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TABLE 5_c°minued Effect of Cycling Time on the Faradaic Efficiencies for CO, and Cl-lzOl-Iofmolybdenum electrodes.” cycling E/ 117 Q/ Emciencyc a carbon dioxide saturated 0.2 M Na2SO4, pH :42 aqueous solution one can see, Table 6, that methanol and traces of carbon monoxide are indeed formed. To show that reduction was not occurring by residual di time/min SCE “Acm-z coul co 25 _0 8 no 18 27 CHSOH 5 thioniteabsorbedtothepowderasolutionofdithionite 61 was saturated with carbon dioxide or methanol was 4,609,441 9 10 "All11a 1am 10 d+0zv SCEbef 111' formed(Table6)' CBCTO 85WBI‘Bcyce eween — . an t VS OTBeCCX‘Oys1s. ElectrolysesinCO2saturated0.2M NagSQ;aqueoussolution. bAveragecurentdensity. 2%faradaiceficiency. Same data as presented in experiment 2A of Table 2. em)=no,datum /This entry is a continuation of the previous experiment. Values are cumulative. gN'M' : “m measured‘ The effects of longer cycling times are seen in FIG. 7 which shows a different electrode cycled for 25 min utes.Thecathodicpeakat-—0.85V eventuallydisap pears after l0 —l5 cycles. The peak due to cathodic formation of metal from metal ocide at 0.6 V, and the 1 - . . The reduction of carbon dloxlde by molybdenum dioxide (eq 4 )to form carbon monoxide and 10 MoOz+COg+HgO H2Mo04+CO (4) molybdic acid (H2moO4) cannot be ruled out since itis downhill(AtlasofElectrochemicalEquilibriainAque 15 ous Solutions, M. Pourbaix, National Association of Corrosion Engineers, Houston, Texas) by 192 kJ mol-l. Ifmolybdenum dioxide powder issuspended in acarbondioxidesaturated0.2M Na2SO4,pH4.2aque ous solution small amounts of carbon monoxide are complimentary anodic peak at -0.55 V, grow with timethoughtherateofgrowthslowswithlongercy 20formedandnomethanolisdetected(Table6).Even cling times. Also, the current due to oxidation of the molybdenum electrode at +0.1 V declines. The effect of cycling time on the faradaic efficiency for methanol production can be seen from the data in Table 5. Apparently there is an optimum cycling time and ifan electrode iscycled too long itbecomes passiv ated. Comparison of the data in Table 5 and in FIGS. 6 and 7 indicates that the > 100% efficiencies are associ though molybronzes can reduce carbon dioxide to methanol it will be shown below that there would be insuf?cient bronze formed on the electrode surface to account for the extra methanol. 25 Molybdenumdioxidecanonlydonatetwoelectrons for carbon dioxide reduction while the molybdenum bronze can supply about 1 electron per molybdenum center. Since oxidation of molybdenum metal to melyb denum dioxide isa four electron process the amount of atedwiththedeclineinthepeakat—0.85V.Thepas 30chargeavailableforreductionofcarbondioxidebya sivation caused by extended cycling is associated with the growth in the molybdenum dioxide peak at —0.6 V (FIG.7)indicatingthatitisthecoverageofthesurface with a passivating oxide layer that causes passivation. surface ?lm can be greater than the anodic charge passed creating molybdenum dioxide. The ?fth entry in Table 5 shows the data for an electrode that was cycled for 15 minutes and showed a faradaic 'ef?ciency of Since more methanol isproduced than ispossible on 35 246% when used for electrolysis. The anodic charge thebasisofthechargepassedduringelectrolysisthere passedduringcyclingthiselectrodewasonly~0.5 must be a source of electrons in addition to those sup plied by the external circuit. The only two reasonable sources are from the “storage” of reducing equivalents coul, far less than the 15 coulombs needed to account for the methanol produced in excess of 100% faradaic ef?ciency. Values in the 0.1 to 0.2 coul range for the on the surface through the formation of a reducing 40 anodic charge passed during cycling are typical for surface ?lm during the cycling pretreatment, or from corrosionofthemetal.A reducingsurfacecouldoccur by the formation ofmolybdenum dioxide on the surface oftheelectrodeduringtheanodicportionofthecycle which isthen reduced during cycling to a species capa ble of reducing carbon dioxide to methanol. A likelycandidateistheformationofamolybdenum bronze. Molybdenum bronzes are formed from the par tial reduction, either chemical or electrochemical, of suchexperiments.Sincethecoulombsmeasuredduring cycling cannot account for the extra methanol formed the coulombs must come from the molybdenum metal. Thus, the effect of cycling is to activate the molybde 45 nummetalforcorrosion.Consistentwiththeseresultsis the fact that, although uncycled molybdenum elec trodes will not make methanol at open circuit, open circuitcorrosionofcycledmolybdenum wirewillmake methanol (Table 3 ).The free corrosion current density molybdenumtrioxidewhichcanbeformed,alongwith 50 isalsohigherforcycledelectrodesandthereisnoindi molybdenumdioxide,duringtheelectrochemicaloxida cation of passivation against corrosion with time. TABLE 6 Chemical Reduction of carbon dioxide.” Time/ CH3OH/ CO/ Reductant Amount hr pmol [.LmOlg-lhr-1 umol pmolg-lhr_1 M002!’ 2 g Molybronzeb 1g Na2S2O4 0.05 M 24 ND." 0 0.025 40 18 0.4 0.044 61 ND.‘ 0 N.D.‘-‘ 0.0005 0.001 0 "In carbon dioxide saturated aqueous solution. In all cases no methane was detected. bln0.2M Na2S04pH = 4.2. ‘ND. = notdetected. tion of molybdenum metal. Such materials are metallic or semimetallic in character and contain large amounts of hydrogen (R. Schollhorn and R. Kuhlmann, Mat. CONCLUSIONS Res.Bull.11(1976)83)makingthemplausiblecandi-65 Ithasbeenshownthatcarbondioxidecanbecon datesforthereductionofcarbondioxide.Ifasampleof verted to methanol with good selectivity and yield molybdenumbronzepowder(producedfromdithionite underappropriateconditions.Thefaradaicef?ciency reduction of molybdenum trioxide powder) is added to depends on several factors including chemical surfacePDF Image | ELECTROCHEMICAL REDUCTION OF AQUEOUS CO2 TO METHANOL
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