Orange II Degradation by Wet Peroxide Oxidation Using Au

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Orange II Degradation by Wet Peroxide Oxidation using Au Nanosized Catalysts – Effect of the Support Carmen S.D. Rodrigues, Sonia AC Carabineiro, Francisco Jose Maldonado-Hodar, and Luis Miguel Miguel Madeira Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04673 • Publication Date (Web): 06 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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Industrial & Engineering Chemistry Research

Orange II Degradation by Wet Peroxide Oxidation using Au Nanosized Catalysts – Effect of the Support

Carmen S.D. Rodrigues1, Sónia A.C. Carabineiro2, Francisco J. Maldonado-Hódar3, Luís M. Madeira1,*

1

LEPABE – Laboratório de Engenharia de Processos, Ambiente, Biotecnologia e Energia,

Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal. 2

LCM – Laboratório de Catálise de Materiais, Laboratório Associado LSRE/LCM,

Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal. 3

Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Avenida de

Fuente Nueva, 18071 Granada, Spain.

*

Corresponding author: Tel. + 351-22-5081519; Fax: + 351-22-5081449; E-mail: [email protected]

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ACS Paragon Plus Environment


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Abstract The degradation of Orange II was evaluated by wet peroxide oxidation using gold nanoparticles supported on Fe2O3, TiO2, ZnO and Al2O3 as catalysts, in a slurry batch reactor. Materials were prepared by the same deposition-precipitation method, which yielded welldispersed nanosized gold particles (2.2 to 5.5 nm). A commercial catalyst (Au/Fe2O3 supplied by the World Gold Council) was used as reference, for comparison. It was demonstrated that the efficiency of wet peroxide oxidation for the Orange II removal and organics mineralization depends on the type of oxide used and the loading and diameter of gold. The Au/Al2O3 material, with the highest BET surface area, showed the highest turnover frequency (TOF) value, and also higher total organic carbon (TOC) and Orange II removals. The catalysts were reused for consecutive cycles, with no Au leaching being detected into the solution, showing their high stability. This stability was confirmed by textural and chemical characterization of the fresh and used materials.

Keywords: Gold; nanoparticles; iron oxide; titanium dioxide; zinc oxide; alumina oxide; wet peroxide oxidation; heterogeneous catalysis; Orange II.

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1. Introduction Advanced oxidation processes (AOPs) are chemical treatment procedures designed to remove organics from water using highly oxidative hydroxyl radicals. AOPs are thus very attractive as they can be applied in the treatment of wastewaters containing refractory and/or toxic compounds 1, 2. AOPs are able to efficiently degrade pollutants, instead of simply separating or transferring them to another phase, as it happens with other technologies like filtration, adsorption and coagulation/flocculation. Wet peroxide oxidation (WPO) is one of those processes, and consists in the oxidation of water pollutants using hydrogen peroxide 3. In this process, the generation of the hydroxyl radicals is facilitated in the presence of a metallic catalyst, which can be either dissolved in the aqueous phase or supported in a solid matrix (respectively homogeneous and heterogeneous WPO). The use of solid catalysts in heterogeneous AOPs, and namely in the WPO, is particularly attractive, as the catalyst separation and recovery are facilitated. Thus, the development of efficient and stable catalysts remains a challenge. Gold supported catalysts are particularly attractive for this process, due to their low (or even negligible) leaching, efficient hydrogen peroxide consumption and adequate stability

4-9

.

Han et al. used a Fenton-like system with Au

on hydroxyapatite and Au-exchanged zeolites to remove phenol, ethanol, formaldehyde, and acetone in aqueous solution 7. Other authors tested gold on TiO2, carbon and Fe2O3 in the wet peroxide oxidation of phenols and benzyl alcohol 4-6, 8. To the best of our knowledge, Au/Al2O3 and Au/ZnO were never tested on the heterogeneous catalytic wet oxidation of dissolved organics with hydrogen peroxide (except the first, in a recent paper of the authors 10, which evaluated the effect of several operating conditions in the efficiency of Orange II degradation and TOC removal). Apart from that, analyzing the effect of the support nature in gold-based catalysts for this AOP is of utmost importance and is the main goal of this study. Materials consisting of Au/TiO2 and Au/Fe2O3, after the same preparation procedure has been followed in all of them equally (deposition-precipitation

11

), were also

tested for comparison, along with the reference catalyst Au/Fe2O3, supplied by the World Gold Council (WGC). Orange II dye was used as model compound because it is a dye commonly used in the cosmetics, food and textile fibers dyeing industry. Dyes are colored substances, generally toxic, carcinogenic and non-biodegradable, which need to be removed from effluents before their discharge into water bodies

12

. Biological treatment processes are not suitable for these

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compounds, so the use of AOPs, particularly of heterogeneous catalytic WPO, is of vital importance.

2. Experimental 2.1. Materials The following commercial supports were used: aluminum oxide (Al2O3) from Aldrich, iron oxide (Fe2O3) from Sigma Aldrich, titanium dioxide (TiO2) from Evonik Degussa (P25), and zinc oxide (ZnO) from Evonik Degussa (AdNano VP 20). Au was loaded on the oxide supports (1% wt.) by the same procedure in all cases, i.e., through the Deposition-Precipitation (DP) method 13-15, which consisted in raising the pH of a HAuCl4 solution until 9 with a NaOH solution (0.1 M), followed by addition of the support. After aging for 12 h, the precipitate was filtered, washed and dried in the oven at 110 ºC overnight. The reaction of HAuCl4 with NaOH causes dissociation of the Au salt, and by reaction with water, it generates hydroxyl ions, which hydroxylate the Au3+ species of the gold precursor. Au3+ can be reduced to Au+ or Au0 by electron transfer from coordinated OH¯ ions on the surface of the supports, as reported by other authors 16. The obtained powder catalysts were compared with the Au/Fe2O3 Type C reference material (labelled as 5 wt. % Au, but having an Au loading of 4.4%, determined by the manufacturers by inductively coupled plasma, as reported in the data sheet of the material), prepared by coprecipitation 17, and supplied by the WGC.

2.2. Catalysts characterization The materials were analyzed in terms of porosity by adsorption of N2 at -196ºC, in a Quantachrom NOVA 4200e apparatus. All samples were previously degassed under high vacuum at 160 °C and for 5 h before analysis. The specific surface area (SBET) was calculated by the Brunauer–Emmett–Teller (BET) equation 18. X-ray photoelectron spectroscopy (XPS) analyses were performed on a VG Scientific ESCALAB 200A spectrometer using Al Kα radiation (1486.6 eV) to determine Au oxidation states. The charge effect was corrected taking the C1s peak as a reference (binding energy of 285 eV). CASAXPS software was used for data analysis. High resolution transmission electron microscopy (HR-TEM) was used to examine the Au dispersion on catalyst samples and was carried out with a Phillips CM-20 equipment. Magnification was 600,000×, with maximum resolution of 0.27 nm between points and 0.14 4


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nm between lines. For the analysis, the powders were dispersed in ethanol and homogenized in an ultrasonic bath before use. A sample of catalysts particles was collected from the dispersion, and allowed to dry at ambient conditions before analysis. Nanoparticle sizes were measured from HR-TEM images, using the ImageJ program. Histograms were drawn from measurements made on 100–1200 nanoparticles, depending on the sample. Average nanoparticle sizes were calculated for all samples. The dispersion (DM) of gold particles, defined as the ratio between the number of surface metal atoms to the total number of metal atoms, was calculated according to equation 1, assuming that metal particles are spherical 19, 20: % =

6 ∗ ∗ ∗ 1000 ∗ 100 1 ρ ∗ N ∗ dp

where ns is the number of atoms at the surface per unit area (1.15 × 1019 m-2 for Au) 21, MM is the molar mass of gold (196.97 g/mol), ρ is the density of gold (19.5 g/cm3), N is the Avogadro’s number (6.022×1023 mol-1), and dp is the average gold particle size (nm).

2.3. Catalytic activity In adsorption or catalytic experiments, it was used 2 g/L of support (Fe2O3,TiO2, ZnO or Al2O3) or gold catalyst (Au/Fe2O3, Au/TiO2, Au/ZnO or Au/Al2O3) and the initial Orange II concentration was 0.1 mM; this concentration of dye is typically found in textile effluents 22, 23. The runs were carried out in a stirred slurry batch reactor with a recirculating water jacket, linked to a thermostatic bath (Hubber, polystat cc1) to keep the temperature constant at 30±1.0 ºC. After the dye solution (50 mL) has reached the desired temperature, pH was adjusted at 3.0 (with 1 N sulfuric acid, from Labchem); then the support or catalyst (2 g/L) was added, this being considered the time zero for the adsorption runs. For oxidation experiments, immediately after the catalyst or support has been added, insertion of 6 mM of hydrogen peroxide (30%, LabChem) was carried out, this being considered the initial instant (t = 0) of the reaction experiments. The value of pH, concentration of hydrogen peroxide and catalyst dose were fixed in the best operating conditions obtained previously 10. Decolorization along the experiments was followed online by the continuous absorbance measurement using a Thermo Electron Corporation UV/VIS spectrophotometer (model γ). For that purpose, recirculation was carried out using a peristaltic pump and a flow-through cell. This way the volume of reaction mixture was kept constant throughout all the reaction time.

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Absorbance was monitored at 486 nm (characteristic wavelength of the Orange II molecule), and the values were registered using a Labview 9.0 interface. In the end of either adsorption or reaction experiments, samples were taken for measuring the total organic carbon (TOC), gold leaching and hydrogen peroxide concentration. In the case of TOC analysis, to the sampling flasks it was previously added excess of sodium sulfite (Fisher Chemical), which instantaneously quenches remaining hydrogen peroxide. For assessing gold concentration, the samples were acidified until pH ~1.0 with concentrated nitric acid (LabChem) to keep the gold dissolved in solution.

2.4. Analytical methods The total organic carbon (TOC) was measured according to the standard method 5310 D

24

,

using a Shimadzu TOC analyzer (model TOC-L) in samples previously filtered with nylon filter membranes (0.45 µm of pore diameter). The gold leaching into the solution was measured by flame atomic absorption spectrometry (AAS) - Method 3111 B 24, using an AAS UNICAM spectrophotometer (model 939/959), after filtrating the samples through nitrate cellulose membranes with 0.45 µm of porosity. The gold loading in the solid catalysts was also determined by AAS, after digesting the solid Au/oxide samples in a mixture of concentrated nitric (65%, LabChem) and chloride (37%, Sigma Aldrich) acids at 140 ºC during 2 h. The quantification of residual hydrogen peroxide was performed as described by Sellers 25. The method is based on the measurement of the intensity of the yellow-orange color resulting from the reaction of hydrogen peroxide with titanium oxalate. The samples were previously filtered through nylon filter membranes with pore diameter of 0.45 µm. In order to eliminate the possible contribution of the dye in the quantification of hydrogen peroxide, the absorbance of a blank solution (with only water) was measured and discounted in the absorbance of the solution. In order to assess the formation kinetics of hydroxyl radicals in solution, 1,5-diphenylcarbazide (Sigma Aldrich) in the presence of catalysts/support and H2O2 was used. The first is oxidized into 1,5-diphenylcarbazone by these radicals. The 1,5-diphenylcarbazone was extracted by a mixed solution of benzene and carbon tetrachloride (50:50 % v/v) and the absorbance was measured at 563 nm 26.

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3. Results and discussion 3.1. Materials characterization 3.1.1. High Resolution Transmission Electron Microscopy (HR-TEM) Gold loaded materials were analyzed by HR-TEM. Figure 1 depicts some of the images. Gold nanoparticles are seen as darker small spots in Figures 1a,c,e,g,i. The corresponding histograms, obtained by analysis of several images, are depicted in Figures 1b,d,f,h,j. Table 1 summarizes the results obtained, namely the average nanoparticle size and gold size range. Table 1. Characterization of the Au supported materials: average gold nanoparticle sizes and size ranges, gold oxidation state, gold loading and dispersion of gold nanoparticles. Samples

Average particle size (nm)

a

Gold size range (nm)

Gold oxidation a

state

b

Gold loading (%)

c

Dispersion (%)

+

0.8

50

Au/Fe2O3

2.3

1-7

Au

Au/Fe2O3

3.6

1-12

Au0

4.0

32

Au/TiO2

2.2

1-12

Au+

1.6

53

Au/ZnO

5.5

1-10

Au0

1.2

21

Au/Al2O3

3.6

1-20

Au0

0.7

32

WGC

a

- determined by HR-TEM; b - determined by XPS of Au 4f (and XPS of Au 4d for Au/ZnO); c

-determined by AAS. The Au-dispersion/particle size will depend on the textural properties of the supports, the interactions between the precursors and the supports, the metal loading and the pre-treatment conditions. In this catalyst series, ZnO provides the highest average Au-particle size (5.5 nm). TiO2 and Fe2O3 provided gold nanoparticles with similar average sizes – 2.2 and 2.3 nm, respectively. Both Au/Al2O3 and Au/Fe2O3 WGC have 3.6 nm average size. On the other hand, gold on Fe2O3 had the smallest range (1-7 nm), while Au/Al2O3 has the largest (1-20 nm) and Au/TiO2 and Au/Fe2O3 WGC have an intermediate range of 1-12 nm, and Au/ZnO of 1-10 nm. The WGC sample has 4% Au (by weight), as determined by AAS. Au/Fe2O3 has the lowest loading in this study (0.8% wt.), while Au/TiO2 has the largest (1.6% wt.) and Au/ZnO has an intermediate value (1.2% wt.). 7



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Figure 1 - HRTEM images and gold nanoparticle size distribution histograms of Au/Fe2O3 (a, b), Au/Fe2O3 WGC (c, d), Au/TiO2 (e, f), Au/ZnO (g, h) and Au/Al2O3 (i, j), respectively. 8


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The dispersion (DM) of gold particles is correlated with the gold particles size (cf. Eq. 1). As expected, the catalysts with a smaller gold size have the highest value of DM (Au/Fe2O3 and Au/TiO2); the Au/Al2O3 and Au/Fe2O3 WGC catalysts have the same values of DM because have the same gold particle size and the Au/ZnO catalyst, with largest gold particle size, has the smaller dispersion.

3.1.2. X-Ray Photoelectron Spectroscopy (XPS) In order to obtain information about the gold oxidation state, Au 4f XPS measurements were also performed. Figure 2a and Table 1 shows that gold was in the Au+ state on Fe2O3 and TiO2, confirming the results obtained by TPR (data not shown). For Au/ZnO sample, there is overlapping of the Zn 3p peak, making it difficult to determine the oxidation state of gold by the analysis of Au 4f. However, a low intensity peak at 83.5 eV is observed over Au/ZnO, corresponding to metallic gold. That is confirmed by the Au 4d spectrum, depicted in Figure 2b. Gold was also on the metallic state on Al2O3 and on Fe2O3 WGC.

a

Au+ Au0

Au+ Au0 Au3+

Au3+

7700

Intensity (a.u.)

6700

Zn 3p

5700 O Au/Al 2 3 4700 Au/ZnO 3700 2 Au/TiO 2700 2O3 WGC Au/Fe

Au/Fe2O3 1700

92

91

90

89

88

87

86

85

84

83

82

81

Binding energy (eV)

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


640

Au3+

Au+

Au0

Au3+

590

Au0

540 490

Au/ZnO 440

360

355

350

345

340

335

330

Binding energy (eV)

Figure 2. Au 4f XPS spectra of Au supported on Fe2O3, TiO2, ZnO and Al2O3, (a) and Au 4d XPS spectra of Au/ZnO (b). 9



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3.1.3. BET Surface Area (SBET) Table 2 shows that among all supports used, Fe2O3 has the lowest surface area (6 m2/g), while Al2O3 shows the highest value (211 m2/g). TiO2 and ZnO have intermediate values (51 and 26 m2/g, respectively). The BET surface area of oxides did not change much upon Au addition (Table 3). This is most likely due to the low loading and low particle size of gold (as seen before), resulting in a negligible porosity blocking.

3.2. Catalytic Activity Measurements 3.2.1. Adsorption and reaction In order to determine which gold-based catalyst had the best performance in Orange II dye degradation by wet peroxidation, some catalytic runs were performed, including preliminary (blank) experiments to evaluate the effect of the oxidant (hydrogen peroxide) per se, the adsorption contribution in the overall process and the catalytic role of the support. Thus, five runs were carried out for each material type, all in a dye-containing solution: i) with the oxidant only, ii) with the support, iii) with the gold catalyst; iv) with the support and oxidant and v) with the gold catalyst and oxidant. Figure 3 shows that hydrogen peroxide alone has a low removal efficiency (values of 13.8% for dye and 6.1% for TOC removal have been reached after 16 h of reaction), due to its low oxidation potential (1.77 vs. NHE – normal hydrogen electrode). However, some dye removal occurs by adsorption, which is more relevant for the supports than for the gold catalysts (except for the Fe2O3 materials). Taking into account the original similar BET surface areas of both the supports and the Au catalysts (see Tables 2 and 3), the different adsorptive performance of these materials should be related to a larger difficulty of dye diffusion in the gold catalysts than in the supports. This is corroborated by the stronger decrease and smaller values of the BET surface areas obtained for the supports after adsorption (see Table 2) as compared to the BET areas of gold catalysts after adsorption (see Table 3).

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b

a

80

H2O2

Au-Fe2O3

Fe2O3

Au-Fe2O3 + H2O2

100

Fe2O3 + H2O2

TOC Dye

80

Removal (%)

Dye Removal (%)

100

60

40

60 40 20

20

0

0 0

2

4

6

8

10

12

14

16

H2O2

Fe2O3

Au-Fe2O3 Fe2O3 Au-Fe2O3 +H2O2

t (h)

c

Fe2O3 WGC + H2O2

80

TOC Dye

Au-Fe2O3 WGC

H2O2

Fe2O3 WGC

Au-Fe2O3 WGC + H2O2

60

40

+ H2O2

d

100 80

Removal (%)

Dye Removal (%)

100

60 40 20

20

0 0 0

2

4

6

8

10

12

14

H2O2

16

Fe2O3 WGC

Au-Fe2O3 Fe2O3 Au-Fe2O3 WGC

t (h)

WGC +H2O2

Au-TiO2

TiO2 + H2O2

100

Au-TiO2 + H2O2

TiO2

WGC + H2O2

f

e H2O2

100

TOC Dye

80

80

Removal (%)

Dye Removal (%)

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


60

40

60 40 20

20

0

0 0

2

4

6

8

10

12

14

16

H2O2

t (h)

TiO2

Au-TiO2

TiO2 +H2O2

11


Au-TiO2 + H2O2


h

g 100

H2O2

Au-ZnO

ZnO + H2O2

80

TOC Dye

80

Removal (%)

Dye Removal (%)

100

Au-ZnO + H2O2

ZnO

60

40

60 40 20

20

0

0 0

2

4

6

8

10

12

14

16

H2O2

ZnO

H2O2

Au-Al2O3

100

Al2O3 + H2O2

Au-Al2O3 + H2O2

Al2O3

Au-ZnO

ZnO +H2O2

t (h)

Au-ZnO + H2O2

j

i 100

TOC Dye

80

80

Removal (%)

Dye Removal (%)

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

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60

40

60 40 20

20

0

0 0

2

4

6

8

10

12

14

16

H2O2

Al2O3

Au-Al2O3 Al2O3 +H2O2

t (h)

Au-Al2O3 + H2O2

Figure 3. Dye removal as a function of time and dye and TOC removals after 16 h for Fe2O3 and Au/Fe2O3 (a, b), for Au/Fe2O3 from WGC (c, d), for TiO2 and Au/TiO2 (e, f), for ZnO and Au/ZnO (g, h) and for Al2O3 and Au/Al2O3 (i, j) (pH=3.0, T= 30 ºC, [H2O2] = 6 mM, [Support or catalyst] = 2.0 g/L and [Orange II] = 0.1 mM).

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1 2 3 4 5 6 Table 2. BET surface areas of the fresh supports materials, after adsorption and after reaction. 7 8 Fe2O3 TiO2 ZnO Al2O3 9 Fresh Adsorption Reaction Fresh Adsorption Reaction Fresh Adsorption Reaction Fresh Adsorption Reaction 10 BET surface area 11 6

Orange II Degradation by Wet Peroxide Oxidation Using Au

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