Introduction

Supported metal catalysts are of great importance in both industrial and fundamental catalysis1. The methods for generating active sites on metal oxide carriers include chemical or physical deposition techniques (e.g., impregnation method2, deposition-precipitation method3, co-precipitation process4, atomic layer deposition5, spraying6 and so on7. Needless to say, those methods have significantly promoted the synthetic chemistry of supported metal catalysts. For example, the control of metal nanocrystal size by modified deposition method reveals the attractive role of corner atoms, which offers a theoretical guidance for catalyst design8. Nevertheless, the deactivation of supported metal catalysts, especially those metal nanoparticles (NPs) with low tamman temperatures (e.g., Cu9, Ag10, Au11) and high surface energies12, is often inevitable by sintering13 under long-term exposure to the high temperature or redox environments. For example, Au/TiO2 catalysts afforded an ultralow temperature for CO oxidation14, while particle growth leading to the activity loss often occurred during high temperature operation15.

Early in this century, a self-regenerative strategy by in situ exsolution of active species from host matrix was presented by Nishihatas’ group16, which can greatly prolong the lifetime of three-way catalysts. The emergence of perovskite-based (ABO3) self-regenerative catalysts exploits a useful approach to optimize catalyst activity and stability16,17,18,19,20,21. To our knowledge, the control of nonstoichiometry18 (A-site defect interactions), judicious choice of B-site composition (e.g., Pd17, Pt22, Ni23, CoFe alloy24) and the surface modification (polished or pre-treated perovskite) are frequently used to tailor B-site metals exsolution. This type of catalytic materials can not only in situ generate active sites, but also accomplish reintegration of those species back to parent perovskites during oxidation-reduction cycles16,17,18,19,20,21. Meanwhile, introducing high-valence cations (Nb25, Ti26, Ga27, etc.) into ABO3 structure can improve the structural stability of the parent perovskites, which contributes to the reintegration process during catalysis19. This process effectively suppresses the sintering or agglomeration of the supported metals16. However, these types of host matrix are limited to perovskite crystal structures and the state-of-art self-generation systems are perovskite-based catalysts16,17,18,19 (Fig. 1a), which cannot meet the broad requirements by different catalytic processes. To date, the proposal of a self-regenerative principle for the design of sintering-resistance catalysts is still desired.

Fig. 1: Comparison of self-regenerative materials.
figure 1

a Representative perovskite-based self-regenerative materials. b i. The configurational entropy formula, and the selected metals for high entropy ZrO2. ii. Calculated values of configurational entropy for Zr0.5(M1-Mn)0.5Ox, n = 1–5 (for example, n = 2, Zr0.5(M1M2)0.5Ox). iii. Schematic representation of in situ exsolution and dissolution of CoFeCuNi alloys in the Zr0.5(NiFeCuMnCo)0.5Ox during redox processes. For simple, oxygen ions were omitted.

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Since the discovery of high entropy alloys in 200428 and the pioneering introduction of high-entropy oxides (HEOs: oxide solid solutions contain five or more distinct metal cations) in 201529, entropy-stabilized materials such as oxides30, sulfides31, phosphide32, nitrides33, carbides34, and borides35, are flourishing in the library of advanced materials during past five years. Inspired by the principle of thermodynamics29,30 (Entropic contribution could predominate the thermodynamic landscape), applying the theory of high entropy materials to trigger the self-regeneration during catalytic redox processes may be a choice. First, based on the configurational entropy34 (it refers to the number of conformations of a molecule and the number of ways that atoms or molecules pack together in the oxide) formula (Fig. 1.bi), more disorder of system and higher randomness of structure with a lower Gibbs free energy can contribute to the stability of host structure, especially during high temperature catalysis (ΔG = H-TΔS)31,36 (Fig. 1bii). From another perspective, the reintegration process can be promoted due to an entropy-added motivation by the oxidative transformation of isolated metals back into parent metal oxides. The theoretical conjecture stimulates us to carry out a number of representative experiments.

In this work, we verify the entropy-driven principle for self-regenerative catalyst by high entropy ZrO2. A single-phase cubic Zr0.5(NiFeCuMnCo)0.5Ox, whose entropic contribution (TΔSconfig = T*12.7 J mol−1 K−1) is predominant in ∆G28,29,31, was prepared and rationally selected as a model catalyst. Interestingly, reductive H2 treatment could induce the exsolution of CoFeCuNi alloy NPs on the surface of parent high entropy ZrO2 NPs. With high entropy Zr0.5(NiFeCuMnCo)0.5Ox as the host matrix, the exsolution and reintegration of CoFeCuNi alloy were observed during three redox cycles (H2 600 °C + Air 550 °C), an interesting feature that may confine metal species under dynamics catalysis (Fig. 1biii). As a proof of principle, the entropy-driven self-regenerative property endows Zr0.5(NiFeCuMnCo)0.5Ox catalyst good stability during CO2 hydrogenation reaction. The Zr0.5(NiFeCuMnCo)0.5Ox catalyst functioned well at 400 °C during 500 h continuous hydrogenation, and at the same time no phase separation or sintering were found on the reused catalyst. The self-regenerative talent of high entropy material may open an alternative for the design of sintering-resistance catalysts.

Results

The formation of high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox

Inspired by the principle of the high entropy-stabilized structure, the synthesis of high entropy ZrO2 catalyst was carefully investigated by a mechanochemical process (Fig. 2). Five common transition metal salts (Ni, Fe, Cu, Mn, Co) were mixed with ZrCl4 and NaOH, followed by calcination in air and sodium salt removal. Definitely, for transition metal dopants, the matching radius (the rationally selected five metals \({{\mbox{|}}}\frac{{{{\mbox{r}}}}_{{{\mbox{Zr}}}}{{\mbox{-}}}{{{\mbox{r}}}}_{{{\mbox{x}}}}}{{{{\mbox{r}}}}_{{{\mbox{Zr}}}}}{{\mbox{|}}}\) < 20%37: Ni, Fe, Cu, Mn, Co) was a crucial factor for achieving the highly doped ZrO2 solid solution. Binary doped ZrO2 samples were initially prepared. Impure phases were found (Supplementary Fig. 1) in Zr0.5Cu0.5Ox, Zr0.5Mn0.5Ox, Zr0.5Co0.5Ox, and Zr0.5Ni0.5Ox, presumably due to the limited solubility of the dopants in ZrO238. At current stage, it is still difficult to incorporate 50 mol% dopants into monocrystalline ZrO239. In addition, a series of Zr0.9M0.1Ox (M: Co, Ni, Mn, Cu, Fe) materials were constructed by the same synthesis conditions. Meanwhile, the doped metal content is the same as Zr0.5Ni0.1Fe0.1Cu0.1Mn0.1Co0.1Ox sample. Deserved to be mentioned, the Zr0.9Cu0.1Ox, Zr0.9Ni0.1Ox exhibited obvious impurity peaks, which could be ascribed to the CuO, NiO, respectively (Supplementary Fig. 2a, b). It seems reasonable that 10% Cu-doped and 10% Ni-doped ZrOx materials would present obvious diffraction peaks of NiO and CuO. The enhancement of entropic contribution may compensate the formation enthalpy40, and then a detailed study for constructing high entropy Zr0.5(NiFeCuMnCo)0.5Ox was carried out.

Fig. 2: The synthetic route of entropy-stabilized Zr0.5(NiFeCuMnCo)0.5Ox.
figure 2

i Mixing metal chlorates ZrCl4, MnCl2, NiCl2, FeCl3, CoCl2, CuCl2, NaCl and NaOH by ball milling. ii Calcination of the intermediate in air. iii Sodium salt removal from the composite by washing.

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Firstly, the calcination temperature was adjusted to explore the phase evolution of Zr0.5(NiFeCuMnCo)0.5Ox precursors. As shown in the Fig. 3a, obvious diffraction peaks for the typical cubic ZrO2 phase started to form at 550 °C. Notably, no other impurity peaks were observed at an elevated temperature (e.g., 600 °C). This phenomenon indicated the possible formation of a high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox. Then, the solubility of five metals (Ni, Fe, Cu, Mn, Co) within ZrO2 solid solution was carefully investigated by adjusting their total ratio (0.3–0.9) (Fig. 3b). The single cubic structure of ZrO2 was well maintained in Zr0.7(NiFeCuMnCo)0.3Ox and Zr0.5(NiFeCuMnCo)0.5Ox, while impure reflections of other crystal structures were detected with a higher ratio (0.7–0.9) of doped metals (Fig. 3b). The molar ratio (Zr:Ni:Fe:Cu:Mn:Co= 5:1:1:1:1:1) was then fixed, and a relative high configurational entropy (Sconfig = 12.47 J mol−1 K−1) could be expected.

Fig. 3: The synthesis of a cubic phase Zr0.5(NiFeCuMnCo)0.5Ox.
figure 3

a The phase evolution of Zr0.5(NiFeCuMnCo)0.5Ox precursors under different calcination temperatures. b The P-XRD patterns of ZrO2 doped with different total ratio (0.3–0.9) of five metals. c P-XRD patterns together with Rietveld fits of the Zr0.5(NiFeCuMnCo)0.5Ox sample prepared at 550 °C. d Structural view of cubic Zr0.5(NiFeCuMnCo)0.5Ox. arb. units: arbitrary unit.

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Rietveld refinement was then performed to determine the structural parameters of Zr0.5(NiFeCuMnCo)0.5Ox. The initial structural model was built based on the cubic ZrO2 (a = b = c = 5.100 Å). The calculated lattice constant of Zr0.5(NiFeCuMnCo)0.5Ox was a = b = c = 5.097 Å (Fig. 3c, d). The difference in atomic sizes between Zr and transition metals would cause the shrinkage of the lattice of ZrO2 matrix41. However, it should be noted that the abundance of oxygen vacancies in doped ZrO2 may enlarge the ZrO2 lattice42. Meanwhile, the R-factor (Rp (%)=4.6), weighted profiles R-factor (Rwp (%)=4.7) and chi square (χ2 = 1.330) of the HE ZrO2 were quite good (Fig. 3c). The simulated PXRD pattern of the high entropy ZrO2 was in good agreement with the experimental one. Those XRD results suggest the formation of cubic Zr0.5(NiFeCuMnCo)0.5Ox.

With the option temperature (550 °C) and composition in hand, a high specific surface area (84 m2 g−1) was obtained by optimizing the dosage of NaCl additive (Supplementary Table 1. Supplementary Fig. 3). To the best of our knowledge, the high temperature for phase transformation (e.g., 900–1300 °C) in entropy-driving force (TΔS) would cause the collapse of pores, leading to HEOs with low surface areas, such as, NiMgCuZnCoOx by solid-state combustion (2 m2 g−1)43, NiMgCuZnCoOx by citric acid-based sol–gel method (28 m2 g−1)43, (MgTiZnCuFe)3O4 by solid-state combustion (12 m2 g−1)44, CoNiCuMgZnOx with graphene oxide as a sacrificial template (42 m2 g−1)45. The current porosity (84 m2 g−1) may be attributed to the particle breakage by mechanochemistry46, and meanwhile a mild crystallization temperature (550 °C) could effectively avoid the excessive growth of fine NPs and interstitial porosity was retained47.

In order to observe the morphology of the Zr0.5(NiFeCuMnCo)0.5Ox sample, the high-angle annular dark-field signal (HAADF) and energy dispersive spectrometer (EDS) elemental maps were performed (Fig. 4). Zr0.5(NiFeCuMnCo)0.5Ox was composed of small NPs (e.g., 10–30 nm), which aggregated together and resulted in abundant interstitial porosity. This feature was in agreement with the found N2 sorption isotherm of Zr0.5(NiFeCuMnCo)0.5Ox (SBET = 84 m2 g−1). Moreover, the EDS elemental maps of individual metals showed that the five dopants were homogeneously distributed in the ZrO2 backbone, suggesting that five metals-doped ZrO2 solid solution was constructed. Meanwhile, the molar ratio of elements (Zr:Ni:Fe:Cu:Mn:Co) in Zr0.5(NiFeCuMnCo)0.5Ox material was confirmed to be 5.7:1:1.1:1.1:1.1:1.1 by inductively coupled plasma mass spectrometer (Supplementary Table 2), which matched well with the theoretical formula. These investigations encouraged us to explore its self-regeneration possibility during redox cycles.

Fig. 4: The characterization of freshly made Zr0.5(NiFeCuMnCo)0.5Ox.
figure 4

STEM-HAADF image and EDS elemental maps of Zr0.5(NiFeCuMnCo)0.5Ox. Scale bar, 200 nm.

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The exploration of self-regeneration process

To verify the exsolution and reintegration behavior, several exploratory experiments were carried out with high entropy Zr0.5(NiFeCuMnCo)0.5Ox as a model. The exsolution of metals was determined by the reducibility of transition metal oxides48, and it was closely related to co-segregation energy (transition metal accompanying with oxygen vacancies)49. The ZrO2 doped with low valence metals could contribute to produce oxygen vacancies50, which promoted the exsolution of metals51 under reductive atmosphere. Subsequently, the H2-TPR spectrum was tested to probe the reduction behavior of Zr0.5Co0.5Ox, Zr0.5Ni0.5Ox, Zr0.5Cu0.5Ox, Zr0.5Fe0.5Ox, Zr0.5Mn0.5Ox, Zr0.5Mn0.25Cu0.25Ox (Supplementary Fig. 4) and pristine Zr0.5(NiFeCuMnCo)0.5Ox samples (Supplementary Fig. 5). As shown in Supplementary Fig. 4a, the peak at 256 °C resulted from the reduction of CuO to Cu52. For Zr0.5Mn0.5Ox sample (Supplementary Fig. 4b), the two reduction peaks were considered as MnO2 or Mn2O3 to Mn3O4 (318 °C), and Mn3O4 to MnO (420 °C)53. For Zr0.5Mn0.25Cu0.25Ox sample (Supplementary Fig. 4c), the reduction peaks at 269 °C and 325 °C were attributed to the reduction of the CuO to Cu0 and manganese oxides to MnO, respectively54. All three types of nickel oxide (363, 467, 570 °C) were reduced to the Ni0 (Supplementary Fig. 4d)55. For Zr0.5Fe0.5Ox sample (Supplementary Fig. 4e), the TPR profile showed two distinct reduction peaks. The peak centered at 401 °C was assigned to the reduction of Fe2O3 to Fe3O4 and the peak at 566 °C was ascribed to the reduction of Fe3O4 to Fe0 56. For cobalt-doped sample (Supplementary Fig. 4f), the reduction peaks around 295 °C and 408 °C were attributed to the transitions from Co3+ to Co2+ and Co2+ to Co0 57. Indeed, the addition of CuO could decrease the reduction temperature (Supplementary Fig. 4a, b, c)54. Interestingly, only one strong reduction peak was observed at ~279 °C for Zr0.5(NiFeCuMnCo)0.5Ox sample (Supplementary Fig. 5). Then, with CuO as the standard reference, the H2 consumption of Zr0.5(NiFeCuMnCo)0.5O1.1+y sample (1.1 ascribes to unreducible ZrO258, and MnO49) can reach 3.63 mmol g−1. Then, the y value is calculated to be 0.3659. In addition, the y values of other samples were provided (Supplementary Table 3). Based on this result, the Zr0.5(NiFeCuMnCo)0.5O1.46 formula suggests that significant amount of oxygen vacancies were created. Hence, these preliminary studies encourage further research about the exsolution process.

Subsequently, the pristine high entropy Zr0.5(NiFeCuMnCo)0.5Ox was treated in reduction condition (H2) at appointed temperatures for 2 h. As shown in Fig. 5a, the early stage of phase transformation was not clear at 400 °C. When the reduction temperature raised to 500–600 °C, the diffraction pattern of the exsolution phase showed a broad peak at 44 °C, which could be assigned to the reflection of transition metals (111). At the same time, typical peaks for cubic ZrO2 were still preserved during H2 treatment. In sharp contrast, the exsolution diffraction peaks of as-prepared binary Zr0.5Cu0.5Ox were sharp, revealing the excessive growth of Cu particles (average particle size by Reitveld refinement analysis: 37.7 nm)60 (Fig. 5b, Supplementary Fig. 6). Meanwhile, the exsolution peaks of quaternary Zr0.5(NiCuCo)0.5Ox presented two different phases (Fig. 5b). The multi redox behavior of Zr0.5(NiFeCuMnCo)0.5Ox was detailedly studied in the following (Fig. 5c). Therefore, high-entropy Zr0.5(NiFeCuMnCo)0.5Ox as a host matrix could somewhat suppress the growth or agglomeration of exsolved metal NPs even under harsh reductive conditions (e.g., 600 °C), which stood on the surface of parent high entropy ZrO2 NPs.

Fig. 5: The XRD patterns of doped ZrOx samples during redox processes.
figure 5

a The P-XRD patterns of Zr0.5(NiFeCuMnCo)0.5Ox treated at 10% H2 balance with N2 at appoint temperature (400, 500, 600 °C) for 2 h. b The P-XRD patterns of Zr0.5Cu0.5Ox, Zr0.5(CuCoNi)0.5Ox, and Zr0.5(NiFeCuMnCo)0.5Ox treated at 10% H2 balance with N2 at 600 °C for 2 h. c the P-XRD patterns of phase evolution in multi cycles between reductive and oxidative conditions. arb. units: arbitrary unit.

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Then, the Zr0.5(NiFeCuMnCo)0.5Ox (treated at 400 °C, H2) was investigated by STEM and elemental mapping analysis. Firstly, STEM was employed to provide insights into the exsolved NPs in Zr0.5(NiFeCuMnCo)0.5Ox under reducing atmosphere. As shown in Supplementary Fig. 7, reductive H2 treatment induced the exsolution of grains growing on the surface of parent high entropy ZrO2 NPs. Meanwhile, elemental mapping analysis exhibited aggregation of some elements (Supplementary Fig. 7b). To further confirm the exsolved metal species, the high resolution elemental mapping images at the edge of the sample revealed the formation of the CoFeCuNi alloy NPs (Fig. 6). To further study clearly the metal exsolution, the O elemental mapping was added (Supplementary Fig. 8). A typical particle on the edge was highlighted by yellow cycle. It showed a relatively low O content, close to the background. This result somewhat demonstrated the formation of the metallic CoFeCuNi particle.

Fig. 6: The characterization of H2-reduced Zr0.5(NiFeCuMnCo)0.5Ox.
figure 6

STEM-HAADF images and EDS elemental maps of Zr0.5(NiFeCuMnCo)0.5Ox (treated at 400 °C, H2). Yellow scale bar, 30 nm. White scale bar, 10 nm.

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To further understand current observation, in situ high-resolution XPS (Fig. 7a, b) was carried out to investigate the surface state of the Zr0.5(NiFeCuMnCo)0.5Ox during H2 treatment from RT to 873 K. As shown in Fig. 7c, the peak centered at 778.3 eV was assigned to the characteristic Co 2P3/2 peak of Co0 61. Meanwhile, the sharp peak at 852.7 eV resulted from surface Ni0 (Fig. 7d)61. Since the Binding Energy of Cu+ generally overlaps with Cu0 in the Cu 2 P core level (Fig. 7e, peak at 932.6 eV), the X-ray induced Auger electron spectra in the kinetic energy region of 928–906 eV is provided in Fig. 7f. As shown in Fig. 7f, the kinetic energy at 918.6 eV could be contributed to Cu0 61. A signal peak is located at 706.9 eV, which is ascribed to the Fe 2P3/2 metal (Fig. 7g)61. As expected, there is no peak for Mn0 at 638.8 eV61. Meanwhile, MnO 2P3/2 (at 641.0 eV) peaks show their characteristic satellite peak at 636.9 eV (Fig. 7h)62. Indeed, manganic oxide is more difficult to be reduced to Mn0 63. Under H2 treatment, several transition metal ions (Co, Ni, Cu, Fe) seem to in situ migrate and separate from the bulk of high entropy Zr0.5(NiFeCuMnCo)0.5Ox, while the original cubic ZrO2 matrix was well maintained (Fig. 5a). The H2-induced formation of exsolved CoNiCuFe NPs is different with the Cantor alloy (FeCrMnNiCo) by melting process64 and those CoNiCuFe NPs could not be described as high entropy alloys65. As illustrated in Supplementary Fig. 5, only one strong reduction peak was observed at ~279 °C, which could be assigned to the produce of the CoFeCuNi alloy. Based on these results, the exsolution phenomenon of alloys occurred under reductive atmosphere, which was just the first step of self-regenerative process.

Fig. 7: In situ XPS measurements of Zr0.5(NiFeCuMnCo)0.5Ox.
figure 7

The sample was treated with X-ray exposure under UHV (traditionally ultrahigh vacuum) conditions (0.2 mbar H2). a Photograph of Zr0.5(NiFeCuMnCo)0.5Ox under NAPXPS test at room temperature. b Photograph of Zr0.5(NiFeCuMnCo)0.5Ox under NAPXPS test at 873 K. c Co 2p region. d Ni 2p region. e, f Cu 2p region. g Fe 2p region. h Mn 2p region. i Zr 3d region.

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To figure out the self-regeneration possibility of high entropy Zr0.5(NiFeCuMnCo)0.5Ox, the dissolution process was subsequently explored. The reduced Zr0.5(NiFeCuMnCo)0.5Ox (treated at 600 °C, H2) was further exposed to oxidative environment (550 °C, air). After this cycle, the obvious XRD peaks for CoFeCuNi alloy phase disappeared (Fig. 5c). The interesting exsolution-reintegration behavior of the high entropy ZrO2 material was further proved by three redox cycles (Fig. 5c). The oxidized sample after three cycles was observed by STEM images. The elemental mapping images in different scale bars, such as 50 nm, 200 nm, 50 μm, have been taken. As shown in Fig. 8 and Supplementary Fig. 9, all five heterometal species were evenly distributed in ZrO2 matrix, which was the same as the pristine Zr0.5(NiFeCuMnCo)0.5Ox (Fig. 4). As expected, the entropy-added motivation may drive the reintegration of CoFeCuNi alloy NPs into the parent ZrO2 matrix in air, which contributed to the reversible self-regenerative process. All those results above suggested the feasibility of the high entropy ZrO2 as a host matrix for self-regenerative process.

Fig. 8: The characterization of Zr0.5(NiFeCuMnCo)0.5Ox after three redox cycles (600 °C H2/550 °C, Air).
figure 8

a–h STEM-HAADF image and EDS elemental maps of Zr0.5(NiFeCuMnCo)0.5Ox after three redox cycles. Scale bar, 200 nm. i–p STEM-HAADF image and EDS elemental maps of Zr0.5(NiFeCuMnCo)0.5Ox after three redox cycles. Scale bar, 50 nm.

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The catalytic performance of high entropy ZrO2 catalyst

Then, the activity and thermal stability of high entropy Zr0.5(NiFeCuMnCo)0.5Ox were explored in high temperature catalysis (Fig. 9). CO2 hydrogenation (reverse water gas shift, RWGS), with an enthalpy ΔH of 41.3 kJ mol−1, is a typical endothermic reaction66. From the sight of both kinetics and thermodynamic, high temperature is more conducive to accelerating the reaction rate and moving the equilibrium in the direction of CO generation67. In other words, RWGS reaction is usually performed at high temperatures (e.g., 300–500 °C), whose catalysts are easy to be sintered68. In addition, a redox reaction mechanism of active sites was proposed to explain this process69. Therefore, RWGS reaction was applied as a model process to evaluate the high entropy ZrO2 catalyst.

Fig. 9: Catalytic performance of doped ZrO2 catalysts.
figure 9

a catalytic performance of Zr0.5Cu0.5Ox, Zr0.5Mn0.5Ox, Zr0.5Co0.5Ox, Zr0.5Ni0.5Ox, Zr0.5Fe0.5Ox, Zr0.5(CuMn)0.5Ox, and Zr0.5(NiFeCuMnCo)0.5Ox in CO2 hydrogenation at 400 °C. b P-XRD patterns of reused Zr0.5(NiFeCuMnCo)0.5Ox samples after 100 h and 500 h on steam. c The P-XRD patterns of phase evolution between H2/CO2 conditions. d Thermal stability tests of Zr0.5(NiFeCuMnCo)0.5Ox and Zr0.5(CuMn)0.5Ox in CO2 hydrogenation. arb. units: arbitrary unit.

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Compared with binary and ternary doped ZrO2 catalysts (Zr0.5Cu0.5Ox, Zr0.5Mn0.5Ox, Zr0.5Co0.5Ox, Zr0.5Ni0.5Ox, Zr0.5Fe0.5Ox, Zr0.5(CuMn)0.5Ox) (Supplementary Fig. 10a–d), the Zr0.5(NiFeCuMnCo)0.5Ox afforded better catalytic activity during CO2 hydrogenation (300–400 °C). When the temperature reached 400 °C, the CO2 conversion was ~29% by Zr0.5(NiFeCuMnCo)0.5Ox and the selectivity for CO is over 90% (Fig. 9a, Supplementary Fig. 10e). Then, the long-term stability of Zr0.5(NiFeCuMnCo)0.5Ox during CO2 hydrogenation reaction was studied. No obvious loss of catalytic activity and selectivity was found during continuous CO2 hydrogenation for 500 h (Fig. 9b). In sharp contrast, severe deactivation by the ternary doped Zr0.5(MnCu)0.5Ox catalyst was observed in relatively short reaction time (40 h) (Fig. 9b). Meanwhile, the XRD pattern of spent Zr0.5(NiFeCuMnCo)0.5Ox catalyst showed little tendency towards segregation or sintering, further illustrating the advantage of high entropy (Zr0.5(NiFeCuMnCo)0.5Ox) as the host structure (Fig. 9c). In addition, the dissolution of metal alloys somewhat underwent during CO2 atmosphere (600 °C), as suggested by the diminished XRD peaks for metallic species (Fig. 9d). Furthermore, control experiments were carried out for comparison. First, the 5%(NiFeCuCo)/ZrOx and 10%(NiFeCuCo)/ZrOx were synthesized by wet-impregnation method. The long-term stability of supported NiFeCuCo catalyst during CO2 hydrogenation reaction was studied at the same condition (Supplementary Fig. 11). Unfortunately, severe deactivation was observed in relatively short reaction time (1 h and 3 h). Compared with the high entropy Zr0.5(NiFeCuMnCo)0.5Ox material (>500 h), 5%(NiFeCuCo)/ZrOx and 10%(NiFeCuCo)/ZrOx exhibited poor stability. Based on those results, unique properties of the high entropy structure indeed promoted the thermal stability of doped ZrO2 catalysts.

Discussion

In summary, a cubic high-entropy Zr0.5(NiFeCuMnCo)0.5Ox was designed and simply prepared. The contribution of high entropy for the sintering-resistance ability of supported transition metals was figured out, while severe particle growth occurred on Zr0.5Cu0.5Ox and Zr0.5(CuCoNi)0.5Ox treated in H2. Meanwhile, the reversible exsolution-dissolution of fine CoFeCuNi alloys on high entropy Zr0.5(NiFeCuMnCo)0.5Ox was observed in H2/Air treatments. Those results somewhat validate that entropy-driving force may contribute to self-regenerative process and sintering-resistance performance of supported transition metals. During high temperature catalysis (400 °C), no obvious loss of catalytic activity and little tendency towards segregation were found during continuous CO2 hydrogenation for 500 h, once again arguing for the excellent sintering-resistance ability of high entropy catalysts.

The high entropy oxide was invented in 201529 and related catalysis has been unveiled in around 201870, and their unexpected talent for heterogeneous catalysis is a young topic63, yet full of promise. The idea of considering entropic contribution for stabilized catalyst is just the beginning, and more unique materials for heterogeneous catalysis would be inspired in the near future.

Methods

Chemicals involved in the manuscript are used directly without purification. ZrCl4 (>99.9%, Aladdin), NiCl2 (>99%, Adamas), FeCl3 (>99%, Adamas), CuCl2·2H2O ( > 99%, Greagent), MnCl2 (>99%, Greagent), CoCl2·6H2O ( > 98%, Adamas), NaOH (>96%, Greagent), NaCl (>99.5%, Greagent).

The preparation of entropy-stabilized, highly doped microstructure Zr0.5(NiFeCuMnCo)0.5Ox is as follows. ZrCl4 (2.5 mmol, 5 eq.), NiCl2 (0.5 mmol, 1 eq.), FeCl3 (0.5 mmol, 1 eq.), CuCl2•2H2O (0.5 mmol, 1 eq.), MnCl2 (0.5 mmol, 1 eq.), CoCl2•6H2O (0.5 mmol, 1 eq.), and NaCl (2 g) were whisked together in a 50 mL ZrO2-milling equipment with three ball-bearings (1× diameter 1.2 cm, 2× diameter 0.8 cm). The mixtures were ball milled for 1 h. Then the NaOH (15.5 mmol) was added into the system and the mixtures were ball milled for another 1 h. Calcination of the as-synthesized powder was performed into a muffle oven in air at 400 °C, 500 °C, 550 °C, 600 °C for 3 h (2 K min−1 to appointed temperature) and then cooled down to room temperature. The powder was washed three times by deionized water, and then washed with ethanol. Then as-made samples (Zr0.5(NiFeCuMnCo)0.5Ox) were put into a vacuum drying oven and dried at 70 °C for 12 h. The synthesis of other doped ZrO2 materials for comparative experiments can be found in Supplementary Methods section. The obtained samples were referred to as Zr0.5Cu0.5Ox, Zr0.5Mn0.5Ox, Zr0.5Co0.5Ox, Zr0.5Ni0.5Ox, Zr0.5Fe0.5Ox, Zr0.5(CuMn)0.5Ox, Zr0.5(NiFeCuMnCo)0.5Ox.

The powder X-ray diffraction (D8 Advance, Bruker) operated at 40 kV and 40 mA and the pattern was recorded in the range of 20–80°. The material was characterized by N2 adsorption isotherms (Tristar II 3020 S1N1878) at 77 K. NAPXPS measurements were carried out on an SPECS system equipped with a differentially pumped Phoibos hemispherical electron energy analyzer using monochromatic Al Kα radiation (1486.6 eV). The in situ characterization was performed in the 873 K 0.2 mbar H2 reduction condition. The heating rate was 5 K min−1 from 301 K to 873 K. HAADF-STEM (Talos F200X G2, FEI). Leica EM TXP (Mingrui GC2060, China). Inductive Coupled Plasma Emission Spectrometer (iCAP7600, made by Thermo., USA).

The as-made catalysts (75 or 50 mg) were used for the H2-TPR experiments (Micromeritics Autochem II 2920). The material was pretreated in Ar (30 mL min−1) at 300 °C for 60 min and then cooled to the room temperature. Then, the 5% H2 in Ar (30 mL min−1) was switched on in a heating rate of 10 °C min−1. Finally, the H2 consumption of the as-made materials was monitored by a TCD detector.

The RWGS reaction worked at atmospheric pressure in a fixed-bed quartz reactor. Firstly, 30 mg catalyst and 100 mg quartz sand were mixed together well. Then, the catalyst with quartz was exposed to a stream of feed gas (H2:CO2 = 3:1, 10 mL min−1). When the catalyst reached the appointed temperature, the gaseous products were analyzed by gas chromatograph with a TCD and FID detector.

Detailed procedures are in the Supplementary Methods section.