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Overviews of the Preparation and Reactivity of Transition Metal-Dinitrogen Complexes

Yoshiaki Tanabe and Yoshiaki Nishibayashi

Department of Systems Innovation, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

1.1 Introduction

Nitrogen, the fifth most abundant element in the solar system, is the most abundant element in the atmosphere of Earth [1] as well as the fourth most abundant element in cellular biomass [2]. However, it is rather a trace element in the lithosphere of Earth [3]. Thus, utilization of chemically inert gaseous molecular dinitrogen (N2) that exists in the atmosphere of Earth as the primary nitrogen source is inevitable in both biogeography and industry. Indeed, fixation of atmospheric nitrogen can be achieved by the conversion of molecular dinitrogen into ammonia (NH3) containing the most reduced form of nitrogen (-3) that can be a convenient precursor for several nitrogen-containing compounds and has been the most fundamental reaction pathway of the global nitrogen cycle [4, 5]. Industrially, NH3 is one of the 10 largest commodity chemical products and has been produced by the Haber-Bosch process in which atmospheric dinitrogen reacts with gaseous dihydrogen (N2?+?3 H2??2 NH3) since the early twentieth century [6-14]. Haber and van Oordt in 1904 first succeeded in the conversion of the mixture of N2 and H2 into NH3 in the presence of transition metal catalyst (Fe or Ni) at a high temperature in a laboratory [15-17]. Later, modification of the reactors and catalysts was achieved, and 90?g of ammonia was shown to be obtained every hour by using an osmium-based catalyst with the total yield of ammonia up to 8?vol% at 550?°C and a total pressure of 175?atm of a stoichiometric mixture of dinitrogen and dihydrogen (1 : 3) in an experimental lecture held in Karlsruhe on 18 March 1909 [18-20]. Further modification of the catalysts for industrialization was investigated by Mittasch and coworkers in BASF, leading to the discovery of the combination of iron, K2O, and Al2O3 as one of the most active catalysts by 1910 [ 6, 21]. The first commercial plant for ammonia synthesis at Oppau began its operation by 1913 in collaboration with Bosch and coworkers at BASF, while the earlier commercial methods to fix atmospheric nitrogen such as Frank-Caro cyanamide process (CaC2?+?N2??CaCN2?+?C) and Birkeland-Eyde electric arc process (N2?+?O2??2 NO) were gradually replaced by the Haber-Bosch ammonia process [ 6- 14]. Typical reaction conditions of the Haber-Bosch process are shown in Figure 1.1a [6], where the reaction is carried out under high temperature and high pressure in the presence of heterogeneous solid-state catalysts prepared from magnetite (Fe3O4) with the addition of alumina (Al2O3), silica (SiO2), or alkaline earth metal oxide (CaO) as a "structural" promoter and alkaline metal oxide (K2O) as an "electronic" promoter.

Figure 1.1 (a) Prototype Haber-Bosch process operated at the first BASF's Oppau plant. H2 is originally obtained from steam reforming of coal. (b) Kellogg advanced ammonia process with methane steam reforming.

Although formation of NH3 from N2 and H2 is thermodynamically favored under standard conditions (?rH° = -45.90?kJ?mol-1, ?rG° = -16.37?kJ?mol-1 at 1?bar and 25?°C), this conversion can hardly occur at ambient reaction conditions because the dissociation energy of the dinitrogen triple bond is high (D0° = 945.37?kJ?mol-1) [22]. To lower and surmount the activation energy of this conversion, elevated pressure and temperature as well as heterogeneous solid-state catalysts are necessary, where bond-breakings upon chemisorption on the surface of solid-state catalysts were experimentally observed by Ertl and coworkers, who clarified the surface reaction pathway of the Haber-Bosch process as shown in Figure 1.2 [23-29]. Activation energy and turnover frequency of the catalytic ammonia synthesis are highly dependent not only on the catalyst but also on temperature, pressure, and the ratio of the substances and products, where the logarithm of the equilibrium constant for the reaction of N2?+?3 H2 = 2 NH3 at 1?bar becomes zero theoretically at 456?K [22]. For example, the apparent activation energy for the catalytic ammonia synthesis on the Fe(111) surface of an iron single crystal at around 748?K and a total pressure of 20?atm of a stoichiometric mixture of dinitrogen and dihydrogen (1 : 3) was determined by Somorjai and coworkers as 81.2?kJ?mol-1 with an initial turnover frequency of 12.7?±?2.0 molecules of ammonia per C4 surface iron atom per second [30].

Figure 1.2 Potential energy diagram for ammonia synthesis on the surface of iron, via stepwise hydrogenation or via formation of radicals.

A more improved method such as Kellogg advanced ammonia process () uses ruthenium-based catalyst supported on graphite-containing carbon copromoted with barium, cesium, or rubidium performed at comparably lower pressure and temperature, the stoichiometry of which can be expressed as Figure  1.1b, when natural gas steam reforming is applied to ammonia production without the separation of dinitrogen from air [8- 14, 31-35]. In this reaction, methane is the main hydrogen source of ammonia, and the gaseous ammonia obtained from the stoichiometry in Figure  1.1b theoretically contains 20.8?GJ per metric ton or 355?kJ?mol-1 as chemical energy calculated based on the heat of combustion of methane in the lower heating value () (?cH° = -802.3?kJ?mol-1, ?cG° = -800.8?kJ?mol-1) or 18.6?GJ per metric ton based on that of ammonia (?cH° = -316.8?kJ?mol-1, ?cG° = -326.5?kJ?mol-1) if full recovery of the reaction heat is assumed (?rH° = -37.8?kJ?mol-1 and ?rG° = -27.5?kJ?mol-1 per NH3 for Figure  1.1b) [22]. A classical BASF-type Haber-Bosch process that uses coke consumes chemical energy of 100?GJ?per metric ton of NH3 in 1920 [6], which is much more efficient than the Birkeland-Eyde electric arc process (600?GJ per metric ton of fixed nitrogen) or the Frank-Caro cyanamide process (190?GJ per metric ton of NH3 derived from the decomposition of CaCN2 with H2O) [12], whereas the most efficient ammonia plant with the ruthenium-based catalyst and methane steam reforming consumes as low as 27.2?GJ per metric ton or 463?kJ?mol-1 of NH3, where energy efficiency of around 75% with respect to the stoichiometric methane demand is achieved, which also means that additional chemical energy of 108?kJ?mol-1 is required for the industrial synthesis of NH3 as represented in Figure  1.1b [13]. In an exergy analysis of a low-energy ammonia process to obtain the liquefied ammonia at -33?°C (20.14?GJ per metric ton or 343?kJ?mol-1) by Dybkjaer under a model reaction at 140?kgf?cm-2 in an indirectly cooled two-bed radial converter using pure methane, cooling water available at 30?°C, a steam to the carbon ratio of 2.5, and so forth, a total exergy of 30.69?GJ per metric ton or 523?kJ?mol-1 is consumed with an exergy loss of 10.55?GJ per metric ton or 180?kJ?mol-1 corresponding to a thermodynamic efficiency of 66% for the production of NH3, where the biggest loss of exergy occurs at methane steam reforming sections with rather a slight loss made during the actual ammonia synthesis (1.70?GJ per metric ton or 29?kJ?mol-1) [ 13, 36]. Further improvement of Haber-Bosch catalysts is still in progress, especially in the development of electronic and structural promoters. For example, Hosono and coworkers have developed ruthenium-loaded electrode catalysts, which show higher catalytic performance than the conventional ruthenium catalysts at lower temperatures and pressures [37-40].

In total, the Haber-Bosch process annually produces more than 170?million metric tons of NH3 [41], consumes fossil fuels as the hydrogen source of NH3, corresponding to 1-2% of the world's annual primary energy supply, and is responsible for the emission of more than 450?million metric tons of CO2 [42-44]. This pollution can be reduced by using renewable energy sources for producing dihydrogen from water, but it should be more convenient to use water as a proton source for...