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Co3O4 nanocomposites derived from MOFs for CO oxidation
Applied Catalysis A: General 487 (2014) 189–194
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High and stable catalytic activity of porous Ag/Co3 O4 nanocomposites derived from MOFs for CO oxidation Shouxin Bao a , Nan Yan a , Xiaohui Shi a , Ren Li a , Qianwang Chen a,b,∗ a Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science & Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China b High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Science, Hefei 230026, China
a r t i c l e
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Article history: Received 6 June 2014 Received in revised form 8 September 2014 Accepted 10 September 2014 Available online 20 September 2014 Keywords: Self-reduction Nanojunctions Synergetic effect Catalysis
a b s t r a c t High-dispersed, well-knit and porous Ag/Co3 O4 nanocomposites are prepared via a novel “in situ selfreduction” route, calcining Ag3 [Co(CN)6 ] nanoparticles at different temperatures in air. All samples exhibit an excellent catalytic performance for CO oxidation due to the synergetic effect of nanojunctions between Ag and Co3 O4 nanoparticles. The sample prepared at 200 ◦ C possesses Ag/Co3 O4 nanojunctions at a maximum with the smallest particle size of Ag, displaying the best catalytic activity and excellent stability with a complete CO conversion temperature (T100 ) of 100 ◦ C. The activity was maintained for more than 18 h and even after the catalyst was exposed to air for two months. Without any pretreatment, Ag/Co3 O4 catalysts prepared in this paper show super catalytic performance compared with other oxide supported-Ag catalysts reported in recent years. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the preparation of supported noble-metal composites is constantly explored to develop catalysts with an enhanced activity for catalytic oxidation of CO to CO2 [1–8]. It is found that the catalytic activities of supported noble-metal catalysts are higher than those of unsupported ones. For example, Sandoval et al. [4] prepared Au–Ag bimetallic catalysts supported on TiO2 by sequential deposition–precipitation method. The composite catalyst gave the highest activity of 100% CO conversion at the temperature lower than 100 ◦ C. Pt–Cu/Al2 O3 catalysts with a low Pt loading showed an excellent activity and selectivity (gave 100% conversion of CO to CO2 at about 120 ◦ C and maximum selectivity at 80 ◦ C) [7]. Chen et al. [3] loaded Pt and Pd on TiO2 , and both showed high catalytic activity for low-temperature CO oxidation. Oxide supports in the composites allow a high loading of highly dispersed metal particles and induce interactions between noble-metals and supports [1,9,10]. During the catalytic oxidation of CO, CO and O2 molecules are generally considered to be adsorbed on the metal particles and supports, respectively, and then react
∗ Corresponding author at: Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science & Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China. E-mail address: [email protected] (Q. Chen). http://dx.doi.org/10.1016/j.apcata.2014.09.015 0926-860X/© 2014 Elsevier B.V. All rights reserved.
at their interfaces to form CO2 [11,12]. Therefore, creating large amounts of such interfaces is very important for improving catalytic performance. To fabricate noble-metal supported catalysts with more effective interfaces, various methods have been proposed, such as impregnation [13], cation-exchange [14], mixing [11,15], combustion [16], simple solid-state reactions [12], supercritical fluid deposition [17,18], post-synthesis grafting [5,19], direct synthesis [9,20,21], coprecipitation [22–24], and deposition–precipitation [4,25]. Most of them are prepared via either mixing noble-metal compounds with different supports, then reducing to form metal particles, or first reducing then loading. However, catalysts prepared in this way suffer some shortcomings. First, and most importantly, metal particles and supports are not in close contact, which goes against with the redox taking place [8]. Besides, metal particles are apt to fall off from the support during ultrasonic washing [4] or aggregate after reaction, which will decrease the amount of interfaces. Second, noble-metal particles can only be loaded on the surface or several nanometers to the interior of the support [1,3,26]. Therefore, CO can only be oxidized on the surface of the catalyst. Third, a reducing agent is always used to reduce metal ions in most methods, which always exists as a resident in the catalyst and difficult to be removed completely. The residual reductant may decrease the catalytic activity by blocking some active sites or reacting with catalyst. Fourth, most conventional preparation methods are extremely complicated [26–29]. Some even need sophisticated equipments and harsh chemical environments [5].
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In this article, we aim to develop a novel approach to solve these problems. Ag catalyst, a relative cheap one in noble metals, has been used as catalysts in many reactions for years, such as 4-nitrophenol reduction [30], ethylene epoxidation [31] and methane oxidation [32]. Besides, it is also active for CO catalytic oxidation due to its good chemical adsorbability to O2 [33]. Co3 O4 has also known to be active in CO oxidation, in which Co3+ is more active than Co2+ , mainly because it can adsorb CO easily [34]. And the amount of Co3+ is more than that of Co2+ in Co3 O4 (the mole ratio is 2:1). So the obstacle of improving the catalytic performance of Co3 O4 is boosting the ability of adsorbing O2 . Therefore, creating junctions between Ag and Co3 O4 nanoparticles may generate enhanced performance in catalytic CO oxidation reaction. During the past decade, metal-organic frameworks (MOFs) have attracted much attention for its unique structure, containing metal ions linked by coordinated ligands into an infinite array. As a kind of MOFs, Prussian blue analogs (PBA), in which metallic ions are bridged by CN groups, are considered as a potential precursor for nanoporous metal oxides with high surface areas. What is more, different particles obtained in this way contact closely to form nanojunctions, which is in favor of the catalytic reaction. Very recently, several metal oxides have been successfully fabricated simply by calcination of PBA [35–38(a)], while no single metal has been prepared by this method. Herein, we demonstrate a novel “in situ self-reduction” route for the fabrication of a nanocomposite consisted of Ag/Co3 O4 nanojunctions without using any reductant. Previously, our group has developed a general way to prepare a series of PBA with geometrical morphologies and homogeneous dimensions [38]. As a type of PBA, Ag3 [Co(CN)6 ] was fabricated by mixing AgNO3 with Co3 [Co(CN)6 ]2 ·nH2 O dispersed in distilled water at room temperature. As precursors and templates, the truncated nanocubes’ geometrical shape and uniform size of Co3 [Co(CN)6 ]2 ·nH2 O maintained. Excitedly, our method provides a new way to the fabrication of PBA with designed structure and morphology. After calcining the uniform Ag3 [Co(CN)6 ] nanocubes at different temperatures in air, porous Ag/Co3 O4 nanocomposites with abundant well-knit Ag/Co3 O4 nanojunctions were obtained. The prepared samples exhibit excellent catalytic activity and stability for CO catalytic oxidation reaction. 2. Experimental 2.1. Preparation of Ag/Co3 O4 nanocomposites The Ag/Co3 O4 nanocomposites were synthesized by a novel “in situ self-reduction” route. All chemicals are of analytical grade and used without purification. The Co3 [Co(CN)6 ]2 ·nH2 O nanoparticles were synthesized following the previous reports of our group [39(a)]. 25 mg of as-prepared Co3 [Co(CN)6 ]2 ·nH2 O was dispersed in 20 mL distilled water under agitated stirring at room temperature. Then 20 mL aqueous solution of AgNO3 (60 mg) was added quickly. After 20 min, the reaction was aged at room temperature without any interruption for 24 h. The resulting milky precipitate was filtered and washed several times with distilled water and finally dried in air at 60 ◦ C. Thermal decomposition of Ag3 [Co(CN)6 ] nanocubes was proceed at 150 ◦ C for 240 h, 200 ◦ C for 48 h, 250 ◦ C for 8 h, 300 ◦ C for 2 h, 400 ◦ C for 1 h and designated as C150, C200, C250, C300, C400, respectively. 2.2. Characterization The powder X-ray diffraction (XRD) patterns were collected on a Japan Rigaku D/MAX-cA X-ray diffractometer equipped
with Cu K␣ radiation over the 2 range of 30–90◦ . Scanning electron microscopy (SEM) images were performed on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. The FT-IR spectrum was obtained using a Magna-IR 750 spectrometer in the range of 500–4000 cm−1 with a resolution of 4 cm−1 . Specific surface areas were calculated from the results of N2 physisorption at 77 K (Micromeritics ASAP 2020) by using the BET (Brunauer–Emmet–Teller) and BJH (Barrett–Joyner–Halenda) methods. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 transmission electron microscope, which was operated at 200 kV. The distribution of the elements was characterized using a scanning transmission electron microscopy (STEM, JEM 2100F) with energy dispersive X-ray (EDX). X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB-250 spectrometer (Thermo Corp.) equipped with an Al K␣ X-ray exciting source (1486.6 eV) operated at 195 W with a 14.8 kV acceleration voltage. 2.3. Catalytic performance measurements The activity of Ag/Co3 O4 nanocomposites for catalytic oxidation of CO was evaluated with a fixed-bed flow reactor. The weight of catalyst was 50 mg and the reaction gas consisting of 1% CO and 99% dry air was fed at a rate of 30 mL min−1 . The catalyst was heated to the desired reaction temperature at a rate of 2 ◦ C min−1 and then kept there for 20 min until the catalytic reaction reached a steady state. Then the composition of effluent gas was analyzed with an online GC-14B gas chromatograph. The conversion of CO was calculated from the change in CO concentration of the inlet and outlet gases. 3. Results and discussion 3.1. Preparation and characterization of Ag3 [Co(CN)6 ] Fig. 1a schematically illustrates the procedure for generating Ag/Co3 O4 nanocomposites. Firstly, Co3 [Co(CN)6 ]2 ·nH2 O nanoparticles were fabricated at room temperature following the previous method of our group [39(a)]. Then Ag3 [Co(CN)6 ] nanoparticles were prepared by substituting Ag+ for Co2+ in Co3 [Co(CN)6 ]2 ·nH2 O. Finally, heterojuncted Ag/Co3 O4 nanocomposites were produced by calcining Ag3 [Co(CN)6 ] at different temperatures in air. During calcinations, large amounts of Ag/Co3 O4 nanojunctions were generated and dispersed uniformly to form a porous nanocomposite due to gas (COx and NOx ) release. This structure allows small reactive molecules such as CO, O2 to get into the inner channels of catalyst, resulting in the catalytic reaction occurring on both the exterior and interior surfaces. The crystalline nature and microstructure of the as-prepared Ag3 [Co(CN)6 ] are shown in Fig. S1. Ag3 [Co(CN)6 ] keeps the cube shape and dispersibility after substituting Ag+ for Co2+ in Co3 [Co(CN)6 ]2 ·nH2 O. But the surfaces become rough and the interior become porous after calcination, which is attributed to the evaporation of generated gases. 3.2. Characterization of Ag/Co3 O4 nanocomposites The crystallographic structure of the Ag/Co3 O4 nanocomposites was analyzed by X-ray powder diffraction (XRD). As shown in Fig. 2, all of the peaks correspond well to face-centered-cubic Ag (JCPDS card no. 89-3722). C150 to C400 were prepared at 150 ◦ C for 240 h, 200 ◦ C for 48 h, 250 ◦ C for 8 h, 300 ◦ C for 2 h, 400 ◦ C for 1 h, respectively. Along with the increasing of calcination temperature from
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Fig. 1. (a) Schematic illustration of the formation process of Ag/Co3 O4 nanocomposites consist of Ag/Co3 O4 junctions, (b) Adsorption and reaction of O2 and CO molecules on the surface of a nanojunction between Ag and Co3 O4 nanoparticle.
Fig. 3. HRTEM image of C200.
Fig. 2. XRD patterns of Ag/Co3 O4 nanocomposites C150–C400.
200 ◦ C to 400 ◦ C, a sharpening of the peaks is observed, indicating the improvement of crystallinity. Peaks at 32.3◦ are hard to be identified. To understand the influence of calcination temperature on the growth of crystallite size, the mean crystallite sizes of Ag are estimated from the (1 1 1) peak with the Scherrer formula, D = 0.89/(B cos ) (D, average dimension of crystallites; , the X-ray wavelength; B, the pure diffraction broadening of a peak at half-height; , the Bragg angel). The sizes from C150 to C400 are 10.9, 6.9, 10.3, 24.2 and 28.6 nm, respectively (the instrumental error has been deducted). As can be seen, C200 has the smallest crystallite size of Ag resulting in the best catalytic performance for CO oxidation, which will be discussed in the following. However, C150 prepared at a lower temperature has a larger crystallite size. Such phenomenon is due to the long calcination time (240 h) during which small particles aggregate into large particles via Ostwald ripening [39(a)]. The FT-IR spectrum demonstrates the generation of Co3 O4 (Fig. S2). Two strong bands at 670 and 587 cm−1 associated to the divalent cobalt ions in octahedral and trivalent cobalt in tetrahedral sites, respectively [40]. The absence of bands at 2170 cm−1 related to CN [38(b)] in C150 to C400 illustrates the complete decomposition of Ag3 [Co(CN)6 ]. The band at 1380 cm−1 becomes weak as the temperature increasing, illustrating the existence of nitrate. It disappears in C250 could be due to the high temperature at local parts and a long time of calcination (8 h), for silver nitrate would decompose above 300 ◦ C [41]. The reasons are our speculation, which need further work to confirm, and not the focus of this paper. So we feel really sorry that we could not do more in-depth discussion. Fig. 3 shows the HRTEM image of heterojunctions of Ag and Co3 O4 . The measured interlayer distance of 0.235 nm is consistent with the (1 1 1) planes of Ag, while the interlayer distance of 0.156 nm and 0.288 nm agrees with the (5 1 1) and (2 2 0) planes of Co3 O4 , respectively. The interlaced boundaries marked with red
circles demonstrate the generation of Ag/Co3 O4 nanojunctions. In the nanojunctions, Ag and Co3 O4 particles closely connect to each other on a nano-level. Such junctions could keep Ag nanoparticles from falling off from the Co3 O4 support while ultrasonic cleaning and aggregation during the catalytic reaction. Crystallite size distribution of Ag from HRTEM in C200 is also calculated. As shown in Fig. S6, crystallite size of Ag ranges from 3 to 7 nm mainly with an average size 5.0 nm. The result also illustrates the good homodispersity of Ag in the catalyst. XPS characterization was performed to investigate the surface composition and chemical state of the Ag/Co3 O4 nanocomposites (Fig. 4). For C200 and C400, two peaks of each sample are similar, centered at 780.0 and 795.0 eV, corresponding to the Co 2p3/2 and Co 2p1/2 . The gap between the peaks is about 15 eV (spin orbit splitting), which corresponds to the standard Co3 O4 spectra [39(a and b)]. The almost complete absence of the Co2+ shake-up satellites (at 787.0 and 804.0 eV) confirms generation of Co3 O4 rather than CoO [39(c)]. The core level binding energy of Ag3d5/2 in C200 and C400 are 368.9 and 368.8 eV, respectively, which are both higher than that of bulk metallic Ag (368.1 ± 0.1 eV). Binding energy of Ag3d3/2 in C200 and C400 are 374.9 and 374.8 eV, respectively, and are also higher than that of bulk metallic Ag (374.2 ± 0.1 eV) [42]. Such a positive shift is attributed to the Ag/Co3 O4 junctions (shown in Fig. 3), in which electrons transfer from Ag particles to Co3 O4 supports. C200 with the smaller particle size shows higher binding energy compared with C400. The positive shift of binding energy with the decrease of Ag size has also been observed by other groups [43,44], and it was often interpreted in terms of final state effects, which arise from screening of the core level hole created by photoemission. For example, electrons relax to screen the hole, resulting in a photo-emitted electron with a higher kinetic energy and therefore lower binding energy. For smaller clusters, the positive hole can be less efficiently screened, causing a positive core level shift. Dark-field STEM and EDX mapping of a particle (Fig. 5) show the spatial distributions of Ag, Co and O in a nanoparticle of C200. All the elements distribute dispersedly without any aggregation, which further suggests the presence of large amounts of
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Fig. 6. Conversion as a function of temperature for CO oxidation over Ag/Co3 O4 nanocomposites. Table 1 Comparison of the activity for CO oxidation on different oxide supported Ag catalysts.
Fig. 4. XPS spectra of as-prepared Ag/Co3 O4 nanocomposites: (a) Ag (3d) binding energy spectrum, (b) Co (2p) binding energy spectrum.
Ag/Co3 O4 junctions. The uniform distributions of Ag and Co3 O4 demonstrate the highly dispersed Ag/Co3 O4 junctions, which make sure the catalytic reaction take place throughout the porous nanocomposites. 3.3. Catalytic performance To study the performance of Ag/Co3 O4 nanocomposites prepared at different temperatures, CO oxidation as a typical probe reaction was carried out to test the catalytic activity. Fig. 6 shows the conversion of CO as a function of reaction temperature for the prepared samples. T100 (temperature for CO 100% conversion) for C200 and other samples are 100 ◦ C and 120 ◦ C, respectively, and the complete conversion remains unchanged as the reaction temperature continues to rise. All the samples show high catalytic activity for oxidation of CO especially C200, which is more active than most supported Ag catalysts reported [4,5,9,10,12,15,16,19,25,43,45,46]. The highest activity of C200 is due to the Ag/Co3 O4 junctions at a
Catalysts
Pretreatment
T100 (◦ C)a
Year
Reference
Ag/CeO2 Ag/SiO2 Ag/TiO2 Ag/Al2 O3 Ag/TiO2 Ag/Fe2 O3 Ag/Co3 O4
– – H2 , 550 ◦ C H2 , 300 ◦ C H2 , 550 ◦ C O2 , 300 ◦ C – – – – –
275 160 330 150 250 275 120 100 120 120 120
2000 2009 2011 2011 2013 2013 2014 2014 2014 2014 2014
[16] [5] [4] [15] [52] [53] This work This work This work This work This work
C150 C200 C250 C300 C400
a Temperature for CO 100% conversion. – No pretreatment.
maximum with the smallest crystallite size of Ag. On the junctions, abundant O2 and CO are adsorbed on Ag and Co3 O4 nanoparticles (Fig. 1b), which make the catalytic reaction occur more easily compared with separate particles. Several oxide supported Ag catalysts have been listed in Table 1. Without any pretreatment, all the Ag/Co3 O4 nanocomposites prepared in this work show much higher activity than those pretreated ones. Significantly, the catalytic activity of C200 is very stable during the reaction time without any decline (Fig. 7a), while this phenomenon always appears in other supported Ag catalysts [10–12,45]. What is more, the high activity can be maintained even after being exposed to air for more than two months (Fig. 7b) and Ag remains in metallic form (Figs. S4 and S5). After catalysis, the core level binding energy
Fig. 5. (a) Dark-field STEM image of a nanoparticle in C200. (b–d) elemental mapping of the same nanoparticle, indicating spatial distribution of Ag (b), Co (c), O (d), respectively.
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Fig. 1b). Therefore, excellent catalytic performance will be achieved as more Ag/Co3 O4 nanojunctions and effective interfaces are generated. Herein, the homodispersed Ag particles in Co3 O4 of C200 with smallest particle size provide sufficient well-knit Ag/Co3 O4 nanojunctions for the reaction. Besides, the Ag/Co3 O4 nanocomposites are polyporous (Fig. S3) with a calculated BET surface area of 20.6 m2 g−1 , which illustrates that CO molecules were catalytically oxidized not only on exterior but also interior surfaces, as a result, display the best catalytic performance. 4. Conclusions In summary, we have successfully fabricated porous Ag/Co3 O4 nanocomposites by calcining Ag3 [Co(CN)6 ] nanoparticles at moderate temperatures in air. The as-prepared samples show high catalytic activity, especially C200 with a complete CO conversion temperature (T100 ) of 100 ◦ C. Moreover, it could maintain high catalytic performances even after 18 h and being exposed to air for two months. The presence of abundant and highly dispersed Ag/Co3 O4 junctions in the nanocomposites was suggested to be responsible for the excellent catalytic performance. On the junctions, O2 and CO adsorb on Ag and Co3 O4 , respectively and react with each other more easily. Moreover, Ag particle aggregation can be avoided during a catalytic reaction due to the highly dispersed Ag/Co3 O4 junctions, which results in excellent catalytic activity and stability. All these advantages are attributed to the simple “in situ self-reduction” route, which could serve as a general method for preparing other noble-metal supported catalysts. Acknowledgement Financial support from the National Natural Science Foundation of China (21271163, U1232211). Fig. 7. Stability of C200, reaction performance with time on stream (100 ◦ C, flow rate = 50 mL min−1 ), and catalytic activity of C200 exposed in air for more than two months.
of Ag3d5/2 changed from 368.9 eV to 368.7 eV in C200, which was still higher than that of bulk metallic Ag (368.3 eV). Such a minor negative shift could be due to the electron transfer between Ag and Co3 O4 . The excellent stability was attributed to the uniformly dispersed Ag/Co3 O4 junctions, which prevents Ag nanoparticles from aggregation and oxidation during reaction and storage. In the case of Co3 O4 -catalyzed CO oxidation, CO species are adsorbed on the surface-exposed Co3+ sites and react with weakly bound oxygen species such as superoxide ion (O2− ) to form CO2 [34]. The active oxygen species are formed on surface oxygen vacancies which are increased by pretreatment in a reducing or oxidizing atmosphere [34,47]. Ag is known for adsorbing and activating oxygen [4]. In the reaction of CO oxidation, an electron transfer from Ag to the anti-bonding orbital of the O2 molecule would weaken the O O bonding and form an O2− , thus improving the oxygen activation. With a neighboring adsorbed CO, the oxygen transfer reaction would then occur easily [45]. In addition, lattice oxygen is generally considered to participate in the reaction [12,34,48–50]. Based on these reasons, a possible mechanism is proposed: O2 is adsorbed on the surface of Ag particles. Some of them reacts with the neighbor CO adsorbed on Co3 O4 particles. Others turns into lattice oxygen by entering the oxygen vacancies in the adjacent Co3 O4 , and then reacts with the adsorbed CO on the surface. In any way, the CO oxidation is more likely to take place at the interface between Ag and Co3 O4 nanojunctions just as other metal-oxide [51] and bimetal [30] catalysts (as shown in
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
High and stable catalytic activity of porous Ag/Co3 O4 nanocomposites derived from MOFs for CO oxidation Shouxin Bao a , Nan Yan a , Xiaohui Shi a , Ren Li a , Qianwang Chen a,b,∗ a Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science & Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China b High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Science, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 6 June 2014 Received in revised form 8 September 2014 Accepted 10 September 2014 Available online 20 September 2014 Keywords: Self-reduction Nanojunctions Synergetic effect Catalysis
a b s t r a c t High-dispersed, well-knit and porous Ag/Co3 O4 nanocomposites are prepared via a novel “in situ selfreduction” route, calcining Ag3 [Co(CN)6 ] nanoparticles at different temperatures in air. All samples exhibit an excellent catalytic performance for CO oxidation due to the synergetic effect of nanojunctions between Ag and Co3 O4 nanoparticles. The sample prepared at 200 ◦ C possesses Ag/Co3 O4 nanojunctions at a maximum with the smallest particle size of Ag, displaying the best catalytic activity and excellent stability with a complete CO conversion temperature (T100 ) of 100 ◦ C. The activity was maintained for more than 18 h and even after the catalyst was exposed to air for two months. Without any pretreatment, Ag/Co3 O4 catalysts prepared in this paper show super catalytic performance compared with other oxide supported-Ag catalysts reported in recent years. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the preparation of supported noble-metal composites is constantly explored to develop catalysts with an enhanced activity for catalytic oxidation of CO to CO2 [1–8]. It is found that the catalytic activities of supported noble-metal catalysts are higher than those of unsupported ones. For example, Sandoval et al. [4] prepared Au–Ag bimetallic catalysts supported on TiO2 by sequential deposition–precipitation method. The composite catalyst gave the highest activity of 100% CO conversion at the temperature lower than 100 ◦ C. Pt–Cu/Al2 O3 catalysts with a low Pt loading showed an excellent activity and selectivity (gave 100% conversion of CO to CO2 at about 120 ◦ C and maximum selectivity at 80 ◦ C) [7]. Chen et al. [3] loaded Pt and Pd on TiO2 , and both showed high catalytic activity for low-temperature CO oxidation. Oxide supports in the composites allow a high loading of highly dispersed metal particles and induce interactions between noble-metals and supports [1,9,10]. During the catalytic oxidation of CO, CO and O2 molecules are generally considered to be adsorbed on the metal particles and supports, respectively, and then react
∗ Corresponding author at: Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science & Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China. E-mail address: [email protected] (Q. Chen). http://dx.doi.org/10.1016/j.apcata.2014.09.015 0926-860X/© 2014 Elsevier B.V. All rights reserved.
at their interfaces to form CO2 [11,12]. Therefore, creating large amounts of such interfaces is very important for improving catalytic performance. To fabricate noble-metal supported catalysts with more effective interfaces, various methods have been proposed, such as impregnation [13], cation-exchange [14], mixing [11,15], combustion [16], simple solid-state reactions [12], supercritical fluid deposition [17,18], post-synthesis grafting [5,19], direct synthesis [9,20,21], coprecipitation [22–24], and deposition–precipitation [4,25]. Most of them are prepared via either mixing noble-metal compounds with different supports, then reducing to form metal particles, or first reducing then loading. However, catalysts prepared in this way suffer some shortcomings. First, and most importantly, metal particles and supports are not in close contact, which goes against with the redox taking place [8]. Besides, metal particles are apt to fall off from the support during ultrasonic washing [4] or aggregate after reaction, which will decrease the amount of interfaces. Second, noble-metal particles can only be loaded on the surface or several nanometers to the interior of the support [1,3,26]. Therefore, CO can only be oxidized on the surface of the catalyst. Third, a reducing agent is always used to reduce metal ions in most methods, which always exists as a resident in the catalyst and difficult to be removed completely. The residual reductant may decrease the catalytic activity by blocking some active sites or reacting with catalyst. Fourth, most conventional preparation methods are extremely complicated [26–29]. Some even need sophisticated equipments and harsh chemical environments [5].
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In this article, we aim to develop a novel approach to solve these problems. Ag catalyst, a relative cheap one in noble metals, has been used as catalysts in many reactions for years, such as 4-nitrophenol reduction [30], ethylene epoxidation [31] and methane oxidation [32]. Besides, it is also active for CO catalytic oxidation due to its good chemical adsorbability to O2 [33]. Co3 O4 has also known to be active in CO oxidation, in which Co3+ is more active than Co2+ , mainly because it can adsorb CO easily [34]. And the amount of Co3+ is more than that of Co2+ in Co3 O4 (the mole ratio is 2:1). So the obstacle of improving the catalytic performance of Co3 O4 is boosting the ability of adsorbing O2 . Therefore, creating junctions between Ag and Co3 O4 nanoparticles may generate enhanced performance in catalytic CO oxidation reaction. During the past decade, metal-organic frameworks (MOFs) have attracted much attention for its unique structure, containing metal ions linked by coordinated ligands into an infinite array. As a kind of MOFs, Prussian blue analogs (PBA), in which metallic ions are bridged by CN groups, are considered as a potential precursor for nanoporous metal oxides with high surface areas. What is more, different particles obtained in this way contact closely to form nanojunctions, which is in favor of the catalytic reaction. Very recently, several metal oxides have been successfully fabricated simply by calcination of PBA [35–38(a)], while no single metal has been prepared by this method. Herein, we demonstrate a novel “in situ self-reduction” route for the fabrication of a nanocomposite consisted of Ag/Co3 O4 nanojunctions without using any reductant. Previously, our group has developed a general way to prepare a series of PBA with geometrical morphologies and homogeneous dimensions [38]. As a type of PBA, Ag3 [Co(CN)6 ] was fabricated by mixing AgNO3 with Co3 [Co(CN)6 ]2 ·nH2 O dispersed in distilled water at room temperature. As precursors and templates, the truncated nanocubes’ geometrical shape and uniform size of Co3 [Co(CN)6 ]2 ·nH2 O maintained. Excitedly, our method provides a new way to the fabrication of PBA with designed structure and morphology. After calcining the uniform Ag3 [Co(CN)6 ] nanocubes at different temperatures in air, porous Ag/Co3 O4 nanocomposites with abundant well-knit Ag/Co3 O4 nanojunctions were obtained. The prepared samples exhibit excellent catalytic activity and stability for CO catalytic oxidation reaction. 2. Experimental 2.1. Preparation of Ag/Co3 O4 nanocomposites The Ag/Co3 O4 nanocomposites were synthesized by a novel “in situ self-reduction” route. All chemicals are of analytical grade and used without purification. The Co3 [Co(CN)6 ]2 ·nH2 O nanoparticles were synthesized following the previous reports of our group [39(a)]. 25 mg of as-prepared Co3 [Co(CN)6 ]2 ·nH2 O was dispersed in 20 mL distilled water under agitated stirring at room temperature. Then 20 mL aqueous solution of AgNO3 (60 mg) was added quickly. After 20 min, the reaction was aged at room temperature without any interruption for 24 h. The resulting milky precipitate was filtered and washed several times with distilled water and finally dried in air at 60 ◦ C. Thermal decomposition of Ag3 [Co(CN)6 ] nanocubes was proceed at 150 ◦ C for 240 h, 200 ◦ C for 48 h, 250 ◦ C for 8 h, 300 ◦ C for 2 h, 400 ◦ C for 1 h and designated as C150, C200, C250, C300, C400, respectively. 2.2. Characterization The powder X-ray diffraction (XRD) patterns were collected on a Japan Rigaku D/MAX-cA X-ray diffractometer equipped
with Cu K␣ radiation over the 2 range of 30–90◦ . Scanning electron microscopy (SEM) images were performed on a JEOL JSM-6700 M scanning electron microscope. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. The FT-IR spectrum was obtained using a Magna-IR 750 spectrometer in the range of 500–4000 cm−1 with a resolution of 4 cm−1 . Specific surface areas were calculated from the results of N2 physisorption at 77 K (Micromeritics ASAP 2020) by using the BET (Brunauer–Emmet–Teller) and BJH (Barrett–Joyner–Halenda) methods. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEOL-2010 transmission electron microscope, which was operated at 200 kV. The distribution of the elements was characterized using a scanning transmission electron microscopy (STEM, JEM 2100F) with energy dispersive X-ray (EDX). X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB-250 spectrometer (Thermo Corp.) equipped with an Al K␣ X-ray exciting source (1486.6 eV) operated at 195 W with a 14.8 kV acceleration voltage. 2.3. Catalytic performance measurements The activity of Ag/Co3 O4 nanocomposites for catalytic oxidation of CO was evaluated with a fixed-bed flow reactor. The weight of catalyst was 50 mg and the reaction gas consisting of 1% CO and 99% dry air was fed at a rate of 30 mL min−1 . The catalyst was heated to the desired reaction temperature at a rate of 2 ◦ C min−1 and then kept there for 20 min until the catalytic reaction reached a steady state. Then the composition of effluent gas was analyzed with an online GC-14B gas chromatograph. The conversion of CO was calculated from the change in CO concentration of the inlet and outlet gases. 3. Results and discussion 3.1. Preparation and characterization of Ag3 [Co(CN)6 ] Fig. 1a schematically illustrates the procedure for generating Ag/Co3 O4 nanocomposites. Firstly, Co3 [Co(CN)6 ]2 ·nH2 O nanoparticles were fabricated at room temperature following the previous method of our group [39(a)]. Then Ag3 [Co(CN)6 ] nanoparticles were prepared by substituting Ag+ for Co2+ in Co3 [Co(CN)6 ]2 ·nH2 O. Finally, heterojuncted Ag/Co3 O4 nanocomposites were produced by calcining Ag3 [Co(CN)6 ] at different temperatures in air. During calcinations, large amounts of Ag/Co3 O4 nanojunctions were generated and dispersed uniformly to form a porous nanocomposite due to gas (COx and NOx ) release. This structure allows small reactive molecules such as CO, O2 to get into the inner channels of catalyst, resulting in the catalytic reaction occurring on both the exterior and interior surfaces. The crystalline nature and microstructure of the as-prepared Ag3 [Co(CN)6 ] are shown in Fig. S1. Ag3 [Co(CN)6 ] keeps the cube shape and dispersibility after substituting Ag+ for Co2+ in Co3 [Co(CN)6 ]2 ·nH2 O. But the surfaces become rough and the interior become porous after calcination, which is attributed to the evaporation of generated gases. 3.2. Characterization of Ag/Co3 O4 nanocomposites The crystallographic structure of the Ag/Co3 O4 nanocomposites was analyzed by X-ray powder diffraction (XRD). As shown in Fig. 2, all of the peaks correspond well to face-centered-cubic Ag (JCPDS card no. 89-3722). C150 to C400 were prepared at 150 ◦ C for 240 h, 200 ◦ C for 48 h, 250 ◦ C for 8 h, 300 ◦ C for 2 h, 400 ◦ C for 1 h, respectively. Along with the increasing of calcination temperature from
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Fig. 1. (a) Schematic illustration of the formation process of Ag/Co3 O4 nanocomposites consist of Ag/Co3 O4 junctions, (b) Adsorption and reaction of O2 and CO molecules on the surface of a nanojunction between Ag and Co3 O4 nanoparticle.
Fig. 3. HRTEM image of C200.
Fig. 2. XRD patterns of Ag/Co3 O4 nanocomposites C150–C400.
200 ◦ C to 400 ◦ C, a sharpening of the peaks is observed, indicating the improvement of crystallinity. Peaks at 32.3◦ are hard to be identified. To understand the influence of calcination temperature on the growth of crystallite size, the mean crystallite sizes of Ag are estimated from the (1 1 1) peak with the Scherrer formula, D = 0.89/(B cos ) (D, average dimension of crystallites; , the X-ray wavelength; B, the pure diffraction broadening of a peak at half-height; , the Bragg angel). The sizes from C150 to C400 are 10.9, 6.9, 10.3, 24.2 and 28.6 nm, respectively (the instrumental error has been deducted). As can be seen, C200 has the smallest crystallite size of Ag resulting in the best catalytic performance for CO oxidation, which will be discussed in the following. However, C150 prepared at a lower temperature has a larger crystallite size. Such phenomenon is due to the long calcination time (240 h) during which small particles aggregate into large particles via Ostwald ripening [39(a)]. The FT-IR spectrum demonstrates the generation of Co3 O4 (Fig. S2). Two strong bands at 670 and 587 cm−1 associated to the divalent cobalt ions in octahedral and trivalent cobalt in tetrahedral sites, respectively [40]. The absence of bands at 2170 cm−1 related to CN [38(b)] in C150 to C400 illustrates the complete decomposition of Ag3 [Co(CN)6 ]. The band at 1380 cm−1 becomes weak as the temperature increasing, illustrating the existence of nitrate. It disappears in C250 could be due to the high temperature at local parts and a long time of calcination (8 h), for silver nitrate would decompose above 300 ◦ C [41]. The reasons are our speculation, which need further work to confirm, and not the focus of this paper. So we feel really sorry that we could not do more in-depth discussion. Fig. 3 shows the HRTEM image of heterojunctions of Ag and Co3 O4 . The measured interlayer distance of 0.235 nm is consistent with the (1 1 1) planes of Ag, while the interlayer distance of 0.156 nm and 0.288 nm agrees with the (5 1 1) and (2 2 0) planes of Co3 O4 , respectively. The interlaced boundaries marked with red
circles demonstrate the generation of Ag/Co3 O4 nanojunctions. In the nanojunctions, Ag and Co3 O4 particles closely connect to each other on a nano-level. Such junctions could keep Ag nanoparticles from falling off from the Co3 O4 support while ultrasonic cleaning and aggregation during the catalytic reaction. Crystallite size distribution of Ag from HRTEM in C200 is also calculated. As shown in Fig. S6, crystallite size of Ag ranges from 3 to 7 nm mainly with an average size 5.0 nm. The result also illustrates the good homodispersity of Ag in the catalyst. XPS characterization was performed to investigate the surface composition and chemical state of the Ag/Co3 O4 nanocomposites (Fig. 4). For C200 and C400, two peaks of each sample are similar, centered at 780.0 and 795.0 eV, corresponding to the Co 2p3/2 and Co 2p1/2 . The gap between the peaks is about 15 eV (spin orbit splitting), which corresponds to the standard Co3 O4 spectra [39(a and b)]. The almost complete absence of the Co2+ shake-up satellites (at 787.0 and 804.0 eV) confirms generation of Co3 O4 rather than CoO [39(c)]. The core level binding energy of Ag3d5/2 in C200 and C400 are 368.9 and 368.8 eV, respectively, which are both higher than that of bulk metallic Ag (368.1 ± 0.1 eV). Binding energy of Ag3d3/2 in C200 and C400 are 374.9 and 374.8 eV, respectively, and are also higher than that of bulk metallic Ag (374.2 ± 0.1 eV) [42]. Such a positive shift is attributed to the Ag/Co3 O4 junctions (shown in Fig. 3), in which electrons transfer from Ag particles to Co3 O4 supports. C200 with the smaller particle size shows higher binding energy compared with C400. The positive shift of binding energy with the decrease of Ag size has also been observed by other groups [43,44], and it was often interpreted in terms of final state effects, which arise from screening of the core level hole created by photoemission. For example, electrons relax to screen the hole, resulting in a photo-emitted electron with a higher kinetic energy and therefore lower binding energy. For smaller clusters, the positive hole can be less efficiently screened, causing a positive core level shift. Dark-field STEM and EDX mapping of a particle (Fig. 5) show the spatial distributions of Ag, Co and O in a nanoparticle of C200. All the elements distribute dispersedly without any aggregation, which further suggests the presence of large amounts of
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Fig. 6. Conversion as a function of temperature for CO oxidation over Ag/Co3 O4 nanocomposites. Table 1 Comparison of the activity for CO oxidation on different oxide supported Ag catalysts.
Fig. 4. XPS spectra of as-prepared Ag/Co3 O4 nanocomposites: (a) Ag (3d) binding energy spectrum, (b) Co (2p) binding energy spectrum.
Ag/Co3 O4 junctions. The uniform distributions of Ag and Co3 O4 demonstrate the highly dispersed Ag/Co3 O4 junctions, which make sure the catalytic reaction take place throughout the porous nanocomposites. 3.3. Catalytic performance To study the performance of Ag/Co3 O4 nanocomposites prepared at different temperatures, CO oxidation as a typical probe reaction was carried out to test the catalytic activity. Fig. 6 shows the conversion of CO as a function of reaction temperature for the prepared samples. T100 (temperature for CO 100% conversion) for C200 and other samples are 100 ◦ C and 120 ◦ C, respectively, and the complete conversion remains unchanged as the reaction temperature continues to rise. All the samples show high catalytic activity for oxidation of CO especially C200, which is more active than most supported Ag catalysts reported [4,5,9,10,12,15,16,19,25,43,45,46]. The highest activity of C200 is due to the Ag/Co3 O4 junctions at a
Catalysts
Pretreatment
T100 (◦ C)a
Year
Reference
Ag/CeO2 Ag/SiO2 Ag/TiO2 Ag/Al2 O3 Ag/TiO2 Ag/Fe2 O3 Ag/Co3 O4
– – H2 , 550 ◦ C H2 , 300 ◦ C H2 , 550 ◦ C O2 , 300 ◦ C – – – – –
275 160 330 150 250 275 120 100 120 120 120
2000 2009 2011 2011 2013 2013 2014 2014 2014 2014 2014
[16] [5] [4] [15] [52] [53] This work This work This work This work This work
C150 C200 C250 C300 C400
a Temperature for CO 100% conversion. – No pretreatment.
maximum with the smallest crystallite size of Ag. On the junctions, abundant O2 and CO are adsorbed on Ag and Co3 O4 nanoparticles (Fig. 1b), which make the catalytic reaction occur more easily compared with separate particles. Several oxide supported Ag catalysts have been listed in Table 1. Without any pretreatment, all the Ag/Co3 O4 nanocomposites prepared in this work show much higher activity than those pretreated ones. Significantly, the catalytic activity of C200 is very stable during the reaction time without any decline (Fig. 7a), while this phenomenon always appears in other supported Ag catalysts [10–12,45]. What is more, the high activity can be maintained even after being exposed to air for more than two months (Fig. 7b) and Ag remains in metallic form (Figs. S4 and S5). After catalysis, the core level binding energy
Fig. 5. (a) Dark-field STEM image of a nanoparticle in C200. (b–d) elemental mapping of the same nanoparticle, indicating spatial distribution of Ag (b), Co (c), O (d), respectively.
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Fig. 1b). Therefore, excellent catalytic performance will be achieved as more Ag/Co3 O4 nanojunctions and effective interfaces are generated. Herein, the homodispersed Ag particles in Co3 O4 of C200 with smallest particle size provide sufficient well-knit Ag/Co3 O4 nanojunctions for the reaction. Besides, the Ag/Co3 O4 nanocomposites are polyporous (Fig. S3) with a calculated BET surface area of 20.6 m2 g−1 , which illustrates that CO molecules were catalytically oxidized not only on exterior but also interior surfaces, as a result, display the best catalytic performance. 4. Conclusions In summary, we have successfully fabricated porous Ag/Co3 O4 nanocomposites by calcining Ag3 [Co(CN)6 ] nanoparticles at moderate temperatures in air. The as-prepared samples show high catalytic activity, especially C200 with a complete CO conversion temperature (T100 ) of 100 ◦ C. Moreover, it could maintain high catalytic performances even after 18 h and being exposed to air for two months. The presence of abundant and highly dispersed Ag/Co3 O4 junctions in the nanocomposites was suggested to be responsible for the excellent catalytic performance. On the junctions, O2 and CO adsorb on Ag and Co3 O4 , respectively and react with each other more easily. Moreover, Ag particle aggregation can be avoided during a catalytic reaction due to the highly dispersed Ag/Co3 O4 junctions, which results in excellent catalytic activity and stability. All these advantages are attributed to the simple “in situ self-reduction” route, which could serve as a general method for preparing other noble-metal supported catalysts. Acknowledgement Financial support from the National Natural Science Foundation of China (21271163, U1232211). Fig. 7. Stability of C200, reaction performance with time on stream (100 ◦ C, flow rate = 50 mL min−1 ), and catalytic activity of C200 exposed in air for more than two months.
of Ag3d5/2 changed from 368.9 eV to 368.7 eV in C200, which was still higher than that of bulk metallic Ag (368.3 eV). Such a minor negative shift could be due to the electron transfer between Ag and Co3 O4 . The excellent stability was attributed to the uniformly dispersed Ag/Co3 O4 junctions, which prevents Ag nanoparticles from aggregation and oxidation during reaction and storage. In the case of Co3 O4 -catalyzed CO oxidation, CO species are adsorbed on the surface-exposed Co3+ sites and react with weakly bound oxygen species such as superoxide ion (O2− ) to form CO2 [34]. The active oxygen species are formed on surface oxygen vacancies which are increased by pretreatment in a reducing or oxidizing atmosphere [34,47]. Ag is known for adsorbing and activating oxygen [4]. In the reaction of CO oxidation, an electron transfer from Ag to the anti-bonding orbital of the O2 molecule would weaken the O O bonding and form an O2− , thus improving the oxygen activation. With a neighboring adsorbed CO, the oxygen transfer reaction would then occur easily [45]. In addition, lattice oxygen is generally considered to participate in the reaction [12,34,48–50]. Based on these reasons, a possible mechanism is proposed: O2 is adsorbed on the surface of Ag particles. Some of them reacts with the neighbor CO adsorbed on Co3 O4 particles. Others turns into lattice oxygen by entering the oxygen vacancies in the adjacent Co3 O4 , and then reacts with the adsorbed CO on the surface. In any way, the CO oxidation is more likely to take place at the interface between Ag and Co3 O4 nanojunctions just as other metal-oxide [51] and bimetal [30] catalysts (as shown in
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