Synthesis of ammonia on ruthenium

In this case, the influence of the alkali promoter is even more pronounced than with Fe, and also the nature of the support exerts a marked influence. This becomes evident from Fig. 6.9 in which the steady-state yields of Cs-promoted and unpromoted Ru catalysts on AI2O3 and MgO supports at atmospheric pressure are plotted [45]. 

The reaction proceeds along the same elementary steps as with the Fe catalyst, and again successful microkinetic modeling on the basis of experimentally derived parameters could be achieved [46]. Again, dissociative nitrogen adsorption is rate limiting, where the sticking coefficient is markedly affected by the presence of atomic steps [47] whose role as active sites had been discussed in Chapter 5. 

On the Ru(OOOl) surface, the N2 molecule is terminally bonded on top with its molecular axis perpendicular to the surface [48]. After dissociation, the N atoms are located in threefold coordinated sites and form two ordered phases with 

FIGURE 6 9 Steady-state ammonia yields of various Ru catalysts as a function of temperature at atmospheric pressure [45]. 

FIGURE 6.10. Ordered phases formed by adsorbed N atoms on a Ru(OOOl) surface at coverages 0 = 0.25 and 0.33 [49]. 

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M. W. Kellogg has developed a new technology in the synthesis of ammonia. They employ a ruthenium on graphite as the catalyst on Kellogg Advanced Ammonia Process (KAAP). The process is the first to employ a non-iron based catalyst and was co-developed with British Petroleum Ventures. The KAAP has been commercialized since 1994, and has been used in an increasing number of projects. 

As early as the middle of 1960s, BP successfully developed a kind of oleophylic graphite with excellent adsorption ability, and then a kind of graphited carbon in 1974, which could be used as supports for various catalysts. From 1978 to 1984, BP claimed a series of patents on ruthenium catalysts for ammonia synthesis. In 1979, ° a novel catalyst for ammonia synthesis was prepared by loading carbonyl compound of ruthenium on carbon containing graphite in laboratory. This kind of catalyst, with graphited carbon as support and Rus (CO) 12 as the precursor, possessed some special features that may be summarized as follows ... 

In contrast, the use of carbonyl-derived ruthenium catalysts on different supports has been explored in ammonia synthesis [120-122], The use of K2[Ru4(CO)i3] as ruthenium precursor on MgO or carbon yields especially effective catalysts for low-temperature ammonia synthesis [120, 122],... 

Ruthenium supported on oxides is a catalyst of various reactions. It is active in methanation reactions [e.g. 1, 2, 3], in Fischer-Tropsch synthesis [e.g. 4, 5, 6], in CO oxidation [7, 8], in the synthesis of methyl alcohol [9], 1" the redu ction of NO to nitrogen CIO] and in hydrogenolysis of ethane [11] and of butane [12]. Ru supported on carbon is supposed to replace the iron in ammonia synthesis [13]. Lately ruthenium supported on oxides is intensively investigated as a potential... 

These simplifying assumptions must be adapted to some extent to explain the nature of some reactions on catalyst surfaces. The case of ammonia synthesis on supported ruthenium described in Example 5.3.1 presents a situation that is similar to rule 1, except the rate-determining step does not involve the mari. Nevertheless, the solution of the problem was possible. Example 5.3.2 involves a similar scenario. If a mari cannot be assumed, then a rate expression can be derived through repeated use of the steady-state approximation to eliminate the concentrations of reactive intermediates. 

Comparison of calculated and measured ammonia concentrations at the effluent of a steady-steady ammonia synthesis reactor containing ruthenium particles supported on magnesia and promoted by cesium. [Adapted from O. Hinrichsen. F Rosowski, M. Muhler, and G. Ertl, The Microkinetics of Ammonia Synthesis Catalyzed by Cesium-Promoted Supported Ruthenium, Chem. Eng. Sci., 51 (1996) 1683, copyright 1996, with permission from Elsevier Science.]... 

Because of the interest in ruthenium as a potential catalyst for ammonia synthesis from N2 and H2, the hydrogenation of Nads on Ru surfaces has been investigated (86). Both NHads and NH2,ads were found, in addition to NHs ads- Dietrich ei nl (86) reported that the thermal stability of NHads is the highest of the three N Ha, ads species (x = 1. 2. and 3) on Ru(OOOl) and on Ru(1121) at temperatures up to 400-450 K. On Ru (1010), however, the thermal stability of NH2,ads is higher than that of NHad., and much higher than those on other Ru surfaces. The reason for the enhanced thermal stability of NH2,ads on Ru(lOlO) is not clear. It was also found that coadsorbed N has a positive effect on the thermal stability of NHads on Ru(OOOl), whereas NHads is stable at temperatures up to 400 K, and NHads in the N/NH coadsorbate is stable at temperatures up to 460 K. 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

Ruthenium has long been known to be an effective catalyst for ammonia synthesis. However, compared to the traditional iron-based catalysts, studies on ruthenium-based catalysts are limited. The rate determining step of ammonia synthesis, the dissociative adsorption of dinitrogen, has been shown to extremely structure sensitive on both iron and mthenium catalysts. To study this structure sensitivity on ruthenium, density functional theory calculations were performed on Ru(OOl) and Ru(llO) clusters. End-on, side-on, and dissociated adsorption states were investigated on both surfaces. While the Ru(llO) cluster could stabilize aJl three adsorption modes, a minimum energy structure for the side-on adsorption on Ru(OOl) could not be found. It is likely that this side-on mode can provide a low energy pathway to the dissociated state, thereby resulting in faster dissociative adsorption on Ru(llO). 

In some cases the so-called "Steady-State Isotopic Transient Kinetic Analysis" (SSITKA) was used for detailed investigations of reaction mechanisms. Shannon and Goodman [123] present an extensive review of this subject. Hinrichsen et al. [124] employed temperature programmed desorption to study the ammonia synthesis on ruthenium catalysts. 

Like iron catalyst, dissociative adsorption of N2 is also the rate determining step on ruthenium catalyst. The difference is that the absorption of H2 strongly inhibits the adsorption of N2, while the inhibition effect for the production of NH3 is not apparent on ruthenium catalyst.The latter is an advantage of ruthenium catalyst, so that the ruthenium catalyst can be placed behind iron catalyst in synthesis ammonia process, e.g., KAAP process.The former effect is still a problem that needs to be solved for the ruthenium catalysts. 

In 1990, the Pacific ammonia synthesis project was initiated. At the same year, Engelhard Corporation obtained the production license of the catalyst. In 1991, Kellogg obtained the catalyst technology from BP Company. In November 1992, Kellogg announced that the first KAAP started up successfully based on ruthenium ammonia s mthesis catalysts at Pacific Ammonia. 

Ruhler et al. considered that Ba-Ru/MgO catalysts have high activities and stabilities. The researchers in Tops Company, Denmark developed series of ruthenium based ammonia synthesis catalysts support on different supports, with electronic and structural promoters. In these studies, the ruthenium catalysts supported on the Mg Al spinel and high surface area graphite show promising activity (see Table 6.6). However, the stability has some problems under industrial conditions. ... 

In ammonia synthesis reaction, the dissociative adsorption of dinitrogen is the rate determining step. The electron donor type promoter, such as alkali metals, which can provide electrons to the Ru, is more propitious to activate the N=N bond in N2 molecule and thus enhance the dissociative adsorption of dinitrogen and increase the reaction rate of ammonia synthesis. The acidity and basicity of supports have important influence on the activity of the ruthenium catalysts without promoter. Acidic supports can easily accept electrons and decrease the activity of catalysts. Basic supports can easily provide electrons and enhance the dissociative... 

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