Article pubs.acs.org/est
Nitrite Reduction Mechanism on a Pd Surface Hyeyoung Shin,∥,† Sungyoon Jung,∥,‡ Sungjun Bae,§ Woojin Lee,*,‡ and Hyungjun Kim*,† †
Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-Gu, Daejeon 305-701, Korea ‡ Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-Gu, Daejeon 305-701, Korea § ́ Ecole Nationale Supérieure de Chimie de Rennes, UMR CNRS 6226, 11 Allée de Beaulieu, 35708 Rennes Cedex 7, France S Supporting Information *
ABSTRACT: Nitrate (NO3−) is one of the most harmful contaminants in the groundwater, and it causes various health problems. Bimetallic catalysts, usually palladium (Pd) coupled with secondary metallic catalyst, are found to properly treat nitrate-containing wastewaters; however, the selectivity toward N2 production over ammonia (NH3) production still requires further improvement. Because the N2 selectivity is determined at the nitrite (NO2−) reduction step on the Pd surface, which occurs after NO3− is decomposed into NO2− on the secondary metallic catalyst, we here performed density functional theory (DFT) calculations and experiments to investigate the NO2− reduction pathway on the Pd surface activated by hydrogen. Based on extensive DFT calculations on the relative energetics among ∼100 possible intermediates, we found that NO2− is easily reduced to NO* on the Pd surface, followed by either sequential hydrogenation steps to yield NH3 or a decomposition step to N* and O* (an adsorbate on Pd is denoted using an asterisk). Based on the calculated high migration barrier of N*, we further discussed that the direct combination of two N* to yield N2 is kinetically less favorable than the combination of a highly mobile H* with N* to yield NH3. Instead, the reduction of NO2− in the vicinity of the N* can yield N2O* that can be preferentially transformed into N2 via diverse reaction pathways. Our DFT results suggest that enhancing the likelihood of N* encountering NO2− in the solution phase before combination with surface H* is important for maximizing the N2 selectivity. This is further supported by our experiments on NO2− reduction by Pd/TiO2, showing that both a decreased H2 flow rate and an increased NO2− concentration increased the N2 selectivity (78.6−93.6% and 57.8−90.9%, respectively).
■
copper (Cu) or nickel (Ni) to form a bimetallic catalyst.11−13 From previous studies investigating the catalytic role of Pd,1,14−16 it has been shown that the Pd surface cannot solely activate NO3−.17 Instead, the Cu or Ni reduces NO3− to NO2− coupled with metal oxidation, and then the converted NO2− is transferred to the adjacent Pd surface and further reduced to N2 or NH3 using surface hydrogens dissociated from H2 gas molecules on the Pd surface.14,15,18 Thus, it has been widely known that the selectivity toward N2 production versus NH3 production is mostly determined by the NO2− reduction stage on the Pd surface.17 To improve the N2 selectivity, it is important to elucidate the entire reduction mechanism of NO2− on the Pd surface and to identify key intermediates and reaction steps determining the reaction pathways. Many experimental studies have investigated the NO 2 − reduction mechanism on Pd-based catalysts,11,12,19−24 for which nitric oxide (NO) was widely
INTRODUCTION The removal of nitrate (NO3−) from contaminated groundwater is one of the most important environmental challenges. The process is complicated by the fact that NO3− can be easily converted into nitrite (NO2−) or N-nitroso compounds, which are known to be hazardous chemicals to human beings and cause serious health problems, such as blue baby syndrome, cancer, and hypertension.1−3 A variety of techniques, such as electrodialysis,4 reverse osmosis,5 ion exchange,6 and biological treatment,7,8 have been applied to the treatment of NO3− and NO2−; however, their technical drawbacks have been frequently reported (for example, NO3− condensed effluent and a huge amount of sludge1,2,9,10). Recently, the removal of NO3− and NO2− using noble metal-based catalysts has attracted attention as a promising solution not only because of the simple synthetic procedure of the catalyst but also because of the high activity and selectivity in producing harmless nitrogen gas (N2) over harmful ammonia (NH3) in the course of NO3− and NO2− reduction.9,10 Among the various noble metals, palladium (Pd) has been demonstrated to most effectively and selectively reduce NO3− to N2 when combined with a secondary metallic catalyst such as © 2014 American Chemical Society
Received: Revised: Accepted: Published: 12768
August 2, 2014 October 2, 2014 October 3, 2014 October 3, 2014 dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
Environmental Science & Technology
Article
discussed as one of the most important intermediates.20−24 Recently, Ebbesen et al. showed the presence of NO, amidogen (NH2), and ammonium cation (NH4+) on a Pd/Al2O3 surface using attenuated total reflection infrared spectroscopy and proposed that the N2 generation pathway is via the reduction of NO and that the NH3 (NH4+) production pathway is via the hydrogenation of NH2.22,23 On the other hand, using the Pd/ TiO2 surface, Sa and Anderson proposed that the reduction of nitrous oxide (N2O) mediates the N2 production, whereas the reduction of NO mediates the NH3 production, based on Fourier transform infrared spectroscopy measurements.24 Density functional theory (DFT), the practical method of choice for accurate quantum mechanics calculations on large systems, is now regarding as an indispensable tool to supplement experiments. It provides ground state total energies of reactants, intermediates, and products as a functional of the electron density, which can be employed to investigate full atomistic details of reaction pathways in wide applications.25−27 Therefore, the objectives of the present research are to provide (1) more comprehensive insight into reliable pathway of NO2− reduction and (2) further clear guideline to improve the selectivity toward N2, based on reaction energetics precisely determined from DFT calculations. In this study, we performed extensive DFT calculations to identify the reaction steps and intermediates for the NO2− reduction to N2 or NH3. We investigated the possible reaction pathways, mostly based on the relative energetics of the intermediate states, and we discussed that maintaining a low surface concentration of hydrogen atoms is important to enhance the N2 selectivity. We note that, to the best of our knowledge, this is the first theoretical study elucidating the full details of NO2− reduction on a Pd surface. We then experimentally examined the effect of the H2 flow rate and the NO2− concentration on the N2 selectivity.
adsorption sites on the Pd surface: the on-top, bridge, fcc, and hcp sites (SI Figure S1b). This leads to 104 (=26 × 4) sets of independent DFT calculations. We note that the intermediate states discussed below are the cases for which they are located at the most favorable adsorption sites, whereas all of the detailed results are listed in the SI Table S1. Catalyst Preparation. The Pd-based catalyst supported by TiO2 (Pd/TiO2) was synthesized by the impregnation method described in a previous study.14 The procedure can be briefly explained as the following: (1) deionized water (DI, 18 MΩ· cm) containing the support material (TiO2 (anatase), 99.7%, Sigma-Aldrich) was mixed with the Pd precursor solution (1 wt % of PdCl2−Pd, 99.9%, Sigma-Aldrich); (2) it was stirred vigorously for 2 h; (3) it was dried in an oven at 105 °C for 24 h; (4) it was calcined at 350 °C for 2 h; and (5) it was reduced by the dropwise addition of NaBH4 (0.01 M). The synthesized catalyst was immediately introduced into a glass batch reactor (500 mL, equipped with a mechanical stirrer, gas inlet, gas outlet, and two ports for injection and sampling) for the catalytic NO2− reduction test. Catalytic Reduction Test. The catalytic NO2− reduction test proceeded in the following manner: (1) deaerated deionized water (DDI, prepared by purging DI water with argon gas for 4 h) containing the Pd/TiO2 catalyst (0.12 g) was introduced into the glass batch reactor; (2) the reduction test was initiated by injecting various amount of the NO2− stock solution (20 000 mg/L as NO2−-N) to the reactor to obtain various concentrations of 10, 20, 30, and 50 mg/L as NO2−N; (3) H2 at various flow rates (10−300 mL/min) was continuously introduced into the solution during the reduction of NO2− and CO2 was provided at 40 mL/min to keep suspension pH at approximately 6; and (4) 4 mL of the samples were taken from the solution at each sampling time and used for measurement of the NO2− and NH3 concentrations by ion chromatography (IC, Metrohm) and UV/vis spectrophotometry (DR-5000, HACH). In this study, we assumed that most of the N2O can be rapidly and completely reduced to N2,31,32 and therefore we considered N2 and NH3 to be the final products. The N2 selectivity was calculated by subtracting the final concentration of the NO2− and NH3 from the initial concentration of NO2−. The morphological characteristics of the Pd/TiO2 catalyst were investigated by transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX, Tecnai G2 F30 S-twin, FEI). The results showed amorphous Pd/TiO2 catalysts with sizes ranging from 20 to 100 nm and ubiquitously dispersed nanoscale particles on the TiO2 support (SI Figure S2a and b). The EDX analysis demonstrated that the particles were Pd (SI Figure S2c), and the STEM-EDX mapping images indicate that the Pd particles were deposited successfully onto the surface of the TiO2 support (SI Figure S2d−g).
■
MATERIALS AND METHODS Computational Details. Density functional theory (DFT) calculations were performed to provide a mechanistic understanding of the reduction pathway of NO2− on the Pd surface. We used the Vienna Ab-initio Software Package (VASP) program.28 The Perdew-Burke-Emzerhof exchange-correlation functional coupled with an empirical van der Waals correction (PBE-ulg)29 was used to consider the London dispersion forces between adsorbates and the Pd surface. We performed dipole correction along the z-direction. We modeled the catalyst using a (3 × 3) Pd (111) surface with three layers, for which the bottom layer was fixed at a lattice point to approximate the bulk, whereas the upper two layers were allowed to relax. The simulation cell consists of a vacuum slab of 15 Å with the lattice constants (a, b) chosen from the bulk structure calculation of Pd, as described in the SI Figure S1a. We used a (4 × 4 × 1) Monkhorst set for the k-space integration and an energy cutoff of 450 eV for the plane wave basis set. We further adopted an implicit Poisson−Boltzmann (PB) solvation method,30 using a dielectric constant of 80 to describe the water-solvation energies. For the anion intermediate of NO2−, we included H3O+ in the simulation cell to neutralize the system. To examine the possible pathways for catalytic NO2− reduction on the Pd surface, we considered 26 possible intermediate species of the reduction steps. We then explored their adsorbed states (denoted using asterisks*) by comparing the adsorption energies for four possible high-symmetric
■
RESULTS AND DISCUSSION Initial Reaction Step. From the DFT calculations, we found that the adsorption of NO2− onto the Pd surface is energetically favored by 1.72 eV, yielding NO2−*, as denoted in eq 1. Then, N−O bond in NO2−* can be broken by H* (generated from H2 on the Pd surface), yielding H2O [ΔE = −4.08 eV; eq 2]. This initial reaction step corresponds to the IR spectroscopic experimental results22,24 reporting that NO2− can be easily adsorbed and converted into NO on the Pd surface. We further note that NO is quite stable as adsorbed on the Pd 12769
dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
Environmental Science & Technology
Article
surface forming NO* (the calculated binding energy is −2.55 eV), which supports the previous discussions that NO* is a key intermediate species with a substantial lifetime.22−24
the mostly energy consuming step, and the most probable pathway can be summarized as
NO2− + H3O+ + * → NO2−* + H3O+ ΔE = −1.72 eV (1)
NO2 *− + H3O+ + H* → NO* + 2H 2O + * ΔE = −4.08 eV
(2)
NO* + H* → HNO* + * ΔE = 0.97 eV
(3)
HNO* + H* → H 2NO* + * ΔE = 0.19 eV
(4)
H 2NO* + H* → H 2NOH* + * ΔE = −0.01 eV
(5)
H 2NOH* + H* → NH 2* + H 2O + * ΔE = − 0.68 eV (6)
Hydrogenation Pathways of NO on the Pd Surface. We investigated the sequential hydrogenation pathways of NO*. It is assumed that the source of hydrogen is the H atom dissociated from H2 on the Pd surface, that is, H*. Depending on the hydrogenation site between the N and O of NO*, two possible intermediates can be formed: HNO* and NOH*. The DFT energetics found that the formation of both species is endothermic; the formation of HNO* requires lower energy (ΔE = 0.97 eV) than the formation of NOH* (ΔE = 1.60 eV), indicating that the more probable pathway is via the hydrogenation of the N site. Figure 1 shows the subsequent hydrogenation pathways (including possible intermediates) and their relative DFT
NH 2* + H* → NH3* + * ΔE = −0.29 eV
(7)
NO Dissociation on the Pd Surface. Although N* formation originated from the hydrogenation of NO* is unfavorable, due to the highly unstable intermediate species of NOH* (see Figure 1), we found that the decomposition reaction of NO* to N* and O* is energetically probable (ΔE = 0.33 eV), for which O* can be further reduced to H2O by using two H* species (ΔE = 0.30 eV). It is notable that the NO dissociation pathway is supported by a previous experimental observation.33 After N* is formed, the sequential hydrogenations are mostly energetically favorable (Figure 2): NO* + * → N* + O* ΔE = 0.33 eV
(8)
O* + 2H* → H 2O + 3* ΔE = 0.30 eV
(9)
N* + H* → NH* + * ΔE = −0.21 eV
(10)
NH* + H* → NH 2* + * ΔE = 0.04 eV
(11)
NH 2* + H* → NH3* + * ΔE = −0.29 eV
(12)
Therefore, the sequential hydrogenations of N* after its formation from NO* dissociation (eqs 8−12) form a more reliable NH3 formation pathway compared to the sequential hydrogenations of NO* (eqs 3−7).
Figure 1. Reaction energy diagram for the hydrogenation pathway of NO* on the Pd (111) surface. This pathway preferentially yields NH3. The inset shows the side view (top) and top view (bottom) of each intermediate. Blue, red, white, and green atoms represent N, O, H, and Pd atoms, respectively. Figure 2. Reaction energy diagram for the decomposition of NO* followed by sequential hydrogenations on the Pd (111) surface. Although the direct combination of two N* can yield N2, the hydrogenations of N* to yield NH3 is more plausible because the migration barrier of N* is substantial, whereas that of H* is low (see Figure 3). The inset shows the side view (top) and top view (bottom) of each state. Blue, red, white, and green atoms represent N, O, H, and Pd atoms, respectively.
energetics. After the formation of HNO*, the second hydrogenation step forming H2NO* or HNOH* is also slightly uphill by 0.19−0.26 eV, whereas the remaining hydrogenation steps are mostly downhill, eventually yielding NH3*. Thus, over the course of the NH3 production pathway via the hydrogenation of NO*, the first hydrogenation step is 12770
dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
Environmental Science & Technology
Article
Figure 3. Energy barriers for the surface migration of (a) N*, (b) H*, and (c) NO* on the Pd (111) surface, from the most favorable adsorption site, fcc (white ball in Figure 3d), to the second most favorable adsorption site, hcp (yellow ball in Figure 3d). H* is a highly migratory species, whereas N* is not. (d) The migration pathway of each adsorbate, where green, orange, and gray balls represent the top, middle, and bottom Pd atoms, respectively.
N2 Formation Pathways. The key question in understanding the N2 formation path is when the N−N bond will be developed over the course of the entire pathway. The direct combination of two N* species formed by two independent NO* dissociations (eq 8) provides the simplest path to yield N2 that is energetically favorable: N* + N* → N2* + * ΔE = −0.94 eV
(13)
This pathway should also involve the surface diffusion of N*. We thus calculated the migration barrier (ΔEa) of N* from the most stable 3-fold site ( fcc) to the next most stable 3-fold site (hcp) using the nudged elastic band (NEB) theory.34 ΔEa for N* diffusion is calculated as high as 0.75 eV, whereas ΔEa = 0.10 eV for H* diffusion (Figure 3). This implies that the direct combination pathway is feasible only when the distance between the two N* species is much smaller than the distance between N* and H*. Otherwise, the combination of N* with the migratory H* will kinetically dominate over the combination of the two less-mobile N* to form N2. Therefore, a low surface concentration of H* is required to make this pathway effective. We further investigated the feasibility of N−N bond formation between two independent NO* species, yielding ON2O*. We note that the migration barrier of NO* is moderate: ΔE a = 0.34 eV (Figure 3). However, the intermolecular interaction between two NO* molecules on the Pd surface is calculated as repulsive, as shown in Figure 4, resulting in no DFT optimized ON2O* structure on Pd even with various initial configurations, due to its highly disfavored energetics. The remaining possibility of N−N bond formation from N* or NO* is the combination of N* and NO*. The DFT energetics determined that this is also less probable, requiring a high energy of 1.14 eV: NO* + N* → N2O* + * ΔE = 1.14 eV
Figure 4. Interatomic potential energy between two NO* species adsorbed on the Pd (111) surface. The NO* species are repulsive to each other, and thus no N−N bond formation between the NO* species is expected.
combination of two N* species, which is kinetically not very favorable. We thus considered another N−N bond formation possibility, via a redox reaction. The NO2− species should be reduced on the Pd surface during the entire reaction pathway. Considering that eq 1 is a large downhill process, NO2− reduction is expected to occur rather stochastically via nonspecific binding on the Pd surface. We considered the reaction pathway in which the NO2− in the solution phase is reduced near the N* by consuming H* on the surface and the proton in the solution phase (Figure 5): N* + NO2− + H3O+ + H* → N2O* + 2H 2O + * ΔE = −3.56 eV
(15)
This pathway is computed as energetically favored by 3.56 eV from the DFT energetics, and it produces N2O* as an intermediate species. This pathway explains the previous labeling experiment showing the complete N2O reduction to N2,31 which implies that N2 can be formed via the N2O intermediate. When N2O* is formed, it can be further reduced to N2 by a series of hydrogenation with H* (Figure 6):
(14)
Accordingly, our DFT energetics rule out all of the considered N−N bond formation possibilities via surface reactions between the stable intermediate species having non-hydrogenated N centers, i.e., the N* and NO* species, except for the direct 12771
dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
Environmental Science & Technology
Article
To enhance the selectivity toward N2 formation, our mechanistic understanding indicates that minimizing the surface concentration of H* is important for developing an N−N bond prior to hydrogenation of the N*. Additionally, the concentration of NO2− impacts its reduction selectivity to N2 by participating in the formation of N2O, which is a key intermediate. Catalytic NO2− Reduction via Pd/TiO2. We experimentally examined the effect of the concentrations of H* and NO2− to support the theoretical DFT calculation results by conducting the catalytic NO2− reduction by Pd/TiO2 at various H 2 flow rates (30−300 mL/min) and NO 2 − concentrations (10−50 mg/L) with a constant CO2 flow rate (40 mL/min). First, the selectivity toward NH3 showed an increasing trend (6.4−21.4%) at 50% of NO2− conversion as the H2 flow rate increased, whereas the N2 selectivity decreased (93.6−78.6%) (Figure 7a). When the H2 flow rate increased, it
Figure 5. Reaction diagram for the reduction of NO2− (solution phase) in the vicinity of N* to yield N2O* on the Pd (111) surface. The net reaction consumed one hydronium ion (H3O+) and one H* while producing two water molecules. Blue, red, white, and green atoms represent N, O, H, and Pd atoms, respectively.
N2O* + H* → N2* + OH* ΔE = −1.70 eV
(16)
OH* + H* → H 2O + 2* ΔE = 0.34 eV
(17)
Figure 7. Experimental transformation product (N2 and NH3) selectivity at 50% of NO2− conversion under various (a) H2 flow rates (30, 50, 100, 200, and 300 mL/min) maintaining the initial NO2− concentration as 30 mg/L, and (b) NO2− concentrations (10, 20, 30, 50 mg/L); the Pd loading and CO2 flow rate were constant at 1 wt % and 40 mL/min, respectively. A decrease in the H2 flow rate increased the N2 selectivity (78.6−93.6%) and an increase in the NO2− concentration also increased it (57.8−90.9%).
Figure 6. Reaction energy diagram for the reduction of N2O* on the Pd (111) surface. This reaction preferentially produces N2. The inset shows the side view (top) and top view (bottom) of each state. Blue, red, white, and green atoms represent N, O, H, and Pd atoms, respectively.
elevates the surface concentration of H* on the Pd surface because H2 is dissociatively adsorbed on the Pd surface with a low energetic barrier.35 The increase in the amount of H atoms can cause a lower ratio of N species to H atoms (N:H ratio) on the Pd surface and provide greater opportunity for the N species to combine with the highly mobile H* which has migration barrier of 0.10 eV forming NH3 rather than with another less mobile N* species which has migration barrier of 0.75 eV forming N2. The experimental results can also support the theoretical demonstration that the reduction of NO* to
Another reaction pathway toward NH3* formation that hydrogenates the N atom of N2O*, yielding HN2O* and then NH* + NOH*, is also investigated using DFT energetics (Figure 6). Although the DFT energetics for this pathway shows that this pathway is also plausible, the thermodynamic driving force toward the N2 formation pathway is much larger, as shown in Figure 6. 12772
dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
Environmental Science & Technology
■
NH3* can occur under the H*-rich condition on the Pd surface by the entropic effect. Second, a decreasing trend in the NH3 selectivity (42.2−9.1%) was observed at 50% of NO2 − conversion as the NO2− concentration increased, whereas the N2 selectivity significantly increased (57.8−90.9%) (Figure 7b). The increase in the NO2− concentration can imply that more NO2− molecules are on the Pd surface, which can cause a higher N:H ratio and generate greater possibility for the N species to bind with other N species to form N2. In the theoretical section, we also confirmed that NO2− can favorably react with N* to form N2O with the negative reaction energy (ΔE = −3.56 eV). The generated N2O can be further reduced to N2 by the consecutive reactions with H* on the Pd surface rather than to NH3 due to the very negative reaction energy for the transformation to N2. Caveats. In this study, we discussed extensively the thermodynamics of all possible stable ground states, but we did not perform transition state calculations to determine the energy barrier of each reaction due to the limited computational cost except for the migration barriers of N*, H*, and NO*. Because the pathways do not include transition state energies, there could be additional barriers and local minima between neighboring ground states, and thereby, one should use care when interpreting the decomposition reaction pathways suggested based on chemical and energetic proximity of intermediate states. However, we note that similar approach has been successful in understanding catalytic reaction pathways of hydrocarbon decomposition on Pt surface25,26 and water decomposition on GaP surface.27 Environmental Implications. The NO2− reduction pathway usually has been proposed based on the measurement of detectable intermediates, relying on diverse assumptions.11,12,19−24 However, we herein provide an essential understanding of NO2− reduction on the Pd surface by demonstrating each reduction step with computational methods; NO* is the key intermediate and further dissociated to N* and O*, and N* is more favorably combined with highly mobile H* to form NH3 rather than combined with less mobile N* to form N2. Especially, we note that it is first time to suggest the N2 formation through N2O* which formed by the direct combination of N* and NO2− by consuming H* and proton. Based on the theoretical results, we also provide a guideline that maintaining low concentration of H* and high concentration of NO2− is important for maximizing selective and effective NO3− and NO2− reduction to N2 because of the difference in mobility of H* having migration barrier of 0.10 eV and that of N* having migration barrier of 0.75 eV. Furthermore, we experimentally confirmed the suggested guideline by controlling important environmental factors (i.e., the NO2− concentration and H2 flow rate). Recently, many studies have been undertaken to enhance the N2 selectivity, but they still report the production of a harmful final product (NH3).11,36,37 This could be a critical limitation for the application of Pd-based catalysts to the treatment of real groundwater contaminated with NO3− and NO2−. This research can provide a breakthrough to overcome the current environmental problem and can be applied to the development of smart Pd-based catalysts for the selective and effective reduction of NO3− and NO2− to N2.
Article
ASSOCIATED CONTENT
S Supporting Information *
Computational and experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*(W.L.) E-mail:
[email protected]. *(H.K.) E-mail:
[email protected]. Author Contributions ∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was partially supported by the National Research Foundation (NRF) funded by the Korean Ministry of Education (2012-C1AAA001-M1A2A2026588) and the GAIA project funded by the Korean Ministry of Environment (ARQ201202076). We also appreciate for the support by the Integrated Water Technology (IWT) Project (2012M1A2A2026588) funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea, and the support by the Global Frontier R&D Program (2013M3A6B1078884) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.
■
REFERENCES
(1) Gao, W. L.; Guan, N. J.; Chen, J. X.; Guan, X. X.; Jin, R. C.; Zeng, H. S.; Liu, Z. G.; Zhang, F. X. Titania supported Pd-Cu bimetallic catalyst for the reduction of nitrate in drinking water. Appl. Catal., B 2003, 46, 341−351. (2) Chen, Y. X.; Zhang, Y.; Chen, G. H. Appropriate conditions or maximizing catalytic reduction efficiency of nitrate into nitrogen gas in groundwater. Water Res. 2003, 37, 2489−2495. (3) Gulis, G.; Czompolyova, M.; Cerhan, J. R. An ecologic study of nitrate in municipal drinking water and cancer incidence in Trnava District, Slovakia. Environ. Res. 2002, 88, 182−187. (4) El Midaoui, A.; Elhannouni, F.; Taky, M.; Chay, L.; Sahli, M. A. M.; Echihabi, L.; Hafsi, M. Optimization of nitrate removal operation from ground water by electrodialysis. Sep. Purif. Technol. 2002, 29, 235−244. (5) Schoeman, J. J.; Steyn, A. Nitrate removal with reverse osmosis in a rural area in South Africa. Desalination 2003, 155, 15−26. (6) Samatya, S.; Kabay, N.; Yuksel, U.; Arda, M.; Yuksel, M. Removal of nitrate from aqueous solution by nitrate selective ion exchange resins. React. Funct. Polym. 2006, 66, 1206−1214. (7) Park, J. Y.; Yoo, Y. J. Biological nitrate removal in industrial wastewater treatment: which electron donor we can choose. Appl. Microbiol Biot 2009, 82, 415−429. (8) Bhatnagar, A.; Sillanpaa, M. A review of emerging adsorbents for nitrate removal from water. Chem. Eng. J. 2011, 168, 493−504. (9) Xie, Y. B.; Cao, H. B.; Li, Y. P.; Zhang, Y.; Crittenden, J. C. Highly Selective PdCu/Amorphous Silica-Alumina (ASA) Catalysts for Groundwater Denitration. Environ. Sci. Technol. 2011, 45, 4066− 4072. (10) Matatov-Meytal, Y.; Barelko, V.; Yuranov, I.; Kiwi-Minsker, L.; Renken, A.; Sheintuch, M. Cloth catalysts for water denitrification II. Removal of nitrates using Pd-Cu supported on glass fibers. Appl. Catal., B 2001, 31, 233−240. (11) Horold, S.; Tacke, T.; Vorlop, K. D. Catalytical removal of nitrate and nitrite from drinking-water 0.1. Screening for hydrogenation catalysts and influence of reaction conditions on activity and selectivity. Environ. Technol. 1993, 14, 931−939. 12773
dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
Environmental Science & Technology
Article
(12) Horold, S.; Vorlop, K. D.; Tacke, T.; Sell, M. Development of catalysts for a selective nitrate and nitrite removal from drinking-water. Catal. Today 1993, 17, 21−30. (13) Soares, O. S. G. P.; Orfao, J. J. M.; Pereira, M. F. R. Bimetallic catalysts supported on activated carbon for the nitrate reduction in water: Optimization of catalysts composition. Appl. Catal., B 2009, 91, 441−448. (14) Jung, J.; Bae, S.; Lee, W. Nitrate reduction by maghemite supported Cu-Pd bimetallic catalyst. Appl. Catal., B 2012, 127, 148− 158. (15) Bae, S.; Jung, J.; Lee, W. The effect of pH and zwitterionic buffers on catalytic nitrate reduction by TiO2-supported bimetallic catalyst. Chem. Eng. J. 2013, 232, 327−337. (16) Liou, Y. H.; Lin, C. J.; Weng, S. C.; Ou, H. H.; Lo, S. L. Selective decomposition of aqueous nitrate into nitrogen using iron deposited bimetals. Environ. Sci. Technol. 2009, 43, 2482−2488. (17) Prusse, U.; Vorlop, K. D. Supported bimetallic palladium catalysts for water-phase nitrate reduction. J. Mol. Catal. a-Chem. 2001, 173, 313−328. (18) Guy, K. A.; Xu, H. P.; Yang, J. C.; Werth, C. J.; Shapley, J. R. Catalytic nitrate and nitrite reduction with Pd-Cu/PVP colloids in water: composition, structure, and reactivity correlations. J. Phys. Chem. C 2009, 113, 8177−8185. (19) Matatov-Meytal, Y.; Barelko, V.; Yuranov, I.; Sheintuch, M. Cloth catalysts in water denitrificationI. Pd on glass fibers. Appl. Catal., B 2000, 27, 127−135. (20) Gao, W. L.; Chen, J. X.; Guan, X. X.; Jin, R. C.; Zhang, F. X.; Guan, N. J. Catalytic reduction of nitrite ions in drinking water over Pd-Cu/TiO2 bimetallic catalyst. Catal. Today 2004, 93−5, 333−339. (21) Ilinitch, O. M.; Nosova, L. V.; Gorodetskii, V. V.; Ivanov, V. P.; Trukhan, S. N.; Gribov, E. N.; Bogdanov, S. V.; Cuperus, F. P. Catalytic reduction of nitrate and nitrite ions by hydrogen: investigation of the reaction mechanism over Pd and Pd-Cu catalysts. J. Mol. Catal. A: Chem. 2000, 158, 237−249. (22) Ebbesen, S. D.; Mojet, B. L.; Lefferts, L. In situ ATR-IR study of nitrite hydrogenation over Pd/Al2O3. J. Catal. 2008, 256, 15−23. (23) Ebbesen, S. D.; Mojet, B. L.; Lefferts, L. Effect of pH on the nitrite hydrogenation mechanism over Pd/Al2O3 and Pt/Al2O3: details obtained with ATR-IR spectroscopy. J. Phys. Chem. C 2011, 115, 1186−1194. (24) Sa, J.; Anderson, J. A. FTIR study of aqueous nitrate reduction over Pd/TiO2. Appl. Catal.,B 2008, 77, 409−417. (25) Kua, J.; Goddard, W. A. Chemisorption of organics on platinum. 2. Chemisorption of C2Hx and CHx on Pt(111). J. Phys. Chem. B 1998, 102, 9492−9500. (26) Jacob, T.; Goddard, W. A. Chemisorption of (CHx and C2Hy) hydrocarbons on Pt(111) clusters and surfaces from DFT studies. J. Phys. Chem. B 2005, 109, 297−311. (27) Jeon, S.; Kim, H.; Goddard, W. A.; Atwater, H. A. DFT study of water adsorption and decomposition on a Ga-rich GaP(001)(2 × 4) surface. J. Phys. Chem. C 2012, 116, 17604−17612. (28) Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169−11186. (29) Kim, H.; Choi, J. M.; Goddard, W. A. Universal correction of density functional theory to include London Dispersion (up to Lr, Element 103). J. Phys. Chem. Lett. 2012, 3, 360−363. (30) Fishman, M.; Zhuang, H. L. L.; Mathew, K.; Dirschka, W.; Hennig, R. G. Accuracy of exchange-correlation functionals and effect of solvation on the surface energy of copper. Phys. Rev. B 2013, 87, 245402. (31) Zhang, R.; Shuai, D. M.; Guy, K. A.; Shapley, J. R.; Strathmann, T. J.; Werth, C. J. Elucidation of nitrate reduction mechanisms on a Pd-In bimetallic catalyst using isotope labeled nitrogen species. ChemCatChem. 2013, 5, 313−321. (32) Jung, S.; Bae, S.; Lee, W. Development of Pd-Cu/hematite catalyst for selective nitrate reduction. Environ. Sci. Technol. 2014, 48, 9651−9658.
(33) Obuchi, A.; Naito, S.; Onishi, T.; Tamaru, K. Mechanism of catalytic reduction of No by H2 or Co on a Pd Foil - Role of Chemisorbed Nitrogen on Pd. Surf. Sci. 1982, 122, 235−255. (34) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 2000, 113, 9901−9904. (35) Dong, W.; Hafner, J. H2 dissociative adsorption on Pd(111). Phys. Rev. B 1997, 56, 15396−15403. (36) Zhang, F. X.; Miao, S.; Yang, Y. L.; Zhang, X.; Chen, J. X.; Guan, N. J. Size-dependent hydrogenation selectivity of nitrate on Pd-Cu/ TiO2 catalysts. J. Phys. Chem. C 2008, 112, 7665−7671. (37) Matatov-Meytal, Y.; Shindler, Y.; Sheintuch, M. Cloth catalysts in water denitrification III. pH inhibition of nitrite hydrogenation over Pd/ACC. Appl. Catal., B 2003, 45, 127−134.
12774
dx.doi.org/10.1021/es503772x | Environ. Sci. Technol. 2014, 48, 12768−12774
