Synthesis and structure investigation of Pt UHL-SACsGiven the wide applications of Pt-based catalysts33, UHL-SACs of Pt are initially investigated. The PCN is used as the substrate firstly, which exhibits characteristic diffraction peaks at 2θ of 27.6 and 60.0° (Supplementary Fig. 1). The C 1 s and N 1 s XPS spectra provide additional confirmation of the formation of C-N bonding (Supplementary Fig. 2) with a N/C atomic ratio of 1.05, establishing ample coordinate nodes for UHL-SAC fabrication. The platinum-based UHL-SACs, denoted as Pt SACs/PCN, were synthesized by impregnating chloroplatinic acid onto PCN and subsequent annealing in a vacuum. To elucidate the crucial role of the negative pressure environment, a reference sample was prepared by annealing in Ar flow at 101 KPa (Pt NPs/PCN). The annealing pressure shows negligible impact on the apparent morphology and chemical constitution (Supplementary Figs. 3–5) of Pt-based catalysts compared with the PCN substrate. Moreover, the color of these two samples turns black from yellow following Pt deposition (Supplementary Fig. 6). Based on ICP analysis, the Pt contents are measured as 41.8 wt% and 40.9 wt% for Pt SACs/PCN and Pt NPs/PCN, respectively (Supplementary Table 2).The TEM image of the Pt NPs/PCN (Fig. 1c) illustrates an accumulation of the metal particles with sizes around 10–50 nm. XRD pattern of Pt NPs/PCN (Fig. 1d) showcases the distinctive diffraction peaks characteristic of crystalline Pt, revealing the formation of Pt particles during annealing under ambient pressure. On the contrary, an absence of discernible diffraction peaks related to crystalline Pt was observed for Pt SACs/PCN. Additionally, the TEM image of Pt SACs/PCN (Fig. 1e and Supplementary Fig. 7) reveals no observable Pt particles, underscoring the effective prevention of metal aggregation under vacuum annealing conditions. To delve into the local structure of Pt sites in Pt SACs/PCN, an AC HAADF-STEM measurement was employed. Figure 1f, g reveal dense bright spots assigned to isolated Pt atoms are uniformly distributed over PCN, affirming the atomic dispersion of Pt sites on PCN randomly (Fig. 1h). Moreover, the average areal density of isolated Pt is estimated to 6.5 atoms/nm2 based on the measured BET surface area of the PCN (Supplementary Figs. 8, 9), similar with the pixel statistics of Fig. 1g (5.6 atoms/nm2). These results identify the Pt SACs/PCN as one of the catalysts with the highest density of isolated Pt sites. Furthermore, EDS element mapping (Fig. 1i–j) affirm the uniform distribution of Pt, reinforcing the accuracy of these statistical findings. Figure 1k presents the XPS results of Pt SACs/PCN, which reaffirms the ultra-high Pt loading (survey) with a positive oxidation state evidenced by the Pt 4f7/2 binding energy of 72.7 eV. The XANES spectrum of Pt SACs/PCN, positioning the white line intensity between Pt foil and PtO2, further verifies the partially positive oxidation state of Pt (Fig. 1l). The FT EXAFS spectra (Fig. 1m) are exploited to elucidate the coordination environment of Pt sites. The Pt SACs/PCN exhibits a dominant peak assigned to Pt-N coordination at 1.6 Å, with the absence of Pt-Pt coordination at 2.5 Å. This result aligns seamlessly with wavelet transformation results (Fig. 1n), identifying the N-coordinated single-atom Pt sites in Pt SACs/PCN. These findings serve as conclusive evidence for successfully fabricating ultra-high loading single-atom catalysts via the negative pressure annealing approach.To shed light on the formation of Pt SACs/PCN, the structure evolution of Pt species during the annealing process was investigated by temperature-dependent in-situ AC HAADF-STEM, ex-situ XAFS, and XPS, in vacuum and Ar condition respectively. The temperature-dependent in-situ AC HAADF-STEM images in vacuum conditions are shown in Fig. 2a and Supplementary Fig. 10. Dense bright spots are observed at 20 °C, revealing the uniform distribution of Pt precursor on the substrate. Moreover, no clusters and particles are generated along the temperature increasing from 20 to 400 °C, even after the sample is kept at 400 °C for 359 s. On the contrary, when annealing in the Ar flow, the atomic Pt can only be stabled below 300 °C (Fig. 2b). Observable Pt particles are generated when the temperature reaches to 300 °C. These particles grow bigger when the temperature is further increased to 400 °C. The corresponding EDS element measurements at different temperatures show a faster increase of Pt/Cl atomic ratio in vacuum than in Ar (Supplementary Table 3), demonstrating the accelerated Cl removal from Pt precursor in negative pressure conditions.Fig. 2: Insight of the Pt species transformation during the generation of Pt SACs/PCN and Pt NPs/PCN.Temperature-dependent in-situ aberration-corrected HAADF-STEM images of (a) Pt SACs/PCN and (b) Pt NPs/PCN, the scale bar represents 5 nm. c, d Pt L-edge FT EXAFS spectra and (e, f) XANES spectra of samples annealed at different temperatures in (c, e) vacuum and (d, f) Ar. g Pt 4 f XPS spectra of samples annealed at different temperatures in vacuum and Ar.The coordination changes are investigated by XAFS. Figure 2c shows the Pt L-edge FT EXAFS spectra in vacuum conditions at different temperatures. Pt-Cl coordination is detected at 200 °C, which transfers to Pt-N coordination at 300 °C. Moreover, no Pt-Pt coordination is detected even at 400 °C, excluding the formation of Pt-Pt bonding. For samples annealed under Ar environment (Fig. 2d), the dominant Pt-Cl coordination at 200 °C is significantly decreased at 300 °C, companying with the formation of Pt-Pt coordination, which further takes the predominance at 400 °C. The FT EXAFS results are in good agreement with the in-situ AC HAADF-STEM and EDS results, demonstrating the different transformation pathways of Pt precursor in vacuum and Ar environment. The oxidation states of Pt under different annealing pressures show down-hill tendencies along the increasing temperature (Fig. 2e, f), which may due to the loss of Pt-Cl bonding and the formation of Pt-N or Pt-Pt coordination. However, for the sample annealed in Ar, the Pt oxidation state is lower than that annealed in vacuum (Fig. 2g). This can be attributed to the weaker electronegativity of Pt than N. These results illustrate the evolution of Pt UHL-SACs, that is Pt-Cl coordination rapidly dissociates at relatively low temperatures to generate active Pt species, and the vacuum condition greatly suppresses the metal aggregation via promoting the Pt-N coordination at relatively high temperatures, thus enables the formation of high-density Pt SACs.Universally preparing metal UHL-SACsTo reveal the generality of the negative pressure annealing approach, this synthetic process is extended to 12 other single-atom metal sites on PCN (M SACs/PCN, M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ir and Au, Fig. 3a). The metal loadings of all these catalysts are measured between 27.3 wt% to 44.8 wt%, and the metal areal density are confirmed at a high level (Fig. 3b, Supplementary Table 2, and Supplementary Fig. 9). The white-line intensities from the XANES spectra indicate the positive oxidation state of the metal sites in M SACs/PCN (Supplementary Fig. 11). The AC HAADF-STEM (Fig. 3c) and FT-EXAFS spectra (Fig. 3d) identify the N-coordinated single-atom metal sites, and the absence of the metal-metal coordination excludes the formation of the metal clusters. The characterizations of XRD, TEM, EDS mapping, and XPS are shown in Supplementary Figs. 12–24, which further indicate the uniform distribution of the positive-charged isolated metal sites on the PCN substrate, and no aggregation of the metal is detected.Fig. 3: The universal preparation of UHL-SACs on PCN (M SACs/PCN).a The metal elements used for fabricating M SACs/PCN. b The metal content in the as-prepared catalysts. c Aberration-corrected HAADF-STEM and (d) FT-EXAFS spectra of various M SACs/PCN.The adaptability of the negative pressure annealing approach on different substrates is also investigated. The NC obtained via the pyrolysis of guanine is used instead of PCN to prepare UHL-SACs (M SACs/NC, M=Pt, Fe, Co, Ni, and Cu). The XRD of the NC substrate reveals the structure of graphitic carbon (Supplementary Fig. 25). The XPS results confirm the doping of N on carbon (Supplementary Fig. 26). The characterizations, including AC HAADF-STEM, FT-EXAFS, EDS element mapping, XANES, XRD, and XPS (Fig. 4a–e and Supplementary Figs. 27–34), identify the N-coordinated single-atom metal sites with positive oxidation states in these M SACs/NC. The mental contents are measured as high as 34.1 wt% (Pt), 21.5 wt% (Fe), 19.6 wt% (Co), 17.7 wt% (Ni), and 29.8 wt% (Cu) (Supplementary Table 2), demonstrating the obtain of UHL-SACs with high metal areal density on NC substrate (Supplementary Figs. 35, 36). Moreover, high-entropy single atoms (HESACs) containing Pt, Fe, Co, Ni, and Cu are also prepared on the NC. As shown in Fig. 4f, g and Supplementary Fig. 37, all five metals distribute uniformly on the NC as N-coordinated isolate metal sites. The metal contents are 15.6, 3.1, 4.1, 2.3, and 7.3 wt% for Pt, Fe, Co, Ni, and Cu, respectively, resulting in an overall metal content of 32.4 wt% (Fig. 4h, Supplementary Table 2). The positive oxidation states of the metal sites are confirmed (Supplementary Figs. 38, 39). Furthermore, no condensed matter of any metal is detected (Fig. 4i and Supplementary Figs. 40, 41). Although a limited number of metals were tested on NC, we speculate that the formation of SACs on NC follows the same evolution pathway as that on PCN, demonstrating the versatility of this synthetic method on different N-containing carbon substrates for preparing SACs with multiple metals. Together, this work provides a universal synthetic strategy to fabricate various UHL-SACs and even HESACs.Fig. 4: Fabricating UHL-SACs on N-doped Carbon (NC) substrate.The aberration-corrected HAADF-STEM images and FT-EXAFS spectra of NC supported UHL-SACs with (a) Pt, (b) Fe, (c) Co, (d) Ni, and (e) Cu. f TEM and (g) Aberration-corrected HAADF-STEM images of the UHL HESACs. h The metal content in the HESACs. i FT-EXAFS spectra for the metals in HESACs.Catalytic evaluation of Pt SACs/PCNThe partial oxidation of propane to valuable liquid oxygenates represents a novel strategy to utilize this class of light alkane34. Among the several current strategies, such as electrocatalysis35, photocatalysis36,37, thermal-derived homogeneous38, and thermal-derived heterogeneous catalysis, the exploitation of heterogeneous catalyst shows the greatest application potential39,40,41. However, the consumption of costly oxidants poses an obstacle to it. To address this issue, a catalytic process that can transfer propane to oxygenates with low-cost oxidants is urgently needed. Inspired by the molecular oxygen activation capacity of isolated Pt sites2, Pt SACs/PCN is evaluated in the oxidation of propane with oxygen in this work.The reaction is performed at 175 °C in a 240 mL autoclave, with propane (5 bar) as reactant, oxygen (6 bar) as oxidant, and acetonitrile (70 ml) as solvent (Fig. 5a). The decreased pressure and the gas chromatography (GC) results indicate the consumption of propane, and the gas production is identified as CO (Supplementary Fig. 42). Interestingly, liquid productions are detected in acetonitrile, which is dominated by oxygenates (acetone, acetic, and methanol, et al.), revealing the capacity of Pt SACs/PCN in transforming propane into valuable liquid productions (Fig. 5b, c and Supplementary Fig. 43). These liquid productions are further quantitatively analyzed via the external standard method (Supplementary Figs. 44, 45). As shown in Fig. 5d, the liquid product is confirmed as 37.1 mmol/gcat at 3 h, which increases with the reaction time, and reaches 71.9 and 107.6 mmol/gcat at 6 and 9 h, surpassing the low-loading Pt SACs/PCN, Pt nanoparticles (Pt NPs/PCN) and commercial Pt/C catalyst (Fig. 5e). To reveal the intrinsic activity of Pt SACs/PCN, the turnover frequency (TOF) and mass-specific activity are confirmed as 1.6 × 10−3 molpro·molPt−1 · s−1 and 12.0 mmol/gcat/h (Fig. 5f, g and Supplementary Table 4), well-placed among select prior reports of propane activation performance with oxygen. Interestingly, among the catalysts working with oxygen, only Pt SACs/PCN selects the pathway toward oxygenates (Fig. 5g). This may be the first observation of heterogeneously catalytic oxidation of propane to oxygenates with oxygen, which provides a strategy to utilize propane for harvesting valuable liquid productions. Moreover, the catalytic performance of Pt SACs/PCN shows insignificant decay after be reused five times (Fig. 5h), and the used catalyst maintains the dense isolated Pt sites (Supplementary Fig. 46), confirming its stability. To clarify the effect of the substrates, Pt SACs on NC were also evaluated in the propane oxidation (Fig. 5e). Following a similar trend with Pt SACs/PCN, Pt SACs/NC with higher Pt loading shows better activity than those with lower Pt loading and Pt particles, and the productions are dominated by oxygenates. These catalyst evaluations demonstrate the potential application of high-loading Pt SACs in activating light alkanes.Fig. 5: Catalytic evaluation of Pt SACs/PCN in propane oxidation.a Illustration of the reaction condition. b Gas chromatographic profile and (c) mass spectra of the liquid productions measured by GC-MS. d Catalytic performance of Pt SACs/PCN over reaction time. e Catalytic performance of various catalysts at 6 h. f TOF value of the catalyst used in this work. g Comparison of the propane oxidation performance with previous reports. h Stability test. The maximum measurement error for (d, e) is ±3.8%, which represent the standard deviation of 3 replicates at least.In conclusion, we report a general negative pressure annealing strategy to fabricate ultrahigh-loading single-atom catalysts across a broad range of transition metals. Besides monometallic SACs, high-entropy single-atom catalysts that contain multiple metal single atoms with high metal contents can also be obtained, proving the general applicability of the pressure annealing method. In-situ microscopic studies combined with ex-situ XAFS reveal the pivotal role of the vacuum annealing condition in suppressing the aggregation of metal species, enabling the formation of dense N-coordinated Pt sites. Furthermore, UHL Pt SACs/PCN exhibits superior catalytic performance in the oxidation of propane towards valuable liquid production. These findings provide valuable guidance for preparing a wide range of high-density SACs and show great potential use in efficient catalytic transformations.
