Catalyst Deactivation

1. Catalyst Deactivation 朱信 Hsin Chu Professor Dept. of Environmental Eng. National Cheng Kung University

2. 1. Introduction • Deactivationa. 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

3. 2. 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).

5. 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.

6. 2.1 Sintering of the Catalytic Component • Next slide (Fig. 5.2)Sintering by growth of catalyst crystalsThis condition can be measured by selective chemisorptions techniques in which a thermally aged catalyst adsorbs much less adsorbate than when it was fresh. • StabilizerCertain 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.

8. 2.2 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

11. Second mechanism for carrier change in crystal structure γ-Al2O3 →α-Al2O3150 m2/g < 5 m2/gAnatase TiO2 Rutile TiO260 m2/g < 10 m2/g • StabilizerBaO, La2O3, SiO2, or ZrO2 can retard the rate of sintering in certain carriers.They are believed to form solid solutions with the carrier surface, decreasing their surface reactivity, which leads to sintering.

12. 2.3 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.

13. 3. Poisoning • Selective poisoningA chemical directly reacts with the active site or the carrier, rendering it less or completely inactive. • Nonselective poisoningDeposition of fouling agents onto or into the catalyst carrier, masking sites and pores, resulting in a loss in performance.

14. 3.1 Selective Poisoning • Next slide (Fig. 5.5)A poison directly reacts with an active site • Permanent deactivationPb, Hg, and Cd react directly with Pt, forming a catalytically inactive alloy. • Reversible deactivationSO2 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.

16. When active sites are directly poisoned, there is a shift to high temperature but with no change in the slope of conversion profile since the remaining sites can function as before with no change in activation energy. • When the carrier reacts with a constituent in the gas stream to form a new compounds, as in the case of Al2(SO4)3, pores are generally partially blocked, resulting in increased diffusion resistance. This will cause a decrease in the activation energy.

17. SO2 chemisorbs onto Pd/γ-Al2O3, causing deactivation for methane oxidation. Some of the adsorbed SO2 is converted to SO3, which spills over, forming Al2(SO4)3.Using nonsulfating carriers such as ZrO2 or SiO2 leads to a faster rate of deactivation since no reservoir is available for spillover. • With the Pt/γ-Al2O3 catalyst the SO2 is readily converted to SO3, which rapidly desorbs and reacts with the Al2O3, forming Al2(SO4)3, which slowly causes pore plugging.By using nonsulfating carriers, the Pt catalyst can be made resistant to deactivation.

18. 3.2 Nonselective Poisoning • Aerosol or high-molecular-weight material from upstream equipment physically deposit onto the surface of the washcoat to cause deactivation is referred to as “fouling” or “masking”. • Reactor-scale metals (Fe, Ni, Cr, etc.) resulting from corrosion, silica/alumina-containing dusts, phosphorous from lubricating oils, and similar compounds are good examples. • Next slide (Fig. 5.6)Masking or fouling of a catalyst washcoat • Second slide (Fig. 5.7)SEM of fresh and aged Pt/Al2O3surfaces

21. 4. Washcoat Loss • Attrition or ErosionIrreversible deactivationa. high linear velocities of gas flowb. thermal expansion differences between the washcoat and the monolith, especially metal substrates