Calcium-containing materials as alternative catalysts in advanced oxidation process

Fuel xxx (2016) xxx–xxx

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Calcium-containing materials as alternative catalysts in advanced oxidation process Manuel Sánchez-Cantú a,⇑, Ma. de Lourdes Ruiz Peralta a,⇑⇑, Alejandra B. Galindo-Rodríguez a, Edgar Puente-López a, Efraín Rubio-Rosas a, Claudia M. Gómez b, Francisco Tzompantzi c a b c

Benemérita Universidad Autónoma de Puebla, Facultad de Ingeniería Química, Avenida San Claudio y 18 Sur, C.P. 72570 Puebla, Puebla, Mexico Universidad de Guanajuato, Col. Noria Alta SIN, C.P. 36050 Guanajuato, Gto., Mexico UAM-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco # 186, C.P. 09340 México, DF, Mexico

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Four calcium-containing materials

were evaluated in Rhodamine 6G photodegradation.
Pure calcium hydroxide was identified as the most active photocatalyst.
More than 60% mineralization was achieved within four hours.
Ca(OH)2 photocatalytic activity was attributed to an indirect dye sensitization.

a r t i c l e

i n f o

Article history: Received 5 May 2016 Received in revised form 18 November 2016 Accepted 22 November 2016 Available online xxxx Keywords: Advanced oxidation process Photocatalysis Sensitization Ca(OH)2 Rhodamine 6G

a b s t r a c t In this work, four calcium-containing materials (calcium hydroxide, commercial hydrated lime, calcium oxide and calcium carbonate) were evaluated as catalysts in the photodegradation of Rhodamine 6G (Rh6G) under UV radiation. The effect of catalyst concentration, reaction time and pH were evaluated. Materials were characterized by X-ray powder diffraction, thermogravimetric analysis, Scanning Electron Microscopy and Diffuse Reflectance Spectroscopy. The X-ray powder patterns showed that Ca (OH)2, CaO and CaCO3 samples presented pure crystalline phases while commercial hydrated lime consisted of a mixture of calcite and calcium hydroxide. Materials evaluation indicated that among the calcium-containing catalysts Ca(OH)2 was the most active material showing a degradation and mineralization percentages of 50 and 61, respectively. Since Ca(OH)2 band gap value was 5.69 eV, characteristic of an insulator material, the photocatalytic activity was attributed to an indirect dye sensitization. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In general, semiconductor materials have been reported in the literature as effective photocatalysts and among them TiO2, in its ⇑ Corresponding author. ⇑⇑ Corresponding author. E-mail addresses: [email protected] (M. [email protected] (Ma. de Lourdes Ruiz Peralta).

Sánchez-Cantú),

anatase phase, is the most frequently used semiconductor. This and other semiconductor materials have been doped [1] or dyesensitized [2] to improve their photocatalytic behavior. However, these methods increase their cost restricting their industrial application. Thus, the search for novel, cheap and available photocatalysts is of paramount importance. Recently, materials not considered as potential photocatalysts due to their dielectrical nature have been evaluated successfully

http://dx.doi.org/10.1016/j.fuel.2016.11.092 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

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in the photodegradation of distinct organic molecules. Such materials include Al2O3 [3,4], layered double hydroxides [5] and spinel [6]. It is worth mentioning that among these non-semiconductor compounds the calcium-containing catalysts have attracted especial attention due to their low cost, availability and their photoactivity. For instance, Zhang [7] evaluated Ca(OH)2 as photocatalyst against methylene blue (MB) aqueous solution under visible light radiation. It was proposed that reaction was conducted by indirect dye photosensitization. In this sense, also CaO has been used in the photodegradation of methylene blue [8], Violet GL2B [9], indigo carmine [10] and Rhodamine B [11]. It is worth mentioning that despite some works have been published with either Ca(OH)2 or CaO there are still some uncertainties regarding the reaction mechanisms of these materials and although their insulator nature is well known in some works they have been considered as semiconductors. Besides, some reaction mechanisms consider that CaO phase remained after their contact with water without considering the possibility of its hydration or reaction with atmospheric CO2 producing calcium carbonate. Thus, in this work, CaCO3, CaO, Ca(OH)2 and commercial hydrated lime were evaluated in the photodegradation of Rhodamine 6G (Rh6G). The effect of the catalyst nature, amount, reaction time and pH was studied. 2. Experimental 2.1. Materials’ synthesis Commercial hydrated lime (CHL) purchased from Cales Santa Emilia located at Perote, Veracruz, México, sold in 25 kg paper sacks, was used without further purification. CaO was obtained from CHL annealing at 800 °C. The annealing temperature was determined from the thermogravimetric analysis (see Section 3). Pure Ca(OH)2 was prepared as follows: 2 g of CHL was calcined at 800 °C for two hours. Then, the resulting material was hydrated at room temperature with 200 mL of decarbonated water under a static nitrogen atmosphere and vigorous stirring for one hour. Then, the suspension was filtered and dried at 120 °C for 20 min. Calcium carbonate was synthesized by precipitation as follows: 13.8 g of K2CO3 and 11.8 g of Ca(NO3)24H2O were dissolved by separate in 100 mL of deionized water. Then both solutions were added to a reaction vessel maintaining a constant pH of 11. Then, the slurry was aged at 60 °C for 17 h under vigorous stirring. Afterwards, the solid was washed with hot deionized water and dried at 120 °C for one hour. 2.2. Analytical methods 2.2.1. X-ray powder diffraction The X-ray diffraction patterns of the solid samples were acquired in a D8 Bruker Discover Series 2 diffractometer with Cu Ka radiation. The samples were measured between 5 and 70°, with a 2h step of 0.04° and a counting time of 0.6 s per point. Average crystal sizes were determined by the Scherrer equation form the most intense reflection: L(hkl) = Kk/(B(h)cos h) where K is the shape factor (a value of 0.9 was used), L is the average crystal size, k is the wavelength of Cu Ka radiation, B(h) is the Full Width at Half Maximum (FWHM), h is the diffraction angle and h k l are the Miller indices. The crystalline phases were identified by means of the JCPDS (Joint Committee of Powder Diffraction Standards) database. 2.2.2. Thermogravimetric analysis Thermogravimetric analyses (TGA) were carried out using a TGAi 1000 series system which was operated under an air flow

(20 mL min1) at a heating rate of 20 °C min1 from room temperature to 1000 °C. In the determination, 40 mg of finely powdered dried sample was used. 2.2.3. Scanning electron microscopy Scanning electron microscopy (SEM) analysis was carried out in a JEOL JSM-6610 LV with an acceleration voltage of 20 keV. Images of the uncovered samples were taken using secondary electrons signals. 2.2.4. Diffuse reflectance spectroscopy Diffuse reflectance spectra of the different samples were recorded on a Cary-5000 Varian (Agilent) spectrophotometer equipped with an integration sphere; polytetrafluoroethylene (Halon material) was used as reference standard. The band gap value was calculated using the Kubelka-Munk formalism [12]. 2.3. Photocatalytic experiments In order to evaluate the photocatalytic activity of the samples, Rhodamine 6G, hereafter called Rh6G (C28H31N2O3Cl; 99%, Aldrich) was chosen as a representative dye pollutant. For a typical procedure a specific amount of the catalyst was dispersed in 150 mL of a 5 ppm Rh6G aqueous solution. The ultraviolet light was provided by a 15 W Hg-lamp UVP-XX-15S (254 nm short wavelength; 5.3 W m2). Prior to photoreaction, air was pumped into the reactor and the catalyst was magnetically stirred in the dark for 30 min to reach an adsorption/desorption equilibrium. After the mixture was irradiated for a given time an aliquot was collected at regular intervals and centrifuged to separate the solid from solution; the quantitative determination of Rh6G was performed measuring its UV–vis absorption spectrum using a Varian Cary100 UV–vis spectrophotometer. The photodegradation of Rh6G dye was followed analyzing the evolution of the optical absorbance at k = 526 nm. The degradation efficiency was calculated using the following Eq. (1):

g¼

C0  C  100% C0

ð1Þ

where C0 is the initial concentration of the dye and C is the concentration at certain time. The photocatalytic variables were catalyst nature, amount, reaction time and pH. The amount of organic carbon present in the solution after reaction was determined with a TOC-V CSH/CSN Shimadzu Analyzer. 3. Results and discussion 3.1. Catalyst characterization The X-ray powder diffraction patterns of the samples are presented in Fig. 1. JCPDS files #88-1807, 84-1263, 78-0649 were used for calcium carbonate (asterisk), calcium oxide (letter x) and calcium hydroxide (circle) identification, respectively. It was evidenced that these samples presented pure crystalline phases. On the other hand, commercial hydrated lime (CHL) exhibited the characteristic reflections of calcium hydroxide and calcite as the main and secondary crystalline phases. Calcite presence in CHL was attributed to CO2 contamination during storage or in the course of CaO rehydration process [13]. Cell parameters and average crystal sizes are presented in Table 1. Cell parameters values of CaCO3 identified in pure calcite and that found in CHL were very close to the reported ones in the JCPDS file (a = 4.988 Å, c = 17.061 Å). In the same way, CaO showed a cell parameter value a = 4.805 Å which is identical to

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100

o

** o*

*

*

*

90

x CaO o *

x o** o

Ca(OH)2

80 70 60 50

CHL

40

1st. derivative (dM/dT)

o

CHL Ca(OH)2 CaCO3

x

Mass %

Relative Intensity (a.u.)

* CaCO3 o Ca(OH)2 x CaO

0.0 -0.1 -0.2 -0.3 -0.4

CHL Ca(OH) 2 CaCO3

-0.5 -0.6

30

CaCO3 10

20

30

40

50

200

400

600

800

1000

Temperature, (°C)

200

400

600

800

1000

Temperature, (°C)

60

2-Theta (degrees)

Fig. 2. Thermogravimetric analysis results.

Fig. 1. X-ray diffraction patterns of the samples.

the reported value of the JCPDS file. Regarding CHL and Ca(OH)2 cell parameters, the former exhibited similar values to those found in the JCPDS file (a = 3.589 Å, c = 4.911 Å) while the latter presented lower values. This effect was assigned to vacancy production during CHL annealing-hydration (see experimental section) causing a lattice contraction. On the other hand, average crystal size values of the samples oscillated between 25 and 66 nm (see Table 1). Fig. 2 exhibits the thermogravimetric analysis (TGA) results of CHL, Ca(OH)2 and calcium carbonate. From the first derivative of both CHL and Ca(OH)2 thermograms (see Fig. 2 insert), three thermal transitions were identified. The first mass loss (25–401 °C) was assigned to hydration water (11%). Then, a second mass loss (401–505 °C) was noticed which was attributed to calcium hydroxide dehydroxilation (17.3 and 20.7% for CHL and Ca(OH)2, respectively). The third mass loss (505–700 and 505–800 °C for Ca(OH)2 and CHL, in that order) corresponded to calcite decomposition and the complete dehydroxylation of calcium hydroxide [14]. This analysis revealed that although a pure calcium hydroxide crystalline phase was identified by the XRD analysis its contamination by CO2 adsorption is very fast and samples should be evaluated as soon as possible to avoid calcite formation and, by consequence, a possible interference during its evaluation. On the other hand, pure CaCO3 sample presented only one mass loss (44.4%) between 580 and 800 °C which corresponded to calcite’s decarbonation. An important issue that must be stressed is that the maximum decarbonation temperature increased from Ca (OH)2, CHL and CaCO3 samples (667, 738 and 763 °C, respectively). A plausible explanation is given in terms of calcite’s crystal sizes where in Ca(OH)2 should be smaller than those found in CHL and CaCO3 (see Table 1) requiring less energy to decompose.

The Scanning Electron Microscopy images of the catalysts are shown in Fig. 3. Regarding CHL and Ca(OH)2 samples, they consisted of irregular-shaped particles. CaO sample exhibited similar morphologies to those reported previously by Milne et al. [15]. Finally, CaCO3 exhibited well-defined rhombic crystals of calcite [16]. Fig. 4 shows the diffuse reflectance spectrum of CHL and Ca (OH)2 materials. The spectra exhibited adsorption edges in the UV region giving Eg values of 5.79 and 5.69 eV for CHL and Ca (OH)2, respectively; suggesting an insulator behavior. XPS measurements and theoretical studies (analysis of the partial density of states) indicate that the upper part of the valence band is composed of O-2p orbitals since the lower section of the conduction band arise from Ca-3d orbitals in the Ca(OH)2 structure [17]. 3.2. Photocatalytic activity Photocatalytic activity evaluation of the calcium-containing samples (CaCO3, CaO, Ca(OH)2 and commercial hydrated lime) were investigated by Rh6G degradation of in aqueous solution. The adsorption experiment indicated that although adsorption was noticed in the first 30 min, adsorption-desorption equilibrium was reached after one hour and not concentration changes where observed afterwards. The photolysis experiment was carried out to determinate dye’s degradation under UV irradiation. As shown in Fig. 5A, not concentration changes of Rh6G were observed; therefore, dye’s stability in such conditions ensures that the degradation process is due to the catalyst’s influence. Fig. 5 presents Rh6G decoloration curves where CaO, CHL and CaCO3 did not exhibit relevant photoactivity giving an efficiency of 33, 25 and 13%, respectively. However, Ca (OH)2 showed an efficiency of 50%. As evidenced by the XRD analysis (see Fig. 1), although hydrated lime is generally considered as

Table 1 Samples’ cell parameters and average crystal sizes.

a

Sample

a, Å

c, Å

L(101), nm

L(200), nm

L(104), nm

CHL Ca(OH)2 CaO CaCO3a CaCO3

3.585 3.579 4.805 4.984 4.983

4.899 4.880 – 17.061 17.045

25 35 – – –

– – 44 – –

– – 49 66

Contained in hydrated lime.

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A

B

5 μm

5 μm

C

D

5 μm

5 μm

Fig. 3. Scanning Electron Microscopy images: (A) CHL, (B) Ca(OH)2, (C) CaO and (D) CaCO3.

(a) 1/2

[F(R )hv)]

8

[F(R )hv)]

(b)

2

1/2

2

1

E g =5.75 eV

1

E g =5.69 eV

0

8

0 1

2

3

4

5

6

7

Band gap (eV)

1

2

3

4

5

6

7

Band gap (eV)

Fig. 4. Diffuse reflectance spectra of (a) CHL and (b) Ca(OH)2.

Ca(OH)2, CHL consisted of a mixture of calcite and Ca(OH)2 crystalline phases and the former’s presence diminished Ca(OH)2 photoactivity. In this sense, CaCO3 activity was attributed to its hydration generating surface hydroxyl groups which, as will be further shown, are responsible for these materials’ activity. It is important to remark that although calcite presented certain photocatalytic activity its presence should be avoided in Ca (OH)2 since it hinders its catalytic activity. For instance, by analyzing the degradation curves of Ca(OH)2 and CHL it was verified that both exhibited a fast degradation rate in the first 90 min. Then, a slope change was noticed which is indicative of a mechanism change which was attributed to CaCO3 formation which is in good agreement to the pH and TOC analysis (see Figs. 5A and 6). As Rh6G degradation using pure Ca(OH)2 was fast, CaCO3 could have been easily formed covering not only material’s surface but its pores diminishing its catalytic activity. On the other hand, as CaCO3 was initially identified in CHL it

caused a slow Rh6G decomposition allowing Rh6G diffusion throughout the entire available surface giving rise to a continuous dye degradation. As pure Ca(OH)2 exhibited the highest photoactivity the next experiments were conducted using this catalyst. To evaluate the effect of Ca(OH)2 amount in the photocatalytic degradation efficiency (see Fig. 5B), different catalyst amounts of were proved (0.030 to 0.090 g). An increase in the degradation efficiency from 19 to 50% was observed when the catalyst mass increased reaching a maximum value with 0.075 g. The degradation enhancement was attributed to an increment of the active sites’ number as observed similarly in TiO2 [18]. Nevertheless, at higher mass values, the photocatalytic efficiency decreased. This behavior was assigned to a ‘‘shadowing effect” since the mixture turbidity causes light’s penetration depth diminution. Also, at high catalyst loading, particle-particle aggregation may also reduce the catalytic activity due to scattering effects [19].

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5

1.0

1.0

0.8

0.8

0.6

C/Co

Photolysis CHL CaCO3

0.4

Dark

0.2 0.0 0

100

A 0.0 150

200

250

Time (min)

Photolysis 0.030 g 0.042 g 0.053 g 0.075 g 0.090 g

0.4 0.2

CaO Ca(OH)2

50

0.6

Dark

C/C0

M. Sánchez-Cantú et al. / Fuel xxx (2016) xxx–xxx

0

50

100

B 150

200

250

Time (min)

Fig. 5. (a) Photoactivity comparison of the Ca-containing catalysis and (b) effect of catalysis dosage in Rh6G photocatalytic degradation.

1.0

10

0.8

0.6

8

TOC

pH

12

0.4

6 0

50

100

150

200

250

300

Time (min) Fig. 6. TOC and temporal evolution of pH values as a function of time during the photocatalytic dye degradation.

3.2.1. Effect of pH A change on the pH value was monitored during Rh6G degradation experiments (see Fig. 6). It was observed that the 5 ppm Rh6G aqueous solution showed an initial pH of 6.5 and when Ca(OH)2 was added the pH value of the suspension increased, reaching a maximum pH = 12.08. To verify if the degradation process was carried out in catalyst surface and not by the produced pH an additional experiment was carried out adjusting the 5 ppm Rh6G aqueous solution with a concentrated NaOH solution until a pH  12.08 was reached. Then, UV irradiation was conducted and the aliquots were analyzed by the procedure described earlier. Our results proved that after 90 min the NaOH-adjusted experiment did not exhibited any pH modification while the Ca(OH)2 suspension presented a pH value of 11.86 in the same period. Moreover, Rh6G degradation was observed after 10 min of irradiation while in the NaOH-adjusted experiment it was noticed after 60 min. From these results we concluded that reaction was initially conducted on the solid’s surface. It was evidenced that a pH decrease occurred during the photocatalytic degradation which is indicative of a mineralization process. Moreover, the pH diminished to 8.24 after 270 min under irradiation and according with TOC results a 61% mineralization was reached. These results are of great importance since Rh6G dye (considered extremely difficult to degrade and mineralize) could be photodegraded by using a cheap, non-toxic and available material. Although semiconductor materials such as TiO2 [20,21], Degussa P-25 and ZnO [22] have been evaluated in the photodegradation of rhodamine dyes reaching a good photocatalytic efficiency (more than 90%), their electron-hole recombination rate, high cost and complex synthesis methods hinder their potential application in industrial processes.

It is known that the pH affects the adsorption properties of organic compounds on the catalyst surface during the photocatalytic process and also represents an important feature for the effective photocatalyitic degradation. Depending of the pH value the surface can bear net negative, positive or possess not charge. In this context, the point zero charge (PZC) is defined as the pH at which the surface has a net neutral charge [23]; however, if the pH is lower than the PZC (basic conditions), the surface positively charged and anions may be absorbed. Under these conditions Rh6G losses a Cl in water and its absorption on Ca(OH)2 surface induces high degradation rate [24] promoted by the amount of dye absorbed. This surface modification has been also reported by other authors, where the ionization state of organic molecules has been modified due to the alkaline conditions that contributes to the formation of OH radicals by the reaction between OH and H+ [25]. Since the principal influence of basic pH is the modification of surface adsorption properties of organic dyes and their ionization state change, the presence of OH groups in Ca(OH)2 support the formation of oxidant species, and by consequence, dye degradation.

3.2.2. Photodegradation mechanism Although several mechanisms have been proposed to explain the photodegradation processes in semiconductors, in materials with different electronic properties such as those evaluated in this work (insulators), such information is limited. For example, in Zn/ Fe layered double hydroxides the photocatalytic process was explained as a semiconductor material in which the generation of the electron hole pairs were responsible of the degradation of methyl violet and malachite green [26]. In the case of Ag-AgI supported in mesoporous alumina two electron transfer process were proposed, from the excited nanoparticles to AgI and from 2+ chlorophenol to Ag nanoparticles, the O 2 and h were the active species in the degradation reaction [27]. In extreme cases, some authors have proposed that insulator materials behave as semiconductors [8–10] explaining their findings in such terms. Due to its wide band gap (5.69 eV), Ca(OH)2 would not be excited by UV irradiation. By consequence, an indirect dye sensitization [28,29] was the proposed mechanism that explains Rh6G degradation where the electrons are injected from excited-states dye bound to the material [30]. Under UV irradiation, Rh6G presents three absorptions bands: the band at 526 nm is assigned to aromatic ring absorption connected by azo groups and is responsible of dye’s color. The other bands, located at 245 nm and 275 nm are assigned to the absorption of benzene and naphthalene like structures present in the Rh6G molecules, respectively [31].

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O2

E g= 5.69 eV

UV photon

eRh6G

Ca(OH)2 Fig. 7. Proposed Rh6G photocatalytic degradation diagram.

The photodegradation of Rh6G is attributed to an electron transfer where the dye acts as sacrificial agent (electron donor), injecting electrons into the conduction band of the insulator [2] that then interacts with the oxygen absorbed in Ca(OH)2 surface (see Fig. 7) to produce superoxide radical (Eq. (2)) [7]. Since the insulator is not exited the degradation process may occur by an oxidation reaction where the superoxide radical induces dye’s degradation. However, a OH radical could be formed in Ca(OH)2 surface which may also contribute to Rh6G dye degradation (Eq. (3)).

e þ O2 ! O 2

ð2Þ

CaðOHÞ2ðaqÞ ! Ca2þ þ 2OH

ð3Þ

4. Conclusions In this work, four calcium-containing non-semiconductor materials were evaluated as catalysts in Rhodamine 6G photodegradation. It was demonstrated that among the evaluated materials, pure calcium hydroxide presented the best photocatalytic results. An indirect dye sensitization mechanism was proposed to explain the photoactivity of our not-semiconductor materials where the irradiated dye is excited and acts as electron donor, injecting electrons in the insulator producing oxidative species with the oxygen adsorbed in Ca(OH)2 surface. These results are of great importance since Rh6G dye (considered extremely difficult to degrade and mineralize) could be photodegraded by using a cheap, non-toxic and available materials which could be used as a sustainable alternative in Advanced Oxidation Process. Acknowledgements We thank BUAP-Vicerrectoría de Investigación y Estudios de Posgrado for the financial support and for A. Galindo y E. Puente scholarships. Furthermore, authors thank BUAP-CUVyTT for the help given in catalysts’ analysis. References [1] Bhatia V, Dhir A. Transition metal doped TiO2 mediated photocatalytic degradation of anti-inflammatory drug under solar irradiations. J Environ Chem Eng 2016;4(1):1267–73. [2] Chen F, Zou W, Qu W, Zhang J. Photocatalytic performance of a visible light TiO2 photocatalyst prepared by a surface chemical modification process. Catal Commun 2009;10:1510–3.

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Please cite this article in press as: Sánchez-Cantú M et al. Calcium-containing materials as alternative catalysts in advanced oxidation process. Fuel (2016), http://dx.doi.org/10.1016/j.fuel.2016.11.092