J. Phys. Chem. C 2009, 113, 10233–10241
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Reactivity of C2Cl6 and C2Cl4 Multilayers with Fe0 Atoms over FeO(111) Gareth S. Parkinson, Zdenek Dohna´lek,* R. Scott Smith, and Bruce D. Kay* Chemical and Materials Sciences DiVision, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352 ReceiVed: February 4, 2009; ReVised Manuscript ReceiVed: April 10, 2009
The interaction of Fe0 atoms with C2Cl6 and C2Cl4 multilayers over FeO(111) has been investigated using the “atom dropping” preparation technique and a combination of temperature-programmed desorption and X-ray photoelectron spectroscopy. The reactivity and reaction products are strongly dependent on the Fe0 coverage. On C2Cl6 multilayers, submonolayer Fe0 doses lead to high reactivity and primarily FeCl3 and C4Cl6, whereas multilayer Fe0 doses lead to the production of FeCl2 and C2Cl4 with much lower Fe0 reactivity. The data are consistent with a model where Fe atoms form intermediate species at low coverage, which consist of an Fe atom inserted into a C-Cl bond. When two Fe atoms react with C2Cl6, a different intermediate species is formed that produces the alternative reaction pathway and the formation of C2Cl4. Similar atom dropping experiments demonstrate that C2Cl4 is also reactive toward Fe0 atoms at low Fe0 dose, leading to the production of one FeCl2 molecule per C2Cl4 molecule reacted. At higher coverages, Fe atoms form clusters that are much less reactive toward C2Cl4. 1. Introduction The potential use of zerovalent iron (ZVI, Fe0) nanoparticles for environmental remediation applications has led to significant interest in recent years. In particular, Fe0 nanoparticles have been investigated for the cleanup of chlorinated hydrocarbons (e.g., refs 1-4), which have arisen in the environment primarily as a result of industrial activity. The presence of these materials in groundwater is considered a threat to public health, and Fe0 nanoparticles are currently thought to present a cost-effective solution for their remediation.5 Several field tests have been conducted showing the viability of the Fe0 solution.6 Much of the work reported in the literature regarding Fe0 nanoparticles focuses on measuring the reaction rates of particles in a solution containing a particular contaminant species. Unfortunately, many of these experiments are unable to pinpoint the origin of variations in the reactivity because several properties vary between different batches of particles. Recently, an additional layer of complexity has arisen because of the addition of a second metallic component, such as Pd or Al, to form “bimetallic” particles in an attempt to try to increase the reactivity.7,8 However, as it stands the fundamental reaction mechanism by which Fe0 interacts with chlorinated hydrocarbons is not well understood. This problem is currently hindering the design of optimum particles for groundwater remediation. In a recent study, we performed novel “atom dropping” experiments in UHV to shed light on the fundamental reaction mechanism between Fe0 and a common contaminant, CCl4.9 These experiments isolated the specific reaction of interest by utilizing deposition of Fe0, from isolated atoms through to nanoparticles, directly into a CCl4 multilayer film grown at low temperature (∼35 K) on FeO(111). Novel intermediate species containing Fe-C-Cl bonds were determined to be necessary to explain the experimental data.9 Such species were calculated to be extremely stable on the basis of density functional theory calculations performed by our collaborators,10 and independently * To whom correspondence should be addressed. E-mail: Bruce.Kay@ pnl.gov;
[email protected].
by another group.11 The intermediate species further reacted to form a range of products that were observed to desorb in temperature-programmed desorption (TPD) experiments at higher temperatures. Two of these final reaction products were C2Cl4 and C2Cl6, which are themselves common environmental contaminants.12,13 In the present work, we investigate the interactions of Fe0 atoms and clusters with C2Cl6 and C2Cl4 multilayers using the Fe0 atom dropping preparation technique over a FeO(111) thin film substrate. Using a combination of TPD and X-ray photoelectron spectroscopy (XPS) experiments, we demonstrate that the major reaction products in the Fe + C2Cl6 reaction are C2Cl4, C4Cl6, FeCl2, and FeCl3. The distribution of these species is strongly dependent on the Fe0 concentration. As C2Cl4 is a major reaction product of the Fe + C2Cl6 reactions, we also investigated the interaction of Fe0 with C2Cl4 multilayers. In the corresponding Fe + C2Cl4, experiments we find that the reactivity is similar to that of C2Cl6, but that the reaction pathway is different, with FeCl2 being the only Fe-containing species created at all coverages. The only precedent for the study of chlorinated C2 species interacting with Fe surfaces in UHV is a study utilizing Fe(110) by Smentkowski et al.14 In that article, C2Cl4 is reported to adsorb molecularly at 90 K, but dissociatively above 325 K. Only C2Cl4, FeCl2, and an unidentified high mass iron chloride species were found to desorb in TPD. In a subsequent article, the authors discuss the role of surface defects in enhancing the reactions of several chlorinated hydrocarbons with Fe(110).15 2. Experimental Section A. Description of Experimental Approach. The experiments were performed in an ultra high vacuum (UHV) system with a base pressure of ∼2 × 10-10 Torr. The system provides the capability for surface analysis using Auger electron spectroscopy (AES), XPS, infrared reflection-absorption spectroscopy (IRAS), low-energy electron diffraction (LEED), and TPD using a UTI-100C quadrupole mass spectrometer (QMS). The Pt(111) sample, a disk 1 cm in diameter and 1-mm thick, was
10.1021/jp901040f CCC: $40.75 2009 American Chemical Society Published on Web 05/14/2009
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spot-welded to a 1-mm Ta wire that was clamped to a goldplated Cu jig. The Cu jig was attached to a closed-cycle He cryostat that facilitated the cooling of the sample to a base temperature of ∼35 K. The sample temperature was monitored via a C-type thermocouple spot-welded directly to the back of the Pt disk and was controlled by computer from 35 to 1300 K by heating resistively through the Ta wire. The absolute temperature was calibrated using the multilayer desorption of various gases (Kr, Ar, H2O) from the sample surface.16 The resulting uncertainty in the absolute temperature was estimated to be (2 K. The Pt(111) sample was cleaned by cycles of Ne+ sputtering (1.5 keV, 20 min at 300 K), O2 annealing (2 × 10-7 Torr, 5 min, 1200 K), and vacuum annealing (300 s, 1300 K). The surface purity and order were checked by AES and LEED, respectively. The freshly prepared Pt(111) surface exhibited small amounts of Fe (
