Chapter 1
Heterogeneous Catalysis

1.1 What is Heterogeneous Catalysis?

The phenomenon of catalysis applies to a wide range of chemical reactions. By using a catalyst, chemical reactions that are non-selective can be made selective and those that require a high temperature can be conducted at a lower temperature (see Figure 1.1). In this and the next chapter, we will describe the discovery of this phenomenon in the nineteenth century, the understanding of catalysis that followed, and the main chemical industrial processes that developed in the twentieth century.

Figure 1.1 Non-catalytic versus catalytic processes. Non-catalytic processes are usually non-selective and require a high temperature of reaction. Catalytic processes are selective and can be conducted at lower temperatures. They save energy, reduce waste, and produce materials that are otherwise not available.

There are many chemical compounds that show catalytic action. In this book, we will mainly discuss heterogeneous catalysts. These are solid materials that are positioned in a reactor where they are exposed to reacting gasses or liquids. The reagents flow over the catalyst and are converted by it into products. These products and the unconverted reagents are continuously removed from the reactor, while the catalyst remains in the reactor and becomes exposed to fresh reactants. This continuous process operation is possible because the catalyst is in a separate phase.

We will focus primarily on inorganic materials, which were also the main focus of the early exploratory catalytic studies. When the molecular principles of heterogeneous catalyst activity were gradually understood, it became apparent that similarities exist between biochemical enzymatic processes and molecular organometallic catalysts. The knowledge of the molecular chemistry of the catalytic action of enzymes in biochemical systems as well as information about the action of molecular organic and metal-organic complexes has been of great importance to the further development of the molecular scientific basis of heterogeneous catalytic systems discussed later in this book.

Heterogeneous inorganic catalysts are often quite robust, which makes them especially useful for chemical reactions that operate under hostile conditions.

A specific advantage of the heterogeneous catalyst is that it can be used in a continuous process operation, instead of in a batch-type process where it must be separated in an individual step from the reaction product. Continuous process operation made the development of the large-scale chemical process industry possible once heterogeneous catalysis was discovered early in the previous century. Heterogeneous catalysis has become basic to more than 80% of current bulk chemical processes used in the chemical and petrochemical industries [1].

The modern science of heterogeneous catalytic chemistry developed as the understanding of its chemical basis at a molecular level increased. The current understanding of the catalytic reaction mechanism at the physical and chemical levels and its relation to the structure and composition of heterogeneous catalysts is based to a large extent on the computational catalytic results obtained during the past decades. The ultimate aim of catalysis science is to predict the proper catalytic material for a given chemical conversion reaction. This is still far off due to the complexity of experimental catalytic systems, but the large body of empirical chemical information currently available about the reactivity of different catalytic materials is helpful. Computational methods provide tools for estimating the rate and selectivity of a catalytic reaction as a function of catalyst composition. In addition to spectroscopic characterization of a catalyst, this has become indispensable for research.

In this chapter, we will describe the discovery of the basic principles of heterogeneous catalysis. The next chapters will provide an introduction to current catalytic processes, followed by chapters on the principles of physical chemistry and inorganic chemistry of heterogeneous catalysis that are the basis of modern catalysis science.

The discovery of quantum mechanics provided the chemical sciences a predictive foundation, which led to the development of computational catalysis. Combined with the application of advanced spectroscopic techniques and advances in material synthesis, this discovery has provided the molecular foundation of heterogeneous catalysis that is the main topic of part II of this book.

1.2 Early Developments

In its infancy, the science of chemistry was highly exploratory and focused primarily on the categorization of chemical materials and their properties. Heterogeneous catalysis science started in that early period of modern chemistry, before the understanding of the molecular nature of matter. It has its origin in the early part of the nineteenth century, after the founding of modern chemistry by Lavoisier and colleagues at the end of the eighteenth century. They introduced the law of conservation of mass, and with it an accurate understanding of the nature of oxygen. This knowledge sparked the development of the significant heterogeneous catalytic oxidation processes of the nineteenth century.

The discovery by Davy and Döbereiner of catalytic oxidation contributed to the earlier recognition by Berzelius of catalysis as a separate phenomenon, and to the development of two important heterogeneous catalytic processes: oxidation of HCl to produce chlorine and oxidation of SO2 to yield oleum, a highly concentrated sulfuric acid.

A second period at the end of the nineteenth century was marked by the development of chemical thermodynamics, which aided in the definition of catalysis by Ostwald and provided its physical chemical foundation. During this period, the rise of coal gasification for domestic lighting and heating made hydrogen readily available. This allowed Sabatier's discovery of catalytic hydrogenation to develop into a practical technology.

Sabatier also discovered an important law of catalysis that defines the condition of maximum activity of a catalyst. The three catalysis laws of Berzelius, Ostwald, and Sabatier are the main topics of this chapter.

This chapter closes with the invention of the Haber-Bosch ammonia synthesis process that is the culminating technological success of this period. As the source for artificial fertilizer, it made a significant impact on the world's growing need for fertilizers. It contributed to chemical technology by creating the continuous heterogeneous catalytic process, which can be operated at high pressure. The development of this process provided the basis for many other modern heterogeneous catalytic processes.

1.2.1 Early Nineteenth Century Discoveries

The early discoveries of heterogeneous catalytic reactions were concurrent with electrocatalytic exploration, which significantly improved the understanding of catalytic reactivity [2]. We will occasionally refer back to this parallel development in the physical chemistry of heterogeneous catalysis and electrocatalysis.

It took some time to recognize the chemical nature of the effect of metals on decomposition reactions as displayed by the decomposition of ammonia or alcohols. These were previously attributed to the effect of heat or electrochemical action, as the latter had been demonstrated in the electrolysis of water [3-5]. A crucial observation by H. Davy was published in 1817. During the course of his studies on the miner's safety lamp, he discovered that a Pt wire remained hot in the presence of coal gas and air. In this experiment, the flame heating the wire was extinguished by exposure to gas enriched with coal gas that is essentially methane. He concluded that the Pt wire assisted in the burning of the gas by air without a flame [3, 6]. In 1823, Döbereiner, another famous chemist from that age, discovered that Pt sponge would react at room temperature with hydrogen to produce a flame. As previously shown by Thénard in the decomposition of ammonia, the effect is material-dependent and could only be observed for particular metals [7].

Based on his discovery, Döbereiner designed a lamp (Figure 1.2) that generates a flame. The hydrogen is produced by the reaction of sulfuric acid with zinc, which over time is converted into ZnSO4. This lamp became widely used in industry and even in households, until it was replaced by matches and other lighters. Döbereiner's invention can be considered a first practical application of catalysis [7].

Figure 1.2 An early design of Döbereiner's lamp. This lamp contains hydrogen gas that is ignited using platinum as a catalyst. In a bottle filled with sulfuric acid, zinc metal (e) reacts with sulfuric acid to produce hydrogen gas. Zn + H2SO4 Zn2+ + SO42- + H2. When the stopcock (e) is opened, the hydrogen is directed by a thin tube onto a platinum sponge (g), and then a flame lights instantaneously. The flame goes out when the stopcock is closed. Hydrogen production ceases as gas pressure builds. (a) Glass cylinder, (b) open bottle, (c) wire, (d) zinc, (e) stopcock, (f) nozzle, (g) platinum sponge [7, 8].

Twenty years after Davy's initial experiment, the electro-catalytic analogue of the Döbereiner reaction was discovered by Grove who designed a fuel cell in 1839. Grove ultimately proved that electrical energy, instead of heat, could be produced from an electrochemical reaction between hydrogen and oxygen over a platinum electrode. He...