SURFACE CHEMICAL REACTIONS

 

1.  Types of reactions There are three categories:

 

A.  Corrosion reactionsMaterials in the gas phase interact with the surface to produce new species (products) that include atoms from the surface.  We can further subdivide this into two general classes depending upon what happens to the new product.

 

Volatilization (or etching) reactions: The product species returns to the gas phase and the surface is progressively consumed.  e.g.

 

H2O (g) + C (s) → CO (g) + H2 (g)

Cl2 (g) + Ni (s) → NiCl2 (g)

3O2 (g) + 2Mo (s) → 2MoO3 (g)

 

Corrosion layer formation: The product is non-volatile and the surface is progressively deposited with the product species.  e.g.

 

O2 (g) + Fe (s) → FeOx (s)

S (g) + Ni (s) → NiS (s)

 

B.  Crystal growth reactions Material is deposited on the surface (with or without a decomposition reaction) to extend the surface or to form the solid phase of a new material.  Much of these are being used for the development of nanostructured materials.

 

Physical vapor deposition (controlled condensation), including Pulsed Laser Deposition: e.g. 

Ag (g) → Ag (s)

 

  

Molecular beam epitaxy (MBE): e.g. 

2Ga (g) + As2 (g) → 2GaAs (s)

 

Chemical vapor deposition (CVD): e.g.

NiCl2 (g) → Ni (s) + Cl2 (g)

 

 

C.  Catalytic reactions Material from the surface is not directly involved in the species produced or decomposed in the reaction.  The surface provides the “sites” to promote the reaction (i.e. to increase the reaction rate relative to that in the gas-phase).

 

Exchange reactions: e.g. H2 (g) + D2 (g) ↔ 2HD (g)

 

Recombination reactions: e.g. H (ads) + H (ads) ↔ H2 (g)

 

Unimolecular decomposition reactions: e.g.

N2O (g) ↔ N2 (g) + O (ads)

CHOOH (g) ↔ CO + CO2 + H2 + H2O

 

Bimolecular reactions: e.g.

2CO (g) + O2 (g) ↔ 2CO2 (g)

CO + 2H2 ↔ CH4 + H2O

 

 

2.   Reaction sequences

 

Like any reaction, we can envision all the surface reactions (discussed above) in terms of a sequence of steps (what synthetic chemists call a reaction mechanism).  In general, we may write the following steps:

 

Adsorption of the “reactants” from the gas phase into a molecular precursor state (or physiadsorption)

Chemisorption of the “reactants”

Surface migration of the “reactants” to the reaction site

The reaction

Surface diffusion of the “products” away from the reaction site

The “products” change from the chemisorbed state to a physisorbed state

Desorption of the “products” into the gas-phase

 

Clearly, not all the steps are necessarily involved in a given reaction.  Each of these steps will have its own “rate”.  In many cases, only one of these steps will control the overall reaction rate (the rate-determining step).

 

 

3.  Jargons

 

Most of these steps, with the exception of the “the reaction”, can be treated using advanced theromodynamics.  For the surface reaction step, it is useful to define some common jargons.

 

·                 Structure sensitivity Reaction rate changes as a function of surface perfection or particle size of the catalytic material.  Conversely, the reaction is structure insensitive if the rate depends only on the amount of surface involved. 

·                 Importance of chemisorption before the reaction A bimolecular reaction that involves chemisorption of both reactants before the reaction is said to follow the Langmuir-Hinshelwood mechanism.  A bimolecular reaction between one chemisorbed species and a second species impringing directly from the gas-phase is said to follow the Eley-Rideal mechanism.

 

 


To give a measure of the efficiency of a catalytic surface, we define:

 

·                 Rate is the number of product molecules per catalyst area available per time,

 

= (kcat )(function of concentration or pressure)

 

where kcat = A exp[- E*cat/(RT)]

 

 

·                 Turnover number is the number of product molecules formed per available surface (catalyst) site per unit time.  TN typically 0.01 to 100/sec at 400-800 K.

 

·                 Unit collision reaction probability is the ratio of the number of product molecules to the total number of reactant molecules striking the surface.

 

= TN / (rate of molecular incidence)

 

where rate of molecular incidence = P/[2 p MRT]½

 

·                Active sites for reaction (e.g. ledge or kink).


 

 

 

 

4.  More on catalysis

 

·                 First catalyst discovered by Berzelius in 1836, who defined a catalyst as a substance that accelerates chemical reactions without visibly changing itself.

 

 

EXAMPLE :  Fischer & Tropsch (1926) – Brennst. Chim. 7, 97 – showed:

CO + H2 → various high molecular weight hydrocarbons, at 1 atm and 200 ºC

 

Used Fe, Co, K2CO3 and Cu as promoters

 

 

·                 Catalysts can be heterogeneous or homogeneous, depending whether they are in different or same phases as the reactants. [REF: G.A. Somorjai, Surf. Sci. 89 (1979) 496]

 

 

 

·                 Catalysts are structure dependent

 

 

 

 

 

·                Catalysts have large surface areas (textured promoters, zeolites (0.8 - 2 nm), supports such as oxides, etc.)

·                Surface intermediates (with “intermediate” bond strengths)

·                Coadsorbates, particularly carbonaceous layer or activated surface “C”, could be very important to enhancing the catalystic activity of real-life catalysts