Catalysisand-Catalyst.ppt

1、1,Mechanisms of Catalytic Reactions and Characterization of Catalysts,2,What is a Catalyst ?,Catalyst is a substance that increases the rate of the reaction at which a chemical system approaches equilibrium , without being substantially consumed in the process.,Catalyst affects only the rate of the

2、reaction,i.e.Kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.,3,Thermodynamics says NOTHING about the rate of a reaction. Thermodynamics : Will a reaction occur ? Kinetics : If so, how fast ?,Thermodynamic,Chemical Reaction,4,A reaction may have a larg

3、e, negative DGrxn, but the rate may be so slow that there is no evidence of it occurring.,Kinetic Vs. Thermodynamic,Conversion of graphite to diamonds is a thermodynamic favor process (G -ve ). C (graphite) C (diamond) Kinetics makes this reaction nearly impossible (Requires a very high pressure and

4、 temperature over long time),5,Reaction path for conversion of A + B into AB,Kinetic Vs. Thermodynamic,Reaction Profile,6,Activation Energy,Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol Ea or G,The Activation Energy (Ea) determines how fast a reacti

5、on occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction,Definition,7,Catalyst lowers the activation energy for both forward and reverse reactions.,Activation Energy,Catalyst Affect,8,Activation Energy,Catalyst Affect,This mean

6、s , the catalyst changes the reaction path by lowering its activation energy and consequently the catalyst increases the rate of reaction.,9,How a Heterogeneous Catalyst works ?,Substrate has to be adsorbed on the active sites of the catalyst,10,Absorption and Adsorption,H2 adsorption on palladium,S

7、urface process,bulk process,11,Adsorption,In physisorption The bond is a van der Waals interaction adsorption energy is typically 5-10 kJ/mol. ( much weaker than a typical chemical bond ) many layers of adsorbed molecules may be formed.,12,Adsorption,For Chemisorption The adsorption energy is compar

8、able to the energy of a chemical bond. The molecule may chemisorp intact (left) or it may dissociate (right). The chemisorption energy is 30-70 kJ/mol for molecules and 100-400 kJ/mol for atoms.,13,Characteristics of Chemi- and Physisorptions,DE(ads) DE(ads) Physisorption Chemisorption small minima

9、large minima weak Van der Waal formation of surface attraction forces chemical bonds,14,Adsorption and Catalysis,Adsorbent: surface onto which adsorption can occur. example: catalyst surface, activated carbon, alumina Adsorbate: molecules or atoms that adsorb onto the substrate. example: nitrogen, h

10、ydrogen, carbon monoxide, water Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate. Coverage: a measure of the extent of adsorption of a specie onto a surface,15,Adsorption Mechanisms,Langmuir-Hinshelwood mechanisms: 1. Adsorption from the gas-phase 2. Desorption

11、to the gas-phase 3. Dissociation of molecules at the surface 4. Reactions between adsorbed molecules,Two Questions: Is the reaction has a Langmuir-Hinshelwood mechanism? What is the precise nature of the reaction steps?,Cannot be solved without experimental or computational studies,16,ExampleThe Rea

12、ction A2 + 2B = 2AB,may have the following mechanism A2 + * = A2*A2* + * = 2A*B + * = B*A* + B* = AB* + *AB* = AB + *,Langmuir-Hinshelwood mechanisms,17,Adsorption Mechanisms,Eley-Rideal mechanism: Adsorption from the gas-phase Desorption to the gas-phase Dissociation of molecules at the surface Rea

13、ctions between adsorbed molecules Reactions between gas and adsorbed molecules,The last step cannot occur in a Langmuir-Hinshelwood mechanism,18,Example The reaction A2 + 2B = 2AB,may have the following Eley-Rideal mechanism A2 + * = A2*A2* + * = 2A*A* + B = AB + *,where the last step is the direct

14、reaction between the adsorbed molecule A* and the gas-molecule B.,Eley-Rideal mechanism,19,Eley-Rideal or Langmuir-Hinshelwood?,For the Eley-Rideal mechanism: the rate will increase with increasing coverage until the surface is completely covered by A*.,For the Langmuir-Hinshelwood mechanism: the ra

15、te will go through a maximum and end up at zero, when the surface is completely covered by A*.,This happens because the step B + * = B* cannot proceed when A* blocks all sites.,The trick is that the step B + * = B* requires a free site.,20,Catalyst Preparation,(1) Unsupported Catalyst Usually very a

16、ctive catalyst that do not require high surface area e.g., Iron catalyst for ammonia production (Haber process),(2) Supported Catalyst requires a high surface area support to disperse the primary catalyst the support may also act as a co-catalyst (bi-functional) or secondary catalyst for the reactio

17、n (promoter),21,Supported Catalyst,Nickel clusters,SiO2,Highly dispersed metal on metal oxide,22,Molecules in Zeolite Cages and Frameworks,23,What is ZSM-5 Catalyst ?,It is an abbreviation for (Zeolite Scony Mobile Number 5 ) First synthesized by Mobil Company in 1972 It replaces many Homogeneous Ca

18、talysts were used in many petrochemical processes ZSM-5 has two diameters for its pores : d1= 5.6 , d2= 5.4 Where as, Zeolite Y has a diameter = 7.4 ,24,Properties ZSM-5,The ZSM-5 zeolite catalyst is used in the petroleum industry for hydrocarbon interconversion. ZSM-5 zeolite is a highly porous alu

19、minosilicate with a high silica/alumina ratio.,It has an intersecting two-dimensional pore structure. The aluminum sites are very acidic. The acidity of the zeolite is very high. The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.,25,Structure of ZSM-5,26,Aromatic Isomerization

20、,Observed by Haag and Co-workers P-Selectivity is achieved due to the high diffusion coefficient of P-Substituted Molecules Relative to that of Ortho or Meta.,A side reaction in homogenous solution phase isomerization is the Bimolecular disproportionation of Xylene.,This Can not occur when using zeo

21、lite such as ZSM-5 since the pore size will not allow for the bulky bimolecular transition state necessary for disproportionation.,27,28,Selectivities of acidic zeolite for disproportionation and isomerization of Xylene. The HZSM-5 Catalyst is preferred, minimizing the bimolecular disproportionation

22、 reaction by virtue of restricted transition state selectivity.,29,Schematic diagram of product shape selectivity: Para-xylene diffuses preferentially out of the zeolite channels,30,Acidity of the Catalyst Vs. Acid Sites,Theoretically, As the Acid Sites increase the Acidity increases,Experimentally,

23、 As the Acid Sites increase the Acidity decreases,How we can understand this behavior?,31,Acidity of the Catalyst Vs. Acid Sites,Electronegativity Effect,Al is the Acidic Site But, Si is more electronegative than Al,Si Makes the release of the H+ faster since it attracts Electrons toward it which ma

24、ke the proton away of it,32,Acidity of the Catalyst Vs. Acid Sites,It is possible to conclude that: the acid strength of a site will increase when there is a decrease in the number of Al atoms in the Next Nearest Neighbor position of the Al atom.,So, the strongest type of framework Bronsted site is

25、A completely isolated Al tetrahedron which have zero NNN or (0NNN),33,Acidity Characterization of a Catalyst,Acidity of the catalysts can be assess by:,I. Temperature Programmed Desorption (TPD) II. Fourier Transformation Infrared spectroscopy ( FTIR) III. Induced Laser spectroscopy (IL),34,Pure car

26、rier gas (typically helium) flows over the sample as the temperature is raised to desorb the previously adsorbed gas e.g. NH3 2. This characteristic fingerprint for each catalyst,used to determine: the distribution of acid-site strength if ammonia is the sorbed gas, or the distribution of basic site

27、s if carbon dioxide is the sorbed gas.,I. Temperature Programmed Desorption (TPD),35,Time(min),Temp.,I. Temperature Programmed Desorption (TPD),36,II. Fourier Transformation Infrared spectroscopy ( FTIR),By this method we assess acidic site: Bronsted acidic site or Lewis acidic site,For example: if

28、pyridine is the probe molecule, It will give peaks at: 1. 1540 cm-1 for Bronsted site 2. 1450 cm-1 for Lewis site,37,II. Fourier Transformation Infrared spectroscopy ( FTIR),38,II. Fourier Transformation Infrared spectroscopy ( FTIR),39,III. Induced Laser spectroscopy (IL),To assess the acidity by IL: 1. Probed molecule has to be prepared in different acidic concentration solutions 2. Life time