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Evolution of Catalysts Design and Synthesis: From Bulk Metal Catalysts to Fine Wires and Gauzes, and that to Nanoparticle Deposits, Metal Clusters, and Single Atoms

Wey Yang Teoh1,2

1 University of Malaya, Centre for Separation Science and Technology, Department of Chemical Engineering, 50603 Kuala Lumpur, Malaysia

2 The University of New South Wales, School of Chemical Engineering, Sydney 2052, Australia

1.1 The Cradle of Modern Heterogeneous Catalysts

The modern discovery of heterogeneous catalysts stretches as far back as 1800 when Joseph Priestley and Martinus van Marum reported the dehydrogenation of alcohol over a heated metal catalyst, although not too much was thought about the role of the metal catalyst at that time except as a heating source. Then in 1813, Louis Jacques Thénard of École Polytechnique in Paris discovered the decomposition of ammonia to nitrogen and hydrogen over "red-hot metals" and recognized that the phenomenon was due to some catalytic reaction [1, 2]. The concept was followed up by Humphry Davy and Michael Faraday at the Royal Institution of London who, in 1817, reported the flameless catalytic combustion of coal gas and air over heated platinum wire producing bright white ignition. Their results were reproducible when using palladium, but not on copper, silver, iron, gold, and zinc [1, 3]. These experiments made clear that there was some form of catalytic role associated with the different metals. The discovery soon became the basis for the invention of the coal mine safety lamp, also known as the Davy lamp - although mysteriously but rather practically, the use of inefficient steel iron rather than platinum gauze became the standard for Davy lamps. At around the same time, Thénard and Pierre Dulong found that the catalytic ammonia decomposition rates decrease in the following order: iron, copper, silver, gold, and platinum, marking the first recognition of the kinetics of different metal catalysts. The importance of catalytic surface area, as we now know to be one of the most important governing factors in heterogeneous catalysis, was discovered by Edmund Davy (cousin to Humphry Davy) at the University College Cork in the 1820s, who found that finely divided platinum could catalyze the oxidation of alcohol as well as the oxidation of hydrogen at room temperature [4].

In 1831, a little-known gentleman by the name of Peregrine Phillips, Jr., patented sulfuric acid production by oxidizing sulfur dioxide in air over platinum packed in porcelain tubes heated to "strong yellow heat". The resultant sulfur trioxide forms sulfuric acid fume upon contact with water, hence earning its name as the Contact Process [5]. Ironically, despite the high importance of this catalytic process, not much is known about Phillips except that he was son of a tailor and was born in Bristol [1]. A large-scale manufacturing of sulfuric acid using the Contact Process and platinum catalyst was realized many years later in 1875 by Rudolph Messel, a German-born and naturalized English industrial chemist. Messel himself was very much involved in the studies of the kinetics as well as the problematic poisoning of platinum catalysts by arsenic trioxide. In 1913, BASF was granted patents on a new catalyst based on the more versatile supported vanadium pentoxide and alkali oxide on porous silica [6, 7]. The first manufacturing plant based on this new catalyst was commissioned in 1915. Improvement in the activity of the supported vanadium pentoxide catalyst through the addition of potassium sulfate promoter was invented in Germany and the United States between 1916 and 1919. It was only in 1988 that Haldor Topsoe and Anders Nielsen revealed that the addition of cesium or rubidium promoter, rather than potassium, was more efficient in enhancing the activity of sulfur dioxide oxidation. With a typical lifetime of up to 10?years, the industrial catalyst composition for the Contact Process has been largely unchanged even to this day [8].

Going back to 1838, just a few years after the discovery of the Contact Process, Frédéric Kuhlmann discovered the production of nitric acid from the oxidation of ammonia in air over platinum sponge at 300?°C and filed a patent on this [9]. Based on the discovery, he later founded the Etablissements Kuhlmann company, which still exists to this day as part of the Pechiney SA. Despite being an important chemical commodity for the use in fertilizers and explosives manufacturing, the interest in Kuhlmann reaction was not immediately of interest since Chile saltpetre (a naturally occurring mineral of alkali metal nitrate precursor found at the Atacama desert repository) was widely available. In his vision, Kuhlmann stated that "If in fact the transformation of ammonia to nitric acid in the presence of platinum and air is not economical, the time may come when this process will constitute a profitable industry."

Indeed, the Kuhlmann reaction picked up interest toward the end of the century as part of the solution to "The Nitrogen Problem." In 1901 and building on Kuhlmann's earlier findings, Wilhelm Ostwald of the University of Leipzig investigated the production of nitric acid using supported platinum on asbestos before moving to coiled platinum strips that gave higher conversion [9]. A large-scale nitric acid manufacturing plant went into operation at Gerthe in 1908 with an output of 3?tons nitric acid per day using 50?g of corrugated platinum catalyst of 2?cm wide. Given the short catalyst lifetime of no more than six weeks, it was soon realized to be a costly operation. To tackle the problem, Karl Kaiser of Technische Hochschule, Charlottenburg, developed the platinum gauze catalyst in 1909, consisting of 0.06?mm diameter wires woven to 1050 mesh/cm2, that gave a higher surface-to-bulk ratio and uninterrupted production of nitric acid of up to six months [9]. But because the source of ammonia at that time was derived from gas works liquors containing impurities such as arsenic and sulfur that deactivate the platinum catalyst, the really large industrial-scale production was only possible after the implementation of the Haber-Bosch process that provided clean ammonia. The present-day nitric acid catalyst is based on rhodium-platinum gauze (5-10% Rh) [10].

Further advancement in the design of bulk metal catalysts was evident from the work of Murray Raney on the synthesis of skeletal nickel, which was granted US patent in 1925 [11]. The Raney catalyst was prepared by first forming a Ni-Al alloy and ground into small particles, followed by the selective leaching of Al in caustic brine (such as NaOH) to yield the skeletal structure. The resultant Raney catalyst is composed of finely divided nickel so fine that it is pyrophoric and hence requiring storage under deionized water. Initially, the Raney Ni was used as an industrial catalyst for the hydrogenation of vegetable oil (to make butter substitutes) but later proved to be useful for a range of other hydrogenation reactions. Other forms of Raney catalysts including those of metallic cobalt, copper, palladium, silver, and ruthenium were later developed and found applications in methanol synthesis, conversion of furfural into furfural alcohol, and the hydrogenation of acrolein to allyl alcohol, among others [12, 13].

1.2 The Game Changer: High-Pressure Catalytic Reactions

The implementation of high-pressure reactor technologies pioneered by Robert Le Rossignol (assistant to Fritz Haber) [14] and later by Carl Bosch [15] was one of the most important milestones in the advancement of heterogeneous catalysis. Their breakthroughs enabled a series of high-pressure catalytic reactions that include the ammonia synthesis and methanol synthesis, which to this day rank among the most important industrial catalytic reactions. High-pressure conditions are particularly useful in overcoming reaction dilemma that under ambient pressure could obtain high selectivity but at extremely sluggish rates and vice versa at high temperatures. By carrying out the same reaction under high-pressure conditions, one can shift the equilibrium line to higher selectivity even at high temperatures, thus allowing high yield of the desired product. Chapter 35 is devoted to this topic.

Haber in one of his earlier efforts in synthesizing ammonia by N2 fixation (through reaction with H2) under ambient pressure could only obtain 0.005% yield when using iron catalysts at 1000?°C [16]. A year later, in 1906, Walther Nernst at the University of Berlin reported favorable conversion at 1000?°C when using iron catalysts in a ceramic apparatus that allowed him to perform the reaction at 75?bar. Unfortunately, the reactor and the extreme condition were far too impractical for industrial-scale implementation. Haber, who became professor at the Karlsruhe Technische Hochschule, used a steel-based reactor but this time working with Le Rossignol (who actually built the bench-scale high-pressure reactor, equipped with a high-pressure and high-temperature valve, now known as the Le Rossignol valve). With the new reactor, they were able to screen a number of catalytic materials ranging from iron, chromium, nickel, manganese, osmium, and uranium (as uranium carbide) at 200?atm and in excess of...