J. Phys. Chem. B 2001, 105, 2773-2778
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Facile Reduction of NO via Dinitrosyl on Highly Oxidized Mo(110): Sensitivity to Local Structure and Defects F. C. Nart† and C. M. Friend* Department of Chemistry, HarVard UniVersity, 12 Oxford St. Cambridge, Massachusetts 02138 ReceiVed: September 25, 2000; In Final Form: January 26, 2001
The formation of mono- and dinitrosyl species on highly oxidized Mo(110) surfaces is investigated as a model for NO reduction induced by metal oxides. Defects and oxygen vacancies are shown to determine the mechanism for dinitrosyl formation and subsequent reduction to N2O. On a highly defective oxide surface prepared by oxidation at 1200 K, mononitrosyl species are exclusively detected at low NO coverage using reflection absorption infrared spectroscopy (RAIRS). No low-temperature reduction of NO is observed under these conditions; instead, NO desorbs below 300 K. A dinitrosyl species, i.e., a species where two NO molecules are bound to the same metal center, is formed at higher coverages on the defective oxide. Notably, the dinitrosyl is not formed by addition to mononitrosyl species. The low-temperature reduction of NO to N2O occurs only when the dinitrosyl is present. On a less defective thin-film oxide prepared at a surface temperature of 800 K, an NO overlayer consisting almost exclusively of dinitrosyls is formed at saturation coverage. These dinitrosyl species undergo competing reactions: reduction to N2O and decoupling to gaseous NO. Infrared spectroscopy is used to show that monomeric NO and the dinitrosyl species occupy different sites on the defective thin-film oxide on the basis of changes in the ModO stretch region of the spectrum. The changes in the ModO stretch intensities are attributed to the displacement of specific types of terminal oxygen (at steps and on terraces) by NO. These results indicate that NO creates its own adsorption sites. This characteristic is probably related to the defect density of the oxide.
Introduction Reduction of nitric oxide to environmentally benign products is of utmost importance in pollution control because NOx is produced during the combustion of fossil fuels. In automotive exhausts, the so-called three-way catalyst, made from platinum group metals, is generally used; however, this catalyst operates only in a narrow range of conditions. Accordingly, there is increasing interest in the development of new catalytic materials. Metal oxides are being investigated as potential catalysts or promoters.1 Specifically, the addition of MoO3 increases the conversion of NOx to N2O and N2 by supported metal catalysts,2 and the ability of MoO3 to form dinitrosyl species is believed to be responsible for the increase in the catalyst performance.3 Nevertheless, a direct correlation between low-temperature reduction and the presence of dinitrosyls is lacking. Furthermore, the role of different surface sites for the stabilization of dinitrosyls is of crucial importance to improving the catalyst action for NO reduction. In this study, we demonstrate that defects and oxygen vacancies affect the mechanism for dinitrosyl formation and that the dinitrosyl is necessary for low-temperature reduction to N2O on oxidized Mo(110). We further show that monitrosyl and dinitrosyl species bind to different sites on the surface. The adsorption and reactivity of NO on oxygen-modified Mo(110)4,5 has been studied previously; however, the role of defects in thin-film oxides prepared by oxidizing Mo(110) was not specifically considered. Remarkably, many of the adsorption characteristics of NO on oxidized Mo(110) are insensitive to * Corresponding author. † On leave from the Instituto de Quimica de Sa ˜ o Carlos, Universidade de Sa˜o Paulo, C. P. 780 - 13560-970 Sa˜o Carlos, SP - Brazil.
the overall degree of oxidation of the surface. In all cases studied, both monomeric NO and dinitrosyl species are detected. A vibrational peak at 1820 cm-1 is present on all surfaces studied and is assigned to the symmetric NdO stretch of a dinitrosyl species on the basis of isotopic labeling studies.4 The band characteristic of dinitrosyl is surprisingly insensitive to the surface preparation in that it was nearly the same for very different surfaces, ranging from nearly zerovalent Mo, containing only chemisorbed oxygen, to a highly oxidized thin-film. Indeed, a band at 1820 cm-1 has been observed for NO bound to supported catalysts containing tungsten,6 chromium,7 and molybdenum.3,6,8-10 Furthermore, a similar band is reported for dichlorodinitrosylmolybdenum complexes.11 In addition to the dinitrosyl species, several monomeric NO moieties are detected on oxidized Mo(110) using infrared spectroscopy. A band characteristic of mononitrosyl at ∼1720 cm-1 is present in the infrared spectra for NO bound to all oxidized Mo(110) surfaces. The position of this band depends weakly on the degree of surface oxidation, shifting to 1728 cm-1 for a defective thin-film oxide. Additional bands at 1558 and 1860 cm-1 are observed for the thin-film oxide. The former resembles the frequency measured for NO bound to hollow sites on Ni(111) surfaces;12 however, this species is unstable for temperatures above 180 K on oxidized Mo(110).5 The second at 1860 cm-1 is attributed to a second layer of weakly bound NO.13 This weakly bound NO desorbs from the surface at low temperature.13 While the binding of dinitrosyl species to oxidized molybdenum is rather insensitive to the molybdenum oxidation state, the vibrational frequencies for monomeric NO are strongly dependent on the surface preparation. The surface preparation
10.1021/jp003475l CCC: $20.00 © 2001 American Chemical Society Published on Web 03/20/2001
2774 J. Phys. Chem. B, Vol. 105, No. 14, 2001 can affect the number and types of oxygen vacancies present in the oxide. The possible roles of oxygen vacancies in determining the reactivity of Mo oxides have been widely discussed. Indeed, oxygen vacancies are thought to be necessary for NO to bind to MoO3. For example, recent theoretical studies of the interaction of NO with MoO3 showed that creation of defects on the surface is necessary for NO binding.14 Thus, we study the binding of NO on thin-film oxides with different degrees of perfection in order to further explore the role of defects in NO reduction. Previously, we showed that a more uniform oxide could be grown by decreasing the temperature used for oxidation to 800 K.15 The surface of the oxide prepared at 800 K does not exhibit significant step bunching and has a single, characteristic ModO stretch frequency of 999 cm-1. In contrast, oxidation at 1200 K is accompanied by the formation of step bunches and two distinct types of terminal oxygen. A ν(ModO) at 1016 cm-1 is attributed to ModO bound to step edges, whereas the ν(ModO) at 999 cm-1 is ascribed to ModO on terraces. Unfortunately, it is not possible to determine the exact oxidation states present because the oxide is only a few atomic layers and oxygen distribution is a function of depth. In this work, we show that it is possible to isolate mononitrosyl species and sequentially populate dinitrosyl species at higher coverages on a defective oxide on the basis of the strong binding of the NO. Low-temperature reduction is only observed when dinitrosyl is present. These experiments unequivocally demonstrate that monomeric nitrosyl species are not involved in the low-temperature reduction of NO to N2O; rather, the dinitrosyl is necessary for this process. Furthermore, we show that monitrosyl and dinitrosyl species occupy different sites on the highly defective oxide and that they displace terminal oxygen. Experimental Section Experiments are performed in a stainless steel ultrahigh vacuum chamber, described in detail elsewhere.16 The chamber contains low-energy electron diffraction (LEED) optics to check surface order, an Auger-electron spectrometer to determine surface composition and cleanliness, a quadrapole mass spectrometer, and a Fourier transform infrared spectrometer (Nicolet, model 800). The protocols for dosing and data collection have been described previously.4 The methods for preparing the highly oxidized surfaces have also been reported earlier.5,15 Briefly, the clean Mo(110) surface is exposed to O2 (1 × 10-9 Torr), while the crystal is held at 1200 K for 5 min to prepare the defective oxide. Alternatively, the crystal is exposed to O2 (1 × 10-9 Torr) at 800 K for 10 min to yield a more uniform oxide.15 Subsequently, the crystal is cooled to 450 K in the presence of oxygen. After reaching ∼450 K, the chamber is evacuated. These protocols produce a thin-film oxide with several types of oxygen coordination, including terminal oxygen. The defective oxide, grown at 1200 K, has two vibrational signatures at 990 and 1016 cm-1 that are characteristic of terminal oxygen bound to terraces and at steps. The more uniform oxide, grown at 800 K, has a single band at 999 cm-1, that is attributed to terminal oxygen bound to terraces.15 In addition, there are peaks at 767, 623, and 406 cm-1 in the electron energy loss spectra of both oxides that are assigned respectively to lattice oxygen and the ν(Mo-O) and δ(Mo-O) of oxygen in high coordination sites.17 Results NO Adsorption on the Defective Oxide: Infrared Spectroscopy Study. Four distinct NO species are formed on the
Nart and Friend
Figure 1. Infrared spectra obtained following exposure of a thin-film molybdenum oxide grown at 1200 K to NO for various dosing times. The relative exposures are (a) 0.025, (b) 0.07, (c) 0.1, (d) 0.5, and (e) 1, where 1 is defined as the NO flux required for a saturation coverage. The integrated NO flux is assumed to be linear with time. The shaded peak at 1821 cm-1 corresponds to the dinitrosyl species on the basis of isotopic labeling studies.
defective oxide surface, the populations of which depend on NO coverage (Figure 1). At low coverage, two mononitrosyl species, signified by NO stretch vibrations at 1716 and 1557 cm-1, are observed (Figure 1 a,b). These features were previously identified as monomeric NO on the basis of isotopic labeling studies.4 The 1716 cm-1 peak predominates at the lowest coverage. As the coverage is increased, a peak at 1557 cm-1 grows in intensity relative to the peak at 1716 cm-1. At coverages above 7% of saturation, two new peaks are detected at 1870 and 1824 cm-1. As the NO coverage is increased up to saturation, the 1870 cm-1 peak continues to grow, whereas the peak at 1557 cm-1 is essentially constant. The feature at 1820 cm-1 is characteristic of dinitrosyl species, in which two NO molecules are bound to the same metal center, whereas the feature at 1870 cm-1 is a weakly adsorbed NO forming a dimeric species. Note that the 1720 cm-1 band shifts from 1717 to 1722 cm-1 and broadens at higher NO coverage. The position and width of this band are sensitive to the surface preparation and NO coverage. The mononitrosyl species formed at low coverage are not converted to dinitrosyl upon additional exposure to NO. Instead, dinitrosyl species form on other sites. This phenomenon is observed when 15NO is dosed onto a surface containing only monomeric 14NO (1557 and 1717 cm-1) and weakly adsorbed dimeric 14NO (1870 cm-1), as in Figure 1c. After sequential adsorption of 15NO onto an 14NO overlayer containing only mononitrosyls, new features develop at 1787, 1693, and 1540 cm-1 (Figure 2b). As is evident from the comparison with the spectrum obtained for the equivalent overlayer containing only
Facile Reduction of NO on Highly Oxidized Mo(110)
Figure 2. Infrared spectra showing that monomeric NO is not converted to dinitrosyl on the defective oxide grown at 1200 K. (a) A pure 14NO layer, (b) sequential dosing of 15NO on a layer containing only monomeric 14NO, and (c) a pure 15NO layer. 15NO (Figure 2c), the dinitrosyl species formed are isotopically pure 15NO-15NO (band at 1787 cm-1). Formation of isotopically mixed 15NO-14NO species would be signified by a band centered at ca. 1807 cm-1 (Figure 2).5 In addition to the formation of (15NO)2, a monomeric 15NO species is populated, as signified by the peak at 1540 cm-1, which corresponds to the expected isotopic shift for the 14NO peak at 1557 cm-1. The band centered around 1716 cm-1 is split into two bands; however, the band expected for monomeric 15NO is not very intense. Interestingly, three peaks are observed in the region assigned to the weakly adsorbed dimer. The bands at 1870 and 1839 cm-1 are readily attributed to (14NO)2 and (15NO)2, respectively. The additional band at 1846 cm-1 has been characterized previously as the isotopically mixed 14NO15NO dimer.13 Changes in the regions of the spectra associated with the ModO moieties induced by adsorption of NO indicate that these terminal oxygen species are extremely labile. Furthermore, the different NO species affect different ModO moieties, indicating that they are associated with different environments. Specifically, the ModO stretch at 1016 cm-1 selectively diminishes in intensity when only monomeric NO is present on the surface. This is clearly seen by comparing the relative intensities for the two bands in Figure 3a (oxide without adsorbed NO) and the bands in Figure 3b (oxide covered with monomeric NO). In previous work, we assigned the peak at 1016 cm-1 to ModO species bound at steps.15 When the surface is fully covered by monomeric NO and dinitrosyl, the ν(ModO) disappears completely (Figure 3d). Notably, no oxygen leaves the surface under these conditions; thus, the oxygen must migrate to another coordination site and/or into the lattice. Unfortunately, the
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2775
Figure 3. Infrared spectra of the ν(ModO) region showing (a) the thin-film molybdenum oxide grown at 1200 K (no NO adsorption), (b) a layer containing only monomeric NO (0.1 saturation), showing selective depletion of the 1016 cm-1 peak, (c) recovery of ν(ModO) intensity by taking the ratio of the spectrum in panel b to that obtained after heating to 760 K to desorb all NO, and (d) the nearly complete depletion of ν(ModO) intensity after adsorption of a saturation amount of NO, forming monomeric NO and dinitrosyl species (Figure 1e). Spectra a, b, and d are ratioed against a spectrum obtained after heating to 1400 K so as to deplete all terminal via diffusion into the bulk.
vibrational bands for oxygen bound to high coordination sites are outside the range of our detector. The depletion of the terminal oxygen upon NO adsorption is reversible, as illustrated by the reappearance of these peaks upon heating to 760 K, a temperature at which all NO species leave the surface (Figure 3c) (note that the reference spectrum is 760 K and not 1400 K, as in Figure 3 a,b). Isotopic labeling experiments, in which N18O is reacted on the thin-film oxide, show that the oxygen deposition associated with N2O formation is not the origin of the observed repopulation. Specifically, only Mod16O was detected after heating an N18O layer adsorbed on an 16O-oxide to 500 K, indicating that the 18O left behind when N2O is formed mainly goes to high-coordination sites. In summary, it is evident from the data presented here that monitrosyl preferentially binds to steps and displaces terminal oxygen, indicated by the depletion of the 1016 cm-1 ν(ModO) band. NO Adsorption on the Thin-film Oxide grown at 800 K: Infrared Spectroscopy Studies. The NO species present on the oxide prepared at 800 K depend on the total NO coverage. At low exposures, there are four different NO stretching vibrations at 1719, 1754, 1869, and 1895 cm-1 (Figure 4 a). The peaks at 1719 and 1869 cm-1 are similar to those observed for the thin-film oxide prepared at 1200 K (Figure 1). No bands comparable to those observed at 1754 and 1895 cm-1 are observed on the thin-film oxide grown at 1200 K. For the higher NO exposures, the four bands increase in intensity, and a new weak band at 1823 cm-1 is observed (Figure 4b). This band at 1820 cm-1 has been assigned as the symmetric stretching of a dinitrosyl species on oxygen modified Mo(110).4,5
2776 J. Phys. Chem. B, Vol. 105, No. 14, 2001
Figure 4. Infrared spectra of the ν(NO) region obtained following NO adsorption on a thin-film oxide grown on Mo(110) at 800 K as a function of NO exposure at (a) 0.03, (b) 0.06, (c) 0.33, (d) 0.7, and (e) 1, where unity is defined as the time required for saturation coverage.
The peak at 1823 cm-1 grows rapidly in intensity upon further NO dosing, and it is the main species at saturation coverage (Figure 4e). As the NO coverage is increased, the bands at ca. 1750, 1870, and 1895 cm-1 all diminish. These bands seem to be due to two NO species that are converted to dinitrosyls, since they disappear at higher coverages (Figure 4 d,e). At saturation coverage, there is no detectable ν(ModO), demonstrating again that NO displaces terminal oxygen. Note that the more uniform oxide is characterized by a single ModO stretch at 999 cm-1. Infrared spectra obtained for a mixture of 15NO and 14NO confirm the assignment of the 1823 cm-1 band to a dinitrosyl species (Figure 5). Scrutiny of the data shows the dinitrosyl band consists of two features, signified by the peak at 1822 and a shoulder at 1816 cm-1 (Figure 5b). A similar shoulder is apparent for NO adsorbed on the defective oxide (Figure 1d). Both of these species are identified as dinitrosyls on the basis of the appearance of two new peaks in the infrared spectra obtained for a mixture of 15NO and 14NO (Figure 5a). There are two additional bands at 1794 and 1804 cm-1 in the infrared spectrum obtained for a 1:1 mixture 15NO:14NO that correspond to 15NO-14NO dinitrosyls. The bands at 1777 and 1782 cm-1 are attributed to 15NO-15NO dinitrosyls and those at 1812 and 1821 cm-1 to 14NO-14NO dinitrosyls on the basis of a comparison with spectra for pure 15NO and 14NO, respectively (Figure 5a). These results confirm that the majority of the species present on the oxide prepared at 800 K are dinitrosyls. Reactivity of Defective Thin-Film Oxide: TemperatureProgrammed Reaction Studies. The ability to selectively populate monomeric NO at low coverage is used to demonstrate that low-temperature N-N coupling to produce N2O only occurs when the dinitrosyl is present. Specifically, no N2O formation is detected in temperature-programmed reaction experiments for coverages where only the monitrosyl is present. Population of dinitrosyl species results in 15N2O production signified by the feature in the m/z ) 46 spectrum (Figure 6b). Isotopically labeled 15NO for the high (not saturation) coverage experiment further demonstrates that the dinitrosyl,
Nart and Friend
Figure 5. Infrared spectra used to identify the dinitrosyl on the oxide grown at 800 K. Lower panel: two independent spectra obtained after exposure of the more uniform thin-film oxide to 15NO and 14NO in separate experiments at saturation coverage. Upper panel: an equimolar mixture of 15NO and 14NO.
formed at high coverage, leads to N2O formation. Specifically, only m/ z )46, due to pure 15N2O, is detected in experiments in which 15NO is sequentially dosed on the surface containing 14NO monomers. No m/z ) 45, which would correspond to 15N14NO, is detected. In addition, no 14N or 14N15N was 2 observed at low temperature. A small amount of 15N2 is probably formed; however, it has the same mass as 14N16O and, therefore, cannot be unequivocally identified. In addition to the formation of N2O, NO desorbs from the defective oxide. Approximately 47% of the NO forms N2O, whereas the majority of the remaining amount leaves the surface as NO.18 Only a minor amount of N2 is formed at low temperature. As reported previously, there is no detectable N2 evolution via nitrogen atom coupling, which would occur at temperatures near 1000 K.18 Reactivity of NO on the Thin-Film Oxide grown at 800 K. The temperature-programmed reaction data for the dinitrosyl species formed at saturation NO coverage on the oxide grown at 800 K also undergoes competing reactions to evolve gaseous N2O and NO (Figure 7). There is also a minor amount of N2 at low temperature (Figure 7). The reduction product, N2O, accounts for about 47% of the NO, the same fraction as that for the thin-film oxide grown at 1200 K. There are three NO desorption peaks at 130, 240 and 290 Kssimilar to the desorption profile obtained for NO on the defective oxide. There are minor differences in the peak temperatures and the fraction of NO that desorbs in each peak on the two oxides. Thus, while the presence of the dinitrosyl is necessary for low-temperature reduction, it is not sufficient. We attribute the relatively low conversion of dinitrosyl to reduction products to an absence of oxygen vacancies, in particular at high coordination sites. Such vacancies are necessary to accept the oxygen formed in the reduction process.
Facile Reduction of NO on Highly Oxidized Mo(110)
Figure 6. Temperature-programmed reaction of the following: (a) a pure 15NO (0.1 saturation) showing m/e 31 (15NO) and m/e 46 (15N2O) traces and (b) a layer prepared by the sequential dosing 15N16O on a pure 14N16O layer containing only mononitrosyls. The total NO coverage is below saturation coverage. Note that the m/e ) 31 traces were corrected for cracking of N2O. The m/e 30 spectrum may also have a contribution from evolution of 15N2.
Discussion The ability to isolate dinitrosyl species from monomeric NO on these thin-film oxides of Mo(110) has provided a means of definitively demonstrating that dinitrosyls are necessary for the low-temperature reduction of NO to N2O. While adsorbed dinitrosyls and/or dimers have been proposed as intermediates in NO reduction, it has not been possible to make such a direct correlation previously. Dinitrosyls have been previously proposed as intermediates leading to NO reduction on oxygenmodified Mo(110)5,4 and on supported catalysts.19,3 Dimeric NO has also been proposed as an intermediate in NO reduction on other metals, such as Ag(111).20,21 However, dinitrosyls always coexisted with monomeric NO, rendering it difficult to establish that a coupled (NO)2 entity is necessary for reduction. Our studies further show that terminal ModO moieties on
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2777 the defective oxide surface are particularly sensitive to NO adsorption. The adsorption of NO clearly induces the displacement of terminal oxygen to other coordination sites, based on the decrease in the ν(ModO) peaks (Figure 3). These results indicate the ModO entities on both oxides studied are extremely labile, consistent with our previous studies of their thermal behavior.15 There is precedent for displacement of oxygen by NO on, for example, Pd(100).22 Defects, i.e., vacancies and steps, almost certainly play an important role in the ability of NO to displace terminal oxygen on our thin-film oxides. Notably, NO does not adsorb at room temperature on alumina-supported Mo(VI) oxide catalysts. Reduction of the catalyst to expose Mo(IV) ions is required for NO adsorption.23 Both nitrosyl and dinitrosyl species, which have been identified through the IR bands at 1710 and 1815 cm-1, respectively, are formed on the reduced catalyst.23 The need for oxygen vacancies is further demonstrated in recent theoretical studies of NO bound to MoO3-based surfaces.14 NO does not bind to the stoichiometric MoO3 surface; however, both mono- and dinitrosyls can form when there are vacancies at terminal sites. The fact that terminal oxygen is reversibly displaced by NO adsorption is strong evidence that a major fraction of the NO molecules bind to single Mo atoms, i.e., that vacancies at these terminal sites are needed for NO binding. The creation of vacancies at terminal sites requires vacancies at other sites that can accept oxygen. The displacement of terminal oxygen should also be most facile since it is more weakly bound than oxygen in high coordination sites. The assertion of stronger oxygen binding at high coordination sites is based on the fact that they are preferentially populated upon oxidation of Mo(110). While it is clear that the dinitrosyl is necessary for NO reduction and that exposed terminal sites are required for NO binding, our studies also show that vacancies at high coordination sites are necessary for conversion of the dinitrosyl to reduction products. The similar yields of N2O and N2 relative to NO desorption on the two different oxides despite the significant differences in the relative populations of dinitrosyls is evidence that their formation is necessary but not sufficient for low-temperature reduction. The specific need for vacancies at high coordination sites, not terminal sites, is illustrated by the fact that the oxygen left behind in the reduction process does not appear at terminal sites. Since only Mod16O and no Mod18O is detected following the reaction of N 18O on an 16Ooxide, oxygen left behind in the reduction must occupy high coordination sites. Ideally, there would be vacancies at both high coordination and terminal sites in close proximity. However, a high density of vacancies at high coordination sites also opens the NO dissociation pathway, which leads to high-temperature reduction via nitrogen atom diffusion and N2 formation. An alternative approach would be to use an oxygen scavenger in order to improve the dinitrosyl conversion. Indeed, this approach has been used in studies of the reduction of NO in the presence of CO,24,25 CH4,26,27 H2,28 and NH3.2 Studies of the CO + NO reaction have also been investigated on rhodium29 and NiO(100),30 for example. Conclusions Oxygen vacancies are shown to be important in determining the mechanism of dinitrosyl formation and subsequent reduction to N2O on highly oxidized Mo(110) surfaces. On a highly defective surface, prepared by oxidation at 1200 K, monitrosyl species are formed at low coverage. On a less defective thin-
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Nart and Friend tion to N2O and decoupling to NO take place. While the presence of dinitrosyl on the surface is necessary for lowtemperature reduction, it is not sufficient. Oxygen vacancies, probably at high coordination sites, are necessary for reduction. This is attributed to the fact that additional oxygen must be accommodated on the surface in order for reduction to proceed. Acknowledgment. The authors are grateful to the Department of Energy, Office of Basic Sciences for support of this work under Grand DE-FG02-84-ER 13289. F.C.N. is also grateful to the Fundac¸ ao de Amparto a Pesquisa do Estado de Sa˜o Paulo (FAPESP) from Brazil for a scholarship. References and Notes
Figure 7. Temperature-programmed reaction of a saturated layer of 15NO on thin-film oxides prepared by oxidation at (a) 800 and (b)1200 K. Masses m/e ) 30 and 31 are corrected from N2O fragmentation.
film oxide, prepared at 800 K, dinitrosyl are the primary species detected at saturation coverages. Dinitrosyl species must be present on the surface in order for low-temperature reduction to occur. Pure mononitrosyl species do not react to produce N2O at low temperature. Rather, NO is desorbed below 300 K. Low-temperature N2O evolution occurs only when dinitrosyl is present on the surface. On a nearly pure dinitrosyl overlayer, competing reactions of reduc-
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