Regulating oxygen vacancies by Zn atom doping to anchor and disperse promoter Ba on MgO support to improve Ru-based catalysts activity for ammonia synthesis

In heterogeneous catalysis, surface defects are widely regarded as an effective means to enhance the catalytic performance of catalysts. In this study, the oxygen vacancy-rich Mg(1-X)ZnXO solid solution support was successfully prepared by doping a small amount of Zn into MgO nanocrystals. Based on this support, Ru/Ba-Mg(1-X)ZnXO catalyst for ammonia synthesis was prepared. Characterization using TEM, EPR, XPS, and DFT calculations confirmed the successful substitution of Zn atoms for Mg atoms leading to the formation of more oxygen vacancies (OVs). N2-TPD, SEM and TEM analyses revealed that a small amount of Zn had minimal influence on the surface morphology and the size of Ru nanoparticles. The abundance of OVs in the support was identified as the primary factor enhancing the catalytic activity. XPS, H2-TPD and kinetics experiment studies further elucidated the mechanism by which OVs promote the reaction, with OVs serving as an anchor point for the promoter Ba on the MgO support and promoted the dispersion of Ba. This anchoring effect not only enhanced the electron density on Ru, favoring the dissociation of the N[triple bond, length as m-dash]N bond, but also mitigated hydrogen poisoning. As a result,the ammonia synthesis rate reached 1.73 mmol g-1 h-1. Furthermore, the CO2-TPD and H2-TPR analyses indicated that Zn doping effectively promotes the metal-support interaction (MSI) and surface alkalinity. The findings of this study offers valuable insights for the design of defective modified catalyst supports.

Synergistic effect of coordinating interface and promoter for enhancing ammonia synthesis activity of Ru@N-C catalyst

Triruthenium dodecacarbonyl (Ru3(CO)12) was applied to prepare the Ru-based ammonia synthesis catalysts. The catalyst obtained from this precursor exhibited higher activity than the other Ru salts owing to its unique atomic reorganization under mild temperatures. Herein, Ru3(CO)12 as a guest metal source incorporated into the pore of ZIF-8 formed the Ru@N-C catalysts. The results indicated that the Ru nanoparticle (1.7 nm) was dispersed in the confined N coordination environment, which can increase the electron density of the Ru nanoparticles to promote N[triple bond, length as m-dash]N bond cleavage. The promoters donate the basic sites for transferring the electrons to Ru nanoparticles, further enhancing ammonia synthesis activity. Ammonia synthesis investigations revealed that the obtained Ru@N-C catalysts exhibited obvious catalytic activity compared with the Ru/AC catalyst. After introducing the Ba promoter, the 2Ba-Ru@N-C(450) catalyst exhibited the highest ammonia synthesis activity among the catalysts. At 360 °C and 1 MPa, the activity of the 2Ba-Ru@N-C(450) is 16 817.3 µmol h-1 gRu-1, which is 1.1, 1.6, and 2 times higher than those of 2Cs-Ru@N-C(450) (14 925.4 µmol h-1 gRu-1), 2K-Ru@N-C(450) (10 736.7 µmol h-1 gRu-1), and Ru@N-C(450) (8604.2 µmol h-1 gRu-1), respectively. A series of characterizations were carried out to explore the 2Ba-Ru@N-C(450) catalysts, such as H2-TPR, XPS, and NH3-TPD. These results suggest that the Ba promoter played the role of an electronic and structural promoter; moreover, it can promote the NH3 desorption from the Ru nanoparticles.

A spin promotion effect in catalytic ammonia synthesis

The need for efficient ammonia synthesis is as urgent as ever. Over the past two decades, many attempts to find new catalysts for ammonia synthesis at mild conditions have been reported and, in particular, many new promoters of the catalytic rate have been introduced beyond the traditional K and Cs oxides. Herein, we provide an overview of recent experimental results for non-traditional promoters and develop a comprehensive model to explain how they work. The model has two components. First, we establish what is the most likely structure of the active sites in the presence of the different promoters. We then show that there are two effects dictating the catalytic activity. One is an electrostatic interaction between the adsorbed promoter and the N-N dissociation transition state. In addition, we identify a new promoter effect for magnetic catalysts giving rise to an anomalously large lowering of the activation energy opening the possibility of finding new ammonia synthesis catalysts.

A minireview on catalysts for photocatalytic N2 fixation to synthesize ammonia

Ammonia (NH3) is an important feedstock in chemical industry. Nowadays NH3 is mainly produced via the industrialized Haber-Bosch process, which requires substantial energy input, since it operates at high temperatures (400-650 °C) and high pressures (20-40 Mpa). From the energy conservation point of view, it is of great significance to explore an alternative avenue to synthesize NH3, which is in line with the concept of sustainable development. Very recently, photocatalytic N2 fixation (PNF) has been discovered as a safe and green approach to synthesize NH3, as it utilizes the inexhaustible solar energy and the abundant N2 in nature to synthesize NH3 under mild conditions. A highly efficient catalyst is the core of PNF. Up to now, extensive studies have been conducted to design efficient catalysts for PNF. Summarizing the catalysts reported for PNF and unraveling their reaction mechanisms could provide guidance for the design of better catalysts. In this review, we will illustrate the development of catalysts for PNF, including semiconductors, plasmonic metal-based catalysts, iron-based catalysts, ruthenium-based catalysts and several other catalysts, point out the remaining challenges and outline the future opportunities, with the aim to contribute to the development of PNF.

Applications of rare earth oxides in catalytic ammonia synthesis and decomposition

Due to their unique structural and electronic properties, rare earth oxides have been widely applied as supports and promoters in catalytic ammonia synthesis and decomposition.

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study

In situ studies on the physical and chemical properties of Au in inverse ceria alumina supported catalysts have been conducted between 295 and 623 K using high energy resolved fluorescence detection X-ray absorption near edge spectroscopy and X-ray total scattering. Precise structural information is extracted on the metallic Au phase present in a 0.85 wt% Au containing inverse ceria alumina catalyst (ceria/Au/alumina). Herein evidence for the formation of an Au hydride species at elevated temperature is presented. Through modelling of total scattering data to extract the thermal properties of Au using Grüneisen theory of volumetric thermal expansion it proposed that the Au Hydride formation occurs synergistally with the formation of a cerium oxyhydride. The temperature reversible nature, whilst remaining in a reducing atmosphere, demonstrates the activation of hydrogen without consumption of oxygen from the supporting ceria lattice.

Effect of nitrogen co-doping with ruthenium on the catalytic performance of Ba/Ru–N-MC catalysts for ammonia synthesis

A highly active and stable mesoporous Ba/Ru–N-MC catalyst for ammonia synthesis was prepared by an in situ thermal carbonization method with nitrogen co-doping with ruthenium NPs.

Efficient ammonia synthesis over a Ru/La0.5Ce0.5O1.75 catalyst pre-reduced at high temperature

A Ru/La_0.5Ce_0.5O_1.75 catalyst pre-reduced at an unusually high temperature (650 °C) catalyses ammonia synthesis at a high rate under mild conditions.

Ammonia is an important feedstock for producing fertiliser and is also a potential energy carrier. However, the process currently used for ammonia synthesis, the Haber–Bosch process, consumes a huge amount of energy; therefore the development of new catalysts for synthesising ammonia at a high rate under mild conditions (low temperature and low pressure) is necessary. Here, we show that Ru/La_0.5Ce_0.5O_1.75 pre-reduced at an unusually high temperature (650 °C) catalysed ammonia synthesis at extremely high rates under mild conditions; specifically, at a reaction temperature of 350 °C, the rates were 13.4, 31.3, and 44.4 mmol g^–1 h^–1 at 0.1, 1.0, and 3.0 MPa, respectively. Kinetic analysis revealed that this catalyst is free of hydrogen poisoning under the conditions tested. Electron energy loss spectroscopy combined with O₂ absorption capacity measurements revealed that the reduced catalyst consisted of fine Ru particles (mean diameter < 2.0 nm) that were partially covered with partially reduced La_0.5Ce_0.5O_1.75 and were dispersed on a thermostable support. Furthermore, Fourier transform infrared spectra measured after N₂ addition to the catalyst revealed that N₂ adsorption on Ru atoms that interacted directly with the reduced La_0.5Ce_0.5O_1.75 weakened the N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bond and thus promoted its cleavage, which is the rate-determining step for ammonia synthesis. Our results indicate that high-temperature pre-reduction of this catalyst, which consists of Ru supported on a thermostable composite oxide with a cubic fluorite structure and containing reducible cerium, resulted in the formation of many sites that were highly active for N₂ reduction by hydrogen.

The ethanol mediated-CeO2-supported low loading ruthenium catalysts for the catalytic wet air oxidation of butyric acid

The Ru/CeO₂-A catalyst shows the higher activity for CWAO of butyric acid because of the adding of absolute ethanol.

New insights into the support morphology-dependent ammonia synthesis activity of Ru/CeO2catalysts

Different morphologies ceria (nanocubes, nanorods and nanoparticles) were synthesized and exhibited significant support-morphology-dependent catalytic activity towards ammonia synthesis.