7065
J. Phys. Chem. 1995, 99, 7065-7074
Very Low Temperature Surface Reaction: NzO Formation from NO Dimers at 70 to 90 K on Ag{ 111} W. A. Brown, P. Gardner,? and D. A. King* Department of Chemistry, University of Cambridge, L.en@eld Road, Cambridge CB2 IEW, U.K. Received: December 5, 1994; In Final Form: February 7, 1995@
When Ag{ 11l} is exposed to NO at 40 K, monolayer adsorption is followed by multilayer (N0)z formation. Heating to 60 K results in the desorption of the dimer multilayer, leaving a monolayer on the surface. Using isotopes, the monolayer has also been characterized as dimers, which have the N-N axis coplanar with the surface. At 70 to 90 K, N20 is formed from (N0)2 on the Ag{ 1111 surface. Further heating to 120 K results in the desorption of the NzO. The reactive species is thus shown to be the adsorbed dimer, which cannot be formed at higher temperatures on exposure to NO because the surface coverage is too low. The monomer, adsorbed NO, is thus weakly adsorbed, in agreement with expectation.
1. Introduction There is currently a striking controversy about the strength and reactivity of the interaction of NO with Ag{ 111). Molecular beam scattering studies conducted at room temperature have shown that the interaction potential is very weak,' and yet, at lower temperatures, reaction products such as N20 and NO2 have been observed on the surface. The room temperature data confirm the general conclusion that dissociative adsorption of NO (and of CO) occurs on transition metals to the left of the periodic table, and nondissociative adsorption to the right. For example, CO and NO dissociate on Mo and W, but do not on Pd and Ir; and CO is only weakly adsorbed on Cu and Ag. The low-temperature reactivity of NO on Ag{ 111) is therefore all the more surprising. In the present paper, a thorough reflection absorption infrared spectroscopy (RAIRS) study of the adsorption of NO and N20 on Ag{ 111) at 40-300 K is reported and the controversy appears to have been resolved. NO monomers are indeed weakly adsorbed on Ag{ 1 11). The adsorption of NO on Ag{ 111) has been studied previously by a variety of methods.'-* Edamoto et aL2performed an angle-resolved UPS study at 150 K and found that NO was adsorbed molecularly in an upright geometry. No species were observed on the surface at 300 K. In contrast to this, Goddard et aL3performed a LEED, AES, and TPD study of this system and reported that at 300 K, nondissociative adsorption of NO occurred with a saturation coverage of -5% and an initial sticking probability of -0.1. In the presence of Na, a strongly bound NO state was observed that underwent reaction to form Nz, N20, and 0 2 . Behm and Brundle4 studied the interaction of NO with Ag(111) at 25 K with photoelectron spectroscopy. At this temperature, three different NO states were observed: bridgebonded NO (possibly bent), physisorbed NO, and a third NO species that was assigned to NO interacting with adjacent adsorbed 0 atoms. The physisorbed NO was assigned to NO dimers, which are known to exist in solid NO, and was seen to desorb at 60 K. The existence of dimers in the multilayer was supported by theoretical calculations performed by Nelin et aL5 The NO said to be interacting with adsorbed 0 atoms was the only species that remained after annealing the surface to 180300 K. +Present address: Department of Chemistry, UMIST, P.O.Box 88, Manchester M60 lQD, U.K. Abstract published in Advance ACS Abstracts, April 1, 1995. @
Behm and Brundle4 also monitored the desorption and decomposition of NO by X P S and thermal desorption spectroscopy. Peaks in the desorption at 100, 124, 220, and 415 K were observed. The peak at 100 K was assigned to NO desorption, and the rest were assigned to N2O desorption. No evidence for adsorbed N atoms was observed in the X P S , and hence it was speculated that the observed N20 was formed directly from two adjacent NO molecules or from an unstable intermediate species, rather than via dissociation of the adsorbed NO. An HREELS study of NO/Ag{ 111) at 80 K was performed by So et aL6 The products of adsorption were found to be complex, with evidence for the coexistence of many surface species. From the data, it was inferred that at low coverages, NO adsorbed both dissociatively and molecularly in upright and bent 3-fold hollow coordinations. The 3-fold sites gave infrared frequencies of 1282 and 1153 cm-', and desorption occurred at 100 K. At medium coverages NO was also adsorbed in an upright, on-top, geometry, indicated by a peak at 1862 cm-'. This species desorbed at -90 K. The formation of N20 on the surface was also observed, indicated by the observation of the N-N stretch of NzO at 2241 cm-'. At saturation coverage, a new on-top NO species was observed at 1749 cm-' and the amount of N20 observed increased. No evidence for the formation of NO2 or (NO)z dimers was observed. Thermal desorption spectra showed desorption of NO at 90, 110, 190, and 400 K. The 400 K peak was assigned to the recombinative desorption of N 0 to give NO. The desorption of N2, N20, and 0 2 was also observed. So et al. suggested that the observed N20 was formed via the dissociative adsorption of A theoretical ab initio cluster calculation of NO/Ag{ 111) was performed by Bagus and Illas' in response to the above study. They suggested that the EELS peak observed at 1282 cm-' was in fact NO adsorbed with the 0-end down in a 3-fold hollow site, and not due to NO adsorbed N-end down or due to N20. There is, however, no experimental evidence for 0-end down NO adsorbed on Ag{ 111). A very detailed study of the NO/Ag{lll} system was recently made by Ludviksson et a1.* using thermal desorption of 14N160and 15N180adsorbed on Ag layers on Ru(OO1). They reported that adsorption of NO between 75 and 100 K produced adsorbed N20 and 0 atoms. The N20 formed from the reaction of NO on the Ag surface desorbed at 125 K, higher than NzO adsorbed on a clean, or oxygen-covered,surface. N20 adsorbed
+
0022-365419512099-7065$09.0010 0 1995 American Chemical Society
Brown et al.
7066 . I Phys. . Chem., Vol. 99, No. 18, 1995 on a clean Ag surface at 70 K desorbed at 85 K, and on an 0-precovered surface it desorbed at 95 K. Hence they suggested that some other species on the Ag surface was stabilizing the N20, possibly an NO2 species that was formed at low temperature. It was suggested that NO2 was formed from adsorbed NO and adsorbed 0 and that it accounted for the observed hightemperature NO desorption peak. This contradicts the N 0 recombination suggested by So et ~ 1or. N20 ~ decomposition suggested by Behm and Brundle4 to account for the hightemperature NO desorption peak. When Ludviksson et ~ 1 dosed . ~ 14N160 and 15N180,no isotopic scrambling was observed in either the NO or the N20 thermal desorption spectra and hence no dissociation of NO was occurring on the surface. On formation of N20 from two NO molecules, one of the NO species remained intact. In addition to this, they were able to verify that N20 that formed on the Ag{ 11l} surface was formed from the same dose of NO, even if NO from a previous dose was present on the surface. Hence dosing of NO onto the Ag{lll} surface resulted in N ~ o ( ~ dNO2(ad), ), O(ad), and NO(ad). The adsorbed NO did not ' react with incoming NO in subsequent doses. They concluded that there are two possible mechanisms for the formation of N20: via dissociative adsorption of NO or via the formation of an (N0)2 dimer. The former mechanism has been supported on Cu{ 100},9J0Cu{ 111},l1 and W{ 110}.l2 However, Ludviksson et al. favored the dimer reaction pathway for the formation of N20 on Ag{ 11l}. This was partly due to the fact that no NO dissociation was observed, but also due to the fact that N20 formation was hindered when NO was readsorbed after heating to 180 K to desorb N20 and NO. This was the opposite to the promotional effect observed on W{ 1lo} and suggested that adsorbed N is not important in the formation of N20. The dimer mechanism was also supported by Baldwin and Friend for N20 formation from NO on W{ 100}.13 The existence of NO dimers in the solid, and in condensed multilayers on the Ag{ 11l} surface, is known435.14J5although there is no evidence for the existence of (N0)2 dimers in the monolayer remaining after the multilayer desorption at 60 K. The dimer mechanism for N20 formation was ruled out by So et ~ 1 but. fits ~ in with the unstable intermediate suggested by Behm and B r ~ n d l e . ~ Several theoretical studies have also been made of the interaction of NO with Ag{lll}. DePristo and Alexander16 calculated a potential energy surface for NO adsorbed on Ag clusters which showed the strongest NO-binding geometry for NO lying flat on the surface. There is no experimental evidence for such a species. In several recent papers, Gates et al.17J8have computed the vibrational excitation occurring when NO is scattered from Ag{ 11l}, based on an empirical potential energy surface with NO chemisorbed on Ag{ 11l} in an on-top position with its axis perpendicular to the surface as suggested by So et ~ 1 It. is~ obvious that the correct binding geometry and adsorption potential for NO on Ag{ 11l} is essential for such calculations to be meaningful. In contrast to the adsorption of NO, there are few direct studies of N20 adsorption on Ag{ 111). Ludviksson et aL8 have studied adsorbed N20 formed during the reaction of NO on the Ag{ 111) surface, as discussed earlier. In summary, on a clean surface N20 is weakly adsorbed. It will only adsorb at temperatures below 70 K and desorbs at 80-90 K. N20 adsorbed onto an 0-precovered Ag{ 111) surface desorbs at the somewhat higher temperature of 95 K. However, N20 formed by the reaction of NO on Ag{ 111) desorbs at 125 K. This is in complete agreement with an earlier study made by the same
+
authors where the N20 was observed to be a molecular adsorbate that would only adsorb at temperatures less than 70 K.19 Study of the interaction of N20 with a polycrystalline Ag surface20 showed that there was no dissociative adsorption of N20 up to 900 "C and that the only observed N20 on the surface was adsorbed onto defects. From an XPS study of N20 on Ag{ 11l} at 30 K2' it was found that molecular N20 was adsorbed in the first layer, and subsequent adsorption resulted in the formation of a multilayer of N20 on the surface. When the adsorbate layer was heated, the multilayers desorbed at 100 K and left behind adsorbed 0 and NO3.
2. Experimental Section All of the RAIRS studies described here were performed with the novel RAIRS setup described elsewhere.22 The UHV chamber was designed for sensitive infrared emission experim e n t ~ and , ~ ~a few minor modifications were made to allow RAIRS experiments to be performed. The UHV chamber had a base pressure of 1 2 x mbar. The Ag{ 11l} crystal was cleaned by repeated cycles of argon ion sputtering at -600 K and annealing at 825 K until good NO spectra were obtainable with the RAIRS system. The crystal was cooled to -40 K by the use of a continuous flow liquid helium cryostat attached to the sample holder. Liquid nitrogen cooling was achieved by filling the same cold finger with liquid nitrogen. The temperature was measured with an N-type (nickel-chromium-siliconhickel-silicon) thermocouple attached to the sample holder next to the crystal. The RAIR spectra were taken with a Mattson instruments RS 100 series FTIR spectrometer. All spectra were taken with a resolution of 4 cm-I. Either 200 scans (taking -1 min) or 800 scans (taking -4 min) were taken depending on the required signal-to-noise ratio. The detector used was a narrow band, liquid nitrogen-cooled mercury cadmium telluride (MCT). Dosing of 14N0 and l5NO was achieved by the use of a precision leak valve mounted on the chamber. Doses could not be accurately recorded as, in a liquid helium experiment, significant quantities of NO adsorbed onto the cold finger before it could reach the sample. This meant that the pressure read by the ionization gauge was larger than that experienced by the crystal. The difference was calibrated by observation with a "standard" system. Using NO on Cu{llO}, we estimate a correction factor of l o x . This has been applied to all reported exposures, but it has to be recognized that they are all approximate. The pressure was measured without correction of the ion gauge sensitivity to NO. The 1:l mixtures of 14N0 (>99% pure, Air Products PLC) and 15N0 ('99% pure, Masonlite Limited) were mixed in the gas line first and then dosed into the chamber where the composition was recorded with a mass spectrometer. A new mixture was made each day to counteract the effect of any decomposition. Impurities (15N20) in the 15N0 were removed by the use of a dry ice and liquid nitrogen bath. The N20 ('99% pure, Distillers MG Limited) was dosed in a similar manner to the NO. During sample heating cycles the sample was flashed to the appropriate temperature and held there for 10 s before cooling back down and taking a spectrum. 3. Results and Discussion Adsorption of 14N0. Figure la-e shows RAIR spectra following adsorption of 14N0 on the Ag{ 11l} surface at 45 K. Initially only one peak is observed, at 1865 cm-I. Further
J. Phys. Chem., Vol. 99, No. 18, 1995 7067
N2O from NO Dimers at 70-90 K on Ag{ 11l}
1863
I
I '
( f ) 103K
I
5 YO
g) Heated to 63 K
I . : , . ' ; , : ; 18%
1800 1600 1400 1200 Wavenumbers Figure 1. Sample RAIR spectra following first the adsorption of 14N0 on Ag{ 111) with increasing exposure at 45 K, and then the heating of the adsorbed layer. The adsorption spectra are the result of exposure to (a) 3.1 L, (b) 6.1 L, (c) 9.1 L, (d) 12.1 L, and (e) 1.8 x lo-' mbar ambient pressure of NO. The desorption spectra are the result of heating to (f) 57 and (g) 63 K. 2200
2000
exposure causes this band to move to 1863 cm-', and a band at 1776 cm-' grows into the spectrum. The 1776 cm-' band moves up to 1788 cm-' with increasing exposure and dominates the spectrum. Increasing exposure at this temperature leads to the growth of multilayers of solid NO, hence the large, sharp bands. The spectrum at saturation coverage shows typical frequencies for (N0)2 dimers which are known to exist in solid N0.435J4J5The frequencies for (N0)2 dimers in the gas phase are 1860 cm-' for the symmetric stretch and 1788 cm-' for the asymmetric stretch.24 This is extremely close to the frequencies observed here. The presence of (N0)2 dimers was also inferred by Behm and Brundle4 from their XPS study at 25 K. It is noted here that the monolayer band at 1865 cm-' observed on initial exposure to NO has the same frequency as the symmetric stretch of the (N0)z dimer. This band was assigned to on-top NO by So et ~ l .even , ~ though the frequency of this band is extremely close to that of gas phase NO at 1876 ~ m - ' . This ~ ~ suggests that if it were due to molecular NO, it is unperturbed by the surface. It is simply assigned to some form of NO containing species at this stage, the exact assignment is made later. In addition, recent studies on Ni{lll} have thrown some doubts on band assignments for N0.26927 Figure lf,g show the results of heating the (N0)2 multilayer formed at 45 K. The multilayer can be seen to desorb between 57 and 63 K, in complete agreement with the 60 K multilayer desorption temperature reported e l ~ e w h e r e .The ~ band that is left after desorption of the multilayer is at 1863 cm-'. Figure 2 shows a heating sequence for the same system, after desorption of the multilayer. As the sample is heated from -70 to 90 K the 1860 cm-' band becomes attenuated, and in its place two bands at -2229 and -1250 cm-' grow into the spectrum. On heating above 110 K, both of these bands disappear together. The bands at -2229 and 1250 cm-' are assigned to the N-N
1800 1600 1400 1200 Wavenumbers Figure 2. Further heating of the same NO layer adsorbed at 45 K to (a) 63, (b) 73, (c) 80, (d) 87, (e) 92, (f) 103, (g) 110, and (h) 117 K. As the adlayer is heated from 63 to 90 K, N2O is formed from the adsorbed NO. All samples were flashed to the appropriate temperature and then cooled down to 45 K before the spectrum was taken. 2200
2000
stretch and N-0 stretch of N20 respectively. So et ~ 1 assigned . ~ the 1250 cm-' band in part to N20 but mostly to NO in a 3-fold site, while Bagus and Illas7 suggest that the 1250 cm-' band is due to NO bonded with the 0-end down. Both of these assignments are incorrect, as readily demonstrated by N20 adsorption. Proof that these bands are due to N20 formation can be seen in Figure 3. At the top is a spectrum for N20 adsorbed onto a clean Ag surface at 67 K. Two bands at 2249 and 1296 cm-' are observed. These are not quite the same frequencies as the bands observed for the N20 formed on the A g i l l l } surface. However, the bottom spectrum shows N20 adsorbed onto a surface with preadsorbed NO. The surface was then flashed to 150 K to remove NO and N20, and on recooling, N20 was adsorbed. In this case adsorbed N20 gives two bands at 2233 and 1252 cm-'; exactly the same frequencies as for the N20 formed on the surface from NO, shown in the central spectrum of the figure. There is obviously a residue on the surface that shifts the frequency of the N20 absorption bands. This is discussed in more detail later. The adsorption and desorption of N20 is also discussed later. We have therefore shown in Figure 2 that NO reacts to form N20 on Ag{ 11l} at 70-90 K, an extremely low temperature for a chemical reaction. The possibility cannot be ruled out that some of the NO desorbs, while the rest of it reacts to form N20. This would then account for the 90 K desorption peak seen by So et aL6 The adsorption of NO at 45 K has thus allowed the exact temperature at which N2O is formed on the Ag{ 11l} surface to be determined accurately. N20 formed by reaction on the surface desorbs at 110-117 K. This is much lower than the 190 K desorption observed by So et ~ 1 but. in~ general agreement with the 125 K N2O desorption observed by Ludviksson et aL8 In the present study, nothing was observed on the surface above the N20 desorption temperature, in contradiction with the UPS2 and LEED3 studies performed at 150 and 300 K, respectively. In a further series of experiments, the Ag{ 11l} surface was exposed to NO at a temperature of 90 K. A spectrum at
7068 J. Phys. Chem., Vol. 99, No. I S , 1995
Brown et al. TABLE 1: Frequency of the NCO Species for Different Supported Catalyst and Single-Crystal Systems system frequency (NCO), cm-' 2190-2201 2264 2180 2148-2238 2259-2267 2170-2 190 2240-23 10 2239-2267 2190 2280 2148 2267 2180 2160 2170
I
I2233
2200
2000
1800
1600
1400
1200
Wavenumbers Figure 3. Proof that the bands at 2233 and 1252 cm-' are due to the formation of N20: (a) the spectrum obtained after the adsorption of N2O onto a clean Ag{ 11 1) surface at 67 K; (b) the spectrum for the N20 formed from the reaction of NO on the Ag{lll} surface after heating to 70-90 K; (c) a spectrum for N20 adsorbed onto a Ag{ 1 1 l} surface at 86 K which has been pretreated by the adsorption of NO and then been flashed to 150 K to desorb any NO and N20 from the surface.
1249
I
2185
2200
(d) 150K
2000
1800
1600
1400
1200
Wavenumbers Figure 4. A heating sequence for NO adsorbed onto Ag{ 1 1 1) at 90 K: (a) the spectrum for a saturated adlayer at 90 K; (b) the spectrum for an adlayer heated to 99 K; (c) the spectrum for an adlayer heated to 115 K; and (d) the effect of heating to 150 K. All samples were flashed to the appropriate temperature and then cooled back down to 90 K before a spectrum was taken.
saturation coverage is shown in Figure 4a. In this spectrum, four main bands are observed, at 2228, 2180, 1859, and 1253 cm-'. The two bands at 2228 and 1253 cm-' are assigned to the N-N and N-0 stretch of N20 respectively, as for the spectra shown in Figure 2. The band at 1859 cm-' is again assigned to a possible molecular NO species, the exact nature of which
is not known at present. An additional band at 2180 cm-I is observed in this spectrum that was not seen in the adsorption at 45 K. This is assigned to an NCO (isocyanate) species. The frequencies for isocyanate formed on supported catalyst and single-crystal systems as a result of the reaction of NO and CO are shown in Table 1.28 For the catalyst systems, the higher frequency bands shown in the table are attributed to NCO bonded to the support, and the lower frequency bands to NCO bonded to the supported metal atoms. The agreement between the frequency of the observed band and the frequencies shown here led to the conclusion that the band observed at 2180 cm-I is due to the formation of NCO on the Ag{ 111) surface. NCO is formed by the reaction of CO with partially dissociated NO on Ru{001),28 and hence the presence of NCO on the Ag{lll) surface at 90 K suggests that some NO dissociative adsorption, as well as molecular adsorption, takes place at higher temperatures. This is in agreement with the observations of So et aL6 The source of CO is clearly the background in the chamber. No NCO formation was observed on adsorption of NO at 45 K, suggesting that no NO dissociation occurs at that temperature. However, we note that CO is only adsorbed on clean Ag{ 111) at temperatures 1 2 0 K.29 The reaction to form NCO must occur through a short-lived adsorbed state, possibly stabilized by coadsorbed N,O, species. Figure 4a-d shows sample spectra from a desorption sequence for NO adsorbed at 90 K, from which it is seen that the NO species at -1860 cm-' desorbs at 90-99 K, NzO desorbs at 99-115 K, and the NCO is removed by -150 K. These fiist two desorption temperatures are in general agreement with previous observations.6,8 When NO was adsorbed at 150 K, no peaks were observed in the RAIR spectrum, apart from a small one at 2180 cm-' (the NCO frequency). This may suggest that some dissociative NO adsorption occurs at 150 K, although we were not able to see any evidence for this with RAIRS. After exposure to NO at 150 K, the surface was cooled to 90 K and an attempt was made to adsorb NO. No adsorption was noted suggesting that something does adsorb at 150 K, as suggested by UPS2 and LEED,3 which blocks further NO adsorption at 90 K. However there is no positive evidence for the nature of this species from infrared. If it were upright NO as suggested by Edamoto et a1.,2it should have been observable with the RAIRS system, and hence this is ruled out. Adsorption of Isotopic Mixtures of 14N0 and I5NO. In order to identify the nature of the species giving rise to the -1860 cm-' NO band, adsorption of a mixture of the two NO isotopes, I4NOand I5NO,was studied. In particular this would enable a distinction to be made between monomeric NO, which
J. Phys. Chem., Vol. 99, No. 18, 1995 7069
NzO from NO Dimers at 70-90 K on A g ( l l l }
(0
2000
1900
1800 1700 Wavenumbers
1600
Figure 5. Saturation coverage spectrum of an NO multilayer at 45 K. The surface was exposed to an ambient pressure of 1.1 x mbar of
1 4 ~ 0 ~ 5 ~ 0 .
should yield two bands of equal intensity with a 1:l mixture of 14N0 and 15N0 and adsorbed dimers, which should yield three bands. The frequencies of the two bands from monomeric NO would be determined by the reduced mass, p, of the species involved; with the 14N0 stretch at 1860 cm-l, the 15N0 band is thus predicted to be at 1795 cm-'. If the 1860 cm-l NO species were due to an (NO);! dimer, three bands would be expected on adsorption of an isotopic mixture of NO. These would be for (14NO);!, (l4N0l5N0),and (15NO)2. In a system with no dipole coupling, these would have an intensity ratio of 1:2:1. In the multilayer, we have already shown the presence of NO dimers through the appearance of both in-phase and outof-phase modes at 1863 and 1788 cm-'. With the isotopic mixture, these bands should each split into three sub-bands. Figure 5 shows a spectrum of an NO multilayer formed from an equal mixture of 14N0 and 15N0 at 45 K. The presence of three bands in each region of the spectrum clearly confirms that the NO multilayer on Ag{ 11l} is composed of (NO);! dimers. The symmetric stretch of the (NO);! dimer shows the expected 1:2:1 ratio for the three observed bands, but the asymmetric stretch shows a completely different ratio of 4:3:1. This is due to dipole coupling, as discussed in more detail later. Figure 6 shows an adsorption sequence for increasing exposure of the Ag{ 111) surface to an equal mixture of 14N0 and 15N0 at 65 K, which is above the multilayer desorption temperature. Three bands can be seen in the dimer symmetric stretching region, at 1856, 1847, and 1828 cm-l, with an intensity ratio -4:3:1. Despite the relatively poor signal-tonoise ratio, it is obvious that three bands are present. This is the first direct evidence for the existence of (NO);! dimers on Ag( 11l} above the multilayer desorption temperature. Further evidence that these bands are due to dimers in the sub-monolayer regime is provided by the fact that no band is observed at 1795 cm-', as would be expected for monomeric 15N0. The observed 4:3: 1 intensity ratio is due to dipole coupling, as discussed later. . Figure 7 shows an adsorption sequence for 14N0 and I5NO at 55 K, Le., below the multilayer desorption temperature, and again three bands are observed in the (NO)z symmetric stretching region (spectrum e) before the onset of multilayer growth. Spectra f and g show the initial stages of multilayer growth. The region around 1860 cm-' shows the same three monolayer bands at 1855, 1849, and 1833 cm-', in transition from a dipole-
1 1950
I
1856
1900
1850 1800 1750 1700 Wavenumbers Figure 6. The adsorption of a 1 :1 mixture of I4NO and 15N0at 65 K. The given exposures are (a) 1.1, (b) 1.7, (c) 2.6, (d) 3.3, and (e) 4.5 L.
I
2000
,
I
1900
1800 1700 1600 1500 Wavenumbers Figure 7. The adsorption of a 1:l mixture of 14N0 and l5NO at 55 K.
The given exposures are (a) 1.8, (b) 2.2, (c) 2.4, (d) 3.0, (e) 3.2, (f) 4.2, and (g) 33 L.
coupling-dominated band, into the multilayer band in which there is no dipole coupling. This provides direct information about the orientation of the (NO);! dimers in the monolayer and multilayer regimes, as discussed later. In contrast, the band in the dimer asymmetric stretching region shows no dipole coupling in the sub-monolayer regime, and then coupling
Brown et al.
7070 J. Phys. Chem., Vol. 99, No. 18, 1995
0
Figure 9. Schematic diagram showing the structure and vibrations of the dimer. The observed infrared stretches are due to the symmetric (vl) and asymmetric (v5)vibrational modes. 1845
I-
*I
2000
I
I
I
I
I
I
1900
1800 Wavenumbers
1700
1600
Figure 8. Spectra showing that (NO)* dimers exist on the surface even at 90 K: (a) the spectrum for I4NO adsorbed onto Ag{ 111) at 90 K; (b) the spectrum for an equal mixture of I4NO and 15N0 at 90 K; and (c) the spectrum for ISNO adsorbed at 90 K.
1
develops as the multilayers build up. The final spectrum in this adsorption sequence is the multilayer spectrum already shown in Figure 5, where the monolayer bands are completely swamped. Final evidence for the presence of an (N0)2 dimer on the Ag{ 11l} surface is provided by exposing the crystal to equal amounts of 14N0 and 15N0 at 90 K. As we have shown, at this temperature low exposures of NO produce adsorbed N20, and higher exposures show the presence of a stable NO species with a band at 1858 cm-' (Figure 4). The spectra shown in Figure 8 now show that this species is also the dimer, (N0)z. Spectrum a in Figure 8 is from 14N0 adsorbed at 90 K and shows a band with a frequency of 1862 cm-'. Spectrum c is from 15N0 and has a band frequency of 1824 cm-' (not 1795 cm-' as for monomer NO). Spectrum b is from an equal mixture of 14N0 and 15N0 and shows three bands, at 1855, 1845, and 1822 cm-I, with an intensity ratio of roughly 1:2:1. These are the same frequencies observed in the spectra taken at 55 and 65 K shown earlier. Dipole coupling is reduced here, presumably due to the presence of N20 and 0 in the mixed adlayer. The observation of these three bands, even at 90 K, is clear evidence for the presence of the (N0)2 dimer species on the surface. This conclusion contradicts that reached by So et a1.,6 who assigned the 1860 cm-' band to upright NO using HREELS. They actually observed, at high coverages, the asymmetric dimer band as well as the symmetric band, but ruled out the presence of (N0)2 dimers on the surface. Our result also demonstrates that the potential energy surfaces used by Gates et al.,1731* based on the assumption that the NO on the Ag{ 11l} surface was a monomer in an upright geometry, are not relevant to this system.
As far as we are aware, this is the first unequivocal demonstration of (N0)2 dimers on a metal surface above the multilayer desorption temperature. The presence of the (N0)2 on Ag{ 111) in preference to the monomer also demonstrates that NO monomers are much more weakly adsorbed than the dimer on Ag{ 111). Some evidence for the presence of (N0)2 dimers on Cu{ 11l} has very recently been seen by Suhren et u ~ . , ~ and O also by ourselves on Cu{ 1 although in this case, the monomer is the most stable species on the surface at low coverages. Orientation of the Dimers in the Monolayer and the Multilayer. The orientation of the (N0)2 dimers in the monolayer and in the multilayer can be determined both by examination of the allowed infrared modes for the different dimer geometries and from the observed dipole coupling. The structure of gas phase (N0)2 is indicated in Figure 9. Bonding occurs between N atoms, and the weakness of the interaction is indicated by the length of the N-N bond. Structure a is rather more stable than structure b. Suzanne et ~ 1performed . ~ ~a neutron diffraction study of NO on graphite and discovered two different solid NO phases, depending on both temperature and coverage. At 10 K, the (N0)2 is first observed lying down on the surface, but at higher coverage it stands up with the crystal-packing structure shown in Figure 10. The N-N axis is normal to the surface plane. The two vibrations of the (N0)2 dimer that are observed in the multilayer on Ag{lll} are the symmetric ( V I ) and the asymmetric ( ~ 5 stretches ) of the molecule illustrated in Figure 9a. In the monolayer regime only the symmetric stretch is observed. For the symmetric stretch ( V I ) , the net dipole moment is perpendicular to the direction of the N-N bond, and for the asymmetric stretch the net dipole moment is parallel to the N-N bond direction, as indicated in Figure 10. Symmetry arguments can be used to determine the orientation of the dimer in the monolayer regime, and in the multilayer. The possible orientations for an (N0)2 dimer on the surface are shown in Figure 11. In the gas phase, the point group of the (N0)2 dimer is CzV and hence the symmetries of the symmetric and asymmetric stretches are A1 and B2, respectively. When the dimer bonds to the Ag{ 11l} surface, the symmetry is reduced to Csl (ax. mirror plane retained) for the tilted and lying down species and to C? (ayzmirror plane retained) for the end-on species. The upright dimer species has the same CzVsymmetry as the gas phase dimer. From correlation tables the changes in symmetry of the symmetric and asymmetric stretches of the dimer with point group are obtained, as shown in Table 2. A vibrational mode is only allowed in infrared spectroscopy of adsorbates on metal surfaces if it is totally symmetric, or contains the totally symmetric transformation. Hence for the lying-down species, the upright species, and the tilted species
J. Phys. Chem., Vol. 99, No. 18, 1995 7071
N20 from NO Dimers at 70-90 K on Ag{ 111)
(a)
I/ 0 0
Nitrogen
Oxygen
Figure 10. A schematic diagram of the crystal-packing structure of NO/graphite observed by Suzanne et ~ 2 1 at. ~high ~ coverages. (a) An on-top view of the structure and (b) a side view of the overall 3D structure. It also shows the arrangement of dipole moments for (a) the symmetric stretch of the (N0)2 dimer and (b) the asymmetric stretch on the Ag( 11l} surface.
only the symmetric stretch is dipole allowed. We can distinguish between these, however, as it is unlikely that the lyingdown species would yield significant intensity in the symmetric stretch. Secondly, recent N E W S data33suggest that the dimer is tilted at an angle of -53’ to the surface normal. Hence the symmetric stretch seen in the monolayer and sub-monolayer regime is assigned to a dimer in a tilted geometry on the surface. Note that the diagram in Figure 11 shows the dimer bonded to the surface via the N-N bonding pair, although bonding via the 0 atoms of the dimer cannot be ruled out. The only geometry in which both the symmetric and asymmetric stretches are allowed is the end-on geometry. Thus, in contrast with the monolayer, the multilayer structure is assigned to a dimer in an end-on configuration. With growth of the multilayer, the asymmetric stretch intensity grows considerably and the symmetric stretch only grows by a small amount. This is further evidence for an end-on orientation of the dimer, as it might be expected that the intensity of the symmetric stretch would be small as it is approximately parallel to the surface. A plausible structure of the solid (N0)2 multilayers which satisfies these symmetry arguments is shown in Figure 10 for solid monolayers adsorbed on graphite. When a 1:l mixture of 14N0 and 15N0 is dosed onto the surface, in the absence of dipole coupling, the symmetric and asymmetric dimer bands would both be expected to split into three bands in the intensity ratio 1:2:1, corresponding to (14NO)2, (14N015N0)/(15N0i4NO), and (l5N0)2. However, dipole coupling will result in “intensity borrowing” into the highfrequency bands. In the spectra shown in Figures 6 and 7, there is clear evidence that dipole coupling is taking place. In the
(b)End on
Upright
(c) Lyingdown
(d) Tilted
Figure 11. Schematic diagram showing the possible geometries for an (N0)2 dimer on a surface. The point groups for each geometry are also given. The data support the tilted structure (d), for the submonolayer to monolayer regime, and the end-on structure (b), for the multilayer.
TABLE 2: Transformation of Symmetric and Asymmetric Stretches of the Dimer under the New Point Groups Generated when the Dimer Bonds to the Surface VI
A1
VS
B2
A’ A“
A‘ A‘
monolayer, with the upright (N0)2 dimer, dipole coupling becomes stronger with increasing coverage. This is indicated by the greater intensity of the high-frequency band with respect to the lower frequency bands. On changing from the monolayer to the multilayer, the symmetric stretch is only loosely coupled in the second layer, and becomes completely uncoupled as the multilayer grows, as seen in Figure 5. The asymmetric stretch, on the other hand, is initially uncoupled (at low multilayer concentration) and becomes strongly coupled as thick multilayers build up on the surface. This unusual behavior is satisfied in the multilayer regime with the structure of solid monolayer (N0)2 proposed on graphite.32 This packing arrangement would give the arrangement of dipoles for the symmetric and asymmetric stretches shown in Figure 10. For the symmetric stretch, neighboring dipoles are orthogonal, and very little dipole coupling occurs. For the asymmetric stretch, the dipole moments are all parallel to one another and are therefore strongly coupled. It is noted here that dipole coupling is reduced for the (N0)z dimers seen adsorbed on the surface at 90 K, from which we conclude that the coadsorbed N20 separates and screens the dimers from each other. In conclusion, the combined use of symmetry arguments and a detailed study of the dipole coupling observed has allowed a determination to be made of the exact geometry of the structure and orientation of NO dimers in both the monolayer and multilayer regime. Mechanism of N20 Formation. There are two competing mechanisms by which N20 could be formed on the Ag{ 111) surface. The first is via dissociative adsorption of NO:
Brown et al.
7072 J. Phys. Chem., Vol. 99, No. 18, 1995
--
-*-
0.0 -
1860 cm-1band 1250 cm-1 an
2229 cm-1bani
0
-1.0-
-2.0 -
s
-3.0-
c
c.
-4.0-
-5.0-6.0-7.0
0
1
I
I
I
1
I
I
1
0.010 0.011 0.012 0.013 0.014 0.015 0.016 0.017
60
70
80
90
100
110
120
Temperature (K) Figure 12. A plot of the normalized absorbance of the 1860 ((N0)2 asymmetric stretch), 2229 (N-N vibration of NzO), and 1250 cm-' (N-0 vibration of N20) bands observed on Ag{ 111) versus temperature.
2N0
-
Nad -k
oad-k NOad
1m (K-1) Figure 13. An Arrhenius plot for the formation of N20 from (N0)z dimers on Ag{ 11I}. A small amount of dissociative adsorption of NO does occur on Ag{lll}, confirmed by the appearance of NCO on the surface. However, from the above it is likely that this is also produced by an alternative reaction pathway, from (N0)2 dimers. Using the graph in Figure 12, the apparent activation energy for the formation of N20 from (NO)? dimers via the reaction
The second mechanism is via the formation of (N0)2 dimers:
2No
-
The observation of the (N0)2 dimer on the surface at all temperatures, both above and below the multilayer desorption temperature, provides strong support for the second mechanism. Further support for this mechanism is provided by the graph shown in Figure 12. The figure shows a plot of the amount of (NO)z and N20 on the surface versus temperature, as determined by integrating the areas under the 1860 cm-' ((N0)2) and the 2229 and 1250 cm-' (N20) infrared bands that were observed when a monolayer of NO adsorbed at 45 K was heated (Figure 2). The absorbances were then normalized so that the maximum peak intensity was equal to an absorbance of 1. An assumption had to be made that the total integrated area under the peaks was proportional to the amount of the appropriate species on the surface. Orientational changes, for example, would introduce nonlinearity. However, we note that band frequencies are independent of coverage which supports the assumption made. As can be seen in Figure 12, the N20 bands grow in as the (N0)2 band decreases, and in fact the curves are mirror images of each other. This is evidence that the (NO)? reacts directly to form N20. This pathway for N20 formation is also favored by Ludviksson et aL8 The dimer mechanism is further supported by the following experiments. NO was adsorbed onto a surface on which NO was previously adsorbed prior to flashing to 150 K to desorb any NO and N20, leaving behind any dissociated N and 0 atoms. Adsorption at 45 K produced very little N20 as the sample was heated, showing that something hindered its production. The same occurred on adsorption at 90 K after the NO pretreatment: no N20 bands were observed. If the dissociative reaction was the path for N20 formation, it would be expected that N20 production would be enhanced by this treatment because of the N atoms on the surface. On W{ 1lo}, where the dissociative mechanism occurs, the production of N20 is enhanced by this procedure.12
can be calculated, assuming the rate expression
and assuming that the rate constant, k, is independent of surface coverage. An Arrhenius plot obtained from the data in Figure 12 is shown in Figure 13. The gradient yields an apparent activation energy of E, = 7.3 kJ mol-', and from the intercept the preexponential v , in the rate constant given by
is -3500 s-'. This very low frequency factor may arise from a large configurational term or from contributions to the overall rate from a multiplicity of pathways. The activation barrier is also very low, as would be expected for a reaction that takes place at 70 K. The loss of an 0 atom from the dimer is therefore a facile process, which may involve thermal excitation of the hindered rotation about the N-N axis, bringing one 0 atom into contact with the bare Ag surface. The Adsorption of NzO. Figure 14a-d shows an adsorption sequence for the adsorption of N20 on clean Ag{ 1111 at 67 K. Two bands at 2228 and 1277 cm-' initially grow into the spectrum. With increasing exposure, these bands are replaced by the bands for multilayer NzO which grow in at 2244 and 1294 cm-' and shift to 2251 and 1296 cm-', respectively, with increasing exposure. These are assigned to the N-N and N-0 stretches of N20, respectively. At very high coverages, the overtone of the 1296 cm-' band is observed at 2580 cm-'. The frequencies of these bands are not identical to those for the N20 formed on the Ag{lll} surface by the reaction of adsorbed NO; the products of the reaction influence the frequencies, and the desorption temperature, of adsorbed N20. Figure 14e-g shows a desorption sequence for N20 adsorbed onto clean Ag{ 11l} at 67 K. The multilayer desorbs at 78-83 K and the monolayer at 83-88 K. This is much lower than the observed
J. Phys. Chem., Vol. 99, No. 18, 1995 7073
N20 from NO Dimers at 70-90 K on Ag{ 11l}
(a)
2228
12%
2249
J
-
1277
(9) Heated to 88 K I ,
'
,
,
I
I
,
2000 1800 1600 1400 1200 Wavenumbers Figure 14. Spectra showing the adsorption and desorption of N20 onto Ag{ 11l} at 67 K. The exposures are (a) 0.1 L, (b) 1.3 L, (c) 6.3 L, and (d) 1.3 x lo-' mbar ambient. The sample was heated to (e) 78, (0 83, and (g) 88 K. 2600
2400
strates the presence of a residue on the surface after NO and N20 desorption at 150 K, although this study provides no evidence for the nature of this residue. Figure 15a-e shows a heating sequence for N20 adsorbed on a pretreated Ag{ 111) surface. Here, N20 desorbs at much higher temperatures, from 107 to 114 K. No evidence for dissociation is observed, in agreement with Grimblot et aLZ1 The residue formed on the Ag{lll} surface when NO is adsorbed at temperatures above 70 K is not adsorbed 0 (although this is definitely present on the surface), since Ludviksson et aL8report that N20 adsorbed in the presence of 0 desorbs at only 90 K. They speculate that it is NO2 which interacts with the N20 and causes it to desorb at a higher temperature. However, no evidence was found for the presence of NO2 on the surface. The only species observed in infrared spectroscopy were (N0)2, N20 and NCO. The possibility remained that NO2 was adsorbed as a flat-lying species with undetectable weak infrared bands. A study of the adsorption of NO2 on the surface was therefore carried out and is reported elsewhere;34infrared spectra were readily observed from a range of reaction products including (N0)2, N203, and Nos. NO2 itself is unstable on the Ag{ 11l} surface, and we can therefore rule its presence out.
2200
2235
-
2234 c
k
-
I
12233
I2233 I
(e) 114-2233 I
'
"
'
'
I
'
I
'
'
,
2600 2400 2200 2000 1800 1600 1400 1200 Wavenumbers Figure 15. A desorption sequence for N20 adsorbed onto a pretreated Ag{ 11l} surface at 86 K. The surface was heated to (a) 86 (saturation coverage spectrum), (b) 92, (c) 100, (d) 107, and (e) 114 K.
desorption temperature (-115 K) for N20 formed during the reaction of NO on Ag{ 11l}. Figure 15a shows a saturation spectrum for N20 adsorbed onto a Ag{ 111) surface at 86 K after pretreatment by adsorption of NO, and then flashing to 150 K to remove any NO and N20. Two bands are observed at 2235 and 1253 cm-'. The multilayer is not significantly built up here because of the higher adsorption temperature. However, the frequencies of these bands are now exactly the same as those observed when N20 is formed on the Ag{ 11l} surface by the reaction of NO. This clearly demon-
4. Conclusions
1. Adsorbed NzO has been observed to form from NO on Ag{ 11l} at 70-90 K. This is the first direct observation of the surface reaction as it is taking place. 2. (N0)2 dimers have been seen on the Ag{ 111) surface in the condensed multilayer phase and also in the monolayer regime. The monolayer dimers are stable on the surface at temperatures up to 70 K. Monomer species are not observed and are therefore only very weakly bound to the surface. 3. The reaction to N20 on Ag{ 11l} takes place directly from the (N0)2 dimer species on the surface. 4. The activation energy of the reaction in which adsorbed N20 is formed from (N0)2 on Ag{ 111) is 7.3 kJ mol-'. 5. The (N0)2 dimer is observed to bond in a tilted geometry with the N-N bond parallel to the surface at low coverage, and with the N-N bond perpendicular to the surface in the multilayer. 6. NCO formation is observed at 90 K, indicating that a small amount of dissociative adsorption of NO occurs on Ag{ 111}, and reaction occurs with very low background pressures of CO or C02. 7. N20 adsorption onto a clean surface at 70 K gives rise to the desorption of an N20 species at 90 K, but N20 formed on the Ag{ 11l} surface from exposure to NO desorbs at -1 15 K. This desorption temperature is higher than that for N20 adsorbed onto an 0-precovered surface, and in agreement with Ludviksson et a1.,8 we conclude that there is something on the Ag surface, in addition to the 0, that affects the adsorption of N20. This has been assigned previously to N O Z , but ~ we find no infrared evidence for this. 8. No infrared evidence was found for any species on the Ag{ 11l} surface other than those mentioned above, Le., (N0)2, N20, and small amounts of NCO. Furthermore, there were no species observable with RAIRS at 150 K. This is in contrast to the work of Edamoto et al.2 and Goddard et aL3 Recent N E W S data indicate that there is something remaining on the surface after heating the adsorbate layer to > 150 K.35 Our inability to observe these species by infrared study may arise from the selection rules or from severely broadened bands.
Brown et al.
7074 J. Phys. Chem., Vol. 99, No. 18, 1995
Acknowledgments. The EPSRC is acknowledged for a studentship to W.A.B., a postdoctoral assistantship to P.G., and an equipment grant. The Royal Society is acknowledged for a supplementary grant to set up the RAIRS experiments, and Unicam (U.K.) is acknowledged for the loan of a Mattson RSlOO FTIR spectrometer and a CASE award to W.A.B. References and Notes (1) Kleyn, A. W.; Luntz, A. C.; Auerbach, D. J. Phys. Rev. Lett. 1981, 47, 1169. ( 2 ) Edamoto, K.; Maehama, S.; Miyazaki, E.; Migahana, T.; Kato, H. Sug. Sci. 1988, 204, L739. (3) Goddard, P. J.; West, J.; Lambert, R. M. Surf. Sci. 1978, 71, 447. (4) Behm, R. J.; Brundle, C. R. J . Vac. Sci. Technol. 1984, A3, 1040. (5) Nelin, C. J.; Bagus, P. S.; Behm, J.; Brundle, C. R. Chem. Phys. Lett. 1984, 105, 58. (6) So, S. K.; Franchy, R.; Ho, W. J . Chem. Phys. 1989, 91, 5701. (7) Bagus, P. S.; Illas, F. Chem. Phys. Lett. 1994, 224, 576. (8) Ludviksson, A.; Huang, C.; Jbsch, H. J.; Martin, R. M. Surf. Sci. 1993, 284, 328. (9) Wendelken, J. F. Appl. Sug. Sci. 1982, 11/12, 172. (10) Johnson, D. W.; Matloob, M. H.; Roberts, M. W. J . Chem. Soc., Chem. Commun. 1978, 40. (11) Johnson, D. W.; Matloob, M. H.; Roberts, M. W. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2143. (12) Masel, R. I.; Umbach, E.; Fuggle, J. C.; Menzel, D. Surf. Sci. 1979, 79, 26. (13) Baldwin, E. K.; Friend, C. M. J. Phys. Chem. 1985, 89, 2576. (14) Tonner, B. P.; Kao, C. M.; Plummer, E. W.; Caves, T. C.; Messner, R. P.; Salaneck, W. R. Phys. Rev. Lett. 1983, 51, 1378. (15) Kao, C. M.; Caves, T. C.; Messner, R. P. J . Vac. Sci. Technol. 1984, A2, 922. (16) DePristo, A. E.; Alexander, M. H. J. Chem. Phys. 1991,94,8454.
(17) Gates, G. A,; Holloway, S. Surf. Sci. 1994, 307/309, 132. (18) Gates, G. A,; Darling, G. R.; Holloway, S. J . Chem. Phys. 1994, 101, 6281. (19) JBhsch, H. J.; Huang, C.; Ludviksson, A,; Rocker, G.; Redding, J. D.; Metiu, H.; Martin, R. M. Sufi. Sci. 1989, 214, 377. (20) Lefferts, L.; Vanommen, J. G.; Ross, J. R. H. J . Catal. 1988, 114, 303. (21) Grimblot, J.; Alnot, P.; Behm, R. J.; Brundle, C. R. J . Electron. Spectrosc. Relat. Phenom. 1990, 52, 175. (22) Gardner, P.; Brown, W. A,; King, D. A. In preparation. 123) Brown, W. A,; Sharma. R. K.: Gardner, P.; King, - D. A.; Martin, D. H. Surf. sci., in press. (24) Diner", C. E.; Ewing, G. E. J . Chem. Phys. 1976, 53, 626. (25) Herzberg, G. Molecula-r Spectra and Molecular Structure; Van Nostrand-Reinhold: New York, 1945. (26) Ascensio, M. C.; Woodruff, D. P.; Robinson, A. W.; Schindler, K.-M.; Gardner, P.; Bradshaw, A. M.; Consea, J. C.; Gonzalez-Elipe, A. R. Chem. Phys. Lett. 1992, 192, 259. (27) Mapledoram, L. D.; Wander, A,; King, D. A. Chem. Phys. Lett. 1993, 208, 259. (28) Kostov, K. L.; Jakob, P.; Rauscher, H.; Menzel, D. J. Phys. Chem. 1991, 95, 7785. (29) Dumas, P.; Tobin, R. G.; Richards, P. L. Surf. Sci. 1986,171, 579. (30) Suhren, M.; Dumas, P.; Hirschmugl, C. I.; Chabal, Y. J.; Williams, G. P. In preparation. (31) Haq, S.; Brown, W. A.; Sharma, R. K.; Gardner, P.; King, D. A. In preparation. (32) Suzanne, J.; Coulomb, J. P.; Bienfait, M.; Matecki, M.; Thomy, A,; Croste, B.; Marti, C. Phys. Rev. Lett. 1978, 41, 760. (33) Brown, W. A.; Gardner, P.; Perez-Jigato, M.; King, D. A. J . Chem. Phys., in press. (34) Brown, W. A.; Gardner, P.; King, D. A. Surf. Sci., in press. (35) P6rez-Jigato, M.; Gardner, P.; Surman, M.; Walter, W. K.; Rassias, S.; King, D. A. In preparation.
JP9432231
