High Surface Area M (M = La, Pr, Nd, and Pm)-Doped Ceria

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High surface area M (M = La, Pr, Nd, and Pm)-doped ceria nanoparticles: Synthesis, characterization and activity comparison for CO oxidation Amit Singhania Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03143 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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Industrial & Engineering Chemistry Research

High surface area M (M = La, Pr, Nd, and Pm)-doped ceria nanoparticles: Synthesis, characterization and activity comparison for CO oxidation Amit Singhaniaa* a

Department of Chemical Engineering, Indian Institute of Technology, Delhi, New Delhi, INDIA

*

Corresponding author email: [email protected]

Abstract High surface area M (M = La, Pr, Nd, and Pm)-doped ceria nanoparticles have been synthesized by the citric acid-aided sol-gel method to check their catalytic activity for carbon monoxide (CO) oxidation. BET analysis revealed high specific surface areas of M-doped CeO2 catalysts (~ 190– 198 m2 g-1). XRD and TEM showed 3-6 nm size of particles of M-doped CeO2 catalysts. Raman results showed the highest amount of oxygen vacancy possessed by La-doped CeO2 compared to other dopants. The catalytic activity for CO oxidation follows the order: La-CeO2 > Pr-CeO2 > Nd-CeO2 > Pm-CeO2 > CeO2. La-doped CeO2 exhibited highest catalytic activity (T50 - 170°C) due to the higher quantity of oxygen vacancies, larger specific surface area and smaller particle size and enhanced oxygen storage capacity (OSC) comparison to other dopants. This catalyst also showed an excellent stability for CO oxidation reaction for a time-on-stream of 50 h.

Keywords: Ceria; Lanthanum; Praseodymium; Neodymium; Promethium; CO oxidation

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1. Introduction Nanomaterials are used frequently in different sectors such as energy, medicine, chemical industries, catalysis, semiconductor, magnetic, and electrical field1, 2. This is may be due to their different remarkable properties at the nanoscale in comparison to the bulk. These remarkable properties attracted the researchers to use these nano-size materials for their different applications. Ceria (CeO2) is one of the rare earth materials which are used regularly by researchers for different applications such as catalysis, fuel cells, sun-blockers, and sensors 3-7. It has been used as a catalyst (especially, at nanoscale) and/ or support for the carbon monoxide (CO) oxidation process due to its redox behavior, which changes Ce4+ to Ce3+

8-10

. Its catalytic

activity comes from the terminated surface oxygen and therefore, if somehow its active oxygen concentration is increased, it will promote its catalytic activity. This active oxygen depends on the various factors such as surface defects, surface areas, reactive facets, and surface elemental composition11-13. The major source of air pollution comes from the vehicle emissions. In vehicles, the main pollutants consist of CO, nitrogen oxides (NOx), and particulate matter (PM). CO is a colorless, odorless and toxic gas to the human body as it has a high affinity with hemoglobin. Therefore, it is necessary to remove this highly toxic pollutant. Among different methods, catalytic route for CO oxidation is an efficient method to convert it into a less toxic carbon dioxide (CO2). Almost 30 years back, Haruta et al.14 revolutionized the world by converting toxic CO gas into a less toxic CO2 by using small gold (Au) nanoparticles. Since then, a lot of work has been done by researchers using Au nanoparticles on different supports such as CeO2, Al2O3 (alumina), activated carbon (AC), titania (TiO2) and so on, for CO oxidation15-18. Au is not considered as a good option for CO oxidation as it is very expensive. It is also found that supported Au 2 ACS Paragon Plus Environment

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nanoparticles sintered during oxidation reaction and lead to bigger particles, which showed that these are not good for practical applications19. Among different supports, CeO2 is considered as a special interest due to its high oxygen storage capacity and high oxygen vacancies which enhance the oxidation rate of the reaction. The ionic radii of Ce4+ is 0.97 Å and the addition of dopants of size lower or greater than the Ce4+ can produce distortions in the fluorite structure of CeO2. This can result into new properties into the doped CeO2 material such as resistance to sintering at high temperatures, improvement in oxygen diffusivity and reducibility of the CeO2 material and consequently increases the overall catalytic activity of the material20. Doping in CeO2 by rare earth metals such as La, Pr, Nd, and Pm could generate oxygen vacancies, which is an important factor in the CO oxidation process. The doping by these rare earth metals also stabilizes the CeO2 against sintering21-23. Recently, Jampaiah et al.24 presented the high catalytic activity of Cu-doped CeO2 and Mn-doped CeO2 due to the creation of oxygen vacancies in CeO2 by the incorporation of Cu and Mn. Similarly, Yang et al.

25

showed high catalytic activity of Cu-doped CeO2 which is also due to

the oxygen vacancies created in CeO2 by the incorporation of Cu. Different methods have been used for the synthesis of doped-CeO2 materials such as sol-gel, co-precipitation, combustion, hydrothermal, micro-emulsion, sonochemical, and solvothermal route26-30. Among these, sol-gel is one of the methods which can produce homogeneous high specific surface area nanoparticles (~ 200 m2 g-1). It is time-efficient, economic, and environment friendly technique. It is a known fact that the higher is the specific surface area, the faster is the catalytic reaction. By optimizing the parameters such as the ratio of gelating agent and metal precursors, and calcination temperatures, it is possible to achieve homogeneous high specific surface area particles.



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In the present paper, high surface area M (M = La, Pr, Nd, and Pm)-doped CeO2 nanoparticles are synthesized by the citric-aided sol-gel method. The idea is to investigate the effect of incorporation of different rare earth metals into CeO2 on CO oxidation activity and stability. The data of characterizations by XRD, BET, ICP-AES, TEM, and RAMAN spectroscopy is also reported in this paper.

2. Experimental work 2.1. Synthesis of M (M = La, Pr, Nd, and Pm)-doped CeO2 The M (M = La, Pr, Nd, and Pm)-doped CeO2 catalysts were synthesized by the citric-aided solgel method. In this method, lanthanum nitrate [La(NO3)3.6H2O], praseodymium nitrate [Pr(NO3)3.6H2O], neodymium nitrate [Nd(NO3)3.6H2O], promethium nitrate [Pm(NO3)3.6H2O], and cerium nitrate [Ce(NO3)4.6H2O] were used as La, Pr, Nd, and Ce precursors [(mol ratio of M(NO3)3.6H2O:Ce Ce(NO3)4.6H2O = 20:80)]. Citric acid was used as a gelating agent. The required amount of metal nitrates and citric acid were dissolved in propan-2-ol (mol ratio of metal nitrates to citric acid is 1.5). After mixing, the above solution was heated at around 80oC on a magnetic stirrer till it produced spongy yellow-gel. The spongy gel was cooled at room temperature and then dried in an oven at 110oC for 15 h. The obtained product was crushed properly and calcined at 300oC for 4 h. The calcined samples were designated as La-CeO2, PrCeO2, Nd-CeO2, and Pm-CeO2. 2.2. Catalyst characterization


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The specific surface area of the M-doped CeO2 materials were calculated using BET instrument, Micrometrics, ASAP 2010. The structural analysis of the doped materials was determined on a Rigaku X-ray diffractometer (Cu-Kα radiation) at 2θ = 20-80°. The elemental composition of rare earth metals was confirmed with the help of ARCOS, Simultaneous ICP-AES spectrometer. The morphology and particle size of the prepared samples were calculated using a Tecnai G2-20 Twin (FEI) transmission electron microscope. The phases and oxygen vacancies were confirmed using 514 nm laser source Horiba JY Lab RAM HR 800 Raman spectrometer. The H2-TPR experiments were conducted for CeO2 and doped-CeO2 materials using 10% H2 and 90% Ar. 2.3. CO oxidation activity The CO oxidation reaction was carried out in a fixed-bed quartz reactor of I.D. 14 mm under atmospheric pressure. 0.5 g of doped-CeO2 catalyst was put into a quartz reactor to check their catalytic activity at different temperatures. The catalyst particles of average diameter of 0.5 mm were employed thereby making the catalyst bed height to 25 mm. To minimize channeling and avoid back-mixing two conditions has to be satisfied: (a) ratio of catalyst bed height to average diameter of catalyst particle (L/Dp) ≥ 20 and (b) ratio of internal diameter of the tube to average diameter of catalyst particle (d/Dp) ≥ 10. In the present study, L/Dp and d/Dp values were maintained to 50 and 28, respectively. A thermocouple was placed near the catalyst bed to measure the temperature. Figure S1 shows the schematic of experimental set-up for CO oxidation. A mixture chamber was used to simulate off-gas mixture. The overall flow rate of gas mixture (20% O2 balance by Ar gas and 500 ppm CO) was maintained at 100 ml/min. Gas chromatograph (Nucon-5765) was used to analyze effluent stream. The CO conversion was measured as follows:



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% =

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(1)

The oxygen storage capacity (OSC) of the different samples was calculated using thermogravimetry (TG) method. The experiment was done under repeated thermal treatments in the range of 100-500oC. The OSC was calculated on the basis of the quantity of released oxygen under thermal treatments. The weight loss was measured by TG analyzer under flowing nitrogen gas. The sample was heated to 500oC and cooled to 100oC and then again heated to 500oC. To calculate the oxygen released properties, weight loss of the catalyst was used during the second stage of heat treatment. An isotopic oxygen tracer has been done to confirm that CO binds to the lattice oxygen. This experiment was conducted at 150oC with He as a carrier gas (50 mL/min.). The reactant mixture of CO/18O2/Ar (1:10:39) was pulsed into the system till the equilibrium has reached. The loop volume was taken to be 1 mL. The analysis of the effluent gases was done by online Hiden Analytical mass spectrometer.

3. Results and discussion 3.1. Characterization of M-doped CeO2 catalysts

Table 1. Physicochemical properties of the M-doped CeO2 materials. Material

Theoretical

Actual Ma Specific

code

M (mol %)

(mol %)

Crystallite

Lattice

OSC/µ

surface areab* sizec

constantsd moles

(m2 g-1)

(nm)

(nm)


O2/g CeO2

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CeO2

-

-

184.3

6.2

0.541

45

La-CeO2

20.0

19.8

197.6

3.3

0.549

282

Pr-CeO2

20.0

19.9

193.8

4.7

0.547

267

Nd-CeO2

20.0

19.6

191.5

5.1

0.544

253

Pm-CeO2

20.0

19.7

189.2

5.7

0.543

241

a

Material composition was determined by ICP-AES. bCalculated from N2 adsorption data. cCalculated using Scherrer

equation due to (111), (200), (220) and (311) planes respectively. dCalculated using Bragg’s Law due to (111), (200), (220) and (311) planes respectively. *error ± 1.

The specific surface area of the M-doped CeO2 catalysts is shown in table 1. The sol-gel method derived CeO2 showed a high specific surface area of 184.3 m2 g-1. Incorporation of rare earth metals such as La, Pr, Nd, and Pm resulted in an increase in specific surface area of CeO2 which is good for the catalytic CO oxidation process. The specific surface area of M-doped CeO2 i.e. La-CeO2, Pr-CeO2, Nd-CeO2, and Pm-CeO2 were 197.6, 193.8, 191.5, and 189.2 m2 g-1, respectively. The results of OSC are given in table 1. Among the different dopants, La-CeO2 and PmCeO2 showed highest and lowest value for OSC. The OSC value of La-CeO2 increased by 4-5 times in comparison to CeO2. This improved value may be due to delocalized oxygen vacancies, weakly bound oxygen and oxygen intestines in their defect crystal structures31. One oxygen vacancy is created by the addition of every 2 La3+ ions to CeO232. Thus, the incorporation of La into CeO2 created the labile oxygen vacancies leading to high mobility of bulk oxygen vacancies within the CeO2 lattice and resulted in increase in OSC value.



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Figure 1. Powder XRD of (a) CeO2, (b) La-CeO2, (c) Pr-CeO2, (d) Nd-CeO2, and (e) Pm-CeO2.

The powder XRD patterns of the M-doped CeO2 materials are shown in figure 1. Pure CeO2 showed sharp peaks at 28.6°, 33.1°, 47.5°, and 56.4° which corresponds to (111), (200), (220), and (311) planes, respectively. Incorporation of rare earth metals M resulted in shifting of CeO2 peaks to lower diffracting angles. No other peaks of rare earth metals M were found in the XRD diffraction patterns (figure 2b to 2e). These XRD results indicate the formation of M-Ce solid solutions. The crystallite sizes of M-doped CeO2 materials were calculated with the help of Debye Scherrer equation (table 1). The crystallite sizes of CeO2, La-CeO2, Pr-CeO2, Nd-CeO2, and Pm-CeO2 were 6.2, 3.3, 4.7, 5.1, and 5.7 nm, respectively. Table 1 showed an increase in the lattice constant values of M-doped CeO2 materials in comparison to CeO2 which indicates the partial substitution of Ce4+ by M3+. This is due to the larger ionic radii of rare earth metals La3+ (1.15 Å), Pr3+ (1.13 Å), Nd3+ (0.99 Å), and Pm3+ (0.98 Å) in comparison to Ce4+ (0.97 Å). The 8 ACS Paragon Plus Environment

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above partial substitution will result into the generation of oxygen vacancies due to charge compensation and lattice distortion in CeO2. These oxygen vacancies will enhance the catalytic activity of M-doped CeO2 materials for CO oxidation process. TEM micrographs of the M-doped CeO2 materials and their particle size distribution are shown in figure 2A and 2B. All the materials revealed spherical particles with homogeneous distribution. Pure CeO2 showed an average size of 6.3 nm. The addition of rare earth metals, M showed a decrease in average particle sizes of M-doped CeO2 materials. The average particle sizes of La-CeO2, Pr-CeO2, Nd-CeO2, and Pm-CeO2 were 3.7, 5.1, 5.4, and 5.9 nm, respectively. Both XRD and TEM results showed similar results. Table 2 shows the comparison of particle sizes of M-doped CeO2 materials by XRD and TEM analysis.

Figure 2A. TEM micrographs of (a) CeO2, (b) La-CeO2, (c) Pr-CeO2, (d) Nd-CeO2, and (e) PmCeO2.



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Figure 2B. TEM micrographs of (a) CeO2, (b) La-CeO2, (c) Pr-CeO2, (d) Nd-CeO2, and (e) PmCeO2.

Table 2. Comparison of particle sizes of M-doped CeO2 materials by XRD and TEM. Material

XRD (average crystallite size)

TEM (average particle size)

(nm)

(nm)

CeO2

6.2

6.3

La-CeO2

3.3

3.7

Pr-CeO2

4.7

5.1

Nd-CeO2

5.1

5.4

Pm-CeO2

5.7

5.9

The Raman spectra of the prepared M-doped CeO2 samples are shown in figure 3. Pure CeO2 sample showed a sharp intensity peak at around 466 cm-1 which corresponds to an F2g 10 ACS Paragon Plus Environment

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vibration mode33. No peaks of rare earth dopants were observed in the spectra of M-doped CeO2 samples which are consistent with the XRD results. Incorporation of rare earth metals into CeO2 lattice resulted in shifting of the peaks to lower frequency bands which indicates the formation of M-Ce solid solutions. In M-doped CeO2 spectra, there was a small peak at around ~ 580 - 600 cm-1 which corresponds to the oxygen vacancy defects13, 34. The oxygen vacancy concentrations can be determined by the intensity ratio of ID (oxygen vacancy peak) to IF2g peak 13, 35. Table S1 shows the ratio of ID/IF2g values which indicates that among all dopants, La-CeO2 exhibited highest ratio ID/IF2g. The intensity ratio, ID/IF2g follows the order: La-CeO2 (0.38) > Pr-CeO2 (0.20) > Nd-CeO2 (0.11) > Pm-CeO2 (0.07) > CeO2 (0.06). This means that the La-doped CeO2 material had the highest amount of oxygen vacancies than the other used dopants.

Figure 3. Raman spectra of (a) CeO2, (b) La-CeO2, (c) Pr-CeO2, (d) Nd-CeO2, and (e) Pm-CeO2.



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The H2-TPR experimental results of the CeO2 and doped-CeO2 materials are shown in figure 4. In all CeO2 materials, two peaks were observed at 485°C and 741°C. The former peak can be attributed to removal of surface oxygen and the latter peak related to reduction of oxygen species in bulk CeO236. The incorporation of different transition metals into the CeO2 material resulted in decrease in reduction temperatures which indicates the higher mobility of surface oxygen at lower temperatures. In La-CeO2, the peaks were decreased to 440°C and 694°C. The order of reduction temperatures of doped materials is as follows: La-CeO2 > Pr-CeO2 > Nd-CeO2 > Pm-CeO2.

Figure 4. TPR spectra of (a) CeO2, (b) La-CeO2, (c) Pr-CeO2, (d) Nd-CeO2, and (e) Pm-CeO2.

The XPS data of Ce 3d and O 1s core levels of the prepared CeO2 and doped-CeO2 materials are shown in figure 5A and 5B. The XPS data of La 3d, Pr 3d, and Nd 3d core levels are shown in figure S2. Pure CeO2 showed characteristic peaks of Ce4+ at binding energy of 12 ACS Paragon Plus Environment

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882.2, 889.0, 898.2, 900.8, 907.4, and 916.8 eV 34-36. Also, peaks at around 885.2 and 904.1 eV was observed corresponding to Ce3+ 3d

36

. In doped samples, the positions of Ce4+ peaks were

shifted to lower binding energies which indicate the introduction of more oxygen vacancies after the addition of dopants. In figure 5B, two peaks were observed at 529.1 and 531.1 eV corresponds to the lattice oxygen and oxygen species of the surface carbonates and hydroxide36. After the addition of dopants, the lattice oxygen peaks were shifted to lower binding energies which indicate that the environment of the lattice oxygen in doped-sample is quite different from the un-doped CeO2 sample. This shows that the different rare earth metals are incorporated into the CeO2 lattice.

Figure 5. XPS data of (A) Ce 3d core levels and (B) O 1s core levels of (a) CeO2, (b) La-CeO2, (c) Pr-CeO2, (d) Nd-CeO2, and (e) Pm-CeO2.

3.3. Activity of M-doped CeO2 catalysts



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The catalytic activity of M-doped CeO2 materials for CO oxidation reaction is shown in figure 6. The catalytic order of M-doped CeO2 for CO conversion followed the order: La-CeO2 > Pr-CeO2 > Nd-CeO2 > Pm-CeO2 > CeO2. Among different rare earth metal dopants, La-doped CeO2 showed highest CO conversion with T50 (temperature at which 50% CO conversion is achieved) value at 170°C. Similarly, Pr, Nd, and Pm-doped CeO2 catalysts showed higher oxidation temperatures (T50 – 181, 198, and 212°C). The light-off temperature (T50) of La-doped catalyst decreases by 42°C (from 212°C to 170°C) when compared to Pm-doped CeO2. The high catalytic activity of La-doped CeO2 catalyst was due to the higher quantity of oxygen vacancies, larger specific surface area, and smaller particle size and enhanced OSC in comparison to other rare earth dopants. The La-CeO2 sample showed T100 at 293°C whereas Pm-doped CeO2 exhibited T100 at 331°C.

Figure 6. Catalytic CO conversion over M-doped CeO2 catalysts (Conditions: catalyst - 0.5 g, CO - 500 ppm, O2 - 20% Ar balance, time-on-stream - 1 h, GHSV – 30,000 h-1).


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The results obtained from the sol-gel derived high surface area M-doped nanocatalysts were compared to other similar rare earth metal doped CeO2 for CO oxidation reaction. Table S2 shows the comparison of M-doped CeO2 nanocatalysts with similar rare earth metal doped CeO2 reported in literature. It is found that the sol-gel derived high surface area rare earth metal doped CeO2 nanocatalysts showed the better catalytic activity than the other similar reported catalysts in literature37-39. It can be concluded that the better specific surface area of the M-doped CeO2 nanocatalysts might be responsible for better CO conversion. The smaller particle size particles provide higher specific surface area than the larger size particles. Generally, higher is the specific surface area of the catalyst higher is the activity for the catalytic reaction. In the present case among different dopants, La-CeO2 possessed smallest particle size and Pm-CeO2 has biggest particle size. Thus, La-CeO2 showed higher catalytic activity than Pm-CeO2 for CO oxidation. From isotropic oxygen tracer experimental results, the signals 44 (m/e) (high intensity), 46 (m/e) (medium intensity), and 48 (m/e) (very low intensity) were appeared for La-CeO2 catalyst (figure S3). The signal 44 corresponds to C16O2 which is generated because of reaction of pulsed C16O and the lattice oxygen (16O) of the catalyst, whereas, the signal 46 corresponds to C16O18O which is generated due to reaction between C16O and 18O. So, the main product here is C16O2 which indicate that CO binds to the lattice oxygen. Similarly, other doped-CeO2 catalysts showed similar behavior (figure S3). No C18O2 was detected. The role of vacancy sites on the surface of material CeO2 in CO oxidation process over M-doped CeO2 nanoparticles can be explained by Scheme 1 and equations given below24: 2Ce4+ + OL → 2Ce3+ + Vo + ½ O2(g)

(1) 15

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CO(g) → CO(ads)

(2)

CO(ads) + OL → CO(ads)-OL

(3)

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CO(ads)-OL → vacancy site + CO2(g) + Vo (4) O2(g) + Vo → 2OL

(5)

(Here, OL and Vo represent lattice oxygen and oxygen vacancy)

Scheme 1. Proposed CO oxidation mechanism over M-doped CeO2 catalysts.

In equation 1, vacancy sites were generated because of change of Ce4+ to Ce3+. The created oxygen vacancies can be used as active sites for adsorption of CO and also to activate lattice oxygen (equation 2 – equation 5)40. Incorporation of metal dopants increased the amount of oxygen vacancies in order to maintain the charge neutrality. Therefore, the catalytic activity of M-doped CeO2 catalysts for CO oxidation was enhanced as the quantity of oxygen vacancies increased.


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The long time-on-stream stability test (time-on-stream ~ 50 h) was conducted over a Ladoped CeO2 catalyst (figure 7). In this test, 0.5 g of catalyst was put into the heating furnace maintained at 170°C and 1 atmospheric pressure for CO oxidation reaction. The GHSV was maintained at 30,000 h-1. Figure 7 showed a constant 50% CO conversion over a doped-CeO2 catalyst during a time-on-stream of 50 h. There was no deactivation observed during the stability test. This confirmed that the La-doped CeO2 catalyst is highly active and stable for CO oxidation process.

Figure 7. Time-on-stream CO conversion stability test over La-doped CeO2 (Conditions: catalyst – 0.5 g, temp. - 170°C, CO - 500 ppm, O2 - 20% Ar balance, time-on-stream ~ 50 h, and GHSV – 30,000 h-1).

The kinetics of the CO oxidation process over M-doped CeO2 catalysts was conducted. A differential mode of reactor was considered by taking CO conversion from a linear region < 20% 17 ACS Paragon Plus Environment


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CO conversion. Among different dopants, the minimum apparent activation energy was possessed by La-CeO2 material which was equivalent to 63.41 kJ mol-1 (figure 8). This apparent activation energy is very well matched with the literature value41. The order of apparent activation energy was as follows: CeO2 > Pm-CeO2 > Nd-CeO2 > Pr-CeO2 > La-CeO2. Lesser is the activation energy, faster is the reaction. These results further indicate that the La-CeO2 will show higher catalytic activity than other rare earth metal dopants. The catalytic CO oxidation results showed similar results.

Figure 8. Arrhenius plot for M-doped CeO2 catalysts.

The powder BET, XRD, and TEM analysis were conducted over the used-La-CeO2 catalyst in order to find any change in specific surface area, phase and/ or particle size of the catalyst after CO oxidation reaction. BET results revealed no appreciable change in the specific surface area of the used-La-CeO2 sample. The fresh and used-La-CeO2 catalysts showed a 18 ACS Paragon Plus Environment

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specific surface area of 197.6 and 194.1 m2 g-1, respectively. Figure 9(I) shows the XRD of the fresh and used-La-doped CeO2 catalyst. The diffraction patterns of both the fresh and used-LaCeO2 sample were similar. No change in crystallite size (~ 3.4 nm) was observed in the used sample after CO oxidation reaction. Figure 9(II) showed a TEM micrograph of used- La-doped CeO2 sample. It also showed no change in average size of particles of La-doped CeO2 (~ 3.9 nm). All these results confirmed the high activity and stability of La-doped CeO2 nanocatalyst for CO oxidation reaction.

Figure 9. (I) Comparison of powder XRD of (a) fresh-La-CeO2 and (b) used-La-CeO2 and (II) TEM micrograph of used-La-CeO2.

4. Conclusion Rare earth metal, M-doped CeO2 catalysts were successfully prepared by the sol-gel method. XRD and TEM results revealed around 3-6 nm average size particles of M-doped CeO2 nanoparticles. BET results showed a very high specific surface area of M-doped CeO2 catalysts



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(~ 190 – 198 m2 g-1). Raman results showed that the La-doped CeO2 possessed highest amount of oxygen vacancy among other dopants. CO oxidation results also showed that the La-CeO2 catalyst is a better catalyst than other doped metals. T50 and T100 values of La-CeO2 catalyst were 170 and 293°C. It also showed an excellent time-on-stream stability for a time-period of 50 h. BET, XRD and TEM characterizations further confirmed the stability of the La-CeO2 catalyst. BET results showed no appreciable change in the used catalyst. No extra peaks were found in the diffraction pattern of used-La-CeO2. TEM revealed similar average particle size of used-LaCeO2 as compared to fresh one.

Conflict of Interest Author declares that there is no conflict of interest.

Acknowledgements The author wants to thank the IIT Delhi for using the characterization instruments.

Supplementary Information Some data are supplied as Supplementary Information. It is listed as follows: Figure S1. Schematic diagram of experimental set-up for CO oxidation. Figure S2 (a). XPS data of La 3d core levels of La-CeO2. Figure S2 (b). XPS data of Pr 3d core levels of Pr-CeO2. Figure S2 (c). XPS data of Nd 3d core levels of Nd-CeO2. 20 ACS Paragon Plus Environment

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Figure S3 (a). Products of IOT experiment on La-CeO2. Figure S3 (b). Products of IOT experiment on Pr-CeO2. Figure S3 (c). Products of IOT experiment on Nd-CeO2. Figure S3 (d). Products of IOT experiment on Pm-CeO2. Table S1. Intensity and the ratio between the different Raman vibrational modes in M-doped CeO2 catalysts. Table S2. Comparison of M-doped CeO2 nanocatalysts with similar rare earth metal doped CeO2 reported in literature.

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High Surface Area M (M = La, Pr, Nd, and Pm)-Doped Ceria

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