Selective catalytic oxidation of ammonia to nitric oxide via chemical looping

Plasma-assisted manipulation of vanadia nanoclusters for efficient selective catalytic reduction of NOx

Supported nanoclusters (SNCs) with distinct geometric and electronic structures have garnered significant attention in the field of heterogeneous catalysis. However, their directed synthesis remains a challenge due to limited efficient approaches. This study presents a plasma-assisted treatment strategy to achieve supported metal oxide nanoclusters from a rapid transformation of monomeric dispersed metal oxides. As a case study, oligomeric vanadia-dominated surface sites were derived from the classic supported V2O5-WO3/TiO2 (VWT) catalyst and showed nearly an order of magnitude increase in turnover frequency (TOF) value via an H2-plasma treatment for selective catalytic reduction of NO with NH3. Such oligomeric surface VOx sites were not only successfully observed and firstly distinguished from WOx and TiO2 by advanced electron microscopy, but also facilitated the generation of surface amide and nitrates intermediates that enable barrier-less steps in the SCR reaction as observed by modulation excitation spectroscopy technologies and predicted DFT calculations.

New Triclinic Perovskite-Type Oxide Ba5CaFe4O12 for Low-Temperature Operated Chemical Looping Air Separation

We present the discovery of Ba5CaFe4O12, a new iron-based oxide with remarkable properties as a low-temperature driven oxygen storage material (OSM). OSMs, which exhibit selective and rapid oxygen intake and release capabilities, have attracted considerable attention in chemical looping technologies. Specifically, chemical looping air separation (CLAS) has the potential to revolutionize oxygen production as it is one of the most crucial industrial gases. However, the challenge lies in utilizing OSMs for energy-efficient CLAS at lower temperatures. Ba5CaFe4O12, a cost-competitive material, possesses an unprecedented 5-fold perovskite-type A5B5O15-δ structure, where both Fe and Ca occupy the B sites. This distinctive structure enables excellent oxygen intake/release properties below 400 °C. This oxide demonstrates the theoretical daily oxygen production rate of 2.41 mO23 kgOSM-1 at 370 °C, surpassing the performance of the previously reported material, Sr0.76Ca0.24FeO3-δ (0.81 mO23 kgOSM-1 at 550 °C). This discovery holds great potential for reducing costs and enhancing the energy efficiency in CLAS.

Simple and Low-Cost Electroactive Membranes for Ammonia Recovery

Ammonia is considered a contaminant to be removed from wastewater. However, ammonia is a valuable commodity chemical used as the primary feedstock for fertilizer manufacturing. Here we describe a simple and low-cost ammonia gas stripping membrane capable of recovering ammonia from wastewater. The material is composed of an electrically conducting porous carbon cloth coupled to a porous hydrophobic polypropylene support, that together form an electrically conductive membrane (ECM). When a cathodic potential is applied to the ECM surface, hydroxide ions are produced at the water-ECM interface, which transforms ammonium ions into higher-volatility ammonia that is stripped across the hydrophobic membrane material using an acid-stripping solution. The simple structure, low cost, and easy fabrication process make the ECM an attractive material for ammonia recovery from dilute aqueous streams, such as wastewater. When paired with an anode and immersed into a reactor containing synthetic wastewater (with an acid-stripping solution providing the driving force for ammonia transport), the ECM achieved an ammonia flux of 141.3 ± 14.0 g.cm^-2.day^-1 at a current density of 6.25 mA.cm^-2 (69.2 ± 5.3 kg(NH₃-N)/kWh). It was found that the ammonia flux was sensitive to the current density and acid circulation rate.

In Situ Spectroscopic Studies of NH3 Oxidation of Fe-Oxide/Al2O3

Simple temperature-regulated chemical vapor deposition was used to disperse iron oxide nanoparticles on porous Al₂O₃ to create an Fe-oxide/Al₂O₃ structure for catalytic NH₃ oxidation. The Fe-oxide/Al₂O₃ achieved nearly 100% removal of NH₃, with N₂ as a major reaction product at temperatures above 400 °C and negligible NO_x emissions at all experimental temperatures. The results of a combination of in situ diffuse reflectance infrared Fourier-transform spectroscopy and near-ambient pressure-near-edge X-ray absorption fine structure spectroscopy suggest a N₂H₄-mediated oxidation mechanism of NH₃ to N₂ via the Mars-van Krevelen pathway on the Fe-oxide/Al₂O₃ surface. As a catalytic adsorbent-an energy-efficient approach to reducing NH₃ levels in living environments via adsorption and thermal treatment of NH₃-no harmful NO_x emissions were produced during the thermal treatment of the NH₃-adsorbed Fe-oxide/Al₂O₃ surface, while NH₃ molecularly desorbed from the surface. A system with dual catalytic filters of Fe-oxide/Al₂O₃ was designed to fully oxidize this desorbed NH₃ to N₂ in a clean and energy-efficient manner.

Designing Reactive Bridging O2- at the Atomic Cu-O-Fe Site for Selective NH3 Oxidation

Surface oxidation chemistry involves the formation and breaking of metal-oxygen (M-O) bonds. Ideally, the M-O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O^2- species within the atomic Cu-O-Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O^2- has a lower antibonding orbital energy and thus weaker Cu-O/Fe-O strength. In selective NH₃ oxidation, the weak Cu-O/Fe-O bond enables fast Cu redox for NH₃ conversion and direct NO adsorption via Cu-O-NO to promote N-N coupling toward N₂. As a result, 99% N₂ selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O^2- in catalysis.

Nitric Oxide Decomposition via Selective Catalytic Reduction by Ammonia on a Transition-Metal Cluster of W2TcO6

Decomposition of nitric oxide (NO) gas on a reactive transition-metal cluster of W₂TcO₆ has been examined and investigated via selective catalytic reduction by ammonia (NH₃-SCR) using the M06-L density functional method. The transition-metal cluster of W₂TcO₆ can be employed to transform NO to N₂ gas efficiently over an active site of tungsten (W). A reaction mechanism of NO conversion based on the NH₃-SCR process has been elucidated by a potential energy surface along the reaction pathways. The reaction pathways of this NH₃-SCR process begin with adsorption of NH₃, adsorption of NO to the cluster, formation of nitrosamine (NH₂NO) and NHNO/NHNOH intermediates, and rearrangement of NHNO/NHNOH to obtain N₂ and H₂O, respectively. Notably, a significant NH₂NO as a key intermediate, namely, "nitrosamine", must be formed before further steps can take place in the generation of N₂ from NO, followed by the involvement of the NHNO or NHNOH intermediate. From our calculated results, the NHNO intermediate via TS3a is found in pathway a, while NHNOH is found in pathway b via TS3b. Pathway b has a lower energy barrier of 35.1 kcal/mol than pathway a with an energy barrier of 41.8 kcal/mol, indicating that pathway b should be more energetically favorable. The step for NHNO intermediate rearrangement is a rate-determining step for the reaction occurring through pathway a, which is found to be more difficult in accordance with a difficult N-H bond cleavage to form the NNOH intermediate before N₂ formation. The overall reaction is an exothermic process with thermodynamic and kinetic favors. Thus, this bimetallic W₂TcO₆ cluster could be used as a promising and active catalyst for NO decomposition via the NH₃-SCR process to an eco-friendly gas, that is, N₂.