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Hydroxyapatite-supported rhodium catalysts for N2O decomposition
Journal of Molecular Catalysis A: Chemical 400 (2015) 90–94
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Hydroxyapatite-supported rhodium catalysts for N2 O decomposition Chengyun Huang a , Zhen Ma b,∗ , Pengfei Xie a , Yinghong Yue a , Weiming Hua a,∗ , Zi Gao a a
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, PR China Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, PR China b
a r t i c l e
i n f o
Article history: Received 13 August 2014 Received in revised form 6 February 2015 Accepted 11 February 2015 Available online 12 February 2015 Keywords: Decomposition of nitrous oxide Hydroxyapatite Rhodium
a b s t r a c t Several hydroxyapatite (HAP) supports were prepared by precipitation under different pH values (8.5, 9.5, or 10.5), and rhodium was loaded onto the supports by impregnation followed by calcination at 500 ◦ C. The pH value during the precipitation was found to play an important role in determining the catalytic activity of the resulting catalysts in N2 O decomposition. The Rh/HAP catalyst prepared under optimal conditions (pH 10.5) showed superior catalytic activity than Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 , due to the rich basic sites of the HAP support and the dispersion of ultra small rhodium particles on HAP. A pretreatment of Rh/HAP in 4% H2 at 400 ◦ C can further improve the activity owing to the partial reduction of the rhodium species. The catalysts/supports were characterized by X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), CO2 temperature-programmed desorption (CO2 -TPD), and X-ray photoelectron spectroscopy (XPS). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nitrous oxide (N2 O) is a potent greenhouse gas and may cause ozone-layer depletion. Anthropogenic N2 O comes from adipic acid production, nitric acid production, and fossil fuels/biomass burning. The concentration of N2 O has increased at an annual rate of 0.2 − 0.3% since the industrial revolution [1]. Thus, it is important to eliminate the emission of N2 O. Methods for N2 O removal include thermal decomposition [2], selective catalytic reduction [3,4], and direct catalytic decomposition [5,6]. Among them, direct catalytic decomposition of N2 O into N2 and O2 is the most attractive, because this process has low operating costs and produces no harmful gases [7]. Catalysts used for N2 O decomposition include supported metal catalysts [8,9], metal oxides [10–13], and zeolite-based catalysts [14–18]. Rh- [8,1–24], Ru- [25,26], and Ir-based [27] catalysts have been reported to be active for N2 O decomposition. In particular, supported rhodium catalysts show high activities at relatively low temperatures [8,19,20]. The catalytic activity of rhodium catalysts was found to be significantly influenced by the supports [28,29]. Supports may influence the dispersion of rhodium [30,31],
∗ Corresponding authors. Tel.: +86 21 65642409; fax: +86 21 65641740. E-mail addresses: [email protected] (W. Hua), [email protected] (Z. Gao). http://dx.doi.org/10.1016/j.molcata.2015.02.011 1381-1169/© 2015 Elsevier B.V. All rights reserved.
the reducibility of rhodium species [32], and the pathway of reaction [8]. Thus, it is important to find suitable supports for making Rh catalysts, in order to improve the activity in N2 O decomposition. Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ), a non-reducible and insoluble salt, has good thermal stability, moderate alkalinity, and rich surface hydroxyls. In addition, its redox and acid/base properties can be tuned by ion exchange [33] or atom defects [34]. Organic molecules can be grafted onto the HAP surface [35]. HAP can be used as a catalyst for some reactions such as the conversion of ethanol into 1-butanol [34], Knoevenagel condensation [36], aldol condensation [37], Friedel-Crafts reaction [38], Michael addition [39], and combustion of formaldehyde [40]. Alternatively, it can be used for making supported catalysts. For instance, Ru/HAP can be used in the water-gas reaction [41], the mild racemization of alcohols [42], cis-dihydroxylation and oxidative cleavage of alkenes [43]. Rh/HAP was reported for the hydrogenation of aromatics [44]. Au/HAP can catalyze CO oxidation [45–48], the water-gas shift reaction [49], and organic synthesis [50–53]. To the best of our knowledge, the decomposition of N2 O on Rh/HAP was not reported. Here we found that Rh/HAP showed much higher activity than conventional Rh/TiO2 , Rh/Al2 O3 , and Rh/SiO2 for N2 O decomposition. This is ascribed to the better basicity of the HAP support and high dispersion of rhodium species on HAP.
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2. Experimental 2.1. Preparation Several HAP supports were prepared by precipitation under different pH values. Typically, 7.9 g (NH4 )2 HPO4 was dissolved in 100 ml deionized water, and 23.2 g Ca(NO3 )2 ·4H2 O was dissolved in 100 ml 50% aqueous ethanol solution. The first solution was then added, with stirring, into the second solution (in a beaker) at a rate lower than 3 ml/min. Aqueous ammonia (25 wt%) was added into the mixed solution to a certain pH value (8.5, 9.5, or 10.5). After aging at 80 ◦ C for 2 h, the precipitate was washed with water, filtered, dried at 110 ◦ C overnight, and calcined at 550 ◦ C for 4 h. The obtained samples are denoted as HAP-pH value. Commercial Al2 O3 (␥-Al2 O3 ), TiO2 (P25), and SiO2 were selected as common supports for comparison. Supported rhodium catalysts based on the above-mentioned supports were prepared by wet impregnation, using Rh(NO3 )3 as the precursor. The Rh content was controlled at 1 wt%. In a typical synthesis, 1.98 g support was mixed with 10 ml aqueous Rh(NO3 )3 solution (containing 0.02 g Rh) in an agante mortar, and the mixture was grinded under an infrared lamp till dry. The impregnated samples were calcined in a muffle furnace at 500 ◦ C for 4 h with flowing air.
Fig. 1. Conversion of N2 O on Rh/HAP-10.5, Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 catalysts as a function of reaction temperature.
2.2. Characterization
3. Results
XRD patterns were recorded on a MSAL XD2 X-ray diffractometer using CuK␣ radiation at a scanning speed of 4o /min. BET surface areas were measured using a Micromeritics Tristar 3000 instrument. The samples were pretreated at 300 ◦ C in vacuum for 3 h, followed by N2 adsorption at −196 ◦ C. Surface basicity was characterized by CO2 -TPD in a flow-type fixed-bed reactor at ambient pressure. 0.3 g sample was preheated at 550 ◦ C for 2 h, and then cooled to 80 ◦ C in flowing He. At 80 ◦ C, sufficient CO2 pulses were injected till adsorption saturation, followed by purging the sample with He for 2 h. The temperature was then raised from 80 to 600 ◦ C at a rate of 10 ◦ C/min. Due the limitation of our instrument, the amount of CO2 was not determined simultaneously when the desorption profile was recorded. The CO2 desorbed was collected in a liquid N2 trap and detected by GC afterwards (after suddenly warming up the loop that collects the liquid CO2 ). SEM images were recorded on a Philips XL 30 operating at 30 kV. TEM data were recorded with a JEM-2011F instrument. XPS spectra were recorded on a PerkinElmer PHI 5000 C spectrometer with MgK␣ radiation as the excitation source. To calibrate the binding energy, the C 1 s line (284.6 eV) was used as the reference.
Fig. 1 shows the influence of different supports on the activity of supported Rh catalysts (with the same Rh content, 1 wt%) in N2 O decomposition. The activity, in terms of N2 O conversion at the same reaction temperature, follows the sequence of Rh/HAP > Rh/Al2 O3 > Rh/TiO2 > Rh/SiO2 . Here HAP refers to the hydroxyapatite precipitated under pH 10.5. The N2 O conversions of these catalysts at 275 ◦ C are 97%, 27%, 10%, and 7%, respectively. On the other hand, the T50 values (temperatures of 50% conversion) of these catalysts are 250, 289, 310, and 324 ◦ C, respectively. Apparently, HAP is the best support to load rhodium for N2 O decomposition. Fig. 2 shows the XRD patterns of Rh/HAP and neat HAP support. Here HAP refers to the hydroxyapatite precipitated under pH 10.5. The standard XRD data for HAP, Rh, and Rh2 O3 are also depicted in the figure for comparison. The HAP support (without rhodium at this stage) synthesized by us can be indexed as a pure hexagonal
refers to the N2 O concentration or peak area at an elevated temperature. If pretreatment is needed, then the catalyst was pretreated in situ at 400 ◦ C in a specific gas flow (4% H2 -He or He) at a flowing rate of 30 ml/min for 2 h. After pretreatment, the catalyst was cooled down to room temperature, under the same ambient, and the ambient was changed to 60 ml/min 0.5% N2 O-He to start the reaction testing.
2.3. Catalytic tests Catalytic activity of N2 O decomposition was tested in a flowtype fixed-bed reactor. 0.5 g catalyst was packed in an U-shaped glass tube (7 mm diameter) sealed by quartz wool. A gas stream of 0.5% N2 O (balanced by He) flowed through the catalyst at a rate of 60 ml/min. The catalyst (initially kept at near room temperature) was exposed to the gas stream for 1 h during which the existing stream was periodically analyzed by a gas chromatograph (GC, Agilent 7890 A) that can separate N2 O, O2 , and N2 . The reaction temperature was then varied using a furnace and kept at various elevated temperature for 30 min in each temperature step. The exhaust was again periodically analyzed by the GC, and the conversion of N2 O was calculated according to X = ([N2 O]in − [N2 O]out )/[N2 O]in , where [N2 O]in refers to the N2 O concentration or peak area at room temperature, and [N2 O]out
Fig. 2. XRD patterns of Rh/HAP-10.5 catalyst and HAP-10.5 support, together with the standard patterns of hydroxyapatite (PDF#09-0432), Rh (PDF#05-0685), and Rh2 O3 (PDF#41-0541).
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Fig. 3. Conversion of N2 O on Rh/HAP catalysts synthesized at different initial pH values as a function of reaction temperature.
phase with a P63 /m space group. The main diffraction peaks located at 25.9, 31.7, 32.2, 32.9, and 34.0o correspond to the (0 0 2), (2 1 1), (1 1 2), (3 0 0), and (2 0 2) crystal faces, and are consistent with the standard XRD data (PDF#09-0432). When rhodium is loaded onto HAP to make the Rh/HAP catalyst, the XRD pattern exhibits only characteristic HAP peaks but neither metallic Rh (PDF#050685) nor Rh2 O3 (PDF#41-0541) can be observed. The HAP peaks of Rh/HAP become slightly sharper than those of neat HAP, due to the additional calcination step (at 500 ◦ C) after the impregnation of HAP by rhodium. Fig. S1 in the Supporting information shows the XRD patterns of Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 catalysts. Neither metallic Rh nor Rh2 O3 peaks can be observed, consistent with the phenomenon in Fig. 2. This is due to the low loading of rhodium species. Because HAP was found to be the support of choice in our study, attempts were then made to investigate the influence of preparation details on the nature of support and the catalytic performance. Fig. 3 shows the catalytic activities of Rh supported on three supports precipitated under different pH values (8.5, 9.5, or 10.5). These catalysts are denoted as Rh/HAP-pH value for convenience, although our XRD data (Fig. 4) show that the support synthesized under pH 8.5 is actually a mixture of HAP and Ca2 P2 O7 (PDF#170499). As shown in Fig. 3, the activity follows the sequence of Rh/HAP-10.5 > Rh/HAP-9.5 > Rh/HAP-8.5. The N2 O conversions of these catalysts at 275 ◦ C are 97%, 37%, and 9%, respectively. One problem in the synthesis of HAP support is that the pH value sometimes slowly drops during the precipitation, due to the consumption of aqueous OH− to form HAP. In the preparation of HAP-10.5, the amount of ammonia added is nearly nine times that of the stoichiometric value required for the synthesis of HAP, therefore the pH value only decreases slightly from 10.5 to 10.2. However, when less amount of aqueous ammonia is introduced, the pH value decreases obviously. The pH value drops to slightly above 7 in the preparation of HAP-9.5, and it deceases to below 7 in the preparation of HAP-8.5. The low pH value during the synthesis means that the amount of OH− is insufficient to generate HAP, consistent with the formation of Ca2 P2 O7 (Fig. 4). To overcome the above problem, the synthesis procedure was modified, i.e., aqueous ammonia was continuously added during the precipitation to keep the pH value constant (at 7.5, 8.5, or 9.5). The supports thus obtained are denoted as HAP-pH-C. Fig. S2 shows the SEM images of these supports, together with that of
Fig. 4. XRD patterns of Rh/HAP catalysts synthesized at different initial pH values and standard patterns of Ca2 P2 O7 (PDF#17-0499), Rh (PDF#05-0685), and Rh2 O3 (PDF#41-0541).
HAP-10.5 (without pH adjusting) for comparison. For HAP-10.5 and HAP-9.5-C prepared at higher pH values, the particles show sheetlike morphology. For HAP-8.5-C and HAP-7.5-C prepared at lower pH values, most of the particles are approximately sphere-shaped. The obtained supports show XRD patterns similar to HAP-10.5 (Fig. S3), and the activities of the resulting Rh catalysts improve significantly compared to the ones precipitated at the same pH value but without pH adjusting (Fig. S4). For example, both Rh/HAP-9.5C and Rh/HAP-8.5-C give a T50 value of 266 ◦ C, which is obviously lower than those of Rh/HAP-9.5 and Rh/HAP-8.5 (286 and 350 ◦ C, respectively). Results from these control experiments underscore the importance of maintaining the high pH level (more OH− ) during the precipitation synthesis of HAP. The question now arises as to why HAP outperforms other common supports. As shown in Table 1, the surface areas of HAP (synthesized under pH 10.5), Al2 O3 , TiO2 , and SiO2 are 58, 114, 48, and 303 m2 /g, respectively, inconsistent with the activity trend seen in our reaction testing. Therefore, surface area of the support is apparently not the determining factor. On the other hand, the basicity of the support seems to be essential. The amounts of basic sites of several supports were measured by CO2 -TPD. Fig. S5 shows the CO2 -TPD profiles of several typical samples. As shown in Table 1, the amounts of basic sites of HAP (synthesized under pH 10.5), Al2 O3 , TiO2 , and SiO2 are 42.7, 14.3, 7.7, and 5.1 mol/g, respectively, consistent with the activity trend. Within the three samples denoted as HAP-pH value, i.e., HAP-10.5, HAP-9.5, and HAP-8.5, the amounts of basic sites also correlate with the catalytic activity of the supported catalysts, further proving the important role of basic sites in enhancing activity in N2 O decomposition. The catalysts collected after the reaction testing were characterized by TEM (Figs. 5 and S6–S11). The characterization of a spent catalyst is assumed to provide information on a system that is closer
Table 1 Surface area and amount of basic sites of the supports and rhodium particle size of the supported catalysts.
Surface area of support (m2 /g) Amount of support’s basic sites (mol/g) Average size of rhodium particles (nm)
HAP-10.5
HAP-9.5
HAP-8.5
Al2 O3
TiO2
SiO2
58 42.7 0.78 ± 0.22
46 9.5 1.12 ± 0.33
36 5.2 1.33 ± 0.50
114 14.3 0.77 ± 1.56
48 7.7 1.70 ± 0.63
303 5.1 2.25 ± 0.67
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Fig. 6. Conversion of N2 O on the Rh/HAP-10.5 catalyst pretreated in situ at 400 ◦ C under different ambient as a function of reaction temperature.
Fig. 5. TEM images of Rh/HAP-10.5 collected after reaction testing. Top: raw data; Bottom: enlarged portion.
to a catalyst under steady state reaction conditions than a fresh catalyst. For each sample, different areas of the same specimen were examined, and the average particle sizes were obtained by measuring the sizes of 150 particles with the aid of Digital Micrograph. The results are shown in Table 1. As shown in Figs. 5 and S6, the rhodium particles are ultra small on HAP-10.5 support. The average size is as small as 0.78 nm. On the other hand, the average sizes of rhodium particles on Al2 O3 , TiO2 , and SiO2 are 0.77, 1.70, and 2.25 nm, respectively (Figs. S7–S9). For the Rh/HAP series catalysts (Figs. 5, S6, S10, S11), the average particle sizes increase (0.78, 1.12, 1.33) as the pH value decreases (10.5, 9.5, 8.5), consistent with the trend observed in catalytic tests. In the above catalytic experiments, the catalysts were calcined at 500 ◦ C ex situ, and no additional in situ pretreatment was applied before conducting catalytic reactions. The influence of pretreatment conditions on the performance of Rh/HAP-10.5 was studied. As shown in Fig. 6, the activity is virtually not changed if the catalyst is pretreated in He at 400 ◦ C. However, the pretreatment in 4% H2 increases the activity significantly. After reaching a reaction temperature of 400 ◦ C, the catalyst was cooled down to room temperature, and the catalytic activity was re-tested as a function of reaction temperature. As shown in Fig. S12, the conversions curves for the first cycle and second cycle of reaction testing are almost the same. Fig. 7 shows the XPS spectra of Rh/HAP-10.5 pretreated ex situ in different atmosphere. There are two sets of Rh peaks at 306 − 311 eV and 311 − 317 eV, due to the splitting of the 3d orbit of Rh into Rh 3d5/2 and Rh 3d3/2 . The ratio of these two peaks is about 1.5. Our following discussion will be focused on the Rh 3d5/2 peak. The binding energy of Rh3+ is 308.3 − 310.5 eV whereas that of Rh0 is 307.0 − 307.7 eV [54,55]. The Rh/HAP-10.5 catalyst calcined at 500 ◦ C (with no additional pretreatment) and the one pretreated in He at 400 ◦ C contains mainly Rh3+ , alone with an ignorable amount of Rh00 (below 4%). In contrast, the catalyst pretreated in 4% H2 at
Fig. 7. XPS spectra of the Rh/HAP-10.5 catalyst pretreated ex situ under different conditions (before reaction) and the one collected after reaction.
400 ◦ C contains a portion of Rh0 (about 32%). After the reaction, the amount of Rh0 increases a bit. Thus, it is clear that the presence of metallic Rh is more advantageous for the N2 O decomposition. As a proof, the pretreatment of Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 in 4% H2 also leads to more active catalysts, but Rh/HAP-10.5 pretreated by 4% H2 is still the most active (Fig. S13). 4. Discussion The most essential finding of this study is that Rh/HAP is more active than Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 in N2 O decomposition. To the best of our knowledge, this finding was not reported in the literature. In addition, HAP has seldom been used for making supported catalysts. As for what makes HAP the support of choice, several reasons may be considered and discussed. First, the surface areas of HAP-10.5, Al2 O3 , TiO2 , and SiO2 are 58, 114, 48, and 303 m2 /g, respectively, inconsistent with the trend seen in catalytic tests. Therefore, surface area is not the determining factor. Second, the amounts of basic sites of these supports are 42.7, 14.3, 7.7, and 5.1 mol/g, respectively, consistent with the trend seen in catalytic tests. It is recognized that basic sites are advantageous for N2 O decomposition [30]. Increasing the basicity of the support could enhance the rhodium dispersion, and hence increase the activity
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of rhodium catalysts [30]. HAP support can lead to small rhodium particles (average size 0.78 nm) on surface, but that is not the sole factor contributing to the high activity since the average size of rhodium particles on Al2 O3 is comparable (average size 0.77 nm). Within the Rh/HAP family, the pH value during precipitation preparation of the support is found to be important to guarantee the purity of the HAP support. Precipitation at an initial pH value of 8.5 leads to a mixture of HAP and Ca2 P2 O7 (Fig. 4). The low-pH precipitation can also lead to decreased surface area, less basic sites, bigger rhodium particles (Table 1), as well as decreased activity (Fig. 3). The data again demonstrate the importance of the basic sites of supports for rhodium catalysts in N2 O decomposition. 5. Conclusions Several hydroxyapatite (HAP) supports was synthesized via a precipitation method, using Ca(NO3 )2 and (NH4 )2 HPO4 as precursors. Rhodium was loaded onto HAP by impregnation and calcination. The resulting Rh/HAP prepared under optimal conditions (pH 10.5) is more active than Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 in N2 O decomposition. The use of HAP support is advantageous for this reaction, because HAP can provide more basic sites and stabilize ultra small rhodium particles. The pH value during the precipitation synthesis of HAP has to be kept at 10.5 to guarantee the high activity of the resulting catalyst. A pretreatment of Rh/HAP in 4% H2 can lead to the reduction of the rhodium species, thus further increasing the activity. Acknowledgments W. Hua and Y. Yue thank the financial support by the National Natural Science Foundation of China (20773027 and 21273043), the Ph.D. programs foundation of the Ministry of Education in China (20100071110008), and the Science and Technology Commission of Shanghai Municipality (13DZ2275200). Z. Ma thanks the financial support by the National Natural Science Foundation of China (21177028 and 21477022). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2015.02.011. References [1] J. Pérez-Ramírez, F. Kapteijn, K. Schöffel, J.A. Moulijn, Appl. Catal. B 44 (2003) 117–151. [2] M. Galle, D.W. Agar, O. Watzenberger, Chem. Eng. Sci. 56 (2001) 1587–1595. [3] G. Centi, F. Vazzana, Catal. Today 53 (1999) 683–693. [4] S. Kameoka, K. Kita, T. Takeda, S. Tanaka, S. Ito, K. Yuzaki, T. Miyadera, K. Kunimori, Catal. Lett. 69 (2000) 169–173. [5] F. Kapteijn, J. Rodriguez-Mirasol, J.A. Moulijn, Appl. Catal. B 9 (1996) 25–64. [6] S. Kannan, Catal. Surv. Asia 10 (2006) 117–137. [7] N. Imanaka, T. Masui, Appl. Catal. A 431 (2012) 1–8. ˜ [8] S. Parres-Esclapez, I. Such-Basanez, M.J. Illán-Gómez, C.S. Lecea, A. Bueno-López, J. Catal. 276 (2010) 390–401. [9] V.G. Komvokis, M. Marti, A. Delimitis, I.A. Vasalos, K.S. Triantafyllidis, Appl. Catal. B 103 (2011) 62–71. [10] K. Asano, C. Ohnishi, S. Iwamoto, Y. Shioya, M. Inoue, Appl. Catal. B 78 (2008) 242–249.
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Hydroxyapatite-supported rhodium catalysts for N2 O decomposition Chengyun Huang a , Zhen Ma b,∗ , Pengfei Xie a , Yinghong Yue a , Weiming Hua a,∗ , Zi Gao a a
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, PR China Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3 ), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, PR China b
a r t i c l e
i n f o
Article history: Received 13 August 2014 Received in revised form 6 February 2015 Accepted 11 February 2015 Available online 12 February 2015 Keywords: Decomposition of nitrous oxide Hydroxyapatite Rhodium
a b s t r a c t Several hydroxyapatite (HAP) supports were prepared by precipitation under different pH values (8.5, 9.5, or 10.5), and rhodium was loaded onto the supports by impregnation followed by calcination at 500 ◦ C. The pH value during the precipitation was found to play an important role in determining the catalytic activity of the resulting catalysts in N2 O decomposition. The Rh/HAP catalyst prepared under optimal conditions (pH 10.5) showed superior catalytic activity than Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 , due to the rich basic sites of the HAP support and the dispersion of ultra small rhodium particles on HAP. A pretreatment of Rh/HAP in 4% H2 at 400 ◦ C can further improve the activity owing to the partial reduction of the rhodium species. The catalysts/supports were characterized by X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), CO2 temperature-programmed desorption (CO2 -TPD), and X-ray photoelectron spectroscopy (XPS). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nitrous oxide (N2 O) is a potent greenhouse gas and may cause ozone-layer depletion. Anthropogenic N2 O comes from adipic acid production, nitric acid production, and fossil fuels/biomass burning. The concentration of N2 O has increased at an annual rate of 0.2 − 0.3% since the industrial revolution [1]. Thus, it is important to eliminate the emission of N2 O. Methods for N2 O removal include thermal decomposition [2], selective catalytic reduction [3,4], and direct catalytic decomposition [5,6]. Among them, direct catalytic decomposition of N2 O into N2 and O2 is the most attractive, because this process has low operating costs and produces no harmful gases [7]. Catalysts used for N2 O decomposition include supported metal catalysts [8,9], metal oxides [10–13], and zeolite-based catalysts [14–18]. Rh- [8,1–24], Ru- [25,26], and Ir-based [27] catalysts have been reported to be active for N2 O decomposition. In particular, supported rhodium catalysts show high activities at relatively low temperatures [8,19,20]. The catalytic activity of rhodium catalysts was found to be significantly influenced by the supports [28,29]. Supports may influence the dispersion of rhodium [30,31],
∗ Corresponding authors. Tel.: +86 21 65642409; fax: +86 21 65641740. E-mail addresses: [email protected] (W. Hua), [email protected] (Z. Gao). http://dx.doi.org/10.1016/j.molcata.2015.02.011 1381-1169/© 2015 Elsevier B.V. All rights reserved.
the reducibility of rhodium species [32], and the pathway of reaction [8]. Thus, it is important to find suitable supports for making Rh catalysts, in order to improve the activity in N2 O decomposition. Hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ), a non-reducible and insoluble salt, has good thermal stability, moderate alkalinity, and rich surface hydroxyls. In addition, its redox and acid/base properties can be tuned by ion exchange [33] or atom defects [34]. Organic molecules can be grafted onto the HAP surface [35]. HAP can be used as a catalyst for some reactions such as the conversion of ethanol into 1-butanol [34], Knoevenagel condensation [36], aldol condensation [37], Friedel-Crafts reaction [38], Michael addition [39], and combustion of formaldehyde [40]. Alternatively, it can be used for making supported catalysts. For instance, Ru/HAP can be used in the water-gas reaction [41], the mild racemization of alcohols [42], cis-dihydroxylation and oxidative cleavage of alkenes [43]. Rh/HAP was reported for the hydrogenation of aromatics [44]. Au/HAP can catalyze CO oxidation [45–48], the water-gas shift reaction [49], and organic synthesis [50–53]. To the best of our knowledge, the decomposition of N2 O on Rh/HAP was not reported. Here we found that Rh/HAP showed much higher activity than conventional Rh/TiO2 , Rh/Al2 O3 , and Rh/SiO2 for N2 O decomposition. This is ascribed to the better basicity of the HAP support and high dispersion of rhodium species on HAP.
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2. Experimental 2.1. Preparation Several HAP supports were prepared by precipitation under different pH values. Typically, 7.9 g (NH4 )2 HPO4 was dissolved in 100 ml deionized water, and 23.2 g Ca(NO3 )2 ·4H2 O was dissolved in 100 ml 50% aqueous ethanol solution. The first solution was then added, with stirring, into the second solution (in a beaker) at a rate lower than 3 ml/min. Aqueous ammonia (25 wt%) was added into the mixed solution to a certain pH value (8.5, 9.5, or 10.5). After aging at 80 ◦ C for 2 h, the precipitate was washed with water, filtered, dried at 110 ◦ C overnight, and calcined at 550 ◦ C for 4 h. The obtained samples are denoted as HAP-pH value. Commercial Al2 O3 (␥-Al2 O3 ), TiO2 (P25), and SiO2 were selected as common supports for comparison. Supported rhodium catalysts based on the above-mentioned supports were prepared by wet impregnation, using Rh(NO3 )3 as the precursor. The Rh content was controlled at 1 wt%. In a typical synthesis, 1.98 g support was mixed with 10 ml aqueous Rh(NO3 )3 solution (containing 0.02 g Rh) in an agante mortar, and the mixture was grinded under an infrared lamp till dry. The impregnated samples were calcined in a muffle furnace at 500 ◦ C for 4 h with flowing air.
Fig. 1. Conversion of N2 O on Rh/HAP-10.5, Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 catalysts as a function of reaction temperature.
2.2. Characterization
3. Results
XRD patterns were recorded on a MSAL XD2 X-ray diffractometer using CuK␣ radiation at a scanning speed of 4o /min. BET surface areas were measured using a Micromeritics Tristar 3000 instrument. The samples were pretreated at 300 ◦ C in vacuum for 3 h, followed by N2 adsorption at −196 ◦ C. Surface basicity was characterized by CO2 -TPD in a flow-type fixed-bed reactor at ambient pressure. 0.3 g sample was preheated at 550 ◦ C for 2 h, and then cooled to 80 ◦ C in flowing He. At 80 ◦ C, sufficient CO2 pulses were injected till adsorption saturation, followed by purging the sample with He for 2 h. The temperature was then raised from 80 to 600 ◦ C at a rate of 10 ◦ C/min. Due the limitation of our instrument, the amount of CO2 was not determined simultaneously when the desorption profile was recorded. The CO2 desorbed was collected in a liquid N2 trap and detected by GC afterwards (after suddenly warming up the loop that collects the liquid CO2 ). SEM images were recorded on a Philips XL 30 operating at 30 kV. TEM data were recorded with a JEM-2011F instrument. XPS spectra were recorded on a PerkinElmer PHI 5000 C spectrometer with MgK␣ radiation as the excitation source. To calibrate the binding energy, the C 1 s line (284.6 eV) was used as the reference.
Fig. 1 shows the influence of different supports on the activity of supported Rh catalysts (with the same Rh content, 1 wt%) in N2 O decomposition. The activity, in terms of N2 O conversion at the same reaction temperature, follows the sequence of Rh/HAP > Rh/Al2 O3 > Rh/TiO2 > Rh/SiO2 . Here HAP refers to the hydroxyapatite precipitated under pH 10.5. The N2 O conversions of these catalysts at 275 ◦ C are 97%, 27%, 10%, and 7%, respectively. On the other hand, the T50 values (temperatures of 50% conversion) of these catalysts are 250, 289, 310, and 324 ◦ C, respectively. Apparently, HAP is the best support to load rhodium for N2 O decomposition. Fig. 2 shows the XRD patterns of Rh/HAP and neat HAP support. Here HAP refers to the hydroxyapatite precipitated under pH 10.5. The standard XRD data for HAP, Rh, and Rh2 O3 are also depicted in the figure for comparison. The HAP support (without rhodium at this stage) synthesized by us can be indexed as a pure hexagonal
refers to the N2 O concentration or peak area at an elevated temperature. If pretreatment is needed, then the catalyst was pretreated in situ at 400 ◦ C in a specific gas flow (4% H2 -He or He) at a flowing rate of 30 ml/min for 2 h. After pretreatment, the catalyst was cooled down to room temperature, under the same ambient, and the ambient was changed to 60 ml/min 0.5% N2 O-He to start the reaction testing.
2.3. Catalytic tests Catalytic activity of N2 O decomposition was tested in a flowtype fixed-bed reactor. 0.5 g catalyst was packed in an U-shaped glass tube (7 mm diameter) sealed by quartz wool. A gas stream of 0.5% N2 O (balanced by He) flowed through the catalyst at a rate of 60 ml/min. The catalyst (initially kept at near room temperature) was exposed to the gas stream for 1 h during which the existing stream was periodically analyzed by a gas chromatograph (GC, Agilent 7890 A) that can separate N2 O, O2 , and N2 . The reaction temperature was then varied using a furnace and kept at various elevated temperature for 30 min in each temperature step. The exhaust was again periodically analyzed by the GC, and the conversion of N2 O was calculated according to X = ([N2 O]in − [N2 O]out )/[N2 O]in , where [N2 O]in refers to the N2 O concentration or peak area at room temperature, and [N2 O]out
Fig. 2. XRD patterns of Rh/HAP-10.5 catalyst and HAP-10.5 support, together with the standard patterns of hydroxyapatite (PDF#09-0432), Rh (PDF#05-0685), and Rh2 O3 (PDF#41-0541).
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Fig. 3. Conversion of N2 O on Rh/HAP catalysts synthesized at different initial pH values as a function of reaction temperature.
phase with a P63 /m space group. The main diffraction peaks located at 25.9, 31.7, 32.2, 32.9, and 34.0o correspond to the (0 0 2), (2 1 1), (1 1 2), (3 0 0), and (2 0 2) crystal faces, and are consistent with the standard XRD data (PDF#09-0432). When rhodium is loaded onto HAP to make the Rh/HAP catalyst, the XRD pattern exhibits only characteristic HAP peaks but neither metallic Rh (PDF#050685) nor Rh2 O3 (PDF#41-0541) can be observed. The HAP peaks of Rh/HAP become slightly sharper than those of neat HAP, due to the additional calcination step (at 500 ◦ C) after the impregnation of HAP by rhodium. Fig. S1 in the Supporting information shows the XRD patterns of Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 catalysts. Neither metallic Rh nor Rh2 O3 peaks can be observed, consistent with the phenomenon in Fig. 2. This is due to the low loading of rhodium species. Because HAP was found to be the support of choice in our study, attempts were then made to investigate the influence of preparation details on the nature of support and the catalytic performance. Fig. 3 shows the catalytic activities of Rh supported on three supports precipitated under different pH values (8.5, 9.5, or 10.5). These catalysts are denoted as Rh/HAP-pH value for convenience, although our XRD data (Fig. 4) show that the support synthesized under pH 8.5 is actually a mixture of HAP and Ca2 P2 O7 (PDF#170499). As shown in Fig. 3, the activity follows the sequence of Rh/HAP-10.5 > Rh/HAP-9.5 > Rh/HAP-8.5. The N2 O conversions of these catalysts at 275 ◦ C are 97%, 37%, and 9%, respectively. One problem in the synthesis of HAP support is that the pH value sometimes slowly drops during the precipitation, due to the consumption of aqueous OH− to form HAP. In the preparation of HAP-10.5, the amount of ammonia added is nearly nine times that of the stoichiometric value required for the synthesis of HAP, therefore the pH value only decreases slightly from 10.5 to 10.2. However, when less amount of aqueous ammonia is introduced, the pH value decreases obviously. The pH value drops to slightly above 7 in the preparation of HAP-9.5, and it deceases to below 7 in the preparation of HAP-8.5. The low pH value during the synthesis means that the amount of OH− is insufficient to generate HAP, consistent with the formation of Ca2 P2 O7 (Fig. 4). To overcome the above problem, the synthesis procedure was modified, i.e., aqueous ammonia was continuously added during the precipitation to keep the pH value constant (at 7.5, 8.5, or 9.5). The supports thus obtained are denoted as HAP-pH-C. Fig. S2 shows the SEM images of these supports, together with that of
Fig. 4. XRD patterns of Rh/HAP catalysts synthesized at different initial pH values and standard patterns of Ca2 P2 O7 (PDF#17-0499), Rh (PDF#05-0685), and Rh2 O3 (PDF#41-0541).
HAP-10.5 (without pH adjusting) for comparison. For HAP-10.5 and HAP-9.5-C prepared at higher pH values, the particles show sheetlike morphology. For HAP-8.5-C and HAP-7.5-C prepared at lower pH values, most of the particles are approximately sphere-shaped. The obtained supports show XRD patterns similar to HAP-10.5 (Fig. S3), and the activities of the resulting Rh catalysts improve significantly compared to the ones precipitated at the same pH value but without pH adjusting (Fig. S4). For example, both Rh/HAP-9.5C and Rh/HAP-8.5-C give a T50 value of 266 ◦ C, which is obviously lower than those of Rh/HAP-9.5 and Rh/HAP-8.5 (286 and 350 ◦ C, respectively). Results from these control experiments underscore the importance of maintaining the high pH level (more OH− ) during the precipitation synthesis of HAP. The question now arises as to why HAP outperforms other common supports. As shown in Table 1, the surface areas of HAP (synthesized under pH 10.5), Al2 O3 , TiO2 , and SiO2 are 58, 114, 48, and 303 m2 /g, respectively, inconsistent with the activity trend seen in our reaction testing. Therefore, surface area of the support is apparently not the determining factor. On the other hand, the basicity of the support seems to be essential. The amounts of basic sites of several supports were measured by CO2 -TPD. Fig. S5 shows the CO2 -TPD profiles of several typical samples. As shown in Table 1, the amounts of basic sites of HAP (synthesized under pH 10.5), Al2 O3 , TiO2 , and SiO2 are 42.7, 14.3, 7.7, and 5.1 mol/g, respectively, consistent with the activity trend. Within the three samples denoted as HAP-pH value, i.e., HAP-10.5, HAP-9.5, and HAP-8.5, the amounts of basic sites also correlate with the catalytic activity of the supported catalysts, further proving the important role of basic sites in enhancing activity in N2 O decomposition. The catalysts collected after the reaction testing were characterized by TEM (Figs. 5 and S6–S11). The characterization of a spent catalyst is assumed to provide information on a system that is closer
Table 1 Surface area and amount of basic sites of the supports and rhodium particle size of the supported catalysts.
Surface area of support (m2 /g) Amount of support’s basic sites (mol/g) Average size of rhodium particles (nm)
HAP-10.5
HAP-9.5
HAP-8.5
Al2 O3
TiO2
SiO2
58 42.7 0.78 ± 0.22
46 9.5 1.12 ± 0.33
36 5.2 1.33 ± 0.50
114 14.3 0.77 ± 1.56
48 7.7 1.70 ± 0.63
303 5.1 2.25 ± 0.67
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Fig. 6. Conversion of N2 O on the Rh/HAP-10.5 catalyst pretreated in situ at 400 ◦ C under different ambient as a function of reaction temperature.
Fig. 5. TEM images of Rh/HAP-10.5 collected after reaction testing. Top: raw data; Bottom: enlarged portion.
to a catalyst under steady state reaction conditions than a fresh catalyst. For each sample, different areas of the same specimen were examined, and the average particle sizes were obtained by measuring the sizes of 150 particles with the aid of Digital Micrograph. The results are shown in Table 1. As shown in Figs. 5 and S6, the rhodium particles are ultra small on HAP-10.5 support. The average size is as small as 0.78 nm. On the other hand, the average sizes of rhodium particles on Al2 O3 , TiO2 , and SiO2 are 0.77, 1.70, and 2.25 nm, respectively (Figs. S7–S9). For the Rh/HAP series catalysts (Figs. 5, S6, S10, S11), the average particle sizes increase (0.78, 1.12, 1.33) as the pH value decreases (10.5, 9.5, 8.5), consistent with the trend observed in catalytic tests. In the above catalytic experiments, the catalysts were calcined at 500 ◦ C ex situ, and no additional in situ pretreatment was applied before conducting catalytic reactions. The influence of pretreatment conditions on the performance of Rh/HAP-10.5 was studied. As shown in Fig. 6, the activity is virtually not changed if the catalyst is pretreated in He at 400 ◦ C. However, the pretreatment in 4% H2 increases the activity significantly. After reaching a reaction temperature of 400 ◦ C, the catalyst was cooled down to room temperature, and the catalytic activity was re-tested as a function of reaction temperature. As shown in Fig. S12, the conversions curves for the first cycle and second cycle of reaction testing are almost the same. Fig. 7 shows the XPS spectra of Rh/HAP-10.5 pretreated ex situ in different atmosphere. There are two sets of Rh peaks at 306 − 311 eV and 311 − 317 eV, due to the splitting of the 3d orbit of Rh into Rh 3d5/2 and Rh 3d3/2 . The ratio of these two peaks is about 1.5. Our following discussion will be focused on the Rh 3d5/2 peak. The binding energy of Rh3+ is 308.3 − 310.5 eV whereas that of Rh0 is 307.0 − 307.7 eV [54,55]. The Rh/HAP-10.5 catalyst calcined at 500 ◦ C (with no additional pretreatment) and the one pretreated in He at 400 ◦ C contains mainly Rh3+ , alone with an ignorable amount of Rh00 (below 4%). In contrast, the catalyst pretreated in 4% H2 at
Fig. 7. XPS spectra of the Rh/HAP-10.5 catalyst pretreated ex situ under different conditions (before reaction) and the one collected after reaction.
400 ◦ C contains a portion of Rh0 (about 32%). After the reaction, the amount of Rh0 increases a bit. Thus, it is clear that the presence of metallic Rh is more advantageous for the N2 O decomposition. As a proof, the pretreatment of Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 in 4% H2 also leads to more active catalysts, but Rh/HAP-10.5 pretreated by 4% H2 is still the most active (Fig. S13). 4. Discussion The most essential finding of this study is that Rh/HAP is more active than Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 in N2 O decomposition. To the best of our knowledge, this finding was not reported in the literature. In addition, HAP has seldom been used for making supported catalysts. As for what makes HAP the support of choice, several reasons may be considered and discussed. First, the surface areas of HAP-10.5, Al2 O3 , TiO2 , and SiO2 are 58, 114, 48, and 303 m2 /g, respectively, inconsistent with the trend seen in catalytic tests. Therefore, surface area is not the determining factor. Second, the amounts of basic sites of these supports are 42.7, 14.3, 7.7, and 5.1 mol/g, respectively, consistent with the trend seen in catalytic tests. It is recognized that basic sites are advantageous for N2 O decomposition [30]. Increasing the basicity of the support could enhance the rhodium dispersion, and hence increase the activity
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of rhodium catalysts [30]. HAP support can lead to small rhodium particles (average size 0.78 nm) on surface, but that is not the sole factor contributing to the high activity since the average size of rhodium particles on Al2 O3 is comparable (average size 0.77 nm). Within the Rh/HAP family, the pH value during precipitation preparation of the support is found to be important to guarantee the purity of the HAP support. Precipitation at an initial pH value of 8.5 leads to a mixture of HAP and Ca2 P2 O7 (Fig. 4). The low-pH precipitation can also lead to decreased surface area, less basic sites, bigger rhodium particles (Table 1), as well as decreased activity (Fig. 3). The data again demonstrate the importance of the basic sites of supports for rhodium catalysts in N2 O decomposition. 5. Conclusions Several hydroxyapatite (HAP) supports was synthesized via a precipitation method, using Ca(NO3 )2 and (NH4 )2 HPO4 as precursors. Rhodium was loaded onto HAP by impregnation and calcination. The resulting Rh/HAP prepared under optimal conditions (pH 10.5) is more active than Rh/Al2 O3 , Rh/TiO2 , and Rh/SiO2 in N2 O decomposition. The use of HAP support is advantageous for this reaction, because HAP can provide more basic sites and stabilize ultra small rhodium particles. The pH value during the precipitation synthesis of HAP has to be kept at 10.5 to guarantee the high activity of the resulting catalyst. A pretreatment of Rh/HAP in 4% H2 can lead to the reduction of the rhodium species, thus further increasing the activity. Acknowledgments W. Hua and Y. Yue thank the financial support by the National Natural Science Foundation of China (20773027 and 21273043), the Ph.D. programs foundation of the Ministry of Education in China (20100071110008), and the Science and Technology Commission of Shanghai Municipality (13DZ2275200). Z. Ma thanks the financial support by the National Natural Science Foundation of China (21177028 and 21477022). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2015.02.011. References [1] J. Pérez-Ramírez, F. Kapteijn, K. Schöffel, J.A. Moulijn, Appl. Catal. B 44 (2003) 117–151. [2] M. Galle, D.W. Agar, O. Watzenberger, Chem. Eng. Sci. 56 (2001) 1587–1595. [3] G. Centi, F. Vazzana, Catal. Today 53 (1999) 683–693. [4] S. Kameoka, K. Kita, T. Takeda, S. Tanaka, S. Ito, K. Yuzaki, T. Miyadera, K. Kunimori, Catal. Lett. 69 (2000) 169–173. [5] F. Kapteijn, J. Rodriguez-Mirasol, J.A. Moulijn, Appl. Catal. B 9 (1996) 25–64. [6] S. Kannan, Catal. Surv. Asia 10 (2006) 117–137. [7] N. Imanaka, T. Masui, Appl. Catal. A 431 (2012) 1–8. ˜ [8] S. Parres-Esclapez, I. Such-Basanez, M.J. Illán-Gómez, C.S. Lecea, A. Bueno-López, J. Catal. 276 (2010) 390–401. [9] V.G. Komvokis, M. Marti, A. Delimitis, I.A. Vasalos, K.S. Triantafyllidis, Appl. Catal. B 103 (2011) 62–71. [10] K. Asano, C. Ohnishi, S. Iwamoto, Y. Shioya, M. Inoue, Appl. Catal. B 78 (2008) 242–249.
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