Carbides and Nitridesâ ed. S. T. Oyama, Blackie A&P 1. (1996). 2. C. Angelkort, H. Lewalter, P. Warbichler, F. Hofer, W. Bock and B. O. Kolbesen, Spectro. Acta.
In-Situ XAFS Characterization for Nitriding Process of Silica Supported Nb Catalysts Under N2-H2 Gas Nobuyuki Ichikuni,*1 Hiroari Matsumoto,1 Hidenori Haneishi,1 Kyoko K. Bando,2 Shogo Shimazu1 1
Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba University, Chiba 263-8522, JAPAN 2 National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, JAPAN
Abstract. Fe-Nb/SiO2 catalyst was prepared from NbCl5 or peroxoniobic acid as Nb precursors. These precursor catalysts were nitrided by the TPR method under N2-H2 passage (without using NH3). Nb K-edge in-situ XAFS measurements were carried out during the nitriding process and revealed that the Nb species was more nitrided in the FeNb/SiO2 catalyst prepared from peroxoniobic acid than in that prepared from NbCl5 as Nb precursor. Keywords: in-situ XAFS, Nb nitride, Catalyst PACS: 61.10.Ht
INTRODUCTION Early transition metal nitride (ETMN) was attractive as a new catalyst material due to the resemblance in physical properties to those of the group 8-10 metals [1]. In usual cases, ETMN was prepared from corresponding metal oxide by temperature programmed reaction (TPR) method under NH3 gas. We have tried to convert ETM oxide into ETMN without using NH3 gas by the addition of Fe species on ETM oxide. The reduction and the conversion from the oxide to the nitride may proceed around the Fe additive, and hence, the structural change could be easily investigated. In this study, the formation process of small Nb nitride cluster was investigated. It is reported that the nitriding of bulk Nb2O5 into NbN requires high temperatures as 1470 K [2]. The high temperature caused a sintering process and diminished a surface area. It is reported that the decomposition temperature of peroxoniobic acid (PNA) is lower than that of Nb2O5 [3]. Thus, we used PNA as Nb precursor instead of using NbCl5 to prepare Nb/SiO2 catalysts.
EXPERIMENTAL Catalyst Preparation 3 wt% Nb/SiO2 was prepared from SiO2 (Aerosil, #200) and NbCl5/methanol solution or PNA aqueous
solution by impregnation method. Catalysts are denoted as Nb/SiO2(NPC) and Nb/SiO2(PNA), respectively. Fe was added by using Fe(NO3)3·9H2O aqueous solution. Nitriding of Nb species was performed by TPR method. The precursor oxides were treated in N2-H2 mixed gas stream to produce NbN samples by TPR process; the samples were heated at a linear rate of 10 K·min-1 or 5 K·min-1 to the final temperature, and kept it for 1 to 3 h.
XAFS Measurements The XAFS measurements were performed at BL10B of IMSS-PF, Tsukuba, Japan (Proposal No. 2002G116, 2005G214). Nb K-edge XAFS spectra were taken in a transmission mode with Si(311) channel cut monochromator by using two ionchambers filled with (50% Ar+50% N2) and (100% Ar) for I0 and I, respectively. The monochromator was stepwise scanned for 1 sec dwelling per each point. Catalyst was pressed into self-supporting disk and placed in the in-situ XAFS cell designed for high temperature treatment same as for the carburization [4]. The cell has two acrylic windows (3 mm thickness) at the both end of the X-ray path. The absorption of Xrays by the windows seemed to be sufficiently small enough at Nb K-edge [4,5]. Nitriding by TPR process was performed from room temperature to the final temperature under a flow of N2 (50 cm3·min-1) and H2 (150 cm3·min-1) at the heating rate of 5 K·min-1.
The collected data were analyzed by using program REX2000 (Rigaku Co.). Curve-fitting (CF) analysis was conducted on k3χ(k) in k-space, where k indicated the wave number of a scattered photoelectron and χ represented normalized EXAFS oscillations. Model parameters for curve-fitting analysis (back scattering amplitude and phase shift) were extracted from an EXAFS oscillation observed for bulk NbN (Nb-N: n =6, r =0.220 nm; Nb-Nb: n =12, r =0.311 nm).
second stage. However, the band is not clearly observed in Fe-Nb/SiO2(NPC). On the other hand, in the case of FeNb/SiO2(PNA), NbN band (at around 19050 eV) gradually grew at the end of TPR stage, and the peak is more distinctly observed at the end of temperature maintaining stage, as shown in Fig. 3.
RESULTS AND DISCUSSION abs. / arb. units
Figure 1 shows the Nb K-edge XANES spectra for reference Nb materials. The peak intensity at around 19050 eV in Nb nitride is clearly observed as compared to that in Nb oxide. Thus, the γ-peak can be used as a fingerprint for the formation of NbN.
normalized absorption
α β
Nb foil γ
δ
bulk NbN bulk Nb2O5
19000
19050
19000 19100 photon energy / eV
FIGURE 2. In-situ XANES spectra collected during TPR nitriding process (dotted lines represent retention stage) for Fe-Nb/SiO2(NPC).
abs. / arb. units
18950
18900
19100
photon energy / eV
FIGURE 1. Nb K-edge XANES spectra for Nb compounds.
Nb oxide in Nb/SiO2 (without Fe addition) was not nitrided but only reduced by treating with N2-H2 gas under TPR up to 1273 K. By introducing 5-20 mol% Fe into Nb/SiO2 catalysts and treated with N2-H2 gas at 1273 K led to the formation NbN species on SiO2. In this case, XRD results (not shown) revealed that Fe was reduced from oxide to metal state. Nb K-edge in-situ XANES spectra during TPR nitriding process for Fe-Nb/SiO2(NPC) and FeNb/SiO2(PNA) were shown in Fig. 2 and Fig. 3, respectively (Fe/Nb=0.10 in molar ratio). It takes ca. 10 min to collect the 1 spectrum. Although there is no band growth at around 19050 eV for FeNb/SiO2(NPC) during the first TPR process (solid lines), the gradual band growth is observed for the second stage of TPR (retaining at 1193 K, dotted lines) as observed in Fig. 2. Thus, it can be said that Nb2O5 on the SiO2 support was only reduced to NbO2 in the first stage of TPR and converted into NbN in the
18900
19000 19100 photon energy / eV
FIGURE 3. In-situ XANES spectra collected during TPR nitriding process (dotted lines represent retention stage) for Fe-Nb/SiO2(PNA).
To confirm the conversion from Nb-oxide to NbN, CF analysis was carried out by using empirically extracted Nb-N and Nb-Nb parameters (from bulk NbN). The second stage of TPR kept the high temperature (1193 K) under N2-H2 flow. To remove the thermal effect on Debye-Waller factor, the nitrided samples were cooled under N2 flow, followed by XAFS data collection at room temperature (FT data are shown in Figs. 4 and 5).
|FT|
TABLE 1. Curve-fitting results for Fe-NbN/SiO2. catalyst coordination CN r / nm R / % Nb-N 2.8 0.226 4.2 Fe-NbN/SiO2 (NPC) Nb-Nb 6.0 0.311 3.0 Nb-N 4.8 0.224 4.9 Fe-NbN/SiO2 (PNA) Nb-Nb 8.9 0.311 2.0 CN: coordination number, r: coordination distance, R: R-factor
(c) (b)
CF results for Fe-NbN/SiO2 catalysts of the final form (measured at RT) are shown in Table 1. CNs of Nb-Nb are 6.0 and 8.9 for Fe-NbN/SiO2(NPC) and FeNbN/SiO2(PNA), respectively. The degree of nitridation cannot be directly determined from the CN of Nb-Nb, because the larger NbN particle also shows the large Nb-Nb CN. Thus, the CN ratio of sample’s Nb-N to bulk Nb-N is compared. The ratio is 0.47 and 0.80 for Fe-NbN/SiO2(NPC) and Fe-NbN/SiO2(PNA), respectively. From this result, the Nb in FeNbN/SiO2(PNA) was more nitrided than that in FeNbN/SiO2(NPC). In other words, the use of PNA as a Nb precursor is an effective way of preparing Nb nitride under relatively mild conditions. On the other hand, CN of Nb-N for Fe-NbN/SiO2(NPC) is very small as 2.8, and hence, the degree of nitridation seems not to be enough. It was supposed that there still exists a small amount of Nb-oxide on the NPC catalyst after the N2-H2 treatment at 1193 K.
(a) 0.0
0.2
0.4
0.6
r / nm FIGURE 4. FT of Nb K-edge in-situ EXAFS spectra for Fe-Nb/SiO2(NPC); (a) second TPR stage at 10 min, (b) second TPR stage at 130 min, (c) final product (measured at RT).
ACKNOWLEDGMENTS The present work is supported by the Grant-in-Aid for Scientific Research (KAKENHI) in Priority Area “Molecular Nano Dynamics” from Ministry of Education, Culture, Sports, Science and Technology.
|FT|
REFERENCES
(c) (b) (a) 0.0
0.2
0.4
0.6
r / nm FIGURE 5. FT of Nb K-edge in-situ EXAFS spectra for Fe-Nb/SiO2(PNA); (a) second TPR stage at 10 min, (b) second TPR stage at 90 min, (c) final product (measured at RT).
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