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Vibrational spectra of ammonia chemisorbed on platinum (111)
Surface Science 99 (1980) 523-538 0 North-Holland Publishing Company
VIBRATIONAL SPECTRA OF AMMONIA CHEMISORBED ON PLATINUM (111) I. Identification of chemisorbed states Brett A. SEXTON * and Gary E. MITCHELL Physical Chemistry Department, Michigan 48090, USA
General Motors Research Laboratories,
Received 19 February 1980; accepted for publication
Warren,
15 May 1980
Ammonia chemisorption on Pt(ll1) has been studied with high resolution electron energy loss spectroscopy (EELS), combined with thermal desorption spectroscopy (TDS). We detect two distinct molecular states of ammonia at different coverages. Near saturation monolayer coverage, ammonia is weakly chemisorbed (Mads u 9 kcal mol-‘) and coordinated to the metal surface via the nitrogen atom. The vibrational frequencies are shifted from the gas phase values, but not as strongly perturbed as in stable platinum-ammine complexes. Below 40% saturation coverage, a new molecular ammonia state is detected, which has a distinct vibrational and thermal desorption spectrum. This state has a considerably reduced v(Pt-N) intensity, and the other frequencies are closer to those in solid ammonia, indicating a weaker interaction with the Pt surface. The thermal desorption spectrum of this lower coverage state is broad, from 160 to 450 K, and coverage dependent. Conversion between the two molecular states appears to be only a function of coverage. We propose that the two molecular states have different adsorption sites, and convert from one form to the other as the coverage is changed. No evidence is found for significant dissociation of ammonia on the Pt surface. At very low temperatures (100 K), solid ammonia multilayers may be grown.
1. Introduction The interaction of ammonia with single crystal and metal catalyst surfaces has been studied with a wide range of experimental methods. Despite the large amount of data in the literature, the details of bonding of ammonia to clean surfaces, and fragmentation to form NH, species, are not well understood. Recent work by Gland [ 1,2] and Schmidt et al. [3,4] on platinum foils and single crystals examined the decomposition of ammonia and ammonia oxidation. The adsorption and decomposition of ammonia on iron single crystal planes has recently been studied by Ertl’s group [5,6]. Seabury et al. [7] have identified molecular ammonia on Ni(ll1) with angle resolved photoemission, and Purtell et al. [B] have identified the adsorption site of NHJIr(ll1). Other electron spectroscopic studies have found * Present address: CSIRO, Materials Science Division, University of Melbourne, Parkville, Victoria, Australia 3052. 523
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G.E. Mitchell / Vibrational spectra ofNHs/Pt(l
I I). I
evidence for NH, species during ammonia adsorption on clean metals [9,10]. In catalytic studies, infrared spectroscopy has been used to study ammonia adsorption [ 11,121 and oxidation [ 121 on supported platinum catalysts. The question of associative versus dissociative behavior, and the structure of the surface ammonia complexes may be readily answered with high resolution electron loss spectroscopy [ 131. In previous work [14-161, we examined the structure of polyatomic surface intermediates, and used deuterium isotope shifts to identify the hydrogen atoms in the surface complexes. The vibrational spectra of ammonia in its different phases, and the spectra of platinum-ammine complexes are known [ 17,181 and may be used as a convenient reference to interpret the surface spectra. In particular, the ammonia bending, stretching and rocking frequencies are very sensitive to the strength of the metal-nitrogen interaction [ 181 and should provide a way of assessing the degree of coordination to the metal surface. In this paper we examine two major questions relating to ammonia adsorption on Pt(l11). In this paper (Part I), we identify two distinct molecular states of ammonia on the platinum surface, and show that they differ in the degree of coordination to the metal. In the second part (Part II), we examine the question of the electron scattering mechanism in this study. We demonstrate that both dipole [13] and impact [19] scattering processes contribute to the observed intensities, and that they may be distinguished by angle and energy dependent measurements of the cross section. In addition, UPS spectra of NH,/Pt(l 11) have been measured and will be published separately [20].
2. Experimental The experiments were conducted in an EELS-Auger-LEED system described previously [21]. A clean Pt(ll1) surface was prepared by heating in oxygen to remove carbon (1 X 10e8 Torr 0,) 1200 K) and annealing at 1400 K. Surface calcium was below the Auger detection limit in all of the experiments. The Pt(ll1) crystal was cooled to 100 K and high purity ammonia gas (>99%) was dosed directly on the front face of the crystal with a multichannel array doser. It was found experimentally that reproducible thermal desorption spectra were obtained only if this dosing method was used. Conventional dosing with a leak valve saturated surfaces adjacent to the crystal and resulted in spurious desorption while the crystal was being ramped. These molecular beam exposures were converted to equivalent exposures in langmuirs (10 -6 Torr s) with reference to a set of energy loss spectra taken as a function of background ammonia pressure. Energy loss spectra were run with beam energies near 5.0 eV impact energy, or
B.A. Sexton,
G. E. Mitchell / Vibrational spectra
of NH3/Pt(l I I). I
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II of this paper, off-specular spectra were measured. As the analyzers are nonrotatable [2 I], off-specular spectra were taken by rotating the Pt(ll1) crystal off-axis. This resulted in a slight variation in the incident beam angle. The resolution of the instrument was set at 8-10 meV, although some broadening was noted at impact energies
3. Results 3.1. Molecular ammonia at high coverage (6 > 0.4) The vibrational spectra were found to be coverage dependent, and in this section, only the high coverage case (0 > 0.4) is discussed. The maximum monolayer coverage, prior to multilayer formation, was found to occur near 1 L exposure for both NH, and ND,. We have no estimate of the absolute ammonia coverage but will define the monolayer point (0 = 1) to be the point just prior to the appearance of a multilayer feature in the thermal desorption spectrum. In figs. la and lb are shown the energy loss spectra after exposure of the clean Pt(ll1) surface to 1 L of NH3 and NDa, respectively at 100 K. The frequencies, together with reference frequencies of ammonia and Pt-NH, complexes are listed in table 1. The spectra in figs. la and 1b clearly show the presence of molecular ammonia, coordinated to the Pt surface via the nitrogen atom. Both coordinated NH3 and ND3 have six vibrational bands which are assigned in table 1. These are, in order of increasing frequency, v(Pt-N), p,(NHa), G,(HNH), 6d(HNH), v,(NH), and vd(NH). As can be seen from table 1, the four highest frequency bands are the molecular ammonia normal modes, and the two lower frequency modes, v(P-N) and pr(NHa) are extra modes caused by coordination to a metal atom, or the surface. The assignment of the modes was made by comparing the surface frequencies with the ammonia reference compounds, and in comparing surface NH3 with surface NDa. All of the frequency shifts from NHa to ND3 are consistent with coordinated NH3 vibrations, although the @t-N) mode has a larger frequency shift than expected. It appears that coupling of the v(Pt-N) mode with another mode of A, symmetry has occurred. We can conclude from the number of modes, and the frequencies, that the high coverage state (0 > 0.4) of NHJPt(l11) is coordinated to
a b c d
s = symmetric,
11) c
350 720 1190 1630 3150 3320
NH3/Pt(lll)
d
270 570 930 1180 2350 2500
11) d
complexes
ND3/Pt(l
111) with NH3 and PtGNH3
d = degenerate.
1140 1600 3240 3340
1060 1646 3223 3378
950 1628 3331 3414
NH3/Pt(l
_ _
b
NH3(solid)
) of NHs/Pt(
_ _
b
(cm-’
frequencies
_ _
NH&as)
of the vibrational
v = stretch, 6 = bend, p = rocking, Ref. [18] Low coverage state (0 < 0.4). High coverage state (0 > 0.4).
pr(NH3) G,(HNH) 6 d(HNH) v,(NW ud(NH)
v(Pt-N)
Vibrational mode a
Table 1 Comparison
510 842 1325 1563 3156 3236
Pt(NH3j4C12
b
530 950 1370 1565 3050 3150
Pt(NH&$&
b a
5 2
2 3
G w 2 0
$ \ S P 9 E g
3 ;;:
-3 cl .?
a
s
co
.b ?
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH,jPt(lII).
I
527
Z
I
I
I
I
I
I
1000 2000 3000 Energy Loss (cm-l)
4000
-270
Pt(111)+ND3 (b’
I
I
0
1L
I I I 1000 2000 3000 Energy Loss (cm -1)
1OOK
I 4000
Fig. 1. (a) Electron energy loss spectrum of NH3 adsorbed on Pt(ll1) at 100 K (1 L exposure). The impact energy was 5 eV. (b) Electron energy loss spectrum of ND3 adsorbed on Pt(ll1) at 100 K (1 L exposure). The impact energy was 5 eV. For the frequency scale, 1 meV = 8.07 cm-‘.
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B.A. Sexton,
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I I). I
the Pt surface in a manner similar to the known Pt-NH, coordination complexes. The extra band near 1460 cm-’ m the spectrum of fig. lb was caused by some exchange of the deuterated ammonia with background ammonia in the vacuum system. This band is a 6,(NDzH) vibration, and can be ignored for the purpose of analyzing the spectra. A comparison of the frequencies in table 1 suggests some interesting trends. The bending, stretching and rocking frequencies of NH, change in a uniform way across the table, from NH3 (gas) to Pt(NH3)6C14. In particular, the G,(HNH) band shifts from 950 cm-’ to 1370 cm-‘, whereas the u,(NH) band decreases from 3337 to 3050 cm-‘. The surface values for NHJPt(l11) are seen to fit between the values for solid NH3 and Pt(NHs)4C1,. This suggests that the Pt-N interaction is not as strong as in the stable platinum complexes, but the ammonia has been perturbed more than in a condensed layer. In another report, thermal desorption spectra will be presented to show that the heat of adsorption of this high coverage state is estimated to be approximately 9 kcal mol-’ , which is essentially weak chemisorption [24]. Ammonia represents an unusual case in that the molecular frequencies are very sensitive to the chemical environment. The downshift of the v(NH) frequencies from the gaseous values upon coordination is indicative of a weakening of the N-H bonds. The value of 350 cm-’ for the v(Pt-N) stretch is significantly lower than the platinum coordination complexes, in agreement with the postulate of a weak Pt-N bond. All of the frequencies support a model of a weakly chemisorbed ammonia molecule, bound to the surface via the nitrogen atom. The surface frequencies are intermediate between a solid ammonia layer and a Pt(NH,)a2 complex. The most likely symmetry for the Pt-NH3 surface complex is Csv. In Part II of this paper we will discuss the molecular orientation and the EELS activity of the different modes. 3.2. The low coverage molecular state (0 < 0.4) The spectra were found to be very coverage dependent. In fig. 2 we show the results of an experiment in which a large dose of NH3 (2 L) was annealed to various temperatures to sequentially remove the layer and produce spectra at different coverages. We also show a thermal desorption spectrum for that coverage, which was generated by the front face of the crystal only, as outlined in the experimental section. The results were independent of the time at each annealing temperature, and the surface coverage was progressively reduced with the sequence of increasing temperatures. Estimation of the relative surface coverages for the high and low coverage states was done by integrating the areas under the two peaks (120-160 K) and (160-500 K). At 100 K, exposures of >1.5 L produced multilayer films of solid ammonia on the cold surface. These could be detected in two ways; the appearance of a sharp peak in the thermal desorption between 100 and 120 K, which continued to grow
B.A. Sexton, G.E. Mitcl?ell / Vibrdunal spectra of NH$Pt(l I I). I
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Pt(lf1)+NW32L Thermat Desorption Mass 17
IQQK
.L.~ Q 1QQQ 2000 3QQQ 4uw Energy Loss (crnef 1 Fig. 2. JZlectron energy loss spectra of NHa/Pt(lll) after annealing a 2 L dose to the tgmperatures specified. The thermal desorptian spectrum for the 2 L dose is also shown. The initial adsorption temperature was 100 K, and the impact energy 5 eV.
with exposure, and an extra band near 1090 cm-’ in the EELS spectrum. When the surface was annealed to 120 K, the aroma f&n desorbed, the 1090 em-’ band disappeared and the EELS spectrum at 120 R in fig. 2 was identical to the 1 L spectrum in fig. la. To produce the high coverage molecular state in fig. la, therefore, the surface could either be exposed to a large dose at 120 K to prevent rn~l~ayer formation, or a reduced dose ($3 .S L) at 100 K. The spectra were iden~~~ in both cases, neglecting minor frequency shifts in the S,(HNH), v(Pt-N), and p,(NH3) vibrations. The appearance of a 1090 cm-r band for the condensed layer is not su~~ri~g~ as the solid arnrno~~a S,(HNH) frequency is 1060 cm-r (table I), whereas the other solid vibrations would be obscured by the high coverage molecular state peaks. If a very thick layer of solid ammonia was grown on the surface, we observed bands close to the solid frequencies in table I_ A more dramatic change in the EELS spectra of fig. 2 occurred when the tem-
530
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G.E. Mitchell / Vibrational spectra of NH3/Pt(ll
j---t210
3150
I). I
Pt(ffl)
i 3320
120
K
t40K
Xl
x3
250
1
1000
I 2000 Energy
I 3000
K
I 4000
Loss (cm-‘)
Fig. 3. Electron energy loss spectra of NHs/Pt(lll). A saturation dose at 120 K was annealed to 140 and 250 K to show the conversion between the high coverage (120 K) and low coverage (250 K) forms of NH3 on the surface. The impact energy was approximately 1 eV, and the elastic peak is not shown as it was too intense to be measured accurately.
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531
perature was raised sequentially from 120 to 300 K. The thermal desorption spectrum in fig. 2 shows essentially two features, a distinct desorption state between 120 and 160 K, and a very broad desorption regime between 160 and 450 K. The EELS spectra in fig. 2 show a splitting of the G,(HNH) band near 140 K, and a disappearance of the @t-N) stretch, and changes in the structure of the v(NH) region as the temperature is raised to 160 K. From 160 to 450 K the vibrational spectra were identical, although attenuation of the intensities occurred as ammonia was removed from the surface. All of the spectra in fig. 2 were taken with 5 eV impact energy, and the surface reflectivity and signal-to-noise of the spectra were low. In order to enhance the intensities, and examine the spectral changes in more detail, we recorded spectra with
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G.E. Mitchell / Vibrational spectra of NH3jPt(ll
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Pt(lll)+NH3
120K
140K
200K
250K
I
I
2500 Energy
3000 Loss (cm-l)
I 3500
Fig. 4. Electron energy loss spectra of the u(NH) stretching region, as a function of annealing temperature. A saturation dose of NHs/Pt(lll) at 120 K was annealed to the temperature specified. The conversion between the high coverage (120 K) and low coverage (>160 K) forms can be seen. The beam energy was approximately 1 eV.
range (0 < 0.4). Near saturation coverage, it is possible that some of the low coverage state could also coexist with the high coverage species (0 > 0.4). Since the v(NH) intensities for low coverage ammonia are weaker than the high coverage state, and the other frequencies are close to the high coverage frequencies, the possibility exists for a coexistence of both states at 0 > 0.4. Independent evidence from UPS also suggests coexistence of the two states near saturation [20], although the relative amounts of each are not easily estimated.
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(lIl).
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3.3. Variation of coverage at 100 K To test the hypothesis of a coverage dependent transformation, and rule out the possibility of a temperature effect, we show data in fig. 5 taken at 100 K. The clean Pt(ll1) surface was dosed with varying amounts of ammonia, and the EELS spectra measured at 100 K. These layers were then thermally desorbed to generate the thermal desorption spectra on the right side of fig. 5. As can be seen from fig. 5, low coverage doses of NH3 produced similar loss and thermal desorption spectra to those measured in the heating experiments. As the amount of ammonia on the surface increases at 100 K, the spectra show the low coverage state, converting to the high coverage state (0 > 0.4) and finally the multilayer features. In fig. 6 is shown a spectrum of a 0.2 L dose of NH,/Pt(l 11) at 100 K. This is almost identical to the heated layer in fig. 3, showing that the low cover-
hermal
Desorption
(4
(a)
&I Xl
I
0 1000 2000 3000 Energy Loss (cm-l )
I
I
100 200
I
300
I 400
II 500
T(K)
Fig. 5. Variation of ammonia surface coverage at 100 K. At 100 K, the Pt(ll1) surface was dosed with the following exposures: (a) 0.1 L, (b) 0.33 L, (c) 0.6 L, (d) 1.0 L, (e) 10.0 L. EELS spectra and the corresponding thermal desorption spectrum were measured. The beam energy was 5 eV.
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B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(ll
Pt (111) + NH3
I]. I
0.2L. 100 K
x 10 3240 1
I
I I I 2000 3000 4000 Energy Loss (cm-l) Fig. 6. Electron energy loss spectrum of a low exposure (0.2 L) of NH3 on the Pt(ll1) surface at 100 K. The 2220 cm-’ band is an overtone of the 1130 cm-l band. This spectrum is very similar to the low coverage 250 K spectrum of fig. 3. The impact energy was approximately 1 eV, and the elastic intensity is not shown as it was too intense to be measured. 0
I
1000
age state depends only on coverage, not temperature. In fig. 6, a small intensity in the v(Pt-N) region was seen, which was absent in the spectrum of fig. 3, and the relative intensity of the G,(HNH) band near 1580 cm-’ was larger than that shown in fig. 3. The frequencies, however, especially the 100 cm-’ splitting in the v(NH) region, are in good agreement. We can conclude that the ammonia spectra depend primarily on coverage, and low coverage spectra produced by smaller doses at 100 K are essentially identical to low coverage spectra after heating from 160 to 450 K.
4. Discussion The results presented here show a clear distinction between two chemisorbed forms of ammonia on Pt(l1 l), and solid ammonia multilayers. We believe that the vibrational spectra are the first complete spectra (100-4000 cm-‘) published of an adsorbed ammonia molecule, and demonstrate the applicability of EELS to the study of catalytically important species. The interpretation of the low coverage state as a molecular species depends on rejecting a dissociative model of amrnonia adsorption. The vibrational spectra of NH and NH, species would be expected to be very similar to an adsorbed ammonia
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(ll
IJ. I
535
spectrum. For adsorbed NH, chemisorbed with C, symmetry, three modes would be active: v(Pt-N), &(NH), and v(NH). We would expect the spectrum to look similar to that of OH/Pt(lll) (22), the strongest band being the 6(NH) frequency. For an NH2 species with Czv symmetry, four modes would be observed: Q-N), s(HNH), v,(NH), v,(NH). In this case the &(HNH) frequency would be higher than the bending mode of NH, and the NH stretching modes would be split into a doublet with a closer spacing than NH3 [ 181. For the low coverage ammonia case, we observed a spectrum which is incompatible with NH or NH2 alone. In fig. 6, the doublet in the v(NH) stretching region could be due to NH 2, but two bands are observed near 1130 and 1580 cm-‘. For NH2 alone, only a single band near 1580 cm-’ would be observed [ 181. This is the case with adsorbed H,O/Pt(l 11) which has a 6(HOH) bending frequency near 1600 cm-’ [23]. The strong band near 1130 cm-’ could be interpreted as a bending mode of NH species, but only a single v(NH) stretch would be observed, not the doublet of figs. 4 and 6. A final possibility is that the low coverage ammonia consists of a mixture of NH and NH2 species. The spectral intensities and frequencies would be compatible with this assumption, but there is other evidence to suggest this is not the case. First, there is no significant change in the ratios of the intensities of the low coverage ammonia bands with coverage. This is more consistent with a single species than the case would be if dissociation occurred. Secondly, in independent experiments we have observed little, if any, hydrogen and nitrogen desorption from adsorbed ammonia on platinum (Ill) in the temperature range studied [24]. In addition, exchange experiments with D, gas show that the exchange mechanism for ammonia does not proceed via a dissociative intermediate such as NH or NH, [24]. Other data, including UPS and work function measurements, support the idea of two molecular states of ammonia on Pt(l11) [20]. In recent papers Ertl and co-workers have reported the identification of NH [6] and NH2 [S] species in ammonia adsorption on Fe surfaces. The main evidence for these species was UPS, although the spectra presented show only weakly resolved features. The thermal desorption spectra for NH,/Fe look very similar to our Pt(ll1) spectra. We believe that the EELS spectra are a more positive identification of the surface layer compared with UPS, and it would be interesting to examine the iron-ammonia system if NH and NH2 can be individually isolated. The two molecular states of NH3 on Pt(l11) are both only weakly chemisorbed, as evidenced by the small frequency shifts from the gas phase. In table 1 we observed frequencies for both states intermediate between ammonia gas and Pt(NHa)r, indicating weak overlap of the nitrogen lone pair 3A, orbital with the surface. The data also indicates weaker coordination at low coverage than at high coverage, a result contrary to the thermal desorption which shows higher desorption temperatures for the low coverage ammonia. In addition, the high coverage NH3 had strong v(Pt-N) and p,(NHa) bands, whereas these were absent in the low coverage case. This latter observation suggests that the two interaction potentials of the ammonia states are quite different. The EELS intensity depends on the
536
B.A. Sexton, G.E. Mitchell / Vibrational spectra of NHs/Pt(ll
I). I
dipole derivative, or dynamic dipole moment of a vibration [ 181, which is the change in dipole moment during vibration. The ~brational frequency depends on the force constant, which is dependent on the curvature of the potential well ]lS] . We can explain the absence of a @t-N) vibration in the low coverage NHa by assuming that the potential well of the low coverage adsorption site has a “flat bottom”, resulting in a very low force constant for the Pt-N vibration, perhaps below 200 cm-‘, where it would not easily be resolved. Alternatively the dipole derivative could be zero for this state, however most adsorbate-substrate vibrations show significant EELS intensities. We propose, therefore, that the low coverage ammonia is chemisorbed in a site with a deep, but “flat bottomed” potential well. ~~0~~ the ~monia is bound to the surface more strongly than at high coverage, the small curvature of the well potential results in a lowering of the v(Pt-N) frequency below the limits of detection (100-200 cm-“), Such a site could be a threefold site on the (111) surface where the atom in the second layer is missing. Coordination of NH3 with Cav symmetry in this site can explain the spectra. For the high coverage case, a more likely site is an “on-top” site where the nitrogen atom coordinates directly to a Pt atom, analogous to known Pt-NH, complexes. The similarity of the spectra for high coverage NH3 and Pt-NH3 complexes supports a linear coordination model, again with Cav symmetry. This model for low coverage ammonia on Pt(lll) is essent~~y in agreement with the one proposed by Purtell et al. [8] for NH,/IR(l 11). In their case, angle resolved photoemission was used to determine the adsorption site as a threefold hollow with the atom missing in the second layer. The only fact which remains to be explained is the apparent “weakening” of the coordination in the low coverage state, as evidenced by shifting of the frequencies toward the gas phase. It may be incorrect to assume that a simple frequency shift model derived from linear Pt-NH, complexes should apply to NH3 chemisorbed in a threefold hole on a surface. Further data is needed on other surfaces to resolve this problem. Further evidence, presented elsewhere [20], shows that the low coverage NH3 has a large work function change of 3 .O V, whereas the high coverage state has only a small contribution up to saturation. UPS spectra of the two states are also quite different 1201. Infrared studies of ammonia adsorption on supported platinum were carried out by Morrow and Cody [12] and Griffiths et al. [ll]. Morrow and Cody reported values of 3220 and 3360 cm-’ for room temperature adsorption of ammonia on Pt/SiOs. This is in excellent agreement with our values of 3240 and 3340 cm-’ for low coverage NH,. Morrow and Cody afso conducted isotopic exchange measurements and concluded that room temperature adsorption produced molecular NH3 on the Pt surface, not NH or NT&. This was based on the observation of two partially labeled species after interaction of Hz with ND3. We support the results of Morrow and Cody and believe that room temperature adsorption on Pt/SiO:! produces the same low coverage state we observed on Pt( 111) in this study.
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G.E. Mitchell / Vibrational spectra of NH3/Pt(lll).
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4. Conclusions (1) There are two molecular states of ammonia chemisorbed on Pt(ll1) between 120 and 450 K. Below 120 K, solid ammonia multilayers may be formed. (2) Above 40% saturation coverage, the ammonia is coordinated to the Pt surface via the nitrogen atom, and the surface frequencies indicate weak overlap of the 3Ar orbital with the surface. Thermal desorption shows a weakly chemisorbed state (-9 kcal mol-r). The most likely coordination is to an “on-top” site. (3) Below 40% saturation, a second molecular state of ammonia is detected. The four normal modes of the molecule are shifted closer to the gas phase values than the high coverage state, indicative of weaker coordination. The v(Pt-N) band is not observed for this state, and it is possible that this form of ammonia is chemisorbed in a threefold site with a “flat bottomed” potential well, where the Q-N) frequency is very low ( 0.4, both states may coexist, and cannot be resolved with EELS. (5) No evidence is found for significant dissociation of ammonia on Pt(l11) under low pressure conditions between 100 and 450 K.
Acknowledgments The authors wish to thank John Gland for useful discussions.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [IO] [ll] [12] [ 131 [14] [15] 1161
J.L. Gland, Surface Sci. 71 (1978) 327. J.L. Gland, .I. Catalysis 53 (1978) 9. D.G. Loffler and L.D. Schmidt, J. Catalysis 41 (1976) 440. D.G. Loffler and L.D. Schmidt, Surface Sci. 59 (1976) 195. M. Grunze, F. Boszo, G. Ertl and M. Weiss, Appl. Surface Sci. 1 (1978) 241. M. Weiss, G. Ertl and F. Nitschke, Appl. Surface Sci. 3 (1979) 614. C.W. Seabury, T.N. Rhodin, R.J. PurteIl and R.P. Merrill, Surface Sci. 93 (1980) 117. R.J. Purtell, R.P. Merrill, C.W. Seabury and T.N. Rhodin, Phys. Rev. Letters, in press. K. Kishi and W.M. Roberts, Surface Sci. 62 (1977) 252. L.R. Danielson, M.J. Dresser, E.E. Donaldson and J.T. Dickinson, Surface Sci. 71 (1978) 599. D.W.L. Griffiths, H.E. HaIlam and W.J. Thomas, Trans. Faraday Sot. 64 (1968) 3361. B.A. Morrow and I.A. Cody, J. Catalysis 45 (1976) 151. H. Ibach, H. Hopster and B.A. Sexton, Appl. Surface Sci. 1 (1977) 1. B.A. Sexton, Chem. Phys. Letters 65 (1979) 469. B.A. Sexton, Surface Sci. 88 (1979) 299. B.A. Sexton, Surface Sci. 88 (1979) 319.
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[17] K.H. Schmidt and A. Muller, Coord. Chem. Rev. 19 (1976) 41. [ 181 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination (Wiley, New York, 1978). [19] W. Ho, R.F. Willis and E.W. Plummer, Phys. Rev. Letters 40 (1978) 1463. [20] G.B. Fisher, to be published. [21] B.A. Sexton, J. Vacuum Sci. Technol. 16 (1979) 1033. [22] G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 (1980) 683. [23] B.A. Sexton, Surface Sci. 94 (1980) 435. [24] J.L. Gland, to be published.
I
Compounds
VIBRATIONAL SPECTRA OF AMMONIA CHEMISORBED ON PLATINUM (111) I. Identification of chemisorbed states Brett A. SEXTON * and Gary E. MITCHELL Physical Chemistry Department, Michigan 48090, USA
General Motors Research Laboratories,
Received 19 February 1980; accepted for publication
Warren,
15 May 1980
Ammonia chemisorption on Pt(ll1) has been studied with high resolution electron energy loss spectroscopy (EELS), combined with thermal desorption spectroscopy (TDS). We detect two distinct molecular states of ammonia at different coverages. Near saturation monolayer coverage, ammonia is weakly chemisorbed (Mads u 9 kcal mol-‘) and coordinated to the metal surface via the nitrogen atom. The vibrational frequencies are shifted from the gas phase values, but not as strongly perturbed as in stable platinum-ammine complexes. Below 40% saturation coverage, a new molecular ammonia state is detected, which has a distinct vibrational and thermal desorption spectrum. This state has a considerably reduced v(Pt-N) intensity, and the other frequencies are closer to those in solid ammonia, indicating a weaker interaction with the Pt surface. The thermal desorption spectrum of this lower coverage state is broad, from 160 to 450 K, and coverage dependent. Conversion between the two molecular states appears to be only a function of coverage. We propose that the two molecular states have different adsorption sites, and convert from one form to the other as the coverage is changed. No evidence is found for significant dissociation of ammonia on the Pt surface. At very low temperatures (100 K), solid ammonia multilayers may be grown.
1. Introduction The interaction of ammonia with single crystal and metal catalyst surfaces has been studied with a wide range of experimental methods. Despite the large amount of data in the literature, the details of bonding of ammonia to clean surfaces, and fragmentation to form NH, species, are not well understood. Recent work by Gland [ 1,2] and Schmidt et al. [3,4] on platinum foils and single crystals examined the decomposition of ammonia and ammonia oxidation. The adsorption and decomposition of ammonia on iron single crystal planes has recently been studied by Ertl’s group [5,6]. Seabury et al. [7] have identified molecular ammonia on Ni(ll1) with angle resolved photoemission, and Purtell et al. [B] have identified the adsorption site of NHJIr(ll1). Other electron spectroscopic studies have found * Present address: CSIRO, Materials Science Division, University of Melbourne, Parkville, Victoria, Australia 3052. 523
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G.E. Mitchell / Vibrational spectra ofNHs/Pt(l
I I). I
evidence for NH, species during ammonia adsorption on clean metals [9,10]. In catalytic studies, infrared spectroscopy has been used to study ammonia adsorption [ 11,121 and oxidation [ 121 on supported platinum catalysts. The question of associative versus dissociative behavior, and the structure of the surface ammonia complexes may be readily answered with high resolution electron loss spectroscopy [ 131. In previous work [14-161, we examined the structure of polyatomic surface intermediates, and used deuterium isotope shifts to identify the hydrogen atoms in the surface complexes. The vibrational spectra of ammonia in its different phases, and the spectra of platinum-ammine complexes are known [ 17,181 and may be used as a convenient reference to interpret the surface spectra. In particular, the ammonia bending, stretching and rocking frequencies are very sensitive to the strength of the metal-nitrogen interaction [ 181 and should provide a way of assessing the degree of coordination to the metal surface. In this paper we examine two major questions relating to ammonia adsorption on Pt(l11). In this paper (Part I), we identify two distinct molecular states of ammonia on the platinum surface, and show that they differ in the degree of coordination to the metal. In the second part (Part II), we examine the question of the electron scattering mechanism in this study. We demonstrate that both dipole [13] and impact [19] scattering processes contribute to the observed intensities, and that they may be distinguished by angle and energy dependent measurements of the cross section. In addition, UPS spectra of NH,/Pt(l 11) have been measured and will be published separately [20].
2. Experimental The experiments were conducted in an EELS-Auger-LEED system described previously [21]. A clean Pt(ll1) surface was prepared by heating in oxygen to remove carbon (1 X 10e8 Torr 0,) 1200 K) and annealing at 1400 K. Surface calcium was below the Auger detection limit in all of the experiments. The Pt(ll1) crystal was cooled to 100 K and high purity ammonia gas (>99%) was dosed directly on the front face of the crystal with a multichannel array doser. It was found experimentally that reproducible thermal desorption spectra were obtained only if this dosing method was used. Conventional dosing with a leak valve saturated surfaces adjacent to the crystal and resulted in spurious desorption while the crystal was being ramped. These molecular beam exposures were converted to equivalent exposures in langmuirs (10 -6 Torr s) with reference to a set of energy loss spectra taken as a function of background ammonia pressure. Energy loss spectra were run with beam energies near 5.0 eV impact energy, or
B.A. Sexton,
G. E. Mitchell / Vibrational spectra
of NH3/Pt(l I I). I
525
II of this paper, off-specular spectra were measured. As the analyzers are nonrotatable [2 I], off-specular spectra were taken by rotating the Pt(ll1) crystal off-axis. This resulted in a slight variation in the incident beam angle. The resolution of the instrument was set at 8-10 meV, although some broadening was noted at impact energies
3. Results 3.1. Molecular ammonia at high coverage (6 > 0.4) The vibrational spectra were found to be coverage dependent, and in this section, only the high coverage case (0 > 0.4) is discussed. The maximum monolayer coverage, prior to multilayer formation, was found to occur near 1 L exposure for both NH, and ND,. We have no estimate of the absolute ammonia coverage but will define the monolayer point (0 = 1) to be the point just prior to the appearance of a multilayer feature in the thermal desorption spectrum. In figs. la and lb are shown the energy loss spectra after exposure of the clean Pt(ll1) surface to 1 L of NH3 and NDa, respectively at 100 K. The frequencies, together with reference frequencies of ammonia and Pt-NH, complexes are listed in table 1. The spectra in figs. la and 1b clearly show the presence of molecular ammonia, coordinated to the Pt surface via the nitrogen atom. Both coordinated NH3 and ND3 have six vibrational bands which are assigned in table 1. These are, in order of increasing frequency, v(Pt-N), p,(NHa), G,(HNH), 6d(HNH), v,(NH), and vd(NH). As can be seen from table 1, the four highest frequency bands are the molecular ammonia normal modes, and the two lower frequency modes, v(P-N) and pr(NHa) are extra modes caused by coordination to a metal atom, or the surface. The assignment of the modes was made by comparing the surface frequencies with the ammonia reference compounds, and in comparing surface NH3 with surface NDa. All of the frequency shifts from NHa to ND3 are consistent with coordinated NH3 vibrations, although the @t-N) mode has a larger frequency shift than expected. It appears that coupling of the v(Pt-N) mode with another mode of A, symmetry has occurred. We can conclude from the number of modes, and the frequencies, that the high coverage state (0 > 0.4) of NHJPt(l11) is coordinated to
a b c d
s = symmetric,
11) c
350 720 1190 1630 3150 3320
NH3/Pt(lll)
d
270 570 930 1180 2350 2500
11) d
complexes
ND3/Pt(l
111) with NH3 and PtGNH3
d = degenerate.
1140 1600 3240 3340
1060 1646 3223 3378
950 1628 3331 3414
NH3/Pt(l
_ _
b
NH3(solid)
) of NHs/Pt(
_ _
b
(cm-’
frequencies
_ _
NH&as)
of the vibrational
v = stretch, 6 = bend, p = rocking, Ref. [18] Low coverage state (0 < 0.4). High coverage state (0 > 0.4).
pr(NH3) G,(HNH) 6 d(HNH) v,(NW ud(NH)
v(Pt-N)
Vibrational mode a
Table 1 Comparison
510 842 1325 1563 3156 3236
Pt(NH3j4C12
b
530 950 1370 1565 3050 3150
Pt(NH&$&
b a
5 2
2 3
G w 2 0
$ \ S P 9 E g
3 ;;:
-3 cl .?
a
s
co
.b ?
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH,jPt(lII).
I
527
Z
I
I
I
I
I
I
1000 2000 3000 Energy Loss (cm-l)
4000
-270
Pt(111)+ND3 (b’
I
I
0
1L
I I I 1000 2000 3000 Energy Loss (cm -1)
1OOK
I 4000
Fig. 1. (a) Electron energy loss spectrum of NH3 adsorbed on Pt(ll1) at 100 K (1 L exposure). The impact energy was 5 eV. (b) Electron energy loss spectrum of ND3 adsorbed on Pt(ll1) at 100 K (1 L exposure). The impact energy was 5 eV. For the frequency scale, 1 meV = 8.07 cm-‘.
528
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NHsfPt(l
I I). I
the Pt surface in a manner similar to the known Pt-NH, coordination complexes. The extra band near 1460 cm-’ m the spectrum of fig. lb was caused by some exchange of the deuterated ammonia with background ammonia in the vacuum system. This band is a 6,(NDzH) vibration, and can be ignored for the purpose of analyzing the spectra. A comparison of the frequencies in table 1 suggests some interesting trends. The bending, stretching and rocking frequencies of NH, change in a uniform way across the table, from NH3 (gas) to Pt(NH3)6C14. In particular, the G,(HNH) band shifts from 950 cm-’ to 1370 cm-‘, whereas the u,(NH) band decreases from 3337 to 3050 cm-‘. The surface values for NHJPt(l11) are seen to fit between the values for solid NH3 and Pt(NHs)4C1,. This suggests that the Pt-N interaction is not as strong as in the stable platinum complexes, but the ammonia has been perturbed more than in a condensed layer. In another report, thermal desorption spectra will be presented to show that the heat of adsorption of this high coverage state is estimated to be approximately 9 kcal mol-’ , which is essentially weak chemisorption [24]. Ammonia represents an unusual case in that the molecular frequencies are very sensitive to the chemical environment. The downshift of the v(NH) frequencies from the gaseous values upon coordination is indicative of a weakening of the N-H bonds. The value of 350 cm-’ for the v(Pt-N) stretch is significantly lower than the platinum coordination complexes, in agreement with the postulate of a weak Pt-N bond. All of the frequencies support a model of a weakly chemisorbed ammonia molecule, bound to the surface via the nitrogen atom. The surface frequencies are intermediate between a solid ammonia layer and a Pt(NH,)a2 complex. The most likely symmetry for the Pt-NH3 surface complex is Csv. In Part II of this paper we will discuss the molecular orientation and the EELS activity of the different modes. 3.2. The low coverage molecular state (0 < 0.4) The spectra were found to be very coverage dependent. In fig. 2 we show the results of an experiment in which a large dose of NH3 (2 L) was annealed to various temperatures to sequentially remove the layer and produce spectra at different coverages. We also show a thermal desorption spectrum for that coverage, which was generated by the front face of the crystal only, as outlined in the experimental section. The results were independent of the time at each annealing temperature, and the surface coverage was progressively reduced with the sequence of increasing temperatures. Estimation of the relative surface coverages for the high and low coverage states was done by integrating the areas under the two peaks (120-160 K) and (160-500 K). At 100 K, exposures of >1.5 L produced multilayer films of solid ammonia on the cold surface. These could be detected in two ways; the appearance of a sharp peak in the thermal desorption between 100 and 120 K, which continued to grow
B.A. Sexton, G.E. Mitcl?ell / Vibrdunal spectra of NH$Pt(l I I). I
529
Pt(lf1)+NW32L Thermat Desorption Mass 17
IQQK
.L.~ Q 1QQQ 2000 3QQQ 4uw Energy Loss (crnef 1 Fig. 2. JZlectron energy loss spectra of NHa/Pt(lll) after annealing a 2 L dose to the tgmperatures specified. The thermal desorptian spectrum for the 2 L dose is also shown. The initial adsorption temperature was 100 K, and the impact energy 5 eV.
with exposure, and an extra band near 1090 cm-’ in the EELS spectrum. When the surface was annealed to 120 K, the aroma f&n desorbed, the 1090 em-’ band disappeared and the EELS spectrum at 120 R in fig. 2 was identical to the 1 L spectrum in fig. la. To produce the high coverage molecular state in fig. la, therefore, the surface could either be exposed to a large dose at 120 K to prevent rn~l~ayer formation, or a reduced dose ($3 .S L) at 100 K. The spectra were iden~~~ in both cases, neglecting minor frequency shifts in the S,(HNH), v(Pt-N), and p,(NH3) vibrations. The appearance of a 1090 cm-r band for the condensed layer is not su~~ri~g~ as the solid arnrno~~a S,(HNH) frequency is 1060 cm-r (table I), whereas the other solid vibrations would be obscured by the high coverage molecular state peaks. If a very thick layer of solid ammonia was grown on the surface, we observed bands close to the solid frequencies in table I_ A more dramatic change in the EELS spectra of fig. 2 occurred when the tem-
530
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(ll
j---t210
3150
I). I
Pt(ffl)
i 3320
120
K
t40K
Xl
x3
250
1
1000
I 2000 Energy
I 3000
K
I 4000
Loss (cm-‘)
Fig. 3. Electron energy loss spectra of NHs/Pt(lll). A saturation dose at 120 K was annealed to 140 and 250 K to show the conversion between the high coverage (120 K) and low coverage (250 K) forms of NH3 on the surface. The impact energy was approximately 1 eV, and the elastic peak is not shown as it was too intense to be measured accurately.
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(l I I). I
531
perature was raised sequentially from 120 to 300 K. The thermal desorption spectrum in fig. 2 shows essentially two features, a distinct desorption state between 120 and 160 K, and a very broad desorption regime between 160 and 450 K. The EELS spectra in fig. 2 show a splitting of the G,(HNH) band near 140 K, and a disappearance of the @t-N) stretch, and changes in the structure of the v(NH) region as the temperature is raised to 160 K. From 160 to 450 K the vibrational spectra were identical, although attenuation of the intensities occurred as ammonia was removed from the surface. All of the spectra in fig. 2 were taken with 5 eV impact energy, and the surface reflectivity and signal-to-noise of the spectra were low. In order to enhance the intensities, and examine the spectral changes in more detail, we recorded spectra with
532
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3jPt(ll
I). 1
Pt(lll)+NH3
120K
140K
200K
250K
I
I
2500 Energy
3000 Loss (cm-l)
I 3500
Fig. 4. Electron energy loss spectra of the u(NH) stretching region, as a function of annealing temperature. A saturation dose of NHs/Pt(lll) at 120 K was annealed to the temperature specified. The conversion between the high coverage (120 K) and low coverage (>160 K) forms can be seen. The beam energy was approximately 1 eV.
range (0 < 0.4). Near saturation coverage, it is possible that some of the low coverage state could also coexist with the high coverage species (0 > 0.4). Since the v(NH) intensities for low coverage ammonia are weaker than the high coverage state, and the other frequencies are close to the high coverage frequencies, the possibility exists for a coexistence of both states at 0 > 0.4. Independent evidence from UPS also suggests coexistence of the two states near saturation [20], although the relative amounts of each are not easily estimated.
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(lIl).
I
533
3.3. Variation of coverage at 100 K To test the hypothesis of a coverage dependent transformation, and rule out the possibility of a temperature effect, we show data in fig. 5 taken at 100 K. The clean Pt(ll1) surface was dosed with varying amounts of ammonia, and the EELS spectra measured at 100 K. These layers were then thermally desorbed to generate the thermal desorption spectra on the right side of fig. 5. As can be seen from fig. 5, low coverage doses of NH3 produced similar loss and thermal desorption spectra to those measured in the heating experiments. As the amount of ammonia on the surface increases at 100 K, the spectra show the low coverage state, converting to the high coverage state (0 > 0.4) and finally the multilayer features. In fig. 6 is shown a spectrum of a 0.2 L dose of NH,/Pt(l 11) at 100 K. This is almost identical to the heated layer in fig. 3, showing that the low cover-
hermal
Desorption
(4
(a)
&I Xl
I
0 1000 2000 3000 Energy Loss (cm-l )
I
I
100 200
I
300
I 400
II 500
T(K)
Fig. 5. Variation of ammonia surface coverage at 100 K. At 100 K, the Pt(ll1) surface was dosed with the following exposures: (a) 0.1 L, (b) 0.33 L, (c) 0.6 L, (d) 1.0 L, (e) 10.0 L. EELS spectra and the corresponding thermal desorption spectrum were measured. The beam energy was 5 eV.
534
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(ll
Pt (111) + NH3
I]. I
0.2L. 100 K
x 10 3240 1
I
I I I 2000 3000 4000 Energy Loss (cm-l) Fig. 6. Electron energy loss spectrum of a low exposure (0.2 L) of NH3 on the Pt(ll1) surface at 100 K. The 2220 cm-’ band is an overtone of the 1130 cm-l band. This spectrum is very similar to the low coverage 250 K spectrum of fig. 3. The impact energy was approximately 1 eV, and the elastic intensity is not shown as it was too intense to be measured. 0
I
1000
age state depends only on coverage, not temperature. In fig. 6, a small intensity in the v(Pt-N) region was seen, which was absent in the spectrum of fig. 3, and the relative intensity of the G,(HNH) band near 1580 cm-’ was larger than that shown in fig. 3. The frequencies, however, especially the 100 cm-’ splitting in the v(NH) region, are in good agreement. We can conclude that the ammonia spectra depend primarily on coverage, and low coverage spectra produced by smaller doses at 100 K are essentially identical to low coverage spectra after heating from 160 to 450 K.
4. Discussion The results presented here show a clear distinction between two chemisorbed forms of ammonia on Pt(l1 l), and solid ammonia multilayers. We believe that the vibrational spectra are the first complete spectra (100-4000 cm-‘) published of an adsorbed ammonia molecule, and demonstrate the applicability of EELS to the study of catalytically important species. The interpretation of the low coverage state as a molecular species depends on rejecting a dissociative model of amrnonia adsorption. The vibrational spectra of NH and NH, species would be expected to be very similar to an adsorbed ammonia
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(ll
IJ. I
535
spectrum. For adsorbed NH, chemisorbed with C, symmetry, three modes would be active: v(Pt-N), &(NH), and v(NH). We would expect the spectrum to look similar to that of OH/Pt(lll) (22), the strongest band being the 6(NH) frequency. For an NH2 species with Czv symmetry, four modes would be observed: Q-N), s(HNH), v,(NH), v,(NH). In this case the &(HNH) frequency would be higher than the bending mode of NH, and the NH stretching modes would be split into a doublet with a closer spacing than NH3 [ 181. For the low coverage ammonia case, we observed a spectrum which is incompatible with NH or NH2 alone. In fig. 6, the doublet in the v(NH) stretching region could be due to NH 2, but two bands are observed near 1130 and 1580 cm-‘. For NH2 alone, only a single band near 1580 cm-’ would be observed [ 181. This is the case with adsorbed H,O/Pt(l 11) which has a 6(HOH) bending frequency near 1600 cm-’ [23]. The strong band near 1130 cm-’ could be interpreted as a bending mode of NH species, but only a single v(NH) stretch would be observed, not the doublet of figs. 4 and 6. A final possibility is that the low coverage ammonia consists of a mixture of NH and NH2 species. The spectral intensities and frequencies would be compatible with this assumption, but there is other evidence to suggest this is not the case. First, there is no significant change in the ratios of the intensities of the low coverage ammonia bands with coverage. This is more consistent with a single species than the case would be if dissociation occurred. Secondly, in independent experiments we have observed little, if any, hydrogen and nitrogen desorption from adsorbed ammonia on platinum (Ill) in the temperature range studied [24]. In addition, exchange experiments with D, gas show that the exchange mechanism for ammonia does not proceed via a dissociative intermediate such as NH or NH, [24]. Other data, including UPS and work function measurements, support the idea of two molecular states of ammonia on Pt(l11) [20]. In recent papers Ertl and co-workers have reported the identification of NH [6] and NH2 [S] species in ammonia adsorption on Fe surfaces. The main evidence for these species was UPS, although the spectra presented show only weakly resolved features. The thermal desorption spectra for NH,/Fe look very similar to our Pt(ll1) spectra. We believe that the EELS spectra are a more positive identification of the surface layer compared with UPS, and it would be interesting to examine the iron-ammonia system if NH and NH2 can be individually isolated. The two molecular states of NH3 on Pt(l11) are both only weakly chemisorbed, as evidenced by the small frequency shifts from the gas phase. In table 1 we observed frequencies for both states intermediate between ammonia gas and Pt(NHa)r, indicating weak overlap of the nitrogen lone pair 3A, orbital with the surface. The data also indicates weaker coordination at low coverage than at high coverage, a result contrary to the thermal desorption which shows higher desorption temperatures for the low coverage ammonia. In addition, the high coverage NH3 had strong v(Pt-N) and p,(NHa) bands, whereas these were absent in the low coverage case. This latter observation suggests that the two interaction potentials of the ammonia states are quite different. The EELS intensity depends on the
536
B.A. Sexton, G.E. Mitchell / Vibrational spectra of NHs/Pt(ll
I). I
dipole derivative, or dynamic dipole moment of a vibration [ 181, which is the change in dipole moment during vibration. The ~brational frequency depends on the force constant, which is dependent on the curvature of the potential well ]lS] . We can explain the absence of a @t-N) vibration in the low coverage NHa by assuming that the potential well of the low coverage adsorption site has a “flat bottom”, resulting in a very low force constant for the Pt-N vibration, perhaps below 200 cm-‘, where it would not easily be resolved. Alternatively the dipole derivative could be zero for this state, however most adsorbate-substrate vibrations show significant EELS intensities. We propose, therefore, that the low coverage ammonia is chemisorbed in a site with a deep, but “flat bottomed” potential well. ~~0~~ the ~monia is bound to the surface more strongly than at high coverage, the small curvature of the well potential results in a lowering of the v(Pt-N) frequency below the limits of detection (100-200 cm-“), Such a site could be a threefold site on the (111) surface where the atom in the second layer is missing. Coordination of NH3 with Cav symmetry in this site can explain the spectra. For the high coverage case, a more likely site is an “on-top” site where the nitrogen atom coordinates directly to a Pt atom, analogous to known Pt-NH, complexes. The similarity of the spectra for high coverage NH3 and Pt-NH3 complexes supports a linear coordination model, again with Cav symmetry. This model for low coverage ammonia on Pt(lll) is essent~~y in agreement with the one proposed by Purtell et al. [8] for NH,/IR(l 11). In their case, angle resolved photoemission was used to determine the adsorption site as a threefold hollow with the atom missing in the second layer. The only fact which remains to be explained is the apparent “weakening” of the coordination in the low coverage state, as evidenced by shifting of the frequencies toward the gas phase. It may be incorrect to assume that a simple frequency shift model derived from linear Pt-NH, complexes should apply to NH3 chemisorbed in a threefold hole on a surface. Further data is needed on other surfaces to resolve this problem. Further evidence, presented elsewhere [20], shows that the low coverage NH3 has a large work function change of 3 .O V, whereas the high coverage state has only a small contribution up to saturation. UPS spectra of the two states are also quite different 1201. Infrared studies of ammonia adsorption on supported platinum were carried out by Morrow and Cody [12] and Griffiths et al. [ll]. Morrow and Cody reported values of 3220 and 3360 cm-’ for room temperature adsorption of ammonia on Pt/SiOs. This is in excellent agreement with our values of 3240 and 3340 cm-’ for low coverage NH,. Morrow and Cody afso conducted isotopic exchange measurements and concluded that room temperature adsorption produced molecular NH3 on the Pt surface, not NH or NT&. This was based on the observation of two partially labeled species after interaction of Hz with ND3. We support the results of Morrow and Cody and believe that room temperature adsorption on Pt/SiO:! produces the same low coverage state we observed on Pt( 111) in this study.
B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NH3/Pt(lll).
I
537
4. Conclusions (1) There are two molecular states of ammonia chemisorbed on Pt(ll1) between 120 and 450 K. Below 120 K, solid ammonia multilayers may be formed. (2) Above 40% saturation coverage, the ammonia is coordinated to the Pt surface via the nitrogen atom, and the surface frequencies indicate weak overlap of the 3Ar orbital with the surface. Thermal desorption shows a weakly chemisorbed state (-9 kcal mol-r). The most likely coordination is to an “on-top” site. (3) Below 40% saturation, a second molecular state of ammonia is detected. The four normal modes of the molecule are shifted closer to the gas phase values than the high coverage state, indicative of weaker coordination. The v(Pt-N) band is not observed for this state, and it is possible that this form of ammonia is chemisorbed in a threefold site with a “flat bottomed” potential well, where the Q-N) frequency is very low (
Acknowledgments The authors wish to thank John Gland for useful discussions.
References [l] [2] [3] [4] [5] [6] [7] [8] [9] [IO] [ll] [12] [ 131 [14] [15] 1161
J.L. Gland, Surface Sci. 71 (1978) 327. J.L. Gland, .I. Catalysis 53 (1978) 9. D.G. Loffler and L.D. Schmidt, J. Catalysis 41 (1976) 440. D.G. Loffler and L.D. Schmidt, Surface Sci. 59 (1976) 195. M. Grunze, F. Boszo, G. Ertl and M. Weiss, Appl. Surface Sci. 1 (1978) 241. M. Weiss, G. Ertl and F. Nitschke, Appl. Surface Sci. 3 (1979) 614. C.W. Seabury, T.N. Rhodin, R.J. PurteIl and R.P. Merrill, Surface Sci. 93 (1980) 117. R.J. Purtell, R.P. Merrill, C.W. Seabury and T.N. Rhodin, Phys. Rev. Letters, in press. K. Kishi and W.M. Roberts, Surface Sci. 62 (1977) 252. L.R. Danielson, M.J. Dresser, E.E. Donaldson and J.T. Dickinson, Surface Sci. 71 (1978) 599. D.W.L. Griffiths, H.E. HaIlam and W.J. Thomas, Trans. Faraday Sot. 64 (1968) 3361. B.A. Morrow and I.A. Cody, J. Catalysis 45 (1976) 151. H. Ibach, H. Hopster and B.A. Sexton, Appl. Surface Sci. 1 (1977) 1. B.A. Sexton, Chem. Phys. Letters 65 (1979) 469. B.A. Sexton, Surface Sci. 88 (1979) 299. B.A. Sexton, Surface Sci. 88 (1979) 319.
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B.A. Sexton,
G.E. Mitchell / Vibrational spectra of NHJ/Pt(lIl).
[17] K.H. Schmidt and A. Muller, Coord. Chem. Rev. 19 (1976) 41. [ 181 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination (Wiley, New York, 1978). [19] W. Ho, R.F. Willis and E.W. Plummer, Phys. Rev. Letters 40 (1978) 1463. [20] G.B. Fisher, to be published. [21] B.A. Sexton, J. Vacuum Sci. Technol. 16 (1979) 1033. [22] G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 (1980) 683. [23] B.A. Sexton, Surface Sci. 94 (1980) 435. [24] J.L. Gland, to be published.
I
Compounds