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École FRANCO-VIETNAMIENNE 2005 de CATALYSE CINETIQUE et RAFFINAGE Hanoi, 18-22 Avril 2005 Institut de Chimie Industrielle Hélène Provendier Institut de Recherches sur la Catalyse Université Claude Bernard Lyon 1 1 Ecole Franco-Vietnamienne 2005 de Catalyse Cinétique et Raffinage Hanoi, 18-22 Avril 2005 Physical and chemical techniques for catalyst characterization Dr. Hélène Provendier Institut de Recherches sur la Catalyse Villeurbanne France Centre National de la Recherche Scientifique ICI 2 Contents Part 1 : Adsorption techniques Part 2 : Diffraction techniques Part 3 : Spectroscopies 3 Part 1 : Adsorption techniques Part 1 : Characterization of porous solids using adsorption techniques 4 Outline I. Introduction to solid surface and porosity 1) Porous solids 2) Some definitions 3) Qualitative description of a porous solid 4) Texture of a solid 5) Classification of pores 6) Characterization methods II. Nature of Adsorption 1) Definition of adsorption 2) Two types of adsorption 3) Transition physisorption-chemisorption 4) Activation energy III. Physisorption 1) General 2) Theoretical approach 3) Porous volume measurements 4) Pore size distribution IV. Chemisorption 1) General 2) Chemisorption isotherms V. Experimental part 5 I. Introduction to solid surface and porosity 1) Porous solids Most materials are to some extent porous : they contain empty cavities. Physical properties : - density, - thermal conductivity - strength depend on the pore structure of a solid The control of porosity is of great industrial importance for example in the design of catalysts, industrial adsorbents, membranes and ceramics. Porosity influences : • the chemical reactivity (activity and selectivity) of solids • the physical interaction of solids with gases and liquids • mass and heat transfers in the solid In catalysis, porosity determines the accessible surface to the reactant (gas or liquid) It is thus important to characterize porous solids in catalysis 6 2) Some definitions Porous solid : a solid with pores, i.e. cavities, channels or interstices,which are deeper than they are wide. Pore volume Vp : volume of the pores, as measured by a given method which must be stated, (together, for instance, with the nature of the probe-molecule, the wavelength of the radiation used or the ultimate intrusion pressure ...). Pore size (generally, pore width) : the distance between two opposite walls of the pore (diameter of cylindrical pores, width of slit-shaped pores). Porosity e : ratio of the total pore volume Vp to the apparent volume V of the particle or powder (excluding interparticle voids). e = VP / V Roughness (or rugosity) factor : ratio of the external surface area to the area of the geometrical envelope of the particles. Surface area : extent of the total surface as determined by a given method under stated conditions. It is essential to state the method used. 7 3) Qualitative description of a porous solid * Closed pores : (a). They influence such macroscopic properties as bulk density, mechanical strength and thermal conductivity, but are inactive in such processes as fluid flow and adsorption of gases. * Open pores : (b) (c) (d) (e) and ( f ) . They have a continuous channel of communication with the external surface. - blind (i.e. dead-end, or saccafe) pores : (b) (f) opened only at one end - through pores : (e) opened at two ends * Classification according to the shape : - cylindrical (either open (c) or blind ( f ) ) , - ink-bottle shaped (b), - funnel shaped (d) or slit-shaped. Close to, but different from porosity is the - roughness of the external surface (g). f Schematic cross-section of a porous solid 8 4) Texture of a solid Texture : detailled geometry of empty spaces in particles. It includes : - intergranular spaces in agglomerates - intragranular pore ditribution - particle shape and external surface - pore shape and porous volume - accessibility of gases to internal surface Parameters describing the texture of a catalyst - specific surface area (accessible per gram of solid) - porosity • pore shape • pore size distribution • mean pore size - granulometry • particle size distribution • shape and size of agglomerates 9 5) Classification of pores 1) By size - macroporous samples  > 50 nm - mesoporous samples 2 nm <  < 50 nm - micro-porous samples  < 2 nm 2) By shape - cylindrical - slit-shaped (parallel or not) - funnel (entonnoir) - spherical - ink-bottle shaped - waved (vague) 10 6) Characterization methods Specific surface area Porosity Particle size Adsorption (BET method) Adsorption (capillary condensation) Hg porosimetry Thermoporosimetry Density measurements Permeability (for membranes) Transmission microscopy Sieving Sedimentation Light diffusion X-ray diffraction Electronic Microscopies (Scanning and Transmission) 11 II. Nature of Adsorption 1) Definition of adsorption Gas • Solid surfaces show strong affinity towards gas molecules that it comes in contact with and some of them are trapped on the surface • the process of trapping or binding of molecules to the surface is called adsorption • Desorption is removal of these gas molecules from the surface Solid 2) Two types of adsorption – Physical adsorption : * Van der Waals forces * bond energy is less than 50 kJ/mole – Chemical adsorption : * bond energy is more than 50 kJ /mole * direct chemical bond 12 Adsorption and Reaction at Surfaces 13 -5. Exothermic like condensation Activated desorption in original form Rapid equilibration, transport limited 14 -50….. Energy similar to chemical reaction Depends on reactivity of adsorbent and adsorptive 15 3) Transition physisorption-chemisorption When P increases, the volume of adsorbed gas Vads increases. When T increases, the volume of physisorbed or chemisorbed gas globaly decreases. Physisorption takes place at lower temperature than chemisorption and allows the molecule to approach the surface without high energy requirement. Physisorption predominates at low temperature and chemisorption at elevated temperature. Vads P = cte Ex : O2 adsorption on Ni At P=1 atm Physisorption Chemisorption T = - 200°C physisorption T = 25 °C chemisorption T = 800°C oxidation (NiO) T If the molecule is chemically transformed before desorption, there is contact catalysis 16 Example of H2 adsorption on Ni catalyst 1) Physisorption is the first step of chemisorption Distance from surface : r (Ni…H) = 3.2Å 2) Then chemisorption must be activated to form the transition state (energy required) H2 + 2 Ni 2 Ni-H DHC = 120 kJ.mol-1) Distance from surface : r (Ni-H) = 1.6Å 17 4) Activation energy Molecule A-B approaches the surface S of an adsorbent along the distance axis r. The first interaction process is physisorption at equilibrium position rP : exothermic process EP is evolved. Next is the endothermic stage where activation Ea is input to form the transition state. This displaces the molecule toward equilibrium position rC and Ec is evolved during chemisorption. Chemisorption is always an activated process. EP = Energy evolved during physisorption (exothermic) EC = Energy evolved during chemisorption (globaly exothermic) Ea = Activation energy (transition state) Ed = Energy required for desorption 18 III. Physisorption 1) General -Physisorption is not adsorbent or adsorptive specific. This is a process similar to condensation of gas on a surface. -The quantity of physisorbed molecules depends on the accessible surface area and not on its chemical nature. -Physisorbed molecules progressively form successive layers when the gas pressure P increases. When P reaches the vapor pressure P0, there is condensation on the surface. -However there can be capillary condensation in the solid pores for P < P0 depending on the pore size. 19 2) Theoretical approach a) Langmuir theory (monolayer adsorption) 1) Thermodynamic Derivation For molecules in contact with a solid surface at a fixed temperature, the Langmuir Isotherm, developed by Irving Langmuir in 1916, describes the partitioning between gas phase and adsorbed species as a function of applied pressure. The adsorption process between gas phase molecules, A, vacant surface sites, S, and occupied surface sites, SA, can be represented by the equation, assuming that there are a fixed number of surface sites present on the surface. An equilibrium constant, K, can be written : 20
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