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Full Paper Preparation, Characterization, and Catalytic Activity of Nanosized NiO and ZnO: Part 74 Dinesh Kumar,a Inder P. S. Kapoor,a Gurdip Singh,*a Prem Felix Siril,b Alok Mani Tripathic a

b c

Department of Chemistry, DDU Gorakhpur University, Gorakhpur-273009 (India) e-mail: [email protected] Chemistry Department, NIT Hamirpur, Hamirpur-177005 (HP) (India) Department of Energy, Tezpur University, Tezpur, Assam (India)

Received: February 2, 2010; revised version: August 1, 2010 DOI: 10.1002/prep.201000013

Abstract Nanocrystals of NiO and ZnO were prepared by a novel refluxing method and characterized by XRD and TEM. The average particles size of NiO and ZnO were found to be 6 and 31 nm, respectively, from the XRD patterns. These Transition Metal Oxide Nanocrystals (TMONs) were found to catalyze the thermal decomposition of 5-nitro-1, 2, 4-triazol-3-one (NTO). Between the two TMOs, ZnO was found to have better catalytic activity than NiO. Kinetic parameters for the isothermal decomposition of NTO in presence and absence of these metal oxides were obtained using a model-free isoconversional method. The activation energy for NTO, NTO + 1 %NiO, and NTO + 1 %ZnO was found to be 60.1, 52.1, and 47.9 kJ mol1 and a similar order was found from the explosion studies (36.1, 31.6, and 27.4 kJ mol1, respectively). Keywords: Catalytic Activity, Nanocrystals, Novel Refluxing Method, TG, XRD

1 Introduction Transition metal oxide nanocrystals (TMONs) find applications in solar energy transformation [1], magnetic materials [2], rechargeable lithium batteries [3], and also as catalysts [4, 5]. Almost all of the TMONs have been found to exhibit super magnetic or paramagnetic behavior. Transition metal oxides (TMOs) are widely used as burning rate additives in composite solid propellants (CSPs). Our group is involved in the development of burning rate modifiers for CSPs. Catalytic activity, being a surface property, generally increases with the increase of surface area of the catalysts [6]. As the size reduction results in the enhancement of surface area, nanomaterials have better catalytic activity over their bulk counterparts 268

[7]. This has been found to be true in our recent studies where a number of TMONs have been developed and were found to have superior catalytic activities for the common oxidizers that are used in CSPs such as ammonium perchlorate [8, 9]. Nanomaterials can be prepared through either “top down” or “bottom up” strategies [10]. Top down methods are often costly and are energy intensive while nanomaterials can be prepared with comparatively less energy input using the “bottom up” methods [10]. We have recently synthesized nanocrystals of Fe, Cr, and Mn using the “bottom up” strategy [8, 9]. Here we are reporting the preparation of nanocrystals of NiO and ZnO using a simple refluxing method. A number of methods have been reported to prepare the nanocrystals of NiO and ZnO [11–17]. It has been reported that porous metal oxides of various morphologies can be prepared in presence of hexamethylenetetramine (HMTA). The objective of the present work was thus to prepare porous nanocrystals of NiO and ZnO and determine their catalytic activities for thermal decomposition of 5-nitro-1, 2, 4-triazol-3one (NTO, C2H2N4O3). NTO is an insensitive high energetic compound that has potential applications as an oxidizer in composite energetic materials [18, 19]. The catalytic activities of the prepared TMONs were evaluated using various thermoanalytical techniques and the results are reported in this communication.

2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Propellants Explos. Pyrotech. 2011, 36, 268 – 272

Preparation, Characterization, and Catalytic Activity of Nanosized NiO and ZnO: Part 74

2 Experimental Part 2.1 Materials Nickel nitrate and zinc nitrate (Merck), ethanol (s.d. fine) HMTA (Lancaster) and methanol (Ranbaxy) were used as received.

2.2 Preparation Ethanolic solutions of metal nitrates and HMTA (1 : 2 ratios by weight) were refluxed for 1 h in a round bottom flask. Distilled water was then added to the solution and the contents refluxed for 20 min. This solution was kept for 2 h and metal oxide nanocrystals were filtered, washed with methanol in order to remove unreacted precursors.

2.3 Characterization X-ray diffraction (XRD) measurements were performed on NiO and ZnO (Figure 1) by using Rigaku Miniflex Xray diffractometer using Cu Ka radiation (l = 1.5418). The XRD pattern of both the samples shows the crystalline nature of the material and crystallite size was calculated by applying Scherers formula [20] (Table 1). Transmission electron microscopy (TEM) images on these samples were obtained using Hitachi (H-7500) instrument (Figure 2).

Figure 2. (a) TEM image of NiO and (b) TEM image of ZnO.

Table 1. Crystal size of oxide nanocrystals. Metal oxide nanocrystal

Average crystallite size from XRD (nm)

Particle size from TEM (nm)

NiO ZnO

6 31

3.2 25.4

2.4 Catalytic Activity

Figure 1. (a) XRD plot of NiO and (b) XRD Plot of ZnO.


The catalytic activities of NiO and ZnO nanocrystals on the thermal decomposition of NTO were investigated using TG, DTA, and explosion delay (DE) measurements. Non-isothermal TG and DTA thermal curves (Figure 3) were recorded on NTO with and without catalysts (~ 25 mg) using a NETZSCH STA 409C/CD instrument in flowing N2 atmosphere at a heating rate of 10 8C min1. Isothermal TG curves (a plot of alpha, i.e. conversion degree vs. time/min) as shown in Figure 4, were recorded under static air atmosphere using an indigenously fabricated TG apparatus [21]. Kinetics of their thermal decomposition were investigated as reported earlier [22]. The calculated rate of decomposition of pure NTO and NTO with catalyst is reported in Table 2. The catalytic ac-


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Figure 3. TG-DTA traces for (a) NTO, (b) NTO + 1 %NiO, and (c) NTO + 1 % ZnO in nitrogen.

tivities were analyzed using isothermal TG with 1 % catalysts and NTO (Table 2).

Figure 4. Isothermal TG of (a) NTO, (b) NTO + 1 %NiO, and (c) NTO + 1 % ZnO in static air.

2.5 Explosion Delay Measurements Explosion delay of NTO with and without nanocrystals was measured using a tube furnace technique (TF) [23]. Approximately 20 mg sample was taken in an ignition tube clamped with bent wire and inserted manually into the furnace. The time interval between the insertion of ignition tube into the TF and the moment of visible explosion i.e. the time of explosion (DE) was noted with the help of a stopwatch (Table 3). The traces of lnDE versus 1/T are presented in Figure 5.

Table 2. Decomposition rate and catalytic activity of NTO, NTO + oxide nanocrystals (1 % by weight). Sample

Decomposition rate (mass loss/min) up to 90 % decomposition at 290 8C

Catalytic activity (CA)

NTO NTO + 1 % NiO NTO + 1 %ZnO

1.24 2.34

– 1.9

2.84

2.3

3 Results and Discussion

ðCH2 Þ6 N4 þ 6H2 O ! 6HCHO þ 4NH3

ð1Þ

It is inferred that ammonia formed by the decomposition of HMTA may accept a proton to generate OH ions which then reacts with Zn2 + and Ni2 + ions to form, zinc oxide and nickel oxide, respectively [24]:

NH3 þ H2 O ! NHþ4 þ OH

ð2Þ

2OH þ Zn2þ ! ZnOðsÞ þ H2 O

ð3Þ

270




Preparation, Characterization, and Catalytic Activity of Nanosized NiO and ZnO: Part 74 Table 3. Explosion delay for NTO and NTO + catalyst activation energy for ignition (E*) and correlation coefficient (r). Sample NTO NTO + 1 %NiO NTO + 1 %ZnO

DE/s at temperature/8C 260 1

270 1

280 1

290 1

300 1

61 51 44

56 42 38

47 36 34

41 33 32

35 31 28

Figure 5. Plot of lnDE versus 1/T.

The characteristic peaks in XRD (Figure 1) for NiO are quite broad indicating very small particle size. However, XRD pattern for ZnO showed relatively sharper peaks that are characteristic of wurtzite type ZnO. NiO and ZnO samples have an average particle size of 6 and 31 nm, respectively. To get more insight into the microstructure of these nanocrystals, TEM imaging was performed. TEM images for NiO and ZnO are shown in Figure 2(a) and (b), respectively. TEM imaging revealed that the NiO sample consists of very small (3 nm) spherical particles with uniform particle size. The ZnO sample contained relatively larger particles (25 nm) when compared to that of NiO. Thus particle size calculated from XRD pattern and from TEM imaging was found to be in agreement. In the case of high energetic compounds, the early thermal decomposition reactions are very important from the viewpoint of understanding the mechanism of explosion. Rothgery et al. [25] identified the major gaseous products in accordance with the equation C2 H2 N4 O3 ðsÞ ! 1=6C2 H3 N3 OðsÞ þ 1=6H2 OðlÞ þ1=3CO2 þ 1=2N2 OðgÞ þ 3=2COðgÞ

ð4Þ

þ4=3N2 ðgÞ þ 1=6NOðgÞ þ 7=12H2 ðgÞ Formation of NO2 radical by CNO2 cleavage, with the rupture of the adjacent CN bond appears to be the probable mechanism in the thermal decomposition of NTO. The oxidative attack of the .NO2 radical on the ring fragment accounts for the formation of gaseous products such as CO2, NO, and N2O. Propellants Explos. Pyrotech. 2011, 36, 268 – 272

E*/kJ mol1

r

36.1 31.6 27.4

0.9928 0.9821 0.9935

The non-isothermal TG-DTA thermal curves (Figure 3a) in N2 atmosphere for NTO clearly indicated that thermal decomposition takes place in a single exothermic step at around 272 8C as reported in the literature [18, 19]. A single step exothermic mass loss was observed (Figure 3b and c) for NTO samples containing 1 % TMONs also. Interestingly the mass loss for the NTO samples containing the TMONs was found to take place at much lower temperatures than that for pure NTO clearly indicating catalyzed decomposition. Corresponding DTA and DTG peak temperatures for the catalyzed NTO samples were observed at much lower temperatures. DTG peaks were observed at 270, 246, and 234 8C for NTO, NTO + 1 %NiO, and NTO + 1 %ZnO, respectively. Similar trends were observed in DTA thermal curves also. Thus among the two newly prepared TMONs, ZnO seems to have better catalytic activity than NiO, in terms of lowering the decomposition temperature of NTO. Kinetics of the thermal decomposition of NTO with and without catalysts were evaluated by using model-free isoconversional method from isothermal TG data (Figure 4) [26]. The corresponding values of Ea are 60.1, 52.7, and 47.9 kJ mol1, respectively. These kinetic results clearly show that, in the presence of catalysts, a noticeable decrease in the activation energy of thermal decomposition of NTO occurred. In order to quantify the catalytic activity of these catalysts, isothermal TG traces of NTO with and without catalysts (1 % by mass) were recorded (Table 2). The catalytic activity (CA) of these catalysts has been calculated by using following equation. Catalytic activityðCA Þ ¼

C C0

ð5Þ

where C0 and C are the rate of decomposition of NTO and NTO with 1 % catalysts, respectively. On the basis of these measurements, the catalytic activity for ZnO is better than NiO. To understand the explosive behavior of NTO and NTO mixed with ZnO and NiO, explosion delay measurements were performed (Figure 5). It is inferred that activation energy for explosion of NTO is lowered by incorporation of these catalysts (Table 3). Thus, slow thermal decomposition studies (TG-DTA and isothermal TG) and fast thermolysis studies (explosion delay measurements) invariably established that NiO and ZnO nanocrystals catalyze the thermal decomposi-



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tion of NTO. However, the parameters that govern the catalytic activity and the order of the observed catalytic activity are unclear. We have previously synthesized transition metal salts of NTO including Ni and Zn salts and studied their thermal behavior in comparison to NTO and found that the thermal stability of the transition metal salts of NTO is generally lower than that of NTO [26]. Thus, lower decomposition temperatures and lower activation energies are expected for thermal decomposition if the transition metal salts of NTO are formed on heating the NTO + TMON mixture initially. However, DTA thermal curves for the mixture samples do not identify any thermal signatures corresponding to any such salt formation reaction. On the other hand if the catalytic thermal decomposition of NTO in presence of NiO and ZnO proceeds with adsorption of NTO on the surface of the metal oxides, then the surface area of the TMON particle can be a determining factor for the catalytic activity. In that case, NiO having much smaller particle size and in turn is expected to have much higher surface area and thus must have better catalytic activity than ZnO. However, it is also reported that highly porous nanoparticles are formed when ZnO is prepared in presence of HMTA and thus it is possible that the prepared ZnO nanoparticles may have higher surface area than the prepared NiO.

4 Conclusion NiO and ZnO nanocrystals were prepared by novel refluxing method in presence of HMTA. These TMONs were found to catalyze the thermolysis of NTO and ZnO is a better catalyst than NiO.

Acknowledgements Thanks are due to Head, Chemistry Department DDU Gorakhpur University, Gorakhpur for lab facilities and IIT Madras for TG-DTA analysis. Financial assistance by UGC to Dinesh Kumar and CSIR to Prof. Gurdip Singh (Emeritus Scientist) are highly appreciated.

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