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Catalytic Nitrate and Nitrite Reduction with Pd-Cu/PVP Colloids in Water: Composition, Structure, and Reactivity Correlations Kathryn A. Guy,† Huiping Xu,‡ Judith C. Yang,‡ Charles J. Werth,§ and John R. Shapley*,† Department of Chemistry and WaterCAMPWS, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Department of Materials Science and Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261, and Department of CiVil and EnVironmental Engineering and WaterCAMPWS, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 ReceiVed: NoVember 14, 2008; ReVised Manuscript ReceiVed: April 4, 2009
A set of bimetallic Pd-Cu/PVP (PVP ) poly(N-vinylpyrrolidone)) colloids, with copper proportions ranging from 0 to 50 atom %, has been examined as catalysts in a batch reactor with flowing hydrogen for the reduction of aqueous nitrate and/or nitrite. The encapsulated Pd-Cu nanoparticles were characterized by powder XRD, TEM, EDX, and IR of adsorbed CO. A significant decrease in average particle diameter and changes in the Pd-Cu crystallinity occurred above ca. 30% copper content, and this transition corresponded with a significant increase in observed nitrate reduction rates. The strong dependence on composition suggests that specific Cun ensembles on the surface of the Pd-Cu nanoparticles are needed for effective nitrate-to-nitrite conversion. In contrast, nitrite reduction rates were only minimally enhanced by the presence of copper. Increasing pH had little effect on the nitrate reduction rates, but it strongly inhibited the rate of nitrite reduction. The requisite protonation of a palladium-nitrite surface intermediate is proposed. Introduction
SCHEME 1: Stepwise Reduction of Nitrate to Dinitrogen
Elevated levels of nitrate are often found in drinking water supplies, especially in agricultural areas using nitrate rich fertilizers.1-4 To reduce the possibility of adverse health effects, such as methemoglobinemia in infants, the EPA and various governmental agencies have set limits on the amounts of nitrate and nitrite in drinking water.1,2,5-8 Interestingly, however, recent research has shown that increased nitrate and nitrite levels in the blood serum of adults may actually be beneficial by reducing the amount of muscle damage accrued during a heart attack.9,10 The removal of nitrate from drinking water is challenging due to its high solubility and high stability.3,5,6,11 Current technologies for removing nitrate include ion exchange,4,12,13 reverse osmosis,14 and biological denitrification.15 However, these techniques often generate cost issues due to specific requirements for supervision and maintenance or disposal of concentrated brines.3,14-17 The selective reduction of nitrate by using metal catalysts is a promising alternative for effective removal of this contaminant from drinking water.18 In the late 1980s, Vorlop and co-workers discovered that nitrate could be reduced primarily to dinitrogen with supported bimetallic hydrogenation catalysts; nitrite was an observed intermediate and ammonia was a coproduct.19,20 Palladium was found to be the most active and selective metal for the reduction of nitrite, but the reduction of nitrate required a second metal as a cocatalyst.19,20 The stepwise mechanism shown in Scheme 1 has been proposed for the reduction of nitrate through the formation of nitrite and other intermediates.19 Although only * To whom correspondence should be addressed. Phone: 217-333-0297. Fax: 217-244-3186. E-mail:
[email protected]. † Department of Chemistry and WaterCAMPWS, University of Illinois at Urbana-Champaign. ‡ Department of Materials Science and Engineering, University of Pittsburgh. § Department of Civil and Environmental Engineering and WaterCAMPWS, University of Illinois at Urbana-Champaign.
NO2- and N2O have actually been detected as intermediates,21-23 the formation and further reaction of NO adsorbed on the catalyst surface has commonly been suggested as the key step in determining the selectivity for dinitrogen vs. ammonia.19,24,25 The most frequently examined system for catalytic nitrate reduction is the combination of palladium and copper deposited on alumina or other supports.18-32 Such studies have shown that both the preparation method and the operating conditions affect the activity and selectivity of bimetallic Pd-Cu nitrate reduction catalysts.21,23,26,29,32 The distribution of palladium and copper on the support surface ranges from isolated phases to alloys depending on the preparation method.26 It has been suggested that mixed Pd-Cu sites show higher catalytic activity compared to the separated metal sites,23,24,33 but the relationship between the catalyst structure on the atomic level and the overall activity/ selectivity characteristics is not well established. The formation of nanoparticles offers a way to prepare bimetallic clusters with narrow size and composition distributions.34-40 Bimetallic Pd-Cu colloidal materials were prepared by Esumi and co-workers in 1990 by thermal decomposition of palladium and copper acetates in high-boiling organic solvents.41 Later researchers modified this method to use 2-ethoxyethanol as the solvent and reducing agent.34 Additionally, the reduction of the metal acetates was carried out in the presence of a protecting polymer such as poly(N-vinylpyrrolidone) (PVP) to control particle size and to increase stability. These polymer-protected bimetallic Pd-Cu colloids, which are dispersible in water, raised the possibility of studying how the nanoparticle composition and metal distribution affect catalytic nitrate reduction. We have adopted these procedures to prepare a set of Pd-Cu/PVP colloids with Pd:Cu atomic ratios ranging from 50:50 to 100:0,
10.1021/jp810049y CCC: $40.75 2009 American Chemical Society Published on Web 04/22/2009
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and we have examined this set of colloidal materials as catalysts for aqueous nitrate and nitrite reduction with dihydrogen. By probing the structures of the encapsulated bimetallic nanoparticles with various techniques, we have sought also to illuminate structure-reactivity correlations for the nitrate reduction process. Experimental Section Chemicals. Palladium(II) acetate and copper(II) acetate were purchased from Sigma-Aldrich and Mallinckrodt, respectively. Poly(N-vinylpyrrolidone) (PVP, MW ) 40 000) was purchased from Alfa Aesar. The solvents 2-ethoxyethanol (Sigma-Aldrich) and acetone (Fisher Scientific) were used as received. Tanks of hydrogen (99.95%) were supplied by Linde Gas. Carbon dioxide (99.9%) was supplied by S.J. Smith Welding Company. All reduction experiments were conducted in 18.2 MΩ cm-1 Milli-Q water generated in a Synergy 185 Millipore with Simpak2 purifying system. Instruments. Elemental composition of the catalysts was determined by ICP-MS in the Microanalysis Laboratory of the School of Chemical Sciences at UIUC. Infrared (IR) spectra were collected in a 1 mm solution IR cell with KBr windows with a Perkin-Elmer 1710 Infrared Fourier Transform Spectrometer. Powder X-ray data were collected on a Bruker general area detector diffraction system (GADDS). Electron microscopy images were recorded with a Jeol 2010 FEG TEM and a Philip Tecnai 20 FEG TEM operated at 200 Kv. Nitrate and nitrite levels were monitored with a Metrohm Basic 792 ion chromatograph with a Cetac AN1-SC column, equipped with a suppressed anion conductivity detector. Measurements of pH were taken with a ThermoOrion 420 meter and a standard pH electrode. Preparation of Pd/PVP Colloid. In a typical preparation, 0.898 g (4.00 mmol) of palladium(II) acetate and 1.700 g of PVP were transferred to a 100-mL round-bottomed Schlenk flask (equipped with a reflux condenser and a magnetic Teflon-coated stirring bar) and put under a nitrogen atmosphere. The solvent, 50 mL of 2-ethoxyethanol, was added by syringe to the solids, which resulted in a palladium concentration of 80 mM. The flask was then lowered into a 145((5) °C oil bath and the solution was brought to reflux. The solution began as a clear golden brown, but the reaction mixture turned opaque dark brown/black within 5 min of being heated. Heating was continued for 2 h to ensure complete reduction of the metal salts. The resulting colloidal dispersion was cooled to room temperature and stored under nitrogen until needed. Preparation of Pd-Cu/PVP Colloids. A set of bimetallic Pd-Cu colloids was prepared in the same manner as above, with the total metal element concentration kept at 80 mM. For a Pd50Cu50 colloid, 0.449 g (2.00 mmol) of palladium(II) acetate, 0.399 g (2.00 mmol) of copper(II) acetate, and 1.700 g of PVP were combined with 50 mL of 2-ethoxyethanol under a nitrogen atmosphere and heated at reflux for 2 h. The solution began as a clear blue-green, but it turned the characteristic opaque dark brown/black after a few minutes. The dark mixture was cooled to room temperature and stored under nitrogen until used. Colloids with Pd:Cu atomic ratios of 60:40, 70:30, 80:20, and 90:10 were similarly prepared. Samples from each of the colloid dispersions were filtered through a 0.2 µm nylon syringe filter and then dried under vacuum at room temperature for elemental analysis by ICP-MS. Elemental analysis results: Pd50Cu50: found N, 7.72; Cu 10.4; Pd 14.5; Pd:Cu 46:54. Pd60Cu40: found N, 8.67; Cu, 4.2; Pd, 9.35; Pd:Cu 58:42. Pd70Cu30: found N, 8.47; Cu, 3.2; Pd, 11.6; Pd:Cu 68:32. Pd80Cu20: found N, 8.72; Cu,
Guy et al. 2.3; Pd, 14.1; Pd:Cu 78:22. Pd90Cu10: found N, 8.62; Cu, 1.2; Pd, 16.0; Pd:Cu 89:11. Powder X-ray Diffraction. Samples for X-ray analysis were prepared by first filtering the colloidal dispersions through 0.2 µm nylon syringe filters and then removing the solvent under vacuum for 72 h. The dried residue was scraped off the glassware and transferred to a 0.7 mm capillary tube for analysis. Data were collected over the 2θ range of 0-100°. A reference spectrum of pure PVP was recorded and used to subtract out the intensities arising from the PVP in the Pd-Cu/PVP spectra. Background corrections for the glass capillary were also made on all spectra. The data were evaluated by using the Topas3 software package. The lines were fit from 30° to 100° with a split-pseudo-Voight function using a fourth order Chebychev polynomial background. Only the values for position, area, and Scherrer crystal size were allowed to vary. Electron Microscopy. Portions of the colloidal dispersions were first filtered through a 0.2 µm syringe filter and dried by removing the solvent under vacuum overnight. Samples for TEM were then prepared by suspending the dried colloidal material in ethanol under ultrasonic vibration. Some drops of the suspension were dropped onto a holey carbon film on a Ni grid. Structural characterization of the Pd-Cu/PVP colloids was performed by high spatial-resolution transmission electron microscopy (TEM), high-resolution TEM (HREM), Z-contrast imaging, electron diffraction, and energy-dispersed spectroscopy (EDS) techniques. Infrared Spectra of Adsorbed CO. Byproducts of the colloid synthesis and excess PVP were removed by washing the colloid prior to obtaining spectra. Thus, a 3 mL sample of the colloidal dispersion as prepared was filtered through a 0.2 µm nylon syringe filter (13 mm, Millex) and dried under vacuum at room temperature overnight in a 5-mL round-bottomed flask. The colloid was redispersed in 1 mL of water, then the addition of 4 mL of acetone caused the colloid to aggregate. The aggregated colloid particles were filtered by passing the mixture through a bed of Celite (Fisher) on a glass frit. The colloid was then rinsed from the Celite into a clean flask by nanopure water until the water came through colorless. Again the colloid was dried under vacuum at room temperature. A 3 mL aliquot of dichloromethane was used to disperse the colloid. After obtaining an initial IR spectrum of the dispersion, carbon monoxide (99.99% pure) was added. A stream of CO was passed through a cold trap of silica gel in a slurry of ethyl acetate and liquid nitrogen to remove any Fe(CO)542 and then through a dichloromethane trap to saturate the stream with solvent prior to bubbling through the colloidal dispersion. A second IR spectrum was collected following 30 min of CO addition. Subtraction of the initial IR spectrum revealed peaks due to CO absorption to the metal surface. Catalytic Experiments. The reduction of nitrate and nitrite were studied by using the set of colloids prepared above as catalysts. Prior to a reduction run, 1.32 mL (0.106 mmol M) of the colloid dispersion was filtered through a 0.2 µm nylon syringe filter, then dried overnight under vacuum at room temperature to remove the 2-ethoxyethanol solvent. The residue was redispersed in 149 mL of nanopure water in a three-necked 200-mL round-bottomed flask. The flask was placed in a water bath and temperatures were recorded over the period of the reaction. The reaction mixture was continuously mixed with a Teflon coated magnetic stir-bar. Hydrogen was introduced through a glass tube into the catalyst solution at a rate of 60 mL/min starting 20 min before the addition of nitrate or nitrite. Sodium nitrate and sodium nitrite concentrated solutions were
Catalytic Nitrate and Nitrite Reduction prepared so that 1 mL in 150 mL total volume would yield 150 mg/L of the anion. To start the reaction, 1 mL of the appropriate concentrate was injected into the colloidal dispersion. The final metal concentration was 0.704 mmol/L. Initial and final pH values were recorded by removing 10 mL of the reaction solution, then adding a small amount of KCl in each case to increase ionic strength for the pH probe. Periodically over the reaction time, 1.5 mL of reaction solution was removed. In total, 15 mL of the reaction solution was removed, accounting for 10% of the total initial volume; the catalyst concentration was unchanged. A 3-fold excess of acetone was added to each aliquot to aggregate the colloidal particles. Each sample was filtered through Celite in a pipet filter, which cleanly removed the colloid materials. The sample filtrates were completely dried in a 110 °C oven, then the residue was dissolved in 4 mL of nanopure water for analysis by ion chromatography. Buffered reductions were carried out in a similar manner. The colloidal materials were filtered, dried, and dispersed as above. Hydrogen (60 mL/min) was bubbled through the solution for 10 min. An additional 15 mL/min of CO2 was then added to the hydrogen stream to create a H2CO3/HCO3- buffer in solution. The mixed gas flow was bubbled through the solution for 10 min prior to the addition of the nitrate or nitrite salts and continued throughout the reaction run. A 20 mL sample of the final reaction mixture of select experiments was kept to determine the amount of ammonia formed over the course of the reaction. Ammonia measurements were made by using a gas-sensitive electrode (Orion 9512) after raising the pH of the sample above 13 by the addition of NaOH. The millivolt readings were compared to a calibration plot with ammonia levels ranging from 0.5 to 10 ppm NH3-N. pH Studies. To look at the effect of pH on nitrite reduction more closely, reactions were carried out with the 60:40, 80:20, and Pd only catalysts over a pH range of ca. 5.5-7.5. Prior to the reaction, the colloids were prepared as aqueous dispersions as follows. A 50 mL quantity of the colloidal dispersion in 2-ethoxyethanol was filtered through a 0.2 µm nylon syringe filter, and the filtrate was dried under vacuum at room temperature for 2 days. The dried residue was then dispersed in 10 mL of nanopure water, and 40 mL of acetone was added to aggregate the colloidal particles, which were isolated by filtering through a bed of Celite on a glass frit. The Celite was rinsed with fresh water until all the colloid was removed (∼100 mL), and the aqueous dispersion was dried under vacuum for 2 days. Finally, the dry colloid residue was redispersed in 50 mL of nanopure water. Nitrite reductions were carried out in three batches of four sequential reactions. For the initial batch of reactions, 178 mL of nanopure water and 1.2 mL of the aqueous dispersion of washed colloid were mixed in a three-necked 200-mL roundbottomed flask containing a magnetic stir bar. A stream of H2 was bubbled through the solution continuously at a rate of 60 mL/min. After the colloidal dispersion had been treated with H2 for 10 min, CO2 (15 mL/min) was added to the gas stream for an additional 10 min prior to the start of the reaction. Then, 0.6 mL of NaNO2 concentrate was injected to start the reaction. The initial NO2- concentration was 50 mg/L. A sample was withdrawn for an initial pH reading. Periodic samples were taken as usual. After a period of 30-60 min, the pH was again recorded and nitrite concentrate was added to reestablish a solution concentration of 50 mg/L of NO2-. This procedure was carried out two more times. The pH of the reaction solution increased with each reaction run.
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Figure 1. Powder X-ray diffraction traces of Pd-Cu/PVP colloids with PVP background subtracted.
The second and third batches were carried out in a similar fashion, only at higher starting pH values. To achieve the higher staring pH values, 1.07 M NaOH was added (0.6 mL for the second batch, and 1.9 mL for the third batch) simultaneously with the colloid. Additionally, the time between CO2 introduction and the start of the reaction was increased to 30 min to allow all of the NaOH to be converted into HCO3-. All other parameters remained constant. Results Synthesis of Pd-Cu/PVP Colloids. Elemental analysis results for the filtered and dried Pd-Cu/PVP materials confirmed that the metal atom ratios of the prepared colloidal products were near the theoretical values calculated from starting metal acetate amounts. The colloidal dispersions were stable in 2-ethoxyethanol under a nitrogen atmosphere with no settling of particles observed during several months of storage. The solvent could be removed under vacuum to leave a dry powder that could subsequently be fully redispersed either in water or in dichloromethane, but not in acetone, due to the ambiphilic character of PVP. Powder X-ray Diffraction. Dried colloidal samples were analyzed by powder X-ray diffraction (XRD) and the resulting traces are shown in Figure 1. For the pure palladium colloid, the five peaks typical of fcc palladium were observed. As the copper content increased, these peaks broadened and shifted positions. At 60% copper content the fcc (111) and (200) peaks were merged into one another and intensity appearing near 75° in 2θ suggested the formation of a new phase. As the copper content reached 50%, the fcc structure was no longer evident, and a broad peak in the range of 70-80° in 2θ was clearly seen. Values of the lattice parameter, a, were calculated from the assignable (hkl) peaks for the colloidal materials with e30% copper by using eq 1, where λ ) λCu ) 1.54 Å.
a)
(h2 + k2 + l2)λ 2 sin θ
(1)
Average particle diameters were calculated by using Scherrer’s formula shown in eq 2, where β is the line broadening (in radians).43 Table 1 summarizes the powder XRD results.
d)
λ β cos θ
(2)
For Pd/PVP, the calculated lattice parameter was 3.90 Å, which is in good agreement with the values of 3.89-3.91 Å reported in the literature for bulk Pd materials.44 As the copper
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TABLE 1: Parameters Obtained from Powder XRD on Pd-Cu/PVP Colloids Pd/PVP
Pd90Cu10/PVP
Pd80Cu20/PVP
Pd70Cu30/PVP
(hkl)
2θ (deg)
β (rad)
a (Å)
davg (nm)
111 200 220 311 222 111 200 220 311 222 111 200 220 311 222 111 200 220 311
40.06 46.51 68.06 81.99 86.34 40.45 47.06 68.8 83.17 87.54 40.65 47.2 69.13 83.71 87.84 40.81 47.17 69.31 83.87
1.876 2.698 2.137 2.537 3.926 2.226 2.935 2.646 3.419 2.604 2.619 3.771 3.298 3.824 3.241 2.884 3.310 3.751 3.556
3.89 3.90 3.89 3.89 3.90 3.86 3.86 3.85 3.85 3.86 3.84 3.85 3.84 3.83 3.85 3.83 3.85 3.83 3.82
4.45
3.94
3.25
Figure 3. Large area EDX of the Pd70Cu30 colloid sample with inset table showing Pd and Cu content.
3.09
content increased, the peaks shifted to higher 2θ values with correspondingly smaller lattice parameter values indicating the insertion of copper into the palladium lattice (Table 1). The lattice parameters were compared to a Vegard’s law plot using literature values for bulk Pd-Cu alloys, as shown in Figure 2.44 There is close agreement between our lattice parameters and those for bulk Pd-Cu alloys, which suggests that the crystalline regions of the bimetallic nanoparticles retain a nearly nominal composition over this range (0-30 atom %) of copper content. The average particle size determined by XRD for Pd/PVP was 4.5 nm. This is in the range of 4.0-6.0 nm reported for similarly prepared palladium nanoparticles.34,45,46 In our case, the average particle size for the bimetallic Pd-Cu nanoparticles varied as a function of composition, decreasing with increasing amounts of copper. Electron Microscopy. Representative large area (>100 particles) energy dispersive X-ray spectroscopy (EDX) results are shown in Figure 3 for a Pd70Cu30/PVP colloid sample. The nominal average compositions of the nanoparticles were in good agreement with the bulk elemental analyses. HREM images of individual nanoparticles with low copper content (10-20%) showed evidence of diffraction patterns that could be indexed as fcc, in agreement with the XRD observations. A representative image for a Pd90Cu10 sample is shown in Figure 4. No clear diffraction patterns could be obtained for nanoparticles with high copper content. Narrow size distributions were observed for the
Figure 2. Vegard’s law comparison of lattice parameters for bulk Pd-Cu alloys and for Pd-Cu/PVP colloids (0-30 atom % Cu).
Figure 4. Electron diffraction image from HREM showing the fcc structure of a single Pd90Cu10/PVP nanoparticle.
Figure 5. TEM image and particle size distribution for a Pd-Cu/ PVP colloid with Pd:Cu ) 60:40.
different metal compositions. Particle size information for the 60:40 Pd:Cu colloidal material, as an example, is shown in Figure 5. IR Characterization. Solution infrared spectra were collected for each of the Pd-Cu/PVP colloids dispersed in dichloromethane after exposure to carbon monoxide. The resulting spectra in the CO stretching region are shown in Figure 6. The two distinct peaks are assigned to CO bound to palladium centers, either as bridging CO near 1945 cm-1 or as terminal CO near 2050 cm-1. These peak assignments are in agreement with those previously reported for Pd-Cu/PVP colloids.34,43,46-48 The values are slightly shifted from those reported for Pd-Cu particles on alumina (1980 and 2065 cm-1).49 While the peak heights are comparable for the majority of the colloidal materials, the total intensity of the 50:50 spectrum is decreased,
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Figure 8. Sample data from a single batch of nitrite reduction experiments with use of a Pd/PVP catalyst. The lines are first-order fits to the data points. Figure 6. Infrared spectra of adsorbed CO on Pd-Cu/PVP colloids in CH2Cl2 solution. The inset expands the higher frequency region.
Figure 7. Concentration data obtained for nitrate reduction with a 60: 40 Pd-Cu/PVP colloid highlighting the different levels of nitrite observed between buffered (right) and nonbuffered (left) conditions.
TABLE 2: Kinetic Parameters and Ammonia Formation from Nitrate and Nitrite Reductiona Catalyst
NOx-
kobs (h-1)
Pd50Cu50/PVP Pd60Cu40/PVP Pd70Cu30/PVP Pd80Cu20/PVP Pd90Cu10/PVP Pd/PVP
NO2 NO2NO2NO2NO2NO2-
2.1 (1.2) 2.8 (0.7) 2.9 (1.3) 6.9 (1.1) 3.5 (2.6) 0.2 (0.1)
Pd50Cu50/PVP Pd60Cu40/PVP Pd70Cu30/PVP Pd80Cu20/PVP Pd90Cu10/PVP
NO3NO3NO3NO3NO3-
0.26 (0.06) 0.30 (0.09) 0.24 (0.09)
