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7 Cards in this Set
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Define the terms catalyst, heterogeneous catalyst & adsorption ? |
Catalyst – a substance, which speeds up a reaction without being consumed in the process. Heterogeneous catalysis – the catalyst (solid) and the reactants (gas or liquid) are in differentphases. Adsorption - the process of attachment of atoms or molecules onto a solid surface. |
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Define the term turnover frequency ? |
Turnover frequency – the number of reactant molecules converted (turned over) per active site inunit time for a fixed set of experimental conditions (temperature, pressure or concentration,reactant ratio). The highest values for solid catalysts are in the range 105– 106s-1 |
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Define the term selectivity ? |
Selectivity – the ability of a catalyst to induce the formation of particular products. In the case ofreactant A undergoing various parallel reactions one of which yields a particular product B, theselectivity Sb=no of mol of A converted to B / total number mol of A consumed *100 |
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Define the terms poisoning, fouling, sintering and loss of active species ? |
Poisoning – impurities in the feed steam reduce the catalytic activity by blocking of activecatalytic sites. Example: sulphur and metallic catalysts. Fouling – a physical blockage, which prevents reactants from reaching the catalyst surface.Example: coke formation. Sintering – loss of surface area (catalyst support or active phase) at high temperatures. Loss of active species – active species are converted to another form which is less active or lessselective. Example: formation of volatile MoO3 from Fe2(Mo4)3 catalyst. |
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Define catalyst regeneration ? |
Catalyst regenerationThe regeneration treatment depends on the causes for deactivation. Examples include heating toburn off the carbon or reduction in hydrogen to redisperse the metal particles. |
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Define the term physisorption ? |
Physisorption is caused by weak van der Waals forces, it is always exothermic, completelyreversible, rapid (no activation energy required), multilayered, and relatively non-specific (it willoccur with all gases and on all surfaces). |
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Define the term chemisorption ? |
Chemisorption is characterised by the formation of new bonds between adsorbent and adsorbate, it is usually exothermic but endothermic chemisorption has also been observed. Chemisorption is often activated and therefore proceeds at an appreciable rate only above certain minimum temperatures, it may or may not be reversible, it is limited to a maximum of one layer and it is highly specific (it will only occur if the adsorbate is capable of forming a chemical bond with the adsorbent). Chemisorption can either take place non-dissociatively where no bonds are broken in the molecule or dissociatively where fragments of the original molecule are adsorbed |
