Catalyst Deactivation

1. 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

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).

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.

2.1 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.

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) Second slide (Fig. 5.4) Conversion profiles for various deactivation modes

Second mechanism for carrier change in crystal structure γ-Al2O3 → α-Al2O3 150 m2/g < 5 m2/g Anatase TiO2 Rutile TiO2 60 m2/g < 10 m2/g Stabilizer BaO, 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.

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.

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

3.1 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.

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. 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.

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/Al2O3 surfaces

4. Washcoat Loss Attrition or Erosion Irreversible deactivation a. high linear velocities of gas flow b. thermal expansion differences between the washcoat and the monolith, especially metal substrates

5- Catalyst Deactivation Modeling

Types of catalyst deactivation Deactivation by Sintering (aging) Deactivation by Coking or Fouling Deactivation by Poisoning The algorithm to solve reactor design with decaying catalyst Mole Balance Reaction rate law Decay rate law Stoichiometry Combine and solve Numerical Evaluation

Deactivation by Sintering (Aging)

Deactivation by Coking or Fouling This decay is common to reactions involving hydrocarbons. It result from carbonaceous (coke) material deposited on the surface of the catalyst. The amount of coke deposit on the catalyst surface found to obey the empirical relationship CC=At n Fresh catalyst (t = 0) t = t

Different functions between activity and coke on the surface been observed

Deactivation by poisoning Occurs when poisoning molecules become irreversibilly chemisorbed to active sites, thereby reducing the total number of active sites. The poisoning molecules maybe reactants, products or impurities

Deactivation by poisoning

Functional from activity Empirical decay laws Integral form Deferential form Decay reaction order Functional from activity

Temperature time trajectories To maintain constant conversion in the reactor, the temperature of the reaction is varied as a function of time

6- Moving bed reactors Feed + Fresh catalyst air For continuously regeneration and or replacement of the catalyst two types of reactors are used Moving Bed Reactors Straight-through transport reactor W ΔW

Catalyst Deactivation