Modulating the microenvironment of AuPd nanoparticles using metal–organic frameworks for selective methane oxidation

Abstract

Direct and selective oxidation of methane (CH₄) into methanol (CH₃OH) under ambient conditions remains a grand challenge because of the high energy barrier of CH₄ activation and the complicated processes involved. Herein, by incorporating AuPd alloy nanoparticles (NPs) into a series of Cu²⁺-doped metal–organic frameworks (MOFs), namely AuPd@Cu-UiO-66_x, efficient and direct catalytic conversion of CH₄ to CH₃OH can be achieved at mild temperature (70 °C) with H₂O₂ as an oxidant. The Cu-UiO-66 serving as the microenvironment parameter not only regulates the electronic state of AuPd NPs to improve the CH₄ adsorption and activation, but also affects the generation of hydroxyl radicals (˙OH) in H₂O₂ reaction pathways, which consequently results in a volcano-type dependency of the CH₃OH selectivity on the Cu contents. This work represents the first finding on achieving direct catalytic oxidation of CH₄ to CH₃OH under mild conditions by modulating the microenvironment of catalytic centers based on a MOF platform.

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Introduction

The direct conversion of methane, the major component of natural gas, to olefins, aromatics, oxygenates, and other products, has been recognized as a potentially economical and environmentally friendly way to generate value-added chemicals from abundant and inexpensive sources.^1–3 Among various routes of methane conversion, the direct and selective oxidation of methane to methanol, known as the ‘Holy Grail’ reaction in catalysis science,^4,5 is particularly promising. However, this transformation has long been challenged by the stable and inert C–H bonds (ΔH_C–H = 104 kcal mol⁻¹), low polarizability, and negligible electron affinity of methane.^4–8 Therefore, energy-intensive routes are conventionally required for CH₄ activation.^2,3 In addition, the challenges in the CH₄ to CH₃OH process are also tangled by the fact that CH₃OH can be more easily oxidized than CH₄, which leads to unexpected peroxidation products.^6,7 Therefore, the development of suitable systems for efficient and selective direct conversion of CH₄ to CH₃OH under mild conditions is highly desirable yet challenging.

To achieve selective oxidation of CH₄ to CH₃OH under mild conditions, environmentally benign hydrogen peroxide (H₂O₂) has been adopted as the oxidant.^4,6,9–11 Unfortunately, this may lead to the formation of methyl-hydroperoxide (CH₃OOH) as the main product by the combination of ˙CH₃ with dissolved O₂ originating from the decomposition of H₂O₂,^9,12 or produce a significant amount of byproducts due to the overoxidation of methanol by ˙OH originating from the homocleavage of H₂O₂.^10,11 Although a few systems, including metal-modified zeolites and metal nanoparticle-based heterostructures,^5,11–14 have been employed, the rational control of the catalytic performance is still far from satisfactory to achieve both high selectivity and activity. The above situation is likely rooted in the complex and highly interconnected processes involved in catalytic CH₄ oxidation using H₂O₂, such as the activation of CH₄ and H₂O₂, the degradation of H₂O₂, overoxidation of methanol, and more.^15–17 Evidently, the complicated nature of the H₂O₂-participated CH₄ oxidation requires a precise balance of these processes for achieving high activity and selectivity. A platform that allows the sophisticated control of H₂O₂ homocleavage by microenvironment modulation is thus highly desirable.

Based on the above analysis, it is assumed that metal–organic frameworks (MOFs), a class of porous crystalline materials featuring high surface areas and tailorable structures,^17–20 would be an ideal platform for creating a suitable microenvironment for hosted active centers. Different components involved in catalytic sites and the microenvironment can work synergistically to target optimal catalytic performance. Specifically, the permanent porosity of MOFs can provide congenital advantages for stabilizing active metal nanoparticles (NPs).^21–25 The high tailorability of MOFs makes it feasible to introduce extra auxiliary components through various modification strategies, enabling the microenvironment modulation for metal NPs in a rational manner.¹⁹

In this work, the representative MOF, UiO-66 with defects in varying degrees, denoted as UiO-66_x (x represents the molar ratio of (acetic acid)/(the linker), x = 0, 50, 100, 150, and 200), has been fabricated. Upon furnishing Cu²⁺ onto the defect sites of Zr-oxo clusters of UiO-66, AuPd NPs are further incorporated to afford AuPd@Cu-UiO-66_x composite (Fig. 1). AuPd NPs were utilized in our study for their capability in partial methane oxidation.¹¹ Remarkably, anchoring Cu species to the Zr-oxo clusters around AuPd NPs effectively tunes the selectivity to CH₃OH with a volcano-type dependency on the Cu loadings, where the optimized CH₃OH selectivity is as high as 80.97%. The regulated selectivity arises from the combined effect of the enhanced CH₄ adsorption and the weakened ˙OH formation accompanied by the Cu²⁺ anchoring. Although MOFs have been reported in the quasi-catalytic CH₄ oxidation under high-temperature in the gas phase,^26,27 this study, integrating AuPd active sites and the microenvironment modulation by systematically tailoring Cu loadings based on a MOF platform, achieves highly selective CH₄ oxidation under mild conditions.

Fig. 1 Schematic illustration showing the synthetic process for AuPd@Cu-UiO-66_x.

Results and discussion

UiO-66_x compounds with different amounts of structural defects were synthesized by reacting ZrCl₄ and terephthalic acid with different equivalents of acetic acid as a modulator in DMF. Powder X-ray diffraction (XRD) patterns suggest that all UiO-66_x compounds maintain similar high crystallinity to UiO-66 (Fig. S1†). Scanning electron microscopy (SEM) images display that the sizes of MOF particles gradually increase and the morphology changes from intergrown prototypes to octahedral nanocrystals, along with more acetic acid being used (Fig. S2†). N₂ sorption isotherms demonstrate that the Brunauer–Emmett–Teller (BET) area for UiO-66_x gradually increases from 736 to 1151 m² g⁻¹ while the pore volume increases from 0.55 to 0.67 cm³ g⁻¹ with increased amounts of acetic acid (Fig. S3†). Cu-UiO-66_x were prepared by a microwave-assisted reaction of CuCl₂·2H₂O and UiO-66_x in acetonitrile. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis demonstrates that the Cu loadings are approximately 0.41–2.42 wt% in Cu-UiO-66_x (Table S1†). To identify the existing form of Cu and its electronic state in the catalysts, Cu-UiO-66₂₀₀ is chosen considering that its higher Cu content than other catalysts would be beneficial to the characterization precision. The diffuse reflectance infrared Fourier transform (DRIFT) spectra show that the intensities of peaks at 3673 and 2769 cm⁻¹, respectively attributed to the terminal –OH/–OH₂ and μ₃-OH on the defect sites, are significantly lower in Cu-UiO-66₂₀₀ than those in UiO-66, which supports the immobilization of Cu atoms to the chelating/defect sites in Cu-UiO-66_x (Fig. S4†).^28–30 The Raman spectrum for Cu-UiO-66₂₀₀ clearly showed the Cu–O vibration peak at 150 cm⁻¹, which also supports that Cu is anchored to UiO-66 (Fig. S5†).³¹ The Cu K-edge X-ray absorption near-edge structure (XANES) spectrum of Cu-UiO-66₂₀₀ displays an absorption threshold analogous to that of CuO, indicating that the oxidation state of Cu is about +2 (Fig. 2a). The Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectrum of Cu-UiO-66₂₀₀ shows only one dominant peak at approximately 1.5 Å, confirming the atomically dispersed Cu sites in Cu-UiO-66₂₀₀ (Fig. 2b).³⁰

Fig. 2 (a) The normalized XANES spectra, (b) FT-EXAFS spectra for Cu-UiO-66₂₀₀, CuO, and Cu foil. (c) TEM and (d) HRTEM images of AuPd@Cu-UiO-66₁₀₀. (e–i) High-angle annular darkfield scanning transmission electron microscopy image of AuPd@Cu-UiO-66₁₀₀, and the corresponding Zr, Au, Pd, and overlapped elemental mapping for the selected area.

To prepare AuPd@Cu-UiO-66_x, the mixture of HAuCl₄ and K₂PdCl₄ aqueous solution was impregnated into Cu-UiO-66_x and subsequently reduced in a H₂/Ar atmosphere at 200 °C. Powder XRD patterns suggest that the crystallinity of Cu-UiO-66_x is maintained after the incorporation of AuPd NPs (Fig. S6†). In addition, the comparison shows that the (200), (220), and (222) diffraction peaks of HKUST-1 were absent in AuPd@Cu-UiO_x, indicating that there is no formation of HKUST-1 in our work when Cu is added to the MOF and subjected to high temperature (Fig. S6†). The contents of Au and Pd determined by ICP-AES are similar across all AuPd@Cu-UiO-66_x samples, which are approximately 3.5 wt% for Au and 1.5 wt% for Pd, respectively (Table S1†). The octahedral shape of UiO-66_x remained after the AuPd incorporation, as demonstrated by the SEM images and the selected area electron diffraction (SAED) pattern has been obtained to demonstrate that UiO-66 is stable under the tested condition (Fig. S7†). In addition, transmission electron microscopy (TEM) observation of AuPd@Cu-UiO-66_x shows that the tiny AuPd NPs with rough sizes of 1.5 nm, matching the pore sizes of defective UiO-66,^32,33 are highly dispersed throughout Cu-UiO-66_x particles (Fig. 2c, S8–S12†). Although the sizes of the AuPd NPs can be larger than the free volume of the octahedron cages (12 Å) in UiO-66, we hypothesize that large-sized NPs may penetrate multiple adjacent cages through the pore aperture or appear where the defects are present. The direct comparison between the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and secondary electron STEM (SE-STEM) images acquired at the same location supports that the AuPd NPs are located inside the crystallites of AuPd@Cu-UiO-66₁₀₀ (Fig. S13†).^34–36 Lattice fringes with a d-spacing of ∼0.23 nm are observed by high-resolution TEM (HRTEM) (Fig. 2d), consistent with the (111) plane of AuPd NPs.³⁷ The energy-dispersive X-ray mapping results suggest homogeneous distributions of Au and Pd elements in the MOF particle (Fig. 2e–i), further supporting the well-dispersed feature of AuPd NPs in UiO-66_x. N₂ sorption isotherms demonstrate that the porous structures of Cu-UiO-66_x are all maintained after the incorporation of AuPd NPs, albeit with slightly decreased BET areas due to the mass and pore occupation by AuPd NPs (Fig. S14†). The pore size distributions of the AuPd@Cu-UiO-66_x are mostly in the range of 7–15 Å, which are still large enough for CH₄, H₂O₂, and intermediates involved in CH₄ oxidation.

The performance of AuPd@Cu-UiO-66_x catalysts for direct catalytic oxidation of CH₄ to CH₃OH has been evaluated in an aqueous H₂O₂ solution (Fig. 3 and Tables S2, S3†). Without the incorporation of AuPd NPs, UiO-66 or Cu-UiO-66 alone fails to catalyze the oxidation of CH₄. The Au@UiO-66 shows good reactivity for CH₄ oxidation to give the oxygenated products CH₃OH, CH₃OOH, and HCOOH, among which CH₃OH only takes up a very small portion with a selectivity of 30.19%. Although Pd@UiO-66 shows a moderately high CH₃OH selectivity of 50.39%, its reactivity toward CH₄ oxidation is very poor since the total oxygenated products are only 39% of those for Au@UiO-66. In contrast to these comparison catalysts, AuPd@UiO-66 gives a reasonable CH₃OH selectivity of 53.84% and a much higher CH₃OH yield (1.61 μmol) than both Au@UiO-66 (0.93 μmol) and Pd@UiO-66 (0.63 μmol), suggesting AuPd NPs are indispensable for high activity and selectivity in the conversion. Despite this, colloidal AuPd NPs present a low CH₃OH selectivity of 23% while CH₃OOH is the major species (64%) among all the oxidation products. The significantly lower CH₃OH yield of colloidal AuPd NPs (0.58 μmol) than that of AuPd@UiO-66 (1.61 μmol) indicates the critical role of the MOF in this reaction.

Fig. 3 Catalytic performance of AuPd@UiO-66, AuPd@Cu-UiO-66_x, UiO-66, Cu-UiO-66, Au@UiO-66, Pd@UiO-66, and colloidal AuPd NPs in the CH₄ oxidation (reaction conditions: catalyst 27 mg, CH₄ pressure 30 bar, H₂O₂ 0.5 M (aq), 70 °C, reaction time 30 min).

Upon introducing Cu species into AuPd@UiO-66, an enhanced CH₃OH selectivity of 54.57% is observed with AuPd@Cu-UiO-66₀ as the catalyst. With increased Cu loading, the selectivity to CH₃OH first exhibits a trend of increase and reaches a maximum of 80.97% with a normalized production rate of 198.52 μmol g_cat⁻¹ h⁻¹ using AuPd@Cu-UiO-66₁₀₀. Further increasing the Cu content leads to a decrease of the selectivity to 76.60% and 72.87% for AuPd@Cu-UiO-66₁₅₀ and AuPd@Cu-UiO-66₂₀₀, respectively. The selectivity of CH₃OH exhibits a volcano-type dependency on the Cu loadings in AuPd@Cu-UiO-66_x (Fig. 3), unambiguously demonstrating that the Cu species effectively regulates the AuPd selectivity in the partial oxidation of CH₄ to CH₃OH. The comprehensive normalized conversions for all the C1 products have also been provided (Table S2†). It is worth noting that by simply increasing the Cu content from 0.4 wt% in AuPd@Cu-UiO-66₀ to 1.4 wt% in AuPd@Cu-UiO-66₁₀₀, a 48% improvement of the methanol selectivity can be achieved. Since Cu in AuPd@Cu-UiO-66_x is anchored to the defect sites of UiO-66, the defect sites exposed in AuPd@Cu-UiO-66_x catalysts will be negligible. Therefore, with other chemical parameters kept the same in AuPd@Cu-UiO-66_x, the observed difference of the selectivity provided by AuPd@Cu-UiO-66_x should be ascribed to their different Cu loadings. Considering that Cu actually accounts for only very small mass percentages in the catalysts, the dependency of selectivity on Cu loading highlights the importance of the precise control of the microenvironment to the optimization of catalytic performance. In addition, a recent report on the use of AuPd NPs encapsulated by ZIF-8 gave a selectivity of 21.9% for CH₃OH.³⁸ Compared with AuPd@Cu-UiO-66₁₀₀, the sharp performance contrast is another sign of the significance of the microenvironment modulation.

To examine the source of the products, the isotope trace experiment by using ¹³CH₄ has been conducted. Synchrotron radiation photoionization mass spectrometry (SR-PIMS) for the reaction mixture gives peaks at m/z = 33 and m/z = 47 at a photon energy of 11.8 eV, which are respectively corresponding to ¹³CH₃OH and H¹³COOH (Fig. 4a). The results unambiguously confirm that CH₄ is the exclusive carbon source of CH₃OH and HCOOH products, which also demonstrate the successful implementation of catalytic CH₄ oxidation over the AuPd@Cu-UiO-66₁₀₀ catalyst. Interestingly, a peak at m/z = 31 assigned to H¹³CHO is also observed (Fig. 4a). HCHO has been confirmed as the primary and active intermediate in the oxidation of CH₃OH.^39,40 However, the low concentration of HCHO in the reaction and the overlap of its chemical shift with solvent make its detection extremely challenging. In our system, the signal of HCHO becomes more apparent when CH₃OH is adopted as the reactant (Fig. S15†). The in situ SR-PIMS test demonstrates the appearance of HCHO after 10 min in the CH₃OH oxidation, prior to the observation of HCOOH (Fig. S16†). HCOOH can be detected when HCHO is used as the reactant (Fig. S17†). These results support that CH₄ is first converted to the desired product CH₃OH, and further overoxidized into HCOOH through the formation of intermediate HCHO (Fig. 4b).

Fig. 4 (a) Photoionization mass spectrum of the products using ¹³CH₄ and ¹²CH₄ as the reactant. (b) Proposed reaction path for methane oxidation over AuPd@UiO-66₁₀₀.

N₂ sorption isotherms (Fig. S18†) and powder XRD pattern of AuPd@Cu-UiO-66₁₀₀ after catalysis manifest that the porosity and crystalline feature of the MOF is maintained (Fig. S19†). No identifiable peak assigned to AuPd NPs can be observed in powder XRD pattern, reflecting that AuPd NPs are not agglomerated during the catalysis, in line with the TEM observation (Fig. S20†). The metal loadings are maintained after the reaction based on ICP-AES results, suggesting the absence of metal leaching during the reaction (Table S4†). The FT-IR, ¹H NMR spectra, and elemental analysis of AuPd@Cu-UiO-66₁₀₀ before and after the reaction are nearly identical, implying that the organic components in the catalysts keep intact during the reaction (Fig. S21, S22, and Table S4†). Furthermore, no apparent activity and selectivity loss occur to AuPd@Cu-UiO-66₁₀₀ in three consecutive cycles (Fig. S23†), demonstrating its good reusability. All the above results evidence the stability of the catalysts under the tested conditions.

To disclose the role of Cu in catalysis, X-ray photoelectron spectroscopy (XPS) has been conducted to investigate the oxidation state of Au and Pd species (Fig. S24†). The AuPd@UiO-66 presents two sets of peaks: one set of peaks are at binding energies of 83.65 and 87.35 eV, corresponding to Au⁰ 4f_7/2 and Au⁰ 4f_5/2, respectively; the other set of peaks at 335.22 and 340.61 eV are ascribed to 3d_5/2 and 3d_3/2 of Pd⁰, respectively.^41–45 Compared to monometallic Au⁰ and Pd⁰, both Au 4f and Pd 3d peaks in AuPd@UiO-66 shift to lower binding energies. The electronic state change of Au and Pd indicates the formation of AuPd alloy, which is supported by the previous reports.^37,41–45 With increased Cu loadings, the Au 4f and Pd 3d peaks gradually shift to higher binding energies in AuPd@Cu-UiO-66_x. These changes may be ascribed to the electron-withdrawing effect of Cu²⁺, leading to electron transfer from AuPd to Cu²⁺.²⁹ To further probe the modulated electronic state of AuPd NPs in AuPd@Cu-UiO-66_x, the carbon monoxide (CO) adsorption analysis using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) has been carried out (Fig. S25†). Both adsorption bands of linearly adsorbed CO on Au (2150–2000 cm⁻¹) and twofold bridge-bonded CO on Pd (2000–1900 cm⁻¹) shift to higher wavenumbers with the change of wavenumber >20 cm⁻¹ when creasing Cu loadings, which further supports enhanced electron transfer from AuPd to the MOFs with increased Cu loadings.^46,47 This phenomenon can be ascribed to the electron-withdrawing effect of Cu²⁺ attached to the Zr-oxo clusters of UiO-66, leading to the electron transfer from Au and Pd to Cu. With the increase of Cu content, the electron-withdrawing effect becomes stronger, resulting in a gradual increase of the binding energy. Taking all these spectroscopic results together, it is suggested that the Cu²⁺ incorporation effectively modulates the AuPd electronic state, which may be responsible for the observed different CH₃OH selectivity of AuPd@Cu-UiO-66_x.^14,48

It is generally believed that CH₄ oxidation using H₂O₂ as an oxidant involves at least three key steps, the adsorption and activation of CH₄ to form active *CH_x species on the surface of catalysts¹³ or through the attack of CH₄ by ˙OH,⁴⁹ the homolytic cleavage of H₂O₂ to generate ˙OH, and the combination of *CH_x and ˙OH (Fig. 4b).⁵ To investigate the adsorption of CH₄ on catalysts, CH₄ temperature-programmed desorption coupled with mass spectrometric analysis (TPD-MS) has been carried out. The results show that the desorption temperature of CH₄ from AuPd@Cu-UiO-66₁₀₀ is approximately 184 °C, in comparison to 158 °C for AuPd@UiO-66 (Fig. 5a, S26†), revealing stronger CH₄ adsorption on AuPd@Cu-UiO-66₁₀₀. The CH₄ sorption displays the higher CH₄ adsorption capacity (17.6 cm³ g⁻¹) of AuPd@Cu-UiO-66₁₀₀ than AuPd@UiO-66 (13.5 cm³ g⁻¹) at 1 bar and 298 K (Fig. S27†). From AuPd@UiO-66 to AuPd@Cu-UiO-66₁₀₀, the increase of CH₄ adsorption is 30%, which is higher than the magnitude of the increase of the BET area, supporting the enhanced CH₄ adsorption of the latter. This enhanced CH₄ adsorption would be beneficial to its activation on AuPd sites and competition with the CH₃OH adsorption to avoid overoxidation,⁵⁰ giving rise to enhanced activity and selectivity toward CH₃OH.

Fig. 5 (a) TPD-MS profiles (m/z = 15) of CH₄ on AuPd@UiO-66 and AuPd@UiO-66₁₀₀. (b) The C 1s XPS spectra of the AuPd@UiO-66₁₀₀ in ultrahigh vacuum and after introducing 1 mbar of CH₄ at different temperatures. (c) DRIFT spectra of CH₄ adsorbed AuPd@UiO-66 and AuPd@UiO-66₁₀₀ after being purged with Ar gas for 30 min. (d) EPR trapping experiment over AuPd@UiO-66 and AuPd@UiO-66₁₀₀ with H₂O₂ as the oxidant and DMPO as the radical trapping agent.

To identify the possible behavior of CH₄ after its adsorption, near-ambient pressure XPS (NAP-XPS) has been conducted. Before CH₄ exposure, the C 1s spectrum of AuPd@Cu-UiO-66₁₀₀ displays two peaks at 284.8 and 288.8 eV attributed to the C=C, C–C, C–H, and C=O bonds in the MOF linkers (Fig. 5b).^51,52 The subsequent exposure of the sample to 1 mbar of CH₄ at 300 K doesn't result in additional peaks, probably due to the overlap of signals between CH₄ and C=C, C–C, and C–H bonds of the MOF. When the temperature is raised to 450 K, deconvolution of the C 1s spectrum gives a new peak at 285.5 eV (Fig. 5b). This peak can be assigned to the *CH_x species, which may include CH₃, CH₂, and CH, inferring the activation of CH₄ on the catalyst.⁵⁰In situ DRIFTS has been further conducted to investigate the transformation of CH_x species on different catalyst surfaces in the presence of H₂O₂. When CH₄ is exposed to the catalyst for 30 minutes, peaks at 1305 and 3015 cm⁻¹ assigned to the typical vibration of absorbed CH₄ are observed (Fig. S28†).^53,54 After the physically adsorbed CH₄ is removed by flushing Ar flow through an H₂O₂ aqueous solution, the peaks in the ranges of 1500–1800 cm⁻¹ and 2700–3000 cm⁻¹ appear (Fig. 5c). The peaks at 1657 and 1745 are assigned to *CO₃²⁻, and the signal at 2828 cm⁻¹ is ascribed to the *CH₃O intermediate.^53–55 The appearance of these peaks indicates the transformation of *CH_x to oxygen-containing species on the catalyst with the participation of H₂O₂. Compared to those in AuPd@UiO-66, the intensity of the peak at 2828 cm⁻¹ (*CH₃O) increases and the intensities at 1657 and 1745 cm⁻¹ (*CO₃²⁻) decrease in the presence of AuPd@Cu-UiO-66₁₀₀, in line with its enhanced CH₃OH selectivity. The above results demonstrate that AuPd NPs play several important roles in the reaction, in which they not only affect CH₄ adsorption but also influence the activation and transformation of CH₄. The modulated electronic state of AuPd by the incorporated Cu²⁺ will, thus, have a profound effect on the CH₃OH selectivity.

Reactive oxygen species originating from H₂O₂, such as superoxide anion radicals (O₂˙⁻) and hydroxyl radicals (˙OH), are the other group of important intermediates in CH₄ oxidation by H₂O₂. Electron paramagnetic resonance (EPR) experiments have been conducted to detect the radical species involved in the reaction. With 5,5-dimethyl-pyrroline-N-oxide (DMPO) as a radical trapping agent, EPR spectra exhibit a characteristic signal of DMPO-OH (Fig. 5d). This result suggests that ˙OH is the main active species in methane oxidation. Compared with AuPd@Cu-UiO-66, the intensity of EPR signals in AuPd@Cu-UiO-66₁₀₀ is lower, indicating its reduced generation of ˙OH. No DMPO-CH₃ peak is observed likely attributed to the fast combination of as-generated ˙CH₃ with the relatively abundant ˙OH.^56,57 When CH₄ is replaced by CH₃OH, products HCOOH and CO₂ are detected (Fig. S29†), implying that ˙OH generated from H₂O₂ also participates in the overoxidation process. Generally, H₂O₂ has two competitive pathways in CH₄ oxidation, the generation of O₂ and H₂O through decomposition and the formation of ˙OH from homolytic cleavage.^15,46–49 It is found that the decomposition rate of H₂O₂ increases with increased Cu content in AuPd@Cu-UiO-66_x (Fig. S30†), probably owing to the catalytic effect of Cu in H₂O₂ decomposition through the Cu^I/Cu^II redox couple.¹⁶ Therefore, with increased Cu loadings in AuPd@Cu-UiO-66_x, the generation of ˙OH will be more or less suppressed due to the enhanced decomposition of H₂O₂. The introduction of Cu at too low loadings will result in the formation of excess ˙OH and lead to over-oxidation during CH₄ oxidation. On the other hand, a too-high loading of Cu²⁺ will reduce the concentration of ˙OH around the catalytic sites, leading to inhibition of oxidation. Therefore, only a suitable content of Cu²⁺ will allow optimal catalytic performance.⁵⁸

The above-mentioned characterization studies and in situ technique shed light on the mechanism of the oxidation of CH₄ to CH₃OH mediated by anchored Cu regarding the activation of CH₄ and the formation of CH₃OH as shown in Fig. 4b, in which the Cu species plays multiple roles as the microenvironment factors for AuPd sites. Firstly, the incorporation of Cu modulates the AuPd electronic state, giving rise to enhanced adsorption of CH₄ on AuPd NPs. Secondly, the presence of Cu regulates H₂O₂ reaction pathways. With increased Cu loadings, the generation of ˙OH is reduced, which weakens both the CH₃OH formation and its overoxidation processes.

Conclusions

In summary, AuPd alloy NPs incorporated into Cu-modified MOF, namely AuPd@Cu-UiO-66_x, have been fabricated and exhibit excellent activity and selectivity in the direct oxidation of CH₄ to CH₃OH in aqueous solution with H₂O₂ as an oxidant under mild conditions. The selectivity presents a volcano-type trend with increased Cu amount in the catalysts, among which AuPd@Cu-UiO-66₁₀₀, featuring an optimized Cu²⁺ content, possesses the best CH₃OH selectivity of 80.97%. A slight change of Cu content from 0.4 wt% in AuPd@UiO-66₀ to 1.4 wt% in AuPd@UiO-66₁₀₀ significantly boosts the CH₃OH selectivity by 48%. Mechanistic investigations reveal that the auxiliary Cu²⁺ on the MOF skeleton, acting as the microenvironment, has a profound influence on AuPd NPs for CH₄ oxidation, where it not only modulates the AuPd electronic state to improve CH₄ adsorption but also regulates the formation of ˙OH in H₂O₂ reaction pathways. This work provides significant insights into the intricate processes involved in CH₄ oxidation toward optimized methanol selectivity, which would open an avenue to microenvironment modulation of active sites for enhanced catalysis.

Author contributions

H.-L. J. conceived the idea and supervised the project. J. S. and M.-L. G. performed the experiments and collected the data. C. L. and Y. P. conducted the synchrotron radiation photoionization mass spectrometry experiments. D. Y. conducted the CH₄ sorption isotherm experiments. H.-L. J., Z. M., and J. S. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2021YFA1500402), the NSFC (U22A20401, 22161142001, and 22101269), the International Partnership Program of CAS (123GJHZ2022028MI), and the Fundamental Research Funds for the Central Universities (WK3450000007 and WK2060000038). We thank the 1W1B station for XAFS measurement at the Beijing Synchrotron Radiation Facility (BSRF).

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Footnotes

† Electronic supplementary information (ESI) available: Materials and instrumentation, experimental section, and supplemental figures and tables. See DOI: https://doi.org/10.1039/d3ta03712f

‡ These authors contributed equally to this work.

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