Modulating nanoceria bandgap via CoO/Pd heterostructure hybrids for efficient light-driven Suzuki–Miyaura coupling reaction

Abstract

The addition of metal oxides as impurities to generate an intermediary energy band near the conduction or valence band to reduce the bandgap is the most distinctive approach to improve the photo-absorption characteristics of the material. Herein, we have reported the synthesis of nanohybrid bimetallic heterostructures by linking the interface of CeO₂ and CoO, which narrows the electronic band structure of CeO₂ from 2.85 eV to 1.5 eV and modulates the distribution of charges at the active sites. The resulting CeO₂/CoO hybrid support enhances the dispersion and stability of Pd NPs, resulting in lowering the activation energy (E_a) barrier of the coupling reaction, thereby significantly enhancing its catalytic efficacy. The E_a value of CeO₂/CoO/Pd (53.7 kJ mol⁻¹) is much lower compared to that of CeO₂/Pd (68.6 kJ mol⁻¹), with excellent catalytic activities (yield: 98%) and exhibiting long-term stability for 5 continuous cycles without any significant loss in activity. Overall, the CeO₂/CoO/Pd hybrid system effectively utilized the photothermal effect to facilitate an effective electron transfer, thereby enhancing the rate of the Suzuki–Miyaura coupling reaction. This study offers a feasible and encouraging prospect to use the heterostructured metal oxide-based catalytic system for efficient Suzuki–Miyaura cross-coupling reaction.

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1. Introduction

Transition-metal-mediated Suzuki–Miyaura coupling reaction (SMC)^1–3 is one of the most commonly used synthetic approaches for convenient C–C bond formation. This reaction serves as a paradigm for the synthesis of a diverse array of complicated molecules with many applications in the fields of medicine, agrochemicals, and materials research.⁴ Usually, this reaction has been conducted using homogeneous Pd catalytic systems with organic ligands that are toxic, expensive, or both. However, the fundamental issues with homogenous Pd catalytic systems are their lack of recyclability, the impurity of end products with catalyst residues (palladium black), the rapid deactivation of catalytic systems, high toxicity, and limited availability of precious metals, which have impeded their widespread use in practical applications. Meanwhile, the durability and economic feasibility of the catalyst are other critical aspects for its industrial application.⁵ In recent decades, the emergence of nanoparticle- and nanocluster-based heterogeneous catalytic systems has made significant advancements owing to their convenient separation and reusability as compared to homogeneous organometallic Pd complexes.⁶ Scientists have created multi-component heterogeneous catalytic systems by depositing Pd metal onto several supports, including carbonaceous materials, zeolite, alumina, and silica.^7–9 The interface between the palladium and the support system, its morphology, and surface chemistry unveil a decisive role in controlling the kinetics and rate of coupling reactions.¹⁰ However, these heterogeneous catalytic systems suffer from inefficient accessibility for reactants due to limited and nonuniform exposed metal active sites.¹¹ Therefore, a thorough understanding of the metal active site and support interaction is a prerequisite to control its catalytic performance.¹² The scientific community is striving hard to develop catalytic systems that can exhibit the characteristics of heterogeneous (excellent durability and ease of separation) and homogeneous (high activity) catalysts.

The application of rare earth (RE) oxides, as support for metal loading, enables highly effective synergistic catalysis due to their unique electronic structures, crystal topologies, and electromagnetic characteristics.^13,14 Collectively, the characteristics of oxide supports, such as their geometric architecture, electrical structure, and defect sites, offer further ways to customize the overall performance of catalysts.¹⁵ Among RE oxides, ceria (CeO₂) exhibits distinct physical and chemical properties as well as redox features, which can be attributed to its electronic configuration, [Xe] 4f² 5d¹ 6s².¹⁶ Besides this, it can stabilize metal nanoparticles through its flexible coordination environment and the formation of oxygen vacancies (OVs), which arise due to reversible valence states between Ce⁴⁺ and Ce³⁺ ions,^17,18 offering additional possibilities for enhancing the stability and equilibrating the positive charges of metal oxide ions, thereby enhancing the catalytic activity.¹⁹ Ceria, being an important semiconductor, has also been explored as a photocatalyst for many chemical processes, i.e., CO₂ redox reaction,^20–22 organic pollutant degradation,^23,24 and air purification.²⁵ The n-type CeO₂ is characterized by a broad bandgap (2.8–3.2 eV) and absorbs radiation in the UV region and to a limited extent in the visible range (400 nm).^26,27

Regrettably, the wide bandgap of CeO₂ hinders its ability to effectively harness abundant solar energy because the visible range comprises ∼50% of the overall solar radiation.²⁸ Additionally, the photo-induced e⁻/h⁺ pairs are prone to recombination,²⁹ resulting in the production of unproductive heat and light, which reduces the photocatalytic efficiency of CeO₂.^30,31 To address the aforementioned problems, various efforts have been made to amend pure CeO₂ in order to improve its photocatalytic potential. A highly effective approach to improve the activities of ceria-based catalytic systems is to engineer the interface by integrating it with other metal oxides. Metal-oxide interfaces offer a novel prospect for enhancing catalytic activity through electrical and chemical interactions at the interface.^24,32 For instance, the interface engineering strategy has helped to reduce the bandgap (E_g) of CeO₂ by raising its valence band (VB) or lowering the conduction band (CB) or executing both processes at once.^33,34 Herein, earth-abundant cobalt oxide was utilized to fabricate the CeO₂ interface to narrow its bandgap. The addition of CoO to CeO₂ can introduce impurity energy levels near the CB and VB within the original bandgap and create lattice defects, which function as recombination centres.^35,36 Besides this, cobalt oxide (CoO) has numerous oxygen vacancies, variable oxidation states, and distinct redox properties, which can operate as active sites and promote electron transport in catalytic reactions. Recent research has shown that the efficiency of transition metal oxide-based catalysts is closely linked to their morphologies and variable electronic structures due to the d orbital, which can adjust their adsorption energies to enhance their catalytic performance.³⁷ Therefore, the integration of CoO with ceria will result in the formation of additional OVs to regulate the electronic structure, enhance charge transfer, and offer the most favorable adsorption–desorption energy for the intermediate species during the reaction.^38,39 The increased number of active sites and profuse OVs close to the interface have a notable impact on improving the catalytic activity.⁴⁰ Wang et al. developed a series of self-assembled cobalt–cerium oxides (CoCe-x) with controlled oxygen vacancies to improve the decontamination of air pollution by low-temperature catalytic oxidation. The oxygen vacancies in the catalyst were optimized through the effective integration of Co atoms into the ceria lattice, thereby enhancing the charge transfer by regulating the redox cycles of Co³⁺/Co²⁺ (ref. 41) and Ce⁴⁺/Ce³⁺.⁴²

Most reported literature on CoO-based catalysts has focused on applications in water splitting, oxygen evolution, and other oxidation processes. Besides this, there are only a limited number of studies examining CoO-based photocatalysts. To our knowledge, the application of CoO-based photocatalysts in coupling reactions, such as the Suzuki–Miyaura reaction, has been rarely investigated. Consequently, the integration of CoO with nanoceria to create a bimetallic oxide support for Pd NPs presents an innovative approach to broaden the catalytic uses of CoO beyond its traditional functions. In this study, the CeO₂/CoO/Pd-based photocatalytic system has been synthesized through the straightforward approach of the calcination-reduction process for catalysing the visible light-stimulated Suzuki coupling. The approach of interface engineering has been utilized to control the electronic band structure of CeO₂ at the interface, including the distribution of charges at the active sites. We have shown that CeO₂/CoO interfaces with distinct spatial orientations have enhanced the photocatalytic SMC reaction by stabilizing the photogenerated electrons/holes in the conduction/valence band upon exposure to visible light. The photogenerated electrons from the CB of CeO₂ are transferred to CoO and then to spatially connected Pd NPs. Mostly, the Pd NPs are disseminated as fine metallic particles (Pd⁰) or oxidized Pd ions (PdO) on the ceria support. However, Pd²⁺ ions are found to be much more abundant on the surface of the CeO₂ support due to more oxygen vacancies as compared to Pd⁰ NPs. Therefore, on exposure to light, the photogenerated electron moves to spatially connected PdO, facilitating the surface-adsorbed Pd²⁺ ion to reduce for the coupling reaction. The bandgap narrowing not only promotes wide light absorption but also improves the usage of photogenerated electrons and holes, thus reducing the energy barrier for the catalytic cycle. The synergistic impact of the bimetallic oxide support (CeO₂/CoO) together with photothermal catalysis has enhanced the rate of the SMC reaction.

2. Experimental section

2.1. Preparation of precursor (CeO₂/CoO)

CeO₂ was synthesized by following the literature procedure.⁴³ In a quartz dish, cerium nitrate hexahydrate (5 g) was subjected to heating at 350 °C for 2 h (5 °C min⁻¹) and then at 600 °C for 5 h in air. The dish was allowed to cool to room temperature, and the off-white powder was collected and stored in a glass vial for further use. 0.58 mmol of CeO₂ powder (100 mg) was put in 50 mL of water and sonicated for 45 min to obtain a homogenous solution. 0.17 mmol of Co(NO₃)₂·6H₂O (50 mg) was dissolved in 20 mL of water and slowly added to the aforementioned CeO₂ solution. The solution was sonicated for 2 h, and then the mixture was dried in an oven at 60 °C. Subsequently, the mixture was calcined at 150 °C (5 °C min⁻¹) for 1 h, followed by heating to 500 °C (5 °C min⁻¹) for 3 h under static air. The dish was cooled to ambient temperature, and the resulting powder was labelled as CeO₂/CoO. The Ce : Co molar ratio in the precursor was approximately 3.4 : 1.

2.2. Preparation of the catalyst (CeO₂/CoO/Pd)

200 mg of CeO₂/CoO was dispersed in 50 mL of water, followed by sonication for 1 h to achieve a homogeneous suspension. Scaling to 200 mg of this precursor equates to 0.77 mmol of Ce and 0.23 mmol of Co. 0.28 mmol of PdCl₂ (50 mg) solution was prepared in 25 mL of 0.1 M HCl solution in a separate vial. This solution was slowly added to the CeO₂/CoO solution, sonicated for 1 h and then placed in an oven at 60 °C to dry under vacuum. Once dry, the dish was transferred to a tube furnace and calcined at 500 °C for 3 h (5 °C min⁻¹) under an Ar atmosphere. The product was cooled to ambient temperature, then thoroughly rinsed using a 1 : 1 (v/v, 5 × 20 mL) EtOH/water solution. The washing was continued until the filtrate was colourless, ensuring the complete removal of unreacted PdCl₂. The resulting catalyst was dried in a vacuum oven at 60 °C and then reduced under an atmosphere of H₂/Ar at 300 °C for 3 h to obtain CeO₂/CoO/Pd.⁴⁴ The overall molar ratio of Ce : Co : Pd in the final composite was roughly 3.3 : 1 : 1.2. The Pd loading was also calculated using inductively coupled plasma mass spectrometry (ICP-MS) and was found to be 0.23 wt% Pd. Other reference catalysts containing CeO₂/Pd, CoO/Pd were also prepared without adding cobalt nitrate (Co(NO₃)₂·6H₂O) and CeO₂, following the above protocol, to investigate the role of CoO and CeO₂ in the enhancement of catalytic activities in the SMC reaction.

2.3. General procedure for the photocatalytic Suzuki–Miyaura cross-coupling (SMC) reaction

The reaction was conducted in a 10 mL photocatalytic sealed apparatus fitted with a water-circulating jacket and irradiated from the top using a PLS-SXE300+ xenon lamp equipped with a UV cut-on filter (λ >420 nm). The reaction vessel was charged with aryl halide (0.5 mmol, 1 equiv.), phenylboronic acid (0.55 mmol, 1.1 equiv.), potassium carbonate (K₂CO₃) (138 mg, 1 mmol, 2 equiv.), nitrobenzene (10 μL; internal standard), and catalyst (20 mg). 2 mL of EtOH and 2 mL of H₂O were added to the reaction vessel, and the vessel was sealed with quartz glass. The reaction concoction was irradiated from the top by a xenon lamp for a specified time at 50 °C. The reaction progress was monitored by TLC and GC (equipped with a FID detector) until the substrate was completely consumed or no further change in the reaction mixture was observed. After cooling to ambient temperature, the catalyst was collected from the reaction concoction by centrifugation, rinsed thoroughly with water and ethanol (3 times each), and dried in a vacuum oven at 60 °C. The filtrate was diluted with ethyl acetate (EtOAc) and water solution (5 mL), followed by the extraction of the aqueous layer with EtOAc (5 mL × 3). The combined organic layers were rinsed with aq. brine solution, dried over anhydrous magnesium sulfate (MgSO₄), filtered, and concentrated under vacuum. The pure product was obtained by column chromatography on silica gel using n-hexane and EtOAc as the mobile phase. Reaction yield and selectivities were assessed by gas chromatography (GC) using 4-nitrobenzene as an internal standard.

2.4. Kinetics studies

A photocatalytic sealed apparatus fitted with a water circulating jacket was filled with 4-bromotoluene (1.0 mmol), phenylboronic acid (1.2 mmol), K₂CO₃ (2 mmol), CeO₂/CoO/Pd catalyst (40 mg), 10 μL nitrobenzene as internal standard and a mixture of EtOH and H₂O (8 mL, v : v = 1 : 1). The reaction temperature was sequentially maintained at 30 °C, 40 °C, 50 °C, 60 °C and 70 °C. A 500 μL aliquot was collected from the reaction mixture every 5 min and poured into a micro vial containing 1 mL of EtOAc and mixed with 1 mL of H₂O, shaken well, and the organic layer was extracted with EtOAc (1 mL × 3). The organic layers were further dried using MgSO₄ (anhydrous) and filtered. The product yield was quantified using gas chromatography (GC). The addition of more equivalents of phenylboronic acid in the reactants, as compared to the aryl halide, allows us to consider the SMC reaction as a pseudo-first-order kinetic reaction.

2.5. Apparent quantum efficiency (AQE)

The AQE was calculated using the formula below:
where n = 1 for the coupling reaction as reported,⁴⁵V is the volume of reaction mixture (4 mL), Y_light and Y_dark are the moles of product molecules (mol mL⁻¹) formed under light irradiation and in the dark, respectively.⁴⁶ The intensity of the light was maintained at 50 mW cm⁻², and the reaction temperature for both dark and light reactions was regulated at 50 °C using a water-circulating system. Different light filters (420 nm, 450 nm, 500 nm, 550 nm, 600 nm) were used to calculate the AQE value of the catalyst.

2.6. Recycling test

A standard reaction of phenylboronic acid and 4-bromotoluene was used to examine the recyclability of the catalyst. After the reaction, the catalyst was filtered off from the reaction concoction and rinsed thoroughly with EtOH and H₂O to remove impurities (organic/inorganic), dried in a vacuum oven at 60 °C, and reused in the next reaction with a new batch of the reaction mixture. Five consecutive reactions were conducted following the above procedure. After completion of each reaction, the filtrate was diluted with EtOAc and H₂O (5 mL), followed by the extraction of the aq. layer with EtOAc (5 mL × 3). The collected organic layers were dried over MgSO₄ (anhydrous), filtered, and the filtrate was concentrated under vacuum. Reaction yields and selectivities were assessed by gas chromatography (GC) using 4-nitrobenzene as an internal standard.

2.7. Heterogeneity analysis

A hot filtration experiment was performed to examine the heterogeneous nature of the catalyst under optimized reaction conditions. After 15 min, which ensured the 50% conversion of 4-bromotoluene into the desired product, the catalyst was filtered off from the mixture. The resultant filtrate was subsequently subjected to stirring again for 1 h. The progression of the reaction was evaluated using gas chromatography (GC) measurements before and after the filtering process.

3. Results and discussion

CeO₂/CoO/Pd is synthesized by a two-step annealing procedure as illustrated in Fig. 1a. First, an aq. solution of CeO₂ powder and Co(NO₃)₂·6H₂O is sonicated for 2 h to homogenize the particles of both compounds in the liquid phase. Then the solution is placed in a vacuum oven to dry, followed by calcination of the dry material in a tube furnace at 500 °C for 3 h. This procedure has facilitated better and uniform dispersion of both metal oxides, resulting in a bimetallic hybrid support with numerous defect sites and oxygen vacancies, thereby presenting ideal support for the stabilization and activation of Pd NPs. This precursor is again dispersed in water followed by the dropwise addition of acidic PdCl₂ solution under continuous stirring, sonicated for 2 h to homogenize the components of the mixture, dried under vacuum, and afterward annealed under an Ar atmosphere at 500 °C for 3 h. The obtained powder is reduced under H₂/Ar for 3 h at 300 °C to obtain CeO₂/CoO/Pd. The annealing process has facilitated the homogenous dispersion of components and transformation of precursor material into a well-defined crystalline composite material having uniform catalytic characteristics throughout the catalytic material, thereby improving its activity and stability. The precursor (CeO₂/CoO) and catalyst (CeO₂/CoO/Pd) are examined by employing XRD, XPS, SEM, and TEM to demonstrate the existence of scattered CoO and Pd NPs on the CeO₂ support. The TEM, HRTEM, and HAADF images of the precursor material CeO₂/CoO and elemental mapping are shown in Fig. S1. Fig. 1b shows the HAADF image of the CeO₂/CoO/Pd hybrid structure and its elemental mapping. HRTEM images (Fig. S2) of the catalyst (CeO₂/CoO/Pd) show small black nanoclusters exhibiting polyhedral morphology corresponding to the formation of CoO and Pd NPs that are well stabilized and anchored on the surface of CeO₂. The HRTEM images of the catalyst show distinct lattice fringes of CeO₂, CoO, and Pd in the catalyst (CeO₂/CoO/Pd) (Fig. S2). The lattice fringes of 0.318 nm identify the {111} facets of CeO₂, and the lattice spacing of 0.249 nm correlates to the CoO {111}.⁴⁷ The loading of Pd in the hybrid structure of precursor CeO₂/CoO gives distinct lattice fringes of Pd at 0.224 nm {111}.^10,48 Furthermore, the elemental mapping images verified the materialization of the nanohybrid structure, affirming that the CeO₂/CoO/Pd catalyst is composed of cerium oxide, cobalt oxide, and palladium. The elemental mapping also reveals a uniform distribution of CoO and Pd NPs over the whole area.⁴⁹ The surface morphology of the precursor and catalyst is also studied by SEM (Fig. S3a and b), which shows the presence of tiny granule-like particles all over the surface. The Pd loading of the catalyst (CeO₂/CoO/Pd) is 0.2 wt% according to EDX analysis (Fig. S3c). The Pd loading is also calculated using inductively coupled plasma mass spectrometry (ICP-MS) and is found to be 0.23 wt% Pd.

Fig. 1 (a) Synthesis of CeO₂/CoO/Pd as a photocatalyst. (b) HAADF images of CeO₂/CoO/Pd and element mapping images of Pd, Ce, Co, and O in CeO₂/CoO/Pd. (c) XRD of CeO₂/CoO and CeO₂/CoO/Pd. XPS of (d) Pd 3d, (e) Ce 3d, (f) Co 3d; f-1 Co 2p_3/2 (778–792 eV), f-2 Co 2p_1/2 (792–808 eV), and (g) O 1s.

XRD and XPS are performed to further characterize the chemical and electronic structure of CeO₂/CoO and CeO₂/CoO/Pd. The XRD scan is shown in Fig. 1c. CeO₂ is identified by eight distinct peaks at 2θ = 28.66° (111), 33.01° (200), 47.38° (220), 56.27° (311), 59.02° (222), 69.57° (400), 76.71° (331), and 79.16° (420) (JCPDS no. 81-0792),⁵⁰ whereas CoO is identified by three distinct peaks at 36.67° (111), 42.8° (200), and 61.72° (220) (JCPDS no. 43-1004).^47,51 The CeO₂/CoO/Pd scan shows two distinct peaks of Pd at 40.16° (111) and 46.68° (200) (JCPDS no. 88-2335), which has affirmed the formation of a nanohybrid heterostructure supported on nanoscale bimetallic oxide support (CeO₂/CoO). The electronic structure of the hybrid catalyst is investigated by X-ray photoelectron spectroscopy (XPS). XPS spectra are obtained for the Ce 3d, O 1s, Co 3d, and Pd 4d for each of the catalysts. The complete survey scan of precursor (CeO₂/CoO) and catalyst (CeO₂/CoO/Pd) is demonstrated in Fig. S4, whereas the elemental XPS of precursor and catalyst are presented in Fig. 1(d–g). Fig. 1d shows the XPS scan of Pd. The smaller peaks at the binding energy of 335.8 and 340.8 eV are ascribed to metallic Pd⁰, whereas the larger peaks at 337.8 and 343.0 eV are ascribed to Pd²⁺.⁵² It has been observed that more Pd NPs on the surface of CeO₂/CoO are dispersed as oxidized Pd²⁺ rather than Pd⁰ NPs.^7,53 The Pd²⁺ ions have also substituted the Ce⁴⁺ ions/ Co²⁺ ions in the composite structure of CeO₂/CoO to ensure the even dispersion of Pd NPs because the introduction of Pd caused a significant shift of the Ce 3d and Co 2p XPS peaks, indicating electronic interaction and charge redistribution between the precursor and Pd NPs. This change indicates a potential replacement of Pd into the lattice positions of Ce or Co rather than simply surface deposition.⁵⁴ The integration of Pd into the CeO₂/CoO lattice induces a charge imbalance, which is compensated by the formation of oxygen vacancies, disturbing the local lattice symmetry. Besides this, the partial reduction of Ce⁴⁺ to Ce³⁺ during Pd inclusion also causes lattice expansion because of its larger ionic radius than Ce⁴⁺. The observed lattice expansion mostly results from the weakening of local lattice symmetry due to oxygen vacancies and the larger ionic radius of Ce³⁺ compared to Ce⁴⁺. The chemical stability and structural durability of this heterogeneous catalytic system are attributed to the formation of Ce–O–Pd or Co–O–Pd in the composite structure, and this strong interaction of Pd²⁺ with the precursor has also inhibited the sintering of Pd NPs in the reaction mixture and prevents catalyst deactivation. The ionic Pd²⁺ formed by Ce–O–Pd/Co–O–Pd has brought much improvement in the catalytic performance of CeO₂/CoO/Pd in the SMC reaction.⁵⁵ In the XPS of Ce, two different forms of Ce are observed: Ce³⁺ and Ce⁴⁺. The peaks at 917.70, 908.45, 902.61, and 899.92. 889.70, and 883.49 eV are attributed to Ce⁴⁺ ions, whereas the peaks at 904.46 and 886.43 eV are assigned to Ce³⁺ ions in the XPS of the precursor material. However, all peaks show slight shifting in the Pd composite structure (CeO₂/CoO/Pd); peaks at 916.51, 908.40, 902.42, 899.34, 887.51, 882.8 eV are associated with Ce⁴⁺ ions, and 905.51, and 885.10 peaks are attributed to Ce³⁺ ions⁵⁶ (Fig. 1e). The shifting of XPS peaks represents the bonding of Pd NPs at the interface of CeO₂/CoO during the annealing process and definitive evidence of a change in the Ce oxidation state resulting from strong metal–support interaction (SMSI). The Ce 3d peaks exhibit a shift towards lower binding energy, representing enhanced electron density due to the reduction of Ce⁴⁺ to Ce³⁺ on the surface of the CeO₂/CoO support and the formation of oxygen vacancies. A single Ce peak (904.1 → 905.0 eV) exhibits a slight shift toward higher binding energy (+0.9 eV), which is ascribed to Ce atoms at the Pd–support interface undergoing local electron withdrawal as a result of Ce–O–Pd interactions. The XPS of CoO shows two prominent peaks at 781.41 eV and 796.8 eV, confirming Co 2p_1/2 and Co 2p_3/2, respectively (Fig. 1f).⁵⁷ Additionally, two shake-up peaks, one at 786.24 eV and another at 802.73 eV, suggest the presence of Co(II) in the CeO₂/CoO/Pd sample.^56,58 The shifts of Co 2p components along with an increase in FWHM in CeO₂/CoO/Pd suggest a more diverse cobalt chemical environment, characterized by the coexistence of Co²⁺ and Co³⁺ states, interface disorder, and defect formation (oxygen vacancies). This behavior aligns with the redox interaction between Co and Pd (Co²⁺ ↔ Co³⁺). The existence of Co²⁺ indicates that CoO is strongly interacting with the reducible ceria support, rather than creating bigger cobalt oxide aggregates. The O 1s XPS scan of the precursor (Fig. 1g) shows peaks at 529.6 and 532.3 eV ascribed to the binding energy of lattice oxygen (O) and defective or surface-adsorbed oxygen (O_ads).⁵⁹ The surface-adsorbed oxygen includes superoxide (O²⁻) and peroxide (O₂²⁻) and is believed to perform a critical role in defining the catalytic activity and reaction rate. The shifting of oxygen species at high binding energies (531.7, 532.5, and 533.6 eV) in the O 1s XPS scan of CeO₂/CoO/Pd signifies the development of increasingly electron-deficient oxygen environments. The 533.3 eV peak probably refers to oxygen atoms interacting with several metal centers (Ce–O–Pd or Co–O–Pd bridges). Besides this, the incorporation of palladium has resulted in the formation of PdO on the catalyst surface and has enhanced the oxygen vacancies, leading to the displacement of the O 1s peak towards higher binding energies (532–533 eV).^47,60 PdO may also interact with CeO₂/CoO, inducing the integration of Pd NPs into the lattices of CeO₂ or CoO, as discussed before in the discussion of Pd XPS. Besides this, the substitution of Pd²⁺ ions into the Ce⁴⁺ ions/Co²⁺ ions during the annealing process under an inert atmosphere would result in the formation of OVs, which also cause the shifting of the O 1s peak to high binding energy. This endorses that the interaction of PdO and CeO₂/CoO is accountable for the formation of abundant O_ads, resulting in better catalytic activity.

The FTIR spectra of CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd are shown in Fig. 2a. The CeO₂/CoO FTIR spectra display two weak signals at 570 cm⁻¹ and 660 cm⁻¹ attributed to Co²⁺ in the composite structure of CeO₂/CoO, thereby confirming the formation of CoO.⁶¹ The small signal at 557 cm⁻¹ represents the stretching vibration of Ce–O in CeO₂. The other stretching bands of Ce–O are visible in the spectra at 663 and 863 cm⁻¹, further revealing the metal–oxygen bond. There are noticeable absorption peaks at 3421 cm⁻¹ and 1630 cm⁻¹ related to –OH stretching vibrations resulting from entrapped water molecules from the environment due to the high surface-to-volume ratio of nanomaterials.⁶²Fig. 2b illustrates the Raman spectra of the CeO₂/CoO composite and CeO₂/CoO/Pd composite. Both samples show intense peaks at 462 and 456 cm⁻¹ corresponding to the F_2g phase of the CeO₂ fluorite structure. The vibrational mode F_2g is usually allied with the elongation of the Ce–O bond in the composite structure, and a slight change in the Ce–O bond position on the addition of Pd has facilitated the shifting of band location.⁶³ Besides this, the peak shifting (F_2g band) to lower wavenumber in the CeO₂/CoO/Pd sample is attributed to various factors such as defect formation, variation in phonon relaxation, and phonon confinement relative to particle size. Other bands in the CeO₂/CoO composite are 620 and 689 cm⁻¹, corresponding to defect-induced (D) mode in the CeO₂ structure due to oxygen vacancies and the valence state of the dopant material (Co). Bands in the bimetallic oxide support of CeO₂/CoO at 480 cm⁻¹ (E_g), 521 cm⁻¹ (F²_2g), 622 cm⁻¹ (F³_2g), and 690 cm⁻¹ (A_1g) are attributed to CoO.⁶² In particular, the band at 521 cm⁻¹ in the CeO₂/CoO composite corresponds to the Co–O stretching frequency, signifying the presence of CoO in the matrix.⁶⁴ However, the weak signal at 1169 cm⁻¹ is associated with the 2nd-order longitudinal optical (2LO) mode. Following the deposition of Pd, new bands at 640 cm⁻¹, attributable to the vibrational mode of the Pd–O bond, emerged in the CeO₂/CoO/Pd catalyst. The literature survey indicates that certain defects may influence the peak intensity of the defect-induced mode, whereas oxygen vacancies in the ceria lattice may enhance the peak intensity. However, the intensity of the CoO band has significantly reduced in the composite catalyst structure of CeO₂/CoO/Pd, representing the addition of Pd NPs.⁶⁵ The textural properties, such as surface area, porosity, and pore volume of the bimetallic oxide support and its catalyst are determined using nitrogen adsorption isotherms (Table 1). Fig. 2c displays each catalyst's nitrogen adsorption and desorption isotherms and pore size distribution. The impregnated samples (CeO₂/CoO and CeO₂/CoO/Pd) exhibited a type IV physisorption isotherm with an H1 hysteresis loop signifying the microporous nature of the catalyst, and nanoclusters/spherical particles are distributed in a homogenous manner.⁶⁶ The pore size distribution indicates uniformity and facile pore connectivity. As indicated in Table 1, the BET surface area of the CeO₂/CoO/Pd has shrunk, while the average pore diameter of the CeO₂/CoO/Pd has enlarged due to Pd NPs. Pd NPs cover the active sites on the CeO₂/CoO surface, reducing the surface area overall. Besides this, the detonation or clogging of the micropores by deposited Pd NPs during the impregnation and calcination process is the cause of the significant drop in the BET surface area of CeO₂/CoO/Pd.^56,67

Fig. 2 (a) FTIR of CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd. (b) Raman spectrum of CeO₂/CoO and CeO₂/CoO/Pd. (c) Nitrogen adsorption and desorption isotherms and c-1; pore size distribution. (d) UV-vis DRS spectra of CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd. (e) Tauc plots of the optical bandgap energy. (f) Schematic illustration of bandgap modulation of CeO₂/CoO/Pd. (g) Electrochemical impedance spectra of CeO₂/CoO, CeO₂/Pd and CeO₂/CoO/Pd. (h) Transient photocurrent of CeO₂/CoO, CeO₂/Pd and CeO₂/CoO/Pd. (i) IR thermography of CeO₂/CoO (sp1, sp2) and CeO₂/CoO/Pd (sp3, sp4, sp5).

Table 1 Textural properties of CeO₂/CoO and CeO₂/CoO/Pd from BET analysis

Catalyst	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (Å)
CeO2/CoO	37.6038	7.6 × 10^−3	8.09
CeO2/CoO/Pd	9.9052	2.7 × 10^−3	11.07

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UV-vis diffuse reflectance spectroscopy is performed to examine the light absorption characteristics of the catalyst (Fig. 2d and S5a). The strong absorption band is seen in the UV range of 300–350 nm. The catalyst bandgap is calculated by using eqn (1):

αhv = A(hv − E_gap)^n/2 (1)

where A represents a constant, h is the Planck constant, v represents the frequency of light, α represents the abs coefficient and E_gap represents the energy difference between the valence band (VB) and the conduction band (CB). The E_gap values obtained from Tauc plots (Fig. 2e and S5b) are 2.85 eV, 2.50 eV, 2.38 eV, 2.12 eV and, 1.50 eV for CeO₂, CoO, CeO₂/CoO, CeO₂/Pd and CeO₂/CoO/Pd, respectively, which indicates that surface coordination of CoO and Pd NPs has modified the electrical and optical properties of CeO₂ by creating structural defects/oxygen vacancy. The calculated bandgap of CeO₂/CoO (2.38 eV) is much lower as compared to that of pristine CeO₂ (2.85 eV), which is further reduced to 1.50 eV with the addition of Pd NPs (Fig. 2f). The band-edge locations of CeO₂, CoO, CeO₂/CoO, CeO₂/Pd and CeO₂/CoO/Pd are also examined using Mott–Schottky (MS) analysis (Fig. S5c and d and S6a–c). CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd are described as n-type semiconductors because of their positive slope. The E_fb (flat band position) of CeO₂, CoO, CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd are −0.23 V, 0.087 V, 0.392 V, 0.142 V, and −0.194 V, respectively, whereas the E_CB (conduction band) positions are −0.33 V, −0.013 V, 0.29 V, 0.042 V, and −0.29 V, respectively (vs. NHE, pH 7). The E_VB (valence band) positions are determined by using the following formula: E_VB = E_g + E_CB. The calculated E_VB of CeO₂, CoO, CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd are 2.52 V, 2.48 V, 2.67, 2.16, and 1.21 eV, respectively (Fig. S7). The integration of CoO into nanoceria markedly reduces its bandgap from 2.85 eV to 2.38 eV, signifying improved visible-light absorption. The lowering of the bandgap is due to the robust electrical interaction between CeO₂ and CoO, introducing additional energy levels and enhancing charge transfer across the heterojunction. Electrochemical impedance spectroscopy (EIS) is used to examine the resistance in charge transfer (Fig. 2g) of CeO₂/CoO, CeO₂/Pd, and CeO₂/CoO/Pd. The equivalent circuit provides quantitative insights into the charge-transfer mechanisms at the catalyst–electrolyte interface. The reduced size of the semicircle diameter in the Nyquist plots suggests that CeO₂/CoO/Pd has a high rate of electron transfer and promotes smooth charge transfer between CeO₂ and Pd with the assistance of CoO. The addition of Pd NPs into the precursor material (CeO₂/CoO) produces more oxygen vacancies to balance the charge difference between the Ce⁴⁺ of the support and the Pd²⁺ of the dopant, which has potentially increased the conductivity of the catalyst.⁶⁸ The pronounced oxygen vacancies increased the rate of electron transmission to the catalyst surface to reduce the surface Pd²⁺ ions, thereby increasing the rate of the SMC reaction. Besides this, the addition of Pd NPs in CeO₂/CoO has considerably increased the photocurrent density, indicating the enhanced rate of charge separation (Fig. 2h). The bimetallic heterojunction of CeO₂/CoO has improved the separation and movement of e⁻/h⁺ pairs produced by light photons, hence reducing their recombination rate and increasing the photocurrent density.⁶⁹ IR thermography measurements have also provided significant insights into the thermal characteristics of the surface of the CeO₂/CoO/Pd catalyst (Fig. 2i). In a heterogeneous catalytic system, the catalytic activity often depends on the surface temperature being close to or equal to the bulk reaction temperature. The temperature monitoring at the surface of the catalyst indicates enthalpy changes, serving as an estimation of the temperature at which the reaction could happen. The recorded surface temperature (46 °C) of CeO₂/CoO/Pd acts as a reference point for comprehending the thermal dynamics of the catalyst inside the reaction mixture. Effective heat transport to and across the catalyst surface has facilitated localized heating at the nanoscale, therefore concentrating energy at the active sites and enhancing its charge transfer efficiency. The IR thermography image shows that the catalyst's surface is thermally active and may facilitate effective catalytic reactions around 50 °C, thereby supplying the necessary thermal energy to commence surface interactions, especially for the adsorption of aryl halides. Reactions conducted below 50 °C exhibited decreased conversion rates as discussed in the next section, suggesting that the catalyst surface did not possess enough energy to facilitate efficient heat transfer and adsorption activities of reactant molecules. The temperature range of 45 to 50 °C guarantees that the activation energy barrier has been adequately surpassed, enabling high TOF (2675 h⁻¹) with 98% yield of the cross-coupled product, signifying that the catalyst's thermal characteristics correspond well with the kinetic energy of the reaction (details are discussed in the next section). Consequently, monitoring the surface temperature indicates the inherent capacity of surface atoms to engage in surface reactions or may correlate with the accessibility of surface atoms having precise energies essential to react with the substrate molecules.

The efficacy of this ternary nanohybrid catalytic system (CeO₂/CoO/Pd) is investigated by catalyzing the SMC reaction of 4-bromotoluene and phenylboronic acid. The reaction conditions for SMC reaction are optimized by testing different solvents and bases under light at 50 °C (Table S1). The electrons are not energetic enough below 50 °C to be injected into adsorbate states, therefore contributing to the augmentation of the reaction rate by means of photothermal effects. The synergistic impact of photothermal catalysis has been operated, which has accelerated the rate of reaction and altered the selectivity pattern, resulting in superior catalytic efficacy. The effect of temperature on the reaction kinetics is discussed in detail in the next section. Following the optimization of reaction conditions, the activities of different catalysts (CeO₂/CoO/Pd, CeO₂/CoO, CeO₂/Pd, CoO/Pd) are evaluated using the standard reaction of 4-bromotoluene and phenylboronic acid under optimized reaction conditions. The results are summarized in Fig. 3a (Table S2). The catalyst CeO₂/CoO/Pd has shown the highest catalytic activity in the SMC reaction compared to the other catalysts (CeO₂/CoO, CeO₂/Pd). The CeO₂/CoO/Pd catalyst has achieved a 98% yield, roughly 3.3 times greater than CeO₂/Pd (30%) and 4.3 times greater than CoO/Pd (23%), whereas the reaction scarcely proceeds without Pd, suggesting that the CeO₂/CoO system lacks catalytic activity. This comparison underscores the synergistic interaction between CeO₂ and CoO in promoting electron transport, stabilizing Pd NPs, and enhancing catalytic efficiency. The CeO₂/CoO/Pd catalyst demonstrates exceptional reactivity and selectivity (99%) in the SMC reaction. The SMC reaction of 4-bromotoluene gives 98% yield of cross-coupled product with 99% selectivity and TOF of 2675 h⁻¹ in only 30 min.⁷⁰ The increased reaction yield is attributed to the decreased bandgap of CeO₂ (2.85 eV to 2.38 eV) as Co is incorporated into the CeO₂ structure, which is further reduced to 1.99 eV with the addition of Pd NPs. Light exposure excites the electrons from the valence shell of CeO₂ and transfers them to the CoO in the composite structure and finally to the surface Pd²⁺ ions to form Pd⁰.⁶⁹ The bimetallic CeO₂/CoO support has enhanced the rate of charge transfer, making CoO a highly active co-catalyst for the SMC reaction.^69,71 The photocatalytic efficacy of CeO₂/CoO/Pd for the SMC process is further examined in the dark (solely thermal reaction) and under light illumination. Control experiments under dark conditions at 50 °C clearly indicate that the catalytic performance is significantly enhanced under visible light irradiation, confirming that the activity arises predominantly from photocatalysis rather than purely thermal contributions (Fig. S8). Furthermore, the correlation between activity and light intensity (Fig. 3b) demonstrates that the contribution of photonic processes to the total reaction efficiency escalates from 43% in the dark (thermal reaction) to 84% at 30 mW cm⁻² to 98% at 50 mW cm⁻². The conversion of 4-bromotoluene in the absence of light irradiation is attributed to the thermal effect; however, only about 43% yield of the coupled product was obtained under thermal conditions in 30 min. The reaction was conducted again at 50 °C under light exposure, yielding about 98% of the coupled product. However, as light intensity increases, the contribution of light becomes more significant. Increasing the irradiation intensity from 10 to 50 mW cm⁻² enhanced the yield of the cross-coupled product from 65% to 98%. The number of photoexcited electrons at elevated energy levels has been augmented by increasing the light intensity. This linear correlation between reaction conversion and light intensity indicates a photoexcited electron-driven chemical process. Furthermore, all light within the wavelength range of 420–600 nm (intensity 50 mW cm⁻²) enhances the yield of the coupled product; however, the yield varies with different wavelengths, and the contribution of the photonic process to reaction efficiency slowly diminishes as the wavelength of the incident light increases (Fig. 3c). Light on/off experiments further confirm that the catalyst activity is induced by light rather than thermal factor (Fig. S9), whereas wavelength-dependent studies indicate that catalytic activity has extended into the visible-light spectrum, aligning with the decreased bandgap energy. In pure CeO₂ (2.85 eV), activity is mostly limited to UV excitation; however, after the inclusion of CoO, the initiation of photocatalytic reaction shifts to longer wavelengths, corresponding to the reduced bandgap of 2.38 eV. The relationship between the absorption edge and the catalytic performance demonstrates that bandgap narrowing facilitates the use of lower-energy photons, therefore improving visible-light-driven activity. The trend of the apparent quantum efficiency (AQE) curve corresponds with the UV-vis absorption spectra of CeO₂/CoO/Pd (Fig. 3d). The action spectrum, representing AQE as a function of excitation wavelength, exhibited elevated AQEs at shorter wavelengths, with an estimated AQE of 6.3% attained under 420 nm light illumination. The AQEs corresponded closely with the absorption spectra of CeO₂/CoO/Pd, therefore validating the role of light absorption amplification in the enhancement of yield in the coupling process.⁴⁶ The significant increase in activity with shorter-wavelength photons suggests that photo enhancement at 420 nm is not only attributable to a basic photothermal effect. Photons of shorter wavelengths are capable of exciting electrons to elevated energy levels, making these electrons more efficient in facilitating coupling processes compared to those stimulated by photons with a larger wavelength of 600 nm. The excited electrons surpassing the energy barrier are introduced into the LUMO of the aryl halide adsorbed on Pd NPs. The contribution from this photoexcitation mechanism intensifies at a low wavelength (420 nm). Meanwhile, electrons stimulated by light of longer wavelengths (>500 nm) lack enough energy for excitation and they may participate in the process via the photothermal effect. Therefore, the nanohybrid heterostructure of the composite catalyst (CeO₂/CoO/Pd) exhibited remarkable photocatalytic activity at low wavelengths due to both augmented light absorption and increased separation of e⁻/h⁺ pairs. The enhanced light absorption is attributed to the narrowing of the bandgap structure of nanoceria by incorporating CoO and Pd, which has kinetically facilitated the promotion of surface charge carriers, whereas thermal energy promotes the rapid formation of the oxidative addition complex, thereby enhancing the reaction rate. The data suggest that photogenerated charge carriers (including electrons and holes) play significant roles in the photocatalytic SMC reaction and photo–thermo catalysis processes function synergistically to augment the total photocatalytic rate. Temperature-dependent reaction kinetics are studied under optimized reaction conditions between phenylboronic acid and 4-bromotoluene in the temperature range of 30–70 °C. A sample from the reaction mixture is taken every 5 min and monitored by GC using nitrobenzene as an internal standard. The product yield of the SMC reaction linearly accelerates with the increase in temperature. According to Fig. 3e, the yield of the cross-coupled product is lowered at temperatures below 50 °C, but it has significantly increased to 98% when the temperature is raised to 50 °C within 30 min. The increase in temperature has resulted in the upgrading of electrons to higher energy levels via channeling through CeO₂ → Co → Pd, which are further elevated in energy upon light absorption when irradiated. These excited electrons under the photothermal effect enhance the probability of rapid activation of aryl halide molecules adsorbed on the Pd NPs' surface, enabling them to overcome the activation barrier and thus initiate the reaction. However, when we further increased the temperature to 70 °C, the yield reached 99% in only 15 min, but the reaction selectivity dropped from 97% to 87% as a result of the temperature rise. Thus, it may be proposed that the rise in temperature has accelerated the rate of the SMC reaction, but this elevated temperature is not conducive to achieving the desired selectivity of the SMC coupled product because more heat energy has stimulated the self-coupling reaction in the solution, indicating that more molecules have crossed the activation barrier, thus increasing the rate of side reaction. To differentiate photocatalytic effects from thermal effects, the activation energy (E_a) for the CeO₂/CoO/Pd catalyst under visible-light irradiation and in the dark at the same reaction temperature is investigated. Then the curves of 4-bromotoluene conversion with respect to time at each temperature are drawn (Fig. S10a–c). The time-dependent ln(C_o/C_t) curve exhibits pseudo-first-order reaction kinetics with respect to the concentration of phenylboronic acid (Fig. S10d–f).⁶⁸ Reaction rate constant (k) values are computed from the slope of ln(C_o/C_t)–time curves. The activation energy (E_a) of the reaction is determined from the Arrhenius plot by drawing ln k versus 1000/T (Fig. 3f). The E_a of CeO₂/CoO/Pd under visible light irradiation (53.7 kJ mol⁻¹) is markedly lower than that under thermal circumstances (76.3 kJ mol⁻¹), indicating that photoexcited charge carriers efficiently reduce the reaction energy barrier accelerating the rate of the coupling process. This observation suggests that the primary factor contributing to the increased reaction rate is photocatalytic rather than solely thermal. In contrast, CeO₂/Pd under illumination also exhibits higher E_a (68.6 kJ mol⁻¹), highlighting the critical role of CoO in facilitating charge separation and reducing the reaction energy barrier.¹¹ The smaller bandgap of CeO₂/CoO/Pd, as shown by UV-vis DRS measurement, has improved the availability of charge carriers, necessitating less activation energy for the photocatalytic reaction, as shown in Fig. 3g. This noticeable relationship between E_g and E_a indicates that bandgap narrowing not only promotes wide light absorption but also improves utilization of photogenerated electrons and holes, thus reducing the energy barrier for the catalytic cycle. The surface-active sites act as entrapping centers for the e⁻/h⁺ pairs, thereby reducing their recombination rate and facilitating their effectual participation in the reaction. This represents that the bimetallic CeO₂/CoO active sites have significantly lowered the bandgap and E_a for the reaction, subsequently promoting the rate of the SMC coupling reaction.⁷¹ Furthermore, the activation energy (E_a) under illumination is considerably lower than that observed in darkness, showing that visible-light photoexcitation dramatically reduces the energy barrier for the reaction and is the primary factor in the improved catalytic performance. Thus, the ability to proficiently integrate thermal and light energy sources to enhance the efficiency of chemical transformations renders this catalytic system superior to traditional photocatalytic systems and conventional thermal catalysts. Furthermore, the rate constant values are used to investigate the initial rate enhancement using CeO₂/CoO/Pd and CeO₂/Pd with the increase in temperature (Table 2). We have observed that the rate enhancement (13.61) is more pronounced at low temperatures as compared to 70 °C (7.66), which signifies the catalyst efficiency at low temperatures. The rate enhancement of CeO₂/CoO/Pd at low temperature is more significant, representing the amplifying role of CoO in the catalytic system as the reaction with CeO₂/Pd is markedly sluggish or insignificant at low temperatures.

Fig. 3 (a) Activities of different catalysts using the standard reaction of 4-bromotoluene and phenylboronic acid. (b) The correlation activity of CeO₂/CoO/Pd and intensity of light for the SMC reaction. (c) The AQE values at different wavelengths at a fixed light intensity of 50 mW cm⁻². (d) Yield of cross-coupled product at different temperatures using CeO₂/CoO/Pd. (e) Arrhenius plots of the reaction using CeO₂/CoO/Pd and CeO₂/Pd. (f) Relationship of E_a and bandgap. (g) Effect of electron and hole scavengers on the reaction rate. (h) Schematic illustration of the role of electrons and holes in the SMC reaction. (i) Mechanistic proposal of the flow of electrons in the composite catalyst (CeO₂/CoO/Pd).

Table 2 Rate enhancement with CeO₂/CoO/Pd with the increase in temperature

Temperature (°C)	k (CeO2/CoO/Pd) (μM min^−1)	k (CeO2/Pd) (μM min^−1)	Rate enhancement
30	0.26	3.54	13.61
40	0.53	6.54	12.34
50	1.51	12.08	7.99
60	4.01	28.61	7.13
70	4.95	37.95	7.66

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To understand the light-stimulated activation of Pd²⁺ ions on the catalyst surface, the coupling reaction has been conducted in the presence of electron and hole scavengers.⁷² The results are described in Table 3. Fig. 3h shows the effect of the electron and hole scavengers on the reaction rate. The SMC reaction is performed under optimized conditions between 4-bromotoluene and phenylboronic acid with benzoquinone (electron scavenger). The conversion of 4-bromotoluene to the cross-coupled product is virtually impeded in the presence of benzoquinone under light-stimulated conditions, demonstrating that the photoexcited electrons initiated the oxidative addition of 4-bromotoluene (R₁–X) to Pd⁰, forming the R₁–Pd–X bond. Fig. 3i illustrates the mechanistic role of photoexcited e⁻/h⁺ in the SMC reaction. Meanwhile, the conversion of 4-bromotoluene to the coupled product is nearly the same when the reaction is conducted at high temperature (100 °C) even in the presence of benzoquinone. The catalyst activity at high temperature in the presence of benzoquinone shows that Pd²⁺ ions capture the more thermally excited electrons moving across the CeO₂ to CoO on the surface to form Pd⁰. Nonetheless, the selectivity of the cross-coupled product decreases from 99% to 67% in the presence of benzoquinone. The decrease in selectivity is attributed to the role of benzoquinone in obstructing the thermally excited electrons from being grasped by the surface of Pd²⁺ ions, and hence the pathway of Pd⁰ activation by electrons is hindered. Therefore, the yield of the self-coupled product of phenylboronic acid is high, which has lowered the selectivity of the reaction.⁷² A control experiment was performed at 100 °C without adding any scavenger to differentiate between photocatalytic and merely thermal effects. The reaction achieved a conversion of almost 99%, representing the thermal effect on the rate of reaction. However, the selectivity decreased to 78%. The decrease in selectivity for the cross-coupled product is attributed to the increased kinetic energy of the reactants at higher temperature, encouraging the formation of the self-coupled product. This outcome verifies that while thermal activation can accelerate the rate of reaction, it challenges the selectivity of the cross-coupling process. This contrast verifies that the light-induced amplification is mostly facilitated by photogenerated electrons rather than being only attributable to localized heating or thermal effects. This result indicates that even under thermal conditions, surface-mediated electron transfer to Pd²⁺ contributes to maintaining catalytic reactivity, whereas the partial suppression of this process by benzoquinone highlights the significance of surface electron dynamics in photothermal contexts.

Table 3 The SMC reaction in the presence of electron and hole scavengers

Entry	Reaction condition	Benzoquinone	TEA	Conversion (%)	Selectivity (%)
Reaction conditions: 4-bromotoluene (0.5 mmol), phenylboronic acid (0.55 mmol), K2CO3 (1.5 mmol), catalyst (20 mg), EtOH/H2O (1 : 1, 4 mL), time 30 min, Xe lamp (150 W) equipped with an optical filter (cut off <420 nm).
1.	Light	50 mg	—	17	84
2.	100 °C	50 mg	—	91	67
3.	Light	—	50 mg	59	47
4.	100 °C	—	50 mg	87	79
5.	100 °C	—	—	99	78

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The next experiment is performed to investigate the function of photogenerated holes in the activation of phenylboronic acid. Phenylboronic acid in the reaction solution reacts with potassium carbonate (base) to produce anionic species, which are adsorbed by electrostatic interaction onto the catalyst's surface (CeO₂/CoO/Pd). On exposure to light, the photogenerated holes diffuse to the adsorption site of aryl boronic acid on the catalyst surface and trigger the oxidation of the adsorbed phenylboronic acid molecules, resulting in the cleavage of the C–B bond. Therefore, in the photocatalytic SMC reaction, phenylboronic acid is activated by holes, and this phenomenon has been experimentally verified by performing the SMC reaction in the presence of triethanolamine (TEA) as a hole scavenger.⁴⁵ The conversion of 4-bromotoluene to the SMC cross-coupled product decreased from 99% to 59%, whereas the appearance of the self-coupled product of 4-bromotoluene shows that phenylboronic acid is unable to couple with the aryl halide in the presence of a hole scavenger (TEA) under light conditions. The impact of holes on the activation of phenylboronic acid in SMC reactions is also investigated by conducting the reaction in the presence of a hole scavenger (TEA) at 100 °C in the absence of light. The comparable yield (87%) and selectivity (79%) of the cross-coupled product in this reaction suggest that TEA has no impact on the reactivity or selectivity of the coupled product in the dark. These results have affirmed that phenylboronic acid undergoes activation by photo-induced holes on the surface of CeO₂/CoO/Pd upon exposure to light irradiation. Fig. 3i also shows the mechanistic proposal of the flow of electrons in the composite catalytic system CeO₂/CoO/Pd. O₂⁻_(ads.) participates in the redox cycle. Surface oxygen donates electrons to reduce Co³⁺ → Co²⁺, generating oxygen vacancies and releasing O₂ on exposure to light. The photogenerated electrons in the CB of Co²⁺ readily move to the surface-adsorbed Pd²⁺, thereby reducing it into Pd⁰ and accelerating it for the oxidative addition step of aryl halide (R₁–X), forming the R₁–Pd–X complex. In addition to activity, recyclability is a crucial element for assessing the performance of catalysts. Following the reaction completion, the catalyst is separated from the concoction using the centrifugal method, washed extensively with water and ethanol to remove impurities, dried in a vacuum oven at 60 °C, and used for the next reaction with a fresh batch of the reaction mixture. Even after five cycles, the yield of the coupled product in the SMC standard reaction is still above 90%. This demonstrates the catalyst's exceptional cycle stability (Fig. S11a). Furthermore, the hot filtration test investigates the heterogeneity of the catalyst. The catalyst is removed from the reaction concoction after 15 min by centrifugation and filtration. The supernatant is collected, poured into the reaction vessel again, and stirred for another 1 h. The reaction aliquots are collected every 15 min, diluted with EtOAc and brine, extracted in EtOAc, and investigated by GC by adding nitrobenzene as an internal standard. It is important to note that no more coupled product is produced even after 1 h of reaction, ensuring that no leached Pd contents are present in the solution after filtration, which can act as a homogenous catalyst during the reaction (Fig. S11b). This experiment has affirmed the catalyst's heterogeneous nature and also shows that the designed catalytic system is extremely stable, in which Pd NPs strongly adhered to the CeO₂/CoO hybrid support without forming palladium black in the solution or palladium agglomerate during the reaction process.⁷³ Furthermore, ICP-MS analysis of the hot filtrate indicated a Pd concentration of 0.004 mg L⁻¹, representing approximately 0.4% of the total Pd initially contained in 20 mg catalyst, thereby confirming that Pd leaching under the reaction conditions is nearly negligible. The morphological characteristics of the recycled CeO₂/CoO/Pd catalyst are examined by XRD and HRTEM. The XRD analysis of CeO₂/CoO/Pd catalyst, both pre- and post-recycled catalyst, showed comparable results, indicating that the structure and oxidation state of the catalyst are essentially the same after the reaction (Fig. S12a). HRTEM is subsequently conducted on the recycled catalyst (after five consecutive cycles), showing the diminutive change in the particle size of Pd NPs that is ascribed to a degree of agglomeration occurring during the reaction process, signifying the slight loss in catalytic activity after every run. Fig. S12c presents the HRTEM image of the CeO₂/CoO/Pd, where lattice fringes of each element are distinctly visible, and the interplanar distance measured 0.317 nm, 0.25 nm, and 0.225 nm, corresponding accurately to the (111) lattice plane of Ce, (111) lattice plane of Co and (111) lattice plane of Pd (Fig. S12b and c). Elemental mapping is conducted on a recycled CeO₂/CoO/Pd sample, revealing a homogeneous distribution of Ce, Co, O, and Pd even after recycling 5 times (Fig. S12d–h). The findings demonstrated that the CeO₂/CoO/Pd catalyst maintained stability following the aforementioned process, exhibiting a slight loss of activity.

The SMC reaction with several other activated and non-activated aryl halide substrates is carried out under optimized reaction conditions (Table 4). Good to excellent yields are achieved using the designed catalyst under light-stimulated conditions. In the activated aryls (having electron-withdrawing groups), entries 1, 2, 4, 9 and 13–15 (Table 4) have given 100% yield under light irradiation conditions with TOF values of 2703, 1802, 5404, and 4095 h⁻¹, respectively.⁷⁴ However, with deactivated aryl halides (having electron-donating groups), i.e., entries 3, 7, 11 and 12, the Pd complex has given good quantitative conversion with 99–96% yield of the coupled product, having a TOF of 1311, 1730, 2675, 2648 h⁻¹ (Table 4). Table S3 shows a detailed comparative analysis of the current work with the literature. To summarize, CeO₂/CoO/Pd has demonstrated excellent catalytic activities in a short reaction time (30 min) and can tolerate a wide range of functional groups.^75,76

Table 4 Substrate scope of Suzuki-Miyaura coupling over CeO₂/CoO/Pd

Entry	X	R	Temp (°C)	Light condition
				Time (min)	Conversion (%)	Selectivity (%)	TOF (h^−1)
Reaction conditions: aryl halide (0.5 mmol), phenylboronic acid (0.55 mmol), K2CO3 (1.5 mmol), catalyst (20 mg), EtOH/H2O, Xe lamp (150 W) equipped with an optical filter (cut off <420 nm).
1	I	–H	25	30	100	99	2703
2	I	–CH3	25	45	100	99	1802
3	I	–OCH3	25	60	97	98	1311
4	I	–CN	25	15	100	99	5405
5	Br	–H	50	20	99	99	4054
6	Br	–CH3	50	30	98	99	2675
7	Br	–OCH3	50	45	96	98	1730
8	Br	–CHO	50	15	99	98	5351
9	Br	–NO2	50	15	100	99	5405
10	Br	–COCH3	50	15	98	98	5297
11	Br	–NH2	50	30	99	99	2675
12	Br	–OH	50	30	98	99	2648
13	Br	–COOH	50	15	100	99	5405
14	Br	–CN	50	20	100	99	4095
15	Br	–CONH2	50	20	100	99	4095
16	Br	–COOCH3	50	20	99	99	4054
17	Cl	–H	50	120	51	82	270
18	Cl	–CH3	50	120	39	88	203
19	Cl	–COOH	50	120	63	90	338
20	Cl	–CN	50	120	70	92	365

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In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) analysis has been further performed to analyze the reaction mechanism. In situ FTIR gives real-time information about molecular dynamics on the catalyst surface and enables continuous monitoring of the reaction progress over time. In situ FTIR has verified the successful coupling of phenylboronic acid and 4-bromotoluene. The reaction intermediates appeared progressively as the time of light exposure increased. The distinct IR absorption signals undergo a shift or weaken the reactant peaks, and the emergence of new signals has verified the formation of the coupled product under light exposure. Fig. 4a illustrates that the peak at 2900 cm⁻¹ along with its shoulder peak at 2980 cm⁻¹ becomes more obvious as the reaction proceeds, indicating the stretching vibration of the C–H bond in –CH₃ in the coupled product. The prominent peak at 1470 cm⁻¹ signifies C–C stretching in the phenyl ring that has shifted to 1450 cm⁻¹ due to the coupling of two phenyl rings (Fig. 4b). Notably, the C–H peak intensity progressively enhanced as the time of light exposure increased, certifying a photocatalytic SMC reaction. The IR signals in the range of 1650–1750 cm⁻¹ demonstrate the stretching vibration of aromatic C=C bonds, which further affirms the formation of the biphenyl product from the SMC reaction. Another noticeable signal at 1050 cm⁻¹ is attributed to the stretching vibration of C–O due to solvent (ethanol) interaction with the coupled product, indicating the deformation vibration of the –CH₃ group in the product due to the solvation effect. Fig. 4c illustrates the plausible pathway for the SMC reaction catalyzed by the CeO₂/CoO/Pd catalyst. The CeO₂/CoO bimetallic hybrid system absorbed photons of light following the electron excitation from VB to CB, generating an electron/hole pair for conducting the SMC reaction. Indeed, the catalytic efficacies of active centers in the SMC reaction are significantly influenced by their electronic states. Since the catalyst must act as a nucleophile during the oxidative addition phase and as an electrophile during both the transmetalation and the reductive elimination phases of the coupling reaction.^71,77 The integration of cobalt oxide into the CeO₂ structure has been proposed to reduce the bandgap and promote the formation of OVs, thus augmenting the concentration of surface-adsorbed oxygen (O_ads) and enhancing the mobility of oxygen, as evidenced by XPS analysis of O 1s spectra. These oxygen vacancies are stabilized by the reversible redox process of Co²⁺ + Ce⁴⁺ ↔ Co³⁺ + Ce³⁺. In this hybrid system, CoO has also enabled the more effective absorption of light due to the narrowing of the bandgap of the bimetallic composite support (CeO₂/CoO) compared to pristine CeO₂, and also acts as an electron shuttle for the transfer of excited electrons between CeO₂ and Pd. The addition of cobalt oxide to CeO₂ has introduced impurity energy levels in between the CB and the VB of the original bandgap of CeO₂, which function as intermediate energy levels to capture photoexcited electrons, thus stabilizing the redox cycle between Co²⁺ and Pd²⁺. The e⁻ emigrated from the O₂⁻(abs) of CeO₂ to the empty 3d orbital of Co, which is lower in position than the CB of Ce 4f, signifying less energy requirement for excitation. The photogenerated electrons in the CB of Co readily move to the surface-adsorbed Pd²⁺, thereby reducing it into Pd⁰ and accelerating it for the oxidative addition step of aryl halide (R₁–X), forming the R₁–Pd–X complex. Conversely, the defective surface structure of CeO₂ hosts several oxidative species (O₂⁻/O²⁻), which are readily transported from the CeO₂ surface to the neighboring interface of CoO, facilitating the redox cycle of the Co³⁺/Co²⁺ (highly active sites) surface. The kinetic energy is provided by the heating effect of the reaction system to the photoexcited electrons, further averting the recombination rate of e⁻/h⁺ pairs. The holes formed in the valence band of ceria (CeO₂) are retained by the VB of O 2p and activate the aryl boronic acid for the coupling cycle. In fact, the aryl boronic acid in the basic media generates anionic species and is adsorbed on the catalyst surface through electrostatic interaction. However, on exposure to light, electron/hole pairs are generated, diffusing the holes to the adsorption site of aryl boronic acid. The migration of holes to the adsorption site of aryl boronic acid results in the release of the oxidized boronic acid complex that reacts with R₁–Pd–X to exchange the halide group with another aryl group from boronic acid and forms the R₁–Pd–R₂ complex. Finally, reductive elimination releases the coupled (R₁–R₂) product. The Pd²⁺ ions on the catalyst surface function as a strong electron acceptor, promoting the rate of transmetalation and reductive elimination step. In this context, the concurrent utilization of photo and thermal energy enhances the pace of the coupling reaction. In brief, the synergistic impact of CeO₂/CoO/Pd in the heterojunction significantly amplifies the photocatalytic activity of the catalytic system, where CeO₂ provides stability and keeps the e⁻/h⁺ pairs separated, and CoO has improved light absorption and provides shuttle service to the electron transfer process between CeO₂ and Pd NPs.

Fig. 4 (a and b) In situ FTIR spectrum of light-stimulated SMC reaction using the CeO₂/CoO/Pd catalyst. (c) Mechanism for the Suzuki coupling reaction catalyzed by the CeO₂/CoO/Pd catalyst.

Conclusions

In summary, we have devised a simple process for synthesizing the heterogeneous binary nanohybrid CeO₂/CoO/Pd system by calcination/annealing following reductive post-treatment. The catalytic potential of the designed catalytic system is investigated in the SMC reaction. The findings indicate that the synergistic effect of CeO₂/CoO has extended the range of light absorption by narrowing the bandgap structure of nanoceria and facilitating charge redistribution at the interface under light conditions. The photogenerated electrons in the CB of Co readily move to the surface-adsorbed Pd²⁺, thereby reducing it into Pd⁰, hence enhancing the catalytic activity in SMC reactions. The narrow bandgap of CeO₂/CoO/Pd also resulted in an increased concentration of e⁻/h⁺ pairs capable of activating aryl halide and phenylboronic acid with less thermal energy input, therefore lowering the overall energy barrier (E_a 53 kJ mol⁻¹) of the SMC reaction.

Author contributions

Sobia Jabeen: project design, experimental design, and writing the manuscript. Yaxi Li, Yuanyuan Cheng, and Yunliang Liu: integration and analysis of data. Zhiquan Lang: revising the manuscript. Naiyun Liu: project administration. Haitao Li: project administration and funding acquisition. All the authors have approved the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data needed to support the conclusions in the paper are presented in the manuscript and/or the supplementary information (SI) material including characterization details for XRD, XPS, Raman, FTIR, in situ FTIR, BET, GC; IR thermography procedure; electrochemical testing procedures; TEM, HRTEM, SEM, XPS survey scan, UV-vis DRS spectra, Mott–Schottky plot images of precursor and catalyst; catalytic activities of catalysts (CeO₂/CoO/Pd, CeO₂/Pd, CoO/Pd) under dark and light-stimulated SMC reaction, time-dependent conversion of 4-bromotoluene and the corresponding ln (C_o/C_t)- time curves for SMC over different catalysts (CeO₂/CoO/Pd-light, CeO₂/CoO/Pd-thermal, CeO₂/Pd) at varied temperatures, recycling efficiency of CeO₂/CoO/Pd in standard SMC reaction, leaching test in standard SMC reaction, XRD, HRTEM of CeO₂/CoO/Pd fresh and recycled sample; tables of screening and control experiments for optimization of reaction conditions for SMC reaction, optimization of reaction conditions for SMC, comparative analysis of current work with literature. Additional data related to this paper may be requested from the corresponding author upon request.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5cy00994d.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 52072152, 51802126), the Jiangsu University Jinshan Professor Fund, the Jiangsu Specially-Appointed Professor Fund, Open Fund from Guangxi Key Laboratory of Electrochemical Energy Materials, Zhenjiang “Jinshan Talents” Project 2021, China Postdoctoral Science Foundation (2022M721372), “Doctor of Entrepreneurship and Innovation” in Jiangsu Province (JSSCBS20221197), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX22_3645).

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