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J. Phys. Chem. C 2009, 113, 1818–1829
Reactivity of Fe0 Atoms, Clusters, and Nanoparticles with CCl4 Multilayers on 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, PO Box 999, Mail Stop K8-88, Richland, Washington 99352 ReceiVed: August 26, 2008; ReVised Manuscript ReceiVed: October 28, 2008
The interaction of Fe0 atoms and clusters with CCl4 multilayers was investigated using a novel “atom dropping” method at 30 K over a FeO(111) thin film. Temperature programmed desorption experiments over a range of Fe0 and CCl4 coverages demonstrate a rich surface chemistry with several reaction products (C2Cl4, C2Cl6, OCCl2, CO, FeCl2, and FeCl3) observed. X-ray photoelectron spectroscopy data show that the initial reactive interaction occurs spontaneously at 30 K, with the experimentally observed reaction products formed at higher temperature, in agreement with the results of theoretical calculations. The formation of OCCl2 and CO is concluded to occur through abstraction of O atoms from the generally inert FeO(111) substrate. The buffer layer assisted growth technique is used to show that the reactivity, and interestingly the reaction products, are determined by the size of Fe0 nanoparticles which interact with CCl4. I. Introduction In recent years it has been shown on numerous occasions that properties of nanoscale particles can be quite different from those of the corresponding bulk materials. In particular, the surface chemistry of metal and metal-oxide nanoparticles has been a subject of much investigation, and the ability to control the rates and products of chemical reactions has emerged as one of the most exciting potential uses of nanotechnology. For example, a concerted effort is underway to develop zerovalent iron (ZVI/Fe0) particles and nanoparticles as remediation agents for the destruction of chlorinated hydrocarbons and reducible inorganic ions in groundwater.1 The appeal of Fe0 nanoparticles to the engineering community is illustrated by the fact that field demonstrations have already been completed and described in the literature.2,3 The interest arises primarily as it has been suggested that Fe0 nanoparticles exhibit high reactivity, can be easily delivered to deep underground contamination, and offer the potential ability to avoid undesirable products via chemical reaction selectivity.4 Indeed, in a recent study, Fe0 nanoparticles with an average diameter of 44 nm were shown to produce significantly less chloroform (CHCl3) in reactions with mixtures of carbon tetrachloride (CCl4) and water than similar experiments conducted with different, on average larger, Fe0 nanoparticles.5 However, several other properties of the particles were identified which might have influenced the branching ratio such as the presence and nature of the oxide-shell, which invariably forms under environmental conditions, and/or the influence of remnant materials left over from the synthesis process. In related work, a recent study reported that magnetite particle size was a critical factor determining the reactivity with CCl4.6 Currently there is little evidence available that would allow identification of the factors that control the reaction pathways for Fe0 nanoparticles. Therefore the interaction of Fe0 nanoparticles with both the target contaminant molecules and other components that make up the natural environment (e.g., H2O) requires investigation on a more fundamental level if a better * To whom correspondence should be addressed. (B.D.K.) Phone: (509) 371-6143. Fax: (509) 371-6145. E-mail:
[email protected]. (Z.D.) Phone: (509) 371-6150. Fax: (509) 371-6145. E-mail:
[email protected].
understanding is to be gained. To that end, this paper presents the results of temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) experiments, conducted in a controlled UHV environment, which trace the evolution of the reaction products formed in the interaction of Fe0 in various regimes with a key contaminant molecule, CCl4. In addition, a theoretical investigation into the reaction mechanism of Fe0 and CCl4 was undertaken by our collaborators, and this work is published as a companion paper to this article.7 There has been prior work reported in the UHV literature investigating the interaction of CCl4 with the single crystal iron surfaces, Fe(110)8,9 and Fe(100).10 TPD investigations of the interaction of CCl4 with the Fe(110) surface conducted by Smentkowski et al. found that CCl4 adsorbs intact upon deposition at 90 K, with well resolved monolayer and multilayer desorption features at 157 and 147 K respectively.8,9 Irreversible dissociative adsorption of CCl4 was found to occur at temperatures greater than 150 K, with the reaction products CCl4, C2Cl4, and C2Cl6 observed in TPD spectra below 200 K. Interestingly, the authors also directly observed desorption of the diradical species, :CCl2, in the TPD spectra (the presence of a •CCl3 radical species was also suspected), the reactivity of which was interpreted as being the mechanism for the formation of the other C containing species. Above 200 K, complete dissociation of CCl4 was observed with C and Cl atoms adsorbed on the surface. The carbon was found to dissolve into the bulk of the Fe crystal around 600 K, whereas Cl remained on the surface and led to the desorption of FeCl2 and another Fe-Cl species (of unknown stoichiometry) at temperatures in excess of 560 K. The earlier work of Jones10 reported dissociative adsorption of CCl4 on Fe(100) at room temperature, resulting in a random array of C atoms located in 4-fold hollow sites with a square array of Cl atoms located above them. A lack of desorption of chlorine containing compounds in TPD was noted, despite the observation of decreasing amounts of surface Cl via Auger electron spectroscopy (AES) as the sample was heated. The interpretation of this result was that Cl must desorb from the Fe(100) surface as atoms, with the C atoms thought to dissolve into the substrate around 500 K.
10.1021/jp8076062 CCC: $40.75 2009 American Chemical Society Published on Web 01/07/2009
Reactivity of CCl4 with Fe0
J. Phys. Chem. C, Vol. 113, No. 5, 2009 1819
In addition to the interest in the reactivity of Fe surfaces and nanoparticles, there is also significant interest in the reactivity of molecules at iron oxide surfaces, in part because of the natural abundance of iron oxides in soils and their applicability as catalysts in an industrial setting.11 Several papers utilizing combinations of TPD,12 AES,13 XPS,14 scanning tunneling microscopy (STM)15-17 and theoretical calculations16 have been published detailing the interaction of CCl4 with the Fe3O4(111) and Fe2O3(0001) single crystal surfaces. The overall conclusions of this work are that reactivity of an iron oxide surface is linked to the presence of iron atoms at the surface, and that oxygen terminated regions are inert to CCl4. CCl4 adsorbed at 100 K onto a surface containing surface Fe atoms leads to dissociative adsorption to form :CCl2 species and Cl atoms on the surface. The formation of the reaction products (CCl4, C2Cl4, and OCCl2) observed in these prior studies are proposed to result from reactions of the adsorbed CCl2 species.12 The resulting surface chemistry is then determined by competition for the CCl2 among three distinct reaction channels. The proposed reactions are summarized by the following schematic chemical equations:12
molecular adsorption: ∆
CCl4(g) 98 CCl4(ad) 98 CCl4(g)
(1)
100K
dissociative adsorption: CCl4(g) 98 : CCl2(ad) + 2Cl(ad)
(2)
100K
surface processes: ∆
98 CCl4(g)
(3a)
∆ 1 f C2Cl4(g) + 2Cl(ad) 2
(3b)
∆
f OCCl2(g) + 2Cl(ad)
(3c)
Essentially, the :CCl2 diradical species can recombine with two adsorbed Cl’s and form CCl4 (reaction 3a), it can undergo dimerization and form C2Cl4 (reaction 3b) and/or it can extract an oxygen atom from the iron oxide crystal lattice and form OCCl2 (reaction 3c). The remaining Cl atoms on the surface lead to desorption of FeCl2 at temperatures in excess of 820 K. It was also noted that the branching ratio for these reactions is different when CCl4 is dosed near room temperature, as this allows the extraction of lattice oxygen atoms (reaction 3c) to compete favorably with the recombination (reaction 3a) and association (reaction 3b) reactions. An activation barrier for production of phosgene, via extraction of a lattice oxygen atom, of 0.16 eV was calculated using density functional theory (DFT).16 A recent paper by Cutting et al., investigating the reactivity of Fe3O4(111) with formic acid, pyridine, and CCl4, has also shown that the reactivity toward CCl4 of an iron oxide surface is largely determined by the surface termination.17 The (111) surface of magnetite (Fe3O4) can be terminated by a number of different atomic arrangements and STM was used to observe the local reactivity of the test molecules on the surface. CCl4 was found to only react significantly on regions of the surface terminated by 1/4 ML of tetrahedraly coordinated Fe ions, where it initially adsorbed intact at low temperature before dissociating to form loosely bound CCl2 species and more tightly bound Cl
Figure 1. Ball model representing the oxygen terminated FeO(111) bilayer structure as grown on Pt(111).
atoms. The formation of the observed reaction products is proposed to be in line with reactions 1-3c. An alternative to using single crystal oxide samples is to grow thin oxide films on metal substrates where there is a good lattice match between the two materials. This prevents problems associated with charging of samples and segregation of impurities, and means that high quality surfaces can be prepared quickly and investigated using the full armory of surface science techniques. Moreover, the need for sputtering to remove surface impurities, which can lead to irreversible damage on oxide surfaces,11 is eliminated. For iron oxide films grown in this manner, Pt(111) is typically the substrate of choice with a lattice mismatch of ∼10%. High quality films of FeO(111), R-Fe2O3(0001), and Fe3O4(111) have been grown and studied in detail by several experimental techniques and theoretical calculations.11,18-22 An extensive review of the preparation, structure and chemistry of the range of iron oxide thin films grown on Pt(111) was published in 2002.11 This review also summarizes the reactivity of the different films with respect to water, pyridine, and ethylbenzene. Again, the general trend is that the reactivity of the iron oxide surface is linked to the termination, with exposed Fe atoms being reactive and O atoms inert. The structure of the FeO(111) thin film, as depicted schematically in Figure 1, is polar, with the surface terminated by a closepacked oxygen layer. The mismatch between the FeO(111) and Pt(111) substrate lattice leads to a (84 × 84) R10.9° commensurate structure and the resulting surface exhibits a Moire` pattern in STM images.11 Prior work done in our group has shown that this FeO(111) surface is inert to chlorinated methane derivatives23 (CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4) and to H2O;24 consistent with the trend for O termination passivity discussed above. Moreover, during the course of the work presented here it was discovered that the FeO(111) surface is also inert to tetrechloroethylene (C2Cl4) and hexachloroethane (C2Cl6). The experimental methods described in this paper rely upon two nanoparticle preparation techniques developed over the past decade. First, the concept of reactive layer assisted deposition (RLAD) or “atom dropping” is used to facilitate the interaction of single Fe0 atoms with CCl4. Essentially, single atoms of Fe0 are deposited directly onto a film of CCl4 which is predeposited onto the FeO(111) surface at 30 K (see Figure 2). The deposition of metal atoms into multilayers of condensed O2 has been used to grow MgO films on polycrystalline Au25 and oxidized Au clusters on Mo(100).26 Similar work utilizing H2O and NO2 multilayers demonstrated the growth of TiO2 nanoparticles on Au(111).27 In other work, Mo atoms were deposited on ethylene on Au(111), resulting in the formation of MoCx nanoparticles.28 Prior to the development of RLAD, a technique called buffer layer assisted growth (BLAG) was developed whereby a layer
1820 J. Phys. Chem. C, Vol. 113, No. 5, 2009
Figure 2. Schematic illustrating the principle of “atom dropping”.
or layers of inert gas are condensed onto a desired substrate at low temperature, with a second material, typically a metal, deposited on top.29-31 Subsequent heating and sublimation of the buffer layer activates cluster formation, agglomeration, and coalescence ultimately resulting in soft landing of the clusters onto the substrate. It has been demonstrated that the average size of nanoparticles arriving at the substrate is directly related to the thickness of the buffer layer. TEM images indicate that final size of particles grown in this way is variable over 3 orders of magnitude (1-1000 nm).32 This technique is utilized here to vary the size of Fe0 nanoparticles, which are subsequently introduced to the reactant CCl4 layers when the buffer layer desorbs. The work presented here demonstrates that the addition of small amounts of Fe0 into CCl4 multilayers condensed on FeO(111) at 30 K, transforms the previously inert CCl4/ FeO(111) system into a reactive system, producing a range of products observable in TPD spectra (CCl4, C2Cl4, C2Cl6, :CCl2, CO, OCCl2, FeCl2, and FeCl3). The reactions involve not only the Fe0 atoms, clusters, and nanoparticles with CCl4, but the FeO(111) surface as well. Interestingly, formation of CO and phosgene (OCCl2) occurs during the TPD temperature ramp as a result of reactive intermediate species attacking the FeO(111) film, facilitating the extraction of lattice oxygen atoms. In addition this paper demonstrates that the size of the nanoparticles deposited into the CCl4 has a direct influence on the reactivity and the reaction products observed in TPD. II. Experimental Section A. Description of Experimental Approach. The experiments were performed in an ultra high vacuum (UHV) chamber with a base pressure of ∼2 × 10-10 Torr. The vacuum chamber provides the capability for surface analysis using AES, XPS, low energy electron diffraction (LEED), and TPD using a UTI100C quadrupole mass spectrometer. The Pt(111) sample, a disk 1 cm in diameter and 1 mm thick, was spot welded to a 1 mm Ta wire which was clamped to a Au plated Cu jig. The Cu jig is attached to a close-cycle He cryostat which facilitates the cooling of the sample to a base temperature of ∼30 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 30-1300 K by heating resistively through the Ta wire. The absolute temperature was calibrated using the multilayer desorption of various gases (Kr, Ar, O2, and H2O) from the sample surface.33 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 at 1200 K), and vacuum annealing (1300 K). The surface purity and order were checked by AES and LEED, respectively. The freshly prepared surface exhibited small amounts of Fe (
