Rare earth valve manipulates dual regulation of electronic states and adsorption geometry in the selective hydrogenation of ethynylbenzene

Abstract

The semi-hydrogenation of ethynylbenzene is a fundamental reaction in the synthesis of polymer precursors. However, achieving a balance between catalytic activity and styrene selectivity remains a significant challenge due to the risk of over-hydrogenation. Herein, we design ternary Pt_2−xCo_xCe rare earth alloys to synergistically regulate electronic states and adsorption geometries, thereby enhancing selective hydrogenation. The optimized Pt_1.5Co_0.5Ce catalyst exhibits remarkable performance, achieving 98.3% conversion of ethynylbenzene, 85.1% selectivity for styrene, and a turnover frequency (TOF) of 1549.6 h⁻¹ under mild conditions, surpassing most reported Pt-based catalysts. In situ spectroscopy, combined with kinetic analysis, demonstrates that the catalyst facilitates the adsorption and conversion of ethynylbenzene. Density functional theory (DFT) calculations reveal that directional electron transfer from Ce (4f) to Pt/Co (5d) via d–f orbital coupling effectively modulates the position of the Pt d-band center, weakening the over-adsorption of the intermediate styrene while preserving optimal activation of ethynylbenzene. Additionally, the large ionic radius of Ce spatially alters the adsorption configuration of styrene, reducing its adsorption energy and increasing the energy barrier to suppress ethylbenzene formation. This study illustrates that rare earth alloy engineering is a universal strategy to address the activity–selectivity trade-off in heterogeneous catalysis.

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New concepts

This work demonstrates a rare earth (RE) alloys manipulation strategy to address the persistent activity–selectivity trade-off in heterogeneous catalysis, where RE elements function as dynamic reaction modulators rather than passive components. Unlike existing RE element-modified catalysts that are limited to electron transfer, our PtCoCe alloy employs Ce's 4f–5d orbital coupling as an electronic switch to optimize the position of Pt d-band center, while the ionic radius serves as a geometric regulator that sterically distorts intermediates. Crucially, although the oxophilicity of RE elements typically hinders alloy synthesis, we leverage the negative formation energy of RE alloys to enhance surface stabilization. By capitalizing on this synergistic regulation, the catalyst exhibits exceptional semi-hydrogenation performance of ethynylbenzene under mild conditions, effectively overcoming the inherent reactivity–selectivity trade-off associated with precious metal systems. These breakthroughs establish that (1) 4f orbitals can couple with d orbitals of transition metals to govern bond activation pathways; (2) ionic radii provide atomic-scale steric control that extends beyond traditional strain effects; and (3) the integration of RE elements introduces two new tunable parameters (orbital coupling and spatial constraint) for precise catalyst design, thereby opening new avenues for manipulating multistep reactions beyond hydrogenation.

Introduction

As a cornerstone of industrial chemistry, styrene supports the synthesis of polymers, pharmaceuticals, and engineered pigments.^1–5 However, styrene derived from naphtha cracking typically contains about 1% ethynylbenzene, which poisons the polymerization catalysts and reduces the quality of polymers.^6–8 Consequently, the selective hydrogenation of ethynylbenzene to styrene is of great importance and has attracted particular attention in both the academia and chemical industry, especially relevant to commodity chemical production.^9–11 Nevertheless, ethynylbenzene could bind strongly to catalyst surfaces via its triple bond, causing high surface coverage that blocks active sites, limiting H₂ activation and slowing reaction kinetics.^12,13 Furthermore, the strong alkene adsorption affinity on the catalyst surface extends intermediate residence time, inevitably leading to over-hydrogenation and thereby suffering from the inherent activity–selectivity trade-offs.^14–16 Therefore, the rational design of selective ethynylbenzene hydrogenation catalysts with high activity, selectivity and stability has become the focus of attention.

It is difficult to achieve high selectivity for styrene due to the fact that the olefin product (styrene) is more prone to undergo further hydrogenation reactions than the alkene substrate (ethynylbenzene).^17,18 To optimize catalytic performance for the highly challenging semi-hydrogenation of ethynylbenzene to styrene, the surface architecture of catalysts requires meticulous design to either lower binding energy/surface coverage or spatially isolate adsorbed molecules, thereby suppressing undesirable side reactions. Various selective regulation strategies have been developed to address this limitation. For instance, poisoning or ligand modification by adding toxic substances (Pb in Lindlar catalysts) or organic ligands can generate a “site-blocking” effect, reducing contiguous active sites and inhibiting over-hydrogenation.^19,20 Additionally, support engineering involves using supports with oxygen vacancies (CeO₂ support) or nitrogen-doped carbon materials to modulate metal–support interactions and enhance preferential adsorption of ethynylbenzene over styrene.^21,22 Nevertheless, maintaining the environmental compatibility and long-term stability of catalysts through controlled carrier sintering or ligand leaching during reaction processes remains a significant challenge. Recent advances in catalyst design have focused on more fundamental tailoring of electronic and geometric structures to enhance activity and selectivity in hydrogenation catalysis.^23–29 Electronic modulation through noble-transition metal alloys can modulate electronic properties, creating isolated active sites with tailored adsorption strengths. For instance, alloying Pt with transition metal Cd generates reconstructed Pt–Cd surfaces that shift the d-band center downward compared to pure Pt, weakening adsorption interactions and enhancing styrene selectivity.³⁰ Geometric engineering strategies, such as anchoring cobaltocene cobalt organometallic fragments (CpCo⁻) on Pt/SiO₂ interfaces, induce interfacial charge transfer to form electron-rich Pt sites, which suppress over-hydrogenation.³¹ The strategic regulation of transition metal alloying in Pt-based catalysts is a critical way for optimizing both electronic and geometric configurations, so that the adsorption strength and spatial distribution of reaction intermediates can be precisely controlled. Consequently, developing tailored alloy architectures to enhance hydrogenation selectivity and activity represents a pivotal topic in selective hydrogenation catalysis.

Rare earth (RE) elements have emerged as pivotal components in advanced catalysis due to their special 4f electron configuration, which can modulate the interaction between active sites and intermediates as electron buffer in specific catalytic systems.^32–34 The unique physicochemical properties of RE elements, including large ionic radii, flexible coordination environments, and variable valence states, endow them with unparalleled capabilities as catalytic promoters.^35–37 Recent research advances demonstrate that regulation of RE 4f electrons can substantially enhance activity and selectivity. Specifically, constructing a d–f electron transition ladder or gradient 3d–2p–4f electronic unit can both facilitate charge transfer and optimize the intermediate adsorption, thereby enhancing the OER activity.^38,39 In addition, engineering d–f orbital coupling to modify electronic structures and surface-bound intermediates, leading to an optimized catalytic selectivity, and promoting 4f electron delocalization synergistically optimize adsorption energetics of reactive intermediates to reduce activation barriers.^40–42 Crucially, RE alloys enable precise tuning of d–f orbital coupling strength while maintaining robustness under harsh catalytic conditions.^43,44 Building on these advancements, we propose that the rational integration of 4f electron characteristics through the tailored RE alloy catalyst design could release novel electronic synergies in selective hydrogenation reactions, thereby developing high-performance and low-cost alternatives to traditional noble metal systems.

Guided by these principles, we firstly develop ternary Pt–Co–Ce RE alloys that integrate 4f orbital coupling with geometric modulation. The prepared catalyst achieves an exceptional balance between catalytic activity and styrene selectivity. Under mild conditions, the ethynylbenzene conversion rate is 98.3%, the styrene selectivity is 85.1%, and the turnover frequency (TOF) is 1549.6 h⁻¹, surpassing most reported Pt-based catalysts. Through synergistic electronic–geometric design, in situ characterization and theoretical analyses reveal a dual-regulation mechanism. Directional electron transfer via d–f coupling modulates the Pt d-band center to differentially activate reactants while suppressing intermediate over-adsorption. Meanwhile, the large ionic radius of RE elements induces steric distortion that alters the geometry of intermediates, reducing adsorption energy and elevating the energy barrier for ethylbenzene formation, thereby breaking the activity–selectivity trade-off. The demonstrated electronic–geometric cooperativity opens avenues for the utilization of 4f orbital characteristics in heterogeneous catalysis, particularly for reactions requiring simultaneous control of adsorption energetics and spatial configurations.

Results and discussion

The ternary Pt_2−xCo_xCe alloy was prepared using the sodium vapor reduction method as shown in Fig. 1a and the ESI.† Among them, chloroplatinic acid hexahydrate, cerous chloride, cobalt chloride hexahydrate, cyanamide, and carbon black were used as metal precursors, dispersant, and carriers, respectively. During thermal activation, CN₂H₂ acts as a bifunctional agent that not only facilitates the formation of a g-C₃N₄ network to confine nanoparticle growth but also creates a reducing atmosphere to accelerate reduction kinetics.^45,46 The Pt_2−xCo_xCe nanoalloy was obtained by annealing the mixture with Na vapor at 600 °C for 2 h in an argon atmosphere. The resulting mixture was washed with isopropanol to remove residual sodium metal and then acid-etched to obtain the carbon-supported Pt_2−xCo_xCe catalyst (referred to as Pt_2−xCo_xCe/C, x = 0, 0.2, 0.5, 0.8, and 1.0). Furthermore, by adjustment of the Co/Pt ratio in the raw materials, the stoichiometric ratio of Co in Pt_2−xCo_xCe nanoparticles (NPs) could be controlled. The Pt/C, PtCo/C (at a Pt/Co molar ratio of 1 : 1), and catalyst were also prepared for comparison using the same conditions, except that no RE elements were added. The Pt/Co/Ce molar compositions of the synthesized samples were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Table S1, ESI†).

Fig. 1 (a) Schematic illustration of the synthesis process for Pt_2−xCo_xCe/C. (b) XRD patterns of Pt_2−xCo_xCe/C samples. (c) Rietveld refinement for the XRD pattern of Pt_1.5Co_0.5Ce/C. (d) Atomic crystal structures of Pt₂Ce and Pt_1.5Co_0.5Ce/C. (e) HRTEM image of Pt_1.5Co_0.5Ce/C. (f) The corresponding FFT and inverse FFT patterns of a₁ (area 1) and a₂ (area 2) in (e). (g) Intensity profiles for the white area in (f). (h) STEM-EDX spectra and (i) elemental mapping images of Pt_1.5Co_0.5Ce/C, showing the distributions of Pt (green), Co (pink), and Ce (blue).

The crystalline phase structure of the synthesized catalysts was characterized by X-ray diffraction (XRD) (Fig. 1b). For Ce-free samples, Pt/C exhibits diffraction peaks matching the face-centered cubic (fcc) Pt reference (PDF #04-0802), while PtCo/C is consistent with PtCo (PDF #65-8970). The Ce-containing Pt₂Ce/C exhibits distinct peaks at 19.9°, 32.9°, 38.7°, 40.5°, and 47.1°, indexed to the (111), (220), (311), (222), and (400) planes of cubic Pt₂Ce (PDF #17-0010), indicating the formation of Pt–Ce alloys.⁴⁷ The XRD patterns of different Pt_2−xCo_xCe/C catalysts show that all of the diffraction peaks are close to those observed for Pt₂Ce, revealing the successful formation of a trimetallic PtCoCe/C alloy structure. In addition, the diffraction peaks of Pt_2−xCo_xCe/C shift slightly to higher diffraction angles compared to those of Pt₂Ce/C (Fig. S1, ESI†), which indicates lattice contraction due to the introduction of Co atoms in the crystal framework.⁴⁸ The structure evolution is further confirmed by Rietveld refinement of the XRD patterns (Fig. 1c and Fig. S2, ESI†). The quantitative lattice parameters (Table S2, ESI†) reveal a reduction in unit cell volume from 445.9 ų (Pt₂Ce) to 439.0 ų (Pt_1.5Co_0.5Ce/C), directly evidencing the lattice distortion induced by Co incorporation. The refinement results explicitly demonstrate that Co partially replaces Pt at the tetrahedral sites of the alloy framework, while Ce atoms remain anchored at the eight corner positions of the unit cell (Fig. 1d). The morphology and dispersion of the obtained Pt, PtCo and Pt_2−xCo_xCe alloy NPs were systematically analyzed by transmission electron microscopy (TEM) analysis. As shown in Fig. S3 (ESI†), the metal nanoparticles over all catalysts are highly dispersed on the carbon support without aggregation. The metal particle size of the Pt_1.5Co_0.5Ce/C is 6.62 nm, which is similar to those of Pt/C, PtCo/C, Pt_1.0Co_1.0Ce/C, Pt_1.2Co_0.8Ce/C, Pt_1.8Co_0.2Ce/C and Pt₂Ce/C catalysts (particle sizes mainly range from 6.62 to 6.72 nm). Furthermore, the Brunauer–Emmett–Teller (BET) surface areas of Pt/C, Pt₂Ce/C, Pt_1.5Co_0.5Ce/C, Pt_1.0Co_1.0Ce/C, and PtCo/C are 373.20 m² g⁻¹, 350.53 m² g⁻¹, 346.39 m² g⁻¹, 329.72 m² g⁻¹, and 349.41 m² g⁻¹, respectively (Fig. S4 and Table S3, ESI†). These similar BET surface areas confirm the uniformity of metal particle dispersion, indicating that the sizes of metal particles in each sample are statistically comparable. This morphological consistency across the alloy series eliminates particle shape/size effects as confounding variables, enabling focused investigation of the composition-dependent structure–activity relationships in RE alloy systems. The high-resolution TEM (HRTEM) and scanning TEM energy-dispersive X-ray spectroscopy (STEM-EDX) results corroborate the successful synthesis of Pt₂Ce and PtCo alloy phases, as evidenced by lattice fringe matching to reference and uniform elemental distribution (Fig. S5 and S6, ESI†). For the Pt_1.5Co_0.5Ce catalyst, the HRTEM image reveals high crystallinity with ordered lattice arrangements (Fig. 1e), verified by the corresponding fast Fourier transform (FFT) patterns (Fig. 1f). Furthermore, the inverse FFT (IFFT) patterns and the corresponding integrated pixel intensities display the lattice spacings of 0.215 and 0.223 nm which correspond to the (222) and (311) planes of Pt₂Ce, respectively (Fig. 1g). Meanwhile, these spacings are slightly smaller than that of Pt₂Ce, which is owing to the lattice shrinkage caused by the difference in ionic radii between Pt (0.62 Å) and Co (0.54 Å),⁴⁹ in agreement with XRD patterns. The atomic ratio of Pt/Co/Ce in Pt_1.5Co_0.5Ce/C is determined to be 1/0.32/0.65 by STEM-EDX (Fig. 1h), showing stoichiometric consistency with ICP-OES measurements. Moreover, the elemental mappings of the Pt_1.5Co_0.5Ce/C catalyst provide the additional confirmation of uniform distribution of Pt, Co and Ce elements throughout the entire NPs, thereby supporting the formation of trimetallic PtCoCe catalysts (Fig. 1i).

To elucidate the structure–activity relationship of the Pt_2−xCo_xCe/C alloy system (x = 0.2, 0.5, 0.8, and 1.0), we systematically evaluated the catalytic performance of the catalysts in ethynylbenzene semi-hydrogenation by using NH₃BH₃ as a hydrogen donor under mild conditions (1.00 mmol ethynylbenzene, 4.00 mL of ethanol, 323.00 K, 3.00 mg of catalyst, and 2.50 mmol mL⁻¹ NH₃BH₃) (Fig. 2a). A volcano-shaped activity trend emerged with Co content variation (Fig. S7, ESI†), where Pt_1.5Co_0.5Ce/C (x = 0.5) achieved conversion above 98% within 5 h. The time-dependent conversion profiles (Fig. 2b) further demonstrate the superior activity of Pt_1.5Co_0.5Ce/C, significantly outperforming Pt₂Ce/C (66.0%), PtCo/C (55.9%), and Pt/C (48.1%) under the same conditions. The enhanced catalytic activity of Pt_1.5Co_0.5Ce/C highlights an optimized balance of geometric and electronic effects conferred by controlled Co incorporation, combined with the synergistic interplay of Pt, Co, and Ce within the alloy structure. In addition, the effect of the NH₃BH₃ amount (0.41–0.33 mmol mL⁻¹) on the ethynylbenzene hydrogenation performance is also discussed (Fig. S8, ESI†), and the optimal addition amount is 2.50 mmol mL⁻¹. Meanwhile, solvent screening identifies ethanol as optimal for maximizing both conversion and styrene selectivity over Pt_1.5Co_0.5Ce/C (Table S4, ESI†). Notably, Pt_1.5Co_0.5Ce/C maintains over 85% selectivity of styrene with high ethynylbenzene conversion at 5 h (Fig. 2c), outperforming Pt/C (61.6%), PtCo/C (26.4%), and Pt₂Ce/C (81.8%) while suppressing ethylbenzene formation. This contrasts sharply with the rapid selectivity degradation observed in PtCo/C (reduced by 53.6%) and Pt/C (reduced by 18.7%) under identical conditions, directly evidencing Ce's critical role in inhibiting over-hydrogenation through electronic and structural modulation. The catalytic performance of Pt_1.5Co_0.5Ce/C is characterized by a product distribution dominated by styrene, achieving an 80.0% yield with a remarkable productivity rate of 230.1 mmol g⁻¹ h⁻¹, with minimal ethylbenzene formation (Fig. 2d and e). The catalytic efficiency was further quantified through intrinsic reaction rate calculations and turnover frequency (TOF) (Fig. 2f), and Pt_1.5Co_0.5Ce/C delivers a hydrogenation reaction rate of 193.9 mmol g⁻¹ min⁻¹. Simultaneously, the catalyst exhibits an ultrahigh TOF value of 1549.6 h⁻¹, which was 4.1, 2.4, and 2.0 times higher than that of Pt/C (379.3 h⁻¹), PtCo/C (640.0 h⁻¹), and Pt₂Ce/C (769.8 h⁻¹), respectively. Recyclability tests under optimal conditions demonstrate outstanding stability, with Pt_1.5Co_0.5Ce/C retaining >95% conversion and >85% styrene selectivity over 10 cycles (Fig. 2g). The XRD pattern of the used Pt_1.5Co_0.5Ce/C exhibits high crystallinity, suggesting no significant structural changes (Fig. S9, ESI†). The TEM image shows that the NPs are uniformly dispersed within the used Pt_1.5Co_0.5Ce/C (Fig. S10, ESI†), with a particle size distribution of approximately 6.87 nm. Furthermore, distinct lattice fringes corresponding to the (311) crystal plane are clearly observed in HRTEM images. Elemental mapping analysis shows that the constituent elements are uniformly distributed, suggesting no detectable phase separation. These observations collectively demonstrate the prominent stability of RE alloy catalysts. Significantly, the catalytic performance of Pt_1.5Co_0.5Ce/C is higher than that of most platinum group metal-based catalysts reported thus far (Fig. 2h and Table S5, ESI†). The outstanding performance stems from the synergistic interplay of the optimized composition and structure. Specifically, Co and Ce incorporation modulates the electronic properties of Pt active sites, enhancing selective hydrogenation of the C≡C bond in ethynylbenzene to the C=C bond in styrene. Additionally, the presence of Ce promotes the stability and dispersion of the active metal sites, further contributing to the high selectivity and productivity of the catalyst.

Fig. 2 Catalytic performance for ethynylbenzene hydrogenation over Pt/C, PtCo/C, Pt_1.5Co_0.5Ce/C, and Pt₂Ce/C catalysts. (a) Schematic of the ethynylbenzene hydrogenation reaction. (b) Time-dependent conversion of the hydrogenation of ethynylbenzene. Reaction conditions: 1.00 mmol ethynylbenzene, 4.00 mL of ethanol, 323.00 K, 3.00 mg of catalyst, and 2.50 mmol mL⁻¹ NH₃BH₃. (c) Styrene selectivity and (d) styrene yield with time on stream. (e) Production rate of styrene at 5 h. (f) TOF and reaction rate values of ethynylbenzene hydrogenation during the initial 20 min. (g) The stability test of Pt_1.5Co_0.5Ce/C for 5 h. (h) Comparison of Pt_1.5Co_0.5Ce/C and other reported Pt-based catalysts for the selective hydrogenation of ethynylbenzene (Table S5, ESI†).

To further elucidate the superior catalytic performance of the Pt_1.5Co_0.5Ce/C catalyst in ethynylbenzene hydrogenation, systematic kinetic analyses were performed. Initial reaction rate data collected at different temperatures facilitated the construction of Arrhenius plots (Fig. 3a and Fig. S11, ESI†), which illustrate the ln(rate) versus 1/T relationship.⁵⁰ Arrhenius analysis reveals a substantial reduction in apparent activation energy (E_a) from 30.79 kJ mol⁻¹ (Pt/C) to 14.82 kJ mol⁻¹ (Pt_1.5Co_0.5Ce/C), demonstrating effective energy barrier lowering through rare-earth alloy engineering. Notably, the binary PtCo/C exhibits an intermediate E_a (26.01 kJ mol⁻¹), highlighting the critical role of Ce in modulating the energy landscape of the rate-limiting step.^51,52 To investigate the factors governing the reaction pathway in the hydrogenation reaction, subsequent evaluation of reaction orders was conducted through ethynylbenzene concentration variation (Fig. S12, ESI†), which demonstrates distinct adsorption characteristics.^51,53 The calculated reaction orders of 0.26 (Pt/C), 0.22 (PtCo/C), and 0.17 (Pt_1.5Co_0.5Ce/C) suggest progressively stronger substrate adsorption on the Pt_1.5Co_0.5Ce/C RE alloy catalyst surface (Fig. 3b), thereby enhancing activation efficiency and pathway selectivity. To clarify the mechanistic origin of the enhanced selectivity in Pt_1.5Co_0.5Ce/C, we performed comparative hydrogenation tests using styrene as a probe reaction. As shown in Fig. 3c, the conversion rate reveals a 3.1-fold lower conversion over the Pt_1.5Co_0.5Ce/C catalyst (24.7%) compared to PtCo/C (76.2%), demonstrating significant suppression of styrene hydrogenation on the ternary RE alloy catalyst. This significant difference highlights the key role of Ce integration in reducing the adsorption strength of styrene intermediates and effectively inhibiting their excessive hydrogenation to ethylbenzene. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) was employed to dynamically monitor the ethynylbenzene hydrogenation processes over Pt_1.5Co_0.5Ce/C, PtCo/C, and Pt/C catalysts. For Pt_1.5Co_0.5Ce/C, the characteristic C≡C stretching vibration of ethynylbenzene at 2150 cm⁻¹ gradually disappears with increasing reaction time,⁵⁴ accompanied by the emergence of a C=C stretching band (1630 cm⁻¹) within 5 minutes,^55,56 confirming rapid and selective hydrogenation to styrene (Fig. 3d, d₁ and d₂). In contrast, PtCo/C exhibits sluggish C≡C bond activation coupled with rapid attenuation of the styrene signal (C=C), indicative of inefficient ethynylbenzene conversion followed by over-hydrogenation to ethylbenzene (Fig. 3e, e₁ and e₂). Notably, the Pt/C catalyst displays negligible activity, with neither C≡C consumption nor C=C formation observed, reflecting its limited capacity for both hydrogen dissociation and product accumulation (Fig. 3f, f₁ and f₂). These spectroscopic results conclusively demonstrate that the ethynylbenzene hydrogenation process proceeds most efficiently on the Pt_1.5Co_0.5Ce/C catalyst, with PtCo/C and Pt/C exhibiting progressively inferior catalytic performance. To further clarify the diversity of the hydrogenation performance of Pt_1.5Co_0.5Ce/C, we expanded the range of reaction substrates under the same ethynylbenzene semi-hydrogenation conditions (Fig. S13, S14 and Table S6, ESI†), demonstrating the wide applicability in hydrogenation reactions of other functional groups.

Fig. 3 (a) Apparent activation energies (E_a) for the hydrogenation of ethynylbenzene and styrene over Pt/C, PtCo/C, Pt_1.5Co_0.5Ce/C, and Pt₂Ce/C. (b) Fitting lines with the reaction order of ethynylbenzene over Pt/C, PtCo/C, and Pt_1.5Co_0.5Ce/C. (c) Product distribution of the styrene hydrogenation reaction under the conditions of styrene solution over PtCo/C and Pt_1.5Co_0.5Ce/C. Spectra and contour maps of (d, d₁ and d₂) Pt_1.5Co_0.5Ce/C, (e, e₁ and e₂) PtCo/C, and (f, f₁ and f₂) Pt/C recorded during hydrogenation of ethynylbenzene at 353.00 K.

X-ray photoelectron spectroscopy (XPS) was conducted to elucidate the electronic interaction and charge transfer mechanism within PtCo/C, Pt₂Ce/C and Pt_2−xCo_xCe/C RE alloy systems. The Pt 4f XPS spectra of these catalysts can be deconvoluted into two sets of doublet peaks, corresponding to the 4f_7/2 and 4f_5/2 electronic orbitals of Pt⁰ and Pt²⁺ (Fig. 4a). Notably, compared to the Pt 4f binding energy of Pt/C (71.8 eV for 4f_7/2), the binding energies of PtCo/C, Pt₂Ce/C, Pt_1.8Co_0.2Ce/C, and Pt_1.5Co_0.5Ce/C exhibited negative shifts of 0.2, 0.2, 0.3, and 0.4 eV, respectively, demonstrating that a large number of electrons are transferred to Pt sites in the alloy.⁵⁷ This systematic decrease in binding energy indicates a progressive electron enrichment at Pt sites, attributed to electron donation from less electronegative Co (electronegativity = 1.88) and Ce (1.12) to Pt (2.28) within the alloy systems,^58,59 where Pt_1.5Co_0.5Ce/C exhibits the highest electron density at Pt sites. Typically, Pt²⁺ gradually converted to Pt⁰ species with Co and Ce incorporation, giving rise to the highest Pt⁰ fraction of ≈71.6% in Pt_1.5Co_0.5Ce/C (Fig. 4b). The Co 2p XPS spectra of PtCo/C, Pt_1.5Co_0.5Ce/C and Pt_1.8Co_0.2Ce/C can be divided into three distinct groups of peaks attributed to the 2p_3/2 and 2p_1/2 orbitals of Co⁰ and Co²⁺ and the satellite peaks (Fig. 4c). The PtCo/C displays three components with the binding energies of Co⁰ (779.1 eV for 2p_3/2 and 794.6 eV for 2p_1/2), Co²⁺ (782.1 eV for 2p_3/2 and 797.9 eV for 2p_1/2), and satellite peaks. It is worth noting that the binding energy of Co⁰ in Pt_1.5Co_0.5Ce/C shows a negative shift of 0.5 eV (778.6 eV) compared to that of PtCo/C, and the Co⁰ fraction is approximately 58.9% (Fig. 4d), suggesting electron accumulation at Co sites. This is in contrast to PtCo/C, where weaker Co–Pt interactions restrict the redistribution of electrons, while the ternary alloy system facilitates enhanced charge transfer through synergistic Co–Ce–Pt interactions, thereby improving catalytic activity.^60,61 The Ce 3d spectra (Fig. 4e) exhibit spin–orbit split peaks at 904.8 eV (Ce 3d_3/2) and 886.5 eV (Ce 3d_5/2), characteristic of metallic Ce⁰ (splitting energy = 18.3 eV) in these alloys.⁶² This splitting value aligns with that of metallic cerium, confirming the dominant metallic valence state of Ce in XPS. Additionally, the binding energy of the Ce 3d_5/2 in Pt_1.5Co_0.5Ce/C (886.7 eV) and Pt_1.0Co_1.0Ce/C (887.0 eV) exhibits positive shifts of +0.2 and +0.5 eV compared to Pt₂Ce/C (886.5 eV). This is due to the electron transfer from Ce atoms to the neighboring Pt/Co atoms, resulting in a decrease of electron density around Ce. Two additional low-intensity peaks at 901.5 eV (Ce 3d_3/2) and 883.2 eV (Ce 3d_5/2) may be caused by the transfer of electrons from Ce to Pt and Co. The XPS valence band spectra of PtCe and PtCoCe catalysts show that the trend of electron density intensity near the Fermi level (E_F) for the samples is Pt₂Ce/C > Pt_1.5Co_0.5Ce/C > Pt_1.0Co_1.0Ce/C, with Pt₂Ce/C exhibiting the highest value (Fig. S15, ESI†). Meanwhile, the d-band of PtCoCe catalysts is altered by valence electrons near E_F (Fig. S16, ESI†). These results highlight the critical role of Ce in increasing electron density at Pt/Co sites and enhancing electron conduction. These observations corporately reveal that there is a significant electronic effect in the PtCoCe alloy, where Ce acts as a primary electron donor to both Pt and Co. The XPS comparative analysis of the states of the catalyst before and after the reaction shows that the electronic structure of the elements remains essentially unchanged (Fig. S17, ESI†). This electronic reconstruction establishes a multistage charge transfer pathway, which correlates with the observed catalytic enhancement. The schematic atomic graph of a representative crystal structure (Fig. 4f) illustrates that the ternary PtCoCe alloy catalytic system employs a rare-earth electronic valve effect (manipulated by Ce), which facilitates directional electron transfer through interfacial charge redistribution. Synergistic electronic coupling between Ce, Co and Pt induces a multistage charge redistribution pathway (Ce–Co–Pt), where Ce acts as an electronic modulator and enhances interfacial electron transfer, optimizing the electronic configuration of Pt. This not only accelerates H₂ dissociation via electron-rich Pt but also suppresses over-hydrogenation by reducing the binding energy of styrene intermediates, thereby improving selectivity. Subsequent analysis further proved the mechanism.

Fig. 4 (a) Pt 4f, (b) amount of different Pt species, (c) Co 2p, (d) amount of different Co species, and (e) Ce 3d XPS spectra of samples. (f) Schematic illustrations of reaction pathways for hydrogenation of ethynylbenzene over the ternary PtCoCe RE alloy. Differential charge density of (g) PtCo and (h) PtCoCe slab models. The yellow and blue isosurfaces present electron accumulation and donation, respectively. (i) The Barder charge of atoms from various structures.

Density functional theory (DFT) calculations were performed to elucidate the intrinsic mechanism underlying enhanced ethynylbenzene hydrogenation performance. The representative Pt, PtCo, and PtCoCe slab models were constructed (Fig. S18, ESI†). The charge density difference analysis of PtCo and PtCoCe alloys further revealed the synergistic electronic coupling between Co and Ce, which collectively modulates the electronic configuration of Pt active sites (Fig. 4g, h and Fig. S19, ESI†). Specifically, Barder charge analysis identified a charge transfer from Co to Pt in the PtCo system (Fig. 4i), where Pt atoms accumulated −0.30 |e⁻|, while adjacent Co atoms exhibited electron depletion (+0.29 |e⁻|). In contrast, the introduction of Ce into the ternary PtCoCe alloy causes a more pronounced charge polarization. Here, Ce acts as the primary electron donor (+1.05 |e⁻|), transferring electrons to both surface Pt (−0.76 |e⁻|) and Co (−0.19 |e⁻|) atoms.

The projected density of states (PDOS) analysis illustrates the pivotal role of Ce in tailoring the electronic structure across alloy configurations (Fig. 5a–c). In the Pt-dominated system (Pt-TDOS), the valence region near the Fermi level (E_F) is predominantly governed by Pt-d orbitals. Upon introducing Co, hybridization between Pt-d and Co-d orbitals emerges, signifying the charge redistribution. In the ternary PtCoCe system, Ce-4f orbitals generate a pronounced electron density near E_F, characterized by sharp, overlapping peaks that enhance interorbital coupling and facilitate efficient electron transfer. Concurrently, the Pt-d band undergoes substantial broadening, while the Co-d orbital splits into dual peaks, reflecting synergistic d–f orbital interactions. This f-orbital dominance near E_F optimizes the d-band center through efficient site-to-site charge transfer between Ce and Pt (Co) atoms. The evolution of the d-band center is quantitatively analyzed by the projected state density (PDOS) of the Pt-d orbitals (Fig. 5d–f). The results reveal that PtCoCe has a moderate d-band center position at −2.13 eV, which is neither too distant nor too close to the E_F compared with PtCo (−1.94 eV) and Pt (−2.21 eV). This suggests that PtCoCe possesses an optimal binding strength for intermediates on the Pt–Co active site and reduces the kinetic barriers for intermediate desorption, thereby enhancing both charge transfer efficiency and reactant adsorption on its surface. These structural modifications underscore the dual role of Ce, which acts as an electronic modulator to tailor orbital coupling and as a charge reservoir to redistribute electron density across Pt and Co active sites. The adsorption configurations and desorption dynamics of styrene (C₈H₈) on Pt, PtCo, and PtCoCe catalysts were systematically investigated to unravel the geometric and electronic modulation effects on catalytic selectivity. The benzene ring and double bond of styrene exhibited distinct variations in bond lengths and angles upon adsorption on Pt, PtCo, and PtCoCe surfaces (Fig. S20, S21 and Tables S7, S8, ESI†). On pristine Pt, the phenyl ring adsorbs in a near-parallel orientation (∠C₁–C₆–Pt = 85.58°), characterized by a Pt–C₆ bond length of 2.24 Å and a C=C–Pt bond angle of 69.54° (Fig. 5g), facilitating strong π-interaction and moderate chemisorption (C=C bond elongation to 1.42 Å).⁶³ In contrast, PtCo exhibits an enhanced metal–substrate interaction, shortening the Pt–C₆ bond to 2.16 Å and the C=C bond to 1.41 Å (Fig. 5h), but introduces a tilted phenyl ring (95.27°) due to Co-driven electronic effects. Remarkably, the ternary PtCoCe catalyst exhibits a distinct geometric distortion due to Ce doping, which induces significant steric hindrance.⁶⁴ This effect forces the phenyl ring into a highly tilted orientation (102.08°) accompanied by a Pt–C₆ bond elongation (3.80 Å) and an adjusted C=C–Pt bond angle (92.65°) (Fig. 5i). These structural modifications disrupt the π-orbital overlap between the aromatic system and the catalyst surface, substantially weakening chemisorption strength. The steric bulk of Ce atoms further destabilizes the adsorbed configuration, lowering the desorption energy barrier to 0.33 eV (vs. 0.83 eV on Pt and 1.18 eV on PtCo). Such geometric modulation not only reduces intermediate over-stabilization but also promotes efficient desorption, thereby aligning with the enhanced catalytic selectivity observed in hydrogenation reactions. The calculated Gibbs free energy (ΔG) reveals a well-tuned binding strength on the geometrically modulated catalyst surface (Fig. 5j). Specifically, PtCo exhibits the strongest adsorption affinity for C₈H₈, with the energy required for *C₈H₈ desorption in the order of PtCo (1.18 eV) > Pt (0.83 eV) > PtCoCe (0.33 eV). This trend indicates that the PtCoCe catalyst facilitates significantly easier desorption of intermediates compared to Pt and PtCo. The lower desorption energy barrier on PtCoCe minimizes intermediate accumulation on the catalyst surface, thereby mitigating surface poisoning and enhancing the overall catalytic cycle efficiency. The Gibbs adsorption free energies (ΔG) for ethynylbenzene semi-hydrogenation on Pt, PtCo, and PtCoCe catalysts were calculated to elucidate the hydrogenation mechanism from the energetic perspective. The adsorption energies and energy barriers for all key steps in the hydrogenation process, as well as the corresponding configurations of the intermediates involved, are provided in Fig. 5k and Fig. S22, Table S9 (ESI†). The results show that the adsorption energy on PtCoCe for C₈H₆ is −1.81 eV, significantly higher than those of Pt (−1.60 eV) and PtCo (−1.77 eV). For the hydrogenation of C₈H₆ to C₈H₈, the stepwise free energy profiles for all three catalysts present a downward trend, indicating the thermodynamically spontaneous reaction pathways. The total free energies for this conversion process are −1.88 eV (Pt), −2.06 eV (PtCo), and −2.26 eV (PtCoCe), among which PtCoCe exhibits the largest energy reduction, revealing a more thermodynamically favorable pathway for C₈H₈ formation relative to Pt and PtCo. In addition, the energy barrier for hydrogenating *C₈H₈ to *C₈H₉ on PtCoCe is 0.70 eV, while the desorption energy of *C₈H₈ is only 0.33 eV. Since the desorption energy is significantly lower than the hydrogenation barrier, *C₈H₈ preferentially desorbs from PtCoCe rather than undergoing further hydrogenation. In contrast, the hydrogenation step on PtCo is thermodynamically favorable (ΔG = −0.29 eV), whereas desorption requires a higher energy (+1.18 eV), forcing the adsorbed *C₈H₈ toward hydrogenation. The subsequent conversion is highly exothermic (ΔG = −0.18 eV), which demonstrates that the styrene is prone to be hydrogenated to the undesired ethylbenzene. Similar results are also seen on the Pt, on which the free energy barrier for the hydrogenation of *C₈H₈ (ΔG = −0.03 eV) is much lower than the desorption free energy (+0.83 eV), favoring hydrogenation over desorption. The final step exhibits a near-neutral free energy change (ΔG = +0.01 eV) and slower kinetics compared to PtCo. Rare earth doping introduces steric hindrance to destabilize alkyne adsorption geometries and enhance olefin desorption kinetics, effectively decoupling activity–selectivity limitations by mitigating site blocking and suppressing over-hydrogenation. These findings underscore the critical role of rare earth elements in tailoring adsorption geometries through steric effects, offering a strategic avenue to optimize catalyst performance by balancing activation and desorption dynamics.

Fig. 5 (a)–(c) The PDOSs of Pt, PtCo, and PtCoCe. (d)–(f) The d orbital states of Pt/C, PtCo/C, and PtCoCe/C. (g)–(i) Adsorption configurations of *C₈H₈ at interfaces of Pt, PtCo, and PtCoCe models. The light pink, brown, grey, blue and yellow balls represent hydrogen, carbon, platinum, cobalt and cerium atoms, respectively. (j) The Gibbs free energy for styrene adsorption over PtCoCe (red line), PtCo (blue line), and Pt (black line). (k) The reaction energies for the hydrogenation of styrene over PtCoCe, PtCo, and Pt.

Conclusions

In summary, a series of ternary Pt_2−xCo_xCe rare earth alloy catalysts have been rationally designed to achieve electronic-geometric synergy, enabling simultaneous optimization of ethynylbenzene activation and styrene selectivity. Comprehensive characterization reveals that directional electron transfer from Ce (4f) to Pt/Co (5d) orbitals via d–f orbital coupling regulates the Pt d-band center to −2.138 eV, precisely balancing C≡C bond dissociation and intermediate styrene desorption. Moreover, steric distortion arising from the large ionic radius of rare earth elements tilts styrene's adsorption geometry, which reduces adsorption energy while elevating the over-hydrogenation energy barrier. The synergistic interplay between electronic optimization of active sites and geometric confinement of intermediates preserves rapid ethynylbenzene conversion while kinetically suppressing ethylbenzene formation. The detailed performance studies revealed that the optimal Pt_1.5Co_0.5Ce/C catalyst exhibits 98.3% conversion, 85.1% selectivity, and a TOF of 1549.6 h⁻¹ under mild conditions, surpassing most previously reported Pt-based catalysts. This study redefines rare earth elements as active modulators of electronic/geometric synergy in catalysis, overcoming classical limitations in industrial hydrogenation and paving the way for sustainable chemical synthesis.

Author contributions

Investigation, K. J.; data curation, K. J.; writing – original draft, K. J.; writing – review and editing, K. J., J. Y., L. Z., and Y. D.; formal analysis, K. J., W. Z., H. L., J. S., S. W., Z. Z., and Y. D.; funding acquisition, Y. D. All authors discussed the results and the manuscript. All authors have agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

We gratefully acknowledge the support from the Advanced Materials-National Science and Technology Major Project (No. 2024ZD0606500), the National Science Foundation for Distinguished Young Scholars of China (22425503), the National Natural Science Foundation of China (22371131), the 111 Project (B18030) from China, and the Key Laboratory of Rare Earths, Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh01041a

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