25-08-2017, 09:32 PM
Catalyst Deactivation
[attachment=45724]
Introduction
Deactivation a. high temperature exposure: automobile catalytic converter, close to 1000℃ b. poisoning: exhaust or process contaminants adsorbing onto or blocking active sites c. attrition and erosion of the washcoat from the support
Model Reaction A convenient tool for studying deactivation and regeneration
Thermally Induced Deactivation
A perfectly dispersed (100% dispersion) catalyst is one in which every atom (or molecule) of active component is available to the reactants. This is shown is Fig. 5.1 (next slide).
Some catalysts are made in this highly active state but are highly unstable, and thermal effects cause crystal growth, resulting in a loss of catalytic surface area.
Additionally, the carrier with a large internal surface network of pores tends to undergo sintering with a consequent loss in internal surface area.
Besides, reactions of the catalytically active species with the carrier, resulting in the formation of a less catalytically active species.
Sintering of the Catalytic Component
Next slide (Fig. 5.2) Sintering by growth of catalyst crystals This condition can be measured by selective chemisorptions techniques in which a thermally aged catalyst adsorbs much less adsorbate than when it was fresh.
Stabilizer Certain rare-earth oxides such as CeO2 and La2O3 have been effective in reducing sintering rates of Pt in the automobile exhaust catalytic converter. It may fix the catalytic components to the surface minimizing mobility and crystal growth.
Carrier Sintering
Within a given crystal structure, such as γ-Al2O3, the loss of surface area is associated with loss of H2O and a gradual loss of the internal pore structure network, as shown in the next slide (Fig. 5.3)
The presence of these phenomena is determined by a progressive decrease in the activation energy of the reaction.
Second slide (Fig. 5.4) Conversion profiles for various deactivation modes
Catalytic Species-Carrier Interactions
Rh2O3 reacts with a high-surface-area γ-Al2O3, forming an inactive compound during high-temperature lean conditions in the automobile exhaust. (for NOx removal)
Therefore, it is better to use carriers such as SiO2, ZrO2, TiO2, and their combinations that are less reactive with Rh2O3 than Al2O3. However, these alternative carriers are not as stable against sintering.
Selective Poisoning
Next slide (Fig. 5.5) A poison directly reacts with an active site
Permanent deactivation Pb, Hg, and Cd react directly with Pt, forming a catalytically inactive alloy.
Reversible deactivation SO2 merely adsorbs onto a metal site (i.e., Pd). Heat treatment, washing, or simply removing the poison from the process stream, often desorbs the poison from the catalytic site and restoring its catalytic activity.
[attachment=45724]
Introduction
Deactivation a. high temperature exposure: automobile catalytic converter, close to 1000℃ b. poisoning: exhaust or process contaminants adsorbing onto or blocking active sites c. attrition and erosion of the washcoat from the support
Model Reaction A convenient tool for studying deactivation and regeneration
Thermally Induced Deactivation
A perfectly dispersed (100% dispersion) catalyst is one in which every atom (or molecule) of active component is available to the reactants. This is shown is Fig. 5.1 (next slide).
Some catalysts are made in this highly active state but are highly unstable, and thermal effects cause crystal growth, resulting in a loss of catalytic surface area.
Additionally, the carrier with a large internal surface network of pores tends to undergo sintering with a consequent loss in internal surface area.
Besides, reactions of the catalytically active species with the carrier, resulting in the formation of a less catalytically active species.
Sintering of the Catalytic Component
Next slide (Fig. 5.2) Sintering by growth of catalyst crystals This condition can be measured by selective chemisorptions techniques in which a thermally aged catalyst adsorbs much less adsorbate than when it was fresh.
Stabilizer Certain rare-earth oxides such as CeO2 and La2O3 have been effective in reducing sintering rates of Pt in the automobile exhaust catalytic converter. It may fix the catalytic components to the surface minimizing mobility and crystal growth.
Carrier Sintering
Within a given crystal structure, such as γ-Al2O3, the loss of surface area is associated with loss of H2O and a gradual loss of the internal pore structure network, as shown in the next slide (Fig. 5.3)
The presence of these phenomena is determined by a progressive decrease in the activation energy of the reaction.
Second slide (Fig. 5.4) Conversion profiles for various deactivation modes
Catalytic Species-Carrier Interactions
Rh2O3 reacts with a high-surface-area γ-Al2O3, forming an inactive compound during high-temperature lean conditions in the automobile exhaust. (for NOx removal)
Therefore, it is better to use carriers such as SiO2, ZrO2, TiO2, and their combinations that are less reactive with Rh2O3 than Al2O3. However, these alternative carriers are not as stable against sintering.
Selective Poisoning
Next slide (Fig. 5.5) A poison directly reacts with an active site
Permanent deactivation Pb, Hg, and Cd react directly with Pt, forming a catalytically inactive alloy.
Reversible deactivation SO2 merely adsorbs onto a metal site (i.e., Pd). Heat treatment, washing, or simply removing the poison from the process stream, often desorbs the poison from the catalytic site and restoring its catalytic activity.