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Adsorption of NO on FeO Films Grown on Ag(111) Vikram Mehar, Lindsay Richard Merte, Juhee Choi, Mikhail Shipilin, Edvin Lundgren, and Jason F Weaver J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01751 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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The Journal of Physical Chemistry

Adsorption of NO on FeOx Films Grown on Ag(111)

Vikram Mehar1, Lindsay R. Merte2, Juhee Choi1, Mikhail Shipilin2, Edvin Lundgren2, Jason F. Weaver1*

1

Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA 2

Division of Synchrotron Radiation Research, Lund University, 22 100 Lund, Sweden

*To whom correspondence should be addressed, [email protected] Tel. 352-392-0869, Fax. 352-392-9513

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Abstract We used temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on crystalline iron oxide films grown on Ag(111), including a Fe3O4(111) layer, an FeO(111) monolayer and an intermediate FeOx multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeOx surfaces, and provides evidence that NO binds more strongly on Fe+2 sites than Fe+3 sites. The NO TPD spectra obtained from the Fe3O4(111) layer exhibit a dominant peak at 380 K, attributed to NO bound on Fe+2 sites, as well as a broad feature centered at ~250 K that is consistent with NO bound on Fe+3 sites of Fe3O4(111) as well as NO adsorbed on a minority FeO structure. The NO TPD spectra obtained from the monolayer FeO(111) film exhibits a prominent peak at 269 K. After growing FeOx multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at ~200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur because the multilayer FeOx islands expose Fe+3 sites that bind NO more weakly than the Fe+2 sites of the FeO monolayer. RAIR spectra obtained from the NO-covered FeOx surfaces exhibit an N-O stretch band that blueshifts over a range from about 1800 to 1840 cm-1 with increasing NO coverage. The measured N-O stretching frequency is only slightly redshifted from the gas-phase value, and lies in a range that is consistent with atop, linearly-bound NO on the Fe surface sites. In contrast to the NO binding energy, we find that the N-O stretch band is relatively insensitive to the NO binding site on the FeOx surfaces. This behavior suggests that π-backbonding occurs to similar extents among the adsorbed NO species, irrespective of the oxidation state and local structural environment of the Fe surface site.


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Introduction Advances in the atomic-scale understanding of iron oxide surfaces are essential for realizing the potential of these materials in catalytic applications. The wide range of structures and compositions in which iron oxides can exist affords flexibility for tailoring catalytic properties while also making it challenging to establish detailed structure-function relationships for iron oxide surfaces. Iron oxides can exist in multiple stoichiometries due to the variability in Fe oxidation states, including FeO, Fe3O4 and Fe2O3. In addition, a given surface structure can present different terminations, depending on the oxidative environment.1-7 For example, the Fe3O4(111) surface can expose up to six terminations, including Fe-rich and O-rich surfaces.1, 4 Researchers have also shown that monolayer FeO(111) films can be grown on crystalline metal surfaces and that these ultrathin oxides can exhibit unique chemical properties due to interactions with the underlying metal substrate.8-21 While several iron oxide surface structures have been reported, the extent to which the structural and compositional variations influence the intrinsic chemical properties of the surface Fe and O atoms is not fully resolved. Investigations of the adsorption properties of well-defined iron oxide surfaces can aid in clarifying how the local structural environment influences the chemical properties of the Fe and O surface sites. Prior investigations of the adsorption of small molecules on well-defined iron oxide surfaces have indeed clarified the chemical affinities of different types of Fe and O surface sites as well as providing insights about the surface structure and composition. For example, based on temperature programmed desorption (TPD) as well as surface infrared spectroscopy experiments, Lemire et al.5 have shown that adsorbed CO molecules experience distinct binding environments on Fe3O4(111), with CO binding more strongly on the Fe+2 sites compared with the Fe+3 sites present on the magnetite surface. The results of Lemire et al.5 also provide information about the 3 ACS Paragon Plus Environment


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likely termination of the Fe3O4(111) surface investigated. Similarly, Henderson and coworkers have shown that the binding of various small molecules, including NO, is sensitive to the types of Fe cation sites present on the surface of an Fe3O4(111)-like termination of crystalline (Fe, Cr)2O3(0001).22-24 These workers specifically show that NO desorbs in multiple features during TPD, and attribute the features observed to NO initially adsorbed on Fe+3, Cr+3 or Fe+2 sites. Recent studies with monolayer FeO(111) films grown on Pt(111) and Ag(100) further demonstrate that the available surface sites can strongly influence the adsorption properties of iron oxides. Freund and coworkers report that a monolayer FeO(111) film on Pt(111) undergoes a structural change when subjected to high O2 partial pressures, with this change promoting facile CO oxidation.25-26 In contrast, negligible quantities of CO or NO adsorb on the FeO monolayer on Pt(111) at 90 K under ultrahigh vacuum (UHV) conditions.26 Prior work shows that the Fe and O atoms of the FeO monolayer can displace perpendicularly to the nominal FeO(111) plane, causing the Fe atoms to move toward the Pt(111) substrate and to thus be inaccessible to CO and NO from the gas-phase.25, 27-28 Recently, Merte et al. have shown that NO adsorbs relatively strongly and achieves high coverages on a FeO(111) monolayer grown on Ag(100) for adsorption at a surface temperature near 90 K.17 The FeO monolayer on Ag(100) exhibits less rumpling compared with the FeO/Pt(111) monolayer,16 and consequently renders the Fe atoms accessible to adsorbing NO molecules. These differences highlight the sensitive interplay between the structures and adsorption properties of FeO layers. In the present study, we used TPD and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on three FeOx structures grown as thin films on a Ag(111) substrate. We find that the binding energy of NO on Fe cation sites is quite sensitive to the cation oxidation state and the local surface structure, whereas the N-O stretching frequency varies to a 4 ACS Paragon Plus Environment

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lesser extent among the various adsorbed NO species. Our results suggest that NO TPD can serve as a more sensitive probe than RAIRS for distinguishing Fe binding sites of FeOx surfaces, in addition to providing new insights about the nature of NO bonding on these surfaces.

Experimental Details The experiments reported in this study were conducted in an ultrahigh vacuum (UHV) chamber with a typical base pressure of 2 × 10-10 Torr.29-30 The UHV chamber is equipped with a four-grid retarding field analyzer for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES), an ion source for Ar+ sputtering, a quadrupole mass spectrometer (QMS) used for TPD experiments and a Fourier transform infrared spectroscopy (FTIR) system for RAIRS measurements. The FTIR system consists of a MIR source (Bruker Tensor 27), a set of mirrors and lenses and an external liquid N2 cooled HgCdTe (MCT) detector. Outside of the UHV chamber, the MIR beam travels within a sealed box which is purged continuously with carbon dioxide and water-free compressed air. In the purge box, a set of flat mirrors are used to direct the MIR beam onto a parabolic mirror that focuses the beam. The focused MIR beam enters the UHV chamber through a differentially pumped KBr window and then reflects from the sample surface at angle of ~80° relative to the surface normal. The reflected MIR beam exits the UHV chamber through another KBr window and is directed onto the MCT detector within a second purge box. We provide more details of the RAIRS measurements performed in this study in the Results and Discussion section. We averaged 512 scans at a resolution of 4 cm-1 for all RAIR spectra reported here.



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The Ag(111) crystal used in the present study was mounted on W wires and attached to a copper sample holder that is in thermal contact with a liquid nitrogen cooled reservoir. A type K thermocouple was clipped to the edge of the crystal to measure the sample temperature. The sample is resistively heated and the temperature is controlled using a PID controller that adjusts the output of a dc power supply. In this setup, we are able to maintain or linearly ramp the sample temperature from 85 to 800 K. Sample cleaning consisted of cycles of sputtering with 1000 to 2000 eV Ar+ ions at a surface temperature of 300 K, followed by annealing at 800 K for 5 minutes. We considered the Ag(111) sample to be clean when we could not detect contaminants with Auger electron spectroscopy (AES) and observed a sharp LEED pattern characteristic of Ag(111). We grew FeOx films on Ag(111) by depositing iron using an electron beam evaporator at various substrate temperatures in a background of O2, with the O2 partial pressures ranging from 2 × 10-7 Torr to 1 × 10-6 Torr. We discuss the conditions used to grow each FeOx film in the Results and Discussion section. We performed TPD experiments as a function of the NO exposure to each FeOx film with the substrate held at 87 K during NO adsorption. After a NO exposure, we collected TPD spectra by positioning the NO-covered surface in front of a shielded quadrupole mass spectrometer (Hiden) at a distance of ~5 mm, and then heating at a constant rate of 1 K/s while simultaneously monitoring the partial pressures of N2, NO, N2O and NO2 using the mass spectrometer. For each of the FeOx films studied, we find that the majority of the NO desorbs during TPD rather than reacting. We observe only small amounts ( 330 K) decreases from 45% to 19% as the NO coverage increases to saturation at ~0.72 ML, demonstrating that NO molecules preferentially adsorb into the more strongly bound state at low coverage. However, a significant quantity of NO also desorbs in the lower temperature peak at low coverage, and both TPD features populate as the NO coverage increases toward saturation. This behavior suggests that the TPD features at ~269 K and 360 K



originate from NO molecules that are adsorbed on distinct types of domains that exist within the FeO layer. We specifically conclude that the peak at 269 K arises from NO molecules that are adsorbed on domains of the majority FeO(111) monolayer structure, while the peak at 360 K arises from NO molecules adsorbed strongly on domains of a minority FeOx structure that is present within the layer. We note that the saturation coverage of NO obtained on the FeO(111) monolayer is about twice that obtained on the Fe3O4(111) layer. This difference is consistent with the higher concentration of Fe cation sites present on the FeO(111) monolayer compared with the Feoct+2/Fetet+3 surface termination of Fe3O4(111) (0.79 ML vs. 0.50 ML).

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Figure 5: NO TPD spectra obtained as a function of the NO coverage prepared at 87 K on a FeO(111) monolayer on Ag(111).


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Figure 6 shows RAIR spectra obtained as a function of the NO exposure to the FeO(111) monolayer on Ag(111) held at 87 K. The RAIR spectra exhibit a single dominant peak that blueshifts from 1802 to 1842 cm-1 as the NO coverage increases to saturation at 87 K. Similar to the RAIRS data obtained from NO-covered Fe3O4(111), the value of the N-O stretch frequency is consistent with linear-bound NO and the band blueshifts with increasing NO coverage on both oxides. The blueshift occurs over a larger range of values for NO on the FeO(111) monolayer compared with Fe3O4(111) (~40 vs. 20 cm-1), likely because the NO coverage spans a larger range on the monolayer oxide. Overall, we conclude that the N-O stretching frequency is modified to similar extents for NO adsorbed on the FeO(111) monolayer and Fe3O4(111), even though TPD reveals differences in the NO binding energies on these surfaces.

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Figure 6: RAIRS spectra obtained as a function of the NO exposure to an FeO(111) monolayer on Ag(111) held at 87 K.



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The RAIR spectra obtained from the NO-covered FeO(111) monolayer exhibit weak intensity near 1885 cm-1, suggesting that NO dimers form in considerably lower quantities on the monolayer oxide compared with Fe3O4(111) under the conditions studied. The weak intensity at 1885 cm-1 is also consistent with the relatively small yield of NO which desorbs below ~200 K during TPD from the FeO monolayer (Figure 5). RAIR spectra obtained from NO-covered Ag(111) also exhibit distinct features that are not observed in the spectra obtained from the FeO(111) monolayer. Specifically, the RAIR spectra obtained after saturating Ag(111) with NO at 87 K exhibits a sharp peak at 1863 cm-1 that arises from NO dimers as well as prominent peaks at 2234 and 1250 cm-1 that originate from the N-N and N-O stretching vibrations of adsorbed N2O, which is produced by the decomposition of adsorbed NO dimers (see SI).47-49 The intensities of such features are very weak in the RAIR spectra obtained from the NO-exposed FeO(111) monolayer, thus providing evidence that the FeO monolayer exposed negligible areas of clean Ag(111). Comparison with recent work demonstrates that NO exhibits very similar properties when adsorbed on FeO(111) monolayers grown either on Ag(111) and Ag(100). Recently, Merte et al. have shown that NO desorbs from an FeO(111) monolayer on Ag(100) in a sharp TPD peak at 282 to 295 K and that the adsorbed NO produces a dominant N-O stretch band in RAIR spectra that blueshifts from 1798 to 1843 cm-1 with increasing NO coverage to saturation.17 These values of the NO desorption temperatures and N-O stretching frequencies agree well with those reported here for NO on FeO/Ag(111); we find that the main NO TPD peak appears between 269 and 297 K and the N-O stretch band appears between 1802 and 1842 cm-1 for NO on FeO/Ag(111) (Figures 5 and 6). This comparison reveals that differences in the Ag(111) and


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Ag(100) surface structures have little influence on the NO binding characteristics of the FeO(111) monolayer films grown on these substrates.

TPD and RAIRS obtained from NO on the FeOx multilayer on Ag(111) Figure 7 shows NO TPD spectra obtained as a function of the NO coverage generated at 87 K on the multilayer FeOx structure on Ag(111). The NO TPD spectra obtained from the FeOx multilayer are broad and consistent with NO desorption from multiple oxide structures. Although the desorption traces exhibit several features, three main desorption maxima are evident at about 207 K, 269 K and 345 K and labeled as γ, β and α, respectively. Additional features are evident at ~150 and 305 K and a sharp peak appears at 98 K when the NO layer approaches saturation at a coverage of 0.47 ML. The main TPD features intensify nearly simultaneously as the NO coverage increases, suggesting that these features originate largely from NO molecules that are adsorbed on different types of crystalline structures that coexist within the FeOx multilayer.



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NO TPD NO + FeOx / Ag(111) Ts = 87 K

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Figure 7: NO TPD spectra obtained as a function of the NO coverage prepared at 87 K on a FeOx multilayer on Ag(111).

Comparison of the NO TPD spectra obtained from monolayer FeO and multilayer FeOx suggests that the α and β peaks originate from structures that are present in both oxide layers. The β peak temperature obtained from multilayer FeOx overlaps well with that of the sharp TPD peak obtained from the monolayer FeO(111) film, and is thus consistent with NO adsorbed on monolayer FeO(111) domains that exist within the multilayer FeOx structure. The β TPD peak obtained from monolayer FeO(111) is 5 to 6 times more intense than that obtained from the multilayer FeOx structure. Similarly, the α TPD peak near 350 K overlaps closely in the NO TPD spectra obtained from the monolayer FeO(111) and multilayer FeOx structures, and exhibits similar intensity in these spectra as well. From integration of the TPD spectra, we estimate that 0.10 to 0.15 ML of NO desorbs in the α TPD peak from both the monolayer and multilayer 22 ACS Paragon Plus Environment

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oxides. The similar desorption yields may indicate that the surface structure responsible for the α TPD peak is present in nearly the same concentrations in the monolayer FeO(111) and multilayer FeOx structures investigated. It is worth noting that we have performed TPD and RAIRS experiments after repeating preparations of the FeO monolayer and FeOx multilayer structures, and find that the main spectral features are reproducible. The γ TPD peak near 210 K accounts for about one third of the NO desorption yield from the multilayer FeOx structure at saturation of the NO layer. Since an analogous feature does not appear in the NO TPD spectra obtained from monolayer FeO(111), it is reasonable to attribute the γ TPD peak to NO desorbing from multilayer FeOx islands, as identified originally by Merte et al.16 In this case, the TPD data reveals that NO binds more weakly on the multilayer islands than on monolayer FeO(111) grown on Ag(111). While several factors could contribute to the relatively weak binding of NO on the multilayer islands, including the surface termination and cation coordination geometry, Merte et al. have shown that the multilayer FeOx islands contain a large concentration of Fe+3 cations whereas the monolayer FeO(111) film nominally contains only Fe+2 cations.16 The weaker binding of NO on the multilayer islands may thus be attributable to NO interacting with Fe+3 sites. Indeed, this interpretation is consistent with that reported by Henderson,22-23 and discussed above, for NO binding on Fe+2 vs. Fe+3 sites of the Fe3O4(111) layer. To summarize, we tentatively assign the γ TPD peak (~210 K) to NO bound on Fe+3 sites of multilayer FeOx islands, the β TPD peak (~268 K) to NO bound on Fe+2 sites in monolayer FeO(111) domains and the α TPD peak (~350 K) to NO bound to Fe cations in a minority oxide structure that is present in both the monolayer and multilayer structures. RAIR spectra clearly demonstrate that NO species coexist in different binding environments on the multilayer FeOx surface after NO adsorption at 87 K. At low NO coverage, the RAIR 23 ACS Paragon Plus Environment


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spectra exhibit two distinct components at ~1814 and 1825 cm-1 (Figure 8a), with the relative intensity of the 1825 cm-1 peak increasing more sharply as the NO coverage initially increases. Although the various NO TPD features intensify nearly simultaneously with increasing coverage, the relative NO desorption yield in the α TPD peak is slightly enhanced at low coverages and decreases with increasing exposure to ~0.5 L. An implication is that NO desorbing in the α TPD peak is responsible for the N-O stretch band at 1814 cm-1 observed at low coverage, while the peak near 1825 cm-1 arises mainly from the more weakly-bound NO that desorbs below ~325 K. As the NO exposure increases above ~0.5 L, the N-O stretch band intensifies and sharpens, and blueshifts to a value of 1830 cm-1 once the NO layer saturates. The concurrent diminution of the spectral intensity near 1814 cm-1 suggests that the observed blueshift arises in large part from an increase in the local NO coverage on the coexisting oxide domains. More specifically, we suggest that the peaks located at 1814 and 1824 cm-1 at low NO coverage originate from NO molecules adsorbed on different oxide domains, and that both of these peaks blueshift with a sufficient increase in the local NO coverage on these domains due to an effect such as dipoledipole repulsion among adsorbed NO molecules.


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Figure 8: a) RAIRS spectra obtained as a function of the NO exposure to the FeOx multilayer on Ag(111) at 87 K, and b) RAIRS spectra obtained from a saturated NO layer on the FeOx multilayer on Ag(111) after adsorption at 87 K and subsequent heating to the temperatures indicated.

To further explore the NO binding, we collected RAIR spectra after heating a saturated NO layer on the multilayer FeOx surface to different temperatures, followed by cooling to 87 K, with the heating temperatures selected to desorb specific fractions of the NO layer. As seen in Figure 8b, heating to 170 K causes the N-O stretch peak to diminish only slightly, demonstrating that NO species which desorb below 170 K contribute weakly to the main RAIRS peak. Heating to 230 K desorbs NO in the γ TPD feature and causes the intensity of the N-O stretch band to decrease. The band also resolves into two components centered at 1824 and 1835 cm-1, with the former being more intense. The decrease in the RAIRS peak intensity indicates that NO adsorbed in the γ state contributes significantly to the peak at 1830 cm-1 observed at high NO coverage. Heating to 290 K causes the 1824 cm-1 peak intensity to decrease significantly, while the peak at



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1835 cm-1 diminishes to a lesser extent and redshifts slightly. Because heating to 290 K causes most of the β NO species to desorb, we assert that the feature that appears at 1824 cm-1 after heating to 230 K arises mainly from NO adsorbed on monolayer FeO(111) domains (i.e., the β NO state). This peak likely emerges because heating to 230 K lowers the local coverage of NO in the β state, resulting in a redshift of the N-O stretch frequency of the NO molecules adsorbed in the FeO monolayer domains. Further heating to 367 K causes the N-O stretch band to diminish and redshift as the NO coverage in the α state decreases. Overall, these heating RAIRS experiments provide further evidence that NO species coexist in different binding environments on the multilayer FeOx surface, and that the different species have similar N-O stretch frequencies that blueshift as the local NO coverage increases.

Discussion The present study provides evidence that NO achieves stronger binding on Fe+2 sites compared with Fe+3 sites among the FeOx surface structures investigated. Other researchers have also shown that both NO and CO bind more strongly on Fe+2 than on Fe+3 sites of Fe3O4(111) surfaces.5, 22-23 The present study confirms this behavior, and further shows that the NO binding strength depends on the nature of the Fe+2 sites present on the FeOx surfaces. Specifically, our results demonstrate that NO achieves higher binding energies on the Fe+2 sites present on Fe3O4(111) compared with the Fe+2 sites of FeO(111) monolayers grown on both Ag(111) and Ag(100). Several factors could influence the bonding of NO on the Fe+2 sites of these surfaces, including the structural environment near the Fe surface sites as well as interactions between the FeO monolayer and the Ag substrates, which are absent for the Fe3O4(111) surface.


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Our results also demonstrate that the N-O stretching frequency of adsorbed NO is relatively insensitive to the NO binding site on the FeOx surfaces, even though the NO binding energies are quite sensitive to the nature of the Fe surface site. For example, at low NO coverage (~0.05 ML), we observe an N-O stretch band at ~1800 ± 2 cm-1 for NO adsorbed on the FeO(111) monolayer on both Ag(111) and Ag(100),17 while the N-O stretch band appears at 1815 cm-1 for low coverages of NO on the Fe3O4(111) surface. This comparison shows that the more stronglybound NO on Fe3O4(111) actually exhibits a slightly higher N-O stretch frequency than NO on FeO(111), which is contrary to the behavior expected if the stronger NO-Fe binding was achieved through enhanced π-backbonding. Similarly, we find that the N-O stretch band remains unchanged at 1833 cm-1 after heating a saturated NO layer on Fe3O4(111) to selectively desorb NO from the Fe+3 sites while maintaining a near-saturation coverage of NO on the Fe+2 sites (Figure 4). This invariance shows that the N-O stretch band is relatively insensitive to the bonding differences experienced at the Fe+2 vs. Fe+3 sites of Fe3O4(111). An implication is that π-backbonding occurs to similar extents for NO bound on the various Fe surface sites, and thereby causes the N-O stretch band to redshift from the gas-phase value by an amount that is nearly the same for the various NO-surface species that form. We thus conclude that factors other than the extent of π-backbonding are responsible for producing the observed differences in the NO binding energies on these FeOx surfaces. As an example, Coulombic interactions could be important to the NO-surface binding, and may make a contribution that depends on the Fe charge state and local environment. Additional experimental and computational work is needed to determine the origin of the differences in the NO binding strength among the FeOx surfaces. Lastly, we make a brief comparison between the N-O stretch bands that we have measured and those reported for metal nitrosyl compounds. Researchers have shown that terminal NO 27 ACS Paragon Plus Environment


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species of transition metal complexes can adopt linear or bent geometries, and report a wide range of N-O stretch frequencies for these compounds. Linearly bound NO ligands exhibit N-O stretch frequencies in a range from ~1820 to 1610 cm-1, while NO bound in a bent geometry exhibits lower N-O stretch frequencies ranging from ~1650 to 1410 cm-1.37-39 The N-O stretch frequencies that we measure for NO adsorbed on FeOx surfaces lie on the high end of the range for linearly-bound NO, thus providing evidence that NO molecules bind on the Fe surface sites in an upright geometry. Further, the measured N-O stretch frequencies are only slightly redshifted from the value for gas-phase NO (~1860 cm-1). An implication is that π-backbonding does strengthen the Fe-NO binding on FeOx surfaces but that the extent of backbonding is relatively limited.

Summary We used TPD and RAIRS to investigate the adsorption of NO on crystalline iron oxide layers grown on Ag(111), including an Fe3O4(111) layer, an FeO(111) monolayer and an FeOx multilayer structure. TPD reveals that the NO binding energy varies sensitively with the charge state and local environment of the Fe surface sites present on the FeOx structures investigated. The NO TPD spectra obtained from the Fe3O4(111) layer exhibits a dominant peak at 380 K, attributed to NO bound on Fe+2 sites, as well as a broad feature centered at ~250 K that is consistent with NO bound on Fe+3 sites of Fe3O4(111) as well as NO adsorbed on a minority FeO structure present within the oxide layer. Desorption of NO from the monolayer FeO(111) film produces a prominent TPD peak at 269 K at saturation of the NO layer. After growing FeOx multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at ~200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur 28 ACS Paragon Plus Environment

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because the multilayer FeOx islands expose Fe+3 sites that bind NO less strongly than the Fe+2 sites of the FeO monolayer. The NO TPD data support previous reports that NO binds more strongly on Fe+2 sites compared with Fe+3 sites,22-23 and further demonstrate that the local structural environment can markedly influence the NO binding strength (e.g., Fe+2 sites of Fe3O4(111) vs. the FeO(111) monolayer). RAIR spectra obtained from NO adsorbed on each of the FeOx surfaces investigated exhibit a N-O stretch band that blueshifts over a range from ~1800 to 1840 cm-1 with increasing NO coverage. The measured N-O stretch frequency is consistent with atop, linearly-bound NO molecules on the Fe surface sites, and the N-O stretch band is only slightly redshifted from the gas-phase value. In contrast to the NO binding energy, we find that the N-O stretch band is relatively insensitive to a change in the surface binding site (e.g., Fe+2 vs. Fe+3). This behavior suggests that π-backbonding occurs to similar extents among the various adsorbed NO species, and thus that other factors are responsible for the differences observed in the binding energies of NO adsorbed on the different types of Fe cation sites present on the FeOx surfaces investigated.

Supporting Information NO TPD spectra obtained as a function of NO coverage from an ~20 Å thick Fe3O4(111) film. NO TPD and RAIR spectra obtained as a function of the NO coverage from clean Ag(111).

Acknowledgements



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We gratefully acknowledge financial support provided by the Department of Energy, Office of Basic Energy Sciences, Catalysis Science Division through Grant DEFG02-03ER15478. We also gratefully acknowledge support from the Swedish Research Council by the RöntgenÅngström cluster “Catalysis on the atomic scale" (Project No. 349-2011-6491) and the Swedish Foundation for International Cooperation in research and Higher Education (STINT) "Training in microscopy, spectroscopy and materials science; Functional materials at work" (Project No. IG

2014-5420).

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ToC Graphic 0.005

+2

(NO)2

NO TPD NO + Fe3O4(111)

+2

+3

RAIRS NO + Fe3O4(111)

NO-Fe /Fe 0.004


NO-Fe Desorption rate (a.u)

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+3

NO-Fe

0.003

0.002

0.001

(NO)2 100

200

300

400

Temperature (K)

500

600

0.000 1950

700

+3

Fe

+2

1900

1850

1800

1750

1700

-1

Wavenumber (cm )

Fe


Adsorption of NO on FeOx Films Grown on Ag(111) - The Journal of

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