Heterogeneous catalysis Lec 9 week 12

Comparison of homogeneous and heterogeneous catalysts

Comparison of homogeneous and heterogeneous catalysts

Heterogeneous Catalysis: Fundamentals Individual Steps in Heterogeneous Catalysis Heterogeneously catalyzed reactions are composed of purely chemical and purely physical reaction steps. For the catalytic process to take place, the starting materials must be transported to the catalyst. Thus, apart from the actual chemical reaction, diffusion, adsorption, and desorption processes are of importance for the progress of the overall reaction.

Steps in catalytic reactions

We will now consider the simplest case of a catalytic gas reaction on a porous catalyst.

In heterogeneous catalysis chemisorption of the reactants and products on the catalyst surface is of central importance, so that the actual chemical reaction step can not be considered independently from adsorption and desorption steps. The measured reaction rate, known as the effective reaction rate, is determined by the most strongly inhibited and therefore slowest step of the reaction sequence. This rate-determining step also determines the reaction order.

The Importance of Adsorption in Heterogeneous Catalysis For the moment, let us focus our attention on gas-phase reactions catalyzed by solid surfaces. For a catalytic reaction to occur, at least one and frequently all of the reactants must become attached to the surface. This attachment is known as adsorprion and takes place by two different processes: physical adsorption (physisorption) and chemical adsorption chemisorption. Physisorption is the result of van der Waals forces, and the accompanying heat of adsorption is comparable in magnitude to the heat of evaporation of the adsorbate. chemisorption, chemical bonds are formed between the catalyst and the starting material. The resulting surface molecules are much more reactive than free adsorbate molecules, and the heats of adsorption are comparable in magnitude to heats of chemical reaction. both types of adsorption are exothermic.

Comparison between physisorption and chemisorption. Physisorption is fast, and equilibrium is rapidly reached, even at low temperature. Chemisorption generally requires high activation energies. The rate of adsorption is low at low temperatures, but the process can be rapid at higher temperatures. The rate of both types of adsorption is strongly dependent on pressure. Chemisorption leads only to a monolayer, whereas in physisorption multilayers can form. The type of adsorption that affects the rate of a chemical reaction is chernisorption

Factors affect the extent of adsorption on the catalyst surfaces: (1) Nature of the adsorbate (gas) and adsorbent (solid) (2) Surface area of the solid adsorbent (3) Effect of pressure on the adsorbate gas (4) Effect of temperature

Factors which affect the extent of adsorption on the catalyst surfaces:  The following are the factors which affect the adsorption, (1) Nature of the adsorbate (gas) and adsorbent (solid) (i) In general, easily liquefiable gases e.g., CO2, NH3, Cl2 and SO2 etc. are adsorbed to a greater extent than the elemental gases e.g. H2, O2, N2, He etc.   (ii) Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks.

Factors which affect the extent of adsorption on the catalyst surfaces: (2) Surface area of the solid adsorbent (i) The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption. (ii) Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface area.  

Factors which affect the extent of adsorption on the catalyst surfaces: (3) Effect of pressure on the adsorbate gas (i) An increase in the pressure of the adsorbate gas increases the extent of adsorption. (ii) At low temperature, the extent of adsorption increases rapidly with pressure. (iii) Small range of pressure, the extent of adsorption is found to be directly proportional to the pressure. (iv) At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting value.

Factors which affect the extent of adsorption on the catalyst surfaces: (4) Effect of temperature (i) As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature.

The important factors influencing the reaction kinetics: 1) Adsorption is a necessary step preceding the actual chemical reaction on solid catalyst surfaces. 2) Heterogeneous catalysis involves Chemisorption, which has the characteristics of a chemical reaction in that the molecules of the starting material react with the surface atoms of the catalyst. 3) Catalyst surfaces have heterogeneous structures, and chemisorption takes place preferentially at active sites on the surface.

Fundamental laws of adsorption Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces. The reverse process of adsorption, i.e. the process in which adsorbed molecules escape from solid surfaces, is called Desorption. Molecules can attach to surfaces in two different ways because of the different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption)

Adsorption process Adsorbent and adsorbate Surface coverage, q Adsorbent (also called substrate) - The solid that provides surface for adsorption high surface area with proper pore structure and size distribution is essential good mechanical strength and thermal stability are necessary Adsorbate - The gas or liquid substances which are to be adsorbed on solid Surface coverage, q The solid surface may be completely or partially covered by adsorbed molecules Adsorption heat Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic) The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of physisorption is in the same order of magnitude of condensation heat.

Adsorption Mechanism

Adsorption Isotherms Data relating adsorbed concentration (g/g of bed weight) to equilibrium gas phase concentration (g/ml of stream) is given in terms of adsorption isotherms. Wads = f (P,T) Three common types of isotherms: Langmuir Freundlich BET

Characterisation of adsorption system Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const. The amount of adsorption as a function of pressure at set temperature Adsorption isobar - (usage related to industrial applications) The amount of adsorption as a function of temperature at set pressure Adsorption Isostere - (usage related to industrial applications) Adsorption pressure as a function of temperature at set volume. Pressure Vol. adsorbed T1 T2 >T1 T3 >T2 T4 >T3 T5 >T4 Temperature P1 P2>P1 P3>P2 P4>P3 V2>V1 V1 V3>V2 V4>V3 Adsorption Isotherm Adsorption Isobar Adsorption Isostere

Langmuir Isotherm The earliest model of gas adsorption suggested by Langmuir (1916). The classical Langmuir model is limited to monolayer adsorption. It is assumed that gas molecules striking the surface have a given probability of adsorption. Molecules already adsorbed similarly have a given probability of desorption. At equilibrium, equal numbers of molecules desorb and adsorb at any time. The probabilities are related to the strength of the interaction between the adsorbent surface and the adsorbate gas.

Langmuir Isotherm (cont’d)  1- air adsorbate Rate of adsorption, Rate of desorption, At equilibrium, where, Wads = the mass of gas adsorbed at pressure P; Wmax = the mass of gas which covers the entire adsorbing surface with a monolayer; P = the partial pressure of interest in the gas phase;  = coverage; C = a constant for the gas/solid combination = ka/kd; ka = the adsorption rate coefficient; kd = the desorption rate coefficient.

Langmuir Isotherm (cont’d) Some physisorption and most chemisoption processes follow this isotherm. It is the one with the best theoretical basis, which assumes that adsorption is limited to one monolayer on the surface. One can obtain the two constants by linearization of the isotherm: take the reciprocal and rearrange  P 1 C P/Wads J = 1/CWmax S = 1/Wmax

Langmuir Isotherm (cont’d) It is particularly suited to represent binary and ternary systems.

Assignment Define the Langmuir Isotherm in case of liquid phase.

Freundlich Isotherm The Fruendlich isotherm model is valid for heterogeneous surfaces, monolayer coverage. Common for most adsorption work since it fits almost all data. It is empirical in nature, although some theoretical foundations do exit.

Freundlich Isotherm The expression： Wads = KF P 1/n (KF and n are experimentally determined parameters) When n = 1, the reaction is linear and called “partitioning”. When n > 1, the reaction is said to be “favorable” as the incremental change in amount sorbed decreases with increasing concentrations. While n < 1 is called “unfavorable” because the reverse is true. Most natural adsorbents exhibit either linear or favorable adsorption. The Langmuir and Fruendlich models for n < 1 are concave downwards, so both models can be calibrated to similar data.. Wads P n = 1 n < 1 n > 1

Freundlich Isotherm (cont’d) Wads = KF P 1/n lnWads = lnKF + 1/n lnP Wads P n = 1 n < 1 n > 1 ln Wads ln P 1/n ln KF log

Freundlich Isotherm Parameters Available for a wide variety of organic vapors on various activated carbon types Wads = KF P 1/n

Brunauer-Emmett-Teller (BET) Isotherm Brunauer, Emmett and Teller (BET) developed several models for gas adsorption on solids which have become the effective standard for surface area measurements. BET isotherm is valid for multiple layers on homogeneous surfaces.

Brunauer-Emmett-Teller (BET) Isotherm The assumptions underlying the simplest BET isotherm are: Gas adsorbs on a flat, uniform surface of the solid with a uniform heat of adsorption due to van der Waals forces between the gas and the solid. There is no lateral interaction between the adsorbed molecules. After the surface has become partially covered by adsorbed gas molecules, additional gas can adsorb either on the remaining free surface or on top of the already adsorbed layer. The adsorption of the second and subsequent layers occurs with a heat of adsorption equal to the heat of liquefaction of the gas. multi-layers adsorption

BET Isotherm (cont’d) Work for almost any type of data on the adsorption of gases on solids. It describes every type of isotherm including the linear, and Langmuir isotherms. The theoretical basis is sound. For single component the equation is, Wads P for n   for finite n Note that n is the number of adsorbed monolayers, and x = P/P0. Where, P is the actual partial pressure of gas in the stream and P0 is the vapor pressure of the pure gas. Note: The BET simplifies to the Langmuir when relative pressure x < 0.01 and C >100 (Valsaraj et al., 1992).

BET Isotherm (cont’d) To obtain the parameters in the BET equation, one needs to linearize the equation: P/Po S = (C-1)/CWmax J = 1/CWmax

The most common isotherm models [Dastgheib and Rockstraw, 2002]