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0.2M Na2SO4 22 0.2MNa2SO4 22 0.05M H2804 22 0.05M H2504 22 0.05M H2804 22 0.05MH2504 52 “All controlled potential electrolysis in CO2 saturated solutions. Average current density. ‘% faradaicefliciency. TABLE 2 FaradaicEfficienciesforCO andCH3OH onKOH/HF pretreated molybdenum electrodes.” Effici Time/ E/ j"/ Q/ cncyc TrialhrVvsSCEuAcm‘zcoulCO CH3OH ld 46.9 —O.52 100 16.9 0.3 77 IO — 1.1 2A 23.3 —0.8 120 14coulombsovertheperiodofonedayhadmade meth anol with a faradaic efficiency of 84%. Continuation of the electrolysis for another 2 days allowed another 22 60 coulombstobepassed.Whilethemethanolef?ciency dropped to 36% during this second period of time, methanol production did not come to a halt with an additional 14 umol ofmethanol being produced. To see ifthe drop in efficiency was due to a change in the 65 electrodesurfacecharacteristicsorduetoachangein the solution composition, another electrode was used to pass 28 coulombs over a two day period, and then used toelectrolyzeafreshsolutionwithoutsurfacepretreat 13.9 N.D.g 84 28‘ 72.5 —0.8 57 21.5 0.4 36 3A 43.4 —0.8 61 27.5 N.D.g 45 4,609,441 34 minutes.TheKOH/HF treatmentgavemorereproduc ible yields. Molybdenum bronzes were prepared ac cording to the method of R. Schollorn, et a1, Mat. Res. Bull.11(1976)83.A molybdenumtrioxidepowderwas stirredina1M solutionofsodiumdithioniteunder nitrogenfora2hourtimeperiodduringwhichthe characteristic blue color formed. The dithionite solu tion was decanted and the powder was then washed several times with water. Equipment Unless otherwise noted all experiments were done at room temperature. Cyclic voltammetry experiments were performed with a Pine Instrument Corporation modelRDE3potentiostat.AP.A.R.model172potenti 15 ostat was used with a model 179 coulometer plugin to measure charge passed during cycling. Electrolyses were performed using an Aardvark model PEC-l potentiostat and Keithly model 616 digital electrometer withastripchartrecorderformeasuringcurrentasa20 function of time. All electrolyses were carried out using a carbon dioxide gas circulating closed system as previ ouslydescribedbyD.Can?eldandK.W. Frese,Jr.,J. Electrochem. Soc. 130 (1983 ) 1772. In all cases electro lytevolumeswereeither25or50ml.Sampleswere25 analyzed on a Gowmac model 750 gas chromatograph with a FID detector. The samples for methanol were collected from the vapor in a 1 ml headspace over a 2 ml aliquot.A 6footPorpakN columnat130°C.wasused formethanolanalysiswhileacolumnofPorpakQ(6ft)30 followed by Porpak R (3 ft) at 50 ° C. was used for methane/carbonmonoxideanalysis.Atomicabsorption analysis for molybdenum was performed by Data Lab Inc.,SanJose,Calif. TABLE 2-continued FaradaicEfficienciesforCO andCH3OH onKOH/HF pretreated molybdenum electrodes.” Effici Time/ E/ j"/ Q/ ency“ Trial hr VvsSCE nAcm_2 coul CO CH3OH as! 69.8 —0.8 33 24.4 N.D.g 15 “AllinCO2saturated0.2M Na2SO4solutionat22°C. bAveragecurrentdensity. "% faradaicefiiciency. "Controlledcurrentelectrolysis. "2B isa continuation of2A aftersampling. Numbers for28 do not include electroly sisbeforesampling. f3B represents the electrolysis of a fresh solution ,with the electrode used in 3A without pretreatment. Numbers for 38 do not include electrolysis before sampling. s'ND. = not detected The faradaic efficiency for methanol formation isgood with values ranging as high as 85%. The reaction is fairly selective with the faradaic efficiency for carbon monoxide formation being, except for an occasional result,generallylessthan5%.AtKOH/HFpretreated electrodes carbon monoxide is consistently formed in lessthan3% faradaicefficiency.Fromthelastentryin Table 1 it can be seen that, in contrast to ruthenium electrodes, K.W. Frese, Jr. and S. Leach, J. Electro chem.Soc.132(1985)259,raisingthetemperatureto 52 ° C. does not change the product distribution signi? cantlyandataltemperaturesmethaneisonlyformedin trace amounts. The lack of a faradiac balance isproba bly caused by hydrogen that is produced during elec trolysisbutnotmeasuredbythe?ameionizationdetec tor on the gas chromatograph. Indeed, bubbles of gas, presumably hydrogen, can be seen forming on the elec trode surfaces. The standard potential for reduction of carbondioxidetomethanolis—0.536V vsSCE atpH 1.ThustheresultsinTable1representthereductionof carbondioxidetomethanolonly160and190mV nega tive of the standard potential for pH’s 4.2 and 1.5 re spectively. Table 2 contains the results for two extended electrol ysisexperimentsin0.2M Na2SO4atpH4.2thatwere conducted to determine ifmolybdenum electrodes con tinued to make methanol over an extended period of time.Entry2A showsthatanelectrodethathadpassed Results and Discussion Electrolysis Experiments 35 In Tables 1 and 2 the faradaic ef?ciencies for the electrolysis of carbon dioxide saturated aqueous solu tionof0.2M Na2SO4(pH4.2)or0.05M H2804(pH1.5 )withanHClpretreatedandKOH/HFpretreated(see40 experimental) molybdenum electrode are listed. As can be seen methanol ismade insigni?cant amounts atboth pH’s. TABLE 1 FaradaicEfficienciesforCH4,CO,andCH3OH onHClpretreatedmolybdenumelectrodes.” Eficiency‘: Electrolyte T/“C. E/VvsSCE jb/pLAcut-2 Q/coul cm co crnon 11.8 2 21 42 8.5 ND 3 55 50 3 1.5 23 —0.70 26 —0.80 50 —0.57 100 —0.68 550 —0.68 310 —O.6O 590 87.6 ND 0.5 21 86 0.15 0.11 3.7 18.7 ND 0.2 46PDF Image | ELECTROCHEMICAL REDUCTION OF AQUEOUS CO2 TO METHANOL
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