Amorphous High‐Entropy Oxides With High‐Valent Metal and Oxygen‐Vacancy Pairs for Thermally Stable Catalytic Oxidation

ABSTRACT

High‐entropy oxides (HEOs) show great promise in heterogeneous catalysis due to their unique structural properties. However, stabilizing the amorphous structure of HEOs under high‐temperature conditions remains challenging. Herein, we propose a thermodynamic synergy‐driven strategy to construct the long‐range disordered structure in HEO, achieving the preservation of defect‐rich sites with high thermal stability. By integrating multiple metal elements with substantial atomic size differences, we construct an amorphous MnFeCoNiCuYZrO _x HEO (MYZrO _x ‐a), in which the high configurational entropy creates a thermodynamic barrier against amorphous‐to‐crystalline transition. This strategy also demonstrates both universality and scalability for synthesizing thermally stable amorphous HEOs. Combined experimental characterization and theoretical calculations reveal that MYZrO _x ‐a retains short‑range disorder and abundant defect sites even after calcination at 600°C. Moreover, the stabilized high‑valence metal–oxygen vacancy (M^δ+–O_v) pairs facilitate the activation of C─H bonds and oxygen species, endowing MYZrO _x ‐a with exceptional methane combustion activity and durability, with stable performance exceeding 200 h even under high‑humidity conditions. This work underscores the pivotal role of configurational entropy in designing amorphous HEOs and expands their potential for advanced thermocatalytic applications.

Amorphous high‐entropy oxides are synthesized through a rapid homogeneous‐mixing strategy, enabling unprecedented structural disorder and abundant defect sites, which lead to high thermal stability and enhanced catalytic activity in methane combustion reactions, demonstrating configurational entropy as a powerful tool for designing stable amorphous thermocatalysts.

1. Introduction

The recent emergence of high‐entropy oxides (HEOs) has attracted widespread research interest due to their unprecedented geometric and electronic diversity, severe lattice distortion, and remarkable structural stability [1]. The high configurational entropy arising from the equimolar mixing of multiple cations effectively stabilizes solid‐solution phases that are otherwise difficult to obtain in traditional single or binary systems [2]. Owing to these unique features derived from the multielement system, HEOs have emerged as promising catalysts for various oxidation reactions under harsh conditions, such as oxygen evolution [3], catalytic combustion [4], and selective oxidation [5]. However, most existing HEO catalysts have been limited to a few crystalline phases, such as rock‐salt, perovskite, fluorite, and spinel structures [6]. While crystalline HEOs provide systematic tuning of lattice parameters and electronic structure, these ordered structures thermodynamically limit the further regulation of defect sites.

Since amorphous structure offers high concentration of coordinatively unsaturated sites, flexible local bonding, and numerous defects, they are increasingly recognized as beneficial for heterogeneous catalysis [7, 8, 9, 10, 11]. Nevertheless, the metastable nature of amorphous structure presents a crucial challenge in maintaining their short‐range disorder under high‐temperature oxidative environments, where their crystallization process leads to defect loss and compromised catalytic performance [12, 13]. So far, only a few reported transition metal oxides are able to maintain their amorphous phase up to 450°C–500°C, severely limiting their practical applications in thermal catalysis [14, 15]. Recently, integrating the concept of high entropy into amorphous structure represents a promising strategy to combine their advantages of high thermal stability and abundant distortion sites [16, 17]. When the number of metal species exceeds seven, the configurational entropy becomes sufficient to overcome enthalpy‐driven ordering preferences, leading to a disordered atomic arrangement [18]. Besides, the sluggish diffusion effect induced by high configurational entropy can confer remarkable thermal stability to metastable materials under harsh conditions [19].

Inspired by the above insights, we develop a thermodynamic synergy‐driven strategy to synthesize the amorphous high‐entropy MnFeCoNiCuYZrO _x oxide (MYZrO _x ‐a) with defect‐rich structure and robust thermal stability. Structural characterizations and thermodynamic analysis reveal that increased configurational entropy suppresses crystallization and stabilizes the defect‐rich structures. To evaluate the catalytic advantages of MYZrO _x ‐a, methane catalytic combustion is selected as a representative high‐temperature oxidation reaction [20]. The MYZrO _x ‐a catalyst not only achieves 90% methane conversion at 488°C but also sustains high activity over 200 h even under high humidity conditions, demonstrating that the amorphization not only improves activity but also enhances stability. Furthermore, mechanistic studies reveal that the abundant M^δ+–O_v active pairs derived from high‐valence metal sites and oxygen vacancies are crucial for activating methane and oxygen, thereby enhancing the catalytic performance.

2. Results and Discussion

2.1. Synthesis and Structural Characterization of Amorphous HEO

Figure 1a depicts the sequential steps involved in the rapid synthesis of amorphous HEOs via the colloid‐mill redox precipitation approach. Initially, mixed redox and co‐precipitation reactions between MnO₄ ⁻/OH⁻ and low‐valence transition metal ions simultaneously generates heteroatom‐doped MnO₆ polyhedral structures and multielement hydroxides. Subsequently, the colloid mill with strong mechanical shearing force simultaneously minimizes phase separation and lowers the free energy barrier by reducing the size of nucleus clusters. This nucleation process can be explained by LM mechanism, which describes a burst nucleation event when the precursor concentration exceeds the critical supersaturation threshold. The ultra‐fast and uniform mixing through the colloid mill ensures that the entire reaction volume reaches this supersaturation nearly simultaneously, leading to a massive nucleation event. However, because the system is continuously driven far from equilibrium under high shear, the subsequent growth of nuclei is kinetically suppressed [21, 22]. Finally, the amorphous high‐entropy septenary metal oxide (MYZrO _x ‐a, M = MnFeCoNiCu) is obtained even after calcination at 600°C. The aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (AC‐HAADF‐STEM) images with sub‐angstrom resolution (Figure 1b) demonstrate no obvious lattice fringes in MYZrO _x ‐a, supporting the disordered atomic structure, while the selected‐area electron‐diffraction (SAED) analysis (Figure 1c) and the corresponding inverse fast Fourier transform (IFFT) image (Figure 1d) further corroborate its amorphous nature [23]. The energy‐dispersive x‐ray spectroscopy (EDS) mappings, as illustrated in Figure 1e and Figure S1, further prove the atomic‐scale mixing of all elements in MYZrO _x ‐a. Consistently, the x‐ray diffraction (XRD) patterns of MYZrO _x ‐a in Figure 1f exhibit broad and weak diffraction features, indicating the absence of long‐range crystallinity. However, the lower‐entropy oxides (MnO₂, MO _x ) prepared by the same method both show a certain degree of crystalline phases with distinct diffraction peaks (Figure S2). Notably, the amorphous property of MYZrO _x ‐a is retained even under scale‐up synthesis conditions (Figures S3 and S4), yielding a quality of up to 640 g, highlighting the excellent practicality and efficiency of this method for larger‐scale catalyst production and potential industrial applications.

FIGURE 1.

Synthesis and morphological characterization of the amorphous oxide. (a) Schematic illustration of the synthesis process for the amorphous oxide. (b) AC‐HRTEM images, (c) SAED patterns, (d) IFFT images, and (e) EDS mapping of MYZrO _x ‐a. (f) XRD patterns of the synthesized catalysts. (g) Synchrotron x‐ray PDF profiles of MYZrO _x ‐a.

Moreover, two additional septenary oxides with identical elemental compositions but distinct mixing scales are synthesized for comparison. The XRD pattern of crystallized septenary oxide (MYZrO _x ‐c) (Figure S5) exhibits high crystallinity and phase separation, accompanied by a nano‐scale mixing of different elements, as confirmed by AC‐HAADF‐STEM, SAED, and EDS analyses (Figure S6). The mechanically mixed septenary oxide (MYZrO _x ‐m) exhibits overlapping diffraction peaks from various oxides (their individual XRD patterns in Figure S7) and clear submicron‐scale phase separation, evidenced by HAADF‐STEM and EDS (Figure S8). Moreover, the corresponding HEOs prepared without KMnO₄ as an oxidizing agent and stirring using magnets all show a certain degree of crystalline phases with distinct diffraction peaks (Figures S9 and S10). These comparisons collectively demonstrate that the amorphous phase of MYZrO _x ‐a is uniquely enabled by homogeneous mixing with the colloid‐mill redox precipitation method.

Synchrotron‐based pair distribution function (PDF) analysis is conducted to validate the short‐range atomic ordering of amorphous structure (Figure 1g and Figure S11). The PDF profile of MYZrO _x ‐a displays only short‐range atomic correlations and rapidly decays to near zero beyond ∼12 Å, suggesting the successful formation of much smaller sub‐nanocrystalline grains. In contrast, septenary oxides synthesized by other methods exhibit sharp peaks and correlations beyond 20 Å, indicating their long‐range ordered structure. Furthermore, fitting of the PDF data reveals that the local coordination microenvironment of MYZrO _x ‐a can be described by tetragonal ZrO₂ and spinel M₃O₄ phases, together with unique metastable Zr₃O structure, highlighting its highly disordered yet partially correlated atomic arrangement (Figure S12). As anticipated, the Raman spectroscopy of MYZrO _x ‐a displays a broadened half‐peak width and reduced intensity of the M─O vibration peak, further confirming its long‐range structural disorder (Figure S13). In addition, MYZrO _x ‐a exhibits a remarkably larger specific surface area than its crystalline counterparts, which provides more exposed active sites and greater surface accessibility for catalytic reactions (Figures S14 and S15, Table S1). Inductively coupled plasma‐optical emission spectrometry (ICP‐OES) results show that the three septenary oxides all have approximately equimolar ratios of each metal element (Table S2).

2.2. The Origin of Amorphous HEO Formation

In situ XRD experiments are performed at elevated temperatures under air atmosphere (Figure 2a,b and Figure S16). MYZrO _x ‐a maintains an absence of pronounced diffraction peaks up to 650°C, indicating strong resistance to crystallization at high temperature, while the crystallization of MO _x into a spinel phase initiates at around 550°C. Meanwhile, the differential scanning calorimetry (DSC) curves in Figure S17 show that both as‐prepared and 600°C‐calcined MYZrO _x ‐a exhibit a distinct exothermic peak at approximately 690°C corresponded to the heat release of phase transition, further indicating that the amorphous structure can be maintained at temperatures of at least 600°C. The high thermal stability of MYZrO _x ‐a is primarily due to the increased elemental diversity induces sluggish atomic diffusion, thereby elevating the energy barrier for crystallization, which is a characteristic feature of high‐entropy systems [24, 25, 26].

FIGURE 2.

Role of entropy in promoting amorphization. In situ heating XRD patterns of (a) MYZrO _x ‐a and (b) MO _x . (c) Gibbs free energy as a function of temperature for the catalysts. (d) Gibbs free energy as a function of O₂ pressure for the catalysts. (e) Calculated average atomic displacement (Δ𝑟̅) of transition metal and oxygen atoms from their original lattice positions in relaxed structures. (f) δ–ΔH _mix diagram showing that larger δ and lower ΔH _mix favor amorphous phase formation. (g) XRD patterns of synthesized HEOs with 8 and 10 elements. (h) Schematic illustration of the relationship between entropy and enthalpy based on the Gibbs equation.

The thermodynamic potential for forming a stable, homogeneous multi‐element oxide can be evaluated using the Gibbs free energy of mixing, defined as ∆G _mix = ∆H _mix − T∆S _mix [3, 27]. Accordingly, the equiatomic incorporation of seven elements in MYZrO _x ‐a leads to a higher configurational entropy (∆S _mix = 1.95 R), which drives ∆G _mix to more negative values compared to MO _x (Figure 2c) [28]. A smaller value of ∆G _mix suggests a stronger thermodynamic driving force for uniform elemental mixing and phase stability, hence boosting the structural stability of MYZrO _x ‐a under high‐temperature conditions [29, 30]. In addition, the negative correlation between the Gibbs free energy and oxygen partial pressure (P_O2) of MYZrO _x ‐a further indicates the enhanced thermodynamic stability in oxidizing environments (Figure 2d) [31]. To further evaluate the structural stability of HEOs, the average atomic displacements (Δ𝑟̅) from their original lattice positions are calculated (Figure 2e). The results reveal that the septenary oxide exhibits more evident Δ𝑟̅ compared to the quinary oxide, reflecting more significant distortion and higher kinetic barriers for atomic diffusion, which contributes to the origin of the amorphous structure that maintain thermal stability in MYZrO _x ‐a [31].

From a crystallographic perspective, the formation of amorphous phases is controlled by the competition between ∆H _mix and ∆S _mix, where a lower or negative ∆H _mix indicates high strain energy, thereby reducing the thermodynamic driving force for crystallization [16]. Besides, atomic size difference (δ) is another critical descriptor for mismatch entropy influencing the degree of lattice distortion and the development of long‐range order [32, 33]. Among the synthesized quinary, senary, and septenary oxides, MYZrO _x exhibits a relatively lower ∆H _mix and higher δ (Figure 2f), indicating a stronger tendency to form an amorphous structure. To demonstrate the generalizability of this synthesis strategy, eight‐ and ten‐element oxides with favorable ∆H _mix and δ values are prepared by the same method, all maintaining amorphous structures after calcination at 600°C (Figure 2g). On the contrary, the prepared MZnCrO _x with a low δ value exhibits a high degree of crystallinity, indicating that the formation of amorphous structures is not solely the result of configurational entropy (Figure S18). Among them, these selected elements are widely recognized active components or promoters in thermocatalytic reactions, which also satisfy critical descriptors for amorphous structure formation.

Taken together, amorphous phase formation is primarily governed by the mismatch entropy and mixing enthalpy, which facilitate rapid supersaturation and hinder long‑range ordering during synthesis. Meanwhile, thermal stabilization against crystallization is enhanced by high configurational entropy, likely through the sluggish diffusion effect, which suppresses atomic rearrangement at elevated temperatures. Overall, the thermodynamic synergy‐driven multiple parameters including configurational entropy, mismatch entropy and mixing enthalpy minimize ∆H _mix and increase ∆S _mix to lower ∆G _mix, thus achieving the high thermal stability of amorphous structures in MYZrO _x (Figure 2h).

2.3. Disorder and Defect‐Rich Structure of Amorphous HEO

X‐Ray absorption near‐edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS) are employed to probe the local coordination environment and electronic structure of catalysts. The Mn K‐edge XANES spectra of MYZrO _x ‐a shift to higher energy, indicative of an increased Mn oxidation state compared to other synthesized catalysts (Figure 3a and Figure S19a) [34]. The Fourier transform of the k²‐weighted Mn K‐edge EXAFS spectra (Figure 3b and Figure S19b) reveals two distinct peaks at ∼1.5 and ∼2.5 Å, corresponding to Mn─O coordination and Mn─M (Metal) bonds in the Mn‐based oxides, respectively. Notably, MYZrO _x ‐a exhibits a significantly attenuated Mn─M peak, unraveling weaker Mn–Metal interactions than MO _x and MnO₂. Combined with the reduced periodicity of the EXAFS oscillation amplitude beyond the second shell, this observation points to a long‐range disorder structure, consistent with the amorphous characteristics of MYZrO _x ‐a [35]. Furthermore, the results of EXAFS curve‐fitting present that Mn─O coordination number of MYZrO _x ‐a is calculated to be 5.3, higher than those of MO _x and MnO₂ (Figure 3c,d, Figure S19c,d, and Table S3). The increased Mn─O coordination and decreased Mn─M coordination number observed in MYZrO _x ‐a likely contribute to pronounced lattice distortion, resulting in a high Mn valence state and abundant metal defects. These findings are further corroborated by wavelet‐transformed EXAFS spectra, which align well with the R‐space analysis (Figure 3e and Figure S20) [36].

FIGURE 3.

Microstructural characterization of the catalysts. (a) Mn K‐edge XANES and (b) EXAFS spectra of different catalysts and reference samples. FT‐EXAFS fitting curves in R space of Mn K‐edge for (c) MYZrO _x ‐a and (d) MO _x . (e) WT‐EXAFS spectra of the catalysts. XPS spectra of (f) Mn 2p and (g) O 1s of the catalysts. (h) Peak‐normalized PALS, with the inset illustrating the schematic of positron annihilation lifetime parameters.

Subsequently, x‐ray photoelectron spectroscopy (XPS) is employed to characterize the electronic states of the catalysts. Significantly, MYZrO _x ‐a exhibits the highest Mn⁴⁺/Mn³⁺ ratio (1.14) compared to MO _x (0.76) and MnO₂ (0.50) (Figure 3f and Figure S21), indicating the existence of a high oxidation state of Mn, consistent with the EXAFS results. Further analysis of Fe 2p, Co 2p, Ni 2p, and Cu 2p spectra (Figure S22) shows that all metal elements in MYZrO _x ‐a possess higher oxidation states than in MO _x (Table S4), which implies the amorphous structure provides a strong basis for enhanced oxidative capability of MYZrO _x ‐a. Meanwhile, the results of O 1s XPS spectra (Figure 3g and Figure S23) illustrate that MYZrO _x ‐a displays a notably higher O_ads/O_latt ratio of 0.88, surpassing the values of 0.62 and 0.47 for MO _x and MnO₂, respectively, proposing a greater concentration of surface oxygen vacancies [37].

To gain deeper insight into the defect characteristics of the catalysts, positron annihilation lifetime spectroscopy (PALS) is employed. Figure 3h elucidates two positron lifetime components (τ ₁ and τ ₂), in which the shorter lifetime, τ ₁, is associated with positron annihilation at small metal vacancies, while the longer component, τ ₂, corresponds to annihilation at vacancy clusters and grain boundaries [38]. According to the positron lifetime parameters (Subfigure in Figure 3h) and the normalized intensity percentage (Table S5), the increased values of both τ ₁ and τ ₂ in MYZrO _x ‐a underscore that the incorporation of more elements leads to a higher concentration of defect structures. Collectively, the high‐entropy amorphous structure of MYZrO _x ‐a promotes the rare coexistence of high‐valent metal species and abundant surface oxygen vacancies, which together generate M^δ+–O_v active pairs that can significantly enhance the activation of non‐polar molecules such as CH₄ and O₂ during catalytic oxidation [39].

2.4. Catalytic Performance in Methane Combustion

Methane catalytic combustion is employed to assess the catalytic performance and thermal stability of the obtained catalysts. The catalytic activity for methane combustion increases notably with the number of constituent elements, with MYZrO _x ‐a displaying the highest performance, achieving 90% methane conversion at 488°C (Figure 4a, Figure S24 and S25). To isolate the influence of individual elements, the catalytic activity of MnMO _x binary oxides (M = Fe, Co, Ni, Cu, and YZr, whose XRD patterns are presented in Figure S26) at 500°C is evaluated and summarized in Figure 4b, confirming that the improved performance arises from the synergistic effect of all incorporated elements. Moreover, the MYZrO _x ‐a catalyst exhibits markedly superior methane conversion efficiency compared to MYZrO _x ‐c and MYZrO _x ‐m (Figure 4c), highlighting the distinct synergistic advantage of atomic‐scale mixing in multi‐element systems. Importantly, the MYZrO _x ‐a catalyst synthesized at large scale also maintains excellent catalytic activity, underlining its strong potential for practical applications, as depicted in Figure S27. Furthermore, comprehensive evaluations of MYZrO _x ‐a against all synthesized samples and previously reported noble metal and transition metal catalysts are compiled in Tables S6 and S7, demonstrating its catalytic performance advantages.

FIGURE 4.

Catalytic performance of CH₄ combustion over different catalysts. (a) Light‐off curves of CH₄ combustion over catalysts with different amounts of elements. (b) CH₄ conversion of different binary catalysts tested at 500°C. (c) Light‐off curves of CH₄ combustion over septenary oxides with different mixed degrees. Test conditions: 1% CH₄, 20% O₂, N₂ as balance, 30 000 mL g⁻¹ h⁻¹. (d) Activation energies of the catalysts calculated from the Arrhenius equation. Dependence of reaction rate on the partial pressure of (e) CH₄ and (f) O₂ over the catalysts at 425°C. (g) Stability test of various catalysts at 500°C for 50 h. (h) Stability test of MYZrO _x ‐a at 500°C with 4.2% H₂O for 200 h.

The kinetic study is performed to further evaluate the reaction process (Figure 4d and Figure S28), revealing that MYZrO _x ‐a features the lowest apparent activation energy (E _a) of 46.1 kJ·mol⁻¹, consistent with its superior catalytic activity. As manifested in Figure 4e,f and Figure S29, the positive reaction orders for CH₄ and O₂ indicate that the activation of both reactants is crucial in catalytic methane combustion [40]. Besides, MYZrO _x ‐a possesses the lowest reaction orders for both CH₄ and O₂, implying enhanced adsorption and activation of the reactants, which can be attributed to the significant role of M^δ+–O_v active pairs in promoting the activation of robust C(sp³)‐H bond and molecular oxygen in methane catalytic combustion [41].

To highlight the impact of structural differences on catalytic stability performance, the stability test is carried out on MYZrO _x ‐a and other control samples. Figure 4g and Figure S30 elucidates that MYZrO _x ‐a and MO _x with high‐entropy structures can maintain stable methane conversion over 50 h, whereas MYZrO _x ‐c and MYZrO _x ‐m exhibit a significant decline in activity from 76% to 59% and 53% to 34%, respectively. Moreover, MYZrO _x ‐a is tested under different gas hourly space velocity (GHSV) conditions with 4.2% water vapor to further assess hydrothermal stability (Figure 4h, and Figure S31). The results show that MYZrO _x ‐a still maintains stable methane conversion without noticeable deactivation for more than 200 h under a GHSV of 30 000 mL·g⁻¹·h⁻¹, confirming its resistance to humid atmosphere. The results of H₂O‐TPD (Figure S32) suggest that the desorption peak of H₂O in MYZrO _x ‐a occurs mainly in the low temperature range (< 300°C). Moreover, XRD patterns and in‐situ Raman analysis of MYZrO _x ‐a during stability test (Figures S33 and S34) reveal that there is almost no change in the microstructural properties, confirming its excellent water‐resistance stability of amorphous structure after long‐term catalytic test.

2.5. Dynamic Characterization of High‐Entropy Effect in Reaction Mechanism

The redox properties and oxygen species mobility are representative features for evaluating the oxidation performance of the catalysts. According to Figure 5a, the hydrogen temperature‐programmed reduction (H₂‐TPR) profile of MnO₂ can be deconvoluted into three sequential reduction peaks corresponding to MnO₂ → Mn₂O₃, Mn₂O₃ → Mn₃O₄, and Mn₃O₄→ MnO [42]. In contrast, due to the synergistic effect within HEOs, both MO _x and MYZrO _x ‐a exhibit three main integrated reduction peaks rather than the mathematical sum of the individual components (Figure S35a). Among all the catalysts, MYZrO _x ‐a features the highest reducibility, as evidenced by the lowest reduction temperatures across all three peaks, significantly lower than those of MO _x and MnO₂, which is likely attributed to the high concentration of extremely reducible high‐valent metal species (Figure S35b and Table S8). Subsequently, the oxygen temperature‐programmed desorption (O₂‐TPD) profiles (Figure 5b and Figure S36a) display three distinct desorption regions: below 300°C for surface adsorbed oxygen, 300°C–700°C for surface lattice oxygen, and above 700°C for bulk lattice oxygen [43]. It is worth mentioning that MYZrO _x ‐a contains a higher amount of chemisorbed oxygen and a lower desorption temperature of surface lattice oxygen, suggesting superior surface oxygen mobility compared to MO _x and MnO₂ (Figure S36b).

FIGURE 5.

Reaction mechanism investigation. (a) H₂‐TPR, (b) O₂‐TPD, and (c) CH₄‐¹⁸O₂‐TPSR profiles of the catalysts. Quasi in situ XPS spectra of (d) Mn 2p and (e) O 1s for MYZrO _x ‐a under different reaction conditions. (f) In situ Raman spectra of MYZrO _x ‐a under reaction gas atmospheres at various temperatures. (g) Schematic illustration of the proposed reaction mechanism.

Besides, the nature of active sites in methane combustion is investigated through a series of experimental tests. First, the results of CH₄‐TPD (Figure S37) reveal that MYZrO _x ‐a demonstrates a notably larger CH₄ desorption amount than other catalysts, implying that high‐valent metal species modify the surface electronic structure to enhance methane adsorption affinity. Concurrently, a higher desorption peak of the generated CO₂ within MYZrO _x ‐a also manifests the enriched involvement of surface lattice oxygen species in CH₄ combustion, which is further proved by the results of CH₄‐TPSR (Figure S38). Subsequently, isotope labeling CH₄+¹⁸O₂‐TPSR was conducted to further demonstrate the role of lattice oxygen in methane combustion (Figure 5c and Figure S39). The curve of C¹⁶O¹⁶O (m/z = 44) is the first to increase compared to the other curves of C¹⁶O¹⁸O (m/z = 46) and C¹⁸O¹⁸O (m/z = 48), which confirms that the reaction proceeds over MYZrO _x ‐a follows the Mars‐van Krevelen (MvK) mechanism dominated by the participation of surface lattice oxygen [36]. Moreover, MYZrO _x ‐a exhibits the greatest weight loss among the catalysts both in thermogravimetric analysis under N₂ (TGA‐N₂) and CH₄ (TGA‐CH₄) atmospheres (Figures S40 and S41), further referring to its excellent active lattice oxygen sites. It is worth mentioning that MYZrO _x ‐a undergoes a more pronounced mass loss under CH₄ atmosphere, pointing out that the reducing environment efficiently facilitates the lattice oxygen mobility via surface reaction [44].

Subsequently, a deeper glimpse into the dynamic reaction process is provided by quasi in situ XPS (quasi in situ XPS) on MYZrO _x ‐a (Figure 5d,e and Figure S42). As the temperature elevated from 300°C to 500°C, a slight decrease in the Mn valence state is observed from Mn 2p XPS spectra, attributed to the consumption of surface lattice oxygen in MYZrO _x ‐a during the oxidation process via MvK mechanism. The participation of surface lattice oxygen leads to electron donation to adjacent Mn atoms, as evidenced by the Mn⁴⁺ peak shifting to lower binding energies. However, both the peak shift and the Mn valence state reverse when the oxygen is introduced, affirming the replenishment of lattice oxygen [45]. In particular, similar valence state variations are observed for other metal elements in MYZrO _x ‐a, including Fe, Co, Ni, and Cu, as summarized in Table S9, indicating their similar redox participation during the reaction and the synergistic behavior of multiple elements in high‐entropy systems. In addition, the O_ads/O_latt ratio derived from O 1s XPS spectra increases from 0.75 to 0.81 with elevated temperature, suggestive of the consumption of surface lattice oxygen due to the oxidation of CH₄. Following the introduction of O₂, the O_latt peak area increases significantly, pointing to effective replenishment of consumed oxygen and reoxidation of the catalyst surface.

Complementarily, in situ Raman spectroscopy (Figure 5f and Figure S43) also demonstrates that the main peak of the Mn‐O‐M stretching vibrations of MYZrO _x ‐a shifts from 580 to 563 cm⁻¹ as the reaction temperature rises from 50°C to 600°C, implying a decrease in the metal–oxygen bond strength and more weakly bound oxygen atoms are predisposed to forming reactive lattice oxygen that actively participates in the oxidation process [46]. In contrast, MnO₂ displays a less stable structure, evidenced by fluctuating peak positions across different reaction temperatures. Altogether, the collective evidence strongly supports that CH₄ combustion over MYZrO _x ‐a proceeds via MvK mechanism, which involves a cyclic interplay between surface lattice oxygen participation and its dynamic replenishment, and is delineated in Figure 5g.

3. Conclusion

In summary, a thermodynamic synergy‐driven synthesis strategy for amorphous HEOs via a facile colloid‐mill redox precipitation method is proposed. Through systematically comparison with lower‐entropy and crystalline counterparts, we explore the formation principle and resultant structural properties of amorphous HEOs. The feasibility of large‐scale synthesis highlights the practical applicability of this strategy, while the successful preparation of a series of amorphous HEOs incorporating more elements attests to its universality. Representatively, the amorphous MYZrO _x ‐a demonstrates superior methane combustion activity and long‐term hydrothermal stability compared to its lower‐entropy and crystalline counterparts, underscoring the critical role of amorphous high‐entropy structures in boosting reactive oxygen mobility and enhancing redox capacity. Furthermore, dynamic mechanistic studies reveal that methane combustion over MYZrO _x ‐a proceeds through the MvK mechanism, wherein high‐valent metal sites and oxygen vacancies synergistically generate M^δ+–O_v active pairs that effectively enhance the adsorption and activation of methane and molecular oxygen. Overall, this work establishes an efficient strategy for constructing thermally stable and defect‐rich amorphous HEOs and broadens their application in heterogeneous thermocatalysis.

4. Experimental Section

4.1. Synthesis of Amorphous HEO

MYZrO _x ‐a was synthesized using a redox precipitation method, of which the details of the synthesis method are as follows. Solution A was prepared by dissolving metal acetates of Fe (II), Co (II), Ni (II), Cu (II), Y (III), and Zr (IV) (2 mmol each) and Mn (II) (1 mmol) in 750 mL of deionized water. Solution B was obtained by dissolving KMnO₄ (1 mmol) and NaOH (20 mmol) in 150 mL of deionized water. Solutions A and B were simultaneously poured into a colloid mill operating at 5000 rpm and mixed for 2 min. The resulting precipitation was aged at room temperature for 30 min, thoroughly washed with deionized water, and then dried overnight at 80°C. Finally, the dried precursor was calcined at 600°C for 3 h under air atmosphere with a heating rate of 5°C·min⁻¹ to obtain the MYZrO _x ‐a catalyst. The synthesis of MYZrO _x ‐a‐stir catalyst is the same as that of MYZrO _x ‐a catalyst, except that the colloidal milling is replaced with magnetic stirring (500 rpm).

4.2. Synthesis of MO _x and MnO₂

MO _x was synthesized following the same procedure as MYZrO _x ‐a, using only Fe, Co, Ni, Cu, and Mn acetates as metal precursors. For MnO₂, only manganese acetate was used. The subsequent steps were kept consistent with the synthesis of MYZrO _x ‐a.

4.3. Synthesis of MYZrO _x ‐c and MYZrO _x ‐m

MYZrO _x ‐c was synthesized via a precipitation mixing method, in which each of the seven metal precursors was individually precipitated by equal amounts of NaOH solution, except that Mn precursor was treated in the same formula as MnO₂. The resulting hydroxide precursors were then combined and homogenized using a colloid mill, then subjected to the same synthesis procedure as used for MYZrO _x ‐a. For MYZrO _x ‐m, single oxides were obtained separately by calcining the hydroxide precursor of single element in the synthesis process of MYZrO _x ‐c at 600°C for 3 h. Subsequently, the mechanical mixing method was employed to physically grind these oxides together in equal molar ratios of each element.

4.4. Characterizations

XRD patterns were collected using a Bruker D8 Advance diffractometer equipped with Cu Kα radiation (40 kV, 40 mA). The data were recorded in a 2θ range of 10°–90° with a step size of 0.02°. In situ XRD measurements were conducted under air flow while heating the samples from room temperature to 700°C at a ramping rate of 5°C·min⁻¹. Diffraction data were collected every 50°C, with each temperature step held for 1 h to ensure signal stability.

AC‐HAADF‐STEM images, SAED patterns, and EDS elemental maps were acquired using a JEOL JEM‐ARM 200F microscope operated at 200 kV and Spectra 300 microscope operated at 300 kV.

PDF analysis was conducted at beamline BL08W of SPring‐8 (Japan) using the total scattering method. x‐Rays with a wavelength of 0.10873 Å and a beam size of 0.5 × 0.5 mm² were employed. Scattering data were collected using a PerkinElmer 1621EN flat‐panel 2D detector, and the resulting data were integrated and processed using the DAWN software to obtain the atomic PDF, G(r).

Raman spectroscopy was carried out using a Renishaw inVia confocal Raman spectrometer with a 785 nm laser source. Spectra were recorded over a range of 100–1200 cm⁻¹ with a spectral resolution of 1 cm⁻¹, an exposure time of 100 s, and detection via a CCD array. Ex situ Raman spectra were obtained at room temperature. For in situ Raman measurements, the catalysts were pretreated under N₂ flow at 300°C for 1 h, then cooled to room temperature. During measurements, the catalyst was exposed to a reaction gas mixture (50 mL·min⁻¹) of 1% CH₄, 20% O₂, 4.2% H₂O (when used) and N₂ for balance, matching the composition used in catalytic performance tests.

XAFS measurements at the Mn K‐edge were conducted at beamline BL14W of the Shanghai Synchrotron Radiation Facility (SSRF), operated at 3.5 GeV. A Si (111) monochromator was used to select the incident x‐ray energy, and Mn foil served for energy calibration. Data at the Mn K‐edge were acquired in transmission mode at room temperature.

XPS analysis was conducted using a Thermo ESCALAB 250 instrument equipped with an Al Kα x‐ray source. Binding energies were calibrated to the C 1s peak at 284.8 eV. Spectra were collected for Mn 2p, Fe 2p, Co 2p, Ni 2p, Cu 2p, Y 3d, Zr 3d, O 1s, and C 1s regions. For quasi in situ XPS analysis, catalyst powders were loaded into the chamber, and the gas mixtures were introduced under controlled conditions. Mn 2p, Fe 2p, Co 2p, Ni 2p, Cu 2p, O 1s, and C 1s spectra were collected after treatment under three gas environments: 1% CH₄/N₂ at 300°C, 1% CH₄/N₂ at 500°C, and 1% CH₄/20% O₂/N₂ at 500°C. The relative content of different oxidation states was determined by peak area analysis.

PALS was performed using a standard fast–fast coincidence lifetime spectrometer with a time resolution of ∼210 ps. A ²²NaCl positron source of around 30 µCi, sealed between two Kapton films (0.0075 × 10 × 10 mm), was placed between two identical catalyst samples, which were compressed from powders into pellets with a diameter of 10 mm and a thickness of 1 mm. The measurements were conducted in a vacuum chamber at room temperature, where the pressure was maintained below 1 × 10⁻³ Pa throughout the experiment using a turbomolecular pump. Each spectrum with total counts of 4 × 10⁶ was analyzed using the LTv9 software.

Isotopic CH₄‐¹⁸O₂‐TPSR experiments were conducted on a Micromeritics Chemisorb 2920 system. A total of 100 mg of catalyst was pretreated in air at 300°C for 1 h under a flow of 50 mL·min⁻¹, followed by purging with N₂ for 30 min. After cooling to 40°C, a gas mixture of 1% CH₄ and 10% ¹⁸O₂ balanced with He (total flow rate: 50 mL·min⁻¹) was introduced into the system. Once the baseline of the signal stabilized, the temperature was ramped from 40°C to 650°C at 10°C·min⁻¹. The reaction products, including—C¹⁸O¹⁸O (m/z = 48), C¹⁶O¹⁸O (m/z = 46), and C¹⁶O¹⁶O (m/z = 44)—were continuously monitored online using Hiden Analytical HPR‐20 R&D mass spectrometer (MS).

Further characterization techniques—including BET, ICP‐OES, XAS data fitting, H₂‐TPR, O₂/CH₄‐TPD, CH₄‐TPSR, and N₂/CH₄‐TGA—are detailed in the Supporting Information.

4.5. Catalytic Performance Measurements for CH₄ Combustion

The catalytic performance for CH₄ combustion was evaluated in a fixed‐bed quartz tube reactor. A total of 100 mg of catalyst was loaded into the reactor, and a gas mixture containing 1% CH₄, 20% O₂, and N₂ as balance gas was passed through at a GHSV of 30 000 mL g⁻¹ h⁻¹. Temperature‐dependent activity was measured from 200°C to 650°C in 25°C intervals. At each temperature, measurements were repeated until CH₄ conversion reached a steady state. The outlet gases were analyzed using an Agilent 7890B gas chromatograph equipped with both a thermal conductivity detector (TCD) and a flame ionization detector (FID). Only CH₄, O₂, CO₂, and N₂ were detected during all tests. Catalyst stability was assessed under dry conditions at 500°C for 50 h using GHSVs of 30 000 mL g⁻¹ h⁻¹, under wet conditions at 500°C for 200 h using GHSVs of 30 000, and under wet conditions at 500°C for 100 h using GHSVs of 15 000 and 120 000 mL g⁻¹ h⁻¹, respectively. CH₄ concentration was continuously monitored over time to evaluate deactivation behavior. Details on the calculations of CH₄ conversion, apparent activation energy (E _a), and the reaction orders concerning CH₄ and O₂ are provided in the Supporting Information.

4.6. Theoretical Calculation Methods

The theoretical configurational entropy of mixing (ΔS _mix) for the multi‐element oxides was calculated using the following equation:

$$\mathrm{\Delta }{S}_{\mathrm{mix}}=-R\sum _{i=1}^N{x}_i\ln \left(x_i\right)$$

where R is the ideal gas constant (0.0000862 eV K⁻¹ metal⁻¹); N represents the number of metal cations in the oxide, and x_i is the molar fraction of the ith metal cation [3].

The mixing enthalpy (ΔH _mix) was determined by the following equation [3, 47]:

$$\mathrm{\Delta }{H}_{\mathrm{mix}}=\sum _{i=1,i\ne j}^N{x}_i{x}_j{\mathrm{\Omega }}_{ij}$$

where ${\mathrm{\Omega }}_{ij}=4\mathrm{\Delta }{H}_{\mathrm{mix}}^{AB}$ is the regular melt‐interaction parameter between ith and jth elements, and $\mathrm{\Delta }{H}_{\mathrm{mix}}^{AB}$ is the enthalpy of mixing for binary liquid alloys, with values sourced from literature [48].

Atomic size difference (δ) was estimated based on atomic radii and elemental percentage as follows [47]:

$$\delta =100\sqrt{{\sum }_{i=1}^N{x}_i{\left(1-r_i/\overline{r}\right)}^2}$$

where x_i is the atomic percentage, r_i is the atomic radius of the ith component, and $\overline{r}={\sum }_{i=1}^N{x}_i{r}_i$ represents the average atomic radius. The atomic radius values of each element were obtained from referenced sources [49].

Additional details of the DFT calculations are provided in the Supporting Information.

Author Contributions

Z.H. and G.L. guided the research, oversaw the work design, and revised the manuscript. B.L. performed the catalyst synthesis, characterization, catalytic performance evaluation, data analysis, and drafted the manuscript. Z.Z., Z.S., and Y.Z. conducted characterization. L.F. provided assistance with the PDF fitting analysis and corresponding data discussion. J.T. contributed to the PDF characterization and related discussion. D.W. provided discussions on the data and offered suggestions that improved the quality of this work and the revision of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma74010‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.