Synthesis and characterization of In-ZSM-5 zeoliteA two-step method has been developed for preparing Pt/In-ZSM-5 catalyst: The first step involved exchanging In+ with BAS in the H-ZSM-5 zeolite, referred to as In-ZSM-5, and the second step introduced Pt into the In-ZSM-5 catalyst. The in-situ reduction of In/H-ZSM-5 resulted in the replacement of BAS by In+, known as the reductive solid-state ion-exchange process40,41. As shown in Fig. 1a, upon dehydration, the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results of In/H-ZSM-5 showed bands at 3735 and 3591 cm−1, attributed to external silanol group (Si-OH) and bridge OH group associated with BAS42, respectively. Upon reduction at 550 °C, the BAS OH group disappeared, indicating the exchange of BAS with In. The difference spectra of before and after reduction confirmed the consumption of the BAS OH group (Fig. 1a). In-situ Fourier transform infrared spectroscopy using pyridine as a probe molecule (pyridine-FTIR) was employed to monitor the density of BAS. As shown in Fig. 1b, pyridine-adsorbed H-ZSM-5 showed three characteristic peaks: 1540, 1490, and 1455 cm−1. The 1540 and 1455 cm−1 features were associated with the protonated pyridinium ion corresponding to BAS and coordinatively adsorbed pyridine on the Lewis acid site (LAS), respectively, while 1490 cm−1 was attributed to the combination of pyridine adsorbed on BAS and LAS42. There was a ~30% decrease in BAS intensity and a minor increase of LAS intensity on calcined In/H-ZSM-5, likely due to the partial ion-exchange between In cations and BAS during the impregnation steps. Upon reduction, the pyridinium band decreased by ~70% compared to parent H-ZSM-5, in parallel with the appearance of pyridine adsorbed on LAS (1455 and 1446 cm−1), attributed to the formation of exchanged In species. Note that although the partial ion-exchange occurred during the impregnation steps, the extent of exchange was no more than 30%. The H2-reduction process was required to promote the migration of surface InOx nanoparticles to neutralize sufficient BAS and produce a large fraction of exchanged In sites.Fig. 1: Characterizations of In/H-ZSM-5 catalyst.a In-situ DRIFTS spectra collected during reduction of In/H-ZSM-5: (i) dehydrated at 550 °C; (ii) reduced by 10 vol. % H2/He at 550 °C; (iii) spectrum (i) subtracted from the spectrum (ii). b Pyridine-FTIR spectra of H-ZSM-5 and In/H-ZSM-5 with calcined and reduced conditions. The calcined sample was dehydrated at 550 °C before cooling to 150 °C for pyridine adsorption. The reduced condition represented that the sample was reduced at 550 °C before cooling to 150 °C for pyridine adsorption. c In-situ In K-edge XANES and (d) Fourier-transformed EXAFS spectra over In/H-ZSM-5 with fresh and reduced conditions. The fresh catalyst referred to the calcined sample measured at room temperature. The reduced condition represented that the sample was reduced at 550 °C. The reduced + oxidized condition represented the sample was reduced at 550 °C and then oxidized by exposing the reduced indium species to the ambient atmosphere. In foil and In2O3 were also measured as standards for reference. e Schematic of the reduction and subsequent oxidation of In/H-ZSM-5 catalyst.The in-situ X-ray absorption near edge structure (XANES) investigation shed light on the oxidation state of reduced In in In/H-ZSM-5 catalyst (Fig. 1c). The In of the fresh In/H-ZSM-5 showed identical absorption energy with In2O3, indicating In being in the +3-oxidation state. Upon reduction, the In K-edge showed a decrease in absorption energy, but different from the In foil (Fig. 1c). There has been an ongoing debate regarding the origin of decreasing absorption energy in metal-zeolites such as Ga/H-ZSM-5 and In-CHA catalysts37,41,43,44,45,46. It was unresolved whether this reduction was due to the formation of a hydride or a reduced In with a lower oxidation state. The H2-D2 exchange experiment on reduced In/H-ZSM-5 revealed the absence of In hydride at 550 °C, which was typically characterized by a band between 1600 and 2000 cm−1 wavenumber range41, as shown in Supplementary Fig. 1. This suggested that In+ instead of In hydride was most likely the species upon reducing In/H-ZSM-5. In addition, the In/H-ZSM-5 did not exhibit any additional scattering peaks in the extended X-ray absorption fine structure (EXAFS) spectrum (Fig. 1d), which was different from In2O3 and the In foil. This distinction became more pronounced when the In/Al ratio was decreased to 0.2, where the In K-edge EXAFS feature of the reduced catalyst disappeared entirely (see Supplementary Note 1, Supplementary Fig. 2). This phenomenon was likely attributed to the formation of extra-framework In+ species. Through linear combination fitting, it was estimated that ~70% of In2O3 was reduced in the In/H-ZSM-5 sample with an In/Al ratio of 1.0. This correlated closely with the consumption of BAS, following stoichiometry where one indium atom replaced one BAS to form In+ species. The significant reduction in the EXAFS scattering peak for the reduced In/H-ZSM-5 aligned with findings reported for the disappearance of In K-edge features in In-CHA41, although the exact reason for the disappearance would need to be further investigated. Upon subsequent oxidation, In+ was transitioned to the +3-oxidation state (Fig. 1d). Importantly, the EXAFS spectrum displayed the In-O feature in the first shell without the In-O-In feature in the second shell (Fig. 1d). This suggested that although In+ was oxidized to +3 upon contact with air, the In remained to be isolated at the extra-framework position within the micropores of zeolites. This was different from the initial In2O3 particles located at the external surface, which showed the In-O feature in both the first and second shell in the EXAFS spectrum (Fig. 1d). Likewise, XRD measurements (Supplementary Fig. 3) showed the crystalline In2O3 particles in the fresh In/H-ZSM-5 and disappeared upon reduction and subsequent oxidation by air, consistent with TEM images that In2O3 particles (~10 nm) observed on the fresh In/H-ZSM-5 catalyst nearly disappeared upon reduction (Supplementary Fig. 4).The examination of In/Al ratios (Supplementary Figs. 2–8) revealed that as the In/Al ratio increased from 0.2 to 1.0, more BAS was replaced by In, but BAS could not be completely replaced even at a high In/Al ratio of 1.5. An excess of In led to the formation of unreduced InOx oligomers. These observations suggested that at an In/Al ratio at or below 1.0, In+ was the dominant species upon reduction of In/H-ZSM-5 catalyst, and dispersed [InO]+ was the species when exposing the reduced indium species to the ambient atmosphere (Fig. 1e). In contrast, when indium was introduced onto siliceous MFI (Si-MFI) and SiO2 supports, the indium oxide exhibited a higher reduction temperature (Supplementary Fig. 8), and furthermore, indium was unable to migrate into the micropores of Si-MFI (Supplementary Figs. 9, 10). This observation emphasized the critical role of BAS in H-ZSM-5, not only in facilitating the reduction of In2O3 but also in stabilizing the formation of extra-framework In+ ions. Using a traditional liquid ion-exchange method, the In-ZSM-5(IE) zeolite attained an indium loading of only 0.8%, with an In/Al ratio of 0.17. TEM images (Supplementary Fig. 11) of the fresh and reduced In-ZSM-5(IE) showed the absence of indium oxide on the external surface, aligning with expectations from the ion-exchange process between indium species and BAS.Propane dehydrogenation performance of Pt/In-ZSM-5 catalystsPt was introduced into In-ZSM-5 to prepare the Pt/In-ZSM-5 catalysts. For comparison, the same metal loadings were used to prepare Pt/H-ZSM-5, PtIn/H-ZSM-5 (one-step method) and PtIn/SiO2. The propane (C3H8) dehydrogenation performance was tested with 20 vol% C3H8 in Ar at 550 °C. As shown in Fig. 2a, the Pt/In-ZSM-5 catalyst showed a stable propane conversion of ~48% without appreciable deactivation within 37 h. The propylene (C3H6) selectivity was higher than 99%. In comparison, PtIn/H-ZSM-5 and PtIn/SiO2 showed much lower propane conversion, indicating that PtIn alloy formed on the external surface of H-ZSM- 5 and SiO2 were less active than the PtIn species inside the micropores. In contrast, Pt/H-ZSM-5 showed a decreasing conversion over time on stream, although the initial propane conversion was higher than other catalysts. This was due to side reactions catalyzed by a large amount of BAS in Pt/H-ZSM-5 (Supplementary Fig. 12), which resulted in a lower propylene selectivity and carbon balance (Fig. 2b, d) compared to other catalysts. These BAS-catalyzed side reactions involved propane cracking, olefin oligomerization, and aromatization. Consequently, Pt/In-ZSM-5 delivered the highest propylene yield among these samples (Fig. 2c). Overall, Pt/In-ZSM-5 showed excellent PDH performance in terms of propane conversion, propylene selectivity, and stability, outperforming other monometallic (Pt, In) and bimetallic (PtIn) with different combinations. Based on the control experiments shown here, In in the Pt/In-ZSM-5 catalyst had at least two roles: (1) neutralizing BAS to avoid side reactions; (2) stabilizing Pt species to serve as the highly active sites for PDH.Fig. 2: Propane dehydrogenation performance of Pt/In-ZSM-5 catalyst.a Propane conversion, b propylene selectivity, c propylene yield, and d carbon balance as a function of time on stream over Pt/In-ZSM-5 and control catalysts. e Comparison of propane conversion as a function of time on stream over Pt/In-ZSM-5 with PtSn/Al2O3, PtSn/SiO2, and Pt/SiO2 catalysts. f Propane dehydrogenation performance of Pt/In-ZSM-5 catalyst for long-term operation. Reaction conditions of (a–f): 550 °C, pressure 20.3 kPa, and WHSV = 5.9 h−1. g Propane dehydrogenation performance of Pt/In-ZSM-5 catalyst with an undiluted propane stream: 580 °C, pressure 101.3 kPa, and WHSV = 4.7 h−1. h Ethane dehydrogenation performance: C2H6 conversion and C2H4 selectivity versus time on stream over Pt/In-ZSM-5 catalyst. Reaction conditions: 550 °C, pressure 20.3 kPa, and WHSV = 5.9 h−1.The PDH performance of Pt/In-ZSM-5 was compared with the benchmark PtSn-based catalysts using SiO2 and Al2O3 as supports24. As shown in Fig. 2e, both PtSn/Al2O3 and PtSn/SiO2 showed much lower propane conversions than Pt/In-ZSM-5, although similar propylene selectivity and carbon balance were observed (Supplementary Fig. 13). In addition, as shown in Fig. 2f, the propane conversion over Pt/In-ZSM-5 maintained at ~40% after 118 h and the propylene selectivity higher than 99.5%. The Pt/In-ZSM-5 catalyst was tested under 580 °C in the pure propane flow (WHSV = 4.7 h−1), as shown in Fig. 2g. Close to equilibrium conversion was obtained with a relatively minor deactivation with propane conversion from 36.2% to 34.7% while the propylene selectivity remained constant at 97.1% within 70 h. In addition, the catalytic performance of Pt/In-ZSM-5 was tested at increased weight hourly space velocities (WHSVs) of 39.3, 118, 236, and 531 h−1 to obtain the C3H6 production rates and apparent rate coefficient. As shown in Supplementary Fig. 14, at WHSVs of 118 h−1 or higher, the C3H8 conversion was below 20%, with propylene selectivity exceeding 99.8%. The net C3H6 formation rates were calculated to be 49.1, 61.6, and 70.0 molC3H6·gPt−1·h−1 at WHSVs of 118, 236, and 531 h−1, respectively. Considering that propane dehydrogenation was a reversible reaction, the forward apparent rate coefficient was derived based on the net C3H6 formation rates, inlet propane partial pressure, equilibrium constant, and propane conversion using the methodology provided in a previous study6. The apparent rate coefficient was determined to be 370 molC3H6·gPt−1·h−1·bar−1 at WHSV of 531 h−1. This value was higher than most of the reported PtGa-, PtZn-, PtIn-, and PtSn-based catalysts21,24,47,48. Using ethane dehydrogenation as another probe reaction, Pt/In-ZSM-5 also showed equilibrium conversion for ethane dehydrogenation with a higher than 99% ethylene selectivity and no deactivation after 80 h (Fig. 2h). These results further confirmed the superior dehydrogenation abilities of the Pt/In-ZSM-5 catalyst.The impact of the In/Al ratio of Pt/In-ZSM-5 catalysts was examined. As shown in Supplementary Fig. 15, as the In/Al ratio increased from 0.2 to 1.0, there was a decrease in the initial propane conversion, and an increase in propylene selectivity, propylene yield, and carbon balance. This was because more In+ neutralized more BAS, and thus led to fewer side reactions. On the other hand, an excess of In (In/Al = 1.5) was also detrimental to the PDH performance of Pt/In-ZSM-5 catalyst. The excess In2O3 could not diffuse into the micropores and located on the external surface as crystalline (Supplementary Figs. 3–8), which prevented the movement of Pt into the micropores of ZSM-5 and instead tended to form PtIn alloy on the external surface upon reduction. The resulting PtIn sites showed a similar performance to the case of PtIn/H-ZSM-5 and PtIn/SiO2. Consequently, selecting the optimal In/Al ratio was crucial: the amount of indium should be sufficient to neutralize BAS without being excessive, yet too much indium would inhibit Pt from migrating into the micropores and instead promote the formation of PtIn alloy on the external surface. Through comparative studies of In-ZSM-5 and Pt/In-ZSM-5 with various In/Al ratios, the optimal In/Al ratio was determined to be 1.0. The propane dehydrogenation performance of the Pt/In-Si-MFI catalyst was also tested, as shown in Fig. 2a–d. Initially, the catalyst achieved approximately 30% propane conversion, but it gradually declined to ~10% within 40 h. This observation suggested that the PtIn species, likely forming a PtIn alloy, not only offered a relatively low propane conversion but also exhibited a fast deactivation within 40 h. PtIn/SiO2 with different ratios (1:3 to 1:8) confirmed the poor propane dehydrogenation performance in terms of low propane conversion or fast deactivation (Fig. 2 and Supplementary Fig. 16). In addition, the prepared Pt/In-ZSM-5(IE) exhibited prominent deactivation, low propylene selectivity (Supplementary Fig. 17).Employing the same method, Pt/Zn-ZSM-5, Pt/Ga-ZSM-5, and Pt/Co-ZSM-5 catalysts were synthesized to examine the influence of the metal cation type. Supplementary Fig. 18 demonstrated a notable contrast with Pt/In-ZSM-5; the three new samples exhibited a decrease in propylene selectivity, although Pt/Zn-ZSM-5 and Pt/Ga-ZSM-5 showed higher propane conversions. As a result, Pt/In-ZSM-5 delivered the highest propylene yield. These results underscored the unique role of In in the Pt/M-ZSM-5 catalysts compared to other metal cations.The influence of Pt loading in Pt/In-ZSM-5 catalysts on PDH was shown in Fig. 3a. Reducing Pt loading to as low as 0.05% led to a marginal decline in initial propane conversion. However, catalysts with lower Pt loading demonstrated deactivation over time on stream. As shown in Supplementary Fig. 19, catalysts with varying Pt loadings exhibited comparable high propylene selectivity. This similarity suggested that active sites, regardless of Pt loading, preferentially favored propylene desorption. The easy desorption of propylene likely prevented its further dehydrogenation on the surface, thereby ensuring high propylene selectivity25. The catalyst deactivation was likely due to the instability of the active sites and the agglomeration at elevated temperatures (550 °C), which was usually a problem with single-atom catalysts1. The deactivation coefficients for varying Pt loadings in Pt/In-ZSM-5 were presented in Fig. 3b. The deactivation coefficient of 1Pt/In-ZSM-5 was 0.0042 h−1 within ~37 h, comparable to the value of 0.0038 h−1 for PtSn/SiO2 catalyst within 8 h and superior to other Pt-, Cr-, and Ga-based catalysts24,49. It was hypothesized that reducing Pt loading from 1% to 0.05% changed Pt from clusters to single atoms; Pt clusters contributed to high propane conversion, propylene selectivity, and durable stability, whereas single Pt atoms exhibited poor stability, and were not promising from a practical application point of view (Fig. 3c).Fig. 3: Impact of Pt loading of the Pt/In-ZSM-5 catalysts on propane dehydrogenation.a Propane conversion over time on stream over Pt/In-ZSM-5 catalysts with different Pt loadings. b Deactivation coefficient of Pt/In-ZSM-5 catalysts with different Pt loadings. Note that the deactivation coefficient was determined within 37 h. Reaction conditions: 550 °C, pressure 20.3 kPa, and WHSV = 5.9 h−1. c Schematic of the possible active species of Pt/In-ZSM-5 with dependence on Pt loading.The coke deposition on spent catalysts was investigated using Raman spectroscopy, temperature-programmed oxidation (TPO), and thermogravimetric analysis (TGA) (Supplementary Figs. 20–22). Notably, the Pt/In-ZSM-5 catalyst exhibited minimal carbon formation, significantly less than that observed on the Pt/H-ZSM-5 catalyst. In contrast, the 0.05Pt/In-ZSM-5 catalyst did not show carbon deposition, suggesting that deactivation resulted from the instability of isolated Pt atoms rather than coke deposition. TGA and TPO analysis further confirmed these observations. Based on this, the Pt/In-ZSM-5 catalyst under harsh conditions (600 °C in the pure propane flow) as well their regeneration ability was further evaluated (Supplementary Fig. 23). The results indicated a very slow deactivation rate over a period of 40 h. This minimal deactivation was attributed to coke formation at high temperatures. The produced coke could be easily regenerated through combustion, effectively restoring it to the original high performance.Determination of active site in Pt/In-ZSM-5 catalystsHigh-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (STEM-EDS) were applied to analyze the reduced Pt/In-ZSM-5 catalysts, providing information about the size of Pt clusters. In the 1Pt/In-ZSM-5 catalyst, HAADF-STEM images revealed Pt clusters averaging 0.7 nm in size (Fig. 4a). In contrast, 1Pt/H-ZSM-5 showed a larger particle size (1.3 nm, Supplementary Fig. 24) than 1Pt/In-ZSM-5 (0.7 nm), suggesting a confinement effect of In-ZSM-5. However, distinguishing between Pt and In elements was challenging due to insufficient contrast. STEM-EDS mapping of 1Pt/In-ZSM-5 (Fig. 4b) confirmed the even distribution of Pt and In within the catalyst. When the Pt loading was decreased to 0.3%, both Pt clusters and single Pt atoms were observed (Fig. 4c). At a lower Pt loading of 0.05%, HAADF-STEM images of 0.05Pt/In-ZSM-5 displayed only isolated Pt atoms (bright spots in Fig. 4d), with zoomed-in areas showing these spots to be ~0.209 nm in diameter and well-separated (Fig. 4e, f). Some less bright spots, potentially isolated In atoms or Pt atoms at varying thicknesses, were also noted (orange circles in Fig. 4e). Although distinguishing Pt from In atoms was difficult, the consistent In loadings across all three samples suggested that the bright spots in HAADF-STEM images should be Pt atoms. STEM-EDS mapping of 0.3Pt/In-ZSM-5 and 0.05Pt/In-ZSM-5 catalysts (Supplementary Figs. 25, 26) further supported the uniform distribution of Pt and In. These results indicated that at the Pt loading of 1%, Pt clusters were predominant, while at the low loading (0.05%), isolated Pt atoms were primarily present. HAADF-STEM images of 1Pt/In-ZSM-5 and 0.05Pt/In-ZSM-5 catalysts after PDH test showed that no particle growth was observed on 1Pt/In-ZSM-5, but isolated Pt atoms agglomerated on the 0.05 Pt/In-ZSM-5 catalyst (Supplementary Fig. 27). Correlating these findings with PDH performance (Figs. 2, 3) led to the conclusion that Pt clusters played a crucial role in the observed superior PDH performance. HAADF-STEM analysis of the Pt/In-Si-MFI and Pt/In-ZSM-5(IE) after reaction revealed Pt with sizes of ~1.2 nm and 1.3 nm, respectively, as shown in Supplementary Fig. 28, 29, similar to the observation of Pt in Pt/H-ZSM-5 catalyst. This indicated that the Pt species were predominantly located on the outer surface of the zeolites, rather than within the micropores. HAADF-STEM analysis of the spent PtIn/SiO2 identified an average size of 3 nm (Supplementary Fig. 30). Moreover, while X-ray diffraction (XRD) measurements did not reveal any active site information for the Pt/In-ZSM-5 due to the small size of the species, the spent PtIn/SiO2 showed PtIn2 as the dominant phase (Supplementary Fig. 31).Fig. 4: HAADF-STEM analysis and STEM-EDS mapping of xPt/In-ZSM-5 catalysts upon reduction.a HAADF-STEM image of 1Pt/In-ZSM-5. Inset in (a) showed the particle size distribution. b STEM-EDS mapping of 1Pt/In-ZSM-5. c HAADF-STEM image of 0.3Pt/In-ZSM-5. Red and blue circles in (a) and (c) showed the single Pt atom and Pt clusters, respectively. d HAADF-STEM image of 0.05Pt/In-ZSM-5. e A zoomed-in image of the orange square in (d). Orange circles in (e) represented the less bright spots compared to the bright spots within the rectangular area. f Intensity profiles along the green area in (e). Note that the samples were reduced at 550 °C and then cooled down to room temperature before the TEM sample preparation.In-situ X-ray absorption fine structure (XAFS) investigations were conducted to elucidate the active site of PtIn-based catalysts. For fresh samples, both Pt and In elements were predominantly in their oxidized forms (see Supplementary Fig. 32 and Supplementary Note 2 for detailed discussion). Upon reduction, as shown in Fig. 5, the In K-edge of all PtIn-related samples exhibited a decrease in absorption energy. Specifically, for Pt/In-ZSM-5 and PtIn/H-ZSM-5, the reduction-induced shift in the In K-edge absorption was similar to that observed in In/H-ZSM-5 catalysts (Supplementary Fig. 2 and Supplementary Note 1), showing a significant decrease in absorption energy, yet distinct from that of the In foil. A closer examination revealed that Pt/In-ZSM-5 exhibited a slightly greater decrease in absorption energy compared to In/H-ZSM-5 sample. This slight variance was attributed to the interaction between Pt species and In+. In the case of PtIn/H-ZSM-5, the presence of an excess amount of In species resulted in the formation of In+ species in the reduced samples, as confirmed by pyridine-FTIR results (Supplementary Fig. 12). Consequently, both Pt/In-ZSM-5 and PtIn/H-ZSM-5 exhibited a nearly complete disappearance of EXAFS features at the In K-edge (Fig. 5b). A careful examination of the Fourier-transformed EXAFS spectra at the In K-edge of the Pt/In-ZSM-5 catalyst (Fig. 5b) showed very minor peaks between 2 and 3 Å, which might be assigned to In-Pt bonds. However, the weak signal hindered further analysis due to the low concentration of In-Pt bonds in all In+ species arising from the Pt/In atomic ratio being close to 1:8. In contrast, Pt/In-Si-MFI and PtIn/SiO2 catalysts only demonstrated a minor decrease in the In K-edge absorption intensity, with EXAFS spectra prominently featuring In2O3, suggesting that only a fraction of In was reduced due to the presence of Pt, while the majority remained as In2O3 nanoparticles. EXAFS fitting data (Supplementary Tables 1, 2, Supplementary Fig. 33-40) further corroborated these findings, revealing a decrease in the coordination number (CN) of In-O and the emergence of In-Pt bonds. For instance, in Pt/In-Si-MFI, the CN of In-O and In-O-In decreased from 5.6 ± 0.5 and 4.7 ± 0.9 in fresh sample to 4.8 ± 0.3 and 4.0 ± 0.5 in reduced sample, respectively, with a minor contribution potentially from the In-Pt bond (CN = 0.13 ± 0.06, d = 2.64 ± 0.04 nm). For PtIn/SiO2, the CN of In-O and In-O-In decreased from 5.8 ± 0.6 and 5.1 ± 1.1 in fresh sample to 5.0 ± 0.2 and 4.1 ± 0.4 in reduced sample, respectively, but with a notable In-Pt bond (CN = 0.43 ± 0.05, d = 2.64 ± 0.01 nm). The more pronounced In-Pt coordination number in PtIn/SiO2 compared to Pt/In-Si-MFI was attributed to the preparation of the former via a one-step impregnation method, which contrasted with the two-step procedures of the latter, resulting in a different extent of alloy formation.Fig. 5: In-situ XANES, EXAFS, and CO-DRIFTS investigations over Pt/In-ZSM-5 catalysts.a In-situ XANES spectra of In K-edge and (b) in-situ Fourier-transformed EXAFS spectra over PtIn-based catalysts. In foil and In2O3 were also measured as standards for reference. c In-situ XANES spectra of Pt L3-edge and d in-situ Fourier-transformed EXAFS spectra over PtIn-based catalysts. e In-situ XANES spectra of Pt L3-edge over xPt/In-ZSM-5 catalysts. The spectra were collected in the presence of 20% H2/He at 550 °C. f CO-DRIFTS of xPt/In-ZSM-5 with different Pt loadings. The catalysts were reduced at 550 °C and then cooled down to room temperature in He gas before CO adsorption.Analysis of the Pt L3-edge XANES and EXAFS provided insights into Pt speciation within PtIn-based catalysts. After reduction, all Pt-containing samples demonstrated absorption features generally similar to the metallic phase, with notable differences upon detailed examination. For Pt/In-Si-MFI, the absorption was nearly identical to that of Pt/H-ZSM-5, indicative of Pt nanoparticles ~1.3 nm in size (CN(Pt-Pt) = 8.7). EXAFS fittings for reduced Pt/In-Si-MFI samples revealed a predominance of Pt-Pt bonds (CN = 7.2 ± 1.7) over Pt-In bonds (CN = 1.0 ± 0.5). Conversely, PtIn/H-ZSM-5 and PtIn/SiO2 displayed slightly higher absorption energies, suggesting primary PtIn alloy formation. This was corroborated by EXAFS fittings showing PtIn/H-ZSM-5 with CN(Pt-Pt) = 4.0 ± 1.0 and CN(Pt-In) = 3.5 ± 0.8, and PtIn/SiO2 with CN(Pt-Pt) = 2.7 ± 0.5 and CN(Pt-In) = 3.4 ± 0.4, confirming that a one-step impregnation method favored PtIn alloy formation. The absorption energy of Pt/In-ZSM-5 fell between that of Pt/H-ZSM-5 and PtIn/SiO2, indicating the interaction between Pt and In species without forming the bulk PtIn alloy. In addition, the absorption energy of Pt within xPt/In-ZSM-5 samples showed a gradual increase with decreasing Pt size from clusters to single atoms (Fig. 5e). This trend was rationalized by the electron transfer (from Pt to In) that occurred when Pt was introduced, resulting in a partially positive charge of Pt and a slight reduction in In. In larger Pt clusters, these positive charges were more evenly distributed, resulting in an average valence state closer to that of the metallic phase. EXAFS fitting of Pt/In-ZSM-5 showed CN(Pt-Pt) = 2.8 ± 0.6 and CN(Pt-In) = 1.6 ± 0.6 for reduced Pt/In-ZSM-5 sample. The ratio of CN(Pt- Pt) to CN(Pt-In) was also between that of the Pt/H-ZSM-5 and PtIn/SiO2 samples, further demonstrating the presence of Pt-In bond, but without bulk PtIn alloys. The difference between Pt/In-ZSM-5 and PtIn/H-ZSM-5 catalysts were discussed in detail, as shown in Supplementary Fig. 41. Therefore, we identified the active center of Pt/In-ZSM-5 catalyst as a Pt4In cluster, primarily due to two reasons: (1) In speciation predominantly existed as In+ in the Pt/In-ZSM-5 catalyst, as evidenced by in-situ XANES, EXAFS, and pyridine-FTIR results. These findings provided the guidance that the active centers should be PtxIn clusters. (2) The HAADF-STEM image of the spent catalyst showed a particle size of 0.7 nm. The total CN of Pt-Pt and Pt-In from the XAFS measurements was 4.4, which was in line with the coordination environments of the Density functional theory (DFT) model of a Pt4In cluster composed of five atoms. It was important to note that, while a cluster of five atoms could be useful for DFT studies to investigate the effects of Pt coordination with In on adsorption and surface reactions, it might not necessarily replicate the precise structure of catalysts used in experiments. As reported previously, the local coordination was critical to control the catalytic activity of an active site50. Although there were minor differences in the EXAFS fittings, possibly due to the lack of a standard Pt-In+ pathway in the zeolite, the collective evidence strongly supported the configuration of the active center as a Pt4In cluster.The CO-DRIFTS results provided additional evidence for the extent of interaction between Pt with In+ in the Pt/In-ZSM-5 catalysts. As shown in Fig. 5f and Supplementary Fig. 42, the lack of a CO adsorption band on reduced In-ZSM-5 implied the negligible interaction between CO and In+. 1Pt/H-ZSM-5 showed an intense peak at 2082 cm−1, characteristic of linearly adsorbed CO on metallic Pt and consistent with the in-situ XANES results (Fig. 5)24. Notably, Pt/In-ZSM-5 exhibited a CO peak at a slightly lower frequency than Pt/H-ZSM-5. The red shift in the CO frequency for Pt/In-ZSM-5 suggested a stronger CO adsorption on this catalyst due to the interaction between In and Pt, resulting in a more effective d-π back-donation from Pt to CO51 and consistent with in-situ XANES results in Fig. 5. A higher In/Al ratio exhibited more pronounced interaction of CO with Pt (Supplementary Fig. 42). With the decrease of Pt loading (Fig. 5f), a notable blue shift in the CO frequency signified a reduced interaction strength between Pt species and CO molecules, aligning with the emergence of isolated Pt atoms that interacted less strongly with CO19, consistent with the in-situ XANES results that a more positively charged Pt was on single Pt atoms (Fig. 5e).Integrating HAADF-STEM images, in-situ XANES and EXAFS characterizations and CO-DRIFTS results led to a comprehensive understanding of Pt/In-ZSM-5 catalysts. At a 1% Pt loading, Pt4 clusters stabilized by In+ within the ZSM-5 zeolite, were predominant. However, reducing the Pt loading to 0.05% favored the formation of single Pt atoms. For these catalysts, In was found to exist primarily in a + 1-oxidation state at extra-framework positions (Fig. 3c), playing a crucial role in stabilizing slightly positively charged Pt atoms. This interaction mechanism was notably distinct from that in conventional PtIn-based catalysts, where In typically existed in a + 3 oxidation state or in a metallic form, forming PtIn alloys48,52.DFT investigations of reaction mechanismsDFT calculations were performed to gain more insights into the structural characteristics of Pt/In-ZSM-5 catalysts. ZSM-5 was described by the periodic MFI framework (Supplementary Fig. 43). Pt1/In1-ZSM-5 and Pt4/In1-ZSM-5 models (Supplementary Figs. 44, 45) were used to describe the isolated single Pt atom and Pt cluster, respectively. In1-ZSM-5, Pt1-ZSM-5, and Pt4-ZSM-5 models (Supplementary Figs. 46–48) were also included for comparison. DFT-calculated partial density of states (PDOS) revealed a slight decrease in the oxidation state of In with an increasing Pt atoms from 1 to 4, evidenced by the downshift of In 5 s and 5p valence states (Fig. 6a). This was consistent with the slight decrease in edge position observed in in-situ XANES (Fig. 5a). By comparison, on interaction with In, Pt of Pt1/In1-ZSM-5 was less positively charged than that of Pt1-ZSM-5 by demonstrating a downshift of d-band, which eventually became more metallic with the increase of size to Pt4/In1-ZSM-5 (Fig. 6b). The formation of the Pt4 cluster in ZSM-5 was more favorable than the formation of Pt1 single atom, where the presence of In species could help stabilize Pt1 and Pt4 clusters compared to the case without the presence of In species in the ZSM-5 framework (Pt4-ZSM-5: Ef = −8.59 eV; Pt4/In1-ZSM-5: Ef = −10.01 eV, Supplementary Fig. 49). These DFT results aligned with the structural characterizations obtained from in-situ XAFS investigations and CO-DRIFTS results (Fig. 5), indicating that In+ in Pt/In-ZSM-5 effectively anchored Pt atom through direct Pt–In interaction, with In being more positively charged than Pt. The differences in the electronic structure between Pt1/In1-ZSM-5 and Pt4/In1-ZSM-5 models should be responsible for the varied PDH performances.Fig. 6: DFT-calculated PDOS for In and Pt orbitals.a In 5 s and 5p orbitals in In1-ZSM-5, Pt1/In1-ZSM-5, and Pt4/In1-ZSM-5 models and (b) Pt 5d orbitals in Pt1-ZSM-5, Pt1/In1-ZSM-5, and Pt4/In1-ZSM-5 models. Middle inset: optimized structures for In1-ZSM-5, Pt1-ZSM-5, Pt1/In1-ZSM-5, and Pt4/In1-ZSM-5.The PDH mechanism was further explored using DFT for Pt1/In1-ZSM-5 and Pt4/In1-ZSM-5 (Fig. 7, Supplementary Fig. 50, 51, and Supplementary Tables 3, 4). As illustrated in Fig. 7a, the Pt cluster site of Pt4/In1-ZSM-5 (solid black line) was active toward the adsorption of CH3CH2CH3 with an adsorption energy (Eads) of −0.82 eV (Supplementary Fig. 50, Supplementary Table 3). The first C-H bond breaking of *CH3CH2CH3 was preferred with a reaction energy (ΔE) of −0.73 eV and an activation energy (Ea) of 0.33 eV. The sequential C-H bond cleavage was also favorable (ΔE = −0.30, Ea = 0.60 eV), where the reaction barrier was lower than that on the Pt(111) surface (Ea = ca. 0.7 ~ 1.1 eV)20,53,54. As a comparison, the single Pt atom site of Pt1/In1-ZSM-5 (Fig. 7a, solid blue) bounded CH3CH2CH3 more strongly (Supplementary Fig. 50, Eads = −1.04 eV). The cleavage of the first C-H bond of *CH3CH2CH3 to *CH3CH1CH3 was slightly less thermodynamically favorable but with a relatively lower barrier (ΔE = −0.31, Ea = 0.26 eV), where a dissociated *H was coadsorbed at the same Pt1 site. The sequential C-H bond cleavage resulted in the bindings of *CH3CH1CH2 via a bidentate conformation together with two *H species at a Pt1 site, which corresponded to a high barrier (ΔE = −0.20 eV, Ea = 1.46 eV) due to the over-coordination of Pt1. This step was much more difficult than that on Pt4/In1-ZSM-5. Alternatively, the second C-H bond scission could undergo a two-step path associated with the synergy between Pt1 and neighboring In1 (Supplementary Fig. 51, solid orange line). Specifically, the *H species on Pt hopped to the neighboring In site (ΔE = 0.92 eV), which was followed by the dehydrogenation of *CH3CH1CH3 species to *CH3CH1CH2 (ΔE = −0.55 eV, Ea = 0.15 eV). However, the H-hopping process on Pt1/In1-ZSM-5 was still more difficult than the direct C-H bond breaking on Pt4/In1-ZSM-5 model, and the corresponding barrier was too high to attribute to the experimentally observed high initial PDH rates (Fig. 3). Our DFT results indicated the superior advantage of the Pt4 cluster site over single Pt atom site. By comparison, the hydrogen bonds provided by the ZSM-5 framework to stabilize the intermediates and transition states were rather limited and the contribution to tune the potential energies (Fig. 7b and Supplementary Fig. 51) was subtle.Fig. 7: Reaction mechanism of propane dehydrogenation over Pt/In-ZSM-5 catalysts.a DFT-calculated potential energy diagram (solid line) and the Gibbs free energy diagram at 550 °C (dashed line) for the dehydrogenation of propane over Pt1/In1-ZSM-5 model and Pt4/In1-ZSM-5 model, also see Supplementary Table 4 for reaction energy and activation energy of each elementary step. b DFT-optimized atomic structures for the reaction intermediates and transition states during PDH on Pt1/In1-ZSM-5 model and Pt4/In1-ZSM-5 models.To ensure the Pt4 cluster rather than Pt1 as the catalytic site that could enable active and selective dehydrogenation as observed experimentally (Fig. 3), the free energy was calculated by including the entropy contribution at 550 °C (Fig. 7a, dashed line). Indeed, Pt4 cluster again displayed a lower apparent barrier of 0.97 eV than Pt1 of 1.37 eV. Finally, as discussed above, the In1-anchored Pt1 site enabled the bidentate binding motif of *CH3CH1CH2 with a strong adsorption energy (Supplementary Fig. 50, Eads = −2.12 eV), while In1-anchored Pt4 site provided a relatively weak binding of *CH3CH1CH2 with two Pt atoms (Eads = −1.77). Yet, the removal of *CH3CH1CH2 was both feasible under the reaction temperature of 550 °C by lowering the adsorption energy to −0.61 eV for Pt1/In1-ZSM-5 and -0.25 eV for Pt4/In1-ZSM-5 (Supplementary Fig. 50, Supplementary Table 3), which was much more favorable than the other steps involved in PDH for each case (Fig. 7b). This was further corroborated by the in-situ DRIFTS investigation (Supplementary Fig. 52) on the flow of propane over reduced Pt/In-ZSM-5. It showed that the C–H stretching band of propane disappeared rapidly upon being purged by He without the associated vibrational features of unsaturated hydrocarbons. In addition, in-situ XAFS investigations revealed that the oxidation states remained unchanged for both Pt and In, from the reduction stage to the reaction feed (Supplementary Fig. 53 and Supplementary Table 2), suggesting no appreciable hydrocarbon coverage at the active center formed upon reduction. Therefore, it was concluded that the desorption of *CH3CH1CH2 was not a rate-limiting step, while the C-H scission, specifically the second C-H scission according to the DFT results, required the highest energy to overcome along the reaction pathway.Without the presence of In species, the formation Pt4 cluster in ZSM-5 was less preferred (Supplementary Fig. 49). This was consistent with the experimental observation that Pt clusters in the ZSM-5 framework were larger than those with the presence of In species (Supplementary Fig. 24). Upon exposure to propane, the first C-H bond breaking was not as favorable as Pt4/In1-ZSM-5 (Pt4-ZSM-5: ΔE = −0.19 eV; Pt4/In1-ZSM-5: ΔE = −0.73 eV, in Supplementary Table 5). While the sequential dehydrogenation to propylene was more favorable for the Pt4-ZSM-5 model (Pt4-ZSM-5: ΔE = −1.17 eV; Pt4/In1-ZSM-5: ΔE = −0.30 eV). This was due to the more weakly anchored Pt4 in Pt4-ZSM-5 than that in Pt4/In1-ZSM-5, enabling structural fluxionality to selective bond-strengthening to propylene via the π-bond (Pt4-ZSM-5: Eb = −2.13 eV; Pt4/In1-ZSM-5: Eb = −1.77 eV in Supplementary Table 6, Supplementary Fig. 54). According to the calculated PDOS (Supplementary Fig. 55), the propylene adsorption introduced more significant change of Pt 5d states for Pt4-ZSM-5 than Pt4/In1-ZSM-5. By comparison, the effect on the binding of *CH3-CH1-CH3 via the σ-bond was much less (Pt4-ZSM-5: Eb = −0.90 eV; Pt4/In1-ZSM-5: Eb = −0.82 eV in Supplementary Table 6, Supplementary Fig. 54).Overall, the DFT calculations (Fig. 7) agreed well with the experimental observations (Figs. 2, 3) that Pt/In-ZSM-5 catalysts were active and selective for PDH. According to the DFT results, the superior dehydrogenation performance of Pt/In-ZSM-5 strongly depended on the size of Pt (single atom or clusters) anchored by In sites. Both the Pt1 atom site and Pt4 cluster site were active enough to enable facile propane adsorption and the first C-H bond cleavage (Fig. 7) by facilitating the electron transfer from Pt to the adsorbates, and moderately enough to allow the removal of the propylene product from the site under the reaction condition. While the situation to break the second C-H bond was rather different. It was hindered by over-coordination of the Pt1 site of Pt1/In1-ZSM-5, but being facilitated by the direct participation of neighbored Pt atoms within of Pt4/In1-ZSM-5under hydrogenation conditions.
