Ammonia synthesis ruthenium

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

Typical catalysts used in both ammonia synthesis and cracking include iron oxide, molybdenum, ruthenium, and nickel. Unlike synthesis, cracking does not require high pressures, and typically it operates at temperatures around 800—900 °C. ... 

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],... 

Metals other than iron which catalyze the ammonia synthesis such as osmium, ruthenium, uranium, and molybdenum can also be promoted by added substances. However, several of these metals do not show improvements of the same magnitude as does iron. Further-... 

A complex nanostructured catalyst for ammonia synthesis consists of ruthenium nanoclusters dispersed on a boron nitride support (Ru/BN) with barium added as a promoter (33). It was observed that the introduction of barium promoters results in an increase of the catalytic activity by 2—3 orders of magnitude. The multi-phase catalyst was first investigated by means of conventional HRTEM, but this technique did not succeed in identifying a barium-rich phase (34). It was even difficult to determine how the catalyst could be active, because the ruthenium clusters were encapsulated by layers of the boron nitride support. By HRTEM imaging of the catalyst during exposure to ammonia synthesis conditions, it was found that the... 

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... 

After C02 removal, final purification includes methanation (8), gas drying (9), and cryogenic purification (10). The resulting pure synthesis gas is compressed in a single-case compressor and mixed with a recycle stream (11). The gas mixture is fed to the KAAP ammonia converter (12), which uses a ruthenium-based, high-activity ammonia synthesis catalyst. It provides high conversion at the relatively low pressure of 90... 

The potential for ruthenium to displace iron in new plants (several projects are in progress [398] of which two 1850 mtpd plants in Trinidad now have been successfully commissioned [1488]) will depend on whether the benefits of its use are sufficient to compensate the higher costs. In common with the iron catalyst it will also be poisoned by oxygen compounds. Even with some further potential improvements it seems unlikely to reach an activity level which is sufficiently high at low temperature to allow operation of the ammonia synthesis loop at the pressure level of the synthesis gas generation. 

From the data presented in the following tables, determine the rates of ammonia synthesis (moles NH3 produced per min per gcat) at 350°C over a supported ruthenium catalyst (0.20 g) and the orders of reaction with respect to dinitrogen and dihydrogen. Pressures are referenced to 298 K and the total volume of the system is 0.315 L. Assume that no ammonia is present in the gas phase. 

Ruthenium has been investigated by many laboratories as a possible catalyst for ammonia synthesis. Recently, Becue et al. [T. Becue, R. J. Davis, and J. M. Garces, J. Catal., 179 (1998) 129] reported that the forward rate (far from equilibrium) of ammonia synthesis at 20 bar total pressure and 623 K over base-promoted ruthenium metal is first order in dinitrogen and inverse first order in dihydrogen. The rate is very weakly inhibited by ammonia. Propose a plausible sequence of steps for the catalytic reaction and derive a rate equation consistent with experimental observation. 

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. 

Table 7.3.2 j A proposed mechanism of ammonia synthesis over ruthenium catalysts. ... 

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.]... 

Describe the main differences in the kinetics of ammonia synthesis over iron catalysts compared to ruthenium catalysts. 

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. 

SYNERGISM OF IRON AND RUTHENIUM IN AMMONIA SYNTHESIS CATALYST AT LOW TEMPERATURE... 

Key words Catalyst catalysis, Iron/Ruthenium catalyst, Ammonia Synthesis... 

Supported ruthenium catalysts have received considerable attention from a fundamental point of view as well as because of their high activity in Fischer-Tropsch synthesis [1], ammonia synthesis [2], hydrogenation of fine chemicals... 

In addition to this electronic interaction between finely divided ruthenium metal particles and cerium hydride, a hydrogen spillover was also proposed to explain the high activity for ammonia synthesis of these catalysts. Since hydrogen in the octahedral sites of CeH2+.t is desorbed at 420°C, atomic hydrogen is certainly present at the temperature of ammonia synthesis (450-550°C). Therefore spillover of atomic hydrogen at the hydride/ transition metal interface could occur. 

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. 

It has been claimed that carbon-supported ruthenium-based catalysts for ammonia synthesis show some important drawbacks, such as high catalyst cost and methanation of the carbon snpport under industrial reaction conditions. This has stimnlated the research for alternative catalysts, although the use of carbon snpports is a common feature. One example of these new catalysts is provided by the work of Hagen et al. [61], who reported very high levels of activity with barinm-promoted cobalt catalysts snpported on Vulcan XC-72. It was demonstrated that althongh cobalt had received little attention as a catalyst for ammonia synthesis, promotion with barium and the nse of a carbon support resulted in very active catalysts with very low NH3 inhibition. 

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