Electrocatalytic Activity of Pt/C Electrodes for Ethanol Oxidation in Vapor Phase

CHEM. RES. CHINESE U. 2005, 21(5), 597-600Electrocatalytic Activity of Pt/c Electrodes forEthanol Oxidation in Vapor PhaseLIANG Hong2"', YE Dai-qi' and LIN Wei-ming 21. School of Biological and Chemical Engineering, Guangzhou UniversityGuangzhou 510091,.r. china2. School of Environment Science and Engineering South China University of TechnologyGuangzhou 510640, P.R. ChinaReceived Nov, 19,2004High performance platinized-carbon electrodes have been developed for the electrocatalyticcetaldehyde in electrogenerative processes. A load current density of the electrode can be achieved as high as 600mA per square centimeter for oxygen reducing in 3 mol/L sulfuric acid with a good stability. With these electrodesand sulfuric acid as an electrolyte in fuel cells ethanol vapor carried by nitrogen gas can be oxidized selectively toacetaldehyde. Selectivity of acetaldehyde depends on the potential of the cell and the feed rate of ethanol vapor andcan be more than 80% under optimized conditions. The initial product of ethanol oxidized on a platinized-carbonelectrode is acetaldehyde and the ethanol oxidation mechanism is discusseds Ethanol, Platinum, Electrocatalytic oxidation, AcetaldehydeArticle|D10059040(2005)05-597404Introductionsubject of numerous investigations with the main purThe selective oxidation of ethanol to acetaldehyde pose to elucidate the mechanism with the double-layerspecial interest in the areas where biomass-based region on platinum, but is has not been well undereconomics are under development since acetaldehyde stood. Most reports about the oxidations of ethanecan be used for the syntheses of other basic chemicals acetaldehyde in literature have dealt with dissolved sub-such as acetic acid, acetic anhydride, n-butanol, strate systems while very few have been devoted to theetc.I-6. With the advancement of fuel cell technology investigation of vapor phase oxidation on gas porousand electrocatalysis stimulated by the world energy and electrodes and the recovery of acetaldehyde hasraw material situation, some special opportunities have pursued in only a few instances[ l0. The work reportedbeen provided for investigating electrosynthesis of or- here is on the electrogenerative oxidation of ethanolganic chemicals not only without requiring a power with air on porous platinized-carbon electrodessource but withgeneratioExperimentalElectrogenerative processes produce desired chemicals 1 Preparation of Catalyst and Electrodeson generating electricity. However, chemical synthe-Active carbon chosen as the catalyst' s support wassis ,i.e. producing desired chemicals, is the primary subjected to repeating gravity separation with a saturatmotivation and electricity is the by-product. Apart from ed calcium chloride solution and" Soxhlet"treatmentenergy recovery there are possibilities of controlling re- with azeotropic hydrochloric acid and the ash contentaction rates as well as selectivity through potentials and of the treated carbon was found to be less than 0. 03%other electrocatalytic means. For its high free energy Then the treated carbon was activated by heating inchange and using ai( O2)as one of the reactants the CO2 atmosphere at 900 C for 4 h. Theelectrogenerative oxidation of ethanol to acetaldehyde is the activated carbon obtained by the bet method wasa very promising electrogenerative process. Qian Yong- found to be more than 1500 m/g. Acetylene black10] studied the electrochemical behavior of used中国煤化Pt/C electrodes. Shen Mu-zhong et al L l investigated hydroC N MH Go subjected to the prethe regulation of power conversion in fuel cells. The treatment with the similar procedure as that for activeelectrooxidation of ethanol on platinum has been the carbon. The BET surface area of gas activated acety∴7摇m时时ted Partially by the Ministry of Education of Chidence should be addressed. E-mail cedaqiye@ scut. edu. enCHEM. RES. CHINESE UVol 21lene black was about 500 m/g. The catalyst was preAir(O3)Ethanol+Npared by depositing platinum from chloroplatinic acidonto the active carbon by sodium formate reduction witha platinum loading of 10%( mass fraction ) The subselent heat treatment was conducted in a quartz tube for3 h at 300C in hydrogen atmosphere. The electrodeswere manufactured by pressing an Ni screen( about 80Electrolytenesh), a diffusion membrane and a catalytic mem-brane together at about 4.9 x 10 Pa for one minuteFig. 2 The electrogenerative cell working atThe diffusion membrane was made by the following pro-room temperature.cedure: a 60%( mass fraction ) Teflon( PtFE) emulthe gas activated acetylene black and an"OP" 1-cm thick electrolyte chamber containing 3 mol/Lemulsifier with the mass ratio of 10: 4. 2 were mixed. H, SO, that could be forced to flow during the measure-Then to it was added a suitable volume of distilled wa- ment. The substrate carrier gas N, passes through a gaster until it became a dough, then it was rolled carefully washing bottle with about 100 mL ethanol ligand repeatedly until a very thin and uniform membrane bubble type flow meter is used to measure the flow ratesas obtained with a thickness of ca. 0.5 mm. The reby diverting streams before and after the cell. The 0sulted film was subjected to repeating" Soxhlet"treat- flow rate at the cathode exceeds the required flow ratement with acetone for 48 h in order to remove the oP for complete combustion at the anode. Analyses of theand to create the fine pores within the membrane. The inlet and outlet streams were carried out by using a Shicatalytic membrane was manufactured by following the madzu 7 A gas chromatograph equipped with a 3 m x3same procedure as that for the diffusion film, but the mm carbowax 1500 column. Prod o r0 was analyzedTeflon content was about 20% and the thickness was with a porapak Q column. Thele polarization curves re-about 0. 2 mm. The resulted platinum loading of cata- ported here are the second ones taken after a period oflytic membrane was found to be about 6-7 mg/cmcell operation. The third polarization experiment gave2 Electrochemical Cellscomparable results. The current and potential valuesThe electrochemical performances of the electrodes were measured with an instrument in the external cir-were tested in an electrochemical cell( Fig. 1) by re- cuit( Fig. 2 )on an operation after about 10 min when aducing O2 at room temperature. The current-potential relative stable potential was reached. The reported cellcurves were obtained galvanostatically and were not IR potential was IR corrected by the cell resistance valuescorrected. Each experimental run was repeated with measured under hydrogen at both the electrodes beforedifferent electrodes of the same type in order to ascer- the polarization experiments were initiatedtain the reproducibility of the data. The lifetime tests Results and Discussionwere conducted at the optimized electrodes loaded withacceptable load currents over an extended duration and 1 Electrochemical Performances of ElectrodesThe current-potential curves for O, reducing in 3acconpanthe electrode potential mol/L H, $ 4 at room temperature are given in Fig 3rere recorded. The electrogenerative cell and the syFour types of electrodes were evaluated. Two of thestem are represented in Fig. 2, which could be operated fPt catalyst and the otherat different temperatures. Two porous electrodes withtwo were not gas activated. Except these differences theabout 5. 5 cm of an exposed area are separated bymanufacture procedure and measurement conditionsAir(Og)were the same. It was clearly shown that the loading ofCounter electrodeelectrodePt enhances the electrode's performance significantly.Pt content, the gas activatedones中国煤化工 mance. The platinizedCN MH Gore cost-effectively byenhancing their catalytic activity via a highly dispersedmol/L H2SO4platinized platinum catalyst though with a relative lowerPt loading. From Fig 3, one can see that when the gasFig. 1 The electrochemical cell for evaluating万方提 erformances of the electrodeactivated high-surface-area active carbon with well dis-persed platinum black was deposited on an electrodeLIANG Hong et alusing pure 0, as reported in literature[ 2 but much lower than the calculated one( 1. 05 V). This is because atan open circuit a significant amount of cell voltage canbe lost initially due to the rest overpotential especiallyat the air electrode. The decrease of the potential with0.2the first current is due to the activation overpotentialHowever, the open circuic rest )overpotential and activation overpotential can be reduced with an effectiveI/(mA·cm-2)electrocatalyst at one or both electrodesig. 3 Current-potential curves for O, reducing in 3Table 1 Selected results of ethanol oxidation at plati-moLH2SO4at20℃.nized-carbon electrodes in 3 moVL H soa. Untreated carbon 6. gas activated carbont/℃R/ΩENI(mA·cm-2)P,/V"F(m:min-1x%yc. 10%Pt on a: d. 10% Pt on b200.580,.4withstand load current up to 600 mA/cm". The current200.580.36200.580.16density output for the electrooxidation of ethanol is also0.74400.520.4320comparable to the value 50 mA/cm)reported in liter400.520.32400.58ature though the electrode has a lower Pt catalyst load0.52ing. The electron micrograph shows that the gas activa600.480.42ted one has uniform pore and Pt particle size distribu600.480.38tion while the inactivated one shows the formation of600.480.160.74platinum metal agglomerates, which results in the rea. P. is the anode potential estimated from the overpotential of theair electrode and the cell's operating voltage b. S is the selectivity ofduction of active surface area and then the poor activi- acetaldehyde from the gas phase productty.The lifetime tests conducted on the optimized eleFigs. 5 and 6 show the effects of the increase oftrode, as shown in Fig. 4, demonstrated a very good substrate's flow rate and cell's temperature on polariza-stability of this type of electrodes. The running hydro- tion curves. Since the reactions take place on the intergen-ai( O2 fuel cell before and after every electrogen- face between the gas-electrocatalyst and the electrolyteerative oxidation of ethanol could give reproducible cur- the basic challenge is to provide effective transportrent and potential outputs indicating that the elec- paths advantageous to reactant to entering removal oftrodes are not affected by experiment time. This result products and current output. Fast flow rates can enagrees with the lifetime test results shown in Fig 4hance mass transfer and the operation performance islence improved significantly as shown in Fig. 5, indicating that the transfer of ethanol molecules to catalystthrough electrode inner pores is the critical factodetermining the whole process rate. A temperature increase is expected to enhance the mass transfer and thereaction rate and to cause a decrease in cell 's ohmicoss, resulting in the increase in the current output asy adsorbe72actants, the reaction rate may significantly increaseFig 4 Lifetime tests for 10 Pt on the gas activawith temperature increasing. However, for reactantsed active carbon electrodeSorbed strongly on the electrocatalyst, the temperaThe measurement conditions are the same as those in Fig 3ture increase will probably have a small effect. Since2 Electrogenerative Oxidation of ethanolthe effect of raising temperature is not so significantThe selected results of several experiments togethV凵中国煤化工 Pt active sites but noter with experimental parameters are listed in TableCNMHG of oxidation in the gasFor the electrogenerative oxidation of ethanol, the phase were acetaldehyde and CO2, while no CO hasmeasured open circuit potential is about 0. 64-0 65 been detected. At the electrolyte side no significantV. By using air instead of pure oxygen as one of the re- by-products such as CH3 COOH and diethyl acetal werctants, the measured open circuit potential values produced. The selectivity of the oxidation of ethanol to(0.. 65 4 are lower than those( 0. 67v)by give acetaldehyde is determined by the cells potentialCHEM. RES. CHINESE UVol 21世gen, and at a little higher potential CO, was producedparallelly. In our experiment the ethanol anode wasoperated in the potential range of 0. 50-0. 80 V( Table1). Because the ohmic and concentration polarizationscan develop along the pores it should work at an lowerpotential than those values. At a potential in thisrange, Pt is known not to undergo surface oxidation orFig 5 Efect of flow rate on operation characteristicsoxide film formation ,i. e., the PtOh or PtO speciesat room temperatureshould not be formed significantly. Based on the factthat etoh will be chemisorbed on pt and on the resultsof oxidation product analyses, for EtOH being oxidizedto acetaldehyde a mechanism consistent with the abovefollowed by a electrochemical step is proposedCH, CH,OH -CH, CHOH+H++e (1)I/mA·cm-2)PtFig 6 Effect of temperature on operation characteris-CH3CHOH—→CH3CHO+H++e(2)ties at a flow rate of 60 mL/ minAs a fast flow rate improves the selectivity of acetthe substrate flow rate and the reaction temperaturealdehyde which has a similar adsorption and oxidationThe higher the temperature, the faster the reactionproperty it seems that it is further oxidized to COrate, while the lower the selectivity because the acetaldehyde is oxidized to CO, before it can be desorbedCH3CHO—+CH3CHO(3)from catalytic sites. The faster the flow rate the fasterCH3CH,+3H2O—+2CO2+10H++10e(4)the mass transfer and the higher the selectivity becausefresh ethanol tends to dislodge and to syReferenceshyde from the catalytic sites. The lower the cell,s volt- 1 1 Palsson B. 0., Fathi-Afahar S.,Rudd D. F,et al.,Scienceage, the larger the current output and then the lower1981,213,5132] Hitmi H. Belgsir E. M., Leger J. M., et al., Electrochimthe selectivity again because acetaldehyde is oxidized tota,1994co before it is desorbed at a faster reaction rate3 Jungwon Shin, Wade J. Tornquist, Card Korzeniewski et alIt is essential to employ porous electrodes sindurface Science, 1996, 364, 122the electrode current is attainable under given condi4] Schmidt V. M., Lanniello R. Pastor E. et al. ,J. PhysChem.,1996,/0045),179tions which is not only a function of the potential but [5] Lanuiello rRodriguez J. L., et aL. ,J.also dependent on the surface available for the reactionElectronal. Chem. 1999/167to occur. However, this leads to the complexity of the 6] Tremiliosi-Filho G., Gouzalez E. R., Motheo A. J, , et al. , Jprocess and makes it difficult to study theElectroanal. Chem. 1998, 444 31mechanism since the reactants entering and product7] Fujiwara Nmming A. U.,J. Electroanal.Chem.,1999,472,120removal through micropores which is the mass transfer [8 Neto A. 0., Giz M. J., Perez J., et al., J. Electrochemor diffusion are necessary and these will influence theprocess significantly. A lot of investigations have been [9 I Ma Guo-xian, Tang Ya-wen, Yang Hui et al. Acta Physdevoted to the study of the ethanol oxidation mechaChem.Sin.,2003,19,100ion that in an acid. 10] Qian Yong-gui, Sun Zi-jie, Hu Jing-bo,et al., Chem. Res. Chiese Uniersities, 2004, 20 1), 103ic solution and at a potential Erhe <0. 9 V, ethanol can [11] Shen Mu-zhong, Zhang J., Scott K.,Chem. Res. Chineseonly be oxidized to acetaldehyde[ 3]. Snell et al. [131Universities,2004,204),466suggested that adsorbed ethanol reacted with the PtOH [12]中国煤化工M.J.,Pnr&l.Cmemor PtO species to give acetaldehyde. However, WillsauCNMH GELectrochim. Acta, 1982 2et al. [14] thought that ethanol was oxidized to acetalde1688hyde directly at a lower potential by the cleavages of 14 Willsou J., Heitbaum J.,J. Electrochem. Soc., 1985, 27one hydrogen of the a-C-atom and the hydroxyl hydro