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: heterogeneous catalysis, 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 ethanol 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 alchemy 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) Wilhelm Ostwald (1853-1932); d) Paul Sabatier (1854-1941). 22 Chimica Oggi - Chemistry Today - vol. 32(6) November/December 2014 Similarly, the production of sulphuric acid in glass apparatus by burning sulphur with nitric acid in humid air (homogeneous catalysis) 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 chemist 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 catalytic cycle 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 ammonia 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 thermodynamics of this process had become well of matter and the decomposition by this matter, catalysis, understood, but only in 2007 the Nobel Prize assigned to just like we understand with the word analysis the Gerhard Ertl recognized his discoveries in the molecular separation of the constituents” (21). Curiously, the creative steps at the solid 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 spectroscopies and advanced scanning probe techniques instance, Faraday studied the catalytic properties of it is possible to study the active sites and molecules in platinum for the oxidation of hydrogen 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 atoms 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 chemical reaction 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 reaction rate 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 Arrhenius equation, spiritual entities, spiritual states and, ultimately, Ea is the activation energy of the reaction (in kJ/mol), R the transmutations (namely chrysopoeia) (17-19). On the other gas 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 ether 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 activation energy (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 adsorption 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 reagent-state to product-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 transition state 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 elementary reaction steps, including adsorption, surface consumed (31, 32). diffusion, 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).