Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion

Preparation and Characterization of Rh/MgSNTs Catalyst for Hydroformylation of Vinyl Acetate: The Rh0 was Obtained by Calcination

A simple and practical Rh-catalyzed hydroformylation of vinyl acetate has been synthesized via impregnation-calcination method using silicate nanotubes (MgSNTs) as the supporter. The Rh0 (zero valent state of rhodium) was obtained by calcination. The influence of calcination temperature on catalytic performance of the catalysts was investigated in detail. The catalysts were characterized in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrometer (XPS), atomic emission spectrometer (ICP), and Brunauer–Emmett–Teller (BET) surface-area analyzers. The Rh/MgSNTs(a2) catalyst shows excellent catalytic activity, selectivity and superior cyclicity. The catalyst could be easily recovered by phase separation and was used up to four times.

Static Regulation and Dynamic Evolution of Single-Atom Catalysts in Thermal Catalytic Reactions

Single-atom catalysts provide an ideal platform to bridge the gap between homogenous and heterogeneous catalysts. Here, the recent progress in this field is reported from the perspectives of static regulation and dynamic evolution. The syntheses and characterizations of single-atom catalysts are briefly discussed as a prerequisite for catalytic investigation. From the perspective of static regulation, the metal-support interaction is illustrated in how the supports alter the electronic properties of single atoms and how the single atoms activate the inert atoms in supports. The synergy between single atoms is highlighted. Besides these static views, the surface reconstruction, such as displacement and aggregation of single atoms in catalytic conditions, is summarized. Finally, the current technical challenges and mechanistic debates in single-atom heterogeneous catalysts are discussed.

Highlights of Major Progress on Single-Atom Catalysis in 2017

Single-atom catalysis has rapidly progressed during the last few years. In 2017, single-atom catalysts (SACs) were fabricated with higher metal loadings and designed into more delicate structures. SACs also found wide applications in C1 chemical conversion, such as selective oxidation of methane and conversion of carbon dioxide. Both experimental characterizations and computational modeling revealed the presence of tunable interactions between single atom species and their surrounding chemical environment, and thus SACs may be more effective and more stable than their nanoparticle counterparts. In this mini-review, we summarize the major achievements of SACs into three main aspects: a) the advanced synthetic methodologies, b) catalytic performance in C1 chemistry, and c) strong metal-support interaction induced unexpected durability. These accomplishments will shed new light on the recognition of single-atom catalysis and encourage more efforts to explore potential applications of SACs.

Isolated Fe Single Atomic Sites Anchored on Highly Steady Hollow Graphene Nanospheres as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The rational design of economical and high-performance nanocatalysts to substitute Pt for the oxygen reduction reaction (ORR) is extremely desirable for the advancement of sustainable energy-conversion devices. Isolated single atom (ISA) catalysts have sparked tremendous interests in electrocatalysis due to their maximized atom utilization efficiency. Nevertheless, the fabrication of ISA catalysts remains a grand challenge. Here, a template-assisted approach is demonstrated to synthesize isolated Fe single atomic sites anchoring on graphene hollow nanospheres (denoted as Fe ISAs/GHSs) by using Fe phthalocyanine (FePc) as Fe precursor. The rigid planar macrocycle structure of FePc molecules and the steric-hindrance effect of graphene nanospheres are responsible for the dispersion of Fe-N x species at an atomic level. The combination of atomically dispersed Fe active sites and highly steady hollow substrate affords the Fe ISAs/GHSs outstanding ORR performance with enhanced activity, long-term stability, and better tolerance to methanol, SO2, and NO x in alkaline medium, outperforming the state-of-the-art commercial Pt/C catalyst. This work highlights the great promises of cost-effective Fe-based ISA catalysts in electrocatalysis and provides a versatile strategy for the synthesis of other single metal atom catalysts with superior performance for diverse applications.

Termination Effects of Pt/v-Ti n+1C nT2 MXene Surfaces for Oxygen Reduction Reaction Catalysis

Ideal catalysts for the oxygen reduction reaction (ORR) have been searched and researched for decades with the goal to overcome the overpotential problem in proton exchange membrane fuel cells. A recent experimental study reports the application of Pt nanoparticles on the newly discovered 2D material, MXene, with high stability and good performance in ORR. In this work, we simulate the Ti _n+1C _nT _x and the Pt-decorated Pt/v-Ti _n+1C _nT _x ( n = 1-3, T = O and/or F) surfaces by first-principles calculations. We focus on the termination effects of MXene, which may be an important factor to enhance the performance of ORR. The properties of different surfaces are clarified by exhaustive computational analyses on the geometries, charges, and their electronic structures. The free-energy diagrams as well as the volcano plots for ORR are also calculated. On the basis of our results, the F-terminated surfaces are predicted to show a better performance for ORR but with a lower stability than the O-terminated counterparts, and the underlying mechanisms are investigated in detail. This study provides a better understanding of the electronic effect induced by the terminators and may inspire realizations of practical MXene systems for ORR catalysis.

Selective mild oxidation of methane to methanol or formic acid on Fe–MOR catalysts

Selective oxidation of methane to methanol or formic acid was achieved using mordenite (MOR)-supported iron catalysts with H₂O₂ as the oxidant.

Identification of active sites in CO oxidation over a Pd/Al2O3 catalyst

The active sites of Pd/Al₂O₃ catalysts for CO oxidations were identified by investigating the dependence of CO oxidation activities on the surface structure and morphology of Pd nanoparticles.

Harnessing the Wisdom in Colloidal Chemistry to Make Stable Single-Atom Catalysts

Research on single-atom catalysts (SACs), or atomically dispersed catalysts, has been quickly gaining momentum over the past few years. Although the unique electronic structure of singly dispersed atoms enables uncommon-sometimes exceptional-activities and selectivities for various catalytic applications, developing reliable and general procedures for preparing stable, active SACs in particular for applications under reductive conditions remains a major issue. Herein, the challenges associated with the synthesis of SACs are highlighted semiquantitatively and three stabilization techniques inspired by colloidal science including steric, ligand, and electrostatic stabilization are proposed. Some recent examples are discussed in detail to showcase the power of these strategies in the synthesis of stable SACs without compromising catalytic activity. The substantial further potential of steric, ligand, and electrostatic effects for developing SACs is emphasized. A perspective is given to point out opportunities and remaining obstacles, with special attention given to electrostatic stabilization where little is done so far. The stabilization strategies presented herein have a wide applicability in the synthesis of a series of new SACs with improved performances.

Ab Initio Molecular Dynamics Simulation Study on the Stereo Reactions between Atomic Oxygen Anion and Methane

Ion–molecule reaction between atomic oxygen anion (O⁻) and methane (CH₄) has been systematically investigated employing the on-the-fly ab initio molecular dynamics simulations. Besides the major H-abstraction process as the exothermic reaction studied widely, an endothermic pathway to produce OCH₃⁻ and H is also observed in this study. Three typical O⁻ attack modes with reference to the pyramid structure of CH₄ fixed in space have been considered. It was found that the internal motions of the radical products are significantly dependent on the O⁻ attack modes. As for the reaction between O⁻ and the thermally vibrating CH₄, the major pathway to produce OH⁻ and CH₃ is preferred by the direct H-abstraction and the minor pathway to produce H and OCH₃⁻ is the roaming reaction via the transient negative ion [HO-CH₃]⁻.

Selective Conversion of Methane by Rh1-Doped Aluminum Oxide Cluster Anions RhAl2O4-: A Comparison with the Reactivity of PtAl2O4. J Phys Chem A 2018;122:3950-3955. [PMID: 29578712 DOI: 10.1021/acs.jpca.8b02483]

Studying the elementary reactions of single-noble-metal-atom-doped species can give theoretical guidance for the design of related single-atom catalysis. Using a combination of mass spectrometry and density functional theory calculations, the reaction of RhAl₂O₄^- with the most stable alkane molecule CH₄ under thermal conditions has been studied. The methane tends to be converted into syngas (free H₂ and adsorbed CO) with activation of four C-H bonds. In sharp contrast, formaldehyde was generated in the previously reported reaction of PtAl₂O₄^- with CH₄. Density functional theory calculations show that the difference in reactivity between RhAl₂O₄^- and PtAl₂O₄^- is found to be due to a higher energy barrier of the third C-H bond activation for the Pt analogue. This work provides the first comparative study on the reactivity of single noble-metal atoms (Rh, Pt) on the same cluster support (Al₂O₄^-) and can be helpful for rational design of single-atom catalysts for selective methane conversion.