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Non-reducible, Basic La2O3 to Reducible, Acidic La2-xSbxO3 with Significant Oxygen Storage Capacity, Lower Band Gap and Effect on the Catalytic Activity Aman Pandey, Gunisha Jain, Divya Vyas, Silvia Irusta, and Sudhanshu Sharma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10821 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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The Journal of Physical Chemistry

Non-reducible, Basic La2O3 to Reducible, Acidic La2-xSbxO3 with Significant Oxygen Storage Capacity, Lower band gap and effect on the catalytic activity Aman Pandey, Gunisha Jain1, Divya Vyas, Silvia Irusta2 and Sudhanshu Sharma* Department of Chemistry, Indian Institute of Technology Gandhinagar, Ahmedabad 382424, India 1

NIBEC, University of Ulster, Shore Road, Newtownabbey-BT37 0QB, United Kingdom

2

Department of Chemical Engineering, Nanoscience Institute of Aragon (INA), University of Zaragoza,50018 Zaragoza, Spain. Corresponding author: Email: [email protected], Phone - 9727749892, Fax- +91-7923972324, 23972583 _____________________________________________________________________________ Abstract: This paper describes one pot solution combustion synthesis of La2-xSbxO3 (0.02 ≤ x ≤ 0.10). Detailed characterization using X-ray diffraction (XRD) and X- ray photoelecron spectroscopy (XPS) is carried out to understand the doping effect and the oxidation state of antimony. Further, temperature programmed desorption (TPD) with CO2 is performed for evaluating the basic property and temperature programmed reduction (TPR) with H2 has been employed to obtain the oxygen storage capacity. The comparative study of La2O3, La2-x SbxO3 (0.02 ≤ x ≤ 0.10) shows that as the concentration of Sb increases, the basicity decreases and the oxygen storage capacity increases. Thus, non-reducible and basic La2O3 can be transformed to significantly reducible and acidic La2-xSbxO3 (0.02 ≤ x ≤ 0.10). Further, solid state UV spectroscopy shows that due to the antimony doping, band gap of La2O3 decreases significantly. Moreover, antimony doping also modifies the support property of La2O3 as demonstrated in the catalytic CO2 methanation reaction in presence of hydrogen. Ru doped La2O3 and La1.96Sb0.04O3 shows different selectivity towards methane formation and the later favours the reverse water gas shift reaction.

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Introduction Lanthanum Oxide (La2O3), is an important metal oxide which has novel electronic1-2 optical3, magnetic3, catalytic4 and mechanical5 properties. It is one of the most effective catalytic materials for exhaust treatment reactions6-9. It is used as an effective support for many hydrocarbon conversion reactions10 due to its high basicity4, 11. It is also found to improve the resistance to coking and sintering during long hour reaction conditions and hence can be potentially useful as the industrial catalyst support12-14. La2O3 is a non-reducible oxide which means that amount of its lattice oxygen is low to be used in an oxidation reaction. Usually, a dopant can drastically activate the lattice oxygen by distorting the lattice structure15-17 . Moreover, dopant can also change the acidic and basic properties of the host oxide. Thus, a dopant can over all affect the catalytic properties by tuning the lattice oxygen concentration18-19 and acidic / basic properties20-21. Activation of lattice oxygen on one hand will directly enhance the oxygen storage capacity (OSC) useful for oxidation reactions such as carbon monoxide22 and hydrocarbon oxidation17, 23. On other hand tuning acidic and basic property will change the adsorptive interaction. In general, lanthanum based perovskites have been explored for catalytic purposes24-26 but doped La2O3 while retaining its hexagonal structure are rarely studied as the catalyst. This is important because if the structure changes completely after adding the secondary metal, for example, if La2O3 changes to LaCoO3, comparison will be between two different structures; hexagonal and cubic perovskites. In that case it will be difficult to ascertain if the changes in the catalytic activity is due to Co addition or due to structural change. Doped La2O3 in the original hexagonal structure has been used as a catalyst for oxidative coupling of methane27-29. Methane oxidation has also been studied on doped La2O330. Density functional theory (DFT) calculations have shown that 2 ACS Paragon Plus Environment

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doping La2O3 with a number of different dopants significantly changes the vacancy formation energy which makes the lattice oxygen more mobile and reactive31. Apart from the catalytic studies, La2O3 along with other dopants extensively explored for luminescent phosphor applications32-34. Being a non-reducible oxide, it’s difficult to find a suitable dopant because the metal cation in the host cannot change its oxidation state. This is different from a reducible oxide which can be doped by a variety of metals. For example, cerium can be doped with a large number of metals19, 35. Metiu and co-workers have carried out DFT calculations on antimony doping in La2O3 along with other dopants and it is shown that vacancy formation energy is greatly affected by Sb5+ substitution in La2O336. However, no experimental study is reported till date. In this work, for the first time, we have carried out antimony (Sb) doping in La2O3 to make La2-x Sbx O3 (0.02 ≤ x ≤ 0.10) solid solutions. Effect of this doping on the band gap, oxygen storage capacity and basic properties of host La2O3 is explored in detail. How the support basicity affects the catalytic activity is also explored. Experimental Synthesis La2O3, Sb2O3 and antimony (Sb) doped La2O3 (La2-x Sbx O3; 0.02 ≤ x ≤ 0.10) were synthesized by solution combustion method11, 17, 37. The precursors used were lanthanum nitrate (La(NO3)3.6H2O), urea (NH2CONH2), antimony chloride (SbCl3) and ruthenium chloride (RuCl3). For La2O3 preparation, 1.733 gm of urea was dissolved in 20 ml of distilled water to which La(NO3)3 was added and dissolved properly. Resulting solution was placed in a muffle furnace at 400 oC. Solution first dehydrated and combusted giving a white powder of mixed phases of La(OH)3 and La2O3. A final calcination step at 800 oC for 10 hours is carried out to 3 ACS Paragon Plus Environment


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make pure La2O3. For the synthesis of Sb2O3 the precursor and fuel used were SbCl3 and urea. Antimony chloride is not soluble in water and hence it needs to be converted into antimony nitrate. For this, 5gm SbCl3 was dissolved in minimum amount of conc. HCl and excess water was added to it to obtain antimony hydroxide in the form of white precipitate. This precipitate was separated from the solution by centrifugation at 2500 rpm for 20 min at room temperature. Now conc. HNO3 in minimum quantity was added to dissolve this precipitate making a clear solution to obtain antimony nitrate. Now, 2.439 gm of urea was dissolved in 20 ml of water and the prepared Sb(NO3)3 solution was added in this solution and placed in the muffle furnace at 400 oC. The solution first boiled and dehydrated followed by an instantaneous combustion reaction giving slightly yellowish colored compound. Finally, calcination at 800 oC for 10 hours makes the final compound. For 1 % antimony doped La2O3 (La1.98Sb0.02O3), 1.7512 gm of urea was dissolved in 20 ml of distilled water to which 5gm La(NO3)3 was added and dissolved properly (solution-1). Antimony nitrate solution using 0.0358gm of SbCl3 was separately prepared as previously described. This solution was added to solution-1 and place into the muffle furnace at 400 oC. The solution first boiled and dehydrated followed by an instantaneous combustion reaction giving white colored compound. A final calcination step at 800 oC for 10 hours is carried out to make the final compound. Same procedure was followed for the 2.5%, 5% Sb doped La2O3, while proportionately changing the amounts of the precursors. Briefly, for 2.5% antimony doping 5 gm of La(NO3)3, 1.776 gm urea and 0.0911 gm SbCl3 was used and for 5% Sb doped La2O3, 5gm of La(NO3)3, 1.825gm of urea and 0.1870 gm of SbCl3 was used . Final calcination conditions remained the same as with 1% Sb doped La2O3. For the catalytic study we synthesized two different catalyst which is 2% Ru doped La2O3 and 2% Ru + 2% Sb doped La2O3. Same precursor and method was followed to synthesize these


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materials. 4.9 ml solution of 1 molar RuCl3 was used as the precursor for ruthenium and required amount of antimony and urea were added and combusted in furnace. Characterization Combustion synthesized La2O3 and La(OH)3 have earlier been fully characterized by Xray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and The BrunauerEmmett- Teller (BET)11 surface area analysis. In this work, we studied the XRD, X-ray photoelectron spectroscopy (XPS), FTIR, Ultraviolet-Vis spectroscopy (UV-Vis) and BET surface area analysis for La2O3 and antimony (Sb) doped La2O3 (La2-x SbxO3; 0.02 ≤ x ≤ 0.10). Powder X-ray diffraction (XRD) was carried out using a D8- Discover diffractometer (Bruker, Germany) using a Cu K-alpha radiation. A scanning range (2θ) from 10 degree to 80 degree was chosen for all the compounds. When required, NaCl was used as an internal standard to confirm if there is any shift in the peak position due to doping. The Rietveld refinement was done using the FullProf-fp2k program varying 17 parameters simultaneously such as overall scale factor and background parameter. Study of the optical properties such as absorbance, reflectance and band gap was carried out using JASCO V-750 UV-Vis spectrophotometer using the wavelength range of 200-900 nm for all the samples. FTIR was carried out by Thermo Scientific Nicolet IS50 FT-IR and BET surface area were estimated using N2 sorption isotherms obtained at 77 K using Micrometrics ASAP 2020 instrument. Samples were degassed at 90 oC for 12 hour in the flow of N2 gas. The X-ray photoelectron spectroscopic (XPS) was performed with Axis Ultra DLD (Kratos Analytical Limited). The spectra were obtained by using the monochromatized AlKα source (1486.6 eV) run at 15 kV and 10 mA. For the individual peak regions, pass energy of 20



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eV was used. Survey spectrum was measured at 160 eV pass energy. Analysis of the peaks was performed with the Casa XPS software, using a weighted sum of Lorentzian and Gaussian components curves after Shirley background subtraction. The binding energies were referenced to the internal C (1s) (284.9 eV) standard. Temperature-programmed

reduction

(TPR)

was

performed

using

a

Thermal

Conductivity Detector (TCD). 50 mg of freshly calcined (at 800 oC) sample with granules size of 250 microns was first placed into a quartz tube (6 mm OD, 4 mm ID and 25 cm length) and heated at 150 oC for 30 minutes in nitrogen gas. This step was carried out to remove moisture or other surface impurities. This was followed by reduction of the sample in a gas mixture of 10% H2 and 90% Ar using a flow rate of 30 ml min-1 from room temperature to 700 oC at a heating rate of 10 oC min-1. The TPR profiles were obtained by monitoring the TCD signal (mV) against the temperature. This procedure was followed for La2O3, 1%, 2.5% and 5% Sb doped La2O3. The oxygen storage capacity (OSC) was then estimated by comparing the area under the curve for various samples. The areas obtained were used to quantify the amount of H2 by comparing the areas obtained using copper oxide as the calibration standard. The effect of doping on the basic property of La2O3 is investigated by using Temperature programmed desorption (TPD) using (CO2) gas. To determine the CO2 desorption ability of the sample, Flame Ionization Detector (FID) is used. 50 mg of sample was placed in a quartz tube of (6 mm OD, 4 mm ID and 25 cm length) and heated in N2 gas from room temperature to 700 oC at a heating rate of 10 oC min-1 in the gas flow rate of 30 ml min-1. After reaching the final temperature, flow of nitrogen gas was stopped and that of CO2 gas was started. Flow rate was maintained at 30 ml min-1. In this condition, sample was allowed to cool down to room temperature. Once it reaches the room temperature the flow of CO2 gas was stopped and 6 ACS Paragon Plus Environment

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the sample was again flushed in nitrogen gas for 30 minutes at room temperature. This step ensures the removal of any gas phase or weakly adsorbed CO2 gas. Thereafter, the sample was heated in presence of nitrogen gas with the same flow rate from room temperature to 700 oC adopting the same heating rate. The peak area recorded for each sample gave an indication of the amount of CO2 adsorbed on the sample. This further indicates the basicity of the material. Catalytic Study The catalytic study was done for the La1.96Ru0.04O3 and La1.92Sb0.04Ru0.04O3 both prepared by solution combustion method. Here, the La2-xSbxO3 (0.02 ≤ x ≤ 0.10) works as support for the methanation reaction. The Temperature Programmed Reaction (TPR) for the hydrogenation of carbon dioxide (CO2) in presence of hydrogen was taken as the model reaction and the catalytic studies were carried out in a packed bed reactor made of quartz tube (6 mm OD, 25 cm length). Catalyst in granule form of size 250 microns was packed inside the quartz tube. The gas flow rates were controlled by mass flow controllers (ALICAT MC Series). The inlet gases were sent to a gas manifold to ensure homogeneous mixing of gases prior to entering the reactor. CO2 methanation reaction was studied to demonstrate the catalytic activity of the materials. 100 mg of the catalyst was used which gave a space velocity of about 20,000 h-1. The temperature inside the catalytic bed was measured by a K-type thermocouple, which was connected to a temperature controller. The outlet gas from the reactor was cooled by a condenser and the composition of the product gases was analyzed by a gas chromatograph (CIC Dhruva, Baroda, India) using both flame ionization detector (FID) and thermal conductivity detector (TCD). All the experiments were carried out at atmospheric pressure. The performance of the catalyst was measured with respect to the percent selectivity of methane and CO formation. Selectivity was calculated as per the following equations:



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S l CH 4 = S l CO =

χ CH 4 × 100 χ CH 4 + χ CO

χ CO × 100 χ CO + χ CH 4

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(1) (2)

Results and Discussion X-Ray Diffraction (XRD) patterns for all the materials are shown in Figure 1. Figure 1a and Figure 1b are for La(OH)3 and La2O3. La(OH)3 is shown for comparison and it is prepared by leaving La2O3 in open atmosphere for 24 hours. Figure 1c, Figure 1d, and Figure 1e are for La1.98Sb0.02O3, La1.95Sb0.05O3 and La1.90Sb0.10O3 calcined at 800 °C respectively. Peaks are intense and sharp suggesting the good crystallanity in all the compounds. All the antimony doped compounds (La2-xSbxO3) show the structure similar to the parent La2O3. No peak related to Sb or its oxide/hydroxide is seen in La2-xSbxO3 (0.02 ≤ x ≤ 0.10) confirming that majority of Sb is doped in La2O3. Antimony oxide is also shown for comparison (Figure 1f). La1.90Sb0.10O3 (Figure 2a) is also hygroscopic just like La2O3 and leaving it in open atmosphere for 24 hours results into La(OH)3 and antimony oxide formation (Figure 2b). Re-calcination at 800 oC for 10 hours is required to get pure La1.90Sb0.10O3 (Figure 2c). Clearly, La2O3 is a suitable host for antimony doping and La(OH)3 does not favor that. This is due to the difference in their crystal structure and hexagonal structure of La2O3 favors the doping of antimony. Similar behavior is seen for other doped compounds as well. Another important feature of the XRD is the significant shift in the peak position towards higher 2θ value in La2-xSbxO3 (0.02 ≤ x ≤ 0.10) in comparison to parent La2O3 (Figure 3a). This shift is not noticeable in La1.98Sb0.02O3 (Figure 3b) but can be easily seen in 8 ACS Paragon Plus Environment

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La1.95Sb0.05O3 (Figure 3c) and La1.90Sb0.10O3 (Figure 3d). This is a clear indication of bulk doping of antimony in La2O3. Figure 3 is the expanded view in small angle range taken from Figure 1. This shifting in XRD towards higher 2θ value suggests that the lattice size is shrinking after the doping. This is obviously due to the smaller ionic radii of antimony (Sb) in both +3 and +5 oxidation state compared to La3+, leading to shrinkage of the lattices upon doping. This reaffirms our claim that Sb has been doped in the La2O3 lattice. Rietveld refinement was carried to estimate the change in the lattice parameter/volume after the doping. Observed, calculated, and difference plot for La2O3 and La1.90Sb0.10O3 is shown in Figure 4a and Figure 4b (other compositions are not shown). Structural parameters as a result of the refinement are given in Table 1. Rietveld refinement has shown considerable decrease in the lattice parameter of La2O3 after the Sb doping. Considering the smaller size of Sb3+/Sb5+ ion than La3+ ion, this agrees well. Most significant decrease is noticed after 1% and 5% antimony doping. However, not much difference is noticed after 2.5% antimony doping in La2O3 compared to 1%. Reason for this is unknown at this stage. As a result of the decrease in the lattice parameter, there is a decrease in the total lattice volume confirming the bulk doping of smaller antimony ion in place of bigger lanthanum ion. A common way of extracting band gap from UV absorption spectra is to get the first derivative of absorbance with respect to photon energy and finding the maxima in the derivative spectra at the lower energy sides.38 The first derivative method is convenient when the absorption peak dominates the spectrum.39 The Optical absorption spectra of all the samples are shown in the Figure 5. Absorbance increases with the increased amount of antimony doping in lanthanum oxide. As shown in calculated parameters after Rietveld refinement (Table 1), here also no significant change is noticed in the absorption spectrum after 2.5% doping compared to



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1% doping. Thus, both refinement and absorption studies shows similar effect after doping and assure the correctness of doping behavior. Further, it can be seen clearly that there is no sharp absorption peaks so the optical absorption coefficient, α, which is the relative rate of decrease in light intensity along its path of propagation, is calculated from the reflectance (inset of Figure 5). To establish the type of band to band transition in these samples, diffuse reflectance data are fitted to Tauc’s equation for both direct and indirect band gap transitions. To calculate the band gap the following relational expression proposed by Tauc, Davis, and Mott is used. (hνα)1/n = A(hν - Eg) Here h: Planck's constant, ν: frequency of vibration, α: absorption coefficient, Eg: band gap, A: proportional constant. Further, to assess the value of α, the acquired diffuse reflectance spectrum is converted to Kubelka-Munk function. In the case of an infinitely thick sample, thickness and sample holder have no influence on the value of reflectance (R). So in this case, the Kubelka-Munk equation at any wavelength becomes:

Where K and S are absorption and scattering coefficient respectively. F(R) is the KubelkaMunk function. When the material scatters the incident light in diffuse manner, which is the case for powder samples, the Kubelka-Munk absorption coefficient ‘K’ becomes equal to 2α. So, K=2α; S is constant with respect to wavelength. Thus, the vertical axis is converted to quantity F(R), which is proportional to the absorption coefficient. Tauc’s equation is substituted with F(R). Thus, in the actual experiment, the relational expression becomes: 10 ACS Paragon Plus Environment

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(hνF(R))1/n = A(hν – Eg) Here, F (R) = (1-R)2/2R . Firstly n=1/2 was taken as it shows direct allowed transitions40. The diffuse reflectance spectra of the samples after Kubelka-Munk treatment are shown in Figure 6. The band gap value Eg=5.2 eV doesn’t change significantly with La1.98Sb0.02O3 and La1.95Sb0.05O3 (figures are not shown here), but in La1.90Sb0.10O3 band gap decreases significantly to 4.9 eV (Figure 6). In addition to the direct band gap, indirect band gap is also estimated by taking n=2. Calculated value of the indirect transition band gap is Eg = 5.0 eV for La2O3 which decreases to Eg = 4.5 eV for La1.90Sb0.10O3 (inset of Figure 6). This is lesser compared to the direct band gap shown in Figure 6. Because band gap energy from indirect transitions relates more to absorbance spectra therefore, this could be the reason to suggest that indirect transition is more favorable in the synthesized samples41. With increasing amount of doping, absorbance also increase probably because of defects. The FTIR spectra of various Lanthanum oxide based compounds are shown in the Figure 7. FTIR spectra of La2O3 indicate presence of different functional groups (Figure 7a). The broad absorption bands at 1630 cm-1, 1489 cm-1 and 1382 cm-1 are attributed to the presence of carbonates10, 42-43. Presence of carbonate is expected due to the high basicity of La2O3 which can form carbonate using CO2 in the atmosphere. FTIR spectrum of La1.90Sb0.10O3 is shown in Figure 7b. Here, all the bands remain same as La2O3 but the intensity of carbonate region diminishes significantly. This can be attributed to the low basicity of this material due to antimony doping. FTIR of La1.90Sb0.10O3 after exposing it to open atmosphere for 24 hours is shown in Figure 7c. Bands at 3611 and 3450 cm-1 are due the OH functional group. According to reported literature the band at 3611 cm-1 is due to tridentate OH groups linked at La+3 and band at 3450 cm-1 is assigned to the OH vibration mode of adsorbed water4. These observations



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confirm the hydroxide formation in La2-xSbxO3 (0.02 ≤ x ≤ 0.10) after exposing to open atmosphere similar to XRD (Figure 2b). FTIR spectrum of the La(OH)3 shown in Figure 7d reveals a sharp band appearing at 3611 cm-1 which is similar to Figure 7c and is attributed to the bulk hydroxyl groups originating from La(OH)3. Peaks at about 1630 cm-1, 1489 cm-1 and 1382 cm-1 are again due to the carbonate groups similar to La2O3. From FTIR studies it is apparent that basic property of La2O3 can be changed by antimony doping as confirmed by the decrease in the carbonate bands after doping. All other band remains approximately unchanged. The Sb(3d) core level spectra from La1.95Sb0.05O3 and La1.90Sb0.10O3 samples show a significant peak related to Sb(3d3/2) at 539.3 eV (Figure 8 ). Peak related to Sb (3d5/2) merges with O(1s) therefore, could not be resolved. Peak at this binding energy is related to the +3 oxidation state of antimony44. Thus, the oxidation state of Sb in La2-xSbxO3 (0.02 ≤ x ≤ 0.10) is +3. Intensity of Sb (3d3/2) peak in La1.90Sb0.10O3 increases in comparison to La1.95Sb0.05O3 due to the high amount of antimony concentration in the former. The BET surface area of the La2O3 was 4.7 m2/g and for La1.9Sb0.1O3 was 9.8 m2/g. Synthesis of these compounds requires high temperature (700−800 °C) calcination for a long time (10 h) causing the surface area to reduce. Due to doping of Sb, the surface area was enhanced slightly in the case of La1.90Sb0.10O3. Temperature Program Desorption (TPD) The CO2-TPD experiment is performed to observe the effect of doping on the basicity of La2O3. The obtained TPD curves are shown in Figure 9. La2O3 shows significant amount of CO2 desorbed from its surface (Figure 9a). La2O3 is a highly basic oxide

4, 8, 11

therefore, it is

expected that it will adsorb large amount of CO2 which is an acidic gas. The basic site strengths are classified according to their carbon dioxide desorption temperature (TD)45 . TPD of La2O3 12 ACS Paragon Plus Environment

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shows two major peaks at temperatures 350 and 600 oC. This indicates that there are two different basic sites, one is weak and other one is comparatively stronger. Different types of basic sites on La2O3 has earlier been reported46. Amount of stronger basic sites are lesser than the weaker sites as depicted by the area under these peaks. Further, the broad nature of the peaks suggest that these basic sites have a broad range of distribution in their strength or they are heterogeneous in nature. 1% antimony doping in La2O3 tremendously affect the amount of CO2 which is desorbed (Figure 9b). Amount of CO2 deceases consinuously in La1.95Sb0.05O3 (Figure 9c) and La1.90Sb0.10O3 (Figure 9d) as the dopingconcentration is increasing. CO2 Importantly, the peak shape of TPD with antimony oxide is also different from La2O3 and La1-xSbxO3 discarding the possibility of any free antimony oxide on La2O3 surface. No effect is seen on the shape of the peak suggesting that the doping homogeneously affects the surface property. In La1.90Sb0.10O3, the desorption peak at high temperature diminishes completely suggesting that only one type of basic site remain. TPD in the case of antimony oxide is shown in Figure 9e for comparison and it is evident that its basicity is lowest among all the compunds and it can be considered as the acidic oxide. Normalazing the peak areas with respect to BET surface areas, amount of CO2 desorbed from La1.9Sb0.1O3 is about 10 times lower lower then La2O3. This is clearly the effect of antimony doping resulting into the significant increase in the acidity or decrease in the basicity of La2O3. This also supports our observations in FTIR spectra (Figure 7). Temperature Programmed Reduction (TPR) TPR is the powerful tool for measuring the oxygen storage capacity (OSC) of the catalytic oxides47-48. Hydrogen is mostly used as a reductant gas in such experiments19. OSC gives information about the available lattice oxygen which can be used in the oxidation reaction



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and it is widely accepted that most of the oxidation and dehydrogenation reactions catalyzed by oxides take place through a Mars-van Krevelen mechanism49-51. The TPR profiles of the synthesized oxides are shown in the Figure 10. For comparison, Sb2O3 and La2O3 are also included in this figure. In La2O3 three reduction peaks were observed at three different temperatures with areas in low range (Figure 10a). This means that the availability of the surface oxygen is low which is obviously due to the non-reducible nature of La2O3. Moreover, there are at least three types of oxygen present on the surface of combustion synthesized La2O3 availability of which is easy, moderate and difficult depending upon the low, moderate and high temperature. Once the dopant (Sb) concentration increases from La1.98Sb0.02O3 (Figure 10b) to La1.95Sb0.05O3 (Figure 10c) and finally to La1.90Sb0.10O3 (Figure 10d), significant changes are observed. Peak in the temperature range of 250 – 400 oC diminishes completely compared to pure La2O3. Peak area in the higher temperature range (400 – 650 oC) increases significantly with increase in the dopant concentration. This implies that OSC increases significantly, though at high temperature. Doping also results in the poorly resolved peaks which may indicate that temperatures to remove various types of oxygen are in a very narrow range. This behavior is different from La2O3 where two distinct peaks appear in this region. Thus, Sb doping is affecting the energetics as well as the availability of surface oxygen. Overall, the areas under each peak (normalized with BET surface areas) are noticeably high and increases almost 5 times in La1.90Sb0.10O3 compared to La2O3 respectively. TPR of antimony oxide (Figure 10e) shows considerable reduction therefore, it is a highly reducible oxide. Its reduction behavior is different from antimony doped compounds; therefore, one can expect that there is no free antimony oxide in La2-xSbxO3 (0.02 ≤ x ≤ 0.10). Thus, temperature programmed reduction (TPR) has indeed shown that non reducible La2O3 can be transformed to give significantly


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reducible La2-xSbxO3 (0.02 ≤ x ≤ 0.10) and the oxygen storage capacity markedly increases as the dopant concentration increases. Effect on the Catalytic Activity Clearly, antimony doping in La2O3 affects the basic properties as well as OSC in comparison to La2O3. Further, if these changes can also account for the change in the catalytic behavior of Ru when it is doped in La2O3 (La1.96Ru0.04O3) or in La1.96Sb0.04O3 (La1.92Sb0.04Ru0.04O3) is examined by taking CO2 methanation reaction (Sabatier reaction) in presence of H2. Ru is a well-known metal used for this reaction43. There are two possibilities during this reaction; formation of CO due to reverse water gas shift reaction52 and normal methanation reaction. Selectivity towards CO and methane is given in Figure 11 as an indicator for the change in the catalytic behavior of La2O3 post antimony doping. The product selectivity’s obtained from La1.96Ru0.04O3 and La1.92Sb0.04Ru0.04O3 catalysts are clearly different. The La1.92Sb0.04Ru0.04O3 catalyst only produced CO and gave no methane reaction at all the reaction temperatures. On the other hand La1.96Ru0.04O3 catalyst exhibited methane production in the temperature range of 200-600 oC. In the case of La1.96Ru0.04O3, reaction started from 200 o

C with CO selectivity reaching 90 % at 300 oC followed by a decrease up to 65% at 400 oC.

During this decrease, methane production starts reaching the maximum selectivity of 30% at 400 oC. Thus, La1.96Ru0.04O3 catalyzes both RWGS and methanation reaction. In the case of La1.92Sb0.04Ru0.04O3 only CO produced at all the temperature making this catalyst 100 % selective towards RWGS. Thus, antimony doping in La2O3 modifies the support properties and changes the catalytic behavior of Ru.



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Conclusion: We synthesized La2O3 and La2-xSbxO3 (0.02 ≤ x ≤ 0.10). XRD of these materials show good crystallinity and pure phases. Consistent shift in the XRD towards the higher 2θ value is noticed indicating that the smaller antimony ion is replacing larger La3+. This confirms the bulk doping of antimony in La2O3 lattice. Rietveld refinement confirmed the decrease in the lattice parameter as well as the lattice volume quantitatively. XPS further confirms that the oxidation state of antimony is +3. Temperature Programmed Desorption (TPD) using CO2 has shown that the basicity of La2O3 is significantly reduced due to the antimony doping. This decrease is consistent with increase in the antimony concentration. Temperature Programmed Reduction (TPR) has indeed shown that non reducible La2O3 can be transformed to give significantly reducible La2-xSbxO3 (0.02 ≤ x ≤ 0.10). Both TPD and TPR also show that behavior of La2xSbxO3

(0.02 ≤ x ≤ 0.10) is different from both La2O3 and antimony oxide. This suggests that

there is negligible possibility of free La2O3 or antimony oxide as the impurity in La2-xSbxO3 (0.02 ≤ x ≤ 0.10). The Kubelka–Munk radiative transfer model, modified to treat optically rough surfaces, has been applied to all the samples of antimony doped lanthanum oxide. The direct optical transitions band gap values in La2O3, and in La1.90Sb0.10O3 is estimated to be 5.2 eV and 4.9 eV respectively. On the other hand, the indirect transition band gap value 5.0 eV in La2O3 decreases to 4.5 eV in La1.90Sb0.10O3. Band gap does not change significantly with low Sb doping. Importantly, antimony doping changes the support properties of La2O3 and thus modifying the catalytic effect of Ru. While 2% Ru doped La2O3 (La1.96Ru0.04O3) favors CO and methane formation during CO2 methanation reaction, 2% Ru doped La1.96Sb0.04O3 (La1.92Sb0.04Ru0.04O3) only produces CO. In conclusion, it can be said that antimony doping


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enhances the oxygen storage capacity (OSC), decreases the basicity of La2O3, decreases the band gap and also modifies its support property to change the catalytic behavior. Acknowledgement: We gratefully acknowledge IIT Gandhinagar, sponsored research project CO2 reforming of methane to generate syngas project no. SB/S1/PC-81/2012 and Ramanujan fellowship SR/S2/RJN-24/2012. Dr. Asha Gupta is greatfully acknowledged for Rietveld refinement. References: (1) Pisecny, P.; Husekova, K.; Frohlich, K.; Harmatha, L.; Soltys, J.; Machajdik, D.; Espinos, J. P.; Jergel, M.; Jakabovic, J. Growth of Lanthanum Oxide Films for Application as a Gate Dielectric in CMOS Technology. Mater. Sci. in Semicond. Process. 2004, 7, 231-236. (2) Mangla, O.; Srivastava, A.; Malhotra, Y.; Ostrikov, K. Lanthanum Oxide Nanostructured Films Synthesized Using Hot Dense and Extremely Non-Equilibrium Plasma for Nanoelectronic Device Applications. J. Mater. Sci 2013, 49, 1594-1605. (3) Sasidharan, M.; Gunawardhana, N.; Inoue, M.; Yusa, S.-I.; Yoshio, M.; Nakashima, K. La2O3 Hollow Nanospheres for High Performance Lithium-Ion Rechargeable Batteries. Chem. Commun. 2012, 48, 3200-3202. (4) Valange, S.; Beauchaud, A.; Barrault, J.; Gabelica, Z.; Daturi, M.; Can, F. Lanthanum Oxides for the Selective Synthesis of Phytosterol Esters: Correlation between Catalytic and Acid-Base Properties. J. Catal. 2007, 251, 113-122. (5) Wei, C.; Fan, J. L.; Gong, H. R. Structural, Thermodynamic, and Mechanical Properties of Bulk La and A-La2O3. J. Alloys. Compd. 2015, 618, 615-622.



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Figure: 1. Wide-angle XRD patterns of all the compounds. XRD of (a) La(OH)3, (b) La2O3, (c) La1.98Sb0.02O3, (d) La1.95Sb0.05O3 (e) La1.90Sb0.10O3 and (f) Sb2O3. No indication of Sb metal or oxide is observed in compounds, (b)-(e).


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Figure: 2. XRD pattern of (a) La1.9Sb0.10O3, (b) La1.9Sb0.10O3 when exposed to open atmosphere for 24 hours and (c) La1.90Sb0.10O3 after re-calcination at 800 oC for 10 hours.



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Figure: 3. Expanded view of XRD patterns in the range of 28°– 32° taken from Figure 1 for compounds, (a) La2O3 and (b) La1.98Sb0.02O3 (c) La1.95Sb0.05O3 (d) La1.90Sb0.10O3.XRD clearly shows that the peaks are shifted towards higher 2θ angle due to doping.


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Table:1. Crystallite structure properties of the samples obtained by Rietveld refinement

Compound

Cell Parameter

Cell volume

a

b

c

La2O3

3.9376

3.9376

6.1307

82.3187

1%Sb/La2O3

3.9309

3.9309

6.1233

81.9404

2.5%/La2O3

3.9307

3.9307

6.1250

81.9577

5%/La2O3

3.9297

3.9297

6.1239

81.8990



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(a)

Figure: 4 (a). Rietveld refined observed and calculated of XRD patterns of La2O3; “|” represents the Bragg position.


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(b)

Figure: 4 (b). Rietveld refined observed, and calculated of XRD patterns of La1.90Sb0.10O3; “|” represents the Bragg position.



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.

Figure: 5. UV-Vis absorbance spectra of all the samples in the range of 200 to 800 nm. Inset image shows the percent reflectance spectra. Change in the band gap after doping is apparent.


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Figure: 6. Kubelka-Munk transformed reflectance spectra of direct transition for La2O3 and La1.9Sb0.10O3. In inset Kubelka-Munk transformed reflectance spectra of indirect transition for La2O3 and La1.9Sb0.10O3.



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Figure: 7. FTIR spectrum of compounds. FTIR of (a) La2O3, (b) La1.9Sb0.1O3, (c) La1.90Sb0.10O3 after exposing to the open atmosphere for 24 hours and (d) La(OH)3.


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Figure: 8. Sb(3d) spectra of (a) La1.95Sb0.05O3 with binding energies of Sb (3d)3/2

539.3 eV and Sb (3d)5/2 529.9 eV related to Sb3+ and (b)

La1.90Sb0.10O3 with binding energies of Sb(3d)3/2 539.3 eV and Sb (3d)5/2 529.9 eV related to Sb3+.



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Figure: 9. CO2-TPD profile of all the compounds. TPD of (a) La2O3 (b) La1.98Sb0.02O3 (c) La1.95Sb0.05O3 (d) La1.90Sb0.10O3 (e) Sb2O3. Graphs suggest that the basicity decreases as dopant concentration increases.


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Figure: 10. H2-TPR profile of all the compounds. TPR of (a) La2O3 (b) La1.98Sb0.02O3 (c) La1.95Sb0.05O3 (d) La1.90Sb0.10O3 (e) Sb2O3. Graphs suggest that oxygen storage capacity (OSC) increases as the dopant concentration increases.



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Figure: 11. Temperature programmed reaction of CO2 in presence of H2.The CO and CH4 selectivity are shown at different temperature for their respective catalyst. Change in the catalytic behavior after doping is apparent.


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Table of Content Graphics


Nonreducible, Basic La2O3 to Reducible, Acidic ... - ACS Publications

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