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1、Mechanisms of Catalytic Reactions and Characterization of Catalysts AYCATLSIS1What 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
2、the reaction,i.e.Kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.2Thermodynamics says NOTHING about the rate of a reaction.Thermodynamics : Will a reaction occur ?Kinetics : If so, how fast ?Chemical Reaction3A reaction may have a large, negative Grxn,
3、 but the rate may be so slow that there is no evidence of it occurring.Kinetic Vs. ThermodynamicConversion 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 temperature over lo
4、ng time)4Reaction path for conversion of A + B into ABKinetic Vs. Thermodynamic5Activation EnergyActivation Energy : The energy required to overcome the reaction barrier. Usually given a symbol Ea or GThe Activation Energy (Ea) determines how fast a reaction occurs, the higher Activation barrier, th
5、e slower the reaction rate. The lower the Activation barrier, the faster the reaction6Catalyst lowers the activation energy for both forward and reverse reactions.Activation Energy7Activation EnergyThis means , the catalyst changes the reaction path by lowering its activation energy and consequently
6、 the catalyst increases the rate of reaction.8How a Heterogeneous Catalyst works ?Substrate has to be adsorbed on the active sites of the catalyst9Absorption and AdsorptionHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH2 absorption palladium hydrideH2 adsorption onpalladiumSurface processbulk process10Adsorpt
7、ionIn physisorption 1.The bond is a van der Waals interaction 2.adsorption energy is typically 5-10 kJ/mol. ( much weaker than a typical chemical bond )3.many layers of adsorbed molecules may be formed. 11AdsorptionFor Chemisorption 1.The adsorption energy is comparable to the energy of a chemical b
8、ond. 2.The molecule may chemisorp intact (left) or it may dissociate (right). 3.The chemisorption energy is 30-70 kJ/mol for molecules and 100-400 kJ/mol for atoms. 12Characteristics of Chemi- and PhysisorptionsdE(d)physisorptionatomic chemisorptionphysisorption/desorptionchemisorptionCO E(ads) 1000
9、0+ p-xyleneZSM-527150000.70Mordenite 1.20.80Effective Diameter of intracrystalline Cavity (nm) 45000HY10000ZSM-4ZSM-5Kdis/Kiso Zeolite Catalyst Type 10000.60Selectivities of acidic zeolite for disproportionation and isomerization of Xylene. The HZSM-5 Catalyst is preferred, minimizing the bimolecula
10、r disproportionation reaction by virtue of restricted transition state selectivity.28p-xylenem-xyleneSchematic diagram of product shape selectivity: Para-xylene diffuses preferentially out of the zeolite channels 29Acidity of the Catalyst Vs. Acid Sites Theoretically, As the Acid Sites increase the
11、Acidity increases Experimentally,As the Acid Sites increase the Acidity decreases How we can understand this behavior?30Acidity of the Catalyst Vs. Acid Sites Electronegativity Effect BCNOFAlSiPSClAl is the Acidic SiteBut,Si is more electronegative than AlSi Makes the release of the H+ faster since
12、it attracts Electrons toward it which make the proton away of it O OAlSiAlH+31Acidity 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,
13、the strongest type of framework Bronsted site isA completely isolated Al tetrahedron which have zero NNN or (0NNN)32Acidity Characterization of a CatalystAcidity of the catalysts can be assess by:I. Temperature Programmed Desorption (TPD)II. Fourier Transformation Infrared spectroscopy ( FTIR)III. I
14、nduced Laser spectroscopy (IL)331.Pure carrier 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 sor
15、bed gas, or the distribution of basic sites if carbon dioxide is the sorbed gas. I. Temperature Programmed Desorption (TPD) 34Time(min)Temp.I. Temperature Programmed Desorption (TPD)35II. Fourier Transformation Infrared spectroscopy ( FTIR)By this method we assess acidic site:Bronsted acidic site or
16、 Lewis acidic site For example: if pyridine is the probe molecule, It will give peaks at:1. 1540 cm-1 for Bronsted site2. 1450 cm-1 for Lewis site 36 II. Fourier Transformation Infrared spectroscopy ( FTIR)37II. Fourier Transformation Infrared spectroscopy ( FTIR)38III. Induced Laser spectroscopy (I
17、L)To assess the acidity by IL:1. Probed molecule has to be prepared in different acidic concentration solutions 2. Life time measurements has to run onto each concentration3. Calibration curve between life time Vs. concentration of the acid 39III. Induced Laser spectroscopy (IL)40ConclusionThere is
18、no single method that can be used to determine all aspects of acidity for a solid i.e. nature, strength and the number of acid sites Each method measures only certain aspects and,therefore, the application of many methods is desirable. THE END41End of the Catalysis and CatalystsTHANK YOU References
19、1.A.Corma,Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. Rev.1995, 95, 559-614.2.Bruce C. gates, Catalytic Chemistry.1992.3.Zaki Seddigi, Characterization of the acidic properties of zeolite and their catalytic behavior in the synthesis of MTBE. 1994.4.Ali El-Rayyes, Study of the photochemical properties of some aromatic compound on molecular sieves using a picosecond pulse laser system. 2001. 5.Keith Laidler, Chemical Kinetics, 1987. 42
