Catalysis

Catalysis is an action by catalyst which takes part in a chemical reaction process and can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions. This definition is one of many definitions about catalysis.

Three key aspects of catalyst action:

1. Taking part in the reaction

     It will change itself during the process by interacting with other reactant/product molecules

2. Altering the rates of reactions

     In most cases the rates of reactions are increased by the action of catalysts; however, in some

     situations the rates of undesired reactions are selectively suppressed

3. Returning to its original form

     • After reaction cycles a catalyst with exactly the same nature is ‘reborn’

     • In practice a catalyst has its lifespan - it deactivates gradually during use

Already from these definitions, it is clear that there is a direct link between chemical kinetics and catalysis, as according to the very definition of catalysis it is a kinetic process. There are different views, however, on the interrelation between kinetics and catalysis. While some authors state that catalysis is a part of kinetics, others treat kinetics as a part of a broader phenomenon of catalysis.

Despite the fact that catalysis is a kinetic phenomenon, there are quite many issues in catalysis which are not related to kinetics. Mechanisms of catalytic reactions, elementary reactions, surface reactivity, adsorption of reactants on the solid surfaces, synthesis and structure of solid materials, enzymes, or organometallic complexes, not to mention engineering aspects of catalysis are obviously outside the scope of chemical kinetics.

Some discrepancy exists whether chemical kinetics includes also the mechanisms of reactions. In fact if reaction mechanisms are included in the definition of catalytic kinetics it will be an unnecessary generalization, as catalysis should cover mechanisms.

Catalysis is of crucial importance for the chemical industry, the number of catalysts applied in industry is very large and catalysts come in many different forms, from heterogeneous catalysts in the form of porous solids to homogeneous catalysts dissolved in the liquid reaction mixture to biological catalysts in the form of enzymes. Catalysis is a multidisciplinary field requiring efforts of specialists in different fields of chemistry, physics and biology to work together to achive the goals set by the mankind. Knowledge of inorganic, organometallic, organic chemistry, materials and surface science, solid state physics, spectroscopy, reaction engineering, and enzymology is required for the advancements of the discipline of catalysis.

Catalyst

Facts and figures about catalysts :
   a. Life cycle on the earth :       - Catalysts (enzyme) participates most part of life cycle, e.g. forming, growing and decaying
      - Catalysis contributes great part in the processes of converting sun energy to various other
           forms of energies, e.g. photosynthesis by plant CO2 + H2O  → C + O2
      - Catalysis plays a key role in maintaining our environment
  b. Chemical industry
       - ca. $2 bn annual sale of catalysts
       - ca. $200 bn annual sale of the chemicals that are related products
       - 90% of chemical industry has catalysis-related processes
       - Catalysts contributes ca. 2% of total investment in a chemical process
                                                                                                                    (Erzeng Xue, 2003)

So?! What is catalyst???

A catalyst was defined by Ostwald as a compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction. This definition allows for the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity is slowly lost. Lets take a look for the example of a catalytic reaction between two molecules A and B with the involvment of a catalyst. From the figure below, we can see that the catalyst unaltered and ready for taking part in a next catalytic cycle after forming the products.

In order to understand how a catalyst can accelerate a reaction, a potential energy diagram should be considered.

The figure above represents a concept for a non-catalytic reaction of Arrhenius, who suggested that reactions should overcome a certain barrier before a reaction can proceed. For catalytic reaction (reaction with catalyst), the change in the Gibbs free energy between the reactants and the products ΔG does not change in case of a catalytic reaction, however the catalyst provides an alternative path for the reaction. See the figure below.

                                                                        (Dmitry Murzin, 2005).

The action of catalyst :
1. Catalyst action leads to the rate of a reaction to change

     - Forming complex with reactants/products, controlling the rate of elementary steps in the process.

       This is evidenced by the facts that :

            a. The reaction activation energy is altered

            b. The intermediates formed are different from those formed in non-catalytic reaction

            c. The rates of reactions are altered (both desired and undesired ones)

    - Reactions proceed under less demanding conditions

       Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat

          sensitive materials

2. The use of catalyst does not vary ΔG & Keq values of the reaction concerned, it merely change

      the pace of the process

      - Whether a reaction can proceed or not and to what extent a reaction can proceed is solely

          determined by the reaction thermodynamics, which is governed by the values of ΔG & Keq,

          not by the presence of catalysts

    - In another word, the reaction thermodynamics provide the driving force for a rxn; the presence

         of catalysts changes the way how driving force acts on that process.

         e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g)   ΔG°373=151 kJ/mol (100 °C)

                                                                             ΔG°973 =-16 kJ/mol (700 °C)

        a. At 100°C, ΔG°373=151 kJ/mol > 0. There is no thermodynamic driving force, the reaction

             won’t proceed with or without a catalyst

       b. At 700°C, ΔG°373= -16 kJ/mol < 0. The thermodynamic driving force is there. However,

             simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a

             proper catalyst heating the mixture in reactor results no conversion of CH4 and CO2 at all.

            When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium

             conversion can be achieved (<100%).

                                                                                                                            (Erzeng Xue, 2003)

Catalysts classification :

1. Based on the its physical state, a catalyst can be :
     a. Gas
     b. Liquid
     c. Solid
2. Based on the substances from which a catalyst is made
     a. Inorganic (gases, metals, metal oxides, inorganic acids, bases, etc.)
     b. Organic (organic acids, enzymes, etc.)
3. Based on the ways catalysts work
     a. Homogeneous
     b. Heterogeneous
4. Based on the catalyst’ action
     a. Acid-base catalysts
     b. Enzymatic
     c. Photocatalysis
     d. Electrocatalysis, etc.
5. According to the preparation procedure as
     a. Bulk catalysts or supports
     b. Impregnated catalysts
6. On this basis the ralative preparation methodes are:
     a. The catalytic active phase is generated as a new solid phase
     b. The active phase is introduced or fixed on a pre-existing solid by a process which intrinsically
            depends on the support surface
7. Bulk catalysts
     a. Precipitation
     b. Gelation / sol-gel
8. Supported catalysts


References : D. murzin and T. Salmi, Catalytic Kinetics, Elsivier, 2005
                    Erzeng Xue, Catalysis and Catalysts, University of Limerick, 2003