Facile Synthesis of Mo2C Nanoparticles from Waste Polyvinyl Chloride.

The resource utilization of waste plastic can not only control environmental pollution but can also ease up the problems of lack of energy resources. In this study, molybdenum carbide (Mo2C) nanoparticles have been synthesized by utilizing waste polyvinyl chloride as a carbon source in a stainless-steel autoclave at 600 °C. X-ray diffraction pattern indicates that the product is orthorhombic phase Mo2C. Electron microscopy photographs show that the obtained Mo2C product consisted of crystalline nanoparticles with an average size of 50 nm. The possible formation mechanisms of Mo2C have been also briefly discussed on the basis of the structures of the products synthesized with different reaction times. The effects of reaction temperature on the crystallinity and microstructure of the obtained products have been investigated. The results show that higher reaction temperature promotes the formation of Mo2C with high crystallinity.

Introduction

Molybdenum carbide is an important ceramic material because of its unique properties, such as high melting point (2770 K), extreme hardness, and high chemical stability.[1−4] Moreover, molybdenum carbide exhibits excellent catalytic properties on ammonia synthesis, H2 production, and alcohol synthesis.[5]

Up to now, several methods have been developed to synthesize molybdenum carbides, such as direct pyrolysis of molybdenum hexacarbonyl,[6] molten salt method,[7] electrochemical method,[8] solution route,[9] carbothermal reduction of molybdenum oxide with carbon at high temperature,[10,11] chemical vapor deposition (CVD),[12−14] solid-state metathesis,[15] thermal reduction of molybdenum chloride and carbon (graphite or carbon nanotube) with metallic sodium,[16] and co-reduction of carbon tetrabromide and molybdenum pentachloride with metallic sodium in benzene.[17] Mo2C nanoparticle-decorated graphitic carbon sheets have been synthesized via a solid-state reaction of (NH4)6Mo7O24·4H2O and sodium alginate under Ar at 900 °C.[18] Recently, Geng and co-workers have synthesized Mo2C/graphene heterostructures by molten copper-catalyzed CVD.[19]

Polyvinyl chloride (PVC) materials have been widely used in the daily life because of their excellent properties such as transparence, chemical stability, and low density.[20] However, the waste PVC in the environment generates a toxic compound dioxin which is very harmful to humans and animals.[21] Developing an effective method of waste PVC disposal can help to reduce the dioxin emissions, which is good for environment protection. Up to now, many treatment methods for waste plastic have been reported.[22−35] Zhang and his co-workers have synthesized carbon-based materials by utilizing waste plastic as a carbon source.[28−33] We have developed a method to synthesize silicon carbide and transition-metal carbides from waste plastic.[34−36] In this study, we have reported a facile method for synthesizing Mo2C nanoparticles through the reactions between metallic sodium, molybdenum sulfide, and waste PVC at relatively low temperature. This work is aimed not only to develop a simple method to synthesize molybdenum carbide but also to explore an effective method of waste PVC disposal.

Results and Discussion

The crystal structures of the obtained products are investigated by X-ray diffraction (XRD). A typical XRD pattern of the obtained product via our designed route is shown in Figure . All the peaks in the Figure can be indexed to orthorhombic phase molybdenum carbide (Mo2C). The calculated lattice parameters a = 4.7311 Å, b = 6.0258 Å, and c = 5.2105 Å are almost consistent with the reported data (JCPDS no. 79-0744, a = 4.7350 Å, b = 6.0250 Å, and c = 5.2100 Å). No other diffraction peaks of byproducts such as MoS2 and metallic Mo are found in the XRD pattern, which indicates that the conversion of waste PVC to molybdenum carbide has been completed via our designed route.

Figure 1

XRD pattern of the obtained Mo2C product.

The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images of the obtained Mo2C product are shown in Figure a,b, respectively. It can be seen that the Mo2C nanoparticles are agglomerated in the obtained product. The average size of Mo2C nanoparticles is about 50 nm. A high-resolution TEM (HRTEM) image of the obtained Mo2C product is presented in Figure c. The neighbor lattice interplanar spacing is about 0.24 nm, which is very close to the distance between two (200) planes in orthorhombic phase Mo2C. A typical energy-dispersive X-ray (EDX) spectrum is shown in Figure d, which displays that the obtained product consists of Mo and C.

Figure 2

(a) FESEM image, (b) TEM image, (c) HRTEM image, and (d) EDX of the obtained Mo2C product.

Magnetization on the obtained Mo2C product is investigated with a superconducting quantum interference device (SQUID) at 10 Oe. Zero field cooling (ZFC) and field cooling (FC) temperature dependencies of magnetization M for the obtained Mo2C product are present in Figure . From Figure , the diamagnetism has been observed under the ZFC and FC conditions. The onset of the strong Meissner effect at about 9.80 K has been observed, which indicates the existence of superconductivity in the Mo2C product. The observation is almost consistent with the results of the previous reported work.[37]

Figure 3

Temperature dependence of magnetization for the obtained Mo2C product.

The effect of reaction time on the formation of Mo2C nanoparticles has been investigated. Figure a shows the XRD pattern of the product obtained from the reaction of molybdenum sulfide, metallic sodium, and waste PVC at 600 °C for 10 min. The diffraction peaks labeled as “●”in Figure a can be indexed as hexagonal phase MoS2 (JCPDS card no. 87-2416), and the other three diffraction peaks labeled as “▼” can be indexed as cubic metallic molybdenum (JCPDS card no. 89-4896). From the XRD result, we found that the product (obtained from the reaction for 10 min) is the mixture of MoS2 and metallic molybdenum, which proves that part of the raw material (MoS2) has been reduced to metallic molybdenum by metallic sodium. The reaction of reducing MoS2 can be expressed as eq

Figure 4

Typical XRD patterns of the products obtained at 600 °C with different reaction times: (a) 10 min, (b) 30 min, (c) 60 min, and (d) 5 h.

According to the calculations of free energy, the reaction 1 at 600 °C is highly exothermic (ΔrHm = −454.3 kJ/mol) and thermodynamically spontaneous (ΔrGm = −384.5 kJ/mol). Meanwhile, waste PVC could be reduced by metallic sodium to activated carbon, which can be expressed as follows

The diffraction peaks of carbon cannot be detected in the XRD pattern (Figure a), probably because the carbon is amorphous. Because of the poor thermal stability of PVC, the waste PVC has been reduced to produce carbon in this process.

Figure b,c shows the XRD patterns of the products obtained from the reaction of molybdenum sulfide, metallic sodium, and waste PVC at 600 °C for 30 and 60 min. The three diffraction peaks labeled as “▼” in Figure b,c can be indexed as cubic metallic molybdenum (JCPDS card no. 89-4896), and the other diffraction peaks labeled as “★” can be indexed as orthorhombic phase Mo2C (JCPDS card no. 79-0744). The results of XRD analysis show that MoS2 has been completely reduced to metallic molybdenum by metallic sodium. Meanwhile, part of the newly formed molybdenum has reacted with the newly formed carbon (produced from Na thermal reduction of waste PVC) to produce Mo2C, which can be expressed as

Figure d shows the XRD pattern of the obtained product from the reaction of molybdenum sulfide, metallic sodium, and waste PVC at 600 °C for 5 h. All the diffraction peaks in Figure d can be indexed as orthorhombic phase Mo2C (JCPDS card no. 79-0744), which reveals that the MoS2 can completely convert into Mo2C under the present experimental conditions for 5 h.

On the basis of the abovementioned experimental results, the chemical reaction of synthesizing Mo2C nanoparticles can be represented as follows

Because the ΔfHm values of Na2S (−364.8 kJ/mol) and NaCl (−411.2 kJ/mol) are much negative, a large amount of heat generated in the process may promote the formation of Mo2C. Besides, the excess metallic sodium could melt (melting point of sodium is 97.8 °C) at the reaction temperature, providing liquid medium for the formation of Mo2C.

Moreover, the effect of the reaction temperature on the formation of Mo2C nanoparticles has been investigated. The reaction temperature is a key factor in the formation of Mo2C. When the temperature is 550 °C, the main product is Mo2C with poor crystallinity (Figure a). When the temperature is below 500 °C, the product is amorphous (Figure b). Therefore, the optimum temperature for synthesis of Mo2C is about 600 °C.

Figure 5

(a) XRD pattern of the product obtained at 550 °C and (b) XRD pattern of the product obtained at 500 °C.

Three typical XRD patterns of the Mo2C products obtained at different temperatures (650, 700, and 800 °C) are shown in Figure A. Curves (a–c) are the XRD patterns of the products obtained at 650, 700, and 800 °C, respectively. All the XRD patterns in Figure A indicate that the obtained products are highly crystalline Mo2C. Along with the reaction temperature increasing, XRD peaks become sharper and the intensities of XRD peaks increase. These results indicate that the higher reaction temperature promotes the formation of molybdenum carbide of high crystallinity. TEM images of the obtained products are shown in Figure B–D. From the TEM images, all the obtained Mo2C products consist of nanoparticles. The sizes of the obtained Mo2C products increase along with the increase of the reaction temperature.

Figure 6

(A) Typical XRD patterns of the Mo2C products obtained at different temperatures for 5 h: (a) 650 (b) 700, and (c) 800 °C, TEM images of Mo2C products obtained at different temperatures (B) 650, (C) 700, and (D) 800 °C.

Conclusions

In this article, Mo2C nanoparticles have been prepared by using waste PVC as a carbon source at 600 °C in an autoclave. The effect of reaction temperature on the crystallinity of the obtained product has been studied. This synthetic method may be hopefully used to synthesize other transition-metal carbides at low temperature.

Experimental Section

Chemicals

Waste PVC used in the experiment was collected from waste PVC hose and cut into foils. Molybdenum sulfide and metallic sodium were purchased from Shanghai Chemical Reagents Company without further purification.

Preparation of Molybdenum Carbide

Typically, molybdenum sulfide (0.32 g), waste PVC (0.10 g), and metallic Na (1.80 g) were put into a stainless-steel autoclave of 20 mL capacity. After the autoclave was sealed and put into an electronic furnace, the electronic furnace was heated from room temperature to 600 °C with a heating rate of 10 °C/min and kept at 600 °C for 10 h, and then cooled to room temperature naturally. The product collected from the autoclave was washed with dilute HCl (1.0 mol/L), distilled water, and alcohol to remove the byproducts. Finally, the obtained product was dried under vacuum at 60 °C for 5 h.

Characterization

The obtained products were investigated by XRD (Philips X’Pert diffractometer with Cu Kα radiation λ = 1.54178 Å), FESEM (JEOL-JSM-6700F), and HRTEM (JEOL-2010, 200 kV). The magnetization measurement was investigated by using a SQUID magnetometer (MPMS, Quantum Design) in the temperature range of 3–25 K with an applied field of H = 10 Oe.