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All That Glitters Is Not Gold: A Probe Into Photocatalytic Nitrate Reduction Mechanism Over Noble Metal Doped and Undoped TiO

2

Swapna Challagulla, Dr. Kartick Tarafder, Ramakrishnan Ganesan, and Sounak Roy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07973 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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The Journal of Physical Chemistry

All that Glitters is not Gold: A Probe into Photocatalytic Nitrate Reduction Mechanism over Noble Metal Doped and Undoped TiO2 Swapna Challagulla1, Kartick Tarafder2, Ramakrishnan Ganesan1*, Sounak Roy1* 1

Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani, Hyderabad

Campus, Jawahar Nagar, Shameerpet Mandal, Hyderabad-500078, India 2. Department of Physics, National Institute of Technology Karnataka, Surathkal, Mangalore - 575 025 Karnataka, India.

Abstract: Photocatalytic reduction of aqueous nitrate has been thoroughly studied over noble metals doped and pristine TiO2 synthesized by a customized single step microwave assisted hydrothermal method. The synthesized catalysts are systematically characterized using XRD, Raman spectroscopy, FE-SEM, HR-TEM, XPS, diffuse reflectance spectroscopy, and PL measurements. The characterization reveals the successful synthesis of highly crystalline doped and undoped nano-TiO2. The photocatalytic rate of aqueous nitrate reduction over undoped TiO2 is found to be higher than that of noble metal doped TiO2. Mechanistic studies of the photocatalytic reduction are carried out with the help of different hole (oxalic acid, and methanol) and electron (sodium persulfate) scavengers, which reveal that the photo-generated electrons are the primary agents towards efficient nitrate photoreduction. Detailed studies have revealed that the noble metal doping in TiO2 helps in efficient photo-generation of H2 compared to the undoped analog, however, the in-situ produced H2 is found to be inefficient in reducing NO3-. The conduction band position from first principle calculations with respect to the nitrate and hydrogen reduction potentials derived from cyclic voltammetry provide insights to the electron transfer process in determining the reaction pathway.

*Corresponding Author(s) Email: [email protected] Email: [email protected]

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Introduction: Due to the excessive industrial effluents, human sewage and nitrogeneous fertilizers from the agricultural sector, the concentration of nitrate is considerably rising in the ground water. As this is the primary source of drinking water, rise in nitrate or nitrite may cause serious global problems

in

health

and

environment.

Excessive

nitrate

in

body

may

cause

methomemoglobinemia in infants and this is potentially carcinogenic.1 Nitrate or nitrite in water can be converted into nitroarenes, which are known to affect the respiratory, cardio-vascular, and central nervous system and also known as endocrine disruptors to human beings.2 Therefore, the World Health Organization recommends that the concentration of NO3- in drinking water should be below 10 ppm and NO2- and NH4+ should be below 0.03 and 0.4 ppm, respectively.3,4 To meet the prescribed limits, there exists a significant importance for denitrification. Several methods have been developed for this purpose, which include ion exchange, adsorption, reverse osmosis, chemical methods, bioremediation, electrochemical, and photocatalytic reduction.5-13 Among them, photocatalytic conversion has been extensively developed in the recent past towards controlling the concentration of inorganic nitrates in drinking and industrial wate.10,14-18 Some of the major advantages of photocatalytic nitrate reduction are clean, room temperature processing, efficient energy management, and cost-effective. In photocatalysis, when light beam of suitable energy is incident on a semiconductor material, electrons from the valence band (VB) jump to the excited conduction band (CB) that produce electron-hole pairs called excitons. The chosen catalytic materials for photo-denitrification are the semiconducting materials like TiO2, ZnO, CdS, BaLa4Ti4O15 etc.19-26 Among these, TiO2 is an attractive material due to its cost-effective, non-toxic and efficient characteristics. TiO2 has a suitable band gap of ~3.2 eV. In addition, its reduction potential corresponding to the VB and CB lie in the optimal position to induce several oxidation and reduction reactions. The VB of TiO2 lies in the suitable range to oxidize species such as hydroxide to hydroxyl radical, whereas the CB lies in the suitable range to reduce various species like nitrate, nitrite etc.27 However, the photogenerated electrons in CB may also reduce water, and because of this competition between water reduction and nitrate reduction, it is always challenging to photocatalytically reduce NO3- in aqueous medium. In the absence of any other photoreaction, the photogenerated electron and hole undergo recombination. To minimize the electron-hole recombination, one of the popular strategies to


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achieve this is through usage of a hole scavenger, which typically scavenges the hole from the VB and thus minimizes the electron-hole recombination.28,29 The hole scavengers like alcohols or organic acids are popular choices. Formic or oxalic acid produce anions, which are believed to scavenge the holes to form CO2•- species. Many authors believe that the CO2•- species have strong reductive ability (E◦ (CO2/CO2•-) = −1.8 V) to reduce nitrate.27,30 Alcohols on the other hand, on scavenging holes, oxidize to aldehydes while producing gaseous hydrogen. It is believed that photochemically produced hydrogen is essential for the effective reduction of NO3−. And metal loading on TiO2 can produce H2 efficiently. Therefore, many metals, such as Pd, Pt, Rh, Ru, Ag along with sacrificial hole scavengers have commonly been used to improve H2 generation.10,14,16,30-34 Researchers have shown that noble metals like Pd, Au or Ag are indispensable for photoreduction of nitrate.35-37 On the other hand, it was also claimed that noble metals convert protons to hydrogen and therefore the conduction band electrons of the supporting semiconductor are not available for nitrate reduction.38,39 Therefore, it is important to understand the excited band position as a function of noble metal doping with respect to the nitrate and hydrogen reduction potentials to gain insights into the reduction mechanism. Hirayama and Kamiya combined a Pt/TiO2 photocatalyst for H2 generation and a SnPd/Al2O3 non-photocatalyst to conventionally reduce NO3- with the in-situ produced H2.40 In the absence of non-photocatalyst, the in-situ produced H2, however, did not reduce the nitrate. Thus, in spite of several exhaustive investigations, there exists a controversy whether the in-situ produced H2 or the photogenerated electrons are the key ones to efficiently reduce the aqueous nitrate. A better understanding of the reaction mechanism would not only help in designing the advanced catalytic materials but also in enhancing the product selectivity. In this work, we have synthesized high surface area nano crystalline pure phase TiO2 through a customized microwave assisted hydrothermal method and studied the aqueous nitrate reduction in the presence of different hole scavengers like methanol and oxalic acid. It may be noted that, in reality, the unconsumed methanol may introduce secondary contamination to drinking water, however, at this stage the present investigation is limited to the mechanistic exploration of photoreduction of nitrate. It can be noted that a prolonged light exposure would completely mineralize the unconsumed methanol. Noble metal doped TiO2 catalysts synthesized by microwave assisted hydrothermal method were explored for understanding the effect of H2



generation on photoreduction of aqueous nitrate. A thorough comprehensive study on reaction mechanism of photocatalytic denitrification was carried out. Materials and Methods: Titanium (IV) isopropoxide, silver nitrate, chloroauric acid, palladium nitrate, chloroplatinic acid hexahydrate, potassium nitrate, salicylic acid were procured from Sigma Aldrich and were used as-received. Oxalic acid, and urea were purchased from SD fine chemicals and used as-received. TiO2 was synthesized through a customized single step microwave assisted hydrothermal method.41 In a typical synthesis, 0.592 mL of titanium (IV) isopropoxide and 0.6 g of urea (in 1:5 ratio) in aqueous solution was placed in a sealed microwave vial. The reaction mixture was subjected to microwave heating with stirring at 130 °C for 2 h in a microwave reactor (Monowave 300, GmbH, Europe) operated at 850 W. After the microwave treatment, the obtained solid product was filtered off, washed with distilled water and dried in a desiccator. The 1% of noble metals such as Ag, Au, Pd and Pt doped TiO2 were also synthesized in a similar manner with the addition of suitable amount of the respective metal precursors to the titanium precursor solution. The actual loading of the noble metals was confirmed by energy dispersive X-ray fluorescence (PANalytical, Epsilon 1), and atomic absorption spectroscopy (Shimadzu AA 7000) of the residual supernatant of the synthesized materials. The noble metal content was found to be in the range of 0.87 - 0.92 wt%. X-ray diffraction (XRD) of the synthesized catalysts was performed with Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at a scan rate of 1° min-1. Raman spectra were measured using UniRAM 3300 Raman microscope with an incident laser wavelength of 532 nm for structural analyses. Field Emission Scanning Electron Microscopy (FE-SEM) fitted with energy dispersive spectroscopy (EDS) (Carl-Zeiss ULTRA-55) and High resolution Transmission Electron Microscopy (HR-TEM) (Jeol, JEM 2100) was employed to analyze the surface morphology of the synthesized catalysts. X-ray photoelectron spectra (XPS) of the synthesized catalysts were recorded on PHI 5000 Versa Prob II (FEI Inc.) spectrometer using Al Kα radiation (1486.6 eV). Binding energies reported are with respect to C (1s) at 285 eV. Solid state UV and photoluminescence (PL) spectra were performed on JASCO V-670 UVvisible and JASCO FP-6300 spectrophotometer, respectively, to study the electronic properties


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of the synthesized materials. The measurements were performed in open atmosphere with the relative humidity of ~60% at room temperature. Photocatalytic reduction of nitrate was performed in a cylindrical annular batch photoreactor fitted with a medium pressure mercury vapor lamp of 125 W. The lamp radiated predominantly at 365 nm. The average energy of the light emitted was 3.5 eV with a corresponding photon flux of 5.86 × 10-6 mol of photons/s. The lamp was surrounded with a double-walled borosilicate immersion well and the set up was fitted inside a reaction vessel. To prevent IR radiation and to maintain constant temperature, water was constantly circulated around the lamp through the double-walled well. In a typical catalytic reaction, 100 mg of catalyst was added into the reactor containing 100 mL of 300 ppm nitrate (KNO3) solution in 50:50 methanol-water or in 4 mM oxalic acid aqueous solution and the pH was adjusted to 3.2. Methanol or oxalic acid was used as a hole scavenger. Argon gas was bubbled throughout the reaction to maintain an inert atmosphere inside the reaction vessel. Before irradiating light, the solution was stirred for 30 min in dark to reach the adsorption-desorption equilibrium. After the reaction was started with the illumination of the lamp, ~1-2 mL of the aliquot suspension was periodically withdrawn and studied for the progress of the nitrate reduction using UV-visible spectrophotometer (JASCO V-650). Nitrate concentration was monitored at 410 nm after complexation with Na-salicylate.42 To understand the relative position of the sub-band formed due to the doped metal atom in TiO2 , we have performed first-principles density functional theory (DFT) calculations using the full-potential Vienna ab-initio simulation package (VASP).43 The applied exchange– correlation functional was the GGA in the Perdew and Wang (PW91) parametrization.44 Projector augmented wave (PAW) potentials,45 were used and the wave functions were expanded in the plane wave basis with a kinetic energy cutoff of 500 eV. In the first step we optimized the TiO2 unit cell in anatase phase. To mimic the 1% doping, we made a 3X3X2 super cell that contains 72 Ti atoms, out of which one was replaced by noble metal atom. The large unit cell geometries were further optimized. In our simulations the forces on each of the atoms were calculated using the Hellmann-Feynman theorem, and were subsequently used to perform a conjugate gradient structural relaxation. The structural optimizations were continued until the forces on the atoms converged to less than 1 meV/Å. Calculated electronic structure of pure TiO2 in bulk anatase phase was compared with literature. It is well known that calculated band gap of



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TiO2 using DFT (GGA) is always less compared to the experimental value. This is due to the electron self-interaction term in DFT. Using exact-exchange method or self-interaction corrected DFT, the problem can be resolved. Using GGA+U method also one can remove this ambiguity. This method essentially prevents the unwanted delocalization of the d- or f-electrons, by adding a term to the Hamiltonian that increases the total energy when d-and f-electrons are located on the same cation. In case of oxidation reactions using TiO2 as catalyst, Ti-atoms change their valences from IV to III and back to IV. Therefore, a suitable U value for Ti is utmost essential. Many different U values are recommended for Ti in different situation ranges from 3 eV to 10 eV. Using 9 eV we found that the calculated band gap is 3.1 eV, which is comparable to the experimental band gap. In our simulation of doped systems, we used the same U = 9 eV value for Ti in all cases, and considered the value between 3 eV to 4 eV for noble metal atoms. Changing the U values of Noble metals, we have not found any changes of relative sub-band position contributed from doped noble metal atoms. The cyclic voltammetry (CV) with a computer controlled potentiostat (AUTOLAB PGSTAT302N, Metrohm Autolab B.V) was used to perform electrocatalytic reductions of aqueous nitrate. The CV setup consisted of three-electrode system, a platinum electrode as a counter electrode, an Ag/AgCl electrode as a reference electrode, and the working electrode. About 100 mg of catalyst with 30 wt % of graphitic carbon was mixed and made pellet in a customized glass jacket and used as working electrode. The electrolyte solution made with NaCl (1 M) in a 100 mL solvent mixture of 50:50 water-methanol containing 5000 ppm concentration of nitrate and pH was maintained at ~1.5 by adjusting with 1M HCl. To remove the dissolved oxygen, the electrolyte solution was purged with argon for 20 min and all the experiments were carried out at room temperature. The reduction potential of nitrate potential was measured in cyclic voltammetry method by varying voltage from 0.1 V to -2.0 V at a scan rate of 10 mV/s. Results & Discussion XRD patterns of the microwave synthesized pristine and noble metals doped TiO2 are shown in Figure 1. As evident from the Figure 1, pristine and noble metal doped TiO2 were found to crystallize in phase pure anatase structure (I41/amd, JCPDS # 89-4921).

The

synthesized catalysts showed broad diffraction peaks, indicating the formation of nanocrystalline domains. The crystallite size was calculated using Debye Scherrer’s formula and in all the cases


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it was found to be ~6 nm (Table 1). The diffraction peaks due to Ag, Pd and Pt were not found in the respective catalysts, which signifies high dispersion of the noble metals in the TiO2 matrix and/or formation of solid solutions.46,47 However, in case of Au/TiO2 the characteristic (200) plane at 44.1° was observed confirming the precipitation of Au nanoparticles. The major characteristic peak of Au at 38.1° due to (111) plane was merged with (004) of TiO2 as evident from the XRD profile. The metallic Au precipitation could be due to rapid reduction of Au precursor during the microwave heating. The Rietveld reﬁnements of the observed powder diffraction patterns of synthesized catalysts were performed using FullProf_Suite program by varying the parameters like overall scale factor, background parameters, unit cell, half width, shape and isotopic thermal parameters along with oxygen occupancy to determine the lattice parameters (Supporting Information, Figure S1). The best fitting was obtained by considering the noble metal substitution in the Ti atomic position, and the refined lattice parameters (in Table 1) were found to be matched well with the anatase phase of TiO2. No significant difference in lattice parameters was observed with metal doping, which could be due to the presence of minute quantity of the dopants. The insignificant change in unit cell volume may also suggest higher dispersion of the noble metals than the lattice substitution. However, a slight deviation of the unit cell volume of Au/TiO2 from pristine TiO2 could be attributed to the noble metal precipitation. Raman spectroscopy was also used to analyze the structural information of the synthesized catalysts and the results are shown in Figure 2. The undoped TiO2, and the Ag, Pd and Pt doped TiO2 showed similar spectra; a prominent band at 145 cm-1 (Eg), and weak bands at 393 (B1g), 505 (A1g), and 626 cm-1 (Eg), corresponding to the Raman active modes of pure anatase phase.48 The characteristic Raman active mode of rutile TiO2 (440 cm-1(Eg)) was not observed, confirming the pure anatase phase formation in all the catalysts. In Au/TiO2, the Eg band at 145 cm-1 was blue-shifted to 154 cm-1and other Eg band at 626 cm-1 was red-shifted to 619 cm-1. This could be attributed to the electronic interference from the plasmonic gold nanoparticles with the excitation laser wavelength due to the strong metal-support electronic interaction as evidenced from the earlier literature.49,50 FE-SEM was employed to study the morphology of the synthesized catalysts and the EDS spectra showed the surface concentration of noble metals are at par with the feed ratios. (Figure S2, and S3, respectively). HR-TEM imaging was performed on the synthesized materials for detailed morphological investigation (Figure 3). The HR-TEM analyses of pristine TiO2



corroborated the particle sizes observed from FE-SEM as in the range of sub-10 nm. The high magnification analysis revealed the lattice fringes of 3.6 Å corresponding to (101) plane of anatase TiO2. The Pd, Pt and Ag doped TiO2 also showed similar surface morphologies. The absence of lattice fringes corresponding to noble metals or noble metal oxides confirm the high dispersion or the formation of solid solution. Interestingly, in Au/TiO2 the lattice fringes due to anatase TiO2 and (111) of Au nanoparticles were observed. This is in accordance with the XRD and Raman studies. The indexed SAED patterns confirm the phase formation. The diffraction ring due to (111) plane of Au was merged with (004) of TiO2 in SAED, as was also observed in XRD. The core level XPS of the synthesized samples revealed the oxidation state of the doped noble metals and are presented in Figure 4. In Figure 4a, the characteristic peaks at 367.9 and 373.9 eV respectively for Ag(3d5/2) and Ag(3d3/2) revealed the Ag in zero valent oxidation state. The narrow scan in Figure 4b represented Au(4f7/2) and Au(4f5/2) peaks at 83.22 and 86.99 eV, respectively indicatingpresence of metallic gold, which was also evident from XRD and Raman analyses. In Pd/TiO2, the Pd(3d5/2) core level binding energy at 336.5 eV and Pd(3d3/2) at 341.8 eV revealed Pd in its +2 oxidation state (Figure 4c). Similarly, the narrow scan of Pt in Pt/TiO2 showed peaks at 73.03 and 76.3 eV corresponding to Pt(4f7/2) and Pt(4f5/2) in mixed valence oxidation states of +2 and +4 (Figure 4d). The presence of Ti4+ was confirmed as Ti(2p3/2) and Ti(2p1/2) peaks were observed at 458.75 and 464.4 eV for all the samples (data not shown). The complete survey spectra of the noble metal doped synthesized catalysts are presented in Supporting Information (Figure S4) and the relative intensities indicate the content of the noble metals in the TiO2 samples. As Pd and Pt have been found in their higher oxidation states, where as Ag and Au are in metallic states, this may influence the band structure of TiO2. To explore this, the VB spectra of the synthesized oxides were obtained and are plotted in Figure 5. The binding energies of the onset edge reveal the energy gap between the VB top and Fermi level, and for all practical reason the VB spectra obtained from XPS coincide with the density of states (DOS).51,52 The TiO2 VB is primarily made up of O(2p) orbitals and the onset edge of the VB for pure TiO2 was obtained at 2.60 eV. A small but significant hump is being observed prior to the onset edge of VB for the metal doped catalysts. For metallic Ag and Au the onset edge was observed at 2.55 and 2.25 eV, respectively, which could be due to the contribution from Ag(4d) and Au(5f) states. The contribution from Pd(4d) and Pt(5f) to O(2p) of TiO2 was noteworthy, as


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significant wide humps with the onset of VB were observed at 1.72 eV for ionic Pd and 2.14 eV for ionic Pt. Apparently, the ionic species doping have modified the TiO2 VB more than the metallic ones. The diffuse reflectance (Tauc plot) and PL spectra of TiO2, and the noble metals doped TiO2 were collected in order to understand the electronic structure and the optical behavior of the materials. The plot of (K × E)1/2 vs E (Figure 6a), obtained from the diffuse reflectance spectra, showed the similar semiconducting band gap of ~ 3.6 eV for TiO2, Ag/TiO2, Pd/TiO2 and Pt/TiO2.On the other hand, Au/TiO2 showed slightly a wider band gap of ~3.68 eV and an additional strong surface plasmonic resonance of Au positioned at 2.27 eV. The PL spectra of the synthesized catalysts measured at room temperature with an excitation wavelength of 330 nm are plotted in Figure 6b. Generally, the higher PL intensity signifies higher electron-hole recombination. Among the lot, pristine TiO2 and Pt/TiO2 showed significantly higher intensity than the rest of the noble metal doped samples, indicating higher electron-hole recombination in these two cases. The Ag/TiO2 and Pd/TiO2 showed relatively lesser intensity signifying that the photoexcited electrons are drawn to the Ag and Pd sites and thus minimizing the electron-hole recombination. In case of Au/TiO2, the photoluminescence intensity was found to be significantly lower, which is in line with the literature.53-56 The Tauc plot of Au/TiO2 showed the presence of a strong Au plasmonic band in between the VB and CB. Therefore, the excited electron in CB of TiO2 might be redistributed to the plasmonic gold domains rather than recombining in a non-radiative thermal fashion to the holes in VB. This could be the reason of significant decrease in PL intensity. The synthesized noble metals doped and pristine TiO2 catalysts were studied for the photocatalytic reduction of nitrate. Initial investigations were performed in order to compare the effect of hole scavenger on nitrate photoreduction. Figure 7a shows the photoreduction studies of 300 ppm nitrate using pure TiO2 in various solvent media such as pure water, aqueous solution of 4 mM oxalic acid, and 50:50 methanol-water mixture. It can be seen from the Figure 7a that the rate of nitrate photoreduction was insignificant with pure water and 4 mM oxalic acid solution. The rate of nitrate reduction calculated at 10% of conversion with 50:50 methanolwater was found to be 4.02 × 10-4 moles lit-1 g-1 min-1. The results are compared with the literature and tabulated in Table 2. A wide range of light sources and hole scavengers have been



used as evident from the Table 2. It is known that TiO2 in water generates reactive radicals such as hydroxyl, superoxide etc, when irradiated with UV light. ௛ఔ

ି TiO2ሱሮ TiO2 + hା ୚୆ + eେ୆ + H2O + hା ୚୆ → H + OH

(1)

.

(2)

.ି O2 + eି େ୆ → Oଶ

(3)

No significant photoreduction of nitrate in water indicates that these reactive radicals generated by TiO2 in pure water had no photocatalytic activity towards nitrate reduction. When two hole scavengers such as oxalic acid and methanol were used, surprisingly, the photoreduction of nitrate occurred only in the presence of methanol but not with oxalic acid. This could be due to the difference in reaction mechanism, which has been addressed later. As methanol was found to be an effective and superior hole scavenger, further photocatalytic nitrate reductions were carried out with 50:50 methanol-water medium. The photoreduction of 300 ppm nitrate over noble metals doped TiO2was performed and compared with undoped TiO2 to understand the effect of these well-known reducing precious metals as dopants (Figure 7b). Apparently, in case of pure TiO2, ~80% of the nitrate was photoreduced in 180 min. The photoreduction of nitrate over Pd/TiO2, Pt/TiO2, and Au/TiO2was observed to be much poorer than the undoped pure catalyst, when compared under identical reaction conditions. Ag/TiO2 was comparatively better than the other noble metal doped catalysts, however, the overall performance was poorer than the undoped TiO2. These observations warranted further detailed mechanistic analyses to uncover the reaction mechanism. In a photocatalytic reaction, the hole scavengers like methanol or oxalic acid, typically scavenge the photogenerated holes (eqn. 4 & 6), and thereby minimizes the electron-hole recombination. This process makes the photogenerated electrons available for further reduction reactions. The various possible photocatalytic pathways are given in the following reactions: + RCH2OH + hା ୚୆ →RCHO + H

(4)

H2C2O4→2H+ + C2O42-

(5)

•C2O42- + hା ୚୆ →CO2 + CO2

(6)

2H+ + 2eି େ୆ → H2

(7)

2NO3− + 12H+ + 10CO2•−→ N2 + 6H2O + 10CO2 -

2NO3 + 5H2→ N2 + 4H2O + 2OH

-

(8) (9)


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2NO3− + 12H+ + 10eି େ୆ →N2 + 6H2O

(10)

The photogenerated electrons either reduce the nitrate (eqn. 10) or reduce the protons to produce H2 (eqn. 7), which eventually may reduce the nitrate (eqn. 9).40 Therefore, to probe which of the mechanisms is working over the microwave synthesized catalysts, we explored a comparative study between the nitrate photoreduction and H2 generation. The H2 generation studies using TiO2 in pure water, 50:50 methanol-water, and 4 mM oxalic acid solution were performed and plotted in Figure 8a. In all these studies, the concentration of nitrate solution and pH were kept as 300 ppm and ~3.2, respectively. As pure water has a poor hole scavenging ability, there was no hydrogen formation with TiO2 even by 180 min. On the other hand, the total amount of H2 produced by 180 min with TiO2 in 50:50 methanol-water and 4 mM oxalic acid solution was found to be 8.9 × 10-3 mmol and 9.5 × 10-3 mmol, respectively. Although the amount of H2 generated in these two solvent media is quite comparable, it is noteworthy that even at a lower concentration, the oxalic acid solution was found to yield larger amount of H2 than the 50:50 methanol-water medium. Comparing the nitrate photoreduction and H2 formation results, it is clear that the hole scavenger that yielded higher amount of H2 under identical conditions resulted in poorer nitrate photoreduction. The H2 production was also performed with the noble metals doped TiO2 and plotted in Figure 8b. Surprisingly, the amount of H2 produced by 180 min with Pd/TiO2and Pt/TiO2 in 50:50 methanol-water was found to be ~ 3.5 mmol, which is 400 fold higher than the corresponding undoped TiO2 catalyst. On the other hand, the H2 production over Au/TiO2 and Ag/TiO2 was found to be 0.6 and 0.3 mmol. The H2 production was highly favored with Pd and Pt incorporation and moderately favored with Au and Ag. The comparison between nitrate photoreduction and H2 formation results showed that the doping of Pd and Pt increased the H2 generation by 400 fold and decreased the rate of nitrate photoreduction by 15 fold compared to the undoped TiO2. These results conclude that there is a competition between in situ H2 production (eqn. 7) and nitrate reduction (eqn. 10). If the rate of reaction in eqn. 7 is faster than that of reaction in eqn. 10, the more H2 will be produced but nitrate reduction will be poorer. Therefore, for better nitrate reduction, H2 generation is not desired. To understand whether any partial nitrate reduction was mediated by the in situ produced H2 (eqn. 9), further studies were performed with purging of H2 gas (Figure S5 in the supporting information). Nonphotocatalytic nitrate reduction experiments in dark were performed using doped and undoped TiO2 catalysts both with and without purging of H2 gas and the results showed that the in-situ



produced H2 has no significant role in the reduction of nitrate. These observations confirm that the photogenerated electrons are the main species that are responsible for the nitrate reduction. To corroborate this, we performed an additional nitrate photoreduction experiment in the presence of an electron scavenger, sodium persulfate. There was no nitrate photoreduction occurred in the presence of persulfate as it scavenged the photogenerated electrons (data not shown). To have additional insight to the energetics of the noble metal doped and pristine TiO2 over the competing reactions between H2 generation and nitrate photoreduction, the CB and the VB positions were estimated by first principle calculations and were compared with the reduction potential of aqueous nitrate obtained from CV. Figure 9 (a-c) shows the DOS of pristine TiO2, and Pt and Ag doped TiO2 as representative cases. The theoretical band gap is very close to the value obtained from the diffuse reflectance experiments. For pristine TiO2, while the VB is primarily made up of O(2p), CB is made up of Ti(3d). For Pt/TiO2 there are additional sub-bands of Pt below the CB minima and above VB maxima. On the other hand, for Ag/TiO2 the sub-bands from Ag are localized only above the VB maxima. The CV measurements (Figure 10) revealed the nitrate reduction potential to be at -0.74 V vs. Ag/AgCl (-0.52 V vs. NHE). The VB and CB energetics of TiO2 obtained through XPS, Tauc plot and DOS are matched against the nitrate reduction potential obtained from CV to construct the energy level diagram as shown in Figure 9(d). It is evident from the Figure 9(d) that the CB minima of pristine TiO2 are favorably positioned above the nitrate reduction potential. Therefore, the photoexcited electron in the CB of pristine TiO2 may easily hop down from the CB minima to nitrate for a facile nitrate reduction, as observed experimentally. With Pt doping, as there are available sub-bands below the CB minima, the photoexcited electron may drain down to the sub-bands positioned below the nitrate reduction potential resulting in poor nitrate reduction efficacy. However, the hydrogen reduction potential is energetically placed below the Pt sub-bands. This could be beneficial for efficient hydrogen formation over Pt/TiO2, which was also observed experimentally. Additionally, the noble metals like Pt and Pd are known for their hydrogen affinity and reducing catalytic properties. For Au/TiO2, we have observed the additional plasmonic band from diffuse reflectance study, which could be responsible for lower efficacy of the catalyst for nitrate reduction due to the non-radiative thermal recombination as discussed earlier. On the other, as


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Page 13 of 33 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 Ag sub-bands are far from the CB, the photoexcited electron prefer to reduce nitrate that could be attributed to the higher nitrate reduction efficacy of Ag/TiO2 among the doped catalysts. We observed that the nitrate reduction is in competition with hydrogen formation, and the photoexcited electrons are the key species responsible for the reduction. Therefore, the energetics of the photoexcited electrons in the available bands played a crucial role. The sub-energy bands created due to the noble metal doping affected the nitrate reduction efficacy. Conclusion Undoped and noble metals doped TiO2 were synthesized by customized microwave assisted hydrothermal method. The XRD and the Raman spectroscopy showed the synthesized catalysts were of anatase phase, whilst FE-SEM and HR-TEM showed sub-10 nm crystalline particles in all the materials. XPS studies revealed that Au and Ag were in their metallic state, whereas Pt and Pd were ionic in nature. Among all the catalysts, Au/TiO2 showed the plasmonic band in Tauc plot. With methanol as a suitable hole scavenger, the photocatalytic nitrate reduction was found to be superior with pristine TiO2 to the noble metals doped TiO2. Interestingly, the nitrate reduction efficiency was inversely proportional to the H2 generation efficacy across the catalysts. Detailed mechanistic investigation revealed the competitive nature between the two aforementioned reactions. It was also found that the in-situ generated H2 was not efficient in reducing the nitrate, but the nitrate reduction was predominantly mediated through the photogenerated electrons. This work demonstrates that the better nitrate photoreduction can be achieved with proper choice of hole scavengers without the necessity to use noble metals. The sub energy bands below the CB created due to the noble metals doping determines the pathway of electron hoping.

Associated Content Supporting Information: Rietveld reﬁnements, FE-SEM images, EDS spectra, complete survey spectra of the noble metal doped synthesized catalysts, Non-photocatalytic nitrate reduction experiments performed using pristine TiO2. Acknowledgement: SR and RG thank the Department of Science & Technology (SERB/F/825/2014-15 and SERB/F/4864/2013-14) for the financial aid. The authors also thank Department of Science and



Page 14 of 33

Technology – fund for improvement of science and technology infrastructure (DST FIST; SR/FST/CSI-240/2012). SR thanks BITS Pilani Hyderabad Campus for the financial assistance under “Additional Competitive Research Grant”. Thanks are due to Dr N. Anbananthan of Ion Cell Volume

Cell Parameters (Å) Sample

χ2

Crystal size (nm)

3

a

b

c

(A )

TiO2

3.791539

3.791539

9.454658

135.95

1.66

6.34

1% Pd/TiO2

3.789084

3.789084

9.456118

135.76

1.57

6.00

1% Pt/TiO2

3.799229

3.799229

9.488122

136.9

1.65

5.56

1% Ag/TiO2

3.790404

3.790404

9.468064

136.02

1.38

6.05

1% Au/TiO2

3.817915

3.817915

9.473905

143.9

2.97

6.15

Exchange (India) Ltd for the discussion on electrochemical study. Table 1: The refined lattice parameters, cell volume, and crystallite size of synthesized catalysts.

Table 2: Comparison of literature on photocatalytic nitrate reduction.

CatalystReference

Condition

Light Source

Hole Scavenger

Comment

(Conc.) 1 % Ag/TiO2 (Hombikat)57

100 ppm of nitrate solution, 0.378 g/L catalyst

UV (110 W Hg lamp)

Formic acid (0.04M)

TiO2 particle size and concentration of hole scavengers played role in catalytic activity. Rate: 0.287 mmol min-1 gcat-1.

1%Au/TiO2 (Hombikat)38

100 ppm of nitrate solution,

UV

Oxalic acid

(400 W)

(0.008M)

0.250 g/L catalyst


Mechanism proposed over competition of conduction band electron between proton and nitrate.

Page 15 of 33 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


Rate: 0.365 mmol min-1 gcat-1. 0.1%Pt/SrTiO3:Rh + (2.3%)Sn(4.2%)Pd/ Al2O358 1%Ag/TiO2 (P90)59

80 ppm of nitrate solution, 2 g/L+ 0.6 g/L catalyst 100 ppm of nitrate solution, 1 g/L catalyst

(5%)Pt(1.25%)Cu/TiO260

100 ppm of nitrate solution, 1 g/L catalyst

1 % Ag/TiO230


CuFe0.7Cr0.3S2 loaded with (0.75%) Pd and (3%) Au35


Ag nanoparticles onto {1 0 1} facets of TiO2 nanocrystals with co-exposed


3 g/L catalyst

0.5 g/L catalyst

H2 produced from photocatalyst used as reductant for non-photocatalytic nitrate conversion.

Visible

Methanol

(300W Xe lamp)

(10 vol%)

UV

Formic acid

(450 W Hg lamp)

(0.04M)

UV

Benzene

(250 W Hg lamp)

(10ppm)

UV

Formic acid (0.04M)

Presence of other anions in drinking water affected the nitrate reduction. Rate: 24 mmolmin-1 gcat-1.

Sodium oxalate

Noble metals like Pd and Au was crucially responsible for photoreduction of nitrate.

(125 W Hg lamp) UV (500 W Hg lamp)

(0.01M)

Visible

Formic acid

(300 W Xe lamp)

(NO3:HCOOH =5:1)

Conduction band electrons rather than in-situ formed radicals were responsible for nitrate reduction. Calcination temperature and ratio of Pt/Cu and loading of metal were responsible for catalytic activity and N2 selectivity.

The selective deposition of silver nanoparticles on {1 0 1} facets of TiO2 decreased the recombination of charge carriers and enhanced the photocatalytic activity.

{0 0 1}/{1 0 1} facets36

Ag-TiO232


(0.5%) Ni BaLa4Ti4O15 layered perovskite26


(1%)Pd(0.5%)Cu/TiO2 P2537


1.43 g/L catalyst


UV

Oxalic acid

(Four 8 W Hg lamp )

(0.012M)

UV

Boric acid

(400 W Hg lamp)

(as Buffer)

UV

Oxalic acid

(400 W Hg lamp)

(0.04M)


Pseudo first order of nitrate reduction was established.

Loading of Ni as cocatalyst and pH controlled by boric acid buffer improved the photocatalytic nitrate reduction. In basic reaction medium noble metal Pd was indispensable for reduction.


(0.5%)Pt/TiO2 +(2.3%)Sn(4.2%)Pd/Al2O340 (1%)Cu/MgTiO3TiO261

1%Pd-1%Cu/TiO262

100 ppm of nitrate solution, 2 g/L+0.4 g/L catalyst 100 ppm of nitrate solution,

Visible (300 W Xe lamp) UV

Glucose (0.001M)

Anions like sulfates silicates and organic compounds in ground water decreased the activity by poisoning the catalysts.

Sodium oxalate

The formation of MgTiO3-TiO2interface helped the photoreduction.

1 g/L catalyst

(125 W Hg lamp)

(0.005M)


UV-Visible (λ≥365 nm)

Formic acid (0.04M)

CO2 and H2wasused as buffer and reducing agent respectively to achieve complete conversion.

UV

Oxalic acid

(125 W Hg lamp)

(0.005M)

Photogenerated holes and electrons are trapped by metals Ni and Cu in the TiO2 surface and prevent recombination reaction.

0.526 g/l catalyst 4%Ni-Cu/TiO2 (3:1)63

Page 16 of 33

100 ppm of nitrate solution, 1g/L catalyst

(Present work)

Photogenerated electrons are liable for reduction of nitrate.

TiO2

Rate: 4.02 × 10-4 mol lit-1g-1min-1 1% Pd/TiO2 1% Pt/TiO2 1% Ag/TiO2


UV

Methanol

(125 W Hg lamp)

1% Au/TiO2

Rate: 2.29 × 10 mol lit g min

-5

-1 -1

-1

-5

-1 -1

-1


-5

-1 -1

-1

-5

-1 -1

-1




Page 17 of 33

(116) (220) (215)

(204)

(200)

(105) (211)

TiO2

(004)

(101)

Figure: 1

1 % Pd/TiO2

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


1 % Pt/TiO2

1 % Ag/TiO2

1 % Au/TiO2 Au (200)

10

20

30

40

50

60

70

80

2θ

Figure 1: XRD patterns of the pristine and noble metals doped microwave synthesized TiO2 catalysts.



145

Figure: 2

626

505

393

TiO2

1 % Pd/TiO2

Intensity (a.u.)

1 % Pt/TiO2

1 % Ag/TiO2

154 0

200

400

619

501

1 % Au/TiO2 392

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 33

600

800 -1

Raman Shift (cm )

Figure 2: Raman spectroscopy of the pristine and noble metals doped TiO2.


1000

Page 19 of 33 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: 3

a

cc

b

(105) (200) (004) (101)

0.36 nm

5 1/nm

5 nm

50

d

50 nm

g

e

f

50 nm

50 nm

h

i

0.36 nm

(105)

0.24 nm

(200) (004)/ (111) (101)

2 nm

50 nm

10 1/nm

Figure 3: HR-TEM images of (a, b) pristine TiO , and (c) its selected area electron diffraction pattern, (d) 2

1% Pd/TiO , (e) 1% Pt/TiO , (f) 1% Ag/TiO , and (g, h) 1% Au/TiO (i) and its selected area electron 2

2

2

diffraction pattern.


2


Figure: 4

380

360

(a)

Ag 3d5/2

(b) Au 4f7/2

340

340

Ag 3d3/2

320

Intensity (a.u.)

Intensity (a.u.)

360

300 280 260

Au 4f5/2

320 300 280 260 240

240 220 360

365

370

375

380

220 80.0

385

82.5

320

(c)

310

87.5

90.0

92.5

95.0

(d)

340

Pd 3d5/2

Pt 4f5/2 Pt 4f7/2

320

Intensity (a.u.)

300

Pd 3d3/2

290

85.0

Binding Energy (eV)

Binding Energy (eV)

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 20 of 33

280 270 260

300 280 260 240 220

250 330

200 335

340

345

350

355

65

70

75

80

Binding Energy (eV)

Binding Energy (eV)

Figure 4: The core level XPS of (a) Ag (3d), (b) Au (4f), (c) Pd (3d), and (d) Pt (4f).


85

90

Page 21 of 33

Figure: 5

TiO2

1 % Pd/TiO2

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


1 % Pt/TiO2

1 % Ag/TiO2

1 % Au/TiO2

0.0

2.5

5.0

7.5

10.0

Binding Energy (eV)

Figure 5: The VB spectra of the pristine and noble metals doped TiO2.


12.5


Figure: 6

13

1/2

(k*hv)

TiO2

(a)

12

1 % Pd/TiO2

11

1 % Pt/TiO2

10

1 % Ag/TiO2

9

1 % Au/TiO2

8 7 6 5 4 1

2

3

4

5

6

7

Energy (eV) 4.0

TiO2

(b)

1 % Pd/TiO2

3.5

1 % Pt/TiO2

3.0

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 22 of 33

1 % Ag/TiO2 1 % Au/TiO2

2.5 2.0 1.5 1.0 0.5 350

400

450

500

550

600

Wavelength (nm) Figure 6: (a) The diffuse reflectance (Tauc plot) and (b) PL spectra of the synthesized catalysts. ACS Paragon Plus Environment

Page 23 of 33

Figure: 7

1.0

(a)

C/C0

0.8

0.6

Dark

50:50-H2O:MeOH

0.4

100 ml H2O 0.2 -30

40 mmol (Oxalic acid) 0

30

60

90

120

150

180

Time (min) 1.0

(b)

0.8

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


Dark 0.6

TiO2 1% Pd/TiO2 1% Pt/TiO2 1% Ag/TiO2 1% Au/TiO2

0.4

0.2 -30

0

30

60

90

120

150

180

Time (min) Figure 7: (a) The nitrate photoreduction studies using pure TiO2 in various reaction media,(b) the photoreduction of nitrate over pristine and noble metals doped TiO2 performed in 50:50 methanol-water mixture.



Figure: 8

10

(a) 50:50-water: MeOH 40 mM Oxalic acid 100 ml Water

H2 (µmol)

8

6

4

2

0 0

H2 (µmol)

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 33

4.5x10

3

4.0x10

3

3.5x10

3

3.0x10

3

2.5x10

3

2.0x10

3

1.5x10

3

1.0x10

3

5.0x10

2

30

60

90

Time (min)

120

150

180

(b) 1% Pd/TiO2 1% Pt/TiO2 1% Ag/TiO2 1% Au/TiO2

0.0 0

20

40

60

80

100 120 140 160 180

Time (min) Figure 8: (a) The H2 generation studies using pure TiO2 in various reaction media and (b) with noble metals doped TiO2 performed in 50:50 methanol-water mixture.


Page 25 of 33 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: 9

Figure 9: (a-c) The DOS of pristine TiO2, and Pt and Ag doped TiO2. (d) The schematic representation of the energy levels of VB and CB of TiO2 in comparison with the nitrate reduction potential.



Figure: 10

0.004 0.002

Current (A)

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 33

0.000 -0.002 -0.004 -

-0.006

No NO3 -

-0.008 -2.0

5000 ppm NO3 -1.5

-1.0

-0.5

0.0

Voltage (V (vs Ag/AgCl))

Figure 10: CV measurements with TiO2 electrode to probe the nitrate reduction potential.


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


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