Mechanistic Insights and Kinetics of CO Oxidation over Pristine and

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Mechanistic Insights and Kinetics of CO Oxidation over Pristine and Noble Metal Modified Fe2O3 using DRIFTS Disha Jain, and Giridhar Madras Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04856 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on February 2, 2017

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Industrial & Engineering Chemistry Research

Mechanistic Insights and Kinetics of CO Oxidation over Pristine and Noble Metal Modified Fe2O3 using DRIFTS Disha Jain, Giridhar Madras* Department of Chemical Engineering Indian Institute of Science, Bangalore 560012, India

*

Corresponding author +91-80-2293 2321: [email protected]

ACS Paragon Plus Environment


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Abstract Fe2O3 is an attractive catalyst for CO oxidation owing to its low cost and reducible nature. Modification in reducible oxide by noble metal enhances its reducibility. In this work, the effect of noble metal (NM - Pt, Pd) impregnation and substitution on the reaction mechanism for CO oxidation has been studied. Detailed ex situ (XRD, XPS, and TEM) and in situ (DRIFTS) characterization were performed along with kinetic studies to develop mechanistic models. Remarkably, Fe2O3-supported NM exhibited superior catalytic activity than the NM-substituted Fe2O3. Consequently, XPS, PL and DRIFTS studies were performed to understand this behavior. Fe2O3-supported NM showed the formation of metal carbonyl bands in DRIFTS studies, however, carbonates were only observed over Fe2O3-supported Pt. Based on spectroscopic evidences and kinetic studies, Eley-Rideal mechanism was proposed for pristine Fe2O3, whereas noncompetitive and competitive Langmuir-Hinshelwood mechanism was proposed for Fe2O3-supported Pt and Pd, respectively. Keywords: CO oxidation; Fe2O3, DRIFTS; Eley-Rideal; Langmuir-Hishelwood; Solution combustion


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1. Introduction CO oxidation is an extensively studied reaction in the field of heterogeneous catalysis because of its great practical importance. CO emission takes place mainly from industries, power plants, automotive exhaust and domestic activities. The concentration of CO released from various sources ranges from 50 to 50,000 ppm. Many studies have been conducted to catalyze CO oxidation over different kinds of materials such as perovskites,1 transition metal2 and their oxides.3, 4 Of these, the transition metals and oxides have been extensively investigated due to the presence of partially filled d-orbital in the transition metals.5 Bonding of CO to the metal (or ion) takes place by the donation of electron density from bonding 5σ orbital of CO to the metal and by receiving electron density from metal to anti-bonding 2π* orbital, which has an opposite effect. The degree of back bonding varies with adsorption site and metal (or ion).6 Based on reducibility, the metal oxides can be classified as reducible (such as CeO2, SnO2, Fe2O3) and non-reducible supports (such as Al2O3, ZrO2, SiO2).7 Reducible supports show superior catalytic activity than inert supports due to their ability to supply active oxygen species like peroxide or superoxide. To further enhance the activity of the reducible supports, modifications have been done by noble or base metal substitution,8 impregnation,9 or by changing the morphology of oxide particles.10 It is evident from the previous studies that substituted oxides exhibit better catalytic activity and stability as compared to the supported oxides.11, 12 This might be due to the creation of distinct active sites for adsorption of CO and O2, which subsequently changes the reaction pathway.13, 14 Metal-support interaction is also an important factor for supported catalysts, which tends to be maximum for high dispersion of metal particles. This interaction influences the heat of adsorption of gas molecules on the metal.15 The mechanism of CO oxidation over different supports with respect to the activation of oxygen and the nature of oxygen species involved is still not clearly understood.16 For instance, over Au/FeOx, Tripathi et al. have proposed redox mechanism (Mars van Krevlen mechanism),17 whereas, Daniells et al. suggested Langmuir Hinshelwood



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mechanism.18 The complexity increases for polycrystalline oxides and oxide-supported metal particles due to the presence of different facets, nature of active sites, defect sites and metal support interactions.19 Fe2O3 is a transition metal oxide that has been investigated for oxidation reactions because of its reducible nature, non-toxicity and low processing cost.20 Among all stable oxides and hydroxides of iron, ferrihydrite and hematite have been found to be catalytically active for the reaction.21 Fe2O3 has been prepared by precipitation method,22 polyvinyl alcohol method,23 while metal supported Fe2O3 was synthesized using co-precipitation method.7, 24-27 Fe2O3 is known to have intrinsic electronic defects like cationic and anionic vacancies and interstitial defects.28, 29 The existence of defects is beneficial for various applications such as the presence of oxide vacancies is favorable for oxidation reactions, as it provides sites for O2 adsorption. It has also been shown that the formation of oxide vacancy is enhanced by the presence of metal on active oxides (CeO2, Fe2O3)26, 30 and substituted in the oxide lattice.11, 13 However, despite several reports on iron oxide, their efficiency can be improved. For instance, iron oxide synthesized by conventional methods shows good catalytic activity only at high temperatures. With this aim, Fe2O3 was synthesized by solution combustion synthesis. Further, it was modified by noble metal (NM- Pt, Pd) substitution and impregnation and the catalytic activity for CO oxidation reaction was investigated. Fe2O3 was substituted with lower-valent ion Pd (2+) and higher-valent ion Pt (4+) to study the effect of dopant on the defect formation. Structural characterization such as X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Photoluminescence (PL) and BET surface area were performed for the synthesized compounds. A comprehensive analysis of the reaction mechanism of CO and O2 on the catalyst was conducted. The active species involved in the reaction were explored using in situ FTIR (DRIFTS) technique to develop a kinetic model for Fe2O3 and NM supported Fe2O3. The experimental data was fitted with the derived rate expression to determine the kinetic parameter.


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2. Experimental 2.1. Materials Commercial Fe2O3 and Fe3O4 (particle size of 100 nm) were obtained from Merck, India and Sigma Aldrich, U.S.A., respectively and are denoted as Fe2O3_C and Fe3O4_C. All other compounds were obtained from S.D. fine Chemicals, India, unless otherwise mentioned. 2.2. Methods 2.2.1. Synthesis of Fe2O3 Fe2O3 was synthesized by a single step solution combustion method. Ferric nitrate (Fe(NO3)3.9H2O) was used as an oxidizer and different fuels such as l-ascorbic acid (C6H8O6), urea (CH4N2O) and glycine (C2H5NO2) were used. Urea and glycine are commonly used in combustion synthesis due to the presence of amino groups, which is responsible for highly exothermic reaction.31 However, l-ascorbic acid complexes with metal ion from COO- end and also, l-ascorbic acid is known to reduce Fe3+ to Fe2+.32 Therefore, the formation of mixed oxide of Fe3+ to Fe2+ is expected when l-ascorbic acid is used as a fuel. The oxidizer to fuel ratio was maintained as unity. In this synthesis method, the oxidizer and fuel were mixed in the stoichiometric ratio in minimum amount of water to ensure a homogeneous solution. The prepared solution was kept in a muffle furnace, preheated at 350±10 °C. In the case where ascorbic acid was used as fuel, the reaction began with evaporation of water, followed by the release of gases, forming a gel like mass. This gel like mass subsequently caught fire, which traveled linearly along the formed gel leading to the formation of a solid product. Besides, when urea and glycine were used as fuel, volume combustion was observed and a voluminous solid product was obtained. The obtained materials were then grinded and calcined for 3 h at 350 °C. The final products obtained when



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ascorbic acid, urea and glycine were used as fuels are designated as Fe2O3_A, Fe2O3_U and Fe2O3_G, respectively. 2.2.2. Synthesis of Fe1.98M0.02Oδ (M = Pd, Pt) Noble metal ion (Pd, Pt) substituted Fe2O3 was synthesized by solution combustion technique. Ferric nitrate (Fe(NO3)3.9H2O), 2 atom% of respective noble metal salt: hexachloroplatinic acid (H2PtCl6, Finar), or palladium nitrate (Pd(NO3)2, Sigma Aldrich, U.S.A.) and l-ascorbic acid (C6H8O6) were taken in the stoichiometric amount. For instance, for the preparation of 2% Pd-substituted Fe2O3, Fe(NO3)3.9H2O, Pd(NO3)2 and ascorbic acid were taken in the ratio of 1.98:0.02:1.495 and dissolved in water to get a clear homogeneous solution. This solution was placed in a preheated muffle furnace maintained at 350±10 °C. After the completion of the reaction, the solid product obtained was grinded and calcined at 350 °C for 3 h. 2.2.3. Synthesis of M/Fe2O3 (M = Pd, Pt) Fe2O3-supported noble metal (Pd or Pt) was prepared by impregnation method. 2 atom% of metal (M = Pd, Pt) was impregnated onto the solution combustion synthesized Fe2O3_A. In this preparation, Fe2O3 was dispersed in water by N2 bubbling, followed by the addition of the respective noble metal precursor to the slurry. The dispersion of Fe2O3 in water was maintained as 0.6 g/L. Hydrazine hydrate (N2H4.H2O) was used as the reducing agent. Noble metal ions were reduced over Fe2O3 by drop wise addition of 0.05 M solution of hydrazine hydrate. The reduction of metal ions over the support was indicated by change in color of solution from light to dark brown. After the completion of the reaction, the addition of hydrazine hydrate was stopped and the mixture was aged for 1 h. The mixture was then filtered and washed several times with ethanol and deionised water until neutral pH was obtained. The solid material recovered after filtration was dried for 8 h at 60 °C. The dried sample was heated in N2


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atmosphere for 3 h at 250 °C to avoid the oxidation of surface Pt or Pd atoms caused by the oxidizing pretreatment. 2.3. Characterization The synthesized materials were characterized by XRD, XPS, TEM and PL. XRD was performed to determine the crystal structure of the sample. The XRD pattern was collected at 28 °C with Rigaku X-ray diffractometer. The diffractometer was equipped with Cu-Kα radiation (λ = 1.5418 Aͦ) and Ni filter. XRD pattern was recorded in the 2θ range from 10° to 80° with steps of 0.02°. The diffracted beam monochromator was placed before the detector to reduce the effect of fluorescence caused by Cu radiation due to the presence of Fe in the samples. X-ray Photoelectron spectra (XPS) were collected on AXIS ULTRA instrument using Al-Kα (1486.6 eV) as source. Pass energy of 20 eV was used while recording the spectra. The sample in pelletized form mounted on carbon tape was introduced into an ultra-high vacuum chamber. XPS was performed to determine different states of element present in the synthesized material. TEM micrographs, High resolution TEM (HRTEM) micrographs and electron diffraction pattern were obtained by Technai F-30, operating at 200 kV. The samples were prepared by drop casting the solution of the material dispersed in ethanol on carbon coated Cu grids (400 mesh size). The particle sizes and crystallinity of the material were determined by TEM analysis. BET surface area was measured by Belsorb surface area analyzer (Smart instruments) using BET nitrogen sorption method at 77 K. The synthesized compounds were regenerated at 150 °C for 3 h. Photoluminescence (PL) was performed at room temperature using LS55 with excitation wavelength of 450 nm. PL was performed to study the nature of defects in the sample.



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2.4. CO oxidation CO oxidation was performed in packed bed flow reactor. The catalyst was made into granules of 150 - 300 µm and loaded in the reactor between two glass wool plugs. The amount of catalyst used was 200 mg and the catalyst bed height was maintained as 1.2 cm. The reactor was positioned in the furnace with a thermocouple placed in the middle of the bed. Isothermal conditions were maintained on the catalyst bed during the experiment using a PID controller within ±1°C. The feed gas mixture containing 2.4% O2 (Air, Noble gases, Bangalore), 2.4% CO (Chemix gases, Bangalore) and rest as N2 (Noble gases, Bangalore) was passed over the catalyst bed from high pressure cylinders. The total gas flow rate was maintained at 100 ml/min using control valves. Thus, the space velocity of 36750 h-1 (on the basis of total bed volume) was kept constant for all the experiments. Gas Chromatograph (Mayura Analytical Ltd., India) equipped with flame ionization detector and methanator was used for analyzing outlet gases from the reactor. CO and CO2 gases were analyzed using flame ionization detector (FID). The methanator uses Ru catalyst at a controlled temperature of 350 °C to convert CO and CO2 to CH4, as CO and CO2 signals are weak in FID. 2.5. DRIFTS studies Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies were performed using Frontier, Perkin Elmer fitted with DTGS detector. The DRIFTS measurements were performed in an in situ DRIFTS cell (Harrick, Model # HVC-DWM-3), fitted with ZnSe windows, which is placed inside Praying Mantis (Model # HVC-DRP-4). This reaction chamber was equipped with a heating cartridge and was connected to a chiller for maintaining temperature. The temperature was controlled using a PID controller (Harrick, Model # ATC-024-4) within ±1°C. The catalyst was placed in the DRIFTS cell and reactant gases were passed in a continuous flow over the catalyst bed. The same operating composition


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(2.4% O2, 2.4% CO and rest N2) was maintained throughout these studies. The spectra were obtained at different temperatures, where, background was recorded at each temperature before starting the analysis with 32 scans and resolution of 4 cm-1. The chamber was purged with N2 while the temperature was increased. DRIFTS study was done for Fe2O3_A, Fe2O3_A-supported noble metal and NM-substituted Fe2O3. 3. Results and discussion 3.1. Structural studies 3.1.1. X-ray Diffraction (XRD) XRD patterns for the synthesized materials and commercial Fe2O3 and Fe3O4 are shown in Figure 1a and 1b. XRD was used to determine various phases present in the synthesized material. Synthesized materials (Figure 1) showed the presence of phases of both α- Fe2O3 (JCPDS file # 01-072-0469) and Fe3O4 (JCPDS file # 01-071-0161). In the substituted Fe2O3 (Figure 1b), no peaks of the noble metals or their oxides were observed. This indicates either the substitution of noble metal in the lattice of Fe2O3_A or the overlapping peak of M/MOx (M = Pd, Pt) with iron oxides. In the XRD pattern of 2% Pt/Fe2O3, an extra peak was observed corresponding to metallic Pt (111). However, in the case of 2% Pd/Fe2O3, no peak was observed other than that of Fe2O3. This may be due to the overlapping peaks of Pd and Fe2O3. The diffraction peaks of metallic Pd are expected at 40.3° (111) and 46.9° (200) (JCPDS# 87-0645). The presence of (111) and (200) phases were identified by TEM analysis. In all the synthesized materials, peak broadening was observed, indicating nanocrystallinity. The specific surface area of Fe2O3_A, Fe2O3-supported Pt and Pd were found to be 17, 33 and 32 m2/g, respectively. The increase in surface area was observed on noble metal impregnation, which can be



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attributed to the presence of NM nanoparticles on the oxide surface. However, there was no significant increase in surface area in case of NM-substituted Fe2O3. Rietveld refinement of XRD pattern of Fe2O3_A, Fe2O3_G and Fe2O3_U was performed using GSAS and EXPGUI. Figure 1c, 1d and 1e show the refined profile of Fe2O3_A, Fe2O3_G and Fe2O3_U, respectively. Since, the synthesized materials contain peaks of both Fe2O3 and Fe3O4; both phases were incorporated while doing the refinement. Fe2O3 crystallizes in the hexagonal crystal system with space group R-3c whereas Fe3O4 crystallizes in the cubic system with space group Fd-3m. Background subtraction was done manually with 18 terms. The profile fitting was done using pseudo-Voigt function. Lattice parameters and phase fractions are determined and the profile refined parameters are tabulated in Table 1. It was observed that Fe2O3_A, Fe2O3_U and Fe2O3_G contained different fractions of phases of Fe2O3 and Fe3O4. Fe2O3_G and Fe2O3_U were found to have Fe2O3 as the major component with negligible amount of Fe3O4. However, Fe2O3_A comprised of Fe2O3 and Fe3O4 in the ratio 0.61:0.36. The remaining ~3% corresponds to the impurities like unreacted nitrates. 3.1.2. X-ray Photoelectron Spectroscopy (XPS) XPS was performed for all the samples before and after the reaction. Due to the presence of reducing (CO) and oxidizing (O2) atmosphere in the reaction mixture, the change in the oxidation state of the elements is probable. Binding energies are calibrated using adventitious carbon i.e. C-C at 284.8 eV. The O 1s spectra comprises of Fe ̶ O (532 eV).33 The relative surface concentrations for noble metal substituted and supported catalyst were determined using the formula

CM =

I M / ( λM σ M DM ) ∑ ( I M / ( λM σ M DM ) )

(1)


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10

(b)

(a)

Fe2O3_A

Intensity (a.u.)

Fe2O3_G

Fe2O3_U Fe3O4_C

2% Pt/Fe2O3

Pt

Fe1.98Pt0.02O3-δ 2% Pd/Fe2O3 Fe1.98Pd0.02O3-δ

Fe2O3_C

40

50

60

70

10

80

20

30

40 50 2θ (deg)

Yobs Ycalc Yobs-Ycalc Bragg position (Fe3O4)

(d)

10

20

40

50

60

70

80

113

10

80

422 116 024

012

119 533

30

70

Bragg Position Fe3O4

400

214 300 440

422 116

211

024

113 400

220

111

012

104

Intensity (a.u)

Bragg position (Fe2O3)

60

Yobs Ycalc Yobs-Ycalc Bragg Position Fe2O3

110 311

311 110

(c)

104

2θ (deg)

20

30

40

50

60

119

30

70

533

20

21-2

10

214 440 300

Intensity (a.u.)

Fe2O3_A

Intensity (a.u.)

80

2θ (deg)

104

2θ (deg)

Yobs Ycalc Yobs-Ycalc Bragg position Fe3O4

311 110

(e)

60

119

214 440 300

422 116

40 50 2θ (deg)

70

533

30

211

220

20

400

111

10

113

012

024

Bragg position Fe2O3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

Figure 1. XRD pattern of (a) Synthesized Fe2O3 and commercial Fe2O3 and Fe3O4, (b) noble metal substituted and impregnated Fe2O3, (c) Rietveld refinement of Fe2O3_A, (d) Rietveld refinement of Fe2O3_G, (e) Rietveld refinement of Fe2O3_U



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Table 1. Profile parameters after Rietveld refinement Fe2O3_A, Fe2O3_G and Fe2O3_U

Fe2O3

Phase fractions

a

b

c

0.61

5.038(3)

5.038(3)

13.759(7)

Fe2O3_A Fe3O4

0.36

8.335(4)

8.335(4)

8.335(4)

Fe2O3

0.95

5.037(3)

5.037(3)

13.760(6)

Fe2O3_G Fe3O4

0.046

8.325(4)

8.325(4)

8.325(4)

Fe2O3

0.95

5.020(7)

5.020(7)

13.711(8)

Fe2O3_U Fe3O4

0.025

8.391(7)

8.391(7)

Rwp

Rp

χ2

12.9

9.7

1.849

10.5

8.2

3.375

10.4

6.9

2.435

8.391(7)

In the above equation, IM is the core level intensity, λM is the inelastic mean free path of respective photoelectron, σM is the photoionization probability and DM is the geometric factor. The value of mean free paths and photoionization probability were taken from literature.34, 35 The geometric factor, which depends on spectrometer, is taken to be unity. The Fe 2p spectra recorded before and after reaction are shown in Figure 2a. Shirley-type correction was applied for background subtraction.36-38 Asymmetry and broadening observed in Fe 2p spectra are because of the existence of both Fe2+ and Fe3+, ligand field effects owing to different Fe environments i.e. octahedral, tetrahedral, crystalline disorder (like oxide vacancy) and multiplet splitting.39 Multiplet splitting occurs as a result of the interaction of unpaired electron in 3d orbital with the 2p unpaired electron generated after photoemission. Besides, broadening can also account to the presence of high-spin Fe3+ and Fe2+ compounds.36


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As evident from XRD, the synthesized materials contain both Fe2O3 and Fe3O4. This suggests the presence of different environment around Fe. In Fe2O3, Fe3+ ions occupy octahedral sites. In Fe3O4, Fe3+ ions occupy both octahedral and tetrahedral sites and Fe2+ ions are present in the octahedral sites. Thus, it is apparent that the binding energies of the three types of Fe3+ ion are slightly different.39 Binding energies and FWHM (full width at half maxima) for different states of Fe are tabulated in Table S1 (see supporting information). In addition to multiplet splitting, Fe 2p displays distinguishable satellite peaks, which is characteristic for different oxidation state of Fe. For Fe3+ state, the satellite peak is observed approximately 8 eV higher than Fe 2p3/2.40 There is a satellite associated with Fe2+ state as well, which appears at ~ 6 eV higher than the main Fe 2p3/2. The relative surface concentration of Fe2+:Fe3+ before and after reaction was found to be 0.31:0.69 and 0.33:0.67, respectively. Therefore, it can be suggested that no considerable change in states was observed. It should be noted that the contributions from ligand field effect and multiplet splitting (in spectral widths) are not taken into account separately in this study. Figure 2b and 2c show the XPS spectra of Pt 4f before and after reaction of Fe2O3_A-supported Pt and Pt-substituted Fe2O3, respectively. As evident from Figure 2b, the signal of Pt 4f in Fe2O3_Asupported Pt is obscure in nature. This may be due to the partial encapsulation of Pt particles with Fe2O3.41 Pt 4f spectra split into 4f7/2 and 4f5/2 due to spin orbit splitting. The energy of separation between 4f7/2 and 4f5/2 is 3.1 eV. The binding energies of Pt 4f7/2 for Pt0, Pt2+ and Pt4+ states are 71, 72.4 and 74.4 eV, respectively.42 The linear background subtraction was done for Pt 4f spectra and the FWHM of all the deconvoluted peaks was maintained as 2 eV.41, 43 In Pt substituted Fe2O3_A, only Pt4+ was observed, indicating the substitution of Pt in the lattice of the oxide. The relative concentration of Fe:Pt was found to be 0.985:0.015. On the contrary, no change in the oxidation state of Pt was observed after the reaction (as reported in the literature).11-13 In doped oxides, noble metal (dopant) acts as CO adsorbing site and the oxygen adsorption takes place on the oxide vacancies formed near the dopant site. Therefore, the decrease



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in oxidation state of Pt is expected after the reaction due to the reducing nature of CO, which is not observed in the present study. In case of Fe2O3_A supported Pt, Pt was observed to be in Pt0 state before reaction. The relative concentration was calculated from the deconvoluted peaks before reaction and found to be 2.4% of Pt with respect to Fe. After reaction, Pt0 and Pt2+ states were present in the ratio of 0.76:0.24. The change in Pt state shows the involvement of metal (Pt) during the reaction.44 Pt has affinity for adsorbing both CO and O2. Oxygen dissociation can occur on the metal-support interface. This results in the oxidation of the Pt present at the interface. DFT calculations suggest similar kind of effect for Au supported on Fe2O3. Oxidized Au linear arrangements are formed, which are stabilized by the interaction of anionic oxygen with Fe3+ ions present on the surface.45 XPS spectra of Pd 3d before and after reaction in Fe2O3_A-supported Pd and Pd-substituted Fe2O3 are shown in Figure 2d and 2e, respectively. The energy of separation between Pd 3d5/2 and 3d3/2 due to spin orbit splitting is 5.4 eV. The binding energy values of Pd5/2 are 335, 337.5 and 338.4 for metallic Pd, Pd2+ and Pd4+, respectively.42 The FWHM of all the component peaks was maintained as 2 eV.46, 47 In case of Pd-substituted Fe2O3_A, Pd was present only in the Pd2+ state indicating the presence of Pd in the lattice of Fe2O3. The relative concentration was determined before reaction and Fe:Pd was found to be 0.984:0.016. There was no change in Pd state after the reaction, which is in contradiction with the literature.11 However, in Fe2O3_A-supported Pd, Pd was present in both Pd0 and Pd2+ state before the reaction. The relative concentration was determined from the deconvoluted peaks and 2.6% of Pd was observed with respect to Fe. The ratio of Pd0:Pd2+ changed from 0.78:0.22 before reaction to 0.58:0.42 after the reaction. We observed an increase in the amount of Pd2+ and appearance of Pd4+ state after the reaction,


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14

after reaction

(a)

(b)

after reaction 0

Pt (4f) 2+

Intensity (a.u.)

Intensity (a.u.)

Pt (4f)

before reaction Fe (II) Tetra Fe (III) Octa Fe (III) Octa

before reaction 0

Pt (4f)

Fe (III) Tetra

Fe (II) Satellite Fe (III) Satellite

740

730

720

710

76

Binding Energy (eV)

(c)

74

72

70

68

Binding Energy (eV)

(d)

after reaction

4+

Pt (4f)

after reaction

0

Pd (3d) 2+

Pd (3d) 4+

Intensity (a.u.)

Pd (3d)

Intensity (a.u.)

before reaction 4+

Pt (4f)

80

76

72

before reaction

0

Pd (3d) 2+

Pd (3d)

344

Binding energy (eV)

340

336

332

Binding energy (eV)

after reaction

2+

(e)

Pd (3d)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2+

before reaction

Pd (3d)

344

340

336

Binding energy (eV)

Figure 2. XP spectra before and after reaction (a) Fe 2p in Fe2O3_A, (b) Pt 4f in 2% Pt/Fe2O3, (c)Pt 4f in Fe1.98Pt0.02O3-δ, (d) Pd 3d in 2% Pd/Fe2O3, and (e) Pd 3d in Fe1.98Pd0.02O3-δ



Page 16 of 53

15

which is similar to that of Fe2O3_A supported Pt. Hence, this may be attributed to the oxidation of Pd nanoparticles by O2 present in the reaction mixture, as Pd is an active site for dissociative adsorption of O2.14 Figure 3 shows the O 1s spectra of Fe2O3, Pt and Pd doped Fe2O3. The O 1s spectra of these materials were deconvoluted into four components i.e. lattice oxygen (OL), oxygen vacancy (OV), chemisorbed or dissociated oxygen (OC) and hydroxyl group (from adsorbed water on oxide surface).48 Further, while deconvoluting, two types of lattice oxygen were considered corresponding to oxygen bonded to Fe2O3 and Fe3O4. Relative composition of OV was determined from the deconvoluted spectra. The binding energy values and the FWHM of the components and the relative percentages of oxygen vacancies in these samples are included in table S2 (see supporting information). It was found that Fe2O3, Pt and Pd doped Fe2O3 contain 14.1%, 15.3% and 16.2% of OV, respectively. Thus, it is apparent that no appreciable change is observed in oxygen vacancy concentration after doping Fe2O3 with noble metal.

(a) OL,Fe O 3

OL,Fe O

4

OV

2

3

Oc -OH

(b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(c)

534

532

530

528

Binding Energy (eV)

Figure 3. Comparison of XP spectra of O 1s of (a) Fe1.98Pd0.02O3-δ, (b) Fe1.98Pt0.02O3-δ, (c) Fe2O3_A


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3.1.3. Transmission Electron Microscopy (TEM) TEM micrographs and diffraction patterns of Fe2O3_A, 2% Pt/ Fe2O3 and 2% Pd/ Fe2O3 are shown in Figure 4. For all compounds, bright field images exhibit that the particle sizes of Fe2O3_A are in the range of 10 to 70 nm and the NM were in the range of 5-20 nm. HRTEM and electron diffraction pattern of Fe2O3_A corroborated the presence of both Fe2O3 (shown in white) and Fe3O4 (shown in orange) phases in the material, as previously observed by XRD and XPS. The HRTEM micrograph and diffraction pattern of Fe2O3-supported Pt showed the presence of (111) phase of Pt (shown in yellow) along with Fe2O3. However, for Fe2O3-supported Pd, HRTEM micrograph revealed the presence of Pd in (111) phase (shown in yellow). SAED pattern indicated the presence of (200) phase of Pd. 3.1.4. Photoluminescence (PL) Bulk Fe2O3 does not exhibit PL due to the local forbidden d-d transition and efficient lattice relaxation.49 However, nano-sized Fe2O3 and Fe3O4 show PL due to the quantum size effect and specific surface area effect.50 In Fe2O3 nanoparticles, Fe-O bond length increases that leads to the enhancement of magnetic coupling of neighboring Fe3+.51 The variation in crystal structure affects the magnetic coupling strength and electronic transition. In the present case, the compounds i.e. Fe2O3, Pt and Pd doped Fe2O3 under observation are synthesized by the same method and have particle sizes in the same range. Therefore, it would be rational to assume that the quantum size effect will have the same influence on all the samples. PL spectrum is also used to obtain the information about the defects such as oxide vacancies, cationic vacancies and other defects. The presence of defects in the semiconductor material would create a corresponding energy level in the band gap. Therefore, there will be change in the photoluminescence with different defects. For instance, as the content of oxygen vacancy and other defects increases the probability of exciton occurrence and the PL signal increases.52



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In the present study, PL was performed to determine the change in the nature and concentration of the defects with the doping of the NM in the Fe2O3 lattice. To determine whether the observed PL signal was due to oxygen vacancies, Fe2O3 was annealed at 700°C in continuous flow of O2. PL spectra of


Page 19 of 53


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Figure 4. Bright field images of (a) Fe2O3, (c) 2% Pt/Fe2O3 and (e) 2%Pd/Fe2O3; HRTEM micrographs of (b) Fe2O3, (d) 2% Pt/Fe2O3 and (f) 2%Pd/Fe2O3

synthesized Fe2O3 and annealed Fe2O3 were compared (Figure 5 inset). It was observed that there was a decrease in the intensity of the PL signal in the annealed sample. This can be explained as annealing of Fe2O3 in the presence of O2 will fill the oxide vacancy present in the sample, thereby resulting in decrease in the PL signal. This illustrates the presence of oxide vacancies in the sample. The existence of Fe vacancies cannot be confirmed by such a procedure, however, according to previous literature, Fe2O3 and Fe3O4 is known to have Fe vacancies.28, 50 Figure 5 shows the comparison of the PL spectra of Fe2O3, Pt and Pd doped Fe2O3. Pd-substituted Fe2O3 shows higher PL intensity compared to Fe2O3 while Ptsubstituted Fe2O3 did not show increase in PL intensity. It is evident from XP spectra of O 1s that there is no significant increase in the oxide vacancy concentration with the doping. The increase in PL intensity in Pd doped Fe2O3 can be explained by the possibility of creation of cationic vacancies on doping. However, for Pt-substituted Fe2O3 there is no formation of defect on doping. These results elucidate the reason for low activity of NM-substituted Fe2O3.



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19

For better understanding of XPS and PL, we have explained the reason for an increase in catalytic activity of doped oxide. In case of reducible oxides, there is an increase in CO oxidation activity on doping, due to the formation of oxide vacancies (created in the vicinity of the dopant) to maintain charge neutrality on the oxide. These oxide vacancies aid in CO oxidation by providing sites for oxygen adsorption and activation. However, in the present system, no synergistic effect was observed on doping on the catalytic activity. Therefore, as apparent from XPS and PL studies, there is a probability of formation of cationic vacancies (or some other defect) on doping NM in Fe2O3.

600

Fe2O3_A Fe2O3_A_700

Intensity (a.u.)

800

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

600 400

400

200

500

550

600

650

Wavelength (nm)

Fe2O3

200

Fe1.98Pt0.02O3-δ Fe1.98Pd0.02O3-δ

0 500

550

600

650

700

750

800

Wavelength (nm)

Figure 5. Photoluminescence spectra of Fe2O3_A, Pd and Pt substituted Fe2O3. The inset shows the PL spectra of Fe2O3_A and Fe2O3_A_700 3.2. Catalytic studies CO oxidation over the synthesized material was performed using 200 mg of catalyst with inlet feed consisting of 2.4% O2, 2.4% CO and rest N2 with total flow rate of 100 ml/min. Figure 6a shows comparison of fractional conversion versus temperature of Fe2O3_A, Fe2O3_U, Fe2O3_G and commercial Fe2O3 and Fe3O4. It is evident that (Figure 6a) combustion synthesized Fe2O3 exhibits better catalytic activity than the commercially available Fe2O3 and Fe3O4. As observed from TEM, synthesized samples have smaller particle size (~20 nm) as compared to commercial samples, which explains the higher


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activity of the combustion synthesized samples. Among Fe2O3 synthesized using different fuels, Fe2O3_A shows better activity as compared to Fe2O3_U and Fe2O3_G. The reason behind the high activity of Fe2O3_A can be understood from the phase ratio of Fe2O3 and Fe3O4 obtained from Rietveld Refinement. All three compounds contain different ratios of the two phases. Fe2O3_A contains phases of Fe2O3 and Fe3O4 in the ratio of 0.61:0.36 whereas in Fe2O3_U and Fe2O3_G, Fe3O4 is present in very less amount. We believe that the higher activity of Fe2O3_A is due to the presence of higher ratio of Fe3O4 compared to Fe2O3_U and Fe2O3_G. The presence of multiple states of Fe creates two different chemical environments around lattice oxygen. Further, the bond length of Fe-O is longer in Fe2O3_A in comparison to Fe2O3.53 These factors make the extraction of the lattice oxygen less energy intensive. Thus, because of this higher catalytic activity, further studies were performed on Fe2O3_A. Figure 6b shows the comparison of fractional conversion versus temperature of Fe2O3_A with NM substituted and supported catalyst. 100% CO conversion was observed at 280 °C over Fe2O3_A, below 150 °C in case of 2% Pd/Fe2O3 and below 200 °C in case of 2% Pt/Fe2O3. Fe2O3_A-supported NM exhibited catalytic activity at low temperature (~40 °C). This is attributed to the presence of –OH species as Fe-OH-Pt at interfaces. Chen et al.54 proposed that CO oxidation at the interface site occurs via formation of –COOH species at low temperatures. This formation of CO2 at lower temperature leads to the formation of oxide vacancy at the interface. Pd and Pt -substituted Fe2O3 does not show much improvement in the activity as compared to Fe2O3_A. Therefore, in case of noble metal modified Fe2O3, supported material exhibits better activity than the substituted ones. This result is not in agreement with the other reducible oxides (CeO2 for instance12) reported in literature. Hence, further attempts were made to investigate the probable reason behind this observation. To understand this behavior, we investigated the change in the nature of defects on NM substitution. It is evident from the XPS spectra of O 1s of pristine and substituted Fe2O3 (Figure 3) that there is no significant change in the number of oxide



Page 22 of 53

1.0

(a) Fe2O3_A

Fractional CO Conversion


21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fe2O3_G

0.8

Fe2O3_U Fe2O3_C

0.6

Fe3O4_C

0.4 0.2 0.0 100

200

300

400 o

Temperature ( C)

500

1.0

(b)

2% Pd/Fe2O3 Fe1.98Pd0.02O3 2% Pt/Fe2O3

0.8

Fe1.98Pt0.02O3 Fe2O3_A

0.6 0.4 0.2 0.0 50

100

150

200

250

300

o

Temperature ( C)

Figure 6. Variation of CO conversion for (a) Synthesized Fe2O3 and commercial Fe2O3 and Fe3O4, (b) Noble metal modified Fe2O3 with Fe2O3_A (reaction condition: catalyst loading-200 mg, GHSV36750 h-1) vacancies in the three materials. PL data shows that there is a possibility of formation of cationic vacancies instead of anionic vacancies (desirable for CO oxidation). Based on these investigations, we believe that in Fe2O3, doping does not result in increasing the catalytic activity due to the formation of cationic vacancies. Experiments were also performed by varying the catalyst loading to determine the kinetic regime in differential reactors. Figure 7 depicts the variation of W/FCO (W = weight of catalyst and FCO = molar flow rate of CO) with fractional conversion of CO for Fe2O3_A, 2% Pd/Fe2O3 and 2% Pt/Fe2O3. At high W/FCO, a change in the slope was observed indicating the change from a kinetic to diffusion regime. In all the cases, rates were determined from the slope of the initial linear region and calculated by rCO = XCO/(W/FCO). Furthermore, Arrhenius plot was used to determine the apparent activation energy (Eaapp) for CO oxidation over the catalysts. The apparent activation energy of Fe2O3_A, 2% Pt/Fe2O3 and 2% Pd/Fe2O3 was found to be 51±1, 40±1 and 31±2 kJ/mol, respectively. The rates and apparent activation energies obtained over different catalysts in the literature and in the present study are compiled in Table S3 (see supporting information). It is observed that rates of CO oxidation over the catalysts in this study


Page 23 of 53


22

are higher than the previous reports. The apparent activation energy is found to be 51 kJ/mol over Fe2O3 that is much lower than the Fe2O3 (100 kJ/mol) synthesized by coprecipitation technique. The stability of Fe2O3_A, 2% Pd/Fe2O3 and 2% Pt/Fe2O3 was tested and no deactivation was observed for Fe2O3-supported NM in 24 h of operation. However, a slight decrease in conversion was observed in the case of Fe2O3.

(a)

140 C o 160 C o 180 C o 200 C o 220 C o 240 C o 260 C o 280 C

1.0 0.8 0.6 0.4 0.2 0.0 0

20

0.35



o

40

60

80 3

100

(b)

o

110 C o 120 C o 130 C o 140 C o 150 C o 160 C o 170 C

0.30 0.25 0.20 0.15 0.10 0.05 0.00

120

0

20

-1

40

60

3

80

100

120

W/FCO (x10 g.s/mol)

W/FCO (x10 g.s mol )

(d)

(c)

0.4

-13.0

o

50 C o 60 C o 70 C o 80 C o 90 C o 100 C

0.3

2% Pd/Fe2O3 2% Pt/Fe2O3 Fe2O3_A

-13.5

ln(rCO)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2

0.1

-14.0

-14.5

0.0 0

20

40

60

80

100

120

-15.0 0.0020

3

W/FCO (x10 g.s/mol)

0.0024

0.0028

0.0032

-1

1/T (K )

Figure 7. Variation of fractional conversion with W/FCO for (a) Fe2O3, (b) 2% Pt/Fe2O3 and (c) 2%Pd/Fe2O3, (d) Arrhenius plot (reaction conditions: catalyst loading-200 mg, GHSV- 36750 h-1)



Page 24 of 53

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3.3. DRIFTS studies The DRIFTS studies were performed to understand the nature of species formed during the reaction. DRIFTS studies were performed for the same temperature range as that of kinetic studies. These studies were conducted in two steps. Firstly, reactant gases (with or without O2) were passed over the catalyst bed for 30 min (as peak intensities were observed to equilibrate in 30 min). In the second step, CO+O2 flow was stopped and N2 was purged over the bed for next 30 min. The rationale behind the second step was to distinguish between the gaseous species (or physisorbed species) and the chemisorbed species. When CO (+O2) flow was stopped and N2 was purged, the chemisorbed species on the surface remained adsorbed, whereas the gaseous or the physisorbed species disappeared. 3.3.1. Fe2O3 Figure 8a shows the DRIFTS spectra recorded at three temperatures over Fe2O3_A viz. 28 °C, 160 °C, and 200 °C in the flow of CO+O2. At 28 °C, a doublet is observed at 2112 and 2172 cm-1 (centered at 2145 cm-1), which corresponds to the gaseous CO peaks.23, 55 When N2 was purged through the system, this doublet disappeared (graph not shown). On further increasing the temperature to 160 °C, in addition to doublet at 2145 cm-1, a new band appeared at 2342 cm-1, which is assigned to asymmetrical gaseous CO2.56 The rotational vibrational bands were observed at 1550 and 1355 cm-1, which correspond to the bidentate and bridged carbonate respectively. At 200 °C (Figure 8b), various carbonate species at 1605 and 1354 cm-1 (carboxylate), 1560 cm-1 (bi-dentate), 1473 cm-1 (monodentate) and 1383 cm-1 (symmetrical) were noted.57, 58 A band appears at 2297 cm-1, which is assigned to linearly adsorbed CO2 .59 At 200 °C, when the CO+O2 flow was turned off (Figure 8c), peaks at 2145 cm-1 and 2342 cm-1 vanished after 30 min, whereas the carbonate bands remained. No bands were observed between 2100-1900 cm-1, which is attributed to CO adsorbed on Fe2+ or Fe3+ sites. There will be shift in CO band even in case of weak


Page 25 of 53


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chemisorption, which is not observed in the present case. This indicates that either CO does not adsorb on the oxide surface or metal carbonyl peaks are overlapping with gaseous CO. During N2 purging, these bands disappeared completely (even at room temperature). From these observations, it can be inferred that CO is not adsorbed on Fe site but instead it is reacting from the gas phase, which is in agreement with the literature reports.23 This finding can be corroborated by XP spectra of Fe 2p, as there was no change in the states of Fe after the reaction. There are no spectator species observed in the present study. 3.3.2. Fe2O3 -supported Pt and Pt-substituted Fe2O3 Figure 9a shows the comparison of the DRIFTS spectra over 2% Pt/ Fe2O3 at 28 °C, 120 °C and 160 °C in presence of CO+O2. Figure 9b and 9c show the spectra in CO alone, CO+O2 atmosphere and N2 purging conditions at 28 °C and 160 °C, respectively. The bands observed at 2173 and 2090 cm-1 are assigned to gas phase CO and linearly adsorbed CO on Pt0, respectively. Qiao et al.25 performed DRIFTS studies over single atom Pt1/FeOx catalysts and observed linearly adsorbed carbonyl on Pt0 at 2030 cm-1. In addition to this, weak bands were observed at 1860 and 1950 cm-1, which were attributed to bridged adsorption of CO on two Pt atoms and CO adsorbed on the interface of Pt and support. However, these bands at 1860 and 1950 cm-1 were only observed at elevated pressures. In the present study no such bands were noted, as all the experiments were performed at 1 atm. DRIFTS studies by Pozdnyakova et al.60 exhibited the band for linear adsorption of CO on Pt0 at 2063 cm-1. A shoulder noted at 2083 cm-1 is ascribed to CO linearly adsorbed on step and terrace Pt0 atoms. It is evident from the literature that the positions of these bands are variable. Kappers and Mass studied the CO stretching frequency (νCO) with respect to cluster size and metal support interactions. The variation in νCO for the same site with different samples might be because of the difference in dipole-dipole interaction of chemisorbed CO molecules with metal cluster size.61 Gruene et al.62 derived an expression for change in stretching frequency of CO



Page 26 of 53

1605 1570 1474 1384 1354

2172 2116

2342 2297

2968 2892

(a) O

(b)

200 C

30 min

Transmittance (a.u.)

1354

1605 1567 1474

2175 2115


2960 2887

2347 2297

25

o 160 C

o 28 C

20 min

10 min

5 min

1 min

3000

2500

2000

1500

3000

1000

2500

-1

2000

1500

1000

-1

Wavenumber (cm )

(c)

1605 1570 1474 1384 1354

2968 2892

Wavenumber (cm )


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 min

20 min

10 min

5 min

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 8. DRIFTS spectra of Fe2O3 (a) at different temperature with CO+O2, (b) 200 °C with CO+O2 switched on, and (c) 200 °C with N2 purging (CO+O2 switched off)

atop Pt with change in cluster size. Hence, as coordination number of Pt increases νCO atop Pt0 is found to increase. The bands at 2096 and 2084 cm-1 are assigned to CO molecule adsorbed on (111) and (100) phases of Pt, respectively.61 The observed Pt0-CO (linear) band at 2090 cm-1 can be associated with (111) phase of Pt and the presence of Pt (111) phase is corroborated by TEM analysis. The bands noted in the vibration region 1700-1000 cm-1 mainly occur due to the formation of carbonates on the oxide surface. The presence of carbonate band on the catalyst surface shows the


Page 27 of 53


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involvement of support in the reaction. The band at 2157 cm-1 corresponding to Pt2+ was not observed at 28 °C and 160 °C. The absence of Pt2+ band at 28 °C can be corroborated by XPS results (Figure 2b). Pt 4f spectra of 2% Pt/Fe2O3 obtained before reaction exhibited only the presence of metallic Pt. At 160 °C, the absence of Pt2+ band may be due to the overlapping of the band with the gas phase CO. The bands occurring at 2297 and 2347 cm-1 are ascribed to linearly adsorbed and gas phase CO2.59 The bands between 3000-2600 cm-1 appeared gradually as the temperature was increased. At 160 °C, in the presence of CO+O2, the bands were observed at 2959 and 2876 cm-1, corresponding to the formate species. However, in the presence of CO alone, very weak bands were observed due to formate species.63 Figure 9d shows the comparison of DRIFTS spectra Fe2O3 -supported Pt and Pt-substituted Fe2O3 collected at 160 °C. For Pt substituted Fe2O3, Pt4+-CO was observed at 2168 cm-1.60 However, no peak was noted at 2095 cm-1, which represents linear Pt0-CO. The carbonate peaks (1558 and 1365 cm-1) observed for Fe2O3 -supported Pt were more intense compared to that of Pt-substituted Fe2O3. As discussed earlier, reaction over Fe2O3_A occurs via formation of carbonates and carbonates observed over Pt-substituted Fe2O3 are ambiguous. This result corroborates our explanation of lower activity of NMsubstituted Fe2O3. Due to the unavailability of oxide vacancies on the surface of Pt-substituted Fe2O3, the formation of carbonate species is reduced resulting in inferior activity of the compound. 3.3.3. Fe2O3 -supported Pd and Pd-substituted Fe2O3 DRIFTS studies were conducted over 2% Pd/Fe2O3 at 28 °C, 60 °C and 80 °C under CO alone, CO+O2 and N2 purging conditions. Figure 10a shows the spectra recorded in CO+O2 flow at different temperatures after 30 min. Figure 10b and 10c show the spectra obtained under different conditions at 28 °C and 80 °C, respectively. Primarily, three bands are observed at 2172, 2099 and 1985 cm-1 at all temperatures (Figure 10a). The band observed at 2172 cm-1 corresponds to gaseous CO, which elapses on



Page 28 of 53


o 160 C

o 120 C

o 28 C

1125

1335

1440

2172 2112 2090

2349

2627

(a)


1221

1419 1364

1562

2090

2384 2365 2347 2307 2297 2173

2958 2876

27

(b)

CO alone

CO+O2

N2 purging

2000

1500

1000

2500

-1

1500 -1

2168 2099

(d)

1000


1440 1368

1562

2383 2349 2297

2959 2876

2175 2090

2000

Wavenumber (cm )

1000

(c) CO+O2

N2 purging

2500

1500

Wavenumber (cm )

CO alone

3000

2000

-1

Wavenumber (cm )

1365

2500

1558

3000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

o

2% Pt/Fe2O3, 160 C

o Fe1.98Pt0.02O3-δ, 160 C

2800

2400

2000

1600

1200

-1

Wavenumber (cm )

Figure 9. DRIFTS spectra of 2% Pt/Fe2O3 (a) at different temperature in CO+O2 flow, (b) 28 °C, (c) 160 °C; (d) Comparison of DRIFTS spectra of 2% Pt/ Fe2O3 and Pt-substituted Fe2O3 at 160 °C

N2 purging, whereas, the bands at 2099 and 1985 cm-1 correspond to CO adsorbed linearly and bridged on Pd0, respectively.64, 65 At room temperature, when CO alone was passed over the catalyst, an additional band was noted at 2157 cm-1, which is ascribed to CO adsorbed on Pd2+ sites. The bands near 2347 and 2297 cm-1 are attributed to the gaseous and linearly adsorbed CO2 (Figure 10b). At 80 °C, the band at 2099 cm-1 is observed to shift to lower wavenumber (Figure 10c). In earlier studies, the bands for linear


Page 29 of 53


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and bridged adsorption of CO on Pd0 were reported at 2082 and 1971 cm-1, respectively.63 However, previous studies64, 65 show that the linear Pd0-CO stretching occurs at 2099 cm-1 and 2097 cm-1, respectively. The reason for the deviation in CO stretching frequency could be due to the variation in Pd crystallite size.15 The reported model calculations suggests that this shift occurs when there is a change in coordination number of the chemisorption site.66 It is also shown that linear adsorption occurs on Pd (111) site whereas bridged carbonyls are noted on (100) phase. The presence of Pd (111) phase is corroborated by HRTEM micrographs. However, no band was observed in the region below 1800 cm-1, indicating the absence of carbonate species.64 This infers that the reaction predominantly occurs on Pd0 site. Figure 10d shows the comparison of DRIFTS spectra Fe2O3 -supported Pd and Pd-substituted Fe2O3 obtained at 80 and 160 °C, respectively. For Pd substituted Fe2O3, Pd2+-CO was observed at 2157 cm-1.63 However, no peak was noted at 2099 cm-1, which corresponds to Pd0-CO. A broad band was observed at 1925 cm-1 is associated with CO species forming bridged bond between Pd (from the carbon atom) and Fe3+ (by oxygen atom).67 The carbonate bands (1512 and 1435 cm-1) were observed for Pdsubstituted Fe2O3; however, no bands were seen in this region for Pd/Fe2O3. The intensity of carbonate bands over Pd-substituted Fe2O3 is lower compared to bare Fe2O3. Therefore, the decrease in oxide vacancies is responsible for lower activity of NM-substituted Fe2O3. 4. Kinetic and mechanism development 4.1. Fe2O3_A It is widely known that Fe2O3 has many defects like oxide vacancies, Fe vacancies and interstitial defects. In particular, oxygen vacancies are desirable for CO oxidation as they are active sites for molecular and dissociative adsorption of O2.68 It is evident from XPS that Fe2O3_A contains Fe3+ and Fe2+



Page 30 of 53

o 60 C

o 28 C

(b)

1089

1262

1985

2175 2159 2099

2384 2347 2297

(a)


1986

2172

o 80 C


2099

29

CO+O2

CO alone

N2 purging

2000

1750

1500

2500

-1

2157 2099

1985

1000

-1


2175 2115 2095

2347

(c) CO+O2

1500

1000 -1

N2 Purging

2000

1500

Wavenumber (cm )

CO alone

2500

2000

(d)

1512 1435

2250

Wavenumber (cm )

1986 1925

2500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

o

2% Pd/Fe2O3, 80 C

o Fe1.98Pd0.02O3-δ, 160 C

2400

Wavenumber (cm )

2000

1600

1200 -1

Wavenumber (cm )

Figure 10. DRIFTS spectra of 2% Pd/Fe2O3 (a) at different temperature in CO+O2 flow, (b) 28 °C, (c) 80 °C; (d) Comparison of DRIFTS spectra of 2% Pd/ Fe2O3 and Pd-substituted Fe2O3 at 80 and 160 °C, respectively

states. There are two types of oxygen present on the surface of oxide: one is bonded to Fe3+ or Fe2+ ions while the other kind is bonded to one Fe2+ and one Fe3+ ion. The presence of different oxygen ions creates different crystal fields, which causes difference in their reactivity. This makes the extraction of oxygen present between Fe2+ and Fe3+ easier compared to the one bonded to same type of Fe.69 DRIFTS studies exhibited that various kinds of carbonates (like monodentate, carboxylate, bi-dentate) are formed on the


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surface of Fe2O3 during the reaction; however, no Fe-CO peaks were noted. Therefore, we propose that CO reacts with the lattice oxygen via carbonate formation. The oxygen vacancies created as a result of the reaction were annihilated by the adsorption of O2. There is an experimental evidence which suggests that oxygen adsorbs as superoxide on the oxide surface.68 Based on all these observations and literature reports, CO oxidation over Fe2O3 is proposed to proceed via following steps: Mechanism A K1 O2 + * ←→ O2*

(2)

K2 O2* + * ←→ 2O *

(3)

K3 CO + O2* ←→ CO3*

(4)

K4 CO + O * ←→ CO2*

(5)

k5 CO3*  → CO2 + O *

(6)

k6 CO2*  → CO2 + *

(7)

Mechanism B k1 O2 + 2 *  → 2O *

(8)

k2 CO + O *  → CO2 *

(9)

k3 CO2 *  → CO2 + *

(10)

where * denotes the oxide vacancy sites on the oxide support. For mechanism A, by pseudo-steady state approximation,



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31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

θO = K1CO θ * 2

(11)

2

K1K2CO2

θO = *

θ

2

(12)

θCO = K1K3CCOCO θ * 3

(13)

2

θ CO = K 4CCO

K1 K 2CO2

* 2

2

θ

(14)

Oxygen ion, oxide vacancy and carbonate species are adsorbed on the same site on the support. Therefore, from site balance

θ + θO + θCO + θCO = 1 *

* 2

(15)

* 3

where θ represent fractional coverage of oxide vacancy on the support, θO* , θ O* are the adsorbed oxygen 2

species and θCO* and θCO* are adsorbed CO2 and carbonate species on the oxide support, respectively. 2

3

Adsorption of CO and O2 in molecular form is assumed as non-activated process. Therefore, for the above mechanism, the final form of rate expression is given as

rCO2 =

(

K1 K 3 k5CCO CO2 + K 4 k6 ( K1 K 2CO2 / 2)1/2

1 + K1CO2 + K1 K 2CO2 / 2

)

1/2

+ K 4CCO ( K1 K 2CO2 )1/ 2 + K1 K 3CCO CO2

(16)

For mechanism B

rCO2 =

2k1k2 k3CCOCO2 2k1k3CO2 + k2 k3CCO + 2k1k2CCOCO2


(17)

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The above reaction steps can be ascribed to Eley-Rideal mechanism.23 Note that while deriving rate expression none of the steps are assumed to be the rate limiting step. Between the two mechanisms, mechanism (A) fits best with the experimental data. The values of rate coefficient are obtained by nonlinear regression using Levenberg-Marquadt algorithm.70 Figure 11 presents the change in reaction rate with partial pressure of CO and O2 and also the fitted curved with the derived rate expression. The parameters obtained after optimization are tabulated in Table 2.

4.2. Fe2O3-supported Pt Noble metals are known for their high catalytic activity for CO oxidation. Noble metals supported on various supports like Al2O3, CeO2, TiO2 have been studied.71 The reaction mechanism over these supports has been investigated, as the nature of the support affects the reaction mechanism. This is due to the reducibility of the support, which plays a significant role in stabilizing various surface intermediates. Many researchers have observed that reaction on noble metal supported on reducible support occurs via bifunctional mechanism. In this model, CO adsorbs on the metal site and adsorption of O2 occurs mainly on the oxygen vacancies at the perimeter site, as a consequence of Schottky junction at the metal support (semiconductor) interface.41, 72 The supported noble metal affects the reducibility of the support by weakening the M-O bond of oxide present in the vicinity of the noble metal site due to the influence of the metal.5, 73 Thus, the extraction of oxygen from the oxide near the Pt site is more probable. Guan et al.72 proved with the help of EPR measurements that TiO2 in the vicinity of Rh nanoclusters was reduced, that resulted in the formation of oxide vacancies. The activation of O2 occurs on these vacant sites via molecular superoxide leading to the formation of Rh-O-O-Ti on the interfacial site. Similar results were obtained by Hoh et al. from DFT studies on Au supported Fe2O3. They suggested that as a result of the reaction, a rim of linear O-Au-O arrangement is formed along the periphery of Au site.45 XANES studies done by Tomita et al.74 suggested that that Fe2+↔Fe3+ redox cycle occurs at the interfacial sites of Pt-



Page 34 of 53

33

3.5 2.5

pO = 0.024 atm

(a)

3.0

2

o

1.5

120 C o 140 C o 160 C o 180 C o 200 C Fitting

rate (µmol/g/s)

2.0

rate (µmol/g/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.0 0.5 0.0 0.00

2.5

o

120 C o 140 C o 160 C o 180 C o 200 C Fitting

(b)

pCO= 0.024 atm

2.0 1.5 1.0 0.5

0.01

0.02

0.03

0.0 0.00

0.01

pCO (atm)

0.02

0.03

0.04

0.05

pO (atm) 2

Figure 11. Variation of rate of CO2 over Fe2O3_A with (a) partial pressure of CO at constant po2, (b) partial pressure of O2 at constant pCO FeOx. These studies show the involvement of lattice oxygen in the reaction. It has been observed that there exist a strong metal support interaction between Pt and FeOx. Liu et al.41 suggested that Pt species were encapsulated with FeOx. However, this encapsulation effect may be promotional for CO oxidation reaction. There is an electron transfer from Fe to Pt atoms, resulting in shift of Fermi level to higher energy that weakens CO adsorption on Pt,41 thus favoring reaction at low temperature. DRIFTS studies exhibited the presence of Pt-CO peaks along with gaseous CO doublet. As evident from literature, oxygen adsorbs on the support at metal-support interface or at the perimeter site. Thus, for the reaction to occur between adsorbed CO and oxygen either oxygen has to spillover from the support to Pt particles or CO has to spillover to the support. However, the surface diffusivity of CO is very high on Pt (T ~ 45-165°C) as compared to oxygen, which is only measureable at high temperatures (T > 330 °C).75 Additionally, carbonates species were also observed on Fe2O3 in DRIFTS studies showing the involvement of support in the reaction. According to the above discussion, we propose that CO adsorbs on the Pt site and O2 adsorbs dissociatively on the oxide vacancies at metal-oxide interface. Therefore, the reaction can be described by following steps:


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Mechanism C K1 CO + Pt ←→ CO − Pt

(18)

K2 O2 + * ←→ O2*

(19)

K3 O2* + * ←→ 2O *

(20)

K4 CO − Pt + O2* ←→ CO3* + Pt

(21)

K5 CO − Pt + O * ←→ CO2* + Pt

(22)

k6 CO3*  → CO2 + O *

(23)

k7 CO2*  → CO2 + *

(24)

Mechanism D K1 CO + Pt ←→ CO − Pt

(25)

k2 O2 + 2 *  → 2O *

(26)

k3 CO − Pt + O *  → CO2* + Pt

(27)

k4 CO2*  → CO2 + *

(28)

For mechanism C, under steady state, the rate of adsorption of CO on Pt site is given by:

θCO − Pt = K1CCOθ Pt

(29)



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K1, θPt and θCO − Pt represent the equilibrium constant for adsorption of CO on Pt site, fraction of unoccupied Pt sites and fraction of Pt sites covered with CO, respectively. The site balance on Pt is as follows:

θ Pt + θCO − Pt = 1

(30)

It is to be noted that O2 does not adsorb competitively with CO on the Pt site. Therefore, the CO coverage over Pt site is,

θ CO − Pt =

K1CCO 1 + K1CCO

(31)

In this study, it was assumed that O2 adsorbs on oxide vacancies present at the interface of the Pt and oxide. By writing the pseudo steady state balance, the final rate expression was obtained as

rCO2 =

(

K1CCO [ K 2 K 4 k6CO2 + K 5 k7 ( K 3 )3/2 ( K 2CO2 / 2)1/2 ]

1 + K 2CO2 + K 3 K 2CO2 / 2

)

1/ 2

+ K 5 K1 ( K 3 )3/ 2 ( K 2CO2 / 2)1/2 CCO + K1 K 2 K 4CCO CO2

(32)

The rate expression for mechanism D is given by,

r=

2 K1k2 k3k4CCO CO2 2k2 k4CO2 (1 + K1CCO ) + 2 K1k2 k3CCOCO2 + K1k3k4CCO CO2

(33)

The initial guess of the parameters was taken from the literature.71 Non-linear regression was performed to determine the values of the rate parameters and it was observed that the best fit was obtained for mechanism C. The experimental data along with the fitted curve is shown in Figure 12 and the optimized rate parameters are mentioned in Table 2.


Page 37 of 53


36

2.0

2.0 110 oC 120 oC

1.6

(a)

pO = 0.024 atm 2

130 oC 140 oC

1.6

150 oC Model fit

rate (µ mol/g/s)

rate (µmol/g/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.2 0.8 0.4 0.0 0.00

0.01

0.02

0.03

0.04

1.2

110 oC 120 oC

(b)

pCO= 0.024 atm

130 oC 140 oC 150 oC Model fit

0.8 0.4 0.0 0.00

0.01

0.02

0.03

0.04

0.05

pO (atm)

pCO (atm)

2

Figure 12. Variation of rate of CO2 over 2% Pt/Fe2O3_A with (a) partial pressure of CO at constant po2, (b) partial pressure of O2 at constant pCO

4.3. Fe2O3-supported Pd Noble metals, especially Pd, exhibit high affinity for CO and O2 chemisorption.76 The literature reports indicate that Pd over various supports shows exceptional catalytic activity for CO oxidation.60 Although, XPS spectra of Fe2O3-supported Pd showed the presence of Pd0 and Pd2+, the effect of the states is not discussed for two reasons: (1) Among all states of Pd, Pd0 is most active for CO oxidation. However, Pd has higher tendency to oxidize than Au or Pt due to difference in affinity of O2 chemisorption.77 (2) XPS study indicated that Fe2O3-supported Pd contains 78% of Pd0, so it is fair to assume CO adsorption takes place on Pd0. This is also corroborated by DRIFTS studies, which clearly exhibit the presence of strong band for Pd0-CO (atop Pd as well as bridged CO over Pd sites) but very small Pd2+-CO band was observed. DRIFTS study showed the absence of carbonate bands (that are usually formed over support) over Fe2O3-supported Pd. The absence of carbonate band indicates that the involvement of support in the reaction is highly unlikely. The kinetic data showed that there is a gradual decrease in reaction rate with



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increasing initial CCO beyond certain CCO. Therefore, it is evident from DRIFTS and kinetic studies that CO and O2 are competing for the same site i.e. Pd. This suggests that the reaction is inhibited by CO as the Pd surface is covered with CO, thus, not allowing oxygen to get adsorbed. Another possible reason for the competitive nature of the reaction over Fe2O3-supported Pd is the unavailability of oxide vacancies at the metal support interface. This may be due to the encapsulation of oxide vacancies by the dispersed Pd atom.41 Therefore, unlike Pt, where oxygen adsorption occurs on the metal support interface, O2 adsorbs on the Pd site, resulting in competitive adsorption of CO and O2 at the same time. Although, the CO and O2 are competing for the same site, CO conversion was observed at low temperatures. The conversions at low temperatures are due to the presence of hydroxyl group (-OH) adsorbed on the surface of the catalyst that is observed during DRIFTS studies.78, 79 Liu et al.41 proposed the existence of metal-support interaction between Pd and Fe2O3, which results in electron transfer between Pd and Fe, thus weakening CO adsorption over Pd. Schauermann et al. showed that decrease in particle size leads to decrease in the CO adsorption heat.80 Based on above discussion, it is believed that the reaction occurs via following steps on the catalyst:

Mechanism E K1 CO + Pd ←→ CO − Pd

(34)

k2 O2 + 2 Pd  → 2O − Pd

(35)

3 CO − Pd + O − Pd k → CO2 + 2 Pd

(36)

Mechanism F (37)

K1 CO + Pd ←→ CO − Pd


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38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

K2 O2 + 2 Pd ←→ 2O − Pd

(38)

3 CO − Pd + O − Pd k → CO2 + 2 Pd

(39)

For mechanism E, The rate of adsorption of CO over Pd, under steady state, is given by,

θCO − Pd = K1CCOθ Pd

(40)

K1, θ Pd , θCO−Pd denote the adsorption equilibrium constant of CO over Pd, the fraction of unoccupied Pd sites and the fractional coverage of CO on Pd sites, respectively. The steady balance of oxygen species can be written as, 2 2k2CO2θ Pd − k3 K1CCOθO − Pd = 0

(41)

k2, k3 and θ O − Pd are adsorption rate constant of O2, reaction rate constant and fractional coverage of oxygen species over Pd sites, respectively. The site balance over Pd site is given by,

θ Pd + θCO− Pd + θO − Pd = 1

(42)

The final rate expression is,

r=

2 2K12 k2 k32CCO CO2

(43)

2 2 (2k2CO2 + k3 K1CCO + k3 K12CCO )

At constant O2 concentration, the rate expression can be rewritten as follows: 1/2

CCO (2k2CO2 ) = r1/2 k3 K1

2 CCO K1CCO + + (2k2CO2 )1/2 (2k2CO2 )1/2


(44)


Page 40 of 53

39

Similarly, at constant CO concentration, equation 43 can be modified as, 1/2

 CO2     r 

=

1 + K1CCO (2k2 )1/2 CO2 + (2k2 )1/2 k3 K1CCO

(45)

For mechanism F, rate expression obtained is,

r=

K1K21/2 k3CCOCO1/22

(46)

(1 + K 21/2CO1/22 + K1CCO )2 Both mechanism E and F were fitted with the experimental data to obtain rate parameters.

However, mechanism F does not fit well with the present experimental data. The optimized values of the rate parameters from mechanism E obtained after regression are mentioned in Table 2. The experimental data and the fitted curve are shown in Figure 13.

(a)

1800

2400

(b)

o

o

o

70 C

o

o

80 C Model fit

o

0.5

70 C o

80 C Model fit

(CO /rate)

0.5

60 C

1800

o

50 C 60 C

50 C

CCO/(rate)

1200

1200

2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

600 600 0 0.4

0.8

1.2

1.6

0.4

3

CCO (mol/m )

0.8

1.2 3 CO (mol/m )

1.6

2.0

2

Figure 13. Fitted rate of CO oxidation over 2% Pd/Fe2O3 to kinetic model as a function of concentration of (a) CO, (b) O2


Page 41 of 53


40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 2. Kinetic parameters obtained after fitting kinetic data

Kinetic

Fe2O3

Pt/Fe2O3

Pd/Fe2O3

(2.28 ± 0.05) x 10-5 √T

(2.15 ± 0.76) x 10-5 √T

(3.91 ± 0.21) x 10-7

parameters K1 (m3/mol)

expቀ

ଵ଻ଽ଺଼±ଵଶ

ቁ

்

expቀ

K2 / K2(m3/mol)/

(4.39 ± 0.12) x 102 √T

k2 (m3/g/s)

expቀ

K3(m3/mol)/ K3

(3.65 ± 0.88) x 106

k3 (mol/g/s)

expቀ−

K4 (m3/mol)/ K4

(3.81 ± 0.02) x 107

ଵଵହ଴଻±ଵଷ

expቀ−

k5 (mol/g/s)/ K5

଺ସଽହ±ଵଵ

ቁ

்

଼ଶଵ଴±ଽସ

ቁ

்

ቁ

்

ଽଽ଻±ଵଶ ்

ቁ

ସଵ଴଴±ଵଶଵ

ቁ

்

(1.91 ± 0.15)√T

ଵ଻଴ଽ±ଷଵ

ቁ

்

ସ଻ହ଴±ଶଶ

ቁ

்

ଶ଼଺ଽ±ଶଵ

ቁ

்

(1.32 ± 0.15) x 106 expቀ−

଻଴ହ଺±ସଶ

ቁ

்

(1.61 ± 0.93) expቀ−

଻଴଴ଷ±ଽ ்

ቁ

(1.56 ± 0.11) x 10-7 expቀ−

଼ଷ଼±଻ହ ்

(7.16 ± 0.58) x 103 expቀ

(1.41 ± 0.84) x 103 expቀ−

଻଴ସ଼±ଵଵ

√T expቀ

(2.46 ± 013) x 103 √T expቀ

(3.32 ± 0.16) x 10-7 √T expቀ−

k7 (mol/g/s)

்

ቁ

்

(3.72 ± 0.23) x 10-7 √T expቀ

(1.23 ± 0.24) x 103 expቀ−

k6 (mol/g/s)

ቁ

଺ଽ଺ଵ±ଷ଺ସ

ቁ


ି଺ଽହଽ±ଶହ ்

ቁ


Page 42 of 53

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5. Conclusions In the present work, Fe2O3 was synthesized by solution combustion synthesis using three different fuels: ascorbic acid (A), glycine (G), urea (U). Among these, Fe2O3_A showed highest CO oxidation activity with Fe2O3:Fe3O4 ratio 0.61:0.36. Higher activity of Fe2O3_A is attributed to the presence of different chemical environments in the vicinity of lattice oxygen, which weakens Fe-O bond. Fe2O3supported NM exhibited higher CO oxidation activity than the substituted catalysts. To understand this behavior, XPS (O 1s) and PL studies were performed. XPS of O 1s showed no significant increase in oxide vacancy on NM doping. However, PL studies indicated the increase in defect concentration on doping was due to the formation of cationic vacancies (or other defects) in lieu of oxide vacancies. DRIFTS studies over NM-substituted Fe2O3 showed the decrease in intensity of carbonate band that is related to the decrease in number of oxide vacancies. Pd/Fe2O3 showed highest catalytic activity and lowest apparent activation energy of 31 ± 2 kJ/mol. DRIFTS studies exhibited the formation of carbonate intermediates with no traces of metal carbonyl bands on Fe2O3_A. However, Fe2O3-supported NM exhibited prominent metal carbonyl bands. Unlike Fe2O3-supported Pt, no carbonate bands were observed for Fe2O3-supported Pd. Based on above observation, the formation of CO2 via carbonate formation over Fe2O3_A and Fe2O3-supported Pt followed Eley-Rideal and Langmuir-Hinshelwood mechanism, respectively. However, in case of Fe2O3-supported Pd competitive Langmuir-Hishelwood mechanism was proposed. The rate expression was derived for the proposed mechanism and kinetic parameters were determined by non-linear regression of the experimental data.

Acknowledgements The authors acknowledge Bharat Heavy Electricals Limited (BHEL) and Gas Authority of India limited (GAIL) for financial support, Micro and Nano fabrication unit, CeNSE, IISc for XRD and XPS


Page 43 of 53


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facility, Advanced Facility for Microscopy and Microanalysis, IISc for TEM facility. The authors would like to thank Neerugatti KrishnaRao Eswar for his help throughout the work and Archana Charanpahari for performing PL measurements. The corresponding author thanks the Department of Science and Technology for the J.C. Bose Fellowship.

Supporting Information The supporting information contains the table of binding energy and FWHM of Fe 2p and O 1s. The comparison of rates and apparent activation energy of reported catalysts with the present study is also included.



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References 1.

Ciambelli, P.; Cimino, S.; De Rossi, S.; Lisi, L.; Minelli, G.; Porta, P.; Russo, G. AFeO3 (A=La, Nd, Sm) and LaFe1-xMgxO3 Perovskites as Methane Combustion and CO Oxidation Catalysts: Structural, Redox and Catalytic Properties. Appl. Catal., B 2001, 29, 239-250.

2.

Engel, T.; Ertl, G. A Molecular Beam Investigation of the Catalytic Oxidation of CO on Pd (111). J. Chem. Phys. 1978, 69, 1267-1281.

3.

Deng, W.; Jesus, J. D.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Low-Content Gold-Ceria Catalysts for the Water–Gas Shift and Preferential CO Oxidation Reactions. Appl. Catal., A 2005, 291, 126-135.

4.

Yu, K.; Wu, Z.; Zhao, Q.; Li, B.; Xie, Y. High-Temperature-Stable Au@SnO2 Core/Shell Supported Catalyst for CO Oxidation. J. Phys. Chem. C 2008, 112, 2244-2247.

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Serre, C.; Garin, F.; Belot, G.; Maire, G. Reactivity of Pt/Al2O3 and Pt-CeO2Al2O3 Catalysts for the Oxidation of Carbon Monoxide by Oxygen: I. Catalyst Characterization by TPR Using CO as Reducing Agent. J. Catal. 1993, 141, 1-8.

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Blyholder, G. Molecular Orbital View of Chemisorbed Carbon Monoxide. J. Phys. Chem. A 1964, 68, 2772-2777.

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Li, L.; Wang, A.; Qiao, B.; Lin, J.; Huang, Y.; Wang, X.; Zhang, T. Origin of the High Activity of Au/FeOx for Low-Temperature CO Oxidation: Direct Evidence for a Redox Mechanism. J. Catal.

2013, 299, 90-100. 8.

McFarland, E. W.; Metiu, H. Catalysis by Doped Oxides. Chem. Rev. 2013, 113, 4391-4427.

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Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. Gold Catalysts Prepared by Coprecipitation for Low-Temperature Oxidation of Hydrogen and of Carbon Monoxide. J. Catal. 1989, 115, 301-309.


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10.

Huang, X.-S.; Sun, H.; Wang, L.-C.; Liu, Y.-M.; Fan, K.-N.; Cao, Y. Morphology Effects of Nanoscale Ceria on the Activity of Au/CeO2 Catalysts for Low-Temperature CO Oxidation. Appl. Catal., B 2009, 90, 224-232.

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Roy, S.; Marimuthu, A.; Hegde, M. S.; Madras, G. High Rates of CO and Hydrocarbon Oxidation and NO Reduction by CO over Ti0.99Pd0.01O1.99. Appl. Catal., B 2007, 73, 300-310.

12.

Roy, S.; Marimuthu, A.; Hegde, M. S.; Madras, G. High Rates of NO and N2O Reduction by CO, CO and Hydrocarbon Oxidation by O2 over Nano Crystalline Ce0.98Pd0.02O2-δ: Catalytic and Kinetic Studies. Appl. Catal., B 2007, 71, 23-31.

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Shinde, V. M.; Madras, G. Kinetics of Carbon Monoxide Oxidation with Sn0.95M0.05O2-δ (M= Cu, Fe, Mn, Co) Catalysts. Catal. Sci. Technol. 2012, 2, 437-446.

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Li, N.; Chen, Q.-Y.; Luo, L.-F.; Huang, W.-X.; Luo, M.-F.; Hu, G.-S.; Lu, J.-Q. Kinetic Study and the Effect of Particle Size on Low Temperature CO Oxidation over Pt/TiO2 Catalysts. Appl. Catal., B 2013, 142–143, 523-532.

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Chou, P.; Vannice, M. A. Calorimetric Heat of Adsorption Measurements on Palladium: II. Influence of Crystallite Size and Support on CO Adsorption. J. Catal. 1987, 104, 17-30.

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Liu, H.; I. Kozlov, A.; P. Kozlova, A.; Shido, T.; Iwasawa, Y. Active Oxygen Species and Reaction Mechanism for Low-Temperature CO Oxidation on an Fe2O3-Supported Au Catalyst Prepared from Au(PPh3)(NO3) and as-Precipitated Iron Hydroxide. Phys. Chem. Chem. Phys.

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Tripathi, A. K.; Kamble, V. S.; Gupta, N. M. Microcalorimetry, Adsorption, and Reaction Studies of CO, O2, and CO+O2 over Au/Fe2O3, Fe2O3, and Polycrystalline Gold Catalysts. J. Catal. 1999, 187, 332-342.



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Mechanistic Insights and Kinetics of CO Oxidation over Pristine and

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