High Metal Loading Research Articles

Emerging Ultrahigh‐Density Single‐Atom Catalysts for Versatile Heterogeneous Catalysis Applications: Redefinition, Recent Progress, and Challenges

In recent years, single‐atom catalysts (SACs) with high metal loading have emerged in different heterogeneous catalysis fields and shown extraordinary catalytic properties. When there are enough coordination atoms (or functional groups) on the supports, it is possible to achieve a limit monolayer atom loading on the surface of supports with ultrahigh atom density (5–15 atoms nm−2) and extremely close site distance (0.2–0.5 nm), by using appropriate synthesis methods and procedures. These high‐density metal atoms usually have no or less metal bonds, which are mostly isolated by support atoms to form 3D foam‐like atomic constructions. Herein, a new notion of metal atomic foam catalysts (AFCs) is propsed to redefine these ultrahigh‐density SACs accommodated by specific supports. This new paradigm of 3D atomic construction for SACs has potential significance for both theoretical research and industrial applications. The latest major advancements in the controllable synthesis of AFCs on various supports (e.g., polymer, carbon, and metallic compound) via different methods (bottom‐up or top‐down approaches) are summarized. The latent catalytic principles and typical application cases of AFCs are emphasized in a wide range of heterogeneous catalysis fields (e.g., thermocatalysis, photocatalysis, electrocatalysis, etc.). The challenges and prospects of this newly 3D ultrahigh‐density AFCs materials in practical industrial application are pointed out as well.

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.

Ir-Pt/C composite with high metal loading as a high-performance anti-reversal anode catalyst for proton exchange membrane fuel cells

Voltage reversal induced by hydrogen starvation can severely corrode the anode catalyst support and deteriorate the performance of proton exchange membrane fuel cells. A material-based strategy is the inclusion of an oxygen evolution reaction catalyst (e.g., IrO 2 ) in the anode to promote water electrolysis over harmful carbon corrosion. In this work, an Ir-Pt/C composite catalyst with high metal loading is prepared. The membrane-electrode-assembly (MEA) with 80 wt% Ir-Pt(1:2)/C shows a first reversal time (FRT) of up to 20 hours, which is about ten times that of MEA with 50 wt% Ir-Pt(1:2)/C does. Furthermore, the MEA with 80 wt% Ir-Pt(1:2)/C exhibits a minimum cell voltage loss of 6 mV@1 A/cm 2 when the FRT is terminated in 2 hours, in which the MEA with 50 wt% Ir-Pt(1:2)/C exhibits a voltage loss of 105 mV@1 A/cm 2 . Further physicochemical and electrochemical characterizations demonstrate that the destruction of anode catalyst layer caused by the voltage reversal process is alleviated by the use of the composite catalyst with high metal loading. Hence, our results reveal that the combination of OER catalyst on the Pt/C with high metal loading is a promising approach to alleviate the degradation of anode catalyst layer during the voltage reversal process for PEMFCs. • The Ir nanoparticles serve as oxygen evolution reaction catalysts. • The carbon support of 80 wt% Ir-Pt (1:2)/C is fully covered by metal nanoparticles. • Metal NPs with high metal loadings form a continuous pathway network for electrons and protons. • A voltage loss of 3.0 mV/h@1 A/cm 2 is observed for MEA with 80 wt% Ir-Pt (1:2)/C. • A voltage loss of 52.5 mV/h@1 A/cm 2 is observed for MEA with 50 wt% Ir-Pt (1:2)/C.

Construction of dual active sites on diatomic metal (FeCo−N/C-x) catalysts for enhanced Fenton-like catalysis

High metal loading of single-atom catalysts enables excellent catalytic activity, but possibly causes serious aggregation problem. Herein, a series of diatomic FeCo−N/C-x (x represents metal content) were skillfully designed and applied to improve the catalytic activity for peroxymonosulfate (PMS) activation toward degrading organic micropollutants. The unprecedented dual active sites, referring to Fe(N3)−Co(N3) moiety and FeCo alloy, are constructed on the obtained FeCo−N/C-x, thereby exhibiting significantly greater performance toward degrading aqueous phenol (e.g., 0.316 min−1 for FeCo−N/C-3) via PMS activation, compared with those of traditional single-atom Co−N/C (0.011 min−1) and Fe−N/C (0.018 min−1). Combined experimental and theoretical calculations demonstrate the independent functions of dual active sites, in which Fe(N3)−Co(N3) and FeCo alloy can decrease the energy barrier of O−O bond cleaving resulting in the formation of high-valent FeCoO reactive species and singlet oxygen, respectively. This study opens up a new platform toward constructing dual active sites for enhanced Fenton-like catalytic activity.

High Oxygen Reduction Activity of Pt-Ni Alloy Catalyst for Proton Exchange Membrane Fuel Cells

In order to fill the research gap of high metal loading of high performance PtNi alloy catalysts, a PtNi/C alloy nano-catalyst with metal loading more than 50 wt.% and core-shell like structure was prepared by ethylene glycol reduction, high temperature annealing, and acid pickling. The electrochemical test results showed that the prepared PtNi alloy catalyst had excellent electrochemical activity: the electrochemical surface area (ECSA) was 63.8 m2·gPt−1, and the mass activity (MA) was 0.574 A·mgPt−1, which is 2.73 times greater than those of the Pt/C JM (Johnson Matthey) catalyst. The durability of the PtNi/C catalyst was further investigated. After 30 K cycles of accelerated durability test, the ECSA and MA of the PtNi/C alloy catalyst decreased by 10.2% and 31.2%, respectively. The PtNi/C alloy catalyst prepared in this study has excellent catalytic activity and overcomes the problem of insufficient durability of traditional alloy catalysts and has the potential for large-scale commercial application.

Oxidative coupling of methane\u2014comparisons of MnTiO3\u2013Na2WO4 and MnOx\u2013TiO2\u2013Na2WO4 catalysts on different silica supports

The oxidative coupling of methane (OCM) converts CH4 to value-added chemicals (C2+), such as olefins and paraffin. For a series of MnTiO3-Na2WO4 (MnTiO3-NW) and MnOx-TiO2-Na2WO4 (Mn-Ti-NW), the effect of loading of MnTiO3 or MnOx-TiO2, respectively, on two different supports (sol–gel SiO2 (SG) and commercial fumed SiO2 (CS)) was examined. The catalyst with the highest C2+ yield (21.6% with 60.8% C2+ selectivity and 35.6% CH4 conversion) was 10 wt% MnTiO3-NW/SG with an olefins/paraffin ratio of 2.2. The catalyst surfaces with low oxygen-binding energies were associated with high CH4 conversion. Stability tests conducted for over 24 h revealed that SG-supported catalysts were more durable than those on CS because the active phase (especially Na2WO4) was more stable in SG than in CS. With the use of SG, the activity of MnTiO3-NW was not substantially different from that of Mn-Ti-NW, especially at high metal loading.

Strong ion interaction inducing ultrahigh activity of NiCoP nanowires for overall water splitting at large current density

The design and development of cheap and efficient electrocatalysts are particularly imperative for renewable energy, but still a challenge for overall water splitting (OWS). Herein, The NiCoP-10 g-125℃ with three-dimensional (3D) nanowire structures grown on nickel foam (NF) was successfully prepared by a novel molten salt method (MSM), serving as an excellent bifunctional electrocatalyst. The catalyst only needs minimal overpotential of 159 and 391 mV to gain the large current density of 400 mA cm−2 for HER and OER in 1 M KOH, respectively. Meanwhile, the alkaline electrolyzer assembled by two electrodes requires a cell voltage of 1.83 V to achieve 400 mA cm−2 for OWS and runs stably at 100 mA cm−2 for 100 h without apparent potential decay. Such impressive catalyticperformance is mainly attributed to the special molten salt synthesis method which provides a liquid environment with strong ions interaction. This special reaction condition contributes to forming 3D nanowire structures with the advantage of high metal loading, large specific surface area, fast charge transfer, and good mechanical strength. Therefore, this work can offer a new avenue for the synthesis of ultrahigh activity bifunctional electrocatalysts.

Superiority of Dual‐Atom Catalysts in Electrocatalysis: One Step Further Than Single‐Atom Catalysts

AbstractIn recent years, dual‐atom catalysts (DACs) have attracted extensive attention, as an extension of single‐atom catalysts (SACs). Compared with SACs, DACs have higher metal loading and more complex and flexible active sites, thus achieving better catalytic performance and providing more opportunities for electrocatalysis. This review introduces the research progress in recent years on how to design new DACs to enhance the performance of electrocatalysis. Firstly, the advantages of DACs in increasing metal loading are introduced. Then, the role of DACs in changing the adsorption condition of reactant molecules on metal atoms is discussed. Moreover, the ways in which DACs can reduce the reaction energy barrier of key steps and change the reaction path are explored. Catalytic applications in different electrocatalytic reactions, including the carbon dioxide reduction reaction, oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and nitrogen reduction reaction are followed. Finally, a brief summary is made and the key challenges and prospects of DACs are introduced.

Facile one-pot synthesis of functional hydrochar catalyst for biomass valorization

Corn stover derived hydrochar (HC) doped with aluminum nanoparticles was synthesized through the facile one-pot hydrothermal carbonization for glucose to fructose isomerization. Amorphous aluminum nanoparticles with crystalline size less than 0.5 nm were uniformly loaded on hydrochar. The phase of the Al-oxide nanoparticles was tailored through aerobic/anaerobic calcination. The results showed that the amorphous aluminum nanocrystals with six coordinated Al-oxide structure were formed under oxygen calcination, which facilitated the high catalytic activity and selectivity of glucose isomerization. It was found that catalysts prepared at hydrothermal temperature of 180℃ and further calcinated at 300℃ under aerobic condition (Al/HC180-300/O) achieved a fructose yield of 42.6% and a selectivity of 83.6% at 180 °C for 5 min. The kinetics, catalytic mechanism, and the deactivation mechanism of the glucose to fructose were further studied using Al/HC180-300/O catalyst. This study provides a high energy efficiency and low metal loading carbon-based catalysts synthetic approach applicable for biorefinery.

An accurate “metal pre-buried” strategy for constructing Ni–N2C2 single-atom sites with high metal loadings toward electrocatalytic CO2 reduction

Ni–N2C2 SACs were obtained by the cleavage of Ni–O bonds and formation of Ni–C bonds.

Sublayer-enhanced atomic sites of single atom catalysts through in situ atomization of metal oxide nanoparticles

The in situ atomization of carbon supported metal oxide nanoparticles provides a novel strategy to synthesize atomic sites supported on highly graphitized carbon materials with high metal loading and controlled atomic layers.

Toxic metals in soil depths from selected abandoned sites: Occurrence, sources, ecological and human health risk

Abstract This study provides a comparative assessment of cadmium (Cd), lead (Pb), chromium (Cr), nickel (Ni), copper (Cu), manganese (Mn), zinc (Zn), and iron (Fe) pollution occurrence, sources, and exposure risk in soils from selected abandoned sites. The concentrations of metals were determined using atomic absorption spectrophotometry. The metals occurrence ranged from 0.02 (Zn) to 16600 mg kg−1 (Fe) in the order of subsoil > topsoil with petroleum tank farm and fuel/gas service station exhibiting high metal loading. The sources of metals are anthropogenic and geologic. The hazard index values for infants’ were higher than that of adults, and the inhalation risk for adults’ was considerably higher than for infants’ exposure. The ecological risk of Cd, Pb, Cr, Ni, and Zn falls in the contamination to pollution index. This study revealed the need for clean-up and restoration of abandoned site soils.

Emerging single-atom iron catalysts for advanced catalytic systems.

Due to the elusive structure-function relationship, traditional nanocatalysts always yield limited catalytic activity and selectivity, making them practically difficult to replace natural enzymes in wide industrial and biomedical applications. Accordingly, single-atom catalysts (SACs), defined as catalysts containing atomically dispersed active sites on a support material, strikingly show the highest atomic utilization and drastically boosted catalytic performances to functionally mimic or even outperform natural enzymes. The molecular characteristics of SACs (e.g., unique metal-support interactions and precisely located metal sites), especially single-atom iron catalysts (Fe-SACs) that have a similar catalytic structure to the catalytically active center of metalloprotease, enable the accurate identification of active centers in catalytic reactions, which afford ample opportunity for unraveling the structure-function relationship of Fe-SACs. In this review, we present an overview of the recent advances of support materials for anchoring an atomic dispersion of Fe. Subsequently, we highlight the structural designability of support materials as two sides of the same coin. Moreover, the applications described herein illustrate the utility of Fe-SACs in a broad scope of industrially and biologically important reactions. Finally, we present an outlook of the major challenges and opportunities remaining for the successful combination of single Fe atoms and catalysts.

Biomineralization by Extremely Halophilic and Metal-Tolerant Community Members from a Sulfate-Dominated Metal-Rich Environment.

The adaptation to adverse environmental conditions can lead to adapted microbial communities that may be screened for mechanisms involved in halophily and, in this case, metal tolerance. At a former uranium mining and milling site in Seelingstädt, Germany, microbial communities from surface waters and sediment soils were screened for isolates surviving high salt and metal concentrations. The high salt contents consisted mainly of chloride and sulfate, both in soil and riverbed sediment samples, accompanied by high metal loads with presence of cesium and strontium. The community structure was dominated by Chloroflexi, Proteobacteria and Acidobacteriota, while only at the highest contaminations did Firmicutes and Desulfobacterota reach appreciable percentages in the DNA-based community analysis. The extreme conditions providing high stress were mirrored by low numbers of cultivable strains. Thirty-four extremely halotolerant bacteria (23 Bacillus sp. and another 4 Bacillales, 5 Actinobacteria, and 1 Gamma-Proteobacterium) surviving 25 to 100 mM SrCl2, CsCl, and Cs2SO4 were further analyzed. Mineral formation of strontium- or cesium-struvite could be observed, reducing bioavailability and thereby constituting the dominant metal and salt resistance strategy in this environment.

A Large-Scale Multiple Genome Comparison of Acidophilic Archaea (pH ≤ 5.0) Extends Our Understanding of Oxidative Stress Responses in Polyextreme Environments.

Acidophilic archaea thrive in anaerobic and aerobic low pH environments (pH < 5) rich in dissolved heavy metals that exacerbate stress caused by the production of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radical (·OH) and superoxide (O2−). ROS react with lipids, proteins and nucleic acids causing oxidative stress and damage that can lead to cell death. Herein, genes and mechanisms potentially involved in ROS mitigation are predicted in over 200 genomes of acidophilic archaea with sequenced genomes. These organisms are often be subjected to simultaneous multiple stresses such as high temperature, high salinity, low pH and high heavy metal loads. Some of the topics addressed include: (1) the phylogenomic distribution of these genes and what this can tell us about the evolution of these mechanisms in acidophilic archaea; (2) key differences in genes and mechanisms used by acidophilic versus non-acidophilic archaea and between acidophilic archaea and acidophilic bacteria and (3) how comparative genomic analysis predicts novel genes or pathways involved in oxidative stress responses in archaea and likely horizontal gene transfer (HGT) events.

Recent progress in electrochemical reduction of carbon dioxide on metal single‐atom catalysts

AbstractElectrochemical reduction reaction of CO2 (CO2RR) is a promising technology for alleviating the global warming caused by the emission of CO2. This technology, however, is still in the stage of finding efficient catalysts. The catalysts must be able to convert CO2 to other carbon‐based products with high activity and selectivity to valuable chemicals. In this review, previous development of heteroatom‐doped metal‐free carbon materials (H‐CMs) is briefly summarized. Recent progress of CO2RR promoted by metal single‐atom catalysts (M‐SACs) is then discussed with emphasis on the synthesis of M‐SACs, the catalytic performance, and reaction mechanisms. The high temperature pyrolysis method and electrodeposition are attracting attentions recently to prepare M‐SACs with high metal loading on N‐doped carbon materials, a very active M‐SACs system for the CO2RR. Theoretical calculations of free energy change on active sites, the Operando X‐ray absorption near edge structure (XANES), and Bader charge analysis reveal a significant role of metal oxidation state and charge transfer between metal atoms and absorbed CO. The challenges and perspectives for the extensive applications of M‐SACs in CO2RR are also discussed in this review.

Race on High‐loading Metal Single Atoms and Successful Preparation Strategies

AbstractAlthough single‐atom catalysts (SACs) show significantly higher catalytic performance compared to conventional and nanoparticle‐based catalysts (NPs) at the same amount of metal loading, their overall catalytic performance may still be unable to compete with the NPs in many applications due to the limited active sites. Generally, trace amounts of metal (less than 1 wt%) can be successfully loaded onto supports in an atomically‐dispersed feature, and higher metal loading usually results in aggregation of the metal atoms. Hence, it is very important to further increase the density of isolated metal atoms in these rising‐star SACs to pave the ways for widespread applications or even to replace the NPs. On the other hand, it is well known that this is one of the most challenging topics on SACs research. In this review, the advanced strategies to overcome this challenge and achieve high loading of isolated metal atoms on various supports will be extensively discussed together with the examples showing the research efforts to enrich the metal active sites from the pioneer works with metal loading below 0.2 wt% to the recently‐reported SACs with metal loading over 20 wt%. Besides, there are still plenty of rooms to further improve the quantity and quality of metal loading on different supports, and outlooks of this scope is provided. We hope that this comprehensive review aids in the development of synthesis techniques for the new‐generation SACs with richer active sites.

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Rationalization on high-loading iron and cobalt dual metal single atoms and mechanistic insight into the oxygen reduction reaction

Rational design of single-atom catalysts (SACs) with high metal loadings is essential to enhance the sluggish kinetics of oxygen reduction reactions in metal-air batteries and proton-exchange membrane fuel cells (PEMFCs). Herein, an effective plasma engineering strategy to construct Fe/Co dual single atoms densely dispersed on porous nitrogen-doped carbon nanofibers (Fe, Co SAs-PNCF) with a high mass loading of 9.8 wt% is proposed without any acid leaching. The electrocatalyst exhibits superior ORR performances in both alkaline and acidic media (e.g., Eonset = 1.04 V and E1/2 = 0.93 V). The N3-Fe-Co-N3 moieties are identified to be the main active sites by X-ray absorption spectroscopy (XAS) and density functional theory calculations. The in situ XAS and Raman spectroscopy quantitively reveal the decrease in oxidation states of Fe/Co and the increase in bond lengths of the Fe-N/Co-N in the N3-Fe-Co-N3 during the ORR. Benefitting from the high loading of single atoms and enhanced activity, the Fe, Co SAs-PNCF endows the Al-air batteries and PEMFCs with excellent discharge performances, demonstrating promising practical applications.

Sacrificial Template‐Assisted Synthesis of Inorganic Nanosheets with High‐Loading Single‐Atom Catalysts: A General Approach

AbstractSingle‐atom catalysts (SACs) supported on inorganic materials have attracted much attention in numerous research fields for their high catalytic performance. However, such SACs have been limited by the low metal loading, especially on different types of inorganic supports. Herein, a general approach is presented for preparing SACs on metallic, metal oxide, and perovskite nanosheet (NS) supports to reach a high metal loading of up to 3.94 wt%, by utilizing N‐doped graphene as a sacrificial template that spatially confines the single atoms (SAs). Specifically, the target support material precursors are adsorbed onto the SAs‐stabilized sacrificial template, followed by subsequent heat treatment to transfer the SAs to the support material and to remove the graphene layer. Pt SAs on oxide support exhibits little to no aggregation throughout &gt;10 000 min of annealing at 275 °C, demonstrating high thermal stability, as also supported by ex situ post‐anneal electron microscopy and X‐ray absorption fine structure studies. As a proof‐of‐concept, Pt SACs on SnO2 NSs exhibit high catalytic activity toward chemiresistive sensing of acetone gas (response = 95.4 at 10 ppm, 7.6‐fold enhancement compared with pristine SnO2 NSs) and unprecedented stability under highly humid conditions (27.4% response deterioration at 95% relative humidity).