Hydrogen Spillover-Driven Low Temperature Synthesis of High-Entropy Alloy Nanoparticles as a Robust Catalyst


 High-entropy alloys (HEAs) have been intensively pursued as potentially advanced materials because of their exceptional properties. However, the facile fabrication of nanometer-sized HEAs over conventional catalyst supports remains challenging, and the design of rational synthetic protocols would permit the development of innovative catalysts with a wide range of potential compositions. Herein, we demonstrate that titanium dioxide (TiO2) is a promising platform for the low-temperature synthesis of supported CoNiCuRuPd HEA nanoparticles (NPs) at 400°C. This process is driven by the pronounced hydrogen spillover effect on TiO2 in conjunction with coupled proton/electron transfer. In this process, Pd nuclei generated in the early stage act as uptake sites to enhance the migration of active hydrogen atoms, and the five component metals are simultaneously reduced by spilled hydrogen on the support rather than via direct reduction by gaseous H2. The CoNiCuRuPd HEA NPs on TiO2 produced in this work were found to be both active and extremely durable during the CO2 hydrogenation reaction. Characterization by means of various in situ techniques and theoretical calculations elucidated the specific mechanism by which the HEA NPs were formed and also established that a synergistic effect was obtained from this combination of elements.


Introduction
In contrast to conventional alloy materials based on single principal elements, high-entropy alloys (HEAs) have recently received signi cant attention in various research elds. These alloys represent a new class of metallic materials in which more than ve near-equimolar components are mixed to form single-phase solid solutions with high mixing entropy values, rather than intermetallic phases. 1,2 Various unique synergistic effects result from such mixtures, including high con guration entropy, lattice distortion, sluggish diffusion and cocktail effects, and endow HEAs with high mechanical strength, good thermal stability and superior corrosion resistance. [3][4][5] To date, several synthetic strategies have been reported, such as bulk melting, 6 solid state processing 7 and additive manufacturing, 8,9 all of which have principally focused on the fabrication of bulk HEAs. However, the development of HEA nanoparticles (NPs) with a mean diameter of less than 10 nm lags signi cantly behind, despite the potential practical applications of these NPs in catalysis, nanoelectronics and material science owing to their large surface area-tovolume ratio and nanoscale size effect. 10 A bottom-up approach to the fabrication of HEA NPs is likely to be more reliable than a top-down approach, because the former would be expected to produce fewer surface defects along with uniform chemical compositions and homogenous size distributions. 11 In an early study, Yao et al. succeeded in the fabrication of HEA NPs containing up to eight elements on conductive carbon nano bers, using a carbothermal shock method based on ash heating (at approximately 10 5 K/s) to approximately 2000 K followed by rapid cooling (at the same approximate rate). [12][13][14] Subsequently, methods incorporating ultrasonication, 15 solvothermal synthesis, 16 polyols in solution 17,18 and fast moving bed pyrolysis 19 were explored as alternative synthetic approaches. Unfortunately, these methods still require the application of high temperatures and special experimental apparatuses. The development of new and simpler techniques for the synthesis of HEA NPs, especially those immobilized on the surfaces of conventional support materials, represents an ongoing challenge. Even so, such research could result in a wider range of industrial uses for these materials and provide a better understanding of the novel functions of nanostructured catalysts.
Hydrogen spillover is a fascinating phenomenon that occurs in sensors, hydrogen storage materials and heterogeneous catalysis. [20][21][22] This process involves the surface migration of dissociated H atoms driven by a concentration gradient. Hydrogen spillover on reducible transition metal oxides such as TiO 2 , WO 3 and MoO 3 proceeds via a set sequence of steps. showing that hydrogen spillover on TiO 2 proceeds ten orders of magnitude faster than on the nonreducible oxide Al 2 O 3 , and that TiO 2 provides longer migration distances from the noble metal proton sources. 26 Our own group has previously demonstrated that TiO 2 is a promising platform for the synthesis of nonequilibrium binary alloy NPs, such as RuNi and RhCu, which are essentially immiscible at equilibrium due to the positive enthalpies of formation of their solid solution alloys. 27,28 However, the highly speci c formation of binary alloy NPs based on combinations of normally immiscible noble and base metals can be achieved with the assistance of the strong spillover effect obtained from TiO 2 . Using this oxide allows spillover hydrogen species with high reduction potentials to be generated from noble metals (Ru or Rh) and to rapidly migrate to and reduce base metals (Ni or Cu) at low temperatures. In the present work, we developed and demonstrated that this facile strategy can be applied to the synthesis of TiO 2 -supported HEA NPs. Speci cally, CoNiCuRuPd HEA NPs on TiO 2 displayed high activity and outstanding stability during the CO 2 hydrogenation reaction. This study also elucidated the speci c mechanism responsible for the formation of HEA NPs, based on in situ characterization techniques. In addition, density functional theory (DFT) calculations were performed to validate both the formation mechanism and to examine the synergistic effects of mixing multiple elements, such as unique catalytic performance and exceptional durability.

Results
Synthesis and characterization of HEA NPs on TiO 2 CoNiCuRuPd HEA NPs supported on TiO 2 (CoNiCuRuPd/TiO 2 ) were synthesized using a simple impregnation method, employing an aqueous solution of the corresponding precursors. This was followed by reduction under a H 2 atmosphere at 400 °C without a speci c calcination step before this reduction. A survey of bulk multi-component alloys determined that the formation of a solid solution HEA required an atomic size difference, δ, of less than 6.6% and an enthalpy of mixing, DH mix , between -11.6 and 3.2 kJ/mol. 29 In the present study, the CoNiCuRuPd combination met the above criteria (δ = 3.9% and DH mix = 1.1 kJ/mol), and so the formation of solid solution CoNiCuRuPd HEA NPs was expected. Figure 1A shows the H 2 temperature programmed reduction (TPR) pro les obtained from the asdeposited mono-and quinary-component samples prior to reduction under H 2 . These data indicate that the single metals generated broad reduction peaks at different temperatures. In addition, the absence of a peak in the case of the Pd/TiO 2 sample suggests the immediate reduction of the deposited Pd 2+ precursor after the switching between H 2 and Ar ows at ambient temperature. 30 The relative trend in the reduction temperatures of these materials is similar to that of the reduction potentials of the respective In contrast, all spectra acquired at 400 °C resembled those of the corresponding foils, suggesting that all the elements were in a metallic state. More detailed inspection of these data also found slight changes in the post-edge region at all Kedges. As an example, the two distinct peaks at approximately 24,390 and 24,415 eV corresponding to the allowed 1s → 5p transition at the Pd K-edge were slightly shifted to higher energy values compared to the Pd foil. This result suggested that the symmetry of the Pd metal face centered cubic (fcc) structure was slightly disordered following integration with the other metals. 31 Fourier transforms of extended X-ray absorption ne structure (FT-EXAFS) data further clari ed the structural transformation during the reduction sequence. The spectra of the as-deposited sample produced a sharp singlet peak in the K-edge region that was attributed to nm ( Figure 2C). The energy dispersive X-ray (EDX) maps of these specimens also con rmed the homogeneous distribution of each element ( Figures 2D-H). In addition, an EDX line analysis showed that all signals appeared in the same area, demonstrating the formation of a solid solution alloy involving all ve elements ( Figure S3). The d ave values for the CoNiCuRuPd/Al 2 O 3 and CoNiCuRuPd/MgO samples were 6.65 nm and 6.73 nm, respectively, ( Figure S4) and partially segregated NPs with a bimodal particle size distribution were observed on the MgO support. These results suggest that the TiO 2 support ensured more rapid and homogeneous reduction at lower temperatures, allowing the formation of nuclei to provide smaller, uniform HEA NPs without segregation.
Formation mechanism driven by hydrogen spillover over TiO 2 Considering the H 2 -TPR and in situ XAFS results, we propose a mechanism for the formation of the HEA NPs on the TiO 2 support in conjunction with hydrogen spillover ( Figure 3A). This mechanism based on the spillover effect was further evaluated by DFT calculations, using rutile TiO 2 (101) as a model because of its thermodynamic stability and Pd 5 clusters as a model for Pd nuclei.
According to the above proposed reaction mechanism, four representative elementary steps were considered for the reduction of metal cations on the TiO 2 through the hydrogen spillover from Pd clusters.
The resulting potential energy pro le is shown in Figure 3B.   Figure S6). It was also calculated that the direct reduction of Co 2+ on TiO 2 (101) by a gaseous H 2 molecule occurred with a barrier of 85.3 kcal/mol ( Figure S7), which was more than six times greater than that for the same process with a spilled H atom. This preliminary analysis further con rmed that spilled H atoms in the presence of Pd clusters promoted the rapid and simultaneous reduction of the Catalytic CO 2 hydrogenation The hydrogenation of CO 2 to high calori c fuels has the potential to alleviate both climate change and future demands for fossil fuels. 34,35 As an example, the endothermic reverse water-gas shift reaction (CO 2 + H 2 → CO + H 2 O, DH = 41 kJ mol -1 ) is one of the most promising means of producing CO as an important feedstock for Fischer-Tropsch processes and as an intermediary step for the further synthesis of fuel and chemicals. 36,37 In addition, the exothermic CO 2 methanation reaction (CO 2 + 4H 2 → CH 4 + 2H 2 O, DH = -165.0 kJ mol -1 ), also known as the Sabatier reaction, has attracted new interest because of the recent development of the power-to-gas concept. 38,39 This reaction is also recognized as an important approach to powering long-term space exploration missions. 40 In the present work, catalytic performance was evaluated based on monitoring the progress of atmospheric pressure CO 2 hydrogenation at temperatures from 300 to 400 ºC, with CO and CH 4 as the major products ( Figure 4A). CoNiCuRuPd/TiO 2 gave the highest yield of hydrogenated products, which was from 2 and 13 times greater, respectively, than those obtained using MgO and Al 2 O 3 as supports. . It should also be noted that the catalytic activity of Pd/TiO 2 prepared by the same method was low compared with that of CoNiCuRuPd/TiO 2 , and that this monometallic sample gave CO as the primary product. As shown in Figure 4B, an apparent activation energy (E a ) of 37.7 kJ/mol was obtained for CoNiCuRuPd/TiO 2 , which was lower than that of 44.2 kJ/mol for Pd/TiO 2 . These results clearly suggest the so-called cocktail effect originating from the synergistic effect obtained from the combination of elements comprising the HEA.
At atmospheric pressure, the most widely accepted mechanism for CO 2 hydrogenation is initiated by the adsorption and activation of CO 2 at the metal/oxide interfaces of the metal-supported catalyst. 41,42 Hydrogenation and/or dissociation subsequently occur to afford a chemically adsorbed CO intermediate that is either desorbed as a product or undergoes further hydrogenation to form CH 4 . Previous studies have demonstrated that both catalytic activity and selectivity are affected by the particle size of the active metal centres and by the metal/support interfaces. 43 Because the particles sizes in CoNiCuRuPd/TiO 2 (d ave = 1.90 nm) and Pd/TiO 2 (d ave = 2.04 nm) were similar ( Figure S9), the different selectivities for CO or CH 4 observed in this study were primarily attributed to the desorption characteristics of CO molecules at metal sites with different binding strengths.
For this reason, the surfaces of the NPs were assessed using temperature programed desorption (TPD) with adsorbed CO, together with Fourier transform infrared spectroscopy (FTIR). In the case of Pd/TiO 2 , a peak assignable to the linear stretching vibration of adsorbed CO (ν CO ) was observed at 2091 cm -1 in association with the initiation of CO desorption at 50 °C ( Figure 5A). In contrast, this ν CO peak was observed at 2070 cm -1 in the spectrum obtained from CoNiCuRuPd/TiO 2 . This shift toward a lower wavenumber occurred together with a change in the CO desorption temperature to above 150 °C ( Figure  5B). These results readily explain the selectivity observed during CO 2 hydrogenation over these materials.
The adsorption sites on CoNiCuRuPd/TiO 2 were de nitely electron enriched compared with those on the monometallic Pd/TiO 2 . This, in turn, delayed the desorption of the CO intermediate owing to the stronger interactions, thus promoting subsequent hydrogenation to form CH 4 . 42 These experimental results were also supported by theoretical DFT calculations. The frequency of CO adsorbed on fcc CoNiCuRuPd was modeled using randomly populated (111) facets of periodically repeating slab models (with the 15 con guration patterns depicted in Figure S10), 44 giving an average ν CO of 2079 cm -1 . The adsorption energies (E ad ) of CO and H on an fcc surface, fcc hollow and hexagonal close packed (hcp) hollow were also calculated for CoNiCuRuPd(111) and for pure metal slabs ( Figure S11). The average E a values for CO and H adsorption on CoNiCuRuPd HEA (denoted as HEA ave (111) in Figure 5C) were determined to be -37.5 and -50.3 kcal/mol, respectively. The average E a for CO adsorption on Pd(111) was substantially lower at -26.2 kcal/mol, while the E a for H (-54.2 kcal/mol) was similar. These results demonstrate that the interaction between CO and the HEA surface was stronger than that with the Pd surface, suggesting that CH 4 and CO would be preferentially formed on the former and latter, respectively. It should be further noted that the average E a for CO and H adsorption on all the pure metals (denoted as Ave CoNiCuRuPd in Figure 5C) was different from the HEA ave (111). This result provided additional evidence for a cocktail effect originating from the synergistic effect of the combined metals, which gives rise to unique electronic properties.

Structural Robustness of HEA NPs
Another crucial phenomenon associated with HEA NPs that affects catalytic performance is the sluggish diffusion effect, which enhances the durability of the catalyst. In trials with Pd/TiO 2 , the catalytic activity during CO 2 hydrogenation was found to gradually decrease with continued use, such that the relative activity was reduced by a factor of 0.76 after a 72 h reaction ( Figure 4C). In contrast, CoNiCuRuPd/TiO 2 retained 96% of its original activity, while keeping constant selectivity. Each of these catalyst specimens was recovered after 72 h and subjected to a TEM analysis ( Figure S12). A substantial enlargement of the NPs was observed in the case of Pd/TiO 2 , such that the average NP diameter was more than doubled to 5.3 nm from 2.0 nm. Conversely, CoNiCuRuPd/TiO 2 exhibited suppressed particle growth and the mean particle diameter was determined to be 2.3 nm ( Figure S13). The homogenous elemental distribution evident in the EDX mapping data also provided strong evidence for the maintenance of the random HEA structure. In addition, EDX line scans con rmed that single NPs contained all the constituent elements.
The structural robustness of the HEA NPs was also con rmed by monitoring radiation damage process using TEM under electron beam irradiation in vacuum. 45,46 Here, the contrast of atomic positions was analysed in the continuous image. As shown in the time-lapsed TEM images, the change of the contrast in the atomic column position is relatively small for the CoNiCuRuPd/TiO 2 even at edge/corner position ( Figure 6A-C), indicating the suppression of structure deterioration by an incident electron beam. Conversely, drastic changes in contrast were observed for Pd/TiO 2 , which is de nitely originated from the atomic displacement induced by the knock-on damage ( Figure 6E,F). 47,48 The temporal changes in intensity of atomic columns at other positions showed similar trend, as summarized in Figure S14. The statistic and precise analysis is indispensable for discussing the number of atoms at an atomic column from the contrast of a TEM image. 49,50 Nevertheless, the stability of the surface atoms in the CoNiCuRuPd NPs has a clear difference from the monometallic Pd NPs.
In an effort to better understand the high robustness of the HEA NPs, theoretical investigations were conducted employing cluster models. DFT calculations demonstrated that the cohesive energy (E c ) of a Co 16 Ni 15 Cu 16 Ru 16 Pd 16 HEA cluster was -3.92 eV, which was higher than the value of -3.09 eV for a Pd 79 cluster ( Figure 7A). Combining these data with molecular dynamics (MD) simulations, diffusion coe cients (D) were determined at 900 K after 0.1 ps ( Figure S15). The results demonstrated that the D values of all metals in a Co 16 Ni 15 Cu 16 Ru 16 Pd 16 HEA cluster were lower than those for the corresponding monometallic clusters (Co 79 , Cu 79 , Ni 79 , Ru 79 or Pd 79 ) ( Figure 7B). As an example, the D for Pd in a Co 16 Ni 16 Cu 15 Ru 16 Pd 16 HEA cluster was calculated to be 1.31×10 -5 m 2 /s, and so was approximately one third lower than the value of 3.43×10 -5 m 2 /s for a Pd 79 cluster. These results provide further evidence that sluggish diffusion in the HEA NPs, originating from the mixing of multiple elements as well as from lattice distortion effects, contributed signi cantly to the high resistance of the HEA NPs against the undesired irreversible agglomeration and radiation damage process.

Discussion
We succeeded in the facile low-temperature synthesis of supported HEA NPs, taking advantage of the hydrogen spillover that proceeds on TiO 2 via a coupled proton-electron transfer mechanism. Both in situ observations and theoretical simulations provided evidence that Pd 2+ ions are rst reduced by H 2 to generate nuclei, after which the dissociation of hydrogen molecules occurs to form active hydrogen atoms that enable the simultaneously reduction of neighboring precursors. A CoNiCuRuPd/TiO 2 catalyst synthesized in this manner exhibited different selectivity and signi cantly improved stability compared with Pd/TiO 2 during the hydrogenation of CO 2 . Theoretical investigations also emphasized that the sluggish diffusion in these CoNiCuRuPd HEA NPs is caused by the combination of multiple metals, and that lattice distortion plays a crucial role in the superior robustness of this material. This study demonstrates not only an ideal heterogeneous catalyst based on HEA NPs with durability that suggests potential practical applications, but also offers advanced insights into an innovative catalyst/photocatalyst architecture providing an essentially unlimited compositional space.

Materials
Rutile TiO 2 (JRC-TIO-6) was supplied by the Catalysis Society of Japan.  These analyses were performed using as-deposited samples before H 2 reduction. A temperature-programmed desorption (TPD) study using adsorbed CO was performed with a JASCO FT/IR-6600 instrument. In addition, in situ X-ray absorption fine structure (XAFS) spectra and X-ray diffraction (XRD) patterns were acquired at the 01B1 beamline station in conjunction with a Si (111) monochromator at SPring-8, JASRI, Harima, Japan (proposal numbers 2019A1048 and 2019B1091). In a typical experiment, spectra were acquired while a pellet sample was held in a batch-type in situ XAFS cell. XAFS data were processed using the REX2000 software program (Rigaku).

Computational method
Adsorption energies, E ad , were calculated using the density functional theory (DFT), employing DMol 3 program 51,52 with Materials Studio 17.2 interface. The generalized gradient approximation (GGA) exchange-correlation functional proposed by Perdew, Burke and Ernzerhof (PBE) 53 was combined with the double numerical plus polarization (DNP) basis sets. A slab consisting of a 4 × 4 surface unit cell was adopted. The slab consists of three atomic (111) layers. The geometry of bottom two layers was fixed at the corresponding bulk positions, and that of top layer and adsorbate was allowed to relax during geometry optimizations. The lattice constant to the surface normal direction was taken to 30Å including the vacuum region. E ad was defined by the equation E ad = E adsorbate/slab -(E adsorbate + E slab ), where E adsorbate/slab , E adsorbate , and E slab are the total energies of adsorbate-slab system, free adsorbate, and bare slab respectively.
Simulations for the formation mechanism of HEA NPs via H 2 spillover were performed using a TiO 2 (101) slab with 2 × 2 surface unit cell and 3-layer thickness was constructed with a vacuum thickness of 20 Å, on which a square pyramidal Pd 5 cluster was loaded. The top-layer atoms allowed to relax during geometry optimizations and the other layers fixed at the corresponding bulk positions.
For the cohesive energy (E c ) calculation of pure metal and HEA clusters composed of Co, Ni, Cu, Ru, and Pd, the plane wave based program, Castep was employed. 54,55 The PBE functional was used together with the ultrasoft-core potentials. 56 The basis set cutoff energy was set to 351 eV. The electron configurations of the atoms were Co: 3d 7 4s 2 , Ni: 3d 8 4s 2 , Cu: 3d 10 4s 1 , Ru: 4s 2 4p 6 4d 7 5s 1 , and Pd: 4d 10 . Sphere like 79 and 81 atom clusters were used for FCC and HCP metals, and the clusters were placed in a cubic cell with a side of 30 Å. For the alloy cluster preparation, Pd atoms in Pd 79 cluster were randomly replaced by Co, Ni, Cu, and Ru atoms, and Co 16 Ni 15 Cu 16 Ru 16 Pd 16 cluster was built. E c was defined by the equation E c = (E cluster -mE atom )/m, where E cluster and E atom are the total energies of the pure metal or alloy cluster, and isolated single atom, respectively. m is the total number of atoms.
DFT based molecular dynamics (MD) calculations were also performed to estimate the difference in diffusion coefficients (D) between pure metal and HEA clusters employing Castep. Firstly, the structures of pure metal and HEA clusters were optimized, and then the optimized structures were subjected to MD calculations. The conditions are NVE ensemble, 900 K, time step: 1 fs, and 100 steps. D was evaluated from the mean-square displacement according to Eq.1.
, where t 1 and t 2 are the initial and final times of simulation interval. The D was evaluated between the start and end (t 1 = 0 and t 2 =100) of simulation. The intermediate D values per each ten steps were also calculated to check the convergence of simulation.

Catalytic activity trials
The performance of each catalyst was evaluated using a fixed-bed reactor system in which a portion of catalyst (50 mg) was placed into a quartz cell with an internal diameter of 17 mm, held within an electric oven. The as-prepared catalyst was pretreated by heating at 5 °C·min −1 to 400 °C in a flow of H 2 (20 mL/min) for 2 h. The sample was subsequently exposed to a N 2 /H 2 /CO 2 mixture having a 4/5/1 composition (total flow of 50 mL·min −1 , SV = 6000 mL·g −1 ·h −1 ). Reaction products were analysed online using a gas chromatograph (Shimadzu GC-14B) equipped with an active carbon column connected to a thermal conductivity detector followed by a flame ionization detector equipped with a methanizer.

ETEM observation
The effect of electron irradiation was monitored using an ETEM apparatus (Titan ETEM G2, Thermo Fisher Scientific Inc., USA) with a Cs-corrector of the objective lens, a monochromator and a K3-IS Direct Detection camera (Gatan, Inc., USA). The accelerating voltage and electron current flux were set at 300 kV and 2 A/cm 2 , respectively. The observation in this condition does not cause serious damage to TiO 2 support. 57 The base pressure around specimen was kept below 1×10 -5 Pa.

Data Availability
All data generated and analysed during this study are included in this article and its Supplementary Information or are available from the corresponding authors upon reasonable request.