In situ- generated metal oxide catalyst during CO oxidation reaction transformed from redox-active metal-organic framework-supported palladium nanoparticles
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The preparation of redox-active metal-organic framework (ra-MOF)-supported Pd nanoparticles (NPs) via the redox couple-driven method is reported, which can yield unprotected metallic NPs at room temperature within 10 min without the use of reducing agents. The Pd@ra-MOF has been exploited as a precursor of an active catalyst for CO oxidation. Under the CO oxidation reaction condition, Pd@ra-MOF is transformed into a PdOx-NiOy/C nanocomposite to generate catalytically active species in situ, and the resultant nanocatalyst shows sustainable activity through synergistic stabilization.
Keywordsredox reaction metal-organic framework CO oxidation metal oxide nanoparticle palladium
Metal-organic frameworks (MOFs) constitute an important class of porous crystalline materials. Modular synthetic routes enable rational design of MOFs with precise control over pore size, connectivity, and functional groups. MOFs have widespread applications, including adsorption and separation[2, 3, 4], catalysis[5, 6, 7, 8], and sensing[9, 10]. In particular, MOFs are highly preferred for catalytic applications because of their large surface area and pore volume, tunable pore size and shape, and flexibility for diverse functionalization. As such, MOF-based catalysis has recently emerged as a burgeoning subfield in heterogeneous catalysis[2, 3, 4]. The active metal sites and/or reactive organic groups that constitute the frameworks of MOFs endow the catalytic functions of the MOFs. In addition, catalytic metal nanoparticles (NPs) incorporated into the cavities of MOFs can also provide catalytic function. The incorporation of metal NPs into porous supports, such as zeolites, mesoporous materials, and MOF, can be achieved by various methods, including solution impregnation[11, 12, 13], chemical vapor deposition[14, 15, 16], and solid grinding. Although these methods have long been useful for generating heterogeneous catalysts, the high-temperature heating steps involved in these methods inevitably yield a wide distribution of particle sizes[18, 19]. Another prominent route to the supported catalysts is by a colloidal deposition method, where pre-synthesized, highly monodisperse colloidal NPs are deposited onto the supports[20, 21]. However, for full catalytic utilization of NP surfaces, this method requires judicious thermal or chemical treatments that can remove surface capping polymers or surfactants that stabilize colloidal NPs.
Recently, the conversion of the MOF into metal oxide nanoparticles has been used as a new strategy for preparing nanoscale functional entities. In this method, the secondary building units of MOF that are mostly composed of metal oxide clusters in angstrom scale were transformed into metal oxide nanomaterials by thermal treatments. For instance, Xu et al. reported the synthesis of Co3O4 nanoparticles converted from cobalt oxide subunits in a cobalt-based MOF, Co3(NDC)3(DMF)4 (NDC = 2,6-naphthalene-dicarboxylate; DMF = N,N’-dimethyl formamide) by pyrolysis in air. The resulting cobalt oxide nanoparticles were utilized for an electrode material for lithium-ion batteries. More recently, MOF-5 was treated at high temperature (over 600°C) under various atmospheric conditions to produce ZnO nanoparticles and ZnO@C hybrid composites. Thermal treatment of MOF-5 under nitrogen generated ZnO@C, whereas MOF-5 heated under air yielded a pure ZnO nanoparticle, indicating the combustion of organic ligands.
Materials and characterization of samples
All chemicals and solvents used in the syntheses were of reagent grade and they were used without further purification. Infrared spectra were recorded with a Thermo Fisher Scientific Nicolet 6700 FT-IR spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). Elemental analyses were performed at the UNIST Central Research Facilities Center in Ulsan National Institute of Science and Technology. Palladium content on ra-MOF was analyzed with an inductively coupled plasma optical emission spectrometer (Varian 720-ES, Varian Inc., Palo Alto, CA, USA). X-ray photoelectron spectroscopy was performed using a Thermo Scientific K-Alpha XPS spectrometer. X-ray diffraction (XRD) patterns were recorded with a Rigaku D/MAZX 2500 V PC diffractometer (Rigaku Corporation, Tokyo, Japan) at 40 kV and 100 mA with Cu-Kα radiations (1.54059 Å) with a scan speed of 2°/min and a step size of 0.02° in 2θ at room temperature. JEOL JEM-2100 F transmission electron microscope (JEOL Ltd., Akishima, Tokyo, Japan) and an Oxford INCA EDS unit (Oxford Instruments, Abingdon, Oxfordshire, UK) were used to examine the morphology of nanostructured catalysts before and after catalytic reactions. Thermogravimetric analysis (TGA) was performed under N2(g) atmosphere at a scan rate of 5°C/min using Q50 from TA instruments (New Castle, DE, USA). N2 sorption isotherms of ra-MOF and Pd@ra-MOF were obtained by BELSORP-max (BEL Japan Inc., Osaka, Japan) at 77 K to estimate the specific Brunauer-Emmett-Teller (BET) surface areas.
Preparation of ra-MOF
[Ni(C10H26N6)](ClO4)2 (C10H26N6 = LCH3) was prepared according to the preparation conditions in a previous report. [NiLCH3(bpdc) (ra-MOF; bpdc2− = 4,4′-biphenyldicarboxylate) was synthesized by the modified method from the previous reports. Synthetic detail is as follows: (NiLCH3)(ClO4)2 (0.80 g, 1.64 mmol) was dissolved in water/pyridine (48 mL, 2:1 v/v), and an aqueous solution (16 mL) of Na2bpdc (0.56 g, 2.10 mmol) was added. The solution was stirred over 20 min at room temperature, and 150 mL of methanol was added to it. The mixture was stirred for 6 h, forming pale purple microcrystalline precipitates which were isolated by filtration, washed with methanol, and dried in air. The as-prepared metal-organic framework was desolvated at 120°C under vacuum for 3 h resulting in a purple color. The yield is 57%.
Preparation of the Pd@ra-MOF
The desolvated solid (0.43 g, 0.81 mmol) was immersed in 3.12 × 10−2 M acetonitrile solution (82 mL) of Pd(NO3)2· × H2O at room temperature and hand-shaken for 10 min, which resulted in a light brown solid. The resulting light brown powder was isolated by filtration, washed with acetonitrile, and dried in air.
Catalytic activity test
The catalytic tests for CO oxidation were performed in a fixed bed reactor at atmospheric pressure, containing 0.06 g of catalyst samples. A feed mixture, prepared using mass flow controllers (MKS Instruments, Inc., Wilmington, MA, USA), contained 3.0% CO and 8.5% O2 and was balanced with He. The total flow rate of the feed mixture was 52 mL min−1, and the gas hourly space velocity was 1,7316 h−1. The effluent gas stream from the reactor was analyzed online by the thermal conductivity detector of parallel gas chromatography (Younglin Instrument Co., Ltd, Anyang, Korea) with a Carboxen 1000 column. In order to determine the conversion, the products were collected during 40 min of steady-state operation at each temperature. The empty reactor (without catalyst) showed no activity under identical conditions.
Results and discussion
[NiLCH3(bpdc), which is composed of Ni(II) hexaaza macrocycle and carboxylate and has one-dimensional channels with a 7.3-Å pore opening (Additional file 1:Figure S1), was selected as the ra-MOF. It has previously been shown that the MOF constructed by Ni(II) macrocyclic complexes and multidentate carboxylate ligands exhibits redox-active properties that originate from the six-coordinated Ni(II) sites[24, 25, 26]. The ra-MOF, [NiLCH3(bpdc) (bpdc = 4,4′-biphenyldicarboxylate), was prepared by the self-assembly of [NiLCH3(ClO4)2 and Na2bpdc in a H2O/pyridine mixture, yielding [(NiLCH3)3(bpdc)3·2pyridine·6H2O, and it was subsequently activated at 120°C under vacuum for 3 h. Pd NPs were spontaneously formed in the ra-MOF solid - without the use of any reducing agent - by soaking the ra-MOF in an acetonitrile solution of Pd(NO3)2 at room temperature for 10 min. Because the oxidation potential of Ni(II) to Ni(III) in the monomacrocyclic complexes ranges from 0.90 to 0.93 V, the redox reaction of a ra-MOF possessing Ni(II) macrocyclic complexes with Pd ions leads to the simultaneous oxidation of Ni(II) to Ni(III) and the reduction of Pd(II) ions to the metallic Pd NPs.
A similar phenomenon occurred in the Pd@ra-MOF catalyst during CO oxidation. The XRD pattern of the catalyst after five consecutive CO oxidation reactions up to 300°C (Figure4 (b)) indicated the formation of NiO and PdO as well as the presence of pre-existing Pd(0) species. The NiO species were formed by the thermal transformation of ra-MOF (as described above), and the PdO NPs were generated by the oxidation of metallic palladium during the reaction. The XRD peaks for NiO and PdO species were broad, and the crystalline sizes determined by the Scherrer equation were 8.9, 9.9, and 10.3 nm for NiO, PdO, and Pd, respectively. The TEM observation of Pd@ra-MOF before and after the CO oxidation (Figure4 (d)) also confirmed the formation of spherical NiO and PdO nanoparticles. Contrary to the complete decomposition of ra-MOF at 500°C, the organic ligands, bpdc2− in Pd@ra-MOF, were partially decomposed to carbogenic supports and partially intact at a relatively low temperature, 300°C as an IR spectrum showed peaks for νO-C=O at 1,593(s) cm−1 and νC=C(aromatic) at 1,528(s) cm−1, respectively (Additional file 1:Figure S7). As shown in Figure4 (e), EDS mapping of the PdOx-NiOy/C catalyst after CO oxidation indicated that the PdOx and NiOy NPs were well dispersed on the carbogenic support. The morphology of the PdOx NPs was maintained after repeated CO oxidation runs, as evidenced by TEM images (Figure4 (d), right).
This might be attributed to the immobilization of the PdOx and NiOy NPs by carboxylate ligands coexisted on the carbogenic support and the consequent suppression of migration and aggregation of NPs, even under the harsh CO oxidation reaction condition.
Changes in the chemical states of Ni and Pd species in the Pd@ra-MOF before and after the catalytic reaction were monitored by XPS. Before the CO oxidation reaction, Pd@ra-MOF reasonably contained N-coordinating Ni(III) species (855.4 eV) as well as Ni(II) (854.7 eV), which were generated by the redox reaction with Pd(II) (Figure5 (c)). However, after the CO oxidation reaction, Ni2O3 (855.9 eV) and NiO (854.1 eV) were formed by the thermal transformation of Ni(II/III) macrocyclic complexes, similar to the case of the as-prepared ra-MOF catalyst (Figure5 (d)). Before the Pd@ra-MOF underwent a catalytic reaction, the small-sized Pd0 clusters (337.7 eV), which were strongly interacting with aromatic ligands of the ra-MOF, coexisted with unreduced Pd(NO3)2 species (338.3 eV) (Figure5 (e)). After the reaction, most of the Pd species were oxidized to PdO (336.9 eV) under a highly oxidative reaction condition, while the rest remained in the reduced Pd0 state (335.3 eV). It is noteworthy that the binding energy of the latter was shifted to a lower energy level compared to that of Pd@ra-MOF before the catalytic reaction (Figure5 (f)). This shift was due to the enlargement of the Pd NPs from approximately 2 nm to approximately 10 nm, which was consistent with the XRD results (Figure4 (b)). Based on the XPS, XRD, and TEM results, the Pd@ra-MOF was transformed into the PdOx-NiOy/C nanocomposite (0 ≤ x ≤ 1, 1 ≤ y ≤ 3/2) catalyst during the catalytic reaction, where each species showed the synergetic catalytic effect to convert CO to CO2 at a very low temperature. In addition, the PdOx-NiOy/C catalyst after the fifth run showed a higher BET surface area (35 m2/g) than that of Pd@ra-MOF, which can result in improved catalytic performance by providing active sites for catalyzing surface reaction (Additional file 1:Figure S2). However, the surface area is still very low, which implies that the significant portion of active sites of PdO and NiO NPs can be buried and covered by carbon support. This might result in the loss of catalytic activity from the potentially expected.
The various metal oxides supporting metal NP catalysts have been investigated as CO oxidation reaction catalysts. Machida et al. reported the CO oxidation activity of metallic Pd NPs supported on CeO2, which was significantly enhanced by thermal aging of the catalyst. It was revealed that the thermal treatment caused the strong metal-support interaction via Pd-O-Ce bonding, which prevented the sintering of Pd oxide species at high temperature and promoted CO adsorption to react with oxygen. Haruta and other groups studied on supported gold catalysts on metal oxides such as TiO2, Fe2O3, Al2O3, CuO, La2O3 NiO, and Y2O3[34, 35, 36]. The activation of O2 molecules has been shown to occur at the perimeter between Au NPs and the metal oxide support, highlighting the importance of metal-metal oxide interface in promoting CO oxidation. In this study, the catalytically active species for the CO oxidation reaction, PdOx-NiOy/C nanocomposite, was generated in situ in the reaction environment, and its stable, sustainable activity under cycled conditions could be ascribed to the synergistic interaction among the PdO, NiO NPs, and carbogenic supports. In addition, compared to the conventional methods, our in-situ generation method of MOx@C catalysts has an advantage due to the simple preparation procedure which can provide a strong metal-metal oxide interaction as well as carbon-metal oxide interaction during the reaction. In general, MOx@C catalysts are synthesized by loading metal precursors on carbon supports and successive pyrolysis, which is mostly conducted at high temperature for a long time. In the present work, the spontaneously formed strong interactions not only provide the enhancement of catalytic properties, but also reduce mobility of metal precursors on the carbon and metal surfaces, which results in small and monodispersed metal oxide nanocrystals.
In conclusion, we reported the preparation of ra-MOF-supported Pd NPs, Pd@ra-MOF, via the redox couple-driven method, which yields unprotected metallic NPs at room temperature within a few minutes without the use of reducing agents. We found that during the CO oxidation reaction, the Pd@ra-MOF was transformed into a PdOx-NiOy/C nanocomposite that showed sustainable and enhanced catalytic activity through the synergistic stabilization of catalytically active PdO species. This study creates new opportunities for taking advantage of the MOF’s vulnerable point to develop a novel class of heterogeneous catalysts by utilization of MOFs or metal NPs@MOF as precursors.
This work was supported by the start-up grant of the UNIST (Ulsan National Institute of Science and Technology) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011–0004358 and 2012–0003813). SHJ is a TJ Park Junior Faculty Fellow supported by the POSCO TJ Park Foundation. JMK also thanks WCU (World Class University, R-31-2008-10029).
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