Synthesis and characterization of HEA NPs on TiO2
CoNiCuRuPd HEA NPs supported on TiO2 (CoNiCuRuPd/TiO2) were synthesized using a simple impregnation method, employing an aqueous solution of the corresponding precursors. This was followed by reduction under a H2 atmosphere at 400 °C without a specific 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, DHmix, between -11.6 and 3.2 kJ/mol.29 In the present study, the CoNiCuRuPd combination met the above criteria (δ = 3.9% and DHmix = 1.1 kJ/mol), and so the formation of solid solution CoNiCuRuPd HEA NPs was expected.
Figure 1A shows the H2 temperature programmed reduction (TPR) profiles obtained from the as-deposited mono- and quinary-component samples prior to reduction under H2. 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/TiO2 sample suggests the immediate reduction of the deposited Pd2+ precursor after the switching between H2 and Ar flows 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 ions (E0(Co2+/Co0) = -0.28 V, E0(Ni2+/Ni0) = -0.26 V, E0(Cu2+/Cu0) = +0.34 V E0(Ru3+/Ru0) = +0.46 V and E0(Pd2+/Pd0) = +0.99 V, all vs. NHE). Interestingly, the quinary-component precursors on TiO2 generated only a single reduction peak with a maximum temperature of approximately 170 °C, which was intermediate between those obtained from the mono-component samples. Thus, the reducibility of each of the Co2+, Ni2+ and Cu2+ ions was improved while those of the Ru2+ and Pd2+ ions were decreased in comparison with the monometallic samples. This simultaneous reduction of the mixed-metal precursors indicates that all atoms were undergoing interactions with one another, leading to the formation of HEA NPs containing all five elements. In contrast, the quinary-component precursors on the non-reducible supports such as MgO and Al2O3 displayed broad reduction peaks ranging from 130 to 250 °C. These reduction profiles indicated that all atoms were not interacting on the MgO and Al2O3 surfaces.
In situ X-ray absorption fine structure (XAFS) analyses conducted under a H2 atmosphere at elevated temperatures further elucidated the reduction sequence (Figures S1, S2 and 1B). X-ray absorption near-edge structure (XANES) spectra confirmed the reduction of all the precursors at 200 °C. The intermediate shapes and edge positions at the Co and Ni K-edges indicated the presence of a mixture of cations and zero valent ions at 200 °C, due to their relatively low reduction potentials. 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 K-edges. 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 fine structure (FT-EXAFS) data further clarified 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 M−O bonds with lengths of approximately 1.7–1.9 Å. In the case of Co and Ni, the peak intensity due to the M−O bonds decreased at 200 °C, whereas another peak attributed to metallic M−M bonds with longer interatomic distances appeared for Cu, Ru and Pd. These transitions demonstrated the reduction of Mn+ ions on the TiO2 support. The bond structure after completion of the reduction revealed that the interatomic metallic M−M bond lengths were significantly different from those for the corresponding bulk references, suggesting that those elements were surrounded by different metallic atoms.
The X-ray diffraction (XRD) pattern for CoNiCuRuPd/TiO2 exhibited new broad peaks at 2θ = 42.2° and 48.9º. These peaks suggest the formation of a single phase with an fcc structure having a lattice parameter (a) of 3.734 Å, which is intermediate between 3.890 Å (for fcc Pd) and 3.524 Å (for fcc Ni) (Figure 2A). No peaks attributable to pure Co, Ni, Cu, Ru or Pd were detected, establishing that these components were homogeneously dispersed in the NPs without segregation. Figures 2B and C present high-angle annular dark field scanning TEM (HAADF-STEM) images showing a lattice fringe spacing of 2.18 Å. From these images, the average size (dave) of the CoNiCuRuPd HEA NPs was estimated to be 1.90 nm (Figure 2C). The energy dispersive X-ray (EDX) maps of these specimens also confirmed 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 five elements (Figure S3). The dave values for the CoNiCuRuPd/Al2O3 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 TiO2 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 TiO2
Considering the H2-TPR and in situ XAFS results, we propose a mechanism for the formation of the HEA NPs on the TiO2 support in conjunction with hydrogen spillover (Figure 3A). In this process, under a H2 atmosphere, the Pd2+ precursors are first partially reduced to generate nuclei that appear to be stabilized at defect sites on the TiO2. Following this, H2 is dissociated on the surfaces of these Pd nuclei to form Pd−H species (Step 1). The reduction of Ti4+ to Ti3+ together with the transfer of H atoms from Pd nuclei at the metal-support interfaces (Step 2) is accompanied by the migration of electrons from Ti3+ ions to neighbouring Ti4+ ions. This promotes the subsequent simultaneous transfer of protons to O2− anions attached to these adjacent Ti4+ ions (Step 3). In this manner, the hydrogen atoms rapidly reach all metal ions by moving over the TiO2 surface (Step 4), such that these ions are all reduced at the same time to form the HEA NPs.
This mechanism based on the spillover effect was further evaluated by DFT calculations, using rutile TiO2 (101) as a model because of its thermodynamic stability and Pd5 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 TiO2 through the hydrogen spillover from Pd clusters. The resulting potential energy profile is shown in Figure 3B. Moving along this profile, the dissociation of H2 on a Pd5 cluster (Step 1) occurs with a barrier of 20.1 kcal/mol. The activation energy (Ea) associated with subsequent H atom transfer from a Pd5 cluster to a neighbouring O atom on the support (Step 2) was estimated to be 27.5 kcal/mol. Owing to the presence of oxygen sites having different coordination numbers, such as 2-coordinated oxygen (O(2)) or 3-coordinated oxygen (O(3)) sites, the migration of a H atom over the TiO2 surface (Step 3) was calculated separately for each scenario. The activation energies for the migration of a H atom from O(2) to O(2), O(2) to O(3) and O(3) to O(2) sites were determined to be 15.0, 37.4 and 12.7 kcal/mol, respectively (Figure S5). These data demonstrate that the participation of O(3) sites in the migration of H atoms over the TiO2(101) is energetically unfavourable, and so this migration preferentially occurs at O(2) sites because of the abundance of such sites on TiO2(101). The reduction of other deposited metal cations by the spilled H atoms (Step 4) was further evaluated by calculating Ea for the attack of a neighbouring H atom on an Mn+−OH species (M=Co2+, Ni2+, Cu2+, Ru3+ or Pd2+) on the support, together with the loss of H2O. These Ea values were estimated to be 12.7, 12.7, 12.6, 8.0 and 7.4 kcal/mol for Co2+, Ni2+, Cu2+, Ru3+ and Pd2+, respectively. Thus, this step had the lowest energy requirement for all cations in the overall reaction.
The dissociation energy of a gaseous H2 molecule on TiO2(101) without Pd clusters was estimated to be 82.3 kcal/mol (Figure S6). It was also calculated that the direct reduction of Co2+ on TiO2(101) by a gaseous H2 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 confirmed that spilled H atoms in the presence of Pd clusters promoted the rapid and simultaneous reduction of the multiple metal precursors at low temperatures on a thermodynamic basis. In comparison, Step 3 on hexagonal Al2O3(100) was found to be thermodynamically unfavorable, with Ea values of 29.3, 41.7 and 43.8 kcal/mol for the transfer pathways from O(2) to O(3), O(3) to O(2) and O(3) to O(3) sites, respectively (Figure S8), which were more than twice as great as those for TiO2(101). Similar calculations were also performed using the γ-Al2O3 model proposed by Digne et al., who reported an Ea for hydrogen migration (38.9 kcal/mol) that was similar to our result for hexagonal Al2O3.32,33 These results clearly suggest that H atom transfer on TiO2 was energetically more likely to proceed than that on Al2O3, hence the rate of hydrogen spillover was faster on the TiO2.
Catalytic CO2 hydrogenation
The hydrogenation of CO2 to high calorific 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 (CO2 + H2 → CO + H2O, 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 CO2 methanation reaction (CO2 + 4H2 → CH4 + 2H2O, 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 CO2 hydrogenation at temperatures from 300 to 400 ºC, with CO and CH4 as the major products (Figure 4A). CoNiCuRuPd/TiO2 gave the highest yield of hydrogenated products, which was from 2 and 13 times greater, respectively, than those obtained using MgO and Al2O3 as supports. This enhanced activity can presumably be ascribed to the formation of a quinary-component HEA NPs solely on the TiO2, as indicated by the H2-TPR data. Specifically, the quinary-component precursors on the MgO and Al2O3 displayed broad reduction peaks ranging from 130 to 250 °C, suggesting the formation of larger segregated NPs rather than smaller HEA NPs. The selectivities for CO and CH4 were also found to vary depending on the catalyst that was employed. CoNiCuRuPd/TiO2 showed relatively high selectivity for CH4 (68.3% CH4 selectivity at 400 ºC), similar to that of the Al2O3 specimen (72.1% CH4 selectivity at 400 ºC) but quite different from that obtained using MgO (75.2% CO selectivity at 400 ºC). It should also be noted that the catalytic activity of Pd/TiO2 prepared by the same method was low compared with that of CoNiCuRuPd/TiO2, and that this monometallic sample gave CO as the primary product. As shown in Figure 4B, an apparent activation energy (Ea) of 37.7 kJ/mol was obtained for CoNiCuRuPd/TiO2, which was lower than that of 44.2 kJ/mol for Pd/TiO2. 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 CO2 hydrogenation is initiated by the adsorption and activation of CO2 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 CH4. 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/TiO2 (dave= 1.90 nm) and Pd/TiO2 (dave= 2.04 nm) were similar (Figure S9), the different selectivities for CO or CH4 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/TiO2, a peak assignable to the linear stretching vibration of adsorbed CO (νCO) was observed at 2091 cm-1 in association with the initiation of CO desorption at 50 °C (Figure 5A). In contrast, this νCO peak was observed at 2070 cm-1 in the spectrum obtained from CoNiCuRuPd/TiO2. 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 CO2 hydrogenation over these materials. The adsorption sites on CoNiCuRuPd/TiO2 were definitely electron enriched compared with those on the monometallic Pd/TiO2. This, in turn, delayed the desorption of the CO intermediate owing to the stronger interactions, thus promoting subsequent hydrogenation to form CH4.42 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 configuration patterns depicted in Figure S10),44 giving an average νCO of 2079 cm-1. The adsorption energies (Ead) 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 Ea values for CO and H adsorption on CoNiCuRuPd HEA (denoted as HEAave (111) in Figure 5C) were determined to be -37.5 and -50.3 kcal/mol, respectively. The average Ea for CO adsorption on Pd(111) was substantially lower at -26.2 kcal/mol, while the Ea 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 CH4 and CO would be preferentially formed on the former and latter, respectively. It should be further noted that the average Ea for CO and H adsorption on all the pure metals (denoted as AveCoNiCuRuPd in Figure 5C) was different from the HEAave (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/TiO2, the catalytic activity during CO2 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/TiO2 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/TiO2, such that the average NP diameter was more than doubled to 5.3 nm from 2.0 nm. Conversely, CoNiCuRuPd/TiO2 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 confirmed that single NPs contained all the constituent elements.
The structural robustness of the HEA NPs was also confirmed 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/TiO2 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/TiO2, which is definitely 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 (Ec) of a Co16Ni15Cu16Ru16Pd16 HEA cluster was -3.92 eV, which was higher than the value of -3.09 eV for a Pd79 cluster (Figure 7A). Combining these data with molecular dynamics (MD) simulations, diffusion coefficients (D) were determined at 900 K after 0.1 ps (Figure S15). The results demonstrated that the D values of all metals in a Co16Ni15Cu16Ru16Pd16 HEA cluster were lower than those for the corresponding monometallic clusters (Co79, Cu79, Ni79, Ru79 or Pd79) (Figure 7B). As an example, the D for Pd in a Co16Ni16Cu15Ru16Pd16 HEA cluster was calculated to be 1.31×10-5 m2/s, and so was approximately one third lower than the value of 3.43×10-5 m2/s for a Pd79 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 significantly to the high resistance of the HEA NPs against the undesired irreversible agglomeration and radiation damage process.