Embedded Ru@ZrO2 Catalysts for H2 Production by Ammonia Decomposition

A Scenario for a Carbon-Neutral Ammonia-Fueled Engine Mediated by Catalytic NH3 Cracking and CO2 Hydrogenation

Utilizing near zero-carbon NH3 as fuel in engines is promising for carbon-neutrality. However, the application of NH3 into the engine suffers from the intrinsic poor combustion characteristics of NH3 and the emission of harmful NOx exhausts. Herein, we proposed and successfully confirmed a novel scenario for converting a conventional "CH4-fueled" engine to "NH3-fueled" engine. Specifically, CH4 was used to power the internal combustion engine and release CO2 as the exhaust. Afterwards, we put forward two routes to convert the exhaust and NH3 into N2 and CH4 for enclosing the carbon cycle. The first "spatially decoupled" route splits the exhaust treatment into NH3 cracking over Ru clusters on the calcined Mg-Al hydrotalcite (Ru/MAO) and CO2 methanation over a commercial Ni/Al2O3. Both NH3 and CO2 were almost completely converted into the target products under their respective optimal conditions. The second "spatially coupled" route refers to an one-pot reaction of NH3 and CO2 into N2, CH4, and H2O. Due to the mismatch of reaction conditions and the competitive adsorption of reactants, the conversions of NH3 and CO2 were lowered to 80.1 % and 49.3 %, respectively, over Ru/MAO under 1 bar (CO2:NH3=3 : 8) at 550 °C.

A General Approach for Metal Nanoparticle Encapsulation Within Porous Oxides

Encapsulation of metal nanoparticles within oxide materials has been shown as an effective strategy to improve activity, selectivity, and stability in several catalytic applications. Several approaches have been proposed to encapsulate nanoparticles, such as forming core-shell structures, growing ordered structures (zeolites or metal-organic frameworks) on nanoparticles, or directly depositing support materials on nanoparticles. Here, a general nanocasting method is demonstrated that can produce diverse encapsulated metal@oxide structures with different compositions (Pt, Pd, Rh) and multiple types of oxides (Al2O3, Al2O3-CeO2, ZrO2, ZnZrOx, In2O3, Mn2O3, TiO2) while controlling the size and dispersion of nanoparticles and the porous structure of the oxide. Metal@polymer structures are first prepared, and then the oxide precursor is infiltrated into such structures and the resulting material is calcined to form the metal@oxide structures. Most Pt@oxides catalysts show similar catalytic activity, demonstrating the availability of surface Pt sites in the encapsulated structures. However, the Pt@Mn2O3 sample showed much higher CO oxidation activity, while also being stable under aging conditions. This work demonstrated a robust nanocasting method to synthesize metal@oxide structures, which can be utilized in catalysis to finely tune metal-oxide interfaces.

Emerging trends in research and development on earth abundant materials for ammonia degradation coupled with H2 generation

Ammonia, as an essential and economical fuel, is a key intermediate for the production of innumerable nitrogen-based compounds. Such compounds have found vast applications in the agricultural world, biological world (amino acids, proteins, and DNA), and various other chemical transformations. However, unlike other compounds, the decomposition of ammonia is widely recognized as an important step towards a safe and sustainable environment. Ammonia has been popularly recommended as a viable candidate for chemical storage because of its high hydrogen content. Although ruthenium (Ru) is considered an excellent catalyst for ammonia oxidation; however, its high cost and low abundance demand the utilization of cheaper, robust, and earth abundant catalyst. The present review article underlines the various ammonia decomposition methods with emphasis on the use of non-noble metals, such as iron, nickel, cobalt, molybdenum, and several other carbides as well as nitride species. In this review, we have highlighted various advances in ammonia decomposition catalysts. The major challenges that persist in designing such catalysts and the future developments in the production of efficient materials for ammonia decomposition are also discussed.

Catalytic ammonia reforming: alternative routes to net-zero-carbon hydrogen and fuel

Ammonia is an energy-dense liquid hydrogen carrier and fuel whose accessible dissociation chemistries offer promising alternatives to hydrogen electrolysis, compression and dispensing at scale. Catalytic ammonia reforming has thus emerged as an area of renewed focus within the ammonia and hydrogen energy research & development communities. However, a majority of studies emphasize the discovery of new catalytic materials and their evaluation under idealized laboratory conditions. This Perspective highlights recent advances in ammonia reforming catalysts and their demonstrations in realistic application scenarios. Key knowledge gaps and technical needs for real reformer devices are emphasized and presented alongside enabling catalyst and reaction engineering fundamentals to spur future investigations into catalytic ammonia reforming.

Ammonia Decomposition over Ru/SiO2 Catalysts

Ammonia decomposition is a key step in hydrogen production and is considered a promising practical intercontinental hydrogen carrier. In this study, 1 wt.% Ru/SiO2 catalysts were prepared via wet impregnation and subjected to calcination in air at different temperatures to control the particle size of Ru. Furthermore, silica supports with different surface areas were prepared after calcination at different temperatures and utilized to support a change in the Ru particle size distribution of Ru/SiO2. N2 physisorption and transmission electron microscopy were used to probe the textural properties and Ru particle size distribution of the catalysts, respectively. These results show that the Ru/SiO2 catalyst with a high-surface area achieved the highest ammonia conversion among catalysts at 400 °C. Notably, this is closely related to the Ru particle sizes ranging between 5 and 6 nm, which supports the notion that ammonia decomposition is a structure-sensitive reaction.

Silica-confined Ru highly dispersed on ZrO2 with enhanced activity and thermal stability in dichloroethane combustion

An efficient strategy (spontaneous deposition to enhance noble metal dispersity and core-shell confinement to inhibit noble metal sintering) is presented to synthesize highly active and thermally stable Ru/ZrO2@SiO2 catalysts for dichloroethane combustion.

Organic-Inorganic Hybrid Materials for Room Temperature Light-Activated Sub-ppm NO Detection

Nitric oxide (NO) is one of the main environmental pollutants and one of the biomarkers noninvasive diagnosis of respiratory diseases. Organic-inorganic hybrids based on heterocyclic Ru (II) complex and nanocrystalline semiconductor oxides SnO2 and In2O3 were studied as sensitive materials for NO detection at room temperature under periodic blue light (λmax = 470 nm) illumination. The semiconductor matrixes were obtained by chemical precipitation with subsequent thermal annealing and characterized by XRD, Raman spectroscopy, and single-point BET methods. The heterocyclic Ru (II) complex was synthesized for the first time and characterized by 1H NMR, 13C NMR, MALDI-TOF mass spectrometry and elemental analysis. The HOMO and LUMO energies of the Ru (II) complex are calculated from cyclic voltammetry data. The thermal stability of hybrids was investigated by thermogravimetric analysis (TGA)-MS analysis. The optical properties of Ru (II) complex, nanocrystalline oxides and hybrids were studied by UV-Vis spectroscopy in transmission and diffuse reflectance modes. DRIFT spectroscopy was performed to investigate the interaction between NO and the surface of the synthesized materials. Sensor measurements demonstrate that hybrid materials are able to detect NO at room temperature in the concentration range of 0.25-4.0 ppm with the detection limit of 69-88 ppb.

A tri-functionalised PtSnOx-based electrocatalyst for hydrogen generation via ammonia decomposition under native pH conditions

Hydrogen is one of the most clean energy carriers because water is only the product of its combustion. The electrolysis of ammonia is expected to offer an attractive alternative to water electrolysis for the production of hydrogen because of the lower thermodynamic energy. However, the synthesis and utilization of high-performance Pt electrocatalysts have encountered challenges related to instability and hydroxyl ion sensitivity. To address these issues, we developed PtSnO_x-based nanoparticles that maintained high electrocatalytic activity and stability for the decomposition of aqueous ammonia to generate hydrogen under native pH conditions which means the acidity/alkalinity is not adjusted. FT-IR, XRD, and XPS evidence showed PtSnO_x was a tri-functionalised electrocatalyst. That is to say, the spherical SnO_x nanoparticles assisted ammonia adsorption and activation, which were accompanied by a hydrogen adsorption on PtSnO_x and hydrogen transfer along the SnOH bond over the electrocatalyst. According to these data of FT-IR, XRD, and XPS before and after reaction, a possible mechanism for the decomposition of aqueous ammonia to produce hydrogen was proposed. This study could pave the way to prospective routes for the selective oxidation of the NH bond to generate hydrogen under mild conditions.

Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic Cycle by Using a Simple Manganese Complex

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar₃ N][X] initiated a nitride coupling reaction to N₂ , a major step toward catalytic ammonia oxidation (salen=N,N'-bis(salicylidene)-ethylenediamine dianion; Ar=p-bromophenyl; X=[SbCl₆ ]^- or [B(C₆ F₅ )₄ ]^- ). N₂ production was confirmed by mass spectral analysis of the isotopomer, 1-¹⁵ N, and the gas quantified. The metal products of oxidation were the reduced Mn^III dimers, [(salen)MnCl]₂ (2) or [(salen)Mn(OEt₂ )]₂ [B(C₆ F₅ )₄ ]₂ (3) for X=[SbCl₆ ]^- or [B(C₆ F₅ )₄ ]^- , respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed-valent Mn^V /Mn^III bridging nitride, [(salen)Mn^V (μ-N)Mn^III (salen)][B(C₆ F₅ )₄ ] (4), was isolated, and its oxidation-state assignment was confirmed by X-ray diffraction (XRD) studies, perpendicular and parallel-mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re-oxidized to 1 in the presence of NH₃ and NaOCl closing a pseudo-catalytic "synthetic" cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH₃ oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.

Nanostructured materials for applications in heterogeneous catalysis

In this review, a brief survey is offered on the main nanotechnology synthetic approaches available to heterogeneous catalysis, and a few examples are provided of their usefulness for such applications. We start by discussing the use of colloidal, reverse micelle, and dendrimer chemistry in the production of active metal and metal oxide nanoparticles with well-defined sizes, shapes, and compositions, as a way to control the surface atomic ensembles available for selective catalysis. Next we introduce the use of sol-gel and atomic layer deposition chemistry for the production and modification of high-surface-area supports and active phases. Reference is then made to the more complex active sites that can be created or carved on such supports by using organic structure-directing agents. We follow with an examination of the ability to achieve multiple functionality in catalysis via the design of dumbbells, core@shell, and other complex nanostructures. Finally, we consider the mixed molecular-nanostructure approach that can be used to develop more demanding catalytic sites, by derivatizing the surface of solids or tethering or immobilizing homogeneous catalysts or other chemical functionalities. We conclude with a personal and critical perspective on the importance of fully exploiting the synergies between nanotechnology and surface science to optimize the search for new catalysts and catalytic processes.

Structural rearrangements of Ru nanoparticles supported on carbon nanotubes under microwave irradiation

The structure evolution of twinned Ru nanoparticles supported on carbon nanotubes rearranging into Ru single nanocrystals under the microwave irradiation and the exposed surface of Ru single crystals were observed, which provided new insights into synthesis and application of metal nanoparticle catalysts.