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Crystalline ordered mesoporous Cu0.25Co2.75O4 prepared with selected mesoporous silica templates and their performances as DeN2O catalysts
Microporous and Mesoporous Materials 221 (2016) 91e100
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Crystalline ordered mesoporous Cu0.25Co2.75O4 prepared with selected mesoporous silica templates and their performances as DeN2O catalysts Tanja Franken, Christian Mbaya Mani, Regina Palkovits* Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, D-52074 Aachen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2015 Received in revised form 8 September 2015 Accepted 11 September 2015 Available online 25 September 2015
Ordered mesoporous crystalline Cu0.25Co2.75O4 spinels were produced via Nanocasting of SBA-15, KIT-6, SBA-16 and FDU-12 as hard templates with different pore geometries. For elucidating the impact of template properties on the replica each hard template was produced at four different hydrothermal temperatures (60 C, 80 C, 100 C and 120 C) to vary the pore sizes of the templates. It was found that the pore size and geometry have a strong influence on the final replica. 3d structured replica were achieved with SBA-15, SBA-16 and FDU-12 templates prepared at hydrothermal temperatures above 100 C. An increase of the hydrothermal temperature goes along with an increase of the pore size of the template. The bigger the pores of the template the more mechanically stable becomes the replica which is accompanied by a loss of specific surface area (SBET). Disordered materials with high specific surface areas up to 138 m2 g1 can be obtained with SBA-16 as template produced at hydrothermal temperatures below 100 C. Only with KIT-6 as template all produced replica yield in ordered mesoporous crystalline Cu0.25Co2.75O4. The pore diameter of KIT-6 determines if one or both of the interpenetrating pore systems of KIT-6 are replicated. The larger the pore size of the template the better is the replication of both pore systems. Replication of both pore systems of KIT-6 yields in overall smaller pore diameters of the ordered mesoporous Cu0.25Co2.75O4 spinel. Beside this, the produced replica were used as catalysts to decompose N2O into N2 and O2 in ideal and realistic reaction conditions according to tail gases of HNO3 production. Thereby, materials with high SBET facilitate superior catalytic activity. By increasing the SBET from 26 m2 g1 to 103 m2 g1 the starting temperature of N2O decomposition can be shifted below 130 C and 210 C in ideal and real reaction conditions, respectively. Furthermore, replica prepared with FDU-12 show increased catalytic activity due to the shaping of the spinel with the template. With these catalysts full conversion could be achieved at temperatures as low as 330 C in ideal and 410 C in real reaction conditions. © 2015 Elsevier Inc. All rights reserved.
Keywords: Nanocasting Hard templating Mesoporous spinel N2O decomposition Environmental catalysis
1. Introduction In various application, heterogeneous catalysis relies on a high specific surface area of the catalytically active material facilitating superior catalytic performance. A mesoporous material structure can increase accessibility of active centers and help avoiding mass transport limitations under reaction conditions. For this reason, strong efforts have been made since the development of ordered mesoporous silica materials of the MCM- [1] and SBA-type [2] in
* Corresponding author. E-mail address: [email protected] (R. Palkovits). http://dx.doi.org/10.1016/j.micromeso.2015.09.019 1387-1811/© 2015 Elsevier Inc. All rights reserved.
the 90th to establish ways for a preparation of crystalline mesoporous metal oxides with high specific surface areas (SBET). Mesoporous silica materials are mostly produced via a soft templating approach [1,2]. During synthesis structure directing agents (SDA) are used as templates to obtain mesoporous silica with a highly ordered pore structure. Tensides or block-copolymers often serve as templates forming a liquid crystalline phase. The silica precursor assembles at the interphase of the liquid crystalline phase and condensates during hydrothermal treatment to form structured silica. Pore geometries and diameter of these silica can be easily tuned due to the use of numerous SDAs, addition of swelling agents or changes of the temperature during the hydrothermal step [2]. In the last decades it was shown that the
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transfer of this soft templating approach is highly challenging for a preparation of ordered mesoporous metal oxides other than SiO2. Especially, crystalline materials with ordered mesoporous pore structures are difficult to produce due to the collapse of the material during template removal [3]. Another possibility to prepare mesoporous crystalline materials offers the so called “Hard Templating” or “Nanocasting” approach [4e7]. Herein, mesoporous silica are used as templates. This synthesis consists mainly of four steps: First, preparation of mesoporous silica templates. Second, infiltration of a metal oxide precursor solution by impregnation into the pores of the silica template. Third, conversion of the precursor into the final metal oxide via thermal treatment within the pores of the silica. And in the last step, dissolving of the silica template to achieve the final mesoporous crystalline metal oxide [8]. For a successful utilization of this technique at least two requirements have to be considered: No solid state reactions should occur between the metal oxide and silica during the conversion of the precursor into its final metal oxide. Furthermore, the silica material should dissolve in conditions in which the final metal oxide stays solid [9]. One of the first published nanostructured metal oxide by Nanocasting was presented by Zhu et al. [10]. They used SBA-15 as a template for Cr2O3 and received a reversed 2-D hexagonal structure of nanorod arrays. In the following years numerous metal oxides were prepared via Hard Templating with plenty of different silica materials. Many of these led to materials with highly enhanced specific surface area and a highly ordered pore system. For a broad overview on non-siliceous mesoporous materials see the two detailed review article by the group of Ferdi Schüth [8,9]. Especially nanostructured Co3O4 and cobaltite based spinels are in the focus of recent research considering their potential as catalysts or electrode materials [10e15]. As described before it is easy to adjust the pore geometry and the porous properties of mesoporous silica templates. In literature, silica materials used as hard templates are often synthesized at hydrothermal temperatures of 100 C or 120 C [10e15]. For good replication a three dimensional porous structure of the silica template is highly useful to achieve porous metal oxides. The pore structures of the templates used in this study are illustrated in Fig. 1. In heterogeneous catalysis and especially in gas phase applications with very short catalyst contact times, e. g. N2O decomposition, mass transport limitations can clearly limit the catalytic performances. Mesoporous materials can certainly assist to overcome such limitations. Additionally, their high specific surface areas result in an increased number of accessible catalytically active surface sites. Therefore, high specific surface areas are often a determining factor for the final catalytic activity. N2O is a highly potent greenhouse gas, which is emitted in large amounts during the production of HNO3. For a more environmentally benign production of HNO3 N2O needs to be decomposed at preferably low temperatures even in the presence of inhibitory gases such as NO, O2 and H2O. In a prior report [16] we could show that the spinel
type Cu0.25Co2.75O4 is a promising catalyst for the decomposition of N2O into N2 and O2 at temperatures below 410 C. Due to the exchange of Co by Cu in the framework of Co3O4 the temperature for 50% conversion could be reduced from 550 C for pure Co3O4 to 345 C for Cu0.25Co2.75O4. However, this spinel exhibits further possibilities for improvement because the crystalline spinel suffers from very low specific surface areas of 26 m2 g1. In order to investigate if an increase in specific surface area is suitable to increase the catalytic activity, we prepared Cu0.25Co2.75O4 catalysts via Hard Templating. We chose this material as model compound for a comparative study in which the hydrothermal temperatures of four silica templates with different pore geometries (SBA-15, KIT-6, SBA-16, FDU-12; Fig. 1) were varied. These materials were used as hard templates to produce Cu0.25Co2.75O4 with enhanced specific surface area. The influences of the specific surface area and pore diameter of the template on the properties of the final replica were investigated. The prepared replicated Cu0.25Co2.75O4 spinels were applied as catalysts in the decomposition of N2O into N2 and O2 in ideal and real reaction conditions to enlighten how the specific surface area, nature of ordered geometry and pore alignment affects the catalytic activity. 2. Experimental 2.1. Material preparation Cu0.25Co2.75O4 spinels were prepared via Hard Templating with mesoporous silica SBA-15, SBA-16, KIT-6 and FDU-12. All investigated mesoporous silica were prepared at four temperatures (60, 80, 100 and 120 C) during hydrothermal treatment to achieve templates with varying pore diameter. SBA-15 and SBA-16 were prepared as described by Zhao et al. [17] In a typical SBA-15 synthesis 6 g P123 was dissolved in 90 mL distilled water and 151 mL of 2 M HCl. Then 12.6 g Tetraethylorthosilicate (TEOS) was added and the mixture was left to stir for 12 h at 35 C. After hydrothermal treatment at a constant temperature for 24 h the product was filtered, washed, and calcined at 500 C for 6 h. For preparation of SBA-16 5 g of F127 was dissolved in 240 mL distilled water and 10.5 g concentrated HCl (37 wt%). Then 15 g butanol and 25.5 mL TEOS was added and the mixture was left to stir at 45 C for 12 h. The samples were aged for 24 h, filtered, washed, and calcined at 500 C for 6 h. KIT-6 samples were prepared as described by Kleitz et al. [18] by dissolving 6 g P123 in 217 mL H2O, 9.9 mL conc. HCl and 6 g butanol. After this 12.9 g TEOS was added at 35 C and the mixture was left to stir at this temperature for 12 h. After aging for 24 h, filtering and washing, calcination of the samples at 500 C for 6 h was performed. A typical synthesis of FDU-12 followed the approach of Fan et al. [19]. Herein, 4 g F127 was dissolved in a mixture of 5.58 mL 1,3,5trimethylbenzene, 10 g KCl and 240 mL 2 M HCl. Then 16.6 g TEOS was added at 35 C and the mixture was stirred for 12 h. The mesoporous silica was achieved after aging for 24 h, filtering,
Fig. 1. Pore structures of the four used silica templates: a) SBA-15, b) KIT-6, c) SBA-16, d) FDU-12.
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washing and calcination. In the following the silica templates are named as follows: SBA-15-60 C which represents SBA-15 prepared at a hydrothermal temperature of 60 C. During Hard Templating of Cu0.25Co2.75O4 the mesoporous silica were impregnated until wetness with a high concentrated aqueous solution of a stoichiometric mixture of Co(NO3)2$6H2O and Cu(NO3)2$3H2O. After that the materials were dried at 100 C and then heated to 600 C to produce the spinel. In order to remove the silica templates, the samples were treated twice in 2 M aqueous NaOH at 60 C, finally washed with H2O and dried at 80 C. The final Cu0.25Co2.75O4 replica are named as follows: REP-SBA-15-60 C. This describes the final replica templated with SBA-15 produced at 60 C. 2.2. Catalytic tests Catalytic N2O decomposition experiments were performed in a continuous fixed bed flow reactor equipped with an online infrared spectrometer (Agilent Cary FTIR 660; Pike 2.4 m gas cell). Before starting the catalysis the materials were treated at 400 C in a flow of nitrogen in order to remove adsorbed species. During catalysis a typical concentration of 1000 ppm N2O (ideal conditions) or 1000 ppm N2O þ 200 ppm NO þ 2% O2 þ 0.5% H2O (real conditions) in N2 as carrier gas and a GHSV of 54,000 h1 was used. The temperature was increased starting from 130 C in steps of 40 K, each temperature was kept constant for 30 min and the N2O conversion was determined via IR-analysis according to X ¼ [c(N2O)in e c(N2O)out]/c(N2O)in$100%. From this first order rate constants were calculated using the following formula: k ¼ ln(1X)$F/Vreactor. A Flow F of 90 mL/min and a reactor volume of 0.2 mL were used in all catalytic runs. 2.3. Characterization X-Ray powder diffraction measurements were performed using a Siemens D5000 power X-ray diffractometer equipped with a CuKa radiation source. N2 physisorption isotherms were collected at 196 C on a Quantachrome Quadrasorb SI instrument. Prior to the analysis the samples were degassed at 100 C in vacuum for 12 h. In order to obtain specific surface areas the BET model in a p/ p0 range of 0.05e0.2 was used. The total pore volume was determined at p/p0 ¼ 0.95. Pore diameter of SBA-15 and KIT-6 were calculated via a DFT method in equilibrium mode, while those of SBA-16 and FDU-12 were determined using the adsorption branch. Elemental analysis ICP-OES was performed on Modell Spectro of Analytical Instruments. TEM-EDX measurements were performed on a FEI Tecnai F20. All TEM-samples were prepared on Ni-grids. 3. Results and discussion 3.1. Characterization of templates and their replica 3.1.1. SBA-15 as template From the results of N2 physisorption isotherms (see Supporting information Fig. S1) specific surface area SBET, pore volumina Vp and
Fig. 2. a) N2-physisorption isotherms of the four Cu0.25Co2.75O4 replica templated with SBA-15 after template removal. An onset of the Isotherms of ~30 units to each other is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with SBA-15 after template removal. Inset: Large angle XRD of the replica confirming the spinel phase.
pore diameter dp (Table 1) were determined of the four SBA-15 templates aged at 60 C, 80 C, 100 C and 120 C. The nitrogen physisorption isotherms show the typical type IV course of porous materials with a narrow pore size distribution. Increasing the hydrothermal temperature results in an increase of the SBET from 332 to 894 m2 g1, Vp from 0.35 to 1.24 cm3 g1 and dp from 5 to 8.2 nm. Via small angle XRD (see Supporting information Fig. S2), the ordered arrangement of the pores within the materials is confirmed as the [100] and [110] reflexes appear. Fig. 2a shows the N2-physisorption isotherms and the pores size distribution of the replicated Cu0.25Co2.75O4 spinel materials after removal of silica with NaOH solutions. The two replica REP-SBA-1560 C and REP-SBA-15-80 C show a typical type II isotherm. While
Table 1 Results from N2 physisorption of SBA-15 templates prepared at four different hydrothermal temperatures and of the corresponding replica. Template
SBA-15
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.60 0.35 1.24 1.15
699 332 894 736
5.0 6.4 8.2 8.2
0.21 0.22 0.18 0.18
94 95 99 91
e e 5.4 5.6
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the isotherms of the two replica REP-SBA-15-100 C and REP-SBA15-120 C exhibit more a type IV character with a clear hysteresis. Analysis of the pore size distributions (inset in Fig. 2a) indicate, that the latter two replica have pores with diameters between 4 and 8 nm, while the former two replica are non-porous. At lower hydrothermal temperatures during template synthesis (60 C & 80 C) interconnecting micropores between the mesopores are unincisive and only present as a corona in the walls of the 2d hexagonal aligned mesopores. With increasing hydrothermal temperature the interconnecting micropores become more distinct and increase in diameter. Therefore, the two replica REP-SBA-15-60 C and REPSBA-15-80 C are achieved as 2d rods without any 3d interconnectivity. With an increasing formation of interconnecting micropores in the SBA-15 template at higher hydrothermal temperatures the achieved replica REP-SBA-15-100 C and REPSBA-15-120 C become 3d cross-linked. Due to a stable 3d structure in the replica the mesoporous character within these materials increases. In the small angle XRD pattern of the replica (Fig. 2b) the reflexes [100] and [110] increase in intensity using templates prepared at higher hydrothermal temperatures. Hence, this indicates an increasingly ordered pore arrangement for REP-SBA-15-100 C and REP-SBA-15-120 C. The two replica REP-SBA-15-60 C and REP-SBA-15-80 C do not show any evidences for ordered porosity. The high angle regime of the XRD patterns (inset in Fig. 2b) exhibits the typical reflexes of the spinel phase. This emphasizes the crystalline character of the mesoporous replica REP-SBA-15-100 C and REP-SBA-15-120 C. The reflexes of all four replica are broadened indicating crystallite sizes of the four replica below 200 nm with a comparable diameter for all materials. The change of the type II to a type IV isotherm and the appearance of the typical reflexes for the ordered pore system can be correlated with the increase of the pore diameter of the template. A collection of the textural properties of the template and the final replica are summarized in Table 1. All four replicated Cu0.25Co2.75O4 reveal specific surface areas in the range of 91e99 m2 g1. Due to Hard Templating the SBET of the replicated spinel is around 4 times higher compared to the SBET of the spinel prepared without templating (SBET ¼ 26 m2 g1). The pore diameter and SBET of the SBA-15 template does not affect the SBET of the final replica. But with increasing interconnectivity of the mesopores by micropores at higher hydrothermal temperatures within the template the three dimensional and ordered mesoporous character of the replica increases. This gives evidence that the replica become more mechanically stable when the interconnectivity of the pores within the template increases. Furthermore, the pore diameter of the replica increases slightly from 5.4 to 5.6 nm with higher hydrothermal temperature. In the replication process the maximal diameter of the material is restricted by the pore sizes of the template, while the wall thickness of the template determines the pore diameter of the pores in the replica. Thus, the materials REPSBA-15-60 C and REP-SBA-15-80 C collapse during template removal due to the absence of a 3d cross-linked pore structure. For this reason, non-porous rodlike nanoparticles are achieved from those two replica. Because of the presence of 2d rods in all cases all four replica exhibit comparable specific surface areas. The smaller diameter of 2d rods in REP-SBA-15-60 C and REP-SBA-15-80 C compensates the effect of higher SBET due to increasing mesoporosity within the material. Combining the results from XRD and N2-physisorption shows that the spinel REP-SBA-15-100 C has the most ordered mesoporous character. TEM micrographs of this material (Fig. 3) display the hexagonal alignment of nanorods of the replicated pore system of SBA-15. The material is ordered over the whole sample and large mesoporous ordered particles close to micrometer size are
Fig. 3. TEM-micrograph of REP-SBA-15-100 C. Inset: close up of REP-SBA-15-100 C with EDX-line scan (black line).
produced. An EDX line scan (inset in Fig. 3 (black line)) of a single layer of ordered spinel clearly indicates that only trace amounts of silica are left in the sample. These trace amounts are not accumulated within the pores of the material. Therefore, these traces are not responsible for the achieved isotherms and mechanical stability of the material. Furthermore, the amount of Co and Cu decreases in the lighter parts of the line scan clearly evidencing the open porous character of this spinel. All samples were prepared on Ni-grids for TEM-measurement. Therefore the detected Cu signals by EDX in this and the following samples fully originate from the replica. This indicates a stable integration of Cu in the spinel structure and only minor amounts were lost due to leaching during template removal. 3.1.2. KIT-6 as template Data from small angle XRD and N2 physisorption (see Supporting information Figs. S3 & S4) of the four produced KIT-6 silica template confirm an ordered mesoporous pore system of the materials with a narrow pore size distribution. Furthermore, the higher the temperature during hydrothermal treatment the higher is the SBET, VP and dp of the KIT-6 template (Table 2). SBET increases up to 871 m2 g1 and the pore diameter raise from 6.4 to 9.4 nm due to the increase of the hydrothermal temperature from 60 C to 120 C. Using the four KIT-6 materials as template for Cu0.25Co2.75O4 results in replica with significantly increased SBET values of up to 117 m2 g1. A general trend is observable for the SBET of the replica: The smaller dp and Vp of the KIT-6 template the higher is the SBET of the final replica. Results from N2 physisorption and XRD (Fig. 4) indicate that with the use of all KIT-6 templates ordered mesoporous Cu0.25Co2.75O4 replica with spinel structure were achieved. A correlation between the SBET of the template and the SBET of the replica could not be found. But the materials REP-KIT-6-60 C, REPKIT-6-80 C and REP-KIT-6-100 C show two hystereses in the isotherms. One at lower and one at higher partial pressure. Therefore, it seems that a bimodal pore system formed in the replica. The pore system exhibits pore diameters of 4.8 nm and 13.8 nm, respectively. The amount of larger pores in the replica
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Table 2 Overview of the N2 physisorption results of four KIT-6 templates produced at different hydrothermal temperatures and results of their corresponding replica. Template
KIT-6
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.70 0.80 1.20 1.3
693 761 871 809
6.2 7.0 8.2 9.4
0.31 0.27 0.21 0.17
117 116 110 99
5.2/13.8 4.8/13.8 4.8/13.8 4.8
decreases with the increase of the pore sizes of the template (inset in Fig. 4a). The template KIT-6-120 C has the biggest pore diameter of 9.4 nm of the KIT-6- templates. With this template a replica with only one pore system of dp ¼ 4.8 nm was obtained. TEM micrographs of the replica REP-KIT-6-120 C confirm the presence of ordered mesoporosity within the compound (Fig. 5). In the micrograph three different orientations of the pores are observable. In area 1 the typical pore system of KIT-6 of two interwoven channels is fully replicated. In area 2 it is observable that only one of the two channel systems is replicated while the second is missing. This explains the discovery of two different pore sizes in N2-physisorption. The smaller pore diameter of 4.8 nm
corresponds to a fully replicated KIT-6 pore system as shown in area 1. Whereby, the pores in the replica result from the free spaces between the two replicated pore systems as shown in area 2. The larger pore diameter of 13.8 nm corresponds to the replication of one of the two interconnected pore systems of the KIT-6-template. In sample REP-KIT-6-120 C mainly both pore systems of KIT-6 template were replicated. Therefore even if small amounts of single replicated material are found these small contributions cannot be detected by N2-sorption. In area 3 multilayers of a fully replicated pore system are visible but the orientation of the material is not along the axis of the pore system. For this reason, the pore system appears disordered. The EDX line scan (inset in Fig. 5 (black line)) shows that the replicated material consists only of Cu, Co and O. In the regions where pores are expected the amount of Cu and Co drops to zero while the amount of Si does not increase. This shows that the silica was fully dissolved out of the material and only trace amounts of Si are left. It can be concluded that the 3d structured crystalline system of Cu0.25Co2.75O4 is stable by its own. These results indicate incomplete filling of the pore system of the KIT-6templates with smaller pore sizes with the metal precursor solution during impregnation. Incomplete replication of both pore systems presents the consequence. Therefore, the amount of larger pores is bigger for smaller pore sizes of the template. 3.1.3. SBA-16 as template XRD of the silica templates (see Supporting information Figs. S5 & S6) show at small angles the typical reflexes [110] and [200] of
Fig. 4. a) N2-physisorption isotherms of the four Cu0.25Co2.75O4 replica templated with KIT-6 after template removal. An onset of the Isotherms of ~30 units to each other is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with KIT-6 after template removal. Inset: Large angle XRD of the replica.
Fig. 5. TEM-micrograph of REP-KIT-6-120 C. Inset: Close up of REP-KIT-6-120 C with EDX-line scan (black line).
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the ordered pore system of SBA-16. N2-Physisorption isotherms exhibit a hysteresis indicating a successful synthesis of the mesoporous silica with a cubic body centered spherical pore structure with interconnecting tubes. Increasing the temperature of the hydrothermal step during the silica synthesis dp increases from 5.2 to 10.4 nm, Vp from 0.34 to 0.93 cm3 g1 and SBET increase from 595 to 1011 m2 g1 (Table 3). For SBA-16-100 C and SBA-16-120 C also pores with a smaller pore diameter appear. These pores correspond to the diameter of the interconnecting tubes between the spherical pores. For the smaller pores the pore diameter increases with increasing hydrothermal temperatures. In order to obtain an open porous structure the connecting micropores need to be present in the templates SBA-16-60 C and SBA-16-80 C, as well. As these micropores decrease in size with lower hydrothermal temperatures, they are not detectable in the measured p/p0 range for these two templates due to the small diameter of the micropores. In the isotherms of the templated Cu0.25Co2.75O4 materials a hysteresis appears only for the two materials REP-SBA-16-100 C and REP-SBA-16-120 C showing an type IV character. In contrast, the two materials REP-SBA-16-60 C and REP-SBA-16-80 C show a typical type II isotherm (Fig. 6a). Therefore, the former two materials appear to be mesoporous, while the latter two materials are non porous. The two mesoporous replica were produced with SBA16 templates with larger pore diameters of 9.4 and 10.5 nm. While the templates of the two replica without porosity have smaller pore sizes of 5.2 and 5.4 nm. A general trend can be observed for the SBET. With decrease of the pore diameter of the template the SBET of the final replica increases from 117 to 138 m2 g1. Small angle XRD only shows the typical reflex [110] of a cubic body centered structure for the materials REP-SBA-16-100 C and REP-SBA-16-120 C (Fig. 6b). Obviously, for these two materials an ordered mesoporous structure is formed. Accordingly, formation of 3d structured spinel materials is facilitated for SBA-16 templates with larger pores. Therefore it is suggested, that the 3d structured materials collapse during template removal if the thickness of the bridging rods in the replica become too thin. The higher specific surface area of the materials REP-SBA-16-60 C and REP-SBA-16-80 C can be thus correlated to the external surface of small particles. In the XRD at higher angles the typical pattern of the spinel phase is present and broadened in the case of all four replica with comparable crystallite size for the four materials. In the TEM-micrographs (Fig. 7) of REP-SBA-16-120 C the spherical shape of the replicated material is notable with an ordered alignment over the whole sample. With a higher resolution even the interconnecting bridges of the spheres are visible confirming the three dimensional character of the replica (inset in Fig. 7). An EDX line scan shows only trace amounts of silica present within the final material. For this material a constant Cu and Co amount was detected over the length of the scan. Restricted by the alignment of the pores in the SBA-16 template (Fig. 1c), the replica consists of a cubic body centered alignment of connected spheres. Therefore, underneath the interstice between four neighboring spheres the spheres from the next layer are arranged. Consequently, the Cu and Co amounts in the EDX line scan stay constant as long as not only one single layer of the material is analyzed.
Fig. 6. a) N2-physisorption isotherms of the four Cu0.25Co2.75O4 replica templated with SBA-16 after template removal. An onset of the Isotherms of ~40 units to each other is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with SBA-16 after template removal. Inset: Large angle XRD of the replica.
3.1.4. FDU-12 as template The typical reflexes [111] and [311] of the pore system of FDU-12 appear in small angle XRD together with a hysteresis in the N2 physisorption (see Supporting information Figs. S7 & S8). This indicates the successful synthesis of mesoporous FDU-12 with a cubic face centered alignment of spherical pores interconnected to each other (Fig. 1d). The increase of the temperature during hydrothermal treatment results in increasing dp, SBET and Vp of the FDU-12 template (see Table 4). By increasing the temperature during the hydrothermal treatment from 60 C to 120 C, dp rises from 10.4 nm to 12.8 nm, SBET from 206 m2 g1 to 423 m2 g1 and Vp from 0.20 cm3 g1 to 0.51 cm3 g1. Furthermore, all four materials show a narrow pore size distribution.
Table 3 Overview of the N2-physisorption results of four SBA-16 templates produced at different hydrothermal temperatures and results of their corresponding replica. Template
SBA-16
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.34 0.42 0.76 0.93
549 644 953 1011
5.2 5.4 2.6/9.4 3.2/10.4
0.29 0.30 0.21 0.20
138 131 117 118
e e 6.0 6.0
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3d structure. Therefore with an increase of the pore size of the template the mechanical stability of the replica increases. In the higher angle regime of the XRD pattern (inset in Fig. 8b) typical reflexes of the spinel phase appear for all replica. These reflexes are slightly broadened compared to the reflexes of the not replicated material, but are less broadened than the reflexes of the replica prepared with SBA-15, KIT-6 and SBA-16 as templates. This indicates that the crystallite sizes of the replica produced with FDU12 as template are bigger than prepared with the other templates. In the TEM-Micrographs of REP-FDU-12-120 C (Fig. 9) the replication of the ordered, spherical pores of the FDU-12 material is easy notable over large regions of the sample. With a closer resolution (inset in Fig. 9) even the replication of the interconnecting pores of the template are visible giving the reason for a stable 3 dimensional structure of the replica. The EDX-line scan shows a successful removal of the silica template. As described for REP-SBA16-120 C it is not expected that the amount of Cu and Co fully drops to zero over the length of the line scan, due to the detection of the next layer of spheres underneath. At the point of the scan where the beam hits no sphere in the next layer the Cu and Co amount drops confirming the open porous structure of the replica.
Fig. 7. TEM-micrograph of REP-SBA-16-120 C. Inset: Close up of REP-SBA-16-120 C with EDX-line scan (black line).
Using the four FDU-12 materials as template for Cu0.25Co2.75O4, the final replica after removal of the template exhibit specific surface areas increasing from 42 m2 g1 (REP-FDU-12-60 C) to 103 m2 g1 (REP-FDU-12-120 C). This trend goes in line with the increase of the SBET, Vp and dp of the template. For the replica REPFDU-12-100 C and REP-FDU-12-120 C the appearance of a hysteresis in the N2-physisorption isotherms indicates the formation of porous materials (Fig. 8a). In line, small angle XRD (Fig. 8b) of these two materials show the typical reflexes [111] and [311] of ordered mesoporous materials with a cubic face centered alignment. The other two replica REP-FDU-12-60 C and REP-FDU-12-80 C do neither show a distinct hysteresis nor reflexes in the small angle XRD region. Accordingly, these two materials are non-porous and do not exhibit an ordered porous structure. Opposite to the trend observed for the other templates, for FDU-12 the specific surface area of the replica increases by using FDU-12 with larger pore diameter. The spherical pores as well as the interconnecting pores increase in diameter with increasing hydrothermal temperature during template synthesis. The interconnecting pores are essential for a stable 3d structured replica. If the interconnecting pores decrease in diameter the replica become mechanically unstable and the 3d structure collapses during template removal. With increasing hydrothermal temperature during FDU-12 synthesis not only the spherical pores increase in diameter but also the interconnecting pores. The bigger these bridging pores the thicker becomes the replica and the more mechanically stable becomes the
3.2. Catalytic activity of replica in N2O decomposition To elucidate the impact of structuring the Cu0.25Co2.75O4 catalysts on the decomposition of N2O into N2 and O2 for every used silica template two replica with the lowest and highest SBET of each series were applied in the reaction. For replica of SBA-15 only the one material with the best 3d structuring was used as catalyst as all REP-SBA-15 samples resulted in materials with comparable SBET values. Comparing the first order rate constants at 290 C k290 (Table 5) for a catalytic run performed in ideal reaction conditions to each other shows that all spinels prepared via Hard Templating reach higher catalytic activities than the conventionally prepared spinel with low SBET. While k290 with around 41.3 min1 is low for the conventional spinel k290 values up to 1187.4 min1 are obtained for REP-FDU-12-120 in the same conditions. For each of the investigated geometries the replica with the higher SBET reaches higher k290 values and therefore higher catalytic activity. Furthermore, catalysts with comparable composition and SBET obtain comparable k290 values. Fig. 10 shows as general trend an increase of k290 values with increasing SBET. This evidences that for the N2O decomposition catalysts with higher specific surface area lead to higher catalytic activities. With a higher SBET an overall higher number of surface atoms on the catalysts are accessible. As it can be observed from elemental analysis all investigated catalysts show comparable ratios between Cu and Co atoms (Table 5). Therefore, the observed differences in catalytic activity are not a result of varying spinel composition. The observed trend in Fig. 10 is not an ideally linear trend. This indicates that also other parameters like geometry or exposed crystal planes, beside the increased SBET, contribute to the observed catalytic activity.
Table 4 Overview of the N2 physisorption results of four FDU-12 templates produced at different hydrothermal temperatures and results of their corresponding replica. Template
FDU-12
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.20 0.32 0.42 0.51
206 309 356 423
10.4 10.4 11.3 12.8
0.11 0.22 0.12 0.27
42 74 64 103
e e 6.2 7.0
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Fig. 9. TEM-micrograph of REP-FDU-12-120 C. Inset: close up of REP-FDU-12-120 C with EDX-line scan (black line).
Table 5 Comparison of first order rate constants at 290 C in ideal (k290) and at 410 C in real reaction conditions (k410) and n(Cu)/n(Co) ratios for the replica used as catalysts in N2O decomposition.
Fig. 8. a) N2-physisorption isotherms of the Cu0.25Co2.75O4 replica templated with FDU-12 after template removal. An onset of the Isotherms of ~25 units is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with FDU-12 after template removal. Inset: Large angle XRD of the replica.
Also in realistic reaction conditions the same trends as in ideal conditions can be observed, but shifted to higher temperatures due to the presence of inhibiting gases (Table 5). Therefore, first order rate constants for the reaction at 410 C (k410) are given for a comparison of the catalytic activities of the different replica. In both reaction conditions, ideal and real, REP-FDU-12-120 shows much higher rate constants than the replica with the other geometries even though SBET and composition are comparable. This reveals that the cubic face centered alignment of the catalyst due to casting of FDU-12 yields into a more active catalyst than with the other geometries. With REP-FDU-12-120 k290 values up to 1187.4 min1 and k410 values up to 1717.2 min1 were obtained under ideal and real reaction conditions, respectively. For this reason the dependency of the N2O conversion on the temperature will be closer discussed for REP-FDU-12-60 (lowest SBET ¼ 42 m2 g1) and REP-FDU-12-120 (highest SBET ¼ 103 m2 g1) in ideal and real reaction conditions (Fig. 11). The material REPFDU-12-60 C exhibits enhanced catalytic activities compared to the conventionally synthesized Cu0.25Co2.75O4 with lower specific surface area. A linear increase of the conversions with temperature from 130 C to 250 C is observable for REP-FDU-12-60 in ideal reaction conditions. Above 250 C a typical S-shaped increase of conversion with temperature according to Arrhenius law is observed. This indicates casting of the spherical pores of FDU-12
Cu0.25Co2.75O4 REP-SBA-15-100 REP-KIT-6-60 REP-KIT-6-120 REP-SBA-16-60 REP-SBA-16-120 REP-FDU-12-60 REP-FDU-12-120
k290/min1
k410/min1
n(Cu)/n(Co)ICP
41.3 540.3 453.7 522.1 670.9 419.0 216.9 1187.4
13.6 465.0 e 1497.6 e 1426.5 e 1717.2
0.090 0.089 0.085 0.087 0.083 0.083 0.093 0.085
Fig. 10. Dependency of the first order rate constants for N2O decomposition at 290 C (k290) in ideal conditions on the SBET of replicated Cu0.25Co2.75O4 catalysts.
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the high specific surface areas of the materials even at high temperatures. Furthermore, additional studies are required enabling a deeper understanding concerning the influence of the structure of the replica on catalytic activity. 4. Conclusions
Fig. 11. N2O conversion against temperature from continuous N2O decomposition experiments of REP-FDU-12-60 C and REP-FDU-12-120 C as catalyst compared to Cu0.25Co2.75O4 without templating in ideal reaction conditions (closed symbols; c(N2O) ¼ 1000 ppm in N2, GHSV ¼ 54,000 h1) and in real reaction conditions (open symbols; c(N2O) ¼ 1000 ppm, c(NO) ¼ 200 ppm, c(O2) ¼ 2%, c(H2O) ¼ 0.5% in N2, GHSV ¼ 54,000 h1).
produces different kinds of surface sites in the templated spinel compared to conventionally synthesized spinels. As the crystallinity is higher for replica prepared with FDU-12 templates than for the other replica the number of atoms on defect positions is lower compared to the other replica. Therefore, it is not likely that these atoms are responsible for the increased catalytic activity. Instead a higher crystallinity results in lager exposed crystal planes. It was shown before that catalytic activities of Co3O4 is significantly affected by the exposed crystal planes. For example in CO-oxidation [20] and also in selective catalytic reduction of NOx by NH3 [21], materials with (110) as exposed crystal plane lead to the best catalytic performances. Therefore, it seems that the pore size and geometry of the FDU-12 template facilitates exposing crystal planes on the surface of the spinel which are more active in catalysis. Why this effect is stronger pronounced for FDU-12 as template than for the other templates even though the composition, SBET and Vp is the same, will be investigated in future studies. A further increase of the specific surface area by using the catalyst REP-FDU-12-120 C reveals much higher catalytic activity. Due to the enhanced SBET the N2O decomposition reaction set off temperature is below 130 C and full conversion is already achieved at 330 C. For the conventional catalyst, the reaction starts at 250 C and full conversion is obtained at temperatures as high as 450 C. In this case, the surface enhanced catalyst REP-FDU-12-120 C performs better in realistic reaction conditions than the conventional Cu0.25Co2.75O4 in ideal reaction conditions. With REP-FDU-12120 C as catalyst first conversions are detected at 210 C and full conversion is already achieved at 410 C even in the presence of inhibitory gases such as NO, O2 and H2O. Compared to this the reaction does not start until 370 C over the not modified Cu0.25Co2.75O4 catalyst under comparable reaction conditions. Full conversion is observed at temperatures as high as 570 C. However, N2-physisorption analysis after catalysis suggests sintering of the templated material during catalysis as the SBET and Vp decrease to 50.4 m2 g1 and 0.13 cm3 g1, respectively. However, after catalysis the SBET of REP-FDU-12-120 C is still double as high as the conventionally synthesized Cu0.25Co2.75O4 catalyst. The presented results illustrate the positive effect of enhanced specific surface areas of spinel materials in N2O decomposition. Nevertheless, sintering occurs for all presented structures at higher temperatures. Therefore, further investigations need to focus on possible ways to decrease sintering and simultaneously preserve
We have shown that ordered porous Cu0.25Co2.75O4 materials can be achieved with the four mesoporous silica SBA-15, KIT-6, SBA-16 and FDU-12 as hard templates. It was also demonstrated that the success of the replication process strongly depends on the synthesis conditions of the silica template and thus on the textural properties of the template. The bigger the pore sizes of the template the more stable become the 3d structured replica and the more distinct is the porous character of the achieved replica. Mechanically stable 3d linked replica can be obtained only with pore diameter of the template larger than 8 nm. The SBET of the final replica show a contrary behavior. The lower the pore diameter of the template the higher is the achieved SBET of the replica. This is mainly due to the decrease in particle size and therefore a higher surface to bulk ratio. In this set of experiments, the highest specific surface area of 138 m2 g1 was achieved with SBA-16 as template synthesized at a hydrothermal temperature of 60 C. Compared to a specific surface area of Cu0.25Co2.75O4 prepared by solution combustion synthesis the obtained SBET is more than 5 times higher. The use of surface enhanced catalysts in the decomposition of N2O confirms the importance of the specific surface area in this reaction. Due to the enhanced surface area, the starting temperatures of the reaction could be reduced below 130 C compared to 250 C for the conventional catalyst with low specific surface area in ideal reaction conditions. The influence of high SBET values of this kind of catalysts for high catalytic performances is even more pronounced in real reaction conditions. Thereby, the light off temperature could be reduced to 210 C and full conversion can be obtained already at 410 C with REP-FDU-12-120 C compared to 370 C and 570 C respectively for Cu0.25Co2.75O4 in the presence of NO, O2 and H2O. Catalysts with comparable specific surface area exhibit comparable catalytic activities. However, REP-FDU-12120 C exhibits higher catalytic activity than the other materials with comparable SBET, an effect which has to be investigated further in future studies. Concerning stability, all templated materials suffered from thermally induced sintering during catalysis. Overall, we showed that surface enhancement and structuring of crystalline materials via Nanocasting is highly valuable for the catalytic decomposition of N2O into N2 and O2 and is expected to show comparable trends in other gas phase applications. Nevertheless, the success of the preparation of a 3d structured crystalline material depends highly on the pore sizes of the used hard template. For stable 3d structured materials larger pore diameter within the used hard template are recommended. Acknowledgment We like to acknowledge the financial support by the Max Buchner foundation (Grant Number: 3334). Part of this work was performed in the frame of the project house “Center for Automotive Catalytic Systems Aachen-ACA” of RWTH Aachen University financed by the Excellence Initiative of the German federal and state governments to promote science and research at German universities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.09.019.
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References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [3] Q. Huo, D. Margolese, U. Ciesla, P. Feng, T. Gier, P. Sieger, R. Leon, P. Petroff, F. Schüth, G.D. Stucky, Nature 368 (1994) 317. [4] P. Llewellyn, M. Keung, S. Kallus, F. Schüth, K.K. Unger, Stud. Surf. Sci. Catal. 84 (1994) 2013. [5] S.M. Ng, S. Ogino, T. Aida, K.A. Koyano, T. Tatsumi, Macromol. Rapid Commun. 18 (1997) 991. € ller, T. Bein, R.X. Fischer, Chem. Mater. 10 (1998) 1840. [6] K. Mo [7] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [8] D. Gu, F. Schüth, Chem. Soc. Rev. 43 (2014) 313. [9] F. Schüth, Chem. Mater. 13 (2001) 3184.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
K.K. Zhu, B. Yue, W.Z. Zhou, H.Y. He, ChemComm (2003) 98. W. Yue, A.H. Hill, A. Harrison, W. Zhou, ChemComm (2007) 1518. J. Zhu, Q. Gao, Microporous Mesoporous Mater. 124 (2009) 144. Y. Cui, Z. Wen, S. Sun, Y. Lu, J. Jin, Solid State Ionics 225 (2012) 598. H. Jin, X. Gu, B. Hong, L. Lin, C. Wang, D. Jin, X. Peng, X. Wang, G. Ge, J. Phys. Chem. C 116 (2012) 13374. J. Rosen, G.S. Hutchings, F. Jiao, JACS 135 (2013) 4516. T. Franken, R. Palkovits, Appl. Catal. B 176 (2015) 298. D. Zhao, Q. Huo, J. Feng, B.B. Chmelka, G.F. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. F. Kleitz, S.H. Choi, R. Ryoo, Chem. Commun. (2003) 2136. J. Fan, C. Yu, F. Gao, J. Lei, B. Tian, K. Wang, Q. Luo, B. Tu, W. Zhou, D. Zhao, Angew. Chem. Int. Ed. 42 (2003) 3146. X. Xie, Y. Li, Z.-Q. Liu, M. Haruta, W. Shen, Nature 458 (2009) 746. B. Meng, Z. Zhao, X. Wang, J. Liang, J. Qiu, Appl. Catal. B 129 (2013) 491.
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Crystalline ordered mesoporous Cu0.25Co2.75O4 prepared with selected mesoporous silica templates and their performances as DeN2O catalysts Tanja Franken, Christian Mbaya Mani, Regina Palkovits* Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, D-52074 Aachen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 May 2015 Received in revised form 8 September 2015 Accepted 11 September 2015 Available online 25 September 2015
Ordered mesoporous crystalline Cu0.25Co2.75O4 spinels were produced via Nanocasting of SBA-15, KIT-6, SBA-16 and FDU-12 as hard templates with different pore geometries. For elucidating the impact of template properties on the replica each hard template was produced at four different hydrothermal temperatures (60 C, 80 C, 100 C and 120 C) to vary the pore sizes of the templates. It was found that the pore size and geometry have a strong influence on the final replica. 3d structured replica were achieved with SBA-15, SBA-16 and FDU-12 templates prepared at hydrothermal temperatures above 100 C. An increase of the hydrothermal temperature goes along with an increase of the pore size of the template. The bigger the pores of the template the more mechanically stable becomes the replica which is accompanied by a loss of specific surface area (SBET). Disordered materials with high specific surface areas up to 138 m2 g1 can be obtained with SBA-16 as template produced at hydrothermal temperatures below 100 C. Only with KIT-6 as template all produced replica yield in ordered mesoporous crystalline Cu0.25Co2.75O4. The pore diameter of KIT-6 determines if one or both of the interpenetrating pore systems of KIT-6 are replicated. The larger the pore size of the template the better is the replication of both pore systems. Replication of both pore systems of KIT-6 yields in overall smaller pore diameters of the ordered mesoporous Cu0.25Co2.75O4 spinel. Beside this, the produced replica were used as catalysts to decompose N2O into N2 and O2 in ideal and realistic reaction conditions according to tail gases of HNO3 production. Thereby, materials with high SBET facilitate superior catalytic activity. By increasing the SBET from 26 m2 g1 to 103 m2 g1 the starting temperature of N2O decomposition can be shifted below 130 C and 210 C in ideal and real reaction conditions, respectively. Furthermore, replica prepared with FDU-12 show increased catalytic activity due to the shaping of the spinel with the template. With these catalysts full conversion could be achieved at temperatures as low as 330 C in ideal and 410 C in real reaction conditions. © 2015 Elsevier Inc. All rights reserved.
Keywords: Nanocasting Hard templating Mesoporous spinel N2O decomposition Environmental catalysis
1. Introduction In various application, heterogeneous catalysis relies on a high specific surface area of the catalytically active material facilitating superior catalytic performance. A mesoporous material structure can increase accessibility of active centers and help avoiding mass transport limitations under reaction conditions. For this reason, strong efforts have been made since the development of ordered mesoporous silica materials of the MCM- [1] and SBA-type [2] in
* Corresponding author. E-mail address: [email protected] (R. Palkovits). http://dx.doi.org/10.1016/j.micromeso.2015.09.019 1387-1811/© 2015 Elsevier Inc. All rights reserved.
the 90th to establish ways for a preparation of crystalline mesoporous metal oxides with high specific surface areas (SBET). Mesoporous silica materials are mostly produced via a soft templating approach [1,2]. During synthesis structure directing agents (SDA) are used as templates to obtain mesoporous silica with a highly ordered pore structure. Tensides or block-copolymers often serve as templates forming a liquid crystalline phase. The silica precursor assembles at the interphase of the liquid crystalline phase and condensates during hydrothermal treatment to form structured silica. Pore geometries and diameter of these silica can be easily tuned due to the use of numerous SDAs, addition of swelling agents or changes of the temperature during the hydrothermal step [2]. In the last decades it was shown that the
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transfer of this soft templating approach is highly challenging for a preparation of ordered mesoporous metal oxides other than SiO2. Especially, crystalline materials with ordered mesoporous pore structures are difficult to produce due to the collapse of the material during template removal [3]. Another possibility to prepare mesoporous crystalline materials offers the so called “Hard Templating” or “Nanocasting” approach [4e7]. Herein, mesoporous silica are used as templates. This synthesis consists mainly of four steps: First, preparation of mesoporous silica templates. Second, infiltration of a metal oxide precursor solution by impregnation into the pores of the silica template. Third, conversion of the precursor into the final metal oxide via thermal treatment within the pores of the silica. And in the last step, dissolving of the silica template to achieve the final mesoporous crystalline metal oxide [8]. For a successful utilization of this technique at least two requirements have to be considered: No solid state reactions should occur between the metal oxide and silica during the conversion of the precursor into its final metal oxide. Furthermore, the silica material should dissolve in conditions in which the final metal oxide stays solid [9]. One of the first published nanostructured metal oxide by Nanocasting was presented by Zhu et al. [10]. They used SBA-15 as a template for Cr2O3 and received a reversed 2-D hexagonal structure of nanorod arrays. In the following years numerous metal oxides were prepared via Hard Templating with plenty of different silica materials. Many of these led to materials with highly enhanced specific surface area and a highly ordered pore system. For a broad overview on non-siliceous mesoporous materials see the two detailed review article by the group of Ferdi Schüth [8,9]. Especially nanostructured Co3O4 and cobaltite based spinels are in the focus of recent research considering their potential as catalysts or electrode materials [10e15]. As described before it is easy to adjust the pore geometry and the porous properties of mesoporous silica templates. In literature, silica materials used as hard templates are often synthesized at hydrothermal temperatures of 100 C or 120 C [10e15]. For good replication a three dimensional porous structure of the silica template is highly useful to achieve porous metal oxides. The pore structures of the templates used in this study are illustrated in Fig. 1. In heterogeneous catalysis and especially in gas phase applications with very short catalyst contact times, e. g. N2O decomposition, mass transport limitations can clearly limit the catalytic performances. Mesoporous materials can certainly assist to overcome such limitations. Additionally, their high specific surface areas result in an increased number of accessible catalytically active surface sites. Therefore, high specific surface areas are often a determining factor for the final catalytic activity. N2O is a highly potent greenhouse gas, which is emitted in large amounts during the production of HNO3. For a more environmentally benign production of HNO3 N2O needs to be decomposed at preferably low temperatures even in the presence of inhibitory gases such as NO, O2 and H2O. In a prior report [16] we could show that the spinel
type Cu0.25Co2.75O4 is a promising catalyst for the decomposition of N2O into N2 and O2 at temperatures below 410 C. Due to the exchange of Co by Cu in the framework of Co3O4 the temperature for 50% conversion could be reduced from 550 C for pure Co3O4 to 345 C for Cu0.25Co2.75O4. However, this spinel exhibits further possibilities for improvement because the crystalline spinel suffers from very low specific surface areas of 26 m2 g1. In order to investigate if an increase in specific surface area is suitable to increase the catalytic activity, we prepared Cu0.25Co2.75O4 catalysts via Hard Templating. We chose this material as model compound for a comparative study in which the hydrothermal temperatures of four silica templates with different pore geometries (SBA-15, KIT-6, SBA-16, FDU-12; Fig. 1) were varied. These materials were used as hard templates to produce Cu0.25Co2.75O4 with enhanced specific surface area. The influences of the specific surface area and pore diameter of the template on the properties of the final replica were investigated. The prepared replicated Cu0.25Co2.75O4 spinels were applied as catalysts in the decomposition of N2O into N2 and O2 in ideal and real reaction conditions to enlighten how the specific surface area, nature of ordered geometry and pore alignment affects the catalytic activity. 2. Experimental 2.1. Material preparation Cu0.25Co2.75O4 spinels were prepared via Hard Templating with mesoporous silica SBA-15, SBA-16, KIT-6 and FDU-12. All investigated mesoporous silica were prepared at four temperatures (60, 80, 100 and 120 C) during hydrothermal treatment to achieve templates with varying pore diameter. SBA-15 and SBA-16 were prepared as described by Zhao et al. [17] In a typical SBA-15 synthesis 6 g P123 was dissolved in 90 mL distilled water and 151 mL of 2 M HCl. Then 12.6 g Tetraethylorthosilicate (TEOS) was added and the mixture was left to stir for 12 h at 35 C. After hydrothermal treatment at a constant temperature for 24 h the product was filtered, washed, and calcined at 500 C for 6 h. For preparation of SBA-16 5 g of F127 was dissolved in 240 mL distilled water and 10.5 g concentrated HCl (37 wt%). Then 15 g butanol and 25.5 mL TEOS was added and the mixture was left to stir at 45 C for 12 h. The samples were aged for 24 h, filtered, washed, and calcined at 500 C for 6 h. KIT-6 samples were prepared as described by Kleitz et al. [18] by dissolving 6 g P123 in 217 mL H2O, 9.9 mL conc. HCl and 6 g butanol. After this 12.9 g TEOS was added at 35 C and the mixture was left to stir at this temperature for 12 h. After aging for 24 h, filtering and washing, calcination of the samples at 500 C for 6 h was performed. A typical synthesis of FDU-12 followed the approach of Fan et al. [19]. Herein, 4 g F127 was dissolved in a mixture of 5.58 mL 1,3,5trimethylbenzene, 10 g KCl and 240 mL 2 M HCl. Then 16.6 g TEOS was added at 35 C and the mixture was stirred for 12 h. The mesoporous silica was achieved after aging for 24 h, filtering,
Fig. 1. Pore structures of the four used silica templates: a) SBA-15, b) KIT-6, c) SBA-16, d) FDU-12.
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washing and calcination. In the following the silica templates are named as follows: SBA-15-60 C which represents SBA-15 prepared at a hydrothermal temperature of 60 C. During Hard Templating of Cu0.25Co2.75O4 the mesoporous silica were impregnated until wetness with a high concentrated aqueous solution of a stoichiometric mixture of Co(NO3)2$6H2O and Cu(NO3)2$3H2O. After that the materials were dried at 100 C and then heated to 600 C to produce the spinel. In order to remove the silica templates, the samples were treated twice in 2 M aqueous NaOH at 60 C, finally washed with H2O and dried at 80 C. The final Cu0.25Co2.75O4 replica are named as follows: REP-SBA-15-60 C. This describes the final replica templated with SBA-15 produced at 60 C. 2.2. Catalytic tests Catalytic N2O decomposition experiments were performed in a continuous fixed bed flow reactor equipped with an online infrared spectrometer (Agilent Cary FTIR 660; Pike 2.4 m gas cell). Before starting the catalysis the materials were treated at 400 C in a flow of nitrogen in order to remove adsorbed species. During catalysis a typical concentration of 1000 ppm N2O (ideal conditions) or 1000 ppm N2O þ 200 ppm NO þ 2% O2 þ 0.5% H2O (real conditions) in N2 as carrier gas and a GHSV of 54,000 h1 was used. The temperature was increased starting from 130 C in steps of 40 K, each temperature was kept constant for 30 min and the N2O conversion was determined via IR-analysis according to X ¼ [c(N2O)in e c(N2O)out]/c(N2O)in$100%. From this first order rate constants were calculated using the following formula: k ¼ ln(1X)$F/Vreactor. A Flow F of 90 mL/min and a reactor volume of 0.2 mL were used in all catalytic runs. 2.3. Characterization X-Ray powder diffraction measurements were performed using a Siemens D5000 power X-ray diffractometer equipped with a CuKa radiation source. N2 physisorption isotherms were collected at 196 C on a Quantachrome Quadrasorb SI instrument. Prior to the analysis the samples were degassed at 100 C in vacuum for 12 h. In order to obtain specific surface areas the BET model in a p/ p0 range of 0.05e0.2 was used. The total pore volume was determined at p/p0 ¼ 0.95. Pore diameter of SBA-15 and KIT-6 were calculated via a DFT method in equilibrium mode, while those of SBA-16 and FDU-12 were determined using the adsorption branch. Elemental analysis ICP-OES was performed on Modell Spectro of Analytical Instruments. TEM-EDX measurements were performed on a FEI Tecnai F20. All TEM-samples were prepared on Ni-grids. 3. Results and discussion 3.1. Characterization of templates and their replica 3.1.1. SBA-15 as template From the results of N2 physisorption isotherms (see Supporting information Fig. S1) specific surface area SBET, pore volumina Vp and
Fig. 2. a) N2-physisorption isotherms of the four Cu0.25Co2.75O4 replica templated with SBA-15 after template removal. An onset of the Isotherms of ~30 units to each other is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with SBA-15 after template removal. Inset: Large angle XRD of the replica confirming the spinel phase.
pore diameter dp (Table 1) were determined of the four SBA-15 templates aged at 60 C, 80 C, 100 C and 120 C. The nitrogen physisorption isotherms show the typical type IV course of porous materials with a narrow pore size distribution. Increasing the hydrothermal temperature results in an increase of the SBET from 332 to 894 m2 g1, Vp from 0.35 to 1.24 cm3 g1 and dp from 5 to 8.2 nm. Via small angle XRD (see Supporting information Fig. S2), the ordered arrangement of the pores within the materials is confirmed as the [100] and [110] reflexes appear. Fig. 2a shows the N2-physisorption isotherms and the pores size distribution of the replicated Cu0.25Co2.75O4 spinel materials after removal of silica with NaOH solutions. The two replica REP-SBA-1560 C and REP-SBA-15-80 C show a typical type II isotherm. While
Table 1 Results from N2 physisorption of SBA-15 templates prepared at four different hydrothermal temperatures and of the corresponding replica. Template
SBA-15
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.60 0.35 1.24 1.15
699 332 894 736
5.0 6.4 8.2 8.2
0.21 0.22 0.18 0.18
94 95 99 91
e e 5.4 5.6
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the isotherms of the two replica REP-SBA-15-100 C and REP-SBA15-120 C exhibit more a type IV character with a clear hysteresis. Analysis of the pore size distributions (inset in Fig. 2a) indicate, that the latter two replica have pores with diameters between 4 and 8 nm, while the former two replica are non-porous. At lower hydrothermal temperatures during template synthesis (60 C & 80 C) interconnecting micropores between the mesopores are unincisive and only present as a corona in the walls of the 2d hexagonal aligned mesopores. With increasing hydrothermal temperature the interconnecting micropores become more distinct and increase in diameter. Therefore, the two replica REP-SBA-15-60 C and REPSBA-15-80 C are achieved as 2d rods without any 3d interconnectivity. With an increasing formation of interconnecting micropores in the SBA-15 template at higher hydrothermal temperatures the achieved replica REP-SBA-15-100 C and REPSBA-15-120 C become 3d cross-linked. Due to a stable 3d structure in the replica the mesoporous character within these materials increases. In the small angle XRD pattern of the replica (Fig. 2b) the reflexes [100] and [110] increase in intensity using templates prepared at higher hydrothermal temperatures. Hence, this indicates an increasingly ordered pore arrangement for REP-SBA-15-100 C and REP-SBA-15-120 C. The two replica REP-SBA-15-60 C and REP-SBA-15-80 C do not show any evidences for ordered porosity. The high angle regime of the XRD patterns (inset in Fig. 2b) exhibits the typical reflexes of the spinel phase. This emphasizes the crystalline character of the mesoporous replica REP-SBA-15-100 C and REP-SBA-15-120 C. The reflexes of all four replica are broadened indicating crystallite sizes of the four replica below 200 nm with a comparable diameter for all materials. The change of the type II to a type IV isotherm and the appearance of the typical reflexes for the ordered pore system can be correlated with the increase of the pore diameter of the template. A collection of the textural properties of the template and the final replica are summarized in Table 1. All four replicated Cu0.25Co2.75O4 reveal specific surface areas in the range of 91e99 m2 g1. Due to Hard Templating the SBET of the replicated spinel is around 4 times higher compared to the SBET of the spinel prepared without templating (SBET ¼ 26 m2 g1). The pore diameter and SBET of the SBA-15 template does not affect the SBET of the final replica. But with increasing interconnectivity of the mesopores by micropores at higher hydrothermal temperatures within the template the three dimensional and ordered mesoporous character of the replica increases. This gives evidence that the replica become more mechanically stable when the interconnectivity of the pores within the template increases. Furthermore, the pore diameter of the replica increases slightly from 5.4 to 5.6 nm with higher hydrothermal temperature. In the replication process the maximal diameter of the material is restricted by the pore sizes of the template, while the wall thickness of the template determines the pore diameter of the pores in the replica. Thus, the materials REPSBA-15-60 C and REP-SBA-15-80 C collapse during template removal due to the absence of a 3d cross-linked pore structure. For this reason, non-porous rodlike nanoparticles are achieved from those two replica. Because of the presence of 2d rods in all cases all four replica exhibit comparable specific surface areas. The smaller diameter of 2d rods in REP-SBA-15-60 C and REP-SBA-15-80 C compensates the effect of higher SBET due to increasing mesoporosity within the material. Combining the results from XRD and N2-physisorption shows that the spinel REP-SBA-15-100 C has the most ordered mesoporous character. TEM micrographs of this material (Fig. 3) display the hexagonal alignment of nanorods of the replicated pore system of SBA-15. The material is ordered over the whole sample and large mesoporous ordered particles close to micrometer size are
Fig. 3. TEM-micrograph of REP-SBA-15-100 C. Inset: close up of REP-SBA-15-100 C with EDX-line scan (black line).
produced. An EDX line scan (inset in Fig. 3 (black line)) of a single layer of ordered spinel clearly indicates that only trace amounts of silica are left in the sample. These trace amounts are not accumulated within the pores of the material. Therefore, these traces are not responsible for the achieved isotherms and mechanical stability of the material. Furthermore, the amount of Co and Cu decreases in the lighter parts of the line scan clearly evidencing the open porous character of this spinel. All samples were prepared on Ni-grids for TEM-measurement. Therefore the detected Cu signals by EDX in this and the following samples fully originate from the replica. This indicates a stable integration of Cu in the spinel structure and only minor amounts were lost due to leaching during template removal. 3.1.2. KIT-6 as template Data from small angle XRD and N2 physisorption (see Supporting information Figs. S3 & S4) of the four produced KIT-6 silica template confirm an ordered mesoporous pore system of the materials with a narrow pore size distribution. Furthermore, the higher the temperature during hydrothermal treatment the higher is the SBET, VP and dp of the KIT-6 template (Table 2). SBET increases up to 871 m2 g1 and the pore diameter raise from 6.4 to 9.4 nm due to the increase of the hydrothermal temperature from 60 C to 120 C. Using the four KIT-6 materials as template for Cu0.25Co2.75O4 results in replica with significantly increased SBET values of up to 117 m2 g1. A general trend is observable for the SBET of the replica: The smaller dp and Vp of the KIT-6 template the higher is the SBET of the final replica. Results from N2 physisorption and XRD (Fig. 4) indicate that with the use of all KIT-6 templates ordered mesoporous Cu0.25Co2.75O4 replica with spinel structure were achieved. A correlation between the SBET of the template and the SBET of the replica could not be found. But the materials REP-KIT-6-60 C, REPKIT-6-80 C and REP-KIT-6-100 C show two hystereses in the isotherms. One at lower and one at higher partial pressure. Therefore, it seems that a bimodal pore system formed in the replica. The pore system exhibits pore diameters of 4.8 nm and 13.8 nm, respectively. The amount of larger pores in the replica
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Table 2 Overview of the N2 physisorption results of four KIT-6 templates produced at different hydrothermal temperatures and results of their corresponding replica. Template
KIT-6
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.70 0.80 1.20 1.3
693 761 871 809
6.2 7.0 8.2 9.4
0.31 0.27 0.21 0.17
117 116 110 99
5.2/13.8 4.8/13.8 4.8/13.8 4.8
decreases with the increase of the pore sizes of the template (inset in Fig. 4a). The template KIT-6-120 C has the biggest pore diameter of 9.4 nm of the KIT-6- templates. With this template a replica with only one pore system of dp ¼ 4.8 nm was obtained. TEM micrographs of the replica REP-KIT-6-120 C confirm the presence of ordered mesoporosity within the compound (Fig. 5). In the micrograph three different orientations of the pores are observable. In area 1 the typical pore system of KIT-6 of two interwoven channels is fully replicated. In area 2 it is observable that only one of the two channel systems is replicated while the second is missing. This explains the discovery of two different pore sizes in N2-physisorption. The smaller pore diameter of 4.8 nm
corresponds to a fully replicated KIT-6 pore system as shown in area 1. Whereby, the pores in the replica result from the free spaces between the two replicated pore systems as shown in area 2. The larger pore diameter of 13.8 nm corresponds to the replication of one of the two interconnected pore systems of the KIT-6-template. In sample REP-KIT-6-120 C mainly both pore systems of KIT-6 template were replicated. Therefore even if small amounts of single replicated material are found these small contributions cannot be detected by N2-sorption. In area 3 multilayers of a fully replicated pore system are visible but the orientation of the material is not along the axis of the pore system. For this reason, the pore system appears disordered. The EDX line scan (inset in Fig. 5 (black line)) shows that the replicated material consists only of Cu, Co and O. In the regions where pores are expected the amount of Cu and Co drops to zero while the amount of Si does not increase. This shows that the silica was fully dissolved out of the material and only trace amounts of Si are left. It can be concluded that the 3d structured crystalline system of Cu0.25Co2.75O4 is stable by its own. These results indicate incomplete filling of the pore system of the KIT-6templates with smaller pore sizes with the metal precursor solution during impregnation. Incomplete replication of both pore systems presents the consequence. Therefore, the amount of larger pores is bigger for smaller pore sizes of the template. 3.1.3. SBA-16 as template XRD of the silica templates (see Supporting information Figs. S5 & S6) show at small angles the typical reflexes [110] and [200] of
Fig. 4. a) N2-physisorption isotherms of the four Cu0.25Co2.75O4 replica templated with KIT-6 after template removal. An onset of the Isotherms of ~30 units to each other is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with KIT-6 after template removal. Inset: Large angle XRD of the replica.
Fig. 5. TEM-micrograph of REP-KIT-6-120 C. Inset: Close up of REP-KIT-6-120 C with EDX-line scan (black line).
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the ordered pore system of SBA-16. N2-Physisorption isotherms exhibit a hysteresis indicating a successful synthesis of the mesoporous silica with a cubic body centered spherical pore structure with interconnecting tubes. Increasing the temperature of the hydrothermal step during the silica synthesis dp increases from 5.2 to 10.4 nm, Vp from 0.34 to 0.93 cm3 g1 and SBET increase from 595 to 1011 m2 g1 (Table 3). For SBA-16-100 C and SBA-16-120 C also pores with a smaller pore diameter appear. These pores correspond to the diameter of the interconnecting tubes between the spherical pores. For the smaller pores the pore diameter increases with increasing hydrothermal temperatures. In order to obtain an open porous structure the connecting micropores need to be present in the templates SBA-16-60 C and SBA-16-80 C, as well. As these micropores decrease in size with lower hydrothermal temperatures, they are not detectable in the measured p/p0 range for these two templates due to the small diameter of the micropores. In the isotherms of the templated Cu0.25Co2.75O4 materials a hysteresis appears only for the two materials REP-SBA-16-100 C and REP-SBA-16-120 C showing an type IV character. In contrast, the two materials REP-SBA-16-60 C and REP-SBA-16-80 C show a typical type II isotherm (Fig. 6a). Therefore, the former two materials appear to be mesoporous, while the latter two materials are non porous. The two mesoporous replica were produced with SBA16 templates with larger pore diameters of 9.4 and 10.5 nm. While the templates of the two replica without porosity have smaller pore sizes of 5.2 and 5.4 nm. A general trend can be observed for the SBET. With decrease of the pore diameter of the template the SBET of the final replica increases from 117 to 138 m2 g1. Small angle XRD only shows the typical reflex [110] of a cubic body centered structure for the materials REP-SBA-16-100 C and REP-SBA-16-120 C (Fig. 6b). Obviously, for these two materials an ordered mesoporous structure is formed. Accordingly, formation of 3d structured spinel materials is facilitated for SBA-16 templates with larger pores. Therefore it is suggested, that the 3d structured materials collapse during template removal if the thickness of the bridging rods in the replica become too thin. The higher specific surface area of the materials REP-SBA-16-60 C and REP-SBA-16-80 C can be thus correlated to the external surface of small particles. In the XRD at higher angles the typical pattern of the spinel phase is present and broadened in the case of all four replica with comparable crystallite size for the four materials. In the TEM-micrographs (Fig. 7) of REP-SBA-16-120 C the spherical shape of the replicated material is notable with an ordered alignment over the whole sample. With a higher resolution even the interconnecting bridges of the spheres are visible confirming the three dimensional character of the replica (inset in Fig. 7). An EDX line scan shows only trace amounts of silica present within the final material. For this material a constant Cu and Co amount was detected over the length of the scan. Restricted by the alignment of the pores in the SBA-16 template (Fig. 1c), the replica consists of a cubic body centered alignment of connected spheres. Therefore, underneath the interstice between four neighboring spheres the spheres from the next layer are arranged. Consequently, the Cu and Co amounts in the EDX line scan stay constant as long as not only one single layer of the material is analyzed.
Fig. 6. a) N2-physisorption isotherms of the four Cu0.25Co2.75O4 replica templated with SBA-16 after template removal. An onset of the Isotherms of ~40 units to each other is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with SBA-16 after template removal. Inset: Large angle XRD of the replica.
3.1.4. FDU-12 as template The typical reflexes [111] and [311] of the pore system of FDU-12 appear in small angle XRD together with a hysteresis in the N2 physisorption (see Supporting information Figs. S7 & S8). This indicates the successful synthesis of mesoporous FDU-12 with a cubic face centered alignment of spherical pores interconnected to each other (Fig. 1d). The increase of the temperature during hydrothermal treatment results in increasing dp, SBET and Vp of the FDU-12 template (see Table 4). By increasing the temperature during the hydrothermal treatment from 60 C to 120 C, dp rises from 10.4 nm to 12.8 nm, SBET from 206 m2 g1 to 423 m2 g1 and Vp from 0.20 cm3 g1 to 0.51 cm3 g1. Furthermore, all four materials show a narrow pore size distribution.
Table 3 Overview of the N2-physisorption results of four SBA-16 templates produced at different hydrothermal temperatures and results of their corresponding replica. Template
SBA-16
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.34 0.42 0.76 0.93
549 644 953 1011
5.2 5.4 2.6/9.4 3.2/10.4
0.29 0.30 0.21 0.20
138 131 117 118
e e 6.0 6.0
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3d structure. Therefore with an increase of the pore size of the template the mechanical stability of the replica increases. In the higher angle regime of the XRD pattern (inset in Fig. 8b) typical reflexes of the spinel phase appear for all replica. These reflexes are slightly broadened compared to the reflexes of the not replicated material, but are less broadened than the reflexes of the replica prepared with SBA-15, KIT-6 and SBA-16 as templates. This indicates that the crystallite sizes of the replica produced with FDU12 as template are bigger than prepared with the other templates. In the TEM-Micrographs of REP-FDU-12-120 C (Fig. 9) the replication of the ordered, spherical pores of the FDU-12 material is easy notable over large regions of the sample. With a closer resolution (inset in Fig. 9) even the replication of the interconnecting pores of the template are visible giving the reason for a stable 3 dimensional structure of the replica. The EDX-line scan shows a successful removal of the silica template. As described for REP-SBA16-120 C it is not expected that the amount of Cu and Co fully drops to zero over the length of the line scan, due to the detection of the next layer of spheres underneath. At the point of the scan where the beam hits no sphere in the next layer the Cu and Co amount drops confirming the open porous structure of the replica.
Fig. 7. TEM-micrograph of REP-SBA-16-120 C. Inset: Close up of REP-SBA-16-120 C with EDX-line scan (black line).
Using the four FDU-12 materials as template for Cu0.25Co2.75O4, the final replica after removal of the template exhibit specific surface areas increasing from 42 m2 g1 (REP-FDU-12-60 C) to 103 m2 g1 (REP-FDU-12-120 C). This trend goes in line with the increase of the SBET, Vp and dp of the template. For the replica REPFDU-12-100 C and REP-FDU-12-120 C the appearance of a hysteresis in the N2-physisorption isotherms indicates the formation of porous materials (Fig. 8a). In line, small angle XRD (Fig. 8b) of these two materials show the typical reflexes [111] and [311] of ordered mesoporous materials with a cubic face centered alignment. The other two replica REP-FDU-12-60 C and REP-FDU-12-80 C do neither show a distinct hysteresis nor reflexes in the small angle XRD region. Accordingly, these two materials are non-porous and do not exhibit an ordered porous structure. Opposite to the trend observed for the other templates, for FDU-12 the specific surface area of the replica increases by using FDU-12 with larger pore diameter. The spherical pores as well as the interconnecting pores increase in diameter with increasing hydrothermal temperature during template synthesis. The interconnecting pores are essential for a stable 3d structured replica. If the interconnecting pores decrease in diameter the replica become mechanically unstable and the 3d structure collapses during template removal. With increasing hydrothermal temperature during FDU-12 synthesis not only the spherical pores increase in diameter but also the interconnecting pores. The bigger these bridging pores the thicker becomes the replica and the more mechanically stable becomes the
3.2. Catalytic activity of replica in N2O decomposition To elucidate the impact of structuring the Cu0.25Co2.75O4 catalysts on the decomposition of N2O into N2 and O2 for every used silica template two replica with the lowest and highest SBET of each series were applied in the reaction. For replica of SBA-15 only the one material with the best 3d structuring was used as catalyst as all REP-SBA-15 samples resulted in materials with comparable SBET values. Comparing the first order rate constants at 290 C k290 (Table 5) for a catalytic run performed in ideal reaction conditions to each other shows that all spinels prepared via Hard Templating reach higher catalytic activities than the conventionally prepared spinel with low SBET. While k290 with around 41.3 min1 is low for the conventional spinel k290 values up to 1187.4 min1 are obtained for REP-FDU-12-120 in the same conditions. For each of the investigated geometries the replica with the higher SBET reaches higher k290 values and therefore higher catalytic activity. Furthermore, catalysts with comparable composition and SBET obtain comparable k290 values. Fig. 10 shows as general trend an increase of k290 values with increasing SBET. This evidences that for the N2O decomposition catalysts with higher specific surface area lead to higher catalytic activities. With a higher SBET an overall higher number of surface atoms on the catalysts are accessible. As it can be observed from elemental analysis all investigated catalysts show comparable ratios between Cu and Co atoms (Table 5). Therefore, the observed differences in catalytic activity are not a result of varying spinel composition. The observed trend in Fig. 10 is not an ideally linear trend. This indicates that also other parameters like geometry or exposed crystal planes, beside the increased SBET, contribute to the observed catalytic activity.
Table 4 Overview of the N2 physisorption results of four FDU-12 templates produced at different hydrothermal temperatures and results of their corresponding replica. Template
FDU-12
Replica
T (hydr.)/ C
VP/cm3 g1
SBET/m2 g1
dP/nm
VP/cm3 g1
SBET/m2 g1
dP/nm
60 80 100 120
0.20 0.32 0.42 0.51
206 309 356 423
10.4 10.4 11.3 12.8
0.11 0.22 0.12 0.27
42 74 64 103
e e 6.2 7.0
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Fig. 9. TEM-micrograph of REP-FDU-12-120 C. Inset: close up of REP-FDU-12-120 C with EDX-line scan (black line).
Table 5 Comparison of first order rate constants at 290 C in ideal (k290) and at 410 C in real reaction conditions (k410) and n(Cu)/n(Co) ratios for the replica used as catalysts in N2O decomposition.
Fig. 8. a) N2-physisorption isotherms of the Cu0.25Co2.75O4 replica templated with FDU-12 after template removal. An onset of the Isotherms of ~25 units is used for better illustration. Inset: Pore size distribution of the replica. b) Small angle XRD of replica templated with FDU-12 after template removal. Inset: Large angle XRD of the replica.
Also in realistic reaction conditions the same trends as in ideal conditions can be observed, but shifted to higher temperatures due to the presence of inhibiting gases (Table 5). Therefore, first order rate constants for the reaction at 410 C (k410) are given for a comparison of the catalytic activities of the different replica. In both reaction conditions, ideal and real, REP-FDU-12-120 shows much higher rate constants than the replica with the other geometries even though SBET and composition are comparable. This reveals that the cubic face centered alignment of the catalyst due to casting of FDU-12 yields into a more active catalyst than with the other geometries. With REP-FDU-12-120 k290 values up to 1187.4 min1 and k410 values up to 1717.2 min1 were obtained under ideal and real reaction conditions, respectively. For this reason the dependency of the N2O conversion on the temperature will be closer discussed for REP-FDU-12-60 (lowest SBET ¼ 42 m2 g1) and REP-FDU-12-120 (highest SBET ¼ 103 m2 g1) in ideal and real reaction conditions (Fig. 11). The material REPFDU-12-60 C exhibits enhanced catalytic activities compared to the conventionally synthesized Cu0.25Co2.75O4 with lower specific surface area. A linear increase of the conversions with temperature from 130 C to 250 C is observable for REP-FDU-12-60 in ideal reaction conditions. Above 250 C a typical S-shaped increase of conversion with temperature according to Arrhenius law is observed. This indicates casting of the spherical pores of FDU-12
Cu0.25Co2.75O4 REP-SBA-15-100 REP-KIT-6-60 REP-KIT-6-120 REP-SBA-16-60 REP-SBA-16-120 REP-FDU-12-60 REP-FDU-12-120
k290/min1
k410/min1
n(Cu)/n(Co)ICP
41.3 540.3 453.7 522.1 670.9 419.0 216.9 1187.4
13.6 465.0 e 1497.6 e 1426.5 e 1717.2
0.090 0.089 0.085 0.087 0.083 0.083 0.093 0.085
Fig. 10. Dependency of the first order rate constants for N2O decomposition at 290 C (k290) in ideal conditions on the SBET of replicated Cu0.25Co2.75O4 catalysts.
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the high specific surface areas of the materials even at high temperatures. Furthermore, additional studies are required enabling a deeper understanding concerning the influence of the structure of the replica on catalytic activity. 4. Conclusions
Fig. 11. N2O conversion against temperature from continuous N2O decomposition experiments of REP-FDU-12-60 C and REP-FDU-12-120 C as catalyst compared to Cu0.25Co2.75O4 without templating in ideal reaction conditions (closed symbols; c(N2O) ¼ 1000 ppm in N2, GHSV ¼ 54,000 h1) and in real reaction conditions (open symbols; c(N2O) ¼ 1000 ppm, c(NO) ¼ 200 ppm, c(O2) ¼ 2%, c(H2O) ¼ 0.5% in N2, GHSV ¼ 54,000 h1).
produces different kinds of surface sites in the templated spinel compared to conventionally synthesized spinels. As the crystallinity is higher for replica prepared with FDU-12 templates than for the other replica the number of atoms on defect positions is lower compared to the other replica. Therefore, it is not likely that these atoms are responsible for the increased catalytic activity. Instead a higher crystallinity results in lager exposed crystal planes. It was shown before that catalytic activities of Co3O4 is significantly affected by the exposed crystal planes. For example in CO-oxidation [20] and also in selective catalytic reduction of NOx by NH3 [21], materials with (110) as exposed crystal plane lead to the best catalytic performances. Therefore, it seems that the pore size and geometry of the FDU-12 template facilitates exposing crystal planes on the surface of the spinel which are more active in catalysis. Why this effect is stronger pronounced for FDU-12 as template than for the other templates even though the composition, SBET and Vp is the same, will be investigated in future studies. A further increase of the specific surface area by using the catalyst REP-FDU-12-120 C reveals much higher catalytic activity. Due to the enhanced SBET the N2O decomposition reaction set off temperature is below 130 C and full conversion is already achieved at 330 C. For the conventional catalyst, the reaction starts at 250 C and full conversion is obtained at temperatures as high as 450 C. In this case, the surface enhanced catalyst REP-FDU-12-120 C performs better in realistic reaction conditions than the conventional Cu0.25Co2.75O4 in ideal reaction conditions. With REP-FDU-12120 C as catalyst first conversions are detected at 210 C and full conversion is already achieved at 410 C even in the presence of inhibitory gases such as NO, O2 and H2O. Compared to this the reaction does not start until 370 C over the not modified Cu0.25Co2.75O4 catalyst under comparable reaction conditions. Full conversion is observed at temperatures as high as 570 C. However, N2-physisorption analysis after catalysis suggests sintering of the templated material during catalysis as the SBET and Vp decrease to 50.4 m2 g1 and 0.13 cm3 g1, respectively. However, after catalysis the SBET of REP-FDU-12-120 C is still double as high as the conventionally synthesized Cu0.25Co2.75O4 catalyst. The presented results illustrate the positive effect of enhanced specific surface areas of spinel materials in N2O decomposition. Nevertheless, sintering occurs for all presented structures at higher temperatures. Therefore, further investigations need to focus on possible ways to decrease sintering and simultaneously preserve
We have shown that ordered porous Cu0.25Co2.75O4 materials can be achieved with the four mesoporous silica SBA-15, KIT-6, SBA-16 and FDU-12 as hard templates. It was also demonstrated that the success of the replication process strongly depends on the synthesis conditions of the silica template and thus on the textural properties of the template. The bigger the pore sizes of the template the more stable become the 3d structured replica and the more distinct is the porous character of the achieved replica. Mechanically stable 3d linked replica can be obtained only with pore diameter of the template larger than 8 nm. The SBET of the final replica show a contrary behavior. The lower the pore diameter of the template the higher is the achieved SBET of the replica. This is mainly due to the decrease in particle size and therefore a higher surface to bulk ratio. In this set of experiments, the highest specific surface area of 138 m2 g1 was achieved with SBA-16 as template synthesized at a hydrothermal temperature of 60 C. Compared to a specific surface area of Cu0.25Co2.75O4 prepared by solution combustion synthesis the obtained SBET is more than 5 times higher. The use of surface enhanced catalysts in the decomposition of N2O confirms the importance of the specific surface area in this reaction. Due to the enhanced surface area, the starting temperatures of the reaction could be reduced below 130 C compared to 250 C for the conventional catalyst with low specific surface area in ideal reaction conditions. The influence of high SBET values of this kind of catalysts for high catalytic performances is even more pronounced in real reaction conditions. Thereby, the light off temperature could be reduced to 210 C and full conversion can be obtained already at 410 C with REP-FDU-12-120 C compared to 370 C and 570 C respectively for Cu0.25Co2.75O4 in the presence of NO, O2 and H2O. Catalysts with comparable specific surface area exhibit comparable catalytic activities. However, REP-FDU-12120 C exhibits higher catalytic activity than the other materials with comparable SBET, an effect which has to be investigated further in future studies. Concerning stability, all templated materials suffered from thermally induced sintering during catalysis. Overall, we showed that surface enhancement and structuring of crystalline materials via Nanocasting is highly valuable for the catalytic decomposition of N2O into N2 and O2 and is expected to show comparable trends in other gas phase applications. Nevertheless, the success of the preparation of a 3d structured crystalline material depends highly on the pore sizes of the used hard template. For stable 3d structured materials larger pore diameter within the used hard template are recommended. Acknowledgment We like to acknowledge the financial support by the Max Buchner foundation (Grant Number: 3334). Part of this work was performed in the frame of the project house “Center for Automotive Catalytic Systems Aachen-ACA” of RWTH Aachen University financed by the Excellence Initiative of the German federal and state governments to promote science and research at German universities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.09.019.
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References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [3] Q. Huo, D. Margolese, U. Ciesla, P. Feng, T. Gier, P. Sieger, R. Leon, P. Petroff, F. Schüth, G.D. Stucky, Nature 368 (1994) 317. [4] P. Llewellyn, M. Keung, S. Kallus, F. Schüth, K.K. Unger, Stud. Surf. Sci. Catal. 84 (1994) 2013. [5] S.M. Ng, S. Ogino, T. Aida, K.A. Koyano, T. Tatsumi, Macromol. Rapid Commun. 18 (1997) 991. € ller, T. Bein, R.X. Fischer, Chem. Mater. 10 (1998) 1840. [6] K. Mo [7] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [8] D. Gu, F. Schüth, Chem. Soc. Rev. 43 (2014) 313. [9] F. Schüth, Chem. Mater. 13 (2001) 3184.
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
K.K. Zhu, B. Yue, W.Z. Zhou, H.Y. He, ChemComm (2003) 98. W. Yue, A.H. Hill, A. Harrison, W. Zhou, ChemComm (2007) 1518. J. Zhu, Q. Gao, Microporous Mesoporous Mater. 124 (2009) 144. Y. Cui, Z. Wen, S. Sun, Y. Lu, J. Jin, Solid State Ionics 225 (2012) 598. H. Jin, X. Gu, B. Hong, L. Lin, C. Wang, D. Jin, X. Peng, X. Wang, G. Ge, J. Phys. Chem. C 116 (2012) 13374. J. Rosen, G.S. Hutchings, F. Jiao, JACS 135 (2013) 4516. T. Franken, R. Palkovits, Appl. Catal. B 176 (2015) 298. D. Zhao, Q. Huo, J. Feng, B.B. Chmelka, G.F. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. F. Kleitz, S.H. Choi, R. Ryoo, Chem. Commun. (2003) 2136. J. Fan, C. Yu, F. Gao, J. Lei, B. Tian, K. Wang, Q. Luo, B. Tu, W. Zhou, D. Zhao, Angew. Chem. Int. Ed. 42 (2003) 3146. X. Xie, Y. Li, Z.-Q. Liu, M. Haruta, W. Shen, Nature 458 (2009) 746. B. Meng, Z. Zhao, X. Wang, J. Liang, J. Qiu, Appl. Catal. B 129 (2013) 491.