tailieunhanh - Chemical Degradation Methods for Wastes and Pollutants - Chapter 2

Xúc tác quang học suy thoái của Chất ô nhiễm trong nước và không khí: Các khái niệm cơ bản và ứng dụng Một số của các quá trình oxy hóa tiên tiến, được mô tả trong cuốn sách này liên quan đến photon được sử dụng để tạo ra các loài oxy hóa, trực tiếp hoặc gián tiếp, từ H2O, H2O2, O3. Xúc tác quang không đồng nhất là một trong những quá trình đó là dựa trên kích thích lượng tử ánh sáng của một chất rắn, dựng nó phức tạp hơn. Xem xét số lượng rất cao của các giấy tờ và bằng sáng. | 2 Photocatalytic Degradation of Pollutants in Water and Air Basic Concepts and Applications Pierre Pichat Ecole Centrale de Lyon Ecully France I. INTRODUCTION Several of the advanced oxidation processes described in this book involve photons used to generate oxidizing species directly or indirectly from H2O H2O2 and O3. Heterogeneous photocatalysis is the only one of these processes that is based on the photonic excitation of a solid which renders it more complex. Considering the very high number of papers and patents in this domain the yearly publication of a bibliography 1 which includes organized references the existence of several review articles . see Refs. 2-6 published since 1997 and the publication of a recent book 7 this chapter cites only some of the studies as a starting point in order to cover the principal issues. The choice of the particular references cited here is somewhat arbitrary and is influenced by the author s knowledge of the individual topics. Certainly excellent reports have not been included but they can be found in Ref. 1 which is a rich source of information. II. BACKGROUND AND FUNDAMENTALS OF THE TECHNIQUE A. General Description 1. Role of Photonic Excitation Electron Transfer and Adsorption The term photocatalysis may designate several phenomena that involve photons and a catalyst 8 . In this chapter the terms heterogeneous photo Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. catalysis and photocatalysis refer only to cases where the photosensitizer is a semiconductor. Moreover only TiO2 is considered among the semiconductors except for the general concepts. Electrons pertaining to an isolated atom occupy discrete energy levels. In a crystal each of these energy levels is split into as many energy levels as there are atoms. Consequently the resulting energy levels are very close and can be regarded as forming a continuous band of energies. For a metal or conductor the highest energy band is half-filled and the .