1、Chapter two: Relationship between surface structures of catalytic materials and their catalytic properties (4 credit hours) By Dr. Yejun Qiu Department of Materials Science and Engineering Spring, 2011. ? Electron structure ? Crystal structure ? Defect structure ? Surface acid or base sites (importa
2、nt acid-base catalysis) ? Skeleton structure (Example: zeolite) ? Size effect (Nano effect) for 1. Electron structure ? Atomic Orbital Theory ( 原子轨道理论); ? Molecular Orbital Theory ( 分子轨道理论); ? Band theory (electron structure of solid). 1.1. Atomic Orbital Theory ? electron cloud and atomic orbital:
3、1 s , 2s , 2px, 2py, 2 pz 2dz dxz dyz dxy 2 2dx -y The atomic orbital model is the currently accepted model of the placement of electrons in an atom. Na: ? The order of Atomic Orbital Energy: ? Energy shell (n = 1、2、3、4、5、6) ? Energy level (s, p, d, f) （1）Same energy shell, nsnpndnf; （2）Same energy
4、level, 1s2s3s4s; （3）Same energy shell, different atomic orbital, 2px=2py=2pz, etc.; （4）Different energy shell and energy level: 1s2s2p3s3p4s3d4p5s4d5p6s4f5d6p7s Energy Pauli approximate energy level diagram ? Filling of orbital ? Pauli exclusion principle: “no two electrons in the same atom can be i
5、n the same quantum state.” One atomic orbital can contain two electrons at most and the spin direction is opposite. ? Hunds rule is an observational rule which states that a greater total spin state usually makes the resulting atom more stable. Every orbital in a subshell is singly occupied with one
6、 electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin. d-orbital p-orbital ? Stable electron configuration: A stable electron configuration refers to an atom in which the outer electron shell is unoccupied, half-full or full. 24Cr：Ar3d5
7、4s1 (3d44s2) 29Cu：Ar3d104s1 (3d94s2) Generally, more unoccupied d-orbitals are beneficial to catalytic reactions. 1.2. Molecular orbital theory ? One of chemical bonding theories ? Molecular orbitals are obtained by combining the atomic orbitals on the atoms in the molecule. Example: the formation o
8、f H-H bond. This diagram suggests that the energy of an H2 molecule is lower than that of a pair of isolated atoms. As a result, the H2 molecule is more stable than a pair of isolated atoms. Examples: O2 and F2 Examples:B2, C2, and N2 Molecular orbitals of benzene ? delocalized molecular orbital: In
9、 the benzene ring, the bonds are delocalized and form around the entire ring, forming a torus (doughnut-shaped) orbital above and below the sigma bond. Examples: CH2=CH-CH=CH2, 1.3. Energy band structure Why bands occur in materials? The electrons of a single isolated atom occupy atomic orbitals, wh
10、ich form a discrete set of energy levels. If several atoms are brought together into a molecule, their atomic orbitals split, as in a coupled oscillation . This produces a number of molecular orbitals proportional to the number of atoms. When a large number of atoms (of order 1020 or more) are broug
11、ht together to form a solid, the number of orbitals becomes exceedingly large. Consequently, the difference in energy between them becomes very small. Thus, in solids the levels form continuous bands of energy rather than the discrete energy levels of the atoms in isolation. However, some intervals
12、of energy contain no orbitals, no matter how many atoms are aggregated, forming band gaps. Metal: Fe, Co, Ni,; Semiconductor: Ge(0.66eV), Si(1.12eV), TiO2(3.23eV), ZnO(3.3eV), GaAs(1.42eV), GaN(3.44eV), Insulator: rubber, ceramics, Comparison of the band gaps for a metal, a semiconductor and an insu
13、lator. Energy Band Structure Density of States (DOS) 1.4. Effect of electron structure on catalysis Example: Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. Band gap:3.23 eV 760nm): 43%; UV-light(400nm): 7%. In photogenerated catalysis, the photocatalytic activit
14、y (PCA) depends on the ability of the catalyst to create electronhole pairs, which generate free radicals (hydroxyl radicals: OH) able to undergo secondary reactions. Eigen value ( E) of the outer orbital of the BO69- (B = Ti - Ni) clusters Photocatalytic activity 文献：桑丽霞 , 傅希贤, 白树林, 杨秋华, 王俊珍, 曾淑兰.感光
15、科学与光化学 , 2001,19(2):109-115 2. Crystal structure To be continued！ Chapter two: Relationship between surface structures of catalytic materials and their catalytic properties (4 credit hours) By Dr. Yejun Qiu Department of Materials Science and Engineering Spring, 2011. 3. Defect structure ? point def
16、ect ? linear defect (dislocation) Impurity Conductive mechanism of ZrO2 electrolyte. (a) Atomically resolved STM image of a defective FeO island (15 nm 15 nm) (Vb = 100 mV, I = 0.02 nA); (b) Zoomed O vacancy clusters in (a); (c) Schematic drawing of missing O atoms (the blue balls with dotted lines)
17、 at fcc sites; (d) STM image from a perfect 1.0 ML FeO film (10 nm 10 nm) (Vb = 800 mV, I = 0.6 nA). 马腾, 傅强, 姚运喜, 崔义, 谭大力, 翟润生, 包信和. 催化学报, 2010, 31(8): 1013-1018. Temperature of the top of catalyst bed (Tb) and furnace temperature (Tf ) 4. Surface acid or base sites ? A Lewis acid is defined to be a
18、ny species that accepts lone pair electrons. ? A Lewis base is defined to be any species that donates lone pair electrons. Example: H+ is a Lewis acid, since it can accept a lone pair, while OH- and NH3 are Lewis bases, both of which donate a lone pair. Me3B + :NH3 Me3B:NH3 Me3B-NH3 -?BF3 + F BF4 3d
19、2 ?2? spSiF4 + 2 F SiF6 RCl +AlCl3 R+ + AlCl4? Lewis diagrams showing the formation of the ammonium ion. IR CCN spectra of pyridine (Py)-adsorbed species established on the various test magnesias following thermoevacution at 10-5 Torr and the temperatures indicated for 5 min. Desorption Thermogravim
20、etry and Calorimetry of Room-Temperature Adsorbed Pyridine (Py) and CO2 Molecules on the Test Magnesias Example of acid catalysis Esterification mechanism Acid and base catalyzed aldol mechanism Aldol condensations are irreversible. Acid catalyzed dehydration Base catalyzed dehydration 5. Skeleton s
21、tructure Zeolites are microporous crystalline aluminosilicates, composed of TO4 tetrahedra (T = Si, Al) with O atoms connecting neighboring tetrahedra. For a completely siliceous structure, combination of TO4 (T = Si) units in this fashion leads to silica (SiO2), which is an uncharged solid. Upon in
22、corporation of Al into the silica framework, the +3 charge on the Al makes the framework negatively charged, and requires the presence of extraframework cations (inorganic and organic cations can satisfy this requirement) within the structure to keep the overall framework neutral. The zeolite compos
23、ition can be best described as having three components: The amount of Al within the framework can vary over a wide range with Si/Al = 1 to . Table: The Evolution of Molecular Sieve Materials Comparison of pore sizes of different framework structures. The construction of four different zeolite framew
24、orks with sodalite or h cages. A pair of TO4 tetrahedra is linked to a single sodalite cage by T-O-T bonds. In a less cluttered representation, the oxygen atoms are omitted and straight lines are drawn connecting the tetrahedral (T) atoms. The sodalite cage unit is found in SOD, LTA, and FAU,-EMT fr
25、ameworks. The cooperative templating effect of amine in the ionothermal synthesis (here the amine is n-DPA). 裴仁彦, 徐云鹏, 魏 莹,温国栋, 李科达, 王 磊,马怀军, 田志坚, 林励吾. 催化学报, 2010, 31(8): 1083-1089. Effect of pore diameter on molecular diffusivity, showing that intrazeolitic diffusion (晶体内的扩散) can span more than 10
26、orders of magnitude. In the range of porosity typically found in zeolites, the intracrystalline diffusivities can change by 12 orders of magnitude depending on the pore size and the size and shape of the molecule diffusing through the zeolite Reactant selectivity Product selectivity Restricted trans
27、ition-state selectivity 6. Size effect (Nano effect) ? Specific surface area increases greatly; ? Melting point decreases obviously; ? Energy levels split. Example: Au ? BET: bulk materials (0.1 m2/g); 2nm (300 m2/g). ? Melting point: bulk materials (1064oC); 2nm (327oC). Example: Growth of carbon n
28、anotubes (a) Typical SEM and (b) AFM images of the as-grown SWNTs on Si/SiO2 substrate covered with a 30-nm-thick sputtering deposited SiORaman spectra 2 film. (c) of the same sample acquired at three positions, with the peaks at 303 cm-1 originating from Si/SiO2 substrate and used for calibration.
29、(d) HRTEM image of three individual SWNTs (indicated by arrows). Raman spectra of (A) SWNTs within the circle trace area and (B) MWNTs or carbon filement inside the circle trace. SEM images of aligned long SWNT arrays from (C) SiO2, (D) Al2O3, (E) TiO2, and (F) Er2O3 Random SWNTs from TiO2 NPs (a), Al2O3 NPs (b) and rare earth oxides Er2O3 NPs
