Dispersed Catalysts Research Articles

Enhancing activity and stability of Fe[sbnd]N[sbnd]C catalysts through co incorporation for oxygen reduction reaction

FeNC single atom catalysts (SACs) have attracted great interest due to their highly active FeN4 sites. However, the pyrolysis treatment often leads to inevitable metal migration and aggregation, which reduces the catalytic activity. Moreover, due to the Fenton reaction caused by FeNC in alkaline and acidic solutions, the presence of Fe and peroxide in electrodes may generate free radicals, resulting in serious degradation of the organic ionomer and the membrane. Herein, we report an original strategy of introducing Co single atoms into FeNC catalysts, forming atomically dispersed bimetallic active sites (FeCoNC) and improving the activity and stability of the catalyst. Benefiting from this strategy, FeCoNC catalyst exhibits excellent oxygen reduction reaction (ORR) activity in alkaline media (E1/2 = 0.88 V) and in acidic media (E1/2 = 0.77 V). As the cathode of Zn-air battery (ZAB), FeCoNC shows an excellent peak power density of 142.8 mW cm−2 and a specific capacity of 806.6 mAh/gZn. This work provides a novel avenue to optimize and enhance the ORR performance of atomic dispersed FeNC catalysts.

Tackling the activity and selectivity challenges of electrocatalysts towards CO2 reduction reaction via atomically dispersed dual atom catalysts

Atomically dispersed catalysts are widely employed in electrochemical CO2 reduction reaction (CO2RR) owing to maximal atomic utilization and outstanding intrinsic activity. Dual atom catalysts (DACs) can provide dispersed active sites and distinct electronic structure, which is conducive to the catalytic performance. In this work, we explored a series of DACs composed of transition metals or main group metals supported on nitrogen-doped graphene (M2–NC) toward high-performance CO2RR. The radius of metal atom is a key factor determining the stability of M2–NC, that metals (M = Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Sn, Pt, and Bi) with small radius can be steadily anchored. Fe2/Co2/Ru2–NC are screened out to activate CO2 because of the strong interaction between metals d orbitals and CO2 2π orbitals. The results show that Fe2/Co2–NC can promote CO2RR to generate HCOOH with limiting potentials (UL) of −0.36 and −0.25 V. Ru2–NC can catalyze the generation of CH3OH and CH4 with UL of −0.28 V. Limited potential difference between CO2RR and hydrogen evolution reaction (HER) show that Fe2/Co2/Ru2–NC can inhibit HER during CO2RR. This work demonstrates Fe2/Co2/Ru2–NC as high-performance CO2RR catalysts and highlights the type of central metal atoms as a vital role for atomically dispersed electrocatalysts.

Highly selective hydrogenation of guaiacol to cyclohexanol over carbon-encapsulated highly dispersed cobalt catalyst

Cyclohexanol is an important chemical intermediate. In recent years, guaiacol has received widespread attention as a lignin model compound for hydrodeoxygenation (HDO) to prepare cyclohexanol. However, it remains a major challenge to prepare a highly active and stable catalyst. In this work, a series of carbon-encapsulated Co-based catalysts with different Co/C ratios were synthesized using an extremely simple approach for the HDO of guaiacol to cyclohexanol. Among them, the Co1@C1 catalyst (with a ratio of 1:1 between glucose and cobalt nitrate) showed the best catalytic activity, achieving almost complete directional conversion of guaiacol to cyclohexanol within 60 min at 200 °C, with a yield of up to 98.5 % for cyclohexanol. In addition, the catalyst also exhibited extremely high stability, and no significant deactivation was observed after 5 cycles under optimal conditions. The research outcomes point out that highly dispersed CoO NPs are the main active centers. The graphite layer ensures the stability of the Cox@Cy catalyst, and effectively avoids the leaching of metals in high-temperature reactions. This work provides important reference for the development of high activity, high stability, and low-cost catalysts for the hydrogenation of phenolic derivatives in the future.

3D Printing for catalytic activity – Ink development using a drop-on-demand cartridge

Powder-bed binder-jetting 3D printing requires the use of a liquid binder or ink. These binders/inks have been widely tailored for individual application. This article covers the proof-of-concept development of an ink for a powder-bed binder-jetting 3D printer. The ink formulation in our work is both catalytically active and is used to bind the catalyst substrate. A modified, commercially available inkjet cartridge is used to spread liquid ink onto a powder surface, forming shapes in a layer-wise fashion to produce green bodies that are heat-treated post-print. The important parameters for manipulating rheology for inkjet printing are explored as a function of ink formulation. Specifically, the relationship between density, viscosity, and surface tension dictates the suitability for printing and those properties can be controlled by using differing amounts of the ink’s constituent components. Relationships correlating density to solvent concentration and viscosity to binder and dispersant concentrations are found, which are used to determine a water-based ink formulation suitable for use in the commercially available ink cartridge. The results of an initial print are presented and show excellent dispersion of the nickel catalyst.

Photochemical tuning of dynamic defects for high-performance atomically dispersed catalysts.

Developing active and stable atomically dispersed catalysts is challenging because of weak non-specific interactions between catalytically active metal atoms and supports. Here we demonstrate a general method for synthesizing atomically dispersed catalysts via photochemical defect tuning for controlling oxygen-vacancy dynamics, which can induce specific metal-support interactions. The developed synthesis method offers metal-dynamically stabilized atomic catalysts, and it can be applied to reducible metal oxides, including TiO2, ZnO and CeO2, containing various catalytically active transition metals, including Pt, Ir and Cu. The optimized Pt-DSA/TiO2 shows unprecedentedly high photocatalytic hydrogen evolution activity, producing 164 mmol g-1 h-1 with a turnover frequency of 1.27 s-1. Furthermore, it generates 42.2 mmol gsub-1 of hydrogen via a non-recyclable-plastic-photoreforming process, achieving a total conversion of 98%; this offers a promising solution for mitigating plastic waste and simultaneously producing valuable energy sources.

Dual-axial engineering on atomically dispersed catalysts for ultrastable oxygen reduction in acidic and alkaline solutions

Atomically dispersed catalysts are a promising alternative to platinum group metal catalysts for catalyzing the oxygen reduction reaction (ORR), while limited durability during the electrocatalytic process severely restricts their practical application. Here, we report an atomically dispersed Co-doped carbon-nitrogen bilayer catalyst with unique dual-axial Co-C bonds (denoted as Co/DACN) by a smart phenyl-carbon-induced strategy, realizing highly efficient electrocatalytic ORR in both alkaline and acidic media. The corresponding half-wave potential for ORR is up to 0.85 and 0.77 V (vs. reversible hydrogen electrode (RHE)) in 0.5 M H2SO4 and 0.1 M KOH, respectively, representing the best ORR activity among all non-noble metal catalysts reported to date. Impressively, the Zn-air battery (ZAB) equipped with Co/DACN cathode achieves outstanding durability after 1,688 h operation at 10 mA cm-2 with a high current density (154.2 mA cm-2) and a peak power density (210.1 mW cm-2). Density functional theory calculations reveal that the unique dual-axial cross-linking Co-C bonds of Co/DACN significantly enhance the stability during ORR and also facilitate the 4e- ORR pathway by forming a joint electron pool due to the improved interlayer electron mobility. We believe that axial engineering opens a broad avenue to develop high-performance heterogeneous electrocatalysts for advanced energy conversion and storage.

Interface effect of MXene/CoP2 on oxygen evolution reaction

Zero/two-dimension MXene/CoP2 was fabricated by hydrothermal and chemical vapor deposition phosphate. MXene nanosheets were employed as substrate to increase dispersion and stability of the multicomponent catalyst. The morphology of CoP2 was regulated by the addtion of Co precursor in hydrothermal process. The catalyst prepared by 40 mg MXene with 0.5 mmol Co precursor (MXene/CoP2-0.5) showed a uniform growth of CoP2 and formed aboundant reaction sites. The interfacial electron transfer between MXene and CoP2 modulated the electron structure of CoP2 and stimulated the oxygen evolution reaction (OER) activity. The optimized MXene/CoP2-0.5 exhibited the OER overpotential of 263 mV and a long-term stability over 16 h. Modulating the interfacial electron transfer of multicomponent catalysts provided a guidance for electrocatalyst design and synthesis.

Design of a highly active TiO2-supported Ni2P@C catalyst with special flower-like radial channels for quick p-cresol hydrodeoxygenation

Efficient hydrodeoxygenation (HDO) catalysts are of great important for converting biomass to value-added chemicals and fuels. Herein, a novel strategy was proposed for preparing a highly dispersed carbon-encapsulated flower-like radial channels TiO2-supported catalysts (Ni2P@C/FLRC-TiO2). The effect of P and pore structure of the TiO2 support on the p-cresol HDO performance was investigated. An appropriate P content improves the HDO performance by increasing the acidity, while the open radial channels of Ni2P@C/FLRC-TiO2 greatly reduces the reaction time and enhance the HDO activity by promoting rapid mass transfer and accessibility of active sites. The highly dispersed carbon-encapsulated Ni2P@C nanoparticles further improve the HDO activity and stability. Among the prepared catalysts, Ni2P@C/FLRC-TiO2-1 (initial NaH2PO2:Ni-MOF/FLRC-TiO2 mass ratio = 1) exhibited the best HDO performance, yielding 100 % of the p-cresol deoxygenation products within 1.5 h under mild conditions. The excellent HDO performance originates from the synergy between FLRC-TiO2 radial channels and the Ni2P@C nanoparticles.

Support effects in vanadium incipient wetness impregnation for oxidative and non-oxidative propane dehydrogenation catalysis

Oxidative propane dehydrogenation with carbon dioxide (OPDH) is an attractive technology, co-producing carbon monoxide and propylene with a potentially negative greenhouse gas balance. Vanadium oxide containing materials are a promising class of catalysts for such process. In this study, we introduce vanadium into a range of micro- and mesoporous aluminosilicate materials by incipient wetness impregnation with NH4VO3 and the complexing agent oxalic acid, targeting a surface density of 0.6 V atoms per nm² of pristine support. Aspects of this impregnation procedure and its interaction with the support are discussed in the context of potential sources of variability in the dispersion of VOx species within the catalyst materials. The results indicate no formation of a monolayer of vanadium species, and differences in catalyst dispersion are discernible through comprehensive characterization techniques, including thermogravimetric analysis (TGA), X-ray diffraction, Raman spectroscopy, H2-TPR and UV-Vis spectroscopy. The materials are evaluated for the catalytic CO2-assisted propane dehydrogenation in two pressure regimes while being compared to classic non-oxidative propane dehydrogenation (PDH). Results are analyzed using a multivariable analysis of reactants and products, which provided valuable insights for catalyst design. The larger pore material with few acid sites (MCM-41 with Si/Al = 180) demonstrates the highest activity in terms of PDH and high pressure OPDH, i.e. high propane conversion and quasi-equivalent propylene production with similar CO2 co-conversion and CO co-production). Yet, in terms of responding to the introduction of CO2, the zeolites seem to respond better in terms of producing less H2 for only a small decrease in propylene. These microporous materials, except for the most acidic aluminous BEA, exhibit a significant increase in their production rates when increasing the total pressure in OPDH.

Biomimetic nucleotide-graphene hybrids for electrocatalytic oxygen conversion: Quantifying biomolecule mass loading

Metal-free electrocatalysts for the electrochemical conversion of gases constitute an important asset for a sustainable energy transition. Nucleotides act as redox mediators in the electron transport chain to reduce oxygen in cellular respiration. The biomimicry of such an efficient natural mechanism could be utilized to address the challenges associated with electrochemical gas conversion technologies, such as sluggish kinetics and high overpotentials. Multiple descriptors are generally reported to benchmark the activity of electrocatalysts where the turnover frequency (TOF) is claimed to be the most accurate criterion. Here, a library of graphene nanosheets-nucleotide hybrid materials was prepared, and the electrocatalytic performance towards ORR/OER reactions of a graphene-flavin mononucleotide hybrid was evaluated by rotating disc electrode experiments and TOF estimation. The determination of catalyst loading and dispersion is especially relevant when assessing the intrinsic activity of a catalyst and, therefore, the amount of nucleotide electrocatalyst loaded into the graphene support was thoroughly quantified by a combination of characterization techniques. Density functional theory calculations supported the observed experimental trends, both on the adsorption rate of a given nucleotide on graphene and the catalytic activity of a specific hybrid material. This work constitutes an avenue to predict nature-mimicking electrocatalysts for efficient energy storage.

Highly Active and Robust Catalyst: Co2B-Fe2B Heterostructural Nanosheets with Abundant Defects for Hydrogen Production.

A high-performance and reusable nonnoble metal catalyst for catalyzing sodium borohydride (NaBH4) hydrolysis to generate H2 is heralded as a nuclear material for the fast-growing hydrogen economy. Boron vacancy serves as a flexible defect site that can effectively regulate the catalytic hydrolysis performance. Herein, we construct a uniformly dispersed and boron vacancy-rich nonnoble metal Co2B-Fe2B catalyst via the hard template method. The optimized Co2B-Fe2B exhibits superior performance toward NaBH4 hydrolysis, with a high hydrogen generation rate (5315.8 mL min-1 gcatalyst-1), relatively low activation energy (35.4 kJ mol-1), and remarkable cycling stability, outperforming the majority of reported catalysts. Studies have shown that electron transfer from Fe2B to Co2B, as well as abundant boron defects, can effectively modulate the charge carrier concentration of Co2B-Fe2B catalysts. Density functional theory calculations confirm that the outer electron cloud density of Co2B is higher than that of Fe2B, among which Co2B with high electron cloud density can selectively adsorb BH4- ions, while the electron-deficient Fe2B is favorable for capturing H2O molecules, therefore synergistically promoting the catalytic NaBH4 hydrolysis to produce H2.

Alkyl C–O bond cleavage assisted by partial C–H activation on atomically dispersed catalysts and metal surfaces

Alkyl C–O bond cleavage assisted by partial C–H activation on atomically dispersed catalysts and metal surfaces

Promotion effect of epoxy group neighboring single-atom Cu site on acetylene hydrochlorination

Carbon materials have been used as the support for catalysts in the field of acetylene hydrochlorination, the influence of inevitable oxygen-containing moieties on the reaction is often ignored and the mechanism of the oxygen-doping structure remains ambiguous. Herein, we explored the effect of the oxygen-containing group (COC) in the support on the activity of single-atom dispersed Cu catalysts. By immersing the Cu single-atom catalyst in an alkaline solution, the epoxy species on the carbon support was cleaved to obtain a pure ether species while the Cu site was modified to a more electron-deficient state. The turnover frequency value of Cu/O-FLP catalyst with epoxy groups was 1.6-fold higher than that of alkaline treated catalyst. Our result indicated that the epoxy groups could assist adjacent single-atom Cu sites to synergistically promote the adsorption and cleavage of the reactant hydrogen chloride toward form COH and CuCl bonds, and reduce the reaction energy barrier. The presence of electron deficient Cu sites and ether species could induce competitive adsorption of the acetylene and hydrogen chloride, thereby reducing the activity of the catalyst. This study highlights the influence of surface oxygen species and the tunability of the support, providing the foundation for the fabrication of higher-activity Cu catalysts for acetylene hydrochlorination.

Atomically dispersed cobalt catalysts for tandem synthesis of primary benzylamines from oxidized β-O-4 segments.

This work presents an innovative approach focusing on fine-tuning the coordination environment of atomically dispersed cobalt catalysts for tandem synthesis of primary benzylamines from oxidized lignin model compounds. By meticulously regulating the Co-N coordination environment, the activity of these catalysts in the hydrogenolysis and reductive amination reactions was effectively controlled. Notably, our study demonstrates that, in contrast to cobalt nanoparticle catalysts, atomically dispersed cobalt catalysts exhibit precise control of the sequence of hydrogenolysis and reductive amination reactions. Particularly, the CoN3 catalyst with a triple Co-N coordination number achieved a remarkable 94% yield in the synthesis of primary benzylamine. To our knowledge, there is no previous documentation of the synthesis of primary benzylamines from lignin dimer model compounds. Our study highlights a promising one-pot route for sustainable production of nitrogen-containing aromatic chemicals from lignin.

Understanding the preparative chemistry of atomically dispersed nickel catalysts for achieving high-efficiency H2O2 electrosynthesis.

Electrochemical hydrogen peroxide (H2O2) production via two-electron oxygen reduction reaction (2e- ORR) has received increasing attention as it enables clean, sustainable, and on-site H2O2 production. Mimicking the active site structure of H2O2 production enzymes, such as nickel superoxide dismutase, is the most intuitive way to design efficient 2e- ORR electrocatalysts. However, Ni-based catalysts have thus far shown relatively low 2e- ORR activity. In this work, we present the design of high-performing, atomically dispersed Ni-based catalysts (Ni ADCs) for H2O2 production through understanding the formation chemistry of the Ni-based active sites. The use of a precoordinated precursor and pyrolysis within a confined nanospace were found to be essential for generating active Ni-N x sites in high density and increasing carbon yields, respectively. A series of model catalysts prepared from coordinating solvents having different vapor pressures gave rise to Ni ADCs with controlled ratios of Ni-N x sites and Ni nanoparticles, which revealed that the Ni-N x sites have greater 2e- ORR activity. Another set of Ni ADCs identified the important role of the degree of distortion from the square planar structure in H2O2 electrosynthesis activity. The optimized catalyst exhibited a record H2O2 electrosynthesis mass activity with excellent H2O2 selectivity.

Unravelling the origin of reaction-driven aggregation and fragmentation of atomically dispersed Pt catalyst on ceria support.

Metal-support interaction plays a crucial role in governing the stability and activity of atomically dispersed platinum catalysts on ceria support. The migration and aggregation of platinum atoms during the catalytic reaction leads to the redistribution of active sites. In this study, by utilizing a multimodal characterization scheme, we observed the aggregation of platinum atoms at high temperatures under reverse water gas shift reaction conditions and the subsequent fragmentation of platinum clusters, forming "single atoms" upon cooling. Theoretical simulations of both effects uncovered the roles of carbon monoxide binding on perimeter Pt sites in the clusters and hydrogen coverage in the aggregation and fragmentation mechanisms. This study highlights the complex effects of adsorbate and supports interactions with metal sites in Pt/ceria catalysts that govern their structural transformations under in situ conditions.

Highly dispersed Pd-based pseudo-single atoms in zeolites for hydrogen generation and pollutant disposal.

Atomically dispersed metal catalysts with excellent activity and stability are highly desired in heterogeneous catalysis. Herein, we synthesized zeolite-encaged Pd-based pseudo-single atoms via a facile and energy-efficient ligand-protected direct H2 reduction method. Cs-corrected scanning transmission electron microscopy, extended X-ray absorption, and pair distribution function measurements reveal that the metal species are close to atomic-level dispersion and completely confined within the intersectional channels of silicalite-1 (S-1) zeolite with the MFI framework. The Pd@S-1-H exhibits excellent activity and stability in methane combustion reactions with a complete combustion temperature of 390 °C, and no deactivation is observed even after 100 h on stream. The optimized bimetallic 0.8Pd0.2Ni(OH)2@S-1-H catalyst exhibits an excellent H2 generation rate from FA decomposition without any additives, affording a superhigh turnover frequency up to 9308 h-1 at 333 K, which represents the top activity among all of the best heterogeneous catalysts under similar conditions. Significantly, zeolite-encaged metal catalysts are first used for Cr(vi) reduction coupled with formic acid (FA) dehydrogenation and show a superhigh turnover number of 2980 mol(Cr2O72-) mol(Pd)-1 at 323 K, surpassing all of the previously reported catalysts. This work demonstrates that zeolite-encaged pseudo-single atom catalysts are promising in efficient hydrogen storage and pollutant disposal applications.

Electrochemically decoupled reduction of CO2 to formate over a dispersed heterogeneous bismuth catalyst enabled via redox mediators.

Electrochemical CO2 reduction is a topic of major interest in contemporary research as an approach to use renewably-derived electricity to synthesise useful hydrocarbons from waste CO2. Various strategies have been developed to optimise this challenging reaction at electrode interfaces, but to-date, decoupled electrolysis has not been demonstrated for the reduction of CO2. Decoupled electrolysis aims to use electrochemically-derived charged redox mediators - electrical charge and potential vectors - to separate catalytic product formation from the electrode surface. Utilising an electrochemically generated highly reducing redox mediator; chromium propanediamine tetraacetate, we report the first successful application of decoupled electrolysis to electrochemical CO2 reduction. A study of metals and metal composites found formate to be the most accessible product, with bismuth metal giving the highest selectivity. Copper, tin, gold, nickel and molybdenum carbide heterogeneous catalysts were also investigated, in which cases H2 was found to be the major product, with minor yields of two-electron CO2 reduction products. Subsequent optimisation of the bismuth catalyst achieved a high formate selectivity of 85%. This method represents a radical new approach to CO2 electrolysis, which may be coupled directly with renewable energy storage technology and green electricity.

Ni-Fe bimetallic catalysts with high dispersion supported by SBA-15 evaluated for the selective oxidation of benzyl alcohol to benzaldehyde.

A wetness impregnation method was used to impregnate the substrate with a substantial quantity of oleic acid together with a metal precursor, leading to significantly dispersed Ni-Fe bimetallic catalysts based on mesoporous SBA-15. Using a wide variety of characterization methods, such as XRD, BET, and TEM Analysis, the physiochemical properties of the catalyst were determined. The addition of the metal does not have any effect on the structural characteristics of the SBA-15 catalyst, as validated by transmission electron microscopy (TEM), which shows that the prepared SBA-15 supported catalyst has a hexagonal mesoporous structure. The catalytic capabilities of the Ni-Fe-SBA-15 catalysts were evaluated in the conversion of BzOH using tert-butyl hydroperoxide (TBHP) as an oxidant and acetonitrile as a solvent. The Ni/Fe-SBA-15 (NFS-15) catalytic composition is the best of the developed catalysts, with a maximum conversion of 98% and a selectivity of 99%. In-depth investigations were conducted into the molar ratio of TBHP to BzOH, the dosage of the catalyst, the reaction rate, temperature, and solvent. The recycling investigations indicate that the synthesized Ni/Fe-SBA-15 (NFS-15) catalyst seems to be more durable up to seven successive cycles.

Atomically dispersed multi-site catalysts: bifunctional oxygen electrocatalysts boost flexible zinc–air battery performance

Based on the advancements in atomically dispersed multi-site catalysts for FZABs, this review discusses the design methodologies to regulate the performance of bifunctional oxygen electrocatalysts from the electronic and geometric structures.