Mn3O4 nanoparticles were prepared by a simple chemical route using cetyltetramethylammonium bromide (CTAB) as a template agent. Mn3O4 nanocrystals present an octahedral shape, and their crystallite size varies between 20 and 80 nm. They were characterized by XRD, SEM, DTA/TG, and IR spectroscopy. XRD studies confirm the presence of a highly crystalline Mn3O4 phase. The Rietveld refinement of the X-ray diffraction data confirms that Mn3O4 nanoparticles crystallize in the tetragonal system with space group I41/amd. DTA/TG and XRD measurements demonstrate the phase transition toward a spinel structure between 25 and 700∘C. The electrical conductivity increases between 80 and 300∘C, suggesting a semiconducting behaviour of Mn3O4. Both dielectric dispersion (ε′) and dielectric loss (ε′′) were investigated from 80 and 300∘C in the frequency range of 10 Hz–13 MHz. The dielectric properties showed typical dielectric dispersion based on the Maxwell-Wagner model.

The electrochemical reduction of nitrate and nitrite on Cu- and Pd–Cu-modified pyrolytic graphite electrodes was studied in neutral and alkaline media. The beneficial effect of Pd on the intrinsic electrocatalytic performance of the modified graphite electrode was demonstrated. In fact, the presence of Pd on the catalytic surface causes a positive potential shift in the onset of nitrate reduction current and a remarkable increase of the faradaic current. The activation energy of the nitrate electroreduction was estimated for the Cu/graphite and Pd–Cu/graphite electrodes at about 32 and <mml:math display="inline"><mml:mrow><mml:mn>18</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:mtext>kJ</mml:mtext><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mrow><mml:mtext>mol</mml:mtext></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>18kJmol−1 , respectively. The reaction rate constants were equal to <mml:math display="inline"><mml:mrow><mml:mn>0.75</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>0.75×10−4 and <mml:math display="inline"><mml:mrow><mml:mn>2.23</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:msup><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mi mathvariant="normal">s</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>2.23×10−4s−1 for the Cu and Pd–Cu surface, respectively, reflecting a more facile electron-transfer process on the latter surface. The beneficial effect of Pd on the catalytic stability is probably related to the existence of a protective Pd layer enveloping the Cu core in the electrode surface structure. The selectivity of the modified electrodes depends on the electrolysis time and potential. In neutral medium, the nitrate electroreduction mainly led to the formation of nitrite at <mml:math display="inline"><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.1</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:mi mathvariant="normal">V</mml:mi></mml:mrow></mml:math>−1.1V and ammonia at <mml:math display="inline"><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.5</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:mi mathvariant="normal">V</mml:mi></mml:mrow></mml:math>−1.5V vs Ag/AgCl. The reaction selectivity was also studied in alkaline medium at <mml:math display="inline"><mml:mrow><mml:mo>−</mml:mo><mml:mn>1.1</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:mi mathvariant="normal">V</mml:mi></mml:mrow></mml:math>−1.1V for the Pd–Cu/graphite electrode only. In this case, the maximal selectivity for the production of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">N</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>N2 was 70% and was achieved with a surface composition of 95 atom % Pd–5 atom % Cu.