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4,609,441 56 ment. During the ?rst period (entry 3 A) methanol was produced with an ef?ciency of 45%. During the second period (entry 3 B) the faradaic ef?ciency declined to 15% indicatingthatthedropinelectrodeef?ciencyis due to a change in the electrode surface characteristics. A possibleexplanationforthedropinef?cienciesisthe depositionofimpurities,suchasHg orAs,fromthe electrolyteduringelectrolysis. FIG. 1a shows the current-time characteristics of electrolysis3A above.Thelargeinitialcurrentdropis characteristicofalelectrolysesatpH 4.2.Itispossible that the drop in current primarily results from the same effect that causes a decrease in the ef?ciency during extendedelectrolysis.InFIG. 1bwe seethecurrent time characteristics of electrolysis 3 B with the elec trode which had previously been used for electrolysis 3 A (representedinFIG.1a)andwasusedagaininafresh electrolyte without pretreatment. The initial drop in current is an order of magnitude less than for a “fresh” electrode indicating that the drop is indeed due to a change in the surface characteristics of the electrode. It is also possible that a decline in the rate of hydrogen evolution,causedbythebuildupofasigni?canthydro genpressure,contributestothecurrentdrop. CorrosionExperiments If a molybdenum wire is placed at open circuit in a carbondioxidesaturated0.2M Na;S04,pH4.2solu tion carbon monoxide isformed. However no methanol is detected (Table 3 ) at uncycled electrodes vide infra. TheKOH/HF pretreatmentleadstofastercarbonmon oxide formation than the HCl treatment, but in both casesthe rate of carbon monoxide formation (expressed in terms of the current necessary to drive the reduction at that rate) is less than 1 uA. The carbon monoxide formation is probably caused by oxidation of the metal tomolybdenum dioxide(eq.2).The reductionofcar bon dioxide to carbon monoxide by molybdenum metal to M0 + 2COg<-_*MoO2+2C0 (2) form molybdenum dioxide is downhill (Atlas of Elec trochemical Equilibria in Aqueous Solutions, M. Pour baix, National Association of Corrosion Engineers, Houston, Texas) by - 18 kJ mol-1 . TABLE 3-continued Open Circuit Corrosion of Molybdenum Wire by Aqueous Carbon Dioxide at Room Temperature” Thepotentiodynamiccharacteristicsofmolybdenum50 rnirrlComparedtothe6V min-1curveinFIG.2 foil electrodes were also used to characterize the open circuit reaction with carbon dioxide. A molybdenum electrode was pretreated by dipping in concentrated HClfor10minutes.Theelectrodewasplacedinanitro gensaturated0.2M Na2SO4,pH=4.2aqueoussolution at open circuit for one hour. Then the cyclicvoltamme try was measured, the results of which are shown in FIG. 2. TABLE 3 Open Circuit Corrosion of Molybdenum Wire by Aqueous Carbon Dioxide at Room Temperature“ Time/ co/ CH3OH/ jcb/ (?fth from the top) it can be seen that the anodic and cathodic currents were increased after exposure to aqueous carbon dioxide at open circuit. The difference inpeakcurrentis0.13mA or0.037mA cm—1.A some whatlargerincreaseof0.12mA wasobservedforthe cathodic peak current at 6 V min-1. In general, in creases in current were observed for all sweep rates in both the anodic and cathodic directions. The reversible potential for the larger anodic peak was again found to 60 correspondtoreaction3.Dissolvedcarbondioxideor carbonic acid thus appear to cause the oxidation of the molybdenum electrode at open circuit. In view of the ?ndingthatCO isformedatopencircuit,thepotentio dynamicdatacon?rmthatCO2reductionandmolybde 65 numoxidationoccursimultaneously.Themolybdenum oxide ?lm thickness and the rate of corrosion by carbon dioxide during the ?rst hour can be estimated as fol lows. Molybdenum has a face centered cubic structure Pretreatment hr umoles HC] 66.2 0.31 KOH/l-IF 46.4 3.1 118 3.1 HCl‘’ 117 N.M.E umoles N.D.C ND.‘ ND.‘ < 1 uA cm_2 0.04 0.8 0.31 _ Pretreatment Time/ 00/ CH3OH/ jcb/ hr umoles umoles uA cm-2 l4] 2.l 6.5 l.5 "All5cm2molybdenumwireina0.2M Na;SO4carbondioxidesaturatedaqueous solution. bAverage free corrosion current for time period indicated. ‘N.D. = none detected. dCycledbetween—1.2and+0.2V vsSCEbeforeexperiment. eN.M. : not measured. The potential was scanned from —0.1 to —0.8 V vs SCEand1.0to10.0V min-1atequalincrementsof sweeprate.A sweeprateof0.5V min—1isalsoshown. The most distinctive feature is the large anodic peak around —0.6 V; a second smaller peak was observed near —0.3 V. In the cathodic direction the complimen tary peaks are poorly resolved, but there are strong suggestions of reduction processes at —0.7 and —0.45 The peak potential of the anodic process near —0.6 V was plotted against the log of the sweep rate (B. E. Conway, H. Angerstein-Kozlowska, and F. C. Hu, J. Vac, Sci,& Tech. 14(1977 )351 ).The reversiblepoten tialwasfoundtobe—0.59V vsSCE.Thenextstepin the analysis is to compare this value with thermody namic data for oxidation of molybdenum species. Ac cording to the thermodynamic data for equation 3, the standardpotentialforthehalfreactionis—0.57V vs SCE Mo + 2H2O2MoO2+4e* +2H+ (3) (B. E. Conway, H. Angerstein-Kozlowska and F. C. Hu,J.Vac,Sci,&Tech.14(1977)351)atpH :42. Theagreementwithourresultof—0.59V suggeststhat we areobservingtheoxidationandreductionofmolyb denum metal and a molybdenum dioxide ?lm. The re sults do not show ifthe oxide originates from the pre treatment with HCl or open circuit corrosion under nitrogen. An electrode scanned immediately after a KOH/HF pretreatment shows much smaller peaks (FIG.5). After a similar pretreatment the same experiment was repeated with nitrogen replaced by carbon dioxide. After one hour at open circuit the cyclic voltammetry shown in FIG. 3 was obtained. The curve with the largest current corresponds to a sweep rate of 6 VPDF Image | ELECTROCHEMICAL REDUCTION OF AQUEOUS CO2 TO METHANOL
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