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One-step synthesis of nickel and cobalt phosphide nanomaterials via decomposition of hexamethylenetetramine-containing precursors
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x
Dispersed pure phases of Ni2P and Co2P nanoparticles with high surface areas were prepared from one-step decomposition of hexamethylenetetramine (HMT)-containing precursors under inert atmosphere. The solids before or after decomposition and the evolution of gas during the processes were studied by various characterization techniques. The HMT precursors underwent three decomposition stages: low-, moderate- and high-temperature stages. The formation of phosphides was observed at high-temperature decomposition stage, in which Ni (Co) and P species were reduced by the decomposition products (C, H2 and CH4) of HMT to yield Ni (Co) phosphides, with the release of COx and H2O. Note that in contrast to traditional H2-temperature-programmed reduction (H2TPR) method, the HMT-based method produced CO as major gas product rather than H2O. The better dispersions and higher surface areas of as-prepared phosphide nanoparticles were achieved probably due to the mitigation of hydrothermal sintering.
1. Introduction Transition metal phosphides are important inorganic materials receiving considerable attention for their widespread applications in various fields, including photonics, electronics, magnetism, catalysis and so on.1-4 In particular, metal phosphides (e.g. Ni2P and Co2P) have shown high catalytic activity for several reactions such as hydrogenation and hydrotreating,5-8 N2H4 decomposition 9,10 and hydrogen evolution reaction.2,11,12 Since the discovery of size-dependent performance of catalytic materials with size scales below 100 nm, there has been a considerable increase in developing routes for the synthesis of nanoscale Ni2P and Co 2P. In recent years, various synthetic methods reported for the preparation of nanosized Ni2P and Co 2P in different forms (solid or hollow nanoparticles, nanowires and nanorods)13 involved direct reduction of phosphate in H2 or H2 plasma, 14-16 reaction of metal/metal oxide with PH3, 17 decomposition of singlesource precursors in solution,18 solid-state reaction method using red or white phosphorus, 19,20 decomposition of highboiling point tri-n-octylphosphine (TOP),21-23 chemical vapor deposition,24 and solvothermal or microwave synthesis.25,26 However, these routes usually involved the toxic starting materials (e.g. PH3, TOP and P 4) or complicated steps that cannot afford the requirements of green chemistry. Among these methods mentioned above, the H2temperature-programmed reduction (H2-TPR) method (Scheme 1), 27 has been considered to be an effective and environment-friendly method to synthesize bulk and supported Ni2P and Co 2P catalysts because of the use of nontoxic phosphorus source ((NH4)2HPO4) and green reducing agent (H2). Since the formation of Ni2P and Co2P from their corresponding oxide precursors by means of H2-TPR method is thermodynamically unfavorable, the forward reaction has to be aided by high temperature and high H2 flow speed.15,28 The synthesis conditions were quite rigid and a large amount of water produced during the reaction process can lead to hydrothermal sintering of phosphide nanoparticles.29,30 Therefore, it is worthwhile to find alternative preparation techniques for the synthesis of metal phosphide catalysts under softer conditions. Recently, hexamethylenetetramine (HMT), an organic compound containing C, H and N elements, has been used
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widely as reducing agent to prepare transition metal nitride and carbide catalysts by a simple one-step decomposition method, 31-38 instead of traditional TPR method. Based on the above ideas, recently we reported a new HMT-based one-step decomposition method for synthesizing metal phosphide (MoP) as a short communication.39 In the current study, it was the first time that Ni2P and Co 2P phosphides were prepared by this convenient route. At the same time the formation processes of phosphides were investigated and the formation mechanism was further perfected.
Scheme 1 A simplified equation for the preparation of M2P (M=Ni, Co) by H2-TPR method 70
2. Experimental 2.1 Sample preparation
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HMT precursors were obtained as follows. The HMT (N4(CH2)6) was mixed with Ni(NO3)2·6H2O or Co(NO3)2·6H2O in an aqueous solution at a mole ratio of M:HMT=1:3 (M=Ni, Co). The solutions were kept under stirring at room temperature (RT) overnight and then dried at 110 oC. The resulting samples were designated as Ni-HMT and Co-HMT, respectively. The metal-HMT solid was subsequently impregnated by an aqueous solution of (NH4)2HPO4 with a atomic ratio of M:P=2:1 (M=Ni, Co), then drying at 110 o C for 12 h. The resultant samples were designated as Ni-HMT-P and Co-HMT-P, respectively. The decomposition of HMT precursors were carried out in a quartz reactor under Ar flow (50 ml min -1). The temperature was increased linearly at a rate of 10 o C min-1 and kept at a given value for 1 h, followed by cooling to RT under Ar flow, and then passivated at RT in a stream of 1%O2/Ar for 2 h. A series of samples were obtained from heating at 500, 600, 700 and 800 oC, designated as M-HMT-T and M-HMT-P-T (M=Ni, Co, T=500, 600, 700 and 800). 2.2 Sample characterizations X-ray diffraction (XRD) was performed by an X-ray diffractometer (X'Pert Pro MPD) equipped with a Cu Kα source. X-ray photoelectron spectroscopy (XPS) investigation Journal Name, [year], [vol], 00–00 | 1
Dalton Transactions Accepted Manuscript
Zhiwei Yao*, Guanzhang Wang, Yan Shi, Yu Zhao, Jun Jiang, Yichi Zhang and Haiyan Wang
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was conducted using a Kratos Axis ultra (DLD) equipped with Al Kα X-ray source. Charging effects were corrected by means of adventitious carbon (284.6 eV) referencing. Fourier
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3.1 Microstructural characterization In order to give insights into the formation mechanism of final phosphide products, the mentioned Ni(Co)-HMT and Ni(Co)HMT-P precursors and the products obtained from thermal decomposition of these precursors needed to be characterized. Fig. 1 shows the XRD patterns of these HMT precursors as well as HMT for comparison. It was clear that the Ni(Co)HMT and Ni(Co)-HMT-P precursors showed similar XRD pattern and they did not contain any known compounds of these elements except HMT itself. The unidentified phases were likely to be monometallic Ni(Co) complexes with HMT, Ni(Co)(HMT)2(H2O)n(NO3)2, because HMT easily formed stoichiometric molecular compounds with transition metals.40 However, the lack of P-containing phases on the XRD patterns of Ni(Co)-HMT-P was probably due to these Pcontaining compounds being fully amorphous. It can be seen that the background features to the precursor XRD patterns did indeed seem consistent with an amorphous component. In order to further determine the formation of Ni(Co)(HMT)2(H2O)n(NO3)2 compounds, FTIR analyses are conducted on these HMT precursors. Fig. 2 shows the FTIR spectra of Ni(Co)-HMT and Ni(Co)-HMT-P precursors as well as HMT for comparison. It can be seen that the bands at 1458, 1370 and 1237 cm-1 had been assigned respectively to νas(CH2), νs(CH2) and ν(C-N) in the free HMT, which was similar to those reported by Agwara and co-workers.41 Nevertheless, in the case of the FTIR spectra of Ni(Co)-HMT and Ni(Co)-HMT-P precursors, these bands (1465, 1385 and 1245 cm-1) showed significant differences in positions and intensities from those of HMT itself. These differences can be regarded as evidence that the ligand was coordinated to the metal ion, as suggested before.42 In addition, the bands at about 1665 cm-1 should be attributed to the coordinated bond between the NO3- and the water molecule.41 The combination of XRD and FTIR analyses allowed us to conclude that the Ni(Co)-HMT-P precursors were composed of HMT, Ni(Co)(HMT)2(H2O)n(NO3)2 and amorphous P-containing compounds. Fig. 3 shows the XRD patterns of the samples obtained from decomposition of M-HMT (M=Ni, Co) at 500 and 800 o C . At 500 o C, the Ni-HMT and Co-HMT precursors were transformed into poorly crystalline Ni (2θ=44.3, 51.7 and 76.1o, corresponding respectively to (111), (200) and (220) 2 | Journal Name, [year], [vol], 00–00
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the samples was characterized by scanning electron microscopy (SEM, Hitachi S-4800) equipped with energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM, Philips Tecnal 10). BET surface area of the samples was measured by a surface area analyzer (NOVA4200). The chemical composition of the samples was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Temperature-programmed decomposition mass spectrometry (TPD-MS) was carried out in a stream of pure Ar gas at a flow-rate of 30 ml min -1. The sample (0.1 g) was heated linearly at 10 o C min -1 from 100 to 800 oC, and then was held at this temperature for 30 min. The Hiden HPR20 mass spectrometer was used for detection.
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transform infrared (FTIR) spectra were recorded on a Nicolet MAGNA-IR 560 spectrometer. The morphology of
planes) and Co (2θ=44.4, 51.6 and 76.1 o, indexing respectively to (111), (200) and (220) planes), respectively. When the decomposition temperature was increased to 800 oC, the two phases became well crystallized with intense diffraction peaks, and a new weak peak observed at about 26.0o should be attributed to graphite carbon. Generally, the carbon species existed as an amorphous form in the metal nitride and carbide products obtained from thermal decomposition of metal-HMT complexes.31,32,37 Here, the formation of graphite carbon was probably due to the fact that metallic Ni and Co catalyzed the graphitization of the amorphous carbon. 43
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Fig. 4 shows the XRD patterns of the products of Ni-HMTP decomposition at different temperatures. After decomposition at 500 oC, the resulting sample was completely amorphous. When the temperature was increased up to 600 o C, Ni3P (2θ=41.8, 43.6, 45.2 and 46.6o, corresponding respectively to (231), (112), (240) and (141) planes) was obtained as the majority phase with a small amount of Ni12P 5 (2θ=38.4, 41.8, 47.0 and 49.0o, indexing respectively to (112), (400), (240) and (312) planes). With the increase of temperature from 600 to 800 oC, it can be seen that an obvious
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Fig. 5 shows the XRD patterns of the products of Co-HMT-P decomposition at different temperatures. Just like the decomposition of Ni-HMT-P, an amorphous compound was observed after decomposition of Co-HMT-P at 500 oC .The XRD pattern of the sample with decomposition at 600 oC showed several weak diffraction peaks within the 40-45o 2θ range, which was probably due to poorly crystalline Co2P. When the thermal decomposition temperature was further increased to 700 and 800 o C, typical diffraction peaks at 40.8, 43.4, 44.2 and 52.1o were observed, corresponding to the diffraction peaks of (121), (211), (130) and (002) planes of Co2P. According to the XRD data, the crystal sizes of as-prepared Ni2P and Co2P samples estimated by the Scherrer method were 36 and 29 nm, respectively. In addition, for the sake of comparison, Table 1 lists the phase structures of the solid products obtained during decomposition processes. It was clear from Table 1 that the thermal decomposition of M-HMT (M=Ni, Co) produced metallic Ni or Co, no matter whether it
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was at 500 or 800 o C. In the case of M-HMT-P (M=Ni, Co), the decomposition products were amorphous phase and phasepure Ni2P (Co2P) at 500 and 800 o C, respectively. However, one cannot exclude the possibility that the amorphous phases contained noncrystalline or very poorly crystalline Ni and Co metals. In order to further confirm the formation of Ni2P and Co 2P, and whether metallic Ni and Co species existed in the form of intermediates during decomposition of metal-HMT-P precursors, subsequent XPS analysis was conducted.
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Table 1 Phase structures of the solid products during decomposition processes. Sample
XRD phases
JCPDS cards
Ni-HMT-500 Ni-HMT-800 Co-HMT-500 Co-HMT-800 Ni-HMT-P-500 Ni-HMT-P-600 Ni-HMT-P-700 Ni-HMT-P-800 Co-HMT-P-500 Co-HMT-P-600 Co-HMT-P-700 Co-HMT-P-800
Ni Ni, C Co Co, C Amorphous Ni3P, Ni12P5 Ni12P5, Ni2P Ni2P Amorphous Co2P Co2P Co2P
65-0380 65-0380, 75-1621 15-0806 15-0806,75-1621 — 34-0501, 74-1381 74-1381, 65-1989 65-1989 — 32-0306 32-0306 32-0306
Fig. 6 shows the XPS spectra of Ni 2p, Co 2p and P 2p levels for the samples obtained from decomposition of Ni (Co)-HMT-P precursors at 500 and 800 oC. On the basis of curve fitting, the binding energies of Ni 2p 3/2, Co 2p 3/2 and P 2p 3/2 and the distribution of these corresponding species are listed in Table 2. As shown in Fig. 6 and Table 2, in the case of phosphide phase samples (Ni-HMT-P-800 and Co-HMT-P800), the surface regions of the two samples were dominated by oxidized species and underlying reduced species related to the phosphide phases. The binding energies of oxidized Ni (855.9 eV), Co (781.5 eV) and P (133.3-133.4 eV) were accordant with assignments by others to Ni2+, Co2+ and P 5+ species,44 respectively. The detection of oxidized species should be attributed to surface oxidation of phosphides during passivation.44 The binding energies of the peaks associated with reduced Ni (853.5 eV), Co (778.3 eV) and P (129.5129.7 eV) species were attributed to Ni2P, Co 2P and phosphide,45 respectively. Note that the XPS Ni 2p 3/2 spectrum Journal Name, [year], [vol], 00–00 | 3
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phase transformation of nickel phosphide from a high Ni/P ratio to a low Ni/P ratio (Ni3P→Ni12P 5→Ni2P) existed in the decomposition process. Expectedly, Ni2P (2θ=40.7, 44.6, 47.4 and 54.2o, indexing respectively to (111), (201), (210) and (300) planes) was obtained as a single phase at 800 oC.
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Table 2 XPS results for the samples obtained in this study. Sample
Ni-HMT-P-500 Ni-HMT-P-800
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Binding energy (eV) Ni 2P3/2 P 2p3/2 Niδ+ Ni2+ PδP5+ —
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Table 3 EDX, ICP-AES and BET results for the phosphides obtained by HMT and H2-TPR methods. Sample
HMT-Ni2P HMT-Co2P H2-Ni2P H2-Co2P 75
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EDX M/P atomic Carbon ratioa (wt %) 2.12 32.5 2.08 25.3 2.09 4.9 2.05 7.1
ICP-AES M/P atomic ratioa 2.08 2.04 — —
BET SBET (m2 g-1) 191.7 43.6 3.8 4.6
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of Ni-HMT-P-800 sample showed only one peak at 853.5 eV for Ni2P species which was far away from the value (852.6 eV) for Ni12P 5, 28 indicating that the Ni-HMT-P-800 sample synthesized was indeed pure Ni2P rather than mixed-phase Ni phosphide (Ni2P and Ni12P 5). In the case of amorphous phase samples (Ni-HMT-P-500 and Co-HMT-P-500), there were no peaks at 852.2 eV for Ni0 46 and 777.9 eV for Co0 47 except the peaks observed at 854.4, 780.5 and 133.0-133.3 eV, which were assigned to Ni2+, Co 2+ and P 5+ species, 48,49 respectively. Note that the binding energies of Ni2+ and Co2+ for Ni(Co)HMT-P-500 were far away from those for Ni(Co)-HMT-P800. This was because the former was usually attributed to metal oxides but the latter was consistent with assignments to metal phosphates.50 These facts indicated that the intermediates involved in the transformations (Ni(Co)-HMT-P →Ni(Co) phosphides) were only the oxidized species without reduced metallic species. Finally, the morphologies of Ni2P and Co 2P (hereafter denoted as HMT-Ni2P and HMT-Co 2P, respectively) obtained by HMT method at 800 o C were investigated by SEM and TEM. For the sake of comparison, the Ni2P and Co2P (hereafter denoted as H2-Ni2P and H2-Co2P, respectively) were prepared by traditional H2-TPR method described elsewhere,27 and their morphologies were also characterized by means of SEM. Figs. 7a-d show the SEM images of Ni2P and Co2P obtained in this study and the corresponding EDX analysis results are listed in Table 3. It can be observed from Figs. 7a and b that the morphologies of HMT-Ni2P and HMT-Co2P were very similar and they were composed of dispersed nanoparticles with size range of around 20-100 nm. Noticeably, it was clear that these divided nanoparticles connected with each other through thin sheets. These sheets might be carbon deposites from HMT decomposition because a large amount of carbon was detected by EDX (Table 3). These results were in good agreement with the observation of TEM images (Fig. 8). The particle sizes observed by SEM and TEM were closed to those estimated by XRD. However, the H2-Ni2P and H2-Co2P consisted of large aggregates of irregularly shaped particles (Figs. 7c and d), which was similar to the results reported by others.28 In addition, the EDX results (Table 3) indicated that the M/P (M=Ni, Co) atomic ratios of HMT-Ni2P and HMT-Co2P were nearly identical to those of H2-Ni2P and H2-Co 2P. These values were close to the expected stoichiometric ratio of 2/1. The M/P (M=Ni, Co) atomic ratios of HMT-Ni2P and HMT-Co2P were further estimated by ICP-AES to be Ni2.08P 1.00 and Co 2.04P 1.00, which showed better agreement with the expected stoichiometry (Ni2P and Co 2P). It was also clear from the EDX data that the HMT-Ni2P and HMT-Co 2P samples showed much higher surface carbon content than the H2-Ni2P and H2Co 2P samples, probably due to the fact that the materials obtained by HMT route contained carbon deposites besides carbon contamination from EDX analysis. 31,32,37 Note that the surface areas of Ni and Co phosphides prepared by HMT route were significantly higher than those of corresponding phosphides prepared by traditional H2-TPR method. In particular, the HMT-Ni2P sample showed a high surface area of 191.7 m2 g-1, which was about fifty times higher than that of H2-Ni2P sample (3.8 m2 g-1). The results were in agreement with the the results of SEM observation (Fig. 7), because the decrease in agglomeration was usually responsible for the increase in the surface area of phosphide, as suggested before. 51
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Fig. 8 TEM images of (a) HMT-Ni2P and (b) HMT-Co2P.
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Fig. 7 SEM images of (a) HMT-Ni2P, (b) HMT-Co2P, (c) H2-Ni2P and (d) H2-Co2P. 40
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To understand the formation mechanism of phosphides, the decomposition of metal-HMT-P precursors was followed by mass spectrometry (MS). Fig. 9 shows the TPD-MS profiles of the metal-HMT-P precursors. According to the previous study on the decomposition of metal-HMT complexes,31-33,39 the masses (M) of 2 (H2), 16 (CH4), 17 (NH3), 18 (H2O), 28 (N2 or CO), 30 (NO) and 44 (CO2) were reported to be featured, thus these masses were monitored mainly in current study. It was obvious from Fig. 9 that three temperature regions can be observed as follows. (I) Low-temperature decomposition stage: 100 oC ≤T< 370 oC. The MS showed the simultaneous signals of all the masses 2-44. Although the peaks at 16 and 17 might be attributed to the fragments derived from NH3 and H2O, respectively, it was possible to deduce that these gas species (H2, CH4, NH3, H2O, N2, CO, NO and CO2) were released simultaneously during decomposition of metal-HMT-P precursors in this temperature range, as suggested by related studies.31-33,39 (II) Moderatetemperature decomposition stage: 370 oC ≤T< 550 oC and 370 o C ≤T< 600 o C for Ni-HMT-P and Co-HMT-P precursors, respectively. In this stage, we observed intense signals of H2 (M=2) , which nearly coincided with the weak signals of CH4 (M=16) and NH3 (M=17). However, there was no sharp increase in the concentration of COx and H2O, indicating no deep reduction of oxidized species by these reductant gases. This was in agreement with the XRD and XPS results (Figs. 4-6) that the samples (Ni(Co)-HMT-P-500) were amorphous phases and their surface regions displayed only oxidized species without phosphided species. Therefore, the formation of phosphides should occur in the (III) high-temperature decomposition stage (550 oC ≤T≤ 800 o C for Ni-HMT-P and 600 oC ≤T≤ 800 oC for Co-HMT-P). In the case of the profile of Ni-HMT-P, it was clear that three signal peaks of M=28 at
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In summary, phase-pure Ni2P and Co2P phosphides were successfully prepared by one-step thermal decomposition of HMT-containing precursors. Investigation of the formation processes indicated that at high-temperature stage the inner reductants (i.e. C, H2 and CH4) which were generated from decomposition of HMT reacted with Ni(Co) and P species, leading to the formation of Ni(Co) phosphides. Unlike the traditional H2-TPR method produced a large amount of H2O in the synthesis process, the HMT-based route produced CO as major gas product, and thus hydrothermal sintering might be mitigated. The dispersions and surface areas of as-prepared Ni2P and Co 2P were superior to those of corresponding phoshides prepared by traditional H2-TPR method. The advantages of the resulting Ni2P and Co2P phosphides were worthwhile for further exploration.
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Acknowledgements
M=16*2
We acknowledge the financial support from the National Natural Science Foundation of China (No. 21276253 and No. 21006032).
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Notes and references M=44
College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Liaoning, PR China 113001 E-mail: [email protected]
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HMT complexes, the formation of metal nitrides and carbides was mainly attributed to carbothermal reduction.31,32,38 In our earlier study,39 we proposed that the HMT-based method yielded molybdenum phosphide via reduction of molybdenum and phosphorus species by CH4 from decomposition of HMT, because Hargreaves and co-workers had proved that CH4 was an effective reductant for the synthesis of phosphides. 53 Based on the results of previous studies as well as the current MS, XRD, XPS and EDX analysis results, the formation mechanism of metal phosphides in the stage III was further perfected as follows: (i) HMT decomposed to H2, C, N2 and CH4; (ii) metal and phosphorus species were reduced by C, H2 and CH4 to produce metal phosphides, with release of COx and H2O. The general observations mentioned above were interesting that the reduction of precursors in the HMT-based route can produce COx as the main gaseous products, followed by a low level of H2O being made. On the contrary, the traditional H2 reduction method yielded metal phosphide with release of a large amount of H2O, with the result that the clustering of phosphide particles occurred due to hydrothermal sintering.29,30 It was reasonable to deduce that the HMT method produced metal phosphides mainly via carbothermal reduction, which can avoid strong hydrothermal sintering of metal phosphide nanoparticles. Therefore, phosphide nanoparticles obtained by HMT-based route showed better dispersions and higher surface areas than those synthesized by traditional H2-TPR method.
4. Conclusions
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620, 685 and 800 oC were observed, which should be attributed to a series of reduction steps (Ni3P→Ni12P 5→Ni2P), as suggested by XRD analyses (Fig. 4). In contrast, the profile of Co-HMT-P showed that the signal of M=28 started to increase at 600 o C and only one signal peak was observed at about 765 oC, accompanied by the weak peaks of H2 (M=2), CH4 (M=16) and CO2 (M=44). The result inferred that the stage III for Co-HMT-P involved a direct reduction from oxidized Co and P to Co2P, which was in accordance with the XRD analyses (Fig. 5). In view of the fact that N2 and CO had the base peak at the same mass number 28 and thus the two gas species generated from decomposition of metal-HMT-P precursors cannot be identified by MS characterization, the gas products released in the stage Ⅲ were further analysed by gas chromatograph (GC). The results (not shown here) indicated that the N2 and CO gases were always released simultaneously in these temperature ranges. Additionally note that there were also weak signals of H2O (M=18) observed together with the intense signals of N2 and CO (M=28) in the stage III (see Fig. 9). However, the delay of H2O responses in reaching the maximum was probably due to the slow desorption of them on the surfaces, as suggested before. 52
Temperature / C Fig. 9 TPD-MS profiles of (a) Ni-HMT-P and (b) Co-HMT-P.
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One-step synthesis of nickel and cobalt phosphide nanomaterials via decomposition of hexamethylenetetramine-containing precursors
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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x
Dispersed pure phases of Ni2P and Co2P nanoparticles with high surface areas were prepared from one-step decomposition of hexamethylenetetramine (HMT)-containing precursors under inert atmosphere. The solids before or after decomposition and the evolution of gas during the processes were studied by various characterization techniques. The HMT precursors underwent three decomposition stages: low-, moderate- and high-temperature stages. The formation of phosphides was observed at high-temperature decomposition stage, in which Ni (Co) and P species were reduced by the decomposition products (C, H2 and CH4) of HMT to yield Ni (Co) phosphides, with the release of COx and H2O. Note that in contrast to traditional H2-temperature-programmed reduction (H2TPR) method, the HMT-based method produced CO as major gas product rather than H2O. The better dispersions and higher surface areas of as-prepared phosphide nanoparticles were achieved probably due to the mitigation of hydrothermal sintering.
1. Introduction Transition metal phosphides are important inorganic materials receiving considerable attention for their widespread applications in various fields, including photonics, electronics, magnetism, catalysis and so on.1-4 In particular, metal phosphides (e.g. Ni2P and Co2P) have shown high catalytic activity for several reactions such as hydrogenation and hydrotreating,5-8 N2H4 decomposition 9,10 and hydrogen evolution reaction.2,11,12 Since the discovery of size-dependent performance of catalytic materials with size scales below 100 nm, there has been a considerable increase in developing routes for the synthesis of nanoscale Ni2P and Co 2P. In recent years, various synthetic methods reported for the preparation of nanosized Ni2P and Co 2P in different forms (solid or hollow nanoparticles, nanowires and nanorods)13 involved direct reduction of phosphate in H2 or H2 plasma, 14-16 reaction of metal/metal oxide with PH3, 17 decomposition of singlesource precursors in solution,18 solid-state reaction method using red or white phosphorus, 19,20 decomposition of highboiling point tri-n-octylphosphine (TOP),21-23 chemical vapor deposition,24 and solvothermal or microwave synthesis.25,26 However, these routes usually involved the toxic starting materials (e.g. PH3, TOP and P 4) or complicated steps that cannot afford the requirements of green chemistry. Among these methods mentioned above, the H2temperature-programmed reduction (H2-TPR) method (Scheme 1), 27 has been considered to be an effective and environment-friendly method to synthesize bulk and supported Ni2P and Co 2P catalysts because of the use of nontoxic phosphorus source ((NH4)2HPO4) and green reducing agent (H2). Since the formation of Ni2P and Co2P from their corresponding oxide precursors by means of H2-TPR method is thermodynamically unfavorable, the forward reaction has to be aided by high temperature and high H2 flow speed.15,28 The synthesis conditions were quite rigid and a large amount of water produced during the reaction process can lead to hydrothermal sintering of phosphide nanoparticles.29,30 Therefore, it is worthwhile to find alternative preparation techniques for the synthesis of metal phosphide catalysts under softer conditions. Recently, hexamethylenetetramine (HMT), an organic compound containing C, H and N elements, has been used
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widely as reducing agent to prepare transition metal nitride and carbide catalysts by a simple one-step decomposition method, 31-38 instead of traditional TPR method. Based on the above ideas, recently we reported a new HMT-based one-step decomposition method for synthesizing metal phosphide (MoP) as a short communication.39 In the current study, it was the first time that Ni2P and Co 2P phosphides were prepared by this convenient route. At the same time the formation processes of phosphides were investigated and the formation mechanism was further perfected.
Scheme 1 A simplified equation for the preparation of M2P (M=Ni, Co) by H2-TPR method 70
2. Experimental 2.1 Sample preparation
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HMT precursors were obtained as follows. The HMT (N4(CH2)6) was mixed with Ni(NO3)2·6H2O or Co(NO3)2·6H2O in an aqueous solution at a mole ratio of M:HMT=1:3 (M=Ni, Co). The solutions were kept under stirring at room temperature (RT) overnight and then dried at 110 oC. The resulting samples were designated as Ni-HMT and Co-HMT, respectively. The metal-HMT solid was subsequently impregnated by an aqueous solution of (NH4)2HPO4 with a atomic ratio of M:P=2:1 (M=Ni, Co), then drying at 110 o C for 12 h. The resultant samples were designated as Ni-HMT-P and Co-HMT-P, respectively. The decomposition of HMT precursors were carried out in a quartz reactor under Ar flow (50 ml min -1). The temperature was increased linearly at a rate of 10 o C min-1 and kept at a given value for 1 h, followed by cooling to RT under Ar flow, and then passivated at RT in a stream of 1%O2/Ar for 2 h. A series of samples were obtained from heating at 500, 600, 700 and 800 oC, designated as M-HMT-T and M-HMT-P-T (M=Ni, Co, T=500, 600, 700 and 800). 2.2 Sample characterizations X-ray diffraction (XRD) was performed by an X-ray diffractometer (X'Pert Pro MPD) equipped with a Cu Kα source. X-ray photoelectron spectroscopy (XPS) investigation Journal Name, [year], [vol], 00–00 | 1
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Zhiwei Yao*, Guanzhang Wang, Yan Shi, Yu Zhao, Jun Jiang, Yichi Zhang and Haiyan Wang
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was conducted using a Kratos Axis ultra (DLD) equipped with Al Kα X-ray source. Charging effects were corrected by means of adventitious carbon (284.6 eV) referencing. Fourier
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3.1 Microstructural characterization In order to give insights into the formation mechanism of final phosphide products, the mentioned Ni(Co)-HMT and Ni(Co)HMT-P precursors and the products obtained from thermal decomposition of these precursors needed to be characterized. Fig. 1 shows the XRD patterns of these HMT precursors as well as HMT for comparison. It was clear that the Ni(Co)HMT and Ni(Co)-HMT-P precursors showed similar XRD pattern and they did not contain any known compounds of these elements except HMT itself. The unidentified phases were likely to be monometallic Ni(Co) complexes with HMT, Ni(Co)(HMT)2(H2O)n(NO3)2, because HMT easily formed stoichiometric molecular compounds with transition metals.40 However, the lack of P-containing phases on the XRD patterns of Ni(Co)-HMT-P was probably due to these Pcontaining compounds being fully amorphous. It can be seen that the background features to the precursor XRD patterns did indeed seem consistent with an amorphous component. In order to further determine the formation of Ni(Co)(HMT)2(H2O)n(NO3)2 compounds, FTIR analyses are conducted on these HMT precursors. Fig. 2 shows the FTIR spectra of Ni(Co)-HMT and Ni(Co)-HMT-P precursors as well as HMT for comparison. It can be seen that the bands at 1458, 1370 and 1237 cm-1 had been assigned respectively to νas(CH2), νs(CH2) and ν(C-N) in the free HMT, which was similar to those reported by Agwara and co-workers.41 Nevertheless, in the case of the FTIR spectra of Ni(Co)-HMT and Ni(Co)-HMT-P precursors, these bands (1465, 1385 and 1245 cm-1) showed significant differences in positions and intensities from those of HMT itself. These differences can be regarded as evidence that the ligand was coordinated to the metal ion, as suggested before.42 In addition, the bands at about 1665 cm-1 should be attributed to the coordinated bond between the NO3- and the water molecule.41 The combination of XRD and FTIR analyses allowed us to conclude that the Ni(Co)-HMT-P precursors were composed of HMT, Ni(Co)(HMT)2(H2O)n(NO3)2 and amorphous P-containing compounds. Fig. 3 shows the XRD patterns of the samples obtained from decomposition of M-HMT (M=Ni, Co) at 500 and 800 o C . At 500 o C, the Ni-HMT and Co-HMT precursors were transformed into poorly crystalline Ni (2θ=44.3, 51.7 and 76.1o, corresponding respectively to (111), (200) and (220) 2 | Journal Name, [year], [vol], 00–00
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the samples was characterized by scanning electron microscopy (SEM, Hitachi S-4800) equipped with energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM, Philips Tecnal 10). BET surface area of the samples was measured by a surface area analyzer (NOVA4200). The chemical composition of the samples was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Temperature-programmed decomposition mass spectrometry (TPD-MS) was carried out in a stream of pure Ar gas at a flow-rate of 30 ml min -1. The sample (0.1 g) was heated linearly at 10 o C min -1 from 100 to 800 oC, and then was held at this temperature for 30 min. The Hiden HPR20 mass spectrometer was used for detection.
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transform infrared (FTIR) spectra were recorded on a Nicolet MAGNA-IR 560 spectrometer. The morphology of
planes) and Co (2θ=44.4, 51.6 and 76.1 o, indexing respectively to (111), (200) and (220) planes), respectively. When the decomposition temperature was increased to 800 oC, the two phases became well crystallized with intense diffraction peaks, and a new weak peak observed at about 26.0o should be attributed to graphite carbon. Generally, the carbon species existed as an amorphous form in the metal nitride and carbide products obtained from thermal decomposition of metal-HMT complexes.31,32,37 Here, the formation of graphite carbon was probably due to the fact that metallic Ni and Co catalyzed the graphitization of the amorphous carbon. 43
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Fig. 4 shows the XRD patterns of the products of Ni-HMTP decomposition at different temperatures. After decomposition at 500 oC, the resulting sample was completely amorphous. When the temperature was increased up to 600 o C, Ni3P (2θ=41.8, 43.6, 45.2 and 46.6o, corresponding respectively to (231), (112), (240) and (141) planes) was obtained as the majority phase with a small amount of Ni12P 5 (2θ=38.4, 41.8, 47.0 and 49.0o, indexing respectively to (112), (400), (240) and (312) planes). With the increase of temperature from 600 to 800 oC, it can be seen that an obvious
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Fig. 5 shows the XRD patterns of the products of Co-HMT-P decomposition at different temperatures. Just like the decomposition of Ni-HMT-P, an amorphous compound was observed after decomposition of Co-HMT-P at 500 oC .The XRD pattern of the sample with decomposition at 600 oC showed several weak diffraction peaks within the 40-45o 2θ range, which was probably due to poorly crystalline Co2P. When the thermal decomposition temperature was further increased to 700 and 800 o C, typical diffraction peaks at 40.8, 43.4, 44.2 and 52.1o were observed, corresponding to the diffraction peaks of (121), (211), (130) and (002) planes of Co2P. According to the XRD data, the crystal sizes of as-prepared Ni2P and Co2P samples estimated by the Scherrer method were 36 and 29 nm, respectively. In addition, for the sake of comparison, Table 1 lists the phase structures of the solid products obtained during decomposition processes. It was clear from Table 1 that the thermal decomposition of M-HMT (M=Ni, Co) produced metallic Ni or Co, no matter whether it
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was at 500 or 800 o C. In the case of M-HMT-P (M=Ni, Co), the decomposition products were amorphous phase and phasepure Ni2P (Co2P) at 500 and 800 o C, respectively. However, one cannot exclude the possibility that the amorphous phases contained noncrystalline or very poorly crystalline Ni and Co metals. In order to further confirm the formation of Ni2P and Co 2P, and whether metallic Ni and Co species existed in the form of intermediates during decomposition of metal-HMT-P precursors, subsequent XPS analysis was conducted.
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Table 1 Phase structures of the solid products during decomposition processes. Sample
XRD phases
JCPDS cards
Ni-HMT-500 Ni-HMT-800 Co-HMT-500 Co-HMT-800 Ni-HMT-P-500 Ni-HMT-P-600 Ni-HMT-P-700 Ni-HMT-P-800 Co-HMT-P-500 Co-HMT-P-600 Co-HMT-P-700 Co-HMT-P-800
Ni Ni, C Co Co, C Amorphous Ni3P, Ni12P5 Ni12P5, Ni2P Ni2P Amorphous Co2P Co2P Co2P
65-0380 65-0380, 75-1621 15-0806 15-0806,75-1621 — 34-0501, 74-1381 74-1381, 65-1989 65-1989 — 32-0306 32-0306 32-0306
Fig. 6 shows the XPS spectra of Ni 2p, Co 2p and P 2p levels for the samples obtained from decomposition of Ni (Co)-HMT-P precursors at 500 and 800 oC. On the basis of curve fitting, the binding energies of Ni 2p 3/2, Co 2p 3/2 and P 2p 3/2 and the distribution of these corresponding species are listed in Table 2. As shown in Fig. 6 and Table 2, in the case of phosphide phase samples (Ni-HMT-P-800 and Co-HMT-P800), the surface regions of the two samples were dominated by oxidized species and underlying reduced species related to the phosphide phases. The binding energies of oxidized Ni (855.9 eV), Co (781.5 eV) and P (133.3-133.4 eV) were accordant with assignments by others to Ni2+, Co2+ and P 5+ species,44 respectively. The detection of oxidized species should be attributed to surface oxidation of phosphides during passivation.44 The binding energies of the peaks associated with reduced Ni (853.5 eV), Co (778.3 eV) and P (129.5129.7 eV) species were attributed to Ni2P, Co 2P and phosphide,45 respectively. Note that the XPS Ni 2p 3/2 spectrum Journal Name, [year], [vol], 00–00 | 3
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phase transformation of nickel phosphide from a high Ni/P ratio to a low Ni/P ratio (Ni3P→Ni12P 5→Ni2P) existed in the decomposition process. Expectedly, Ni2P (2θ=40.7, 44.6, 47.4 and 54.2o, indexing respectively to (111), (201), (210) and (300) planes) was obtained as a single phase at 800 oC.
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854.4
(b)
890
880
870
860 781.5
850 778.3 Co 2p
(c) 780.5
(d)
810
800
790
780
Binding Energy / eV
65
133.4
(a)
P 2p 129.7
133.0
(b)
133.3
129.5
(c) 133.3
(d)
152
148
144
140
136
132
128
Binding Energy / eV Fig. 6 XPS spectra of Ni 2p, Co 2p and P 2p (a) Ni-HMT-P-800; (b) NiHMT-P-500; (c) Co-HMT-P-800; (d) Co-HMT-P-500. 70
Table 2 XPS results for the samples obtained in this study. Sample
Ni-HMT-P-500 Ni-HMT-P-800
Co-HMT-P-500 Co-HMT-P-800
Binding energy (eV) Ni 2P3/2 P 2p3/2 Niδ+ Ni2+ PδP5+ —
854.4
853.5 855.9 Co 2P3/2 Coδ+ Co2+ — 780.5 778.3 781.5
—
133.0
129.7 133.4 P 2p3/2 PδP5+ — 133.3 129.5 133.3
Table 3 EDX, ICP-AES and BET results for the phosphides obtained by HMT and H2-TPR methods. Sample
HMT-Ni2P HMT-Co2P H2-Ni2P H2-Co2P 75
a
EDX M/P atomic Carbon ratioa (wt %) 2.12 32.5 2.08 25.3 2.09 4.9 2.05 7.1
ICP-AES M/P atomic ratioa 2.08 2.04 — —
BET SBET (m2 g-1) 191.7 43.6 3.8 4.6
M=Ni, Co
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of Ni-HMT-P-800 sample showed only one peak at 853.5 eV for Ni2P species which was far away from the value (852.6 eV) for Ni12P 5, 28 indicating that the Ni-HMT-P-800 sample synthesized was indeed pure Ni2P rather than mixed-phase Ni phosphide (Ni2P and Ni12P 5). In the case of amorphous phase samples (Ni-HMT-P-500 and Co-HMT-P-500), there were no peaks at 852.2 eV for Ni0 46 and 777.9 eV for Co0 47 except the peaks observed at 854.4, 780.5 and 133.0-133.3 eV, which were assigned to Ni2+, Co 2+ and P 5+ species, 48,49 respectively. Note that the binding energies of Ni2+ and Co2+ for Ni(Co)HMT-P-500 were far away from those for Ni(Co)-HMT-P800. This was because the former was usually attributed to metal oxides but the latter was consistent with assignments to metal phosphates.50 These facts indicated that the intermediates involved in the transformations (Ni(Co)-HMT-P →Ni(Co) phosphides) were only the oxidized species without reduced metallic species. Finally, the morphologies of Ni2P and Co 2P (hereafter denoted as HMT-Ni2P and HMT-Co 2P, respectively) obtained by HMT method at 800 o C were investigated by SEM and TEM. For the sake of comparison, the Ni2P and Co2P (hereafter denoted as H2-Ni2P and H2-Co2P, respectively) were prepared by traditional H2-TPR method described elsewhere,27 and their morphologies were also characterized by means of SEM. Figs. 7a-d show the SEM images of Ni2P and Co2P obtained in this study and the corresponding EDX analysis results are listed in Table 3. It can be observed from Figs. 7a and b that the morphologies of HMT-Ni2P and HMT-Co2P were very similar and they were composed of dispersed nanoparticles with size range of around 20-100 nm. Noticeably, it was clear that these divided nanoparticles connected with each other through thin sheets. These sheets might be carbon deposites from HMT decomposition because a large amount of carbon was detected by EDX (Table 3). These results were in good agreement with the observation of TEM images (Fig. 8). The particle sizes observed by SEM and TEM were closed to those estimated by XRD. However, the H2-Ni2P and H2-Co2P consisted of large aggregates of irregularly shaped particles (Figs. 7c and d), which was similar to the results reported by others.28 In addition, the EDX results (Table 3) indicated that the M/P (M=Ni, Co) atomic ratios of HMT-Ni2P and HMT-Co2P were nearly identical to those of H2-Ni2P and H2-Co 2P. These values were close to the expected stoichiometric ratio of 2/1. The M/P (M=Ni, Co) atomic ratios of HMT-Ni2P and HMT-Co2P were further estimated by ICP-AES to be Ni2.08P 1.00 and Co 2.04P 1.00, which showed better agreement with the expected stoichiometry (Ni2P and Co 2P). It was also clear from the EDX data that the HMT-Ni2P and HMT-Co 2P samples showed much higher surface carbon content than the H2-Ni2P and H2Co 2P samples, probably due to the fact that the materials obtained by HMT route contained carbon deposites besides carbon contamination from EDX analysis. 31,32,37 Note that the surface areas of Ni and Co phosphides prepared by HMT route were significantly higher than those of corresponding phosphides prepared by traditional H2-TPR method. In particular, the HMT-Ni2P sample showed a high surface area of 191.7 m2 g-1, which was about fifty times higher than that of H2-Ni2P sample (3.8 m2 g-1). The results were in agreement with the the results of SEM observation (Fig. 7), because the decrease in agglomeration was usually responsible for the increase in the surface area of phosphide, as suggested before. 51
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Fig. 8 TEM images of (a) HMT-Ni2P and (b) HMT-Co2P.
3.2 Formation mechanism
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Fig. 7 SEM images of (a) HMT-Ni2P, (b) HMT-Co2P, (c) H2-Ni2P and (d) H2-Co2P. 40
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To understand the formation mechanism of phosphides, the decomposition of metal-HMT-P precursors was followed by mass spectrometry (MS). Fig. 9 shows the TPD-MS profiles of the metal-HMT-P precursors. According to the previous study on the decomposition of metal-HMT complexes,31-33,39 the masses (M) of 2 (H2), 16 (CH4), 17 (NH3), 18 (H2O), 28 (N2 or CO), 30 (NO) and 44 (CO2) were reported to be featured, thus these masses were monitored mainly in current study. It was obvious from Fig. 9 that three temperature regions can be observed as follows. (I) Low-temperature decomposition stage: 100 oC ≤T< 370 oC. The MS showed the simultaneous signals of all the masses 2-44. Although the peaks at 16 and 17 might be attributed to the fragments derived from NH3 and H2O, respectively, it was possible to deduce that these gas species (H2, CH4, NH3, H2O, N2, CO, NO and CO2) were released simultaneously during decomposition of metal-HMT-P precursors in this temperature range, as suggested by related studies.31-33,39 (II) Moderatetemperature decomposition stage: 370 oC ≤T< 550 oC and 370 o C ≤T< 600 o C for Ni-HMT-P and Co-HMT-P precursors, respectively. In this stage, we observed intense signals of H2 (M=2) , which nearly coincided with the weak signals of CH4 (M=16) and NH3 (M=17). However, there was no sharp increase in the concentration of COx and H2O, indicating no deep reduction of oxidized species by these reductant gases. This was in agreement with the XRD and XPS results (Figs. 4-6) that the samples (Ni(Co)-HMT-P-500) were amorphous phases and their surface regions displayed only oxidized species without phosphided species. Therefore, the formation of phosphides should occur in the (III) high-temperature decomposition stage (550 oC ≤T≤ 800 o C for Ni-HMT-P and 600 oC ≤T≤ 800 oC for Co-HMT-P). In the case of the profile of Ni-HMT-P, it was clear that three signal peaks of M=28 at
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In summary, phase-pure Ni2P and Co2P phosphides were successfully prepared by one-step thermal decomposition of HMT-containing precursors. Investigation of the formation processes indicated that at high-temperature stage the inner reductants (i.e. C, H2 and CH4) which were generated from decomposition of HMT reacted with Ni(Co) and P species, leading to the formation of Ni(Co) phosphides. Unlike the traditional H2-TPR method produced a large amount of H2O in the synthesis process, the HMT-based route produced CO as major gas product, and thus hydrothermal sintering might be mitigated. The dispersions and surface areas of as-prepared Ni2P and Co 2P were superior to those of corresponding phoshides prepared by traditional H2-TPR method. The advantages of the resulting Ni2P and Co2P phosphides were worthwhile for further exploration.
M=17
Acknowledgements
M=16*2
We acknowledge the financial support from the National Natural Science Foundation of China (No. 21276253 and No. 21006032).
765
75
M=2
Notes and references M=44
College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Liaoning, PR China 113001 E-mail: [email protected]
M=28 M=30
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HMT complexes, the formation of metal nitrides and carbides was mainly attributed to carbothermal reduction.31,32,38 In our earlier study,39 we proposed that the HMT-based method yielded molybdenum phosphide via reduction of molybdenum and phosphorus species by CH4 from decomposition of HMT, because Hargreaves and co-workers had proved that CH4 was an effective reductant for the synthesis of phosphides. 53 Based on the results of previous studies as well as the current MS, XRD, XPS and EDX analysis results, the formation mechanism of metal phosphides in the stage III was further perfected as follows: (i) HMT decomposed to H2, C, N2 and CH4; (ii) metal and phosphorus species were reduced by C, H2 and CH4 to produce metal phosphides, with release of COx and H2O. The general observations mentioned above were interesting that the reduction of precursors in the HMT-based route can produce COx as the main gaseous products, followed by a low level of H2O being made. On the contrary, the traditional H2 reduction method yielded metal phosphide with release of a large amount of H2O, with the result that the clustering of phosphide particles occurred due to hydrothermal sintering.29,30 It was reasonable to deduce that the HMT method produced metal phosphides mainly via carbothermal reduction, which can avoid strong hydrothermal sintering of metal phosphide nanoparticles. Therefore, phosphide nanoparticles obtained by HMT-based route showed better dispersions and higher surface areas than those synthesized by traditional H2-TPR method.
4. Conclusions
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620, 685 and 800 oC were observed, which should be attributed to a series of reduction steps (Ni3P→Ni12P 5→Ni2P), as suggested by XRD analyses (Fig. 4). In contrast, the profile of Co-HMT-P showed that the signal of M=28 started to increase at 600 o C and only one signal peak was observed at about 765 oC, accompanied by the weak peaks of H2 (M=2), CH4 (M=16) and CO2 (M=44). The result inferred that the stage III for Co-HMT-P involved a direct reduction from oxidized Co and P to Co2P, which was in accordance with the XRD analyses (Fig. 5). In view of the fact that N2 and CO had the base peak at the same mass number 28 and thus the two gas species generated from decomposition of metal-HMT-P precursors cannot be identified by MS characterization, the gas products released in the stage Ⅲ were further analysed by gas chromatograph (GC). The results (not shown here) indicated that the N2 and CO gases were always released simultaneously in these temperature ranges. Additionally note that there were also weak signals of H2O (M=18) observed together with the intense signals of N2 and CO (M=28) in the stage III (see Fig. 9). However, the delay of H2O responses in reaching the maximum was probably due to the slow desorption of them on the surfaces, as suggested before. 52
Temperature / C Fig. 9 TPD-MS profiles of (a) Ni-HMT-P and (b) Co-HMT-P.
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