电化学直接还原

In situ synchrotron diffraction of the electrochemical reductionpathway of TiO 2qR.Bhagat a,*,D.Dye b ,S.L.Raghunathan b ,R.J.Talling b ,D.Inman b ,B.K.Jackson b ,K.K.Rao c ,R.J.Dashwood aaUniversity of Warwick,Coventry CV47AL,UK bImperial College,London SW72AZ,UK cMetalysis,Rotherham S635DB,UKReceived 2November 2009;received in revised form 17May 2010;accepted 20May 2010Available online 16June 2010AbstractDespite over ten years of work into the low-cost electrowinning of Ti direct from the oxide,the reduction sequence of TiO 2pellets in molten CaCl 2has been the subject of debate,particularly as the reduction pathway has been inferred from ex situ studies.Here,for the first time white beam synchrotron X-ray diffraction is used to characterize the phases that form in situ during reduction and with $100l m resolution.It is found that TiO 2becomes sub-stoichiometric very early in reduction,facilitating the ionic conduction of O ions,that CaTiO 3persists to nearly the end of the process and that,finally,CaO forms just before completion of the process.The method is quite generally applicable to the in situ study of industrial chemical processes.Implications for the industrial scale-up of this method for the low-cost production of Ti are drawn.Ó2010Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.Keywords:Electrochemistry;Titanium;Synchrotron radiation;XRD;Phase transformation kinetics1.IntroductionIn the FFC Cambridge process,metal oxides are electro-chemically reduced to metal using a molten chloride flux [1].Currently the Kroll process is used to produce Ti from rutile [2];potentially,the FFC process may lead to a step change in the cost of extraction of this and other alloy systems.The process involves the progressive reduction and deox-idation of a porous TiO 2pellet cathode in a molten halide salt [1].At the cathode TiO 2is reduced to Ti.The oxide ions dissolve into CaCl 2and then migrate to a C anode,formingCO 2and CO.This reduction pathway has been studied using pellet reductions [3],metal-cavity electrode [4–6]and thin-film electrode experimentation [7,8].Using cyclic voltamme-try and X-ray diffraction (XRD)analysis of thin films aftereach event,the electrochemical events C4,C3,C20,C2and C1,corresponding to reactions (1)–(6),respectively,have been identified during reduction.Despite being observed in XRD,the formation of CaTiO 3was not observed on the vol-tammograms and was therefore believed to form via a chem-ical reaction [9].2TiO 2þ2e À!Ti 3O 5þO 2Àð1Þ2Ti 3O 5þ2e À!3Ti 2O 3þO 2Àð2ÞCaTiO 3þ2e À!Ca 2þþTiO þ2O 2Àð3ÞTi 2O 3þ2e À!2TiO þO 2Àð4ÞTiO þ2e À!2TiO þO 2Àð5ÞTiO 2þCa 2þþO 2À!CaTiO 3ð6Þ1359-6454/$36.00Ó2010Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.actamat.2010.05.041qR.J.D.and D.D.conceived and designed the study.B.K.J.,R.J.D.and D.D.designed the apparatus,which B.K.J.built and commissioned.R.B.,R.J.D.,D.D.,S.L.R.,R.J.T.and K.K.R.conducted the experiments.R.B.and S.L.R.analysed the data with support from R.J.T.,R.J.D.and D.D.The paper was written by R.B.with contributions and editing from R.J.D.,D.D.,D.I.,R.J.T.and S.L.R.*Corresponding author.E-mail address:r.bhagat@ (R.Bhagat)./locate/actamatActa Materialia 58(2010)5057–5062The reduction pathway of TiO2can be rationalized using an electrochemical predominance diagram[9](Fig.1a), which indicates the thermodynamically stable species at dif-ferent potential/oxide activities.The electrolyte oxide content after pre-electrolysis is found to be3–4pO2À(Àlog(aO2À)[10].Fig.1a shows that the evolution of phases at a particular electrode potential is dependent on the electrolyte oxide content.For example,reducing TiO2in an oxide-saturated electrolyte(pO2À=0)will lead to a chemical reaction between CaO and TiO2,forming CaTiO3,which would sub-sequently reduce to Ti.If the electrolyte contained lower lev-els of oxide(pO2À=6),then TiO2would reduce through the whilst reducing TiO2,here we are able to present thefirst unequivocal description of the reduction pathway using the crystallographic information gathered(Fig.1b).2.MethodTiO2precursors were formed using reagent-grade TiO2 (99.5%Alfa Aesar),ball milled with ethanol and zirconia spheres for24h.The slurry was dried and the powder ground in a mortar and pestle with a small quantity of dis-tilled water,which acted as a binding agent.0.4g of pow-der was uniaxially compacted at100MPa in a13mm diameter die.The precursors were then drilled to acceptFig.1.(a)Electrochemical predominance diagram for Ca–Ti–O–Cl at1173K.(b)Phase evolution near the pellet surface(1.2mm from the centre).Fig.2.Electrochemical reduction cell used,showing the sampling volumes employed.5058R.Bhagat et al./Acta Materialia58(2010)5057–50621K minÀ1under Ar(BOC pureshield–50mL minÀ1)tothe electrolysis temperature(1173K).The electrolyte was then thermally equilibrated for3h.Prior to experiments the salt was pre-electrolysed for1–3h to remove electroac-tive impurities by polarizing a grade2CP-Ti rod(3mm) vs.the graphite crucible.Following the pre-electrolysis, the precursor was lowered into the molten salt and polar-ized at a rate of0.5mV sÀ1from open circuit to À3100mV vs.the C anode.The ID15A beamline at the ESRF was used to produce an unmonochromated(white)X-ray beam in energy dis-persive mode.A50l mÂ100l m incident beam and5°scattering angle were defined by slits yielding a gauge length of1000l m(Fig.2).The assembly was mounted on a translation stage,allowing nine locations at different heights in the pellet to be studied.Acquisition times were 60s per pattern.The diffraction patterns were analysed by Rietveld refinement using GSAS[11],allowing phases to be identi-fied and atomic fractions obtained.The source spectrum was normalized using both an Al powder and the powder TiO2pattern obtained from the pellet prior to insertion into the cell.Beam hardening within the cell was accounted for by comparison with the initial patterns obtained from the pellet.3.Results and discussionThe scans at the pellet surface(1.2mm)are shown in Fig.1b.Rietveld refinements at key intervals during reduc-tion for position1.2mm are shown in Fig.3.In some cases, due to the small gauge volume,insufficient grains were sampled to produce a powder pattern,resulting in some disparity between observed and calculated peak intensities. Molar fractions of the phases present at each position are shown in Fig.4(e.g.after4h at the1.2mm position, approximately55%CaTiO3and45%TiO2is present;after 10h approximately75%Ti,20%CaTiO3and5%TiO is present).The refined lattice parameters can be used to estimate the interstitial O content in hexagonal close-packed(hcp)a-Ti. The thermal expansion data for the lattice parameters[12], from room temperature to700°C,were combined with the room temperature dependence of O content on the lattice parameter[13].The data used is shown in Table1and the combined thermal and compositional effects are given in Eqs.(7)and(8).The hexagonal a lattice parameter is largely independent of O content,so only the c parameter data was analysed.It was assumed that the Ti thatfirst forms was sat-urated with O(14wt.%O at900°C).The variation in c lat-tice parameter and inferred interstitial O content in the a-Ti phase are shown in Fig.5a.Given the previously discussed assumption made,the inferred interstitial oxygen content (shown in Fig.5a)will have an uncertainty of±0.1at.%O arising from the data showing the dependence of O content on the lattice parameter[13].As such,error bars are not included in Fig.5a as they would not be visible on the scaleR.Bhagat et al./Acta Materialia58(2010)5057–506250595060R.Bhagat et al./Acta Materialia58(2010)5057–5062Fig. 5.(a)Evolution of the a-Ti c-lattice parameter and inferredinterstitial O content and(b)inferred oxygen content of phases presentduring reduction through the pellet.a a Ti¼a0ð1þD T a aÞþCðOÞdadCðOÞð7Þc a Ti¼c0ð1þD T a aÞþCðOÞdcdCðOÞð8ÞAt the beginning of the reduction,TiO2,Ti4O7and CaTiO3 were present at all locations(Fig.4).The presence of Ti4O7is consistent with the general electronic theory of TiO2 [15–17],which states that TiO2will equilibriate in a low oxy-gen atmosphere.This is achieved via the creation of crystal defects,resulting in TiO2being reduced.It should be noted that in addition to Ti4O7,other Magne`li phases(e.g Ti5O9)fit relatively well.Ti4O7is the Magne`li phase most often de-tected during the FFC reduction of TiO2[3,18].The transformation to substoichiometric TiO2phases and the consequent release of O ions into the electrolyte results in the formation of CaTiO3.It has been suggested that the slight reduction of TiO2facilitates the transforma-tion to the CaTiO3perovskite structure[19].Interestingly Ti3O5was not detected during reduction, despite its formation being predicted by Fig.1and by the Ti–O phase diagram[14].Significant quantities were observed previously[3,7]during ex situ analysis of thin films.The narrow homogeneity range of Ti3O5(Fig.1) [14]indicates that significant quantities would only form if the electrode potential were held within the Ti3O5phase field.It is hypothesized that the significant quantities of Ti3O5observed[3]were a result of a disproportionation reaction of Magne`li phases during cooling.Consequently, either the Ti3O5phasefield is traversed rapidly or kinetic considerations prevent its formation.Ti2O3seems to have a significant impact on reduction as it is present throughout the sample.Fig.2shows that Ti2O3 reduces to b-TiO.The monoclinic variants,a-TiO and Ti3O2,were not detected,which is inconsistent with the Ti–O phase diagram[14].The b-TiO then gradually reduces to a-Ti.CaTiO3and CaTi2O4(in the pellet inner regions)were present for most of the reduction.It is believed that the oxy-gen content of CaTiO3does not significantly change until the point of reduction to TiO or Ti.CaTiO3is reported to have a compositional range[20];however,the derived lattice parameters remain quite consistent throughout reduction. The detected CaTiO3is most likely non-stoichiometric,i.e Ca/Ti ratio–1and oxygen sub-stoichiometric(thus CaTiO3could be more accurately referred to as Ca x Ti y O3Àz, where x/y–1and z is related to the number of oxygen vacancies present).It should be noted that between the 1.1h and3.3h pattern in Fig.3,small shifts in the three main CaTiO3peaks to lower d-spacing values can be observed. Such changes suggest limited removal of oxygen ions from the CaTiO3,which distort the crystal structure.Suchfind-ings are consistent with the work of Bak et al.[21],who observed the removal of oxygen ions from CaTiO3to create doubly ionized oxygen vacancies.We previously speculated that CaTiO3may act as a solid electrolyte,allowing the transmission of oxide ions[10],which may account for the consistent stoichiometry.At the surface layers it would appear that CaTiO3is reduced directly to Ti despite the fact that thermodynamically this would only occur at very low pO2À.In the inner layers,CaTi2O4forms at the expense of CaTiO3and TiO.Previously,it was theorized that the for-mation of CaTi2O4was a result of either the electrochemical reduction of CaTiO3or a comproportionation reaction between CaTiO3and TiO.Fig.4shows that the formation of CaTi2O4results in the equimolar consumption of TiO and CaTiO3,indicating that a comproportionation reaction is indeed responsible for its formation(Reaction(9))[3,20]. CaTiO3þTiO!CaTi2O4ð9ÞCaTi2O4has been produced by mixing equimolar or Ti-rich mixtures of CaTiO3and Ti powder with a CaCl2flux at 1000°C[22].When the Ti powder was oxidized to TiO,Ca-Ti2O4was not formed,suggesting the comproportionation reaction has a small positive D G[22].Fig.4shows that the formation of CaTi2O4at the inner layers coincides with the occurrence of Ti in the outer layers(0.6,À0.9and1.2mm). It is speculated that the presence of titanium in close prox-imity,during reduction,makes the comproportionation be-tween TiO and CaTiO3more favourable.Fig.5shows that significant removal of O occurs at events1,2and3.Event1can be identified as the formation of TiO(Fig.4).For the surface layers(1.2,À0.9and 0.6mm),event2was identified as the reduction of TiO to Ti.For the inner layers(0.3and0mm),event2is retarded due to the formation of CaTi2O4.Event3involves the removal of CaO,which occurs at all locations except the pellet centre(0mm).Following the dissolution of CaO,the removal of interstitial O from Ti commences.CaO is detected throughout the pellet at the later stages of reduction and forms when the electrolyte in the pores becomes saturated with oxide ions($22mol.%).The onset of CaO formation correlates with the reduction of CaTi2O4 in the core.This results in a rapid evolution of oxide ions that have to be transported through the pellet pores to the bulk electrolyte.During reduction,the porosity of the pellet(originally60%open porosity)is reduced through a combination of sintering of Ti and formation of titanates. This significantly reduces the pellet permeability,allowing oxide ion saturation to occur.After event2at the inner lay-ers theflux of oxide ions passing through the pellet dimin-ishes.At this point the surface CaO rapidly dissolves, closely followed by the core.We have proposed two mechanisms for the surface for-mation of TiC[10]:(i)A chemical reaction with C dust floating on the electrolyte surface,occurring as the sample is removed.(ii)Electrochemical deposition of C and subse-quent reaction at the cathodic Ti pellet surface.This occurs via the chemical formation of CO2À3via a reaction between CO2formed at the anode and O2Àin the electrolyte(Reac-tion(10)).This complex is then reduced at the cathode forming CaO and C,which chemically reacts with a-Ti to form TiC(Reaction(11)).This is supported by the present in situ data.R.Bhagat et al./Acta Materialia58(2010)5057–50625061CO2þO2À!CO2À3ð10ÞCO2À3þTiþ4eÀ!3O2ÀþTiCð11ÞTiC formation is restricted to the pellet surface and only forms during the later stages of reduction.It is proposed that TiC only forms once the cathode is sufficiently nega-tively polarized for Reaction(11)to occur.The potential at which this occurs is dependent on the activity of boththe CO2À3and more importantly the oxide ions.The oxideion concentration in the region of the pellet will only reduce once reduction is nearly complete,meaning Reac-tion(11)becomes more favourable.There is an increase in c lattice parameter(Fig.5)at the later stages of reduc-tion at positions0.6,0.9,1.2mm,coinciding with the for-mation of TiC.It is suggested that this effect is due to the solubility of interstitial C in a-Ti(approximately $0.5at.%at1373K),leading to an increase in the c parameter.Once the solubility of interstitial C is exceeded, the formation of TiC occurs.4.ConclusionsThe keyfindings of this work can be summarized as fol-lows.Upon entering the low O atmosphere,TiO2quickly forms sub-stoichiometric phases,which significantly improves ionic conductivity over stoichiometric TiO2.In contradiction to previous work[3],CaTiO3forms chemi-cally from the sub-stoichiometric TiO2phases.CaTi2O4 formed from the comproportionation of TiO and CaTiO3 and was detrimental to the reduction process.The phases Ti3O5,a-TiO and Ti3O2were not observed despite being pre-dicted by the Ti–O phase diagram.The formation and even-tual dissolution of CaO was related to the oxideflux within the pellet.The deoxidation of Ti was found to occur only once CaO had been removed locally within the pellet.TiC formed on the reduced pellet surface through an in situ pro-cess,involving the electrochemical deposition of C,but only seemed to occur when the O content in Ti reached approxi-mately$16at.%.The relative ease with which low electrical conductivity TiO2can be reduced via this process can be attributed to the low O2partial pressure.This results in a nearly instan-taneous formation of conductive Magne`li phases.The time required to reduce TiO2pellets appears to be controlled by the formation of Ca compounds(CaTiO3,CaTi2O4and CaO).Their formation reduces the permeability of the pores,effectively causing passivation.Consequently,under-standing the mechanism by which these phases form and how to prevent their formation is key to improving the effi-ciency of the process.The chemical formation of CaTiO3, and the subsequent formation of CaTi2O4could be avoided if lower valence metal oxides(i.e.Ti2O3and TiO)were used as the starting product.Preventing the formation of the titanates will improve permeability and hence decrease the likelihood of oxide saturation and CaO formation. Alternatively,engineering of the preform to maximize ini-tial pore size and permeability could reduce titanate forma-tion.The formation of TiC appears to be unavoidable with the current cell design.Its formation can only be prevented if C-free anodes are employed or a membrane used.AcknowledgementsThe authors gratefully acknowledge the Defence Ad-vanced Research Project Agency(DARPA),the Office of Naval Research and the Engineering and Physical Sciences Research Council(EPSRC)for their support.The authors also thank T.Buslaps,J.Daniels and A.Steuer at the ESRF,together with M.Jackson for their assistance.The results described in this paper were collected in conjunction with data for the reduction of NiTiO3.This work is de-scribed elsewhere[23].References[1]Chen G,Fray DJ,Farthing TW.Nature2000;407:361.[2]Kroll W.Seventy-Eighth General Meeting1940:35.[3]Schwandt C,Fray D.J Electrochim Acta2005;51(1):66.[4]Rao K,Brett D,Inman D,Dashwood RJ.Investigation of thereduction electrochemistry of titanium oxides in molten calcium chloride using cavity electrode.In Niinomi M,et al.,editors.Titanium-2007Science and Technology.Sendai,Japan:The Japan Institute of Metals;2007.p.3.[5]Cha CS,Li CM,Yang HX,Liu PF.J Electroanal Chem1994;368:47.[6]Qiu G,Ma M,Wang D,Jin X,Hu X,Chen GZ.J Electrochem Soc2005;152:E328.[7]Dring K,Dashwood RJ,Inman D.J Electrochem Soc2005;152(3):E104.[8]Chen G,Fray DJ.J Electrochem Soc2002;149:E455.[9]Dring K,Dashwood RJ,Inman D.J Electrochem Soc2005;152(10):D184.[10]Bhagat R.PhD Thesis.London:Imperial College;2008.[11]Larson AC,Von Dreele RB.Los Alamos National LaboratoryReport;2000.[12]Berry RLP,Raynor GV.Research1953;6:21S.[13]David D,Garcia EA,Lucas X,Beranger G.J Less-Common Met1979;65:51.[14]Murray JL,Wriedt HA.Bull Alloy Phase Diagr1987;8:148.[15]Bak T,Nowotny J,Rekas M,Sorrell CC.J Phys Chem Solids2003;64(7):1057.[16]Nowotny J,Radecka M,Rekas M.J Phys Chem Solids1997;58(6):927.[17]Nowotny J,Radecka M,Rekas M,Sugihara S,Vance ER,WeppnerW.Ceram Int1998;24(8):571.[18]Alexander DTL,Schwandt C,Fray DJ.Acta Mater2006;54(11):2933.[19]Jiang K,Hu X,Ma M,Wang D,Qiu G,Jin X,et al.Angew Chem IntEd2006;45(3):428.[20]Bhagat R,Jackson M,Inman D,Dashwood RJ.J Electrochem Soc2009;156(1):E1.[21]Bak T,Nowotny J,Sorrell CC.J Phys Chem Solids2004;65:1229.[22]Rogge MP,Caldwell JH,Ingram DR,Green CE,Geselbracht MJ,Siegrist T.J Solution Chem1998;141:338.[23]Jackson BK,Dye D,Inman D,Bhagat R,Talling RJ,RaghunathanSL,et al.J Electrochem Soc2010;157(4):E57.5062R.Bhagat et al./Acta Materialia58(2010)5057–5062。