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Rui Xi, Kentaro Yonesato, Takafumi Yatabe, Yoshihiro Koizumi, Soichi Kikkawa, Seiji Yamazoe, [Koji Harano](https://orcid.org/0000-0001-6800-8023), Kazuya Yamaguchi, [Kosuke Suzuki](https://orcid.org/0000-0002-8123-1462)

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[Surface‐Exposed Pd Nanocluster Confined within a Ring‐Shaped Polyoxometalate for Selective Hydrogenation](https://mdr.nims.go.jp/datasets/0653f46f-4e9d-4e2b-84b5-1b288e676cd0)

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Surface‐Exposed Pd Nanocluster Confined within a Ring‐Shaped Polyoxometalate for Selective HydrogenationRESEARCH ARTICLEwww.advancedscience.comSurface-Exposed Pd Nanocluster Confined within aRing-Shaped Polyoxometalate for Selective HydrogenationRui Xi, Kentaro Yonesato, Takafumi Yatabe, Yoshihiro Koizumi, Soichi Kikkawa,Seiji Yamazoe, Koji Harano, Kazuya Yamaguchi, and Kosuke Suzuki*Developing efficient catalysts for selective hydrogenation of moleculesbearing multiple reducible functional groups remains a major challenge.Palladium (Pd) nanoclusters are promising candidates owing to their strongH2 activation ability, broad substrate compatibility, and unique surfaceproperties. However, the controlled synthesis of small Pd nanoclusters withaccessible, coordinatively unsaturated active sites remains difficult as they areprone to aggregation. In this study, a strategy is presented to fabricatesurface-exposed Pd nanoclusters confined within a ring-shapedpolyoxometalate (POM) via a mild solid-state reduction process (1 atm H2,≈25 °C). The resulting Pd nanocluster exhibits exceptional chemoselectivityin the hydrogenation of multifunctional substrates by preferentially adsorbingC═C and C≡C bonds on its discrete, exposed Pd surface with a well-definedcoordination environment. Importantly, the rigid POM frameworkconsiderably stabilizes Pd nanocluster, enabling excellent reusability overmultiple catalytic cycles. This study demonstrates a molecular templatingapproach for constructing robust and chemoselective metal nanoclustercatalysts, offering new opportunities in the design of hydrogenation systems.1. IntroductionSelective hydrogenation of molecules containing multiple re-ducible functional groups is a cornerstone of modern chemi-cal manufacturing, playing a crucial role in the production ofR. Xi, K. Yonesato, T. Yatabe, Y. Koizumi, K. Yamaguchi, K. SuzukiDepartment of Applied ChemistrySchool of EngineeringThe University of Tokyo7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JapanE-mail: ksuzuki@appchem.t.u-tokyo.ac.jpS. Kikkawa, S. YamazoeDepartment of ChemistryGraduate School of ScienceTokyoMetropolitanUniversity1-1MinamiOsawa,Hachioji, Tokyo 192-0397, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/advs.202509418© 2025 The Author(s). Advanced Science published by Wiley-VCHGmbH. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properly cited.DOI: 10.1002/advs.202509418petrochemicals, pharmaceuticals, and finechemicals.[1] However, achieving highchemoselectivity in such transformationsis still a longstanding challenge. Undesiredoverhydrogenation and side reactions re-duce product yields and complicate down-stream purification, thereby increasingenergy consumption and environmen-tal burden. Therefore, the developmentof heterogeneous catalysts that combinehigh activity with precise chemoselectivityis of central importance for sustainablecatalysis.[2] Palladium (Pd) nanoparticleson supports are widely employed cata-lysts in hydrogenation reactions owingto their strong ability to activate molec-ular hydrogen (H2) and their affinity forvarious organic substrates, enabling highactivity under mild conditions.[3,4] Never-theless, achieving high chemoselectivityremains difficult. This is largely attributedto the polydispersity and low stability ofPd nanoparticles, which often results inaggregation under reaction conditions.Therefore, nonspecific adsorption of functional groups on ex-tended Pd surfaces frequently leads to overhydrogenation andpoor selectivity (Figure 1a). To overcome these issues, vari-ous strategies have been developed.[5] These include molecu-lar poisoning (e.g., CO, thiols, and amines) to block adjacentK. HaranoCenter for Basic Research on MaterialsNational Institute for Materials Science (NIMS)1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanK. HaranoResearch Center for Autonomous Systems Materialogy (ASMat)Institute of Integrated ResearchInstitute of Science Tokyo4259 Nagatsuda-cho, Midori-ku, Yokohama, Kanagawa 226-8501, JapanK. SuzukiDepartment of Advanced Materials ScienceGraduate School of Frontier SciencesThe University of Tokyo5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, JapanAdv. Sci. 2025, 12, e09418 e09418 (1 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbHhttp://www.advancedscience.commailto:ksuzuki@appchem.t.u-tokyo.ac.jphttps://doi.org/10.1002/advs.202509418http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadvs.202509418&domain=pdf&date_stamp=2025-07-28www.advancedsciencenews.com www.advancedscience.comFigure 1. Schematic representation of the proposed synthesismethod: a) supported Pd nanoparticle catalyst for hydrogenation reactions, b) ring-shapedpolyoxometalate ([P8W48O184]40−, P8W48), and c) preparation of surface-exposed Pd nanocluster catalyst using P8W48 and its selective hydrogenationability through selective adsorption of functional groups.Pd atoms,[6] incorporation of secondary metals to deactivate un-selective Pd sites,[7] and alloying to electronically modulate orgeometrically isolate Pd atoms.[7] A classic example is Lind-lar’s catalyst (Pd/CaCO3–Pb with quinoline), which employs sec-ondarymetal addition andmolecular poisoning to achieve alkynesemihydrogenation.[8] However, these approaches frequently suf-fer from a trade-off: enhancing selectivity often comes at the ex-pense of catalytic activity owing to excessive surface blocking.[9]To overcome this limitation, an ideal catalyst should pos-sess a well-defined Pd architecture with coordinatively unsatu-rated active sites that are accessible and structurally stabilizedagainst aggregation. Metal nanoclusters represent a promisingplatform in this context owing to their discrete electronic struc-tures and tunable reactivity.[10,11] However, their high surface en-ergy renders them thermodynamically unstable, often leadingto aggregation or deactivation. Although stabilization with or-ganic ligands can suppress aggregation,[10c,11c,12] it often comesat the expense of catalytic activity by blocking active sites. Im-mobilization on solid supports can improve thermal stabilityand help to preserve coordinatively unsaturated metal sites, butachieving precise control over cluster size, structure, and elec-tronic state remains challenging—especially for inherently un-stable metal clusters.[13] Therefore, the development of stablePd nanocluster catalysts with well-defined structures and acces-sible, coordinatively unsaturated active sites continues to be akey objective in catalyst design. Polyoxometalates (POMs), an-ionic metal-oxide clusters with well-defined structures and di-verse properties,[14] have emerged as promising molecular tem-plates for constructing metal clusters.[15] In particular, lacunaryPOMs, which contain vacant sites with reactive oxygen atoms,can coordinate metal ions in specific geometries, enabling elec-tronic modulation and structural stabilization.[16,17] These fea-tures allow them to stabilize metal nanoclusters and modu-late their electronic and catalytic properties.[18] Among variousPOM architectures, the ring-shaped [P8W48O184]40− (P8W48), atetramer of the hexavacant Dawson-type [P2W12O48]14−, offers aunique cavity (≈1 nm in diameter) surrounded by multiple coor-dination sites, allowing selective encapsulation and stabilizationAdv. Sci. 2025, 12, e09418 e09418 (2 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 2. a) Anion structure of Pd8 (disordered Pd sites). b) Photographs of Pd8 before and after H2 treatment at room temperature (≈25 °C) for 30min.of metal species.[19] Our group has recently demonstrated thatP8W48 can serve as a versatile platform for synthesizing surface-exposed Ag nanoclusters,[20] Au–Ag alloy nanoclusters,[21] andCu nanoclusters[22] with active surfaces and excellent catalyticperformance and stability.Herein, we report the synthesis of a small, surface-exposed Pdnanocluster confined within the P8W48 framework and its appli-cation as a chemoselective heterogeneous hydrogenation catalyst(Figure 1b). The nanocluster, denoted as Pd8-H2, was preparedvia mild solid-state reduction of a P8W48-incorporated Pd2+ pre-cursor (Pd8) under 1 atm H2 at room temperature (≈25 °C). Insitu X-ray absorption spectroscopy (XAS) revealed a reversible in-terconversion between the Pd2+ complex and the Pd0 nanoclus-ter. Notably, Pd8-H2 features a surface-exposed, discrete Pd siteswith well-defined coordination environment, enabling efficientand highly selective hydrogenation of substrates bearingmultiplereducible groups—for example, the selective conversion of cin-namaldehyde (1a) to hydrocinnamaldehyde (2a), and 1-ethynyl-4-nitrobenzene (1e) to 4-ethylnitrobenzene (2e). Remarkably, thestructural rigidity of the P8W48 framework provides exceptionalthermal and chemical stability, enabling reusability over multiplecatalytic cycles. These findings highlight the potential of POM-based molecular templating to achieve surface control and supe-rior chemoselectivity, offering a powerful strategy for designingnext-generation nanocluster catalysts for diversemolecular trans-formations.2. Results and Discussion2.1. Synthesis of a Small Pd Nanocluster within P8W48The highly reactive O atoms within the P8W48 cavity act asanchoring sites for metal ion coordination.[19–22] To incorporatePd2+ ions, the tetra-n-butylammonium (TBA+) salt of P8W48(TBA-P8W48) was reacted with Pd acetate in acetone at roomtemperature (≈25 °C) over 3 d (see Supporting Informationfor experimental details). Subsequent addition of diethyl etheryielded an orange crystalline product, denoted Pd8, in 37% yield(Figure S1a, Supporting Information). Elemental analysis con-firmed the incorporation of eight Pd2+ ions with the P8W48framework. Single-crystal X-ray diffraction (SCXRD) further re-vealed coordination of Pd2+ ions to the inner O atoms of the cav-ity, although positional disorder was evident (Figures 2a and S2a,and Table S1, Supporting Information).Initial attempts to synthesize Pd nanoclusters within P8W48involved reducing Pd8 in various organic solvents, such as ace-tone, acetonitrile, and N,N-dimethylformamide, using H2 orTBA borohydride (TBABH4) as reducing agents. However, thesesolution-phase reductions consistently yielded black Pd precipi-tates (Figure S3, Supporting Information). Elemental analysis ofthe resulting crystals revealed that only two Pd atoms per P8W48unit remained, indicating that the reduced Pd species had beeneluted from the framework followed by aggregation in the solu-tion. To circumvent this issue, a solid-state reduction approachwas employed.[22] Exposure of Pd8 to H2 gas (1 atm) at roomtemperature (≈25 °C) led to a rapid color change from orangeto blackish (Figure 2b), indicative of Pd2+ reduction to Pd0. Re-crystallization of the resulting blackish solid (Pd8-H2) in a mix-ture of acetone and diethyl ether under an Ar atmosphere yieldedblackish single crystals. Elemental analysis confirmed the reten-tion of all eight Pd atoms within the P8W48 framework, demon-strating that solid-state reduction effectively suppressed Pd elu-tion and aggregation. These findings suggest that Pd nanoclus-ter formation occurred in situ within the P8W48 cavity duringthe solid-state reduction, whereas solution-phase reduction likelyfacilitated Pd detachment through solvent interactions, leadingto uncontrolled aggregation. Thus, solid-state reduction was es-sential for the controlled synthesis of Pd nanoclusters confinedwithin the P8W48 framework.X-ray photoelectron spectroscopy (XPS) of Pd8 revealed twodistinct peaks at 336.8 eV (Pd 3d5/2) and 342.0 eV (Pd 3d3/2), con-sistent with the presence of Pd2+ species (Figure 3a). Follow-ing H2 treatment, the corresponding peaks in Pd8-H2 shifted tolower binding energies—335.7 eV (Pd 3d5/2) and 340.9 eV (Pd3d3/2)—indicating the reduction of Pd2+ to Pd0 (Figure 3b). Thisassignment was further corroborated by Pd K-edge X-ray absorp-tion near-edge structure (XANES) analysis. The Pd8-H2 spec-trum exhibited a diminished white-line intensity (≈24356 eV)and a negative edge energy shift relative to Pd8, closely resem-bling that of Pd foil (Figure 3c).Infrared (IR) spectra of P8W48, Pd8, and Pd8-H2 in the1200–500 cm−1 range confirmed that the P8W48 framework re-mained structurally intact following H2 reduction (Figure S4,Supporting Information). Extended X-ray absorption fine struc-ture (EXAFS) analysis provided further insights into the localcoordination environment of the Pd species. The R-space EX-Adv. Sci. 2025, 12, e09418 e09418 (3 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 3. XPS spectra of a) Pd8 and b) Pd8-H2. c) Pd K-edge XANES spectra, d) R-space Pd K-edge EXAFS spectra, and e) curve−fitting results of Pd8,Pd8-H2, Pd8-H2a, and Pd foil. Fitting parameters of the Pd–Pd interactions (R range = 2.10–2.86 Å and back k range = 3–14 Å−1; Pd foil, R range =1.90–2.85 Å). aPd8-H2 after recrystallization.bCoordination number. f) CO-DRIFTS spectrum of Pd8-H2.AFS spectra exhibited a transition from predominant Pd–O co-ordination in Pd8 to Pd–Pd interactions in Pd8-H2 (Figure 3d),consistent with the formation of metallic Pd clusters. Curve-fitting analysis yielded a Pd–Pd coordination number (CN) of1.8± 0.2 for Pd8-H2—significantly lower than that of bulk Pdfoil (10.6 ± 0.2; Figure 3e) and commercial Pd nanoparticlessupported on carbon (Pd/C, 5.1 ± 0.2; Table S2, SupportingInformation)—supporting the formation of a small Pd nan-ocluster. Notably, the XANES and EXAFS spectra of recrystal-lized Pd8-H2 (CN = 1.9± 0.3) closely matched those of the as-reduced sample, indicating that both the electronic state andcluster size were preserved during recrystallization under an Aratmosphere (Figure 3c–e). SCXRD analysis of Pd8-H2 revealedelectron density consistent with Pd atoms localized at the cen-ter of the P8W48 cavity. No significant electron density corre-sponding to Pd was detected outside the framework, further sub-Adv. Sci. 2025, 12, e09418 e09418 (4 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 4. STEM images of Pd8-H2. a) ADF-STEM image of an aggregate of Pd8-H2 particles on an amorphous carbon film. b) A magnified view of asingle Pd8-H2 particle, showing an encapsulated Pd cluster in the P8W48 cavity (yellow arrow). c) EDS elemental maps of P, Pd, and W. Scale bars: 5 nmfor a, 1 nm for b,c.stantiating the confinement of the Pd nanocluster within theP8W48 cavity (Figure S2b and Table S1, Supporting Informa-tion).To further confirm the spatial confinement of Pd within theP8W48 cavity, atomic-resolution scanning transmission electronmicroscopy (STEM) was performed. Although individual POMmolecules are typically challenging to visualize due to their sen-sitivity to electron beam damage, careful control of beam inten-sity during annular dark-field (ADF) STEM imaging enabled theobservation of a bright contrast feature located within the centralring of the P8W48 framework (Figures 4a,b and S5, SupportingInformation). This contrast is attributed to a Pd species encap-sulated within the cavity. Notably, such atomic-resolution STEMimaging of an isolated POM molecule—without the aid of en-capsulation in carbon nanotubes[23]—is exceedingly rare. To fur-ther validate the presence and localization of Pd, STEM–energy-dispersive X-ray spectroscopy (EDS) elementalmapping was con-ducted. Although the higher beam intensity required for EDSled to some degradation of the molecular structure, the elemen-tal maps clearly showed co-localization of Pd, P, and W withina ≈2 nm region (Figure 4c), providing strong evidence for theformation of a Pd nanocluster confined within the ring-shapedP8W48 framework.The surface structure of heterogeneous catalysts critically in-fluences their catalytic behavior. Carbon monoxide (CO) is com-monly employed as a probemolecule due to its strong adsorptionon metal surfaces and its ability to yield distinct IR signaturesthat reflect local surface environments.[24] Diffuse reflectance IRFourier transform spectroscopy (DRIFTS) of Pd8-H2 after COexposure at room temperature revealed two sharp absorptionbands at 2060 and 1930 cm−1, corresponding to linearly andbridge-bonded CO on Pd, respectively (Figure 3f).[24b] The domi-nance of the linear CO band indicates a high proportion of low-coordination Pd surface sites, a hallmark of small Pd nanoclus-ters. Notably, the absence of a band near 1850 cm−1—typicallyassociated with tri-bonded CO species—suggests the lack of ex-tended Pd surfaces, clearly distinguishing Pd8-H2 from conven-tional Pd nanoparticles.[24b,d,e] The sharpness and symmetry ofthe CO stretching bands further suggest a uniform distributionof surface-active sites. These results collectively indicate that Pd8-H2 possesses a discrete, surface-exposed Pd nanocluster with awell-defined coordination environment, potentially contributingto its unique catalytic properties.2.2. Structural Transformation of Pd8 into Pd8-H2The structural transformation of Pd8 into Pd8-H2 under H2was monitored using a range of in situ characterization tech-niques. Solid-state ultraviolet–visible (UV–vis) spectra of Pd8under H2 flow at 50 °C revealed gradually intensified broadabsorption bands around 650 and 1000 nm, corresponding toW(VI)/W(V) intervalence charge transfer within the P8W48framework (Figure 5a). This behavior suggests partial reductionof W(VI) to W(V). Consistent with this, the W L3-edge XANESspectra showed a slight shift of the white line (10209 eV) towardlower energy (Figure 5b), further supporting the partial reduc-tion of W(VI). Importantly, both the W L3-edge k-space and R-Adv. Sci. 2025, 12, e09418 e09418 (5 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 5. a) In situ diffuse reflectance solid-state UV–vis spectra of Pd8 under H2 gas flow at 50 °C; the interval of each measurement was 2 min. b) Insitu W L3-edgeXANES spectra, c) in situ k3-weighted k-space, and d) R-space W L3-edge EXAFS spectra of Pd8 with a H2 flow at 50 °C; the interval ofeach measurement was 7 min.space EXAFS spectra exhibited no significant structural changesduring the H2 treatment (Figure 5c,d), confirming the preserva-tion of the P8W48 framework. These results indicate that P8W48serves as a robust inorganic protective ligand, facilitating the for-mation of metal nanoclusters under reductive conditions.Interestingly, upon exposure to air at room temperature, thecolor of Pd8-H2 gradually shifted from blackish to brown, and re-verted to blackish upon H2 treatment (Figure S6, Supporting In-formation), indicating reversible changes in the oxidation state ofthe Pd species. To further investigate this behavior, in situ Pd K-edge XAS studies were performed under alternating H2 and O2flow at 50 °C. The Pd K-edge XANES spectra revealed a time-dependent decrease in white-line intensity (≈24356 eV) and acorresponding increase in pre-edge peak intensity (≈24340 eV),both of which reached completion within 10 min (Figure 6a), in-dicating the rapid reduction of Pd2+ into Pd0. The alternating H2and O2 flows demonstrated the reversible redox behavior of Pd.Notably, the XANES spectrum and white-line intensity after O2exposure slightly differed from those of the pristine Pd8, sug-gesting a subtle modification in the electronic states or coordi-nation environments (Figure 6b). Despite repeated cycles, bothPd K-edge XANES and EXAFS spectra of the final state remainedunchanged (Figure S7, Supporting Information), confirming thestability of the reduced state and structural integrity of the Pdnanocluster (Figure 6c). Crystallographic analysis of the reoxi-dized product, Pd8* (obtained via exposing a Pd8-H2 single crys-tal to O2), demonstrated the disappearance of central electrondensity and re-coordination of all Pd atoms to O atoms within theP8W48 framework (Figure S2c and Table S1, Supporting Infor-mation). Although the Pd atoms in Pd8* were severely disorder-ing over multiple positions, the analysis showed that the severalPd sites differed from those in the pristine Pd8, which accountingfor the slight differences observed in the XANES spectra. Theseresults underscore the role of P8W48 as a nanoreactor, facilitat-ing reversible structural transformations of Pd while preservingits confinement within the cavity.2.3. Selective HydrogenationChemoselective hydrogenation of 𝛼,𝛽-unsaturated aldehydes andketones remains a significant challenge due to the concurrentpresence of reducible C═O and C═C functional groups. Al-though Pd catalysts typically exhibit higher reactivity toward C═Cbond hydrogenation, selectively inhibiting C═Ohydrogenation isdifficult, particularly when the C═C bond bears substituents.[25]To evaluate the catalytic performance of Pd8-H2 for hydrogenat-ing 𝛼,𝛽-unsaturated carbonyl compounds, cinnamaldehyde (1a)was chosen as a model substrate (Table 1). When Pd8-H2 wasused as a heterogeneous catalyst in diethyl ether under a 1-atmAdv. Sci. 2025, 12, e09418 e09418 (6 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 6. a) Schematic of reversible structural transformations. b) In situ Pd K-edge XANES spectra of Pd8 under alternating H2 and O2 flow at 50 °C.c) Overall time profile of the normalized absorption intensity (Norm. Abs.) at energy in the pre-edge region (24339 eV).Table 1. Hydrogenation of 1a using different catalysts.Entry Catalyst Conversion [%] Yield [%]1a 2a 3a 4a1 Pd8-H2 >99 92 5 n.d.2 No catalyst <1 n.d. n.d. n.d.3 TBA-P8W48 <1 n.d. n.d. n.d.4 Pd-SiW9 5 n.d. n.d. n.d.5 Pd/C >99 60 31 n.d.6 Lindlar’s catalyst 20 17 2 n.d.7 Pd8 71 66 3 n.d.(Reaction conditions: 1a (0.5 mmol), catalyst (Pd: 0.5 mol%), diethyl ether (3 mL), biphenyl (0.25 mmol), room temperature (≈25 °C), H2 (1 atm), 18 h. Conversions andyields were determined by gas chromatography (GC) using biphenyl as an internal standard (n.d. = not detected).H2 atmosphere at room temperature (≈25 °C), selective hydro-genation of the C═C bond occurred, yielding hydrocinnamalde-hyde (2a) with 92% yield (Table 1, entry 1). Minimal amountsof C═O-reduced products, such as hydrocinnamic alcohol (3a)and cinnamyl alcohol (4a), were detected. In contrast, no reac-tion was observed in the absence of a catalyst (Table 1, entry 2),and TBA-P8W48 alone exhibited no catalytic activity (Table 1, en-try 3), confirming that the Pd species within the P8W48 frame-work serve as the active sites. Previously reported Pd nanoparti-cles stabilized by a lacunary POM [SiW9O34]10− (Pd-SiW9, d: ≈2.4 nm), synthesized by our group,[17c] exhibited negligible activ-ity (Table 1, entry 4), likely due to POMcoverage of the Pd surface,which hindered substrate access. A commercial Pd/C catalyst (d≈ 2.7 nm),[26] displayed high activity but poor selectivity, yield-ing 2a in 60% yield and 3a in 31% upon complete conversionof 1a (Table 1, entry 5). Lindlar’s catalyst (Pd/CaCO3–Pb, with-out quinoline treatment) showed high selectivity toward 2a (90%selectivity) but significantly lower catalytic activity (17% yield of2a) (Table 1, entry 6), likely due to surface poisoning and low Pdatom utilization efficiency. These results underscore the criticalimportance of exposed Pd surfaces in catalytic activity, as passi-vated Pd particles result in poor performance. In contrast, thesuperior performance of Pd8-H2 can be attributed to its smallcluster size and discrete, accessible surface.When Pd8 was used for the reaction under the conditions de-scribed in Table 1, the reaction mixture gradually changed fromAdv. Sci. 2025, 12, e09418 e09418 (7 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comFigure 7. a) Leaching and b) recyclability test of the Pd8-H2 catalyst in hydrogenation of 1a. Reaction conditions: 1a (0.5 mmol), Pd8-H2 (Pd: 0.5 mol%),diethyl ether (3 mL), biphenyl (0.25 mmol), room temperature (≈25 °C), H2 (1 atm), 19 h. Conversions and yields were determined by GC using biphenylas an internal standard.yellow to blackish over 2 h. Although Pd8 exhibited selectiv-ity similar to Pd8-H2, the conversion of 1a was notably lower(Table 1, entry 1 vs entry 7). Reaction profiles indicated a higherinitial rate for Pd8-H2, whereas Pd8 exhibited an induction pe-riod, likely due to the in situ reduction of Pd2+ into Pd0 (FigureS8, Supporting Information). Following this induction period,the reaction rates for both catalysts converged, suggesting theformation of the same active Pd nanocluster in situ. ReoxidizedPd8* also catalyzed the reaction, displaying a shorter inductionperiod than Pd8-H2 (Figure S8, Supporting Information), whichwas consistent with the in situ XANES results indicating a morerapid reduction to Pd0 (Figure 6b). After the induction period,the reaction rate and selectivity for 2a closely aligned with thoseof Pd8-H2, demonstrating the structural robustness and redox re-cyclability of Pd8-H2.The heterogeneous nature of Pd8-H2 during hydrogenationof 1a under the conditions outlined in Table 1 was further ex-amined. Filtration of the catalyst at approximately 30% conver-sion quenched the reaction, with no additional hydrogenationobserved under identical conditions (Figure 7a). Inductively cou-pled plasma-atomic emission spectroscopy confirmed that no de-tectable Pd species were present in the filtrate (below the detec-tion limit), further confirming the truly heterogeneous nature ofPd8-H2. The reusability of Pd8-H2 was then assessed. After thereaction, the catalyst was recovered by filtration, washed with di-ethyl ether, and reused in successive reaction cycles. The recov-ered Pd8-H2 consistently maintained its catalytic performancefor the hydrogenation of 1a over multiple cycles (Figure 7b).Moreover, the selectivity toward 2a remained stable throughoutthe reaction cycles, indicating the structural integrity of the Pdnanocluster. This stability was additionally supported by time-profile data, which showed consistent selectivity during the re-action with the reused catalyst (Figure S9, Supporting Informa-tion).The integrity of the P8W48 framework after catalysis was con-firmed byWL3-edge XAS and IR spectroscopy, which revealed nosignificant spectral changes compared to fresh Pd8-H2 (FiguresS10 and S11, Supporting Information). Further investigation ofthe structure and electronic state of the Pd nanocluster aftercatalysis was conducted using XPS, XANES, and EXAFS anal-yses. The XPS spectrum of the recovered catalyst after hydro-genation of 1a (Pd8-H2-AF) showed only a minor shift in the Pd3d5/2 binding energy from 335.6 eV (fresh Pd8-H2) to 335.5 eV(Pd8-H2-AF), indicating minimal change in the electronic state(Figure 8a,b). Similarly, the Pd K-edge XANES spectra of Pd8-H2-AF and fresh Pd8-H2 were nearly identical (Figure 8c). Minorchanges in the oscillation patterns of the Pd K-edge k-space EX-AFS spectra suggested slight atomic rearrangement (Figure 7d).Additionally, the Pd–Pd CN of 2.2 ± 0.3 in Pd8-H2-AF confirmedthe retention of the small Pd nanocluster within the P8W48framework (Figure 8e and Table S2, Supporting Information).These results collectively demonstrate the exceptional structuraland electronic stability of Pd8-H2 under catalytic conditions.To investigate the origin of the differing reactivity betweenPd8-H2 and Pd/C, we examined the hydrogenation of 2a. When2a was hydrogenated using Pd8-H2, the C═O bond remained in-tact with less than 1% conversion (Table S3, Supporting Infor-mation, entry 1). Similarly, Pd/C produced only a trace amountof 3a (3% yield) by the hydrogenation of 2a (Table S3, Support-ing Information, entry 2). These results suggest that 3a doesnot form through sequential overhydrogenation of 2a. Indeed, 4awas not detected during the reaction (Figure S8, Supporting In-formation), and when 4a was subjected to the same conditions,it quickly converted to 3a (Table S4, Supporting Information).These observations indicate that 4a is the intermediate for form-ing 3a and that the selectivity toward 2a or 3a is determined bythe hydrogenation of the C═C or C═O bond of 1a, respectively(Scheme 1).In the hydrogenation of chalcone (1b), a representative𝛼,𝛽-unsaturated ketone, Pd8-H2 exclusively produced dihy-drochalcone (2b) without reducing the C═O bond, whereasPd/C predominantly yielded over-hydrogenated product 1,3-diphenylpropan-1-ol (3b) (Scheme 2a). Notably, even after pro-longed reaction times, Pd8-H2 retained its selectivity and did notproduce 3b, while Pd/C promoted the overhydrogenation of 2bto 3b (Table S5, Supporting Information). By contrast, in the hy-drogenation of 2-nonenal (1c) and 2-cyclohexen-1-one (1d), bothPd8-H2 and Pd/C selectively hydrogenated the C═C bond to af-ford nonanal (2c) and cyclohexanone (2d), respectively, withoutoverhydrogenation (Scheme 2b,c and Tables S6 and S7, Support-ing Information). These results suggest that the presence of aphenyl group influences product selectivity.Adv. Sci. 2025, 12, e09418 e09418 (8 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comScheme 1. Reaction pathways for the hydrogenation of 1a using Pd8-H2or Pd/C.The adsorption mode of 𝛼,𝛽-unsaturated aldehydes and ke-tones on Pd surfaces is crucial for determining hydrogenationselectivity.[27] Pd K-edge XANES spectra indicated that Pd8-H2and Pd/C share similar electronic states, as evidenced by theiroverlapping white-line peaks (≈24356 eV) (Figure S12a, Support-ing Information,). Curve-fitting analysis of EXAFS data revealedidentical Pd–Pd distances (2.75 Å) in both catalysts; however,Pd/C exhibited a higher CN (5.1 ± 0.2; Figure S12b,c and TableS2, Supporting Information), consistent with its larger particlesize (2.7 nm).[26] These structural differences significantly influ-ence the adsorption behavior on the Pd surfaces. Previous studieshave shown that the 𝜂4 adsorption mode (binding both C═C andC═Obonds) is themost stable configuration for cinnamaldehydeon a Pd(111) surface,[27] facilitating hydrogenation of both func-tional groups. The phenyl ring further stabilizes this 𝜂4 configu-ration through 𝜋 interactions with the extended flat Pd surface,leading to the abovementioned poor selectivity in the hydrogena-tion of 1a and 1b by Pd/C (Figure S13a, Supporting Information).In contrast, the discrete surface structure of Pd8-H2 suppresses𝜂4 adsorption and favors 𝜂2 adsorption of the C═C bond, result-ing in selective hydrogenation of the C═C bond (Figure S13b,Supporting Information). Density function theory studies haveshown that phenyl ring adsorption is suppressed on ultrasmallPd nanoclusters.[28] For 1c, the flexible alkyl chain destabilizes𝜂4 adsorption, making 𝜂2 adsorption favorable even on extendedPd surfaces.[27] As a result, both Pd/C and Pd8-H2 exhibit simi-lar selectivity for the saturated aldehyde 2c in the hydrogenationof 1c (Figure S13c,d, Supporting Information). These findingsunderscore the effects of differences in Pd surface structures onadsorption behavior and hydrogenation selectivity in substratescontaining aromatic groups.The hydrogenation of 1-ethynyl-4-nitrobenzene (1e) was inves-tigated further to evaluate the chemoselectivity of Pd8-H2. Under1-atm H2 in diethyl ether, Pd8-H2 selectively hydrogenated theC≡C bond of 1e to yield 4-nitrostyrene (2e’), which was furtherreduced to 4-ethylnitrobenzene (2e) (Figure 9a). Notably, over-hydrogenated products resulting from nitro group hydrogena-tion, such as 4-ethylaniline (3e), were scarcely detected even af-ter complete conversion of 1e. In contrast, when Pd/C was usedunder identical conditions, the nitro group was rapidly hydro-genated, leading to the selective formation of 3e after 2e’ and2e intermediate formation (Figure 9b). Similar to the selectiveFigure 8. XPS spectra of a) Pd8-H2 and b) recovered Pd8-H2 after the hydrogenation of 1a (Pd8-H2-AF). c) Pd K-edge XANES spectra, d) Pd K-edgek-space, and e) R-space EXAFS spectra of Pd8-H2 and Pd8-H2-AF.Adv. Sci. 2025, 12, e09418 e09418 (9 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comScheme 2. Hydrogenation of a) 1b, b) 1c, and c) 1d using Pd8-H2 and Pd/C. Reaction conditions: substrates (0.5 mmol), catalyst (Pd: 0.5 mol%), diethylether (3 mL), room temperature (≈25 °C), H2 (1 atm), 9 h. n.d. = not detected.hydrogenation of 1a, this difference in selectivity may again beattributed to the phenyl group of 2e. After full conversion of 1einto 2e, the planar interaction between the phenyl ring and theextended Pd surface of Pd/C facilitates the adsorption of 2e, pro-moting its further hydrogenation to form 3e (Figure S13e, Sup-porting Information). In contrast, the discrete surface of Pd8-H2 interacts weakly with the phenyl ring, enabling rapid des-orption of 2e from active sites and preventing overhydrogenation(Figure S13f, Supporting Information). These findings highlightthe unique surface characteristics of Pd8-H2, which favor selec-tive adsorption and hydrogenation of C≡C and C═C bonds, ac-counting for its distinctive catalytic behavior.3. ConclusionIn this study, we developed a strategy to fabricate a surface-exposed, small Pd nanocluster confined within the cavity of aring-shaped POM [P8W48O184]40−. The synthetic approach in-volved preparing a Pd2+ precursor (Pd8), followed by solid-Figure 9. Time profiles of the hydrogenation of 1e catalyzed by a) Pd8 and b) Pd/C. Reaction conditions: 1e (0.5 mmol), catalyst (Pd: 0.5 mol%), diethylether (3 mL), biphenyl (0.25 mmol), room temperature (≈25 °C), H2 (1 atm).Adv. Sci. 2025, 12, e09418 e09418 (10 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.comwww.advancedsciencenews.com www.advancedscience.comstate reduction using H2 gas under mild conditions (1 atm H2,≈25 °C) to form the Pd nanocluster Pd8-H2. Comprehensivecharacterization using SCXRD, XPS, XAS, CO-DRIFTS, atomic-resolution STEM, and UV–vis confirmed that Pd8-H2 consistsof a small Pd nanocluster with an exposed, discrete metal sur-face. This unique surface structure enabled Pd8-H2 to functionas a highly selective heterogeneous catalyst. It demonstrated ex-ceptional chemoselectivity in the hydrogenation of cinnamalde-hyde to hydrocinnamaldehyde and 1-ethynyl-4-nitrobenzene to4-ethylnitrobenzene, outperforming commercial Pd/C catalysts.Recyclability studies and post-reaction analyses demonstrated thehigh structural and electronic stability of the Pd nanocluster dur-ing catalysis. The rigid POM molecular templates not only pro-vided a confined nanospace for cluster formation but also im-parted enhanced thermal and chemical robustness of the result-ing catalyst. This strategy offers a promising approach for design-ing advanced metal nanocluster catalysts, providing precise sur-face control and superior selectivity for diverse molecular trans-formations.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThe authors gratefully acknowledge the financial support from JSTFOREST (JPMJFR213), JST CREST (JPMJCR20B2), JSPS KAKENHI(JP24K01448, JP24H02210, JP24H02217, JP24K01279, JP23K13812,JP22H04971). A part of this work was supported by the Electron Mi-croscopy Unit, National Institute for Materials Science (NIMS). Theauthors thank Dr. Koji Kimoto and Dr. Fumihiko Uesugi (NIMS) fortheir support in STEM imaging and analysis. Single-crystal X-ray crys-tallographic analysis and X-ray absorption spectroscopy measurementswere conducted at SPring-8 with the approval of the Japan SynchrotronRadiation Research Institute (proposal numbers: 2023B1842, 2024A1880,2024B1868, 2022B1860, 2023A1732, 2023B1651, 2024B1879, 2023B2070,2024A1775, 2024B1775).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the Sup-porting Information of this article.Keywordsheterogeneous catalysis, palladium nanoclusters, polyoxometalates, se-lective hydrogenationsReceived: May 23, 2025Revised: June 27, 2025Published online: July 28, 2025[1] a) H. Storch, Chem. Rev. 1941, 29, 483; b) H. Blaser, C. Malan, B.Pugin, F. Spindler, H. Steiner, M. Studer, Adv. Synth. Catal. 2003, 345,103; c) X. Li, P. Jia, T. Wang, ACS Catal. 2016, 6, 7621; d) H. Blaser,H. Steiner, M. Studer, ChemCatChem 2009, 1, 210.[2] a) G. Vilé, D. Albani, N. 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Xia, S. Zhang, F. Liu, Y. Ma, Y. Qu, C. Wu, J. Phys. Chem. C 2019,123, 2182.Adv. Sci. 2025, 12, e09418 e09418 (12 of 12) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH 21983844, 2025, 38, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202509418, Wiley Online Library on [14/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advancedscience.com Surface-Exposed Pd Nanocluster Confined within a Ring-Shaped Polyoxometalate for Selective Hydrogenation 1. Introduction 2. Results and Discussion 2.1. Synthesis of a Small Pd Nanocluster within P8W48 2.2. Structural Transformation of Pd8 into Pd8-H2 2.3. Selective Hydrogenation 3. Conclusion&#x00A0; Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords