II) Stabilizing Raft-Like Pt Clusters: High

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Lattice-Confined Sn (IV/II) Stabilizing Raft-Like Pt Clusters: High Selectivity and Durability in Propane Dehydrogenation Yanru Zhu, Zhe An, Hongyan Song, Xu Xiang, Wenjun Yan, and Jing He ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02264 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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Lattice-Confined Sn (IV/II) Stabilizing Raft-Like Pt Clusters: High Selectivity and Durability in Propane Dehydrogenation Yanru Zhu,† Zhe An, † Hongyan Song, † Xu Xiang, † Wenjun Yan‡ and Jing He*† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China

‡

ABSTRACT: Catalytic dehydrogenation of propane (DHP) to propene is highly endothermic, requiring a high reaction temperature. Under harsh conditions, it has been great challenges to maintain an excellent propene selectivity and as well suppress the irreversible deactivation caused by sintering of metallic active centers. This work reports a highly selective and durable Pt-Sn catalyst for DHP, in which metallic Pt centers are dispersed homogeneously in small raft-like clusters on Mg(Sn)(Al)O and form strong interactions with the SnIV/II sites confined in Mg(Al)O lattices. A propene selectivity of > 99 % at 550 oC with a conversion close to the equilibrium (specific rate of 0.96 s-1 for propene formation) and a propene selectivity of > 98 % (specific rate of 1.46 s-1) even under 600 oC have been produced by highly dispersed Pt sites in Pt/Mg(Sn)(Al)O. The Pt-Sn interactions and SnIV/II confinement were revealed to afford the catalyst with good durability. No visible sintering of Pt clusters was observed in the longterm DHP reaction. KEYWORDS: Stabilized Pt clusters, Lattice-confined SnIV/II, Propane dehydrogenation, High selectivity, Excellent durability

Propene has all along been in short supply with everincreasing global demand as one of the most important raw materials and intermediates in petrochemical industry.1 Steam cracking of oil-based naphtha and fluid catalytic cracking (FFC) of heavy oil, the bulk of the propene production, suffer from high-energy demands and low selectivity toward propene.2 Catalytic dehydrogenation of propane (DHP) is an on-purpose technique that exclusively yields propene instead of a mixture products.3 In recent years, DHP process with its high-profitability has been commercialized to meet the demand of propene.4 The catalyst widely applied in commercial DHP process is alumina-supported PtSn5 (Oleflex process from UOP). The DHP reaction is highly endothermic, and a reaction temperature of > 500 oC is usually needed to achieve a feasible propane conversion.6 Under such a high temperature, Pt sintering, propane hydrogenolysis (or cracking), and coke deposition become much more severe, leading to rapid irreversible deactivation and low propene selectivity, which are the major challenges for commercial DHP processes.7 Great efforts have been devoted to developing effective catalysts for tackling the challenges mentioned above. Several novel catalytic materials for DHP process have been reported,8 including metallic Sn/SiO2,8a Fe:P/Al2O3,8b ZrO2-based bulk oxides,8c In2O3-Ga2O3,8d isolated FeII on silica,8e single-site Zn2+ on silica,8f and isolated Ga sites on silica.8g But the efficient and selective production of propene requires welldefined catalytic sites.9 So it is still of high interest to improve or modify mainstream Pt-based catalysts.3a,10 Promoter addition, affording bimetallic and/or alloying systems such as Pt-Sn,11 Pt-In,12 Pt-Ga,13 Pt-Ir,14 Pt-Cu,15 or even trimetallic PtSnIn,16 has been found to be effective in improving propene

selectivity and suppressing coke formation by increasing Pt dispersion11-12 or diluting Pt ensembles13-14,16, and changing the electronic environment of Pt atoms.15 Modifications of support are alternative methods to enhance propene selectivity.17 For instance, doping of Al2O3 with TiOx17a or Mg(Al)O oxide17b with ZnO weakens the Pt-alkene interaction, and thus suppresses the side reactions. Besides propene selectivity, the catalytic stability is an important issue that attracts much attention.7c,18 Markedly improved catalytic stability has been achieved with CeOx-doped Pt-Ga/Al2O37c and Ca-doped PtSnIn/Al2O318a, where CaO or CeOx species stabilize the catalyst by inhibiting Pt particle from sintering. In the case that Ga3+ species serve as the active sites for actual dehydrogenation while Pt acts as a unique promoter element, the catalyst remains stable for 12 days on stream after a conversion drop in first 2 days.18b With Zn2+ species as the active sites for dehydrogenation while Pt as a promoter, it remain stable over 20 h with ~100% propylene selectivity at 550 °C.18c Recently, great progress has been made with Ptbased catalysts.19 Sn surface-enriched Pt-Sn bimetallic nanoparticles exhibit a propene selectivity of > 99 % and a deactivation rate of 0.0028 h-1 in 90 h at 580 oC.19a The γAl2O3 nanosheet that contains a large amount of pentacoordinate Al3+ ions is exceptionally able of dispersing and stabilizing Pt-Sn clusters.19b Pt-Sn/Al2O3-nanosheet displays high specific activity for propylene formation with a selectivity of > 99 % and a low deactivation rate of 0.007 h-1 in DHP at 590 oC.19b Here this work reports a highly selective and durable Pt-Sn catalyst for DHP, in which Pt centers are dispersed homogeneously in small raft-like clusters on Mg(Sn)(Al)O and interact strong with the SnIV/II sites (SnIV/II-O-M) confined

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in Mg(Al)O lattices. A propene selectivity of > 98 % (specific rate of 1.46 s-1 for propene formation) and a deactivation rate of 0.0045 h-1 have been afforded at 600 oC. Blooming studies have focused on atomic scale metallic catalysts in recent years.20 Single Pt atoms anchored on FeOx support were prepared by a co-precipitation method.20a Atomically dispersed Pd on graphene was fabricated by atomic layer deposition technique.20b Single iron sites were embedded in a silica matrix by melting method.20c While under high operating temperature, single atoms or small clusters tend to get sintered and/or aggregated, durable atomic scale dispersion is of great challenge.21 A sinter-resistant, atomically dispersed Pt was synthesized at high temperature via Pt trapped on CeO2/LaAl2O3.21a The encapsulation of Pt atoms or small clusters in highly siliceous chabazite showed enhanced stability toward sintering in a variety of industrial conditions.21b Stable singleatom silver supported on a hollandite manganese oxide was obtained by anti-Ostwald ripening.21c The highly and stably dispersed Pt on Mg(Sn)(Al)O reported here in this work was produced from the reduction-followed calcination of Pt2+ loaded Sn-containing layered double hydroxides (LDHs), which was developed22 in our group. Mg, Al, and Sn-containing LDHs was synthesized in situ on the surface of γ-Al2O3, affording MgSnAl-LDHs@Al2O3 with a Mg/Sn molar ratio of 12. MgAl-LDH@Al2O3 was also synthesized as control. The reflections characteristic of hydrotalcite structure23 are clearly observed for either MgSnAl-LDHs@Al2O3 or MgAl-LDH@Al2O3 in the X-ray diffraction (XRD) patterns (Figure S1). In our previous work,22 it has been demonstrated that SnIV sites in MgSnAlLDHs are atomically dispered and confined by the lattice of LDHs. From the scanning electron microscope (SEM) images (Figure S2), MgSnAl-LDHs is observed to grow densely on both exterior and interior surfaces of γ-Al2O3 with a thickness of approximately 0.36 µm. The thickness of MgSnAl-LDHs layer can be increased to 0.50 µm by prolonging the crystallization time (Figure S3). Pt2+ was introduced onto MgSnAlLDHs@Al2O3 by incipient wetness impregnation. After calcination in air followed with reduction in H2 at 600 oC, 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 (Pt/Sn molar ratio of 0.6) was produced. According to the high-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) images (Figure 1a) with size distribution as inset, Pt clusters ( d = 1.1 ± 0.3 nm) are homogenously distributed on the support surface. No large Pt nanoparticles are observed. By comparing the brightness intensity of Pt cluster with that of single atom (Figure S4), the Pt clusters are confirmed to be in two-dimensional (2D) raft-like dispersion with a thickness of no more than two atomic layers. This observation is similar to that with PtSn/Al2O3-nanosheet,19b or Pt/FeOx,24 hinting a strong interaction between the support surface and Pt. The energy-dispersive X-ray (EDX) element mapping images (Figure 1a) offer clear evidence for no large aggregation of both Pt and Sn elements. With Pt loading increased to 1 wt% (Pt/Sn = 1.2) or 2 wt% (Pt/Sn = 2.4), Pt nanoparticles get donimated (Figure S5). The particle size distribution for 1.0 wt% Pt/Mg(Sn)(Al)O@Al2O3 and 2.0 wt% Pt/Mg(Sn)(Al)O@Al2O3 show a maximum at d = 1.9 ± 0.5 nm and d = 2.6 ± 0.7 nm. For comparison, control samples were also prepared by impregnating Sn4+-impregnated (Mg/Sn

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Figure 1. HAADF-STEM images with EDX element-mapping analysis of (a) fresh and (b) spent 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. In EDX element-mapping images, green dots represent Pt element while red ones represent Sn element.

= 12) or as-prepared MgAl-LDH/Al2O3 with Pt2+ aqueous solution. 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 and 0.46 wt% Pt/Mg(Al)O@Al2O3 were produced by calcination in air followed with reduction in H2 at 600 oC. Similar to the observation on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3, raft-like Pt clusters are observed (Figure S6), with a maximum at d = 1.2 ± 0.4 nm for 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 and d = 1.2 ± 0.3 nm for 0.46 wt% Pt/Mg(Al)O@Al2O3. The textural properties of the above samples are presented in Table S1. In the quasi-in-situ 119Sn Mössbauer spectra (Figure 2A), 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 presents two doublet bands at isomer shift (IS) of 0.24 mm s−1 and 2.92 mm s−1 with quadrupole splitting (QS) of 0.92 mm s−1 and 2.34 mm s−1 (Table S2). For 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3, besides the doublet bands located at 0.05 mm s−1 (with a QS of 0.51 mm s−1) and 2.71 mm s−1 (with a QS of 2.49 mm s−1), another band at IS of 2.03 mm s−1 is also observed (Figure 2A and Table S2). According to previous reports,25 the doublet bands at the IS of 0.00−0.50 mm s−1 and 2.50−3.50 mm s−1 originate from SnIV and SnII species, while the single band at the IS around 2.00 mm s−1 from Sn0. The SnIV species in SnO2 give

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QS of ~ 0.5 mm s−1 and SnII in SnO give QS of ~ 1.2 mm s−1, and the formation of SnIV-O-M or SnII-O-M (M ≠ Sn) causes an increase in QS.26 The higher QS values for both SnIV and SnII (Figure 2A (a)) in Pt/Mg(Sn)(Al)O@Al2O3 are consistent with the existence of SnIV-O-M or SnII-O-M (M = Mg or Al here) linkages, which gives rise to asymmetric charge distribution of the Sn-O bonds. From the relative area, the SnII-O-M species and SnIV-O-M species were estimated to account for 56 % and 44 %. The Sn species are all present in Sn-O-M linkages (M = Mg or Al), confirming the definite confinement of Sn sites in the Mg(Al)O lattices. In Pt/SnOx/Mg(Al)O@Al2O3, the SnIV species presents a QS identical to SnO2 (Figure 2A (b)). But the QS of SnII species is similar to that for SnII-O-M linkage, which has been proposed25b to originate from the Sn-aluminates formed in the calcination/reduction. Pt/SnOx/Mg(Al)O@Al2O3 was estimated to contain 73 % of SnO2, 19 % of SnII-O-M linkage, and 8 % of Sn0. In the Sn 3d5/2 XPS spectra (Figure S7), the content of SnII-O-M and SnIV-O-M species were estimated to be 51 % and 49 %, in accordance with the 119Sn Mössbauer result (Figure 2A). By decreasing the reduction temperature to 550 oC in the preparation process, SnII-O-M content decreases to 32 %. While increasing the reduction temperature to 650 oC, the SnII-O-M content only slightly increases (56 %, Figure S7). In the H2-TPR profiles (Figure 2B), the H2 consumption at 250−300 °C for Pt-containing sample is assigned to the reduction of PtIV species.16 The H2 consumption at 450−600 °C originates from the reduction of PtOx species strongly interacting with the support and/or the reduction of oxidized Sn species16,25b,27 and the ones at 700−850 °C are attributed to the loss of different types of hydroxyl groups on the Al2O328 and/or Mg(Al)O. The presence of the SnIV component is clearly observed to increase the reduction temperature of PtIV sites, and the SnIV sites confined in the Mg(Al)O lattice increases the reduction temperature more visibly. The difficulty in Pt reduction is possibly result from the interactions between PtIV sites and SnIV species, and the confinement of SnIV sites further facilitates the PtIV-SnIV interactions. In the quasi-in-situ Pt LIII-edge X-ray absorption near edge structure (XANES)

Figure 2. (A) Quasi-in-situ 119Sn Mössbauer spectra of (a) 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 and (b) 0.48 wt% Pt/SnOx/Mg(Al)O @Al2O3. (B) H2-TPR profiles of (a) Pt2+/MgSnAl-LDHs@Al2O3, (b) Pt2+/Sn4+/MgAl-LDH@Al2O3, (c) Pt2+/MgAl-LDH@Al2O3, and (d) MgAl-LDH@Al2O3 after calcination at 600 oC.

spectra (Figure S8), the white line intensity indicates that the electrons are donated from Sn component to Pt, and the Sn species confined in Mg(Sn)(Al)O lattice (0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3) donates more electrons than that in Pt/SnOx/Mg(Al)O@Al2O3. Used as the catalyst for DHP reaction (Table S3), 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 affords a propene selectivity of > 99 % at 550 oC, with a conversion close to the equilibrium (initial conversion of 29 %). Even under 600 oC, a harsher condition, the propene selectivity remains > 98 % with initial conversion increasing to 48 %. The thickness of MgSnAlLDHs layer on Al2O3 made no effects on the propane conversion and propene selectivity (Figure S9). At 550 oC, the initial propane conversion on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 is similar to that on 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 while higher than that on 0.46 wt% Pt/Mg(Al)O@Al2O3. But at 600 oC, the initial propane conversion on 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 decreases to the level similar to that on 0.46 wt% Pt/Mg(Al)O@Al2O3, which is lower than that on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. In the whole on-stream time (Figure 3), 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 affords higher propene selectivity than both 0.48 wt% Pt/SnOx/ Mg(Al)O@Al2O3 and 0.46 wt% Pt/Mg(Al)O@Al2O3. It is notable that the conversion on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 shows only a slight decrease in the first 9 h while with a sharp rise in the propene selectivity (Figure 3). The conversion decreases in only 0.0067 % h-1 at 550 oC in 240 h on stream and 0.11 % h-1 at 600 oC in 48 h. But on 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 (Table S3), the propane conversion drops from initially 29 % to 18 % at 550 o C in 240 h, and from 36 % to 17 % in 48 h at 600 oC. The Snfree 0.46 wt% Pt/Mg(Al)O@Al2O3 displays a dramatic drop of propane conversion from 25 % to 9 % at 550 oC in 240 h and from 38 % to 13 % in 48 h at 600 oC. Usually, the “specific activity”, defined as the moles of propene formed per mol of Pt per second,3a is a comprehensive evaluation of catalytic performance. 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 displays a specific activity of 0.96 s-1 at 550 oC and 1.46 s-1 at 600 oC, higher than those for 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 (0.95 s-1 and 1.01 s-1) or 0.46 wt% Pt/Mg(Al)O@Al2O3 (0.80 s1 and 0.92 s-1), which is to say good yield at short contact time (Table S3). The specific activity on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 shows no any visible drop in the onstream time at both 550 and 600 oC. Deactivation rate (kd) and mean catalyst life (the reciprocal of kd, τ), based on a firstorder deactivation mechanism,29 are also introduced as proxies to evaluate the catalyst stability. 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 exhibits a kd value as low as 3.3×10-4 h-1 (τ = 3062 h) in 240 h reaction at 550 oC and 4.5×10-3 h-1 (τ = 224 h) in 48 h reaction at 600 oC. The mean catalyst life is almost 7-fold or 13-fold higher than that for 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 or 0.46 wt% Pt/Mg(Al)O@Al2O3 at 550 oC (almost 4-fold or 5-fold higher at 600 oC), indicating an excellent durability of 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. To further confirm the stability of 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3, the dehydrogenation/regeneration cycle tests at 600 oC have been performed (Figure S10). In 10 cycles, only small changes in conversion are observed for 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. But 0.48 wt% Pt/SnOx/ Mg(Al)O@Al2O3 suffers from a severe deactivation, the conversion decreasing from 42 % at 1st to 15 % at 10th. In

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Figure 3. Propane conversion and propene selectivity over 0.50 wt% Pt/ Mg(Sn)(Al)O@Al2O3 (red), 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 (bule), and 0.46 wt% Pt/Mg(Al)O@Al2O3 (green). Reaction conditions: 550 oC, atmospheric pressure, C3H8/H2/Ar = 1/0.5/2, and WHSV = 14 h-1.

comparison with the commercial Pt-Sn/Al2O3 reported in literature19b (Table S4), 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 reported in this work possesses much higher specific activity and better durability. In comparison with those of the state-ofthe-art Pt-based catalysts19 (Table S4), the catalyst life reported in this work is superior, and the specific activity is also attractive. In the Sn 3d5/2 XPS spectra (Figure S11), no Sn0 is resolved for as-prepared 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3, while Sn0 is estimated to be present in 9 % for as-prepared 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3, consistent with the observations by119Sn Mössbauer spectra (Figure 2A). After short contact with the reactant gases at either 500 or 600 oC, no change is observed for the Sn 3d5/2 XPS spectra of 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. But the percentage of Sn0 in 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 increase to 27 % after short contact at 600 oC with the reactant gases. The Sn0 species in 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 might act as a poison for the catalytic conversion of propane, which is consistent with the observation in an earlier report30 for supported-PtSn DHP catalysts. The stabilization of SnIV/II sites by the confinement in Mg(Al)O lattices account for the higher conversion on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 than on 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 at harsh condition. DHP reaction was also carried out on 1.0 wt% Pt/Mg(Sn)(Al)O@Al2O3 and 2.0 wt% Pt/Mg(Sn)(Al)O @Al2O3 under the same conditions at 550 oC as used for 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. With Pt dispersion changing from raft-like cluster to nanoparticle, the propene selectivity decreases from 98 % to 85 % at a conversion of 29 % (Figure S12). It is clear that highly dispersed Pt is one important factor to efficiently promote the selectivity to propene. This observation is not surprising because previous researches22a,31 also found that the hydrogenolysis highly depended on the size of the metal centers. But highly dispersed Pt could not account for the higher propene selectivity on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 than on 0.48 wt% Pt/SnOx/Mg(Al)O @Al2O3, which possess similar Pt dispersion. So in situ FT-IR spectra for propene adsorption were recorded (Figure 4A). Only easily evacuated physical adsorption has occurred on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. On 0.48 wt% Pt/SnOx/ Mg(Al)O@Al2O3 however, the chemical adsorption with propene π-bonded to Pt is observed at 1609 cm-1, and still present after evacuation. The weak adsorption and fast desorption of propene on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 is therefore responsible for the excellent propene selectivity. As revealed by XANES characterization, the SnIV/II sites confined

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in Mg(Al)O lattice donates more electrons to Pt. The electronrich Pt sites disfavor the π-bonding of propene to Pt, thus improving the propene selectivity. In the operando FT-IR spectra recorded with 1 % propane in Ar (Figure 4B), the absorption assigned to the ν (C-C) of benzene ring skeleton at 1508 cm-1 is obversed on 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 while not on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. The emergence of aromatic ring indicates the possibility of coke formation. So in situ thermogravimetric analysis (TGA) under reaction conditions was performed at 550°C (Figure S13). The sample weight increases by 25 % in 90 min DHP over 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 while only by 4 % over 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. The excellent coking-resistance of 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 coincides with the higher propene selectivity, which is supposed to result from the weak adsorption and fast desorption of propene observed in Figure 4A. In the DHP process at 550 oC on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3, the propene selectivity was observed to increase (Figure S14) with the increasing SnII-O-M content (Figure S7). The Sn (II) species seems to contribute more to propene selectivity. In the operando FT-IR spectra (Figure 4B), the absorption bands assigned to ν (C=C) and δas (C-H) in adsorbed propene are observed at 1613 and 1442 cm-1 for 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3, and at 1601 and 1452 cm-1 for 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3. The ν (C-H) absorption of methane is observed at 3015 cm-1 in each case. But the intensity of band at 3015 cm-1 for 0.48 wt% Pt/SnOx/Mg(Al)O

Figure 4. (A) FT-IR spectra for 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 and 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 (a) with propene adsorbed at 233 K and (b) evacuated (a) till the absorption bands show no alteration in intensity in each case. The absorption bands at 1662 cm-1, 1642 cm-1, 1446 ± 10 cm-1, and 1376 cm-1 are ascribed , according to previous reports,32 to the ν (C=C) of the gas-phase propene, ν (C=C) of propene physisorbed, δas (C-H) of methyl and δs (C-H) of methyl group, and the absorption at 1609 cm-1 is ascribed to the ν (C=C) of propene π-bonded to Pt. (B) Operando FT-IR spectra recorded at intervals over 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 and 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 by introducing 1 % propane in Ar at 400 o C. The absorption bands at 3015 cm-1, 2900-3000 cm-1, 1605 ± 10 cm-1, 1508 cm-1, and 1446 ± 10 cm-1 are ascribed to the ν (C-H) of methane, ν (C-H) of methyl and/or methene in adsorbed propane, ν (C=C), ν (C-C) from benzene ring skeleton, and δas (C-H) of methyl group.

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@Al2O3 increases visibly with on-stream time, which suggests a gradual severe hydrogenolysis. As mentioned above, poor Pt dispersion might be one cause for hydrogenolysis. So the Pt dispersion in spent 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 (Figure 1b) and 0.48 wt% Pt/SnOx/Mg(Al)O@Al2O3 (Figure S15) are investigated by STEM technique. No visible change in the dispersion and size of Pt clusters in 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 is observed before and after 240-h DHP reaction at 550 oC, as well as no aggregation of both Pt and Sn elements in EDX element-mapping analysis (Figure 1b). This result indicate a superior anti-sintering ability for both of the Pt clusters and the lattice-confined SnIV/II sites in 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3. For 0.48 wt% Pt/SnOx/Mg(Al)O @Al2O3, large Pt nanoparticles (> 10 nm) are observed after 240-h DHP reaction at 550 oC, indicative of severe sintering of Pt (Figure S15). Besides Pt (111) lattice, Pt1Sn1 (102) and Pt1Sn1 (211) lattices are also observed over the spent 0.48 wt% Pt/SnOx/ Mg(Al)O@Al2O3, meaning that the Sn sites, simply supported on the surface of Mg(Al)O@Al2O3, have aggregated together with Pt sites to form Pt-Sn alloy, rather than stabilized the Pt clusters. As reported by the previous literature,17b the surface property of the support could affect the catalytic performance. So in this work, the role of the surface of γ-Al2O3 has been investigated. 0.50 wt% Pt/Mg(Sn)(Al)O (without Al2O3) and 0.50 wt% Pt/SnOx@Al2O3 (without LDHs) were prepared. The surface area of 0.50 wt% Pt/Mg(Sn)(Al)O (without Al2O3) is measured to be 194.6 m2/g (Table S1), similar to that of 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 (195.4 m2/g). But in the 6-h propane dehydrogenation reaction at 600 oC, 0.50 wt% Pt/Mg(Sn)(Al)O (without Al2O3) only exhibits an initial propane conversion of 8 %, much lower than that on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 (47 %), with a similar propene selectivity (95 %) (Figure S16). Without LDHs, 0.50 wt% Pt/SnOx@Al2O3 afforded an initial propane conversion of 46% and a propene selectivity of 94%, which is similar to that for Pt/SnOx/Mg(Al)O@Al2O3 (Figure S17). That means the surface of γ-Al2O3 has no effects on the catalytic performance. So the γ-Al2O3 spherules here are supposed to only serve as a substrate for surface Pt/Mg(Sn)(Al)O. The much higher propane conversion on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 than that on 0.50 wt% Pt/Mg(Sn)(Al)O is supposed to result from the better propane diffusion on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3, which benefits from the wellorganized array of Mg(Sn)(Al)O species on γ-Al2O3 surface. Additionally, the catalysts with γ-Al2O3 spherules as the substrate are beneficial to apply in industry directly. In conclusion, this work reports a supported Pt-Sn catalyst, in which Pt atoms are dispersed in raft-like small clusters and interact strong with the SnIV/II sites confined in Mg(Al)O lattices. In the DHP reaction, the Pt-Sn catalyst developed in this work displayed a propene selectivity of > 99 % at 550 oC with a conversion close to the equilibrium (initial conversion of 29 %); even under 600 oC, a harsher condition, the propene selectivity remains > 98 % with the initial conversion increasing to 48 %. More inspiringly, the confined SnIV/II sites facilitate the Pt clusters with good durability and stability through Pt-Sn interactions. The specific activity on 0.50 wt% Pt/Mg(Sn)(Al)O@Al2O3 retains constant and no visible sintering of Pt clusters is observed in the long-term DHP.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental section, additional characterizations, and catalytic performances (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] or [email protected] (J. He)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial supports by NSFC of China (21521005 and 91534101) and 973 Project (2014CB932104) are gratefully acknowledged.

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