Mild-driven circulate synthesis of acetic acid from methane with chemical looping

Structural characterization of nanocomposites

Within the preparation of nanocomposites, Pd nanoparticles (NPs) are loaded on WO₃ nanosheets (Pd/WO₃), adopted by the additional thermal annealing course of to brighten PdO species on Pd NPs. The obtained samples are denoted as PdO/Pd–WO₃–x the place x = 1−5 by rising the annealing temperature (confer with Strategies). The Pd contents are stored fixed in these samples, that are confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES) (Supplementary Desk 1), to exclude the impact of Pd content material on CH₄ conversion efficiency. Transmission electron microscopy (TEM) photos reveal that the ready WO₃ nanosheets have the sting lengths of ~170 nm (Supplementary Fig. 1), and the nanoparticles in all samples are extremely dispersed on WO₃ substrate (Supplementary Fig. 2). The sizes of Pd NPs improve from Pd/WO₃ to PdO/Pd–WO₃-5 with the annealing temperature raised (Supplementary Fig. 3), implying the incorporation of oxygen atoms into the nanoparticles together with their lattice growth. The samples are additional characterised by X-ray diffraction (XRD) as proven in Supplementary Fig. 4. The diffraction peaks of Pd and PdO are absent within the XRD patterns, indicating that the nanoparticles are extremely dispersed at a low loading quantity.

To look into the detailed constructions, the nanoparticles on WO₃ helps are examined by high-resolution TEM (HRTEM). The Pd NPs are embellished with PdO with completely different oxidation diploma by controlling the annealing temperature. As proven in Fig. 1a, the pristine Pd nanoparticle solely shows the interplanar distance of two.2 Å, comparable to the spacing of Pd (111) planes^24,25,26. After the annealing course of, the brand new lattice fringes with a spacing of two.65 Å seem within the nanoparticles (Fig. 1b and Supplementary Fig. 5), which could be assigned to the (101) planes of PdO^27,28. In the meantime, the Pd (111) planes are nonetheless noticed within the nanoparticles, indicating the existence of Pd/PdO heterostructure in PdO/Pd–WO₃-1 to PdO/Pd–WO₃-4. Because the annealing temperature reaches 450 °C, the Pd NPs are utterly remodeled to PdO NPs (Fig. 1c). Furthermore, the compositions of Pd and PdO species are investigated by X-ray photoelectron spectroscopy (XPS). As proven in Supplementary Fig. 6, the content material of Pd²⁺ will increase by elevating the annealing temperature, in settlement with the findings from HRTEM photos.

a–c HRTEM photos of Pd/WO₃ (a), PdO/Pd–WO₃-2 (b) and PdO/Pd–WO₃-5 (c). d Typical HRTEM picture of PdO/Pd–WO₃-2 pattern displaying Pd/PdO heterostructure. e Corresponding FFT patterns of α and β areas in d. f HAADF-STEM picture of PdO/Pd–WO₃-2 pattern. g EELS spectra collected within the areas 1 and a couple of marked in f. h Structural illustration of PdO–Pd–WO₃ heterointerface.

The Pd–PdO interface is additional resolved meticulously as an instance the lively construction for CH₄ conversion. Taking PdO/Pd–WO₃-2 for example, plentiful Pd/PdO grain boundaries are noticed by the distinguishable lattice parameters of Pd and PdO (Fig. 1d and Supplementary Fig. 7). Determine 1e reveals the quick Fourier remodel (FFT) diffraction patterns obtained from the α and β areas in Fig. 1d. The FFT sample with the labels of (110), (101) and (1(bar{1})2) in α area matches the tetragonal construction of PdO alongside the zone axis of [(bar{1})11]²⁹. In the meantime, in β area, we will additionally get hold of the FFT sample of Pd alongside the zone axis of [011] route belonging to the face-centered cubic (fcc) construction with (200), (1(bar{1})1) and (0(bar{2})2)³⁰. Moreover, the Pd/PdO grain boundary is examined by atomic-resolution high-angle annular dark-field scanning TEM (HAADF-STEM), with their compositions additional analyzed by way of electron energy-loss spectroscopy (EELS). As proven in Fig. 1f and g, along with the detected alerts of Pd–M_4,5 edges at each websites 1 and a couple of, the height of O–Ok edge can be acknowledged at web site 1, comparable to the composition of PdO^31,32. Taken collectively, the aforementioned outcomes display the existence of Pd/PdO heterostructure on WO₃ help. Throughout the annealing course of, the PdO part seems on Pd NPs beneath the cooperation of oxygen and help, as nano-islands quite than core-shell construction³³, forming the PdO–Pd–WO₃ triple interface (Fig. 1h).

Efficiency of light-driven CH₄ conversion

Upon buying the effective constructions, we at the moment are ready to research the efficacy of the PdO/Pd–WO₃ nanocomposites in light-driven CH₄ conversion. The photochemical measurements are performed in a quartz reactor beneath xenon arc lamp irradiation. Pure WO₃ nanosheets present sluggish properties for CH₄ conversion (Supplementary Fig. 8). After Pd NPs deposition, the CH₄ conversion exercise over Pd/WO₃ is enhanced with CH₃OH as the first product (Fig. 2a). Apparently, the photochemical efficiency is considerably altered after the incorporation of PdO species into the Pd/WO₃ construction, which displays a volcano-like relationship with the quantity of PdO. Particularly, the addition of PdO to the samples dramatically boosts the manufacturing of CH₃COOH. Among the many samples, the PdO/Pd–WO₃-2 achieves the best manufacturing charge and selectivity towards CH₃COOH at 62.5 μmol g^–1 h^–1 and 60.2%, respectively. The excellent efficiency of changing CH₄ to CH₃COOH signifies that the Pd/PdO heterostructure permits an environment friendly C–C coupling course of. Nevertheless, the extreme PdO in nanocomposites hinders the Schottky contact between Pd and WO₃, which additional reduces photo-induced cost separation effectivity and considerably suppresses photochemical efficiency (Supplementary Figs. 9 and 10).

a Manufacturing charges for light-driven CH₄ conversion over Pd/WO₃ and PdO/Pd–WO₃-1 to PdO/Pd–WO₃-5 samples. b Time-dependent charges of CH₃COOH and CH₃OH manufacturing in addition to CH₄ conversion over PdO/Pd–WO₃-2 nanocomposite. c Response-regeneration cycles in CH₄ conversion. The error bars signify the usual deviation of the experiments.

To substantiate the carbon supply of the liquid merchandise, the origin of CH₃OH and CH₃COOH, as the principle merchandise, are traced with ¹³C nuclear magnetic resonance (¹³C NMR) spectroscopy through the use of ¹³CH₄ because the reactant. As proven in Supplementary Fig. 11, the peaks at 20.5 and 176.7 ppm are attributed to ¹³CH₃¹³COOH whereas the height at 48.9 ppm is assigned to ¹³CH₃OH. As well as, the management experiments point out that no product could be detected within the absence of nanocomposite, mild irradiation or CH₄ reactant (Supplementary Fig. 12). These outcomes present the proof that the first merchandise certainly originate from light-driven CH₄ conversion.

It’s price stating that differing from the standard CH₄ photooxidation requiring further oxidant addition (e.g., O₂), our response system makes use of the reactants of CH₄ and H₂O, wherein the ·OH radical produced from water oxidation is the best oxidizer for CH₄ activation (Supplementary Figs. 13 and 14)³⁴. As displayed in Supplementary Fig. 15, rising the focus of O₂ won’t promote the era of liquid merchandise however result in CO₂ manufacturing through the CH₄ photooxidation over PdO/Pd–WO₃-2. The activation of CH₄ by ·OH radicals can produce ·CH₃ as detected by in situ electron paramagnetic resonance (EPR) measurement (Supplementary Fig. 16).

To raised consider the effectivity, we conduct time-dependent measurement over PdO/Pd–WO₃-2. As proven in Fig. 2b, the manufacturing of CH₃COOH and CH₃OH step by step will increase within the first 3 h, attaining a powerful CH₄ conversion charge of 181.5 μmol g^–1 h^–1. Nevertheless, the efficiency reveals distinct decay when the response time exceeds 3 h, which ought to be ascribed to the consumption of PdO species after the photochemical course of (Supplementary Figs. 17 and 18). The constructed Pd/PdO heterostructure is step by step destroyed together with the response, which additional reduces the effectivity of C–C coupling towards CH₃COOH manufacturing. Within the meantime, negligible H₂ detection through the response means that the lattice oxygen of WO₃ is consumed for H₂O manufacturing, which additionally results in efficiency decay (Supplementary Figs. 19 and 20). To beat this limitation, we feature out the regeneration course of by heating the nanocomposites in air, wherein the consumed oxygen (i.e., PdO on Pd and lattice oxygen in WO₃) could be replenished to get well exercise (Fig. 2c and Supplementary Fig. 21). As such, a sturdy photochemical CH₄ conversion course of could be established by recycling the photochemical CH₄ conversion and air restoration. The sturdiness measurement signifies that the efficiency of PdO/Pd–WO₃-2 is nicely maintained for 5 cyclic assessments with every cycle lasting 5 h in such a recycling system (Supplementary Fig. 22). Furthermore, the leaching out of Pd through the cyclic assessments can be negligible in response to the outcomes of mass spectrometry (Supplementary Fig. 23).

Response intermediates detection

The knowledge gleaned above has acknowledged the promising efficiency for the conversion of CH₄ to CH₃COOH by modulating Pd/PdO heterostructure. Naturally, a query arises how CH₄ evolves into CH₃COOH over PdO/Pd–WO₃ nanocomposites with out extra carbon sources. To this finish, we examine the response intermediates over the nanocomposites through the photochemical CH₄ conversion course of. Determine 3a reveals the in situ diffuse reflectance-infrared Fourier-transform spectra (DRIFTS) for light-driven CH₄ conversion over Pd/WO₃ pattern. Upon mild irradiation, other than the peaks at 1305 and 3015 cm^–1 comparable to C–H deformation vibration of CH₄, the height at 1439 cm^–1 for CH₂/CH₃ deformation vibration seems step by step, indicating the CH₄ dissociation over the pattern^35,36. Furthermore, the numerous progress of vibrational peak at 1061 cm^–1 and bands at 2927 and 2963 cm^–1, comparable to the methoxy and C–H stretching vibrations in CH₃OH product, could be attributed to the CH₄ activation within the presence of ·OH³⁷. In sharp distinction, PdO/Pd–WO₃ nanocomposites that may produce CH₃COOH by means of light-driven CH₄ conversion exhibit completely different habits in DRIFTS (Fig. 3b). Along with the vibration alerts of CH₃OH noticed over Pd/WO₃, the extra vibrational modes of C = O (1654 cm^–1), C–O (979 cm^–1), C–C (867 cm^–1) and C–H (2858 cm^–1) stretching vibrations could be monitored for the formation of CH₃COOH over PdO/Pd–WO₃-2^38,39. Notably, a broad peak at 2060 cm^–1 in Fig. 3b is noticed with the sunshine irradiation continuing, which could be assigned to the adsorbed *CO on Pd web site (Supplementary Fig. 24)⁴⁰. The vibration alerts of C = O and *CO solely seem with the existence of PdO species, implying that the synergistic impact of Pd/PdO heterostructure within the nanocomposite can facilitate the CH₃COOH manufacturing with *CO as an intermediate.

a, b In situ DRIFT spectra for light-driven CH₄ conversion over Pd/WO₃ (a) and PdO/Pd–WO₃-2 (b). c, d In situ NAP-XPS outcomes of high-resolution C 1 s (c) and O 1 s (d) spectra over PdO/Pd–WO₃-2 nanocomposite.

To additional perceive the method with elemental info, the floor carbon and oxygen species are additionally monitored by in situ close to ambient stress XPS (NAP-XPS) characterization. As proven in Fig. 3c, after introducing the reactant into NPA-XPS chamber, the height of gas-phase CH₄ (287.0 eV) is noticed within the high-resolution C 1 s XPS spectrum (Supplementary Fig. 25). Upon mild irradiation, three C 1 s peaks of floor ·CH_x (285.5 eV), C–O (286.1 eV) and COO (289.1 eV) species seem and improve with the time evolution^41,42,43. In the meantime, the formation of oxygenates from CH₄ oxidation can be verified by amassing the O 1 s spectra in NAP-XPS research. Other than the height of lattice oxygen in pattern (O1, 530 eV), the featured peaks of hydroxyl (O2, 531.1 eV), C–O (O3, 531.8 eV), adsorbed H₂O (O4, 532.5 eV) and C = O (O5, 533.2 eV) species are resolved after mild illumination (Fig. 3d)^44,45,46. Of notice, though the alerts of *CO have been detected by in situ DRIFT and NAP-XPS measurements, gaseous CO is just not noticed as a product. Certainly, earlier work has demonstrated that the adsorption of CO on PdO web site is extraordinarily sturdy in order that *CO can be coupled with different intermediates earlier than desorption⁴⁷. Apparently, the floor ·CH_x, C–O and C = O species are corroborated with the remark of in situ DRIFTS spectra. This means that the co-adsorption of CH₄ and H₂O over Pd/PdO heterostructure can produce numerous floor carbonaceous intermediates together with methyl and carbonyl species and additional generate liquid oxygenates.

Mechanistic research

As revealed by in situ characterizations, the carbonyl species is the important thing intermediate for the conversion of CH₄ to CH₃COOH, which is carefully correlated with the presence of PdO within the ready nanocomposite. The case of Pd/WO₃ reveals that the era of ·OH radicals alone can not result in the formation of C = O within the absence of PdO species (Fig. 3a), implying that the oxygen in C = O is most probably derived from PdO within the nanocomposite. To additional hint the oxygen supply of carbonyl intermediate in CH₃COOH manufacturing, we put together the ¹⁸O-labeled PdO-modified nanocomposite (denoted as Pd¹⁸O/Pd–WO₃-2) by annealing pristine Pd/WO₃ in ¹⁸O₂ ambiance. Subsequently, the light-driven CH₄ oxidation is carried out over Pd¹⁸O/Pd–WO₃-2 and the merchandise are analyzed by gasoline chromatography−mass spectrometry (GC−MS), in reference to Pd¹⁶O/Pd–WO₃-2. In distinction to the case of Pd¹⁶O/Pd–WO₃-2, the peaks at m/z = 45 and 47 by Pd¹⁸O/Pd–WO₃-2 could be ascribed to CH₃C¹⁸O⁺ and ⁺C¹⁸OOH (Fig. 4a), indicating that the O atom in *CO is derived from the lattice oxygen of Pd¹⁸O in pattern. In consequence, the height at m/z = 62 for CH₃C¹⁸OOH could be detected.

a Mass spectra of CH₃COOH product utilizing Pd¹⁶O/Pd–WO₃-2 and ¹⁸O-labeled Pd¹⁸O/Pd–WO₃-2 nanocomposite. b The comparability of PdO contents in samples (50 mg) and the CH₃COOH yields of the corresponding samples. c Schematic illustration for photochemical conversion of CH₄ to CH₃COOH over Pd/PdO heterointerface within the presence of ·OH radicals. The numbers signify the response steps.

To additional perceive the working mechanism of PdO within the manufacturing of CH₃COOH, we quantitatively set up the relation between PdO consumption and CH₃COOH manufacturing. With this objective, the H₂-temperature-programmed discount (H₂-TPR) is employed to find out the content material of PdO in PdO/Pd–WO₃–x nanocomposites (Supplementary Fig. 26). As revealed in Fig. 4b, the quantities of PdO in samples are very near the CH₃COOH yield at low PdO contents (i.e., PdO/Pd–WO₃-1 and PdO/Pd–WO₃-2). On condition that the PdO is totally consumed after photochemical assessments, this additional confirms that the lattice oxygen of PdO solely contributes to the formation of C = O in CH₃COOH throughout photochemical CH₄ oxidation course of. Nevertheless, extreme PdO in nanocomposites will decrease the content material of metallic Pd to type PdO−WO₃ interface, which suppresses cost separation and reduces efficiency^48,49,50. On this case, the low CH₄ photooxidation efficiency permits most lattice oxygen atoms in PdO to stay within the samples (i.e., PdO/Pd–WO₃-3 to PdO/Pd–WO₃-5 in Fig. 4b). It’s price stating that the formation of Pd–PdO interface in nanocomposite is crucial for CH₃COOH era. Our management experiments point out that CH₃COOH can’t be produced by means of merely mixing Pd/WO₃ with PdO, suggesting that Pd–PdO interface is a key issue for CH₃COOH era (Supplementary Fig. 27). Furthermore, the CH₃COOH manufacturing relies on the structural character of Pd/PdO heterointerface (Supplementary Figs. 28 and 29), corroborating the significance of Pd/PdO interface high quality to CH₃COOH synthesis.

Taken collectively, the experimental outcomes have revealed the crucial function of Pd/PdO heterointerface in CH₄-to-CH₃COOH conversion. Determine 4c illustrates the proposed response pathway. CH₄ prefers to be activated at Pd web site within the presence of ·OH and type Pd–CH₃ intermediate (Supplementary Fig. 30). The methyl species could be step by step transformed to Pd−CO intermediate by means of the mix with O atom from PdO and the dehydrogenation by ·OH. Subsequently, the C–C coupling between carbonyl and methyl species generates the Pd–COCH₃ intermediate at Pd−PdO interface, and the additional hydrolysis of Pd–COCH₃ provides CH₃COOH as a product^51,52,53. Of notice, W websites are additionally lively for *CH₃ era by straight oxidizing CH₄ on WO₃. Nevertheless, the *CH₃ fashioned on WO₃ can hardly method the *CO on PdO in order that the excess *CH₃ species would evolve into C₁ oxygenates^54,55.

From the working mechanism of Pd/PdO heterostructure, we will now perceive that the entire consumption of lattice oxygen in PdO throughout light-driven CH₃COOH manufacturing, within the case of PdO/Pd–WO₃-2, will inevitably result in the numerous efficiency decay for CH₃COOH manufacturing after the response (Supplementary Fig. 31). Compared, the CH₃OH manufacturing is just not considerably affected by the oxygen loss in PdO. It’s price noting that the evolution of WO₃ in photochemical CH₄ conversion can be an element for efficiency decay. On condition that no H₂ is detected within the photochemical course of, the WO₃ is inevitably lowered by photo-induced electrons, which is accompanied with gradual lattice oxygen loss, additionally inflicting efficiency decay (Supplementary Fig. 20). However, the quantity of misplaced oxygen atoms in WO₃ is set to be 1.28% by means of the calculation based mostly on the demand of ·OH radical manufacturing, which is negligible as in contrast with the consumption of PdO (taking PdO/Pd–WO₃-2 as instance). Moreover, the misplaced lattice oxygen in WO₃ could be replenished along with that in PdO through the regeneration course of, recovering photochemical exercise.

Following the mechanistic research, our investigation on photochemical CH₄ conversion in gas-solid part signifies that the generated *CH₃ might bear self-coupling to supply C₂H₆ as the first product (Supplementary Fig. 32). For that reason, the controllable utilization of *CH₃ in resolution and gasoline phases is of nice significance to additional enhance the manufacturing charge and selectivity for CH₃COOH.

Photochemical circulate synthesis of CH₃COOH

The important thing to controllable *CH₃ utilization is the environment friendly methyl−carbonyl coupling. Actually, such an environment friendly coupling ought to be based mostly on the availability of enough *CO species. Our management experiments present that the addition of CO to the response system utilizing Pd/WO₃ nanocomposite, within the absence of PdO, can ship related CH₃COOH manufacturing (Supplementary Fig. 33). Compared, the addition of methanol doesn’t clearly promote the CH₃COOH manufacturing over PdO/Pd–WO₃-2 nanocomposite, suggesting that CH₃COOH is just not the product primarily from methanol carbonylation (Supplementary Fig. 34). The outcomes present us the clues for enhancing CH₃COOH manufacturing—the cascade response between *CO and *CH₃ on nanocomposite in steady response channels that may promote the utilization of PdO and *CH₃.

To this finish, we design a photochemical circulate response gadget with arc-shaped circulate channels to additional improve the efficiency of CH₃COOH manufacturing (Supplementary Fig. 35). On this design, the *CH₃ species that haven’t coupled with *CO can migrate alongside the pattern to additional react with the adsorbed *CO and even evolve into *CO on the downstream PdO websites, selling the conversion of CH₄ to CH₃COOH. Particularly, CH₄ and H₂O are premixed to type the monodispersed gasoline bubbles, that are then pumped into the circulate reactor to generate gas-liquid-solid contact in channels (Fig. 5a). Benefitting from the flowing reactants and three-phase interface between CH₄, H₂O and pattern (Fig. 5b, c), the generated *CH₃ in resolution from gas-solid part CH₄ oxidation could be quickly captured by *CO on pattern layer to understand steady synthesis of CH₃COOH. As such, the outstanding selectivity of 91.6% and manufacturing charge of 90.7 μmol g^–1 h^–1 are achieved for CH₃COOH manufacturing over PdO/Pd–WO₃-2 nanocomposite (Fig. 5d). As normalized to the Pd loading weight, the manufacturing charge reaches 1.5 mmol g_Pd^–1, which exceeds the efficiency of current photocatalysts for oxygenates manufacturing beneath gentle situation (Supplementary Desk 2). Moreover, the combination of photochemical CH₄ conversion with regeneration course of additionally demonstrates the reproducibility and sturdiness of the circulate response gadget (Fig. 5e).

a Schematic illustration of photochemical circulate response gadget, together with reactants provider, selfmade reactor and merchandise collector. b, c Facet (b) and high (c) views of the arc-shaped circulate channel in selfmade reactor and the three-phase contact between CH₄, H₂O and pattern. The purple, blue and brown colours signify CH₄, H₂O and nanocomposite, respectively. d Manufacturing charge and selectivity of light-driven CH₄ conversion towards CH₃COOH over PdO/Pd–WO₃-2 nanocomposite utilizing the circulate response gadget or typical gadget for the primary 3 h. e Response-regeneration cycles on PdO/Pd–WO₃-2 pattern by using the circulate response gadget. The error bars signify the usual deviation of the experiments.