Metal-support interaction in platinum and palladium nanoparticles loaded on nitrogen-doped mesoporous carbon for oxygen reduction reaction

A sulfur-modified pore-blocking method to enhance the electrocatalytic stability of carbon-supported platinum nanoparticles

Currently, the durability of electrode materials remains a big obstacle to the widespread adoption of proton exchange membrane fuel cells (PEMFCs). Herein thiourea and sodium dodecyl benzene sulfonate (SDS) were employed as sulfur source and carbon source to modify the pristine carbon black (Ketjen black EC300 J). A highly durable carbon supported Pt nanosized catalyst with higher platinum utilization for oxygen reduction reaction (ORR) in PEMFCs was produced by doping elemental sulfur into carbon supports and decreasing the carbon pore sizes and volume through a successive impregnation technique. The catalyst exhibits an initial activity of 0.167 A mgPt -1 at 0.90 V and demonstrates minimal activity loss after acceleration stress test (30,000 cycles of AST). The half-wave potential loss for representative sample (Pt/S-C-3) is only 14 mV with only 21.8 % ECSA decrease, 27.5 % MA loss and 5.9 % SA loss. A sintering test at various temperature shows a minor average size increase for sulfur-doped carbon (S-C) supported one (from 2.09 to 2.52 nm). In single-cell test, the MEA sample employing the platinum catalyst on modified carbon as cathode exhibited almost negligible performance loss after 30,000 cycles of AST.

Synergistic effects of nitrogen-doped carbon and praseodymium oxide in electrochemical water splitting

Hybrid materials featuring perovskite-type metal oxide in conjunction with heteroatom-doped graphene hold immense promise as alternatives to costly noble metal catalysts for electrochemical water splitting, facilitating the generation of environmentally friendly hydrogen. In this study, perovskite-type oxide containing praseodymium, barium, strontium, cobalt, and iron atoms dispersed in a carbon matrix as a catalyst is synthesized via annealing of the carbon material with substrates for the preparation of perovskite oxide. The mass ratio of reagents regulates the porous structure and elemental composition. The result of the hydrogen evolution reaction (HER), suggests that the hybrid catalysts exhibit intermediate HER kinetics compared to the commercial Pt/C and the catalyst without carbon. The Tafel slope for HER is lower for materials containing carbon, because of the improved reaction kinetics, facilitated proton transfer, and enhanced electrochemical surface area. Therefore, the study provides an effective strategy for the preparation of catalyst and their use as the active catalyst of water splitting.

Photocatalytic Synthesis of Ultrafine Pt Electrocatalysts with High Stability Using TiO2 -Decorated N-Doped Carbon as Composite Support

Commercial Pt/C (Com. Pt/C) electrocatalysts are considered optimal for oxygen reduction and hydrogen evolution reactions (ORR and HER). However, their high Pt content and poor stability restrict their large-scale application. In this study, photocatalytic synthesis was used to reduce ultrafine Pt nanoparticles in-situ on a composite support of TiO2 -decorated nitrogen-doped carbon (TiO2 -NC). The nitrogen-doped carbon had a large surface area and electronic effects that ensured the uniform dispersion of TiO2 nanoparticles to form a highly photoactive and stable support. TiO2 -NC served as a composite support that enhanced the dispersibility and stability of ultrafine Pt electrocatalyst, owing to the presence of N sites and the strong metal-support interaction. Relative to Com. Pt/C, the as-obtained Pt/TiO2 -NC had positive shifts of 44 and 10 mV in the ORR half-wave potential and HER overpotential at -10 mA cm-2 , respectively. After an accelerated durability test, Pt/TiO2 -NC had lower losses in electrochemical specific area (0.7 %) and electrocatalytic activity (0 mV shift) than Com. Pt/C (25.6 %, 22 mV shift). These results indicate that the developed strategy enabled the facile synthesis and stabilization of ultrafine Pt nanoparticles, which improved the utilization efficiency and long-term stability of Pt-based electrocatalysts.

Pt-C interactions in carbon-supported Pt-based electrocatalysts

Carbon-supported Pt-based materials are highly promising electrocatalysts. The carbon support plays an important role in the Pt-based catalysts by remarkably influencing the growth, particle size, morphology, dispersion, electronic structure, physiochemical property and function of Pt. This review summarizes recent progress made in the development of carbon-supported Pt-based catalysts, with special emphasis being given to how activity and stability enhancements are related to Pt-C interactions in various carbon supports, including porous carbon, heteroatom doped carbon, carbon-based binary support, and their corresponding electrocatalytic applications. Finally, the current challenges and future prospects in the development of carbon-supported Pt-based catalysts are discussed.

N/Ce doped graphene supported Pt nanoparticles for the catalytic oxidation of formaldehyde at room temperature

Pt catalysts with nitrogen-doped graphene oxide (GO) as support and CeO₂ as promoter were prepared by impregnation method, and their catalytic oxidation of formaldehyde (HCHO) at room temperature was tested. The Pt-CeO₂/N-rGO (reduced GO) with a mass fraction of 0.7% Pt and 0.8% CeO₂ exhibited an excellent catalytic performance with the 100% conversion of HCHO at room temperature. Physicochemical characterization demonstrated that nitrogen-doping greatly increased the defect degree and the specific surface area of GO, enhanced the dispersion of Pt and promoted more zero-valent Pt. The synergistic effect between CeO₂ and Pt was also beneficial to the dispersion of Pt. Nitrogen-doping promoted the production of more Ce³⁺ ions, generating more oxygen vacancies, which was conducive to O₂ adsorption. As a result, the catalyst exhibited enhanced redox properties, leading to the best catalytic activity. Finally, an attempt to propose the reaction mechanism of HCHO oxidation has been made.

The Effect of a Hydrogen Reduction Procedure on the Microbial Synthesis of a Nano-Pd Electrocatalyst for an Oxygen-Reduction Reaction

Noble-metal electrocatalysts supported by biological-organism-derived carbons have attracted attention from the public due to the growing demands for green synthesis and environmental protection. Carbonization at high temperatures and hydrogen reduction are critical steps in this technical route. Herein, Shewanella oneidensis MR-1 were used as precursors, and the effects of the hydrogen-reduction procedure on catalysts were explored. The results showed that the performances of FHTG (carbonization followed by hydrogen reduction) displayed the best performance. Its ECSA (electrochemical surface area), MA (mass activity), and SA (specific activity) reached 35.01 m2 g−1, 58.39 A·g−1, and 1.66 A cm−2, respectively, which were 1.17, 1.75, and 1.50 times that of PHTG (prepared through hydrogen reduction followed by carbonization) and 1.56, 2.26, and 1.44 times that of DHTG (double hydrogen reduction). The high performance could be attributed to its fine particle size and rich N content, and the specific regulation mechanism was also proposed in this paper. This study opens a practical guide for effectively avoiding particle agglomeration during the fabrication process for catalysts.

Nitrogen doped carbon for Pd-catalyzed hydropurification of crude terephthalic acid: roles of nitrogen species

The purification of crude terephthalic acid was performed by the hydrogenation of 4-carboxybenzaldehyde (4-CBA) over activated carbon (AC) supported Pd catalysts in industry. However, traditional Pd/AC catalysts usually suffer from low hydrogenation activity and poor thermal stability. Herein, nitrogen was incorporated into AC via a simple hydrothermal treatment of AC with urea as the nitrogen resource. The N doped AC contained pyridinic N, pyrrolic N, graphitic N and oxidized N. Wide characterizations revealed that N doping not only effectively improved the dispersion of Pd NPs but also increased the proportion of Pd⁰. In addition, N doping also enhanced the dissociative adsorption capacity of molecular hydrogen. More importantly, the resistance to sintering of Pd NPs was efficiently suppressed after N doping. As a result, N doped AC supported Pd showed both higher activity and better thermal stability than the N-free one.

Pd on N doped activated carbon exhibited increased activity and stability in 4-CBA hydrogenation relative to Pd catalysts without N doping. Higher dispersion of Pd⁰ and facile activation of H₂ accounted for the better activity of Pd/NC.

Testing PtCu Nanoparticles Supported on Highly Ordered Mesoporous Carbons CMK3 and CMK8 as Catalysts for Low-Temperature Fuel Cells

Pt(Cu) nanoparticles supported on CMK3 and CMK8 ordered mesoporous carbons (OMCs) have been synthesized by electroless deposition of Cu followed by galvanic exchange with Pt. The structural characterization by high-resolution transmission electron microscopy and X-ray diffraction showed the formation of Pt(Cu) nanoparticles of 4–5 nm, in which PtCu alloys with contracted fcc Pt lattice and 70–80 at.% Pt was identified. The X-ray photoelectron spectroscopy analyses indicated that the Pt(Cu) nanoparticles were mainly composed of a PtCu alloy core covered by a Pt-rich shell, in agreement with the steady cyclic voltammograms, which did not show any Cu oxidation peaks. Electroactive surface areas up to about 70 m2 gPt−1 were obtained. The onset potentials for CO oxidation and the oxygen reduction reaction were more negative and positive, respectively, as compared to Pt/C, thus indicating higher activity of these Pt(Cu) catalysts with respect to the latter. Based on the corresponding binding energies, these better activities were attributed to the favorable geometric and ligand effects of Cu on Pt, which were able to reduce the adsorption energy of the intermediates on Pt. Pt(Cu)/CMK3 showed competitive mass and specific activities, as well as better stability than Pt/C.

Revealing the Structure Evolution of Heterogeneous Pd Catalyst in Suzuki Reaction via the Identical Location Transmission Electron Microscopy

The mechanism of palladium nanoparticles (Pd NPs)-catalyzed cross-coupling reactions has been the subject of intense debate since the recognition of catalytic active sites involving a wide array of dynamic changed Pd species. Here, through the combination of the hot filtration experiment together with the recently developed identical location transmission electron microscopy (IL-TEM) method, the delicate structure evolution of highly dispersed Pd NPs supported on oxygen-functionalized carbon nanotubes (Pd/oCNTs) as well as the kinetics properties of derived dissolved species in liquid phase were systemically investigated in the Suzuki-Miyaura reaction. The result indicates that the leached Pd components caused by the strong adsorption of reactants might have a significant contribution to the coupling products, and the degree for different substrates follows the order of iodobenzene > phenylboronic acid > bromobenzene. Meanwhile, the typical three sequential behaviors of supported Pd NPs, including dissolution, deposition, and growth, along with the increase of the conversion throughout the reaction were spatiotemporally observed by tracking the evolution of individually identifiable NPs. The performed work not only provides direct evidence for the interaction between Pd NPs surface with reactants on atomic scale but also gives a valuable reference for fundamentally understanding the mechanism of the heterogeneous Pd-catalyzed Suzuki coupling process as well as rational design of next-generation catalysts with high efficiency and reusability for synthetic applications.

Facile d-band tailoring in Sub-10 nm Pd cubes by in-situ grafting on nitrogen-doped graphene for highly efficient organic transformations

We demonstrate for the first time the in-situ synthesis of Pd nanocubes (PdNC) on nitrogen-doped reduced graphene oxide (NRGO) for facile organic transformations wherein the cubic morphology of Pd can only be realized by precision-controlled acid additions in the tune of 0.02 pH variations in the reaction medium. Due to the intimate contact arising from atom-by-atom addition of Pd on NRGO, the composite has exhibited a pronounced catalyst to support charge transfer effect, shift in the d-band center, and lowering of charge-transfer resistance when compared with PdNC-NRGO ex-situ composites prepared by mixing of the preformed components of PdNC and NRGO or PdNCs alone. The activities of these catalysts were tested for the Suzuki coupling and nitroarene reduction reactions using water as an industry-friendly solvent. In both, the in-situ deposited sample exhibited substantially higher catalytic activity as well as stability when compared with an ex-situ sample or pure PdNCs. We show that a very high turnover frequency of ~31300 h^-1 and ~900 h^-1 are achievable by using the in-situ deposited PdNC-NRGO composite for Suzuki coupling reactions and nitroarene reduction respectively, better than the state-of-the-art catalysts developed recently, in addition to high recyclability.

Mixed-metal hybrid ultramicroporous material (HUM) precursor to graphene-supported tetrataenite as a highly active and durable NPG catalyst for the OER

Current interest in investigating non-precious group (NPG) metals for catalyzing the oxygen evolution reaction (OER) has revealed that doping of Ni hydroxides with Fe results in the dramatic enhancement of catalytic activity. Herein, a facile pathway to construct tetrataenite, an NiFe alloy of extraterrestrial origin and to address the limited electrical conductivity of metal oxides/hydroxides by directly constructing them atop graphene sheets is described. In this approach, a one-pot, bottom-up assembly of hybrid ultramicroporous materials (HUMs) was carried out, in the presence of suspended graphene (G), to homogeneously deposit the HUMs on unmodified graphene sheets, affording HUMs@G. Single metal (SIFSIX-3-Ni@G) and mixed metal (SIFSIX-3-NiFe@G) HUMs can be readily synthesized from their respective metal salts to afford a well-designed catalyst for the OER. The pyrolysis of SIFSIX-3-NiFe@G resulted in the deposition of the nanoalloy tetrataenite on G, demonstrating an exceptionally low OER onset potential of 1.44 V vs. RHE and reduced overpotential at 10 mA cm-2 (η10 = 266 mV). The synergy between the composition of the active catalyst and the electronically conductive support was attained by designing a reaction system encoding the self-assembly of a crystalline pre-catalyst on G sheets.

Supporting effects of a N-doped carbon film electrode on an electrodeposited Ni@Ni(OH)2 core-shell nanocatalyst in accelerating electrocatalytic oxidation of oligosaccharides

The supporting effect of a N-doped carbon film induced superior crystallinity in electrodeposited Ni@Ni(OH)2 core-shell nanoparticles. This improvement resulted in a much higher regeneration rate of catalytic sites (NiOOH), leading to higher oxidation currents of sugars. Also, the overpotential of the maltopentaose (G5) oxidation reaction decreased significantly, probably due to the effect of the electrostatic interaction between NPs and G5.

Pd Catalysts Supported on Bamboo-Like Nitrogen-Doped Carbon Nanotubes for Hydrogen Production

Bamboo-like nitrogen-doped carbon nanotubes (N-CNTs) were used to synthesize supported palladium catalysts (0.2–2 wt.%) for hydrogen production via gas phase formic acid decomposition. The beneficial role of nitrogen centers of N-CNTs in the formation of active isolated palladium ions and dispersed palladium nanoparticles was demonstrated. It was shown that although the surface layers of N-CNTs are enriched with graphitic nitrogen, palladium first interacts with accessible pyridinic centers of N-CNTs to form stable isolated palladium ions. The activity of Pd/N-CNTs catalysts is determined by the ionic capacity of N-CNTs and dispersion of metallic nanoparticles stabilized on the nitrogen centers. The maximum activity was observed for the 0.2% Pd/N-CNTs catalyst consisting of isolated palladium ions. A ten-fold increase in the concentration of supported palladium increased the contribution of metallic nanoparticles with a mean size of 1.3 nm and decreased the reaction rate by only a factor of 1.4.

Nanocomposite Cathode Catalysts Containing Platinum Deposited on Carbon Nanotubes Modified by O, N, and P Atoms

Platinum deposited on dispersed materials has so far been the most demanded catalyst for creating cathodes for a wide range of electrochemical power sources. This paper sets out to investigate the effect of carbon nanotube (CNT) modification by O, N, and P atoms on the structural, electrocatalytic, and corrosion properties of the as-synthesized monoplatinum catalysts. The investigated Pt/CNTmod catalysts showed an increased electrochemically active platinum surface area and electrical conductivity, as well as an increased catalytic activity in the oxygen reduction reaction (ORR) in alkaline electrolytes. The improved characteristics of Pt/CNT catalysts are explained by alterations in the composition and number of groups, which are formed on the CNT surface, and their electronic structure. By the sum of the main characteristics, Pt/CNTHNO3+N and Pt/CNTHNO3+NP are the most promising catalysts for use as cathode materials in alkaline media.

Oxygen reduction reaction on Pd nanocatalysts prepared by plasma-assisted synthesis on different carbon nanomaterials

In this work He/H₂ plasma jet treatment was used to reduce Pd ions in the aqueous solution with simultaneous deposition of created Pd nanoparticles to support materials. Graphene oxide (GO) and nitrogen-doped graphene oxide (NrGO) were both co-reduced with the Pd ions to formulate catalyst materials. Pd catalyst was also deposited on the surface of carbon black. The prepared catalyst materials were physically characterized using transmission electron microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy. The plasma jet method yielded good dispersion of small Pd particles with average sizes of particles being: Pd/rGO 2.9 ± 0.6 nm, Pd/NrGO 2.3 ± 0.5 nm and Pd/Vulcan 2.8 ± 0.6 nm. The electrochemical oxygen reduction reaction (ORR) kinetics was explored using the rotating disk electrode method. Pd catalyst deposited on nitrogen-doped graphene material showed slightly improved ORR activity as compared to that on the nondoped substrate, however Vulcan carbon-supported Pd catalyst exhibited a higher specific activity for oxygen electroreduction.

Development of a highly stable Pt-based ORR catalyst over Mn-modified polyaniline-based carbon nanofibers

A highly stable Pt-based ORR catalyst was developed over Mn-modified polyaniline-based carbon nanofibers with a high degree of graphitization.

Reactive Plasma N-Doping of Amorphous Carbon Electrodes: Decoupling Disorder and Chemical Effects on Capacitive and Electrocatalytic Performance

Nitrogen-free amorphous carbon thin films prepared via sputtering followed by graphitization, were used as precursor materials for the creation of N-doped carbon electrodes with varying degrees of amorphization. Incorporation of N-sites was achieved via nitrogen plasma treatments which resulted in both surface functionalization and amorphization of the carbon electrode materials. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to monitor composition and carbon organization: results indicate incorporation of predominantly pyrrolic-N sites after relatively short treatment cycles (5 min or less), accompanied by an initial etching of amorphous regions followed by a slower process of amorphization of graphitized clusters. By leveraging the difference in the rate of these two processes it was possible to investigate the effects of chemical N-sites and C-defect sites on their electrochemical response. The materials were tested as metal-free electrocatalysts in the oxygen reduction reaction (ORR) in alkaline conditions. We find that the introduction of predominantly pyrrolic-N sites via plasma modification results in improvements in selectivity in the ORR, relative to the nitrogen-free precursor material. Introduction of defects through prolonged plasma exposure has a more pronounced and beneficial effect on ORR descriptors than introduction of N-sites alone, leading to both increased onset potentials, and reduced hydroperoxide yields relative to the nitrogen-free carbon material. Our results suggest that increased structural disorder/heterogeneity results in the introduction of carbon sites that might either serve as main activity sites, or that enhance the effects of N-functionalities in the ORR via synergistic effects.

Nitrogen and Oxygen Co-Doping Assisted Synthesis of Highly Dispersed Pd Nanoparticles on Hollow Carbon Spheres as Efficient Electrocatalysts for Oxygen Reduction Reaction

Electrochemical reduction of O₂ (oxygen reduction reaction; ORR) provides an opportunity to achieve the commercial application of clean energy, but it remains challenging, so the rational design of inexpensive and efficient electrocatalysts is required. Palladium-based electrocatalysts have emerged as a class of the most promising candidates for the ORR, which could accelerate O₂ adsorption, dissociation, and electron transfer. However, the metal Pd atoms tend to aggregate into nanoparticles, driven by the tendency of the metal surface free energy to decrease, which significantly reduces the atom utilization efficiency and the catalytic performance. Herein, a facile double solvent impregnation method is developed for the synthesis of highly dispersed Pd nanoparticles supported on hollow carbon spheres (Pd-HCS), which could act as efficient electrocatalysts for the ORR in basic solution. Systematic investigation reveals that the nitrogen-containing and oxygen-containing functional groups (especially -COOH groups) are essential for achieving the homogenous dispersion of Pd nanoparticles. Significantly, the optimized Pd-HCS electrocatalyst with homogeneously dispersed Pd nanoparticles and Pd-N sites delivers high electrocatalytic activity for the ORR and excellent stability, without significant decay in onset potential and half-potential and good resistance to methanol crossover. This work offers a new route for the rational design of efficient ORR electrocatalysts toward advanced materials and emerging applications.

Topographical and compositional engineering of core-shell Ni@Pt ORR electro-catalysts

Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance catalytic performance. We report on the preparation of binary PtNi NPs via a co-thermolytic approach in which we optimize the synthesis variables, which results in significantly improved catalytic performance. We used scanning transmission electron microscopy to characterise the range of morphologies produced, which included spherical and concave cuboidal core–shell structures. Electrocatalytic activity was evaluated using a rotating disc electrode (1600 rpm) in 0.1 M HClO₄; the electrocatalytic performance of these Ni@Pt NPs showed significant (∼11-fold) improvement compared to a commercial Pt/C catalyst. Extended cycling revealed that electrochemical surface area was retained by cuboidal PtNi NPs post 5000 electrochemical cycles (0.05–1.00 V, vs. SHE). This is attributed to the enclosure of Ni atoms by a thick Pt shell, thus limiting Ni dissolution from the alloy structures. The novel synthetic strategy presented here results in a high yield of Ni@Pt NPs which show excellent electro-catalytic activity and useful durability.

Complex faceted geometries and compositional anisotropy in alloy nanoparticles (NPs) can enhance catalytic performance.

Enhancing Oxygen Reduction Activity of Pt-based Electrocatalysts: From Theoretical Mechanisms to Practical Methods

Pt-based electrocatalysts are considered as one of the most promising choices to facilitate the oxygen reduction reaction (ORR), and the key factor enabling their success is to reduce the required amount of platinum. Herein, we focus on illuminating both the theoretical mechanisms which enable enhanced and sustained ORR activity and the practical methods to achieve them in catalysts. The various multi-step pathways of ORR are firstly reviewed and the rate-determining steps based on the reaction intermediates and their binding energies are analyzed. We then explain the critical aspects of Pt-based electrocatalysts to tune oxygen reduction properties from the viewpoints of active sites exposure and altering the surface electronic structure, and further summarize representative research progress towards practically achieving these activity enhancements with a focus on platinum size reduction, shape control and core Pt elimination methods. We finally outline the remaining challenges and provide our perspectives with regard to further enhancing their activities.

Synthesis of carbon-supported sub-2 nanometer bimetallic catalysts by strong metal-sulfur interaction

Small-sized bimetallic nanoparticles that integrate the advantages of efficient exposure of the active metal surface and optimal geometric/electronic effects are of immense interest in the field of catalysis, yet there are few universal strategies for synthesizing such unique structures. Here, we report a novel method to synthesize sub-2 nm bimetallic nanoparticles (Pt-Co, Rh-Co, and Ir-Co) on mesoporous sulfur-doped carbon (S-C) supports. The approach is based on the strong chemical interaction between metals and sulfur atoms that are doped in the carbon matrix, which suppresses the metal aggregation at high temperature and thus ensures the formation of small-sized and well alloyed bimetallic nanoparticles. We also demonstrate the enhanced catalytic performance of the small-sized bimetallic Pt-Co nanoparticle catalysts for the selective hydrogenation of nitroarenes.

High Active PdSn Binary Alloyed Catalysts Supported on B and N Codoped Graphene for Formic Acid Electro-Oxidation

A series of PdSn binary catalysts with varied molar ratios of Pd to Sn are synthesized on B and N dual-doped graphene supporting materials. The catalysts are characterized by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Formic acid electro-oxidation reaction is performed on these catalysts, and the results reveal that the optimal proportion of Pd:Sn is 3:1. X-ray photoelectron spectroscopy (XPS) measurements show that when compared with 3Pd1Sn/graphene, B and N co-doping into the graphene sheet can tune the electronic structure of graphene, favoring the formation of small-sized metallic nanoparticles with good dispersion. On the other hand, when compared with the monometallic counterparts, the incorporation of Sn can generate oxygenated species that help to remove the intermediates, exposing more active Pd sites. Moreover, the electrochemical tests illustrate that 3Pd1Sn/BN-G catalyst with a moderate amount of Sn exhibits the best catalytic activity and stability on formic acid electro-oxidation, owing to the synergistic effect of the Sn doping and the B, N co-doping graphene substrate.

Lateral silicon oxide/gold interfaces enhance the rate of electrochemical hydrogen evolution reaction in alkaline media

The production of solar hydrogen with a silicon based water splitting device is a promising future technology, and silicon-based metal-insulator-semiconductor (MIS) electrodes have been proposed as suitable architectures for efficient photocathodes based on the electronic properties of the MIS structures and the catalytic properties of the metals. In this paper, we demonstrate that the interfaces between the metal and oxide of laterally patterned MIS electrodes may strongly enhance the catalytic activity of the electrode compared to bulk metal surfaces. The employed electrodes consist of well-defined, large-area arrays of gold structures of various mesoscopic sizes embedded in a silicon oxide support on silicon. We demonstrate that the activity of these electrodes for hydrogen evolution reaction (HER) increases with an increase in gold/silicon oxide boundary length in both acidic and alkaline media, although the enhancement of the HER rate in alkaline electrolytes is considerably larger than in acidic electrolytes. Electrodes with the largest interfacial length of gold/silicon oxide exhibited a 10-times larger HER rate in alkaline electrolytes than those with the smallest interfacial length. The data suggest that at the metal/silicon oxide boundaries, alkaline HER is enhanced through a bifunctional mechanism, which we tentatively relate to the laterally structured electrode geometry and to positive charges present in silicon oxide: Both properties change locally the interfacial electric field at the gold/silicon oxide boundary, which, in turn, facilitates a faster transport of hydroxide ions away from the electrode/electrolyte interface in alkaline solution. This mechanism boosts the alkaline HER activity of p-type silicon based photoelectrodes close to their HER activity in acidic electrolytes.

Directing nitrogen-doped carbon support chemistry for improved aqueous phase hydrogenation catalysis

Influencing stability and performance through directing nitrogen-doping in carbon support materials.

Galvanic Exchange as a Novel Method for Carbon Nitride Supported CoAg Catalyst Synthesis for Oxygen Reduction and Carbon Dioxide Conversion

A bimetallic alloy of CoAg nanoparticles (NPs) on a carbon nitride (CN) surface was synthesized using a galvanic exchange process for the oxygen reduction reaction (ORR) and carbon dioxide electrocatalytic conversion. The reduction potential of cobalt is ([Co2+(aq) + 2e− → Co(s)], −0.28 eV) is smaller than that of Ag ([Ag+(aq) + e− → Ag(s)], 0.80 eV), which makes Co(0) to be easily replaceable by Ag+ ions. Initially, Co NPs (nanoparticles) were synthesized on a CN surface via adsorbing the Co2+ precursor on the surface of CN and subsequently reducing them with NaBH4 to obtain Co/CN NP. The Co NPs on the surface of CN were then subjected to galvanic exchange, where the sacrificial Co atoms were replaced by Ag atoms. As the process takes place on a solid surface, only the partial replacement of Co by Ag was possible generating CoAg/CN NPs. Synthesized CoAg/CN bimetallic alloy were characterized using different techniques such as powder x-ray diffraction (PXRD), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron diffraction spectroscopy (EDS) to confirm the product. Both the catalysts, Co/CN and CoAg/CN, were evaluated for oxygen reduction reaction in 1M KOH solution and carbon dioxide conversion in 0.5 M KHCO3. In the case of ORR, the CoAg/CN was found to be an efficient electrocatalyst with the onset potential of 0.93 V, which is comparable to commercially available Pt/C having Eonset at 0.91 V. In the electrocatalytic conversion of CO2, the CoAg/CN showed better performance than Co/CN. The cathodic current decreased dramatically below −0.9V versus Ag/AgCl indicating the high conversion of CO2.

Mesoporous Carbon with Different Density of Thiophenic-Like Functional Groups and Their Effect on Oxygen Reduction

The metal-support interactions between sulfur-doped carbon supports (SMCs) and Pt nanoparticles (NPs) were investigated, aiming at verifying how sulfur functional groups can improve the electrocatalytic performance of Pt NPs towards the oxygen reduction reaction (ORR). SMCs were synthetized, tailoring the density of sulfur functional groups, and Pt NPs were deposited by thermal reduction of Pt(acac)₂ . The extent of the metal-support interaction was proved by X-ray photoelectron spectroscopy (XPS) analysis, which revealed a strong electronic interaction, proportional to the density of sulfur defects, whereas XRD spectra provided evidence of higher strain in Pt NPs loaded on SMC. DFT simulations confirmed that the metal-support interaction was strongest in the presence of a high density of sulfur defects. The combination of microstrain and electronic effects resulted in a high catalytic activity of supported Pt NPs towards ORR, with linear correlations of the half-wave potential E_1/2 or the kinetic current j_k with the sulfur content in the support. Furthermore, a mass activity value (550 A g^-1 ) well above the United States Department of Energy target of 440 A g^-1 at 0.9 V (vs. reversible hydrogen electrode, RHE), was determined.

Oxygen Reduction Reaction Activity of Microwave Mediated Solvothermal Synthesized CeO2/g-C3N4 Nanocomposite

Electrocatalytic active species like transition metal oxides have been widely combined with carbon-based nanomaterials for enhanced Oxygen Reduction Reaction (ORR) studies because of the synergistic effect arising between different components. The aim of the present study is to synthesize CeO₂/g-C₃N₄ system and compare the ORR activity with bare CeO₂. Ceria (CeO₂) embedded on g-C₃N₄ nanocomposite was synthesized by a single-step microwave-mediated solvothermal method. This cerium oxide-based nanocomposite displays enhanced ORR activity and electrochemical stability as compared to bare ceria.

Stabilizing Pt Nanocrystals Encapsulated in N-Doped Carbon as Double-Active Sites for Catalyzing Oxygen Reduction Reaction

Polypropylene fiber, a cheap source of nitrogen-doped carbon, is introduced to design robust nitrogen-doped carbon-encapsulated small Pt nanocrystals with Pt and nitrogen-carbon double-active centers toward oxygen reduction reaction (ORR). Ascribed to the separation effect of the polypropylene fiber, even suffering from a high-temperature carbonization treatment at 720 °C for 90 min, the polypropylene fiber-derived carbon-encapsulated Pt nanocrystal maintains a small particle size (3 nm diameter on average). As expected, its ORR mass activity is up to 116.5 mA/mg at 0.9 V. After 8000 cycles, the half-wave potential of the prepared catalyst declines only by 14 mV compared with 43 mV for the commercial Pt/C catalyst. The significantly improved electrochemical properties of the as-prepared catalyst are resulted from the nitrogen-doped carbon-encapsulated Pt nanocrystal structure, which is benefited to adsorption and activation of oxygen due to the presence of nitrogen-doped carbon as the important active site for ORR besides Pt metal. In addition, the migration, aggregation, and growth of Pt nanoparticles are prohibited in terms of the outer nitrogen-doped carbon protection layer, greatly enhancing the stability of the catalyst.

PdAu alloy nanoparticles supported on nitrogen-doped carbon black as highly active catalysts for Ullmann coupling and nitrophenol hydrogenation reactions

The PdAu/NCB catalyst with a Pd/Au mole ratio of 1/1 shows the highest activity towards both Ullmann coupling reactions of aryl halides and the hydrogenation reaction of nitrophenols.

Enhancement of Electrocatalytic Oxygen Reduction Activity and Durability of Pt-Ni Rhombic Dodecahedral Nanoframes by Anchoring to Nitrogen-Doped Carbon Support

Pt-based nanostructured electrocatalysts supported on carbon black have been widely studied for the oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells. Because sluggish ORR kinetics are known to govern the cell performance, there is a need to develop highly active and durable electrocatalysts. The ORR activity of Pt-based electrocatalysts can be improved by controlling their morphology and alloying Pt with transition metals such as Ni. Improving the catalyst durability remains challenging and there is a lack of catalyst design concepts and synthetic strategies. We report the enhancement of the ORR activity and durability of a nanostructured Pt-Ni electrocatalyst by strong metal/support interactions with a nitrogen-doped carbon (NC) support. Pt-Ni rhombic dodecahedral nanoframes (NFs) were immobilized on the NC support and showed higher ORR electrocatalytic activity and durability in acidic media than that supported on a nondoped carbon black. Durability tests demonstrated that NF/NC showed almost no activity loss even after 50 000 potential cycles under catalytic conditions, and the Ni dissolution from the NFs was suppressed at the NC support, as confirmed by energy dispersive X-ray spectroscopy analysis. Physicochemical measurements including surface-enhanced infrared absorption spectroscopy of surface-adsorbed CO revealed that the strong metal/support interactions of the NF with the NC support caused the downshift of the d-band center position of the surface Pt. Our findings demonstrate that tuning the electronic structure of nanostructured Pt alloy electrocatalysts via the strong metal/support interactions with heteroatom-doped carbon supports will allow the development of highly active and robust electrocatalysts.

Understanding Heteroatom-Mediated Metal–Support Interactions in Functionalized Carbons: A Perspective Review

Carbon-based materials show unique chemicophysical properties, and they have been successfully used in many catalytic processes, including the production of chemicals and energy. The introduction of heteroatoms (N, B, P, S) alters the electronic properties, often increasing the reactivity of the surface of nanocarbons. The functional groups on the carbons have been reported to be effective for anchoring metal nanoparticles. Although the interaction between functional groups and metal has been studied by various characterization techniques, theoretical models, and catalytic results, the role and nature of heteroatoms is still an object of discussion. The aim of this review is to elucidate the metal–heteroatoms interaction, providing an overview of the main experimental and theoretical outcomes about heteroatom-mediated metal–support interactions. Selected studies showing the effect of heteroatom–metal interaction in the liquid-phase alcohol oxidation will be also presented.

Controllable Synthesis of Multiheteroatoms Co-Doped Hierarchical Porous Carbon Spheres as an Ideal Catalysis Platform

The synthesis of porous carbon spheres with hierarchical porous structures coupled with the doping of heteroatoms is particularly important for advanced applications. In this research, a new route for efficient and controllable synthesis of hierarchical porous carbon spheres co-doped with nitrogen, phosphorus, and sulfur (denoted as NPS-HPCs) was reported. This new approach combines in situ polymerization of hexachlorocyclophosphazene and 4,4'-sulfonyldiphenol with the self-assembly of colloidal silica nanoparticles (SiO₂ NPs). After pyrolysis and subsequent removal of the SiO₂ NPs, the resulting NPS-HPCs possess a high surface area (960 m²/g) as well as homogeneously distributed N, P, and S heteroatoms. The NPS-HPCs are shown to be an ideal support for anchoring highly dispersed and uniformly sized noble metal NPs for heterogeneous catalysis. As a proof of concept, Pd NPs are loaded onto the NPS-HPCs using only methanol as a reductant at room temperature. The prepared Pd/NPS-HPCs are shown to exhibit high activity, excellent stability, and recyclability for hydrogenation of nitroarenes.