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Encapsulation of Pt particles in MFI zeolite

Encapsulation of Pt particles in MFI zeolite
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  ARTICLES 1 Instituto de Tecnología Química, Universitat Politècnica de València–Consejo Superior de Investigaciones Científicas, Valencia, Spain. 2 Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Cádiz, Spain. 3 ALBA Synchrotron Light Source, Barcelona, Spain. *e-mail: O ne rontier in the synthesis o zeolites, and microporous materials more generally, consists in locating active sites in the desired ramework or extraramework position 1–4 . For instance, in the case o zeolites containing pores and/or cavities o dierent dimensions, it is clear that locating the active sites selec-tively in one speciic type o pore or cavity is a undamental chal-lenge with important implications or their catalytic application, due to dierences in geometric and coninement eects. For instance, attempts have been reported to preerentially locate ramework Al atoms in the intersectional sites or within the 10MR channels o ZSM-5 (MFI-type zeolite) 5–7 . However, when unctional metals, such as Pt or Pd, have been introduced by including the metal pre-cursor during the synthesis or through interzeolite transormations, nanoparticles o 1–2 nm instead o subnanometric metal species (single atoms or clusters) have been obtained within the MFI zeo-lite 8–11 . As ar as we know, the metal particles show random distribu-tion within the zeolite crystallites. From the materials and catalysis point o view, the ollowing challenges remain unsolved: (1) to gen-erate stable subnanometric metal species within the zeolite crystal-lites, (2) to stabilize them regioselectively within the 10MR channels and (3) to achieve the above goals with purely siliceous MFI zeolite and to avoid the presence o acid sites in the inal material. Note that i these three objectives were achieved a very regular distribu-tion o metal clusters o about 0.5 nm would be obtained, that is with the diameter o the 10MR channels, which would be highly stable against Ostwald ripening sintering, even at high temperature ( > 550 °C) under a reductive atmosphere 12,13 . his type o material should open new possibilities or a number o catalytic applications, including the very relevant activation o alkanes 14–17 .It has been demonstrated that subnanometric Pt clusters show higher reactivity than Pt nanoparticles or propane dehydrogena-tion reaction 18 . However, when Pt clusters are supported on solid carriers with open structures, these Pt clusters may suer ast deactivation, and the subsequent regeneration–reaction cycles will lead to severe sintering o Pt. hereore, i regular Pt clusters can be regioselectively generated within the 10MR channels o a purely siliceous MFI zeolite, the resultant catalyst should present not only high activity, but also an improved stability when working under high-temperature conditions.o achieve this goal, we have carried out one-pot synthesis o Pt-MFI materials. he starting hypothesis was that the template molecules (PA + OH − ) will occupy the intersectional voids, hence limiting the access o Pt species to this position. Considering the larger space in the sinusoidal channels versus the straight channels, Pt species may preerentially be located in the sinusoidal channels. A controllable amount o K +  is introduced to compensate the silanol groups, and to stabilize the subnanometric Pt species 19–21 . Finally, atomically dispersed Sn species can also be introduced to electroni-cally modiy the Pt clusters to increase propylene selectivity or the propane dehydrogenation reaction. Encapsulation of Pt particles in MFI zeolite As illustrated in Fig. 1a,b, by a one-pot synthesis strategy, Pt spe-cies can be encapsulated into purely siliceous MFI zeolite with a Pt loading o about 0.4 wt% (Supplementary able 1). All the Pt-zeolite materials with dierent chemical compositions show typical X-ray diraction patterns o MFI zeolite (Supplementary Fig. 1). Interestingly, the presence o K +  within the synthesis mixture has signiicantly inluenced the size o the encapsulated Pt species. As shown in Fig. 1c,d, Pt mainly exists as nanoparticles o 3–5 nm in the K-ree Pt@MFI sample (Supplementary Fig. 2), while subnano-metric Pt clusters o about 0.4–0.7 nm are the dominant species in the K-Pt@MFI sample (Supplementary Fig. 3). o investigate the role o K, we irst characterized the as-synthesized, non-calcined, Regioselective generation and reactivity control of subnanometric platinum clusters in zeolites for high-temperature catalysis Lichen Liu 1 , Miguel Lopez-Haro 2 , Christian W. Lopes  1 , Chengeng Li 1 , Patricia Concepcion 1 , Laura Simonelli 3 , Jose J. Calvino  2  and Avelino Corma 1 * Subnanometric metal species (single atoms and clusters) have been demonstrated to be unique compared with their nanopar-ticulate counterparts. However, the poor stabilization of subnanometric metal species towards sintering at high temperature ( > 500 °C) under oxidative or reductive reaction conditions limits their catalytic application. Zeolites can serve as an ideal sup-port to stabilize subnanometric metal catalysts, but it is challenging to localize subnanometric metal species on specific sites and modulate their reactivity. We have achieved a very high preference for localization of highly stable subnanometric Pt and PtSn clusters in the sinusoidal channels of purely siliceous MFI zeolite, as revealed by atomically resolved electron microscopy combining high-angle annular dark-field and integrated differential phase contrast imaging techniques. These catalysts show very high stability, selectivity and activity for the industrially important dehydrogenation of propane to form propylene. This stabilization strategy could be extended to other crystalline porous materials. NATURE MATERIALS  |  ARTICLES NATURE MATERIALS Pt-zeolite samples (K-Pt@MFI-SDA and Pt@MFI-SDA), obtained in the presence and absence o K, by electron microscopy. As shown in Supplementary Figs. 4 and 5, only atomically dispersed Pt species are detected in both as-synthesized samples, as conirmed by image simulation (Supplementary Fig. 6). Herein, to identiy the location o Pt atoms in Pt@MFI-SDA and K-Pt@MFI-SDA, we have employed a combination o high-resolution high-angle annular dark-ield scan-ning transmission electron microscopy (HR HAADF-SEM) and integrated dierential phase contrast (iDPC) imaging techniques to simultaneously visualize both Pt atoms and the zeolite structure with atomic resolution 22,23 . As shown in Supplementary Fig. 4, by corre-lating the HR HAADF-SEM and iDPC images, it can be deduced that most Pt atoms are located in the sinusoidal channels o the MFI zeolite structure, whereas a ar smaller number are ound in the straight pore channels or at the intersectional voids. his regioselec-tive distribution o Pt species may be caused by the occupation o the intersectional voids by the template (PA + ), as well as by the avail-ability o larger spaces in the sinusoidal channels compared with the straight channels (Supplementary Fig. 7) 24–26 . he characteristics o the location o Pt atoms described above are the same when the syn-thesis is carried out in the presence o K +  (K-Pt@MFI-SDA sample), indicating that K +  does not play a relevant role in the encapsula-tion o atomically dispersed Pt species during the hydrothermal synthesis. his conclusion is also conirmed rom characteriza-tion by X-ray absorption spectroscopy (Supplementary Fig. 8 and Supplementary able 2). A short-distance contribution at about 2.0 Å can be ascribed to Pt–O/N bonding, while the contribution o Pt–Pt bonding is not observed in either sample.A subsequent calcination in air at 600 °C to remove the organic template in the as-synthesized Pt-MFI materials gives rise to the or-mation o Pt nanoparticles in the K-ree Pt@MFI-air sample, indi-cating the migration and subsequent sintering o Pt (Supplementary Fig. 9). However, as presented in Supplementary Fig. 10, Pt remains atomically dispersed ater calcination in air in the K-Pt@MFI-air sample. Interestingly, its location within the 10MR sinusoidal chan-nels remains almost unchanged when compared with the as-synthe-sized K-Pt@MFI-SDA sample. A main contribution at about 2.0 Å, corresponding to Pt–O bonding, is observed in the extended X-ray absorption ine structure (EXAFS) spectrum o the K-Pt@MFI-air sample (Supplementary Fig. 11 and Supplementary able 3), while a typical Pt–Pt bonding at about 2.7 Å is observed in the K-ree Pt@MFI-air sample. he results rom SEM-iDPC imaging and X-ray g h i jd ec f  20 nm20 nm20 nm20 nmPt@MFI-SDAK-Pt@MFI-airK-Pt@MFIK-Pt@MFI-SDAPt precursor, OSDA, silicon source, K +  Pt precursor, OSDA,silicon sourceCalcinationin airReductionby H 2 Pt@MFI-airPt@MFIHydrothermalcrystallizationCalcinationin airReductionby H 2 Hydrothermalcrystallization ab 30252015105025201510501.    P  e  r  c  e  n   t  a  g  e Particle size (nm)Particle size (nm)Particle size (nm)Particle size (nm)    P  e  r  c  e  n   t  a  g  e   P  e  r  c  e  n   t  a  g  e   P  e  r  c  e  n   t  a  g  e 23456789 d   = 4.5 nm  d   = 0.9 nm  d   = 4.0 nm  d   = 0.8 nm–––– Fig. 1 |   One-pot synthesis of Pt-zeolite materials.   a , b , Schematic illustration of the formation process of Pt@MFI and K-Pt@MFI samples by one-pot synthesis. c – f  , STEM images of Pt-zeolite samples after reduction by H 2  at 600 °C: K-free Pt-MFI ( c ), K-Pt@MFI ( d ), K-free PtSn@MFI ( e ) and K-PtSn@MFI ( f  ). g –  j , The size distributions of Pt particles in different Pt-zeolite materials: K-free Pt-MFI ( g ), K-Pt@MFI ( h ), K-free PtSn@MFI ( i ) and K-PtSn@MFI (  j ). The average particle size is calculated according to d   =   Σ n i d i 3  / Σ n i d i 2 . n i  stands for the number of the particle with a size of d i . NATURE MATERIALS  |  ARTICLES NATURE MATERIALS absorption spectroscopy indicate that the role o K is related to the stabilization o Pt atoms during the calcination process, avoiding their sintering into Pt nanoparticles 19,20 .As shown in Supplementary Fig. 12, the number o –OH groups in the K-ree Pt-zeolite samples measured by inrared spectroscopy decreased signiicantly ater the introduction o K +  (re. 27 ). A plau-sible explanation is that the –OH groups in the K-ree zeolite are replaced by –O − K +  species. Furthermore, the –O − K +  species can interact with the positively charged Pt species by orming stabilized –O–Pt species during the high-temperature calcination in air. I the K +  in the K-Pt@MFI-air sample was exchanged or NH 4 + , Pt atoms sintered into Pt nanoparticles in the subsequent calcination at 600 °C in air and reduction treatment by H 2  at 600 °C (Supplementary Fig. 13 and Supplementary Fig. 14).Most interestingly, when the K-containing K-Pt@MFI-air sam-ple was reduced in H 2  at 600 °C, Pt atoms turned into subnano-metric clusters, as shown in Fig. 1d. he low white-line intensity in the Pt L 3 -edge X-ray absorption near-edge structure (XANES) spectrum o the K-Pt@MFI sample (Fig. 2a) indicates the presence o metallic Pt ater the H 2  reduction treatment. he coordination number o irst-shell Pt–Pt ( N  Pt–Pt ) bonding or the K-Pt@MFI sam-ple is about 6.6 (able 1), corresponding to an average size o about 0.9 nm (Fig. 1h) 28 . Considering that the average size o Pt clusters and Pt nanoparticles is about 0.55 nm and about 2 nm respectively, then about 80% o Pt atoms should be located in the internal space o MFI crystallites as subnanometric clusters while about 20% o Pt atoms exist as Pt nanoparticles (according to the ollowing sim-ple estimation: 80% ×  0.55 +  20% ×  2 ≈  0.85 nm). In the case o the K-ree Pt@MFI sample, the irst-shell N  Pt–Pt  is about 10.6, corre-sponding to an average size o 4–5 nm, as shown in Fig. 1g. Determination of the location of subnanometric Pt clusters Considering the structure o MFI zeolite, one can expect that sub-nanometric encapsulated Pt clusters may distribute in dierent locations (including the straight channels, intersectional voids and sinusoidal channels). hereore, we have analysed the HR HAADF-SEM results in urther detail. For the K-Pt@MFI sample, as dis-played in Fig. 3a,c, 0.4–0.6 nm Pt clusters can be clearly observed as high-intensity areas along the [010] direction, while the 10MR straight channels appear as low-intensity pores. Corresponding HR HAADF-SEM images in greyscale are shown in Supplementary Fig. 17. However, due to the weak contrast o the zeolite ramework and its sensitivity to the electron beam, the detailed structure o the MFI zeolite is not well revealed in the HR HAADF-SEM image. By using the newly developed iDPC technique, the atomic struc-ture o the MFI zeolite can be recorded simultaneously with the HR HAADF-SEM image under low-dose conditions (Fig. 3b) 22,23 . As presented in Fig. 3d, the detailed structure o the MFI zeolite can be clearly identiied in the iDPC image, even the 5 R  units in the rame-work. Since HR HAADF-SEM imaging is more sensitive to heavy Pt@MFIPtSn@MFIK-PtSn@MFI    N  o  r  m  a   l   i  z  e   d      µ   x   N  o  r  m  a   l   i  z  e   d      µ   x   |   F   T   |   (    Å   –   4    )   |   F   T   |   (    Å   –   4    ) R   (Å)Pt foilPtO 2 Energy (eV) R   (Å)Energy (eV)K-Pt@MFIPt@MFIPtSn@MFIK-PtSn@MFIK-PtSn@MFIK-PtSn@MFIK-PtSn@MFI-airSnSnOSnO 2 SnSnOSnO 2 Pt foilPtO 2 K-Pt@MFI11,540 11,560 11,580 11,600 11,620 11,640 0029,180 29,200 29,220 29,240 29,260 1 2 3 4 5 61 2 3 4 5 62 Å –4 5 Å –4 a bdc Fig. 2 |   Characterization of Pt-zeolite materials by X-ray absorption spectroscopy.   a , b , XANES spectra ( a ) and EXAFS spectra ( b ) of the Pt L 3 -edge of different Pt-zeolite samples. c , Sn K-edge XANES spectra of the K-PtSn@MFI-air and K-PtSn@MFI samples. d , Sn K-edge EXAFS spectrum of the K-PtSn@MFI sample. All the samples are reduced in situ by H 2  at 600 °C before the spectrum collection, except for the K-PtSn@MFI-air sample in c , which was measured directly without prereduction by H 2 . The Pt and Sn standard samples were also measured directly. μχ , absorption of X-ray by the sample. NATURE MATERIALS  |  ARTICLES NATURE MATERIALS elements (Pt in this work), and the structural inormation o the zeolite ramework is inely captured by iDPC imaging, the precise location o subnanometric Pt clusters can be reliably identiied by correlating the paired images. he results demonstrate that Pt clus-ters o 0.4–0.6 nm are preerentially located in the sinusoidal chan-nels (see more images in Supplementary Figs. 18–22). he location o Pt clusters has also been conirmed by SEM-iDPC imaging on a zeolite crystallite with the tilted-[010] orientation (see Fig. 3e–h), showing that the subnanometric Pt clusters overlap with the sinu-soidal channels. Synthesis of bimetallic PtSn@MFI Following the same synthesis procedure, a second metal component such as Sn can be introduced together with Pt. Similar to the situa-tion observed beore, ater calcination in air, Pt species in the K-ree PtSn@MFI sample agglomerate into nanoparticles (Supplementary Fig. 23) while the Pt species in the K-PtSn@MFI sample remain atomically dispersed (Supplementary Fig. 24), as in the as-synthe-sized K-PtSn@MFI-SDA sample (Supplementary Fig. 25). Ater reduction by H 2  at 600 °C, large Pt nanoparticles are ormed in the PtSn@MFI sample (Fig. 1e,i and Supplementary Fig. 26), while sub-nanometric Pt clusters (~0.5 nm) are ormed in the K-PtSn@MFI sample (Fig. 1,j and Supplementary Fig. 27).According to the XANES results (Fig. 2a), Pt species also exist in the metallic state in the PtSn@MFI and K-PtSn@MFI samples. As shown in Fig. 2b and able 1, the K-ree PtSn@MFI has a irst-shell N  Pt–Pt  o about 10.7, corresponding to Pt nanoparticles o 4–5 nm, while a irst-shell N  Pt–Pt  o about 6.4 is ound or the K-PtSn@MFI sample, corresponding to an average size o about 0.9 nm. hese results urther conirm the critical role o K +  in stabi-lizing subnanometric Pt species 18–20 .Following the same approach, we have studied the location o subnanometric Pt and Sn species in the K-PtSn@MFI sample by paired HR HAADF-SEM (Supplementary Fig. 28 images in greyscale) and iDPC imaging. As can be seen in Fig. 3i–p, subnano-metric Pt and Sn species are also located in the 10MR sinusoidal channels o the MFI zeolite (see additional images in Supplementary Figs. 29–33). he oxidation state o Sn in the unreduced K-PtSn@MFI-air sample is determined to be Sn 󰁩󰁶. Ater reduction by H 2 , a decrease in the white-line intensity and a redshit o the spectrum is observed in the K-PtSn@MFI sample. As shown in Fig. 2c, the shape o the XANES spectra o the reduced K-PtSn@MFI does not resemble either Sn metal or SnO, suggesting a possible ormation o SnO 4 −   x   species. his can also be supported by the reduction in the irst-shell intensity o |F| (Fig. 2d), indicating the loss o neigh-bouring oxygen. he idea that Sn species are well dispersed within the K-PtSn@MFI sample is supported by the EXAFS spectrum, since no additional higher shells are observed. Furthermore, we have ound that the EXAFS spectrum o K-PtSn@MFI is dierent rom that o Sn-Beta, indicating that Sn exists as an extraramework species (Supplementary Fig. 35) 29,30 .he interaction between Pt and Sn in the K-PtSn@MFI sample has been investigated by CO inrared spectroscopy. As can be seen in Supplementary Fig. 36, the CO adsorption bands at 1,887 and 1,719 cm − 1  can only be observed in the K-PtSn@MFI sample, indicating that the introduction o Sn can modulate the electronic structure o Pt clusters. hese bands are probably related to the or-mation o bimetallic PtSn species 31 .As shown in Fig. 3q–t, the simulated images with Pt or Sn clusters located in the sinusoidal channels are consistent with the experi-mental images. According to our simulation results (Supplementary Figs. 37 and 38), it is possible to dierentiate single Pt and Sn atoms rom the zeolite ramework. Indeed, we have observed a ew singly dispersed atoms (Pt or Sn) in both K-Sn@MFI and K-PtSn@MFI samples (Supplementary Figs. 39 and 40). Considering the average particle size obtained rom the EXAFS results, the number o sin-gly dispersed Pt atoms in the K-Pt@MFI and K-PtSn@MFI samples should be low and Pt should mainly exist as subnanometric clusters (Supplementary Fig. 41). Determination of the Pt and Sn distributions in K-PtSn@MFI As displayed in Fig. 4a– , K, Sn and Pt are homogeneously distrib-uted in the K-PtSn@MFI sample according to the X-ray energy dispersive spectroscopy (XEDS) mapping. However, due to the res-olution limitation, it is not possible to determine the relative spatial relationship between Sn and Pt. o overcome the limitation o XEDS on such beam-sensitive samples, we have attempted, on the basis o the imaging simulation results (Supplementary Figs. 42–44), to dis-criminate Pt and Sn distributions by a k -means clustering analysis o the HR HAADF-SEM images (see Supplementary Inormation or details). As shown in Fig. 4g–i, subnanometric Pt and Sn species can be identiied according to their dierent contrasts in the HR HAADF-SEM images. hen, in the image depicting the distribu-tion o the Pt and Sn species (Fig. 4h) ater k -means clustering, it is possible to pick out the Pt and Sn species that are in contact, simply by selecting those that all at a distance shorter than 0.1 nm in Fig. 4h. Table 1 |  Fit results for the Pt edge and Sn edge of EXAFS spectra of various Pt-zeolite materials Sample  N Pt–Pt  R Pt–Pt  (Å)  σ  2  (Å 2 )  Δ E  0  (eV)  R factor Pt foil122.763 ±  0.0010.0048 ±  0.00016.7 ±  0.50.0017Pt@MFI10.6 ±  0.62.762 ±  0.0010.0051 ±  0.00027.0 ±  0.30.0025PtSn@MFI10.7 ±  0.42.763 ±  0.0010.0050 ±  0.00010.0014K-Pt@MFI6.6 ±  1.22.743 ±  0.0060.0074 ±  0.00100.0049K-PtSn@MFI6.4 ±  0.42.768 ±  0.0010.0049 ±  0.00020.0150 Sample  N Sn–O  R Sn–O  (Å)  σ  2  (Å 2 )  Δ E  0  (eV)  R factor SnO 2 62.055 ±  0.0100.0023 ±  0.00127.3 ±  1.40.0044SnO42.202 ±  0.0010.0071 ±  0.00128.3 ±  0.70.0017K-PtSn@MFI3.2 ±  0.22.061 ±  0.0060.0058 ±  0.00128.1 ±  1.00.0037 R , bonding distance; σ  2 , Debye − Waller factor; Δ E  0 , inner potential correction; R factor , difference between modelled and experimental data. The fits of the Pt edge were performed on the first coordination shell ( Δ R   =  2.0–3.0 Å) over the Fourier transform (FT) of the k  1 k  2 k  3 -weighted χ ( k  ) functions in the range Δ k    =  3.6–16.7 Å − 1 , where Δ k   and Δ R  are the intervals in the k   and R  spaces for the Fourier transformation and the fit, respectively, resulting in several independent parameters of 2 Δ R   Δ k   / π    =  39.5 (7.9 for Pt foil). Independent parameters in this work were obtained from the Artemis software as a result of a co-refinement fit of different spectra. The standard Pt foil was fitted individually while the samples were fitted using a co-refinement approach resulting in one N Pt–Pt , R  and σ  2  for each sample and one common Δ E  0  for all samples. The many-body amplitude reduction factor S 02   =  0.89. The fits of the Pt-edge EXAFS spectra of Pt-zeolite samples are presented in Supplementary Figs. 15 and 16. The fits of the Sn edge were performed on the first coordination shell ( Δ R   =  1.0–2.0 Å) over the FT of the k  1 k  2 k  3 -weighted χ ( k  ) functions in the range Δ k    =  2.8–11.0 Å − 1 , resulting in a number of independent parameters of 2 Δ R   Δ k   / π    =  20.3 for the K-PtSn@MFI sample (5.0 for both SnO 2  and SnO). SnO 2   S 02   =  0.89; SnO S 02   =  1.0. The fit of the Sn-edge EXAFS spectrum of the K-PtSn@MFI sample is presented in Supplementary Fig. 34. NATURE MATERIALS  |  ARTICLES NATURE MATERIALS his distance threshold was selected on the basis o the irst-shell Sn–O distance ound in the EXAFS analysis. As a result, it is possible to estimate the presence o bimetallic PtSn clusters in the K-PtSn@MFI sample that are made up o two neighbouring Pt and Sn units rather than alloyed clusters, as shown in Fig. 4h. he statistical analysis shows that about 40% o the subnanometric metal clusters automatically detected in the experimental image o Fig. 4g all at a distance below 0.1 nm. It should be noted that this is a rough esti-mate, since the image analysis is based on two-dimensional projec-tion images. his may result in an overestimation o the raction o bimetallic clusters, since Pt and Sn at dierent depths may overlap in the SEM image. he complexity o the experimental images also PtPtPtSnSnSnSn10 nm10 nm10 nm10 nm a b c d 1 nm1 nm e f hg 2 nm2 nm5 nm5 nm5 nm5 nm1 nm1 nm2 nm2 nmSnSnSnPtPtPt imk j lponq tsr Fig. 3 |   Identification of the location of subnanometric Pt clusters within the MFI structure.   a – d . Large-area and detailed HR HAADF-STEM image ( a , c ) and the iDPC image of the same area ( b , d ) of the K-Pt@MFI sample in the [010] orientation. e – h , Large-area and detailed HAADF-STEM image ( e , g ) and the iDPC image of the same area ( f  , h ) of the K-Pt@MFI sample in the tilted-[010] orientation. i – l , Large-area and detailed HR HAADF-STEM image ( i , k ) and the iDPC image of the same area (  j , l ) of the K-PtSn@MFI sample in the [010] orientation. m – p , Large-area and detailed HAADF-STEM image ( m , o ) and the iDPC image of the same area ( n , p ) of the K-PtSn@MFI sample in the tilted-[010] orientation. In the HAADF-STEM images, subnanometric Pt clusters (~0.5 nm) are clearly imaged. In the corresponding iDPC images, the atomic structures of the MFI zeolite are also clearly revealed. By combining the images obtained in the two modes, we can identify the precise location of Pt species in the MFI zeolite, corresponding to the sinusoidal channels. q , Structural model of an MFI zeolite containing a single Pt atom, a single Sn atom, Pt clusters and Sn clusters in the sinusoidal channels along the [010] orientation. r , Simulated HAADF-STEM image of the model in q , showing the contrasts of the different types of metal species. s , Model of an MFI zeolite containing a single Pt atom, a single Sn atom, Pt clusters and Sn clusters along the tilted-[010] orientation. t , Simulated HAADF-STEM image of the model in s , showing the contrasts of the different types of metal species. NATURE MATERIALS  |
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