Ruthenium Catalysts For Ammonia Synthesis Research Articles

Carbon-based ruthenium catalyst for ammonia synthesis: Role of the barium and caesium promoters and carbon support

A series of ruthenium catalysts deposited on graphitized carbon and promoted with barium, caesium or both Ba and Cs have been studied in ammonia synthesis. Under experimental conditions (90 bar, H 2:N 2=3:1, 400 °C, 10% NH 3), the reaction rate over the co-promoted catalyst (9.1 wt.% Ru in Ru+C) was found to be higher than those over singly promoted specimens, the overall effect from Ba + Cs in Ba-Cs-Ru/C being almost as high as the sum of individual effects from Ba (Ba-Ru/C) and Cs (Cs-Ru/C), respectively. The co-promoted Ru catalysts, especially that of high ruthenium loading (23.1 wt.% Ru) were shown to be significantly more active in NH 3 synthesis than the conventional fused iron catalyst (KMI, H. Topsoe). The oxygen chemisorption studies have shown that the amounts of O 2 taken up by the Cs-containing samples (Cs-Ru/C, Cs-Ba-Ru/C) are considerably larger than those for Ru/C and Ba-Ru/C, thus indicating caesium to be in a highly reduced, most likely zero valent form, when operating under ammonia synthesis conditions. It is suggested that barium ((Ba+O) adlayer) is located on the ruthenium surface and it acts as a structural or electronic promoter. Caesium (Cs 0) is suggested to be localised on the carbon surface: the Cs promotion occurs at contact points between Ru and the Cs atoms adsorbed on carbon (“hot ring promotion”—electronic).

Surface heterogeneity and ionization of Cs promoter in carbon-based ruthenium catalyst for ammonia synthesis

Second-generation ammonia synthesis cesium-doped ruthenium catalyst supported on turbostratic carbon was investigated by the species resolved thermal alkali desorption method (SR-TAD). Energetic barriers for cesium ions (2.86 eV), ground state (1.96 eV) and electronically excited atoms (5.76 eV) desorbing from the Cs-Ru/C catalyst were determined. In the case of ruthenium-free Cs/C system, cesium desorbs as ground state atoms only, with an energy barrier of 2.87 eV. The work functions determined by the thermionic emission of electrons from Cs/C and Cs-Ru/C were of the same value (2.9 eV). It was concluded that ruthenium induces heterogeneous distribution of cesium on the catalyst surface. The promoter stability is reduced on low work function areas and its surface ionization on high work function areas opens the ionic desorption channel. The Cs desorption from the catalyst is discussed in terms of the literature data for the cesium/graphite system.

Support Effect and Active Sites on Promoted Ruthenium Catalysts for Ammonia Synthesis

The catalytic activities of three supported, barium-promoted ruthenium catalysts for ammonia synthesis are reported. The three supports are silicon nitride (Si3N4), magnesium aluminum spinel (MgAl2O4), and graphitized carbon (C). The effect of the promoter on the activity is strongly dependent on the choice of support material in accordance with several previous observations. Here, this dependence is ascribed to a difference in the affinity of the promoter for the different supports. It is shown how it is possible to image the barium promoter present on the surface of ruthenium crystals in passivated catalysts by conventional high-resolution transmission electron microscopy (HRTEM). By comparison with in situ HRTEM images obtained lately from similar catalysts, and with reference to recent density functional theory (DFT) calculations, we suggest that active B5-type sites on the surfaces of the ruthenium crystals are promoted by nearby promoter atoms via electrostatic interactions.

Atomic-resolution in situ transmission electron microscopy of a promoter of a heterogeneous catalyst.

Insight into the location, state, and function of a promoter in heterogeneous catalysis was obtained through atomic-resolution in situ transmission electron microscopy. In the most active ruthenium catalyst for ammonia synthesis known so far, the barium promoter is shown to be located in two different phases in the catalyst. The increased activity is suggested to be related to a two-dimensional barium-oxygen overlayer on the ruthenium crystals. The possibility for conducting such studies for other reactions could add substantially to our current understanding of heterogeneous catalysis. Heterogeneous catalysis plays an increasingly important role in environmental protection processes, in fuel upgrading, and in providing the majority of the chemical building blocks required by contemporary society. Most heterogeneous catalysts of industrial importance are multicomponent materials that are designed by trial-and-error experimentation. Application of even the most sophisticated physical-chemical characterization techniques is usually not sufficient to obtain a complete understanding of the structure of the active site, the reaction mechanism and kinetics, the structural dynamics, and the specific roles of all catalyst components.

X-ray and IR Spectroscopy of Barium-Promoted, Zeolite-Supported Ruthenium Catalysts for Ammonia Synthesis

Several different zeolite-supported Ru catalysts for ammonia synthesis were characterized by X-ray and infrared absorption spectroscopy. The samples were composed of 1−2 wt % Ru supported on either KX or BaX zeolites. In one case, barium was added to Ru/BaX in excess of ion-exchange capacity. For comparison, cesium-promoted Ru/MgO was also examined. The turnover frequency for ammonia synthesis at 20.7 atm over zeolite-supported Ru increased by over an order of magnitude after adding Ba to the catalysts and approached that of the highly active Cs-promoted MgO catalyst. Analysis of Ru K edge EXAFS recorded at low temperature in H2 indicated that the zeolite-supported Ru clusters were about 1 nm in diameter, regardless of barium loading. The average Ru−Ru coordination number was 4.8 at an interatomic distance of 2.61 A. Adding barium to Ru/BaX beyond ion-exchange capacity caused new features to appear in the Ru EXAFS that were attributed to the interaction of oxygen atoms with Ru clusters at a distance of 1....

Cover Picture

The cover picture shows an arrangement of ruthenium atoms representing the active site of ruthenium catalysts for ammonia synthesis. This B5-type active site consists of three ruthenium atoms in one layer (blue spheres) and two ruthenium atoms in the layer directly above this (red spheres). This arrangement is energetically preferred, because none of the ruthenium atoms is in contact with both of the nitrogen atoms (green spheres) of the adsorbed N2 molecule. Based on knowledge of this active site a number of catalysts were prepared by using [Ru3(CO)12] (upper green sphere) as a precursor. These catalysts were characterized (left red sphere and TEM micrograph in the background), tested using a parallel screening procedure (left and lower blue spheres), and kinetically analyzed (right blue sphere). The resulting barium-promoted Ru/MgO catalyst is the most active catalyst for ammonia synthesis to be described (right red sphere). It can be concluded that the B5-type site also dominates the activity of the promoted catalysts (lower green sphere). The enormous catalytic potential of Ba-Ru/MgO is described by Muhler et al. on pp. 1061 ff.

Effect of Methanation of Active Carbon Support on the Barium-Promoted Ruthenium Catalyst for Ammonia Synthesis

The effect of the induced methanation of hydrogen-treated active carbon (HTAC) support on the structural change and then catalytic activity over Ba-promoted Ru/HTAC catalysts (Ba(NO3)2 and RuCl3 were precursors) for ammonia synthesis was investigated. It was found that a moderate degree of the methanation of HTAC support increased the catalyst surface area and pore volume significantly rather than destroyed the porous structure. But the methanation of carbon support might accelerate the growth of Ru particles, which finally led to the low activity. We suggested that both the dechlorination temperature (refer to the reductive dechlorination of RuCl3 to form Ru on HTAC) and the hydrogenolysis temperature (refer to the hydrogenolysis of Ba(NO3)2 on Ru/HTAC) have binary effects on the catalytic activity, respectively. On the one hand, the dechlorinating effect and the hydrogenolysis effect increased with the increase of H2 treatment temperature. On the other hand, simultaneously the methanation degree of the support increased too in above both processes. The performance of the catalyst was the combined result of the interaction of the amount of residual chlorine, Ru particle size, and the active promoter component. A respective control of the methanation degree during the catalyst preparation and activation processes was suggested to be necessary for the practical use of the promoted Ru/C catalyst.

Structure sensitivity of supported ruthenium catalysts for ammonia synthesis

The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. It is seen that Ru/MgAl 2O 4 is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si 3N 4. The activity differences cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT calculations have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is suggested that special B 5-type sites are primarily responsible for the catalytic activity of the present supported Ru catalysts. It is shown how the number of such B 5-type sites depends on the Ru crystal size for a given crystal morphology. We have found that the activity of the Ru/MgAl 2O 4 catalyst increases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0 nm due to sintering and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in supported ammonia synthesis catalysts. This is in agreement with a low number of B 5-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphology of the Ru crystals and the resulting change in abundance of B 5-type sites is the main cause for the significant activity variations observed for Ru catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring B 5-type sites.

Carbon-supported ruthenium catalyst for the synthesis of ammonia. The effect of the carbon support and barium promoter on the performance

The dispersion and activity of carbon-based ruthenium catalysts for ammonia synthesis were examined. Barium was used as a promoter. Parallel XRD and extensive chemisorption (H 2, O 2 and additionally CO) studies of a series of unpromoted ruthenium/carbon materials showed that a more developed texture (porosity and surface area) of the carbon support results in a significant increase in the ruthenium dispersion. It was found that the presence of ultra small mesopores (diameters<3 nm) in the support results in the formation of ultra fine ruthenium particles. Barium influences the adsorption of the three reactant gases: (i) chemisorption of oxygen increases slightly (15–20%); (ii) hydrogen chemisorption increases significantly (up to 100%); (iii) carbon monoxide chemisorption drops to a negligible value. Barium is supposed to decorate the ruthenium particles and to retard the migration of carbon–hydrogen species onto the metal surface. The promoted ruthenium catalysts exhibit extremely high activities in ammonia synthesis. Under the experimental conditions ( p=6.0 MPa, T=673 K, x NH 3 =8%, H 2:N 2=3:1) the reaction rate referred to the number of Ru surface atoms (TOF) was by an order of magnitude higher than that of a commercial fused iron catalyst. The TOF values are about constant for large Ru crystallites (diameters>3 nm) but they decrease, very likely, for smaller ones.

Effect of ruthenium precursor on hydrogen-treated active carbon supported ruthenium catalysts for ammonia synthesis

Active carbon (A.C.), after treatment at 1123 K with hydrogen for more than 24 h, was found to be very effective as a support for ruthenium catalysis in ammonia synthesis. The activity of barium promoted Ru catalysts supported on this treated A.C. is affected remarkably by the Ru precursor. Ru 3(CO) 12 was found previously to be the most effective Ru precursor for oxide supports such as Al 2O 3, MgO, and CeO 2 in ammonia synthesis. However, in this study, the RuBao/A.C. catalyst prepared from Ru(acac) 3 was found to yield the highest activity, while the catalyst prepared from Ru 3(CO) 12 resulted in the lowest activity among several precursors. Transmission electron microscopy and hydrogen chemisorption showed that the particle size of Ru obtained from the decomposition of Ru(acac) 3 on hydrogen-treated A.C. is smaller than the particle size of Ru obtained from the decomposition of Ru 3(CO) 12. Additionally, RuCl 3 was found to be an effective precursor for Ru/A.C. catalyst. It has been suggested that chlorine ions can be removed easily from A.C. by hydrogen reduction at 773 K, and that this results in the high activity.

The promoter effect of lanthana on MgO supported ruthenium catalysts for ammonia synthesis

When activated at high temperature (873 K) doped lanthana such as La2O3, Ce2O3, Sm2O3 derived from nitrate showed remarkable effect on the MgO-supported ruthenium activity for ammonia synthesis. The activity treated with hydrogen at 873 K became 5 times as active as that treated at 623 K. The activity levels were almost the same as those of the Ru-lanthana (support) system under the same activation condition. It was thought that the lanthana was partly reduced and a strong metal support interaction occurred. The partially reduced lanthana was thought to donate electrons to the contacting Ru atoms forming the active centers. The other characters such as the stability were also discussed by comparing with alkali promoters.

ChemInform Abstract: Ruthenium Catalysts for Ammonia Synthesis at High Pressures: Preparation, Characterization, and Power‐Law Kinetics.

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Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics

Supported Ru catalysts for NH 3 synthesis were prepared from Ru 3(CO) 12 and high-purity MgO and Al 2O 3. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H 2 chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH 3 formation was found: Cs 2CO 3Ru MgO > CsNO 3Ru MgO > Ru MgO > Ru KAl 2O 3 > Ru Al 2O 3 . The Cs-promoted Ru MgO catalysts turned out to be more active than a multiply-promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10 −2 s −1 for the non-aqueously prepared Cs 2CO 3Ru MgO catalyst at 588 K. The strong inhibition by H 2 was found to require a lower molar H 2:N 2 ratio in the feed gas than 3:1 in order to achieve a high effluent NH 3 mole fraction. The optimum ratio for Cs 2CO 3Ru MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH 3 mole fraction which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH 3 synthesis also at high pressure.

Effect of hydrogen treatment of active carbon as a support for promoted ruthenium catalysts for ammonia synthesis

Hydrogen treatment of active carbon (A.C.) at temperatures ranging from 1073 to 1188 K for 12 h is an effective method for eliminating surface impurities, leading to the preparation of useful ruthenium catalysts; for example, 2 mass% Ru–BaO/A.C. (Ba/Ru = 5) yields 2 mmol NH 3 h - 1 g - 1 at 588 K under 1 atmosphere.

A novel basic alkaline-earth zeolite-supported ruthenium catalyst for ammonia synthesis

Ruthenium clusters supported on basic zeolites containing alkaline-earth cations catalyse ammonia synthesis with reaction rates that exceed those for analogous catalysts containing only alkali-metal cations.

Infrared Studies of Adsorbed Dinitrogen on Supported Ruthenium Catalysts for Ammonia Synthesis: Effects of the Alumina and Magnesia Supports and the Cesium Compound Promoter

Infrared Studies of Adsorbed Dinitrogen on Supported Ruthenium Catalysts for Ammonia Synthesis: Effects of the Alumina and Magnesia Supports and the Cesium Compound Promoter

ChemInform Abstract: Promoter Effect of Alkali Metal Oxides and Alkaline Earth Metal Oxides on Active Carbon‐Supported Ruthenium Catalyst for Ammonia Synthesis.

AbstractIn order to prepare an effective catalyst for ammonia synthesis, the promotion behavior of alkali and alkaline earth metal nitrates on carbon‐supported Ru catalysts is investigated.

New type of carbon coated alumina supports for the preparation of highly ctive ruthenium catalysts for ammonia synthesis

In the present work advantage is taken of the beneficial property of carbon, i.e. the electron transport capacity of the graphite lattice, the ionic alkali promoter and the stability of the classical support, Al 2 O 3 , in the preparation of new ammonia synthesis catalysts based on ruthenium. Two carbon-coated aluminas were used as supports for the preparation of the catalysts

Promoter Effect of Alkali Metal Oxides and Alkali Earth Metal Oxides on Active Carbon-Supported Ruthenium Catalyst for Ammonia Synthesis

Abstract Promoter effects of alkali metal nitrates and alkali earth metal nitrates on Ru/A.C. (active carbon) were studied in order to prepare an effective catalyst for ammonia synthesis. Of alkali metal nitrates, RbNO3 was more effective than CsNO3 and KNO3. Alkali earth metal (Mg, Ca, Sr, Ba) nitrates were also found to be effective promoters on Ru/A.C. catalysts. Of them heavier alkali earth metal elements were more effective (Ba&amp;gt;Sr&amp;gt;Ca&amp;gt;Mg). Ba(NO3)2 was as effective as CSNO3, which was not expected from the stand point of electronegativity. The transformation of promoter precursors to active compounds (nitrates to oxides or hydroxides) and the interaction of promoters and ruthenium were discussed. Precursor of ruthenium salt was also studied. RuCl3·3H2O and [Ru(NH3)6]Cl3 gave higher activities than Ru3(CO)12. It was discussed that RuCl3 gave finer Ru particles than Ru3(CO)12 on active carbon. The turn over frequency (TOF) of Ru–Rb+/A.C. was lower than those of RU–Cs+/Al2O3 and of Ru–Cs+/MgO.

Ammonia decomposition over 430-SS etched metal catalysts

In this work, strontium tantalate (Sr2Ta2O7) nanowires were used for the first time as a support of ruthenium catalysts for ammonia synthesis. The highest ammonia synthesis activity of 1582 μmol g−1cat h−1 was observed with Ru/Sr2Ta2O7 catalysts prepared with molar ratio of Sr(OH)2/Ta2O5 at 4. After loading promoters, the ammonia synthesis activity was significantly improved to 4856, 5004 and 5038 μmol g−1cat h−1 for K-, Rb- and Cs- modified Ru/Sr2Ta2O7 catalysts. Characterization and kinetic experiments indicate that the improved ammonia synthesis rate of promoters-Ru/Sr2Ta2O7 catalysts was attributable to the combined effect of alleviated hydrogen poisoning through the promotion of hydrogen spillover from Ru species to support of Sr2Ta2O7 and enhanced activation of N2 due to increased electron density of Ru sites because of electron donation from introduced oxygen vacancies and promoters. The study not only clarifies the role of promoters in ammonia synthesis, but also provides an excellent Ru catalyst on Sr2Ta2O7 nanowires for ammonia synthesis.