Aqueous Phase Reforming over Platinum Catalysts on Doped Carbon Supports: Exploring Platinum-Heteroatom Interactions

A series of platinum catalysts supported on carbon nanofibers with various heteroatom dopings were synthesized to investigate the effect of the local platinum environment on the catalytic activity and selectivity in aqueous phase reforming (APR) of ethylene glycol (EG). Typical carbon dopants such as oxygen, nitrogen, sulfur, phosphorus, and boron were chosen based on their ability to bring acidic or basic functional groups to the carbon surface. In situ X-ray absorption spectroscopy (XAS) was used to identify the platinum oxidation state and platinum species formed during APR of EG through multivariate curve resolution alternating least-squares analysis, observing differences in activity, selectivity, and platinum local environment among the catalysts. The platinum-based catalyst on the nitrogen-doped carbon support demonstrated the most favorable properties for H2 production due to high Pt dispersion and basicity (H2 site time yield 22.7 h-1). Direct Pt-N-O coordination was identified by XAS in this catalyst. The sulfur-doped catalyst presented Pt-S contributions with the lowest EG conversion rate and minimal production of the gas phase components. Boron and phosphorus-doped catalysts showed moderate activity, which was affected by low platinum dispersion on the carbon support. The phosphorus-doped catalyst showed preferential selectivity to alcohols in the liquid phase, associated with the presence of acid sites and Pt-P contributions observed under APR conditions.

Molecular Self-Assembly in Conductive Covalent Networks for Selective Nitrate Electroreduction to Ammonia

Electrochemical nitrate (NO3-) reduction in aqueous media provides a useful approach for ammonia (NH3) synthesis. While efforts are focused on developing catalysts, the local microenvironment surrounding the catalyst centers is of great importance for controlling electrocatalytic performance. Here, we demonstrate that a self-assembled molecular iron catalyst integrated in a free-standing conductive hydrogel is capable of selective production of NH3 from NO3- at efficiencies approaching unity. With the electrocatalytic hydrogel, the NH3 selectivity is consistently high under a range of negative biases, which results from the hydrophobicity increase of the polycarbazole-based electrode substrate. In mildly acidic media, proton reduction is suppressed by electro-dewetting of the hydrogel electrode, enhancing the selectivity of NO3- reduction. The electrocatalytic hydrogel is capable of continuous production of NH3 for at least 100 h with NH3 selectivity of ∼89 to 98% at high current densities. Our results highlight the role of constructing an internal hydrophobic surface for electrocatalysts in controlling selectivity in aqueous media.

Coalescence-Induced Jumping for Removing the Deposited Heterogeneous Droplets: A Molecular Dynamics Simulation Study

The removal of the deposited droplets on a solid surface is crucial to considerable practical applications that require self-cleaning properties. In this work, a strategy of cleaning a deposited droplet ("D-droplet") by coalescing with a heterogeneous and easily jumping droplet ("J-droplet") is proposed. Molecular dynamics simulation studies have shown that the coalescence of these two kinds of droplets would not guarantee the removal of D-droplet, unless the lifting ability of J-droplet is enhanced through the reduction of the solid-liquid interaction. However, this is a bad scenario with low efficiency. Further investigation suggests that by introducing two J-droplets to produce triple-coalescence dynamics, the D-droplet could be successfully jumping from the substrates due to the coalescence-induced effect, which is also verified by the free energy calculation. Moreover, the effects of the size of the droplets and the arrangement mode of these three droplets on the jumping dynamics are both considered. The studies not only help advance our understanding of coalescence-induced jumping of heterogeneous droplets, but also open up new ways to remove the deposited impure droplets, which is expected to guide the fields of self-cleaning.

Wettability of Metals by Water

The wetting behavior of water on metal surfaces is important for a wide range of industries, for example, in the metallurgical industry during the preparation of metallic nanoparticles or electrochemical or electroless coating preparation from aqueous solutions, as well as in the construction industry (e.g., self-cleaning metal surfaces) and in the oil industry, in the case of water–oil separation or corrosion problems. Wettability in water/metal systems has been investigated in the literature; nevertheless, contradictions can be found in the results. Some papers have reported perfect wettability even in water/noble metal systems, while other researchers state that water cannot spread well on the surface of metals, and the contact angle is predicted at around 60°. The purpose of this paper is to resolve this contradiction and find correlations to predict the contact angle for a variety of metals. In our research, the wetting behavior of distilled water on the freshly polished surface of Ag, Au, Cu, Fe, Nb, Ni, Sn, Ti, and W substrates was investigated by the sessile drop method. The contact angle of the water on the metal was determined by KSV software. The contact angle of water is identified as being between 50° and 80°. We found that the contact angle of water on metals decreases linearly with increasing the atomic radius of the substrate. Using our new equation, the contact angle of water was identified on all of the metals in the periodic table. From the measured contact angle values, the adhesion energy of the distilled water/metal substrate interface was also determined and a correlation with the free electron density parameter of substrates was determined.

High-Performance Heterogeneous Thermocatalysis Caused by Catalyst Wettability Regulation

Catalyst wettability regulation has emerged as an attractive approach for high catalytic performance for the past few years. By introducing appropriate wettability, the molecule diffusion of reactants and products can be enhanced, leading to high activity. Besides this, undesired molecules are isolated for high selectivity of target products and long-term stability of catalyst. Herein, we summarize wettability-induced high-performance heterogeneous thermocatalysis in recent years, including hydrophilicity, hydrophobicity, hybrid hydrophilicity-hydrophobicity, amphiphilicity, and superaerophilicity. Relevant reactions are further classified and described according to the reason for the performance improvement. It should be pointed out that studies of utilizing superaerophilicity to improve heterogeneous thermocatalytic performance have been included for the first time, so this is a comparatively comprehensive review in this field as yet.

Oxygen Reduction Reaction with Manganese Oxide Nanospheres in Microbial Fuel Cells

Operating microbial fuel cells (MFCs) under extreme pH conditions offers a substantial benefit. Acidic conditions suppress the growth of undesirable methanogens and increase redox potential for oxygen reduction reactions (ORRs), and alkaline conditions increase the electrocatalytic activity. However, operating any fuel cells, including MFCs, is difficult under such extreme pH conditions. Here, we demonstrate a pH-universal ORR ink based on hollow nanospheres of manganese oxide (h-Mn3O4) anchored with multiwalled carbon nanotubes (MWCNTs) on planar and porous forms of carbon electrodes in MFCs (pH = 3-11). Nanospheres of h-Mn3O4 (diameter ∼ 31 nm, shell thickness ∼ 7 nm) on a glassy carbon electrode yielded a highly reproducible ORR activity at pH 3 and 10, based on rotating disk electrode (RDE) tests. A phenomenal ORR performance and long-term stability (∼106 days) of the ink were also observed with four different porous cathodes (carbon cloth, carbon nanofoam paper, reticulated vitreous carbon, and graphite felt) in MFCs. The ink reduced the charge transfer resistance (R ct) to the ORR by 100-fold and 45-fold under the alkaline and acidic conditions, respectively. The current study promotes ORR activity and subsequently the MFC operations under a wide range of pH conditions, including acidic and basic conditions.

Switchable wetting of oxygen-evolving oxide catalysts

The surface wettability of catalysts is typically controlled via surface treatments that promote catalytic performance. Here we report on potential-regulated hydrophobicity/hydrophilicity at cobalt-based oxide interfaces with an alkaline solution. The switchable wetting of single particles, directly related to their activity and stability towards the oxygen evolution reaction, was revealed by electrochemical liquid-phase transmission electron microscopy. Analysis of the movement of the liquid in real time revealed distinctive wettability behaviour associated with specific potential ranges. At low potentials, an overall reduction of the hydrophobicity of the oxides was probed. Upon reversible reconstruction towards the surface oxyhydroxide phase, electrowetting was found to cause a change in the interfacial capacitance. At high potentials, the evolution of molecular oxygen, confirmed by operando electron energy-loss spectroscopy, was accompanied by a globally thinner liquid layer. This work directly links the physical wetting with the chemical oxygen evolution reaction of single particles, providing fundamental insights into solid–liquid interfacial interactions of oxygen-evolving oxides.

Surface treatments can tune catalysts’ wettability, which can be used to promote their catalytic performance. Now, a potential-dependent dynamic wetting behaviour of cobalt-based oxide catalysts is identified before and during the oxygen evolution reaction.

Controllable Coalescence Dynamics of Nanodroplets on Textured Surfaces Decorated with Well-Designed Wrinkled Nanostructures: A Molecular Dynamics Study

The control of coalescence and motion of droplets play a significant role in advanced technology and our daily life, and one of the most important approaches is surface design. Here, the microtextured surfaces decorated with wrinkled substrates are designed and studied for coalescence behaviors. The simulation results show that the coalescence speed would be slower on such surfaces due to the downward movement of the droplets induced by the penetration of atoms into the grooves that delays their movement along the coalescence direction, which is considered as a restriction effect. With the increase of the angles and the interval distance of wrinkled structures, the coalescence time becomes longer and the coalescing process becomes unfavorable, resulting from the enhanced restriction effect. More importantly, a composite substrate possessing the original and sub-wrinkled structures has been designed to tune the coalescence dynamics. The results of this work may not only help shed light on understanding the coalescence behaviors on the wrinkled substrates but also propose a feasible method for controlling the liquid coalescence behavior, which is expected to provide some useful implications for practical applications.

Designing Undercoordinated Ni-Nx and Fe-Nx on Holey Graphene for Electrochemical CO2 Conversion to Syngas

In this study, we propose a top-down approach for the controlled preparation of undercoordinated Ni-N_x (Ni-hG) and Fe-N_x (Fe-hG) catalysts within a holey graphene framework, for the electrochemical CO₂ reduction reaction (CO₂RR) to synthesis gas (syngas). Through the heat treatment of commercial-grade nitrogen-doped graphene, we prepared a defective holey graphene, which was then used as a platform to incorporate undercoordinated single atoms via carbon defect restoration, confirmed by a range of characterization techniques. We reveal that these Ni-hG and Fe-hG catalysts can be combined in any proportion to produce a desired syngas ratio (1-10) across a wide potential range (-0.6 to -1.1 V vs RHE), required commercially for the Fischer-Tropsch (F-T) synthesis of liquid fuels and chemicals. These findings are in agreement with our density functional theory calculations, which reveal that CO selectivity increases with a reduction in N coordination with Ni, while unsaturated Fe-N_x sites favor the hydrogen evolution reaction (HER). The potential of these catalysts for scale up is further demonstrated by the unchanged selectivity at elevated temperature and stability in a high-throughput gas diffusion electrolyzer, displaying a high-mass-normalized activity of 275 mA mg^-1 at a cell voltage of 2.5 V. Our results provide valuable insights into the implementation of a simple top-down approach for fabricating active undercoordinated single atom catalysts for decarbonized syngas generation.

Surfactant-in-Polymer Templating for Fabrication of Carbon Nanofibers with Controlled Interior Substructures: Designing Versatile Materials for Energy Applications

A simple, scalable, surfactant-in-polymer templating approach is demonstrated to create controlled long-range secondary substructures in a primary structure. A metal bis(2-ethylhexyl) sulfosuccinate (MAOT) as the surfactant is shown to be capable of serving as a sacrificial template and metal precursor in carbon nanofibers. The low interfacial tension and controllable dimensions of the MAOT are maintained in the solid-phase polymer, even during electrospinning and heat-treatment processes, allowing for the long-range uniform formation of substructures in the nanofibers. The MAOT content is found to be a critical parameter for tailoring the diameter of the nanofibers and their textural properties, such as size and volume of interior pores. The metal counterion species in the MAOT determine the introduction of metallic phases in the nanofiber interior. The incorporation of MAOT with Na as the counterion into the polymer phase leads to the formation of a built-in pore structure in the nanofibers. In contrast, MAOT with Fe as a counterion generates unique iron-in-pore substructures in the nanofibers (FeCNFs). The FeCNFs exhibit outstanding charge storage and water splitting performances. As a result, the MAOT-in-polymer templating approach can be extended to combinations of various metal precursors and thus create desirable functionalities for different target applications.

Insights into the Interfacial Effects in Heterogeneous Metal Nanocatalysts toward Selective Hydrogenation

Heterogeneous metal catalysts are distinguished by their structure inhomogeneity and complexity. The chameleonic nature of heterogeneous metal catalysts have prevented us from deeply understanding their catalytic mechanisms at the molecular level and thus developing industrial catalysts with perfect catalytic selectivity toward desired products. This Perspective aims to summarize recent research advances in deciphering complicated interfacial effects in heterogeneous hydrogenation metal nanocatalysts toward the design of practical heterogeneous catalysts with clear catalytic mechanism and thus nearly perfect selectivity. The molecular insights on how the three key components (i.e., catalytic metal, support, and ligand modifier) of a heterogeneous metal nanocatalyst induce effective interfaces determining the hydrogenation activity and selectivity are provided. The interfaces influence not only the H₂ activation pathway but also the interaction of substrates to be hydrogenated with catalytic metal surface and thus the hydrogen transfer process. As for alloy nanocatalysts, together with the electronic and geometric ensemble effects, spillover hydrogenation occurring on catalytically "inert" metal by utilizing hydrogen atom spillover from active metal is highlighted. The metal-support interface effects are then discussed with emphasis on the molecular involvement of ligands located at the metal-support interface as well as cationic species from the support in hydrogenation. The mechanisms of how organic modifiers, with the ability to induce both 3D steric and electronic effects, on metal nanocatalysts manipulate the hydrogenation pathways are demonstrated. A brief summary is finally provided together with a perspective on the development of enzyme-like heterogeneous hydrogenation metal catalysts.

Some Critical Insights into the Synthesis and Applications of Hydrophobic Solid Catalysts

The preparation methods of hydrophobic materials such as zeolites, modified silicas and polymers has been reviewed. Particular attention has been paid to the characterization methods classified according to the surface and bulk composition, on one hand, and to the measure of interactions with water or organic solvents, on the other. Some selected applications are analyzed in order to understand the relevance of the reactants/products adsorption to address activity and selectivity of the reaction. Thus, absorption of a non-polar reactant or desorption of a hydrophilic product are much easier on a hydrophobic surface and can effectively boost the catalytic activity.

Construction of a thermo-responsive copolymer-stabilized Fe3O4@CD@PdNP hybrid and its application in catalytic reduction

A thermo-responsive copolymer stabilized Fe₃O₄@CDs@PdNP hybrid was fabricated as a nanocatalyst for catalytic reduction reaction with excellent recyclability and controllable thermo-responsive catalytic property.