Catalysis for Chemical Engineers A Brief History and Fundamental Catalytic Principles

What is Catalysis? The science of catalysts and catalytic processes. A developing science which plays a critically important role in the gas, petroleum, chemical, and emerging energy industries. Combines principles from somewhat diverse disciplines of kinetics, chemistry, materials science, surface science, and chemical engineering. 2

What is Catalyst? A catalyst is a material that enhances the rate and selectivity of a chemical reactions and in the process is cyclically regenerated. Fe2+ + Ce4+  Fe3+ + Ce3+ (Slow Reaction) 2Fe2+ + Mn4+  2Fe3+ + Mn2+ Mn2+ + 2Ce4+  Mn4+ + 2Ce3+ Fe2+ + Ce4+  Fe3+ + Ce3+ Homogeneous Catalysis (Fast Reaction) CO + H2O  CO2 + H2 @ low temperature (Slow Reaction) S* + H2O  H2 + O-S* O-S* + CO  CO2 + S* (Faster Reaction) Heterogeneous Catalysis CO + H2O  CO2 + H2

What is Catalyst? N2 (Desired Reaction) rD NO rU (Undesired Reaction) NH3 rD Rate of formation of D SD/U = = rU Rate of formation of U Rh SD/U Pt SD/U From http://www.automotivecatalysts.umicore.com

How Important Is Catalysis? Fibers, Plastics, Food, Home Products, Pharmaceuticals Chemicals Raw Materials Heating, Transportation, Power Fuels Four of the largest sectors of our world economy (i.e. the petroleum, power, chemicals, and food industries), which account for more than 10 trillion dollars of gross world product, are largely dependent on catalytic processes.

Development of Important Industrial Catalytic Processes Mittasch investigated over 2500 catalysts compositions!!!

Development of Important Industrial Catalytic Processes It played a vital role as a feedstock for chemicals: 30 million tons per year in 2000

Development of Important Industrial Catalytic Processes Production of Liquid Fuels!!!

Development of Important Industrial Catalytic Processes NO CO CxHy N2 CO2 H2O O2

How to Define Reaction Rate?? 1 dni Reaction Rate (r) = i * Q dt Q = V, W or S.A. of catalyst i = Stoichiometric Coefficient i iMi = 0 involving species Mi (i is negative for reactants and positive for products) e.g. 2NH3 = N2 + 3H2 2 x (NH3) -1 x (N2) -3 x (H2) = 2N + 6H – 2N – 6H = 0 ni = # of moles of species Mi

Chemical Reactions Four Basic Variables to Control Chemical Reactions: Temperature Pressure Conc Contact time Rate of Reaction = K(T) x F(Ci) K(T) = A exp(-E/RT)  i (Ci)i H I H C Cl H C H I Cl Energy Intensive & damaging to equipment and materials & non-selective C H Cl I

Components of a Typical Heterogeneous Catalyst

Pt Nanoparticles on Al2O3 Supports

Heterogeneous Catalysis A (g)  B (g) Support (Al2O3) Active Metals (Pt, Co, MoO2) support Minimize P Minimize Mass Transport Resistances Maximize Activity Minimize Poisoning and Fouling

Components of a Typical Heterogeneous Catalyst

Active Catalytic Phases and Reactions They Typically Catalyze

Typical Physical Properties of Common Carrier (Supports)

Heterogeneous Catalysis A (g)  B (g) Support (Al2O3) Active Metals (Pt, Co, MoO2) support Minimize P Minimize Mass Transport Resistances Maximize Activity Minimize Poisoning and Fouling

Heterogeneous Catalysis Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on temp Steps 3, 4, & 5 are chemical processes => Large dependences on temp For Knudsen Diffusion Order of Magnitude d Phase cm2/s m2/s Temp and Pressure Dependences d <  T2 1.75  T1 For Bulk, Molecular or Fick’s Diffusion d From Elements of Chemical Reaction Engineering, S. Fogler d >  

Heterogeneous Catalysis Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on temp Steps 3, 4, & 5 are chemical processes => Large dependences on temp Given that the rates of the chemical steps are exponentially dependent on temperature and have relatively large activation energies compared to the diffusional process (20~200 kJ/mol Vs. 4-8 kJ/mol), they are generally the slow or rate-limiting processes at low reaction temperatures. As the temperature increases, the rates of chemical steps with higher activation energies increase enormously relative to diffusional processes, and hence the rate limiting process shifts from chemical to diffusional. Kapp(T) = Aapp exp(-Eapp/RT)

Film Mass Transfer Effect on Reaction Rate If Boundary Layer is Too Thick, Reaction Rate = Mass Transfer Rate k A  B -rA = kc (CAb – CAs) where Kc = DAB /  As the fluid velocity (U) increases and/or the particle size (Dp) decreases, the boundary layer thickness () decreases and the mass transfer coefficient (Kc) increases

Internal Diffusion Effect on Reaction Rate k A  B -rA = k η CAS Where η = Effectiveness Factor η = (CA)avg / CAS cosh cosh Φpore (1 - x/L) CA = CAS cosh ( Φpore) L x Φpore (Thiele Modulus) = L (k P / Deff)1/2 η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore

Internal Diffusion Effect on Reaction Rate While the equations above were derived for the simplified case of first-order reaction and a single pore, they are in general approximately valid for other reaction orders and geometry if L is defined as Vp/Sp (the volume to surface ratio of the catalyst particle). Hence, L = z/2, rc/2 and rs/3, respectively, for a flat plate of thickness z, a cylinder of radius rc, and a sphere of radius rs.

Elementary Reaction A + B  C It is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equation A + B  C If it is an elementary reaction, A B C -rA = k [A]1 [B]1

Elementary Reaction It is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equation O3  O2 + O Is this an elementary reaction? If it is an elementary reaction, -rO3 = k [O3]1

Elementary Reaction It is one that proceeds on a molecular level exactly as written in the balanced stoichiometric equation O3  O2 + O On molecular level, what really is really happening is: O2 + O3  O2 +O2 + O -rO3 = k [O3]1 [O2]1 We never really know for sure if we have an elementary reaction based on the balanced stoichiometric equation!!!

Heterogeneous Catalysis A (g)  B (g) Active Metals (Pt, Co, MoO2) support Proposed Reaction Mechanism k1 A + S A-S k-1 k2 A-S B-S k-2 k3 B-S B + S k-3

What If Adsorption Is Rate Limiting Step? Length of Vector Is Proportional to RXN Rate Director of Vector Indicates Direction of RXN Adsorption of A Net RXN of Adsorption Net RXN of Adsorption Surface RXN of A to B Net RXN of Surface RXN Net RXN of Surface RXN Desorption of B Net RXN of Desorption Net RXN of Adsorption = Net RXN of Surface RXN = Net RXN of Desorption Following Approximations Can Be Made: Adsorption of A is almost irreversible Both surface rxn and desoprtion steps are almost at equilibrium

What If Adsorption Is Rate Limiting Step? k1 A + S A-S Where S is a free surface site and A-S is a chemisorbed complex Since it is an elementary reaction, v = the fractional coverage of vacant site -rA = k1 CA CS v = CS / Ctotal How can we experimentally measure Cs ??? Cs = functions of parameters that one can experimentally measure or easily obtain

What If Adsorption Is Rate Limiting Step? Since both surface rxn and desorption steps are in near equilibrium, k2 A-S B-S rnet = k2 CA-S –k-2 CB-S  0 k2 / k-2 = K2 = CB-S / CA-S k-2 k3 B-S B + S rnet = k3 CB-S –k-3 CB CS  0 k3 / k-3 = K3 = CB CS / CB-S k-3 Both K2 and K3 are equilibrium constants which one can obtain: RT ln K = - G Let us do the site balance, Ctotal = CS + CA-S + CB-S = Const. Ctotal K2 = CB-S / CA-S CS = 1 + [ (1 + K2) CB / (K2 K3) ] K3 = CB CS / CB-S

What If Adsorption Is Rate Limiting Step? From the site balance and quasi-equilibrium approximation, Ctotal CS = 1 + [ (1 + K2) CB / (K2 K3) ] From the rate limiting step, k1 Ctotal CA k1 Ctotal CA -rA = k1 CA CS = = 1 + [ (1 + K2) CB / (K2 K3) ] 1 + K’ CB Where K’ = (1 + K2) / (K2 K3) If A and B behave according to the ideal gas law, CA = PA / RT CB = PB / RT

What If Surface Reaction Is Rate Limiting Step? k1 A + S A-S k-1 Rate Limiting Step k2 A-S B-S k-2 k3 B-S B + S k-3 k2 K1 PA -rA = 1 + K1 PA Figure 1.16 from Fundamentals of Industrial Catalytic Processes

What If Desoprtion Is Rate Limiting Step? k1 A + S A-S k-1 k3 K1 K2 PA k2 A-S B-S -rA = k-2 1 + (K1 + K1 K2) PA Rate Limiting Step k3 B-S B + S k-3

Fundamental Catalytic Phenomena and Principles Chemical Properties (Oxidation State, Acidity, Surface Composition) Physical Properties (Surface Area, Pore Structure, Pore Density) Catalyst Design Catalytic Properties (Activity and Selectivity)

Structure Sensitive Reactions

CO Oxidation over Au/TiO2: Particle Size Effect 2 nm 2.5 nm 6 nm Au TiO2

Particle Size Vs. Electronic Structure Change of Au