Hydrothermal synthesis of bismuth vanadate-alumina assisted by microwaves to evaluate the photocatalytic activity in the degradation of methylene Blue

Catalysis Today xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

Hydrothermal synthesis of bismuth vanadate-alumina assisted by microwaves to evaluate the photocatalytic activity in the degradation of methylene Blue ⁎

Rocío Magdalena Sánchez-Alboresa, Bianca Yadira Pérez-Sariñanaa, , C.A. Meza-Avendañob, P.J. Sebastianc, Odín Reyes-Vallejoc, J. Billerman Robles-Ocampoa a b c

Universidad Politécnica de Chiapas, Centro de Investigación y Desarrollo Tecnológico en Energías Renovables, Suchiapa, Chiapas, 29150, Mexico Instituto de Investigación e Innovación en Energías Renovables, IIIER-UNICACH, Tuxtla Gutiérrez, Chiapas, 29039, Mexico Instituto de Energías Renovables UNAM, Materiales Solares, Temixco Morelos, 62580, Mexico

A R T I C LE I N FO

A B S T R A C T

Keywords: BiVO4-Al2O3 photocatalyst Visible light specter Structural characterization Dye degradation

The present study synthesized powders of bismuth vanadate-alumina (BiVO4-Al2O3) by means of the microwaveassisted hydrothermal method. The powders prepared were characterized by X-ray diffraction, scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of the synthesized powders was determined via the oxidative degradation of methylene blue (MB) in aqueous solution under the irradiation of visible light. The effect of parameters such as alumina concentration, synthesis temperature and reaction time was examined in order to determine their influence on crystal size, morphology, photocatalytic activity and the type of crystal structure. The sample prepared under conditions of 160 °C, 60 min and 1.5 wt % Al2O3, presented the highest percentage of degradation and photonic efficient of all samples, which was close to 86% and 4.86 × 10−6 respectively, under the parameters of the experimental design proposed for this research.

1. Introduction Impacting directly on both the economy and health and safety, water is an essential natural resource for human activity. Global water shortages, the growth of the world population and the escalating water pollution crisis is beginning to take its toll in many regions [1,2]. A prominent cause of water pollution is the discharge of untreated textile industry effluent into bodies of water. As untreated textile effluent contains organic dyes such as methylene blue (MB), which can cause severe damage to fertile agricultural land, fisheries and human health [3], it is essential that dyes are removed before being discharged into surface water bodies. Heterogeneous photocatalysis has, in recent years, become one of the most efficient and clean alternative technological solutions for removing organic pollutants (with low biodegradability) from wastewater [4]. Heterogeneous photocatalysis is a process based on the use of a solid semiconductor (usually broadband) capable of directly or indirectly absorbing radiant energy (visible or UV) equal to or greater than its band of forbidden energy. The initial stage of the process consists in the generation of electron-hole pairs in the semiconductor particles, which

⁎

enable the oxidation-reduction reactions to take place on the surface of the photocatalyst [5]. Titanium dioxide (TiO2) has been considered an excellent photocatalyst for the degradation of organic pollutants in air and water due to its non-toxicity and chemical stability [6]. However, it can only absorb ultraviolet light (around 3–5 % of the solar spectrum) and does not absorb visible light, which, due to its large band gap (3.2 eV), greatly limits its efficiency under natural sunlight [7]. In light of the above, several studies have developed photocatalysts, such as bismuth tetroxide (Bi2O3), tungsten trioxide (WO3), Ag3VO4, Bi2WO6 and bismuth vanadate (BiVO4), that present activity in the visible region (400–800 nm) of the solar spectrum [8]. A semiconductor with great interest is the BiVO4 for having properties of interest such as ferroelasticity and ionic conductivity [9]. Recently, the photocatalytic activity of BiVO4 has been studied for the degradation of organic materials [10], organic dyes [11], and for the photocatalytic splitting of the water molecule [12]. The photocatalytic property of the material depends to a great extent on the structure, the BiVO4 has three phases: monoclinic scheelite, tetragonal zircon and tetragonal scheelite [13]. Due to its relatively narrow energy band (2.4 eV), the monoclinic

Corresponding author. E-mail address: [email protected] (B.Y. Pérez-Sariñana).

https://doi.org/10.1016/j.cattod.2019.07.044 Received 17 March 2019; Received in revised form 3 July 2019; Accepted 24 July 2019 0920-5861/ © 2019 Published by Elsevier B.V.

Please cite this article as: Rocío Magdalena Sánchez-Albores, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.044

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

form exhibits the greater photocatalytic activity than the other phases for the chemical reactions induced by the irradiation of visible light [14]. However, the activity of pure BiVO4 needs to be improved, due to its high recombination of the hole-electron pairs and its low absorption capacity [15], recent studies show that aluminum is an excellent dopant in photocatalysts due to its coefficient of stable thermal expansion and physical properties. Yao et al. theoretically, predicted that BiVO4 doped with Al would exhibit a better photoactivity for the division of the water molecule and the H2 evolution, than the pure BiVO4 [16]. However, this has never been supported by experiments. In addition, the morphology, the surface area and the size of the particle directly influence the photocatalytic activity, so there is a growing interest in the controlled synthesis of nanostructures of BiVO4 by different routes. [17]. Unfortunately, the most reported methods have hard experimental conditions such as solid state or high temperature fusion reaction, as well as longer reaction time, which dramatically limit the large scale production of BiVO4. Therefore, it is desirable to develop an easy and environmentally friendly solution for a synthesis route. Recently, the technique of microwave-assisted heating is a simple and economical method compared to the conventional method of heating [18]. Microwave irradiation provides great advantages such as uniformity, rapid dielectric heating without the effects of thermal gradients [19], promotes reaction speed and increase yields [20]. This article reports the synthesis of BiVO4 catalysts with aluminum oxide at different concentrations by the hydrothermal assisted microwave method, as well as the effects of the concentration of Al on the morphology of BiVO4 and its photocatalytic evaluation on the degradation of the organic dye of methylene blue.

Fig. 1. Molecular structure of methylene blue.

carried out by X-ray diffraction using an Ultima IV RIGAKU X-ray diffractometer, with Cu Kα radiation (λ = 0.15406 nm) applied in the 2Ө interval between 10° and 70°, at 40 kV voltage and 44 mA current. The morphologies of the powders were observed with a scanning electron microscope (SEM) used with a Hitachi S-5500 field emission instrument at 3 kV. Diffuse reflectance spectroscopy was measured using a Shimadzu UV-3600 spectrophotometer with an integrating sphere (Shimadzu ISR-3100). 2.3. Photocatalytic degradation of methylene blue (MB) Methylene blue solution was prepared at a concentration of 10 ppm. 0.03 mM of methylene blue powder (≥98.5%) was aggregated in 200 mL deionized water. Finally, the solution was sonicated for 15 min. The photocatalytic activities of the samples, were evaluated via photocatalysis in the discoloration of the MB solution (Fig. 1) under visible light irradiation. A 25 W fluorescent lamp, located 10 cm away from the reactor, was used as a light source. In each experiment, 0.2 g of photocatalyst was added to 200 mL of MB solution (10 mg/L), which, prior to irradiation, was subjected to sonication for 15 min in an ultrasonic bath in order to obtain a homogeneous solution. Finally, magnetically stirred for 30 min in the dark in order to attain adsorption/desorption equilibrium. Aliquots of 3 mL were subsequently taken every 30 min for 6 h. The samples corresponding to the dye were centrifuged for 10 min in order to separate the photocatalyst from the solution. The centrifuged MB solution was analyzed by UV spectroscopy at a wavelength of 664 nm, which is the maximum absorption wavelength of the dye [21]. The percentage of methylene blue degradation was calculated by means of Eq. (1) [22]:

2. Experimental 2.1. Synthesis of BiVO4-Al2O3 BiVO4-Al2O3 powders were synthesized via the microwave-assisted hydrothermal method using an Anton Paar Synthos 3000 reactor, which enables control over both reaction time and temperature and stirring speed. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 98%, SigmaAldrich) and ammonium metavanadate (NH3VO4, 99%, Sigma-Aldrich) were used as bismuth and vanadium precursors, respectively. To prepare the bismuth nitrate pentahydrate solution, 5 mM of Bi(NO3)3·5H2O was dissolved in 50 mL of 2 M HNO3 at 60 °C under stirring, while ammonium metavanadate solution was prepared by dissolving 5 mM of NH4VO3 in 50 mL of 2 M NH4OH at 60 °C under agitation. The Bi(NO3)3·5H2O solution was added to the NH4VO3 solution, forming a yellow suspension, to which different Al2O3 fractions were added (0.6, 1.5, 2.4, 3% wt) after stirring for 20 min, with the pH adjusted to 8.5 using NaOH. The final solution was stirred for 20 min and placed in the microwave, under maximum stirring conditions with a 5 min and 600-W heating ramp. Temperatures of 120, 140, 160 and 180 °C and reaction times of 10, 30, 60 and 90 min were tested in accordance with the proposed experimental design comprised a central composite design rotatable, in order to evaluate the effects of the variables involved (alumina concentration, temperature and reaction time) on the response variable (percentage of MB removal). The resulting precipitate was alternately washed, via centrifugation, with deionized water and ethanol and then dried at 60 °C for 12 h. The samples selected for the optimum morphological and structural characterizations are as follows: a) 160 °C - 1.5 wt % Al2O3 - 10 min; b) 160 °C - 1.5 wt % Al2O3 - 60 min; c) 160 °C - 1.5 wt % Al2O3 - 90 min; d) 160 °C - 0.0 wt % Al2O3 - 60 min; e) 180 °C - 0.6 wt % Al2O3 - 30 min; and, f) 180 °C - 2.4 wt % Al2O3 - 30 min.

% degradation =

Ci − Cf Ci

x 100

(1)

Where, Ci corresponds to the initial concentration and Cf corresponds to the final concentration. 2.4. Statistical analysis Contemporary research on photocatalysis has focused on changing the properties of the material in order to improve photocatalytic activity, while little attention has been paid to optimizing the synthesis conditions of the material in order to improve photocatalytic activity. This has led to the use of statistical tools that enable the variables involved in the process to be correlated and ensure higher performance levels, although, given the diversity of the variables, it is difficult to establish a simple model that predicts the behavior of this type of reaction [23]. However, it is possible to optimize the variables involved through the use of a statistical experimental design that applies response surface methodology (RSM), which has proved to be a powerful statistical technique for obtaining optimal conditions for oxidation processes and evaluating the interactions of the parameters that influence each other, by means of a limited number of experiments [24]. Therefore, the present study applied RSM in order to identify the interaction among the variables (alumina concentration, temperature and

2.2. Material characterization The structural characterization of the synthesized materials was 2

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

Fig. 3. (a) UV–vis absorption spectra of the BiVO4 samples synthesized with different Al2O3 contents.

Fig. 2. XRD patterns of pure BiVO4 and BiVO4-Al2O3 with different Al2O3 contents.

aluminum in the lattice modifying the structure of the material [16]. At long reaction times, cause the unstable structure to disappear, leaving only the monoclinic structure.

reaction time) and establish the optimal conditions of the synthesis process. 3. Results and discussion

3.2. UV–vis analysis 3.1. X-ray diffraction analysis Fig. 3 shows the optical absorption spectra of the synthesized samples; the optical properties of the nanoparticles are determined by their band energy. Diffuse reflectance was analyzed in order to determine the energy gap, which was calculated using the Kubelka-Munk equation (Eq. (2)) [26].

The X-ray diffraction analysis (Fig. 2) revealed that the samples synthesized at 10 and 60 min (Samples a and b) present diffraction peaks corresponding to the tetragonal phase, which are located at 2Θ = 18.23, 24.32, 34.82 and 68.53, corresponding to (1 0 1), (2 0 0), (1 1 2) and (2 2 4) planes respectively, according to the PDF -01-0831812 pattern. In addition, the analysis identified diffraction peaks corresponding to the monoclinic phase located at 2Θ = 28.89, 30.72, 42.57 and 48.59, corresponding to (0 1 3), (0 0 4), (1 0 5) and (3 1 2) planes respectively, according to the PDF-01-074 -4894 pattern, which leads to the conclusion that there is a mixture of both phases. The increasing intensity of the diffraction of the plane (2 0 0) indicates a preferential orientation in this direction, while an improvement in the crystallinity of the samples is also observed at 60 min. However, only one structural phase is observed at 90 min (Sample C). The peaks are indexed to the monoclinic phase with the exception of the planes located at 2Θ = 27.22 (111), 31.56 (200), which are indexed to the aluminum oxide II PDF-01-075-0278. It should be noted that peaks were indexed to aluminum oxide only in this sample, as opposed to the other diffractograms, as the alumina added at the other concentrations is not reflected in the presence of new intensity peaks, which could be an indication that the material is either in the structure or cannot be identified due to the low amounts of alumina. The successive crystalline transitions (monoclinic-tetragonalmonoclinic), identified as the synthesis time increases, indicate the low stability of the tetragonal phase [25]. This leads to the formation of a tetragonal phase that exhibits a weakly stable equilibrium state, which, on being subjected to prolonged reaction times, again forms the monoclinic structure, which is the stable phase of the system. It is also observed that the sample which does not contain alumina (Sample d) only presents diffraction peaks that are indexed to the characteristic peaks of the monoclinic phase. This leads to the conclusion that aluminum oxide has a strong influence on the formation of the monoclinictetragonal phases. This is possibly due to the fact that aluminum can substitute one of the two metals present (Bi + 3, V + 5), or form an AlV bond with oxygen vacancies, this distortion has been studied theoretically by Yao et al., where he proposes three models of substitution of

F (R) =

(1 − R)2 2R

(2)

Where R is the diffuse reflectance obtained, once the factor F(R) is calculated, hv is plotted on the abscissa and [F(R) hv]n in the ordinate, where hv is photon energy, and n is an exponent whose value depends on the characteristics of semiconductor transition (n = 2 for a direct transition semiconductor and n = 1/2 for an indirect transition semiconductor). As BiVO4 is a direct transition semiconductor, the value of n is 2 [27]. The sample that does not contain alumina is observed to have a bandwidth of 2.40 eV, which is similar to those reported by Ying Zhou et al. [28] and is a bandwidth generally reported for the monoclinic phase. The sample containing 1.5 wt % of Al2O3 and 60 min (Sample b) presents a band gap of 2.34 eV, which indicates that there is displacement at the absorption border and is similar to those reported by other authors such as Ying Wang et al. [29]. The reduction of the band gap could be due to different parameters, such as dopant, morphology, particles size and defects in the lattice. In addition, could happen a charge transfer due to the substitution of ions between Al and Bi or V, in the lattice, how is mentioned in X-ray. Which could be causing the shift in the band gap [30]. The spectra obtained show that all the samples present a strong level of absorption in the visible region of the electromagnetic spectrum and the UV region thereof. In addition, the spectra show that the powders studied have different wavelengths, in the range of 500–540 nm, of maximum absorption in the visible region. The absorption identified in the visible region enables the materials to show photocatalytic activity under the irradiation of visible light.

3

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

Fig. 4. Micrographs of the synthesized samples: a) 160 °C - 1.5% Al2O3 - 10 min; b) 160 °C - 1.5% Al2O3 - 60 min; c) 160 °C - 1.5% Al2O3 - 90 min; d) 160 °C - 0% Al2O3 - 60 min; e) 180 °C - 0.6% Al2O3 - 30 min; and, f) 180 °C - 2.4% Al2O3 - 30 min.

3.3. Scanning electron microscopy (SEM) In Fig. 4, the morphologies of the particles were studied by scanning electron microscopy. Fig. 4d showing the SEM images of BiVO4 particles, without the addition of alumina, which have rod-like morphologies an average of 7 μm in length and 1 μm in width, while square particles are also observed. When the alumina is incorporated, it is observed that its morphology changes significantly, with particles forming spherical arrangements of approximately 5 μm, as seen in Fig. 4a–c. Similar structures can be seen during the 60 min synthesis of TiO2/BiVO4 at 200 °C via the microwave-assisted hydrothermal method described by Lili Zhang et al [31]. It can be seen that, at long reaction times (90 min), these spherical conglomerates tend to disappear, forming sets with square architectures. Likewise, it is observed that the variation of the percentage of alumina contributes to the rearrangement of the particles into the spherical segregated conglomerates shown in Figs. 3e and f. This type of arrangement can be compared with other studies which found particles with this arrangement, such as the synthesis of BiVO4 described by the author García Pérez et al., which used Pluronic P-123 as a structuring agent by means of the coprecipitation method at 200 °C for 24 h [32].

Fig. 5. Change in methylene blue concentration during the course of photocatalytic degradation under the irradiation of visible light both with and without the presence of BiVO4-Al2O3. [MB]=10 mg/L, VReactor =250 mL y 200 mg of photocatalyst.

3.4. Methylene blue degradation The photocatalytic activity of as prepared a hydrothermal assisted by microwave the BiVO4 was evaluated by measuring the degradation of MB under visible light irradiation. Wang et al. carried out studies with BiVO4/carbon spheres nanocomposites for MB and RhB dyes, observing a better photodegradation of methylene blue [33]. Whereby, MB was considered for degradation. The concentration of MB (Cf) was measured in periods of 30 min in a UV–vis spectrometer. The variation of concentration of MB (C/Co) with function of time is shown in Fig. 5. Fig. 5 inset shows the adsorption-desorption of the photocatalyst in dark, can be observed that after 30 min, not concentration changes, whereby the adsorption stops. The first test is performed in the absence of the photocatalyst and in the presence of visible light to confirm that photocatalyst is crucial for the degradation of methylene blue. The sample that does not contain alumina is found to present the lowest photocatalytic performance (Sample d). This could be due to the morphology of the particles, as they present a rod shape an average of 7 μm in length and 1 μm in width and square architectures. In addition, to the crystal structure observed in X-ray diffraction shows only the presence of the monoclinic phase compared to the sample that has a better

photocatalytic performance (Sample b) that presents morphology of segregated particles in spherical systems and has a combined structure of the monoclinic-tetragonal phases as well as a band gap of 2.34 eV. In samples e and f, the alumina addition percentage used was 0.6 and 2.4, obtaining degradation percentages of 81.74 and 79%, respectively. The photocatalytic activity was observed to be greater at a lower concentration of alumina and to decrease with increased alumina levels, which can be attributed to the fact that, at high metal concentrations, the semiconductor can degenerate due to the high level of energy introduced into the band gap. There is also the possibility that the photoactivity of the semiconductor is affected by the presence of superficial metal ions that interact with the species produced both on the surface and within the solution, which would easily cause recombination of the photogenerated electron-hole pairs [34]. The photocatalytic degradation can be described by a first-order kinetic equation

− ln

4

CO = −kt C

(3)

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

Fig. 7. Calculated valence and conduction band positions of BiVO4-Al2O3. Table 1 Comparison of degradation percentage of present work with other reported materials. Fig. 6. Relationship between ln (Co/C) vs irradiation time during the photocatalytic decomposition of MB in the presence of BiVO4 and BiVO4-Al2O3. Table 3 CCD design matrix with results for methylene blue degradation. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Factor

Response

Temperature (°C)

Alumina concentration (%)

Reaction time (min)

MB Degradation (%)

160 160 160 160 160 160 120 160 180 140 180 180 140 140 140

1.5 1.5 1.5 1.5 1.5 3 1.5 0 2.4 2.4 0.6 0.6 2.4 0.6 0.6

60 60 60 90 10 60 60 60 30 90 30 90 30 30 90

85.21 86.91 85.02 69.46 68.81 59.08 81.68 64.98 79.01 66.54 81.74 58.30 60.30 56.10 66.11

Materials

Methods

Time (min)

% degradación

Ref.

CdS/ZnO TiO2/SiO2-CdS ZnS-CdS ZnO/NiFe2O4 BiVO4-Al2O3

Precipitation method Hydrotermal method Microwave irradiation State solid Hidrotermal microwave irradiation

300 60 360 70 360

71.1 88 73 87 86

[35] [36] [37] [38] Its work

Table 2 First-order rate constants, half-life (t1/2), photonic efficiency and degradation percentages during the MB degradation in the presence of BiVO4 and BiVO4Al2O3.

Sum of square

Degree of freedom

Mean square

Coefficient – F

P value

Model A- Temperature B- Concentration C- Reaction time AB AC BC A2 B2 C2 Residue Total

1633.25 15.63 17.75 201.36 23.81 384.89 9.73 3.22 880.60 664.68 37.94 1671.19

9 1 1 1 1 1 1 1 1 1 5 14

181.47 15.63 17.75 201.36 23.81 384.89 9.73 3.22 880.60 644.68 7.59

23.91 2.06 2.34 26.54 3.14 50.72 1.28 0.42 116.04 84.95

0.001 0.21 0.19 0.004 0.14 0.009 0.31 0.54 0.0001 0.0003

Kapp × (min−1)

a b c d e f

3.3 5.9 3.3 2.8 5.2 4.9

10−3

t½

(min)

208.77 119.09 208.77 244.06 133.29 141.45

Radj

Photonic efficiency Φλ

Degradation (%) of MB in 360 min

0.958 0.989 0.962 0.944 0.977 0.977

2.72 × 10−6 4.86 × 10−6 2.72 × 10−6 2.31 × 10−6 4.28 × 10−6 4.04 × 10−6

68.90 86.90 69.45 64.97 79.00 81.74

that follows the Langmuir-Hinshelwood model presents a linear adjustment with the slope of the line being the apparent constant of first order speed Kapp with a correlation factor R2∼0.967 confirms the pseudo first order kinetics. The speed constants for the BiVO4 without alumina (Sample d) and with 1.5%wt of alumina (Sample b) were 2.08 × 10−3 and 5.9 × 10−3 (min−1) respectively; where the highest value k indicates a faster degradation. In addition, it can also be observed that the half-life time of sample b is 2.04 times smaller than the sample without alumina. The photonic efficiency, Φλ, can be used to describe the rate of decomposed molecules (M) relative to the total rate of photons incident on the reactor (L). Photonic efficiency was calculated from the following equation:

Table 4 ANOVA analysis. Source of variation

Sample

Φλ=

M L

(4)

The number of pollutant molecules (for a volume of 200 mL) decomposed within time t is given by

R2 = 97.73 % R2adj = 93.64 %.

M=(

Where, Co (mg/L) is the initial concentration of MB, C (mg/L) is the measured concentration at an irradiation time, k is apparent kinetic constant (min−1) and t is the irradiation time (min). Fig. 6 shows the plot of ln(C/Co) vs irradiation time, the kinetics

200 )(CO N (1 − e−kt ) 1000

(5)

Where, Co is the pollutant initial concentration (mol/L), N the Avogadro’s number (6.023 × 1023) and k is the apparent first-order reaction constant (s−1). The number of incident photons within time t is 5

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

Fig. 8. (a) Internally studentized residuals versus normal % probability distribution (b) Experimental MB degradation (%) plotted against the predicted values derived from the RSM model.

and E° (OH- / OH •) = 1.55 V) [32] (Fig. 7), it is considered a good photocatalyst as the photogenerated holes are able to react with the H2O molecule absorbed on the surface and produce OH • radicals, which are powerful agent oxidants capable of degrading most organic components. The crystalline structure of the sample with the best photoactivity presents monoclinic and tetragonal phases, wherein both the monoclinic and tetragonal BiVO4 could promote the generation of free electrons (e−) in the conduction band (CB) and holes in the valence band (h+). However, a disadvantage of BiVO4 is the rapid recombination of pair hole-electron. Whereby decrease the activity photocatalytic of material. The alumina plays a role important, acting as a charge transfer catalyst, where the electrons of BiVO4 can be transferred to the alumina, delaying the time electron-hole pair recombination, favoring the formation of highly reactive species for degradation MB [42]. Irradiating the semiconductor particles with visible light (Equation 5) causes the photogenerated hole in the valence band to react with the water absorbed on the surface to form OH• radicals (Eq. (6)), while the electron in the conduction band can reduce the molecular O2 to form a superoxide anion (O2 •) (Eq. (7)) [43,44], as described below:

given by

L=

Pλ hc

(6)

Where P is the light power (W), λ is the wavelength (m), h is the plank´s constant (6.63 × 10−34 Js) and c is the velocity of light in vacuum (3 × 108 ms−1). The equation obtaining a first approximation on the quantum efficiency of the decomposition process. Considering a decomposed time as quick as 1 s, this efficiency corresponds to the so-called “initial photonic efficiency’’ value. It also constitutes a lower limit of the actual quantum yield, based on the assumption that all the incident photons are absorbed by the photocatalyst [39]. The photonic efficiency values are shown in the Table 2 . The improvement in k, half-life time and photonic efficiency, which are directly related to the degradation percent indicates the sample b for 1.5% wt Al2O3 has an improvement in the photocatalytic performance. Table 2 First-order rate constants, half-life time (t1/2), photonic efficiency and degradation percentages during the MB degradation in the presence of BiVO4 and BiVO4-Al2O3 The photocatalytic process in semiconductor materials involves the formation of electrons in the conduction band and the holes in the valence band, catalytic reactions which occur when the photogenerated charges migrate towards the semiconductor surface.

− + BiVO4 + hv → BiVO4 (eCB ) + BiVO4 (hVB )

H2 O + O2 +

I

The band edge positions of the conduction and valence bands of a semiconductor can be determined using a simple approach, where the edge of the conduction and valence bands at the zero point of charge (pHZPC) can be predicted using Eqs. (3 and 4), respectively [40]: (3)

E0VB = X− EC + 0.5Eg

(4)

→

H+

− BiVO4 (eCB )

(5)

OH •

(6)

→ O2•−

(7)

+

This radical (O2 •) is also responsible for the production of the hydroxyl ion and radicals of hydrogen peroxide [45], a mechanism which can be observed in the following reactions:

I The bottom part of the valence band should be more negative than the redox potential H+/H2 (0 V vs NHE). II The upper part of the valence band should be more positive than the redox potential of O2/ H2O (1.23 V).

0 ECB = X− EC − 0.5Eg

+ hVB

O2•− + 2H+ → HO2•

(8)

2HO2• → O2 + H2 O2

(9)

H2 O2 + O2•− → O2 + OH− + OH •

(10)

The OH• radicals oxidize the molecules of methylene blue absorbed on the surface of the semiconductor and are highly reactive due to the fact that they have a higher bond energy than the bond energies of CC, CN, CH, CO, OH and NH present in organic compounds; therefore, the OH• is able to easily break these bonds [46]. Finally, the active species + (OH • + O2•− + hVB ) can degrade methylene blue such as CO2, H2O and inorganic ions [47].

Where, X is the absolute electronegativity of the 6.035 [41] semiconductor for the BiVO4, EC is the electron free energy in the 4.5 eV hydrogen scale, and Eg is the band gap of the semiconductor (Tables 3 and 4). Calculating the sample with the highest photocatalytic performance (Sample B) revealed that the conduction edge was 0.365 and the edge of the valence band was 2.705 eV. Given that the edge of the valence band is more positive than the oxidation potential of H20 and OH• (E° (H2O / OH•) = 2.38 V vs NHE

+ MB + (OH • + O2•− + hVB ) → CO2 + H2 0 + R•

(11)

3.5. Response surface model analysis Response surface methodology (RSM) was introduced by Box and Wilson in 1951 and further developed by Box and Hunter in 1987 [48] in an effort to make experimental applications more efficient. This 6

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

Fisher’s exact test showed a higher value compared to the critical value, which indicates that the model is highly significant, with a significance value of 0.05. The fit of the model is verified by the coefficient of determination, R2, a value which must be higher than 70%, which is the minimum value suggested for optimization purposes [49]. The result obtained via the polynomial regression equation (Eq. (12)), according to the factors selected for calculating the degradation percentage, was the following:

% Deg = 96.1377 + 1.6412A + 54.50149B + 3.6355C − 0.1219AB − 0.0147AC − 0.0519BC − 0.0015A2 − 11.1214B2 − 0.0115C 2

(12)

Where, A = Reaction temperature, B = Alumina concentration, and C = Reaction time. The purpose of the model represented by Eq. 12 was to reproduce the behavior of the independent variables (factors A, B, and C) compared to the dependent variable (percentage of MB degradation). Fig. 8a shows a minimal difference between the assumptions, with the errors distributed normally and independently, while the variance of the error is homogeneous. The ANOVA reveals an R2 = 0.9773, which adequately explains the real relationship between the response and the variables, when the experimentally measured values are compared with the predicted responses for MB elimination efficiency. This shows a satisfactory correlation between the experimental and predicted values, while the standard deviation between them was minimal, as seen in Fig. 8b. After carrying out all the experiments suggested via Central Composite Design (CCD), the response surface is analyzed in order to investigate the effects of the variables and find optimal conditions for the degradation of methylene blue. The response surface analysis helped to identify the type of interactions among the selected variables. Fig. 9a–c show the effects of each study variable on the surface generated for the degradation of MB (the percentage of degradation was evaluated based on the change in coloration). Fig. 9a the red region represent the combination of time and alumina concentration for having a higher percentage of degradation, is observed than was reached a removal close to 86% of methylene blue with a concentration of alumina of 1.5 wt% and a reaction time of 60 min, as the time and the amount of alumina increases the percentage of removal begins to decrease Fig. 9b shows the response surface graph for alumina concentration and temperature, revealing that, at low concentrations of 0.6 wt% and high concentrations of 2.4 wt% and low temperatures corresponding to 140 °C, there is a decrease in dye removal. However, a concentration of 1.5 wt % and temperatures of 160–180 °C is found to represent the optimal region for obtaining a higher percentage of degradation. Fig. 9c shows the response surface graph for temperature and reaction time, where it is observed that at low concentrations of 0.6 wt % and highs of 2.4 wt % with low temperatures corresponding to 140 °C there is a decrease in the removal of the dye, though, at a concentration of 1.5 wt % and temperatures between 160–180 °C a region with the greater colorant degradation is obtained.

Fig. 9. a) Effect of Al2O3 reaction time and concentration on MB removal, b) Effect of Al2O3 concentration and temperature on MB removal, c) Effect of temperature and reaction time on MB removal.

enables cost reductions in terms of both time and experimentation, which is why RSM is widely applied in the area of photocatalytic photodegradation. Based on the proposed design, Table 1 shows the number of treatments and the corresponding degradation percentage. The experimental design was analyzed using the Design Expert 7.0 software. The response variable (percentage of degradation) showed significant differences among all the treatments (P < 0.05), with runs 1, 2, 3, 7 and 11 presenting the highest percentage of MB removal. With the analysis of variance (ANOVA) at a 95% confidence level (P < 0.05), Table 2 shows the results of the statistical analysis as well as the sum of squares, the mean square, the estimated coefficient, the standard error, the F value, and the P values obtained. In statistics, a model with a value of F (F model = 23.46) and a small P value (< 0.05) is considered significant. The application of

4. Conclusions This research obtained the BiVO4 semiconductor oxide with a monoclinic structure, while materials with monoclinic-tetragonal structures were obtained in the presence of Al2O3. A strong influence on the crystalline structure was observed in the photodegradation process, as only the samples presenting the monoclinic phase obtained a low yield, while the samples presenting monoclinic-tetragonal phases show the greatest level of dye degradation. The optical spectra obtained showed that all the synthesized powders presented a high level of absorption in the visible region of the electromagnetic spectrum, leading 7

Catalysis Today xxx (xxxx) xxx–xxx

R.M. Sánchez-Albores, et al.

to the conclusion that the materials are able to show photocatalytic activity under the irradiation of visible light. The statistical models and the range of study variables (temperature, reaction time and Al2O3 concentration) used in this investigation enable the definition of the most favorable conditions for synthesizing the materials, namely BiVO4-Al2O3. Due to these conditions involved a hydrothermal microwave-assisted approach with a semi-spherical morphology and a high level of photocatalytic activity and obtained the degradation of the model pollutant, methylene blue, in a concentration range of 1.5–2 wt % Al2O3, in times of 50–60 min and at temperatures ranging from 160 to 180 °C.

S. Phanichphant, J. Chem, App. Mater. Today. 1 (2015) 67–73. [19] A. de Hoz, A.D. Ortiz, A. Moreno, Chem. Soc. Rev. 34 (2005) 164–178. [20] O.R. Vallejo, M. Sanchez, M. Pal, R. Espinal, J. Llorca, P.J. Sebastian, Mater. Res. Express 3 (2016) 125010–125017. [21] A.K. Kudo, K. Omori, H. Kato, J. Am. Chem. Soc. 121 (1999) 11459–11467. [22] B.X. Lei, L.L. Zeng, P. Zhang, Adv. Powder Techonology 25 (2014) 946–951. [23] H. Ghasemi, M. Arabieh, A. Malekinejad, T. Yusefi, Energy Environ. Focus. 5 (2016) 187–194. [24] C. Song, X. Li, L. Wang, W. Shi, Sci. Rep. 6 (2016) article 2. [25] S. Tokunaga, H. Kato, A. Kudo, Chem. Mater. 13 (2001) 4624–4628. [26] R. Lopez, R.J. Gomez, J. Sol-Gel. Sci.Technol. 61 (2012) 1–7. [27] Y. Li, J. Zhu, H. Chu, J. Wei, F. Liu, M. Lu, J. Tang, B. Zhang, J. Yao, Z. Huo, L. Hu, S. Dai, Sci-China. Chem 58 (2015) 1489–1493. [28] Y. Zhou, K. Vuille, A. Heel, B. Probast, R. Kontic, G.R. Patzke, App. Catal. A. General 375 (2010) 140–146. [29] Y. Wang, F. Liu, Y. Hua, C. Wang, X. Zhao, X. Liu, H. Li, J. Colloid Interface Sci. 483 (2016) 307–313. [30] Majid Farahmandjou, Sara Motaghi, Opt. Commun. 441 (2019) 1–7. [31] L. Zhang, G. Tan, S. Wei, H. Ren, A. Xia, Y. Luo, Interceram - Int. Ceram. Rev. 39 (2013) 8597–8604. [32] U.M. Garcia-Perez, S. Sepulveda-Guzman, A. Martinez-de la Cruz, J. Peral, Inter. J. Electrochem.Sci 7 (2012) 9622–9632. [33] Xiaotong Wang, Jianqing Zhou, Shuo Zhao, Xiao Chen, Ying Yu, Appl. Surf. Sci. 453 (2018) 394–404. [34] J. Bi, J. Li, L. Wu, H. Zheng, W. Su, Mater. Research Bulletin 47 (2012) 850–855. [35] S.G. Kumar, K.S.R.K. Rao, RSC Adv. 5 (2015) 3306–3351. [36] P.G.C. Pragathiswaran, J. Mater. Sci. Mater. Electron. 28 (2016) 5063–5069. [37] Nayereh Soltani, Elias Saion, Mohd Zobir Hussein, Maryam Erfani, Alam Abedini, Ghazaleh Bahmanrokh, Manizheh Navasery, Parisa Vaziri, Int. J. Mol. Sci. 13 (2012) 12242–12258. [38] J.T. Adeleke, T. Theivasanthi, M. Thiruppathi, M. Swaminathan, T. Akomolafe, A.B. Alabi, Appl. Surf. Sci. 455 (2018) 195–200. [39] D.S. Tsoukleris, A.I. Kontos, P. Aloupogiannis, P. Falaras, Catal. Today 124 (2007) 110–117. [40] T. Saison, N. Chemin, C. Chaneac, O. Durupthy, L. Mariey, F. Mauyge, V. Brezoba, J. Pierre-Jolivet, J. Phys. Chem. C. 119 (2015) 12967–12977. [41] N. Wetchakun, S. Chaiwichain, B. Inceesunguorn, K. Pingmuang, S. Phanichphant, A.I. Minnet, J. Chen, ACS. Apll. Mater. and Interfaces 4 (2012) 3718–3723. [42] Claudia M. Gómez, Esthela Ramos-Ramírez Gloria Del Angel, Israel RangelVázquez, Federico Gnzález, Alma Arrieta, Armando Vázquez-Zavala, Adriana Bonilla-Sánchez, M. Sánchez Cantú, J. Chem. Technol. Biotechnol. 91 (2016) 2211–2220. [43] V. Sivakumar, R. Suresh, K. Giribabu, V. Narayanan, Cogent Chem. 1 (2015) 1–10. [44] R. Venkatesan, S. Velumani, K. Ordon, M. Makowska-Janusik, G. Corbel, A. Kassib, Mater. Chem. Phys. 205 (2018) 325–333. [45] Hinda Lachheb AmmarHouas, Mohamed Ksibi, Elimame Elaloui, Chantal Guillard, Jean-Marie Herrmann, Appl. Catal. B-Environ. 31 (2001) 145–157. [46] A. Sivakumas, B. Murugesan, A. Loganathan, P. Sivakumar, J. Taiwan of Chem. Engineers 45 (2014) 2300–2306. [47] S.K. Lakhera, H.Y. Hafeez, R. Venkataramana, P. Veluswamy, H. Choi, B. Neppolian, Appl. Surf. Sci. 487 (2019) 1289–1300. [48] R.H. Myers, D.C. Montgomery, C.M. Anderson, Response Surface Methology, John Wiley, 2009 8. [49] H. Gutierrez-Pulido, R. de la Vara-Salazar, Analysis and Desing of Experiments, Mc Graw Hill, 2008 402.

Acknowledgement To Mr. José Campos and Mr. Rogelio Moran for the morphological characterization and Ms. María Luisa Ramón and Ms. Edith Ponce for the XRD characterization, and to the National Council of Science and Technology for the financing of this project with grant number 423693 and Registration No. 590851. References [1] V.I. Merupo, S. Velumani, M. Bizarro, A. Kassiba, Mexico City, 28–30 October12th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE)2015, 12th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE) (2015). [2] J.C. Medina, M. Bizarro, P. Silva, M. Giorcelli, Thin Solid Films 612 (2016), pp. 72–81. [3] H. Hassena, Int. J. Mod. Chem. Appl. Sci. 4 (2016) 1–5. [4] S. Obregon, G. Colon, J. Molec. Catal A. Chemical 376 (2013) 40–47. [5] R. Abe, Photochemistry Reviews. 11 (2010) 179–209. [6] A. Kudo, J. of the Ceramic Soc. of Japan 109 (2001) 81–88. [7] N. Wetchakun, S. Chainet, S. Phanichphant, K. Wetchakun, Ceram. Int. 41 (2015) 5999–6004. [8] A. Martinez de la Cruz, U.M. Garcia Perez, Mater. Res. Bull. 45 (2010) 135–141. [9] A.R. Lim, K.H. Lee, S.H. Choh, Solid State Commun. 83 (1992) 185–186. [10] T. Li, H. Wang, Y. Zhu, Z. Bian, Adv. Mat. Res. 1073–1076 (2015) 336–339. [11] Y. Shen, M. Huang, Y. Huang, J. Lin, J. Wu, J. Alloys and Compounds 496 (2010) 287–292. [12] G.P. Nagbhushana, G. Nagaraju, G.T. Chandrappa, J. Mater. Chem. A Mater. Energy Sustain. 1 (2013) 388. [13] L. Zhou, W. Wang, L. Zhang, H. Xu, W. Zhu, J. Phys. Chem. A 111 (2007) 13659–13664. [14] Y. Park, K.J. Mcdonald, K. Shin Choi, Chem. Soc. Rev. 42 (2013) 2321–2337. [15] Y. Wang, F. Liu, Y. Hua, C. Wang, X. Zhao, X. Liu, H. Li, J. Coll and Interface Science. 483 (2016) 307–313. [16] S. Yao, K. Ding, Y. Zhang, Theor. Chem. Acc. 127 (2010) 751–757. [17] W. Shi, Y. Yan, X. Yan, Chem. Eng. J. 215–216 (2013) 740–746. [18] K. Pingmuang, A. Nattestad, W. Kangwansupamonicon, G.G. Wallace,

8