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ARTICLE

Science of Advanced Materials

Vol. 6, pp. 1-9, 2014

(www.aspbs.com/sam) Ce 1-x Co x O 2

Nanorods Prepared by

Microwave-Assisted Hydrothermal Method:

Novel Catalysts for Removal of Volatile

Organic Compounds

Rosana Balzer

1,? , Luiz F. D. Probst 1 , Andrés Cantarero 2 , Maurício M. de Lima Jr 2,3 , Vinícius D. Araújo 4

Maria I. B. Bernardi

4 , Waldir Avansi Jr 5 ,RaulArenal 6,7 ,andHumbertoV.Fajardo 8 1

Departamento de Química, Universidade Federal de Santa Catarina, UFSC, 88040-900, Florianópolis-SC, Brasil

2 Instituto de Ciencia de los Materiales, Universidad de Valencia, E-46071 Valencia, Spain 3 Fundación General, Universitat de Valencia, Valencia, Spain 4

Instituto de Física de São Carlos, Universidade de São Paulo, USP, 13560-970, São Carlos-SP, Brasil

5

Departamento de Física, Universidade Federal de São Carlos, UFSCar, 13565-905, São Carlos-SP, Brasil

6

Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza,

Calle Mariano Esquillor, 50018 Zaragoza, Spain

7 ARAID Fundation, Calle Mariano de Luna, 50018 Zaragoza, Spain 8

Departamento de Química, Universidade Federal de Ouro Preto, UFOP, 35400-000, Ouro Preto-MG, Brasil

ABSTRACT

In this study, nanorods of a Ce

1-x Co x O 2 system, withx=0, 0.05, 0.10, 0.15 and 0.20, were synthesized

employing a microwave-assisted hydrothermal method and their catalytic activity in the total oxidation of three

VOCs (benzene, toluene ando-xylene) was investigated. The physicochemical characterization showed that the

inclusion of cobalt led to an increase in the oxygen vacancies in the system. Also, an increase in the catalytic

activity was observed with the progressive incorporation of cobalt into the ceria matrix. The results revealed

that the Ce 0.80 Co 0.20 O 2 catalyst was able to remove 100% of the benzene, nearly 100% of the toluene and

around 70% of theo-xylene. Increasing the amount of oxygen vacancies could result in an enhancement of

the bulk and surface oxygen mobility. The higher oxidation activity observed for the catalyst with the highest

cobalt load can be attributed to a combination of several factors, including a greater number of active sites of

the cobalt being exposed and greater mobility of the active oxygen species. KEYWORDS:Hydrothermal Method, Nanorods, Catalytic Oxidation, Volatile Organic Compounds.

1. INTRODUCTION

Volatile organic compounds (VOCs) are hazardous highly toxic pollutants that cause a number of environmental and human health problems. They are released during a wide range of industrial, transportation and commer- cial activities and their emissions have reached high levels. 1-3

Several VOCs removal techniques, including

physical, chemical and biological methods, have been described in the literature. Catalytic oxidation has been acknowledged as the most effective approach, mainly due to its high degradation efficiency, low energy cost, the potential for the removal of low concentrations of Author to whom correspondence should be addressed.

Email: rosanabalzer@gmail.com

Received: XX Xxxx XX

Accepted: XX Xxxx XX

VOCs and the low thermal NO

x emissions involved. 2?4-6 The noble metal-based catalysts used in this process fre- quently exhibit better performance when compared to non-noble metal-based catalysts.

3?5?7-9

However, due to

the high cost of the noble metals, which can repre- sent an economic obstacle to the employing the pro- cess, these are increasingly being replaced with cheaper catalysts employing transition metals. 7?10

Of the metal

oxides, cobalt and cerium oxides have been shown to be among the most efficient for the promotion of oxi- dation reactions. 10?11

To be an active catalyst for VOCs

oxidation, cobalt must be well dispersed on the support and highly reducible particles. This can be achieved by using a suitable support. The use of ceria as support offers some advantages due to its redox property which can help the dispersion of the supported metal and ensure Sci. Adv. Mater. 2014, Vol. 6, No. xx1947-2935/2014/6/001/009 doi:10.1166/sam.2014.20591 Ce 1-x Co x O 2 Nanorods Prepared by Microwave-Assisted Hydrothermal MethodBalzer et al.

ARTICLE

more oxygen available for the oxidation reaction. 12-15 Thus, aiming to contribute to the development of non- noble metal-based catalysts to be applied in processes involving oxidation reactions of VOCs, the objective of this study was to investigate the use of well designed bifunctional catalysts, based on cobalt and cerium, for the total oxidation of volatile organic compounds (ben- zene, toluene ando-xylene), which were chosen as model compounds due to their toxic, carcinogenic and molecular characteristics. The Ce 1-x Co x O 2 catalysts were prepared through the microwave-assisted hydrothermal method. This method combines the advantages of both hydrother- mal and microwave-irradiation techniques. Recently, this procedure has attracted the attention of researchers since it offers very short reaction times (e.g.,: Ifrah et al. 16 synthe- sized La 1-x Ag x MnO 3+? during 24 hours in a conventional hydrothermal method whileonly 120 minutes was neces- sary for the microwave-assisted method.), the production of small particles with a narrow particle size distribution and high purity, applicability to samples with different morphologies and structures, and low energy consumption compared with conventional methods. 17-19 Differencesin the activity exhibited by the catalysts have been attributed to their physical and chemical properties (such as specific surface area, morphology, size, dispersion and strength of interaction between metal particle and sup- port) which are related to the method used for their prepa- ration. The importance of these properties is confirmed by the large number of publications dealing specifically with the synthesis of nanostructuredmaterials forcatalytic appli- cations. Compared with the traditional bulk catalysts, the nanostructuredcatalysts havemanyimprovementsconcern- ing the structural aspects, which are expected to contribute to enhance the catalytic activity due to their unique prop- erties. The use of particles in a nanometer scale increases the relative proportion of active sites per unit area of the metal particles, resulting in a large surface area for con- tact between the catalyst active phase and the reactant molecules. This means that more reactions can occur, and thus a more efficient catalyst can be expected. 20?21

In this

regard, the development of nanostructured catalysts which exhibit superior activity in the catalytic oxidation process is an important challenge confronting researchers. To the best of our knowledge, this is the first time that Ce 1-x Co x O 2 nanorods, synthesized via microwave-assisted hydrother- mal method, is used as catalysts for the total oxidation of benzene, toluene ando-xylene.

2. EXPERIMENTAL DETAILS

2.1. Catalyst Preparation

In a typical procedure to obtain Ce

1-x Co x O 2 (x=0,

0.05, 0.10, 0.15 and 0.20) nanostructures, 0.02 mol of the

precursors cerium chloride (CeCl 3

·7H

2

O-Sigma-Aldrich)

and cobalt chloride (CoCl 2

·6H

2

O-Sigma-Aldrich) were

dissolved in 50 mL of distilled water. In the next step,50 ml of a 10 M NaOH solution was added rapidly under

vigorous stirring. The mixed solution was placed in a

110 mL Teflon autoclave (filling 90% of its volume) which

was sealed and placed in a microwave assisted hydrother- mal system, applying 2.45 GHz of microwave radiation at a maximum power of 800 W. The temperature was mea- sured with a temperature sensor (type K thermocouple) inserted inside the vessel. The as-prepared solution was subjected to microwave hydrothermal synthesis with heat- ing to a temperature of 140

C in 1 min, which was held

for 10 min. The sample was then air-cooled to room tem- perature. The as-obtained precipitate powder was washed several times with distilled water and isopropyl alcohol and then dried on a hot plate at 60

C for 24 h. The result-

ing powder was then calcined at 600

Cfor2h.

2.2. Catalyst Characterization

The specific surface area (BET method) was estimated from the N 2 adsorption/desorption isotherms at liquid nitrogen temperature, using a Micromeritics ASAP 2000. The equivalent spherical diameter of the particles,d BET was calculated with the equation,d BET =6/?S s ??,where S s is the specific surface area and?is the density of the material in the particles. The powders were characterized structurally in an X-ray diffractometer (Rigaku, Rotaflex RU200B) with CuK? radiation (50 kV, 100 mA,?=1?5405 Å), using a?-2? configuration and a graphite monochromator. The scan- ning range was between 20 and 90 (2??, with a step size of 0.02 and a step time of 5.0 s. Rietveld analysis was performed using the Rietveld refinement program GSAS. 22

A pseudo-Voigt profile function was used.

Elemental analysis of the catalysts was performed using a Varian AA240FS atomic absorption spectrometer in order to verify their doping levels. Raman spectroscopy was carried out at room tempera- ture in a Jobin-Yvon-T64000 micro-Raman system in the backscattering geometry, using different laser line excita- tions (325, 364, 457, 488, 514 and 647 nm). We used an optical lens with 100X magnification, which supplies an average laser spot size of 1?m. Photoluminescence measurements were carried out in a McPherson—207 monochromator, with 1200 gr/mm gratings equipped with an Andor CCD. The samples were mounted in a closed-cycle Leybold cryostat, which allowed a temperature of 7 K. The 325 nm excitation wavelength of a He-Cd laser (Kimmon Koha) was used, with the nominal output power kept at 5 mW and a spot size of approximately 100?m in diameter.

The different morphologies of the nanocatalysts

were determined by field emission scanning electron microscopy (FE-SEM) using a Zeiss Supra™ 35 FE-SEM microscope and also investigated by transmission elec- tron microscopy (TEM) performed on a JEM 2100 URP (operating at 200 KV) and on an imaging-side aberration- corrected FEI Titan-Cube microscope working at 300 kV,

2Sci. Adv. Mater., 6, 1-9, 2014

Balzer et al.Ce

1-x Co x O 2 Nanorods Prepared by Microwave-Assisted Hydrothermal Method

ARTICLE

equipped with a Cs corrector (CETCOR from CEOS

GmbH).

The Electron paramagnetic resonance (EPR) spectra

were recorded at 20 K in an X-band Bruker ELEXSYS E580 spectrometer. The temperature was controlled by an Oxford ITC503 cryogenic system. The spectra were obtained at a modulation frequency of 100 kHz, modula- tion amplitude of 0.2 mT and microwave power of 1 mW. The O 2 -chemisorption measurements were conducted at 600

C using a ChemBET analyzer (Quantachrome

Instruments).

2.3. Catalytic Tests

The catalytic activity was measured in a fixed bed tubular quartz reactor (39.5 cm length and 9 mm inner diameter) under atmospheric pressure. The following conditions were chosen: catalyst volume 0.11 g, inlet benzene (Vetec concentration 1.2 g·m -3 , toluene (Vetec ?concentration

0.7 g·m

-3 ,o-xylene (Vetec ?concentration 0.5 g·m -3 in air, gas flow rate 20 cm 3

·min

-1 , residence time 0.3 s, gas hourly space velocity 12000 h -1 and temperature range

150-750

C. The reaction data were collected after at least

2 h on-stream at room temperature. The reaction products

were determinedby GC-MS. The reactant and productmix- tures were analyzed using two on-line gas chromatographs equipped with FID and TCD detector and an HP-5 column. The catalytic activity was expressed as the degree of the conversion of benzene, toluene ando-xylene, respectively. The conversion of the BTX compounds (benzene, toluene ando-xylene) was calculated as follows:

BTXs?%?=?BTXs?

in -?BTXs? out ?BTXs? in

×100%

where BTXs (%)=percentage of BTX conversion; [BTXs] in =input quantity and [BTXs] out =output quantity.

3. RESULTS AND DISCUSSION

Table I shows the BET specific surface area of the catalysts and the density and the equivalent spherical diameter of the particles. The specific surface area of the catalysts (S s ?var- ied monotonically with the composition. A linear decrease in the surface area with increasing cobalt content was

Table I.Specific surface area (S

s ?, lattice parameter (a?, crystallite size (D cryst ?, oxygen occupancy factor (O ocup ?, density (??, equivalent spherical diameter (d eq ?and oxygen storage capacity (OSC) of Ce ?1-x? Co ?x? O 2 nanoparticles. S s D cryst ?d eq R exp R Bragg OSC

Sample (m

2 /g)a(Å) 1 (nm)O occup (g/cm 3 1 (nm) (%) (%) (mmol/m 2 ?1 5.4176±0.0001 11 0.954±0.007 7?128 10 6?88 3?42 - Ce 0?95 Co 0?05 O 2

81?9 5.4157±0.0001 11 0.914±0.006 6?914 11 6?81 3?78 3?02

Ce 0?90 Co 0?10 O 2

77?8 5.4161±0.0001 10 0.885±0.006 6?704 12 6?83 4?07 3?96

Ce 0?85 Co 0?15 O 2

72?8 5.4161±0.0001 11 0.887±0.006 6?531 13 6?86 3?15 4?51

Ce 0?80 Co 0?20 O 2

65?7 5.4158±0.0001 11 0.851±0.006 6?336 14 7?04 3?97 4?96

Note: 1

Calculated via Rietveld refinement.

observed. The behavior of the equivalent spherical diame- ter of the particles (d eq ) was the inverse of that observed for the specific surface area, indicating an increase in par- ticle size with increasing cobalt content. It has been shown that an increase in the size of CeO 2 nanostructures pre- pared by the microwave-assisted hydrothermal method can be described by the process of Ostwald ripening coupled with self-assembly. 23?24

For materials prepared under the

same time/temperature conditions, the only factors influ- encing the growth of the particles are the diffusion coeffi- cient and the surface energy. The solubilization of a lower valence dopant in ceria leads to the formation of oxygenquotesdbs_dbs29.pdfusesText_35

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