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The electrochemical promotion of catalytic methane oxidation was studied using a false(CH4,O2,normalArfalse),normalPt| polybenzimidazole false(PBIfalse)­H3PO4|normalPt,false(H2,normalArfalse) fuel cell at 135°C. It has been found that C2H2, CO2, and water are the main oxidation products. Without polarization the yield of C2H2 was 0.9% and the yield of CO2 was 7.3%. This means that C2 open-circuit selectivity was approximately 11%. Open-circuit voltage was around 0.6 V. It has been shown that the CH4→C2H2 catalytic reaction can be electrochemically promoted at negative polarization and exhibits a clear “volcano-type” promotion behavior, meaning that there was a maximum promotion effect at a polarization of −0.15 V, or 0.45 V catalyst potential vs. a hydrogen electrode (3.8% C2H2 yield). The catalytic rate enhancement ratio, rfalse(C2false)/rnormalofalse(C2false), at this maximum was 4.2. There was no C2H2 production at polarization ⩾0.1 and ⩽−0.3 V. The yield of C2H2 decreased with decreasing temperature. Dependence of CO2 yield on polarization also showed a “volcano-type” behavior with maximum yield of 8.3% at −0.15 V polarization. The catalytic rate enhancement ratio for CO2 production, rfalse(CO2false)/rnormalofalse(CO2false), at this maximum was 1.1, which means that this catalytic reaction is only slightly affected by the electrochemical polarization. This indicates that polarization especially affects the C2 selectivity of the catalyst. The obtained data was explained by the electrochemical production of Pt-H active centers at the electrolyte-catalyst-gaseous reactant interface false(λ≫1false). © 2002 The Electrochemical Society. All rights reserved.

The electrochemical promotion of catalytic NO reduction by hydrogen was studied using a (NO, H2, Ar), Pt polybenzimidazole false(PBIfalse)­H3PO4|normalPt, false(H2, Ar) fuel cell at 135°C. A mixture of NO/H2/normalAr was used as the working mixture at one electrode and a mixture of H2/normalAr was used as reference and counter gas at the other electrode. Products of NO reduction false(N2 and H2Ofalse) were analyzed by an on-line mass spectrometer. At high NO+H2+normalAr flow rate (17 mL/min; 17 and 354 mL/min, respectively, at atmospheric pressure) the maximum rate enhancement ratio was 4.65. At low NO+H2+normalAr flow rate (17 mL/min; 17 and 140 mL/min, respectively), NO reduction increased 20 times even without polarization compared to the high gas flow rate. The electrochemical promotion effect occurs at positive polarization with a maximum increase at approximately 0.08 V and with 1.5 times the zero polarization value. The promotion at the negative polarization can be attributed to the electrochemical production of the promoters. At low gas flow rates, a charge-induced change of the strength of chemisorptive bonds can take place. © 2003 The Electrochemical Society. All rights reserved.

The electrochemical behavior of K2S2O7-KHSO4-V205, K2S207-V2O4, and K2S2O7-KHSO4-V204 melts was studied in argon and S02/air atmospheres using a gold electrode. In order to identify the voltammetric waves due to KHSO4, molten KHSO4 and mixtures of K2S2OrKHSO4 were investigated by voltammetry performed with Au and Pt electrodes in an argon atmosphere. It was shown that H reduction took place at 0.26 V vs. an AgVAg reference electrode, i.e., at a potential in between the V(V) -* V(IV) and V(IV) -* V(III) reduction stages. The presence of KHSO4 caused an increased concentration of V(III) species in the V2O5 containing molten electrolytes. This effect may be caused either by protonic promotion of the V(IV) -* V(III) reduction (V02 + 2W + e -V3 + H20) or by chemical reduction of V(IV) complexes with hydrogen, formed from W as the product of the electrochemical reduction. Both the V(V) -,V(IV) reduction and the V(IV) V(V) oxidation remained one-electron electrochemical reactions after the addition of KHSO4 (or water) to the H2S207-V2O5 melt. Water had no noticeable effect on the V(V) -s V(IV) reduction but the V(IV) -V(V) oxidation proceeded at higher polarizations in the water-containing melts in both argon and S03/air atmospheres. This effect may be explained by participation of the water molecules in the V(IV) active complexes.

The electrochemical behavior of the molten V,O,-M,S,O, (M = K, Cs, or Na) system was studied using a gold working electrode at 440°C in argon and air atmosphere. The aim of the present investigation was to find a possible correlation between the promoting effect of Cs + and Na + ions on the catalytic oxidation of SO, in the V,O,-M,S,O, system and the effect of these alkali cations on the electrochemical behavior of V,O, in the alkali pyrosulfate melts. It has been shown that Na + ions had a promoting effect on the V(V) 4 V(IV) electrochemical reaction. Sodium ions accelerate both the V(V) reduction and the V(IV) oxidation, the effect being more pronounced in the case of the V(IV) oxidation. Sodium ions also had a significant (almost 0.2 V) depolarization effect on the V(IV) -V(V) oxidation. The peak current of the V(IV) -V(V) oxidation waves vs. Na,S,0 7 concentration plots had maximums at ca. 8.5 mol % of NaS207 in air atmosphere for all the studied potential scan rates. In the Cs2S20,-K,S20, (1:1) melt the V(IV) -V(V) oxidation was affected by Cs + ions with a depolarization effect of 0.2 V and an even more significant acceleration than the in molten V,O,-Na,S2,O,-K,S,O, system. The V(IV) -V(V) oxidation peak currents were approximately 1.5 times higher than in the V2,O,-K2S20, system at all studied potential scan rates. No correlation has been found between the described effects and the electric conductivity of the systems. The rate-determining stage in the catalytic SO, oxidation most likely is the oxidation of V(IV) to V(V) and the Na + and Cs + promoting effect is based on the acceleration of this stage. It has also been proposed that voltammetric measurements can be used for fast optimization of the composition of the vanadium catalyst (which is approximately V,O,-M2S207) for sulfuric acid production.