Recent achievements in rare earth modified metal oxides for environmental and energy applications: A review
Yicheng Li , Qian Liu , Tianhao Li , Hao Bi , Zhurui Shen
Since the advent of the Industrial Revolution, there has been a continuous and exponential growth in global energy demand. In the initial stages of the industrial era, conventional energy sources, including oil, coal, and natural gas, constituted the primary sources of power [1]. Despite the advent of new energy sources, including wind, solar, and hydrogen, these have yet to make a substantial impact on the contribution of traditional fossil energy sources [2-6]. Currently, human society continues to depend on fossil fuels. However, the reserves of traditional fossil fuels are not only limited in quantity but also unevenly distributed geographically. Moreover, an overreliance on conventional energy sources has resulted in constrained energy supplies and frequent price fluctuations, thereby introducing considerable instability into economic development. Additionally, the utilisation of traditional energy sources is accompanied by significant environmental concerns, particularly in relation to pollution. The combustion of these sources releases considerable quantities of carbon dioxide, which contributes to global warming and endangers the survival of humanity. Furthermore, industrial activities result in the generation of a considerable volume of wastewater, waste gas, and waste residue, which cause significant damage to the atmosphere, water, and soil. To tackle these two major challenges, researchers are constantly on the lookout for alternative, environmentally friendly lifestyles, which is the most pressing issue of our time [7, 8]. The development and utilisation of green and non-polluting renewable energy sources such as light [9-12], heat [13-16], electricity [17-20], and biochemical energy [21-23] for efficient energy conversion and utilisation. Accordingly, the design and preparation of rational catalysts to enhance energy utilisation efficiency represents a crucial solution concept for the advancement of renewable energy sources. Currently, metal oxide catalysts play a significant role in the field of catalysis [24-28].
A metal oxide consists of an oxygen atom bonded with another metal atom to form a compound with stable structure and properties. It is widely used in the field of photoelectrocatalysis [29-32]. Metal oxides typically exhibit multiple oxidation states, and the metal ions on their surfaces can undergo conversion between different oxidation states. For instance, iron ions in iron oxide (Fe2O3) can transition between oxidation states of +2 and +3 [33]. This variability in oxidation states leads to a large number of active sites on the surface of metal oxides. These sites can adsorb and activate H2O2 or persulfate and can be used for the removal of organic pollutants. Moreover, metal oxides often have structural imperfections such as oxygen vacancies and metal vacancies. Such defects can act as active sites for the adsorption and activation of reactant molecules, thereby facilitating the catalytic reaction. For example, the crystal structure of indium oxide (In2O3) contains numerous oxygen vacancies that serve as active sites, significantly enhancing CO2 activation and the formation of *HCOO intermediates [34]. This results in the efficient generation of formic acid with remarkable stability.
In recent years, the development and utilization of rare earth resources have been the subject of considerable attention due to their importance and relative scarcity [35-38]. This has had a significant impact on global economic and scientific development. Rare earths are a group of 17 elements, including scandium, yttrium, and the 15 lanthanides (ranging from lanthanum to lutetium), which display a number of analogous physical properties. The introduction of rare earth atoms into metal oxides can result in the acquisition of a plethora of optical, electrical, magnetic, and catalytic properties, thereby enabling the metal oxides to exhibit enhanced performance [39-42]. In addition, the direct combination of rare earth metals and oxygen results in the formation of rare earth metal oxides that exhibit exceptional catalytic properties. Rare earth metal oxides represent a distinctive category of catalysts that have demonstrated remarkable performance in a wide range of chemical reactions. In the modern field of chemistry, the search for highly effective, environmentally friendly, and widely applicable catalysts remains a pivotal area of investigation. The particular electronic configurations and chemical characteristics of rare earth metal oxides present new prospects and challenges in the field of catalysis. As scientific and technological advancement continues, the requirements for catalysts are increasing. Rare earth metal oxides are distinguished by their ability to catalyze a wide range of reactions due to their diverse oxidation states, exceptional thermal stability and remarkable catalytic activity. Rare metals have considerable potential in the fields of environmental protection and energy conversion [43-45]. In this review, we commence by depicting the properties of rare earth metals, especially their distinctive electronic structure, the modification of metal oxides and their ability to combine with oxygen. Simultaneously, the potential of rare earth metals and their oxides in energy and environmental applications is elaborated in detail. Then, we summarize recent years of research on rare earth modified metal oxides and rare earth metal oxides in the fields of energy and environment. This review will delve into the crucial role of rare earth metals and rare earth metal oxides in catalysis, summarize applications in hydrogen evolution reaction, CO2 reduction, nitrogen reduction, waste water treatment, hydrogen peroxide synthesis and machine learning, analyze their catalytic mechanisms and factors influencing catalytic performance, and provide a theoretical basis and practical guidance for further research and application of rare earth metal catalysts.
Currently, rare earth metal elements can be categorized into light, middle and heavy elements based on the solubility of the corresponding sulfate and the nature of the compound [46, 47]. Light rare earth elements include La, Ce, Pr, and Nd. Middle rare earth elements include Sm, Eu, Gd, Tb, and Dy. Heavy rare earth elements include Ho, Er, Tm, Yb, Lu, and Y. Rare earth elements have unique electronic structures and physicochemical properties [48]. Researchers expect to utilize their unique properties in the design and development of electrode materials to facilitate electrocatalytic reactions.
The atoms of rare earth metal elements have unfilled 4f electron layers [49]. Due to the different ways in which the 4f electrons are filled, rare earth metal elements have a variety of electronic configurations [50]. From lanthanum (La) to lutetium (Lu), the 4f electrons are filled sequentially as the atomic number increases. This variety of electron configurations allows the rare earth metal elements to exhibit certain regular variations in their physical and chemical properties [51, 52]. Specifically, La, Ce, Gd, and Lu metal elements have an electronic structure basis configuration of 4fn−15d16s2, Sc and Y metal elements have an electronic structure basis configuration of 3d14s2 and 4d15s2, respectively. Other rare-earth metal elements have similar electronic structure basis configurations: 4fn6s2 [48]. The unfilled 4f electron layer is in the inner part of the atomic structure, which is shielded by the outer electrons, resulting in a strong coupling between the orbital angular momentum and the spin angular momentum of 4f electrons, leading to a complex electronic structure with a high number of electrons and a high number of electrons stronger, leading to a complex electronic energy level structure. In detail, the 5s and 5p orbitals have lower energies than the 4f orbitals, and the dispersion of the 5s and 5p orbitals is larger than that of the 4f orbitals. As a result, the electron-filled 5s2 and 5p6 closon shell layers remain outside the 4f orbitals, leading to a weak shielding of the 4f electrons [53].
The special electronic structure of rare earth metal elements can be used as "electron modulators" in electrocatalytic reactions. The d-f orbital coupling with the d orbitals of transition metals can establish electron transfer channels and enhance the electrical conductivity and electron transfer ability of the catalysts, thus accelerating the charge transfer process in the electrocatalytic reaction. Fu reported a rare-earth Er-induced electron modulation strategy to enhance the oxygen precipitation electrocatalytic performance of NiFe LDH [50]. The electrocatalytic test results show that the optimized Er-NiFe-LDH@NF exhibits excellent OER activity with an overpotential of only 191 mV@10 mA/cm2, which is superior to that of commercial RuO2 electrocatalysts. Compared with the pristine electrode, Er-NiFe-LDH@NF exhibited better reaction kinetics, higher TOF value and lower activation energy. In situ Raman spectroscopy demonstrated that the NiFe-LDH material after Er doping could promote the conversion of Ni-OOH, which enhanced the kinetics of the OER reaction. Density-functional theory (DFT) calculations show that the doped Er sites optimize the redistribution of surface charge and promote the spin-flip of valence electrons, which is conducive to weakening the spin-flip forbidden effect. The stronger spin-orbit coupling has an important effect on the kinetics of the electrocatalytic reaction. In the electrocatalytic reaction process, the spin state of electrons may affect the formation and transformation of reaction intermediates [54, 55]. The spin-orbit coupling of rare earth elements can promote the spin-flip of electrons and break the spin-flip forbidden effect, so that the electrons can participate in the reaction more freely, thus improving the kinetic speed of the electrocatalytic reaction [56]. Moreover, the presence of rare earth elements can also regulate the adsorption and reactivity of the catalyst to electrons of different spin states, so that the reaction is more inclined to follow specific spin paths, thus improving the selectivity of the electrocatalytic reaction [57].
The 4f electrons can also change the electron distribution and energy level structure on the catalyst surface by interacting with the electrons of other elements to optimize the adsorption energy of the reaction intermediates on the catalyst surface and improve the selectivity and activity of the electrocatalytic reaction. For example, Wan et al. [58] synthesized citrate anion intercalated trimetallic layered double hydroxides NiFeCe-LDH by co-precipitation method, confirming the modulation of electrocatalytic activity by 3d-4f electronic interactions. It is shown that the OER activity of Ni2Fe1−xCex gradually increases with the increase of Ce doping rate, among which Ni2Fe0.7Ce0.3 has the best OER activity, with the lowest overpotential of 224 mV at a current density of 20 mA/cm2, which is much smaller than that of RuO2 (381 mV). Theoretical calculations show that both spin-up and spin-down states are continuously populated near the Fermi energy level, which is mainly attributed to the orbital hybridization of Ce 4f, Fe 3d, and Ni 3d, which provides strong theoretical evidence for the 3d-4f electronic interactions. Reaction potential calculations also showed that the introduction of Ce effectively optimized the adsorption/desorption performance of the oxygenated intermediates, thus enhancing the intrinsic OER activity of the catalyst.
Since the arrival of the industrial era, a multitude of pollutants have been discharged into natural water bodies in various ways as a consequence of industrial activities. Of particular concern are persistent organic pollutants such as pharmaceuticals, cosmetics, dyes, pesticides and endocrine disrupters. The evidence indicates that these pollutants have a detrimental impact on ecosystems and present a risk to human health. For instance, consider the example of antibiotics. Antibiotics are extensively utilized in the treatment of both humans and animals. According to previous studies and reports, the usage of antibiotics in China reached 162,000 tons in 2013, with 52% of this amount being veterinary antibiotics [59]. Over 50,000 tons of antibiotics entered the soil and water environment through human and animal excreta and other channels. Some research has indicated that regions such as the Yangtze River Basin still exhibited elevated concentrations of antibiotics in 2020 [60]. The large-scale production and utilisation of antibiotics is a result of their effectiveness. However, this can also lead to significant environmental contamination and pose substantial risks to human health. Therefore, there is an urgent need to mineralize, remove, transform and reduce antibiotic contaminant molecules in the aquatic environment. Researchers are actively engaged in seeking optimal solutions. For this purpose, a variety of water treatment technologies have been developed, such as coagulation and precipitation [61-64], chemical methods [65-67], physical adsorption [68-71], membrane separation [72-75], advanced oxidation [76-78], and biological processes [79-81]. Among the numerous treatment methods available, the removal of harmful pollutants from wastewater through photoelectrocatalysis has attracted considerable attention. This is because the resulting oxidised substances can be used to break down the pollutants into molecules which are less harmful or even harmless. Advanced oxidation processes (AOPs), as a category of chemical process, are recognised as one of the most promising and effective methods of dealing with chemical and wastewater issues. The multivalent transformation of rare earth metals promotes enhanced electron transfer and transport efficiency during the Fenton reaction. For examples, Chen et al. employed the synergistic effect of iron and cerium to effectively promote the cycling of Fe3+/Fe2+ and Ce4+/Ce3+ redox electron pairs, thereby enhancing the electron transfer and transport efficiency at the material surface and accelerating the generation of •OH [82]. In this work, Fe0-Fe3O4/CeO2/C composite electrode materials were prepared using biomass porous carbon produced as a carrier for dispersed rare earth metal oxides and iron-based nano-catalyst particles. The transmission electron microscope (TEM) morphology of the samples is presented in Figs. 1a–c. The CeO2 nanohollow spheres have an approximate diameter of 320 nm and are surface-loaded with cubic crystal system Fe0-Fe3O4 nanoparticles. The materials were then used for the degradation of 20 ppm antibiotic ceftriaxone sodium, which was degraded by 95.59% within 120 min (Fig. 1d). The composite material, with its hollow structure and rough surface, exhibits a high specific surface area. This provides an effective adsorption capacity and a large number of active sites, which is conducive to the adsorption of pollutants. As can be seen in Fig. 1e, the incorporation of the CeO2 catalyst within the hollow structure facilitates the conversion of H2O2 to •OH. The inclusion of FeO also enhances the effective cyclic conversion of Fe³+/Fe2+, accelerates the generation of ·OH, and improves the electrocatalytic degradation of the composite material. The addition of Fe0 facilitated the effective cyclic conversion of Fe3+/Fe2+ and enhanced the electrocatalytic degradation of the composite. This study presents a novel electro-Fenton catalyst with promising applications across a range of fields. It provides valuable insights and experimental data that can guide the development of effective strategies for the treatment and removal of antibiotic organic pollutants.
Permonosulfate (PMS) exhibits robust oxidising properties, and the activation of PMS is a crucial aspect of AOPs. AOPs based on PMS activation employ PMS in combination with a catalyst to generate highly oxidative reactive oxygen species, including •SO4− and •OH, with the objective of degrading organic pollutants in water. The activation of PMS using metal-based materials represents a promising avenue for the pollutants treatment. However, it is frequently challenging to prevent secondary environmental damage when utilizing conventional transition metals. Rare earth metals with high energy levels, multi-electron configurations and unique 4f orbitals have been employed as catalysts for PMS activation due to their capacity to act as electron transfer bridges, reduce interfacial energy loss and facilitate electron transfer. As shown in Fig. 2a, Pei et al. fabricated LaCO3OH with N and S co-doped graphene (LCOH/GNS) by a combination of calcination and hydrothermal methods, and the resulting catalyst was observed to degrade levofloxacin (LVX) with high efficiency [83]. The findings demonstrated that the removal of LVX reached 89.6%, while the leaching of La³+ was recorded at a minimal level of 1.2 mg/L. The use of LCOH for the activation of PMS represents a novel approach that has been demonstrated to exhibit excellent degradation performance and low ecotoxicity. The doping of N and S in the graphene matrix has been observed to enhance the adsorption of PMS, increase electron transfer, and reduce the leaching of metal ions. It has been demonstrated that this process facilitates the interaction and reaction between LCOH and PMS. The free radical quenching experiment and electron paramagnetic resonance (EPR) techniques have provided evidence to support the hypothesis that the degradation process may involve multiple oxidation pathways. This work proposed an innovative approach for the rational design of rare earth metal-based carbon catalysts, and it is anticipated that LCOH/GNS-PMS will prove to be an effective technology for the degradation of antibiotics in wastewater treatment. Furthermore, as demonstrated in Fig. 2b, Zhao et al. successfully synthesized a Co3O4-La2O3 electrocatalyst containing numerous oxygen vacancies (Ov) [84]. This was accomplished by introducing defects into the Co3O4 matrix through the introduction of La2O3 as the dopant. The findings demonstrated that the catalyst could effectively activate PMS and facilitate the selective generation of 1O2. This resulted in the almost complete removal of tetracycline (TC) within 40 min. A comprehensive study was conducted on the Co3O4-La2O3/PMS electrocatalytic system for TC degradation, and its potential for the effective degradation of other antibiotics was also evaluated. The excellent performance may be attributed to several factors, including the effective cycling of Co(Ⅲ)/Co(Ⅱ), the Ov introduced by La2O3 doping, the activation of PMS and the enhanced driving force of the electrocatalytic system. The addition of Co at a concentration of 50% La resulted in optimal performance of the Co3O4-La2O3/PMS system. Within 40 min, the degradation efficiency of TC exceeding 97.50% and the mineralization rate reaching 62.97%. Consequently, the incorporation of rare earth elements can result in the introduction of corresponding defects within the catalytic system, thereby enhancing the catalytic degradation performance of the system. This approach can serve as an effective strategy for the treatment of antibiotic-containing wastewater while simultaneously reducing energy consumption and pollution. Overall, rare earth metals possess a distinctive electronic configuration, characterized by incompletely filled 4f orbitals. These orbitals are capable of participating in redox reactions, thereby facilitating the transfer of electrons during the catalytic process. This enables the rare earth metals to activate the oxidant with greater efficacy in the advanced oxidation reaction, thereby generating strong oxidizing radicals that are capable of degrading pollutants in water. In addition, the incorporation of rare-earth metals influences the electronic configuration of the catalytic system, thereby promoting the generation and transfer of electrons during the reaction process and facilitating the degradation of pollutants in water.
Hydrogen peroxide (H2O2) is a green oxidising agent, which is environmentally friendly. An aqueous solution of hydrogen peroxide is frequently referred to as simply hydrogen peroxide. Due to its straightforward structure and the absence of toxic by-products, hydrogen peroxide is regarded as the most environmentally friendly chemical product. It plays a significant role in numerous fields, including chemical synthesis, energy production, medical disinfection, sewage treatment, bleaching, and aircraft fuel [85-88]. Currently, in excess of 95% of the industrial production of hydrogen peroxide at home and abroad is dependent on the energy-intensive anthraquinone redox process, otherwise known as the anthraquinone method [89, 90]. It is characterised by a high energy consumption, a multitude of cumbersome steps and a considerable pollution output during the preparation process. Furthermore, the process requires the use of large equipment, which results in the generation of a considerable quantity of organic waste liquid. This presents a significant challenge in meeting the requirements of green and sustainable development. Concurrently, the establishment of a sophisticated logistics system is of the utmost importance in order to provide users with highly concentrated hydrogen peroxide over extended distances. However, this presents a significant risk of explosion during transportation, while also markedly escalating the costs and safety concerns associated with storage, dilution, and utilization of hydrogen peroxide. Consequently, the extensive use of hydrogen peroxide is considerably constrained in numerous industrial sectors. Annually, there are numerous incidents pertaining to the production, transportation, and storage of hydrogen peroxide worldwide. In recent years, rare earth oxides have been employed extensively in a range of applications, including catalysis, accelerators and high-performance light-emitting devices, owing to their distinctive optical and electronic properties. Their excellent corrosion resistance, abundance of oxygen vacancies and high oxygen capacity have attracted considerable attention as potential co-catalysts, with the objective of enhancing the electrocatalytic performance of transition metal active components. Among these, cerium matrix composites, which are abundant, have a stable structure, impressive oxygen storage capacity and excellent co-catalysis capacity, and therefore have considerable potential for a wide range of applications. The electrochemical two-electron oxygen reduction reaction (2e− ORR) using rare earth oxides as catalysts enables the continuous decentralized production of hydrogen peroxide under mild conditions [91-93]. This method has a high efficiency of hydrogen peroxide synthesis efficiency and numerous advantages, including economy, safety, environmental protection and mildness. Of particular significance is the ability to combine this process with inexpensive electricity (including solar or wind power) to facilitate localized, small-scale production of hydrogen peroxide on demand. Additionally, the reaction time can be controlled to produce varying concentrations of hydrogen peroxide, which not only reduces the cost associated with storage and transportation but also mitigates safety concerns. In general, the adsorption and activation capacity for oxygen-containing compounds of CeO2 can be adjusted by modifying the surface oxygen coverage, exposing specific surfaces or introducing surface oxygen deficiency. In particular, CeO2 displays considerable potential for selectively promoting the generation of hydrogen peroxide, attributed to its slow oxygen uptake and weak cleavage of *O-OH. Yuan et al. successfully prepared CeO2 nanoparticles via a hydrothermal method and employed them in 2e− ORR to synthesise hydrogen peroxide, achieving a selectivity of up to 70% for the latter [94]. Subsequently, they combined carbon quantum dots (CDs) with CeO2 in order to further enhance the performance of the hydrogen peroxide synthesis (Fig. 3). The optimised CDs/CeO2 system exhibited a hydrogen peroxide selectivity of 92% and an approximately 1.5-fold increase in activity. Furthermore, the system demonstrated stability for 24 h at a current density of 20 mA/cm2, exhibiting a Faraday efficiency of 97%. The in situ transient potential scan demonstrated that the appropriate addition of CDs markedly enhanced the reaction kinetics of oxygen, facilitated the transfer of electrons, and regulated the distribution of charges within the reaction system, thereby improving the selectivity and activity of CeO2. What is more, Cheng et al. modified the morphology of CeO2 and successfully prepared a novel nanocubic CeO2 (CeO2 NCs), which was then compared with the conventional nanodot CeO2 (CeO2 NDs) [95]. The findings indicated that the CeO2 NCs exhibited considerably enhanced H2O2 activity, with a corresponding H2O2 yield of 632 mmol gcat−1 h−1 at 0.5 V (vs. RHE) and a selectivity reaching 95.3%. Additionally, CeO2 NCs were observed to function as an oxidant for 2e− H2O, resulting in the generation of H2O2 with a yield of 115.2 mmol gcat−1 h−1 and a selectivity of 66%. This property is exceedingly uncommon among other catalysts. It can thus be concluded that CeO2 NCs can be applied in the cathode of a flow cell for the co-synthesis of H2O2, with a yield of 4.6 mol gcat−1 h−1 at 1.5–2.3 V and a Faraday efficiency exceeding 85%. The findings of this study provide definitive evidence that CeO2 NCs are a novel bifunctional catalyst for the efficient electrochemical synthesis of H2O2. This may facilitate the exploration of new avenues for the design and development of other rare earth metal oxide materials for the selective production of H2O2. In addition to CeO2, other rare-earth metal oxides have also been demonstrated to exhibit excellent catalytic performance for H2O2 synthesis. A comparative study of the electrosynthesis of H2O2 with four metal oxides, namely Pr6O11, Nd2O3, Sm2O3, and Gd2O3, was conducted by Hota et al. [96]. Amongst these, Pr6O11 was observed to display the most favorable H2O2 catalytic activity, which can be attributed to the presence of a considerable number of oxygen vacancies (Ov). In contrast, Gd2O3 demonstrated a higher degree of selectivity, reaching up to 80%. Furthermore, the selectivity of H2O2 continued to improve with the addition of carbon black for modification, with a yield of 91.91%. Chen et al. synthesized a series of catalysts with excellent performance for the synthesis of H2O2 by combining rare earth oxides with biomass char (NPC) [97]. A comparative analysis was conducted to assess the performance of the rare earth metals La, Pr, Nd, Sm and Gd when they were doped separately. At 0.6 V, the H2O2 yields were 70% for the La-NPC catalyst, 65% for the Pr-NPC catalyst, and 50% for the Nd-NPC catalyst. To date, the number of reports on the synthesis of hydrogen peroxide from rare earth metals is currently limited, with existing reports predominantly focusing on rare-earth oxides. The distinctive electronic configuration of rare-earth metals may be the underlying reason for the facile electron transfer that enables their participation in the 4e− ORR. Consequently, future research efforts may be directed towards rare-earth oxides, which exhibit a high oxygen vacancy density. Moreover, the high oxygen capacity enables the utilisation of a range of modification techniques to facilitate the exposure of surface active sites or to regulate the adsorption and activation capacity of oxygen, thus enhancing the ability to synthesize H2O2 via the 2e− ORR. The production of hydrogen peroxide at a low cost and on-site is of great importance for both industrial and civil applications, and 2e− ORR is currently the most promising method of H2O2 production. Hence, it serves as an effective remedy for the issues associated with the anthraquinone process for hydrogen peroxide synthesis and is considered a promising hydrogen peroxide production process for the days to come. Table 1 summarizes the results of rare earth metal element modified electrode material systems applied to wastewater treatment and hydrogen peroxide synthesis.
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| Material | Reaction types and applications | Performance | Ref. |
| Fe0-Fe3O4/CeO2/C | Degradation of pollutants | 20 ppm antibiotic ceftriaxone sodium, 95.59%/120 min | [82] |
| Co3O4-La2O3/PMS system | Degradation of pollutants | degrade levofloxacin (LVX), 89.6% | [83] |
| Co3O4-La2O3/Ov | Degradation of pollutants | TC, 97.50% | [84] |
| CDs/CeO2 | Synthesis of H2O2 | Selectivity 92%, FEs 97% | [94] |
| CeO2 NCs | Synthesis of H2O2 | Selectivity 95.3%, FEs > 85% | [95] |
| Gd2O3 | Synthesis of H2O2 | Selectivity 80% | [96] |
Electrocatalytic hydrogen evolution reaction (HER) has become the focus of attention of many researchers as a pathway that can convert electrical energy into chemical energy and prepare hydrogen efficiently [98, 99]. In the field of HER, finding high-performance, low-cost, stable and reliable electrocatalysts has been one of the core tasks. Metal oxides have the advantages of low cost and outstanding stability, but the electrocatalytic activity of pristine metal oxides is often limited [100]. Rare earth elements show great potential in modifying metal oxide electrocatalytic materials to improve HER performance due to their special electronic structure and chemical properties. Water undergoes a reduction reaction at the cathode to produce hydrogen. This process is generally accomplished by the Volmer reaction (where hydrogen ions gain electrons to form adsorbed hydrogen atoms), the Heyrovsky reaction (where adsorbed hydrogen atoms combine with hydrogen ions to form hydrogen), or the Tafel reaction (where adsorbed hydrogen atoms combine with each other to form hydrogen). The efficiency of HER is affected by a variety of factors, such as the number of active sites in the catalyst, the electron-conducting properties, and the surface-absorption energy, among others. An efficient HER catalyst can lower the overpotential of the reaction, speed up the reaction rate and reduce energy consumption. Liu et al. [101] realized the preparation of composite electrodes by using Ti mesh as the substrate and Liu composited rare earth metal oxide CeO2 with porous nanowire arrays with transition state metal oxide MnO2 (shown in Fig. 4a) and chose to utilize oxalic acid for selective etching of MnO2. Under alkaline conditions (with 1 mol/L KOH as the electrolyte), the composite electrode exhibited high HER activity in the three-electrode system. As shown in Fig. 4b, RuO2 exhibits excellent OER activity. The overpotential required for np-CeO2/TM is 279 mV, which is 101 mV lower than that of MnO2-CeO2/TM. The intrinsic catalytic kinetics during the OER process were compared using the Tafel slope (Fig. 4c). As shown in the linear sweep voltammetry (LSV) curve (Fig. 4d), the overpotential required for the HER of the composite electrode was drastically reduced, and a current density of 10 mA/cm2 was realized at 91 mV, which was 93 mV lower compared to the pristine electrode. np-CeO2/TM had a tafel curve slope of only 57 mV/dec (Fig. 4e), whose results demonstrate the rapid discharge of single protons during the Volmer Heyrovsky mechanism compounded with other rate-limiting electrochemistry to provide high resonance energies, thus exhibiting excellent HER activity. Estimate the effective surface area of MnO2-CeO2/TM and np-CeO2/TM through CV curves (Fig. 4f). The electrochemically active area was estimated by CV curves, and the double-layer capacitance values at the electrode/electrolyte interface were 0.26 mF/cm2 and 0.74 mF/cm2 for MnO2-CeO2/TM and np-CeO2/TM, respectively (Fig. 4g), and their electrochemical impedance plots (Fig. 4h) also showed a decrease in the charge-transfer resistance. The excellent electrochemical water decomposition performance and long-term stability were attributed to the porous nanowire array structure and material composite after selective etching.
Du et al. [102] addressed the durability and catalytic activity problems of Pt catalysts and the idea of modifying the support carriers for Pt catalysts. Pt nanoparticles were designed to be anchored to high-entropy rare earth metal vacancies, which were used as electrodes for hydrogen production by water electrolysis (Figs. 5a–d). High-entropy rare earth metals have unique thermodynamic and chemical properties, and the tunability of their compositions and electronic structures enable highly efficient and directionally selected electrochemical conversion effects. Pt-(LaCeSmYErGdYb)O exhibits highly efficient HER performance, and under a wide pH electrolyte environment window, the realization of a high 100 mA/cm2 The current densities were only 12, 57, and 77 mV (Fig. 5e). In addition, the slope of the Tafel curve (Fig. 5f) for Pt(LaCeSmYErGdYb)O was only 10 mV/dec, which was lower than that of the commercial Pt/C catalytic system. The capacitance of the Pt(LaCeSmYErGdYb)O bilayer calculated from the CV (Cyclic Voltammetry) curve (Fig. 5g) was 33.4 mF/cm2, and the large increase in the electrochemically active area corroborated the change in HER activity. The good cycling stability of the electrodes was demonstrated by 5000 CV cycles and a stable current density of more than 100 h, and possessed a turnover frequency (TOF) value of 38.2 s−1@12 mV. Similarly, the charge transfer resistance of the EIS (Electrochemical impedance spectroscopy) (Fig. 5h) was reduced, indicating the facilitating effect of the rare earth metals on the charge transfer at the solid-liquid interface. The enhanced electrochemical HER performance is attributed to the interaction between Pt and rare earth metals that promotes proton binding and water dissociation. The overpotential (57 mV) of Pt(LaCeSmYErGdYb)O at a current density of 100 mA/cm2 in 1.0 mol/L KOH is lower compared to many HER catalysts (Fig. 5i). As shown in Fig. 5j, Pt(LaCeSmYErGdYb)O exhibits an intrinsic HER mass activity of 37.7 A mg−1 Pt, which is superior to many catalysts.
Pradhan and his coworkers reported a bifunctional electrocatalyst in alkaline medium consisting of CeO2 and CuO composite to form an electrode system [103]. As shown in Figs. 6a-d, the microscopic morphology of the composite electrode is a microsphere composed of nanoparticles with a rough surface having a double-layered core-shell structure. This has a significant effect on the electrochemical reaction, and the inner part of the core-shell structure has an obvious positive performance change trend in terms of participation in charge transfer and exposure of catalytic active sites. Raman (Fig. 6e) and XPS (X-ray Photoelectron Spectroscopy) spectra (Figs. 6h, i) were utilized to explain the effect of oxygen vacancy concentration at the phase interface of the composites, as well as the synergistic charge transfer mechanism, suggesting that the composites are capable of bifunctional activity. Fig. 6f shows that the composite material with CeO2/CuO ratio of 1:1 has the lowest η value of 245 mV, which is closest to the η10 value of commercial Pt/C. As shown in Fig. 6g, the tafel slope of the 1:1 CeO2/CuO composite electrode was 108.4 mV/dec, and the HER reaction process of the electrode followed the Heyrovsky–Volmer mechanism with accelerated electrode reaction kinetics and reduced overpotential, proving its superiority for HER reactions. Zhang et al. [104] developed and synthesized a heterostructure material composed of two-dimensional transition state metal oxides (TMOs: Co3O4/NiO) with oxygen rich vacancies and rare earth metal oxides (CeO2) (shown in Fig. 6j). As shown in Figs. 6k–n, v-TMOs/CeO2 heterostructured electrodes exhibited excellent HER activity performance under alkaline medium. The v-TMOs/CeO2 heterostructured electrodes showed lower starting potentials (~37 mV and 38 mV) than the commercial catalysts in the linear voltammetry curves LSV (Fig. 6l). v-TMOs/CeO2 heterostructured electrodes required only low overpotentials of 99 and 265 mV to achieve a current density of 10 mA, which outperforms many metal-based catalysts. Tafel curves were used to evaluate HER kinetics of v-TMOs/CeO2 heterostructured electrodes. The Tafel slope of Co3O4/CeO2 was as low as 48 mV/dec (Fig. 6m), exhibiting faster HER kinetics compared to other electrodes. The vacancy mechanism facilitates the regulation of the electronic structure of the catalyst and affects the catalytic reaction process. The rare-earth genus oxide CeO2 not only possesses a low redox potential, but also the presence of redox couples with different valence cations (Ce4+/Ce3+), which leads to its excellent redox properties.
Du and his colleagues reported the preparation of a composite electrode material system with a dot shaped nanoneedle array structure (Co3O4@CeO2, Fig. 7a) using hydrothermal, air calcination, and selective selenization methods [105]. In alkaline medium, the HER performance exhibited lower overpotential values: 48 mV@10 mA/cm2 and 175 mV@50 mA/cm2 (Fig. 7b). This demonstrated the effectiveness of Se doping in enhancing the HER performance. In addition, as shown in Fig. 7c, the electrochemical surface area value (ECSA) was also obtained by calculating the CV curve of the composite electrode. The slope of the Tafel curve (Fig. 7d) of the composite electrode also indicates its superior electrochemical activity. Fig. 7e shows the overpotential of the samples obtained at different current densities. Moreover, the catalytic activity of the composite electrode was similar to that of the Pt/C electrode at low current densities compared to the recently reported non-precious metal electrode systems. Corresponding theoretical calculations show that Se doping drives the strong adsorption effect of Co3O4 on water, which also indicates the presence of Se as an active center for the HER reaction.
Shibli and coworkers utilized the idea of doping by doping Ni-p with optimal phosphorus content in CeO2-RuO2 mixed oxide system and analyzed its activity change in HER reaction [106]. The crystallinity of CeO2-RuO2 was controlled by using the variation of calcination temperature. The formation of oxygen vacancies in CeO2-RuO2 enhances the electrode redox capacity, and the controlled electronic structure of Ce promotes the catalytic activity. The elevated OCP (Open Circuit Potential) value with the microstructure of electrode remains unchanged after HER reaction ensures the potential of application as a long time HER catalyst. structure promotes the catalytic activity. The increase in OCP value and the unchanged microstructure of the electrode after HER reaction ensure its potential as a long-term hydrogen evolution catalyst. Under alkaline conditions, the overpotential of the modified electrode was as low as 180 mV at a current density of 250 mA/cm2.
In today's global pursuit of sustainable development, reducing carbon emissions and realizing the effective conversion and utilization of carbon resources have become key issues. Electrocatalytic CO2 reduction technology, as a highly promising pathway, has attracted the attention of many researchers [107-109]. This technology not only helps to alleviate the environmental crisis caused by greenhouse gases, but also converts CO2 into valuable chemicals and fuels, such as carbon monoxide, formic acid, methanol, thus realizing the recycling of carbon [110]. The electrocatalytic CO2 reduction reaction is a multi-electron transfer process, which involves complex reaction mechanisms and kinetics [111]. On the one hand, the CO2 molecule is highly stable, and its activation process needs to overcome a high energy barrier, which poses a serious challenge to the catalyst activity. On the other hand, the HER, as a competing reaction, tends to occur more readily in aqueous electrolytes, which severely interferes with the selectivity of CO2 reduction and leads to a lower Faraday efficiency of the target product [112]. Meanwhile, in order to achieve large-scale industrial applications, electrocatalysts not only need to have high activity and selectivity, but also should be cost-effective and stable.
Rare earth elements provide new ideas for improving the electrocatalytic performance of metal oxides due to their special electronic structure and chemical properties. Rare earth elements have unfilled 4f electronic orbitals, which enable them to produce strong electronic interactions with non-precious metal oxides. Doping of rare earth elements can change the energy band structure of the oxides, introduce new electronic conduction channels, and improve the electron mobility. Rare earth elements can modify the surface of metal oxides, modulating the electronic environment and geometrical configuration of the active sites. The special affinity between rare earth elements and reactants can enhance the adsorption of CO2 on the catalyst surface, and help stabilize the reaction intermediates, directing the reaction in the direction of generating a specific target product and improving the selectivity. At the same time, the introduction of rare earth elements can also inhibit the surface hydroxyl groups and other groups that are unfavorable to CO2 reduction, reducing the occurrence of side reactions.
Du et al. [113] focused their modification ideas on building interfaces as a way to effectively regulate electrochemical activity and selectivity. CeO2/Bi3NbO7 fiber tubular structure (Figs. 8a and b) electrode materials were synthesized using electrostatic spinning method. The two sides of the heterogeneous interface where complexes with different spatial structures are in contact with each other usually show different atomic structure arrangements. However, the lattice of coherent contacts near the CeO2/Bi3NbO7 interface was observed by HRTEM without lattice distortion. The heterogeneous interface atoms are in close contact and the interfacial effect plays a modulating role on the electronic structure. The surface roughness of CeO2/Bi3NbO7 is relatively large (Figs. 8c and d), which increases its electrochemical surface area. The selected area electron diffraction (SAED) image shows two distinct diffraction rings (Fig. 8e), and the high-resolution TEM (HRTEM) image shows lattice spacings of 0.312 nm and 0.323 nm, corresponding to the (111) lattice planes of CeO2 and Bi3NbO7, respectively (Fig. 8f). As shown in Figs. 8g–k, the energy color line X-ray spectra also show a uniform distribution of the elements Bi, Nb, Ce and O on the fiber tubular. The CO2RR performance of the samples was systematically evaluated by passing CO2 into a 0.1 mol/L KHCO3 solution and saturated. The LSV spectra (Fig. 8l) showed the highest current density values for the samples with a Ce: Bi of 1:4. The slope value of the CeO2/Bi3NbO7 electrode was measured by the Tafel curve to be 436 mV/dec, which is lower than that of the pristine electrode, suggesting an acceleration of its reaction kinetics. The CO2RR reaction selectivity showed a substantial increase in the selectivity of the structure to formic acid to 84.73% (Fig. 8m), with a corresponding increase in durability. Within a wider test potential window, the current density of CeO2/Bi3NbO7 is higher than that of Bi3NbO7 (Fig. 8n).
Zeng et al. [114] explored and analyzed the reaction mechanism and reaction intermediates of CO2RR by synthesizing Cu/CeO2 catalysts with different morphologies (Figs. 9a–d). Catalysts with nanorod, nanocube, nanoparticle, and nanosphere morphologies were synthesized by controlling the hydrothermal conditions and the addition of surfactants. It is proposed in the paper that the difference in crystallinity and the lattice mismatch in Cu leads to the difference in specific surface area with microstrain generation, which is beneficial for the effective exposure of active sites. The nanorod-like electrode microstructure is not only pore-rich to facilitate the reactive adsorption-desorption process, but also provides coordinated sites for anchoring Cu at the interface. The Cu/CeO2 exhibited the highest current density values in 0.1 mol/L KHCO3 solution with 49.3% FE (Faraday efficiency) of CH4 at an applied bias of 1.6 V (Fig. 9e). The electrode of Cu/CeO2-R possessed the highest CH4 turnover frequency. Similarly, the double layer capacitance values obtained for the CV curves at different scan rates highlight the significant increase in the electrochemically active surface area of Cu/CeO2-R, which also provides more CO2 adsorption/activation sites. The electrode of Cu/CeO2-R has a Tafel slope (Fig. 9f) value of 335 mV/dec, which is beneficial to stimulate the catalytic activity and promote the charge transfer to the adsorption/activation sites of CO2 molecules, accelerating the formation of *CO2 intermediates. Han et al. [115] CeO2 modified CuO to obtain CeO2/CuO composite electrode, which realized the high selectivity effect of C2+ products. The results from XANES (X-ray absorption near edge structure spectra) (Figs. 9g and h) revealed the low valence state Cu as the active phase of the composite electrode, and the interaction between Cu and Ce stabilized the presence of low valence state Cu. The composite electrode enhanced the conversion of CO intermediates in the generation step of intermediate CO, providing more CO precursors. Theoretical calculations revealed that the step of C—C coupling depends on the generation of *CHO. The enhanced water splitting effect with interfacial effects reduced the thermodynamic and kinetic barriers. The current density of the composite electrode was increased to 1.21 mA/cm2 in alkaline medium at 1.12 V applied bias and the FE of C2+ product reached 75.2% (Fig. 9i). It is very impressive that the partial current density of C2+ products over CC20 could reach as high as 0.91 A/cm2 at −1.12 VRHE (Fig. 9j).
As shown in Figs. 10a–c, Yang et al. [116] exploited the properties of bismuth oxide in electrocatalytic CO2 reduction for the generation of formate by complexing CeO2 with Bi2O3 to enhance the catalytic performance and long-term stability of formate generation by CO2 reduction. The microscopic morphology of the electrode showed a loose porous shape with abundant heterogeneous interfaces leading to an increase in the number of catalytically active sites. The LSV results show that the current density of Bi2O3-CeOx is better than that of Bi2O3 (Fig. 10d). The lattice oxygen in CeOx bound to C atoms multiplied Lewis basic sites and Lewis acidic sites bound to Ce3+ ions and O atoms facilitated the adsorption/activation of CO2. At a lower starting potential (−0.3 V), the composite electrode bias current density reached 4.8 mA, achieving 85.6% FE formate selectivity (Figs. 10e and f).
As shown in Fig. 10g, the effect of different atomic dopants on the CO2RR properties of Cu2O was investigated by means of DFT calculations by Han et al. [117] The thermodynamic limiting potential difference between CO2RR and HER was calculated, as well as the effect of different metals doped with Cu2O on the *CO binding energy. On this basis, the Gd/CuOx catalyst was designed after performance screening. The fractional current density of the C2+ product reached 444.3 mA at −0.8 V, and the Faraday efficiency reached 81.4% (Figs. 10h and i). The rare-earth metal Gd, with its unique electronic structure and large ionic radius, not only maintains the stability of Cu+ during the reaction, but also induces tensile strain. The Gd doping not only enhances the activation of the catalyst against CO2, but also lowers the C-C coupling barrier and drives the stabilization of the key intermediate O*CCO. The Gd doped electrodes have a long term stability of more than 40 h.
Wang et al. [118] reported the use of oxygen vacancy defect engineering to control the concentration of oxygen vacancy defects by solvothermal doping of Ce3+ ions into ZnO and adjusting the doping concentration of Ce3+ ions. The CO2RR performance of electrodes with different doping concentrations was tested in an H-type electrolytic cell. The linear voltammetric curves showed that the current density values of Ce0.016Zn0.984O electrode were higher than those of other doping gradients within the potential window of 0.4–1.35 V. The current density values of Ce0.016Zn0.984O electrode were higher than those of other doping gradients. And after constant potential electrolysis, the CO Faraday efficiency of Ce0.016Zn0.984O electrode was 88%, which was 55.0%, 70% and 75% higher than that of ZnO, Ce0.008Zn0.9992O and Ce0.032Zn0.968O, respectively. The Mott–Schottky and Nyquist plots reflected the charge transfer during the reaction process characterization, the test demonstrated that the carrier density of Ce0.016Zn0.984O is 15.4 times higher than that of ZnO, and the charge transfer resistance is also shown to be smaller than that of the pristine electrode. It is shown that the oxygen vacancies induced by Ce doping into ZnO can enhance the efficient conversion of CO2 by promoting the charge transfer in the reaction.
Although the traditional Haber-Bosch method of ammonia synthesis dominates the industry, the method relies on high-pressure and high-temperature conditions and consumes a large amount of fossil energy, which not only brings high economic costs but also raises serious environmental issues [119-121]. As the global pursuit of sustainable development and clean energy becomes more and more urgent, exploring methods to achieve efficient ammonia synthesis under mild conditions has become the focus of scientific research. There are many limitations in electrocatalytic NRR [122, 123]. Firstly, the N2 molecule has an extremely high chemical stability with an N≡N triple bond energy as high as 941 kJ/mol, which makes the activation process of N2 exceptionally difficult [124]. The weak adsorption ability of many metal oxides on N2 cannot effectively break the N≡N bond, resulting in low reactivity. Secondly, during the electrocatalytic reaction, the metal oxide surface is prone to HER, a competitive reaction. Due to the high concentration of H+ in water and a small reduction potential, HER tends to occur more easily than NRR, which greatly reduces the selectivity of NRR and the ammonia yield. In addition, metal oxides usually have poor electrical conductivity, which limits the electron transfer rate during the catalytic reaction and thus affects the overall catalytic efficiency. Rare earth elements, due to their special electronic structure and rich physicochemical properties, provide new ideas to solve the problems faced by metal oxides in NRR. Rare earth elements interact with metal oxides and change the electronic energy band structure of metal oxides. By doping rare earth elements, it is possible to regulate the electron cloud density of metal atoms, optimize the electron transport path inside the material, and improve the conductivity of the material. This optimization of the electronic structure helps to enhance the adsorption capacity of metal oxides on N2 molecules, reduce the energy barrier of N2 activation, and promote the NRR reaction. The electrons of rare earth elements can be involved in the interaction with N2 molecules to form a unique electronic coupling, thus activating N2 molecules.
Rare earth elements can modulate the chemical properties of the metal oxide surface and reduce the adsorption of H+ on the active sites, thus effectively inhibiting the occurrence of HER side reactions. By changing the hydrophilic nature of the surface or modulating the local chemical environment of the active site, rare earth elements can improve the selectivity of the NRR, allowing more electrons and protons to participate in the reduction process of N2, and increasing the efficiency of the generation of the target product. As shown in Figs. 11a–f, Yu et al. [125] constructed a cathode for electrocatalytic NRR by hybridized composite of amorphous Bi4V2O11 and CeO2.The Bi4V2O11/CeO2 electrode showed a hollow nanofiber morphology with a diameter of 60 nm, and CeO2 nanocrystals with a size of about 3 nm were observed to be uniformly dispersed from its inner part. The amorphous Bi4V2O11 has defective state of V4+ ions and CeO2 has oxygen vacancy defects. In addition, the amorphous Bi4V2O11 is enriched with dangling bonds, which increases the active sites of the NRR process and reduces the energy barrier. The type Ⅰ band alignment formed by the Bi4V2O11/CeO2 electrodes promotes the interfacial charge transfer. The excellent electrocatalytic NRR performance of the Bi4V2O11/CeO2 electrodes demonstrates an average NH3 yield of 23.21 mg and a Faraday efficiency of 10.16%, significantly higher than that of the pristine electrode. As shown in Figs. 11g–i, La2O3 nanoplate electrocatalysts for electrochemical conversion of N2 to NH3 were reported by Xiong et al. [126]. The selectivity and long-term stability of the catalysts showed good performance. It possessed a Faraday efficiency of 4.76% and an NH3 yield of 17.04 mg at 0.8 V. The catalysts were characterized by a high efficiency and a low yield of NH3. Relevant theoretical calculations demonstrated the La atom as the active site for adsorption of N2, and N2 exhibited the best adsorbed endo-di structure on the (011) crystalline surface, showing some reducing properties.
The N reduction properties of Y2O3 nanosheets were demonstrated by Li et al. [127] A simple hydrothermal method and synthesized nanosheets of Y2O3. 0.1 mg of Y2O3 was loaded on carbon paper for NRR performance testing, and the yield of NH3 was determined spectrophotometrically. The highest yield of NH3 and reached 1.06 × 10−10 mol s−1 cm−2 at −0.9 V, with a FE of 2.53%, which was much higher than that of the Ru/C electrode. In addition, there was no by-product N2H4 production after constant potential electrolysis, indicating the excellent selectivity of Y2O3.The NH3 yield after multiple cycles of Y2O3 showed less variation from FEs, indicating the excellent electrochemical durability of the electrode in NRR. Sun et al. [128] reported the performance optimization effect of CeO2 nanorods with oxygen-rich vacancies for NRR. The current density values of the r-CeO2/CP electrode climbed significantly in the N2 gas-saturated electrolyte, indicating the generation of electrocatalytic NRR reaction. Within the potential window of −0.3 V to −0.7 V, the NH3 yield and FEs increased with the change of negative potential, and the FEs reached the highest 3.74% at −0.4 V with a yield of 16.4 µg h−1 mgcat−1. Compared with Au nanorods and γ-Fe2O3 electrodes its NRR performance performed Superiority. This is due to the vacancy clusters resulting in Ce3+ and Ce4+ as active sites, which promote charge transfer and N2 molecular activation. Zhang et al. [129] reported a method for NRR in acidic media Au@CeO2 Electrocatalyst. A core-shell structure was prepared by spontaneous oxidation–reduction method at room temperature Au@CeO2 Electrocatalyst. ICP-OES determination Au@CeO2 The loading amount of Au in the electrocatalyst is 3.6 wt%, and the nanoparticle size is less than 10 nm. TEM and BET test results show that Au@CeO2 The electrocatalyst has a surface area of 40.7 m2/g and a porous structure, which is beneficial for electrocatalytic NRR. Under the condition of −0.4 V and acidic medium, Au@CeO2 The NH3 yield of the electrocatalyst is 28.2 µg h− 1 cm−2 The FE is 9.5%. The excellent NRR activity originates from the oxygen rich vacancies in the CeO2 shell, optimizing the adsorption and activation process of N2, and synergistically promoting N2 conversion with Au. Table 2 summarizes the results of rare earth metal element modified electrode material systems applied to HER, CO2RR and NRR.
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| Material | Reaction types and applications | Performance | Ref. |
| np-CeO2/TM | Hydrogen evolution reaction | 91 mV, 10 mA/cm2 | [101] |
| (LaCeSmYErGdYb)O | Hydrogen evolution reaction | 77 mV, 100 mA/cm2 | [102] |
| v-TMOs(TMOs: Co3O4/NiO)/CeO2 | Hydrogen evolution reaction | 99 mV, 10 mA/cm2 | [104] |
| CeO2-RuO2 | Hydrogen evolution reaction | 180 mV, 250 mA/cm2 | [106] |
| CeO2/Bi3NbO7 | CO2 reduction reaction | Formic acid, selectivity 84.73% | [113] |
| Cu/CeO2-R(nanorod) | CO2 reduction reaction | CH4, 49.3% FEs | [114] |
| CeO2/CuO | CO2 reduction reaction | C2+ product, FEs 75.2% | [115] |
| CeO2/Bi2O3 | CO2 reduction reaction | 85.6% FEs formate selectivity | [116] |
| Bi4V2O11/CeO2 | Nitrogen reduction reaction | NH3, FE 10.16% | [125] |
| Y2O3 nanosheets | Nitrogen reduction reaction | NH3, FE 4.76% | [126] |
| Au@CeO2 | Nitrogen reduction reaction | NH3, FE 9.5% | [129] |
In recent years, with the gradual emergence of the digital age, artificial intelligence, in particular machine learning (ML), has emerged as a potent engine for scientific exploration [130-134]. The advent of ML has transformed the conventional research paradigm of scientific inquiry, offering a novel avenue for addressing intricate scientific challenges. It liberates scientists from the necessity of repetitive and onerous experimentation, transforming existing experimental data into a computer-readable format from which patterns may be discerned. Based on specific problems, we have established the search starting point and search direction, thereby establishing the feature extraction strategy of sequence search, and ultimately selected the corresponding features. Once trained on selected features, ML is able to identify patterns in the data and generate models that can be applied to a variety of tasks such as prediction, classification and clustering. This enables the prediction of future events, the making of decisions, and the performance of tasks. For example, Zhou et al. investigated the activation process of methane on a range of metal oxides, utilizing the low-temperature catalytic activation of methane inert C—H bonds as a case study. They proposed a four-dimensional structural descriptor [135]. Subsequently, They built a machine learning model using gradient boosting decision trees. This model is used to express the structural features of the catalytic activity centre and to quantitatively predict the activation energy barrier of the C—H bond. Compared with the conventional BEP scalar relationship, the prediction accuracy of this model is remarkably improved. Furthermore, it can clearly elucidate topology-dependent structure-reaction relationships, as evidenced by its illustration of the structural basis for the exceptional methane activation capacity of rutile IrO2 (110). Concurrently, the team established an automated high-throughput screening process for catalysts based on the machine learning model and materials database. This model is based on support vector machine and is suitable for nonlinear problems with numerous features. This process quickly identified 178 promising catalysts with favourable structural features from 9095 binary and ternary metal oxides. Eventually, 13 catalysts were identified with an activation energy barrier of the C—H bond of less than 0.5 eV, which now await validation in subsequent experiments. The methodology described in this work provides a reference point for the high-throughput screening of high-quality catalysts for other chemical reactions. The advancement of high performance rare earth metal catalysts is currently constrained by a multitude of factors. On the one hand, there is a wide variety of rare earth elements, on the other hand, there are many preparation methods, the ratio of different rare earth elements is complicated, and it is also closely related to the nature of the supports. Moreover, machine learning is able to identify the rare earth metal catalyst formulations and preparation conditions with optimal catalytic performance among numerous combinations, based on a substantial corpus of existing experimental data and theoretical calculations. This significantly boosts the efficiency of research and development processes. For example, by learning from existing rare earth catalytic reaction data, the catalytic activity of different element combinations or doping ratios can be predicted, providing guidance for experimental design. As shown in Fig. 12, Du et al. have successfully constructed a high-throughput synthesis workstation for the synthesis of high-nuclear rare earth-transition metal clusters in a cage-like framework structure [136]. This structure demonstrates a complex and challenging relationship between the synthesis parameters and the structure. Subsequently, the researchers employed the Density-based spatial clustering of applications with noise method to realise the high-throughput automated synthesis of the clusters. Using the high-throughput synthesis workstation, the researchers successfully synthesised two rare earth-transition metal clusters: La74Ni104 with a 5 × 5 × 3 topology and La84Ni132 with a 5 × 5 × 5 topology. These clusters were synthesised in a highly nucleated rare earth transition metal system with a framework structure. Among the synthesised clusters, La84Ni132, consisting of 216 metal ions, represents the rare earth-transition metal cluster with the highest number of nuclei. To elucidate the assembly laws, the spatial phase diagrams of the two clusters were studied based on the high-throughput experimental capability of the synthesis workstation and machine learning. This method revealed the formation and size regulation laws of the two clusters from a data science perspective. This work not only expands the n × m × l rare earth-transition metal cluster family, but also establishes a novel research paradigm for the synthesis of complex clusters by integrating high-throughput synthesis and machine learning. Mikolajczyk et al. reported the successful synthesis of a series of rare-earth metal atom-doped TiO2 (RE-TiO2) using a hydrothermal method, encompassing the elements Tm, Er, Nd, Dy, Lu, La, Ho, Pr, Tb, Sc, Ce, Yb, Sm, Gd, Y and Eu [137]. They employed a combination of experimental characterisation and machine learning algorithms to perform quantitative structure-activity relationship analyses.
Computational studies indicate that the photocatalytic activity of RE-TiO2 under visible light irradiation is attributable to the doping of rare-earth metal atoms on the TiO2 surface, which results in disorder within the TiO2 lattice. Furthermore, the doping of rare-earth metal atoms results in the formation of a novel sub-bandgap state at the rare-earth 4f energy level within the TiO2 bandgap, which may prove to be a pivotal factor in the photocatalytic activity. Sun et al. employed a combination of theoretical calculations and a series of machine learning classification and clustering models to screen all transition and rare-earth metals in diatomic catalysts loaded on graphdiyne [138]. This work presents a novel approach to the rapid and efficient screening of suitable diatomic pairs using theoretical methods, thereby providing a valuable reference for the design of atomic catalysts. In this study, Gaussian process regression algorithms were used to predict the formation energies and electronic structures of 990 diatomic catalysts (Fig. 13). It is shown that potential f-d orbital coupling between rare earths and transition metals reduces orbital repulsion and increases stability during the formation of diatomic molecules. Furthermore, it is difficult to accurately predict the electronic structure based solely on the physicochemical parameters of the material structure. This indicates that the interactions between metal atomic orbitals play a pivotal role in determining the electronic structure, an issue that requires further investigation. This study demonstrates the potential for flexible design and effective prediction of diatomic catalysts through the integration of theoretical calculation and machine learning. The approach provides valuable insights and a theoretical foundation for the development of future diatomic catalysts. In conclusion, ML has the potential to revolutionize the field of catalysis. By using machine learning algorithms related to classification and clustering algorithms, researchers can extract valuable insights from large amounts of data. This can greatly assist in the design of catalysts, the prediction of performance and the exploration of reaction mechanisms. The application of machine learning has great potential to improve the activity, selectivity and stability of catalysts. Despite the promising applications of machine learning in catalysis, it is evident that the field also presents a number of significant challenges. It appears that those working in the field of catalysis may not possess sufficient familiarity with the intricacies of machine learning algorithms. It is possible that researchers may encounter difficulties in accurately understanding the principles and application methods of machine learning algorithms in their actual research work, especially when the samples involve higher dimensional calculations and more complex topological structures. This may result in challenges in preparing datasets that are suitable for machine learning analysis, even when the research is highly relevant. Furthermore, machine learning is a rapidly evolving interdisciplinary field, variants based on several of the most classic regression classification clustering models are constantly emerging, even on a daily basis. It requires significant computational resources, making it a challenging area for chemists to enter. From the perspective of researchers, the continuous updating of machine learning technologies necessitates a commitment to learning and mastering new algorithms and techniques, which requires a significant investment of time and effort.
Machine learning algorithms typically require significant computing resources for data processing and model training. This can be a challenge for research teams with limited financial resources. They may not be able to afford the high cost of computing equipment and software. In addition, they may find it difficult to obtain the services of professional computer technicians for data processing and model optimisation. Despite these difficulties, there is reason to believe that these issues will gradually be resolved as machine learning technology continues to advance and catalysis researchers deepen their understanding of it. In the future, ML is expected to play a more significant role in catalysis, contributing to the sustainable development of the chemical industry.
Rare earth metal element modified metal oxides have shown significant application potential in multiple important fields such as wastewater treatment, and hydrogen peroxide synthesis, HER, CO2RR, NRR. In terms of HER, the electronic structure of the catalyst has been effectively optimized through modification, reducing hydrogen adsorption energy and improving the activity and efficiency of hydrogen evolution reaction, providing a more competitive catalytic pathway for renewable energy hydrogen production. For CO2RR, the use of rare earth elements enhances the adsorption and activation ability of CO2, regulates the adsorption configuration of reaction intermediates, improves product selectivity, and greatly increases the possibility of converting greenhouse gas CO2 into valuable chemicals. In NRR, the electronic conductivity has been improved, competitive reactions such as hydrogen evolution have been suppressed, and the adsorption efficiency of N2 molecules and the generation efficiency of ammonia have been enhanced, opening up new ideas for the synthesis of ammonia under mild conditions. In the field of wastewater treatment, as an efficient electrode material, it can generate highly oxidizing active substances, accelerate the oxidation degradation of pollutants, and effectively improve the effectiveness of wastewater treatment. In the synthesis of hydrogen peroxide, the electronic structure and active sites on the catalyst surface were adjusted, promoting the two electron reduction reaction to generate hydrogen peroxide and improving the synthesis efficiency.
The electrocatalytic mechanism of rare earth metal element modified metal oxides in the above-mentioned applications is common. The unique electronic structure of rare earth elements, such as the unfilled 4f electron layer, enables them to affect catalyst performance in various ways. In terms of electronic structure regulation, the band structure of metal oxides can be changed to adjust the electron cloud density of active sites, thereby optimizing the adsorption energy of reactants on the catalyst surface and promoting electron transfer processes. This plays a key role in reactions such as HER, CO2RR, NRR. At the same time, rare earth elements have a special affinity for reactants and reaction intermediates, which can optimize their adsorption configuration on the catalyst surface, guide the reaction towards the desired direction, and improve the selectivity of specific reactions, such as in CO2RR and NRR. The selection of metal oxide substrates cannot be ignored. Different substrates have different physical and chemical properties, and their synergistic effects with rare earth elements are different. Suitable substrates can better exert their modification effects. The preparation method and process conditions determine the dispersion and chemical bonding mode of rare earth elements in metal oxides, which in turn affect the structural integrity and electrocatalytic performance of the catalyst. In addition, environmental factors such as electrolyte type, concentration, temperature, and applied potential can also affect the electrocatalytic performance, alter ion migration rate and the double-layer structure on the electrode surface, and affect electron transfer and reaction rate.
(1) In terms of mechanism analysis, strengthen the close integration of theoretical calculations and experiments, and verify and provide feedback on theoretical predictions through advanced experimental techniques. Real time monitoring of the structure and chemical state changes of catalysts during electrocatalytic reactions using in-situ characterization techniques (such as in-situ infrared spectroscopy, in-situ X-ray absorption spectroscopy), which are mutually confirmed with theoretical calculation results, further correcting and improving the theoretical model. Meanwhile, the new phenomena and problems discovered in the experiment will also drive the continuous development of theoretical calculation methods.
(2) Future research is expected to further enhance the electrocatalytic performance of rare earth metal element modified metal oxides in various application fields. By precisely controlling the doping type, content, and distribution of rare earth elements, combined with advanced preparation techniques, more refined optimization of the electronic structure and surface properties of catalysts can be achieved, thereby improving reaction activity, selectivity, and stability. In terms of expanding new applications, with the continuous increase in attention to energy and environmental issues, its application in more new energy conversion and environmental remediation fields can be explored, such as new energy storage systems, degradation of other complex pollutants, fully leveraging its unique catalytic advantages.
(3) Interdisciplinary approaches such as machine learning will play an increasingly important role in the research of rare earth metal element modified metal oxides. By utilizing the powerful data analysis and prediction capabilities of machine learning, the understanding of complex electrocatalytic systems can be accelerated, and potentially high-performance catalyst formulations and preparation conditions can be quickly screened. Conducting more in-depth collaborative research and optimizing the entire chain from material design, reaction engineering to practical applications is expected to achieve a leap from laboratory results to industrial applications.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Yicheng Li: Writing – original draft, Conceptualization. Qian Liu: Writing – review & editing, Writing – original draft, Data curation. Tianhao Li: Writing – review & editing. Hao Bi: Software. Zhurui Shen: Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization.
This work was supported by the National Key Research and Development Program of China (No. 2023YFC3708005). The Fundamental Research Funds for the Central Universities, Nankai University (No. 63241208). This work was supported by the National Natural Science Foundation of China (Nos. 21872102 and 22172080).
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Figure 1 (a, b) TEM images of Fe0-Fe3O4/CeO2/C. (c) HRTEM patterns of Fe0-Fe3O4/CeO2/C. (d) Degradation of ceftriaxone sodium of Fe0-Fe3O4/CeO2/C and (e) electrocatalytic reaction mechanism. Reproduced with permission [82]. Copyright 2022, Elsevier.
Figure 3 (a) The schematic diagram for the synthesis of CDs/CeO2. (b) TEM image of CDs/CeO2 at low magnification. (c) TEM image of CDs/CeO2 at high magnification. (d) HAADF-STEM image, and (e-g) Element mapping images of CDs/CeO2. Reproduced with permission [94]. Copyright 2024, Elsevier.
Figure 4 Scanning electron microscope (SEM) for (a) np-CeO2 and TEM for np-CeO2. (b) Polarization curves for MnO2-CeO2/TM, np-CeO2/TM, and RuO2 on TM and bare TM with a scan rate of 5 mV/s. (c) Tafel plots of MnO2-CeO2, np-CeO2 and RuO2. (d) Polarization curves of MnO2-CeO2/TM, np-CeO2/TM and Pt/C with a scan rate of 100 mV/s. (e) Tafel plots of MnO2-CeO2, np-CeO2 and RuO2. (f) CVs of np-CeO2/TM at the different scan rates. The corresponding capacitive currents at 0.08 V vs. Ag/AgCl as a function of scan rates for (g) MnO2-CeO2/TM and np-CeO2/TM. Nyquist plots for (h) EIS (Electrochemical impedance spectroscopy) of MnO2-CeO2/TM and np-CeO2/TM in 1.0 mol/L KOH. Reproduced with permission [101]. Copyright 2020, RSC.
Figure 5 (a–c) High-resolution TEM images of the (LaCeSmYErGdYb)O and Pt-(LaCeSmYErGdYb)O associated with corresponding intensity profiles of the selected areas. (d) HAADF-STEM (high-angle annular dark-field, scanning transmission election microscope) and EDS (Energy dispersive spectrometer) mapping of Pt-(LaCeSmYErGdYb)O. (e) Electrochemical HER activity of catalysts in 1.0 mol/L KOH electrolyte: LSV curves of Pt-HEREOs. (d) the overpotentials at the current densities of 10 mA/cm2. (f) Tafel and (g) Cdl (double-layer capacitance values) values of Pt/C, Pt-CeO2, and Pt-(LaCeSmYErGdYb)O. (h) Nyquist plots. (i) Comparison of the overpotential at the current density of 100 mA/cm2 for this work with reported electrocatalysts. (j) Comparisons of the mass activity and overpotentials of the representative reported HER catalysts. Reproduced with permission [102]. Copyright 2024, ACS.
Figure 6 (a–d) HAADF-STEM image, and corresponding elemental Mapping. (e) Raman spectra of CeO2, CeO2/CuO composites, and CuO. (f) HER polarization curves, (g) Tafel slopes for HER. (h) Cu 2p for the 1:1 CeO2/CuO composite and pure CuO, and (i) Ce 3d for the 1:1 CeO2/CuO composite and pure CeO2. Reproduced with permission [103]. Copyright 2023, ACS. (j) Schematic illustration of the synthetic process for 2D v-TMOs/CeO2 HSs. HER electrocatalytic performance of the as-prepared samples in 1 mol/L KOH: (k) LSV curves of Co3O4/CeO2 HSs, NiO/CeO2 HSs, Pt/C, Co3O4 NFs, NiO NFs and CeO2 NFs, (l) the comparison of HER overpotentials at 10 mA/cm2 for different catalysts, (m) corresponding Tafel slopes derived from the LSV curves, and (n) galvanostatic measurement for Co3O4/CeO2 HSs, NiO/CeO2 HSs, Pt/C, Co3O4 NFs and NiO NFs at the current density of 10 mA/cm2 over 50 h. Reproduced with permission [104]. Copyright 2021, Elsevier.
Figure 7 (a) Synthetic strategy of the SCCN nanostructure. (b) LSV curves with iR-compensation, (c) Cdl (double-layer capacitance values), (d) Tafel slopes and (e) Overpotentials of SCCN (Se-Co3O4@CeO2/NF) -r (r = 0.25, 0.5, 1, 2) for HER progress. Reproduced with permission [105]. Copyright 2021, Elsevier.
Figure 8 Structural characterizations of CeO2/Bi3NbO7. (a) X-ray diffraction (XRD) pattern, (b) SEM image, (c) TEM image, (d) magnified TEM image, (e) SAED (selected area electron diffraction) pattern and (f) HRTEM image and (g) enlarged view of interface and (h-k) corresponding elemental mapping images of CeO2/Bi3NbO7. CO2RR performance characterizations. (l) LSV curves of the CeO2, Bi3NbO7, CeO2/Bi3NbO7 with different ratios and blank carbon paper. (m) FE of formic acid, CO2, and H2 at selected voltages for CeO2/Bi3NbO7. (n) Current density of Bi3NbO7 and CeO2/Bi3NbO7 corresponding to HCOO−. Reproduced with permission [113]. Copyright 2022, RSC.
Figure 9 TEM and HRTEM images of the Cu/CeO2 catalysts with different morphologies: (a) nanorods, (b) nanocubes, (c) nanoparticles, and (d) nanospheres. (Inset: perspective view of (110), (100), (111) and (311) facets of cubic-fluorite CeO2). (e) Faradic efficiencies for C2H4, CH4, CO and H2 on the Cu/CeO2-R catalysts and current density. (f) Tafel plots of the catalysts toward CH4 production. Reproduced with permission [114]. Copyright 2022, Elsevier. (g, h) Operando XANES and the corresponding Fourier transforms of k 3-weighted EXAFS (Extended X-ray absorption fine structure) data at the Cu K-edge at various applied potentials (vs. RHE) over CC20 during CO2RR. (i) The average FE of C2+ products in 1 mol/L KOH over CC0 and CC20, respectively. (j) The partial current density of C2+ products at various potentials in 1 mol/L KOH solution over CC0 and CC20. Reproduced with permission [115]. Copyright 2021, RSC.
Figure 10 Morphology and structure characterizations on Bi2O3-CeOx. (a) XRD patterns of Bi2O3 and Bi2O3-CeOx. (b) SEM image of Bi2O3-CeOx and (c) the corresponding EDS (energy dispersive spectrometer) mapping of Bi, Ce, O, and C. CO2RR Performances of Bi2O3-CeOx. (d) LSV curves of Bi2O3-CeOx and Bi2O3 at a scan rate of 10 mV/s. Product distributions in terms of faradaic efficiency with (e) Bi2O3-CeOx and (f) Bi2O3 in 1 mol/L KOH electrolyte at a CO2 flow rate of 20 sccm (standard cubic centimeter per minute). Reproduced with permission [116]. Copyright 2022, ACS. (g) Schematic diagram of CO2RR catalyst Gd1/CuOx. Electrocatalytic CO2RR performance. (h) FEs for C2+ products over Gd1/CuOx with different Gd contents and CuOx under different potentials. (i) Product distribution over 6.5% Gd1/CuOx and CuOx under different potentials. Reproduced with permission [117]. Copyright 2023, ACS.
Figure 11 Structural and morphologic characterization of BVC-A: (a) SEM image, (b) TEM image, (c) HAADF image. Electrocatalytic NRR of BVC-A at ambient conditions. (d) Yield of NH3 (blue-green) and Faradaic efficiency (red) at each given potential, (e) cycling test of BVC-A, (f) yield of NH3 with different catalysts at −0.2 V vs. RHE. Reproduced with permission [125]. Copyright 2018, Wiley. (g) SEM image for La2O3, and EDX (energy dispersive X-ray spectroscopy) elemental mapping images of La and O elements for La2O3. (h) NH3 yields and FEs for La2O3/CP at a series of potentials in 0.1 mol/L Na2SO4. (i) Amount of NH3 generated with different electrodes at −0.8 V after 2 h electrolysis under ambient conditions. Reproduced with permission [126]. Copyright 2019, Elsevier.
Figure 12 (a–d) Crystal structure and synthetic features of the reported topologies in the n × m × l family. (e) Structural diagram of known and predictable members according to structural features in the n × m × l family. (f) Advantages and challenges faced in the development of the n × m × l family. (g) The combination of a high-throughput synthetic workstation and machine learning was used to accelerate the development of the n × m × l family. Reproduced with permission [136]. Copyright 2023, ACS.
Figure 13 (a–c) The formation energy of 990 combinatorial models of graphdiyne-based diatomic catalysts is demonstrated herein. Reproduced with permission [138]. Copyright 2021, Wiley.
Table 1. Rare earth metal element modified electrodes applied to wastewater treatment and hydrogen peroxide synthesis.
| Material | Reaction types and applications | Performance | Ref. |
| Fe0-Fe3O4/CeO2/C | Degradation of pollutants | 20 ppm antibiotic ceftriaxone sodium, 95.59%/120 min | [82] |
| Co3O4-La2O3/PMS system | Degradation of pollutants | degrade levofloxacin (LVX), 89.6% | [83] |
| Co3O4-La2O3/Ov | Degradation of pollutants | TC, 97.50% | [84] |
| CDs/CeO2 | Synthesis of H2O2 | Selectivity 92%, FEs 97% | [94] |
| CeO2 NCs | Synthesis of H2O2 | Selectivity 95.3%, FEs > 85% | [95] |
| Gd2O3 | Synthesis of H2O2 | Selectivity 80% | [96] |
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Table 2. Rare earth metal element modified electrodes applied to HER, CO2RR and NRR.
| Material | Reaction types and applications | Performance | Ref. |
| np-CeO2/TM | Hydrogen evolution reaction | 91 mV, 10 mA/cm2 | [101] |
| (LaCeSmYErGdYb)O | Hydrogen evolution reaction | 77 mV, 100 mA/cm2 | [102] |
| v-TMOs(TMOs: Co3O4/NiO)/CeO2 | Hydrogen evolution reaction | 99 mV, 10 mA/cm2 | [104] |
| CeO2-RuO2 | Hydrogen evolution reaction | 180 mV, 250 mA/cm2 | [106] |
| CeO2/Bi3NbO7 | CO2 reduction reaction | Formic acid, selectivity 84.73% | [113] |
| Cu/CeO2-R(nanorod) | CO2 reduction reaction | CH4, 49.3% FEs | [114] |
| CeO2/CuO | CO2 reduction reaction | C2+ product, FEs 75.2% | [115] |
| CeO2/Bi2O3 | CO2 reduction reaction | 85.6% FEs formate selectivity | [116] |
| Bi4V2O11/CeO2 | Nitrogen reduction reaction | NH3, FE 10.16% | [125] |
| Y2O3 nanosheets | Nitrogen reduction reaction | NH3, FE 4.76% | [126] |
| Au@CeO2 | Nitrogen reduction reaction | NH3, FE 9.5% | [129] |
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