Abstract
Platinum is a much used catalyst that, in petrochemical processes, is often alloyed with other metals to improve catalytic activity, selectivity and longevity1,2,3,4,5. Such catalysts are usually prepared in the form of metallic nanoparticles supported on porous solids, and their production involves reducing metal precursor compounds under a H2 flow at high temperatures6. The method works well when using easily reducible late transition metals, but Pt alloy formation with rare-earth elements through the H2 reduction route is almost impossible owing to the low chemical potential of rare-earth element oxides6. Here we use as support a mesoporous zeolite that has pore walls with surface framework defects (called ‘silanol nests’) and show that the zeolite enables alloy formation between Pt and rare-earth elements. We find that the silanol nests enable the rare-earth elements to exist as single atomic species with a substantially higher chemical potential compared with that of the bulk oxide, making it possible for them to diffuse onto Pt. High-resolution transmission electron microscopy and hydrogen chemisorption measurements indicate that the resultant bimetallic nanoparticles supported on the mesoporous zeolite are intermetallic compounds, which we find to be stable, highly active and selective catalysts for the propane dehydrogenation reaction. When used with late transition metals, the same preparation strategy produces Pt alloy catalysts that incorporate an unusually large amount of the second metal and, in the case of the PtCo alloy, show high catalytic activity and selectivity in the preferential oxidation of carbon monoxide in H2.
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Data availability
The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.
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Acknowledgements
This work was supported by IBS-R004-D1. We thank M. G. Kim at Pohang Accelerator Laboratory (PAL) for allocating special beam time for XAFS measurements. Electron microscopy experiments were performed with help from H. B. Bae and J. S. Choi at KAIST Analysis Center for Research Advancement (KARA) and C. S. Kim at Korea Institute of Ceramic Engineering and Technology (KICET).
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R.R. initiated and led the project. R.R., J.K. and S.W.H. wrote the manuscript. J.K., C.J., S.W.H., J.-C.K., H.P., J.H. and H.S.S. carried out materials synthesis, characterization and catalytic measurements. J.W.S. and J.-C.K. performed the TEM investigation.
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Extended data figures and tables
Extended Data Fig. 1 Pt3La nanoparticles with an L12 superlattice structure supported on mz-deGa.
a, b, Low-magnification HAADF-STEM images showing uniformly sized metal nanoparticles dispersed on the mesoporous zeolites. c, EDS spectrum taken from the red box in b, indicating the presence of both Pt and La in the same particle. d, AR-HAADF-STEM image of a metal nanoparticle, showing the Pt3La ordered alloy structure with the L12 superlattice. e, FFT image from the HAADF-STEM image of d. The L12 superlattice reflection from the intermetallic compound structure is indicated by a yellow arrow. f, Intensity profile taken along the light-blue box in d. The intensity profile in f shows that Pt and Pt + La atomic columns are alternating.
Extended Data Fig. 2 Structure of mesoporous MFI gallosilicate synthesized using C18-6-4 as a dual micro-/mesopore structure-directing agent.
a, PXRD patterns. b, TEM image. c, N2 adsorption–desorption isotherm. d, BJH pore size distribution derived from the adsorption branch of the N2 isotherm. The mesoporous MFI gallosilicate (mz-Ga; parent zeolite of mz-deGa) exhibited highly mesoporous frameworks built with ultrathin zeolitic walls and a uniform mesopore size distribution centred at about 4 nm.
Extended Data Fig. 3 Structure of mz-deGa obtained by degallation of the mesoporous MFI gallosilicate synthesized using C18-6-4 as a dual micro-/mesopore structure-directing agent.
a, PXRD patterns. b, TEM image. c, N2 adsorption–desorption isotherm. d, BJH pore size distribution derived from the adsorption branch of the N2 isotherm. mz-deGa showed similar structural properties to those of the parent mesoporous MFI gallosilicate.
Extended Data Fig. 4 Atomically dispersed La on mesoporous zeolite with silanol nests.
a, FT-IR spectra of mz, mz-deGa and mz-Ga. The mz-Ga sample shows the FT-IR absorbance band corresponding to isolated Si–OH (about 3,750 cm−1) and Ga–OH (3,600 cm−1). The mz-deGa sample displays increased broad FT-IR adsorption at around 3,500 cm−1, which is assigned to the silanol nests. The mz sample shows one sharp FT-IR peak corresponding to the isolated silanols. b, HAADF-STEM image of LaOx-supported mz-deGa showing no noticeable white dots on the grey zeolite matrix, which can be regarded as LaOx nanoparticles. c, AR-HAADF-STEM image of LaOx/mz-deGa magnified from the STEM image of b, revealing the single-atom-dispersed La species as white dots. d, HAADF-STEM image of LaOx-supported mz, showing the nanoparticle-like LaOx species as white dots. As shown in the HAADF-STEM images and FT-IR spectra, La species can be single-atom-dispersed only if the mesoporous MFI zeolite has sufficient silanol nests. e, FT-IR spectra of La-supported mz-deGa samples with various La loadings. The samples were prepared by incipient wetness impregnation of La nitrate and subsequent heat treatment at 350 °C under an O2 flow. The absorption band corresponding to the silanol nest (~3,500 cm−1; marked with an arrow) was gradually weakened with increasing La loading, indicating that the incorporated La species resulted in the formation of bonds with the silanol nests present in the mz-deGa support.
Extended Data Fig. 5 XANES analysis of PtY/mz-deGa.
a, XANES spectra at the Pt L3 edge. The XANES spectrum of PtY/mz-deGa at the Pt L3 edge is compared with two reference samples of Pt metal foil and Pt/mz. PtY/mz-deGa showed a notable shift in edge energy and white-line region to higher energy compared to the monometallic Pt reference samples. This indicates that electron donation occurred from less electronegative adjacent Y metals to Pt metals in the bimetallic alloy nanoparticles of PtY/mz-deGa. b, XANES spectra at the Y K edge. The PtY/mz-deGa sample is compared with PtY/mz-deGa before H2 reduction and an Y foil as reference samples. The Y XANES spectrum of PtY/mz-deGa exhibited a noticeable shift in edge energy to lower energy after the reduction. This indicates that a sizable portion of oxidic Y was reduced to metallic Y. c–j, Linear-fit analysis of the Y K-edge XANES spectra of PtY/mz-deGa and related samples: PtY/mz-deGa before H2 reduction (that is, sample loaded with metal precursors and calcined with O2 at 350 °C) at 700 °C (c, g); PtY/mz-deGa (d, h); and PtY/mz-deGa after exposure to air (e, i). In c–e, the Y K-edge spectra and their fit results are shown. f shows the metallic and oxidic Y contents determined by linear fitting of the Y K-edge XANES spectra. In g–i, the first derivatives of the Y XANES spectra and their fit results are shown. The deconvolution was performed using the Y XANES spectra of two reference samples: (1) Y metal foil and (2) Y2O3/mz, which was prepared to have 1 wt% Y by impregnation of yttrium nitrate and subsequent O2 calcination at 350 °C. j shows the metallic and oxidic Y contents, determined by linear fitting of first derivatives of the Y K-edge XANES spectra.
Extended Data Fig. 6 Structure of mz siliceous mesoporous MFI synthesized using C18-6-4 as a dual micro-/mesopore structure-directing agent.
a, PXRD patterns. b, TEM image. c, N2 adsorption–desorption isotherm. d, BJH pore size distribution derived from the adsorption branch of the N2 isotherm. The mz zeolite showed an almost identical porous texture to that of the mesoporous MFI gallosilicate in Extended Data Fig. 2. e, Physicochemical properties of various zeolite samples and alumina.
Extended Data Fig. 7 Propane dehydrogenation performance of catalysts using mz-deGa support.
a, Propane conversion as a function of time on stream. b, Propylene selectivity as a function of time on stream. The supported catalysts are composed of either 1 wt% Pt or 1 wt% La. The PDH reaction conditions were as follows: 50 mg catalyst, WHSV = 11 h−1 with pure propane gas flow, and temperature of 580 °C. To determine the effect of remaining Ga species in mz-deGa on PDH performance, Pt/mz-deGa, La/mz-deGa and mz-deGa catalysts were tested. The mz-deGa catalyst showed negligible propane conversion, indicating that the remaining Ga species in the zeolite framework were not effective for PDH. Moreover, the La/mz-deGa sample showed negligible propane conversion, implying that the single atomic La species do not have PDH activity. Both mz-deGa and La/mz-deGa showed much poorer propylene selectivity compared to the Pt–REE/mz-deGa samples in Fig. 2. In the case of the Pt/mz-deGa sample, the initial propane conversion is slightly higher than that of Pt/mz shown in Fig. 2. In addition, the Pt/mz-deGa gave a noticeably lower deactivation rate than the Pt/mz sample. However, the improvement of Pt/mz-deGa in catalytic lifetime was almost insignificant when compared to that of the Pt–REE/mz-deGa catalysts. On the basis of these catalytic results, we can conclude that the remaining Ga species in mz-deGa could somehow promote the PDH performance of the supported Pt catalyst. However, this Ga contribution would be almost insignificant compared to that of Pt–REE alloy formation in the case of the Pt–REE/mz-deGa catalysts.
Extended Data Fig. 8 Pt3Ce nanoparticles with an L12 superlattice structure supported on mz-deGa.
a, b, Low-magnification HAADF-STEM images showing uniformly sized metal nanoparticles dispersed on the zeolite. c, EDS spectrum taken from the red box in b, indicating the presence of both Pt and Ce in the same particle. d, g, AR-HAADF-STEM image of the metal nanoparticle, showing the Pt3Ce ordered alloy structure with the L12 superlattice. e, h, FFT images from the HAADF-STEM image of d and g, respectively. The L12 superlattice reflection from the intermetallic compound structure is indicated by the yellow arrows in e and h. f, i, Intensity profiles taken along the light-blue boxes in d and g, respectively. The intensity profiles in f and i show that Pt and Pt + Ce atomic columns are alternating.
Extended Data Fig. 9 Representative STEM images and preferential CO oxidation tests in a H2-rich stream over supported PtCo catalysts.
a–c, STEM images of PtCo/mz-deGa (a), PtCo/mz (b) and PtCo/alumina (c). d, e, CO conversion (d) and CO2 selectivity (e) for supported PtCo catalysts. f, Amounts of Co species incorporated in Pt alloy nanoparticles supported on various supports measured with STEM-EDS. Depending on the support, there was a remarkable difference in the amounts of reduced Co species incorporated into the Pt alloy nanoparticles. As shown in f, the average Co/Pt molar ratio increased in the order PtCo/alumina (0.32) ≪ PtCo/mz (0.45) < PtCo/mz-deGa (0.50). The distinctively low Co/Pt of PtCo/alumina appears to be the result of the stronger interaction between the cationic Co species and the alumina support, which makes the reduction of Co more difficult than when using siliceous zeolite supports. Of the two siliceous zeolites, mz-deGa exhibited a larger degree of Co incorporation into the Pt nanoparticles. We believe that the silanol nests in mz-deGa could be beneficial to homogeneously distribute the Co species into atomic cations on the zeolite, thereby providing mobility to help atomistic diffusion. Such a benefit might enable better Co incorporation in the case of mz-deGa than in mz with no silanol nests. This difference in Co incorporation led to a variation in the catalytic performance for PROX in a H2-rich stream containing 1.4% CO. The PtCo/mz-deGa catalyst exhibited complete CO conversion over a wide range of operating temperatures (60–120 °C). On the other hand, for the PtCo/mz catalyst complete CO conversion was achieved only above 90 °C. In the case of the PtCo/alumina catalyst, complete CO removal was not achievable, even at high temperatures.
Extended Data Fig. 10 HAADF-STEM images.
a, Pt/mz. b, PtLa/mz. c, PtSn/alumina. All three samples had uniformly dispersed Pt nanoparticles without any bulky sintered particles. These catalysts were tested for the PDH reaction, as shown in Fig. 2. For the preparation of Pt/mz, Pt(NH3)4(NO3)2 was loaded to mz using the incipient wetness impregnation technique. The Pt precursor-loaded mz was heated under an O2 flow at 350 °C for 2 h (ramping rate, 0.8 °C min−1; flow rate, 500 cm3 min−1 gcat−1) and subsequently treated under a H2 flow at 580 °C for 2 h (ramping rate, 0.3 °C min−1; H2 flow rate, 300 cm3 min−1 gcat−1). The obtained Pt/mz contained 1 wt% Pt. For the preparation of PtSn/alumina, the alumina support was purchased from Sasol (PURALOX γ-Al2O3, 98%, BET surface area of 170 m2 g−1). The alumina was prepared by the same procedure as the Pt/mz except for the co-incorporation of H2PtCl6 and SnCl2·2H2O as metal precursors. The PtSn/alumina contained 1 wt% Pt and 1 wt% Sn.
Supplementary information
Video 1
Dynamic motions of atomically dispersed La species supported on the mz-deGa. The video was obtained by combining 66 images of 1 wt% La-supported mz-deGa, which were taken by AR-HAADF-STEM [300 kV acceleration voltage, 50 pA beam current, 5 μs pixel dwell time (512×512 pixels)]. The video shows that La single atoms are dynamically moving on the mz-deGa zeolite surface. Although the STEM investigation was performed under a low-dose condition, there could be some influences from the electron beam on the motions of La species, which would be analogous to those under the H2 reduction process at a high temperature.
Video 2
Stationary La species with nanoparticle-like morphology supported on the mz. The video was obtained by combining 66 images of 1 wt% La-supported mz, which were consecutively taken by HAADF-STEM at 300 kV of acceleration voltage and 50 pA of beam current with 5 μs of a pixel dwell time (512×512 pixels). The video shows that the La-species in the form of nanoparticles are stationary under the same electron beam condition as in the Supplementary Video 1.
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Ryoo, R., Kim, J., Jo, C. et al. Rare-earth–platinum alloy nanoparticles in mesoporous zeolite for catalysis. Nature 585, 221–224 (2020). https://doi.org/10.1038/s41586-020-2671-4
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DOI: https://doi.org/10.1038/s41586-020-2671-4
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