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J. Phys. Chem. C 2009, 113, 4960–4969
Reactivity of Fe0 Atoms and Clusters with D2O over FeO(111) Gareth S. Parkinson, Yu Kwon Kim, 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: NoVember 18, 2008; ReVised Manuscript ReceiVed: January 13, 2009
The interaction of Fe0 atoms with D2O layers on FeO(111) has been investigated using the “atom-dropping” preparation technique and a combination of temperature-programmed desorption, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and infrared reflection-absorption spectroscopy. The data are consistent with a model where isolated Fe atoms form a DFeOD insertion species upon deposition at 35 K, which then dissociates into FeOD and a surface hydroxyl above 150 K. Interestingly, even at very low Fe0 coverages, all of the D2O is perturbed by the presence of the Fe. However, only D2O desorption is observed because the FeOD and the surface hydroxyl species recombine to produce D2O at 360 K, leaving Fe on the surface. At higher (g0.5 ML) coverages, clusters of Fe form that have molecular D2O and OD species adsorbed. Both molecular and recombinative D2O desorption are observed in TPD. In contrast to the low-coverage data, a second reaction pathway emerges at high coverage that leads to desorption of D2 and the formation of stable substoichiometric oxide. The mechanism for this minor channel is concluded to involve a reaction between two (or more) DFeOD complexes. 1. Introduction The potential applicability of zerovalent iron (ZVI) nanoparticles as an agent for the cleanup of contaminated groundwater1 has led to much interest in creating optimized particles that satisfy specific performance criteria. In general, ZVI nanoparticles have been shown to exhibit enhanced reactivity over larger particles with chlorinated hydrocarbons,2 while there have been several studies that demonstrated changing other properties of the particles can enhance the reactivity.3 In addition, it is clear that the final reaction products should be benign and immobile. It has recently been demonstrated that Fe0 particles produced in different ways produce significant variation in the chloroform (CHCl3) yield in reaction with CCl4 in solution.3 However, the reason for the difference is not known, in part because the factors controlling the fundamental reaction pathway between Fe0 and CCl4 are not well understood. Recently, we published a paper describing novel “atomdropping” experiments, where Fe0 atoms are sublimated directly into thin films of a reactant species at low temperature, to investigate the reactivity of ZVI atoms, clusters, and nanoparticles with CCl4.4,5 This work was primarily motivated by the desire to shed light on the mechanism by which ZVI reacts with CCl4 in groundwater remediation applications. Our recent study demonstrated that Fe0 atoms and nanoparticles can dechlorinate CCl4 molecules over FeO(111) in the absence of water, primarily yielding FeCl2, CO, and OCCl2. Of course, the lack of water in the controlled ultrahigh vacuum (UHV) environment means that no oxide shell was formed on the Fe particles and that no H atoms were available for the formation of chloroform. CCl4 is typically a minority species in contaminated groundwater, so it is vital to also understand how ZVI interacts with water and how this affects the interactions with the target molecule. To that end, we describe temperature-programmed desorption * Corresponding authors. Z.D.: phone, (509) 371-6150; fax, (509) 3716145; e-mail,
[email protected]. B.D.K.: phone, (509) 371-6143; fax, (509) 371-6145; e-mail,
[email protected].
(TPD), X-ray photoelectron spectroscopy (XPS), and infrared reflection-absorption spectroscopy (IRAS) Fe0 atom-dropping experiments that elucidate the interaction of ZVI atoms and clusters with water films on a FeO(111) substrate. Potentially relevant information regarding the interaction of Fe0 atoms and clusters with water molecules can be gleaned from combined matrix isolation FTIR and density functional theory (DFT) studies. In those studies, Fe0 atoms are reacted with water molecules in an inert matrix at low temperature. Zhang et al.6 determined the primary reaction products to be an Fe-H2O complex and a “half-dissociated” HFeOH complex, where the Fe atom is inserted into an OH bond within the water molecule. The term “half-dissociated” means that only one of the OH bonds of the original water molecule is broken. In addition, DFT calculations were performed that calculated that formation of the HFeOH complex was favored energetically, located -41.1 kJ/mol in energy below the separated reactant species. More recently, both Mebel and Hwang7 and Gutsev et al.8 have performed DFT calculations to determine the interaction of iron with water molecules. Mebel and Hwang calculated that the initial interaction may create a weakly bound Fe-OH2 complex that can isomerize to HFeOH, overcoming a barrier of 15-33 kcal/mol.7 However, further decomposition into FeO + H2 was found to be hindered by a high barrier of ∼58 kcal/ mol. Gutsev et al. studied Fen, Fen+, and Fen- (n ) 1-4) with water molecules and found the minimum-energy structure to be the half-dissociated HFeOH structure for n ) 1.8 Higher n values led to fully dissociated water complexes, where both of the original OH bonds of the water molecule are broken. The fragmentation energies calculated indicated that fragmentation of HFeOH should lead to desorption of FeO + H2 in all cases, except for the FeH2O+ and Fe4H2O+ cations, for which intact water is predicted (Fe + H2O). Matrix isolation studies determine the reaction between isolated species. In our atom-dropping experiments, Fe0 atoms interact with a body of water molecules over a substrate that
10.1021/jp810143y CCC: $40.75 2009 American Chemical Society Published on Web 02/26/2009
Reactivity of D2O with Fe0
Figure 1. Schematic of an Fe0 atom-dropping experiment. A D2O film is grown on a FeO(111) monolayer on Pt(111) at 35 K. Fe0 atoms are deposited into D2O from a Fe atom beam created using Fe sublimation.
may or may not play a role in defining the reaction mechanism. The FeO(111) thin film was selected as the substrate for this work first because it has been demonstrated to adsorb water molecularly in TPD experiments,9-11 and second because the behavior of Fe atoms and water molecules on an iron oxide surface may be relevant to the more complex issue of iron oxide coated Fe0 nanoparticles in solution. In general, water adsorption on oxide surfaces can be both molecular and dissociative. On the Fe3O4(111) surface, water is found to dissociate regardless of whether the sample is a thin film or single crystal.10,11 Moreover, a very recent paper by Cutting et al.12 has demonstrated that the saturation coverage of dissociated water on the Fe3O4(111) surface is limited to the number of surface Fe sites, indicating that the metal sites are the active sites for water dissociation. In this paper we describe the results of atom-dropping experiments where Fe0 atoms are deposited directly into D2O films preadsorbed on FeO(111) (schematic shown in Figure 1). D2O molecules are found to dissociate upon interaction with isolated Fe0 atoms, yielding a Fe-OD species and a surface hydroxyl species. Interestingly, evidence is found for the existence of a DFeOD precursor that is stable up to around 150-220 K. Increasing the density of the Fe0 atoms leads to the formation of more DFeOD species and the formation of Fe clusters on the surface. D2O desorption is observed to occur from the surface of these clusters. In addition, a second reaction pathway emerges that leads to the desorption of D2 and the formation of stable substoichiometric oxide (FeOx, x < 1) on the surface. 2. Experimental Details The experiments were performed in a UHV chamber with a base pressure of ∼2 × 10-10 Torr. The vacuum chamber provides the capability for surface analysis using Auger electron spectroscopy (AES), XPS, low-energy electron diffraction (LEED), IRAS, 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 spot welded to a 1 mm Ta wire that was clamped to a Au-plated Cu jig. The Cu jig is attached to a close-cycle He cryostat that facilitates 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.13 The resulting uncertainty in the absolute temperature is 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
J. Phys. Chem. C, Vol. 113, No. 12, 2009 4961 surface purity and order were checked by AES and LEED, respectively. The freshly prepared Pt(111) surface exhibited small amounts of Fe (
