Haitao Liab,
Kun Zhaoac,
Shouqin Tiand,
Dawen Zeng*ab,
Aimin Pange,
Xiaoxia Wanga and
Changsheng Xiea
aState Key Laboratory of Materials Processing and Die & Mould Technology, Nanomaterials and Smart Sensors Research Lab (NSSRL), Department of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), No. 1037, Luoyu Road, Wuhan 430074, People's Republic of China. E-mail: dwzeng@mail.hust.edu.cn
bHubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, People's Republic of China
cCollege of Chemical Engineering and Food Science, Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices, Hubei University of Arts and Science, Xiang Yang 441053, People's Republic of China
dState Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, No. 122, Luoshi Road, Wuhan 430070, People's Republic of China
eHubei Institute of Aerospace Chemotechnology, Xiangyang 441003, Hubei, People's Republic of China
First published on 17th August 2017
ZnO nanocrystals as catalysts have been widely employed in the catalytic thermal decomposition of ammonium perchlorate (AP). However, the catalytic mechanism is still controversial and the role of surface lattice oxygen is always ignored. Herein, a classical catalytic mechanism based on the surface lattice oxygen was proposed to reveal AP decomposition promoted by ZnO nanosheets. ZnO and ZnS nanosheets, both of which have the same wurtzite structure and the same (2−1−10) exposed facets, have been synthesized by the calcination of a ZnS(en)0.5 precursor in different conditions. ZnO nanosheets with a smaller surface area showed a better catalytic activity than ZnS nanosheets because the surface lattice oxygen of the ZnO nanosheets can react with NH3 (an intermediate of AP thermal decomposition) to generate oxygen vacancies that can subsequently be recovered, while the surface lattice sulfur of the ZnS nanosheets did not react with NH3. The generation and replenishment of oxygen vacancies on the (2−1−10) exposed facets of the ZnO nanosheets were confirmed by XPS and FTIR results, and thus revealed the origin of the efficient catalytic AP decomposition over (2−1−10) facets of ZnO nanosheets as surface lattice oxygen. Therefore, this work could provide a new insight into the catalytic mechanism of metal oxides to promote AP decomposition.
Among these metal oxides, ZnO is considered as one of the best single metal oxide catalyst and thus used indispensably in the photochemical industry and chemical industry, due to its excellent reactivity, non-toxicity and stability.8,10 In addition, the morphology and structures of ZnO can be tuned more easily by simple methods.13 In this sense, ZnO is the best model material to investigate the underlying relationship between its surface structure and its catalytic performance for AP decomposition. In our previous work, the ZnO nanosheets exposed with more (0001) facets exhibited a better catalytic property than other structures.13 Also, it is found that HClO4 gas (one intermediate of AP decomposition) is preferentially adsorbed on ZnO (0001) facets and NH3 gas (the other intermediate of AP decomposition) preferentially adsorbed on the non-polar facets,14–16 such as (2−1−10) and (10−10), with hydrogen bonds forming between H atoms and the adjacent O atoms as shown in Scheme 1(a). It is traditionally considered that NH3 gas is oxidized by the active oxygen anions17–21 which are diffused from the (0001) facets as shown in Scheme 1(b). However, the oxidation path of the adsorbed NH3 gas is still unclear. Here, two possible oxidation paths are proposed. On one hand, lattice oxygen and interstitial oxygen are possibly transformed to oxygen vacancies and release oxygen to oxidize NH3 as shown in Scheme 1(c). On the other hand, the N atom is adsorbed on the Zn atom and a H atom can form a hydrogen bond with an O atom.15 And a N–H bond can break to form –NH2.13,14,21 Thus, the released H atom will combine with adjacent lattice oxygen atoms or diffused active oxygen anions. That is shown in Scheme 1(d), and the remaining –NH2 bond is faced with the same situation. It can be seen obviously that the two paths are deeply related with surface lattice oxygen. Therefore, a reasonable mechanism (either classical or novel) based on the surface lattice oxygen will be proposed to further reveal the oxidation path of NH3.
Herein, a classical catalytic mechanism was employed to further reveal AP decomposition in the presence of ZnO nanosheets exposed with (2−1−10) facets. ZnO and ZnS nanosheets, both of which had the same wurtzite structure and exposed (2−1−10) facets, were prepared by calcinations of ZnS(en)0.5 precursor with different conditions.20–23 The obtained ZnO nanosheets showed a better catalytic performance than the ZnS nanosheets in the AP decomposition. This indicates that the surface lattice oxygen on ZnO (2−1−10) facets plays a positive role in the catalyst, while the surface lattice sulfur on ZnS (2−1−10) facets cannot do the same. NH3 could be possibly oxidized by the surface lattice oxygen. To prove this view, ZnO and ZnS nanosheets were reacted with NH3 on the same condition that they were used to promote AP decomposition. The concentration of surface lattice oxygen on ZnO (2−1−10) facets was highly decreased, while that of surface lattice sulfur was almost unchanged. Oxygen vacancies were formed due to the reaction and consumption of surface lattice oxygen. But after AP decomposition catalyzed by ZnO, the concentration of the surface lattice oxygen on ZnO was recovered to a normal extent, indicating that the formed oxygen vacancies were replenished subsequently. In this sense, NH3 oxidation by surface lattice oxygen is another path of NH3 oxidation during the catalytic decomposition of AP, compared to the NH3 oxidation by active oxygen anions. This work shows a great significance in the introduction of a catalytic mechanism based on the surface lattice oxygen into the field of the catalytic decomposition of AP.
Fig. 2 shows the SEM images, TEM images, HRTEM images and SAED patterns of ZnS and ZnO nanosheets, respectively. They were similar size about 0.1 × 1 × 2 μm like ZnS(en)0.5 precursor (as shown in Fig. S2†) with a little porous for ZnS and highly porous for ZnO. The facet spacing of 0.31 nm in Fig. 2(c) was the (001) facet of wurtzite ZnS, and that of 0.26 nm in Fig. 2(d) was the (001) facet of wurtzite ZnO. And their SAED patterns show their diffraction spots both were (010) and (002) facets. These results indicated that the facets perpendicular to the incident direction of electrons were (100) i.e. (2−1−10) facets as shown in Fig. 2(d) and (h), and this facet relationship had been demonstrated in the previous work.25 Obviously, these (2−1−10) facets were the exposed facets which were facing us in Fig. 2(a), (b), (e) and (f). Therefore, these ZnO and ZnS nanosheets showed the same exposed (2−1−10) facet, which agreed well with the previous work.20
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Fig. 2 (a) SEM, (b) TEM, (c) HRTEM images and (d) SAED pattern of ZnS; (e) SEM, (f) TEM, (g) HRTEM images and (h) SAED pattern of ZnO. |
To further investigate the porous structures of ZnO and ZnS nanosheets, BET method was employed and the results are shown in Fig. 3. The N2 adsorption–desorption isotherms were displayed in Fig. 3(a), indicating that the BET surface areas of ZnO and ZnS nanosheets were 8.1 and 43.7 m2 g−1, respectively. It can be seen that ZnS nanosheets exhibited a larger surface area than ZnO nanosheets. In addition, ZnO nanosheets presented a wider pore size distribution and a larger average pore size than ZnS nanosheets in Fig. 3(b).
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Fig. 3 (a) N2 adsorption–desorption isotherms and (b) pore size distributions for ZnS (■) and ZnO (●). |
ln(β/Tp2) = ln(AR/Ea) − Ea/RTp. | (1) |
In this correlation, β was the heating rate, Tp was the peak decomposition temperature, R was the ideal gas constant, Ea was the activation energy, and A is the pre-exponential factor. ln(β/Tp2) varied linearly with 1/Tp, and yielded the kinetic parameters of activation energy from the slope of the straight line and of pre-exponential factor from the intercept. As shown in Fig. 4(d), the Ea of AP decomposition was reduced from 151.1 kJ mol−1 to 137.6 kJ mol−1 with ZnS and 111.1 kJ mol−1 with ZnO, respectively. Their N-containing gaseous products, which were discussed in the ESI,† also reflected the differences in their catalytic properties.
Based on the above results, their specific catalytic activities are summarized in Table 1. ΔsT, ΔsH and ΔsEa denote their HTD temperature decreases, heat release increases and activation energy decreases per unit of surface area of catalyst, respectively.26 Obviously, ZnO nanosheets showed a better specific catalytic activity than ZnS nanosheets. It can be seen that the surface area cannot play a critical role in the catalytic activities because that ZnS nanosheets exhibited a larger surface area but a poorer catalytic performance. This was probably related with their other different structures. The obvious difference between ZnO nanosheets and ZnS nanosheets was the surface atoms which can put important effects on the catalytic properties. Thus, the difference of their catalytic activities was probably caused by their different surface atoms (the surface lattice oxygen existed in ZnO nanosheets while the surface lattice sulfur existed in ZnS nanosheets). More experiments were needed to investigate this issue.
ZnO | ZnS | |
---|---|---|
Surface area (m2 g−1) | 8.1 | 43.7 |
ΔsT (K m−1) | 14.94 | 2.70 |
ΔsH (J g−1 m−1) | 67.90 | 9.06 |
ΔsEa (kJ mol−1 m−1) | 4.94 | 0.31 |
To investigate the surface structures of ZnO and ZnS nanosheets after the oxidization of NH3 and decomposition of AP, XPS method was employed. Fig. 5 and 6 shows their XPS results. As shown in Fig. 5, the peak of surface lattice oxygen (Olattice noted as Oa) was at 530 eV, the peak at 531.4 eV was associated with the O2− ions in the oxygen-deficient regions within the ZnO matrix (noted as Ob), and the peak at 532.3 eV was usually attributed to chemisorbed oxygen on the surface of the ZnO, such as –CO3, adsorbed H2O or adsorbed O2 (noted as Oc).28 And as shown in Fig. 6, the peak of Slattice can be divided in two peaks, S 2p3/2 at 162.2 eV and S 2p1/2 at 163.4 eV.29 Their atomic concentrations of Zn, Olattice and Slattice were listed in Table 2. The Zn/Olattice ratio of ZnO–NH3 was as high as 4.31, which indicated a lot of surface lattice oxygens indeed were consumed to oxidize the NH3 as our assumption, leading to the formation of abundant surface oxygen vacancies. Whether these oxygen vacancies can exist until the total catalytic reaction or not? The Zn/Olattice ratio of ZnO–AP was 1.39 which is close to that of ZnO before reaction (1.38). This result indicated that the formed oxygen vacancies were replenished to be surface lattice oxygen and ZnO (2−1−10) facets were recovered. As a comparison, Zn/Slattice ratios of ZnS nanosheets were almost unchanged during the whole catalytic reaction. These results solidly suggested that the surface lattice oxygens on ZnO (2−1−10) facets can oxidize NH3 to form oxygen vacancies and then they were replenished subsequently.
Concentration | Zn | Olattice | Slattice | Zn/Olattice (Slattice) |
---|---|---|---|---|
ZnO | 57.92% | 42.08% | N/A | 1.38 |
ZnO–NH3 | 81.16% | 18.84% | N/A | 4.31 |
ZnO–AP | 58.16% | 41.84% | N/A | 1.39 |
ZnS | 59.39% | N/A | 40.61% | 1.46 |
ZnS–NH3 | 59.53% | N/A | 40.47% | 1.47 |
ZnS–AP | 58.95% | N/A | 41.05% | 1.44 |
To further confirm the generation of oxygen vacancies on the ZnO (2−1−10) facets, ZnO and ZnS nanosheets were used to catalyze NH3 or AP, respectively, and then subjected to the XRD characterization. The results were shown in Fig. 7. Obviously, it can be seen that the phase structure of ZnO and ZnS nanosheets cannot be changed, indicating that ZnO and ZnS nanosheets can be used as catalyst to promote AP decomposition and NH3 oxidation. Interestingly, ZnO and ZnS nanosheets after NH3 oxidation exhibited wider and lower diffraction peaks in the XRD patterns than that after AP decomposition. The change was more obvious for ZnO nanosheets. The wide and low diffraction peak usually means a lower crystallinity, also indicating that more surface defects existed on the nanocrystals.30–33 In this sense, ZnO nanosheets after NH3 oxidation showed more surface defects than that after AP decomposition and the surface defect here is probably oxygen vacancy. The result is in good agreement with the above XPS results.
FTIR spectra of ZnO, ZnO–NH3 and ZnO–AP are shown in Fig. 8, in order to identify the generation of oxygen vacancies on the ZnO (2−1−10) facets further more. According to the previous work,34,35 there were several peaks in all the samples including the peaks at 437 cm−1 associated to the E2 mode of hexagonal ZnO (Raman active), the peaks at 1630 and 1384 cm−1 due to the asymmetrical and symmetrical stretching of the zinc carboxylate, peaks between 2830 and 3000 cm−1 due to C–H stretching vibration of alkane groups, a broad band at 3500 cm−1 assigned to the O–H stretching mode of hydroxyl group. There was only one special peak in ZnO–NH3 sample at 505 cm−1 associated with oxygen deficiency and/or oxygen vacancy (VO) defect complex in ZnO.34,35 This was another evidence for the generation of oxygen vacancies on the ZnO (2−1−10) facets.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra07906k |
This journal is © The Royal Society of Chemistry 2017 |