., H atoms irreversibly adsorbed divided by total metal atoms, is measured experimentally. H/Ms, where Ms refers to surface metal atoms, may be known on the basis of surface science studies [e.g., low-energy electron diffraction (LEED)] and is usually assumed equal to unity. However, we shall discuss some papers that show that H/Ms is also a function of d and other variables. In addition, reversibly adsorbed hydrogen may also be important (97, 98). For interpreting most data, nevertheless, we can only assume that FE = H/M. It seems reasonable to use surface metal atoms as a basis, and not surface sites. The number of surface atoms required to form a site is in many cases a subject of debate, whereas H/M is measured experimentally. 

---