Catalyst Deactivation Mechanisms

• Catalyst deactivation directly impacts reaction rates and catalyst longevity.

Catalyst Activity

• Defined as the ratio of reaction rate at time t to the initial reaction rate at t = 0: a(t) = r'(t) / r'(t=0).
• Defined as the ratio of a catalyst property at time t to its initial value: a(t) = S(t) / S(t=0).

Mechanisms of Catalyst Deactivation

• Poisoning involves strong adsorption of reactants/products or impurities that bond to active sites.
• Selective poisoning occurs when the poisoning agent bonds to specific active sites.
• Fouling is the deposition of materials on the catalyst surface, blocking active sites (e.g., coke formation).
• Sintering involves the loss of catalytic surface area due to high temperatures.
• Surface migration, where surface atoms move, leads to crystal growth causing the catalyst to become less effective.
• Pore mouth blocking occurs as sintering causes the pore structure to collapse.
• Catalyst supports like SiO₂, Al₂O₃, or C can minimize sintering.
• Rare earth oxides can also stabilize the catalyst, reducing sintering.
• Vaporization involves the loss of catalytic components due to their volatility at high temperatures (e.g., MoO₃).

Kinetics of Catalyst Deactivation

• If catalyst decay rate is independent of the main reaction, the rate law is represented by r = r₀ f(Cᵢ) a(t).
• r₀ is the rate of reaction on a fresh catalyst, f(Cᵢ) is a function of reactant concentration, and a(t) is the catalyst activity.
• Rate of catalyst decay (r_d) is -da/dt = r_d = k_d a^(η), where k_d is the deactivation rate constant and η is the order of deactivation.

First Order Deactivation (η = 1)

• The activity a = e^(-k_dt).

Second Order Deactivation (η = 2)

• The activity a = 1 / (1 + k_dt).

n-th Order Deactivation (η = n)

• The activity a = (1 + (n-1)k_dt)^(1/(1-n)).

Types of Catalytic Deactivation

• Parallel deactivation occurs when the reactant A forms the desired product B and an undesired product P that poisons the catalyst.
• The rate laws are r = k C_A a and r_d = k_d C_A a, leading to a = e^(-k_d ∫₀ᵗ C_A dt).
• Series deactivation occurs when the desired product B further reacts to form a poisoning product P.
• The rate laws are r = k C_A a and r_d = k_d C_B a, leading to a = e^(-k_d ∫₀ᵗ C_B dt).
• Side-by-side deactivation occurs when an impurity P in the feed poisons the catalyst.
• The rate laws are r = k C_A a and r_d = k_d C_P a^(η), with solutions depending on the order η.
• For first order deactivation, a = e^(-k_d C_p t).
• For second order deactivation, a = 1 / (1 + k_d C_p t).

Reactor Operation

• In fixed bed reactors, activity varies with position (W) and time (t): a = f(W,t).
• Rapid catalyst decay can cause plugging.
• Temperature runaway should be avoided, especially in exothermic reactions, as it accelerates deactivation.
• Temperature can be programmed as a function of time: T = f(t).
• Moving bed reactors continuously remove, regenerate, and return the catalyst, giving uniform activity: a ≠ f(W,t).
• Moving bed reactors are more complex and expensive than fixed-bed reactors.
• Fluidized bed reactors suspend catalyst particles in the reacting fluid, maintaining uniform catalyst activity: a ≠ f(W,t).
• Temperature is uniform throughout fluidized bed reactors.
• Fluidized bed reactors are well-suited for solid catalyzed reactions.
• Attrition can be a problem in fluidized bed reactors.
• Fluidized bed reactors are more complex and expensive than fixed-bed reactors.