Environ. Sci. Technol. 2010, 44, 5079–5085
Characterization and Reactivity of Iron Nanoparticles prepared with added Cu, Pd, and Ni ‡,§
spike experiments for long-term reactivity demonstrated that thepresenceofthemetaladditivesfacilitatedreductionbyenabling greater utilization of Fe0.
Introduction ‡
CHAN LAN CHUN, DONALD R. BAER, DEAN W. MATSON,‡ JAMES E. AMONETTE,‡ AND R . L E E P E N N * ,† Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, and Pacific Northwest National Laboratory, P.O, Box 999, Richland, Washington 99352
Received October 27, 2009. Revised manuscript received April 22, 2010. Accepted May 7, 2010.
The association of a secondary metal with iron particles affects redox reactivity in engineered remediation systems. However, the structural characteristics of the metal additives and mechanism responsible for changes in reactivity have not been fully elucidated. Here, we synthesized iron nanoparticles with Cu, Pd, and Ni content ranging from 0-2 mol % via a solution deposition process (SDP), hydrogen reduction process (HRP), or hydrogen reduction of ferrihydrite coprecipitated with the metal cations (HRCO). Results from solid-state characterization show that the synthesis methods produced similar iron core/magnetite shell particles but produced substantial differences in terms of the distribution of the metal additives. In SDP, the metal additives were heterogeneously distributed on the surface of the particles. The metal additives were clearly discernible in TEM images as spherical nanoparticles (5-20 nm) on the HRP and HRCO particles. Because the metals were integral to the synthesis process, we hypothesize that the metal additive is present as solute within the iron core of the HRCO particles. Kinetic batch experiments of carbon tetrachloride (CT) degradation were performed to quantitatively compare the redox reactivity of the particles. Overall, metal additives resulted in enhanced pseudo-first-order rate constants of CT degradation (kO,CT) compared to that of the iron nanoparticles. For the bimetallic iron nanoparticles prepared by SDP and HRP, kO,CT increased with the concentration of metal additives. The values of chloroform yield (YCF) were independent of the identity and amount of metal additives. However, both kO,CT and YCF of the HRCO iron particles were significantly increased. Results suggest that it is the distribution of the metal additives that most strongly impacts reactivity and product distribution. For example, for materials with ca. 0.9 mol % Ni, reactivity and YCF varied substantially (HRCO > SDP > HRP), and HRCONiFe resulted in the lowest final chloroform concentration because chloroform was rapidly dechlorinated. In addition, sequential
* Corresponding author phone: (612) 626-4680; e-mail: rleepenn@ umn.edu. § Current address: Department of Microbiology and Immunology, Medical University of South Carolina, Charleston, South Carolina. ‡ Pacific Northwest National Laboratory. † University of Minnesota. 10.1021/es903278e
2010 American Chemical Society
Published on Web 05/28/2010
There has been a growing interest in the use of bimetallic iron for treatment of contaminated groundwater. The presence of metal additives can result in enhanced reaction rates and prevent toxic byproduct formation by promoting complete dehalogenation (1–5). Reactivity could be further improved by using nanoscale particles because nanoscale particles have greater surface area to volume ratios and their surface sites may have greater intrinsic reactivity (6). Also, nanoscale particles can be injected by gravity or under pressure directly into a contaminant source zone (7). Due to such advantages, the engineering community has adopted this technology for various systems (e.g., treatment of chlorinated solvents in groundwater, bioavailability enhancement of polychlorinated biphenyl or chlorobenzene in soil/sediment, and trihalomethane reduction in drinking water systems (8)). To optimize the performance of bimetallic iron for environmental engineering applications, several studies have attempted to understand the role of metal additives in enhancing reactivity and modifying product distributions. The rate enhancement by bimetallic iron has been explained by galvanic corrosion (9, 10) and the production of atomic hydrogen (1, 11–14). Recent studies comparing bimetal reactivity towards water with that towards organohalides (1, 12) have shown that indirect reduction with atomic hydrogen adsorbed on iron, rather than galvanic corrosion, is predominantly responsible for the rate enhancement. While some studies have reported the observation of partly dehalogenated intermediates with the use of iron-based bimetallic particles (e.g., Pt and Au 1, 12, 13), other studies have reported full dehalogenation (14–16). Moreover, less accumulation of the toxic, partly dehalogenated intermediates was observed for nanosized bimetallic particles than for coarser bimetallic particles (14–16). This is due either to a greater reducing power of the nanosized iron particles, enabling reduction of the less reducible intermediates, or a shift in reaction pathways, resulting in less production of the toxic intermediates (14). However, the relationship between the identity of metal additives and product distribution has not been clearly established by previous work. For example, Fennelly and Roberts (2) observed different product distributions between the metal additives Ni and Cu, whereas Lin et al. (13) reported that the halogenated product yield was independent of the metal identity among Pd, Pt, Ru, and Au. To resolve this inconsistency, recent studies (1, 12) have examined trends in product distribution for six metal additives and four organohalides. These studies (1, 12) revealed that the presence of metal additives favored the production of more fully dehalogenated products and the ratios between fully dehalogenated product and halogenated intermediates depended on the reaction rates, suggesting a close relationship between the rate-determining step and the product-determining step. In addition, some studies have examined the structural characteristics of bimetallic iron and attempted to establish the relationship between physical structure and reactivity. Specifically, surface coverage, loading, distribution, and oxidation state of the metal additives have been investigated using surface sensitive techniques (1, 2, 12, 13, 15, 17). One comprehensive study (18) showed that Cu was heterogeVOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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neously distributed on the granular iron surface and proposed that reduction occurred on the Cu surface rather than at the interface between Cu and Fe0 by coupling surface specific information (e.g., two-dimensional surface coverage and loaded mass) with kinetic results. This study is a good example of how information regarding structural characteristics can provide mechanistic insights pertaining to reactive sites and improve our understanding of the molecular level phenomena responsible for the reactivity of bimetallic iron. However, this approach has been mostly applied to granular iron with metal additives and not nanosized iron with metal additives. Despite their potential in environmental engineering applications, our understanding and control of the reactivity of nanoscale bimetallic iron are limited but improving. To date, nanoscale bimetallic iron has been prepared by the borohydride-reduction process (9–11, 14, 16, 19–21) and characterized for bulk composition, morphology, and oxidation state of both the metal additive and iron (14, 16, 20, 21). Two studies (5, 16) attempted to correlate microscopic and spectroscopic characterization of nanoparticles with reactivity. Tee et al. (5) found the optimal Ni/Fe ratios by coupling reaction rate with information obtained from scanning transmission electron microscopy. Zhu et al. (16) compared the results from reactivity experiments with X-ray photoelectron spectrometry(XPS) analyses to show that the observed loss in reactivity of aged Pd/Fe nanoparticles was the consequence of Pd dislodgment caused by iron oxide encapsulation. Currently, how metal additives are distributed on nanoiron and what structural features control reactivity remain unclear. In this study, we synthesized and characterized bimetallic iron nanoparticles to enhance our understanding of properties (e.g., identity, loading, and distribution of metal additives) that influence both rate and reaction pathway for dechlorination of carbon tetrachloride (CT). The iron nanoparticles were prepared by reduction of ferric oxide or oxyhydroxide particles using H2(g) at high temperature (22). Three methods were employed for addition of the Cu, Ni, or Pd. In the first, metals were electroplated onto commercial iron nanoparticles (Toda Kogyo Corp. (22)). In the second, ferric-oxide nanoparticles were added to a solution of a metal salt, allowed to dry, and then reduced. Lastly, ferrihydrite nanoparticles were prepared by coprecipitation from an initially homogeneous solution of ferric and metal salts, and the resulting material was dried and then reduced. Ultimately, we combined controlled synthesis and solid-state characterization with batch kinetic experiments to quantify CT degradation for each material. By this comparison, we hope to correlate chemical reactivity of the bimetallic iron nanoparticles with structural features and the distribution of metal additives and to use such understanding to synthesize nanoparticles with properties optimized for use in the field.
Experimental Methods Chemicals. Chemicals, suppliers, and purities are listed in the Supporting Information (SI). Material Synthesis. Bimetallic iron particles (MFe) were synthesized by three processes: solution deposition (SDP), hydrogen reduction (HRP), and hydrogen reduction of iron oxide coprecipiated with metal additives (HRCO). For SDP, Cu, Ni, and Pd bimetallic iron nanoparticles were prepared in an anaerobic chamber (97% N2/3% H2, Coy Laboratory Products) using a modification of the method of Bransfield et al. (18). Displacement plating (SDP) was conducted with metal sulfate salt on nanosized metallic iron (RNIP-10DP) obtained from Toda Kogyo Corp. (Schaumberg, IL). The HRP and HRCO processes were conducted as described in the SI. Each material is defined hereafter as synthesis process-Fe for a sample without metal additives and synthesis processM(mol %)Fe for samples with metal additives, with “M” 5080
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FIGURE 1. TEM images of Fe and bimetallic Fe particles produced by the SDP and HRP processes. Note the distinctive ICOS structure in the Fe particles. serving to identify the metal added. Because of the differing natures of the metal additions in relation to particle processing, these three processes are likely to produce different distributions of M (Cu, Ni, or Pd) in the iron particles. Materials Characterization. Iron and bimetallic iron nanoparticles were characterized using inductively coupled plasma mass spectrometry (ICP-MS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and XPS. Details for each are provided in the SI. CT Reactions. Kinetic batch experiments were carried out in 124 mL serum bottles without headspace. The assembly of reactors, sampling, and analysis of CT and its degradation products are provided in the SI. Sequential spike experiments were also conducted in order to examine the long-term reactivity of the nanoparticles by exposing them to repeated cycles of aqueous CT (∼100 µM) at 24 h intervals. Each reactor was spiked a total of five or seven consecutive times. Controls were prepared without solid materials to monitor loss of CT.
Results and Discussion Structure and Composition. The amount of metal additive in each final material was measured using ICP-MS and is reported in the SI. XRD analysis shows that SDP-Fe is composed of iron metal (R-Fe0) and magnetite (Fe3O4; ∼3 wt % based on Rietveld refinement analysis of XRD patterns, Figure S1a). No significant differences in the degree of crystallinity, size (peak broadening), and magnetite/Fe ratio for the SDP-Fe and bimetallic materials (SDP-MFe) were observed. Like SDPFe, HRP-Fe also consists of iron metal and magnetite, although HRP-Fe typically contains more magnetite (∼11 wt %). Interestingly, the bimetallic iron particles prepared by HRP (SI Figure S1b; HRP-MFe) and HRCO (SI Figure S3; HRCO-MFe) have much lower magnetite contents than HRPFe, which suggests that metal additives may enhance the efficiency of reduction from hematite/ferrihydrite to iron. In summary, the bimetallic materials produced using these processes have similar overall phase composition (SI Table S1). TEM images (Figure 1 and SI Figure S2) show that both SDP-Fe and HRP-Fe consist of aggregates of small irregular particles (10-100 nm) composed of single-crystal Fe0 and magnetite. Each particle has an Fe0-core-oxide shell (ICOS) structure, and the oxide-shell thickness appears to be rather
0.25 0.29 0.33 0.25 0.29 0.35 6.1((0.3) × 10-3 1.2((0.6) × 10-2 2.0((0.2) × 10-2 0.87 ( 0.10 1.34 ( 0.16 2.29 ( 0.22 3.44 ( 0.36 4.49 ( 0.48 6.47 ( 0.77 SDP-Pd(0.02)Fe SDP-Pd(0.03)Fe SDP-Pd(0.08)Fe
a Overall pseudo-first-order rate constant of CT disappearance. b Pseudo-first-order rate constant of CF formation. c Overall pseudo-first-order rate constant of CF disappearance. Yield of CF ()kF,CF/kO,CT) where kO,CT and kF,CF are pseudo-first order rate constants of overall CT disappearance and CF formation, respectively. e Actual yield of CF at T ) 5 h ()[CF]measured at T ) 5 h/[CT]initial). f Values in the parentheses indicate mol % of metal additives based on ICP-MS analysis (See the SI). g Errors represent 95% confidence limits. h No degradation observed over 50 h. d
0.36 0.26 0.27 HRP. We attribute the greater reactivity of SDP over HRP to smaller particle size of metal additives and the increased surface roughness of SDP-MFe (TEM observation). While the segregation of metal additives in HRP-MFe produces only a minimal increase in additive surface area, the SDP resulted in substantially increased surface roughness, providing the greatest increase in additive surface area. Considering a proposal that reduction occurs on the metal additive surface rather than at the interface between the metal additive and Fe0 (18), SDP-MFe has a greater amount of exposed metaladditive particles than HRP-MFe at similar loading. HRCO-MFe is even more reactive than SDP-MFe, although careful examination of the TEM images and XRD patterns shows no clear difference between HRP-MFe and HRCOMFe. We think that this may be due to the presence of a Ni-Fe solid solution in HRCO-NiFe particles, as proposed above. It has been observed that Ni-substituted ferrite (NiFe2O4) prepared by H2 reduction contained an isomorphous Ni-Fe phase and accelerated the reduction of CO2 to C (28, 29). Similarly, the substitution of Ni into a green rust increased its oxidation rate to goethite due to local distortion in the structure (30). The experiments initiated with CT resulted in the formation of CF, dichloromethane (DCM), CH4 and CO, which is consistent with previously reported work with iron and iron oxides (31–33). In this study, both CO and CH4 were detected using a GC-thermal conductivity detector, but only CF and DCM were continuously quantified. Figure 2 shows representative data tracking the concentration of CT, CF, and DCM versus time. In experiments with SDP-Fe and HRP-Fe, CT was rapidly degraded to CF and other products, which included CO and CH4. However, no significant CF degradation was observed. In contrast, CF initially formed in experiments with SDP-Cu(3.13)Fe, HRP-Cu(1.86)Fe, HRCO-Ni(0.90)Fe, and HRCO-Ni(4.17)Fe, but subsequently degraded. After 2 h,
CO and CH4 were detected as major products and DCM as a minor product (
