Comparison of graphene oxide titania catalysts for their use in photocatalytic ozonation of water contaminants: Application to oxalic acid removal

Journal Pre-proofs Comparison of graphene oxide titania catalysts for their use in photocatalytic ozonation of water contaminants: Application to oxalic acid removal Fernando J. Beltrán, Manuel Checa PII: DOI: Reference:

S1385-8947(19)33337-6 https://doi.org/10.1016/j.cej.2019.123922 CEJ 123922

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

27 September 2019 4 December 2019 22 December 2019

Please cite this article as: F.J. Beltrán, M. Checa, Comparison of graphene oxide titania catalysts for their use in photocatalytic ozonation of water contaminants: Application to oxalic acid removal, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123922

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Comparison of Graphene Oxide Titania Catalysts for their use in Photocatalytic Ozonation of Water Contaminants: Application to oxalic acid removal Fernando J. Beltrán*, Manuel Checa Departamento de Ingeniería Química y Química Física. Instituto Universitario de Investigación del Agua, Cambio Climático y Sostenibilidad. Universidad de Extremadura. 06006 Badajoz. Spain. *[email protected]

Abstract In this work, three catalysts constituted by graphene oxide on titania prepared from different methods for their use in photocatalytic ozonation (PhOz) have been compared through their properties determined from well known characterization techniques (SBET, XRD, Raman, TEM, UVVisDRS, XPS, etc) and efficiency to remove oxalic acid as model end product of ozone processes. Comparison has also been made for transfer ozone demand and RCT and RHO,O3 parameters. By considering the reacting system as one irreversible gas-liquid-solid catalytic reaction, a kinetic study has been developed and activation energies of these reactions calculated for two out of three catalysts. Finally, kinetic modelling was also proposed and solved to calculate TOC and ozone concentrations in the gas leaving the reactor and dissolved in water for the three PhOz processes.

Key words: Photocatalytic ozonation, Graphene Oxide-Titania catalysts, Oxalic acid, water remediation, kinetics

1. Introduction

Photocatalytic oxidation (PhOx) is a well accepted process to remove contaminants from water [1,2]. It requires a semiconductor, the catalyst, and a radiation source with energy high enough to exceed the catalyst band gap and excite electrons from the valence to conduction bands of the catalyst. Created positive holes of the valence band then oxidize adsorbed water pollutants and/or adsorbed hydroxyl groups to generate hydroxyl radicals that become the principal oxidising species [3]. One important inconvenient of PhOx is the electron-hole recombination that inhibits the process. Therefore, an oxidant is also required to trap electrons and avoid this recombination step. So far, titania is the most used catalyst in PhOx due to their many advantages (low cost, no toxicity, no leaching, etc) but its high band gap makes it non active when visible light is an important component, if not the only one, of the radiation source [4]. Present trend in today research concerning PhOx is the use of visible light or visible light emitting diodes (LED) as radiation source that makes the process more environmentally sustainable [5– 8]. Combination of PhOx with ozone which is then called Photocatalytic ozonation, PhOz, is an emerging oxidation process that allows the improvement of contaminant removal because of the synergism effect due to ozone electron trapping at the conduction band of the catalyst or semiconductor [7,9]. This reaction step not only reduces the electron-hole recombination that inhibits the oxidation process but also it increases the formation of hydroxyl radicals, especially when water mineralization is the objective to reach. Although the use of ozone implies consumption of energy, this problem could be solved with the use of solar photovoltaic panels and a system to transform solar in electrical energy [10]. Combination of TiO2 with graphene oxide is another emerging method to obtain catalysts that make TiO2 active under visible light [11–13]. This type of catalysts, GO/TiO2, present lower band gap values because of the high conductivity graphene materials have [14]. In GO/TiO2 composites, new bands of energy between valence and conduction bands of TiO2 allow taking electrons away facilitating the increase of oxidation capacity and making the catalyst active

under visible light [15]. So far, in PhOz three main types of GO/TiO2 catalysts have been prepared to improve the oxidation capacity of the system [12,16,17]. The methods of preparation, that are especified in the supplementary part of this work, are called: Liquid phase deposition (LPD), hydrothermal (HT) and sol-gel (SG) methods. Works where these GO/TiO2 catalysts are applied constitute important contributions to the knowledge of PhOz but a study about comparison of their performances, including activity, estability and kinetics to remove a given compound is missing. Kinetics of PhOz is an aspect of interest that is not usually treated and, in most of cases, it mainly reduces to the determination of pseudo first or zero order rate constants of contaminant removal [9,18–20]. In this work, three GO/TiO2 catalysts have been prepared with the techniques indicated above, characterized and applied to remove a refractory compound: oxalic acid. Oxalic acid was chosen as model compound because it does not react with ozone, does not absorb visible light and its main way of removal from water is by reacting with hydroxyl radicals. Then, direct ozone reaction and direct photolysis contribution can be suppressed in the photocatalytic ozonation kinetic study. Also, oxalic acid is a main end product of advanced oxidation processes of organic water contaminants, especially when ozone is used [21]. The aims of the work were to compare these catalysts about their characterization properties, needs of ozone consumption, generation of free radicals and kinetics.

2. Experimental part 2.1. Materials Graphite (ref.282863), ammonium hexafluorotitanate (ref.204749) titanium (IV) butoxide and isopropanol were obtained from Aldrich; sulphuric acid (95%), ethanol absolute, nitric acid (65%), boric Acid, hydrogen peroxide (33%), oxalic acid 2-hydrate and potassium permanganate from Panreac; chloridric acid (37%) from Fischer Chemical and titanium dioxide P25 from Degussa.

2.2. Catalyst preparation Modified Hummer´s method [12], was selected as preparation method for GO nanosheets and a brief description can be found in the supplementary information.

The three prepared GO/TiO2 catalysts were denominated as Liquid phase deposition (LPD), Hydrothermal (HT) and Sol-Gel (SG) according to the method of preparation. Since the synthesis procedures of the catalysts are well explained in literature, only a brief explanation about their preparation methods are given in the Supplementary section.

2.3. Experimental procedure Experiments were carried out in a tubular glass-made semibatch reactor of 0.5 L of effective volume provided with gas inlet and outlet, temperature and liquid sampling ports. The reactor was placed in a special box-like chamber containing a total of 44 LEDs, as described in a previous work where more details can be found [12]. LEDs emitted irradiance in the range 25–455 W m−2 with a maximum wavelength of 425 nm. Contribution of the UV component emitted (λ < 400 nm) can be estimated between 0.6-0.7%. A schematic view of the reacting system can be found in Figure 1S of supplementary section. In a typical experiment, 0.5 L of a 10 mgL-1 oxalic acid aqueous solution and 0.125 mg of catalyst was placed in the reactor and stirred magnetically for 30 min in absence of light, in order to reach the adsorption equilibrium. Ozone was generated from pure oxygen with a 30/7 Sander laboratory ozonator type. Reaction starts when LED was connected in light involving runs, and ozone-oxygen mixture (gas flow rate: 35 Lh-1, 10 mgL-1 ozone concentration) was bubbled in the reaction media, in ozonation processes. During reaction time, several samples were withdrawn from the reactor, filtered and analysed.

2.4. Catalyst characterization Catalyst structure, morphology and composition were determined by a Talos F200X (TEM/STEM) transmission electron microscope working in different modes (TEM, FEI Company). All samples were mounted on 3 mm and 200 mesh Lacey carbon film copper grids. In transmission mode, high resolution (HRTEM) images were taken at 200 kV with a resolution of 0.25 nm per point. In HRSTEM mode, resolution at 200 kV was 0.16 nm, obtaining High Angle Annular Dark Field (HAADF) images. Spectroscopy X-Ray microanalysis was performed with a super-X windowless EDX system, with 4 SDD detectors (solid angle ≈ 0.7 srad; resolution <136eV Mn-Kα at 10,000 cps; operation voltage: 80-200 kV; X-FEG high bright electron cannon ≥ 7 x 107 A m-2 sr V). Images were acquired with a CMOS Ceta 16M Camera, using Thermo scientific “Velox” software. Composition of the solids were also determined by SEM-EDX, using a QUANTA·D FEG instrument (FEI Company) coupled with an Octane Elect EDX detector and the instrument software “Genesis Spectrum”. X-Ray Diffraction analysis were performed in a powder Bruker D8 Advance XRD diffractometer with a CuKα radiation (λ= 0.1541 nm) and Ge 111 as monochromator. The data were collected in a 2θ range from 20o to 80o at a scan rate of 0.02 s−1 and 1 s per point. A SETSYS Evolution ‐ 16 (TGA, SETARAM – Scientific & Industrial Equipment.) coupled to a mass spectrometer (OmniStarTM – PFEIFFER VACUUM) was used for thermogravimetric analysis. A known weight of the catalyst, 30 mg, was placed in an alumina crucible for TGA–MS analysis and heated at temperatures from 50 to 900 oC at a rate of 10 oC min-1 under a stream of 40 mL min1

of synthetic air in order to measure weight loss, and detect combustion products: H2O and CO2.

An Autosorb iQ2-C Series were used to determine surface areas of the solids, measuring nitrogen adsorption–desorption isotherms obtained at liquid nitrogen temperature, and applying Brunnauer–Emmett–Teller (BET) method. Pretreatment of samples was with 12 h vaccum degassification at 0.1 Pa and 120 °C.

RAMAN analysis was performed in a Thermo Scientific Almega XR dispersive RAMAN spectrometer, equipped with two objectives for sample focus: MPlan 10x BD and MPlan 50x BD. A 630 nm laser beam as radiation source and 1288-92 cm-1 as spectral range fluorescence correction were used, the spot size employed was 2.5 µm. All raman spectra were registered and treated with OMNIC thermo scientific software. FTIR analysis were performed in a Nicolet is10 (Thermo Scientific), in an operation range of 4000-400 cm-1 and a resolution of 4, that involves a data spacing of 0.482 cm-1 and 32 scans. Spectra were registered in a Omnic 3.2v software. UV-vis diffuse reflectance spectra (UVvis-DRS) were collected on a Varian Cary 5000 UV-Vis-NIR spectrophotometer with an integrating sphere. The wavelength range studied was 200-900 nm. The resulting reflectance spectra were transformed into apparent absorption spectra using the Kubelka–Munk function (F(R)). The optical band gap of the materials was determined through the construction of Tauc plots by plotting (F(R)hν)n against E (eV), with n = 1/2 since TiO2 is an indirect semiconductor. The optical band gap was obtained by extrapolating the linear part of this plot to the energy axis. XPS data were recorded on a Kratos AXIS Ultra DLD XPS spectrometer. Samples were successively placed into the vacuum chamber of the spectrometer. Spectra were recorded with monochromatic Al Kα radiation (hν 1486.6 eV) with a selected X-ray power of 150 W. Surface charging effects were compensated by using the kratos coaxial neutralization system. The spectrometer was operated in constant Analyzer Energy mode, with pass energy of 160 eV for low resolution, wide range survey spectra, 20 eV for high resolution and narrow core level spectra. All characterization methods listed have been applied to freshly prepared catalysts while nitrogen adsorption-desorption isotherms, XRD, SEM-EDX, XPS and Raman were also obtained for catalyst used. 2.5. Analytical methods

Oxalic acid concentration was determined by HPLC-DAD. A Hitachi instrument (Elite LaChrom chromatograph) equipped with a Phenomenex C-18 column (5 μm, 150 mm long and 3 mm diameter) was used. An acidified water:acetonitrile solvent mixture (95:5 v/v, 0.1% phosphoric acid) was selected as mobile phase. Operating conditions were 0.4 mL·min−1 isocratic flow and 205 nm as detection wavelength. The detection and quantification limits (DL and QL, respectively) of oxalic acid concentration in water by HPLC were 0.23 and 0.24 mg L -1, respectively. The coefficient of variation (C.V.) of the method was 4.3% (see supplementary part). TOC analysis was performed in a Shimadzu TOC-VSCH analyser. It was observed that concentration of oxalic acid expressed as TOC was similar to experimental determined TOC. This was also seen in a previous work where catalytic ozonation of oxalic acid was studied with an Al2O3/TiO2 catalyst [21]. In the treatment that follows (see Results and Discussion section and Supplementary part) TOC was used to quantify the remaining oxalic acid as organic matter content, that is, mineralization. Detection limit and accuracy of TOC measurements was 0.33 and 0.35 mg L-1. Ozone concentration in the gas was monitored with an Anseros GM-19 ozone detector. Dissolved ozone concentration in water was determined by the Indigo method described by [22], measuring absorbance values at 600 nm in a Hach Lange 2800 Pro spectrophotometer.

3. Results and discussion 3.1. Catalysts characterization A wide range of techniques (see section 2.4), concerning textural, electrical, chemical and structural properties were used for a proper comparison of the catalysts. The main features observed for fresh catalysts can be found in Table 1. As can be expected, preparation of a composite through different methodologies lead to solids with different surface area. In our case, corresponding bare TiO2 for HT, LPD and SG solids exhibited a B.E.T. surface area of 60, 72 and 226 m2g-1, respectively. The effect of GO incorporation, yielded an increment in the surface

area of all solids with the highest one observed in LPD catalyst where an increment of 123 m2g1

was measured. In comparison, GO incorporation to SG and HT solids, only led to increments of

20-30 m2g-1 with respect to their corresponding bare TiO2.

Table 1.

TGA-MS experiments allow discerning the mass losses associated to the combustion of carbonaceous species and structural water desorption, remanent of the synthetic method employed or related to the stability of GO-composite. The patterns obtained for both, bare TiO2 and GO/TiO2 catalysts, are shown in Figure 2S. In a typical TGA-MS experiment, an initial loss of mass associated to desorption of hydratation water from the synthesis is observed. Then, there is a second loss of mass that could be assigned to the combustion of carbonaceous materials or, in this case, to GO. The carbon content as weight percentage that can be assigned to GO is 1.1, 3.0 and 3.2% for SG, LPD and HT catalysts, respectively. These results are coincident with SEMEDX data that show similar trend though slightly higher (2% higher) (see Table 1). In some cases, a peak due to CO2 desorption is observed. This can be due to the presence of strong basic centers in the solid. These centers can trap atmospheric CO2 during the synthesis and can be retained until reaching a temperature needed for desorption that depends on the center strength. Also, the presence of carboxylic groups in GO sheet can explain this peak [23]. The stability of GO along the different synthesis can be followed by RAMAN spectroscopy (Figure 1A). This technique provides information on the chemical environment of carbonaceous materials (Figure 1.A). When these materials are studied in RAMAN, the measure of the bands “D” and “G”, around 1300 and 1600 cm-1 respectively, are critical. As can be found in bibliography, “D” band intensity depends on the disorder degree in the structure meanwhile “G” band is indicative of graphitic like structure [24]. The ratio calculated between the intensity of “D” and “G” bands is a parameter that can be used to compare the oxidation state of the nano

sheet. In our case, the values obtained for each GO-based catalyst were estimated around 1.231.27 and can be assigned to GO nanosheets. Some differences can also be appreciated in FTIR spectra of solids (Figure 1B). Comparing FTIR patterns, it is clear that LPD ad HT solids exhibit more oxygenated organic groups such as COH, COC, C=O, etc, than SG. In the case of SG catalyst, the lower intensity of these bands can be due to the use of 2-propanol during the synthesis, more specifically during the catalyst drying. It is well known that 2-propanol is a reducing agent of this type of groups [25,26].

Figure 1.

In general, the complete FTIR spectra presents some broad bands (Figure 3S), at 1000 cm-1 and in the interval 3000-3500 cm-1, indicative of the presence of water in the solids, despite the drying step during the synthesis, which is consistent with the low-temperature mass-loss observed in TGA experiments (Figure 2S). This latter band can also be associated to O-H stretching, typical of alcohols and carboxylic acid groups (–COOH). These -COOH groups can be easily removed through thermal treatment, explaining the results observed in TGA-MS as a mass-loss associated to a CO2 emission without any remarkable increment in H2O signal. These groups modify the acid-base properties of TiO2 and act as anchor points where compounds of basic nature can be trapped for mineralization. The presence of –COOH groups can also be confirmed by XPS deconvolution of C 1S region (Figure 1.C). In general, three types of XPS bands were observed in all GO/TiO2 composites corresponding to C-C (284.4 eV), C-O (286 eV) and C=O (<288 eV) bonds [27]. In SG and LPD catalysts presence of bands under 284 eV can be assigned to C=C bond of graphene oxide layer. In the case of HT catalyst, the bands are difficult to isolate mainly due to the equipment resolution. Around 286 eV a broad signal assigned to C-O bonds, a combination of both C-OH

(285.7 eV) and C-O-C (286.4 eV) signals can be found. Finally, the presence of signals over 288 eV are indicative of two carbon environment in samples, typical of C=O and COOH groups. Regarding COOH groups, over 288 eV, it is clear that SG catalyst is the solid with the lower signal, followed by LPD and finally HT catalysts. The content of COOH groups follows the order HT>LPD>SG, supporting FTIR results and confirming the hypothesis of COOH groups decomposition as responsible of TGA peak associated to a low or null water mass spectrometer signal. The same order was observed attending to the counts of CO2 detected in each sample. For any material with photocatalytic properties, determination of the optical properties as can be the band-gap energy is mandatory. The analysis through UV-Vis DRS allows the band-gap value be estimated when representing Kubelka–Munk function equation (1) vs Energy (eV).

(1  R)2 F ( R)  2R

(1)

In our case, band-gap value of the catalysts was found to be 3.02-3.12 in bare TiO2 samples and 1.59-2.99 eV in GO/TiO2 composites (Table 1). This decrease of energy means that the prepared catalysts are able to work under wavelength from 414 to 780 nm (Figure 4S). Comparing the catalys studied, LPD attracts much attention due to the low band-gap value observed (1.59 eV). This coiuld be explained as follows: The LPD procedure involves precursors with boron and fluorine elements in their structure that could be incorporated in TiO2 nanocristals as dopants. In fact, bibliography describes both elements as effective dopant agents for TiO2 in photocalysis, increasing TiO2 effectiveness in photocatalysis each one independently [28,29]. Both F and B were detected by XPS in LPD solid, representing 3.6 and 1.3 %, respectively, of the surface. This band-gap reduction of LPD composite could likely be due to a synergic effect between GO, F, B and TiO2 structure but further research is required to confirm it. Crystalline phases of TiO2 in solids would provide valuable information. Figure 2 shows XRD patterns of crystalline phases for the three catalysts.

Figure 2.

As can be seen from this Figure, anatase (main peak at 2θ=25 o, ref. PDF 01-075-8897) was the main phase found in all cases. Other minor peaks of less important phases can also be observed as a consequence of the synthetic method except for GO/TiO2-LPD, which exhibited a 100% of anatase phase. This has sense due to fluorine anion presence in TiO2 structure, that can inhibit the formation of other crystalline phases as Brookite [29]. For GO/TiO2-HT solid, the synthetic method used TiO2-P25 as starting material. The presence of both phases, anatase and rutile (confirmed by the signals at 27o, 36o and 41o 2θ values, corresponding with the planes 110, 101 and 111 respectively, ref. PDF 01-089-8300) indicate that GO incorporation did not affect the structure of the material. Conversely, GO/TiO2-SG catalyst shows some difraction patterns associated to brookite structure (confirmed by the signal at 30o 2θ, corresponding to the diffraction plane 121, ref. PDF 00-016-0617). Despite the same GO was used to prepare the composites, no diffraction peak around 10-15o 2θ values, associated to GO presence, is observed in XRD patterns. This fact could be due to the following reasons: Firstly, as a consequence of the low GO content in the sample the peak is likely strongly smoothed and masked under the signal noise typical to powder samples. Another reason is the reduction or photo-reduction of the GO incorporated in all samples, leading to reduced graphene oxide that exhibits a broad low intensity signal in the same region as anatase 101 plane (25.5o 2θ) and makes difficult to distinguish due to the intensity differences. The same patterns can be used to predict the catalyst particle size, using Scherrer equation, and the crystallinity degree (see Table 1). For particle size purpose, the main diffraction peak: anatase (101), was selected for LPD and SG catalysts and Rutile (110), for the case of HT solid. The particle size observed in both, XRD and TEM, is quite similar (see Table 1). For the HT catalyst, we can observe different particle size depending on the crystalline phase selected, being Rutile the phase that presents the biggest

particles (around 40 nm). As commented above, P25 TiO2 is the starting material for HT synthesis. Concerning P25 TiO2 composition, anatase is the main crystalline domain which supports the results observed by TEM (see Figures 5S to 7S). In terms of nanoparticle distribution, anatase, the main phase in P25 presents an estimated XRD particle size around 20 nm (see Table 1). The heterogeneous distribution observed in TEM images (Figure 7S-A) is a consequence of P25 phase mix, where the particles around 20 nm diameter could be tentatively assigned to anatase phase and the ones around 40 nm could be considered for rutile phase. Attending to the crystalline degree of the solids, this parameter strongly depends on the particle size observed and the presence of impurities in the crystal structure. As expected, the HT catalyst exhibited the highest values because TiO2 employed in the synthesis was P25, a high crystalline nanopowder. For LPD and SG catalysts, the synthesis method leads to a quite low particle size between 3.3 and 4.9 nm, respectively. This directly affects the XRD pattern since the signal to noise ratio is attenuated and the cristallinity degree diminishes. Regarding the influence on phocatalytic ozonation behaviour, different factors should be considered. Firstly, Beltrán et al. [30] demonstrated that a small particle size leads to high catalytic ozone decomposition in water, increasing the amount of HO· radicals available for oxalic acid mineralization. The crystalline degree is also determinant for photocatalytic activity. Literature reports that the crystalline degree increases as a consequence of the thermal treatment of TiO2 and the activity of the material increases until a phase change happens [31]. This last factor should also be considered in our materials, that in all cases, exhibited anatase as main phase, though brookite and rutile were also detected. Rutile phase is present in several publications involving Degussa TiO2-P25, and the synergic effect involving anatase-rutile phases has been demonstrated [32]. This cannot be applied to brookite phase since its presence in the TiO2 structure usually leads to a decay in photocatalytic activity when particle size is too low [33]. In addition, brookite particles appear in a quite low size in the SG catalyst which predicts a lower activity of this solid in PhOz.

According to these comments, different synthesis procedures lead to composites with different properties associated to interactions between GO and TiO2 as expected. The increase in surface area observed in all catalysts is not only due to the presence of GO, in spite that high surface area is one graphene and GO sheets properties. During the synthesis, both HT and LPD catalysts incorporate similar amounts of GO but the increment of surface area observed was much higher with LPD catalyst. This confirms that GO/TiO2 interaction was different depending on the synthesis method. Furthermore, this fact can also justify the band gap value observed for LPD catalyst which drastically decreases compared to that of bare TiO2 not only because of the presence of GO and the small particle size but also because the presence of F and B remanents from the synthesis method. These can act as dopant elements that improve visible light absorption and also could contribute to increment the surface area. Regarding SG catalyst, the presence of brookite could be positive to increase visible light absorption but the small cristal size reduces this advantage. Lower band gap and higher surface of LPD catalyst appoint the LPD synthesis procedure as the most convenient for PhOz application. Also, it should be highlighted the higher presence of surface oxygen groups (SOG), detected by FTIR and XPS, in HT and LPD catalysts. These SOG strongly affect the surface acid-base properties of catalysts and their presence is positive for catalityc ozonations due to the role of hydroxyl groups enhancing HO. radical production from ozone decomposition [34].

3.2. Activity of catalysts In order to compare the activity of the synthesized catalysts a series of experiments to remove oxalic acid from water were carried out. These experiments involved not only PhOz and PhOx runs but also adsorption, direct photolysis, single ozonation and catalytic ozonation. Figure 3, as an example, shows the variation of remaining TOC with reaction time corresponding to Led photolysis, non catalytic and catalytic experiments of oxalic acid, the latter with LPD catalyst.

Figure 3.

It can be seen that adsorption, LED photolysis, single ozonation and ozone photolysis do not lead to any TOC removal after 60 min treatment. This was expected since ozone does not react with oxalic acid [35] and concentration of hydroxyl radicals generated from ozone decomposition, especially at acid pH, is negligible [36]. Ozone does not either absorb light of 425 nm that LEDs emit so that absence of oxalic acid elimination during ozone-visible Led process is also expected. In the case of PhOx and catalytic ozonation, about 14 and 23% TOC removal is observed, respectively. This means that GO activates, though slightly, TiO2 in the presence of visible LED light, that confirms the lower band gap of this catalyst compared to that of TiO2. The catalyst used also slightly activated the ozone decomposition to produce hydroxyl radicals to eliminate oxalic acid. In any case, best results were achieved with PhOz with total TOC consumption after 45 min. Synergism between ozone and photocatalysis and benefficial effect of activated GO/TiO2 LPD significantly improved TOC or oxalic acid removal. Results obtained with the other two catalysts (HT and SG) are shown in Figures 8S and 9S of the supplementary section. Similar behaviour is observed from these figures as far as adsorption, LED photolysis, single ozonation and ozone photolysis are concerned. However, these two catalysts (HT and SG) present poorer activity to decompose ozone since catalytic ozonation hardly has any effect on oxalic acid removal. As commented in the catalyst characterization section, catalyst particle size is a determinant factor for ozone decomposition in water [30]. Among catalysts prepared and tested, LPD catalyst has the lowest particle size (Table 1) and their catalytic results leads to the highest mineralization yields (see Figure 4). SG catalyst exhibited the lowest mineralization in catalytic ozonation which can be explained by assuming the agglomeration of the composite in water, that increases the particle size [37]. In SG catalyst there was a scarce presence of SOG as observed from FTIR results. This means SG catalyst presents a surface of higher hydrophobic

character that likely favours particle agglomeration. Also, comparing PhOz rates, the one observed when LPD catalyst is used is higher than those with HT and SG catalysts. Comparison of the catalytic action of the three tested GO/TiO2 materials can be better observed in Figure 4. Here, it can be seen that the order of catalyst activity is LPD > HT > SG. The behavior of SG catalyst reveals that some sort of deactivation has happened after about 20 min reaction that leads to the inhibition of oxidation. The nanomaterial agglomeration process described previously for this catalyst can explain this sort of inhibition observed. Nevertheless, inhibition due to fouling on the catalyst surface, blocking active centres where oxalic acid reacts, is a problem commonly reported in bibliography [38]. The final performance of LPD and HT catalysts after 1 h treatment is similar though HT catalyst leads to 90% TOC removal and the oxidation rate with LPD catalyst is something higher. Comparison of the three catalysts as far as catalytic ozonation is concerned can be deduced from Figures 3, 8S and 9S but in a more direct way from Figure 10S. From Figure 10S it is seen that LPD catalyst is also the best nanomaterial among the three studied to remove oxalic acid when ozone is also present. TOC removals of 3, 6 and 15% are observed in catalytic ozonation with SG, HT and LPD catalysts, respectively.

Figure 4.

3.3.

Catalyst reusability.

To evaluate the influence of the synthesis method in the stability of the material, a three runs reusability test was performed for each solid in PhOz conditions. After 1 h reaction time, reaction mixture was centrifuged and the solid was washed with ultrapure water previously to the following run. Data of final TOC removal obtained is presented in Figure 5 and Table 2.

Figure 5.

Table 2.

After three runs, activity of LPD and HT solids remains practically constant. In case of SG solid, an activity increment was observed after the first use, reaching similar results to those obtained with the other two catalysts in the second and third runs. This increment could be explained considering the XRD pattern of the solid (Figure 11S.A). In SG material, a light increment in crystallinity degree (69.2 to 74.3 %) and a better definition of the Brookite peak in the used catalyst is observed. This light improvement in cristallitation can justify the activity enhancement, though this effect should be confirmed in further studies. Related to sp2/sp3 Carbon ratio of GO in the catalysts, Raman spectra of the solids do not show any important change respect to the fresh catalysts. Nevertheless, XPS results show some change in peak intensity likely due to a partial photoreduction of GO in all solids (Figure 11S.B and 11S.C). Finally, regarding B.E.T area measurements and Carbon percentage by SEM-EDX, differences between fresh and used catalysts were in the range of experimental error (see Table 2).

3.4. Ozone demand and hydroxyl radical formation Two important parameters in ozonation processes are the consumption of ozone, due to economic reasons, and the concentration of hydroxyl radicals, to speed the oxidation rate of contaminants. In this work, we have first calculated the transfer of ozone dose (TOD) [39], related to the consumtion of ozone, defined as follows:

TOD 

vg Vl

 C t

0

O3 , ga sin



 CO3 , ga sout dt

(2)

where vg and VL are the gas flow rate and liquid reaction volumen, respectively, t the reaction time and CO3gi and CO3go the concentrations of ozone in the feeding and exiting gas, respectively. TOD gives the ozone transfered to the water that can also be considered as the ozone demand since concentration of dissolved ozone is more than one order of magnitude lower than TOD. Figure 12S shows the concentration profiles of ozone in the gas at the reactor inlet and outlet from a given experimental run of PhOz with LPD catalyst. The area of the shaded zone of Figure 12S is the value of the integral of equation (2) after 1 h treatment. Figures 6 and 7 show the variation of TOD with the amount of TOC consumed at different time intervals in catalytic and PhOz, respectively, with the three catalysts used. Firstly, it is seen that for a given consumption of TOC, TOD needed for PhOz is significantly lower than those for catalytic ozonation. For instance, to consume 1 mgL-1 TOC, between 35 to 43 mg/L ozone are needed in PhOz while catalytic ozonation is not able to get more than 0.7 mgL-1 TOC removal with LPD catalyst but with a very high ozone demand of about 100 mgL-1. With the other two catalysts, CatOz hardly achieves 0.05 mgL-1 TOC removal with ozone demands of about 50 mgL-1. If higher TOC removal want to be achieved ozone will be wasted. Focusing on PhOz, from Figure 8, it is seen that LPD and HT catalysts are the most appropriate to reach a given TOC consumption with the lowest ozone transferred demand. Notice that SG catalyst only allows TOC removals lower than 1.25 mgL-1. For TOC removals lower than 1.25 mgL-1, TOD needed in LPD and SG PhOz is similar but reaction time is shorter when LPD is used (see also Figures 3 and 9S).

Figure 6. Figure 7.

Concentration of hydroxyl radicals is the another important parameter in PhOz since the oxidation rate of oxalic acid and many other contaminants, specially ozone refractory

contaminants, is exclusively proportional to the concentration of these free radicals [21]. In this work, concentration of hydroxyl radicals has not been determined but parameters RCT and ROH,O3 [39–41]. Values of these two parameters give an estimation of the hydroxyl radical concentration. Both parameters are deduced from the application of a TOC mass balance that in a semibatch perfectly mixed ozone reactor where PhOz or CatOz is carried out in this work, is as follows:

dTOC  kHOCHOTOC dt

(3)

where kHO and CHO are the rate constant of the reaction between oxalic acid and hydroxyl radicals (kHO=7.7x106 M-1s-1; [42]) and the concentration of these radicals, respectively. Separation of variables and integration for a given time of equation (3), yields:

t  TOC  Ln    k  .  0 CHO dt HO  TOC0 

(4)

By assuming hydroxyl radicals in PhOz or CatOz come from the decomposition of ozone through different mechanisms, RCT definition as reported by Elovitz and von Gunten [40] can be applied, and equation (4) becomes:

t  TOC  Ln    R k  CT HO.  CO 3 dt  TOC0  0

(5)

Equation (5) has then been applied to results of PhOz and CatOz of oxalic acid with the three catalysts used. From the plots of the left hand side of equation (5) against the time of ozone exposure (integral of the right side) values of RCT were obtained. Experimental results fitted

straight lines, as shown in Figures 13S and 14S, though in the case of LPD catalyst experimental results follow two straight linesof different slope. RCT values are presented in Table 3. In any case, RCT values are some order of magnitude higher than others determined from single ozonation processes [40] which also confirms the higher production of hydroxyl radicals with GO/TiO2 catalytic and photocatalytic ozonations.

Table 3.

In this work, the highest RCT values determined corresponded to experiments when LPD catalyst was applied. For CatOz the following order of magnitude was found: LPD > HT > SG while in PhOz, the order was LPD > SG > HT. Notice here that RCT from SG PhOz is higher than when HT catalyst was applied but validity of RCT in the first case is only for the first 20 min reaction, that is, when PhOz SG is effective (see also Figure 4).

ROH,O3 parameter was also determined. This parameter is also an estimation of the concentration of hydroxyl radicals regardless of the ozone concentration applied. For a semibatch ozone reactor, ROH,O3 can be obtained from equation (6) [39] (see section 4.4. of supplementary part):

Ln



TOC   RO3 HO kOH TOD  CO3 TOC0



(6)

but given the negligible value of dissolved ozone concentration (CO3) compared to TOD, equation (6) can be simplified to equation (7):

Ln

TOC   RO3 HO kOH TOD TOC0

(7)

Plots of ln(TOC/TOCo) versus TOD are presented in Figures 8 and 9 for CatOz and PhOz, respectively.

Figure 8.

From these figures It can be observed that experimental points follow straight lines as expected. Known the value of the rate constant of hydroxyl radical-oxalic acid reaction (see above) ROH,O3 values were determined and presented in Table 3. As can be seen from Table 3, R OH,O3 values decrease in the following order depending on the catalyst used: For PhOz: LPD > HT > SG while for CatOz: HT =LPD > SG. Values of ROH,O3 are also some order of magnitude higher than others determined for single ozonation [39,41,43].

Figure 9

3.5. Kinetic study Literature reports two main methods to study the kinetics of photocatalytic oxidation: a rather simple one is used in most of works where apparent kinetics is studied. This method simply determines pseudo first order rate constants for the different oxidation processes [9,19,20]. In a few works, a more rigurous model is followed. This takes into account the radiation transfer equation and needs to solve very complex equations based on numerous parameters that depend on the nature of the catalyst (absorption and scattering coefficients, scattering albedo, optical thickness, etc), geometry of the photoreactor and situation of the radiation source among others [44–46]. In this work an intermediate method has been followed. The kinetics is based on the assumption that an irreversible second order gas-liquid solid catalytic reaction

develops between ozone and oxalic acid (or TOC in this particular case). Accordingly, in the semibatch perfectly mixed photoreactor used in this work, equation (8) represents a TOC mass balance applied to experimental results of PhOz [47,48]:



PO3 dTOC z dt  1  1 1 1  He    ´  kg a  kl a kc ac w  k wTOC 

(8)

where PO3 is the ozone partial pressure at the reactor outlet, He the Henry constant of the water/ozone system, z the stoichiometry of the reaction, kga, kLa and kcac the volumetric mass transfer coefficients of ozone to diffuse through the gas film and the liquid films closed to the gas-water and water –catalyst interfaces, respectively, w the concentration of the catalyst,  the internal effectiveness factor and k’ the catalytic reaction rate constant that depends on the radiation intensity (if present) and nature of catalyst. In the reacting system studied, kga and kcac must have extremely high values since ozone is a rather insoluble gas in water and the catalyst used is in the powder form [47]. Then, first and third terms of the denominator of equation (8) in its right hand side can be removed. These terms are the mass transfer resistances of ozone through the gas film and liquid film closed to the water-catalyst interface. The fourth term of the denominator, which is the resistance due to internal diffusion of ozone and catalytic reaction can also be simplified in this case since the internal effectiveness factor can be taken as 1 because the catalyst is in powder form [47]. By applying these simplifications and after variable separation, rearranging, and integration for a given reaction time, equation (8) reduces to:

ln

 PO TOCi  TOC  TOC  k ' w  z 3  t  ti    TOCi kl a  He 

(9)

where TOC and TOCi represent TOC values at time t and ti, respectively, the latter when ozone partial pressure, PO3, starts to be constant in an ozone process (see for instance Figure 10 where constant PO3 is from about 7 min of reaction). Notice that:

PO3 

CO 3 go RT

(10)

with R and T the gas perfect constant and reaction temperature in Kelvin, respectively. Application of equation (9) requires constant ozone partial pressure at the reactor outlet and constant temperature during the reaction time. In Figure 10, time profiles of TOC, temperature and ozone gas concentration at the reactor outlet are shown corresponding to oxalic acid PhOz with LPD catalyst.

Figure 10.

As it can be seen from Figure 10, from about 7 min reaction, the concentration of ozone in the gas at the reactor outlet does not vary with time but temperature slightly increases from 27 oC up to 33 oC at 45 min when TOC has been completly eliminated. For this reason, the reaction time (7 to 45 min) was divided in three periods through which temperature does not change more than 3.5 oC (see vertical lines in Figure 10 separating these constant temperature periods called T1, T2 and T3). For the experiment of Figure 10 these periods are from 7 to 15 min with temperature nearly constant at 27 oC, a second period from 15 to 26 min where temperature changes from 27 to 29.5 oC (mean temperature: 28.3 oC) and a third period from 26 to 45 min with temperature going from 29.5 to 33 oC (mean temperature: 31.3 oC). In order to apply equation (9), these periods of reaction time were considered of constant temperature, and hence, constant ozone partial pressure (see equation (10)). This procedure was also applied for the case of HT catalyst (see Figure 15S). In the case of SG catalyst, due to the lower TOC

conversión achieved, the valid reaction time interval was from 8 to 15 min with constant temperature of 27 oC (see Figure 16S). Equation (9) was then applied to experimental results obtained during these constant temperature reaction periods. Figure 11 shows as example the application of equation (9) for the case of PhOz with LPD catalyst.

Figure 11.

As It is seen from Figure 11 experimental results of the left side of equation (9) when plotted against the terms in braquets of its right hand side follow straight lines of increasing slope with temperature. Similar behaviour is observed for the case of HT catalyst (see Figure 17S) and also for PhOz with SG catalyst though in this case, only one straight line was obtained corresponding to 27 oC temperature (Figure 18S). (Procedures for calculating parameters needed to check equation (9): Henry constant, kLa and z are shown in the Supplementary Information). From the slopes of straight lines of Figures 11, 17S and 18S obtained from least squares analysis values of rate constants k’ were determined as shown in Table 4. With these values and corresponding temperatures activation energies of ozone photocatalytic reactions with LPD and HT catalysts were calculated by applying Arrhenius plots (see Figure 19S). Activation energies for PhOz LPD and HT were found to be 105,6 and 36.2 kcal mol-1, respectively, which are in the order of magnitude expected for catalytic reactions.

Table 4.

3.6. Kinetic modelling

Finally, in a last step to confirm the validity of rate constants k’ a kinetic model of PhOz process was performed. The model was constituted by mass balance equations of ozone in the gas and water phases and TOC in water. These equations for the semibatch perfectly mixed photoreactor used in this work are:

Ozone gas mass balance:

dCO 3 go dt



 CO 3 go RT  1  mO 3 gi  vg CO 3 go   kL a   CO 3  (11)  (1   )V 1   He 

Ozone water mass balance:

 CO 3 go RT  dCO 3  kL a   CO 3   k 'd CO 3 dt  He 

(12)

TOC mass balance:

dTOC  z dt

CO 3 go RT  1  1 He   ´   kl a k wTOC 

(13)

where  is the liquid holdup (0.98 in this reactor), mO3gi the molar rate of ozone at the reactor inlet and k’d the rate constant that involves all reactions ozone undergoes in water (see Supplementary information). These equations were solved through a modified Euler numerical method [49] as shown in the Supplementary information. The calculated and experimental concentrations of ozone (in gas and water) and TOC are plotted versus reaction time in Figures 12, 20S and 21S for the PhOz with LPD, HT and SG catalysts, respectively. As can be observed from Figure 12 the model perfectly estimates the experimental TOC and ozone concentrations. Similar results were obtained for the case of PhOz with HT and SG catalysts (see Figures 20S and 21S). In this later case, modelling of TOC was good until about 16 min reaction, after which TOC removal rate suddenly decreased. According to these results the kinetic model can be accepted

to simulate PhOz in spite of the numbers of experimental parameters that influence the solution of mass balance equations.

Figure 12.

4. Conclusions In this work three GO/TiO2 catalysts prepared from different methods reported in literature to study GO/TiO2 photocatalytic ozonation of water contaminants have been compared as far as their characterization, activity to remove oxalic acid and kinetics are concerned. Main conclusions are: Regarding characterization properties: Catalyst final properties are strongly dependent on synthesis methodology. Despite SG method was able to obtain Brookite phase, the particles were too small to produce a significant effect in PhOz behaviour and the employment of i-propanol during the synthesis leads to a partialy reduction of incorporated GO. For GO/TiO2-HT, GO interaction with P25, enhances P25 properties as SB.E.T. (from 60 to 93 m2 g-1) or Band gap. GO/TiO2-LPD composite was the catalyst that best combines the properties for PhOz purposes, exhibiting a low TiO2 particle size associated to a large surface area, and adequate interactions between TiO2 and GO nanosheet. Adding to this, remanents F and B induce, with GO presence, a strong narrowing of catalyst band-gap, increasing the effectiveness of the solid in visible photocatalysis. The presence of F and B likely affects the crystalline phases formed during synthesis, inhibiting the formation of other crystalline phases as Brookite. Regarding activity to remove oxalic acid: GO/TiO2-LPD catalyst presented the highest activity among the three catalysts studied. Only 45 min are needed to eliminate from water 10 mgL-1 of oxalic acid (2.67 mgL-1 TOC). A combination

of low particle size and an adequate band-gap value were responsible to improve PhOz efficiency with the use of GO/TiO2-LPD catalyst. GO/TiO2-HT catalyst had some lower activity since it allows 90% oxalic acid or TOC removal in 45 min. Finally, GO/TiO2-SG catalyst presented the lowest activity with 48% TOC removal in 15 min and only 54% after 60 min reaction. Reusability tests indicate that solids are active along 3 complete uses without any substancial change in the catalyst properties. Transfer of ozone dose needed to remove oxalic acid (or TOC) after 30 min reaction were 19, 21 and 27 mgO3 L-1 in PhOz with LPD, HT and SG catalysts, respectively, which also confirm GO/TiO2LPD catalyst as the best catalyst among them. Determination of RCT and ROH,O3 parameters indicated that GO/TiO2-LPD catalyst allows the formation of the highest concentration of hydroxyl radicals. Regarding kinetics and modelling: Rate constants of photocatalytic ozonation were also determined and activation energies were found to be 106 and 46,2 kcal mol-1 for LPD and HT catalysts, respectively. Finally, the kinetic model proposed predicted with accuracy the concentrations of ozone both at the reactor outlet and dissolved in water and TOC. In view of these results, research is ongoing to deepen on the use of GO/TiO2-LPD catalysts to check the viability of treating complex mixtures of contaminants and observe the effect of the water matrix on the PhOz process rate.

Acknowledgement Authors thank Spanish Ministry of Economy and Competitiveness and European Funds for Regional Development for the economic support (Project CTQ2015/64944-R).

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Figure caption list:

Figure 1: RAMAN: D and G bands (A), FTIR (B) and XPS C 1S (C) expanded regions of GO/TiO2 composites.

Figure 2. XRD patters of the GO/TiO2 composites. Detected TiO2 Phases: Anatase ( ), Rutile ( ) and Brookite ( ).

Figure 3. Changes of remaining TOC with time for different experiments: Conditions: COx= 10 mgL-1, w (GO/TiO2-LPD): 0.25gL-1 cat, Radiation intensity: 25.7 Wm-2, CO3gi= 10 mgL-1, vg=35Lh-1. LED ( ), O3+LED ( ), O3 ( ), GO/TiO2–LPD+O2 ( ), GO/TiO2–LPD O2+LED ( ), GO/TiO2–LPD+O3 ( ), GO/TiO2–LPD+O3+LED ( ).

Figure 4. Comparison results for TOC removal with time from ozone involving oxidation processes of oxalic acid with the three tested catalysts. Conditions as in Figures 3, 7S and 8S. O3+LED ( ), O3 ( ), GO/TiO2–SG+O3 ( ), GO/TiO2–HT+O3 ( ),GO/TiO2–LPD+O3 ( ).

Figure 5. TOC removal achieved for each catalyst in different reuses after 60 min of reaction. ( ) GO/TiO2-LPD, (

) GO/TiO2-HT; (

) GO/TiO2-SG. Reaction conditions as described in Figure 3

for PhOz.

Figure 6. Changes of TOD with eliminated TOC during catalytic ozonation of oxalic acid. Conditions as in Figures 3, 7S and 8S. GO/TiO2–SG+O3 ( ), GO/TiO2–HT+O3 ( ),GO/TiO2–LPD+O3 ( ).

Figure 7. Changes of TOD with eliminated TOC during photocatalytic ozonation of oxalic acid. Conditions as in Figures 3, 7S and 8S. GO/TiO2–SG+O3+LED ( ), GO/TiO2–HT+O3+LED ( ), GO/TiO2–LPD+O3 +LED ( ).

Figure 8. Determination of ROH,O3 for catalytic ozonation of oxalic acid. Conditions as in Figures 3, 8S and 9S. GO/TiO2–SG+O3 ( ), GO/TiO2–HT+O3 ( ), GO/TiO2–LPD+O3 ( ).

Figure 9. Determination of ROH,O3 for photocatalytic ozonation of oxalic acid. Conditions as in Figures 3, 8S and 9S. GO/TiO2–SG+O3+LED ( ), GO/TiO2–HT+O3+LED ( ), GO/TiO2–LPD+O3 +LED ( ).

Figure 10. Time profiles of TOC, temperature and ozone gas concentration at the reactor outlet corresponding to photocatalytic ozonation of oxalic acid with LPD catalyst. Conditions as in Figure 3. CO3go ( ), TOC ( ) and Temperature ( ).

Figure 11. Checking equation (9) for rate constant determination in photocatalytic ozonation of oxalic acid with LPD catalyst. 27 oC ( ), 28.3 oC ( ) and 31.3 oC ( ).

Figure 12. Variation with time of experimental and calculated (from kindetic model) TOC and ozone concentrations in the gas leaving the reactor and dissolved in water during PhOz with LPD catalyst. Conditions as in Figure 4.

Table caption list:

Table 1. Main properties of synthesized catalysts Table 2. Textural properties of the reused catalysts. Table 3. Values of RCT and ROH,O3 for catalytic and photocatalytic ozonation Table 4. Values of k’ determined from checking kinetic equation (9)

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Table 1. Main properties of synthesized catalysts SEM-EDX

TGA

S B.E.T.

Band-gap

Raman

Cristallinity degree

Particle size

Catalyst

%C

%C

m2g-1

eV

ID/IG

%

XRD (nm)

TEM (nm)

GO/TiO2-HT

5.5

3.2

93

2.99

1.23

84.0

24.2 (A) 40.2 (R)

19.0

GO/TiO2-SG

3.2

1.1

245

2.85

1.27

69.2

6.1

4.9

GO/TiO2-LPD

6.5

3.0

195

1.59

1.25

77.1

8.5

3.3

A: Anatase. R: Rutile.

Table 2. Textural properties of the reused catalysts. SEM-EDX

S B.E.T.

Raman

Cristallinity degree

Particle size

Used Catalyst

%C

m2g-1

ID/IG

%

XRD (nm)

GO/TiO2-HT

5.2

87

1.25

85.1

24.2 (A) 37.5 (R)

GO/TiO2-SG

3.4

252

1.22

74.3

6.1 (A) 6.1 (B)

7.3

226

1.32

77.0

8.5 (A)

GO/TiO2-

LPD

A: Anatase. R: Rutile. B: Brookite

Table 3. Values of RCT and ROH,O3 for catalytic and photocatalytic ozonation Catalyst Process ROHO3 (s) RCT -4 CatOz 1.4x10 4.2x10-8 GO/TiO2-LPD PhOz 2.8x10-3 1.8x10-7 -5 CatOz 7.1x10 5.2x10-9 GO/TiO2-HT PhOz 2.8x10-3 2.7x10-8 CatOZ 3.3x10-6 9.2x10-10 GO/TiO2-SG PhOz 8.7x10-4 4.8x10-8

Table 4. Values of k’ determined from checking kinetic equation (9) GO-TiO2-LPD GO-TiO2-HT GO-TiO2-SG 5 a 5 a T, K k’ x 10 T, K k’ x 10 T, K k’ x 105 a 300 7.8 300.5 2.2 300 3.8 301 10.9 302 3.8 304.3 46.7 304 4.9

a

Units of k’: m6(mol∙s∙gcat)-1

Highlights:     

GO/TiO2-LPD lead to better textural and optical properties for PhOz processes Low particle size with narrow band gap were key factors for catalyst activity. In PhOz, values ROH,O3 and TOD were calculated for each composite. Kinetic study allows the determination of photocatalytic rate constants for PhOz. A proposed kinetic model predicts TOC and ozone concentrations with accuracy.