because 
 of the high o2-looping capacity per unit mass 
 of zn , the thermochemical zn / zno redox cycle has been considered as 
 an attractive path toward producing solar fuels . in 
 the first step that takes place at temperatures in the vicinity of 
 1800 c ( eq 1 ) , zno is thermally reduced 
 into a gaseous mixture of zn and o2 using concentrated 
 solar radiation as the heat source . upon quenching by inert gas , this mixture yields a powder consisting of zn and zno also known as  solar 
 zn  . in the following step performed 
 at temperatures in the range 3501000 c ( eq 2 ) , the solar zn is oxidized by h2o and/or co2 to produce fuel ( h2 and/or co ) and zno which is 
 then fed into the reduction step to close the cycle.12 the overall cycle efficiency 
 is measured by the heating value of 
 the fuel produced per total energy input . thermodynamic calculations 
 have shown that it increases with an increase in the yield of fuel 
 ( h2 and/or co ) , thus the extent of both reactions 1 and 2 . however , it has also been shown that achieving a high extent of 
 reaction 1 requires ultrafast quenching of the 
 product mixture to suppress the recombination of zn - vapor and o2 . this task is generally performed 
 by mixing substantial amounts of inert gas with the solar reactor 
 product stream , which adversely affects 
 the cycle efficiency due to the additional energy required for purification 
 and recycling of the inert gas quencher . nevertheless , the solid product 
 leaving the quencher as solar zn  generally contains 
 substantial amount of zno . at the same time , 
 the studies of reaction 2 have shown that below 
 its melting temperature ( 420 c ) the 
 pure metallic zn reacts rather slowly with either co2 or h2o . moreover , the complete oxidation is not possible 
 due to the passivation of the zn surface by the impervious zno product 
 scale . for this reason , previous investigators 
 have proposed the use of zn as either liquid or aerosol nanoparticles / droplets precipitated from in situ quenched zn - vapor as the preferred paths toward a high extent of the oxidation . these , 
 however , not only impose unconventional , hard - to - operate oxidizer 
 reactor concepts but they also require the additional energy to melt 
 and/or vaporize zn that has an adverse impact on the overall cycle 
 efficiency . recent research has demonstrated that the zno contamination 
 of 
 zn produced by solar thermal reduction of zno in fact facilitates 
 the oxidation of the metallic zn with either co2 or h2o . it has been shown that the oxidation 
 of solar zn by co2 could be almost completed at temperatures 
 as low as 350 c in the matter of minutes ( 90% conversion 
 in 180 s ) . this reaction time is indeed 
 2-orders of magnitude longer than the residence time reported for 
 the equivalent conversion of zn vapor ( 1 s ) . however , becaue of the difference between the feedstock 
 densities ( 0.7 kg / m for zn - vapor at 1000 c 
 versus 440 kg / m for solar zn containing only 25 
 wt % zn ) the reactor volumes required 
 for oxidizing solar zn and zn - vapor at the equivalent fuel productivity 
 are about the same . proper choice and design of a solar zn oxidizer 
 reactor should 
 be guided by a reliable kinetic model that is based on a plausible 
 reaction mechanism . in particular , such a mechanism must be able to 
 account for the pivotal role of zno in enhancing zn oxidation . this 
 enhancement has been recently attributed to a rather general multireaction 
 scheme postulated by stamatiou et al . the objective of this work is to further refine this scheme and 
 discriminate pertinent reaction pathways . for this purpose the oxidation 
 of solar zn was simulated by the oxidation of zn in blends with zno 
 or al2o3 diluents prepared from well characterized 
 commercial powders . the oxidation of diluted zn particles was explored 
 by thermogravimetry as a function of the specific area and chemical 
 nature of the diluent surface . the refined oxidation mechanism was 
 supported by the evidence provided by transmission electron microscopy 
 ( tem ) , scanning tem ( stem ) , and electron dispersive x - ray spectrometry 
 ( edx ) . the solar zn 
 surrogates were prepared by blending commercially available zn ( sigma - aldrich , 
 > 98% purity ) and zno powders ( strem chemicals or grillo ) . in some 
 experiments , al2o3 powder ( strem chemicals ) was used as an alternative to zno to assess the specific role of the 
 chemical nature of the diluent surface . to further investigate the 
 effect of the diluent surface , some blends were prepared from ingredients 
 that were immersed in water or ethanol and then dried at room temperature . all blends were prepared by shaking equivalent 89 mg aliquots 
 of the zn and a diluent in a covered petri dish for 3 min with occasional 
 stirring and breaking agglomerates with a spatula . no segregation 
 of the ingredients was observed . the raw materials were analyzed 
 for particle size distribution ( horiba la-950 laser scattering analyzer ) , 
 brunauer  emmett  teller ( bet ) equivalent specific surface 
 area ( micrometrics tristar 3000 n2 absorption analyzer ) 
 and particle morphology ( scanning electron microscopy ( sem ) , zeiss 
 supra 55vp and hitachi tm-1000 ) . the volume - based mean particle sizes d and the bet - equivalent surface areas of the materials 
 are listed in table 1 . figure 1 indicates broad size distributions of al2o3 and zno ( strem ) particles and narrower , rather similar distributions 
 of the zn and zno ( grillo ) particles . the sem pictures of zno ( strem ) 
 ( figure 2c , d ) and al2o3 powders ( figure 2e , f ) reveal the origin of 
 their large bet areas in spite of the relatively large mean particle 
 sizes : both materials comprised porous micron - sized agglomerates of 
 finer nanosized grains . the al2o3 bet area is 
 additionally enlarged by a porous structure of the grains ( average 
 pore diameter of 2.8 nm as reported by the manufacturer ) . the bet 
 areas of the round , smooth zn particles ( figure 3 ) and the needle - like zno ( grillo ) agglomerates ( figure 2a , b ) were about 2 orders of magnitude smaller . reacted 
 samples were examined by tem , stem , and edx performed with a fei tecnai 
 f30st instrument . the tem sample was prepared by dispersing the investigated 
 particles on a carbon - sputtered cu mesh holder . as reported by the manufacturer . volume - based particle size distributions of the used zn , zno , and 
 al2o3 powders . sem images of the materials used as zn diluents : zno ( grillo ) ( a , 
 b ) , zno ( strem ) ( c , d ) , and al2o3 ( strem ) ( e , 
 f ) . oxidation of solar zn 
 surrogate blends was studied isothermally in a thermogravimetric analyzer 
 ( netzsch 409 sta ) at temperatures between 350 and 400 c ( below 
 the melting point of zn ) . gas mixtures of ar ( messer 5.0 ) with co2 ( messer 4.8 ) or h2o ( generated by adrop dv2 steam 
 generator ) served as the reacting gases . the composition of the product 
 gas was analyzed by a gas chromatograph ( gc varian cp4900 with molsieve-5a / poraplot - u 
 columns ) . the tg setup has been attested for measuring the kinetics 
 free of mass and heat transfer intrusion as described elsewhere . a cylindrical alumina crucible containing 
 the sample was positioned on an alumina shielded s - type thermocouple 
 inside the tg furnace and heated in ar to a designated oxidation temperature 
 at a rate of 10 k / min . after the sample temperature reached steady - state , 
 the oxidizing gas flow was started at the balance reading designated 
 as the initial sample mass m0 . once asymptotic 
 conversion was attained under isothermal conditions , the sample was 
 heated up to 800 c at 10 k / min in 15% co2ar 
 and kept at this temperature until all the remaining zn was oxidized 
 as reflected by a constant final sample mass mf . the total mass of zn available for the oxidation ( mzn , tot ) and temporal zn conversion ( x ) were calculated as3and4where mi and m(t ) designate molecular 
 mass of species i and temporal sample mass , respectively . this method for determining mzn , tot and x eliminates errors potentially brought in by the loss of 
 zn during sample handling and desorption of impurities during the 
 heat - up to the reaction temperature . the implementation of the method 
 was reflected by remarkable repeatability of the results and good 
 agreement with previous work . the work by stamatiou et al . has reported the facilitating effect of zno 
 diluents on the zn oxidation with co2 as a function of 
 the mass fraction of different zno sources that , however , had similar 
 specific surface areas and mean particle sizes . as the mass fraction 
 of a zno diluent in a homogeneous blend with zn powder ( wzno ) increases so do both the surface area of the zno 
 per amount of zn and the degree of the dispersion of the zn particles . therefore , the reported facilitating effect of an increase in wzno on both oxidation rate and asymptotic conversion 
 of zn could be attributed to either a 
 more pronounced reaction path taking place on the zno surface or suppressed 
 sintering of the zn particles . in order to distinguish between these 
 two effects , the previously reported co2 oxidation kinetics 
 obtained with zno ( grillo ) diluent particles ( d = 6.5 m , ssa = 2.9 m / g ) was compared to the oxidation kinetics of zn blended with larger 
 but higher ssa zno ( strem ) particles ( d = 
 33.9 m , ssa = 95 m / g ) at the same wzno = 0.5 . the results shown in figure 4 indicate that at the same reaction conditions ( 375 c , 
 15% co2ar ) zn - zno ( strem ) blends attained almost 
 complete zn conversion after 10 min , whereas zn in a blend with zno 
 ( grillo ) reached the asymptotic conversion of only 0.9 after 
 25 min . moreover , at 350 c the zn - zno ( strem ) blend performed 
 similarly to the zn - zno ( grillo ) blend but at the temperature 25 c 
 higher , demonstrating also remarkable repeatability not attainable 
 at such a low temperature with the zn - zno ( grillo ) blend . as smaller zno ( grillo ) particles should favor 
 better dispersion of the zn particles , the improvements brought in 
 by larger but higher ssa zno ( strem ) particles imply that higher zn 
 oxidation rates and asymptotic conversions both stem from the direct involvement of the zno surface in 
 a reaction path parallel to the one leading to the passivation of 
 the zn surface . zn conversion during reaction in 15% co2ar 
 of 
 50 wt % zn - zno blends prepared with zno ( strem ) ( open symbols ) and 
 zno ( grillo ) ( filled squares ) . the dashed 
 line designates a repeated experiment . when using h2o as an oxidant 
 instead of co2 , the repeatability of experiments was poor . 
 additionally , overall conversion leveled off at lower values . this 
 can be seen in figure 5 showing zn conversion 
 curves for three repeated reactions of a 50 wt % zn - zno ( grillo ) blend 
 in 8% h2o  ar at 400 c . the repeatability of 
 the results could not be improved by switching the zno source from 
 grillo to strem or changing reaction conditions ( sample mass , h2o concentration , temperature ) . in contrast to the homogeneous 
 appearance of products obtained by oxidation with co2 , 
 the samples reacted with h2o exhibited a compacted brittle 
 white / yellow top crust covering the gray remainder of the material 
 underneath . as indicated by the sem pictures of a 50 wt % zn - zno ( strem ) 
 sample reacted at 400 c in 8% h2o  ar shown 
 in figure s1 in the supporting information , the top crust ( figure s1a in the supporting 
 information ) consisted of larger agglomerates and the hollow 
 shells that are typical for reacted zn particles while the powder collected from the bottom of the crucible 
 ( figure s1b in the supporting information ) consisted of smaller particles and spherical zn particles that 
 appeared barely reacted . zn conversion during reaction of a 50 wt % zn - zno 
 ( grillo ) blend 
 in 8% h2o  ar at 400 c . in order to investigate if the zn remaining in the bottom 
 layer 
 of the samples that had been exposed to steam was passivated , a reacted 
 50 wt % zn - zno ( strem ) sample that had reached asymptotic conversion 
 of 0.3 at 375 c in 30% h2o  ar was 
 homogenized with a spatula and subjected again to the same conditions . the second run demonstrated additional conversion of 0.05 
 calculated based on the zn content at the beginning of the first run 
 ( figure s2 in the supporting information ) . conversely , zn that reached asymptotic conversion in co2ar atmospheres could further react only when heated above 
 400 c . the limited zn conversion 
 in h2o atmospheres is thus attributed to the top crust 
 formed during the experiments that restricted the access of h2o to the zn in the bottom layer of the sample . the crust was 
 likely formed by sintering of zn and/or zno particles induced by the 
 presence of h2o in the atmosphere . a similar problem has been encountered while carrying the same reaction 
 in a packed bed reactor . however , the 
 tg oxidation of a real solar zn in a h2o atmosphere demonstrated 
 the complete conversion of zn indicating 
 that the sintering of the material may be specific to the investigated 
 zn - zno blends . although scrutinizing the zno diluent effect 
 on the zn oxidation 
 with h2o was precluded by a poor repeatability of the results , 
 the zn asymptotic conversions of 0.50.7 obtained with zn - zno 
 ( grillo ) at 400 c in 8% h2o  ar ( figure 5 ) are considerably higher than those reported for 
 pure zn having a similar particle size and reacting at the same temperature . therefore , the well documented favorable effect of the zno diluent 
 on the zn oxidation with co2 pertains to reactions with 
 h2o as well . it is therefore reasonable to assume that 
 both oxidation reactions follow similar mechanisms involving the zno 
 surface . the next step in elucidating the mechanism of zn 
 oxidation in the presence of a diluent was to identify the type of 
 reactions occurring on the diluent surface . the effect of the diluent 
 surface chemistry was therefore examined by switching diluent from 
 zno to al2o3 as well as by surface modifications 
 of the ingredients in zn - diluent blends by ethanol- or water - pretreatment . the temporal conversions of zn blended with al2o3 or zno ( strem ) are compared in figure 6 for oxidations with co2 or h2o at 375 c . in contrast to the zn blended with zno , the zn blended with al2o3 exhibited no measurable conversion in co2 . this finding is in agreement with previously reported results 
 obtained with larger , segregating al2o3 diluent 
 particles ( d = 81 m ) . at the same time , however , the al2o3 diluent facilitates the oxidation of zn with h2o performing 
 similarly to zno . zno ( strem ) versus al2o3 ( strem ) 
 diluent 
 effect on zn oxidation with co2 or h2o at 375 
 c . the diluent effect shown in figure 6 implies 
 that the zno diluent facilitates zn oxidation with either h2o or co2 while the al2o3 contributes 
 to the reaction only in the presence of h2o . to clarify 
 the origin of this difference , the effects of the diluent surface 
 modifications was explored through the influence of powder pretreatments 
 on the zn oxidation with co2 . figure 7 compares the oxidation performance of a standard 50 wt % zn - zno 
 ( strem ) blend in 15% co2ar at 375 c with 
 the performance of the same composition blends prepared with zno ( strem ) 
 powders that were prior to reaction subjected to water - wash , ethanol - wash , 
 or a 4 h drying step at 450 c under ar flow . as seen in this 
 figure , almost complete conversion could be attained with all these 
 blends . compared to the standard blend , ethanol - pretreated zno contributed 
 to a slightly faster oxidation rate ; the water - pretreated zno demonstrated 
 the fastest decay in the oxidation rate underperforming even the dried 
 out zno at zn conversions greater than 0.4.conversely , as illustrated in figure 8 the performance of zn al2o3 blends was substantially affected by either water- or ethanol - pretreatment 
 of al2o3 contributing to the asymptotic conversions 
 as high as 0.9 and 0.75 , respectively . as the zn particles 
 pretreated with ethanol , dried at room temperature , and then blended 
 with the standard al2o3 powder did not show 
 any reactivity under the same reaction conditions ( run not shown ) , 
 the enhancements by the water- and ethanol - pretreatments demonstrated 
 in figure 8 have been attributed solely to 
 the surface modifications of al2o3 . effect of zno 
 ( strem ) pretreatment on the zn conversion during 
 reaction of 50 wt % zn - zno blends at 375 c in 15% co2ar . effect of al2o3 pretreatment on conversion 
 of zn during reaction of 50 wt % zn understanding 
 why untreated al2o3 does not promote zn oxidation 
 with co2 and how it is activated by the ethanol- or water - pretreatment 
 is essential to elucidating reaction steps involving the diluent surface . 
 to test if the lack of its chemical affinity toward co2 was the reason , tg chemisorption experiments were performed at 375 
 c using 15% co2ar . results shown in figure 9 indicate that both ethanol - treated and untreated 
 al2o3 demonstrated essentially the same co2 chemisorption kinetics and equilibrium capacity as the zno 
 ( strem ) ; the zno ( grillo ) sample showed no measurable co2 chemisorption . this implies that the amount of co2 chemisorbed 
 on a diluent surface is not affecting the zn oxidation performance . relative 
 mass change during tg chemisorption of co2 from 
 15% co2ar on zno ( strem ) , zno ( grillo ) , ethanol - treated 
 al2o3 , and untreated al2o3 materials at 375 c . a clue to the reaction mechanism responsible for enhanced 
 oxidation 
 in the presence of pretreated al2o3 is revealed 
 by the temporal h2 concentrations in the effluent gas acquired 
 during preheating of the sample to the reaction temperature in ar . figure 10 compares these alongside relative 
 sample masses and temperatures for two tg runs performed with blends 
 containing either untreated ( figure 10a ) or 
 ethanol - pretreated al2o3 as diluent ( figure 10b ) . both blends were subjected to the identical 
 standard experimental schedule . in the case of ethanol - pretreated 
 al2o3 , a significant h2 peak is observed 
 during the initial heating in ar and the reaction begins immediately 
 upon exposure to co2 . in contrast , in the presence of untreated 
 al2o3 no h2 evolution is observed 
 during preheating and the reaction does not proceed upon exposure 
 to co2 . the later h2 peak observed during the 
 onset of the reaction with co2 are attributed to trace 
 h2o impurities in the zn material used as they are common for all experiments regardless of whether 
 or which diluent was used . temporal h2 concentration in effluent 
 gas ( filled symbols ) , 
 sample mass ( open symbols ) , and temperature ( dashed line ) during temperature - programmed 
 tg heat - up in ar ( shaded area ) followed by oxidation in 15% co2ar of 50 wt % zn al2o3 blends prepared with ( a ) untreated and ( b ) ethanol - pretreated al2o3 . h2 evolution was not observed during preheating 
 blends 
 of untreated al2o3 with ethanol - pretreated zn 
 ( not shown ) . namely , high zn conversion after subsequent exposure 
 to co2 was found to be related solely to the modifications 
 of the al2o3 surface induced by the ethanol 
 pretreatment . moreover , a 50 min thermal treatment of ethanol - pretreated 
 al2o3 at 375 c under ar purge reversed 
 the effect of the pretreatment ; the zn subsequently blended with the 
 equal mass of this thermally treated ethanol - washed al2o3 showed no reactivity with co2 . the 
 above findings imply that the h2 evolution from 
 pretreated samples during preheating could be related to the oxidation 
 of zn on the ethanol pretreated al2o3 surface . the surface hydroxyl groups 
 then react with zn(g ) sublimated from the zn particles at elevated 
 temperatures producing h2 and zno:5 this reaction 
 has also been used to explain the facilitating effect 
 of hydroxylated al2o3 on deposition of various 
 other metals from the vapor phase . once the zno seeds are 
 formed on the al2o3 surface , they facilitate 
 the subsequent reaction of zn with co2 in the same way 
 the zno diluent particles in zn - zno blends do . it is believed that 
 the activation of al2o3 by the water pretreatment 
 demonstrated in figure 8 is caused by the very 
 same effect . if , however , hydroxylated al2o3 is heated to 375 c in the absence of zn(g ) , the hydroxyl groups 
 are desorbed . therefore , the formation of the zno seeds during subsequent 
 heating up in the presence of zn(g ) is precluded . the effect 
 of the zno surface area on the zn conversion and the 
 sensitivity of the reaction on the diluent surface chemistry both 
 imply the growth of zno on the diluent surface as a reaction path 
 parallel to the formation of the impervious zno scale over the surface 
 of the zn particles . in order to provide conclusive evidence for this 
 reaction path , a sample of zn blended with equal weight of ethanol - treated 
 al2o3 was reacted for 3 min in 15% co2ar at 375 c to conversion of x  
 0.5 . the reaction product is then cooled in ar to room temperature 
 and analyzed by sem , tem , and stem - edx . as the same reaction mechanism 
 is assumed to govern the oxidation of both zn - zno ( i.e. , solar zn ) 
 and zn-(ethanol - treated al2o3 ) blends , the ethanol - treated 
 al2o3 was chosen as diluent from two reasons : 
 ( a ) it is chemically distinguishable from the reaction product ( zno ) 
 in edx and ( b ) because of a small size and polyhedral shape of its 
 particles it can be visually differentiated from the larger and round 
 zn particles in sem and tem . the sem image enclosed as figure 11 reveals 
 two types of particles in the postreaction product : the spherical , 
 hollow , ruptured scales that are reminiscent of the original zn particles 
 and agglomerates of smaller polyhedral particles resembling the al2o3 material shown in figure 2e , f . the scales are believed to represent zno ; their appearance is 
 substantially different from the thick , homogeneous , nonruptured scales 
 completely covering the zn particles that were observed by loutzenhiser 
 et al . after reaction of undiluted zn 
 at 400 c in 15% co2ar . as the sample represented 
 by figure 12 reacted only partially ( x  0.5 ) , a certain amount of reactive zn was expected 
 to still remain within the scales . sem image of a 50 wt % zn-(ethanol - treated 
 al2o3 ) blend converted to x  0.5 in 15% 
 co2ar at 375 c . tem image of an al2o3 agglomerate from a 
 50 wt % zn-(ethanol - treated al2o3 ) blend reacted 
 to x  0.5 ( left ) and the results of the edx 
 elemental analysis over the segment outlined by the white square ( right ) . the region of the sample outlined 
 by the white square in figure 12 was further 
 analyzed with stem - edx . this particular 
 particle agglomerate was targeted for these analyses because it resembled 
 the alumina particles ( figure 2e , f ) . moreover , 
 it was reasonably isolated from the surrounding particles resembling 
 the original zn particles that were thus likely to contain both the 
 zn and zno phases . the latter was of a particular interest as the 
 detection of spurious x - rays generated outside the investigated area 
 due to electron scattering or x - ray fluorescence has been a known 
 shortcoming of edx analysis . the 
 acquired high resolution tem images of several al2o3 particle agglomerates did not reveal the presence of 
 the distinctive zno and al2o3 phases . although 
 they did indicate some variability in the contrast across the investigated 
 region ( figure s3 in the supporting information ) , this variability was rather gradual thus implying the spatial 
 distribution of the sample thickness rather than the differences in 
 the atomic numbers . on the other hand , 
 the stem - edx analysis ( figure 12 , right ) did 
 reveal peaks of o , al , and zn together with those of c and cu . the 
 latter two peaks are the artifacts caused by the sample - supporting 
 film and the cu - grid over which the sample was suspended . the strong 
 al peak confirmed that the visual resemblance to an al2o3 agglomerate had merit in choosing the sample region 
 to be analyzed . the o peak was thus at least partly caused by the 
 al2o3 agglomerate . whether it also reflected 
 the zno deposited over the al2o3 surface depended 
 merely on ruling out that the zn peak originated from spurious reflections 
 by the zn and/or zno phases located outside the scanned region . the 
 latter was achieved by comparing the intensities of the cu and zn 
 peaks while considering that substantially more cu than zn was present 
 by mass in the surroundings of the scanned sample region . as the cu zn 
 cliff - lorimer factor is of the order of 1 , if the analysis were falsified by spurious radiation from the surrounding 
 zn particles the intensity of the zn peak would not be as strong compared 
 to the intensity of the cu peak . therefore , it has been concluded 
 that the acquired zn peak reflected the presence of the zno layer 
 uniformly distributed over the al2o3 diluent 
 surface by the growth from the seed zno sites formed per eq 5 . the experimental findings and the analytical evidence 
 presented in the previous sections provide a basis for refining the 
 mechanism of zn oxidation by h2o and co2 in 
 the presence of zno stipulated by stamatiou et al . namely , a gas - phase reaction of zn(g ) with either of the 
 oxidants producing zno nanoclusters is ruled out as a pertinent reaction 
 path because it would lead to high conversions irrespective of the 
 diluent employed in the experiments . instead , as illustrated by figure 13a for the case of co2 as oxidant , the 
 zno product is formed by parallel reactions around the zn particles 
 ( solid lines ) and on the zno seed sites at the surface of the diluent 
 particles ( dashed lines ) . in the early stage of the reaction ( t1 ) , a thin zno scale quickly develops over the 
 surface of the zn particles leaving apertures and crevices for the 
 passage of the sublimated zn(g ) toward the zno diluent and oxidants 
 toward the reminder of the zn particles ( t2 > t1 ) . the lateral scale growth eventually 
 blocks the zn surface , thereby inhibiting further zn sublimation ( t3 > t2 ) . in the 
 absence of a zno diluent ( figure 13b ) , the 
 zn sublimation is substantially slower , thus less zn escapes through 
 the apertures before they close . the enclosed zn may further react 
 only via a rather slow solid - state diffusion through the zno scale . therefore , the proposed mechanism stipulates 
 that the maximum conversion of zn is governed by the ratio of the 
 zno scale growth and the zn sublimation rates and that the presence 
 of a zno diluent facilitates the latter . schematic of the mechanism 
 proposed for the oxidation of metallic 
 zn with co2 in the presence ( a ) and the absence ( b ) of 
 zno diluent . a similar reaction mechanism is expected for the oxidation 
 with h2o . for a given temperature , pressure , and coverage of the zn 
 surface , the only remaining parameter controlling the zn sublimation rate is 
 the difference between the equilibrium vapor pressure of zn at the 
 zn surface and the vapor pressure of zn in the surrounding gas . in 
 spite of low equilibrium zn vapor pressures for the investigated temperature 
 range , it has been shown that the sublimation rate of zn can be rather 
 fast in high vacuum ( 30 s for a 2 m particle ) . therefore , the zn sublimation may be sustained 
 by an effective sink of zn(g ) outside the zn particles that is provided 
 by a fast reaction at the zno sites on the diluent particles . although 
 the information available at present is not sufficient to discriminate 
 among potential surface reactions involving adsorbed zn and/or oxidants , 
 the following scenarios are consistent with both experimental findings 
 of this work and the literature . the observed difference 
 in the effects of zno ( facilitating ) and al2o3 ( no effect ) as diluents in the zn oxidation with co2 may 
 be supported by the difference in the interaction of zn(g ) with zno 
 and al2o3 surfaces . the crystal structure of 
 the zno surface inherently provides sites for zn(g ) deposition . furthermore , 
 zno shows nonstoichiometric behavior permitting the presence of excess 
 zn in the crystal which may facilitate incorporation of metallic zn 
 from zn(g ) on its surface . on the other 
 hand , it has been reported that zn(g ) does not interact with al2o3 at temperatures within the investigated range . because an al2o3 diluent does not facilitate the zn oxidation with co2 , this hypothesis implies that the co2 chemisorbs differently 
 on the zno and al2o3 surfaces , yielding the 
 active oxygen on the zno only . on the other hand the surface hydroxyls may then 
 react with zn(g ) to provide the initial zno sites on the al2o3 . once these sites are formed , they participate in the 
 further zn oxidation in the same way the zno diluent does . this work investigates 
 the mechanism of oxidation of metallic zn 
 in blends with zno or al2o3 diluents prepared 
 from well - characterized commercial powders . the effects of diluent 
 surface area and ethanol- or water - pretreatment of the starting powders 
 were explored within 350400 c by thermogravimetry using 
 co2 or h2o in mixtures with ar as oxidants . the presence of a zno diluent facilitates oxidation by either co2 or h2o . because of substantial sintering of samples , 
 the repeatability of experiments performed in 830% h2o  ar was poor . the participation of the pretreated al2o3 in 
 the oxidation by co2 has been attributed to the formation 
 of the zno seed sites on its surface . it is believed that these sites 
 were formed in situ during heat - up to the reaction temperature in 
 ar by the reaction of zn vapor with hydroxilated al2o3 surface . electron microscopy and edx spectroscopy have supported 
 this hypothesis by revealing substantial deposition of produced zno 
 on the surface of pretreated al2o3 . the 
 refined zn oxidation mechanism stipulates that the maximum 
 achievable conversion of zn is governed by the ratio of the zno scale 
 growth over the zn surface and the zn sublimation rates . the presence 
 of a zno diluent facilitates the latter by serving as an effective 
 sink for zn(g ) , thereby increasing the driving force for the diffusion 
 of sublimated zn from the surface of the solid zn . it is not clear 
 whether the zno surface provides the sink for zn(g ) by its affinity 
 for zn atoms or by its interaction with oxidants that activates them 
 for a fast reaction with zn(g ) .
in this work we investigate the mechanism 
 of zn oxidation with 
 co2 and/or h2o to produce solar derived fuels 
 ( co and/or h2 ) as part of the zn / zno thermochemical redox 
 cycle . 
 it has been observed that the zno contamination of zn produced 
 by solar thermal reduction of zno ( solar zn ) facilitates oxidation 
 of the metallic zn by co2 and h2o , allowing 
 for nearly complete conversion at temperatures as low as 350 c . 
 reaching the same reaction extent starting with pure zn requires considerably 
 higher temperatures which imposes use of unconventional hard - to - operate 
 reaction configurations utilizing zn as vapor . 
 the mechanism of this 
 enhancement is investigated by studying the oxidation of solid zn 
 diluted with zno or al2o3 at 350400 
 c utilizing thermogravimetry . 
 it is found that zno acts as the 
 site for the oxidation of zn originating from the vapor phase , thereby 
 serving as a sink for zn vapor and maintaining the driving force for 
 sustainable zn sublimation . 
 as this zn sublimation competes with the 
 growth of an impervious zno scale over the surface of the remaining 
 solid zn , the presence of the zno increases the reaction extent according 
 to the magnitude of its surface area . 
 this mechanism is supported 
 by energy - dispersive x - ray ( edx ) spectroscopy , revealing a substantial 
 deposition of produced zno over the surface of the zno - seeded al2o3 diluent .