DOI: 10.1002/chem.201303741
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& Supported Catalysts
Polymeric Carbon Nitride/Mesoporous Silica Composites as Catalyst Support for Au and Pt Nanoparticles Ping Xiao,[a] Yanxi Zhao,[a] Tao Wang,[a] Yingying Zhan,[b] Huihu Wang,[c] Jinlin Li,[a] Arne Thomas,[d] and Junjiang Zhu*[a]
Abstract: Small and homogeneously dispersed Au and Pt nanoparticles (NPs) were prepared on polymeric carbon nitride (CNx)/mesoporous silica (SBA-15) composites, which were synthesized by thermal polycondensation of dicyandiamide-impregnated preformed SBA-15. By changing the condensation temperature, the degree of condensation and the loading of CNx can be controlled to give adjustable particle sizes of the Pt and Au NPs subsequently formed on the composites. In contrast to the pure SBA-15 support, coating
Introduction The preparation of metal nanoparticles (NPs) with controlled shape and size is of great interest in many fields, including electronics,[1] biological labeling,[2] magnetic devices,[3] information storage,[4] and especially catalysis.[5] The intrinsic properties of metal NPs depend mainly on their size, shape, composition, crystallinity, and structure,[6] and various efforts to tailor these properties have been made.[6–9] In catalytic applications, small particle size and good dispersion of the catalytic NPs in general lead to high activity due to the high ratio of surface to bulk atoms For use as catalysts, metal NPs can be suspended in solution or supported on a solid. The preparation of suspended metal NPs is simple, and very small NPs can be obtained, but the use of a protective agent is essential to prevent agglomeration of
[a] P. Xiao, Dr. Y. Zhao, T. Wang, Prof. J. Li, J. Zhu Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs & Commission Ministry of Education South-Central University for Nationalities Wuhan 430074 (P. R. China) E-mail: [email protected] [b] Dr. Y. Zhan National Engineering Research Center of Chemical Fertilizer Catalyst Fuzhou University, Fuzhou 350002 (P. R. China) [c] Dr. H. Wang School of Mechanical Engineering Hubei University of Technology Wuhan 430068 (P. R. China) [d] Prof. A. Thomas Department of Chemistry, Technische Universitt Berlin Hardenbergstr. 40, 10623 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303741. Chem. Eur. J. 2014, 20, 2872 – 2878
of SBA-15 with polymeric CNx resulted in much smaller and better-dispersed metal NPs. Furthermore, under catalytic conditions the CNx coating helps to stabilize the metal NPs. However, metal NPs on CNx/SBA-15 can show very different catalytic behaviors in, for example, the CO oxidation reaction. Whereas the Pt NPs already show full CO conversion at 160 8C, the catalytic activity of Au NPs seems to be inhibited by the CNx support.
the particles.[10–12] For practical applications, supported metal NPs on mostly porous supports are preferred. However, for supported metal NPs it is still a rather challenging task to ensure stability of the NPs and prevent their agglomeration and ripening during the reaction.[13–15] Among the materials used for supporting metal NPs, ordered mesoporous silicas such as SBA-15[16, 17] have received a lot of attention due to their straightforward synthesis and highly defined textural properties, such as high surface area, ordered pore structure, and controllable pore size. However, since the surface of silicas is relatively inert, the formation of small and stable metal NPs on the silica surface is difficult to achieve. Therefore organic compounds bearing, for example, amino or thiol groups, are often used to functionalize the SBA15 surface before impregnation with metal ions and subsequent formation of the metal NPs.[18–20] However, such functional groups can be problematic if the final catalyst is used at higher temperatures, as they can be thermally cleaved with high energy release[18] which again would result in agglomeration and leaching of the NPs during the reaction.[18, 21] K. P. de Jong et al.[22] found that small transition metal (e.g., Cu) NPs supported on SBA-15, prepared by incipient wetness impregnation, can be stabilized by controlling the collective properties (e.g., the relative spatial location) of metal NPs. Herein, we report that small noble metal (e.g., Au and Pt) NPs with controlled particle size and high stability can be prepared by a facile impregnation method using polymeric carbon nitride/mesoporous silica composite as support. Carbon nitrides and nitrogen-doped carbons have been found to stabilize metal NPs when used as support.[23–26] Since they also have high thermal stability (up to 600 8C) and stability against oxidation, such materials are interesting supports for ensuring that the metal NPs are applicable in elevated-temperature reactions
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Full Paper without agglomeration or leaching. However, polymeric carbon nitrides have low surface areas when prepared in bulk, and hence laborious templating methods are used to increase the accessible surface and make them suitable catalysts or catalyst supports.[27–30] In our approach, polymeric carbon nitrides (CNx) are used to functionalize porous silica supports to stabilize metal NPs. SBA15 was first impregnated with a layer of CNx before being used to support Pt or Au NPs. The CNx was prepared by polycondensation of dicyandiamide that was impregnated into the pores of SBA-15, similar to that reported elsewhere.[31] The formation of CNx from dicyandiamide is briefly illustrated in Scheme 1. Depending on the treatment temperature, the
Scheme 1. Reaction path for the formation of polymeric graphitic carbon nitride (g-CNx) starting from dicyandiamide.
degree of polycondensation of CNx formed on the surface of SBA-15 changes from melamine to melem, melon, and a more condensed polymeric carbon nitride. As the ratio of primary to tertiary amine changes during this reaction, the functionality of the support can be tailored by simple heat treatment. Furthermore, the amino functional groups on the surface of CNx are stable up to 600 8C, which is much higher than for any grafted amines or other functional groups. Therefore, CNx could provide stable functionality even at high temperatures. Supported Au and Pt NPs have been reported to be good heterogeneous catalysts.[15, 32, 33] Therefore, the catalytic performance of the Au and Pt NPs were also tested, by using CO oxidation as a model reaction. Catalytic CO oxidation is not only a way of removing the poisonous gas CO, but also a widely used test reaction to evaluate catalytic performance of catalysts. Our results indicate that the Pt-NPs on CNx/SBA-15 are highly active catalysts for CO oxidation, whereas the Au NPs on CNx/SBA-15 show negligible activity.
Results and Discussion The wide-angle XRD patterns of a series of SBA-15-supported CNx materials treated at different temperatures, CNx/SBA-15_T (T = temperature), show an intense peak at 2 q 26.78 and broad peak at 2 q 228 (Figure 1 A) By comparison with the diffraction peaks of SBA-15, the broad peak at 2 q 228 is attribChem. Eur. J. 2014, 20, 2872 – 2878
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Figure 1. Characterization of SBA-15 and CNx/SBA-15_T. A) Wide-angle XRD patterns. B) Small-angle XRD patterns. C) N2 adsorption/desorption isotherms. D) TGA curves and the weight losses.
uted to amorphous silica.[16, 34] Thus, the intense peak at 2 q 26.78 is assignable to CNx. With increasing temperature, the peak intensity of CNx decreased compared to that of amorphous silica SBA-15, due to the weight loss of CNx discussed below. To confirm the existence of CNx, we chose CNx/SBA-15_ 500 for further investigation. Polycondensation of dicyandia-
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Full Paper mide at 500 8C was reported to form highly condensed heptazine units, known as melon, showing a diffraction peak at 2 q 27.58, which can be assigned to graphitic stacking of the layered polymer.[35, 36] Although the weak diffraction peak observed from CNx/SBA-15_500 is shifted slightly towards smaller angles, a comparable diffraction peak was found for mesoporous CNx that was obtained by removing the SBA-15 support with a 4 m aqueous solution of NaOH (see Figure S1 in the Supporting Information). This confirms that the CNx of CNx/ SBA-15_500 exists in a highly condensed polymeric form. At smaller angles, all samples show three diffraction peaks assignable to SBA-15 (Figure 1 B), indicating that the porous structure of SBA-15 remains unchanged irrespective of the formation of CNx. The diffraction peaks of CNx/SBA-15_T appear at the same positions, but small shifts to higher angles relative to those of the parent SBA-15 indicate slight shrinkage of the porous structure due to the extra calcination step. Figure 1 C shows the N2 adsorption/desorption isotherms of SBA-15 and CNx/SBA-15_T. All samples show a type IV isotherm with H1-type hysteresis loop corresponding to an ordered mesoporous structure, which implies that the framework of SBA-15 is not destroyed after formation of CNx, in agreement with the XRD results in Figure 1 B. Smaller overall pore volumes and surface areas of the loaded samples show the presence of a nonporous CNx phase. The surface areas and pore volumes of the samples increased again after the formation of metal NPs (Figure S2 in the Supporting Information) because of the extra treatment steps. The loading of CNx on CNx/SBA-15_T prepared at different temperatures was evaluated by thermogravimetric analysis (TGA, Figure 1 D). As expected, the loading of CNx decreases with increasing temperature. Considering the weight loss (8.4 %) for SBA-15, which is caused by removal of water and gases physically adsorbed on SBA-15, the loading of CNx was calculated to be 30, 19.6, and 5.6 wt % for CNx/SBA-15_T treated at 300, 400, and 500 8C, respectively. Energy-dispersive X-ray mapping images of the carbon and nitrogen elements (Figure 2) showed that CNx is homogeneously dispersed on SBA-15, in accordance with what was observed for SBA-15-supported Fe/g-C3N4.[37] The polymeric CNx can in principle be located on the outer surface or inside the pores of SBA-15. In earlier work[31] using a similar impregnation method, removal of the porous silica clearly revealed that CNx is formed inside the mesopores of SBA-15. In this work, a decrease in pore size was observed when SBA-15 was impregnated with polymeric CNx (Figure S2 in the Supporting Information), which suggests homogeneous coating of the pore walls. A slight increase in pore size with increasing temperature (i.e., from CNx/SBA-15_300 to 500) indicates condensation and shrinkage of the polymeric CNx within the pores. This also explains the increase in surface area and pore volume with increasing treatment temperature (Figure 1 C). However, an additional layer of polymeric CNx on the outer surface of SBA-15 cannot be excluded. Figure 3 A and B show TEM images of Au/SBA-15 and Au/ CNx/SBA-15_500 prepared by the wet-impregnation (WI) method, respectively. No significant change in the texture of Chem. Eur. J. 2014, 20, 2872 – 2878
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Figure 2. TEM and EDX mapping of the C and N elements of CNx/SBA-15_ 500. TEM = transmission electron microscopy, EDX = energy-dispersive X-ray spectroscopy.
Figure 3. TEM images of A) Au/SBA-15 and B) Au/CNx/SBA-15_500. C) Dependence of particle size of Au NPs on the treatment temperature for CNx/ SBA-15_T. The average particle size was calculated by counting at least 1500 Au particles.
SBA-15 before and after loading with CNx is observed (Figure S3 in the Supporting Information), that is, impregnation with CNx does not influence the porous structure of SBA-15. The Au particles are severely agglomerated and confined in a small area on Au/SBA-15 (for more images, see Figure S3 of the Supporting Information), but are highly dispersed and randomly distributed on Au/CNx/SBA-15_500; thus, the presence of CNx helps to disperse and stabilize the Au NPs. Moreover,
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Full Paper the Au NPs are located both on the surface and inside the pores. The average particle size of the Au NPs increases from 3.2 nm for Au/CNx/SBA-15_300 to 4.7 nm for Au/CNx/SBA-15_ 500 (Figure 3 C). We suggest that the larger amount of lesscondensed CNx in SBA-15 provides more sites for metal adsorption and stabilization, and finally results in better dispersion and smaller particle size of the Au NPs. Thus, the Au particle size is indeed partially controllable by adjusting the loading of CNx or the condensation temperature of dicyandiamide. In contrast the Au particle size on pure SBA-15 (i.e. Au/SBA-15) is hard to measure, as the Au particles are severely agglomerated. The very weak and broad Au diffraction peaks in the XRD patterns of Au/CNx/SBA-15_T (Figure S1 of the Supporting Information) indicate that, on average, very small particles were formed. The formation of smaller and better-dispersed Au NPs on CNx/SBA-15_T as opposed to pure SBA-15 indicates that the CNx plays an important role in the formation process of Au NPs. Similar to Au NPs, a significant decrease in particle size and improvement in dispersion were observed for Pt NPs on going from Pt/SBA-15 to Pt/CNx/SBA-15 T (Figure 4 A and B; for others, see Figure S4 of the Supporting Information). The Pt
Figure 4. TEM images of fresh samples of Pt/SBA-15 (A) and Pt/CNx/SBA-15_ 500 (B) and of the used sample of Pt/CNx/SBA-15_500 (C).
NPs in Pt/SBA-15 are severely agglomerated, and individual particles are hard to separate. Chen et al.[14] reported that the Pt particles of Pt/SBA-15 prepared by direct deposition of H2PtCl6 species on pure SBA-15 by an impregnation method is severely sintered and shows particle size of about 18 nm, consistent with our results. Thus, the presence of CNx is indeed crucial in the stabilization of Au and Pt NPs. We expected that other porous silicas besides SBA-15 could also lead to small and size-controlled metal NPs. Indeed, Pt Chem. Eur. J. 2014, 20, 2872 – 2878
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NPs with controllable particle size could also be synthesized on SBA-16-supported CNx (see Figure S5 of the Supporting Information). This suggests that the present method of impregnating SBA-15 with CNx could be general method of preparing small and highly dispersed metal NPs, also on industrially relevant porous silica supports. Such small and highly dispersed noble metal NPs are generally good heterogeneous catalysts. For example, Haruta et al.[32] found that a nanosized Au catalyst showed a significant increase in the activity for CO oxidation compared to bulk Au. On the other hand, the high thermal stability of CNx and the strong interaction between CNx and metal NPs may guarantee that the Au or Pt NPs endure high temperature without significant agglomeration under reaction conditions. Consequently, we suspected that the herein-prepared small and stable Au or Pt NPs could even be good catalysts for reactions conducted at elevated temperature. To prove this, we tested the catalytic performances of Pt and Au NPs supported on CNx/SBA-15_500 in the CO oxidation reaction. This catalyst was used because the CNx will not undergo further structural changes during the catalytic reaction, as the highest temperature applied in CO oxidation was less than 500 8C. The reactant (0.5 % CO and 7.5 % O2 in argon) was passed through 30 mg of catalysts at a flow rate of 50 mL min 1 and a heating rate of 5 8C min 1 at each temperature stage. The catalysts were activated at 350 8C in the reactant for 30 min before starting the measurement, to remove any water and gases (e.g., CO2) that were possibly adsorbed on the catalyst. The Pt/CNx/SBA-15_500 catalyst showed 100 % CO conversion already at 160 8C (Figure 5 C). Because of the small particle size, high dispersion, and possibly new electronic properties caused by the interaction with CNx, Pt/CNx/SBA-15_500 is more active and shows lower ignition temperature for CO oxidation than comparable Pt/SBA-15 catalysts that were reported previously (Table S2 of the Supporting Information). On the other hand, Au/CNx/SBA-15_500 showed no catalytic activity for CO oxidation (Figure 5 A), and thus no change in activity was observed compared to the pure support (Figure 5 B). This is contradictory to previous reports on CO oxidation over an Au/ SBA-15 catalyst,[19, 38] whereby good CO oxidation activity could be obtained even below 40 8C. Considering that the average particle size (4.6 nm) is within the range of active Au catalysts for CO oxidation, the low activity must be ascribed to the presence of CNx, which seems to inhibit the reaction. To verify that the low activity is not due to the morphology of Au NPs or the preparation method, but to the suppressing effect of CNx, we synthesized a new Au/CNx/SBA-15_500 catalyst by a deposition–precipitation (DP) method from HAuCl4 as gold precursor, NaOH as precipitant (pH 10), and NaBH4 as reductant (for details, see Supporting Information). Au catalysts prepared by the DP method usually exhibit better activity than those prepared by the WI method applied here.[39, 40] However, this catalyst also showed very low activity for CO oxidation (Figure 5 A), which confirmed that the presence of CNx indeed suppresses the ability of Au NPs to catalyze CO oxidation irrespective of the preparation method.
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Full Paper dized (Figure S7 of the Supporting Information). The presence of oxygen-containing species in the partially oxidized Pt NPs may play important role in the reaction. The importance of oxygen-containing species or the oxidation states of noble metals to catalytic performances in CO oxidation has been reported.[23, 26, 41–44] Guzman and Gates[41] found that the CO oxidation activity increases with increasing of amount of Au + from 15 to 60 %. Nieuwenhuys et al.[42] reported that active oxygen is supplied from the oxidized support of the Au catalyst. It was also suggested that oxygen-containing species are the site of oxygen activation on Au-based catalysts.[26] In addition, Lambert et al. reported that Au0 NPs with particle size above 2 nm supported on an oxygen-free BN support are inactive to molecular oxygen and show negligible activity in oxidation reactions,[45] which proves the importance of oxygen-containing species on the catalyst for the reaction and supports our results, as CNx is also an oxygen-free material. For Pt NPs this support effect may be negligible, as they are readily oxidized even when stored in air at room temperature.[46] To further test the feasibility of the CNx/SBA-15 as support for noble metal NP catalysts, we tested the long-term stability of Pt/CNx/SBA-15_500 for CO oxidation at 160 8C. The catalyst still showed full conversion of CO after 30 h (Figure 5 D) and this confirms the stability of the catalyst. To further verify the stability of the Pt NPs, TEM images of Pt/CNx/SBA-15_500 were recorded after three runs of the reaction (Figure 4 C). The Pt particle size increased from 4.85 nm for the fresh catalyst (Figure 4 B) to 5.48 nm for the used catalyst, while the Pt particles remained highly dispersed. We can therefore conclude that CNx/SBA-15 500 indeed stabilizes the metal particles even at high temperature.
Conclusion
Figure 5. Catalytic activity of CO oxidation on A) Au/CNx/SBA-15_500 prepared by WI and DP methods, B) various supports and in the blank experiment (no catalyst), and C) Pt/SBA-15 and Pt/CNx/SBA-15_500. D) Long-term stability of Pt/CNx/SBA-15_500 in CO oxidation at 160 8C.
To elucidate the difference in activity of Au and Pt catalysts in CO oxidation, we performed X-ray photoelectron spectroscopic measurements and found that Au NPs on CNx exist exclusively as Au0, whereas the Pt NPs on CNx are partially oxiChem. Eur. J. 2014, 20, 2872 – 2878
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We have reported a new way of preparing small Au and Pt NPs with high stability and controlled particle size by means of CNx. The CNx is prepared by a polycondensation route and its amount can be controlled by adjusting the treatment temperature. Because of the high thermal stability of CNx and its strong interaction with the metal NPs, the prepared metal NPs show strong resistance to agglomeration. Catalytic studies demonstrated that Au/CNx/SBA-15_500 with metallic Au NPs shows low activity, whereas Pt/CNx/SBA-15_500 with partially oxidized Pt NPs exhibits good activity for CO oxidation, presumably because of their ability to activate oxygen. The current method of using CNx as coating material can be applied to prepare small and stable metal NPs supported on other solids (e.g., SBA-16). We believe that the herein-reported method could be a general route for synthesizing highly stable and size-controlled metal NPs, and the composites could be suitable materials for identifying the oxygen-activation route of oxidation reactions in heterogeneous catalysis.
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Full Paper Experimental Section Synthesis of CNx/SBA-15_T Dicyandiamide (3.0 g) was dissolved in ethanol/water (40/10 mL) by heating to 80 8C and kept at this temperature for 5 min to ensure all the dicyandiamide was dissolved. To this solution preformed SBA-15 (3.0 g) was added and the mixture stirred vigorously. After that, the mixture was heated at 100 8C until the ethanol and water were evaporated and a white solid was obtained. The resulting material was then dried at 100 8C in an air oven overnight and thermally treated in N2 atmosphere at different temperatures (300, 400, and 500 8C) for 4 h to yield the SBA-15 supported carbon nitrides CNx/SBA-15_T, where T is the treatment temperature.
Synthesis of Au/CNx/SBA-15 T and Pt/CNx/SBA-15_T The supported Au or Pt nanoparticles were prepared by wet impregnation of 1.0 g CNx/SBA-15_T in 50 mL of a 0.001 m aqueous solution of HAuCl4 or H2PtCl6 with vigorous stirring for about 3 h. Then, the solution was filtered, and the solid dried in a vacuum oven at 60 8C for 6 h and reduced in H2 atmosphere at 300 8C for 4 h to afford Au/CNx/SBA-15_T or Pt/CNx/SBA-15_T. The loadings calculated for Au and Pt NPs supported on CNx/SBA-15_500 were 0.95 and 0.98 wt %, respectively
Catalytic tests Catalytic CO oxidation was carried out in a fixed-bed reactor. Approximately 30 mg of sample was loaded into a quartz reactor blocked with quartz wool at the bottom, and the feed gas (0.5 % CO + 7.5 % O2 in Ar) was passed through the reactor at a flow rate of 50 mL min 1, corresponding to a space velocity of 100 000 mL h 1 gcat1 . The temperature was first raised to 350 8C and maintained for 30 min to activate the catalyst, then decreased to 150 8C in steps of 25 8C (temperature ramp: 5 8C min 1) to measure the catalytic activity. Koros–Nowak tests indicated that the reactions were in the kinetic regime and were not affected by heat- or mass-transport limitations (Table S1 of the Supporting Information). The composition of the effluent gas was analyzed by an on-line Agilent 7890 GC equipped with a thermal conductivity detector, Propack Q and 5 molecular sieve columns. CO conversion was calculated as conversion/% = {[CO]in [CO]out}/[CO]in 100, where [CO]in and [CO]out are the inlet and outlet concentration of CO, respectively.
Acknowledgements Financial support from the National Science Foundation of China (21203254), the Special Fund of the Central University Research, South-Central University for Nationalities (CZZ11007, 12ZNZ006), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (BZY11055), the Natural Science Foundation of Fujian Province, China (2012J01042), and the Cluster of Excellence “Unifying Concepts in Catalysis” (EXL 31411) is gratefully acknowledged. Keywords: heterogeneous catalysis · oxidation · platinum · supported catalysts
nanoparticles
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Received: September 24, 2013 Published online on February 4, 2014
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& Supported Catalysts
Polymeric Carbon Nitride/Mesoporous Silica Composites as Catalyst Support for Au and Pt Nanoparticles Ping Xiao,[a] Yanxi Zhao,[a] Tao Wang,[a] Yingying Zhan,[b] Huihu Wang,[c] Jinlin Li,[a] Arne Thomas,[d] and Junjiang Zhu*[a]
Abstract: Small and homogeneously dispersed Au and Pt nanoparticles (NPs) were prepared on polymeric carbon nitride (CNx)/mesoporous silica (SBA-15) composites, which were synthesized by thermal polycondensation of dicyandiamide-impregnated preformed SBA-15. By changing the condensation temperature, the degree of condensation and the loading of CNx can be controlled to give adjustable particle sizes of the Pt and Au NPs subsequently formed on the composites. In contrast to the pure SBA-15 support, coating
Introduction The preparation of metal nanoparticles (NPs) with controlled shape and size is of great interest in many fields, including electronics,[1] biological labeling,[2] magnetic devices,[3] information storage,[4] and especially catalysis.[5] The intrinsic properties of metal NPs depend mainly on their size, shape, composition, crystallinity, and structure,[6] and various efforts to tailor these properties have been made.[6–9] In catalytic applications, small particle size and good dispersion of the catalytic NPs in general lead to high activity due to the high ratio of surface to bulk atoms For use as catalysts, metal NPs can be suspended in solution or supported on a solid. The preparation of suspended metal NPs is simple, and very small NPs can be obtained, but the use of a protective agent is essential to prevent agglomeration of
[a] P. Xiao, Dr. Y. Zhao, T. Wang, Prof. J. Li, J. Zhu Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs & Commission Ministry of Education South-Central University for Nationalities Wuhan 430074 (P. R. China) E-mail: [email protected] [b] Dr. Y. Zhan National Engineering Research Center of Chemical Fertilizer Catalyst Fuzhou University, Fuzhou 350002 (P. R. China) [c] Dr. H. Wang School of Mechanical Engineering Hubei University of Technology Wuhan 430068 (P. R. China) [d] Prof. A. Thomas Department of Chemistry, Technische Universitt Berlin Hardenbergstr. 40, 10623 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303741. Chem. Eur. J. 2014, 20, 2872 – 2878
of SBA-15 with polymeric CNx resulted in much smaller and better-dispersed metal NPs. Furthermore, under catalytic conditions the CNx coating helps to stabilize the metal NPs. However, metal NPs on CNx/SBA-15 can show very different catalytic behaviors in, for example, the CO oxidation reaction. Whereas the Pt NPs already show full CO conversion at 160 8C, the catalytic activity of Au NPs seems to be inhibited by the CNx support.
the particles.[10–12] For practical applications, supported metal NPs on mostly porous supports are preferred. However, for supported metal NPs it is still a rather challenging task to ensure stability of the NPs and prevent their agglomeration and ripening during the reaction.[13–15] Among the materials used for supporting metal NPs, ordered mesoporous silicas such as SBA-15[16, 17] have received a lot of attention due to their straightforward synthesis and highly defined textural properties, such as high surface area, ordered pore structure, and controllable pore size. However, since the surface of silicas is relatively inert, the formation of small and stable metal NPs on the silica surface is difficult to achieve. Therefore organic compounds bearing, for example, amino or thiol groups, are often used to functionalize the SBA15 surface before impregnation with metal ions and subsequent formation of the metal NPs.[18–20] However, such functional groups can be problematic if the final catalyst is used at higher temperatures, as they can be thermally cleaved with high energy release[18] which again would result in agglomeration and leaching of the NPs during the reaction.[18, 21] K. P. de Jong et al.[22] found that small transition metal (e.g., Cu) NPs supported on SBA-15, prepared by incipient wetness impregnation, can be stabilized by controlling the collective properties (e.g., the relative spatial location) of metal NPs. Herein, we report that small noble metal (e.g., Au and Pt) NPs with controlled particle size and high stability can be prepared by a facile impregnation method using polymeric carbon nitride/mesoporous silica composite as support. Carbon nitrides and nitrogen-doped carbons have been found to stabilize metal NPs when used as support.[23–26] Since they also have high thermal stability (up to 600 8C) and stability against oxidation, such materials are interesting supports for ensuring that the metal NPs are applicable in elevated-temperature reactions
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Full Paper without agglomeration or leaching. However, polymeric carbon nitrides have low surface areas when prepared in bulk, and hence laborious templating methods are used to increase the accessible surface and make them suitable catalysts or catalyst supports.[27–30] In our approach, polymeric carbon nitrides (CNx) are used to functionalize porous silica supports to stabilize metal NPs. SBA15 was first impregnated with a layer of CNx before being used to support Pt or Au NPs. The CNx was prepared by polycondensation of dicyandiamide that was impregnated into the pores of SBA-15, similar to that reported elsewhere.[31] The formation of CNx from dicyandiamide is briefly illustrated in Scheme 1. Depending on the treatment temperature, the
Scheme 1. Reaction path for the formation of polymeric graphitic carbon nitride (g-CNx) starting from dicyandiamide.
degree of polycondensation of CNx formed on the surface of SBA-15 changes from melamine to melem, melon, and a more condensed polymeric carbon nitride. As the ratio of primary to tertiary amine changes during this reaction, the functionality of the support can be tailored by simple heat treatment. Furthermore, the amino functional groups on the surface of CNx are stable up to 600 8C, which is much higher than for any grafted amines or other functional groups. Therefore, CNx could provide stable functionality even at high temperatures. Supported Au and Pt NPs have been reported to be good heterogeneous catalysts.[15, 32, 33] Therefore, the catalytic performance of the Au and Pt NPs were also tested, by using CO oxidation as a model reaction. Catalytic CO oxidation is not only a way of removing the poisonous gas CO, but also a widely used test reaction to evaluate catalytic performance of catalysts. Our results indicate that the Pt-NPs on CNx/SBA-15 are highly active catalysts for CO oxidation, whereas the Au NPs on CNx/SBA-15 show negligible activity.
Results and Discussion The wide-angle XRD patterns of a series of SBA-15-supported CNx materials treated at different temperatures, CNx/SBA-15_T (T = temperature), show an intense peak at 2 q 26.78 and broad peak at 2 q 228 (Figure 1 A) By comparison with the diffraction peaks of SBA-15, the broad peak at 2 q 228 is attribChem. Eur. J. 2014, 20, 2872 – 2878
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Figure 1. Characterization of SBA-15 and CNx/SBA-15_T. A) Wide-angle XRD patterns. B) Small-angle XRD patterns. C) N2 adsorption/desorption isotherms. D) TGA curves and the weight losses.
uted to amorphous silica.[16, 34] Thus, the intense peak at 2 q 26.78 is assignable to CNx. With increasing temperature, the peak intensity of CNx decreased compared to that of amorphous silica SBA-15, due to the weight loss of CNx discussed below. To confirm the existence of CNx, we chose CNx/SBA-15_ 500 for further investigation. Polycondensation of dicyandia-
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Full Paper mide at 500 8C was reported to form highly condensed heptazine units, known as melon, showing a diffraction peak at 2 q 27.58, which can be assigned to graphitic stacking of the layered polymer.[35, 36] Although the weak diffraction peak observed from CNx/SBA-15_500 is shifted slightly towards smaller angles, a comparable diffraction peak was found for mesoporous CNx that was obtained by removing the SBA-15 support with a 4 m aqueous solution of NaOH (see Figure S1 in the Supporting Information). This confirms that the CNx of CNx/ SBA-15_500 exists in a highly condensed polymeric form. At smaller angles, all samples show three diffraction peaks assignable to SBA-15 (Figure 1 B), indicating that the porous structure of SBA-15 remains unchanged irrespective of the formation of CNx. The diffraction peaks of CNx/SBA-15_T appear at the same positions, but small shifts to higher angles relative to those of the parent SBA-15 indicate slight shrinkage of the porous structure due to the extra calcination step. Figure 1 C shows the N2 adsorption/desorption isotherms of SBA-15 and CNx/SBA-15_T. All samples show a type IV isotherm with H1-type hysteresis loop corresponding to an ordered mesoporous structure, which implies that the framework of SBA-15 is not destroyed after formation of CNx, in agreement with the XRD results in Figure 1 B. Smaller overall pore volumes and surface areas of the loaded samples show the presence of a nonporous CNx phase. The surface areas and pore volumes of the samples increased again after the formation of metal NPs (Figure S2 in the Supporting Information) because of the extra treatment steps. The loading of CNx on CNx/SBA-15_T prepared at different temperatures was evaluated by thermogravimetric analysis (TGA, Figure 1 D). As expected, the loading of CNx decreases with increasing temperature. Considering the weight loss (8.4 %) for SBA-15, which is caused by removal of water and gases physically adsorbed on SBA-15, the loading of CNx was calculated to be 30, 19.6, and 5.6 wt % for CNx/SBA-15_T treated at 300, 400, and 500 8C, respectively. Energy-dispersive X-ray mapping images of the carbon and nitrogen elements (Figure 2) showed that CNx is homogeneously dispersed on SBA-15, in accordance with what was observed for SBA-15-supported Fe/g-C3N4.[37] The polymeric CNx can in principle be located on the outer surface or inside the pores of SBA-15. In earlier work[31] using a similar impregnation method, removal of the porous silica clearly revealed that CNx is formed inside the mesopores of SBA-15. In this work, a decrease in pore size was observed when SBA-15 was impregnated with polymeric CNx (Figure S2 in the Supporting Information), which suggests homogeneous coating of the pore walls. A slight increase in pore size with increasing temperature (i.e., from CNx/SBA-15_300 to 500) indicates condensation and shrinkage of the polymeric CNx within the pores. This also explains the increase in surface area and pore volume with increasing treatment temperature (Figure 1 C). However, an additional layer of polymeric CNx on the outer surface of SBA-15 cannot be excluded. Figure 3 A and B show TEM images of Au/SBA-15 and Au/ CNx/SBA-15_500 prepared by the wet-impregnation (WI) method, respectively. No significant change in the texture of Chem. Eur. J. 2014, 20, 2872 – 2878
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Figure 2. TEM and EDX mapping of the C and N elements of CNx/SBA-15_ 500. TEM = transmission electron microscopy, EDX = energy-dispersive X-ray spectroscopy.
Figure 3. TEM images of A) Au/SBA-15 and B) Au/CNx/SBA-15_500. C) Dependence of particle size of Au NPs on the treatment temperature for CNx/ SBA-15_T. The average particle size was calculated by counting at least 1500 Au particles.
SBA-15 before and after loading with CNx is observed (Figure S3 in the Supporting Information), that is, impregnation with CNx does not influence the porous structure of SBA-15. The Au particles are severely agglomerated and confined in a small area on Au/SBA-15 (for more images, see Figure S3 of the Supporting Information), but are highly dispersed and randomly distributed on Au/CNx/SBA-15_500; thus, the presence of CNx helps to disperse and stabilize the Au NPs. Moreover,
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Full Paper the Au NPs are located both on the surface and inside the pores. The average particle size of the Au NPs increases from 3.2 nm for Au/CNx/SBA-15_300 to 4.7 nm for Au/CNx/SBA-15_ 500 (Figure 3 C). We suggest that the larger amount of lesscondensed CNx in SBA-15 provides more sites for metal adsorption and stabilization, and finally results in better dispersion and smaller particle size of the Au NPs. Thus, the Au particle size is indeed partially controllable by adjusting the loading of CNx or the condensation temperature of dicyandiamide. In contrast the Au particle size on pure SBA-15 (i.e. Au/SBA-15) is hard to measure, as the Au particles are severely agglomerated. The very weak and broad Au diffraction peaks in the XRD patterns of Au/CNx/SBA-15_T (Figure S1 of the Supporting Information) indicate that, on average, very small particles were formed. The formation of smaller and better-dispersed Au NPs on CNx/SBA-15_T as opposed to pure SBA-15 indicates that the CNx plays an important role in the formation process of Au NPs. Similar to Au NPs, a significant decrease in particle size and improvement in dispersion were observed for Pt NPs on going from Pt/SBA-15 to Pt/CNx/SBA-15 T (Figure 4 A and B; for others, see Figure S4 of the Supporting Information). The Pt
Figure 4. TEM images of fresh samples of Pt/SBA-15 (A) and Pt/CNx/SBA-15_ 500 (B) and of the used sample of Pt/CNx/SBA-15_500 (C).
NPs in Pt/SBA-15 are severely agglomerated, and individual particles are hard to separate. Chen et al.[14] reported that the Pt particles of Pt/SBA-15 prepared by direct deposition of H2PtCl6 species on pure SBA-15 by an impregnation method is severely sintered and shows particle size of about 18 nm, consistent with our results. Thus, the presence of CNx is indeed crucial in the stabilization of Au and Pt NPs. We expected that other porous silicas besides SBA-15 could also lead to small and size-controlled metal NPs. Indeed, Pt Chem. Eur. J. 2014, 20, 2872 – 2878
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NPs with controllable particle size could also be synthesized on SBA-16-supported CNx (see Figure S5 of the Supporting Information). This suggests that the present method of impregnating SBA-15 with CNx could be general method of preparing small and highly dispersed metal NPs, also on industrially relevant porous silica supports. Such small and highly dispersed noble metal NPs are generally good heterogeneous catalysts. For example, Haruta et al.[32] found that a nanosized Au catalyst showed a significant increase in the activity for CO oxidation compared to bulk Au. On the other hand, the high thermal stability of CNx and the strong interaction between CNx and metal NPs may guarantee that the Au or Pt NPs endure high temperature without significant agglomeration under reaction conditions. Consequently, we suspected that the herein-prepared small and stable Au or Pt NPs could even be good catalysts for reactions conducted at elevated temperature. To prove this, we tested the catalytic performances of Pt and Au NPs supported on CNx/SBA-15_500 in the CO oxidation reaction. This catalyst was used because the CNx will not undergo further structural changes during the catalytic reaction, as the highest temperature applied in CO oxidation was less than 500 8C. The reactant (0.5 % CO and 7.5 % O2 in argon) was passed through 30 mg of catalysts at a flow rate of 50 mL min 1 and a heating rate of 5 8C min 1 at each temperature stage. The catalysts were activated at 350 8C in the reactant for 30 min before starting the measurement, to remove any water and gases (e.g., CO2) that were possibly adsorbed on the catalyst. The Pt/CNx/SBA-15_500 catalyst showed 100 % CO conversion already at 160 8C (Figure 5 C). Because of the small particle size, high dispersion, and possibly new electronic properties caused by the interaction with CNx, Pt/CNx/SBA-15_500 is more active and shows lower ignition temperature for CO oxidation than comparable Pt/SBA-15 catalysts that were reported previously (Table S2 of the Supporting Information). On the other hand, Au/CNx/SBA-15_500 showed no catalytic activity for CO oxidation (Figure 5 A), and thus no change in activity was observed compared to the pure support (Figure 5 B). This is contradictory to previous reports on CO oxidation over an Au/ SBA-15 catalyst,[19, 38] whereby good CO oxidation activity could be obtained even below 40 8C. Considering that the average particle size (4.6 nm) is within the range of active Au catalysts for CO oxidation, the low activity must be ascribed to the presence of CNx, which seems to inhibit the reaction. To verify that the low activity is not due to the morphology of Au NPs or the preparation method, but to the suppressing effect of CNx, we synthesized a new Au/CNx/SBA-15_500 catalyst by a deposition–precipitation (DP) method from HAuCl4 as gold precursor, NaOH as precipitant (pH 10), and NaBH4 as reductant (for details, see Supporting Information). Au catalysts prepared by the DP method usually exhibit better activity than those prepared by the WI method applied here.[39, 40] However, this catalyst also showed very low activity for CO oxidation (Figure 5 A), which confirmed that the presence of CNx indeed suppresses the ability of Au NPs to catalyze CO oxidation irrespective of the preparation method.
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Full Paper dized (Figure S7 of the Supporting Information). The presence of oxygen-containing species in the partially oxidized Pt NPs may play important role in the reaction. The importance of oxygen-containing species or the oxidation states of noble metals to catalytic performances in CO oxidation has been reported.[23, 26, 41–44] Guzman and Gates[41] found that the CO oxidation activity increases with increasing of amount of Au + from 15 to 60 %. Nieuwenhuys et al.[42] reported that active oxygen is supplied from the oxidized support of the Au catalyst. It was also suggested that oxygen-containing species are the site of oxygen activation on Au-based catalysts.[26] In addition, Lambert et al. reported that Au0 NPs with particle size above 2 nm supported on an oxygen-free BN support are inactive to molecular oxygen and show negligible activity in oxidation reactions,[45] which proves the importance of oxygen-containing species on the catalyst for the reaction and supports our results, as CNx is also an oxygen-free material. For Pt NPs this support effect may be negligible, as they are readily oxidized even when stored in air at room temperature.[46] To further test the feasibility of the CNx/SBA-15 as support for noble metal NP catalysts, we tested the long-term stability of Pt/CNx/SBA-15_500 for CO oxidation at 160 8C. The catalyst still showed full conversion of CO after 30 h (Figure 5 D) and this confirms the stability of the catalyst. To further verify the stability of the Pt NPs, TEM images of Pt/CNx/SBA-15_500 were recorded after three runs of the reaction (Figure 4 C). The Pt particle size increased from 4.85 nm for the fresh catalyst (Figure 4 B) to 5.48 nm for the used catalyst, while the Pt particles remained highly dispersed. We can therefore conclude that CNx/SBA-15 500 indeed stabilizes the metal particles even at high temperature.
Conclusion
Figure 5. Catalytic activity of CO oxidation on A) Au/CNx/SBA-15_500 prepared by WI and DP methods, B) various supports and in the blank experiment (no catalyst), and C) Pt/SBA-15 and Pt/CNx/SBA-15_500. D) Long-term stability of Pt/CNx/SBA-15_500 in CO oxidation at 160 8C.
To elucidate the difference in activity of Au and Pt catalysts in CO oxidation, we performed X-ray photoelectron spectroscopic measurements and found that Au NPs on CNx exist exclusively as Au0, whereas the Pt NPs on CNx are partially oxiChem. Eur. J. 2014, 20, 2872 – 2878
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We have reported a new way of preparing small Au and Pt NPs with high stability and controlled particle size by means of CNx. The CNx is prepared by a polycondensation route and its amount can be controlled by adjusting the treatment temperature. Because of the high thermal stability of CNx and its strong interaction with the metal NPs, the prepared metal NPs show strong resistance to agglomeration. Catalytic studies demonstrated that Au/CNx/SBA-15_500 with metallic Au NPs shows low activity, whereas Pt/CNx/SBA-15_500 with partially oxidized Pt NPs exhibits good activity for CO oxidation, presumably because of their ability to activate oxygen. The current method of using CNx as coating material can be applied to prepare small and stable metal NPs supported on other solids (e.g., SBA-16). We believe that the herein-reported method could be a general route for synthesizing highly stable and size-controlled metal NPs, and the composites could be suitable materials for identifying the oxygen-activation route of oxidation reactions in heterogeneous catalysis.
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Full Paper Experimental Section Synthesis of CNx/SBA-15_T Dicyandiamide (3.0 g) was dissolved in ethanol/water (40/10 mL) by heating to 80 8C and kept at this temperature for 5 min to ensure all the dicyandiamide was dissolved. To this solution preformed SBA-15 (3.0 g) was added and the mixture stirred vigorously. After that, the mixture was heated at 100 8C until the ethanol and water were evaporated and a white solid was obtained. The resulting material was then dried at 100 8C in an air oven overnight and thermally treated in N2 atmosphere at different temperatures (300, 400, and 500 8C) for 4 h to yield the SBA-15 supported carbon nitrides CNx/SBA-15_T, where T is the treatment temperature.
Synthesis of Au/CNx/SBA-15 T and Pt/CNx/SBA-15_T The supported Au or Pt nanoparticles were prepared by wet impregnation of 1.0 g CNx/SBA-15_T in 50 mL of a 0.001 m aqueous solution of HAuCl4 or H2PtCl6 with vigorous stirring for about 3 h. Then, the solution was filtered, and the solid dried in a vacuum oven at 60 8C for 6 h and reduced in H2 atmosphere at 300 8C for 4 h to afford Au/CNx/SBA-15_T or Pt/CNx/SBA-15_T. The loadings calculated for Au and Pt NPs supported on CNx/SBA-15_500 were 0.95 and 0.98 wt %, respectively
Catalytic tests Catalytic CO oxidation was carried out in a fixed-bed reactor. Approximately 30 mg of sample was loaded into a quartz reactor blocked with quartz wool at the bottom, and the feed gas (0.5 % CO + 7.5 % O2 in Ar) was passed through the reactor at a flow rate of 50 mL min 1, corresponding to a space velocity of 100 000 mL h 1 gcat1 . The temperature was first raised to 350 8C and maintained for 30 min to activate the catalyst, then decreased to 150 8C in steps of 25 8C (temperature ramp: 5 8C min 1) to measure the catalytic activity. Koros–Nowak tests indicated that the reactions were in the kinetic regime and were not affected by heat- or mass-transport limitations (Table S1 of the Supporting Information). The composition of the effluent gas was analyzed by an on-line Agilent 7890 GC equipped with a thermal conductivity detector, Propack Q and 5 molecular sieve columns. CO conversion was calculated as conversion/% = {[CO]in [CO]out}/[CO]in 100, where [CO]in and [CO]out are the inlet and outlet concentration of CO, respectively.
Acknowledgements Financial support from the National Science Foundation of China (21203254), the Special Fund of the Central University Research, South-Central University for Nationalities (CZZ11007, 12ZNZ006), the Scientific Research Foundation for Returned Scholars, Ministry of Education of China (BZY11055), the Natural Science Foundation of Fujian Province, China (2012J01042), and the Cluster of Excellence “Unifying Concepts in Catalysis” (EXL 31411) is gratefully acknowledged. Keywords: heterogeneous catalysis · oxidation · platinum · supported catalysts
nanoparticles
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Received: September 24, 2013 Published online on February 4, 2014
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