Microwave assisted technique and reaction model

In this paper, we made an effort of using microwave heating to purify the as-prepared MWCNTs and the results showed that it was an extremely highly efficient method to eliminate metallic catalysts embedded in carbon nanotubes. Reactions of metal catalysts of La and Ni with nitric acid are

O In comparison with those traditional methods which need longer treatment time even 2 or 3 days, the microwave-assisted purification method can remove metallic particles in relatively short time, only 15 min to remove almost 85 wt% catalysts in the as-prepared carbon nanotubes. The possible mechanism to explain why microwave-assisted purification can achieve such high efficiency is discussed below.

It is well known that microwaves are electromagnetic waves including the components of electric and magnetic field. These two components of electromagnetic waves will rapidly change their molecular motion in directions and furthermore result in the effect of warming and heating on objects. This is because molecules, semi-solid or liquid, cannot immediately respond to the variation of the electromagnetic fields in direction and hence creates the effect of molecular friction. Microwave dielectric heating, also referred to as microwave heating or dielectric heating, has been reported that the significant accelerations in reaction rate were achieved in a conventional microwave oven [182,183]. These results have attracted considerable attention on reactions accelerated by dielectric heating and thereby more multi-function microwave ovens were designed for industry or laboratory. Organic reactions, such as hydrolysis, esterification, etherification, substitution reactions and Diels-Alder reactions have been extensively investigated by microwave heating process.

Additionally, various aspects of inorganic and polymer chemistry have also been studied. Recently, rapid synthesis of metallic nanostructures in solution under microwave heating has attached considerable attention as a new promising synthesis method. Gold spherical nanoparticles, [184] polygonal plates, [185,186] nanorods and nanowires [187,188] were efficiently synthesized under microwave heating owing to thermal and non-thermal effects caused by microwave irradiation. In addition, many kinds of silver nanostructure were also successfully produced by microwave heating method [186,189].

The reason why microwave-assisted heating could achieve such high reaction rate than traditional methods might be due to thermal and non-thermal effects. Thermal effects have been described as rapid heating, hot spots or hot surface at solid-liquid interface. In the case of rapid heating effects, the acceleration in reaction rate can be significantly increased if the microwave energy is mostly absorbed by reactants

themselves, for example, metal catalysts, not by absorbents, such as solvent. It means that the selective heating of metal catalysts embedded in the tube’s tip can be achieved in microwave-assisted heating to accelerate the etching rate of catalysts in acid faster than that in other methods.

In other cases with solids involved in the reaction system, there are local hot spots or hot surfaces on solid-liquid interfaces while the bulk temperature still remained low.

The formation of hot spots or hot surfaces has been reported to accelerate the reduction of metallic precursors and the nucleation of nano-particles metal in the synthesis of metallic nanostructure [190]. Non-thermal effects were reported by Laurent et al. that microwave energy might lower the activation Gibbs free energy of reactions [191]. According to these, microwave-assisted method heats reactants rapidly and creates hot surface on solid-liquid interfaces while the bulk temperature still remained low to achieve high chemical reaction rate.

Although microwave heating can accelerate the reaction rate, another important factor in our experiment to significantly accelerate the reaction rate might be that catalysts at the apex of CNTs can be heated by themselves to higher temperature than tubes or solvents by absorbing the energy of microwave radiation. The temperature of unpurified CNTs could reach approximately 1850°C after 4 seconds of microwave radiation; however, carbon black and purified CNTs only reached 500-650 °C after 10 seconds of radiation when microwave radiation was applied both on purified and unpurified CNTs in ultrahigh vacuum [192]. It is commonly known that the temperature increase is accomplished by the reaction rate increase, because higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. In general, most chemical reactions approximately doubles its rate per 10°C increase in temperature. For this reason, the higher the temperature at the apex of CNTs, the higher purification rates of metal catalysts. In addition to the absorption

of microwave energy by metal catalysts embedded in unpurified CNTs, the pure cobalt and copper powders can also be heated up very fast in the microwave oven, about 700°C in 1-2 minutes, but there is no temperature rise for solid copper bar, even exposed in microwave field for 10 minutes. Nickel powders can be heated up to 384°C within 1 minute in microwave [193,194]. These results imply that the smaller the size of metallic particles, the higher the temperature of them when exposed in microwave field. This can not only ensure the previous explanation that metallic nanoparticles might be heated in microwave field but also attract a lot of interests on microwave sintering of metallic powders [195].

Microwave adsorption in metal resulting in rapid heating might be mainly due to magnetic resonance and interfacial electric polarization [196,197]. Magnetic resonance adsorption occurs when microwave field couples with internal magnetic moments of ferromagnetic particles, such as Fe, Co, and Ni [196]. Interfacial electric polarization adsorption occurs when microwave radiation interacts with charge multiples at the interface [196,197]. Therefore, it is believed that Ni catalysts in CNTs are ferromagnetic and adsorb microwave via magnetic resonance to reach higher temperature in extremely short time than traditional boiling method and result in high reaction rate. Interfacial dipoles within boundaries between catalysts and carbon shells absorb more microwave energy when crystallites are nanometer-size instead of micron size, since the smaller size of catalysts would have greater interfacial polarization effects [198].

According to the former discussion and experiments, we conclude that metallic nano-particles in microwave digestion system can absorb electromagnetic wave energy and be heated by themselves to form a local hot area at the tip end and by combining with hot-surface effects around the CNTs’ tip-liquid interface, result in significant reaction rate acceleration to etch away the cap of tubes and metal catalysts

to accomplish the purification of CNTs.

Although the actual temperature gradient at the tip of individual CNT in acid solution is hard to measure at present and still needs further investigation, it is believed that the temperature at the apex of unpurified CNTs might reach at least three times higher than that of side wall of CNTs or solvent and result in the purification rate acceleration. We suggest a possible reaction model of microwave assisted purification, as shown in Fig. 4-22(a)-(e). Figure 4-22(a) represents the state of CNTs without microwave irradiation applied on them, where the temperature of CNTs remain low without hot surfaces around the apex of CNTs. As microwave is applied on CNTs (Fig. 4-22(b)), the nano-sized metallic catalysts in the tip absorb the energy of microwave irradiation and the temperature of catalysts increase to higher temperature than that of the bulk. The local hot area is also produced near the tip at the same time because of the formation of hot surface in the solid-liquid interface.

These two phenomena result in local high temperature around the tip of CNTs, and the local high temperature results in reaction rate acceleration of pentagonal rings and metal catalyst with nitric acid in Fig. 4-22(c) and (d). Finally, as shown in fig. 4-22(e), the tip of CNTs is opened and the metallic catalyst embedded in tubes is also eliminated in this process, but the opening of CNTs increases with increasing treatment time in microwave-assisted purification system (shown in Fig. 4-22(f)).

Fig. 4-22(a)-(f) Reaction model of purification assisted by microwave dielectric heating for the CNTs.

4.4.4 Summary

A high-efficiency and low-temperature purification processing technique for the raw MWCNTs sample has been significantly developed. The contents of metallic catalysts in the as-prepared MWCNTs can be effectively eliminated from 10.39 wt%

to 1.515 wt% within 15 minute purification time at 120°C. A possible reaction model was apparently proposed to describe this reaction, that is, the nano-scale metallic catalysts embedded at the tip end of MWCNTs could absorb microwave radiation energy in electromagnetic field by magnetic resonance and interfacial electric polarization, and then form a localized hot area to combine with hot-surface effects around the tip-liquid interface of CNTs and significantly accelerate the reaction rate in the wall of CNTs near the tip.

Although microwave technology has been extensively applied in our daily life, there might be a great potential in microwave chemistry to apply microwave radiation

energy on traditional chemical reaction processes. However, further intensive studies are expected to acquire complete understandings of the microwave-assisted chemical reaction.

Chapter 5 Coating Pt Particles on CNTs as DMFC