Presentation on theme: "2003.01.24 Dong Won Hwang, Jae Sung Lee Catalysis & Ecofriendly Science Lab. Chemical Engineering, POSTECH The Electronic Structure of Layered Perovskites."— Presentation transcript:
2003.01.24 Dong Won Hwang, Jae Sung Lee Catalysis & Ecofriendly Science Lab. Chemical Engineering, POSTECH The Electronic Structure of Layered Perovskites and Its Photocatalytic Water Splitting Activity 제 2 회 광촉매 연구모임
Alternative Energy -Driving Forces Sustainability Renewability Pollution Reduction Total Energy Consumed at 1989; 1.0 10 17 kcal The amount of solar radiation for 60 min -Types and Amounts (108 kcal/sec) Solar (420,000) Wind (880) Hydro (4.8) Biomass (250) Geothermal (77) Ocean (Thermal, Tidal) (7.2) etc. (50)
Photocatalytic Water Splitting - CB VB Cat 1 Cat 2 H2H2 2OH - + O 2 + 2H + 2H + hv C.B. : more negative than the reduction potential V.B. : more positive than the oxidation potential H + /H 2 OH - /O 2 Electrochemical Requirements of photocatalyst for water splitting E G >> 2.43 eV
Factors which have been considered in Photocatalysis of Water Splitting Band gap and band position Band Bending Cocatalysts; Junction structure Surface area Defects in crystal structure and composition Solution pH; External additive Stability against photocorrosion
Photocatalytic Water Splitting Activity electronic structure defect/surface Factors to Photocatalysis in Water Splitting bulk structure
Perovskite 1 StructureCation Valency Activity 2 (µ mol H 2 /gcat.h ) SrTiO 3 bulk+68 Sr 3 Ti 2 O 7 Layered+611 Sr 2 Nb 2 O 7 Layered+7402 Sr 2 Ta 2 O 7 Layered+7796 La 2 Ti 2 O 7 Layered+7474 LaTiO 3 bulk+7137 1 All the samples were prepared by solid-state reaction at 1273 K for10 h. 2 At room temperature; catalyst 0.3 g; water 500 cc; 450 W medium-pressure Hg lamp. Photocatalytic Activity of Various Perovskites Electronic Structure of Layered Perovskite All of the layered perovskites do not have good photocatalytic activity!
Ln 2 Ti 2 O 7 (Ln = La, Pr, Nd) How does the electronic structure of layered perovskite work? Elucidation of Relationship between Electronic Structure of Layered Perovskites and Its Photocatalytic Activity
Catalyst Preparation La 2 O 3, Pr 6 O 11, Nd 2 O 3 + TiO 2 Macromixing Grinding Calcination (Air, 1150 o C) Ln 2 Ti 2 O 7 Reduction (H 2, 500 o C) Oxidation (Air, 200 o C) Ni-Nitrate NiO/Ln 2 Ti 2 O 7
c = 13.01 Å b = 5.544 Å a = 7.81 Å La 2 Ti 2 O 7 with Layered Structure
SEM & TEM image of La 2 Ti 2 O 7 10 nm 3 nm La 2 Ti 2 O 7 sintered at 1423K for 10 h 200nm Ni Particle
Crystal Structure of Ln 2 Ti 2 O 7 All samples were calcined at 1423K for 10h. monoclinic P21
Calculation of Electronic Structure local spin density approximation + Kohn-Sham equation FLAPW Method DFT basis set I II Wien97
Electronic Structure of La 2 Ti 2 O 7 Valence Band Conduction Band wave vector E g = 3.2eV direct band gap
Dependence of photocatalytic activity on the electronic band structure as well as bulk structure of photocatalyst materials The valence band consists of mainly O2p, which is hybridized a little with Ti3d, while the conduction band consists of mainly Ti3d. The Ln4f level in Ln 2 Ti 2 O 7 was shifted to lower energy as the number of Ln4f electron increased. Both bulk structure and electronic structure was not sufficient to determine the photocatalytic activity. Therefore, other factors such as crystal defect and surface property should be considered, as shown in Ba-doped La 2 Ti 2 O 7. defect/surface Summary Until Present Study
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