Effects of Ba Adsorption Layer on NO Electrochemical Reduction Reaction Mechanism

This study investigated the electrochemical reduction of NO and O₂ on Pt symmetric electrode by adding NOx adsorbents for the removal of NO from the exhausts of diesel engine or gasoline engine. O₂ is excess when compared with NO. Thus, a large mount of external electric field should be consumed by electrochemical decomposition of O₂ which gives rise to a low selectivity towards NO. Electrochemical reduction of NO has confronted with the difficult of improving NO selectivity in NO/O₂ mixtures.

Two cells were prepared and tested: single Pt electrode, Pt electrode with Ba(NO₃)₂ impregnation. Linear sweep voltammetry(0V~2V), cyclic voltammetry(CV:0.5V~-0.5V) and electrochemical impedance spectroscope(EIS) measurements were carried out between 350℃ and 550℃. The Pt electrode was covered with fine particles on the impregnated electrode surface by scanning electron microscope(SEM). It could be observed from the polarization curves that the Ba impregnated electrode showed higher electrochemical performance than the single Pt electrode especially at low temperatures and high voltages (1.25V to 2V). It can be obtained from the CV tests that the Ba impregnated electrode was superior to the Pt electrode in 800ppm NO. When scanned from 0.5V to -0.5V, the electrochemical performance of NO with O₂ was far better than NO especially at high voltages and was close to the performance of O₂. While changed the sweep direction backward, NO showed the best electrochemical performance than NO with O₂ and O₂ near open circuit voltage.  

EIS results at open circuit voltage revealed that the Ba impregnated electrode exhibited lower resistance than the single Pt electrode due to the decreased polarization resistance in the low-frequency region that dominated the impedance spectra in all atmospheres. The polarization resistances in 800ppm NO showed the smallest while in 8% O₂ showed the largest in both electrodes possibly resulted from the generated NO₂ formed by NO oxidation on Pt sites, the storage of NOx in the form of Ba(NO₃)₂ in active sites or the direct electrolysis of Ba(NO₃)₂ in external electric field that enhanced the reactivity and selectivity of NO reduction.

From the impedance analysis using by equivalent circuit model R(CR)(CR)(CR) consisted of serial resistance R_s(related with the conductivity of the electrolyte) and 3 (CR) elements. Results showed that the activation energy of R_s for the two electrodes were nearly the same. The activation energy of the total polarization resistance R_p­ for the Ba impregnated electrode and Pt electrode were 0.874eV and 1.396eV respectively in 800ppm NO and 8% O₂. The results were very close to the results obtained in the pure 8% O₂ atmosphere. And the activation energy were 0.92eV and 1.50eV for the Ba impregnated electrode and Pt electrode. This was very close to the results of O₂ reduction on Pt electrode conducted by Bauerle^[1] with the activation energy between 1.43eV and 1.78eV demonstrated that the dissociative adsorption of O₂ can be the controlling reaction step. The activation energy in the low frequency(0.01Hz-2Hz) for the three electrodes were close to the results showed by Bauerle resulted from the adsorption, surface diffusion, transfer of O₂ and NOx intermediates near/at the triple phase boundary(TPB) and the dissociative adsorption of O₂^[1]. The activation energy for the Ba impregnated electrode and Pt electrode in the high frequency were 0.608eV and 0.646eV respectively. The high frequency arc indicated the diffusion of oxide ions to the electrode/electrolyte interface and charge transfer of oxide ions from electrode/electrolyte interface to the electrolyte^[2-3]. The activation energy in the middle frequency may be attributed to the charge transfer reactions in the electrode and the dissociative adsorption of O₂. And the activation energy were 0.657eV and 1.06eV for the Ba impregnated electrode and Pt electrode. Results showed that the Ba impregnated electrode exhibited lower activation energy in middle and low frequency region indicating the improvement mechanism.

Reference

[1]J.E. Bauerle, J. Phys. Chem. Solids, 30, 2657 (1969).

[2]M. J. Jørgensen, M. Mogensen, J. Electrochem. Soc., 148, A438 (2001).

[3]M. L. Traulsenz, K. K. Hansen, J. Electrochem. Soc., 158, P156 (2011).