Rate-determining Step Of Ammonia Synthesis Research Articles

Barium Oxide Encapsulating Cobalt Nanoparticles Supported on Magnesium Oxide: Active Non-Noble Metal Catalysts for Ammonia Synthesis under Mild Reaction Conditions

To realize a carbon-free society, catalysts are needed for the synthesis of ammonia under mild reaction conditions (<400 °C, <10 MPa) that use hydrogen produced from renewable energy. Ru-based catalysts are currently the most promising candidates; however, Ru is expensive and of low abundance. Here, we discovered that the encapsulation of Co nanoparticles with BaO enhanced the ammonia synthesis activity of Co and that a simple Ba-doped Co/MgO catalyst prereduced at an unusually high temperature of 700 °C (Co@BaO/MgO-700red) showed outstanding ammonia synthesis activity. The ammonia synthesis rate (24.6 mmol gcat–1 h–1) and turnover frequency (0.246 s–1) of the catalyst at 350 °C and 1.0 MPa were about 80 and 250 times higher, respectively, than those of the nondoped parent catalyst. At the same temperature but higher pressure (3.0 MPa), the ammonia synthesis rate was increased to 48.4 mmol gcat–1 h–1, which is higher than that of active Ru-based catalysts. In addition, at 1.0 MPa, our catalyst produced ammonia even at temperatures as low as 150 °C. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy investigations revealed that after reduction at 700 °C, the Co nanoparticles had become encapsulated by a nanofraction of BaO. The mechanism underlying the formation of this unique structure was considered to comprise reduction of oxidic Co to metallic Co, decomposition of BaCO3 to BaO, and migration of BaO and Co nanoparticles. Spectroscopic and density functional theory investigations revealed that adsorption of N2 on the Co atoms at the catalyst surface weakened the N2 triple bond to the strength of a double bond due to electron donation from Ba2+ of BaO via adjacent Co atoms; this weakening accelerated the cleavage of the triple bond, which is the rate-determining step for ammonia synthesis.

Altering the rate-determining step over cobalt single clusters leading to highly efficient ammonia synthesis.

Activation of high-energy triple-bonds of N2 is the most significant bottleneck of ammonia synthesis under ambient conditions. Here, by importing cobalt single clusters as strong electron-donating promoter into the catalyst, the rate-determining step of ammonia synthesis is altered to the subsequent proton addition so that the barrier of N2 dissociation can be successfully overcome. As revealed by density functional theory calculations, the N2 dissociation becomes exothermic over the cobalt single cluster upon the strong electron backdonation from metal to the N2 antibonding orbitals. The energy barrier of the positively shifted rate-determining step is also greatly reduced. At the same time, advanced sampling molecular dynamics simulations indicate a barrier-less process of the N2 approaching the active sites that greatly facilitates the mass transfer. With suitable thermodynamic and dynamic property, a high ammonia yield rate of 76.2 μg h–1 mgn}{}^{-1 }_{rm cat.} and superior Faradaic efficiency of 52.9% were simultaneously achieved.

Low-Temperature Synthesis of Perovskite Oxynitride-Hydrides as Ammonia Synthesis Catalysts.

Mixed anionic materials such as oxyhydrides and oxynitrides have recently attracted significant attention due to their unique properties, such as fast hydride ion conduction, enhanced ferroelectrics, and catalytic activity. However, high temperature (≥800 °C) and/or complicated processes are required for the synthesis of these compounds. Here we report that a novel perovskite oxynitride-hydride, BaCeO3-xNyHz, can be directly synthesized by the reaction of CeO2 with Ba(NH2)2 at low temperatures (300-600 °C). BaCeO3-xNyHz, with and without transition metal nanoparticles, functions as an efficient catalyst for ammonia synthesis through the lattice N3- and H- ion-mediated Mars-van Krevelen mechanism, while ammonia synthesis occurs over conventional catalysts through a Langmuir-Hinshelwood mechanism with high energy barriers (85-121 kJ mol-1). As a consequence, the unique reaction mechanism leads to enhancement of the activity of BaCeO3-based catalysts by a factor of 8-218 and lowers the activation energy (46-62 kJ mol-1) for ammonia synthesis. Furthermore, isotopic experiments reveal that this catalyst shifts the rate-determining step for ammonia synthesis from N2 dissociation to N-H bond formation.

La2Ce2O7 supported ruthenium as a robust catalyst for ammonia synthesis

La2Ce2O7 nanoparticles were prepared by citric acid complexation method followed by calcination at varied temperatures. Then, supported with 4 wt% Ru, they were evaluated as the catalysts for ammonia synthesis under conditions similar with industry. With La2Ce2O7 being calcinated at 700 or 800 °C, the experimental results indicate that the Ru/La2Ce2O7 catalyst exhibits much higher ammonia concentration or ammonia synthesis rate than that of Ru/CeO2 and Ru/La2O3. In addition, Ru/La2Ce2O7 possesses high stability under over-heating test. In the absence of any promotor, ammonia concentration of Ru/La2Ce2O7 catalyst approaches 14% at 450 °C, GHSV of 10000 h−1 and pressure of 10 MPa. The rate-determining step of ammonia synthesis, dissociation of N2 is significantly facilitated by the strong metal–support interaction (SMSI) between Ru and La2Ce2O7. Due to the interaction, La2Ce2O7 tends to donate electrons to Ru, resulting in the high electron density over the surface of Ru active sites which is favorable for the dissociation of N2. Consequently, high activity is achieved.

Kinetic evidence: the rate-determining step for ammonia synthesis over electride-supported Ru catalysts is no longer the nitrogen dissociation step

The rate-determining step for ammonia synthesis over Ru catalysts supported by electrides, such as [Ca24Al28O64]4+(e−)4 and Ca2N:e−, is suggested to be the surface reactions of N and H adatoms, in which case the Langmuir–Hinshelwood model should be used to describe the kinetics.

A low-crystalline ruthenium nano-layer supported on praseodymium oxide as an active catalyst for ammonia synthesis.

Ammonia is a crucial chemical feedstock for fertilizer production and is a potential energy carrier. However, the current method of synthesizing ammonia, the Haber-Bosch process, consumes a great deal of energy. To reduce energy consumption, a process and a substance that can catalyze ammonia synthesis under mild conditions (low temperature and low pressure) are strongly needed. Here we show that Ru/Pr2O3 without any dopant catalyzes ammonia synthesis under mild conditions at 1.8 times the rates reported with other highly active catalysts. Scanning transmission electron micrograph observations and energy dispersive X-ray analyses revealed the formation of low-crystalline nano-layers of ruthenium on the surface of Pr2O3. Furthermore, CO2 temperature-programmed desorption revealed that the catalyst was strongly basic. These unique structural and electronic characteristics are considered to synergistically accelerate the rate-determining step of NH3 synthesis, cleavage of the N[triple bond, length as m-dash]N bond. We expect that the use of this catalyst will be a starting point for achieving efficient ammonia synthesis.

Synthesis and Characterization of Ru Cubic Nanocages with a Face-Centered Cubic Structure by Templating with Pd Nanocubes.

Nanocages have received considerable attention in recent years for catalytic applications owing to their high utilization efficiency of atoms and well-defined facets. Here we report, for the first time, the synthesis of Ru cubic nanocages with ultrathin walls, in which the atoms are crystallized in a face-centered cubic (fcc) rather than hexagonal close-packed (hcp) structure. The key to the success of this synthesis is to ensure layer-by-layer deposition of Ru atoms on the surface of Pd cubic seeds by controlling the reaction temperature and the injection rate of a Ru(III) precursor. By selectively etching away the Pd from the Pd@Ru core-shell nanocubes, we obtain Ru nanocages with an average wall thickness of 1.1 nm or about six atomic layers. Most importantly, the Ru nanocages adopt an fcc crystal structure rather than the hcp structure observed in bulk Ru. The synthesis has been successfully applied to Pd cubic seeds with different edge lengths in the range of 6-18 nm, with smaller seeds being more favorable for the formation of Ru shells with a flat, smooth surface due to shorter distance for the surface diffusion of the Ru adatoms. Self-consistent density functional theory calculations indicate that these unique fcc-structured Ru nanocages might possess promising catalytic properties for ammonia synthesis compared to hcp Ru(0001), on the basis of strengthened binding of atomic N and substantially reduced activation energies for N2 dissociation, which is the rate-determining step for ammonia synthesis on hcp Ru catalysts.

Probing the Electronic Effect of Carbon Nanotubes in Catalysis: NH3 Synthesis with Ru Nanoparticles

Carbon nanotubes (CNTs) have been shown to modify some properties of nanomaterials and to modify chemical reactions confined inside their channels, which are formed by curved graphene layers. Here we studied ammonia synthesis over Ru as a probe reaction to understand the effect of the electron structure of CNTs on the confined metal particles and their catalytic activity. The catalyst with Ru nanoparticles dispersed almost exclusively on the exterior nanotube surface exhibits a higher activity than the CNT-confined Ru, although both have a similar metal particle size. Characterization with TEM, N(2) physisorption, H(2) chemisorption, temperature-programmed reduction, CO adsorption microcalorimetry, and first-principles calculations suggests that the outside Ru exhibits a higher electron density than the inside Ru. As a result, the dissociative adsorption of N(2), which is an electrophilic process and the rate-determining step of ammonia synthesis, is more facile over the outside Ru than that over the inside one.

The Kinetics of Ammonia Synthesis over Ru-Based Catalysts: 1. The Dissociative Chemisorption and Associative Desorption of N2

The dissociative chemisorption of N2is generally accepted to be the rate-determining step of ammonia synthesis over Ru-based catalysts. The interaction of N2with the following three Ru catalysts has been studied: Ru supported on Al2O3(Ru/Al2O3) and on MgO (Ru/MgO), and Ru/MgO promoted with cesium (Cs–Ru/MgO). Temperature-programmed N2adsorption and desorption experiments and the isotopic exchange reaction14N14N+15N15N⇌214N15N were performed in a microreactor flow system. A microkinetic analysis based on the Langmuir–Hinshelwood Hougen–Watson mechanism has been applied to these kinetic experiments yielding the rate constants of dissociative chemisorption (kads) and associative desorption (kdes). The dissociation of N2was indeed found to be a slow and activated process. Ru/Al2O3was found to be rather inactive for N2dissociation. Ru/MgO turned out to be a heterogeneous system with respect to the interaction with N2due to the presence of promoted active sites which dominate the rate of N2dissociation. Promotion by cesium was observed to enhance bothkadsandkdessignificantly and rendered the Ru metal surfaces uniform toward the interaction with N2. The initial sticking coefficient and the rate of desorption of N2derived from the microkinetic models are in good agreement with results obtained with Ru single crystal surfaces under ultra-high vacuum conditions.

Activation of nitrogen by alkali metal-promoted transition metal: VI. Hydrogen effect on isotopic equilibration of nitrogen and rate-determining step of ammonia synthesis on potassium-promoted ruthenium catalysts

The effect of hydrogen on isotopic equilibration of nitrogen was studied on ruthenium catalysts with and without potassium at P N 2 = 40–150 mm Hg, P H 2 = 7–180 mm Hg and 300–450 °C. The equilibration rate is increased on pure Ru but is decreased on RuK by the addition of hydrogen. The rate of ammonia synthesis is correspondingly faster than equilibration on pure Ru but is slower on RuK. The equilibration rate on RuK is decreased by condensing the ammonia produced in the presence of hydrogen and by increasing the H 2 N 2 ratio and approaches negligible value at H 2 N 2 = 3 , demonstrating that the rate-determining step of ammonia synthesis is the dissociation of nitrogen molecule. The equilibration rate on RuK in the presence of ammonia also decreases as hydrogen pressure increases, whereas the reaction order with respect to nitrogen pressure is higher than that in the absence of hydrogen, suggesting that the adsorbed hydrogen exhibits a retarding effect on the adsorption of nitrogen. The surface area of Ru-K was found to increase on hydrogen treatment of nitrogen covered surface, suggesting a corrosive adsorption of nitrogen to form a nitride on RuK.