Hybrid Composite of Subnanometer CoPd Cluster ‐ Decorated Cobalt Oxide ‐ Supported Pd Nanoparticles Give Outstanding CO Production Yield in CO 2 Reduction Reaction

: Catalytic carbon dioxide (CO 2 ) hydrogenation to carbon monoxide (CO) via reverse water–gas shift (RWGS) reaction is of particular interest due to its direct use in various industrial processes as feedstock. However, the competitive CO 2 methanation process severely limits the RWGS reaction in a lower temperature range. In this context, we propose a novel nanocatalyst (NC) comprising oxygen vacancy ‐ enriched subnanometer ‐ scale CoPd hybrid cluster (CoO xV Pd) ‐ anchored Pd nanoparticles (NPs) on cobalt oxide support underneath (denoted as CP ‐ CoO xV Pd) by using a galvanic replacement reaction ‐ assisted wet chemical reduction method. As ‐ developed CP ‐ CoO xV Pd NC initiated the RWGS reaction at 423 K temperature while showing an optimum CO production yield of ∼ 3414 μ mol g − 1catalyst and a CO selectivity as high as ∼ 99% at 523 K in the reaction gas of CO 2 :H 2 = 1:3. The results of physical characterizations along with electrochemical and gas chromatography (GC) suggest that abundant oxygen vacancies in the surface ‐ anchored CoO xV Pd clusters are vital for CO 2 adsorption and subsequent activation, while neighboring Pd domains facilitate the H 2 dissociation. The obtained results are expected to provide a feasible design of Co ‐ based NCs for the RWGS reaction.


Introduction
The extensive consumption of conventional fossil fuels results in massive CO2 emissions, which later cause alarming energy and environmental issues. To this end, CO2 sequestration is crucial for global sustainability; however, technological and economic bottlenecks are prompting the scientific community to recycle CO2 into value-added fuels or chemicals [1]. CO2 is a potential one-carbon (C1) source for producing commodity fuels such as formic acid (HCOOH), methane (CH4), and carbon monoxide (CO), etc. via CO2 hydrogenation [2]. Nevertheless, CH4 is closest to meeting the commercial demands due to its well-explored industrial application [3]. Multidirectional engineering steps have been demonstrated for direct conversion of CO2 into methane via CO2 methanation (i.e., CO2 to CH4 conversion); however, due to the high activation temperature of CO2 molecules, the production yield is severely limited [4]. Alternatively, the catalytic transformation of CO2 to CO via reverse water-gas shift (RWGS) reaction has gained more attention, because unlike CO2, which is thermodynamically more stable, CO is highly active and its further conversion to value-added commodities is well known [5,6]. It is a well-known fact that the RWGS reaction is endothermic and thus more favored at higher temperatures, while the CO2 methanation is thermodynamically more favored at lower temperatures due to its exothermic nature. Therefore, it can be concluded that the competitive CO2 methanation process suppresses the CO production yield at lower temperatures [7]. Consequently, developing highly efficacious catalysts for RWGS reaction with high CO selectivity and low-temperature operation is needed.
It is an undeniable fact that understanding reaction mechanisms is imperative before designing high-performance catalysts. Therefore, the in-depth study of RWGS reaction carried out in past decades and two possible reaction mechanisms (redox and association) have been proposed [8]. The redox mechanism is two reaction steps, where hydrogen molecules (H2) dissociate in the first step to *H atoms, which further react with surface *O to create oxygen vacancies (O V ). In the second step, the CO2 molecule dissociates at the O V sites to produce *CO. The *H-assisted reduction of surface oxygen species is the ratedetermining step (RDS) in the redox mechanism [9]. Generally, the reducible oxides follow the redox mechanism [10]. On the other hand, in the associative mechanism, the adsorbed CO2 molecules react with dissociated *H atoms to form intermediate species such as carboxyl (*COOH), and formate (*HCOO), etc., finally decomposing to produce CO and H2O, where, the adsorption and dissociation, respectively, of CO2 and H2 molecules are determined as key elementary steps [11,12]. The association mechanism is mainly followed by the reducible oxide-supported noble metal nanoparticles (NPs) [13]. The aforementioned discussion suggests that catalyst material has a direct influence on the pathway of RWGS reaction. In addition, it can be concluded that two essential reaction sites for CO2 adsorption and H2 dissociation are required in an ideal RWGS reaction catalyst. It is frequently reported in the literature that noble-metal NPs, such as Pd or Pt, facilitate the H2 dissociation, while O V sites favour CO2 adsorption [14,15]. For instance, Arenas et al. elucidated the role of O V in CO2 methanation. They carried out CO2 methanation on two different catalyst systems: one with O V (Ni-CeO2) and the other without O V (Ni-Al2O3), where the O V -enriched Ni/CeO2 catalyst achieved higher methanation yield and selectivity. They demonstrated that CO2 molecules are reduced at the OV sites of CeO2 support, while H2 molecules dissociated at the metallic Ni sites [16]. Bahmanpour et al. also found that O V sites are vital for CO2 adsorption and activation [17]. Our previous works have outlined the role of synergistic collaboration between two or more reaction sites in bimetallic and trimetallic catalyst systems for CO2 methanation and RWGS reactions, respectively. Pd domains promoted CO2 dissociation and late transition metal oxides (Ni, Cu, Co) were reduced by H2 at higher temperature range [18,19].
In line with recent advancements, herein we prepared a heterogeneous nanocatalyst (NC) comprising cobalt oxide-supported Pd NPs with O V -enriched CoPd hybrid cluster (denoted CoOx V Pd) decoration (CP-CoOx V Pd) using a galvanic replacement reactionassisted wet chemical reduction method. Such a material delivers a CO production yield of ~3414 μmol g −1 catalyst with CO selectivity of ~99% at 523 K in the ambient of the CO2 and H2 mixture. Results of the physical inspections along with the electrochemical analysis ambiguously suggest that the impressive CO production yield and high CO selectivity of CP-CoOx V Pd NC can be attributed to the presence of O V reaction sites on the material's surface, which are vital for CO2 adsorption and subsequent activation. On top of that, adjacent Pd atoms facilitate the H2 dissociation. Consequently, multiple reaction sites in the CP-CoOx V Pd simultaneously facilitate all the reaction pathways.  Figure S1. For easy clarification, an HRTEM image of carbon-supported CoOx NPs (i.e., CoOx/AC) is shown in Figure S2. Accordingly, the CoOx NPs are grown with short-range ordered polycrystalline structure and can be consistently confirmed by the presence of ring-like FFT patterns [20]. Figure  1a shows an HRTEM image of bare Pd NPs (i.e., Pd/AC), where Pd NPs are grown in a drop-like structure with severe surface defects (denoted by the yellow circles in the digital image-processing results in Figure a-1) and a typical size range of ~3 to 4 nm. An even closer inspection of Figure 1a reveals that Pd NPs exhibit a long-range ordered atomic structure in three-dimensional space that is consistently confirmed with the hexagonally symmetric FFT pattern (Figure a-2). The fuzzy surface (denoted by the red triangles) can be attributed to some extent of surface oxidation in the absence of steric protection. These rationales can be further confirmed by the IFT (Figure a-3)-derived line histograms (insets) of interplanar spacing of Pd NPs at the surface, as well as subsurface regions. Accordingly, the Pd NPs exhibit random lattice spacing of 0.253 nm and 0.222 nm in the surface and subsurface regions, respectively. These observations integrally indicate a certain extent of lattice expansion at the Pd surface due to possible oxidation. Figure 1b presents the HRTEM image of CoOx-supported Pd NPs with CoPd hybrid cluster decoration (i.e., CP-CoOx V Pd NC), where it can be seen that the surface and subsurface atomic arrangements and the particle size of the Pd NPs (denoted by blue circle) are severely changed compared to that of bare Pd NPs. The size of CoOx-supported Pd NPs is increased by 30% (~4-5 nm) compared to that of Pd NPs on the carbon support (Figure 1a), and can be attributed to the limited extent of heteroatomic intermixing (i.e., CoPd alloy formation) at the CoOx-to-Pd interface, as well as surface-anchored CoPd species. These scenarios are consistently confirmed by the increased lattice spacing of 0.246 nm (i.e., lattice relaxation), which is a typical trademark of alloy formation and consistently proved by spectroscopic analysis in later sections. Of utmost importance, the reduced surface defects ( Figure 2a shows the X-ray diffraction (XRD) patterns of CP-CoOx V Pd NC and reference samples (AC, CoOx/AC and Pd/AC), where the diffraction peaks "O 1 ," "O 2 ," and "O 3 ," respectively, centered at ~8.45°, ~11.44°, and ~19.18° correspond to the carbon support. The suppressed and broad diffraction (blue lines) confirm the formation of shortrange ordered polycrystalline structure, consistent with former HRTEM observations. The Pd/AC and CP-CoOx V Pd NCs show three characteristic peak-A, B, and C-centered at about ~17.50°, 20.10°, and 28.80°. These peaks can be assigned to the diffraction signals from (111), (200), and (220) facets of metallic face-centered cubic Pd crystal [21]. Notably, compared to Pd/AC, the diffraction peaks of CP-CoOx V Pd NC are significantly broadened, showed higher background (h) and shifted to lower angles (i.e., left side) (Figure 2b), suggesting reduced coherent length (particle size), considerable out-of-phase scattering from its surface with high roughness (due to presence of decorated CoPd species) and higher lattice constant (i.e., expansive strain), respectively. These observations are in good agreement with former HRTEM findings. The presence of peak X (corresponding to the Co3O4 (311)) in the XRD pattern of CP-CoOx V Pd NC confirms that the Co is present in form of Co3O4 and exposed to the surface. In addition, the CP-CoOx V Pd NC demonstrates a suppressed peak Y, corresponding to the Co2Pd (1 0 -1 3) plane, implying the existence of subnanometer Co2Pd alloys. To get more insights into the local atomic and electronic structure of Co atoms in CP-CoOx V Pd NC, X-ray absorption spectroscopy (XAS) at Co K-edge was performed. Figure  3a shows the normalized X-ray absorption near edge structure (XANES) spectra of CP-CoOx V Pd NC compared with reference samples (CoOx/AC), while the XANES spectra of standard CoO and Co3O4 are also compared. In the Co K-edge XANES spectra, the preedge "M" and the absorption edge "N" are corresponding to the 1s-to-3d and 1s-to-4p electron transitions, respectively [22]. Generally, the pre-edge elucidates the local symmetry of target atoms. Accordingly, the suppressed pre-edge intensities of standard samples (i.e., CoO and Co3O4) suggest their octahedral Co-configuration. CoOx/AC and CP-CoOx V Pd NCs exhibit intense pre-edge intensity, suggesting tetrahedral coconfiguration (i.e., loss of O-atoms (presence of O V )) in these samples [22]. The inflection point (X) position indicates the oxidation state of the target atoms [23]. Notably, the inflection point of CP-CoOx V Pd NC undergoes the lowest energy position among the samples under investigation, implying the lowest oxidation state of Co atoms in CP-CoOx V Pd NC and therefore increased O V [24]. With the steric protection of Pd atoms to Co domains underneath, the higher white-line intensity (HM) of CP-CoOx V Pd NC compared to CoOx/AC indicates the electron relocation from Co-to-Pd atoms. The Fouriertransformed extended X-ray absorption fine structure (FT-EXAFS) analysis further confirms the existence of O V sites in CP-CoOx V Pd NC. Figure 3b shows the FT-EXAFS spectra of experimental samples at Co K-edge, while the model-simulated fitting curves are shown in Figure S3 and corresponding structural parameters summarized in Table 1 [25]. In addition, the existence of a small CN for the Co-Pd bond pair (CNCo-Pd = 0.43) reveals a certain extent of Co-to-Pd heteroatomic intermixing and is consistent with former observations.

Physical Characterizations
The results of Pd K-edge XAS analysis complementarily confirm the aforementioned scenarios. Figure 3c shows the normalized XANES spectra of experimental NCs at Pd Kedge, where the similar position of inflection point to Pd foil confirms the metallic state of Pd atoms in Pd/AC and CP-CoOx V Pd NC. The two absorption edges "A" and "B" correspond to the electron transition from 1s to unoccupied 5p and 4f orbitals, respectively [25]. Accordingly, compared to Pd/AC, the suppressed intensity of peak "A" implies a slight extent of electron relocation from Co-to-Pd atoms in CP-CoOx V Pd NC and is consistent with Co-K-edge XANES analysis. The FT-EXAFS spectra of CP-CoOx V Pd NC are shown in Figure 3d, while the corresponding structural parameters are listed in Table  1 and the fitting curves shown in Figure S4. Consistent with XRD and Co K-edge FT-EXAFS results, the presence of a small CN for the Pd-Co bond pair (CNPd-Co = 0.55) confirms the formation of subnanometer CoPd alloys CP-CoOx V Pd NC. In addition, the higher CN for the Pd-Pd bond pair compared to Pd-O complementarily proves the metallic characteristic of Pd atoms in CP-CoOx V Pd NC. These scenarios are further confirmed by wavelet transformation (WT) patterns of the EXAFS spectrum at Co ( Figure  S5) and Pd K-edges ( Figure S6) and discussed in the supplementary information. In the FT-EXAFS spectra at Co K-edge, peak Co Octa -Co Octa is the contribution of coordinated Co atoms at octahedral symmetry from the 2nd coordination shell in CoO lattice. The peak of Co Tetra -Co Octa is the contribution of coordinated Co atoms (tetrahedral symmetry) in the 2nd coordination shell of CoOx lattice. *The intensity of Pd foil is reduced by 50% for better representation. X-ray photoelectron spectroscopy (XPS) was used to elucidate the oxidation states of constituting elements and to prove the presence of O V in CP-CoOx V Pd NC. The XPS spectra of CoOx/AC and CP-CoOx V Pd NC at Co-2p orbital are, respectively, shown in Figure 4a,b, whereas the corresponding deconvolution results are shown in Table S2. Previously published literature reported that a higher Co 2+ /Co 3+ ratio implies the presence of O V [26]. Accordingly, compared to CoOx/AC (1.11), the higher Co 2+ /Co 3+ (1.29) confirms the abundant O V sites in CP-CoOx V Pd NC. Notably, with the similar Co content, a higher Co 2+ /Co 3+ ratio indicates that the majority of O V sites are present in surface-anchored CoOx V Pd clusters. The XPS spectra at the O-1s orbital further confirm these observations.  Table S2. Accordingly, the higher area under O V peak suggests the high density of O V sites in CP-CoOx V Pd NC. The XPS analysis at Pd-3d orbital confirms these characteristics. Figure S7a,b, respectively, depict the XPS spectra of Pd/AC and CP-CoOx V Pd NC at the Pd-3d orbital. Accordingly, the high Pd 0 /Pd 2+ ratio (Table S2) confirms the metallic state of Pd atoms in Pd/AC and CP-CoOx V Pd NC, which is in good agreement with XAS results. In contrast to XAS observations, the binding energies of Pd 0 and Pd 2+ are higher in CP-CoOx V Pd NC than CoOx/AC (Table S2); however, this can be attributed to the electron transfer to surface O V sites.
Cyclic voltammetry (CV) and CO-stripping analysis have been used to confirm the surface chemical identities and geometric configuration of CP-CoOx V Pd NC. As shown in Figure 5a, the two characteristic peaks O ads−1 and O ads−2 at the higher potential region in the forward sweep, respectively, correspond to the formation of Co hydroxide and oxide in CoOx/AC, while the presence of the broad O des−1 peak in the backward sweep can be assigned to the desorption of oxygenated species from these reaction sites. The smeared peak profile (H des-Pd ) in the forward sweep of the hydrogen underpotential deposition (HUPD) region confirms the high affinity towards proton adsorption for Pd/AC [27]. Notably, the CP-CoOx V Pd NC shows a similar (smeared) peak profile to Pd/AC in HUPD region and to CoOx/AC (presence of peaks O ads−1 and O ads−2 in the forward sweep and peaks O des−1 in the backward sweep) in higher potential regions, which confirms the formation of CoOx-supported Pd NPs and is consistent with our experimental design. In addition, both the Pd/AC and CP-CoOx V Pd NC show an obvious oxide reduction peak O des-Pd in the backward sweep, which corresponds to the oxide reduction from the Pd surface [28]. Figure 5b shows the CO-stripping curves of CP-CoOx V Pd NC compared with reference samples, where the flat CO-stripping curve of CoOx/AC shows its inert nature towards CO oxidation. The strong CO oxidation peak (O) indicates a high density of reaction sites for CO adsorption and its subsequent reduction on the surface of Pd/AC [29]. For CP-CoOx V Pd NC, an offset of the CO oxidation peak O* to lower potential indicates the reduced energy barrier for CO oxidation and can be attributed to the certain extent of charge localization (i.e., electron transfer) from Co-to-Pd atoms, which is consistent with former findings. The presence of a suppressed and broad peak "P" at lower potentials can be assigned to the current response due to the CO oxidation at the low-energy barrier reaction sites of the Co2Pd alloy. Of utmost importance, compared to Pd/AC, the suppression of the main CO oxidation peak "O" indicates the reduced reaction sites for CO oxidation on the surface of CP-CoOx V Pd NC and can be attributed to the presence of inert CoOx species in form of CoPd hybrid clusters on the surface of CP-CoOx V Pd NC. Based on the aforementioned physical and electrochemical results, the geometric configuration of CP-CoOx V Pd NC is proposed and represented in Figure 5c.  Inspired by the potential geometric and electronic properties, the CO2 hydrogenation performance of CP-CoOx V Pd NC was evaluated by a previously reported protocol [18,19], where the gaseous products were analyzed at ambient pressure using a gas chromatography (GC) system equipped with a PDHID detector. Figure 6a shows the CO production yield of CP-CoOx V Pd NC and reference samples in the reaction gas (CO2:H2 = 1:3), while corresponding performance parameters are listed in Table S3. Accordingly, CoOx/AC, Pd/AC, and the physical mixture of CoOx+Pd are chemically inert toward CO2 below 423 K temperature, while the CO production yield for CP-CoOx V Pd NC is 68.2 μmol g −1 catalyst at 423 K, which continuously increased with raising the temperature to 573 K, and the highest CO production yield is ~3414 μmol g −1 catalyst at 573 K. This value is ~90% and ~57% improved compared to the CoOx/AC (~348 μmol g −1 catalyst) and Pd/AC (~1466 μmol g −1 catalyst), respectively. In addition, the CP-CoOx V Pd NC achieved CO selectivity as high as ~99%. Such high activity and CO selectivity of CP-CoOx V Pd NC can be attributed to the presence of O V sites and adjacent Pd domains in subnanometer-scale CoPd clusters. Previously published literature reported that Pd atoms weakly adsorb CO2, while O V sites are favorable for adsorption and subsequent reduction of CO2 molecules [14,15]. These scenarios are consistently proved by assessing the CO2 hydrogenation performance of experimental NCs in the pure CO2 ambient. As shown in Figure S8a, both the CoOx/AC and Pd/AC are inactive toward CO2 in the pure CO2 ambient across the temperature range, while the CP-CoOx V Pd NC shows a significant CO production yield in the hightemperature range, confirming that O V sites promote CO2 activation. These observations confirm that the O V and Pd sites synergistically trigger the CO production yield of CP-CoOx V Pd NC, where O V sites and adjacent Pd domains, respectively, boost the CO2 activation and H2 dissociation. Finally, the much lower CH4 production yield in pure CO2 ambient ( Figure S8b) and the reaction gas (Figure 6b) confirms the suppression of the competitive CO2 methanation process and thus high CO selectivity. The proposed mechanism of CO2 hydrogenation on the CP-CoOxVPd NC is shown in Figure 6c.

Materials and Methods
Cobalt oxide-supported Pd NPs with O V -enriched CoPd hybrid cluster decoration were prepared via a galvanic replacement reaction-assisted wet chemical reduction method. Before synthesis, the catalyst carrier (i.e., carbon black; UR-XC72, UniRegion Bio-Tech, Taipei, Taiwan) was surface-functionalized with an adequate protocol for increasing the metal-support interaction [30]. The surface-functionalized carbon black was denoted as active carbon (AC) in the following of this article. Subsequently, 12 g (0.5 wt.% solution in D.I. water; i.e., the actual amount of AC is 60 mg) of AC was dispersed in 3.06 g of 0.1 M aqueous solution of cobalt (III) chloride (99%, CoCl3, Sigma-Aldrich Co, St. Louis, MO, USA.) and stirred at 400 rpm for 4 h at room temperature (solution A) in the 1st step. Solution A (i.e., Co 3+ adsorbed AC) contains a 30 wt.% weight ratio of Co to AC. In the 2nd step, 0.12 g of sodium borohydride (NaBH4; 99%, Sigma-Aldrich Co, St. Louis, MO, USA.) dissolved in 20 mL of D.I. water was instantly dropped into solution A and stirred at 400 rpm for 10 s to grow Co NPs on the AC surface (solution B). These Co NPs were partially oxidized in the ambient and resulting in a mixed metallic and oxide phase of Co (i.e., Co/CoOx). After that, in the 3rd step, 3.06 g of 0.1 M Pd precursor solution was mixed into solution B to grow the Pd NPs over CoOx support. In this step, the Pd 2+ ions were reduced by the excessive amount of NaBH4 added in the 2nd step. The Pd precursor solution was prepared by dissolving palladium chloride (PdCl2, 99%, Sigma-Aldrich Co, St. Louis, MO, USA.) in 1.0 M of HCl(aq). Herein, it is worth noting that the electronegativity of Pd atoms is higher than Co atoms and thus the galvanic replacement reaction between Pd 2+ ions and metallic Co atoms is obvious. In this event, subnanometer-scale CoPd hybrid clusters are formed on the surface of Pd NPs due to galvanic replacement between Pd 2+ ←→ Co followed by spontaneous deposition of residual Pd 2+ and Co 3+ ions. Generally, the galvanic replacement reaction is dominated by the electronegativity difference between two atoms. In this case, the electronegativity of Pd is higher than Co and thus, the galvanic replacement reaction between Pd 2+ ions and Co 0 is obvious. Finally, the as-prepared material was washed with acetone and D.I. water, centrifuged, and dried at 100 °C overnight. In the last step, the O V sites in the as-prepared sample were created by exposure to the ambient environment. Hereafter, the cobalt oxide-supported Pd NPs with O Venriched CoPd hybrid cluster decoration are denoted as CP-CoOx V Pd.

Conclusions
We have reported a novel heterogeneous nanocatalyst (NC) consisting of CoOxsupported Pd nanoparticles decorated with an oxygen vacancy-enriched subnanometer CoPd hybrid cluster (CP-CoOx V Pd). The prepared material actively suppressed the competitive CO2 methanation reaction and thus achieved CO selectivity as high as ~99%. CP-CoOx V Pd NC initiated the CO production at 423 K temperature and achieved an optimum CO production yield of ~3414 μmol g −1 catalyst at 573 K in the reaction gas of CO2:H2 = 1:3. The cross-referencing results of physical characterizations, electrochemical analysis, and gas chromatography (GC) indicate that such a high CO2 hydrogenation performance of CP-CoOx V Pd NC at low onset temperature originated from the synergistic cooperation between oxygen vacancies and adjacent Pd domains, where oxygen vacancies promoted the CO2 adsorption and neighboring Pd sites favored H2 splitting. In addition, the CoPd alloys at the heterogeneous interface of CoOx-to-Pd offered ideal desorption energy for subsequent reaction steps.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.