BAE 820 Physical Principles of Environmental Systems

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1 BAE 820 Physical Principles of Environmental Systems Catalysis of environmental reactions Dr. Zifei Liu

2 Catalysis and catalysts Catalysis is the increase in the rate of a chemical reaction due to the participation of an additional substance called a catalyst. With a catalyst, reactions occur faster and with less energy. Catalyst has been defined as a substance that changes the rate of reaction but that is not itself consumed in the process. In fact, catalysts participate in a reaction, but are eventually regenerated in the system such that there is no net concentration change. Catalyzed reactions are prevalent in the natural environment and pollution prevention processes. Most industrial reactions are run with catalysts. Over 90% of chemical and petroleum products are made via catalytic processes. 2

3 Reactivity and selectivity Sometimes the selectivity to form a desired product is more important than reactivity; a catalyst can be used provide a lower barrier for the desired reaction, leaving the undesired reaction rate unchanged. The search for this active and selective catalyst in biology or in chemical synthesis is perhaps the central goal in most biological and physical sciences research. 3

4 Homogeneous and heterogeneous catalysis Homogeneous catalysis: The catalysts are in the same phase as the reactants. Heterogeneous catalysis: The catalysts are in a separate phase. They act at the boundary of the phase of reactants (usually a gas or liquid solution) to promote reactions (e.g. solid particles in water). Most surface reactions belong to the heterogeneous catalysis. 4

5 Examples of homogeneous and heterogeneous catalysis in environmental engineering Reaction Application Catalysis type Enzyme-catalyzed biodegradation Aquatic, soil chemistry, waste treatment Homogeneous Ozone destruction in gas phase Atmospheric chemistry Homogeneous Hydroxylation of N 2 O over zeolite catalysts Oxidation of organics in water on TiO 2 NO x formation in combustion Acid and base hydrolysis of pesticides and esters Removal of N 2 O Removal of organic pollutants Atmospheric chemistry Aquatic, soil and sediment chemistry Heterogeneous Heterogeneous Homogeneous and heterogeneous Homogeneous and heterogeneous 5

6 General rate expressions for catalyzed reactions Consider reactant A and catalyst X. The first step is the formation of a complex Z. The complex Z further reacts with another reactant C to give desired products and regenerate the catalyst X. A(reactant)+X(catalyst) Z(complex)+C k 2 Z(complex)+B products + X(catalyst) The overall equilibrium constant for the formation of a complex Z is K eq = k f k b = C Z C B C X C A = C Z C B (C X0 C z )(C A0 C z ) The rate of product formation is proportional to the concentration of the complex Z. -r = k 2 C Z C C 6

7 When the reactant is in large excess When C A0 >>C X0, C A C A0, K eq C Z C B C X0 C z C A0 If K eq C A0 CB, C z K eq C X0 C A0 K eq C A0 + CB C z K eq C X0 C A0 K eq C A0 = C X0 -r = k 2 C Z C C k 2 C X0 C C In this case, the rate of product formation is linear to the concentration of the catalyst and independent of C A0. 7

8 When the catalyst is in large excess When C A0 <<C X0, C X C X0, K eq C Z C B C A0 C z C X0 If K eq C X0 C B, C z K eq C X0 C A0 K eq C X0 +C B C z K eq C X0 C A0 K eq C X0 = C A0 -r = k 2 C Z C C k 2 C A0 C C In this case, the rate of product formation is linear to C A0. 8

9 Catalysis and activation energy Catalyzed reactions have a lower activation energy (rate-limiting free energy of activation) than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. 9

10 Rate of enhancement with catalysts Consider the decomposition of hydrogen peroxide in the aqueous phase. Under normal condition, the activation energy is 76 kj/mol. Presence of bromide can reduce it to 57 kj/mol. The enhancement in rate at 298K can be estimated from Reaction E 1 57 RT ) k 1 = exp( RT ) = exp( k 2 exp( E 2 RT ) exp( 76 Catalyst E a uncatalyzed Kcal/mol RT ) = 2140 E a catalyzed Kcal/mol Rate of enhancement calculated at 500K H 2 +I 2 2HI Pt N 2 O 2N 2 +O 2 Au

11 Catalysis and reaction temperature From a practical standpoint, one of the key roles of a catalyst is to lower the temperature required for a reaction. E.g. a reaction has an activation barrier of 53 kcal/mol in the absence of a catalyst, and 38 kcal/mol in the absence of a catalyst. With an activation barrier of 53 kcal/mol you need to run the reaction at about 800K, while with an activation barrier of 38 kcal/mol you can run the reaction at about 600K. T minute = E a 15 K mol/kcal In which, T minute is the temperature needed for a reaction to get a half-life of about one minute. 11

12 Acids or bases catalysis The most prevalent reactions in the environment are catalyzed by acids or bases. If a catalyzed reaction is carried out at a high enough [H + ] such that [OH - ] is negligible, the rate of reaction will be directly proportional to [H + ] and C A. -r= k H [H + ]C A Similarly, the rate of a base-catalyzed reaction is given by -r= k OH [OH - ]C A The overall rate of an acid-base catalyzed reaction is therefore -r = (k 0 +k H [H + ]+ k OH [OH - ])C A 12

13 Enzymes Enzymes are a very important class of homogeneous catalysts. Action of enzymes is an important aspect of environmental bioengineering. Enzymes are complicated proteins. They are usually classified according to what they do rather than how they are constructed. Oxidoreductasea are enzymes that promote oxidation-reduction reactions Transferases are enzymes that promote transfer of functional groups from a donor molecule to a acceptor molecule. Hydrolases are enzymes that promote hydrolysis reactions. Enzymes work principally by four routes: Bind to the reactants in a way that key bonds in the reactants are stretched. That makes bonds easier to break. Lower the energy of the transition state. Stabilize key intermediates. Push the reactants together for a bond-forming reaction. 13

14 Heterogeneous catalysis (surface reactions) Many reactions in environment occur at the surfaces of either solids or liquids, and they are significantly influenced by the nature and property of the surface. Particulate-mediated catalysis plays a large role in many atmospheric photochemical reactions. Removal of VOCs from automobile exhaust involves the use of sophisticated catalysts. Many waste treatment processes rely on reactions as surfaces. 14

15 General mechanisms of surface catalysis A surface reaction involves a series of successive steps. The first step is bulk diffusion of reactant, which is generally fast. Then the process of adsorption, reaction, and desorption is usually considered a single rate-limiting step. A heterogeneous surface reaction mechanism involves postulating a surface-adsorbed molecule which further becomes aa activated complex that then breaks down to give the products. A(reactant)+X(surface) Z(complex) Z(complex) products + X A Adsorption Z Reaction P P Desorption 15

16 Reaction rate of surface catalysis The rate of conversion of the adsorbed complex to products is K L C A -r= ks 0 θ A = ks 0 1+K L C A In which, θ A is surface coverage of the reactant A, which is estimated using the Langmuir isotherm; K L is Langmuir adsorption constant; S 0 is the total binding sites available on the surface. At high reactant concentration, K L C A >>1, -r ks 0 and is independent of the concentration of A. At low reactant concentration, K L C A <<1, -r ks 0 K L C A and the rate is first order in A. 16

17 Two competing species Consider the Langmuir isotherm for two competing species A and B. If only A reacts, and B is nonreactive, it can act as an inhibitor since it reduces the surface coverage of A. If K L,B C B >>1+ K L,A C A, -r= ks 0 θ A = ks 0 K L, A C A 1+K L, A C A +K L, B C B -r ks 0 K L, A C A K L, B C B The rate is first order in A and inversely proportional to concentration of B. If Both A and B react, -r= ks 0 θ A θ B = ks 0 K L, A K L, B C A C B (1+K L, A C A +K L, B C B )2 The equation indicates the competition for surface sites between A and B. If C A is held constant, the rate will go through a maximum as C B is varied. 17

18 Heterogeneous catalysts Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. The total surface area of solid has an important effect on the reaction rate. The smaller the catalyst particle size, the larger the surface area for a given mass of particles. Heterogeneous catalysts are very important in industry. Heterogeneous catalysts are generally powders or pellets that one can add to a reacting mixture to speed up the reaction. They are used extensively in chemical processing because heterogeneous catalysts are easier to separate from the products of a reaction mixture than a homogeneous catalyst. 18

19 Surface reaction rate The rate of a heterogeneously catalyzed reaction is usually proportional to the surface area of the catalysts. The reaction rate scales as the surface area of the catalyst, not the volume of the reactor. So it is common to express the rate in units of mol per unit time per surface area. r = surface area volume In which r is the surface reaction rate in units of mol per unit time per surface area. r is a pseudohomogeneous rate in units of mol per unit time per volume. surface area = S volume g ρ(1 ε) In which S g is the surface area per unit weight of catalyst; ρ is density of the catalysts; ε is porosity, which is given by ε = void volume/total volume r 19

20 Surface area of catalysts The highest possible area is desired to attain the highest rate with minimum total reactor volume. This is usually achieved by formulating the catalyst as powder, which is then pressed into porous pellets that are packed into the packed bed reactor. A number of special materials (e.g. activated carbon) have been developed to squeeze as much surface area as possible in a certain volume. It is possible to squeeze the surface area of 5000 m 2 (about the area of a football field) into 100 cm 3 or less of material! 20

21 Supports of catalysts Heterogeneous catalysts are typically "supported," which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes their cost. Supports prevent or reduce agglomeration of the small catalyst particles, exposing more surface area, thus catalysts have a higher specific activity (per gram) on a support. Support can be merely a surface on which the catalyst is spread to increase the surface area. Supports can be porous materials with a high surface area, most commonly alumina, zeolites or various kinds of activated carbon. Specialized supports include silicon dioxide, titanium dioxide, calcium carbonate, and barium sulfate. 21

22 Catalytic reactors Packed bed reactor: This is typically a tank or tube filled with catalyst pellets with reactants entering at one end and products leaving at the other end. Slurry reactor and fluidized bed reactor The fluid and the catalyst are stirred instead of having the catalyst fixed in a bed. The stirring must be fast enough to mix the fluid and particles. It is called a slurry reactor if the fluid is a liquid. It is called a fluidized bed reactor if the fluid is a gas and it flows such that the particles are lifted and gas and particles swirl around the reactor. 22

23 Limitations of catalysts Catalysts do not work over a wide range of conditions. At very high temperature, catalysts may slow down the reaction by promoting termination reactions. The effect of a catalyst may vary due to the presence of other substances known as inhibitors (if reversible) or poisons (if irreversible) which reduce the catalytic activity. The opposite of a catalyst, a substance that reduces the rate of a reaction, is an inhibitor. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid liquid system or sublimate in a solid gas system. 23

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