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4,609,441 12 ELECTROCHEMICAL REDUCTION OF AQUEOUSCARBONDIOXIDETOMETHANOL TECHNICAL FIELD The invention relates to the electrochemical reduc tion of aqueous carbon dioxide to form methanol. BACKGROUND The reduction of carbon dioxide at semiconductor electrodes has been the focus of study in recent years with the goal of developing a system for the storage of solarenergyascarbonbasedfuel.One suchfuelismeth anol which can be used to replace hydrocarbon fuels without major adjustments. Metal electrodes have also been studied since they offer the possibility of superior catalytic properties and can be coupled to solid state photoelectrical cells for solar energy storage. The re duction of carbon dioxide to methanol has been demon strated on p-GaP, n- and p-GaAs, n-Si and P-InP (M. Halmann,Nature275(1978)115,T.Inoue,etal,Nature 277 (1979 ) 637, D. Can?eld, et al, J. Electrochem. Soc. 130 (1983 ) 1772 and D. Can?eld, et al, J. Electrochem. Soc. 131 (1984) 2518. Reduction atn-GaAs isselective with nearly 100% faradaic efficiencies. Unfortunately, atpH4.2apotentialof—1.2V to~—1.4V vsSCEis necessary to drive the reduction with high faradaic efficiencies although the standard potential for the re duction of carbon dioxide to methanol, derived from the value for heat of formation found in Selective Val ues of Chemical Thermodynamic Properties, United States National Bureau of Standards, Technical Note No.290-2,atpH4.2isonly-—O.563V vs.SCE.Semi conductor electrodes are also inherently susceptible to corrosion (A. J. Bard, et al, J. Electrochem. Soc. 124 (1979) 1706 and H. Gerischer, J. Electroanal. Chem. 82 (1977 ) 133 ). Indeed, some of the materials that have been shown to make methanol suffer from corrosion in parallel with the electrocatalytic reduction of carbon 40 dioxide. FIG. 1 illustrates current/time plots for controlled potential electrolysis of carbon dioxide in 0.2 NagSO4, pH4.2at—0.8Vvs.SCEat(a)aKOH/HFpretreated molybdenum electrode and (b) unpretreated molybde num electrodepreviouslyusedforelectrolysiswiththe electrode area being 2.9 cm2; FIG. 2 illustrates cyclic voltammetry for molybde numelectrodeinnitrogensaturated0.2M Na2SO4,pH 4.2 aqueous solution, electrode being HCl pretreated and allowed to sit at open circuit for one hour before scanning,sweepratebeing0.5to10V perminuteand theelectrodeareabeing3.4cm2; FIG. 3 illustrates cyclic voltammetry for a molybde numelectrodeincarbondioxidesaturated0.2M Na; S04, pH 4.2 aqueous solution, electrode being HCl pretreated and allowed to sit at open circuit for one hourbeforescanning,sweepratebeing0.5to6V per minute and the electrode area being 3.4 cm2; FIG. 4 illustrates free corrosion potential versus time for a KOH/HF pretreated molybdenum electrode in carbon dioxide saturated aqueous solution having a pH 4.2andbeing0.2M inNa;S04; FIG.5illustratescyclicvoltammetryat2.5V per minute in nitrogen saturated, pH 4.2, 0.2 M Na2SO4 aqueoussolutionof(a)KOH/HF pretreatedmolybde numelectrodeand(b)KOH/HF pretreatedmolybde num electrodeusedforelectrolysisofacarbondioxide saturated,pH4.2,0.2M Na2SO4aqueoussolutionfor76 hoursat—0.8V vs.SCE; FIG. 6illustratescyclicvoltammetryofaKOH/HF pretreated, 2.9 cm2 molybdenum electrodes being cy cledinacarbondioxidesaturated,pH 4.2,0.2M Nag S04 aqueous solution for 7 minutes; and 7 FIG. 7 illustrates cyclic voltammetry of a KOH/I-IFv pretreated,3.0cm2molybdenumelectrodebeingcycled inacarbondioxidesaturated,pH4.2,0.2M Na2SO4 aqueous solution for 25 minutes. BEST FOR CARRYING OUT INVENTION The reductionofcarbondioxidetomethanolisrepre sentedbytheequation: Previous electrolytic reduction of carbon dioxide at metal electrodes has led to reduction of the carbon dioxide to formic acid or carbon monoxide when the electrodeswerePd,PtandHg.(W.Paik,etal,Electro chimica Acta 19 (1969 ) 1217, V. Kaiser, et al, Br. dB. Gesellschaft 77 (1973 ) 818, T. N. Andersen, et al, Stud. Trop. Oceanogr. 5 (1965 ) 229, P. G. Russel, et al, J. Electrochem. Soc. 124 (1977 ) 1329 and S. Kapusta, et al, J. Electrochem. Soc. 130 (1983 ) 607 ). The present invention isdirected to overcoming one or more of the problems as set forth above. DISCLOSURE OF INVENTION In accordance with the present invention a method is set forth of producing methanol from carbon dioxide comprisingelectrolyzingasolutionofcarbondioxidein an aqueous solvent having an electrolyte dissolved thereinandutilizingacathodewhichcomprisesmolyb denurn. The method assetforthabovehasfaradaicefficien ciesofoverabout50% andattimesnearly100%.The potentialsfortheelectrolysisaregenerallylessthan200 mV negativeofthestandardpotentialcorrectedforpH. DESCRIPTION OF DRAWINGS The invention will be better understood by reference tothe?guresofthedrawingswherein: c02+6H++6e-.—_>cH30H+I-l20- (1) In accordance with the present invention the above 45 reactioniscarriedoututilizingamolybdenumcathode. A molybdenum cathode can reduce carbon dioxide to methanol selectively and with up to 80 to 100% faradaic efficiency. Such reductions can occur, for example, at -—O.7V vs.SCEatpH4.2,only160mV negativeofthe standardpotentialcorrectedforpH. The followingexperimentaldataalrepresentactual experimentswillservetogiveabetterunderstandingof theinvention. EXPERIMENTAL Materials Allsolutionswereeither0.2M reagentgradesodium sulfateor0.05M reagentgradesulfuricacidindistilled 60 deionized. Electrodes were prepared by mounting ei thermolybdenumfoilorwireattheendofaCuwirein a glass tube and sealing with epoxy. The electrodes were cleaned either by dipping in concentrated hydro chloric acid or by rubbing the electrode with a cotton 65 tipapplicatorsaturatedwithconcentratedsodiumhy droxide several times, followed by sonicating the elec trode in the sodium hydroxide solution for ten minutes and?nallysoakingin30% hydro?uoricacidforfivePDF Image | ELECTROCHEMICAL REDUCTION OF AQUEOUS CO2 TO METHANOL
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