- Email: [email protected]
Role of metal oxide supports in NH3 decomposition over Ni catalysts
Accepted Manuscript Title: Role of metal oxide supports in NH3 decomposition over Ni catalysts Author: Isao Nakamura Tadahiro Fujitani PII: DOI: Reference:
S0926-860X(16)30273-3 http://dx.doi.org/doi:10.1016/j.apcata.2016.05.020 APCATA 15884
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
29-3-2016 19-5-2016 21-5-2016
Please cite this article as: Isao Nakamura, Tadahiro Fujitani, Role of metal oxide supports in NH3 decomposition over Ni catalysts, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.05.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
・The essential role of metal oxide supports for NH3 decomposition over Ni catalysts is clarified. ・The volcano-curve relationship between turnover frequency and Ni–N binding energy is obtained for Ni catalysts supported on various metal oxides. ・The rate-determining step for NH3 decomposition reaction on Ni is changed by the Ni–N binding energy.
1
TOF (molecules/site-Ni/s)
*Graphical Abstract
Ni/ZrO2 Ni/CeO2 Ni/La2O3
Ni/Y2O3
Ni/MgO Ni/Al2O3
Unsupported Ni
N2 desorption temperature (oC)
Role of metal oxide supports in NH3 decomposition over Ni catalysts Isao Nakamura, Tadahiro Fujitani* Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Abstract The decomposition of NH3 over Ni catalysts supported on various metal oxides (Y2O3, CeO2, MgO, La2O3, Al2O3, and ZrO2) was investigated, and the Ni/Y2O3 catalyst was found to show the highest activity. The rate of H2 formation per exposed Ni atom (turnover frequency) for the supported Ni catalysts depended strongly on the type of metal oxide support, indicating that the active sites for the Ni catalysts were not identical in nature. Investigation of the associative desorption of atomic nitrogen formed by NH3 decomposition over the Ni catalysts revealed that the N2 desorption temperature depended on the type of support. This result indicates that the Ni–N binding energy varied among the metal oxide supports, which would be attributable to the change of the Ni electronic state by the supports. The volcano-curve relationship was obtained between turnover frequency and N2 desorption temperature, indicating that the NH3 decomposition activity was dominated by the Ni–N binding energy. We considered that the metal oxide supports changed the rate-determining step for NH3 decomposition on Ni.
Keywords: NH3 decomposition, Ni catalyst, Metal oxide support, Active site
*Corresponding author, E-mail: [email protected] 2
1. Introduction If the use of hydrogen as a fuel is to become widespread, infrastructure for transport, storage, and distribution of large amounts of hydrogen will be indispensable. As a potential method for hydrogen transport, the use of ammonia has received attention because its hydrogen mass density (about 18 wt%) is higher than that of other hydrogen storage materials and it is readily liquified (its vapor pressure is 0.8 MPa at 20 °C) [1,2]. Ammonia must be decomposed into H2 and N2, and the use of a catalyst is the most effective method for this purpose [3]. The NH3 decomposition performances of catalysts based on metals such as Fe, Ni, Pt, Ru, Co, Ir, Pd, and Rh have been extensively investigated [4–12]. Of the metals that have been studied, Ru is the most active, but this noble metal is very expensive. Therefore, for practical applications, the amount of Ru in the catalyst must be reduced, or Ru must be replaced by a non-noble metal. Among the non-noble metal alternatives, Ni, which is more active than Fe [13], has attracted attention [4,5], and research aimed at improving the performance of Ni catalysts has focused on the selection of a support [14–20], the addition of a promoter element [21–25], and the formation of alloys or bimetallic compounds [26–28]. In these previous reports, the nature of the active sites and the role of the support are discussed. Metallic Ni is generally considered to be the active site for NH3 decomposition, and the supports and additives are thought to improve the Ni dispersion [14,15,24,25]. Muroyama et al. [15] investigated the influence of the support on the NH3 decomposition activity of Ni catalysts supported on various metal oxides and found that Ni/Al2O3 catalysts show the highest NH3 conversion, owing to the high surface area of the Al2O3 support; and these investigators concluded that a high dispersion of Ni particles over the support is crucial for superior catalytic activity. Liu and co-workers 3
[14] reported that small Ni particle size, high dispersion, and high resistance to sintering improve the performance and stability of Ni catalysts supported on SBA-15, which has well-ordered hexagonal mesoporous silica structures. Furthermore, these investigators showed that the NH3 decomposition activity of the Ni/SBA-15 catalyst is enhanced by the addition of a Ce or La oxide; the enhancement is due to increases in the Ni surface area, the Ni dispersion, and the reducibility of the catalyst [24]. Introduction of an appropriate amount of Ce to Ni encapsulated in SiO2 results in high-surface-area catalysts with small Ni particles, which show enhanced catalytic activity for NH3 decomposition compared to the activity of Ni catalysts with no additive [25]. Metal oxide supports and additives have been reported to have other roles, in addition to increasing the Ni surface area. For example, Deng et al. [16], who investigated NH3 decomposition over a Ni catalyst supported on a high-surface-area Ce0.8Zr0.2O2 solid solution, speculated that hydrogen spillover from the Ni surface to the support via a Ni–O–Ce species plays an important role promoting the decomposition reaction. The presence of a La promoter in a Ni/Al2O3 catalyst has been shown to reduce the stability of reaction intermediates in the decomposition of NH3 [22]. Furthermore, the addition of CeO2 to a Ni/Al2O3 catalyst reportedly enlarges catalyst pores, moderates the interaction between Ni and the alumina, and improves the recombinative desorption of nitrogen adatoms from the Ni0 surface [23]. Thus, there are still various theories about the roles of supports and additives, despite the fact that their favorable effects on Ni dispersion have long been generally known. In this study, we carried out NH3 decomposition reactions over Ni catalysts supported on various metal oxides, with the goal of shedding light on the role of the supports and on the nature of the active sites.
4
2. Experimental Ni/MxOy (M = Y, Ce, Mg, La, Al, or Zr) catalysts were prepared by a co-precipitation method from solutions of the corresponding metal nitrates (Ni(NO3)2·6H2O + MxOy(NO3)z·nH2O) with potassium carbonate (K2CO3). The precipitates were filtered, washed twice with distilled water, and dried at 110 °C for 17 h. After drying, the samples were calcined at 500 °C for 2 h in air. The Ni loading for all catalysts was fixed at 30 wt%. The N2 adsorption isotherms of the catalysts at liquid N2 temperature (–196 °C) were
measured with an automated physisorption instrument (ASAP
2020,
Micromeritics). Prior to the measurement, each catalyst (0.3 g) was degassed in a vacuum at 350 °C for 10 h to remove physically adsorbed components. The specific surface area was determined from the linear portion of the Brunauer–Emmett–Teller plot. The pore size and pore volume were calculated from the desorption branch of the N2 adsorption isotherm by means of the Barrett–Joyner–Halenda formula. The dispersion of metallic Ni for each catalyst (0.03–0.2 g) was evaluated by the H2 pulse method (BELCAT-B, BEL Japan) at 50 °C after reduction with 10.1% H2/Ar gas at 400 or 600 °C, where Ni dispersion is defined by a ratio of surface-exposed Ni metal atoms to the supported Ni atoms. The pulse adsorption of H2 was detected by a thermal conductivity detector. The X-ray diffraction (XRD) patterns of the supported Ni catalysts after reduction with 10.1% H2/Ar gas at 600 °C were obtained with an X-ray diffractometer (RINT2000, Rigaku) operated at 40 kV and 40 mA using Cu Kα monochromatized radiation (λ = 0.154178 nm). Temperature-programmed reduction (TPR) and desorption (TPD) experiments were carried out in a BELCAT-B apparatus. TPR measurement for each catalyst (0.03 g) was performed under a flow of 10.1% H2/Ar (50 ml min-1) from 30 to 900 °C at a 5
constant heating rate of 10 °C min-1. The consumption of H2 was monitored with a thermal conductivity detector. Prior to the TPD measurements, each catalyst (0.2 g) was pre-reduced in situ with 10.1% H2/Ar at 600 °C. The reduced catalyst was exposed to a flow of 10% NH3/He (40 ml min-1) at 280 °C for 30 min, and then the flow gas was switched to He (40 ml min-1). After holding in He at 280 °C for 20 min, the temperature of the catalyst bed was raised under a flow of He (40 ml min-1) to 500 °C at a rate of 10 °C min-1. The desorption products were analyzed by means of an online mass spectrometer. The N2 desorption was observed without desorption of NH3 and H2, indicating that the dehydrogenation of NH3 completely proceeded, and only the atomic nitrogen was remained on the catalyst surface. Ammonia decomposition reactions were carried out in a continuous-flow fixed-bed quartz reactor under a flow of pure NH3 (40 ml min-1) at atmospheric pressure and a temperature range of 450–600 °C. The reaction temperature was monitored by means of a thermocouple placed in the catalyst bed. Prior to each reaction, the catalyst (0.2 g, 22–42 mesh) was pre-reduced in situ with 20% H2/N2 gas at 600 °C. After reduction, the bed was allowed to cool to 450 °C under a H2/N2 flow, and then the flow gas was switched to NH3. The catalytic activities were measured at 450, 500, 550, and 600 °C after each temperature had been maintained for at least 30 min. The reaction products (N2 and H2) were analyzed with an online gas chromatograph equipped with a thermal conductivity detector. The catalytic activity was evaluated on the basis of the H2 production.
3. Results and discussion We carried out NH3 decomposition reactions over Ni catalysts supported on various metal oxides and compared the H2 yields obtained at reaction temperatures of 6
450–600 °C (Table 1). At 450 °C, the Ni/CeO2 catalyst exhibited the highest activity. The order of activities at 500 and 550 °C was as follows: Ni/Y2O3 > Ni/CeO2 > Ni/MgO > Ni/La2O3 > Ni/Al2O3 > Ni/ZrO2. No significant differences in the H2 yields were observed at 600 °C, because the activities of all the supported Ni catalysts were close to the equilibrium value for NH3 decomposition. As previously mentioned, the dispersion of metallic Ni is known to be an important determinant of the NH3 decomposition activity of supported Ni catalysts [14,15,24,25]. Furthermore, Zhang et al. [22] reported that experimental data obtained for NH3 decomposition over Ni/Al2O3 and Ni/La-Al2O3 catalysts show an excellent fit to the Temkin–Pyzhev equation, which assumes that Ni metal is the active site. Thus, the dependence of NH3 decomposition activity on the type of metal oxide support may be due to differences in the degree of reduction of supported NiO. To evaluate this possibility, we began by using TPR to investigate the influence of the support on the reduction behavior of NiO (Fig. 1). For unsupported NiO, the reduction peak was observed at 369 °C, which is in agreement with the previously reported reduction temperature for NiO [29]. For the NiO/Y2O3 catalyst, reduction peaks were observed at 426 and 503 °C; both of these temperatures are higher than that of NiO alone. These peaks may be attributable to the reduction of two types of NiO, one type that interacts weakly with Y2O3 and another that interacts strongly. The reduction of strongly interacting NiO species may be related to the formation of NiO–Y2O3 solid solution [30]. In contrast, the NiO/CeO2 catalyst showed only one reduction peak, at 354 °C, a temperature that was slightly lower than that observed for NiO alone. This result is consistent with previous reports indicating that the reduction of NiO species is promoted by its interaction with the CeO2 support [31,32], that is, by the formation of an –O–Ni–O–Ce 7
superstructure that is reducible at low temperature [33]. For NiO/MgO, there was a small peak at 357 °C, but most of the reduction occurred at 603 °C. The peak at this temperature was assigned to the reduction of Ni2+ ions in the surface and subsurface layers of the MgO [34]. NiO/La2O3 showed two reduction peaks, at 395 and 598 °C, which were ascribed to the reduction of NiO interacting with La2O3 and the reduction of LaNiO3 [35,36], respectively. In the case of NiO/Al2O3, the only peak appeared at 595 °C, which was attributed to the reduction of the precursor of the NiAl2O4 spinel structure [21]. For NiO/ZrO2, peaks were observed at 398 and 508 °C. The lower and higher peaks may be due to the reduction of NiO interacting with the ZrO2 support and diffused into ZrO2 lattice, respectively [37]. In summary, NiO was reduced to metallic Ni below 600 °C on all of the metal oxide supports; that is, all of the Ni in the catalysts was in the metallic state as a result of treatment of the catalysts with H2 at 600 °C before the activity measurements. These results indicate that the support-dependence of the NH3 decomposition activity was not due to the extent of reduction of the supported NiO. We also evaluated the Ni dispersion and the physical properties (specific surface area, pore size, and pore volume) of the catalysts (Table 2). After reduction with H2 at 400 °C, the Ni dispersions for the Ni/MgO and Ni/Al2O3 catalysts were clearly lower than those of the other catalysts, owing to incomplete reduction of NiO. However, the dispersions for Ni/MgO and Ni/Al2O3 after reduction at 600 °C were higher than at the lower temperature, a result that we attributed to complete reduction of NiO at the higher temperature. These behaviors are consistent with the TPR results (Fig. 1). In contrast, the Ni dispersions for the Ni/Y2O3, Ni/CeO2, Ni/La2O3, and Ni/ZrO2 catalysts after H2 reduction at 600 °C were 40–60% lower than those after 400 °C reduction, owing to sintering of the Ni particles. The Ni dispersion after 600 °C 8
reduction decreased in the order Ni/Y2O3 > Ni/MgO > Ni/Al2O3 > Ni/CeO2 > Ni/La2O3 > Ni/ZrO2. This order was not the same as the order based on specific surface area, pore size, or pore volume, indicating that the Ni dispersion was not affected by the physical properties of the metal oxide support. To determine whether Ni dispersion and NH3 decomposition activity were correlated, we plotted H2 formation rate at 500 °C (as determined from the H2 yields listed in Table 1) as a function of Ni surface area after 600 °C reduction (Fig. 2). The data did not fit on a straight line passing through the origin, suggesting that the NH3 decomposition activity was not determined solely by Ni surface area. To investigate this possibility, we estimated the H2 formation rate per exposed Ni atom (turnover frequency, TOF) at 500 °C (Table 3). The TOFs for all the Ni catalysts should be equal if the active sites for NH3 decomposition of the catalysts are identical in nature and differ only in number. We found that the TOFs for the supported Ni catalysts depended strongly on the type of metal oxide support, indicating that active sites with electronic states other than Ni0 or with different morphologies were formed on the supports. Danielle et al. [38] calculated the catalytic activity for NH3 decomposition on various single-metals such as Co, Pt, Pd, Ni, Ru, Rh, Ir, Re, and Mo using microkinetic modeling. These investigators observed a volcano-curve relationship between the calculated conversion of NH3 and the nitrogen-binding energies of the transition metals, and Ru exhibited the highest activity among the single-metal catalysts studied. From a sensitivity analysis for the elementary reaction steps on each metal, the removal of the second hydrogen (NH2 → NH + H) is shown to be the rate-determining step for metals with lower nitrogen-binding energies, whereas the removal of the first and second hydrogens (NH3 → NH2 + H and NH2 → NH + H) and nitrogen desorption (N + N → N2) are kinetically significant steps for metals with higher nitrogen-binding energies. 9
This analysis underscores the fact that the kinetically significant step and dominant coverage may be changing along a volcano curve. In this study, to determine whether NH3 decomposition activity was related to nitrogen-binding energy of the supported Ni, we measured the associative desorption behaviors of atomic nitrogen formed by the NH3 dehydrogenation. It has been generally well known that the NH3 is adsorbed on Ni in N coordination, and its decomposition proceeds consecutively in the mechanism of NH3 → NH2 → NH → N, and finally N2 is produced by associative desorption of atomic nitrogen adsorbed on Ni [39–41]. Thus, the N2 desorption temperature reflects the Ni–N binding energy. We firstly determined the NH3 exposure temperature to produce atomic nitrogen over Ni catalysts. It has been reported that the formation of atomic nitrogen by the NH3 dehydrogenation over Ni single crystals proceeds around 230 °C [39,42]. Then, we exposed the Ni catalysts to NH3 at various temperatures, and found that only the N2 desorption was observed by the exposure of NH3 above 250 °C, clearly indicating that the NH3 dehydrogenation step completely proceeded, and only the atomic nitrogen was remained on the Ni surface. Thus, we adopted 280 °C as the NH3 exposure temperature. Fig. 3 shows the N2 TPD spectra measured after NH3 exposure to Ni catalysts at 280 °C. For unsupported Ni, the N2 desorption peak was observed at 324 °C, whereas the N2 desorption peaks for the supported Ni catalysts were observed in the temperature range between 346 and 420 °C; that is, the N2 desorption temperature depended strongly on the type of support. This result indicates that the Ni–N binding energy varied with the metal oxide support. We also investigated the relationship between TOF and N2 desorption temperature for each catalyst, and the results for the decomposition reaction at 500 °C are shown in Fig. 4. The TOF increased with increasing N2 desorption temperature up to approximately 400 °C, and then decreased. Thus, a volcano-curve relationship between 10
TOF and Ni–N binding energy was obtained for Ni catalysts supported on various metal oxides. It has been reported by Duan et al. [43] that the NH3 decomposition over Ni catalysts supported on mobil crystalline material 41 (MCM-41) exhibits a volcano relationship between TOF and Ni particle size. They also demonstrated by means of first-principles calculations that the rate constants for the associative desorption of atomic nitrogen and NH3 dehydrogenation are the smallest among elementary steps for NH3 decomposition on Ni(211) and Ni(111), respectively. Thus, the existing optimal Ni particle size can be explained that smaller Ni particles have more step sites which are easily blocked by strongly adsorbed nitrogen atoms, and hence present the low catalytic activity, while larger Ni particles have less step sites which are favorable to NH3 dehydrogenation, and also present the low catalytic activity. It was concluded that the NH3 decomposition activity is sensitive to the ratio of Ni step sites to Ni terrace sites, where the optimal ratio is obtained around 10 nm of particle size. In contrast, the Ni particle size for the supported Ni catalysts in this study was estimated to be 15–60 nm, which was much larger than that for the Ni/MCM-41 catalysts. It has been reported that the surface for our particle sizes has a low step density, and is almost occupied by the terrace site [44]. That is, the ratio of step sites to terrace sites is hardly changed between these Ni particles. Furthermore, we investigated the crystal structure of Ni particles for the supported Ni catalysts after H2 reduction by means of XRD. As a result, the relative intensity ratio of the diffraction peaks from each crystal face to the (111) peak for metallic Ni was almost same in each Ni catalyst, clearly indicating that the surface structure of the Ni particles is not changed. On the other hand, we examined a correlation between TOF and Ni particle size for the supported Ni catalysts, but a volcano relationship was not confirmed. Therefore, the variation of the N2 desorption 11
temperature observed for the supported Ni catalysts in this study would not be due to the structural change of Ni. We consider that the difference in the N2 desorption temperature is attributable to the change of the Ni electronic state by the metal oxide supports. It has been reported that the addition of alkali onto the Ru surface destabilizes the adsorption of atomic nitrogen because of the electron transfer from alkali to Ru [45]. That is, the binding energy of atomic nitrogen is changed by the electronic state of metal. On the other hand, it has been demonstrated from density functional theory calculations that the deposition of Ni on stoichiometric ceria results in the electron transfer from Ni to CeO2 [46], and the electrons of Ni cluster are transferred to γ-Al2O3 when Ni cluster was supported on γ-Al2O3(100) [47]. We thus considered that the Ni supported on metal oxide support is in a cationic state, resulting in a strong Ni–N binding energy, which was consistent with the results in Fig. 3. We believe that the extent of the electron transfer depends on the type of support, and the more electron transfer from Ni to support produces more cationic Ni, resulting in a stronger Ni–N binding energy. It is generally known that the rate-determining step for NH3 decomposition is the associative desorption of atomic nitrogen [4,5]. However, we have previously found by the kinetic analysis that the overall rate of NH3 decomposition over a Ni/MgO catalyst is controlled by the NH3 dehydrogenation step [48]. As shown in Fig. 4, the Ni/MgO catalyst corresponds to the region of lower Ni–N binding energy, which is consistent with that the reaction of NH2 → NH + H is the rate-determining step in the region of lower nitrogen-binding energies [38]. We thus considered that the volcano-curve relationship in Fig. 4 indicated the change of the rate-determining step depending on the Ni–N binding energy. That is, the NH3 dehydrogenation is the rate-determining step for the supported Ni catalysts with low Ni–N binding energy, whereas the associative 12
desorption of atomic nitrogen is the rate-determining step in the case of high Ni–N binding energy. We clearly indicated that the metal oxide supports changed the rate-determining step for NH3 decomposition on Ni.
4. Conclusions We investigated the NH3 decomposition activities and physicochemical properties of Ni catalysts supported on various metal oxides. The TOFs for the supported Ni catalysts depended strongly on the type of metal oxide support, indicating that the active sites for the Ni catalysts were not identical in nature. TPD measurements indicated that the N2 desorption temperature also depended on the type of support; that is, the Ni–N binding energy varied with the metal oxide support, which was considered to be due to the change of the Ni electronic state by the supports. Furthermore, we observed a volcano-curve relationship between TOF and N2 desorption temperature, indicating that the NH3 decomposition activity was dominated by the Ni–N binding energy. This would be attributable to that the rate-determining step for NH3 decomposition reaction was different with the Ni–N binding energy, which was changed by the metal oxide supports.
Acknowledgement This work was supported by the Cross-ministerial Strategic Innovation Promotion Program (SIP) of the Cabinet Office, Government of Japan.
13
References [1] A. Klerke, C.H. Christensen, J.K. Nørskov, T. Vegge, J. Mater. Chem. 18 (2008) 2304–2310. [2] R. Lan, J.T.S. Irvine, S.W. Tao, Int. J. Hydrogen Energy 37 (2012) 1482–1494. [3] A. Ozaki, K. Aika, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Springer–Verlag, New York, 1981, pp. 87–158. [4] S.F. Yin, B.Q. Xu, X.P. Zhou, C.T. Au, Appl. Catal. A: Gen. 277 (2004) 1–9. [5] F. Schüth, R. Palkovits, R. Schlögl, D.S. Su, Energy Environ. Sci. 5 (2012) 6278–6289. [6] D.C. Huang, C.H. Jiang, F.J. Liu, Y.C. Cheng, Y.C. Chen, K.L. Hsueh, Int. J. Hydrogen Energy 38 (2013) 3233–3240. [7] A.D. Carlo, L. Vecchione, Z.D. Prete, Int. J. Hydrogen Energy 39 (2014) 808–814. [8] D. Varisli, E.E. Elverisli, Int. J. Hydrogen Energy 39 (2014) 10399–10408. [9] K. Nagaoka, T. Eboshi, N. Abe, S. Miyahara, K. Honda, K. Sato, Int. J. Hydrogen Energy 39 (2014) 20731–20735. [10] G. Li, H. Nagasawa, M. Kanezashi, T. Yoshioka, T. Tsuru, J. Mater. Chem. A 2 (2014) 9185–9192. [11] Y.Q. Gu, Z. Jin, H. Zhang, R.J. Xu, M.J. Zheng, Y.M. Guo, Q.S. Song, C.J. Jia, J. Mater. Chem. A 3 (2015) 17172–17180. [12] A.K. Hill, L.Torrente-Murciano, Appl. Catal. B: Environ. 172–173 (2015) 129–135. [13] S.F. Yin, Q.H. Zhang, B.Q. Xu, W.X. Zhu, C.F. Ng, C.T. Au, J. Catal. 224 (2004) 384–396. [14] H. Liu, H. Wang, J. Shen, Y. Sun, Z. Liu, Appl. Catal. A: Gen. 337 (2008) 138–147. 14
[15] H. Muroyama, C. Saburi, T. Matsui, K. Eguchi, Appl. Catal. A: Gen. 443–444 (2012) 119–124. [16] Q.F. Deng, H. Zhang, X.X. Hou, T.Z. Ren, Z.Y. Yuan, Int. J. Hydrogen Energy 37 (2012) 15901–15907. [17] R. Atsumi, R. Noda, H. Takagi, L. Vecchione, A.D. Carlo, Z.D. Prete, K. Kuramoto, Int. J. Hydrogen Energy 39 (2014) 13954–13961. [18] H. Zhang, Y.A. Alhamed, Y. Kojima, A.A. Al-Zahrani, H. Miyaoka, L.A. Petrov, Int. J. Hydrogen Energy 39 (2014) 277–287. [19] J. Ji, T.H. Pham, X. Duan, G. Qian, P. Li, X. Zhou, D. Chen, Int. J. Hydrogen Energy 39 (2014) 20722–20730. [20] J. Yang, A.F.S. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, ACS Appl. Mater. Interfaces 7 (2015) 7406–7412. [21] J. Zhang, H. Xu, X. Jin, Q. Ge, W. Li, Appl. Catal. A: Gen. 290 (2005) 87–96. [22] J. Zhang, H. Xu, W. Li, Appl. Catal. A: Gen. 296 (2005) 257–267. [23] W. Zheng, J. Zhang, Q. Ge, H. Xu, W. Li, Appl. Catal. B: Environ. 80 (2008) 98–105. [24] H. Liu, H. Wang, J. Shen, Y. Sun, Z. Liu, Catal. Today 131 (2008) 444–449. [25] L.F. Zhang, M. Li, T.Z. Ren, X. Liu, Z.Y. Yuan, Int. J. Hydrogen Energy 40 (2015) 2648–2656. [26] S.B. Simonsen, D. Chakraborty, I. Chorkendorff, S. Dahl, Appl. Catal. A: Gen. 447–448 (2012) 22–31. [27] A.S. Chellappa, C.M. Fischer, W.J. Thomson, Appl. Catal. A: Gen. 227 (2002) 231–240. [28] H. Silva, M.G. Nielsen, E.M. Fiordaliso, C.D. Damsgaard, C. Gundlach, T. Kasama, I.B. Chorkendorff, D. Chakraborty, Appl. Catal. A: Gen. 505 (2015) 548–556. 15
[29] G. Li, L. Hu, J.M. Hill, Appl. Catal. A: Gen. 301 (2006) 16–24. [30] Y.J.O. Asencios, C.B. Rodella, E.M. Assaf, Appl. Catal. B: Environ. 132–133 (2013) 1–12. [31] Sk. Mahammadunnisa, P.M.K. Reddy, N. Lingaiah, Ch. Subrahmanyam, Catal. Sci. Technol. 3 (2013) 730–736. [32] W. Zou, C. Ge, M. Lu, S. Wu, Y. Wang, J. Sun, Y. Pu, C. Tang, F. Gao, L. Dong, RSC Adv. 5 (2015) 98335–98343. [33] P. Pal, R.K. Singha, A. Saha, R. Bal, A.B. Panda, J. Phys. Chem. C 119 (2015) 13610–13618. [34] Y.H. Wang, H.M. Liu, B.Q. Xu, J. Mol. Catal. A: Chem. 299 (2009) 44–52. [35] E. Ruckenstein, Y.H. Hu, J. Catal. 161 (1996) 55–61. [36] K. Sutthiumporn, S. Kawi, Int. J. Hydrogen Energy 36 (2011) 14435–14446. [37] Y. Wang, W. Wang, X. Hong, B. Li, Catal. Commun. 10 (2009) 940–944. [38] D.A. Hansgen, D.G. Vlachos, J.G. Chen, Nature Chem. 2 (2010) 484–489. [39] I.C. Bassignana, K. Wagemann, J. Küppers, G. Ertl, Surf. Sci. 175 (1986) 22-44. [40] D. Chrysostomou, J. Flowers, F. Zaera, Surf. Sci. 439 (1999) 34-48. [41] X. Duan, G. Qian, C. Fan, Y. Zhu, X. Zhou, D. Chen, W. Yuan, Surf. Sci. 606 (2012) 549–553. [42] C.W. Seabury, T.N. Rhodin, R.J. Purtell, R.P. Merrill, Surf. Sci. 93 (1980) 117–126. [43] X. Duan, G. Qian, Y. Liu, J. Ji, X. Zhou, D. Chen, W. Yuan, Fuel Process. Technol. 108 (2013) 112–117. [44] M. Mavrikakis, P. Stoltze, J.K. Nørskov, Catal. Lett. 64 (2000) 101–106. [45] W. Raróg-Pilecka, D. Szmigiel, Z. Kowalczyk, S. Jodzis, J. Zielinski, J. Catal. 218 (2003) 465–469. 16
[46] M. Shishkin, T. Ziegler, J. Phys. Chem. C 114 (2010) 21411–21416. [47] J. Li, E. Croiset, L. Ricardez-Sandoval, J. Phys. Chem. C 117 (2013) 16907–16920. [48] A. Takahashi, T. Fujitani, J. Chem. Eng. Jpn. 49 (2016) 22–28.
17
Table 1 H2 yields for NH3 decomposition over unsupported Ni and over Ni catalysts supported on various metal oxides. Catalysts
H2 yield (%) 450 °C
500 °C
550 °C
600 °C
Ni
0.3
1.0
4.7
12.4
Ni/Y2O3
23.4
60.8
98.3
99.9
Ni/CeO2
28.6
60.0
92.9
99.0
Ni/MgO
20.9
51.2
87.2
98.2
Ni/La2O3
21.2
48.6
84.3
98.0
Ni/Al2O3
16.9
42.3
84.0
98.0
Ni/ZrO2
14.2
31.4
71.8
95.0
18
Table 2 Physicochemical properties of NiO and Ni catalysts supported on various metal oxides.
NiO
400 °C reduction 0.66
600 °C reduction 0.16
Specific surface area (m2/g) 18.7
Ni/Y2O3
11.3
6.4
Ni/CeO2
9.3
Ni/MgO
Ni dispersion (%)
Pore size (nm)
Pore volume (cm3/g)
21.2
0.112
43.0
17.8
0.192
4.0
88.2
17.1
0.413
1.9
4.9
177.8
5.1
0.311
Ni/La2O3
8.9
3.5
65.6
20.6
0.377
Ni/Al2O3
0.03
4.7
289.5
4.5
0.467
Ni/ZrO2
4.3
1.7
85.7
4.6
0.138
Catalysts
19
Table 3 Turnover frequencies (TOFs) for H2 formation at 500 °C over unsupported Ni and over Ni catalysts supported on various metal oxides.
Ni
TOF (molecules/site-Ni/s) 0.27
Ni/Y2O3
0.42
Ni/CeO2
0.66
Ni/MgO
0.46
Ni/La2O3
0.61
Ni/Al2O3
0.39
Ni/ZrO2
0.81
Catalysts
20
Figure captions
Fig. 1 Temperature-programmed reduction profiles of unsupported NiO and NiO supported on various metal oxides.
Fig. 2 Rates of H2 formation via NH3 decomposition over unsupported Ni and supported Ni catalysts at 500 °C as a function of surface area of Ni.
Fig. 3 Temperature-programmed desorption spectra of N2 after exposure of NH3 to unsupported Ni and to Ni catalysts supported on various metal oxides at 280 °C.
Fig. 4 Turnover frequencies of H2 formation at 500 °C for unsupported Ni and supported Ni catalysts as a function of N2 desorption temperature.
21
Fig. 1
22
Fig. 2
23
Fig. 3
24
Fig. 4
25
S0926-860X(16)30273-3 http://dx.doi.org/doi:10.1016/j.apcata.2016.05.020 APCATA 15884
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
29-3-2016 19-5-2016 21-5-2016
Please cite this article as: Isao Nakamura, Tadahiro Fujitani, Role of metal oxide supports in NH3 decomposition over Ni catalysts, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.05.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights
・The essential role of metal oxide supports for NH3 decomposition over Ni catalysts is clarified. ・The volcano-curve relationship between turnover frequency and Ni–N binding energy is obtained for Ni catalysts supported on various metal oxides. ・The rate-determining step for NH3 decomposition reaction on Ni is changed by the Ni–N binding energy.
1
TOF (molecules/site-Ni/s)
*Graphical Abstract
Ni/ZrO2 Ni/CeO2 Ni/La2O3
Ni/Y2O3
Ni/MgO Ni/Al2O3
Unsupported Ni
N2 desorption temperature (oC)
Role of metal oxide supports in NH3 decomposition over Ni catalysts Isao Nakamura, Tadahiro Fujitani* Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Abstract The decomposition of NH3 over Ni catalysts supported on various metal oxides (Y2O3, CeO2, MgO, La2O3, Al2O3, and ZrO2) was investigated, and the Ni/Y2O3 catalyst was found to show the highest activity. The rate of H2 formation per exposed Ni atom (turnover frequency) for the supported Ni catalysts depended strongly on the type of metal oxide support, indicating that the active sites for the Ni catalysts were not identical in nature. Investigation of the associative desorption of atomic nitrogen formed by NH3 decomposition over the Ni catalysts revealed that the N2 desorption temperature depended on the type of support. This result indicates that the Ni–N binding energy varied among the metal oxide supports, which would be attributable to the change of the Ni electronic state by the supports. The volcano-curve relationship was obtained between turnover frequency and N2 desorption temperature, indicating that the NH3 decomposition activity was dominated by the Ni–N binding energy. We considered that the metal oxide supports changed the rate-determining step for NH3 decomposition on Ni.
Keywords: NH3 decomposition, Ni catalyst, Metal oxide support, Active site
*Corresponding author, E-mail: [email protected] 2
1. Introduction If the use of hydrogen as a fuel is to become widespread, infrastructure for transport, storage, and distribution of large amounts of hydrogen will be indispensable. As a potential method for hydrogen transport, the use of ammonia has received attention because its hydrogen mass density (about 18 wt%) is higher than that of other hydrogen storage materials and it is readily liquified (its vapor pressure is 0.8 MPa at 20 °C) [1,2]. Ammonia must be decomposed into H2 and N2, and the use of a catalyst is the most effective method for this purpose [3]. The NH3 decomposition performances of catalysts based on metals such as Fe, Ni, Pt, Ru, Co, Ir, Pd, and Rh have been extensively investigated [4–12]. Of the metals that have been studied, Ru is the most active, but this noble metal is very expensive. Therefore, for practical applications, the amount of Ru in the catalyst must be reduced, or Ru must be replaced by a non-noble metal. Among the non-noble metal alternatives, Ni, which is more active than Fe [13], has attracted attention [4,5], and research aimed at improving the performance of Ni catalysts has focused on the selection of a support [14–20], the addition of a promoter element [21–25], and the formation of alloys or bimetallic compounds [26–28]. In these previous reports, the nature of the active sites and the role of the support are discussed. Metallic Ni is generally considered to be the active site for NH3 decomposition, and the supports and additives are thought to improve the Ni dispersion [14,15,24,25]. Muroyama et al. [15] investigated the influence of the support on the NH3 decomposition activity of Ni catalysts supported on various metal oxides and found that Ni/Al2O3 catalysts show the highest NH3 conversion, owing to the high surface area of the Al2O3 support; and these investigators concluded that a high dispersion of Ni particles over the support is crucial for superior catalytic activity. Liu and co-workers 3
[14] reported that small Ni particle size, high dispersion, and high resistance to sintering improve the performance and stability of Ni catalysts supported on SBA-15, which has well-ordered hexagonal mesoporous silica structures. Furthermore, these investigators showed that the NH3 decomposition activity of the Ni/SBA-15 catalyst is enhanced by the addition of a Ce or La oxide; the enhancement is due to increases in the Ni surface area, the Ni dispersion, and the reducibility of the catalyst [24]. Introduction of an appropriate amount of Ce to Ni encapsulated in SiO2 results in high-surface-area catalysts with small Ni particles, which show enhanced catalytic activity for NH3 decomposition compared to the activity of Ni catalysts with no additive [25]. Metal oxide supports and additives have been reported to have other roles, in addition to increasing the Ni surface area. For example, Deng et al. [16], who investigated NH3 decomposition over a Ni catalyst supported on a high-surface-area Ce0.8Zr0.2O2 solid solution, speculated that hydrogen spillover from the Ni surface to the support via a Ni–O–Ce species plays an important role promoting the decomposition reaction. The presence of a La promoter in a Ni/Al2O3 catalyst has been shown to reduce the stability of reaction intermediates in the decomposition of NH3 [22]. Furthermore, the addition of CeO2 to a Ni/Al2O3 catalyst reportedly enlarges catalyst pores, moderates the interaction between Ni and the alumina, and improves the recombinative desorption of nitrogen adatoms from the Ni0 surface [23]. Thus, there are still various theories about the roles of supports and additives, despite the fact that their favorable effects on Ni dispersion have long been generally known. In this study, we carried out NH3 decomposition reactions over Ni catalysts supported on various metal oxides, with the goal of shedding light on the role of the supports and on the nature of the active sites.
4
2. Experimental Ni/MxOy (M = Y, Ce, Mg, La, Al, or Zr) catalysts were prepared by a co-precipitation method from solutions of the corresponding metal nitrates (Ni(NO3)2·6H2O + MxOy(NO3)z·nH2O) with potassium carbonate (K2CO3). The precipitates were filtered, washed twice with distilled water, and dried at 110 °C for 17 h. After drying, the samples were calcined at 500 °C for 2 h in air. The Ni loading for all catalysts was fixed at 30 wt%. The N2 adsorption isotherms of the catalysts at liquid N2 temperature (–196 °C) were
measured with an automated physisorption instrument (ASAP
2020,
Micromeritics). Prior to the measurement, each catalyst (0.3 g) was degassed in a vacuum at 350 °C for 10 h to remove physically adsorbed components. The specific surface area was determined from the linear portion of the Brunauer–Emmett–Teller plot. The pore size and pore volume were calculated from the desorption branch of the N2 adsorption isotherm by means of the Barrett–Joyner–Halenda formula. The dispersion of metallic Ni for each catalyst (0.03–0.2 g) was evaluated by the H2 pulse method (BELCAT-B, BEL Japan) at 50 °C after reduction with 10.1% H2/Ar gas at 400 or 600 °C, where Ni dispersion is defined by a ratio of surface-exposed Ni metal atoms to the supported Ni atoms. The pulse adsorption of H2 was detected by a thermal conductivity detector. The X-ray diffraction (XRD) patterns of the supported Ni catalysts after reduction with 10.1% H2/Ar gas at 600 °C were obtained with an X-ray diffractometer (RINT2000, Rigaku) operated at 40 kV and 40 mA using Cu Kα monochromatized radiation (λ = 0.154178 nm). Temperature-programmed reduction (TPR) and desorption (TPD) experiments were carried out in a BELCAT-B apparatus. TPR measurement for each catalyst (0.03 g) was performed under a flow of 10.1% H2/Ar (50 ml min-1) from 30 to 900 °C at a 5
constant heating rate of 10 °C min-1. The consumption of H2 was monitored with a thermal conductivity detector. Prior to the TPD measurements, each catalyst (0.2 g) was pre-reduced in situ with 10.1% H2/Ar at 600 °C. The reduced catalyst was exposed to a flow of 10% NH3/He (40 ml min-1) at 280 °C for 30 min, and then the flow gas was switched to He (40 ml min-1). After holding in He at 280 °C for 20 min, the temperature of the catalyst bed was raised under a flow of He (40 ml min-1) to 500 °C at a rate of 10 °C min-1. The desorption products were analyzed by means of an online mass spectrometer. The N2 desorption was observed without desorption of NH3 and H2, indicating that the dehydrogenation of NH3 completely proceeded, and only the atomic nitrogen was remained on the catalyst surface. Ammonia decomposition reactions were carried out in a continuous-flow fixed-bed quartz reactor under a flow of pure NH3 (40 ml min-1) at atmospheric pressure and a temperature range of 450–600 °C. The reaction temperature was monitored by means of a thermocouple placed in the catalyst bed. Prior to each reaction, the catalyst (0.2 g, 22–42 mesh) was pre-reduced in situ with 20% H2/N2 gas at 600 °C. After reduction, the bed was allowed to cool to 450 °C under a H2/N2 flow, and then the flow gas was switched to NH3. The catalytic activities were measured at 450, 500, 550, and 600 °C after each temperature had been maintained for at least 30 min. The reaction products (N2 and H2) were analyzed with an online gas chromatograph equipped with a thermal conductivity detector. The catalytic activity was evaluated on the basis of the H2 production.
3. Results and discussion We carried out NH3 decomposition reactions over Ni catalysts supported on various metal oxides and compared the H2 yields obtained at reaction temperatures of 6
450–600 °C (Table 1). At 450 °C, the Ni/CeO2 catalyst exhibited the highest activity. The order of activities at 500 and 550 °C was as follows: Ni/Y2O3 > Ni/CeO2 > Ni/MgO > Ni/La2O3 > Ni/Al2O3 > Ni/ZrO2. No significant differences in the H2 yields were observed at 600 °C, because the activities of all the supported Ni catalysts were close to the equilibrium value for NH3 decomposition. As previously mentioned, the dispersion of metallic Ni is known to be an important determinant of the NH3 decomposition activity of supported Ni catalysts [14,15,24,25]. Furthermore, Zhang et al. [22] reported that experimental data obtained for NH3 decomposition over Ni/Al2O3 and Ni/La-Al2O3 catalysts show an excellent fit to the Temkin–Pyzhev equation, which assumes that Ni metal is the active site. Thus, the dependence of NH3 decomposition activity on the type of metal oxide support may be due to differences in the degree of reduction of supported NiO. To evaluate this possibility, we began by using TPR to investigate the influence of the support on the reduction behavior of NiO (Fig. 1). For unsupported NiO, the reduction peak was observed at 369 °C, which is in agreement with the previously reported reduction temperature for NiO [29]. For the NiO/Y2O3 catalyst, reduction peaks were observed at 426 and 503 °C; both of these temperatures are higher than that of NiO alone. These peaks may be attributable to the reduction of two types of NiO, one type that interacts weakly with Y2O3 and another that interacts strongly. The reduction of strongly interacting NiO species may be related to the formation of NiO–Y2O3 solid solution [30]. In contrast, the NiO/CeO2 catalyst showed only one reduction peak, at 354 °C, a temperature that was slightly lower than that observed for NiO alone. This result is consistent with previous reports indicating that the reduction of NiO species is promoted by its interaction with the CeO2 support [31,32], that is, by the formation of an –O–Ni–O–Ce 7
superstructure that is reducible at low temperature [33]. For NiO/MgO, there was a small peak at 357 °C, but most of the reduction occurred at 603 °C. The peak at this temperature was assigned to the reduction of Ni2+ ions in the surface and subsurface layers of the MgO [34]. NiO/La2O3 showed two reduction peaks, at 395 and 598 °C, which were ascribed to the reduction of NiO interacting with La2O3 and the reduction of LaNiO3 [35,36], respectively. In the case of NiO/Al2O3, the only peak appeared at 595 °C, which was attributed to the reduction of the precursor of the NiAl2O4 spinel structure [21]. For NiO/ZrO2, peaks were observed at 398 and 508 °C. The lower and higher peaks may be due to the reduction of NiO interacting with the ZrO2 support and diffused into ZrO2 lattice, respectively [37]. In summary, NiO was reduced to metallic Ni below 600 °C on all of the metal oxide supports; that is, all of the Ni in the catalysts was in the metallic state as a result of treatment of the catalysts with H2 at 600 °C before the activity measurements. These results indicate that the support-dependence of the NH3 decomposition activity was not due to the extent of reduction of the supported NiO. We also evaluated the Ni dispersion and the physical properties (specific surface area, pore size, and pore volume) of the catalysts (Table 2). After reduction with H2 at 400 °C, the Ni dispersions for the Ni/MgO and Ni/Al2O3 catalysts were clearly lower than those of the other catalysts, owing to incomplete reduction of NiO. However, the dispersions for Ni/MgO and Ni/Al2O3 after reduction at 600 °C were higher than at the lower temperature, a result that we attributed to complete reduction of NiO at the higher temperature. These behaviors are consistent with the TPR results (Fig. 1). In contrast, the Ni dispersions for the Ni/Y2O3, Ni/CeO2, Ni/La2O3, and Ni/ZrO2 catalysts after H2 reduction at 600 °C were 40–60% lower than those after 400 °C reduction, owing to sintering of the Ni particles. The Ni dispersion after 600 °C 8
reduction decreased in the order Ni/Y2O3 > Ni/MgO > Ni/Al2O3 > Ni/CeO2 > Ni/La2O3 > Ni/ZrO2. This order was not the same as the order based on specific surface area, pore size, or pore volume, indicating that the Ni dispersion was not affected by the physical properties of the metal oxide support. To determine whether Ni dispersion and NH3 decomposition activity were correlated, we plotted H2 formation rate at 500 °C (as determined from the H2 yields listed in Table 1) as a function of Ni surface area after 600 °C reduction (Fig. 2). The data did not fit on a straight line passing through the origin, suggesting that the NH3 decomposition activity was not determined solely by Ni surface area. To investigate this possibility, we estimated the H2 formation rate per exposed Ni atom (turnover frequency, TOF) at 500 °C (Table 3). The TOFs for all the Ni catalysts should be equal if the active sites for NH3 decomposition of the catalysts are identical in nature and differ only in number. We found that the TOFs for the supported Ni catalysts depended strongly on the type of metal oxide support, indicating that active sites with electronic states other than Ni0 or with different morphologies were formed on the supports. Danielle et al. [38] calculated the catalytic activity for NH3 decomposition on various single-metals such as Co, Pt, Pd, Ni, Ru, Rh, Ir, Re, and Mo using microkinetic modeling. These investigators observed a volcano-curve relationship between the calculated conversion of NH3 and the nitrogen-binding energies of the transition metals, and Ru exhibited the highest activity among the single-metal catalysts studied. From a sensitivity analysis for the elementary reaction steps on each metal, the removal of the second hydrogen (NH2 → NH + H) is shown to be the rate-determining step for metals with lower nitrogen-binding energies, whereas the removal of the first and second hydrogens (NH3 → NH2 + H and NH2 → NH + H) and nitrogen desorption (N + N → N2) are kinetically significant steps for metals with higher nitrogen-binding energies. 9
This analysis underscores the fact that the kinetically significant step and dominant coverage may be changing along a volcano curve. In this study, to determine whether NH3 decomposition activity was related to nitrogen-binding energy of the supported Ni, we measured the associative desorption behaviors of atomic nitrogen formed by the NH3 dehydrogenation. It has been generally well known that the NH3 is adsorbed on Ni in N coordination, and its decomposition proceeds consecutively in the mechanism of NH3 → NH2 → NH → N, and finally N2 is produced by associative desorption of atomic nitrogen adsorbed on Ni [39–41]. Thus, the N2 desorption temperature reflects the Ni–N binding energy. We firstly determined the NH3 exposure temperature to produce atomic nitrogen over Ni catalysts. It has been reported that the formation of atomic nitrogen by the NH3 dehydrogenation over Ni single crystals proceeds around 230 °C [39,42]. Then, we exposed the Ni catalysts to NH3 at various temperatures, and found that only the N2 desorption was observed by the exposure of NH3 above 250 °C, clearly indicating that the NH3 dehydrogenation step completely proceeded, and only the atomic nitrogen was remained on the Ni surface. Thus, we adopted 280 °C as the NH3 exposure temperature. Fig. 3 shows the N2 TPD spectra measured after NH3 exposure to Ni catalysts at 280 °C. For unsupported Ni, the N2 desorption peak was observed at 324 °C, whereas the N2 desorption peaks for the supported Ni catalysts were observed in the temperature range between 346 and 420 °C; that is, the N2 desorption temperature depended strongly on the type of support. This result indicates that the Ni–N binding energy varied with the metal oxide support. We also investigated the relationship between TOF and N2 desorption temperature for each catalyst, and the results for the decomposition reaction at 500 °C are shown in Fig. 4. The TOF increased with increasing N2 desorption temperature up to approximately 400 °C, and then decreased. Thus, a volcano-curve relationship between 10
TOF and Ni–N binding energy was obtained for Ni catalysts supported on various metal oxides. It has been reported by Duan et al. [43] that the NH3 decomposition over Ni catalysts supported on mobil crystalline material 41 (MCM-41) exhibits a volcano relationship between TOF and Ni particle size. They also demonstrated by means of first-principles calculations that the rate constants for the associative desorption of atomic nitrogen and NH3 dehydrogenation are the smallest among elementary steps for NH3 decomposition on Ni(211) and Ni(111), respectively. Thus, the existing optimal Ni particle size can be explained that smaller Ni particles have more step sites which are easily blocked by strongly adsorbed nitrogen atoms, and hence present the low catalytic activity, while larger Ni particles have less step sites which are favorable to NH3 dehydrogenation, and also present the low catalytic activity. It was concluded that the NH3 decomposition activity is sensitive to the ratio of Ni step sites to Ni terrace sites, where the optimal ratio is obtained around 10 nm of particle size. In contrast, the Ni particle size for the supported Ni catalysts in this study was estimated to be 15–60 nm, which was much larger than that for the Ni/MCM-41 catalysts. It has been reported that the surface for our particle sizes has a low step density, and is almost occupied by the terrace site [44]. That is, the ratio of step sites to terrace sites is hardly changed between these Ni particles. Furthermore, we investigated the crystal structure of Ni particles for the supported Ni catalysts after H2 reduction by means of XRD. As a result, the relative intensity ratio of the diffraction peaks from each crystal face to the (111) peak for metallic Ni was almost same in each Ni catalyst, clearly indicating that the surface structure of the Ni particles is not changed. On the other hand, we examined a correlation between TOF and Ni particle size for the supported Ni catalysts, but a volcano relationship was not confirmed. Therefore, the variation of the N2 desorption 11
temperature observed for the supported Ni catalysts in this study would not be due to the structural change of Ni. We consider that the difference in the N2 desorption temperature is attributable to the change of the Ni electronic state by the metal oxide supports. It has been reported that the addition of alkali onto the Ru surface destabilizes the adsorption of atomic nitrogen because of the electron transfer from alkali to Ru [45]. That is, the binding energy of atomic nitrogen is changed by the electronic state of metal. On the other hand, it has been demonstrated from density functional theory calculations that the deposition of Ni on stoichiometric ceria results in the electron transfer from Ni to CeO2 [46], and the electrons of Ni cluster are transferred to γ-Al2O3 when Ni cluster was supported on γ-Al2O3(100) [47]. We thus considered that the Ni supported on metal oxide support is in a cationic state, resulting in a strong Ni–N binding energy, which was consistent with the results in Fig. 3. We believe that the extent of the electron transfer depends on the type of support, and the more electron transfer from Ni to support produces more cationic Ni, resulting in a stronger Ni–N binding energy. It is generally known that the rate-determining step for NH3 decomposition is the associative desorption of atomic nitrogen [4,5]. However, we have previously found by the kinetic analysis that the overall rate of NH3 decomposition over a Ni/MgO catalyst is controlled by the NH3 dehydrogenation step [48]. As shown in Fig. 4, the Ni/MgO catalyst corresponds to the region of lower Ni–N binding energy, which is consistent with that the reaction of NH2 → NH + H is the rate-determining step in the region of lower nitrogen-binding energies [38]. We thus considered that the volcano-curve relationship in Fig. 4 indicated the change of the rate-determining step depending on the Ni–N binding energy. That is, the NH3 dehydrogenation is the rate-determining step for the supported Ni catalysts with low Ni–N binding energy, whereas the associative 12
desorption of atomic nitrogen is the rate-determining step in the case of high Ni–N binding energy. We clearly indicated that the metal oxide supports changed the rate-determining step for NH3 decomposition on Ni.
4. Conclusions We investigated the NH3 decomposition activities and physicochemical properties of Ni catalysts supported on various metal oxides. The TOFs for the supported Ni catalysts depended strongly on the type of metal oxide support, indicating that the active sites for the Ni catalysts were not identical in nature. TPD measurements indicated that the N2 desorption temperature also depended on the type of support; that is, the Ni–N binding energy varied with the metal oxide support, which was considered to be due to the change of the Ni electronic state by the supports. Furthermore, we observed a volcano-curve relationship between TOF and N2 desorption temperature, indicating that the NH3 decomposition activity was dominated by the Ni–N binding energy. This would be attributable to that the rate-determining step for NH3 decomposition reaction was different with the Ni–N binding energy, which was changed by the metal oxide supports.
Acknowledgement This work was supported by the Cross-ministerial Strategic Innovation Promotion Program (SIP) of the Cabinet Office, Government of Japan.
13
References [1] A. Klerke, C.H. Christensen, J.K. Nørskov, T. Vegge, J. Mater. Chem. 18 (2008) 2304–2310. [2] R. Lan, J.T.S. Irvine, S.W. Tao, Int. J. Hydrogen Energy 37 (2012) 1482–1494. [3] A. Ozaki, K. Aika, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Springer–Verlag, New York, 1981, pp. 87–158. [4] S.F. Yin, B.Q. Xu, X.P. Zhou, C.T. Au, Appl. Catal. A: Gen. 277 (2004) 1–9. [5] F. Schüth, R. Palkovits, R. Schlögl, D.S. Su, Energy Environ. Sci. 5 (2012) 6278–6289. [6] D.C. Huang, C.H. Jiang, F.J. Liu, Y.C. Cheng, Y.C. Chen, K.L. Hsueh, Int. J. Hydrogen Energy 38 (2013) 3233–3240. [7] A.D. Carlo, L. Vecchione, Z.D. Prete, Int. J. Hydrogen Energy 39 (2014) 808–814. [8] D. Varisli, E.E. Elverisli, Int. J. Hydrogen Energy 39 (2014) 10399–10408. [9] K. Nagaoka, T. Eboshi, N. Abe, S. Miyahara, K. Honda, K. Sato, Int. J. Hydrogen Energy 39 (2014) 20731–20735. [10] G. Li, H. Nagasawa, M. Kanezashi, T. Yoshioka, T. Tsuru, J. Mater. Chem. A 2 (2014) 9185–9192. [11] Y.Q. Gu, Z. Jin, H. Zhang, R.J. Xu, M.J. Zheng, Y.M. Guo, Q.S. Song, C.J. Jia, J. Mater. Chem. A 3 (2015) 17172–17180. [12] A.K. Hill, L.Torrente-Murciano, Appl. Catal. B: Environ. 172–173 (2015) 129–135. [13] S.F. Yin, Q.H. Zhang, B.Q. Xu, W.X. Zhu, C.F. Ng, C.T. Au, J. Catal. 224 (2004) 384–396. [14] H. Liu, H. Wang, J. Shen, Y. Sun, Z. Liu, Appl. Catal. A: Gen. 337 (2008) 138–147. 14
[15] H. Muroyama, C. Saburi, T. Matsui, K. Eguchi, Appl. Catal. A: Gen. 443–444 (2012) 119–124. [16] Q.F. Deng, H. Zhang, X.X. Hou, T.Z. Ren, Z.Y. Yuan, Int. J. Hydrogen Energy 37 (2012) 15901–15907. [17] R. Atsumi, R. Noda, H. Takagi, L. Vecchione, A.D. Carlo, Z.D. Prete, K. Kuramoto, Int. J. Hydrogen Energy 39 (2014) 13954–13961. [18] H. Zhang, Y.A. Alhamed, Y. Kojima, A.A. Al-Zahrani, H. Miyaoka, L.A. Petrov, Int. J. Hydrogen Energy 39 (2014) 277–287. [19] J. Ji, T.H. Pham, X. Duan, G. Qian, P. Li, X. Zhou, D. Chen, Int. J. Hydrogen Energy 39 (2014) 20722–20730. [20] J. Yang, A.F.S. Molouk, T. Okanishi, H. Muroyama, T. Matsui, K. Eguchi, ACS Appl. Mater. Interfaces 7 (2015) 7406–7412. [21] J. Zhang, H. Xu, X. Jin, Q. Ge, W. Li, Appl. Catal. A: Gen. 290 (2005) 87–96. [22] J. Zhang, H. Xu, W. Li, Appl. Catal. A: Gen. 296 (2005) 257–267. [23] W. Zheng, J. Zhang, Q. Ge, H. Xu, W. Li, Appl. Catal. B: Environ. 80 (2008) 98–105. [24] H. Liu, H. Wang, J. Shen, Y. Sun, Z. Liu, Catal. Today 131 (2008) 444–449. [25] L.F. Zhang, M. Li, T.Z. Ren, X. Liu, Z.Y. Yuan, Int. J. Hydrogen Energy 40 (2015) 2648–2656. [26] S.B. Simonsen, D. Chakraborty, I. Chorkendorff, S. Dahl, Appl. Catal. A: Gen. 447–448 (2012) 22–31. [27] A.S. Chellappa, C.M. Fischer, W.J. Thomson, Appl. Catal. A: Gen. 227 (2002) 231–240. [28] H. Silva, M.G. Nielsen, E.M. Fiordaliso, C.D. Damsgaard, C. Gundlach, T. Kasama, I.B. Chorkendorff, D. Chakraborty, Appl. Catal. A: Gen. 505 (2015) 548–556. 15
[29] G. Li, L. Hu, J.M. Hill, Appl. Catal. A: Gen. 301 (2006) 16–24. [30] Y.J.O. Asencios, C.B. Rodella, E.M. Assaf, Appl. Catal. B: Environ. 132–133 (2013) 1–12. [31] Sk. Mahammadunnisa, P.M.K. Reddy, N. Lingaiah, Ch. Subrahmanyam, Catal. Sci. Technol. 3 (2013) 730–736. [32] W. Zou, C. Ge, M. Lu, S. Wu, Y. Wang, J. Sun, Y. Pu, C. Tang, F. Gao, L. Dong, RSC Adv. 5 (2015) 98335–98343. [33] P. Pal, R.K. Singha, A. Saha, R. Bal, A.B. Panda, J. Phys. Chem. C 119 (2015) 13610–13618. [34] Y.H. Wang, H.M. Liu, B.Q. Xu, J. Mol. Catal. A: Chem. 299 (2009) 44–52. [35] E. Ruckenstein, Y.H. Hu, J. Catal. 161 (1996) 55–61. [36] K. Sutthiumporn, S. Kawi, Int. J. Hydrogen Energy 36 (2011) 14435–14446. [37] Y. Wang, W. Wang, X. Hong, B. Li, Catal. Commun. 10 (2009) 940–944. [38] D.A. Hansgen, D.G. Vlachos, J.G. Chen, Nature Chem. 2 (2010) 484–489. [39] I.C. Bassignana, K. Wagemann, J. Küppers, G. Ertl, Surf. Sci. 175 (1986) 22-44. [40] D. Chrysostomou, J. Flowers, F. Zaera, Surf. Sci. 439 (1999) 34-48. [41] X. Duan, G. Qian, C. Fan, Y. Zhu, X. Zhou, D. Chen, W. Yuan, Surf. Sci. 606 (2012) 549–553. [42] C.W. Seabury, T.N. Rhodin, R.J. Purtell, R.P. Merrill, Surf. Sci. 93 (1980) 117–126. [43] X. Duan, G. Qian, Y. Liu, J. Ji, X. Zhou, D. Chen, W. Yuan, Fuel Process. Technol. 108 (2013) 112–117. [44] M. Mavrikakis, P. Stoltze, J.K. Nørskov, Catal. Lett. 64 (2000) 101–106. [45] W. Raróg-Pilecka, D. Szmigiel, Z. Kowalczyk, S. Jodzis, J. Zielinski, J. Catal. 218 (2003) 465–469. 16
[46] M. Shishkin, T. Ziegler, J. Phys. Chem. C 114 (2010) 21411–21416. [47] J. Li, E. Croiset, L. Ricardez-Sandoval, J. Phys. Chem. C 117 (2013) 16907–16920. [48] A. Takahashi, T. Fujitani, J. Chem. Eng. Jpn. 49 (2016) 22–28.
17
Table 1 H2 yields for NH3 decomposition over unsupported Ni and over Ni catalysts supported on various metal oxides. Catalysts
H2 yield (%) 450 °C
500 °C
550 °C
600 °C
Ni
0.3
1.0
4.7
12.4
Ni/Y2O3
23.4
60.8
98.3
99.9
Ni/CeO2
28.6
60.0
92.9
99.0
Ni/MgO
20.9
51.2
87.2
98.2
Ni/La2O3
21.2
48.6
84.3
98.0
Ni/Al2O3
16.9
42.3
84.0
98.0
Ni/ZrO2
14.2
31.4
71.8
95.0
18
Table 2 Physicochemical properties of NiO and Ni catalysts supported on various metal oxides.
NiO
400 °C reduction 0.66
600 °C reduction 0.16
Specific surface area (m2/g) 18.7
Ni/Y2O3
11.3
6.4
Ni/CeO2
9.3
Ni/MgO
Ni dispersion (%)
Pore size (nm)
Pore volume (cm3/g)
21.2
0.112
43.0
17.8
0.192
4.0
88.2
17.1
0.413
1.9
4.9
177.8
5.1
0.311
Ni/La2O3
8.9
3.5
65.6
20.6
0.377
Ni/Al2O3
0.03
4.7
289.5
4.5
0.467
Ni/ZrO2
4.3
1.7
85.7
4.6
0.138
Catalysts
19
Table 3 Turnover frequencies (TOFs) for H2 formation at 500 °C over unsupported Ni and over Ni catalysts supported on various metal oxides.
Ni
TOF (molecules/site-Ni/s) 0.27
Ni/Y2O3
0.42
Ni/CeO2
0.66
Ni/MgO
0.46
Ni/La2O3
0.61
Ni/Al2O3
0.39
Ni/ZrO2
0.81
Catalysts
20
Figure captions
Fig. 1 Temperature-programmed reduction profiles of unsupported NiO and NiO supported on various metal oxides.
Fig. 2 Rates of H2 formation via NH3 decomposition over unsupported Ni and supported Ni catalysts at 500 °C as a function of surface area of Ni.
Fig. 3 Temperature-programmed desorption spectra of N2 after exposure of NH3 to unsupported Ni and to Ni catalysts supported on various metal oxides at 280 °C.
Fig. 4 Turnover frequencies of H2 formation at 500 °C for unsupported Ni and supported Ni catalysts as a function of N2 desorption temperature.
21
Fig. 1
22
Fig. 2
23
Fig. 3
24
Fig. 4
25