贮氢合金用作硝基苯电解加氢的催化电极研究

A Investigation of Hydrogen-storage Alloy as Catalytic Electrode for Hydrogenation of Nitrobenzene

贮氢合金用作硝基苯电解加氢的催化电极研究卢世刚，杨汉西，王长发（武汉大学化学系，武汉１３００７２）混合稀土贮氢合金是近年来发展起来的新型功能材料。在室温下它可以吸收比自身体积大几百倍的氢原子形成金属氢化物．这种金属氢化物可作为氢源提供高反应活性的氢原...

Since hydrogen.storage alloy ean not only absorb hydrogen atoms to form highlyreactive metal hydride as hydrogen source by electrolysis but also possesses a certain catalytic activityfor hydrogenation,it is thus feasible to utilize the allov as a novel electrOde for electrolvtichydrogenation in organic electrored uction. This work employed misch metal AB_5 type alloy as acatalytic electrode and studied the electrochemical hydrogenation of nitrobenzene. The hydrogen-storage electrede(MH)was a plastic bonded electrede sheet made from a paste ofMm(NiCoMnAl)_5 alloy powder and teflon emulsion. The electrolysis was performed at constantcurrent and controlled potential in a three electrede cell of H-type configuration. The electrolyte was0.5 mol/L KOH aqueous solution and 0.5 mol/L KOH in CH_3OH+H_2O(80:20 by vol.). Changes in the concentrations of nitrobenzene(NB)and its reaction pred ucts were monitored bysampling at different d uration of electrolysis and then analysed by HPLC and UV spectroscopy. The CV cruve of a powder microelectrode of hydrogen-storage alloy in alkaline solution showed apair of distinct oxidation-reduction current peaks of hydride formation at E=-1.1V and hydrogendesorption at E=-0.9V.In addition of NB, a strong reduction peak appeared at E=-0.7 V oncathodic scan,and the anodic peak of hydride oxidation completely disappeared on reverse scan tosuggest the participation of absorbed hydrogen in the NB red uction(Fig.3). In the electrolysis of NB-saturated alkaline solution,we used UV spectroscopy monitoring the compositional changes of theelectrolvte. It was found that with increasing the electrolysis time the concentration of NB dropedsteadily and instead the concerntration of the main product aniline (AN)went up contineously as seenby the decreasing 278 nm and increasi ng 230nm UV peaks both characteristic of nitrobenzene andaniline. By HPLC seperation and analyse of the electrolyte after electrolysis,the electrolysis yield ofAN reached up to 90%and the current efficiency calculated also exceeded 85%. Except for a verysmall amount of azoxvbenzene and azobenzene(totally about 2%content)there were no any otherside reaction products or intermediates found in the course of electrolysis,indicating the high selectivityof MH electrede for nitrobenzene hydrogenation. In order to increase the dissolubility of NB in electrolyte,we tried to use the methanol-watermixed solvent instead of aqueous solvent for the electrolytic hydrogenation of NB. In the case of usingthe mixed solvent,the main electrolysis preduct was still AN,and the AN yield and current efficiencywere 86%and 85%both almost equal to the values measured in alkaline solution. Comparising of theMH electredes before and after electrolvsis there were no any discernible changes observed in thehydrogen-storage capacity and charge-discharge properties,suggesting a quite catalytic stability in theorganic media. In summay it could be concluded that the use of MH electrode in the electrochemicalhydrogenation of nitrobenzene offers a number of advantages of high selectivity,high currentefficiency and excellent catalytic stability,this shows a great promise for ind ustrial applications.