Layered double hydroxides supported noble-metal single-atom catalysts: precise synthesis, microenvironment regulation, and diverse applications
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Abstract
Noble-metal single-atom catalysts (SACs) have arisen as a research hotspot in heterogeneous catalysis resulting from superior noble-metal-atom utilization, well-defined catalytic centers, and tunable microenvironments. Recently, the advent and rise of noble-metal SACs supported by layered double hydroxides (LDHs) have injected fresh vitality and vigor into this research field. LDHs offer distinct advantages as the support of SACs, such as an ordered and adjustable crystal structure, a two-dimensional layered structure possessing a large specific surface area, facile synthesis with cost-effectiveness, and strong co-catalytic metal-support interaction between LDHs and noble-metal single atoms. In this review, we classified and comprehensively outlined the current synthesis strategies of noble-metal SACs supported by LDHs, and conducted an in-depth analysis of the specific mechanisms underlying each strategy. Subsequently, considering the critical role of the microenvironment of SACs in affecting their catalytic-related properties, we discussed the current microenvironment regulation strategies of LDH-supported noble-metal SACs. We also provide an introduction to the characterization techniques for
Keywords
INTRODUCTION
Catalysis is pivotal to the modern chemical industry, with noble metals being highly esteemed for their exceptional catalytic activity[1-9]. However, the suboptimal utilization efficiency of noble-metal atoms and the ambiguous composition of catalytic centers present formidable challenges[10,11]. The advent of single-atom catalysts (SACs) offers an enticing solution to augment atom utilization and establish precise identification of active sites. First presented by Qiao et al. in 2011, the concept of single-atom catalysis has swiftly garnered significant attention, establishing itself as an emerging research field in catalysis[12-18]. SACs represent a unique class of catalysts, where the active metal atoms are coordinated with atoms from the supports and dispersed as isolated entities on the supports, effectively bridging the gap between heterogeneous and homogeneous catalysis[19-23]. They possess advantages including well-defined and uniform active sites, 100% theoretical utilization of active sites, and adjustable coordination numbers of metal atoms[24-28]. Consequently, SACs present a significant opportunity to minimize the usage of noble metals and maximize catalytic efficiency through precise control over noble-metal active centers. In recent years, significant achievement has been gained in the advancement of SACs.
Since the noble-metal atoms incline toward aggregating because of the high surface free energy, choosing the proper supports is a crucial prerequisite to achieving efficient noble-metal SACs. The utilization of layered double hydroxides (LDHs) as noble-metal SAC supports has recently attracted widespread attention[29,30]. LDHs are a type of two-dimensional layered material composed of positively charged laminates and negatively charged interlayer anions[31]. The chemical formula of LDHs is
In this review, we first comprehensively outline the reported synthetic strategies of LDH-supported
Figure 1. A summary of the key themes in this review. XAS: X-ray absorption spectroscopy; DFT: Density functional theory;
THE SYNTHESIS STRATEGIES OF LDH-SUPPORTED NOBLE-METAL SACS
Due to the distinctive structural characteristics of LDHs, noble metals can be anchored as isolated atoms, either residing on the surface or embedded within the LDH laminates. In 2008, Yan et al. utilized DFT technology to explore the feasibility of incorporating various metal cations into LDH laminates[38]. They found that the distortion angle (θ) of octahedral coordination hexahydrate cations {[M(H2O)6]n+} related to the preferential coordination environment of metal ions significantly influenced the formation of LDH laminates. With an increasing distortion angle, the challenge of incorporating metal cations into LDHs also escalated. Consequently, only a portion of noble-metal ions, such as Ru, Rh, Ir, and Os (Type I, θ < 10°), could enter the LDH laminates and form well-organized LDHs [Figure 2][36,38]. Conversely, other
Figure 2. The distortion angle (θ) of the [M(H2O)6]n+.
Co-precipitation strategy
The co-precipitation strategy involves the direct incorporation of noble-metal salts into the raw materials of LDH during synthesis, enabling the simultaneous formation of LDHs and anchoring of noble-metal atoms within their structure. For Type I noble-metal atoms, namely Ru, Rh, Ir, and Os, which are easy to enter the LDH laminates, there are two possible anchoring positions: (1) within the LDH laminates; (2) on the surface of LDH laminates. For example, Liu et al.[39] employed a co-precipitation method to synthesize
Figure 3. (A, B) Synthesis process (A) and Rh K-edge EXAFS fitting result (B) of Rh/NiFe-x[39]. Copyright 2021, Royal Society of Chemistry; (C, D) Synthesis process (C) and Ru K-edge EXAFS spectra of Ru/Ni3V-LDH with proposed model (D)[40]. Copyright 2022, American Chemical Society; (E, F) Synthesis process (E) and Au L3-edge EXAFS fitting results (F) of Au1-MgAl-LDH[41]. Copyright 2023, Wiley-VCH; (G, H) The synthesis process (G) and Pt L3-edge EXAFS spectra (H) of PtSA-NiCo-LDH/NF[42]. Copyright 2023, Elsevier. LDHs: Layered double hydroxides; EXAFS: Extended X-ray absorption fine structure; PtSA-NiCo-LDH/NF: Pt single-atom sites (PtO5) anchored NiCo-LDH supported by nickel foam.
Type II noble-metal atoms, including Pd, Ag, Pt, and Au, with larger ion sizes, tend to anchor on the surface of LDH laminates by the co-precipitation strategy. A notable example can also be found in the research conducted by Shen et al.[41]. They successfully synthesized MgAl-LDH-supported Au SACs
Compared to other synthesis strategies, the co-precipitation strategy has unique advantages in its simple synthesis process. The co-precipitation strategy is straightforward and scalable, making it suitable for
Impregnation strategy
Impregnation strategy refers to immersing the pre-synthesized LDHs in a solution containing noble-metal salts so that noble-metal atoms gradually anchor to the surface of LDH laminates. A reduction process is usually required to enhance the anchoring strength of noble-metal single-atom sites. Given that the surface of LDH laminates is enriched with hydroxyl groups, noble-metal atoms could be anchored on the surface as single atoms by these surface hydroxyl groups easily by this strategy. This method serves as a versatile synthetic strategy that can be applied to both type I and type II noble metals. By this impregnation strategy, our group successfully anchored several noble-metal atoms, involving Ru and Pt single-atom sites, onto the surface of LDH laminates[43-46].
For instance, Ru1/LDH was synthesized by immersing MgAl-LDH in RuCl3 aqueous solution to anchor Ru single atoms and subsequent reduction by H2/Ar at 100 °C [Figure 4A and B]. The fitting results of EXAFS spectra indicated that the Ru single-atom sites were anchored on the surface of MgAl-LDH in the form of RuO3. Adding alkali in the impregnation solution can also effectively promote the anchor of noble-metal atoms on the surface of LDH laminates. For example, CoFe-LDH-supported Ru SACs were synthesized by Li et al. using this strategy [Figure 4C and D], revealing a valence state between 0 and +3, and a specific coordination environment of RuO4 for Ru species on the surface of LDH laminates[47]. Hu et al.[48] utilized this strategy to anchor Ir single-atom sites on the surface of NiFe-LDH laminates at 80 °C using H2IrCl6 as a noble-metal source [Figure 4E and F]. XNANES and EXAFS spectra showed the oxidation state (> +3) and specific configurations of the Ir as IrO6, while the DFT calculation results indicated that the Ir atoms were placed at the top of the Fe sites. In addition, Chen et al.[49] introduced PtCl62- in the interlayer of Ni3Fe-LDH via the impregnation strategy and anchored Pt single-atom sites on the Ni3Fe-LDH laminates
Figure 4. (A, B) Synthesis progress (A) and Ru K-edge EXAFS fitting curve (B) of Ru1/LDH[43]. Copyright 2023, Wiley-VCH; (C, D) Synthesis progress (C) and Ru K-edge EXAFS fitting curve (D) of Ru/CoFe-LDH[47]. Copyright 2019, Springer Nature; (E, F) Synthesis progress (E) and Ir L3-edge EXAFS fitting curve (F) of IrSAC-NiFe-LDH[48]. Copyright 2023, American Chemical Society; (G, H) Synthesis progress (G) and Pt L3-edge EXAFS spectra (H) of Ni3Fe-CO32- LDH-SA[49]. Copyright 2021, Royal Society of Chemistry. LDHs: Layered double hydroxides; EXAFS: Extended X-ray absorption fine structure.
The impregnation method offers several advantages in material preparation, such as simplicity, broad applicability, and strong controllability. By adjusting solution concentration and impregnation time, the loading of active components can be precisely controlled. However, this method also has drawbacks, including the potential uneven distribution of active components, the need for strict control of solvent selection, and the complexity and time costs associated with multiple steps. Therefore, achieving consistency and reproducibility in large-scale production can be challenging.
Ion exchange strategy
The ion exchange strategy involves the substitution of metal cations in the LDH laminates with noble-metal cations by post-synthesis strategies, which anchor noble-metal atoms within the LDH laminates. During a cation exchange reaction, the exchanged cations undergo migration from the host lattice and subsequent dissolution in the solvent, whereas the substituted cations migrate into the host lattice[50]. The cation exchange reaction possesses two main driving forces: (1) mass-driven law caused by the concentration of highly substituted cations (Le Chatelier’s principle); (2) the difference in solubility between the reactant and product[51]. For instance, Mu et al.[52] harnessed the Ru to replace a portion of transition-metal atoms within FeCo-LDH laminates (Ru1SACs@FeCo-LDH) by soaking the FeCo-LDH in ethanol solution with a high concentration of RuCl3 [Figure 5A and B]. Ru1SACs@FeCo-LDH not only retained the porous structure of FeCo-LDH but also exhibited a distinctive symmetry-breaking structure formed by the asymmetric coordination of Fe/Co atoms around Ru atoms. The ion exchange strategy diverges from the impregnation strategy mentioned earlier in its demand for a notably elevated concentration of noble-metal salts to facilitate the cation exchange reaction.
Figure 5. (A, B) Synthesis progress (A) and AC HAADF-STEM image with linear scanning analysis (B) of Ru1SACs@FeCo-LDH[52]. Copyright 2022, Royal Society of Chemistry; (C, D) Synthesis progress (C) and Ag K-edge EXAFS fitting curve (D) of
In addition, transition-metal atoms within LDH laminates have been found to detach into the solution during the electrochemical process in prior studies[53,54]. Although this phenomenon results in the loss of active sites[55], it also presents an opportunity for the incorporation of noble-metal atoms into LDH laminates. Interestingly, this strategy can achieve ion exchange between type II noble-metal ions and transition metal ions within the LDH laminates. For instance, He et al.[56] achieved ion exchange of Ni and Ag in NiCo-LDH (AgSA-NiCo-LDH/CC) via the cyclic voltammetry (CV) method [Figure 5C and D]. The Ag single-atom sites were dispersed in the NiCo-LDH laminates in the valence state between 0 and 1 and the coordination form of AgO6. Wang et al.[57] also anchored Au single-atom sites onto Fe vacancies in MnFeCoNiCu-LDH using the electrochemical CV method [Figure 5E and F]. They confirmed the atomic dispersion and coordination environment (AuO4) of Au species via the inductively coupled plasma-optical emission spectroscopy (ICP-OES), AC HAADF-STEM, EXAFS, and DFT techniques. We speculate that the main reason for Au and Ag entering LDH laminates is due to the surface defects and strain in the LDH laminates caused by the separation of transition metal cations during the electrochemical CV process. The surface defects and strain are considered to be ideal high-energy sites for cation exchange reactions[58]. The ion exchange strategy offers significant advantages, such as high selectivity, allowing for the precise anchor of noble-metal atoms within LDH laminates, thereby achieving uniform distribution of noble-metal
THE MICROENVIRONMENT REGULATION STRATEGIES OF LDH-SUPPORTED NOBLE-METAL SACS
The microenvironment of SACs refers to the localized, specific, and isolated chemical environment surrounding the central metal atoms. This includes factors such as coordination environments and electronic configuration[59-63]. Precisely regulating these microenvironments is of utmost importance, as it holds the key to unlocking unprecedented catalytic efficiency and selectivity, thereby paving the way for sustainable and environmentally friendly chemical processes[64-67]. The regulation of microenvironments is pivotal in deciding the catalytic performance of SACs by influencing the electronic state of the active metal, thus impacting the overall catalytic activity. Inheriting the highly tunable structure of LDHs, LDHs are not only excellent supports for SACs but also are beneficial for precise regulation of the microenvironments of noble-metal single-atom sites. In this section, we have summarized the reported microenvironment regulation strategies of LDH-supported noble-metal SACs [Figure 6], including defect engineering strategy and axial coordination engineering strategy.
Figure 6. The microenvironment of noble-metal single-atom sites. LDHs: Layered double hydroxides.
Defect engineering strategy
In SACs, the pronounced interaction or charge transfer between the metal atoms and the supports, known as the metal-support interaction, is evident[68]. The strong metal-support interaction can capture and stabilize metal atoms, and also provides a new opportunity to regulate the electronic structure and coordination environment of metal atoms, thereby optimizing their catalytic performance[69]. An effective strategy for regulating metal-support interactions is introducing defects into the supports, known as the defect engineering strategy. Defects are widely present in nanomaterials, serving as effective anchoring sites for metal atoms and an important component of the microenvironment of central metal atoms[61,70].
Recently, our group has developed a defect engineering strategy to construct noble-metal single-atom sites with a specific microenvironment directionally. Specifically, divalent/trivalent cation vacancies on
Figure 7. (A, B) Synthesis progress (A) and Ru K-edge EXAFS fitting curves (B) of Ru1/LDH-VIII and Ru1/LDH-VII[44]. Copyright 2022, Wiley-VCH; (C, D) Synthesis progress (C) and Pt L3-edge EXAFS fitting curve (D) of Pt1/LDHv[45]. Copyright 2024, Wiley-VCH; (E, F) Synthesis progress (E) and Ru K-edge EXAFS fitting curves (F) of Ru1/D-NiFe-LDH[74]. Copyright 2021, Springer Nature. LDHs: Layered double hydroxides.
The noble metals that pose challenges in entering the LDH laminates can also be precisely anchored on the surface of LDH laminates using this defect engineering strategy. For instance, Pt single-atom sites were precisely anchored onto the top of the divalent cation vacancies of CoFe-LDH with divalent cation vacancies (Pt1/LDHv) [Figure 7C and D] by a similar process[45]. The Pt atoms in the Pt1/LDHv were coordinated with three oxygen atoms, while Pt1/LDH prepared by impregnation method on CoFe-LDH without vacancies only coordinated with one oxygen atom. These structural differences resulted in significant differences in their intrinsic properties. The defect engineering strategy can achieve targeted anchoring of noble-metal atoms with a specific microenvironment, providing a novel route for the precise synthesis of SACs with bright development prospects. Besides the targeted anchoring of noble-metal atoms, defect engineering strategies can serve as a post-treatment approach to regulate the interaction between noble-metal atoms and LDH laminates. For example, Zhai et al.[74] employed an alkaline etching method to remove Al from NiFeAl-LDH anchored with Ru single-atom sites [Figure 7E and F], yielding a defective (D-) NiFe-LDH anchored with RuO4 sites (Ru1/D-NiFe-LDH). Experimental results demonstrated that the introduction of metal defects significantly enhanced the interaction between Ru atoms and LDH laminates.
Axial coordination engineering strategy
The coordination structure and geometric configuration of SACs are pivotal factors influencing the catalytic activity of central metal atoms[75]. Axial coordination engineering is a novel approach for adjusting the local microenvironment of central metal atoms in SACs[76]. By incorporating one or more ligands at the axial positions of the central metal atoms, the planar symmetry of SACs is intentionally broken, achieving changes and adjustments in the electron distribution of central metal atoms[77]. Recently, axial coordination engineering has also been applied in LDH-supported noble-metal SACs.
We have developed a simple irradiation impregnation method to directionally manipulate the axial ligands on the Pt single-atom sites (labeled as X-Pt/LDH, X = F, Cl, Br, I, and OH) supported on the surface of NiFe-LDH [Figure 8A and B][46]. Specifically, by immersing the synthesized Cl-Pt/LDH in KOH aqueous solution with white light irradiation, HO-Pt/LDH was obtained. When immersing the obtained
Figure 8. (A, B) Synthesis progress (A) and Pt L3-edge EXAFS fitting curves (B) of Cl-Pt-LDH[46]. Copyright 2022, Springer Nature;
Besides these two widely studied strategies, the microenvironment of LDH-supported noble-metal SACs can be regulated by substituting the metal in LDH laminates. For example, Duan et al.[79] substituted Ni2+ in NiFe-LDH with Fe2+ (NiFe2+Fe-LDH) to modulate the interaction between Ru atoms and the LDH laminates. For comparison, they anchored Ru single-atom sites on NiFe2+Fe-LDH (Ru/NiFe2+Fe-LDH) and NiFe-LDH (Ru/NiFe-LDH) under the same conditions. Experimental results demonstrated that the reducibility of Fe2+ enhanced the anchoring ability of the LDH supports to Ru atoms, resulting in Ru loading on Ru/NiFe2+Fe LDH being three times higher than that on Ru/NiFe LDH. Moreover, the reducibility of Fe2+ decreased the oxidation state of Ru (from 2.58 to 2.37) and kept it at a lower level throughout the entire working process. Despite recent attention from researchers on microenvironment regulation in
THE CHARACTERIZATION TECHNIQUES OF LDH-SUPPORTED NOBLE-METAL SACS
The significant advancements in characterization techniques have been crucial in driving the success of SACs in the field of catalysis. Confirming the existence of noble-metal single-atom sites and acquiring detailed information about their microenvironment are essential for unveiling the structure-performance relationship in LDH-supported noble-metal SACs and for the subsequent development of
AC HAADF-STEM technique
Since the 1990s, advancements in electron microscopy have led to significant improvements in
XAS technique
Beside visualizing individual metal atoms, understanding how their local microenvironment, including coordination and support interactions, is also crucial to SACs, as it impacts their activity and stability. XAS is widely utilized due to its element-specific nature and ability to investigate the electronic and atomic structure of examined atoms[81]. It can be divided into XANES and EXAFS regions. Details about the oxidation state, electronic properties, and bonding geometry of absorbing atoms are generally derived from analyzing the XANES region. Concurrently, analysis of the EXAFS region provides in-depth information on the microenvironment of single-atom sites, including the types, numbers, and bond lengths of neighboring atoms, typically within a range of ~ 5Å. However, XAS technology also has its limitations. The limitations of EXAFS fitting primarily stem from its indirect nature and inherent subjectivity. Structural information is inferred by fitting with a hypothesized model rather than being directly obtained. The choice of the structural model and the fitting process may involve subjective judgments, which can undermine the reliability of the results. Furthermore, coordination number errors in the fitting process can reach up to 10%, further reducing the accuracy of the structural interpretation[82]. Identifying specific coordination elements in unknown samples, especially when peaks with similar bond lengths are present in the R space, can be particularly difficult[19]. And the reliability of fitting results can be compromised by the low quality of EXAFS in many XAS datasets. Furthermore, as a bulk-averaging technique, XAS provides information that represents an average value[83]. However, this value is often assumed to reflect the unique configuration of single-atom sites within the sample, though it may be an average of different configurations of single-atom sites. XAS technology is frequently paired with DFT calculations to determine the microenvironmental structure of single-atom sites, enhancing the reliability of the results.
DFT calculations
Compared to AC HAADF-STEM and XAS techniques, DFT calculations offer the advantage of investigating catalysts at a more microscopic scale. Typically, DFT is employed to predict the possible adsorption sites of metal atoms, which are generally the local coordination structures with the lowest formation energy and highest stability[84]. By integrating these predictions with the coordination information obtained from XAS, the most likely configurations of single-atom sites can be inferred. Additionally, DFT calculations can simulate the adsorption of reaction intermediates and the energy barriers of different reaction pathways to analyze reaction mechanisms and assess charge transfer during catalytic processes. Combining DFT calculations with experimental characterization enhances the accuracy and reliability of research conclusions. However, DFT simulations rely on idealized models that do not fully capture the complexities of real-world conditions. As a result, current findings in SAC research retain a degree of uncertainty, primarily due to the limitations of existing technologies. Nevertheless, it is anticipated that as characterization techniques continue to advance, SAC research will experience significant breakthroughs. Following our overview of the characterization techniques of LDH-supported noble-metal SACs, we also provide a review of the applications of LDH-supported noble-metal SACs.
THE APPLICATIONS OF LDH-SUPPORTED NOBLE-METAL SACS
LDH-supported noble-metal SACs embody the combined strengths of LDHs and noble metals, showing excellent performance in electrocatalysis, thermal catalysis, photocatalysis, etc.[85,86]. This section discusses the LDH-supported noble-metal SACs used in electrocatalytic water splitting, hybrid water electrolysis, hydrogenation, selective oxidation, hydrosilylation, photocatalytic benzene oxidation, and enzyme-like applications.
Electrocatalysis
Excessive reliance on fossil fuels has brought increasingly severe energy crises and environmental issues for humanity[87]. Hence, the exploration and development of renewable energy conversion devices have emerged as a research hotspot to address these challenges. Electrocatalysis is extensively used in producing clean fuels, green chemical synthesis, and reducing carbon emissions[88,89]. Currently, LDH-supported
Electrocatalytic water splitting to produce green hydrogen stands out as one of the most promising approaches for efficiently utilizing intermittent renewable energy and achieving decarbonization in the future[90,91]. The water splitting involves two crucial half-reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER)[92]. Noble-metal-based catalysts involving Pt, Ru, and Ir are commercial electrocatalysts for water splitting[85,93]. However, their heavy expense and limited availability hamper the prevalent commercial deployment of such technologies. Decreasing the size of noble-metal catalysts from particles to individual atoms can effectively boost the utilization of noble metals to offset their high prices. Therefore, LDH-supported noble-metal SACs have attracted great attention[94]. Up to now, LDH-supported noble-metal SACs have achieved rapid development in electrocatalytic water splitting.
As the anode reaction, the OER is vital in the water splitting. Zhang et al.[95] synthesized NiFe-LDH with surface-anchored Au single-atom sites (AuO), named sAu/NiFe-LDH, which exhibited a 6-fold OER activity improvement (an overpotential of 198 mV at 129.8 mA cm-2) by 0.4 wt% Au loading [Figure 9A-E]. In-situ Raman characterizations revealed that the active sites were attributed to Ni rather than Au as only the Ni sites underwent electrochemical reconstruction during the CV cycle. The introduction of Au led to the redistribution of charges between Au and LDH with a net charge difference of 0.32e, which was subsequently transferred to neighboring O, Ni, and Fe atoms. This facilitated the adsorption of OH- ions and modified the adsorption strength of O* and OOH* intermediates, leading to decreased energy barriers during the rate-limiting step from O* to OOH*.
Figure 9. (A) CV curves of different catalysts; (B) Overpotential and Tafel slope of different catalysts; (C) LSV curves of sAu/NiFe-LDH before and after 2000 cycles and the time-dependent current density; (D) Raman spectra of sAu/NiFe-LDH at various potentials; (E) Differential charge densities of NiFe-LDH with/without Au atom[95]. Copyright 2018, American Chemical Society; (F) The model of Ru/CoFe-LDH; (G) Differential charge density of elements in CoFe-LDH and Ru/CoFe-LDH; (H) The OER curves of different catalysts; (I) In-situ XANES of Ru/CoFe-LDH[47]. Copyright 2019, Springer Nature. LDHs: Layered double hydroxides; CV: cyclic voltammetry; RHE: reversible hydrogen electrode; LSV: linear scan voltammetry.
Besides the role in regulating the active sites of OER, noble-metal atoms are widely recognized as the main catalytic active sites. Li et al. anchored Ru single-atom sites (RuO4) on the surface of CoFe-LDH
Furthermore, precise manipulation of the axial ligands in noble-metal SACs offers a powerful means to
Figure 10. (A) The model of Cl-Pt/LDH; (B) HER curves of different catalysts; Computational models and localized electric field distributions of (C) Cl-Pt-LDH and (D) HO-Pt-LDH; (E) Energy barriers for water dissociation and (F) adsorption-free energies of H* on different catalysts[46]. Copyright 2022, Springer Nature; LDHs: Layered double hydroxides; HER: hydrogen evolution reaction; RHE: reversible hydrogen electrode.
Apart from serving as single-functional catalysts for OER or HER, LDH-supported noble-metal SACs also present the potential to catalyze both OER and HER concurrently. It is ascribed to the presence of multiple catalytic centers, including noble metal atoms and non-noble-metal atoms within the LDH laminates. These multiple catalytic sites may possess strong electronic interactions with each other, leading to unique catalytic activity and stability. As an example, Sun et al.[40] modified Ru atoms (RuO4) on the surface of
LDH-supported noble-metal SACs for electrocatalytic water splitting
| Noble metal | LDH type | Application | Overpotential (mV) | Stability | Ref. |
| Pt (0.72 wt%) | NiFe-LDH | HER | η10: 25.2 | No evident decay over 100 h test at 500 mA cm-2 | [46] |
| Ru (12.8 wt%) | NiV-LDH | HER | η10: 6 | No evident decay after 5,000 CV cycles | [96] |
| Ru (0.45 wt%) | FeCo-LDH | OER | η10: 198 | 99% current remained after 24 h test at initial 200 mA cm-2 | [47] |
| Ir (8.47 wt%) | NiFe-LDH | OER | η10: 194 | No evident decay over 120 h test at 200 mA cm-2 | [48] |
| Ag (0.50 wt%) | NiCo-LDH | OER | η10: 192 | 94.7% current remained after 500 h test at initial 100 mA cm-2 | [56] |
| Au (1.10 wt%) | MnFeCoNiCu-LDH | OER | η10: 213 | 93.6% current remained after 700 h test at initial ~ 100 mA cm-2 | [57] |
| Ru (1.32 wt%) | NiFe2+Fe-LDH | OER | η10: 194 | Increase of 16mV after 100 h test at 100 mA cm-2 | [79] |
| Au (0.40 wt%) | NiFe-LDH | OER | η10: 237 | No evident decay after 2,000 CV cycles | [95] |
| Ir (1.59 wt%) | NiCr-LDH | OER | η10: 232 | No evident decay over 36 h test at 1.48 V (vs. RHE) | [97] |
| Ru (9.45 wt%) | NiV-LDH | HER OER | η10: 21 η10: 225 | No evident decay over 240 h test at 10 mA cm-2 in the system of overall water splitting | [40] |
| Pt (9.70 wt%) | NiFe-LDH | HER OER | η10: 45 η10: 218 | No evident decay over 12 h for HER | [49] |
| Ru2 (1.53 wt%) Ru1 (0.51 wt%) | FeCo-LDH | HER OER | η500: 84 η10: 194 | No evident decay over 1,000 h test at 1 A cm-2 in the system of overall water splitting | [52] |
| Ru (1.20 wt%) | NiFe-LDH | HER OER | η10: 18 η10: 189 | No evident decay over 100 h test at initial ~ 100 mA cm-2 in the system of overall water splitting | [74] |
| Ru (5.23 wt%) | CoV-LDH | HER OER | η10: 28 η25: 263 | Increase of 12 mV in η10 after 2,000 CV cycles in the system of overall water splitting | [98] |
In traditional water splitting, the sluggish kinetics of OER and low-added-value products pose challenges in reducing the cost of hydrogen through this process. Consequently, researchers have proposed hybrid water electrolysis systems that substitute OER with other reactions, such as alcohol oxidation, which are thermodynamically and economically more favorable[99]. This approach not only decreases the energy consumption and costs in hydrogen production but also yields high-value added oxidation products[100]. In recent years, this emerging technology has garnered significant attention from researchers and has made rapid progress and breakthroughs. LDH-supported noble-metal SACs have also been documented for their application in hybrid water electrolysis systems.
The oxidation of 5-hydroxymethylfurfural (HMF) holds immense significance as it marks a vital transformation process in converting biomass into valuable chemicals, thereby promoting sustainable production of renewable fuels and materials. Xu et al.[101] achieved efficient HMF oxidation (HMFOR) via Ru single-atom sites (RuO4) anchored on the surface of NiFe-LDH (Ru0.3/NiFe) via electrodeposition strategy. Compared with OER [1.13 V vs. reversible hydrogen electrode (RHE)], Ru0.3/NiFe possessed a lower onset potential at 0.1 mA cm-2 (0.91V) in HMFOR with the oxidation product of 2,5-furan dicarboxylic acid (FDCA), at a yield of 98.68%. Moreover, both experiment and DFT results suggested that the incorporation of Ru single-atom sites improved the adsorption energy of HMF, thus enhancing the capacity of FeNiOOH to capture protons and electrons.
Bifunctional catalysts for LDH-supported noble-metal SACs applied in hybrid water electrolysis systems have also been reported. For example, Yu et al.[42] developed Pt single-atom sites (PtO5) anchored
The urea oxidation reaction (UOR) is fascinating due to the low thermodynamic equilibrium potential of 0.37 V vs. RHE, which is not only energy-efficient for hydrogen production but also provides a chance for pacificating urea-rich wastewater, demonstrating great promise for real-world applications. Sun et al.[102] anchored Rh single-atom sites (RhO4) onto Ni-V hollow sites in NiV-LDH (Rh-NiV-LDH) to achieve bifunctional catalysts for UOR and HER. In the UOR process, a potential of 1.33 V vs. RHE sufficed to attain 10 mA cm-2, lower than the potential needed for NiV-LDH (1.35 V vs. RHE), and marking a reduction of 200 mV compared to the potential required for the OER process. Rh-NiV-LDH also possessed outstanding HER activity, requiring only an overpotential of 12 mV to attain 10 mA cm-2, lower than that of NiV-LDH (155 mV). DFT calculations indicated that Rh single-atom sites optimized the electronic structure of NiV-LDH, effectively lowering the reaction barriers in HER. Additionally, these sites optimized the adsorption and activation of urea, facilitating the desorption of CO* and NH*, significantly reducing the reaction barrier for UOR. Besides, Khalafallah et al.[103] dispersed Pt species as isolated atoms on NiCo-LDH
The theoretical potential of the hydrazine oxidation reaction (HzOR) is -0.33 V vs. RHE, which is 1.56 V lower than that of OER[104]. Additionally, the product of the HzOR is the inert gas N2[36], rendering it highly safe. Wang et al.[105] found that Ru atoms in the form of RuO4 coordination above the Fe sites of monolayer NiFe-LDH (Ru1/mono-NiFe-1.6) are an efficient catalyst towards the HzOR. Ru1/mono-NiFe-1.6 catalyst demonstrated a potential of just 1,260 mV at 10 mA cm-2, significantly better than other catalysts including Ru1/mono-NiFe-0.3 (1,301 mV), mono-NiFe (1,364 mV), and bulk-NiFe (1,394 mV). This catalyst also maintained its catalytic activity and stability over 600 cycles, with XPS signals of Ru, Fe, and Ni showing minimal changes after hydrazine electrooxidation, highlighting its durability. DFT calculations indicated that the addition of Ru increased the valance of Fe and stabilized the intermediates containing unpaired electrons (*N2H3 and *N2H), leading to a transition in the rate-determining step (RDS) of HzOR from *N2H2 dehydrogenation to *N2H to the formation of *N2H2 specie, thereby reducing the energy barrier of the HzOR. To achieve a thorough understanding of the present research advancements, we have meticulously summarized the various applications of LDH-supported noble-metal SACs utilized for the anode reaction in hybrid water electrolysis [Table 2].
LDH-supported noble-metal SACs for the anode reaction of hybrid water electrolysis
| Noble metal | LDH type | Application | Performance (V) | Selectivity | Stability | Ref. |
| Pt (5.64 wt%) | NiCo-LDH | GOR | E100: 1.298 | 88.7% (formate) | The current density is stable after 24 h | [42] |
| Ru (0.30 wt%) | NiFe-LDH | HMFOR | Eonset: 0.91 | 99.24% (FDCA) | Over 85% in FDCA selectivity after five cycles | [101] |
| Rh (6.59 wt%) | NiV-LDH | UOR | E100: 1.33 | - | ~ 90% urea removal rate remained after three cycles | [102] |
| Pt (1.78 wt%) | NiCo-LDH | UOR | E10: 1.25 | - | No evident decay after 50 h test at different potentials | [103] |
| Rh (5.40 wt%) | NiFe-LDH | HzOR | E10: 1.38 | 95% (N2) | No evident decay after 1,000 CV cycles | [39] |
| Ru (1.60 wt%) | NiFe-LDH | HzOR | E10: 1.26 | 98% (N2) | No evident decay after 600 CV cycles | [105] |
Thermal catalysis and photocatalysis
Thermal catalytic reactions are central to modern industrial chemical processes, involving the acceleration of chemical reactions through heating and the use of catalysts. In thermal catalysis, catalysts are typically employed to lower the activation energy of reactions, enabling them to proceed at lower temperatures or at an increased rate[106]. Catalysts used in these processes can be either homogeneous (dissolved in the reaction medium) or heterogeneous (typically solid catalysts). Enhancing the activity and selectivity of industrial heterogeneous catalysts holds important significance from both profitable and environmental standpoints. The homogeneous distribution of active sites in SACs typically leads to high performance. However, until now, only a limited number of LDH-supported noble metal SACs have demonstrated remarkable performance in organic synthesis reactions [Table 3], including the hydrogenation of CO2, selective benzyl alcohol oxidation, and anti-Markovnikov hydrosilylation of olefins. Given the crucial role of organic synthesis in the industry, there is a strong incentive to explore efficient LDH-supported noble metal SACs that can be applied in this field.
LDH-supported noble-metal SACs for thermal catalysis and photocatalysis
| Metal type | LDH type | Application | Selectivity | Stability | Ref. |
| Ru (1.10 wt%) | NiFe-LDH | Benzyl alcohol oxidation (thermal catalysis) | ~ 100% (benzaldehyde) | Slightly decreased after five cycles | [44] |
| Pt (0.35 wt%) | CoFe-LDH | Alkene hydrosilylation (thermal catalysis) | 99% (tri ethoxy(octyl)silane) | 96.6% of original activity remained after ten cycles | [45] |
| Ru (0.36 wt%) | MgAl-LDH | Hydrogenation of CO2 (thermal catalysis) | > 99% (formic acid) | 90% of original activity remained after three cycles | [107] |
| Au (3.27 wt%) | MgAl-LDH | Benzene oxidation (photocatalysis) | 99% (phenol) | No evident decay after five cycles | [41] |
The hydrogenation of CO2 to formic acid is vital for promoting sustainability, as it offers a renewable hydrogen storage solution while helping to reduce greenhouse gas emissions. Mori et al.[107] developed isolated Ru single-atom sites with an octahedral coordination geometry, consisting of one OH and two H2O ligands, anchored to three O atoms derived from the original OH group on the surface of MgAl-LDH
The selective aerobic oxidation reaction of benzyl alcohol to benzaldehyde represents a crucial transformation in organic synthesis, as it enables the efficient and environmentally friendly production of benzaldehyde, a valuable chemical intermediate with widespread industrial applications. Recently, our group[44] systematically investigated the microenvironment influence of NiFe-LDH-supported Ru SACs on the catalytic performance of benzyl alcohol oxidation [Figure 11A-C]. Three kinds of Ru SACs,
Figure 11. (A) The reaction for benzyl alcohol oxidation; (B) Plots of conversion versus time and (C) TOF value and selectivity of different catalysts[44]. Copyright 2022, Wiley-VCH; Simulated models and spatial electric field distributions of Pt1/LDHV (D) and Pt1/LDH (E); (F) Graphs of conversion over time for Pt-SACs and LDH supports; (G) Arrhenius plots on Pt1/LDHV and Pt1/LDH; (H) Graphs of conversion over time for Pt-SACs prior to and following hot filtration[45]. Copyright 2024, Wiley-VCH. LDHs: Layered double hydroxides; SACs: single-atom catalysts; TOF: turnover frequency.
The process of alkene hydrosilylation, involving the adding of Si-H to the C=C, is a pivotal industrial reaction widely used in the production of functional organosilicon compounds vital to various fields such as materials science, iatrochemistry, and medicinal chemistry, making it one of the most important large-scale reactions. We recently prepared an efficient catalyst for this reaction by anchoring Pt single-atom sites onto the divalent cation vacancies of CoFe-LDH (Pt1/LDHV) in the form of PtO3 coordination [Figure 11D-H][45]. The Pt1/LDHV exhibited efficient catalytic activity in the alkene hydrosilylation reaction of 1-octene and triethoxysilane, achieving a high conversion rate of ≈ 99%, and only the anti-Markovnikov addition product was detected. Additionally, the catalytic performance of Pt1/LDHV (with a TOF value of 130,000 h-1) surpassed that of other alternative catalysts and most reported noble-metal SACs. Notably, this Pt1/LDHV catalyst demonstrated remarkable stability and activity retention after ten reaction cycles, with catalytic efficiency loss lower than most reported noble metal SACs. The superior performance and stability of
Besides the above thermal catalytic organic synthesis, the LDH-supported noble metal SACs also demonstrated potential in selective organic synthesis powered by light. Shen et al.[41] reported that
Enzyme-like applications
The biosafety of LDH-supported noble-metal SACs confers them with potential for enzyme-like applications. Recently, our group[43] first found that MgAl-LDH with Ru single-atom sites (RuO3) on the surface (Ru1/LDH) can serve as an artificial nanoenzyme to achieve efficient scavenging of reactive radicals. Ru1/LDH demonstrated excellent peroxidase (POD)-like catalytic performance, exhibiting 20 times higher catalytic efficiency (kcat/Km = 1.69 × 104 S-1 M-1) than Ru NCs/LDH (Ru nanoparticles anchored on
Figure 12. (A) Assessment of ROS scavenging capacity of Ru1/LDH and LDH using fluorescence microscopy; (B) Time-dependent intracellular antioxidant activities and average intracellular fluorescence intensity of Ru1/LDH and LDH; (C) Normalized flow cytometry analysis of intracellular ROS after 48 h of Rosup stimulation with different enzymes; (D) Cell viability following 12 h incubation with LDH and Ru1/LDH[43]. Copyright 2023, Wiley-VCH. LDHs: Layered double hydroxides; ROS: reactive oxygen species.
The LDH-supported noble-metal SACs also exhibited excellent activity in the anti-bacterial field. Zhou et al.[108] developed a Ru SAC (RuO4) supported on the surface of NiFe-LDH (Ru/NiFe-LDH) in the activation of peroxymonosulfate (PMS) to deactivate Escherichia coli (E. coli). With the combined action of low concentrations of Ru/NiFe-LDH (40 mg/L) and PMS (5 mg/L), full inactivation of 7 log E. coli was achieved in 90 seconds. The Ru/NiFe-LDH activated PMS, generating key reactive oxygen species, notably electrophilic 1O2, serving as the main active species responsible for the rapid deactivation of E. coli.
CONCLUSION AND PERSPECTIVE
In summary, LDH-supported noble-metal SACs have garnered significant attention from researchers and have undergone rapid developments recently. LDHs exhibit an adjustable structure, offering
The precise tuning of noble-metal single-atom sites
Precise tuning is essential for optimizing the catalytic properties of noble-metal SACs. Due to the unique and adjustable structure of LDHs, noble-metal single-atom sites tend to stabilize at the position with the lowest formation energy, making directional anchoring on LDHs possible. However, most of the reported LDH-supported SACs mainly focus on LDH with laminates composed of two kinds of non-noble metals. Introducing a third or more non-noble metals into LDH laminates may further regulate the electronic structure of the laminates, thereby obtaining stronger directional anchoring ability of noble-metal atoms. In addition, the modification of axial ligands and the coordination number regulation of noble-metal atoms also face significant challenges. The modulation of the microenvironments of noble-metal single-atom sites can further adjust the dynamic changes of active sites and the adsorption behavior of intermediates during the catalytic process, thereby imparting distinct catalytic stability, activity, and selectivity to the catalysts. Precise tuning allows for a deeper understanding of catalytic mechanisms, which paves the way for designing catalysts with improved efficiency and selectivity. Therefore, despite challenges, precise tuning of LDH-supported SACs holds promise for catalytic advancements.
The high loading of noble-metal single-atom sites
Achieving the high loading of noble-metal single-atom sites is tricky due to the high surface energy. The strong co-catalytic metal-support interactions between LDHs and noble-metal atoms provide feasibility for achieving high loading of noble-metal single-atom sites. However, most of the reported LDH-supported SACs currently have low noble-metal loading (< 3 wt%). Therefore, developing specific synthesis strategies to increase the loading of noble-metal single-atom sites is the focus of future research work. Improving the loading of noble-metal single atoms is the key to optimizing the catalytic performance of LDH-supported noble-metal SACs.
The role of interlayer anions in LDHs
The interlayer anions, such as CO32-, Cl-, NO3-, SO42-, OH-, PO43-, CH3COO-, etc., influence the overall chemical environment of LDHs, thereby shaping the properties and behavior of LDH-supported catalysts. The introduction of a variety of interlayer anions has not only enriched the catalog of LDH-based catalysts but also offered enhanced structural flexibility for fine-tuning their performance. Currently, understanding how interlayer anions alter the characteristics of LDHs remains a focal point of scientific inquiry. However, despite the recognized significance of interlayer anions, existing research on LDH-supported noble-metal SACs has predominantly focused on CO32- intercalation, neglecting the potential contributions of other interlayer anions. The exploration of diverse interlayer anions promises to endow LDH-supported
The stability of LDH-supported noble-metal SACs
The high surface energy of single-atom sites significantly enhances their mobility on the support. During catalysis, when reactants adsorb onto these single-atom sites, the interaction between single-atom sites and the support weakens, leading to their migration and aggregation into clusters or particles, or even detachment from the support. This issue is particularly pronounced under harsh operating conditions, where stability is further compromised. Although most reported LDH-supported noble-metal SACs demonstrate good stability, maintaining high activity after several hundred hours or multiple cycles, they remain distant from industrial-scale application. Enhancing the interaction between noble-metal
The development of DACs/MACs
As the field of SACs progresses, their inherent limitations have become increasingly apparent to researchers. For instance, SACs, which only feature one metal center, face difficulties in the reactions based on multi-site adsorptions that are common in numerous catalytic reactions. This constraint poses substantial challenges to the widespread exploration and application of SACs across various catalytic domains. In light of these restrictions, the advent of DACs and MACs, evolving from SACs, has generated significant excitement among the scientific community. Previous investigations have revealed that interactions between two or more metal single-atom sites, including synergistic effects, distance-enhancement effects (also known as geometric effects), and electronic effects, can effectively address some of the limitations encountered in SAC applications. Furthermore, the steric hindrance effect among different atoms serves to efficiently hinder the aggregation of identical metal atoms, thereby enabling DACs and MACs to accommodate higher metal atom loadings compared to SACs. It is worth noting that, to date, there have been no reports on
The application of in-situ characterization
In the aforementioned research, the anchoring form and coordination environment of noble-metal single-atom sites are usually determined by AC-STEM and XAS techniques. The above information obtained is used to construct theoretical models for DFT, revealing the intrinsic mechanisms between the microenvironment of noble-metal single-atom sites and catalytic performance. However, the electronic structure and coordination of noble-metal single-atom sites tend to change during electrochemical processes. The inability of these characterizations to capture such changes in the working state leads to differences between DFT computational results and the actual reaction mechanism. Therefore, employing advanced in-situ characterization techniques, such as in-situ XAS and single-atom electron spectroscopy, to further capture the changes in the electronic structure, the coordination environment of noble-metal
The establishment of advanced intelligent synthesis platforms
The application exploration and performance optimization of SACs is mainly based on a terrific amount of repetitive experiments. However, with the rapid development of artificial intelligence (AI) and robot technology, researchers are expected to break free from the above inefficient and labor-intensive process. Researchers can use AI combined with DFT to establish predictive catalyst models and achieve rapid reaction parameter screening and catalyst preparation via high-throughput screening and laboratory robot technology. Notably, the highly ordered and adjustable crystal structure and the relatively facile synthesis step of LDH-supported noble-metal SACs provide favorable conditions for the realization of intelligent synthesis. The establishment of advanced intelligent synthesis platforms can significantly improve the work efficiency of researchers, allowing them to have more time to focus on more valuable work.
Further exploration in application fields
LDH-supported noble-metal SACs hold immense potential across various fields, including electrocatalysis, thermal catalysis, photocatalysis, energy storage, and medical treatment. However, current research on these materials primarily focuses on a limited scope within electrocatalysis, specifically in water splitting and hybrid water electrolysis. Notably, LDH-supported materials and noble-metal-based catalysts have already demonstrated diverse applications in other electrocatalytic reactions, such as carbon dioxide, nitrate, and nitrogen reduction reactions. Moreover, there is a noticeable scarcity of reports on the use of
Large-scale industrial production
LDHs have achieved large-scale industrial production and application due to their facile synthesis,
DECLARATIONS
Authors’ contributions
Made the literature review and drafted the original version: Liu, D.
Literature survey and organization: Liu, D.; Zhang, T.; Gu, X.; Yang, X.
Revised the manuscript: Liu, D.; Zhang, T.; Han, A.; Liu, J.
Conceived and supervised the project: Han, A.; Liu, J.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was financially supported by the Beijing Natural Science Foundation (Z240027), the National Natural Science Foundation of China (NSFC), and the Fundamental Research Funds for the Central Universities.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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