Chemistry
Catalysts
Catalysts are substances that speed up chemical reactions by lowering the activation energy required for the reaction to occur. They do not undergo any permanent change themselves during the reaction and can be used repeatedly. Catalysts provide an alternative reaction pathway, allowing reactions to occur more readily and at lower temperatures.
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9 Key excerpts on "Catalysts"
eBook - PDFGreen Chemical Engineering
An Introduction to Catalysis, Kinetics, and Chemical Processes
- S. Suresh, S. Sundaramoorthy(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
Later, it was J. Liebig and W. Ostwald who noted that the catalyst increases the rate of a chemical reaction without getting consumed by the reaction. A catalyst is a substance that alters the rate of a reaction but remains unchanged after the reaction. Many chemical reactions that are not feasible under normal conditions can be carried out in the presence of a catalyst. It is dif fcult to exaggerate the importance of catalysis since many life processes and industrial processes would not be possible with-out it. Several industrially important reactions that drive the large-scale production of chemical products such as sulphuric acid, agricultural fertilisers, plastics and fuels are catalytic reactions. Catalytic cracking, catalytic reforming, aromatisation, isomerisation and so on in petrochemical industries, hydrogenation of vegetable oil, enzymatic reac-tions in food processing, conversion of sulphur dioxide into sulphur trioxide, ammonia synthesis, etc., are some of the common examples of industrially important catalytic chemical processes. Catalysts change reaction rates by promoting different mechanisms for the reactions. It is to be noted that even though a catalyst is not consumed in the reaction, it does take part in the chemical reaction but is not observed in the overall reaction. Catalysts activate mol-ecules and reduce the activation energy necessary for reactions to occur. Catalysts do not change the state of equilibrium; they only act to increase the rates at which the equilibrium state is attained. Catalysts can affect yield and selectivity of a chemical reaction because of their ability to change the reaction mechanism. Catalysis is an active area of research. Figure A.1 shows the yearly progress in the num-ber of research publications appearing in the area of catalysis.
eBook - PDF- Ryan Richards(Author)
- 2006(Publication Date)
- CRC Press(Publisher)
majority of current industrial chemical processes use Catalysts, and most of these processes — in terms of quantity of Catalysts, quantity of products, and financial value in chemical 196 SURFACE AND NANOMOLECULAR CATALYSIS industry — are heterogeneously catalyzed processes in petrochemical industry. This sector of the chemical industry is closely related to the energy sector for which it produces various kinds of fuels derived from petroleum oil and natural gas as two nonrenewable (fossil) primary fuel resources. Most of the fuels produced are used to feed internal combustion engines (ICEs), on the use of which the progress of our civilization has been based since the dawn of the 20th century. These industrial chemical processes are the result (and the best illustration) of the very close link between (hetero-geneous) catalysis and chemical reaction engineering. In the absence of aging (deactivation) of the catalyst, a phenomenological definition of catal-ysis would be enhancement of chemical reactions or a change of their rate under the influence of substances — Catalysts — which several times enter into transient chemical interactions with re-action participants and then, after each cycle of transient interactions, regenerate their chemical identity [1]. A catalyst is a substance that increases the rate at which a chemical reaction reaches equilib-rium. Catalysis is the word used to describe the action of the catalyst. Heterogeneous catalysis describes the enhancement in the rate of a chemical reaction brought about by the presence of an interface between two phases [2]. Historically, catalysis has developed within the domain of physical chemistry.
eBook - PDFCatalysis for Sustainability
Goals, Challenges, and Impacts
- Thomas P. Umile, Ph.D, Thomas P. Umile(Authors)
- 2015(Publication Date)
- CRC Press(Publisher)
1 1 Catalysts and Sustainability Thomas P. Umile 1.1 INTRODUCTION Catalysts are compounds that increase the rate of chemical reactions without them-selves being consumed in the process. Accordingly, this enhancement makes them attractive for the manufacture of chemical products. Because most of the goods and energy sources we use require such chemical processing, Catalysts are fundamental in our daily lives. Additionally, enzymes are nature’s biological Catalysts, making the biochemical transformations necessary for life possible. Indeed, Catalysts have a consistent and broad effect on our daily lives. Beyond simply “speeding up a chemical reaction,” Catalysts have an intrinsic potential to address our concerns and needs for a sustainable human existence. Because the world’s population is exponentially growing, we are faced with our civi-lization’s greatest challenge: to satisfy the needs of our current way of life without jeopardizing our ability to do the same tomorrow. Catalysts’ chemical rate enhance-ment means that they can contribute to overcoming this challenge by lowering the energy costs of chemical reactions, making such reactions more selective and less wasteful, and offering opportunities for developing new chemical reactions that make efficient use of consumed materials. Thus, Catalysts can have dramatic, benefi-cial effects on the use of environmental resources, the development of new products and technological solutions, and the financial concerns of people, businesses, and manufacturers. In this chapter, we will explore the needs of a sustainable society with special respect to the responsibilities of science, technology, and especially catalysis, review the fundamentals of catalytic chemistry and how such processes are CONTENTS 1.1 Introduction ...................................................................................................... 1 1.2 Sustainability and Green Chemistry ................................................................
eBook - PDF- Arthur Scott(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
It seems almost certain that we know a much larger fraction of those reactions which proceed spontaneously than of those which require Catalysts since reactions in the latter category must be both more numerous and more difficult to discover. The following is from Definitions, Terminology, and Symbols in Colloid and Surface Chemistry—Part II. Heterogeneous Catalysis, International Union of Pure and Applied Chemistry (1976): Catalysis is the phenomenon in which a relatively small amount of a foreign material, called a catalyst, augments the rate of a chemical reaction without it-self being consumed. A catalyst provides for sets of elementary processes (often called elementary steps) which link reactants and products and which do not occur in the absence of the catalyst. For example, suppose the reaction A = C to proceed at some rate which might be measurable but might be essentially zero. The addition of X might now provide a new pathway involving the inter-mediate B, A + X-> B B-+C + X If reaction by this pathway proceeds at a rate significant with respect to the uncatalyzed rate such that the total rate is increased, than X is a catalyst. In this sense, a catalytic reaction is a closed sequence of elementary steps similar to the propagation steps of a gas-phase chain reaction. The catalyst enters into reaction but it is regenerated at the end of each reaction cycle. Thus, if the catalyst can be protected against poisoning and deactivation, one unit of catalyst results in the conversion of many units of reactants. Catalysis is usually divided into three categories: homogeneous, hetero-geneous, and enzyme. In homogeneous catalysis, all the reactants and the catalyst are molecularly dispersed in one phase. In heterogeneous catalysis, the catalyst constitutes a separate phase. In the usual case, the catalyst is a solid and the reactants and products are in one or more gaseous or liquid phases.
eBook - PDF- Gerard V. Smith, Ferenc Notheisz(Authors)
- 1999(Publication Date)
- Academic Press(Publisher)
1 CHAPTER 1 Introduction to Catalysis 1.1. Definitions and Language of Catalysis 1.2. Special Considerations for Heterogeneous Catalysis in Liquids 1.3. Drawing and Naming Surface Species in Organic Reactions on Surfaces 1.4. Hydrogen Sources 1.5. Books on Heterogeneous Catalysis of Organic Reactions References 1.1. DEFINITIONS AND LANGUAGE OF CATALYSIS In any field, certain definitions and language must be understood, and the field of catalysis is no exception. Thus we start with some definitions before describing organic reactions on surfaces. 1.1.1. W HAT I S A C ATALYTIC R EACTION ? A catalytic reaction is one in which more than one turnover or event occurs per reaction center or catalytically active site (that is, the turnover number [TON] is greater than 1). Thus a reaction is not catalytic if it is stoichiometric or if its TON is less than 1. A reaction might indeed involve a true catalyst and under some circumstances be catalytic, but if one or fewer turnovers occur per active site, it is not a catalytic reaction. 2 Chapter 1 Introduction to Catalysis 1.1.2. W HAT I S A C ATALYST ? A catalyst is a substance that increases the rate of a chemical reaction without itself being changed in the process. That is, the substance called a catalyst is the same after the reaction as before. During the reaction, it may become a dif-ferent entity, but after the catalytic cycle is complete, the catalyst is the same as at the start. A catalyst is not light or heat or any sort of electromagnetic radiation. These are not substances in the ordinary sense and therefore are not Catalysts. What a catalyst does is change the reaction pathway to one with a lower energy; however, one must remember that the rate of a chemical reaction depends on two things: the rate constant, which contains energy terms (both enthalpy and entropy), and concentration terms.
eBook - PDF- R.E. Hayes, J.P. Mmbaga(Authors)
- 2012(Publication Date)
- CRC Press(Publisher)
313 7 Introduction to Catalysis In Chapters 3 and 4, we briefly considered some catalytic systems, especially in the example problems, without going into very much, if any, detail. The remainder of this book is devoted to catalytic systems, with the goal of providing the reader with a solid foundation for further study. In Chapter 1, the plethora of possible products that could be made from a stream of natural gas was discussed (see Figure 1.1). Although not emphasized in that chapter, it is important to state that most of the reactors shown in Figure 1.1 require the presence of a catalyst for optimal efficiency. Indeed, if one were to undertake a survey of all industrial reactors cur-rently used to manufacture useful products, it would be apparent that the vast majority of reactions are conducted in the presence of a catalyst. For most reactions of industrial signifi-cance, the noncatalyzed reaction either is not sufficiently speedy or produces an abundance of unwanted side reactions. Indeed, many reactions are simply not feasible without the presence of a catalyst. To give the simplest definition, a catalyst is a substance that increases the rate of reaction by providing reaction pathways that have an activation energy that is lower than the one for the noncatalytic reaction. Note here that we have specifically stated that a catalyst increases the reaction rate. Although it is actually possible for a catalyst to decrease the reaction rate (the so-called negative catalyst), such Catalysts are of little practical interest, and for all intents and purposes, we are interested in increasing reaction rates. The purpose of this chapter is to provide a general introduction to the field of catalysis, provide some historical perspective, and to show the versatility of catalytic systems. It is also intended to give a somewhat brief overview of some of the complexities of catalytic systems.
eBook - PDF- Dmitry Murzin(Author)
- 2013(Publication Date)
- De Gruyter(Publisher)
1 The basics 1.1 Catalytic concepts 1.1.1 Definitions Catalysis is a phenomenon related to the acceleration of rates of chemical reactions. The available data on the conversion of starch to sugars in the presence of acids, com-bustion of hydrogen over platinum, decomposition of hydrogen peroxide in alkaline and water solutions in the presence of metals, and were summarized by a Swedish scientist J. J. Berzelius in 1836 (1), who proposed the existence of a certain body, which “effecting the (chemical) changes does not take part in the reaction and remains unal-tered through the reaction”. Catalytic power or force, according to Berzelius, meant that “substances (Catalysts) are able to awaken affinities which are asleep at this temperature by their mere presence and not by their own affinity”. This concept was immediately criticized by Liebig, as this theory was placing cataly-sis somewhat outside of other chemical disciplines (2). A catalyst was later defined by Ostwald as “a compound that increases the rate of a chemical reaction, but which is not consumed by the reaction” (3). This definition allows for the possibility that small amounts of the catalyst are lost in the reaction or that the catalytic activity slowly declines. From these definitions, a direct link between chemical kinetics and catalysis is apparently clear as, accordingly, catalysis it is a kinetic process. There are, however, many issues in catalysis that are not directly related to kinetics, such as mecha-nisms of catalytic reactions, elementary reactions, surface reactivity, adsorption of reactants on solid surfaces, synthesis and structure of solid materials and catalytic engineering. An important issue in catalysis is selectivity towards a particular reaction. For exam-ple, transformation of synthesis gas (a mixture of CO and hydrogen) can lead either to methanol (on copper) or high alkanes (on cobalt). For consecutive reactions it could be desirable to obtain an intermediate product.
eBook - PDF- Harold Schobert(Author)
- 2013(Publication Date)
- Cambridge University Press(Publisher)
Chapter 2 introduced the concept of catalysis, and focused on homogeneous catalysis. For large-scale production of commodities such as fuels, a homogene- ous catalyst requires separation and recovery steps downstream of the reactor, unless the catalyst either is thrown away or is allowed to dilute or contaminate the product. This adds to the complexity and expense of a process. Heterogeneous Catalysts are favored by industry, especially for production of commodities. In part, this derives from a very easy, even non-existent, separation from the process stream. Many heterogeneous Catalysts can withstand more severe conditions of temperature and pressure than homogeneous Catalysts, especially enzymes. Heterogeneous Catalysts work well for gas-phase reactions, where it might be difficult to select a homogeneous catalyst [A]. Several criteria must be fulfilled by any heterogeneous catalyst used in industrial processing. It must provide a rate of reaction high enough to achieve conversions needed for a commercially viable process. It should show good selectivity, which means that it should induce formation of desired products and suppress, or at least not enhance, formation of those that are not desired. The catalyst must have sufficient stability to withstand temperatures, pressures, and mechanical stresses in the reactors in which it is likely to be employed. 13.1 Catalytic materials 13.1.1 The active species In fuel chemistry, the kinds of reaction for which Catalysts are most often used are shuttling hydrogen into or out of molecules, changing the size of molecules by cracking large ones or putting together small ones, or changing the shapes of molecules through carbocation rearrangement. Metals tend to be good Catalysts for hydrogenation and dehydrogenation. In some processes, metal sulfides can be useful, especially when the process stream is contaminated with sulfur.
eBook - PDFFine Chemicals Manufacture
Technology and Engineering
- A. Cybulski, M.M. Sharma, R.A. Sheldon, J.A. Moulijn(Authors)
- 2001(Publication Date)
- Elsevier Science(Publisher)
Although applied in large quantities in bulk chemicals production, this technology is not up to modern standards. In fact, a trend is to replace these systems by solid Catalysts (see Chapter 4). Here, we will confine ourselves to homogeneous Catalysts based on transition metals. These Catalysts are becoming increasingly important and have already found a large number of industrial applications. Homogeneous transition-metal catalysis is applied in polymerization, in the synthesis of bulk chemicals (solvents, detergents, plasticizers), and in fine chemistry and pharmaceuticals production. It is useful to compare homogeneous and heterogeneous catalysis (Moulijn et al., 2001). In homogeneous catalysis the reaction mixture contains the catalyst complex in solution. This means that all of the metal is exposed to the reaction mixture. In heterogeneous catalysis, on the other hand, only the surface atoms are active. Thus, in terms of activity per metal centre, homogeneous Catalysts are often more active (at lower temperature), i.e. metal utilization and productivity are higher for homogeneous Catalysts. The high dispersion of homogeneous Catalysts also minimizes the effect of catalyst poisons; in homogeneous catalysis one poison molecule only deactivates one metal complex, whereas in heterogeneous Catalysts a poison molecule can block a pore containing many active sites (pore plugging). Homogeneous Catalysts are also capable of being much more selective; while with homogeneous Catalysts there is usually only one type of active site, heterogeneous Catalysts contain many different types of active sites, some of which could catalyse undesired reactions. The discrete metal complexes in homogeneous systems provide a well-defined catalyst system, which is highly specific and is easy to study by conventional techniques (infi'ared spectroscopy, NMR).
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