Cr Ternary Composites by Aerosol Flame

Subscriber access provided by University of Otago Library

Article

Stabilization of Cr in Ti/Si/Cr ternary composites by aerosol flame spray-assisted synthesis for visible-light-driven photocatalysis Siva Nagi Reddy Inturi, Thirupathi Boningari, Makram Suidan, and Panagiotis G. Smirniotis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02947 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 5, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Stabilization of Cr in Ti/Si/Cr ternary composites by aerosol flame spray-assisted synthesis for visiblelight-driven photocatalysis Siva Nagi Reddy Inturi,ǂ Thirupathi Boningari,ǂ Makram Suidan,*† Panagiotis G. Smirniotis*ǂ ǂ Chemical Engineering Program, College of Engineering & Applied Science , University of Cincinnati, Cincinnati, OH 45221-0012, USA.

† Engineering College and Architecture, American University of Beirut, Beirut1107-2020, Lebanon.

KEYWORDS: Flame Spray Pyrolysis (FSP), SiO2-TiO2-Cr, Photocatalytic degradation, Liquid Phase, Stability.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 47

ABSTRACT We have successfully stabilized Cr leaching in Ti/Cr/Si ternary composites by adopting Flame Spray Pyrolysis (FSP) synthesis technique and to achieve higher photocatalytic activity in visible light region. Initially, we have prepared a series of catalysts with different Si/Ti atomic ratios (80, 75, 70, 60 and 50) as well as (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30) and investigated their visible light photocatalytic activity. The effect of the Si and Cr atoms on the structural, optical, and photocatalytic properties of TiO2 has been investigated. Our XRD studies reveal that the (Si+Ti)/Cr ratios and Si/Ti atomic ratios alter the TiO2 lattice structure, with increase in the Si ratio in the catalyst reduction of rutile phase was observed. Increase in Si/Ti atomic ratio from 50 to 75 monotonically transformed the rutile phase into anatase phase. Introduction of Cr induced a substantial amount of disorder into the TiO2 lattice and altered the crystallinity. High amount of Cr in the catalyst led to the transition from the tetrahedral coordination of CrO3 to hexagonal coordination of Cr2O3 oxide implies the segregation of chromium out of the TiO2 lattice. Dropping the Cr amount (increase in (Si+Ti)/Cr atomic ratio) to within solubility limits would solve the difficulties accompanying with Cr2O3 segregation out of TiO2. The addition of SiO2 into the titania lattice greatly promoted the surface texture by enhancing the surface area by four times to the commercial TiO2. The introduction of Cr extended the visible light absorption of catalyst to 550 nm. Our H2-TPR results suggest that the increase in Si/Ti atomic ratio greatly promoted the suppression of Cr2O3 phase in the catalyst along with an increase in reduction potential of Cr6+. It is found in our studies that Cr has a imperative role in the photocatalytic activity while the SiO2 was found to be responsible for the stability of the photocatalyst. We have observed that the optimal flame-made Cr/Ti/Si catalyst demonstrated extremely low level of Cr and Ti leaching throughout photocatalytic degradation reaction, therefore inhibiting the contamination of the treated waste water due to secondary metal leaching. We observed a stable


2

Page 3 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr (IV) concentration for the reused catalyst, which play an important role in improved and stable photocatalytic activity in visible region for the materials. Unlike the Cr/TiO2 systems, optimal flame-made Cr/Ti/Si catalyst established high efficiency in visible photocatalysis, stability and reusability without any Cr leaching even after five consecutive cycles.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 47

Introduction The use of semiconductor for the heterogeneous photocatalytic degradation of organic contaminants in waste water and air streams had gained significant attention for its low toxicity, high economy, environmental restoration and their high efficiency.1–4 Among various semiconductors, TiO2 is the well-studied photocatalyst due to its abundant obtainability, ease of use, non-toxic nature, non- corrosive nature, chemical and physical stability. The band gap of TiO2 is about 3.2 eV (3.0 eV for rutile and 3.2 eV for anatase), which allows TiO2 be activated by only UV light resulting in lower photoquantum efficiencies.5, 6 This relatively higher band gap (∼3.2 eV) and the high frequency of recombination of the photogenerated hole-pair and electron impairs its real-world applications.7-8 One of the methods widely applied for improving the activity of TiO2 under visible region is by surface modification or incorporation of TiO2 with transition metal/metal oxides.9-10 The incorporation of transition metal (Fe, Cr, V, Mn, Cu, Ce, Co, W, etc.) oxides into the TiO2 lattice structure will improve the visible light absorption of the materials due to the formation of intermediate energy levels between conduction and valence energy bands. The transition metal dopants shown that they reduce the recombination of the photogenerated charge as the metal/metal oxide sites are observed to accept the electrons generated from the TiO2 valance band by acting as a trapping site .10-15 In our previous work and from literature studies of transition metal incorporated TiO2,16-17 we have observed Cr had superior visible light photocatalytic activity compared to other transition metal modified TiO2.13-23 The photocatalytic activity of Cr/TiO2 for the degradation of the organic pollutants under visible region has drawn a significant interest for further studies in visible light photocatalytic degradation of organic contaminants. However, the Cr/TiO2 systems as visible light photocatalysts may lead to Cr leaching in the fluid solution. There is a possibility


4

Page 5 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of reduction of Cr(VI) to Cr(III) hindering the reusability and stability of the photocatalyst.22 There are few studies available for investigation of the metal leaching in the solution and causing the secondary contamination and their effect due to the catalyst preparation process. Furthermost, among these studies frequently observed the deterioration in the photocatalytic efficiency of the catalyst with time on stream analysis and leaching of the Cr ion into fluid stream causing secondary contamination.23 The use of supported titanium dioxide could enhance and stabilize the photo degradation activity when compared with the pure TiO2.24-27 The benefits of using silica supports for the titania in photocatalysis are associated with numerous advantages such as increased adsorption, enhanced electron-abstraction, high surface area of ultrafine titania particles, and lower UV-light scattering.28-31 However, the synthesis method for mesoporous SiO2 supported TiO2 relied highly on expensive equipment and lengthy complicated experimental process, which hinder their wider applications. In our present studies, Flame Spray Pyrolysis (FSP) method was employed for production of the catalyst. FSP method is a single step and economical synthesis process that can be used for bulk synthesis of the catalyst in potential industrial applications. The current work is focused on the investigation of the impact of SiO2 in Cr/TiO2 nanoparticles for the visible light (400-800 nm) photocatalytic degradation of phenol in the liquid stream. The present work aimed at developing a high visible light photocatalytic active catalyst with superior stability of the repeated runs. For this purpose, the catalyst with Si/Ti atomic ratios of 80, 75, 70 60, and 50 were synthesized by keeping the ratio of M/Cr of 20 (where M=Si+Ti). To optimize the ratio of Cr, catalysts with different (Si+Ti)/Cr (optimal Si/Ti) atomic ratios also synthesized and investigated for the visible photocatalytic activity. We also synthesized Cr/TiO2 by flame spray pyrolysis method for comparison studies. To study the


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 47

catalyst stability of the optimal catalyst, we have used the catalyst for five consecutive runs under cyclic operation. The secondary contamination caused by leaching of the Cr and Ti ions was studied for the wide range of pH (3-11For the photocatalytic studies we have taken phenol as the probe molecule. It is often found in various industrial and municipal wastewaters streams.32 A comparative catalytic studies, reusability, and stability studies were performed by altering the silica, titania, and chromium contents in the catalyst. In the present work, we have successfully stabilized Cr leaching in Ti/Cr/Si ternary composites by adopting Flame Spray Pyrolysis (FSP) synthesis technique and attained enhanced visible light photocatalytic activity. The enhanced photocatalytic activity under visible light irradiation, reusability, and stability of our catalyst formulations will definitely advance the development of continuous photocatalytic degradation processes. The developed formulations can be employed for the photocatalytic degradation of organics in industrial applications. These stable catalysts could be immobilized on a surface for the photocatalytic degradation of industrial organic waste and indoor air purification. In addition, the effect of the Si and Cr atoms on the structural, optical, and photocatalytic properties of TiO2 has been investigated by means of various characterization techniques.

2. Experimental 2.1. Materials O-xylene (Sigma–Aldrich Reagent, 99%), titanium-tetraisopropoxide (TTIP, Sigma-Aldrich, purity N97%), acetonitrile (Sigma–Aldrich Reagent), Chromium (III) 2-ethylhexanoate (Strem, 70% in mineral spirits Cr), and phenol were purchased from Sigma-Aldrich. Oxygen (1.5 bar, Wright Brothers, 99.98%) was used as received. Deionized water was obtained from a local


6

Page 7 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


supplier and used in the whole experiment. All of the chemicals were of analytical grade and used without further purification. 2.2. Synthesis of the photocatalysts using Flame Spray Pyrolysis synthesis technique The single step liquid flame spray pyrolysis was employed to synthesis a series of Si/Ti/Cr photocatalyst. The elaborate description of the catalyst synthesis process by FSP was given elsewhere.33-36 Briefly, the solvent solution containing a mixture of 1:3 volumetric ratio of acetonitrile: o-xylene is used All the catalysts synthesized in the work have maintained uniform total metal molar concentration i.e (Si molarity + Ti molarity + Cr molarity should be equal to 0.3 M. For Cr/TiO2 Chromium (III) 2-ethylhexanoate and TTIP was mixed with solvent solution such that Ti/Cr atomic ratio was 40 and molar concentration of (Ti +Cr) was 0.3 M. For Cr/Ti/Si synthesis with different ratios TEOS as the silicon source was used. During FSP catalytic synthesis, we use a syringe pump (Cole Parmer, 74900 series) to feed the liquid precursor of flow rate 3 mL/min through the spray nozzle. The 5 L/min oxygen gas flow was employed as dispersion gas to atomize the precursor solution. Surrounding supporting flame (premixed 1.0 L min-1 O2/1.0 L min-1 CH4) was used for the combustion of the dispersed droplets. Additionally, 3 L/min sheath O2 was passed through the outermost metal ring. A flat glass fiber filter (Whatman GF/A, 150 mm in diameter) was used to collect the as-synthesized fine nanoparticles leaving the flame. The collected catalyst were used directly without any additional treatment. The collected catalyst was denoted with corresponding elemental ratios. 2.3. Characterizations Micromeretics (ASAP 2010) was used to study specific surface area and porosity of the catalysts. For the BET analysis of the as-prepared catalysts with different Si/Ti atomic ratios (80,


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 47

75, 70, 60 and 50) as well as (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30) by using the saturated liquid N2 adsorption equilibrium isotherm analysis with continuous gas sorption in automated system. Preceding the analysis, degaussing of the 0.050±0.005 g catalyst was performed under He atmosphere at 200 °C for 3 hours. The nitrogen adsorption data were obtained at 77 K temperature with relative pressure from 0.05-0.99. The crystalline phases of the as synthesized catalyst were obtained from powder X-ray diffraction (XRD) data. Phillips Xpert diffractometer was employed to record XRD patterns using a scintillation counter detector and nickelfiltered CuK (wavelength 0.154056 nm) radiation. The as prepared catalyst samples are supported on an aluminum holder. With a step time of 0.5 seconds and step size of 0.025° over a range of 10° to 80° the intensities of the assynthesized catalyst were collected. The obtained XRD intensities data is used to identify the crystalline phases of the catalyst samples by comparing with the ICDD (International Center for Diffraction Data) files. Rutile (ICDD 03-065-1118) (XR) and anatase (ICDD 03-065-5714) (XA) phase composition was determined using the relative intensity ratio method. UV-vis absorption and transmittance spectrum was obtained by Shimadzu 2501PC with an ISR1200 sphere attachment. The UV-vis analysis was carried out in the range of 300 – 800 nm with slit width of 0.1mm and sample interval of 1 sec. For the background barium sulfate is employed as the standard foe the following analysis. Philips CM 20 electron microscope was employed for transition electron microscope (TEM) analysis. For the TEM analysis the assynthesized catalysts were dispersed in the solvent (isopropyl alcohol) and sonicated for 10 min. The dispersed sample was transferred onto carbon-Cu grid and the isopropyl alcohol was evaporated. The catalyst particles in the carbon grid are analyzed under the TEM with the


8

Page 9 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


applied accelerating voltage was 200 keV, with a LaB6 emission current and a point-to-point resolution of 0.27 nm. H2TPR of as-prepared catalysts with different Si/Ti atomic ratios (80, 75, 70, 60 and 50) as well as (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30) were studied with the help of Micromeritics automated catalyst systems (AutoChem2910). 50 mg of the catalyst was used for the H2-TPR analysis. Initially the sample was treated with 30 mL/min of UHP He (ultra-high pure) at 250 °C for 2 hours. After preheating at 250 °C, we returned to 50 °C then the gas was changed to 10% H2 in He used to reduce the catalyst. The temperature was linearly increased to 800 °C from 50 °C. The temperature programmed reduction runs were conducted with 20 mL/min flow of 10% H2 in He.

The H2 consumption of the catalyst was measured with the help of thermal

conductivity detector. Leaching of metal ions was studied by using ICP-MS analysis. For the leaching analysis of the catalyst, 100 mg of the catalyst was used in 100 ml of the DI water in beaker. To achieve the various pH (3 to 11) of the solution, we have added carefully dropwise the dilute solution of HCl and NaOH. Each beaker was labelled when the expected pH of the solution was achieved and the solution was stirred for 24 hours in dark. After the 24 hour stirring in dark each solution was filtered with Cameo 25P polypropylene syringe filters (OSMONICS, Cat# DDP02T2550). Inductively coupled plasma mass-spectrometry (ICP-MS) was used to accurately analyze the metal ion concentration in the solution. State-of-the-art ICP-MS (Agilent 8800 ICP-QQQ) was employed for the analysis of the filtered samples for metal ion concentration. The standard plasma condition with the conventional Meinhard nebulizer and the Peltier collard spray chamber (2 °C) was employed with a shield torch for the sample introduction and analysis.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 47

2.4. Visible light photocatalytic degradation tests The as-synthesized catalysts are tested for the visible light photocatalytic degradation of phenol under visible light using a flat plate reactor with round bottom in batch process described elsewhere.23 For ultraviolet radiation filter, a thin double layer acrylic OP-2 sheet (Museum quality) is used between the reactor and the light source for conducting visible-light experiments. To effectively eliminate any IR part of light source effect on the photocatalytic reaction the reactor temperature was maintained at 25 °C with the filtered water flowing through the cooling jacket around the reactor. 6 F8T5 ww lamps surrounding the reactor are the light source. 400 mg of catalyst was mixed with 500 mL of 500 µm phenol. There was no interference for the pH of the reactant solution. The catalyst was sonicated for 15 min to equally distribute and remove any agglomerate in the solutions. A continuous flow of 500mL/min of oxygen (Wright Brothers, 99.9%) was supplied to the aqueous solution which has the suspended catalyst After 30 min in dark the lights were switched on and at the given intervals sample of the reaction suspension were collected by using a syringe at given time intervals and Cameo 25P polypropylene syringe filters (Cat# DDP02T2550, OSMONICS) were used to remove the catalyst suspended in the sample. Total organic carbon analyzer (TOC-VCSH Shimadzu) was used to analyze the filtered sample solution. For the cyclic experiments, we have used the cellulose nitrate membrane filter (0.2 mm in the pore size, MFS, A020A090C) for separation of the used catalyst from the remaining aqueous solution after each run. The visible light photocatalytic degradation phenol by using the used


10

Page 11 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst was performed with the same experimental set-up as stated early. We have employed a prefix R for the catalyst which are reused.

3.

Results and discussion The X-ray diffraction (XRD) profiles of Cr/Ti/Si ternary oxides with varying Si/Ti atomic

ratios and Cr/Ti/Si ternary oxides with different (Si+Ti)/Cr atomic ratios have shown in Figure 1a and Figure 1b, respectively. In our earlier studies, we observed the strong characteristic peaks of anatase titania and rutile titania.20 In Figure 1a, the intrinsic Cr/TiO2 samples showed diffraction peaks at 2θ = 25.3, 36.9, 37.8, 48.0, 54.2, 55.1, 62.6, 68.9, 70.1, and 74.9o corresponding to the (101), (103), (004), (200), (105), (211), (204), (116), (220), and (215) planes of anatase titania (JCPDS card #84-1286), respectively. In addition, peaks at 2θ = 27.3, 36.0, 41.1, 43.9, 56.6, 64.2, and 69.8o can be ascribed to the (110), (101), (111), (210), (220), (002), and (301) planes which can be well indexed to rutile phase with tetragonal structure (JCPDS card #75- 1753). To study the effect of the Si addition and Cr amount for the anatase and rutile phase transformation, we have calculated the ratio of anatase and rutile phase (we have not observed brookite phase) with the respective peak intensities by equation 1:

𝑋𝑅 % = (1 −

1 ) ∗ 100 1.26 ∗ 𝐼𝑅 1+ 𝐼𝐴

𝐸𝑞 1

Where, IR is the intensity of rutile [110] plane peak and IA is the intensity of anatase [101] plane peaks. The relative percentages of rutile and anatase ratios are presented in Table 1.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 47

We can observe from Figure 1a, intensity of the all the diffraction peaks decreased with increase in Si/Ti atomic ratio due to the reduction in the crystallinity nature of the samples. This phenomenon can be attributed to the formation of Si—O networks by excessive SiO2, which prevents the formation of bigger crystallites.37 The sensitized low crystalline catalyst plays a role to achieve the performance in visible light. We can also observe the broadening of the (101) diffraction peak with the increase in Si content this may be attributed to the incorporation of peak corresponding to SiO2 centered at 2θ = 23◦ and the formation of structural defects.38 Reduction in the crystallinity of the catalyst along with the reduction of the peak intensities are due to these structural defects. These defects also hinder the crystallite growth. For the non-crystalline samples, recombination methods are dominated by the charge-trapping sites accompanied with a low crystalline matrix and structural defects induced by the chromium atoms. As one can see from Figure 1a, introduction of silica into the Cr/TiO2 has an interesting effect on the phase transformation from rutile to anatase. As the Si/Ti atomic ratio increased from 50 to 75, the rutile phase monotonically transformed into the anatase phase, further increase in Si/Ti atomic ratio had minimal effect (Figure 1a and Table 1). We have calculated the inter plane spacing (d) and lattice strain of our catalysts and included in Table 1. Among all the samples, Cr/Ti/Si catalyst with Si/Ti atomic ratio = 75 exhibited the maximum rutile to anatase transformation. The XRD analysis of the as-synthesized ternary (Cr/Ti/Si) oxides implies that the introduction of Si atoms into the TiO2 greatly induced a suppressed preference for the rutile phase in the titania lattice. These results also endorse that the addition of SiO2 to TiO2 improves the thermal stability of the TiO2 catalysts , thus decreasing the anatase to rutile phase transformation in the resulting catalyst. The reduction in the anatase-rutile phase transformation


12

Page 13 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is because of the presence of SiO2, which inhibits formation of rutile phase by preventing the direct (mutual) contact of Ti atoms (Ti—Ti) while forming the (Ti—O—Si) linkage.39 Conversely, this transformation is evident in Figure 1b for the ternary (Cr/Ti/Si) oxides with different M/Cr atomic ratios as a result of the lower amounts of Cr concentration and its interactions with SiO2 along with TiO2. The crystallite size of anatase calculated for these samples was in the range of 33-56 nm according to the Debye–Scherrer equation (Table 1).40 As can be seen from Figure 1b, we have observed no chromium related diffraction peaks in the asprepared catalysts with the (Si-Ti)/Cr atomic ratio = 20 and 25 as well as in R5 (Si-Ti)/Cr =25, which suggests that all the chromium atoms successfully inserted into the TiO2 lattice matrix. However, the fractions of chromium related impurities might exist over the grain or on the surface border which could be above the detection limit of X-ray diffraction technique. With increase in chromium content in the catalyst, new sharp peaks evolved at 2θ = 24.5, 33.6, 36.0, 65.1o which can be attributed to the formation of (012), (104), (110), and (300) planes of highly crystalline Cr2O3 with eskolaite structure (JCPDS card # 38-1479) (Figure 1b). These results suggest that the transition from the tetrahedral coordination of CrO3 to hexagonal coordination of Cr2O3 oxide implies the segregation of chromium out of the TiO2 lattice. Compared with Cr/TiO2, the introduction of Si along with Cr led to the shift in diffraction angle of the (101) peak of anatase TiO2 shift to a higher angle, which indicates that the addition of Si and Cr with the certain atomic ratios could induce crystal lattice defects in TiO2 (Figure 1a and Figure 1b). As is shown in Figure 1b, either the Cr2O3 oxide or Cr-T-Si mixed phases diffraction peaks do not appear for the samples with low concentration of chromium [(Ti+Si)/Cr =20 and 25]. This is due to the fact that the concentration of doped chromium is so very low and cannot be identified by the X-ray diffraction. Moreover, the atomic radius sizes of Ti4+ (0.061 nm) and Cr6+


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 47

(0.058) as well as Cr3+ (0.062) are nearly the same, for this reason Crn+ ions insert into the structure of TiO2 and occupy the lattice sites to form a ternary solid solution.41 Diffractogram patterns in Figure 1b demonstrate that all samples with low Cr amount or in other words with high (Si+Ti)/Cr ratio show only TiO2 peaks with dominant anatase phase.42 Our XRD results illustrate that the Si and Cr introduce a substantial amount of disorder into the TiO2 lattice and altering the crystallinity and these observation are in correspondence with the visible light photocatalytic activity results. Our Cr/Ti/Si ternary catalysts do reveal enriched photocatalytic activity and stability under the visible light irradiation compared to the intrinsic Cr/TiO2. To get insights into the effect of SiO2 and Cr on the textural properties of FSP TiO2, we investigated the differences in the N2 adsorption–desorption isotherms of the selected photocatalysts. Figure 2 illustrates the N2 adsorption–desorption isotherms and hysteresis curves of as-synthesized flame-made TiO2, Cr/TiO2, and Cr/Ti/Si composite matrix. As we can see from Figure 2., all the catalysts undergo a gradual increase in the adsorption volume at high relative pressure and along formed adsorption-desorption hysteresis curve. The hysteresis cycle showed in Figure 2, represents the capillary condensation due to the presence of microspores in the catalysts. Due to the presence of the loosely coherent particles, the hysteresis loop at high relative pressures shows no limiting adsorption of N2. The observed hysteresis loops and adsorption-desorption isotherms for our catalysts would be attributed to type H3 and type IV isotherms from the IUPAC classification, respectively. 43 The surface area calculated from BET analysis of the synthesized catalyst was shown in Table 2. The BET surface area of the FSP TiO2 catalyst was 93 m2/g and the BET surface area of commercial P25 was 50 m2/g. The addition of SiO2 to the catalyst has almost doubled the surface area (140 m2/g -209 m2/g) of the catalyst, including the pore volume. The increased BET surface


14

Page 15 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


area of our catalysts offers with more active sites on the surface for the adsorption of the reactants and molecules, thus improving the efficiency of the photocatalytic degradation process.44 Figure 3a demonstrates the UV–vis spectra of samples with altered Si/Ti atomic ratios with (Si+Ti)/Cr ratio of 20 and Figure 3b shows samples with different M(Si+Ti)/Cr atomic ratios with Si/Ti ratio of 75. The absorption spectra and band gap energies were obtained by analyzing the reflectance measurement with Kubelka-Munk emission function. For pristine TiO2 made by flame spray pyrolysis, there was only one band at 388 nm corresponding to the anatase TiO2 (3.19 eV) because of high amount of anatase phase in it (~80 wt. %). In Figure 3a, one could see two absorption bands at ca. 378 and 472 nm which indicate the presence of monochromatic (Cr6+) species.45-47 In Figure 3a, no peak was observed at 609 nm for primitive TiO2, Cr/TiO2, and ternary oxides with Si/Ti atomic ratio 50 and 60. A new peak evolved at 609 nm band with further increase in Si/Ti atomic ratio from 70 to 80. Evolution of new band at 609 nm is a result of d–d transitions of Cr3+ species.48 We have not observed any red-shift adsorption in spectrum due the presence of the chromate species in highly dispersed form in the photocatalysts. From Figure 3b, one can observe that the peak intensity of the 609 nm band monotonically increased with decrease in (Si+Ti)/Cr atomic ratio in the catalyst (top to bottom). In other words, intensity of the 609 nm band increased with increase in the Cr content, indicating the rise of the Cr3+ concentration. Our X-ray diffraction analysis also suggested the transition of tetrahedral coordination of CrO3 to hexagonal coordination of Cr2O3 and the segregation of chromium out of the TiO2 lattice with increase in Cr loading (Figure 2b). As shown in Figure 3b insert, Cr/Ti/Si ternary metal oxide with ((Si+Ti)/Cr atomic ratio = 20 shows four UV–vis absorption bands near 260 (1A1 → 1T2 transition), 350 (1A1 → 1T2 transition), 470 (1A1 → 1T1 transition), and 600 nm


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 47

(A2g → T2g), first three bands are corresponding to the charge transfer from O2- to Cr6+ of tetrahedral coordinated chromium oxide.

22, 23,49-51

As we can see from Figure 3b, octahedral

coordinated Cr3+ is very low in samples with (Si+Ti)/Cr = 25 and 20 atomic ratio. These results imply that the Cr6+ moieties are dominant in the corresponding catalysts, whereas the concentration of octahedral Cr3+ (of Cr2O3) increased when Cr loading was enriched for the given Si/Ti ratio of 75. Further, we investigated the as-prepared Cr/Ti/Si ternary oxides for the temperature programmed reduction (H2-TPR) in order to find out the reduction profiles of chromium (Figure 4a and Figure 4b). In Figure 4a, reduction behavior of chromium is altered to some extent yet the amount of chromium is same in the catalyst. All the catalysts with different Si/Ti atomic ratio exhibited two major peaks, low-temperature peak (T1) at 464 °C can be ascribed to the reduction of Cr6+ to Cr3+, and the peak (T2) around 700 °C is can be attributed to the reduction of Cr3+ to Cr2+. This high-temperature peak is also related to the direct reduction of bulk chromia species. 52 The increased reduction temperatures of Cr are due to the lower concentrations of chromium and higher degree of its interaction with the titania and silica loading. From Figure 4a, we can clearly observe that the intensity of the high-temperature reduction peak decreasing with increase in Si/Ti atomic ratio. Moreover, the reduction temperatures of second peak shifted to higher temperatures due to the decrease in reduction potential of Cr2O3 in the catalyst. These results suggest that the increase in Si/Ti atomic ratio greatly promoted the suppression of Cr2O3 phase in the catalyst. The reduction potential of Cr6+ and Cr3+ in the catalyst plays a vital role in reactions relating to a redox mechanism. As shown in Figure 4a, as the ratio of Si/Ti increases from 50 to 80, the first reduction peak slightly shifts towards the lower temperatures, suggests the increase in reduction potential of Cr6+. As shown in Figure 4b, the samples with (Si+Ti)/Cr atomic ratio =


16

Page 17 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8, 10, and 15 (with high chromium) show two peaks at 310 °C and 430°C to 550 °C, which represents the existence of Cr6+ (T1) along with Cr3+ (T2). The XRD results and the above TPR analysis are in agreement, where we observed formation of highly crystalline Cr2O3 phase in these catalysts. We observed only one broad reduction peak for the (Si+Ti)/Cr ratios of 20, 25 and 30, which indicates the existence of Cr6+. The broadening of the peak indicates the stronger interactions between Cr-Ti and Cr-Si. Figure 4b shows the two reduction peaks for the assynthesized FSP Cr/Ti/Si. Figure 5 shows TEM and SAED images of the FSP TiO2, Cr/TiO2 (Ti/Cr =40), Fresh Cr/Ti/Si (with Si/Ti=75 and (Ti+Si)/Cr=25), and used Cr/Ti/Si (with Si/Ti=75 and (Ti+Si)/Cr= 25), respectively. With respect to the catalyst particles shapes the as-synthesized photocatalyst are mostly spherical while pure TiO2 contains round edges but have irregular shapes. The particle size of as-synthesized TiO2 was 14.6 nm to 11 nm which is in similar to the calculated particle diameter from BET analysis (Table 2), However we have observed that higher particle diameter calculated from XRD and BET analysis compared to the TEM analysis for Cr/TiO2. Figure 5c and 5d shows TEM images of fresh and used FSP Cr/Ti/Si catalysts, respectively. We can see that there is no substantial variation in particle size for the pure and reused catalysts samples, this is also consistence with the calculated particle diameter from BET and XRD analysis. We can observe the aggregation of particles in used Cr/Ti/Si and the particles have more spherical outline when compared with FSP Cr/TiO2. We have not found any separate crystalline phase of SiO2 electron diffraction patterns which is also evident in our XRD. It is interesting to observe the SAED patterns of catalyst powders in Figure 5, which shows that the brightness and intensity of polymorphic ring is becoming weak from pure TiO2 to Cr/TiO2 to fresh Cr/Ti/Si to used


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 47

Cr/Ti/Si (Figure 5a−Figure 5d). These results show that the addition of Si enriched the amorphous matrix and structural defects in the catalyst. In the current studies, we have investigated the photocatalytic degradation of phenol under visible region. We have operated the semi-batch rector with 500 mL of 500 µm phenol in aqueous phase and the 800 mg/L of catalyst in solution with steady air flow rate of 500 mL min1

. We have used double layer sheet of acrylic OP-2(Museum quality) between the reactor and

the light source to filter any UV radiation to the reactor. The total organic concentration was continuously monitored to study the photocatalytic degradation of phenol in the present work. We have the fractional conversion (reduction of TOC concentration) as dependent variable and initial total organic carbon concentration (CA0) and time as independent variable. The energy required for the creation of electron-hole pair in the catalyst is provided by the visible light source. Cr(IV) provides the intermediate energy level which will enhance the electron-hole generation under visible light. The charge separation with visible light is due to the sensitization of TiO2 by Cr ion in the tetrahedral coordinated structure, the electron is released in conduction band and hole which can scavenge electron which is created in the valance band. These hole and electron pair created in the valance band and conduction band respectively, react with the surface adsorbed oxygen and H2O to form the hydroxyl and reactive oxygen species which in return react with the organic (phenol). Figure 6, shows the curves of the relative total organic carbon (TOC) concentration with respect to time for the visible light photodegradation of phenol. The activity of catalyst increases with the increase in the ratio of Si/Ti ranging from 50 to 75 and then decreases with further Si incorporation. This increase in the photocatalytic activity could be attributed to the increase in Cr6+ concentration and structural defects along with the enriched specific surface area. Even though further increase of Si increases the surface area of


18

Page 19 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the material to 214 m2/g, the reduction in the catalytic activity for the sample with Si/Ti ratio 80 is due to decrease in Cr6+ concentration as confirmed by TPR analysis. These results illustrate that the visible light induced photocatalytic activity not only depends on the surface area but also on the oxidation state of Cr and the interaction of Cr with Si and Ti. Further, to optimize the Cr content in the catalyst, we investigated our catalysts by altering the (Si+Ti)/Cr atomic ratio for the visible light photodegradation of phenol. Figure 7 shows that 0.5-1% of the phenol adsorbed over the catalyst surface before switching on the light source. As can be seen from the Figure 7, the visible photocatalytic activity monotonically increased with increase in (Si+Ti)/Cr atomic ratio from 8 to 25 and then decreased for (Si+Ti)/Cr ratio =30. We observed an interesting relation between the visible light photocatalytic activity of the catalyst, with the presence Cr6+ content in the catalyst and surface are of the catalyst. Catalysts with high concentration of chromium ((Si+Ti)/Cr = 8, 10, and 15) demonstrated poor performance due to the existence of bulk crystalline Cr2O3 as observed from XRD results (Figure 2b). The formation of the agglomerates in the catalysts and reduction in the Cr6+ concentration as suggested by the TPR results have hindered the photocatalytic activity of the catalyst. From our TPR studies, one can clearly notice that the presence of Cr(IV) for the active photocatalyst is considerably higher than Cr(III) species indicating that the hexavalent chromium promotes the photocatalytic activity.53 For liquid phase photocatalysis reactions, there could be secondary contamination of the water stream due to the leaching of the metal ions in the environmental applications.

23, 54

The

waste water stream from various sources are contaminated by various pollutants which results in the change of pH for the water stream, so accordingly we have used the wide pH range from 3 to 11. After 24 hrs stirring at the room temperature, the Si, Ti, and Cr ion concentration in the


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 47

aqueous solutions were evaluated by ICP-MS analysis and the results are shown in Figure 8a and Figure 8b, respectively. The concentration of the ions in Figure 8a and Figure 8b shows the extent of the leaching of the catalyst. In Figure 8a, we can observe the influence of synthesis technique on chromium leaching with respect to change in the pH of aqueous solution. There is an increase in the leaching of chromium with the increase in the pH. The catalyst prepared from co-precipitation method has showed higher leaching, namely 6.8 ppm for a pH of 11 and minimum of 4.1 ppm at pH of 3. The catalyst synthesized from sol-gel (Sol-Gel) method has lower leaching compared to co-precipitation (Co-Precp) and higher than FSP Cr/TiO2. The Cr leaching of catalysts prepared from various synthesis techniques at different (3-11) pH conditions, decreased in the following sequence: Cr/TiO2 (Co-Precp) >>> Cr/TiO2 (Sol-Gel) >> FSP Cr/TiO2 > FSP Ti/Si/Cr (M/Cr =25). We have found the extremely low level of Cr metal leaching in the FSP made catalysts compared to other synthesis techniques. The leaching of Cr in FSP Cr/Ti/Si (M/Cr =25) is negligible (50 – 70 ppb) in the pH range of 3-11, respectively. These results suggest that the developed FSP Cr/Ti/Si (M/Cr =25) catalyst showed extremely lower level of chromium and titania ions leaching in aqueous solution for the wide range of pH, which indicates the negligible secondary metal ion contamination caused to the waste water stream. We can observe from Figure 8b that the leaching of Ti with respect to pH is not a linear relation unlike Cr leaching. As we can see from Figure 8b, the Ti leaching (ppb) is considerably lower than that of Cr leaching (ppm) in Cr-TiO2 samples. We observed a negligible amount (1-3 ppb) of Ti leaching for FSP Cr/Ti/Si (M/Cr =25) compared to other catalysts. Due to the reduction of the hexavalent chromium(yellow) to trivalent chromium(pale green) in the FSP Cr/TiO2 catalyst under visible light photocatalysis, we have observed the deactivation of the catalyst. To compare the stability of the catalyst, we have synthesized FSP Cr/TiO2 with


20

Page 21 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr/Ti ratio of 40 (the optimal Cr/Ti ratio) and tested the stability of the catalyst by reusing the catalyst for the cyclic photocatalytic activity runs. Figure 9 shows the visible light photocatalytic degradation of the phenol with the FSP Cr/TiO2 and the influence of recycled catalyst. In Figure 9, we can observe the stability and the activity of the catalyst was decreasing from 1st cycle to 2nd and 3rd cycles. The three mechanisms which could cause the reduction in activity are 1) change in the oxidation state of Cr6+ which could be observed by TPR analysis, 2) formation of agglomerates and reduction in the surface area thus reducing the active surface, and 3) leaching of the metal ions which further can cause the secondary contamination.24 Figure 9 shows the visible light photocatalytic degradation of the phenol with the optimal Cr/Ti/Si (Si/Ti ratio of 75 and M/Cr ratio of 20) and the effect of reuse of the catalyst without any reactivation of the spent photocatalyst. We have studied the catalyst for five cycles. This catalyst has similar activity for all the five cycles, which is attributed to the stability of the photocatalytic activity and assures no leaching. R5 FSP M/Cr 25 represents the catalyst after the five cycles. We can also notice the stability in the surface area of the fresh and catalyst after five cycles (Table 2). These results demonstrated that the structure of our optimized catalyst was stable. We also have observed the similar band- gap energy for the fresh and R5 FSP M/Cr 25 catalyst and these results illustrate the stability of the optimal catalyst.

4. Conclusions A series of Cr/Ti/Si composite photocatalysts have been prepared by using an aerosol flame spray assisted method and evaluated for the degradation of phenol under visible light irradiation. The acquired composites demonstrate large surface areas, substantial disorder in the TiO2 lattice and high visible-light-harvesting proficiency. Among all the synthesized catalysts, Cr/Ti/Si


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 47

composites with Si/Ti atomic ratio = 75 and (Si+Ti)/Cr atomic ratio = 20 and 25 illustrated an impressive performance for the visible light induced photocatalysis. Catalysts with high concentration of chromium ((Si+Ti)/Cr = 8, 10, and 15) demonstrated poor performance due to the existence of bulk crystalline Cr2O3 as observed from XRD. Increase in the Cr content in the catalyst led to the segregation of Cr2O3 crystallites, and thus decrease in photocatalytic activity. Dropping the Cr amount (increase in (Si+Ti)/Cr atomic ratio) to within solubility limits would solve the difficulties accompanying with Cr2O3 segregation out of TiO2. The optimal flame-made Cr/Ti/Si photocatalyst has also shown very negligible amounts of metal ion leaching thus prevent the formation of secondary metal contamination to the aqueous stream. Pristine Cr/TiO2 showed poor activity and stability in photocatalysis recycle reactions. Incorporation of SiO 2 into Cr/TiO2 with optimal atomic ratios (Si/Ti = 75 and (Si+Ti)/Cr = 25) promoted the catalytic activity as well as stability.

AUTHOR INFORMATION Corresponding Authors Tel.: (513) 556-1474. Fax: (513) 556-3473 E-mail: [email protected] (Panagiotis G. Smirniotis) E-mail: [email protected]; [email protected] (Makram Suidan)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


22

Page 23 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors wish to acknowledge EPA/Pegasus contract (contract number EP-C-11-006) for financial support of this work through the scholarship to Siva Nagi Reddy Inturi.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 47

References (1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature. 1972, 238, 37. (2)

Peral, J.; Domenech, X.; Ollis, D. F. Heterogeneous Photocatalysis for Purification,

Decontamination, and Deodorization of Air, J. Chem. Technol. Bio. 1978, 70, 117. (3)

Ollis, D.; Pichat, P.; Serpone, N. TiO2 photocatalysis-25 years, Appl. Catal. B. 2010, 99,

377. (4)

Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental

Applications of Semiconductor Photocatalysis, Chem. Rev. 1995, 95, 69. (5)

Matthews, R. W. Purification of Water with Wear U.V.

Illuminated Suspensions of

Titanium Dioxide, Water Res. 1990, 24, 653. (6)

Sun, B.; Vorontsov, A. V.; Smirniotis, P. G. Role of Platinum Deposited on TiO2 in Phenol

Photocatalytic Oxidation, Langmuir. 2003, 19, 3151. (7)

Anpo, M.; Takeuchi, M. The Design and Development of Highly Reactive Titanium Oxide

Photocatalysts Operating Under Visible Light Irradiation, J. Catal. 2003, 216, 505. (8)

Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in

Nitrogen-Doped Titanium Oxides, Science. 2001, 293, 269. (9)

Zhu, J. F.; Zheng, W.; Ben, H. E. Characterization of Fe-TiO2 Photocatalysts Synthesized

by Hydrothermal Method and their Photocatalytic Reactivity for Photodegradation of XRG dye Diluted in Water. J. of Mol. Cata. A-Chemical, 2004, 216, 35. (10) Chen, X.; Li, H.; Sun, T. One-step Fame-Assisted Pyrolysis Synthesis of Fe Doped Carbon Incorporated TiO2 and its photocatalytic activity. Chem. J. of Chin. Uni. 2013, 34, 2855.


24

Page 25 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Butler, E. C.; Davis, A. P. Photocatalytic Oxidation in Aqueous Titanium Dioxide Suspensions: the Influence of Dissolved Transition Metals, J. Photochem. Photobiol. A: Chem. 1993, 70, 273. (12) Paola, A. D.; Lopez, E. G.; Ikeda, S.; Marci, G.; Ohatani, B.; Palmisano, L. Photocatalytic Degradation of Organic Compounds in aAqueous Systems by Transition Metal Doped Polycrystalline TiO2, Catal. Today. 2002, 75, 87. (13) Serpone, N.; Lawless, D. Spectroscopic, Photoconductivity, and Photocatalytic Studies of TiO2 Colloids: Naked and with the Lattice Doped with Cr3+, Fe3+, and V5+ Cations, Langmuir. 1994, 10, 643. (14) Grzybowska, B.; Słoczynski, J.; Grabowski, R.; Samson, K.; Gressel, I.; Wcisło, K.; Gengembre, L.; Barbaux, Y. Effect of Doping of TiO2 Support with Altervalent ions on Physicochemical and Catalytic Properties in Oxidative Dehydrogenation of Propane of Vanadia– Titania Catalysts, Appl. Catal. A: Gen. 2002, 230, 1. (15) Karakitsou, K. E.; Verykios, X. E. J. Effects of Altervalent Cation Doping of Titania on its Performance as a Photocatalyst for Water Cleavage, Phys. Chem. 1993, 97, 1184. (16) Xu, Y. M.; Langford, C. H. Enhanced Photoactivity of a Titanium(IV) Oxide Supported on ZSM5 and Zeolite A at Low Coverage, J. Phys. Chem. 1995, 99, 11501. (17) Yu, J. C.; Li, G. S.; Wang, X. C.; Hu, X. L.; Leung, C. W.; Zhang, Z. D.; An Ordered Cubic Im3m Mesoporous Cr–TiO2 Visible Light Photocatalyst, Chem. Commun. 2006, 2717. (18) Davydov, L.; Reddy, E. P.; France, P.; Smirniotis, P. G. Transition-Metal-Substituted Titania-Loaded MCM-41 as Photocatalysts for the Degradation of Aqueous Organics in Visible Light, J. Catal. 2001, 203, 157.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 47

(19) Reddy, E. P.; Sun, B.; Smirniotis, P. G.Transition Metal Modified TiO2-Loaded MCM-41 Catalysts for Visible- and UV-Light Driven Photodegradation of Aqueous Organic Pollutants, J. Phys. Chem. B. 2004, 108, 17198. (20) Inturi, S. N. R.; Boningari, T.; Suidan, M.; Smirniotis, P. G. Transition Metal Modified TiO2-Loaded MCM-41 Catalysts for Visible- and UV-Light Driven Photodegradation of Aqueous Organic Pollutants, Appl. Catal. B: Environ. 2014, 144, 333. (21) Inturi, S. N. R.; Boningari, T.; Suidan, M.; Smirniotis, P. G. Flame Aerosol Synthesized Cr Incorporated TiO2 for Visible Light Photodegradation of Gas Phase Acetonitrile, J. Phys. Chem. B. 2014, 118, 231. (22) Sun, B.; Reddy, E. P.; Smirniotis, P. G. Visible Light Cr(VI) Reduction and Organic Chemical Oxidation by TiO2 Photocatalysis, Sci. Technol. 2005, 39, 6251. (23) Inturi, S. N. R.; Suidan, M.; Smirniotis, P. G. Influence of Synthesis Method on Leaching of the Cr-TiO2 Catalyst for Visible Light Liquid Phase Photocatalysis and Their stability, Appl. Catal. B: Environ. 2016, 180, 351. (24) Sun, B.; Reddy, E. P.; Smirniotis, P. G. TiO2-loaded Cr-Modified Molecular Sieves for 4Chlorophenol Photodegradation under Visible Light, J. Catal. 2006, 237, 314. (25) Awate, S. V.; Jacob, N. E.; Deshpande, S. S.; Gaydhankar, T. R.; Belhekar, A. A.Synthesis, Characterization and Photocatalytic Degradation of Aqueous Eosin Over Cr Containing Ti/MCM-41 and SiO2-TiO2 Catalysts Using Visible Light, J. Mol. Catal. A. 2005, 226, 149. (26) Elias, V. R.; Vaschetto, E. G.; Sapag, K.; Crivello, M. E.; Casuscelli, S. G.; Eimer, G. A. Synthesis and Photocatalytic Activity of Titania-Loaded Transition Metal-Modified MCM-41 Molecular Sieves, Top. Catal. 2011, 54, 277.


26

Page 27 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Marques, F. C.; Canela, M. C.; Stumbo, A. M. Use of TiO2/Cr-MCM-41 Molecular Sieve Irradiated with Visible Light for the Degradation of Thiophene in the Gas Phase, Catal. Today. 2008, 133, 594. (28) Herrmann, J. M.; Matos, J.; Disdier, J.; Guillard, C.; Laine, J.; Malato, S.; Blanco, J. Solar Photocatalytic Degradation of 4-Chlorophenol using the Synergistic Effect between Titania and Activated Carbon in Aqueous Suspension, Catal. Today. 1999, 54, 255. (29) Xu, Y.; Langford, C. H. Enhanced Photoactivity of a Titanium(1V) Oxide Supported on ZSM5 and Zeolite A at Low Coverage, J. Phys. Chem. 1995, 99, 11501. (30) Xu, Y.; Langford, C. H. Photoactivity of Titanium Dioxide Supported on MCM41, Zeolite X, and Zeolite Y, J. Phys. Chem. 1997, 101, 3115. (31) Cho, C. H.; Kwak, J. H.; Ryoo, R.; Ahn, W. S.; Jung, K. Y.; Park, S. B. Photocatalytic Decomposition of Tricholoethylene over Aluminosilicate Zeolites with Isomorphous Incorporation of Titanium, Stud. Surf. Sci. Catal. 1996, 105, 1617. (32) Eriksson, E.; Baun, A.; Mikkelsen, P. S.; Ledin, A. Risk Assessment of Xenobiotics in Stormwater Discharged to Harrestrup Å, Denmark Desalination, 2007, 215, 187. (33) Pratsinis, S. E. Flame Aerosol Synthesis of Ceramic Powders, Prog. Energ. Combust. 1998, 24, 197. (34) Madler, L.; Kammler, H. K.; Mueller, R.; Pratsinis, S. E.

Controlled Synthesis of

Nanostructured Particles by Flame Spray Pyrolysis, Aerosol Science. 2002, 33, 369. (35) Teoh, W. Y.; Madler, L.; Beydoun, D.; Pratsinis, S. E.; Amal, R.; Direct (one-step) Synthesis of TiO2 and Pt/TiO2 Nanoparticles for Photocatalytic Mineralisation of Sucrose. Chem. Eng. Sci. 2005, 60, 5852.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 47

(36) Stark, W. J.; Wegner, K.; Pratsinis, S. E.; Baiker, A. Flame Aerosol Synthesis of Vanadia– Titania Nanoparticles: Structural and Catalytic Properties in the Selective Catalytic Reduction of NO by NH3, J. Catal. 2001, 197,182. (37) Cheng, P.; Zheng, M.; Jin, Y.; Huang, Q.; Gu, M. Preparation and Characterization of Silica-doped Titania Photocatalyst Through sol–gel Method, Mater Lett. 2003, 57, 2989. (38) Zang, L.; Lange, C.; Abraham, I.; Storck, S.; Maier, W. F.; Kisch, H. Amorphous Microporous Titania Modified with Platinum(IV) Chloride A New Type of Hybrid Photocatalyst for Visible Light Detoxification, J. Phys. Chem. B. 1998, 102, 10765. (39) Shifu, C.; Gengyu, C. The Effect of Different Preparation Conditions on the Photocatalytic Activity of TiO2·SiO2/beads, Surf. Coat. Technol,. 2006, 200, 3637. (40) Jenkins, R.; Snyder, R. L. Introduction to X-ray Powder Diffractometr; Wiley & Sons, Inc. 1996. (41) Shannon, R. D.; Handbook of Chemistry and Physics; CRC Press:Boca Raton, FL, 1974. (42) Chen, R.; Liu, H. Preparation of Cr-doped TiO2/SiO2 Photocatalysts and their Photocatalytic Properties, J. Chin. Chem. Soc. 2011, 58, 947. (43) Thommes, M. Physical Adsorption Characterization of Nanoporous Materials, Chem. Ing. Tech. 2010, 82, 1059. (44) Yu, J. G.; Wang, W. G.; Cheng, B. Synthesis and Enhanced Photocatalytic Activity of a Hierarchical Porous Flowerlike p–n Junction NiO/TiO2 Photocatalyst, Chemistry. 2010, 5, 2499. (45) Gaspar, A. B.; Brito, J. L. F.; Dieguez, L. C. Characterization of Chromium Species in Catalysts for Dehydrogenation and Polymerization, J. Mol. Catal. A. 2003, 203, 251.


28

Page 29 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(46) Jimeńez-Lo´pez, A.; Rodrigues-Castelloń, E.; Maireles-Torres, P.; Diaz, L.; Me´ridaRobles, J. Chromium Oxide Supported on Zirconium- and Lanthanum-doped Mesoporous Silica for Oxidative Dehydrogenation of Propane, Appl. Catal. A. 2001, 218, 295. (47) Zhu, Z.; Chang, Z.; Kevan, L. Synthesis and Characterization of Mesoporous ChromiumContaining Silica Tube Molecular Sieves CrMCM-41, J. Phys. Chem. B. 1999, 103, 2680. (48) Takehira, K.; Ohishi, Y.; Shishido, T.; Kawabata, T.; Takaki, K.; Zhang, Q.; Wang, Y. Behavior of Active Sites on Cr-MCM-41 Catalysts During the Dehydrogenation of Propane with CO2, J. Catal. 2004, 224, 404. (49) Weckhuysen, B. M.; Verberckmoes, A. A.; Buttiens, A. L.; Schoonheydt, R. A. Diffuse reflectance spectroscopy study of the thermal genesis and molecular structure of chromiumsupported catalysts, J. Phys. Chem. 1994, 98, 579. (50) Lannibello, A.; Marengo, S.; Tittarelli, P.; Morelli, G.; Zecchina, A. Spectroscopic study of the structure of chromium(VI), molybdenum(VI) and tungsten(VI) oxospecies anchored on aluminium oxide, J. Chem. Soc. Faraday Trans. I. 1984, 80, 2209. (51) Chmielarz, L.; Kustrowski, P.; Kruszec, M.; Dziembaj, R.; Cool, P.; Vansant, E. F.; Nitrous Oxide Reduction with Ammonia and Methane Over Mesoporous Silica Materials Modified with Transition Metal Oxides, J. Porous Mater. 2005, 12, 183 (52) Ramesh, Y.; Thirumala Bai, P.; Hari Babu, B.; Lingaiah, N.; Rama Rao, K. S.; Sai Prasad, P. S. Oxidative dehydrogenation of ethane to ethylene on Cr2O3/Al2O3–ZrO2 catalysts: the influence of oxidizing agent on ethylene selectivity, Appl. Petrochem. Res. 2014, 4, 247. (53) Nahar, S.; Hasegawa, K.; Kagaya, S. Photocatalytic degradation of phenol by visible lightresponsive iron-doped TiO2 and spontaneous sedimentation of the TiO2 particles, Chemosphere. 2006, 65, 1976.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 47

(54) Awate, S.V.; Jacob, N.E.; Deshpande, S.S.; Gaydhankar, T.R.; Belhekar, A.A. Synthesis, characterization and photo catalytic degradation of aqueous eosin over Cr containing Ti/MCM-41 and SiO2-TiO2 catalysts using visible light. J. Mol. Catal. A: Chem. 2005, 226, 149.


30

Page 31 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure and Table captions

Figure 1. Powder X-ray diffraction patterns of the catalysts (a) with different Si/Ti atomic ratios (80, 75, 70, 60, and 50) and (b) with different (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30); A = anatase, R = rutile, and ∆ = Cr2O3. Figure 2. N2 adsorption–desorption isotherms of selected as-synthesized catalysts. Figure3. UV-vis diffuse reflectance spectra of as-synthesized and reactivated catalysts (a) with different Si/Ti atomic ratios (80, 75, 70, 60, and 50) and (b) with different (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30). Figure 4. H2-TPR patterns of all the prepared catalysts (a) different Si/Ti atomic ratios, and (b) different M/Cr (M= Si+Ti) atomic ratios. Figure 5. TEM and SAED patterns of as-prepared flame-made catalysts (a) FSP TiO2, (b) Cr/TiO2 (Ti/Cr =40), (c) Fresh Cr/Ti/Si (with Si/Ti=75 and M (Ti+Si)/Cr= 25), and (d) used Cr/Ti/Si (with Si/Ti=75 and M(Ti+Si)/Cr= 25). Figure 6. The time course photocatalytic conversion over FSP Cr/Ti/Si for visible light photocatalysis of 500 mL of phenol with 500 µm initial concentration (with M(Si+Ti)/Cr = 20) with 400 mg of catalyst at 25± 2 °C. Figure 7. The time course photocatalytic conversion over FSP Cr/Ti/Si for visible light photocatalysis of 500 mL of phenol with 500 µm initial concentration (with Si/Ti ratio of 75) with 400 mg of catalyst at 25± 2 °C. Figure 8. Effect of pH on (a) Cr leaching and (b) Ti leaching in Cr/TiO2 FSP, Cr-TiO2 sol-gel, Cr-TiO2 co-precipitation (Ti/Cr =40) and FSP Cr/Ti/Si (M/Cr =25) catalysts (pH range 3-11). Figure 9. The effect of reusability and time course photocatalytic conversion over FSP Cr/TiO2 (Ti/Cr =40; red color filled circle) and FSP Cr/Ti/Si (M/Cr =25; green color filled square) of 500 mL of phenol with 500 µm initial concentration with 400 mg of catalyst in visible light region at 25± 2 °C.

Table 1.

XRD and UV-vis Spectroscopy characterization results for the studied catalysts.

Table 2. Specific surface area and pore volume of Cr/Ti/Si composites as a function of Si/Ti and (Si+Ti)/Cr atomic ratios.


31


AR

A A

(116) (301) (220)

(200)

R

(204) (002)

A

AA

(105) (211) (220)

R AR R (210)

(101) (110)

R

R A

(215)

A

A

(101) (103) (004) (111)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 47

Si/Ti = 80 Si/Ti = 75 Si/Ti = 70 Si/Ti = 60 Si/Ti = 50

R3 Cr/TiO2

Cr/TiO2 20

30

40

50

60

70

80

2 (°)


32

A

AA

A Anatase R Rutile  Cr2O3

A (204)

(104)

R

(105) (211)

(101) (110)



R

(200)

A

(110)

(b)

Intensity (a.u.)

(Si+Ti)/Cr =30

R5 (Si+Ti)/Cr =25 (Si+Ti)/Cr =25

(Si+Ti)/Cr =20 (Si+Ti)/Cr =15

(300)

 (012)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(111)

Page 33 of 47

(Si+Ti)/Cr =10 (Si+Ti)/Cr =8 20

30

40

50

60

70

80

2 (°)

Figure 1. Powder X-ray diffraction patterns of the catalysts (a) with different Si/Ti atomic ratios (80, 75, 70, 60, and 50) and (b) with different (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30); A = anatase, R = rutile, and ∆ = Cr2O3.


33


350

3

Quantity absorbed (cm /g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 47

300

250

FSP TiO2 FSP Cr/TiO2

200

FSP Si/Ti/Cr

150

100

50

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 2.

N2 adsorption–desorption isotherms of selected as-synthesized catalysts.


34

Page 35 of 47

(a)

Visible Absorbance

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FSP Si/Ti=80 FSP Si/Ti=75 FSP Si/Ti=70 FSP Si/Ti=60 FSP Si/Ti=50 FSP Cr/TiO2 FSP TiO2

UV 300

400

500

600

700

Wavelength(nm)


35

Visible Absorbance

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a.u.)


Page 36 of 47

(Si+Ti)/Cr = 20

250 300 350 400 450 500 550 600 650 700

Wavelength (nm)

R5 FSP M/Cr=25 FSP M/Cr=30 FSP M/Cr=25 FSP M/Cr=20 FSP M/Cr=15 FSP M/Cr=10

(b)

FSP M/Cr=8

UV 300

400

500

600

700

Wavelength(nm) Figure3. UV-vis diffuse reflectance spectra of as-synthesized and reactivated catalysts (a) with different Si/Ti atomic ratios (80, 75, 70, 60, and 50) and (b) with different (Si+Ti)/Cr ratios (8, 10, 15, 20, 25, and 30).


36

Page 37 of 47

(a)

T.C.D Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FSP Si/Ti=80

FSP Si/Ti=75

FSP Si/Ti=70

FSP Si/Ti=60

FSP Si/Ti=50

100

200

300

400

500



600

700

Temperature ( C)


37


(b) FSP M/Cr=30 T.C.D Signal (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 47

FSP M/Cr=25 FSP M/Cr=20 FSP M/Cr=15

FSP M/Cr=10 FSP M/Cr=8

100

200

300

400

500

600

700

800



Temperature ( C)

Figure 4. H2-TPR patterns of all the prepared catalysts (a) different Si/Ti atomic ratios, and (b) different M/Cr (M= Si+Ti) atomic ratios.


38

Page 39 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 nm

20 nm

10 nm

20 nm

Figure 5. TEM and SAED patterns of as-prepared flame-made catalysts (a) FSP TiO2, (b) Cr/TiO2 (Ti/Cr =40), (c) Fresh Cr/Ti/Si (with Si/Ti=75 and M (Ti+Si)/Cr= 25), and (d) used Cr/Ti/Si (with Si/Ti=75 and M(Ti+Si)/Cr= 25).


39


100

50 (Si/Ti) 60 (Si/Ti) 70 (Si/Ti) 75 (Si/Ti) 80 (Si/Ti)

90 80 70

C/C0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 47

60 50 40 30 20 10 -50

0

50

100 150 200

250 300 350

400

Time (min) Figure 6. The time course photocatalytic conversion over FSP Cr/Ti/Si for visible light photocatalysis of 500 mL of phenol with 500 µm initial concentration (with M(Si+Ti)/Cr of 20/1) with 400 mg of catalyst at 25± 2 °C.


40

Page 41 of 47

100

8 (M/Cr) 10 (M/Cr) 15 (M/Cr) 30 (M/Cr) 20 (M/Cr) 25 (M/Cr)

90 80 70

C/C0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20 10 -50

0

50

100

150

200

250

300

350

400

Time (min)

Figure 7. The time course photocatalytic conversion over FSP Cr/Ti/Si for visible light photocatalysis of 500 mL of phenol with 500 µm initial concentration (with Si/Ti ratio of 75) with 400 mg of catalyst at 25± 2 °C.


41


FSP

10000

Sol-Gel

Co-Precp

Ti/Si/Cr

(a) Cr concentration in solution (ppb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 47

1000

100

10

3

5

7

9

11

pH


42

Page 43 of 47

FSP 1000

Ti concentration in solution (ppb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sol-Gel

Co-Precp

Ti/Si/Cr

(b)

100

10

1

0.1

3

5

7

9

11

pH

Figure 8. Effect of pH on (a) Cr leaching and (b) Ti leaching in Cr/TiO2 FSP, Cr-TiO2 solgel, Cr-TiO2 co-precipitation (Ti/Cr =40) and FSP Cr/Ti/Si (M/Cr =25) catalysts (pH range 311).


43


100

1

2

200 400 0

200 400 0

4

3

5

80

C/C0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

60

40

20

0

200

400 0

200

400 0

200

400

Time (min)

Figure 9. The effect of reusability and time course photocatalytic conversion over FSP Cr/TiO2 (Ti/Cr =40; red color filled circle) and FSP Cr/Ti/Si (M/Cr =25; green color filled square) of 500 mL of phenol with 500 µm initial concentration with 400 mg of catalyst in visible light region at 25± 2 °C.


44

Page 45 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. XRD and UV-vis Spectroscopy characterization results for the studied catalysts. Catalyst a

XAnatase dAb, (nm) , (%)

dRb, (nm)

DXRDc, (nm)

d (inter Lattice plane strain spacing, Å)d

Band gap, (eV )

FSP TiO2

81

27

130

46

3.512

-

3.10

FSP Cr/TiO2

61

23

120

61

3.509

0.0008

2.61

Used R3 Cr/TiO2

FSP 59

34

103

62

3.525

0.0007

2.59

FSP Si/Ti=50

56

33

59

45

3.524

0.0011

2.22

FSP Si/Ti=60

66

39

96

58

3.508

0.0017

2.21

FSP Si/Ti=70

79

38

61

43

3.488

0.0021

2.14

FSP Si/Ti=75

94

35

122

40

3.505

0.0026

2.11

FSP Si/Ti=80

91

25

133

34

3.508

0.0034

1.96

FSP M/Cr=30

93

42

8

40

3.513

0.0018

1.90

FSP M/Cr=25

86

37

2

32

3.498

0.0024

1.86

FSP M/Cr=15

70

43

527

186

3.501

0.0018

1.73

FSP M/Cr=10

55

56

600

303

3.491

0.0028

2.00

FSP M/Cr=8

53

55

324

181

3.514

0.0032

2.08

Used R5 M/Cr=25

FSP 88

47

547

107

3.489

0.0023

1.88

a

Xanatase (%) = 100 – Xrutile (%). dA and dR are the crystallite sizes of anatase and rutile, respectively. c DXRD is calculated based on mass weight average density of anatase and rutile phases determined by XRD. d in anatase (101) plane. b


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 47

Table 2. Specific surface area and pore volume of Cr/Ti/Si composites as a function of Si/Ti and (Si+Ti)/Cr atomic ratios. Catalyst

Isotherm

SSA (m2/g)

typea

dBETb

Pore volume Pore Diameter

(nm)

(cm3/g)

(nm)

FSP TiO2

IV

98.6

15

0.28

11.8

FSP Cr/TiO2

IV

56.7

26

0.17

11.6

Used R3 FSP Cr/TiO2

IV

52.8

28

0.30

20.0

FSP Si/Ti=50

IV

166

09

0.48

12.8

FSP Si/Ti=60

IV

172

09

0.90

22.1

FSP Si/Ti=70

IV

192

08

0.60

14.0

FSP Si/Ti=75

IV

221

07

0.80

15.5

FSP Si/Ti=80

IV

250

06

1.10

20.1

FSP M/Cr=30

IV

235

07

0.90

17.9

FSP M/Cr=25

IV

215

07

0.65

12.8

FSP M/Cr=15

IV

168

09

0.70

17.7

FSP M/Cr=10

IV

158

09

0.57

15.9

FSP M/Cr=8

IV

153

10

0.70

19.6

Used R5 FSP M/Cr=25 IV

209

07

0.50

10.0

a

Isotherm type is based on IUPAC nomenclature. dBET is calculated based on mass weight average density of anatase and rutile phases determined by XRD. b


46

Page 47 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical Abstract

Stabilization of Cr in Ti/Si/Cr ternary composites by aerosol flame spray-assisted synthesis for visible-light-driven photocatalysis Siva Nagi Reddy Inturi,ǂ Thirupathi Boningari,ǂ Makram Suidan,*† Panagiotis G. Smirniotisǂ ǂ Chemical Engineering Program, College of Engineering & Applied Science , University of Cincinnati, Cincinnati, OH 45221-0012, USA. † Engineering College and Architecture, American University of Beirut, Beirut1107-2020, Lebanon.


47

Cr Ternary Composites by Aerosol Flame

Recommend Documents