Preparation and characterization of chlorine-free ruthenium catalysts and the promoter effect in ammonia synthesis. 1. An alumina-supported ruthenium catalyst

Co nanoparticles supported on mixed magnesium-lanthanum oxides: effect of calcium and barium addition on ammonia synthesis catalyst performance

The synthesis of ammonia in the Haber-Bosch process produces millions of tons of ammonia annually needed for producing fertilisers required to feed the growing population. Although this process has been optimised extensively, it still accounts for about 2% of global energy consumption. It is, therefore, desirable to develop an efficient ammonia synthesis catalyst. Over the last decades, many attempts have been made to improve the ammonia synthesis catalyst efficiency under mild conditions. Here, we studied the effect of adding Ca and Ba to the cobalt ammonia synthesis catalyst. The combination of the different experimental results allows concluding that Ca served as an inactive additive, whereas Ba served as an electronic promoter. The Ca addition did not change the textural, structural, and chemisorptive properties of the Ca-doped Co catalyst. On the other hand, the Ba addition had a major effect on the nature of active Co sites. It contributed to the formation of new active sites for hydrogen and nitrogen adsorption and dissociation. Barium addition also contributed to the generation of new basic sites, particularly the strong ones. These unique characteristics were ascribed to the formation of Co(core)-BaO(shell) structures. It is likely that the donation of electrons from BaO to N₂ via Co markedly promoted ammonia synthesis. This catalyst exhibited ammonia synthesis activity 4 times higher than that of the undoped Co catalyst and 2 times higher than that of the industrial Fe catalysts under identical conditions.

Ammonia from dinitrogen at ambient conditions by organometallic catalysts

Fixation of atmospheric dinitrogen in plants by [Mo-Fe] cofactor of nitrogenase enzyme takes place efficiently under atmospheric pressure and normal temperature. In search for an alternative methodology for the highly energy intensive Haber-Bosch process, design and synthesis of highly efficient inorganic and organometallic complexes by mimicking the structure and function of [Mo-Fe] cofactor system is highly desirable for ammonia synthesis from dinitrogen. An ideal catalyst for ammonia synthesis should effectively catalyse the reduction of dinitrogen in the presence of a proton source under mild to moderate conditions, and thereby, significantly reducing the cost of ammonia production and increasing the energy efficacy of the process. In the light of current research, it is evident that there is a plenty of scope for the development and enhanced performance of the inorganic and organometallic catalysts for ammonia synthesis under ambient temperature and pressure. The review furnishes a comprehensive outlook of numerous organometallic catalysts used in the synthesis of ammonia from dinitrogen in the past few decades.

Regulating the interaction of Ru nanoparticles and an Eu2O3 support achieves enhanced activity for ammonia synthesis

A Ru/Eu2O3 catalyst with enhanced metal–support interaction is successfully prepared for highly efficient ammonia synthesis from nitrogen and hydrogen.

Liberating Active Metals from Reducible Oxide Encapsulation for Superior Hydrogenation Catalysis

The strong metal-support interaction (SMSI) is of significant importance to heterogeneous catalysis. The electronic modification and encapsulation of active metals by reducible supports are the intrinsic properties of the SMSI, where the latter would decrease or even cease the catalytic activity of transition metals. Here, we demonstrate for the first time that alkalies are the functional additives that can effectively manipulate the SMSI for better hydrogenation catalysis. Specifically, both thermodynamic analyses and experimental results show that the addition of alkalies to the Ru/TiO₂ catalyst could form a titanate top layer that effectively hampers the migration of TiO_2-x to the surface of Ru nanoparticles. In the meantime, a substantially enhanced reduction of the support is achieved, leading to an even stronger electron donation from the support to Ru. The alkali-modified Ru/TiO₂ exhibits superior low-temperature catalytic activity in the hydrogenation of aromatics, which is ca. an order of magnitude higher than that of the commercial Ru/Al₂O₃ catalyst and is in clear contrast to that of the neat Ru/TiO₂ catalyst that shows negligible activity due to the severe encapsulation of Ru by TiO_2-x.

Carbon-free H2 production from ammonia triggered at room temperature with an acidic RuO2/γ-Al2O3 catalyst

Ammonia has been suggested as a carbon-free hydrogen source, but a convenient method for producing hydrogen from ammonia with rapid initiation has not been developed. Ideally, this method would require no external energy input. We demonstrate hydrogen production by exposing ammonia and O₂ at room temperature to an acidic RuO₂/γ-Al₂O₃ catalyst. Because adsorption of ammonia onto the catalyst is exothermic, the catalyst bed is rapidly heated to the catalytic ammonia autoignition temperature, and subsequent oxidative decomposition of ammonia produces hydrogen. A differential calorimeter combined with a volumetric gas adsorption analyzer revealed a large quantity of heat evolved both with chemisorption of ammonia onto RuO₂ and acidic sites on the γ-Al₂O₃ and with physisorption of multiple ammonia molecules.

Influence of Ru precursors on the activity of Ru/Al2O3–TiO2 catalysts for catalytic wet air oxidation of high concentration organic compounds

The nature of the Ru precursors affected the performance of Ru catalysts with those prepared from chloride-free Ru precursors being more active than those prepared from chlorine-containing Ru precursors.

Extended study of ammonia conversion to N2 using a Ru/0.2TiZrO4 catalyst via catalytic wet air oxidation

During the (NH₄)₂SO₄ degradation, both the NO₂⁻ and NO₃⁻ generated can react with NH₄⁺ in the solution and produce N₂ as the only final product.

Electride support boosts nitrogen dissociation over ruthenium catalyst and shifts the bottleneck in ammonia synthesis

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. It is accepted that the key is the development of a high-performance catalyst that significantly enhances dissociation of the nitrogen-nitrogen triple bond, which is generally considered a rate-determining step. Here we examine kinetics of nitrogen and hydrogen isotope exchange and hydrogen adsorption/desorption reactions for a recently discovered efficient catalyst for ammonia synthesis--ruthenium-loaded 12CaO·7Al2O3 electride (Ru/C12A7:e(-))--and find that the rate controlling step of ammonia synthesis over Ru/C12A7:e(-) is not dissociation of the nitrogen-nitrogen triple bond but the subsequent formation of N-Hn species. A mechanism of ammonia synthesis involving reversible storage and release of hydrogen atoms on the Ru/C12A7:e(-) surface is proposed on the basis of observed hydrogen absorption/desorption kinetics.

Pd-supported catalysts: evolution of support porous texture along Pd deposition and alkali-metal doping

Adsorption of N2 at 77 K and scanning electron microscopy have been used to measure the changes in the support morphology, at nano- and microscale level, along the processes involved in the preparation of a supported Pd catalyst: Pd deposition, doping, and thermal treatments. Among the investigated supports, viz., activated carbons, gamma-Al2O3, SiO2, and SiO2-Al2O3 (SA), the SA one was found particularly sensitive to these processes, as a result of its high plasticity and reactivity. Involved processes can be summarized as follows: (i) During the Pd deposition, the support itself is partially dissolved and removed as a result of both the basicity of the precipitating agent and the final washing. (ii) When the undoped sample is thermally treated up to 823 K, only modest phenomena are observed. (iii) Upon doping with potassium carbonate, the support dissolution continues, and the greater the carbonate concentration, the greater the dissolution extent. In this case the dissolved material is not removed, but reprecipitates (partially outside the pores), during the subsequent drying at 393 K. (iv) When doped samples are thermally treated, the reaction between carbonate and support causes the mobilization of the support itself, with sintering phenomena that can reach the total collapse of the porous structure. The starting temperature of the pore collapse decreases with increasing potassium carbonate concentration. The modification of the support influences, directly or indirectly, the surface properties and the availability of Pd particles that can be doped or even covered by materials from support and made more or less accessible or even inaccessible by pore narrowing, widening, or blocking.

Compositional Effects in Ru, Pd, Pt, and Rh-Doped Mesoporous Tantalum Oxide Catalysts for Ammonia Synthesis

A series of early metal-promoted Ru-, Pd-, Pt-, and Rh-doped mesoporous tantalum oxide catalysts were synthesized using a variety of dopant ratios and dopant precursors, and the effects of these parameters on the catalytic activity of NH3 synthesis from H2 and N2 were explored. Previous studies on this system supported an unprecedented mechanism in which N-N cleavage occurred at the Ta sites rather than on Ru. The results of the present study showed, for all systems, that Ba is a better promoter than Cs or La and that the nitrate is a superior precursor for Ba than the isopropoxide or the hydroxide. 15N-labeling studies showed that residual nitrate functions as the major ammonia source in the first hour but that it does not account for the ammonia produced after the nitrate is completely consumed. Ru3(CO)12 proved to be a better Ru precursor than RuCl(3).3H2O, and an almost linear increase in activity with increasing Ru loading level was observed at 350 degrees C (623 K). However, at 175 degrees C (448 K), the increase in Ru had no effect on the reaction rate. Pd functioned with comparable rates to Ru, while Pt and Rh functioned far less efficiently. The surprising activities for the Pd-doped catalysts, coupled with XPS evidence for low-valent Ta in this catalyst system, support a mechanism in which cleavage of the N-N triple bond occurs on Ta rather than the precious metal because the Ea value for N-N cleavage on Pd is 2.5 times greater than that for Ru, and the 9.3 kJ mol-1 Ea value measured previously for the Ru system suggests that N-N cleavage cannot occur at the Ru surface.

Electroactive mesoporous tantalum oxide catalysts for nitrogen activation and ammonia synthesis

A new mesoporous Ta oxide catalyst for conversion of dinitrogen to ammonia shows strong evidence for a novel mechanism involving low valent Ta on the surface, supporting recent work in organometallic chemistry using low valent early transition metals for dinitrogen cleavage.

Heterogeneously catalyzed aerobic oxidative biaryl coupling of 2-naphthols and substituted phenols in water

The oxidative coupling reaction can efficiently be promoted by supported ruthenium catalyst Ru(OH)x/Al2O3. A variety of 2-naphthols and substituted phenols can be converted to the corresponding biaryl compounds in moderate to excellent yields using molecular oxygen as a sole oxidant in water without any additives. The catalysis is truly heterogeneous in nature, and Ru(OH)x/Al2O3 can easily be recovered after the reaction. The catalyst can be recycled seven times with the maintenance of the catalytic performance, and the total turnover number reaches up to 160. The results of competitive coupling reactions suggest that the present oxidative biaryl coupling reaction proceeds via the homolytic coupling of two radical species and the Ru(OH)x/Al2O3 catalyst acts as an one-electron oxidant. Two radical species are coupled to give the corresponding biaryl product, and the one-electron reduced catalyst is reoxidized by molecular oxygen. The amounts of O(2) uptake and H(2)O formation were almost one-quarter and one-half the amount of substrate consumed, respectively, supporting the reaction mechanism. The kinetic data and kinetic isotope effect show that the reoxidation of the reduced catalyst is the rate-limiting step for the coupling reaction.