Synthesis and characterization of highly dispersed bimetallic Au-Rh nanoparticles supported on titanate nanotubes for CO oxidation reaction at low temperature

Low-temperature CO oxidation was carried out by using rhodium incorporated into titanate nanotubes (Rh/NTs) prepared by the sol-gel and hydrothermal methods; otherwise, gold nanoparticles were deposited homogeneously onto the Rh/NT surface through the deposition-precipitation with urea (DPU) method. The Au-Rh/NT sample exhibited high metal dispersion (55%), outstanding CO oxidation at low temperature, and better resistance to deactivation than the monometallic Rh/NT and Au/NT samples. The characterization of bimetallic samples, with particle sizes from 1 to 3 nm, revealed the remarkable presence of interacting Au and Rh species in metallic state. In this way, Au⁰ and Rh⁰ were answerable for the higher catalytic activity observed in the bimetallic samples. The interaction between Au and Rh in the nanoparticles of Au-Rh/NT promoted a synergistic effect on the CO oxidation reaction, explained by the creation of new CO adsorption sites.

High-Pressure Scanning Tunneling Microscopy

This is a Review of recent studies on surface structures of crystalline materials in the presence of gases in the mTorr to atmospheric pressure range, which brings surface science into a brand new direction. Surface structure is not only a property of the material but also depends on the environment surrounding it. This Review emphasizes that high/ambient pressure goes hand-in-hand with ambient temperature, because weakly interacting species can be densely covering surfaces at room temperature only when in equilibrium with a sufficiently high gas pressure. At the same time, ambient temperatures help overcome activation barriers that impede diffusion and reactions. Even species with weak binding energy can have residence lifetimes on the surface that allow them to trigger reconstructions of the atomic structure. The consequences of this are far from trivial because under ambient conditions the structure of the surface dynamically adapts to its environment and as a result completely new structures are often formed. This new era of surface science emerged and spread rapidly after the retooling of characterization techniques that happened in the last two decades. This Review is focused on the new surface structures enabled particularly by one of the new tools: high-pressure scanning tunneling microscopy. We will cover several important surfaces that have been intensely scrutinized, including transition metals, oxides, and alloys.

Palladium supported on mixed-metal-organic framework (Co-Mn-MOF-74) for efficient catalytic oxidation of CO

Successful monometallic and bimetallic metal-organic frameworks with different Co/Mn ratios have been synthesized under solvothermal conditions. The as-synthesized MOFs followed by deposition of Pd nanoparticles with 0.5 to 7 wt%. The XRD, BET, SEM, TEM, EDAX and FT-IR characterization results reveal that bimetallic MOFs and Pd nanoparticles were finely dispersed on the prepared MOFs surfaces. XRD results confirm the formation of the desire MOFs and show the high degree of dispersion of Pd nanoparticles. TEM images show that Pd nanoparticles are nano-sized with almost uniform shape. EDAX shows that Pd nanoparticles successfully loaded on Co0.5-Mn0.5-MOF-74 catalyst. CO oxidation as a model reaction was then used to assess the catalytic performance of the prepared catalysts. The catalytic activity results show enhancement in the catalytic activities of monometallic MOFs after introducing another metal in the same framework and show an excellent improvement in CO conversion after loading with Pd nanoparticles. Furthermore, the samples that contain Pd nanoparticles exhibits higher catalytic activities which raised with increasing the content of Pd nanoparticles.

Phase junction-confined single-atom TiO2–Pt1–CeO2 for multiplying catalytic oxidation efficiency

The phase junction confinement within the TiO₂–Pt₁–CeO₂ ensemble leads to 5 times higher CO oxidation efficiency under 300 K.

Pt–Pd bimetallic effect in PtxPd1−x/(Ce,Zr,La)O2 catalysts for NOx, HC and CO elimination

The bimetallic catalysts show the alloy structure and the surplus precious metals presented as isolated PMO_x species.

One-pot hard template synthesis of mesoporous spinel nanoparticles as efficient catalysts for low temperature CO oxidation

Co-Fe, Cu-Cr, and Co-Mn mixed oxide catalysts were prepared using a one-pot hard template synthesis method, and their catalytic performance was investigated before and after the rearrangement of the template. To evaluate the structural properties of the catalysts, various analyses were employed, including the BET, XRD, H₂-TPR, FE-SEM, EDX, and X-ray digital mapping of the elements. The results indicated that the rearrangement of the catalyst structure had a profound effect on the structural and catalytic properties, so that in all three synthesized catalysts, the specific surface and the reducibility increased significantly, and the crystalline structure and morphology of the catalysts changed remarkably. The specific surface area of the CoFe, CuCr, and CoMn catalysts increased from 3.5, 1.1, and 72.9 m²/g to 151.3, 52.8, and 108.0 m²/g, respectively. These structural changes significantly increased the catalytic performance. The results indicated that the 100% conversion temperature of the CoMn catalyst as the optimal sample after rearrangement was reduced from 250 to 125 °C. Also, the stability of the CoMn catalyst in dry and wet conditions was investigated and the results indicated that the presence of water vapor reduced the activity and stability of the catalyst. The activation energy was also calculated on Co-Mn catalyst (59.5 kJ/mol) and the results confirmed that the most probable mechanism for this reaction was the MVK mechanism.

Oxidation of methane to methanol over Pd@Pt nanoparticles under mild conditions in water

Pd@Pt core–shell colloidal nanoparticles efficiently catalyse the direct oxidation of methane to methanol with high selectivity using H₂O₂ in water.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure-Property-Function Relationships for Faceted Nanoscale Metal Oxides

Nanoscale metal oxides (NMOs) have found wide-scale applicability in a variety of environmental fields, particularly catalysis, gas sensing, and sorption. Facet engineering, or controlled exposure of a particular crystal plane, has been established as an advantageous approach to enabling enhanced functionality of NMOs. However, the underlying mechanisms that give rise to this improved performance are often not systematically examined, leading to an insufficient understanding of NMO facet reactivity. This critical review details the unique electronic and structural characteristics of commonly studied NMO facets and further correlates these characteristics to the principal mechanisms that govern performance in various catalytic, gas sensing, and contaminant removal applications. General trends of facet-dependent behavior are established for each of the NMO compositions, and selected case studies for extensions of facet-dependent behavior, such as mixed metals, mixed-metal oxides, and mixed facets, are discussed. Key conclusions about facet reactivity, confounding variables that tend to obfuscate them, and opportunities to deepen structure-property-function understanding are detailed to encourage rational, informed design of NMOs for the intended application.

Smart Designs of Anti-Coking and Anti-Sintering Ni-Based Catalysts for Dry Reforming of Methane: A Recent Review

Dry reforming of methane (DRM) reaction has drawn much interest due to the reduction of greenhouse gases and production of syngas. Coking and sintering have hindered the large-scale operations of Ni-based catalysts in DRM reactions at high temperatures. Smart designs of Ni-based catalysts are comprehensively summarized in fourth aspects: surface regulation, oxygen defects, interfacial engineering, and structural optimization. In each part, details of the designs and anti-deactivation mechanisms are elucidated, followed by a summary of the main points and the recommended strategies to improve the catalytic performance, energy efficiency, and utilization rate.

Soft-template-assisted synthesis: a promising approach for the fabrication of transition metal oxides

The past few decades have witnessed transition metal oxides (TMOs) as promising candidates for a plethora of applications in numerous fields. The exceptional properties retained by these materials have rendered them of paramount emphasis as functional materials. Thus, the controlled and scalable synthesis of transition metal oxides with desired properties has received enormous attention. Out of different top-down and bottom-up approaches, template-assisted synthesis predominates as an adept approach for the facile synthesis of transition metal oxides, owing to its phenomenal ability for morphological and physicochemical tuning. This review presents a comprehensive examination of the recent advances in the soft-template-assisted synthesis of TMOs, focusing on the morphological and physicochemical tuning aided by different soft-templates. The promising applications of TMOs are explained in detail, emphasizing those with excellent performances.

Ambient Temperature CO Oxidation Using Palladium–Platinum Bimetallic Catalysts Supported on Tin Oxide/Alumina

A series of Pt-based catalysts were synthesised and investigated for ambient temperature CO oxidation with the aim to increase catalytic activity and improve moisture resistance through support modification. Initially, bimetallic PtPd catalysts supported on alumina were found to exhibit superior catalytic activity compared with their monometallic counterparts for the reaction. Following an investigation into the effect of Pt/Pd ratio, a composition of 0.1% Pt/0.4% Pd was selected for further studies. Following this, SnO2/Al2O3 supports were synthesised from a variety of tin oxide sources. Catalytic activity was improved using sodium stannate and tin oxalate precursors compared with a traditional tin oxide slurry. Catalytic activity versus tin concentration was found to vary significantly across the three precursors, which was subsequently investigated by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX).

Synthesis, Characterization and Catalytic Activity of Ternary Oxide Catalysts Using the Microwave-Assisted Solution Combustion Method

Ni-Co-Al, Ni-Cu-Al and Co-Cu-Al ternary oxide catalysts, with a fixed 5 wt% transition metal loading, were prepared by the microwave-assisted solution combustion method and tested in CO oxidation. The bulk and surface properties of the catalysts were investigated, using XRD, N2 adsorption-desorption, SEM, XPS and TEM techniques. XRD, XPS and TEM results revealed that nickel and cobalt were present as spinels on the surface and in the bulk. Differently, copper was preferentially present in "bulk-like" CuO-segregated phases. No interaction between the couples of transition metal species was detected, and the introduction of Cu-containing precursors into the Ni-Al or Co-Al combustion systems was not effective in preventing the formation of NiAl2O4 and CoAl2O4 spinels in the Ni- or Co-containing catalysts. Copper-containing catalysts were the most active, indicating that copper oxides are the effective active species for improving the CO oxidation activity.

Copper catalysis at operando conditions-bridging the gap between single nanoparticle probing and catalyst-bed-averaging

In catalysis, nanoparticles enable chemical transformations and their structural and chemical fingerprints control activity. To develop understanding of such fingerprints, methods studying catalysts at realistic conditions have proven instrumental. Normally, these methods either probe the catalyst bed with low spatial resolution, thereby averaging out single particle characteristics, or probe an extremely small fraction only, thereby effectively ignoring most of the catalyst. Here, we bridge the gap between these two extremes by introducing highly multiplexed single particle plasmonic nanoimaging of model catalyst beds comprising 1000 nanoparticles, which are integrated in a nanoreactor platform that enables online mass spectroscopy activity measurements. Using the example of CO oxidation over Cu, we reveal how highly local spatial variations in catalyst state dynamics are responsible for contradicting information about catalyst active phase found in the literature, and identify that both surface and bulk oxidation state of a Cu nanoparticle catalyst dynamically mediate its activity.

Identifying the catalyst chemical state and adsorbed species during methanol conversion on copper using ambient pressure X-ray spectroscopies

Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can generate a hydrogen-containing gas mixture. Understanding the chemical state of the catalyst over which these reactions occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions. Here we study polycrystalline Cu foils using ambient pressure X-ray spectroscopies to reveal the Cu oxidation state and identify the adsorbed species during partial oxidation (CH₃OH + O₂), steam reforming (CH₃OH + H₂O), and autothermal reforming (CH₃OH + O₂ + H₂O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen-rich conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu₂O. We rationalise this behaviour on the basis of the shift in equilibrium of the CH₃OH* + O* ⇌ CH₃O* + OH* reaction step caused by the addition of H₂O.

Exsolution of Catalytically Active Iridium Nanoparticles from Strontium Titanate

The search for new functional materials that combine high stability and efficiency with reasonable cost and ease of synthesis is critical for their use in renewable energy applications. Specifically in catalysis, nanoparticles, with their high surface-to-volume ratio, can overcome the cost implications associated with otherwise having to use large amounts of noble metals. However, commercialized materials, that is, catalytic nanoparticles deposited on oxide supports, often suffer from loss of activity because of coarsening and carbon deposition during operation. Exsolution has proven to be an interesting strategy to overcome such issues. Here, the controlled emergence, or exsolution, of faceted iridium nanoparticles from a doped SrTiO₃ perovskite is reported and their growth preliminary probed by in situ electron microscopy. Upon reduction of SrIr_0.005Ti_0.995O₃, the generated nanoparticles show embedding into the oxide support, therefore preventing agglomeration and subsequent catalyst degradation. The advantages of this approach are the extremely low noble metal amount employed (∼0.5% weight) and the catalytic activity reported during CO oxidation tests, where the performance of the exsolved SrIr_0.005Ti_0.995O₃ is compared to the activity of a commercial catalyst with 1% loading (1% Ir/Al₂O₃). The high activity obtained with such low doping shows the possibility of scaling up this new catalyst, reducing the high cost associated with iridium-based materials.

Efficient Noble-Metal-Free Catalysts Supported by Three-Dimensional Ordered Hierarchical Porous Carbon

Development of heterogeneous catalysts has attracted increasing attention, owing to their remarkable catalytic performance and recyclability. Herein, we report well-developed heterogeneous catalysts with a three-dimensional ordered hierarchical structure, constructed from nickel or cobalt nanoparticles embedded in porous carbon. The obtained catalysts were fully characterized by several techniques. On account of the uniform distribution of metal nanoparticles in the porous carbon matrix and large diffusion channels that allow for effective mass transport, the catalysts exhibited superior catalytic performance for styrene epoxidation reaction. In particular, the catalysts showed good catalytic activity, high selectivity and excellent recyclability toward the styrene epoxidation. Thus, this facile approach developed allows for fabricating advanced heterogeneous catalysts with high catalytic activities for useful practical applications.

5-Hydroxymethyl-2-Furfural Oxidation Over Au/CexZr1-xO2 Catalysts

A series of gold catalysts supported on pure CeO₂, ZrO₂, and two different Ce-Zr mixed oxides have been prepared and tested in the 5-hydroxymethyl-2-furfural oxidation reaction. All catalysts show high catalytic activity (100% conversion) and important selectivity (27–41%) to the desired product i.e., 2,5-furandicarboxylic acid at low base concentration. Products selectivity changes with the support nature as expected, however, the observed trend cannot be related neither to gold particle size, nor to catalyst reducibility and oxygen mobility. An important relation between the FDCA selectivity and the support textural properties is observed, conducing to the general requirement for optimal pore size for this reaction.

Influence of the Fuel/Oxidant Ratio on the Elaboration of Binary Oxide Catalyst by a Microwave-Assisted Solution Combustion Method

Three series of binary metal oxide catalysts containing Ni, Cu, or Co oxides were prepared, fully characterized and tested in CO oxidation. The catalysts, with a constant transition metal loading of 10 wt%, were prepared from metal nitrates and urea mixtures by a microwave-assisted solution combustion method. The RV/OV ratio, corresponding to the stoichiometry of the reactants, calculated from their elemental oxidizing and reducing valences, was varied. In order to modify the redox character of the synthesis medium from the quantity of urea; an excess of urea was used for attaining reducing conditions, while a deficit of urea shifted the medium to oxidizing conditions. Three RV/OV ratios (0.9, 1.0, and 1.1) were selected to elaborate the different binary metal oxide catalysts, nine oxide catalysts were synthesized. Then, the influence of the stoichiometry (RV/OV ratio) on the bulk and surface properties of the binary metal oxide catalysts was investigated. Similarly, the influence of the RV/OV ratio on the CO oxidation activity was discussed and the optimal value of RV/OV ratio was identified. The results show that the increasing of the RV/OV ratio from 0.9 to 1.1, particularly in Ni- and Co-containing catalysts, induces stronger metal-aluminum interactions, in the form of aluminates phases, and that are correlated to the dramatic reduction of the CO oxidation activity. The best physicochemical properties and highest catalytic activities were achieved with the catalysts prepared in redox systems stoichiometrically balanced (RV/OV = 1). The Cu-containing catalysts presented the best catalytic activities in CO oxidation.

2D molybdenum disulphide nanosheets incorporated with single heteroatoms for the electrochemical hydrogen evolution reaction

2D nanosheets give enhanced surface area to volume ratios in particle morphology and they can also provide defined surface sites to disperse foreign atoms. Placing atoms of catalytic interest on 2D nanosheets as Single Atom Catalysts (SAC) represents one of the novel approaches due to their unique but tunable electronic and steric characteristics. Here in this mini-review, we particularly highlight some recent and important developments on heteroatom doped MoS₂ nanosheets (SAC-MoS₂) as catalysts for the electrochemical hydrogen evolution reaction (HER) from water, which could lead to opening up to a flagship of important renewable technologies in future. It is shown that the nature of dopants, doping positions and the polytypes of MoS₂ nanosheets are the determining factors in the overall catalytic abilities of these functionalised nanosheets. This may serve to obtain atomic models which lead to further understanding of the 'metal-support interaction' in catalysis.

Dispersion of copper oxide species on nanostructured ceria

Copper oxides species deposited on ceria rods, particles, and cubes were examined for low-temperature oxidation of CO. It was found that the shape of ceria altered the dispersion and chemical state of copper species considerably. CuO_x monolayers and bilayers were formed on ceria rods and particles, while multilayers and faceted particles co-existed on ceria cubes. The formation of Cu⁺ species at the copper-ceria interface involved a significant charge transfer from copper oxides to the ceria surface via a strong electronic interaction, which was more pronounced on ceria rods. The concentrations of surface Cu⁺ and oxygen vacancies followed the order rods > particles > cubes, in line with their catalytic activity for CO oxidation at 343 K.