Regulating the digital construction by cost redistribution in dense single-atom catalysts for enhanced alkene epoxidation

Normal synthesis and characterizations of densely populated steel SACs

A two-step technique incorporating polycondensation and subsequent pyrolysis procedures was developed to synthesize the densely populated steel SACs (Supplementary Fig. 1, particulars are proven within the Experimental Part)³². The important thing success of this technique depends on the controllable polycondensation throughout step one, whereby the small molecules (melamine, cyanuric acid, l-alanine and phytic acid) are spontaneously polymerized in water to kind two-dimensional nanosheets (Supplementary Fig. 2). On the identical time, steel ions are complexed in it by the encircling heteroatoms (N/O). Steel SACs with N/O-coordination construction are produced after pyrolysis at a average temperature (700 °C) in an Ar ambiance. As a result of the precursor comprises plentiful websites for anchoring steel ions, densely populated steel SACs with loadings as much as 35.5 wt% might be achieved.

As proven in Fig. 1a, seven various kinds of densely populated steel SACs have been produced. In precept, this environment friendly artificial strategy might be prolonged to supply different steel SACs. In accordance with inductively coupled plasma atomic emission spectroscopy (ICP-AES) outcomes, the steel loadings have been all increased than 10 wt% (Supplementary Desk 1). X-ray diffraction (XRD) patterns present solely a sole broad characteristic at ∼22°, comparable to the interlayer distance of the carbon matrix, excluding the existence of any crystalline species in these densely populated steel SACs (Supplementary Fig. 3). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and corresponding energy-dispersive X-ray spectroscopy (EDS) mapping photos reveal that every one of those steel species are extremely dispersed all through the entire two-dimensional laminar structured carbon matrix doped with heteroatoms (Supplementary Figs. 4–9).

a Steel loadings in M-SACs and schematic diagram of ready M-SACs. b–g AC HAADF-STEM photos and (h–m) okay³-weighted Fourier rework spectra of (b, h) Fe₁/NOC, (e, okay) Ni₁/NOC, (c, i) Cu₁/NOC, (f, l) Zn₁/NOC, (d, j) Ru₁/NOC, and (g, m) Ir₁/NOC.

The atomic dispersion of steel species was then straight visualized by using AC HAADF-STEM. Because of the heavy Z-contrast, the brilliant single dots ascribing to steel single atoms might be clearly seen of their typical photos (Fig. 1b–g). Furthermore, Fourier-transformed okay³-weighted prolonged XAFS (FT-EXAFS) spectra present just one outstanding peak at ~1.6 Å (with out section shift) comparable to metal-N/O scattering path and the absence of metal-metal scattering, additional confirming the atomically dispersed steel atoms in all SACs samples (Fig. 1h–m). Wavelet rework (WT) photos additionally present just one depth of M-N/O-coordination scattering (Supplementary Fig. 10). Additional quantitative EXAFS becoming outcomes prompt a median coordination quantity (CN) of ~4 for all steel SACs (Supplementary Desk 2). As a result of N and O atoms are tough to distinguish in EXAFS, X-ray photoelectron spectroscopy (XPS) was employed to research the metal-N/O bonds. N 1 s and O 1 s XPS spectra present the existence of metal-N and metal-O peaks (Supplementary Figs. 11and 12). These outcomes prompt steel single atoms have been dual-coordinated with N and O atoms in these SACs, inside a complete CN of 4. For comfort, these densely populated steel SACs have been denoted as M₁/NOC, M represents steel.

Controllable synthesis and characterizations of Co SACs with varied densities

Steel SACs with totally different densities may also be simply obtained by adjusting the quantities of steel precursor based mostly on the proposed technique, which supplies a dependable solution to examine the location interplay impact in densely populated SACs. Taking Co for example, three Co SACs with loadings of 5.4, 10.9, and 21.2 wt% have been ready (Supplementary Desk 3). The corresponding samples have been designated as Co₁/NOC-5, Co₁/NOC-11 and Co₁/NOC-21, respectively. SEM photos reveal the same morphology of those Co₁/NOC samples (Supplementary Fig. 13). XRD, TEM and corresponding EDS mapping characterizations confirmed extremely dispersed Co species in these Co SACs samples (Supplementary Figs. 14–17). As proven in Fig. 2a–c, AC HAADF-STEM photos current that Co species are all atomically dispersed, and the density of Co single atoms turns into increased from Co₁/NOC-5 to Co₁/NOC-21. This may most likely induce loads of adjoining single atoms and end in totally different digital construction in densely populated Co SACs.

a–c AC HAADF-STEM photos of 5.4 wt%, 10.9 wt%, and 21.2 wt% Co SAC samples. Co single atoms are marked in purple circles. d Normalized Co Okay-edge XANES spectra. e okay³-weighted Fourier rework spectra of Co₁/NOC-x and reference samples. f Corresponding wavelet rework (WT) evaluation. g The Co Okay-edge XANES experimental spectra (strong traces) and the theoretical spectra (dots) calculated with the depicted constructions (insert) of Co_1/NOC-x samples.

XAFS measurements have been then carried out to research the digital properties and coordination constructions of those Co SACs with totally different loadings. Determine 2nd exhibits the normalized Co Okay-edge XANES spectra. It may be seen that the absorption threshold positions have been positioned between CoO and Co foil, suggesting that the valences of Co species have been all between 0 and +2. Nevertheless, the place of the rising edge exhibited a downshift from Co₁/NOC-5 to Co₁/NOC-21, indicating that the Co oxidation state grew to become decrease with rising Co density. Co 2p XPS spectra additionally exhibited an analogous tendency (Supplementary Fig. 18). These variations counsel the digital constructions of central Co atoms are totally different. The corresponding FT-EXAFS spectra displays solely a outstanding peak at ~1.52 Å of Co-N/O and with out Co–Co peak at ~2.2 Å (Fig. 2e), confirming the atomically dispersed steel atoms in Co_1/NOC-x samples. WT photos additionally present just one most depth at ~5 Å⁻¹ of Co-N/O scattering path (Fig. 2f). Furthermore, a small k-value up-shift might be noticed on the WT contour plots of samples in comparison with CoPc. This may be ascribed to the introduction of O within the first-coordination shell of Co atoms, implying that Co single atoms have been coordinated with N/O dual-atoms. In the meantime, the first-coordination shell of Co-P was excluded (Supplementary Fig. 19). Quantitative EXAFS curve-fitting outcomes present the common CNs of Co single atoms in all SACs are estimated to be 3.8–4.2 (Supplementary Desk 4). Ingredient evaluation exhibits that the ratios of N to O are additionally saved as ~3 in Co_1/NOC-x (Supplementary Desk 5). Moreover, the Raman and C 1 s XPS spectra of Co_1/NOC-x reveal an analogous native construction of helps (Supplementary Fig. 20). These outcomes counsel that these Co SACs may preserve the equivalent Co₁-N₃O₁ coordination construction with various web site density. That’s to say, the shift of Co valence states and totally different digital constructions of central Co atoms ought to be attributed to the totally different web site density in these Co SACs, relatively than coordination construction or coordination numbers.

Theoretical understanding the distinction of Co SACs with varied densities

With the intention to higher perceive the affect of web site density on the digital constructions of Co single atoms from theoretical, totally different numbers of Co₁-N₃O₁ single websites have been evenly embedded in the identical 6*6 cell of N/O doped graphene to symbolize Co SACs with totally different densities (Supplementary Fig. 21). The utmost of 4 Co₁-N₃O₁ websites might be accommodated in such a grid space. Below this case, the corresponding theoretical mass loading of Co atom was calculated as 22 wt%, which was fairly near the precise loading of as-synthesized Co₁/NOC-21 pattern (Supplementary Fig. 21a). Particularly, when the variety of Co₁-N₃O₁ single websites have been set to 1 and two, respectively, the theoretical Co loadings have been calculated to be 6 wt% and 12 wt%, which additionally agreed effectively with the experimental outcomes of Co₁/NOC-5 and Co₁/NOC-11 samples (Supplementary Fig. 21b, c). Thus, we have now successfully correlated the Co steel loading and Co single-atom density, which is very helpful in guiding us within the building of environment friendly SACs. These fashions with totally different numbers of Co₁-N₃O₁ single websites are dependable to symbolize Co SACs with totally different densities for theoretical investigation. DFT calculation outcomes additionally indicated the excessive stability of those configurations after optimization, with very adverse adsorption energies in principle (Supplementary Fig. 22). Furthermore, XANES simulation curves with these fashions matched effectively with Co₁-N₃O₁ coordination configuration (Fig. 2g), additional confirming the opportunity of Co₁-N₃O₁ coordination construction in all Co SACs.

In accordance with the optimized fashions, the cost density variations of Co single websites have been then calculated. As proven in Fig. 3a, it may be seen that cost was transferred from Co atoms to the supported matrix. With rising variety of Co₁-N₃O₁ websites from 1 to 4, the entire help grew to become extra cost connections, which resulted within the cost redistribution. Bader cost evaluation additional prompt that the common cost switch of Co atom grew to become regularly much less from 4-Co₁-N₃O₁ to 1-Co₁-N₃O₁ (Supplementary Desk 6), which was in step with the XAFS and XPS outcomes. These variations in cost density confirmed that the digital constructions of Co atoms might be altered by cost redistribution in densely populated Co SACs. Additional projected density of states (pDOS) calculations outcomes confirmed that the Co d-band heart of 4-Co₁-N₃O₁ was up-shifted and regularly closed to the Fermi degree (E_F) compared with 1-Co₁-N₃O₁, in in step with Bader cost evaluation outcomes (Fig. 3b, c).

a Prime and facet views of differential cost density of x-Co₁-N₃O₁ fashions. Isosurface: 0.005 e/Å^–3. Coloration legend for isosurface: blue, cost depletion; yellow, cost accumulation. b Projected density of state (pDOS) for the Co 3d orbital. c The connection between Bader charger and d-band heart in varied x-Co₁-N₃O₁ fashions. d The plot of spin second with respect to Co single-atom density. Inset: The spin density isosurfaces of 0.005 e/Å^–3.

As well as, as proven in Supplementary Fig. 23, due to the asymmetry from the spin-up to spin-down channel, the bottom state of 1-Co₁-N₃O₁ was calculated to be ferromagnetic with a complete spin second of 0.797 μ_B, indicating a weak interplay amongst Co₁-N₃O₁ moieties in low-density Co SAC. Quite the opposite, due to the marginally symmetrical distribution in spin-up and spin-down channel, the bottom state of 4-Co₁-N₃O₁ displays a lowered spin second of 0.310 μ_B equally distributed on all Co atoms, suggesting very sturdy interplay between adjoining Co₁-N₃O₁ moieties in high-density Co SAC (Supplementary Desk 7). The lower of spin second from 1-Co₁-N₃O₁ to 4-Co₁-N₃O₁ might be ascribed to the downshift of vitality of the Co 3d orbital as adjoining Co atoms get nearer (Fig. 3d), which was in step with the reported ends in literatures^21,33. To sum up, from each experimental and theoretical evaluation, we verified that the interplay amongst these single websites in densely populated Co SACs might certainly alter the cost density and digital construction of Co atoms by cost redistribution, which might additional have an effect on their catalytic efficiency.

Catalytic performances of Co SACs with varied densities for trans-stilbene epoxidation

Epoxides are of significance in advantageous chemical trade and natural synthesis^34,35,36. In present processes for alkene epoxidation, a lot of costly oxidants or co-reagents are normally required, which inevitably results in excessive value^37,38. To handle this downside, a number of SACs (Fe, Ag, Pt, Pd, and many others) had been developed and exhibited glorious catalytic performances, permitting O₂ to be the oxidant with none co-reagents^{29,37,39,40,41}. Impressed by these findings, we explored the potential utility of as-synthsized M₁/NOC samples in trans-stilbene (SB) epoxidation beneath 1 atm O₂. First, we investigated the efficiency of assorted M₁/NOC samples. Co₁/NOC SAC can convert as much as 96.5% of SB after 1 h, however different transition steel SACs can solely convert as much as 54.4%, displaying that Co SACs have the best catalytic efficiency in SB epoxidation (Fig. 4a). Subsequently, we targeted on the Co SACs samples and disclosed the affect of density on the catalytic property.

a The catalytic efficiency of M₁/NOC samples in trans-stilbene epoxidation inside 1 h. b The dynamics plots of trans-stilbene conversion towards response time over varied samples. Response situation: 1 mmol trans-stilbene epoxidation, containing 0.01 mmol Co in every catalyst (10 mg help), 10 mL solvent, 140 °C, O₂ balloon. c TOF and mass-specific exercise (MSA) of assorted Co₁/NOC samples. d Optical picture of as-prepared gram-scale Co SAC pattern. e The dynamics plots of SB conversion towards response time over gram-scale Co SAC pattern. f ¹H NMR spectra of generated SBO. (400 MHz, D₆-DMSO) δ 7.34–7.40 (m, 10H), 4.07 (s, 2H). Inset: molecular construction and optical picture of product SBO.

As proven in Fig. 4b, there was no response detected with P-doped help alone, indicating that Co atoms have been the lively species in all samples. Nevertheless, notable variations within the catalytic exercise of Co SACs with totally different density might be noticed beneath the identical response circumstances. Morevoer, the calculated turnover frequency (TOF) was considerably enhanced by 10 instances with rising Co density from Co₁/NOC-5 to Co₁/NOC-21, whereas the selectivity all the time remained at a passable degree (>98%), suggesting the excellent lively websites in high-density Co₁/NOC-21 (Fig. 4c). Additional obvious activation energies (E_a) exams present that Co₁/NOC-21 has an E_a of 58.2 kJ mol⁻¹, which is way decrease than that of Co₁/NOC-11 and Co₁/NOC-5, confirming the wonderful catalytic efficiency of densely populated Co SACs (Supplementary Fig. 24). Significantly, the mass-specific exercise is extra necessary for industrial functions. Because of the excessive steel loading and intrinsic exercise, the mass-specific exercise of Co₁/NOC-21 reached as excessive as 193 mol·g⁻¹·h⁻¹, which was practically 30 instances and even 15 instances to that of Co₁/NOC-5 and Co nanoparticles (Co NPs), respectively. Such excessive mass-specific exercise was additionally a lot increased than the values reported in literatures^37,40. Furthermore, Co₁/NOC-21 can tolerate a broad scope of substrates in alkene epoxidation (Supplementary Desk 8) and displays excellent catalytic stability after seven consecutive cycles (Supplementary Fig. 25). The microstructure and digital construction of Co single atoms after re-used additionally maintained practically the identical and no leaching of Co species was noticed (Supplementary Figs. 26 and 27 and Supplementary Desk 3).

Due to the facile synthesis technique and glorious catalytic efficiency, we will readily obtain gram-scale manufacturing of densely populated Co SACs for SB epoxidation (Fig. 4d). XAFS measurement demonstrated the atomically dispersed state of gram-scale Co SAC with no considerable distinction in contrast with Co₁/NOC-21 (Supplementary Fig. 28). Below optimized response circumstances, the gram-scale Co SAC was evaluated to be efficient for SB epoxidation with a excessive yield of 92.6% (Fig. 4e). The obtained SB epoxidation product (SBO) was characterised by nuclear magnetic resonance (NMR, Fig. 4f). Within the ¹H NMR spectrum, the triplet peak at 4.07 ppm corresponds to the hydrogen adjoining to the epoxy group, and the a number of peak with a chemical shift at 7.34−7.40 ppm corresponds to the fragrant hydrogen, indicating the excessive purity of SBO (>96%) in contrast with business manufacturing (Supplementary Fig. 29). The height splitting and peak space coupled with ¹³C NMR have been all in step with the SBO (Supplementary Fig. 30). General, the densely populated Co SAC displayed superior catalytic efficiency, displaying the potential for industrial utility in stilbene epoxidation.

Theoretical understanding the density impact on the catalytic efficiency

As proven in above experimental outcomes, Co SACs with totally different densities exhibited totally different catalytic efficiency for SB epoxidation. To disclose the response mechanism and the affect of digital constructions of Co single atoms induced by the density on the catalytic efficiency, DFT calculations have been additional carried out. We selected 1-Co₁-N₃O₁ and 4-Co₁-N₃O₁ to symbolize the bottom density and highest density Co SACs, respectively. From the kinetic curves, it may be discovered that SB epoxidation was the consecutive response, implying the activation of O₂ was step one. Furthermore, the calculated adsorption energies prompt that Co websites in x-Co₁-N₃O₁ have been extra favorable for the O₂ adsorption relatively than SB molecule (Fig. 5a). As well as, we investigated the opportunity of O₂ adsorption on a faulty N₃O₁ emptiness. The calculated O–O bond distance after adsorption on emptiness was just one.25 Å (equal to that of free O₂), implying O₂ molecules can’t be activated on faulty N₃O₁ vacancies (Supplementary Fig. 31). Subsequently, we targeted on the digital construction of Co atoms and O₂ molecules to grasp the activation mechanism.

a The calculated energies for adsorption O₂ and SB, in addition to SBO desorption energies. b The connection between Bader cost and catalytic exercise. c Cost density variations after O₂ adsorption on varied x-Co₁-N₃O₁ fashions. d Vitality profiles of trans-stilbene epoxidation response over 1-Co₁-N₃O₁, 2-Co₁-N₃O₁, and 4-Co₁-N₃O₁ fashions. Inset: the configurations of intermediates of 4-Co₁-N₃O₁.

As proven in Fig. 5b, we will discover that it presents a constructive correlation between the catalytic exercise and common Bader cost of Co atoms, the place a pointy enhancement of TOF worth realizes with the lower of Bader cost, suggesting that the Bader cost could be a key descriptor in trans-stilbene epoxidation. Subsequently, we investigated the cost density distinction and Bader cost switch between Co atom and O₂ (Fig. 5c), which revealed that *O₂ gained 0.36 e, 0.38 e and 0.44 e from Co atom in 1-Co₁-N₃O₁, 2-Co₁-N₃O₁, and 4-Co₁-N₃O₁, respectively, suggesting extra electrons have been crammed into the O₂ 2π* orbital to lively O₂ on 4-Co₁-N₃O₁ (Supplementary Fig. 32)⁴². In contrast with O–O bond size of 1.29 Å on 1-Co₁-N₃O₁, it was enlarged to 1.31 Å on 4-Co₁-N₃O₁, indicating Co single atoms with increased cost density in 4-Co₁-N₃O₁ was extra favorable for activation of O₂ molecule. Moreover, we investigated the spin moments of Co atoms after O₂ adsorption (Supplementary Fig. 33 and Supplementary Desk 9). It was discovered that there was extra pDOS overlap and a beer vitality splitting between bonding and antibonding orbitals (Supplementary Fig. 34). 4-Co₁-N₃O₁ confirmed the most important spin second of 0.443 μ_B. In the meantime, the adsorbed O₂ additionally exhibited the most important atomic or molecular spin moments, which was in step with earlier work⁴³. The bigger variations in spin second of O₂ in 4-Co₁-N₃O₁ earlier than and after oxygen adsorption demonstrates essentially the most intensive electron switch from Co to O₂ (Supplementary Desk 10). As a consequence, when O₂ was adsorbed on high-density Co with the bottom activation vitality, the largest enhance within the O–O bond distance occurred.

The response pathways and the corresponding vitality profiles over these three Co₁-N₃O₁ fashions have been lastly investigated utilizing DFT calculations (Fig. 5d and Supplementary Figs. 35–37). The O₂ molecules have been firstly coated on all Co single atoms, with exothermic values of −1.10, −1.12 and −1.15 eV over 1, 2, 4-Co₁-N₃O₁ fashions, respectively (II). Subsequently, an SB molecule attacked one O₂ molecule pre-adsorbed Co₁ web site (III) for activation and remodeled to SBO with vitality obstacles (TS-I) of 0.79, 0.71 and 0.61 eV on 1, 2, 4-Co₁-N₃O₁, respectively (Supplementary Desk 11). The residual oxygen might be transformed to a brand new one-coordinated oxygen atom by overcoming relative low-energy obstacles (IV to V) after which participated within the subsequent steps for SB epoxidation response (VI to VIII). Equally, the vitality barrier (TS-II) on 4-Co₁-N₃O₁ was additionally decrease than that on 1-Co₁-N₃O₁ and 2-Co₁-N₃O₁. Upon the desorption of the second SBO molecule, in addition to the adsorption of one other O₂ molecule, the catalytic response cycle began once more. From the entire profile, it may be seen that Co single atom in 4-Co₁-N₃O₁ with increased cost density was extra favorable for the activation of O₂ and SB substrates (Supplementary Desk 12).

Furthermore, we additionally calculated the vitality modifications of the response over 4-Co₁-N₃O₁ mannequin by which the 2 benzene rings work together with two adjoining Co websites concurrently (Supplementary Fig. 38). The vitality barrier for the oxidation of the benzene ring by two adjoining Co websites was calculated to be 2.02 eV (formation of epoxy benzene) or 2.60 eV (formation of phenol), which was a lot increased than the corresponding worth of the SB epoxidation path (0.61 eV, Fig. 5d). Subsequently, the interplay of SB with two adjoining Co websites might be excluded as the principle response path in SB epoxidation.

Lastly, we used DFT calculations to grasp the trans-stilbene epoxidation response pathways of Co NPs. Each XRD and HRTEM characterizations revealed that Co NPs uncovered with Co(111) crystal faces (Supplementary Figs. 39 and 40). Subsequently, we used Co(111) to symbolize Co NPs/NC pattern for calculations. As proven in Supplementary Fig. 41, an O₂ molecule was adsorbed and dissociated to O* on Co(111) with a excessive exothermic of −4.56 eV (III). The SB molecule was then activated and transformed to SBO with an vitality barrier of 1.21 eV (IV–V). By overcoming one other vitality barrier of 1.36 eV (V to VI), the remaining oxygen might be remodeled to a brand new one-coordinated oxygen atom and subsequently take part within the subsequent step of the SB epoxidation cycle. The best vitality barrier for SBO manufacturing reached to 1.78 eV (VII to VIII). It may be seen that the vitality barrier on Co NPs was a lot increased than on Co₁/NOC-x samples, ensuing within the lowest response price in trans-stilbene epoxidation, which was in step with experimental outcomes.