a current 
 trend in catalysis sees a refocus on the catalytic action 
 of the individual parts of a more complex catalyst entity . as many 
 catalyst systems represent a combination of ( noble ) metals and ( oxidic ) 
 supporting materials , the latter are increasingly studied with respect 
 to their intrinsic surface reactivity . however , due to the inherent 
 structural and electronic complexity of oxides , the identification 
 of e.g. a 
such 
 complex systems may either be two - phase systems such as mechanically 
 mixed oxide powders(e.g .
, being promising methanol - to - gasoline 
 catalysts ) or single - phase binary oxides 
 adopting a distinct crystallographic structure . a well - known 
 example of
the latter are perovskite - type materials with the general 
 formula abo3 , where a and b are cationic metal species 
 with variable composition .
their use 
 as ferroelectrica , high - temperature superconductors , or solid oxide fuel cell ( sofc ) cathodes is already well - known .
potential applications 
 in catalysis are reported for environmentally relevant denox processes , diesel exhaust catalysis , polymerization 
 of olefins , total oxidation of hydrocarbons , or dry reforming of methane .
specifically , also the water - gas shift reactivity has recently been 
 in the focus of research .
a comprehensive review of the 
 salient aspects of the structure and known catalytic performance of 
 perovskites has been given by e.g. by pena et al . or royer et al . a rather new field of catalytic research deals with the replacement 
 of the nowadays common ni / ysz sofc anodes by acceptor - doped mixed 
 conducting perovskite - type anodes .
for this application a combination 
 of good ionic and electronic conductivity , as well as high ( electro)catalytic activity and high resilience 
 against carbon deposition , is necessary .
acceptor - doped oxides under 
 reducing conditions , however , often exhibit a relatively high ionic 
 conductivity , but their rather low electronic conductivity may be 
 problematic for an application as sofc anode materials . in the case of model - type
thin film electrodes , this poor electronic 
 conductivity can lead to an electronic sheet resistance and thus to 
 inhomogeneous electrochemical polarization . to avoid artifacts in electrochemical measurements , metallic thin 
 film
current collectors can be applied on such thin film electrodes , where the electrochemical surface activity of perovskite - type electrode 
 materials can be separated from resistances due to poor current collection . 
 moreover , catalytic activity in carbon - based fuel reforming and hydrogen 
 oxidation on ni / ysz electrodes is provided by the ni phase .
however , 
 also perovskite - type lanthanum chromite / ferrite - based sofc anodes 
 have been shown to exhibit good catalytic activity for methane steam 
 reforming and methane oxidation .
the 
 understanding of the surface catalytic properties of such acceptor - doped 
 perovskites in reducing atmospheres , however , is still in its infancy . 

with respect to fuel cell research and oxide surface chemistry , investigations 
 on water - gas shift reactivity and hydrocarbon reforming are hence 
 an obvious research field .
the water - gas shift reaction is known to be 
 catalyzed by redox - active 
 oxidic materials , in particular also by more complex mixed systems 
 such as cuo / zno / al2o3 , fe2o3/cr2o3 , or even perovskite 
 materials .
although there appears to be increasing interest in the general 
 use of perovskite materials as valuable catalysts , also including 
 hydrocarbon conversion reactions , direct correlations of catalytic properties and associated structural 
 changes for eventual determination of the catalytically active site 
 still remain scarce due to the inherent complexity of these materials . 

this is a particular pity since e.g. hydrocarbon conversion is usually 
 carried out at very high temperatures ( t > 600 
 c ) , 
 which might give rise to an array of structural changes , including 
 ( but not limited to ) surface reconstruction , altering bulk and/or 
 surface terminations , and/or chemical segregation of individual atom 
 species . especially the surface structure and chemistry , e.g. ,
the 
 cation concentration or the formation and amount of oxygen vacancies , 
 are in a dynamical state depending on the experimental conditions . for oxide and sofc - related research
this basically refers to the 
 hydroxylation degree of the surface , high temperatures , and the oxygen 
 partial pressure and reactivity .
thus , surface and bulk structure 
 and composition might significantly deviate from one another and need 
 to be separately assessed .
regarding chemical alterations of perovskite - type 
 materials , cation segregation is already a well - known phenomenon especially 
 under operational conditions of an sofc .
moreover , reversible in situ segregation of electrolysis - promoting 
 metallic fe species under reducing ( cathodic ) conditions was recently 
 reported as well as growth of defined 
 nanoparticles by controlled perovskite nonstoichiometry . in order to fill this knowledge gap and 
 to bridge the gap between 
 sofc - relevant reactivity and  conventional  catalysis , 
 which also includes a cross - correlation between catalytic application 
 and electrocatalysis , the present contribution aims at ( i ) characterizing 
 the electrochemical surface activity of perovskite - type materials 
 ( in particular lsf ) in both oxidizing and reducing atmosphere by means 
 of impedance spectroscopy . to account for the low electronic conductivity 
 of acceptor - doped mixed conductors in reducing atmosphere ( see above ) , 

model - type thin film electrodes with a current - collecting platinum 
 thin film grid are used in these investigations .
( ii ) as the specific 
 oxygen reactivity is suspected to play a major role in eventually 
 steering the catalytic reactivity , the main goal of this study is 
 to highlight reactivity differences for the ( inverse ) water - gas shift 
 ( ( i-)wgs ) and methane conversion on two representative perovskite - type 
 materials , la0.6sr0.4feo3 ( lsf ) and srti0.7fe0.3o3 ( stf ) , respectively .
the latter two are deemed relevant materials 
 due to their different iron content . as iron
is mostly suspected to 
 be the center of catalytic reactivity due to its pronounced redox 
 behavior , we aim at providing data to what extent this is true . in 
 fact , combined structural characterization ( mostly by tem ) and catalytic 
 testing will indicate that this is only part of the truth , and a more 
 complex behavior , especially with respect to the quality of oxygen 
 vacancies , must be assumed .
in due course , the stability of the perovskite 
 ( surface ) structure during a full catalytic cycle including preoxidation 
 and reduction will be monitored .
therefore , catalytic measurements 
 in a recirculating batch reactor setup especially designed for the 
 measurement of small reaction rates , structure - determining 
 methods ( analytical high - resolution electron microscopy and x - ray 
 diffraction ) , and raman spectroscopic measurements were jointly applied .
la0.6sr0.4feo3 was 
 prepared via the pecchini route : respective 
 amounts of la2o3 , srco3 , and metallic 
 fe ( all three at least 99.95% pure , sigma - aldrich ) were dissolved 
 in hno3 ( double distilled , sigma - aldrich ) .
all three solutions 
 were mixed , and citric acid ( 99.9995% pure , sigma - aldrich ) was added 
 to the solution in a 20% excess with respect to the cations .
the solution 
 was heated , and after the evaporation of water the mixture formed 
 a highly viscous foam , which finally decomposed in a spontaneous combustion . 

the obtained powder was calcined for 2 h at 1000 c in air ; the 
 product was ground in a mortar and checked for phase purity by x - ray 
 diffraction ( xrd ) .
the srti0.7fe0.3o3 powder was obtained by a solid state reaction 
 from srco3 , tio2 , and fe2o3 ( all three at least 99.98% pure , sigma - aldrich ) .
the educts were 
 thoroughly mixed , calcined at 1000 c for 2 h , ground in a mortar , 
 and calcined a second time at 1250 c for 2 h. the product was 
 again ground and also checked for phase purity by xrd .
surface areas 
 using the bet method were determined for both samples to be around 
 0.4 m g. bet surface areas were measured 
 with a quantachrome nova 2000 surface and pore size analyzer . for catalytic measurements
the powders were used as obtained by 
 the procedures described above . for pulsed laser deposition ( pld ) 
 of thin films , targets of lsf
were prepared by isostatically pressing 
 the powders and sintering the green bodies at 1250 c for 5 h 
 in air .
thin films of lsf were deposited on 
 yttria - stabilized zirconia ( 100 ) single crystals ( ysz , 9.5 mol % y2o3 , crystec , germany ) by pld . for electrochemical 
 measurements in reducing conditions a current collecting thin film 
 grid
was deposited prior to the pld process by sputtering 10 nm ti 
 on ysz followed by 100 nm of pt .
this thin film was micropatterned 
 by a photolithography process and ar - ion beam etching .
for the subsequent 
 pld process a krf excimer laser ( lambda compexpro 201f ) with 248 nm 
 wavelength was used to ablate the lsf target .
the deposition was done 
 in 4  10 mbar of pure oxygen with a laser 
 pulse repetition rate of 5 hz , a nominal laser pulse energy of 400 
 mj , and a ysz substrate temperature of 650 c ( controlled by 
 a pyrometer ; heitronics , germany ) .
current collectors circular 
 microelectrodes were produced by again carrying out a photolithography 
 process and an ar - ion beam etching step .
a scanning electron microscopy 
 ( sem ) image of a circular lsf electrode with buried ti / pt current 
 collector is shown in figure s1 , and a 
 schematic depiction of the thin film electrode setup is shown in figure s2 . in a previous impedance study on stf 
 electrodes with buried pt current collectors , it was shown that the surface exchange resistance and the chemical 
 capacitance of the electrode can be extracted from the low - frequency 
 part of the impedance spectra .
impedance spectroscopy measurements 
 on these thin film electrodes were carried out in a home - built microcontact 
 setup .
it allows contacting of individual electrodes by pt / ir tips , 
 which can be accurately positioned by piezoelectric high - precision 
 actuators ( newport agilis ) .
the atmosphere in the setup was either 
 synthetic air or 2.5% h2/2.5% h2o / balance ar , and the electrode temperature was about 610620 
 c .
impedance 
 measurements were conducted with an alpha - a high performance frequency 
 analyzer with pot / gal 30 v 2a test interface ( both novocontrol , germany ) 
 in a frequency range between 10 mhz and 1 mhz at an ac voltage of 
 10 mv ( root - mean - square , rms ) .
impedance data analysis was done by 
 the complex nonlinear least - squares fitting software z - view 3.1 ( scribner , 
 usa ) .
the catalytic 
 measurements on lsf and stf powder samples were exclusively performed 
 in a recirculating batch reactor ( ca .
detection of 
 the reactants and products was carried out by a quadrupole mass spectrometer 
 ( qms ) attached to the reaction chamber via a capillary leak . for a 
 typical inverse water - gas shift experiment ,
a 1:4 mixture of co2 and h2 , delivering the right stoichiometry for 
 a potential methanation reaction , as well as 25 mbar of h2o to ensure thermodynamically stable conditions , was filled into 
 the reaction chamber of the batch reactor .
for the water - gas shift 
 reaction , a 1:1 ratio of co and h2o ( 25 mbar each ) was 
 used .
subsequently , the mixture was heated with 5 c min from room temperature to 600 c , followed by 
 isothermal periods of variable duration before the final cooldown . 

ar ( 7.5 mbar ) was additionally admitted to all reaction mixtures 
 to correct all other signals for the continuous gas withdrawal through 
 the leak .
the mass - to - charge ratios m / z = 2 ( h2 ) , 15 and 16 ( ch4 ) , 18 ( h2o ) , 28 ( co ) , 40 ( ar ) , and 44 ( co2 ) were routinely monitored . 

the co signal was corrected for the contribution from co2 fragmentation in the mass spectrometer .
the qms intensities were 
 calibrated ( i.e. , converted into partial pressure given in mbar ) using 
 external calibration standards .
a brief account of the mass 
 and heat transport limitations should be given at this point .
we exclude 
 effects of mass transport and pore diffusion limitation , since the 
 samples are used in quite small amounts ( 100200 mg ) of loose 
 powder , which can be quickly penetrated by the reaction gas .
note 
 that the same catalyst setup that has been used for the studies on 
 comparable systems , where much higher reaction rates were accurately 
 measured .
heat transfer limitation is also judged to play a minor 
 role since local temperature effects are excluded due to the low reaction 
 rates , and generally , heat transfer via the gas phase is enhanced 
 due to the deliberate admission of he to the reaction mixture ( 1 bar 
 total pressure ) .
the volumetric 
 adsorption measurements were conducted in an all - quartz apparatus 
 described in detail elsewhere ( reactor volume : 32 ml ) , additionally using a balzers qma125 quadrupole 
 mass analyzer and a linn furnace operating up to 1220 c .
all 
 perovskite powder samples were subjected to a pretreatment in flowing 
 oxygen up to 600 c , with an isothermal period at 600 c 
 for 1 h before finally cooling down .
after this oxidation step , the 
 system was evacuated at room temperature to a base pressure of about 
 5  10 mbar after the aforementioned oxidation 
 treatment , and a defined amount of 100 mbar of h2 was admitted .
subsequently , the samples were heated to 600 c 
 at a rate of 10 c min , followed by an isothermal 
 period at 600 c for 1 h before cooling to room temperature . 
 for better comparison also to literature - reported data , all mass spectrometer 
 data ( in mbar )
were converted into micromoles on the basis of the 
 ideal gas equation , with subsequent normalization to sample mass and 
 surface area .
cleaning of the gases was performed using either a liquid 
 n2 ( for h2 ) or a liquid n2/ethanol 
 cooling trap ( for o2 ) .
note that the term  adsorption  
 ( better viewed as  uptake  ) in this case also includes 
 actual hydrogen  consumption 
reactions , such as bulk 
 reduction or hydrogen insertion into the perovskite lattice . as a 
 zeolite trap is used to quantitatively remove reaction - formed water 
 from the volumetric chamber , accurate absolute quantification of the 
 second mechanism is presently not possible . however , as will be shown 
 later , different behavior of both stf and lsf in their oxidized and 
 reduced states , respectively , is an experimental matter of fact . although 
 the exact bulk oxygen and hydrogen stoichiometry of the samples in 
 different states of reduction is not known due to the above - mentioned 
 obstacle , we nevertheless decided to use a specific partially reduced 
  prereaction  state prior to catalytic testing ( see 
 section 2.3 ) .
prereduction at 873 k in dry 
 hydrogen was on the one hand done gently enough to avoid reductive 
 decomposition of the samples .
on the other hand , substantial formation 
 of additional oxygen vacancies ( beyond those already induced by the 
 prereduction ) during e.g. a subsequent temperature - programmed water - gas 
 shift reaction experiment could be largely suppressed by the reductive 
 pretreatment and by performing the catalytic experiment at t  tprereduction .
thus , 
 true catalytic experiments with complete reactant / product mass balance 
 became feasible .
structural characterization of the samples was carried out by two 
 types of electron microscopes .
bright - field ( high - resolution ) imaging 
 and high - angle annular dark - field imaging were carried out using a 
 200 kv fei tecnai f20 stwin analytical ( scanning ) transmission electron 
 microscope ( ( s)tem ) equipped with a tridiem energy filter .
high - resolution , 
 high - angle annular dark - field imaging eel and edx spectroscopy were 
 performed using an aberration - corrected fei titan microscope operating 
 at 300 kv .
electron - energy loss spectra are background - corrected and 
 corrected for plural scattering .
prior to imaging , the samples were 
 sputter - cleaned to remove surface carbon impurities .
for the high - resolution 
 images , the stf and lsf samples were suspended in acetone and cleaned 
 in an ultrasonic bath prior to mounting on a holey carbon film . in 
 the case of haadf images to be taken , the samples were heated to 80100 
 c under high - vacuum conditions ( 4  1010 mbar ) prior to imaging .
x - ray powder diffraction 
 data were collected at ambient conditions with a bruker axs d8-advance 
 powder diffractometer using cu k1 and cu k2 radiation ( 1 = 1.5406  ; 2 = 1.5444  ; 40 kv ; 40 ma ) .
data acquisition was performed in the 2 
 range between 2 and 90 with a step width of 0.02 
 and a counting time of 3 s under sample rotating conditions .
the sample 
 itself was prepared on a si single crystal holder with suppressed 
 background intensity .
confocal raman 
 spectra of the polycrystalline samples in the range of 503800 
 cm were recorded with a horiba jobin yvon labram - hr 
 800 raman microspectrometer .
the samples were excited using the 532 
 nm ( 2.33 ev ) emission line of a frequency - doubled 25 mw nd : yag laser 
 under an olympus 100 objective lens with a numerical aperture 
 of 0.9 and additionally he  ne laser ( 633 nm ) for detection 
 of luminescence effects .
the size of the laser spot on the surface 
 was approximately 1 m in diameter .
the scattered light was 
 dispersed by an optical grating with 1800 lines mm and collected by a ( peltier - cooled ) 1024  256 open - electrode 
 ccd detector confocally coupled to the focal point of the sample . 
 the spectral resolution , determined by measuring the fwhm of the rayleigh 
 line , was below 2 cm ( using a slit width of 100 
 m ) .
the accuracy of the raman line shifts , calibrated by measuring a silicon 
 standard ( 522 cm ) , was in the order of 0.5 cm .
spectra were recorded from a representative grain 
 of the samples as a single measurement ( 10 s integration time ) .
impedance spectra obtained on lsf electrodes 
 with and without buried ti / pt current collectors are depicted in figure 1 .
both types of electrodes were characterized in 
 air ( see figure 1a ) as well as in reducing 
 2.5% h2/2.5% h2o / ar atmosphere 
 ( cf .
the impedance spectra 
 consist of three relatively well separated features . in the high - frequency 
 range
( see inset in figure 1a ) a real axis 
 intercept can be observed , which can be attributed to the spreading 
 resistance of ion conduction in ysz . this 
 resistance ,
together with the size of the circular microelectrodes , 
 was used to obtain the true temperature of the measured microelectrodes 
 ( for details regarding this calculation please refer to refs ( 77 and 78 ) ) .
the arcs in the medium- and 
 low - frequency region can be related to an ionic transfer resistance 
 at the electrode / electrolyte interface and to the oxygen incorporation 
 resistance at the electrode surface , respectively . in the case of the electrode with buried current collector 
 the interfacial resistance and the dominating surface resistance
are 
 increased , which can be easily explained by the ion blocking character 
 of the current collectors .
thus , only a smaller part of the electrode / electrolyte 
 interface as well as of the electrode surface is available .
taking 
 the geometry of the grid into account ( 3/4 of the interface covered 
 with metal , 1/4 free interface ) , the electrode kinetics of both types 
 of electrodes ( with and without grid ) in air can be regarded as very 
 similar . in reducing atmosphere
figure 1b ) . here , the spectrum measured 
 on lsf without current collector is both qualitatively and quantitatively 
 significantly different than the spectra obtained in oxidizing atmosphere . 

first , the high - frequency axis intercept is about 2 orders of magnitude 
 larger than one might expect from the nominal size of the electrode . 

second , in the medium - frequency regime an additional shoulder appears , 
 and third , the total polarization resistance of the electrode is more 
 than 1 order of magnitude larger than in air ( cf .
the finding 
 that the high - frequency intercept  which is expected to be the 
 spreading resistance of ion conduction in ysz  is much too large 
 for the nominal electrode size is a strong indication that owing to 
 a high sheet resistance in the lsf film only a rather small part of 
 the electrode around the tip contact is electrochemically active .
this interpretation is further supported 
 by the fact that in the 
 case of lsf electrodes with buried current collector the high - frequency 
 real axis intercepts are close to those of spectra measured in air 
 ( see inset in figure 1c ) . moreover ,
the total 
 electrode impedance of lsf with current collector measured in h2/h2o is within the same order of magnitude as the 
 electrode resistance in air ( see figure 1c ) . 

considering the active areas of the two electrodes ( 0.79  10 cm for lsf in air and 3.14  10 cm for lsf in h2/h2o ) , one can even calculate almost identical area - specific polarization 
 resistances of 16 and 15 cm for lsf under 
 oxidizing and reducing conditions , respectively.a since the inverse polarization resistances are a direct measure 
 for the exchange current and thus the exchange rates at the electrode 
 surfaces , at ca .
620 c the catalytic activitiy of the lsf surface 
 for oxygen reduction and hydrogen oxidation is rather similar .
in a previous impedance study on stf model electrodes in reducing 
 atmosphere a surface resistance of ca .
this comparison reveals 
 that lsf is significantly more active for h2 oxidation 
 than stf .
corresponding differences in the  conventional  
 catalytic behavior of the two materials will be discussed below on 
 the basis of their specific oxygen vacancy / lattice oxygen reactivities . 
 in particular , vacancies in lsf are shown below ( section 3.2 ) to be much more reactive with respect to quenching 
 by water splitting ( the reverse process of hydrogen oxidation by lattice 
 oxygen ) , and the associated water affinity of the vacancies is clearly 
 higher than on stf .
taking into account the above - mentioned 
 area - related polarization 
 resistance , an application of lsf in porous sofc anodes may lead to 
 electrodes with polarization resistances comparable to nowadays used 
 porous paste cathodes .
this potential applicability , however , raises 
 the question of compatibility of lsf anodes with carbon based fuels 
 such as methane .
thus , besides the fundamentally interesting questions 
 regarding surface activity of perovskite - type catalysts ( as discussed 
 in the introduction ) , the catalytic activity 
 of lsf for reactions such as water - gas shift and methane activation 
 needs to be investigated in detail as shown in the following sections .
impedance 
 spectra ( nyquist plots ) measured on 200 m thick 
 lsf microelectrodes with and without buried ti / pt current collectors . 

the insets in the top right corners show magnifications of the high - frequency 
 region of the spectra .
the measurements were performed in air ( a ) 
 as well as in reducing atmosphere , 2.5% h2/2.5% 
 h2o / balance ar ( b ) .
the spectra measured on lsf with current 
 collectors in air ( from a ) and in reducing atmosphere ( from b ) are 
 compared in ( c ) . before focusing on 
 the structural changes occurring on the stf 
 and
lsf perovskite - type systems upon oxidative and reductive pretreatments 
 or during a catalytic cycle , the activities for the water - gas shift 
 and methane conversion reaction are outlined .
this also includes a 
 discussion of temperature - programmed hydrogen uptake experiments , 
 which very much adds to the understanding of the associated temperature - programmed 
 catalytic reaction profiles .
the term  adsorption  in 
 this respect includes all hydrogen uptake or consumption processes 
 lowering the hydrogen concentration in the gas , including adsorption , 
 absorption in the oxide , but also water formation by oxide reduction .
figure 2 shows two sets of temperature - programmed 
 inverse water - gas shift reaction runs on stf with different sample 
 pretreatments .
a preoxidation at 400 c in oxygen for 1 h was 
 applied in both experiments to ensure identical starting conditions . 

the experiment shown in panel a was done directly after this oxidation 
 step ; i.e. , it refers to the fully oxidized stf catalyst . in the case 
 of the catalyst shown in panel b ,
a reductive treatment at 600 c 
 ( 1 h , 1 bar hydrogen ) was performed for the above - mentioned reasons 
 ( see section 2.4 . ) after the preoxidation step 
 and before the catalytic experiment .
the mass spectrometer signals 
 of h2 , ch4 , co , and co2 were recorded . 
 as can be clearly seen in both panels ,
the reduction / reaction starts 
 in the case of the fully oxidized sample at 430 c , and 
 on the prereduced one at around 500 c , as indicated by the pronounced 
 decrease of the h2 signal .
as the inverse water - gas shift 
 reactionalso consumes 
 co2 and produces 
 co in a fixed stoichiometric amount , the profile shown in panel b 
 is that of a  true 
inverse water - gas shift reaction 
 ( i - wgsr ) with stoichiometric reactant consumption and product formation 
 ( the water signal is not displayed since a zeolite trap was used in 
 the tpr experiments ; see section 2.4 ) .
in contrast , 
 stoichiometric co formation is apparently not the case for the fully 
 oxidized sample , given the substantially higher hydrogen consumption 
 shown in panel a , largely exceeding the stoichiometric amount with 
 respect to co. neither is co2 simultaneously consumed nor 
 sufficient co produced alongside hydrogen being reacted off .
this 
 behavior can only be consistently interpreted in terms of a reduction 
 of the perovskite catalyst by hydrogen forming oxygen vacancies in 
 the perovskite lattice , which remain mostly unreactive toward co2 , and releasing 
 h2o into the reaction mixture ; in krger  vink 
 notation this reaction readsfrom a catalytical point of view , 
 the water - gas 
 shift equilibrium is obviously disfavorably shifted toward co2 due to the ongoing hydrogen depletion of the reaction gas 
 mixture at these elevated temperatures .
this behavior can be regarded 
 as a strong indication that a suitable prereduction of the perovskite - type 
 catalyst is essential for generating sufficiently reactive vacancies 
 capable of stoichiometric co2 reduction and thus stoichiometric 
 turnover of the reactants toward the water - gas shift equilibrium .
a similar catalytic inverse water - gas shift reaction profile was 
 observed for the corresponding lsf powder sample after a reductive 
 pretreatment : under comparable experimental conditions , the reaction 
 of h2 and co2 to co ( and h2o ) starts 
 at around 500 c . as observed for stf , also the prereduced lsf 
 shows a stoichiometric inverse water - gas shift reaction profile ( cf . 

figure 6 a ) ; further details of the lsf catalyst 
 will be discussed below .
temperature - programmed reaction profiles of 
 the inverse water - gas 
 shift reaction on stf without ( panel a ) and with ( panel b ) prereduction 
 in hydrogen at 600 c ( 1 h , 1 bar )
. heating rate : 5 c min , 1:4 starting mixture of co2 and h2 including 25 mbar of h2o , 7.5 mbar of ar , and 
 he added to 1 bar total pressure . to further shed light on the hydrogen reducibility of lsf 
 and stf , 
 temperature - programmed hydrogen consumption measurements after oxidation 
 at 600 c were performed ( see figure 3 ) .
according to these measurements , on lsf hydrogen starts to be 
 steadily consumed at around 200 c with a notable rate increase 
 around 350 c .
at least two discernible steps in the hydrogen 
 consumption are visible . a very similar tpr pattern was observed on 
 stf , although the onset temperature is shifted by about 60260 
 c .
the h2 consumption appears to proceed at a slower 
 rate compared to lsf , and no clearly discernible steps were observed . 

saturation of hydrogen uptake is apparently not even reached at the 
 highest studied temperatures for both perovskite samples , as can be 
 deduced from the quite pronounced isothermal uptake , accounting for 
 yet another 200300 mol m hydrogen 
 consumption at 600 c within 60 min .
most notable , 
 however , is the outstanding amount of consumed hydrogen in the course 
 of the experiment .
after the isothermal period , a total amount of 
 3500 mol m consumed hydrogen on lsf and 
 about 2000 mol m on stf has been determined , 
 which is approximately 300 times the amount measured on highly reducible 
  simple  oxides , such as in2o3 , under comparable experimental conditions . volumetric 
 hydrogen uptake measured on stf and lsf . heating rate 
 from room temperature to 600 c at 10 c min followed by an isothermal perion at 600 c for 1 h. initial 
 hydrogen pressure : 100 mbar .
the measurement was performed using a 
 zeolite trap in the cold part of the reaction chamber for effective 
 removal of water resulting from the oxide reduction . as already mentioned above , in the case of both 
 perovskite - type 
 catalyst powders
the reductive treatment leads to an oxygen nonstoichiometry , 
 i.e. , to a formation of oxygen vacancies and the corresponding modification 
 of the electronic charge carrier concentrations . from the above - mentioned 
 amounts of consumed hydrogen
the degree of reduction  of the 
 two perovskite - type catalysts ( i.e. , their oxygen nonstoichiometry 
 in the prereduced state ) can be estimated . for la0.6sr0.4feo3 and srti0.7fe0.3o3 values of ca .
0.1 
 were obtained , respectively ( only in rough approximation , i.e. , upon 
 excluding the formation of bulk hydrogen species obscuring this mass 
 balance ; compare section 2.4 ) .
it is already 
 worth to mention that effects of pronounced reduction of perovskites 
 via reaction of lattice oxygen with h2 are also visible 
 in the respective raman spectra and x - ray diffractograms ( cf .
this indicates that the perovskite structural entity as a 
 whole is stable , but nevertheless structurally slightly altered by 
 the hydrogen treatment , at least at temperatures t  600 c .
in previous studies on simpler oxides , 
 like in2o3 or ga2o3 , the reactivity 
 in the ( inverse ) water - gas 
 shift equilibrium was determined to be strongly dependent on the prereduction 
 degree of surface and bulk regions .
thus , associated tests on the 
 oxygen vacancy quenching ability were performed on lsf and stf powders 
 by exposing the prereduced catalysts to 25 mbar of h2o 
 vapor and performing a tpr experiment ( figure 4 ) .
interestingly , no reaction of h2o on a hydrogen prereduced 
 sample has been observed for stf ( inset : the small water uptake at 
 low temperatures is an experimental artifact ) , in contrast to lsf 
 ( main panel ) , where a clear hydrogen signal , likely arising from the 
 quenching of the introduced vacancies via dissociative decomposition 
 of h2o , is observed .
the potential implications of this 
 result on the promotion of electrochemical water splitting are already 
 mentioned in section 3.1 ; those regarding the 
 activity toward attaining the ( inverse ) water - gas shift equilibrium 
 are jointly discussed in the context of figure 6 ( see below ) .
note that under the chosen experimental reduction conditions 
 ( static reduction ) , no signs of segregation of metallic iron are visible , 
 although we can not exclude the formation of undetectable modest amounts 
 of metallic iron .
tem experiments show no pronounced iron oxide segregation 
 after room temperature exposure of a reduced sample to ambient ( redissolution 
 of ionic iron in the perovskite lattice is therefore excluded ) . reactivity 
 of water with oxygen vacancies introduced by hydrogen 
 reduction as shown in figure 3 .
both samples were prereduced 
 in hydrogen at 600 c ( 1 bar , 1 h ) , recooled to room temperature , 
 and exposed to about 25 mbar of h2o vapor , before a temperature - programmed 
 reaction ( tpr ) was started .
the heating rate was set to 10 c 
 min . in figure 5 the activity of lsf and 
 stf 
 in the inverse water - gas shift reaction
is depicted ( after prereduction 
 at 600 c ) . in both panels a and b
, the time - dependent conversion 
 of co2 , normalized to the sample surface , as well as the 
 reaction temperature is plotted versus the reaction time .
the latter 
 is composed of a heating period ( 5 c min ) followed by a prolonged isothermal period in order to quantify 
 the respective isothermal reaction rate at a given conversion from 
 the respective slope of the conversion curve .
panel a shows isothermal 
 experiments at 500 c and panel b those at 450 c . for both 
 temperatures and at low conversion values , the normalized co2 conversion rate on lsf is always higher compared to that on stf . 
 at conversions approaching the water - gas shift equilibrium ,
a readout 
 of the slope makes no more sense anyway . as during the isothermal 
 periods increasing conversion
is observed , this allows the determination 
 of surface area - normalized and thus comparable reaction rates ( at 
 the same co2 conversion ) , given in mbar min m. the extracted reaction rates from the isothermal 
 range of the conversion vs time plots are summarized in table 1 .
note that the co2 conversion was calculated 
 as the ratio of the co2 pressure and total pressure of 
 co2 and co , which is equivalent to the co2 pressure 
 in the beginning .
the temperature - dependent water - gas shift equilibrium 
 for the chosen experimental conditions is shown in figure s3 of the supporting information and very much coincides 
 with the experimental values .
the rates were 
 determined in 
 both cases between 20 and 30% conversion . at 500 c , co2 conversions of about 
 40% and 60% 
 are obtained on stf and lsf , respectively , whereby the isothermal 
 reaction time of 1 h at 500 c is sufficient to approach 
 the ( i-)wgsr equilibrium only on lsf .
a qualitative comparison with 
 the water - gas shift reaction on other perovskite - type oxides , namely 
 la1xcexfeo3 and la0.9sr0.1cr1+xmnxcoxo3 , reveals that especially on the latter ( again measured at 450 and 
 500 c ) much shorter contact times ( of a few minutes ) with the 
 catalyst are sufficient to achieve equilibrium conversion , especially 
 if the chromium content is high .
we therefore conclude that our specific 
 activities may rather compare to the respective mn / co - rich samples .
note also that in combination with the hydrogen tpr experiments 
 shown in figure 3 the reaction profiles in 
 figures 2 and 4 generally 
 suggest a potential influence of the vacancies ( formed by reduction ) 
 on the catalytic properties in the water - gas shift equilibrium . in 
 general , wgsr activity of different perovskite materials is well - known and almost always directly linked to the number of oxygen vancancies , 
 as derived from chemical looping pulse experiments with sequential 
 pulsing of reactants .
note in this respect , however , that the wgsr 
 reaction mixture is therefore not admitted simultaneously , and generally , 
 no difference is made regarding the qualitative nature of the oxygen 
 vacancies .
vacancy formation is in due course usually connected 
 with the amount 
 of the iron content of the material .
however , correlating the reactivities 
 to the amount of the initial number of vacancies introduced by the 
 hydrogen treatment ( and therefore to the amount of iron ) is not feasible : 
 in doing so , one has to assume that ( i ) all the hydrogen consumption 
 leads to water formation ( hence , the amount of vacancies represents 
 only the upper limit ) and ( ii ) the amount of vacancies does not change 
 during the reaction .
as discussed above , ( i ) is clearly not fulfilled . 
 in view of figure 4 , it is also obvious that 
 ( ii ) does not hold , since the reactivity toward water is different 
 on stf and lsf . as for the direct correlation to the bulk iron content , 
 note also that the surfaces of both perovskites after the catalytic 
 runs are predominantly sro - terminated ( cf .
in particular , our results illustrate the qualitative differences 
 of surface ( near ) defect / vacancy reactivity on these materials and 
 the necessity to link the  apparent  activation barriers 
 ( see below ) to the specific oxygen affinity of such ( to some extent 
 likely fe- or ti - containing ) redox - active centers
. moreover , it appears 
 mandatory to link the surface area normalized  macroscopic  
 catalytic activity to their effective ( i.e. , not bulk - proportional ) 
 surface concentration , which is determined by the specific segregation 
 chemistry of the surfaces under realistic reaction conditions . eventually , an arrhenius estimation of the apparent activation 
 energy for the i - wgsr based on the initial rates at low conversions 
 yielded ca . 90 and ca . 130
surface - area - normalized temperature - programmed reaction 
 profiles 
 of the inverse water - gas shift reaction shown as co2 conversion 
 vs time plots at two different temperatures ( 450 c : panel b ; 
 500 c : panel a ) with extended isothermal periods for both fully 
 oxidized stf and lsf to reveal the inherently different catalytic 
 activity .
heating rate : 5 c min , 1:4 starting 
 mixture of co2 and h2 with 25 mbar of h2o , 7.5 mbar of ar , and he added to 1 bar total pressure .
figure 6 summarizes 
 a set of reactivity data necessary for a thorough understanding of 
 the ( inverse ) water - gas shift and methane reactivity . for direct comparison , 
 panels a and e again
show the inverse water - gas shift reactivity profiles 
 on lsf and stf after prereduction ( note that figure 6e corresponds to figure 2a ) . for comparison 
 to the inverse water - gas shift experiments , panels b and f
experiments have been 
 conducted both on fully oxidized ( o2 , 400 c , panels 
 d and h ) and prereduced ( h2 , 600 c , panels b and 
 f ) samples . as for the water - gas shift activity
, co is reacted off 
 with simultaneous co2 and h2 production on both 
 perovskites , if the perovskite powder is prereduced ( cf .
the onset temperature of reaction 
 is at about 300 c , therefore between the one typically observed 
 for conventional low - temperature copper - based wgs catalysts like cu / zno / al2o3 and iron - based 
 catalysts used for higher temperatures .
on fully oxidized 
 samples only co2 formation without h2 production 
 was observed ( cf .
figure s2 ) . apparently , 
 in the latter case , total oxidation of co to co2 is the 
 only observed reaction channel , occurring via reduction of reactive 
 lattice oxygen by co , but not primarily via a catalytic water - gas 
 shift reaction cycle .
this is directly proven by the simple reduction 
 of the fully oxidized perovskite systems by co without water ( figure 6 , panels d and h ) , which both show direct transformation 
 of co into co2 .
note in this respect that the start temperature 
 of the water - gas shift reaction on both catalysts is lower as compared 
 to the inverse direction ( compare panels a and b as well as e and 
 f in figure 6 ) .
additional  
 low - temperature prereduction by co ( thus forming co2 ) takes 
 place ( see also the  two - step  wgsr reaction profile 
 on stf , figure 6f ) .
obviously , the catalyst 
 adapts itself to the equilibrium state in a kinetically different 
 manner depending on the starting conditions ( co2 and h2 for the inverse ; co and h2o for the water - gas 
 shift reaction ) .
as it is well - known from experiments on simpler 
 oxide systems ( e.g. , 
 ga2o3 or in2o3 ) , the ( inverse ) 
 water - gas shift reaction can in principle follow different distinct 
 reaction pathways , depending on the temperature - dependent relative 
 contribution of surface adsorbate and defect chemistry ( i.e. , oxygen 
 affinity of vacancies / reduced surface centers vs stability of adsorbed 
 intermediates ) : a vacancy - mediated mechanism ( vmm ) , a formiate - mediated 
 mechanism ( fmm ) , or following a pathway including co dissociation . 

on ga2o3 , fmm and vmm pathways were observed , 
 depending on the degree of reduction ( i.e. , the number of vacancies ) , 
 degree of surface hydroxylation , and temperature . on in2o3 , due to its outstanding reducibility , the vm mechanism 
 was strongly dominating already at low temperatures .
in due course , 
 the question arises , which pathway is predominantly followed on the 
 more complex perovskite systems ?
our results suggest that on lsf the 
 vm mechanism is predominantly observed but appears to play a minor 
 role on stf , as can be deduced from the water - quenching experiments 
 shown in figure 4 . at the present stage of 
 experiments
, we can not exclude a possible contribution of an fm mechanism 
 on lsf , even more on stf , where future directed in situ ftir experiments will definitely show if , and in which temperature 
 regime , the reaction profile observed in panel f follows the latter 
 mechanism . as can be deduced from the inability of the vacancies to 
 dissociate water ( figure 4 ) , the vm mechanism 
 is at least not predominantly prevailing on stf .
most important in 
 this respect is a comparison of the  depth  of oxide 
 reduction and further reducibility in h2 at 600 c , 
 i.e. , a prereduced state compared with the fully oxidized state in 
 figure 2 : upon deep reduction higher temperatures 
 are required to ( re)establish reactive reduced centers in course of 
 the catalytic inverse water - gas shift reaction cycle ( figure 6a , e ) . in line with our results , recent studies by 
 diebold et al . on the wgsr mechanism on fe3o4
revealed that indeed the fmm is dominant at low temperatures via 
 oh groups and formates from oh and co. at higher temperatures ( t > 520 k ) , a defect - mediated water - splitting mechanism 
 ( mars  van krevelen type ) is shown by stm .
co dissociation , on the contrary , is certainly correlated 
 with the presence of metallic iron .
if the hydrogen partial pressure 
 and the temperature are high enough , the presence of iron carbide / metallic 
 iron can not be excluded , but if then the mechanism is still the same , 
 remains an open question . note
that after prereduction at 600 
 c the hydrogen is pumped 
 off and the sample recooled in vacuum  this treatment most probably 
 removes the major part of the adsorbed hydrogen .
admission of the 
 inverse water - gas shift reaction mixture and heating to about 300 
 c for lsf results in a readsorption of hydrogen between 300 
 and 470 c , without simultaneously observing inverse 
 water - gas shift reactivity . only above 470 c ,
a similar behavior , albeit at slightly different 
 temperatures and less pronounced , is also observed for stf ( figure 6e ) . in order to determine the rate - limiting step 
 of the reaction , thus the depth of reduction in the active working 
 state of the catalysts and the associated specific reactivity of the 
 reduced centers at the surface has to be considered .
these appear 
 clearly different from those obtained after prereduction of a fully 
 oxidized sample . in this respect ,
the water - gas shift reaction on 
 fully oxidized perovskite samples only yields total oxidation to co2 ( cf .
comparative 
 experiments on the methane reactivity clearly indicate 
 that on fully oxidized catalyst samples in the absence of water as 
 well as under steam reforming conditions methane is always totally 
 oxidized to co2 ; the corresponding catalytic results are 
 depicted in figure 6 , panels c and g , as well 
 as in figure s5 of the supporting information , respectively .
most importantly , on prereduced perovskite catalysts 
 ( h2 , 600 c , 1 h ) no methane reactivity at all was 
 observed under otherwise comparable experimental conditions ( cf . supporting information figure s6 ) .
this indicates 
 that the fully oxidized states of lsf and stf exhibit reactive oxygen 
 species capable of fully oxidizing methane  in contrast to a 
 deeply reduced state , where the remaining oxygen species obviously 
 are unreactive toward methane .
hence , also no methane reforming activity 
 is observed . especially for lsf , partial reoxidation by water has 
 been observed ( see cf .
figure 4 ) , which is 
 anyway not enough to set in methane oxidation .
this can be understood 
 on the basis that water obviously dissociatively adsorbs on the surface 
 through partial refilling of vacancies . as this does not lead to methane 
 reactivity
, we might infer that methane reactivity is hindered because 
 of missing methane activation .
this might be partially connected to 
 the iron content of the sample and the associated redox chemistry 
 ( cf .
the results on methane 
 oxidation are in good agreement with reports 
 by belessi for the reaction of co and no and spinicci et al . for the total oxidation of methane using lafeo3 , who observed a significant influence of the nonstoichiometry 
 on the activity between compositions of la0.95feo2.85 and la0.7feo2.55 .
( partial ) oxidation of methane is generally a reaction that has 
 been vastly studied on many different perovskite systems , but almost always using a chemical looping pulse technique at higher 
 reaction temperatures . summarizing  in close correlation to 
 the results presented here 
oxygen nonstoichiometry has been 
 elucidated as the key parameter governing the activity and stability . 

however , structure  activity correlations appear scarce especially 
 regarding reforming reactions , and research is therefore focused on 
 methane combustion .
it is clear , nevertheless , that upon presence 
 of water the complexity is much higher and qualitative differences 
 in the oxygen vacancies between perovskite systems may arise  as 
 can be seen in figures 4 and 6 .
h ) . panels a and e : i - wgsr , co2 + h2  co + h2o , 20 mbar of co2 and 80 mbar of h2 including 25 mbar of h2o ( preoxidation at 400 c , prereduction at 600 c ) .
panel 
 e , for the sake of clarity , again shows the reactivity profile of 
 figure 2a ; panels b and f : wgsr , co + h2o
 co2 + h2 25 mbar co and 25 mbar 
 h2o ( preoxidation at 400 c , prereduction at 600 c ) . 

panels c and g : methane oxidation on initially fully oxidized samples , 
 ch4  co2 + h2o , 20 mbar of 
 ch4 ( preoxidation 400 c ) . panels
d and h : co oxidation 
 on initially fully oxidized stf and lsf , co  co2 + v , 25 mbar of co ( preoxidation at 400 c ) . for establishment 
 of unambiguous structure 
activity correlations , a clear picture 
 of structural and morphological changes occurring upon catalyst pretreatments 
 as well as during reaction must evolve .
this is even more imperative 
 given the complexity of the perovskite systems under investigation . 

in due course , the catalysts were characterized by electron microscopy , 
 x - ray diffraction , and raman spectroscopy after each step of a full 
 inverse water - gas shift reaction cyle including preoxidation and prereduction . 

scanning high - resolution transmission electron microscopy was used 
 to determine the surface termination of both perovskites to detect 
 eventual irreversible structural changes in the catalytically relevant 
 surface regions .
x - ray diffraction and raman spectroscopy were then 
 used to investigate perovskite lattice stability and to eventually 
 corroborate the above - discussed tpr profiles , especially regarding 
 the influence of prereduction .
this in due course also raises the 
 question of full reversibility of the changes introduced both by the 
 specific pretreatments and the subsequent catalytic reaction . in order 
 to serve as a valuable ( electro)catalyst ,
all structural changes must 
 be fully reversible upon reoxidation after reduction and reaction . 
 especially for more complex systems , this is by no means a trivial 
 question and needs to be accurately assessed .
figure 7 highlights 
 the morphology of a representative 
 stf grain in the as - grown state ( corresponding high - resolution tem 
 images are highlighted in figure s7 of the supporting 
 information ) .
panel a shows a bright - field transmission electron 
 microscopy ( tem ) image of a micrometer - sized grain with some internal 
 structure being mostly due to mass  thickness contrast , whereas 
 panel b reveals that stf is basically chemically homogeneous after 
 the catalytic reaction and no pronounced chemical segregation takes 
 place . for this purpose , the corresponding ti  k ( a ) , fe 
l ( c ) , and o  k edges are mapped over the indicated 
 squared region by energy - dispersive x - ray spectroscopy ( edx ) . before 
 performing the edx experiments , a high - angle annular dark - field ( haadf ) 
 image of the grain was taken , which also shows no substantial variation 
 in chemical composition .
note that the haadf contrast is basically 
 proportional to z ( with z denoting the atom number ) , thickness t , and material 
 density . thus , regions with greater z or 
 higher thickness appear brighter in the corresponding images . by combination 
 of haadf images and edx measurements , the contrast variations in the 
 former image
bright - field 
 overview tem image of a representative stf grain ( panel 
 a ) and stem - haadf image of an extended region on the edge of a stf 
 grain to emphasize the chemical homogeneity of the sample ( panel b ) . 

l ( c ) , 
 and o  k ( d ) edges are shown as false color insets and were 
 collected in the squared region as indicated . to further shed light on the structural changes on the atomic 
 level 
 during each step of a catalytic cycle , ( scanning )
high - resolution 
 electron microscopy imaging was performed for visualizing changes 
 in the ordering of the samples , but most importantly , for changes 
 in the bulk structure and surface termination .
the corresponding 
 surface termination after performing the active 
 inverse water - gas shift state is subsequently shown in the atomically 
 resolved stem - haadf images of figure 8 ( panel 
 a , stf ; panel b , lsf ) .
ball models of the respective structures are 
 shown as large insets , with the different colors denoting the different 
 atoms present in the structures .
furthermore , due to the resolved 
 atom rows , it is possible to assign the individual atoms to the intensity 
 profile in the haadf image .
it should be noted that , despite difficulties 
 to distinguish fe from ti and sr from la , the a and the b cations 
 can clearly be distinguished .
the sr atoms , as the heaviest atoms , 
 appear correspondingly bright in these images .
thus , the surfaces 
 can clearly identified to be predominantly ( sr , la)o - terminated after 
 the catalytic run , and no fe / ti segregation was observed for any of 
 the perovskite structures . in figure 8 , panel 

b , the ( sr , la)o termination of the lsf surface is best visible at 
 the bottom left side , where a side edge of the crystallite is shown . 

these results are in line with recent results on the surface composition 
 of lsf in different gas atmospheres , obtained by in situ x - ray photoelectron spectroscopy studies , which also strongly indicate 
 a sr 
high - resolution 
 stem - haadf images of stf ( above ) and lsf ( below ) 
 with ball models of both crystallographic structures as overlayers 
 to visualize the exact position of each atom in the structure and 
 at the surface .
x - ray diffraction 
 has already been shown to be a valuable tool to investigate the bulk 
 structural changes of several perovskite systems , especially regarding 
 changes in crystallite size , lattice parameters , oxygen vacancy concentration , 
 or valence state as well as structural transformations upon oxidative 
 and reductive treatments at elevated temperatures . in due course , we will show if and how the bulk structure 
 is affected by the reductive or catalytic ( pre)treatments .
the red diffractogram in panel 
 a , corresponding to the initial , untreated sample , is in agreement 
 with the cubic structure of srfe1xtixo3 reported e.g. by adler 
 et al . , whose structure data can be used as starting values for refining .
the obtained lattice parameter of a = 3.899  ( from fitting of the peaks including the split peaks 
 of k1 and k2 at higher angles  see 
 also the inset ) corroborates this conclusion .
after reduction , however , 
 a significant shift of the peaks to lower diffraction angles is observed , 
 which can be most clearly seen in the inset highlighting the group 
 of peaks around 2 = 77. this shift has also been observed 
 previously and has , in combination with xanes spectra , been assigned 
 to formation of oxygen vacancies and the corresponding changes in 
 the fe valence state , both causing a change in the lattice parameter .
most notably , as highlighted in the green xrd 
 pattern , this shift is not reversed during prolonged contact to the 
 inverse water - gas shift reaction mixture but can only be reversed 
 by reoxidative treatment at 400 c ( see also the corresponding 
 raman spectroscopy experiments in the following section ) . for lsf 

the xrd experiments show a very similar behavior as can be seen by 
 comparing the red and black xrd pattern in figure 9 , panel b , denoting the initial state and the state after 
 final reoxidation at 400 c with oxygen , respectively .
the initial 
 diffractogram ( black ) again corresponds well to literature - reported 
 data on the rhombohedral sr0.4la0.6feo2.8 structure . using co / co2 mixtures 
 of varying content , corresponding to different reduction potentials , 
 a slight shift of the peaks as well as a corresponding intensity increase 
 of the peak at lower 2
the same features are observed : 
 the intensity of the peak at higher 2 ( at 68.4 ) can not 
 be distinguished from the background noise upon reduction ( blue trace ) 
 but more or less re - established after the catalytic reaction ( green 
 trace ) and after reoxidation . in summary ,
also the xrd experiments 
 allowed for monitoring the 
 structural changes after each step of the catalytic cycle and yielded 
 direct information with respect to the formation and reactivity of 
 oxygen vacancies after the prereduction procedures .
our xrd experiments 
 also corroborated the above - mentioned differences between stf and 
 lsf with respect to the reactivity of oxygen vacancies toward water 
 ( cf .
the changes introduced by stf 
 prereduction could not be reversed by either water formed during catalytic 
 reaction or a direct treatment but only by reoxidation in molecular 
 oxygen . in contrast , in the case of lsf the initial state is almost 
 re - established even by water formed during the catalytic reaction 
 ( and even more by a direct treatment , cf .
consequently , this again implies a different extent of 
 reduction of stf and lsf under comparable reaction conditions and 
 also the presence of more reactive defects toward h2o in 
 the case of lsf .
( a ) x - ray diffraction patterns of stf collected after 
 different 
 steps of a catalytic cycle .
initial state ( red ) , after reduction at 
 600 c in 1 bar of hydrogen ( blue ) and after catalysis as described 
 before ( i - wgsr , green ) .
the inset shows a close - up view of the peaks 
 at around 2 = 77. ( b ) x - ray diffraction patterns of lsf 
 collected after different steps of a catalytic cycle .
initial state 
 ( black ) , after reduction at 600 c in 1 bar of hydrogen ( blue ) , 
 after i - wgsr ( green ) and after reoxidation at 400 c in 1 bar 
 of oxygen ( red ) .
the inset shows a close - up view of the peaks at around 
 2 = 68. raman spectroscopy 
 has already been proven to be an indispensable tool to characterize 
 the structure of various perovskites , including stf and lsf , not only 
 in the initial state but also after various annealing treatments .
moreover , the raman spectrum of the archetypical 
 srtio3 perovskite is already well - understood , as are the 
 effects of temperature , grain boundaries , and sizes or phase transitions .
stf and lsf have also been characterized 
 regarding the effects of anodic or cathodic polarization or charge - disproportionation 
 phase transitions .
hence , as the method 
 is already well - established and a sufficiently large data set is already 
 available , it is a promising methodical approach to study the catalytic 
 effects on the structure of the perovskite systems described in the 
 preceding sections .
the experiments have been in this case restricted 
 to the stf material , as lsf shows pronounced luminescence effects , 
 rendering the measurements of real raman shifts difficult . in 
 close correlation to the other experiments , figure 10 now summarizes the raman spectra acquired 
 after different steps of catalyst pretreatment and reaction .
the black 
 spectrum refers to the initial , untreated stf perovskite , which in 
 due course will be used as reference material for all structural changes . 
 for full appreciation of the spectral changes , it is useful to give 
 a brief overview of the raman spectrum of the untreated stf state . 

the spectrum between 200 and 2000 cm reveals several 
 pronounced peaks at 125 , 180 , 325 , 480 , 540 , and 770 cm and a number of very broad signals above 1000 cm . except the one at 700 cm , these correspond 
 ( with increasing wavenumber ) to the to1 , lo1/to2 , lo2/to3 , lo3 , to4 , 
 and lo4 transversal and longitudinal optical phonon modes , respectively .
most importantly , the lo4 mode at 770 cm is hardly present in the spectrum of untreated stf . 

the most pronounced feature at 700 cm and the 
 associated shoulder at lo4 770 cm have been observed 
 by vracar et al . to be strongly affected by variations in the iron 
 content of the sample .
the latter was 
 found to be very pronounced for low iron contents but getting progressively 
 weaker in relation to the feature at 700 cm with 
 increasing iron content .
hence , peak shifts or intensity variations 
 of these peaks might be used as an indicator for local changes in 
 the structural environment of fe - containing centers . after reduction 
 at 600 c in hydrogen ( blue spectrum ) the 
 overall features do not change substantially , with the one exception 
 of an even more suppressed intensity of the lo4 mode at 770 cm . however , after the catalytic reaction ( green spectrum ) , 
 the changes are much more pronounced .
this basically refers to intensity 
 changes of the to4 and lo4 modes and to the appearance of several 
 new peaks , especially in the high wavenumber region .
new features 
 are now found at 1050 and 1500 cm , and the lo4 
 mode , previously only present as a suppressed shoulder , has now gained 
 intensity and exceeds the corresponding to4 mode .
o stretching vibration and is connected with the features at 1050 and 
 1500 cm .
the peak at 1500 cm ( a 2lo4 overtone ) gains only intensity , if the associated lo4 mode 
 gets progressively more pronounced .
the peak at 1050 cm most probably is a superposition of the overtones of the 2lo2 and 
 2to4 modes .
note also the pronounced 
 peak shift between the spectrum of the untreated perovskite and the 
 reduced state , which is practically reversed upon treatment in the 
 catalytic inverse water - gas shift reaction mixture .
after reoxidation 
 at 400 c ( red spectrum ) , all previously introduced changes are 
 completely reversed and the initial state is clearly re - established . 

taking the xrd result into consideration , the following correlation 
 arises : from the xrd measurements after prereduction , the creation 
 of an increasing number of oxygen vacancies has been inferred after 
 reduction , which is now accompanied by the almost total suppression 
 of the lo4 mode and the red - shifted peaks .
after catalysis , the xrd 
 measurements did not indicate reversal of the bulk structural changes 
 introduced by the reduction but showed that the inverse water - gas 
 shift reaction atmosphere is not able to fully replenish especially 
 the bulk vacancies .
in contrast , the raman experiments after catalysis 
 show that part of the introduced changes can be indeed reversed , as 
 indicated by the shift of lo4 peak position back to the  oxidized  
 state ( note , however , that there are still some differences between 
 the raman spectra of the untreated and  oxidized  stf 
 sample , which indicate that some irreversible structural changes are 
 still present ) .
most importantly , as the changes in the spectrum mainly 
 affect the peaks sensitive to the fe content , we might conclude that 
 these changes not only mainly affect surface - near regions but also 
 appear to be at least in part associated with a local concentration 
 change of fe - containing centers .
although the stem images showed that 
 these changes obviously do not alter the surface termination of stf , 
 we nevertheless may assume that the stf surface - near regions are in 
 a highly dynamic state , adapting easily to the reductive or oxidative 
 environments of catalyst pretreatment or reaction .
raman spectra collected 
 on stf after various catalytic treatments : 
 black , untreated sample ; blue , after reduction at 600 c in hydrogen 
 ( 1 h ) ; red , after reoxidation at 400 c ( 1 h ) ; green , after i - wgsr 
 up to 600 c ( 1 h ) .
vertical lines have been added to distinct features as a guide to 
 the eye .
in the present study the ( electro)catalytic properties of the reducible 
 perovskite - type oxides la0.6sr0.4feo3 and srti0.7fe0.3o3 were investigated .
the activity of lsf for the ( electro)oxidation 
 of hydrogen was studied by means of impedance spectroscopy experiments 
 on model - type thin film electrodes .
interestingly , the surface resistance 
 ( which is a measure of the surface activity ) at ca .
610 c in 
 a h2/h2o atmosphere was very similar to the 
 surface resistance in synthetic air . compared to stf ,
the activity 
 of lsf for the h2 oxidation reaction is significantly higher . 
 catalytic - wise
, the presented results allow the correlation of a catalytic 
 profile measured on a perovskite system to bulk and surface structural 
 changes occurring during each step of catalytic pretreatments and 
 catalytic reaction . for this correlation two different types of experiments 

were necessary : ( 1 ) catalytic measurements identifying the activity 
 of the perovskite - type 
 catalysts .
exemplarily shown for the inverse water - gas shift equilibrium , 
 a pretreatment of the perovskite powders in hydrogen is a prerequisite 
 for a successful use of lsf and stf as wgsr catalysts . using this 
 pretreatment and combining xrd and catalytic experiments ( for stf 
 also raman measurements ) , a clear difference in the surface and bulk 
 reactivity of the two perovskite - type oxides
thus , as 
 the most important parameter highly influencing the reactivity , the 
 reduction degree and depth could be determined : it controls the oxygen 
 reactivity and the specific oxidation and reduction capability of 
 the active sites in the working state of the catalysts .
especially , 
 the methane activation / oxidation ability strongly depends on the chosen 
 reduction depth .
( 2 ) by means of high - resolution tem measurements 
 on both catalyst 
 powders a predominant sro  or ( sr , la)o  surface termination 
 could be verified after a catalytic reaction , indicating that a bulk - proportional 
 surface contribution of e.g. redox - active iron centers in both catalysts 
 is unlikely and a more complex scenario , especially with regard to 
 qualitatively differently oxygen - affine surface - near vacancies and 
 a reaction- and segregation - specific concentration of such redox - active 
 sites at the surface , must be assumed .
 comparative ( electro)catalytic , structural , 
 and spectroscopic studies 
 in hydrogen electro - oxidation , the ( inverse ) water - gas shift reaction , 
 and methane conversion on two representative mixed ionic  electronic 
 conducting perovskite - type materials la0.6sr0.4feo3 ( lsf ) and srti0.7fe0.3o3 ( stf ) were performed with the 
 aim of eventually correlating ( electro)catalytic activity and associated 
 structural changes and to highlight intrinsic reactivity characteristics 
 as a function of the reduction state . 
 starting from a strongly prereduced 
 ( vacancy - rich ) initial state , only ( inverse ) water - gas shift activity 
 has been observed on both materials beyond ca . 
 450 c but no 
 catalytic methane reforming or methane decomposition reactivity up 
 to 600 c . 
 in contrast , when starting from the fully oxidized 
 state , total methane oxidation to co2 was observed on both 
 materials . 
 the catalytic performance of both perovskite - type oxides 
 is thus strongly dependent on the degree / depth of reduction , on the 
 associated reactivity of the remaining lattice oxygen , and on the 
 reduction - induced oxygen vacancies . 
 the latter are clearly more reactive 
 toward water on lsf , and this higher reactivity is linked to the superior 
 electrocatalytic performance of lsf in hydrogen oxidation . 
 combined 
 electron microscopy , x - ray diffraction , and raman measurements in 
 turn also revealed altered surface and bulk structures and reactivities . 
Introduction
Experimental Methods
Results
and Discussion
Conclusions