Carbon-supported Metal Catalysts Research Articles

Fundamental Insights into the Direct Electron Transfer Mechanism on Ag Atomic Cluster.

The nonradical oxidation pathway for pollutant degradation in Fenton-like catalysis is favorable for water treatment due to the high reaction rate and superior environmental robustness. However, precise regulation of such reactions is still restricted by our poor knowledge of underlying mechanisms, especially the correlation between metal site conformation of metal atom clusters and pollutant degradation behaviors. Herein, we investigated the electron transfer and pollutant oxidation mechanisms of atomic-level exposed Ag atom clusters (AgAC) loaded on specifically crafted nitrogen-doped porous carbon (NPC). The AgAC triggered a direct electron transfer (DET) between the terminal oxygen (Oα) of surface-activated peroxodisulfate and the electron-donating substituents-containing contaminants (EDTO-DET), rendering it 11-38 times higher degradation rate than the reported carbon-supported metal catalysts system with various single-atom active centers. Heterocyclic substituents and electron-donating groups were more conducive to degradation via the EDTO-DET system, while contaminants with high electron-absorbing capacity preferred the radical pathway. Notably, the system achieved 79.5% chemical oxygen demand (COD) removal for the treatment of actual pharmaceutical wastewater containing 1053 mg/L COD within 30 min. Our study provides valuable new insights into the Fenton-like reactions of metal atom cluster catalysts and lays an important basis for revolutionizing advanced oxidation water purification technologies.

Molecular Dynamics Simulations Guide the Gasification Process of Carbon-Supported Nickel Catalysts in Biomass Supercritical Water.

In this study, the Density Functional Theory (DFT) Calculations for Molecules and Clusters-ADF module is employed to model carbon-supported nickel catalysts and lignin monomers, integrating the ReaxFF module to simulate molecular dynamics under supercritical water conditions, with a focus on lignin decomposition reactions. Molecular dynamics models for supercritical water gasification are established under various conditions such as catalyst presence, water molecule quantities, and reaction temperature. By comparing simulation systems under different conditions, the yields of and variations in combustible gases (hydrogen and carbon monoxide) are summarized. Introducing heteroatoms into the lattice of the carbon support can alter the electronic structure within graphene, thereby influencing its electrical and electrochemical properties, increasing the number of active sites, and significantly enhancing electrocatalytic activity. Simulation results indicate that carbon-supported nickel metal catalysts can promote the cleavage of C-C bonds in lignin monomers, thereby increasing the rate of water-gas shift reactions and boosting hydrogen production in the system by 105%. Increasing water molecule quantities favored water-gas shift reactions and hydrogen generation while lowering carbon monoxide formation. Moreover, elevating reaction temperatures led to increased hydrogen and carbon monoxide production rates, which were particularly pronounced between 2500 K and 3500 K. These findings offer crucial theoretical insights for advancing efficient hydrogen production through biomass supercritical water gasification.

Contrast variation method applied to structural evaluation of catalysts by X-ray small-angle scattering

In the process of developing carbon-supported metal catalysts, determining the catalyst particle-size distribution is an essential step, because this parameter is directly related to the catalytic activities. The particle-size distribution is most effectively determined by small-angle X-ray scattering (SAXS). When metal catalysts are supported by high-performance mesoporous carbon materials, however, their mesopores may lead to erroneous particle-size estimation if the sizes of the catalysts and mesopores are comparable. Here we propose a novel approach to particle-size determination by introducing contrast variation-SAXS (CV-SAXS). In CV-SAXS, a multi-component sample is immersed in an inert solvent with a density equal to that of one of the components, thereby rendering that particular component invisible to X-rays. We used a mixture of tetrabromoethane and dimethyl sulfoxide as a contrast-matching solvent for carbon. As a test sample, we prepared a mixture of a small amount of platinum (Pt) catalyst and a bulk of mesoporous carbon, and subjected it to SAXS measurement in the absence and presence of the solvent. In the absence of the solvent, the estimated Pt particle size was affected by the mesopores, but in the presence of the solvent, the Pt particle size was correctly estimated in spite of the low Pt content. The results demonstrate that the CV-SAXS technique is useful for correctly determining the particle-size distribution for low-Pt-content catalysts, for which demands are increasing to reduce the use of expensive Pt.

Tuning Active Site Flexibility by Defect Engineering of Graphene Ribbon Edge-hosted Fe-N3 Sites.

Nitrogen-doped, carbon-supported transition metal catalysts are excellent for several reactions. Structural engineering of M-Nx sites to boost catalytic activity is rarely studied. Here, we demonstrate that the structural flexibility of Fe-N3 site is vital for tuning the electronic structure of Fe atoms and regulating the catalytic transfer hydrogenation (CTH) activity. By introducing carbon defects, we construct Fe-N3 sites with varying Fe-N bond lengths distinguishable by X-ray absorption spectroscopy. We investigate the CTH activity by density-functional theory and microkinetic calculations and reveal that the vertical displacement of the Fe atom out of the plane of the support, induced by the Fe-N3 distortion, raises the Fe orbital and strengthens binding. We propose that the activity is controlled by the relaxation of the reconstructed site, which is further affected by Fe-N bond length, an excellent activity descriptor. We elucidate the origin of the CTH activity and principles for high-performing Fe-N-C catalysts by defect engineering.

(Invited) Interface Engineering of Nanocarbon-Supported Catalysts for Direct Ethanol Fuel Cells

A new strategy to controllably regulate the local bonding states and local coordination environment (LCE) in the classic carbon-supported metal catalysts for direct ethanol fuel cells will be discussed. Particularly, we use F-doped Pd-X-C (X = N, P, S, B) as model systems to study the LCE effects on oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR). The N-rich Pd surface with regulated LCE not only boosts the activity of Pd but also enhances the catalyst stability by inhibiting the migration and agglomeration of Pd as well as endowing carbon support with long-term anti-corrosion properties. As a result, an unbeatable power density of 0.57 W cm-2 with more than 5,900 hours of operation was achieved in direct ethanol fuel cells using the proposed LCE-regulated catalysts, remarkably outperforming benchmarking catalysts. In an extended study, the concept of F-induced LCE regulation can be applied to other Pd-X-C catalysts (X = P, S, B) with drastically improved activities and stabilities

Enhanced stability of nitrogen-doped carbon-supported palladium catalyst for oxidative carbonylation of phenol

Enhancing the stability of supported noble metal catalysts emerges is a major challenge in both science and industry. Herein, a heterogeneous Pd catalyst (Pd/NCF) was prepared by supporting Pd ultrafine metal nanoparticles (NPs) on nitrogen-doped carbon; synthesized by using F127 as a stabilizer, as well as chitosan as a carbon and nitrogen source. The Pd/NCF catalyst was efficient and recyclable for oxidative carbonylation of phenol to diphenyl carbonate, exhibiting higher stability than Pd/NC prepared without F127 addition. The hydrogen bond between chitosan (CTS) and F127 was enhanced by F127, which anchored the N in the free amino group, increasing the N content of the carbon material and ensuring that the support could provide sufficient N sites for the deposition of Pd NPs. This process helped to improve metal dispersion. The increased metal-support interaction, which limits the leaching and coarsening of Pd NPs, improves the stability of the Pd/NCF catalyst. Furthermore, density functional theory calculations indicated that pyridine N stabilized the Pd2+ species, significantly inhibiting the loss of Pd2+ in Pd/NCF during the reaction process. This work provides a promising avenue towards enhancing the stability of nitrogen-doped carbon-supported metal catalysts.

Enhanced Catalytic Hydrodeoxygenation of Activated Carbon-Supported Metal Catalysts via Rapid Plasma Surface Functionalization.

We employ a nonthermal, He/O2 atmospheric plasma as an efficient surface functionalization method of activated carbons. We show that plasma treatment rapidly increases the surface oxygen content from 4.1 to 23.4% on a polymer-based spherical activated carbon in 10 min. Plasma treatment is 3 orders of magnitude faster than acidic oxidation and introduces a diverse range of carbonyl (C═O) and carboxyl (O-C═O) functionalities that were not found with acidic oxidation. The increased oxygen functionalities reduce the particle size of a high 20 wt % loading Cu catalyst by >44% and suppress the formation of large agglomerates. Increased metal dispersion exposes additional active sites and improves the yield of hydrodeoxygenation of 5-hydroxymethyl furfural to 2,5-dimethyl furan, an essential compound for biofuel replacement, by 47%. Surface functionalization via plasma can advance catalysis synthesis while being rapid and sustainable.

Dispersed Pd/alumina catalyst with finite iodine entry for boosted CO purification and dimethyl carbonate synthesis

Sintering caused catalyst deactivation resulting from the agglomeration of active metal sites. CH3I/CO based approaches are attractive owing to their effectiveness and mildness in rejuvenating sintered catalysts. However, few investigations have been reported for their application in oxides-supported catalysts. Herein, a landscape of reversible structural transformation of Pd/γ-Al2O3 catalyst is depicted. The scenario of Pd redispersion in γ-Al2O3 is significantly different from previously reported activated carbon-supported metal catalysts. Time-controlled CH3I/CO treatment facilitates the rapid redispersion of aggregated Pd particles on γ-Al2O3 surface. The redispersion of Pd promotes the reactivity of Pd/γ-Al2O3 catalyst. For dimethyl carbonate synthesis, the space-time yield increases from 556.9 gDMC·kgcat.−1·h−1 to 1008.9 gDMC·kgcat.−1·h−1. For CO purification, 100% conversion of H2 impurities and the rise of 27.4% in selectivity are achieved. The intrinsic oxidation rate is elevated from 607.8 mmolH2·gPd−1·h−1 to 1166.5 mmolH2·gPd−1·h−1. However, complete oxidation and atomic dispersion of Pd cannot be achieved through further treatments. Combined characterizations verify the particle reconstruction of Pd and the evolution of anchoring sites on γ-Al2O3via strong metal-iodine binding. It reveals a double-edged-sword character of CH3I for Pd redispersion, wherein the excessive treatment results in re-aggregated Pd particles and inferior reactivity.

Efficacy and mechanism of peroxymonosulfate activation by single-atom transition metal catalysts for the oxidation of organic pollutants: Experimental validation and theoretical calculation

Single-atom catalysts can activate peroxymonosulfate (PMS) to enhance its oxidation of organic pollutants in water treatment. We synthesized a series of carbon-supported single-atom transition metal catalysts (MnN@C, FeN@C, CoN@C, NiN@C, and CuN@C) with similar compositions and structures. Their catalytic activity toward PMS activation and oxidation mechanisms were investigated using acid orange 7 (AO7) as a model pollutant. The degradation rate (min−1·mol−1·g·m−2) of AO7 followed order: FeN@C/PMS (7.576 × 103) > MnN@C/PMS (5.104 × 103) > CoN@C/PMS (1.919 × 103) ≫ NiN@C/PMS (0.058 × 103) > CuN@C/PMS (0.035 × 103). Electron transfer mediated by surface-activated PMS was found to be the main regime of AO7 oxidation in the catalytic systems. Density functional theory calculations indicated that the degradation of AO7 was promoted by the intense adsorption of PMS and the electron transfer between AO7 and the surface-activated PMS on the catalyst. The cleavage of the naphthalene ring and the azo group was the primary degradation pathway. The toxicity of the products was significantly reduced. This research provides valuable findings for preparing highly efficient single-atom transition metal catalysts for PMS-based degradation of toxic and refractory organic pollutants from water.

Metal–Support Interaction of Carbon–Based Electrocatalysts for Oxygen Evolution Reaction

Metal–support interaction (MSI) is considered a key effect of electronic and geometric structures of catalysts on tuning catalytic performance. The oxygen evolution reaction (OER) is a crucial process during energy conversion and storage. However, the OER process requires the help of noble metal catalysts to reduce the reaction overpotential, enhance reactivity with intermediates, and maintain good operating stability. Carbon–supported metal catalysts have been considered candidates for noble metal catalysts for OER. MSI occurs at the interface of carbon supports and metals, affecting the catalytic performance through electronic and geometric modulation. MSI can influence the catalytic performance and change reaction pathways from charge redistribution, electron transfer, chemical coordination and bonding, and steric effect. Connecting MSI effects with the OER mechanism can provide theoretical guidance and a practical approach to the design of efficient catalysts, including the modulation of particle size, morphology, heteroatom doping, defect engineering, and coordination atom and number. Advantage can be taken of MSI modulation between metal compounds and carbon supports to provide guidance for catalyst design.

Catalytic effect of bamboo-like carbon nanotubes loaded with NiFe nanoparticles on hydrogen storage properties of MgH2

Catalyst doping modification has become an important strategy to solve the high desorption temperature and sluggish kinetics issues of magnesium hydride (MgH2) for further commercial application. Herein, we report a novel strategy by exploiting a catalyst with the “magnesophilic” transition metal and “magnesiphobic” carbon material to construct a MgH2-catalysts-carbon layer hydrogen storage material, where the bimetallic NiFe nanoparticles are supported atop the bamboo-like carbon nanotubes (NiFe@CNT) via calcination. The experimental results show that the built MgH2-NiFe@CNT composite can absorb 4.06 and 3.25 wt% H2 at 373 and 348 K, respectively, while the milled-MgH2 almost no longer absorbs H2 and takes only 0.82 wt% even at 423 K. More importantly, the initial desorption temperature of the MgH2-NiFe@CNT composite reduces to 498 K by 122 K in contrast to the milled-MgH2, and the dehydrogenation activation energy decreases from 151.8 to 49.7 kJ mol−1. Ex situ structural characterization and theoretical calculation show that the synergistic effects of the “hydrogen pump” role of Mg2Ni/Mg2NiH4 and “hydrogen gateway” role of α-Fe, as well as the good dispersion function of carbon nanotubes generated in situ from the NiFe@CNT, contribute the excellent hydrogen storage properties of the MgH2-NiFe@CNT composite. This study provides new insights into the modification of MgH2 by carbon-supported transition metal catalysts.

The phenomenon of “dead” metal in heterogeneous catalysis: opportunities for increasing the efficiency of carbon-supported metal catalysts

The key factors contributing to the formation of “dead” metal – the fraction of metal in supported catalysts that remains inaccessible to reactants – are examined. Strategies to minimize the formation of “dead” metal are analyzed.

Activating nitrogen-doped carbon nanosheets by KOH treatment to promote the Fischer-Tropsch synthesis performance

Both appropriate support and promoter decoration are vital to improve the performance of Fe-based catalysts in Fischer-Tropsch Synthesis (FTS). In this work, KOH chemical activation was applied to modify N-doped carbon nanosheets supported Fe catalysts, which achieves support modification and promoter introduction simultaneously. The influence of KOH activation on pore structure, N configuration and residual K effect were discussed on Fe-based FTS catalyst. Upon KOH activation, the micro-porosity was enriched which improved the Fe dispersion. Meanwhile, the N configuration was regulated through KOH in-situ reacting with carbon matrix. Further investigation on residual K revealed a combined effect of K and N. The physical structure, electronic properties and phase compositions after activation were characterized by N2-physisorption, XRD, Raman, TEM, XPS and Mössbauer spectroscopy. Combing with the catalytic performance, we demonstrated that the pyrrolic N sites and suitable K residuals contribute to electron donation to Fe species, collectively promoting Fe carburization accompanied by higher C2–C4= productivity. This finding is expected to offer an activation method for preparation of N-doped carbon supported metal catalysts.

Redispersion strategy for high-loading carbon-supported metal catalysts with controlled nuclearity

Redispersion strategy for high-loading carbon-supported metal catalysts with controlled nuclearity

Enhanced interfacial interaction of mesoporous N, S co-doped carbon supported WO3-WS2 for green and selective oxidation of alcohols

WO3-WS2 supported on mesoporous N, S co-doped carbon were prepared via one-step molten-salt strategy, and an enhanced interfacial interaction between metal and carbon was formed, which boosted the selective oxidation of benzyl alcohol to benzaldehyde. Under green conditions (solvent: H2O, oxidant: H2O2, no base-additives), 96.2 % conversion and 97.0 % selectivity were achieved within 2 h. It was found that N, S co-doped carbon with abundant mesopores served as an excellent support, the pyrrolic and graphitic N species of carbon provided main anchoring sites for metal nanoparticles. Additionally, the N, S co-doped carbon support could effectively promote the distribution of metal species, and thereafter induced the formation of metal–carbon interfacial interaction. Such advantages resulted in the satisfactory reusability (negligible activity loss after six cycles) and remarkable catalytic activity. The scavenger tests and EPR results disclosed that ·OH was the main oxidative species for this reaction. DFT calculations revealed that co-doping of N, S into carbon promoted the adsorption of benzyl alcohol, and the enhanced metal–carbon interaction reduced migration energy barrier of ·OH on catalyst surface. In sum, this work provided some new insights into the interfacial interaction of mesoporous carbon supported metal catalyst for boosting the selective oxidation of alcohols.

Well-dispersed cobalt nanoparticles encapsulated on ZIF-8-derived N-doped porous carbon as an excellent peroxymonosulfate activator for sulfamethoxazole degradation

The practical application of carbon materials supported cobalt (Co)-based heterogeneous materials as excellent peroxymonosulfate (PMS) activators has attracted wide attention in recent years because of their low particle self-agglomeration and low Co leaching. Herein, we developed a new synthetic strategy for designing Co nanoparticles (Co NPs)-encapsulated zeolitic imidazolate frameworks-8-derived porous carbon (NC) (denoted as Co-N@NC-x) by calcinating NC with a polydopamine-chelating Co coating. It was demonstrated that NC could effectively prevent Co agglomeration because it provided rich load sites for Co. Raman and X-ray photoelectron spectroscopy results indicated that the addition of Co could increase the surface defects of NC. Co-N@NC-90 (0.1 g/L) efficiently catalyzed PMS to degrade >96.91 % of sulfamethoxazole (SMX, 20 mg/L) within 90 min with an initial pH of 3.65–9.05 based on the synergistic effect between Co NPs and NC. It was demonstrated that Co NPs were the main active sites, and the defects and carbonyl (CO) group active sites also contributed to SMX degradation by catalyzing PMS. Graphitic N was an adsorption site for SMX. A singlet oxygen (1O2)-dominated non-radical mechanism has been proposed. Additionally, Co-N@NC-90 exhibited excellent stability resulting from NC encapsulation. This work provides a novel approach for the application of metal organic framework-derived carbon-supported metal catalysts in wastewater purification.

Bottom-Up Synthesis Strategies Enabling the Investigation of Metal Catalyst-Carbon Support Interactions

The structural versatility and vibrant surface chemistry of carbon materials offer tremendous opportunities for tailoring the catalytic performance of supported metal nanoparticles through the modulation of interfacial metal-support interactions (MSI). MSI’s geometric and structural effects are well documented for these materials. However, other potential support effects such as electronic metal-carbon interactions remain poorly understood. Such limitations are tied to constraints intrinsic to commonly available carbon materials such as activated carbon (e.g., microporosity) and the top-down approach that is often used for their synthesis. Nonetheless, it is crucial to understand the interplay between the structure, properties, and performance of carbon-supported metal catalysts to take steps toward rationalizing their design. The present study investigates promising and scalable bottom-up synthesis approaches, namely hydrothermal carbonization (HTC) and evaporation-induced self-assembly (EISA), that offer great flexibility for controlling the carbon structure. The opportunities and limitations of the methods are discussed with a particular focus on harnessing the power of oxygen functionalities. A remarkable production yield of 32.8% was achieved for mesoporous carbons synthesized via EISA. Moreover, these carbon materials present similar external surface areas of 316 ± 19 m2/g and average pore sizes of 10.0 ± 0.1 nm while offering flexibility to control the oxygen concentration in the range of 5–26 wt%. This study provides the cornerstone for future investigations of metal-carbon support interactions and the rational design of these catalysts.

Porous chitosan-derived activated N-doped carbon-supported Pd nanoparticles encaged in Al, Fe pillared montmorillonite as novel heterogeneous catalysts

Novel pillared montmorillonite encaging porous chitosan derived activated N-doped carbon-supported Pd nanoparticles catalyst (Pd@Al, Fe-Mt/N-C) was synthesized by ion-exchange of the chitosan/Pd precursors into the layered-nanospaces of Al, Fe-pillared montmorillonite and subsequent confinement carbonization. N2 adsorption-desorption study indicated that the resulting nanocomposites have rich mesoporous structures. Effective encagement and in-situ reduction of Pd species led to a decrease in surface area and N2 adsorption, respectively. TEM images of the prepared Pd@Al, Fe-Mt/N-C showed that most of the Pd species with a size of 1–3 nm were well dispersed in the interlayer spaces of the pillared montmorillonite. The catalysts showed high catalytic efficiency for the Sonogashira reactions of aryl iodides or bromides with terminal aryl alkynes, and can be recycled 20 runs with no significant loss of activities. The unique encagement of the porous carbon-supported metal catalysts in the layered nanospaces of pillared montmorillonite was further elucidated by the positron annihilation spectroscopy analysis and other structural characterization methods. This layered-nanospace confinement strategy achieves a perfect combination of the advantages of activated carbon and montmorillonite as metal nanoparticle carrier.

A general strategy for overcoming the trade-off between ultrasmall size and high loading of MOF-derived metal nanoparticles by millisecond pyrolysis

Metal-organic frameworks (MOFs) have flourished as a library of promising precursors for synthesizing carbon-supported metal catalysts by pyrolysis, but it is extremely difficult to simultaneously achieve a high metal loading and an ultrasmall size, particularly for non-noble metal (Fe, Co, Ni, etc.) that are highly active and have a strong tendency to coarsen. Here, we report a general strategy for controllable synthesize thermodynamically metastable sub-3 nm non-noble metal nanoparticles (NPs) with ultrahigh metal loading up to 41.0 wt% (12.8 at%) by rapid pyrolysis of MOF (e.g., ~ 1000 °C in 0.3 s), at least four-fold higher than the reported strategy where ultrasmall NPs were obtained but with a significant sacrifice of metal loading (usually less than 10 wt%). Furthermore, we found that the formation of metal NPs during high-temperature pulse agrees with the LaMer model (sigmoidal coarsening kinetics), in which rapid pyrolysis triggers only the initial nucleation and avoids Ostwald ripening or further coalescence. We also demonstrate the generality of our strategy in synthesizing other MOF-derived ultrasmall NPs, including non-noble metal NPs (Ni), metallic compound (CoS2), and alloy (CoPd). As a demonstration, the obtained CoPd-based catalyst showed high activity and robust stability during prolonged catalytic reactions. Therefore, our strategy and mechanistic insights enable the rational design and controlled synthesis of advanced catalysts with a good balance between ultrasmall size and a high metal loading, from more than 100,000 types of MOFs.

Redispersion strategy for high-loading carbon-supported metal catalysts with controlled nuclearity.

Supported low-nuclearity metal catalysts integrating single atoms or small clusters have emerged as promising materials for diverse applications. While sophisticated synthetic methods provide a high level of nuclearity control in the subnanometre regime, these routes do not fulfil the requirements for translation into industrial practice of (i) effectiveness for high metal contents and (ii) facile scalability. Herein, we present a gas-phase redispersion strategy consisting of sequential C2H2 and HCl treatments to gradually disperse Ru, Rh and Ir nanoparticles supported on commercial activated carbon with metal content up to 10 wt% and initial average sizes of ≈ 1 nm into small clusters and eventually single atoms. Avoidance of nanoparticle surface overchlorination, which hinders C2H2 adsorption, is identified as key for the redispersion process, as demonstrated by the inefficacy of both C2H2–HCl cofeeding and inverse sequence (i.e., HCl first) treatments. Precise size control (±0.1 nm) is enabled by regulating the number of C2H2–HCl cycles. Detailed characterisation by X-ray absorption spectroscopy, electron paramagnetic resonance and time-resolved mass spectrometry reveals that the redispersion occurs via a layer-by-layer mechanism. Specifically, the migration of surface chlorinated metal species to the carbon support is induced by the C2H2 treatment, depleting accessible surface Cl atoms, while the subsequent HCl treatment rechlorinates the cluster surface. The strategy paves the way for the generation of high-density metal sites with tuneable nuclearity for tailored applications.