Supporting Information

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1 Supporting Information Splitting of CO 2 by Manganite Perovskites to Generate CO by Solar Isothermal Redox Cycling Sunita Dey and C. N. R. Rao* Chemistry and Physics of Materials Unit, Sheikh Saqr Laboratory, International Centre for Materials Science (ICMS), and CSIR Centre of Excellence in Chemistry. Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore , India. * cnrrao@jncasr.ac.in Reactivity Tests: 1. Reduction time variation: To check the role of reduction time on fuel productivity of LSM50, the reduction duration (t red ) was increased as 15 min 20 min 25 under sweep gas ( =10-5 atm) flow of 0 ml/min/g. The oxidation time (t oxd ) was 5 min under CO 2 ( =1 atm) flow of 0 ml/min/mg. T I was 1773K. 2. Sweep gas flow rate variation during reduction: Sweep gas flow rate ( =10-5 atm) was varied as 1500ml/min/g 0ml/min/g 600ml/min/g 200ml/min/g during reduction whereas CO 2 flow rate ( =1 atm) was kept constant at 800 ml/min/g. t red (15 min) was more than t oxd (5 min). T I was 1773K. 3. CO 2 flow rate variation during oxidation: CO 2 flow rate ( =1 atm) was varied as 1400ml/min/g 0ml/min/g 800ml/min/g 600ml/min/g 400ml/min/g 200ml/min/g during oxidation whereas sweep gas ( =10-5 atm) flow rate was kept constant at 0ml/min/g. t oxd (15 min) was more than t red (10 min). T I was 1773K. 4. Multiple cycling: During each cycle LSM50 was maintained at 1773K for t red of 10 min during reduction ( =10-5 atm) and t oxd of 5 min under oxidation ( =1 atm) with flow rates of sweep gas and CO 2 are being 0 and 400 ml/min/g respectively. S1

Yobs Ycal CeO2 Intensity (a.u.) Yobs-Ycal Bragg positions 5 µm 20 40 60 80 2 ( ) 120 Figure S1. Lebail fitted PXRD patterns and FESEM image of as synthesized CeO2.

2 Yobs Ycal CeO2 Intensity (a.u.) Yobs-Ycal Bragg positions 5 µm ( ) 120 Figure S1. Lebail fitted PXRD patterns and FESEM image of as synthesized CeO2. Lattice parameters are summarized in Table S1. Yobs Ycal Intensity (a.u.) LSM 30 Yobs-Ycal Bragg positions LSM 40 Yobs-Ycal Bragg positions ( ) 80 YSM 50 (d) Yobs Ycal Yobs-Ycal Bragg positions Yobs Ycal Intensity (a.u.) LSM 50 Intensity (a.u.) ( ) 2 ( ) (c) Yobs Ycal Intensity (a.u.) Yobs-Ycal Bragg positions ( ) Figure S2. Lebail fitted PXRD patterns of LSM30 LSM40 (c) LSM50 and (d) YSM50. LSM 30 and LSM 40 crystallize in the rhombohedral structure (Space group R-3c) whereas LSM50 adopts tetragonal symmetry (S.G. I4/mcm). YSM50 crystallizes in orthorhombic symmetry (S.G. Pnma). Lattice parameters are summarized in Table S1. S2 120

3 Counts (a.u.) Counts (a.u.) x=0.5 x=0.4 x=0.3 YSM50 LSM ( ) ( ) Figure S3. Magnified range of to show the systematic change in PXRD due to increasing x in La 1-x Sr x MnO 3 (x=0.3, 0.4 and 0.5) and the shifts in Bragg peak of YSM50 than LSM50. Table S1. Summary of Space group (S.G.), crystalline sizes (calculated using Debye-Scherrer equation based on PXRD peak at 2 ~ 33 ) and lattice parameters (obtained from Lebail fitting, see Figure. S1) of CeO 2, LSM30, LSM40, LSM50 and YSM50. materials Crystal S. G. Cell size a (Å) b (Å) c (Å) vol. (Å 3 ) CeO Fm-3m (1) (1) (1) La 0.7 Sr 0.3 MnO 3 (LSM 30) 81 R-3c (1) (1) (4) La 0.6 Sr 0.4 MnO 3 (LSM 40) 87 R-3c (2) (2) (7) La 0.5 Sr 0.5 MnO 3 (LSM 50) 52 I4/mcm (4) (4) (6) Y 0.5 Sr 0.5 MnO 3 (YSM 50) 62 Pnma (4) (4) (4) S3

4 Figure S4. EDAX (X and Y axis are Energy in KeV and counts respectively) of as synthesized LSM50 and YSM50. Table S2. Atomic % of La, Y, Sr, Mn and O present in La 1-x Sr x MnO 3 { x=0.3, x=0.4, (c) x=0.5} and Y 0.5 Sr 0.5 MnO 3 as calculated from EDS analysis. La L Y L Sr L Mn K O K Atomic Atomic Atomic Atomic Atomic Compound % % % % % LSM LSM LSM YSM S4

5 5 µm 5 µm (c) (d) 5 µm 5 µm (e) (e) 20 µm 20 µm Figure S5. FESEM images of the porous structural features of LSM30, LSM40, (c) LSM50 and (d) YSM50 before high temperature thermogravimetric cycles. (e) Overall porus structure morphology in lower magnification for LSM50. S5

6 3.0 LSM LSM (c) LSM K 973K 1073K 1173K 1273K log po 2 (atm) K 973K 1073K 1173K 1273K log po 2 (atm) K 973K 1073K 1173K 1273K log po 2 (atm) Figure S6. Oxygen nonstoichiometry as a function of temperature (873K-1273K) for different La 1-x Sr x MnO 3 (x=0.3, x=0.4 and x=0.5) compositions as experimentally measured by Tagawa et. al and Mizusaki et.al (adapted from refs. 31 and 32 of main text). These values are extrapolated to obtain the oxygen nonstoichiometry at high temperatures (1673K and 1773K) as mentioned in detail in the main text. Thermodynamic calculations: In Figures 1a and b we have showed the thermodynamically calculated oxygen nonstoichiometry isotherms of LSM30, LSM40 and LSM50 at 1673K and 1773K respectively. To obtain these, we have utilized the low temperature (873K-1273K) oxygen nonstoichiometry data sets of Mizusaki and Tagawa et. al (Figure S6). For ABO 3 type perovskites, the reduction reaction can be explained as, ( ) ( ) The Gibbs free energy for this redox reaction is given by, ( ) ( ) ( ) Hence, is the equilibrium constant, R the universal gas constant and and are the standard enthalpy and entropy of redox reactions. Taking activities of solids as unity and O 2 as an ideal gas, in the limit of eqn. (S1) gives, S6

7 ln (po 2 /atm) ln (po 2 /atm) ln (po 2 /atm) ( ) ( ), where is the oxygen partial pressure with respect to standard state. Combining eqns. (S1) to (S3) gives, ( ) ( ) ( ) ( ) The Arrhenius plots of as a function T -1 for a set of are obtained considering and to be independent of temperature (Figure S7). From the linear plots we obtain the standard enthalpy change ( ) and entropy change ( ) at different values (Figure S8). These and values can be used to extrapolate the datas at higher temperatures through numerical treatment of eqn. (S4). A significant decrease in reduction enthalpy ( ) occurs with increasing Sr content (Figure S8), suggesting the ease in reducibility with higher Sr content LSM LSM (c) LSM /T (K -1 ) /T (K -1 ) /T (K -1 ) Figure S7. Arrhenius type plots of oxygen partial pressure as a function of temperature for selected δ values for different La 1-x Sr x MnO 3 (x=0.3, x=0.4 and x=0.5) compositions derived from the isotherm of Figure S6. The slope and intercept of each straight line defines the molar enthalpy and molar entropy at that particular δ. S7

8 150 LSM50 LSM40 LSM LSM50 LSM40 LSM30 H 0 red (KJ/mol) S 0 red (J/mol/K) Figure S8. Variations of thermodynamic functions, namely partial molar enthalpy and partial molar entropy of reduction of La 1-x Sr x MnO 3 (x=0.3, x=0.4 and x=0.5) as a function of oxygen nonstoichiometry δ LSM LSM log po 2 (atm) log po 2 (atm) Figure S9. Theoretically obtained oxygen nonstoichiometry of present work as a function of oxygen partial pressure at 1673K (green curve) and 1773K (black curve) is presented in comparison with the data points from refs. 22 and 30. The red curve corresponds to data from ref 22 (1673K). The orange (1673K) and blue curve (1773K) corresponds to data from ref 30. S8

9 CO yield ( mol/g) LSM atm 0.5 atm 0.2 atm K T I (K) 1773K Figure S10. Variations in equilibrium CO productivity of LSM50 with several values as shown for two different isothermal CO 2 splitting temperatures (1673K and 1773K). Detailed calculations of TGA analysis: The weight loss in TG measurements are converted to the amount of O 2 released per gram of composites by the following equation, n O2 Δm [M. m ] (S5) Δm is the mass loss observed by TG during reduction, M is the molecular weight of O 2 and m the mass of the reactive substrate during the TG experiment. The CO produced (mol/g) is calculated as follows, n CO Δm [M. m ] (S6) Δm is the mass gain observed by TG during CO 2 splitting and M is the atomic weight of O. S9

10 3- Weight (%) 3.00 i eq oxd (i) (ii) red Time (min) Time (min) Figure S11. Representative TG plots (oxygen nonstoichiometry as a function of time) to mark the oxygen nonstoichiometry during initial equilibration (δ eq ), after reduction (δ red ) and after oxidation (δ oxd ) where (i) difference between δ red and δ eq is the reduction yield and (ii) difference between δ red and δ oxd is the oxidation yield. Actual TG plot (% weight vs time) obtained from the instrument is given for reference. Table S3. Summarize the values of oxygen nonstoichiometry at initial equilibration (δ eq ), after reduction (δ red ) and reduction yield ( δ) as a function of temperature and composition of perovskites. materials T I = 1673K T I = 1773K δ eq δ red δ δ eq δ red δ La 0.7 Sr 0.3 MnO 3 (LSM 30) La 0.6 Sr 0.4 MnO 3 (LSM 40) La 0.5 Sr 0.5 MnO 3 (LSM 50) S10

11 3- Temperature (K) 3- Temperature ( C) CeO 2 T I =1773K Time (min) 5 µm Figure S12. Representative TGA of isothermal CO 2 splitting of CeO 2 at 1773 K. t red and t oxd are 15 min and 10 min respectively. =10-5 atm during reduction and = 1 atm during oxidation with flow rates of gases being 0ml/min/g K (i) eq oxd 2.92 red (ii) (iii) Time (min) Figure S13. Representative TG plot (oxygen nonstoichiometry as a function of time) of LSM40 to mark the oxygen nonstoichiometry during initial equilibration (δ eq ), after reduction (δ red ) and after oxidation (δ oxd ). (i) LSM40 is first heated at 1673K under CO 2 and maintains for 15 mins to reach δ eq (ii) CO 2 (g) is replaced with sweep gas, reduces for 15 mins and reaches δ red (iii) oxidation under CO 2 is carried out isothermally. S11

12 CO yield ( mol/g) GC intensity yield ( mol/g) 1.6x10 4 CO 2 (g) x x x10 3 CO(g) CO ( mol/g) Time (min) 60 TGA GC-TCD LSM30 LSM40 LSM50 Figure S14. CO and CO 2 signal in GC-TCD and Thermo gravimetrically estimated CO in comparison with TCD detected CO of LSM40 at 1673K K K K K O x=0.3 x=0.4 x=0.5 x in La 1-x Sr x MnO 3 Figure S15. Thermo gravimetrically measured and theoretically estimated O 2 and CO yield of La 1-x Sr x MnO 3 (x=0.3, 0.4 and 0.5). Theoretical calculations estimated the maximum yield possible under reduction at =10-5 atm and oxidation under infinite supply of CO 2 at = 1 atm (no time bounds). Experiments are undertaken in 15 min and 10 min of t red ( and t oxd ( = 1 atm) respectively with the flow rates being 0 ml/min/g of each gases. S12 =10-5 atm)

13 Weight (%) O 2 and CO produced ( mol/g) Table S4. Experimentally obtained CO yield of perovskites are presented along with theoretically estimated values. Values are in µmol/g. materials 1673K 1773K theoretical experiment % obtained theoretical experiment % obtained LSM LSM LSM t red = 25 mins t red = 20 mins t red = 15 mins O 2 CO t red (min) t oxd (min) 99.9 t oxd =5 mins LSM50 T I =1773 K Time (min) Figure S16. O 2 production (CO yield) varies as a function of reduction time (t red ) for LSM50 during ITCS at 1773K. t red was ascended from 15 min to 25 min keeping the t oxd constant at 5 min. Flow rates of sweep gas ( =10-5 atm) and CO 2 ( = 1 atm) are being 0 ml/min/g. Corresponding histogram quantifies the total O 2 and CO yield as observed during TG measurement S13

14 Overall CO production rate ( mol/g/hr) CO yield ( mol/g) Table S5. Conversion % of CO (experimentally) as a function of reduction time (t red ). t red (min) Experimental Predicted % conversion CO yield (µmol/g) CO yield (µmol/g) Reduction time (t red ) in min Total cycle time (min) 220 Figure S17. Estimated rate of overall CO production with total cycle time (blue curve) and CO yield as a function of t red (black curve) for LSM 50. The total cycle time is the addition of t red and t oxd. t oxd is 5 min for all the measurements. T I is 1773K. Flow rates of sweep gas and CO 2 are being 0ml/min/g. S14

15 Weight (%) N 2 flow rate (ml/min/g) 1: 1500 ; 3: 600 2: 0 ; 4: time avg O 2 production rate ( mol/min/g) Total O 2 and CO yield ( mol/g) O 2 CO Sweep gas flow rate (ml/min/mg of sample) Time (min) Sweep gas flow rate (ml/min/g of sample) Figure S18. Representative TGA (weight % vs time) and average rate of O 2 production (µmol/min/g) of LSM50 over 9.9 min (T I is 1773K) as a function of sweep gas ( =10-5 atm) flow rates of 1500 to 200 ml/min/g. In Fig a, the points marked by arrow numbered 1-4 specifies the entry timings of sweep gas at various flow rates. Inset quantifies the total O 2 and CO yield as a function of variable sweep gas flow rates. t red (15 min) is greater than t oxd (5 min) during this experiment with constant CO 2 ( = 1 atm) flow rates of 800 ml/min/g during oxidation. S15

16 Weight (%) 98.8 CO 2 flow rate (ml/min/g) 1: 1400 ; 4: 600 2: 0 ; 5: 400 3: 800 ; 6: time avg CO production rate ( mol/min/g) Time (min) 50 (c) CO 2 flow rate (ml/min/g of sample) CO production rate ( mol/min/g) Total O 2 and CO yield ( mol/g) CO 2 flow rate (ml/min/g) Time (min) (d) O 2 CO CO 2 flow rate (ml/min/mg of sample) Figure S19. Representative TGA (weight % vs time) global rate of CO production (µmol/min/g) (c) average rate (initial 3 min of oxidation) of CO production (µmol/min/g) and (d) corresponding total O 2 and CO yield (µmol/g) of LSM50 (T I is 1773K) as a function of CO 2 ( = 1 atm) flow rates of 1400 to 200 ml/min/g. In Fig. a, the points marked by arrow numbered 1-6 specifies the entry timings of oxidant at various flow rates. t oxd (15 min) is greater than t red (10 min) during this experiment with constant flow of sweep gas ( =10-5 atm) of 0 ml/min/g during reduction. S16

Counts (a.u.) Counts (a.u.) LSM50-after TGA LSM50-before TGA LSM40-after TGA LSM40-before TGA LSM30-after TGA 20 µm LSM30-before TGA 20 40 60 80 120 2 ( ) Figure S20.

17 Counts (a.u.) Counts (a.u.) LSM50-after TGA LSM50-before TGA LSM40-after TGA LSM40-before TGA LSM30-after TGA 20 µm LSM30-before TGA ( ) Figure S20. PXRD patterns of La 1-x Sr x MnO 3 (x=0.3, x=0.4 and x=0.5) after high temperature thermogravimetric analysis in comparison with as synthesized perovskites. FESEM image of LSM50 after multiple cycles of thermogravimetric CO production. ** * YSM50-after TGA_1773 K (iii) YSM50-after TGA_1673 K (ii) YSM50-before TGA (i) ( ) Figure S21. PXRD patterns of YSM50 (i) as synthesized and after isothermal TG measurement at (ii) 1673K and (iii) 1773K. The black circular area highlighted the difference in diffraction S17

18 pattern due to the formation of YMnO 3 phase after fuel production at 1773K.The impurity peaks are marked with (*). (c) (d) Figure S22. FESEM images of LSM40 after high temperature thermogravimetric cycles. Magnifications are x, x, (c) 000 x and (d) x. S18

Chapter 17.3 Entropy and Spontaneity Objectives Define entropy and examine its statistical nature Predict the sign of entropy changes for phase

Chapter 17.3 Entropy and Spontaneity Objectives Define entropy and examine its statistical nature Predict the sign of entropy changes for phase changes Apply the second law of thermodynamics to chemical