CATALYSIS

MARCO PIUMETTI Politecnico di Torino, Department of Applied Science and Technology, Corso Duca degli Abruzzi 24, I-10129, Turin, Italy

Marco Piumetti

A brief history of the science of catalysis - I: from the early concepts to single-site heterogeneous catalysts

KEYWORDS: , history of catalysis, active sites

A brief history of heterogeneous catalysis is reported in this work, with a focus on key catalytic discoveries Abstractthat have led to the development of the science of catalysis. However, this is not a comprehensive review on the history of catalysis and hence many concepts and discoveries have not been reported, for sake of brevity. The emerging picture is a survey of some ideas and innovations that have occurred in catalysis over the last few centuries, from early observations of alchemists to recent developments of molecular catalysis.

INTRODUCTION For a long time (~ 104 years), humanity thought that chemical changes took place for mysterious reasons, The Bible mentions fermentation for the production of sometimes caused by acts of Gods. Catalysis has made bread and wine and most probably the first catalytic great steps forward in the last two centuries, moving from application was the production of by being an empirical subject, strongly linked to mystic and fermentation. Empirical investigations of the natural world divine ideas, to a Galilean-type Science (3-6). have been described since classical antiquity and Catalysis as a scientific discipline originated in the early was practiced in several civilizations for many part of the last century, defined as “the age of the centuries (1, 2). molecularization of the sciences” (7). However, it took

Figure 1. a) Jöns Jacob Berzelius (1779-1848); b) Michael Faraday (1791-1867); c) (1853-1932); d) (1854-1941).

22 Chimica Oggi - Today - vol. 32(6) November/December 2014 Similarly, the production of sulphuric in glass apparatus by burning sulphur with in humid air () was also developed in these early times (20). It was only from the beginning of the 19th century, after ages of hypotheses and empirical notions about chemical transformations, that the Swedish Jöns J. Berzelius (Figure 1a) introduced the concepts of “catalytic force” and “catalysis” (8). Indeed, Berzelius from 1821 Figure 2. a) The Sabatier principle; b) The and its elementary steps. onwards summarized and critically reviewed the main scientific works in his “Annual almost a century before the molecular basis of some Report” (Jahresberichte). In his annual review of 1835 catalytic processes, now widely applied at large scale, Berzelius considered a number of reactions that occur in became understood (8). For instance, the production of the presence of a substance which remains unaffected according to the Haber-Bosch process was (21, 22). He wrote “In order to avail myself of a derivation discovered early in the 20th century once the well-known in chemistry I will call both the catalytic force of this process had become well of matter and the decomposition by this matter, catalysis, understood, but only in 2007 the assigned to just like we understand with the word analysis the recognized his discoveries in the molecular separation of the constituents” (21). Curiously, the creative steps at the surface for this reaction (9-10). work of Michael Faraday (Figure 1b) was not mentioned Nowadays, thanks to recent progress in in-situ by Berzelius in his survey of the catalytic phenomena. For and advanced scanning probe techniques instance, Faraday studied the catalytic properties of it is possible to study the active sites and in for the oxidation of and also complex environments and follow their changes over time, investigated both the effects of pretreatment and reaching the femtosecond scale (10-15 s), the scale for poisoning, making a significant contribution to and molecules in motion (11). The active sites are heterogeneous catalysis (8, 23). In 1931, S.H. Taylor wrote dynamic entities that can continuously modify their (24) “Faraday was the first to indicate the zone of properties depending on the operating conditions (12-14). adsorbed material as the reaction space of a Moreover, it has been established that active sites (or heterogeneous catalytic action” and recently, G.A. regions) and molecules at the solid surfaces may interact Somorjai pointed out that (25) “Faraday was the first with each other through transport phenomena and scientist who studied catalytic reactions”. surface flexibility (13-15). These interactions lead to self- Berzelius has been credited with introducing the concept organization phenomena, in which both structural and of “catalysis”, the decomposition of compounds by kinetic effects play a role (14-16). catalytic force. He also applied the concept of catalysis to those phenomena in which the typical barriers to were removed (8, 9). However, these FROM ALCHEMY TO EARLY CONCEPTS novel concepts attracted much criticism that was directed, for the most part, not at catalysis, but rather at For ages, civilizations believed that substance the concept of catalytic force (8, 26). During the fifty transformations took place for mysterious reasons related years following the concepts of Berzelius, many examples to mythology, and spirituality. Early alchemists, such as of catalytic action were discovered but little progress was Zosimos of Panopolis (~ 300 AD), emphasized the spiritual made in understanding them. After a period of empirical nature of the alchemical quest and used classical figures evidence, usually based on trial-and-error approach, from Greek, Roman and Egyptian mythology to allegorize catalysis as a branch of chemical science made great alchemical transmutations (5, 17, 18). Indeed, alchemy progress at the end of the 19th century (8). In 1884, J. H. included various philosophical traditions spanning some van’t Hoff described the formal dependence of the four millennia and three continents. These traditions on temperature (namely K = Ae -Ea / k∙T), continued in the Middle Ages, as metaphysical aspects, while S. Arrhenius in 1895 understood the physical basis of substances, physical states, physico-chemical phenomena this empirical relationship and consequently the equation and materials processing were used as metaphors for is now attributed to Arrhenius (27, 28). In the , spiritual entities, spiritual states and, ultimately, Ea is the energy of the reaction (in kJ/mol), R the transmutations (namely chrysopoeia) (17-19). On the other constant (8.314 J/K∙ mol) and T the absolute hand, in the 16th century the use of sulphuric acid to catalyze temperature. The quantity A represents the collision the synthesis of diethyl was already well known, as frequency and is called “the frequency factor”. revealed by written records dating back to 1552 (8). According to Arrhenius, only the collisions between

Chimica Oggi - Chemistry Today - vol. 32(6) November/December 2014 23 molecules spatially oriented in a suitable way and with formates (representing the stability of the surface energy higher than the (Ea) may give compound). On the other hand, at high ΔHf values, the rise to products (29, 30). Arrhenius introduced a reaction rate is also low and corresponds to the hypothetical body, now known as the “activated desorption rate, which increases with decreasing ΔHf. complex”, a key concept in the theory of reaction rates. Therefore, a maximum in the rate of reaction takes place

This interpretation displayed that catalysts do not only at an intermediate value of ΔHf, which is neither a pure affect the energetic barrier (reflected by kT) but also the rate of nor a pure rate of desorption but which effective collisions (factor A) among molecules. However, depends on both (32, 33). the rate constant (K) does not offer a detailed molecular description of reactions. In fact, K is an average of the microscopic -state to -state rate coefficients over all possible encounters. Therefore, a new way was needed to describe the process itself of catalytic reactions (29).

THE ONSET OF APPLIED CATALYSIS

The state of knowledge on the topic of catalysis was described in detail by Wilhelm Ostwald (Figure 1c) in his work “Ältere Lehre von den Berühungswirkungen” which appeared in 1897/1898 (22). Ostwald first recognized catalysis as a kinetic phenomenon and his thoughts on the nature of catalysis were presented at the meeting of the Figure 3. Polanyi’s representation of his theory Gesellschaft Deutscher Naturforscher und Ärzte on (redrawn from ref. 35). September 26, 1901: “A catalyst is a material that changes the rate of a chemical reaction without appearing in the final product”. He also considered the catalytic Moreover, Sabatier’s principle provided a kinetic phenomenon as the acceleration of chemical reactions understanding of the catalytic cycle and its corresponding by the presence of foreign substances which are not steps, including adsorption, surface consumed (31, 32). , surface reaction and desorption (Figure 2b). In Like Liebig before him, Ostwald explained the catalytic other words, his work provided the foundation necessary process by analogy: “a catalyst acts like oil on a machine, to begin the establishment of molecular level theories and or as with a whip on a tired horse”, although he still had no kinetic models (31, 33, 34). For instance, Michael Polanyi clear perception on the actual origin of this phenomenon formulated his “transition state” theory of catalysis by (8, 22). His great work on catalysis and investigations into considering Sabatier’s principle, assuming that free the fundamental principles governing chemical equilibria homopolar valencies protrude from the surface of a solid, and reaction rates were recognized by the Nobel Prize he and that a reaction AB + CD = AC + BD takes place on the received in 1909. surface of the catalyst (Figure 3) (35). Similarly, the Meanwhile, the chemist Paul Sabatier (Figure 1d) showed assumption that an unstable surface intermediate, that not only platinum but also other finely divided requiring chemical bonding of reactants to the catalyst transition may be active in surface (or surface atoms) led to the concept of “active reactions (8, 32). This finding dominated the field of sites” (32). heterogeneous catalysis during the first part of 20th A remarkable step in catalysis took place in 1909 when the century and new catalytic materials were developed, German chemist (Figure 4) proposed the including the catalysts for the CO oxidation, the synthesis of ammonia from and hydrogen, using methanation catalysts and the Fisher-Tropsch catalysts as a new catalyst. The prospects for industrial active toward the (4n+2)H2 + nCO → CnH2n+2 + nH2O production were not favourable, but the challenges could reaction. In 1912, Sabatier received the Nobel Prize for his work focusing mainly on the hydrogenation of and CO over Ni and Co catalysts. A novel molecular view of the catalytic process was introduced, namely the “Sabatier principle”, which relates catalytic activity with the free energy associated with reactant-catalyst interactions; according to this model, the catalytic rate reaches a maximum at optimum adsorption strength (Figure 2a) (28, 33, 34). Specifically, plotting a reaction property (e.g. the rate of decomposition of formic acid on various surfaces) versus the strength of adsorption (e.g. the heat of formation of metallic formates, ∆Hf) yields a volcano- shaped curve. At low ΔH values, the reaction rate is low Figure 4. a) Fritz-Haber (1868–1934); b) Apparatus for the synthesis f of ammonia from nitrogen and hydrogen by Haber and Le and corresponds to the rate of adsorption, which Rossignol (1909) kept at the Fritz-Haber Institute, Berlin. increases with increasing heat of formation of the

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Interested in more information about tBu3P catalysts? Please visit http://jmcct.com be overcome by working at high pressures and by the development of a new apparatus in which the gas could be circulated over the catalyst for the continuous separation of ammonia. In fact, on July 2, 1909 Haber and Le Rossignol were able to produce 90 g of ammonia per hour at a pressure of 185 atm with about 100 g of catalyst (osmium) (36). Haber’s laboratory apparatus, developed while he was still in Karlsruhe, was adapted by Carl Bosch to large-scale industrial production, hence the name Haber–Bosch process. Haber approached BASF (Badische Anilin und Figure 5. a) Hugh Stott Taylor (1890-1974); b) Pictorial representation of a catalyst Sodafabrik) and they decided to start a large surface (in Taylor’s sense). programme in which Carl Bosch also became involved. Many experiments were carried out in order to find a suitable catalyst. Over a two-year period, occurred from 1900 to 1930 encouraged scientists to more than 6500 experiments with about 2500 different investigate the catalytic surfaces. In a short time, catalysts were conducted in a systematic way, truly an concepts like “active site”, “particle morphology”, example of an Edisonian approach. As a result, in 1913 “exposed faces”, “surface defects”, “chemical/physical ammonia was produced on an industrial scale for the first adsorption” became progressively popular and new time using catalysis. The social relevance of the Bosh- physico-chemical techniques were developed for the was enormous for various reasons, including characterization of solid surfaces (30). political ones, since the production of ammonia was A significant step in happened in 1925 when directly related to the explosives industry. Moreover, the H.S. Taylor (Figure 5a) focused much attention on the large-scale production of ammonia, forming the basis of concept of active sites, displaying the existence of surface the development of the industry, led to an atoms in varying degrees of saturation by neighbouring atoms increase in agricultural productivity to feed the increasing (Figure 5b); his simple model was based on the heterogeneity world population (30). World production of cereals (e.g. surface irregularities, defects, etc.) of solid surfaces, and increased seven-fold during the 20th century allowing on the fact that the total surface would not be equally active humanity to survive a quadrupling of the global in effecting chemical reactions (39, 40). Indeed, the previous population from 1.6 billion in 1900 to today’s 7 billion. Langmuir “checkerboard model” (41) with a uniform Nowadays, most ammonia is produced on a large scale distribution of static sites that do not interact with one by the Haber-Bosch process with capacities of up to 3,300 another, was strongly contrasted by Taylor’s active site metric tons per day and about 90 percent of ammonia is hypothesis. used for the production of (36). However, despite Taylor’s view interested many scientists and researchers at its great industrial importance and the numerous studies the beginning of the 20th century and in a short time, thanks that were done, the actual mechanism of this catalytic reaction remained unclear until a few decades ago (37). Over the years, it has been observed the structure-sensitivity of ammonia synthesis over iron surfaces and considerable effort has been expended on the kinetics of dissociative nitrogen adsorption. In 1974, P.H. Emmett argued (38) “the experimental work of the past 50 years leads to the conclusion that the rate-limiting step in ammonia synthesis over iron catalysts is the of nitrogen”. Then, G. Ertl (9, 10) achieved a more detailed insight into the dynamics of energy exchange between the solid and chemisorbed species by application of ultrafast (femtosecond) laser techniques. Hence, the kinetic parameters related to the individual reaction steps have allowed to calculate the steady-state yield of ammonia formation for given external parameters (10).

Figure 6. Fractions of surface atoms (N /N ), where N and N are the number of THE DEVELOPMENT OF SURFACE CATALYSIS s T s T surface atoms and the total number of atoms, respectively, as a function of the number of atoms lying on an equivalent edge (n) (calculated for a cubo- The discovery of the catalytic properties of octahedron structure) (43). finely dispersed transition metals that

26 Chimica Oggi - Chemistry Today - vol. 32(6) November/December 2014 to the development of new characterization techniques CONCLUDING REMARKS and of computational methods, great progress was made in the study of reactions at solid surfaces (11). It has been The brief discussion of the history of catalysis in this work has shown that surfaces of solid catalysts usually contain been limited to some fundamental concepts and relevant terraces, ledges, kinks and vacancies with sites having discoveries in heterogeneous catalysis. The author’s purpose different coordination environments (namely coordination was not to be comprehensive, but to lay out advances in numbers, CN). A number of studies concerning small metal catalysis over the last few centuries. The science of catalysis particles demonstrated that surface atoms having lower has made great steps forward in the last 300 years, from CN usually exhibit higher chemical (e.g. corners) being an empirical subject, strongly linked to mystic and and are therefore the preferred active centres for many divine ideas, to becoming a Galilean-type Science. reactions (13,14,16,42). However, the fraction of surface sites varies considerably with the particles’ size, as shown in Figure 6 (43). In fact, as a particle grows in size, some atoms ACKNOWLEDGEMENTS become completely surrounded by neighbouring atoms and are thus no longer on the surface. Based on the The author is very grateful to the reviewer’s valuable Taylorian view, M. Boudart et al. (44, 45) introduced the comments that improved the manuscript. concepts of “structure-sensitive” and “structure-insensitive” reactions. 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