Chapter-4shodhganga.inflibnet.ac.in/bitstream/10603/44773/9/09_chapter 4.pdf · , Sm-CeO 2,...

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Chapter-4

Sm-CeO2 supported gold nanoparticle catalyst for benzyl

alcohol oxidation using molecular O2

Nanocrystalline ceria supported gold catalysts are suitable candidates for alcohol

oxidation reaction under mild condition. In this study, ceria based mixed oxides were

prepared by modified sol–gel method using triethanolamine/water mixture where the defect

sites were created by doping Sm3+

cation to cerium oxide. The catalysts were characterized

by XRD, FT-RAMAN, TPR, TPD, XPS, XAFS, and HRTEM techniques. Strong gold–

support interaction facilitates the easy removal of capping oxygen ions present in the Sm3+

doped cerium oxide surface as observed by XPS and TPR studies. The acid–base property

of the CeO2 oxide after incorporation of samarium and gold deposition was observed from

TPD studies. The Au/Sm-CeO2 catalyst was active for benzyl alcohol oxidation reaction

using molecular O2 under mild condition.

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Chapter-4: Sm-CeO2 supported gold nanoparticle………..

4.1. Introduction

Gold nanoparticles were deposited on rare earth metal (samarium) doped cerium oxide

support material. The samarium doped cerium oxide supports were prepared by a non-

hydrothermal sol-gel method resulting in agglomerated primary nanoparticles in the 10-15

nm range on which gold particles of 2-5 nm size were deposited by means of deposition-

precipitation method. The doped cerium oxide-supported Au catalysts with a noble metal

loading of 2.33 wt% were investigated concerning their structural properties and tested in

the liquid phase oxidation of benzyl alcohol to benzaldehyde by using molecular O2 in mild

condition. The catalytic tests showed that the activity of these catalysts strongly depends on

the composition of the support and different parameters during gold nanoparticles

preparation method. The enhancement of the catalytic activity was observed for samarium

doped cerium oxide supported gold nanoparticles catalyst compared to cerium oxide

supported gold nanoparticles catalyst. Extension of the catalytic tests to a variety of

structurally different alcohols indicated that samarium doped cerium oxide supported gold

nanoparticles catalyst possesses catalytic properties in the aerobic oxidation of a broad

range of structurally different alcohols. XPS spectra revealed that both metallic and oxidic

Au species are present on the samarium doped cerium oxide supported gold catalyst.

RAMAN spectra of different catalysts (CeO2, Sm-CeO2, Au/Sm-CeO2) shows that after

samarium doping oxide vacancies are created in the CeO2 lattice gold nanoparticles are

deposited on oxide vacant sites.

4.2. The objective of the Present Work

Nanocrystalline CeO2 has drawn its attention as the physicochemical property of its surface

changes distinctly from its bulk counterpart. Several studies have demonstrated that doping

of CeO2 with elements of different ionic radii or oxidation state improves the exchange of

oxygen in the oxide network by decreasing the energy barrier for oxygen migration [1].

The synergy between the ceria and the modifier can be achieved throughout different

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mechanisms, such as creation of oxide vacancy, the formation of solid solution [2], the

integration between segregated oxides, and the deposition of metallic particles over the

surface.

In this work it is hypothesized that the samarium doping (rare earth metal) induces oxide

vacancy in the cerium oxide lattice. In this oxide vacant site gold nanoparticles nucleate

allowing the growth of small and well-dispersed Sm-CeO2 clusters, which facilitate the

reactivity of the catalyst. It is also assumed that the oxide vacancies in CeO2 lattice where

gold nanoparticles will reside facilitate gold-support interaction more strongly. This strong

gold-support interaction promotes changes in the electronic environment of Sm-CeO2

supported gold nanoparticles its catalytic performance towards alcohol oxidation reaction.

Though several reports have been published in the literature for samarium doped ceria (as

both have close ionic radii) catalyst for other oxidation reactions, no study has been carried

to see the effect of oxide ion vacancies on supported gold nanoparticle catalyst for benzyl

alcohol oxidation.

4.3. Experimental Section

4.3.1. Preparation of Sm-CeO2 mixed oxide support

The support material i.e. samarium doped cerium oxide was prepared by the non-

hydrothermal sol-gel method [2]. In this procedure a solution of Ce(NO3)3, 6H2O (99%,

Aldrich) was added to triethanolamine (99+%, Acros) under stirring condition at room

temperature. After complete addition of cerium nitrate solution, a quantitative amount of

deionized water (Milipore water) and samarium nitrate hexahydrate (99.9%, Acros) in solid

form was added to it with constant stirring. This total content was stirred for 10 min after

which tetraethylammonium hydroxide (20% aqueous solution, Merck Germany) was added

dropwise with constant stirring condition. The resulting final gel composition (molar ratio)

of the mixture is triethanolamine: Ce(NO3)3,6H2O: H2O: Sm(NO3)3,6H2O:

tetraethylammonium hydroxide is 0.2: 0.1: 1.1: 0.004: 0.1. This whole content was aged

for 24 h with continuous stirring condition. The gel material was dried at 110 °C for 24 h

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in a static air oven and the dried material was collected and calcined at 700 °C for 10 h in a

static air muffle furnace with temperature increasing rate 1°C/ min.

4.3.2. Deposition of gold on Sm-CeO2 support by Deposition Precepitation method

The gold nanoparticles were deposited on the support by the procedure given by Haruta et

al. [3]. In this typical procedure 150 ml deionized water was taken in a beaker and the

required amount of HAuCl4, 3H2O (Aldrich) was added to it. Then the solution was heated

to 70 °C from the initial temperature 30 °C. After attaining the temperature, the pH was

raised to 7.0 from the initial pH 2.87 by adding aqueous NaOH solution dropwise. After

attaining a temperature of 70 oC and pH 7.0, the support material (0.75 g) was added and

the whole content was stirred for 1 h. Then the material was filtered off, washed with 100

ml deionized water and dried in vacuum at -5 °C temperature for 12 h. The dried material

was calcined at 300 °C for 4 hours in a static air muffle furnace with temperature increasing

rate 1 °C/min. For some samples the catalysts were calcined at 400 °C and 500 °C as

mentioned in the activity study.

4.3.3. Catalyst characterization

EDXRF

An EDXRF instrument has been developed for versatile analytical applications. The

spectrometer is basically built together with a 3 kW long-fine-focus 42Mo-anode X-ray

diffraction tube and a 4 kW X-ray generator procured from Pan Analytic, the Netherlands. A

collimated beam from the X-ray tube is made to excite the target. A Si(Li)/LEGe detector

(FWHM = 160 eV at Mn K X rays, Canberra, US) in the horizontal configuration coupled with

a PC based multichannel analyzer is used to collect the fluorescent X-ray spectra. The annular

sources of 241

Am (3.7 GBq, AEA technology QSA GmbH, Germany) radioisotope used.

The 241

Am source is incorporated in a ceramic enamel and sealed in a welded model

capsule with brazed beryllium window; the active components are recessed into a stainless

steel support with tungsten alloy backing.

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X-ray diffraction

XRD patterns are recorded at room temperature on a D8 ADVANCE (BRUKER AXS,

Germany) diffractometer using CuKα radiation with parallel beam (Gobel Mirror). The

catalysts are ground to fine powder prior to measurement. The scans are recorded in the 2θ

range between 10 and 75° using a step size of 0.02° and a scan speed of 2s/step. Peaks are

identified by a search match technique using DIFFRACplus

software (BRUKER AXS,

Germany) with reference to the JCPDS database. The software TOPAS 3.0 from Bruker

AXS (2005) is used for refinement of the cerianite diffraction peak (111) located at 2θ =

28.68o to determine the average crystallite size, in which it is possible to make all of the

corrections and refinement processes. The software package TOPAS 3.0 uses the

fundamental parameter approach (FPA), and is therefore capable of estimating the

instrumental influence. The Double-Voigt approach in TOPAS is used with both calculated

(FPA) and measured instrument functions. Employing integral breadth βi, Stokes &

Wilkinson have developed a more generalized treatment (βi = λ/LVolCosθ; λ: wave-length

of X-ray, LVol: Volume weighted crystallite size, θ: angle of diffraction angle) of domain-

size broadening which is independent of the crystallite shape. This conception directly

leads to a formula identical with the Scherrer equation except that the constant assumes a

value of unity.

Raman Analysis

The Raman spectra was recorded at room temperature on an HR 800 Raman

spectrophotometer (Jobin Yvon Horiba, France) using monochromatic radiation emitted by

a He-Ne laser (633nm), operating at 50 mW and with a spectral resolution of 0.3 cm-1

.

Electron microscope Analysis

The HRTEM investigation was done on JEOL JEM 2100 microscope operated at 200 KV

acceleration voltage using lacey carbon coated Cu grids of 300 mesh size. STEM-EDS used

was from Oxford Instruments (model x-sight).

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X-ray photoelectron (XPS) Analysis

X-ray photoelectron spectroscopy (XPS) was obtained with a PHI-5500 spectrometer with

Al K radiation (1253.6 eV). The energy was calibrated with a C 1s peak.

X-ray absorption fine edge structure (XAFS) Analysis

XAFS measurements were carried out at beam line 7C (BL7C) of Photon Factory (PF), in

the Institute of Materials Structure Science (IMSS), High Energy Accelerator Research

Organization (KEK) in Japan, and at beam line 01B1 (BL01B1) at SPring 8 in Japan. The

XAFS spectra were observed in transmittance and fluorescence mode and the collected data

were analyzed by the software program REX2000 (Rigaku Co.). Curve fitting analysis was

carried out with parameters obtained with FEFF8 (Univ. of Washington).

Temperature programmed reduction (TPR) & Temperature programmed oxidation

(TPO) analysis

TPR and TPD profiles of the catalysts are obtained with ChemiSorb 2720 (Micrometrics,

USA) equipped with a TCD detector. The H2-TPR profiles are obtained by reducing the

catalyst samples (0.1 g) with 10% H2 in Ar at a flow rate of 20 ml/min and the temperature

is increased from ambient to 1000 °C at a rate of 10 °C/min. For TPO, the catalysts were

first reduced by a TPR run till 700 °C. The gas was then switched to He and allowed to

cooled to ambient temperature. Then the TPO was run with 5.2 % O2 in He from ambient

temperature to 700 °C. Hydrogen and oxygen consumptions in the profiles were evaluated

by peak area of CuO TPR and Cu TPO calibrations, respectively.

4.3.4. Benzyl alcohol oxidation reaction

The selective oxidation of benzyl alcohol was performed in a 50 ml two necked round

bottom flask fitted with a water condenser and an O2 bubbler. The reaction mixture was

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prepared by taking 4 ml of dry toluene, 4 mmol of benzyl alcohol (99%, Merck India) and

0.1 ml of dodecane (99%, Acros) as an internal standard. This total mixture was shaken

vigorously for homogenization and 0.1 g catalyst was added to this solution. The oxygen

flow was maintained at 50 ml/min and bubbled through the reaction mixture. The reaction

mixture was kept in a thermostatic oil bath at 90 °C temperature and the rotation speed of

the magnetic stirrer was maintained at 260 rpm speed for 3 h. After that the sample was

collected and centrifuged. The supernatant liquid was collected and analyzed by a Gas

Chromatograph (CIC-India) equipped with SE-30 coloumn. During GC analysis the oven

temperature was maintained at 210 °C while injector and detector temperature was kept at

235 °C. The conversion and selectivity was calculated by using the following equations.

Conversion (%) =

Selectivity (%) =

Here [Alcohol](t=0) is the initial concentration (molar) of benzyl alcohol and [Alcohol](t) is the final

concentration (molar) of benzyl alcohol.

4.4. Results & Discussion

4.4.1. Characterization of supported gold nanoparticle catalysts

X-Ray Diffraction

The X-ray diffraction patterns of CeO2, and samarium doped CeO2 are presented in Fig. 1.

The peaks of all the samples could be indexed as (111), (200), (220), (311), (222), (400)

planes of cubic fluorite structure (Space Group: Fm3m, JCPDS 78-0694) of CeO2.

However, mixed oxides of Ce and Sm, SmxCe1-xO2, are reported [4] to have the same

crystal structure, Fm3mj, and very similar unit cell dimensions to pure CeO2. Therefore,

the existence of some SmxCe1-xO2 could not be ignored based on the XRD data. The

crystallite sizes of both the ceria and samarium-incorporated ceria calculated by the Scherer

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formula from the powder XRD data are 49.5 nm and 36 nm. The decrease in size of

samaria doped ceria particles may be ascribed to decrease in the concentration of surface

hydroxyls as low valent samarium replaces the higher valent cerium in the lattice [5]. The

unit cell parameter a increases with the Sm content (Table 1) as expected from effective

ionic radii (rCe4+

= 0.1110 nm; rSm3+

= 0.1219 nm) in accordance to Vegard‟s rule [6].

Tsunekawa et al. investigated the lattice expansion of the CeO2 nanoparticles, and they

found that “the reduction of the valence induces an increase in the lattice constant due to

the decrease in electrostatic forces” [7]. Also the diffractograms show no peak due to gold

nanoparticles indicating high dispersion of gold nanoparticles over the Sm-doped ceria.

However, to know whether samarium is dispersed on ceria surface and to get an idea about

its local chemical environment, the XPS characterization was undertaken as describe later.

Fig. 1. XRD pattern of the (a) CeO2, (b) Sm-CeO2 (Sm/Ce = 4/100), (c) Au/Sm-

CeO2 (Sm/Ce = 4/100)

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Table 1: Calculation of crystallite size from XRD analysis using Debye-Scherer

formula

Catalyst Phase Crystallite Size Cell Parameters

(nm) (nm)

CeO2 Cerianite 49.5 a = 0.540176

Sm-CeO2 (Sm/Ce = 4/100) Cerianite 36.2 a = 0.541726

Au/Sm-CeO2 (Sm/Ce = 4/100) Cerianite 29.6 a – 0.541428

Raman Spectra

Raman spectroscopy was used to elucidate the structure of samarium doped cerium oxide

and gold deposited samarium doped cerium oxide. As observed from Fig. 2a and 2b, only

one peak appeared at 466.77 cm-1

for cerium oxide, which is due to the Raman mode with

F2g symmetry [8]. This peak (466.77 cm-1

) also reflects the fluorite structure of cerium

Fig. 2a. Raman Spectra of (a) CeO2 (b) Sm-CeO2 (Sm/Ce = 4/100) (c) Au/Sm-CeO2

(Sm/Ce = 4/100)

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Fig. 2b. Raman Spectra of (a) CeO2 (b) Sm-CeO2 (Sm/Ce = 4/100) (c) Au/Sm-CeO2

(Sm/Ce = 4/100)

oxide [9] and symmetric breathing mode of the oxygen atoms around Ce 4+

ions [10, 11]. In

case of Sm doped cerium oxide, two peaks are observed; one at 464.77 cm-1

and the other

at 608.40 cm-1

. These two peaks are assigned to F2g symmetry mode in cerium oxide of

small particles, which is related to the presence of bulk oxygen vacancies [12]. Samarium

doping does not distort the crystal structure of cerium oxide but induces oxygen vacancies

in the bulk of the cerium oxide material. This result is in good agreement with that of the

XRD analysis. The position and width of the peak at 464.77 cm-1

for two samples, i.e.

cerium oxide and samarium doped cerium oxide (Sm/Ce = 4/100) are almost the same.

However, in the case of the gold deposited samarium doped cerium oxide catalyst, the band

of F2g symmetry broadens and shifts towards 440.87cm-1

. For this peak shifting, several

factors could be responsible such as, the particle size, the size distribution, and the number

and nature of defects [13]. The band at 608.40 cm-1

almost disappeared in the gold

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deposited samarium doped cerium oxide, which may be attributed to the residence of gold

nanoparticle to the vacant sites of Sm-CeO2.

HRTEM Analysis

Since lattice parameter of ceria fluorite structure is not significantly altered, we carried out

HRTEM image analysis for the Au/Sm-CeO2 catalyst only. At higher magnification,

crystalline particles with a particle size of 30–40 nm and 8–12 nm are visible in Fig. 3. The

Fig. 3. HRTEM image of Au/Sm-CeO2 (Sm/Ce = 4/100) at different scales

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crystallite sizes are similar as per the XRD observation in this study. The gold nanoparticles

are visible even in the small scale. A close look at the surface in the higher resolution

shows that there are gold nanopaticles of 10 nm in diameter and the lattice fringes are

distinctly evident. The particle shows a large contact area with the support. In these

conditions, it is possible to transfer an electron from the metal to the support as revealed

from XPS observations described later.

HAADF-STEM with atomic imaging

To find out the distribution of the components in the Au/Sm-CeO2 (Sm/Ce = 4/100) high-

angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Fig.

4a.) and atomic imaging analysis (Fig. 4b.) were carried out. The bright field images

indicate cerium and oxygen. The distribution of cerium shows that there is no growth of the

particles. The distribution of gold nanoparticles is homogeneous as evidenced also from the

atomic mapping image (Fig. 4b.).

Fig. 4a. HAADF-STEM spectra of Au/Sm-CeO2 catalyst (Black colour is Cerium, Green

colour is Oxygen and red colour is Gold)

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Fig. 4b. Elemental mapping of Cerium, Gold and Oxygen in the Au/Sm-CeO2 catalyst

(Sm/Ce = 4/100)

XPS Spectra

Fig. 5. shows XPS spectra of Ce (3d5/2) for the samples; CeO2, Sm-CeO2 (Sm/Ce = 2/100),

Sm-CeO2 (Sm/Ce = 4/100), and Au/ Sm-CeO2 (Sm/Ce = 4/100). The results of quantitative

analysis of XPS are shown in Table 2. Cerium compounds are known to exhibit rather

complex features due to hybridization of the valence 4f orbital with ligand orbitals [14, 15].

The peaks observed at 882.0, and 888.5 eV (labeled as V0, and V1 in Fig. 5. respectively)

can be attributed to the Ce (3d5/2) of CeO2. The peak at 882.0 can be assigned for Ce 3d9 4f

1

states of Ce (IV). V1 is a features resulting from the transfer of one electron from a filled

O(2p) orbital to an empty Ce (4f) orbital, i.e. Ce 3d9 4f

2 of Ce(IV) in the final state [16].

While the peaks V0 and V1 are due to cerium ions in the +4 oxidation state, U1 reflects Ce3+

ions. The presence of the U1 peak in the spectrum indicates that the sample contains some

oxygen vacancies and is in a partially reduced state [12]. It is also proposed that the

concentration of oxygen vacancies in ceria is related to the particle sizes, and smaller

nanoparticles possess a higher concentration of oxygen vacancies. As the hierarchically

structured ceria fabricated in this work consists of nanosized crystals of ceria as building

units, a high concentration of oxygen vacancies would be expected for the samarium-

incorporated nanocrystalline CeO2, which corresponds to the observations with Raman

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characterization. But after gold deposition the intensity ratio between Ce3+

/Ce4+

peaks

decreases substantially. Also the binding energy (885.0) of Ce3+

shifts to higher values after

Fig. 5. Ce (3d) core level XPS spectra of (a) CeO2, (b) Sm-CeO2 (Sm/Ce = 2/100), (c) Sm-

CeO2 (Sm/Ce = 4/100),(d) Au/Sm-CeO2 (Sm/Ce = 4/100 )

samarium and gold doping which indicates the depletion of negative charge density over

the CeO2 lattice. This may be due to the covalent interaction of samarium-cerium and

cerium -gold via surface hydroxyl groups.

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In all of the analyzed samples, the O (1s) spectra (shown in Fig. 6.) consisted mainly of

three peaks; a dominant one at energy 529 eV, another at 532 eV along with a peak at 528

eV, which can be assigned as Oa, Ob, and Oc respectively. The peak Ob is due to Ce2O3.

Fig. 6. Oxygen 1s Core level XPS spectra of (a) CeO2, (b) Sm-CeO2 (Sm/Ce = 2/100), (c)

Sm-CeO2 (Sm/Ce = 4/100), (d) Au/Sm-CeO2 (Sm/Ce = 4/100)

The second one (Oa) is oxygen present in CeO2 lattice and the weak third peak (Oc) at a

higher binding energy. After samarium incorporation the lattice oxygen becomes less

tightly bound showing lower binding energy of the peak at 529.0 eV, which may be due to

change of lattice potential due to incorporation of samarium [17]. Interestingly, when gold

was deposited then there is an interaction between gold and lattice oxygen thereby shift of

the binding energy towards higher values.

The Sm (3d) core level spectra (Fig. 7.) were deconvoluted and two peaks at 1082. eV

and 1110 eV, which are mainly due to a Sm3+

cation. With increase in the loading of

samarium the Sm/Ce ratio is increased which emphasizes that samarium resides mostly on

the surface of the catalyst.

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Fig. 7. Samarium 3d core level XPS spectra of (a) Sm-CeO2 (Sm/Ce = 2/100), (b) Sm-

CeO2 (Sm/Ce = 4/100),(c) Au/Sm-CeO2 (Sm/Ce = 4/100)

Fig. 8. Au (4f) core level XPS spectra of Au/Sm-CeO2 (Sm/Ce = 4/100)

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The emission of 4f photoelectrons from Au is identified in six spin-orbit coupled peaks of

the XPS spectra (Fig. 8.), four of which are assigned to Au0 (4f7/2 at 83.79 and 82.05 eV)

and two are due to Au+ (4f7/2 at 85.22 eV) [18]. The decrease in the XPS peak area due to

Ce3+

(U1) and the increase in the peak area due to Ce4+

(V0) after the gold doping and the

presence of peaks due to an oxidic gold (85.22 eV) strongly emphasizes the metal–support

interaction for the gold deposited catalyst.

Table 2: XPS quantitative analysis

Catalyst Ce(3d5/2)

V0 V1 U1

Position Area Position Area Position Area

CeO2 881.95 9598 888.13 2781 885.59 2781

Sm-CeO2 881.99 7314 888.59 3080 885.26 4058

(Sm/Ce = 2/100)

Sm-CeO2 882.4 4998 888.67 3023 885.29 4632

(Sm/Ce = 4/100)

Au/Sm-CeO2 883 12200 888.63 2768 886.66 467

(Sm/Ce = 4/100)

Catalyst O(1s)

Oa Ob Oc

Position Area Area% Position Area Area% Position Area Area%

CeO2 529.4 3221 42 531.8 1191 16 528.4 3082 41

Sm- CeO2 529.3 5124 67 531.89 1601 21 528.3 877 12

(Sm/Ce = 2/100)

Sm- CeO2 529.2 4843 66 531.8 1394 19 527.5 1083 15

(Sm/Ce = 4/100)

Au/Sm-CeO2 529.6 6007 52 532.3 1156 10 527.9 4355 38

(Sm/Ce = 4/100)

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Catalyst Sm(3d5/2) Sm(3d3/2)

Position Area Position Area

Sm-CeO2 1082.6 520 1110.0 347

(Sm/Ce = 2/100)

Sm-CeO2 1082.4 1085 1110.0 723

(Sm/Ce = 4/100)

Au/Sm-CeO2 1082.98 1239 1110.4 826

(Sm/Ce = 4/100)

Catalyst Au (4f7/2) Au (4f5/2)

Position Area Position Area

Au/Sm-CeO2 83.79 919 87.46 690

(Sm/Ce = 4/100)

82.05 434 85.34 326

85.22 80 88.89 60

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge

structure (XANES) analysis

To provide a further, and unambiguous, evidence for the oxidation state of the metals in our

materials, we turned to XANES spectroscopy, since it is well known that spectral features

of the near-edge region of the X-ray absorption spectra are affected by the oxidation state

of the element studied and that the method provides a qualitative fingerprint of electronic

structure and local atomic environment.

As shown in Fig. 9 (a), the Au LIII-edge XANES has a characteristic profile of metallic

clusters. Fig. 9(b-d) shows the Fourier-transform k3 weighted Au LIII-edge EXAFS spectra

of the Au/Sm-CeO2 (Sm/Ce = 4/100) along with the curve-fitted result. The gold-gold

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coordination number is 9.6, which emphasizes the presence of gold nanocluster in the

Au/Sm-CeO2 catalyst. The presence of gold particles is clearly indicated by the observed

low Au–Au signal at ~2.85 ±0.02 Å. The observed Au–Au in the gold foil is ~2.85 Å. The

coordination number is a strong and nonlinear function of the particle diameter, and the

accuracy of this method is higher for smaller metal clusters since the average coordination

number for bigger particles approaches that of bulk [19]. The oxidation state of Au in

oxidation reaction has been the subject of various studies aimed at identifying the

catalytically active species, with some reports emphasizing the importance of the metal

Fig. 9. (a) XANES spectra of Au LIII-edge, (b-d) Fourier-transform k3 weighted

Au LIII-edge EXAFS spectra

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being in a positive oxidation state [20] and, in contrast, other reports identifying zero-valent

Au as the active species. Most of the contributions analyzing the origin of Au as an active

catalyst have dealt with gas-phase reactions, while Baiker et al. reported that metallic gold

was responsible for liquid-phase alcohol oxidation using in-situ XANES spectroscopy [21].

Temperature programmed reduction (TPR) & Temperature programmed oxidation

(TPO) analysis

To investigate the reactivity changes of the ceria support upon gold addition, the H2

temperature programmed reduction (TPR) was performed on CeO2, Sm-CeO2 (Sm/Ce =

4/100) and Au/Sm-CeO2 (Sm/Ce = 4/100) samples. The TPR patterns of the analyzed

samples are shown in Fig. 10. The H2-TPR profile of pure CeO2 is usually characterized by

reduction peaks at 500 and 800 °C, which are assigned to the surface capping oxygen ions

and the bulk oxygen of CeO2, respectively [22]. The surface capping oxygen ions can

Fig. 10. H2-TPR of (a) CeO2, (b) Sm-CeO2 (Sm/Ce = 4/100), (c) Au/Sm-CeO2 (Sm/Ce =

4/100)

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exhibit varied level of coordinations depending on the method of the preparation of the

ceria and the presence of impurities or dopant [23]. Thus, the pattern and hydrogen

consumptions of the profiles provide a cursory measure of levels of coordination and

availability of the surface capping oxygen ions in the catalysts. It is also observed from Fig.

10. that the pattern of H2-TPR profile of ceria is virtually unaltered by Sm doping, but the

presence of Au lowers the temperature of removal of surface capping oxygen ions. The

ionic radii of Sm3+

(0.096 nm) and Ce4+

(0.103 nm) are similar [24]. The presence of the

Sm in the ceria did not cause significant distortion of the fluorite structure of ceria. The

small changes observed in the distribution may be due to reduction of the framework Sm3+

and its hydrogen spillover effect on the ceria. The hydrogen spillover effect is further

enhanced by the presence of Au. As reported by Andreeva et al. [25] the peaks assigned to

the ceria surface layer reduction were shifted to in the range of 120–150 oC temperature.

They have also described that the gold deposited CeO2–Al2O3 had more oxygen storage

capacity as revealed by TPR studies. In accordance with their observation we have found a

similar trend for Au/Ba-CeO2, Au/Mg-CeO2 and Au/Ca-CeO2 catalysts [26]. The H2

consumption of the prepared catalysts is shown in Fig. 12. In the case of Sm-CeO2 (Sm/Ce

= 4/100) catalyst H2 consumption is more comparable to CeO2, with gold deposition on

Sm-CeO2 (Sm/Ce = 4/100) H2 consumption was further increased. This H2 consumption

profile clearly indicates that gold nanoparticles increases surface reducibility of the catalyst.

Oxygen uptake of the catalysts was determined by temperature programmed oxidation

(TPO) after TPR run from ambient temperatures to 700 °C. The TPO profiles are presented

in Fig. 11. The patterns of the TPO profiles are almost similar for the three catalysts. The

profiles show small peaks at temperature below 100 °C. The profiles also indicate that there

is oxygen uptake continued at above 100 °C till around 300 °C after that the profile curves

are almost straight lines. The amount of oxygen uptake below 100 °C can be quantified by

integration because the peaks are discernible, where as the oxygen uptake above 100 °C

cannot be determined because of difficulties in defining peaks. Hence, discussion on

oxygen uptake of the catalysts is restricted to the small peaks below 100 °C. Fig. 12 shows

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Fig. 11. O2-TPO of (a) CeO2, (b) Sm-CeO2 (Sm/Ce = 4/100), (c) Au/Sm-CeO2 (Sm/Ce = 4/100)

Fig. 12. Gas uptake of the catalysts

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the oxygen uptakes from ambient temperature to 100 °C. It is observed that the Sm doping

to ceria increases oxygen uptakes, however, the Au impregnated sample showed a lower

oxygen uptake. The oxygen uptake in the small peaks can be attributed to healing of the

oxygen vacancy on ceria [27]. Substitution of Ce4+

with Sm3+

in the fluorite structure has

been reported to increase the oxygen vacancy and promote the oxide ion conductivity in

ceria [28]. The Raman and XPS studies suggest the presence of oxide vacancies which

agrees with the increased in oxygen uptake for the Sm doped cerium oxide (Sm/Ce =

4/100). The observed decrease of oxygen uptake in the Au deposited catalyst may be due to

filling of oxide vacancy with Au nanoparticles as observed in the Raman study. The

hydrogen consumption renders these sites unavailable for oxygen uptake. This may account

for the Au adsorption on surface capping oxygen or oxygen vacancies on ceria which can

compensate for the similar hydrogen consumption and oxygen uptake of the Au deposited

sample to the Sm doped ceria catalysts.

4.4.2. Catalytic activity studies

The catalytic activity for benzyl alcohol oxidation using molecular O2 was measured by

using CeO2, Sm/CeO2 (Sm/Ce = 4/100), Au/CeO2, Au/Sm-CeO2 (Sm/Ce = 4/100) catalysts

(Table 3). The effects of Sm and Au are observed very clearly. The activity of Au/Sm-

CeO2 was higher than Au/CeO2, which unambiguously demonstrates that the support plays

an important role in the catalytic oxidation reaction via more beneficial metal-support

interaction as evidenced by FT-Raman, XPS and TPR/TPO studies. This type of

interaction has been widely documented by both theoretical and experimental studies

finding that the oxygen vacancies energetically favors and modifies the electronic

properties of the deposited metal [29]. It is found from XPS that Au was composed of three

kinds of electronic states. In order to confirm the role samarium for Au/Sm-CeO2 (Sm/Ce =

4/100) catalyst an experiment was carried to check the catalytic activity for the benzyl

alcohol oxidation reaction by keeping the substrate:Au ratio almost similar for Au/Sm-

CeO2 (Sm/Ce = 4/100) and Au/CeO2 (Table 4). The enhanced activity Au/Sm-CeO2

121


(Sm/Ce = 4/100) established the synergistic role of samarium and gold to the benzyl

alcohol oxidation reaction. A recent work reported by Santra et al. [26] demonstrated that

only gold content could not explain the difference in activity for Au/CeO2 and Au/Ba-CeO2

for the benzyl alcohol oxidation reaction. It is observed that more reducibility of Au/Ba-

CeO2 was responsible for higher catalytic activity.

The different reaction parameters like temperature and the flow of oxygen were varied.

The conversion of benzyl alcohol was varied due to change in the flow of oxygen and

benzyl alcohol conversion increased with the flow of oxygen up to 50 ml/min (Table 3). As

reported by Baiker et al. [30] at low oxygen flow the active metal center might be poisoned

by decomposed products whereas higher oxygen flow can cause over-oxidation. An

important role of oxygen is to remove the poisoning species and thus increase the number

of active MO sites available for alcohol dehydrogenation. During development of an

alcohol oxidation process, the aim is to operate the reactor at around region by fine-tuning

the rate of oxygen supply to the actual rate of alcohol oxidation.

As far as the turn over number (TON) is concerned, we selected the 2.33 wt% Au

catalyst for correlation of gold nanocluster size with the benzyl alcohol oxidation activity. It

is observed by several studies by Haruta et al. the size of gold nanoparticle depends on

several parameters, e.g. reducing agents and calcination temperature after gold deposition

[31]. The catalytic activity is less in the case of higher temperature (400 °C, 500 °C)

calcined samples (Table 5). Also among different precipitating agents, e.g. NaOH and

NaHCO3 (Table 6), the earlier agent shows better activity.

The catalytic activity of the Au/Sm-CeO2 (Sm/Ce = 4/100) catalyst was evaluated for

other substrates e.g. cyclopentanol, cyclohexanol, 2-octanol, cinnamyl alcohol and p-

methyl benzyl alcohol and shown in Table 8. As expected the similarity of electronic and

steric factor of cyclopentanol and cyclohexanol the activity was almost the same for the

oxidation reaction over the Au/Sm-CeO2 catalyst. The oxidation of cinnamyl alcohol is

often used as a model reaction for alcohol oxidation reaction. Baiker et al. [30]

122


demonstrated the existence of a complex reaction network. The target product is

cinnamaldehyde; however, side reactions due to transfer hydrogenation, hydrogenolysis

and decarbonylation also can be possible, depending on the reaction conditions and metal

catalyst used. The activity data given in Table 8 shows high conversion of cinamyl alcohol

compare to other substrate. Octanol was used as a model for linear, long-chain, aliphatic

alcohol. It has been reported [30, 32] that the oxidation of octanol is much more difficult

than the oxidation of cinnamyl and benzyl alcohol; thus, octanol is a more demanding

substrate. It was reported [33] that on gold catalyst aromatic alcohols are more easily

oxidized than aliphatic alcohols, which was attributed to the hydrophobic character of the

former, which displaced water molecules from the catalyst surface, leaving it more

accessible to the reactant. The high reactivity of aromatic alcohol may also be explain by

conjugation between the carbonyl group (Product molecule) and the C=C bond of the

aromatic ring, respectively. Therefore, the equilibrium between the aldehyde and the

alcohol is shifted to the aldehyde side [34].

Table 3: Result of catalytic activity of (a) CeO2 (b) Sm-CeO2(Sm/Ce = 4/100) (c)

Au/CeO2 (d) Au/Sm-CeO2(Sm/Ce = 4/1000 towards benzyl alcohol oxidation reaction

Catalyst Au loading Au loading O2 flow Conversion Selectivity

(wt%) (wt%)a (ml/min) (%) (%)

CeO2 - - 50 2.75 > 99

Sm-CeO2 - - 50 8.94 > 99

(Sm/Ce = 4/100)

Au/CeO2 2.33 0.7859 50 12.54 > 99

Au/Sm-CeO2 2.33 1.7749 50 27.41 > 99

(Sm/Ce = 4/100)

Au/Sm-CeO2 2.33 1.7749 30 12.72 > 99

(Sm/Ce = 4/100)

Au/Sm-CeO2 2.33 1.7749 70 10.23 > 99

(Sm/Ce = 4/100)

(a) Weight% of Gold was determined from EDXRF measurement

Reaction conditions: Toluene solvent, 4 ml; benzyl alcohol, 4 mmol; catalyst, 100 mg;

reaction temperature, 90 °C; reaction time, 3 h; dodecane used as an internal standard

123


Table 4: Result of catalytic activity of (a) Au/CeO2 (b) Au/Sm-CeO2 (Sm/Ce = 4/100

towards benzyl alcohol oxidation reaction by keeping same Substrate:Gold ratio

Catalyst Au loading Au loading Temperature Time Conversion Selectivity

(wt%) (wt%)a (°C) (h) (%) (%)

Au/CeO2 2.33 0.2698 90 3 5.43 > 99

Au/Sm-CeO2 2.33 0.705 90 3 16.59 > 99

(Sm/Ce = 4/100)

(a) Weight% of Gold was determined from ICP measurement

Reaction conditions: Toluene solvent, 4 ml; benzyl alcohol, 4 mmol; catalyst, O2, 50

ml/min; reaction time, 3 h; dodecane used as an internal standard.

Substrate(Benzyl alcohol):Gold ratio (mol) for these reaction is 4×10-3

:1.36×10-6

Table 5: Result of catalytic activity of 2.33 wt% Au/Sm-CeO2 (Sm/Ce = 4/100) with

different calcination temperature towards benzyl alcohol oxidation.

Catalyst Calcination Au loading Au loading Conversion Selectivity

Temperature (°C) (wt%) (wt)a (%) (%)

Au/Sm-CeO2 300 °C 3.33 1.7749 27.41 > 99

Au/Sm-CeO2 400 °C 2.33 1.3996 12.16 > 99

Au/Sm-CeO2 500 °C 2.33 1.6459 5.29 > 99

(a)- Weight% of Gold were determined from EDXRF measurement Reaction condition: Toluene solvent, 4 ml; benzyl alcohol, 4 mmol; catalyst, 100 mg; O2,

50 ml/min; reaction time, 3 h; reaction temperature, 90 °C; dodecane used as an internal

standard

Table 6: Result of catalytic activity of 2.33 wt% Au/Sm-CeO2 (Sm/Ce = 4/100)

prepared by using different reducing agent towards benzyl alcohol oxidation.

Catalyst Precipitating Au loading Au loading Conversion Selectivity

Agent (wt%) (wt)a (%) (%)

Au/Sm-CeO2 NaOH 2.33 1.7749 27.41 > 99

Au/Sm-CeO2 NaHCO3 2.33 1.1829 12.73 > 99

(a)- Weight% of Gold was determined from EDXRF measurement

Reaction conditions: Toluene solvent, 4 ml; benzyl alcohol, 4 mmol; catalyst, 100 mg; O2,

50 ml/min; reaction time, 3 h; reaction temperature, 90 °C; dodecane used as an internal

standard

124


Table 7: Effect of concentration of solution during deposition-precipitation method on

catalytic performance of 2.33 wt% Au/Sm-CeO2 (Sm/Ce = 4/100), calcination

temperature is 300 °C

Catalyst Water taken during Au loading Au loading Conversion Selectivity

DP method (ml) (wt%) (wt)a (%) (%)

Au/Sm-CeO2 150 2.33 1.7749 27.41 > 99

Au/Sm-CeO2 100 2.33 1.4027 19.32 > 99

Au/Sm-CeO2 50 2.33 1.5083 9.59 > 99

(a)- Weight% of Gold was determined from EDXRF measurement Reaction conditions: Toluene solvent, 4 ml; benzyl alcohol, 4 mmol; catalyst, 100 mg; O2,

50 ml/min; reaction time, 3 h; reaction temperature, 90 °C; dodecane used as an internal standard

It is remarkable that all the catalysts displayed high selectivity to benzaldehyde. This

suggests that the active sites on the catalysts are synergistically involved in the transition

states for conversion of the substrate. Moreover, the active sites are common to all the

catalysts and the conversion displayed by a catalyst is most probably a reflection of the

sites densities. Activity of Sm-CeO2 (Sm/Ce = 4/100) is higher than that of CeO2. In our

study, as the defects such as oxygen vacancies of Sm-CeO2 support expose exclusively

Ce3+

ions, it may act as a more efficient support to activate oxygen molecules and

simultaneously as a hydrogen scavenger from the metal surface. Although XRD result

indicated that Sm-doping did not significantly altered the lattice parameter of the fluorite

structure of CeO2 however formation of oxygen vacancies was observed from RAMAN

study. Significantly, TPR/TPO results suggest that the presence of Sm into the lattice of

CeO2 increases the density of easily removed surface oxygen ions. Expectedly, Au

deposition enhances the catalytic performance of the doped and undoped CeO2 catalysts

however the activity was inferior compare to the Au/CeO2 catalyst reported by Corma et al.

[35]. It is widely accepted that the oxidative dehydrogenation is responsible for the reaction

mechanism of alcohol oxidation [36-38] although the precise reaction pathway is still under

debate because the nature and concentration of adsorbed species are not known. In our

study, it is found that the catalytic activity depends strongly on the gold content. We have

varied the calcination temperature, precipitating agent, concentration of gold precursor

125


Table 8: Catalytic data obtained from the oxidation of various alcohols by

using Au/Sm-CeO2 (Sm/Ce = 4/100)

Reactant Product Conversion Selectivity

(%) (%)

OH O 6.95 > 99

OH O 5.4 > 99

(CH2)5

OH

(CH2)5

O

8.91 > 99

CH2OHCHO

31.85 > 99

CH2OH

Me

CHO

Me

17.37 > 99

Reaction conditions: Toluene solvent, 4 ml; alcohol, 4 mmol; catalyst, 100 mg; O2, 50

ml/min; reaction time, 3 h; reaction temperature, 90 °C; dodecane (for cyclopentanol,

cyclohexanol, 2-octanol) and tridecane (for cinnamyl alcohol, p-methyl benzyl alcohol)

used as an internal standard

126


during the preparation of gold catalyst. It is found that the supported gold catalyst shows

different activity as depicted in Tables 3–7. Strong gold-support interaction has been

evidenced from XPS investigation, resulting different electronic states of gold

nanoparticles. Au nanoparticles most probably constitute the secondary active sites here

(involved in the complete catalytic cycle) while the acid-base and redox sites on the CeO2

play a complementary role.

4.5. Conclusion

In conclusion we have successfully synthesized samarium incorporated nanocrystalline

cerium oxide by non hydrothermal sol-gel method using triethanolamine/water mixture as a

solvent. The XRD study reveals the decrease in crystallite size of cerium oxide after

samarium incorporation which is mainly due to surface restructuring of the hydroxyl

groups. The Raman study depicts the formation of oxygen vacancies due to samarium

doping. The HRTEM investigation shows that 2-5 nm gold nanoparticles are deposited on

25-30 nm size support oxide nanoparticles. The STEM-HAADF analysis shows the

homogeneous distribution of gold nanoparticles on the oxide surface. A thorough XPS

investigation proves the strong gold-support interaction on the catalyst surface. The XAFS,

XANES study strengthen the presence of gold nanocluster (< 3 nm) in the Au/Sm-CeO2

(Sm/Ce = 4/100). TPR studies show the high reducibility of the Au/Sm-CeO2 catalyst

whereas TPO studies strengthen the residence of gold nanocluster on the oxide vacancy of

Sm-CeO2 catalyst. TPR studies show the high reducibility of the Au/Sm-CeO2 catalyst.

The improved catalytic activity of Au/Sm-CeO2 catalyst compare to Sm-CeO2 catalyst is

mainly due to easy surface redox functionality as observed by TPR studies.

127


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