Zumin
Wang
a,
Lin
Gu
b,
Li
Song
c,
Hao
Wang
d and
Ranbo
Yu
*ad
aDepartment of Physical Chemistry, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China. E-mail: ranboyu@ustb.edu.cn; Fax: +86-10-62332525; Tel: +86-10-62332525
bInstitute of Physics, Chinese Academy of Sciences, Beijing 100190, China
cNational Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China
dCentre for Future Materials, University of Southern Queensland, Queensland, Australia
First published on 12th March 2018
Catalysts based on extremely small (single atoms or pseudo single atoms) noble metals can lead to much more efficient use through enhanced reactivity and selectivity. However, fabrication of practical and stable (pseudo) single-atom catalysts on various supports remains a significant challenge. Herein, we report a facile co-precipitation and conversion strategy to fabricate a series of gold pseudo single-atom supported metal organic framework catalysts (PSAC Au/MOF) on a large scale. By well controlled synthesis of precursors, up to 1.0 wt% surface clean Au atoms can be atomically dispersed and stabilized on the support. The PSAC Au/MOF exhibited superior catalytic activity in hydrogenation of p-nitrophenol, exceeding the turnover frequency of most reported catalysts by more than one order of magnitude. No obvious decline in activity was observed after 5 cycles.
Recent developments of SACs have attracted extensive attention both experimentally3–6 and theoretically.7–9 However, two major challenges still remain in the field of SACs: (i) to prepare them with a loading content high enough for practical applications; (ii) to keep the individual metal sites from sintering/reunion under catalytic conditions. A possible effective solution to resolve these problems is to prepare pseudo single atom catalysts (PSACs) instead of SACs. PSACs consist of atomically dispersed single atoms and slightly clustered ones (like dimers, and trimers). As a result, PSACs can contain a higher metal loading density, which is enough for practical applications, while still maintaining the majority of metal species in single atom form to avoid the formation of NPs. It is a compromise between size and loading amount, which comes to a harmonious balance in PSACs. So compared with SACs, PSACs may show similar catalytic performance but more potential for promising practical industrial applications.
On the other hand, for both SACs and PSACs, the supports, which offer many anchoring sites to stably bind the metal through strong interactions for atomically dispersed catalysts, are deliberately chosen. However there are only a few known examples of supported single-atom catalysts and pseudo single atom catalysts so far, and most of them have focused on supporting single-atoms on oxides,1,5,6,9,10 zeolites11 or carbon based materials.2,3,12,13 This very limited selection of supported materials presents a significant obstacle to the application of SACs and PSACs for specific reactions. There is thereby an urgent need but it is still a significant challenge to prepare SACs and PSACs containing a larger amount of single atom active sites with greater scope of typical supports.
Currently, metal−organic frameworks (MOFs) have been emerging as a class of promising multifunctional materials owing to their nanosized channels and cavities, ultrahigh surface area, chemical tunability and structure flexibility.14 Compared with other porous materials, MOFs show tremendous advantages as platforms of supported catalysts.15–18 To name a few, loading metal species into the pores of MOFs is expected to control the migration and growth of active sites in the confined cavities and produce stable catalytic active centers, which could further increase their catalytic activities and stabilities.19,20 By incorporating functional moieties with high affinities for certain molecules, modifying the pore chemistry can be easily achieved.21,22 Thus, the selectivity of reactant adsorption and product or intermediate desorption can be tuned.23,24 Moreover, the three-dimensional topological pore structures and interactions of metal sites make MOFs possess unique advantages in terms of chemoselectivity, especially for shape selective catalysis.25–27 The introduction of MOFs into the field of SACs may open the door to effective selectivity regulators for a wide variety of important yet challenging transformations, which paves the way for future research.
Moreover, although various synthetic protocols have been applied to prepare SACs and PSACs, such as mass-selected soft landing,28 high temperature vapor transport,10 atomic layer deposition13 and combustion/pyrolysis synthesis,2 they are far from satisfactory due to two problems: (1) too low loadings limit overall performance; and (2) too complex steps and professional equipment are hard to achieve. Thus, it still remains a big challenge to develop an easy method to construct SACs/PSACs with a loading content high enough for practical applications. Herein, we report a room-temperature co-precipitation and conversion strategy to fabricate a high yield, atomically dispersed Au catalyst (pseudo single atom Au/MOF) on a series of conventional MOFs with Au loading up to about 1.0 wt%. This unique method demonstrates its effectiveness for producing ultra-small atoms on MOF supports and provides a powerful route to create clean and neat active surfaces for both metal and support without further treatment. The PSACs exhibit extremely high catalytic activities and stabilities in hydrogenation of p-nitrophenol. A turnover frequency (TOF) greater than that of surface Au atoms on reported analogous catalysts by a factor of >55 was demonstrated on the PSACs in the hydrogenation of p-nitrophenol at room temperature. And no decay in the catalytic activity was observed during catalysis.
Scheme 1 Schematic illustration of the preparation procedures and plausible mechanism of the formation of PSAC Au/MOF. |
It should be noted that the whole process was continuous and tandem, and carried through in one reactor. We describe this process in three divided steps only to clearly define the purpose of each operation. Moreover, as shown in Fig. S1 (ESI†), this approach is very productive (gram level production per synthetic batch) and has a good yield rate (>62%). Notably, our clean and fast method can easily realize the above procedure on a large scale, just with larger volumes of the solutions being used. The other conditions were the same as in the above procedures. And it is universal for many other Au/MOF catalysts (namely, CoBTC, NiBTC, CuBTC, as describe in the ESI†). It is a promising synthesis route due to its fast kinetics, mild conditions, high space–time–yields and low waste generation, which makes it a remarkably accessible, affordable and environmentally-friendly process. A patent application has been submitted for the synthesis method for catalyst preparation (the application number is CN2018102016905).
The scanning electron microscopy (SEM) images in Fig. S2a (ESI†) show that the as-obtained Au/MOF hybrids exhibited nanosized (∼250 nm) crystal aggregates with no residual crystals of precursors or impurities observed. The TEM image (Fig. S2, ESI†) also showed the typical MOF morphology; no Au NPs beneath the loose structure can be observed by their deeper contrast from MOFs. So to verify the existence of Au atoms, high-resolution high-angle annular dark-field (HAADF-STEM) imaging (Fig. 1a) and EDS element mapping (Fig. 1b) have been carried out, which revealed that single or pseudo single Au atoms were evenly dispersed in the sample. The PXRD patterns of the materials (Fig. 1c) matched well with the simulated result, indicating the lack of the presence of any other phrases, and the framework structure was retained upon the loading of Au species. On the other hand, peaks due to Au NPs at 38.27°, 44.60° and 64.68° were also not detected, indicating the tiny size of the Au species. The permanent porosity of all these samples is preserved as confirmed by measurement of the N2 gas-adsorption isotherm (Fig. 1d), which exhibits a Type I behavior similar to the reported results. The corresponding Langmuir surface areas of the MOF and Au/MOF were calculated to be 723 and 500 m2 g−1, respectively; values that are also similar to those found in previous reports.35 ICP-AES revealed that the amounts of Au for all samples were found to be around 1.0 wt%. To further clarify the existing state of Au species in the sample, we performed X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectrometry (Fig. 2a and b), considering the poor resolution of X-ray photoelectron spectroscopy (XPS) signals for the detection of a small amount of rather tiny Au species (Fig. S3, ESI†).37 For comparison, an Au nanoparticle supported catalyst was also synthesized (described in the ESI†) and tested. There was only one notable peak (with a certain degree of separation as a result of the relativistic effect caused potential well distortion) in the region 2 to 3 Å from the Au–Au contribution, and no peak in the region before and after the Au–Au bond from the Au–Fe, Au–C, and Au–Cl contribution, confirming the absence of any other atoms around the dispersed Au atoms in the samples. The occurrence of Au–Au long coordination at a distance of about 2.57 Å may suggest the presence of loosely clustered Au single atoms as observed in the HAADF-STEM images. Obviously, the shapes of the Au peaks of the PSAC and NP supported one were quite different, suggesting the difference in their coordination environment and electronic state distribution. Therefore, the structure and behaviour of the as-prepared PSACs, in essence, resembles those of catalysts with metal atoms in totally isolated single form, and therefore they can be considered as PSACs. Moreover, Fig. 2b demonstrated that the white-line intensity of the XANES spectra of the pseudo single atom and nanoparticle supported catalysts were almost exactly the same. Both of them matched well with Au foil (only very slightly higher), suggesting that the Au species were in a zero valent state and consisted of uncharged Au atoms. The similarity between the Au foil and our sample strongly suggested that the Au atoms were surface naked and in pure metallic form.
Fig. 1 (a) High-resolution high-angle annular dark-field (HAADF) STEM image; (b) EDS elemental mapping; (c) XRD; and (d) N2 gas-adsorption isotherm of the as-prepared PSAC Au/FeBTC. |
Fig. 2 (a) FT-EXAFS spectra of Au/MOF and bulk Au foil at the Au K-edge, showing the surrounding atoms adjacent to Au atoms; (b) normalized XANES spectra at the Au LIII-edge of the Au/MOF samples. |
TG-DTA analysis (Fig. S4, ESI†) indicated that the product was thermally stable until nearly 320 °C. And the product obtained from this synthesis process was extremely clean with no mass loss before the decomposition temperature, owning to the absence of high boiling point organic solvents during all the synthesis procedures. This meant that there were no cumbersome and repeated solvent exchanges or calcination procedures necessary to remove solvents and unreacted materials and activate internal surfaces and tunnels.
The conversion of layered hydroxides into MOF based materials remains a promising synthesis route due to its fast kinetics, mild conditions, high space–time–yields and low waste generation, which makes it a remarkably accessible, affordable and environmentally-friendly process. The most exciting thing is that the obtained hybrid materials possessed a clean porous scaffold and bare active sites (ultra small pseudo single Au atoms) via an in situ redox reaction, which made them ideal choices for catalytic reactions.
In order to study the activities of all catalysts, the absorption effect of catalysts and non-catalytic reactions should be eliminated. Therefore, a set of control experiments were conducted. First of all, an experiment with all the same parameters except that no catalyst but 20 μL of water was added instead was carried out. As shown in Fig. S4 (ESI†), no obvious changes in the respective absorbance could be detected for as long as 2 h. So we could safely conclude that the non-catalytic hydrogenation reaction was rather slow and it could be neglected compared with the fast catalytic reaction. Secondly, a series of experiments was devised to eliminate the pure adsorption of the catalysts. So p-nitrophenol solution and catalyst solution were mixed without a hydrogen source (NaBH4), and NaOH was added to maintain the same alkalinity conditions. From the data observed (Fig. S5, ESI†), despite the ultra-large surface area of all the catalysts, the velocities of adsorption were at least two orders of magnitude lower than those of the catalytic reactions. So we can safely conclude that the observed fast decrease of the absorption peaks resulted mainly from the catalytic reaction triggered by our catalysts.
To obtain the activation energy (Ea) of reduction of p-nitrophenol catalyzed by PSACs Au/FeBTC, the catalytic experiments were carried out at temperatures ranging from 0 to 60 °C (Fig. S8, ESI†), and Ea was determined to be 15.9 kJ mol−1, which is among the best results of the reported values.38,39 In order to evaluate the basic properties of the catalysts, not only the kinetic rate constant but also the turnover frequency (TOF) was studied. The rate constant of PSACs Au/FeBTC hybrid was 2.49 min−1 which was among the highest values ever reported under the exact same conditions (at 25 °C). And the approximate turnover frequency (TOF),40 defined as moles of p-aminophenol molecules generated per moles of noble metal atoms per minute (as described by eqn (1)) during the first 2 min (conversion reached >90%), has been used to compare the catalytic activities of the samples.
(1) |
Additional comparison about the catalytic activity has been made between the present PSACs Au/MOF and other reported catalyst systems for the reduction of 4-NP (Table S1, ESI†). Compared with other traditional noble metal supported catalysts, the TOF of the metal/MOF hybrid catalyst was 581 min−1, which varied from hundreds to thousands of times the reported results. The detailed comparison is listed in Table S1 (ESI†). We believed that the strong synergistic interactions between the Au atoms and the MOF support were significantly important in achieving superior activity.
In order to demonstrate superiority of the fast conversion and in situ redox approach, we compared our samples with those synthesized by using traditional pre-synthesis and post-synthesis methods (see detailed information of those samples in Fig. S7 and S8, ESI†). On one hand, in terms of pre-synthesized Au NP-loaded MOF catalysts, it is always necessary to use surface protecting agents (or so-called stabilizers) to reduce the Au particle size and to prevent NPs aggregation. The presence of stabilizer (such as PolyVinyl Pyrrolidone or PVP and Sulfur Alcohol) inhibits the active sites of the metal NPs, resulting in low activity of the catalysts.41 On the other hand, in terms of post-synthesized Au NPs in and on MOF catalysts, the formation of Au NPs within the MOF matrix causes structure destruction, and the uncontrollability of Au nucleation and growth results in a larger average size and broader size distribution of Au NPs, which jeopardizes the catalytic performance. As for our in situ redox Au atoms, because of the clean surfaces and the strong binding force between the active sites and supports, no surface reconstruction was needed, and thus no induction time was observed (Table 1).38
Noble metals loaded catalysts | T (K) | k (min−1) | TOF (min−1) | t ind (min) |
---|---|---|---|---|
a Traditional pre-synthesis Au stabilized with PVP. b Traditional pre-synthesis Au NPs stabilized with sulfur alcohol.46 c Post-synthesized Au NPs in and on MOF catalysts. | ||||
Catalyst as-prepared | 298 | 2.49 | 581 | 0 |
Catalyst method 1a | 298 | 0.0014 | 5.6 | 6.1 |
Catalyst method 2b | 298 | 0.0051 | 10.5 | 2.3 |
Catalyst method 3c | 298 | 0.638 | 163 | 1.5 |
Finally, stability tests were carried out in terms of durability in five cyclic usages. As shown in Fig. 3d, there was a slight decrease of activity after five cycles, but the conversion rate within two minutes was not changed. Additionally, after 10 cycles of catalytic reaction, the catalyst kept its original morphology and activity (Fig. S9, ESI†), indicating its excellent stability and long catalytic lifetime. Obviously, the as-synthesized Au/MOFs catalysts have shown excellent catalytic performance, for the following reasons:
(1) First, this novel synthetic process without using high-temperature solvent leads to clean MOFs with high nanoporosity and open pore network, which permits full accessibility and fast molecule diffusion of 4-NP as well as the products on and off the Au surface.
(2) Second, MOFs offer preferable specific adsorption for the enrichment of π-rich 4-NP from solution via π–π stacking interaction,42,43 leading to a high local concentration of reactants.
(3) Third, the pseudo single Au atoms on the MOF hybrid catalysts were synthesized in situ without any stabilizer or surfactant; therefore, the unsaturated surface atoms of Au species may coordinate in a better manner and adsorb 4-NP, thus speeding up the reaction.42,44,45 This well-studied reaction can be modelled by classical Langmuir–Hinshelwood kinetics, in which the surface coverage of metals by adsorbed reactants influenced the total activity greatly.38 So a clean and neat metal surface is the key to improvement of catalytic performance.
(4) Finally, the porous surface structure of the MOF may offer steric restriction to confine and effectively suppress the aggregation of Au atoms and their leaching and dissolution into a reaction mixture.
This unusual atomic constitution of supported metals is suggestive of a new approach to prepare extremely efficient pseudo single-atom catalysts.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00081f |
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