Heterogeneous Photocatalysis

Relationships with Heterogeneous Catalysis and Perspectives

Giuseppe Marcì, Leonardo Palmisano

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780444640154
• eBook ISBN: 9780444640161
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Heterogeneous Photocatalysis: Relationships with Heterogeneous Catalysis and Perspectives highlights the differences between thermal-catalysis and photo-catalysis and indicates borderlines, in particular, the possible synergism between them. The book outlines the basic aspect of thermal- and photo-catalysis, along with the most important characterization techniques. In addition, it presents case studies of thermal-catalytic and photo-catalytic or thermal-photo-catalytic reactions and includes a comparison between the results obtained using an inorganic solid as thermal catalyst and photocatalyst for the same reaction, and in the same setup. Final sections offer information on the preparation methods of (photo)catalysts, various techniques used for their characterization, engineering and economical aspects.This book will be a valuable reference source for students and researchers involved in heterogeneous photocatalysis and catalysis, chemistry, chemical engineering, materials science, materials engineering, environment engineering, nanotechnology and green chemistry.- Provides selective methods for the preparation of microcrystalline/nanocrystalline solids or films used in catalytic and photocatalytic processes- Describes (photo)reactions that can be carried out catalytically and/or photocatalytically- Outlines the different mechanisms, yields and experimental conditions under which photocatalytic reactions can take place- Describes various (photo)reactors and set ups under which the photacatalytic reactions can be carried out- Provides an economic assessment to understand the feasibility of some photocatalytic reactions

Chapter 1

Heterogeneous Photocatalysis and Catalysis

An Overview of Their Distinctive Features

Vincenzo Augugliaro¹, Giovanni Palmisano², Leonardo Palmisano¹ and Javier Soria³, ¹“Schiavello-Grillone” Photocatalysis Group, Dipartimento di Energia, Ingegneria dell'informazione e modelli Matematici (DEIM), University of Palermo, Palermo, Italy, ²Department of Chemical Engineering, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates, ³Institute of Catalysis and Petrochemistry, CSIC, C/ Marie Curie, 2. Cantoblanco, 28049 Madrid, Spain

Abstract

General definitions of heterogeneous catalysis and photocatalysis are reported together with parameters to assess the catalytic and photocatalytic performances. Some features of catalytic and photocatalytic reactions, the importance of amorphous and crystalline phases of the solid photocatalysts, and reactors and photoreactors with the most important kinetic aspects are presented.

Keywords

General definitions; catalyst; photocatalysts; TiO₂; reactors; photocatalysts

1.1 Introduction

This introductory chapter aims to discuss the characteristic features of heterogeneous photocatalysis and catalysis in a comparative way, to shade light on similarities and differences (if any) between them; the analysis will outline the main characteristics of both processes without discussing them in great detail, as that will be done in the following chapters of this book. To reach this objective, however, it is necessary that preliminarily the conceptual meanings attributed to these two terms are clearly recognized and defined independently from the conceptual contexts of researchers involved in those fields [1]. While in the field covered by the catalysis concept there is undoubtedly total agreement in the scientific world [2], for the photocatalysis concept agreement has been reached only in the last few years as the field has matured enough [3–5]. Indeed, when the term photocatalysis started to be used, researchers involved in the photochemistry field gave emphasis on the photo term and for them photocatalysis indicated a process accelerated by light; on the other hand, researchers in catalysis emphasized the catalysis term and for them photocatalysis indicated a catalytic process occurring in the presence of light. Depending on the researchers involved in investigating processes occurring in the presence of a solid and irradiation, the emphasis was put on the photochemical or catalytic aspects of those processes.

1.2 Definitions

1.2.1 Catalysis

The term catalysis was coined by Berzelius in 1835 to describe the property of substances that speed up chemical reactions without being consumed during their occurrence. A more precise, and still valid definition of catalysis, is from Ostwald (1895) and it may be expressed in the following way: “a process where a substance participates in modifying the rate of a chemical transformation of the reactants without being altered or consumed in the… end; this substance is known as the catalyst which increases the rate of a reaction by reducing the activation energy without affecting the position of the equilibrium.” Therefore, the main task of the catalyst is the acceleration of a specific reaction and, at the end of the reaction step, its state is, strictly speaking, equal to that at the beginning of the reaction. In terms of free energy changes, reactants undergo an overall change as they convert to products while the catalyst does not change it.

The assumption that the catalyst remains unmodified throughout the occurrence of the reaction is satisfied only by an ideal catalyst, but this is not the case in practice. In fact, the catalyst may be effective only if it gives rise to interactions that are neither too strong nor too weak with the reactants during the catalytic process. At the end of each catalytic cycle, the catalyst recovers, but not completely, its initial state; as a consequence, it undergoes small chemical changes that eventually decrease its activity (catalyst deactivation) so that, after a certain time, the catalyst must be regenerated or replaced.

Apart from increasing the reaction rate, another important property of catalysts is that they can affect the selectivity of chemical reactions, that is, that different catalytic systems may produce different products with the same starting reactants. From an industrial point of view, this property is sometimes more important than the rate increase. In any case, the activity and selectivity properties and deactivation behavior are the parameters to be taken into account in choosing an industrial catalyst.

1.2.2 Photocatalysis

The term photocatalysis appeared for the first time in 1921 in the title of a paper by Baly et al. [6] but the meaning given to it was connected to phenomena in which light accelerated a reaction. The first papers in which the term photocatalysis was used as a label to indicate that the combination of light and a solid catalyst was able to affect a reaction were published in 1964 by Doerffler and Hauffe [7,8]. The very inception of photocatalysis investigation occurred in 1972 when Fujishima and Honda published a paper on the photocatalytic splitting of water on TiO₂ electrodes [9]. This paper has been fundamental in promoting research on heterogeneous catalysis in the presence of irradiation; ever since, research efforts in understanding the fundamentals of this combined process and in enhancing its efficiency have involved chemists, physicists, and chemical engineers.

In irradiated catalysts, two types of chemical reactions can take place in the presence of suitable radiation of the catalytic surface depending on the substance involved in the radiation absorption (photoexcitation) [10]. Indeed the photoexcitation may involve (1) an adsorbed species, whose internal bonds are modified by the chemical adsorption as it occurs in catalysis (this process is called catalyzed photoreaction); or (2) the catalyst, generating charge carriers that are transferred to the ground state of a photoadsorbed species (the process is also known as sensitized photoreaction).

After Fujishima and Honda’s paper a high number of papers followed in the photocatalysis field, but no distinction was done between photochemical and catalytic phenomena. This fact determined the need to have an agreed glossary of terms and definitions, to correctly communicate and understand the photocatalytic investigations [11–14]. Today, according to the Glossary of Terms Used in Photocatalysis and Radiation Catalysis (IUPAC recommendations 2011) [15] the accepted definition of photocatalysis is “change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance, the photocatalyst, that absorbs light and is involved in the chemical transformation of the reaction partners” and photocatalyst is defined as a “substance able to produce, by absorption of ultraviolet, visible, or infrared radiation, chemical transformations of the reaction partners, repeatedly coming with them into intermediate chemical interactions and regenerating its chemical composition after each cycle of such interactions.”

The principles of photocatalysis may be briefly explained in the following way: an electron in an electron-filled valence band (VB) is excited by photoirradiation to an empty conduction band (CB), which is separated by a forbidden band, a bandgap, from the VB, leaving a positive hole in the VB. These electrons and positive holes drive reduction and oxidation, respectively, of compounds adsorbed on the surface of a photocatalyst.

In this book, with the word photocatalysis we will refer always to the process in which sensitized photoreactions are occurring, that is, chemical reactions induced by a solid material, or “photocatalyst,” which absorbs a suitable radiation and remains unchanged during the reaction. In other words, the solid acts catalytically (without measurable changes at the end of each photocatalytic cycle) under light absorption; photocatalysis will be the conceptual name for photocatalytic reactions. On the basis of this definition, the occurrence of a photocatalytic process may be proved by measuring the consumption of the starting substrate(s) or the appearance of reaction product(s) after the starting of irradiation and by checking whether the photocatalyst features have been modified during the photoreaction. Even if this procedure may seem very easy, different problems are encountered when trying to prove that a given process is really photocatalytic.

The first studies on photocatalysis were devoted to renewable energy and energy storage but, owing to intrinsic difficulties in reaching a satisfactory performance, the photocatalytic processes have gained importance in the treatment of contaminated gaseous and liquid streams. In fact, it has been proven that the photocatalytic method is able to perform the complete mineralization of dangerous species at mild conditions of temperature and pressure. A major advantage of the photocatalytic processes is the possibility of using sunlight or near UV light with great economic savings especially for large-scale operations.

The relationship between photocatalysis and catalysis was questioned by Childs and Ollis in 1980, just a few years after the Fujishima and Honda discovery, in a paper titled “Is photocatalysis catalytic?” [16]. The authors, by considering that the reported photocatalytic reactions were characterized by small quantities of product, posed the following two central questions: Has the reaction been demonstrated to be catalytic? and How is the fundamental activity of different truly catalytic materials to be compared? In analogy with the turnover number (TON) of heterogeneous catalysis and with the quantum efficiency of photochemistry, the authors proposed the “photocatalytic turnover number” as a parameter to assess the activity of different photocatalysts.

In the following we will examine the reaction steps occurring in a catalytic and photocatalytic process and on the main components needed to carry out those processes...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Preface
• Chapter 1. Heterogeneous Photocatalysis and Catalysis: An Overview of Their Distinctive Features
• Chapter 2. Preparation of Catalysts and Photocatalysts Used for Similar Processes
• Chapter 3. Bulk and Surface Characterization Techniques of TiO2 and TiO2-Doped Oxides
• Chapter 4. (Photo)catalyst Characterization Techniques: Adsorption Isotherms and BET, SEM, FTIR, UV–Vis, Photoluminescence, and Electrochemical Characterizations
• Chapter 5. Photocatalytic and Catalytic Reactions in Gas–Solid and in Liquid–Solid Systems
• Chapter 6. Special Needs and Characteristic Features of (Photo)catalytic Reactors with a Review of the Proposed Solutions
• Chapter 7. Kinetic Aspects of Heterogeneous Catalytic Versus Photocatalytic Reactions
• Chapter 8. Economic Assessment and Possible Industrial Application of a (Photo)catalytic Process: A Case Study
• Index