Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts

Abstract

•Constructing selective and ultrastable single-site intermetallic catalysts•Achieving the highest propylene productivity at the lowest deactivation rate•Revealing active site structures under representative dehydrogenation reaction Propane dehydrogenation (PDH) is a commercial propylene production technology, but high reaction temperatures result in cracking and coking. Isolating active metal components into single atoms can maximize atomic efficiency, and the resulting single-atom catalysts (SACs) could inhibit structure-sensitive side reactions with unprecedented selectivity. Herein, we report a single-site [PtZn4] catalyst by assembling atomically ordered PtZn intermetallic alloys, which enables more than 95% selectivity of propylene over a broad temperature range of 520°C to 620°C. No obvious deactivation is observed within the 160-h industrial PDH test. We reveal that the surface [PtZn4] ensembles in PtZn IMAs serve as the stable active site structures, wherein the geometry-isolated and electron-rich Pt in [PtZn4] ensembles readily promotes desorption of surface-bounded propylene and improves the stability by prohibiting coke side reactions. Propane dehydrogenation (PDH) is a commercial propylene production technology that has received much attention, but high reaction temperatures result in a decrease of propylene selectivity and catalyst stability. This paper describes a single-site [PtZn4] catalyst by assembling atomically ordered intermetallic alloy (IMA) as a selective and ultrastable PDH catalyst. The catalyst enables more than 95% of propylene selectivity from 520°C to 620°C. No obvious deactivation is observed within the 160-h test, superior to PtSn/Al2O3 and state-of-the-art Pt-based catalysts. Based on in situ X-ray absorption fine structure, X-ray photoelectron spectroscopy measurements, and density functional theory calculations, we reveal that the surface [PtZn4] ensembles in PtZn IMAs serve as the key active site structures, wherein the geometry-isolated and electron-rich Pt1 site in [PtZn4] ensembles readily promotes the first and second C–H cleavage of propane but inhibits further dehydrogenation of surface-bounded propylene. This significantly improves the selectivity and stability by prohibiting coke side reactions. Propane dehydrogenation (PDH) is a commercial propylene production technology that has received much attention, but high reaction temperatures result in a decrease of propylene selectivity and catalyst stability. This paper describes a single-site [PtZn4] catalyst by assembling atomically ordered intermetallic alloy (IMA) as a selective and ultrastable PDH catalyst. The catalyst enables more than 95% of propylene selectivity from 520°C to 620°C. No obvious deactivation is observed within the 160-h test, superior to PtSn/Al2O3 and state-of-the-art Pt-based catalysts. Based on in situ X-ray absorption fine structure, X-ray photoelectron spectroscopy measurements, and density functional theory calculations, we reveal that the surface [PtZn4] ensembles in PtZn IMAs serve as the key active site structures, wherein the geometry-isolated and electron-rich Pt1 site in [PtZn4] ensembles readily promotes the first and second C–H cleavage of propane but inhibits further dehydrogenation of surface-bounded propylene. This significantly improves the selectivity and stability by prohibiting coke side reactions. A change of feedstock from naphtha to light alkanes, due to the emergence of shale gas and the increasing availability of natural gas, has led to an increasing development of on-purpose propane dehydrogenation (PDH) technologies.1Sattler J.J. Ruiz-Martinez J. Santillan-Jimenez E. Weckhuysen B.M. Catalytic dehydrogenation of light alkanes on metals and metal oxides.Chem. Rev. 2014; 114: 10613-10653Crossref PubMed Scopus (930) Google Scholar, 2Grant J.T. Carrero C.A. Goeltl F. Venegas J. Mueller P. Burt S.P. Specht S.E. McDermott W.P. Chieregato A. Hermans I. Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts.Science. 2016; 354: 1570-1573Crossref PubMed Scopus (339) Google Scholar, 3Chen S. Zeng L. Mu R. Xiong C. Zhao Z.J. Zhao C. Pei C. Peng L. Luo J. Fan L.-S. Gong J. Modulating lattice oxygen in dual-functional Mo-V-O mixed oxides for chemical looping oxidative dehydrogenation.J. Am. Chem. Soc. 2019; 141: 18653-18657Crossref PubMed Scopus (52) Google Scholar PDH is an extremely energy-intensive process largely due to the high positive enthalpy of the reactions (ΔrH° = 124 kJ mol−1), necessitating high reaction temperatures,4Liu G. Zhao Z.-J. Wu T. Zeng L. Gong J. Nature of the active sites of VOx/Al2O3 catalysts for propane dehydrogenation.ACS Catal. 2016; 6: 5207-5214Crossref Scopus (111) Google Scholar, 5Xiong H. Lin S. Goetze J. Pletcher P. Guo H. Kovarik L. Artyushkova K. Weckhuysen B.M. Datye A.K. Thermally stable and regenerable platinum-tin clusters for propane dehydrogenation prepared by atom trapping on ceria.Angew Chem Int Ed Engl. 2017; 56: 8986-8991Crossref PubMed Scopus (176) Google Scholar, 6Hu Z.-P. Yang D. Wang Z. Yuan Z.-Y. State-of-the-art catalysts for direct dehydrogenation of propane to propylene.Chin. J. Catal. 2019; 40: 1233-1254Crossref Scopus (62) Google Scholar which presents grand challenges for alkene selectivity and catalyst stability.7Sun G. Zhao Z.J. Mu R. Zha S. Li L. Chen S. Zang K. Luo J. Li Z. Purdy S.C. et al.Breaking the scaling relationship via thermally stable Pt/Cu single atom alloys for catalytic dehydrogenation.Nat. Commun. 2018; 9: 4454Crossref PubMed Scopus (198) Google Scholar Especially, it has been shown that larger ensembles of active sites catalyze structure-sensitive side reactions, including cracking and deep dehydrogenation, leading to the production of C1 and C2 molecules and coke.8Zhu J. Yang M. Yu Y. Zhu Y. Sui Z. Zhou X. Holmen A. Chen D. Size-dependent reaction mechanism and kinetics for propane dehydrogenation over Pt catalysts.ACS Catal. 2015; 5: 6310-6319Crossref Scopus (109) Google Scholar To tackle these challenges, the “active site isolation” strategy has been proved to be effective.9Marcinkowski M.D. Darby M.T. Liu J. Wimble J.M. Lucci F.R. Lee S. Michaelides A. Flytzani-Stephanopoulos M. Stamatakis M. Sykes E.C.H. Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C-H activation.Nat. Chem. 2018; 10: 325-332Crossref PubMed Google Scholar, 10Qiao B. Wang A. Yang X. Allard L.F. Jiang Z. Cui Y. Liu J. Li J. Zhang T. Single-atom catalysis of CO oxidation using Pt1/FeOx.Nat. Chem. 2011; 3: 634-641Crossref PubMed Scopus (3115) Google Scholar, 11Wegener E.C. Bukowski B.C. Yang D. Wu Z. Kropf A.J. Delgass W.N. Greeley J. Zhang G. Miller J.T. Intermetallic compounds as an alternative to single-atom alloy catalysts: geometric and electronic structures from advanced X-ray spectroscopies and computational studies.ChemCatChem. 2020; 12: 1325-1333Crossref Scopus (16) Google Scholar Isolating the active metal components into the single-atom site and the resulting single-atom catalysts (SACs) could inhibit the structure-sensitive side reactions.2Grant J.T. Carrero C.A. Goeltl F. Venegas J. Mueller P. Burt S.P. Specht S.E. McDermott W.P. Chieregato A. Hermans I. Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts.Science. 2016; 354: 1570-1573Crossref PubMed Scopus (339) Google Scholar,9Marcinkowski M.D. Darby M.T. Liu J. Wimble J.M. Lucci F.R. Lee S. Michaelides A. Flytzani-Stephanopoulos M. Stamatakis M. Sykes E.C.H. Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C-H activation.Nat. Chem. 2018; 10: 325-332Crossref PubMed Google Scholar,12Li Z. Yu L. Milligan C. Ma T. Zhou L. Cui Y. Qi Z. Libretto N. Xu B. Luo J. et al.Two-dimensional transition metal carbides as supports for tuning the chemistry of catalytic nanoparticles.Nat. Commun. 2018; 9: 5258Crossref PubMed Scopus (90) Google Scholar, 13Khorshidi A. Violet J. Hashemi J. Peterson A.A. How strain can break the scaling relations of catalysis.Nat. Catal. 2018; 1: 263-268Crossref Scopus (138) Google Scholar, 14Mehta P. Barboun P. Herrera F.A. Kim J. Rumbach P. Go D.B. Hicks J.C. Schneider W.F. Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis.Nat. Catal. 2018; 1: 269-275Crossref Scopus (186) Google Scholar, 15Greiner M.T. Jones T.E. Beeg S. Zwiener L. Scherzer M. Girgsdies F. Piccinin S. Armbrüster M. Knop-Gericke A. Schlögl R. Free-atom-like d states in single-atom alloy catalysts.Nat. Chem. 2018; 10: 1008-1015Crossref PubMed Scopus (170) Google Scholar For example, the isolated Pt1 site in Pt/Cu single-atom alloy (SAA) can only interact with a single Pt atom for the deep dehydrogenated C3H5, a model precursor of coke formation, instead of three more stable Pt–C interactions on a 3-fold Pt3 hollow site over Pt (111), leading to less carbon deposition. However, the long-term stability of Pt/Cu SAAs is disillusionary due to its thermal and chemical instabilities.9Marcinkowski M.D. Darby M.T. Liu J. Wimble J.M. Lucci F.R. Lee S. Michaelides A. Flytzani-Stephanopoulos M. Stamatakis M. Sykes E.C.H. Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C-H activation.Nat. Chem. 2018; 10: 325-332Crossref PubMed Google Scholar,16Kyriakou G. Boucher M.B. Jewell A.D. Lewis E.A. Lawton T.J. Baber A.E. Tierney H.L. Flytzani-Stephanopoulos M. Sykes E.C. Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations.Science. 2012; 335: 1209-1212Crossref PubMed Scopus (854) Google Scholar,17Wang A. Li J. Zhang T. Heterogeneous single-atom catalysis.Nat. Rev. Chem. 2018; 2: 65-81Crossref Scopus (1340) Google Scholar Although single-site Zn(II),18Schweitzer N.M. Hu B. Das U. Kim H. Greeley J. Curtiss L.A. Stair P.C. Miller J.T. Hock A.S. Propylene hydrogenation and propane dehydrogenation by a single-site Zn2+ on silica catalyst.ACS Catal. 2014; 4: 1091-1098Crossref Scopus (166) Google Scholar Co(II),19Hu B. “Bean” Getsoian A. Schweitzer N.M. Das U. Kim H. Niklas J. Poluektov O. Curtiss L.A. Stair P.C. Miller J.T. Hock A.S. Selective propane dehydrogenation with single-site CoII on SiO2 by a non-redox mechanism.J. Catal. 2015; 322: 24-37Crossref Scopus (132) Google Scholar Ni(II),20Zhang G. Yang C. Miller J.T. Tetrahedral nickel(II) phosphosilicate single-site selective propane dehydrogenation catalyst.ChemCatChem. 2018; 10: 961-964Crossref Scopus (19) Google Scholar and Ga(III)21Searles K. Siddiqi G. Safonova O.V. Copéret C. Silica-supported isolated gallium sites as highly active, selective and stable propane dehydrogenation catalysts.Chem. Sci. 2017; 8: 2661-2666Crossref PubMed Google Scholar on silica have been developed, these active sites suffer from low intrinsic activity and reduction deactivation. The development of highly efficient and stable SACs, therefore, remains grand challenges in the field of alkane dehydrogenation at elevated temperatures. Atomically ordered Pt1M1 intermetallic alloy (IMA) (molar ratio of Pt/M = 1) contains one active metal Pt and another inert component M, which can disperse the active sites into the isolated Pt1 site by the neighboring M atoms.11Wegener E.C. Bukowski B.C. Yang D. Wu Z. Kropf A.J. Delgass W.N. Greeley J. Zhang G. Miller J.T. Intermetallic compounds as an alternative to single-atom alloy catalysts: geometric and electronic structures from advanced X-ray spectroscopies and computational studies.ChemCatChem. 2020; 12: 1325-1333Crossref Scopus (16) Google Scholar,22Zhang L. Zhou M. Wang A. Zhang T. Selective hydrogenation over supported metal catalysts: from nanoparticles to single atoms.Chem. Rev. 2020; 120: 683-733Crossref PubMed Scopus (304) Google Scholar,23Nakaya Y. Hirayama J. Yamazoe S. Shimizu K.I. Furukawa S. Single-atom Pt in intermetallics as an ultrastable and selective catalyst for propane dehydrogenation.Nat. Commun. 2020; 11: 2838Crossref PubMed Scopus (61) Google Scholar Additionally, this highly ordered crystalline structure provides active sites with higher thermal stability compared with conventional single-atom alloys.11Wegener E.C. Bukowski B.C. Yang D. Wu Z. Kropf A.J. Delgass W.N. Greeley J. Zhang G. Miller J.T. Intermetallic compounds as an alternative to single-atom alloy catalysts: geometric and electronic structures from advanced X-ray spectroscopies and computational studies.ChemCatChem. 2020; 12: 1325-1333Crossref Scopus (16) Google Scholar For example, PtZn IMA has been reported in ethane dehydrogenation.24Cybulskis V.J. Bukowski B.C. Tseng H. Gallagher J.R. Wu Z. Wegener E. Kropf A.J. Ravel B. Ribeiro F.H. Greeley J. Miller J.T. Zinc promotion of platinum for catalytic light alkane dehydrogenation: insights into geometric and electronic effects.ACS Catal. 2017; 7: 4173-4181Crossref Scopus (88) Google Scholar However, the loading of Pt is relatively high with about 9.53 wt %, which is far higher than that of the industrial Pt-based catalysts with the Pt loading of about 0.3 wt % in PtSn/Al2O3. This induces low olefin productivity and atom utilization. Recently, novel preparation methods have been developed to improve the propylene productivity by constructing ultrasmall PtZn bimetal nanoclusters by embedding them inside S-1 zeolites.25Wang Y. Hu Z.-P. Lv X. Chen L. Yuan Z.-Y. Ultrasmall PtZn bimetallic nanoclusters encapsulated in silicalite-1 zeolite with superior performance for propane dehydrogenation.J. Catal. 2020; 385: 61-69Crossref Scopus (37) Google Scholar,26Sun Q. Wang N. Fan Q. Zeng L. Mayoral A. Miao S. Yang R. Jiang Z. Zhou W. Zhang J. et al.Subnanometer bimetallic platinum-zinc clusters in zeolites for propane dehydrogenation.Angew. Chem. Int. Ed. Engl. 2020; 59: 2-11Crossref Google Scholar However, due to the very complex coordination structures in zeolites, e.g., the coexistence of Pt–O, Pt–Pt, and Pt–Zn coordinations, it is very difficult to identify the active site structures, regardless of experimental and theoretical simulations. Therefore, further revealing the unclear active site structures and promoting propylene productivity is highly desired. This paper describes a selective and ultrastable single-site [PtZn4] catalyst by assembling atomically ordered PtZn IMA with trace Pt loadings of 0.1 wt %, which enables more than 95% selectivity of propylene over a broad temperature range of 520°C to 620°C and the highest propylene productivity of 83.2 mol C3H6 gPt−1 h−1. No obvious deactivation was observed within 160-h under industrial operating conditions, superior to PtSn/Al2O3 and state-of-the-art Pt-based catalysts. Based on in situ X-ray absorption fine-structure spectroscopy measurements, X-ray photoelectron spectroscopy measurements, and density functional theory calculations, it is further revealed that surface [PtZn4] ensembles in Pt1Zn1 (110) serve as the stable active site structures, wherein the geometry-isolated and electron-rich Pt in [PtZn4] ensembles readily promote desorption of the surface-bounded propylene and improve the stability by prohibiting coke formation and segregation of Pt and Zn atoms via structural ordering. By direct H2 temperature-programmed reduction strategy (see Experimental Procedures), we successfully constructed PtZn IMAs (molar ratio of Pt:Zn = 1) supported on SiO2, where the H2-rich atmosphere at high temperatures (HT) of 600°C contributes to the transformation of disordered PtZn nanoparticles (NPs) into structurally ordered PtZn IMAs (Figure 1A).27Qi Z. Xiao C. Liu C. Goh T.W. Zhou L. Maligal-Ganesh R. Pei Y. Li X. Curtiss L.A. Huang W. Sub-4 nm PtZn intermetallic nanoparticles for enhanced mass and specific activities in catalytic electrooxidation reaction.J. Am. Chem. Soc. 2017; 139: 4762-4768Crossref PubMed Scopus (180) Google Scholar We monitored the formation and transformation from Pt to PtZn IMAs by X-ray diffraction (XRD) patterns at different reduction temperatures (Figure S1). After 600°C reduction, a set of new diffraction peaks (31.1°, 40.8°, and 44.8°) appeared, which are ascribed to (110), (111), and (200), respectively, corresponding well with the standard PXRD patterns (PDF#06-0604) of intermetallic Pt1Zn1 (P4/mmm) (Figure 1C).24Cybulskis V.J. Bukowski B.C. Tseng H. Gallagher J.R. Wu Z. Wegener E. Kropf A.J. Ravel B. Ribeiro F.H. Greeley J. Miller J.T. Zinc promotion of platinum for catalytic light alkane dehydrogenation: insights into geometric and electronic effects.ACS Catal. 2017; 7: 4173-4181Crossref Scopus (88) Google Scholar,27Qi Z. Xiao C. Liu C. Goh T.W. Zhou L. Maligal-Ganesh R. Pei Y. Li X. Curtiss L.A. Huang W. Sub-4 nm PtZn intermetallic nanoparticles for enhanced mass and specific activities in catalytic electrooxidation reaction.J. Am. Chem. Soc. 2017; 139: 4762-4768Crossref PubMed Scopus (180) Google Scholar This is definitely different from the diffraction peaks (39.8°, 46.2°), which are ascribed to (111) and (200) of Pt (Fm3̅m) (PDF#04-0802). Notably, the crystalline phase has been transformed from the face-centered cubic (fcc) of Pt to body-centered tetragonal (bct) Pt1Zn1 IMA (Figure 1B). The average particle sizes of 0.1Pt0.17Zn alloy (additions of 0.1 wt % Pt and 0.17 wt % Zn) supported on SiO2 significantly decreased from 2.1 nm (0.1Pt/SiO2, additions of 0.1 wt % Pt) to 0.9 nm due to the ensemble effect between Pt and Zn atoms (Figure S2). Aberration-corrected high-angle annular dark-field scanning transmission electron microscope (AC-HAADF-STEM) imaging (Figure 1E and S3) revealed the formation of atomically ordered PtZn IMAs, featuring the alternating bright Pt column and dark Zn column contrast on [110] planes (Figure 1E). Energy dispersive spectrometry (EDS) elemental mappings of the PtZn nanoparticle indicated the homogeneous distribution of Pt and Zn atoms (Figure 1G) with an approximately 1:1 ratio. Considering that the Zn content in this PtZn nanoparticle is lower than those fed in the catalyst (1 wt % Pt and 1.7 wt % Zn), a part of Zn may present on SiO2 support. The fast Fourier transform (FFT) image of the PtZn nanoparticle in Figure 1E shows the superlattice diffraction patterns, which further implies the formation of ordered Pt1Zn1 IMAs (Figure 1F). The atomic plane distances were measured as 0.286 nm, agreeing with a lattice spacing of intermetallic Pt1Zn1 along with the [110] directions, which implies the highly exposed (110) facets in these PtZn IMAs. At the surface of PtZn (110), the [PtZn4] ensembles (rectangle-framed motif) periodically formed, in which Pt atom was isolated by 4 neighboring Zn atoms, leading to the formation of single-atom Pt1 site. Comparatively, for Pt/SiO2, AC-HAADF-STEM imaging (Figure S4) showed that two atomic plane distances were measured as 0.229 and 0.199 nm (Figure 1H), which were in agreement with the lattice spacing along the [111] and [200] directions. On the surface of Pt (111), the [Pt3] ensembles (triangle-framed motif) periodically formed (Figure 1J), indicating the formation of surface Pt3 sites over Pt/SiO2. To further obtain information about different surface Pt ensembles over Pt/SiO2 and PtZn/SiO2, we performed Fourier transform infrared (FTIR) spectroscopy with CO adsorption. For Pt/SiO2, one CO adsorption peak appeared in CO-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Figure S5) at 2,075 cm−1, which was assigned to stretching vibration of CO adsorbed on Pt ensembles with on-top modes. Comparatively, for PtZn/SiO2, upon the increase of reduction temperatures, the CO adsorption band corresponding to Pt ensembles (2,075 cm−1) disappeared with an appearance of a new band feature at around 2,047 cm−1 (Figure S6). Noticeably, no position changes of this band was observed with the decrease of CO coverage during the 60 min purge (Figure 1D), indicating that the band at 2,047 cm−1 might have been assigned to the on-top CO adsorbed on the single-atom Pt1 site.28Shi L. Deng G. Li W. Miao S. Wang Q. Zhang W. Lu A. Al2O3 nanosheets rich in pentacoordinate Al3+ ions stabilize Pt-Sn clusters for propane dehydrogenation.Angew. Chem. Int. Ed. Engl. 2015; 54: 13994-13998Crossref PubMed Scopus (193) Google Scholar,29Han Z. Li S. Jiang F. Wang T. Ma X. Gong J. Propane dehydrogenation over Pt-Cu bimetallic catalysts: the nature of coke deposition and the role of copper.Nanoscale. 2014; 6: 10000-10008Crossref PubMed Google Scholar These results clearly demonstrate the formation of a single-atom Pt1 site in PtZn IMAs supported on SiO2. Kinetic studies of the catalysts showed that H2 response in terms of the rate-determining initial C–H cleavage in C3H8 temperature-programmed surface reaction (TPSR) starts at about 181°C on Pt/SiO2, 214°C on PtZn/SiO2, and 500°C on Zn/SiO2, respectively (Figure S7), revealing that the Pt atom might serve as the active component and Zn functions as the promoter, which slightly increases the C–H activation temperature. The formation of CH4 as a result of cracking and coke side reactions at HT was significantly inhibited with the addition of Zn over PtZn/SiO2, which might signify the higher propylene selectivity and anti-coke resistance. According to Pt dispersions (Figure S8) and specific activity at 600°C, the calculated turnover frequency (TOF)C3H6 (6.8 s−1) over PtZn/SiO2 was much higher than that over Pt/SiO2 (4.6 s−1) (Table S1), suggesting the superior ability of propylene production over PtZn/SiO2 than Pt/SiO2. When testing for PDH, we evaluated the influence of Pt and Zn additions, H2 pressures, weight hourly velocity (WHSV) of propane, and reaction temperatures on propane conversion and propylene selectivity. As a result, increasing Zn additions and H2 pressures contribute to the improvement of propylene selectivity (Figures S9 and S10). Increasing Pt loadings (Figure S11) and reaction temperatures (Figure S12) increases the conversion of propane, while higher WHSV of propane (Figure S13) leads to the loss of conversion. Optimally, the PtZn/SiO2 with a low loading of 0.1 wt % Pt and 0.17 wt % Zn obtained about 48% of propane conversion and 96% of propylene selectivity at 600°C and WHSV of propane = 4 h−1 (C3H8/H2 = 1/1), which is significantly higher than that over Pt/SiO2 (42% conversion, 67% selectivity) (Figure 2A) and Zn/SiO2 (4% conversion, 60% selectivity) (Figure S14). In addition, supports could influence the structures of PtZn alloys and thus dehydrogenation performance. We found that PtZn supported on SiO2 exhibited better both propane conversion and propylene selectivity than Al2O3 (Figure S15), despite the similar sizes of PtZn particles (Figure S16). The reason may be that SiO2 interacts weakly with the metals, allowing for a greater extent of metal interaction and alloying between Pt and Zn, which may be the significant support effect for the IMA formation.22Zhang L. Zhou M. Wang A. Zhang T. Selective hydrogenation over supported metal catalysts: from nanoparticles to single atoms.Chem. Rev. 2020; 120: 683-733Crossref PubMed Scopus (304) Google Scholar,30Baria O.A. Holmen A. Blekkan E.A. Propane dehydrogenation over supported Pt and Pt–Sn catalysts: catalyst preparation, characterization, and activity measurements.J. Catal. 2018; 158: 1-12Crossref Scopus (296) Google Scholar Consequently, there was a lower CO adsorption band position (2,047 cm−1) over PtZn/SiO2 than that over PtZn/Al2O3 (2,069 cm−1) (Figure S5). Additionally, the stronger acidity of Al2O3 supports than SiO2 (Figure S17A) would lead to coking and catalyst deactivation. Raman spectra of the spent catalysts (Figures S17B and S17C) further indicated that the coke amount and graphitization degree (the calculated ratio of ID/IG) of PtZn/SiO2 was significantly lower than that over PtZn/Al2O3. Therefore, the weak acid SiO2 in this work was selected as a support to promote the formation of highly alloying PtZn IMA and suppress the influence of support acidity on coking behaviors. Consequently, SiO2-supported PtZn IMAs showed ascendant temperature tolerance ranging from 520°C to 620°C, enabling over 95% selectivity of C3H6 (Figure 2B), indicating superior selectivity of this PtZn IMA during the wide range of HTs. Moreover, the PtZn IMA catalysts ran more than 160 and 100 h of continuous operation with little deactivation at 520°C (Figure S18) and 575°C (Figure S19), respectively. To further evaluate the stability of this PtZn IMA catalyst, we performed the long-term test under industrial operating conditions (600°C, WHSV of C3H8=11 h−1, C3H8/H2 = 2, no inert gas dilution) and compared with the representative PtSn/Al2O3 catalysts (see Experimental Procedures). After 160 h, dehydrogenation reaction showed a very slight decrease in the conversion of 0.04% per hour, suggesting that only slight deactivation had occurred. Meanwhile, the propylene selectivity was as high as 97.2% after a 160-h test (Figure 2C). In contrast, for the PtSn/Al2O3 reference catalyst, propane conversion dramatically dropped from an initial 34.8% to 11.7% after 100 h. A first-order deactivation model was used to evaluate the catalyst stability (for details, see Experimental Procedures).28Shi L. Deng G. Li W. Miao S. Wang Q. Zhang W. Lu A. Al2O3 nanosheets rich in pentacoordinate Al3+ ions stabilize Pt-Sn clusters for propane dehydrogenation.Angew. Chem. Int. Ed. Engl. 2015; 54: 13994-13998Crossref PubMed Scopus (193) Google Scholar A comparatively lower deactivation rate of 0.002 h−1 and a catalyst lifetime of 160 h for the PtZn/SiO2 IMA quantitatively demonstrate its high stability compared with the PtSn/Al2O3 catalyst (0.01 h−1, 100 h). The highest productivity of 83.2 mol C3H6 gPt−1 h−1, more than 5 times of PtSn/Al2O3 catalysts, at the lowest deactivation rate (0.002 h−1) clearly demonstrates the superior reactivity and stability of the PtZn/SiO2 IMA catalyst in the PDH reaction, which is superior to PtSn/Al2O3 and those of state-of-the-art Pt-based catalysts (Figure 2D; Table S2). Additionally, during 4 successive regeneration cycles, the propane conversion and propylene selectivity remained slightly changed as compared with the initial run (Figure S20), further indicating the good regenerability of this PtZn IMA. To further investigate the stability of PtZn IMA catalysts under PDH reaction conditions, in situ X-ray absorption near-edge structure (XANES) spectra and Fourier transforms (FTs) of the extended X-ray absorption fine-structure (EXAFS) spectra were collected. As shown in Figure 3A, after H2 reduction, all Pt atoms were in their metallic states, as evidenced by the white line intensity relative to that of the Pt foil. The edge energy of the PtZn/SiO2 was shifted to 11,565 eV, indicating the formation of bimetallic PtZn particles. Additionally, in situ Pt LIII EXAFS revealed a single broad peak in R space centered around 2.2 Ǻ in PtZn/SiO2, whereas Pt/SiO2 showed three peaks in R space, characteristic of Pt–Pt scattering (Figures 3B and S21). For PtZn/SiO2 catalysts, the model gave high quality fits with the crystal structure of the β1-Pt1Zn1 IMA. The FT peak at 2.57 Å could be unambiguously ascribed to Pt–Zn coordination whose number (CN) was determined to be about 4 (Table S3) lower than the values (8 zinc neighbors and 4 Pt neighbors) in bulk PtZn IMA. This is consistent with the consensus that intrinsically stable single-atom site is usually formed by 4 coordinate complexes containing a metal with tetrahedral or square-planar coordination geometry. Considering the under coordinated surface atoms in this sub-1 nm PtZn IMA particle, the surface coordination of Pt–Zn ensembles could be approximately identified as this obtained coordination environment of [PtZn4] ensembles.17Wang A. Li J. Zhang T. Heterogeneous single-atom catalysis.Nat. Rev. Chem. 2018; 2: 65-81Crossref Scopus (1340) Google Scholar,31Dvořák F. Farnesi Camellone M. Tovt A. Tran N.D. Negreiros F.R. Vorokhta M. Skála T. Matolínová I. Mysliveček J. Matolín V. Fabris S. Creating single-atom Pt-ceria catalysts by surface step decoration.Nat. Commun. 2016; 7: 10801Crossref PubMed Scopus (262) Google Scholar Therefore, as the illustrations shown in Figure S22, the [PtZn4] coordination structure exists on the surface of PtZn IMAs, wherein single-atom Pt1 site forms and it is isolated by 4 neighboring Zn atoms in [PtZn4] ensembles. Notably, the XANES and EXAFS spectra of PtZn/SiO2 under PDH reaction conditions are similar with H2 reduction (Figures 3C and S23), and the calculated CN of Pt–Zn remained about 4 (Table S3), indicating the superior stability of PtZn coordination ensembles in PtZn IMAs under PDH reaction conditions. In situ X-ray photoelectron spectroscopy (XPS) was further employed to monitor the electronic states of this PtZn ensembles. After H2 reduction, for PtZn/SiO2, Pt was reduced to metallic Pt0 and the binding energy (BE) of Pt0 in Pt 4f7/2 peak was 71.1 eV (Figures 3D and S24), which was lower than that over Pt/SiO2 (71.3 eV) (Figure S25), indicating the electron transfers from Zn to the 5d band of Pt and formation of electron-rich Pt in PtZn ensembles.24Cybulskis V.J. Bukowski B.C. Tseng H. Gallagher J.R. Wu Z. Wegener E. Kropf A.J. Ravel B. Ribeiro F.H. Greeley J. Miller J.T. Zinc promotion of platinum for catalytic light alkane dehydrogenation: insights into geometric and electronic effects.ACS Catal. 2017; 7: 4173-4181Crossref Scopus (88) Google Scholar,32Yu C. Xu H. Ge Q. Li W. Properties of the metallic phase of zinc-doped platinum catalysts for propane dehydrogenation.J. Mol. Catal. A. 2007; 266: 80-87Crossref Scopus (126) Google Scholar Combining spectra of Zn 2p and Zn LMM Auger (Figures 3E and S26), it was found that Zn2+ species (985.9 eV) were partially reduced to metallic Zn0 (991.9 eV) and some nonreducible ZnOx. Therefore, we infer that partial Zn species are alloyed with Pt to form PtZn alloys, while some nonreducible ZnOx species strongly interact with SiO2 supports to form ZnOx-Si (ZnOx bridging with SiO2 support, 986.9 eV). Through peak fitting, the percent of surface composition