Communication Cite This: Chem. Mater. 2018, 30, 5534−5538
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Atomically Dispersed Pt/Metal Oxide Mesoporous Catalysts from Synchronous Pyrolysis−Deposition Route for Water−Gas Shift Reaction Long Kuai,*,⊥,† Shoujie Liu,⊥,‡ Sufeng Cao,⊥,§ Yiming Ren,† Erjie Kan,† Yanyan Zhao,∥ Nan Yu,‡ Fang Li,† Xingyang Li,† Zhichuan Wu,† Xiong Wang,† and Baoyou Geng*,‡ †
School of Chemical and Biological Engineering, Anhui Polytechnic University, Beijing Middle Road, Wuhu 241000, China College of Chemistry and Materials Science, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Center for Nano Science and Technology, Anhui Normal University, No. 1 Beijing East Road, Wuhu 241000, China § Department of Chemical Engineering, East China University of Science and Technology, Xuhui, Shanghai 200237, China ∥ Department of Chemistry, Merkert Chemistry Center, Boston College, Newton, Massachusetts 02467, United States
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‡
S Supporting Information *
R
Scheme 1. SPDR Processes for the Synthesis of Atomically Dispersed Pt/Metal Oxide Mesoporous Catalysts
ecently, reactions on atomically dispersed noble metals have brought new insights to supported catalysts beyond the nature of 100% metal dispersion and atomic efficiency.1−8 A range of work has demonstrated that the active center of the supported catalyst is the metal−support interface instead of the metal nanoparticles, and the isolated metal species from the metal−support interaction are responsible to the activity.9−12 Moreover, the synergistic effect between the metal atoms and nearby support atoms is found to remarkably boost the activity of some reactions.13,14 Therefore, the atomically dispersed catalysts are attractive due to their maximized utilization of metal, high selectivity to catalytical reactions, and exclusive metal species to better understand catalysis, leading to the synthesis of atomically dispersed catalysts is of importance and interest. It is challenging to obtain atomically dispersed catalysts from chemical routes. Especially, studies should overcome the metal aggregation with high metal loading because the atomically dispersion of metal atoms is a thermodynamic disadvantage compared to their aggregate forms such as clusters and nanoparticles.15 The general chemical strategy for atomically dispersing of metal atoms is by adhering them on the supports with large surface areas by the strategies of precipitation− deposition,16−18 impregnation−deposition13,14 and so forth, reviewed by Liu.19 In fact, the challenge is still faced that the metal loading is often sacrificed for acquiring atomically dispersion of metal atoms, usually hinders these materials’ catalytical performance. Recently, Zheng’s group successfully accessed atomically dispersed Pd/TiO2 catalyst with metal loading over 1 wt % by photodeposition route.20 This work demonstrates a remarkable catalytic performance of high metal loaded atomically dispersed catalysts and motivates people to explore more atomically dispersed catalysts for widely applications. Therefore, it is a highly vital challenge to develop a general method to prepare versatile atomically dispersed catalysts with high metal loading. Herein, a facile synchronous pyrolysis−deposition route (SPDR, Scheme 1) is developed for atomically dispersing Pt atoms on mesoporous metal oxides. The proposed SPDR is universal to versatile metal oxide supports, demonstrated by © 2018 American Chemical Society
the synthesis of Pt1/MnOx, Pt1/FeOx, Pt1/CeO2, and Pt1/ Al2O3 catalysts. It is valuable to mention that supports are rarely reported mesoporous structure for atomically dispersed catalysts, which brings benefits of the mass-transfer and active site exposure. The superior performance of the catalysts from SPDR was confirmed with low-temperature WGS reaction. Typically, the as-prepared atomically dispersed Pt1/MnOx shows more than 2 times’ activity of Pt/MnOx catalyst from conventional immersion methods. As illustrated in Scheme 1, the SPDR starts from the homogeneous precursor solution containing [Pt(NH3)4](NO3)2, nitrates (e.g., Mn(NO3)2·4H2O) and mesoporous temperate of Pluronic F-127 (PE106PO70EO106). The solution was ultrasonic-sprayed (right) into a heating zone (middle) with 3.7 ± 0.2 s’ residence time by a vacuum pump (left). In the heating zone with a temperature of 600 °C, after the evaporation of solvent (H2O), nitrates and [Pt(NH3)4](NO3)2 in the microdrops are immediately decomposed to MOx (metal oxides) and single-site Pt species to form Pt/MOx composite particles. Meanwhile, the F-127 micelles and Pt/ Received: May 22, 2018 Revised: August 15, 2018 Published: August 16, 2018 5534
DOI: 10.1021/acs.chemmater.8b02144 Chem. Mater. 2018, 30, 5534−5538
Communication
Chemistry of Materials
Figure 1. XRD profiles (a), AC-HAADF-STEM image (b) and low-resolution AC-HAADF-STEM image (c), and low (d) and high (e) magnification EDX-mapping of Mn, O, and Pt elements of atomically dispersed 0.5 at. % Pt1/MnOx mesoporous catalysts. The inset of panel c is the corresponding TEM images.
MOx blocks self-assemble into microspheres.21,22 In the microdrops, the salts decomposition and Pt deposition are completed quickly and synchronously, so the Pt aggregation gets suppressed. Thus, the Pt atoms are atomically anchored on MOx particles. Based on X-ray photoelectron spectra (XPS, Figure S1), the absence of N signal reveals that the raw materials are completely converted into Pt/MOx products after pyrolysis−deposition. Finally, after removal of F-127 template and 5% H2/Ar activation, the atomically dispersed Pt/MOx mesoporous catalysts (Figure S2) are obtained. The synthesis of atomically dispersed Pt1/MnOx catalysts with a Pt loading of 0.5 at. % (∼1.2 wt %) confirmed our SPDR proposal. The XRD patterns (Figure 1a) show that the MnOx support appears in Mn3O4 phase, and no crystallized Pt or PtO2 signal was detected although the sample was obtained with 5% H2/Ar treatment at 200 °C. The aberration-corrected high-angle annular darkfield-STEM (AC-HAADF-STEM) image (Figure 1b) reveals that all the Pt atoms (the individual brighter points) are anchored on the MnOx surface and no Pt subnanoclusters or nanoparticles are formed (Figure 1c and S3). Based on the statistics Pt atoms on random surface of more than 800 nm2, the frequency of Pt on the supports is 0.34 Pt atoms per nm2, which well matches the estimation value (0.38 Pt atoms per nm2) from the metal loading and the BET surface area (96 m2/g) of MnOx support determined by N2 sorption isotherm (Figure S4a). In this regard, ∼89% of Pt atoms are dispersed in the surface of support and partial Pt atoms embed in the metal oxide bulks (e.g., doping). However, more careful analysis based on selective surface-leaching should be further developed to directly and solidly identify the Pt on the surface because our result based on Pt sites’ counting is higher than the reported value (69%) in 1.0 wt %
Pd/TiO2 in the work by Fujiwara et al.23,24 Unfortunately, the reported leaching approach10 did not work for the acid-soluble supports in this work, so some future work is demanded. The MnOx supports are mesoporous structures with pore size of 20−40 nm (inset of Figure 1c), which is consistent with the Barrett−Joyner−Halenda BJH pore size distribution (Figure S4b). Herein, the F127 template is essential to the well mesoporous structure. The pore size and frequency of the product without F127 is rather poor (Figure S5). Furthermore, both the high and low-magnification EDX mapping (Figure 1d,e) of Pt1/MnOx catalyst indicates that Pt atoms are uniformly distributed in the MnOx matrix and no visible Pt particle presents. The X-ray photoelectron spectroscopy (XPS, Figure S6a) is carried out to study the oxidation state of atomically dispersed Pt. For comparison, we also studied the XANES of Pt in cluster-sized 0.5 at. % Pt/MnOx-R400 (Figure S7) catalyst by 5% H2/Ar treatment at 400 °C. The catalyst before H2 reduction (Pt/MnOx-A400) presents total PtIV state. The H2 reduction at 200 °C causes a mixture of PtII and PtIV. Whereas, the Pt in Pt/MnOx-R400 is further reduced to PtII and Pt0. The changes of oxidation state of Pt are further confirmed by the X-ray absorption near-edge structure (XANES, Figure S6b). The binding energy of Pt in Pt1/ MnOx is positively shifted in comparison to Pt foil and Pt/ MnOx-R400. Moreover, the intensities of the white line follow the order of Pt1/MnOx > Pt/MnOx-R400 > Pt foil. Therefore, the atomically dispersed Pt are positively charged with a mixture oxidation state of PtII and PtIV. The high-magnification AC-HAADF-STEM and extended X-ray absorption fine spectra (EXAFS) are used to study the structure of single Pt sites in 0.5 at. % Pt1/MnOx catalyst. As depicted in Figure 2a, Pt atoms (lighter points) mainly located 5535
DOI: 10.1021/acs.chemmater.8b02144 Chem. Mater. 2018, 30, 5534−5538
Communication
Chemistry of Materials
Figure 2. (a) High-resolution AC-HAADF-STEM image of atomically dispersed 0.5 at. % Pt1/MnOx, (b) FT-EXAFS spectra of 0.5 at. % Pt1/ MnOx (red), Pt/MnOx-R400 (black) and Pt foil (blue), (c) H2-TPR profiles of 0.5 at. % Pt1/MnOx (red) and pristine MnOx (black).
O−Mn sites.27,28 Therefore, the Pt−O−Mn configuration is the structure of single Pt sites on the surface of Pt1/MnOx. To evaluate the advantages of SPDR for preparing atomically dispersed mesoporous catalysts, the 0.5 at. % Pt/ MnOx catalysts from both conventional coprecipitation (Pt/ MnOx-CP) and immersion (Pt/MnOx-Im) methods were studied. All subsequent treatment is the same as that of Pt1/ MnOx. As shown in Figure S10, the metallic Pt−Pt signal obviously appears in both Pt/MnOx-Im and Pt/MnOx-CP catalysts, suggesting a mixture of Pt aggregations and failure of the conventional immersion and coprecipitation methods for acquiring atomically dispersed catalysts with high Pt loading. Mesoporous structure supports can be obtained from this onepot SPDR synthesis. It is typically difficult to control the morphology of the support by the conventional precipitation method. Thus, the SPDR provides a new method to prepare atomically dispersed catalysts with high Pt loading and mesoporous structure. To further demonstrate the SPDR, the atomically dispersed 0.5 at. % Pt1/FeOx catalyst was prepared. As shown in Figure S11a, no detectable crystallized Pt appears in the XRD pattern (blue). Furthermore, the effect of Pt on the property of FeOx is visible and the support FeOx is magnetic Fe3O4 phase (blue) whereas the pristine FeOx exhibits hematite phase (black) prepared under the same conditions and processes. The HAADF-STEM images (Figure S11b,c) confirm the atomically dispersion (circles). Different from the Pt1/MnOx catalyst, partial Pt atoms tend to move close enough to form 2D rafts (squares) and subnanoclusters (triangle). The aggregation may stem from the serious phase change of Fe2O3 to Fe3O4 during reduction process. Luckily, SPDR is further applicable to other samples such as 1.0 at. % Pt1/CeO2 (Figure S12) and even 0.6 wt % Pt1/Al2O3 with typical irreducible metal oxide support (Figure S13). Moreover, all the catalysts are mesoporous. Therefore, SPDR is successful in preparing versatile atomically dispersed Pt/metal oxide mesoporous catalysts with significant Pt loading. The low-temperature WGS test was performed to check the performance of the obtained atomically dispersed Pt catalysts from SPDR. As a case study, the Pt1/MnOx catalyst was studied in a fixed-bed reactor with feeds of 10% CO, 3% H2O, and 87% balance He. As shown in Figure 3a, the reactivity (denoted by H2 production and normalized with catalyst loading) was dominant in the low-temperature condition (below 483 K). Typically, the reactivity at 473 K reaches as high as 9.6 μmol H2/min/gcat and accordingly presents H2O conversion of 7.8% and turnover frequency of 0.04 s−1. During the cycling test from 483 to 443 K, a slight drop of activity
at lattice line of surface Mn, suggesting a Pt−O−Mn structure of single Pt sites, where Pt is anchored at the site of Mn vacancy. The EXAFS of the Pt L3-edge (Figure S8 and S9) further supports this point. As shown in the R-spaced FTEXAFS spectra (Figure 2b) and Table 1, no Pt−Pt bond signal Table 1. EXAFS Fitting Results of 0.5 at. % Pt1/MnOx and Pt/MnOx-R400 Catalysts Sample
Pt1/MnOx Pt/MnOxR400
Shell
Na
R (Å)
σ2 (×10−3 Å2)
E0 (eV)
Rfactor
Pt−O Pt−Mn Pt−O
3.5 1.1 1.5
1.99 2.98 2.00
3.5 3.8 5.2
5.3 5.3 7.0
0.002
Pt−Pt Pt−Mn
1.0 0.8
2.74 2.95
5.6 7.0
6.9 6.9
0.003
a
N, Coordination number; R, distance between absorber and backscatter atoms; σ2, change in the Debye−Waller factor value; E0, inner potential correction.
was detected in Pt1/MnOx (red) with reference of Pt foil (blue), indicating Pt atoms are atomically dispersed,25,26 which is consistent with the observation of AC-HAADF-STEM. Moreover, the R value of 1.99 Å is attributed to the Pt−O bond with a notable coordination number of 3.5, revealing the significant Pt−O coordination for Pt1/MnOx catalyst. The much longer Pt−Mn coordination (∼2.98 Å) than Pt−Mn alloy suggests that Pt is not directly coordinated with Mn. Therefore, the Pt−O−Mn sites are reasonable for Pt1/MnOx catalyst. As a supplement, the Pt/MnOx-R400 (black, Figure 2b) was studied and a significant Pt−Pt bond was detected, which is consistent with the results of HRTEM observation (Figure S7). Although the Pt−O and Pt−Mn bonds were also detected, the coordination number (1.5) of Pt−O is much lower than that of Pt1/MnOx (3.5), suggesting the lower frequency of atomically dispersed Pt in Pt/MnOx-R400 catalyst. To further confirm the formation of Pt−O−Mn sites, H2-temperature-programmed reduction (TPR) profiles were collected. As shown in Figure 2c, an overlapped sharp reduction peak is observed at ∼170 °C for Pt1/MnOx (red) while there appear two obvious peaks for pristine MnOx sample (black). Moreover, the reduction temperature is decreased about 200 °C compared to MnOx. The overlapped peak and notable temperature decrease suggest that the Mn3+ is easily reduced to Mn2+ by the spillover hydrogen from Pt− 5536
DOI: 10.1021/acs.chemmater.8b02144 Chem. Mater. 2018, 30, 5534−5538
Communication
Chemistry of Materials
Figure 3. (a) Cycling stability test on 0.5 at.% Pt1/MnOx catalyst, (b) Arrhenius plots of WGS on 0.5 at. % Pt1/MnOx (black), Pt1/FeOx (red), Pt/ MnOx-Im (blue), and Pt/MnOx-R400 (pink) (conversion was controlled below 15% to stay in kinetic region), (c) reactivity of WGS vs Pt loading for Pt1/MnOx at T = 473 K.
catalysts and make them more practical but also sheds light on the fundamental knowledge in the atomically dispersed Pt catalysts.
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