1、1,Catalyst Characterization,朱信 Hsin Chu Professor Dept. of Environmental Eng. National Cheng Kung University,2,1. Introduction,Physical properties: pore size, surface area, and morphology of the carrier; and the geometry and strength of the support Chemical properties: composition, structure, and na
2、ture of the carrier and the active catalytic components Changes during the catalysis process: deactivation,3,2. Physical Properties of Catalysts,2.1 Surface Area and Pore Size of the Carrier Surface area Pore size Pore size distribution Pore structure Pore volume The size and number of pores determi
3、ne the internal surface area. It is usually advantageous to have high surface area (large number of small pores) to maximize the dispersion of catalytic components. However, if the pore size is too small, diffusional resistance becomes a problem.,4,2.2 surface Area and Pore Size Measurements A stand
4、ardized procedure for determining the internal surface area of a porous material with surface area greater than 1 or 2 m2/g is based on the adsorption of N2 at liquid N2 temperature onto the internal surfaces of the carrier. Each adsorbed N2 molecule occupies an area of the surface comparable to its
5、 cross-sectional area (16.22) By measuring the number of N2 molecules adsorbed at monolayer coverage, one can calculate the internal surface area. Next slide (Fig. 3.1a) BET gas adsorptin plot Flatten part: monolayer Second rise: multiple layers,6,The Brunaure, Emmett, and Teller (BET) equation desc
6、ribes the relationship between N2 volume adsorbed at a given partial pressure and the volume adsorbed at monolayer coverage: where P = partial pressure of N2 P0 = saturation pressure at the experimental temperature V = volume adsorbed at P Vm= volume adsorbed at monolayer coverage C = a constant Nex
7、t slide (Fig. 3.1b) Linear form of the BET equation The most reliable results are obtained at relative pressure (P/P0) between 0.05 and 0.3.,8,The same equipment can be used to determine the pore size distribution of porous materials with diameters less than 100, except that high relative pressures
8、are used for condensing N2 in the catalyst pores. Next slide (Fig. 3.1c) Pore size distribution measurement The procedure involves measuring the volume adsorbed in either the ascending or descending branch of the BET plot at relative pressures close to 1. This plot shows the carrier having a signifi
9、cant volume of mesopores (diameters between 20 and 500),10,Capillary condensation occurs in the pores in accordance with the Kelvin equation: where = surface tension of liquid nitrogen = contact angle V = molar volume of liquid nitrogen r = radius of the pore R = gas constant T = absolute temperatur
10、e P = measured pressure P0 = saturation pressure The form of the Kelvin equation describes the desorption isotherm, and it is the preferred one for calculation of pore size distribution. (desorption requires a lower pressure compared to adsorption),11,2.3 Pore Size by Mercury Intrusion For materials
11、 with pore diameters greater than about 30, the mecury intrusion method is preferred. The penetration of mercury into the pores of a material is a function of applied pressure. The Washburn equation: where d = pore diameter, nm p = applied pressure, atm = wetting or contact angle, between the mercur
12、y and the solid is usually 130o = the surface tension of the mercury, 0.48 N/m Next slide (Fig. 3.2) The mercury intrusion method: 30-106 the BET method: 100,13,2.4 Particle Size Distribution of the Carrier Sieves of various mesh sizes Reliable only for particles larger than about 40m (#400=38 m) Li
13、ght scattering, image analysis, sedimentation, centrifugation, and volume exclusion (coulter counter) Reliable for finer particles Laser techniques (more recently) Next slide (Fig. 3.3) Laser-generated particle size distribution for an Al2O3 carrier F: fraction U: cumulation,15,2.5 Washcoat Thicknes
14、s Optical microscopy is the most commonly used method of obtaining washcoat thickness. 2.6 Mechanical Strength of a Monolith Monoliths, particularly when used in a stacked mode (e.g., in stationary pollution abatement) must resist crushing axially. For automobile and truck application, and for ozone
15、 abatement in aircraft, resistance to vibration and radial strength is important. 2.7 Adhesion Erosion Thermal shocks (start up and shutdown),16,2.8 Location and Analysis of Species within the Catalyst Scanning electron microscope (SEM) is equipped with an energy dispersive analyzer (EDX) or wavelen
16、gth dispersive analyzer (WDS). The bombardment of a sample with electrons generates X rays characteristic of the elements present. Next slides (Fig. 3.4 a Fig. 3.4 d) WDS analysis for catalysts deposited on double-layered washcoats (a)(b): topcoat Pt dispersed on bottom coat SiO2 (c)(d): topcoat Pd
17、dispersed on bottom coat Al2O3,21,Next slide (Fig. 3.5) A electron microscope line profile of a poisoned automobile exhaust catalyst P and Zn, originating from the lubricating oil, are concentrated on the edge of the washcoat indicating they physically deposited as aerosols. Gaseous sulfur compounds
18、 such as SO2/SO3, penetrate deeply and more uniformly into the washcoat.,23,3. Chemical Properties,3.1 Elemental Analysis Small amounts of promoter oxides intentionally added (often 0.1 %) can influence catalyst activity, selectivity, and life. Carriers are derived from raw materials, which contain
19、various impurities such as alkali and alkaline-earth compounds, if used in excess, causing sintering or loss of surface area in Al2O3. When added in the proper amount, the same impurities can enhance stability against sinterting or, in some cases, improve selectivity. Therefore, the quantitative pro
20、cedures used to analyze the composition of catalysts are important.,24,3.2 Thermal Gravimetric Analysis (TGA) TGA: a useful technique to follow microscopic weight changes A few milligrams of contaminated catalyst can be loaded into a quartz pan suspended in the microbalance. A controlled gas flow an
21、d temperature ramp is initiated and a profile of weight change versus temperature is recorded. Frequently, TGA units are equipped with a mass spectrometer so that the offgases from the catalyst can be measured as a function of temperature. The weight-temperature profile is helpful in establishing pr
22、ocedures for regenerating the catalyst and other processes in the process reactor.,25,3.3 Analysis by X-Ray Diffraction (XRD) Provided a material is sufficiently crystalline to diffract X rays and is present in an amount greater than 1%, XRD can be used for qualitative and quantitative analysis. The
23、 angles of diffraction differ for the various planes within the crystal. Thus, every compound or element has its own somewhat unique diffraction pattern. 3.4 Structural Analysis: structure of Al2O3 carriers Next slide (Fig. 3.6) XRD patterns of amorphous -Al2O3 and -Al2O3,27,-Al2O3: the high-surface
24、-area, lower - temperature structure -Al2O3: produced at high temperature and has low surface area Amorphous: Materials with crystallites smaller than 50. A well-defined X-ray pattern will not be obtained.Need to be characterized by other techniques listed below.,28,3.5 Dispersion or Crystallite Siz
25、e of Catalytic Species,3.5.1 Chemisorption When a structure has a definite XRD pattern, it usually has less than optimum activity. This is because most catalytic reactions are favored by either amorphous materials or extremely small crystallites. Frequently, the purpose of the preparation technique
26、is to disperse the catalytic components to maximize their availability to reactants: % dispersion = When this is done effectively, only small crystals are present and the diffraction of X rays is minimized. Selective chemisorption can be used to measure the accessible catalytic component on the surf
27、ace by noting the amount of gas adsorbed per unit weight of catalyst.,29,One assumes that the catalytic surface area is proportional to the number of active sites. A gas that will selectively chemisorb only onto the metal and not the support is used under predetermined conditions. Hydrogen and CO ar
28、e most commonly used as selective adsorbates for many supported metals. The measurements are usually carried out in a static vacuum system similar to that used for BET surface area measurements. Next slide (Fig. 3.7a) Chemisorption isotherm (monolayer coverage, 1 H per metal site)One can determine t
29、he catalytic surface area by multiplying molecules adsorbed by cross-sectional area of the site and dividing by the weight of catalyst, e.g., the cross-sectional area of Pt is 8.92 and Ni, 6.52.,31,The static vacuum technique is time-consuming. Alternatively, a dynamic pulse technique has been used
30、in which a pulse of adsorbate such as H2 or CO is injected into a stream of inert gas and passed through a bed of catalyst. Next slide (Fig. 3.7b) Pulse chemisporption profiles The static method measures only species that are strongly adsorbed. The dynamic method, performed under equilibrium conditi
31、ons, measures strong and weakly chemisorbed species. Thus, static techniques usually give better dispersion results.,33,3.5.2 Transmission Electronmicroscopy (TEM) A thin sample is subjected to a beam of electrons. The dark spots on the positive of the detecting film correspond to dense areas in the
32、 sample that inhibit electron transmission. These dark spots form the outline of metal particles or crystallites and, hence, their sizes can be determined. Next slide (Fig. 3.8) A TEM of sintered Pt, dispersed on TiO2 (500) Following slide (Fig. 3.9) Crystallites of Pt about 100 in size, dispersed o
33、n CeO2,36,3.5.3 X-Ray Diffraction The larger the crystals of a given component, the sharper the peaks on the XRD pattern for each crystal plane. The scherrer equation: Where B = the breadth at half-peak height of an XRD line L = the size of the crystallites = X-ray wavelength = diffraction angle k =
34、 a constant usually equal to 1 As the crystallite size increases, the line breadth B decreases. Next slide (Fig. 3.10) XRD profile for different crystallite sizes of CeO2,38,3.6 Surface Composition of Catalysts,XRD and TEM measure the structure and/or chemical composition of catalysts extending belo
35、w the catalytic surface. The composition of the surface is usually different from that of the bulk. It is on these surfaces that the active sites exist and where chemisorption, chemical reaction, and desorption take place. The tools available for surface composition characterization are X-ray photoe
36、lectron spectroscopy (XPS), Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS), and secondary-ion mass spectroscopy (SIMS).,39,XPS is used more widely than the others for studing the surface composition and oxidation states of industrial catalysts. XPS refers to the technique of bo
37、mbarding the surface with X-ray photons to produce the emission of characteristic electrons. These are measured as a function of electron energy. Because of the low energy of the characteristic electrons, the depth to which the analysis is made is only 40 . The composition of this thin layer as a fu
38、nction of depth can be determined by removing or sputtering away top layers and analyzing the underlying surfaces. This technique can provide properties including oxidation state of the active species, interaction of a metal with an oxide carrier, and the nature of chemisorbed poisons and other impurities.,
